This article describes some issues that must be considered when selecting a ceramic for use in a kiln furniture application.

With the wide variety of materials available in today's marketplace, it is essential to select the material that is most suitable for the application. This can be achieved by balancing the properties of the chosen material with the design of the part to ensure maximum life. When selecting a ceramic material for use in a kiln furniture application, thermal shock resistance, along with the maximum operating temperature, determines a material's ability to perform in a certain applications.

For ceramics, thermal conductivity and thermal expansion are the key material properties that determine a material's ability to resist thermal shock and, ultimately, failure. Thermal shock is a common failure mode for ceramics and results from either a rapid heat-up or cool-down of the ceramic body. This can be a significant problem to PM parts producers since metal parts are not as sensitive to thermal shock as ceramics. Also, efficiency in manufacturing requires processing with a quick turn-around rate; consequently, during what is believed to be a routine cycle time for metal parts, the ceramic may prematurely fail due to thermal shock.

Maximum Use Temperature
The maximum use temperature of a ceramic is very important and indicates the temperature above which the body will perform poorly in most applications. As a ceramic material is heated during use, it loses strength as liquid phases develop within the microstructure. As the liquid phases cool they form glassy phases within the matrix that, when cycled, cause premature cracking. These glass phases are susceptible to thermal shock.

Processes that require high maximum temperatures also require a ceramic material that can withstand those temperatures without deforming or failing. Most kiln furniture is made out of alumino-silicate ceramic materials, meaning they are primarily comprised of alumina and silica. Silica transforms into glass at a lower temperature than does alumina, so the amount of silica tends to limit the maximum use temperature. For example, a typical mullite is comprised of 26% to 30% silica and thus has a lower maximum use temperature than does a nominal 90% alumina (see Table I).

What Causes Thermal Shock?
Thermal conductivity and thermal expansion within the ceramic body contribute to the phenomenon of thermal shock. Thermal conductivity is the ability of a body to transport heat. A material with a high thermal conductivity will transfer heat quickly across a body, lowering the overall temperature of the body and decreasing any thermal gradients present. A material with a high thermal conductivity, such as silicon carbide, is less prone to thermal shock since it does not allow for the gradients to build up.

Thermal expansion is perhaps the most significant contributor to thermal shock. All materials expand or contract as the temperature changes. Ceramics typically expand upon heating and contract upon cooling; however, ceramics expand at a lesser rate than metals. If a ceramic body is heated slowly and evenly, no problems will result. If a body is heated or cooled very quickly or unevenly across the body, failure of the part due to thermal shock is likely.



Fig. 2 A schematic of the failure mode associated with the development of thermal gradients within a ceramic tile in relation to the thermal expansion of the material.

PRACTICAL APPLICATIONS

Tile in a Pusher Furnace
As a rectangular tile travels through a pusher furnace, the tile is heated unevenly (Fig. 2). The front half of the tile experiences a higher temperature than the back half and expands more due to a higher thermal expansion that correlates to the hotter temperature. At the same time, the back section of the tile experiences lesser amounts of thermal expansion. This difference in thermal expansion causes cracking as the tile literally pulls itself apart.



Fig. 3 Schematic representation of thermal expansion slots cut into a tile prior to firing.

There are two ways to overcome this problem: (1) make the tile shorter in the push direction, or (2) add thermal expansion slots. When you shorten the tile in the push direction, you lessen the thermal gradient between each end of the tile. This lowers the amount of stress within the tile due to thermal expansion. However, there are many instances where the length of the tile cannot change. In this case, the addition of thermal expansion slots can help. These slots are either cut into the tile or made during the formation of the part. They allow for more even expansion within the tile and reduced thermal gradients. In a sense, the slots allow the tile to act as a smaller tile of the dimensions between the slots (Fig. 3).



Fig. 4 Schematic representation of the failure mode of a tile as is passes through a periodic kiln.

Tile in a Periodic Furnace
In a periodic kiln, there tends to be even heating rates in all directions. Tiles often fail in periodic kilns due to corner cracks (Fig. 4). Corner cracks result from even expansion rates where the thermal stresses build up in the corners of the tile. The best way to prevent corner cracking is to minimize the stresses by rounding off the corners.

Saggers in a Periodic Furnace
Periodic kilns heat evenly from all directions to eliminate the uneven heating that contributes to thermal shock. When stacking saggers (boats) in a periodic kiln, they are stacked typically as high and as close together as possible in order to maximize throughput. Collectively, the saggers represent a large thermal mass in the kiln that requires more time to heat up and retains more heat upon cool down. The amount of heat retained within the parts can be misleading when using thermocouples to determine when to open kiln doors. The actual temperature of the ceramic parts often times is much higher than the thermocouple reading, causing thermal shock upon opening the kiln doors.

There are two ways to overcome this problem-either change the process or redesign the part. Changing the process requires little more than waiting to open the kiln doors until the temperature is lower. Alternatively, adding space between columns of stacked saggers or alternate stacking of the saggers in a brick pattern will allow airflow through the kiln. A fan should never be placed on hot ceramics as this will result in thermal shock. When redesigning a part, it is best to reduce the mass of each individual sagger by using the thinnest wall thickness possible. Also, adding vents to the sagger walls to promote airflow, and adding radii on the corners to relieve stress will also promote long life of the ceramic kiln furniture.



AVAILABLE MATERIALS

Among the common materials used in the PM industry for kiln furniture, each offers certain advantages and disadvantages due to their respective thermal properties. Common kiln furniture materials used in the PM industry are cordierite, fused silica, mullite, alumina, and clay-bonded silicon carbide. The thermal properties of these materials vary resulting in different degrees of thermal shock resistance (Table I).

Cordierite
Cordierite is commonly used as kiln furniture due to its low cost and thermal shock resistance. Cordierite, a magnesia-alumina-silicate, has a low thermal expansion coefficient, which provides for excellent thermal shock resistance. However, other considerations must be taken into account. Cordierite's maximum use temperature is limited to 2400¯F. Firing cordierite above 2400¯F will cause glass formations and reduce its thermal shock resistance and life. Reactions with parts are also a concern with cordierite; for example, ferrites react with cordierite causing spalling and premature failure of the refractory.

Fused Silica
Fused silica offers excellent thermal shock resistance due to its low thermal expansion coefficient. Fused silica is typically composed of 72 to 76% SiO2 and 21 to 25% Al2O3 by weight and is an exceptional alternative to cordierite for kiln furniture. Compared to cordierite, fused silica offers strength advantages and better thermal cycling. However, fused silica is also limited by its maximum use temperature of 2400¯F when cycling to room temperature and cannot be used in dry hydrogen atmospheres above 2200¯F due to silica dissociation. Silica contamination can also be a concern when firing materials that are basic in nature and readily react with silica.

Mullite
Mullite offers better thermal shock resistance than alumina due to its lower thermal expansion coefficient, but is not as thermally shock resistant as fused silica or cordierite. Mullite is composed of approximately 66 to 70% Al2O3 and 26 to 30% SiO2. Mullite has higher strength at elevated temperatures compared to fused silica or cordierite. Mullite also has a higher maximum use temperature of 2750¯F. Mullite makes an excellent choice when firing to temperatures that exceed the use temperature of fused silica and cordierite when thermal shock resistance is required.

Alumina
Alumina offers the greatest maximum use temperature of 3200¯F to 3400¯F. Alumina has high strength at elevated temperatures with good thermal shock resistance. High alumina kiln furniture typically refers to a nominal 90% Al2O3 material with the balance comprised of silica, which is generally in the form of mullite. Alumina is an excellent material choice when silica contamination is a concern due to alumina's chemical inertness with most materials. Alumina is also well suited for use in dry hydrogen and vacuum atmospheres due to its lower silica content. If silica dissociation or contamination is a problem with the 90% alumina, the alumina percentage can be increased to 95, 97 or 99%. However, it should be noted that by increasing the alumina percentage thermal shock resistance decreases. A 95% alumina has fair thermal shock resistance while the 97 and 99% alumina materials have poor thermal shock resistance. If a higher alumina percentage is to be used, the heating rates should be slowed accordingly to avoid thermal shock failures.

Clay-Bonded Silicon Carbide
Clay-bonded silicon carbide has a high thermal conductivity and low thermal expansion, which gives it excellent thermal shock resistance. Clay-bonded silicon carbide is composed of roughly 73 to 77% SiC, 17 to 21% Al2O3, and 4 to 6% SiO2. Silicon carbide also offers high strength at elevated temperatures. The maximum use temperature of silicon carbide is limited to 2450¯F in oxidizing atmospheres due to the oxidation of SiC to SiO2. Silicon carbide is a superb choice when thermal shock resistance is needed in highly reducing atmospheres that will degrade fused silica, cordierite or mullite.



SUMMARY

The ceramic materials commonly used for kiln furniture have been described in this article. A material's resistance to thermal shock is the key factor in determining its applicability in most instances. A more detailed knowledge of the mechanical characteristics involved in this property can aid in the selection of the proper material for a specific application. IH