Spatial characteristics of discharge phenomena in plasma electrolytic oxidation of aluminium alloy

This paper reports the first results of microdischarge studies performed using real time imaging of the plasma electrolytic oxidation process. It is shown that small microdischarges with a cross-sectional area of approximately 0.02 mm2 dominate throughout the oxidation process. With the oxide coating growth, the percentage of large (area 0.37 mm2) and extra-large microdischarges (area >0.87 mm2) in the total microdischarge population increases from 0 to 8% and 4%, respectively, at the end point. At the same time the microdischarge current density falls from 50 to 18 kA m−2, due to the increase in total testpiece area exposed to microdischarges from 3 to 6%. The microdischarge contribution to coating growth is evaluated to be in the range of 8–10 nm per a microdischarge event.