Electrochemical Corrosion Behavior of Ta(2)N Nanoceramic Coating in Simulated Body Fluid

In order to improve the corrosion and wear resistance of biomedical Ti-6Al-4V implants, a Ta(2)N nanoceramic coating was synthesized on a Ti-6Al-4V substrate by the double glow discharge plasma process. The Ta(2)N coating, composed of fine nanocrystals, with an average grain size of 12.8 nm, improved the surface hardness of Ti-6Al-4V and showed good contact damage tolerance and good adhesion strength to the substrate. The corrosion resistance of the Ta(2)N coating in Ringer’s physiological solution at 37 °C was evaluated by different electrochemical techniques: potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), potentiostatic polarization and capacitance measurements (Mott-Schottky approach). The evolution of the surface composition of the passive films at different applied potentials was determined by X-ray photoelectron spectroscopy (XPS). The results indicated that the Ta(2)N coating showed higher corrosion resistance than both commercially pure Ta and uncoated Ti-6Al-4V in this solution, because of the formed oxide film on the Ta(2)N coating having a smaller carrier density (N(d)) and diffusivity (D(o)) of point defects. The composition of the surface passive film formed on the Ta(2)N coating changed with the applied potential. At low applied potentials, the oxidation of the Ta(2)N coating led to the formation of tantalum oxynitride (TaO(x)N(y)) but, subsequently, the tantalum oxynitride (TaO(x)N(y)) could be chemically converted to Ta(2)O(5) at higher applied potentials.