GROWTH KINETICS, PHASE-STRUCTURAL STATE AND HARDNESS OF MICRO-ARC OXIDE COATINGS ON CAST ALUMINUM ALLOYS AL2, AL9, AL25

Abstract

Purpose. The aim of the research was to study the effect of electrolysis conditions during microarc oxidation in an alkaline silicate electrolyte on the regularities of the growth kinetics, structural-phase state, and hardness of MAO coatings on casting alloys of different compositions. Methodology. To achieve the goal of the research, microarc oxidation techniques were used with different compositions of alkali silicate electrolytes and at different current densities. The study of the formed surface was carried out by the method of scanning electron microscopy, the phase-structural analysis was carried out by the X-ray diffraction method, the data of which were compared with the results of measuring the microhardness. Findings. The possibility of forming high-density MAO coatings on cast alloys with a silicon content of 9…13 % has been determined. The regularities of the kinetics of the growth of MAO coatings on casting alloys AL2, AL9, and AL25 are revealed depending on the current density and the composition of the electrolyte. It is shown that the phase composition of MAO coatings on casting alloys includes: γ-А12О3, α-А12О3 and the mullite phase (3Al2O3 • 2SiO2). The initial stage of coating growth is characterized by the predominant formation of γ-Al2O3 and 3Al2O3 • 2SiO2 phases. With an increase in the duration of oxidation, the formation of the α- Al2O3 phase occurs, the content of which does not exceed 20 % at the longest (120 min) oxidation duration. The hardness of the coatings increases with an increase in the content of α-Al2O3 and reaches 17 000 MPa at the highest content (18 %). Originality. It was found that for a process duration of more than 60 minutes, the kinetic dependence of the change in the coating thickness on the time of the MAO process is close to linear with a growth rate of about 0,5 μm/min. This growth rate is more than two times lower than the growth rate of coatings on deformed aluminum alloys (for example, D16 and AMg6) under similar oxidation conditions. With a formation time of less than 60 minutes, the growth rate depends on both the alloy type and the electrolyte composition. The highest growth rate of 0.7 μm/min is achieved on the AL25 alloy in electrolysis 2 g/L KOH + 12 g/L Na2SiO3. A change in the lattice period of the γ-Al2O3 phase with an increase in the oxidation duration was revealed, from values less than the tabulated values (0,790 nm) with an oxidation time of less than 60 minutes to a value significantly higher than the tabulated value with a long oxidation time (more than 120 min). A model is proposed in accordance with which a decrease in the lattice period is associated with the replacement of Al ions by Si, and an increase in the period is determined by the different valences of the replacing ions and the formation of additional vacancies in this connection. Practical value. The study has shown the possibility of forming a high-hardness (HV ≈ 17 000 MPa) coating on a cast aluminum alloy, which is a condition for ensuring high wear resistance in most types of wear.