Study Of Preparation And Performance Of Ceramic Coatings Formed By Micro-arc Oxidation On Aluminum Alloys

In this paper, a comprehensive introduction of micro-arc oxidation, including its mechanism, influence factors and preparation methods was described. By means of the micro-arc oxidation (MAO) technique, ceramic coatings with high micro-hardness, strong adhesion, high strength and high wear resistance can be fabricated.The water solution composed of H3BO3, KOH and Na2WO4 was used as the electrolyte for MAO derived coatings on aluminum alloy substrates. The electrolyte composition was optimized by means of orthogonal design from both technical and economic points of view, and the stability was evaluated. The MAO process was studied by measuring the voltage as a function to time. The coatings fabricated in different technical parameters were characterized by XRD and SEM with respect to the phases and microstructures, and by testing micro-hardness and the wear resistance with respect to mechanical properties. Final, the samples were compared with hard anodic oxidation and others fabricated in other solutions.The results showed that electrolyte containing H3BO3 10 g/1, KOH 2 g/1 and Na2WO4 2 g/1 was the most optimal in terms of the hardness and thickness measurements. This solution was more stable than Na2SiO3 solution. The ceramic coating in this electrolyte mainly consisted of -Al2O3 phase, -A12O3 phase and a small amount of W. The concentration of Na2WO4 had direct effects on the behavior of MAO process and the quality of the MAO coatings as well. With decreasing Na2WO4 concentration in the electrolyte, the number of the discharge centers on the aluminum alloys surface decreased, the working voltage at the micro-arc discharge stage increased, the thickness and the content of -Al2O3 phase in the coating both increased. The micro-hardness and the wear resistance were both enhanced as the content of -Al2O3 phase increased.The control of electrical parameters in MAO plays an important role in the coating growth rate and phase composition. The more energy density acts on theceramic coatings using positive half wave and stronger currents, faster the coatings grow and harder the coatings are.The different solutions influence the growth rate, surface morphology and phase composition of the coatings. But the porosity and corrosion-resistance are not influenced. The micro-arc oxidation is super to hard anodic oxidation in anti-wear and corrosion-resistance.