Sintering is a process and phenomenon in which the ceramic body is subjected to high temperature to increase the strength of the material, and is usually a process of densification of the material. During the sintering process, with the increase of temperature and the prolongation of time, under the action of particle surface energy, the contact area between solid particles increases continuously, and mass transfer and grain boundary movement of the contact particles occur.
As the crystal grows, the voids (pores) and grain boundaries gradually decrease, the total volume of the green body shrinks, the density increases, and finally becomes a dense and hard ceramic material with a certain microstructure. The macroscopic changes of the green body after sintering are volume shrinkage, increase in density, and increase in strength. Therefore, the ratio of shrinkage, porosity or bulk density to theoretical density of the green body is often used to reflect the degree of sintering.
(1) Sintering method: According to whether there is external pressure during sintering, the sintering method can be divided into normal pressure sintering and pressure sintering. Pressure sintering can be divided into hot pressing sintering and hot isostatic pressing sintering.
Hot-pressing sintering is to heat the powder and pressurize it to increase the contact area between the powder particles, increase the power of densification, and make the particles rearrange through plastic flow to improve the stacking condition. In hot press sintering, the pressure increases the energy of densification by a factor of about 20. Hot pressing is generally carried out at a temperature of 1/2 of the melting point temperature (thermodynamic temperature, unit K) of the material, which is lower than the normal pressure sintering temperature and shorter time. Therefore, the grains of the obtained material are relatively small, and the mechanical properties are improved.
In the process of dry pressing, Tianshi ceramic binder mainly plays the role of bonding, increasing the particle size of false particles and increasing the strength of particles. If the addition is improper, it will be difficult to obtain a high-density green body during dry pressing.
Tianshi ceramic molding binder wax-based binder L466X plays a very important role in ceramic molding, which directly affects the rheology, formability, and product dimensional accuracy of the powder.
|Appearance||white micro powder|
|Particle Size Dv50||20-40|
|Melting Point ℃||65-75|
1. Effectively prevent the segregation of powder components and reduce the dust on the powder mixing site;
2. It can improve green strength and excellent bonding ability;
3. There is no residue after sintering, and there is no black spot on the surface of the workpiece.
Atmospheric pressure sintering is also called pressureless sintering, that is, under atmospheric pressure, the green body is sintered freely. In the absence of external power, the temperature at which the material starts to sinter usually needs to reach 0.5 to 0.8 times the melting point of the material. The key to the normal pressure sintering process is to control the sintering system.
The hot isostatic pressing sintering process is to put the powder compact or the powder packed into the package into a high-pressure container, and sinter it into a dense ceramic body under the action of high temperature and balanced gas pressure. This method can produce high-quality workpieces with uniform and isotropic grains, almost no pores, and a density close to the theoretical density. At the same time, the limitations of the hot pressing method are also solved, and the final shape of the part can be obtained. Due to the complex process and high cost of hot isostatic pressing, its application is limited.
According to whether there is an atmosphere during sintering, it can be divided into ordinary sintering and atmosphere sintering. Ordinary sintering is sometimes called normal pressure sintering. For products that are difficult to sinter in air, such as non-oxide ceramics or light-transmitting ceramics, in order to ensure the composition, structure and performance of the products, the blanks must be sintered in a special atmosphere, which is called atmosphere sintering. Various atmospheres such as vacuum, hydrogen, nitrogen and inert gases (such as argon) are commonly used.
According to the internal state of the green body during sintering, it can be divided into gas phase sintering, solid phase sintering, liquid phase sintering, activation sintering and reaction sintering. In addition, some sintering methods have been developed, such as plasma sintering, electric spark sintering, electric field sintering, high temperature self-propagating sintering, ultra-high pressure sintering, etc.
(2) Sintering process: According to the different physical and chemical changes that occur during the sintering process, the sintering process of ceramic materials is divided into three stages: heating stage, heat preservation stage and cooling stage.
The heating stage is manifested in the discharge of water from the green body, the decomposition and oxidation of organic binders, the generation of liquid phase, and the rearrangement of grains. Macroscopically, the green body shrinks and the density increases, so as to achieve the required performance of the product. At this stage, the heating rate is an important parameter.
The heat preservation stage promotes the diffusion and recrystallization of the ceramic material, so that the temperature and performance of the product are kept uniform. The holding time is determined according to the specifications and dimensions of the ceramic material, and cannot be too long, otherwise the grains will grow twice or recrystallization will occur.
The cooling stage is the process in which the ceramic material is lowered from the highest temperature to room temperature. The cooling process is accompanied by physical and chemical changes such as liquid phase solidification, crystallization, and phase transformation. Therefore, the cooling method and cooling speed have a great influence on the composition, structure and properties of the final phase of the ceramic material. Different types of ceramic materials, different sizes, different temperature stages, etc., have different cooling rates.
(3) Factors affecting the quality of sintering: ① The particle size of the material The smaller the particle size of the material, the larger the specific surface area and the larger the surface free energy, which increases the driving force for sintering and improves the solubility of fine particles in the liquid phase. Thereby, the sintering temperature is lowered and the sintering process is accelerated.
②Crystallization of materials: chemical properties The crystallization and chemical properties of materials are the key internal factors that determine the difficulty of sintering. When the lattice energy is large and the bond force is strong, the particles are combined firmly, and it is difficult for the particles to move at high temperature, which is not conducive to sintering. Therefore, the lattice energy, which characterizes the size of the crystal bond force, is an important parameter that determines the difficulty of sintering and recrystallization.
Different structure types also have an important influence on sintering. The cation polarity is low, the structure type is stable, the quality defects are few, the particles in the material are not easy to move, and the sintering is difficult.
③The role of additives: During the sintering process, a small amount of additives and the sintered phase can produce a series of reactions and changes, thereby promoting sintering. The additive forms a solid solution with the sintered phase, resulting in lattice defects and activation of the lattice. For example, adding TiO2 to alumina forms a small amount of solid solution, the lattice is distorted, and the sintering temperature can be lowered. Additives can promote the formation of the liquid phase and change the properties of the liquid phase to promote sintering. In addition, the additives can also inhibit grain growth.
④Sintering temperature and holding time: Sintering temperature and holding time are important external conditions that affect sintering. Within a certain range, the increase of the sintering temperature and the prolongation of the holding time are beneficial to the sintering. The following table shows the properties of corundum-mullite-bonded corundum products at different sintering temperatures.
⑤ Sintering atmosphere:
The sintering atmosphere generally includes oxidizing, reducing and neutral atmosphere. Different materials have different requirements for atmosphere under different conditions.
⑥Contact and pressing of material particles When pressing and forming, the powder particles can be brought into contact with each other by external pressure, the pores can be reduced, and the residual stress can be generated and retained at the contact points between the particles. This residual stress becomes the driving force for the migration of solid diffusion species during sintering. Anything that interferes with particle-to-particle contact is not conducive to sintering or densification of the ceramic material.
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