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Oxide Ceramics

Component made of oxide ceramics
© Fraunhofer-Center HTL

Component made of oxide ceramics

Components made of aluminium oxide, produced with 3d-printing
© Fraunhofer-Center HTL

Components made of aluminium oxide, produced with 3d-printing

In the production of oxide ceramics, the heat treatment is an important step in the production process, which determines the product quality but also adds significantly to the manufacturing costs. Consequently, the process parameters of heat treatment must be optimized. For the drying and debinding processes, methods have to be identified which enable a maximum throughput but at the same time ensure that the components are not damaged. In the sintering process, near-net-production with tight production tolerances and good material properties are aimed for. The output in the sintering furnace has to be maximized and energy and maintenance costs must be minimized.

The optimization of the heat treatment can be carried out for all types of oxide ceramics at Fraunhofer-Center HTL, e.g. structural ceramics such as alumina, zirconium oxide, ZTA, steatite or functional ceramics such as lead zirconate titanate (PZT), barium titanate or zinc oxide. The methods used at the HTL are suitable for components on a millimeter scale as well as for large components with dimensions in the meter range. ThermoOptical measurement systems (TOM) for a range of atmospheres are available for recording the necessary in-situ measurement data: air, inert gas, vacuum, overpressure and atmospheres, like they are present in gas-fired ovens. Depending on the atmosphere, maximum temperatures between 1500 °C and 2200 °C are feasible. All the material properties necessary for the optimization of the heat treatment of oxide ceramics can be measured in-situ. The in-situ measurements are carried out on small samples with a volume of approx. 1 cm³ to 30 cm³. The upscaling to the component scale is carried out with the help of Finite-Element (FE) methods. For all essential heat treatment steps in oxide ceramics, corresponding FE models are available. The process parameters are optimized and experimentally verified using FE simulation. In the final step, the optimized process parameters are transferred to the production furnace.

During the process of drying, the solvent or water in the green bodies must be removed without damaging the components. In the case of oxide ceramics, considerable drying stresses can occur due to the small pores. Too fast or uneven drying leads to cracking or deformation. The local drying rates are strongly dependent on the relative humidity, the temperature and the flow rate of the drying gas. The HTL has a special device for weighing samples under controlled humidity, temperature and gas flow during drying. Using this device, the drying rate can be determined during the various drying stages. In the critical stages, the drying rate can be lowered in a defined manner. The deformation of the components by uneven drying is simulated in FE models, which take into account the gas flow and the local moisture gradient. For the optimization process, results from moisture and flow measurements in the industrial drying units are taken into account.

During the debinding process, the organic additives contained in the ceramic green bodies, such as binders, plasticizers or dispersants, are generally removed thermally. The organic additives are burnt out in an oxygen-containing atmosphere. Heat is released locally during the burn-out of the binder. The resulting temperature differences cause thermal stresses, which in turn can lead to cracks or to the destruction of the green bodies. If the binder is removed too fast, the resulting gases cannot be transported quickly enough through the pore channels to the component surface. The resulting overpressure in the pore channels can also lead to component damage. In addition, there are other undesirable phenomena which may occur during debinding, e.g., the segregation of the liquid binder in the pore channels or the inflammation of low-temperature gases in the furnace chamber. Similar to drying, it is important to find the fastest yet safe temperature profile with which the debinding can be carried out without defects. The flow velocity and the composition of the furnace gases can be varied in many cases.

Analogous to drying, debinding experiments are carried out at the HTL in controlled atmosphere and temperature. The degree of debinding is monitored by measuring the sample mass. Crack formation is detected by sound emission analysis. The optimization of the debinding parameters takes place analogously to drying by means of FE simulation and the verification of the optimized conditions by means of additional debinding experiments.

During sintering, densification with a defined residual porosity has to be achieved. The structure must be defect-free and homogeneous, and the component must not be deformed. The high costs associated with the finishing of oxide ceramics require as near-net-production as possible, in which the sintering shrinkage is included in the dimensioning of the green bodies. Deviations from the desired geometry can occur during sintering due to an uneven distribution of the porosity in the green body (see below) as well as the influence of friction, gravitation and temperature differences during the heat treatment. Temperature differences within the components lead to deformation; larger temperature differences in the components can also trigger crack formation or lead to the destruction of the components. Temperature differences in the furnace lead to scattering in the residual porosity and the component dimensions. Even in the microstructure, unwanted processes can occur during sintering: During the densification, thermodynamic effects can lead to locally stronger sintering or phase separation in an originally homogeneous green body. At the end of the densification step, increased grain growth occurs, which - particularly in the so-called overcritical grain growth - as well as the thermodynamic effetcs mentioned above, deteriorates the material properties. Other phenomena, which must be controlled during sintering, are gas-phase processes which are caused by reactions of the firing material with the furnace atmosphere or by gas releases from the material.
The sintering shrinkage is measured at highest precision using the TOM systems at HTL. In addition, other important material properties concerning heat transport and creep deformation can be recorded during sintering. By means of coupled FE models, the thermomechanical effects in the component are simulated, and the process parameters are optimized.
Even more specific heat treatment processes, such as hot pressing or sintering, hot isostatic pressing (HIPen), or the densification by melt filtration can be optimized with the methods at HTL. When the optimized process parameters are transferred to larger industrial furnaces, possible temperature gradients in the furnace are taken into account. FE methods can be used to simulate the temperature distribution in furnace systems.
Defects resulting from the raw material quality, the raw material preparation or the forming process can generally not be compensated for by the heat treatment process. Therefore, before optimizing the heat treatment, a careful assessment of the quality of the green body is recommended in cases of doubt. Even small differences in porosity in the green body will inevitably lead to deformation during sintering, because the local shrinkage in the component will differ. Even large pores, insufficiently pressed granules, cracks or inclusions lead to the deterioration of the quality of the sintered parts. Special methods for the green body assessment are available at HTL, which are carried out ahead of the optimization of the heat treatment if necessary.
In addition, HTL offers a simulation of the application performance of the ceramic components by means of FE methods. From this, an optimized ceramic-compatible design of the components can be derived. Prototypes can be produced by means of 3D printing without costly shaping molds. If necessary, new oxide ceramics and ceramic composites with improved material properties can be developed.

Contact us for further information

Holger Friedrich

Contact Press / Media

Dr. Holger Friedrich

Fraunhofer-Center for High Temperature Materials and Design
Gottlieb-Keim-Str. 62
95448 Bayreuth, Germany

Phone +49 921 78510-300

Fax +49 921 78510-001

Marina Stepanyan

Contact Press / Media

Marina Stepanyan

Fraunhofer-Center for High Temperature Materials and Design
Gottlieb-Keim-Str. 62
95448 Bayreuth, Germany

Phone +49 921 78510-310

Fax +49 921 78510-001