Analysis of the current density and electrolyte composition influence on the quality of micro-arc coatings

The results of an experimental study of oxide coatings obtained using an automated system developed by the authors in the anode-cathode mode at a ratio of the anode current to the cathode current equal to 1, at low current (0.125 A or 0.25 A) with different electrolyte composition are presented. An analysis of the dependence of the quality of the obtained coatings on the molding curves, which characterize the change in the electrical voltage on the samples depending on the time of their processing was made. It has been established that with increasing current density, the process is more intense, microdischarges begin to burn earlier, and, in general, the coating quality increases. However, at too high current densities (50 A/dm2), it is undesirable to carry out micro-arc oxidation treatment due to an increase in the coating roughness. In particular, it is expedient to use the results obtained for quality control of the technological mode under production conditions: the coating has inadequate quality if the molding curve has a distorted shape. The results obtained have been tested in the implementation of the method of formation by microarc oxidation of oxide coatings with specified properties.


Introduction
Varying the technological modes of the microarc oxidation method makes it possible to achieve the required indicators of wear resistance, microhardness and corrosion resistance of coatings [1][2][3][4].In works [5][6][7], methods for obtaining coatings with a given porosity are considered, which allows them to be used in medical devices due to high biocompatibility.Coatings on instrument-making products and the aerospace industry are subject to the requirements of high temperature resistance [9,10], resistance to radiation exposure, which is considered in [11].However, the considered oxidation method is characterized by heterogeneous factors that collectively affect the quality parameters of coatings [12], which indicates the need for their analytical description in order to establish optimal control technological effects.In addition, the imperfection of technological equipment and means of measuring the parameters of coatings in real time limit the widespread industrial introduction of microarc oxidation [13,14].This article presents the results of an analysis of the current density and electrolyte composition effect, which made it possible to develop criteria for assessing the coatings quality by analyzing the electrical characteristics of the micro-arc oxidation method.

Materials and experimental technique
The purpose of the experiment is to identify and formalize the relationship between the current density and the composition of the electrolyte: • determination of the CN/CK concentrations (where CN and CK are the concentrations of Na2SiO3 and NaOH, respectively) ratio, at which the micro-arc oxidation (MAO) process is fundamentally possible; • selection of electrolyte composition for the stage of experimental studies; • development of criteria for assessing the coatings quality by analyzing the electrical characteristics of the MAO process.Pieces of aluminum wire with a diameter of 1.8 mm were used as samples.During the experiment, the lowest value of the electric current was established, at which a micro-arc process is possible and a high-quality coating for various electrolyte compositions is obtained.The MAO process possibility was assessed visually according to the following criteria: • the microdischarges appearance on the sample surface; • coating formation.
Since the experiment is preliminary, the coating quality was also determined visually according to the following criteria: • lack of metallic luster; • the absence (or minimum amount) of plaque on the sample surface; • color uniformity; • surface smoothness.
Coatings were obtained using the developed automated system [15] (Figure 1) in the anode-cathode mode when the anode and cathode currents are equal, at a low current (0.125 A or 0.25 A).Different current densities were obtained by varying the sample surface area (wire length).The processing time was 500 seconds or less (if there was a significant drop in the forming stress).For each sample, the forming curve was measured.The experimental conditions for some samples, including the concentrations of NaOH (CK) and Na2SiO3 (CN) are presented in Table 1.

The discussion of the results
Visual inspection of the samples allows to conclude that not under any conditions of the MAO process, high-quality coatings are obtained.An analysis of the molding curves (Figure 2) of the MAO process made it possible to identify the following reasons for the formation of low-quality MAO coatings: • the coating destruction by powerful arc discharges (curve 2, sample n25k0.5j50); • low growth rate of the forming stress at the beginning of the process and fast transition to the stage of arc discharges (curve 3, sample n25k0.5j25);• low forming stress (curves from 4 to 6, samples n25k0.5j20,n30k0.5j10,n40k0.5j05).A high-quality MAO coating was obtained, for example, on a sample n50k0.5j10or n30k0.5j20,which corresponds to forming curve 1 (Figure 2).The foregoing makes it possible to establish a criterion for the quality of MAO coatings, which can be used for express assessment of the technological mode suitability under production conditions: the coating will be of inadequate quality if the forming curve has a distorted shape.In addition, there is an increase in the roughness of the coating in the flesh before the formation of "teeth" during oxidation at high current densities.This is due to the presence of high plasma discharge power, and is also associated with the intensification of coating dissolution processes in the cathode half-life.The forming curve in this case has an oscillatory character at the stage of micro-arc discharges (Figure 3).The forming curves of the samples, on which a high-quality MAO coating is formed, are approximated by piecewise linear functions of the following form: , where the coefficients U1-U3 describe the initial displacement, v1-v3 are the angular coefficients of the forming curves during anodizing, discharges at the stages of sparking and discharges; tan, tis, tmr are the moments of time corresponding to the beginnings of these stages; tdr is the moment of time corresponding to the beginning of the arc stage or the moment of the measurement end of the forming curve.The coefficients of the approximating piecewise linear functions for the test sample n30k0.5j50are shown in Table 2.The forming curves of samples are obtained in the same electrolyte at different current densities (Figure 4).The forming curves of samples are obtained in the same electrolyte at different current densities (Figure 4).Forming curves of the MAO process in an electrolyte composition of 0.5 g/l NaOH and 30 g/l Na2SiO3 and electric current density: curve 1 corresponds to 15A/dm 2 ; curve 2 corresponds to 20 A/dm 2 ; curve 3 corresponds to 25 A/dm 2 ; curve 4 corresponds to 50 A/dm 2 .
The rate of the moulding voltage rise at the anodization stage, which is described by the slope of the initial segment of the forming curve, increases with increasing current density (coefficients v1 in the table 2).The only exception is sample n30k0.5j50,however, the moulding voltage at the beginning of the forming curve measurement (processing time 10 seconds) for it is already about 300 V, which indicates a higher rise rate than for other samples.Thus, with an increase in the current density, the process proceeds more intensively, the microdischarges begin to burn earlier, and, in general, the coating quality increases.Due to the increase in the roughness of the coating at high current densities (50 A / dm2), it is undesirable to carry out MAO treatment.
Forming curves for samples obtained at two current densities in electrolytes with different Na2SiO3 concentrations are shown in Figures 5 and 6.  Figure 5 shows that the forming curves for a higher electrolyte concentration are located to the left than the forming curves for a low concentration, although their slopes are quite close.It may indicate that not so much the angular coefficient as the initial stress depends on the sodium silicate concentration.

Conclusion
Analysis of the results of experimental studies of aluminum samples allowed us to formulate criteria for assessing the quality of oxide coatings by analyzing the electrical characteristics of the MAO process and to establish the causes of the formation of low-quality coatings.It is shown that the criterion for the deterioration of the quality of coatings is a change in the graph of the molding curve, which in this case deviates from the piecewise linear form.
It is shown that an increase in the density of the process current leads to a more intensive process of micro-arc oxidation, microcharges begin to burn earlier, and, in general, the quality of the coating increases.However, when the current density increases above 50 A/dm 2 , the roughness of the coating surface increases.
It has been established that the forming curves for a higher electrolyte concentration are located to the left than the forming curves for a low concentration, although their slopes are quite close.It may indicate that not so much the angular coefficient as the initial stress depends on the sodium silicate concentration.
The obtained results were tested in the technological process of microarc oxidation when obtaining a protective coating with the specified properties.

Figure 4 .
Figure 4. Forming curves of the MAO process in an electrolyte composition of 0.5 g/l NaOH and 30 g/l Na2SiO3 and electric current density: curve 1 corresponds to 15A/dm 2 ; curve 2 corresponds to 20 A/dm 2 ; curve 3 corresponds to 25 A/dm 2 ; curve 4 corresponds to 50 A/dm 2 .

Table 2 .
Coefficients of approximating piecewise linear functions.