Phys. Rev. B 80, 060103 (2009)

Physicists recently discovered that materials known as transition-metal diborides rival the hardness of diamonds in one direction but are relatively soft in another. Such materials have the potential to revolutionize industrial processes — if scientists can work out what makes them so tough in certain orientations.

Theorists had thought that the strength came from strong vertical bonds within the crystal, similar to the ridges in a sheet of corrugated cardboard. But Antonín Šimůnek of the Academy of Sciences of the Czech Republic in Prague thinks it might be just the opposite. He proposes a model in which strong horizontal bonds between surface atoms act as a tough shell that can resist denting by a sharp tip. The theory could lead to development of new super-hard materials.