An investigation of ceramic coating growth mechanisms in plasma electrolytic oxidation (PEO) processing

To further our understanding of the plasma electrolytic oxidation (PEO) process, and to aid in the optimization of the process, it is important to identify the mechanisms of coating formation. In the present work, coatings up to 110μm thick were produced on an AJ62 Mg-alloy substrate using the PEO process. Optical emission spectroscopy (OES) was employed to follow the microdischarges and substrate and electrolyte elements present in the plasma discharge during the coating growth, and to determine plasma electron temperatures. During PEO processing of magnesium, some of the metal cations are transferred outwards from the substrate and react with anions to form ceramic coatings. Also, due to the high electric field in the discharge channels, oxygen anions transfer toward the magnesium substrate and react with Mg2+ cations to form a ceramic coating. In PEO process, the ceramic coating grows inwards to the alloy substrate and outwards to the coating surface simultaneously. The total coating thickness variation compared with the geometrical dimensions of the uncoated and coated samples were investigated. For the coating growth, there are three simultaneous processes taking place, namely the electrochemical reactions, the plasma chemical reactions and thermal diffusion. Oxygen diffusion occurring during PEO processing is discussed in terms of coating growth mechanisms.