Hardmetals

K. J. A. Brookes, World Director and Handbook ofHardmetals and Hard Materials, 5th ed.. International Carbide Data, East Barnet, Hertfordshire, UK, 1992. T. E. Chung, D. S. Coleman, A. G. Dowson, and B. WiUiams, eds.. Proceedings of the Pecent Advances in Hardmetal Production, Loughborough University of Technology, pubHshed by Metal Powder Report, Sept. 17—19, 1979. 

Fig. 12.12. Cracks in specimens of WC-6%Co hardmetal held under load at 850°C ...

K. J. A. Brookes, World Directory and Handbook of Hardmetals and Hard Materials, 5th ed., International Carbide Data, Pub. East Barnet Hertfordshire, UK, 1992. 

Brookes, K.J.A. World Directory and Handbook of Hardmetals and Hard Materials, 5th ed. 

Matsumoto and co-workers showed that the deposition of thick nano-cBN layers is possible and the adhesion on Si substrates is of acceptable quality [230]. The one major question is whether this process is also suitable for hardmetal (WC-Co) substrates or not, and whether there might be any problems with the Co binder phase and the BF3 in the gas phase ... 

PcBN compacts are often bonded to a hardmetal or hard steel alloy for manufacturing a tool. This composite can be produced by direct sintering of WC-Co-PcBN mixtures or by brazing. 

Onto the surface of the sintered part (e.g. hardmetal) the mixture of c-BN and/or diamond powder is bonded during high-pressure high-temperature sintering [267]. 

Co hardmetals, it is possible to improve both properties at the same time (Cherradi et al., 1994) (Fig. 22.13). This can be achieved by a gradation in the binder content from the centre to the surface of the tool. The improved mechanical properties of the FGM-cemented carbide are due to compressive stresses near the surface, which enable a reduction of the Co-phase content from 6 to 3 wt% without any loss in apparent fracture toughness. 

The preparation of TiC, VC, NbC, TaC, M02C, WC, and the chromium carbides are important technical processes for the production of carbide powders for hardmetals. Generally, the carbides are prepared by the reduction of oxides with carbon only M02C and WC are manufactured by reaction of the metal powders with graphite or carbon black. 

Solid solutions of transition metal carbides and carboni-trides, which also play an important role in the hardmetal industry, can be advantageously manufactured by the simultaneous reduction of oxide or oxide-carbide mixtures in the presence of carbon (equation 3). 

In the systems Co-C and Ni-C and in the other transition metal-carbon systems not mentioned so far, no stable carbide phases are observed. The carbon solnbilities in the metals are of importance for the fabrication and properties of hardmetals (see Section 9.1.1). The phase diagrams are of the entectic type. Metastable carbide phases have been reported in rapidly qnenched Co-C and Ni-C alloys. 

There are several families of ternary carbide phases with transition metals and main gronp elements. Several of them are of structnral interest snch as the H-phases, the filled /3-manganese type carbides and the /c-carbides. The rj-phases are formed in carbon-deficient hardmetals and canse embrittlement. 

Shortly after the invention of WC o hardmetals, TiC-based hardmetals were proposed as cntting tools bnt have not found extended use because of their comparative brittleness. Only in the last two decades, TiC-based hardmetals have found wider use when titanium nitride, 5-TiN, was... 

Figure 15 Microstructure of a coarse WC-Co hardmetal with gray WC particles embedded in a Co matrix, the size of the largest grains is about 20 xm...

Figure 16 Micro structure of a titanium carbonitride hardmetal (cermet) showing the core-and-rim type structure of hard particles...

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... 

Table 5 Characteristics of hardmetal and cermet grades and their applications...

Nitrides and carbonitrides in form of powders play some role in hardmetals (cemented carbides). The reader is referred to the article on Carbides of this Encyclopedia (see Carbides Transition Metal Solid-state Chemistry). There, also nitrided hardmetals to form functionally graded hardmetals (FGHM) are discussed. 

Figure 18 CVD multilayer coating on WC-Co hardmetal with TiNi j as top and bottom layer...

It was Krupp, a company experienced in steel technology and machining problems, who really recognized the potential of cemented carbides (hardmetals). 

Since 1930, cemented carbides (also called hardmetals) steadily attained a greater share in tungsten consumption. It is of interest to ask why the demand for cemented carbide grew so rapidly. Table 2.9 shows a chronological table indicating the most important events in cemented carbide research and development, a process which is still under way today. We recognize that what we call cemented carbides or hardmetals are in reality a very wide palette of materials with different properties. Cemented carbide properties can be adjusted by several variations and combinations of the components, as shown in Table 2.10. Hence cemented carbides could be applied widely. Figure 2.10 presents a breakdown of the fields of application of cemented carbides. 

The binary tungsten-carbon system (Fig. 4.1) [4.13-4.15] is of high technical importance. It contains three intermediate compounds W2C(P), WCi ((y), and WC(5), the latter being the main constituent in most of the commercial cemented carbides (hardmetals see Chapter 9). Besides, ternary and even more complex compoimds exist which are of interest in alloyed steels and cemented carbides. Tungsten hexacarbonyl is an important precursor for organic synthesis. 

The extremely high modulus of elasticity of WC (only exceeded by diamond and W2B5), and the high electrical and thermal conductivity are further important criteria for its use in hardmetals. The latter two properties also reflect the strong metallic component of the mixed covalent (W5d-C2p) metallic bonds in the carbide. Fermi surface properties of WC and electronic band structure calculations can be found elsewhere [4.23, 4.24]. 

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