The Refractory Carbides

The refractory carbides are hard and wear resistant, have high melting points, and are chemically inert. In a relatively short time, they have become major industrial materials with numerous applications such as cutting and grinding tools, bearings, textile-machinery components, oxidation-resistant gas burners, and many others. 

This chapter is a general review of the structural characteristics of the refractory carbides, their classification, and general features. These materials can be divided into two major types the interstitial carbides reviewed in Chs. 3 to 6, and the covalent carbides, reviewed in Chs. 7 and 8. 

---

Compounds such as the refractory carbides, nitrides, and oxides have extremely high boiling points and generally dissoci-... 

Thermal spray is a well-established, relatively low-cost, industrial processes which is used widely for the deposition of metals and compounds, including the refractory carbides and nitrides. Examples are coatings of tungsten carbide with a cobalt binder which are of major industrial importance. PI... 

E. K. Storms, ed., The Refractory Carbides, Vol. 2, Refractory Materials, Academic Press, Inc., New York, 1970. 

Figure 3.11 Comparison of the chemical diffusion coefficients of UCj 0 and the refractory carbides at C/metal ratio of 0.98.

E.K. Storm. The refractory carbides. (Academic Press, New York, 1967). 

The refractory carbides NbC and TaC and the nitride NbN, each with the sodium chloride structure, are also similar to the corresponding zirconium and hafnium compounds. Finally, it may be noted that the same group of complexing agents form complexes with Nb(V) and Ta(V) as with Zr(IV) and Hf(IV) (for example, fluoride, hydrogen peroxide, acetylacetone, and 1,2-dihydroxybenzene). 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

Sch] Schenk, H., Steinmetz, E., GohUce, R., Contribution to the Study of the Chemical Activity of the Elements P, S, Si, Cu and Cr in Molten Iron Solutions Saturated with Carbon (in German), Zrc/ . Eisenhuettenwes.,37(12), 919-924 (1966) (Thermodyn., Calculation, 30) [1966Yas] Yasinskaya, G.A., Wetting of the Refractory Carbides, Borides and Nitrides by Molten Metals (in Russian), Poroshk. Metall. (Kiev), 43(7), 53-55 (1966) (Interface Phenomena, Experimental, 5)... 

Wettability of the refractory carbide WC by the molten iron was investigated by [1966Yas] using the resting drop technique described by [1958Ere]. This type of wettability was characterized by the contact angle... 

Refractory carbides and nitrides are useful materials with numerous industrial applications and a promising future, in addition to being materials of great interest to the scientific community. Although most of their applications are recent, the refractory carbides and nitrides have been known for over one hundred years. Titanium and tungsten carbides were extracted from steel and properly identified around the middle of the nineteenth century. In 1890, E. G. Acheson produced the first silicon carbide, trademarked Carborundum, and by 1900 the French chemist Moissan had s)mthesized most other refractory carbides in his electric arc-furnace. Titanium carbonitride was first described in 1822 and identified by chemical analysis in 1850. Additional notes of historical interest will be presented in the relevant chapters. 

The industrial importance of the refractory carbides and nitrides is growing rapidly, not only in the traditional and well-established applications based on the strength and refractory nature of these materials such as cutting tools and abrasives, but also in new and promising fields such as electronics and opto-electronics. Some typical applications are as follows ... 

Although carbides and nitrides as a group form the most refractory compounds, they are certainly not the only ones. Several borides, oxides, phosphides, silicides, and metals meet the refractory requirements mentioned above. To some degree, these materials complement the refractory carbides and nitrides and may be considered as competitors. 

Why should the refractory carbides and nitrides be reviewed together in one book ... 

---