UHMWPE wear response to apposing nitrogen S-phase coated and uncoated orthopaedic implant grade stainless steel

The use of ultra-high molecular weight polyethylene (UHMWPE) components for the articulating surfaces of total hip replacements can be problematic because of their proneness to wear which leads to osteolysis, implant loosening and failure. The ability of the metallic counterface (femoral head) to avoid damage by third body particles is crucial because it is the scratching of this surface that is responsible for the wearing of the UHMWPE (acetabular cup). In the present study the tribological appropriateness of using nitrogen S-phase coatings for the protection of Ortron 90 femoral heads and their suitability for making sliding contact with UHMWPE cups was evaluated. Three S-phase coating materials, containing ∼8, 19 and 26 at.% N, were deposited on to orthopaedic implant grade austenitic stainless steel (Ortron 90) test plates using magnetron sputter deposition and then polished. The UHMWPE wear response to the coated test plates was compared to that of uncoated Ortron 90 with a similar surface roughness (Ra ∼ 0.009 μm or better). The results demonstrated that S-phase coated Ortron 90 caused slightly greater UHMWPE wear, even though the Ra for the coatings was lower than the uncoated Ortron 90. This was attributed to subtle differences in the micrometre and sub-micrometre topography between the coated and uncoated test plate countersurfaces, which resulted in a greater frequency of asperity shearing when UHMWPE was slid against the coated plates.