Sliding friction dynamics of hard single asperities on soft substrates

The sliding friction of hard, micron-sized single asperities sliding on soft polyester films was studied. Transitions from steady sliding to so-called "stick-slip" or nonstationary motion occur for decreasing driving speed, decreasing driving spring stiffness, increasing normal load, decreasing tip radius, and decreasing crosslink density. Normal displacements of the tip during sliding were studied in some detail. It is argued these play an important role in the dynamics of the system, being the dominant factor in determining the contact area between asperity and substrate. A rather simple model is proposed that is related to rate-and-state descriptions of stick-slip phenomena. In this particular description the normal displacement plays a part analogous to that of the state parameter. In a limited comparison of experiment and numerical results we find qualitative agreement on all measured trends.