Alumina ceramics are currently divided into two types: high-purity type and ordinary type. High-purity alumina ceramics are ceramic materials with an Al2O3 content of more than 99.9%. Because of its sintering temperature as high as 1650-1990°C and a transmission wavelength of 1-6μm, it is generally made into molten glass to replace platinum crucibles: use its light transmittance and It can resist the corrosion of alkali metal and be used as a sodium lamp tube; it can be used as an integrated circuit substrate and high-frequency insulating 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. Sometimes those with Al2O3 content of 80% or 75% are also classified as ordinary alumina ceramic series. Among them, 99 alumina porcelain materials are used to make high-temperature crucibles, refractory furnace tubes and special wear-resistant materials, such as ceramic bearings, ceramic seals and water valve plates, etc.; 95 alumina porcelain is mainly used for corrosion-resistant and wear-resistant parts; 85 porcelain Because some talc is often mixed in, the electrical properties and mechanical strength are improved, and it can be sealed with metals such as molybdenum, niobium, and tantalum, and some are used as electric vacuum devices. Its production process is as follows:

1. Powder preparation:

The alumina powder entering 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. If the high-purity alumina ceramic products are manufactured, in addition to the alumina purity of 99.99%, ultra-fine pulverization is required to make the particle size distribution uniform. When extrusion molding or injection molding is used, binders and plasticizers need to be introduced into the powder, generally thermoplastics or resins with a weight ratio of 10-30%. The organic binder should be mixed with alumina powder at 150-200 Mix evenly at ℃ temperature to facilitate molding operation. Powder raw materials formed by hot pressing process do not need to add binder. If semi-automatic or fully automatic dry pressing is used, there are special process requirements for the powder, and the powder needs to be processed by spray granulation to make it appear spherical, so as to improve the fluidity of the powder and facilitate the automatic filling of the mold during molding. wall. In addition, in order to reduce the friction between the powder and the mold wall, it is necessary to add 1 to 2% lubricant? Such as stearic acid? and binder PVA.

When dry pressing is required, the powder needs to be sprayed and granulated, and polyvinyl alcohol is introduced as a binder. In recent years, a research institute in Shanghai has developed a water-soluble paraffin used as a binder for Al2O3 spray granulation, which has good fluidity under heating. The powder after spray granulation must have good fluidity, loose density, and flow angle friction temperature less than 30°C. Particle gradation ratio is ideal and other conditions to obtain greater green body density.

2. Molding method:

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

1 Dry pressing molding: The dry pressing molding technology of alumina ceramics is limited to objects with a simple shape, an inner wall thickness exceeding 1mm, and a ratio of length to diameter not greater than 4:1. The molding method is uniaxial or bidirectional. There are two types of presses, hydraulic and mechanical, which can be semi-automatic or fully automatic. The maximum pressure of the press is 200Mpa. The output can reach 15-50 pieces per minute. Due to the uniform stroke pressure of the hydraulic press, the height of the pressed parts is different when the powder filling is different. However, the pressure applied by the mechanical press varies with the amount of powder filling, which can easily lead to differences in dimensional shrinkage after sintering and affect product quality. 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 control of the dimensional accuracy of the manufactured alumina ceramic parts. Powder particles larger than 60 μm and between 60 and 200 mesh can obtain the maximum free flow effect and the best pressure forming effect.

2 Grouting molding method: Grouting molding is the earliest molding method for alumina ceramics. Due to the use of plaster molds, the cost is low and it is easy to form large-sized and complex-shaped parts. The key to slip casting is the preparation of alumina slurry. Usually, water is used as the flux medium, and the debonding agent and binder are added, exhausted after fully grinding, and then poured into the plaster mold. Due to the adsorption of moisture by the capillary of the gypsum mold, the slurry is solidified in the mold. During hollow grouting, when the mold wall absorbs the slurry to reach the required thickness, it is necessary to use a hook and a hook N.

Organic additives also need to be added to the alumina ceramic slurry to form an electric double layer on the surface of the slurry particles so that the slurry can be suspended stably without sedimentation. In addition, binders such as vinyl alcohol, methyl cellulose, and alginate amine and dispersants such as polyacrylamine and gum arabic must be added to make the slurry suitable for grouting.

3. Firing technology:

The technical method of densifying the granular ceramic body and forming a solid material is called sintering. Sintering is a method to eliminate the voids between particles in the green body, 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 most widely used heating device for firing is an electric furnace. In addition to atmospheric pressure sintering? That is, pressureless sintering, there are also hot pressing sintering and hot isostatic pressing sintering. Although continuous hot pressing sintering increases the output, the cost of equipment and molds is too high. In addition, due to the axial heating, the length of the product is limited. Hot isostatic pressing uses high-temperature and high-pressure gas as the pressure transmission medium, which has the advantage of uniform heating in all directions, and is very suitable for sintering products with complex shapes. Due to the uniform structure, the material performance is 30-50% higher than that of cold press sintering. It is 10-15% higher than that of general hot-pressing sintering. Therefore, at present, some high value-added alumina ceramic products or special parts required for national defense and military industry, such as ceramic bearings, mirrors, nuclear fuel and gun barrels, etc., are fired by hot isostatic pressing.

In addition, microwave sintering method, arc plasma sintering method and self-propagating sintering technology are also being developed and researched.

4. Finishing and packaging process:

Some alumina ceramic materials need to be finished after sintering. For example, products that can be used as artificial bone require a high surface finish, like a mirror surface, to increase lubricity. Due to the high hardness of the alumina ceramic material, it needs to be finished with a harder grinding and polishing brick material. Such as SiC, B4C or diamond etc. Usually, it is grinded step by step from coarse to fine abrasives, and the final surface is polished. Generally, Al2O3 micropowder or diamond paste <1μm can be used 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.

Alumina ceramic strengthening process

In order to strengthen alumina ceramics and significantly improve their mechanical strength, a new strengthening process for alumina ceramics has been introduced abroad. The process is novel and simple, and the technical method adopted is to use electron ray vacuum coating, sputtering vacuum coating or chemical vapor phase steaming on the surface of alumina ceramics. Heat treatment at 1580°C to toughen alumina ceramics.

The mechanical strength of the strengthened alumina ceramics can be greatly increased on the original basis, and alumina ceramics with ultra-high strength can be obtained.