Breakthrough Studies on the Plasma Electrolytic Oxidation (PEO) Coating Process

Lead Research Organisation: University of Cambridge
Department Name: Materials Science & Metallurgy


Plasma Electrolytic Oxidation (PEO), also known as Micro Arc Oxidation (MAO), Spark Anodising and Microplasma Oxidation, is a processing technique in which the surfaces of metals such as Al, Mg and Ti are converted into oxide coatings, ranging from tens to hundreds of microns in thickness. Coating growth occurs via large numbers of short-lived sparks (electrical discharges), caused by local dielectric breakdown. The resultant coatings can be highly resistant to wear and corrosion, and adhere exceptionally well to the substrate. Until recently, however, the nature of these plasma discharges, and the links between this and the resultant coating microstructure, have been very poorly understood. Recent work at Sheffield and Cambridge has produced new information about the temperature, density, resistivity, spatial distribution, frequency and duration of these discharges, and also about the influence that these characteristics have on the coating microstructure. The proposed project is aimed at utilising and expanding the techniques that have been developed in this work, employing the researchers primarily responsible for these advances, and also benefitting from the input of experienced plasma physicists based in Southampton. One of the objectives will be to create a new process model. This will give quantitative insights into the interplays between electrical circuitry, electrolyte composition, plasma discharge characteristics and coating microstructure. This should assist in the aim of improving the energy efficiency of the process. The enhanced understanding provided by this modelling will then be utilised to explore the potential for using PEO-like processing to implant small atoms, such as carbon and boron, into metals such as steels, giving increased surface hardness. Preliminary reports of this possibility are encouraging. If it does prove to be viable, then it would offer major energy-saving benefits in competition with conventional carburizing, which requires components to be held at high temperatures for extended periods. The work will be carried out in collaboration with two UK SMEs in the PEO field, and should thus lead to substantial and relatively short term benefits to UK industry.

Planned Impact

Technological Impact and Economic Benefits * Optimisation of the PEO process will allow more efficient production, leading to cost benefits, reduced energy consumption and higher coating quality. Development of PEISA processing could also lead to large energy savings compared with conventional carburizing. This is in line with government priorities - see - and would also be of direct benefit to various firms involved in development and use of PEO processing. * New surface treatment techniques, such as surface hardening by the plasma electrolytic implantation of small atoms such as carbon and nitrogen, may prove to be of commercial value to a range of firms, and could open up a new field for scientific and technological development. * There are already strong links between the academic and industrial partners. The project would enhance these, and lead to new avenues of collaboration. Scientific Impact and Knowledge Development * New characterisation techniques expected to emerge, such as applied plasma diagnostics, will be used for purposes such as monitoring of discharges from cryogenic liquids in superconducting transformers, and monitoring of insulation ageing in high voltage power systems (via detection of discharges and analysis of physical and chemical changes in the insulation). Such advances will be transferred to Airbus UK, National Grid and IET's Power Academy, via existing links with Southampton University. * Improved understanding of discharge events in PEO, and resultant microstructures, will promote scientific and technological developments, creating benefits for researchers and industrialists. Skills Development * The four named researchers, Dr. Chris Dunleavy, Dr. Kevin Goddard, Mr. Nick Kasch and Dr. Aleksey Yerokhin, will have the opportunity to enhance their expertise in important areas of physical science, and the project will be a major boost to all of their careers. Specialists in the areas concerned (plasma physics, advanced materials processing, surface engineering etc) are in very high demand, in both academia and industry. Collaboration, Communications and Engagement with Beneficiaries * The research will be carried out in close collaboration with two UK SMEs, both of which are at the forefront of PEO technology development. One of these is Keronite Ltd, which is located close to Cambridge. A scientist in the Keronite team, James Curran, will spend one day a week on the project. The other industrial partner is Plasma Coatings Ltd, an SME operating in biomedical and aerospace coating sectors. It has recently made significant investments in state-of-the-art 300 kW commercial PEO equipment, which will be made available to the Sheffield group. * The sectors that could benefit from expected developments include biomedical, aerospace, defence, automotive and general engineering. Dissemination to these sectors will be promoted by the industrial partners, and also to MoD, via the DSTL contact (Prof. Richard Jones). * Agreements covering sensitive information will be established, covering prior proprietary knowledge and results obtained during the project. Most of these are expected to be suitable for wide dissemination, which will be effected via conferences, publications and a dedicated website. * Good communications between the partners will be ensured via quarterly meetings, attended by all of the academics involved, by representatives of both industrial firms and by Richard Jones (DSTL). * One of the Investigators (to be decided shortly) will be the Organiser of a PEO Symposium at Euromat 2011 (Montpellier). * A workshop will be held in Cambridge at Easter 2013, for dissemination of information emerging from the project, with invited attendees from around the world. The arrangements will be similar to those of the CAMTEC Symposia (


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Dunleavy C (2011) Self-similar scaling of discharge events through PEO coatings on aluminium in Surface and Coatings Technology

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Robinson HJ (2011) Cell adhesion to plasma electrolytic oxidation (PEO) titania coatings, assessed using a centrifuging technique. in Journal of the mechanical behavior of biomedical materials

Description This work has led to considerably improved understanding of the fundamentals of the PEO process. Using innovative high speed logging of electrical signals associated with individual discharges (which typically have lifetimes of a few tens of microseconds), synchronised with high speed photography (exposure periods ~ few microseconds), the relationships between charge transfer, energy injected, supply frequency and coating microstructure have become much clearer. This has already led to a number of process improvements and there is scope for this to expand considerably.
Exploitation Route The techniques developed in the project are now finding more widespread usage. For example, some joint work has been undertaken recently with the main PEO group in France (at Nancy), and some very useful sharing of expertise concerning electrical and photographic monitoring of individual discharges has taken place.
Sectors Aerospace, Defence and Marine,Energy,Leisure Activities, including Sports, Recreation and Tourism,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

Description Outcomes from this research has been of considerable use to Keronite plc in developing their commercial products. In particular, improved understanding of the complex relationships between individual PEO discharge characteristics, energy consumption rates and coating microstructures have led to improvements in the range and performance of several individual products.
First Year Of Impact 2015
Sector Aerospace, Defence and Marine,Leisure Activities, including Sports, Recreation and Tourism,Manufacturing, including Industrial Biotechology,Transport
Impact Types Economic

Title Curvature-based method for study of residual stresses in PEO coatings 
Description Production of PEO coatings on one side only of a thin strip of substrate, and measurement of the resulting curvature, is used to infer the misfit strain associated with the PEO process, and hence the residual stress levels to be expected when the coating is produced on a thick substrate. 
Type Of Material Improvements to research infrastructure 
Year Produced 2014 
Provided To Others? Yes  
Impact This methodology has promoted a rethink about the origins and nature of residual stresses in PEO coatings, which have previously been measured only via X-ray diffraction. Results obtained using the curvature methodology have confirmed the sign of the residual stress created in the coating (compressive), but have indicated that the magnitude is probably smaller than had originally been thought. A paper describing these results will be published shortly. 
Title High speed synchronised electrical/optical monitoring of the PEO process, using parallel small area samples 
Description The electrical discharges that take place during PEO are central to understanding and control of the process. It's now known that they have a very short lifetime (~tens or hundreds of microseconds), but tend to occur in "cascades" that persist in the same location for hundreds or thousands of discharges, with a relatively short "incubation period" between each one (that probably corresponds to the period required for the large void left at the site concerned after collapse of the plasma to refill with electrolyte). This understanding has largely arisen from the technique developed in the Gordon Lab. for synchronised monitoring of the electrical current through, and the light emission from, a small area sample (~1 mm diameter) connected in parallel with the main sample. At any given time, there will be no more than one discharge occurring on the small sample, so the associated characteristics can be monitored. 
Type Of Material Improvements to research infrastructure 
Year Produced 2015 
Provided To Others? Yes  
Impact This technology has led to detailed study of various characteristics of the PEO process, including the incidence of cathodic discharges, the "soft regime" etc. 
Description Keronite International Ltd 
Organisation Keronite International
Country United Kingdom 
Sector Private 
Start Year 2002
Description PEO link with U. of Lorraine 
Organisation University of Nancy
Country France 
Sector Academic/University 
PI Contribution Work in Cambridge on the PEO process, particularly concerning the characteristics of individual discharges, attracted the attention of researchers in Nancy.
Collaborator Contribution Personnel at the University of Nancy have spent a week in Cambridge, using a high speed camera which they brought with them. This has been used, in conjunction with specialized PEO equipment here, to obtain valuable information about the spatial and temporal distributions of discharges.
Impact This is ongoing work, but a paper is already in preparation based on the work done during the visit. Further visits are planned
Start Year 2013