Stable Discharge Mechanism in Microarc Oxidation and Processing in Phosphate Electrolytes

Microarc oxidation (MAO) is a popular surface treatment process to generate oxide coatings with excellent mechanical properties on valve metals. As a plasma discharge technique, the discharge mechanism in MAO is different from that in the bipolar plate. Due to the alternating arcs and multiple electrolytes in MAO, it is difficult to control and optimize the coating properties. Based on the arcing mechanism and ion motion, the boundary conditions of no-arc discharge, alternating arc discharge, and continuous arc discharge are derived, and the relationship between the discharge current and breakdown current and sustained arc current is investigated. For the same electrolyte, the boundary conditions are determined by equivalent concentration and conductivity. The results show that in the stable alternating arc discharge regime, the higher the concentration and smaller the conductivity, the more intense is the discharge. According to the boundary conditions, MAO experiments are designed using phosphate electrolytes to improve the hardness of oxide coatings. By selecting electrolyte characteristics close to the continuous arc discharge boundary in the stable alternating arc discharge regime, the arc discharge is most intense and brightest and the largest energy is transferred to the Al₂O₃ coating. Consequently, the crystallinity and mechanical properties are improved significantly.