Cellular Ceramics / 5

439

5.3

Kiln Furnitures

Andy Norris and Rudolph A. Olson III

5.3.1

Introduction

A definition for kiln furniture might be movable articles, similar to chairs and tables, which are necessary and useful for firing materials at high temperature. At the simplest level, when a ceramic or metal component is fired, it must sit on something, and this something is usually kiln furniture.

When a ceramic or powdered metal component is manufactured, it typically undergoes a multistep heating process called a thermal cycle, an example of which is shown in Fig. 1. Initially, the green component (prior to firing) is typically heated to an intermediate temperature to allow organic binder or chemically bound water to exhaust from the component at an acceptable rate. Next, the component is heated to relatively high temperature to sinter or chemically bond the material together. Finally, the component is returned to room temperature, possibly with another hold at some other temperature for further processing; this last step might be annealing or exposure to a gas such as oxygen, argon, or hydrogen, to control redox conditions in the final component. The component will often shrink during the cycle, by as much as 30–40 %. The final microstructure, geometry, composition, properties, and performance of the component are a function of the firing process.

One of the main reasons kiln furniture is used to support ware in a kiln is that manufacturers are trying to meet product design specifications and reduce variation in their processes. The goal is to manufacture the component to fall within certain quality-, property-, and performance-based tolerances. Well-designed kiln furniture facilitates this because it provides an inert, nonstick, thermally stable, flat (or supportive) substrate on which a component can sit during its thermal cycle. If the furniture provides these functions without breaking or deforming during thermal cycling, then its continued use will tend to reduce variability in the characteristics of the ware, and the final product will more closely meet the design specifications of the product. For these reasons, kiln furniture is an essential part of many ceramic and metal powder manufacturing processes, but with this vital association come some limitations.

Traditional kiln furniture tends to be dense, which is attractive from the standpoint of mechanical strength, but its high mass is unfavorable with respect to energy consumption, weight, ergonomics, and thermal shock resistance in relatively fast

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

440 Part 5 Applications

Time (hours)

Fig. 1 Example of a thermal cycle for processing a green ceramic component.

thermal cycles. Dense kiln furniture also tends to have relatively high silica content to lower thermal expansion and enhance thermal shock resistance [1]. This may be unacceptable in situations where silica is a contaminant to the ware. Silica also migrates quickly under reducing conditions, as solid SiO2 converts to vaporous SiO when reduced. Making the kiln furniture highly porous can lessen these limitations. This type of product is usually termed cellular ceramic or ceramic foam.

The pore size used for kiln furniture generally ranges from about 5 mm to 5 mm, depending on the application, and the volume fraction of porosity tends to be between 50 and 85 %. Just about any pore size or density can be manufactured within this range by using fugitive pore formers, but these processes typically have trouble manufacturing material suitable for use as kiln furniture at large pore size and high volume fraction of porosity. The pore size of reticulated foam manufactured by the foam replication process ranges from about 200 mm to 5 mm and the volume fraction of porosity can be very high (85 %). Processes that generate foam through the use of stable air bubbles or self-expanding slurries containing gas-generating materials can make foam with finer pore size averaging between about 50 mm and 1 mm, but can have difficulty making foam pores in the 1–5 mm range. Casting foam in molds by gelation or coagulation processes produces foams with an average pore size in the range of 50–300 mm. Freeze casting of ceramic slurries generates 5–50 mm sized pores through the formation of ice crystals and the associated 9 % expansion. Vacuum-formed fiber compacts also tend to have pores in this size range. In all of these processes, the pore structure within the foam tends to be interconnected, which is different from structures such as brick containing bubbled alumina, in which the pore size may average hundreds of micrometers, but the pores are virtually disconnected.

The characteristics of lower density, good refractoriness, thermal stability, and improved thermal shock resistance can give cellular ceramics enhanced performance over traditional dense kiln furniture. In addition, the enhanced thermal

5.3 Kiln Furnitures 441

shock resistance allows furniture to be made essentially silica-free, so reactivity with ware tends to be lower. With these benefits comes one obvious trade-off: lower mechanical strength. For instance, the modulus of rupture (MOR) of reticulated ceramic kiln furniture tends to be 3–7 MPa, whereas the MOR for dense furniture tends to be 30–70 MPa or more. Thus, cellular ceramic furniture may not be able to support a particularly heavy load across a span, or display enough resiliency to function as a pusher plate. Due to its lower strength, the maximum load cellular kiln furniture can support without creep (deformation under mechanical load at high temperature) is generally lower than that of dense furniture. Taken as a whole, these drawbacks must be weighed against the benefits when evaluating cellular ceramics for application as kiln furniture. Examples where cellular kiln furniture performs extremely well are stackable 7 0 20 0 1 cm setters for firing oxygen sensors, 20 0 25 0 1.3 cm plates for firing powder metal components, stackable 10 0 10 0 1 cm tworail setters for firing dielectric components, and stackable 10 0 15 0 20 cm saggers for firing pelletized ceramic media. There are many niches in this traditional market segment where the performance of a cellular ceramic adds value to an operation. These performance benefits as applied to kiln furniture are the main topic of this chapter.

5.3.2

Application of Ceramic Foam to Kiln Furniture

Advantages of ceramic foam for use as kiln furniture can include 1) longer life, 2) better uniformity of atmosphere surrounding the fired ware, 3) reduction of frictional forces generated during ware shrinkage, 4) chemical inertness such that it does not react with atmosphere or fired ware, and 5) cost benefits.

5.3.2.1

Longer Life

Rapid advances in the areas of ceramics and powder metallurgy have resulted in increased complexity and decreased size and mass of the manufactured components. The same level of attention has not been applied to the kiln furniture on which the components are fired. Relatively large and heavy furniture is still commonly used to fire very small, low-mass components. The refractory nature of the furniture also means it tends to have a high coefficient of thermal expansion (CTE) and low thermal conductivity, which translate to poor thermal shock resistance. In many cases the kiln furniture, and not the component, limits the thermal cycle, which is an intolerable fact given the constant pressures of productivity improvement in the current manufacturing environment.

Cellular kiln furniture tends to have longer life than traditional dense furniture when an aggressive firing profile is employed, as is commonly used in the manufacture of electrical or powder metal components. There is a continuous desire to speed the thermal cycle for these products to enhance productivity, reduce cycle time, and

442 Part 5 Applications

lower cost. A standard cycle for such products might be 2–3 h to 1150 C with hold, then cooling over 1–1.5 h to about 150 C, followed by immediate removal from the kiln. The fast cooling rate is especially harsh on ceramic kiln furniture, as it generates tensile stresses in the body, and it is well known that ceramics are generally an order of magnitude weaker in tension than compression. Figure 2 demonstrates how tensile stresses develop in kiln furniture during cooling. The CTE of most materials is positive, that is, they expand as the temperatures increases, and contract as it decreases. When kiln furniture is cooled, the exterior cools faster than the interior, which causes the exterior to contract faster than the interior, and induces tensile stress on the surface. Under similar thermal conditions, materials with higher CTE will tend to generate larger tensile stresses than those with much lower ones (e.g., alumina and zirconia both have CTEs of about 9 0 10–6 mm mm C–1, whereas that of mullite is about 4.5 0 10–6 and that of cordierite less than 2 0 10–6). Ultimately, the most important question is performance-based: how long will the kiln furniture last?

Tatmosphere

Tinterior

compressive stresses

Fig. 2 Schematic cross section of a cooling piece of kiln furniture with a plate configuration. The temperature Tatmosphere represents the kiln atmosphere. Tatmosphere < Texterior < Tinterior. In this representation, tensile stresses develop in the exterior, whereas the interior is in compression.

Ceramic foam furniture is weaker than dense ceramic furniture, but this can be used to advantage to gain longer lifetimes in applications having aggressive firing cycles. Hasselman [2–4] developed several thermal shock resistance parameters that characterize the behavior of ceramics subjected to thermal stress. These parameters represent two main criteria: 1) resistance to crack initiation, and 2) resistance to crack growth, which is correlated with minimization of the release of stored elastic strain energy.

If cracks are to be completely avoided in the furniture and not allowed to initiate, the following parameter must be considered [3]:

where R is the maximum temperature gradient a component can withstand before cracks are initiated (the higher the value, the greater the resistance), E is Young’s modulus (Pa), m Poisson’s ratio, ry yield strength (Pa), and a coefficient of thermal expansion (mm mm C–1). The units of a can be understood from its definition (Eq. (2))

To completely avoid crack initiation with every thermal cycle, the furniture must have a composition with sufficiently high yield strength, low Young’s modulus, and low thermal expansion while remaining sufficiently inert and retaining its refractoriness. In this case, ceramic foam is not very different from dense ceramic, as discussed below.

At densities between about 10 and 25 %, the yield strength of reticulated ceramic foam follows the relationship

where is the density of the foam (g cm–3), C a constant, and s and rs are the density and yield strength of the solid ceramic material, respectively. The value of n is typically around 2, regardless of whether testing is performed in compression or bending. A similar relationship is found for the dependence of Young’s modulus on density:

where Es is the Young’s modulus of the solid ceramic material. Experimental evidence from Gibson and Ashby [5], as well as a rigorous finite-element modeling approach by Roberts and Garboczi [6], has shown that n is also about 2 in this relationship. Because E and ry display nearly the same dependence on density, according to Eq. (1), these parameters do not significantly influence the resistance to crack initiation with decreasing density from dense solid to porous foam. Note that Poisson’s ratio is relatively constant with change in density [5, 6], so it would also have minimal influence on R. The one parameter in Eq. (1) that is indirectly influenced by density is a, because the thermal gradient across the foam is dependent on thermal conductivity, which is influenced by the presence of porosity. More detail on thermal shock resistance as influenced by thermal conductivity is provided in Refs. [2–4] and Chapter 4.3 of this book.

Once cracks or flaws have been nucleated in an article of furniture, the dominant parameter becomes resistance to crack propagation, and it is then desired to reduce the amount of stored elastic strain energy released upon failure. The following equation can be used to estimate this behavior [3]:

R¢¢¢ is also considered a resistance parameter, as it is inversely related to the extent of damage incurred at failure; the larger the R¢¢¢ value, the less energy is released on failure and the lesser the extent of crack propagation, so the resistance to crack propagation is increased. In this equation, the yield strength and Young’s modulus are inverted from Eq. (1), and the yield strength is now a squared term. Because E and

446 Part 5 Applications

Projection of the strength to 10 000 cycles suggests that ZTA and Mg-PSZ would be too weak to handle, yet YZA is four times stronger than the others and would still have some integrity. The experiment was also repeated on ten samples of YZA with a higher density of 20–21 %. As shown in Fig. 5, the data still fall on the same curve as those for 14 % dense material. This is in agreement with Vedula et al. [7–9], who stated that thermal shock resistance of ceramic foam is not a strong function of density. The projected strength of the higher density YZA at 10 000 cycles is over 700 kPa.

Under these extreme conditions, ceramic foam performs relatively well, and even the most aggressive powder-metal or electrical-component thermal cycles cannot match the severity of that used in this experiment. The better performance of YZA is due to the microstructural and material-dependent properties of the composite (of important note is that YZA is entirely composed of highly inert, refractory materials and does not contain silica to reduce its CTE). When properly designed and implemented, the lifetimes of cellular kiln furniture in these applications tend to be on the order of months to years.

Table 1 Modulus of rupture (MOR) for thermally shocked ceramic foams.

* SD = standard deviation. ** Projected.

5.3.2.2

More Uniform Atmosphere Surrounding the Fired Ware

The open structure of cellular ceramic provides better access to the bottom of the part in contact with the setter, allowing the gas in the atmosphere to interact more uniformly with the bottom as well as the sides and top. For example, certain tita- nate-based electrical ceramics must be fired in an oxidizing atmosphere to finely tune desired electrical characteristics such as dielectric constant and loss tangent. In

5.3 Kiln Furnitures 447

some cases, easily reduced dopants such as Sn or Zn are used in the component to achieve desired effects. Firing on a dense surface may sometimes result in reducing conditions at the bottom of the component in contact with the setter, and the desired electrical characteristics are not achieved because the redox chemistry of the component is compromised. Promotion of gas flow to the bottom of the component helps to prevent reducing conditions. Typical kiln furniture for this particular application is made of calcia-stabilized zirconia. A similar situation arises in the manufacture of translucent alumina components. These are also fired on reticulated calcia-stabi- lized zirconia foam plates in an oxidizing atmosphere to more effectively burn out organic binder prior to sintering in hydrogen.

5.3.2.3

Reduction of Frictional Forces during Shrinkage

The sintering of powder metal components is another process where the properties of cellular kiln furniture are utilized. Metal injection molded (MIM) parts experience more extensive binder burn out and have greater shrinkage than more traditional parts (> 20 %). At firing temperatures of 1100 C and below, powder metal parts are typically sintered on a metal mesh belt or on dense cordierite plates, but at higher temperatures, the refractoriness, chemical inertness, and thermal shock resistance of cellular ceramics is required. The furniture must be stable in hydrogen atmosphere and must not react with the metal; hence, silica-bearing furniture is usually avoided.

In such cases of high shrinkage, frictional forces between the component and the furniture must be kept low to prevent the part from failing. When the part is shrinking at high temperature, the organic binder is gone, and the component is simply held together by interparticle contacts, so it does not have considerable strength. Such interparticle contacts between the component and the furniture create friction as the part shrinks. If the frictional forces exceed the strength of the component at any time in the cycle, the part tears or cracks. The porosity at the surface of the furniture minimizes the point contacts between the component and the furniture, inhibiting reaction between the two and reducing frictional forces during shrinking.

5.3.2.4

Chemical Inertness

As extremes are approached in temperature and dimensions for a particular application, recrystallized silicon carbide is often preferred for strength and low mass, yet even in these cases, a cellular plate may be used as a barrier between the component and the SiC kiln plate. For example, slip-cast ceramic sputtering targets are sintered on relatively thin, porous plates of zirconia-toughened alumina stacked on thin SiC plates to maximize kiln loading; the SiC plates provide the necessary support and the ceramic foam protects the part from silica contamination from the SiC plate.

In some cases, the ware contains elements such as zinc or manganese that tend to be very mobile at high temperature and have an undesirable propensity to migrate from the ware. These elements are commonly used in electrical compo-

448 Part 5 Applications

nents to finely tune certain properties, and when they are leached out, the properties degrade. The component may also be composed of a very reactive ceramic, such as iron oxide used to make ferrite components. To combat this effect, the composition of the cellular furniture is designed to more closely match that of the ware to reduce the chemical potential between the two. For example, one cellular ceramic manufacturer markets porous zinc oxide kiln furniture for firing of zinc oxide electronic components for use in computers and cell phones. The kiln furniture must retain the other primary properties that allow it to function and, depending on the composition, it can be easier to manufacture such custom formulations in the form of a cellular ceramic than in dense form.

5.3.2.5

Cost Benefits

Cellular kiln furniture can also have cost benefits. The 50–85 % lower mass compared to dense kiln furniture translates to much less material to heat in each thermal cycle and lower energy costs. Lighter loads also translate to ergonomic benefits, which can save money through greater productivity and fewer injuries, or may enable the use of robots to load kilns.

Alumina is fairly cheap, so dense alumina kiln furniture is relatively economical, but it can react detrimentally with certain materials. Firing on zirconia can minimize reactions, but in dense form the cost is typically prohibitive because it is many times more expensive than alumina ($ 0.70/kg versus $2.50 kg for fine powder). The combination of a zirconia cellular ceramic set onto a dense alumina-based setter provides a sacrificial barrier in a cost effective fashion, combining the high strength of the dense alumina setter with the low weight of the cellular zirconia plate. Compositions and uses of reticulated ceramic kiln furniture are listed in Table 2.

Table 2 Compositions and uses of atypical reticulate ceramic kiln furniture.