Researchers at the U.S. Department of Energy’s Ames Laboratory have made a precious, if not accidental, discovery: the second hardest known bulk substance after diamond.
By introducing a small amount of silicon into an alloy of aluminium, magnesium and boron, they created a material slightly harder than cubic boron-nitride, the material now ranked second.
‘This was the first additive we tried and it produced a material that’s right up there with cubic boron-nitride,’ said Bruce Cook, an associate scientist and lead investigator on the project.
Cook and his colleagues tested samples of the alloy on several different instruments, all of which measured the hardness of the material at approximately 46 gigapascals (the equivalent of 6.67 million psi), slightly higher than cubicboron-nitride’s hardness of about 45 GPa (6.53 million psi). By contrast, diamond’s hardness is estimated at between 70 and 100 GPa (10.15-14.5 million psi).
The aluminium-magnesium-boron compound could also be the least expensive of the three materials.
Cook estimated its cost at around $700 per pound, compared to cubic boron-nitride’s price tag of up to $7,000 per pound and diamond’s price of $2,000 per pound, making the material an attractive to manufacturers.
Cook said diamond isn’t an option for cutting and grinding steel because it reacts by turning into graphite when brought into contact with iron-based materials at high temperatures. Cook said preliminary tests indicate that the Ames Lab compound doesn’t react with iron in the same way as diamonds.
Cook’s discovery of the hardness of the compound came about by accident. He was researching its thermoelectric properties in 1992 when he found that he couldn’t cut the samples he’d made.
Strangely enough, the material is an unlikely candidate for a hard material because of the structure of its unit cell. ‘A diamond has eight carbon atoms in a unit cell. It’s a very simple, highly symmetric structure,’ Russell explained. ‘This material has 64 atoms in the unit cell.’
Cook said 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 said.