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A major reason for choosing ceramics is their high-temperature performance. In Fig. 3.4, the specific strength of CFCCs is compared to other high-temperature materials. The various types of CFCCs and their processes are given in Section 3.3.

Continuous fiber reinforcement is used as monofilament or multifilament tows. A composite using fiber tows costs less because it is easier to process into complex shapes. Some of the more common fibers include oxides (alumina and mullite) and nonoxides (silicon carbide and silicon nitride). Where application temperatures are below 1100°C (2012°F) or the exposure time is limited, the oxide fiber mullite is most widely used because of its lower cost. Silicon carbide is favored where engineers desire a stronger, harder, stiffer composite with superior thermal stability.

CFCC Thermal Stability

CFCC Thermal Stability

FIGURE 3.4 CFCCs operate beyond temperature range of best metals.

Ceramic matrices used in CFCCs are either metal oxides or nonoxides. Oxides are desired for their inherent oxidative stability. Oxide matrices are alumina, silica, mullite, barium aluminosilicate, lithium aluminosilicate, and calcium alu-minosilicate. Alumina and mullite are the most frequently used oxides because of their thermal stability, chemical resistance, and their compatibility with various fiber reinforcements. Although the oxide matrices have a longer history, nonoxide CFCCs are stronger, harder, stiffer, and uniquely resist certain environments. Common nonoxides include silicon carbide and silicon nitride.

Composites of fiber and matrix can be mixed and matched, that is, oxide matrix reinforced with nonoxide fibers, and visa versa, as well as oxide fibers with oxide matrix and nonoxide fibers with nonoxide matrix. The most widely used reinforcement is silicon carbide fiber because of its compatibility with a wide range of oxide and nonoxide matrices. Table 3.3 shows the variety of CFCC materials.

TABLE 3.3 CFCC Materials and Processes

Process

Matrix

Fiber

Manufacturer

Chemical vapor infiltration (CVI)

Direct metal oxidation (DMO)

Polymer impregnation pyrolysis Melt infiltration Reaction bonding Sol-gel

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