Boron carbide hardness range

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

Boron carbide is very hard, refi-actory solids (m.p.>2400 °C). And its thermoelectric properties are unconventional in high temperature range above... 

Silicon carbide (SiC) is the most widely used nonoxide ceramic. Its major application is in abrasives because of its hardness (surpassed only by diamond, cubic boron nitride, and boron carbide). Silicon carbide does not occur in nature and therefore must be synthesized. It occurs in two crystalline forms the cubic P phase, which is formed in the range 1400-1800°C, and the hexagonal a phase, formed at >2000°C. 

A superfinishing abrasive stone is generally vitrified bonded and has a hardness range of HRH20-70. White aluminum oxide, silicon carbide, cubic boron nitride, or diamond abrasive is used. Abrasive sizes in the JIS 300- 500 range are used for coarse finishing, while JIS 600- 1500 abrasives are used for a fine finish (Matsui and Nakasato 1965, Onchi et al. 1995, Varghese et al. 1998). 

Boron carbide is often represented as B4C, but this is only a rough empirical formula. The compound is thought to consist of dodecahedral clusters of 12 boron atoms associated with linear rods of 3 carbon atoms, but the stoichiometry is highly variable and can range from BeCs to B4C. Boron carbide is extremely hard (9.3 on the Mohs scale) and is used as an abrasive. 

In equation (1.2), is the measured hardness, is the hardness at zero porosity, and 0 is the fractional porosity. Typically B is less than unity and for boron carbide, B4C, it has the value 0.35 for 0 in the range 0-1.0. The bifunctional dependence of hardness on porosity contrasts with equations used to relate ceramic porosity and strength, for example, equation (1.3),... 

The abrasives used for coatings are diamond, aluminum oxide (corundum), silicon carbide, boron carbide, boron nitride, emery, flint (quartz), and garnet. These range in hardness and cost. Typically the superabrasives, synthetic diamond and cubic boron nitride, do not compete easily with the standard abrasives. 

However, on the one hand, low ESR signals alone are a weak argument for the assumption of hole bipolarons. On the other hand, several experimental results are in contradiction of this model. For example, (a) the electrical conductivity of boron carbide is maximum at the minimum concentration of BnC icosahedra in the homogeneity range (b) polaron-type effects are restricted to one electron per icosahedron and no corresponding electron-phonon interaction with holes, in particular not with hole pairs in icosahedra, has been proved experimentally (c) the distortion of the icosahedra in boron carbide depends to only a small degree on electron-phonon interaction and (d) the electronic transport in boron-rich solids is due to classical band-type conduction and hopping processes side by side. Hence, the hole bipolaron theory for boron-rich solids can hardly be maintained. 

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