In the production of formed refractory materials through the powder route, the heat treatment is an important step in the production process which characterizes the product quality but also contributes significantly to the production costs. Accordingly, the process parameters must be optimized during heat treatment. When drying and debinding, methods have to be identified which, on the one hand, enable as high a throughput as possible, but, on the other hand, ensure that the components remain damage-free. The same is true at higher temperatures for the quartz conversion, the carbonate decomposition and the dewatering of clay minerals - if the corresponding constituents are present in the green body. During sintering, near-net-shape production with tight production tolerances and good material properties is strived for. The throughput through the furnace is to be maximized and its energy and maintenance costs minimized. A particular challenge arises from the product sizes customary in refractory materials, which - without optimization – often lead to particularly long temperature cycles.
The optimization of the heat treatment can be carried out at the Fraunhofer-Center HTL for all types of refractory materials, e.g., for silicic, silica or basic products, chamottes or special products, if these are produced via the powder route. The methods used at the HTL are particularly suitable for upscaling to large components with characteristic lengths in the decimeter to meter range. ThermoOptical measurement systems (TOM) with all common atmospheres are available for the recording of the necessary in-situ measurement data: air, inert gas and fuel gas atmospheres as are present in gas-fired furnaces. Depending on the atmosphere, maximum temperatures between 1700 °C and 2200 °C can be produced. All material properties required for the optimization of the heat treatment of refractory materials can be measured in situ. The in-situ measurements are carried out on samples with a volume of approximately 30 cm3 to 100 cm3. The upscaling to the component scale is carried out with Finite-Element (FE) models. For all essential heat treatment steps on refractory materials, corresponding FE models are available. By means of FE simulation, the process parameters are optimized and subsequently verified experimentally in the furnace under defined conditions. In the last step, the optimized process parameters are then transferred to the production furnace.
During the process of drying the water still present in the green bodies must be removed without damage to the components. In the case of refractory materials, considerable drying stresses can occur due to the component sizes. Too fast or uneven drying leads to cracks 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 device for weighing samples under controlled humidity, temperature and gas flow during drying. In this way, the drying rate can be determined during the various drying stages. In the critical stages, the drying rate can be lowered in a targeted 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. During the optimization, moisture and flow measurements are taken into account in the industrial drying units.
In the debinding process, the organic forming additives contained in the green bodies, such as binders, plasticizers or dispersants, are thermally removed. The organic additives are burnt out in an oxygen-containing atmosphere or pyrolyzed in a reducing or inert atmosphere. Heat is released locally during the burn-out of the binder - while heat is consumed during pyrolysis. The resulting temperature differences cause thermal stresses, which in turn can lead to cracks or to the destruction of the green bodies. If the debinding is 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 also leads to component damage. In addition, there are other undesirable phenomena that may occur during debinding, e.g., the inflammation of sulfur 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 composition of the furnace gases can be varied in many cases.
Analogous to drying, debinding experiments are carried out at the HTL in controlled atmospheres and temperatures. The degree of debinding is monitored by the measurement of the sample mass; crack formation is detected by sound emission analysis. The optimization of the debinding parameters takes place analogously to the drying by means of FE simulation, the verification of the optimized conditions by means of additional debinding experiments. The same is also the case with other thermal reactions which free gases, e.g., in the case of dewatering or carbonate decomposition.