Design and Characterization of Lubricants Based on Functionalized Nanoparticles

INTERNATIONAL DOCTORAL SCHOOL OF THE USC Fátima Mariño Fernández PhD Thesis Design and Characterization of Lubricants Based on Functionalized Nanoparticles Santiago de Compostela, 2023 Doctoral Programme in Fluid Thermodynamics Engineering DOCTORAL THESIS DESIGN AND CHARACTERIZATION OF LUBRICANTS BASED ON FUNCTIONALIZED NANOPARTICLES Fátima Mariño Fernández INTERNATIONAL PHD SCHOOL OF THE UNIVERSITY OF SANTIAGO DE COMPOSTELA PHD PROGRAMME IN FLUID THERMODYNAMICS ENGINEERING SANTIAGO DE COMPOSTELA 2023 DECLARACIÓN DEL AUTOR/A DE LA TESIS D./Dña. Fátima Mariño Fernández Título de la tesis: Design and characterization of lubricants based on functionalized nanoparticles Presento mi tesis, siguiendo el procedimiento adecuado al Reglamento y declaro que: 1) La tesis abarca los resultados de la elaboración de mi trabajo. 2) De ser el caso, en la tesis se hace referencia a las colaboraciones que tuvo este trabajo. 3) Confirmo que la tesis no incurre en ningún tipo de plagio de otros autores ni de trabajos presentados por mí para la obtención de otros títulos. 4) La tesis es la versión definitiva presentada para su defensa y coincide la versión impresa con la presentada en formato electrónico. Y me comprometo a presentar el Compromiso Documental de Supervisión en el caso que el original no esté depositado en la Escuela. En Santiago de Compostela, 25 de agosto de 2023. Firma electrónica AUTORIZACIÓN DEL DIRECTOR / TUTOR DE LA TESIS Design and characterization of lubricants based on functionalized nanoparticles D./Dª. Josefa Fernández Pérez D./Dª. Enriqueta R. López Iglesias INFORMAN: Que la presente tesis, se corresponde con el trabajo realizado por Dª. Fátima Mariño Fernández, bajo nuestra dirección, y a utorizamos su presentación , considerando que reúne los r equisitos exigidos en el R eglamento de Estudios de Doctorado de la USC, y que como directoras de esta no incurre en las causas de abstención establecidas en la Ley 40/2015. De acuerdo con lo indicado en el Reglamento de Estudios de Doctorado, declaramos también que la presente tesis doctoral es idónea para ser defendida en base a la modalidad de Monográfica con reproducción de publicaciones, en los que la participación de la doctoranda fue decisiva para su elaboración y las publicaciones se ajustan al Plan de Investigación. En Santiago de Compostela, 23 de Agosto de 2023 I ABSTRACT The main objective of this PhD Thesis is to design and characterize efficient nanolubricants based on three polyalphaolefins (PAOs) and functionalized nanoparticles (NPs) for wind turbine gearboxes and transmissions of electric vehicles, EV. Preliminary tests and an in-depth study of the literature on time stability of nanodispersions containing chemically modified nanoadditives were carried out to analyze factors such as morphology, size and coating type, concluding that organic acid coated spherical metallic or ceramic oxide NPs with diameters lower than 20 nm show longer stability times and enhanced tribological results. Another analyzed factor was preparation method of the nanolubricant, finding that the evaporation method leads to long-lasting dispersions. Therefore, three NPs (ZnO-OA, 10 nm, TiO2, 5 nm, and SiO2, 8 nm) and two modifying agents (oleic acid, OA, and stearic acid, SA) were selected, to synthetize three coated spherical NPs: ZnO-OA, TiO2-OA and SiO2-SA NPs. All the coated NPs were morphologically characterized (TEM or SEM) and the coatings were verified by Fourier transform infrared spectroscopy and Raman. Polyalphaolefin base oils were selected for the preparation of the nanolubricants due to the excellent physical and chemical properties compared to other types of base oils. The specific PAOs used were chosen to match the needed working conditions of the applications; a highviscosity PAO, PAO40, for the gearboxes, and low-viscosity PAOs, PAO6 and PAO8, for the EV electric drivetrains. Tribological and thermophysical properties of PAO40 + ZnO-OA NPs nanolubricants were evaluated. The OA coating made possible the preparation of homogeneous nanodispersions at different concentrations (0.10-1.00 wt%) of ZnO-OA NPs in PAO40 oil, reaching 29 days of stability (0.25 wt%). Both density and viscosity values increased with the concentration of NPs, furthermore, the experimental results were compared with predictive models. Pure-sliding tribological tests were performed at 80 ºC using an Anton Paar MCR 302 rheometer equipped with a tribological ball-on-three-pins configuration testing module. The optimal concentration was 0.25 wt% of ZnO-OA, with 25 % coefficient of friction (COF) reduction and 82 % crosssectional area reduction, respect to those obtained with the neat base oil. Rolling, mending, polishing, and tribofilm formation mechanisms owing to the spherical shape of the nanoadditives, the reduced roughness and the presence of PAO40, ZnO-OA NPs and iron oxides, that was evidenced from confocal Raman microscopy on the worn surfaces, explain the better tribological performance of the optimal nanolubricant with respect to that of neat PAO40. PAO8 nanodispersions of TiO2-OA were formulated with concentrations between 0.10 wt% and 0.50 wt% of TiO2-OA NPs, all of them also contained 0.20 wt% OA as a dispersant to enhance stability. The OA coating and dispersant increased the stability times from 24 h (TiO2 NPs) to at least 4 weeks (until 0.35 wt% TiO2-OA). The thermophysical analysis of PAO8 + 0.35 wt% + 0.20 wt% OA showed almost no variation on the density values, and an increment up to 15 % on the dynamic viscosity due to the addition of both additives. Tribological experiments were performed under pure sliding and rolling-sliding conditions at 120 °C. All FÁTIMA MARIÑO FERNÁNDEZ II the nanolubricants showed lower friction coefficients than that obtained with the PAO8 base oil, reaching maximum reductions for the 0.35 wt% TiO2-OA nanolubricant, for both types of test conditions. Under pure sliding conditions the highest wear reductions were 26 %, 65 % and 73 %, in wear track width (WTW), wear track depth (WTD) and cross-sectional area, respectively. Through confocal Raman microscopy and roughness study of the worn samples, it can be inferred that tribofilm, mending and polishing mechanisms occur. On the other hand, nanodispersions of (0.05–0.30 wt%) SiO2-SA NPs in PAO6, that also contained SA as a dispersant, were studied. The stability reached up to 100 days for the nanodispersion of PAO6 + SiO2-SA NPs + SA, while only having 48 h stability the nanodispersions of PAO6 + SiO2 NPs. Concerning the thermophysical characterization, the PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA had no effect on the density of PAO6 and a maximum increment of 13 % in the dynamic viscosity. The same tribological tests as in the TiO2-OA work were performed. The optimum concentration for COF reduction was 0.30 wt% for both tribological conditions. The best antiwear results were achieved with PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA with reductions of 55%, 86% and 92%, in WTW, WTD and wear area, respectively. Tribological mechanisms of the nanoparticles have been analyzed through roughness measurements, concluding that polishing, mending, and tribofilm formation occur. Moreover, the effect of the coating and the dispersant were evaluated separately, concluding that the SA dispersant can act as an antifriction additive but does not reduce wear, additionally, there is a synergistic effect between the SA and the SiO2-SA NPs; concerning the effect of the coating, it was verified that the presence of a SA layer can help the SiO2 NPs enhance the tribological properties. Finally, the nanodispersion PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA, was compared to PAO6 + 0.20 wt% zinc dialkyl dithiophosphate, proving that the nanodispersion has better tribological properties than the mixture containing the commercial additive. In conclusion, this thesis fulfilled its objectives, developing potential nanolubricants based on PAOs formulated NPs chemically modified with amphiphilic molecules that improved the stability of the nanodispersions compared to the corresponding ones with bare NPs and effectively reduce friction and wear in tests under laboratory conditions. It is also found that the addition of the same organic acid as dispersant as well as modifying agent leads to excellent synergies for tribological enhancements and long-lasting stabilities. ABBREVIATURE LIST IX SPAN 80 Sorbitan monooleate SRR Slide-to-roll ratio TEM Transmission electron microscope TGA Thermogravimetric analysis TMPTO Trimethylolpropane trioleate TOA N,N-dioctyl-1-octanamine U Expanded uncertainties Us Entrainment speed USC University of Santiago de Compostela UV-Vis Ultraviolet-visible VFT Vogel-Fulcher-Tammann VI Viscosity index Wrate Wear rate WTD Wear track depth WTW Wear track width WVol Wear volume XRD X-ray diffraction ZDDP Zinc dialkyl dithiophosphate η Dynamic viscosity ϕ Volume fraction  Mass fraction Λ Specific film thickness ρ Density XI INDEX 1. INTRODUCTION, STATE OF ART AND OBJETIVES………………………. 1 1.1. Introduction……………………………………………………………………. 1 1.2. Nanodispersions and their stability……………………………………………. 5 1.2.1. Techniques used to improve the nanolubricant stability………………… 5 1.2.2. Techniques to evaluate the nanolubricant stability……………………….. 7 1.2.3. Role of the nanoparticles on the lubrication mechanism……………........ 8 1.3. Chemically modified nanomaterials as lubricant additives: time stability, friction, and wear………………………………………………………………….... 9 1.3.1. Stability…………………………………………………………………... 11 1.3.2. Tribology…………………………………………………………………. 19 1.3.3. Overview…………………………………………………………………. 28 1.4. Thesis framework……………………………………………………………… 29 1.5. Objectives……………………………………………………………………… 32 1.6. Thesis structure………………………………………………………………... 32 1.7. References……………………………………………………………………... 32 2. MATERIALS AND METHODS………………………………………………... 45 2.1. Materials………………………………………………………………….…… 46 2.1.1. Selection of materials…………………………………………………….. 46 2.1.2. Synthesis and functionalization of nanoparticles………………….…....... 48 2.2. Characterization techniques………………………………………………........ 50 2.2.1. Fourier transform infrared spectroscopy…………………………….……. 50 2.2.2. Electron microscopy…………………………………………………....... 50 2.2.3. Confocal Raman microscopy……………………………………….…….. 51 2.2.4. X-ray diffractometer…………………………………………………....... 51 2.2.5. Elemental analysis………………………………………………………... 52 2.3. Preparation and characterization of the nanodispersions………………………. 53 2.3.1. Preparation method………………………………………………….……. 53 2.3.2. Stability of the nanodispersions…………………………………….…….. 55 2.3.3. Thermophysical characterization……………………………………........ 55 2.4. Tribological characterization………………………………………………….. 56 2.4.1. Anton Paar T-PTD200 tribological cell…………………………….…….. 56 2.4.2. EHD2 tribometer…………………………………………………………. 58 2.4.3. Hommelwerke contact profilometer…………………………………........ 61 2.4.4. 3D optical profilometer Sensofar S Neox……………………………....... 61 2.5. References…………………………………………………………………….. 63 SECTION I. NANOLUBRICANTS FOR GEARBOXES………………………… 67 3. ZINC OXIDE NANOPARTICLES COATED WITH OLEIC ACID AS ADDITIVES FOR A POLYALPHAOLEFIN LUBRICANT……………………... 67 3.1. Nanoparticle synthesis and characterization…………………………….…….. 68 3.1.1. Synthesis and functionalization of nanoparticles……………………........ 68 FÁTIMA MARIÑO FERNÁNDEZ XII 3.1.2. Characterization of nanoparticles……………………………………........ 69 3.2. Stability results………………………………………………………….……... 72 3.3. Thermophysical properties……………………………………………………. 73 3.4. Tribological results……………………………………………………………. 77 3.4.1. Pure sliding tests…………………………………………………….……. 77 3.4.2. Wear surface characterization…………………………………….………. 78 3.5. Conclusions……………………………………………………………………. 83 3.6. References……………………………………………………………………... 83 SECTION II. NANOLUBRICANTS FOR ELECTRIC DRIVETRAINS IN ELECTRIC VEHICLES…………………………………………………………… 87 4. TITANIUM OXIDE NANOPARTICLES COATED WITH OLEIC ACID FOR TRIBOLOGICAL ENHANCEMENT FOR ELECTRIC VEHICLE LUBRICANTS……………………………………………………………………... 87 4.1. Nanoparticle synthesis and characterization…………………………….……. 88 4.1.1. Functionalization of nanoparticles……………………............................... 88 4.1.2. Characterization of nanoparticles………………………………….…….. 89 4.2. Stability results……………………………………………………………........ 90 4.3. Thermophysical properties…………………………………………………….. 93 4.4. Tribological results……………………………………………………………. 94 4.4.1. Pure sliding tests…………………………………………………….…… 94 4.4.2. Wear surface characterization……………………………………….…… 95 4.4.3. Rolling-sliding conditions………………………………………………... 99 4.5. Conclusions……………………………………………………………............ 101 4.6. References…………………………………………………………………….. 102 5. STEARIC ACID COATED SILICON OXIDE NANOPARTICLES AS ADDITIVE FOR A LOW VISCOSITY PAO LUBRICANT……………………… 105 5.1. Nanoparticle synthesis and characterization…………………………………… 106 5.1.1. Functionalization of nanoparticles…………………….............................. 106 5.1.2. Characterization of nanoparticles……………………………….………... 107 5.2. Stability results……………………………………………………………........ 109 5.3. Thermophysical properties…………………………………………………….. 111 5.4. Tribological results…………………………………………………………….. 112 5.4.1. Pure sliding tests………………………………………………….……… 112 5.4.2. Wear surface characterization…………………………………….……… 112 5.4.3. Rolling-sliding conditions………………………………………………... 119 5.5. Conclusions……………………………………………………………............. 121 5.6. References……………………………………………………………………... 122 6. CONCLUSIONS AND FUTURE WORK………………………………............ 125 APPENDIX A. PUBLICATIONS AND CONFERENCES………………………... 129 APPENDIX B. PRELIMINARY RESULTS………………………….…………… 135 APPENDIX C. INDEX OF TABLES AND FIGURES………………….………… 137 APPENDIX D. RESUMO………………………………………………….………. 143 1 1 INTRODUCTION, STATE OF ART AND OBJETIVES The results presented in this chapter are mainly related to the following publication (the publisher authorization for the use of this publication is in the Appendix A): F. Mariñoa, J. M. Liñeira del Ríoa,b, E. R. Lópeza, J. Fernándeza. Chemically modified nanomaterials as lubricant additive: Time stability, friction, and wear. Journal of Molecular Liquids, (2023) 382, 121913. (Open access) https://doi.org/10.1016/j.molliq.2023.121913 a Laboratory of Thermophysical and Tribological Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics and Institute of Materials (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain b Unidade de tribologia, vibraçoes e manutençao industrial, INEGI, Universidade do Porto, Porto, Portugal The main contributions of the PhD student to this study are explicitly indicated below: Experimental: Data curation, Investigation, Methodology Manuscript: Writing original draft, Writing – review & editing, Formal analysis Some conclusions of this chapter related with low viscosity nanolubricants are included in: F. Mariñoa, J. M. Liñeira del Ríoa,b, E. R. Lópeza, J. Fernándeza. Influence of the nanoparticle coating agent on stability time and tribological performance on potential e-transmission nanofluids, Proceedings of the eightth “International Conference on Lubrication, Maintenance and Tribotechnology”, LUBMAT 2023, Preston, UK, 17th - 19th July 2023. a Laboratory of Thermophysical and Tribological Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics and Institute of Materials (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain b Unidade de tribologia, vibraçoes e manutençao industrial, INEGI, Universidade do Porto, Porto, Portugal The main contributions of the PhD student to this study are explicitly indicated below: Experimental: Data curation, Investigation, Methodology Manuscript: Writing original draft, Writing – review & editing, Formal analysis 1.1 INTRODUCTION At present, around 23 % of the total energy consumed in the world is owing to friction and wear occurring in the tribological contacts of mechanical elements [1]. Of that, around 87 % is used to overcome friction, and around 13 % is used to remanufacture worn parts and spare equipment owing to the wear. According to Holmberg and Erdermir [1] calculations, worldwide, the overall economic costs due to tribological contacts in all societal sectors in 2017 were around FÁTIMA MARIÑO FERNÁNDEZ 2 2500 billion euros/year of that around ¾ are owing to friction and ¼ is owing to wear. Furthermore, the amount of CO2 emissions generated from friction and wear was estimated in more than 8000 MtCO2/year. The friction loss of the total energy used in the transportation sector is the greatest, being about 30 % of the total energy use, in comparison with 20 % in the manufacturing and power-generating industry and just 10 % in the residential sector. Moreover, friction also causes several problems, such as environmental contamination, and might contribute to increased corrosion. Consequently, the reduction of friction and wear plays a crucial role in the extension of mechanical equipment service life, in saving energy and decreasing emissions [1]. Lubrication is considered one of the most effective ways to save energy and increase the effectiveness of devices used in industries of different sectors [1,2]. Thus, lubrication is the procedure used to reduce friction and wear between two surfaces that move at a very short distance from each other, through the interposition of a substance called lubricant. A lubricant is a multicomponent chemical mixture of different base oils and additives in a proportion around 90 % and 10 %, respectively. Many researchers considered diverse technologies to develop new procedures to substitute traditional environmental harmful additives that cause adverse emissions and contain sulfur or phosphorous (for instance zinc dialkyldithiophosphate) studying other additives like many nanoparticles (NPs) [3-8]. However, unlike traditional additives currently used in the field of lubrication, information about the effect of NPs on the environment is still lacking. Numerous studies show that several NPs used as additives have better tribological properties than traditional additives in lubricant bases used in different applications, such as wind turbines [9,10], automobile engines [11-15], transmission fluids for electric vehicles [16,17], cutting tools [18], among others. In fact, adding a small quantity of NPs to a lubricant usually has favorable effects on both friction and wear reductions. Such improvement is owing to their small size, since nanoadditives can enter the contact area, causing a positive lubrication effect [15]. One of the main benefits of NPs as lubricant additive is their low volatility that prevents NP losses under high temperature conditions [19]. Furthermore, NPs are less chemically reactive than common additives, since their films are mechanically formed, so they do not react with other additives and, therefore, both the NPs and the other additives will be more durable [19]. Nevertheless, NPs still present several problems that limit their use in real applications in the industry. One of the main challenges of using NPs as additive is to achieve homogeneous and stable dispersions over time. The stability of a nanolubricant is the time that the NPs are suspended in the base oil without settling. Stability is a drawback that restricts both development and technical application of nanolubricants. Preparation of stable nanolubricants is a key step to achieve their use in industrial applications, such as in gearboxes or bearings of wind turbines or for lubrication of hybrid or electrical vehicles since long-term stability is needed [10,16,20-22]. After aggregation, sedimentation occurs. NPs tend to aggregate due to attractive van der Waals forces acting on each other [6,11]. To obtain a stable nanofluid, it is essential to overcome the attractive interaction between particles, which must be compensated with other types of forces [12]. A common solution is the addition of surfactants which work through two types of mechanisms: electrostatic stabilization with ionic surfactants that prevents agglomeration due to the presence of a double layer of electrical charges around the NPs, and steric stabilization in which surfactant molecules with long hydrocarbon chains surround the NPs create a steric barrier that separates the NPs from each other [23-26]. Electrostatic stabilization is the dominant mechanism of water-based dispersions. The use of surfactants may hinder the reduction of friction and wear produced by NPs [27], changes the formulation of the Chapter 1 3 lubricant, and also does not usually lead to long-term stability (several months) [28]. Regarding oils, in addition to the use of non-ionic surfactants, there is another strategy to increase the stability time of the nanodispersions, which consists of the chemical modification of the NPs [29-31]. Of these two existing strategies [28], in this work we focus on the modification through covalent bonds of the surface of NPs with molecules of different type as organic acids, amines, silanes, organic phosphates, polymers, alcohols, among others (Figure 1.1), the first three being the most common (Figure 1.2). In all these cases, the molecules are chemically bonded to the surface of the NP and, in addition to their functional group that reacts with its surface, have long nonpolar chains at the other end of the molecule, which are capable of stabilizing the NPs in the oil [28]. Among the main conclusions of Chen et al. [28] review is that chemical coating with amphiphilic molecules is appropriate for stabilizing NPs with size smaller than 50 nm whereas for larger NPs, alkoxysilanes are better. These authors also indicate, based on theoretical analysis, that steric stabilization plays a more important role than other mechanisms because of the nonpolar nature of most of lubricating oils. On the other hand, the way most solvents perform is like dissolves like, thus it is generally agreed that a nonpolar compound will better solubilized in a nonpolar base fluid rather than in a polar one [23], although this is not exactly the case of nanomaterials, dispersing rather than dissolving in the base fluid. This rule has been modified to like seeks like, then including interactions with surfaces [32]. Thus, this rule, referred to nanomaterials, means that solvents will better disperse nanomaterials with similar surface chemistry [33]. Figure 1.1 Chemical structure of different NP modifying agents used in the literature. The names of the most common of each type are underlined FÁTIMA MARIÑO FERNÁNDEZ 4 Figure 1.2 Number of publications (those until February 2022 that include information and evidence about stability time using the search engine Google Scholar) for each type of modifying agent based on the publications evaluated in Section 1.3 The base oils are categorized by the American Petroleum Institute (API) into five categories depending on their physical properties and composition [34]. The API groups I, II and III include mineral base oils refined from crude petroleum oil and are differentiated by viscosity index (VI), content of saturated carbon chains, and sulfur content. The API group I oils are obtained by solvent refining or separation processing, meanwhile those of groups II and III are obtained by conversion or hydroprocessing technologies or by a combination of solvent and hydroprocessing technologies. The API group IV designates polyalphaolefins (PAOs), which are synthetic oils. Finally, the API group V is assigned to all the base oils not covered in the other categories, including naphthenic base oils, esters, silicones, glycols, polyalkylene glycols, vegetable oils, etc. PAOs have been used in lubrication since the 1950s and are the most common synthetic base oils nowadays [35]. They are prepared under controlled conditions from synthetic alphaolefins, mainly 1-decene. These base oils are named by their acronym followed by their viscosity grade, which is close to the kinematic viscosity in centistokes at 100 ºC, i.e., PAO40 is a polyalphaolefin with a kinematic viscosity around 40 cSt at 100 ºC. PAOs can overcome the limitations of mineral oils, so the industry is shifting from mineral oils to these synthetic alternatives. Moreover, PAOs have better physical and chemical properties and a wider temperature operating range comparing with those of mineral oils with similar viscosities at 100 ºC. Among these properties are lower volatility, higher flash point, lower pour point, higher thermal stability, and higher oxidative stability. In addition, PAOs are non-toxic to aquatic environments (up to 49500 ppm) and low viscosity PAOs are quite biodegradable (PAO2-4) and non-toxic to mammals (PAO2-10) [36]. Regarding API group V base oils, this introduction focuses on esters, polyalkylene glycols and vegetable oils. Firstly, ester oils can be a natural or a synthetic product, natural ester oils have been used in lubrication since ancient times [37]. Esters can be synthesized by chemical reaction 0 2 4 6 8 10 12 14 16 Number of publications Chapter 1 5 of a carboxylic acid and an alcohol, both can be mono-, bior polyfunctional acids or alcohols, thus a broad variety of esters can be obtained. Generally, ester lubricants have long hydrocarbon groups and their properties depend on the number and structure of these groups [38,39]. The molecular weight and relative concentration of the ester groups determine the viscosity and polarity of the ester oil. Secondly, polyalkylene glycols (PAGs) are oil soluble glycols, which make them desirable components for hydrocarbon-based lubricants [40]. The VI of polyalkylene glycols are typically higher than conventional API Group I-III mineral oils with similar viscosity grades and show more favorable low-temperature properties than API group III oils. Nevertheless, antioxidant additives are required due to the poor thermal oxidative stability. In addition, compared to API groups II, III and IV base oils, the PAGs show higher volatilities for similar viscosities. Thirdly, vegetable oils are also used as lubricants [41] due to the excellent lubricity, good anticorrosion capabilities, great viscosity–temperature behavior and low evaporation losses. Important advantages of these oils are their biodegradability and environmental safety compared to mineral or synthetic oils. Nevertheless, the use of vegetable oils as base stocks is limited due to their poor cold-flow behavior and low thermal oxidative stability [42]. 1.2 NANODISPERSIONS AND THEIR STABILITY Long-term stability is a fundamental requirement for nanolubricants to be used in real applications. To obtain stable suspensions of NPs in lubricant oils, there are some strategies presented below. In addition, techniques for evaluating stability time are discussed. 1.2.1 Techniques used to improve the nanolubricant stability -Preparation of nanodispersions: It is generally considered that there are two main methods for preparing nanolubricants: single-step method and two-step method [43-45]. In the former, the production of NPs and their dispersion in the liquid occur simultaneously [11]. Vapor deposition is the single-step method most frequently used: the raw material is heated and evaporated in a resistively heated crucible, the vapor is condensed into NPs when it contacts with a flowing low vapor pressure liquid [43,46]. If the raw material is a liquid or solid this procedure is based in physical vapor deposition (PVD), and it is named one-step physical method. With this method only small amounts of NPs can be synthesized, which limits their application on an industrial scale. Zhu et al. [47] developed the one-step chemical method by preparing nanofluids under microwave irradiation. Mineral-oil-based nanofluids including silver NPs with a narrow size distribution (9.5 ± 0.7 nm) were prepared by one-step chemical method, but surfactants were needed to stabilize the nanofluid [48]. The one-step chemical method combines the synthesis of NPs with the preparation of nanofluids. With this method it is possible to control the particle size, reduce the agglomeration of NPs, and produce nanofluids containing metallic NPs [49]. However, it is difficult to prepare nanofluids with a high concentration of NPs using this last method. Nevertheless, with the single-step methods good stability results as well as a small agglomeration of the NPs are usually achieved but they are quite expensive and if the reactants are not completely transformed into products, there will be impurities in the nanofluid [45], making difficult to clarify the effect of NPs on the nanodispersion properties without removing impurities. In addition, these residues could negatively affect the tribological behavior and dispersion stability of the formulated lubricating oil. Alternatively, in the two-step method, the NPs in dry powder form are incorporated into the base oil and the resulting dispersion is homogenized (Figure 1.3). Such homogenization is FÁTIMA MARIÑO FERNÁNDEZ 6 usually carried out by ultrasonic stirring. For this purpose, an ultrasound tip or/and bath are used and different parameters such as power, shaking mode or sonication time are modified until the optimal conditions are found. An advantage of this method is the easy control of size of the NPs, since a wide variety of NPs with different sizes are commercially available. The two-step method is the most broadly used to formulate nanolubricants and thermal nanofluids as it is the most economical [23]. Figure 1.3 Two-step method for the preparation of nanodispersions For nanodispersions containing ILs and NPs as hybrid additives of oils, Sanes et al. [50] designed a modified two-step method. In this case, nanopowders are added to the IL in an agate mortar being both mechanically mixed for 5 min and then mixed with the base oil. Finally, the nanodispersions are homogenized by means of an ultrasound bath. Using this method, Nasser et al. [51] obtained stabilities longer than eight months. Ali and Xianjun [52] proposed another method that includes mechanical stirring to mix the base oil and the IL with a magnetic stirrer and then adding the NPs to the liquid mixture using a probe sonicator and a ultrasonic vibration bath to disperse them. Stability times up to 70 days were obtained for the lowest concentrations of NPs. Another interesting method to improve the dispersion stability is the four-step method reported by Sui et al [29,53-55], which is based on an azeotropic distillation (Figure 1.4). Firstly, a dispersion of ethanol and NPs is mixed with toluene and, after the evaporation of ethanol at 70 ºC, PAO is added to the nanodispersion with fast stirring. Finally, the dispersion of the NPs in PAO is obtained by removing toluene by evaporation in an oven [55]. The stability results obtained through this method by Sui et al. [53] were better than those of the same dispersions prepared by the two-step method. Figure 1.4 Scheme of four-step method based on azeotropic distillation Recently, Liñeira del Río et al. [56] designed an efficient method to prepare nanodispersions (Figure 1.5), in which superparamagnetic NPs, suspended in cyclohexane after the synthesis, are transferred to another highly volatile solvent by centrifugation. Subsequently, the NP concentration was obtained by thermogravimetry, and the dispersion was added to the base oil and blended with an ultrasonic bath and an ultrasonic probe sonicator for a total time of 15 min. Then, the solvent was evaporated with a rotary evaporator, obtaining the superparamagnetic Chapter 1 13 Table 1.1 Stability of nanolubricants based on modified surface carbon-based nanomaterials Nanoadditive Functionalization Concentration Base oil Stability Ref. Graphene Nanosheets Fluorine NA GTL-8 7 days [110] GO DDA 0.1 wt% 5W30 32 days [80] GO ODA 0.04 g L-1 10W40 1 month [104] GO DDP 0.1–0.5 g L-1 Mineral oil 10 days [111] GO OA 0.5-2 wt% PAO12 7 days [112] rGO ODA 0.04 g L-1 10W40 >1 month [65] rGO ODA 0.05wt% + 1 wt% IL Ester Oil 28 days [113] rGO PEI-PAA or PEI-PSS 1 g L-1 Paraffin oil 2 months [105] rGO OTS 0.04 wt% Polyol ester 20 days [102] rGO Octadecyl alcohol 0.01 wt% Mineral oil 1 month [106] rGO Triazole 0.01 wt% 500N 1 month [114] rGO Reduction 0.01 wt% Mineral oil 45 days [95] rGO Reduction 0.50 wt% PAO6 4 days [68] rGO Reduction 0.25 wt% TMPTO and PAO40 10 days [67] MWCNT (20-40 nm) SA 0.45 wt% Paraffin oil 6 months [103] CNPs (25-35 nm) OM 1 wt% PAO10 3 months [115] CDs (4.2 nm) HDA 1 wt% PAO4 3 months [107] CDs (4-12 nm) N-OM N 1wt% 1 wt% PAO PEG 1 month 1 month [108] [108] CQDs (2 nm) DPA 0.2 wt% Castor Oil 5 months [109] CQDs (4 nm) DPA and OM 2 wt% PAO6 < 3 months [109] Functionalization of carbon nanomaterials leads to improve the stability of nanodispersions from days to months, mostly dispersed in mineral oils but also in ester oils, PAOs and formulated oils. Furthermore, usually reducing GO the stability increases [67,113]. Chen et al. [103] reported the highest stability time of all the nanolubricants from Table 1.1 (6 months). Nevertheless, in their article [103] the photographs included were taken two months after preparation. However, those photographs evidence that the modification of MWCNTs leads to a more stable nanodispersion, although their length/diameter ratio. This improvement can be explained by the appearance of favorable steric hindrance force, due to the alkyl chains, which reduces the unfavorable van der Waals interaction between MWCNTs. Functionalized metal NPs: these nanoadditives were functionalized with dialkyldithiophosphate, dodecanethiol, OM or OA (Table 1.2). Li et al. [116] synthetized surface modified Ag and Cu NPs, using dialkyldithiophosphate as a coating agent, indicating that their paraffin oil dispersions (0.5 wt%) kept unchanged for several months in ambient conditions or for 3 days at 140 ºC (no images are reported). Kumara et al. [117] synthetized dodecanethiol coated silver and palladium NPs finding that the dispersions with 0.5 wt% concentration in PAO4 (17 cSt at 40 ºC) are stable for several months, whereas at higher concentrations, up to 2 wt%, the time stability is only several days (no images are reported). Briefly, all these authors got dispersions with stabilities of several months for the NPs with diameters lower than 7 nm [116,117] whereas for dialkyldithiophosphate functionalized Ag NPs (15 nm) dispersions in a paraffin oil the stability is only five days [118]. FÁTIMA MARIÑO FERNÁNDEZ 14 Table 1.2 Stability of nanolubricants based on modified metal NPs Nanoadditive Functionalization Concentration Base oil Stability Ref. Ag (4 nm) Dialkyldithiophosphate 0.5 wt% Paraffin oil Several months (r.t.)* 3 days at 140 ºC. [116] Ag (3-6 nm) Dodecanethiol 0.5 wt% PAO4 Several months [117] Ag (15 nm) Dialkyldithiophosphate 0.5 wt% Paraffin oil 5 days [118] Cu (5 nm) Dialkyldithiophosphate 0.5 wt% Paraffin oil Several months (r.t.) 3 days at 140 ºC. [116] Pd (2-4 nm) Dodecanethiol 1.0 wt% PAO4 Several months [117] Ni (7 nm) OM and OA 0.025 wt% PAO6 1 month [119] *r.t.: room temperature Functionalized metal oxide NPs: Regarding these NPs, oleic acid is the most common functionalization although poly(lauryl methacrylate) (PLMA), 3glycidoxypropyltrimethoxysilane (GOPS) or 2-octyldodecyl gallate are also used (Table 1.3). Wright et al. [120] studied three dispersions in PAO4 of hairy TiO2 NPs densely grafted with PLMA which remained transparent and stable after 56 days at three different temperatures, - 20, 22, and 100 ºC, but the dispersion color at 100 ºC changed, which was attributed by the authors to the oxidation of a residual catalyst used in the functionalization of the hairy NPs. Liñeira del Río et al. [56] found the best stability among metal oxides, achieving stable TMPTO dispersions of OA modified Fe3O4 for at least 11 months due to the mutual affinity of the base oil which contains three oleate groups and the OA coating as well as the preparation method of the nanodispersions (Figure 1.5). Table 1.3 Stability of nanolubricants based on modified metal oxide NPs Nanoadditive Functionalization Concentration Base oil Stability Ref. TiO2 (15 nm) PLMA 1.0 wt% PAO4 56 days [120] TiO2 (20-25 nm) 2-octyldodecyl gallate 1.0 wt% PAO10 10W30 3 days 5 days [121] Al2O3 (78 nm) GOPS 0.05 wt% N.A.** 50 days [122] ZnO (10-30 nm) OA 0.5 wt% 60SN 12 hours [123] Fe3O4 (6.3, 10 nm) OA 0.015 wt% TMPTO 11 months [56] **Not available Functionalized metal sulfide NPs. Different dispersions of metal sulfides with some modifying agents were studied, such as silanes (OTS; dodecyltrichlorosilane, DTS; hexyltrichlorosilane, HTS), and OM (Table 1.4). Shahar et al. [124] investigated the dispersibility in a paraffin oil of inorganic fullerene-like tungsten disulfide (IF-WS2) NPs with the three different alkylsilane surface modifiers: OTS, HTS and DTS. The different silanized IF-WS2 NPs (size <150 nm) were dispersed in a paraffin oil at 1 wt%. Surprisingly, the dispersion of IF-WS2 functionalized with the silane with the longest alkyl chains, OTS (C18 chain) is stable only for 4 days, whereas that of IF-WS2 functionalized with DTS (C12 chain) is stable for more than 14 days and that with HTS (C6 chain) remained stable for 8 days. This could be due to differences in grafting-density. Jiang et al. [125] studied 2 wt% dispersions of OM modified WS2 nanosheets (size 6-8 nm with 0.276 nm between nanosheets) in PAO6 (30.6 cSt at 40 ºC), at ambient conditions the dispersions remain stable up to 6 months and, at 160 ºC, up to 7 days. Another article from Jiang et al. [126] reports studies on ultrathin WS2 nanosheets Chapter 1 15 capped by OM and maleic anhydride dodecyl ester (MADE), being their 2 wt% dispersions in dioctyl sebacate (DIOS, 11.5 cSt at 40 ºC) stable for 6 months at room temperature. Table 1.4 shows again the relevance of the size of the NPs in the stability of the nanodispersions. Table 1.4 Stability of nanolubricants based on modified metal sulfide NPs Nanoadditive Functionalization Concentration Base oil Stability Ref. IF-WS2 (< 150 nm) OTS DTS HTS 1.0 wt% Paraffin oil 4 days >14 days 8 days [124] WS2 (6-8 nm) OM 2.0 wt% PAO6 6 months [125] WS2 (6-8 nm) OM and MADE 2.0 wt% DIOS 6 months [126] Other functionalized NPs: other compounds used as nanoadditives for lubricants are silica (SiO2) NPs, h-BN, rare-earth compounds like LaF3 and mineral oxides like ZnAl2O4. Several modifying agents are used, such as organic acids (benzoic acid, BA; succinic acid, ScA or lauric acid, LA), amines (N,N-dioctyl-1-octanamine, TOA and ODA), OA, SA, polymers (PLMA, poly(alkyl)methacrylate) and silanes (octadecyltriethoxysilane, OTES; N1-(3trimethoxysilylpropyl)diethylenetriamine, DETAS and 3-(trimethoxysilyl)propyl methacrylate, 3-MPS) (Table 1.5). Liñeira del Río et al. [56] found the best stability, achieving stable TMPTO dispersions of OA modified Nd alloy NPs (Nd2Fe14B) for at least 11 months due to the mutual affinity of the base oil which contains three oleate groups and the OA coating as well as the preparation method of the nanodispersions (Figure 1.5). Li et al. [127] obtained a stability time even longer than 5 months for nanolubricants of three different silane-modified SiO2 NPs (APTES, GOPS or 3-MPS) in a gas mobile oil (0.3 wt%). Sui et al. [55] reported the stability of a hairy SiO2 NPs (HSNs) dispersed in PAO100 using an aminosilane ended in NH2: DETAS. This aminosilane is not amphiphilic. After two months, the nanodispersion with HSNs remained stable but not that of unmodified SiO2 NPs. Sui et al. [29,53] also investigated the effect of three different end modifications of DETAS HSNs adding BA, SA or ScA finding that their nanodispersions in PAO100 are also stable after 2 months standing. The nanodispersion of the DETAS HSNs modified with SA (18 C) presents the best optical transparency, which can be attributed to excellent affinity of the nonpolar functional groups of the NPs with PAO100. In addition, Sui et al. [54] analyzed the advantages of using silica NPs functionalized with both alkyl and amino terminated organic silanes (OTES and DETAS). The dispersibility characterized with DLS showed that as the amount of OTES increased, the dispersibility of these NPs in PAO100 improved. Seymour et al. [73] studied a series of PLMA HSNs and the effect of alkyl pendant length (6, 8, 12, 13, 16 and 18 C atoms) on their stability in PAO4. The unmodified silica NPs were not stable in this PAO. All the HSNs with alkyl pendant length of more than eight carbons were readily dispersed in PAO and presented stability at room or high temperatures such as 80 ºC for up to 60 days. However, C6and C8-grafted silica NPs (1 wt%) are not readily dispersible in PAO4 at room temperature. The C8 HSNs suspensions in PAO4 became clear and homogeneous upon heating at 80 °C. Interestingly, this cloudy-to-clear transition is reversible. These changes in the stability behavior can be described in terms of favorable brush solvation forces, which increase with the alkyl pendant length of the HSNs as well as the unfavorable interactions a) between PAO4 and silica NPs and b) between hairy NPs. Li et al. [116] synthesized NPs of dialkyldithiophosphate modified lanthanum trifluoride, which were dispersed in a paraffin oil (viscosity 36 mPa·s at 40 ºC) leading to a transparent lubricant with stabilities of several months in ambient conditions and for 3 days at 140 ºC. FÁTIMA MARIÑO FERNÁNDEZ 16 Literature results, showed in Table 1.5, bring out that SiO2 NPs with sizes from 15 to 200 nm led to quite stable nanodispersions when are functionalized with silanes or polymers. Generally, the longest the alkyl chain of the functionalization is, the most stable the nanodispersions. Furthermore, it is important to modulate the polarity of the coating depending on that of the base oil. Table 1.5 Stability of nanolubricants based on other modified NPs Functionalized nanocomposites: The modifying agents are organic acids (OA, SA), silanes (GOPS and APTES), and triazole (Table 1.6). Jiao et al. [132] compared uncoated alumina/silica (Al2O3/SiO2) composite NPs with GOPS modified Al2O3/SiO2 NPs as additives of a mineral oil and after 3 months the uncoated nanocomposites were mostly precipitated, and the modified ones stayed homogenously dispersed in the oil. Two main factors affect the best time stability results corresponding to GOPS modified Al2O3/SiO2 NPs: first the adequacy of the functionalization method (silanization) to the size of the NPs (70 nm) [28] and second, the fact that the surface properties of Al2O3/SiO2 nanocomposite changed, after modification, from hydrophilicity to lipophilicity [132], showing affinity for nonpolar oils. Using click chemistry, Farsadi et al. [133] synthesized a novel friction modifier: a nanocomposite of MoS2 NPs and a reduced graphene oxide functionalized with triazole (rGO-T). This nanocomposite was dispersed in a group II 500 N petroleum-based oil (viscosity 93 cSt at 40 ºC) remaining uniformly dispersed for one month. Regarding the lubricants containing nanocomposites, it has not broadly been investigated the stability of the nanodispersions with a given nanocomposite compared to that of the dispersions containing the corresponding separated NPs. Interestingly, Farsadi et al. [133] found that a MoS2 Nanoadditive Functionalization Concentration Base oil Stability Ref. LaF3 (8 nm) Dialkyldithiophosphate 0.5 wt% Paraffin oil Several months (r.t.) 3 days (140 ºC) [116] SiO2 (15-20 nm) APTES 0.3 wt% Gas mobile oil 5 months GOPS 0.3 wt% 5 months [127] 3-MPS 0.3 wt% 5 months SiO2 (<100 nm) DETAS 1.0 wt% PAO100 2 months [55] SiO2 (<200 nm) DETAS 0.5 wt% PAO100 < 2 months DETAS-SA 0.5 wt% > 2 months [29,53] DETAS-BA 0.5 wt% 2 months DETAS-ScA 0.5 wt% < 2 months SiO2 (100 nm) DETAS and OTES NA PAO100 4 months [54] SiO2, (24 nm) PLMA 1.0 wt% PAO4 55 days [120] SiO2 (23 nm) PLMA 1.0 wt% PAO4 2 months [73] SiO2 (58 nm) OA NA Paraffin oil 30 days [128] h-BN OA NA PAO8 3 days [129] h-BN (70 nm) BA ODA TOA NA NA 0.25 g L-1 SN500 5 days 1 day 6 days [130] ZnAl2O4 (95 nm) OA 0.5 wt% Lubricating oil Several days (70 ºC) [131] Nd alloy (19 nm) OA 0.015 wt% TMPTO 11 months [56] Chapter 1 17 nanodispersion shows sedimentation meanwhile the nanodispersion of MoS2/FrGO nanocomposite remains stable, so, the triazole modified rGO, rGO-T, is capable of stabilizing the MoS2 NPs. Table 1.6 Stability of nanolubricants based on modified nanocomposites Nanoadditive Functionalization Concentration Base oil Stability Ref. Al2O3/SiO2 (70 nm) GOPS 1 wt% Lubricating oil 3 months [132] MoS2 (50 nm)/rGOT Triazole NA Group II oil 1 month [133] ZnO/Al2O3 (63 nm) OA 1 wt% Mineral oil 28 days [134] Graphene/MTT APTES 0.4 g/L 15W40 1 month [135] Cu (5 nm)/GO SA 0.05 wt% Paraffin oil 10 days [136] Cu/rGO (50 nm) OA 0.5 wt% PAO10 7 days [137] Considering only those publications with information on the size of the NP, it is possible to relate the effect of their size, and the type of modifying agent with the stability time of the nanodispersions (Figure 1.7). For this purpose, five types of modifying molecules were considered: organic acids, amines, silanes, combinations of two types and other modifying agents. Most nanodispersions included in Tables 1.1 to 1.6 show stability times longer than 4 weeks (28 days). Among the nanodispersions with lower stabilities, more than half have NPs with sizes above 50 nm. Silanes are the most effective modifying agents for increasing the stability times for nanodispersions of NPs above 70 nm, achieving a maximum stability of 120 days for a dispersion containing OTES and DETAS silanized SiO2 NPs (100 nm) dispersed in PAO100 [54]. Despite the small size, some of the NPs with sizes below 30 nm have poor stability times, such as 2-octadecyl gallate modified TiO2 NPs (20-25 nm) dispersed in PAO10 or 10W30 [121], and dialkyldithiophosphate modified Ag NPs (15 nm) dispersed in paraffin oil [118], all of them with less than 5 days of stability. It is very unlikely that the three close hydroxyl groups of the 2-octadecyl gallate react with the NP surface, so the formed coated NPs have OH groups, i.e., higher polarity than PAO10 and 10W30. The polar groups of 2-octadecyl gallate and dialkyldithiophosphate are bulkier than those of organic acids or amines, which can lead to lower grafting densities. Most of the nanodispersions with stability times greater than 4 weeks contain NPs with sizes smaller than 30 nm with organic acids or amines as modifying agents, reaching stabilities of 11 months (330 days), in the case of an ester oil containing OA modified Fe3O4 NPs (6.3 or 10 nm) or Nd alloy NPs (19 nm) [56]. Other nanolubricants with stability times longer than 5 months are: paraffin oil + (MWCNT-SA (20-40 nm), Agdialkyldithiophosphate (4 nm), Cu-dialkyldithiophosphate (5 nm)), castor oil + CQD-DPA (2 nm), PAO4 + (Ag-dodecanethiol (3-6 nm), Pd-dodecanethiol (2-4 nm)), PAO6 + WS2-OM (68 nm) and DIOS + WS2-OM-MADE (6-8 nm) and gas mobile oil + SiO2-APTES, GOPS, or 3MPS (15-20 nm). Except MWCNT-SA, all the last nanoadditives are spherical (Tables 1.1, 1.2, 1.4 and 1.5). FÁTIMA MARIÑO FERNÁNDEZ 18 Figure 1.7 Effect of the size of the NPs and their modifying agent on the stability of their nanodispersions Another relevant factor to choose the appropriate modifying agent is the similarity in the polarities of the agent and of the base oil. For instance, an N-doped modification of CDs is more suitable for PEG (stability time of one month) meanwhile a further modification with OM to the N-doped CDs was needed for PAO4 (stability time of one month) [108]. Similarly, in the case of OM modified WS2 NPs [125,126] with a non-polar base oil like PAO6 a stability time of 6 months was achieved. A further modification of OM-WS2 NPs with MADE is needed to get the same stability time with a more polar ester base oil [126]. Figure 1.8 presents the effect of the type of NPs on the stability of the nanodispersions with stabilities longer than 28 days. Metal oxide NPs, specifically Fe3O4-OA NPs, and Nd alloy-OA NPs show the best stability times, more than 11 months [56]. Metal sulfide NPs, specifically OM or OM/MADE modified WS2 NPs, is the next type with the best stability, up to 180 days [125,126]. Small carbon nanomaterials, such as amine modified CQDs, also show some promising results, reaching up to 150 days of stability [109]. Other NPs, like silane modified SiO2 NPs, can achieve long stability times remaining stable for 150 days [127]. Figure 1.8 Effect of the size and type of the NPs on the stability of their nanodispersions Regarding the base oil and limiting the analysis to those nanodispersions with stabilities longer than 28 days, Figure 1.9 shows that PAOs are the most used base oil in the study of nanolubricants. However, nanodispersions based in PAOs show poorer stabilities compared to ester oils. Mineral oil nanolubricants show similar stability results to those of PAOs. Vegetable oil nanolubricants are scarce in the literature. The best results were obtained with an ester oil, specifically TMPTO, which contains oleate, thus having great compatibility with the oleic acid coating of the nanoparticles used in these cases [56]. 0 50 100 150 200 250 300 350 050 100 150 200 250 Stability time (days) NP size (nm) Organic acid Amine Silane Two types Others 0 50 100 150 200 250 300 350 050 100 150 200 250 Stability time (days) NP size (nm) Carbon nanomaterial Metal NPs Metal oxide NPs Metal sulfide NPs Other NPs Chapter 1 19 Figure 1.9 Effect of the size of the NPs and the base oil on the stability of their nanodispersions Another factor affecting the nanolubricant stability related to the base oil is its viscosity. Figure 1.10 presents the literature stability results that include information about the dynamic viscosity of the base oil at 40 ºC. The base oils used in most of these articles have low viscosities (less than 100 mPa s at 40 ºC) and correspond to the most stable nanolubricants [56,125,126]. That of high viscosity (1100 mPa s at 40 ºC) was PAO100 and its nanodispersions remained stable up to 120 days[29,53-55]. This trend could be related to the conclusion from Figure 1.9, PAO nanolubricants show lower stabilities compared to ester oils. Figure 1.10 Effect of the dynamic viscosity at 40 ºC of the base oils and the NP type on the stability of their nanodispersions In conclusion, multiple factors play a role in the stabilization of the nanoparticles, like modifying agent and base compatibility, type of base oil and its viscosity, as well as size, morphology, and concentration of the NPs. Thus, all these factors must be evaluated to reach the best possible stability results. 1.3.2 Tribology The stability of the coated NPs in the lubricants is not the only challenge, but also their tribological behavior, which is affected by many factors such as grafting density, size, shape, concentration, or composition [116,117,119,120] of the NPs. Furthermore, when the NP coating has polar groups at its external ends, tribochemical reactions can occur between those polar moieties and the positively charged metallic surface, facilitating the adsorption of the NPs onto the metallic surface [29,114,120]. This section reviews the tribological results reported in the literature for nanolubricants, containing chemically modified NPs, with stabilities of at least four weeks, discussed in Section 1.3.1, with the objective of providing knowledge on the characteristics that the NPs must have to give rise to strong reductions in friction and wear. This is one of the tools of this PhD thesis to contribute to cover this gap in science. Thus, the friction and wear reductions due to nanoadditives of carbon and its derivatives (Table 1.7), metals (Table 1.8), metal oxides (Table 1.9), metal sulfides (Table 1.10), nanocomposites (Table 1.11) and other NPs (Table 1.12) are summarized. 0 100 200 300 400 050 100 150 200 250 Stability time (days) NP size (nm) PAO Ester Mineral Vegetal 0 100 200 300 400 0 200 400 600 800 1000 1200 Stability time (days) η (mPa s) Carbon nanomaterial Metal NPs Metal oxide NPs Metal sulfide NPs Other NPs FÁTIMA MARIÑO FERNÁNDEZ 20 The different experimental conditions of the tribological experiments significantly affect both wear and friction values and highly difficult the comparison of the literature results. Standard measurement conditions should be defined by international associations of tribologists to better compare the antiwear and antifriction capabilities of the additives and lubricants from a global perspective. However, it is very difficult to establish standards that can generalize results because most of the tribologists make friction tests with conditions as close as possible to those of the real applications, taking into account also the characteristics of their tribometers. This issue can be partially arranged because in this paper friction and wear reductions with respect to those of the base oils were used. The dependence of the friction and wear reductions on the nanoparticle concentration, frequently shows a maximum at the usually named optimal concentration; for lower concentrations, the friction and wear reducing mechanisms due to NPs can partially (or even cannot) act since the quantity of NPs is insufficient, being the tribological behavior governed mainly by the base oil, while if the concentration is higher than the optimum one, the NPs can act as debris particles increasing friction and wear [114]. Wear can be characterized by different parameters such as wear volume (WVol), wear track width (WTW), wear track depth (WTD), or wear rate (Wrate). There are several tribological mechanisms by which nanoadditives improve lubrication [138], such as the formation of physical or chemical tribofilms leading to the protection of the tribopair from wear and a reduction of the COF [31,120,126]. The other mechanisms that reduce friction and wear are rolling effect, which only occurs when the NPs are spherical or cylindrical, mending effect, which consists in the introduction of NPs in micropits and grooves present on the metallic surface, and polishing effect, through which the lubricated surface roughness is reduced by abrasion due to NPs [4]. Carbon and its derivatives (Table 1.7): Sun and Du [139] studied the mechanisms by which graphene derivatives with two-dimensional structures, as GO or rGO, act as antifriction and antiwear nanoadditives. According to these authors, these NPs 1) can easily enter the friction pair contact region, being able to be effectively involved in lubrication because they easily shear; 2) can form friction transfer films and 3) can both fill the surface concave area, and adsorb at the roughness peaks, thus reducing the friction between the rubbing surfaces. The tribological performance of the lubricants containing this type of additives not only depends on the morphology and the type of coating but also on number of layers, and their size [140]. Paul et al. [80] analyzed the effect of applied load for a 5W30 oil and three of its nanodispersions containing DDA functionalized GO (0.01, 0.05, 0.10 wt%) obtaining the optimal concentration 0.10 wt% with reductions of 40% in COF and 50% in WTW under a load of 10 N, the lowest load analyzed. Mungse et al. [104] studied the antifriction and antiwear properties of GO functionalized with ODA as an additive for a 10W40 oil, presenting a 25 % reduction in both COF and WTW compared to those of 10W40 oil. Several articles shown the tribological properties of lubricants using rGO as additives. Samanta and Sahoo [105] examined the effect of a polymer-grafted rGO on the tribological behavior at different contact pressures and on load-bearing capacity of a paraffin oil. The highest COF reductions achieved under 147 N were 75 % and 50 % for rGOPEI-PSS and rGO-PEI-PAA respectively being the WVol reductions 69 % and 53 % compared to that obtained with the paraffin oil additivated with GO. Due to the higher packing density and the dispersion stability, the rGO modified with polymer brushes can more effectively reduce stress, compression, and shear than unmodified rGO, leading to improvements in antifriction, antiwear and load-bearing capabilities. Chapter 1 21 Ismail et al. [114] studied the tribological behavior of a base oil (group II 500 N petroleumbased oil) containing triazole ring-decorated rGO, FrGO, at three different concentrations, by means of four-ball tests. The nanodispersion with 0.01 wt% of FrGO led to the greatest COF and wear reductions, being respectively 16 % and 30 % (WTW) when compared to those of neat base oil. The aforementioned results show that several functionalities (ODA, PEI-PSS, octadecyl alcohol and triazole) of GO or rGO, enhance the stability of the nanoadditives in the oil and also lead to excellent tribological properties. On the other hand, Patel et al. [95] used a ball-on-disc instrument to measure the wear preventive characteristics of rGO dispersed at three different concentrations in a group II oil; the nanolubricant containing 0.05 wt% rGO nanoplatelets showed the greatest wear reduction, 52 % compared to the group base oil, as well as a reduction in friction of around 40 % at 60 rpm. Chen et al. [103] modified MWCNTs with SA (20-40 nm), dispersed them in a liquid paraffin (being the concentration 0.45 wt%), and studied the effect of the mass ratio of SA to MWCNTs (SA:MWCNTs) on tribological properties under a load of 1000 N, finding 2:1 as the optimum ratio to improve the friction reduction and antiwear capacity of base lubricant. These could be explained because SA is also a good lubricant; during the wear test, SA forms an effective film, but, due to the high temperature reached during the tests, for high SA concentrations it is decomposed and oxidized easily, increasing COF and wear. Furthermore, these authors also examined the effect of load on tribological properties using modified and unmodified MWCNTs nanodispersions (0.45 wt% and a SA:MWCNT mass ratio 2:1) obtaining the highest friction reductions and the best antiwear capabilities with the modified MWCNTs. This fact can be explained by the better dispersibility of modified MWCNTs; the tribopair surfaces were easily filled with the dispersed modified MWCNTs during the tribological tests, and then the NPs on the wear surface could help as spacers, avoiding rough contact between the two mating surfaces, thus greatly reducing the wear loss significantly (45 % at 1000 N). Nevertheless, the friction reductions are quite small: 9 % at 500 N and 4 % at 1000 N.Liang et al. [74] analyzed the tribological behavior of CDs-HDA (4.2 nm) as additives of PAO4, observing that for the load of 20 N and the nanolubricant with the optimal concentration (1 wt%), the COF and WVol decreased by 27 % and 46 %, respectively. These authors proposed that boundary lubrication films containing iron oxides and CDs-HDA are formed on rubbing surfaces by absorption and deposition but also act through rolling, mending and polishing mechanisms. Furthermore, Shang et al. [75] analyzed the tribological behavior of dispersions of N-CDs in a PEG as well as OM-N-CDs in a PAO finding that the COF and WVol of PEG with 1.0 wt% N-CDs are reduced up to 76 % and 83 %, respectively. In addition, for the OMN-CDs (1.0 wt% in PAO) 23 % and 46 % reductions in COF and wear were achieved. These authors suggested that N-CDs act as rolling ball bearings and could be irregularly deposited on the worn surface with mending effect and the formation of a tribochemical film which contains also Fe2O3 and nitrogen element, which prevented direct contact of the metal surfaces. Likewise, Lu et al. [76] studied the mass concentration effect of OM-CDs on tribological properties of PAO10, concluding that the 1.0 wt% OM-CDs nanolubricant showed the best friction-reducing and antiwear properties. Specifically, the COF and diameter of the worn scar were reduced by 47 % and 30 %, respectively. These authors suggested that the synergistic effect of a formed tribofilm which includes PAO and the OM-CDs might account for the good antiwear and antifriction capabilities under boundary lubrication. Finally, Ye et al. [109] evaluated the tribological performance of castor oil and PAO nanooils containing N-CQDs as additives, finding that for both oils the COF is not improved by adding these nanoadditives. FÁTIMA MARIÑO FERNÁNDEZ 22 Nevertheless, the wear is reduced by 45 % for the optimal concentration of 0.2 wt% N-CQDs (in comparison to the CO base oil) and 29 % for optimal addition of 0.5 wt% OM-N-CQDs (in comparison to PAO). These authors proposed that CQDs are deposited unevenly on the worn surface and form a thin protective film also containing iron oxides and nitrogen. In conclusion, it can be inferred that nanolubricants based on functionalized carbon derivatives act as contact mitigators between rubbing surfaces through the formation of effective tribofilms, which reduce friction and protects the surfaces against undesirable wear, especially when their coating contains polar groups, which promote tribochemical reactions with the positively charged metal surfaces. Table 1.7 Friction and wear reduction of nanolubricants based on modified carbon NPs Nanoparticle Functionalization Best concentration Base oil Friction reduction Wear reduction Ref. GO DDA 0.1 wt% 5W30 40 % ~50 % WTW [80] GO ODA 0.02 g L-1 10W40 25 % 25 % WTW [104] rGO ODA 0.05wt% + 1wt% IL Ester Oil 34 % 34 % WTW [113] rGO PEI-PSS PEI-PAA 0.5 wt% 0.5 wt% Paraffin oil Paraffin oil 75 % 50 % 69 % WVol 53 % WVol [105] [105] rGO Octadecyl alcohol 0.005 wt% Hydraulic oil 10 % 44 % WVol [106] rGO Triazole 0.01wt% Group II oil 16 % 30 % WTW [114] rGO Reduction 0.050 wt% Group II oil 40 % 52 % Wrate [95] MWCNT (2040 nm) Stearic acid 0.45 wt% Paraffin oil 4 % 45 % wear loss [103] CNPs (25-35 nm) OM 1 wt% PAO10 47 % 30 % WTW [115] CDs (4.2 nm) HDA 1 wt% PAO4 27 % 46 % WVol [107] CDs (4-12 nm) N 1 wt% PEG 72 % 83 % WVol [108] CDs (4-12 nm) N-OM 1 wt% PAO 23 % 46 % WVol [108] CQDs (2 nm) DPA 0.2 wt% Castor Oil COF increases 45 % WTW [109] CQDs (4 nm) DPA and OM 0.5 wt% PAO6 COF increases 29 % WTW [109] Functionalized metal NPs (Table 1.8): Li et al. [116] found improvements in the antiwear properties of a paraffin oil through the addition of dialkyldithiophosphate-coated copper or silver NPs. Wear analysis showed significant reductions with the addition of these NPs compared to that obtained using the base oil, especially at high loads. Specifically, paraffin oil nanodispersions containing 0.5 wt% dialkyldithiophosphate coated copper or silver NPs lead to WTW reductions of 58 % or 59 %, respectively. Antiwear properties of the dispersions containing dialkyldithiophosphate coated Cu NPs are better than those containing only zinc dialkyldithiophosphate (ZDDP), especially at lower concentrations. Moreover, Kumara et al. [117] performed tribological tests with dodecanethiol-modified silver or palladium NPs (Ag and Pd NPs) dispersed in PAO4 for a contact formed by a steel ball and a cast iron plate. Both Pd and Ag NPs led to substantial reductions of the COF (25–40 %) and wear of the cast iron plate (90–97 %) with the best reductions in both properties being obtained with Pd NPs, Chapter 1 29 1.4 PHD THESIS FRAMEWORK This PhD Thesis was carried out in the Laboratory of Thermophysical and Tribological Properties (LTTP) at the Applied Physics Department of the Universidade de Santiago de Compostela. This Laboratory is part of the Nanomaterials, Photonics and Soft Matter, NaFoMat, research group (GI-1488) of the Universidade de Santiago de Compostela. The NaFoMat group has received funding from the Consellería de Cultura, Educación e Ordenación Universitaria (Xunta de Galicia) as competitive reference group (GRC ED431C 2016/001 and GRC ED431C 2020/10). Since January 2019 NaFoMat participated in the Galician Strategic Grouping of Materials, AeMAT (AGRUP2018708), collaborating through the project “Development of nanolubricants based on functionalized 1D and 2D nanoparticles and nanomaterials (nanoLUBs)” with the NANOMAG laboratory. As a result of the achievements of AeMAT, in December 2020 the Institute of Materials of the University of Santiago de Compostela (iMATUS) was created. Among the topics of the iMATUS, the LTTP group works in the research line “Efficient energy systems: nanolubricants and advanced electrochemical storage devices”. This PhD Thesis was developed in the framework of two national research projects. The first one, titled “Development of hybrid nanofluids, nanolubricants and nano-enhanced Phase Change Materials for the transfer, storage and production of energy” (AdLuTer, 201830/9/2021, call “RETOS 2017”) coordinated together with the University of Vigo. The LTTP group specifically developed the subproject 2 “Advanced nanoadditive based lubricants for gears and motors” (ENE2017-86425-C2-2-R). This project was supported by the Spanish Ministry of Science, Innovation and Universities and the European Regional Development Fund (ERDF, FEDER in Spanish). The main objective of the AdLuTer project is the proposal of new advanced materials focused on the field of lubrication, storage, and transfer of thermal energy guide to renewable energies (solar, wind, hydraulic and geothermal) and for automotive applications. The main objective of the subproject 2 is to propose advanced lubricants based on nanodispersions for gears and motors in the field of wind, hydraulic and automotive transmissions, with attention on the aggregation of nanoadditives and on the control of the stability. The second is “Nanomaterials for Electric Vehicles: Transmission Fluids and coatings”, subproject 2 of the coordinated project “Innovative applications for nanofluids and nanomaterials in renewable energies and electric vehicles (NanoGeoVe)” awarded in the call RETOS 2020 (PID2020-112846RB-C22). One of the main goals of this subproject is the development of new nanolubricants formulated for the transmission systems of electric and hybrid vehicles. An important factor in the development of these new nanolubricants is the improvement of their stability through the chemical modification of the surface of the NPs using various modifying agents. Part of the work presented in this PhD Thesis (synthesis and functionalization of some nanoparticles) was performed in collaboration with two research groups from iMATUS, specifically, NANOMAG and R+D Pharma. Moreover, a part of this PhD Thesis was conducted during a stay of three months in the Instituto de Ciência e Inovação em Engenharia Mecânica e Engenharia Industrial (INEGI) of the Faculdade de Engenharia da Universidade do Porto (FEUP) under the supervision of Prof. Seabra. This stay has been financed by the IACOBUS 2021 Program, which focuses on promoting cooperation between higher education institutions of the Galicia - North Portugal Euroregion. In the framework of the above projects, this PhD Thesis focuses in two applications: gearboxes for wind turbines and automatic transmissions for electric and hybrid vehicles. In FÁTIMA MARIÑO FERNÁNDEZ 30 both cases, in the selection of the optimal lubricant, many factors must be considered, such as working temperature range, load, speed, materials [141]. The use of renewable energy sources, like wind or solar power, has gained relevance in the development of more efficient and environmentally friendly energy production systems [142]. Net power generation from wind power worldwide rose by more than 437 % over the 20102021 period [143]. New designs of larger and more powerful wind turbines have been developed, which need multi-stage gearboxes to multiply the speed until it is suitable for the generator [144]. Wind turbine gearboxes are mechanical components that rarely reach their design life (20 years) [145,146] because they are prone to pitting and fatigue failure [35]. According to the analytical study from 2008 to 2016 of the National Renewable Energy Laboratory, 76 % of the wind turbine failures occurred in the gearbox [147]. Due to the remote location of wind turbines and their large size, repair and maintenance costs for the gearbox are high [148], and during those procedures, wind turbines remain inactive for several days [149]. Selecting the right lubricants can increase the power output of wind turbines while reducing capital costs. Wind companies are demanding lubricants with longer life. Offshore wind farms require lubricants that are resistant to water contamination and perform well over a wide range of temperatures [150]. Approximately 30 % of the energy losses in wind turbine gearboxes occur in the rolling bearings. At high torque conditions, the friction between the meshing teeth causes higher energy losses. One of the aims of the selection of the gear oil is to prevent these losses as well as help dissipate heat, which further enhances the efficiency of the wind turbines. Excellent resistance to aging and oxidation for extended service intervals. All types of oils degrade with use, so it is important to maintain the lubrication conditions optimal for their operation; excellent resistance to aging and oxidation is required for extended service intervals. For this reason, it is important to improve the time of use of the lubricants to have a positive impact on maintenance costs. This is an important factor affecting the oil change interval, which is three times higher for PAOs than for mineral oils. Additionally, a reduction in wear also leads to an extension of oil service life; the presence of metal particles from the gearbox in the oil can act as catalysts in the oxidation reaction [151]. Nowadays, PAOs are the most widely used lubricants in wind turbine gearboxes [152-154]. PAO oils are considered to be the best option for gearboxes due to their great low-temperature properties, wide operating temperature range, high-load performance, and high oxidation resistance, withstanding temperatures as high as 250-300 ºC [35,151]. Another important factor in choosing the appropriate lubricant is its viscosity and anti-scuffing properties at temperatures above 80 ºC [155,156]. To effectively maintain the lubricant film when the working temperature is above 80 ºC, the use of synthetic oils is preferable [157]. In summary, the selected gearbox lubricant must offer a) excellent scuffing resistance at high loads; b) great micropitting resistance against fatigue damage; c) good compatibility with other materials (paints, elastomers, seals…); d) oxidative resistance for increased service life; e) appropriate viscosity to ensure sufficient lubricant film at elevated temperatures; f) good pumpability; g) wide operating temperature range [150]. It is a major challenge to reduce the wind energy losses by improving the tribological behavior of the gear oils, for this aim, new high-performance nanolubricants for gearboxes are being studied [10,51,60,150,152]. The electric vehicle (EV) has become an alternative transportation to reduce related emissions, which are 14 % of the total emitted greenhouse gasses, as stated by the Intergovernmental Panel on Climate Change [158]. The average energy efficiency of EVs generated from the electrical grid is significantly higher (77 %) than that from the total fuel Chapter 1 31 energy provided from internal combustion engine (ICE) vehicles (21 %) [2]. The increased performance of EVs results in a substantial reduction in CO2 emissions [159], particularly when using electricity produced from renewable sources [160]. Comparing to traditional ICE vehicles, mechanical parts of EVs work at higher speeds, loads, and temperatures, as well as, under electromagnetic fields [81,161]. One of the challenges of electric vehicles (EVs) is to extend their driving range, which could be done by improving the performances of the battery and lubricant [162]. The electric motor (EM) can be located in different positions depending on the type of EV [163,164]. If the EM is located within the transmission of the EV (that is, electric drivetrain), the electric transmission fluids (ETFs) interact with the EM, which requires that they meet several requirements: a) ability to limit corrosion of copper and compatibility with polymers of the housing and of the electronic components; b) low viscosity; c) suitable electric and thermal properties [164-166]. An adequate gear lubrication is key for transmission operation. Firstly, the right viscosity must be selected depending on the specific application [167]. The reason behind using low viscosity lubricants is the high torque and operational speeds of tribological elements in EVs. Electric motors used in EVs operate at high speeds, from 3000 to 16000 rpm, and high-performance motors reach over 20000 rpm [167]. In addition, by reducing the viscosity of the oil, viscous heating decrease and heat transfer is increased [81]. Reducing the lubricant viscosity leads to a shift from full film to boundary lubrication, which can lead to severe surface wear and affects the fatigue life of mechanical parts such as gears and bearings, decreasing the component durability [168]. Thus, improved anti-wear and anti-friction properties are needed. One of the most successful methods to reduce friction and wear is the use of nanoadditives in lubricants [22,169]. As far as we know, fluids for automatic transmissions (ATF), for automatic continuously variable transmissions (CVT), for automatic dual-clutch transmissions (DCT), and for automatic direct-shift gearboxes (DSG) are based on similar low-viscosity oils with different additive packages and could be used as starting point to develop transmission oils for electrified drivetrains. According to McCoy [170], the current standard for lubricating electrified vehicle drivetrains is automatic transmission fluids (ATFs), which is not optimized for EVs. Some properties of these ETFs would be common to those of ATFs, such as efficiency, durability, seal compatibility, or wide operating range, among others. However, other properties gain significance in ETFs, like oxidation stability, copper compatibility and electrical conductivity. Others, like traction properties (critical for wet clutches) lose importance in ETFs [167]. Nanoadditives used in conventional ICE lubricants reduce friction and wear [80,171,172]. However, studies on their performance under the tribological conditions of the ETFs in EVs are limited. Hence, Mustafa et al. [81] reviewed articles on low-viscosity nanolubricants for EVs, among them those based on PAOs (20-100 cSt at 40 ºC). These PAOs can withstand high temperatures, making them desirable for EV application not only as base lubricants but also as oil-coolants, which support temperatures up to 150 ºC in the transmission and EM housing [173]. In addition, the thermal conductivity and heat capacity of PAOs are 10 % higher than the corresponding mineral oils [35]. FÁTIMA MARIÑO FERNÁNDEZ 32 1.5 OBJECTIVES The main goal of this PhD Thesis is to contribute to the design and characterize efficient nanolubricants based on three PAOs and functionalized NPs for wind turbine gearboxes and EV transmissions. To achieve this aim, the following specific objectives are considered: 1. Selection, synthesis, and characterization of functionalized nanoadditives to design new potential nanolubricants. 2. Obtaining long-term stable nanolubricants of coated nanoadditives in PAOs. 3. Evaluation of thermophysical properties of the potential nanolubricants. 4. Evaluation of the tribological behavior under pure sliding conditions of nanolubricants and influence of concentration, type of functionalized nanoadditive, dispersant and comparison with commercial additives. 5. Study of the tribochemical processes occurring in the interface between the nanolubricant and the worn lubricated surfaces. 6. Evaluation of the tribological behavior of EV nanolubricants under rolling-sliding conditions. Suitable base oils have been chosen for these two applications and the temperature of the tribological tests has been adapted to each application, but many studies would be needed to propose specific formulations for EVs and gearboxes, since other properties would have to be measured, new tribological experiments would have to be carried out with more suitable speed and SRR ranges and other additives would have to be used. This is far from the objectives of this thesis, among other reasons because we do not have the necessary apparatus. 1.6 PHD THESIS STRUCTURE To describe the steps needed to achieve its goals this PhD Thesis is structured as follows. Once the motivation and background of this research is explained, the chapter 2 describes the procedure used to select the base oil, NPs, and functionalization agent, as well as the methodology used in this work. The results are presented in two sections depending on the application of the designed nanolubricant. The first section, Chapter 3, is focused on lubricants for wind turbine gearboxes. The second section, Chapters 4 and 5, is centered on transmission fluids for electric or hybrid vehicle transmissions. Finally, Chapter 6 presents the general conclusions of this doctoral thesis and future work related to it. Additional information is provided at the end of this dissertation in four appendices. 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Ul Haq, Tribological performance of avocado oil containing copper nanoparticles in mixed and boundary lubrication regime, Industrial Lubrication and Tribology 70 (2018) 865-871, https://doi.org/10.1108/ILT-06-2017-0166. [85] J. Taha-Tijerina, L. Peña-Paras, T.N. Narayanan, L. Garza, C. Lapray, J. Gonzalez, E. Palacios, D. Molina, A. García, D. Maldonado, P.M. Ajayan, Multifunctional nanofluids with 2D nanosheets for thermal and tribological management, Wear 302 (2013) 1241-1248, https://doi.org/10.1016/j.wear.2012.12.010. [86] J. Viesca, A.H. Battez, R. González, R. Chou, J. Cabello, Antiwear properties of carboncoated copper nanoparticles used as an additive to a polyalphaolefin, Tribology international 44 (2011) 829-833, https://doi.org/10.1016/j.triboint.2011.02.006. 45 2 MATERIALS AND METHODS The purpose of this section of the PhD Thesis is to summarize the materials and methods used during the research process. The main materials selected are base oils, NPs, and modifying agents. Regarding the methods can be summarized as techniques used to characterize the selected materials, the nanodispersions and the worn surfaces, as well as those used to the preparation of nanodispersions, the device used to thermophysical characterization and the tribometers. The techniques used and the procedure followed in this PhD Thesis are summarized in the flowchart in Figure 2.1. Some of the information presented in this chapter is related to the following publications (the publisher authorizations for the use of these publications are in the Appendix A): F. Mariñoa, E. R. Lópeza, A. Arnosab, M. A. González Gómez b, Y. Piñeirob, J. Rivasb, C. Alvarez-Lorenzoc, J. Fernándeza. ZnO nanoparticles coated with oleic acid as additives for a polyalphaolefin lubricant. Journal of Molecular Liquids, 348, (2022) 118401. (Open access) https://doi.org/10.1016/j.molliq.2021.118401 a Laboratory of Thermophysical and Tribological Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics, and Institute of Materials (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain b NANOMAG Laboratory, Department of Applied Physics, Faculty of Physics, and Institute of Materials (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain c Departamento de Farmacología, Farmacia y Tecnología Farmacéutica, R+D Pharma Group (GI-1645), Facultad de Farmacia, Instituto de Materiales (iMATUS) and Health Research Institute of Santiago de Compostela (IDIS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain The main contributions of the PhD student to this study are explicitly indicated below: Experimental: Investigation, Formal analysis. Manuscript: Writing – original draft. J. M. Liñeira del Ríoa,b, F. Mariñoa, E. R. Lópeza, D. E. P. Gonçalvesb, J. H. O. Seabrac, J. Fernándeza. Tribological enhancement of potential electric vehicle lubricants using coated TiO2 nanoparticles as additives. Journal of Molecular Liquids, 371, (2022) 121097. (Open access) https://doi.org/10.1016/j.molliq.2022.121097 a Laboratory of Thermophysical and Tribological Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics, and Institute of Materials (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain b Unidade de tribologia, vibraçoes e manutençao industrial, INEGI, Universidade do Porto, Porto, Portugal c FEUP, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal The main contributions of the PhD student to this study are explicitly indicated below: Experimental: Investigation, Methodology, Conceptualization Manuscript: Writing – review & editing FÁTIMA MARIÑO FERNÁNDEZ 46 Figure 2.1 Flowchart of the techniques and procedures used in this PhD Thesis 2.1 MATERIALS 2.1.1 Selection of materials The selection of base oils, NPs, and modifying agents was the first task of this work (Figure 2.1). As it was thoroughly discussed in Chapter 1, overall, polyalphaolefins (PAOs) have better physical and chemical properties, such as enhanced thermal and oxidative stabilities, compared to base oils belonging to groups I-III and V, being then more durable [1]. Thus, three PAOs were chosen as base fluids of the nanolubricants for the referred applications because are the most convenient base oils due to the working conditions required [2], as it was explained in Section 1.4. So, the specific polyalphaolefins shown in Table 2.1 were selected considering their application and availability: - Wind turbine gearboxes: a high viscosity PAO, PAO40. - Electric Drivetrains: two low viscosity PAOs, PAO8 and PAO6 [3]. Table 2.1 Thermophysical properties of the selected base oils Base oil Supplier Density (g/cm3) at 40 ºC Viscosity (mPa·s) at 40 ºC VI PAO40 Repsol 0.8345 332.53 150 PAO8 0.8163 39.47 138 PAO6 0.8114 24.84 138 The selection of the NPs was carried out taking into account all the literature reviewed in the previous chapter, as well as the preliminary work described in Appendix B. From Figure 1.7, and Tables 1.1-1.6 it was concluded that the best stability results are obtained when spherical NPs, smaller than 20 nm, are chemically modified with organic acids or amines with alkyl chains longer than 12C. Figure 1.8 and table 1.3 show that some coated metal oxides NPs smaller than 20 nm, lead to nanolubricants with long stabilities in PAO, ester and mineral oils. Furthermore, Figure 1.9 shows the stability results from the literature for PAOs for NP sizes up Chapter 2 47 to 200 nm, with the largest NPs being silane-coated and used for a high viscosity base oil (PAO100). Moreover, from our previous results reported in Appendix B, we concluded that silanization via sCO2 was quite unsuccessful, so the use of silanized NPs was discarded. Additionally, we also discarded carbon nanomaterials (graphene nanoplatelets and GO) due to poorer stability results (specially GO) even after their functionalization. From a tribological point of view, the effect of the nanoparticle and the modifying agent was also presented in Figure 1.11. Again, the best results for friction and wear reduction are obtained when the nanoparticle size is smaller than 20 nm. In addition, from the preliminary results (Appendix B) we also discarded other carbon nanomaterials such as modified GnPs due to their poorer tribological capabilities compared to the functionalized metal oxides NPs tested. Regarding the nature of the nanoparticle, metal or ceramic oxides are advantageous due to the ease with which they can react with organic acids or other functional groups such as amines or silanes. In presence of water, the oxidized surface of the NPs provides highly reactive hydroxyl groups that can react with the modifying agents. From Table 1.3 and 1.9, we chose two metal oxide NPs. One of them, TiO2 NPs, was selected because of the promising stability and tribology results shown in the Wright et al. article [4] but using a different low-viscosity PAO and a different coating agent. The other one, ZnO NPs, was selected to test whether similar ZnO-OA NPs could be dispersed for a longer period in PAO instead of a mineral oil [5]. From Table 1.5 and 1.11, we selected SiO2 NPs as a ceramic oxide due to the promising stability and tribology results obtained by other authors with polymer grafted SiO2 NPs in low-viscosity PAOs [4,6]. Thus, in this PhD Thesis, three types of spherical NPs, metal, or ceramic oxides, with sizes between 5 and 10 nm were selected as bare nanoadditives as indicated in Table 2.2. TiO2 and SiO2 NPs were purchased from US Research Nanomaterials, Inc. (Houston, TX, USA), whereas ZnO NPs were synthesized during this PhD Thesis. Table 2.2 Technical information about the selected NPs Nanoparticle Supplier CAS Number Purity Average size Morphology ZnO None* 1314-13-2 - 10 nm Spherical TiO2 US Research Nanomaterials, Inc. 1317-70-0 99.9 % 5 nm SiO2 7631-86-9 99 % 8 nm * Synthesized in the laboratory (Section 3.1.1) All the previously mentioned nanoadditives were functionalized before their addition to the PAO base oil to improve their dispersibility in the oil and the time stability of the nanodispersions. According with the literature reviewed in Chapter 1, the most common modifying agents are organic acids, amines, and silanes (Figure 1.2). For all these cases, the modifying agent is formed by a polar group and a non-polar aliphatic chain with at least ten carbon atoms. Organic acids, also known as fatty acids [7], are organic molecules formed by a carboxylic acid and an aliphatic chain which can be saturated or unsaturated. Taking into account the conclusions described in the previous chapter, in this PhD Thesis we used a saturated fatty acid, stearic acid (SA), an unsaturated fatty acid, oleic acid (OA). The molecular structure and technical information of the two modifying agents are indicated in Figure 2. and in Table 2.3. The chemically modified NPs that we have analyzed in this PhD Thesis are summarized in Table 2.4. FÁTIMA MARIÑO FERNÁNDEZ 48 Table 2.3 Technical information about the selected modifying agents Modifying agents Supplier CAS Number Purity Oleic acid (OA) Sigma-Aldrich 112-80-1 90 % Stearic acid (SA) 57-11-4 95 % Figure 2.2 Chemical structure of the modifying agents Table 2.4 Summary of the functionalized nanoparticles Bare nanoparticles Modifying agent Average size Coated nanoparticles ZnO OA 10 nm ZnO-OA TiO2 OA 5 nm TiO2-OA SiO2 SA 8 nm SiO2-SA 2.1.2 Synthesis and functionalization of nanoparticles Once the nanoparticle and the modifying agent have been selected, the next step was the synthesis and functionalization of those NPs to obtain the chemically modified NPs (Figure 2.1). All synthesis carried out follow some general guidelines for the synthesis of bare ZnO NPs and for the functionalization of the three NPs of Table 2.2: a fume hood (Figure 2.a) was used to carry out the reactions safely, an assembly for refluxing reactions (Figure 2.b), a round bottom flask coupled to a condenser that contains the reaction stirred with a magnetic bar in a silicone bath and a heating plate with magnetic stirring (Figure 2.3c,d). The solid reagents were weighted with a high precision Sartorius MC 210P Microbalance (Figure 2.4a) and the reaction products were separated and washed with the aid of a MPW M-Universal centrifuge (Figure 2.4b). In the case of ZnO NPs synthesis and functionalization, a Perkin Elmer Pirys 7 TGA of NANOMAG research group from the USC (Figure 2.4c) was used to know the concentration of NPs dispersed in a solvent. To know the amount of modifying agent on the surfaces of SiO2 (Figure 2.4d), a TGA/DSC 1 (Mettler Toledo) was used. In all the other cases, due to the unavailability of the TGA, a stove Memmert BM400 (Figure 2.4e) was used with a similar strategy, 1 mL of nanodispersion was weighted before and after evaporating the solvent. Chapter 2 49 Figure 2.3 Material used for the synthesis: (a) Fume hood; (b) Reaction set-up; (c) Magnetic hot plate LBX H03D; (d) Magnetic hot plate IKA RCT basic (NANOMAG) Figure 2.4 Used equipment during the synthesis and functionalization of the NPs: (a) High precision Sartorius MC 210P Microbalance; (b) Centrifuge MPW M-Universal; (c) Perkin Elmer Piryis THA (NANOMAG); (d) TGA/DSC 1 Mettler Toledo (RIAIDT of the USC); (e) Stove Memmert BM400 FÁTIMA MARIÑO FERNÁNDEZ 50 2.2 CHARACTERIZATION TECHNIQUES In this section, a description of several techniques used for the characterization of base oils, modifying agents and NPs, as well as worn steel pins used in the tribological measurements are presented. Most of these techniques belong to the Network of Infrastructures to Support Research and Technological Development (RIAIDT) of the University of Santiago de Compostela. 2.2.1 Fourier transform infrared spectroscopy Infrared Spectroscopy is a non-destructive technique based on the absorption of infrared radiation and the conversion of this energy in characteristic molecular vibrations for each chemical bond present on the tested compound [8]. A Fourier transform infrared (FTIR) spectrum can be displayed as a graph of absorbance (or transmittance) of infrared light on the vertical axis versus frequency, wavenumber, or wavelength on the horizontal axis. Thus, a FTIR spectrometer VARIAN 670-IR (Figure 2.5), coupled to an attenuated total reflectance accessory (ATR) was used to record the absorbance spectra of base oils, modifying agents and NPs. The most relevant application of FTIR in this PhD Thesis is to verify the efficiency of the surface functionalization of the NPs through the appearance of new chemical bonds between the nanoparticle and the functionalization molecules, as well as the disappearance of the hydroxyl peak of the carboxylic group of the fatty acids. Figure 2.5 Fourier transform infrared spectrometer: FTIR VARIAN 670-IR (RIAIDT of the USC) 2.2.2 Electron microscopy A scanning electron microscope (SEM) scans the analyzed surface with a focus beam of electrons. Due to the interaction of this beam with the sample secondary electrons are emitted and reflected from the surface, which are detected by a secondary electron detector producing signals of different intensities depending on the surface geometry, chemical characteristics, and bulk chemical composition [9]. The obtained image is therefore a distribution map of the intensity of the emitted signals from the scanned area [10]. The spatial resolution of a conventional SEM is 50-100 nm [9]. The instrument used to observe the morphology of the commercial TiO2 and SiO2 NPs, as well as the worn pin surface of those pins lubricated with PAO6 and its formulated lubricants, was a field emission scanning electron microscope (FESEM) which differs from a traditional SEM in the electron generation system, and provides images with a spatial resolution three to six times better [9]. The used Zeiss FESEM Ultraplus (Figure 2.6a) has an acceleration voltage range from 0.02 to 30 kV and resolutions of 1.0 nm/15 kV, 1.7 nm/1 kV and 4.0 nm/0.1 kV. A transmission electron microscope (TEM) is another electron microscope in which a beam of electrons is transmitted through the sample to form an image using a detector [11]. The image Chapter 2 51 formation is due to the interaction of the electrons with the sample during the scan and can reach atomic resolution [12]. To further analyze the morphology and size of the NPs a highresolution TEM JEOL JEM-1011 was used (Figure 2.6b) due to its enhanced capability to magnify the samples compared to SEM. To perform these analyses dry NPs were dispersed in a volatile solvent and placed in copper grids with carbon films. Subsequently, the TEM was configured with a high brightness lanthanum hexaboride (LaB6) source and an accelerating voltage between 40 and 100 kV. Figure 2.6 Electron microscopes (RIAIDT of USC): (a) Zeiss FESEM Ultra Plus SEM; (b) JEOL JEM-1011 TEM 2.2.3 Confocal Raman microscopy Raman spectroscopy is a commonly used technique in chemical characterization that provides a fingerprint of the analyzed samples from the vibrational modes of molecules [13]. A confocal Raman microscope combines the Raman spectral information with the spatial filtering of a confocal microscope, eliminating out of-focus signals [14], for high resolution chemical mappings of samples [15]. This device was used to characterize the PAOs and coated NPs through Raman spectroscopy, as well as to perform elemental mappings of the worn surfaces to identify the tribological mechanisms during the rubbing process, i.e., analyze the role the NPs have in the wear reduction by detecting the components present on the worn surface after a cleaning step with hexane solvent to remove the excess nanodispersion. A Witec alpha300R+ confocal Raman microscope (Figure 2.7) operating at 532 nm was used with two scanning systems: a coarse motorized stage for large areas and a piezo stage for subnanometer adjustments. This device gives chemical information and a mapping of the components of the scanned area. Figure 2.7 Confocal Raman Microscope: WITec alpha300R+ (RIAIDT of the USC) 2.2.4 X-ray diffractometer X-ray diffraction is an analytical technique that determines the three-dimensional geometry of crystalline materials using electromagnetic radiation (X-rays) [16]. The atomic structures of crystals are regular and repetitive through the whole structure. When the X-rays travel through FÁTIMA MARIÑO FERNÁNDEZ 52 this type of structure, diffraction occurs through the atomic or molecular layers conforming the crystal. These diffracted rays interfere with each other through constructive and destructive interferences. The patterns formed after the diffraction of those X-rays can be evaluated and areas specific for each material. A Philips PW1710 diffractometer (Figure 2.8) with a graphite diffracted beam monochromator and a copper radiation source (λ (Cu Kα) = 1.5406 Å), at 40 kV and 30 mA was used to determine the crystal structure of the synthesized NPs. Measurements were carried out for a 2θ angle from 10° to 80° every 0.02° and 10 s/step on the powder sample. Figure 2.8 X-ray diffractometer: Philips PW1710 (RIAIDT of the USC) 2.2.5 Elemental analysis Inductively coupled plasma mass spectrometry (ICP-MS) is an inorganic elemental and isotopic analysis technique able to determine and quantify most of the elements of the periodic table [17]. The procedure consists in the transformation of a liquid sample into ions that differ by their mass. For this PhD Thesis, the ICP-MS was used to quantitatively determine the concentration of Zn in the nanodispersion containing ZnO NPs modified with OA before and after a tribological test to identify the role of these NPs in the reduction of wear. A reduction in the NP concentration means that part of the nanoadditive remains on the worn surface by physical or chemical adsorption in the tribological process, while no changes in the NP concentration would indicate that there was no NP adsorption. An Agilent 7900x ICP-MS (Figure 2.9a) was used after a microwave-assisted acid digestion (Figure 2.9b) which was necessary due to the viscous nature of the sample. Figure 2.9 Equipment for elemental analysis (RIAIDT of the USC): (a) Inductively coupled plasma mass spectrometer Agilent 7900x; (b) Milestone ultraWAVE3 microwave digestion system Chapter 2 53 2.3 PREPARATION AND CHARACTERIZATION OF THE NANODISPERSIONS After the characterization of the base oils, NPs and modifying agents, as well as the chemically modified NPs, the next step is the preparation and characterization of the nanodispersions (Figure 2.1). For this purpose, several procedures were selected taking into account the conclusions of Chapter 1. The preparation method of Liñeira del Rio et al. [18] (Figure 1.5) was adapted for the preparation of most of the nanodispersions. The stability of the nanodispersions with time was evaluated using two methods: visual control and refractometry. Moreover, density, viscosity, and viscosity index at atmospheric pressure of the base oils and nanodispersions were measured at several temperatures. 2.3.1 Preparation method Most of the nanodispersions were prepared with similar procedures (Figure 2.10) with some differences according with the availability of some of the devices (Figures 2.11), such as the rotary evaporator or the TGA. In the case of OA modified ZnO NPs (ZnO-OA NPs), homogeneous dispersions in PAO40 were prepared following the procedure indicated in Figure 2.10a. At the end of the synthesis procedure, ZnO-OA NPs were dispersed in chloroform (Section 3.1.1) to avoid agglomeration being the concentration determined by TGA (2.12 wt%). Known amounts of this ZnOOA/chloroform dispersion and PAO40 were mixed. Subsequently, this blend was homogenized with both an OVAN ultrasonic bath (Figure 2.11a) for 15 min and a Branson ultrasonic Sonifier S-250A probe sonicator (Figure 2.11b) for 5 min at 200 W of effective power and 60 Hz sonication frequency with a 20 % amplitude. The next step was to evaporate the chloroform with a rotary evaporator (Figure 2.11e) at 60 ºC for 30 min. The obtained PAO40 + 1.00 wt% ZnO-OA dispersion was then further sonicated for 240 min using a Fisherbrand ultrasonic bath FB11203 (Figure 2.11c), in a continuous shaking mode at 180 W effective power and 37 kHz sonication frequency. Two replicates were prepared, one for the stability study, and the other for thermophysical and tribological characterizations. Diluted nanodispersions (0.10, 0.25, 0.50, 0.75 wt%) were obtained from the PAO + 1.00 wt% ZnO-OA nanodispersion adding PAO40. All the nanodispersions were further sonicated in the Fisherbrand ultrasonic bath (Figure 2.14 c) under the same conditions. To determine the optimal conditions for the rotary evaporator step, several tests were previously carried out to find the time and temperature necessary to remove completely the chloroform. For this task, chloroform was mixed with PAO40 and afterwards evaporated using the rotary evaporator at different temperatures during several times. After each temperature– time test, the viscosity of the sample was measured with an Anton Paar Stabinger SVM3000 rotational viscosimeter (Figure 2.13). The viscosity values, from 5 to 100 °C, were compared with those of neat PAO40, considering as optimal temperature–time conditions those for which viscosity values of neat PAO40 and of the sample were closest, being these conditions: 60 °C and 30 min. For other nanodispersions (OA modified TiO2 NPs, TiO2-OA, and SA modified SiO2, SiO2SA) the rotary evaporator was substituted by a magnetic hot plate LBX H03D (Figure 2.2c), being the procedure summarized in Figure 2.10b. The optimal time and temperature conditions for the complete evaporation of hexane (the volatile solvent for the nanodispersions based on PAO6 or PAO8) was determined similarly using the same viscosimeter. To carry out this task, previously a known concentration of NP/hexane dispersion (known using the stove, Figure 2.4e) was mixed with an appropriate amount of PAO6 or PAO8 using the Fisherbrand ultrasonic bath for 15 min, followed by the evaporation of hexane in the hot plate with magnetic agitation FÁTIMA MARIÑO FERNÁNDEZ 54 for 60 min at 90 ºC, obtaining a concentrated nanodispersion in PAO. Dilutions of this dispersion were carried out. To improve the dispersibility of NPs in the PAO6 or PAO8, OA or SA was added to each nanodispersion of TiO2-OA and SiO2-SA, respectively. Figure 2.10 Schematic procedure for preparing nanodispersions: (a) using a rotary evaporator; (b) using a magnetic hot plate Figure 2.11 Ultrasonication devices: (a) OVAN ultrasonic bath (NANOMAG); (b) Branson ultrasonic Sonifier S-250A probe sonicator (NANOMAG); (c) Fisherbrand ultrasonic bath FB11203; (d) ultrasonic HD 2200 Sonopuls disruptor; (e) Büchi Rotavapor R-210 (NANOMAG) Chapter 2 61 2.4.3 Hommelwerke contact profilometer The surface roughness for each steel disc used in the EHD2 tribometer was previously measured using a Hommelwerke LV50 profilometer equipped with a TKL300/17 probe (Figure 2.18), which has an amplitude of 300 μm, a diamond cone of 90º and a radius tip of 5 μm. The average roughness (Ra) is obtained by calculating the arithmetic mean value of all measured heights (valleys and peaks) in the analyzed surface area. The Hommelwerke contact profilometer allows to measure in an area around 7x1.5 mm every 1 μm [36]. Figure 2.18 Contact profilometer Hommelwerke LV50 2.4.4 3D optical profilometer Sensofar S Neox The worn pins from the tribological tests from the Anton Paar tribological module were analyzed using a 3D Sensofar S Neox optical profilometer (Figure 2.19a). Table 2.8 summarized its technical specifications and measured parameters. The profilometer has three optical lenses, 10x, 20x and 50x and can operate in three different modes: confocal, interferometry and focus variation mode; the vertical and lateral resolutions are 0.01 nm and 0.1 μm, respectively. In this PhD Thesis the measurements were performed using the confocal mode, which allows the measurement of surface heights, with the 10x optical lens being optimal for the sample size. The observed worn track produced by the ball on the pins has a concave shape and its generated 3D image is obtained with the SensoScan Software (Figure 2.19b). These images are then transferred to SensoMap Software which is based on Mountains technology from Digital Surf for a precise analysis of the surface topology (Figure 2.19c). Several wear parameters (Figure 2.19d-f) can be determined using the SensoMap Software, such as: wear track width (WTW), wear track depth (WTD), maximum transversal worn area (area) and worn volume (WVol). In this PhD Thesis the WVol was not determined due to its higher expanded uncertainty compared to the other parameters. The values of these parameters are automatically calculated by the software after the manual selection of the limits in the profile. In Figure 2.19d, for the determination of WTW and WTD, the limits are indicated by the user through the vertical lines marked with 0 and 1 inside squares. The same selection is used for the maximum transversal area (Figure 2.19e) that is indicated by the solid black lines. In the case of WTD, the user should also indicate the deepest point of the maximum transversal area. For the worn volume (WVol), the limits (contour) are indicated by the user as shown by the dots surrounding its highest horizontal surface in Figure 2.19f. The average values of the parameters (WTW, WTD and area) and their standard deviations were obtained from the worn FÁTIMA MARIÑO FERNÁNDEZ 62 track profiles of the nine pins tested with each lubricant. Finally, the expanded uncertainties were determined from the standard deviations of the average and k = 2. The uncertainty contribution of type B has not been calculated due to the lack of information about the uncertainties of the properties affecting the measurement of the wear parameters [29]. Table 2.8 Profilometer specifications and measured wear parameters S Neox (Sensofar) Specifications Measured parameters Mode: confocal Magnification objective: 10× Software: • SensoScan • SensoMap Wear Track Width (WTW) Wear Track Depth (WTD) Worn area (Area) Wear volume (WVol) Surface roughness (Ra, Rq): • ISO 4287 standard • Gaussian filter wavelength cut-off: 0.025 or 0.08 mm Skewness (Rsk) and Kurtosis (Rku) Figure 2.19 Non-contact 3D optical profilometer: (a) Sensofar S Neox, (b) 3D image of the worn surface in SensoScan Software (c) topographic layer of the worn surface in SensoMap Software, (c) WTW and WTD of the worn profile, (d) area of the worn profile, (e) WVol. There are several roughness parameters to characterize the surface topography, and the most used is the arithmetic average height (Ra) which is defined as the average absolute deviation of the roughness anomalies from the mean line over the sampled surface [37]. However, Ra is not sensitive to small changes in the profile. Another parameter with better sensitivity than Ra is the root mean square roughness (Rq) and represents the standard deviation of the distribution of surface heights [37]. Other interesting parameters to characterize a surface are the skewness (a) (b) (c) (d) (e) (f) Chapter 2 63 (Rsk) and the kurtosis (Rku). The Rsk is used to measure the symmetry of the profile about the mean line [37], and it sensitive to deep valleys or high peaks. A positive Rsk means that the profile has more valleys than peaks, meanwhile a negative Rsk would have more peaks than valleys. A Rsk value of zero would mean that the surface has as many peaks as valleys. The Rku describes the sharpness of the probability density of the profile [37]. If the Rku value is lower than three, then its considered platykurtoic and has few high peaks and low valleys. If the Rku value is higher than 3, then it is leptokurtoic and has many high peaks and low valleys. 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NANOLUBRICANTS FOR GEARBOXES 3 ZINC OXIDE NANOPARTICLES COATED WITH OLEIC ACID AS ADDITIVES FOR A POLYALPHAOLEFIN LUBRICANT The results presented in this section are related to the following publication (the editorial authorization for the use of this publication is in the Appendix A): F. Mariñoa, E. R. Lópeza, A. Arnosab, M. A. González Gómez b, Y. Piñeirob, J. Rivasb, C. Alvarez-Lorenzoc, J. Fernándeza. ZnO nanoparticles coated with oleic acid as additives for a polyalphaolefin lubricant. Journal of Molecular Liquids, 348, (2022) 118401. (Open access) https://doi.org/10.1016/j.molliq.2021.118401 Elsevier, ISSN: 0167-7322 a Laboratory of Thermophysical and Tribological Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics and Institute of Materials (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain b NANOMAG Laboratory, Department of Applied Physics, Faculty of Physics and Institute of Materials (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain c Departamento de Farmacología, Farmacia y Tecnología Farmacéutica, I+D Farma Group (GI-1645), Facultad de Farmacia, Instituto de Materiales (iMATUS) and Health Research Institute of Santiago de Compostela (IDIS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain The main contributions of the PhD student to this study are explicitly indicated below: Experimental: Investigation, Formal analysis. Manuscript: Writing – original draft. Among the most critical challenges in science, technology and economy is planning long-term resource consumption and societal trend and trying to balance them in the best possible way [1]. Nanoadditives have proven to be an effective way to improve antifriction and antiwear capabilities of lubricants, which is of great significance for diminishing energy consumption and emission reduction [2]. Among the possible nanoadditives, in this work ZnO NPs were selected, because they are currently used in commercial applications due to their photolytic properties [3], as a photocatalyst [4] and as an additive in wastewater treatment and cosmetics [5]. Comparative studies have shown that metal oxide NPs, among them ZnO NPs (ZnO NPs), when used as additives, improve the lubricating properties of the base oils [6]. Xue et al. [7] analyzed the effect of bare ZnO NPs (50 nm) in a mineral oil (Yubase-6) and compared the effect of these NPs with zinc dialkyl dithiophosphate (ZDDP), and obtained similar antioxidant, antifriction and antiwear behaviors, with reductions up to 45 % (1.0 wt% ZnO) for COF and 40 % (0.5 wt% ZnO) for WSD. Nevertheless, there was no information about the stability of these nanodispersions. FÁTIMA MARIÑO FERNÁNDEZ 68 Regarding the use of dispersants to enhance the stability (section 1.2.1) of ZnO NPs, Hernández Battez et al. [8] analyzed the tribological behavior of uncoated ZnO NPs (20 nm) as additives of a polyalphaolefin (PAO6) with two non-ionic commercial dispersants (OL100 and a poly12-hydroxy stearic acid) using four-ball test, showing good results in extreme pressure conditions. Nevertheless, the stability of NP dispersions containing the dispersants was still poor, especially for OL100. Gara and Zou [9] used oleic acid (OA) to disperse ZnO NPs (diameter 40–100 nm) into a paraffinic mineral oil, claiming improvements in the stability of the dispersions in comparison with those without OA. In addition, these authors show that oleic acid as a dispersant reduces COF to some extent. Regarding the chemical modification of ZnO to enhance the stability of their nanodispersions, Ran et al. [10] dispersed OA surface-modified ZnO NPs (core diameter 10–30 nm) in a mineral oil (60SN base oil) obtaining a stability of 12 h for concentrations up to 0.5 wt% and reductions around 29 % and 15 % for COF and WTW, respectively, in comparison to the base oil. Wu et al. [11] modified ZnO NPs with OA and tested them as lubricant additives in a PAO, likely a mixture of PAO2, PAO3 and PAO4, and in diisooctyl sebacate, claiming that the coating of the ZnO NPs with OA significantly improved their dispersibility in both oils. In addition, tribological performance of OA-modified ZnO (4.04 nm) as additives in PAO (optimal concentration in NPs 1.2 wt%) was better than in sebacate, with reductions of up to 9.9 % for the COF and up to 31.2 % for the WTW compared to those of neat PAO. The aim of this chapter is to obtain homogeneous and efficient nanolubricants of ZnO NPs coated with oleic acid, ZnO-OA, in PAO40, which is an appropriate base oil for wind turbine gearboxes that has been previously studied for this purpose by Gutierrez et al. [12]. For this purpose, it is necessary to know a) the optimum concentration of the ZnO-OA NPs synthesized by us for friction and wear reduction b) the stability of the nanodispersion at the optimum concentration c) the variations of viscosity, density and viscosity index of the nanodispersions with respect to PAO 40 d) the empirical equations that predict well the density and viscosity of the nanodispersions e) the mechanisms of the nanoadditives for friction and wear reduction. Thus, tribological tests were performed at 80 °C using the Anton Paar MCR 302 rheometer equipped with the ball-on-three-pin tribological module. This temperature was chosen because in wind turbine gearboxes the oil can reach temperature peaks around 80 ºC [13,14]. The tribological mechanisms of the dispersions were analyzed by 3D profilometry and confocal Raman microscopy on the worn tracks of the tested pins. 3.1 NANOPARTICLE SYNTHESIS AND CHARACTERIZATION 3.1.1 Synthesis and functionalization of nanoparticles The reagents used in the synthesis of ZnO NPs were: zinc acetate dihydrate (≥98 %), potassium hydroxide (≥85 %) and oleic acid (90 %) from Sigma-Aldrich (Saint Louis, MO, USA); acetone (reagent grade) and ammonium hydroxide (25 %) from Fisher (Madrid, Spain); hydrochloric acid (37 %) used to neutralize the ZnO dispersion (Acros Organics, Geel, Belgium) and absolute ethanol used as the reaction solvent (Scharlau, Barcelona, Spain). All these products were used without further purification. ZnO NPs were prepared using Shamhari et al. [15] method through the following reaction: Zn(CH3COO)2·2 H2O + 2KOH 60 ℃, EtOH → ZnO + 2CH3COOK + 2H2O (3.1) Briefly, Zn(CH3COO)2·2H2O (1.49 g) was dissolved in absolute ethanol (64 mL) in a Schott bottle provided with a magnetic stirrer, the blend being heated to 60 ºC. KOH (0.79 g) was dissolved separately in absolute ethanol (30 mL) under the same conditions, the solution being slowly added dropwise to the Zn(CH3COO)2·2H2O solution at 60 ºC. After 3 hours of SECTION I. Chapter 3 69 vigorous stirring (500 rpm), ZnO precipitated and it was collected by centrifugation (4000 rpm) for 10 minutes, washed twice with acetone and once with ultrapure water. Finally, ZnO was dispersed in ultrapure water for storage, the dispersion concentration being determined by TGA using the Perkin Elmer Pirys 7 TGA. The measurements were made from 50 to 850 ºC at a heating rate of 10 ºC/min under a nitrogen flow of 20 mL/min. The ZnO NPs were coated with OA summarized in the reaction: ZnO NH4OH, OA 1) 60 ℃, 30 min 2) 95 ℃, reflux, 2 h → ZnO-OA (3.2) The as prepared ZnO water dispersion was sonicated for 15 minutes in an ultrasonic bath. Then, 100 mg (12.9 mL) of this dispersion were introduced in a round-bottom flask, provided with a magnetic stir bar, and heated to 60 ºC. Once the setting temperature was reached, NH4OH (2.4 mL) was added, and 1 minute later OA (300 mg, 0.27 mL) was also added. The mixture was kept at 60 ºC for 30 minutes and then the temperature was raised to 95 ºC with reflux for 2 hours. The excess of NH4OH was neutralized with HCl (9 vol%) and the precipitate (ZnO-OA) was collected by centrifugation under the same conditions as ZnO NPs and washed with ultrapure water and hexane. The ZnO-OA NPs were dispersed in chloroform for storage and to avoid agglomeration. The concentration of this dispersion was determined also by TGA (2.12 wt%). 3.1.2 Characterization of nanoparticles One of the techniques used to confirm the effectiveness of the chemical modification of the NPs is the thermogravimetric analysis of the modified NPs dispersed in a solvent. Thus, the thermogram (Figure 3.1) gives information about the degradation temperature of the OA coating on the ZnO NPs, and the mass of each constituent (OA coatings and ZnO NPs). From these experimental data, it is possible to obtain the coupling density of OA molecules per surface unit of a ZnO nanoparticle. The first mass loss (until 90 ºC) is due to the evaporation of the organic solvent (chloroform) used to disperse the NPs after reaction 3.2. The thermal degradation of the OA takes place from around 250 ºC to 550 ºC, the remaining mass is uncoated ZnO NPs. The TGA determined that the content of OA on the NPs was 65 wt% for the ZnO-OA NPs determined and the number of OA molecules per NP surface area unit in nm2 was 91. The experimental diffraction pattern of the ZnO-OA NPs, obtained with XRD, together with the theoretical pattern of a hexagonal phase of zincite [16] (JCPDS card No. 36-1451) is shown in Figure 3.2. The main diffraction peaks, (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4) and (2 0 2), perfectly matched each other. Figure 3.1 Thermogram of ZnO-OA NPs dispersed in chloroform 0 0.5 1 1.5 2 40 140 240 340 440 540 640 740 840 Mass/mg Temperature/ºC FÁTIMA MARIÑO FERNÁNDEZ 70 Figure 3.2 Experimental XRD pattern of ZnO-OA NPs (black line) with the main peaks labelled with their corresponding Miller indices, and theoretical reflections (red bars) for zincite phase (JCPDS card No. 361451) TEM micrograph (Figure 3.3a) of the oleic acid coated ZnO NPs, dispersed after synthesis in chloroform, revealed spherical NPs with relatively inhomogeneous size distribution (medium size distribution of around 10 nm, Figure 3.3b). Size distribution was performed using the ImageJ software for a sample of 200 NPs. Figure 3.3 TEM micrograph (a) and the size distribution (b) of the ZnO-OA NPs As aforementioned, the ZnO NPs were coated with OA to facilitate their dispersibility in the base oil. The effective surface functionalization of these NPs was also analyzed by FTIR and Raman spectroscopies. The FTIR spectrum (Figure 3.4) presents an absorption band at 3410 cm-1, corresponding to the stretching vibrations of the O-H group attributed to the presence of hydroxyl residue, probably due to atmospheric moisture. In addition, the peaks appearing at 2923, 2853, and 722 cm-1 correspond to the C–H stretching vibration bands of – CH3 and –CH2, and the -CH2 bending vibration, respectively. The peaks that appear at 1543 and 1435 cm-1 were assigned to the asymmetric and symmetric stretching vibrations of –COO– , respectively. The peaks at 1543 and 1435 cm-1 were assigned to the asymmetric and symmetric stretch vibrations of –COO–, respectively. Finally, the vibration band observed at 468 cm-1 corresponds to the stretching vibration of the zincite structure (Zn-O) [17]. These findings confirmed that ZnO NPs were successfully coated with oleic acid. It is worth mentioning that the FTIR spectrum of Figure 3.4 is very similar to that reported by Wu et al. [11] for other ZnOOA NPs. SECTION I. Chapter 3 77 As in the case of densities, the nanodispersion viscosities have been predicted by three empirical models: the Einstein equation [28] (eq. 3.4), the Pak and Cho equation [23] (eq. 3.5) and the Chen et al. equation [29] (eq. 3.6): ηnd=ηPAO40·(1+2.5φ) (3.4) ηnd=ηPAO40·(1+39.11φ+533.9φ2) (3.5) ηnd=[1+10.6φ+(10.6φ)2]·ηPAO40 (3.6) Despite its simplicity, the equation due to Einstein predicts experimental viscosities with AAD% lower than 2.9 %. The worse results are those provided by Pak and Cho model [23] (AAD% lower than 7.3 %). The best results are those provided by Chen et al. equation [29], with AADs lower than 0.87 %. In Figure 3.11 the predicted viscosities are plotted together with the experimental ones. Figure 3.11 Viscosities of neat PAO40 and its ZnO-OA nanodispersions at 25, 40 and 100 ºC against ZnO-OA volume fraction. Symbols: experimental data. Solid lines: predictions of the Pak and Cho empirical equation [23]. Dashed lines: predictions of the Chen et al. equation [29]. According with the Carreau parameters reported for PAO40 [30], up to very high shear rates at 0.1 MPa (from 2·106 s−1 at 20 ºC to 3·107 s−1 at 100 ºC), its behavior is Newtonian. These shear rates are much higher than those applied when using SVM3000 apparatus, which remain between 10 and 103 s−1, being dependent on the fluid viscosity [31]. So, taking these facts into account and the small influence of the addition of NPs on the viscosity (lower than 4 %) in the range of concentrations analyzed, it is expected that the nanodispersions do not display any shear thinning in the shear conditions of the viscosity measurements. Finally, the measured viscosity indices (VI) of the nanodispersions were slightly lower than that of PAO40 (VI 151) and remained approximately constant regardless of the composition (VI 149). 3.4 TRIBOLOGICAL RESULTS 3.4.1 Pure sliding tests The friction results of the ball-on-three-pins tests for PAO40 and all the ZnO-OA nanodispersions are summarized in Table 3.3 and presented in Figure 3.12. The five studied dispersions decreased the coefficient of friction compared to the neat PAO40, ranging from 14 % to 25 %. The lowest friction value was achieved when 0.25 wt% ZnO-OA was added to PAO40; a further increase on nanoparticle concentration resulted in worse friction reduction. Therefore, based on these results, the optimal ZnO-OA concentration for COF reduction was 0 100 200 300 400 500 600 700 800 900 1000 0 0.0005 0.001 0.0015 0.002 0.0025 η/mPa s ϕ 40 ºC 25 ºC 100 ºC FÁTIMA MARIÑO FERNÁNDEZ 78 0.25 wt%. Minima in the friction-concentration curves for several nanolubricants were previously reported by different authors [10,11,32-35]. Table 3.3 Average friction coefficient values, COF, at 80 ºC and the expanded uncertainties, U, for PAO40 base oil and PAO40+ wt% ZnO-OA lubricants at different NP weight percentages, wt% Lubricant COF U Reduction % PAO40 0.0938 0.0027 - + 0.10 wt% ZnO-OA 0.0721 0.0028 23 + 0.25 wt% ZnO-OA 0.0701 0.0022 25 + 0.50 wt% ZnO-OA 0.0757 0.0032 19 + 0.75 wt% ZnO-OA 0.0765 0.0012 18 + 1.00 wt% ZnO-OA 0.0802 0.0023 14 Figure 3.12 Average friction coefficient, COF, (blue) and wear track depth, WTD, (orange) with the expanded uncertainty bars for PAO40 and PAO40 + wt% ZnO-OA nanodispersions for different NP weight percentages, wt% (the orange line is a guide for the eye) 3.4.2 Wear surface characterization Regarding the WTD at the surface of the pins (Figure 3.13), the best antiwear performance was also obtained for the nanolubricant prepared with 0.25 wt% ZnO-OA NPs. Dispersions with lower concentrations did not have enough NPs to minimize friction and wear. On the other hand, for higher NP concentrations, ZnO-OA NPs or aggregates could act as debris particles, increasing friction and wear [36]. Average values of various wear parameters and the respective expanded uncertainties, such as wear track width (WTW), depth (WTD), and cross-sectional area of the worn pins are summarized in Table 3.4. The greatest reductions for these wear 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0.02 0.04 0.06 0.08 0.1 0.12 0.00 0.10 0.25 0.50 0.75 1.00 WTD (µm) COF wt% SECTION I. Chapter 3 79 parameters were also found with the PAO40 + 0.25 wt% ZnO-OA nanodispersion, being 38 %, 68 % and 82 %, in WTW, WTD and cross-sectional area, respectively. Table 3.4 Average values of the width, WTW, depth, WTD, and cross-sectional area of the wear track and the expanded uncertainties, U, for PAO40 + wt% ZnO-OA lubricants for different NP weight percentages, wt% Lubricant WTW/µm U/µm WTD/µm U/µm Area/µm2 U/µm2 PAO40 370 7 1.50 0.17 411 54 + 0.10 wt% ZnO-OA 269 15 0.67 0.10 119 36 + 0.25 wt% ZnO-OA 230 14 0.48 0.07 75 18 + 0.50 wt% ZnO-OA 241 15 0.57 0.07 78 14 + 0.75 wt% ZnO-OA 289 29 0.69 0.07 96 22 + 1.00 wt% ZnO-OA 259 25 0.65 0.03 119 31 2D images and 3D profiles of the worn pins are shown in Figure 3.13. Wear was strongly reduced as evidenced when comparing a pin tested with PAO40 (Figure 3.13a and 3.13b) with a pin tested with PAO40 + 0.25 wt% ZnO-OA (Figure 3.13c and 3.13d). Furthermore, the wear reductions were also evident in the cross-sectional profiles of those same worn pins in Figure 3.14. Figure 3.13 2D Images and 3D profiles of the wear tracks in the pins tested with neat PAO40 (a, b) and with its nanolubricant containing 0.25 wt% ZnO-OA (c, d) FÁTIMA MARIÑO FERNÁNDEZ 80 Figure 3.14 Cross-sectional profiles of the wear tracks on the pins lubricated with neat PAO40 (a) PAO40 + 0.25 wt% ZnO-OA (b) The roughness (Ra) of wear tracks was another parameter measured by 3D profilometry to analyze the anti-wear capability of the nanolubricants. The surface roughness of the untested pins was also characterized. The average values of the roughness are shown in Figure 3.15 for worn and untested pins. All nanodispersions, except for 1.00 wt% ZnO-OA in PAO40, provided worn surfaces with lower Ra values than those of the untested pins, however, only the dispersions with the two lowest NP concentrations (0.1 and 0.25 wt%) led to lower Ra values than neat PAO40, obtaining the optimal Ra value for 0.25 wt% ZnO-OA dispersion, which reduced the Ra value by 67 % compared to that of the untested pin surface and 43 % compared to that lubricated with PAO40. Figure 3.15 Roughness (Ra) of the untested pin surface (orange) and of the worn pin surfaces (blue) with the expanded uncertainty bars tested with PAO40 and PAO40 + wt% ZnO-OA nanodispersions with different NP weight percentages, wt% Raman spectra of ZnO-OA (Figure 3.16) and of PAO40 (Figure 3.17) are very similar due to the structural similarity of both compounds containing C-C bonds and -CH2 groups. Nevertheless, there were two clear differences in the case of oleic acid (Figure 3.16): there was 0 2 4 6 8 10 12 14 16 18 PAO40 0.10 wt% ZnO-OA 0.25 wt% ZnO-OA 0.50 wt% ZnO-OA 0.75 wt% ZnO-OA 1.00 wt% ZnO-OA Pin Ra/nm SECTION I. Chapter 3 81 a C = C signal at 1663 cm−1 due to the vibration of the double bond and another signal at 3008 cm−1 corresponding to vibration = C-H, which did not appear in the PAO40 spectrum. Raman maps of the worn pin surfaces were also recorded. To distinguish between PAO40 and ZnOOA in the Raman maps of these surfaces, the dissimilar peak at 1663 cm−1 of the ZnO-OA spectrum was used to identify the presence of these NPs. Similar procedure has been used previously by Nasser at al. [37-39] to discriminate between the PAO32 Raman spectrum and those of three ionic liquids. Figure 3.16 Raman spectrum of ZnO-OA NPs Figure 3.17 Raman spectrum of PAO40 From the Raman map corresponding to the worn pin surface lubricated with neat PAO40 (Figure 3.18), the presence of PAO40 (red area), iron oxides (blue area) as well as small and scarce carbon areas (yellow) were evidenced. These results agree with those found by Ratoi et al. [40] from XPS measurements on a disk wear track obtained with PAO6 from ball-on-disk tribological tests at 120 ºC, using a similar tribo-pair. These authors concluded that the tribofilm built up on the wear track was mainly formed by carbon, iron, and oxygen, and thus the smooth FÁTIMA MARIÑO FERNÁNDEZ 82 wear could be due to oxide formation and lubricant degradation on the wear track among other effects. In addition, recently, from confocal Raman microscopy, Nasser et al. [37] also found for a similar pin worn surface tested with PAO32 at 80 ºC; namely, the boundary tribofilms were composed of iron oxides, carbon, and the oil itself. Figure 3.18 Raman map of the worn pin surface tested with PAO40 and spectra of the components present on this surface: PAO40 (red), carbon (yellow) and iron oxides (blue) Raman map on the worn surface tested with PAO40 + 0.25 wt% ZnO-OA (Figure 3.19) shows the presence of PAO40 (red color), oleic acid, i.e., ZnO-OA NPs, (green color) and iron oxides (blue color). The obtained signals of the ZnO-OA NPs in the worn scar overlapped with those of another component, probably carbon, which appeared also in the worn surfaces of neat PAO40 (Figure 3.18, yellow color). The presence of the ZnO-OA NPs in the worn scar tribofilm was also shown in the ICP-MS analysis of the nanolubricant collected after the tribotest. This nanolubricant contains 0.21 wt% of ZnO-OA NPs, i.e., a 16 % reduction. Figure 3.19 Raman map of the worn pin surface tested with PAO40 + 0.25 wt% ZnO-OA and spectra of the components present on that surface: PAO40 (red), ZnO-OA (green) and iron oxides (blue) Regarding the tribological mechanisms, both rolling and mending effects could be behind the better tribological performance of nanolubricants with respect to that of neat PAO40. Rolling may transform sliding friction into rolling friction [41,42] as a result of the spherical shape of ZnO-OA NPs (Figure 3.3). The mending effect was evidenced by the presence of small SECTION I. Chapter 3 83 areas of ZnO-OA NPs in the Raman maps of the contact surfaces and by the decrease of their roughness. Raman mapping also evidenced the presence of large PAO40 tribofilms. 3.5 CONCLUSIONS The features achieved in this chapter can be summarized as: • The synthesis and coating with oleic acid of ZnO NPs were carried out successfully. The oleic acid coated ZnO (ZnO-OA) NPs, with an average diameter around 10 nm, were characterized by XRD (the nanoparticle core) and FTIR (the coating). • The OA coating of the ZnO NPs enable their dispersion in the PAO40, reaching stability times up to 29 days for the 0.25 wt% nanodispersion. • Using as neat base oil PAO40, five nanolubricants with ZnO-OA concentrations ranging from 0.10 to 1.00 wt% were designed as well as thermophysically and tribologically characterized. • Density increases roughly linearly with the mass concentration of NPs, reaching 0.5 % for nanodispersion with 1.00 wt% ZnO-OA. Dynamic viscosities also increase with the concentration of NPs, with relative increases of up to 4.0 % for the base oil additivated with 1 wt% of ZnO-OA. 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Chapter 4 93 Figure 4.7 Refractive index evolution for 0.10 wt% TiO2 and TiO2-OA nanolubricants 4.3 THERMOPHYSICAL PROPERTIES PAO8 base oil and the PAO8 + 0.35 wt% TiO2-OA nanodispersion were thermophysically characterized with the SVM3000 Stabinger. These results are presented in Table 4.1. Comparing with the corresponding PAO8 property, the nanolubricant density showed a maximum variation of 0.06%, close to the measurement uncertainty, and the dynamic viscosity shows an increment up to 15 %. Finally, the VI value of this nanodispersion (VI 149) is 7 % higher than that of PAO8 base oil (VI 138). Table 4.1 Densities (ρ) and dynamic viscosities (η) of PAO8 and 0.35 wt% TiO2-OA nanolubricant measured at atmospheric pressure Lubricant PAO8 0.35 wt% TiO2-OA PAO8 0.35 wt% TiO2-OA T/ºC ρ/g cm-3 ρ/g cm-3 η/mPa.s η/mPa.s 5 0.8375 0.8378 253.1 289.2 10 0.8345 0.8347 184.3 210.1 15 0.8315 0.8317 137.1 156.1 20 0.8285 0.8287 103.6 117.9 25 0.8255 0.8257 79.77 90.78 30 0.8224 0.8227 62.13 70.77 35 0.8194 0.8197 49.18 56.04 40 0.8163 0.8167 39.47 44.99 45 0.8133 0.8136 32.09 36.60 50 0.8102 0.8106 26.41 30.13 55 0.8072 0.8076 21.98 25.10 60 0.8041 0.8045 18.50 21.12 65 0.8010 0.8014 15.71 17.94 70 0.7979 0.7984 13.46 15.38 75 0.7948 0.7953 11.63 13.30 80 0.7918 0.7922 10.13 11.60 85 0.7887 0.7891 8.893 10.18 90 0.7856 0.7860 7.860 9.006 95 0.7825 0.7829 6.992 8.017 100 0.7794 0.7798 6.262 7.184 1.462 1.464 1.466 1.468 020 40 60 n Time / h 0.10 wt% TiO2-OA 0.10 wt% TiO2 FÁTIMA MARIÑO FERNÁNDEZ 94 The viscosities measured in the Stabinger SVM3000 were correlated with the Vogel-FulcherTammann (VFT) equation [14]. This equation was chosen because of its capability to extrapolate at temperatures out of the measurement range [dx.doi.org/10.1021/je200883w]. The VFT correlation was used to obtain the dynamic viscosity of each nanolubricant and the base oil (PAO8) at 120 °C, since the viscosimeter does not allow to measure above 100 °C. This viscosity value is required for the film thickness estimation needed to plot the Stribeck curve of the rolling-sliding tribological tests performed at 120 ºC. 4.4 TRIBOLOGICAL RESULTS 4.4.1 Pure sliding tests Average friction coefficients obtained under pure sliding conditions for PAO8 and the four nanolubricants composed by TiO2-OA NPs and PAO8 base oil are presented in Figure 4.8 and Table 4.2 It can be obviously seen that for all the TiO2-OA nanolubricants the achieved coefficients of friction are lower than that of PAO8 without additives, being the greatest friction decrease achieved for the nanolubricant containing 0.35 wt% TiO2-OA NPs. Specifically, the lowest average coefficient of friction is 0.098, obtained with this last nanolubricant, whereas the one obtained for the base oil is 0.139. Thus, a maximum 30 % friction reduction owing to the TiO2-OA nanoadditives is achieved. Regarding the other nanolubricants, friction reductions of 16 %, 24 % and 23 % were obtained for 0.10 wt% TiO2-OA, 0.25 wt% TiO2-OA and 0.50 wt% TiO2-OA nanolubricants, respectively. Figure 4.8 Average friction coefficient, COF, and WTW with the expanded uncertainty bars of PAO8 and its nanolubricants containing TiO2 nanoparticles coated with oleic acid (the orange line is a guide for the eye) 100 200 300 400 500 0.06 0.08 0.10 0.12 0.14 0.16 WSD/µm COF SECTION II. Chapter 4 95 Table 4.2 Average coefficients of friction, COF, and mean wear parameters, including the expanded uncertainty, U, for the tested PAO8 lubricants Lubricant COF U WTW/m U/m WTD/m σ/m Area/ m2 U/ m2 PAO8 0.1392 0.0021 392 14 2.35 0.17 605 52 + 0.10 wt% TiO2-OA 0.1163 0.0015 359 14 2.11 0.18 527 41 + 0.25 wt% TiO2-OA 0.1061 0.0015 336 12 1.85 0.12 459 39 + 0.35 wt% TiO2-OA 0.0978 0.0011 289 11 0.82 0.11 163 18 + 0.50 wt% TiO2-OA 0.1066 0.0013 318 12 1.04 0.14 245 17 4.4.2 Wear surface characterization To compare qualitatively 3D Profiles, worn areas and cross section profiles of the pins tested in pure sliding conditions with the PAO8 base oil and with the optimal TiO2-OA nanolubricant are shown in Figures 4.9 and 4.10. Average wear produced in pins after tribological tests was quantified by means of different parameters: WTW, WTD and worn area, which results are reported in Table 4.2 for each lubricant tested. As it can be observed, for all the tested TiO2OA nanolubricants, the three wear parameters were smaller than those found with PAO8 oil. The maximum wear reductions were found for 0.35 wt% TiO2-OA nanolubricant with reductions of 26 %, 65 % and 73 % for WTW, WTD and worn area, respectively. These impressive wear reductions can be clearly observed in Figures 3.2.9 and 3.2.10 for the nanolubricant with the optimal concentration (0.35 wt% TiO2-OA). Wear results show a good correlation with friction, as it can be observed in Figure 4.8. Figure 4.9 3D images and worn areas of worn tracks lubricated with PAO8 oil and with the optimum TiO2OA nanolubricant (0.35 wt%) FÁTIMA MARIÑO FERNÁNDEZ 96 Figure 4.10 Profiles comparison of worn tracks lubricated with PAO8 oil and with the 0.35 wt% TiO2-OA nanolubricant Furthermore, roughness (Ra) of worn tracks of pins was investigated to obtain more information about the antiwear properties of TiO2-OA NPs. Worn tracks lubricated with TiO2OA nanolubricants are less rough than those lubricated with PAO8 without additives (Table 4.3). In particular, a Ra value of 18.8 nm was found for the worn track lubricated with PAO8 while for the surface lubricated with the 0.35 wt% TiO2-OA nanolubricant a smaller Ra was reached (12.8 nm), which leads to a 32 % roughness reduction. Therefore, it can be suggested that polishing and/or mending effects occur owing to the presence of TiO2-OA NPs. Due to the mending effect, NPs can fill the grooves and scars of the rubbing surface developing an improved surface finish [15,16]. To quantify better the effect of the TiO2-OA NPs in the roughness, other two parameters were analyzed: skewness, Rsk, and kurtosis, Rku. The untested surface has a Rsk higher than 0 (1.51), which means that the surface contains more peaks and asperities than valleys [17]. The decrease in the Rsk parameter due to the friction process to a negative value closer to zero (-0.76 − -0.35) confirms that the worn surfaces lubricated with base oil and nanolubricants are very flat, most of the material being concentrated around the valleys. As regards Rku, the value before tribological tests is higher than 3 (5.54), which indicates the presence of very high peaks and/or deep valleys (this parameter does not distinguish between peaks and valleys) [17]. After friction tests this value decreases (2.98– 2.41), which implies low both peaks and valleys. Comparing the average Ra, Rsk and Rku values of the worn pins lubricated with PAO8 with those of worn pins lubricated with PAO8 + 0.35 wt% TiO2-OA, it can be concluded that the last pins have flatter worn surfaces. These facts confirm that polishing and/or mending effects takes place. Table 4.3 also shows that the worn surfaces lubricated with PAO8 + 0.5 wt% TiO2-OA are flatter than those tested with the optimal nanolubricant (PAO8 + 0.35 wt% TiO2-OA). -2.5 -2 -1.5 -1 -0.5 0 0.5 1 200 400 600 800 1000 1200 1400 1600 Vertical distance/m Horizontal distance/m PAO8 PAO8 + 0.35 wt% TiO2-OA SECTION II. Chapter 4 97 Table 4.3 Average roughness parameters, Ra, Rsk and Rku and the expanded uncertainties, U, in worn pins tested with PAO8 lubricants (Gaussian filter: 0.08 mm cut-off) Lubricant Ra/nm U Rsk U Rku U PAO8 18.8 1.7 -0.562 0.048 2.98 0.21 + 0.15 wt% TiO2-OA 15.7 1.5 -0.434 0.055 2.44 0.18 + 0.25 wt% TiO2-OA 14.4 1.7 -0.512 0.045 2.41 0.17 + 0.35 wt% TiO2-OA 12.8 1.2 -0.356 0.052 2.46 0.18 + 0.50 wt% TiO2-OA 13.3 1.1 -0.762 0.062 2.52 0.15 Finally, with the aim of obtaining information about the nanoparticle distribution in worn tracks after pure sliding tests, Raman mappings of the worn surfaces were recorded. First, Raman spectra of the PAO8 base oil (Figure 4.11), oleic acid (Figure 4.12) and the TiO2 nanopowders (Figure 4.13) were obtained to identify the components in mapping. Therefore, mapping of the worn pin lubricated with the optimal TiO2-OA nanolubricant (Figure 4.14) was performed to identify the role that NPs play in the wear decrease. Figure 4.14 shows the relevant areas in green and blue color, that coincide with the spectra obtained for TiO2 and OA (Figures 4.12 and 4.13), respectively. This fact indicates that tribofilms containing OA and TiO2 were produced on the worn surface during the friction test. Considering the Raman and roughness results, it seems that the main tribological mechanisms that occur are tribofilm formation and mending and polishing effects. Figure 4.11 Raman Spectrum of PAO8 base oil 450 650 850 1050 1250 0 1000 2000 3000 Intensity Wavenumber / cm-1 1086 cm-1 1306 cm-1 1448 cm-1 2897 cm-1 2853 cm-1 FÁTIMA MARIÑO FERNÁNDEZ 98 Figure 4.12 Raman spectrum of oleic acid Figure 4.13 Raman Spectrum of TiO2 nanoparticles 450 650 850 1050 0 1000 2000 3000 Intensity Wavenumber / cm-1 1090 cm-1 1306 cm-1 1658 cm-1 2853 cm-1 2907 cm-1 1444 cm-1 450 950 1450 1950 2450 0 1000 2000 3000 Intensity Wavenumber / cm-1 150 cm -1 401 cm-1 519 cm-1 644 cm-1 SECTION II. Chapter 4 99 Figure 4.14 Elemental mapping and Raman characterization of worn surface obtained with the PAO8 + 0.35 wt% TiO2-OA nanolubricant 4.4.3 Rolling-sliding tests Rolling-sliding tribological measurements for PAO8 base oil and the designed TiO2-OA nanolubricants were carried out at the same temperature than pure sliding tests (120 °C) and with a SRR of 5 %. Results are reported as Stribeck Curves (Figures 4.15 and 4.16) obtained by plotting the coefficient of friction versus specific film thickness Λ. To cover the different lubrication regimes, three discs with different roughness were tested (Table 2.6). The Stribeck curves are presented in Figures 4.15 and 4.16 for PAO8 base oil and TiO2-OA nanolubricants. As usual, friction tests carried out with the rough discs showed higher friction values than those made with the smooth disc. Figure 4.15 clearly shows that, for each tested disc, the friction coefficients are quite lower for all the TiO2-OA nanolubricants than for the PAO8 base oil. Once again, the best friction behavior was found for the 0.35 wt% TiO2-OA nanolubricant and that is why only the full Stribeck curves of PAO8 base oil and 0.35 wt% TiO2-OA nanolubricant are presented in Figure 4.16. The results presented in Figure 4.15 for each disc are very interesting since it can be observed that for high entrainment speeds (right part of the curves) and consequently higher specific film thickness, the friction coefficient is quite similar for all nanolubricants and base oil. However, at low entrainment speeds the effect of TiO2-OA NPs is crucial, contributing to greatly reduce the friction when the hydrodynamic effect is poor (low speeds). FÁTIMA MARIÑO FERNÁNDEZ 100 Figure 4.15 Stribeck curves of PAO8 and PAO8 + TiO2-OA nanolubricants tested with each disc at 120 °C and 5 % SRR SECTION II. Chapter 4 101 Regarding the effect of NPs concentration in friction behavior, the friction coefficient decreases as the concentration of NPs increases, until the optimal value of 0.35 wt% is reached. This result is in perfect agreement with the friction results obtained for the pure sliding tests, where the 0.35 wt% TiO2-OA concentration was also the optimal ones. A higher nanoparticle content leads to higher friction values, which means that there should be a saturation point (optimal concentration) above which the nanoparticle content cannot provide further friction reduction. In Figure 4.16 the COF values of PAO8 and its 0.35 wt% TiO2-OA nanodispersion are plotted against Λ, defined by eq. 2.4. In the case of the nanodispersion, it can be observed that for Λ < 0.07 (approximately) the COF is almost constant (around 0.05) which is typical of the boundary film lubrication regime of lubricants containing additives. For 0.07 < Λ < 1 (approximately) the COF decreases as the Λ values increase, which is typical of the mixed film lubrication regime. The smooth disc shows a different behavior to the rough 1 and rough 2 discs for the PAO8 base oil: the difference between COF values of the PAO8 and the nanolubricant (observed at Λ > 0.3) for the smooth disc is higher than that observed for the two rougher discs. After reaching Λ > 2, the COF decreases with Λ more smoothly for PAO8 and its nanolubricant, and there are no significant differences between both Stribeck curves, meaning that the influence of the NPs and of the surface roughness are no longer significant, which is typical of full film lubrication regime. The values of Λ0  0.07 and Λ1  1 observed for this nanolubricant in this case are not general (Section 2.4.2), they fit the contact geometry considered (ball-on-disc), the roughness parameter (Ra) considered to define Λ and the test temperature (120 C in the present case). Figure 4.16 Full Stribeck curves of PAO8 (green) and PAO8 + 0.35 wt% TiO2-OA nanolubricant (red) tested with each disc (squares smooth, circles rough 1, triangles rough 2) at 120 °C and 5 % SRR 4.5 CONCLUSIONS Four nanolubricants based on a low-viscosity oil, PAO8, TiO2-OA NPs as antifriction and antiwear additives, and oleic acid as dispersant were tribologically characterized. The conclusions of this chapter can be briefly expressed as: 0 0.02 0.04 0.06 0.08 0.01 0.1 1 10 COF Λ Smooth_PAO8 Base Rough1_PAO8 Base Rough2_PAO8 Base Smooth_0.35wt%TiO2-OA Rough1_0.35wt%TiO2-OA Rough2_0.35wt%TiO2-OA Λ0 = 0.07 Λ1 = 1 FÁTIMA MARIÑO FERNÁNDEZ 102 • The OA coating of the commercial TiO2 NPs was proven successful by FTIR analysis. • Both the OA coating and dispersant improved the time stability from 48 h, for the nanolubricant with uncoated TiO2 NPs, to at least 1 month for nanodispersions up to 0.35 wt% of TiO2-OA, higher concentrations (0.5 wt%) sedimented after 4 weeks. • Friction coefficients attained with TiO2-OA nanolubricants are smaller than those observed for the neat PAO8 oil for all tribological tests (pure sliding or rolling/sliding contacts). • In pure sliding conditions, for all the TiO2-OA nanolubricants the wear observed in pins is much lower than the wear found with the PAO8 base oil, the highest reductions reaching 26 %, 65 % and 73 % in width, depth, and area, respectively, for PAO8 + 0.35 wt% TiO2-OA nanolubricant. • From roughness measurements and Raman mappings of worn pins tested in pure sliding conditions, it is concluded that the lubrication mechanism can be described by adsorbed tribofilms as well as the polishing and mending effects. • In the rolling/sliding tests (small slide to roll-ratio SRR), the antifriction capability of TiO2-OA NPs is more important at low speeds, which shows that the use of NPs is particularly important when operating in the boundary lubrication regime. 4.6 REFERENCES [1] R. Shah, B. Gashi, A. Rosenkranz, Latest developments in designing advanced lubricants and greases for electric vehicles—An overview, Lubrication Science 34 (2022) 515526 https://doi.org/10.1002/ls.1605. [2] W. Ahmed Abdalglil Mustafa, F. Dassenoy, M. Sarno, A. Senatore, A review on potentials and challenges of nanolubricants as promising lubricants for electric vehicles, Lubrication Science 34 (2022) 1-29 https://doi.org/10.1002/ls.1568. [3] Understanding Base Oils and Lubricants for Electric Drivetrain Applications. [4] Y. Chen, S. Jha, A. Raut, W. Zhang, H. Liang, Performance characteristics of lubricants in electric and hybrid vehicles: a review of current and future needs, Frontiers in Mechanical Engineering 6 (2020) 571464 https://doi.org/10.3389/fmech.2020.571464. [5] Y. Chen, P. Renner, H. Liang, Dispersion of nanoparticles in lubricating oil: A critical review, Lubricants 7 (2019) 7 https://doi.org/10.3390/lubricants7010007. [6] C. Cerrillo, G. Barandika, A. Igartua, O. Areitioaurtena, G. Mendoza, Towards the standardization of nanoecotoxicity testing: Natural organic matter ‘camouflages’ the adverse effects of TiO2 and CeO2 nanoparticles on green microalgae, Science of The Total Environment 543 (2016) 95-104 https://doi.org/10.1016/j.scitotenv.2015.10.137. [7] S.J. Klaine, P.J. Alvarez, G.E. Batley, T.F. Fernandes, R.D. Handy, D.Y. Lyon, S. Mahendra, M.J. McLaughlin, J.R. Lead, Nanomaterials in the environment: behavior, fate, bioavailability, and effects, Environmental Toxicology Chemistry: An International Journal 27 (2008) 1825-1851 https://doi.org/10.1897/08-090.1. [8] C. Birleanu, M. Pustan, M. Cioaza, A. Molea, F. Popa, G. Contiu, Effect of TiO2 nanoparticles on the tribological properties of lubricating oil: an experimental investigation, Scientific Reports 12 (2022) 5201 https://doi.org/10.1038/s41598-022-09245-2. [9] V. Cortes, K. Sanchez, R. Gonzalez, M. Alcoutlabi, J.A. Ortega, The performance of SiO2 and TiO2 nanoparticles as lubricant additives in sunflower oil, Lubricants 8 (2020) 10 https://doi.org/10.3390/lubricants8010010. [10] Y. Gao, G. Chen, Y. Oli, Z. Zhang, Q. Xue, Study on tribological properties of oleic acid-modified TiO2 nanoparticle in water, Wear 252 (2002) 454-458 https://doi.org/10.1016/S0043-1648(01)00891-2. SECTION II. Chapter 5 109 Figure 5.4 Raman spectra of uncoated SiO2 NPs (blue), SA (orange) and SiO2-SA NPs (grey). Inset detail of Raman spectra for SiO2 at low wavenumbers 5.2 STABILITY RESULTS PAO6 oil was characterized by means of FTIR (Figure 5.5) and by Raman spectroscopy (Figure 5.13). Regarding the FTIR results, the observed peaks are the same than those present in PAO8 from Chapter 5 (Figure 4.4). This is to be expected because all PAOs have the same basic chemical structure, saturated hydrocarbon chains. Thus, the only observed peaks are from C―H and ―CH2 bond vibrations. Figure 5.5 FTIR spectrum of PAO6 0 0.2 0.4 0.6 0.8 1 1.2 300800130018002300280033003800 Absorbance Wavenumber/cm-1 2940 νas C―H 2919 νs C―H bond in ―CH3 2852 νs C―H bond in ―CH2 1461 δ ―CH2 1376 δs ―CH3 721 ϒ ―CH2 30 cm-1 600 cm-1 FÁTIMA MARIÑO FERNÁNDEZ 110 The dispersion method of the SiO2-SA NPs in PAO6, summarized in Figure 5.6, is the same as the used for TiO2-OA NPs (Figure 4.1) and has been further described in Subsection 2.3.1 from Chapter 2 (Figure 2.10b). After the esterification reaction, the SiO2 NPs were washed with the aid of a centrifuge. The collected NPs were redispersed in n-hexane for storage and subsequent handling. After this purification process, the concentration of SiO2-SA NPs was obtained by difference in weight of 1 mL of the nanodispersion before and after the evaporation of n-hexane. Then, a known amount of n-hexane nanodispersion was added to a given amount of PAO6. This new dispersion was homogenized and then heated up to 95 ºC, remaining at this temperature for 60 min to remove the n-hexane, obtaining a 4.7 wt% SiO2-SA + PAO6 nanodispersion. Finally, this dispersion was diluted to obtain nanodispersions with concentrations of 0.3, 0.2, 0.1 and 0.05 wt% in SiO2-SA. In order to further enhance the stability time of these last nanodispersions, the same wt% of SA as of SiO2-SA was added as surfactant. Figure 5.6 Scheme of the dispersion method The stability of the nanodispersions was assessed by visual observation over time until sedimentation was detected. The images are shown in Figure 5.7. Just after their preparation, the SiO2 and SiO2-SA + SA nanodispersions presented a high transparency (Figures 5.7a and 5.7d). The nanodispersion with uncoated SiO2 NPs sedimented after 24 h. Thus, a yellow-white turbidity appeared in the bottom of the flask (Figure 5.7b), turning more yellow and compact over time (Figure 5.7c). In contrast, the SiO2-SA nanodispersion kept the same slightly white and transparent appearance even after 100 days (Figure 5.7f). Therefore, the nanodispersion containing the modified SiO2 NPs showed an important enhancement of the stability compared to that containing unmodified SiO2 NPs. Figure 5.7 Photographs of PAO6 + 0.2 wt% SiO2 and PAO6 + 0.2 wt% SiO2-SA + 0.2 wt% SA dispersions at 0 h (a,d), 24 h (b,e) and 2400 h (c,f) after their preparation SECTION II. Chapter 5 111 5.3 THERMOPHYSICAL PROPERTIES Density and dynamic viscosity of the base oil and the nanolubricant PAO6 + 0.20 wt% SiO2SA + 0.20 wt% SA were measured at 0.1 MPa from 5 to 100 ºC using a rotational viscosimeter Stabinger SVM3000 (Anton Paar, Graz, Austria). The experimental results are presented in Table 5.2. The density of this nanodispersion had an average absolute variation of 0.012 %, with respect to PAO6 (Table 5.2), which is lower than the density uncertainty (0.06 %) meaning that the addition of SiO2-SA and SA additives does not affect the density value of PAO6 at the studied temperature range and 0.1 MPa. Regarding the dynamic viscosity (Table 5.2), the effect of both additives (SA and SiO2-SA) is greater, leading to increments around 9 % and up to a 13 % at higher temperatures. Furthermore, viscosity index was also measured, which is for the nanodispersion (VI 154) 12 % higher to that of PAO6 (VI 0.138). Table 5.2 Density (ρ) and dynamic viscosity (η) of PAO6 and PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA nanodispersion as a function of temperature, T, at 0.1 MPa PAO6 0.20 wt% SiO2-SA + 0.20 wt% SA PAO6 0.20 wt% SiO2-SA + 0.20 wt% SA T/ºC ρ/g cm-3 ρ/g cm-3 η/mPa·s η/mPa·s 5 0.8329 0.8328 137.9 151.3 10 0.8298 0.8297 102.8 113.1 15 0.8268 0.8267 78.15 86.19 20 0.8238 0.8237 60.36 66.73 25 0.8207 0.8206 47.55 52.67 30 0.8176 0.8175 37.70 41.86 35 0.8145 0.8144 30.40 33.84 40 0.8114 0.8114 24.84 27.71 45 0.8084 0.8083 20.54 22.95 50 0.8053 0.8052 17.17 19.22 55 0.8021 0.8021 14.50 16.26 60 0.7990 0.7989 12.36 13.89 65 0.7959 0.7958 10.63 11.96 70 0.7928 0.7927 9.207 10.39 75 0.7897 0.7896 8.040 9.102 80 0.7866 0.7864 7.070 8.041 85 0.7834 0.7833 6.262 7.063 90 0.7803 0.7802 5.582 6.300 95 0.7772 0.7770 5.008 5.659 100 0.7740 0.7739 4.521 5.111 Similarly to Chapter 4, to obtain the Stribeck curves of the rolling-sliding tribological tests at 120 °C, the dynamic viscosity of each nanolubricant and the base oil (PAO6) at this temperature is needed. For this reason, the experimental viscosities were correlated with the VFT equation [18]. FÁTIMA MARIÑO FERNÁNDEZ 112 5.4 TRIBOLOGICAL RESULTS 5.4.1 Pure sliding conditions Table 5.3 and Figure 5.9 summarize the average values of the COF for PAO6 and for the studied lubricants. All the nanodispersions (PAO6 + SiO2-SA + SA) lead to coefficients of friction lower than that measured for the PAO6 base oil, the reductions varying from 15 to 56 %, depending on the concentration. The mass concentrations of the dispersions leading to the most significant COF reductions are (0.2 and 0.3) wt% SiO2-SA + (0.2 or 0.3) wt% SA in PAO6. Comparing the coefficients of friction of the PAO6 + 0.2 wt% SiO2-SA + 0.2 wt% SA nanodispersions with the PAO6 + 0.2 wt% SA mixture, the former showed a 17 % COF reduction compared to the latter. SA and SiO2-SA NPs seem to have a synergistic effect: the reduction of COF of the combined additives (53 %) is higher than the reductions of friction obtained when only one of the additives is present, the PAO6 + SA mixture shows a 44 % reduction and the PAO6 + SiO2-SA nanodispersion a 15 % reduction, both compared to the base oil. In addition, the SA coating on the SiO2 NPs surfaces leads to a higher COF reduction than uncoated SiO2 NPs, which did not have any COF reduction effect on the PAO6 base oil. Besides, the PAO6 + 0.2 wt% SiO2-SA + 0.2 wt% SA nanodispersion has shown higher COF reduction than that of PAO6 + 0.2 wt% ZDDP mixture, which led to 33 % reduction compared to PAO6. Table 5.3 Average coefficients of friction, COF, at 120 ºC and the expanded uncertainty, U, for PAO6 base oil and all studied lubricants Lubricants COF U Reduction % compared to PAO6 PAO6 0.1542 0.0026 + 0.05 wt% SiO2-SA + 0.05% SA 0.1316 0.0021 15 + 0.10 wt% SiO2-SA + 0.10 % SA 0.1264 0.0014 18 + 0.20 wt% SiO2-SA + 0.20 % SA 0.0719 0.0015 53 + 0.30 wt% SiO2-SA + 0.30 % SA 0.0680 0.0003 56 + 0.2 wt% SA 0.0864 0.0048 44 + 0.2 wt% SiO2 0.1554 0.0039 0 + 0.2 wt% SiO2-SA 0.1303 0.0063 15 + 0.2 wt% ZDDP 0.1034 0.0022 33 5.4.2 Wear Surface characterization The topology of the worn surfaces of the pins tested in the friction tests was analyzed using a 3D profilometer. Table 5.4 summarizes the average values of WTW, WTD and worn area and the expanded uncertainties. The WTD values are plotted in Figure 5.8 for the studied lubricants. All the nanodispersions (SiO2-SA + SA) lead to lower WTD values than PAO6. In what regards to wear, the optimum nanolubricant leading to the highest reductions for all the parameters (55 %, WTW; 86 %, WTD; and 92 %, worn area) is the PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA nanodispersion. SECTION II. Chapter 5 113 Table 5.4 Average values of the width, WTW, depth, WTD, and area of worn tracks and the expanded uncertainties, U, for PAO6 and all studied lubricants Lubricants WTW/ µm U/ µm WTD/ µm U/ µm Area/ µm2 U/ µm2 PAO6 435 10 2.60 0.19 806 65 + 0.05 wt% SiO2-SA + 0.05 wt% SA 256 14 0.99 0.12 200 32 + 0.10 wt% SiO2-SA + 0.10 wt% SA 268 19 0.55 0.05 104 18 + 0.20 wt% SiO2-SA + 0.20 wt% SA 195 17 0.35 0.06 40 9 + 0.30 wt% SiO2-SA + 0.30 wt% SA 240 18 0.51 0.07 61 12 + 0.20 wt% SA 422 19 2.54 0.25 723 76 + 0.20 wt% SiO2 471 21 3.01 0.22 967 69 + 0.20 wt% SiO2-SA 342 14 1.82 0.21 353 44 + 0.20 wt% ZDDP 232 12 0.64 0.11 81.2 13 In Figure 5.9 the profiles of the worn surface obtained with PAO6 and each nanodispersion (PAO6 + SiO2-SA + SA) are shown. The largest groove is obtained using the base oil, and the smallest one using PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA nanodispersion. Comparing the PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA nanodispersion with the PAO6 + 0.20 wt% SA mixture, the former showed an 86 % reduction in WTD compared to the latter, which only led to a wear reduction of 3 % in WTD compared to PAO6. The reductions in WTD obtained with PAO6 + 0.20 wt% SiO2-SA was also lower (30 %) than using combination of both additives (86 %), which indicates a great synergy among both additives. This trend is similar to that obtained for the COF results. Hence, the nanodispersion containing both SiO2-SA and SA showed an enhanced tribological behavior. Regarding the SA coating effect on the SiO2 antiwear results, the commercial SiO2 NP was the only studied nanodispersion that worsen the PAO6 wear results, increasing a 15 % the WTD compared with PAO6. Furthermore, the optimal nanodispersion of PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA also has a better wear reduction capability compared to the PAO6 + 0.20 wt% ZDDP, which had a 75 % WTD reduction compared to PAO6 base oil. In view of these results, from now on only PAO6 + SiO2SA + SA nanodispersions will be considered. Figure 5.8 Average coefficients of friction, COF, (blue) and wear track depth, WTD, (orange) with the expanded uncertainty bars for PAO6 and the studied lubricants based on PAO6 (the orange line is a guide for the eye) 0 0.5 1 1.5 2 2.5 3 3.5 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 WTD/µm COF FÁTIMA MARIÑO FERNÁNDEZ 114 Figure 5.9 Profile images of worn tracks lubricated with PAO6 (blue) and PAO6 + wt% SiO2-SA + wt% SA nanodispersions (0.05 wt%, orange; 0.10 wt% grey; 0.20 wt%, yellow; 0.30 wt%, green) The arithmetic average roughness, Ra, was also measured through 3D profilometry, considering a gaussian filter 0.25 mm. In Table 5.5 the average values of Ra are plotted for all the worn surfaces lubricated with PAO6 and PAO6 + SiO2-SA + SA nanodispersions, as well as for the unworn surface of a pin. All the nanodispersions led to worn tracks with smaller Ra than those of the unworn surface and of the worn pins lubricated with PAO6. The lowest Ra is that of the worn surface lubricated with PAO6 + 0.3 wt% SiO2-SA being 41 % lower than that of the unworn surface, and 53 % of the worn surface lubricated with PAO6. However, 0.10 and 0.20 wt% nanodispersions exhibit very similar results with more than a 30 % reduction compared to the unworn surface and a 45 % compared to the worn surface from PAO6. Another parameter used to characterize the topology of the surfaces is the root mean square roughness (Rq), which is more sensitive to large deviations from the mean line than Ra [19]. The same trend with lubricants and pins, for Ra and Rq, have been found, as can be seen in Table 5.5. Table 5.5 Average values of the Ra and Rq of untested pin surface, worn tracks and the expanded uncertainties, U, for PAO6 and PAO6 + wt% SiO2-SA + wt% SA nanodispersions Ra/nm U/nm Rq/nm U/nm PAO6 11.2 1.4 15.3 2.4 + 0.05 wt% SiO2-SA + 0.05 wt% SA 8.5 0.6 10.4 0.7 + 0.10 wt% SiO2-SA + 0.10 wt% SA 6.1 1.0 8.8 2.5 + 0.20 wt% SiO2-SA + 0.20 wt% SA 6.2 1.3 7.3 1.5 + 0.30 wt% SiO2-SA + 0.30 wt% SA 5.2 0.7 6.2 0.9 Untested Pin 8.9 0.6 10.4 0.6 3D images of the worn tracks of the steel balls used in the friction tests are shown in Figure 5.11. The nanolubricant containing SiO2-SA and SA as dispersant showed the lowest wear on the ball, 172.6 µm for WTW. Interestingly, the absence of SA dispersant (PAO6 + 0.20 wt% SiO2 and PAO6 + 0.20 wt% SiO2-SA) leads to WTW values similar or higher than that obtained with neat PAO6 base oil. However, the use of only SA as additive generated a slightly higher WTW value (201 µm) compared to the optimal nanodispersion, which is coherent with the wear results from the pins, thus a synergistic effect of both additives is observed. The ball lubricated -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 0200 400 600 800 1000 1200 1400 µm µm PAO6 0.05 wt% SiO2-SA + 0.05 wt% SA 0.10 wt% SiO2-SA + 0.10 wt% SA 0.20 wt % SiO2-SA + 0.20 wt% SA 0.30 wt% SiO2-SA + 0.30 wt% SA SECTION II. Chapter 5 115 with PAO6 containing ZDDP as additive showed good wear reduction, although the WTW value was higher (198 µm) than that obtained for the optimal nanodispersion. Figure 5.10 3D images of the worn ball surfaces for (a) PAO6, (b) PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA, (c) PAO6 + 0.20 wt% SA, (d) PAO6 + 0.20 wt% SiO2, (e) PAO6 + 0.20 wt% SiO2-SA, (f) PAO6 + 0.20 wt% ZDDP, and their WTW values As regards to Raman microscopy, for the assignation of the components in the worn surface, the signals were separated using the software Project FIVE. Figures 5.12 and 5.13 show the Raman mappings of the worn tracks of pins with distinctive areas indicated with several colors corresponding to different compounds. Regarding the worn pins tested with the base oil (Figure 5.11), the presence of PAO6 (blue), as well as of iron oxides (yellow) and of carbon (green) coming from the lubricant degradation were observed (Figure 3.18). On the other hand, the Raman mapping from the 0.20 wt% SiO2-SA + 0.20 wt% SA nanodispersion in PAO6 (Figure 5.12) indicates the presence of SiO2 NPs (red), as well as of PAO6 or SA (blue) and of carbon (green). Regarding the blue areas in the Raman mapping in Figure 5.13, the corresponding intense peaks around 3000 cm-1 are characteristic C-H signals. Both PAO6 and SA have saturated alkyl chains which present very similar Raman spectra (Figure 5.13) with the most intense peaks being the 3000 cm-1 ones. Thus, it is not possible to differentiate those compounds in the Raman mapping of the pin lubricated with the nanodispersion. The presence of SiO2 NPs is confirmed by the appearance of an intense and sharp peak close to 500 cm-1 [20]. This peak does not appear in the Raman spectrum of iron oxide, which instead presents peaks of similar intensity in the lower region of the spectrum (Figure 5.11) [21], i.e., with the addition of SiO2SA NPs, iron oxides were not observed. On the other hand, in the Raman spectra of the mapping of Figure 5.12, the lack of SA signal in the SiO2 spectrum (red areas in the mapping) reveals that, during the tribo-tests, some tribo-chemical reactions occur promoted by the high both temperatures and pressures due to the friction process. Thus, these conditions cause the breaking of the bonds between SA and the SiO2 NPs, similarly to what was found by Zhang et al. [22] for SA modified TiO2 NPs through XPS. Assuming the same hypothesis as these authors [22], the uncoated SiO2 NPs are easily adsorbed on the worn surface, generating a boundary lubricating film. Moreover, SA could be physically adsorbed on the steel surface during the FÁTIMA MARIÑO FERNÁNDEZ 116 tribotests [23-25] and be partially removed with the hexane solvent in the cleaning process of the worn pins before the confocal Raman analysis. Figures 5.11 and 5.12 show that the chemical components are located in the grooves formed during the tribological experiments. When adding the SiO2-SA NPs and SA to the PAO6 lubricant the tribofilm created in the worn track is mainly composed by SiO2 NPs and carbon, being the tribological mechanisms governed by the NPs rather than the base oil or the SA dispersant. Considering the roughness values and these Raman mappings it can be assumed that on the worn surface tribofilm formation, polishing and/or mending effects occur. Figure 5.11 Elemental mapping and Raman spectra of worn pins tested with PAO6 base oil Figure 5.12 Elemental mapping and Raman spectra of worn pins tested with 0.20 wt% SiO2-SA + 0.20 wt% SA nanolubricant SECTION II. Chapter 5 117 Figure 5.13 Raman spectra of PAO6 (orange) and SA (blue) Regarding the SEM micrographs of the worn pins (Figure 5.14), an inspection of these images reveals that abrasive wear in the sliding direction is the main wear mechanism in the case of the worn surface lubricated by PAO6, with evident ploughing even at the lowest magnification (500x). This type of wear is slightly reduced when using SA as the only additive. The addition of SiO2 or SiO2-SA NPs reduce the grooves in the worn surface but micro-cracks on the surfaces appears. In addition, when ZDDP is used as additive to PAO6, although the wear track is considerably reduced, the surface presents similar damage. The least damaged surface is the one obtained with PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA, which again highlights the positive synergies between both additives, that increase scuffing resistance and reduce wear. These results agree with those obtained with profilometry for the pins. 400 600 800 1000 1200 1400 1600 1800 2000 0 500 1000 1500 2000 2500 3000 3500 Wavenumber/cm-1 PAO6 SA FÁTIMA MARIÑO FERNÁNDEZ 118 Figure 5.14 SEM micrographs at three magnifications of the worn pin surfaces lubricated by PAO6 and lubricants based on PAO6 125 6 CONCLUSIONS AND FUTURE WORK This work was carried out, under two national research projects, to contribute to the development of nanolubricants for two key applications: gearboxes in wind turbines and electric drivetrains systems in electric vehicles. The main conclusions of this PhD Thesis are: 1. Selection, synthesis, and characterization of functionalized nanoadditives to design new potential nanolubricants: From the preliminary work performed during this PhD Thesis and the in-depth analysis of the literature in the field of tribology of oil-based nanolubricants, we concluded that the best stability times and tribological results were achieved with spherical NPs with sizes below 20 nm coated with organic acids that contain aliphatic chains longer than 12 carbon atoms. Hence, spherical NPs of metal and ceramic oxides with diameters between 5 and 10 nm were selected. Two types of them were purchased, TiO2 NPs (5 nm) and SiO2 NPs (8 nm); and the others were synthesized, ZnO NPs (10 nm). The selected modifying agents were oleic acid (OA) and stearic acid (SA), both with an aliphatic chain of 18 carbon atoms. The selected NPs were surface modified on iMATUS laboratories, obtaining the following coated NPs: ZnO-OA, TiO2-OA, and SiO2-SA. The chemical modification of all the NPs weas obtained by esterification reaction, which was proven successful by subsequent analysis of the coated NPs via FTIR. To complete the characterization of these NPs, other techniques were used: the morphology was identified by SEM or TEM, and, in the case of ZnO NPs, the chemical composition was also verified by XRD. 2. Obtaining long-term stable nanolubricants of coated nanoadditives in PAOs: After preliminary work and the analysis of the literature on preparation methods, the evaporation method was chosen for all the nanodispersions. Without any dispersant, for the PAO40 + 0.25 wt% ZnO-OA nanolubricant a stability time of 29 days was obtained, against no dispersibility for the uncoated ZnO NPs. Using OA as dispersant at a concentration of 0.20 wt%, stability times are four weeks for the three PAO8 nanolubricants containing TiO2-OA up to 0.35 wt%, in contrast to 48 h stability for the PAO8 + TiO2 nanodispersions. In this PhD Thesis, the longest stability time was obtained for PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA, being 100 days, whereas for the PAO6 + 0.20 wt% SiO2 the stability time was less than 24 h. The effectiveness of the coating in increasing the stability time has been verified. 3. Evaluation of thermophysical properties of the potential nanolubricants: the density and dynamic viscosity at atmospheric pressure and temperatures from 5 to 100 ºC, as well as the viscosity index, of the PAO base oils and the studied nanodispersions were analyzed: - Regarding the density results, all nanodispersions studied led to low density variations with respect to the corresponding base oil. The density showed a concentration dependence: the higher the NP concentration, the higher the density value, being 0.5 % the highest average absolute deviation with respect to PAO40 for the PAO40 + 1.00 wt% ZnO-OA nanodispersion. Moreover, for these nanodispersions, the experimental results were compared with two predictive models: the Pak and Cho model provided the closest predictions, with a maximum FÁTIMA MARIÑO FERNÁNDEZ 126 of 0.28 % average absolute deviation. All the other nanodispersions (PAO8 + 0.35 wt% TiO2OA + 0.20 wt% OA; and PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA) led to much lower deviations from net oil behavior (0.06 % and 0.012 %), which are close or lower than the density uncertainty of the set-up. - For the dynamic viscosities the largest variations (around 15 %) were obtained for the nanodispersions also containing a dispersant (PAO8 + 0.35 wt% TiO2-OA + 0.20 wt% OA or PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA). The nanodispersions containing only the functionalized NPs (PAO40 + ZnO-OA) showed smaller increases in the dynamic viscosity, around 4 %, showing a dependence on the concentration of the nanoadditive (similarly to the density behavior). As regard to the two predictive models applied to PAO40 + ZnO-OA, the best estimates were obtained with the Chen et al. equation (0.87 % AAD). - The effect of the additive on the viscosity index was quite small for all the studied nanodispersions: the addition of ZnO-OA NPs resulted in a slight decrease of the viscosity index (4 %) and for the other studied nanodispersions a slight increment was found, reaching up to a 7% for PAO8 + 0.35 wt% TiO2-OA + 0.20 wt% OA. 4. Evaluation of the influence of concentration, type of functionalized nanoadditive, dispersant and comparison with commercial additives on the tribological behavior under pure sliding conditions: All the tests performed under pure sliding conditions were carried out at high temperatures, 80 ºC for lubricants containing PAO40 and 120 ºC for those containing PAO8 or PAO6, and for sliding distances of 180 m for the former and 340 m for the latter. All other parameters (speed, distance, tribo-contact materials) remained constant. The coefficient of friction and wear parameters vary according to different specifications such as: - Concentration effect: all the nanodispersions studied showed optimum concentrations at which the highest friction or wear reductions were obtained. For ZnO-OA and TiO2-OA nanodispersions, the highest reductions of friction and wear were obtained at the same optimum concentration (0.25 wt% ZnO-OA and 0.35 wt% TiO2-OA), whereas for SiO2-SA nanolubricants, the highest reduction for COF corresponds to the optimum concentration 0.30 wt% SiO2-SA and for all the wear parameters was 0.20 wt% SiO2-SA. At the optimum concentrations, the COF reduction goes from 25 % (for ZnO-OA NPs) to 56 % (for SiO2-SA NPs + SA), and the WTD reductions from 65 % (for TiO2-OA + OA) to 86 % (for SiO2-SA NPs + SA). - The effect of the modifying agent was evaluated in the case of PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA nanodispersion. The corresponding unmodified SiO2 did not improve the antifriction capability of the base oil. The SA coating was shown to improve the antifriction capability of the nanoparticle with a friction reduction of 15% compared with the COF of the nanodispersion containing unmodified SiO2 NPs. Furthermore, uncoated SiO2 NPs worsen wear compared to PAO6, whereas the SiO2-SA NPs improve the antiwear capability of the base oil: 22 % in WTW, 30 % in WTD, and 50 % in cross-sectional area. - The effect of the dispersant in the PAO6 + 0.20 wt% SiO2-SA nanolubricant was also determined experimentally. The PAO6 + 0.20 wt% SA reduced the COF by up to 44 % compared to PAO6, but only reduced WTD by 2 %. In addition, the PAO6 + 0.20 wt% SiO2SA + 0.20 wt% SA nanolubricant reduced COF by up to 45 % compared to PAO6 + 0.20 wt% SiO2-SA, and by 65 % WTD. Hence, there is a synergistic effect between the SiO2-SA NPs and the SA dispersant, with the combination of both additives showing the greatest reductions of friction and wear. Chapter 6 127 - Furthermore, the tribological performance of PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA nanolubricant was compared with that of PAO6 + 0.20 wt% ZDDP, showing that the former lubricant led to the following reductions with respect to the latter: 30 % in COF, 16 % in WTW, 45 % WTD, and 51 % in cross-sectional area. Thus, the combined use of SiO2-SA NPs and SA has proven to be an alternative to ZDDP. 5. Study of the tribochemical processes occurring in the interface between the nanolubricant and the worn lubricated surfaces: to identify the tribological mechanisms, the roughness and the mapping composition of the worn areas were analyzed using 3D profilometry and confocal Raman microscopy, respectively. For the nanolubricants at the optimal concentration, the Ra values of the worn pins were lower than that of the pin lubricated with the corresponding base oil and also lower than that of the unworn surface. Hence, the three NPs lead to polishing and/or mending mechanisms. According to the confocal Raman microscopy results, the presence of NPs is observed in the mappings, through the identification of OA (distinguishing it from PAOs due to its double bond) or through the presence of peaks corresponding to the NPs (SiO2). During the tribotests, the NPs are also adsorbed on the rubbing surface through tribofilm formation. Due to the sphericity of all the used NPs, it is likely that rolling mechanism occurs at boundary conditions. Furthermore, for the PAO40 + ZnO-OA nanolubricant, it was proven by ICP-MS that the amount of Zn present in the sample after the tribological test was lower than before, i.e., part of the ZnO-OA NPs is adsorbed on the steel surface. 6. Evaluation of the tribological behavior of EV nanolubricants under rolling-sliding conditions: From the Stribeck curves obtained for PAO8 + TiO2-OA +OA and PAO6 + SiO2SA + SA nanodispersions at 120 ºC and with a SRR of 5%, the conditions of lubrication regimes were identified. In the case of the optimal nanolubricants, PAO8 + 0.35 wt% TiO2-OA + 0.20 wt% OA and PAO6 + 0.30 wt% SiO2-SA + 0.30 wt% SA, boundary behavior occurs up to the specific film thickness value, Λ, of 0.07 for the first one, and 0.08 for the latter, whereas the mixed regime appears from 0.07 to 1, and from 0.08 to 0.6, respectively. For the last nanolubricant important friction reductions up to Λ=0.6 were found. Up to our knowledge so strong reductions have not been reported in the literature, in similar sliding-rolling conditions. The reductions found at sliding-rolling conditions is coherent with the 56% COF reduction found in pure sliding conditions which is due to the combined effect of the SiO2-SA NPs and the SA dispersant. Taking into account the tribological and stability results PAO6 + 0.20 wt% SiO2-SA + 0.20 wt% SA nanolubricant is the best nanolubricant designed in this PhD Thesis. Taking into consideration the above conclusions, the following suggestions are proposed as future work: 1. 1. Based on the literature review presented in Chapter 1, NPs of diameters between 5 and 10 nm were analyzed in this PhD Thesis. It is needed to analyze even smaller NPs, such as carbon dots or other types of quantum dots. 2. The chemical modification of the NPs surface with amphiphilic molecules has been shown to improve stability in all the studied cases, further studies with different modifying agents are needed. Moreover, the synthesis of the NPs prior to (or at the same time as) the modification reaction instead of using commercial NPs should be further investigated to avoid aggregation and thus the stability of the nanodispersions. FÁTIMA MARIÑO FERNÁNDEZ 128 3. The addition of SA as a dispersant showed an interesting synergy with the SiO2-SA nanodispersion in PAO6; an in-depth study of this type of synergies of dispersants (amines or organic acids) and coated NPs (with the same amines or organic acids) should be performed. 4. NPs of metal and ceramic oxides have proven to be excellent antifriction and antiwear additives in PAOs, the combination of two or more of these NPs should be considered to search possible synergies. Additionally, the tribological study of these kind of NPs in formulated oils should be performed, especially with the SiO2-SA NPs. 5. For the development of lubricants for gearboxes for wind turbines, additional tribological (elastohydrodynamic lubrication, Stribeck curves, film thickness and friction torque) experiments with loads and speeds conditions similar to those of these gearboxes should be envisaged. In addition, compatibility with other additives and with other materials as well as the oxidative resistance of the formulated nanolubricants should be evaluated. The environmental effect of PAO40 + ZnO-OA should also be investigated. 6. For the development of lubricants for electric drivetrains, it is needed to analyze other properties that were not considered as specific objectives of this PhD Thesis, such as thermal or electrical conductivities of the nanolubricants or the compatibility with components of the transmission system like copper or polymers. The toxicity of these new nanomaterials should also be investigated. 129 APPENDIX A. PUBLICATIONS AND CONFERENCES List of publications used in this thesis with quality index and publication rights 1. Mariño, F., López, E. R., Arnosa, Á., Gómez, M. A. G., Piñeiro, Y., Rivas, J., ÁlvarezLorenzo, C., & Fernández, J. ZnO nanoparticles coated with oleic acid as additives for a polyalphaolefin lubricant. Journal of Molecular Liquids, 2022, 348, 118401. https://doi.org/10.1016/j.molliq.2021.118401 Research article, ELSEVIER, ISSN; 01677322, EISSN: 1873-3166, Open Access Journal quality index: • Journal Rank (JCR in 2022): Physics, Atomic, Molecular & Chemical, 4/35 Q1 (6.0) • Field-weighted citation impact*: 4.37 • Number of citations (publication): 16 (25/08/2023) * Field-Weighted Citation Impact shows how well this document is cited when compared to similar documents. A value greater than 1.00 means the document is more cited than expected. Publication rights: FÁTIMA MARIÑO FERNÁNDEZ 130 2. Liñeira del Río, J. M., Mariño, F., López, E. R., Gonçalves, D. E., Seabra, J. H., & Fernández, J. Tribological enhancement of potential electric vehicle lubricants using coated TiO2 nanoparticles as additives. Journal of Molecular Liquids, 2023, 371, 121097. https://doi.org/10.1016/j.molliq.2022.121097 Research article, ELSEVIER, ISSN; 01677322, EISSN: 1873-3166, Open Access Journal quality index: • Journal Rank (JCR in 2022): Physics, Atomic, Molecular & Chemical, 4/35 Q1 (6.0) • Field-weighted citation impact*: 4.69 • Number of citations (publication): 3 (25/08/2023) Publication rights: 3. Mariño, F., Liñeira del Río, J. M., López, E. R., & Fernández, J. Chemically Modified Nanomaterials as lubricant additive: time stability, friction, and wear. Journal of Molecular Liquids, 2023, 382, 121913. https://doi.org/10.1016/j.molliq.2023.121913 Review article ELSEVIER, ISSN; 0167-7322, EISSN: 1873-3166, Open Access Journal quality index: • Journal Rank (JCR in 2022): Physics, Atomic, Molecular & Chemical, 4/35 Q1 (6.0) • Field-weighted citation impact*: 4.69 • Number of citations (publication): 1 (25/08/2023) APPENDIX A 131 Publication rights: 4. F. Mariño, J. M. Liñeira del Río, D. E. P. Gonçalves, J. H. O. Seabra, E. R. López, J. Fernández. Effect of the addition of coated SiO2 nanoparticles on the tribological behavior of a low-viscosity polyalphaolefin base oil. Wear, 2023, 350-351, 205025. (Open Access) https://doi.org/10.1016/j.wear.2023.205025 Research article, ELSEVIER, ISSN; 0043-1648, EISSN: 1873-2577 Journal quality index: • Journal Rank (JCR in 2022): Engineering, Mechanical, 23/135 Q1 (5.0) • Number of citations (publication): 0 (25/08/2023) Publication rights: FÁTIMA MARIÑO FERNÁNDEZ 132 5. F. Mariño, J. M. Liñeira del Río, E. R. López, J. Fernández. Influence of the nanoparticle coating agent on stability time and tribological performance on potential e-transmission nanofluids, Proceedings of the eighth “International Conference on Lubrication, Maintenance and Tribotechnology”, LUBMAT 2023, Preston, UK, 17th to 19th of July 2023. Complementary publications 6. Villamayor, A., Guimarey, M. J., Mariño, F., Liñeira del Río, J. M., Urquiola, F., Urchegui, R., Comuñas, P. M. J., & Fernández, J. High-Pressure Thermophysical Properties of Eight Paraffinic, Naphthenic, Polyalphaolefin and Ester Base Oils. Lubricants, 2023, 11 (2), 55. https://doi.org/10.3390/lubricants11020055 Research article This complementary publication evaluates the thermophysical properties at high pressure of four mineral (paraffinic and naphthenic) and four synthetic (polyalphaolefin and ester) base oils. The two PAOs are PAO4 and PAO16, which could be used as transmission fluids in EVs. Journal quality index: • Journal Rank (JCR in 2022): Engineering, Mechanical, 46/135 Q2 (3.5) Conference contributions 1. Mariño, F., Yañez S., Gómez, M. A. G., Piñeiro, Y., Rivas, J., Fernández, J., R López, E. R., Comuñas, P. M. J.; Estudio tribológico de dispersiones de ZnO con recubrimiento de ácido oleico en PAO 40 (poster); VII Encontro da Mocidade Investigadora. Santiago de Compostela (Spain), 27th -29th May 2019. 2. Mariño, F., Fernández, J., R López, E. R.; Nanolubricants based on silane-coated nanoparticles using supercritical CO2 (poster); 1º Encontro Ibérico de Fluidos Supercríticos (EIFS2020). Santiago de Compostela (Spain), 18th -20th February 2020. 3. Mariño, F., Fernández, J., R López, E. R.; Design and characterization of functionalized nanoparticle-based lubricants (oral presentation); 10th International Seminar on Thermodynamic Engineering of Fluids. Tarragona (Spain) and online, 22nd-23rd July 2021. 4. Mariño, F., Fernández, J., R López, E. R.; Estudio tribológico de nanolubricantes (oral presentation); IX Encontro da Mocidade Investigadora. Santiago de Compostela (Spain), 20th - 22nd Jun 2022. APPENDIX A 133 5. Mariño, F., Fernández, J., R López, E. R.; Lubricantes basados en nanopartículas funcionalizadas: diseño y caracterización (oral presentation); Thesis Pitch, iMATUS. Santiago de Compostela (Spain), 21st July 2022. 6. Mariño, F., Fernández, J., R López, E. R.; ZnO nanoparticles coated with oleic acid as additives for a polyalphaolefin lubricant (oral presentation); IBERTRIB 2022. Setúbal (Portugal), 6th -7th October 2022. 7. Mariño, F., Liñeira del Río, J. M., López, E. R., Fernández, J. Reviewing Stability over Time and Tribological Performance of Non-Aqueous Lubricants containing Chemical Modified Nanoadditives (oral presentation); IBERTRIB 2022. Setúbal (Portugal), 6th -7th October 2022. 8. Liñeira del Río, J. M., Mariño, F., Gonçalves, D. E. P., Seabra, J. H. O., López, E. R., Fernández, J.; Tribological behavior enhancement using coated TiO2 nanoparticles as PAO8 additives (oral presentation); IBERTRIB 2022. Setúbal (Portugal), 6th -7th October 2022. 9. Liñeira del Río, J. M., Castro Currás, A., Somoza, V., Mariño, F., Guimarey, M. J. G., Comuñas, M. J. P., Fernández, J.; Effect of SiO2 and coated SiO2-SA nanoparticles on the lubricant properties of a paraffinic base oil (poster); IBERTRIB 2022. Setúbal (Portugal), 6th -7th October 2022. 10. Guimarey, M. J. G., Liñeira del Río, J. M., Mariño, F., Castro Currás, A., Somoza, V., Comuñas, M. J. P., Fernández, J.; The role of chemical modified SiO2 nanoparticles in the tribological performance of a paraffinic oil (poster); XXVI Encontro Galego-Portugués de Química. Santiago de Compostela (Spain), 16th-18th November 2022. 11. Liñeira del Río, J. M., Guimarey, M. J. G., Mariño, F., Comuñas, M. J. P., López, E. R., Fernández, J.; Nanolubricants stability: surface modification of nanoparticles (poster); XXVI Encontro Galego-Portugués de Química. Santiago de Compostela (Spain), 16th-18th November 2022. 12. Mariño, F., Liñeira del Río, J. M., López, E. R., & Fernández, J.; Influence of the nanoparticle coating agent on stability time and tribological performance on potential etransmission nanofluids (oral presentation); LUBMAT 2023, Preston (United Kingdom), 17th19th July 2023. 13. Mariño, F., López, E. R., & Fernández, J.; On the stability of non-aqueous lubricants containing functionalized nanoparticles (oral presentation); 12th International Seminar on Thermodynamic Engineering of Fluids. Tarragona (Spain) and online, 20th-21st July 2023. 14. Mariño, F., Liñeira del Río, J. M., López, E. R., & Fernández, J.; Effect of the size and coating agents of the NPs and the dispersion method on the stability time of non-aqueous nanofluids (accepted as oral presentation); ECTP 2023, Venice (Italy), 10th-13rd September 2023. Awards Prize for 1st Thesis Pitch (iMATUS) for the oral presentation of Lubricantes basados en nanopartículas funcionalizadas: diseño y caracterización in Santiago de Compostela (Spain), 21st July 2022. https://imatus.usc.es/en/we-already-have-the-imatus-thesis-pitch-winners-heldon-july-21/