Kiln technology has rapidly evolved during the past few decades. Originally, firing ceramic tiles in a chamber kiln took as long as 38 hours. This firing time (cold to cold) was reduced to approximately 22 hours in a tunnel kiln, and then the trend of using longer rollers with increasingly smaller diameters to obtain higher thermomechanical load capacity led to the introduction of the roller-hearth kiln by the Italian company, Siti. In this type of kiln, firing times were dramatically decreased to less than one hour.
Roller-hearth kilns are mainly used to manufacture tiles, porcelain and sanitary ware, and have many advantages including energy efficiency and high flexibility due to short firing cycles. Additional examples of support roller applications include firing bricks (Fig. 1) and oxide ceramics (Fig. 2), and annealing glass (Fig. 3).
In these kilns, aluminosilicate-base standard kiln rollers have traditionally been used in applications having operating temperatures to 2460 F (1350 C), while synthetically produced oxide-ceramic and silicon-carbide roller materials have been used for particularly high loads or applications reaching temperatures above 2460 F. However, with new application possibilities for roller-hearth kilns, and the rising market demand for roller materials to cope with the sometimes-extreme application conditions, roller producers must modify and further develop the materials they have used in the past. Such new developments have created a fourth group of basic support roller types.
Basic support-roller types
Traditional ceramic support rollers are classified into 3 basic systems:
- Al2O3 - SiO2 (- Al2O3) - The ceramic structure consists of aluminum oxide, mullite and a vitreous phase. Sillimantin (SM 60, SM 65, SM 60 NG, and SM 65 NG) (porous) and Pythagoras (dense) belong to this group of standard types.
- SiC - The SiC-base ceramic structure is recrystallized or R-SiC (Halsic-R/-RX) compact SiC matrix with open-porosity, silicon-infiltrated reaction bonded SiC, or SiSiC (Halsic-I) pressureless sintered SiC, or SSiC (Halsic-S).
- Amorphous SiO2 - The ceramic structure consists exclusively of amorphous, sintered SiO2; that is, a so-called vitreous fused silica (porous).
Demands of modern support rollers include:
- Homogeneity of structure: Support rollers must be homogeneous over their entire length to ensure that mechanical properties are uniform.
- Dimensional accuracy: With ever-increasing roller lengths, tolerances of the outer diameters and roller sagging (TIR, or total indicator reading; i.e., deviation of outer diameter before default after a 360 rotation) are decreasing to ensure directional stability of the ware during firing.
- Thermomechanical stability: As a result of greater roller lengths in wider kilns, the thermomechanical properties must be very good, at low creep propensity.
- Thermal-shock resistance: All support rollers must have excellent thermal-shock resistance. Most roller kilns are operated continuously, and rollers need to be replaced for cleaning or due to failure without interrupting production. The rollers must, therefore, be able to withstand a hot change, allowing the kiln to remain at its production temperature.
- Chemical stability: Rollers must be able to maintain chemical stability, withstanding contact reactions and remaining unchanged by a flux-containing kiln atmosphere. Stability can be considerably improved by a well-targeted use of rollers that were specially developed for the specific purpose.
- Surface finish: A smooth and clean roller surface decreases contamination hazard or sticking of caking. In special cases, the surface will have to be polished to avoid imprints by an uneven roller surface on soft glass plates that are being annealed.
Reasons for developing new materials
New applications using high-performance ceramic rollers require higher maximum application temperatures to 3000 F (1650 C) at high thermomechanical constant loads with extreme thermal-shock resistance. In addition, application temperatures to 3000 F are prerequisite for efficient sintering of a large number of oxide ceramics, such as spark plugs or substrates. A roller-kiln installation will become more efficient with higher roller loads. The RSiC-base rollers currently used for extreme temperatures oxidize at these application temperatures over time, resulting in reactions with the oxide-ceramic firing material by releasing SiO2. Therefore, operating at constant extreme temperatures demand high-temperature materials.
New high-temperature applications also require rollers having improved corrosion resistance, such as against metallic components in oxidizing and inert or H2 containing atmospheres. Improved corrosion resistance primarily is required for thermal treatment of metals where high-alloy steels are heat treated at temperatures to 2280 F (1250 C) under a reducing atmosphere. The problem in this case would be direct contact of ceramic with metal, where the metal oxides represent the very problem. For instance, presence of nitrogen and hydrogen can lead to formation of ?-Si3N4 on recrystallized SiC. Slag formation can develop even on plasma-coated, recrystallized SiC. Various silicides can develop on SSiC. On mullite-containing Sillimantin, there can be a reaction between Fe and SiO2. Figure 4 shows an example of a Sillimantin 65 roller that has been operating for one year in a steel-hardening furnace chamber, in a reducing atmosphere at temperatures below 2190 F (1200 C). The cavities in the structure are not a consequence of chemical attack, but are pores within the structure, which deliberately were produced to give the ceramic material sufficient thermal-shock resistance.
New group of support rollers
Material developments in response to customer demand for increased roller efficiency in new applications focused on discovering an oxidation-resistant material that does not age. This led to the development of new engineered high-temperature, all synthetic-based materials. These materials show good creep resistance because of the absence of a vitreous phase, and have a good (or even very good) thermal-shock resistance due to the appropriate microstructure. This material group represents a new, fourth group of basic support roller types:
A new, porous CM (corundum-mullite) roller material having considerably less creep propensity and good thermal-shock resistance was created by combining Al2O3 and mullite. Extensive tests proved that this material, consisting of a mullite matrix and fused corundum, creeps considerably less than the pure starting materials. Here, the low creep propensity of mullite compared with Al2O3 was used. A prerequisite for this method was using high-purity synthetic raw materials and firing temperatures far beyond 3090 F (1700 C). The porous ceramic structure, practically free from vitreous phase and contamination, is critically important for high refractoriness and excellent thermal-shock resistance. Compliance with the correct ratio of mullite/corundum is also a basic precondition for high-temperature resistance; deviations from the correct ratio result in deterioration of material properties.
Corundum-zirconium and corundum-zirconium-mullite
CM roller material was further modified by incorporating ZrO2 to make CZ (corundum-zirconium) and CZM (corundum-zirconium-mullite) materials. CZ was specifically developed for use in chemically problematic applications. In this case, the mullite component was replaced with ZrO2. Thus, the material is free of SiO2, so it represents roller material having the highest chemical resistance. The mullite matrix, previously responsible for a good thermal-shock resistance, is missing in this combination, and is replaced by open porosity and the zirconium oxide that is incorporated in the corundum matrix. The change in the composition is made at certain reduction in thermal-shock resistance, which, however, is still good. These support rollers have the highest chemical durability, and can be used at temperatures to 2820 F (1550 C) under load, and even to 2910 F (1600 C) under smaller loads. They are suitable for use in metal-treating furnaces in direct contact with dross and in the presence of Na2O, because no nepheline or albite can form, which might, due to its very low melting point, lead to premature support roller failure.
CZM roller material excels due to its excellent thermal-shock resistance, which allows changing rollers at temperatures above 2740 F (1500 C). In addition, its excellent high-temperature resistance under load makes it suitable for use in high-temperature kilns at temperatures to 2910 F. Here, roller rotation needs to be increased with increasing weight of the firing ware. No substances will evaporate from the roller material, and, therefore, these support rollers also can be used to fire parts used in the semiconductor industry. The ceramic structure of CZM is resistant to corrosive attack. Currently, developments are being worked on to optimize the ratio, the grain size of the synthetic raw materials and the firing process to achieve operating temperatures to 3000 F (1650 C) for higher loads, without sacrificing thermal-shock resistance.
Figure 5 shows the performance of the new roller materials compared with traditional oxide support rollers. The values were determined by mechanically loading rollers in a high-temperature roller test mode. Note the considerably higher operating temperatures of the new materials. Table 1 lists important characteristics and properties of the new, oxide high-temperature rollers.