Brief introduction of alumina ceramic production process

Among them, 99 alumina porcelain material is used to make high temperature crucible, furnace tube and special wear-resistant materials, such as ceramic bearings, ceramic seals and water valves, etc. 95 alumina porcelain is mainly used as corrosion-resistant and wear-resistant parts; The electrical properties and mechanical strength of 85 porcelain are improved due to the addition of talc, which can be sealed with molybdenum, niobium, tantalum and other metals, and some of them are used as electric vacuum devices. Its production process is as follows:

One, powder preparation:

The alumina powder is prepared into powder material according to different product requirements and different molding processes. Powder size in 1μm? Micron? Below, if the manufacture of high purity alumina ceramic products in addition to alumina purity in 99.99%, but also need to ultrafine grinding and make its particle size distribution uniform. When extrusion or injection molding is used, binder and plastic agent should be introduced into the powder. Thermoplastic or resin with a weight ratio of 10-30%? The organic binder should be mixed with alumina powder at 150-200 ℃ to facilitate the molding operation. The powder materials formed by hot pressing process do not need to add binder. If the use of semi-automatic or automatic dry pressing molding, there are special technical requirements for the powder, the need to use spray granulation method to treat the powder, make it appear spherical, in order to improve the fluidity of the powder to facilitate automatic filling mold wall molding. In addition, in order to reduce the friction between the powder and the die wall, it is necessary to add 1 ~ 2% lubricant. Like stearic acid? And binder PVA.

In order to dry pressing molding, spray granulation of powder is required, in which polyvinyl alcohol is introduced as binder. In recent years, a research institute in Shanghai has developed a water-soluble paraffin as a binder for spray granulation of Al2O3, which has good fluidity under heating. The powder after spray granulation must have good fluidity, loose density and friction temperature of flow Angle less than 30℃. In order to obtain higher green density, particle size ratio is ideal.

Two, forming method:

The molding methods of alumina ceramic products include dry pressing, grouting, extrusion, cold isostatic pressing, injection, flow casting, hot pressing and hot isostatic pressing. In recent years at home and abroad also developed pressure filter molding, direct solidification injection molding, gel injection molding, centrifugal injection molding and solid free molding molding technology methods. Different shapes, sizes, complex shapes and precision of products require different molding methods. Abstract its common molding introduction:

1, dry pressing molding: alumina ceramic dry pressing molding technology is limited to simple shape and wall thickness of more than 1mm, length and diameter ratio is not greater than 4∶1 objects. The forming method has uniaxial or bidirectional. The press has hydraulic, mechanical two kinds, can be semi-automatic or automatic molding. The maximum pressure of the press is 200Mpa. The output can reach 15 ~ 50 pieces per minute. Due to the uniform stroke pressure of hydraulic press, the height of pressing parts is different when the powder filling is different. However, the pressure applied by mechanical press varies with the amount of powder filling, which will easily lead to the difference in size shrinkage after sintering and affect the quality of products. Therefore, the uniform distribution of powder particles in dry pressing process is very important for mold filling. Whether the filling quantity is accurate or not has great influence on the dimensional precision control of manufactured alumina ceramic parts. The maximum free flow effect can be obtained when the powder particles are larger than 60μm and between 60 ~ 200 mesh, and the best pressure forming effect can be obtained.

2, grouting molding method: grouting molding is the earliest use of alumina ceramics molding method. Due to the use of gypsum mold, low cost and easy to shape large size, complex appearance of the parts. The key of grouting molding is the preparation of alumina slurry. Usually with water as the flux medium, and then add the glue dissolving agent and binder, fully after grinding exhaust, and then poured into the plaster mold. Due to the adsorption of water by the capillary of the gypsum mold, the slurry is solidified in the mold. Hollow grouting, in the mold wall adsorption slurry thickness up to the required, but also need to pour out the excess slurry. In order to reduce the shrinkage of the body, high concentration slurry should be used as far as possible.

Organic additives should be added to the alumina ceramic slurry to form a double electric layer on the surface of slurry particles so that the slurry can be suspended stably without precipitation. In addition, it is necessary to add vinyl alcohol, methyl cellulose, alginate amine and other binder and polypropylene amine, Arabic gum and other dispersants, the purpose is to make the slurry suitable for grouting molding operation.

Three, firing technology:

The technical method of densifying granular ceramic body and forming solid material is called sintering. Sintering is the method of removing the void between the particles in the body of the billet, removing a small amount of gas and impurities from the organic matter, so that the particles grow together and form new substances.

The heating device used for firing is the most widely used electric furnace. In addition to atmospheric sintering? That is, press-free sintering? Outside, there are hot pressing and hot isostatic pressing sintering. Continuous hot pressing increases production, but the cost of equipment and molds is too high, and the length of products is limited because of axial heating. Hot isostatic pressure sintering adopts high temperature and high pressure gas as pressure transfer medium, which has the advantage of uniform heating in all directions and is suitable for sintering of complex products. Because of the uniform structure, the properties of the material are increased by 30 ~ 50% compared with cold pressing sintering. 10 ~ 15% higher than ordinary hot pressing sintering. Therefore, at present, some high value-added alumina ceramic products or special parts needed by the national defense military industry, such as ceramic bearings, mirrors, nuclear fuel and gun barrel products, field using hot isostatic pressure firing method.

In addition, microwave sintering, arc plasma sintering and self-propagating sintering are also under development.

Four, finishing and packaging process:

Some alumina ceramic materials need to be finished after sintering. Such as can be used as artificial bone products require a high surface finish, such as mirror, to increase lubricity. Because of the higher hardness of alumina ceramic material, it is necessary to use harder grinding and polishing brick material for finishing. Such as SiC, B4C or Diamond, etc. The final surface polishing is usually performed by progressive grinding from coarse to fine abrasives. Generally, < 1μm? Micron? Al2O3 powder or diamond paste for grinding and polishing. In addition, laser processing and ultrasonic processing grinding and polishing methods can also be used.

Some alumina ceramic parts need to be packaged with other materials.