Contact mechanics of high-density polyethylene: Effect of pre-stretch on the frictional response and the onset of wear

Nowadays, in many applications metal parts are replaced by light-weight polymer products. As a result of the processing history, these polymer fabricates are, more often than not, anisotropic, leading to a direction dependent mechanical performance. Recently we showed the frictional response of isotactic polypropylene is improved by pre-stretching the crystalline network. In the present work, the scratch response of isotropic high-density polyethylene (HDPE) is compared with that of several pre-stretched samples of the same material, subjected to a single-asperity contact with a rigid diamond indenter. The surface penetration and lateral force are measured in-situ for a range of applied loads and sliding velocities. In the direction perpendicular to the orientation, the observed response is comparable to that of isotactic polypropylene (iPP). Contrary, in the direction parallel to the oriented crystals abrasive wear is observed in HDPE already for relatively low applied loads. As the amount of anisotropy increases, the wear-rate also increases, leading to a decrease in global scratch resistance of these materials. The discrepancies between iPP and HDPE are explained by the intrinsic material behaviour; the lack of strain softening in HDPE prevents strain localization, hence the ever increasing local stress reaches its maximum value and brittle machining is observed