## 3 Results and Discussion
### Mechanisms of Friction Reduction and Anti-Wear  
Figure 7b is the SEM image of the wear surface with pure lubricant (no nanoparticles), the roughness of the surface is high, the image shows deep wear tracks along the sliding direction. The measured COF was also high. In Figure 7c-f, the wear tracks on the collar surfaces obviously decreased corresponding to an increase in the concentration of nanoparticles in the aqueous glycerol solution, as compared to the wear tracks in Figure 7b. The scratches on the collar surfaces in Figure 7d,e, with the presence of 0.5 wt % and 0.75 wt % of nanoparticles, exhibit relative smoothness and the depth of tracks are less than other friction surfaces shown in Figure 7c,f. Therefore, the surface roughness of the collars for both test 14 and 18 are low, revealing the remarkable reduction in the COF and wear rate. The EDS analysis (Figure 8b) of the wear surface (Figure 8a) of optimal concentration parameters shows the tribo-film contains aluminum formed on the collar surface. The film formed by the nanoparticles deposited on the friction surface acts like a protection film that reduces the direct \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Response**} & \multicolumn{2}{c}{**Optimum Conditions**} & \multirow{2}{*}{**Predicted**} & \multirow{2}{*}{**Experimental**} & \multirow{2}{*}{**\%**} \\ \cline{2-2} \cline{5-6}  & **Nanoparticles (wt \%)** & & & & **Water (wt \%)** & **Value** & **Value** & **Error** \\ \hline COF & 0.6667 & 2 & 10 & 0.0494 & 0.051 & 3.13 \\ Wear rate (cm\({}^{3}\)/N-m) & 0.6667 & 2 & 10 & \(7.8155\times 10^{-10}\) & \(8.0128\times 10^{-10}\) & 2.46 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: The results of the optimization for responses. Figure 7: SEM micrographs of collar surface. (**a**) Collar surface before test; (**b**) collar surface after test with only 10 wt % water content in glycerol of trial 1; (**c–f**) Collar surfaces after test with different concentrations of aluminum nanoparticles at 0.25, 0.5, 0.75, and 1.0 wt % of experiment number 10, 14, 18, and 22, respectively. contact of two mating surfaces. Another wear reduction mechanism is in the form of the aluminum nanoparticles acting like bearing balls to prevent the collision of the thrust and collar surfaces. Table 11 lists the \(Ra\) of the collars before and after each experiment. The highest roughness is 0.187 \(\upmu\)m for test 1, for which the lubricant has no nanoparticles. The experiments that had lower surface roughness exhibited tiny wear traces, thus suggesting lower COF values. The combined effects of the tribo-film and the ball-bearing of nanoparticles prevent the development of wear tracks. The same results have been reported by [PERSON] et al. (2016) and [PERSON] et al. (2017). The COF curves of friction tests of 120 min are displayed in Figure 9; the COF was the highest for the lubricant with 10 wt % water content of glycerol among the three experiments. The influence of nanoparticles in lubricant became apparent upon observing the results of 0.6667 wt % aluminum nanoparticles and 10 wt % water content of glycerol, whereby the COF dropped to 0.0549. With optimal concentration levels of 0.6667 wt % nanoparticles, 2 wt % of SDS, and 10 wt % water content of glycerol, the COF further reduced to 0.051. Figure 10 shows the comparison of wear rate between predicted and measured values for different lubricating conditions. It can be seen that the wear rate decreased with the presence of nanoparticles in the lubricant. At the optimal concentration, the measurement value of wear rate achieved 8.0128 \(\times\) 10\({}^{-10}\) (cm\({}^{3}\)/N\(\cdot\)m), 42.4% less than that of glycerol with 10 wt % of water. The kinematic viscosity of lubricants was measured before and after each experiment following ASTM D445 at 40 \({}^{\circ}\)C, as shown in Figure 11. The values indicated that there was not a notable change in kinematic viscosity before and after each experiment.