Traditional ceramics handle high heat very well. Temperatures over 1,000 degrees Celsius are generally not an issue, but they have one fatal flaw: they are incredibly brittle. Drop a ceramic coffee mug on a hard floor, and it will shatter into dozens of pieces. In aerospace design, that kind of catastrophic failure isn’t acceptable. That’s why jet engines and spacecraft shields are not made of traditional ceramics.
Until recently, engineers have relied on heavy nickel-based super-alloys for high-temperature applications in aerospace. These metals are tough. They can handle the heat, but they are also very heavy and require complex cooling systems to keep them from melting.
Is there a better material choice? Absolutely. High-temperature CMCs (ceramic matrix composites) combine ceramics with engineered fibers to create a class of materials that can handle the heat of a volcano but still resist shattering. The two CMC heavyweights aerospace engineers prefer are SiC/SiC and Oxide/Oxide composites.
Traditional ceramics fail under high-performance aerospace applications because of the way they hold together. In a traditional piece of ceramic, its atoms are locked together in very tight and rigid bonds. But when the material is stressed, microscopic cracks form. There is nothing to stop the energy that created a crack, so it rips through the entire material very quickly. In the industry, this is called ‘catastrophic brittle failure’.
CMCs are designed around the principle of strength-in-numbers reinforcement. Think of how nature makes a single piece of straw. One piece of straw is easy to snap, but mix it with mud and you get a very tough building material that will hold up to just about anything. In reality, the building material is mostly mud. But it is mud reinforced by the hay. CMCs work on the same basic principle.
CMCs offer three distinct components:
When a microscopic crack forms in a CMC matrix, it doesn’t continue splitting the material. Instead, the energy is transferred into the nearest fiber. Meanwhile, the interphase allows the fiber to move just a fraction of a millimeter, absorbing the energy and forcing the crack to move along a long, winding path. Even though the material might crack internally, the entire piece holds together. This is what we call ‘graceful failure’.
Every high-temperature CMC is engineered to achieve specific properties on a per-application basis. Although they may differ from one application to the next, all feature the three previously mentioned components.
The ceramic fibers embedded in a high-temperature CMC are microscopically thin. They are just a fraction of the width of a human hair, yet they possess incredible tensile strength. Manufacturers take advantage of that strength by weaving the fibers into two-dimensional fabrics or three-dimensional braided structures. When an external force pulls or bends the CMC, its fibers absorb the weight and energy.
In terms of volume, the majority of a CMC consists of its matrix. The matrix is a continuous ceramic phase that holds the woven fibers together and fills the gaps between them. Its primary job is to give shape to the component and handle the high compression loads it experiences. The matrix also protects fibers from oxidation, moisture, chemical erosion, and other environmental hazards.
Combining ceramic fibers with a matrix creates an incredibly tough material, but it is still a material that can fail. A crack in the matrix could slice right through the nearest fiber and then continue on its way. To solve that problem, engineers coat the fibers with a thin layer of material that is typically boron nitride or carbon. This material is the interphase.
The interphase creates an intentional weak bond between itself and the fiber. This bond allows the fiber to de-bond and slip slightly when a crack occurs. The result is energy being deflected away from the fiber.
High-temperature CMCs can be as different as the applications for which they are created. Generally speaking, the industry divides CMCs into two primary families based on chemistry, fibers, and matrices: SiC/SiC and Oxide/Oxide. An engineer’s choice will depend entirely on the operating temperature and environment.
SiC/SiC composites are made using silicon carbide fibers embedded in a silicon carbide matrix. These are the CMCs engineers prefer for applications involving the hottest zones of a next-generation commercial or military jet engine.
Their biggest advantage is the ability to withstand temperatures that would melt metal super-alloys. In addition, they weigh roughly one-third as much as a typical nickel alloy, allowing an engine to run hotter while still reducing fuel consumption and carbon emissions.
Oxide/Oxide CMCs are made with oxide-based ceramics – like alumina or aluminosilicate – for both fiber and matrix. Their biggest advantage is that they are already fully oxidized. That means they will not burn, rust, or degrade, even when exposed to oxygen at high temperatures.
Both types of CMCs come with their challenges. For example, Oxide/Oxide CMCs cannot handle the same amount of heat as their SiC counterparts. But SiC/SiC CMCs require a highly specialized environmental barrier coating to protect them from the harsh atmosphere inside a jet engine.
High-temperature CMCs used to be a laboratory novelty. They are not anymore. Instead, they are actively reshaping aerospace, defense, and energy.
By overcoming the brittleness challenges of traditional ceramics, CMCs allow engineers to design lighter aircraft that fly farther on less fuel, support hypersonic vehicles capable of withstanding the extreme stresses of atmospheric re-entry, and enable industrial gas turbines to operate with greater efficiency. As advanced applications increasingly call for materials that can perform under extreme temperatures and demanding conditions, Aerodine is expanding its manufacturing capabilities beyond traditional carbon fiber and prepreg materials to include CMCs. This enables Aerodine to manufacture high-performance CMC components and parts tailored to the specific requirements of its customers’ applications.
Why are traditional ceramics impractical for jet engines?
Although traditional ceramics can handle extreme heat very well, they are too brittle. One crack could result in catastrophic brittle failure.
What protects high-temperature CMCs from this same type of failure?
All three components of a CMC – fiber, matrix, and interphase – work together to stop energy from traveling through a material and causing it to fail.
How does graceful failure work in a CMC?
Graceful failure occurs when a CMC incurs a crack but still holds together. CMCs are purposely designed to fail in this way.
How do high-temperature CMCs reduce carbon emissions?
In aircraft engines, CMCs reduce carbon emissions in two ways. The first is by reducing weight. Less weight translates into less fuel burned and fewer emissions. The second is thermal efficiency. Engines can operate at much higher temperatures without requiring heavy cooling systems. This further reduces weight, fuel consumption, and emissions.
