The review describes recent progress on understanding and quantification of the various phenomena that take place during plasma electrolytic oxidation, which is in increasing industrial use for production of protective coatings and other surface treatment purposes. A general overview of the process and some information about usage of these coatings are provided in the first part of the review. The focus is then on the dielectric breakdown that repeatedly occurs over the surface of the work-piece. These discharges are central to the process, since it is largely via the associated plasmas that oxidation of the substrate takes place and the coating is created. The details are complex, since the discharge characteristics are affected by a number of processing variables. The interrelationships between electrical conditions, electrolyte composition, coating microstructure and rates of growth, which are linked via the characteristics of the discharges, have become clearer over recent years and these improvements in understanding are summarised here. There is considerable scope for more effective process control, with specific objectives in terms of coating performance and energy efficiency, and an attempt is made to identify key points that are likely to assist this.

International audienceInformation is presented from high speed video imaging of the free surface of coatings being grown on aluminium substrates by PEO processing. The exposure time during image capture ranged down to 5.5 mu s, while the linear spatial resolution of the images ranged upwards from about 12 mu m. The area being viewed was about 2.4 mm(2), which was taken to be representative of the substrate area as a whole (similar to 129 mm(2)). PEO processing was carried out at 50 Hz AC. The periods over which image sequences were captured was about 100 ms, covering several cycles of variation of the applied potential. This operation was repeated periodically while the coating thickness increased from a few microns to several tens of microns. During the imaging periods, it was typically observed that tens or hundreds of individual discharges were occurring, all of them readily distinguishable from the background light levels. Their duration was of the order of several tens of microseconds. It was noticeable that they tended to occur in ``cascades'' at particular locations, each sequence comprising tens or hundreds of individual discharges, with an ``incubation'' period between them of the order of several hundreds of microseconds. It seems likely that they all occurred during the positive (anodic) half-cycle, while the applied voltage was sufficiently high. An individual cascade tended to persist (at the same location) over several voltage cycles. As the coating became thicker, these characteristics broadly persisted, although individual discharges became longer-lived and more energetic. An attempt is made to relate these observations to the overall picture of how coating growth takes place during PEO processing, and also to th

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International audienceSynchronised electrical current and high speed video information are presented from individual discharges on Al substrates during PEO processing. Exposure time was 8 mu s and linear spatial resolution 9 mu m. 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 mu m, within periods of 100 mu s, 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

The preparation of single grain, Y-Ba-Cu-O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa 2 Cu 3 O 7−δ (Y-123) and Y 2 BaCuO 5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40 wt% Y-211 in the vicinity of the seed, which decreased to 30 wt% and then 20 wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30 wt% Y-211 to enable comparison. The field trapping ability, T c and J c , of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

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