Debris expulsion as a rate determining process in fretting - The effect of slip amplitude on debris expulsion behaviour and rates

In contrast to the well-established understanding of sliding wear, recent work has clearly demonstrated that the instantaneous wear rate in fretting of metals is dependent upon the contact size due to the fact that transport of species in and out of the contact are normally the rate-determining processes (RDPs), the processes being oxygen transport and debris transport respectively. In this work, non-conforming (cylinder-on-flat) contacts were fretted with a range of slip amplitudes between similar to 5 mu m and 80 mu m, and the contact size-dependent wear coefficient was determined in each case. It was observed that although a slip amplitude threshold (below which wear does not proceed) may exist, this threshold is below the lowest slip amplitude examined in this work. However, as the slip amplitude was decreased, the number of cycles for wear to be initiated increased; for example, at the lowest slip amplitude examined in this work, measurable wear was not observed after 5 million fretting cycles whilst it was observed after 10 million cycles. The development of wear scar profile as a function of slip amplitude was also examined. It was observed that at a fixed number of cycles, smaller slip amplitudes resulted in W-shaped wear scars whereas larger slip amplitudes resulted in U-shaped scars. It was also observed that at a fixed slip amplitude, the scar shape developed from being U-shaped in the initial stages of fretting to W-shaped as the test proceeded. These changes in scar geometry are interpreted bearing in mind the fact that the scar size is also a function of changes in these parameters due to the non-conforming nature of the contact. Scanning electron microscopy was employed to aid understanding with the observations being interpreted in the framework of debris-ejection as the rate determining process. Reasons for the strong dependence of wear rate on slip amplitude are proposed.