The effect of relative humidity on wear of a diamond-like carbon coating

Sliding wear behaviour of a diamond-like carbon (DLC) coating deposited using the combined closed field unbalanced magnetron sputter ion plating and plasma assisted chemical vapour deposition technique has been investigated against tungsten carbide balls in air with various relative humidity (RH) levels as well as in water. A smooth transfer layer was always formed on the ball surface and wear of the ball was negligible. The wear rate of the coating decreased significantly with increase in RH, with sliding in water showing the lowest wear rate. When sliding in dry air or in a vacuum, the coating failed very quickly, producing a wear rate of approximately two orders that for sliding in humid air. Under the investigated conditions, sliding speed (0.25 and 0.05 m s^-1) had no significant effect on the wear behaviour of the DLC coating in the various environments. The role of humid air in reducing wear rate of the DLC coating has been attributed to the formation of low energy surfaces by the adsorption of water at the dangling carbon bonds on the edge-faces of the sp² clusters in the DLC coating.