Why do silicon nitride products sometimes undergo sintering densification?

In our modern lives, many essential items rely on chemical substances, and silicon nitride products are also made using chemical components. These products are not only exceptionally tough, but also boast impressive strength and hardness. However, some people wonder why silicon nitride products sometimes experience densification during sintering. Let's discuss this issue.

Many silicon nitride products are sintered using methods such as gas pressure, hot pressing, and hot isostatic pressing. While other methods exist, these three are the most common and frequently used by many manufacturers.

Hot isostatic pressing allows silicon nitride products to achieve higher performance. However, using it for product development requires advanced equipment and hardware, which increases costs for many manufacturers, hindering large-scale adoption. Therefore, larger manufacturers often choose this sintering method.

Common pneumatic pressing methods can suppress the decomposition of silicon nitride powder due to high temperatures. By introducing nitrogen gas at a certain pressure during sintering, higher density silicon nitride can be obtained. However, its effect on density promotion is limited.

If the sintering temperature is continuously increased, diffusion and dissolution mass transfer processes can be facilitated, significantly reducing the viscosity of the system and increasing fluidity, thus promoting densification. However, excessively high temperatures not only waste energy but also lead to excessive liquid phase and excessively low viscosity, causing product deformation, performance deterioration, and decreased density. This makes controlling the appropriate sintering temperature and holding time a crucial consideration in most research.

Currently, hot pressing is a widely chosen method. By continuously applying pressure from ambient temperature, combined with constant mechanical pressure and external pressure, additional driving force can be provided. Compared to traditional sintering processes, the temperature of silicon nitride products can be reduced by 100–200°C, the sintering time is significantly shortened, and the density can reach over 99%.

However, this external pressure is not a panacea; it has an upper limit. Furthermore, the strength of the graphite mold itself is limited, making it difficult to easily achieve the downloading problem under high pressure. In addition, current pressure-assisted crafts are basically produced under constant pressure, which leads to the densification problem of silicon nitride products mentioned above. Solving this problem is not easy and requires professional operation. If the operation is not standardized or the personnel do not understand the process, densification will occur, which will also cause certain losses.