New Superhard Ternary Borides in Composite Materials

Superhard substances are those a hardness above 20 GPa, i.e. higher than that of corundum (Kislii et al.,1988) or with a Vickers hardness Hv exceeding 40 GPa (Sologenko&Gregorianz, 2005). The ten utmost non-metal substances and refractory compounds form a "hardness pyramid" (Kislii et al.,1988) diamond being on top, followed by cubic-boron nitride and boron carbide. Metal-like ones form a similar pyramid, the transition metal borides occupying its top. However, the maximum hardness found for them is inferior to that of the non-metal substances. The interest in borides is due to their extraordinary hardness (up to 1873 0C) as compared to other refractory compounds. The hardest boride (B12C3) is used as a wear resistant polycrystalline material, armor tiles, nuclear industry, etc. (Anderson, 2002).However, applications are restricted by its high brittleness due to the strong covalent bonds in its crystal lattice. It has been established (Zakhariev&Radev, 1988) that superhard ternary compounds based on boron carbide with dissolved IV-VI group metals (B12-nC3Men) can be obtained by sintering CMC composite materials without pressure. Sintering of TiB2-MeC-Me systems with no pressure applied is a new trend in the field of superhard boride composite materials. The binding metal (e.g. Ni or Co) in TiB2-Me system reacts with the boride and forms a low-melting phase (Lecrivan&Provost, 1968); (Fitzer, 1973). The growing interest in borides comes from their high-temperature behavior: high melting point, hardness, wear resistance and chemical inertness. These allows using them to produce cathodes for electrolytic aluminium, first wall coatings and neutron absorbers for nuclear technology, valve components in cool liquefaction plants and crucible materials for metal evaporation. Studies on the TiB2-TiC and TiB2-TaC systems show that the carbides inhibits the grain growth of the boride phase (Murata et al., 1967).The TiB2-MeC-Me materials have the following advantages as compared with the binary systems mentioned (Petzow&Telle, 1984): application of hot pressing is not necessary, the grain growth is completely inhibited, the mechanical properties are improved due to the small grain size (about 1μm) (Zakhariev et al., 1993). WС-Со is the source material in the field of the metal-working whereas superhard ternary boride (WCoB) coating entails a sharp risе in wear resistance of the composite materials (Zakhariev et al., 1987). The micro hardness of synthetic polycrystalline WСоВ amounts to