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Ultrahard Nanotwinned Cubic Boron Nitride

2023-05-07 12:40:02  Knowledge

Cubic boron nitride (c-BN) is a superhard mineral that is second only to diamond in hardness. It's also four times as strong as traditional abrasive and is used in high-tech cutting tools, aerospace applications and thermal spraying to hardened steel. Until recently, though, the only c-BN available was man-made in labs and was much softer than diamond. Scientists now have made the first c-BN that is actually harder than diamond. How they did it is a fascinating story.

The scientists started with "onion-like boron nitride particles shaped a bit like a flaky rose," and compressed them to 15 gigapascals. This caused the crystals to reorganize into a new, nanotwinned structure. The term comes from the way adjoining atoms in this new structure mirror each other, rather like apartments in an apartment building. This is a very powerful form of deformation called nanotwinning, which the scientists believe is responsible for boosting the new c-BN's strength.

After the samples were compressed, the team analyzed them with a scanning electron microscope and a focused ion beam. They found that the c-BN exhibited nanotwinning and had very strong, uniform sutures between diamond and c-BN domains, as shown in a series of electron micrographs, or TEMs, taken on a FEI Nova NanoSEM 430.

The atomic structure of the new material is very stable, even at high temperatures and under very high pressures. That means it should last a long time, and the sp3-hybridized c-BN bonds make the new c-BN remarkably tough. This finding puts the new c-BN in a class of materials that could be used in everything from cutting tools to drilling for resources.

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