Kiln Furniture

Kiln furniture is used in heating processes in the production of individual pieces such as ceramic or metal components. The kiln furniture carries the charge and is intended to ensure that the components neither deform nor stick to one another. In order to achieve a high product quality and a low reject rate, the charge is to be heated and cooled as homogeneously and uniformly as possible, regardless of the position in the furnace. The kiln furniture plays an important role in the heat management of the furnace. The interaction of the charge with the furnace atmosphere is also strongly influenced by the kiln furniture. In order to control the gas exchange, in addition to different carriers and plates, capsules are often required, in which the heating material is placed.

In addition to the material, the kiln furniture must always be heated during the heat treatment processes. As a result, energy consumption rises – in case of small components, it might even be doubled. Kiln furniture components are wearing parts, which must be replaced regularly because of the thermomechanical and chemical stresses during use. Their production causes additional energy expenditure and increases the operating costs of the furnace plants. In order to increase the energy efficiency of the heat treating processes and to reduce the costs, the heat capacity of the kiln furniture must be reduced and their life cycle increased.

In order to reduce the heat capacity, the materials are specifically optimized for its use as kiln furniture at the Fraunhofer Center HTL. The reduction in the mass of the kiln furniture is achieved by a thinner wall design and / or an increase in its porosity. However, both approaches have a negative impact on the load-bearing capacity. While at low temperatures increased stresses more likely lead to faiure by fracture, increased deformation due to creep occurs at higher temperatures. Kiln furniture materials with a high load bearing capacity and a low mass are a particular challenge in material development.

For the best possible orientation in this trade-off, Finite Elements (FE) methods are available at the HTL for the material and component design. However, in simple cases, as in the example mentioned, so-called material indices can also be very helpful. All material properties and boundary conditions relevant for the specific application are combined to form a single index, the material index, which must then be optimized^[1]. E.g., in order to be able to lay out a plate-shaped kiln furniture material with low creep deformation while simultaneously achieving the lowest possible heat capacity, its density, specific heat capacity and uniaxial viscosity are combined to form a material index. With this approach, benchmark tests can be performed using a single key figure. At the HTL, porous kiln furniture materials have been developed with significantly increased performance compared to commercial products.

The first step in the development of kiln furniture is the development of the base materials. The ceramic raw materials are carefully prepared and mixed as homogeneously as possible using various additives. The ceramic masses are further processed by various forming processes, such as, for example, slip casting or wet pressing. It is important to dry the bodies gently without any defects. The quality of the resulting green bodies is a prerequisite for a high-quality end product. In order to be able to evaluate the quality of the green bodies, special analysis methods are used at the HTL. Forming, drying and debinding are followed by the sintering of the kiln furniture. The sintering process is adjusted with the methods developed at the HTL in such a way that the above-mentioned material properties are matched optimally. For the determination of the material, different analytical methods are available at the HTL. For a well-aimed optimization of the kiln furniture, it is particularly important to know the properties of the materials under operating conditions. For this purpose, ThermoOptical measurement systems (TOM) have been developed at HTL for the characterization of high temperature properties. For the evaluation of kiln furniture, thermal shock and thermal cycling behavior, thermal conductivity, mechanical properties and creep deformation are determined up to very high temperatures.

At the HTL, kiln furniture is developed from different ceramics, produced as prototypes, and tested for their operating behavior. These may have oxidic compositions, such as alumina, mullite, cordierite or zirconia, or non-oxidic compositions, e.g., SiC or SiSiC. In addition to pores, ceramic fibers can also be integrated in the kiln furniture materials in order to improve the application properties. For the development of prototypes with complex geometry, 3D printing is used. Ceramic protective coatings are developed at the HTL for corrosion-resistant kiln furniture. These are applied via wet chemical coating techniques such as dipping, spraying or brushing and then burned.

^[1] Raether, F.: Ceramics Facing Competition with other Materials, Ceramic Applications, 4/2016, pp. 57-61