"Toughness" has been improved! Eight tips for toughening silicon nitride ceramics- Zhejiang Shangguijuli Special Material Technology Co., Ltd.

As an important structural ceramic material, Si₃N₄ ceramics have good mechanical properties and thermal shock resistance (heated to more than 1000℃ in air, it will not break even if it is suddenly cooled or heated). It is considered to have a good comprehensive performance at present and has been widely used in metallurgy, aerospace, energy, machinery, military industry, optics, glass industry, and other fields.

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

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

However, the low fracture toughness of Si₃N₄ ceramics and the sensitivity to local cracks inside the material have become the fatal shortcomings of Si₃N₄ ceramics, which seriously affect its service life and reliability and greatly limit its application range.

Does the raw material powder affect its fracture toughness?

Since the preparation process of Si₃N₄ ceramics mainly uses powder as raw material, a dense ceramic body is obtained after pressing and sintering. Therefore, the characteristics of Si₃N₄ powder play a vital role in the sintering process and performance. Si₃N₄ powder mainly includes two types: α-Si₃N₄ phase and β-Si₃N₄ When the β-phase content in the powder is >30vol.%, the driving force decreases during the sintering dissolution and reprecipitation stage, and the densification process of silicon nitride ceramics is inhibited; and the microstructure of the ceramic is mainly composed of finer equiaxed crystals, which is not conducive to obtain high fracture toughness.

Using α-Si₃N₄ as the initial powder is more conducive to the preparation of high-strength and toughness Si₃N₄ ceramics because α-Si₃N₄ is formed by dissolution precipitation reaction during liquid phase sintering β-Si₃N₄, and in the subsequent grain coarsening stage, the anisotropic growth of β-Si₃N₄ can form a self-toughening microstructure, improving the density and toughness of Si₃N₄ ceramics.

In terms of oxygen content, toughness increases as the oxygen content of the powder decreases. This is because when using powders with low surface oxygen content, less liquid phase is produced during sintering, resulting in fewer nucleation sites, and fewer nuclei, and the crystal form changes from semi-axial to axial. β-Si₃N₄ is in the form of long rods, with a higher aspect ratio and higher fracture toughness.

In addition, Si₃N₄ powders 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 generate CO and SiO, the formation of the liquid phase is inhibited, which is not conducive to the densification process of Si₃N₄.

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