During hot-pressing of a-Si3N, powders, the equiaxed a microstructure gradually transforms into a structure characterized by needle-shaped prkmatic grains which are closely entangled and linked together. With increasing amounts of the p fraction, the bend strength, fracture toughness, and work of fracture increase significantly, then decrease as grain growth occurs.The K,, improves by a factor of >2 and the change in yF by a factor of >4. The crack resistance to achieve the same crack velocity in materials of different / 3 contents shows a similar trend. The dependence of the mechanical properties on the microstructure is explained by linking and pullout of the /3 crystals and by grain coarsening.
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