Onset of nanoscale wear of metallic implant materials: Influence of surface residual stresses and contact loads

Nanoscale wear response of cobalt–chromium alloy (ASTM F-75) was investigated as a function of contact load and surface residual stress in order to identify mechanism governing onset of surface damage in modular implants. A unique loading configuration was utilized to apply range of known in-plane stress states to the specimen surface in order to simulate different residual stress levels. Using the tip of a scanning force microscope as a well characterized “asperity,” controlled contact forces were applied on the specimen to mechanically stimulate the loaded surface. Volume of material removed was measured to characterize the wear rate as a function of the contact loads and surface stress state. Experimental measurements of material removal indicate that a critical level of contact pressure is required to initiate wear of the cobalt-chromium surface and as expected higher contact pressures accelerate the wear process. At a constant contact pressure, wear rate is accelerated by compressive in-plane stress while tensile in-plane stresses tend to suppress the surface wear. A surface damage mechanism based on successive damage/delamination of native oxide covered surface due to single asperity contact and repassivation of exposed surface is proposed to elucidate the experimental observations.