PEO processing of AZ91Nd/Al2O3 MMC-the role of alumina fibers

This work reports the influence of alumina fiber reinforcement of an AZ91Nd MMC (metal matrix composite) on the PEO coating formation process in a sodium phosphate-based electrolyte. By comparison with the pure AZ91Nd, the evolution of alumina fiber during the processing and the characteristics of the resultant PEO coating were investigated. The voltage response as a function of processing time was changed. Lower voltage in the presence of alumina fiber is responsible for the lower coating thickness. The morphology and phase composition of the coatings are also influenced by the incorporation of the fiber. Firstly, the fiber is embedded in the coating and interrupts the continuity of the coating. With increasing processing time, the fiber is found to be reactively incorporated in the coating. The intention to produce a MgAl2O4 containing coating is achieved and it is mainly accumulated near the coating surface. However, due to the low number of fibers, the Al content is overall still low and only near to the fibers the MgAl2O4 spinel phase can form.