PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Vacuum Sintering of Fine Ceramics

With the development of science and technology, especially since the 1950s, the rapid rise and development of high-tech industries, traditional ceramics have become increasingly unable to meet the needs of chemical industry, metallurgy, energy, communications, electronics, bioengineering, military construction and New materials required for the development of aerospace and other space technology equipment – fine ceramics. Although the entire development history of fine ceramics is less than a century, the rapid development and popularization and application of fine ceramics, especially the application of monocrystalline silicon, optical fibers and superconducting materials, have greatly promoted the electronic industry, communication, energy, Development of space technology equipment such as bioengineering, national defense and military construction, and aerospace.

Fine ceramics are also called advanced ceramics, special ceramics, high-performance ceramics and high-tech ceramics. It can be divided into two categories: structural ceramics and functional ceramics. Structural ceramics refer to materials with high mechanical strength, wear resistance, corrosion resistance and high temperature stability; functional ceramics refer to materials used in special electrical, magnetic, optical, chemical and biological materials. In short, fine ceramics refer to materials composed of precisely controlled chemical structures and precisely designed microstructures with special mechanical, optical, thermal, chemical, electrical, magnetic and acoustic properties and functions. Rare materials that are used in a wide range of applications.

Advanced ceramic preparation process:

Raw material batching → wet ball milling → sieving → iron removal → spray drying and granulation → aging → molding → drying → green body processing → sintering → finishing → inspection → packaging and storage

Wet ball mill mixing:

Wet grinding is a production method in which materials are ground into a slurry by adding water. The fineness of the wet milled product is uniform, the energy consumption per unit weight of the product is low, there is no dust flying field, and the noise is low. The main function of wet grinding is to prepare powder raw materials with fixed components, and through this process, it will have a mixed slurry with a certain particle size and uniform distribution of each component.

Compression molding:

Compression forming is the main forming method of fine ceramics at present, which has the advantages of simple production process and good forming quality. Molding can be divided into dry pressing and cold isostatic pressing. The main factors affecting fine ceramic molding are the grade and proportion of ceramic powder particles, as well as fluidity. Among them, the ceramic powder obtained by the spray drying process has good fluidity and can be quickly filled into any part of the mold cavity, and it is easy to obtain a compact body with an isotropic structure.

Sintering:

After the fine ceramic body is processed by the green body, a series of physical and chemical reactions will occur under the action of high temperature, and finally its apparent porosity is close to zero, and the process of achieving complete densification is called sintering (also called sintering).

The sintering of fine ceramic products is done in a kiln. Kilns can be divided into batch kilns and continuous kilns. Continuous kilns are usually used for the sintering of low-temperature ceramic products in mass production. Fine ceramic products are usually sintered by electric heating furnace. The sintering methods mainly include: atmospheric pressure sintering, vacuum sintering, hot pressing sintering, hot isostatic pressing sintering, liquid phase sintering, reaction sintering, microwave sintering, arc plasma sintering, self-propagating sintering and other sintered. Among them, vacuum sintering is that the green body is in a vacuum state. Free sintering, widely used in the sintering of cermets and carbide ceramics.

Vacuum sintering is currently mostly batch sintering. Sintered parts heat up, sinter and cool with the furnace. The position of the product does not change during the sintering process, so the various stages of sintering can only be carried out by adjusting the heating rate, temperature and time. Therefore, it must be set according to the sintering requirements. Generally, the heating curve as shown in the figure can be used:

1.Degreasing stage

The first stage is the delubricant or forming agent stage which may also be referred to as the burn-in stage. At this stage, the temperature should be raised slowly. The decomposition temperature of the lubricant and the forming agent is mostly about 300 °C. Therefore, the temperature should be raised as slowly as possible at around 300 °C, and there should be enough time to remove the lubricant. The first stage should be kept at a certain temperature for a period of time. The purpose is: one is to fully remove the lubricant, and the other is to carry out its own redox reaction. If the sintered parts contain carbon, a carbon-oxygen reaction will occur above 700°C. The time required for the first stage depends on the amount of lubricant added to the part and the size of the part. The first stage of pre-burning should allow sufficient removal of lubricant or forming agent decomposition gases and oxygen. Whether these gases are fully eliminated can be observed by the degree of vacuum. If the degree of vacuum is stable at a certain value, it means that it has been eliminated.

2.Sintering stage

The temperature set in the sintering stage is the temperature required for sintering. Since vacuum sintering has the effect of activating sintering, the sintering temperature is 50-100°C lower than that of atmosphere sintering. If liquid phase sintering is performed, the sintering temperature should be specified at a temperature slightly higher than the melting point of the liquid phase metal. Sintering between powder particles and alloying between alloying elements will occur at this stage. At the same time, too high vacuum should not be used in this stage, because the higher the vacuum, the greater the loss of liquid metal. In order to reduce the volatilization loss of metals, certain gases, such as nitrogen, argon and hydrogen, are often filled in the sintering process.

3.Cooling stage

The cooling of vacuum sintering includes direct power-off cooling or stepwise current-reducing cooling, depending on the cooling requirements. Since it is cooled with the furnace, the cooling rate is slower than that of atmosphere sintering. Filled with protective gas, the cooling rate can be increased.

Finishing:

When fine ceramic products are sintered, shrinkage and deformation usually occur, and fine ceramic products must be finished to meet the requirements of shape position accuracy, dimensional accuracy and surface finish. Because fine ceramic products are particularly hard and wear-resistant, they must be processed with high-hardness wear-resistant tools such as cubic boron nitride and diamond. Obviously, the finishing takes a lot of time, so the cost of finishing usually accounts for about 30% of the total production cost of fine ceramic products. 30% to 40%.

Although fine ceramic products can be processed by mechanical, thermal or chemical methods, the most commonly used processing method is still mechanical grinding. The commonly used abrasives are mainly zirconia, alumina, silicon carbide, boron carbide, cubic boron nitride and For diamonds, etc., in order to ensure that the surface of the product is not damaged due to processing, it is usually necessary to use coarse to fine abrasives for gradual grinding.

Selection of sintering equipment: RVS series vacuum sintering furnaces produced by SIMUWU are suitable for vacuum sintering furnaces including boron carbide and silicon carbide ceramics. It has the advantages of good temperature uniformity, good sealing and high degree of vacuum. It is an ideal product for processing boron carbide ceramics.