Modification of the microstructure of ultra-high molecular weight polyethylene (UHMWPE) is essential for improving the wear resistance of orthopaedic implants. A common approach is to crosslink the polymer by gamma irradiation. The objective of this study was to examine the tribological behaviors of untreated and gamma irradiated UHMWPE under physiologically relevant contact conditions. Emphasis was given on the identification of the dominant wear mechanisms in the early stage of polymer wear. The irradiation dose exhibited as strong effect on the tribological properties of UHMWPE sliding against Co-Cr alloy in a bath of bovine serum. Transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) were used to examine the microstructure and morphology of the worn surfaces. Regularly spaced folds with average spacing depending on the irradiation dose (i.e., crosslink density) formed on the wear tracks. Surface folding was related to plastic flow and the degree of mobility of the crystalline lamellae. SEM and TEM results elucidated the roles of the crosslink density and crystalline lamellae in the wear process. Based on the experimental evidence, a deformation model was obtained that provides explanation for the dependence of surface folding on the crosslink density and lamellae reorientation during sliding.

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