Superhard materials

Superhard materials are materials possessing hardness exceeding that of commercial polycrystalline cubic boron nitride (cBN), i.e. with Vickers hardness H_V > 40 GPa.Fact|date=August 2008

Superhard materials are widely used in many applications, from cutting and polishing tools to wear-resistant coatings [Wentorf et al. (1980), Leger & Haines (1997), Brook (1999), Brazhkin et al. (2002), and McMillan (2002)] . The hardest of them, diamond (H_V = 70-150 GPa), found wide application in modern science and technology due to its unique properties [Harlow (1998)] .

At the same time, diamond is non-resistant to oxidation at high temperature [Miyata & Kobashi (1996)] and reactive with ferrous metals [Ong & Shing (1997)] . The growing demand for superhard, diamond-like compounds in electronic [Isberg et al. (2002)] and electrochemical [Koppang et al. (1999) and Yano et al. (1999)] applications, cutting and shaping hard metals and ceramics [Novikov (2005)] stimulated the search for novel advanced superhard phases that are more thermally and chemically stable than pure diamond.

The high pressure synthesis of diamond in 1953 in Sweden [Liander (1955)] and in 1954 in the USA [Bundy et al. (1955)] , made possible by the development of new apparatus and techniques, became a milestone in synthesis of artificial superhard materials; clearly showed the potential of high-pressure applications for industrial purposes; and stimulated the growing interest in the field. Four years after the first synthesis of artificial diamond, cubic boron nitride cBN was obtained that was found to be second hard phase [Wentorf (1957) and Bundy & Wentorf (1963)] .

Recently the high-pressure synthesis gave rise to cubic boron carbo-nitrides, c BCxN [Solozhenko et al. (2001a), Solozhenko (2002), Zhao et al. (2002), Hokamoto et al. (2003), and Komatsu (2004)] , that are superhard phases with diamond-related structure. Also, the syntheses of diamond-like boron oxide, d B2O [Endo et al. (1987)] , and cubic carbon nitride, c C3N4, [Ming et al. (2006)] were claimed; however, these experiments were not yet reproduced.

The phases with diamond-like structure are not the only candidates to be superhard. Boron displays some of the most remarkable physical and chemical properties of any element [Bullett (1982)] . Boron-rich solids give a rise to a large group of hard refractory compounds, i.e. B6O, B4C etc., with unique crystal structures and interesting physical and chemical properties related to their strongly covalent and electron-deficiency character [Rizzo et al. (1962), Lundstrom & Bolmgren (1994), and Lundstrom & Andreev (1996)] . The important combination of extremes of lightness, hardness, strength and stability has suited boron and boron-rich compounds to a variety of technological applications [Emin (1987), Kimura (1993), McMillan (2002), and Emin (2006)] .

Only nano-diamond and aggregated diamond nanorods have been proven to be harder than diamond. The aggregated boron nitride nanocomposites are the hardness comparable to that of diamond. Beta carbon nitride, if it could be synthesized, is predicted to be harder than diamond.