A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals

Abstract

The review describes recent progress on understanding and quantification of the various phenomena that take place during plasma electrolytic oxidation, which is in increasing industrial use for production of protective coatings and other surface treatment purposes. A general overview of the process and some information about usage of these coatings are provided in the first part of the review. The focus is then on the dielectric breakdown that repeatedly occurs over the surface of the work-piece. These discharges are central to the process, since it is largely via the associated plasmas that oxidation of the substrate takes place and the coating is created. The details are complex, since the discharge characteristics are affected by a number of processing variables. The interrelationships between electrical conditions, electrolyte composition, coating microstructure and rates of growth, which are linked via the characteristics of the discharges, have become clearer over recent years and these improvements in understanding are summarised here. There is considerable scope for more effective process control, with specific objectives in terms of coating performance and energy efficiency, and an attempt is made to identify key points that are likely to assist this.

Cathodic discharges during high frequency plasma electrolytic oxidation

“…It can be seen that the periods during which there are current pulses correspond with images in which light is being emitted from two distinct points on the surface of the sample, corresponding to two active discharge cascades. Observations of this type, which have been reported previously in some detail [7,8,35], confirm that the anodic current pulses arise from cascades of discharges, which tend to occur in particular locations. These characteristics, including the absence of well-defined discharges in the cathodic half-cycle, are consistent with the majority of previous observations and reports.…”

“…Interest in Plasma Electrolytic Oxidation (PEO) continues to grow, and the technical attractions of the process are well established, but the mechanisms of oxide growth remain quite poorly understood. The key to improved understanding certainly lies in detailed study of the individual discharges, and indeed considerable progress has been made recently as a result of such investigations [1][2][3][4][5][6][7][8]. It's now clear that discharges have a strong tendency to occur in sequences ("cascades") at particular locations, with each discharge being of relatively short duration (few tens to few hundred µs), but the periods between them are somewhat longer (several hundred µs to a few ms) and the cascades can persist over many cycles of the applied potential.…”

“…For example, Stojadinovic et al [21] reported cathodic PEO of Mo. Recognizing that coating creation (substrate oxidation) during PEO predominantly occurs during cooling of a plasma containing both substrate atoms or ions and oxidizing agents of some sort [8], it seems likely that this can take place as readily in a cathodic discharge as in an anodic one.…”

Plasma Electrolytic Oxidation of Titanium in H2SO4–H3PO4 Mixtures

“…The formation of a new plasma is prevented while the gas is inside and above the channel. When the gas escapes, the electrolyte fills the void [3]. As a consequence, the next discharge is created in the same channel.…”

“…The morphology and thickness suggest a high surface-to-bulk ratio, which is beneficial for gas sensor technology. In general, breakdowns during PEO promote the formation of crystalline titanium dioxide phases, namely, anatase and the high-temperature phase rutile [3][4][5]. Both phases differ in significant properties (e.g., band-gap energy and electron-hole recombination rate) for potential application as a gas sensor material [6].…”