Preparation Process of Silicon Nitride Ceramics
The types of preparation process of silicon nitride ceramics are mainly distinguished by the difference of synthesizing, forming and sintering methods and order.
- Reaction Sintering (RS)
Reaction sintering to prepare silicon nitride goes in the following steps. After Si powder or the mixture of Si powder and Si3N4 powder is molded, put it pre-nitrided at about 1200 ℃ with nitrogen gas, then mechanically process it into the desired items, and finally go through the final nitride sintering about 1400 ℃. In this process, there is no need to add sintering aids, etc., so the strength of material at high temperature will not be significantly reduced. Meanwhile, the reaction-sintered silicon nitride has shrinkage characteristics, so parts of complex shapes can be prepared. But since the products have low density, 70% to 90%, there is a large number of pores, the mechanical properties can be greatly affected.
- Pressureless Sintering (PLS)
The pressureless sintered silicon nitride is prepared from mixing the silicon nitride powder with high purity, ultra fineness, and high levels of α-phase with a small amount of sintering aids by forming, sintering and other processes. During the sintering process, αopposite liquid phase dissolves and then changes into β-Si3N4 when deposited on the β-Si3N4 crystal nuclei, which is conducive to the sintering process. During sintering, nitrogen gas must be ventilated to inhibit pyrolysis of Si3N4. Ceramics with complex shapes and excellent performance can be obtained by pressureless sintering. The disadvantage the relatively large sintering shrinkage, typically 16% to 26%, making products easy for crack and deformation.
- Post Sintering (PS)
Put the reaction-sintered Si3N4 sintered compact, in the presence of sintering aids, into the silicon nitride powder, re-sintered at high temperatures to obtain the dense Si3N4 products. The sintering aid can be introduced in the ball milling of silicon powder, or can also be added by the impregnation method after the reaction sintering. Since pre-processing is available during the reaction sintering process, the shrinkage in the re-sintering process is only 6% to 10%, and components with complex shapes as well as excellent performance can be prepared.
Also in recent years, a number of other sintering and densification processes have been developed, such as ultra-high pressure sintering, chemical vapor deposition, and explosion forming, etc.