1. Field of the Invention
The present invention generally relates to ceramic components, and in particular, silicon carbide-based ceramic components that find particular use in refractory applications.
2. Description of the Related Art
As is well understood in the area of ceramic processing, ceramic bodies are typically heated or fired at relatively high temperatures, such as on the order of 1,000° C. and higher. Such firing operations require use of refractory materials and refractory components that are resistant to such high temperatures, and which maintain structural integrity not only at high temperatures, but upon repeated heating and cooling cycles during the service life of the furnace or kiln. Such refractory components include furnace or kiln furniture utilized in connection with holding or supporting work pieces during firing operations, as well as refractory lining materials and structural walls defining the furnace heating area.
Silicon carbide-based ceramic materials have been utilized in refractory applications for their various high temperature properties, including corrosion resistance, strength, thermal shock resistance, thermal conductivity, and other properties. Among various types of silicon carbide-based ceramics, dense silicon carbide ceramics such as sintered silicon carbide, hot pressed silicon carbide, and hot isostatically pressed silicon carbide have particularly robust characteristics. However, because of the production complexity and costs associated with fabrication of highly dense silicon carbide components (such as Hexoloy®), such components are not widely used as refractory components, and only used under the most severe service conditions. On the other hand, more cost effective but relatively porous silicon carbide materials such as nitride bonded silicon carbide (known by acronyms such as NBSC and NSIC) have found practical use in refractory applications.
Nitride bonded silicon carbide tends to be a comparatively porous material, oftentimes having a porosity within a range of about 10 to about 15 volume percent. These components are manufactured from a green body containing silicon carbide and silicon, and sintering the green body in a nitrogen containing atmosphere at temperatures on the order of 1,500° C. While nitride bonded silicon carbide has desirable high temperature properties, it unfortunately suffers from poor oxidation resistance when used in oxidizing conditions, due in part to its intrinsic porosity. This particular characteristic has been addressed in the past by re-firing nitride bonded silicon carbide components in an oxidizing atmosphere to form a thin oxide layer of amorphous or glassy silica, which functions to passivate and seal the outer surface of the component. However, the present inventor has recognized that the outer passivation layer formed by an oxidation process does not adequately protect the component from excessive oxidation during use, particularly in demanding firing operations, which can lead to premature component failure. Multiple mechanisms have been identified in connection with the unwanted oxidation.
Foremost, during use of such components, the outer passivation layer may be damaged, permitting a pathway for deep oxidation. Further, phase changes in the passivation layer may cause tensile stresses and subsequent initiation and propagation of cracks in the layer. In addition, the presence of water vapor may cause blistering or bubbles to form in the passivation layer, leading to failure of the passivation layer and undesirable oxidation of the component.
Other techniques have focused on forming an outer, protective layer by firing a glass former, such as a silica-containing coating or a silica precursor, that is coated on the component. However, unfortunately, such processing pathways tend to form porous layers that have a propensity to crack and spall during use, rendering the outer protective layer of limited effectiveness.
In view of the state of the art of silicon carbide-based refractory materials, and in particular, nitride bonded silicon carbide components, there is a need in the art for improved components, particularly components having improved oxidation resistance in practical use.