Density is a core indicator for measuring the quality of wear-resistant ceramics, and its calculation formula is: Density = Mass ÷ Volume. In the high-temperature sintering process of wear-resistant ceramics, the mass of the green body only changes slightly due to the volatilization of a small amount of water and impurities, while the volume shrinkage rate can reach over 40%. This characteristic of "slight mass change and sharp volume reduction" directly drives a significant increase in the density of wear-resistant ceramics. Therefore, volume shrinkage is a key factor driving the increase in density of wear-resistant ceramics. So, why does wear-resistant ceramic exhibit such significant volume shrinkage during the sintering stage? The specific reasons can be summarized as follows:
Pore Elimination and Gas Escape
The main raw material of wear-resistant ceramics is alumina powder. After the powder is formed into a green body through dry pressing, slip casting, and other molding processes, it is filled with a large number of pores – including open pores formed by particle accumulation and closed pores enclosed by particles. At the same time, the surface of the powder particles also adsorbs gases such as air and water vapor. When the sintering temperature rises to the high-temperature range of 1600℃, the pores inside the green body expand due to heat. The originally isolated closed pores gradually connect to form pore channels; as the temperature continues to rise, the gases quickly escape along the channels, and a large number of pores are gradually eliminated. Meanwhile, the alumina particles, without the support of the pores, constantly move closer and pack tightly under the driving force of surface energy, directly leading to a significant shrinkage in the volume of the green body, laying the foundation for increased density.
Water Evaporation and Impurity Decomposition
Even with high-purity raw materials, trace amounts of water and impurities will remain in the powder, although the impurity content is far lower than that of ordinary raw materials. During the sintering heating process, the free water in the green body is the first to evaporate; as the temperature further increases, the trace impurities such as carbonates and sulfates in the powder undergo decomposition reactions, converting into gases such as carbon dioxide and sulfur dioxide, which are then expelled from the green body. Water evaporation and impurity decomposition not only reduce the "ineffective space" inside the green body but also allow the alumina particles to overcome the obstruction of impurities, resulting in a tighter bond between them, thus further exacerbating volume shrinkage.
Particle Rearrangement and Structural Densification
When the sintering temperature reaches the sintering activity range of the alumina powder, the atomic kinetic energy of the particles significantly increases, and their fluidity is enhanced. In some localized areas of the green body, a small amount of liquid phase is formed due to the action of sintering aids. Driven by both surface energy and capillary forces, the alumina particles migrate, slide, and rearrange, spontaneously filling the pores created by gas escape, water evaporation, and impurity decomposition. Simultaneously, the contact between particles gradually changes from point contact after molding to surface contact, the crystal structure is continuously optimized, and the grains begin to grow, forming a continuous grain boundary network. This process not only serves as the core driving force for volume shrinkage but also significantly increases the density of the wear-resistant ceramic green body, ultimately resulting in the finished product having excellent hardness and wear resistance.
In summary, during the sintering process of wear-resistant ceramics, although gas escape, water evaporation, and impurity decomposition may lead to a slight decrease in the mass of the green body, their impact is almost negligible compared to the volume shrinkage of up to 40%. It is this significant volume shrinkage that enables a dramatic increase in the density of wear-resistant ceramics. Therefore, density is not only an important indicator for measuring the quality of wear-resistant ceramic products but also a core basis for determining whether the sintering degree meets the standards and whether the internal structure is dense.
Your message must be between 20-3,000 characters!
