The contact size dependence of wear rate in fretting: Understanding and rationalising data from tests with flat-on-flat, cylinder-on-flat and sphere-on-flat contact configurations

In fretting contacts between metals such as steels, wear normally takes place by the formation of an oxide debris in a closed contact and its subsequent expulsion. There are three stages to this process: (i) the transport of oxygen into the closed contact to form the oxide debris; (ii) the formation of the oxide within the fretting contact; (iii) the expulsion of this debris from the closed contact. In the steady state, the observed wear rate will be the rate of the slowest of these three processes which is known as the Rate Determining Process (RDP). Previously, it has been proposed that this dependence of the observed wear rate on the transport of species into and out of the contact results in the observed wear rate being inversely proportional to a critical dimension of the contact, with the instantaneous wear rate changing throughout tests conducted with non-conforming contact configurations (due to the growth in contact size as the test proceeds). In this paper, for the first time, equations for a contact-size dependent specific wear rate, k s d (with typical units of mm 4 MJ -1 ) are presented for three contact configurations commonly utilised in fretting tests, namely flat-on-flat, cylinder-on-flat and sphere-on-flat contacts. These are discussed in terms of physical understanding of the fretting wear process and the ability to compare data from tests conducted with different contact configurations more robustly than has been previously possible. In addition, for each of these commonly utilised contact configurations, the equations describing the development of wear scar size, contact pressure and wear depth throughout fretting wear tests are also derived, and the significance of differences in the evolution of these throughout a test for the different contact configurations is discussed.