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Energy Sector Analysis

CMC components enable gas turbines to operate at higher temperatures and with higher efficiency than their metal counterparts.

Ceramic-matrix composites take the heat By Melissae Fellet Feature Editor Wolfgang Rossner

U

S air carriers consume tens of billions of gallons of fuel a year. Though their fuel consumption dropped 15% between 2000 and 2014, rising oil prices meant that fuel costs increased more than 300% during that time. Even when oil prices drop, airlines still want to improve efficiency. One way to increase fuel efficiency, and thus decrease fuel costs, is to increase the engine operating temperature. However, temperatures inside current aircraft turbines are already approaching the melting point of the metal superalloys used to make the engine components. These superalloys show surface softening at temperatures 15% below their melting point; normal materials, on the other hand, show surface softening at temperatures 30% below their melting temperature. Engine makers like General Electric (GE) and Rolls-Royce are now replacing some metal parts in their aircraft turbines with ones made from ceramic-matrix composites (CMCs). CMC components can withstand higher temperatures than ones made from current superalloys. They are also about a third as dense as their metal counterparts, providing additional fuel savings by reducing weight, particularly for rotating parts. A GE engine with a CMC component will appear on narrow body commercial aircraft starting in 2016. According to the company, this new engine uses 15% less fuel than its predecessor. Since aircraft turbines are structurally similar to land-based turbines that produce electricity, GE engineers hope to build CMC parts for other turbines too. Ceramics researchers have dreamed of using CMCs in gas turbines for more than 40 years, said Krishan Luthra, chief scientist for manufacturing, chemical, and materials technologies at GE Global Research. Incremental improvements in metal superalloys have taken about three decades of research to increase turbine operating temperature from 950°C to around 1100°C today. With CMC components, engines can run at temperatures closer to 1300°C, gaining a 150°C increase in one step. “That’s why they’re revolutionary,” Luthra said. Ceramics are typically brittle materials that shatter under stress. Fiber-reinforced CMCs, however, often fail like wood. When stressed, these materials fi rst develop microcracks in the ceramic matrix, then they slowly bend, and eventually, they completely break. Woven fi bers in CMCs bridge the microcracks and provide damage tolerance by hindering crack opening and further propagation.

Wolfgang Rossner, Siemens, Germany Melissae Fellet, [email protected]

916

MRS BULLETIN

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VOLUME 40 • NOVEMBER 2015

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www.mrs.org/bulletin • Energy Quarterly

Today’s best-performing CMCs are based on siliconcarbide (SiC) constituents. These SiC/SiC materials are typically made from nearly polycrystalline SiC fibers that are first covered with a boron nitride coating an