Gross slip fretting wear performance of a layered thin W-DLC coating: Damage mechanisms and life modelling

Fretting damage, in the form of wear and fatigue-cracking, is a common cause for concern in a wide range of dynamically loaded mechanical joints, and consequently, control of the rates of such damage is commonly addressed either by design or by the choice of materials with enhanced performance. Such material enhancements may be made to the material bulk, or just to the surface regions through surface engineering. In this work, a thin PVD W-DLC coating with a CrN interlayer was deposited onto both a high-strength steel and a titanium alloy. A cylinder-on-flat fretting configuration was employed under gross slip fretting conditions. It was observed that whilst a low coefficient of friction (COF) was measured throughout the wear of the DLC layer, there were variations in the rates of wear which could be correlated with compositional variations within the film. As wear progressed through the carbon-rich and tungsten-rich upper sub-layers of the W-DLC layer, the wear coefficient decreased. With further wear into the CrN underlayer, the wear coefficient and the COF increases. An incremental finite element (FE) wear model based on the Archard wear equation has been used to simulate the coating wear process, with particular emphasis on evolution of the contact geometry for the W-DLC layer. The predicted coating wear life is then shown to correlate, across a range of load–displacement combinations, with a single predictive coating life equation.