A Study of the Phase-Structural Engineering Possibilities of Coatings on D16 Alloy During Micro-Arc Oxidation in Electrolytes of Different Types

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