Refractory metal carbides

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

Structure. The structure of the refractory-metal carbides increases in complexity with increasing group number. Thus the carbides of Group IVa are characterized by a single cubic monocarbide. In those of Group Va, a M2C phase exists as well as the monocarbide. The carbides of Group Via are far more complex and have several compositions. 

Carbides may also be prepared, either by direct carburizing, as in the case of steel, in which a surface carbide film dissolves into the substrate steel, or by refractory metal carbide formation as in the cases when one of the refractory metal halides is mixed with methane in the plasma gas. 

Figure 10.26. Correlation of the diamond nucleation density after 60 min of BEN with the heat of formation of refractory metal carbides [241],...

Refractory metal carbides (TaC, WC, M02C) and some covalent carbides (SiC, B4C) have a positive effect on diamond nucleation, while effects of ionic carbides (AI4C3, liquid salts, etc.) are still less known.l ... 

Storms, E. K., The Refractory Metal Carbides, Academic Press, New York (1967)... 

Lowther, J. E., Molecidar Orbital Studies of Refractory Metal Carbides, in Institute of Physics Conf, Series No. 75, Ch. 1, Adam Hilger Ltd., London (1986)... 

The CVD of other refractory metal carbides is essentially similar to that of TiC. The metal halide is reacted with a hydrocarbon, usually methane, although propane, propene and toluene have been used also. Pressure varies from 1 kPa to 1 atm. (composition closest to stoichiometry are usually obtained at the lower pressures). 

Pierson, H. O., The CVD of Refractory Metal Carbides, High-Temp Materials and Processes, 11(1-4) (1993)... 

Y Ando, R Ueda. Preparation of ultrafine particles of refractory metal carbides by a gas-evaporation method. J Cryst Growth 52 178, 1981. 

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Stable oxides, such as those of clrromium, vanadium and titanium cannot be reduced to the metal by carbon and tire production of these metals, which have melting points above 2000 K, would lead to a refractoty solid containing carbon. The co-reduction of the oxides widr iron oxide leads to the formation of lower melting products, the feno-alloys, and tlris process is successfully used in industrial production. Since these metals form such stable oxides and carbides, tire process based on carbon reduction in a blast furnace would appear to be unsatisfactory, unless a product samrated with carbon is acceptable. This could not be decarburized by oxygen blowing without significairt re-oxidation of the refractory metal. 

Above 1000°C Refractory metals Mo, W, Ta Alloys of Nb, Mo, W, Ta Ceramics Oxides AI2O3, MgO etc. Nitrides, Carbides SiaN., SiC Special furnaces Experimental turbines... 

Handbook of Chemical Vapor Deposition 1.1 Refractory-Metal (Interstitial) Carbides... 

The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

The carbothermic reduction processes outlined so far apply to relatively unstable oxides of those metals which do not react with the carbon used as the reductant to form stable carbides. There are several metal oxides which are intermediate in stability. These oxides are less stable than carbon monoxide at temperatures above 1000 °C, but the metals form stable carbides. Examples are metals such as vanadium, chromium, niobium, and tantalum. Carbothermic reduction becomes complicated in such cases and was not preferred as a method of metal production earlier. However, the scenario changed when vacuum began to be used along with high temperatures for metal reduction. Carbothermic reduction under pyrovacuum conditions (high temperature and vacuum) emerged as a very useful commercial process for the production of the refractory metals, as for example, niobium and tantalum, and to a very limited extent, of vanadium. 

Titanium Carbide. Carbides of transition metals are known for their hardness, wear resistance and also for their high electrical conductivity, which makes them attractive as a refractory coating material for cutting tools or bearings. Only little work has been done on the electrochemical stability of transition metal carbides with the exception of TiC, where a corrosion and passivation mechanism was suggested by Hintermann et al. [119,120]. This mechanism was confirmed on amorphous TiC produced by metal-... 

Figure Plasma reactor for growing single crystals of refractory metals, alloys and high melting point carbides.

Uses. Amorphous boron is used as an addictive in pyrotechnic mixtures, solid rockets propellants, explosives, etc. Refractory metal borides are used as addic-tives to cemented carbides. High purity boron is used in electronics as a dopant to... 

In a pilot plant [2,13], superalloy scrap containing Mo, W, Cr, Fe, Co, and Ni is pretreated in a furnace with carbon to transfer refractory metals (Mo, W, etc.) into carbides. The melt is granulated and the resulting material is charged into titanium baskets. Diaphragm-type electrolytic cells are used for anodic dissolution of the granulated material. Fe, Co, Ni, and small amounts of Cr are dissolved into a calcium chloride solution by the current. The metal carbides are not dissolved and remain as an anodic residue in the baskets. 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

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