What High-Temperature Materials Are Being Used in Aviation?
The jet engine remains the pinnacle of modern aviation technology. With its extremely complex design and the need to consistently function without failure, aerospace engineers are constantly looking for ways to improve their performance and safety. In this domain, one of the biggest focuses is the material used in jet engine construction. Traditionally, each section of the engine was made of different materials due to the varying heat and mechanical stress. Today, several novel materials and composites are being tested to replace the heterogeneous design.
The biggest obstacles for jet engine materials to overcome are heat and shear stress. In the aerospace realm, the four classes of materials that can withstand such factors are composites, intermetallic alloys, ceramics, and refractory materials. Composites are being used to replace traditional materials on several aircraft sections, including the wings, interiors, nacelles, and more. They are advantageous due to their ability to maintain lighter weight and optimal fuel efficiency without sacrificing strength. Currently, the majority of composite materials are made from metal matrix composites (MMCs), ceramic matrix composites (CMCs), and fiber-reinforced polymers (FRPs).
CMCs have recently been commercialized and widely implemented in next-generation aircraft, particularly military models. Their most noticeable benefits are that they can withstand extreme operating temperatures and are around 30% lighter than similar MMCs. They can also readily combust fuel due to their ability to run at high heat, leading to increased efficiency and decreased emissions. MMCs, on the other hand, are generally made from a pure metal or an alloy, and therefore are much stronger than polymer matrices and CMCs. While the MMC's operating temperature is lower than its ceramic counterpart, they offer incredible longevity and are cheaper to fabricate.
Carbon-carbon composites have long been implemented in re-entry heat shields and gas turbine engines. They are generally lightweight and can be exposed to moderately high temperatures for a long period of time while still retaining durability. Meanwhile, sandwich composites are designed with a lightweight yet thick core and two sheets of a thin, rigid material. This composition allows them to be incredibly lightweight while maintaining a moderate degree of durability and corrosion resistance. Finally, braided composites account for many propellers, stator vanes, and fan-blade containment cases in modern aircraft due to their superior strength and stiffness.
Other composites such as auxetics, nanomaterials, and hybrid multiscales are being used in experimental applications but have yet to be widely adopted. These composites feature outstanding energy absorption, superior crack resistance, and a high shear modulus. In addition, many nanomaterial composites have the added benefit of being electrical and thermal conductors.
Intermetallic materials have been studied for several decades, but fail before ever reaching commercial application. While less dense and inherently oxidation resistant, intermetallic alloys generally have a very narrow window of operating temperatures, usually below that of a jet engine. However, some nickel-based superalloys maintain the favorable properties mentioned while also featuring a higher melting point.
Pure ceramics feature the highest degree of heat resistance but are much more brittle than any other materials considered in jet engine development. As such, the lighter, more durable ceramic-metal mixes combine the best of both materials to create a high-heat, high-strength composite. Although pure ceramics are sometimes used to mold fasteners in the hottest engine areas, there are currently no examples of commercially available components made from this material.
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