Functional Surfaces via Electrical Discharge Methods

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.

The impact of this project will be felt in the development of new tools, coatings and coating methodologies for the aerospace, biomedical and tool making industries. Gains made in the technologies which underpin these industries will enhance the competitiveness of companies who adopt these technologies at the earliest stage possible. Since this research has a broad reach the technology also has potential applications in a wider variety of industrial sectors, though specific requirements for surfaces within each sector will differ. Improved methods of surface finishing and functionalisation are sought by a range of industries, which provides a market pull and opportunities for commercialisation.

Industrial developments in this area have a direct societal benefit. Through the development of superior products the finite lifetime of key engineering products can be extended. By augmenting the lifetime of aero-engine components, for example, less planned maintenance is required making the cost of air travel lower. Furthermore, any technique which allows aero-engines to operate at high temperatures enhances efficiency and hence air transport is also greener while being cheaper. It is expected that by 2030 the global fleet of passenger planes will more than double from current figures to 27,800 (Airbus, 2011). Much of this demand will be based in developing nations such as Brazil, Russia, India and China (BRIC). Given the rapid increase in demand for air transport, technologies which enhance the sustainability of this growth will be key.

With respect to bio-implant applications there is also significant opportunity to affect a change. In the UK alone there are approximately 10,000 artificial metal-on-metal surface replacement hip implants in service (Arthritis Research UK, 2013). These are monitored through life and often subject to replacement/revision due to wear. Enhanced coatings techniques have the potential to ameliorate this growing problem which is set to affect more people mean population age and life expectancy increase. Similarly to changes in the aerospace market so too will new opportunities for makers of biotech goods emerge in the BRIC nations.

The tooling industry within the UK is hugely important to the domestic economy. Currently valued at £1.19 Billion (IBISworld industry report, 2013) the industry hosts many SMEs who are significant employers and wealth creators. They will commonly be engaged in supplying tools to larger manufacturing companies. To maintain the competitiveness of the industry improved tooling is continually developed. This generally relates to increasing the time that tools can stay in service or the ability to repair them when they become worn. Both of these activities reduce material consumption and hence the environmental burden of the industry.

Recently the UK Knowledge Transfer Network (KTN) established the Surface Engineering and Advanced Coatings Special Interest Group (SEAC SIG). Demonstrating that this technology area is critical to the core manufacturing base within the UK with key players such as Rolls-Royce, Curtiss Wright and Bodycote all boasting a significant presence in the UK. European Union estimates suggest that the UK employs 143,500 in forging, metal coating and general mechanical engineering (NACE Rev 1.1, 2002). Therefore the value of technologies associated with surface improvement to the UK is significant.

A multidisciplinary approach has been taken to address the challenges associated with this project. In doing so, contributions to the knowledge base associated with EDM, manufacturing process modelling and more generically manufacturing technology will made. The competencies which will make this project a success will allow knowledge contributions to be made to these fields and advance the understanding the scientific and industrial communities currently hold.