Scientists seek to recreate the strength of diamonds in artificial materials.
In a strongman contest among Earth's natural materials, nothing competes with the diamond. With a hardness of 96 gigapascals, diamonds demonstrate more than twice the hardness of second-place finisher boron nitride and nearly 100 times the hardness of stainless steel.
Theoretical physicists Chong Long Fu, the late Gayle Painter and postdoctoral researcher Xing-Oiu Chen have been trying to unlock the secret of diamond's strength. They are among a number of theorists, experimentalists and engineers at Oak Ridge National Laboratory, working to understand, develop and test new breeds of materials that mimic the strength and toughness found in nature—without having to recreate the forces that formed them.
Diamonds are, of course, made of carbon. But, Painter pointed out, so is graphite, diamond's near polar opposite on the hardness and toughness scale. Thus, the answer lies not simply in the atoms themselves but in their structure, alignment and the bonds among them.
"The big difference between carbon in graphite and carbon in diamond is that in diamond a large linking structure exists that goes all the way through, and each of those links is a strong bond," he said. "In graphite, the structure is similar to a honeycomb with very weak forces between the rows. The explanation lies not just in how the electrons move but also in which directions they move." In other words, diamonds feature a three-dimensional lattice with very strong bonds that makes the material virtually impenetrable—and the choice for products such as cutting tools and precious jewelry. Other elements, such as metals, have less sturdy structures, with weaker bonds that are easier to break apart.
The theorists have been working on the idea that carbon and boron, elements used to create diamonds and boron nitride in nature, could be incorporated into metals to create materials that could vie for diamond's strength. They also look for materials that can be made at zero pressure or high pressure, producing different properties.
"We are looking at the transition metals that have "holes" boron and carbon can easily fill and form strong bonds with," Fu says. "Ultimately, we are looking for a material that has high strength and high hardness and is easy to make."
Using supercomputing capabilities at the National Energy Research Scientific Computing Center in Berkeley, California, Fu and his colleagues have predicted the enhanced mechanical strength and hardness of the metals tungsten, hafnium, tantalum, rhenium, osmium and iridium when reinforced with boron atoms. Combining these elements could produce alloys that feature the same strong covalent bonds and lattice structures found in diamonds, boron nitride and other superstrong natural materials.
Fu shows a diagram of the network of atoms and bonds, arrayed in tinker-toy-like fashion, that forms when boron is incorporated into a theoretical sample of the metal tungsten. The resulting tungsten boride has the incompressibility of a diamond, at 1,000 gigapascals, and the hardness of its runner-up, boron nitride, at about 45 gigapascals. Tungsten, by itself, although one of the hardest metals, measures just 10 gigapascals in that category and 520 gigapascals in incompressibility.
The next step is to discover how to synthesize the new tungsten boride alloy under ambient conditions in a laboratory. Potential applications could include highly wear-resistant cutting blades for tools, bearings and bearing sleeves and a variety of other components for use in industries from aviation to the military. The work also fits in neatly with the Department of Energy's larger research objectives.
"This research ties in wonderfully with energy," Painter said. "The goal is simple. If things do not wear out, energy is not expended to make their replacements nor are processes shut down to replace parts."
Future efforts are being made to explore the potential of marrying transition metals with carbon and nitrogen both in zero pressure and high-pressure scenarios.
"Under high pressure the bonds become shorter, and when the bonds become shorter they become stronger," Fu says. "We will add pressure and see what happens."—Larisa Brass
Note: Gayle Painter died March 26, 2008, following a brief illness. He was a senior member of the Materials Theory group in ORNL's Materials Science & Technology Division. He worked at ORNL for 39 years.
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