Physical boron carbides

Small single crystals, such as those of potassium titanatc, are being used at an annual rate of over 10,000 tons for the reinforcement of nylon and other thermoplastics. These composites are replacing die-cast metals in many applications. Another microfiber, sodium hydroxycarbonate (Dawsonite), also improves the physical properties and flame resistance of many polymers. Many other single crystals, called whiskers, such as alumina, chromia, and boron carbide, have been used for making high-performance composites. 

The carbides and nitrides are well known for their hardness and strength, and this section will briefly compare a number of these properties with those of the pure metals. Concentration will be placed here on the first row compounds, since these constitute a complete series, and Mo and W, since these are the most commonly studied metals. As will be shown, the physical and mechanical properties of carbides and nitrides resemble those of ceramics not those of metals. Comparisons will be made with boron carbide (B4C), silicon carbide (SiC), aluminium nitride (AIN), silicon nitride (Si3N4), aluminium oxide (A1203), and diamond, as representative ceramic materials. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

The influence of the laser and plasma parameters (such as wavelength, laser power density, pulse length, plasma temperature, electron and ion density and others) on the physical and chemical processes in a laser induced plasma with respect to the formation of polyatomic and cluster ions has been investigated for different materials (e.g. graphite, boron nitride, boron nitride/graphite mixture, boron carbide, tungsten oxide/graphite mixture and superconductors ). 

Telle R (1990) Structure and properties of Si-doped boron carbide. In Freer R (ed) The Physics and Chemistry of Carbides, Nitrides and Borides. Kluwer Academic Publishers, Dordrecht, p 249... 

Abzianidze, T., G. Karumidze, L. Kekelidze, E. Tabatadze, L. Shengelia, G. Bokuchava, and B. Shirokov. 2004. Possibilities of rising in the radiation stability and thermoelectric efficiency of boron carbide. In Proceedings of the 16th International Conference on Physics Radiation Phenomena and Radiative Materials Science, Kharkov, Ukraine, p. 88. 

Silicon carbide is noted for its extreme hardness [182-184], its high abrasive power, high modulus of elasticity (450 GPa), high temperature resistance up to above 1500°C, as well as high resistance to abrasion. The industrial importance of silicon carbide is mainly due to its extreme hardness of 9.5-9.75 on the Mohs scale. Only diamond, cubic boron nitride, and boron carbide are harder. The Knoop microhardness number HK-0.1, that is the hardness measured with a load of 0.1 kp (w0.98N), is 2600 (2000 for aAl203, 3000 for B4C, 4700 for cubic BN, and 7000-8000 for diamond). Silicon carbide is very brittle, and can therefore be crushed comparatively easily in spite of its great hardness. Table 8 summarizes some typical physical properties of the SiC ceramics. 

The physical and chemical properties of boron carbide have been reviewed by Lipp [159], Thevenot and Bouchacourt [256], Thevenot [164,165], and Schwetz (1999) [223]. Special problems while presenting the physical properties arise from the large homogeneity range of boron carbide. Furthermore, its poor sinterability requests additives that are usually unspecified and results in residual porosity and various grain sizes which are often also not considered in the publications. Most variation and discrepancies in the properties reported come from the undefined composition of the materials studied. 

As introduced above, the reaction of boron carbide with metal carbides can be used to fabricate metal borides or metal boride/boron carbide composites in a controlled way during densification if boron carbide or free boron is used in excess, or if carbon is bonded by another additive. Although the incompatibility of B4C and metal carbides is well known, many attempts have been undertaken to produce composites or coatings thereof but failed as soon as equilibrium conditions were approached. Physical or chemical vapor deposition of B4C on hard metal substrates, or WC coatings on boron carbides are typical problems (e.g., [252]). In both cases, interlayers of graphite form and hence result in an unsatisfactory adhesion of the deposited coating to the substrate. 

The sintered boron carbide-aluminum control material has good physical properties, is a good heat transfer media and can be fabricated to close tolerances. The flat rod tips require these properties. The powdered boron carbide used in the round tips is not required to have the attributes listed above. A sufficient quantity of boron must be contained in the rod tip to compensate for that burned up during the tiime the rod is performing its control function. Since boron gives off helium after capturing a neutron, the poison volume of the rod tip must be vented to prevent a pressure buildup. 

Table 4.9 Physical properties of boron carbide ceramics used in armor applications. After Karandikar et al. [576]. 

Alexander, M. N., Nuclear Magnetic Resonance Studies of the Structure of Boron Carbides, in Boron Rich Solids, Am. Inst, of Physics Conf. Proc. 140, New York (1986)... 

Because the solubility coefficients of carbon in the solid and the liquid phase are almost the same, zone melting, which is used to prepare high-purity crystals of many other elements, is not suitable in the case of boron. Technical boron, which is often taken as the ingredient for the preparation of boron compounds, contains up to about 0.5% carbon. However, in several preparative methods for boron compounds the carbon content may be reduced by secondary chemical or physical reactions. The purest P-rhombohedral boron crystals that have become available up to now were produced by Wacker-Chemie, Munich, FRG. Despite the claimed purity of 99.9999% with respect to other elements, even this high-purity boron contains carbon in concentrations of typically 30 to 80 ppm. Therefore, apart from boron carbide containing carbon as a determining bonding partner, in the assessment of the properties of boron and boron compounds attention must be paid to the fact that a certain, usually unknown carbon content could have influenced the properties determined. 

Quantitative investigations of the effect of the carbon content on the structure and physical properties of boron-rich solids have been restricted to p-rhombohedral boron and boron carbide (56-61). Up to carbon contents of about 1 at.% in p-rhombohedral boron, the carbon atoms substitute statistically for boron atoms at the polar sites of the Bjj icosahedra in the structure with a maximum of one carbon atom per icosahedron. Compared with the boron atom, the... 

For the influence of the carbon content on the physical properties of P-rhombohedral boron and boron carbide, see later. 

---