MACROSCOPIC AND MICROSCOPIC WEAR MECHANISMS IN ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE

Studies of the wear of ultra-high molecular weight polyethylene sliding on relatively smooth metallic and ceramic counterfaces under a wide range of tribological conditions in pin-on-disc and pin-on-plate tests, hip joint simulators and components taken from patients, have demonstrated evidence for two separate types of wear processes. Microscopic wear processes were associated with the very small asperities or the smooth counterfaces (less than 0.2 mum). Macroscopic polymer asperity wear processes were associated with stress concentrations under the much larger peaks in the polymer surface (amplitude less than 10 mum). For constant load tests with rougher counterfaces, microscopic asperity wear dominated. However for smooth counterfaces (R(a)<0.02 mum) under constant load, the contribution of microscopic asperity wear was small, and incremental increases in wear rates were caused by removal of macroscopic polymer asperities due to high subsurface strains and failure. In artificial joints under dynamic loading, the macroscopic polymer asperity wear processes were accelerated by subsurface cracking and fracture producing high wear factors on smooth counterfaces.