Scientists and engineers still debate the relative contributions of roughness and interlocking of asperities, adhesion and interatomic forces, and elastic and plastic deformation in friction phenomena. However a central point of agreement does exist: Friction involves energy dissipation, or the conversion of mechanical work into heat. Therefore any mechanism that allows for this energy dissipation during sliding interactions of materials deserves consideration, whether it occurs strictly at the nominal sliding interface or within material at some distance from that interface.
If the interactions are limited to atomic distances, it is entirely appropriate to focus attention on atoms at the two surfaces (see the articles by M.O. Robbins and J. Krim and J.A. Harrison and S.S. Perry in this issue). Friction is then a surface phenomenon. However if the sliding conditions lead to extensive plastic deformation of either material—over distances readily detectable by eye or by optical microscopy—then the associated energy dissipation is likely to account for most of the frictional losses. In such cases, friction involves more than processes occurring at the surface.