Interstitial nitride, carbide

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. 

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. 

At elevated temperatures it combines with most nonmetals. With oxygen it gives V205 contaminated with lower oxides, and with nitrogen the interstitial nitride VN. Arsenides, silicides, carbides, and other such compounds, many of which are interstitial and nonstoichiometric, are also obtained by direct reaction of the elements. 

Although it is more stable than Ti3N4, strong heating converts it to ZrN. The interstitial nitrides, like the carbides TiC and ZrC, have the NaCl structure alternatively they may be regarded as cubic close-packed arrangements of Ti atoms with nitrogens in the octahedral holes. As the metals have h.c.p. lattices, these have not been simply expanded to admit the N atoms. 

A. .. A etc. contacts between the layers (see later). The reason for the great importance of the most closely packed structures is that in many halides, oxides, and sulphides the anions are appreciably larger than the metal atoms (ions) and are arranged in one of the types of closest packing. The smaller metal ions occupy the interstices between the c.p. anions. In another large group of compounds, the interstitial borides, carbides, and nitrides, the non-metal atoms occupy Interstices between c.p. metal atoms. 

In the so-called interstitial nitrides the metal atoms are approximately, or in some cases exactly, close-packed (as in ScN, YN, TiN, ZrN, VN, and the rare-earth nitrides with the NaCl structure), but the arrangement of metal atoms in these compounds is generally nor the same as in the pure metal (see Table 29.13, p. 1054), Since these interstitial nitrides have much in common with carbides, and to a smaller extent with borides, both as regards physical properties and structure, it is convenient to deal with all these compounds in Chapter 29. 

The most important difference from titanium is that lower oxidation states are of minor importance. There are few authenticated compounds of these elements except in their tetravalent states. Like titanium, they form interstitial borides, carbides, nitrides, etc., but of course these are not to be regarded as having the metals in definite oxidation states. Increased size also makes the oxides more basic and the aqueous chemistry somewhat more extensive, and permits the attainment of coordination numbers 7 and, commonly, 8 in a number of compounds. 

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. 

This chapter is a review of the general characteristics of the refixictory nitrides and their classification. Like the refiactory carbides (see Ch. 2), the refractory nitrides can be divided into two major types the interstitial nitrides reviewed in Chs. 10 and 11 and the covalent nitrides, reviewed in Chs. 12 and 13. 

This difference is large with the interstitial nitrides (Ti-N 1.5, V-N 1.4, Zr-N 1.6, Nb-N 1.4, Hf-N 1.7, Ta-N 1.5) but less pronounced with the covalentnitrides(B-N 1.0, Al-N 1.5, Si-N 1.2). Since nitrogen has a higher electronegativity than carbon, refractory nitrides show a greater electronegativity difference than the equivalent carbides. 

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. 

As mentioned in Ch. 9, the refractory nitrides consist of two structurally different types generally known as interstitial and covalent nitrides. This chapter provides a general review of the structural characteristics and composition of the interstitial nitrides and follows the outline of Ch. 3, Interstitial Carbides Structure and Composition. Some of these interstitial nitrides, titanium nitride in particular, are major industrial materials. 

Interstitial nitrides are crystalline compounds of a host metal and nitrogen where the nitrogen atom occupies specific interstitial sites in the metal structure which is generally close packed (see Ch. 3, Sec. 1.1 for a similar definition of the interstitial carbides). This places a lower limit on the size of the metal atom in order for the nitrogen atom to fit in the available sites of the metal structure. 

Interstitial nitrides are similar to interstitial carbides in structure and composition, and the two groups of materials closely resemble each other. The nitrides however are not as refractory. In fact, only the nitrides of Group IV and V have melting points above 1800°C. Those of Group VI, i.e., chromium, molybdenum, and tungsten nitrides, have lower melting (or decomposition) points and dissociate rapidly into N2 and the pure element at high temperature ( 1000"C). Their chemical stability is relatively poor and they do not therefore meet the refractory criteria. They are mentioned in this chapter for reference purposes. 

As mentioned in Ch. 9, Sec. 3.2, the nitrogen atom is smaller than the carbon atom and interstitial nitrides are formed more readily than the corresponding carbides (see Ch. 3). As shown in Table 10.1, the nine early... 

Like the bonding of the interstitial carbides, the bonding of the interstitial nitrides is still not completely understood. Their characteristics... 

The properties of interstitial nitrides have not been studied as extensively as those of the interstitial carbides and many gaps remain, particularly in determining the effects of composition and impurities, the thermodynamic functions, and the mechanical properties. 

The density and melting point of interstitial nitrides are shown in Table 11.1 and compared with the values for corresponding carbides and host metals. 

Table 11.4 Electrical Properties of Group IV and V Interstitial Nitrides and Carbides at 20°C.

As shown in the above table, the interstitial nitrides arc relatively good electrical conductors although with a resistivity slightly higher than that of the corresponding carbides and the parent metals, but still reflecting the essentially metallic character of these compounds. The electrical resistivity of TiN (and presumably of the other interstitial nitrides) increases almost linearly with temperature as shown in Fig. 11.3.1 " ... 

The observations on failure mechanism, ductile-brittle transition, and hardness of the interstitial carbides (Ch. 4, Secs. 4.3 and 4.4) are applicable to the interstitial nitrides. These materials have a ductile-brittle transition temperature of approximately 800°C. 

Little information is available on the mechanical properties of the interstitial nitrides and what has been published is summarized in Table 11.5 and compared with properties of the equivalent interstitial carbides. The values are averages reported in the recent literature.Ii][ W7][i4][i5]... 

As shown in Table 11.5, the hardness of the interstitial nitrides is somewhat lower than that of the corresponding carbides. The Group IV nitrides generally have higher hardnesses than those of Groups V. This reflects the greater contribution of M-N bonding found in these compounds. 

Hardness vs. Composition. Hardness varies with composition as shown in Fig. 1The hardness of the interstitial nitrides of Group IV (TiN, ZrN, and presumably HfN) reaches a maximum at stoichiometry while the maximum hardness of the nitrides of Group V (NbN, TaN, and presumably VN) occurs before stoichiometry is reached. A similar behavior is observed for the corresponding carbides (see Fig. 4.5 of Ch. 4). 

The interstitial nitrides are chemically stable and have a chemical resistance similar to that of the Group IV and V carbides. 

Like the covalent carbides, the covalent nitrides have a relatively simple crystal structure and an atomic bonding which is less complex than the interstitial nitrides. The bonding is mostly covalent by the sharing of electrons and is achieved by the hybridization of the respective electron orbitals. 

Thermal Conductivity. The thermal conductivity K) of the covalent nitrides, like that of the covalent carbides but unlike that of the interstitial nitrides and carbides, decreases with increasing temperature as shown in Fig. 13.2.1 1 The thermal conductivity of the single crystals of c-BN and AIN are extremely high (1300 and 320 W/m-K respectively) and comparable to that of the best conductors such as Type II diamond (2000 W/m K), silver (420 W/m-K), copper (385 W/m-K), and beryllium oxide (260 W/mK).l l... 

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