Exploring the development of self-toughening technology for silicon nitride ceramics

What is self-toughening technology?

Self-toughening is a new process developed in recent years that can effectively improve the fracture toughness of ceramics. It is mainly controlled by process factors to make ceramic grains have a large aspect ratio in situ. To a toughening and reinforcing effect similar to whiskers.

Since the silicon nitride crystal has growth anisotropy, the α phase can change to the β phase at a high temperature, and the β-Si3N4 crystal will continue to grow to change its microstructure. Therefore, the desired microstructure can be obtained by controlling the nucleation and growth of the β-Si3N4 grains to form a self-toughened Si3N4 ceramic.

The essence of self-toughening is to obtain the ideal microstructure through reasonable composition design and optimal process conditions, so as to achieve the purpose of improving fracture toughness.

Preparation method of self-toughening Si3N4 ceramic

Hot press sintering

Hot press sintering is a sintering process performed simultaneously with press molding and pressure sintering. The hot pressing technology was first used in the preparation of dense parts of tungsten carbide and tungsten powder. It has been widely used in the production of ceramics, powder metallurgy and composite materials. Hot press sintering is a one-way or two-way pressurized pressure sintering method. The hot pressing sintering process is dry pressing at high temperature, that is, only the mold is heated together with the sample, and a certain pressure is applied, and the powder during hot pressing sintering does not need to be formed. The heating method in hot press sintering is still an electric heating method, and the pressing method is an oil pressure method, and a mold or an alumina mold can be used according to different requirements of the mold. The commonly used mold must be used in a non-oxidizing atmosphere at pressures up to 70 MPa. Graphite molds are simple to make and cost less.

Hot isostatic pressing

Hot isostatic pressing sintering combines the advantages of both hot press sintering and pressureless sintering. Compared with conventional pressureless sintering and ordinary hot press sintering, hot isostatic pressing can increase the density and suppress the crystal as in hot press sintering. Granular growth, improve the performance of the product, and can produce a very complicated product like pressureless sintering, and can also achieve metal-ceramic sealing, if the sealing is properly obtained, the product with high surface finish can be reduced or Avoid machining.

Spark plasma sintering

Discharge plasma sintering is a pressure sintering method in which direct current pulse current is directly used for sintering, and the heating rate and sintering temperature are controlled by adjusting the magnitude of the pulsed direct current. The entire sintering process can be carried out in a vacuum environment or in a protective atmosphere. During the sintering process, the pulsed direct current passes directly through the upper and lower indenters and the sintered powder or graphite mold. Therefore, the heat capacity of the heating system is small, and the temperature rise and heat transfer speed are fast, thereby making rapid temperature rise sintering possible.

Microwave sintering

Microwave sintering is a new sintering method that uses microwaves to directly interact with material particles (molecules, ions) and utilizes the dielectric loss of the material to directly absorb the microwave energy of the sample. The microwave sintering method can sinter many high-tech ceramic materials such as alumina, yttria, stabilized zirconia and mullite.

Microwave sintering characteristics: The heating mechanism is unique. Microwave sintering is heated by the direct action of microwave and material. The sample itself can be regarded as a heat source. In the thermal process, the sample absorbs microwave energy on the one hand, and loses energy by surface radiation on the other hand. The unique heating mechanism makes the temperature rise of the material not only depends on the characteristics of the microwave system, such as frequency, but also related to the material properties of the material, such as dielectric loss. The higher the dielectric loss, the faster the heating rate; the microwave sintering can reduce the sintering temperature. It has the advantages of suppressing grain growth, but the higher heating rate results in uneven temperature distribution, large thermal effect, and limited sample size.

The essence of microwave sintering heating is the interaction of molecules or ions in the material with microwave electromagnetic fields. Under the action of high-frequency alternating electric field, the polar molecules, dipoles, ions, etc. inside the material move violently with the change of electric field, and the internal friction of collision and friction between the components makes the microwave energy transform into thermal energy.

Reaction sintering

Reaction sintering is also called activated sintering or reinforced sintering. A sintering method in which the reaction is carried out simultaneously with sintering by the action of the additive. The characteristics of reaction sintering: improve the quality of the product, the fired product does not shrink, the size does not change; the reaction speed is fast, the mass transfer and heat transfer process are carried out in the whole process of sintering.

There are many preparation methods for self-toughening Si3N4, such as pressureless sintering, hot pressing sintering and gas pressure sintering. The purpose is to effectively control the grain size of each Si3N4 by reasonable selection and optimization of process conditions, so as to obtain ideal microscopic. structure. For a particular sintering process, changes in sintering temperature and time will cause changes in the microstructure. Increasing the temperature or prolonging the time can increase the mass transfer of the material, thereby promoting crystal growth and changing the aspect ratio. Temperature is the main parameter under many conditions.

At present, research on silicon nitride ceramic materials pays more attention to the microstructure, bending strength, fracture toughness, hardness, thermal expansion coefficient, thermal conductivity and friction coefficient of the material. Silicon nitride ceramics have excellent physical and mechanical properties and chemical properties, and are widely used in high temperature, chemical, metallurgical, aerospace and other fields. Although the silicon nitride ceramics have relatively high fracture toughness in structural ceramics, in order to further broaden the application field of the silicon nitride ceramics and improve the reliability of its use, improving the fracture toughness has been an important issue of the material.