When firing silicate ceramics, a compaction with defined residual porosity should be achieved. The microstructure should be free of defects and homogeneous, and the component must not warp. If glazes are used, they must be free of defects such as pinholes or bubbles. Deviations from the target geometry occur during firing due to an uneven distribution of porosity in the body as well as the influence of friction, gravity and temperature differences during heat treatment:
- The uneven porosity distribution in the body can be caused by the shaping - e.g. by sedimentation effects in slip casting. It is detected at Fraunhofer-Center HTL with very sensitive measuring methods.
- Friction and gravity lead to distortion of the cullet if its stability is too low. Particularly critical is the range above 1000°C, where a lot of melt phase can form and stabilising reactions such as the formation of secondary mullite still insufficiently counteract.
- Temperature differences within the components lead to distortion during firing. Larger temperature differences in the components can also trigger cracks or lead to the destruction of the components. Temperature differences in the kiln lead to scattering in the residual porosity and the component dimensions.
If iron oxide is contained in the raw materials, the conversion of the trivalent to the bivalent iron, which takes place in air at approx. 1200°C, must also be taken into account. The oxygen released during this conversion must be able to leave the components through the pore channels. If the pores are already closed due to strong sintering shrinkage, the cullet will swell. As a countermeasure, reducing atmospheres must be used to lower the temperature of the iron oxide transformation. Gas phase processes often lead to glaze defects. By means of optical dilatometry and weighing, the sintering and degassing behaviour of glazes can be measured and optimised at the HTL.
The sintering shrinkage of the cullet is precisely measured with the ThermoOptic Measuring (TOM) equipment. The atmosphere of gas-heated kilns is precisely simulated. In addition, other important material properties for heat transport and creep deformation during firing are recorded. Coupled FE models are used to simulate the thermomechanical effects in the component and optimise the process parameters. In addition to the furnace atmosphere and the firing cycle, kiln furniture can also be taken into account, which has a major influence on the temperature distribution in the cullet and may have to be used to support cullet with low stability. The optimisation is possible for annealing and glost firing processes as well as for fast firing. The verification of the optimised process parameters can be carried out on large silicate ceramic components with the ThermoOptic measuring system TOM_gas.