Beyond Superalloys: Why CMCs are the Future of Hypersonic Flight

The latest frontier in aerospace engineering is hypersonic flight. Aerospace engineers are pushing vehicles to the limits, reaching speeds of Mach 5 – five times the speed of sound – and beyond. But moving that fast means fighting the laws of both physics and chemistry. A big concern from our end is a thermal environment that makes traditional aircraft construction materials wholly inappropriate.

Nickel-based superalloys have been the go-to choice for decades. These materials have set the standard for the high-heat environments of the jet age, proving themselves to be the best choice for what is often the hottest section of an aircraft: the turbine. Unfortunately, we have reached a thermal ceiling as we press forward with hypersonic flight. Nickel-based superalloys will not work for next-gen propulsion systems.

The solution? Ceramic matrix composites (CMCs). Here at Axiom Materials, we see the shift toward CMCs as a milestone, and our engineers will press forward designing and building the aircraft of the future. We couldn’t be more pleased. The CMC space is one of our specialties.

The Thermal Ceiling Problem

As useful as nickel-based superalloys have been over the years, they have certain limitations that make them inappropriate for achieving hypersonic flight. The metals are marvels of engineering, no doubt. But we have pushed them to operating at temperatures that are very close to their own melting points. This is a dangerous place to be in the hypersonic arena.

Most high-performance superalloys begin losing structural integrity in the neighborhood of 1,000°C. At this temperature, they are susceptible to a condition we call ‘creep’, meaning they start to demonstrate permanent deformation under stress. Practically speaking, creep leads to the metals becoming soft in engine components and on leading edges – where temperatures can easily exceed 1,200°C.

Engineers can still utilize superalloys, but they need to employ two workarounds:

• Heavy Cooling Systems – Systems capable of pumping cold air into complex channels built within the parts themselves.
• Thermal Barrier Coatings – Ceramics that shield metal components, thereby helping to keep temperatures down.

Both solutions work. However, they add significant weight to an aircraft. Deploying the workarounds also adds to system complexity and the resulting maintenance costs. But weight is still the big issue. Designers would rather avoid the extra weight so that it can be dedicated to increasing fuel or payload capacity.

How CMCs Solve the Problem

Nickel-based superalloys are monolithic materials, while CMCs are composites. Therein lies the major difference that gives the advantage to ceramic matrix composites. Manufacturers, like Axiom, combine high-strength ceramic fibers and a ceramic matrix to create a material boasting exceptionally high heat resistance without the brittleness ceramics have historically been known for.

Comparing CMC performance against that of nickel superalloys at 1,200°C clearly reveals the advantages of choosing CMCs:

1. Exceptional Heat Tolerance

A superalloy will struggle to remain rigid at 1,200°C. Meanwhile, a high-quality CMC can offer it comfortably at temperatures that would render a nickel alloy structurally useless. The exceptional heat tolerance CMCs offer make them ideal for hypersonic engines designed with greater thermodynamic efficiency and increased thrust.

2. Significantly Less Weight

Weight is the ultimate tax in aerospace design. Where nickel superalloys are dense and heavy, CMCs are just the opposite. On average, a CMC weighs just one-third of the metal it replaces. This is everything in hypersonic flight. Less weight is a force multiplier that accommodates higher speeds, longer range, and the ability to carry more advanced components with no additional weight.

3. Dimensional Stability

Heat forces metals to expand and soften. Under normal flight conditions, the heat is manageable. But at hypersonic speeds, heat becomes a real problem for nickel superalloys. On the other hand, CMCs are incredibly stable. They maintain both shape and stiffness even under the extreme conditions of hypersonic maneuvering. This dimensional stability ensures that aerodynamic surfaces remain precise throughout flight.

Preventing Oxidation With Oxide/Oxide

Many types of ceramic matrix composites could be used in aerospace design. But specifically where hypersonic design is concerned, we have found that Oxide/Oxide (Ox-Ox) composites are the way to go. Why? Because Earth’s atmosphere is incredibly oxygen rich.

Carbon/Carbon composite can oxidize (burn away) if their coatings are damaged. This is a problem in hypersonic flight. But Ox-Ox composites resist oxidation. They do not require protective coatings to handle high temperatures. And even if a component is impacted or scratched, the material beneath the surface remains stable. This makes Ox-Ox composites the wise choice for reducing the risk of catastrophic failure.

The Bottom Line for Decision-Makers

As the push toward faster hypersonic speeds continues, decision-makers need to determine how they are going to push the envelope both safely and cost-effectively. Here is the bottom line in terms of CMCs:

• Simplified Design – CMCs eliminate the need for complex cooling systems. This allows engineers to focus on cleaner, simpler engines and airframes. Assembly and maintenance are also simplified because the total number of parts needed to complete a system is reduced.
• Increased Payload – Every bit of weight saved by switching from nickel-based superalloys to ceramic matrix composites can be dedicated to increasing payload or preserving fuel.
• Mission Safety – There is no room for error in the hypersonic space. Using materials designed specifically for the hypersonic environment, like CMCs, greatly increases both mission safety and the likelihood of success each time an aircraft takes to the skies.

The transition to hypersonic flight requires a new way of thinking. Heat is no longer a problem to be managed. It creates an environment to be mastered. We take that as a personal challenge here at Axiom Materials.

The era of superalloys as the primary material for the hottest parts of hypersonic aircraft is quickly coming to an end. We need materials that perform better at high temperatures in order to get to Mach 6 and beyond. Those materials are ceramic matrix composites. They are leading the way to higher speeds, increased payloads, and greater fuel efficiency – even at breathtaking temperatures.

FAQs

Why is 1200°C the threshold when choosing certain materials?

Most nickel-based superalloys reach their physical limits at that temperature. CMCs can perform flawlessly at much higher temperatures.

How do CMCs overcome the brittleness of traditional ceramics?

The secret is in the composite design. CMCs comprises a ceramic matrix with ceramic fibers embedded within. The fibers reinforced the material to overcome brittleness.

Does using CMCs require a complete redesign of the engine or airframe?

In some cases, CMCs can be utilized as drop-in replacements. But to make the best use of what CMCs bring to the table, it is best to design around them.

How do CMCs impact a hypersonic aircraft’s fuel burning?

Weight reduction increases fuel efficiency. In addition, CMC thermodynamic efficiency allows an engine to run hotter, thereby ensuring that fuel is burned more efficiently and completely.

Are CMCs susceptible to moisture and salty air?

The Ox-Ox composites we specialize in are exceptionally resistant to both. They do not corrode or oxidize, making them perfect for coastal and maritime operations.