Analyses of reinforcement phases during plasma electrolytic oxidation on magnesium matrix composites

A plasma electrolytic oxidation (PEO) process on SiCP/AZ31 and Al2O3-SiO2(sf)/AZ91 magnesium metal matrix composites (MMCs) in phosphate electrolyte was carried out to reveal the evolution of SiCP and Al2O3-SiO2(sf) reinforcement phases in relation to coating growth. Optical emission spectroscopy (OES) was used to identify the elements presented in the plasma discharge process and evaluate the plasma electron temperature. XRD, SEM, EDS and ICP-AES were employed to characterize the microstructure, composition and phase constituents of the PEO coatings and the dissolved quantity of MMC in the electrolyte. The results show that the elements from the reinforcement phases can be found in the OES spectrum, which indicates that the reinforcement phases are involved in the plasma discharge, and the average electron temperature is about 5000 K-7000 K during the plasma discharge. The microstructure and composition suggest that MgSiO3, Mg2SiO4 and MgAl2O4 oxide layers are formed at the reinforcement/coating interface due to the local high temperature of the plasma discharge. It is believed that most of the reinforcement phases are molten firstly and then react with oxygen and magnesium oxide to form other oxides. However a few reinforcement phases still remain in the coating close to the MMC/coating interface.