Patent Application
20170167276
This disclosure generally relates to materials and articles for service in high-temperature applications such as, for example, turbomachinery. More specifically, this disclosure relates to articles that include ceramic matrix composites and associated protective coatings, and methods for making these articles.
Ceramic matrix composite (CMC) materials offer the potential for higher operating temperatures than do metal alloy materials due to the inherent high-temperature material properties of ceramic materials. In applications such as gas turbine assemblies, this capability may be translated into a reduced cooling requirement which, in turn, may result in higher power, greater efficiency, and/or reduced emissions from the machine. However, CMC materials that include significant amounts of silicon-bearing materials, such as silicon carbide, are susceptible to attack and rapid recession by water vapor at elevated service temperatures. Environmental barrier coatings (EBC) have been developed to inhibit this degradation mechanism.
EBCs applied to silicon-bearing materials, for example, are designed to be relatively stable chemically in high-temperature, water vapor-containing environments. One illustrative conventional EBC system, as described in U.S. Pat. No. 6,410,148, comprises a silicon or silica bond layer (also referred to herein as a “bondcoat”) 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, but opportunities for improvement in coating performance exist. For instance, the presence of free silicon in the bond layer may restrict the maximum rated material temperature for a coated component to avoid melting of the silicon and resultant mechanical instability of the bond layer. Improvements in the quality of engineered structural materials, such as silicon-bearing ceramics and ceramic matrix composites, have enhanced the high temperature capability of these materials to the point where the melting point of the silicon in the EBC bond layer has become a limiting factor for the use of such materials in high-temperature structural applications.
Therefore, there remains a need in the art for EBC bond layers with temperature capability that exceeds that of conventional bond layers. There is also a need for machine components employing coating systems that incorporate an improved bond layer to enhance their high-temperature capability.
Embodiments of the present invention are provided to meet this and other needs. One embodiment is an article. The article comprises a substrate comprising a ceramic matrix composite; a first layer disposed over the substrate, the first layer comprising an interconnected first silicide, and a second phase; and a second layer disposed over the first layer, the second layer comprising a second silicide in mass transfer communication with the first silicide.
Another embodiment is an article. The article comprises a substrate comprising a ceramic matrix composite; a first layer disposed over the substrate, the first layer comprising an interconnected first silicide phase, and a second phase, the second phase comprising silicon carbide, wherein a volume fraction of silicide in the first layer is in the range from about 10 percent to about 40 percent; a second layer disposed over the first layer, the second layer comprising a second silicide in mass transfer communication with the first silicide; and a top layer disposed over the second layer, the top layer comprising an oxide.
Another embodiment is an article. The article comprises a substrate comprising a ceramic matrix composite; a first layer disposed over the substrate, the first layer comprising an interconnected first silicide phase, and a second phase, the second phase comprising silicon nitride, wherein a volume fraction of silicide in the first layer is in the range from about 10 percent to about 50 percent; and a second layer disposed over the first layer, the second layer comprising a second silicide in mass transfer communication with the first silicide; and a top layer disposed over the second layer, the top layer comprising an oxide.
Another embodiment is an article. The article comprises a substrate comprising a ceramic matrix composite; a first layer disposed over the substrate, the first layer comprising an interconnected first silicide phase, and a second phase, the second phase comprising a rare earth disilicate, wherein a volume fraction of silicide in the first layer is in the range from about 10 percent to about 40 percent; a second layer disposed over the first layer, the second layer comprising a second silicide in mass transfer communication with the first silicide; and a top layer disposed over the second layer, the top layer comprising an oxide.
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 drawing in which like characters represent like parts, wherein:
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, and “substantially” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged; such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.
In the following specification and the claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the term “or” is not meant to be exclusive and refers to at least one of the referenced components being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise.
As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.
The techniques described herein address the temperature limitations described above for conventional silicon bondcoats through the use of silicide materials that have substantially higher melting temperatures than elemental silicon, but possess the ability to form a protective silica layer in the presence of an oxidative environment in a similar manner to conventional silicon bondcoats. However, silicides tend to have significantly higher coefficient of thermal expansion than substrate materials of interest, such as silicon carbide, silicon nitride, and CMC's based on one or more of these; this mismatch in thermal expansion coefficient may lead to spallation of silicide coatings due to high thermal stresses. Reducing the thickness of the silicide will reduce the thermal stress to some degree, but such a reduction reduces the amount of material available to protect the substrate, leading to unacceptable service life.
Embodiments of the present invention are designed to deal with the oxidation issue through the application of silicide materials, using a coating architecture that mitigates the thermal expansion mismatch, through the use of a mixture of silicides (comparatively high thermal expansion coefficient) with comparatively low-thermal-expansion-coefficient compounds such as silicon nitride, while maintaining excellent high temperature oxidation resistance.
Referring to
Substrate 101 may be made from any suitable material, such as a ceramic or an intermetallic material. The substrate, in one embodiment, includes a ceramic, for example an oxide, nitride, or carbide. In a particular embodiment, substrate 101 includes a material that comprises silicon, such as silicon nitride, or silicon carbide. This material, in some embodiments, includes a ceramic-matrix composite (CMC) material, such as a material made of a matrix phase and a reinforcement phase. The matrix phase, the reinforcement phase, or both of these phases includes, in certain embodiments, a silicon-bearing ceramic, such as silicon carbide (SiC) or silicon nitride. In one embodiment, the article 100 is a component for a gas turbine assembly, such as, for example, a combustion liner, transition piece, shroud, vane, or blade.
First layer 110 includes at least two materials: a first silicide, and a second phase. The second phase includes material having a lower coefficient of thermal expansion than the first silicide, and in particular embodiments has a coefficient of thermal expansion less than or equal to that of silicon carbide. Including this low-expansion material as the second phase in first layer 110 reduces the effective thermal expansion coefficient of the layer 110 and mitigates thermal stresses that would otherwise arise through the presence of silicide materials. Examples of such low-expansion materials include ceramic materials, such as those including carbon, e.g., carbides (for example, silicon carbide); those including nitrogen, such as nitrides (for example, silicon nitride); those including oxygen, such as oxides (for example, titanates, and rare-earth silicates, including rare-earth disilicates); and those including more than one of these elements, such as an oxynitride (for example, SiAlON).
First layer 110 also includes a first silicide, as stated above. It should be noted that the use of the term “first silicide” is intended only to delineate silicide material present in first layer 110 from silicide material present in second layer 120. No limitation on the composition of the material is implied. Indeed, in some embodiments, described below, the “first silicide” includes more than one silicide composition (such as silicides of the same metal element but having different silicon content levels), and in some embodiments, a silicide in first layer 110 is nominally identical to a silicide present in second layer 120.
The first silicide, that is, any silicide material present in first layer 110, may serve at least two functions during service. The first silicide may provide a source of silicon for transport to the surface 140 of second layer 120 to participate in the formation of protective TGO 150.
Additionally, and importantly, the first silicide provides a pathway for silicon, liberated from its source materials within first layer 110, to diffuse rapidly from the interior of first layer 110 to second layer 120. For this reason, the first silicide is generally situated within first layer 110 in a morphology that provides silicon from first layer 110 with a substantially continuous transport pathway via solid state diffusion through first silicide to second layer 120. This morphology is referred to herein as an “interconnected first silicide,” to distinguish it from arrangements that would not provide a continuous diffusion path for silicon through silicide, such as discrete, unconnected particles of silicide dispersed within a second phase. However, it is not necessary that the “interconnected” first silicide be completely interconnected. For example, multiple networks of first silicide may be present instead of one completely connected network; moreover, for instance, a series of parallel plates of first silicide disposed to span through the cross section of first layer 110 may also provide the required level of mass transport for silicon, depending on plate spacing and other factors. These arrangements, and any others that provide a substantially continuous mass transport pathway through the first layer 110 via first silicide, fall within the scope of the term “interconnected” for the purposes of this description.
The selection of relative amounts of first silicide and second phase results from a consideration of competing factors. Mass transport of silicon from first layer 110 to second layer 120 through the first silicide is more easily facilitated by having higher levels of connectivity among branches of the first silicide network, which tends to favor relatively high levels of silicide content, depending on the morphology of the silicide within the layer. On the other hand, if the amount of first silicide in first layer is too high, the effective thermal expansion coefficient of first layer 110 may exceed levels desirable to mitigate thermal stress. Clearly, the use of a second phase material having comparatively low thermal expansion coefficient, such as silicon nitride, may allow for the use of a higher content of silicide than would the use of a second phase having a higher thermal expansion coefficient. In one embodiment, the first silicide is present in first layer 110 in a range from about 10 volume percent to about 70 volume percent, and in particular embodiments, the range is from about 20 volume percent to about 40 volume percent.
Where the second phase includes silicon carbide, in some embodiments the volume fraction of first silicide is desirably, but not necessarily, maintained in a range from about 10 percent to about 40 percent. Where the second phase includes silicon nitride, which has a lower thermal expansion coefficient than silicon carbide, in some embodiments the volume fraction of first silicide is desirably, but not necessarily, maintained in a range from about 10 percent to about 50 percent. A somewhat higher volume fraction may be possible here compared to the silicon carbide example, due to the lower coefficient of thermal expansion for the silicon nitride. Where the second phase is a rare earth disilicate, in some embodiments the volume fraction of first silicide is desirably, but not necessarily, maintained in a range from about 10 percent to about 40 percent.
In a particular embodiment, the materials and amounts selected for first layer 110 are such that the first layer 110 has an effective coefficient of thermal expansion below about 6 parts per million per degree Celsius; such a value is desirable for maintaining thermal stresses at practical levels on CMC substrates containing silicon carbide, for example. Where the CMC substrate includes substantial amounts of silicon nitride, the effective coefficient of first layer may be designed to be even lower than this, because silicon nitride has a lower thermal expansion coefficient than silicon carbide.
The selection of silicide materials for use in first layer 110 depends in part on a number of factors. For example, the silicide should have a high melting temperature and at most minimal reactivity with the second phase at the high temperatures inherent to service conditions for gas turbine components. The silicide in first layer 110 should also readily transport silicon via solid state diffusion at these high temperatures. Examples of silicides that may be suitable for use in first layer 110 include silicides of molybdenum, tantalum, tungsten, niobium, titanium, and ruthenium, along with silicides that include combinations of one or more of these elements.
Illustrative examples of silicide types that may be included in first layer 110 include disilicide compounds, such as molybdenum disilicide (MoSi2), lower silicide compounds (that is, silicides such as Mo5Si3 that have lower silicon content than disilicides), and combinations of disilicide and lower silicide. As noted previously, the “first silicide” term is used herein to encompass not only embodiments where only one silicide compound is present in first layer 110, but also embodiments in which the first silicide includes a combination of more than one silicide compound. Disilicides, being higher in silicon content level than lower silicides, may provide a ready source for silicon through decomposing to a lower silicide and silicon. Lower silicides, in some cases, may have lower coefficients of thermal expansion than their corresponding disilicides, providing the potential to use thicker layers and/or higher silicide contents in first layer 110 than thermal stress considerations may allow for higher silicides, such as disilicides. In some embodiments, the first silicide includes a disilicide of one or more metal elements, and a lower silicide of the same one or more metal elements; use of the same family of silicides in such a mixture may provide good chemical stability between the two compounds of the first silicide. The term “first silicide” encompasses silicide material present in first layer 110 initially and at any time thereafter, including silicide that forms as a result of chemical changes within first layer 110 that transform one silicide to a different silicide.
Thickness of first layer 110 depends in part upon materials selection for substrate 101, and the first silicide and second phase of first layer 110; the thermal expansion properties of these materials contribute to the thermal stress generated within first layer 110. Generally, the higher the mismatch between the effective thermal expansion coefficient of first layer 110 and substrate 101, the thinner the layer that can be accommodated. In some embodiments, first layer has a thickness in the range from about 25 micrometers to about 500 micrometers. In certain embodiments, this range is from about 25 micrometers to about 250 micrometers, and in particular embodiments it is from about 25 micrometers to about 100 micrometers.
First layer 110, a layer comprising at least two materials, may be fabricated using techniques available in the art. The fabrication method disposes the second phase within an interconnected first silicide to arrive at the structure described above. For instance, where a comparatively high total silicide content, such as 25 volume percent or higher, can be tolerated, layer 110 may be fabricated by plasma spraying, including, for instance, liquid injection plasma spray techniques, with expectations of suitable degree of interconnection of the silicide material to provide adequate mass transport of silicon to second layer 120. Optimizing spray parameters such as particle size distributions for silicide and second phase feedstock may provide connectivity at silicide levels even lower than 25 percent by volume under some conditions.
However, as silicide levels decrease, plasma spray techniques may not provide desired levels of silicide connectivity in first layer 110. Moreover, plasma spraying may not provide an effective means for depositing nitrides, as silicon nitride, for instance generally decomposes before it melts.
As an alternative to plasma spray techniques, vapor deposition techniques, combined with masking, may be applied to ensure adequate connectivity for the first silicide. For instance, chemical vapor deposition (CVD), or a physical vapor deposition (PVD) method such as sputtering, may be applied to deposit second phase material, such as silicon carbide or silicon nitride, onto substrate 101. The deposition may be performed through a mask that creates a pattern having regions where second phase material is deposited, with adjacent spaces between regions, such as a grid pattern or a series of parallel lines. These spaces may then be filled with silicide, again using a CVD or PVD process. Depending on the desired thickness and/or volume of material being deposited, a mask may be applied in this step as well, to allow selective deposition of silicide within the spaces. Second layer 120 may then be applied over the finished first layer 110.
Second layer 120 includes a second silicide, and this second silicide is disposed in mass transfer communication with the first silicide (i.e., the silicide material found in first layer 110). “Mass transfer communication” as used herein means that the materials are situated so as to allow solid state diffusion of silicon through the first silicide into the second silicide. Typically, this means the materials are disposed in physical contact with one another, but mass transfer communication may also be maintained through an intervening layer of material provided that intervening layer does not have a substantially lower diffusion rate for silicon than the first silicide and the second silicide. The term “second silicide” is used herein only to delineate silicide material present in second layer 120 from silicide material present in first layer 110. No limitation on the composition of the material is implied. Indeed, in some embodiments, described below, the “second silicide” includes more than one silicide composition, and in some embodiments, a silicide in first layer 110 (“first silicide”) is nominally identical to a silicide present in second layer 120 (“second silicide”). Moreover, the term “second silicide” encompasses silicide material present in second layer 120 initially and at any time thereafter, including silicide that forms as a result of chemical changes within second layer 120 that transform one silicide to a different silicide. Second layer 120 includes at least about 80% by volume second silicide, and in particular embodiments at least about 95% by volume second silicide. In certain embodiments, second layer 120 consists essentially of the second silicide, meaning that second layer 120 includes second silicide and no other material that significantly alters the ability of second layer 120 to perform the functions described herein.
Silicide in second layer 120, that is, the second silicide, performs at least two functions. First, it isolates the second phase of first layer 110 from oxidative species diffusing from the service environment, which prevents undue and potentially damaging oxidation of the second phase. It should be noted that previous proposed coatings that purport to use layers including a mixture of a silicide with a second, low-thermal-expansion phase do not describe the use of anything akin to second layer 120 described herein, the absence of which would leave these proposed systems vulnerable to environmental degradation and substantially reduced effectiveness. Second, silicide in second layer 120 provides a pathway for silicon to diffuse to surface 140, whereupon the silicon is available to participate in the formation of the protective silica TGO 150. Also, depending on the specific material used, the second silicide may also serve as a source for silicon to feed the TGO formation reaction; as a non-limiting example, where the second silicide and the first silicide are identical disilicides, the second silicide will convert to a lower silicide and supply silicon to the TGO reaction until all of the disilicide of the second layer 120 is converted to lower silicide. As explained in more detail, below, proper material selection may provide a remarkable ability for second layer 120 to serve as a “membrane” across which silicon diffuses to the surface 140 for TGO formation, without the membrane itself being structurally consumed.
The material(s) selected for the second silicide desirably is chemically stable at temperatures in the relevant use range for article 100, such as temperatures of 1200 degrees Celsius or greater for the case of gas turbine components. Moreover, at such temperatures it is desirable that the second silicide not substantially react with the second phase of first layer 110. In some embodiments, the second silicide includes a metal element, such as molybdenum, tantalum, tungsten, niobium, titanium, ruthenium, or a combination including one or more of the aforementioned elements. Generally, the diffusion rate of silicon through the silicide is desirably higher at service temperatures (such as temperatures higher than 1200 degrees Celsius) than the diffusion rate of the constituent metal element(s) through the silicide at the same temperature, to promote desirable silicon oxidation at surface 140 over potentially less desirable metal oxide formation.
Moreover, the silicide is typically selected such that, again at relevant service temperatures, silicon diffusion through the silicide exceeds the rate at which oxygen diffuses through silica; in some cases this difference may be by a factor of ten or more, and in certain instances it may differ by more than a factor of 100. This comparatively rapid rate of silicon transport through the second silicide enables a remarkable ability to generate the protective TGO 150 without substantially consuming (that is, completely oxidizing) the second silicide, because silicon from first layer 110 is able to move quickly to interface 140 to feed the TGO formation reaction before oxygen penetrates through the TGO to substantially attack second layer 120. First layer 110 thus serves as a “reservoir” of silicon that travels via solid state diffusion through the first silicide in first layer 110 and the second silicide in second layer 120 to arrive at surface 140 and there participate in the formation of TGO 150. Because second layer 120 is not substantially consumed in service, it may be disposed as a relatively thin layer, and so may not generate undesirable levels of thermal stress even where its silicide has a rather substantial mismatch of coefficient of thermal expansion relative to that of substrate 101.
The second silicide may include one silicide constituent, or it may include two or more silicide constituents. In one embodiment, the second silicide includes a disilicide, such as a disilicide of molybdenum, tantalum, tungsten, niobium, titanium, ruthenium, or a combination including one or more of these elements. As noted previously, a disilicide may advantageously provide a source for silicon to be used in formation of the TGO 150, but at the cost of a higher level of thermal expansion coefficient mismatch with substrate 101. In some embodiments, the second silicide includes a silicide having lower silicon content than a disilicide, referred to herein as a “lower silicide,” such as a lower silicide that exists in chemical equilibrium with a given disilicide, i.e., the next silicide to the disilicide in a descending order list of stable silicides for a given silicon-metal element equilibrium system. In particular embodiments, the second silicide consists essentially of one or more of such lower silicides. Examples of such lower silicides include, without limitation, TiSi, RuSi, and Z5Si3 where Z is, for instance, molybdenum, niobium, tantalum, and/or tungsten. As noted previously, lower silicides tend to have smaller coefficients of thermal expansion than their corresponding disilicides (meaning, disilicides of the same metal element), providing a better thermal expansion match with low-expansion CMC substrates such as CMC's containing silicon carbide or silicon nitride. The use of lower-expansion materials may allow the use of a thicker second layer 120, which may provide a more robust coating that better withstands mechanisms operative at high temperature, such as Oswaldt ripening, that may damage thinner coatings over time. In some embodiments, second layer 120 has a thickness up to about 250 micrometers. Generally, the thickness of the second layer 120 is designed to be little or no thicker than is needed to withstand these operative damage mechanisms for a desired lifetime. Accordingly, in certain embodiments, this thickness is in a range from about 1 micrometer to about 50 micrometers. Where a thinner layer is desired, second layer may be in a range from about 1 micrometer to about 25 micrometers, and in particular embodiments the range is from about 1 micrometer to about 5 micrometers.
In some embodiments, the second silicide includes a lower silicide, which may allow for a thicker second layer 120 to be applied than if a higher-thermal-expansion-coefficient silicide, such as a disilicide, were used. In particular embodiments of this type, the metal element components of first silicide and the second silicide are identical, that is, the first silicide and the second silicide are compounds of the same metal element, which may mitigate thermodynamic instability issues within the layers.
Second layer 120 may be deposited over first layer using any suitable coating technique, such as by plasma spraying, CVD, and/or PVD techniques.
Other coating layers may be present in article 100. As shown in
Another example of an optional layer present in article 100 addresses the issue of crystallization of the TGO (150,
Doping second layer 120 with aluminum may be one means of promoting aluminum uptake into the TGO, though this may not be efficient because the second silicide, present in second layer 120, is not substantially consumed by TGO formation. In one embodiment, as shown in
Combinations of the various functionalities described above for layers 201, 301 are also encompassed within the scope of this description. For instance, in some embodiments, layer 201 (
Top layer 130 may include various sub-layers. Use of the term “top layer” does not signify that layer 130 must be the outermost layer of article 100, although in some embodiments top layer is outermost. Top layer 130, typically including some form of oxide material, may be disposed to provide thermal insulation (a thermal barrier coating), environmental protection (an environmental barrier coating), or a combination of these functions. The selection of a suitable material will depend on the type of environment the article is to be exposed to, the composition of the underlying coatings and substrate, the cost of processing, and other factors. The top layer 130 may be a ceramic material including, but not limited to, a silicate (such as an aluminosilicate or rare earth silicate), and yttria-stabilized zirconia. Top layer 130 may include a rare earth monosilicate and/or rare earth disilicate. Top layer 130 may be a dual-layer coating, with an outer layer of rare earth monosilicate and an inner layer of rare earth disilicate. The rare earth elements associated with these monosilicate and disilicate materials include, for example, one or more of yttrium, ytterbium, lutetium, and scandium. In one embodiment, the outer layer of top layer 130 includes yttrium monosilicate and the inner layer of top layer 130 includes a rare earth disilicate (such as yttrium disilicate, for example).
Any of various techniques may be used for fabrication of top layer 130 and will be well known to practitioners in the art. Plasma spray, sol-gel, electrophoretic, and slurry-based deposition techniques are non-limiting examples of methods commonly used to apply coatings of this type.
The following examples are presented to further illustrate non-limiting embodiments of the present invention.
One illustrative example of an embodiment of the present invention is an article 100 that includes a substrate 101 comprising a ceramic matrix composite, such as a CMC including silicon carbide. A first layer 110 is disposed over substrate 101, and this layer 110 includes a first silicide and a second phase. The first silicide is disposed to have an interconnected morphology, and the second phase includes silicon carbide. The volume fraction of silicide in the first layer 110 is in the range from about 10 percent to about 40 percent, such as about 20 percent, for instance. A second layer 120 is disposed over the first layer 110. The second layer 120 comprises a second silicide. This second silicide is disposed in mass transfer communication with the first silicide. A top layer 130, comprising an oxide such as a mono- or di-silicate or yttria-stabilized zirconia, for example, is disposed over second layer 120.
Another illustrative example of an embodiment of the present invention is an article 100 that includes a substrate 101 comprising a ceramic matrix composite, such as a CMC including silicon carbide. A first layer 110 is disposed over substrate 101, and this layer 110 includes a first silicide and a second phase. The first silicide is disposed to have an interconnected morphology, and the second phase includes silicon nitride. The volume fraction of silicide in the first layer 110 is in the range from about 10 percent to about 50 percent, such as about 30 percent, for instance. A second layer 120 is disposed over the first layer 110. The second layer 120 comprises a second silicide. This second silicide is disposed in mass transfer communication with the first silicide. A top layer 130, comprising an oxide such as a mono- or di-silicate or yttria-stabilized zirconia, for example, is disposed over second layer 120.
Another illustrative example of an embodiment of the present invention is an article 100 that includes a substrate 101 comprising a ceramic matrix composite, such as a CMC including silicon carbide. A first layer 110 is disposed over substrate 101, and this layer 110 includes a first silicide and a second phase. The first silicide is disposed to have an interconnected morphology, and the second phase includes a rare earth disilicate. The volume fraction of silicide in the first layer 110 is in the range from about 10 percent to about 40 percent, such as about 20 percent, for instance. A second layer 120 is disposed over the first layer 110. The second layer 120 comprises a second silicide. This second silicide is disposed in mass transfer communication with the first silicide. A top layer 130, comprising an oxide such as a mono- or di-silicate or yttria-stabilized zirconia, for example, is disposed over second layer 120.
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.
