Nickel superalloys are tough. Seriously tough. For decades they have been the workhorses of jet engine hot sections, withstanding temperatures that would melt most metals into puddles. They’ve earned their reputation.
But they’ve hit a wall.
The hottest bits of a modern jet engine whiz past 2,200°F; nickel alloys can take that, just about, but only with elaborate cooling systems that bleed air out of where it might be making thrust. That air that’s being cooled is lost efficiency. Lost fuel savings. Lost money. And tomorrow’s engines must run even hotter to extract the performance improvements that airlines, militaries and the planet are requesting.
So engineers went in search of something better. They discovered it in ceramic matrix composites. And what these materials can do verges on ridiculous.
What Ceramics Like This Bring to the Fight
Here’s the basic pitch. Silicon carbide fibers inside a silicon carbide matrix. SiC/SiC, if you want to be cryptic. These composites are around a third the weight of the nickel superalloys they replace. They put up with temperatures 500°F higher, running happily at 2,400°F. And because they don’t require quite as much cooling air, the engine can stuff that airflow into making thrust instead of babysitting hot metal.
One-third the weight. Higher temperature capability. Better efficiency. That’s not an incremental improvement. That’s a separate category of material.
But the really clever bit is what they do with damage. Ceramics, by themselves, are brittle. Drop a coffee mug on tile and you know this. Plant a turbine blade made of pure ceramic, and the first time something goes wrong it will just break. Which, in a jet engine rotating at tens of thousands of RPM, is catastrophic.
CMCs have a trick up their sleeves borrowed from nature to solve this. When a crack appears in the ceramic matrix, it doesn’t shoot straight through the fiber. Instead, it bounces off along the fiber-matrix boundary losing energy as it travels. The crack spreads sidewise, not through. Instead of shattering, the material absorbs the damage. It’s the same principle that makes plywood sturdier than a solid plank. Layers redirect force.
The result is a ceramic with toughened action. Not brittle. Tough.
Elements of Engines, Parts and Flights
This isn’t theory anymore. These materials are taking a flight right now, on commercial aircraft with paying customers.
The CFM LEAP engine used on the Airbus A320neo and Boeing 737 MAX features 18 SiC/SiC ceramic matrix composite turbine shrouds per engine. The LEAP entered commercial service in 2016 and quickly became the best-selling jet engine in the world, with a backlog that has topped more than 15,500 orders. To Date the manufacturer has dispatched more than 100,000 CMC turbine shrouds, which by 2021 had amassed more than 10 million flight hours of service.
Ten million hours. On a material that some engineers still consider “experimental.” It’s not experimental anymore. It’s infrastructure.
The LEAP is 15 percent more fuel efficient, with 15 percent lower CO2 emissions than the CFM56. CMCs are only one part of that improvement, along with composite fan blades and improved aerodynamics. But they’re a significant piece.
Then there’s the GE9X. The world’s largest commercial jet engine at 134 inches across the fan, built for the Boeing 777X, producing over 100,000 pounds of thrust. The GE9X doesn’t just use one CMC component. It uses five: inner and outer combustor liners, high-pressure turbine Stage 1 shrouds, and Stage 1 and Stage 2 nozzles. During testing, CMC components ran through 2,800 endurance cycles and came out in what GE described as pristine condition.
The CMC parts in the GE9X need 20 percent less cooling air than equivalent metal parts. That reclaimed air goes straight into thrust production. Which means you’re getting more push from the same fuel burn. Airlines notice that kind of thing. Quickly.
The Weight Savings Trick That Multiplies Itself
Replace a nickel superalloy turbine component with CMC, and the part itself weighs about half as much. That’s the obvious savings. But here’s what most people miss: lighter rotating parts produce lower centrifugal force, which means the shaft holding them can be smaller, which means the structure around the shaft can be lighter, which means…
The savings cascade. GE estimates this multiplier effect can triple the initial weight reduction. So a 10-pound weight saved on the part itself might mean 30 pounds saved on the entire engine assembly.
That matters more than it sounds. Every single percentage of fuel reduction saves a commercial airline more than a million dollars per year. Rolls-Royce reported that composite fan blades and a composite fan case on their UltraFan demonstrator cut 700 kilograms from a twin-engine aircraft. That’s seven passengers with luggage. Seven seats’ worth of fuel saved on every flight, forever.
Change that out over a fleet of 200 planes, operating four flights a day, and those numbers become very real very quickly.
The Achilles Heel (and the Fix)
There’s a catch. There’s always a catch.
Water vapor is generated as fuel burns inside a jet engine combustor. That water vapor interacts with the protective silica layer that naturally develops on the outer surface of SiC-based composites. The reaction produces a gas that slowly devours the material. Without the proper protection, CMC components would wear and ultimately fail.
NASA Glenn Research Center has termed the rapid surface recession as the Achilles heel of CMCs. And that’s not an exaggeration.
The answer lies in environmental barrier coatings, or EBCs. Very thin, carefully engineered coatings on the CMC surface that prevent water vapor from penetrating through to the silicon carbide. First-generation EBCs employed a three-layer structure consisting of a silicon bond coat, mullite middle layer, and barium-strontium-aluminosilicate top coat. Second-generation coatings transitioned to rare earth silicates, especially ytterbium disilicate which showed improved performance at high temperatures. Third-generation architectures are five-layer systems aimed at a capability of approximately 2,700°F with even more exotic chemistries. NASA has demonstrated in excess of 500 hours of steam oxidation survivability at 2,700°F with advanced EBCs.
But real-world surprises still happen. In one instance, a LEAP engine in commercial service showed coating flaking on the CMC turbine shrouds and a design change was needed to correct it in October 2017. That’s not a shortcoming of the technology. It’s a reminder that engineering at these temperatures is hard in practice.
Constructing These Things Presents a Challenge in Itself
You don’t stamp out a CMC part like a metal component. It is slow, prohibitively expensive and unforgiving. More than $1.5 billion in CMC technology and established a vertically integrated supply chain spanning five sites across the United States. The melt infiltration process turns SiC fiber into a finished part in less than a month, which seems quick until you do the math compared to how fast you can make metal parts.
By 2020, about 750 employees were manufacturing as much as 20 tons of CMC prepreg, 10 tons of SiC fiber and more than 50,000 engine parts annually.
The main barrier to wider adoption of CMCs outside of premium jet engine applications remains cost. But the gap is closing. And the performance gains are big enough that engine manufacturers continue to put money into it.
Where This Goes Next
The industry’s ultimate goal: 2,700°F for SiC/SiC composites. Achieving this goal will be as hard, Oak Ridge National Laboratory has said, as it was to create the very first ceramic composite. According to a study by the National Academy of Sciences, this ability could greatly decrease, or even eliminate, the need for cooling air in some engine sections.
Beyond that, materials scientists are developing ultra-high temperature ceramics for scenarios where even SiC fails. Zirconium diboride, hafnium diboride, and hafnium carbide have melting points exceeding 3,000°C, with hafnium carbide near a record-setting 3,958°C. These are under development for hypersonic vehicles and next-generation propulsion systems.
The progression is clear. Metals pushed jet engines as far as they could go. Ceramics are taking them further. And the materials after ceramics? Those are still being invented.
That is the beauty of high temperature composites. The story is far from over. It is still only a few pages into the first chapter.
WHAT CMCs DELIVER
→ 1/3 the weight of nickel superalloys at 500°F higher operating temperature
→ Less need for cooling air which equates directly to additional thrust from the engine
→ Crack deflection mediated by fiber-matrix interface damage tolerance
THE BOTTOM LINE
Metals pushed jet engines as far as they could go. Ceramics are taking them further. The story is hardly through its first chapter.
Mentis Sciences operates at the intersection of advanced composites, aerospace materials and defense engineering. Whether they are ceramic matrix composites or hypersonic thermal protection, the materials that survive extreme environments are the materials that matter. www.mentissciences.com
