On the running-in behaviour of diamond-like carbon coatings under the ball-on-disk contact geometry

Abstract

The running-in behaviour of a diamond-like carbon (DLC) coating deposited on M42 tool steel substrate has been investigated on a ball-on-disk wear rig in dry air. During sliding, a classical running-in wear curve with a transition in wear rate from a high initial rate to a low rate with sliding was observed. Variations in friction coefficient with sliding distance were not related with this transition in wear rate. A transfer film was rapidly developed on the uncoated ball surface and wear of the ball was negligible. According to Talysurf measurements and optical observations, the width of the wear track on the disk specimen kept almost invariant with increase in sliding distance after the very initial sliding stages. Major change to the wear track cross-section profiles with sliding was the deepening of the wear track. A simulation model for the variation in wear volume and wear track profiles as a function of sliding distance is presented based on the assumption that there are two different dominant wear regimes with respectively a high wear rate and a low wear rate at contact pressures above and below some critical contact pressure, p c. The model has been assessed using the Hertzian elastic contact analysis and the elastic foundation contact model respectively to analyse contact pressures between the rubbing surfaces. It is shown that the running-in feature of wear results mainly from the presence of this transition in wear rate at different contact pressures. Linear increase in wear volume is expected if the Archard's specific wear rate is assumed to be the same at any contact pressure. Reasonably good agreement between the simulated results and experimental observations has been obtained. The elastic foundation model can be applied to simulating wear processes under any contact conditions.