Contact mechanics and friction: Role of adhesion

This paper presents an overview of the role of adhesion in various tribological phenomena. We discuss (1) adhesion and adhesive hysteresis in rough contacts, (2) the adhesive contribution to dry friction, (3) the properties of adhesive contacts under tangential loading, (4) "negative adhesion" and superlubricity and (5) adhesive wear. Based on theoretical considerations, simulations with the Boundary Element Method and experiments, we argue that the key process underlying all these phenomena are jump-like changes of the contact boundary. These jumps are an essential property of adhesive contacts and are solely responsible for energy dissipation in both adhesive hysteresis and adhesive friction. On the mesoscale, the aforementioned instabilities give rise to boundary line friction, which forms a convenient tool for understanding the properties of adhesive contacts both under normal and tangential loading, including changes of contact area and the phenomenon of the "sticking zone". On the macroscale, the concept of boundary line friction can be approximated by a simple adhesive contact with two different works of adhesion – a smaller one for closing the adhesive crack (attachment) and a larger one for opening it (detachment). The well-known equivalence between the adhesive contact boundary and the Griffith crack also leads to application of the same ideas to wear. In this context we discuss the modified Rabinowicz criterion for wear particle formation and argue that the adhesive nature of wear all but rules out Archard's law. Finally, we note that adhesive forces are not necessarily attractive and discuss how "negative", i.e. repulsive, adhesion can account for the phenomenon of superlubricity.