Titanium compounds carbide

Annual world production of titanium carbide is 1200—1500 metric tons. On an iadustrial scale, it is produced most often through the reaction of Ti02 with carbon black (see Titaniumand titanium alloys Titanium compounds). 

Lung fibrosis caused by titanium and some of its compounds (carbide, hydride) is included in the list of the German Occupational Disease Regulations . Since Ti02 tends to accumulate in the tissues, persons heavily exposed to this material at their workplace should undergo periodical medical examinations, including chest X-radiog-raphy and ventilatory function tests (Sibum 2002). 

Transition element carbides and nitrides are applied as cutting tools because of their extreme hardness and wear resistance. In some cases nitrides and carbides (e.g., of titanium) form solid solutions over the entire compositional range other transition metal nitrides and carbides exhibit fairly different structures and are not completely soluble. Carbon contents within the range of few percentage points usually do not influence the mechanical properties of transition metal nitrides, and vice versa. Hence, completely carbon-free nitrides or nitrogen-free carbides are not required, especially for the titanium compounds. 

Both covalent carbides have high melting points which are slightly lower than the titanium compounds but higher than silicon and boron. Under most conditions, the thermal decomposition of SiC may occur well below its intrinsic melting poind l and decomposition can become significant at approximately 1700°C (see Sec. 3.7 and Fig. 7.8 of Ch. 7). The density of SiC is closer to that of diamond than it is to graphite, which can be expected since SiC has the structure of diamond. 

It is possible to make electrolithically deposited coatings in special cells and in the cells under operation [123, 124]. Thermodynamically, it is possible to receive the deposited coatings, combining the addition of titanium compounds in electrolyte, and boron oxides to the carbon anode material. Metal oxides dissolve in electrolyte the ions of metals discharge at the cathode and deposit on the cathode as titanium boride and titanium carbide. The problem involves the poor controllability of the process and the need to fulfill the required purity of aluminium (in titanium and boron content). Once small amounts of boron oxide and titanium (in the form of oxide of salt) are added, it is possible to obtain the metal of required purity and quality, but the coating process lasts for a long time and is poorly ccmtroUed. 

In the p bands of TiO the admixture of partial t/-like DOS is small, so very little pd bonding occurs. The main covalent bonding contribution is brought about by the dd bonds in the occupied range of the d bands. In the titanium compounds considered, the covalency decreases from the carbide to the oxide. The ionicity—actually the localization of the charge on the nonmetal atom—increases in that direction, whereby the charge transfer remains almost constant, as already discussed in Sec. II.E.2. 

When steel samples are reacted with U-Bi fuels containing zirconium and magnesium, the x-ray patterns of the surface are those for pure or very nearly pure nitrides or carbides. When graphite is contacted with the fuel, however, solid solutions of the carbide and nitride are often found. The unit cells vary from 4.567 Kx to 4.685 Kx for the zirconium compounds and from 4.237 Kx to 4.320 Kx for the titanium compounds. These parameters are low for complete carbon carbide structures. 

Processing. Tungsten carbide is made by heating a mixture of lampblack with tungsten powder in such proportions that a compound with a combined carbon of 6.25 wt % is obtained. The ratio of free-to-combined carbon is of extreme importance. Tantalum and titanium carbides are made by heating a mixture of carbon with the metal oxide. Multicarbide powders, such as M02C—WC, TaC—NbC, and TiC—TaC—WC, are made by a variety of methods, the most important of which is carburization of powder mixtures. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Titanium carbide may also be made by the reaction at high temperature of titanium with carbon titanium tetrachloride with organic compounds such as methane, chloroform, or poly(vinyl chloride) titanium disulfide [12039-13-3] with carbon organotitanates with carbon precursor polymers (31) and titanium tetrachloride with hydrogen and carbon monoxide. Much of this work is directed toward the production of ultrafine (<1 jim) powders. The reaction of titanium tetrachloride with a hydrocarbon-hydrogen mixture at ca 1000°C is used for the chemical vapor deposition (CVD) of thin carbide films used in wear-resistant coatings. 

Next to Cr C2, TiC is the principal component for heat and oxidation-resistant cemented carbides. TiC-based boats, containing aluminum nitride, AIN, boron nitride, BN, and titanium boride, TiB2, have been found satisfactory for the evaporation of metals (see Boron compounds, refractory boron compounds Nitrides). 

In the case of TiC, preferential evaporation of titanium leads to a change in the stoichiometry of the compound towards the carbon-rich end, the excess carbon being left diffuses into the carbide phase, and so the flux ratio of the two elements changes widi time until congruent vaporization is achieved. 

Other Binary Compounds.—Scandium nitride and zirconium and titanium carbide do not conform with the theoretical radii. It is possible that these crystals do not consist essentially of Sc+3, N 3, Ti+4, Zr+4 and C-4 ions, especially since zirconium and titanium nitride, ZrN and TiN, also form crystals with the sodium chloride structure but possibly also the discrepancy can be attributed to deformation of the anions, which have very high mole refraction values. 

The deposition of a binary compound can be achieved by a coreduction reaction. In this manner, ceramic materials such as oxides, carbides, nitrides, borides, and silicides can be produced readily and usually more readily than the parent metal. A common example is the deposition of titanium diboride ... 

Other compounds have been deposited by fluidized-bed CVD including zirconium carbide (from ZrCl4 and a hydrocarbon), hafnium carbide (from HfC and methane or propylene), and titanium carbide (from TiCl3 and propylene). 

Zirconium carbide is a highly refractory compound with excellent properties but, unlike titanium carbide, it has found only limited industrial importance except as coating for atomic-fuel particles (thoria and urania) for nuclear-fission power plants.l " ] This lack of applications may be due to its high price and difficulty in obtaining it free of impurities. 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

---