Three critical factors determine the success of any new material technology: material properties, design practices (i.e., how to properly use and apply the material) and a supply chain that can produce the material at specification. Apart from these development and production challenges, a business case needs to be made in which the risk and cost of introducing a new material technology is outweighed by the benefits (i.e., lower weight, lower manufacturing and/or support costs, and higher performance).
Since the early 1990s, a nondescript GE Aviation microfactory hidden in the corner of a Newark, Del., industrial park has pioneered the successful introduction of ceramic matrix composites (CMCs) in the aerospace industry, culminating in the first commercial aircraft engine with CMCs—CFM International’s LEAP engine. The LEAP is scheduled to enter service in 2016 and will power the new Airbus A320neo, Boeing 737 MAX and COMAC (China) C919 aircraft. Like any new material integration, GE’s two-decades-plus CMC journey has been long and arduous, with more than one million hours of testing completed at GE Aviation facilities across the world to mature the technology.
Soon after the Space Shuttle Columbia broke up on descent from orbit in February 2003, material scientists and engineers at GE’s Newark plant started building a set of repair kits long thought impossible. Columbia suffered a crack in its left wing by a briefcase-sized insulating foam fragment that fell from a fuel tank during takeoff. During the shuttle’s return, superheated air entered the spacecraft through the wound and ripped the shuttle apart 15 minutes before touchdown. To prevent future disasters, the GE team—in collaboration with NASA and industry partners—helped design and fabricate patches to plug up similar damage on the shuttle’s wings and belly while in space.
The team designed the patches from a special ceramic composite material that could survive wild temperature swings, from -250°F in orbit to a 3,000°F inferno caused by the drag of Earth’s atmosphere during the shuttle’s 17,000 mph descent.
“You could bolt it on the wing leading edge in space and cover the damaged portion,” says Robert Klacka, technology marketing manager at GE Ceramic Composite Products in Delaware. “The repair kit had 30 different patches that could cover a hole located on over 80% of the wing leading edge surface. The thin, flexible panels used a high-temperature toggle bolt to attach it through the hole on the wing. Thankfully, we never had to use them.”
The uses for commercial and military engines have only just begun, however. In addition to the LEAP engine, GE’s adaptive cycle engine for the next-generation fighter application is running successfully with CMCs throughout the hot section. The GE9X, GE’s replacement for its GE90 engine powering Boeing’s 777, will incorporate CMCs in the combustor and high-pressure turbine. Today, the F414 engine powering the F-18 includes external flaps and seals comprised of CMCs.
CMCs are made of silicon carbide ceramic fibers and ceramic resin, manufactured through a highly sophisticated process and further enhanced with proprietary coatings. The CMC manufacturing process starts with yarns of thin fibers imported from Japan; these fibers are a fraction of the thickness of human hair. The fibers are strengthened by a proprietary coating that is then passed through a “slurry bath,” melding the fibers into a rigid tape.
The tape is cut into precise 3D shapes that are sent to an autoclave to be heated at temperatures up to 2900°F. The autoclave heats up the material and sets the geometry of the part. Organic compounds are baked out of the part, leaving a carbon-rich matrix surrounding the fibers. Then the parts are shipped to the “melt infiltration” chamber, where they’re baked to extreme temperatures, allowing the silicon to react with the remaining carbon to finally form the CMC. Following this process, the CMC is 98% dense, giving the part its unmatched strength at the high temperatures needed for improved engine efficiency and
precious fuel savings.
Silicon-containing materials are subject to loss of material at high temperature due to a reaction with water vapor. This recession is a known chemical reaction, and is mitigated by an appropriately designed multi-layer coating, known as an environmental barrier coating (EBC). Part of GE’s materials development effort involves this coating where recession is predictable, not a sudden failure mode.
CMCs are lighter, stronger and more heat-resistant than conventional nickel super-alloys, leading to benefits in both fuel burn and performance. CMCs are one-third the weight of traditional alloys, trimming hundreds of pounds off future engine applications.
Their heat-resistant nature allows for the diversion of less cooling air into the engine’s hot section, thereby improving overall engine efficiency. By using this precious cooling air in the engine flow path instead, the engine can run more efficiently at higher thrust.
In jet engine propulsion history, the average rate of technology progress for turbine engine material temperature capability increased 50° per decade. With the introduction of CMCs, GE increased its capability by more than 150° this decade (3x the traditional rate). Because of the increased temperature capability, GE’s advanced CMCs could increase engine thrust by 25% and improve specific fuel consumption by 10% in the coming decade.
With the growing need for more fuel-efficient, low-emission engines, demand for CMCs is expected to grow tenfold over the next decade. The year 2013 marked an important year for the infrastructure investment needed to support that demand, as GE committed to doubling the size of its Newark facility, adding up to 70 jobs and investing more than $25 million. This will allow the microfactory to continue to prove out manufacturing processes for additional CMC components.
With more than 5,000 LEAP engines on order, however, the facility will be too small to handle the booming manufacturing load. As a result, GE has committed to opening the first CMC mass-production manufacturing plant in Asheville, N.C. The $125 million facility will manufacture the CMC shroud for the LEAP.
For additional information, visit www.ge.com/aviation.