Experimental and Numerical Modeling of Variable Friction Between Nanoregions in Conventional and Crosslinked UHMWPE

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Abstract
Recently, highly crosslinked UHMWPE components have been promoted for their high abrasive wear resistance over conventional UHMWPE (PE) in total joint replacement (TJR) prostheses to minimize osteolysis and consequent implant loosening. This study was aimed at investigating the role of friction gradients induced by localized coefficients of friction at both crystalline and amorphous nanoregions in PE, and crystalline and crosslinked nanoregions in crosslinked UHMWPE (XPE), in submicron wear debris generation. An abrasive wear study performed on both XPE and PE using atomic force microscopy (AFM) illustrated that the onset of plastic deformation for XPE occurred at a normal load that was approximately 3 times higher when compared to PE. Coefficients of friction μd of 0.2, 0.35, and 0.61, experimentally derived using AFM, were used as representative μd for crystalline, amorphous, and crosslinked nanoregions, respectively, in a numerical Hertzian model. An increase in μ (0.2±0.02, 0.35±0.01 and 0.6±0.04) was observed with a decrease in crystallinity and storage modulus at 22°C. Using the Hertzian contact model, it was observed that variability in friction between nanoregions contributed to higher magnitude stresses for XPE (0.2 to 0.61; maximum σeff=2.8) compared to PE (0.2 to 0.35; maximum σeff=1.1) over a negligible thickness of the interfacial zone (IZ) between nanoregions. The experimentally observed increase in abrasive wear resistance of XPE could be attributed to an increase in the thickness of the interfacial zone between nanoregions with μ changing gradually from crystalline to crosslinked nanoregions, a situation that may not be observed with PE. This would cause a decrease in the friction gradient and resulting stresses thereby agreeing with the observed experimental higher abrasive wear resistance for XPE. However, in both PE and XPE, the presence of stress concentrations over a period of time could lead to irreversible damage of the material eventually generating submicron wear debris. Hence, semicrystalline, inhomogenous UHMWPE with several nanoregions (amorphous and crystalline) would be at a disadvantage for bearing application in terms of abrasive wear resistance compared to UHMWPE with relatively lower number of nanoregions and crosslinked nanoregions.