## 3 Results
### Characterization of PS/OMMT Nanocomposite
#### 3.2.7 Friction Performance Analysis  
Figure 11 shows the coefficient of friction (COF) versus time of PAO and PAO with PS/OMMT nanocomposite samples. As it can be seen from the figure, the COF of PAO was around 0.095 at the beginning and then increased dramatically to about 0.125 within 1000 s, followed by a stable period of approximately 2700 s. The COF then increased from 0.120 to about 0.145 in the rest of the test. For PAO-PS/P-MMT, the COF rose up sharply from 0.082 to 0.110, dropped drastically to 0.090 within 250 s, and then experienced a stable period with fluctuation around 1500 s. The COF then increased slowly to 0.115 at the end of the test. The COF of PAO-PS/P-SLS-MMT started at 0.070, increased slowly to about 0.100 around 2200 s, and then remained steady during the rest of the test. The COF of PAO-PS/P-KH570-MMT was 0.070 at the beginning, increased to 0.085 within 250 s, and then rose  
Figure 10: TGA and DTG curves of PS, PS/P-MMT, PS/P-SLS-MMT, and PS/P-KH570-MMT.  
up slowly. At the end of the test, the COF reached 0.090. It can be found from the test results that the adding PS/OMMT nanocomposites to the PAO decreased the COF values and clearly enhanced the friction reduction properties. Furthermore, with the same content of nanocomposite particles, PS/P-KH570-MMT performed the lowest COF value and the best friction reduction properties.  
To further characterize the tribological properties for the PS/OMMT nanocomposites, we examined the wear surfaces of the PAO and PAO-PS/OMMT nanocomposites using SEM. Figure 12A-D and Figure 13A-D show the SEM morphology of the wear surfaces of the upper and lower ball of the four-ball machine for PAO, PAO-PS/P-MMT, PAO-PS/P-SLS-MMT, and PAO-PS/P-KH570-MMT, respectively. From Figure 12A, a wide and deep pluging groove was found on the wear surface, indicating serious abrasive wear. This worn morphology was also observed in Figure 13A. However, when the PS/OMMT nanocomposites were added into the PAO, as shown in Figure 12B-D and Figure 13B-D, the grooves on the wear surface became shallower and narrower, which indicated that the surface roughness was improved and the COF was reduced. These results indicated that the PS/OMMT nanocomposites exhibited excellent anti-wear and friction reduction properties in their tribological properties in a lubricant, which were consistent with the results of Figure 11. During the lubrication process, the nanocomposite particles were able to fill in the microgrooves on the wear surfaces and reduce the roughness of the metal surfaces to reduce friction and wear [43]. Furthermore, the nanocomposite particles could support the metal surfaces and prevent them from contacting each other and roll like a bearing ball, thereby reducing the COF and wear [44]. Among the wear surfaces, there was a smoother wear surface with microgrooves and nanogrooves in Figures 12D and 13D. This result suggested that the best anti-wear and friction reduction properties among all the type of nanocomposites were observed with the PS/P-KH570-MMT nanocomposite in this study. This may be due to the smaller PS/P-KH570-MMT nanocomposite particle filling the grooves more effectively than the larger particles. The nanocomposite particles with smaller diameters could interact with the metal surface more effectively and form the transfer film more easily to protect the metal surface and reduce the wear. Meanwhile, with higher thermal stability, the PS/P-KH570-MMT nanocomposite particles might roll better between two wear surfaces and reduce the friction. The above results indicated that the improvements of tribological properties of PS/OMMT nanocomposite particles showed promising applications in the drilling fluid lubrication.  
Figure 11: The COF versus time of PAO, PAO-PS/P-MMT, PAO-PS/P-SLS-MMT, and PAO-PS/P-KH570-MMT.