Pore formation and pore inter-connectivity in plasma electrolytic oxidation coatings on aluminium alloy

The porosity and microstructure of plasma electrolytic oxidation (PEO) coatings are key factors in determining their properties and applications. Despite advances in understanding the PEO process, the mechanisms driving pore formation and their correlation with process parameters remain unclear due to the complex interplay between these variables. This study investigates the effects of treatment time and duty cycle on the microstructure of PEO coatings produced on an aluminium alloy in an alkaline electrolyte, with a particular focus on pore formation. Our findings reveal that longer treatment durations lead to the significant development of sub-surface pores at the interface between the inner and outer layers. Additionally, a lower duty cycle leads to an increase in sub-surface pores, while a higher duty cycle favours the formation of surface pores. Morphological, 3D microstructural mapping, and chemical analyses reveal that pore formation is driven by the micro-discharges, gas generation, and the preferred gas escape path within the micro-melt pools formed during the PEO formation process. The preferred gas escape path is closely linked to the characteristics and lifetime of local micro-melt pools, elucidating the mechanisms behind pore formation.