29 Particlereinforced Ceramic Composites

The options for achieving a composite microstructure by self-reinforcement are limited. Another approach is to add particles of a second ceramic into a ceramic matrix during the fabrication process. This opens up many additional options for making ceramic matrix composites.

Most powders that are added to a ceramic for toughening are prepared by crushing and grinding or by chemical processes. These powders are typically equiaxed (roughly spherical), like a grain of sand, and between 0.5 and 40 ^m in diameter. The composite is prepared by mixing the reinforcing powder with the matrix ceramic powder and compacting the powders into the desired shape by a conventional ceramic fabrication processes such as pressing. The compact is then placed in a high-temperature furnace and sintered the same way that the matrix would be sintered if the reinforcing powder had not been present. This conventional sintering works for small to moderate volume fraction of reinforcing particles, generally up to about 15-20%. For larger volume fraction of particles, hot pressing or postsintering hot isostatic pressing may be required if a pore-free composite is desired. Both are more costly than conventional sintering.

An important ceramic matrix composite with equiaxed particle reinforcement is Al2O3 with a dispersion of nominally 30-35% titanium carbide (TiC) particles. This material was initially developed as a cutting tool insert. Alumina without reinforcement had been used intermittently as a cutting tool since the 1920s, but only with limited success. The alumina-TiC had higher hardness and slightly higher toughness and could cut a wider range of alloys including hardened steel, chilled cast-iron, and cast-iron with an abrasive surface scale. It had improved reliability and could even survive interrupted cuts.

An important early success of alumina-TiC cutting tool inserts was in the steel industry [10]. Large steel rolls (typically over 4 m long and 75 cm in diameter) in steel rolling mills require frequent refurbishing. This refurbishing was previously done using an expensive ceramic grinding wheel and required 14-18 h per roll. Use of alumina-TiC cutting tool inserts reduced the refurbishing time to 5 h per roll [10] and became standard practice.

Alumina-TiC also has become important in the computer industry as the substrate material for read-write heads. Its attributes for this application are light weight, high stiffness, and ability to be machined chip-free to a precision smooth surface.

Other particles have been added to alumina in efforts to increase toughness. A 10 vol% of 30-^m-diameter flat plates of Ba-mica was reported to increase the toughness to 8.6 MPa ■ m1/2 [27]. A 5 vol % of titanium diboride particles was reported to result in toughness of 6.5 MPa ■ m1/2 [28]. Addition of dispersions of particles to SiC and Si3N4 also have resulted in increase in toughness. Examples are listed in Table 2.6 and the strength and toughness values compared with in situ reinforced silicon nitride, particle-reinforced alumina, whisker-reinforced ceramics, and a couple of ductile metal-reinforced ceramics.

The particulate-reinforced ceramics listed in Table 2.6 resulted in increased toughness primarily due to the mechanism of crack deflection. Addition of transformation-toughened zirconia particles to other ceramics can result in toughness increase by the mechanism of crack shielding. Toughening occurs if the particles are small (usually under 0.5 ^m), if the host ceramic is strong enough to prevent the particles from transforming during cooling for the sintering temperature, and if there is no chemical interaction between the materials. Alumina with 15-20% addition of transformation-toughened zirconia particles has been reported to have toughness between 6.5 and 15 MPa ■ m1/2 and flexure strength between 480 and 1200 MPa [38]. These values of toughness and strength are comparable to values reported for pure transformation-toughened zirconia, and the transformation-toughened alumina (TTA) has higher hardness and is thus

TABLE 2.6 Comparison of Strength and Toughness for Various Ceramic Matrix Composites


Flexural Fracture Strength Toughness (MPa) (MPa • m1/2) Reference

Alumina with 30 wt % TiC particles Si3N4 with 30 vol% 8-^m SiC particles SiC with 16 vol% TiB2 particles Alumina with 30 vol% 30-^m Ba-mica Alumina with 30 vol % SiC whiskers Si3N4 with 30 vol% 0.5-^m SiC whiskers Si3N4 with 30 vol% 5-^m SiC whiskers Si3N4 with 30 vol % BN-coated Si3N4 whiskers MoSi2 with 20 vol % SiC whiskers In situ reinforced silicon nitride In situ reinforced silicon nitride In situ reinforced silicon nitride ZrC-ZrB2 with 24.2 vol % Zr metal ZrC-ZrB2 with 2.5 vol% Zr metal

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