We report the friction behavior of graphene edges within a carbon film, which encompasses structures ranging from amorphous carbon (a-C) to graphene nanocrystalline carbon (GNC). Structural characterization revealed that vertically growing graphene nanocrystallites were implanted into the a-C structure, exposing high-density layer edges on the film surface. Atomic force microscopy (AFM) nanofriction tests highlighted the nature of graphene edge friction. Firstly, the edge friction of GNC films was tested in a critical-contact state, and the results showed that graphene edges exhibited lower friction forces than did a-C edges. Secondly, the surface friction of GNC films was investigated in a full-contact state, revealing that the edge friction of graphene nanocrystallites regulated the surface friction of GNC films. As the edge density of graphene nanocrystallites increased, the nanofriction force of GNC films decreased. Finally, the mechanism of the regulated friction behavior was attributed to the number of edges of the graphene nanocrystallites, which provided plentiful sp² C dangling bonds with weak bonding interactions and edge quantum wells with low surface potentials for lowering friction. These findings shed light on the importance of graphene-related materials and their high-density edges in the structural design and nanofriction application of carbon films.

In this work, we proposed a method for coating the whole surfaces of bearing balls uniformly by carbon film with a rotatable ball clamp. We studied the carbon/carbon friction with a self-designed current- carrying ball bearing friction test system. A notable and instant friction force drop of 28% and significant carbon film wear alleviation were found when currents were applied. By using TEM-, SEM-, and EDS-analysis, special carbon stacks with a mixture of large wear particles and oxide were found in the wear areas under current applied condition. We elucidated the current-carrying friction mechanisms as follows: (1) wear particles formation; (2) wear particles charged by tribomicroplasma; (3) formation of surface passivated carbon stacks under electric force; (4) sliding between passivated carbon surfaces. This work may facilitate the development of novel solid-lubricated ball bearings and lay some foundations for current-carrying rolling friction.