Cermets: Down to Earth and Microstructure
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Cermets: Down to Earth and Microstructure Forty years ago an exciting new class of materials that researchers hoped would combine the. best properties of ceramics and metals began to emerge from academic and industrial research laboratories. The new materials were called cermets. The advent of jet aircraft inspired much of the early cermet work. The optimum o p e r a t i n g t e m p e r a t u r e of a jet e n g i n e (1800-2400°F) was above the practical limit of high temperature metal alloys in use at the time (1500°F). An ideal material would combine the high temperature strength and oxidation resistance of ceramics and the ductility and toughness of metal alloys. The s e m a n t i c s of this research area would never be crystal clear. The new materials were variously called ceremetallics, ceramuls, ceramals, metalceramics, and ceremets before the community agreed with the suggestion of A.L. Berger, chief of the Supercharger Branch of the U.S. Air Force's Wright Field Power Plant Laboratory, that the new materials be termed simply "cermets." Cermets were defined broadly in 1950 by John B. Campbell, associate editor of the t r a d e j o u r n a l Materials & Methods, as "refractory metallic compounds combined with metals and fabricated primarily by powder metallurgy methods." He listed three kinds — oxide-metal, boride-metal, a n d carbide-metal c o m b i n a t i o n s — but wrote that "the latter are nothing more nor less than cemented carbides, an old and respected family hardly in need of a more glamorous title." For a decade and a half or so from the mid 1940s through the 1950s, researchers made and tested an extensive variety of c o m b i n a t i o n s of m e t a l s w i t h ceramic oxides, carbides, and borides through techniques of slip casting, sintering, infiltration, and explosive forming. First on the scene was Carborundum's ceramic/metal mix called Durhy and Dug, the aluminairon material made during the World War II by German researchers to substitute for scarce high strength metal alloys. The German work was revealed in postwar Army intelligence reports. "In the early days, the problem (of making a cermet turbine blade) was not looked upon as being particularly difficult, but merely a matter of a little time to find 'that combination,'" wrote J.R. Tinklepaugh and W.B. Crandall of Alfred University in the 1960 book they edited, Cermets. Moreover, although test parts were expensive to make, observers were confident that if the other problems associated with cermet properties would be overcome, making the use of these materials in large quantities practical, mass machining techniques would also be perfected.
But, it was not to be. Various teams produced cermet turbine blades (also called buckets) that proved individually strong, but all fell prey to their inherently low impact strength. The blades would shatter when struck by any piece of debris passing through the rotor. Pratt & Whitney and the National Advisory Committee for Aeronautics (the predecessor to the National Aeronautics and Space Administr
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