Friction and Viscoelastic Properties of Polymeric Solids

Abstract

Abstract When a hard slider slides over a rubber surface, there is a close relation between sliding friction at various speeds and temperatures and viscoelastic properties of the rubber. If friction is due to an adhesion mechanism this may be explained in terms of the way in which area of contact A and shear strength s vary with speed and temperature. Area of contact is determined by the elastic modu us of the rubber; during sliding the deformation process is a relatively low frequency one. In contrast, shear strength s involves very high strain rates in a thin surface layer; this corresponds to a very high frequency process. By combining these two effects the observed friction behavior may be explained. Behavior of polymers below their glass transition temperature is very different. Correlation between sliding friction and various viscoelastic peaks is not at all strong. On the other hand, conventional viscoelastic data correlates well with rolling friction. This shows that high local pressures at the contact regions do not substantially change the viscoelastic characteristics. In sliding, however, there are very high shear strains in the surface and appreciable tearing and transfer of polymer; presumably these violent deformations, although in themselves speed and temperature dependent, bear little relation to deformation processes involved in conventional low strain viscoelastic studies. The good correlation with rubber suggests that, even at the high strains involved in sliding, slip of chain segments over one another still remains the basic mechanism of shear. With PTFE, sliding friction shows a dependence on speed and temperature which resembles a viscoelastic peak. However, it is probable that this refers to gliding of crystallites of PTFE over one another where the effective strains are enormous and not to conventional viscoelastic peaks determined under conditions where strains are very small. Apart from its bearing on friction studies the work described in this paper suggests that more information is needed on viscoelastic properties of polymers at high pressure and at large strains.