Alumina ceramic is a kind of widely used ceramics, because of its superior performance, has been more and more widely used in modern society, to meet the needs of daily use and special performance.
1.1 preparation of powder
Aluminum oxide powder is prepared into powder material according to different product requirements and different molding process. If the particle size of the powder is less than 1 muon, in addition to the purity of aluminum oxide at 99.99%, the production of high purity alumina ceramic products also need to be superfine grinding and make the particle size distribution uniform. When using extrusion molding or injection molding, the powder should be introduced into the binder and plasticizer, generally in the weight ratio of 10-30% thermoplastic plastic or resin, organic binder should be mixed with alumina powder at 150-200℃, to facilitate the molding operation.
The raw material of powder formed by hot pressing process does not need to add binder. If semi-automatic or full-automatic dry pressing molding is adopted, there are special technological requirements for the powder. Spray granulation method should be adopted to treat the powder and make it appear spherical, so as to improve the fluidity of the powder and facilitate the automatic filling of mold wall in the molding.
Powder spray granulation is required when dry pressing, in which polyvinyl alcohol is introduced as binder. In recent years, a water-soluble paraffin has been developed in a research institute in Shanghai as a binder for Al2O3 spray granulation. The powder after spray granulation must have good fluidity, loose density and flow Angle friction temperature less than 30℃. The ratio of particle size is ideal, so as to obtain a larger blank density.
1.2 molding method
Alumina ceramic products molding methods are dry pressure, grouting, extrusion, cold isostatic pressure, injection, flow, hot and hot isostatic pressure molding and other methods. In recent years, some new forming technologies have been developed at home and abroad, such as pressure filtration molding, direct solidification injection molding, gel injection molding, centrifugal injection molding and solid free molding. Different product shapes, sizes, complex shapes and precision require different molding methods.
2.1 dry pressing
The dry pressing technology of alumina ceramics is limited to the simple shape, inner wall thickness of more than 1mm, length to diameter ratio of not more than 4:1. The forming method is uniaxial or bidirectional. There are two types of press: hydraulic type and mechanical type. The maximum pressure of the press is 200Mpa, and the output can reach 15 ~ 50 pieces per minute.
Because the stroke pressure of the hydraulic press is even, the height of the pressing parts is different when the powder is filled differently. However, the pressure applied by mechanical press varies with the amount of powder filling, which will easily lead to the difference of 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 a great influence on the dimension accuracy control of alumina ceramic parts. The maximum free flow effect and the best pressure forming effect can be obtained by the powder particles with the size of over 60 microns and between 60 ~ 200 mesh.
2.2 grouting molding method
Grouting molding is the earliest molding method of alumina ceramics. Because of the use of gypsum mold, low cost and easy to form large size, complex shape parts, the key to grouting molding is the preparation of alumina paste. Water is usually used as the flux medium, and then a degluing agent and binder are added. After being fully ground, the exhaust is discharged and then poured into the gypsum mold. Because of the adsorption of water by the capillary of gypsum mold, the slurry was solidified in the mold. When hollow grouting, 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 green body, high concentration slurry should be used as far as possible.
It is also necessary to add organic additives in alumina ceramic slurry to form a double electric layer on the surface of slurry particles so that the slurry can be suspended without precipitation. In addition, binder such as ethylene alcohol, methyl cellulose, alginate amine and dispersant such as polypropylene amine and gum Arabic are also added to make the slurry suitable for grouting molding.
2.3 firing technology
Sintering is a process of densification of granular ceramic bodies to form solid materials. Sintering is a method of removing the intergranular cavity in the body of billet, removing a small amount of gas and impurity organic matter, and making the particles grow and combine with each other to form a new substance.
The heating device used in firing is the most widely used electric furnace. In addition to normal pressure sintering, that is, no pressure sintering, hot pressure sintering and hot isostatic pressure sintering. Continuous hot-pressed sintering increased production, but the cost of equipment and molds was too high, and because of the axial heat, the length of the product was limited. The hot isostatic firing adopts high temperature and high pressure gas as the pressure transmission medium, which has the advantages of uniform heating in all directions, and is very suitable for sintering complex products. Due to the uniform structure, the material performance is improved by 30 ~ 50% compared with the cold-pressed sintering. Compared with general hot-pressed sintering, it is improved by 10 ~ 15%. In addition, microwave sintering, arc plasma sintering and self-propagating sintering are also under development. [2]
2.4 finishing and packaging procedures
Some alumina ceramic materials still need finishing after sintering. If used as artificial bone, the surface is required to have a high degree of smoothness, such as mirror, to increase lubrication. Because the hardness of alumina ceramic material is higher, a harder abrasive polishing brick material is needed for its finishing. Such as SiC, B4C or diamond, etc. The final surface is polished by grinding from coarse to fine abrasive. Generally, we can use Al2O3 powder <1 mum 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.
2.5 alumina ceramic strengthening process
In order to enhance the mechanical strength of alumina ceramics, a new strengthening technology of alumina ceramics was developed. The process is novel and simple, the technology adopted is on the surface of alumina ceramics, using electron ray vacuum coating, sputtering vacuum coating or chemical vapor deposition method, plating a layer of silicon compound film, heating treatment at 1200℃ ~ 1580℃, the alumina ceramics toughened.