How Wurtzite Boron Nitride Is Harder Than Diamond

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There are two materials that could be considered to be the hardest ever made: diamonds and wurtzite boron nitride. Wurtzite boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with a carbon lattice. It has a number of crystalline forms, including cubic and rhombohedral. A form of it known as lonsdaleite is very rare and is sometimes found in meteorites that have hit Earth and in volcanic eruptions, but large amounts can’t be made in the lab.

Computer simulations have suggested that lonsdaleite and the cubic form of wurtzite boron have an indentation strength far greater than diamonds, but physical experiments to confirm this have been difficult because the substances are so hard to make. But now researchers have shown that if the indentation is applied at just the right temperature and force, a phase transformation occurs that increases the material’s strength even further.

This results in a new phase of wurtzite boron with a hexagonal crystal structure. It’s able to resist 18% more stress than diamond, and 58% more pressure. This makes it a promising candidate to replace the tungsten carbide tip on cutting and drilling tools that work at high temperatures or as an anti-wear coating for space vehicles.

The research also demonstrates that atomically thin monolayers of wurtzite boron are harder than bulk material. This is due to the flexibility of bonds between the atoms, which allows some to relax and relieve tension when they are stressed. This can be detected by observing the Raman spectrum of monolayers, which is affected by strain.