Growth Kinetics of Microarc Oxidation TiO₂ Ceramic Film on Ti6Al4V Alloy in Tetraborate Electrolyte

The growth kinetics of microarc oxidation TiO₂ ceramic film on Ti6Al4V alloy in 0.1 M Na₂B₄O₇·10H₂O electrolyte were studied using scanning electron microscopy, transmission electron microscope, atomic force microscope, X-ray diffraction, and potentiodynamic polarization. It was found that the discharge sizes increased gradually from anodic oxidation, spark discharge, and microarc to arc discharges in the microarc oxidation process, and reached the maximum value without anymore increases. The microarc oxidation coating was mainly composed of nanocrystalline rutile TiO₂ grains with very little amorphous titanium dioxide, and was made up of three layers, namely an inner dense thin layer, a middle dense thick layer, and a porous outer layer. The content of the rutile phase increased with the increasing discharge size, and when the discharge size reached a maximum, the increase rate decreased slightly. The surface roughness, R_v, and corrosion resistance of the coating increased with the increasing discharge size until the discharge size reached a maximum. It was concluded that the thickness of two dense layers depended on the discharge size, with any changes in it revealing the growth kinetics of the coating. A new method was developed to obtain the effective thickness of the coatings by measuring the thickness of the two dense layers. The results demonstrated that the growth kinetics of the two dense layers are linear during spark and microarc discharges, of which the growth trend is the same as that of the voltage. A linear relationship for the voltage and thickness of the two dense layers was obtained.