Adding surface value to composites
Applying surface-enhancing coatings to carbon fibre reinforced polymers (CFRP) using thermal spray techniques is the focus of research at TWI in Great Abington, UK. This could extend the use of these materials in high temperature or corrosive environments in helicopter blades, and heat ducts.
Engineers have coated the composite with functional materials such as zirconia for thermal management, aluminium for conductivity and tungsten carbide-cobalt-chromium for wear resistance. Attempts to apply protective coatings to composites have previously proved difficult, as the low thermal tolerance of the materials means they are easily damaged if heated too much during application.
Traditional methods of metalising composites involve incorporating metal meshes into components, explains Dr Melissa Riley, Senior Project Leader for TWI’s Metallurgy, Corrosion and Surfacing Technology Group. ‘[However], this often causes voids or defects, which affects their performance and leads to an unacceptable reject rate during manufacture,’ she says. ‘Spraying avoids these issues.’
By modifying thermal spraying techniques, multilayers of functional coatings are being applied without damaging the substrate (the Group could not reveal the exact modifications made). While the temperature of the spray is generally between 1,000-3,000ºC, ‘the heat content of individual particles is low, [so] the thermal input to the composite can be controlled’, explains Riley.
Adhesion is sensitive to surface preparation, but the TWI group has achieved sucess in this area by using grit blasting. The multiple layers ‘are built up by a series of particle splats’, she adds. ‘The overall thickness can be tailored and is controlled by the selection of consumables, substrate and spraying process [typical coatings are 50-750µm thick].’
The group has demonstrated the use of thermal spraying on the CFRP composite, and has created reflective and thermal barrier coatings that can manage thermal radiation on surfaces, as well as layers exhibiting good electrical conductivity.
‘The potential for [this] technology is good,’ says Tom Corden, Applied R&D Engineer at the Advanced Composites Group in Heanor, UK. ‘It gives you all the benefits of composites – lightweight, stiff, corrosion resistant, easy to manufacture – with added benefits (hardness, abrasion resistance, polishable, etc).’
Possible applications include longer lasting helicopter blades and heat ducts that display better heat transfer for Formula One cars. Corden says other organisations are applying coatings to composites using thermal spraying, but complications exist, including coating adhesion, getting a uniform thickness and avoiding damage to the composite.
Thus far, the Group has created high-strength coatings that can survive 500,000 bending cycles at 1.06% strain. It is working on coatings for specific applications, such as lightning strike protection, as well as looking at applications on other carbon and glass based composites that incorporate thermoset systems, such as epoxy or phenolic resins.
Further information: TWI
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