A study of the nanotribological fatigue of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) provides a low friction, high toughness interface in artificial knees and hips. Micron-sized wear debris forms over time in these transplants leading to osteolysis and poor clinical outcomes. Using the atomic force microscope (AFM) as a model single asperity contact, tribological studies were performed on nanometer smooth samples of UHMWPE under dry conditions to elucidate the mechanisms of debris formation. Low loads produced no changes in friction or topography despite repeated scanning. Above a critical load, polymer accumulated at the perimeter of the scan and led to the formation of a wear debris particle. Plastically deformed material exhibited a surprisingly high friction compared to surrounding pristine areas, and may partially explain macroscale observations of adhesive wear. In contrast, the polymer in the interior of the scanned area exhibited a friction identical to pristine polymer. These data link strain-softening and delamination of the surface to the formation of wear debris.