Analysis on the problems in the firing of zirconia ceramic

The ceramic is composed of many individual solid particles before sintering. There are a large number of pores. The porosity is generally 35%~60% (that is, the relative density of the ceramic is 40%~65%). The value depends on the characteristics of the powder, the molding method and technology. When the solid billet is heated in high temperature, the particles in the billet will migrate. After reaching a certain temperature, the billet will shrink, and crystal grains will grow, accompanied by the elimination of pores, and finally at a temperature below the melting point (generally 0.5 to 0.7 times the melting point) the ceramic becomes a dense polycrystalline ceramic material. This process is called sintering.

Sintering is the last process of ceramic forming. A large part of the performance of ceramic products is determined by sintering. Zirconia ceramic is sintered with high density and uniformity. Not only is the degreasing step of the previous processing procedure essential, but also affected by factors such as powder, additives, sintering temperature and time, pressure and sintering atmosphere.

During the specific sintering process, the zirconia ceramic body may have problems such as deformation, cracking, and abnormal growth of crystal grains. What are the possible reasons? Let's analyze the specific problems in detail below.

1. Deformation:

The deformation of zirconia ceramics during the sintering process may be due to the excessively 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 inconsistency of ceramic shrinkage:

①The furnace temperature is not uniform, and the ceramic body shrinks inconsistently;

②The heating speed is fast, and the temperature conduction produces a gradient. The closer the ceramic body is to the surface, the faster the shrinkage, and the closer the center is, the slower the shrinkage.

③There is a density gradient. During molding, the internal shrinkage ratio of the blank is inconsistent due to factors such as pressure and filler.

2: rupture

The main reason for the cracking of the sintered ceramic body is that there are defects in the ceramic body, and it is also related to the shrinkage of the green body. The reason for the inconsistent shrinkage of the green body is referred to the analysis of the reasons for 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 propagation causes the green body to crack.

3. Abnormal growth of grains

When the grains grow abnormally, there are often a large number of pores in these excessively large grains, which are difficult to be discharged from the grains to the grain boundaries, which will make it difficult for the ceramic material to achieve higher density and many properties of the material. Deterioration, especially mechanical properties (fracture toughness, bending strength, etc.).

The main reasons for the abnormal growth of crystal grains are as follows:

①The particle size distribution range of the original powder is too wide, that is, the largest particle in the powder is larger or much larger than 2 times the average particle size;

②The density of the green body is uneven during molding (powder agglomeration, the pressing pressure of the green body has a gradient, and the additive is uneven), and uneven densification occurs in the sintering;

③ Too high sintering temperature and too long holding time.