In this article, we review the present status of experimental and theoretical work on friction at the interface between extended macroscopic bodies, rough on the micrometer scale. We show that systematic detailed studies of low-velocity friction using dynamical systems analysis, together with their shear response in the static state, provide a tool for investigating the physical processes taking place on the mesoscopic scale of real contacts between rough surfaces. This approach should shed light on the enduring question of the relationship between macroscopic friction and microscopic dissipative mechanisms. This still open issue has come back to the fore during the last decades, following considerable progress due to the development of “molecular tribometers.
Bowden and Tabor pointed out that, because nominally flat surfaces are in general rough on small scales, the real area of contact Ar (Figure 1) is only a small fraction ϕ of apparent contact area A0. On the other hand, they postulated the existence of a stressσs characteristic of the shear strength of the interface between a given couple of solids. Hence the friction force:
In this framework, the Amontons-Coulomb (AC) law F = μFN amounts to stating that Ar is proportional to the normal load FN where μ is the coefficient of friction.
When considering soft metals, Bowden and Tabor noticed that ϕ ≪ 1 entails that the nominal local pressure p on the real contacts—of the order of FN/(ϕA0)–generally overcomes the yield strength Y so that the contacting asperities flow plastically until p = H≈ 3Y, the “hardness” of the (softer) material. So, Ar = FN/H.