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Thermal sprayed coatings for composites

 

Connect, no. 154, May/June 2008, p.3

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The use of composites as structural components in aircraft and in the automotive industry is increasing due to good strength to weight ratios. However their limited surface properties prevent their use in applications where wear resistance, thermal management or electrical conductivity are required. To extend composite applications, coatings are required to provide protection and increase functionality of the surface.

Thermal spraying offers the prospect of producing a wide range of coatings with increased functionality. Due to the low melting point of composite resins, and the high temperatures associated with thermal spraying processes, it is difficult to deposit well adhered coatings on composite substrates without damaging them.

The Surface Engineering group at TWI has successfully demonstrated the feasibility of coating composite materials, such as carbon fibre reinforced polymers (CRFP), using various thermal spraying processes.

Maximum adhesion is achieved through careful selection of coating materials and control of the surface preparation. Bond strengths up to 11MPa can be achieved for coatings based on zinc and aluminium alloys, exceeding that of other coatings, such as paints, and approaching levels exhibited by arc sprayed aluminium on steel.

TWI has also developed novel spraying techniques to deposit graded coatings incorporating functional top coats.

Hard coatings, such as WC-Co-Cr, can be deposited on CFRP following application of a bond coat. Other functional layers may also be incorporated, for example to provide thermal insulation for protecting composites in high temperature applications.

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Initial tests have shown, that in four point bending, coatings remain attached even after failure of the underlying composite. Promising fatigue results have also been achieved, with coatings surviving 0.5million cycles through significant deflection in four point bending.

The technology shows promise for a number of applications within the military, aerospace and motorsport industries, such as providing wear resistance or thermal insulation or as reflective coatings and primers for subsequent evaluation of paints. Further work is required to develop coatings for specific applications and validate coating performance in service. A number of single client projects are currently in progress to develop the technology.

For more information on thermal sprayed coatings, please contact Melissa Riley. surfacing@twi.co.uk

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contactus@twi.co.uk