Forming Process and Sintering Method of Alumina Ceramics
Molding is one of the important processes that directly affect the sintering process and the properties of sintered body. Relevant experiments have verified that at the same sintering temperature, if the relative density of blank is higher, the relative density of sintered body is also higher.
Therefore, in order to ensure that the density is higher, the general molding pressure is larger. At present, high performance alumina ceramic molding methods are divided into dry and wet two types.
1. Dry molding
In dry molding, we will first think of cold isostatic pressing. The isostatic pressing method is to pack alumina powder into an elastic mold to seal, and then put the mold into a container of high pressure gas or liquid. After sealing, the powder is pressed into a green body with higher density by utilizing the characteristics of three-dimensional equal force.
In order to further improve the density of the forming body, the high pressure forming and ultra high pressure forming are developed on the basis of the ordinary isostatic forming. By using these two forming methods, relatively high compactness ceramic billets have been prepared.
2. Wet Molding
For wet forming, it has developed rapidly in recent years because it can control the agglomeration and impurity content of particles in the green body, reduce the defects of the green body, and form ceramic parts with complex shapes.
For example, centrifugal grouting is to uniformly disperse the powder in the liquid by adjusting the pH value and other means, and then high-speed centrifugation to settle the particles to obtain a green body; there is also direct gel molding that enables the powder preparation and molding process to be completed in one go; The organic monomer is polymerized to complete the gel casting molding of the green body and the infiltration curing method that relies on chemical potential molding.
These methods have been well used in the synthesis of certain nanomaterials.
The quality of sintering directly affects the microstructure and a series of mechanical properties of ceramics.
However, using the traditional sintering method, it is difficult to suppress the growth of the grains, and the excessive growth of the grain size may have a negative impact on the performance of the ceramic.
Therefore, some special sintering methods must be adopted, such as pressureless sintering and pressure sintering. Pressureless sintering is further divided into reaction sintering and atmosphere sintering, and pressure sintering is further divided into hot pressing, hot isostatic pressing, ultra-high pressure sintering and so on.
1. Hot Pressing Sintering
Hot pressing sintering starts from the driving force of sintering, and the combined action of the external pressure (10~40MPa) and the surface energy of the powder promotes the plastic flow and rearrangement of the green particles, so that the pores in the green body are reduced, the green body shrinks, and the density is increased. The process of increasing the bonding strength between particles and increasing the mechanical strength.
Compared with the common sintering method (pressureless sintering), the alumina ceramics prepared by this method have higher density and less grain growth, and can obtain fine-grained alumina ceramic materials. common sintering method.
2. Vacuum Sintering
Vacuum sintering the samples are sintered under low pressure by evacuating the vacuum furnace.
Compared with ordinary solid-phase sintering, vacuum pressureless sintering can not only suppress the growth of grain size but also effectively discharge the gas in the green body under a longer holding time, and the obtained grain size is more uniform.
3. Hot Isostatic Pressing Sintering (HIP)
Hot isostatic pressing sintering is essentially a special hot pressing sintering method.
It refers to placing the sintered body in a gas medium under high temperature conditions, so that it is subjected to equal pressure in all directions, thereby promoting the densification process of ceramic materials.
Hot isostatic pressing is an advanced material densification process, which has the characteristics of low sintering temperature, short sintering time and uniform shrinkage of the green body. High-performance and complex-shaped ceramic parts with uniform microstructure and almost no pores can be prepared.
4. Atmosphere Sintering Method
Atmosphere sintering mostly adopts inert gas atmosphere. When sintering alumina ceramics, a higher concentration of oxygen defects will be formed inside the green body, thereby promoting the diffusion of atoms, increasing the density of the sintered body, and reducing the sintering temperature.
5. Microwave Sintering
Microwave sintering has the advantages of high energy efficiency, no pollution, rapid overall heating, low sintering temperature, and uniform material microstructure.
The overall heating method of microwave can make the ceramic body reach the sintering temperature quickly. The microwave radiation causes the crystal grain to vibrate with the frequency of the electromagnetic wave. Greatly improve the sintering efficiency.
6. Spark Plasma Sintering
Spark plasma sintering has the characteristics of hot pressing sintering, and at the same time introduces the effect of electric field, so the driving force of sintering comes not only from high temperature and external mechanical force, but also from electric field and intergranular plasma, so it can greatly promote sintering.
This method can effectively reduce the sintering temperature, greatly shorten the sintering time, and reduce the grain size in the sintered body.
