Wednesday, 03 September 2014
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Ceramic nanoparticle coating protects aero engines from heat

Researchers at University West in Sweden are using nanoparticles in the heat-insulating surface layer that protects aircraft engines from heat.

In tests, this is said to have increased the service life of the coating by 300 per cent and it is hoped that components with the new thermal barrier coating will be in production within two years.

To increase the service life of aircraft engines, a heat-insulating surface layer is sprayed on top of the metal components. The goal of the University West research group was to control the structure of the surface layer in order to increase its service life and insulating capability.

The thermal barrier coating is manufactured using thermal spray application, whereby a ceramic powder is sprayed onto a surface at a very high temperature –7,000 to 8,000 degrees C – using a plasma stream. The ceramic particles melt and strike the surface, where they form a protective layer that is approximately half a millimetre thick.

‘The base is a ceramic powder, but we have also tested adding plastic to generate pores that make the material more elastic,’ said Nicholas Curry, who has presented his doctoral thesis on the subject.

The ceramic layer is subjected to stress due to changes in temperature that make the material alternately expand and contract. Making the layer elastic became paramount and over the last few years the researchers have focused on further refining the microstructure in order to make the layer useful for industry.

‘We have tested the use of a layer that is formed from nanoparticles. The particles are so fine that we aren’t able to spray the powder directly onto a surface. Instead, we first mix the powder with a liquid that is then sprayed. This is called suspension plasma spray application,’ Curry said in a statement.

Curry and his colleagues have since tested the new layer thousands of times in thermal shock tests to simulate the temperature changes in an aircraft engine. Results showed that the new coating layer lasts at least three times as long as a conventional layer while it has low heat conduction abilities.

‘An aircraft motor that lasts longer does not need to undergo expensive, time-consuming service as often; this saves the aircraft industry money. The new technology is also significantly cheaper than the conventional technology, which means that more businesses will be able to purchase the equipment,’ said Curry.

One of the most important issues for the researchers to solve is how they can monitor what happens to the structure of the coating over time, and to understand how the microstructure in the layer works.

‘A conventional surface layer looks like a sandwich, with layer upon layer. The surface layer we produce with the new method can be compared more to standing columns. This makes the layer more flexible and easier to monitor. And it adheres to the metal, regardless of whether the surface is completely smooth or not. The most important thing is not the material itself, but how porous it is,’ said Curry.

The research was conducted in collaboration with GKN Aerospace and Siemens Industrial Turbomachinery.

Readers' comments (4)

  • Read this article twice just to be sure.... It makes no mention at all where in an Aircraft engine this coating is used. Just says " the heat-insulating surface layer that protects aircraft engines from heat".
    Meaningless, so this otherwise interesting article is rendered more or less useless.

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  • We put your query to Dr Curry who says: The coatings are...commonly used on combustion chamber walls, inlet guide vanes, turbine blades and vanes. These being the so –called hot section components that see high temperatures due to combustion gasses.

  • Thank you for the response. I hope the process is sufficiently cost effective to make engines more robust and engine maintenance much easier.
    I am curious though about it's use on 'hot-section' turbine blades. I believe blades in many large aircraft engines are grown crystals (RR Trent?) and very effectively internally air-cooled so does this process replace or supplant this technology. A 0.5mm thick layer on any 'hot-section' blade seems a lot given the ultra tight tolerances on these items. The article claims the coating "... has low heat conduction abilities...". Would adding such a layer to a blade reduce its thermal transfer capability and could it interfere with the thermal co-efficient of the blade material? I can understand perhaps it's use on the inner surfaces of the Hot chambers but not so much on the turbine blades themselves. I would have thought anything that interfered with taking heat away from these items would be detrimental, or maybe the opposite is true, stopping the blades heating so much or so quickly?........
    Lots of questions remain for me and I'm still curious but as I don't fully understand the intricacies of cooling in such areas I suspect I will remain blissfully ignorant.

    A response/comment from one of the engine manufacturers themselves could be enlightening though.

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  • Hope this response from Dr Curry helps with your query: ‘Just to clarify: on a blade for a flying turbine we are talking closer to 200-250 microns; thicker coatings on combustor parts.

    ‘Yes its true that on most engines the blades are single crystal with a very good internal cooling. However, Rolls Royce introduced a thermal barrier coating on high pressure turbine blades using older technology in the mid-1980’s. It is my understanding that practically all modern commercial engines use thermal barrier coatings in order to lower the temperature of the turbine blade and increase component life.

    ‘This new coating technique may allow more components to be coated that were not previously due to cost or practicality considerations. The coatings will not supplant the single crystal blade technology, more it will possibly allow it to be used for longer before the engine manufacturers have to look to new materials for the blade itself.
    The coating relies on having the internal cooling in the blade to extract some of the heat. This sets up a thermal gradient across the ceramic coating that can be as much as 200 degrees Celsius. This is then able to exploit the insulating properties of the coating.
    The metal blade is then able to operate at 1,000 Celsius or there abouts…. with a surface temperature of perhaps 1,200 Celsius or greater on the ceramic layer. In this way the blade can operate in an environment that would cause it to soften and fail without the coating. Its also possible to reduce the amount of cooling air required in this case which also helps efficiency.'

  • Brilliant. Thank you both for your responses.

    Dr Curry - Given your explanation I wish you well in your endeavours and look forward to seeing the results implemented and enabling significant reductions in Engine 'down time'.

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  • Nowhere in the article does it discuss the coefficient of thermal expansion of the coating material and how this would affect its adhesion to different substrates such as metals and composites. Please elaborate.

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