A discussion of oxidation, oxide thickness and oxide transfer in wear

Wear is defined as a set of processes of which adhesion, transfer, abrasion, fatigue and oxidation may be some of the constituent processes; these effect transformations of matter from one state to another. The final transformation in wear is attrition in which free debris particles are formed. Oxidation in the wear of metals is discussed, in particular for circumstances in which an oxide layer thick enough for the oxidation rate to be inversely dependent on oxide thickness is formed i.e. the standard relationship (outside the wear situation) between the oxide thickness and elapsed time is parabolic. It is shown that the basic factors determining the wear process are the mechanical characteristics of the oxide layer and that the oxidation process, while important, is a secondary influence. Theoretical considerations show that the oxidation temperature may be lower than the contact temperature, even for oxidation nominally at the contact regions. Also, an oxidation rate in wear may decrease with increasing oxidation temperature, as is sometimes observed in practice. This may be due to an increase in the thickness to which the oxide layer can grow in wear, when the oxidation temperature increases.