Dynamic Evolution of MgO ₂ and Polymorphs in Plasma Electrolytic Oxidation Coatings on AZ91D: A Study of Phase Transformations and Elemental Distribution due to non-equilibrium phase generation

Plasma Electrolytic Oxidation (PEO) has achieved remarkable significance in the field of protective coating technology. Although non-equilibrium rapid cooling presents a challenge in predicting the properties of the coatings, it has rarely been studied. Therefore, Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Energy-dispersive X-ray Spectroscopy (EDS) was employed to explore the impact of thermodynamic non-equilibrium on the deposited coating. The XRD analysis of the coating unveiled for the first time the presence of rare and volatile MgO2 phases, polymorphs, and unidentified peaks resulting from non-equilibrium phase formation. Notably, semi-quantitative analysis of the cubic phases revealed a variation in MgO2 presence, ranging from 64.4% to 28.3%, with increased operation time. Rapid cooling facilitated the freezing of different coating phases with slightly different crystal structures, leading to polymorphism. Additionally, Point and Line Energy-dispersive Xray Spectroscopy (EDS) analysis indicated the atomic percentage of oxygen in six EDS points varied from 27.82% to 41.30%, indicating non-equilibrium nature of the coating. This study is the first to explore how non-equilibrium phase formation affects PEO coating properties, addressing a key gap in our understanding of coating behaviour.