Sliding friction dynamics of hard single asperities on soft surfaces

The sliding friction dynamics of hard single asperities on soft polyester s urfaces has been studied. Derailed time-resolved measurements of the slidin g dynamics are presented. Of particular interest are normal displacements d uring "stick-slip". Furthermore, the usual transitions from "stick-slip" to steady sliding behaviour are measured for increasing driving speed and inc reasing driving spring stiffness. Similar transitions were encountered for decreasing asperity radius and increasing normal force, that are taken to r eflect the single-asperity nature of the contact. It is argued that the nor mal component of displacement is crucial for the sliding dynamics. Based on this observation a simple description in a dynamical system is proposed an d its relation to rate-and-state formulations discussed. It is concluded th at, apart from being relevant for friction and wear of polymers in general, the system is also interesting as a model system for the study of rate-and -state dynamics.