keywords： Alumina ceramic Zirconia ceramics
Before sintering, ceramic green body is composed of many single solid particles. There are a lot of pores in the green body. The porosity is generally 35% - 60% (i.e. the relative density of green body is 40% - 65%). The specific value depends on the characteristics of powder itself and the molding method and technology used. When the solid billets are heated at high temperature, the particles in the billets migrate. When the solid billets reach a certain temperature, the billets shrink, and the grains grow up. With the elimination of pores, the billets become dense polycrystalline ceramic materials at a temperature lower than the melting point (generally 0.5-0.7 times of the melting point). This process is called sintering.
Sintering is the last process of ceramic body forming. A large part of the performance of ceramic products is determined by sintering. In order to obtain high density and uniformity in the sintering process, it is not only important to degrease the previous process, but also affected by factors such as powder, additive, sintering temperature and time, pressure and sintering atmosphere.
In the process of sintering, some problems such as deformation, cracking and abnormal grain growth may occur in the green body of zirconia ceramics? Let's make a specific analysis according to the specific problems.
The deformation of zirconia ceramics in the sintering process may be caused by the wide particle size distribution of the powder, the unreasonable selection and addition of additives in the powder, and the inconsistent shrinkage of the ceramics.
There are three reasons for the inconsistent shrinkage of ceramics:
① The furnace temperature is not uniform, and the ceramic body shrinks unevenly;
② The faster the temperature rise, the gradient of temperature conduction, the faster the ceramic body shrinks near the surface and the slower the center shrinks.
③ There is a density gradient. During the forming process, because of the pressure and packing, the shrinkage ratio of the blank body is inconsistent.
The main reason for the fracture of the sintered ceramic body is that there are defects inside the ceramic body, which is also related to the shrinkage of the body. For the reasons of the inconsistent shrinkage of the body, please refer to the analysis of the causes of sintering deformation.
When the shrinkage is inconsistent, if there are defects (holes, dark cracks, etc.), the defects become the source of fracture, and the crack diffusion leads to the billet cracking.
3、 Abnormal grain growth
When the grains grow abnormally, there are often a lot of pores in these oversized grains that are difficult to be discharged from the grains to the grain boundary, which will make the ceramic materials difficult to reach a higher density, and many properties of the materials deteriorate, especially the mechanical properties (fracture toughness, bending strength, etc.).
The main reasons for abnormal grain growth are as follows:
① The particle size distribution range of the original powder is too wide, that is, the maximum particle size in the powder is greater than or far greater than 2 times of the average particle size;
② In the process of forming, the density of green body is not uniform (powder agglomeration, pressing pressure gradient of green body, additive unevenness), and sintering takes place unevenly densification;
③ High sintering temperature and long holding time.