Experimental evidence of micro-EHL lubrication in rough soft contacts

An experimental investigation of the lubricated steady-sliding contact for elastically soft solids is reported. We show that the anisotropic character of the surface roughness has a strong influence in determining the transition from boundary lubrication, where the normal load is fully supported by the asperity-asperity interactions, to the hydrodynamic regime, where a thin fluid layer completely separates solid surfaces from direct contact. In particular, tests have been carried out using a ball-on-flat test configuration to measure the friction coefficient at the contact between a smooth steel ball and a rough PDMS as a function of the sliding speed. The most noteworthy result is that the presence of roughness anisotropy strongly modifies the classical shape of the Stribeck curve as a consequence of local micro-EHL conditions which occur at the contact interface.