Claims:
1. An article, comprising:
a substrate; and
a plurality of coatings disposed on the substrate, the plurality of coatings comprising:
a thermal barrier coating disposed on the substrate; and
an overlay coating disposed over the thermal barrier coating, the overlay coating comprising an oxide of nominal composition AaA’bMO7 ;
wherein A and A’ respectively and independently comprise one or more elements selected from the group consisting of yttrium, scandium, indium, and the lanthanide series of elements, provided that at least one of A and A’ comprises a lanthanide series element;
wherein the sum of a and b is 3;
wherein M comprises a pentavalent cation of one or more element selected from the group consisting of ruthenium, tantalum, niobium, antimony, rhenium, osmium, and iridium; and
wherein the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm.
2. The article of claim 1, wherein A comprises at least one element from the lanthanide series having an atomic number in the range from 57 to 66; A’ comprises yttrium, scandium, indium, or a combination of these; a = 1; b = 2; and M comprises tantalum, niobium, or a combination of these.
3. The article of claim 2, wherein A comprises gadolinium.
4. The article of claim 2, wherein A’ comprises yttrium.
5. The article of claim 2, wherein A comprises gadolinium and A’ comprises yttrium.
6. The article of claim 1, wherein A comprises lanthanum; A’ comprises europium, gadolinium, terbium, or a combination of these; a=1; b=2; and M comprises ruthenium, niobium, tantalum or a combination of these.
7. The article of claim 6, wherein A’ comprises gadolinium.
8. The article of claim 1, wherein A and A’ are the same lanthanide element, and wherein M comprises ruthenium, antimony, rhenium, osmium, iridium, or a combination including any of these.
9. The article of claim 1, wherein the overlay coating further comprises a secondary oxide phase, the secondary oxide phase comprising an orthorhombic weberite crystal structure of nominal composition X3ZO7
wherein X comprises a lanthanide element; and Z comprises niobium, tantalum, or a combination of these.
10. The article of claim 9, wherein X comprises gadolinium.
11. The article of claim 1, wherein the overlay coating comprises a first oxide and a second oxide, each of the first oxide and the second oxide independently having a fluorite-related crystal structure of space group C2221 or Cmcm, and each of the first oxide and second oxide independently having said nominal composition.
12. The article of claim 11, wherein
the first oxide has a nominal composition of AaA’bMO7 wherein A comprises at least one element from the lanthanide series having an atomic number in the range from 57 to 66; A’ comprises yttrium, scandium, indium, or a combination of these; a = 1; b = 2; and M comprises tantalum, and
the second oxide having a nominal composition of AaA’bMO7 wherein A comprises lanthanum; A’ comprises europium, gadolinium, terbium, or a combination of these; a=1; b=2; and M comprises ruthenium.
13. The article of claim 1, wherein the overlay coating has a thickness in a range from about 10 microns to about 1000 microns.
14. The article of claim 1, wherein the overlay coating consists essentially of the oxide.
15. The article of claim 1, wherein the thermal barrier coating comprises yttria-stabilized zirconia, scandia-stabilized zirconia, calcia-stabilized zirconia, hafnia-stabilized zirconia, magnesia-stabilized zirconia, or combinations thereof.
16. The article of claim 1, wherein the oxide is present in the plurality of coatings in an amount in a range from about 10 volume percent to about 75 volume percent of the plurality of coatings.
17. The article of claim 1, wherein the substrate comprises a superalloy material.
18. The article of claim 1, further comprising a bond coating disposed between the substrate and the thermal barrier coating.
19. A turbine engine component comprising the article of claim 1.
20. The turbine engine component of claim 18, wherein the article is a combustor component, a turbine blade, a shroud, a nozzle, a heat shield, or a vane.
21. An article, comprising:
a substrate; and
a plurality of coatings disposed on the substrate, the plurality of coatings comprising:
a thermal barrier coating disposed on the substrate; and
an overlay coating disposed over the thermal barrier coating, the overlay coating comprising an oxide of nominal composition GdY2MO7 ;
wherein the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm,and wherein M comprises niobium, tantalum, or a combination of these.
22. An article, comprising:
a substrate; and
a plurality of coatings disposed on the substrate, the plurality of coatings comprising:
a thermal barrier coating disposed on the substrate; and
an overlay coating disposed over the thermal barrier coating, the overlay coating comprising an oxide of nominal composition LaGd2MO7 ;
wherein the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm, and wherein M comprises tantalum, niobium, or a combination of these.
, Description:ARTICLE FOR HIGH TEMPERATURE SERVICE

BACKGROUND OF THE INVENTION
[0001] The invention relates generally to articles including protective coatings for thermal barrier coatings. More particularly, the invention relates to articles including protective coatings for thermal barrier coatings, such that the protective coatings are calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive.
[0002] Thermal barrier coatings are typically used in articles that operate at or are exposed to high temperatures. Aviation turbines and land-based turbines, for example, may include one or more components protected by the thermal barrier coatings. Under normal conditions of operation, thermal-barrier coated components may be susceptible to various types of damage, including erosion, oxidation, and attack from environmental contaminants.
[0003] For turbine components, environmental contaminant compositions of particular concern are those containing oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. These oxides combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide systems (Ca--Mg--Al--SiO), hereafter referred to as "CMAS." At the high turbine operating temperatures, these environmental contaminants can adhere to the heated or hot thermal barrier coating surface, and thus cause damage to the thermal barrier coating. For example, CMAS can form compositions that are liquid or molten at the operating temperatures of the turbines. The molten CMAS composition can dissolve the thermal barrier coating, or can infiltrate its porous structure by infiltrating the pores, channels or other cavities in the coating. Upon cooling, the infiltrated CMAS composition solidifies and reduces the coating strain tolerance, thus initiating and propagating cracks that may cause delamination and spalling of the coating material. This may further result in partial or complete loss of the thermal protection provided to the underlying metal substrate of the part or component. Further, spallation of the thermal barrier coating may create hot spots in the metal substrate leading to premature component failure. Premature component failure can lead to unscheduled maintenance as well as parts replacement resulting in reduced performance, and increased operating and servicing costs.
[0004] Thus, there is a need for improved coating systems that provide protection to thermal barrier coatings from the adverse effects of environmental contaminants, when operated at or exposed to high temperatures. In particular, there is a need for improved coating systems that provide protection to thermal barrier coatings from the adverse effects of deposited CMAS.
BRIEF DESCRIPTION OF THE INVENTION
[0005] One embodiment is directed to an article including a substrate and a plurality of coatings disposed on the substrate. The plurality of coatings comprises a thermal barrier coating disposed on the substrate, and an overlay coating disposed over the thermal barrier coating. The overlay coating comprises an oxide of nominal composition AaA’bMO7 wherein A and A’ respectively and independently comprise one or more elements selected from the group consisting of yttrium, scandium, indium, and the lanthanide series of elements, provided that at least one of A and A’ comprises a lanthanide series element; wherein the sum of a and b is 3, wherein M comprises a pentavalent cation of one or more element selected from the group consisting of ruthenium, tantalum, niobium, antimony, rhenium, osmium, and iridium, and wherein the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm.
[0006] Another embodiment of the invention is directed to an article including a substrate and a plurality of coatings disposed on the substrate. The plurality of coatings comprises a thermal barrier coating disposed on the substrate and an overlay coating disposed over the thermal barrier coating. The overlay coating comprises an oxide of nominal composition LaGd2MO7, wherein the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm, and wherein M comprises niobium, tantalum, or a combination of these.
[0007] Another embodiment of the invention is directed to an article including a substrate and a plurality of coatings disposed on the substrate. The plurality of coatings comprises a thermal barrier coating disposed on the substrate and an overlay coating disposed over the thermal barrier coating. The overlay coating comprises an oxide of nominal composition GdY2MO7, wherein the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm, and wherein M comprises niobium, tantalum, or a combination of these.
DRAWINGS
[0008] 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:
[0009] FIG. 1 is a schematic cross section of one embodiment of the invention;
[0010] FIG. 2 is a schematic cross section of another embodiment of the invention; and
[0011] FIG. 3 is a schematic cross section of yet another embodiment of the invention.
DETAILED DESCRIPTION
[0012] 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.
[0013] 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.
[0014] 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.
[0015] As used herein, the term “coating” refers to a material disposed on at least a portion of an underlying surface in a continuous or discontinuous manner. Further, the term “coating” does not necessarily mean a uniform thickness of the disposed material, and the disposed material may have a uniform or a variable thickness. The term “coating” may refer to a single layer of the coating material or may refer to a plurality of layers of the coating material. The coating material may be the same or different in the plurality of layers.
[0016] As used herein, the term “disposed on” refers to layers or coatings disposed directly in contact with each other or indirectly by having intervening layers there between, unless otherwise specifically indicated. The term “adjacent” as used herein means that the two layers or coatings are disposed contiguously and are in direct contact with each other.
[0017] As mentioned earlier, thermal barrier coatings are susceptible to molten CMAS compositions at high turbine operating temperatures. The molten CMAS composition can dissolve the thermal barrier coating, or can infiltrate its porous structure by infiltrating the pores, channels or other cavities in the coating. Upon cooling, the infiltrated CMAS composition solidifies and reduces the coating strain tolerance, thus initiating and propagating cracks that may cause delamination and spalling of the coating material. Previous methods to protect the thermal barrier coatings include use of CMAS-reactive materials, which react with the CMAS to produce reaction products with higher melting point or higher viscosity than standard CMAS and thereby arrest liquid infiltration of the coating, or CMAS-resistant materials, which act as a physical barrier to the liquid CMAS. However, the previously known CMAS-reactive compositions may not provide the desired level of CMAS-reactivity, or may be costly and/or difficult to manufacture.
[0018] Embodiments of the invention described herein address the noted shortcomings of the state of the art. Some embodiments present an article including a substrate and a plurality of coatings disposed on the substrate. The plurality of coatings includes a thermal barrier coating disposed on the substrate, and a protective overlay coating including a CMAS-reactive material disposed on the thermal barrier coating. The CMAS-reactive material is present in the plurality of coatings in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition.
[0019] In accordance with some of the embodiments of the invention, the protective overlay coating may protect the thermal barrier coating by undergoing chemical changes, physical changes, or combinations of these when in contact with a CMAS composition. The protective coating may be disposed on the thermal barrier coating such that the protective coating overlies the thermal barrier coating. In certain embodiments, this overlay coating is disposed adjacent to the thermal barrier coating.
[0020] The term “CMAS” or “CMAS composition” as used herein refers to a contaminant composition including, though not necessarily limited to, calcium, magnesium, aluminum and silicon. In some embodiments, the CMAS composition primarily includes a mixture of magnesium oxide, calcium oxide, aluminum oxide and silicon oxide. A non-limiting example of a CMAS composition includes calcium oxide present in an amount in a range from about 1 weight percent to about 60 weight percent of the total CMAS composition; magnesium oxide present in an amount in a range from about 0 weight percent to about 20 weight percent of the total CMAS composition; aluminum oxide present in an amount in a range from about 10 weight percent to about 30 weight percent of the total CMAS composition; and silicon oxide present in an amount in a range from about 20 weight percent to about 80 weight percent of the total CMAS composition.
[0021] In some embodiments, other elements, such as nickel, iron, titanium and chromium, may also be present in the CMAS composition. In such instances, the additional elements may be present in a small amount, for example, less than about 10 weight percent of total amount of CMAS composition present. In some such instances, the CMAS composition may include about 29 wt percent calcium oxide, about 7 wt percent magnesium oxide, about 11 wt percent aluminum oxide, and about 43 wt percent silicon oxide. Further, the composition may include about 2 wt percent nickel oxide, about 8 wt percent iron oxide, and small amounts of titanium oxide and chromium oxide, such that the total weight percentage of these elements is less than 10 wt percent. The CMAS composition may have a melting temperature less than about 1315° C (2399° F) in some embodiments, and less than about 1227° C (2240° F) in some other embodiments.
[0022] The particular compositional characteristics of the CMAS composition may depend on the source of the environmental contaminants and the reaction temperature. The CMAS composition is typically formed at operational temperatures of about 1000° C. (1832° F) or more. Sources of CMAS composition include, but are not limited to, sand, dirt, volcanic ash, fly ash, cement, runway dirt, fuel and air sources, oxidation and wear products from engine components, or combinations of any of these.
[0023] As used herein, the term "CMAS-reactive material" refers to a material capable of reacting with a CMAS composition to form a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition. In some instances, the reaction product may form a glassy (typically thin) protective layer that the CMAS deposits are unable (or less able) to adhere to and/or penetrate.
[0024] In accordance with embodiments of the invention, a CMAS-reactive material included in the aforementioned overlay coating is a complex oxide having a fluorite-related crystal structure of space group C2221 or Cmcm. The oxide further has a nominal composition represented by the formula AaA’bMO7. This representation is referred to as a “nominal” composition because those skilled in the art will recognize that small deviations in stoichiometry from the nominal formula may be possible to formulate through the use of dopants and/or charge compensation agents, in keeping with standard known techniques in the art of ceramic science. Indeed all chemical formulae provided in this description should be understood as nominal compositions and inclusive of small stoichiometric deviations from the specified formulae.
[0025] In embodiments encompassed by the above formula, A and A’ respectively and independently comprise one or more elements selected from the group consisting of yttrium, scandium, indium, and the lanthanide series of elements. These moieties A and A’ are said to “respectively and independently comprise” the specified alternatives because each of A and A’ comprises one or more element from the specified collection of alternatives, and A and A’ may include one or more identical or different element from this collection. The lanthanide series of elements is the series of elements having atomic number in the range from 57 (lanthanum) to 71 (lutetium).
[0026] In the above formula, M comprises a pentavalent cation of one or more element selected from the group consisting of ruthenium, tantalum, niobium, antimony, rhenium, osmium, and iridium. To satisfy the requirements of charge balance, generally the sum of a and b in the above formula is 3.
[0027] Oxides having the characteristics set forth above have been observed to be to be highly reactive with CMAS materials, as noted in more detail in the Examples section of this disclosure, below. The behavior of these oxides is consistent with experience with similar compositions having an orthorhombic weberite crystal structure, such as those described in commonly assigned United States Patent Application serial number 14/525,586, filed 28 October 2014. Like weberites, the oxides having the C2221 or Cmcm space groups are orthorhombic superstructures of cubic fluorite. The attractive levels of reactivity with CMAS observed for the presently disclosed C2221 or Cmcm oxides suggest that the presently described materials have high potential effectiveness as protective coatings in embodiments described herein.
[0028] In some embodiments, the A and A’ moieties are identical, that is, they are the same lanthanide element; furthermore, in such embodiments M comprises ruthenium, antimony, rhenium, osmium, iridium, or a combination including any of these. Oxides of this composition having a C2221 or Cmcm crystal structure have been characterized and, in some cases, studied for their magnetic properties. Non-limiting examples of such oxide compounds described as having the Cmcm structure include Ln3RuO7 (where Ln represents a lanthanide having an atomic number in the range from 57 to 65); Ln3ReO7 (where Ln represents a lanthanide having an atomic number in the series including 59, 60, and 62-65), Ln3OsO7 (where Ln represents a lanthanide having an atomic number in the series including 59, 60, and 62-65), Ln3IrO7 (where Ln represents a lanthanide having an atomic number in the series including 59, 60, 62, and 63), and Pr3SbO7. Non-limiting examples of such oxides having the C2221 structure include Ln3SbO7 (where Ln represents yttrium, dysprosium, and/or holmium); and Ln3ReO7 (where Ln represents dysprosium and/or holmium). Moreover, in some embodiments M comprises tantalum and/or niobium. Examples of such oxides include Ln3TaO7 (where Ln represents yttrium, samarium, europium, gadolinium, terbium, dysprosium, and/or holmium), which has been characterized as having a Cmcm form; and Gd3NbO7, which has been characterized as having a C2221 form.
[0029] In some embodiments, A comprises at least one element from the lanthanide series having an atomic number in the range from 57 to 66 (that is, from lanthanum through dysprosium); A’ comprises yttrium, scandium, indium, or a combination of these; a = 1; b = 2; and M comprises tantalum, niobium, or a combination of these. Oxides of this type, such as LnY2TaO7 (Ln signifying one or more element from the series of lanthanum through lutetium, in order of atomic number) have previously been described as having a C2221 structure. The crystal structure and associated atomic positions described for these compounds may allow for enhanced reactivity with CMAS material. The inclusion of tantalum and/or niobium may provide a stability advantage in that some complex oxides, such as weberites, that include these elements have shown desirable amenability to processing by plasma spraying and electron beam physical vapor deposition (EBPVD), along with desirable stability in steam exposure. Gadolinium-bearing complex oxides in particular have shown the ability to react with CMAS to form phases such as apatite with comparatively high melting points relative to CMAS. Accordingly, in some embodiments, A comprises gadolinium. Yttrium is also a CMAS reactive element. It has been shown that high-yttrium containing zirconia (e.g. zirconia with 55 percent yttria addition by weight) also forms apatite phases upon reaction with CMAS. Accordingly, in some embodiments A’ comprises yttrium. Furthermore, yttrium also stabilizes the formation of highly stable rare-earth silicate garnet. Though the garnet phase is slower to form than apatites, the garnets are found to be stable equilibrium phases that confine additional calcia and silica from the CMAS in solid phases when they eventually form. Gadolinium, on the other hand, is not as efficient at producing garnet phases. Therefore, combining gadolinium and yttrium is expected to have both the benefits of fast apatite formation to stop CMAS infiltration as well as slow garnet formation for complete crystallization of the residual CMAS. In one illustrative embodiment that potentially combines the advantages described above, A comprises gadolinium and A’ comprises yttrium, for instance, in a composition having a nominal formula GdY2TaO7 .
[0030] In some embodiments, A comprises lanthanum; A’ comprises europium, gadolinium, terbium, or a combination of these; a=1; b=2; and M comprises ruthenium, niobium, tantalum, or a combination of these. Oxides of this type, such as LaLn2RuO7 (Ln = europium, gadolinium, terbium) have previously been described as having a Cmcm structure. As noted above, the crystal structure and associated atomic positions described for these compounds may allow for enhanced reactivity with CMAS material. The inclusion of tantalum and/or niobium at the M sites in the oxide may provide stability advantages noted previously. The gadolinium-bearing compositions of this type may offer particular advantages in CMAS reactivity, and thus, in some embodiments, A’ comprises gadolinium.
[0031] The overlay coating that includes the AaA’bMO7 complex oxide as described above, in some embodiments, includes other oxide constituents. For example, in certain embodiments the overlay coating further comprises a secondary oxide phase. The secondary oxide phase includes an oxide having a different crystal structure, a different composition, or some other discernable difference from the AaA’bMO7 complex oxide. One illustrative secondary oxide phase comprises an orthorhombic weberite crystal structure of nominal composition X3ZO7, where X signifies one or more lanthanide elements, such as gadolinium, for instance; and Z signifies niobium, tantalum, or a combination that includes one or both of these elements. Weberites of this type, as noted in the aforementioned United States Patent Application serial number 14/525,586, have shown desirable levels of reactivity with CMAS materials and may be useful additions to the overlay coating described herein.
[0032] Another embodiment illustrative of the inclusion of multiple oxide constituents in the overlay coating has the overlay coating including a first oxide and a second oxide. Both the first oxide and the second oxide independently have the AaA’bMO7 nominal composition described above, and both independently have a fluorite-related crystal structure selected from the types noted previously, that is, each independently has a structure of space group C2221 or Cmcm. For example, in one embodiment the first oxide has a nominal composition of AaA’bMO7 , wherein A comprises at least one element from the lanthanide series having an atomic number in the range from 57 to 66 (that is, lanthanum through dysprosium); A’ comprises yttrium, scandium, indium, or a combination of these; a = 1; b = 2; and M comprises tantalum; and the second oxide has a nominal composition of AaA’bMO7 , wherein A comprises lanthanum; A’ comprises europium, gadolinium, terbium, or a combination of these; a=1; b=2; and M comprises ruthenium.
[0033] As noted earlier, the CMAS-reactive material is present in the plurality of coatings in an effective amount to react with the CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition.
[0034] The term “effective amount” as used herein refers to an amount (for example, volume) of the CMAS-reactive material sufficient to effectively increase one or both of the melting temperature and viscosity of the reaction product formed.
[0035] The term “operating temperature” of the thermal barrier coating refers to the temperature that the thermal barrier coating is exposed to in the turbine. In some embodiments, the operating temperature of the thermal barrier coating refers to the surface temperature of the thermal barrier coating. The term “reaction product” as used herein refers to a product or a mixture of products formed by reacting the CMAS-reactive material with the CMAS composition. In certain embodiments, the reaction product may include a mixture of products. Accordingly the terms “reaction product” and “reaction product mixture” are used herein interchangeably. In some such instances, one or more products in the reaction product mixture may include crystalline phases that have a melting temperature greater than that of the CMAS composition. Further, in some such instances, the reaction product mixture may have a viscosity greater than that of the CMAS composition.
[0036] In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount such that the melting temperature of the reaction product is increased at least to the surface temperature of the thermal barrier coating. In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount such that the melting temperature of the reaction product increases by at least about 10° C. above the surface temperature of the thermal barrier coating during its operation. In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount such that the melting temperature of the reaction product increases by about 40°C to about 100° C above the surface temperature of the thermal barrier coating during its operation. Thus, by way of an example, if the surface temperature of the thermal barrier coating during operation is about 1230° C, then the CMAS-reactive material is present in amount such that the melting temperature of the reaction product increases to at least about 1240° C.
[0037] In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount such that the viscosity of the reaction product increases by at least about 10 centipoise above the viscosity of the CMAS composition, at the operating temperature of the thermal barrier coating. In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount such that the viscosity of the reaction product increases by about 10 centipoise to about 1000000 centipoise above the viscosity of the CMAS composition, at the operating temperature of the thermal barrier coating.
[0038] In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount in a range from about 10 volume percent to about 75 volume percent. In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount in a range from about 10 volume percent to about 50 volume percent. In some embodiments, the CMAS-reactive material is present in the plurality of coatings in an amount in a range from about 10 volume percent to about 25 volume percent. The selected amount may vary from case to case depending in part on the temperature of the application, the nature of the substrate, the material selection for the thermal barrier coating, the particular composition of the CMAS encountered in the application, the reactivity of the CMAS-reactive material with that CMAS composition, and other factors.
[0039] The protective overlay coating may be further characterized by the thickness, and may have a thickness such that an effective amount of the CMAS-reactive material is present in the plurality of coatings. In some embodiments, the overlay coating has a thickness in a range from about 10 microns to about 1000 microns. In some embodiments, the overlay coating has a thickness in a range from about 25 microns to about 500 microns. In some embodiments, the overlay coating has a thickness in a range from about 50 microns to about 100 microns. Thickness selection for the protective coating may also be affected by the material selection of the substrate and thermal barrier coating, the temperature expected for the application, and other factors.
[0040] The protective coating may include the CMAS-reactive material in an amount of up to 100 weight percent, and sufficient to protect the thermal barrier coating at least partially against deposited CMAS. In some embodiments, the protective coating consists essentially of the CMAS-reactive material. The term “consists essentially” as used herein means that the overlay coating includes less than 10 volume percent of material other than the CMAS-reactive material that may alter certain important properties of the protective coating (for example, coefficient of thermal expansion, overall reactivity with CMAS). In certain embodiments, the overlay coating includes less than about 10 volume percent of the thermal barrier coating material (for example, ceramic thermal barrier coating material).
[0041] As used herein, the term "thermal barrier coating" refers to a coating including a material capable of substantially reducing heat flow to the underlying substrate of the article, that is, forming a thermal barrier. In some embodiments, the thermal barrier coating includes a material having a melting point greater than about 1090 °C. In some embodiments, the thermal barrier coating includes a material having a melting point greater than about 1200 °C. In some embodiments, the thermal barrier coating includes a material having a melting point in a range from about 1200 °C to about 1930 °C.
[0042] In some embodiments, the thermal barrier coating includes a ceramic thermal barrier material. Suitable ceramic thermal barrier coating materials include various zirconias, in particular chemically stabilized zirconias (for example, metal oxides blended with zirconia), such as yttria-stabilized zirconias, ceria-stabilized zirconias, calcia-stabilized zirconias, scandia-stabilized zirconias, magnesia-stabilized zirconias, india-stabilized zirconias, ytterbia-stabilized zirconias, lanthana-stabilized zirconias, gadolinia-stabilized zirconias, as well as mixtures of such stabilized zirconias.
[0043] In certain embodiments, the thermal barrier coating includes yttria-stabilized zirconia. Suitable yttria-stabilized zirconia may include from about 1 weight percent to about 20 weight percent yttria (based on the combined weight of yttria and zirconia), and more typically from about 3 weight percent to about 10 weight percent yttria. An example yttria-stabilized zirconia thermal barrier coating includes about 7 percent yttria and about 93 percent zirconia. These chemically stabilized zirconias may further include one or more of a second metal (e.g., a lanthanide or actinide) oxide such as dysprosia, erbia, europia, gadolinia, neodymia, praseodymia, urania, and hafnia to further reduce thermal conductivity of the thermal barrier coating. In some embodiments, the thermal barrier coating may further include an additional metal oxide, such as titania.
[0044] Suitable ceramic thermal barrier coating materials may also include pyrochlores of general formula A2B2O7 where A is a metal having a valence of 3+ or 2+ (e.g., gadolinium, aluminum, cerium, lanthanum or yttrium) and B is a metal having a valence of 4+ or 5+ (e.g., hafnium, titanium, cerium or zirconium) where the sum of the A and B valences is 7. Representative materials of this type include gadolinium-zirconate, lanthanum titanate, lanthanum zirconate, yttrium zirconate, lanthanum hafnate, cerium zirconate, aluminum cerate, cerium hafnate, aluminum hafnate and lanthanum cerate.
[0045] The thermal barrier coating may include the ceramic thermal barrier coating material in an amount of up to 100 weight percent. In some embodiments, the thermal barrier coatings includes the ceramic thermal barrier coating material in a range from about 95 weight percent to about 100 weight percent and more particularly from about 98 weight percent to about 100 weight percent. The composition of the thermal barrier coating in terms of the type and amount of the ceramic thermal barrier coating materials may depend upon one or factors, including the composition of the adjacent bond coat layer (if present), the coefficient of thermal expansion (CTE) characteristics desired for the thermal barrier coating, and the thermal barrier properties desired for the thermal barrier coating.
[0046] The thickness of the thermal barrier coating may depend upon the substrate or the component it is deposited on. In some embodiments, the thermal barrier coating has a thickness in a range from about 50 percent to about 90 percent of the total thickness of the plurality of layers. In some embodiments, the thermal barrier coating has a thickness in a range of from about 25 microns to about 2000 microns. In some embodiments, the thermal barrier coating has a thickness in a range of from about 25 microns to about 1500 microns. In some embodiments, the thermal barrier coating has a thickness in a range of from about 25 microns to about 1000 microns.
[0047] As noted earlier, the thermal barrier coatings typically include pores, channels or other cavities that, during service, may be infiltrated by molten environmental contaminants, such as CMAS. In some instances, these pores, channels, or cavities may be created by environmental damage or the normal wear and tear during operation of the thermal barrier coatings. In some instances, the pores, channels or other cavities in the thermal barrier coating surface may result due to the deposition processes. For example, thermal barrier coatings that are deposited by plasma spray techniques, such as air plasma spraying, may result in a sponge-like porous structure of open pores in at least the surface of the coating. Similarly, thermal barrier coatings that are deposited by vapor deposition techniques, such as physical vapor deposition, may result in a porous structure including a series of columnar grooves, crevices or channels in at least the surface of the coating. The porous structure is generally thought in the industry to be one of the factors that provides for strain tolerance in thermal barrier coatings during thermal cycling. Further, the porous structure may further provide for stress reduction due to the differences between the coefficient of thermal expansion (CTE) of the coating and the CTE of the underlying bond coat layer/substrate.
[0048] The thermal barrier coating may be disposed over the afore-mentioned bond coat or directly onto the substrate depending upon the desired application. The type of substrate may depend in on part on the turbine component. Non-limiting examples of suitable substrates include metals, metal alloys, or combinations thereof. In certain embodiments, the substrate includes an alloy of nickel, cobalt, iron, or combinations thereof. For example, the substrate may include a high temperature, heat-resistant alloy, e.g., a superalloy. Non-limiting examples of suitable high temperature nickel-based alloys include Inconel®, Nimonic®, Rene® (e.g., Rene® 80, Rene® 95 alloys), Udimet®, or combinations thereof.
[0049] The article may further include a bond coating disposed between the substrate and the thermal barrier coating. The bond coating may be formed from a metallic oxidation-resistant material that protects the underlying substrate and enables the thermal barrier coating to more tenaciously adhere to substrate. Suitable materials for the bond coating include M1CrAlY alloys, where M1 represents a metal such as iron, nickel, platinum or cobalt. Non-limiting examples of other suitable bond coat materials include metal aluminides such as nickel aluminide, platinum aluminide, or combinations thereof. A typical, but not limiting, range of bond coat thickness is from about 25 microns to about 500 microns.
[0050] In some embodiments, the protective coating may be the outermost layer (sometimes also referred to as “top coat layer”) in the article. In some other embodiments, the article may further include one or more additional layers disposed on the protective coating to form the top coat layer. Non-limiting examples of suitable top-coat layers include erosion-resistant layers, such as aluminum oxide, for example.
[0051] Referring now to Fig. 1, an article 100 includes a substrate 110 and a plurality of coatings 120 disposed on the substrate 110. The plurality of coatings includes a thermal barrier coating 122 disposed on the substrate 110, and a protective overlay coating 124 disposed on the thermal barrier coating 122. In the embodiment illustrated in Fig. 1, the protective overlay coating 124 overlies and is disposed adjacent to the thermal barrier coating 122. Details and illustrative examples of the characteristics of protective overlay coating 124, thermal barrier coating 122, and substrate 110 are described above.
[0052] Fig. 2 illustrates another embodiment similar to Fig. 1, with the addition of a bond coating 130 disposed between the substrate 110 and the thermal barrier coating 122. In the embodiment illustrated in Fig. 2, the thermal barrier coating 122 overlies and is disposed adjacent to the bond coating 130. Details and illustrative examples of the characteristics of bond coating 130 are described above.
[0053] Fig. 3 illustrates another embodiment of the invention similar to Fig. 3, with the addition of a top-coat layer 140 disposed over the protective overlay coating 124. As mentioned previously the top-coat layer 140 may include an erosion-resistant material in some embodiments.
[0054] The articles 100 described herein may be useful in a wide variety of turbine components (e.g., turbine engine components) that are operated at, or otherwise exposed to, high temperatures. Non-limiting examples of suitable turbine engine components include turbine airfoils such as blades and vanes, turbine shrouds, turbine nozzles, buckets, combustor components such as liners and deflectors, heat shields, augmentor hardware for gas turbine engines, and the like. The coatings systems 120 employed in the presently described articles 100 may be disposed over a portion or over all of the metal substrate 110. For example, with regard to airfoils such as blades, the coating systems 120 are typically used to protect, cover or overlay portions of the metal substrate 110 of the airfoil other than solely the tip thereof, for example, the thermal barrier coatings may cover the leading and trailing edges and other surfaces of the airfoil.
[0055] In an illustrative embodiment, an article 100 includes a substrate 110 and a plurality of coatings 120 disposed on the substrate 110. The plurality of coatings 120 includes a thermal barrier coating 122 disposed on the substrate and an overlay coating 124 disposed over the thermal barrier coating 122. The overlay coating 124 includes an oxide of nominal composition GdY2MO7, where the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm. M includes niobium, tantalum, or a combination of these. As noted previously, the inclusion of gadolinium in an oxide of this crystal structure is believed to promote reactivity with CMAS materials, and niobium and/or tantalum may promote desirable stability in the oxide.
[0056] In another illustrative embodiment, an article 100 includes a substrate 110 and a plurality of coatings 120 disposed on the substrate 110. The plurality of coatings 120 includes a thermal barrier coating 122 disposed on the substrate and an overlay coating 124 disposed over the thermal barrier coating 122. The overlay coating 124 includes an oxide of nominal composition LaGd2MO7, where the oxide has a fluorite-related crystal structure of space group C2221 or Cmcm. M includes niobium, tantalum, or a combination of these. As noted previously, the inclusion of gadolinium in an oxide of this crystal structure is believed to promote reactivity with CMAS materials, and niobium and/or tantalum may promote desirable stability in the oxide.
[0057] Oxides having the composition and crystal structure suitable for use in overlay coating 124 can be made, for example, by performing a solid-state chemical reaction between appropriate amounts of binary oxides of the various metal element constituents. Such reactions are well known and readily developed by those of ordinary skill in the art. Examples of typical synthesis reactions for materials of the type described herein are set forth in Y. Hinatsu et al., Journal of Solid State Chemistry 233 (2016) 37–43.
[0058] An illustrative method for manufacturing article 100 includes disposing thermal barrier coating 122 over substrate 110; and disposing overlay coating 124 including a calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive material over thermal barrier coating 122. As mentioned earlier, the CMAS-reactive material included in the aforementioned overlay coating 124 is a complex oxide having a fluorite-related crystal structure of space group C2221 or Cmcm. The oxide further has a nominal composition represented by the formula AaA’bMO7 where A and A’ respectively and independently include one or more elements selected from the group consisting of yttrium, scandium, indium, and the lanthanide series of elements, provided that at least one of A and A’ includes a lanthanide series element; the sum of a and b is 3; and M includes a pentavalent cation of one or more element selected from the group consisting of ruthenium, tantalum, niobium, antimony, rhenium, osmium, and iridium. The CMAS-reactive material is present in the plurality of coatings 120 in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating 122, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition.
[0059] The thermal barrier coating 122 may be deposited or otherwise formed on a bond coating 130 (if present) or on the substrate 110 directly by any of a variety of conventional techniques, including vapor disposition, such as physical vapor deposition (PVD), electron beam physical vapor deposition (EBPVD); plasma spray, such as air plasma spray (APS), suspension plasma spray (SPS), and vacuum plasma spray (VPS); other thermal spray deposition methods such as high velocity oxy-fuel (HVOF) spray, detonation, or wire spray; chemical vapor deposition (CVD), sol-gel method, or combinations of two or more of the afore-mentioned techniques.
[0060] The particular technique used for applying, depositing or otherwise forming the thermal barrier coating 122 may depend on one or more of its composition, thickness, and desired physical structure. In certain embodiments, the thermal barrier coating 122 is disposed on the substrate 110 using a plasma spray technique. Various types of plasma-spray techniques are well known to those skilled in the art, and any one or combination of these techniques may be used to dispose thermal barrier coating 122.
[0061] In some embodiments, thermal barrier coating 122 may be disposed on bond coating 130. In such instances, the bond coating 130 may be applied, deposited, or otherwise formed on the substrate by any of a variety of conventional techniques including vapor disposition, such as physical vapor deposition (PVD) or electron beam physical vapor deposition (EBPVD); plasma spray, such as air plasma spray (APS), suspension plasma spray (SPS), or vacuum plasma spray (VPS); other thermal spray deposition methods such as high velocity oxy-fuel (HVOF) spray, detonation, or wire spray; chemical vapor deposition (CVD); sol-gel methods; or combinations of two or more of the aforementioned techniques. In some embodiments, a plasma spray technique, such as that used for the thermal barrier coating 122, may be employed to dispose bond coating 130 on substrate 110.
[0062] The method further includes disposing protective coating 124 on thermal barrier coating 122. Protective coating 124 may be disposed on thermal barrier coating 122 using one or more of the aforementioned techniques used to dispose thermal barrier coating 122. In embodiments where the protective coating 124 further includes a ceramic thermal barrier material, the CMAS-reactive material and the ceramic thermal barrier material may be co-deposited on the thermal barrier coating 122. In some embodiments, co-depositing may be achieved by blending, mixing or otherwise combining the CMAS-reactive material and ceramic thermal barrier coating material together (for example, as powders) to provide a mixture that is then deposited onto the thermal barrier coating 122. In some embodiments, co-depositing may be achieved by separately depositing onto the thermal barrier coating 122 (e.g., as separate plasma spray streams) the respective CMAS-reactive material and ceramic thermal barrier coating material in a manner such that these materials blend, mix, or otherwise combine together to form a mixture.
[0063] The method of the present invention is particularly useful in providing protection or mitigation against the adverse effects of environmental contaminant compositions for newly manufactured articles. However, the method of the present invention is also useful in providing such protection or mitigation against the adverse effects of environmental contaminant compositions for refurbished articles.
[0064] According to embodiments of the invention, articles are provided with at least partial protection against, or mitigation of, the adverse effects of environmental contaminant compositions, such as CMAS materials, that can deposit on the surface of articles during normal turbine operation. The CMAS-reactive material present in protective overlay coating 124 reacts with the CMAS deposits to form a reaction product having a higher melting point that does not become molten, or alternatively has a viscosity such the molten reaction product does not flow readily at higher temperatures normally encountered during turbine engine operation. In some instances, this combined reaction product may form a glassy (typically thin) protective layer that CMAS deposits are unable or less able to adhere to. As a result, these CMAS deposits may be unable to infiltrate the normally porous surface structure of the thermal barrier coating, and thus may not cause undesired partial (or complete) delamination and spalling of the coating.
EXAMPLES
Example 1
[0065] GdY2TaO7 powder compositions (C2221 structure) formed via solid state reaction of constituent oxides at temperatures greater than 1300 degrees Celsius were contacted with nominal CMAS composition for 15 minutes at one of two nominal temperatures (either 1260 degrees Celsius or 1400 degrees Celsius). The volume ratio of GdY2TaO7 oxides to the CMAS composition was 1:3. X-ray diffraction analysis was conducted to analyze the phases of the reaction products. At both temperatures, the GdY2TaO7 substantially reacted completely with the CMAS to form a reaction product having an apatite-type structure with a nominal composition Ca2(Gd,Y)8(SiO4)6O2.
Example 2
[0066] LaY2TaO7 and GdY2TaO7 pellets were contacted with nominal CMAS by placing CMAS-bearing tapes on the surface of the pellets. The pellets and CMAS tapes were heat treated at 1400 degrees Celsius for 15 minutes. Top-down scanning electron micrograph images of reaction products were observed to analyze the morphology of the phases. Both of these oxides formed needle-shaped precipitates that are commonly found to have an apatite type structure with a nominal composition Ca2(Gd,Y)8(SiO4)6O2.
[0067] The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, while remaining within the scope of the invention and the appended claims.