Roles of phase transition and surface property evolution in nanotribological behaviors of H-DLC: Effects of thermal and UV irradiation treatments

Abstract

Hydrogenated diamond-like carbon (H-DLC) films that behave super-low friction in vacuum are effective lubricants for aerospace moving parts; however, their tribological properties strongly depend on space environments. In this study, the transition of substrate phase and the evolution of outermost surface properties caused by simulated space conditions, involving thermal treatments and ultraviolet (UV) irradiation, were investigated; their effects on the tribological behaviors of H-DLC films were qualified at nanoscale by controlling the interface deformation and substrate wear at single-asperity contact using atomic force microscopy (AFM). Although thermal treatments in vacuum cannot change the outermost surface properties, they can induce the phase transition of H-DLC substrates above a critical temperature, which results in the degradation of tribological performance. UV irradiation in moist air varies the material phase of surfacial layer accompanied by surface oxidation, which not only increases the nanowear and wear-related friction but also enhances the wearless-related interfacial adhesion and friction interactions. Results of this study can provide new insight into the mechanism of space environments affecting the tribological reliability of H-DLC and help for developing a new coating for minimizing interfacial interaction and surface damage in space applications.