Detailed explanation of the three stages of solid phase sintering of zirconia ceramics

The sintering method of zirconia ceramics generally adopts atmospheric pressure sintering, and there is another sintering method---solid phase sintering, which has three stages, because sintering is an important stage before ceramic processing, so the following Kezhong Ceramics The factory will bring you a detailed explanation of these three stages.

Zirconia ceramic products will undergo many changes during the solid phase sintering process. Figure 3-13 shows the changes in the solid phase sintering process. Among these changes, the most important are:

① Dimensional shrinkage and density increase;

② Significant increase in strength;

③ Alloying;

④ grain growth and so on.

Solid phase sintering is roughly divided into three stages:

The first stage is the bonding process. The original contact points or faces between particles are transformed into crystal bonds, that is, sintered necks are formed through processes such as nucleation and grain growth. At this stage, the grains in the particles do not change, the shape of the particles basically remains unchanged, the entire sintering does not shrink, and the density increase is very small, but the strength and conductivity of the sintered body are due to the increase in the contact surface of the particles. There is a noticeable increase.

The second stage is the sintering neck growth stage. In this stage, the sintered body expands, the distance between particles shrinks, a continuous closed-pore network is formed, a large number of pore necks disappear, the sintered body shrinks, and the density and strength increase significantly.

In the third stage, the pores are spheroidized and shrunk, and the entire sintered body may still shrink slowly, which is mainly achieved by the disappearance of pores and the reduction of the number of pores, but a small amount of closed pores still remains.

During the sintering process, as the grain grows, the driving force of sintering is weakened, so the sintering speed decreases. Coarse grains are prone to defects at the grain boundaries, which can easily reduce the strength, so it is not desirable for general structural materials. Therefore, how to suppress the excessive growth of crystal grains and make the density close to the theoretical value has become the key technology.

No matter in theory or through experimental observation, it can be found that the particles with small average particle size shrink and become smaller, while the large particles merge and become larger. There is a free energy difference on both sides of the curved grain boundary around the large particle, which is the driving force for the grain boundary migration. Therefore, the grain boundaries migrate toward the center of curvature and the particles grow. Some theories point out that the grain boundary migration speed increases exponentially with the increase of temperature, and is proportional to the curvature of the grain boundary at a certain temperature.

However, in the case of many oxides, even if the raw material purity is very high, there is a considerable width of lattice defects and space charge layers near the grain boundaries. Due to the segregation of impurities to the grain boundaries, in practically applied ceramic materials, if the grain boundary rate decreases to a certain extent, the grain boundary migration will slow down sharply.

Website: https://www.zhengtaifeng.com/