What part does interfacial energy play in the deformation and adhesion of metals?

Abstract

Many surface-mechanical phenomena, like the bulk strength properties of crystals, are dominated by the behaviour of imperfections. However, recent work in various laboratories suggests that the study of fundamental thermodynamic properties can usefully contribute to our understanding of adhesion, deformation and friction.Although accurate values of surface and interfacial energies, and hence of the Dupré energy of adhesion y, can seldom be calculated from first principles, y for smooth elastomers in contact has been derived from experimental measurements of adherence (at equilibrium the stress intensity factor for the crack at the periphery of the contact area is proportional to y12). With hard solids the problem of how to determine the real area of contact may be attacked with the help of specially-designed ‘mechanical microprobes’. A sufficiently clean metal-metal interface is generally stronger under tension than at least one of the bulk metals, in which case the practical significance of y disappears except that plastic deformation can occur even at zero applied load, as an indirect consequence of surface forces. This process may decisively influence the very early stages of sintering, according to the balance between the surface, elastic, grain and twin boundary energies. For thin elastic films subjected to peeling or scratch tests, given the value of y it is possible to predict the mode of detachment.Although for unlubricated metals friction involves energy-dissipative shearing of adhesive micro-contacts, large values of the thermodynamic quantity y may be associated with an increase in real contact area and/or interfacial shear strength, as with polymer monofilaments and carbon fibres. For the elastic case, there is a transition from peeling to sliding at a critical value of stress intensity factor. The plastic case has yet to be analysed but for metals, y should become significant in friction when its value approaches the product of hardness and standard deviation of asperity heights.