Designers of aero engines and industrial gas turbines need to know if high temperature materials are working close to their maximum temperature capabilities.
Thermochromic paints can help with this process because, if applied correctly, they give an inspector a visual indication of temperature contour patterns on that engine part.
Legislation, however, is set to greatly restrict the use of thermal paints that contain chromate and Pigment Red, substances deemed toxic under the EU’s REACH regulations.
Dr Jörg Feist, managing director of Sensor Coating Systems (SCS) is confident that his company’s Thermal History Coating (THC) - shortlisted in 2011 for The Engineer’s Technology & Innovation Awards - will fill the gap and simultaneously bring about efficiency gains.
‘Thermal paints are being used in the design process for more energy efficient gas turbines,’ explained Feist via email. ‘However, using the thermal paints is not just toxic, but very laborious and a failed paint test for a component can easily be worth, for example, £25,000 when the paint comes off or when the paint is used a few minutes longer than anticipated. The readout time after a successful test is also very time consuming. Our technology will be faster and more robust.’
SCS has now established an industry User Club to further the development of THC and members so far include Alstom (Switzerland), MAN Diesel & Turbo, Germany, and SNECMA, France.
‘In principal the development programme is co-financed by the club members and in return they receive commercial benefits when the technology is being introduced into their design processes,’ said Feist.
Testing
THC is based on the light emitting properties of a class of ceramic materials, which, when exposed to particular levels of temperature, undergo irreversible changes in the material structure or chemistry.
When excited with a probing light the material starts to phosphoresce and this can be observed with specialised optical components to establish a correlation between the observed light and the past temperature. The readout device can be bench based or hand held, the latter enabling in-situ temperature profiling on a component.
Feist explained that SCS has developed two types of THC coating.
‘The paint coating is water based and can be applied using a commercially available [hand] air-spray-gun. However, durability of paints is limited time wise and in relation to high temperatures,’ he said. ‘The second coating solution is based on atmospheric plasma spraying (APS) or electron physical vapour deposition.
‘These techniques are well established and THCs have been already produced using a commercial production line. The longest living THC has been applied in Didcot power station for 4,500 hrs.
‘The base material is made of an oxide ceramic and features optical active elements also found in mobile phones displays or energy efficient light bulbs and LEDs.’
Feist said an OEM speculated that - per engine development programme - costs of the order of £1.5m to £3m could be saved and that THC would similarly speed up development times and hence the route to market.
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