Plasma electrolytic oxidation, or PEO, is a surface modification process for the production of ceramic oxide coatings upon substrates of metals such as aluminium, magnesium and titanium. Two methodologies for the quantitative study of electrical breakdown (discharge) events observed during plasma electrolytic oxidation processes were developed and are described in this work. One method presented involves direct measurement of electrical breakdowns during production of an oxide coating within an industrial scale PEO processing arrangement. The second methodology involves the generation and measurement of electrical breakdown events through coatings pre-deposited using full scale PEO processing equipment. The power supply used in the second technique is generally of much lower power output than the system used to initially generate the sample coatings. The application of these techniques was demonstrated with regard to PEO coating generation on aluminium substrates. Measurements of the probability distributions of discharge event characteristics are presented for the discharge initiation voltage; discharge peak current; event total duration; peak instantaneous power; charge transferred by the event and the energy dissipated by the discharge. Discharge events are shown to increase in scale with the voltage applied during the breakdown, and correlations between discharge characteristics such as peak discharge current and event duration are also detailed. Evidence was obtained which indicated a probabilistic dependence of the voltage required to initiate discharge events. Through the scaling behaviour observed for the discharge events, correspondence between the two measurement techniques is demonstrated. The complementary nature of the datasets obtainable from different techniques for measurement of PEO discharge event electrical characteristics is discussed with regards to the effects of interactions between concurrently active discharge events during large scale PEO processing.

This work presents information obtained from high-speed video and electrical monitoring of electrical breakdown (discharge) events during plasma electrolytic oxidation (PEO) of aluminium alloy substrates. Discharges were found to occur in extended sequences termed "cascades" at particular locations. This was a feature common to all the substrates and processing frequencies investigated. As the coating thickness increases, the characteristics remained broadly similar, although discharges become more energetic and longer-lived. Short PEO treatments were applied to existing PEO coatings in order to investigate the microstructural effects of discharge cascades. It was found that cascades persist at particular locations due to the residual deep pore channel left by previous discharges in the cascade. Observations were made of the way the coating was restructured around a cascade location. Samples were illuminated with very high intensity flashes during PEO processing, revealing that relatively large (1 mm diameter) bubbles form where a discharge emerges from the surface of a coating. Analysis of the overall energy consumption, as well as the energetic processes occurring within an individual discharge, indicate that the bubble growth occurs due to rapid volatilisation of water originating from the electrolyte. It is postulated that the growth of this bubble causes the electrical resistance to rise and is responsible for the termination of the discharge current. Investigations of high frequency (2,500 Hz) processing lead to the discovery of discharges occurring during the cathodic half-cycle, after a certain coating thickness had been achieved. Cathodic discharges were more energetic than anodic discharges, and created large craters in the coatings. Gas evolution was found to exceed the electrochemical Faraday yield, and was similar at low and high frequency initially. Once cathodic discharges began, the gas evolution rate increased and the coating mass gain levelled off.

L’oxydation par plasma électrolytique (ou oxydation micro-arc) est un procédé de traitement des alliages légers (Al, Mg, V, Ti, etc.) apte à pallier les limites de l’anodisation, en particulier au regard des contraintes environnementales. Bien que connu depuis de nombreuses années, les mécanismes sous-jacents à ce procédé assisté par des micro-décharges restent peu ou mal compris. L’objectif de ce travail est de cerner les mécanismes de formation et de développement des micro-décharges et d’associer leurs caractéristiques aux propriétés des couches d’oxyde élaborées sur l’alliage d’aluminium Al2214.La démarche adoptée consiste à associer étroitement l'étude des micro-décharges, la caractérisation des couches élaborées, et les mécanismes de claquage de la couche d'oxyde en cours de croissance. A l’aide de moyens originaux de vidéo rapide (> 125 000 images/s) et d'ombroscopie, la dépendance de l’évolution des micro-décharges aux paramètres macroscopiques du procédé a clairement été établie. L’importance de la présence et de la position de contre-électrodes a été mise en évidence et étudiée. Il est également montré que le choix judicieux de la fréquence et de la densité de courant anodique améliore la qualité des couches obtenues. Une fréquence de l’ordre du kHz semble la mieux appropriée.Enfin, à partir de mesures synchrones, un retard à l’apparition des micro-décharges par rapport au front montant des impulsions de courant a été mis en exergue. Très sensible aux paramètres du procédé, ce retard est probablement lié aux mécanismes de claquage de la couche d'oxyde isolante. Des scénarios concernant ces mécanismes ont ainsi été proposés. / Plasma electrolytic oxidation is a surface treatment process applied to light weight alloys (Al, Mg, V, Ti, etc.) which may advantageously replace conventional anodizing, especially regarding environmental issues. Though this process has been known for many years, the underlying mechanisms that govern this micro-discharge assisted process remain poorly understood. This work aims at better identifying the breakdown and development mechanisms of the micro-discharges and at correlating the micro-discharge characteristics to the properties of the layers grown onto Al2214 aluminium alloy samples. The approach consists in coupling the study of the micro-discharges, the characterization of the grown layers and the breakdown mechanisms. By means of high rate video recording (> 125 000 frames/s) and shadowgraph techniques, the dependence of the evolution of the micro-discharges with the macroscopic process parameters has been clearly established. The important role of counter-electrodes and their respective position with respect to the sample have been identified and studied. It is also shown that the suitable choice of current frequency and anodic current density may greatly improve the quality of the resulting oxide layers. Current frequency in the kHz range seems most appropriate to grow thick and defect-free homogeneous layers.Finally, from synchronous measurements, it has been pointed out a delay in the onset of micro-discharges with respect to the rising edge of the current pulses. Besides this delay is strongly sensitive to the process parameters, it is probably related to the breakdown mechanisms of the insulating layer. Scenarios for these mechanisms have been proposed.

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