Asperity-asperity contact mechanisms simulated by a two-ball collision apparatus

Abstract

Wear of materials is controlled by asperity-asperity contacts. Since the asperities are of uneven height, the stresses imposed by different pairs of asperities necessarily are unequal. When two surfaces come together, there is a distribution of the contact stresses. In a wear experiment, we measure wear by measuring the cumulative wear across the interface from the beginning of the experiment, and we measure friction by measuring the average friction of all the asperity contacts across the interface at the particular instance in time. The friction and wear characteristics at the asperity level have interested the tribology community for a long time. Data at the level, however, are difficult to obtain and the conditions that lead to a single asperity wear event are not understood. This paper describes the design and construction of a two-ball collision test apparatus to simulate asperity contacts. Materials used include steels brass, and aluminum of different hardness and elasticity. Dynamic friction and wear of the collision were measured under dry, paraffin oil lubricated conditions. The results suggested that at a coefficient of 0.4, wear of the asperity contact occurred. A simple plain strain model confirmed the observations that under certain conditions, asperity wear should occur at a coefficient of 0.39.