Electrochemical properties of a novel β-Ta₂O₅ nanoceramic coating exposed to simulated body solutions

Abstract

To enhance the corrosion resistance, biocompatibility and mechanical durability of biomedical titanium alloys, a novel β-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 β-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 β-Ta2O5 coating were evaluated by nanoindentation. Additionally, scratch tests were performed to evaluate the adhesion strength between the β-Ta2O5 coating and the Ti-6Al-4V substrate. The β-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 °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 β-Ta2O5 coating possesses a more positive Ecorr and lower icorr in both aqueous solutions, which is attributed to the thicker and denser β-Ta2O5 coating that provides more effective protection against corrosive attack. In addition, the β-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 β-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 × 1019 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 β-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.