Mechanical Properties of Hot‐Pressed Silicon Nitride with Different Grain Structures

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