P-Si3N4 crystals

Fig. 2. Crystal structures of a-(left) and P-Si3N4 (right). All of the nitrogen atoms form (N ) bridges between three neighboring silicon atoms.

Most well-known hard materials have surprisingly simple crystal structures. Here we give an overview of the crystal chemistry of hard materials. Different aspects of the various crystal structures are discussed, such as close-packed structures with filled octahedral voids like titanium carbide, the tetrahedral arrangements in SiC or the three-dimensional inorganic networks in a- and P-Si3N4. Also we briefly mention the synthesis and some applications of various hard materials. 

The microhardness was determined for the different crystal planes of p-Si3N4. The data differ widely in different orientations. 

We have discussed the formation of P-SigN crystals. However, silicon nitride has another modification - -Si3N4, which exists till 1500 C. 

The density of P-Si3N4, as calculated from the structural data of the single crystal, was 3.214gcm [14], while that for polycrystaDine P-Si3N4 was 3.202 gcm [44]. 

The rare earth-doped oxynitride glasses demonstrated higher densities and thermal expansion coefficients, but a substantially lower hardness, compared to the a- and P-Si3N4 single crystals. 

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