Before sintering, the ceramic green body is composed of many individual solid particles. There are a large number 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 specific value depends on the characteristics of the powder itself and the molding method and technology used. When the solid green body is heated at high temperature, the particles in the green body migrate. After reaching a certain temperature, the green body shrinks, the grains grow, and the pores are eliminated. Finally, at a temperature below the melting point (generally 0.5~0.7 times the melting point), the green body becomes a dense polycrystalline ceramic material. This process is called sintering.
Sintering is the last process of ceramic green body molding. The performance of ceramic products is largely determined by sintering. Zirconia ceramics must be sintered with high density and uniformity. Not only is the degreasing link in the previous processing step crucial, but it is 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 deform, crack, and have abnormal grain growth. What are the possible reasons? Let’s analyze it according to specific problems.
Deformation
Zirconium oxide ceramics deform during the sintering process. The reasons may be that the powder particle size distribution is too wide; the selection and addition of additives in the powder are unreasonable; the shrinkage of the ceramic is inconsistent, etc.
If the shrinkage of the ceramic is inconsistent, there are generally three aspects:
1 The furnace temperature is uneven, and the ceramic body shrinks inconsistently;
2 The temperature rises too fast, and the temperature conduction produces a gradient. The closer the ceramic body is to the surface, the faster it shrinks, and the slower it shrinks in the center.
3 There is a density gradient. During molding, due to factors such as pressure and filler, the shrinkage ratio inside the body is inconsistent.
Fracture
The main reason for the rupture of the ceramic body after sintering is that there are defects inside the ceramic body, which is also related to the shrinkage of the body. The reasons for the inconsistent shrinkage of the body refer to the analysis of the causes of sintering deformation.
Abnormal grain growth
When the grains grow abnormally, there are often a large number of pores in these oversized grains that are difficult to reach the grain boundary and be discharged from the grains, which will make it difficult for the ceramic material to reach a higher density, and many properties of the material will deteriorate, especially the mechanical properties (fracture toughness, bending strength, etc.).
The main reasons for abnormal grain growth are as follows:
1. The distribution range of the original powder particle size is too wide, that is, the largest particle in the powder is larger than or much larger than 2 times the average particle grain size;
2. The density of the green body is uneven during molding (powder agglomeration, green body pressing pressure gradient, uneven additives), and uneven densification occurs during sintering;
3. Excessively high sintering temperature and too long insulation time.
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