Physical analysis of the state- and rate-dependent friction law: Static friction

We report the first part of an extensive study of friction at a multicontact interface between nominally flat bodies, rough on the micrometer scale, made of polymer glasses. Temperature is controlled between 20 $\ifmmode^\circ\else\textdegree\fi{}$C and the glass transitions of poly(methyl metacrylate) and polystyrene. In this article, we focus on static friction force. The static friction threshold to macroscopic slip increases with the contact time. Increasing temperature is found to accelerate markedly the aging process. This effect is correlated with the strong increase of the sensitivity of plastic creep to the strain rate on approaching the glass transition. A constitutive law for creep is established, taking into account the triaxial nature of the deformations at microcontacts between asperities. This is included into a simple model which provides a satisfactory account for the aging process in terms of creep growth of the overall contact area. A strong effect of the shear force sustained by the interface during the aging process is evidenced and discussed. The possible role of healing due to chain diffusion across the interface is considered as a concomitant strengthening process in the glass transition region.