Refractory-Metal Interstitial Nitrides

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. 

Interstitial nitrides are crystalline compounds of a host metal and nitrogen, where the nitrogen atom occupies specific interstitial 

The interstitial nitrides have several important characteristics in common with the interstitial carbides. 

More so than the carbides, the interstitial nitrides are susceptible to the presence of even minute amounts of impurities such as hydrogen and particularly oxygen which tend to distort the structure. To avoid such harmful contamination, it is necessary to maintain a deposition system that is completely free of oxygen and hydrogen. 

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However, its reactions with heavy metals at elevated temperatures form hard refractory interstitial nitrides of nonstoichiometric compositions. 

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. 

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. 

The CVD of refractory interstitial nitrides other than TiN remains mostly on an experimental basis. The principal reaction is that of the metal chloride with nitrogen (or ammonia) in excess hydrogen at low pressure ( 1 kPa) (see Sec. 3.2 for a discussion on the metal halides). A typical reaction is ... 

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. 

Only the early transition metals (Groups IV, V, and VI) have a host lattice that is large enough for the nitrogen atom to fit readily and so form stable interstitial compounds, as shown in Table 9.1. As mentioned previously, only the nitrides of Group IV and V are considered refractory. 

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