Preparation, microstructure and properties of silicon nitride ceramics with aligned whisker seeds

Effect of tape casting variables on the degree of alignment of whisker seeds were studied. Shear stress developed during the casting played a major role in rotating individual seeds in the casting direction. Simple fluid dynamics predict a high degree of alignment of the whiskers with high aspect ratio, and that was confirmed by the results obtained from the experiments with different seeds. Microstructural observation of the sintered samples revealed that the large elongated grains growing from the seeds were highly aligned. Some of those grains had so close orientations that glassy grain boundary between them was replaced by a low angle boundary. The properties including the flexural strength, the fracture toughness and the wear rate showed a high anisotropy. Low wear rate was measured from the surface normal to the casting direction.