Reactive-sputter-deposited β-Ta₂O₅ and TaON nanoceramic coatings on Ti-6Al-4V alloy against wear and corrosion damage

Abstract

In this study, two different types of nanoceramic coatings; namely, β-Ta2O5 and TaON, were synthesized by double glow discharge plasma technique on Ti-6Al-4V alloy to improve its wear and corrosion resistance. The new coatings were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The two coatings were found to have a thickness of about ~25 μm and exhibit similar microstructural characteristics, i.e., composed of equiaxed grains with an average grain size of ~15 nm. They are tightly adhered to the Ti-6Al-4V substrate. Nanoindentation was used to determine the hardness and elastic modulus of the two coatings and the adhesion strength between the coatings and substrate was also evaluated by scratch testing. Dry sliding wear tests were performed using a ball-on-disk type tribometer, in which the two coatings were slid against Si3N4 ceramic balls under applied loads ranging from 2.3 N to 5.3 N at room temperature. Compared with the Ti-6Al-4V, the coated samples showed a decrease of two orders of magnitude in the specific wear rate, and the specific wear rates of the two coatings were also insensitive to the applied normal loads. The electrochemical behavior of the two coatings was characterized by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis in a naturally aerated 5 wt.% HCl solution. While both types of coatings were able to greatly improve the corrosion resistance of Ti-6Al-4V alloy, the TaON coating exhibit a higher corrosion resistance than the β-Ta2O5, due primarily to the TaON coating with a lower carrier density.