Funct. Mater. 2019; 26 (4): 752-758.
Increase of the α-Al2O3 phase content in MAO-coating by optimizing the composition of oxidated aluminum alloy
1National Technical University "Kharkiv Polytechnic Institute", 2 Kyrpychov Str., 61002 Kharkiv, Ukraine
2Al-Zaytoonah University, 594 Queen Alia Airport Str., 11733 Amman, Jordan
By the method of microarc oxidation of technically pure aluminum and aluminum doped with copper, vanadium and zinc (in an alkali silicate electrolyte at a current density of ~ 20 A/dm2), the resulting coating is about 100 μm thick. The nonmonotonic dependence of the phase composition and hardness on the doping level of aluminum (Al + Cu, Al + Zn, and Al + V systems) is revealed. It was established that the degree of influence of alloying elements on the transformation process of γ-Al2O3 -> α-Al2O is determined by their crystal-chemical characteristics (charge, ionic radius). Therefore, the mechanism of formation of the phase composition should be associated with the stabilization and destabilization of the γ-Al2O phase. The results indicate that Cu2+ cations contribute to destabilization of the γ-Al2O3 phase, while Zn2+ and V5+ cations stabilize the γ-Al2O3 phase (with a Zn and V content of more than 3 wt.%). A model is proposed for explaining the results obtained, which is based on the formation of the γ-Al2O phase in the initial period of time and the appearance of the γ-Al2O3 phase in subsequent periods (as a result of an increase in the power of microdischarges). It is shown that the highest hardness of coatings on aluminum alloys (about 16 GPa) is achieved with a copper content in the range of 3-4 wt.%.
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