# Synergistic Effects of CuO and ZnO Nanoadditives on Friction and Wear in Automotive Base Oil +
## 3 Results
### Scanning Electron Microscopy Results
This supports the stability of the Cu-based tribofilm across the entire wear track. The surface roughness of the 2:1 CuO-ZnO formulation increased slightly, and some localized disruptions became visible. While copper remained dominant, the zinc signal began appearing primarily as isolated clusters rather than continuous layers. The partial overlap between Cu and Zn may indicate competition for surface adsorption sites, with Cu still providing a degree of surface protection. When testing the 1:2 CuO-ZnO sample, the wear track displayed more pronounced micro-abrasive features, with shallower but less uniform grooves. Copper was sparsely distributed, while zinc was dominant across the surface. However, the Zn signal again lacked coherence, showing no signs of organized film formation, suggesting ZnO particles remained dispersed or aggregated, contributing little to surface protection. The pure ZnO (0.3 wt%) sample revealed a rough and irregular surface, with evident abrasion and micropitting. EDX mapping showed dense but chaotic Zn distribution, likely due to particle accumulation and incomplete dispersion. As with previous regions, no tribofilm-like morphology was observed, and the surface appeared heavily affected by unprotected shear contact. In summary, CuO additives ensured consistent surface coverage and film stability, even in low-pressure, high-speed regions. As the proportion of ZnO increased, the protective effect diminished, and the nanoparticles tended to agglomerate rather than integrate into the boundary layer. ZnO alone proved ineffective in forming a stable, shear-resistant layer under these conditions. Figure 13 compares EDX spectra in the Cu (L\(\alpha\) = 0.93 keV) and Zn (L\(\alpha\) = 1.01 keV) energy regions for five lubricant formulations tested at different points along the wear scar. The spectra are color-coded:  
Figure 12: SEM and EDX elemental maps of Cu (yellow) and Zn (orange) at the outer edge of the wear scar (high sliding speed, low contact pressure) for various nanoadditive formulations: 0.3 wt% CuO, 2:1 CuO-ZnO, 1:2 CuO-ZnO, and 0.3 wt% ZnO. The spectral data correlate well with the spatial element maps shown earlier. As expected, no Cu or Zn peaks are observed in the reference sample, validating the additive-free condition. The pure CuO formulation (red) shows a distinct Cu peak with no Zn presence, confirming selective incorporation of copper species. Interestingly, in the case of nanoparticle mixtures, the Cu signal intensity increases compared to pure CuO, suggesting that the presence of ZnO enhances CuO incorporation into the tribological interface. The 2:1 and 1:2 CuO-ZnO mixtures exhibit comparable peak intensities for Cu and Zn, supporting the similarity in their observed tribological performance (friction and wear volume). However, the increase in Cu content in the presence of Zn appears to be a key factor in improved film formation and surface protection. This points to a synergistic effect where Zn facilitates the deposition or retention of Cu at high-pressure regions, without excessively promoting Zn incorporation itself. In contrast, the pure ZnO formulation (dark green) exhibits a strong Zn peak and the absence of Cu, consistent with high Zn incorporation but lacking any balancing effect from CuO. This supports earlier findings of severe abrasive wear, as excessive Zn-containing accumulations lead to unstable and damaging particle-surface interactions without a Cu-based film. These results reinforce that tribofilm composition and structure are strongly influenced by additive interactions. ZnO alone behaves abrasively, but when co-applied with CuO, it contributes to controlled CuO deposition and limits ZnO overaccumulation. The observed synergy extends beyond friction reduction, manifesting in wear mechanism modulation and nanomaterial integration into the tribofilm. Figure 14 shows the atomic percentages of Cu and Zn detected at three characteristic regions (DC, WTHP, WTLP) for each lubricant formulation. The reference sample showed negligible Cu and Zn, as expected.