Effects of Adhesion and Transfer Film Formation on the Tribology of Self-Mated DLC Contacts†

Diamond and diamondlike carbon (DLC) films exhibit a wide range of sometimes contradictory tribological behavior. Experimentally, isolating the influences of factors such as film structure, testing conditions, and environmental effects has proven difficult. In this work, molecular dynamics simulations were used to examine the effects of film structure, passivation, adhesion, tribochemistry, and load on the tribology of self-mated DLC contacts. Addition of hydrogen to a DLC film causes a large decrease in the unsaturated carbon bonds at the interface of the film when compared to both the bulk and non-hydrogenated films. These unsaturated carbon atoms serve as initiation points for the formation of covalent bonds between the counterface and the film. These adhesive interactions cause an increase in friction during sliding. The formation and breaking of covalent bonds during sliding results in the formation of a transfer film. When covalent bonds break, friction decreases and there is a concomitant increase in the local temperature emanating from the interface. These simulations reveal that reducing unsaturated atoms, both spand sp2-hybridized carbon, at the sliding interface reduces the number of adhesive interactions, alters the transfer film formed, and reduces friction. In addition, these simulations support and elucidate the passivation hypothesis for DLC friction.