Hardness of diamond

Very high hardness with values of 8 to 10 on the Mohs scale in some cases they approach the hardness of diamond (e.g. W2C). 

Figure 5.14 Normalized temperature dependence of the hardnesses of diamond, Si, and Ge. Note that the hardnesses divide by the low temperature hardnesses begin to decrease at the respective Debye temperatures (0).

The great hardness of diamond results from the fact that the entire diamond crystal is actually one very large molecule. 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

The element carbon occurs in nature in two so-called allotropic forms, different crystal structures with the same chemical formula. In Fig. 3.13 the crystal structure of diamond and graphite have been represented. In diamond the C atoms are closely packed and each C atom is linked with four other C atoms. Thus a tight network of atoms is formed which, together with the binding strength, is responsible for the extreme hardness of diamond. Graphite has a layered structure and the space between the layers is relatively large. 

Silicon dioxide (silica, Si02) has a similar 3D structure and properties. The hardness of diamond enables it to be used as the leading edge on cutting tools. 

The exceptionally high hardness of diamond, the easy cleavage along the three main lattice planes of the crystal, its high refractive index and its optical dispersion makes diamonds a valuable raw material for brilliants. The gem diamonds utilized are still exclusively natural diamonds. 

On the other hand, diamond, a form of solid carbon, is one of the hardest substances known and has an extremely high melting point (about 3500 °C). The incredible hardness of diamond arises from the very strong covalent carbon-carbon bonds in the crystal, which lead to a giant molecule. In fact, the entire crystal can be viewed as one huge molecule. A small part of the diamond structure is represented below. 

Compounds intermediate in composition between diamond and cubic boron nitride, such as BC2N, have recently been synthesised, as well as other iso-electronic compounds such as B2CO and B5NO2, but none, so far, has the hardness of diamond. 

Figure 1. Vickers hardness of selected materials in comparison with that of the heterostructures and nanocrystalline composites. Notice that the hardness of diamond depends on the quality and purity of the crystal (see text).

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