What is the production process of alumina wear-resistant ceramics?

At present, it is divided into two types: high purity type and ordinary type. High purity alumina ceramic series Al2O3 content of more than 99.9% of the ceramic material, due to its sintering temperature as high as 1650-1990 ℃, transmission wavelength of 1 ~ 6μm, generally made of molten glass to replace the platinum crucible: the use of its light transmission and alkali metal corrosion resistance as sodium lamp tube; It can be used as integrated circuit board and high frequency insulation material in the electronics industry.

Ordinary alumina ceramics are divided into 99 porcelain, 95 porcelain, 90 porcelain, 85 porcelain and other varieties according to the Al2O3 content, and sometimes the Al2O3 content is also classified as ordinary alumina ceramics series. Among them, 99 alumina porcelain materials are used to make high temperature crucible, refractory furnace pipe and special wear-resistant materials, such as ceramic bearings, ceramic seals and water valves; 95 alumina porcelain is mainly used as corrosion resistance and wear resistance parts; The 85 porcelain is often mixed with talc to improve the electrical properties and mechanical strength, and can be sealed with molybdenum, niobium, tantalum and other metals, and some are used as electric vacuum devices. The production process is as follows:

1. Powder preparation

The aluminum oxide powder in the factory is prepared into powder materials according to different product requirements and different molding processes. The particle size of the powder is less than 1μm micron, if the manufacture of high-purity alumina ceramic products in addition to the purity of alumina in 99.99%, but also need to ultrafine grinding and make its particle size distribution uniform. When the wear-resistant plate is extruded or injection molded, the binder and plasticizer need to be introduced into the powder, generally thermoplastic or resin with a weight ratio of 10-30%, and the organic binder should be evenly mixed with the alumina powder at 150-200℃ to facilitate the molding operation.

The powder material formed by hot pressing process does not need to add binder. If semi-automatic or automatic dry pressing molding is used, the powder has special process requirements, and it is necessary to use spray granulation method to treat the powder and make it appear spherical, so as to improve the fluidity of the powder and facilitate the automatic filling of the mold wall in the molding. In addition, in order to reduce the friction between the powder and the mold wall, it is necessary to add 1 to 2% of the lubricant, such as stearic acid and binder PVA. To dry press molding, powder spray granulation is required, in which polyvinyl alcohol is introduced as a binder.

2. Molding method

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

◆ Grouting molding method: grouting molding is the earliest molding method used for alumina ceramics. Due to the use of gypsum mold, the cost is low and it is easy to form large size and complex shape parts. The key of grouting molding is the preparation of alumina slurry. Usually water is used as the flux medium, and then the degumming agent and the binder are added, after full grinding, the gas is discharged, and then the plaster mold is poured backwards. Due to the absorption of water by the capillaries of the plaster mold, the slurry is solidified in the mold. When the wear-resistant plate is hollow grouting, the excess slurry should be poured out when the slurry adsorbed on the mold wall reaches the required thickness. In order to reduce the shrinkage of the body, high concentration slurry should be used as far as possible.

◆ Dry press molding: alumina ceramic dry press molding technology is limited to simple shape and inner wall thickness of more than 1mm, length to diameter ratio is not greater than 4:1 objects. The molding methods are uniaxial or bidirectional. The press has two kinds of hydraulic type and mechanical type, which can be semi-automatic or automatic forming method. The maximum pressure of the press is 200Mpa. The output can reach 15 ~ 50 pieces per minute. Because of the uniform stroke pressure of the hydraulic press, the height of the pressing parts is different when the powder filling is different. The size of the pressure applied by the mechanical press varies due to the amount of powder filling, which is easy to lead to the difference in the size shrinkage after sintering and affect the quality of the product.

Therefore, the uniform distribution of powder particles in the dry pressing process is very important for mold filling. Whether the filling amount is accurate or not has a great influence on the dimensional accuracy control of manufactured alumina ceramic parts. Powder particles with more than 60μm, between 60 ~ 200 mesh can obtain the maximum free flow effect, to achieve the best pressure molding effect.

Organic additives should also be added to the alumina ceramic slurry to form a double electric layer on the surface of the slurry particles so that the slurry is stably suspended without settling. In addition, binders such as vinyl alcohol, methyl cellulose, alginate amine and dispersants such as polyacrylamide and gum Arabic are added to make the slurry suitable for grouting molding operation.

3. Firing technology

The technical method of densifying the granular ceramic body and forming a solid material is called sintering. The method of sintering is to remove the void between the particles in the body of the billet, remove a small amount of gas and impurity organic matter, and make the particles grow and combine with each other to form new substances.

The heating device used for firing is the most widely used electric furnace. In addition to atmospheric pressure sintering, that is, non-pressure sintering, there are hot pressure sintering and hot isostatic pressure sintering. Although continuous hot pressing sintering increases the output, the equipment and mold costs are too high, and the length of the product is limited due to axial heating. Hot isostatic pressing sintering uses high temperature and high pressure gas as the pressure transfer medium, has the advantage of uniform heating in all directions, and is suitable for sintering of complex shaped products. Due to the uniform structure, the properties of the material are 30 ~ 50% higher than that of cold pressing sintering. It is 10 ~ 15% higher than the general hot pressing sintering.

Therefore, at present, some high value-added alumina ceramic products or special parts needed for national defense, such as ceramic bearings, mirrors, nuclear fuel and gun barrels and other products, the field uses hot isostatic pressing sintering method.

In addition, there are microwave sintering method, arc plasma sintering method, self-spreading sintering technology.

4, finishing and packaging processes

Some materials need to be finished after sintering. Products that can be used as artificial bones require a high surface finish, such as a mirror, to increase lubricity. Due to the high hardness of alumina ceramic materials, wear-resistant plates need to be finished with harder grinding and polishing brick materials. Such as SiC, B4C or diamond.

It is usually used from coarse to fine abrasive grinding step by step, the final surface polishing. Generally can be used <1μm micron Al2O3 powder or diamond paste 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.