# Third-Body and Dissipation Energy in Green Tribology Film
## 3 Results and Discussion
### Friction and Wear Properties of the HPMC Composites  
The effect of nanoparticles on the friction coefficient of HPMC/MoS\({}_{2}\)/additive composites is shown in Figure 6. The HPMC/MoS\({}_{2}\)/additive composites exhibited a lower friction coefficient compared to the HPMC/MoS\({}_{2}\) coating, revealing that the addition of nanoparticles can effectively improve the tribology property of the composite. The addition of the metal particles, Al and Cu, is effective in reducing the coefficient of friction (COF), and the COF continuously decreases with an increase in the metal particle concentration. The oxide additives, Al\({}_{2}\)O\({}_{3}\) and CuO, can achieve a lower COF relative to  
Figure 4: SEM observation of HPMC/MoS\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) composites: (**a**) HPMC/MoS\({}_{2}\), (**b**) Al\({}_{2}\)O\({}_{3}\) 1 wt.%, (**c**) Al\({}_{2}\)O\({}_{3}\) 2 wt.%, and (**d**) Al\({}_{2}\)O\({}_{3}\) 3 wt.%.  
Figure 5: SEM observation of HPMC/MoS\({}_{2}\)/CuO composites: (**a**) HPMC/MoS\({}_{2}\), (**b**) CuO 1 wt.%, (**c**) CuO 2 wt.%, and (**d**) CuO 3 wt.%.  
the metal additive, and the COF reaches a stable state after adding the oxide additive. The lowest COF in this study was obtained by HPMC/MoS2/CuO 2 and 3 wt.%.  
The wear volume of HPMC/MoS2/additive composites is shown in Figure 7. Nano-additives can effectively reduce the wear volume of the coating, except for the higher alumina additive concentration group. The addition of high concentration of alumina may cause embrittlement of the composite film, resulting in an increase in the wear volume. On comparing all additives, HPMC/MoS2/CuO 2 wt.% was observed to have the best wear-reducing ability, for which 90% reduction of wear rate can be achieved compared to the HPMC/MoS2 coating.  
The worn surfaces of the HPMC/MoS2 and HPMC/MoS2/additive 2 wt.% composites coating after tribology test (50 m) are shown in Figure 8. Plucking and ploughing marks can be observed for the HPMC/MoS2 coating. Slight scuffing was observed on the HPMC/MoS2/Al composite. The worn surface of HPMC/MoS2/Cu was smoother than the others; however, it still had wear marks. Thus, the nanoparticles agglomerated in the wear scar on the HPMC/MoS2/Al2O3 composite, which caused abrasive wear. Further, fine wear debris stacked on the surface of the HPMC/MoS2/CuO composite. CuO particles are accumulated on the surface because of the abrasion. These hard particles appear on the surface of the coating and they provide good load capacity and anti-wear behavior.  
Figure 6: Average coefficient of friction of the HPMC/MoS2/additive composites (sliding distance 100 m).  
Figure 7: Wear volume of the HPMC/MoS2/additive composites (sliding distance 100 m). f represents the failure of the coating.  
In the previous results (Figures 6-8), the HPMC/MoS2/CuO composites showed good wear resistance and COF reduction behavior. Therefore, a long-distance wear test (1000 m) was performed to study the tribology mechanism of the composite coating.  
The wear surface of the substrate and the counter ball is shown in Figure 9. The transfer film on the substrate and the counter ball is the third-body layer formed by abrasion, and it is evident in Figure 9a,b. Thus, owing to the synergistic effect of MoS2 and CuO nanoparticles, a thin transfer film was formed, and it helped minimize the friction effect between the substrate and the grinding ball.