Structural recovery (physical ageing) of the friction coefficient of polymers

Most of the mechanical properties of the polymers are subject to structural recovery (physical ageing). The aim of this work was, therefore, to evaluate the sensitivity of the friction coefficient to physical ageing. The apparent friction coefficient is the ratio of the tangential force to the normal load applied to a moving tip in contact with the surface of a material. This coefficient includes a true friction at the interface and a geometrical friction, which is the lowing and bulk dissipative effect. In the case of solid polymers, the material underneath the moving tip may display various types of behavior: elastic, viscoelastic, elastoplastic (elastic and plastic strains are present in the contact area), or fully plastic. Scratching and sliding experiments were performed on PMMA to determine the true friction coefficient over a wide range of contact deformations. An analysis of the true friction coefficient as a function of the physical ageing (structural recovery) showed that the friction decreases with increasing structural recovery of the polymer in the case of sliding. Whatever the level of physical ageing, at a high contact strain (fully plastic contact), the true friction coefficient tended to a unique and high value independent of the thermal history of the polymer. (C) 2008 Wiley Periodicals, Inc.