Refractory-Metal Interstitial Carbides

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

Structure. The structure of the refractory-metal carbides increases in complexity with increasing group number. Thus the carbides of Group IVa are characterized by a single cubic monocarbide. In those of Group Va, a M2C phase exists as well as the monocarbide. The carbides of Group Via are far more complex and have several compositions. 

The interstitial carbides have the following general characteristics  

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Handbook of Chemical Vapor Deposition 1.1 Refractory-Metal (Interstitial) Carbides... 

The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

In this section and the next three, the properties and characteristics of the interstitial carbides of Group IV are reviewed and compared with those of the host metals, the corresponding interstitial nitrides, as well as those of another refractory group the borides of the Group IV metals. The values given are those for composition as close to stoichiometry as possible. 

As mentioned in Ch. 2, the refractory carbides include two structurally different types (a) the interstitial carbides of the transition metals of Group IV, V, and VI (reviewed in Ch. 3, 4, 5, and 6), and (b) two covalent carbides boron carbide and silicon carbide. The structural characteristics of these two carbides are reviewed in this chapter and their properties and general characteristics in the following chapter. 

The stmctures of the so-called interstitial carbides (formed by heating C with fi -block metals having > 130 pm, e.g. Ti, Zr, V, Mo, W) may be described in terms of a close-packed metal lattice with C atoms occupying octahedral holes (see Fig. 6.5). In carbides of type M2C (e.g. V2C, Nb2C) the metal atoms are in an hep lattice and half of the octahedral sites are occupied. In the MC type (e.g. TiC and WC), the metal atoms adopt a cep stmcture and all the octahedral holes are occupied. These interstitial carbides are important refractory materials they are very hard and infusible, have melting points >2800 K and, in contrast to the acetylide... 

The interstitial or refractory carbides are the most important of the three classes these are formed by the transition metals, the most stable being the carbides of the metals in Groups IVa, Va, and Via. Such carbides may be made from the elements at high temperatures under pressure and resemble the metals themselves. They tend, however, to be much harder and higher melting than the parent metals. The melting points, (admittedly approximate) of tantalum carbide, TaC, (4200° C) and zirconium carbide, ZrC, (3800° C) may be compared with those of tungsten... 

Metallic nitrides, sometimes termed interstitial compounds, are formed from combinations of N with transition metals of groups IVA, VA, and VIA. As the name implies, they exhibit electrical conductivity and most of the general characteristics associated with standard metals. They are also refractory and hard, and usually depart from the ideal stoichiometry ratios displayed above (see 17.3.9, Table 1). They readily form solid solutions with carbides and oxides, which gives rise to problems when it is necessary to obtain nitrides in pure form. Included in this category are numerous ternary nitrides of a transition metal with a group B metal. 

Although rather unreactive at ordinary temperatures, titanium combines directly with most non-metals, for example, hydrogen, the halogens, oxygen, nitrogen, carbon, boron, silicon, and sulfur, at elevated temperatures. The resulting nitride, TiN, carbide, TiC and borides, TiB and TiB2, are interstitial compounds which are very stable, hard and refractory. 

The third factor governing the structure of nitrides is the nature of the bond between the nitrogen atom and the other element forming the compound. As mentioned in Ch. 2, the bond is the force of attraction that holds together the atoms of a molecule.l The bonds in refractory carbides can be ionic (saltlike nitrides), covalent (covalent nitrides), or a combination of metallic, covalent, and ionic (interstitial nitrides) (for a discussion of electronic bonding, see Ch. 2, Sec. 5.0). 

Unlike the interstitial nitrides, the covalent nitrides are not metallic compounds. The differences in electronegativity and atomic size between the nitrogen and the other element are small and their electronic bonding is essentially covalent. In this respect, they are similar to the covalent carbides. They include the nitrides of Group mb (B, Al, Ga, In, Tl) and those of silicon and phosphorus. Of these, only three are considered refractory boron nitride, silicon nitride, and aluminum nitride. These are reviewed in Chs. 12 and 13. 

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