Influence of micro-arc oxidation discharge sparks on the surface roughness of aluminum alloy coating

In order to research the changes of the surface roughness of aluminum alloy micro-arc oxidation (MAO) coating during the reaction process, different stages of the discharge sparks were investigated when the voltage increased during the oxidization process and the influences of the sparks on the surface roughness of the coating were also studied. The reaction process was divided into four stages; the shape, thickness, surface roughness and the elemental composition of the coating formed in these stages were analyzed. The reasons why the surface roughness of the coating changed were discussed on the basis of the spark status at each stage. The results are shown as follows: in the stage without the spark, the surface roughness of the coating was influenced greatly by the matrix; in the stage with the white small spark, a compact oxide coating formed and the surface roughness decreased gradually; in the stage with the yellow small spark, the growth rate of the coating increased and oxide gathered on the coating surface and formed a volcano-like bulge. For this reason, the surface roughness began to increase. When the MAO reaction moved into the orange big spark stage, more volcano-like bulges appeared. In addition, the surface roughness of the coating increased quickly.