Before sintering, the green ceramic body consists of many individual solid particles. There are many pores in the green body, and the porosity is generally 35% to 60% (that is, the relative density of the green body is 40% to 65% ). The exact value depends on the powder and molding characterization method and the technology used. When the solid green body is heated at high temperature, the particles in the green body migrate, and when it reaches a certain temperature, the green body shrinks, the particles grow, the pores are eliminated, and the green body is below the melting point (usually 0.5~0.7 times the melting point) ) into a dense polycrystalline ceramic material. This process is called sintering.

Sintering is the last process of forming the zirconia ceramic body, and the performance of ceramic products depends to a large extent on sintering. In order to sinter high-density, high-uniformity zirconia ceramics, in addition to the very important degreasing step in the previous process, the influence of powder, additives, sintering temperature and time, pressure and sintering atmosphere should also be considered.

During the specific sintering process, the zirconia ceramic body may have problems such as deformation, cracking, abnormal grain growth and so on. What are the possible reasons? Let's analyze it according to the specific problem.

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, etc.

There are two reasons for the inconsistent shrinkage of ceramics: the furnace temperature is not uniform, and the shrinkage of the ceramic body is inconsistent; the temperature rises rapidly, and the temperature conduction produces a gradient. The more rapidly the ceramic body shrinks near the surface layer, and the more slowly it shrinks in the center, there is a density gradient. Due to factors such as pressure and filler, the shrinkage rate of the green body is inconsistent.

2. Rupture

The main reason for the fracture of the sintered ceramic body is the internal defects of the ceramic body, which is also related to the shrinkage of the body. See Sintering Deformation Analysis for the reasons for the inconsistent shrinkage of the green body. When the shrinkage is inconsistent, if there are defects (holes, dark cracks, etc.), the defects become the source of fracture, and the crack propagation causes the green body to crack.

3. Abnormal growth of grains

When the grains grow abnormally, these oversized grains usually have a large number of pores, which are difficult to discharge from the grains to the grain boundaries, which will make it difficult for the zirconia ceramic material to obtain higher density and make the material's Many properties deteriorate, especially mechanical properties (fracture toughness, flexural strength, etc.).

There are three main reasons for abnormal grain growth:

The particle size distribution range of the original powder is too wide, that is, the larger particles in the powder are larger or much larger than twice the average particle size; the density of the green body is not uniform during the molding process (powder agglomeration, green body pressing pressure gradient and additives are not uniform), The densification is uneven during the sintering process; the sintering temperature is too high, and the holding time is too long.

The above problems are all likely to occur, so in the process of sintering zirconia ceramics, we need to pay attention to the above three problems.