The role of tribologically transformed structures and debris in fretting of metals

In fretting wear of metals, debris is commonly in the form of oxide. Wear occurs when such debris is expelled from the contact, the observed wear rate depending upon: (i) oxygen transport into the closed contact; (ii) oxide debris formation as a result of the tribological and environmental exposure; (iii) debris transport out of the contact. If oxygen is not supplied into the contact fast enough to satisfy the demand, then tribologically transformed structures (extensive subsurface damage of the first bodies) are observed. In contrast, oxide debris may act to protect the first bodies from further damage during the period that the debris is retained in the contact. The observed wear rate will be the smallest of the rates of the three key processes—with this process itself being termed the rate-determining process. The key role of transport processes means that the rates of fretting wear are contact size dependent.