New roller materials are enabling roller hearth kilns to provide reduced firing times, increased flexibility and other benefits in high-tech applications.



Kiln technology has evolved rapidly over the last few decades. At one time, firing ceramic tile in a chamber kiln took as long as 38 hours. Tunnel kilns reduced this firing time (cold to cold) to approximately 22 hours. Companies eventually discovered that using longer rollers with increasingly smaller diameters provided higher thermomechanical load capacity, and this discovery led to the introduction of the roller hearth kiln by the Italian company Siti in the early 1970s. In this type of kiln, firing times for tile were dramatically decreased to less than one hour.

The short firing cycles of roller hearth kilns enable them to provide advantages such as energy efficiency and high flexibility. However, these kilns have primarily been relegated to applications in the traditional ceramic industry, such as tile, porcelain and sanitaryware, due to the limitations of conventional rollers. Standard aluminosilicate-based kiln rollers have been used for applications in temperatures up to 2460°F (1350°C), while synthetically produced oxide-ceramic or silicon carbide roller materials have been used for particularly high loads or applications reaching temperatures above 2460°F (1350°C).

Today, new application possibilities for roller hearth kilns in advanced areas such as firing oxide ceramics, annealing glass and heat treating steel are creating a demand for roller materials that can cope with the extreme conditions that are often found in these areas. As a result, roller producers have begun modifying and further developing materials for these high-tech applications.

Limitations of Conventional Rollers

Traditional ceramic support rollers can be classified into three basic systems: Al₂O₃-SiO₂ (-Al₂O₃), SiC and amorphous SiO₂. In the Al₂O₃-SiO₂ (-Al₂O₃) system, the ceramic structure consists of aluminum oxide, mullite and a vitreous phase.¹ SiC rollers are typically classified as recrystallized SiC (RSiC), which is a compact SiC matrix with open porosity; silicon-infiltrated reaction-bonded silicon carbide (SiSiC); or pressureless sintered silicon carbide (SSiC).² Amorphous SiO₂ features a ceramic structure consisting exclusively of amorphous, sintered SiO₂, which is also known as "vitreous fused silica" (porous).

Although all of these rollers work well in kilns firing traditional ceramics, such as tile and sanitaryware, they cannot handle the high application temperatures up to 3000°F (1650°C) required for efficiently sintering a large number of oxide ceramics, such as spark plugs or substrates. RSiC rollers that are currently used in these applications tend to oxidize over time and release SiO₂, which reacts with the oxide ceramic products being fired.

Conventional rollers are also susceptible to corrosion in certain atmospheres. For instance, in the thermal treatment of metals, where high-alloy steels are heat-treated at temperatures up to 2280°F (1250°C) in a reducing atmosphere, the presence of nitrogen and hydrogen can lead to the formation of ß-Si₃N₄ on RSiC rollers. Slag formation can develop even on plasma-coated RSiC, and various silicides can develop on SSiC. On mullite-containing Al₂O₃-SiO₂ (-Al₂O₃) rollers, a reaction between Fe and SiO₂ can occur.

In some cases, these rollers can be tailored to meet the demands of certain applications. For example, the SEM images above show an Al₂O₃-SiO₂ (-Al₂O₃) roller that has been operating for one year in a steel-hardening furnace chamber in a reducing atmosphere below 2190°F (1200°C). The cavities in the structure are not a result of chemical attack, but are pores within the structure that were deliberately produced to give the ceramic material sufficient thermal-shock resistance. For many high-tech areas, however, new roller materials are needed in order for roller hearth kilns to be efficiently used.

Roller Criteria

The ideal support rollers must show a homogeneous structure over their entire length to ensure that the mechanical properties are uniform. Additionally, as ever-increasing roller lengths are needed to fit increasingly wider kilns, the tolerances of the outer diameters and roller sagging (measured through the total indicator reading [TIR]*) must continually decrease to ensure the directional stability of the ware during firing.

The rollers must also possess good thermomechanical properties, high creep resistance and excellent thermal shock resistance. Since most roller hearth kilns are operated continuously, the rollers need to be replaced for cleaning or maintenance without interrupting production. They must therefore be able to withstand a hot change, which allows the kiln to remain at its production temperature.

The rollers must also be able to maintain chemical stability, withstand contact reactions and remain unchanged by a flux-containing kiln atmosphere. Additionally, a smooth, clean roller surface is needed to decrease contamination hazards, sticking or caking. In some cases, the surface might even need to be polished-for instance, to avoid imprints on soft glass plates annealed by an uneven roller surface.

Other criteria include higher maximum application temperatures of up to 3000°F (1650°C) at high thermomechanical constant loads with extreme thermal-shock resistance; and improved corrosion resistance, such as against metallic components in oxidizing and inert or H₂-containing atmospheres. *The total indicator reading is the deviation of the outer diameter before default after a 360° rotation.

Developing New Roller Materials

Although some of the foregoing roller criteria involve careful forming and finishing techniques, many others rely on the material formulation. In response to the demand for increased roller efficiency in advanced applications, researchers have focused on discovering an oxidation-resistant material that does not age. Recently, these efforts have resulted in the development of several new high-temperature, synthetic-based materials for rollers. These materials exhibit good creep resistance due to the absence of a vitreous phase, as well as good thermal-shock resistance due to their carefully controlled microstructure.

One of the new materials is a porous corundum-mullite (CM) formulation based on Al₂O₃ and mullite. Extensive tests have proved that this material, which consists of a mullite matrix and fused corundum, creeps considerably less than the pure starting materials due to the low creep propensity of mullite against Al₂O₃. The prerequisites for this product were the use of high-purity synthetic raw materials that could be processed at temperatures beyond 3090°F (1700°C).

The CM material also contains virtually no vitreous phase or contamination. As a result, rollers made using the material exhibit high refractoriness and excellent thermal shock resistance. (It is important to note, however, that achieving high temperature resistance requires the correct ratio of mullite to corundum. Deviations will result in a deterioration of material properties.)

Researchers also discovered that the CM formulation could be further modified by incorporating another component, ZrO₂, to create two other roller materials-corundum-zirconium (CZ) and corundum-zirconium-mullite (CZM). CZ was specifically developed for applications in which chemical interactions typically create firing problems. To create this material, researchers replaced the mullite matrix that is responsible for good thermal shock resistance in the corundum-mullite material with open porosity and ZrO₂. Although this change provides a slight reduction in thermal shock resistance, support rollers made with the CZ material exhibit extremely high chemical durability (due to the lack of SiO₂) and can be used at up to 2820°F (1550°C) under high loads and even to 2910°F (1600°C) under lesser loads. They are suitable for use in metal-treating furnaces in direct contact with dross, and they can also be used in the presence of Na₂O because they prevent nepheline or albite from forming in the kiln. (With conventional rollers, the formation of these materials often leads to premature roller failure.)

CZM features excellent thermal shock resistance, which allows rollers made with the material to be changed at temperatures above 2739°F (1500°C). (The roller rotation needs to be increased as the weight of the fired ware increases.) Its excellent high-temperature resistance under load also makes it suitable for use in high-temperature kilns up to 2910°F (1600°C). Because no substances will evaporate from CZM rollers, they can be used to fire products for the semiconductor industry. The ceramic structure of the rollers is also resistant to corrosive attack. Developments are currently under way to further optimize the material ratio, the grain size of the synthetic raw materials and the firing process to achieve operating temperatures of up to 3000°F (1650°C) for higher loads without sacrificing thermal shock resistance.

Figure 1 shows the stress behavior of the new roller materials compared to the traditional oxide support rollers. The values were determined by mechanically loading the rollers in a high-temperature roller test mode. Note the considerably higher operating temperatures of the new materials. The main physical parameters of the new materials are listed in Table 1.

Looking Ahead

For more than 30 years, roller kilns have been limited by the use of traditional materials based on aluminosilicates for standard roller applications up to 2460°F (1350°C). The development of corundum-mullite, corundum-zirconium and corundum-zirconium-mullite as support roller materials is revolutionizing traditional methods by providing increased temperature resistance, longevity, low creep propensity, increased thermal shock resistance and outstanding chemical resistance for a variety of new uses. However, with the increasing number of new applications for roller kilns and the ongoing demand for improved performance, roller producers must continue to develop new materials to keep up with the expanding market.

For more information about the new roller materials discussed in this article, visit Morgan Advanced Ceramics' website at www.morganadvancedceramics.com or contact the company at one of the following locations:

• North America: 100 Cooper Circle, Peachtree City, GA 30269; (800) 433-0638; fax (770) 261-4801; or e-mail sales@morganadvancedceramics.com .
• UK and Ireland: Bewdley Road, Stourport, Worcestershire DY13 8QR; (44) 1299-872210; fax (44) 1299-872218; or e-mail info@mac-sales.com .
• Mainland Europe: Bahnhofstrasse 16, D-85774 Munich-Unterföhring,Germany; (49) 89-416098-0; fax (49) 89-416098-41; or e-mail info@mac-eu.com .
• Asia: 158 Jiajian Road, Jiading, Shanghai, 201818, People's Republic of China; (86) 21-5951-0809; fax (86) 21-5951-1241; or e-mail mmsh@public.sta.net.cn .

Links

• Morgan Advanced Ceramics