Nanotwinned Cubic Boron Nitride

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nanotwinned cubic boron nitride is a material with remarkable hardness and toughness. It is a good candidate for superhard applications such as cutting, grinding and drilling in high-temperature environments.

The crystalline structure of nt-cBN is based on the interlocking of boron and nitrogen atoms in a latticework, much like neighboring apartments in a building. The boron and nitrogen atoms in the latticework are also joined to each other by shared bonds, similar to a hydrogen bond.

But the latticework is much smaller than that of diamond. It’s about 30 nm in diameter and consists of just a few layers of boron and nitrogen atoms, compared to diamond’s hundreds of layers of atoms.

This small size, combined with a high number of neighboring boron and nitrogen atoms, makes the crystals harder. This makes it more resistant to pressure from a pyramid-shaped piece of diamond called an indenter.

Researchers used this test to gauge the hardness of Tian’s nanotwinned boron nitride, which had a Vickers number of 108 GPa, far surpassing synthetic diamond (100 GPa) and more than double the hardness of commercial forms of cubic boron nitride. But the process for measuring a material’s hardness is a tricky one.

To accurately measure the hardness of a material, scientists usually take an even harder substance, shape it into a pyramid, and see how much pressure it takes to drive that pyramid into the more hardened material. But when you take something as small as 10 nm and use that as a reference, there’s a problem: the inherent defects and distortions in that tiny grain are nearly as big as the grains themselves, reducing their strength by about a factor of two.

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