Differences in the tribological mechanisms when using non-doped, metal-doped (Ti, WC), and non-metal-doped (Si) diamond-like carbon against steel under boundary lubrication, with and without oil additives

In this paper, we report on the tribological performance and mechanisms of the boundary-lubricated contacts of steel against diamond-like carbon (DLC) coatings, i.e., steel/DLC, using the same materials, oils, additives and conditions as we have previously reported for DLC/DLC contacts. We present and compare the behaviour of two non-doped, two metal-doped (Ti, WC) and one non-metal-doped (Si) DLC coatings in contact with steel surfaces in reciprocating sliding, lubricated with a paraffinic mineral base oil with and without anti-wear and extreme-pressure additives. Friction behaviour similar to that of the steel/steel surfaces is observed in all the steel/coating contacts, irrespective of the coating type. However, the doped and non-doped coatings showed different wear behaviours. The non-doped DLC coatings provided the lowest wear with the base oils, due to the absence of adhesion between the steel and the DLC coating. In contrast, additivation of the oil was necessary for the adhesion prevention and successful operation of the steel/doped-DLC contacts.