Fiber ceramics for aviation engines

Lightweight, temperature-resistant, robust—ceramic matrix compounds are the ideal materials for making engines. These new materials have the potential to reduce weight, optimize combustion and raise efficiency.

09.2018 | Text: Monika Weiner

Text:
Monika Weiner has been working as a science journalist since 1985. A geology graduate, she is especially interested in new developments in research and technology, and in their impact on society.

The Porsche 911 GT2 has them and so do the Ferrari 488 GTB and the Lamborghini Aventador: lightweight brake discs made of ceramic matrix compounds. These discs don’t rust, don’t abrade and don’t smolder even when the driver slams on the brakes at 300 kilometers an hour.

What has proved itself in automotive manufacturing is now set to become an asset in aviation. “Ceramic matrix compounds open the door to significant weight reductions. Their lightweight properties alone make them extremely attractive for engine construction,” says Dr. Bertram Kopperger, head of compound materials at MTU Aero Engines. “Another benefit is that they are highly temperature-resistant. This means we can develop new, powerful and efficient engines with material temperatures of up to 1,400 degrees Celsius.” Ceramic matrix compounds, or CMCs for short, require less cooling than metals. This means air that would previously have been compressed and fed through cooling channels is now available to aid propulsion, which in turn makes the engine more efficient. Kopperger sees these new materials as an aid to achieving the European Commission’s Flightpath 2050 targets, which MTU maps out in its Claire initiative. Claire stands for Clean Air Engine. Through Claire, MTU is striving to reduce fuel consumption by 40 percent by the year 2050, compared with today’s V2500 engine.

“Ceramic fibers embedded in a ceramic matrix—what are known as CMCs—don’t have the brittleness of traditional ceramics and as such can be used in structures that have to withstand high loads—such as aircraft engines. Compared to metal materials, they can tolerate significantly higher temperatures, which in turn raises the turbine’s efficiency.”

Dr. Friedrich Raether, director of the Fraunhofer Center for High Temperature Materials and Design HTL in Bayreuth, Germany.

Eight times thinner than a human hair

These materials, the stuff of dreams for engine developers, are reinforced with ceramic fibers in order to give them the required robustness. These fibers are eight times thinner than a human hair and are characterized by their extremely high break resistance. Also the ceramic matrix in which the fibers are embedded is robust and virtually non-malleable. Amazingly, the combination of these two ceramic components produces materials that can cope with high loads. Although small cracks do appear in the material, these cannot spread because they are diverted by many thin fibers and are robbed of the energy they would need to grow. The key to this behavior lies in the way the fibers bond with the matrix, what’s known as the interface, where interactions happen that must be properly “adjusted,” as the material specialists say.

“Since ceramic fibers embedded in a matrix don’t have the brittleness of traditional ceramics, they can be used in structures that have to withstand high loads—such as aircraft engines,” says Dr. Friedrich Raether, director of the Fraunhofer Center for High Temperature Materials and Design HTL in Bayreuth, Germany.

“Producing CMCs still represents a major challenge,” explains Katrin Schönfeld from the Fraunhofer Institute for Ceramic Technologies and Systems IKTS, who is developing new CMCs for the aviation industry. “Humans have thousands of years of experience working with metals, and we know all about what they can do. But with CMCs, we’re only at the beginning of the story: We have to establish and optimize new production processes; it’s about finding out what the materials can withstand and how to use them in practice.”

Ceramic fiber-compound materials

A fiber-compound material is made up of two main constituents: an embedding material (matrix) and the fibers embedded in it. Through interactions at the interface between the two constituents, the compound material has a much higher damage tolerance than each of the constituents separately. The arrangement and volume of the fibers can be adjusted as needed. In this way, it is possible to set the desired component characteristics.

The challenge of production

Even the embedding of the fibers in the ceramic matrix is tricky enough. It starts with a basic structure of fibers, which is encased in a liquid smelt that then hardens. Imagine pouring concrete onto a framework of steel supports. This method is called liquid-phase infiltration. To produce a compound that is good enough to be used in an engine, this process must be repeated multiple times. An alternative is chemical-vapor infiltration, in which the fiber framework is placed in a reactor and bathed in the matrix in gas phase. Here, the ceramic matrix accumulates around the fibers, one layer of atoms at a time. But this takes time: it can take months to produce a single part.

What counts for the properties of the finished workpiece is the chemistry of the ingredients: if aluminum oxide fibers are encased in an aluminum oxide matrix, the result is oxidic CMC—what the developers also call Ox/Ox. This is very robust because neither air nor aggressive chemicals can harm it. While this “white CMC” can be produced relatively cheaply, it can withstand temperatures of only 1,200 degrees Celsius. More temperature-resistant—up to 1,400 degrees Celsius—and more robust is non-oxidic “black CMC.” This is made of silicon carbide fibers in a silicon carbide matrix, or SiC/SiC for short. Since this combination does not have sufficient corrosion protection when oxygen penetrates the surface, parts must also receive a protective layer known as the environmental barrier coating. Production is therefore complex and expensive.

“Both white and black CMCs are suitable for manufacturing turbines. Which material gets used depends on the environmental conditions,” Kopperger explains. “Being particularly temperature-resistant and able to withstand mechanical loads, non-oxidic SiC/SiC materials are used for components such as blades. In contrast, gas-feeding housing parts can be manufactured from oxidic CMC.”

(strich:Heat test) A development part made from oxidic CMC is being tested in an oven heated to as much as 1,100 degrees Celsius to see how it holds up under extreme conditions. Hover over the image for a bigger view

Heat test A development part made from oxidic CMC is being tested in an oven heated to as much as 1,100 degrees Celsius to see how it holds up under extreme conditions.

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Heat test A development part made from oxidic CMC is being tested in an oven heated to as much as 1,100 degrees Celsius to see how it holds up under extreme conditions.

(strich:Research for aviation) After testing, the parts are thoroughly inspected to ascertain how the manufacturing processes can be further optimized. Hover over the image for a bigger view

Research for aviation After testing, the parts are thoroughly inspected to ascertain how the manufacturing processes can be further optimized.

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Research for aviation After testing, the parts are thoroughly inspected to ascertain how the manufacturing processes can be further optimized.

(strich:Ultra-thin) The ceramic fibers—seen here magnified 1,000 times—are eight times thinner than a human hair. Reinforcing the matrix with the fibers increases its break resistance, allowing for the material’s use in engine construction. Hover over the image for a bigger view

Ultra-thin The ceramic fibers—seen here magnified 1,000 times—are eight times thinner than a human hair. Reinforcing the matrix with the fibers increases its break resistance, allowing for the material’s use in engine construction.

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Ultra-thin The ceramic fibers—seen here magnified 1,000 times—are eight times thinner than a human hair. Reinforcing the matrix with the fibers increases its break resistance, allowing for the material’s use in engine construction.

(strich:Special properties) White CMC cannot be used at temperatures as high as black (SiC/SiC) CMC can, but it doesn’t have to be protected from oxygen when exposed to high temperatures. Hover over the image for a bigger view

Special properties White CMC cannot be used at temperatures as high as black (SiC/SiC) CMC can, but it doesn’t have to be protected from oxygen when exposed to high temperatures.

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Special properties White CMC cannot be used at temperatures as high as black (SiC/SiC) CMC can, but it doesn’t have to be protected from oxygen when exposed to high temperatures.

Start your engines

The first CMC parts have already passed muster in engine construction. CFM is using SiC/SiC sealing rings. The GE9X engine made by GE Aviation is also equipped with CMC parts. And Boeing tested a CMC acoustic exhaust nozzle, designed to reduce noise, for the Trent 1000 from Rolls-Royce.

For its part, MTU wants to first use the new materials in the further development of today’s geared turbofan engines. CMCs are to be used in the manufacture of moving and static turbine blades as well as housing parts. “These materials are not available off the rack, so we work with our partners in research and industry to come up with new materials,” Kopperger reports. Partners include BJS Ceramics in Gersthofen, the Schunk Group in Heuchelheim, DLR in Stuttgart, the Fraunhofer Center for High Temperature Materials and Design HTL in Bayreuth and the Fraunhofer Institute for Ceramic Technologies and Systems IKTS in Dresden.

“Our goal is to expand on MTU’s expertise in designing suitable engine components and to establish an accessible supply chain for manufacturing said components,” Kopperger explains.

Development in the EU project

MTU engineers have already gathered key experience in the EU Clean Sky technology project. In the initial phase, protective layer segments for the interior coatings for housings were produced and tested. The project partners are currently working on parts for flowpath hardware. Kopperger notes that the design and manufacture both of the demonstrator engine and of the test run are of particular interest, because this is where they can test how to produce hardware that meets to aviation regulations and how ceramic parts interact with metal ones: “Metals expand when they are heated, much more so than ceramics. If we’re looking to replace more and more metal parts with CMC ones, we need to be prepared for the kinds of design solutions this might entail.”

In the future, a significant number of turbine components could be made from CMCs. But does this make economic sense? At the moment, it costs much more to make a part out of CMCs than out of metal. “But the cost will go down once the materials and parts manufacturing are scaled for mass production,” Kopperger is sure. And: “The reduction in fuel consumption brought about by greater efficiency will justify the extra costs for ceramic matrix compounds.”

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