Microstructure and phase composition of microarc oxidation coatings formedon Ti-6Al-4V alloy in aluminate solution

A dense ceramic oxide coating, about 50 mum thick, was prepared on a Ti-6Al -4V alloy by alternating current microarc oxidation in an aluminate solutio n. Its microstructure and phase composition were investigated using X-ray d iffraction (XRD) and scanning electron microscopy (SEM) with energy dispers ive spectroscopy (EDS). The oxide coating contains two layers: an outer lay er and a dense internal layer. The coating is mainly composed of TiO(2)ruti le and TiAl2O5 spinel phases. In the outer layer coating, the TiAl2O5 fract ion is much higher than the rutile fraction. However, in the internal layer , rutile becomes the main phase while the TiAl2O5 fraction decreases signif icantly. During microarc oxidation, local melt-solidification processes tak e place in the coating, however, the microstructure of the Ti-6Al-4V substr ate near the coating/metal interface is not changed. Aluminum from. the sol ution is enriched in the outer layer coating, although it has also diffused into the interior of the coating.