25 Silicon Carbide

Another ceramic that is well established in the marketplace is silicon carbide (SiC). Silicon carbide has many of the same applications as aluminum oxide and silicon nitride. It is more expensive than alumina and has lower toughness than silicon nitride, so it is not the optimum material for all corrosion or wear applications. But where it can be used, it normally provides superior wear resistance and long life. Table 2.4 identifies some of the production applications of silicon carbide.

Silicon carbide also is important for tooling in the semiconductor industry, for laser mirrors, as a substrate for wear-resistant diamond coatings, as an abrasive and grinding wheel, as heating elements and igniters, as an additive for reinforcement of metals, and for numerous refractories applications.

Like silicon nitride, silicon carbide is a family of materials each with its special characteristics. Most of the silicon carbide materials have very high hardness (harder than alumina and silicon nitride) and thus have superior wear resistance. Most have unusually high thermal conductivity for a ceramic, low thermal expansion compared to metals, and very high temperature capability. Some actually increase in strength at elevated temperature, such as sintered silicon carbide from Saint-Gobain Advanced Ceramics Corporation that has room temperature flexure strength slightly above 413 MPa (60,000 psi) and that increases in strength to around 580 MPa (80,000 psi) at 1800°C.

Relatively pure SiC also has excellent resistance to corrosion in the presence of hot acids and bases. In one series of tests reported by Saint-Gobain Advanced Ceramics Corporation in one of its product brochures, dense SiC was immersed in different acids and bases for 125-300 h. For 98% sulfuric acid at 100°C, the SiC lost only 1.8 mg/cm2/year compared to >1000 for tungsten carbide with 6% cobalt and 65 for 99% pure alumina. For 50% NaOH at 100°C, the SiC

TABLE 2.4 Production Applications of Silicon Carbide

Seals

High-temperature liners, refractories Heat exchanger tubes Thermocouple protection tubes Links for high-temperature belt furnace Bearings in magnetic drive pumps Grit blast nozzle liners

Thrust bearings Valves

Pump parts

Cyclone liners Radiant burners

FIGURE 2.6 Silicon carbide seal and pump parts. (Photo courtesy of Saint-Gobain Advanced Ceramics Corporation, Niagara Falls, NY).

lost only 2.5 mg/cm2/year compared to 5 for WC-Co and 75 for alumina. The SiC exhibited even less weight loss (>0.2 mg/cm2/year) for exposures in highly concentrated hot nitric and phosphoric acids and room temperature HCI and HF. Because of the high corrosion resistance combined with high wear resistance, SiC is important for seals and pump components, as illustrated in Figure 2.6.

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