Abstract
Coatings with a thickness of about 100 μm were obtained by microarc oxidation of technically pure aluminum and aluminum alloyed with copper and zinc in an alkaline silicate electrolyte at a current density of ~20 A/dm2. The results of studying the surface morphology, phase composition, and hardness of MAO coatings are presented. The change parameters were the electrolyte composition and the concentration of alloying (Cu and Zn) elements. This study was carried out because the currently available data are not enough to understand the nature of the influence of the chemical composition of the aluminum alloy and electrolysis conditions (in particular, electrolyte composition) on the mechanism and kinetics of γ→α transformation. Without understanding this, a directed change in the structural state and properties of MAO coatings becomes impossible. As a result of the studies, it was found that during microarc oxidation of aluminum alloys in an alkaline electrolyte with the addition of liquid glass (Na2SiO3) of various concentrations, the strengthened layer consists of oxides α-A12O3, γ-A12O3 and mullite 3Al2O3·2SiO2. The data of x-ray diffraction analysis of the coatings indicate the crystal structure of the coatings. It was established that aluminum alloying with copper and zinc significantly affects the phase composition of the coating, changing the quantitative ratio of the phases in a nonlinear manner. The highest content of the α-A12O3 phase (up to 60 vol. %) is achieved by Cu doping. The highest hardness of MAO coatings is achieved using an electrolyte with a composition of 1 g/l KOH and 6 g/l Na2SiO3 in aluminum alloys with a copper content of more than 3 %, and zinc – 2–3 %. It is established that the mechanism of formation of the phase composition should be associated with stabilization and destabilization of the γ-A12O3 phase. Therefore, to achieve high hardness, it is necessary to choose those alloying elements that affect the destabilization of γ-A12O3, which ensures the formation of the α-A12O3 phase (corundum). In this regard, it was revealed that Cu2+ cations contribute to the destabilization of the γ-А12О3 phase, and Zn2+ cations lead to stabilization of the γ-А12О3 phase at a Zn content g3 %
Highlights
The surface condition of functional materials largely determines their properties [1, 2]
The analysis of the results shows that the mechanism of formation of the phase composition should be associated with stabilization and destabilization of the γ-A12O3 phase
A comparison with the results of hardness measurements (Fig. 4) shows that the achievement of a high hardness of microarc oxidation (MAO) coatings on aluminum alloys is associated with the need to ensure a high α-A12O3 phase content
Summary
The surface condition of functional materials largely determines their properties [1, 2]. One of the most promising methods in this direction is plasma oxidation [15, 16] The use of this method shows the greatest efficiency for obtaining highly hard protective coatings on valve materials (Al, Ti, Mg, Ta, Nb, Zr) [17, 18]. Such a process is called microarc oxidation (MAO) [19, 20]. There has been an increased interest to light metal elements (to a large extent this relates to Al and Ti) as structural elements of aerospace equipment and engines of various modifications In this regard, the techno logies that lead to an increase in the strength and hardness of these materials are relevant and demanded by the industry.
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