Metal carbides sintering

The industrial practice for the production of tantalum consists of two steps. In the first, the carbide is made by charging a graphite crucible with an intimate, pelletized mixture of lamp black and tantalum pentoxide and heating it in a high-frequency furnace under a dynamic vacuum (10 torr). In the next step, the ground carbide and the requisite amount of tantalum pentoxide are mixed, palletized, and fed to a reduction furnace where the reduction to the metal occurs. The formation of tantalum carbide as well as the reduction to the metal occur at about 2000 °C. The product leaving the reduction furnace is in the form of pellets or roundels (small cylinders) of porous metal, usually sintered together. 

Cemented carbides are sintered materials consisting of very hard and high melting point metal carbides in a matrix of low melting point bonding metals of the iron group, particularly cobalt. 

Pellet F, Perdrix A, Vincent M and Mallion JM (1984) Biological determination of urinary cobalt. Significance in occupational medicine in the monitoring of exposures to sintered metallic carbides. Arch Mai Prof Med Trav Secur Soc 45 81-85. Perdrix A, Pellet F, Vincent M, De Gaudemaris R and Mallion JM (1983) Cobalt and sintered metal carbides. Value of the determination of cobalt as a tracer for exposure to hard metals. Toxicol Fur Res 5 233-240. 

Which physical property makes transition metal carbides and nitrides a good component for cutting tools In this respect why is cobalt used as a matrix material for their sintering ... 

Alumina is the most important oxidic abrasion-resistant material. Metal carbides are in some ways superior to oxides with respect to hardness and melting point, but they are much more brittle than the oxides and are only used in isolated instances as wearing bodies. Silicon carbide is characterized by its low thermal expansion and high thermal conductivity and has proved to be more resistant to thermal shock than oxides. Zirconia is tougher than alumina its modulus of elasticity is only about half as large, and it is comparable with that of steel. Zirconia is therefore very suitable for compound structures with steel. At present, the applications of ceramic sintered materials in chemical plant construction are slide rings, pump parts, and slide bearings. 

The most widely used transition metal carbide is tungsten carbide, hexagonal WC, which is employed as the hard constituent in WC-Co hardmetals. Such hardmetals are sintered composite materials with 80-90% of hard particles such as WC embedded in a ductile binder phase such as Co. For these apphcations WC combines a number of... 

In a similar multiphase system, transition metal carbides were used as additives for pressureless sintering of TiB2, yielding composites of binary and ternary borides [218,296]. Attrition milled powder mixtures of TiB2 with 3-10 mass-% Co or Ni and 20-35 mass-% WC have been sintered in a vacuum at temperatures between... 

A nozzle opening of a sandblaster may suffer severe wear during operation. This can be countered by using hot-pressed boron carbide (B4C), which is expensive but is very hard and has a long lifetime in this application. Sintered alumina or hardmetal (a cermet of tungsten carbide sintered with some cobalt or nickel metal) show more wear but are suitable as well and cheaper to make. 

Al alloys are frequently used to prepare composites with p- and y-BN [9 to 12]. This can be done by sintering the reaction mixture originating from preformed p-BN particles or from a-BN powder. The aluminium is often combined with transition metal carbides or nitrides (especially TiC and TiN) in order to form the binding phase [13 to 39]. However, TiC, TiN, and TiBg also form composites with the hard BN phases without the addition of aluminium [40 to 46] addition of TaN has also been reported [47]. 

The mechanical properties and the failure mechanisms of transition-metal carbides are reviewed in detail by Toth.t l Generally, large spreads in the reported values found in the literature are common. This is particularly true in older reports which were mostly performed on sintered materials. More recently, testing has been switched to single crystals or polycrystalline materials obtained from the melt or by thin-film deposition. These are believed to yield more accurate and consistent information. Yet, any test must be carefully characterized in order to be meaningful. The following factors influence mechanical testing.l H ... 

Transition metal carbides are often processed by sintering with a metal binder such as cobalt and nickel. The mechanical properties of such composites are often quite different from those of single crystal or polycrystalline materials. ) This often adds to the confusion when quoting property values (see Ch. 17). 

The most important bulk material is tungsten carbide sintered with a metallic binder which is usually cobalt. It is known as cemented carbide or hard metal (see Ch. 6, Sec. 8.0). Many combinations of carbides and binders are possible and it is estimated that 20,000 tons of these materials are produced annually throughout the world. An unusual and beneficial feature of WC is that it maintains its high hardness value at high temperature (see Ch. 6, Sec. 8.0)... 

In principle all materials available in a sinterable powder can be mixed with an appropriate binder and processed on injection moulding machines. Therefore in addition to the traditional oxide ceramics it is also possible for example to use metals, carbides and nitrides. Some typical materials are shown in Table 7.11. 

Sintering is the basic technique for the processing of ceramics, but other materials can also use it metals, carbides bound by a metalhc phase and other cermets, as well as natural materials, primarily snow and ice. 

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