Fraunhofer Institute for Silicate Research ISC, Center HTL, Bayreuth
In the production of non-oxide ceramics, the heat treatment is an important step of the production process. It determines the product quality, but also adds to manufacturing costs. Thus, the process parameters must be optimized thoroughly with regard to heat treatment. For drying and debinding, methods have to be identified which, on the one hand, enable a maximum throughput but on the other hand, it has to be ensured that the components do not get damaged. During sintering, near-net-production with tight tolerances and good material properties is aimed for. The output has to be maximized, and energy and maintenance costs must be minimized.
At the Fraunhofer-Center HTL, the optimization of heat treatment processes can be carried out for all types of non-oxide ceramics. These include SiC, AlN and Si3N4 ceramics as well as hardmetals or cermets. The methods used at the HTL are suitable for components on a millimeter scale as well as for large components with characteristic lengths in the meter range. ThermoOptical measurement systems (TOM) with a range of different atmospheres are available during the recording of the in-situ measurement data. Sintering can be carried out in graphite, molybdenum disilicide or tungsten-heated measuring furnaces. Inert gases, oxidic or reducing gases - even 100% hydrogen - can be used as furnace atmospheres. Some TOM systems can be operated with vacuum or overpressure (up to 30 bar). Depending on the atmosphere, maximum temperatures between 1500 °C and 2200 °C can be achieved. All the material properties necessary for the optimization of the heat treatment of nonoxide 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 Finite-Elements (FE) methods. For all essential heat treatment steps on nonoxide ceramics, corresponding FE models are available. The process parameters are optimized and experimentally verified using FE simulation. In the last step, the optimized process parameters are transferred to the production furnace.
During the process of drying, the solvent in the green parts must be removed without damaging the components. In the case of nonoxide 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 (solvent concentration), the temperature and the flow rate of the drying gas. The HTL possesses a device that can be used to weigh samples under controlled solvent concentration in the atmosphere, temperature and gas flow during drying. Using this device, the drying rate can be determined during the different drying stages. In the critical stages, the drying rate can be lowered in a defined manner. Crack formation is detected by means of sound emission analysis. The deformation of the components by uneven drying is simulated in FE models, which take into account the gas flow and the local moisture or solvent concentration gradient. In the optimization, moisture or solvent concentration and flow measurements in the industrial drying units are taken into account.
During the debinding process, the organic process additives of the ceramic green bodies, such as binders, plasticizers or dispersants, are generally removed thermally. The organic additives are pyrolyzed or - in an oxygen-containing atmosphere - burnt out. However, the use of oxygen is only possible to a very limited extent, as otherwise oxidation of the nonoxide ceramics or damage to the sintering furnace occurs. During the endothermic pyrolysis, heat is consumed locally. 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 released gases cannot be transported quickly enough through the pore channels to the component surface. The resulting overpressure in the pore channels also can also lead to component damage. There are other undesirable phenomena which may occur during debinding, e.g. the absorption of low temperature gases by colder green bodies in continuous furnaces or the segregation of the liquid binder in the pore channels. It is also necessary to ensure that the pyrolysis has no undesirable effects on the sintering process or the product properties. Similar to drying, it is important to find the fastest yet safe temperature profile with which the debinding can be carried out without producing 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 in a controlled atmosphere and temperature at the HTL. The degree of debinding is determined 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.
