Features and technical indicators
Alumina ceramics are currently classified into high-purity and ordinary types.
High-purity alumina ceramics have a content of 99.9% or more of Al2O3. Since the sintering temperature is as high as 1650-1990 ° C and the transmission wavelength is 1 to 6 μm, it is generally made into molten glass instead of platinum crucible: using its light transmittance and Alkali metal corrosion resistance is used as a sodium lamp; it can be used as an integrated circuit substrate and high frequency insulation material in the electronics industry.
Ordinary alumina ceramics are classified into 99 porcelain, 95 porcelain, 90 porcelain, 85 porcelain, etc. according to the content of Al2O3. Sometimes the content of Al2O3 is 80% or 75%, which is also classified into ordinary alumina ceramic series.
1. High hardness
According to the Shanghai Institute of Ceramics of the Chinese Academy of Sciences, its Rockwell hardness is HRA80-90, and its hardness is second only to diamond, far exceeding the wear resistance of wear-resistant steel and stainless steel.
2. Excellent wear resistance
According to the Institute of Powder Metallurgy, 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 tracking survey for more than ten years, we can extend the service life of the equipment by at least ten times under the same working conditions.
3. Light weight
The alumina ceramic density is 3.5g/cm3, which is only half of that of steel, which can greatly reduce the equipment load. The performance conforms to the technical standard of Q/OKVL001-2003. The main technical indexes of wear-resistant ceramics are alumina content ≥95%, density ≥3.5 g/cm3, Rockwell hardness≥80 HRA, compressive strength≥850 Mpa, fracture toughness KΙC ≥4.8MPa · m1/2 , bending strength ≥ 290 MPa, thermal conductivity 20 W / mK, thermal expansion coefficient: 7.2 × 10-6 m / mK.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 sheets; 95 alumina ceramics are mainly used as corrosion-resistant and wear-resistant parts; In addition, due to the incorporation of part of talc, 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 device devices.
The alumina powder entering the plant is prepared into a powder material according to different product requirements and different molding processes. The particle size of the powder is below 1 μm. If the purity of the alumina is 99.99%, the ultra-fine pulverization is required to make the particle size distribution uniform.
When extrusion molding or injection molding is used, a binder and a plasticizer are introduced into the powder, generally a thermoplastic plastic or resin having a weight ratio of 10-30%, and the organic binder should be 150-200 with the alumina powder. Mix evenly at °C to facilitate molding operations.
The powder raw material formed by the hot pressing process does 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. It is necessary to spray the granulation method to treat the powder to make it spherical, so as to improve the fluidity of the powder and facilitate the automatic filling of 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 a lubricant such as stearic acid and a binder PVA.
Powder spray granulation is required for dry press molding, in which polyvinyl alcohol is introduced as a binder. In recent years, a research institute in Shanghai has developed a water-soluble paraffin wax 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 a flow angle friction temperature of less than 30 °C. The particle gradation ratio is ideal and the like to obtain a larger bulk density.
Alumina ceramic products are formed by various methods such as dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing. In recent years, molding techniques such as pressure filtration molding, direct solidification injection molding, gel injection molding, centrifugal grouting molding and solid free forming have been developed at home and abroad. Different product shapes, sizes, complex shapes and precision products require different molding methods.
Picking up its usual molding introduction:
1. Dry pressing: The alumina ceramic dry pressing technology is limited to objects with a simple shape and an inner wall thickness of more than 1 mm and a length to diameter ratio of not more than 4:1. The molding method is uniaxial or bidirectional. The press is available in hydraulic or mechanical form 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. Since the hydraulic press has a uniform stroke pressure, the height of the pressed parts is different when the powder filling is different. However, the pressure applied by the mechanical press varies depending on how much the powder is filled, which may cause a difference in dimensional shrinkage after sintering, which affects product quality. Therefore, the uniform distribution of powder particles during dry pressing is very important for mold filling. The accuracy of the filling has a great influence on the dimensional accuracy control of the manufactured alumina ceramic parts. The powder particles have a maximum free-flowing effect of more than 60 μm and between 60 and 200 mesh, and the best pressure forming effect is obtained.
2, grouting molding method: grouting molding is the earliest molding method used in alumina ceramics. Due to the use of plaster molds, the cost is low and it is easy to form large-sized, complex-shaped parts. The key to grout molding is the preparation of alumina slurries. Usually, water is used as a flux medium, and then a debonding agent and a binder are added, fully ground, and then vented, and then poured into a plaster mold. The slurry is solidified in the mold due to the adsorption of moisture by the gypsum mold capillary. In the hollow grouting, when the mold wall adsorbs 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.
An organic additive is also added to the alumina ceramic slurry to form an electric double layer on the surface of the slurry particles so that the slurry is stably suspended without precipitation. In addition, a binder such as vinyl alcohol, methyl cellulose, alginic acid amine, or a dispersing agent such as polyacrylamide or gum arabic is added, and the purpose is to make the slurry suitable for the slurry molding operation.
The technical method of densifying a granular ceramic body and forming a solid material is called sintering. Sintering is to remove the voids between the particles in the body, remove a small amount of gas and impurity organic matter, and make the particles grow and combine with each other to form a new substance.
An electric furnace is most widely used for heating devices used for firing. In addition to normal pressure sintering, that is, pressureless sintering, there are also hot press sintering and hot isostatic pressing. Although continuous hot press sintering increases the yield, the cost of equipment and mold is too high, and the length of the product is limited due to the axial heat. The hot isostatic pressing uses high-temperature and high-pressure gas as the pressure transmission medium, and has the advantages of uniform heating in all directions, and is suitable for sintering of complex shapes. Due to the uniform structure, the material properties are improved by 30 to 50% compared to cold press sintering. It is 10 to 15% higher than ordinary hot press sintering. Therefore, some high-value-added alumina ceramic products or special parts required for defense industry, such as ceramic bearings, mirrors, nuclear fuel and barrels, and hot isostatic pressing methods.
In addition, microwave sintering, arc plasma sintering, and self-propagating sintering are also under development.
Finishing and packaging process:
Some alumina ceramic materials need to be finished after sintering. For example, articles that can be used as artificial bones require a high surface finish, such as a mirror surface, to increase lubricity. Due to the high hardness of the alumina ceramic material, it needs to be finished with a harder abrasive polishing brick material. Such as SiC, B4C or diamond. Grinding is carried out step by step from coarse to fine abrasive, and the final surface is polished. Grinding and polishing can generally be carried out using <1 μm micron Al2O3 fine powder or diamond paste. In addition, laser processing and ultrasonic processing methods of polishing and polishing can also be used. Some alumina ceramic parts need to be packaged with other materials.
Alumina ceramic strengthening process, in order to enhance alumina ceramics, significantly improve its mechanical strength, a new alumina ceramic strengthening process. The process is novel and simple, and the technical means adopted is to apply a layer of silicon compound film on the surface of the alumina ceramic by electron beam vacuum coating, sputtering vacuum coating or chemical vapor deposition, and heat at 1200 ° C to 1580 ° C. Treatment to temper alumina ceramics. The mechanical strength of the reinforced alumina ceramic can be greatly increased on the basis of the original, and an alumina ceramic having an ultrahigh strength is obtained.