Embedding Wear Models into Friction Models

Frictional behavior in dry or boundary-lubricated tribosystems is commonly time-dependent. Examples include phenomena like running-in, scuffing initiation, adhesive transfer, coating wear-through, and lubricant starvation. Fundamental models for the sliding friction coefficient usually focus either on determining a steady–state value or on predicting periodic behavior like stick-slip. They often neglect the details of long- and short-period frictional transients, some of which are quite repeatable. In addition to generating heat, frictional work is known to be dissipated in several ways, including roughness changes, wear particle generation, tribomaterial evolution, and microstructural alteration. Pairs of materials can display identical average friction coefficients but significantly different wear processes because frictional work is dissipated differently from one pair of materials to the next. The attributes of friction-versus-time behavior for combinations of metals, ceramics, and polymers can be comprised of stages whose understanding may require the development of piecewise friction models that include wear. This paper discusses past work on the subject, exemplifies embedding a simple wear model into a friction-versus-time model, and indicates how friction process diagrams can play a role.