Potentiodynamic and Potentiostatic Characterization of CVD Alumina Coating for Orthopaedic Implant Wear Reduction

As orthopaedic implant manufacturers improve longevity of hip and knee prostheses, new coatings are being developed to minimize in-vivo wear against a polyethylene bearing.  In this investigation, a multilayer coating consisting of CVD Al₂O₃/TiCN/TiN on a Co-28Cr-6Mo implant substrate is evaluated in terms of corrosion resistance of its components in undamaged and scratch-damaged conditions.  Oxidized Zr-2.5Nb, which approximates Oxinium material manufactured by orthopaedic implant manufacturer Smith & Nephew, is selected for comparison because of its similar oxide surface/metal substrate composite structure.

Single sweep potentiodynamic scans performed in Hanks solution at 37°C, followed by SEM surface inspection suggest that at potentials up to +2.2 V (vs. Ag/AgCl), the as-deposited Al₂O₃, TiCN and TiN components of the multilayer coating exhibit excellent corrosion resistance, with much lower corrosion currents and better resistance to dissolution with increasing anodic potential than the Co-28Cr-6Mo substrate.  Cyclic scans performed in Hanks solution with 25% bovine serum on scratched multilayer CVD Al₂O₃/TiCN/TiN  coating to reverse potentials of +1.1V and +2.2V (vs. Ag/AgCl), followed by SEM cross section inspection, exhibit no hysteresis characteristic of crevice corrosion or pitting.  Potentiostatic testing of scratched CVD Al₂O₃/TiCN/TiN coating and scratched oxidized Zr-2.5Nb at +0.5V and +1.0V (vs. V_oc) for 72 hours reveal current decrease to approximately 1 nA, suggesting that passivation occurs in the scratched CVD  Al₂O₃/TiCN/TiN  coated Co-28Cr-6Mo under these conditions.