Rate effects on the microindentation-based mechanical properties of oxidized, crosslinked, and highly crystalline...

The surface micromechanical properties of ultrahigh-molecular-weight polyethylene (UHMWPE) are critical in determining the wear, deformation, and fracture in the surface region. These properties have not been accessible to simple mechanical testing on a spatial scale relevant to these mechanical processes until recently. The structural factors associated with surface mechanical properties (crosslinking, oxidation state, local orientation of polymer, crystallinity, etc.) can be highly variable and localized and may vary on micron spatial scales or smaller. Furthermore, time/frequency-dependent behavior of the surface may have an important role in the overall surface mechanical behavior. Recent work has shown the utility of depth sensing microindentation/nanoindentation testing to interrogate local surface mechanical properties. The goal of the present study was to measure the effect of loading rate on the depth-sensing microindentation testing of UHMWPE. Three different UHMWPE materials (Hylamer, a large crystal material; GUR 1020, a standard medical-grade material; and Marathon, a crosslinked material) were tested using a microindentation method at loading rates ranging from 0.01 to 1 Hz. Similarly, a gamma-irradiated in air and 15-year shelf-aged tibial component was tested through its cross-section to assess the variations in mechanical properties with location and to compare the micromechanical profile with the oxidation profile. It was found that rate of testing affected the microhardness of each material, however, only GUR 1020 and Hylamer showed rate-dependent behavior for modulus and energy dissipation factor. Micromechanical profiles through oxidized regions of the tibial component showed a high correlation with the oxidation profile. Increases in modulus, hardness, and energy dissipation factor were seen with increasing oxidation and each property was loading-rate dependent. These results show that depth-sensing microindentation/nanoindentation testing on the micron scale provides highly consistent and reproducible measurements of surface mechanical properties. This scale of testing minimizes the potential variations caused by local heterogeneity in crystallinity, surface orientation, and other submicron structural features.