Burning issues...
..metal-to-composite coating solves insulation problem
Flame sprayed aluminium found an unusual application recently when TWI was presented with a far from straightforward thermal protection problem.
Its client, a top racing vehicle manufacturer in the British Sportscar Championship league, faced a huge heat insulation issue and asked TWI to solve it under the National Composites Network programme funded by the DTI and industry.
The exhaust system of its endurance cars, which regularly perform at Le Mans and Nürburgring, passes within millimetres of the vehicle's carbon fibre composite honeycomb floor. Exposure at up to 600°C for hours at a time radically compromises the structural integrity of the floor.
'The flooring is a Nomex honeycomb core sandwiched between composite skins' says project leader Paul Burling. 'Gold and silver reflective tape used in the original design breaks down and delaminates. Somehow we had to apply an aluminium reflective coating to the composite surface to reflect the heat back into the engine bay and stop the composite from burning.'
So the objective of the work was to achieve by spraying, a combined thermal barrier and reflective coating on carbon fibre composite material.
In fact TWI's surfacing team used a wire feed flame spray system to deposit a thin bond coat to improve the adhesion of a secondary thermal barrier layer. The high heat input and short stand-off distance associated with spraying ceramics demanded that the team used a ceramic metal blend. This reduced the heat input during spraying to an acceptable level.
A separate part of the work looked at using a pure aluminium reflective layer on top of the thermal barrier layer. This created a semi-matt finish, which could be buffed to a highly reflective finish.
The completed work showed that it is feasible to deposit both reflective layers and thermal barrier layers onto carbon fibre composites provided that the surface of the composite is first treated with a bond layer.
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