Effects of cathode pulse at high frequency on structure and composition of Al₂TiO₅ ceramic coatings on Ti alloy by plasma electrolytic oxidation

The aim of this work is to investigate the effects of cathode pulse under high working frequency on structure and composition of ceramic coatings on Ti-6Al-4V alloys by plasma electrolytic oxidation (PEO). Ceramic coatings were prepared on Ti alloy by pulsed bi-polar plasma electrolytic oxidation in NaAlO₂ solution. The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The coating was mainly composed of a large amount of Al₂TiO₅. As the cathode pulse was increased, the amount and grain size of Al ₂TiO₅ were first increased, and then decreased. γ-Al₂O₃ in the coating was gradually decreased to nothing with the increase in the cathode pulse whereas rutile TiO₂ began to form in the coating. As opposed to the single-polar anode pulse mode, the cathode pulse reduced the thickness of the coatings. However, as the cathode pulse intensity continued to increase, the coating then became thicker regardless of cathode current density or pulse width. In addition, the residual discharging channels were reduced and the density of the coating was increased with the appropriate increase of the cathode pulse.