Tribological enhancement of potential electric vehicle lubricants using coated TiO2 nanoparticles as additives

Tribological enhancement of potential electric vehicle lubricants using coated TiO 2 nanoparticles as additives José M. Liñeira del Río a,b, ⇑ , Fátima Mariño a , Enriqueta R. López a , David E.P. Gonçalves b , Jorge H.O. Seabra c , Josefa Fernández 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 FEUP, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal article info Article history: Received 22 September 2022 Revised 7 December 2022 Accepted 15 December 2022 Available online 17 December 2022 Keywords: Low-viscosity lubricants Transmission fluids Nanoparticles coating Friction Wear abstract This work presents the antifriction and antiwear properties of TiO 2 nanoparticles coated with oleic acid, TiO 2 -OA, as additives of a low viscosity polyalphaolefin base oil, PAO8. To find the optimal concentration of nanoadditives that minimize friction and wear, four PAO8 based nanodispersions were formulated: PAO8 + 0.10 wt% TiO 2 -OA, PAO8 + 0.25 wt% TiO 2 -OA, PAO8 + 0.35 wt% TiO 2 -OA and PAO8 + 0.50 wt% TiO 2 -OA. Tribological experiments were performed under pure sliding and rolling-sliding conditions at 120 °C, with the four formulated nanolubricants and with PAO8 base oil. All the nanolubricants showed lower friction coefficients than that obtained with the PAO8 base oil, reaching maximum reductions for the 0.35 wt% TiO 2 -OA nanolubricant, for both types of test conditions. The tribological specimens tested under pure sliding conditions with the nanolubricants showed fewer wear than those tested with PAO8, finding the highest wear decreases also with the PAO8 + 0.35 wt% TiO 2 -OA nanolubricant, being 26 %, 65 % and 73 %, in wear track width, depth and area, in that order. Through Raman microscopy and roughness study of the worn samples, it can be inferred that tribofilm, mending and polishing mechanisms occur. Moreover, the thermal conductivity of the optimal nanolubricant (0.35 wt%) was measured at 20, 30, 40 and 50 °C. Ó2022 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/). 1. Introduction Globally, transportation is one of the main contributors to CO 2 emissions, producing 28 % of total emissions [1]. In this regard, it is estimated that the CO 2 global emissions produced by vehicles with internal combustion engines (ICE) are between 4 and 5 times higher than by electric vehicles (EVs) when the electrical energy is provided through renewable sources [2]. Electric vehicles (EVs) have dramatically increased in popularity and are leading to a greener future for the automotive industry [1]. A remarkable example is the fact that United Kingdom’s government is set to prohibit new fossil fuel vehicle manufacture by 2030 [2]. It is estimated that this would help decrease car gas emissions, only in that country, to the equivalent of almost 50 million tons of CO 2 [2]. Although EVs have a substantial high efficiency in terms of energy consumption, there is a challenge to improve the efficiency even more. Thus, being very efficient and generating very little gas emissions, the effectiveness and resilience 2 problems of EVs lie in the moving components and, consequently, in their tribology. By decreasing friction in elements like gears and bearings, tribology can increase the EVs range of driving. According to the hardware designs of hybrid electric vehicles (HEVs) and full EVs, the electric motor can be placed in different positions [3,4]. If it is integrated within the transmission housing, the lubricant is in contact with the motor and must satisfy different requirements, such as compatibility with the materials of the motor structure and the housing, magnetic compatibility, prevention of cooper corrosion, and good heat transfer characteristics to cool the electric motor [3,5,6]. In comparison to traditional ICE vehicles, mechanical elements of EVs work at higher loads, speeds, temperatures, and under electromagnetic fields [2,7]. These facts imply that electrical and thermal properties of the transmission fluids should be taking into account apart from traditional fluid properties [7]. These peculiarities require the use of a low-viscosity lubricant to act the high speeds of the electric motor [2]. All this, without losing sight of the fact that an effective electric vehicle transmission oil must provide high antiwear performance, at extremely low viscosities. Today, in https://doi.org/10.1016/j.molliq.2022.121097 0167-7322/Ó2022 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/). ⇑ Corresponding author. E-mail address: [email protected] (J.M. Liñeira del Río). Journal of Molecular Liquids 371 (2023) 121097 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq EV and HEV models conventional automatic transmission fluids for ICE vehicles are being used with acceptable performance, but these fluids have not been specifically formulated to verify the HEV/EV requirements [8]. Thus, in order to achieve the requirements for coming EVs lubricants, it is essential to use efficient additives to base oils [9]. In this regard, the study of nanomaterials as additives can lead to the advancement of a recent production of lubricants with low viscosity particularly designed to the needs of the new and future EV electric drivelines. Nanomaterials have good tribological properties, high thermal conductivity and can be ecologically friendly in contrast to other conventional additives [10]. Several nanomaterials used as additives of conventional ICE lubricants have shown success in decreasing the friction and wear [11–13]. Nevertheless, studies on their performance in terms of EVs’ tribological conditions are scarce. In this aim, Mustafa et al. [2] reviewed some few articles on low-viscosity nanolubricants, among them those based on low-viscosity polyalphaolefins (PAOs). Kalin et al. [14] examined the antifriction and antiwear performance of PAO using MoS 2 nanotubes like additives, discovering significant improvements in friction and wear. PAOs are usually utilized in numerous industrial areas, as automotive engine oils, vehicle gear oils, dielectric fluids, electronics cooling and automotive automatic transmission fluids. In comparison to mineral oils, PAOs show greater chemical and physical properties as better fluidity at low temperature, lower volatility, higher viscosity index, a lower pour point, improved oxidative/thermal stability, lower toxicity, and are biodegradable for low viscosity grades. In spite of the several developments that have been reached with nanopowders in the area of lubricants, there is yet an important difficulty with the nanodispersions stability due to sedimentation, because nanopowders have a tendency to agglomerate each other [15]. This fact is even more accentuated in low-viscosity lubricants. Chen et al. [15] reviewed the temporal stability of several nanolubricants. They study various characteristics of nanoparticles like their size or the surface modification agent, concluding that the surface modification is vital to efficiently disperse nanopowders in a lubricant oil. Given those requirements, it is crucial to develop and study possible lubricants formed by low-viscosity base oils and modified nanoparticles as additives. In this work, the key objective is to analyze the tribological properties of TiO 2 nanoparticles with an oleic acid coating, TiO 2 -OA, as additives of a low-viscosity base oil (PAO8). Non-coated TiO 2 nanoparticles have shown good tribological properties as additives of an engine lubricant and a vegetable oil [16,17]. Thus, Birleauno et al. [16] recently examined the addition of TiO 2 nanoparticles and a surfactant to 10W30 engine oil, observing that the nanoparticles can improve the friction reduction and the antiwear properties of the neat lubricant. Moreover, Cortes et al. [17] researched the lubrication behavior of a sunflower oil containing non-coated SiO 2 and TiO 2 nanopowders. They observed that the friction coefficient was reduced thanks to adding SiO 2 and TiO 2 nanoparticles around 80 % and 95 %, respectively comparing to the sunflower oil and that the loss of volume was dropped by around 75 % and 70 %, respectively. 2. Material and methods 2.1. Materials Polyalphaolefin 8, PAO8, was kindly supplied by REPSOL. This sample possesses a dynamic viscosity and density at 313.15 K of 39.47 mPas and 0.8163 gcm 3 , respectively, as well as a 138 viscosity index. Infrared spectroscopy (FTIR) was used to characterized PAO8. Fig. 1 reveals a weak peak at 770 cm 1 associated to CAC alkyl chains, a peak at 1510 cm 1 owing to the CAH bending, two robust peaks around 2850 and 2920 cm 1 which correspond to the CAH stretching [25,35]. It can be noted that there is no presence of signals attributed for C@C bonds. Regarding the nanoadditives, TiO 2 nanoparticles coated with oleic acid (TiO 2 -OA) were synthesized using commercial TiO 2 , purity: 99.9 % and diameter of 5 nm, which were provided by US Research Nanomaterials, Inc. The TiO 2 nanopowders have been coated with OA following the subsequent reaction: . 200 mg of commercial TiO 2 nanopowder were dispersed in distilled H 2 O (20 mL) in a round bottom flask that contains a magnetic stir bar. This blend was heated to 60 °C under stirring (400 rpm) until this temperature was reached. After that, 5 mL of NaOH (0.05 M) were added and after one minute, oleic acid, OA (0.6 mL), was also added. This blend remained at 60 °C for 30 min and then temperature is increased to 95 °C and was refluxed for 1 h 30 min, observing aggregation. The excess of NaOH was neutralized using HCl and the formed precipitate (TiO 2 -OA) was isolated by centrifugation under 4000 rpm for 10 min and washed two times with ultrapure water and hexane. Subsequently, the obtained nanodispersion (hexane with TiO 2 nanoparticles coated with OA) was added to PAO8 using an ultrasonic bath to homogenize the new dispersion. Finally, a hot plate was used to evaporate the hexane (low boiling point) thus obtaining the desired PAO8 + TiO 2 -OA nanolubricant (3 wt% in TiO 2 -OA). Fig. 2 presents the preparation scheme of the nanodispersions of TiO 2 -OA nanoparticles in PAO8. To confirm that the OA coating is properly linked to the TiO 2 nanopowders, FTIR analyses of synthetized TiO 2 -OA nanopowders were carried out with a spectrometer VARIAN 670-IR. Fig. 3 provides the FTIR spectrum of OA, TiO 2 and coated TiO 2 -OA nanoparticles. The peaks at 2910 and 2846 cm 1 for the TiO 2 -OA nanoparticles evidence that CH 3 and CH 2 of OA are present in the coated TiO 2 molecular structure [18]. Furthermore, peaks at 1490 and 1452 cm 1 prove that the –COO functional group of OA is present [18]. In this regard, the disappearance of the intense peak at 1708 cm 1 implies that the –COOH of OA has totally reacted with TiO 2 nanoparticles to form the coated TiO 2 -OA nanopowders [18]. Furthermore, an absorption band for the nano-TiO 2 core is identified at low frequencies, because of the peak around 540 cm 1 is associated to the TiAOAO bond vibration [19]. Considering these FTIR results, it is confirmed that the OA coating was carried out successfully. Bare TiO 2 nanoparticles were also characterized by means of scanning electron microscopy (SEM) to obtain information about its shape. Fig. 4 shows the TiO 2 nanoparticles with two magnifications observing that these nanoparticles have roughly spherical spongy shape. 2.2. Formulation of nanodispersions Nanolubricants were prepared with various weight percentages of TiO 2 -OA (0.10, 0.25, 0.35 and 0.50 wt%) in PAO8. To prepare these nanodispersions, dilutions of the previously obtained 3 wt% TiO 2 -OA nanolubricant were carried out adding PAO8. Furthermore, to improve the dispersibility of nanoparticles in the PAO8 oil, oleic acid was added for each TiO 2 -OA nanodispersion to obtain an OA mass concentration of 0.2 wt%. Finally, the nanodispersions were homogenized by means of an ultrasonic bath (Fisherbrand FB11203), operating in continuous sonication mode (4 h, 180 W and 37 kHz). It should be noted that during the sonication procedure the temperature was under control to prevent overheating José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 2 of samples. Furthermore, nanodispersions stability was examined by sediment photo capturing of lubricants and by measuring the evolution of the refractive index during time by means of a Mettler Toledo refractometer. 2.3. Thermal conductivity of lubricants Thermal conductivities of PAO8 base oil and the optimal TiO 2 -OA nanolubricant (0.35 wt%) were measured at 20 °C, 30 °C, 40 °C and 50 °C using a Tempos thermal properties analyzer (METER Group, USA). This device is based on the principle of the transient hot wire method [20]. This analyzer was used together with a KS-3 sensor, which consists of a single-needle (60 mm length and 1.3 mm diameter) with a heating element and a temperature sensor inside. The accuracy of thermal conductivity measurements is around 10 % in the range (0.02–2.00) Wm 1 K 1 .A lubricant volume of around 30 mL is introduced in a sealed glass vial. The probe (needle) is placed in the middle of the vial. Subsequently, the vial is fully submerged and vertically placed in a temperature-controlled water bath. Measurements have been performed in low-power mode, which helps to prevent free convection in liquid samples. Five replicates of thermal conductivity measurements were performed for each temperature. Verification of the sensor was achieved by measuring pure glycerin, which is a thermal conductivity standard. 2.4. Tribological tests 2.4.1. Pure sliding conditions Friction tests were performed at pure sliding conditions with a MCR 302 rheometer from Anton-Paar provided with a tribology cell T-PTD 200, using a H-PTD 200 Peltier hood for an accurate control of temperature. In this work, ball-on-three-pins test configuration was used. In this setup, the ball is placed on a shaft and is set to rotate by the rheometer motor, while is pressed against the pins. The axial force of the rheometer (in this case 20 N) is transmitted into a normal force acting vertical to the contact points on each pin (9.43 N resulting in a maximum contact pressure of approximately 0.8 GPa). Tests were made at a rotational speed of 213 rpm during 3400 s at 120 °C. The ball has 12.7 mm diameter and a roughness of 0.02 l m, while the pins have sizes of 6 mm in diameter and height and a roughness of 0.05 l m. Both specimens are made of hardened 100Cr6 steel and have a hardness of 62–66 Rockwell C. Pins and balls were washed with hexane prior to the tribological tests. Pins were fully flooded by adding around 1.2 mL of each tested lubricant. To achieve representative values, at least 3 replicates were performed with each nanolubricant. More details about this device can be found in previous articles [21–25]. With the aim of analyzing the worn surface after tribological tests, a 3D Optical Profilometer was utilized to measure the wear produced on the pins. Fig. 1. FTIR spectrum of PAO8 base oil. Fig. 2. Preparation scheme of the nanodispersions of TiO 2 -OA nanoparticles in PAO8. José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 3 Wear scar diameter (WSD), wear track depth (WTD) and worn area were the three wear parameters evaluated in the three pins in a confocal mode with a 10X objective. Furthermore, the roughness (Ra) of worn pins were also measured with the same 3D profiler to identify the tribological mechanisms due to the modified nanoparticles as additives to PAO8. The roughness was evaluated according to ISO4287 standard utilizing a Gaussian filter with a cut-off of 0.08 mm. With the aim of obtaining information about the distribution of nanoparticles and oil on the worn pins, a WITec alpha300R+ confocal Raman microscope at 532 nm was utilized. Prior to the Raman analyses, the worn surfaces were cleaned with hexane. 2.4.2. Rolling-sliding conditions: Stribeck curves A ball-on-disc tribometer, EHD2, from PCS Instruments (London, UK) was used to determine the friction behavior of a contact pair consisting of a carbon chrome steel ball with a 19.05 mm diameter and a rotating carbon chrome steel disc. Disc and ball are run with two electric motors to perform the tests in rollingsliding motion. The selected normal load is automatically applied with a load cell. The friction force is determined on the ball by means of a torque cell installed on the ball shaft, firstly when the disc is rotating quicker than the ball, and then, for the identical entrainment speed when the ball turning faster than disc. Subsequently, coefficient of friction is determined with normal force and friction forces. More information about this tribometer can be observed in previous articles [22,24]. It should be noted that these friction tests were performed for the PAO8 base oil and the TiO 2 -OA nanolubricants under fully flooded lubrication (around 130 mL of lubricant) at the operating temperature of 120 °C, under a constant 50 N load that generates a maximum Hertz pressure of 0.7 GPa, and a 5 % slide-to-roll ratio (SRR): SRR %ðÞ¼2ðU disc U ball Þ ðU disc þU ball Þ100 ð1Þ U disc and U ball are the speeds of the contact points of the disc and the ball respectively, whereas the entrainment speed (U s ) is: U S ¼ðU disc þU ball Þ 2ð2Þ Fig. 3. FTIR spectrum of TiO 2 nanoparticles, OA, and coated TiO 2 -OA nanoparticles with their characteristic vibrational bands. Fig. 4. SEM images of TiO 2 nanopowders. José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 4 For the three discs (two rough and one smooth, Fig. 5) friction properties of TiO 2 -OA nanolubricants and PAO8 base oil were analyzed by means of Stribeck curves for a 5 % SRR value. For each test, the same ramp of entrainment speed was used: 0.05 m/s to 2 m/s. Specifically, the lowest speed of ball is 0.047 m/s and the highest is 1.997 m/s for all the tests whereas, the lowest speed of disc is 0.052 m/s and the highest is 2.101 m/s. During tests, the EHD2 tribometer regulates automatically disc and ball speeds, being the speed of disc faster to reach positive SRR whereas the entrainment speed remains constant. The friction coefficient is obtained as the average of those achieved from two different friction tests, one ramp raising speed and the other one reducing speed. Ball and disc properties (Table 1) were supplied by the producer while the surface roughness of the ball and the three discs was measured with a Profiler Hommelwerke (Table 1). 3. Results 3.1. Stability of the nanolubricants The temporal stability of TiO 2 -OA nanolubricants against sedimentation was investigated through visual observation as well as with refractometry. Fig. 6 evidence that for the four designed TiO 2 -OA nanolubricants no signs of sedimentation have appeared for at least three weeks just after homogenization. Nevertheless, for the most concentrated (and less stable) TiO 2 -OA nanolubricant (0.50 wt%), it seems that sedimentation starts to occur at 4 weeks after sonication. It should be noted that the visual stability of uncoated TiO 2 nanolubricants was<48 h (Fig. 7), therefore an important stability improvement was achieved with the OA coating. Thus, the stability time of prepared TiO 2 -OA nanolubricants is much greater than that required to carried out pure sliding and rolling-sliding tribological tests, around 8 h. The other method used to test the temporal stability of TiO 2 -OA nanolubricants against sedimentation is refractometry. Fig. 8 presents the time evolution of the refractive index for the TiO 2 -OA nanolubricant and for the bare TiO 2 nanolubricant, both with a NP concentration of 0.10 wt%. It can be clearly observed that for the 0.10 wt% TiO 2 nanolubricant after the first ten hours the nanoparticles are full sedimented, whereas in the case of the 0.10 wt% TiO 2 -OA nanolubricant the sedimentation is slower. Specifically, for the former nanolubricant the refractive index increased 0.27 % after 10 h while for the TiO 2 -OA nanolubricant it raised just 0.06 %, which indicates that the oleic acid coating of TiO 2 nanoparticles clearly enhances the stability of nanolubricants. Moreover, the TiO 2 -OA refractive index evolution shows a significant enhancement in the stability compared to other earlier measured nanolubricants [25]. In particular, with trimethylolpropane trioleate based nanolubricants (higher viscosity oil) containing graphene oxide nanoadditives, refractive index raises 0.44 % after 50 h [25], whereas for the same time (50 h) and with a very low viscosity oil, the refractive index evolution shows increases of 0.32 % for 0.10 wt% TiO 2 -OA nanolubricant. It should be noted that for low viscosity oils is more difficult to find good stabilities, since the nanoparticles have less resistance to drop in oil. Considering this fact, in this research a great stability was achieved owing to the coating with oleic acid of the TiO 2 nanoparticles. 3.2. Thermal conductivity results Experimental thermal conductivity measurements of PAO8 base oil and 0.35 wt% TiO 2 -OA nanolubricant are shown in Fig. 9 from 20 °Cto50°C. The measurements indicate that for both lubricants the thermal conductivity values remains almost constant when the temperature rises (Fig. 9). The 0.35 wt% TiO 2 -OA nanolubricant has approximately the same thermal conductivity values than PAO8 being around 0.15–0.16 W/m K at all the evaluated temperatures. 3.3. Friction results of pure sliding tests Mean friction coefficients ( l ) obtained under pure sliding conditions for PAO8 and the four nanolubricants composed by Fig. 5. Smooth and rough discs used in rolling-sliding tribological tests. Table 1 Main physical characteristics of ball and discs used in rolling-sliding tribological tests. Parameters Steel Ball Steel Discs Smooth Rough 1 Rough 2 Elastic modulus / GPa 210 210 210 210 Poisson coefficient 0.29 0.29 0.29 0.29 Diameter / mm 19.05 100 100 100 Surface roughness, Ra / nm 20 20 100 340 Hardness (Rockwell C) 62–66 62–66 62–66 62–66 José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 5 TiO 2 -OA with PAO8 base oil are presented in Fig. 10 and Table 2.It can be obviously seen that for the all TiO 2 -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% TiO 2 -OA nanoparticles. 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 TiO 2 -OA nanoadditives is achieved. Regarding the other nanolubricants, friction reductions of 16 %, 24 % and 23 % were obtained for 0.10 wt% TiO 2 -OA, 0.25 wt% TiO 2 -OA and 0.50 wt% TiO 2 -OA nanolubricants, respectively. Fig. 6. Visual stability of TiO 2 -OA nanolubricants. José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 6 3.4. Surface analysis of worn pins To compare qualitatively 3D Profiles, worn areas and cross section profiles of the pins tested with the PAO8 base oil and with the optimal TiO 2 -OA nanolubricant for pure sliding tests are shown in Figs. 11 and 12. Average wear produced in pins after tribological tests was quantified by means of different parameters: WSD, WTD and worn area, which results are reported in Table 2 for each lubricant tested. As it can be observed, for all the tested TiO 2 -OA nanolubricants, the three wear parameters were smaller than those found with PAO8 oil. The maximum wear reductions were found for 0.35 wt% TiO 2 -OA nanolubricant with reductions of 26 %, 65 % and 73 % for WSD, WTD and worn area, respectively. These impressive wear reductions can be clearly observed in Figs. 10 and 11 for the nanolubricant with the optimal concentration (0.35 wt% TiO 2 -OA). Wear results show a good correlation with friction, as it can be observed in Fig. 10. Furthermore, roughness (Ra) of worn tracks of pins was investigated to obtain more information about the antiwear properties of TiO 2 -OA nanoparticles. Worn tracks lubricated with TiO 2 -OA nanolubricants are less rough than those lubricated with PAO8 without additives (Table 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% TiO 2 -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 TiO 2 -OA nanopowders. Due to the mending effect, nanoparticles can fill the grooves and scars of the rubbing surface developing an improved surface finish [26,27]. To quantify better the effect of the TiO 2 -OA nanoparticles 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 [28]. The decrease in the Rsk Fig. 7. Visual stability of uncoated TiO 2 nanolubricants. José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 7 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) [28]. After friction tests this value decreases (2.98–2.41), which implies low both peaks and valleys. Comparing the mean Ra, Rsk and Rku values of the worn pins lubricated with PAO8 with those of worn pins lubricated with PAO8 + 0.35 wt% TiO 2 -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 3 also shows that the worn surfaces lubricated with PAO8 + 0.5 wt% TiO 2 -OA are flatter than those tested with the optimal nanolubricant (PAO8 + 0.3 5 wt% TiO 2 -OA). 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 (Fig. S1), oleic acid (Fig. S2) and the TiO 2 nanopowders (Fig. S3) were obtained to identify the components in mapping. Therefore, mapping of the worn pin lubricated with the optimal TiO 2 -OA nanolubricant (Fig. 13) was performed to identify the role that nanoparticles play in the wear decrease. Fig. 13 shows the relevant areas in green and blue color, that coincide with the spectra obtained for TiO 2 and OA, Figs. S3 and S4, respectively. This fact indicates that tribofilms containing OA and TiO 2 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, mending and polishing. 3.5. Rolling-sliding tribological results: Stribeck curves Rolling-sliding tribological measurements for PAO8 base oil and the designed TiO 2 -OA nanolubricants were carried out at the same Fig. 8. Refractive index evolution for 0.10 wt% TiO 2 and TiO 2 -OA nanolubricants. Fig. 9. Effect of temperature on thermal conductivity of PAO8 base oil and 0.35 wt% TiO 2 -OA nanolubricant. José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 8 Fig. 10. Mean friction coefficient, l , and WSD with PAO8 and its nanolubricants containing TiO 2 nanoparticles coated with oleic acid. Table 2 Mean coefficients of friction, l , and mean wear parameters, including their standard deviations, for the tested PAO8 lubricants. Lubricant l r WSD/ l m r / l m WTD/ l m r / l m Area/ l m 2 r / l m 2 PAO8 0.139 0.002 392 14 2.35 0.17 605 52 + 0.10 wt% TiO 2 -OA 0.116 0.002 359 14 2.11 0.18 527 41 + 0.25 wt% TiO 2 -OA 0.106 0.002 336 12 1.85 0.12 459 39 + 0.35 wt% TiO 2 -OA 0.098 0.001 289 11 0.82 0.11 163 18 + 0.50 wt% TiO 2 -OA 0.107 0.001 318 12 1.04 0.14 245 17 Fig. 11. 3D images and worn areas of worn tracks lubricated with PAO8 oil and with the optimum TiO 2 -OA nanolubricant (0.35 wt%). José M. Liñeira del Río, Fátima Mariño, E.R. López et al. Journal of Molecular Liquids 371 (2023) 121097 9