The project will involve high speed synchronised electrical and optical monitoring of individual discharges, in combination with gas composition measurement, during PEO of several metals. This will be done using a range of waveforms and supply frequencies. The focus will be on mechanisms of energy consumption, with growth of water vapour bubbles at the top of plasma channels known to be a prime source of energy absorption. It's also known that high frequencies, in the kHz range (such that the half-cycle time starts to approach the period between discharges), can promote formation of discharges during the cathodic part of the cycle. PEO is not a conventional electrochemical process, and sparking simply exposes the metal to oxidizing agents in the plasma, so cathodic discharges could improve the energy efficiency. Correlations will be explored between PEO conditions and coating microstructure, using X-ray tomography to capture 3-D pore architectures.