This article presents an overview of use, advantages, and challenges related to ceramic gas turbines. The research shows that if these ceramic parts perform as promised, both operationally and economically, it could radically alter the jet engine industry. The promise of ceramics is that by taking advantage of their lower weight and superior high temperature properties, one could replace the complex air-cooled metal components with simpler ceramic components more tolerant of high temperatures. However, one difficulty engineers have had in developing ceramic components is the inability to put promising designs in production gas turbines for a true ‘beta’ test. GE plans to expand its application of ceramic matrix composites use in its 100,000-pound thrust GE9X engine, now under development for Boeing’s 777X airframe and scheduled to enter service in 2020. It will feature CMC combustion liners, high-pressure turbine stators, and first-stage shrouds. The jet engine industry has since developed successful composite fans; however, the inaugurating company got off to a rocky start.
Animal, vegetable, or mineral
Those are the classic choices from the 20 questions game. But materials science presents a wider array of options for making advanced products. Carbon-based materials—in the form of fibers, nanotubes, or sheets of graphene—are full of promise. Aerogels, in which the liquid component of a conventional gel is replaced by a gas, are remarkably lightweight for a solid and have extremely low thermal conductivity.
But as David Richerson of the University of Utah has pointed out, most solid materials that aren’t metal, plastic, or derived from plants or animals, are ceramics. Traditional ceramics such as pottery, tile, bricks, stone, and glass that date from antiquity. Today, sporting goods stores sell ceramic knives, and bullet-proof vests have ceramic plate armor. High-temperature superconductors are made from complicated ceramics, and ceramic materials are used in aerospace applications for heat shielding.
The makers of gas turbines have—from the beginning—looked at ceramics and wondered how to make use of them. Ceramic materials have many favorable characteristics. Compared to metals now used in gas turbines, they often can have superior corrosion resistance and hardness, lower density, and higher temperature capability. It seems obvious that gas turbines ought to be made with ceramic parts.
I remember a story told to me by Dick Goldstein, a past president of ASME, about his late colleague at the University of Minnesota, Ernst Eckert. Eckert was a famous heat transfer expert and was also a colleague of jet engine pioneer Ernst Schmidt at the German Aviation Research Institute in Braunschweig in the late 1930s and early 1940s. Eckert was working on gas turbine designs, and while carefully doing calculations and design of metal air-cooled turbine blades, a coworker (possibly Schmidt) remarked to him that his design was probably done in vain. Ceramics soon would be taking over, negating the need for the air cooling of the blades.
That confident prediction did not come to pass.
The main drawback for ceramics, which over the first 75 years of gas turbine technology had never been sufficiently overcome, is comparatively low toughness, and the resultant possibility to fracture in a catastrophic brittle mode.
Toughness is a measure of load or stress needed to drive a crack through a material. A china dinner plate is not easy to break in half, but if it has a slight crack, fragmentation is easy, compared to say, an identical ductile metal plate. Ceramics subjected to compressive stresses, where crack defects are made smaller, are very strong. But when ceramics are subject to tensile or bending stresses (such as in rotating turbine blades), any crack defects are pulled apart and can cause sudden failure.
So it is news indeed that commercial jet engines with ceramic components in their hot section are coming to market. After more than a billion dollars in research and development, General Electric has recently opened a $125 million plant in Asheville, N.C., and is planning a $200 million factory in Huntsville, Ala., to mass produce gas turbine components out of an advanced material called ceramic matrix composites.
These components will operate at a temperature that would soften or melt the superalloys currently used in jet engines.
If these ceramic parts perform as promised, both operationally and economically, it could radically alter the jet engine industry. But with the price for failure so catastrophic, that isn’t a prediction that can be made with confidence.
The promise of ceramics is that by taking advantage of their lower weight and superior high temperature properties, one could replace the complex air-cooled metal components with simpler ceramic components more tolerant of high temperatures.
Turbine inlet gas temperatures can reach 3,600 ̊F (1,982 ̊C) in advanced military jet engines and 2,700 ̊F (1,482 ̊C) in stationary gas turbines. Those all require air-cooled hardware, since superalloy metals soften and melt in the 2,200-2,500 ̊F (1,204-1,371 ̊C) range.
By contrast, a ceramic formed by silicon carbide decomposes at 4,950 ̊F (2,730 ̊C), well above current turbine gas path temperatures. Thus, a replacement SiC part would save weight and reduce the need for cooling air, which would increase gas turbine efficiency and reduce fuel consumption.
But it wasn’t simply ceramics’ material properties that enamored the gas turbine pioneers; it is also a function of place and time. After the first jet-powered flight, near the Baltic city of Rostock, Germany, on August 27, 1939, work commenced on six jet engine programs at the German companies of Heinkel, Junkers, BMW, and Daimler Benz. That put the Germans years ahead of other countries in the establishment of jet engine technology and production.
It was somewhat natural that ceramics would be seriously considered for German jet engine hot section components. German ceramic technology at such companies as Siemens Neuhaus and Degussa was well advanced, going back to 1709 when Europe's first successful porcelain, or bone china, was developed at Meissen in Saxony. Also, with the Second World War just beginning, Germany had an increasingly difficult time importing raw material of strategic value, including high temperature alloy metal such as nickel. German scientists looked to the possible use of ceramics for engine hardware.
Before the Nazi regime took power in Germany, Ernst Schmidt was a leading thermodynamics researcher and academic; the dimensionless number formed by the ratio of momentum and mass diffusivities is named after Schmidt. With the war on, Schmidt led a program at the top-secret German Aviation Research Institute—which in German is compounded into Luftfahrtforschungsanstalt and abbreviated LFA—to adapt ceramics to jet engine construction.
Schmidt quickly abandoned any effort to use ceramic rotor blades because of their brittleness in tension and problems associated with their attachment. He also considered, briefly, the construction of a turbine in which the casing rotated and the rotor was stationary, much like the WWI French Gnome rotary piston engine. In that way the rotating airfoils would be put under secure compression instead of tenuous tension.
The ceramic gas turbine work at LFA did lead to turbine stator development, with Degussa and Siemens Neuhaus furnishing alumina stators. Those, however, proved to be relatively sensitive to heat shock.
Thus, despite the pioneering German work on gas turbine ceramics, there is no evidence that ceramics were used in the gas paths of any of the approximately 7,000 production jet engines built by Germany by the end of WWII. Turbine airfoils were made of metal alloys, and air-cooled when necessary.
A ceramic formed by silicon carbide decomposes at 4,950 ̊F, well above current turbine gas path temperatures.
The challenges involved in developing ceramic rotating parts for a gas turbine have persisted in the decades since. As David Richerson pointed out in a 2004 paper, ceramics introduce a wide spectrum of challenges in the high temperature ranges found within gas turbines. For instance, how do you go about designing and fabricating components using these brittle materials in such high stress and possibly high impact applications? There's virtually no margin for error: a cracked rotating ceramic turbine blade can suddenly fail, taking out other blades and causing total engine failure.
Also, under high temperature and high pressure gas path conditions, some advanced ceramics can oxidize and react to water vapor, causing strength degradation.
Of course, high melting point ceramics have always served as cores and molds for investment casting of advanced superalloy turbine blades and vanes which can have very intricate geometries. And current engines in production use ceramic thermal barrier coatings, called TBCs, which raise the high-temperature capacity of a metal part by hundreds of degrees and increase its life. Critical component loading is carried by the coated metallic part itself, so that the brittle coating isn’t subjected to high stresses.
One difficulty engineers have had in developing ceramic components is the inability to put promising designs in production gas turbines for a true “beta” test. For example, the very extensive and far-reaching high-temperature automotive ceramic gas turbine program run by the Ford Motor Co. centered around an experimental automotive single-shaft high-temperature ceramic gas turbine.
The Ford ceramic gas turbine program began in 1967 with the promise of low exhaust emissions, an advantage the continuous combustion of a gas turbine had over the intermittent combustion of the piston engine. Designated as Model 820, the turbine comprised an all-ceramic rotor with a radial compressor and an axial flow turbine part of the all-ceramic hot flow gas path. The ceramic rotor was the biggest challenge, and it was successfully tested, proving that a low-cost one-piece low inertia ceramic rotor for production would be possible.
But the Model 820 Program never made it to the production test phase. While the program progressed over the years, reciprocating engines were developed with direct fuel injection and computer-aided electronic controls. Those new engines successfully met emission standards, and since the company had a high capital investment in building reciprocating engines, it didn’t make financial sense to make an equally large investment to continue developing and then manufacture gas turbines.
The turbine program was folded around 1990.
While researchers and developers have explored the use of ceramics in gas turbine high-temperature gas path applications, none have gone into a major production engine. But General Electric is now changing this, by using gas-path parts made from ceramic matrix composites in the company's new LEAP production engines.
CFM International, jointly owned by GE Aviation and SNECMA of France, is developing the LEAP—short for Leading Edge Aviation Propulsion—high-bypass turbofan engine as a successor to the very successful CFM56, the top-selling commercial jet engine in the world. (Some 28,000 of these 19,000-to-27,000-pound thrust engines have been delivered since 1994.) In development since 2008, the LEAP is scheduled to enter into service this year and already has more than 8,000 orders.
The ceramic matrix composites used in the LEAP's hot section have one-third the density of conventionally used nickel-cobalt superalloys. CMCs have more heat resistance and require less cooling air. Such properties promise to enhance engine durability, fuel economy, and performance. GE's first use of CMCs is as the shroud of the first-stage high- pressure turbine, which, as the inner structure of the engine casing, provides the closest stationary surface to the rotating first stage turbine blade tips.
The CMCs in use there are a composite consisting of fine intertwined ceramic silicon carbon fiber, embedded in and reinforcing a continuous silicon carbon-carbon ceramic matrix. The SiC fibers are continuous, reaching more than 5 cm in length while being just a fraction of a human hair in diameter, and relatively free of oxygen (which can degrade high-temperature properties). The resulting intertwined fiber reinforcers are covered with a multi-layer coating based on boron nitride.
The shroud itself also has an environmental barrier coating to protect the CMC from chemical reactions with turbine gases.
To get around the potential for sudden and catastrophic failure of the ceramic part, the fiber-reinforced CMCs have a unique failure mechanism, dubbed a “graceful failure” mode. As the SiC-C matrix cracks develop under imposed thermal or mechanical stresses, the load is transferred to the reinforcing SiC fibers. Their multi-layer boron nitride coating then permits the fibers to slide in the matrix, allowing load transfer and energy absorption. Instead of one micro-crack quickly becoming a point of failure, multiple micro-cracks can build up prior to actual fracture, resulting in increased toughness that imitates the ductile behavior of a metal. This mitigating crack tolerance that resists the classic brittle failure of a pure ceramic, should also yield gas turbine parts that are not highly sensitive to manufacturing flaws.
GE plans to expand its application of ceramic matrix composites use in its 100,000 pound thrust GE9X engine, now under development for Boeing's 777X airframe and scheduled to enter service in 2020. It will feature CMC combustion liners, high-pressure turbine stators, and first stage shrouds. Early last year, GE ran tests on a turbine rotor with CMC blades—the ultimate structural test of this new material.
One factor the company must work on is cost. Currently, CMCs are very expensive, hundreds to thousands of dollars per kilogram. GE is counting on cost reduction by process scale-up, automation and improved machining.
But overall, GE's use of CMC gas path parts looks very promising. CMC's graceful failure mechanism will allow the use of this promising composite ceramic, with its light weight and high-temperature characteristics. The company estimates an advantage of at least 180-360 Fahrenheit degrees (100-200 Celsius degrees) in comparison to metals currently in use. That means that CMC parts could operate at about 2,400 ̊F (1,315 ̊C), well above the softening and melting point of superalloys. Competitor Pratt & Whitney estimates a CMC operating temperature at 2,700 ̊F.
This brief account describing the management of tensile cracking does not do justice to the research, analysis, and testing GE and others have done to develop CMCs for gas turbines. Trying to manufacture a ceramic material structure that can imitate what nature provides in a ductile metal is an enormous challenge.
It should be noted, however, that success does not always favor the pioneer. In the late 1960s, for instance, Rolls-Royce attempted to first use a composite ducted fan on its then new RB211 engines, for the Lockheed L-1011 airframe. The fan, using Hyfill, a carbon fiber composite, failed final testing. This setback ultimately led to the bankruptcy of the company in 1971.
The jet engine industry has since developed successful composite fans, but the inaugurating company got off to a rocky start.
Coming after seven decades of development, it may seem that the day for ceramic gas turbine components has finally arrived. Who will reap the rewards of that breakthrough remains to be seen.