Probing the low-friction mechanism of diamond-like carbon by varying of sliding velocity and vacuum pressure

The lubrication of diamond-like carbon (DLC) films has commonly been ascribed to two hypotheses, i.e., 'friction-induced graphitization' mechanism and passivation mechanism. To clarify the primary low-friction mechanism of DLC, we investigate the effects of sliding velocity and vacuum pressure on the friction behavior of DLC film. Counterintuitively, examination of wear tracks by Raman spectroscopy reveals that a higher friction coefficient is accompanying with a higher degree of graphitization. We therefore claim that it is the higher friction force that results in the higher degree of graphitization, which subverts the concept of a higher degree of graphitization leading to a lower friction coefficient. Besides, the friction coefficient is found to depend on the ratio of ambient pressure to rotating speed, indicating that the passivation mechanism is at play. Besides, the additional slide-hold-slide test in room air also gives evidence that cannot be understood in terms of the 'friction-induced graphitization' mechanism.