Superhard compounds

Superhard compounds are obviously formed by a combination of the low atomic number elements boron, carbon, silicon, and nitrogen. Carbon-carbon as diamond, boron-nitrogen as cubic boron nitride, boron-carbon as boron carbide, and silicon-carbon as silicon carbide, belong to the hardest materials hitherto known. Because of their extreme properties and the variety of present and potential commercial applications, silicon carbide (SiC) and boron carbide (B4C) are, besides tungsten carbide-based hard metals, considered by many as the most important carbide materials. 

This section deals with high pressure phases of two silicon based compounds Si02 and Si3N4, which previously did not belong to the family of superhard materials. 

However, the experimental evidence for the synthesis and structure of dense superhard C3N4 phases reported in the literature is not unambiguous. Thus, future research should be focused on the synthesis of sufficient amounts of these new superhard materials and on their structural characterization. The structures of all C3N4-polymorphs reported so far still remain to be verified by single crystal X-ray or electron diffraction. Furthermore, the difficulties in the determination of reliable hardness values of these new materials not only at room temperature but also at elevated temperatures have to be overcome. Diamond will not lose its reputation as the hardest material until the newly developed phases and compounds have been characterized in detail and can be produced in reasonable amounts. 

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