A theory of friction and wear based on a new characterisation of asperity interactions

Abstract

The occurrence of adhesive transfer and wear in sliding systems indicates that some asperity interactions produce junctions with a strength at least as great as that of the bulk material. However, the coefficient of friction measured in most environments requires that most junctions must be very much weaker than the bulk. This observation suggests that the adhesive theory of friction should be extended to allow for this variation in junction strengths. A simplifying assumption of only two levels of adhesion (welded and non-welded) occurring in an asperity interaction allows the concept of a probability of welding. Linked with the statistical properties of surfaces this concept leads to a theory of friction and wear which predicts: 1. (a) a unique relation between the coefficient of friction and the probability of welding (for a given pair of materials and surface roughnesses); 2. (b) that the coefficient of friction is a statistical variable, the variance of which is inversely proportional to the mean number of asperity interactions; and 3. (c) a linear relation between the coefficients of friction and wear.