A tribochemical evaluation of a WC–DLC coating in EP lubrication conditions

Improvement of fuel economy and component durability as well as reduction of dependence on some harmful components of lubricants are the key concerns for automotive industries. In recent years, efforts have been made to use surface engineered surfaces in automotive components to produce environmentally friendly vehicles as well as to increase fuel economy. Since commercially available lubricants are mainly optimised for effective lubrication of ferrous materials, effective lubrication of novel materials, especially in the boundary lubrication/EP regime, is not well understood. Published research has also indicated that lubricant chemistry could have a detrimental impact on the intrinsic coating properties, changing their mechanical properties. Tribological systems working in boundary lubrication/EP conditions rely on lubricant–additive interactions to initiate surface reactions. The performance (friction-related efficiency and wear-dependent reliability) of tribological systems depends on the time taken to initiate surface reactions, the nature of the surface reactions and the stability of the films formed as a result of the surface reactions. This paper is a fundamental study of the lubrication and wear mechanisms of a WC-doped diamond-like-carbon (DLC) coating when lubricated in the extreme pressure (EP) regime.In the present work results from a tribochemistry study to understand the tribofilm evolution mechanisms for a DLC interface lubricated with a model oil containing a typical antiwear and corrosion inhibitor additive are presented. The performance was correlated with the wear performance. The wear measurements were carried out at small time-intervals along with the examination of the chemistry of the tribofilm using surface sensitive techniques; XPS (X-ray photoelectron spectroscopy) and Raman spectroscopy. It was observed that surface reactions between the additive-coating surface varied considerably with time and these reactions significantly influence the tribological performance.

► The tribofilm evolution mechanisms for a DLC interface was studied.
► Friction/wear and chemistry of the tribofilm present between the contacting surfaces varies with time.
► The chemistry of tribofilm significantly influences tribological wear performance.
► The wear-reducing compounds like zinc sulphide, tungsten carbide and potentially zinc tungstate as well as friction reducing-compounds like tungsten sulphide are able to form as a result of coating/lubricant interactions.
► Ordered amorphous nano-crystalline carbon, indication coating graphitisation, was more prominent after the initial running period had finished.