The technical method of densifying granular ceramic body and forming solid material is called sintering. Sintering is a method of removing voids between particles in the billet, removing a small amount of gas and impurity organic matter, so as to make the particles grow and combine with each other to form new substances.
Electric furnaces are widely used as heating devices for firing. In addition to atmospheric pressure sintering, i.e. pressureless sintering, there are hot pressing sintering and hot isostatic pressing sintering. Although continuous hot pressing sintering increases the output, the cost of equipment and die is too high. In addition, due to axial heating, the length of products is limited. Hot isostatic pressing sintering uses high temperature and high pressure gas as pressure transfer medium, which has the advantage of uniform heating in all directions. It is very suitable for the sintering of products with complex shapes. Due to the uniform structure, the material properties are 30 ~ 50% higher than those of cold pressing sintering. It is 10-15% higher than that of general hot pressing sintering. Therefore, some high value-added alumina ceramic products or special parts for national defense and military industry, such as ceramic bearings, mirrors, nuclear fuel and gun barrel, adopt hot isostatic pressing firing method.
In addition, microwave sintering, arc plasma sintering and self propagating sintering are also being developed and studied.
Finishing and packaging process
Some alumina ceramic materials need finishing after sintering. For example, products that can be used as artificial bones require high surface finish, such as mirror, to increase lubricity. Due to the high hardness of alumina ceramic materials, it is necessary to finish them with harder grinding and polishing brick materials. Such as SiC, B4C or diamond. It usually adopts step-by-step grinding from coarse to fine abrasive and surface polishing. Generally < 1 can be used μ M 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.
Strengthening process of alumina ceramics
In order to strengthen alumina ceramics and significantly improve their mechanical strength, a new strengthening process of alumina ceramics is introduced abroad. The process is novel and simple. The technical means adopted is to coat a layer of silicon compound film on the surface of alumina ceramics by electron ray vacuum coating, sputtering vacuum coating or chemical vapor evaporation, and heat it at 1200 ℃ ~ 1580 ℃ to toughen alumina ceramics.
The mechanical strength of strengthened alumina ceramics can be greatly increased on the original basis to obtain alumina ceramics with ultra-high strength.