The sintering process of zirconia ceramics is a complex and delicate process that determines the performance and quality of the final product.
Initial sintering, mid-sintering and late sintering. In the early sintering, the ceramic particles begin to bond and form sintering necks, but the shape of the particles and the internal grains do not change significantly. As the sintering enters the middle stage, the sintering necks gradually grow, the distance between the particles continues to shrink, the grain size increases, and the density and strength are improved. In the late sintering, the pores between the particles continue to spheroidize and shrink, forming closed pores, and the materials on the grain boundaries continue to migrate to the pores, which promotes the continued growth of the grains, reduces the porosity, and improves the density.
The particle size of the original powder has a significant effect on the sintering process of zirconia ceramics. Fine particles increase the driving force of sintering, shorten the atomic diffusion distance, and increase the solubility of particles in the liquid phase, thereby accelerating the sintering process. However, if the original powder particles are severely agglomerated and have poor compactness, it will lead to uneven density and composition of the various parts of the green blank, and then produce uneven densification during sintering.
In the sintering process of zirconia ceramics, additives (also known as sintering aids) play a vital role. In solid-phase sintering, a small amount of additives can form a solid solution with the main crystal phase, promote the increase of defects, and thus accelerate sintering. In liquid-phase sintering, additives can change the properties of the liquid phase (such as viscosity, composition, etc.), further promoting sintering. However, the selection and addition of additives need to be reasonably controlled, otherwise it may lead to abnormal grain growth or deterioration of sintered body performance.
Sintering temperature and holding time are two key factors affecting the sintering quality of zirconia ceramics. The sintering temperature depends on the bonding of the crystals. For crystals with strong ion bonding, a higher sintering temperature is required. The holding time determines the degree of sufficient migration and diffusion of material particles. With the increase of ambient temperature and the extension of holding time, the migration and diffusion of material particles are more sufficient, the green body continues to shrink, and the volume density continues to increase. However, too high a sintering temperature and too long a holding time may lead to over-burning phenomena such as coarse grains and decreased volume density.
Molding pressure also has a certain influence on the sintering process of zirconia ceramics. During the sintering process, if the migration direction of particles or vacancy defects is appropriate, the greater the speed and efficiency, the more conducive to sintering. Molding pressure reduces the diffusion resistance during sintering by increasing the close contact conditions between particles. Generally, the greater the molding pressure, the closer the contact between particles, and the more favorable it is for sintering.
The sintering atmosphere also has an important influence on the sintering process of zirconia ceramics. Different sintering atmospheres may cause changes in the properties of ceramic materials. In addition, degreasing is also one of the important steps before sintering. Incomplete degreasing may cause organic matter and other impurities in the ceramic body, affecting the sintering quality and final performance.
During the sintering process of zirconia ceramics, problems such as cracking, deformation, and abnormal grain growth may occur. These problems are usually related to factors such as internal defects of the body, inconsistent shrinkage, too wide distribution of powder particle size, and improper selection of additives. To solve these problems, measures such as optimizing the powder processing process, reasonably selecting additives, accurately controlling the sintering temperature and holding time, and increasing the molding pressure can be taken.
The sintering process of zirconia ceramics is a complex process affected by many factors. By precisely controlling these factors, the sintering process can be optimized and the performance and quality of zirconia ceramics can be improved.
