The material is a boron nitride “nanocomposite.” This means that, rather than consisting of one large continuous crystal, it is made of crystalline boron-nitride grains that are each a few to several nanometers in size. Although research groups have previously reported boron carbonitride materials, claimed to be the second and third hardest materials after diamond, the particular versions, or “phases,” of those materials were unstable at high temperatures.
Single-crystal diamond, the hardest type, has a hardness of about 100 GPa. The boron nitride nanocomposite displayed a maximum hardness of 85 GPa at a grain size of about 14 nanometers, and is thermally stable up to 1600 degrees Kelvin (about 2400 degrees Fahrenheit). Prior to this research, the next hardest known material after single-crystal diamond was cubic boron nitride, a single-crystal phase of the material, which has a Vickers hardness of 50 GPa.