Whether traditional ceramics or precision structure ceramics can not be separated from sintering, it can be said that sintering is the last process of ceramic body molding, the performance of ceramic products is largely determined by sintering, how to burn high density, uniform firing, and reduce manufacturing costs? In fact, its previous process degreasing link is very important, in addition, we also need to understand the impact of several major factors: powder, additives, sintering temperature and time, pressure and sintering atmosphere.
What is ceramic sintering?
The ceramic green body is composed of many single solid particles before sintering. There are a lot of pores in the green body, and the porosity is generally 35% ~ 60% (i.e., the relative density of green body is 40% ~ 65%). The specific value depends on the characteristics of the powder itself and the molding methods and techniques used. When the solid green body is heated at high temperature, the particles in the green body migrate, and when the green body reaches a certain temperature, the green body shrinks, the grains grow up, the porosity is eliminated, and the green body finally becomes a dense polycrystalline ceramic material at a temperature lower than the melting point (usually 0.5 ~ 0.7 times the melting point). This process is called sintering.
The driving force of sintering is the decrease of the system surface energy of the powder body, and the sintering process is driven by the decrease of the total boundary area caused by the substitution of the low energy grain boundary for the high energy grain surface and the volume shrinkage of the body. The sintering mechanism for the densification of the powder body includes evaporation-condensation, lattice diffusion, grain boundary diffusion, viscous flow and other mass transfer modes.
II. Three Changes in Sintering Process
There are usually three main changes during sintering:
1. The increase of grain size and density;
2. The change of stomatal shape;
3. Changes in the size and number of pores usually result in a decrease in porosity.
For dense ceramics, the relative density can reach more than 98%, but for transparent ceramics, the porosity of sintered ceramics tends to zero.
What are the five factors that affect the sintering of ceramics?
The factors that affect ceramic sintering include pressure, atmosphere, powder, temperature, additives and so on. But the most important one is its previous process: degreasing process. Let's analyze it as follows:
Degrease link done, cover sintering on the success of more than half!
How do ceramics burn well? In response to this problem, many people have ignored a link, is its previous process: degreasing, simply by heating or other methods to separate organic matter from the ceramic body. Degreasing and sintering is a very important thermal process in the whole process, even can be said, sintering good or not, depends on whether degreasing thoroughly!
The degreasing process is very important, If the control is not good, such as uneven temperature, inadequate degreasing, degreasing time is not enough, will lead to a series of problems, including pores, micro-cracks and so on, but these problems are invisible from the degreasing furnace, must go through the sintering process, these defects will be exposed, but it is already late, because the sintering process is irreversible. Especially in mass production, the requirement of degreasing is very high.
In addition, what other factors affect the sintering of ceramics? Let's see:
1. Raw powder
Generally speaking, the higher the dispersion of the material, the higher the surface energy. Therefore, it has a strong force to promote migration and diffusion, which is beneficial to sintering. At the same time, in the process of full pulverization, the defects in the interior and surface of the particles increase, and the particle mobility is effectively improved. Whether solid or liquid sintering, fine particles increase the sintering speed because of increasing the driving force of sintering, shortening the atomic diffusion distance and improving the solubility of particles in the liquid phase. However, when the agglomeration of raw powder particles is serious and the compactness is poor, and the pressure gradient distribution and the admixture can not be mixed with the powder, the density and composition of each part of the green body will be uneven, so the green body will be uneven densified during sintering.
2. Additive
Usually the addition of admixtures will restrain the grain boundary movement of the sintered material and inhibit the grain growth, but in some cases, the results are just the opposite. For example, in solid state sintering, a small amount of additives (also called sintering aids) can form solid solution with the main crystal phase to promote the increase of defects. In liquid phase sintering, additives can change the properties of liquid phase (such as viscosity, composition, etc.), thus playing a role in promoting sintering.
3. Sintering temperature and holding time
The environmental temperature and the holding time at high temperature are the important external factors for the complete sintering of the green body. The larger the lattice energy is, the stronger the ion binding is, the more difficult the ion diffusion is, and the higher the sintering temperature is. Because of the different bonding conditions of different crystals, the sintering temperature varies greatly, and even for the same crystal, the sintering temperature is not a fixed value. With the increase of ambient temperature and holding time, the migration and diffusion of particles are sufficient, the green body is shrinking and the bulk density is increasing. Usually, the porosity, density and mechanical strength of the green body are changed,
The temperature at which it stabilizes is called the sintering temperature. The holding time at the highest temperature is called holding time. When the ambient temperature exceeds a certain quantity limit, the bulk density and mechanical strength of the green body decrease without increasing, which is called overfiring. The above temperature range is often referred to as the firing temperature rang
Burning temperature is less than, the heat preservation time will be burnt again long; If the sintering temperature is too high, or if the sintering temperature is too high, or if the holding time is too long, the grain size will be coarse and the bulk density will be decreased, which will lead to the decrease of the mechanical strength of the sintered body. Therefore, it is very important to determine the sintering temperature and holding time of the sintered body under the condition of stabilizing the blending process.
Step 4: Forming pressure
In the sintering process, if the migration direction of particle or vacancy defects is appropriate, the greater the velocity and efficiency, the more conducive to sintering. The effect of promoting particle or vacancy migration and diffusion efficiency is called migration driving force, including surface energy, ambient pressure and grain boundary free energy. Isostatic pressure sintering method can effectively promote sintering process by increasing the environmental pressure factor of sintering process. The energy consumption of sintering process is reduced, the density of sintered body is increased, and the sintered body can be qualified for sintering of green body which can not be sintered at normal temperature. Generally speaking, the higher the molding pressure, the closer the contact between particles, the more favorable for sintering.
5. Sinter atmosphere
Different sintering atmosphere will have different effects on the sintering process of ceramics, or promote sintering, or hinder sintering. For example, ZnO powder doped with a small amount of Al2O3, its green body sintered in an oxidizing atmosphere, ZnO grain growth will be inhibited, grain morphology is fine and uniform. On the other hand, ZnO grains will grow rapidly after sintering in reducing atmosphere after adding a small amount of Li2O into ZnO powder, and abnormal growth of ZnO grains may occur at last.