Growth mechanism of oxide layer on Ti-6Al-4 V substrate with different surface topographies during the early stage of micro-arc oxidation

The aim of this work was to explore the effect of microscopic surface topographies of the Ti-6Al-4 V alloy on the coating growth behaviour during the initial stages of micro-arc oxidation (MAO). Firstly, MAO treatments were carried out on the surfaces of Ti-6Al-4 V alloys with different surface morphologies for 5, 10 and 15 min respectively. Second, the effects of substrate topographies on the roughness, porosity, thickness, and micro-structure of the coatings were studied. Then, the corrosion resistance of the MAO-treated titanium alloys was assessed by electrochemical analysis. Finally, the formation mechanisms and morphological results of the oxide layer were discussed and illustrated graphically. The results showed that the surface morphologies of the sub-strate affected the micro-arc discharge behaviour during the process of oxidation. Notably, rough areas on the substrate surface facilitated the breakdown of the passivation film, which was evidenced by the early appearance of plasma discharges in these areas. However, surface morphology with uneven distribution of pores observed in the oxide layer formed by the rough substrate. The coatings grown on the grooves were more porous than those grown in other areas, due to the non-uniformity of the micro-discharge on the rough substrate surface.