To increase the service life of aircraft engines, a heat-insulating surface layer is sprayed on top of the metal components. Thanks to this extra layer, the engine is shielded from heat. The temperature can also be raised, which leads to increased efficiency, reduced emissions, and decreased fuel consumption.
The goal of the University West research group is to be able to control the structure of the surface layer in order to increase its service life and insulating capability. They have used different materials in their work.
"The base is a ceramic powder, but we have also tested adding plastic to generate pores that make the material more elastic," says Nicholas Curry, who has just presented his doctoral thesis on the subject.
Great stress on the material
The ceramic layer is subjected to great stress when the enormous changes in temperature make the material alternately expand and contract. Making the layer elastic is therefore important. Over the last few years, the researchers have focused on further refining the microstructure, all so that the layer will be of interest for the industry to use.
"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.
Shock tests simulate temperature changes
Dr Curry and his colleagues have since tested the new layer thousands of times in what are known as "thermal shock tests" to simulate the temperature changes in an aircraft engine. It has turned out 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.
Research at University West is conducted in close collaboration with aircraft engine manufacturer GKN Aerospace (formerly Volvo Aero) and Siemens Industrial Turbomachinery, which makes gas turbines. The idea is that the new layer will be used in both aircraft engines and gas turbines within two years. What happens to the material over longer periods of time?
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," Dr Curry says.
How thermal spray application works
The surface layers on aircraft engine and gas turbines are called the thermal barrier coating and they are manufactured using a method called thermal spray application. 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 millimeter thick.