Only very few candidates do exist, i.e. those
which form pure or nearly pure covalent bonds. Metallic
or dipolar contributions to the bonding must be very
small or zero. Diamond is an example. The isolated
carbon atom has a 2s 2 2p 2 outer shell configuration,
which hybridizes in the solid state to sp 3, i.e. the four
electrons each form a strong, well-localized ~ bond with
four neighbour atoms. The short bond length of 1.54
and the tetrahedral configuration are responsible for the
high density of the crystal and its high Young's modulus.
The hardness is of the order of 10000kgfmm -2.
Similarly, cubic boron nitride and silicon carbide are
tetrahedral, sp3-bonded materials. Only the four elements
boron, carbon, nitrogen and silicon can form such
strong a bonds. As soon as arc bond is involved (sp 2
hybridization), soft materials such as graphite or hexagonal
boron nitride evolve.
Some mechanical properties of different materials are
listed in Table 1. B4C and fl-Si3N 4 are not listed --
although they reveal hardnesses close to 4000 VH --
since only a few papers have been published on physical
vapour deposition (PVD) coatings of these materials.