Growth regularity of ceramic coating on magnesium alloy by plasma electrolytic oxidation

The voltage–time response and phenomenon occurred on the metal surface in plasma electrolytic oxidation (PEO) process were analyzed, there was a relationship between them. The PEO process was divided into three stages according to the voltage–time response and four stages by the phenomenon occurred on the metal surface, which was correlated to the breakdown and critical voltage. The electron current density associated with the sparking results in the transformation of the slope of voltage–time response. The film growth mechanism was also investigated. The experimental results indicate that the weight gain increases linearly. At the initial stage, the ceramic coating increases inwards to the alloy substrate and outwards to the coating surface simultaneously, subsequently mainly grows towards the alloy substrate and the density of ceramic coating increases gradually, which results in the decrease of the total thickness as PEO time increases.