Abstract Friction models that relate normal and friction forces at sliding contacts are widely used in the design and analysis of many mechanical systems and components. Most models include many hidden assumptions and tend to be valid under rather limited sets of conditions. Studies of unsteady sliding, (i.e., when some combination of sliding speed, applied load, friction force or friction coefficient vary with time), can reveal some of the limitations of simplified models. In this paper we summarize a series of studies of unsteady sliding at a liquid lubricated line contact. The unsteadiness arises due to changes in sliding velocity or normal load, and sometimes includes stick-slip. The lubrication takes place in the mixed or boundary regimes, so that the frictional resistance is developed due to a combination of fluid shear over the entire contact and solid friction at localized asperity contacts. A number of parameters are varied and the key elements of any dynamic friction model are idendfied. It is suggested that a geometric approach to the problem, wherein the instantaneous friction is related to the instantaneous separation of the sliding bodies and the sliding velocity, can capture most aspects of friction behavior.
