▎ 摘　　要

Graphitization and tribo-oxidation are two common phenomena found at the friction interfaces of carbon-based films in atmospheric environment. However, the specific contributions of them to the low friction behavior are not well clarified. Here, we show two kinds of low friction behaviors of graphene nanocrystalline carbon (GNC) films doped with different elements (Ti, Al, Si). The promotion effects of graphene nanocrystallites on the graphitization and different doping elements on the tribo-oxidation were detailed analyzed for uncovering the low friction mechanisms. The results suggest that the relationship between graphitization and tribo-oxidation could be competitive or coexisting. For competitive type, the friction coefficient of Ti-doped GNC film first decreased to a low-friction stage due to graphitization, and then gradually increased into a steady-friction stage by tribo-oxidation. For coexisting type, the graphitization and tribo-oxidation co-induced the steady-low-friction behaviors of Al-doped and Si-doped GNC films. This work provides new insights into the friction mechanism, and the guidance for the design of doped GNC films with desired friction behaviors.