Metal-metalloid compounds, hard

Also metal-metalloid compounds tend to retain their hardnesses as temper-atures become elevated. These compounds have been discussed in Chapter 10. 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

The structures of the prototype borides, carbides, and nitrides yield high values for the valence electron densities of these compounds. This accounts for their high elastic stiffnesses, and hardnesses. As a first approximation, they may be considered to be metals with extra valence electrons (from the metalloids) that increase their average valence electron densities. The evidence for this is that their bulk modili fall on the same correlation line (B versus VED) as the simple metals. This correlation line is given in Gilman (2003). 

Covalent Carbides Compounds composed of carbon and low-electronegativity nonmetals or metalloids are covalent carbides. The most important covalent carbide is silicon carbide (SiC), a very hard material. Over 500,000 tons of silicon carbide are produced annually, mostly for use as an abrasive material in the cutting and polishing of metals. In a process analogous to the formation of calcium carbide, silicon carbide forms by the reaction of silicon oxide with coke at high temperatures. 

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