Experimental study of the effect of dwell time and normal load on static friction in creeping elastic-plastic polymer spherical contact

An experimental study of the effect of dwell time and normal load on static friction in a spherical polymer contact is presented. The measurements are done for different diameters of spherical tip specimens made of polytetrafluoroethylene (PTFE). These specimens are loaded against a flat glass substrate by different normal loads and under different dwell times prior to applying tangential loading. It is found that the experimental immediate friction coefficient (after zero dwell time) is about an order of magnitude smaller than the theoretical results of a model proposed by Malamut et al. [S. Malamut, Y. Kligerman, I. Etsion, Tribol. Lett. 35 (2009) 159-170]. However, the creep components of the static friction force and static friction coefficients fairly agree with the model predictions.