Patent Application
20090155554
This invention relates to high-temperature machine components. More particularly, this invention relates to coating systems for protecting machine components from exposure to high-temperature environments. This invention also relates to methods for protecting articles.
Silicon-bearing materials, such as, for example, ceramics, alloys, and intermetallics, offer attractive properties for use in structures designed for service at high temperatures in such applications as gas turbine engines, heat exchangers, and internal combustion engines, for example. However, the environments characteristic of these applications often contain water vapor, which at high temperatures is known to cause significant surface recession and mass loss in silicon-bearing materials. The water vapor reacts with the structural material at high temperatures to form volatile silicon-containing species, often resulting in unacceptably high recession rates.
Environmental barrier coatings (EBC's) are applied to silicon-bearing materials susceptible to attack by high temperature water vapor, and provide protection by prohibiting contact between the water vapor and the surface of the material. EBC's are designed to be relatively stable chemically in high-temperature, water vapor-containing environments and to minimize connected porosity and vertical cracks, which provide exposure paths between the material surface and the environment. Cracking is minimized in part by minimizing the thermal expansion mismatch between the EBC and the underlying material, and improved adhesion and environmental resistance can be achieved through the use of multiple layers of different materials. One exemplary conventional EBC system, as described in U.S. Pat. No. 6,410,148, comprises a silicon or silica bond layer applied to a silicon-bearing substrate; an intermediate layer comprising mullite or a mullite-alkaline earth aluminosilicate mixture deposited over the bond layer; and a top layer comprising an alkaline earth aluminosilicate deposited over the intermediate layer. In another example, U.S. Pat. No. 6,296,941, the top layer is a yttrium silicate layer rather than an aluminosilicate.
The above coating systems can provide suitable protection for articles in demanding environments. However, cracking, spalling, volatilization, and other mechanisms operating on localized areas of the EBC top layer expose the underlying material—a silicon bond coat, for instance—to the environment, leading to rapid oxidation and volatilization. Once the bondcoat is locally removed by these mechanisms, rapid recession of the underlying silicon-bearing structural component ensues. Recession and perforation of the silicon-bearing component can lead to both component and system failure, as neighboring metallic parts not designed for high temperature service become directly exposed to a corrosive high-temperature environment. Therefore, there is a need to provide articles with robust environmental barrier coating systems that have the capability to reliably withstand long-term exposure to high-temperature environments containing water vapor.
Embodiments of the present invention are provided to address these and other needs. One embodiment is an article comprising a substrate, an intermediate coating system disposed over the substrate, and a topcoat disposed over the intermediate coating system.
The intermediate coating system comprises a separator layer comprising an oxygen getter material, and a barrier layer disposed over the separator layer, the barrier layer comprising a ceramic composition. The topcoat also comprises this ceramic composition.
Moreover, at least about 50% by volume of the ceramic composition present in the barrier layer is a metastable precursor material that tends to transform over time into a product material. At least about 75% by volume of the ceramic composition present in the topcoat is the product material, and up to about 25% by volume of the ceramic composition present in the topcoat is the metastable precursor material.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Referring to
Embodiments of the present invention provide enhanced resistance to mechanical damage, such as cracking, due to the presence of layers containing materials that may resist the formation of cracks, or heal existing cracks, much more readily relative to the materials found in systems as described above.
Referring to
Substrate 202 comprises silicon in certain embodiments, including, for example, substrates comprising ceramic compounds, metal alloys, intermetallic compounds, or combinations of these. Examples of intermetallic compounds include, but are not limited to, niobium silicide and molybdenum silicide. Examples of suitable ceramic compounds include, but are not limited to, silicon carbide and silicon nitride. Embodiments of the present invention include those in which the substrate comprises a ceramic matrix composite (CMC) material, including in which the CMC comprises a matrix phase and a reinforcement phase, both of which phases comprise silicon carbide. Regardless of material composition, in some embodiments substrate 202 comprises a component of a turbine assembly, such as, among other components, a combustor component, a shroud, a turbine blade, or a turbine vane.
Intermediate coating system 204 contains multiple layers. A separator layer 208 comprises an oxygen getter material, and serves to inhibit the movement of oxidizing species through the coatings by chemically combining with them and thus binding them before they can arrive at and react with the substrate. As used herein, an “oxygen getter material” means a substance having a high affinity for oxygen atoms or molecules. In certain embodiments, the oxygen getter material comprises silicon. Suitable examples of an oxygen getter material include elemental silicon (including, for example, industrially pure silicon) and a silicide (meaning a compound of silicon and one or more additional chemical elements). The separator layer 208, in some embodiments, has a thickness of up to about 250 micrometers. In certain embodiments, this thickness is in the range from about 50 micrometers to about 150 micrometers, and in particular embodiments, the thickness is in the range from about 80 micrometers to about 120 micrometers. Separator layer 208 in some instances is disposed in contact with substrate 202, where it serves as a traditional bondcoat as that term is understood in the art; thus the term “bondcoat” can be used herein to refer to a “separator layer” that is disposed directly on substrate 202.
Intermediate coating system 204 also includes a barrier layer 210 that comprises a ceramic composition and is disposed over separator layer 208. Barrier layer 210, as that term is used herein, means a coating that is, among other things, designed to provide resistance to recession in high temperature, water-vapor-containing environments, and further to inhibit the penetration of water vapor to underlying layers and substrate. At least about 50% by volume of the ceramic composition present in barrier layer 210 is a metastable precursor material having the tendency to transform over time into a product material. In many ceramic coating systems, this metastable material is an amorphous (glassy) phase with a higher degree of strain compliance than the product material into which it transforms (which may inhibit the formation of cracks during service), or with a sufficiently low viscosity to allow for the material to flow into and at least partially fill defects such as cracks and pores, thereby “healing” accumulated damage to mitigate risks of catastrophic failure.
Barrier layer 210 has a thickness of up to 750 micrometers in some embodiments. In certain embodiments, this thickness is in the range from about 75 micrometers to about 500 micrometers. In particular embodiments, the thickness is in the range from about 75 micrometers to about 125 micrometers. Selection of barrier layer thickness will depend on a number of design considerations, including, for instance, the nature of the expected service environment, the material selected for use as barrier layer 210, and the desired service life.
Topcoat 206 also comprises the ceramic composition found in barrier layer 210, but while the overall chemical makeup of the ceramic composition is the same in both topcoat 206 and barrier layer 210, the constituent phase content of the ceramic composition in these coating layers is somewhat different. For example, as described above, half or more of the ceramic composition volume contained in barrier layer 210 is a metastable precursor material, whereas in topcoat 206, only up to about 25% of the volume is this metastable precursor material. In fact, at least about 75% of the volume of the ceramic composition present in the topcoat is the product material into which the metastable material transforms with time. Thus, the topcoat ceramic composition is primarily made of the more stable phases formed when the metastable phases transform. The topcoat composition is typically selected for recession resistance with suitable erosion and thermal expansion properties to meet design life requirements. The thickness of topcoat 206 is selected in accordance with similar considerations as described above for barrier layer 210, and thus the thickness of the topcoat may be in a similar range of alternatives as described above for the barrier layer 210.
In an exemplary embodiment, the ceramic composition includes an aluminosilicate; that is, a compound or mixture of oxides of aluminum, silicon, and other metal or semi-metal elements. Examples of aluminosilicates include, but are not limited to, barium aluminosilicate, strontium aluminosilicate, and barium strontium aluminosilicate. The metastable precursor material of the barrier layer 210, in some embodiments using aluminosilicate coatings, includes a hexacelsian aluminosilicate phase, an amorphous aluminosilicate phase, or, in some cases, a mixture including these two phase types. These metastable precursor phases are known to transform over time at elevated temperatures to at least a monoclinic celsian aluminosilicate phase. It is this latter phase that, in some embodiments, makes up a significant majority of the aluminosilicate volume in the topcoat, often 80% of the volume or more, and 95% of the volume or more in some embodiments. Monoclinic celsian has a significantly closer CTE match to many silicon-bearing ceramic substrate materials than the metastable hexacelsian phase has.
In some embodiments, as shown in
Other layers may be applied to the article 200. In some embodiments, a top separator layer 212 is disposed between the intermediate coating system 204 and topcoat 206. Top separator layer 212, in some embodiments, is accompanied by a transition layer 214 disposed between top separator layer 212 and topcoat 206. Top separator layer 212 comprises an oxygen getter material, in like manner to separator layer 208 described previously, and its thickness may be generally comparable to that of separator layer 208. Transition layer 214 is often applied to limit chemical interactions occurring at an interface. For example, silicon in a separator layer 212 may react with oxygen to form silica. This silica, if in contact with an aluminosilicate coating (such as topcoat 206), may quickly react with the aluminosilicate and further deplete silicon from the separator layer 208, thereby degrading its performance. A transition layer 214 comprising mullite, or barium strontium aluminosilicate mixed with mullite, for instance, may inhibit the deleterious interaction by reducing the amount of aluminosilicate in contact with the silica. For this reason, in some embodiments, a transition layer 214 is disposed between separator layer 208 and barrier layer 210.
The following exemplary embodiment is provided to further illustrate the above description. Referring to
As described previously, other coatings may be applied in this exemplary embodiment. In some embodiments article 300 further includes multiple transition layers 214 respectively disposed between the bondcoat 220 and the intermediate coating system 204 and between the intermediate coating system 204 and the topcoat 206. Transition layers 214 comprise mullite, barium strontium aluminosilicate, or mixtures thereof, to limit chemical interactions between the silicon-bearing coatings and the aluminosilicate coatings. In embodiments (not shown) where article 300 comprises multiple barrier layers 210, a separator layer comprising elemental silicon or a silicide is disposed between the adjacent members of each pair of barrier layers, and each separator layer further may be accompanied by a transition layer 214 as described previously.
All of the coatings described herein may be deposited by any of various manufacturing processes, including but not limited to spray processes such as plasma spraying, that have the potential to deposit metastable forms of materials. In an exemplary embodiment, referring again to
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following example is included to provide additional guidance to those skilled in the art in practicing the claimed invention. The example provided is merely representative of the work that contributes to the teaching of the present application. Accordingly, this example is not intended to limit the invention, as defined in the appended claims, in any manner.
An article in accordance with the embodiments described above was fabricated using plasma spray to deposit all coating layers. The substrate was silicon carbide, upon which was deposited a silicon bondcoat, from about 75 micrometers to about 125 micrometers thick. A first transition layer made of a mixture of barium strontium aluminosilicate (BSAS) and mullite, from about 100 micrometers to about 150 micrometers thick, and a barrier layer of BSAS, from about 200 micrometers to about 250 micrometers thick, were deposited over the bondcoat. A silicon separator layer of similar nominal thickness to the bondcoat and a second transition layer of BSAS and mullite of similar nominal thickness to the first transition layer was deposited over the barrier layer, and a topcoat of BSAS, from about 200 micrometers to about 250 micrometers thick was deposited over the second separator layer.
In the as-deposited condition, the BSAS in both the topcoat and the barrier layer was present predominantly in the form of glassy phase. The article was then heat treated in air at a nominal temperature of about 1250 degrees Celsius. This heat treatment converted most of the glassy BSAS material in the topcoat into monoclinic celsian phase, but the glassy phase in the underlying barrier layer remained largely unconverted (though there was some crystallization into the metastable hexacelsian phase), resulting in an article having a compliant barrier layer underlying a topcoat made mostly of monoclinic BSAS.
An article processed as described above was exposed for 500 hours of cyclic steam exposure (250 cycles) in a 90% H2O-10% O2 environment, which has a water vapor partial pressure approximately similar to that of a typical gas turbine. The coating system accumulated minimal damage during this exposure. The separator layer closest to the topcoat was observed to have a thin oxide layer on it, much as would be observed for a similarly exposed silicon bondcoat in a conventional EBC system (
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
