Design of Uniformly Textured Polymer Surfaces for Hard-soft Bearing Systems
The aim of this study is to investigate the effect of surface texturing for improving the tribological performance of metal-ultra high molecular weight polyethylene (UHMWPE) bearing systems during intermittent stop-dwell-start (SDS) motion. Numerical and experimental studies were conducted to analyze the role of texture parameters for hemispherical segments (dimples) on the tribological characteristics of two lubricated surfaces in contact, with the objective to minimize contact stress in the polymeric component. A finite element study was undertaken to determine the effects of three texture design parameters: dimple diameter, d, dimple centre-to-centre spacing, c, and contact area radius, a. Results indicated that lower stress values were evident in the region between dimples. When dimple features were distributed with the centre-to-centre spacing from 20% to 60% of the dimple diameter, the polymeric contact stress was reduced by 10%-45% compared to the average stress without texturing. In a second study, a computational technique was proposed to calculate the desired dimple stress, σd, for given contact region radius, a, average applied contact load, F, dimple diameter, d, and dimple spacing ratio, c/d. It was demonstrated that an efficient dimple texture design could be realized through an optimal combination of the texture spacing ratio, contact region radius and dimple diameter. It was observed that the contact stress in the plastic region is only dependent on the dimple spacing ratio, c/d, for a given load. However, dimple stress increased as the dimple diameter is enlarged for a fixed contact area and spacing ratio, c/d. In the final study, a method to fabricate textured surfaces is described that uses hot embossing. The protocol was optimized to produce textured test samples that achieve 95.6% of their target dimensions. Pilot studies of pressure distribution under quasi-static load using a flat cylindrical punch show trends consistent with model predictions. The results from this work have clinical relevance in the selection of dimple dimensions and lubrication fluid for application in total joint arthroplasty design. However, future studies should consider the effects of lubricant fluid viscosity and the change in material properties of UHMWPE with hot embossing.