## Result and Discussion
### Friction Performance Analysis  
The coefficient of friction (COF) versus the experiment time for PAO and PAO with 1 wt % (based on the weight of PAO) of the PS-OMMT nanocomposite samples is shown in Figure 10.  
In Figure 10, the COF of PAO initially was approximately 0.10. However, it subsequently increased to approximately 0.14 within 500 s and then experienced a stable period of time between approximately 500 s and 700 s. The COF then decreased slightly from 0.14 to approximately 0.13 within 2500 s, followed by an increase to 0.15 with fluctuations. However, for the PAO-PS-OMMT nanocomposites, the COF values were clearly decreased compared with those of PAO because when the PS-OMMT nanocomposite particles were taken into the contact area by the lubricant, they were likely to support the two counterparts and prevent them from contacting each other and rolling like bearing ball in bearing between two metal surfaces to reduce the COF value. For the PAO-PS-OMMT-1 sample, the COF increased with fluctuations from 0.08 at the beginning to 0.11 until approximately 2000 s and then remained relative steady for the rest of the test. For the PAO-PS-OMMT-3 sample, the COF started at 0.078 and increased to approximately 0.095 within 500 s. It then remained relatively stable, and at the end of the test, the COF was approximately 0.09. For the PAO-PS-OMMT-5 sample, the COF increased sharply from 0.083 to approximately 0.10 near 300 s and then increased slowly to approximately 0.12 at the end of the test. It could be found that with the same content of nanocomposite particles, PAO-PS-OMMT-3 demonstrated the best friction reduction and the lowest COF value. This result indicated that the PS-OMMT-3 nanocomposite particles with a higher degree of exfoliation and better dispersion of MMT can effectively support the two counterparts and prevent contact more efficiently than the other studied materials. Figure 9: SEM images of (**A**) PS, (**B**) PS-OMMT-1, (**C**) PS-OMMT-3 and (**D**) PS-OMMT-5. To further investigate the variations in the tribological behavior due to the addition of the PS-OMMT nanocomposite particles, the wear surface morphologies of PAO and the nanocomposites were examined by SEM. SEM images of the wear surfaces of the upper and lower ball of the four-ball machine for PAO, PAO-PS-OMMT-1, PAO-PS-OMMT-3 and PAO-PS-OMMT-5 are shown in Figure 11A-D and Figure 12A-D, respectively. As shown in Figure 11A, a deep and wide groove formed on the wear surface and can be associated with the effect of the presence of hard particles between the contact surfaces. Figure 12A further verified the existence of this worn morphology. It can be seen from Figure 11B that the grooves on the wear surface were shallow in depth and narrow in width compared with those in Figure 11A, suggesting that the addition of PS-OMMT-1 nanocomposite particles could improve the surface roughness and reduce the COF. A smooth wear surface with microgrooves and nanogrooves was observed for the PAO-PS-OMMT-3 sample, as shown in Figures 11C and 12C. However, the wear surface of the nanocomposites containing 5 wt % OMMT show deep scratch-like lines and coarse surfaces, which indicates that the plowing phenomenon occurred more severely on the surface compared to the composite containing 3 wt % OMMT. This result was due to the aggregation of the OMMT layers in the PS-OMMT-5 samples, which led to poor dispersion and caused three-body abrasion, reducing the lubrication effect of the nanocomposite particles. As shown in Figure 12, the wear scar diameters (WSDs) for the PAO, PAO-PS-OMMT-1, PAO-PS-OMMT-3 and PAO-PS-OMMT-5 samples were 958.14 \(\pm\) 47.91 \(\upmu\)m, 993.70 \(\pm\) 44.71 \(\upmu\)m, 929.00 \(\pm\) 32.51 \(\upmu\)m and 1600.00 \(\pm\) 62.40 \(\upmu\)m, respectively. This result revealed that the PS-OMMT-3 nanocomposite particles showed the best antiwear properties among all the types of nanocomposite particles in this study.