Silicon nitride (Si₃N₄) is extremely hard. Only diamond and cubic boron nitride (cBN) are harder. Due to its hardness, silicon nitride is used for ball bearings, cutting tools, and engine parts in the automotive and aircraft industries. The desired components are produced by pressing the powdered starting material into the corresponding form under pressure and heat. This produces the hexagonal silicon nitride.

By increasing pressure (15.6 GPa) and temperature (1800 °C), Norimasa Nishiyama, Deutsches Elektronen-Synchrotron DESY, Hamburg, and colleagues developed transparent cubic silicon nitride (c-Si₃N₄): a superhart and extremely heat-resistant polycrystalline material.

The transformation is similar to that of carbon, which also has a hexagonal structure under normal conditions and transforms under high pressure into a cubic variant, the diamond. c-Si₃N₄ has very fine grain boundaries, which explain its transparency. The transitions between individual granules in the material are smaller than a nanometer. Furthermore, in the high-pressure phase, oxygen impurities are distributed throughout the material and do not accumulate at the grain boundaries as under normal conditions. This is also crucial for transparency. Diamond is stable in air only up to about 750 °C; c-Si₃N₄ is stable up to 1400 °C.

The raw material is cheap. Production, however, requires about twice as much pressure as required to produce artificial diamond. Materila diameters of about 5 mm can quite easily be realized according to the researchers. For special applications, for example in space travel, the superhard, transparent material could be very interesting.

• Transparent polycrystalline cubic silicon nitride,
  Norimasa Nishiyama, Ryo Ishikawa, Hiroaki Ohfuji, Hauke Marquardt, Alexander Kurnosov, Takashi Taniguchi, Byung-Nam Kim, Hidehiro Yoshida, Atsunobu Masuno, Jozef Bednarcik, Eleonora Kulik, Yuichi Ikuhara, Fumihiro Wakai, Tetsuo Irifune,
  Scientific Reports 2017.
  DOI: doi: 10.1038/srep44755