Effect of individual discharge cascades on the microstructure of plasma electrolytic oxidation coatings

Abstract

Short duration (∼1s) PEO treatments have been applied to aluminium alloy samples on which coatings of thickness ∼100μm had previously been created. This was done using the small area electrical monitoring system previously developed in the Gordon Laboratory in Cambridge. Voltage supply frequencies of 50Hz and 2.5kHz were employed. Fairly high resolution SEM micrographs were taken, covering the whole surface of small area samples (ie over a circular area of diameter about 0.9mm). This was done both before and after the 1s PEO treatments. X-ray tomographic data were also obtained in the vicinity of a recently-completed set of discharges. The outcomes of these observations were correlated with synchronised high speed electrical monitoring and video photography, carried out during the PEO treatment periods. Localised cascades (comprising hundreds of individual discharges) were observed in all cases, persisting throughout the 1s periods and also reappearing in the same location when a second 1s PEO treatment was applied to the same sample. This repetition of discharges at the same location is apparently due to the deep pores associated with these sites, creating a pathway of low electrical resistance, even after appreciable oxidation has occurred in the vicinity. Observations were made of the way in which the surfaces were reconstructed locally as discharge cascades occurred. With the high frequency voltage supply, discharge lifetimes were limited to the half-cycle period (of 200μs), but in other respects the cascades were similar to those with the lower frequency. However, some discharges occurred during cathodic half-cycles with the high frequency supply, at the same location as the anodic discharges in the cascade concerned.