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Structured aluminium oxide coating provides effective heat shield

12 November 2014

A coating of micro-scaled hollow aluminium oxide spheres offers effective heat insulation at the sort of temperatures present in incinerators and gas turbines.

Hollow spheres of aluminium oxide filled with gas provide effective insulation at high temperatures (image: Fraunhofer ICT)
Hollow spheres of aluminium oxide filled with gas provide effective insulation at high temperatures (image: Fraunhofer ICT)

Gases don’t conduct heat as well as solids; cellular or aerated concretes take advantage of this effect, called 'gas-phase insulation'. The heat barrier is achieved by air encased in the cavities of the concrete.

But gas-phase insulation has wider uses, as researchers at the Fraunhofer Institute for Chemical Technology ICT in Pfinztal have demonstrated in a laboratory setting. They’ve designed a coating that consists of an outer topcoat made from conjoined aluminium oxide spheres, which are hollow and filled with gas.

When the outer side of a part is exposed to temperatures of 1,000°C, these gas-filled spheres reduce temperatures on the part’s inner side to below 600°C.

Conventional thermal barrier techniques – most of which are based on ceramic materials – are expensive. The process that the Fraunhofer scientists have adapted was originally designed to protect metallic components from oxidation.

“We’ve optimised the technique so that the coat not only retains its oxidation protection, but furthermore protects against heat,” says ICT's Dr Vladislav Kolarik. The basic coating layer forms by interaction of aluminum particles and the metallic component. This is done by depositing aluminium powder on the surface of the metal and heating it to a certain temperature over several hours, resulting in a final aluminium-rich coating.

With the new procedure an additional topcoat of hollow aluminium oxide spheres is formed. “Up to now, it never occurred to anyone to use these spheres to produce another coating layer – they were just a waste product,” says Dr Kolarik.

Now the scientists have refined the process so they can produce both coating layers in the required thickness. The way it works is to take aluminium particles and mix them with a viscous liquid bonding agent. This produces a substance similar to a paint or slurry, which the scientists then manually paint, spray or brush onto the metallic component.

“All that’s left is to add a fair bit of heat,” says Dr Kolarik. But it’s easier said than done: Dr Kolarik and his team have had to precisely fine tune the size and size distribution of the aluminium particles, the temperature and duration of the heating stage and the viscosity of bonding agents.

For more information about this new coating, click here.

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