Microscale and nanoscale surface strain mapping of single asperity wear in ultra high molecular weight polyethylene: Effects of materials, load, and asperity geometry

Wear of ultra high molecular weight polyethylene (UHMWPE) is a limiting factor in longevity of joint replacements. Therefore, there is a desire to create new materials and enhance processing conditions associated with current materials to reduce wear. This requires understanding the effects of processing on performance of implants and of micron-scale wear mechanisms ongoing. Our goal is to generate detailed understanding of the micron-scale deformation- structure-properties relationships associated with UHMWPE subject to asperity wear processes and ultimately to be predictive of material success, in vivo. In this work, a surface strain analysis technique is developed and used to measure permanent strain from asperity deformations on the nano and micro scales. Deformation was applied to four material types (GUR 1050, GUR 1020, Hylamer, and Marathon) varying in molecular weight, crystallinity, and crosslinking. Surface strains were determined by mapping surface deformation fields and were compared across loading conditions and spatial scale, with variations in tip geometry and size, contact load, and material. Surface strains increased with asperity load for a fixed tip and were dependent on UHMWPE material, with a highly crystalline form exhibiting the most plastic strain and a crosslinked form exhibiting the least. Different asperity geometry [spherical microindenters with radius of 20 and 1500 μm, and a nanoindenter (Berkovich-type)] resulted in different surface strains (e.g., Berkovich vs. spherical were not similar) even when the nominal contact stress was similar. Finally, the extent of deformation during asperity wear correlates to the level of viscoelastic recovery of the materials observed after indentation testing.