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The transition in wear rate from a high value to a low value for metals after some time of sliding is a well known phenomenon. However, few models have been presented to account for such a transition. In this paper, a mathematical model, based on experimental observations that the transition is caused by the development of wear protective layers on the rubbing surfaces, is proposed. The protective layers are developed mainly from accumulated wear debris particles retained within the wear tracks; these can have various characteristics, depending on the experimental conditions and the properties of the metal, particularly the oxidation conditions and the contact between the rubbing surfaces. There is broad agreement between reported experimental observations and calculated predictions based on this model. For example, the development of protective layers occurs very quickly once the transition time/distance …