Effects of cathode pulse at low frequency on the structure and composition of plasma electrolytic oxidation ceramic coatings

The aim of this work is to investigate the effects of the cathode pulse under the low working frequency on the structure and the composition of the 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. The coating was mainly composed of a large amount of Al₂TiO₅ and a little α-Al₂O₃ and rutile TiO₂. Increasing the cathode pulse, the amount of rutile TiO₂ was increased while the amount of Al₂O₃ was decreased; and decreasing the cathode pulse, the amount of Al₂O₃ was increased while the amount of rutile TiO₂ was decreased. The thickness of the coatings was increased and then decreased with the increase of the cathode pulse. The grain sizes of Al₂TiO₅ were increased with the cathode current densities, but changed little with the cathode pulse width. The grain size of α-Al₂O₃ was decreased with the decrease of the cathode pulse, while the grain size of TiO₂ was increased with the increase of the cathode pulse. The proper cathode pulse was helpful to reduce the roughness and to increase the density of the coatings.