Phenomena Associated with Individual Discharges during Plasma Electrolytic Oxidation
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.