Hybrid Tribofilm Formation at Lubricated Sliding Interfaces Induced by Cu and SiO₂ Nanoparticles Dispersed in Oil

This study evaluates the tribological performance of copper (Cu) and silica (SiO2) nanoparticles used individually and in combination as hybrid additives in PAO-4 oil. The 0.5 wt % Cu + 0.5 wt % SiO2 formulation demonstrated superior dispersion stability, favorable rheological properties, and excellent tribological behavior. At room temperature, the hybrid nanolubricant reduced friction and wear by ∼52% and ∼45%, respectively, while, at 100 °C, reductions improved to ∼68% and ∼88%. TOF-SIMS, cross-sectional TEM-EDS, and APT analyses of worn surfaces revealed the formation of hybrid tribofilms, with thicknesses of 300 ± 31nm at room temperature and 168 ± 61 nm at 100 °C. The enhanced performance is attributed to the synergistic interaction between the additives and temperature-dependent variations in tribofilm composition and formation.