SILICON CARBIDE

Silicon carbide (SiC) is not found in any appreciable quantities in nature but is one of the most widely used synthetic technical minerals. The market for SiC focuses on its hardness and refractoriness, but SiC is also used as a source of silicon in the metallurgical processing of iron. SiC’s hardness and high-temperature stability make it as widely used as alumina as an abrasive grain. For higher-performance applications, the higher-purity (green) SiC powder is used, and for lesser requirements the lower-purity (black) SiC powder is used. For advanced ceramic applications such as armor, only the high-purity green materials are used. Other applications of high-purity SiC include space-based mirrors, semiconductor processing equipment, wire-impregnated saws for silicon wafer cutting, and automobile catalysts. These markets have driven the world supply of green SiC to more than 1 million tons per year. Armor ceramics make up less than 1 percent of the world market for high-purity SiC.7

There are many methods for producing SiC, including carbothermic reduction of silica, chemical vapor-phase reactions, and electrothermal techniques. The Acheson process, which dates from 1893, places electrodes into a graphite core laid within a mixture of reactant carbon, salt, and sand. The electric current resistively heats the graphite and in turn the surrounding reactants, resulting in the formation of a hollow cylinder of SiC and the evolution of carbon monoxide (CO) gas.8 The chemical reaction that Acheson described for the manufacture of SiC from silica sand and carbon is as follows:

SiO2 + 3C → SiC + 2CO

Within the ceramic-grade zone, both green SiC (>99 percent SiC) and black SiC (95-98 percent SiC) can be found, with metallurgical SiC (80-94 percent SiC) making up the remainder of the reaction zone. The boundary between unreacted materials and the reaction zone is marked by a layer of condensed impurities. This layer is discarded, but the unreacted precursors can be used again.

The formation of SiC is the result of four subreactions, each of which provides vapor-phase mass transport:9

C + SiO2 → SiO(g) + CO(g)
SiO2 + CO(g) → SiO + CO2(g)
C + CO2(g) → 2CO(g)
2C + SiO→ SiC + CO(g)

The exact kinetics of the reaction are highly dependent on carbon source, particle size, mixing uniformity, and packing of the silica and the carbon. During the heating of the graphite core, silica can react with carbon at temperatures as low as 1527°C to create b-SiC. At temperatures about 1900°C, the b-SiC converts to α-SiC. The various polytypes formed are dependent not only on temperature but also on the presence of impurities. For example, for α-SiC the 6H polytype is most prevalent. However, in the presence of aluminum, either intentionally or as an impurity, the 4H polytype becomes dominant. This change in polytype alters not only the shape of the resultant particles but also the microhardness, with the 4H being less hard.10

Today’s Acheson furnaces are very large. The first commercial furnace was 2 meters long and had a power input rate of 58 kW; today the largest furnace has a 240-ton capacity and a power input rate of nearly 6 MW! Aside from SiC processing being a tremendous consumer of electricity, for every pound of SiC produced, 1.4 pounds of CO are emitted. Both electricity costs and environmental concerns shifted the manufacturing of powder offshore to the extent that today, the United States accounts for less than 5 percent of the world’s production of SiC, whereas China accounts for more than 60 percent. However, that 5 percent produced in the United States supplies the abrasives and metallurgical markets, meaning that there was no supplier in 2010 providing SiC for advanced ceramics, including armor.

Work by Choi et al.11 indicated that SiC sintered with AlN and oxide additives could have a marked effect on the mechanical properties of the resulting SiC. Zhou et al.12showed the strong influence of rare-earth additions and resulting intergranular properties on the mechanical properties of SiC. Thus a better understanding of the role of intergranular phases could be used to engineer high-performance armor materials.

BORON CARBIDE

Worldwide, 1,000 to 2,000 metric tons of boron carbide are produced annually. The boron carbide market is driven by the use of boron carbide based on selected properties, such as its hardness—for example, as an abrasive grit or powder; its neutron absorption capacity (for use as control rods and shielding in pressurized water nuclear reactors, among other applications); and its specific hardness—as an armor

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7Moores, S. 2007. Energy prices prune SiC bloom. Industrial Minerals 475: 28-35.

8Guichelaar, P. 1977. Acheson process. Pp. 115-128 in Carbide, Nitride and Boride Materials Synthesis and Processing, A.W. Weimer, ed. London, U.K.: Chapman and Hall.

9Weimer, A., K. Nilsen, G. Cochran, and R. Roach. 1993. Kinetics of carbothermal reduction synthesis of beta silicon carbide. AIChE Journal 39(3): 493-503.

10Poch, W., and A. Dietzel. 1962. Formation of silicon carbide from silica and carbon. Berichte der Deutschen Keramischen Gesellschaft 39(8): 413-426 (in German).

11Choi, H-J., Y-W. Kim, M. Mitomo, T. Nishimura, J-H. Lee, and D-Y. Kim. 2004. Intergranular glassy phase free SiC ceramics retain strength at 1500°C. Scripta Materialia 50(9):1203-1207.

12Zhou, Y., K. Hirao, M. Toriyama, Y. Yamauchi, and S. Kanzaki. 2001. Effects of intergranular phase chemistry on the microstructure and mechanical properties of silicon carbide ceramics densified with rare-earth oxide and alumina additions. Journal of the American Ceramic Society 84(7): 1642-1644.



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