Silicon nitride powder and sintering aid were placed in graphite mold and unidirectional pressure sintering was carried out at high temperature. As the external pressure increases the sintering driving force, the a- B transition and densification speed are accelerated. High strength silicon nitride ceramics with a density of more than 95% can be obtained by hot pressing method, which has high performance and short manufacturing cycle. However, this method can only manufacture products with simple shapes, and the processing cost is high for parts with complex shapes.Moreover, due to unidirectional pressure, the organization has a preferred orientation, so that the performance is different in parallel and vertical directions with the hot pressing surface.
The powder mixture of silicon nitride and firing aid is encapsulated into a metal or glass envelope, which is then vacuumed and sintered at high temperature by highpressure gas. The commonly used pressure is 200MPa and the temperature is 2000C. Hot isostatic pressure silicon nitride can reach the theoretical density, but its process is complicated and its cost is high. In recent years, other sintering and densification processes have been developed, such as ultra-high pressure sintering, chemical vapor deposition, explosive forming, etc.