Key points of preparation technology of silicon nitride ceramics
Silicon nitride (Si3N4) is a compound of nitrogen and silicon. In nature, nitrogen and silicon are very common elements, which are ubiquitous in the world we live in. However, up to now, no compound of these two elements has been found in nature. At present, silicon nitride is synthesized under artificial conditions.
Silicon nitride ceramics are made by sintering inorganic powders, and the research process is very difficult. Silicon nitride was synthesized directly more than 140 years ago, but it was not until the mid-1970s that high-quality, low-cost and widely used silicon nitride ceramic products were truly made. In the preparation of Si 3N 4 ceramics, the preparation of Si 3N 4 ceramic powder, sintering and toughening are the key technical points.
Preparation of silicon nitride powder
Due to the different preparation processes, different types of silicon nitride ceramics have different microstructures (such as porosity and pore morphology, grain morphology, intergranular morphology and the content of the second phase). Therefore, the performance is very different. In order to obtain silicon nitride ceramic materials with excellent properties, high quality silicon nitride powder should be prepared first. The quality of silicon nitride powder prepared by different methods is not the same, which leads to the difference in its use.
Many properties of silicon nitride are attributed to its structure. There are three kinds of crystal structures of silicon nitride: α, β and γ. The phase transformation of α phase will take place at high temperature, which is irreversible and beneficial to sintering. Therefore, high-quality silicon nitride powder should have the characteristics of high content of α phase, uniform composition, few impurities, uniform distribution in ceramics, small particle size, narrow particle size distribution and good dispersion. The α phase should account for at least 90% of the good silicon nitride powder. This is because part of the α phase will change into β phase in the sintering process of silicon nitride, but not enough α phase content will reduce the strength of ceramic materials.
Sintering of silicon nitride ceramics
The sintering technology of silicon nitride ceramics is developing rapidly, its diffusion coefficient, volume diffusion, grain boundary diffusion speed and sintering driving force are very small, which determines that pure silicon nitride can not achieve densification by conventional solid-phase sintering. At present, the main sintering methods of silicon nitride ceramics are: atmospheric pressure sintering, reaction sintering, hot pressing sintering, air pressure sintering and so on. Among them, reactive sintering is the first method to produce silicon nitride ceramics in industrial production. It is necessary to add sintering aids to hot pressing sintering and atmospheric pressure sintering. Different sintering aids have different effects on sintered silicon nitride ceramics. The sintering methods of advanced ceramics include different properties of silicon nitride ceramics sintered by different sintering methods, so different sintering methods and sintering aids can be selected according to different product requirements.
Toughening and strengthening of silicon nitride ceramics
The inherent brittleness of silicon nitride ceramics limits its application, so improving its toughness and reliability has been an important research direction of silicon nitride ceramics. At present, there are many ways to toughen silicon nitride ceramics, such as particle dispersion toughening, whisker or fiber toughening, ZrO2 phase transformation toughening, self toughening with columnar β - Si3N4 grains and Composite Toughening with layered structure.
Particle dispersion toughening is to add particles with certain size and high elastic modulus into silicon nitride ceramics. At present, the most common method is to introduce SiC into silicon nitride. The self toughening of columnar β - Si3N4 grains means to cultivate columnar β - Si3N4 grains in the sintering of silicon nitride ceramics through reasonable selection of composition and process. This technology has been paid more and more attention, It is becoming a new way to improve the fracture toughness of silicon nitride ceramics.