Discharge characterization in plasma electrolytic oxidation of aluminium

Abstract

Digital video imaging of the plasma electrolytic oxidation (PEO) of aluminium has been performed, which allowed evaluation of both dimensional characteristics of individual microdischarges appearing at the oxide–electrolyte interface and their collective behaviour throughout the oxidation process. It has been shown that the microdischarge cross-sectional dimensions vary within the range 0.01–1.35 mm2. In the course of PEO processing, small localized events (<0.03 mm3) always dominate in the microdischarge spatial distribution and the relative proportion of medium-sized to very large microdischarges is gradually redistributed in favour of the latter. Temporal dependences have been found for the fraction of surface area instantaneously experiencing the discharge, as well as for the spatial and current densities of the microdischarge. Discharge mechanisms occurring during PEO are discussed and a model of microdischarge formation is suggested, assuming the possibility of free-electron generation and glow discharge ignition in the gaseous media developed at the oxide–electrolyte interface. First approximation evaluations of thermal processes in the oxide layer under the discharge conditions have been considered. The estimated ranges of the microdischarge current density (50–18 kA m−2) and duration (0.25–3.5 ms) sufficient for initiating phase transitions (e.g. γ–α transformation and melting) in the surface oxide layer are shown to be in good agreement with experimental data.

Evolution of the Three-Dimensional Structure and Growth Model of Plasma Electrolytic Oxidation Coatings on 1060 Aluminum Alloy

“…In general, mechanisms such as "dielectric breakdown", "discharge-in-pore", and "contact glow discharge electrolysis" were the mainstream views about the plasma discharges during the PEO process [35]. Various growth models were proposed based on these mechanisms to describe the formation of the PEO coating [30][31][32]36].…”

Quantitative Parameters and Definition of Stages of Anodic-Cathodic Microplasma Processes on Aluminum Alloys

“…This criterion creates favorable conditions for the immediate establishment of a gas-phase discharge on the oxide surface at the moment when the voltage drop across the gas bubble reaches the critical breakdown value during positive biasing''. 20) This statement must be verified through detailed research because the experiments in this study show that the negative current has no influence (Fig. 5) on the parameters U spark1 , U spark2 , and U microarc and on the duration of the initial stages.…”

“…This fact was also observed by other researchers. 20) They obtained even the data of the fractional distribution of the cross-sectional areas of the microdischarge population for the different stages of the microplasma process. They detected four categories of microdischarge sizes in the population.…”

“…A number of models have already been offered by different authors, 19,20) but this problem has not been fully addressed.…”

Influence of Solvent and Electrical Voltage on Cathode Plasma Electrolytic Deposition of Al2O3 Antioxidation Coatings on Ti-45Al-8.5Nb Alloys

“…PE processes comprise anode and cathode plasma electrolysis, depending on the discharge electrode. As micro-arc oxidation (MAO) technique, anode plasma electrolysis had been widely used for coating metal substrates [23,24]. Nevertheless, the resulting composition and structure of coatings are subject to the substrate's influence.…”