The growth behavior and performance of microarc oxidation coating on AZ91/Ti composite: Influence of Ti-reinforcement phase and electrolyte

Magnesium matrix composites with both high strength and ductility have been achieved by introducing pure Ti particles. However, the properties of the surfaces of the composites need to be improved by surface technology, such as micro-arc oxidation (MAO). In this study, we investigated the influence of the Ti-reinforcement phase on coating growth and evolution by subjecting both AZ91 alloy and AZ91/Ti composite to MAO treatment using silicate-based and phosphate-based electrolytes. Results revealed that the Ti-reinforcement phase influenced the MAO process, altering discharge behavior, and leading to a decreased cell voltage. The vigorous discharge of the Ti-reinforcement phase induced the formation of coating discharge channels, concurrently dissolving and oxidizing Ti-reinforcement to produce a composite ceramic coating with TiO2. The MAO coating on the AZ91/Ti composite exhibited a dark blue macromorphology and distinctive local micromorphological anomalies. In silicate electrolyte, a "volcano-like" localized morphology centered on the discharge channel emerged. In contrast, treatment in phosphate-based electrolyte resulted in a coating morphology similar to typical porous ceramic coatings, with visible radial discharge micropores at the reinforcement phase location. Compared to the AZ91 alloy, the coating on the AZ91/Ti composite exhibited lower thickness and higher porosity. MAO treatment reduced the self-corrosion current density of the AZ91/Ti surface by two orders of magnitude. The silicate coating demonstrated better corrosion resistance than the phosphate coating, attributed to its lower porosity. The formation mechanism of MAO coatings on AZ91/Ti composites in phosphate-based and silicate-based electrolytes was proposed.