Functional Surfaces via Electrical Discharge Methods

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Electrical discharge machining (EDM) is an excellent process of realising complex features in tough materials. EDM makes use of repeated electrical discharges to remove material from a workpiece. Each discharge constitutes a plasma channel which results in removal of a small amount of material from both the EDM machine's electrode ('the tool') and the work piece. Machine parameters and spark conditions are optimised such that under typical conditions more material is removed from the work piece when compared to the tool per discharge. When the plasma arc strikes a localised region is rapidly heated and liberated from the bulk. This 'debris' is then normally evacuated from the spark gap by the so called flushing mechanism. The spark gap is in an incredibly tempestuous place and hence very difficult to understand. However, knowledge of this behaviour is crucial to advancing our understanding of ED techniques for industrial applications.It has been shown that under the correct physical conditions in the spark gap the debris created by these discharges can be used to apply a coating and build low aspect ratio surface structures. This presents an interesting opportunity to high value manufacturers who are often tasked with machining precision features and then applying precision coatings or, perhaps, repairing worn regions in high value components. However, the physics of this process is not yet fully understood and there remains a requirement to advance this technology significantly from fundamental principles.This project will explore the technology from a new, multidisciplinary perspective which will incorporate both experimental and modelling activities. The investigators will present a solid understanding of the debris dynamics electrical discharge coating techniques to the community and will use this new knowledge to create complex multi-layer coatings which will have applications in the aerospace, biomedical and tool making industries. Companies operating in these industries thrive on the technological advantage their products possess over competitors. For example all of these products require advanced tribological properties amongst other complex surface characteristics. There is an array of surface modification techniques available to manufactures such as laser cladding, cold/plasma spray, PVD amongst others which have developed rapidly in the last 20 years. However, none of these have been directly integrated into a process which can also remove material. The rapid adoption of all of these coating techniques has been borne out of a solid understanding of the process mechanics, materials science and optimisation for application. In all of the high value applications for these coating techniques surface integrity is critical since failures which occur as a result of corrosion, fatigue or high temperature effects emanate from the surface or near surface. Enhancing surface integrity is the core rationale for surface treatment although coatings are also often applied for aesthetic purposes also.EDM itself has developed significantly in recent years with modern machine makers claiming to have developed processes capable of the so called 'near zero' recast layer. This is in response to the extensive studies which have been undertaken on the surface damage that results from EDM and is known to induce deleterious tensile stresses. However, no solid solution to this has been developed which means the confidence in EDM surface layers is currently low. However, the application of consolidated coatings may be able to tackle this problem and will be investigated in this project. The ability to remove material and apply a coating within the same process presents significant advantages. Therefore the use of ED coating methods has great potential to enhance high value manufacturing in terms of enhanced product performance but also through process efficiency savings.