Surface Forces and Adhesion

Abstract

This chapter is concerned, not with the controversial topic of the adhesion component of friction (covered later in this book), but with how surface forces may influence a wide range of friction processes. This influence is either direct, or as a result of changes in the real area of contact.

First it is necessary to clarify some confusing terminology associated with the word “adhesion”, and to list current techniques used to measure surface forces. The chapter then describes the analysis of the elastic contact between a sphere and a flat, as a first step in understanding the adhesion of solids. Useful approximations are given by the “fracture mechanics” or “energy balance” model described by Sperling and independently by Johnson, Kendall and Roberts (JKRS), and the “deformed profile” model due to Derjaguin, Muller and Toporov (DMT). The JKRS approximation assumes a value of the work of adhesion or Dupre adhesion energy. It takes into account the additional deformation near the contact periphery resulting from surface forces, over and above the Hertzian deformation that would be given by the external load alone, and is valid in cases of strong adhesion, large radius and low elastic modulus. In the opposite situation, the DMT approximation applies: an arbitrary deformed profile (e.g. Hertzian), and the appropriate intermolecular force law, are assumed, and the force of attraction outside the contact zone is obtained by integration.

The effective force of attraction between the surfaces (not the same as the pull-off force required to separate them) can give rise to plastic as well as elastic increases in contact area. A size effect operates here, in that for smaller radii of curvature, the force required to initiate plastic deformation decreases faster than the force of attraction: this is opposed by a compressive reaction which therefore will produce plastic deformation if the scale of the contact region is small enough. The conditions for this “adhesion-induced plastic deformation”, and for ductile or brittle failure of an adhesive contact, have been summarized in the form of maps.

There are two at first sight contradictory effects of roughness on adhesion. For two surfaces glued together, roughness increases the force needed to peel them apart. However, for two solids placed in contact, roughness reduces the pull-off force. The classical analysis of this effect, for elastic and plastic contact, is reviewed: recent theoretical and experimental work has described an additional phenomenon known as avalanching, in which enlarged asperity junctions are formed when adhesion energy is released. This leads to the topic of energy dissipation in static contact, the associated hysteresis involving mechanical, chemical, or bulk effects (plasticity, viscoelasticity). As will be evident from later chapters, corresponding processes are involved in sliding contact.