Ceramic matrix composite (CMC) materials are composed of coated ceramic fibers surrounded by a ceramic matrix. These materials are lightweight, tough, and can handle temperatures between 300-400 °F which is hotter than what metal alloys can endure.
If some components were manufactured using CMC materials instead of metal alloys, the turbine engines of power plants and aircraft could function more efficiently at higher temperatures, combusting fuel more completely and releasing fewer pollutants.
About 25 years ago, the U.S. Department of Energy initiated a program to support U.S. development of CMC materials. The program was led by DOE’s Oak Ridge National Laboratory. In 2016, a new aircraft engine called LEAP became the first extensively deployed CMC-containing product. CFM International, a 50/50 joint venture of Safran and GE, built LEAP.
The engine has one CMC component, a turbine shroud covering the hottest region, thus enabling it to function at up to 2400 °F. The CMC requires less cooling air than nickel-based super-alloys and is part of a collection of technologies that add to 15% fuel savings for LEAP compared to its predecessor, the CFM 56 engine.
Presales to airlines eager to lower fuel costs are astounding—$140 billion at list price for more than 11,000 engines. In August, the first LEAP engine began flying commercially on Airbus A320neo. Other LEAP engines will fly on the Boeing 737 MAX in 2017.
The materials developed in the DOE program became the foundation for the material now going into aircraft engines.
Krishan Luthra, who led GE Global Research’s development of CMCs for 25 years.
GE’s CMC is composed of silicon carbide (SiC) ceramic fibers (containing silicon and carbon in equal measures) coated with a proprietary material consisting of boron nitride. The coated fibers are shaped into a “preform” that is embedded in SiC comprising of 10–15 % silicon.
ORNL’s Rick Lowden conducted foundational work in the 1980s that facilitated the DOE programs. The solution was coating the ceramic fibers.
“A ceramic matrix composite is different than almost all other composites because the matrix is ceramic and the fiber is ceramic,” Lowden said. Normally, blending two brittle materials produces a brittle material, he said. But modifying the bond between fiber and matrix allows the material to behave more like a piece of wood. Cracks do not propagate into the fibers from the matrix around them. The fibers grip the material together and bear the load while gradually pulling from the matrix, increasing toughness.
DOE’s Continuous Fiber Ceramic Composite (CFCC) program continued from 1992 to 2002 and supported industrial development of CMCs by AlliedSignal, Amercom, Alzeta, Babcock and Wilcox, DuPont-Lanxide Composites, Dow Chemical, Dow Corning, GE and Textron. Its budget averaged $10 million annually, and industry split the costs.
CFCC funded companies to create composites and national labs and universities to describe the properties of the materials. Efforts were coordinated and funded via ORNL. Lowden wrote the program plan with Scott Richland of DOE and Mike Karnitz of ORNL and co-led support to companies with ORNL’s Karren More, Pete Tortorelli and Edgar Lara-Curzio and Argonne National Laboratory’s Bill Ellingson. The U.S. Advanced Ceramics Association represented industry in informing Congress of the advantages of CMCs.
“We were looking at different fibers and different interfacial coatings and different matrices,” More said of ORNL’s role. “We were involved in understanding the degradation mechanisms and down-selection of the more promising composites and cost-effective techniques for preparing them.”
We were working toward a common goal of getting ceramic matrix composites into industrial applications including high-pressure heat exchangers, land-based turbines, carburizing furnaces and radiant burners.
GE’s CFCC project was to create CMCs for industrial gas turbine engines that generate electricity. GE manufactures both propulsion and power turbines. A follow-on DOE program was scheduled for 2005 to fund highly promising CFCC companies to further develop materials and components and if possible, examine them in applications. Total funding was about $15 million, with industry cost-sharing approaching 50%. Under this program, GE field-tested a CMC shroud in a 170-megawatt industrial gas turbine. GE spent $1.5 billion after that to market the technology.
Seed money is critical for high-risk, high-payoff technologies. Material development is a long-term activity, and Oak Ridge tremendously supported the basic research.
Luthra showcased the new CMC factories and jobs today as evidence of success. GE acquired a CMC facility in Newark, Delaware in 2002, which has grown significantly. In 2014, a new GE facility was started in Asheville, North Carolina, for manufacturing shroud components. Additionally, GE is building two adjacent factories in Huntsville, Alabama - one for speeding up fiber production and the other for coating fibers and making tape for processing into components. At full-scale, it is hoped that the Huntsville and Asheville sites would bring 640 high-tech jobs.
In 2019, GE will create an engine, GE9X, with five CMC parts— two nozzles, two combustor liners, and one shroud. Presales are about $29 billion at list prices for 700 engines.
Firing up research of ceramic composites
In the past, before ceramic fibers reinforced ceramic composites, ORNL researchers had coated nuclear fuel with carbon and SiC to trap radioactivity within tristructural-isotropic (TRISO) fuel particles.
During experiments in the 1970s, ORNL’s Jack Lackey understood that the process could be altered to produce ceramic composites more quickly. With assistance from DOE’s Fossil Energy Materials Program, his group pioneered a technique to achieve just that.
“You take a fibrous preform, place it in a furnace, and vapor-deposit solids on and around the fibers,” explained Lowden, who was Lackey’s technician. The deposition process has to be very slow in order to coat the whole object evenly. It may take six months to process a half-inch part.
However, the ORNL team discovered that keeping a fibrous mat on a cold plate, heating the top and forcing gases via the mat accelerated the process from months to hours. “That’s where we got involved in ceramic matrix composites,” Lowden said. ORNL supplied CMCs for years to researchers assessing CMCs for several applications.
Using a melt infiltration process, GE mass-produces CMCs. The production capacity is being scaled to create 36,000 superior quality shroud segments annually by 2020. (Each LEAP engine needs 18 shrouds segments.)
During the CFCC years, the program’s biggest success was an industrial gas turbine placed into operation at the Malden Mills plant in Massachusetts in 1999. The turbine was fitted with a CMC combustor liner—developed by Solar Turbines with input from researchers at ORNL, United Technologies, Argonne, B.F. Goodrich, and DuPont-Lanxide Composites—that helped optimize the efficiency of the turbine. At the time, Energy Secretary Bill Richardson said the Malden Mills plant had “the lowest emissions of any industrialized heat and electric combined facility in the United States.”
Since CFCC, GE has evaluated CMCs for over two million hours, including 40,000 hours in industrial gas turbines. Jim Vartuli of GE’s CMC program said DOE support on large industrial gas turbines to procure those first demonstrators boosted GE confidence that the ceramics could endure high temperatures and stresses in turbines for extended periods.
GE is the only company in the world with both large industrial gas turbines and aircraft engines businesses, and this enables many opportunities for co-development of advanced technology. This is an example of the ‘GE Store’—the transfer of technology and knowledge between GE businesses. The success of the turbine tests convinced our aviation business that CMCs would be successful for aircraft engines, too.
How DOE and its national labs helped industry
CFCC companies took the materials they had created to DOE national laboratories at Argonne for nondestructive assessment and Oak Ridge for microstructural characterization and stress and oxidation tests.
This partnership highlights the value of the national labs. We do work that is fundamental and broad to understand materials’ behaviors. We provide necessary information to help the community make decisions about where to go, how to proceed.
New knowledge regarding how materials deteriorated helped industry speed up improvements and improve manufacturing processes.
Research at ORNL spanned from development by Allen Haynes of environmental barrier coatings that could lengthen the lives of core materials five-fold to nondestructive imaging of materials with thermal cameras by Ralph Dinwiddie.
At Argonne National Laboratory, Bill Ellingson guided the development of broader nondestructive testing techniques to guarantee safe continued use of components by monitoring material degradation after gaps in usage. Without damaging the components, the inspections showed how materials reacted in an environment over time. With ORNL researchers, Argonne scientists created many nondestructive inspection technologies that were helpful in establishing component performance.
ORNL’s Pete Tortorelli and H. T. Lin stressed materials in environmental exposure chambers to study their areas of failure. Lab colleagues Jim Keiser and Irv Federer exposed samples to temperatures up to 2550 °F, corrosive gases, and pressures up to 500 psi in “Keiser rigs” that replicated conditions in turbines. These were also used by More, Tortorelli and Keiser to monitor protective coatings required in combustion environments.
In the meantime, More described structures of stressed materials. “Karren More entered the picture as our microscopist, and that changed our world,” Lowden recalled. “To be able to see what was happening with transmission electron microscopy, and understand what was happening at that level, was incredible.” GE had access to some methods in-house because of its large infrastructure. “But we got invaluable help from Karren on the fiber coatings,” Luthra said. “It helped us develop the fiber coatings faster.”
ORNL’s initial findings supported industry to discard carbon as a fiber coating. Carbon oxidized, turning into carbon dioxide and carbon monoxide, and volatilized, thinning the coating. ORNL engineers proposed oxidation-resistant boron nitride instead.
Furthermore, Edgar Lara-Curzio modeled and analyzed the mechanical performance of CMC materials under a variety of loading conditions and their resistance to creep, fatigue, and rupture in ORNL’s High Temperature Materials Laboratory. In partnership with Matt Ferber and Chun-Hway Hsueh, he executed experimental and analytical techniques to describe the micromechanics of fiber–matrix interfaces. “These measurements were essential to quantify chemical bonding between fibers and matrix, residual stresses experienced by the fibers and friction between the fibers and the matrix during fiber sliding,” said Lara-Curzio, noting CMCs are strong primarily because interfacial coatings allow fibers to slide and bridge matrix cracks. He and Hsueh provided key information regarding how a single fiber slides in a ceramic matrix. Lara-Curzio, Ferber and Lowden then quantified the effect of the thickness of fiber coatings on sliding and found a value that improved mechanical properties. Companies extensively adopted this link to optimize their composites.
Back to the future
Today at GE, Luthra hopes to put CMCs in all the places where the engine gets hot— nozzles, blades, and liners. To realize this vision, the community has several technological hurdles to overcome. One is creating manufacturing processes that, unlike melt infiltration, do not generate excess silicon that can volatilize and develop cracks in the matrix.
“Every decade we have increased [the heat metals can take] by about 50 degrees,” Luthra noted. Today, CMC material can endure up to 2400 °F, but Luthra would like the next generation to achieve 2700 °F. “This is going to be as challenging as the development of the first ceramic composite,” he said.
To emphasize these challenges, the U.S. Advanced Ceramics Association is developing an industry-driven roadmap for the creation of 2700 °F CMCs for advanced gas turbines. This roadmap will update Congress about successes of 2400 °F CMCs, promote investment in the creation of 2700 °F CMCs and showcase the contributions of CMCs to the development of high-paying U.S. manufacturing jobs, national security and the environment. USACA’s roadmap supports findings of a new National Academy of Sciences study that concludes investment in gas turbine materials and coatings should be a high priority and that 2700 °F CMCs could significantly decrease or eliminate the need for cooling in engines, improve efficiency and lower weight. DOE national labs could once again be called upon to help determine high-performance materials and processes that can function at higher temperatures and even more severe environments.
Future CMCs will have to bear extremes on four time scales, according to the application: one hour or less of hot time for launch vehicles; days for accident-tolerant fuels (e.g., if a cooling system fails in a nuclear power plant); thousands of hours, the operating life of aircraft turbines; and more than 30,000 hours for industrial gas turbines for power production.
A land-based gas turbine to produce electricity can be more challenging than an aircraft engine application because it spends much more time operating at high temperature, Luthra said. Progress in the next generation of 2700 °F materials would enable breakthrough improvements in efficiency and emissions that could decrease the cost of electricity.
DOE's Advanced Manufacturing Office (AMO), previously known as the Industrial Technologies Program, supports applied research, development and demonstration of new materials, information and processes that optimize American manufacturing’s energy efficiency, as well as platform technologies for manufacturing clean energy products. This research was funded by AMO.