An Investigation of Inward Ceramic Coating Growth Mechanisms Under Bipolar Plasma Electrolytic Oxidation (PEO) Processing

ABSTRACT Plasma electrolytic oxidation (PEO) coatings commonly suffer from discharge‐induced porosity and cracking, which limit their protective performance. Inward coating growth has been reported as an effective route to improve coating density; however, the governing mechanism of this phenomenon remains unclear. In this study, the effect of bipolar current, with a constant negative current component and dynamically varied positive current density, on PEO coating growth behavior is systematically investigated on AZ91D magnesium alloy. Microstructural observations reveal the formation of dense inward‐grown regions that are free of discharge‐channel‐related porosity. Elemental mapping demonstrates selective inward incorporation of O and F, whereas larger anions such as phosphate remain confined to the outer layer. Voltage–time responses, combined with SEM, EDS, XRD, Raman, and FTIR analyses, indicate that inward growth cannot be explained solely by thermal diffusion or discharge‐channel transport. Instead, the results show that the negative current component actively promotes lattice‐mediated inward anion diffusion through electrostatic assistance in addition to thermal activation. We propose a revised coating growth mechanism, in which repeated cathodic polarization suppresses outward discharge activity and enables electrically assisted inward diffusion through the oxide lattice. These findings provide new insight into inward coating growth during bipolar PEO and demonstrate that negative current is a key parameter for tailoring dense, low‐porosity oxide coatings.