By Susan Dieterle
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 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 boron, aluminum and magnesium.
With test results that confirm the hardness of the material, nicknamed BAM, the researchers 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 BAM 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 fundamental science with an opportunity to improve industrial competitiveness in a number of key areas," 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 in powder form. 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 1,000 C (1,800 F).
When industries need an abrasive material that works fast and cuts deep into specially hardened martensitic steels, they 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. "Cutting iron and steel is an enormous part of the U.S. manufacturing economy."
The BAM 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.
"The consumer doesn't see the costs of these materials up front, but it definitely affects the overall price of the product," Russell says.
Cook discovered the hardness of the boron-aluminum-magnesium 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 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 existence of the baseline compound had been known 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 complexity of the chemical structure offers a number of possibilities to vary its physical properties, such as enhancing the compound's hardness by substituting elements such as silicon. "We thought we might be able to change the bonding environment if we added silicon to the structure, and it worked. It made the material harder," he says.
"We would expect that by tweaking the composition, we may be able to push the hardness up a little higher," he 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 better understand the reasons behind the material's hardness, band-structure calculations are being made by Bruce Harmon, deputy director of the the Ames Laboratory and director of the Lab's Condensed Matter Physics Program, and scientists in his group. With this information, Cook, Russell and Harringa could develop expectations as to which chemical additions would result in the ultimate hardness of the material.
"When the baseline compound was first synthesized in the late 1960s, the computational power to analyze the electronic properties of such a complex structure simply did not exist," Cook explains. "Only recently has the ability to tackle such a complex system existed. Even so, a 64-atom unit cell is an extremely challenging theoretical problem."
To find out how BAM 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 its initial tests, Cook says, adding that the company was especially pleased that the material didn't fracture -- a common problem for many cutting tools, which are often brittle.
Cook adds that hardness is not the only material property of interest. Wear resistance, toughness, hardness at high temperatures and how well it conducts heat are also important factors affecting the viability of BAM in industrial applications.
During 1998, the scientists used a one-year Department of Commerce grant from Iowa State University'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 BAM's preparation and properties. Among their research priorities are a better scientific understanding of the material itself and determining how best to produce large quantities of it.
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."
He says initial tests show that BAM's coefficient of thermal expansion -- the change in a material's length in response to temperature changes -- is close to that of iron, which indicates that a coating of the material might adhere well to steel.
The BAM compound is generating quite a buzz among scientists and manufacturers. Russell notes that researchers from Ames Lab and other facilities are exploring the compound's properties on specialized equipment at Stanford University and DOE's Argonne National Laboratory.
In addition, more than 70 industry representatives have contacted Cook and Russell about a diverse assortment of possible uses for the material, such as testing-probe tips and wire guides for the microprocessor fabrication industry; wear-resistant linings for the petroleum and mining industries; and thin coatings for cutting tools.
"We're excited about the level of interest in this material and the wide range of applications that it could possibly be used for," Cook says. "We want to continue our research so that we can give these companies the information they need in order for them to determine how they could use the material."
Part of that effort involves devising a method for producing larger quantities of the compound instead of the small disks they've been using for research purposes.
"Based on the work we've done to this point, we feel confident that we can resolve the remaining issues and keep moving forward," Cook says.
For more information:
www.external.ameslab.gov/news/boride.html
Research funded by:
U.S. Department of Commerce
Roy J. Carver Trust
DOE Office of Basic Energy Sciences' Process Science Initiative
Last revision: 9/15/00 sd
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