Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-06-27
2003-08-19
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S212000, C428S334000, C428S697000, C428S698000, C428S699000, C428S701000, C428S446000, C416S24100B
Reexamination Certificate
active
06607852
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to coating systems and more specifically to the protection of silicon containing materials, such as ceramic matrix composites (CMC's), in hot, oxidizing environments, such as the combustor or turbine portion of gas turbine engines.
Higher operating temperatures for gas turbine engines are continuously sought in order to improve their efficiency. However, as the operating temperatures increase, the high temperature durability of the components of the engine must also correspondingly increase. Materials containing silicon, particularly those with silicon carbide (SiC) as a matrix material and/or as a reinforcing material, are currently being used for high temperature applications, such as in the components for the combustor and/or turbine sections of gas turbine engines.
Silicon carbide (SiC) has a tendency to oxidize into silica (SiO
2
) and CO
2
at elevated temperatures in the presence of O
2
. Some additional silica may be formed due to the presence of some free silicon. While the silica formed can be an excellent diffusion barrier to prevent diffusion of O
2
, silica is itself subject to deterioration in the presence of water or vapors of water such as steam. Thus, while coating systems have been used to provide some protection for the SiC in high temperature environments that include silica, these systems have been ineffective or deficient in some aspect. Either the silica layer that has been provided is very thin and formed by decomposition of the substrate or inadequate protection of the silica layer from a hydrous atmosphere has been provided. When a coating system is utilized for protection of the underlying SiC in hot, oxidizing environments is formed by preoxidation of the SiC substrate, which is not a preferred method because of the length of time required to form a silica layer of sufficient thickness, there is still a tendency for void formation to occur, even when the silica is protected from deterioration from a hydrous atmosphere. The voids may appear in the thin interfacial silica scale formed by the transformation. The voids can also exist at the interfaces between the silica scale and the substrate. The voids are undesirable as they decrease the environmental protection provided by any external coatings. Not only do the voids break down the SiC, which can adversely affect the mechanical properties of the CMC composite, which are designed around the mechanical properties of its system components, but the voids can also provide a path of least resistance that permits the continued inward diffusion of oxygen to promote further deterioration of the SiC composite. The voids can aggregate during the course of operation at high temperatures and can reduce the life of the external coating by promoting spallation of the applied coating. Alternatively, when inadequate protection of the silica layer is provided, the silica layer quickly deteriorates by contact with a hydrous atmosphere such as may be encountered by turbine engines in marine or coastal environments leading to a breakdown of the protective silica layer by the formation of volatile SiO and silicon hydroxide (Si(OH)
x
) products. This can then result in oxidation of the underlying SiC and rapid deterioration of the substrate in a hydrous oxidizing environment as the SiC decomposes and as any SiO
2
formed during the decomposition of the SiC also decomposes. Thus, any coating system applied to the silicon-containing material should provide environmental protection by inhibiting the degradation of the silicon containing material in a water-containing or steam-containing environment. Thus, a dense layer of silica is effective in protecting a silicon-containing substrate from oxidation only so long as the silica layer is not itself degraded in a hydrous environment, such as those in which gas turbines frequently operate.
Various coating systems are available for the protection of silicon containing materials, such as silicon carbide systems, from oxidation at high temperatures and degradation in a water-containing environment. One type of coating system is discussed in U.S. Pat. Nos. 6,129,954 to Spitsberg et al. and 5,869,146 to McCluskey et al., which disclose techniques for applying a mullite (3Al
2
O
3
.2SiO
2
) coating to a silicon-based ceramic substrate. The mullite coating is used as a thermal barrier coating (TBC) for the silicon-based ceramic substrate. The mullite coating can also serve as a bond coat for the subsequent application of an environmental barrier coating (EBC) such as yttria-stabilized zirconia (YSZ) as there is a mismatch between the coefficients of thermal expansion of YSZ and silicon carbide. However, mullite does not provide adequate protection in high temperature environments containing water vapors because mullite has, thermodynamically, significant silica activity due to the high concentration of SiO
2
in mullite, and volatilizes at high-temperatures in the presence of water or water vapor. Another coating system is discussed in U.S. Pat. No. 5,985,470 to Spitsberg et al., which discloses a thermal/environmental barrier coating system for a silicon-based ceramic substrate. The coating system includes a layer of barium strontium aluminosilicate (BSAS), which serves as a bond coat for a ceramic topcoat. The ceramic topcoat can include a zirconia partially or fully stabilized with yttria (YSZ) and yttrium silicate.
Other coating systems have proposed protecting the silicon-containing substrate by providing environmental protection using barium aluminosilicate and variations of this material and intermediate layers between the barrier layer and the substrate to enhance adherence and prevent reactions between the barrier layer and the silicon-containing substrate. The intermediate layers can include a bond coat of silica formed by preoxidation of the silicon-containing substrate or a layer of silicon applied to the substrate followed by application of the intermediate layer by thermal spray. The intermediate layer may be applied without the formation of a bond coat.
Still other coating systems for protecting CMC's against oxidation include the formation of a thin silica layer to the silicon containing material. To provide additional thermal and environmental protection, a mullite coating and a ceramic topcoat of YSZ can be applied to the silica layer. Still other coating systems can apply a layer of silicon onto the silicon-containing material to improve adhesion, followed by additional thermal and environmental barrier coatings that are then applied onto the silicon layer.
Current coating systems are designed for interface temperatures between the protective coating and the substrate of not more than about 2300° F. What is needed is a protective coating system that can increase this interface temperature into the range of about 2400-2500° F. as the temperatures of the turbine are increased by use of a coating sufficiently thick to improve the temperature capability of the coating system, while eliminating components that may experience incipient melting at these elevated temperatures. The protective coating system ideally forms a barrier to diffusion of oxygen to protect the silicon-containing substrate from degradation, yet is stable in high temperature environments containing water molecules and can be selectively applied. The coating system must be easy to apply to the desired thickness and be capable of maintaining its resistance to oxygen diffusion, in the presence of water molecules and at temperatures of up to 3000° F. Most importantly, the material should be also improve the elevated temperature adherence of thermal barrier coatings such as yttria-stabilized zirconia to the silicon containing substrate by providing an intermediate coefficient of thermal expansion (CTE) so that the component to which it is applied can be used in hydrous environments that experience even higher temperatures, while having excellent environmental properties.
SUMMARY OF THE INVENTION
The present invention is directed t
Nagaraj Bangalore Aswatha
Spitsberg Irene
Wang Hongyu
General Electric Company
Jones Deborah
McNees Wallace & Nurick
McNeil Jennifer
Narciso David L.
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