# Synergistic Effects of CuO and ZnO Nanoadditives on Friction and Wear in Automotive Base Oil +
## 3 Results
### Scanning Electron Microscopy Results  
SEM analysis of the worn disk surface lubricated with the Group III base oil revealed typical abrasive wear features across all regions (see Figure 8). Fine and parallel grooves dominated the surface at the outer edge (WTLP--high sliding speed, low contact pressure), indicating mild abrasion with no evidence of tribofilm formation. The dead center region (low speed, high pressure) showed localized material transfer and slight adhesion-related features, likely due to prolonged static contact. The central zone, characterized by high contact pressure and high speed, exhibited the most severe wear, including deep grooves, micro-cracks, and signs of plastic deformation, indicating a combination of abrasive and adhesive wear mechanisms under boundary lubrication conditions. Figure 7: Disk (DWV), ball (BWV), and total wear volumes (TWV) for Group III base oil with different CuO and ZnO nanoadditive compositions. Error bars represent standard deviation. The reference sample's EDX spectrum (Figure 9) reveals the presence of Fe, Cr, Si, C, and O elements, which correspond to the base elements and alloying components of the 100 Cr6 bearing steel. The detected carbon and oxygen signals may also partially originate from the base oil residue or tribo-induced surface oxidation. No foreign elements related to nanoadditives were observed, confirming the sample's additive-free nature. SEM and EDX analysis were performed at the wear scars' dead center (DC) to evaluate CuO and ZnO nanoparticles' distribution and tribological role in various lubricant  
Figure 8: SEM micrographs of the disk wear scar (Group III base oil) at three characteristic locations: top—outer edge (high speed, low pressure), bottom left—dead center (low speed, high pressure), and bottom right—central zone (high speed, high pressure). Figure 9: EDX spectrum of the wear scar obtained from the reference lubricant (Group III base oil without additives) tested on the 100 Cr6 steel disk. formulations (see Figure 10). The images revealed a clear transition in wear severity and surface topography as the ratio of CuO to ZnO decreased. In the case of the pure CuO formulation (0.3 wt%), the wear surface exhibited shallow, parallel grooves typical of mild abrasive wear. EDX mapping showed a dense and homogeneous distribution of Cu, suggesting the formation or presence of a tribofilm composed of elemental copper due to triboreduction [24], likely contributing to the observed wear protection. Wear marks with the 2:1 CuO-ZnO mixture became slightly rougher, with more localized material displacement. Cu remained the dominant element, although Zn presence increased, appearing more scattered. This suggests partial incorporation of ZnO without entirely disrupting the tribological benefits provided by CuO, indicating limited synergy. For the 1:2 CuO-ZnO formulation, surface degradation was more pronounced, with deeper grooves and signs of adhesive interactions. Zn became the dominant species, while Cu signals weakened significantly. The Zn distribution was more uniform, but the absence of an effective tribofilm structure corresponded with worsened wear performance. Finally, the pure ZnO sample (0.3 wt%) exhibited the most severe wear, characterized  
Figure 10: SEM images and corresponding EDX elemental maps of Cu (yellow) and Zn (orange) at the DC region of worn disk surfaces lubricated with different CuO-ZnO nanoadditive formulations. From top to bottom: 0.3 wt% CuO, 2:1 CuO-ZnO, 1:2 CuO-ZnO, and 0.3 wt% ZnO. by fractured surfaces, irregular debris accumulation, and deep abrasive scars. While Zn was highly concentrated across the wear track, it did not appear in cohesive film-like structures. Instead, it likely acted as an abrasive due to particle agglomeration or poor dispersion stability. The results confirm that CuO nanoparticles form effective protective tribofilms, especially under static high-pressure contact conditions.