Tungsten carbide, sintering

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. 

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)... 

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. 

Other potential applications are ceramic powders coated with their sintering aids, zirconia coated withyttria stabilizer, tungsten carbide coated with cobalt, or nickel, alumina abrasive powders coated with a relatively brittle second phase such as MgAl204 and plasma spray powders without the segregation of alloying elements. 

Natural single-crystal diamond and carbonado can now be replaced in many industrial uses by sintered diamond tool blanks. Such tool blanks are available in disks and cores. The disks, or sectors of disks, consist of a thin (0.5—1.5 mm) layer of sintered diamond up to about 50 mm diameter on a cemented tungsten carbide-base block about 3—6 mm thick. Using diamond abrasive, such blanks can be formed into cutting tools of various shapes. Typical tool blanks are shown in Figure 9. The wire dies have diamond cores up to 10 mm in diameter and 10 mm in length, which are encased in a cemented tungsten carbide sleeve up to 25 mm in diameter. 

The LDPE production with tubular reactors (see Section 5.1) requires some sophisticated control valves [45]. The let-down valve (Fig. 4.2-6 B) controls the polymerization reaction via the pressure and temperature by a high-speed hydraulic actuator (9) together with an electronic hydraulic transducer. The position of the valve relative to the stem is determined by a high-resolution electronic positioner (7). The cone-shaped end of the valve stem (2), as well as the shrunk valve seat (3) are made from wear-resistant materials (e.g., sintered tungsten carbide) in order to tolerate the high differential pressure of around 3000 bar during the expansion of the polymer at that location. 

Parameters of sintered tungsten carbide with variable cobalt content and strength characteristics calculated from Palmquist cracks (specification based on tests of Peters, 1979)... 

Mixtures of powders of two materials sinter very rapidly if one of them melts at the sintering temperature. Initially capillary action causes the liquid phase to rapidly wet the solid phase, causing an initial contraction. Then as the solid phase dissolves in the liquid it is rapidly transported to locations that decrease the pore volume. Carbide tool material is made from a mixture of cobalt and tungsten carbide powders sintered below the melting point of cobalt. The volume fraction liquid must be limited so capillarity can retain the shape during sintering. 

One further condition is necessary that has not been discussed. In the foregoing discussion it was tacitly assumed that the deformation and flow of the container were small compared to those of the sample. If the material in the pressure cavity is harder or stronger than the apparatus, the flow will occur in the apparatus and not in the sample. However, when we attempted to pressure-sinter diamond dust, it was the apparatus that flowed and not the diamond particles. Besides the high pressure, it is essential that the sample be softer than the material of the apparatus. This last limitation precludes very few materials from study, since the parts of the apparatus in contact with the sample are compacted tungsten carbides, such as G. E. Carboloy 999, which is extremely hard. 

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