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Abstract

We report a depth-resolved analysis of the microstructure of a bulge-like failure in a medical grade ultra high molecular weight polyethylene (UHMWPE) implant material induced by a cyclic load. The depth-dependent arrangement of the crystalline lamellae was analyzed by cross-sectional transmission electron microscopy (TEM). The failure emerges at the top wear surface as an amorphous bulge with a maximum thickness of approximately 300 µm. A sharp interface below the bulge consists of an approximately 0.1 µm thick boundary layer with stacked lamellae with an outstanding degree of orientation perpendicular to the wear surface. The boundary layer is separated from the intact, unmodified base material with random lamellar orientation by an approximately 5 µm thick transition zone with a lamellar alignment perpendicular to the wear surface, which decreases toward the center of the UHMWPE test specimen. We further observed an overall decrease of lamellar thickness toward the wear surface within the influence zone. This indicates the strength and heterogeneity of the local dynamic stress field during cyclic wear and the ease for lamellar rearrangements presumably facilitated by a locally elevated temperature. We propose the bulge to originate from either wear material dragged along the wear surface or from a retransfer from the articulating counter surface, and discuss implications on the degree and relevance of adhesive wear. This study stresses the importance of the UHMWPE transfer material under such dynamic load conditions. Our approach seems suitable for investigating the mechanical aspect of failure mechanisms at the UHMWPE polymer biointerface under cyclic load.