Hardness boron carbides

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

Uses. Apphcations for boron carbide relate either to its hardness or its high neutron absorptivity ( B isotope). Hot-pressed boron carbide finds use as wear parts, sandblast no22les, seals, and ceramic armor plates but in spite of its hardness, it finds Httie use as an abrasive. However, this property makes it particulady usehil for dressing grinding wheels. 

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. 

Shielding, armour Hardness, toughness Alumina, boron carbide... 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

On ihe Mohs scale diamond is 10 and quartz 7. An abemative measure is the Knoop hardness (kgmm ) as measured with a 100-g load typical values on this scale are diamond 7000, boron carbide 2750, corundum 2100. topaz 1340, quartz 820. hardened tool steel 740. 

Boron 800-1 050 (Halide) 1. Gaseous 2. Semi- gaseous 3. Pack 4. Salt-bath electrolysis Up to 500 fttn Matrix plus borides plus boron carbide Brittle Hardness up to 2 000HV Heat treatment acceptable ... 

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

Boron carbide (B4C) is extremely hard and is used where maximum resi stance to erosion is required. It has good nuclear properties (see Ch. 9). 

The hardness of boron carbide (carbon hexaboride) is not well defined because it is made as sintered compacts which have variable densities, compositions, and defect densities. It is very hard (up to 4400kg/mm2), and of relatively low density, so it has been used extensively as body-armor (McColm,... 

Boron carbide (B C) is a hard, black crystal that is used as an abrasive powder and as an additive to strengthen composite parts in aircraft. 

Boron Carbide, B4C coml prod called, tNor-bide, mp ca 2375°, d 2.52 is prepd by heating anhyd boric oxide B30, with carbon in graphite resistance furnace at ca 2500°. Its special interest is due to its remade able hardness jwhich lies on the Moh s scale betw thatjof silicon carbide and diamond. Used as an abrasive. Detailed description of this compd is given in Kirk Othmer 2(1948), 830-4(21 refs)... 

Boron can be isolated as a hard, brownisli-black powder. Its carbon compound, boron carbide (B,C). is almost as hard as diamond. 

It is used in antiseptics and medicines, as a flux in smelting, soldering and welding operations, as a deoxidizer in nonferrous metals, as a neutron absorber for atomic energy shields, in rocket fuels, and as extremely hard abrasive boron carbide (harder than corundum). See also Boron. 

When boron is heated to high temperatures with carbon, it forms boron carbide, B12C3, a solid with a high melting point that is almost as hard as diamond. The solid consists of B12 groups that are pinned together by C atoms. When boron is heated to white heat in ammonia, boron nitride, BN, is formed as a fluffy, slippery powder ... 

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. 

B4C boron carbide has a melting point of 2450 °C and a hardness somewhere between those of SiC and diamond. This makes the material a suitable abrasive. It is used in heads of sand blasting equipment, in mortars and in armour plating. For the latter application a B4C plate is provided on both sides with a plastic which has been reinforced with glass fibre. This is done to reduce the risk of splintering. Boron carbide is also used as the raw material for many other boron compounds ... 

From Eq. (11), an obviously desirable characteristic for thermoelectric materials is to have low thermal conductivity k. The thermal diffusivity constant, Dt, of ErB44Si2 has been found to have small values of Dt < 1.1 x 10 2 cm 2/s (Mori, 2006c). These values are significantly smaller than what has been observed for boron carbide samples (Wood et ah, 1985). Although no data exists for the sound velocities of ErB44Si2, the velocities are probably high since borides are typically hard materials. Therefore, the small values of Dt indicate extremely short phonon... 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

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