Role of Al18B4O33 Whisker in MAO Process of Mg Matrix Composite and Protective Properties of the Oxidation Coating

Selective growth of oxidation coating was observed on Mg matrix composite Al18B4O33w/AZ91 (a composite with Al18B4O33 crystal whisker as reinforced phase) when this composite was treated by microarc oxidation (MAO) technique, and then the role of Al18B4O33 whisker in the process of MAO was analyzed. The protective properties of MAO coating also were investigated. Scanning electron microscopy (SEM) was used to characterize the existing state of Al18B4O33 whisker in MAO process and the microstructure of MAO coating. Corrosion resistance of the bare and coated composite was evaluated by immersion corrosion test and potentiodynamic polarizing test. Wear resistance of MAO coating was investigated by a ball-on-disc friction and wear tester. The results showed that sparking discharge did not occur on Al18B4O33 whisker and the whisker existed in the coating as a heterogeneous phase when MAO coating grew on the composite; then the whisker would be covered gradually with growing thick of the coating. Corrosion current density of the coated composite was decreased by 4 orders of magnitude compared with that of the uncoated composite; excellent corrosion resistance was closely related to the compact whisker-coating interface since Al18B4O33 whisker did not induce structural defects. The coating also exhibited high wear resistance and a slight adhesive wear tendency with bearing steel as its counterpart material.