Coatings key to lighter jets

Materials World magazine
,
3 May 2015

Award-winning research into environmental barrier coatings could reduce the emissions, noise and weight of aeroplanes. Simon Frost reports. 

Coatings that improve the durability of ceramic matrix composites (CMCs), a lighter alternative to nickel alloys being incorporated into jet engines, are being developed at Imperial College London. 

CMCs, such as silicon carbide fibre-reinforced silicon carbide, can withstand temperatures 300°C higher than nickel-based alloys, removing the need for high levels of cooling air in turbine components. This, together with their low density compared with conventional metal-based components, leads to weight savings of around 30%, significantly reducing the operational costs.

However, CMCs suffer from poor durability, which means that they require costly environmental barrier coatings (EBCs) to operate under high-velocity combustion. 

Dr Nasrin Al Nasiri explains, ‘In the past 10–15 years, researchers have been working with rare earth disilicate EBCs, containing more silicon oxide than rare earth oxide. Typically, three or four layers of these coatings are applied to prevent cracking, but continuous coatings haven’t been mastered and these disilicates often crack under steam, sometimes through all of the layers to the CMC.’  

Instead, Al Nasiri examined the properties of single-layered monosilicate EBCs. ‘One mole of rare earth oxide and one mole of silicon oxide,’ she says. ‘It hasn’t been done before because no one wants to work with monosilicates. They have a high thermal expansion coefficient, which could cause a thermal expansion mismatch when you go from ambient to high temperature.’ 

However, CMCs are also subject to thermal expansion, so Al Nasiri developed five different monosilicate EBCs to test their compatibility. ‘I looked at erbium, lutetium, gadolinium, yttrium and ytterbium monosilicates. These all have a similar thermal expansion coefficient to the CMC, except for gadolinium, which is around twice as high. Monosilicates are very stable compared with disilicates, which have a tendency to go back to their rare earth oxide when heated, leaving the silicon oxide on top so that it reacts with the oxygen and it’s as though you don’t have any coating.’ Erbium, lutetium and yttrium displayed the best thermal properties. 

Towards production

‘We’re carrying out two kinds of corrosion test. The first is a static test, where we put the coated CMCs into a high-temperature furnace and apply steam, normally 90% water and 10% air. Then we can see if the coating remains as a monosilicate or goes back to the rare earth oxide. The other kind of test is in simulated gas turbine conditions – high-temperature steam at a high velocity. We want to see if the monosilicates are stable, without any cracking. Next year, we will be testing on a larger scale.’ 

The research, fully funded by Rolls-Royce, earned Al Nasiri the Gold Award in Engineering at the 2015 SET for Britain poster exhibition. Rolls-Royce aims to put the EBCs into production in 2020, and is currently patenting the method. 

EBCs are normally applied using a plasma spray. Rare earth powder is loaded into a spray gun and fired at the CMC at high temperature. It’s expensive because the samples need to have a specific roughness, but also because it is inefficient. Plasma spraying isn’t accurate enough to coat very small turbine parts without a lot of waste – about 60% of the powder is lost. ‘We’ve developed another way to apply the coating,’ she says, ‘but that’s a part we’re looking to patent at the moment, so that’s all I can say for now.’ 

Rolls-Royce is not alone in developing CMC technology – GE Aviation announced in March 2015 that it was ground testing an engine containing CMC components for the Boeing 777X, which is scheduled to enter service in 2020.