Influence of friction modifier and antiwear additives on the tribological performance of a non-hydrogenated DLC coating

The use of diamond-like carbon (DLC) coatings in engine tribocomponents is increasingly a strategy to improve fuel economy and component durability to simultaneously provide low friction and excellent wear protection. The influence of conventional additives on the tribological performance of various types of DLC coatings is still poorly understood. This paper addresses the friction and wear performance of a non-hydrogenated DLC (a-C) coating as a function of additive chemistry. Tests were performed in a pin-on-plate tribometer under boundary lubrication conditions. Surface analyses were carried out using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). Results showed that the antiwear additive zinc dialkyldithiophosphate (ZDDP) provided excellent wear protection to a-C coating by forming zinc phosphate in the tribofilm. However, when friction modifier Moly Dimer (MD) or Moly Trimer (MT) was added with ZDDP, zinc phosphate was found to be absent, resulting in higher wear than ZDDP alone. Both friction modifiers decomposed and supplied MoS2 to the tribological interfaces. The transfer of the worn coating material, which was a function of the antiwear performance of additives, was shown to have an influence on friction performance.