Design principles describe new superhard metal borides

The market for hard and superhard materials is more than $25 billion a year. Superhard materials, such as synthetic diamond and cubic boron nitride, are used to cut and polish hard substances like superalloys, but are expensive because they are prepared using high temperature and pressure. Metal borides with increased hardness and increased temperature stability, and made simply, would be extremely useful in a wide range of machining and cutting applications.

Metal borides, the world’s hardest metals, could be affordable alternatives to current commercial superhard materials because they can be prepared at ambient pressure. Thanks to their high electrical conductivity, they can also be easily cut and shaped using an electric discharge machine, a common tool in metal manufacturing. In an APL Materials article, the authors describe ways to design new transition metal borides for increased hardness and temperature stability. They turn to ternary and higher borides, such as YReB₄, Y₂ReB₆, Y₃ReB₇ and YMo₃B₇.

Hardness is influenced not only by atomic bonding within a crystalline material but also by the internal structure of the material, as well as the microstructure. The material becomes harder if the atomic layers in the crystal cannot slide past each other, and one way to prevent the sheets from sliding is by doping a metal into the boride with atoms larger than the host metal.

Doping can also have the benefit of altering the cooling rate of the materials during formation, so that their crystals form nanostructured grains. Hence, there are more grain boundaries to impede the atomic layers from sliding, thus leading to a further increase in the hardness.

Source: “Perspective: Superhard metal borides: A look forward,” by Georgiy Akopov, Lisa E. Pangilina, Reza Mohammadi, and Richard B. Kaner, APL Materials (2018). The article can be accessed at https://doi.org/10.1063/1.5040763 .