Tribological Performance of a Paraffinic Base Oil Additive with Coated and Uncoated SiO2 Nanoparticles

Citation: Liñeira del Río, J.M.; Guimarey, M.J.G.; Somoza, V.; Mariño, F.; Comuñas, M.J.P. Tribological Performance of a Paraffinic Base Oil Additive with Coated and Uncoated SiO 2 Nanoparticles. Materials 2024,17, 1993. https://doi.org/10.3390/ ma17091993 Academic Editor: Csaba Balázsi Received: 15 March 2024 Revised: 11 April 2024 Accepted: 22 April 2024 Published: 25 April 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Tribological Performance of a Paraffinic Base Oil Additive with Coated and Uncoated SiO2Nanoparticles JoséM. Liñeira del Río 1, * , María J. G. Guimarey 1 , Vanesa Somoza 1 , Fátima Mariño 2 and María J. P. Comuñas 1 1Laboratory of Thermophysical and Tribological Properties, Nafomat Group, Department of Applied Physics, Faculty of Physics and Instituto de Materiais (iMATUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; [email protected] (M.J.G.G.); [email protected] (V.S.); [email protected] (M.J.P.C.) 2School of Engineering, University of the Basque Country UPV/EHU, Plaza Ingeniero Torres Quevedo 1, 48013 Bilbao, Spain; [email protected] *Correspondence: [email protected] Abstract: Electric vehicles (EVs) have emerged as a technology that can replace internal combustion vehicles and reduce greenhouse gas emissions. Therefore, it is necessary to develop novel lowviscosity lubricants that can serve as potential transmission fluids for electric vehicles. Thus, this work analyzes the influence of both SiO 2 and SiO 2 -SA (coated with stearic acid) nanomaterials on the tribological behavior of a paraffinic base oil with an ISO VG viscosity grade of 32 and a 133 viscosity index. A traditional two-step process through ultrasonic agitation was utilized to formulate eight nanolubricants of paraffinic oil + SiO 2 and paraffinic base oil + SiO 2 -SA with nanopowder mass concentrations ranging from 0.15 wt% to 0.60 wt%. Visual control was utilized to investigate the stability of the nanolubricants. An experimental study of different properties (viscosity, viscosity index, density, friction coefficient, and wear) was performed. Friction analyses were carried out in pure sliding contacts at 393.15 K, and a 3D optical profilometer was used to quantify the wear. The friction results showed that, for the SiO 2 -SA nanolubricants, the friction coefficients were much lower than those obtained with the neat paraffinic base oil. The optimal nanoparticle mass concentration was 0.60 wt% SiO 2 -SA, with which the friction coefficient decreased by around 43%. Regarding wear, the greatest decreases in width, depth, and area were also found with the addition of 0.60 wt% SiO 2 -SA; thus, reductions of 21, 22, and 54% were obtained, respectively, compared with the neat paraffinic base oil. Keywords: friction; wear; nanolubricants; nanoparticle surface modification 1. Introduction Energy needs are constantly increasing; consequently, the natural environment is significantly impacted. This is the case with the transport sector, as it is responsible for a large part of carbon dioxide, CO 2 , gas emissions, and climate change due to fuelpowered machinery [ 1 ]. For this reason, automotive industries need to develop new technologies to produce highly efficient vehicles for individual and public mobility [ 2 , 3 ]. Thus, electrification designs (hybrid or electric vehicles (EVs)) have emerged as an optimal solution for new propulsion systems to reduce greenhouse gas emissions [ 4 ], although a specific analysis of each country is necessary to ensure proper emissions reduction [ 5 ]. These greenhouse gas reductions, particularly that of CO 2 , greatly depend on the source of the electricity [ 6 ]. When electricity originates from renewable energy sources, the CO 2 emissions of an EV are 4.5 times less than those of a combustion engine car [ 6 ]. Although EVs are very efficient and produce very low exhaust emissions, they have efficiency and endurance issues that affect the moving components and thus their tribology. Thus, tribological solutions such as new materials or optimized lubricants can help to Materials 2024,17, 1993. https://doi.org/10.3390/ma17091993 https://www.mdpi.com/journal/materials Materials 2024,17, 1993 2 of 12 increase the driving range of EVs since tribology can help to enhance the efficiency by lowering friction in elements like gears and wheel bearings [7]. Even though EVs exhibit significantly elevated efficiency in terms of energy use, there are still challenges related to the need to further enhance the efficiency; hence, the improvement of new fluids and materials [ 7 ] and the progression of new batteries [ 8 ] are being promoted. EVs require transmission oil lubricants with greater technical requirements [ 9 ] than those of internal combustion motors; this is because during the operation process contact is made with copper wires, sensors, and circuits [ 10 ]. Moreover, the high rotation rates of the electric motor require the use of lubricants with very low viscosity. If the oil viscosity is reduced, viscous drag and viscous heating drop and, therefore, the heat transfer is raised [ 9 , 11 ]. However, if the viscosity of a lubricant is lowered a shift from full film to boundary lubrication occurs and a more critical surface contact and wear is produced. This circumstance indicates that enhanced anti-wear and anti-friction properties are required. Therefore, to meet the needs of future EV lubricants, it is fundamental to use advanced additives [ 12 ]. The best conventional lubricants used for ICEVs, with the chance to be used in EV automotive elements, are made of mineral-based oils prepared with several additives to meet the rigorous requirements [7]. Recently, nanotechnology-based anti-friction and anti-wear additives were suggested for transmission fluids for EVs [ 7 ]. Then, the research on nanoparticles as oil additives was able to support the advance of a novel production of lubricants with low viscosity that were specifically modified to meet the necessities of EVs (electrified transmissions), owing to their outstanding anti-friction and anti-wear capacities, which can lead to an extended life in the operating conditions of EVs [ 12 – 14 ]. Furthermore, nanomaterials are more ecologically friendly than other conventional additives [ 15 , 16 ]. A crucial part of achieving a proper nanolubricant is the creation of temporal stability for an extended time; the sedimentation of nano-additives can lead to a decrease in efficiency and system damage owing to the abrasive wear [ 17 ]. To improve the stability of nanolubricants, different procedures can be carried out, using surfactants, physical treatment, or chemical surface modification [ 18 , 19 ]. The requirement of stable nanolubricants is particularly important for those lubricants composed of oils with low viscosity owing to the poor stability of the nanoparticles in such fluids. Even though lubricants with additives containing nanomaterials have exhibited good anti-friction and anti-wear performances in conventional lubricants [ 13 , 20 – 22 ], there is scarcely any investigation on nanolubricants with regard to EVs’ tribological needs. Mustafa et al. [ 9 ] have recently reviewed the tribological performance of several lowviscosity lubricants, based on different polyalphaolefin low-viscosity base oils and water. For instance, Chou et al. [ 23 ] analyzed the effect of adding Ni nanoparticles (20 nm) on the tribological activity of PAO6 base oil; they observed a reduction in friction between 7% and 30% and in wear between 5% and 45%, and they achieved the highest friction and wear reductions with the PAO6 + 0.5 wt% Ni nanolubricant. Because of the needs, it is necessary to improve and study potential stable lubricants formed through base oils with low viscosity and nano-additives. In this investigation, a paraffinic G-III base oil was selected to meet those qualities. The nano-additives used in this work, SiO 2 nanoparticles, have exclusive physical and chemical characteristics; therefore, they can be used in several fields, such as in adhesives, the textile industry, and lubrication [ 24 ]. In fact, SiO 2 as a lubricant additive usually shows an excellent anti-wear property, due to the fact that SiO 2 has hydroxyl and unsaturated bonds and can form a solid chemical adsorption film to protect the metallic surface, significantly improving the friction performance of the lubricating oil. Furthermore, it has good electrical, optical, and magnetic properties and has received considerable interest in terms of applications such as those of catalysis, pharmaceuticals, drug delivery, and pigments. SiO 2 nanopowder is a solid and colorless crystalline substance, which does not react with water and is resistant to acids. Furthermore, in this research, commercial SiO 2 nanoparticles were chemically functionalized with stearic acid (SA) through an esterification process to enhance their stability in G-III base oil. SiO 2 nanoparticles were studied as lubricant additives and demonstrated good ability as Materials 2024,17, 1993 3 of 12 friction and wear modifiers [ 25 – 30 ]. For instance, Cortés et al. [ 25 ] studied the tribological performance of non-coated SiO 2 nanoparticles, such as the additives of a vegetable oil, achieving decreases of up to 77% in terms of friction and 74% in terms of wear volume. Additionally, some authors [ 17 , 26 ] functionalized the SiO 2 nanoparticle surface with the aim of enhancing the temporal stability of the nano-dispersions. For instance, Peng et al. [ 17 ] coated the SiO 2 nanoparticles with oleic acid (SiO 2 -OA), reaching a temporal stability of about one month for a paraffin oil, with the mass percentages shifting from 0.05 to 1.0 wt% of the SiO2-OA. In this article, we focus our attention on the use of commercial SiO 2 and stearic acidcoated SiO 2 (SiO 2 -SA) nanoparticles as additives of a G-III base oil, and these nanolubricants were tribologically analyzed at high temperature (393.15 K) in pure sliding contacts. 2. Materials and Methods 2.1. Base Oil and Nanoparticles The paraffinic G-III base oil was provided by Repsol S.A. (Madrid, Spain); it possesses a dynamic viscosity and density of 28.9 mPa and 0.8234 g · cm −3 at 313.15 K, respectively, and a 133 viscosity index. This oil was previously fully characterized through infrared spectroscopy (FTIR) and Raman spectroscopy; peaks associated with CH 3 and CH 2 stretching were observed using FTIR, and others attributed to C-H and C-C stretching were found using Raman spectroscopy [ 31 ]. Regarding the nano-additives, two different types of SiO 2 nanoparticles were used. The first ones were commercial SiO 2 nanoparticles provided by the company US Research Nanomaterials, Inc. (Houston, TX, USA), with a purity of 99% and a diameter of 8 nm. The second ones were the same SiO 2 nanoparticles but coated in our laboratory with stearic acid (SiO 2 -SA). The SiO 2 nanopowders were characterized by means of transmission electron microscopy (TEM); it can be seen in Figure 1a that the studied SiO2NPs have a roughly spherical shape. Through the TEM characterization, the calculation of the average particle size was performed using Image J software (version 1.54h). Thus, as shown in Figure 1b, average sizes of around 11 nm were reached and were similar to the average size provided by the manufacturer (8 nm). Furthermore, in a previous work [ 32 ], infrared spectra of SA, uncoated SiO 2 NPs, and SiO 2 -SA NPs were also reported; it was observed, among other information, that the characteristic peaks of SA also appear in the spectrum of SiO 2 -SA, evidencing a proper SA coating with the SiO 2 nanoparticles. Materials 2024, 17, x FOR PEER REVIEW 3 of 12 demonstrated good ability as friction and wear modifiers [25–30]. For instance, Cortés et al. [25] studied the tribological performance of non-coated SiO2 nanoparticles, such as the additives of a vegetable oil, achieving decreases of up to 77% in terms of friction and 74% in terms of wear volume. Additionally, some authors [17,26] functionalized the SiO2 nanoparticle surface with the aim of enhancing the temporal stability of the nano-dispersions. For instance, Peng et al. [17] coated the SiO2 nanoparticles with oleic acid (SiO2-OA), reaching a temporal stability of about one month for a paraffin oil, with the mass percentages shifting from 0.05 to 1.0 wt% of the SiO2-OA. In this article, we focus our attention on the use of commercial SiO2 and stearic acidcoated SiO2 (SiO2-SA) nanoparticles as additives of a G-III base oil, and these nanolubricants were tribologically analyzed at high temperature (393.15 K) in pure sliding contacts. 2. Materials and Methods 2.1. Base Oil and Nanoparticles The paraffinic G-III base oil was provided by Repsol S.A. (Madrid, Spain); it possesses a dynamic viscosity and density of 28.9 mPa and 0.8234 g·cm−3 at 313.15 K, respectively, and a 133 viscosity index. This oil was previously fully characterized through infrared spectroscopy (FTIR) and Raman spectroscopy; peaks associated with CH3 and CH2 stretching were observed using FTIR, and others attributed to C-H and C-C stretching were found using Raman spectroscopy [31]. Regarding the nano-additives, two different types of SiO2 nanoparticles were used. The first ones were commercial SiO2 nanoparticles provided by the company US Research Nanomaterials, Inc. (Houston, TX, USA), with a purity of 99% and a diameter of 8 nm. The second ones were the same SiO2 nanoparticles but coated in our laboratory with stearic acid (SiO2-SA). The SiO2 nanopowders were characterized by means of transmission electron microscopy (TEM); it can be seen in Figure 1a that the studied SiO2 NPs have a roughly spherical shape. Through the TEM characterization, the calculation of the average particle size was performed using Image J software (version 1.54h). Thus, as shown in Figure 1b, average sizes of around 11 nm were reached and were similar to the average size provided by the manufacturer (8 nm). Furthermore, in a previous work [32], infrared spectra of SA, uncoated SiO2 NPs, and SiO2-SA NPs were also reported; it was observed, among other information, that the characteristic peaks of SA also appear in the spectrum of SiO2-SA, evidencing a proper SA coating with the SiO2 nanoparticles. Figure 1. Image of TEM (a) and mean size particle distribution of SiO2 NPs (b). 2.2. Formulation of Nanolubricants The uncoated SiO2 nanolubricants were formulated with different mass concentrations of SiO2 (0.15, 0.30, 0.45 and 0.60 wt%) in G-III base oil. For this purpose, a conventional two-step method and a Sartorius MC 210P microbalance (±0.00001 g) were utilized. Furthermore, an ultrasonic method (Ultrasonic bath FB11203 Fisherbrand from Fisher Figure 1. Image of TEM (a) and mean size particle distribution of SiO2NPs (b). 2.2. Formulation of Nanolubricants The uncoated SiO 2 nanolubricants were formulated with different mass concentrations of SiO 2 (0.15, 0.30, 0.45 and 0.60 wt%) in G-III base oil. For this purpose, a conventional two-step method and a Sartorius MC 210P microbalance ( ± 0.00001 g) were utilized. Furthermore, an ultrasonic method (Ultrasonic bath FB11203 Fisherbrand from Fisher Scientific, Hampton, VA, USA) was used for 4 h to homogenize the SiO 2 -based nanolubricants. On the Materials 2024,17, 1993 4 of 12 other hand, to prepare the SiO 2 -SA nano-dispersions, commercial SiO 2 nanopowders were coated with SA following the chemical reaction given in Figure 2a and then the dispersion method displayed in Figure 2b, to finally obtain a 4 wt% SiO 2 -SA nanolubricant. More details about a similar functionalization process can be seen in our previous article [32]. Materials 2024, 17, x FOR PEER REVIEW 4 of 12 Scientific, Hampton, VA, USA) was used for 4 h to homogenize the SiO 2 -based nanolubricants. On the other hand, to prepare the SiO 2 -SA nano-dispersions, commercial SiO 2 nanopowders were coated with SA following the chemical reaction given in Figure 2a and then the dispersion method displayed in Figure 2b, to finally obtain a 4 wt% SiO 2 -SA nanolubricant. More details about a similar functionalization process can be seen in our previous article [32]. Figure 2. Scheme of the nanoparticle functionalization (a) and dispersion method (b). Therefore, dilutions of the achieved 4 wt% SiO 2 -SA nanolubricant were performed by adding G-III base oil, until reaching the desired (0.15, 0.30, 0.45, and 0.60 wt%) SiO 2 -SA nanolubricants. After the dilutions, the nanolubricants were also homogenized via an ultrasonic bath, as in the case of the bare SiO 2 nanolubricants. Furthermore, the temporal stability of the nanolubricants was evaluated by visual control and refractive index evolution of the samples over time. 2.3. Thermophysical Characterization The density of the nanolubricants was examined from 278.15 to 373.15 K, utilizing a vibrating densimeter Anton Paar (Graz, Austria) SVM 3000 Stabinger. The expanded (k = 2) uncertainty of the density measurements was 0.0005 g cm −3 . The viscosity at atmospheric pressure and the viscosity index (VI) of the nanolubricants were also analyzed with the aforementioned densimeter. This device can measure kinematic and dynamic viscosities between 278.15 and 373.15 K. A relative expanded (k = 2) uncertainty of 1% was calculated for the dynamic viscosity. 2.4. Tribological Characterization Friction tests were carried out in pure sliding contacts with a rheometer MCR 302 from Anton-Paar, kitted with a tribology unit T-PTD 200 and utilizing a Peltier hood HPTD 200 for an ideal temperature control. In this research, a ball-on-three-pins test disposition was utilized; the ball is put on a shaft and set to turn by the rheometer motor, while being pushed at the same time against the three pins. The rheometer axial force is transferred into a normal force which proceeds perpendicularly to the contact positions on the pins. In this case, the ball turns on the pins below a 20 N normal force, resulting in a load of 9.43 N in each pin, which corresponds to a maximum contact pressure of around 0.8 GPa. Friction experiments were conducted at a constant rotational speed of 213 rpm and for 3400 s at 393.15 K. The specimens tested were polished AISI 52100 (100Cr6) steel balls (Ra = 20 nm) and pins (Ra = 50 nm) with a hardness of 62–66 HRC. The ball had a 12.7 mm diameter, and the cylindrical pins had a diameter and height that were both 6 mm. The balls and pins were cleaned with acetone/hexane and dried with air prior to the tribological tests. The pins were completely flooded by adding over 1.2 mL of each tested nanolubricant or base oil. At least three replicates were tested for each concentration of lubricant to obtain representative values. More information involving this tribological Figure 2. Scheme of the nanoparticle functionalization (a) and dispersion method (b). Therefore, dilutions of the achieved 4 wt% SiO 2 -SA nanolubricant were performed by adding G-III base oil, until reaching the desired (0.15, 0.30, 0.45, and 0.60 wt%) SiO 2 -SA nanolubricants. After the dilutions, the nanolubricants were also homogenized via an ultrasonic bath, as in the case of the bare SiO 2 nanolubricants. Furthermore, the temporal stability of the nanolubricants was evaluated by visual control and refractive index evolution of the samples over time. 2.3. Thermophysical Characterization The density of the nanolubricants was examined from 278.15 to 373.15 K, utilizing a vibrating densimeter Anton Paar (Graz, Austria) SVM 3000 Stabinger. The expanded (k = 2) uncertainty of the density measurements was 0.0005 g cm −3 . The viscosity at atmospheric pressure and the viscosity index (VI) of the nanolubricants were also analyzed with the aforementioned densimeter. This device can measure kinematic and dynamic viscosities between 278.15 and 373.15 K. A relative expanded (k = 2) uncertainty of 1% was calculated for the dynamic viscosity. 2.4. Tribological Characterization Friction tests were carried out in pure sliding contacts with a rheometer MCR 302 from Anton-Paar, kitted with a tribology unit T-PTD 200 and utilizing a Peltier hood H-PTD 200 for an ideal temperature control. In this research, a ball-on-three-pins test disposition was utilized; the ball is put on a shaft and set to turn by the rheometer motor, while being pushed at the same time against the three pins. The rheometer axial force is transferred into a normal force which proceeds perpendicularly to the contact positions on the pins. In this case, the ball turns on the pins below a 20 N normal force, resulting in a load of 9.43 N in each pin, which corresponds to a maximum contact pressure of around 0.8 GPa. Friction experiments were conducted at a constant rotational speed of 213 rpm and for 3400 s at 393.15 K. The specimens tested were polished AISI 52100 (100Cr6) steel balls (Ra = 20 nm) and pins (Ra = 50 nm) with a hardness of 62–66 HRC. The ball had a 12.7 mm diameter, and the cylindrical pins had a diameter and height that were both 6 mm. The balls and pins were cleaned with acetone/hexane and dried with air prior to the tribological tests. The pins were completely flooded by adding over 1.2 mL of each tested nanolubricant or base oil. At least three replicates were tested for each concentration of lubricant to obtain representative values. More information involving this tribological machine can be obtained from an earlier article [ 31 ]. To inspect the worn pins after the tribological studies, a 3D Optical Profiler was employed to measure the wear created in the pins for diverse parameters, such as wear scar diameter (WSD), wear track depth (WTD), or worn area. These parameters were analyzed in the three different pins by means of a confocal Materials 2024,17, 1993 5 of 12 mode (10 × objective). Moreover, a WITec alpha300R+ confocal Raman microscope (Oxford Instruments, Abingdon, UK) was utilized to obtain knowledge regarding the spreading of the nanoparticles in the worn pins. 3. Results 3.1. Stability of the Dispersions The stability of the SiO 2 and SiO 2 -SA nanolubricants was checked using two different techniques: visual observation and temporal evolution of the refractive index using a Mettler Toledo RA-510 M refractometer (Columbus, OH, USA). Figure 3a reveals that sedimentation does not happen for the first 96 h after the nanolubricant formulation, for the coated SiO 2 -SA nanolubricants. Conversely, in the case of the uncoated SiO 2 nanolubricants, it can be observed in Figure 3a that 24 h after the preparation, the sedimentation takes place. Thus, through the SA coating of SiO 2 nanoparticles a better stability is reached. Similar stability behavior was observed in other studies using the stearic or oleic acid as the coating of the NPs, and stability improvements were achieved [ 19 , 33 ]. Figure 3b shows the temporal evolution of the refractive index (n) for the base oil and 0.6 wt% SiO 2 -SA nanolubricant. As can be seen, the tendencies of the refractive index evolution for the SiO 2 -SA nanolubricant and base oil are very similar, confirming a good stability against sedimentation. Materials 2024, 17, x FOR PEER REVIEW 5 of 12 machine can be obtained from an earlier article [31]. To inspect the worn pins after the tribological studies, a 3D Optical Profiler was employed to measure the wear created in the pins for diverse parameters, such as wear scar diameter (WSD), wear track depth (WTD), or worn area. These parameters were analyzed in the three different pins by means of a confocal mode (10× objective). Moreover, a WITec alpha300R+ confocal Raman microscope (Oxford Instruments, Abingdon, UK) was utilized to obtain knowledge regarding the spreading of the nanoparticles in the worn pins. 3. Results 3.1. Stability of the Dispersions The stability of the SiO 2 and SiO 2 -SA nanolubricants was checked using two different techniques: visual observation and temporal evolution of the refractive index using a Mettler Toledo RA-510 M refractometer (Columbus, OH, USA). Figure 3a reveals that sedimentation does not happen for the first 96 h after the nanolubricant formulation, for the coated SiO 2 -SA nanolubricants. Conversely, in the case of the uncoated SiO 2 nanolubricants, it can be observed in Figure 3a that 24 h after the preparation, the sedimentation takes place. Thus, through the SA coating of SiO 2 nanoparticles a better stability is reached. Similar stability behavior was observed in other studies using the stearic or oleic acid as the coating of the NPs, and stability improvements were achieved [19,33]. Figure 3b shows the temporal evolution of the refractive index (n) for the base oil and 0.6 wt% SiO 2 -SA nanolubricant. As can be seen, the tendencies of the refractive index evolution for the SiO 2 - SA nanolubricant and base oil are very similar, confirming a good stability against sedimentation. Figure 3. (a) Visual stability observation and (b) temporal evolution of the refractive index for SiO 2 - SA nanolubricant and base oil. 3.2. Thermophysical Results The experimental densities and dynamic viscosities acquired for the base oil and SiO 2 and SiO 2 -SA nanolubricants are reported in Tables S1 and S2 (Supplementary Materials). Figure 4a shows the relative variation in the densities of the nanolubricant concentration with respect to the neat paraffinic base oil. For the SiO 2 nanolubricants, a clear increase in density variation is observed as a function of the mass concentration of the nanoparticle; the higher the concentration, the higher the density of the nanolubricant. Thus, the SiO 2 nanolubricants at 0.15, 0.3, 0.45 and 0.6 wt% increase relatively with respect to the neat base oil densities of 0.10, 0.20, 0.29 and 0.34%, respectively. The rise in nanolubricant density with the nanoparticle concentration is attributable to the agglomeration phenomenon [34]. However, for the SiO 2 -SA nanolubricants, the relative density increase is similar (around 0.03%) for all the concentrations of the functionalized nanoparticles. The relative viscosity variation in the SiO 2 and SiO 2 -SA nanolubricants compared to the neat G-III Figure 3. (a) Visual stability observation and (b) temporal evolution of the refractive index for SiO2-SA nanolubricant and base oil. 3.2. Thermophysical Results The experimental densities and dynamic viscosities acquired for the base oil and SiO 2 and SiO 2 -SA nanolubricants are reported in Tables S1 and S2 (Supplementary Materials). Figure 4a shows the relative variation in the densities of the nanolubricant concentration with respect to the neat paraffinic base oil. For the SiO 2 nanolubricants, a clear increase in density variation is observed as a function of the mass concentration of the nanoparticle; the higher the concentration, the higher the density of the nanolubricant. Thus, the SiO 2 nanolubricants at 0.15, 0.3, 0.45 and 0.6 wt% increase relatively with respect to the neat base oil densities of 0.10, 0.20, 0.29 and 0.34%, respectively. The rise in nanolubricant density with the nanoparticle concentration is attributable to the agglomeration phenomenon [ 34 ]. However, for the SiO 2 -SA nanolubricants, the relative density increase is similar (around 0.03%) for all the concentrations of the functionalized nanoparticles. The relative viscosity variation in the SiO 2 and SiO 2 -SA nanolubricants compared to the neat G-III paraffinic base oil is shown in Figure 4b. The dynamic viscosity rises as the concentration of SiO 2 nanoparticles grows from 1% to 12%. Regarding the SiO 2 -SA nanoparticles, the growth in viscosity varies between 12 and 18% for the 0.15 and 0.3 wt% SiO 2 -SA nanolubricants, respectively. Materials 2024,17, 1993 6 of 12 Materials 2024, 17, x FOR PEER REVIEW 6 of 12 paraffinic base oil is shown in Figure 4b. The dynamic viscosity rises as the concentration of SiO 2 nanoparticles grows from 1% to 12%. Regarding the SiO 2 -SA nanoparticles, the growth in viscosity varies between 12 and 18% for the 0.15 and 0.3 wt% SiO 2 -SA nanolubricants, respectively. Figure 4. Relative increase in the densities (a) and viscosities (b) with the mass concentration with respect to the neat paraffinic base oil. Additionally, with the aforementioned SiO 2 and SiO 2 -SA nanolubricants, the impact of concentration on the viscosity index (VI) was analyzed, as shown in Figure 5. A suitable viscosity index (VI) is essential in a lubricant since it helps to avert collisions and friction among the mechanical device components during operation, while also enhancing the machine’s efficiency [35]. It can be observed that all the samples have a higher viscosity index (VI) than the neat base oil, which confirms that the nanolubricant remains useful even at elevated temperatures with the preservation of the thickness of the oil film. The results show that VI increased from 3% to 13% and from 11% to 15% for the SiO 2 and SiO 2 - SA nanolubricants, respectively, compared with the neat base oil. Figure 5. Viscosity index (VI) for the neat paraffinic base oil and for the SiO 2 and SiO 2 -SA nanolubricants. 3.3. Tribological Results Figure 6 and Table 1 present the mean values of the coefficient of friction (µ) for all the tested lubricants based on G-III paraffinic base oil. The friction coefficients found for all the uncoated SiO 2 nanolubricants are quite similar to that reached for the neat G-III base oil (without additives). Nonetheless, for the coated SiO 2 -SA nanolubricants the obtained friction coefficients are much lower than that previously reported using the neat GIII base oil [31]. Specifically, the optimal nanoparticle concentration was attained for the 0.60 wt% SiO 2 -SA nanolubricant, with a friction decrease of around 43% (µ of 0.077 was Figure 4. Relative increase in the densities (a) and viscosities (b) with the mass concentration with respect to the neat paraffinic base oil. Additionally, with the aforementioned SiO 2 and SiO 2 -SA nanolubricants, the impact of concentration on the viscosity index (VI) was analyzed, as shown in Figure 5. A suitable viscosity index (VI) is essential in a lubricant since it helps to avert collisions and friction among the mechanical device components during operation, while also enhancing the machine’s efficiency [ 35 ]. It can be observed that all the samples have a higher viscosity index (VI) than the neat base oil, which confirms that the nanolubricant remains useful even at elevated temperatures with the preservation of the thickness of the oil film. The results show that VI increased from 3% to 13% and from 11% to 15% for the SiO 2 and SiO2-SA nanolubricants, respectively, compared with the neat base oil. Materials 2024, 17, x FOR PEER REVIEW 6 of 12 paraffinic base oil is shown in Figure 4b. The dynamic viscosity rises as the concentration of SiO 2 nanoparticles grows from 1% to 12%. Regarding the SiO 2 -SA nanoparticles, the growth in viscosity varies between 12 and 18% for the 0.15 and 0.3 wt% SiO 2 -SA nanolubricants, respectively. Figure 4. Relative increase in the densities (a) and viscosities (b) with the mass concentration with respect to the neat paraffinic base oil. Additionally, with the aforementioned SiO 2 and SiO 2 -SA nanolubricants, the impact of concentration on the viscosity index (VI) was analyzed, as shown in Figure 5. A suitable viscosity index (VI) is essential in a lubricant since it helps to avert collisions and friction among the mechanical device components during operation, while also enhancing the machine’s efficiency [35]. It can be observed that all the samples have a higher viscosity index (VI) than the neat base oil, which confirms that the nanolubricant remains useful even at elevated temperatures with the preservation of the thickness of the oil film. The results show that VI increased from 3% to 13% and from 11% to 15% for the SiO 2 and SiO 2 - SA nanolubricants, respectively, compared with the neat base oil. Figure 5. Viscosity index (VI) for the neat paraffinic base oil and for the SiO 2 and SiO 2 -SA nanolubricants. 3.3. Tribological Results Figure 6 and Table 1 present the mean values of the coefficient of friction (µ) for all the tested lubricants based on G-III paraffinic base oil. The friction coefficients found for all the uncoated SiO 2 nanolubricants are quite similar to that reached for the neat G-III base oil (without additives). Nonetheless, for the coated SiO 2 -SA nanolubricants the obtained friction coefficients are much lower than that previously reported using the neat GIII base oil [31]. Specifically, the optimal nanoparticle concentration was attained for the 0.60 wt% SiO 2 -SA nanolubricant, with a friction decrease of around 43% (µ of 0.077 was Figure 5. Viscosity index (VI) for the neat paraffinic base oil and for the SiO 2 and SiO 2 -SA nanolubricants. 3.3. Tribological Results Figure 6and Table 1present the mean values of the coefficient of friction ( µ ) for all the tested lubricants based on G-III paraffinic base oil. The friction coefficients found for all the uncoated SiO 2 nanolubricants are quite similar to that reached for the neat G-III base oil (without additives). Nonetheless, for the coated SiO 2 -SA nanolubricants the obtained friction coefficients are much lower than that previously reported using the neat G-III base oil [ 31 ]. Specifically, the optimal nanoparticle concentration was attained for the 0.60 wt% SiO 2 -SA nanolubricant, with a friction decrease of around 43% ( µ of 0.077 was found against 0.134). This promising friction performance can be explained by the synergetic effect between the SiO2nanoparticles and the coating of stearic acid. Materials 2024,17, 1993 7 of 12 Materials 2024, 17, x FOR PEER REVIEW 7 of 12 found against 0.134). This promising friction performance can be explained by the synergetic effect between the SiO2 nanoparticles and the coating of stearic acid. Figure 6. Mean friction coefficients, µ, for the prepared SiO2 and SiO2-SA nanolubricants and for the neat G-III base oil [31]. As cited previously, the wear formed in the pins after the friction tests was estimated through many parameters of the wear track: width, depth, and area. For this goal, crosssection profiles and 3D mappings of the wear tracks were taken. The WSD, WTD, and transversal area mean values were taken from the profiles of the worn tracks on the pins tested with the nanolubricants and base oil. The values are reported in Table 1. Table 1. Average coefficients of friction, µ, and mean parameters of wear with their standard deviations for the studied G-III base oil nanolubricants at 393.15 K. Sample μ σ WSD/μm σ/μm WTD/μm σ/μm Area/μm2 σ/μm2 G-III base oil [31] 0.1351 0.0014 366 18 2.11 0.19 607 44 +0.15 wt% SiO2 0.1319 0.0011 380 23 2.76 0.72 571 31 +0.30 wt% SiO2 0.1370 0.0010 422 53 3.37 0.67 806 76 +0.45 wt% SiO2 0.1410 0.0011 352 31 2.61 0.45 476 55 +0.60 wt% SiO2 0.1332 0.0012 372 8.3 2.67 0.46 726 49 +0.15 wt% SiO2-SA 0.0989 0.0014 356 23 2.00 0.49 439 99 +0.30 wt% SiO2-SA 0.0905 0.0011 371 18 1.60 0.19 401 62 +0.45 wt% SiO2-SA 0.0927 0.0010 296 54 1.80 0.38 279 98 +0.60 wt% SiO2-SA 0.0766 0.0011 289 25 1.64 0.47 281 84 As with the friction results, the SiO2 nanolubricants revealed similar wear results to those of the G-III base oil. Nonetheless, for all the SiO2-SA-based nanolubricants, the produced wear was greatly inferior to that achieved with neat G-III base oil, particularly in the case of the worn areas (Figure 7). Furthermore, the additive mass concentration used in the nanolubricant design considerably influenced the lubrication performance. Specifically, the greatest decreases in width and area were reached with the G-III base oil + 0.60 wt% SiO2-SA nanolubricant (Table 1), with reductions of 21 and 54%, respectively (Figure 8). Similar improved tribological performances with SiO2 NPs were previously obtained by other authors. Thus, Sanukrishna et al. [29] studied the tribological properties of SiO2 NPs as additives of a PAG lubricant, observing friction reductions of around 38% and Figure 6. Mean friction coefficients, µ , for the prepared SiO 2 and SiO 2 -SA nanolubricants and for the neat G-III base oil [31]. Table 1. Average coefficients of friction, µ , and mean parameters of wear with their standard deviations for the studied G-III base oil nanolubricants at 393.15 K. Sample µ σ WSD/µmσ/µm WTD/µmσ/µm Area/ µ m 2σ/µm2 G-III base oil [31] 0.1351 0.0014 366 18 2.11 0.19 607 44 +0.15 wt% SiO20.1319 0.0011 380 23 2.76 0.72 571 31 +0.30 wt% SiO20.1370 0.0010 422 53 3.37 0.67 806 76 +0.45 wt% SiO20.1410 0.0011 352 31 2.61 0.45 476 55 +0.60 wt% SiO20.1332 0.0012 372 8.3 2.67 0.46 726 49 +0.15 wt% SiO2-SA 0.0989 0.0014 356 23 2.00 0.49 439 99 +0.30 wt% SiO2-SA 0.0905 0.0011 371 18 1.60 0.19 401 62 +0.45 wt% SiO2-SA 0.0927 0.0010 296 54 1.80 0.38 279 98 +0.60 wt% SiO2-SA 0.0766 0.0011 289 25 1.64 0.47 281 84 As cited previously, the wear formed in the pins after the friction tests was estimated through many parameters of the wear track: width, depth, and area. For this goal, crosssection profiles and 3D mappings of the wear tracks were taken. The WSD, WTD, and transversal area mean values were taken from the profiles of the worn tracks on the pins tested with the nanolubricants and base oil. The values are reported in Table 1. As with the friction results, the SiO 2 nanolubricants revealed similar wear results to those of the G-III base oil. Nonetheless, for all the SiO 2 -SA-based nanolubricants, the produced wear was greatly inferior to that achieved with neat G-III base oil, particularly in the case of the worn areas (Figure 7). Furthermore, the additive mass concentration used in the nanolubricant design considerably influenced the lubrication performance. Specifically, the greatest decreases in width and area were reached with the G-III base oil + 0.60 wt% SiO2-SA nanolubricant (Table 1), with reductions of 21 and 54%, respectively (Figure 8). Similar improved tribological performances with SiO 2 NPs were previously obtained by other authors. Thus, Sanukrishna et al. [ 29 ] studied the tribological properties of SiO 2 NPs as additives of a PAG lubricant, observing friction reductions of around 38% and wear reductions of 41%. Also, Rastogi et al. [ 30 ] studied the effect of SiO 2 nanoparticles on the tribological characteristics of jatropha oil, obtaining important friction and wear reductions for different normal loads. Materials 2024,17, 1993 8 of 12 Materials 2024, 17, x FOR PEER REVIEW 8 of 12 wear reductions of 41%. Also, Rastogi et al. [30] studied the effect of SiO2 nanoparticles on the tribological characteristics of jatropha oil, obtaining important friction and wear reductions for different normal loads. Figure 7. Mean worn areas, found for all the tested SiO2 and SiO2-SA nanolubricants and for the neat G-III base oil. Figure 8. Mean reductions in WSD and worn area, obtained for all the tested SiO2 and SiO2-SA nanolubricants. Additionally, it can be clearly observed in the worn profiles in Figure 9 that the optimal SiO2 nanolubricant that contains the stearic acid coating presents considerably better anti-wear capacities with respect to the G-III base oil and the optimal uncoated SiO2 nanolubricant. Figure 7. Mean worn areas, found for all the tested SiO 2 and SiO 2 -SA nanolubricants and for the neat G-III base oil. Materials 2024, 17, x FOR PEER REVIEW 8 of 12 wear reductions of 41%. Also, Rastogi et al. [30] studied the effect of SiO2 nanoparticles on the tribological characteristics of jatropha oil, obtaining important friction and wear reductions for different normal loads. Figure 7. Mean worn areas, found for all the tested SiO2 and SiO2-SA nanolubricants and for the neat G-III base oil. Figure 8. Mean reductions in WSD and worn area, obtained for all the tested SiO2 and SiO2-SA nanolubricants. Additionally, it can be clearly observed in the worn profiles in Figure 9 that the optimal SiO2 nanolubricant that contains the stearic acid coating presents considerably better anti-wear capacities with respect to the G-III base oil and the optimal uncoated SiO2 nanolubricant. Figure 8. Mean reductions in WSD and worn area, obtained for all the tested SiO 2 and SiO 2 -SA nanolubricants. Additionally, it can be clearly observed in the worn profiles in Figure 9that the optimal SiO 2 nanolubricant that contains the stearic acid coating presents considerably better anti-wear capacities with respect to the G-III base oil and the optimal uncoated SiO 2 nanolubricant. Furthermore, the Raman spectra of the nanolubricant components evidence the fact that characteristic areas of these elements appear in the worn surfaces of the pins (Figure 10). Thus, in Figure 10a blue areas of Raman mapping are associated with iron oxides, green areas with the base oil, and red areas with the burned oil. In Figure 10b, it can be seen from the presence of blue areas associated with the SiO 2 -SA nanoparticles that the spectrum of this area coincides with the SiO 2 -SA Raman spectrum [ 32 ]. Considering these Raman analyses, a protective tribofilm from the SiO 2 -SA in the tribo-contact can be a possible tribological mechanism that participates in the decrease in friction and wear. Thus, the spherical SiO 2 NPs that are dispersed in the paraffinic lubricant with the high contact pressure can enter into the interspace of contact surfaces and progressively deposit on surfaces, causing the creation of a physical film. This tribofilm can separate the two metal surfaces and prevent direct contact [ 36 ]. Furthermore, some tribochemical reactions can Materials 2024,17, 1993 9 of 12 occur, boosted by the high temperatures and pressures caused by the friction process. Hence, these conditions could cause the breaking of the bonds between SA and the coated SiO 2 NPs, as was previously pointed out by Zhang et al. [ 37 ] for SA-modified TiO 2 NPs. SiO 2 NPs can easily be adsorbed on the worn surface, generating a boundary-lubricating film, whereas the SA can also be physically adsorbed on the steel surface during the tribotests, generating good lubricant properties [ 38 , 39 ]. Likewise, owing to the spherical nature of SiO 2 NPs, they are more likely to roll between two surfaces, reducing the friction coefficient and wear. Therefore, rolling and tribofilm formation are the two possible tribological mechanisms. Similar results were previously obtained by Xie et al. [ 40 ] for SiO 2 NPs dispersed in engine oil. Materials 2024, 17, x FOR PEER REVIEW 9 of 12 Figure 9. Three-dimensional (a) and two-dimensional (b) profiles of worn pins tested with neat GIII base oil [31], 0.60 wt% SiO 2 , and 0.60 wt% SiO 2 -SA. Furthermore, the Raman spectra of the nanolubricant components evidence the fact that characteristic areas of these elements appear in the worn surfaces of the pins (Figure 10). Thus, in Figure 10a blue areas of Raman mapping are associated with iron oxides, green areas with the base oil, and red areas with the burned oil. In Figure 10b, it can be seen from the presence of blue areas associated with the SiO 2 -SA nanoparticles that the spectrum of this area coincides with the SiO 2 -SA Raman spectrum [32]. Considering these Raman analyses, a protective tribofilm from the SiO 2 -SA in the tribo-contact can be a possible tribological mechanism that participates in the decrease in friction and wear. Thus, the spherical SiO 2 NPs that are dispersed in the paraffinic lubricant with the high contact pressure can enter into the interspace of contact surfaces and progressively deposit on Figure 9. Three-dimensional (a) and two-dimensional (b) profiles of worn pins tested with neat G-III base oil [31], 0.60 wt% SiO2, and 0.60 wt% SiO2-SA.