Surface tension, wettability and tribological properties of a low viscosity oil using CaCO3 and CeF3 nanoparticles as addiitives

Journal of Molecular Liquids 391 (2023) 123188 Available online 26 September 2023 0167-7322/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Surface tension, wettability and tribological properties of a low viscosity oil using CaCO 3 and CeF 3 nanoparticles as additives Jos´ e M. Li˜ neira del Río a , b , * , Alonso Alba a , María J.G. Guimarey a , Jose I. Prado c , Alfredo Amigo d , Josefa Fern´ andez a 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 CINBIO, Universidade de Vigo, Grupo GAME, Departamento de Física Aplicada, 36310 Vigo, Spain d Laboratory of Thermophysical and Surface Properties of Liquids, Department of Applied Physics, Faculty of Physics, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain ARTICLE INFO Keywords: Lubricants EV systems Nanoparticles Transmissions ABSTRACT The rise of the electric vehicles as a more sustainable transport alternative makes it necessary to study new transmission fluids that adapt to their needs and improve their efficiency. The thermophysical properties (contact angle, surface tension and rheology) and tribological properties (friction and wear) of potential PAO8 transmission nanofluids using CaCO 3 and CeF 3 nanoparticles as additives are evaluated at mass concentrations of 0.05, 0.10, 0.15 and 0.20 wt%. Low contact angle and surface tension values are obtained, indicating good wettability. When the nanoparticles are used as PAO8 additives, small reductions in surface tension are achieved, but no differences in contact angle are observed. All nanolubricants and base oil exhibit Newtonian behavior. Concerning the tribological behavior, reductions in the friction coefficient are achieved for both types of nanoparticles, with the largest reductions comparing to PAO8 base oil being 13 % and 10 % for nanolubricants containing 0.05 wt% CaCO 3 and 0.10 wt% CeF 3 nanoparticles (NPs), respectively. In terms of antiwear performance, maximum reductions are obtained for the CaCO 3 nanolubricant of 28 % (0.15 wt%), 41 % (0.10 wt%) and 59 % (0.15 wt%) and for the CeF 3 nanolubricant of 19 % (0.20 wt%), 53 % (0.10 wt%) and 58 % (0.20 wt%) for the parameters of diameter, depth, and area of the worn track, respectively. Through Raman microscopy, the tribological mechanisms of tribofilm formation, repairing, and rolling can be proposed. A discontinuous tribofilm formed by the CaCO 3 and CeF 3 nanoparticles reduces the contact area between the two surfaces protecting them. 1. Introduction Nowadays, electric vehicles (EVs) have become increasingly important for reducing fuel use as well as pollution of air. To enhance more this positive outcome, it is crucial to improve their behavior. One possible way is to enhance the tribological performance of the mechanical elements and of the lubricants of the transmission power system. In many EVs, the electric motor and transmission are in the same cover, in which case the electric transmission fluids (ETF) must a) have a low viscosity; b) have capability to do not allow copper corrosion and also be compatible with polymers, and c) exhibit suitable electrical properties [1–4]. Low viscosity lubricants are used owing to the high operating speeds and torque of mechanical components in EVs. Viscous drag as well as viscous heating reduce and heat transfer improves when the oil viscosity is reduced [5,6]. Nevertheless, if the viscosity of the lubricant decreases, the change from full film to boundary lubrication could occur, probably leading to critical surface contact and cause significant wear. Therefore, advanced lubricants with good anti-wear and antifriction properties are required. In this vein, an efficacious method to decrease friction and wear is using nanomaterials as lubricant * Corresponding author at: 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. E-mail address: [email protected] (J.M. Li˜ neira del Río). Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq https://doi.org/10.1016/j.molliq.2023.123188 Received 12 July 2023; Received in revised form 17 September 2023; Accepted 25 September 2023 Journal of Molecular Liquids 391 (2023) 123188 2 additives [7–12]; achieving decreases in losses of energy, wear as well as pollutant emissions. There are numerous reasons for using nanoparticles (NPs) as lubricant additives but most important is that because of their small size, NPs can enter in the contact area, resulting in positive lubrication effects [13]. In addition, NPs as lubricant additives can act through different lubrication mechanisms: rolling effect and tribo-film formation owing to direct effect of the nanoparticle on the surface and mending and polishing effects due to surface enhancement [14]. Moreover, the suitable nanoparticles used as additives should be less chemically reactive than traditional additives, as their films are formed mechanically, so they will be more durable and less reactive with other additives [15]. Another advantage of using NPs as lubricant additives is their low volatility, which avoids losses at high temperature conditions [15]. Another important quality of transmission fluids is their ability to spread on a metal surface, and their wettability. Wettability is usually characterized through property measurements such as contact angle or surface tension, being relevant to study its dependence on temperature. This type of studies can be found for different fluids such as ionic liquids [16] or lubricants [17]. Currently, there are hardly any studies on the effect of the nanoadditives on the wettability properties (contact angle and surface tension) of nanolubricants [18], although there are studies of NPs in water [19,20]. It is therefore of particular interest to known how these properties are affected by the addition of different concentrations of NPs. Polyalphaolefins (PAOs) are widely used in various applications owing to their excellent competence as base oils and their thermal and oxidative stability, which leads to enhanced performance in comparison with mineral oils [5]. Thus, Thampi et al. [21] and Mari˜ no et al. [22] reviewed the effect of NPs and chemically modified NPs, respectively, on the tribological properties of various lubricating oils, including PAOs, observing important tribological improvements with different NPs. Specifically, Mari˜ no et al. [22] found that the best tribological performance in PAOs was obtained with Pd [23], TiO 2 [24] and SiO 2 [25] NPs with friction and wear reductions around 40 % and 90 %, respectively. Furthermore, Mustafa et al. [5] reviewed some few articles on lowviscosity nanolubricants, including those based on low-viscosity PAOs, finding remarkable results in friction and wear. Mustafa et al. [5] and Thampi et al. [21] observed that the best tribological performance was obtained by Kalin et al. [26] using MoS 2 NPs as additives of PAO6, reducing the friction coefficient by more than two times, while the wear was reduced by five-nine times. In this work, polyalphaolefin 8 (PAO8) was selected as the base oil because its performance for electric transmissions and for its low viscosity [27]. Regarding the nanoadditives, CaCO 3 and CeF 3 NPs were used as additives of the PAO8 base oil. CaCO 3 NPs are environmentally friendly and verify the European Ecolabel criteria for use as lubricant additives [57]. CaCO 3 NPs are generally used in the automotive industry, in construction, in plastic seals, adhesives, paints, and inks. There are numerous studies on the use of calcium carbonate NPs as grease additives [29–31]. However, the literature on CaCO 3 NPs as lubricating oil additives is scarce [32–34]. Zhang et al. Table 1 Main physical properties of PAO8 [38]. Nature Density at 313.15 K / g cm −3 Dynamic Viscosity at 313.15 K / mPa s Viscosity index Synthetic 0.8163 39.47 138 Fig. 1. SEM images of a) CaCO 3 and b) CeF 3 nanopowders. J.M. Li˜ neira del Río et al. Journal of Molecular Liquids 391 (2023) 123188 3 [35] studied the tribological behavior of CaCO 3 NPs as an environmentally friendly additive to PAO10 base oil, showing that CaCO 3 NPs can considerably enhance the load carrying capability and the anti-wear and anti-friction-reducing properties of a PAO10 base oil (88 % reduction in wear volume). Kulkarni et al. [36] have investigated green nanolubricants dispersing different mass concentrations of CaCO 3 NPs in jojoba oil, finding that the addition of CaCO 3 NPs in the oil reveals remarkable anti-wear and extreme-pressure properties, with maximum decreases in coefficient of friction and wear scar diameter of balls around 35 % and 40 %, respectively. It is worth mentioning that currently there are hardly any literature articles that use CeF 3 NPs as lubricant additives [37]. Sunqing et al. [37] synthesized this type of NPs using a microemulsion of water, cyclohexane and polybutene diamide, achieving NPs with an average size of 25 nm, which were added to a 500SN oil-based lubricant. Tribological results showed a significant reduction in the coefficient of friction (about 25 %) and a slight reduction in wear. Considering all the above, it is important to develop new potential low viscosity nanolubricants particularly adapted to EVs and CeF 3 and CaCO 3 NPs may be excellent candidates to be used as additives for further study. Thus, in this work surface tension, wettability and tribological properties of potential electric transmission nanofluids based on PAO8 oil containing CaCO 3 or CeF 3 NPs as additives are evaluated at mass concentrations of 0.05, 0.10, 0.15 and 0.20 wt%. 2. Material and experimental methods 2.1. Materials The polyalphaolefin 8 base oil, PAO8, was provided by REPSOL. As shown in Table 1, this base oil has a dynamic viscosity and density at 313.15 K of 39.47 mPa s and 0.8163 g cm −3 , respectively, and a viscosity index of 138 [38]. The PAO8 base oil was previously characterized using Fourier-transform infrared spectroscopy (FTIR) as well as Raman spectroscopy [38]. Concerning the nanoadditives, CaCO 3 and CeF 3 NPs were delivered by US Research Nanomaterials, Inc. (Houston, TX USA). The CaCO 3 NPs have a purity of 98 %, an average diameter of 50 nm, and a specific surface area of 40–80 m 2 /g. Furthermore, CeF 3 NPs have a size of 20 nm and a purity of 99.99 %. Scanning Electron Microscopy (SEM, Zeiss FESEM Ultra Plus) and Transmission Electron Microscopy (JEOL JEMF200CF-HR) were used to know the aspect and dimension of both Fig. 2. TEM images and size nanoparticles distribution of a) CaCO 3 and b) CeF 3 nanopowders. J.M. Li˜ neira del Río et al. Journal of Molecular Liquids 391 (2023) 123188 4 types of NPs. As Figs. 1 and 2 illustrate, both NPs (CaCO 3 and CeF 3 ) have a spherical shape, especially in the case of CaCO 3 NPs. Regarding the nanoparticle sizes, it can be observed that CaCO 3 NPs have a larger size than CeF 3 NPs. Specifically, Fig. 2 shows the size distribution of both NPs (obtained with ImageJ), observing that the average sizes for the CaCO 3 and CeF 3 NPs are around 91 nm and 30 nm, respectively. The CaCO 3 and CeF 3 NPs were also characterized by FTIR to determine the bonds associated with each NP. The FTIR spectra of CaCO 3 and CeF 3 NPs are shown in Fig. 3. The first one shows three prominent peaks around 710 cm −1 , 875 cm −1 and 1405 cm −1 corresponding respectively to a deformation in the plane, to a deformation outside in-plane and doubly degenerate in-plane strain typical of CaCO 3 [39,40]. On the other hand, the spectrum of CeF 3 NPs shows peaks detected at 736 cm −1 , 1494 cm −1 , 1637 cm −1 and 3384 cm −1 [41]. Raman spectroscopy of both nanopowders was used to identify the characteristic absorption bands, Fig. 4. The CaCO 3 spectrum, Fig. 4a, shows a very strong Stokes band at 1087 cm −1 and weaker ones at 160 cm −1 , 286 cm −1 , 716 cm −1 and 2950 cm −1 , which were also found in a spectrum of the corresponding bulk material, CaCO 3 [42]. On the other hand, Fig. 4b shows the Raman spectrum of the CeF 3 NPs. In this spectrum there are two pronounced bands at 309 cm −1 and 566 cm −1 and other less intense bands at 993 cm −1 and 1094 cm −1 , which match with the spectrum of the corresponding bulk material, CeF 3 [41]. CaCO 3 and CeF 3 NPs were also characterized by X-Ray diffraction (XRD) to determine the crystalline form of both NPs. For this aim, a Bruker D8 Advance with Cu-K α radiation (λ =1.5418 nm) was used at room temperature. The XRD patterns were carried out between 2θ =380◦. The Debye-Scherrer’s formula was applied to calculate the crystallite size of the nanoparticles based on the following equation [43]: Dp =Kλ/(Bcosθ)(1) where Dp, K, λ, B, and θ are the average crystallite size, the Scherrer constant, the entire width at the half summit of the diffraction peak, and the Bragg angle of the peak. Fig. 5 shows the XRD pattern of the CaCO 3 and CeF 3 NPs. In the CaCO 3 XRD pattern, Fig. 5a, peaks are found at 2θ =23.1◦, 29.41◦, 35.98◦, 39.42◦, 43.16◦, 47.52◦and 48.52◦, corresponding to (012), (104), (110), (113), (202), (018) and (016) crystal planes [44]. The experimental XRD pattern agrees with the ICSD Fig. 3. FT-IR spectrum of CaCO 3 (a) and CeF 3 (b) NPs. J.M. Li˜ neira del Río et al. Journal of Molecular Liquids 391 (2023) 123188 5 Card No: 98–004-0107 (Calcite) [44]. The crystallite size of CaCO 3 NPs determined from the above peaks using the Eq. (1) is 96.68 nm. Regarding the XRD pattern of CeF 3 NPs (Fig. 5.b), peaks are found at 2θ =24.38◦, 24.93◦, 27.81◦, 43.93◦, 45.10◦, 50.92◦, 52.80◦and 64.80◦. The experimental XRD pattern is in accordance with the ICSD Card No: 98–006-4720 (Fluocerite) [45]. The crystallite size of CeF 3 NPs calculated from these peaks using the Eq. (1) is 25.01 nm. 2.2. Preparation of nanolubricants Nanolubricants were made according to the conventional two-step procedure [22]. Firstly, dry nanopowders (CaCO 3 or CeF 3 NPs) were mixed with PAO8 base oil, determining the mass concentrations through a Sartorius MC 210P balance. For this purpose, the base oil was weighed and then the amount of nanopowder required to achieve the desired mass concentrations was also weighed. Secondly, the homogenization of nanodispersions was carried out first with a mechanical stirrer (Fisherbrand Vortex) for 2 min and finally with an ultrasonic bath (Fisherbrand FB11203 from Fisher Scientific, USA) for 4 h. It should be noted that the preparation process the nanolubricants (formulation and homogenization) is relatively short (around 6 h). To find out the optimum mass concentration with the highest tribological behavior, eight nanodispersions of PAO8 were prepared with the following nanoadditives: + 0.05 wt% CaCO 3 , +0.10 wt% CaCO 3 , +0.15 wt% CaCO 3 , +0.20 wt% CaCO 3 , +0.05 wt% CeF 3 , +0.10 wt% CeF 3 , +0.15 wt% CeF 3 and + 0.20 wt% CeF 3 . 2.3. Stability of nanolubricants To control the stability of the NPs within the oil, a simple setup is used, where the samples of nanolubricants are placed with good lighting on a background of the opposite color to that of the sample for better Fig. 4. Raman spectrum of CaCO 3 (a) and CeF 3 (b) NPs. J.M. Li˜ neira del Río et al. Journal of Molecular Liquids 391 (2023) 123188 6 visualization. The process consists in that once the nanodispersion is prepared and homogenized, it is not manipulated, and a series of photographs are taken periodically at room temperature, to detect when the NPs are deposited. In this case, a black background is used, and photographs were taken approximately every 24 h. Furthermore, to analyze stability more quantitative, the evolution of the refractive index over time was measured by means of a refractometer (Mettler Toledo RA-510 M). 2.4. Surface tension, contact angle and rheology To carry out the surface tension measurements, a Lauda TVT 2 Drop Volume Tensiometer was used, which allows measuring the surface tension through the volume of the falling drop following Tate’s law [46]. The fluid sample is introduced into a syringe of 2.5 mL volume, being expelled from it through a needle of 1.370 mm inner radius. The temperature of the lubricant sample in the syringe is controlled with a thermal bath to maintain it at 298.15 or 313.15 K. The uncertainty in the measured temperature was u(T)/K =0.03. For each surface tension experiment, 30 to 40 drops are used to obtain a mean volume value with good reproducibility. The uncertainty in surface tension measurements depends on the nanolubricant ranging between 0.05 and 0.2 mN m −1 . A Phoenix MT-A analyzer was utilized to achieve the contact angle of all the lubricants on AISI 420 stainless steel plates. For the calibration of the equipment camera, a glass plate supplied by the manufacturer is used, which contains reference graduation marks for different contact angles. The lubricant is introduced into a syringe which, with the help of the equipment’s motor, makes a drop of lubricant fall on the steel plate previously cleaned with acetone. It should be noted that this plate is placed on a surface that is thermostated with a bath to keep the desired temperature during the analysis time. The uncertainty in the measured temperature was u(T)/K =0.5. The obtained result is the evolution of the contact angle over time, specifically its value every second for 1 min (a series of 60 values). Several series of measurements are carried out for each oil, ensuring the correct repeatability of the contact angles, taking the average of all the values. The estimated uncertainty in the contact angle measurements is 2 %. The rheological behavior of the PAO8 base oil and its nanolubricants containing CaCO 3 and CeF 3 NPs was investigated with an Anton Paar MCR 101 rheometer (Graz, Austria) at 293.15, 333.15, and 363.15 K. The rheometer is equipped with a cone-plate geometry with a cone diameter of 50 mm and a cone angle of 1◦. The cone went down to an imposed gap of 0.102 mm from the plate and covered the whole sample for all tests. Furthermore, the temperature is controlled by means a Peltier P-PTD 200 (Anton Paar, Graz, Austria), set at the lower plate, with a diameter of 56 mm. Further details of this apparatus can be found in a previous article [47]. Flow curves of tested lubricants were recorded in the shear rate range from 10 to 1000 s −1 . The expanded uncertainty of Fig. 5. XRD pattern of CaCO 3 (a) and CeF 3 (b) NPs. J.M. Li˜ neira del Río et al. Journal of Molecular Liquids 391 (2023) 123188 7 viscosity measurement is estimated to be better than 5 % (k =2) [47]. It should be noted that an amount of 0.59 mL of sample was considered for the analysis and was placed on the Peltier plate. 2.5. Tests of friction and wear examination Ball-on-three pins friction tests with PAO8 base oil and with the eight CaCO 3 and CeF 3 nanolubricants were performed through a Peltier heated T-PTD200 tribology unit in an Anton Paar MCR 302 rheometer (Graz, Austria). The ball is placed in a vertical tube that is powered by the motor of the rheometer, while the pins, situated at the base of the holder, are touching the ball at an angle of 45◦with the tube. During the friction tests, the ball that supports a vertical axial force applied by the rheometer, turns on the pins. This force (20 N) gives rise to three identical normal forces (9.43 N) that action perpendicularly on the surfaces of the three pins. In each pin, the maximum Hertzian pressure corresponds to 1.1 GPa [48]. Pins with 6 mm radius and 6 mm height, and balls with 12.7 mm diameter were utilized, being both components of 100Cr6 hardened steel (hardness Rockwell C between 66 and 62). The friction tests were performed at rotational speed of 213 rpm (ball surface velocity being 0.1 m s −1 ), for 3400 s for a 393.15 K temperature. Approximately 1 mL of lubricant was utilized in each test in order to cover the contact surface. More information on this tribological apparatus can be found in earlier articles [49,50]. The temperature was selected since for great operation of EV motors, a high-power density is necessary and so the heat generation in the coil improves. Nonetheless, to avoid demagnetization, the temperature must be below 150 ◦C. More information on these issues was reported by Rodriguez et al. [4]. When the friction tests for each lubricant were completed (3 repeats for each one), the wear was estimated in pins, to investigate which nanolubricant offers the best anti-wear performance. Pins were cleaned with a hexane stream and dried with hot air before the wear analysis. A 3D profilometer (Sensofar, S-Neox) was used to measure various worn track parameters: wear scar diameter (WSD), wear track depth (WTD), worn area and roughness. It should be noted that the worn area was determined using the section profile by subtracting from the worn area the sum of areas of the profiles for the material displaced on both sides of the worn track. All the parameters were determined in all the pins tested with all the lubricant samples to obtain appropriate mean values. Moreover, a confocal Raman microscope (WITec alpha300R+) was used to obtain information of the chemical organization in the worn scar and Fig. 6. Stability control performed on the different nanolubricants of CaCO 3 and CeF 3 . Fig. 7. Refractive index evolution for 0.05 wt% CaCO 3 and CeF 3 nanolubricants. J.M. Li˜ neira del Río et al. Journal of Molecular Liquids 391 (2023) 123188 8 to gain insight into the tribological mechanisms that may happen. 3. Results and discussion 3.1. Stability results The photos corresponding to the visual stability control of the nanodispersions are shown in Fig. 6. Both NPs are stable in the PAO8 base oil for at least 48 h, which is a sufficient stability time to carry out the tribological and thermophysical tests. It should be noted that Fig. 6 also shows that the stability decreases for both NPs when the mass concentration increases. The other method used to analyze the stability of the formulated nanolubricants against sedimentation is refractometry. Fig. 7 shows the evolution of the refractive index for the CaCO 3 and CeF 3 nanolubricants, both with a NP concentration of 0.05 wt%. It can be clearly observed that for the 0.05 wt% CeF 3 nanolubricant after the first ten hours NPs are full sedimented, whereas in the case of the 0.05 wt% CaCO 3 nanolubricant the sedimentation is quite gradual. More precisely, for the first nanolubricant the refractive index increased 0.28 % after 10 h and for the CaCO 3 nanolubricant it increased 0.19 %. It should be noted that for low viscosity oils is more difficult to find good stabilities than for viscous oils, since the nanoparticles have less resistance to falling in oil. 3.2. Surface tension, contact angle and rheology results Fig. 8 and Table 2 show the values of surface tension, γ, obtained for the nanolubricants with the four mass concentrations CaCO 3 or CeF 3 NPs and the PAO8 base oil at temperatures of 298.15 K and 313.15 K. It can be observed that the surface tensions of the nanodispersions are slightly lower than those of PAO8 at both temperatures. In particular, for the samples containing CeF 3 NPs the variation is almost negligible. For the case of 0.15 wt% decreases of 0.16 % for T =298.15 K and 0.26 % for T =313.15 K were found. On the other hand, for the nanolubricants containing CaCO 3 NPs, the difference is greater, showing a minimum at 0.10 wt% with reductions of 0.64 % for T =298.15 K and 0.67 % for T = 313.15 K. Currently, there are hardly any studies related to wettability properties (contact angle and surface tension) when nanoparticles are added to lubricants [18], although there are several studies of nanoparticles in water [19,20]. Therefore, it is critical to study how these properties are affected by the addition of different concentrations of nanoparticles. Regarding the contact angle, Fig. 9 shows the measurements of the nanolubricants with CaCO 3 and CeF 3 NPs and of the base PAO8 without additives. No clear trend is observed in any of the cases that could indicate a significant change in the angle values of contact by the addition of NPs. It is worth mentioning that the differences are within of the uncertainty of the measurements. Furthermore, it is also observed that when the temperature is increased, the values of contact angle drops. Thus, at working temperature of the EV transmissions (around 393 K), the contact angle will be lower implying a superior wettability and the creation of a tribo-film that may prevent direct surface contact [51]. However, the effects of nanoadditives on wetting have not yet been explained and need to be better understood. Marques et al. [17] previously studied the surface tension and contact angle of four PAO base oils (PAO6, PAO20, PAO32 and PAO40) at 293.15 to 323.15 K, finding the same trends for viscosities, surface tensions and contact angles of PAOs. Kalin et al. [52,53] also investigated the wetting properties between some engineering surfaces (steel and several types of DLC coatings) and PAO9. These authors obtained a steady-state contact angle of 11.3◦for PAO9 on the surface of steel AISI 52100 at room temperature. In the current work, a lower contact angle value has been obtained for PAO8 at room temperature (298 K), around 8◦. So, the higher the molecular mass (or the viscosity grade) of the PAOs, the greater the contact angle. This result is in line with that reported by Marques et al. [17]. Concerning the rheological tests, the flow curves for the PAO8 base lubricant as well as for both types of nanolubricants (Fig. 10) at the studied mass fractions (0.05, 0.10, 0.15 and 0.20 wt%) and temperatures show linear dependence of the shear stress ( τ ) and shear rate (ɣ), the shear viscosity being the slope of such relationship. Thus, the base oil and the nanolubricants exhibit Newtonian behavior. Consequently, there is no influence of the shear rate on the viscosity for each lubricant, i.e., it is possible to determine a single viscosity value at each studied temperature without considering the flow conditions. As can be seen in Fig. 10, there is a slight increment of the viscosity with the mass fraction, Fig. 8. Surface tension obtained for the base oil and its nanolubricants containing CaCO 3 or CeF 3 NPs at different mass fractions, wt%, at 298.15 and 313.15 K. Table 2 Surface tension, γ, and its uncertainties, σ , of the formulated nanolubricants and the PAO8 base oil. 298.15 K 313.15 K γ/mN m −1 σ /mN m −1 γ/mN m −1 σ /mN m −1 PAO8 Base 29.61 0.07 28.33 0.07 +0.05 wt% CaCO 3 29.51 0.10 28.24 0.08 +0.10 wt% CaCO 3 29.42 0.09 28.14 0.09 +0.15 wt% CaCO 3 29.53 0.17 28.17 0.10 +0.20 wt% CaCO 3 29.57 0.07 28.27 0.06 +0.05 wt% CeF 3 29.58 0.09 28.27 0.08 +0.10 wt% CeF 3 29.56 0.04 28.28 0.06 +0.15 wt% CeF 3 29.56 0.12 28.26 0.07 +0.20 wt% CeF 3 29.57 0.05 28.28 0.04 J.M. Li˜ neira del Río et al. Journal of Molecular Liquids 391 (2023) 123188 9 which can be considered negligible considering the studied range of the nanoparticle concentrations. As expected, temperature has a strong effect on viscosity, decreasing by up to one order of magnitude between 293.15 and 363.15 K. 3.3. Friction and wear results Friction tests were carried out in the tribometer for the nanolubricants of PAO8 with 0.05 wt%, 0.10 wt%, 0.15 wt% or 0.20 wt% in CaCO 3 or CeF 3 NPs. Mean values of the coefficient of friction, μ , were obtained (Fig. 11 and Table 3) for each of the studied lubricants. The μ value for the PAO8 base oil has been previously measured [38], for the same operating conditions. As it can be observed in Fig. 11 and in Table 3, the addition of NPs as base oil additives leads to a reduction in the friction coefficient with respect to the oil without additives, reaching decreases of 13 % for the optimal concentration of CaCO 3 NPs (0.05 wt %) and 10 % for that of CeF 3 NPs (0.10 wt%). It is evidently seen that for both types of nanolubricants the NP concentration influences the antifriction behavior. Thus, at concentrations higher than the optimum for both NPs, the stability of nanolubricants is poorer and may cause problems of nanoparticle agglomeration at the contact area, because if the nanoparticle concentration is too high, nanoparticle deposition may create new asperities and thus increase the friction [28]. Concerning the wear produced during friction tests, the profile measurements show notable wear improvements in tested pins lubricated with nanolubricants with respect to those lubricated with PAO8 without additives (Fig. 12 and Table 3). Specifically, for nanolubricants containing CaCO 3 NPs, improvements of 28 % in WSD (0.15 wt%), 41 % in WTD (0.10 wt%) and 59 % in the area of the wear track (0.15 wt%) are achieved; while that for lubricants PAO8 +CeF 3 NPs, improvements of 19 % in WSD (0.20 wt%), 53 % in WTD (0.10 wt%) and 58 % for the area of the worn tread (0.20 wt%) are obtained. Fig. 13 shows the 3D profile and its vertical section obtained for the PAO8 base oil [38], its optimal nanolubricants with CaCO 3 NPs (0.15 wt%) or with CeF 3 NPs (0.20 wt%). Excellent wear reduction can be observed for both nanolubricants. Thus, it can be concluded that both NPs have good antiwear ability. Moreover, the roughness (Ra) of worn tracks of pins was examined to get more information about the antiwear properties of CaCO 3 and CeF 3 NPs. Worn pins lubricated with both types of nanolubricants are less rough than those lubricated with PAO8 without additives (Table 4). Specifically, a Ra value of 18.8 nm was observed for the worn track lubricated with PAO8, whereas the smallest Ra values (16.8 and 16.9 nm) were obtained for the surface correlated with the 0.15 wt% CaCO 3 and 0.20 wt% CeF 3 nanolubricants, resulting in a roughness reduction of about 10 %. These results suggest that both nanopowders improve the surface contact area compared to the base oil, which may be due to a surface repair mechanism. Finally, to obtain information on the distribution of NPs in worn tracks to detect the function that NPs play in reducing wear, after Fig. 9. Contact angle measurements obtained for the base oil and CaCO 3 and CeF 3 nanolubricants at 298 and 313 K. J.M. Li˜ neira del Río et al.