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A Diamond Double

Hardness has made diamond, especially synthetic diamond, a great tool for cutting all sorts of things in an industrial setting, but it isn¿t an all-purpose tool. Synthetic diamond, which is created by compressing such carbon-rich materials as graphite, decomposes into carbon dioxide when heated by friction. It also dissolves in iron if it is heated. These two qualities make it useless for tasks like cutting steel. But now a group of scientists from France, Germany and the Ukraine has created an artificial material that is almost as hard as diamond and doesn¿t share its drawbacks. Their findings were published in yesterday¿s Applied Physics Letters.

The team looked at cubic boron nitride (cBN)¿an artificial material initially created as an alternative to synthetic diamond, though about half as hard. Like the carbon in diamond, both boron and nitrogen in cBN tend to form short, strong chemical bonds, which is why they create such hard materials. So the scientists considered how they might combine all three elements. The results were boron carbonitrides (BC2N and BC4N). Like graphite, they are initially soft but can be compressed into harder materials. Squeezing BC2N at about 1.8 million times atmospheric pressure produces its cubic form, which pushes cubic boron nitride to third place on the list of hardest materials. Diamonds still hold first place.

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