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.