Effect of water adsorption on microtribological properties of hydrogenated diamond-like carbon films

Microtribological properties of diamond-like carbon (DLC) films in the presence of water molecule have been studied. The amount of water adsorbed on DLC films was measured by the quartz crystal microbalance (QCM), while microtribological properties of the film were evaluated by an environment-controlled microtribometer. The hydrogenated-DLC showed an advancing water contact angle of 97degrees, indicating a relatively hydrophobic surface. No hysteresis in adsorption and desorption isotherms were observed. This suggests an absence of micropores at the DLC surface. The greatest adhesion and friction forces between a W-tip and DLC films deposited onto QCM were observed at a water coverage of 2-3 monolayers, which corresponds to a relative humidity of 70-80%. This abrupt increase in adhesion was explained by the generation of a meniscus from the condensed water between tip and DLC surface. The atomically. at DLC surface showed maximum adhesion and friction forces at lower humidity than those of DLC on OCM, implying the importance of surface texturing, which reduces the effect of water condensation.