Electrochemical Corrosion Behavior of Ta₂N Nanoceramic Coating in Simulated Body Fluid

In order to improve the corrosion and wear resistance of biomedical Ti-6Al-4V implants, a Ta₂N nanoceramic coating was synthesized on a Ti-6Al-4V substrate by the double glow discharge plasma process. The Ta₂N 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 Ta₂N 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 Ta₂N 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 Ta₂N coating having a smaller carrier density (N) and diffusivity (D) of point defects. The composition of the surface passive film formed on the Ta₂N coating changed with the applied potential. At low applied potentials, the oxidation of the Ta₂N coating led to the formation of tantalum oxynitride (TaO_xN_y) but, subsequently, the tantalum oxynitride (TaO_xN_y) could be chemically converted to Ta₂O₅ at higher applied potentials.