# Synergistic Effects of CuO and ZnO Nanoadditives on Friction and Wear in Automotive Base Oil +
## 4 Discussion and Conclusions  
This study demonstrates that combining CuO and ZnO nanoparticles in lubricating oil formulations significantly enhances the tribological performance of Group III base oils. The results confirm a clear synergy between the two metal oxides: CuO predominantly contributes to wear reduction through stable tribofilm formation, while ZnO reduces friction and improves dispersion stability through nano-rolling and tribosintering effects. Among the tested formulations, the 2:1 CuO-ZnO ratio offered the most balanced tribological performance, reducing average FAI by up to 27% and static friction by 17% relative to the reference oil, while achieving a 54% wear reduction on the disk surface (DWV). Elemental analysis of the tribofilm composition by EDX supports these functional roles: the Cu content within the wear scar region correlated positively with reduced friction and improved wear protection, while Zn presence was associated with smoother friction profiles but often accompanied by increased localized wear. This suggests that partial substitution of CuO by ZnO is feasible, but with trade-offs. Specifically, ZnO incorporation reduces the protective Cu-rich tribofilm density, increasing wear on sacrificial components. Therefore, this strategy may be ideal for systems where one component must be preserved at all costs, and the other can tolerate increased wear or be more easily replaced. Compared to the literature, these findings reinforce and refine several established trends:  
Figure 14: Normalized atomic percentages of Cu and Zn at three wear scar regions, showing synergistic incorporation in CuO-ZnO mixtures and excessive Zn uptake in the absence of Cu. * The tribofilm-forming role of CuO is again confirmed, aligning with other research works [9], where CuO lowered wear by over 40% [8]. * The observed friction reduction is similar in scale to previous ZnO-based findings, where up to 22-23% reductions were achieved [11; 19], though here they were maximized only when ZnO was combined with CuO. * The synergistic behavior noted in [PERSON] and [PERSON]'s study [14] and hypothesized in major reviews [10; 20] is validated and quantified with a practical formulation. Importantly, this study addresses gaps in the literature regarding the need for systematic studies on CuO-ZnO mixtures, including ratio optimization and real-surface analysis. Unlike many earlier works focused on friction alone, this investigation incorporates dynamic friction metrics and direct wear volume measurement on dual counterfaces, providing a more complete tribological picture. The results also show that ZnO reduces friction and may enhance CuO deposition; however, the exact mechanism behind this potential cross-effect remains hypothetical. Based on the observed microstructural features and supporting literature [15; 16], it can be speculated that ZnO, through its nano-rolling and heat dissipation effects, modifies local contact conditions in a way that facilitates CuO triboreduction and the incorporation of Cu into the tribofilm. Further studies will be needed to confirm this mechanism. To capitalize on these findings, future research should  
* investigate the surface modification of CuO and ZnO to prevent agglomeration and enhance dispersion stability;
* study the tribochemical pathways that govern synergistic tribofilm formation, especially in Zn-rich environments;
* evaluate the long-term performance and oxidative stability of these mixtures under thermomechanical stress;
* explore integration with existing additive packages, as compatibility and cumulative effects are not yet well understood [17; 19]; and
* extend testing to realistic engine and aerospace scenarios, where fluctuating loads and temperatures require advanced lubricating behavior. In conclusion, the optimized 2:1 CuO-ZnO formulation demonstrates a compelling balance between friction reduction and wear protection. Its tunable synergy allows strategic prioritization of component preservation versus overall efficiency.