- Mar 26, 2010
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A 2-millimeter chunk of nanotwinned cubic boron nitride.
How do you design industrial tools that can top the most heavy-duty diamond-tipped devices? Easy: you create a new material that’s even harder than diamond.
Yes, it’s an oft-misstated “fact”: Diamond is the hardest material in the world. That title has been contested for some time now, and a paper published this month in Nature offers yet another contender.
“Ultrahard nanotwinned cubic boron nitride,” describes how researchers from the University of Chicago, the University of New Mexico, Yanshan University, Jilin University, and Hebei University of Technology compressed a form of boron nitride particles to an ultrahard version.
The transparent nuggets that resulted rivaled — and even exceeded — diamond in their hardness, according to tests run by the researchers. With a Vickers score of 108 GPa, it surpasses synthetic diamond (100 GPa) and more than doubles the hardness of commercial forms of cubic boron nitride.
The secret is in the nanostructure. Yongjun Tian and the other researchers started with onion-like boron nitride particles shaped a bit like a flaky rose — or, as Tian describes them, like Matryoshka dolls. When they compressed them at 1,800 Celsius and 15 GPa (around 68,000 times the pressure in a car tire), the crystals reorganized and formed in a nanotwinned structure.
In a nanotwinned crystalline structure, neighboring atoms share a boundary, the way neighboring apartments do. And like some apartments, the twins mirror each other. Typically, to make a substance harder, scientists decrease the size of the grains, which makes it harder for anything to puncture it — small grains equals less space between them for any point to enter. But the process hit a wall: in anything smaller than about 10 nm, inherent defects or distortions are nearly as big as the grains themselves, and thus weakens the structure.
But the nanotwinning also makes substances harder to puncture, and in the case of boron nitride, maintained that characteristic strength at sizes averaging about 4 nm, explains Tian. And as a bonus, the cubic boron nitride was stable at high temperatures as well.
“In our nanotwinned cBN, the excellent thermal stability and chemical inertness are maintained with hardness competitive to or even more than diamond, making it the most desirable tool material for industry,” says Tian.
He anticipates that, with further research, the product will be comparable in price to the softer, commercial forms of cubic boron nitride that are currently available. Probable uses include machining, grinding, drilling and cutting tools, as well as scientific instrumentation.
Of course, the problem is, to accurately measure the hardness of a material, scientists take an even harder substance, shape it into a pyramid, and see how much pressure is required to drive that pyramid into the material. That doesn’t work unless you have something you’re sure is harder, so the Vickers number for Tian’s cubic boron nitride is not necessarily the final word on the measurement, notes crystallographer Natalia Dubrovinskaia in Scientific American.