1. Introduction

Metal-mechanic industry deals daily with issues concerning tooling and components in constant contact, friction and wear and the effects of high temperatures and loading pressures; particularly in deep-drawing processes (Vallavi, Subramanian, Das, & Nachimuthu, 2017). Metal-forming fluids provide reduction in friction and wear among components under extreme pressure (EP) conditions, as well as serving as cooling media. (Canter, 2009) With aid of proper lubrication and components design, manufacturing processes have been improved under extreme working conditions.

Lubricant properties and materials performance have been widely studied, and nowadays through Nano-technology, next generation greases and lubricants have been able to excel the required working conditions; reducing friction and wear. (Chou & Lee, 2010; Dai, Kheireddin, Gao, & Liang, 2016; Demas, Timofeeva, Routbort, & Fenske, 2012; Ettefaghi, Ahmadi, Rashidi, Mohtasebi, &· Alaei, 2013; Hernandez Battez et al., 2008; Peña-Parás et al., 2015; Laura Peña-Parás et al., 2014; Ran, Yu, & Zou, 2016; Sayuti, Sarhan, & Salem, 2014; Younes, Christensen, Groven, Hong, & Smith, 2016; Zhang, Simionesie, & Schaschke, 2014).

Several types of nanoparticle additives such as nitrides, oxides, and sulfides have shown to improve the load-carrying capacity of conventional metal-forming lubricants. (Alves, Barros, Trajano, Ribeiro, &· Moura. 2013; Choi & Jung, 2012; Laura Peña-Parás et al., 2016: Laura Peña-Parás, Taha-Tijerina, Maldonado-Cortés, et al., 2017; Laura Peña-Parás, Taha-Tijerina, García-Pineda, Maldonado-Cortés, &· Garza, 2017; Thakre & Thakur, 2015) Nanolubricants are prepared by homo geneously dispersing nanoparticles within conventional lu bricants. A mechanism commonly present in nanolubricants states that nanoparticles penetrate in the contact areas, and are deposited on surfaces due their small or similar size as the surface roughness. (Mishina et al., 1993; Taha-Tijerina et al., 2013; Jaime Taha-Tijerina, Laura Peña-Parás, Maldonado, & Cortés, 2016) According to this, friction and wear behavior exhibit different tendencies depending on the nanoparticle used, based in 3 aspects: 1) morphology: nanoparticle shape that allows free movement among gaps in the oil film.; 2) size: larger nanoparticle sizes limit their movements and their ability to be paced in the gaps of the lubricant film; 3) hardness: harder material suffer lower wear and friction.

In recent years, tribology has evolved and with the nanomaterials and additives incursion, novel and more efficient nanolubricant fluids have been developed. Many researchers have investigated the relationship among lubricants and nanoparticles, and it has been established that this interactions could represent the difference between conventional fluids and nanofluids with improved tribological performance. (Choi & Jung, 2012; Gao et al.. 2012; Peña-Parás et al., 2015; Laura Peña-Parás et al.. 2014; Taha-Tijerina et al., 2013). These nanolubricants possess superb characteristics, such as improvements on thermal dissipation, anti-wear and friction reduction properties and EP performance. (Angayarkanni & Philip. 2015; Choi, 2009; Dharmalingam, Sivagnanaprabhu. Senthil Kumar, & Thirumalai, 2014; Jiang et al., 2015; Sharma, Tiwari, & Dixit, 2016; Sun, Zhou, Zhang, & Dang, 2004; Taha-tijerina, Narayanan, Avail, &· Ajayan, 2012; Jaime Taha-Tijerina, Sakhavand, Kochandra, Ajayan Pulickel, & Shahsavari, 2017; Tang & Li, 2014), In recent years, the application of nanolubricants in tribological applications has received significative attention to reduce friction and wear, (Hernandez Battez et al., 2006: Hernández Battez et al., 2008, 2010, Laura Peña-Parás et al., 2014, 2016; Taha-Tijerina et al., 2013) also preventing temperature rise during loading and reduction of shear stress; (Chou & Lee, 2008) which mainly depends upon the size, (Beck, Yuan, Warrier, & Teja, 2009; Chopkar. Das, & Manna, 2006; Gu, Li, Gu, & Zhu, 2008; Kim, ChoL & Kim, 2007; Lou, Zhang, & Wang, 2015) and shape of nanoparticles, (Elias et al., 2013; Kalin, Kogovšek, & Remškar, 2012; Nesappan, Palanisamy, &· Chandran. 2014; Qi, Jia, Yang, & Fan, 2011; Wan, Jin, sun, & Ding, 2015; Zeinali Heris, Razbani, Estelle, & Mahian, 2014) viscosity, (Chou & Lee, 2008; Elias et al., 2013; Gulzar et al., 2016; Havet, Blouet, Robbe Valloire, Brasseur, & Słomka, 2001; Lee et al., 2009; McCants, Ali, & Khan, 2009; Nabeel Rashin & Hemalatha, 2013; Syam Sundar, Singh, & Sousa, 2013; Taha-Tijerina et al., 2014; Jaime Taha-Tijerina et al., 2012; Wan et al., 2015; Zeinali Heris et al., 2014) filler fraction, (Peña-Parás et al., 2015; Laura Peña-Parás et al., 2014, 2015, 2016; Ran et al., 2016) temperature, additives or surfactants usage, (Gara & Zou, 2013; Hernandez Battez et al., 2006; Hernández Battez et al., 2008; Wu et al., 2016) and the defects on the rubbing surfaces. (Antusch et al., 2010; Luo, Wei, Zhao, Cai, & Zheng, 2014; Peng, Kang, Chen, Shu, & Chang, 2010).

For instance, in order to minimize wear, Caixiang et al. dispersed nanoparticles within lubricating oil and observed they deposited at the rubbing surfaces, which were "welded" and reacted to form a tribo-sintered layer. (Gu et al., 2008) It has been also found that nanoparticles suspension increases the load-carrying capacity and decreases the coefficient of friction of the lubricating oil, (Hu, Dong, Chen, & He, 2000; Hu et al., 2002) reducing the contact pressure on the rubbing surfaces. (Padgurskas. Rukuiza, Prosyčevas, &· Kreivaitis, 2013). Also, Chinas-Castillo et al. investigated the phenomenon of nanoparticles behavior within lubricating oils. In their study, it was reported that dispersed nanoparticles can penetrate and strengthen the rubbing contacts by mechanical entrapment. (Chiñas-Castillo & Spikes, 2003). Furthermore, it has been found that the scuffing resistance and frictional power of lubricating oil is enhanced when it is dispersed with 3.0vol.% of nanodiamond structures. (Chu, Hsu, & Lin, 2010).

Nanoparticles based on oxides have shown significant improvements in the tribological properties of nanolubricants. For instance, CuO nanoparticles (<50nm) added (at 0.01wt.%) to a metal-cutting lubricant fluid demonstrated a reduction on the wear scar diameter (WSD) and the coefficient of friction (COF) by 86% and 7%, respectively, in comparison to the base fluid. ( Laura Peña-Parás et al., 2014) Similarly, Alves, et al. (Alves, Mello, Faria, & Camargo, 2016) studied CuO nanoparticles as EP additives in a PAO oil, obtaining reductions in friction and wear properties under boundary conditions.

Hernandez-Battez et al. studied ZnO nanoparticles (20nm) dispersed within poly-alpha-olefin (PA06) lubricant using a four-ball tribotester. The effect of ZnO nanostructures was not significant as an anti-wear agent under certain conditions, however, in EP conditions from the initial seizure load, a decrease in wear was achieved., due to reduction in metal-metal contact (Hernandez Battez et al., 2006) Ran et al. worked on 60SN oil reinforced with ZnO nanoparticles (10 ֊ 30nm), it was found that nanolubricants at 0.2wt.% of ZnO with addition of 10wt.% oleic acid (OA) achieved the best friction-reducing and anti-wear properties; mainly due to the contribution of the nanoparticles acting as nanosized bearings. ( Ran, Yu, Wang, & Xiao, 2015) On investigations performed by Gara et al., (Gara & Zou. 2013) it was demonstrated that oil-based nanolubricants reinforced with ZnO nanoparticles also reduced friction and wear.

In this work, Zn and ZnO nanoparticles (< 50nm) were homogeneously dispersed within a conventional metal-forming synthetic fluid at various filler fractions (0.01, 0.05 and 0.10wt.%). Tribological investigations were performed with a four-ball tribotester, based on the Polish method for testing lubricants under scuffing conditions, developed by the Institute for Sustainable Technologies - National Research Institute (ITEePib). It has been de monstrated that this method is very sensitive to the type and concentration of EP additives. (Szczerek & Tuszyński, 2002) The main benefits of this method are the cost-time relation, due to the test duration (18 seconds) more ex periments are performed in less time, allowing us to reduce the standard deviation. ( Laura Peña-Parás et al., 2014).

2. Materials and methods

2.1 Materials

The studied nanoparticles were Zn and ZnO (See Table 1), supplied by Sigma-Aldrich. The investigated lubricant is a metal-forming synthetic lubricant fluid (See Table 1), which is a medium viscosity fluid used in metal-forming applications such as stamping, drawing and punching operations; providing a light but effective film to reduce wear in tools. Other advantages are the excellent heat transfer, it reduces fractures and it's completely biodegradable. ( Laura Peña-Parás et al., 2014; Ran et al.. 2015).

2.2 Nanolubricants preparation

Zn and ZnO nanoparticles (described in Table 1) were homogeneously dispersed within synthetic lubricant at different concentrations: 0.01, 0.05 and 0.10wt.% for each material. Extended water bath sonication (~3֊4 h) was first used; the water bath was maintained at room temperature (25 °C) to avoid possible nanoparticles agglomeration. Subsequently, a Metason 120T bath sonicator (output power of 70W) was used, according to a previous study by our group. ( Jaime Taha-Tijerina et al.. 2012).

2.3 Tribological evaluation

Tribological properties of base oil and the nanofluids at various filler fractions were measured using a four-ball tribotester. Using this tribotester, the ITEePib Polish method for testing lubricants under scuffing conditions (Michalczewski, Piekoszewski, Tuszyński, Szczerek, & Wulczynski, 2011) can be used to determine the friction torque, the maximum applied load, and the temperature of the lubricants; the limiting pressure of seizure (p_oz) was also evaluated. These evaluations help to identify the EP properties of NFs, namely the time and load when the wear and the loss of film lubricant occur.

The conditions of this test are as follows: an upper ball rotates (See Figure 1) in a perpendicular axis with a speed of 500 rpm against three stationary balls covered with -10mL of lubricant, with a linearly increasing load of 0-7200 N at room temperature (25 ^๐C), over the course of 18 s. The load increases until seizure occurs and the lubricant film is destroyed (at 10 N.m), this load is denoted as p_oz. If a torque of 10 N.m is not reached and no seizure occurs 7200 N is taken as P_oz. For every test, the limiting pressure of seizure, Ροζ, is calculated with the following equation (Eq. 1) (Michalczewski et al., 2011):

The WSD is calculated by averaging the wear scar from the three stationary lower-balls, with minimum of 5 tests for each nanofluid, and following the Dixon probabilistic methodology (Dean & Dixon, 1951; Rorabacher, 1991) to generate statistical reliable results. In general, the addition of nanoparticles resulted in a significant decrease on these parameters. (Taha-Tijerina et al., 2013) Furthermore, the greater the P_oz is, the best tribological characteristics the lubricant has.

2.4 Worn surface analysis

Surface roughness of wear scars of steel balls were characterized with an Alicona IF-EdgeMaster optical 3D surface measurement system. Energy Dispersive Spectroscopy (EDS) analysis was performed with a JEOL Scanning Electron Microscope (SEM) using 15kV and 10mm working distance. Three specimens were analyzed, finding a variation of 1 ֊ 2% among them, reporting the most representative value.

3. Results and discussion

Figure 2 shows the friction torque at increasing load over time for Zn and ZnO nanolubricants. It can be observed that the unfilled synthetic lubricant presents seizure before the end of the run, at -3800N. The addition of both nanoparticles delayed seizure at 0.01wt, with loads of -4500N and -4800N for Zn and ZnO nanoparticles, respectively. Higher concentrations showed no lubricant seizure, thus a P_t of 7200N is taken, with the best performance provided by ZnO. This is due to the nanoparticle 'S higher hardness (4.5 Mohs) compared to Zn (2.5 Mohs) that allows for a higher load-carrying capacity. Pressure loss limit was then calculated considering the WSDs of the three lower balls of each test (Figure 3). A comparison between WSDs is shown for pure lubricant and 0.10wt.% ZnO nanolubricants (see Figure 4).

It is also observed that during the homogeneous dispersion process, a thin tribofilm of ZnO was formed within the fluid and components surfaces. The tribofilm between rubbing surfaces increase the load-carrying capacity of the lubricant:-This effect has been also observed by other authors. (Alves et al., 2016; Chen, Liu, Liu, Gunsel, & Luo, 2015; Peng, Chen, Kang, Chang, & Chang, 2010; Laura Peña-Parás et al., 2014) Chen et al. observed an enhancement on the non-seizure load of liquid paraffin due to the formation of a tribofilm (ultrathin MoS₂ nanoparticles). (Chen et al., 2015) Similarly, ՏiՕ₂ nanoparticles dispersed in liquid paraffin formed a tribofilm between rubbing surfaces increasing the load-carrying capacity of the lubricant. (Peng, Chen, et al.. 2010).

Roughness was also measured from the average diameter of the three stationary lower-balls, with an Alicona IF-EdgeMaster optical 3D surface measurement system. Table 2 shows the R_a: average roughness (pm). R_q: root-mean-square roughness (pm), and R_z: maximum different roughness (pm). Here, the initial roughness for the steel balls is 0.280 (m).

In our results high Ra, R_q, and R_z values are obtained for the worn surfaces due to the EPs of the test; a decrease in these values mean lower metal-metal contact due to the presence of the nanoparticle additives. The worn surface with the synthetic base fluid showed Ra of 3.42μm. The nanolubricant containing 0.05wt.% of Zn nanoparticles showed a decrease of 41%, compared to the synthetic fluid. Moreover, the nanolubricant with 0.10wt.% ZnO had the highest decrease in Ra of 65% due to a tribofilm formation, reducing the contact area between moving surfaces.

Figure 5 shows the EDS analysis of steel balls after tribological evaluation. The EDS spectrum and SEM image of a steel ball is presented in Figure 5a as a reference. EDS spectra in Figure 5b and Figure 5c reveal the presence of Zn on the worn surfaces tested with Zn and ZnO nanolubricants. Nanoparticle tribosinterization is also evident in the SEM insets for Figure 5b and Figure 5c, which formed a tribofilm. This tribofilm that prevented metal-metal contact between rubbing surfaces explains the increase in tribological and surface roughness properties shown in Figure 2, Figure 3, and Table 2. Additionally, the presence of residual elements like potassium (K) and calcium (Ca) are expected since potassium salicylate and calcium chloride are used as additives within conventional metal-mechanic lubricants (Brinksmeier, Meyer. Huesmann-Cordes, & Herrmann, 2015), US8299007 B2, 2010.

4. Conclusions

After an evaluation of various concentrations of Zn and ZnO nanoparticles homogeneously dispersed within a conventional synthetic fluid, it can be concluded that these additives provide significant enhancements on EP properties. The highest improvement was shown by the 0.10wt.% ZnO nanofluid, preventing lubricant seizure and with a superb impact in p_oz of up to 250%. It is also worth noting that all the other concentrations of nanoparticles increased tribological properties. These small nanostructures were tribosintered onto the surfaces forming a tribofilm, reducing the contact area, surface roughness, and wear of materials, and potentially be employed as EP lubricant additives for metal-forming processes.

Acknowledgments

Authors acknowledge the support from CONACYT, as well as the support from UdeM, and Metalsa for supplying the lubricants and materials.

References

Alves, S. M., Barros, B. S., Trajano, M. F., Ribeiro, K. S. B., & Moura, E. (2013). Tribological behavior of vegetable oil-based lubricants with nanoparticles of oxides in boundary lubrication conditions. Tribology International, 65, 28-36. https://doi.org/10.1016/j.triboint.2013.03.027

Alves, S. M., Mello, V. S., Faria, Ε. Α., & Camargo, Α. P. P. (2016). Nanolubricants developed from tiny CuO nanoparticles. Tribology International , 100, 263-271. https://doi.org/10.1016/j.triboint.2016.01.050

Angayarkanni, S. Α., & Philip, J. (2015). Review on thermal properties of nanofluids: Recent developments. Advances in Colloid and Interface Science, 225, 146-176. https://doi.org/10.1016/jxis.2015.08.014

Antusch, S., Dienwiebel, M., Nold, E., Albers, P., Spicher, U., & Scherge, M. (2010). On the tribochemical action of engine soot. Wear, 269(1-2), 1-12. https://doi.org/10.1016/j.wear.2010.02.028

Beck, M. P., Yuan, Y., Warrier, P., & Teja, Α. S. (2009). The effect of particle size on the thermal conductivity of alumina nanofluids. Journal of Nanoparticle Research, 11(5), 1129-1136. https://doi.org/10.1007/sll051-008-9500-2

Brinksmeier, E., Meyer, D., Huesmann-Cordes, A. G., & Herrmann, C. (2015). Metalworking fluids - Mechanisms and performance. CIRP Annals - Manufacturing Technology, 64(2), 605-628. https://doi.org/10.1016/j.cirp.2015.05.003

Canter, N. (2009). Challenges in formulating metal-forming fluids. Tribology & Lubrication Technology, 65(3), 56. Retrieved from http://www.metalworkingfluid.com/mwf/docs/tlt_ChallengesMFF.pdf

Carey, J. T., Galiano-Roth, A. S., & Dietz, T. G. (2010, October 28). US8299007 B2. United States. Retrieved from https://www.google.com/patents/US8299007

Chen, Z., Liu, X., Liu, Y, Gunsel, S., & Luo, J. (2015). Ultrathin MoS2 Nanosheets with Superior Extreme Pressure Property as Boundary Lubricants. Scientific Reports, 5, 12869. https://doi.org/10.1038/srepl2869

Chiñas-Castillo, F., & Spikes, H. A. (2003). MechanismofAction of Colloidal Solid Dispersions. Journal of Tribology, 125(3) , 552-557. https://doi.org/10.1115/l.1537752

Choi, C, & Jung, M. (2012). Extreme pressure properties of multi-component oil-based nanofluids. Journal of Nanoscience and Nanotechnology, 12(4), 3237-41. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/228 49096

Choi, S. U. S. (2009). Nanofluids: From Vision to Reality Through Research. Journal of Heat Transfer, 131(3), 33106. https://doi.org/10.1115/l.3056479

Chopkar, M., Das, P. K., & Manna, I. (2006). Synthesis and characterization of nanofluid for advanced heat transfer applications. Scripta Materialia, 55(6), 549-552. https:// doi.org/10.1016/j.scriptamat.2006.05.030

Chou, C-C., & Lee, S.-H. (2010). Tribological behavior of nanodiamond-dispersed lubricants on carbon steels and aluminum alloy. Wear, 269(11-12), 757-762. https://doi.org/10.1016/j.wear.2010.08.001

Chou, C. C, & Lee, S. H. (2008). Rheological behavior and tribological performance of a nanodiamond-dispersed lubricant. Journal of Materials Processing Technology, 201(1-3), 542-547. https://doi.org/10.1016/j.jmatproteci2007.11.169

Chu, H. Y., Hsu, W. C, & Lin, J. F. (2010). The anti-scuffing performance of diamond nano-particles as an oil additive. Wear, 268(7-8), 960-967. https://doi.org/10.1016/j.wear.2 009.12.023

Dai, W., Kheireddin, B., Gao, H., & Liang, H. (2016). Roles of nanoparticles in oil lubrication. Tribology International, 102, 88-98. https://doi.org/10.1016/j.triboint.2016.05.020

Dean, R. B., & Dixon, W. J. (1951). Simplified Statistics for Small Numbers of Observations. Analytical Chemistry, 23(4), 636-638. https://doi.org/10.1021/ac60052a025

Demas, N. G., Timofeeva, E. v., Routbort, J. L., & Fenske, G. R. (2012). Tribological effects of BN and MoS 2nanoparticles added to polyalphaolefin oil in piston skirt/cylinder liner Tests. Tribology Letters, 47(1), 91-102. https://doi.org/10.1007/sll249-012-9965-0

Dharmalingam, R., Sivagnanaprabhu, K. K., Senthil Kumar, B., & Thirumalai, R. (2014). Nano materials and nanofluids: An innovative technology study for new paradigms for technology enhancement. Procedia Engineering, 97, 1434 -1441. https://doi.org/10.1016/j.proeng.2014.12.425

Elias, M. M., Miqdad, M., Mahbubul, I. M., Saidur, R., Kamalisarvestani, M., Sohel, M. R., ... Amalina, M. A. (2013). Effect of nanoparticle shape on the heat transfer and thermodynamic performance of a shell and tube heat exchanger. International Communications in Heat and Mass Transfer, 44, 93-99. https://doi.org/10.1016/j.ich eatmasstransfer.201 3.03.014

Ettefaghi, E., Ahmadi, H., Rashidi, A., Mohtasebi, S., & Alaei, M. (2013). Experimental evaluation of engine oil properties containing copper oxide nanoparticles as a nanoadditive. International Journal of Industrial Chemistry, 4(1), 28. https://doi.org/10.1186/2228-5547-4-28

Gao, G., Gao, W., Cannuccia, E., Taha-Tijerina, J., Balicas, L., Mathkar, A., ... Ajayan, P. M. (2012). Artificially stacked atomic layers: Toward new van der waals solids. Nano Letters, 12(7). https://doi.org/10.1021/n1301061b

Gara, L., & Zou, Q. (2013). Friction and Wear Characteristics of Oil-Based ZnO Nanofluids. Tribology Transactions, 56(2), 236-244. https://doi.org/10.1080/10402004.2012.740148

GU, C, Li, Q., GU, Z., & ZHU, G. (2008). Study on application of CeO2 and СаСОЗ nanoparticles in lubricating oils. Journal of Rare Earths, 26(2), 163-167. https://doi.org/10.1016/S1002-0721(08)60058-7

Gulzar, M., Masjuki, H. H., Kalam, Μ. Α., Varman, M., Zulkifli, N. W. M., Mufti, R. Α., & Zahid, R. (2016). Tribological performance of nanoparticles as lubricating oil additives. Journal of Nanoparticle Research, 18(8), 223. https://doi.org/10.1007/s11051-016-3537-4

Havet, L., Blouet, J., Robbe Valloire, F., Brasseur, E., & Słomka, D. (2001). Tribological characteristics of some environmentally friendly lubricants. Wear, 248(1-2), 140- 146. https://doi.org/10.1016/S0043-1648(00)00550-0

Hernandez Battez, Α., Fernandez Rico, J. E., Navas Arias, Α., Viesca Rodriguez, J. L., Chou Rodriguez, R., & Diaz Fernandez, J. M. (2006). The tribological behaviour of ZnO nanoparticles as an additive to PA06. Wear, 261(3-4), 256-263. https://doi.org/10.1016/j.wear.2005.10.001

Hernández Battez, Α., Gonzalez, R., Viesca, J. L., Fernández, J. E., Díaz Fernández, J. M., Machado, A., ... Riba, J. (2008). CuO, ZrO2 and ZnO nanoparticles as antiwear additive in oil lubricants. Wear, 265(3-4), 422-428. https://doi.org/10.1016/j.wear.2007.11.013

Hernández Battez, Α. , Viesca, J. L. , Gonzalez, R. , Blanco, D., Asedegbega, E., & Osorio, Α. (2010). Friction reduction properties of a CuO nanolubricant used as lubricant for a NiCrBSi coating. Wear, 268(1-2), 325-328. https://doi.org/10.1016/j.wear.2009.08.018

Hu, Z., Dong, J., Chen, G. & He, J. (2000). Preparation and tribological properties of nanoparticle lanthanum borate. Wear, 243(1-2), 43-47. https://doi.org/10.1016 /S0043-1648(00)00415-4

Hu, Z. S., Lai, R., Lou, F., Wang, L. G., Chen, Z. L., Chen, G. X., &· Dong, J. Χ. (2002). Preparation and tribological properties of nanometer magnesium borate as lubricating oil additive. Wear, 252(5-6), 370-374. https://doi.org/10.1016/S0043-1648(01)00862-6

Jiang, X.-F., Weng, Q., Wang, X.-B., Li, X., Zhang, J., Golberg, D., & Bando, Y. (2015). Recent Progress on Fabrications and Applications of Boron Nitride Nanomaterials: A Review. Journal of Materials Science & Technology, 31(6), 589-598. https://doi.org/10.1016/j.jmst.2014.12.008

Kalin, M., Kogovšek, .J., & Reinškar, M. (2012). Mechanisms and improvements in the friction and wear behavior using MOS2 nanotubes as potential oil additives. Wear, 280-281, 36-45. https://doi.org/10.1016/j.wear.2012.01.011

Kim, S. H., Choi, S. R., & Kim, D. (2007). Thermal Conductivity of Metal-Oxide Nanofluids: Particle Size Dependence and Effect of Laser Irradiation. Journal of Heat Transfer , 129(3), 298. https://doi.org/10.1115/l.2427071

Lee, C-G., Hwang, Y.-J., Choi, Y.-M., Lee, J.-K., Choi, C, & Oh, J.-M. (2009). A study on the tribological characteristics of graphite nano lubricants. International Journal of Precision Engineering and Manufacturing, 10(1), 85-90. https://doi.org/10.1007/S12541-009-0013-4

Lou, J. -f., Zhang, H., & Wang, R. (2015). Experimental investigation of graphite nanolubricant used in a domestic refrigerator. Advances in Mechanical Engineering, 7(2), 16 87814015571011.https://doi.org/10.1177/1687814015571011

Luo, T., Wei, X., Zhao, H, Cai, G., & Zheng, X. (2014). Tribology properties of Аl2O3/ТiO2nanocomposites as lubricant additives. Ceramics International, 40(7), 10103 10109. https://doi.org/10.1016/j.ceramint.2014.03.181

McCants, D. A., Ali, M Y., & Khan, J. (2009). Effective Viscosity Measurement of CuO and ZnO Nanofluids. In ASME 2009 Second International Conference on Micro/Nanoscale Heat and Mass Transfer, Volume 1 (pp. 607-615). ASME. https://doi.org/10.1115/MNHMT2009-18492

Michalczewski, R., Piekoszewski, W., Tuszyński, W., Szczerek, M., & Wulczynski, J. (2011). The new methods for scuffing and pitting investigation of coated materials for heavy loaded lubricated elements. In Kuo C.-H. (Ed.), Tribology - Lubricants and Lubrication (pp. 305-320). Croatia: InTech. Retrieved from http://cdn.intechopen.com/pdfs-wm/21939.pdf

Mishina, H., Kohno, A., Kanekama, U., Nakajama, K., Mori, M., &· Iwase, M. (1993). Lubricity of the Metallic Ultrafine Particles. Jap. Journ. of Trib, 38, 1109-1120.

Nabeel Rashin, M., & Hemalatha, J. (2013). Viscosity studies on novel copper oxide-coconut oil nanofluid. Experimental Thermal and Fluid Science, 48, 67-72. https://doi.org/10.1016/j.expthermflusci.2013.02.009

Nesappan, S., Palanisamy, N., & Chandran, M. (2014). Tribological Investigation of Copper (Cu) and Copper Oxide (CuO) Nanoparticles Based Nanolubricants for Machine Tool Slideways. In Volume 2B: Advanced Manufacturing (p. V02BT02A020). ASME. https://doi.org/10.1115/IMECE2014-37707

Padgurskas, J., Rukuiza, R., Prosyčevas, I., & Kreivaitis, R. (2013). Tribological properties of lubricant additives of Fe, Cu and Co nanoparticles. Tribology International, 60, 224֊ 232. https://doi.org/10.1016/j.triboint.2012.10.024

Peng, D., Chen, C, Kang, Y., Chang, Y., & Chang, S. (2010). Size effects of SiO2 nanoparticles as oil additives on tribology of lubricant. Industrial Lubrication and Tribology, 62(2), 111-120. https://doi.org/10.1108/00368791011025656

Peng, D., Kang, Y., Chen, S., Shu, F., & Chang, Y. (2010). Dispersion and tribological properties of liquid paraffin with added aluminum nanoparticles. Industrial Lubrication and Tribology, 62(6), 341-348. https://doi.org/10.1108/ 00368791011076236

Peña-Parás, L., Taha-Tijerina, J. , García-Pineda, P., Maldonado-Cortés, D., & Garza, G. T. (2017). Temperature dependence of the extreme-pressure behavior of CuO and TiO2 nanoparticle additives in metal-forming polymeric lubricants. Industrial Lubrication and Tribology , 69(5), 730-737. https://doi.org/10.1108/ILT-02-20f6-0023

Peña-Parás, L. , Taha-Tijerina, J. , Garcia, A., Maldonado, D., Gonzalez, J. Α., Molina, D., ... Cantil, P. (2014). Antiwear and Extreme Pressure Properties of Nanofluids for Industrial Applications. Tribology Transactions, 57(6), 1072֊1076. https://doi.org/10.1080/10402004.2014.933937

Peña-Parás, L. , Taha-Tijerina, J. , Garcia, A. , Maldonado, D. , Nájera, Α., Cantil, P. , & Ortiz, D. (2015). Thermal transport and tribological properties of nanogreases for metal-mechanic applications. Wear, 332-333, 1322-1326. https://doi.org/10.1016/j.wear.2015.01.062

Peña-Parás, L. , Taha-Tijerina, J. , Garza, L., Maldonado-Cortés, D. , Michalczewski, R., & Lapray, C. (2015). Effect of CuO and AI2O3 nanoparticle additives on the tribological behavior of fully formulated oils. Wear, 332-333, 1256-1261. https://doi.org/10.1016/j.wear.2015.02.038

Peña-Parás, L. , Taha-Tijerina, J. , Maldonado-Cortés, D. , García-Pineda, P. , Garza, G. T. , Irigoyen, M., ... Sanchez, D. (2016). Extreme pressure properties of nanolubricants for metal-forming applications. Industrial Lubrication and Tribology , 68(1), 30-34. https://doi.org/10.1108/ILT-05-2015-0069

Peña-Parás, L. , Taha-Tijerina, J. , Maldonado-Cortés, D. , García, P., Irigoyen, M. , &· Guerra, J. (2017). Tribological performance of halloysite clay nanotubes as green lubricant additives. Wear, 376-377(Ρart A), 885-892. https://doi.org/10.1016/j.wear.2017.01.044

Qi, X., Jia, Z., Yang, Y., & Fan, B. (2011). Characterization and auto-restoration mechanism of nanoscale serpentine powder as lubricating oil additive under high temperature. Tribology International , 44(7-8), 805-810. https://doi.org/10.1016/j.triboint.20U.02.001

Ran, X., Yu, X. Y., Wang, Y., & Xiao, Z. (2015). Tribological Properities of Oil-Based ZnO Nanofluids. Key Engineering Materials, 645-646, 437-443. https://doi.org/10.4028/www.scientific.net/KEM.645-646.437

Ran, X., Yu, X., & Zou, Q. (2016). Effect of Particle Concentration on Tribological Properties of ZnO Nanofluids. Tribology Transactions , https://doi.org/10.1080/10402004.2016.1154233

Rorabacher, D. B. (1991). Statistical treatment for rejection of deviant values: critical values of Dixon's "Q" parameter and related subrange ratios at the 95% confidence level. Analytical Chemistry , 63(2), 139-146. https://doi.org/10.102f/ac00002a010

Sayuti, M., Sarhan, A. A. D., & Salem, F. (2014). Novel uses of SiO2 nano-lubrication system in hard turning process of hardened steel AISI4140 for less tool wear, surface roughness and oil consumption. Journal of Cleaner Production, 67, 265-276. https://doi.org/10.1016/j.jcle pro.2013.12.052

Sharma, A. K., Tiwari, A. K, & Dixit, A. R. (2016). Rheological behaviour of nanofluids: A review. Renewable and Sustainable Energy Reviews, 53, 779-791. https://doi. org/10.1016/j.rser.2015.09.033

Sun, L., Zhou, J., Zhang, Z., & Dang, H. (2004). Synthesis and tribological behavior of surface modified (NH4)3PMol2O40 nanoparticles. Wear, 256(1-2), 176-181. https://doi.org/10.1016/S0043-1648(03)00386-7

Syam Smidar, L., Singh, M. K., & Sousa, Α. C. M. (2013). Investigation of thermal conductivity and viscosity of Fe3O4 nanofluid for heat transfer applications. International Co mmunications in Heat and Mass Transfer, 44, 7֊ 14. https://doi.org/10.1016/j.icheatmasstransfer.2013.02.014

Szczerek, M., & Tuszyński, W. (2002). A method for testing lubricants under conditions of scuffing. Part I. Presentation of the method. Tribotest, 8(4), 273-284. https://doi.org/10.1002/tt.3020080402

Taha-Tijerina, J. J. , Narayanan, T. N., Tiwary, C. S., Lozano, K., Chipara, M., & . Ajayan, P. M. (2014). Nanodiamond based thermal fluids. ACS Applied Materials & Interfaces, 6, 4778-4785. https://doi.org/10.1021/am405575t

Taha-Tijerina, J., Laura Peña-Parás, Maldonado, D. , & Cortés. (2016). 2D-Based Nanofluids: Materials Evaluation and Performance. In P. K. Nayak (Ed.), Two-dimensional Materials-Synthesis, Characterization and Potential Applica tions. 153-198. InTech. https://doi.org/10.5772/64760

Taha-tijerina, J., Narayanan, T. N. , Avail, S., & Ajayan, P. M. (2012). 2D Structures-based Energy Management Nanofluids. In ASME 2012 International Mechanical Engineering Congress & Exposition IMECE 2012 (p. IMECE 2012-87890). Houston, TX. https://doi.org/10.1115/1МЕСЕ2012-87890

Taha-Tijerina, J. , Narayanan, T. N. , Gao, G. , Rohde, M., Tsentalovich, D. Α., Pasquali, M., & Ajayan, P. M. (2012). Electrically insulating thermal nano-oils using 2D fillers. ACS Nano, 6(2), 1214-1220. https://doi.org/10.1021/nn 203862p

Taha-Tijerina, J. , Peña-Paras, L., Narayanan, T. N. , Garza, L. , Lapray, C, Gonzalez, J., ... Ajayan, P. M. (2013). Multifunctional nanofluids with 2D nanosheets for thermal and tribological management. Wear, 302(1-2), 1241-1248. https://doi.org/10.1016/j.wear.2012.12.010

Taha-Tijerina, J. , Sakhavand, N., Kochandra, R., Ajayan Pulickel, M., & Shahsavari, R. (2017). Theoretical Prediction of Physical Properties (Viscosity) on 2D-based Nanofluids. Ingeniería. Investigación Y Tecnología, 18(1), 101-109. Retrieved from http://www.redalyc.org/html/404/40449649009/

Tang, Z., & Li, S. (2014). A review of recent developments of friction modifiers for liquid lubricants (2007-present). Current Opinion in Solid State and Materials Science, 18(3),119-139. https://doi.org/10.1016/j.cossms.2014.02.002

Thakre, Α. A., & Thakur, A. (2015). Study of behaviour of aluminium oxide nanoparticles suspended in SAE20W40 oil under extreme pressure lubrication. Industrial Lubrication and Tribology , 67(4), 328-335. https://doi.org/10.1108/ ILT-06-2014-0057

Vallavi, Α., Subramanian, M., Das, M., & Nachimuthu, G. (2017). Assessment of cutting force and surface roughness in LM6 I SiC P using response surface methodology. Journal of Applied Research and Technology, 15(3), 283- 296. https://doi.org/10.1016/J.JART.2017.01.013

Wan, Q., Jin, Y., Sun, P., & Ding, Y. (2015). Tribological behaviour of a lubricant oil containing boron nitride nanoparticles. Procedia Engineering , 102, 1038-1045. https://doi.org/10.1016/j.proeiig.2015.01.226

Wu, L., Zhang, Y., Yang, G., Zhang, S., Yu, L., & Zhang, P. (2016). Tribological properties of oleic acid-modified zinc oxide nanoparticles as the lubricant additive in poly-alpha olefin and diisooctyl sebacate base oils. RSC Adv., 6(74), 69836-69844. https://doi.org/10.1039/C6RA10042B

Younes, H., Christensen, G., Groven, L., Hong, H., & Smith, P. (2016). Three dimensional (3D) percolation network structure: Key to form stable carbon nano grease. Journal of Applied Research and Technology , 14(6), 375-382. http://doi.org/10.1016/j.jart. 2016.09.002

Zeinali Heris, S., Razbani, Μ. Α., Estelle, P., & Mahian, O. (2014). Rheological Behavior of Zinc-Oxide Nanolubricants. Journal of Dispersion Science and Technology, 36(8), 1073-1079. https://doi.org/10.1080/01932691.2014.945595

Zhang, Ζ. J., Simionesie, D., & Schaschke, C. (2014). Graphite and Hybrid Nanomaterials as Lubricant Additives. Lubricants, 2(2), 44-65. https://doi.org/10.3390/hibricants2020044

Glossary

EP: Extreme pressure

P_oz: Limiting pressure of seizure

COF: Coefficient of friction

PAO: Poly-alpha-olefin

OA: Oleic acid

ITEePib: Institute for Sustainable Technologies-National Research Institute

WSD: Wear scar diameter

R_a: Average roughness (μm)

R_q: Root-mean-square roughness (μm)

R_z: Maximum different roughness (pm)

EDS: Energy dispersive spectroscopy

Notes

Author notes

^aCorresponding autor: E-mail address: jose.taha@udem.edu

Conflict of interest declaration