Kinetics of the Microarc Oxidation Coating Growth in the No-Bath Process

Abstract

The authors consider theoretical foundations of the coating formation when using the no-bath method of microarc oxidation (MAO) as applied to a point counter electrode. The relationships between the growth rate of the coating thickness and the electrical, geometric and chemical parameters of the MAO process have been mathematically determined. An algorithm for calculating the productivity of the MAO process, due to the growth rate of the coating thickness has been developed. The off ered methodology for the experimental selection of modes of the no-bath MAO of aluminum alloys with an electrically neutral nozzle was verifi ed to obtain a coating on a small area of a part with a fl at surface. In the experiment, with a constant “Nozzle-Workpiece” distance equal to 10 mm, the distance “Electrode-Workpiece” took the values of 5, 10, 20, 30 mm. As a “fl at” surface, the authors took the end face of a 50 mm rod made of the D16T alloy based on aluminum, and a rod made of the 08Kh18H10T steel with a diameter of 6 mm served as an electrode. Use was made of the composition applied in the bath method which included an electrolyte: 8 g of KOH, 30…35 g of Na2SiO3, 1 g of artifi cial diamond nanopowder per 6 liters of distilled water. The duration of the MAO process was 120 min. It has been experimentally established that an increase in the “Electrode-Workpiece” distance decreases the potential for coating formation, which decreases the current in the electrochemical circuit. The analysis of the MAO coating thickness has revealed that 5…15 mm is the optimal distance from the electrode and nozzle to the workpiece, which ensures a stable coating thickness of more than 100 microns on an area equal to or greater than the cross-sectional area of the nozzle supplying electrolyte. The expediency of using the developed mathematical model and the methodology for selecting the process modes with the no-bath method of microarc oxidation has been experimentally confi rmed. The experimental studies have established that the proposed scheme of the MAO process provides an increase in the productivity (the thickness growth rate) of coating deposition by 20% and a decrease in energy consumption by 25%.

Application features of microarc oxidation technology

“…In the aerospace industry, the maximum reduction in the mass of the structure being developed is important, so even the replacement of individual elements is of great importance. In particular, it has been shown that replacement of steel or titanium in the spacecraft opening jaws by micro-arc oxidationmodified aluminium alloy AMg6M is justified [9]. Hardened parts made of lightweight structural material provide high reliability of the unit operation under conditions of vibration loads, increased wear, low temperatures and vacuum.…”

“…Modification by microplasma electrolytic oxidation of aluminum parts to protect them from aggressive environments: chemically active solutions, seawater, extreme weather conditions, etc. [3,9,10,18]. It was found in [19] that, at temperatures up to 870°C, MAO coatings on titanium alloy Ti-6Al−4V 45-50 µm thick, retained high adhesion to the substrate and surface density without signs of pitting.…”

“…The oxidation process consists in the formation of a hardened ceramic layer under the action of high stress on the surface of metals with the strongly marked valve properties (Al, Mg, Ti, Zr, Nb, Ta, Be and alloys based on them). The technology of microplasma modification of the surface of parts is in demand for the modification of tribounits in the machine building, oil and gas industry, instrument making, radio electronics, transport, aviation and space, and other industries [1][2][3][4][5][6][7][8][9].…”