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Harder than diamond?

A nice article in New Scientist about the search for substances harder than diamond. This is important for nanomechanical engineering because hardness translates into properties useful in machine parts at the nanoscale.

A nanocrystalline form of diamond, sometimes called aggregated diamond nanorods, was described in 2003 by Tetsuo Irifune and his colleagues at Ehime University in Japan. Since then, Natalia Dubrovinskaia and her colleagues at the University of Bayreuth in Germany have found that a tip made of these nanorods could scratch regular diamond, seemingly indicating a greater hardness.

Meanwhile, attention was switching back to the lighter end of the periodic table, home to many elements that can form short, strong bonds. One such is boron, which sits just one berth over from carbon. The idea that boron has superhardness potential goes back at least to 1965, when Robert Wentorf, one of the General Electric team that made synthetic diamond, claimed to have made superhard crystals of boron at a pressure of 100,000 atmospheres and a temperature of 1500 °C. He couldn’t work out what the material’s structure was, though, and the idea was shelved for 40 years.

It was only in February this year that a team led by Oganov published a structure for the superhard boron crystal – a repeating pattern of 28 boron atoms they called B28 (Nature, vol 457, p 863). In May, Dubrovinskaia and her team announced that they had made large crystals of B28 that were about half as hard as diamond (Physical Review Letters, vol 102, p 185501). Close, but still no diamond necklace.

Pure boron is not the last word, though. Boron nitride – in which boron is combined with nitrogen – forms analogues of all the known carbon phases. There is a soft variant called h-BN, which is made of sheets of hexagonal rings just like graphite, and finds similar use as a lubricant. Then there’s cubic boron nitride, or c-BN, which has a structure similar to diamond, and for a long time has played second fiddle only to diamond in hardness.

There is a third version, too, known as wurtzite or w-BN, which is comparable to a diamond-like form of carbon known as lonsdaleite.

w-BN may be inherently hard, because it can transform into another, stronger structure when another material presses into it. The pressure causes chemical bonds to flip into a different arrangement which looks like that of c-BN, but has its network of atomic bonds ideally positioned to resist stress (Physical Review Letters, vol 102, p 055503).

“It’s a bit like someone changing their body posture in response to applied stress so that they can carry a higher load,” says Chen. As a result, the material becomes even harder than diamond, at least in theory.

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