283 Microstructure Containing Elongated Intertwined Grains

Flat platelets in the microstructure, such as mica crystals and aluminate crystals, cause a crack to deflect only in a single plane. Elongated rod-shaped grains in the microstructure force a crack to deflect in more than one plane to get around the grain. This requires more energy, so highly elongated grains have the potential to achieve higher toughness than platelets. Faber and Evans [22] predicted and verified experimentally that a dispersion of disk-shaped particles or grains can increase toughness by a factor of 3 and rod-shaped ones by a factor of 4.

FIGURE 2.9 Elongated, intertwined microstructure of high-strength, high-toughness AS-800 silicon nitride material manufactured by Honeywell Engines, Systems, and Services, Phoenix, AZ. (Photo courtesy of George Graves, University of Dayton Research Institute, Dayton, OH).

Microstructures with elongated grains have been achieved for some silicon nitride materials. These are referred to as self-reinforced silicon nitride [23] and have been reported to have fracture toughness values ranging from about 6-14 MPa ■ m1/2 [24-26]. Alpha phase Si3N4 powder is blended with MgO, Y2O3, Al2O3 + Y2O3, or other oxide sintering aids. At high-temperature, the oxide reacts with a thin layer of SiO2 that coats each Si3N4 particle to form a liquid-phase. The a-Si3N4 particles dissolve and recrystallize as elongated P-Si3N4 grains. By control of chemical composition, temperature, and time at temperature, an intertwined structure such as that shown in Figure 2.9 results. The high toughness of this type of intertwined structure has been a significant factor in the success of silicon nitride in surviving applications that prior ceramics have not been able to survive.

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