Silicon nitride (Si3N4) is a compound of nitrogen and silicon. In nature, nitrogen and silicon are extremely common elements, and they are ubiquitous in the world we live in. However, no compound of these two elements has yet been found in nature. At present, all silicon nitrides are synthesized under artificial conditions. Silicon nitride ceramics are prepared by sintering inorganic powder, and the research process is very difficult. As early as 140 years ago, people directly synthesized silicon nitride, but it was not until the mid-1970s that they really produced high-quality, low-cost, widely used silicon nitride ceramic products. In the preparation of silicon nitride ceramics, the preparation, sintering of silicon nitride ceramic powders and the toughening and strengthening of ceramic bodies are the key technical points in the preparation of silicon nitride ceramics.
Due to different preparation processes, various types of silicon nitride ceramics have different microstructures (such as porosity and pore morphology, grain morphology, intergranular morphology, and intercrystalline second phase content, etc.). Therefore, the performance varies greatly. To obtain silicon nitride ceramic materials with excellent performance, first of all, high-quality silicon nitride powder should be prepared. The quality of silicon nitride powder prepared by different methods is not exactly the same, which leads to the difference in its use.
Many properties of silicon nitride are due to its structure. The crystal structure of silicon nitride has three kinds of crystal structures: α, β and γ. The α phase will undergo a phase change at high temperature. This phase change is irreversible and is beneficial to sintering. Therefore, high-quality silicon nitride powder should have the characteristics of high alpha phase content, uniform composition, less impurities and even distribution in ceramics, small particle size and narrow particle size distribution, and good dispersion. The α phase of good silicon nitride powder should account for at least 90%. This is because during the sintering process of silicon nitride, part of the α phase will be transformed into β phase, and if there is not enough α phase content, it will reduce the strength of the ceramic material. the reason.
The sintering technology of silicon nitride ceramics has developed rapidly, and its diffusion coefficient, volume diffusion necessary for densification and grain boundary diffusion speed, and sintering driving force are very small, which determines that pure silicon nitride cannot be densified by conventional solid phase sintering . The current silicon nitride ceramic sintering process methods are: normal pressure sintering, reaction sintering, hot press sintering, air pressure sintering, etc. Among them, reaction sintering is the earliest method used to manufacture silicon nitride ceramics in industrial production. Hot pressing sintering and normal pressure sintering need to add sintering aids. Different sintering aids have different effects on sintered silicon nitride ceramics. The sintering methods of advanced ceramics include different properties of sintered silicon nitride ceramic products. Therefore, different sintering methods and sintering aids can be selected according to different product requirements.
The inherent brittleness of silicon nitride ceramics limits its application, so improving its toughness and reliability has always been an important direction of silicon nitride ceramics research. At present, there are many ways to toughen silicon nitride ceramics, mainly including particle dispersion toughening, whisker or fiber toughening, ZrO2 phase transformation toughening, self-toughening using columnar β-Si3N4 grains, and layered structure compound toughening Tenacity etc.
Particle dispersion toughening is to add a certain size of particles with high elastic modulus to silicon nitride ceramics; at present, the introduction of SiC in silicon nitride is the most common method; self-toughening using columnar β-Si3N4 grains It means that through reasonable selection of ingredients and processes, silicon nitride ceramics can cultivate columnar β-Si3N4 grains during sintering. This technology has been increasingly valued by people and is becoming a new way to improve the fracture toughness of silicon nitride ceramics.
How to sinter silicon nitride ceramics?
2020 06/08