Abstract
The effect of electrolysis conditions with different electrolyte compositions on the growth kinetics, phase-structural state, and hardness of coatings obtained by microarc oxidation (MAO) on the D16 aluminum alloy (base – aluminum, main impurity Cu) was studied. An analysis of the results obtained showed that the choice of the type of electrolyte and the conditions for the MAO process makes it possible to vary the growth kinetics and phase-structural state of the coating on the D16 aluminum alloy within a wide range. For all types of electrolytes, with an increase in the content of KOH, Na 2 SiO 3 , or KOH+Na 2 SiO 3 , the growth rate of MAO coatings increases. It was found that in MAO coatings obtained in an alkaline (KOH) electrolyte, a two-phase (γ−Al 2 O 3 and α−Al 2 O 3 phases) crystalline state is formed. An increase in the KOH concentration leads to an increase in the relative content of the α–Al 2 O 3 phase (corundum). During the formation in a silicate electrolyte, the phase composition of MAO coatings with an increase in the content of liquid glass (Na 2 SiO 3 ) changes from a mixture of the γ−Al 2 O 3 phase and mullite (3Al 2 O 3 ∙2SіO 2 ) to an X-ray amorphous phase. The use of a complex electrolyte leads to a two-phase state of the coating with a large (compared to an alkaline electrolyte) shift of the γ−Al 2 O 3 →α−Al 2 O 3 transformation towards the formation of the α−Al 2 O 3 phase. It was determined that the value of hardness correlates with the content of the α−Al 2 O 3 phase in the MAO coating, reaching the maximum value of 1620 kg/mm 2 at the highest content (about 80 vol. %) of the α−Al 2 O 3 phase. Two types of dependences of the coating thickness on the amount of electricity passed were revealed. For the amount of passed electricity 10–50 A-h/dm 2 , the thickness dependence is determined as 4.2 μm/(A-h/dm 2 ), which suggests the basic mechanism of electrochemical oxidation during the formation of a coating. For the amount of electricity transmitted 50–120 A-hour/dm 2 , the thickness dependence is determined by a much smaller value of 1.1 μm/(A-hour/dm 2 ). This suggests a transition to a different mechanism of coating formation − the formation of a coating with the participation of electrolysis components
Highlights
Coatings are the basis of most modern technologies in mechanical engineering, aircraft construction and medical technology [1, 2]
As follows from [41], such regularities should be based on the influence of the type and composition of electrolytes and treatment modes on the kinetics of microarc oxidation (MAO) coatings growth, the quantitative ratio of different phases and their structural states in them
The aim of the study was to examine the effect of the type (KOH, Na2SiO3 and KOH+Na2SiO3 solutions) and composition of the electrolyte on the growth kinetics of MAO coatings on the D16 alloy, the formation of their phase-structural state, and the effect of the phase composition on hardness
Summary
Coatings are the basis of most modern technologies in mechanical engineering, aircraft construction and medical technology [1, 2]. Materials science (Al, Ti, Mg, Nb, Zr, etc.), which transforms surface layers into oxide ceramic coatings [13, 14]. This process was originally called plasma electrolytic oxidation (PEO) [15, 16], and somewhat later – anodic spark deposition (ASD) [17]. As a result of the MAO process, coatings with a thickness of 100–300 μm are formed on the surface with a unique combination of physical and mechanical properties (high hardness, wear resistance, heat resistance, heat strength, corrosion resistance, etc.) [20, 21]. At present, the industrial implementation of this technology is very promising, and the development of the method of structural engineering to achieve the required properties in coatings of this type is an urgent and demanded task
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