Fraunhofer Institute for Silicate Research ISC, Center HTL, Bayreuth
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.
