Sintering Mechanism of Advanced Ceramics

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1. Definition of ceramic sintering:

After the powder is formed, a green body with a certain shape is formed. At this time, the green body contains a large amount of gas (35%~60%), and the particles are in point contact.

Toothed zirconia ceramic water nozzle

2 Concepts related to ceramic sintering

(1) Sintering and sintering are processes that heat treat silicate products under certain conditions to undergo a series of physical and chemical changes, forming the expected mineral composition and microstructure, thereby achieving a fixed appearance and achieving the required performance.

(2) Both melting and sintering processes are caused by atomic thermal vibrations, but during melting, all components are transformed into a liquid phase, while during sintering, at least one component is in a solid state.

(3) The similarities between Dry media reaction and sintering are that they are both carried out below the melting point or melting temperature of the material, and at least one phase of the process is solid all the time.

3. The driving force of ceramic sintering process

After the powder material is pressed and formed, the particles are only in point contact and can be tightly combined into hard objects without chemical reactions. This process must have a driving force at play. According to modern sintering theory research, it is believed that the surface energy of powder materials is greater than the grain boundary energy of polycrystalline sintered bodies, which is the driving force of sintering. After sintering, the system reduces energy and grain boundary energy replaces surface energy, which is the reason for the stable existence of polycrystalline materials.

4. Material transfer during ceramic sintering process

In addition to having a driving force during the sintering process, there must also be a material transfer process in order to gradually fill the pores and make the green body become dense from loose. Many scholars have studied the mode and mechanism of material transfer in the sintering process, and proposed viewpoints include: ① evaporation and condensation mass transfer; ② Diffusion mass transfer; ③ Viscous flow and plastic flow mass transfer; ④ Dissolution and precipitation. In fact, the material transfer phenomenon during the sintering process is very complex and cannot be a mechanism at work. In reality, there may be several mass transfer mechanisms at work during the sintering process, but under certain conditions, a certain mechanism will dominate. When the conditions change, the dominant mechanism will also change accordingly.

(1) Evaporation condensation mass transfer

In the process of high temperature, due to the different surface curvature of solid particles, the Vapor pressure at different parts of the system must be different, so there is a mass transfer trend through the gas phase, and the particles realize mass transfer through vapor phase evaporation and re condensation to promote sintering.

(2) Diffusion mass transfer

Diffusion mass transfer refers to the mass transfer process in which particles (or vacancies) migrate with the help of concentration gradients.

(3) Flow mass transfer

The basic principle of liquid phase sintering is similar to that of solid phase sintering, where the driving force is surface energy. The sintering process is also composed of stages such as particle rearrangement, pore filling, and grain growth.

a. Viscous flow mass transfer

During liquid phase sintering, the liquid content is very high, and the mass transfer caused by the Newtonian flow of viscous liquid at high temperatures is called viscous flow mass transfer.

b. Plastic flow mass transfer

When the liquid phase content in the green body decreases and the solid phase content increases at high temperatures, the mass transfer during sintering cannot be regarded as a Newtonian fluid, but as a plastic flow fluid, and the driving force of the process is still surface energy.

(4) Dissolution precipitation mass transfer

In sintering with solid and liquid phases, the following mass transfer processes occur between the solid and liquid phases: small particles (with high solubility) and the surface of general particles

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