The small
piece of material resting in associate scientist Bruce Cook's palm isn't much to look at.
If anything, it appears to be a shiny, charcoal-gray shirt button without any holes.
But this rather drab-looking compound has one dazzling characteristic -- it is the second-hardest bulk substance after diamond.
And better still, it is expected to be less expensive than the material that now ranks a close third but is nearly as costly as gold.
Cook and two Ams Laboratory colleagues, associate scientist Alan Russell and assistant scientist Joel Harringa, developed the new material by introducing a small amount of silicon and other additives to an alloy of aluminum, magnesium and boron. With a recent round of tests that confirmed the hardness of the material, they now hope to expand their research on two fronts. First, they want to further investigate the material's properties and find out whether other combinations of additives might make it even harder. And second, they want to determine how the material can be produced in large quantities for industries that use ultrahard materials for coating, grinding and machining applications.
"What we're trying to do is combine science with an opportunity to address some industrial concerns," Cook says. "We're hoping that this material will do that."
If the Ames Laboratory scientists succeed in resolving some of the questions surrounding the compound, the discovery could mean huge savings for manufacturers who use these types of materials in abrasives and cutting tools.
Diamond, which has a measured hardness of between 70 and 100 gigapascals (the equivalent of 10.2-14.5 million pounds per square inch), costs about $2,000 per pound. But it can't be used for cutting and grinding steel because it reacts by slowly dissolving into the iron when brought into contact with iron-based materials at high temperatures. In the high-speed grinding that takes place in the auto industry, for example, friction between the steel and the tool produces surface temperatures as high as 1000 C (1800 F).
To cut and grind hardened steel, most industries rely on cubic boron-nitride, which has a hardness of about 45 GPa (6.5 million psi). Cubic boron-nitride doesn't have the iron reactivity problem of diamond, but it costs anywhere from $1,500 to $7,000 per pound because it is produced under conditions of extremely high temperatures and pressures.
"The fact that industries are willing to pay that price for certain uses gives some insight into what a critical industrial process this is," says Russell, a co-investigator on the project and an associate professor of materials science and engineering at Iowa State University. "Cutting iron and steel is an enormous part of the U.S. manufacturing economy."
The aluminum-magnesium-boron compound measures slightly harder than cubic boron-nitride at about 46 GPa (6.7 million psi) and is estimated to cost around $700 per pound. That could substantially reduce the cost of the cutting and grinding tools, enabling manufacturers to trim their production costs.
"You don't see the costs of these materials upfront, but it definitely affects the overall price of the product," Russell says.
Cook discovered the hardness of the aluminum-magnesium-boron compound by accident. He was researching its thermoelectric properties in 1992 when he discovered that he couldn't cut the samples he'd made. "We have high-speed, precision diamond saws in the lab that can cut virtually anything, and we weren't able to cut this material," Cook says. "That caught our attention."
Although the aluminum-magnesium-boron compound had been around for awhile, the Ames Lab scientists found that the material's mechanical properties hadn't been fully investigated. "When Bruce discovered the hardness, it was unexpected and something that no one had thought to look for previously," Russell says.
It was also an unlikely candidate for a hard material because of the structure of its unit cell, or fundamental building block. "A diamond has eight carbon atoms in a unit cell. It's a very simple, highly symmetric structure," Russell explains. "This material has 64 atoms in the unit cell. If you gave this structure to a panel of experts and asked if it would be hard, they'd say, 'Nah, the crystal structure is all wrong.' But it's extremely hard. And that's the kind of thing that gets scientists salivating."
Cook says the complex chemical structure makes it possible to enhance the compound's hardness by substituting other elements, such as silicon. "We thought we could change the bonding environment if we added silicon to the structure, and it worked. It made the material harder," he says.
"We think that by tweaking the composition, we may be able to push the hardness up a little higher," Cook adds. "This was the first additive we tried and it produced a material that's right up there with cubic boron-nitride. But there may be other variations that could further increase the hardness of this material."
To find out how the material would fare in a real-world setting, the Ames Lab scientists sent samples to Autodie International, a Michigan company that manufactures tools, dies and molds for the automotive industry. Autodie reported favorable results in their initial tests, Cook says, adding that the company was especially pleased that the material didn't fracture -- a common problem for many abrasive materials, which are often brittle.
Cook adds that hardness is not the only material property of interest. Wear resistance, toughness, hardness at high temperatures and thermal conductivity are also important factors affecting the viability of the material in industrial applications.
During 1998, the scientists used a one-year Department of Commerce grant from ISU's Center for Advanced Technology Development to study the material and possible additives to enhance its hardness. They also received a small grant through the Roy J. Carver Trust.
The researchers are now looking for additional funding for a more extensive study of the material's preparation and properties. Among their research priorities are a better scientific understanding of the material itself and figuring out the best, most inexpensive way to produce large quantities of the compound.
They also want to investigate the possibility of producing the material as a uniform powder that could be deposited as a wear-resistant coating on surfaces such as bulldozer blades and mining tools. "We know that the two other hardest materials won't tolerate it," Russell says. "This one might."
Last revision: 1/26/00 sd