Friction and wear properties in dry, water- and oil-lubricated DLC against alumina and DLC against steel contacts

Diamond-like carbon (DLC) films can be divided into two major categories according to their hydrogen content. These categories have similarities in tribological performance, but the films also behave in a different manner in different tribological conditions. The results of amorphous hydrogenated carbon films (a-C:H) and hydrogen-free hard carbon films (a-C) are reported in this study. The a-C:H films were deposited using the radio frequency (rf) plasma technique, and the hydrogen-free hard carbon films using pulsed vacuum arc. The coatings were characterized and investigated with respect to their tribological performance in dry (50% RH), water-lubricated and oil-lubricated slow sliding conditions (0.004 m s−1). The a-C and a-C:H films had a low friction coefficient in dry sliding conditions (0.15 to 0.22), which was further decreased by 10–40% under boundary lubrication. The a-C:H(Ti) films exhibited good self-lubricating properties (0.10) in dry sliding conditions and the a-C films had the lowest friction coefficient in water- (0.03) and oil-lubricated (0.08) conditions. The hydrogen-free hard carbon films showed excellent wear resistance in dry, water- and oil-lubricated conditions, but hydrogenated a-C:H films suffered from severe wear in aqueous conditions. The performance of a-C:H films could be improved by titanium alloying. In dry sliding conditions, the tribolayer formation of DLC films influenced the friction and wear performance, but in oil-lubricated conditions boundary lubrication layers were formed, which governed the tribological mechanisms in the contact.