Cathodic discharges during high frequency plasma electrolytic oxidation

Using small area electrical data logging, high speed photography, sample mass gain monitoring, gas evolution measurement and microstructural examination, a study has been made of the formation and effect of cathodic discharges during PEO of Al substrates. Discharge formation during the cathodic half-cycle is promoted by high frequency, thick coatings and high pH. They form (under constant current conditions) when the voltage during cathodic polarization reaches a value (~250 V in the work described here) sufficient to cause dielectric breakdown across thin residual oxide layers on the substrate. This occurs when the normal cathodic process of proton flow through electrolyte channels in the coating can no longer deliver the required current. Cathodic discharges tend to carry higher currents, and emit more light, than anodic ones. Gas evolution rates during PEO are well above the Faraday yield level. This is due to water entering discharge plasmas, breaking down into ionized species and failing to recombine completely during subsequent collapse and quenching. It is reported here that rates of gas evolution rise as discharges start to take place in the cathodic part of the cycle, as well as in the anodic part. Rates of substrate oxidation (coating growth), however, drop off, rather than rise, when cathodic discharges start. Evidence is presented here suggesting that this is associated with their highly energetic nature, causing substantial amounts of oxide to be expelled into the electrolyte during cathodic discharges. This is also apparent in the coating microstructure, where recent cathodic discharge sites are identifiable as large, highly porous regions.EPSRC (grant number EP/I001174/1) Sims Scholarship (Cambridge University

High speed video evidence for localised discharge cascades during plasma electrolytic oxidation

Information 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 μs, while the linear spatial resolution of the images ranged upwards from about 12 μm. The area being viewed was about 2.4 mm2, which was taken to be representative of the substrate area as a whole (~ 129 mm2). 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 the overall energy consumption.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.This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S0257897215000778#

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

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

Short duration (~1 s) PEO treatments have been applied to aluminium alloy samples on which coatings of thickness ~100 $\mu$m 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 50 Hz and 2.5 kHz 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 0.9 mm). This was done both before and after the 1 s 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 1 s periods and also reappearing in the same location when a second 1 s 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 200 $\mu$s), 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.Engineering and Physical Sciences Research Council (Grant ID: EP/I001174/1), Sims Scholarship, Keronite plc, Fundacion Banco Santander (Research Mobility Scholarship)This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.apsusc.2016.07.10

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

Short duration (~1 s) PEO treatments have been applied to aluminium alloy samples on which coatings of thickness ~100 $\mu$m 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 50 Hz and 2.5 kHz 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 0.9 mm). This was done both before and after the 1 s 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 1 s periods and also reappearing in the same location when a second 1 s 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 200 $\mu$s), 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.Engineering and Physical Sciences Research Council (Grant ID: EP/I001174/1), Sims Scholarship, Keronite plc, Fundacion Banco Santander (Research Mobility Scholarship)This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.apsusc.2016.07.10

The incorporation of particles suspended in the electrolyte into plasma electrolytic oxidation coatings on Ti and Al substrates

© 2020 Elsevier B.V. This investigation concerns the mechanisms by which (fine) particles become incorporated into plasma electrolytic oxidation (PEO) coatings when added to the electrolyte. Three different types of particle have been used, covering a wide size range, and processing has been carried out with both Al and Ti substrates. For some of these combinations, the particulate was chemically similar to the expected PEO product, while for others it was different. The power supply was 50 Hz AC, with a pre-selected current density. It has been established that, where such reactions are chemically favoured, phase changes can occur that must have involved the particulate reaching very high temperatures. From this and other evidence, it is concluded that the main incorporation mechanism involved is that of (fine) particulate being swept into the pores associated with active discharge sites, while they are being refilled with electrolyte immediately after collapse of the plasma. They are then likely to become entrapped, and in many cases to be strongly heated as the plasma is created during the next discharge cycle. Typical pore sizes are such that particles (or particulate clusters) above about 10 μm in size would be unlikely to enter them. While particles a few microns in diameter can become incorporated, it takes place more readily with sub-micron particles. It is also concluded that electrophoretic forces are unlikely to play any significant role in the incorporation process

Development and assessment of photo-catalytic membranes for water purification using solar radiation

This paper describes a novel set-up for characterization of the performance of membranes designed for purification of water. It involves a recirculatory system, with continuous monitoring of the concentration in the water of a representative pollutant (Methylene Blue). Pressures, flow rates and temperatures are also measured. Results, in the form of rate constants for reduction in pollutant concentration, are presented for three different types of membrane, all of which incorporate relatively high surface areas of titania and have permeability values in a range making them suitable for this type of processing (~10-11 m2). These results are rationalized in terms of the surface areas of the membranes, and the likely water flow characteristics within them. It is concluded that all of the titania surfaces within them have similar efficiencies for photo-catalytic oxidation of pollutants, but there are significant differences in the ways that the water is exposed to these surfaces, and hence in the pollutant oxidation rates. These points are relevant to the optimization of membrane design for this purpose

The processing and properties of single grain Y-Ba-Cu-O fabricated from graded precursor powders

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 YBa2Cu3O 7-δ (Y-123) and Y2BaCuO5 (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 Jc, 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. © 2013 IOP Publishing Ltd