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Is wurtzite boron nitride harder than diamond?

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Wurtzite-like, wurtzite boron nitride has recently announced as a material harder than diamond. Such "recentness" is explained by the fact that it is relatively rare in nature, and these comparisons have practically not been made earlier.

 


Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the following crystal structures:

- BNw (wurtzite structure),
- BNcub (cubic BN), softer than diamond, but its thermal and chemical resistance is higher, used as an abrasive;
- BNhex (hexagonal BN), the most stable and mild among the BN polymorphs, is used as a lubricant and additive in cosmetic products;
- rhombohedral.

   The BNw structure is metastable under any conditions. BNw (wurtzite structure, also known as g-BN) was first synthesized in 1963. As a rule, BN crystals with the symmetry of wurtzite (wurtzite boron nitride powder) are very small (fractions of a micron), highly defective, and contain other phases. In other words, it is practically impossible to measure the strength of these crystals and compare them with diamond: this material is rare in nature, and, according to my data, it has not yet been possible to synthesize a crystal suitable for measuring hardness.

   At the same time, another material related to wurtzite boron nitride, cubic boron nitride, is the second hardest material after diamond, with a fracture force of about 50 GPa; despite the fact that for diamond it is 70–150 GPa.
    However, wurtzite boron nitride and cubic boron nitride have the same wavelength and elastic moduli, which suggests that they can exhibit the same hardness as well. In fact, this "opportunity" is of no value, since it is impossible to apply it in practice due to the size of the wurtzite boron nitride crystal. In other words, it is impossible to make an abrasive wheel or paste suitable for anything other than fine polishing, the breadth of which, although significant (eg in optics), is rather limited compared to sharpening and grinding.

   However, wurtzite-like boron nitride is a promising wide-gap group III-V material for modern electronic devices because it has many properties superior to gallium nitride (GaN) and aluminum nitride (AlN), such as higher thermal conductivity and greater spontaneous polarization.


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