Your Expert for Innovative Ceramic-to-Metal Integrated Systems:
Aluminum oxide ceramic the polycrystalline material
Alumina is the English word for alumina (Al2O3)
Due to its availability and properties, Al2O3 is very often used for the manufacture of ceramic components. Alumina is used in various modifications. The most commonly used for technical ceramics is the trigonal α-Al2O3 (corundum). In order to produce aluminum oxide, the natural raw material bauxite is digested in caustic soda using the Bayer process to produce aluminum hydroxide. This is deprived of water by sintering or calcination in rotary kilns, and aluminum oxide is obtained.
Due to its availability and properties, Al2O3 is very often used for the manufacture of ceramic components. Alumina is used in various modifications. The most commonly used for technical ceramics is the trigonal α-Al2O3 (corundum). In order to produce aluminum oxide, the natural raw material bauxite is digested in caustic soda using the Bayer process to produce aluminum hydroxide. This is deprived of water by sintering or calcination in rotary kilns, and aluminum oxide is obtained.
Due to its hardness and high wear resistance, alumina is often used as a grinding or polishing agent. As the melting temperature is above 2000 ° C, the material is ideal for use as a refractory material. In electrical engineering Al2O3 is used because of the low dielectric loss factor and the high electrical insulation and dielectric strength. The applications range from sealing discs in taps to implants to ballistic protection made of aluminum oxide.
The properties of the material aluminum oxide are strongly influenced by the purity and the manufacturing process. As the proportion of alumina increases, the hardness, corrosion resistance, dielectric strength and thermal conductivity of the material increase. The thermal shock resistance decreases, however.
Alumina Systems GmbH uses two ceramic materials according to DIN EN 60672 due to its excellent electrical properties and good metallizability: C795 with 96% Al2O3 and C799 with 99.7% Al2O3.
The properties of the material aluminum oxide are strongly influenced by the purity and the manufacturing process. As the proportion of alumina increases, the hardness, corrosion resistance, dielectric strength and thermal conductivity of the material increase. The thermal shock resistance decreases, however.
Alumina Systems GmbH uses two ceramic materials according to DIN EN 60672 due to its excellent electrical properties and good metallizability: C795 with 96% Al2O3 and C799 with 99.7% Al2O3.
Production of Al2O3 ceramic pressed granules by spray drying
At Alumina Systems, the alumina granules are prepared by spray-drying. Here, small droplets of a suspension of ceramic with water and organic binder are dried by hot air to pellets.
The production can be divided into three steps:
Since the drying takes place continuously in the so-called spray tower, large quantities can be produced inexpensively and with consistent quality in a relatively short time.
The production can be divided into three steps:
- Atomization of the ceramic suspension (slip)
- Drying in a tempered air stream
- Separation of the dry granules from the drying air.
Since the drying takes place continuously in the so-called spray tower, large quantities can be produced inexpensively and with consistent quality in a relatively short time.
Ceramic shape of the Al2O3 ceramic
Cold Isostatic Pressing (CIP Cold Isostatic Pressing)
In cold isostatic pressing, the ceramic granulate is filled into a dense elastic mold and pressed uniformly from all sides over the water pressure (1000 to 2000 bar). The contour of the component can only be partially molded here. Therefore, the pressed product must still be brought into the desired shape by processing before firing (green processing).
In cold isostatic pressing, the ceramic granulate is filled into a dense elastic mold and pressed uniformly from all sides over the water pressure (1000 to 2000 bar). The contour of the component can only be partially molded here. Therefore, the pressed product must still be brought into the desired shape by processing before firing (green processing).
Uniaxial dry pressing
Uniaxial pressing is a process for the near-net-shape production of final contour accuracy of series products. The granules are compacted in a steel mold, which is profiled according to the part to be produced. The high cost of the tool (often made of carbide in ceramics) is usually only economical for large series. Disadvantage of the method compared to the isostatic pressing is the different compression of the compact due to friction between granules and tool and not all-round compression.
Uniaxial pressing is a process for the near-net-shape production of final contour accuracy of series products. The granules are compacted in a steel mold, which is profiled according to the part to be produced. The high cost of the tool (often made of carbide in ceramics) is usually only economical for large series. Disadvantage of the method compared to the isostatic pressing is the different compression of the compact due to friction between granules and tool and not all-round compression.
Form processing - green processing of oxide ceramics
After pressing, Alumina Systems usually does the greening. Often not all details, such as grooves or holes perpendicular to the uniaxial pressing direction, of the component can be imaged by the pressing. This is why processing then takes place by turning, milling and drilling using diamond-coated tools. Although the strength of the green body is comparable to that of a piece of chalk, the alumina particles are still very hard and abrasive.
A green body or green body refers to an unfired ceramic blank, which is still easy to work. The blank contains not only the ceramic powder but also binders which are necessary for bonding the ceramic particles during pressing. The processing is done.
A green body or green body refers to an unfired ceramic blank, which is still easy to work. The blank contains not only the ceramic powder but also binders which are necessary for bonding the ceramic particles during pressing. The processing is done.
Firing = sintering of the aluminum oxide high-temperature ceramic
Sintering or firing of Al2O3 high-temperature ceramics.
The term sintering of alumina high temperature ceramics refers to the solidification and densification of a green body to a dense material by high temperatures (1500 to 2000 ° C). In this case, the ceramic grains should not completely melt, so that the outer shape is maintained. The compaction should lead to the most uniform and reproducible shrinkage of the component.
The component is about 20% smaller by the fire. It should nevertheless be a dense component without cracks. The driving force is the minimization of surface and interfacial energy through grain growth and pore shrinkage. This means that the ceramic particles merge and thus fill the interstices. At the same time you approach, which makes the component smaller overall. It is dwindling. Due to the high shrinkage of the ceramic much effort must be operated to keep the manufacturing tolerances small. Alumina Systems still manages to maintain tolerances below 1% in the series.
The term sintering of alumina high temperature ceramics refers to the solidification and densification of a green body to a dense material by high temperatures (1500 to 2000 ° C). In this case, the ceramic grains should not completely melt, so that the outer shape is maintained. The compaction should lead to the most uniform and reproducible shrinkage of the component.
The component is about 20% smaller by the fire. It should nevertheless be a dense component without cracks. The driving force is the minimization of surface and interfacial energy through grain growth and pore shrinkage. This means that the ceramic particles merge and thus fill the interstices. At the same time you approach, which makes the component smaller overall. It is dwindling. Due to the high shrinkage of the ceramic much effort must be operated to keep the manufacturing tolerances small. Alumina Systems still manages to maintain tolerances below 1% in the series.
Glazing the Al2O3 engineering ceramics
A ceramic has certain surface roughness, which can promote soiling.
By applying a glaze, the surface is smoothed. It improves many properties such as electrical behavior, mechanical strength and chemical resistance.
Ceramics are also glazed to increase the strength of the component through a compressive stress in the surface. At the same time, the smooth surface minimizes contamination and wetting by liquids. As a result, associated leakage currents are reduced.
Glazing is done in two steps. The glaze suspension is sprayed onto the alumina ceramic at Alumina Systems. After drying, the glaze is baked at about 1400 ° C. Since the glaze contracts more during cooling, like the ceramics, the desired compressive stresses develop on the surface.
By applying a glaze, the surface is smoothed. It improves many properties such as electrical behavior, mechanical strength and chemical resistance.
Ceramics are also glazed to increase the strength of the component through a compressive stress in the surface. At the same time, the smooth surface minimizes contamination and wetting by liquids. As a result, associated leakage currents are reduced.
Glazing is done in two steps. The glaze suspension is sprayed onto the alumina ceramic at Alumina Systems. After drying, the glaze is baked at about 1400 ° C. Since the glaze contracts more during cooling, like the ceramics, the desired compressive stresses develop on the surface.
Grinding - Hard machining of aluminium oxide ceramics
Hard machining on fired components, which have already been brought as close as possible to their final dimensions by pressing and green machining. The tightest tolerances can be achieved by grinding. Diamond tools are used here because of the great hardness of the aluminium oxides.
Due to the low stock removal rate - approx. 50 times lower than grinding steel - the costs of hard machining of ceramics are often very high. Therefore, Alumina Systems always tries to minimize this process step in the interest of the customer.
Due to the low stock removal rate - approx. 50 times lower than grinding steel - the costs of hard machining of ceramics are often very high. Therefore, Alumina Systems always tries to minimize this process step in the interest of the customer.