New Ultrahard Materials

Diamond s combination of properties make it a unique material. Although hardness is its primary characteristic, thermal conductivity, compressive strength, refractive index, spectral transmittance, and chemical stability are either the highest or among the highest found in nature. 

This combination of properties can be traced to the same structural characteristics of diamond that give rise to its high hardness. It is therefore reasonable to expect that other ultrahard materials would also exhibit such a suite of properties. This would make them also desirable for a number of industrial applications. 

New ultrahard materials might possess new attributes other than a higher hardness that would make them more attractive than diamond in some cases. A different chemical composition would give rise to different interaction of cutting tool and workpiece as is the case for cBN, as would a different crystal structure and ultrahard particle morphology. 

The search for new ultrahard materials is motivated largely by these considerations, but also from purely economic ones. It might be possible to synthesize such materials through routes cheaper than those involved in the case of diamond, thus accessing new applications and new markets. In this paragraph we briefly review the basic ideas behind the search for new ultrahard materials, as well as the latest developments in the search for some of the identified cases. 

Hardness is a measure of a material s ability to resist elastic and plastic deformation. The hardness of non-ideal material is determined by the intrinsic stiffness of the material, as well as by the nature of its defects, be they point defects, dislocations, or macroscopic defects such as microcracks etc. For ideal systems, the hardness of a material will scale with its bulk modulus. 

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The search for new ultrahard materials has also been extended into nonboron based oxides. The expectation was that high pressure allotropes would exhibit a high... 

R. S. Sussmann, J. R. Brandon, S. E. Coe, C. S. J. Pickles, C. G. Sweeney, A. Wasenczuk, C. J. H. Wort, and C. N. Dodge, CVD Diamond, A, New Engineering Material for Thermal Dielectric and Optical Applications, Proceedings of the Industrial Diamond Association of America meeting on Ultrahard Materials, Windsor, Ontario 28-30 May 1998, and Indust. Diamond... 

This introduction deals with some of the latest experimental and theoretical developments in the field of novel boron- and carbon-based ultrahard materials as well as with new observations on a class of silicon-based compounds which previously were not classified as ultrahard. 

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