Automobile engine cylinders could last much longer if they were formed from composites of metals and ceramics. But the ceramic used to make the most wear-resistant metal-ceramic or ceramic-ceramic composites must meet two requirements. It must be one of the hardest known ceramics, such as tungsten carbide, which is used for tough coatings and cutting tools. And it must have an appropriate shape, such as that of a nail or a needle, so that it can reinforce the metal to make a very hard composite.
Carlos Bamberger, a chemist at the Department of Energy's Oak Ridge National Laboratory , has discovered a chemical method for producing tungsten carbide needles from needle-shaped crystals of sodium tungsten bronze, an inexpensive material. The chemical reaction used is a "pseudomorphic reaction" in which the final product has the same shape as the material initially used in the reaction.
"The performance of a material is often strongly dependent on its shape, or morphology," Bamberger says. "For example, whiskers of silicon carbide, a very hard and durable material, have been used to strongly reinforce other ceramics.
"Because the morphology of a material during manufacture cannot be predicted, this work is valuable because it shows for the first time that needles of tungsten carbide can be made from needles of another starting material."
Bamberger, a senior staff member of ORNL's Chemical and Analytical Sciences Division, believes that properly shaped tungsten carbide could be used to form ceramic composites and ceramic-metal composites that have industrial use.
To produce a material that is about 75 to 80 percent tungsten carbide and about 20 to 25 percent tungsten metal, Bamberger reacted solid crystals of sodium tungsten bronze with various hydrocarbon gases at high temperatures. He found that the proportions of tungsten carbide and tungsten metal could be altered by varying the temperature, flow rate, and composition of the hydrocarbon gas.
"This experiment," Bamberger concluded, "showed that, by a simple reaction with a common gas, an inexpensive material could be converted into an industrial product of potential high value."
The work was supported by DOE's Office of Energy Research, Basic Energy Sciences, Materials Sciences.
ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Lockheed Martin Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.