Analysis of the effect of nano-roughness on the friction of a vitreous polymer

The rheological behavior at the interface between a polymer surface and a rigid indenter during dry friction is linked to the local pressure and the roughness of the surface of the indenter. This study analyzed experimental results for the friction coefficient μ versus the mean contact pressure pmean obtained at ambient temperature on polycarbonate. The rigid indenters consisted of spherical glass tips of several radii having surfaces of two types: polished to tight tolerance (optical lenses) or nano-roughened (chemical etching). Below the yield stress (pmean<100 MPa), the intrinsic friction coefficient followed a master curve for smooth indenters whereas nano-roughnesses reduced the friction (from 5.5 to 140 nm Rrms). To explain these experimental results an original model based on the classical laws of contact mechanics was developed and provides a basis to study the rheology of confined polymer layers.