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13/05/2010
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Tom Shelley reports on remarkable things that can be made by thermally spraying metal on to composite.
 Metal spraying allows a small fuel cell to be made that is embedded in carbon fibre composite. Apart from the multiplicity of applications for the base manufacturing technique, the integrated fuel cell has the potential to reduce the weight of kit that has to be borne by infantry soldiers, or, possibly, to allow even more electrically powered equipment to be carried, as well as opening up aerospace and top end automotive applications. It is, however, just one of a family of products being developed that incorporate electrical conductors in composites.
Key to the development, which has come from TWI, is its long-established thermal spraying technology. Provided suitable attention is paid to choice of coating material and surface preparation, it possible to achieve bond strengths of up to 11MPa that allow zinc and aluminium alloys coatings to be firmly attached to composites.
The method of attachment is mechanical. As each hot particle impinges on the substrate, it deforms it locally and buries itself in the composite surface. The deformed composite partially wraps itself round the particle, securing it in place. Subsequent metal particles weld to those already there.
The prototype carbon fibre based composite fuel cell delivers 600mW if fed with hydrogen and oxygen and 200mW if fed with hydrogen and air. It weighs 225g. The cell should last, indefinitely, but, says TWI's Lawrence Dingle: "We do expect some degradation over time, but more than five years seems likely." Machined composite plates can be used as bipolar plate electrodes as an alternative to graphite, combining both structural strength and electrical function. Use of a core material such as a gas or liquid transfer mechanism allows composite to be used as membrane material also.
Composite fuel cells are a subject of much research worldwide, not only to integrate them into already existing composite structures – the next generation of airliners will largely be made out of carbon fibre – but also as a way of making fuel cells much less expensive than at present. Furthermore, since hydrogen fuel tanks in experimental fuel cell-powered vehicles are usually made out of composite to save weight, it makes sense to integrate structure, fuel tank and power cell, using the same basic materials.
Another more immediate application area for sprayed on metal coatings is to provide lightning protection for carbon composite aircraft wings and fuselages. The current fashion is to use metal foils to protect the wings and embedded wire mesh to protect the fuselage. Sprayed on metal should be less expensive and interferes less with structural strength. In automotive, sprayed on, thick metal tracks offer an alternative to wire conductors.
Because TWI has many years experience with thermal spraying metal coatings onto a wide variety of substrates, the base technologies are all well tried and tested. The Institute has four high-velocity oxy fuel spraying systems, as well as air plasma, twin wire arc, flame spraying and cold spraying equipment. Hard coatings, such as tungsten carbide – cobalt – chrome, can be deposited on carbon fibre reinforced plastics following the application of a bond coat.
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Author
Tom Shelley
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