Origin of superlubricity of diamond-like carbon (DLC)

Hydrogenated diamond-like carbon (H-DLC) is produced as a coating or thin film through plasma-enhanced chemical vapor deposition. H-DLC is relatively hard and well known to exhibit superlubricity. Is superlubricity an intrinsic property of the H-DLC? This paper argues that H-DLC is not intrinsically superlubricious, but it has an ideal structure that allows transition of the interface region to a superlubricious structure upon frictional shear in proper conditions. Thus, its superlubricity is an extrinsic property. This argument is made by comparing frictional behaviors of three allotropes of carbon materials – graphite, amorphous carbon, and diamond – and carefully scrutinizing the run-in behavior as well as environment sensitivity of H-DLC friction. The superlubricious structure is generally known to be graphitic, but its exact structure remains elusive and is subject to further study. Nevertheless, accurate knowledge of how superlubricity is induced for H-DLC can guide engineering design to achieve superlubricious behaviors with other carbon materials produced via different synthetic routes.