The influence of anti-wear additive ZDDP on doped and undoped diamond-like carbon coatings

Diamond-like carbon (DLC) coatings are recognised as a promising way to reduce friction and improve wear performance of automotive engine components. DLC coatings provide new possibilities in the improvement of the tribological performance of automotive components beyond what can be achieved with lubricant design alone. Lubricants are currently designed for metallic surfaces, the tribology of which is well defined and documented. DLC does not share this depth of tribological knowledge; thus, its practical implementation is stymied. In this work, three DLC coatings are tested: an amorphous hydrogenated DLC, a silicone-doped amorphous hydrogenated DLC and a tungsten-doped amorphous hydrogenated DLC. The three coatings are tested tribologically on a pin-on-reciprocating plate tribometer against a cast iron pin in a group III base oil, and a fully formulated oil that consists of a group III base oil and contains ZDDP, at 100°C for 6 h and for 20 h in order to determine whether a phosphor-based tribofilm is formed at the contact. The formation of a tribofilm is characterised using atomic force microscopy and X-ray photoelectron spectroscopy techniques. The main findings of this study are the formation of a transfer film at the undoped, amorphous hydrogenated DLC surface, and also the tungsten amorphous hydrogenated DLC having a significant wear removal during the testing. The three coatings were found to have differing levels of wear, with the tungsten-doped DLC showing the highest, the silicon-doped DLC showing some coating removal and the amorphous hydrogenated DLC showing only minimal signs of wear.