The Effect of Ni Nanoparticles on the Lubrication of a DLC-Based Solid–Liquid Synergetic System in All Lubrication Regimes

Ni nanoparticles (with average diameter 7 nm) capped with oleylamine and oleic acid were added to a DLC/DIOS solid–liquid synergetic lubricating system. The tribological behavior rubbed with steel balls in three lubricating regimes (boundary, mixed and elasto-hydrodynamic lubrication) has been systematically studied by surface analyses techniques such as SEM, EDX and XPS. It was found that, in all the lubricating regimes, the system’s lubricating performance was substantially improved because of the addition of the Ni nanoparticles. For all the lubrication regimes, the COF decreased by 10.3–19.1%. In the boundary lubrication regime, the wear rate of DLC could be decreased by 50%. Ni nanocores facilitate the adequate oxidation of the steel surface and the transformation of C=C (sp2) to C–C (sp3) of the DLC sliding surface. The tribo-film containing ferrous oxides, Ni and its oxides, presents low shearing strength and good cohesion to the DLC coating, which greatly improves the lubricating behaviors of the DLC/DIOS solid–liquid synergetic system. In the elasto-hydrodynamic lubrication regime, the Ni nanoparticles cause the occurrence of obstructed or plug flow in the oil films, which in turn reduces friction by reducing the layers of lubricant molecules to slide on each other.