Wear of materials used for artificial joints in total hip replacements

Two tribological pairs used for artificial joints were studied. Wear tests were performed in a pin-on-disc type machine modified to allow lubrication with bovine serum. The pins were made of either AISI F138 stainless steel or ASTM F136 titanium alloy, while the discs were manufactured with ultra-high molecular weight polyethylene (UHMWPE). The tested pairs were ASTM F138 stainless steel–UHMWPE and ASTM F136 titanium alloy–UHMWPE. The sliding velocity was fixed to 0.58 m/s for all the tests and the variation of friction force was registered as a function of time and normal load. Also, an electrochemical setup allowed monitoring the variation of the corrosion potential between the metallic pins and the bovine serum. The lower values of friction coefficient were measured when a thin film of polymer was transferred to the metallic surface, which led to a smooth interface and avoided debris generation. The viscoelastic behavior of UHMWPE was responsible for the increase in friction coefficient after a testing period, which depended on the normal load and the specific pair tested.