Alumina ceramics is a widely used ceramics. It is a ceramic material mainly composed of alumina (Al2O3) corundum, and has good conductivity, mechanical strength and high temperature resistance. High-purity alumina ceramic Al2O3 content of 99.9% or more, the sintering temperature up to 1650 °C ~ 1990 °C. Ordinary type alumina ceramics are divided into 99 porcelains, 95 porcelains, 90 porcelains, 85 porcelains, etc. according to the content of Al2O3. Sometimes the content of Al2O3 is 80% or 75%, which is also classified as ordinary alumina ceramics. Due to its many classifications, alumina ceramics are used more widely.
Among them, 99 alumina ceramic 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 discs; 95, 92 alumina ceramics are mainly used as ceramic grinding bodies, etc. to resist corrosion , Wear parts; 85 porcelain often incorporate part of the talc, improve the electrical properties and mechanical strength, and some used as electrical vacuum device devices.
Alumina powders that are introduced into the plant are prepared into powder materials according to different product requirements and different molding processes. The powder size is below 1μm. If high-purity alumina ceramic products are to be produced, the purity of the alumina is required to exceed 99.99%. Fine pulverization and uniform particle size distribution. When using extrusion molding or injection molding, binder and plasticizer should be introduced into the powder, generally 10% to 30% by weight of thermoplastic plastic or resin, and the organic binder should be at 150°C with alumina powder. Mix evenly at ~200°C to facilitate molding operation. Powdered raw materials formed by hot pressing process do not need to be added with a binder.
If semi-automatic or fully-automatic dry pressing is used, there is a special process requirement for the powder. The powder must be treated with a spray granulation method to make it spherical, so as to improve the powder flowability, and facilitate automatic filling during molding. Mold wall. In addition, in order to reduce the friction between the powder and the mold wall, it is also necessary to add 1% to 2% of a lubricant, such as stearic acid and a binder PVA.
For dry compression molding, the powder needs to be spray granulated, and polyvinyl alcohol is introduced as a binder. A research institute in Shanghai has developed a water-soluble paraffin wax that is used as a binder for Al2O3 spray granulation and has good fluidity under heating conditions. The powder after the spray granulation must have good fluidity, loose density, flow angle friction temperature less than 30°C, ideal particle size distribution ratio, etc. to obtain a larger green density. Alumina ceramic dry press molding methods are uniaxial or bidirectional. The presses are hydraulic and mechanical. They can be semi-automatic or fully automatic. The maximum pressure of the press is 200MPa, and the output can reach 15 to 50 pieces per minute.
In the dry pressing process, the uniform distribution of powder particles is very important for mold filling. The accuracy of the filling amount has a great influence on the dimensional accuracy control of the manufactured alumina ceramic parts. Powder particles larger than 60μm, between 60-200 mesh, can get the maximum free flow effect, get the best pressure molding effect.
The technique of densifying a granular ceramic body and forming a solid material is called sintering. Sintering will eliminate the inter-particle voids in the billet, eliminate a small amount of gas and impurities organics, and enable the particles to grow and combine with each other. After a reasonable thermal system control, a corundum-based microcrystalline ceramic structure is formed.
Microcrystalline alumina ceramic grinding body has the following characteristics, especially suitable for cement ball mill application technology requirements.
1. Hardness: As determined by Shanghai Institute of Ceramics, Chinese Academy of Sciences, its Rockwell hardness is HRA 80 ~ 90. Hardness is second only to diamond and far exceeds the wear resistance of wear-resistant steel and stainless steel.
2. Excellent wear resistance: As determined by Institute of Powder Metallurgy, Central South University, its wear resistance is equivalent to 266 times of manganese steel and 171.5 times of high chromium cast iron. According to the customer follow-up survey, under the same conditions of cement grinding, it can at least extend the service life of the grinding body by more than 10 times.
3. Light weight: its density is 3.6 ~ 3.8g/cm3, only half of steel, can greatly reduce the equipment load.
4. The main technical indicators are as follows:
Alumina ceramic content ≥92%
Density ≥3.6g/cm3
Rockwell hardness ≥ 80HRA
Compressive strength ≥850MPa
Fracture toughness KΙC≥4.8MPa·m1/2
Flexural strength ≥290MPa
Thermal conductivity 20W/m·K
Thermal expansion coefficient 7.2×10-6m/m·K
5. Development trend: Alumina ceramics is one of the most widely used materials in advanced ceramics. With the development of the entire industry, the following trends have emerged.
(1) The level of technology and equipment will rapidly increase: The development of computer technology and digital control technology will promote the technological progress and rapid development of the advanced ceramic material industry. Such as automatic control of continuous sintering kiln, high-power large-capacity grinding equipment, high-performance milling and granulation equipment, and other advanced pressure molding equipment and other advanced complete sets of equipment, effectively promoted the industry's overall level of improvement, at the same time, in the production efficiency, Product quality has also improved significantly.
(2) The level of product quality has been continuously improved: domestic microcrystalline alumina ceramic products have grown from scratch, the industry scale has grown from small to large, and product quality has been from low to high, experiencing a rapid development process.
(3) The scale of the industry will rapidly expand: Microcrystalline alumina ceramics, as the basic material in other industries or fields, will be affected and limited by the level of development of other industries. From the perspective of the application of alumina ceramics, the application range is wider and wider, and the amount is increasing. This is particularly evident in cement grinding systems and architectural ceramics used in new dry process production.
