Simulation of synthesis of oxide-ceramic coatings in discharge channels of a metal-electrolyte system

Abstract

The formation of oxide-ceramic coatings on rectifying metals in the metal-electrolyte system under the action of an external electric field includes several stages. The first stage consists in the formation of the primary oxide film according to the electrochemical mechanism. The electrochemical activity of the rectifying metals decreases in the row Mg, A1, Ti, Nb, Ta, and these metals remain at the beginning of the passivity row. A characteristic feature of the anodic behavior of the rectifying metals is the absence of the active region of dissolution, which is due to their electronic structure. In the metal-electrolyte systems, they give off their d-electrons and transform into ions with rearranged d-levels. These ions react with the oxygen dissolved in electrolyte, OH radicals, or water, forming surface oxide films. Anodic oxide films are amorphous and poreless, and they have ionic conductivity and high electrical resistance. They are classified [1] as n-semiconductors with a wide forbidden band. The electrons are not transferred through the passive layers on the rectifying metals [2,3]. The only reaction on the anode at a voltage of up to 100 V is the anodic growth of oxide. In this case, the overvoltage of the oxide formation is less than the voltage of the decomposition of the electrolyte. When the latter is reached, the injection of electrons into the oxide layer takes place, and oxygen begins to evolve on the anode [4]. This corresponds to the beginning of the second stage of the formation of the spark discharge channel in the metal-oxide-electrolyte system. The mechanisms of breakdown and the parameters of the discharge channel can be different [5,6], depending on the material of the electrode and the nature of the interelectrode space. In the case of realization of the spark discharge in the metal-oxide-electrolyte system under the action of a direct-current electric field, the metal serves as an anode, and the electrolyte serves as a cathode. The interelectrode space consists of the oxide and the double electric layer on the oxide-electrolyte boundary. The main drop of the potential is at the oxide interlayer where the electric field intensity is high [7]. At as low electric field intensity as 105 V/m, the high-energy electrons can cause the galvanoluminescence of the oxides [8]. For the electrons to have, under the action of the electric field, the acceleration necessary for breaking down the wide-band oxide, their energy must be higher than 3Eg/2 (Eg is the width of the forbidden band). These electrons have energy sufficient for ionizing the atoms, which results in the appearance of new electrons. When the coefficient of reproduction exceeds unity, electronic avalanche arises. The ionized atoms that remain behind the avalanche create a practically stationary spatial charge which decelerates the electrons. Since, as was mentioned above, the potential drop is localized mainly in the oxide, and the oxides of the rectifying metals show the properties of wide-band semiconductors of n-type, for determining...

Characterisation of discharge events during plasma electrolytic oxidation

“…It was suggested by Klapkiv [9] that there are several distinct zones, including a "core" plasma channel and a surrounding gas bubble. Emissions from hydrogen and the recombination continuum are postulated to come mostly from low density plasma in bubbles.…”

“…Of these three investigations, only that of Klapkiv et al was carried out using a spectrograph with sufficient resolution to allow estimates of the plasma electron temperature to be made from broadening of spectral lines. They estimated typical plasma temperatures to be~6800-7800 K. The physical properties of the plasma within the discharge were investigated further by Klapkiv in a subsequent paper [8] and the chemical reactions taking place within a discharge, and the internal structure of the plasma, were also later discussed by him [9]. The plasma structure proposed by Klapkiv comprises a central core, at 7000 K, containing free electrons, ionic and atomic species, surrounded by lower temperature regions, in which different plasma components and reactions are found, depending on the temperature.…”

Plasma Electrolytic Oxidation of High-Strength Aluminium Alloys—Substrate Effect on Wear and Corrosion Performance

“…Thus, alloying elements in the substrate play a crucial role during the formation of the oxide coating and hence affect the structure of the coating, its composition, and its properties. Generally, PEO coatings on aluminium are comprised of various modifications of alumina (α-, γ-, δ-, η-Al 2 O 3 or amorphous Al 2 O 3 ) [4][5][6]. Further phases also occur, which can include electrolyte components (e.g., mullite from silicate-containing electrolytes) [7][8][9].…”

Gibbs energy calculation of electrolytic plasma channel with inclusions of copper and copper oxide with Al-base

“…It was found previously that in the discharge channel there were 4 regions that differed in both temperature and composition [12][13][14][15][16]. The temperature in the central zone of the discharge channel is 7.000 -10.000 K and decreases to 600 K in the adjacent areas.…”

“…CuO g +2AlO  -Al 2 O 3cr l +Cu g (14); CuO cr l +2AlO  -Al 2 O 3cr l +Cu g (15). (12); (13); (14); (15).…”