In 2001, scientists for the Institute for Defense Analysis assessed the huge challenges required to industrialize ceramic matrix composites (CMCs) for aviation. They starkly concluded: “There may be more pigs flying than ceramics in the future.”
Today, CMC technology leaders at GE Aviation, as well as the corporation’s Global Research Centers, hold up that report and smile. What a difference two decades of concerted effort by hundreds of GE technologists, including more than $1.5 billion in investments, have made. CMCs represent one of GE Aviation’s most aggressive technology efforts in its long history. The payoff is nothing short of transformative.
GE turbine shrouds made of CMCs now successfully operate in the hottest section of the best-selling LEAP turbofan, produced by CFM International, (a 50/50 joint company of GE and Safran Aircraft Engines), which is powering hundreds of single-aisle commercial jetliners.
CMC components are benefitting GE’s military designs, including a demonstrator engine that achieved the highest jet-engine temperatures ever. Also, GE has successfully tested CMC rotating parts.
With the 2018 grand opening of GE Aviation’s CMC operation in Huntsville, Alabama, the company celebrated another CMC milestone: a decade-long effort to establish America’s first fully-integrated CMC supply chain, which includes a network of four interrelated GE production sites. Meanwhile, GE’s new CMC component-assembly plant in Asheville, North Carolina, has produced more than 40,000 CMC turbine shrouds. The plant also fabricates five different CMC hot-section components for the GE9X high-thrust engine.
CMC technology is a centerpiece of GE’s jet propulsion strategy for the 21st century. There is no turning back.
It has been a long journey already. CMCs are made of silicon carbide (SiC), ceramic fibers and ceramic resin, manufactured through a sophisticated process and further enhanced with proprietary coatings. CMCs are one-third the density of metal alloys and one-third the weight. Because they are more heat resistant than metal alloys, CMCs require less air from the flow path of a jet engine to be diverted to cool the hot-section components. By keeping more air in the flow path instead of cooling parts, the engine runs more efficiently at higher thrust. In total, CMCs bring better fuel efficiency, lower emissions, and greater durability.
During jet propulsion’s history, the average rate of increase for turbine engine material temperature capability has been about 50 degrees Fahrenheit each decade. With CMCs, GE increased jet engine temperatures by 150 degrees Fahrenheit in one decade. As CMCs further populate the core of GE engines, they are expected to increase engine thrust by 25 percent and improve fuel burn by 10 percent.
The CMC benefits are seductive, but industrializing this sophisticated material system has posed a huge challenge to private industry for decades. Difficult to fabricate, CMCs also have brittle properties. The U.S. government has funded CMC research since the early 1970s, and GE scientists have wrestled with the technology ever since then. In the 1980s, GE pursued CMCs for large ground-based gas turbines and filed for its first CMC patent in 1986. Within 25 years, the company successfully ran CMC turbine shrouds in multiple industrial gas turbine applications.
By the mid-2000s, GE’s Global Research Center (GRC) shifted its CMC focus to jet engines. “There was a steering of the ship to jet engines as we progressed the technology,” recalls Sanjay Correa, GRC’s former head of Energy and Propulsion Technology, and, later, head of GE Aviation’s CMC program. “Because they reduce weight, CMCs held even greater potential for flying engines.”
During the same period, GE Aviation committed to demonstrating CMCs in engines and building the supply chain. By 2018, the company established CMC sites in Evendale, Ohio (component development); Newark, Delaware (low-rate production); Asheville, North Carolina; (full-rate production); and Huntsville, Alabama (raw materials). GE and Safran’s joint venture with Nippon Carbon of Japan, a leading producer and innovator of CMC raw material, is instrumental in establishing the Huntsville site.
The most recent of GE’s CMC production sites, the Huntsville complex is comprised of two factories on 100 acres. One produces SiC ceramic fiber, the first high-volume production operation in the US. Supported by USAF funding, this plant increases US capability to produce SiC ceramic fiber capable of temperatures of 2,400 degrees Fahrenheit. The adjacent factory uses SiC ceramic fiber to make unidirectional CMC prepreg, a reinforcing fabric which has been pre-impregnated with a resin system, necessary to fabricate CMC components.
The demand for CMCs for GE and CFM engines has grown twentyfold over the course of a decade. And that is just the beginning.
With an established supply chain, GE Aviation continues to increase CMC production rates and improve shop-floor productivity, both key factors in driving down the overall cost curve at a rapid rate. GE’s advancements in CMC production, castings, and coatings will facilitate a greater CMC presence in new engines, as well as in replacement parts for the massive GE and CFM base of jet engines in commercial and military service.
In the same way digital analytics is driving jet propulsion efficiencies, the same technology is refining GE’s CMC production processes, says Jonathan Blank, CMC leader at the Evendale lab. “We will institutionalize our learning, further develop the robustness of the material and process models, and drive digital tools deeper into our processes to make analytics a way of life for this vertically integrated technology,” Blank says.
GE’s CMC story parallels the historic aviation narrative. The continuous advancement of materials dates back to the Wright Brothers with the first powered-aircraft comprised of wood, steel, and canvas. Faster and more capable airplanes drove the introduction of metal alloys, such as aluminum, titanium, and other high-temperature metals.
In the jet propulsion industry, GE has introduced some of the world’s most advanced metal alloys, including single-crystal alloys, inside the jet engine core. “Both the chemistry and processing of new materials are stepping-stones for advancements in thermal efficiency, fuel burn, and emissions,” says Gary Mercer, vice president of engineering. During the 1990s, when GE introduced carbon-fiber composite fan blades for the GE90 turbofan, aircraft manufacturers more aggressively pursued large composite structures to reduce aircraft weight and increase durability.
With CMCs, GE’s journey will extend beyond the jet engine to support broader aerospace requirements, including space travel, where operating environments can be most extreme. The journey will “rocket” GE Aviation into the role of world-class CMC provider for a fast-evolving aerospace environment well into the twenty-first century.
“We are at generation one with CMCs,” says Mercer. “As you think of the future of flight, light and hotter are two constants. With the reemergence of supersonic, hypersonic, and reusable space vehicles, it is easy to see how CMCs will add value to future propulsion and airframes alike.”