The effects of Na2WO4 concentration on the properties of microarc oxidation coatings on aluminum alloy

Alumina coating, approximately 30 μm, was deposited on an LY12 Al alloy substrate using a microarc oxidation (MAO) process in a H3BO3-KOH electrolyte solution with the Na2WO4 addition varying from 0 to 6 g/l. The MAO process was studied by measuring the voltage as a function of time. The coating layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with respect to the phases and microstructures and by measuring microhardness and wear resistance. The results show that the concentration of Na2WO4 has direct effects on the behavior of MAO process and the quality of the MAO coatings as well. The final phases in the coating were found to be α-Al2O3 and both γ-Al2O3 and a small amount of W. Without an addition of Na2WO4, the MAO coating process could not successfully proceed. With increasing the Na2WO4 concentration in the electrolyte, the working voltage at the microarc discharge stage decreased, the thickness and the content of α-Al2O3 phase in the coating both reduced. The microhardness and the wear resistance were both enhanced as the content of α-Al2O3 phase increased.