Si3N4 is a compound mainly based on covalent bonds. The bonds are strong and the directionality of the bonds is strong. The formation and migration of defects in the structure require a lot of energy, that is, the defect diffusion coefficient is low (disadvantages) and it is difficult to sinter. Among them, the covalent bond Si- The N component is 70% and the ionic bond is 30%. At the same time, because the structure of Si3N4 itself is not dense enough, in order to improve the performance, a small amount of oxide sintering aid needs to be added, and it is densified through liquid phase sintering.
Si3N4 contains two crystal forms, one is α-Si3N4, acicular crystals, white or off-white, and the other is β-Si3N4, darker in color, in the form of dense granular polyhedrons or short prisms. Both are hexagonal crystal systems, and both are three-dimensional spatial networks composed of [SiN4]4-tetrahedrons sharing vertex corners.
At high temperatures, the β phase is thermodynamically more stable, so the α phase undergoes a phase change and turns into the β phase. Therefore, when Si3N4 powder with high α phase content is sintered, fine-grained, long columnar β-Si3N4 grains can be obtained, which improves the fracture toughness of the material. However, the abnormal growth of particles must be controlled during ceramic sintering, causing pores, cracks, and dislocation defects to appear and become the source of material fracture.
