281 Multiphase Microstructure

An example that illustrates a ceramic composite with a multiphase microstructure achieved in situ during sintering is shown in Figure 2.8. This ceramic composite

FIGURE 2.8 Microstructure showing aluminate platelets formed in situ during sintering. (Photograph courtesy of Raymond Cutler, Ceramatec Inc., Salt Lake City, UT).

consists of CeO2-doped transformation-toughened ZrO2 containing an interlacing network of platelets of strontium aluminate [20]. Two wt% SrZrO3 and 30 vol% Al2O3 powders were mixed with a coprecipitated powder of ZrO2-12 mol% CeO2 and sintered at 1500°C. Thin platelets of strontium aluminate about 0.5 ^m wide and 5-10 ^m long formed throughout the zirconia matrix during sintering. The resulting strength (in four-point bending) was 726 MPa, and the fracture toughness was 11.2 MPa ■ m1/2. The mechanisms of toughening were a mixture of transformation toughening and crack deflection.

A crack passing through a monolithic ceramic cuts across grains, goes around grains, or follows along natural crystallographic cleavage planes. The crack's path is relatively short, so that the total energy needed to drive the crack through is generally small. The resulting fracture toughness is typically 1-2 MPa ■ m1/2 for a single-crystal ceramic and 2-3.5 MPa ■ m1/2 for a polycrystalline ceramic. The aluminate platelets deflect the crack, forcing it to follow a longer and more tortuous path through the ceramic. This increases the amount of energy required to drive the crack through the material, resulting in higher fracture toughness.

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