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Saint-Gobain Ceramic Materials recently introduced enhanced silicon carbide refractory products for the metallurgical, kiln furniture and waste-to-energy markets. These new products offer greater oxidation protection throughout the high-temperature ranges and environmental conditions for these process industries.
Silicon carbide is a building block for many electrical products, shaped and unshaped refractories, and wear-resistant products for a wide range of process industry applications. Since its early commercial development in the late 1800s as an abrasive material, its high hardness, high thermal conductivity at elevated temperatures, corrosion resistance and electrical properties have made silicon carbide useful for applications as diverse as kiln furniture, rocket nozzles, body armor, radiant burner tubes, metallurgical furnace linings, and heating elements, among others.
Industrial-grade silicon carbide raw materials are produced in large volumes in high-temperature electric furnaces using sand and petcoke as raw materials. Considering the temperature gradients and variability in the process conditions during the high-temperature reactions that form silicon carbide, the silicon carbide grains made in these furnaces are separated according to grades (e.g., metallurgical, refractory, abrasive and electrical) and are crushed and sized for end-use applications.
Challenges for Refractory Applications
While silicon carbide performs well in a wide range of applications in harsh environmental conditions, one weakness in process industry applications has been in its oxidation resistance, particularly in the temperature range of approximately 900-1100°C. In refractory applications, the oxidation of silicon carbide leads to volume and linear expansion, which, if severe enough, can result in cracking and failure of the refractory products.
As is the case with many materials exposed to air, silicon carbide forms a very thin passivation layer of silica on grain surfaces, even at room temperature. As temperatures increase, the thickness of the oxide layer increases, providing a glassy protective layer that inhibits accelerated oxidation up to about 1600°C. Due to the changing characteristics of the silica layer, silicon carbide is most sensitive to oxidation in the temperature range of 900-1100°C. Steam, alkalis, carbon dioxide and other chemicals in the process environment also affect the composition and viscosity of the glassy layer at elevated temperatures, particularly in the range of 900-1100°C. When silicon carbide refractories have to operate in this temperature range, performance and refractory life can be affected unless the silicon carbide oxidation process is inhibited through design and engineering approaches.
The mechanisms and kinetics of silicon carbide oxidation have been extensively studied, particularly for the electronics industry. In the oxidation of silicon carbide refractories, cristobalite (a crystalline phase of silicon dioxide) can form on the surface of the refractory shapes and in the open porosity. Since cristobalite undergoes a phase change related to volume expansion, thermal cycling results in internal expansion stresses that can cause cracking and failure of the silica passivation layer, leading to accelerated damage of the refractory products and/or wastage of the refractory surfaces.
Innovation through Observation
The Saint-Gobain Ceramic Materials research and development team at CREE (Centre de Recherche et d’Etudes Européen) in Cavaillon, France, had been following developments in ceramic matrix composites and were aware that various non-oxide compositions could be modified to enhance the composites’ oxidation resistance in high-temperature aerospace applications. Considering the well-known concern about silicon carbide oxidation, the research staff conducted a project to investigate enhancements to nitride-bonded and oxynitride-bonded silicon carbide refractories.
New silicon carbide refractory compositions were developed using the oxidation enhancement technology and tested in the laboratory for properties including density, porosity and microstructure development. ASTM C 863-00 Standard Test Method for Evaluating Oxidation Resistance of Silicon Carbide Refractories at Elevated Temperatures was used to compare existing and new compositions for oxidation resistance. The ASTM C 863-00 standard is an accelerated oxidation test conducted on test samples in a steam environment at temperatures in the range of interest for up to 500 hours. Test samples are withdrawn from the furnace at 100-hour intervals, photographs are taken, and measurements are made for changes in weight and volume.
Figure 1 illustrates test results for comparison of a standard nitride-bonded silicon carbide composition to an enhanced version of that composition for metallurgical applications. The standard composition experienced nearly 8.5% volume gain over 500 hours in steam at 900°C, while the enhanced composition only experienced about 0.5% volume gain under the same test conditions. Volume changes above 3% typically lead to significant cracking and disintegration of test samples, which has generally translated well into an understanding about whether products that pass laboratory tests will work well in industrial applications.
After observing favorable improvements in test samples’ appearance and properties, the first industrial application addressed for the new silicon carbide refractory compositions was for bricks used in aluminum reduction cells. In this application, silicon carbide refractory bricks are used for aluminum reduction cell sidewalls.
In addition to oxidation testing, cryolite corrosion tests were conducted at about 950°C to make sure that the silicon carbide compositions could withstand the molten cryolite. The new compositions performed well in the cryolite corrosion tests and were released for manufacture of bricks for field testing in sidewalls of aluminum reduction cells. ADVANCAL® enhanced silicon carbide refractories are now widely used in the aluminum market.
Ongoing Application Development
Based on the successes of the enhanced silicon carbide compositions in the aluminum market, brick shapes were manufactured for testing in the copper shaft market, where oxidation of silicon carbide bricks was known to limit furnace campaign life. Figure 2 shows standard silicon carbide brick sidewalls in a copper shaft furnace after one year of operation. Figure 3 focuses on the same area of the copper shaft furnace after 2½ years of operation with the enhanced silicon carbide bricks (except for the damaged area in the top section of the photograph, which was where the standard silicon carbide bricks were used for comparison to the performance of the enhanced silicon carbide bricks).
The standard bricks have clearly experienced significant volume change during the furnace campaign and the mechanical strength of the standard bricks has been compromised. The enhanced bricks, however, had very little evidence of oxidation. CRYSTON® Cu enhanced silicon carbide refractories are now used extensively in the copper shaft market.
While following the results for nitride-bonded silicon carbide compositions in metallurgical applications, the research team studied oxynitride-bonded silicon carbide compositions that had experienced some oxidation issues in the waste-to-energy market. Silicon carbide tile shapes are commonly used to protect water tube walls from corrosion in large boilers that process domestic waste. Oxynitride-bonded silicon carbide compositions have been shown to have slightly better oxidation and corrosion resistance in this environment compared to nitride bonded compositions, but can experience accelerated oxidation when operating in the temperature range of 900-1100°C.
In recent years, environmental concerns have resulted in changes in operating conditions to reduce emissions, which often affect all structural materials in the furnaces. In many waste-to-energy applications, operating temperatures have increased and furnace gas compositions have changed, adversely affecting refractory life. Figure 4 details oxidized tiles in a boiler tube wall installation after three years. Tiles experience a useful life of 3-10 years, depending on their position in the boiler, since the atmosphere and temperatures vary considerably throughout the boiler.
The enhanced oxynitride-bonded silicon carbide tiles were introduced to the waste-to-energy market and were tested side-by-side on boiler tube walls against existing products. The enhanced tile compositions have exhibited far fewer cracks and failures than standard products, particularly in zones where operating temperatures were in the critical oxidation range of 900-1100°C. The challenge of harsher operating conditions has been addressed with the introduction of the new compositions (CRYSTON CN790 and REFRAX® PLUS) for this market.
Another benefit of the development of enhanced silicon nitride-bonded and oxynitride-bonded silicon carbide is improved performance in kiln furniture applications. Greater demands have been placed on kiln furniture as high-temperature firing technologies have evolved. Kiln cycles are shorter, loads are heavier and furniture design is more complex. In addition, the trend continues toward refractory products having lower mass and thinner cross-sections. Silicon carbide refractories are necessary for these types of applications based on high strength, high thermal conductivity (for thermal shock resistance) and, ultimately, oxidation resistance for the longest possible life in kiln firing campaigns.
Today’s silicon carbide kiln furniture offers many years of service, especially for advanced silicon carbide products based on finer grain sizing and advanced bonds and coatings. However, traditional silicon carbide kiln furniture is susceptible to increased oxidation rates in the temperature range of 900-1100°C, especially in applications where moisture in the furnace atmosphere accelerates oxidation rates.
Enhanced oxidation-resistant compositions for both advanced and traditional nitride-bonded silicon carbide kiln furniture have substantially improved oxidation resistance throughout operating temperature ranges. Many applications using these newly developed silicon carbide refractory products (e.g., N-DURANCE®, CRYSTON X-TREME and REFRAX X-TREME) are experiencing more than twice the life of kiln furniture products based on previous silicon carbide compositions.
LO-MASS® silicon carbide kiln furniture is thinner, lighter and significantly stronger than other types of kiln furniture. This results in more kiln capacity, less energy consumption and lower firing costs. The additional life of the new family of enhanced nitride-bonded silicon carbide products offers further firing cost savings. These savings have been extended to related applications such as furnace muffles, furnace hearth plates and skid rails for heat treating furnaces.
Based on the success of the oxynitride-bonded compositions in several applications, other metallurgical applications have been developed, such as immersion tubes for molten aluminum and zinc galvanizing furnaces, and brick bottoms for aluminum melting furnaces. These new silicon carbide compositions significantly extend product life compared to previous compositions.
Testing continues across additional markets and applications for enhanced oxidation-resistant silicon carbide products. U.S. Patents 8,003,557 and 8,097,547 and 8,076,254 have been granted to Saint-Gobain for compositions based on this new technology for enhanced silicon carbide refractories.
Author’s note: Eric Jorge, Nancy Levoy, Julio Spadaccia and Mike Arbini of Saint-Gobain made significant contributions to this article.