Hydrostatic stress as indicator for wear initiation in polymer tribology

Over the last two decades the study of friction has been an important topic in polymer tribology. The obtained knowledge about friction phenomena enables to take the next step towards understanding wear in polymers. When increasing the amount of local deformation in sliding friction experiments, the onset of failure is obtained, i.e. periodic cracks are initiated. Comparing the location of these cracks for a range of polymers with large differences in their intrinsic deformation response, suggests that different kinds of wear mechanisms are important. The relation between these mechanisms and the intrinsic properties are explained by the subtle interplay between intrinsic strain softening and strain hardening in the material. The critical locations for crack initiation are for polystyrene behind the indenter tip, at the centerline of the scratch, while for high-density polyethylene the cracks are initiated in the bow wave in front of the indenter. In finite element scratch simulations, the position of the maximum hydrostatic stress appears to be identical to the experimentally observed crack location. This suggests that crack initiation is related to a critical positive hydrostatic stress, which is known to be an intrinsic parameter for failure initiation in bulk polymers.