# Synergistic Effects of CuO and ZnO Nanoadditives on Friction and Wear in Automotive Base Oil +
## 3 Results
### Scanning Electron Microscopy Results
In contrast, ZnO lacks film-forming ability under these conditions, and its presence, particularly in higher proportions, correlates with increased surface damage. The transition from copper-dominant to zinc-dominant systems reflects a shift from protective to abrasive wear behavior. To assess tribofilm behavior under the most severe operating conditions, SEM and EDX mapping were conducted at the wear scar's central region, where contact pressure and sliding speed are at their maximum (WTHP). Figure 11 shows that the topographical and elemental trends confirm that additive composition plays a decisive role in surface protection at this location. Figure 11: SEM and EDX elemental maps of Cu (yellow) and Zn (orange) at the central region (WTHP) of the wear scars (maximum pressure and speed) for four different lubricant formulations: 0.3 wt% CuO, 2.1 CuO-ZnO, 1.2 CuO-ZnO, and 0.3 wt% ZnO. In the case of 0.3 wt% CuO, the surface exhibited regular abrasive grooves with moderate wear severity. EDX analysis revealed a homogeneous and widespread distribution of copper, particularly along the edges of wear grooves and material flow lines. This suggests the formation of a tribochemically anchored Cu-based film that mitigates wear under high stress. In the 2:1 CuO-ZnO formulation, the WTHP region showed a notably high Cu signal, indicating the presence of a Cu-rich tribofilim. Flattened, smoother areas were visible, suggesting regions covered by Cu-induced tribofilms, which are known to promote lower friction (FAI). While primary abrasion grooves were still observable, particularly in the upper part of the image, signs of self-repairing effects could also be identified, where the tribofilm partially overlaid the abrasion grooves. The distribution of the tribofilm was less uniform across the width of the wear track but showed more pronounced extension along the sliding direction. In contrast, in the 1:2 CuO-ZnO mixture, more intense abrasive features and some crater-like fatigue damage began to appear. Importantly, when interpreting the behavior of CuO-ZnO mixtures, it must be considered that tribofilm quantity does not necessarily equal quality; even though a higher Cu signal may be present, the resulting tribofilm may be thinner, patchier, and less coherent. Additionally, ZnO particles can increase local abrasive stress, damaging the surface despite higher Cu incorporation. Cu may accumulate in localized clusters at higher ZnO ratios rather than forming a uniform protective layer. Overall, the interplay between oxides is complex: chemical film formation and mechanical abrasion occur simultaneously, and their combined effect is not necessarily additive. The 0.3 wt% ZnO formulation displayed the most severe wear morphology, characterized by deep cracks, delamination zones, and massive surface damage. While Zn was highly concentrated across the surface, it was present in a fragmented, non-film-forming pattern. These features suggest abrasive behavior caused by particle agglomeration or unstable surface interactions. In summary, CuO-containing systems could form effective tribofilms under extreme contact conditions. As the ZnO ratio increased, the surface protection deteriorated, and wear severity increased. The findings underline that CuO is essential for maintaining tribological stability, while ZnO alone or in excess disrupts film integrity and increases abrasive wear. SEM and EDX investigations are shown in Figure 12 from the outer edge of the wear scar (WTLP), where sliding velocity is at its maximum and normal load is minimal. This region is typically characterized by lower mechanical stress but higher shear rates, making it sensitive to film stability and additive dispersion. In the case of 0.3 wt% CuO, the surface showed fine, regular abrasion marks without signs of severe damage. Copper was uniformly distributed across the scanned area, indicating good dispersion of the CuO nanoparticles and the likely presence of a thin tribofilm even under low-pressure conditions.