Synchronised Electrical Monitoring and High Speed Video of Bubble Growth associated with Individual Discharges During Plasma Electrolytic Oxidation

Synchronised electrical current and high speed video information are presented from individual discharges on Al substrates during PEO processing. Exposure time was 8 μs and linear spatial resolution 9 μm. Image sequences were captured for periods of 2 s, 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 500 μm, within periods of 100 μs, with peak growth velocity about 10 m/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.This work has been supported by EPSRC (grant number EP/I001174/1), by a Sims Scholarship (for SCT) in Cambridge University and by Keronite plc. The research also forms part of the activities of the COST TD 1208 Network. Thanks are due to Steve Hutchins and Suman Shrestha, of Keronite, for many helpful discussions. The technical assistance of Fréderic Brochard (Nancy) and Kevin Roberts (Cambridge) is also gratefully acknowledged.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.apsusc.2015.10.124 In compliance with EPSRC requirements, raw data in the form of selected video and discharge current files are available at www.ccg.msm.cam.ac.uk/publications/resources, and are also accessible via the University repository at http://www.data.cam.ac.uk/repository