Possibilities of phase-structural engineering and properties of microarc oxide coatings on the AMг3 alloy

Abstract

Purposeof the work: to establish the regularities of the alkali-silicate electrolyte composition influence and the conditions of electrolysis during microarc oxidation of the AMг3 aluminum alloy on the kinetics of the oxide coating formation, its structure and the conditions for the formation of the α-Al2O3 phase. Results. The possibility of forming high-density MAO coatings on the AMг3 aluminium alloy during electrolysis in alkali-silicate electrolytes has been  determined. The regularities of the kinetics of MAO coatings growth at different electrolyte composition and oxidation time were revealed. It was found that the phase composition of MAO coatings consists of the following phases − α-Al2O3, γ-Al2O3, and mullite (3Al2O3•2SiO2), the ratio of which changes with a change in the amount of sodium silicate (Na2SiO3). It was found that addition of an inorganic soluble salt dopant to the electrolyte, which contains oxygen (K2Cr2O7), leads to a qualitative change in the phase composition − an increase in the amount of
α-Al2O3 phase in the coating composition and an increase in hardness. In this case, it becomes possible to obtain MAO coatings of small thickness (up to 90 μm) with a high content of the α-Al2O3 phase (up to 40 %), which cannot be carried out in an alkali-silicate electrolyte. Analysis of the effect of high-temperature annealing of MAO coatings on the shape of the diffraction spectrum made it possible to revealthe appearance of tetragonality in the crystal lattice of the
γ-А12О3 phase. The structural state with a tetragonally distorted lattice is a stage of γ-А12О3 → α-А12О3 transformation. Scientific novelty. It was found that when using an alkaline-silicate electrolyte for electrolysis, the addition of liquid glass (Na2SiO3) leads to an increase in the growth rate of the coating, but at the same time it stimulates the formation of mullite (3Al2O3 × 2SiO2) as a phase component. An increase in the alkaline (KOH) component leads to a decrease in the growth rate (to 0,6…0,7 μm/min) and stimulates the formation of the γ-Al2O3 phase. The formation of the α-Al2O3 (corundum) phase is stimulated with a long process duration, when the thickness of the dielectric layer and the breakdown power increase. An increase in the amount of α-Al2O3 leads to an increase in the hardness of the coatings. Isothermal annealing at temperatures exceeding 1 000 °C stimulates the γ-Al2O3 → α-Al2O3 polymorphic transformation. The initial stage of such a transformation is the appearance of tetragonality in the crystal lattice of the γ-Al2O3 phase. Practical value. This study allows us to propose an additive to the alkali-silicate electrolyte in the form of a K2Cr2O7 salt, which increases the rate of formation of MAO coatings to 1,4 μm/min and, at the same time, qualitatively and quantitatively changes the phase composition of the coatings, increasing its hardness.