Evaluation of plasma temperature during plasma oxidation processing of AZ91 Mg alloy through analysis of the melting behavior of incorporated particles

Plasma electrolytic oxidation (PEO) process was used to incorporate TiO2 and ZrO2 particles into a magnesium oxide layer. Whereas incorporated ZrO2 particles were clearly observed in the PEO-treated oxide layer, TiO2 particles were scarce in the oxide layer because the temperature of the plasma was between the melting temperature of the TiO2 particles and that of the ZrO2 particles. XRD patterns, TEM images, and SAED patterns also provided strong evidence that the TiO2 particles were melted or transformed, which generated a new ternary oxide phase, MgxTiyO. Because cubic ZrO2 was not observed and could possibly exist at a temperature greater than 2643 K, which was the temperature for the phase transformation to the cubic structure, the temperature of the oxide melted by plasma was estimated to be in the range of 2116–2643 K.