Wear of Ultra High Molecular Weight Polyethylene Sliding Against a CoCrMo Coating Applied by Physical Vapor Deposition

摘要

The UHMWPE (Ultra-High-Molecular-Weight-Polyethylene) -CoCrMo wear couple is used in most artificial joints despite the well-established problems of wear debris induced osteolysis and subsequent implant loosening. In this study, the wear behavior of UHMWPE sliding against a cast CoCrMo alloy, a forged CoCrMo alloy and a CoCrMo-coating obtained by magnetron sputtering Physical Vapor Deposition was investigated. The evaluation of UHMWPE wear was performed on a two-station Reciprocating Pin-On-Flat (RPOF) machine according to ASTM F732. The wear of the UHMWPE pins was measured by weighing up to one million cycles, and the surface of the alloys and the PVD coating was analyzed by SEM, AFM and laser profilometry. The results show that the UHMWPE wear was least for the cast CoCrMo and the forged CoCrMo alloy, whereas the CoCrMo-PVD coating caused significantly higher UHMWPE wear. This result is remarkable, because in the as-received state the forged CoCrMo alloy had a three-times higher mean surface roughness compared to the CoCrMo coating. It is assumed that small parts of the coating have delaminated during the test leading to the formation of a large number of microscratches (third-body wear). Additionally, ridges perpendicular to the sliding direction have formed on the coating surface. Both the microscratches and the ridges increase the surface roughness and cause high UHMWPE wear. No difference was found between the cast and the forged CoCrMo alloy with respect to UHMWPE wear. The authors believe that the reciprocating motion could have been responsible for this effect. The main result of this study suggests that the UHMWPE wear is not only determined by the mean surface roughness of the metallic counterpart, but also by the surface finishing.