"Resilience" up! Eight Tips for Toughening Silicon Nitride Ceramics

2022-10-21


As an important structural ceramic material, Si3N4 ceramics have excellent mechanical and thermal shock resistance properties (when heated to over 1000 ℃ in air, they will not crack even when subjected to sudden cooling and heating). They are considered to have the best comprehensive properties at present, and have been widely used in fields such as metallurgy, aerospace, energy, machinery, military technology, optics, and glass industry.

Restricted by the "common fault of ceramics" - high brittleness

Si3N4 is a strong covalent bond compound with high atomic bonding strength and excellent comprehensive properties. In addition, due to the directionality and saturation of covalent bonds, there are few slip systems in Si3N4 ceramics composed of covalent bonds, which usually break before slip occurs, resulting in significant brittleness of Si3N4 ceramics.

However, the low fracture toughness of Si3N4 ceramics and their sensitivity to local cracks within the material have become a fatal disadvantage of Si3N4 ceramics, seriously affecting their service life and reliability, greatly limiting their application range.

Does raw material powder have an impact on its fracture toughness?

Due to the preparation process of Si3N4 ceramics mainly using powder as raw material, dense ceramic bodies are obtained after pressing and sintering. Therefore, the characteristics of Si3N4 powder play a crucial role in the sintering process and final performance. Si3N4 powder mainly includes α- Si3N4 phase sum β- There are two types of Si3N4 phase. When in powder β Phase content>30 vol.% During the sintering dissolution re precipitation stage, the driving force decreases and the densification process of silicon nitride ceramics is inhibited; Moreover, the microstructure of ceramics is mainly composed of finer equiaxed grains, which is not conducive to obtaining high fracture toughness.

use α- As an initial powder, Si3N4 is more advantageous for preparing Si3N4 ceramics with high strength and toughness, because α- Formation of Si3N4 by dissolution precipitation reaction during liquid phase sintering β- Si3N4, in the subsequent grain coarsening stage, β- The anisotropic growth of Si3N4 can form a self toughening microstructure, improving the density and toughness of Si3N4 ceramics.

1

In terms of oxygen content, toughness increases as the oxygen content of the powder decreases. This is because the use of powders with low surface oxygen content produces less liquid phase during sintering, resulting in a reduction in nuclear sites, a reduction in the number of crystal nuclei, and the transformation of the crystal shape from a semi axial shape to a long rod shape, β- Si3N4 has a higher aspect ratio and improved fracture toughness.

In addition, Si3N4 powder with high carbon content will inhibit the densification process of silicon nitride. Because carbon reacts with silicon dioxide (SiO2) on the surface of Si3N4 powder to form CO and SiO, inhibiting the formation of liquid phases, which is detrimental to the densification process of Si3N4.

Therefore, in the Si3N4 ceramic raw material powder α The phase content, oxygen content, and carbon content all affect the fracture toughness of Si3N4 sintered body. Select High α The key factors for obtaining high fracture toughness Si3N4 ceramics are phase, low oxygen, low carbon content, and appropriate specific surface area of Si3N4 powders.

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