Superhard materials

New Scientist magazine has a review of materials that are nearly as hard as or harder than diamond.

Wurtzite or w-BN, which is comparable to a diamond-like form of carbon known as lonsdaleite. It had been made since the 1970s by using high pressure or explosive shock waves to squeeze h-BN, but had only been fabricated in quantities too small for its hardness to be measurable by any conventional means. In 2007, however, Dubrovinskaia and her colleagues succeeded in making a mosaic of w-BN crystals which they claimed had a hardness comparable to that of diamond.

Changfeng Chen of Jiao Tong University in Shanghai, China, and his colleagues think that 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. Their research indicates that wurtzite boron nitride is harder than diamond.

Simulation showed that wurtzide boron nitride would withstand 18% more stress than diamond, and lonsdaleite 58% more. Mineral lonsdaleite, or hexagonal diamond is made from carbon atoms just like diamond, but they are arranged in a different shape. If the results are confirmed with physical experiments, both materials would be far harder than any substance ever measured. Wurtzite boron nitride may turn out most useful of the two, because it is stable in oxygen at higher temperatures than diamond

Mao and theorist Yanming Ma and colleagues at Jilin University in Changchun, north-east China, recently proposed that transformed graphite has a structure they call monoclinic carbon. This M-carbon forms when graphite sheets buckle and form extra chemical bonds between the layers (Physical Review Letters, vol 102, p 175506). The resulting structure, they calculated, should be almost as hard as diamond.

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