Mechanical properties and hardness of boron and boron-rich solids

A brief review of the recent studies of the crystal and electronic structure and its stability under large strain-shear and tensile deformation of selected boron compounds, which have been predicted to be superhard on the basis of their high elastic moduli, is presented. It will be shown that in many cases, the materials undergo electronic instability and transformation to softer phases with a lower shear resistance than the original equilibrium structure. Therefore, high values of elastic moduli (“low compressibility”) do not guarantee high hardness. These results also challenge the recent “models of theoretical hardness of an ideal crystal”, which are based on the equilibrium electronic properties. It is shown that appropriately nanostructured materials open the way to the design of superhard materials.