- THE MAGAZINE
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 RollersTraditional 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 CriteriaThe 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 MaterialsAlthough 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).
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.)
Looking AheadFor 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:
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