Metal Carbides
The carbides constitute a particularly interesting family of compounds in that they were the first man-made refractories and, unlike oxides and silicates, are extremely uncommon in nature. Iron carbide, Fe3C, and later TiC and WC were identified in and extracted from steels in the mid 1800’s. By 1900, the French chemist Moisson had synthesized a number of refractory carbides in his new arc furnace and had studied their properties.
Characteristically, most of these carbides have high hardness, good electrical and thermal conductivity, and high stability.
Characteristically, most of these carbides have high hardness, good electrical and thermal conductivity, and high stability. These properties account for the principal applications: structures resistant to chemical reaction, uses in which wear resistance is of major importance, and high- temperature radiant-energy sources. The brittleness of carbides, however, has prevented their use as single-phase materials in highly stressed structural applications and has led to the development of metal-bonded composites.
The carbides vary widely in their chemical inertness, particularly with respect to their attack by oxidizing atmospheres. At sufficiently high temperatures all are attacked quite rapidly, although some are more resistant than metals of comparable melting point.
Until recently, adequate attention had not been given to the importance of structure in determining properties. Not only microstructure but also lattice-defect structure and dislocation substructure are important. Especially among interstitial compounds, details of structure and chemical composition exert an important influence on mechanical and physical properties. Consequently, few properties of the carbides are known with precision and their future development is, as yet, unbounded.
Carbides may be grouped according to the periodic classification of the metal constituents. The carbides of boron, silicon, and the transition metals (including the rare-earth and actinide series) are of greatest interest and utility.
The manufacture of carbides involves two groups of processes: those resulting in synthesis of the compound and those involving the purification and consolidation of the compound into an integral body. In some preparative techniques, such as vapor plating, these groups may be combined.
The synthesis techniques may be subdivided as shown in Table 1. A specific example of each is shown. The exact nature of the reaction, however, seldom is known in detail and is strongly dependent on temperature and other variables. Although the reactions usually are classified as noted above, it must be remarked that even in solid-solid or solid-liquid reactions the presence of a vapor phase is often critical for completion or rapid rate of reaction.
The processes of greatest present commercial importance are the solid-state carburization of the metal and the carburization of the oxide, but some of the other reactions are inherently capable of yielding products of higher purity and, hence, are of greater interest for research purposes and special applications.
The principal consolidation techniques have been sintering, hot pressing, and surface deposition. Most sintering of transition-metal carbides has been carried out in the presence of a liquid metallic phase. Relatively little has been done on the sintering of carbides without the use of additives which form a liquid phase.
The usual procedure for sintering unadulterated transition-metal carbides is to fire the pressed compact in the best available atmosphere, well gathered with raw carbide or reactive metal, at the highest feasible temperature. Final sintering at extremely high temperatures is accomplished by passing an electric current directly through the presintered bar.
The very high sintering temperatures required result in some purification by volatilization of impurities. Temporary liquid-phase formation with small metal additions has sometimes been used, the metallic phase disappearing at the end of the sintering operation by either solid-solution formation or volatilization.
Little has been done on solid-state sintering of carbides with "activating additions," as has been extensively employed in oxide and metal sintering. Hot pressing allows densification of unadulterated carbides at lower temperatures than does solid-state sintering. A variety of surface-deposition methods has been examined in recent years for providing wear-resistant, corrosion-resistant, or special purpose coatings of carbides on different substrates. Among these are the vapor-deposition techniques, flame spraying, and plasma jets.
It now appears, however, that electronic as well as size factors must be considered and the carbides more properly may be regarded as metallic lattices stabilized by electron transfer from the carbon atom. The viewpoint that an optimum number of bonding electrons exists for a given coordination number finds support in the extraordinary stability of these carbides and in several other observations: true stoichiometry is some- times energetically unfavorable [e.g., TaC and TiC-WC solid solutions, melting-point maxima are observed in some quasi-binary systems half of the tetrahedral sites formed by cubic close packing of the other type. Various analogous hexagonal and rhombohedral arrays of tetra-hedrally coordinated carbon and silicon are all known as a-SiC. Although SiC itself is practically stoichiometric, some A14C3, which in some respects is similar to SiC, is soluble in it.
Boron carbide, B4C, is similar to the rhombohedral modification of boron in having icosahedra of 12 boron atoms centered on the lattice points of a deformed face-centered cubic lattice and bonded together along the diagonals of the lattice. Centered on each octahedral interstice, making a rhombohedral NaCl lattice, are three carbon atoms in a chain. The end carbon atoms in every chain are bonded to three boron atoms, one in each of the three closest icosahedra.
The resulting structure has exceptional hardness and stability, although the central carbon atom can be removed easily by neutron irradiation and can be replaced by boron to yield B13C2 at high temperatures. Both titanium and silicon dissolve in the structure in small amounts. The lattice is capable of retaining more helium, resulting from neutron irradiation, than can pure boron.
Other carbides of interest include the strongly ionic MC2 structures (Group II, rare-earth, and actinide series) and Be2C.
The most stable transition-metal carbides are composed of close-packed layers of metal atoms stacked together in several arrangements. Carbon sites are either trigonal prismatic, octahedral, or tetrahedral holes formed between the metal layers: WC; W2C ; MoC; UC2; ,S-SiC; a-SiC; and Be2C.
In the more complex transition-metal carbides, carbon usually occupies sites similar to those in the structures described above. In Cr3C2, carbon occupies trigonal prismatic holes, as in WC, except that adjacent sites are occupied to form parallel zigzag chains of carbon through the structure. The nearest neighbors of carbon in the MeC "eta " carbides are identical to those in W2C, but the structure as a whole is considerably more complex, with 112 atoms in the unit cell. In (Cr, Fe, W)23Ce, the carbon atoms lie among 8 metal atoms instead of 6, but the M-C bond distances are the same.
Two of the most important carbides, SiC and B4C, have strongly directed, covalent bonds and are not interstitial compounds, although structural similarities are evident.