Composition and structure of coatings on Ti-6Al-4V alloy by micro-plasma oxidation in NaAlO2 solution

Compound ceramic coatings on Ti-6Al-4V alloy were prepared for different time by pulsed bi-polar micro-plasma oxidation (MPO) in NaAlO2 solution. The phase composition was studied by XRD. And the electrochemical impedance spectra (EIS) of the coatings were measured to study the structure of the coatings, combined with the surface morphology. Using the proper EIS interpreting software, the "equivalent circuit" of the coatings was established, and meanwhile fitting values of equivalent element were obtained. The results show that the coating is composed of Al2 TiO5, α-Al2O3 and rutile TiO2, of which Al2 TiO5 is the main crystalline. Increasing the MPO time, the contents of Al2TiO5, rutile TiO2 and α-Al2O3 in the surface of the coating increase. And the EIS analysis and the surface morphology of the coatings illustrates the double-layer structure of the coatings, and the outer layer is loosen and the inner layer is compact. With the increase of the oxidizing time, the surface coarse degree of the coatings and the porosity of the outer layer of the coating are increased while the compactness of the inner layer of the coating is improved.