production process of alumina ceramics
1.Features and technical indicators:
Alumina ceramics are currently divided into two types: high-purity type and ordinary type. High purity alumina ceramic series Al2O3
Ceramic materials with a content of more than 99.9%, because their sintering temperature is as high as 1650-1990 ℃, and the transmission wavelength is 1 to 6 μm, they are generally made into molten glass to replace platinum crucibles: use its light transmittance and alkali metal corrosion resistance as Sodium lamp tube; 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 different Al2O3 content. Sometimes those with 80% or 75% Al2O3 content are also classified as ordinary alumina ceramic series. 1. Large hardness
As determined by the Shanghai Institute of Ceramics, Chinese Academy of Sciences, its Rockwell hardness is HRA80-90, which is second only to diamond and far exceeds the wear resistance of wear-resistant steel and stainless steel. 2. Excellent wear resistance
Measured by the Institute of Powder Metallurgy of Central South University, its wear resistance is equivalent to 266 times that of manganese steel and 171.5 times that of high-chromium cast iron. According to our customer follow-up survey over the past ten years, under the same working conditions, the service life of the equipment can be extended by at least ten times.
The density of alumina ceramics is 3.5g/cm3, which is only half of that of steel, which can greatly reduce the equipment load. The performance meets the Q/OKVL001-2003 technical standard. The main technical indicators of wear-resistant ceramics are alumina content ≥95%, density ≥3.5g/cm3, Rockwell hardness ≥80HRA, compressive strength ≥850?Mpa, fracture toughness KΙC≥4.8MPa •M1/2, bending strength ≥290MPa, thermal conductivity 20W/mK, thermal expansion coefficient: 7.2×10-6m/mK.
Among them, 99 alumina porcelain is used to make high temperature crucibles, refractory furnace tubes and special wear-resistant materials, such as ceramic bearings, ceramic seals and water valve discs; 95 alumina porcelain is mainly used as corrosion-resistant and wear-resistant parts; 85 porcelain Because it is often mixed with some talc to improve the electrical performance and mechanical strength, it can be sealed with metals such as molybdenum, niobium, and tantalum, and some are used as electrical vacuum devices. The production process is as follows:
2. 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 below 1μm. If high-purity alumina ceramic products are manufactured, in addition to the alumina purity of 99.99%, ultrafine pulverization and uniform particle size distribution are required. When using extrusion molding or injection molding, a binder and a plasticizer should be introduced into the powder, generally a thermoplastic plastic or resin with a weight ratio of 10-30%. The organic binder should be at 150-200℃ with alumina powder. Mix uniformly at temperature to facilitate the molding operation. The powder raw material formed by the hot pressing process does not need to add a binder. If semi-automatic or fully automatic dry pressing is used, there are special technological requirements for the powder, and the powder needs to be processed by spray granulation to make it spherical, so as to improve the fluidity of the powder and facilitate the automatic filling of the mold during the 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% of lubricants, such as stearic acid and binder PVA.
When dry pressing is desired, the powder must 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 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℃. Ideal particle gradation ratio and other conditions to obtain a larger green density.
3. Forming method:
Alumina ceramic products molding methods include dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing. In recent years, molding technology methods such as filter press molding, direct solidification injection molding, gel injection molding, centrifugal grouting and solid free molding have been developed at home and abroad. Different product shapes, sizes, complex shapes and precision products require different molding methods. Excerpt from its commonly used molding introduction:
3.1 Dry pressing: The aluminum oxide ceramic dry pressing technology is limited to objects with a simple shape, an inner wall thickness of more than 1mm, and a length to diameter ratio of not more than 4:1. The forming method is uniaxial or bidirectional. There are two types of presses: hydraulic and mechanical, and 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 part is different when the powder filling is different. However, the pressure applied by the mechanical press varies with the amount of powder filling, which easily leads to differences in size shrinkage after sintering, which affects product quality. Therefore, the uniform distribution of powder particles during dry pressing is very important for mold filling. Whether the filling amount is accurate or not has a great influence on the dimensional accuracy control of the manufactured alumina ceramic parts. The powder particles larger than 60μm and between 60-200 mesh can obtain the maximum free flow effect, and achieve the best pressure forming effect.
3.2 grouting molding method: grouting molding is the earliest molding method used 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 grouting is the preparation of alumina slurry. Usually water is used as the flux medium, and then the debonding agent and the binder are added, and the gas is exhausted after being fully ground, and then poured into the plaster mold. Due to the absorption of water by the capillary of the plaster mold, the slurry solidifies in the mold. When hollow grouting, when the mold wall absorbs the slurry to the required thickness, the excess slurry needs to be poured out. In order to reduce the shrinkage of the green body, a high-concentration slurry should be used as much as possible.
4. Four firing technology
The technical method of densifying the granular ceramic body and forming a solid material is called sintering. Sintering is a method of removing the voids between particles in the green body, removing a small amount of gas and impurity organic matter, so that the particles can grow and combine with each other to form a new substance.
The heating device used for firing is the most widely used electric furnace. In addition to atmospheric sintering, namely pressureless sintering, there are also hot pressing sintering and hot isostatic pressing sintering. Although continuous hot-pressing sintering increases the output, the equipment and mold costs are 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-pressed sintering. 10-15% higher than general hot pressing sintering. Therefore, some high value-added alumina ceramic products or special parts required by the national defense industry, such as ceramic bearings, reflectors, nuclear fuel and gun barrels, are fired by hot isostatic pressure.
In addition, microwave sintering, arc plasma sintering, and self-propagating sintering technologies are also under development and research.
5. Finishing and packaging process
Some alumina ceramic materials still need finishing after sintering. For example, products that can be used as artificial bones require a high surface finish, like a mirror surface, to increase lubricity. Due to the high hardness of alumina ceramic material, it is necessary to use harder grinding and polishing tile materials for finishing. Such as SIC, B4C or diamonds and so on. Usually use coarse to fine abrasives to grind step by step, and finally the surface is polished. Generally, Al203 powder or diamond paste of <1μm micron can be used for grinding and polishing. In addition, laser processing and ultrasonic processing, grinding and polishing methods can also be used.
6. Alumina ceramic strengthening process
In order to strengthen alumina ceramics and significantly improve its mechanical strength, a new alumina ceramic strengthening process has been introduced abroad. The process is novel and simple. The technical method adopted is to use electron beam vacuum coating, sputtering vacuum coating or chemical vapor deposition on the surface of alumina ceramics to coat a layer of silicon compound film and heat it at 1200℃～1580℃ Treatment to temper the 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.