Ceramic hot-section parts to debut this year in GE aero engines, 7F gas turbines next

General Electric Co is developing hot-section ceramic components for future versions of its popular 7F gas turbines, reports Lee S Langston, professor emeritus, UConn, who began his career as a research engineer at Pratt & Whitney more than 50 years ago. Among the first will be a ceramic matrix composite (CMC) shroud for the first stage on new 7F.04 and 7F.05 machines; it also is an option on full Advanced Gas Path upgrades to the earlier 7F.01, .02, and .03 models.

CMCs, at about one-third the weight of conventionally used hot-section nickel/cobalt alloys they replace, are more heat resistant and require less cooling air. These properties promise enhanced engine durability, life, fuel economy, and performance.

Ceramics have many favorable characteristics. Compared to metals now used in gas turbines, they often can have superior corrosion resistance, hardness, lower density, and high-temperature capability. Their main drawbacks are comparatively low toughness and the 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, Langston points out, 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. Ceramics subject to tensile or bending stresses (such as in rotating turbine blades), where crack defects are made larger, can cause sudden failure. CMCs have been developed to alleviate this characteristic ceramic brittle behavior.

Ceramics figGraceful failure. GE is introducing CMCs both in its 7F gas turbines and latest jet engine for single-aisle commercial aircraft—such as the Boeing 737 and the Airbus A320. Called LEAP (Leading Edge Aviation Propulsion), it is scheduled to enter into service this year. Since the LEAP CMC program is further along than that for the 7F, let’s review it.

CMC first use in LEAP is as the shroud for the first-stage high-pressure turbine (HPT). Note that the shroud is the segmented inner structure of the engine casting and the closest stationary surface to the rotating first-stage turbine blade tips. There are 18 segments in the LEAP engine casting’s inner structure.

GE will expand its application of CMC use in the company’s 100,000-lb-thrust GE9X engine, now under development for Boeing’s 777X airframe and scheduled to enter service in 2020. It will feature CMC combustion liners, HPT stators, and first-stage shrouds. Early in 2015, GE ran tests on a turbine rotor with CMC blades—the ultimate structural test of this new material. One can speculate that GE’s use of CFMs in its 7F gas turbine will follow a similar path taken for the GE9X.

The LEAP CMC first-stage turbine shroud is a composite consisting of fine intertwined ceramic silicon carbide (SiC) fiber, embedded in, and reinforcing, a continuous silicon carbide-carbon (SiC-C) ceramic matrix. The shroud also has an environmental barrier coating to protect the CMC from chemical reactions with the turbine gases.

The CMC SiC fibers are continuous (greater than about 2 in. long), a fraction of a human hair in diameter, and relatively free of oxygen (which can degrade its high-temperature properties). The resulting intertwined fiber reinforcers are covered with a multi-layer coating based on boron nitride.

The fiber-reinforced CMC has a unique failure mechanism, which Langston refers to as 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. Thus multiple micro-cracks build up, prior to actual fracture, resulting in increased toughness, and imitating the ductile behavior of a metal.

This crack-mitigation tolerance, which resists the classic brittle failure of a pure ceramic, should also yield gas-turbine parts that are not highly sensitive to manufacturing flaws.

In sum, 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. GE estimates an advantage of at least 180 to 360 deg F compared to metals currently in use.

This means CMC parts could operate at about 2400F, well into and above the softening/melting point of superalloys. (Pratt & Whitney estimates a CMC operating temperature at 2700F.) 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.

CMC future. All-in-all, GE has been working on CMCs for two decades, has spent over $1 billion on the technology, and recently opened a $125-million plant in Asheville, NC, to mass-produce CMC gas-turbine components. Just last October, the company announced an investment of over $200 million to create factories in Huntsville, Ala, to mass produce silicon carbide materials and manufacture CMCs for both aviation and land-based GTs.

The brief account given here to describe 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, says Langston. Trying to manufacture a ceramic material structure which imitates what nature provides in a ductile metal is a challenge.

However, success does not always favor the pioneer. For instance, in the late 1960s, Rolls-Royce attempted to be the first to use a composite ducted fan on its then new RB211 engines, for the Lockheed L-1011 airframe. The fan, using a carbon-fiber composite Hyfil, failed final testing, contributing to the 1971 bankruptcy of the company. The jet engine industry has since developed successful composite fans, but the inaugurating company got off to a rocky start.

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