Many innovations do not make it from the lab to the factory for 10 to 15 years, but one ORNL ceramic became a commercial product three years after it was discovered. This hall-of-fame ceramic is a composite of aluminum oxide (alumina) and microscopic silicon carbide (SiC) whiskers made from the common rice hull.
In 1981-82 ORNL's George Wei, Terry Tiegs, and Paul Becher discovered this material, which has high strength and fracture toughness. By 1985 the technology for making the patented composite was being used by Advanced Composite Materials Corporation and Greenleaf Corporation for commercial cutting tools. SiC-whisker-reinforced alumina cutting tools can machine jet engine components ten times faster than tools made of competing materials. Annual sales of these tools in the United States are over $20 million. Sandvik, a Swedish company, sells the cutting tools worldwide.
SiC whiskers were eventually found to be a health hazard if inhaled, but the ORNL concept of whisker reinforcement was key to the development of improved silicon nitrides in which seed crystals grow inside the material as it sinters, making it tougher. Research at ORNL and the National Aeronautics and Space Administration, with heavy industry investment, spawned a strong, fracture-resistant silicon nitride, processes to produce this ceramic, and advanced designs for turbines using silicon nitride components. The result was a significant improvement in the performance, emissions, and durability of small gas turbines.
Under the leadership of Ray Johnson, ORNL researchers Vic Tennery, Matt Ferber, and Ken Liu tested and characterized silicon nitride samples from different suppliers over several years to determine their resistance to fast fracture, fatigue, and creep at high temperatures. For this work, they used the sophisticated equipment at the High Temperature Materials Laboratory. ORNL's feedback to ceramic manufacturers helped them dramatically improve the properties of silicon nitride so it would not fail under various conditions.
This tougher silicon nitride—originally developed for the Department of Energy's industry program to design advanced gas turbines to power cars—is being used in commercial components aboard aircraft and in diesel trucks. Honeywell found that silicon nitride components survive airport dust and grit better than metal parts in auxiliary power units (APUs) that generate electricity for air conditioning, electronics, and certain nonessential services on airplanes parked at the gate. Since 1992, APUs with silicon nitride parts have been flying on Boeing and Airbus aircraft.
Since 1996, Honeywell and Kyocera have fabricated durable silicon nitride oil seals that more effectively prevent leakages from fuel pumps in small business jets. These seals were installed in 7000 jet engines that have more than 6 million accumulated flight hours.
Silicon nitride parts are being in-corporated into next-generation, high-efficiency, low-emission microturbines, as well as larger gas turbines, for stationary power generation. Current metal microturbines operate at efficiencies of about 25%; the incorporation of ceramics allowing hotter operation should raise that efficiency to 35 or 40%. Honeywell is making prototype ceramic wheels for a microtur-bine manufacturer, using the gelcasting process developed by ORNL's Mark Janney and Omats Omatete for fabricating large, complex-shaped components.
Improved silicon nitride also is being used to help reduce exhaust emissions from large diesel engines in highway trucks. To meet Environmental Protection Agency standards, diesel fuel-injection systems must be run at high pressures to reduce emissions. Because cam-roller followers are part of the mechanical chain that creates high fuel pressures, they must withstand high stresses. Silicon nitride produced by Ceradyne has been used to make cam roller-followers for Detroit Diesel Corporation. Kyocera has sold more than 3 million of the parts for diesel trucks.
Another ceramic that ORNL helped improve is transformation-toughened zirconia (TTZ), made by CoorsTEC and sold to Cummins Engine Company for diesel trucks. Cummins has experienced virtually no problems with the millions of TTZ plungers it makes for its high-pressure fuel injectors.
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