Cellular Ceramics / 5

5.3 Kiln Furnitures 449

5.3.3

Manufacture of Kiln Furniture

5.3.3.1

Foam Replication Process

The foam replication process [10] (see also Chapter 2.1) manufactures cellular ceramics to a selected pore size and geometric shape, and thus makes design work economical and flexible. Molds or forming dies, which tend to be expensive, are not used. Design and construction of molds and dies are also time-consuming. Dimensional adjustments with foam are fast and easy. In the process, polyurethane foam of desired pore size and geometry is first selected and shaped; this material functions as the precursor structure to the ceramic foam. The final pore size of the ceramic is determined by the pore size of the polyurethane foam and the firing shrinkage of the ceramic. The pore size range for replication-type cellular ceramic is about 200–5000 mm. Dimensional extremes are also material dependent, ranging up to about 60 mm for the thickness and 500 mm for the length of planar structures. Minimum dimensions are set by the pore size of the chosen precursor and the need to maintain sufficient mechanical strength. Once the polyurethane foam is prepared, it is coated with slurry of appropriate composition and rheology, dried, and fired.

Between drying and firing, the foam can be further processed:

Integrated posts or rails can be incorporated into the furniture for stacking purposes. Some applications require very large components with dimensions greater than 1 m. These cannot be manufactured in one piece due to limitations in the manufacturing process, but can be produced by using modular design, where components are interlocked by bevels or shiplap joints and bonded after firing. Figures 6 and 7 show examples of shapes created by the foam replication process for use in specialty kiln applications; these represent only a small portion of the potential variations.

The porosity in reticulated kiln furniture provides several advantages, but also presents one disadvantage in some cases. When the component being fired has fine geometric detail, undergoes very high shrinkage, and/or tends to slump at high temperatures during sintering, it can get caught in the porosity at the surface (ware grabbing) or slump slightly into the face of the furniture, and this creates an imprint of the foam on the part. Reducing the pore size can help to inhibit these defects, but as the pore size is decreased, grinding the reticulated foam to a smooth finish becomes increasingly difficult. The struts themselves are much smaller and weaker as a result, and cannot hold up to the stresses generated in the grinding process, so they tend to break instead of being polished. In addition, the thermal shock resis-

5.3 Kiln Furnitures 451

tance of the foam is reduced as the pore size is decreased, as shown by Vedula et al. [7–9], and this is an undesirable characteristic. For these cases, experimental reticulated kiln furniture is being tested. A novel concept involves applying a continuous, thin “skin” of dense ceramic on one or more sides of a ceramic foam article, as shown in Fig. 8 [11]. This solves some of the problems described above without entirely losing the low density of ceramic foam. The foam can be skinned on two sides, creating a sandwich and providing a flat surface on the bottom for sliding or additional strength. The skin can also be a different composition from the foam [12].

5.3.3.2

Foams Manufactured by using Fugitive Pore Formers

Cellular ceramics made by using fugitive pore formers can achieve much smaller pore sizes than those from reticulated foam, typically between 5 and 200 mm. These products are generally pressed, cast, or extruded. Micromass is a commercial product of this type having 100 mm pores connected by 10 mm necks [13–15]. A polished cross section is shown in Fig. 9. The standard commercial composition is zirconiatoughened alumina, but other compositions have been developed for special applications. The pores are formed by using polymeric spheres. The porosity is about 70 %, and as a result the product can be easily machined with relatively good precision. For a 17 0 28 0 1 cm plate, a flatness of 500 mm is achievable, which is measurably better than is achieved when grinding reticulated foam. The fine porosity inhibits ware grabbing and imprinting, and the good machinability and flatness help to meet tight tolerances in the fired ware. Strength is well retained on cycling, as demonstrated by data in Fig. 10, which compares the MOR values of Micromass plates that were cycled in a commercial production facility for electrical components for several months with those of new uncycled plates.

Fig. 9 Scanning electron micrograph of a polished cross section of zirconia-toughened alumina (Micromass).

Fig. 10 Comparison of the MOR of Micromass kiln furniture plates cycled for several months at a production facility for electrical components with an aggressive thermal cycle versus those for new uncycled plates over a broad density range. The curve represents a power-law fit to the NEW data.

5.3.4

Summary

Performance benefits of cellular ceramic kiln furniture can include: