Effect of ultrasonic on microstructure and growth characteristics of micro-arc oxidation ceramic coatings on 6061 aluminum alloy

•Ultrasonic was introduced into the MAO process for the first time.•A concept, breakdown resistance, was defined.•Ultrasonic action increased the thickness and micro-hardness of coatings.•Ultrasonic promoted the formation of α-Al2O3 instead of γ-Al2O3.•Micro discharges became uniform and MAO coatings were improved.

Non-uniform microstructure and low growth rate of ceramic coatings in the mid-to late period of micro-arc oxidation (MAO) are the problems that limit the development of micro-arc oxidation technology. In this paper, the MAO process on 6061 aluminum alloy was carried out under ultrasonic condition, and the effects of ultrasonic vibration on microstructure and growth characteristics of MAO coatings were investigated. The results showed that the ultrasonic action obviously promoted the formation of α-Al2O3 phase, reduced the number of large discharge pores and promoted the increase of the thickness, uniformity and hardness of the coatings. A concept, breakdown resistance, was defined to discuss the results conveniently.