Role of wear particles in severe–mild wear transition

Severe–mild wear transition is one of the most interesting phenomena in adhesive wear, because the wear rate decreases drastically after the transition. This paper is aimed at clarifying the mechanism of wear mode transition from severe to mild. A copper pin was rubbed against rotating iron disk in laboratory air. Sliding conditions such as load and sliding velocity were changed. To focus on the role of wear particles, they were removed by air cleaner. To control the behavior of wear particles, the set position of pin-on-disk tester was changed. The inner structures of severe and mild wear particles were also examined. As the wear mode changed from severe to mild, morphologies of wear particles also changed and resulted in smaller particle sizes. However it is to be noted that some mild wear particles were noticeable even in severe wear mode and that some severe wear particles were also found at the early stage of mild wear mode. Large black wear particles produced in mild wear mode were composed of severe wear particles in the core, which was covered by fine black mild wear particles. When small wear particles are formed they are easily oxidized and transform to mild wear particles. However if they were removed from the surface, mild wear was not observed. The attachment of mild wear particles to the worn surface formed a protective oxide film, and induced mild wear. As the sliding velocity increased, the sliding distance needed for severe–mild wear transition increased proportionally at relatively low load conditions. It indicates the existence of characteristic time for the transition. It is the necessary time for the production of adequate amount of mild wear particles and their attachment on the surface.