Synchronised electrical monitoring and high speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation

Abstract

Synchronised electrical current and high speed video information are presented from individual discharges on Al substrates during PEO processing. Exposure time was 8μs and linear spatial resolution 9μm. Image sequences were captured for periods of 2s, during which the sample surface was illuminated with short duration flashes (revealing bubbles formed where the discharge reached the surface of the coating). Correlations were thus established between discharge current, light emission from the discharge channel and (externally-illuminated) dimensions of the bubble as it expanded and contracted. Bubbles reached radii of 500μm, within periods of 100μs, with peak growth velocity about 10m/s. It is deduced that bubble growth occurs as a consequence of the progressive volatilisation of water (electrolyte), without substantial increases in either pressure or temperature within the bubble. Current continues to flow through the discharge as the bubble expands, and this growth (and the related increase in electrical resistance) is thought to be responsible for the current being cut off (soon after the point of maximum radius). A semi-quantitative audit is presented of the transformations between different forms of energy that take place during the lifetime of a discharge.