Electrochemical Properties of a Novel β-Ta2O5 Nanoceramic Coating Exposed to Simulated Body Solutions

To enhance the corrosion resistance, biocompatibility and mechanical durability of biomedical titanium alloys, a novel beta-Ta2O5 nanoceramic coating was developed using a double glow discharge plasma technique. The surface morphology, phase composition and microstructure of the as deposited coating were examined by atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coating exhibits a striated structural pattern along the growth direction, which consists of equiaxed beta-Ta2O5 grains, 15-20 nm in diameter in cross-section, showing a strong (001) preferred orientation. The mechanical properties and contact damage resistance of the beta-Ta2O5 coating were evaluated by nanoindentation. Additionally, scratch tests were performed to evaluate the adhesion strength between the beta-Ta2O5 coating and the Ti-6Al-4V substrate. The beta-Ta2O5 coating shows high hardness combined with good resistance to both indentation and scratch damage, thus favoring it for long-term load-bearing application in the human body. Electrochemical behavior of the coating was analyzed in both a 0.9 wt % NaCl solution and Ringer's solution at 37 degrees C, by various electrochemical analytical techniques, including potentiodynamic polarization, electrochemical impedance spectroscopy, potential of zero charge and Mott-Schottky analysis. Compared with uncoated Ti-6Al-4V and commercially pure tantalum, the beta-Ta2O5 coating possesses a more positive E-corr and lower i(corr) in both aqueous solutions, which is attributed to the thicker and denser beta-Ta2O5 coating that provides more effective protection against corrosive attack. In addition, the beta-Ta2O5 coating shows stable impedance behavior over 5 days immersion under both simulated body solutions, corroborated by the capacitance and resistance values extracted from the EIS data. Mott-Schottky analysis reveals that the beta-Ta2O5 coating shows n-type semiconductor behavior and its donor density is independent of immersion time in both aqueous solutions. Its donor density is of the order of 1 X 10(19) cm(-3), which is an order of magnitude less than that of the passive films formed on either commercially pure Ta or uncoated Ti-6Al-4V. Moreover, according to the differences between corrosion potential and potential of zero charge, the beta-Ta2O5 coating exhibits a greater propensity to repulse chloride ions than both commercially pure Ta and uncoated Ti-6Al-4V. Therefore, the newly developed coating could be used to protect the surface of biomedical titanium alloys under harsh conditions.