WC-Co carbide metal

Grinding Diamond 20 Film Water 20 120 3 until flat 

Lapping Diamond 9 Grooved composite disk Suspension 20 150 8 

Polishing Diamond 3 Perforated synthetic fiber cloth Suspension 20 150 8 

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Figure 131. WC-Co carbide metal, BF. Reactive sputtering with Fe/02- Monochromatic light X = 520 pm. The WC phase has a bright appearance, while the Co is dark gray and the Ti(Ta)C is dark.

Figure 133. WC-Co carbide metal, POL. The Ti(Ta)C and Co are dark. Grains of the WC phase with different orientations have become visible in the polarized Ught.

Figure 134. WC-Co carbide metal, BF. Etched by chemical method (see Table 60). Grain boundaries have been rendered visible...

MWCNTs have been tested to reinforce various matrices because they have many unique mechanical and physical properties.14,15 However, these nanotubes become corroded with metals (such as iron, cobalt, and aluminum) at temperatures above 850°C. These shortcomings limit the applications of MWCNTs as nano-reinforcements. The SiC coating can effectively protect the diamond from molten cobalt, thus allowing dense SiC-coated diamond-dispersed cemented carbide composites to be successfully fabricated at lower pressures. If MWCNTs can be coated with the same SiC layer, more stable MWCNTs would be produced and expected to be used as nano-reinforcements for various matrices. The development of SiC-coated MWCNTsAVC-Co composites has potential to extend functions of both MWCNTs and WC-Co. 

Wear-resistant layers of transition metal carbides that have been deposited by CVD (see Chemical Vapor Deposition) and PVD processes on the surface of WC-Co hardmetals further reduce the wear in cutting applications. About 80% of... 

The most important property of tungsten carbide in its utilization in cemented carbides is its ability to dissolve partially in compressed powder mixtures of WC and ferrous metals, particularly cobalt, at 1300 to 1500°C. In the case of sintering with a liquid phase, WC partly crystallizes out of the binder phase of the WC-Co-alloy upon cooling. It becomes embedded in the tough but hard (not brittle) binder phase. 

Carbothermal reduction of tungsten oxides with carbon monoxide [3.47], or gas mixtures of CO/CO2, CO/H2, CH4/H2 [3.50], C2H4/H2, and C2H4/H2 [3.51], as well as by reaction between metal oxide vapor and solid carbon [3.52] have recently attracted attention for producing high surface area tungsten carbides (up to lOOm /g), for use as catalyst (see Section 10.4), and for nanophase WC/Co composite powders (see also Section 9.2.1.4) [3.53]. 

In several cases, materials for combined erosive and corrosive conditions have been evaluated on the basis of separate erosion and corrosion studies and data, with the consequence that the synergistic effects are left out of the evaluation. Since one or the other of these effects may be large, the conclusions may be quite wrong. For materials fliat usually are passive due to a dense oxide film, such as stainless steels, Wc is by definition very low. But since sand erosion more or less destroys the passive film, the corrosion rate increases strongly and may reach very high values, i.e. the contribution of Wce may be particularly high for these materials. The other synergy effect, Wec, is most pronounced for ceramic-metallic materials in which the metallic phase has inferior corrosion resistance, e.g. for a cemented carbide with a metallic phase of cobalt (WC-Co). 

The deterioration mechanism for WC-Co (and similar materials) is assumed to be as follows the binder phase of metal around the carbide particles corrodes away so fliat the WC particles are more easily removed by erosion. The transition from the unexposed state to a corroded and eroded state is schematically illustrated in Figure 7.43. It should be noticed that under extremely erosive conditions or if die metal phase is highly corrosion resistant in the actual environment, erosion is dominating and the effect of die shown mechanism may be insignificant. 

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... 

Schreiner, M., Schmitt, Th., Lassner, E. and Lux, B., On the origins of discontinuous grain growth during liquid phase sintering of WC-Co cemented carbides, Powder MetalL Inter., 16, 180-83, 1984. 

A semicircular crack profile appears up to a load of 100 N. As the load increases beyond 200 N, the crack depth is approximately 20% less than the surface crack length. Large lateral cracks are clearly masking crack growth at greater depths. Other crack paths develop in relatively tough materials (for example, carbide metals such as WC-Co) at relatively low loads. This crack system, known as Palmqvist cracks , is shown in Fig. 150. The surface exhibits radial cracks that do not extend deep into the material. 

Ceramics as Valves Alumina or silicon carbide is fhe material for valves. Gate valves used in an irrigation ditch are made of metal or concrete. Slide-gate valves in ladles holding molten steel are made of ceramics. Alumina is the material used to make oxygen valves of respirators used in hospitals. Ball-and seat valves, made of WC-Co, transformation-toughened zirconia, or silicon nitride, are used in downhole pumps required for deep oil wells. 

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