Combined Effects of Structural Transformation and Hydrogen Passivation on the Frictional Behaviors of Hydrogenated Amorphous Carbon Films

Tribological behaviors of hydrogenated amorphous carbon (a-C:H) films under single asperity contact are investigated by molecular dynamics (MD) simulations. Hydrogen concentration and normal load are found to play essential roles in the frictional behavior of a-C:H films. With low hydrogen concentration, the a-C:H film shows high adhesion and friction even at very low normal loads (1.75 nN). The sp3-to-sp2 rehybridization is observed in the a-C:H films with all studied hydrogen concentrations, which is greatly enhanced with increasing normal load. At high normal loads, formation of nanocrystalline graphene-like lamellar structures is observed locally, usually beneath the instantaneous contact area, acting as a lubricating agent, demonstrating local graphitization due to combined effects of the compression and shear process. With high hydrogen concentration, the friction shows a linear increase along with the normal load and higher load bearing capability. Hydrogen atoms accumulate on the film surface during the sliding process, reducing friction significantly. Hydrogen passivation becomes more obvious with higher hydrogen content especially at low normal loads, where a hydrogen-rich monolayer is found to attach onto the asperity, functioning as a lubricating transfer layer. This work shows the combined effects of structural transformation and surface passivation on the frictional behaviors of a-C:H films and may shed light on the well-known but not well-understood superlubricity mechanism of a-C:H films.