In order to sintering zirconia ceramics with high density and high uniformity, not only the debonding step of the previous process is very important, but also the effects of powder, additive, sintering temperature and time, pressure and sintering atmosphere should be considered.
Before sintering, the ceramic blank consists of many single solid particles. There are many 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 specific value depends on the characteristics and methods of the powder and molded products and the technology used.
When the solid billet is heated at the high temperature, the particles in the billet migrate, and when it reaches a certain temperature, the billet shrinks, the particles grow and the pores are eliminated. Finally, the green body becomes a dense polycrystalline ceramic material at a temperature below the melting point (usually 0.5-0.7 times the melting point), a process called sintering.
Sintering is the last process of zirconia ceramic body forming, and the performance of ceramic products depends on sintering to a large extent.
In order to sintering zirconia ceramics with high density and high uniformity, not only the debonding step of the previous process is very important, but also the effects of powder, additive, sintering temperature and time, pressure and sintering atmosphere should be considered.
In the specific sintering process, the zirconia ceramic body may have some problems, such as deformation, cracking, abnormal grain growth and so on. Why do these problems arise?
1. deformation.
The deformation of zirconia ceramics during sintering may be due to the wide particle size distribution of the powder, unreasonable selection and dosage of additives in the powder, inconsistent shrinkage of ceramics, and so on.
There are three reasons for the inconsistent shrinkage of ceramics:
a. The temperature of the furnace is not uniform and the shrinkage of the ceramic body is not uniform.
b. The temperature rises rapidly and the temperature conduction produces a gradient. The more rapidly the ceramic shrinks near the surface, the more slowly it shrinks at the center.
c. There is a density gradient, which leads to the inconsistency of green body shrinkage due to pressure, filler, and other factors.
2. Rupture
The main reason for the fracture of the sintered ceramic body is the internal defect of the ceramic body, which is also related to the shrinkage of the ceramic body. For the reasons for the inconsistency of green body shrinkage, see the analysis of sintering deformation.
If there are defects (holes, dark cracks, etc.) when the shrinkage is inconsistent.). The defect becomes the source of fracture, and the crack diffusion leads to the cracking of the green body.
3. Abnormal grain growth
When the grains grow abnormally, there are usually a large number of pores in these oversized grains, which are difficult to discharge from the grains to the grain boundaries, which will make it difficult for zirconia ceramics to obtain higher density and worsen many properties of the materials. especially the mechanical properties (fracture toughness, bending strength, etc.).
There are three main reasons for the abnormal growth of crystal grains:
a. The particle size distribution of the original powder is too wide, that is, the largest particle in the powder is more than or much more than twice the average particle size.
b. The green body density is not uniform in the forming process (powder agglomeration, green body pressing pressure gradient and additive is not uniform), and the densification is not uniform in the sintering process.
c. The sintering temperature is too high and the holding time is too long.
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