Assessment of S-phase coated medical grade stainless steel (Ortron 90) for use in the human joint replacement corrosion–wear environment

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Hard and corrosion resistant coating materials such as S-phase, a nitrogen supersaturated Fe–Ni–Cr–Mo phase, have the potential to limit the corrosion–wear (tribo-corrosion) damage that can affect the metallic bearing surfaces of joint replacement devices in vivo, but hitherto have not been much investigated. To test their suitability, a range of S-phase coatings (with five nitrogen concentrations [N]) was applied to several polished Ortron 90 (biomedical grade austenitic stainless steel) test pieces via magnetron sputter deposition. Crevice attack, via intergranular corrosion, was found to be worse for most S-phase coated and uncoated Ortron 90 materials when tested in bovine serum compared with testing in 0.89% NaCl solution. Measurements of the breakdown potentials and the anodic passive current density, for a given test solution, revealed the crevice corrosion response of uncoated Ortron 90 to be more severe than the S-phase coated materials, although this was not unequivocally supported by 2D profilometry measurements. Corrosion–wear tests made using a sliding contact Al2O3 counterface showed the surface degradation to be more aggressive in saline solution than in bovine serum. All S-phase coatings were very effective in mitigating corrosion–wear of Ortron 90 in bovine serum solution, but only one S-phase coating composition, containing 21 at% N, gave improved resistance when tested in 0.89% NaCl solution.