Growth mechanism of titania on titanium substrate during the early stage of plasma electrolytic oxidation

In the present work, the early-stage growth behaviour of titania on titanium (Ti) substrate during plasma electrolytic oxidation (PEO) process in an aqueous electrolyte rich in Ca and P was investigated. Microstructural characterization reveals the dramatic evolutions of morphology, porosity, phase component and chemical composition of titania during the early stage of PEO process. The PEO coating initially exhibits ultrafine pores with a completely amorphous structure after oxidation for 5 s, which displays a relatively low Ca/P ratio. With the extension of oxidation periods, titania gradually transforms to grooved morphology with an increased Ca/P ratio, which mainly consists of crystalline anatase phase. Finally, with the prolongation of oxidation period to 1 min, titania exhibits porous morphology typical for PEO coating, which comprises of both rutile and anatase crystals. Along with the prolongation of oxidation periods, the surface wettability of titania remains relatively stable whereas the corrosion resistance changes dramatically, which is closely associated with the combined effects of its porosity and thickness. The results of our present work advance the understanding of early-stage growth of titania during the PEO process, which offer theoretical guidance for the controllable synthesis of PEO coating on Ti and its alloys.