Wire and textile thread drawing cones, pulleys,rings and guidesALUMINA CERAMICS
This is ceramics produced of aluminum oxide (alumina) having the Al2O3 content of even up to 99.99%.
There are various technologies used for fabrication of ceramic products and the most frequently used ones are pressing, single-and double side and isostatic, then extruding and casting. Choice of technology depends on the desired shape and quality of products. The pressed green parts ere strong enough to be machined to the final shape and approximate dimensions.
The most important operation in production of ceramic parts is high-temperature sintering. Ceramics is sintered at temperature of 1873-1973 °K and fine polycrystalline structure is achieved.
Owing to such a high purity and manufacturing technology, ceramics has excellent mechanic, electric, thermal and chemical characteristics. Ceramic products produced in this way have dimensional deviations of about 2%, as defined by JUS B.D7.030 or DIN 40680 standards. Therefore, an effort should be made that design tolerances are within the allowed limits. The best effects are achieved by a mutual agreement between a manufacturer and customer.
If requirements upon dimensional preciseness of the ceramic products are more demanding, these can be met by subsequent machining (grinding, lapping, polishing), what enables the dimensional accuracy equivalent to metallic parts.
Diamond tools do subsequent machining and polishing pastes so that high quality of surface finish is obtained:
Surface roughness
| - | As fired surface | Ra | = 0.4 - 1.6 µm |
| - | Ground and lapped surface | Ra | = 0.4 - 0.8 µm |
| - | Polished surface | Ra | = 0.1 - 0.4 µm |
For aesthetic reasons and easier maintenance of ceramic products, glazing of the sintered parts is done. This should be avoided with tight tolerances, sharp angles and irregular shapes.
Should a part of intricate shape be produced, it is made up of several simple elements joint together by different sealing techniques, depending on the work piece operating conditions.
Ceramics - to - ceramics or metal-to-ceramics assemblies are formed by:
- gluing - for lower loads and temperatures
- mechanical bonding - for higher loads and temperatures
- metallization - where vacuum tightness should be achieved for high voltages and temperatures.
CHARACTERISTICS OF CERAMICS
Characteristics of alumina ceramics are dependant on quality and content of alumina and technological procedure of preparation, sintering and fabrication.
Depending on operating conditions, ceramics of different properties are produced. The most significant properties of ceramics are the following:
| TABLE OF ENKER ALUMINA CERAMICS |  |
ELECTRICAL INSULATION PROPERTIES
Due to its chemical analysis and structure after sintering, ceramics is a good electrical insulator even at high temperatures.
Products of this ceramics have high volume resistance and high dielectric strength at varying frequency and voltage, low dielectric loss factor and coefficient of capacitance, as well as low dielectric constant.
CHEMICAL PROPERTIES
Depending on Al2O3 content, ceramics is resistant to chemical and corrosion attacks. Even under various thermal and mechanical influences, it is resistant to organic and inorganic acids, low alkali solutions, dissolved salts and others.
MECHANICAL PROPERTIES
The widest application of ceramics comes from its excellent mechanical properties. High hardness and wear resistance, modulus of elasticity, compressive and flexural strengths has enabled the application of ceramic parts there where high mechanical stresses are experienced.
Such a high strength of ceramics is maintained at high temperatures, what makes it an excellent replacement for majority of metals, glasses, plastics, etc.
THERMAL PROPERTIES
Ceramics is a good heat conductor compared to porcelain, glass and other ceramics and is not far behind metals. Both its heat conductivity and temperature field of application depend on Al2O3 content, shape, etc. This ceramics maintains its mechanical properties at maximum operating temperatures, higher than for metals, and demonstrates very high durability in oxidizing and reducing atmospheres and high vacuum.
Due to such electrical and thermal properties, ceramics is suitable for metallization.
Besides this non-porous ceramics, there is also porous ceramics produced of corundum combined with mullites, exhibiting porosity of 15 - 25% and density of 2.4 - 3 g/cm3. This material is stable at high temperatures (its operating temperature is as high as 2073 K) and such as that is suitable for fabrication of ceramics sintering saggers, metal and glass casting elements, heater supports, etc.
Apparently, ceramics is a very durable material under extremely severe operating conditions, what makes its application highly economical.
Since ceramics may be applied in various industries, we will state only the most common (typical) products: