What is Stronger Than Diamond?

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For centuries, diamonds have held a reputation for being the hardest material on Earth. But according to new research, a mineral called lonsdaleite may be even stronger. It has the same atomic makeup as a diamond, but unlike diamonds, it has a hexagonal design, which makes it 58% harder. Lonsdaleite was found in meteorites that landed in Africa, and scientists believe it came from a dwarf planet billions of years ago.

To determine a material’s hardness, researchers press a pyramid-shaped indenter against it and record how much force is required to leave an impression on the surface of the material. This technique is known as the Vickers test and has long been used to measure the hardness of a material. In a recent study, scientists compared the indentation hardness of diamonds, lonsdaleite and wurtzite boron nitride. They found that under the same indentation conditions, w-BN and lonsdaleite had far greater strength than diamonds. Their enhanced strength was due to a structural phase shift in which the materials flipped their atomic bonds, while still retaining their crystal volume.

This bond-flipping is what makes lonsdaleite so strong. But it is not the only thing that gives lonsdaleite its edge over diamonds. Unlike diamonds, which are polycrystalline, lonsdaleite has a face-centred hexagonal structure. This type of structure is more compact than the tetrahedral crystal structures that give diamonds their unique hardness. Interestingly, a team at North Carolina State University recently discovered another form of carbon that is not only harder than diamonds, but also glows when exposed to low levels of energy. This form of carbon is known as Q-carbon and, if it exists in the natural world, may be found deep inside some planets. It is similar to graphite but much more rigid. This discovery could lead to super-strong and lightweight screens for electronics.