OF SOLID STATE Investigation of the influence of electrolyte composition on the structure and properties of coatings obtained by microarc oxidation

Aluminum alloys, along with positive properties - low density, high specific strength, electrical conductivity, ductility, viscosity and others, have disadvantages: low hardness, modulus of elasticity, wear resistance and high chemical activity in many inorganic acids. The method of microarc oxidation (MAO) can eliminate these shortcomings. The transformation of the surface layers of the workpiece into high-temperature oxides of aluminum will strengthen the surface and increase its protective properties. The properties of MAO coatings depend on many factors, one of which is the composition of the electrolyte. According to the literature, the most widely used alkaline-silicate electrolytes, which have shown the greatest efficiency in microarc oxidation of aluminum alloys. The data available in the literature on the use of multicomponent electrolytes containing sodium aluminate NaAlO 2 and sodium hexametaphosphate Na 6 Р 6 O 18 do not allow to elucidate their role in the formation of MAO coatings. Research of the use of hexametaphosphate to alkaline silicate electrolyte shown that Na 6 Р 6 O 18 promotes the formation of a thicker coating. The rate of thickness formation in the absence of hexametaphosphate is 0,5 ÷ 0,7 μm/min, and at a hexametaphosphate content of 10 g/l - 0.9 ÷ 1.1 μm/min. As for the effect on the phase composition, the effect was not detected. It was investigated that the addition of aluminate-silicate electrolytes of sodium aluminate in the amount of up to 13 g/l does not have a significant effect on the thickness of the coating, but affects the phase composition of the coating.


Introduction
Aluminum alloys, along with positive propertieslow density, high specific strength, electrical conductivity, ductility, viscosity and others, have disadvantages: low hardness, modulus of elasticity, wear resistance and high chemical activity in many inorganic acids [1].
The method of microarc oxidation (MAO) can eliminate these shortcomings. The transformation of the surface layers of the machined aluminum part into hightemperature aluminum oxides will strengthen the surface and increase its protective properties [2][3].
MAO is an electrochemical process that is carried out in electrolytes of different composition and with different electrical parameters of formation [3]. The properties of MAO coatings depend on many factors, one of which is the composition of the electrolyte. According to the literature [4][5][6], the most widely used alkaline silicate electrolytes containing (1 ÷ 3) g/l KOH + (3 ÷ 12) g/l Na2SiO3 (liquid glass), which showed the greatest efficiency in microarc oxidation of aluminum alloys.
Attempts have been made to use multicomponent electrolytes containing sodium aluminate NaAlO2 [7] and sodium hexametaphosphate Na6Р6O18 [8]. The data available in the literature do not allow us to determine the role of these soluble inorganic compounds in the formation of MAO coatings.

I. Goal
The goal of this work is to study the influence of electrolyte composition on the kinetics of coating thickness formation, their phase composition and hardness.

II. Experimental methods
The study was carried out on samples of AV alloys. The chemical composition of the alloys is given in table 1.
Microarc oxidation was carried out in the anodiccathodic mode on an installation with a capacitor-type power supply. The duration of treatment was 1 hour at a current density of 20 A/dm 2 .
The phase composition of the coating was determined from the diffractograms obtained on a DRON-3 diffractometer in Кα-Cu radiation. The survey was carried out in a point-by-point mode with a step of 2θ = 0.1°. The quantitative content of the phases was determined by the method of quantitative X-ray analysis using a previously constructed calibration curve according to the data of standard mixtures.
The microhardness was determined using a PMT-3 device. The coating thickness was determined using a VT-10 NTs.

III. Results and Discussion
The formation of MAO coatings leads to the formation of a developed surface relief, which has a significant roughness. The relief and surface roughness are approximately the same for all coatings with a thickness of ⁓ 50 μm studied. Typical surface morphology is shown in Fig.1. The study of transverse sections of coated samples (Fig. 2) revealed a layered structure of coatings -the technological layer and the main. The basic layer which is monolithic, firm and wearproof has practical value. Technological layer -highly porous, with low hardness, used as corrosion and electrical protection, as well as a sublayer when applying additional protective and decorative coatings. When high hardness and wear resistance are required from the surface, then the technological layer is removed. The results below (hardness and phase composition) refer to the base layer.
The process of microarc oxidation was performed in alkaline-silicate electrolyte (base electrolyte) and with the addition of NaAlO2 (sodium aluminate) and Na6Р6O18 (sodium hexametaphosphate).
The results of the study of coatings formed in the base electrolyte are given in table. 2 and in Fig. 3 The kinetics of formation of the coating thickness and their microhardness are shown in Fig.3 As can be seen from the above data, increasing the concentration of Na2SiO3 increases the thickness of the coating, but the hardness of the main coating layer decreases (Fig. 3). This result can be explained by the change in the phase composition of the coating.
Thus, the results of deciphering the diffraction patterns of the base layer coatings showed that the diffraction maxima belong to the phases γ-Al2O3 and mullite (3Al2O3 2SіO2) (Fig. 4), the phase α-Al2O3 (corundum) is absent. The presence of clear diffraction peaks on the diffraction pattern indicates the crystal structure of the main coating layer. A clear texture of the detected phases was not detected, which indicates the chaotic orientation of the crystals of the main coating layer.
The formation of the mullite phase 3Al2O32SiO2 (Fig. 5), which has a relatively low hardness, and its increase to 85% reduces the hardness of the coating. However, certain adjustments must be made to the porosity of the coatings.   The obtained data indicate that the phase formation of the coating in the alkaline-silicate electrolyte occurs from the formation of the γ-Al2O3 phase. In the process of increasing the coating and increasing the power of micro-discharges, the γ-Al2O3 phase interacts with the electrolyte components. When Na2SiO3 is dissolved in water, hydrolysis occurs by the reaction: Na2SіO3 + Н2О → 2 NaОН + SіO2. The existing phase of γ-Al2O3 interacts with SіO2 with the formation of the mullite phase: 3Al2O3 + 2SіO2 = 3Al2O3 2SіO2.
Thus, the increase in the electrolyte of liquid glass (Na2SiO3) promotes the formation of mullite, which does not provide the formation of a coating with high hardness. According to the obtained results, the content of liquid glass should be 3 -6 g/l (see Fig. 3), which will ensure maximum hardness of the coating.
The study of the influence of the content of sodium hexametaphosphate Na6Р6O18 was carried out in the basic electrolytes of the composition: 1 g/l КОН + 3 g/l Na2SіO3, 1 g/l КОН + 6 g/l Na2SіO3, 1 g/l КОН + 12 g/l Na2SіO3, to which was added Na6Р6O18 in an amount of 1 to 10 g/l.
The results of the study of thickness and phase composition are shown in table 3 and in Fig. 6. It was investigated that the addition of sodium hexametaphosphate in the amount of up to 2 g/l to the base electrolytes affects the phase formation, at a content of ˃ 2 g/l the phase composition practically does not change. The increase in the thickness of the coating with increasing content of Na6Р6O18 can be explained by the participation of the components of the additive in the formation of the oxide coating.
The detected dependences are equally manifested for different composition of the basic alkaline-silicate electrolyte.
The results of the study of the effect of sodium aluminate on the characteristics of the coating are given in table. 4 and in fig. 7.

Table 3
The results of the research of the Na6Р6O18 effect on the properties of coatings. a -base composition of 1 g/l KOH + 3 g/l Na2SіO3, b -base composition of 1 g/l KOH + 6 g/l Na2SіO3, c -base composition of 1 g/l KOH + 12 g/l Na2SіO3 Fig. 6. The effect of sodium hexametaphosphate on the thickness and phase composition of MAO coatings (Cm -content of 3Al2O32SiO, Cγcontent of γ-Al2O3).
a -base composition of 1 g/l KOH + 3 g/l Na2SіO3, b -base composition of 1 g/l KOH + 6 g/l Na2SіO3, c -base composition of 1 g/l KOH + 9 g/l Na2SіO3, d -base composition of 1 g/l KOH + 12 g/l Na2SіO3 Fig. 7. The effect of sodium aluminate on the characteristics of MAO coatings.
The research of the effect of NaAlO2 on the formation of coatings revealed (see table. 4): Addition of alkali-silicate electrolyte NaAlO2 has virtually no effect on the thickness of the coating; NaAlO2 affects the phase composition differently depending on the composition of Na2SіO3 in the base electrolyte: in electrolytes 1 g/l KOH + 3 g/l Na2SіO3, NaAlO2 stimulates the formation of the phase α -Al2O3, mullite is not formed (samples 1A -13A); in electrolytes 1 g/l KOH + 6 g/l Na2SіO3 addition of NaAlO2 provides the formation of the α-Al2O3 phase (⁓ 10%), Na2SіO3 promotes the formation of mullite (samples 14A -18A); in electrolytes 1 g/l KOH + 9 g/l Na2SіO3 and 1 g/l KOH + 12g/l Na2SіO3 no effect of NaAlO2 addition on the phase composition of the coating was detected (samples 19A -23A and 24A -28A).
Thus, the effect of adding sodium aluminate depends on the composition of Na2SіO3 in the alkaline-silicate electrolyte. To ensure maximum hardness, it is recommended to use an electrolyte of 1 g/l KOH + (3 ÷ 6) g/l Na2SіO3 with the addition of ⁓ 4 g/l NaAlO2.

Conclusions
1. It is determined that the phase formation of the coating in the alkaline-silicate electrolyte occurs from the formation of the γ-γ-Al2O3 phase. The increase in the electrolyte of liquid glass (Na2SiO3) promotes the formation of mullite, which does not provide the formation of a coating with high hardness.
2. It is shown that hexametaphosphate promotes the formation of a thicker coating, the addition of sodium hexametaphosphate in the amount of ⁓ 2 g/l to the base electrolytes promotes the formation of mullite. At a content of 2 to 10 g/l the process of mullite formation is stabilized; 3. It was investigated that the addition of sodium aluminate to the base electrolytes in the amount of up to 13 g/l does not have a significant effect on the thickness of the coating. The effect of NaAlO2 in different ways affects the phase composition of the coating -it depends on the composition of the alkaline-silicate electrolyte.