Friction of diamond-like carbon: Run-in behavior and environment effects on superlubricity

This chapter reviews the structure and surface chemistry of diamond-like carbon (DLC) films and explains the origin of the run-in process of DLC films before superlubricity is achieved in inert environmental conditions and the environmental effect on the friction and wear properties of DLC films. The high friction and sp3-to-sp2 phase transformation during the run-in process and the loss of superlubricity in the presence of water or oxygen are elaborated based on the facts that DLC is an amorphous material with a broad distribution of C–C bond parameters and thus its surface can be easily oxidized upon exposure to ambient air. In addition, it also discusses that the achievement of superlubricity is dependent more on the degree of phase transformation from amorphous structure to graphitic structures at the shear plane, rather than the thickness of the transfer film.