Characteristics of grain growth of microarc oxidation coatings on pure titanium

Grainy titania coatings are prepared by microarc oxidation on pure titanium (TA2) substrates in a Na$_{2}$SiO$_{3}$--NaF electrolytic solution. The coating thickness is measured by an optical microscope with a CCD camera. Scanning electron microscope (SEM) and x-ray diffraction (XRD) are employed to characterize the microstructure and phase composition of coatings. The results show that the coating thickness increases linearly as the treatment time increases. The coatings are mainly composed of anatase and rutile (TiO$_{2})$. With the increase of treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the oxidizing process. Meanwhile, the size of grains existing on the coating surface increases and thus the surface becomes much coarser.