Micro-structures and growth mechanisms of plasma electrolytic oxidation coatings on aluminium at different current densities

In this study, plasma electrolytic oxidation (PEO) coatings were produced on pure aluminum in an in-house developed composite electrolyte at three current densities of 2.2 A/dm2, 4.4 A/dm2 and 8.8 A/dm2, respectively. The coatings were investigated based on the observation and measurement of the working voltage, surface microdischarges and morphologies of surface, polished cross-section, fracture cross-section and inner surface of the coatings, by using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), data acquisition module, single-lens reflex camera. Especially, an in-house developed electrochemical detachment technique of the coatings was employed to observe some details of the internal micro-structure and coating/substrate interface morphology of the coatings. It was noted that the results from the above investigations are much related to the current densities going through the PEO coatings and the different growth mechanisms of coatings at low and high current densities were proposed and discussed.