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Posted: February 20, 2009

Diamond no longer nature's hardest material

(Nanowerk News) Diamond lost its title of the "world's hardest material" by 58% to a rare natural substance, according to a new research by Chinese scientists. Pan Zicheng at Shanghai Jiao Tong University and colleagues simulated how atoms in two substances believed to have promise as very hard materials would respond to the stress of a finely tipped probe pushing down on them (Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite).
Extreme conditions
The first, wurtzite boron nitride has a similar structure to diamond, but is made up of different atoms.
The second, the mineral lonsdaleite, or hexagonal diamond is made from carbon atoms just like diamond, but they are arranged in a different shape.
Only small amounts of wurtzite boron nitride and lonsdaleite exist naturally or have been made in the lab so until now no one had realised their superior strength. The simulation showed that wurtzide boron nitride would withstand 18% more stress than diamond, and lonsdaleite 58% more. If the results are confirmed with physical experiments, both materials would be far harder than any substance ever measured.
Doing those tests won't be easy, though. Because both are rare in nature, a way is needed to make enough of either of them to test the prediction.
Rare mineral lonsdaleite is sometimes formed when meteorites containing graphite hit Earth, while wurtzite boron nitride is formed during volcanic eruptions that produce very high temperatures and pressures.
Flexible friend
If confirmed, however, wurtzite boron nitride may turn out most useful of the two, because it is stable in oxygen at higher temperatures than diamond. This makes it ideal to place on the tips of cutting and drilling tools operating at high temperatures, or as corrosion resistant films c on the surface of a space vehicle, for example.
Paradoxically, wurtzite boron nitride's hardness appears to come from the flexibility of the bonds between the atoms that make it up. When the material is stressed some bonds re-orientate themselves by about 90º to relieve the tension.
Although diamond undergoes a similar process, something about the structure of wurtzite boron nitride makes it nearly 80% stronger after the process takes place, says study co-author Changfeng Chen at the University of Nevada, Las Vegas, an ability diamond does not have.
Single crystals
Natalia Dubrovinskaia from the University of Heidelberg in Germany has carried out similar research.
"This is important because any attempt to give an insight into the mechanism that improves a material's property, especially hardness, is technologically extremely significant," she told New Scientist.
The more that is understood about what influences the hardness of materials, the more it will become possible to design hard materials to order, she explains.
However, she points out that in order to prove the theory, single crystals of each material would be needed. So far there are no known ways to isolate or grow such crystals of either material.
Source: Chinese Academy of Sciences