210 Whiskerreinforced Ceramic Matrix Composites

Ceramic whiskers are usually single crystals that have grown preferentially along a specific crystal axis under vapor or liquid-vapor conditions. The whiskers typically range in size from 0.5 to 10 ^m in diameter and a few microns to a few centimeters in length and can have very high strength. Some silicon carbide whiskers have been reported with a strength of 21,000 MPa and a Young's modulus of 840 GPa [40].

Whiskers are more difficult than particles to disperse uniformly in a ceramic matrix. Furthermore, the matrix is more difficult to densify to a pore-free condition; the whiskers form an infrastructure that inhibits shrinkage of the ceramic during sintering. For whisker volume fraction greater than about 10%, either hot pressing or liquid-phase sintering are generally required. Table 2.6 included some examples of whisker-reinforced ceramic matrix composites.

The most extensive development has been conducted on the addition of SiC whiskers to aluminum oxide [41, 42]. Figure 2.10 illustrates improvement in strength with different volume percent of SiC whiskers. The whiskers increase the high-temperature strength as well as the room-temperature strength and also improve the creep resistance, the stress rupture life, the thermal shock resistance, and the Weibull modulus. A data sheet from Advanced Composite Materials Corp., Greer, South Carolina, reported that alumina with SiC whiskers survived quenching from 900°C into room temperature water, while unreinforced alumina

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