Properties
The hardness of ReB2 exhibits considerable anisotropy because of its hexagonal layered structure (see structure model). Its value (HV ~ 22 GPa) is much lower than that of diamond and is comparable to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride.
ReB2 slowly reacts with water, converting into a hydroxide.
Two factors contribute to ReB2's high hardness: a high density of valence electrons, and an abundance of short covalent bonds. Rhenium has one of the highest valence electron densities of any transition metal (476 electrons/nm3, compare to 572 electrons/nm3 for osmium and 705 electrons/nm3 for diamond). The addition of boron requires only a 5% expansion of the rhenium lattice, because the small boron atoms fill the existing spaces between the rhenium atoms. Furthermore, the electronegativities of rhenium and boron are close enough (1.9 and 2.04 on the Pauling scale) that they form covalent bonds in which the electrons are shared almost equally.
Read more about this topic: Rhenium Diboride
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