Before being sintered, the ceramic green body is composed of many single solid particles. There are a lot of pores in the green body, and the porosity is generally 35%~60% (that is, the relative density of the green body is 40%~65%). The value depends on the characteristics of the powder itself and the molding method and technology used. When the solid green body is heated at a high temperature, the particles in the green body migrate, and the body shrinks after reaching a certain temperature, and the grain grows, accompanied by the elimination of pores, and finally at a temperature below the melting point (generally at (0.5 to 0.7 times the melting point) The green body becomes a dense polycrystalline ceramic material. This process is called sintering.
Conventional sintering
The conventional heating method is generally used, which is carried out in a traditional electric furnace, and it is currently the most commonly used sintering method in the production of ceramic materials.
Because pure ceramic materials are sometimes difficult to sinter, some sintering aids are usually introduced under the conditions that allow performance, in order to form part of a low-melting solid solution, glass phase or other liquid phase to promote the rearrangement of particles and viscous flow to obtain Dense products can also reduce the sintering temperature.
Hot press sintering
Hot pressing sintering adopts a special hot pressing machine, which is completed by single-phase or two-phase pressing at high temperature. The interaction of temperature and pressure strengthens the viscosity and plastic flow of the particles, which is conducive to the compaction of the blank, and products with almost no pores can be obtained. At the same time, the sintering time is short, the temperature is low, the grain growth is suppressed, and the product performance is improved. .
However, hot-press sintering can only produce products with simple shapes. At the same time, the microstructure after hot-press sintering has anisotropy, resulting in an anisotropy in use performance, which limits its scope of use.
Hot isostatic pressing
The hot isostatic pressing process is an inert gas such as argon gas as the pressure transmission medium. The product is placed in a closed container. Under the combined action of a certain temperature and pressure, the product is applied with equal pressure in all directions to the product. The technology of pressing and sintering treatment, due to the hot isostatic pressing sintering technology, has higher requirements on the cladding materials and technology.
Therefore, it is usually used to manufacture products with simple shapes and low production efficiency. However, ceramic products sintered by hot isostatic pressure sintering gas pressure do not need to be wrapped.
Microwave sintering
Microwave heating is different from the conventional heating mode. It uses the dielectric loss of the ceramic material in the microwave electromagnetic field to bring the material to the sintering temperature to achieve the sintering and densification of the ceramic. During microwave sintering, the material absorbs microwaves and converts the kinetic energy and potential energy of the molecules inside the material, so that the whole material is heated uniformly, the internal temperature gradient is small, the thermal stress is small, and the heating and sintering speed is fast. It can realize rapid sintering at low temperature, and significantly improve the mechanical properties of ceramic materials.
Since most ceramic materials have good microwave transmission, microwave heating is uniform. However, in the actual heating process, there is heat dissipation on the surface of the sample. If there is no suitable insulation device, the temperature difference between the inside and outside of the heating body is extremely large, which may cause uneven sintering of the sample. Therefore, the insulation layer should be designed reasonably to minimize heat loss.
Sintering is the final process of ceramic body molding. A large part of the performance of ceramic products is determined by sintering. For zirconia ceramics to be sintered with high density and uniformity, not only is the degreasing step of the previous processing procedure essential, but it is also affected by factors such as powder, additives, sintering temperature and time, pressure and sintering atmosphere.
What is the sintering method of zirconia ceramics?
2020 06/02