Understanding and modelling wear rates and mechanisms in fretting via the concept of rate-determining processes - Contact oxygenation, debris formation and debris ejection

Abstract

A new framework which describes the role of three key processes in fretting wear of metals is proposed, with these three processes being: (i) oxygen transport into the contact; (ii) formation of oxide-based wear debris in the contact and (iii) ejection of the wear debris from the contact. Based upon a physical understanding, rate equations for the three key processes are proposed, which provide a basis for the influence of test parameters (such as contact geometry, applied load, slip amplitude, fretting frequency etc) on the three rates to be understood. To maintain system equilibrium in steady-state fretting, the three processes must operate at the same rate as each other (debris cannot be ejected from the contact faster than it is formed, and debris cannot be formed faster than it is ejected). Accordingly, the observed wear rate is the rate of the process with the lowest rate of the three processes, with this process being termed the rate-determining process. The effect of test parameters on the three key processes differs, and thus the effect of changes in any test parameter on the observed rate of wear will itself be dependent upon which of the three processes is rate-determining.A number of assumptions have been made in deriving the equations which describe the key processes and it is recognised that these equations themselves may be refined in light of future research; however, any such revised equations can simply replace those proposed as part of the rate-determining process framework.The framework can be applied to both conforming and non-conforming contact geometries. In tests with non-conforming contact geometries, the contact size increases with wear; since it is proposed that the rates of two of the three processes (oxygen transport and debris ejection) are dependent upon the size of the contact, the rate determining process can change during such a test and the rate of wear will continually fall when either of these processes are rate-determining. This complexity means that the evolution of the wear volume can only be evaluated numerically through the use of a time-marching analysis in such cases. In contrast, in tests with conforming contact geometries, the contact size does not change as wear occurs, and so there will be no changes in the rate-determining process as wear proceeds.It is recognised that this framework addresses wear under steady-state conditions and does not consider the initial period of exposure where steady-state conditions are being developed. In the case of conforming contacts where the contact size is large, the duration of the initial transient period may be substantial and may form a significant proportion of the test duration or component lifetime. This needs to be recognised both in the design of tests and in the application of this framework.