Claims:1. An article (10) comprising:
a superalloy substrate (14);
a bond coating (16) disposed on the superalloy substrate (14); and
a thermal barrier coating (20) disposed on the bond coating (16),
wherein the thermal barrier coating (20) comprises an inner ceramic layer (22) and an outer ceramic layer (24), wherein the outer ceramic layer (24) comprises a material comprising a composition of formula I,
(alkaline earth metal)x(rare earth metal)y(Zr, Hf, Ti)zOd (I)
wherein x>0, y>0, z>0, d >0, and [y/(x+y+z)] = 0.28, and
wherein the composition of formula I has a cubic fluorite structure.
2. The article (10) of claim 1, wherein the composition has a formula (Ca, Mg)x(Y, Gd)y(Zr, Hf, Ti)zOd.
3. The article (10) of claim 2, wherein the composition has a formula CaxYyZrzOd.
4. The article (10) of claim 3, wherein the composition has a formula CaY8Zr9O31.
5. The article (10) of claim 1, wherein the [y / (x+y+z)] is in a range from about 0.29 to about 0.40.
6. The article (10) of claim 1, wherein the [y / (x+y+z)] is in a range from about 0.40 to about 0.57.
7. The article (10) of claim 1, wherein [x / (x+y+z)] > 0.02.
8. The article (10) of claim 1, wherein 0.30 < [(x+y) / (x+y+z)] < 0.60.
9. The article (10) of claim 1, wherein the material has a lower compositional density compared to the compositional density of 6-9 weight % yttria-stabilized zirconia.
10. The article (10) of claim 1, wherein the material further comprises a composition of formula II,
(alkaline earth metal)p(rare earth metal)q(Zr, Hf, Ti)r O3±ß (II)
wherein p=1, q=0, r=1, and ß = 0, and wherein the composition of formula II has an orthorhombic structure.
11. The article (10) of claim 10, wherein the composition of formula II is present in the material in an amount that is less than 80 weight percent of the material.
12. The article (10) of claim 10, wherein the composition of formula II comprises calcium zirconate, magnesium zirconate, or a combination thereof.
13. The article (10) of claim 10, wherein the composition of formula II comprises the rare earth metal solid solution.
14. The article (10) of claim 1, wherein the inner ceramic layer comprises 6-9 weight % yttria-stabilized zirconia.
15. The article (10) of claim 1, wherein the article is a gas turbine engine component.
16. An article (10) comprising:
a nickel-based superalloy substrate (14);
a bond coating (16) disposed on the nickel-based superalloy substrate (14); and
a thermal barrier coating (20) disposed on the bond coating (16),
wherein the thermal barrier coating (20) comprises an inner ceramic layer (22) and an outer ceramic layer (24), wherein the outer ceramic layer (24) comprises a material comprising a composition of formula (Ca, Mg)x(Y, Gd)yZrzOd, wherein x>0, y>0, z>0, d >0, and [y / (x+y+z)] = 0.28, and wherein said composition has a cubic fluorite structure.
17. The article (10) of claim 16, wherein the [y / (x+y+z)] is in a range from about 0.29 to about 0.40.
18. The article (10) of claim 16, wherein the [y / (x+y+z)] is in a range from about 0.40 to about 0.57.
19. The article (10) of claim 16, wherein [(x+y) / (x+y+z)] > 0.35.
20. The article (10) of claim 16, wherein the article is a gas turbine engine component.
, Description:BACKGROUND
[0001] The invention relates generally to thermal barrier coatings including materials of low thermal conductivity. More particularly, the invention relates to articles comprising thermal barrier coatings having a layered structure comprising an inner ceramic layer and an outer ceramic layer.
[0002] Thermal barrier coatings (TBCs) are typically used in articles that are exposed to high temperatures. For example, one or more components of gas turbines or jet engines such as combustors, high pressure turbine (HPT) blades, vanes and shrouds may be protected by thermal barrier coatings. The thermal insulation provided by a TBC enables these components to survive higher operating temperatures, increases component durability, and improves engine reliability.
[0003] Examples of materials used for thermal barrier coatings include rare earth-stabilized zirconia materials such as yttrium-stabilized zirconia (YSZ). Rare earth stabilized zirconia materials have a low thermal conductivity of about 2.2 W/m-K when evaluated as a dense sintered compact and also a higher toughness. Furthermore, these materials have a large linear thermal expansion coefficient (CTE) that matches with the CTE of typical substrates used for TBCs. The YSZ is widely used as a TBC material in gas turbines because of its high temperature capability, low thermal conductivity, and relative ease of deposition. Typically, the zirconia that is used in TBCs is stabilized to inhibit a tetragonal to monoclinic crystal phase transformation at about 1000°C, which results in a volume change that can cause spallation.
[0004] In recent years, there has been a growing demand for further improvements in the thermal barrier properties to decrease the overall weight, thickness and amount of materials used to form TBCs. Increasing the thickness of thermal barrier coating on a component to reduce the thermal conductivity makes the coating more susceptible to cracking and peeling.
[0005] Thus, TBCs exhibiting lower thermal conductivity than conventionally used rare earth stabilized zirconia and having good phase stability at operating temperatures of the components are desired. Further, it is desirable that the materials of the TBC have melting points and CTEs that are similar to, or higher than, the melting points and CTEs of a rare earth stabilized zirconia. Furthermore, it is desirable to have a high erosion resistance and good sintering resistance for the materials to be used in the TBCs. Another desired property of a material for the TBC is low compositional density. Low compositional density reduces the weight of TBC on the components and hence reduces the overall weight of the components. Accordingly, it would be desirable to provide a thermal barrier coating material with a low thermal conductivity, low compositional density, high melting point, high coefficient of thermal expansion, high erosion resistance, and /or high sintering resistance. More specifically, it would be desirable to provide materials having a lower thermal conductivity than the conventionally used TBC materials (e.g., YSZ), to obtain a thermal barrier coating having superior thermal barrier properties without increasing the thickness of the coating.
BRIEF DESCRIPTION
[0006] Embodiments of the present disclosure are provided to meet this and other needs. One embodiment is directed to an article comprising a superalloy substrate with a thermal barrier coating. A bond coating is disposed on the superalloy substrate and a thermal barrier coating is disposed on the bond coating, wherein the thermal barrier coating comprises an inner ceramic layer and an outer ceramic layer. The outer ceramic layer includes a material comprising a composition of formula I,
(alkaline earth metal)x(rare earth metal)y(Zr, Hf, Ti)zOd (I)
wherein x>0, y>0, z>0, d >0, and [y / (x+y+z)] = 0.28, and composition of formula I has a cubic fluorite structure.
[0007] Another embodiment of the disclosure is directed to an article comprising a nickel-based superalloy substrate, a bond coating disposed on the nickel-based superalloy substrate, and a thermal barrier coating disposed on the bond coating. The thermal barrier coating comprises an inner ceramic layer and an outer ceramic layer. The outer ceramic layer includes a material comprising a composition of formula (Ca, Mg)x(Y, Gd)yZrzOd, wherein x>0, y>0, z>0, d >0, and [y / (x+y+z)] = 0.28 and the composition has a cubic fluorite structure.
DRAWINGS
[0008] Various features, aspects, and advantages of the present disclosure 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. Unless otherwise indicated, the drawings provided herein are meant to illustrate only key features of the disclosure. These key features are believed to be applicable in a wide variety of systems which comprises one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for practicing the disclosure.
[0009] FIG. 1 illustrates a schematic cross-section of an article including a substrate having thermal barrier coatings in accordance with some aspects of the present disclosure.
[0010] FIG. 2 is an X-Ray Diffraction (XRD) pattern of an example composition of the present disclosure.
[0011] FIG. 3 is a schematic representation of an average of the thermal conductivity data of some of the alkaline earth-substituted rare earth zirconates, measured at 1000°C and 1100°C in accordance with some aspects of the present disclosure.
[0012] FIG. 4 illustrates thermal expansion data of an example composition in accordance with some aspects of the present disclosure.
DETAILED DESCRIPTION
[0013] 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 “about” may not be limited to the precise value specified, and may include values that differ from the specified value. In at least 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, and such ranges include all the sub-ranges contained therein unless context or language indicates otherwise. In the following specification and the claims, 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] To more clearly and concisely describe and point out the subject matter, the following definitions are provided for specific terms, which are used throughout the following description and the appended claims, unless specifically denoted otherwise with respect to particular embodiments.
[0015] As used herein, the term “coating” or “coat” refers to a layer of material disposed on at least a portion of a surface in a continuous or discontinuous manner. The term “coating” may refer to a single layer of the material or may refer to a plurality of layers of the material. The coating may have a uniform or a variable thickness. The material used for the coating may be the same or different in the plurality of layers. The surface employed may or may not be an even surface. As used herein, a coating is “disposed on a substrate” if the coating is either laid directly in contact with the substrate or laid indirectly by having intervening layers between the coating and the substrate, unless otherwise specifically indicated. As used herein, the term "thermal barrier coating" or “TBC” refers to a coating that includes a material capable of reducing heat flow to an underlying substrate, thereby forming a thermal barrier.
[0016] As used herein, an atomic ratio of a rare earth element or an alkaline earth element in a formula is measured as a ratio of the number of that particular element to the total number of cationic elements of the formula. The atomic ratio calculation thus excludes the anions (such as, for example, oxygen). By way of an example, an atomic ratio of yttrium in the formula CaY8Zr9O31 is calculated by excluding number of oxygen atoms from the ratio calculation. Thus, the atomic ratio of yttrium in this example is 8 out of 18, that is, 0.44. Further, an atomic percent of an element in a formula is calculated as a percentage of the atomic ratio. Therefore, an atomic percent of yttrium in CaY8Zr9O31 is 44 percent.
[0017] Embodiments of the present disclosure are generally applicable to turbine components subjected to high temperatures, and particularly to components such as the high and low pressure turbine vanes (nozzles) and blades (buckets), shrouds, combustor liners and augmentor hardware of gas turbine engines. While the advantages of this disclosure will be described with reference to gas turbine engine components, the teachings of the disclosure are generally applicable to any component on which a TBC may be used to protect the component from a high temperature environment.
[0018] According to the aspects of the present disclosure, an article includes a superalloy substrate, a bond coating disposed on the superalloy substrate, and a thermal barrier coating disposed on the bond coating. The thermal barrier coating includes an inner ceramic layer and an outer ceramic layer. The outer ceramic layer comprises a material that includes a composition of formula (alkaline earth metal)x(rare earth metal)y(Zr, Hf, Ti)zOd, wherein x>0, y>0, z>0, d >0, and [y / (x+y+z)] = 0.28. The composition of this formula is present in a cubic fluorite structure in the outer ceramic layer.
[0019] FIG. 1 is a cross-sectional view of an article 10, in accordance with one or more aspects of the present disclosure. The article 10 may be employed for use with high temperature components such as gas-turbine engines. In some embodiments, the article 10 may be the blades, vanes, combustor liners, or shrouds of the gas turbine engines. In some embodiments, the article is a gas turbine engine component.
[0020] In the illustrated figure, a superalloy substrate 14 is provided. The superalloy substrate 14 is typically the superalloy base material of the article 10 protected by a thermal barrier coating (TBC) 20. In some embodiments, the superalloy substrate 14 includes an alloy of nickel, cobalt, iron, or combinations thereof. For example, the substrate may include a high temperature, heat-resistant alloy. In some embodiments, the superalloy substrate 14 includes high temperature nickel-based superalloys, such as, for example, nickel-chromium-based superalloys with optional additions of at least one of titanium, aluminum, molybdenum, copper, cobalt, iron, or boron. In some embodiments, a nickel-based superalloy has nickel as a major constituent. In some embodiments, nickel-based superalloys includes nickel in an amount greater than 50 wt.%.
[0021] In the illustrated figure, the substrate 14 is a protected by TBC 20 by depositing the TBC 20 indirectly on the substrate via an intervening bond coating 16 between the TBC 20 and the substrate 14. The bond coating 16 is disposed on the surface of the superalloy substrate 14. Coating materials that may be used for the bond coating 16 include, but are not limited to, overlay alloy coatings such as MCrAlX, where M is iron, cobalt and/or nickel and X is hafnium, zirconium, yttrium, tantalum, platinum, palladium, rhenium, silicon or a combination thereof. A suitable thickness for the bond coating 16 may be in the range from about 100 micrometers to about 300 micrometers, though lesser and greater thicknesses are foreseeable as long as the bond coating 16 is capable of providing the desired function of anchoring the TBC 20. Oxidation of one or more elements present in the bond coating 16 may form a thermally grown oxide scale 18 above the bond coating 16. For example, aluminum present in the bond coating may develop an aluminum oxide (alumina) scale, which is thermally grown by oxidation of the bond coating 16.
[0022] In the illustrated figure, the TBC 20 is a multilayer coating overlying the bond coating 16. The TBC 20 may have two or more layers. In FIG. 1, the TBC 20 comprises an inner ceramic layer 22 that has been disposed on the bond coating 16 so as to overlie the bond coating 16, and an outer ceramic layer 24 that has been disposed directly on the inner ceramic layer 22 so as to overlie the inner ceramic layer 22. In some embodiments, the outer ceramic layer 24 defines the outermost surface 26 of the TBC 20 and article 10. In some embodiments, a material of the inner ceramic layer 22 is different from a material of the outer ceramic layer 24. The material of the outer ceramic layer 24 may include one or more chemical compositions. In some embodiments, a composition of the outer ceramic layer 24 has a higher atomic percent of rare earth metals compared to the atomic percent of rare earth metals in a composition of the inner ceramic layer.
[0023] According to some embodiments of the disclosure, the inner ceramic layer 22 is formed of rare earth metal stabilized zirconia systems, and the atomic percent of the rare earth metal in the outer ceramic layer 24 of the TBC 20 is higher than the atomic percent of the rare earth metal content in the inner ceramic layer 22. In some embodiments, the inner ceramic layer 22 is formed of one or more YSZ materials having less than 10 weight% of yttria (Y2O3) content. In these embodiments, the atomic percent of yttrium in the YSZ material of the inner ceramic layer 22 may be less than 10.7. In certain embodiments, the inner ceramic layer 22 comprises 6-9 weight% YSZ, with the atomic percent of yttrium in the range from about 6.5 to about 9.7. In some embodiments, the composition of 6-9 weight% YSZ in the inner ceramic layer 22 has a tetragonal structure.
[0024] The outer ceramic layer comprises a material comprising a composition of formula I,
(alkaline earth metal)x (rare earth metal)y (Zr, Hf, Ti)z Od, (I)
wherein x>0, y>0, z>0, d >0, and [y / (x+y+z)] = 0.28. That is., the outer ceramic layer 24 includes a material, and the material includes an oxide of formula I that has at least one alkaline earth metal, at least one rare earth metal, and at least one of the zirconium, hafnium, or titanium, such that an atomic ratio of the rare earth metal in the composition of formula I is equal to or greater than 0.28. Thus, the disclosed composition of formula I has at least 28 atomic percent of the rare earth metal. Non-limiting examples of suitable alkaline earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), or combinations thereof. In some embodiments, the composition of formula I has at least 2 atomic percent of the alkaline earth metal. In some embodiments, the composition of formula I has greater than 2 atomic percent of the alkaline earth metal. In some embodiments, an atomic percent of the alkaline earth metal in the composition of formula I is in a range from about 2 atomic percent to about 32 atomic percent. Non-limiting examples of suitable rare earth metals in the composition of formula I in the material of the outer ceramic layer 24 include scandium, yttrium, lanthanum, cerium, gadolinium, praseodymium, neodymium, promethium, samarium, europium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or combinations thereof. In some embodiments, an atomic percent of the rare earth metal in the composition of formula I is in a range from about 29 atomic percent to about 58 atomic percent. In some embodiments, a combined atomic percent of zirconium, hafnium and titanium in the composition of formula I is in a range from about 40 atomic percent to about 70 atomic percent. In some embodiments, an atomic percent of zirconium in the composition of formula I is in a range from about 40 atomic percent to about 65 atomic percent, and the atomic percent of a combination of hafnium and titanium, if any of the hafnium or titanium are present, is less than 5 atomic percent.
[0025] The composition of formula I, present as a part of the material of the outer ceramic layer 24 of the TBC 20 has a cubic fluorite structure. In some embodiments, greater than 95 weight % of the outer ceramic layer 24 is made up of disclosed material. In some embodiments, greater than 98 weight % of the outer ceramic layer 24 is made up of the disclosed material. In some embodiments, the outer ceramic layer 24 consists essentially of a material that is consisting essentially of the composition of formula I. Therefore, in some embodiments, the outer ceramic layer 24 may not contain any other material constituents other than the material of the composition of formula I, that would materially affect the basic and novel characteristics of lowering thermal conductivity of the outer ceramic layer 24. In some embodiments, the material of the outer ceramic layer 24 includes a secondary phase having an orthorhombic structure, in addition to the composition of formula I having the cubic fluorite structure. The “cubic fluorite structure” as used herein is the typical calcium fluorite (CaF2) crystal structure having a cubic arrangement of anions with about 50% of the cubic sites filled. The cations in the structure have a coordination number of 8 and the anions in the structure have a coordination number of 4, with an anion to cation ratio of about 2. In the rare earth-substituted zirconias, a pyrochlore structure is favored by larger lanthanide ions. Substitutions of larger sized lower valent ions such as calcium for the rare earth tend to destabilize the pyrochlore structure to form other phases such as perovskites and fluorites. Therefore, in some embodiments, the cubic fluorite structure is favored for larger electropositive cations such as calcium. The cubic fluorite structure is made up of a FCC lattice of cations with the tetrahedral sites occupied with anions, with a 1:2 cation to anion ratio. FIG. 2 represents the XRD pattern of an example composition CaY8Zr9O31 of formula I, that shows a face centered cubic crystal structure. This cubic fluorite is significantly different from a pyrochlore structure. A unit cell of the pyrochlore has a double layer structure having a general formula of A2B2X7, wherein A and B are cations and X is an anion. For example, a composition having A and B cations and X anions may be in a pyrochlore structure having two sub lattices of type A2X and B2X6 with cation A having a coordination number of 8 and the cation B having a coordination number of 6.
[0026] The content of the composition of formula I may also be denoted in terms of weight percent of the constituent oxides, such as, for example, weight percent of alkaline earth metal oxide and/or weight percent of rare earth metal oxide present as a part of the composition of formula I. In terms of weight percent of rare earth oxides, such as, for example yttria, the composition of formula I may have from about 27 weight % to about 55 weight percent of rare earth metal oxides. For example, YSZ containing an alkaline earth metal and having a yttria content in the range from about 27 weight % to about 55 weight percent of the composition offers good erosion and spallation resistance. Yttria content from about 27 weight % to about 55 weight percent in the composition of formula I is particularly advantageous in an outer ceramic layer 24 of the TBC 20 as compared to the compositions having lower than 27 weight% yttria or higher than 55 weight % yttria content. In some embodiments, the composition of formula I contains rare earth metal in an amount from about 29 atomic percent to about 40 atomic percent. Thus, in some embodiments, the atomic ratio, [y / (x+y+z)] is in a range from about 0.29 to about 0.40. An example composition for atomic ratio of rare earth metal in a range from about 0.29 to about 0.40 may be a partial alkaline earth metal-substituted 38YSZ (i.e., having alkaline earth metal substitution in the yttrium site in a 38 weight % yttria stabilized zirconia) composition. In some embodiments, the composition of formula I contains rare earth metal in an amount from about 40 atomic percent to about 57 atomic percent. Thus, in some embodiments, the atomic ratio [y / (x+y+z)] is in a range from about 0.4 to about 0.57. An example composition of formula I for the atomic ratio of rare earth metal in a range from about 0.4 to about 0.57 may be a partial alkaline earth metal-substituted 55YSZ composition.
[0027] In some embodiments, the material of the outer ceramic layer 24 includes the composition of formula I having the cubic fluorite structure and further includes a secondary phase having a composition of formula II having orthorhombic structure.
(alkaline earth metal)p(rare earth metal)q(Zr, Hf, Ti)r O3±ß (II)
wherein p=1, q=0, r=1, and ß = 0.
[0028] Thus, in some embodiments, the material of the outer ceramic layer 24 includes both the composition of formula I and the composition of formula II. In some embodiments, the material consists essentially of a combination of composition of formula I and the composition of formula II. Thus, in some embodiments, the outer material in the outer ceramic layer 24 may not contain any other material constituents other than the combinations of the composition of formula I and composition of formula II, that would materially affect the basic and novel characteristics of lowering thermal conductivity of the outer ceramic layer 22. in some embodiments, in some embodiments the material the outer ceramic layer 24 consists essentially of a material that consists essentially of the composition of formula I. In some embodiments, the outer ceramic layer 24 consists essentially of the material consisting essentially of a combination of composition of formula I and the composition of formula II.
[0029] In some embodiments, the outer ceramic layer 24 Depending on the actual percentages of the alkaline earth metal and the rare earth metal present in the material, the material may have a cubic fluorite structure or a mixture of the cubic fluorite structure and the orthorhombic structure. In some embodiments, the formula 1 and the formula II has the same alkaline earth metal. In some embodiments, the orthorhombic structure is present in the material in an amount that is less than 80 weight % of the material. In some embodiments, the amount of orthorhombic structure that is present in the material is less than 25 weight % of the material. In some embodiments, the material is completely in the cubic fluorite structure. In embodiments where q>0 in the formula II, the composition of formula II having the orthorhombic structure comprises a solid solution of a rare earth metal. In some embodiments, the rare earth metal present in formula I and in formula II are the same. Thus, in certain embodiments, the outer ceramic layer 24 has a material that includes the composition of formula I in cubic fluorite crystal structure and the composition of formula II in orthorhombic structure such that the alkaline earth metal and the rare earth metal of both the formula I and formula II are same, but the relative contents of the alkaline earth metal and the rare earth metals in the formula I and the formula II are varied.
[0030] In some embodiments, weight percent of rare earth oxide of the material of the outer ceramic layer 24 is calculated as weight percent of total oxides in the material of the outer ceramic layer, accounting for the oxide of both the cubic fluorite structure and the orthorhombic structure if the orthorhombic structure is present. In some embodiments, the material of the outer ceramic layer 24 includes about 27 weight percent to about 55 weight percent of rare earth metal oxides. Non-limiting example compositions of the material of the outer ceramic layer include alkaline earth metal substituted 38YSZ (i.e., having a partial substitution of alkaline earth metal in the yttrium position of 38 YSZ), alkaline earth metal substituted 44YSZ, alkaline earth metal substituted 55YSZ, or any combinations of the foregoing.
[0031] Rare earth-containing zirconia compositions normally possess anion vacancies, lattice imperfections and defects. These contribute to an enhanced phonon scattering in the system and reduced thermal conductivity. Substitution of size engineered lower valence cations such as alkaline earth metals having 2+ valency in the lattice sites of rare earth metals of 3+ valency or zirconium, hafnium, or titanium of 4+ valency increases the anion vacancy concentration, and thereby enhances the phonon scattering and reduces thermal conductivity. Substitution of alkaline earth metals in the rare earth metal sites also causes disordering by oxygen defects within the structure, which also contributes to increased phonon scattering and lower thermal conductivity. Further, partial substitution of alkaline earth metals in certain lower amounts, such as, for example, 15 mole percent of calcium in gadolinium zirconates, destabilizes pyrochlore structure and stabilizes cubic fluorite structure. These effects are observed to be more pronounced when the rare earth metal content is high, such as, in the composition of formula I. Thus, a cubic fluorite structure represented by composition of formula I has increased phonon scattering. This increased phonon scattering may be due to, among other reasons, a disordering of lattice structure caused by oxygen defects. Increased phonon scattering decreases the thermal conductivity. The thermal conductivity of the material having formula I is significantly lower than that of a conventional rare earth-stabilized zirconia.
[0032] In some embodiments, the thermal conductivity of the composition of the formula I is less than 1.8 W/m-K at 1000°C. In some embodiments, the thermal conductivity of the composition of formula I is in a range from about 1 W/m-K to about 1.6 W/m-K at 1000°C. Further, the thermal conductivity of the composition of the formula I decreases with increasing content of alkaline earth metal substitution. Thus, a combination of high rare earth metal content and alkaline earth metal substitution decreases the thermal conductivity of the compositions of formula I.
[0033] Additionally, partial substitution of alkaline earth metals having low atomic weight in the site of rare earth metal and /or zirconium having high atomic weight in the rare earth metal zirconate systems particularly aid in decreasing the compositional density of the formula I and weight of the outer ceramic layer 24, thereby reducing the overall weight of the TBC 20 on the article 10 and enhancing the performance of the article 10. In some embodiments, the material of the outer ceramic layer 24 has a lower compositional density in comparison to the compositional density of 6-9 weight % yttria stabilized zirconia of the inner ceramic layer.
[0034] Alkaline earth metal substitution in zirconia, such as, for example, calcium substitution in zirconia or calcium substitution in typical 7-8 wt.% YSZ systems were earlier known to be having inferior high temperature withstanding capability when compared to the unsubstituted 7-8 wt.% YSZ systems. Further, due to a faster tetragonal-monoclinic transformation of the calcium-substituted 7-8 wt.% YSZ, the calcium substituted 7-8 wt.% YSZ showed an inferior cyclic capability compared to the 7-8 wt.% YSZ systems. Thus, alkaline earth metal substitution along with rare earth metal substitution in zirconia or hafnium doped zirconia were not considered in the past for thermal barrier applications as the thermomechanical properties of these materials were known to be compromised, in comparison to the 7-8 wt.% YSZ systems. However, with engineered substitutions of alkaline earth metals in highly stabilized, high rare earth-substituted zirconia systems, unexpected reductions in the high temperature thermal conductivity are obtained in the present disclosure. This reduction in thermal conductivity may be due to favorable contributions toward defect structure formation, phonon scattering, and vacancy population and distribution.
[0035] In some embodiments, the composition of formula I in the material of the outer ceramic layer 24 contains alkaline earth metal in an amount greater than 2 atomic percent. Thus, in some embodiments, the atomic ratio, [x / (x+y+z)] is greater than 0.02. The amount of alkaline earth metal that may be effectively used for obtaining the cubic fluorite structure and a low thermal conductivity for the outer ceramic layer 24 may also depend on the amount of rare earth metal present in the composition. In some embodiments, it is desirable to have the composition of the formula I in the material of the outer ceramic layer 24 such that the total amount of the alkaline earth metal and the rare earth metal in the composition having formula I is greater than 30 atomic percent of the composition. Thus, in some embodiments, in the composition having formula I, the ratio (x+y) / (x+y+z) is greater than 0.30. In some embodiments, in the composition having formula I, the ratio (x+y) / (x+y+z) is greater than 0.35. In certain embodiments, in the composition having formula I, the amounts of alkaline earth element and the rare earth elements are such that the ratio (x+y) / (x+y+z) is greater than 0.30 but less than 0.60.
[0036] The presence of a secondary phase of orthorhombic structure in the material depends on the solubility limit of the alkaline earth metal in the cubic fluorite structure. Above the alkaline earth metal solubility limit in the cubic fluorite structure, the orthorhombic structure is often observed as the secondary phase. In some embodiments, the presence of orthorhombic structure in the material of the outer ceramic layer 24, in addition to the presence of cubic fluorite structure, is controlled by the combination of alkaline earth metal content and the rare earth metal content in the material of the outer ceramic layer 24. For example, it was observed that a rare earth content of greater than 55 atomic percent in the composition of formula I formed a secondary phase of orthorhombic structure when the alkaline earth content is greater than 5 atomic %. In contrast, when a rare earth content was about 30 atomic percent in the composition of formula I, the secondary phase of orthorhombic structure did not form even when the alkaline earth content was 12 atomic percent. These observations were from the samples that are sintered and aged at a temperature greater than 1700°C for a time duration less than 20 hours. Longer time duration of aging may slightly change the onset of second phase appearance at a particular atomic percent substitution of alkaline earth metal for both the high rare earth content and low rare earth content compositions.
[0037] In some embodiments, the composition of the formula I in the material of the outer ceramic layer 24 is predominantly zirconates where a suitable amount of the zirconium is substituted by hafnium, titanium, or combination of hafnium and titanium. Several characteristics of the material of the outer ceramic layer 24 can be improved by the incorporation of hafnium or titanium into the alkaline earth rare earth zirconates. Such improved characteristics include thermal conductivity reduction and sintering rate reduction. Presence of hafnium oxide and /or titanium oxide increases phonon scattering in the yttria-zirconia system, and decreases thermal conductivity. The presence of hafnium and/or titanium also reduces the oxygen ionic conductivity of the high-rare earth content of the outer ceramic layer 24, which in turn reduces the sintering rate of the layer 24. In some embodiments, the material of the outer ceramic layer 24 may have less than 25 atomic percent of hafnium. In some embodiments, the composition/s of the material of the outer ceramic layer 24 has hafnium in an amount from about 2 atomic percent to about 10 atomic percent. In some embodiments, hafnium is present in the material of the outer ceramic layer 24 only as incidental impurities. In some embodiments, an amount of titanium in the material of the outer ceramic layer 24 is less than 5 atomic percent. In some embodiments, titanium is present in the material only as incidental impurities.
[0038] In some embodiments, the rare earth metal of the material of the outer ceramic layer 24 is yttrium, gadolinium, or a combination thereof. In some embodiments, the alkaline earth metal of the material of the outer ceramic layer 24 is calcium, magnesium, or a combination thereof. In certain embodiments, the composition of the formula I in the material of the outer ceramic layer 24 has a formula (Ca, Mg)x(Y, Gd)y(Zr, Hf, Ti)zOd.. In these embodiments, the contents of calcium, magnesium, yttrium, gadolinium are such that x>0, y>0, z>0, and d >0. Thus, at least one of calcium or magnesium, at least one of yttrium or gadolinium, and at least one of zirconium, hafnium, or titanium are present in the composition of the formula I in the material of the outer ceramic layer. Further, the composition of formula I contains a high amount of the rare earth metal (yttrium and/or gadolinium) such that the atomic ratio [y / (x+y+z)] is equal to or greater than 0.28.
[0039] In some embodiments, the material of the outer ceramic layer 24 includes the composition of the formula CaxYyZrzOd. The value of z, y, z, and d are such that the composition has calcium, yttrium, zirconium, and oxygen in non-zero amounts. Further, yttrium is present in the composition in an amount that is equal to or greater than 28 atomic percent. An example for this composition is CaY8Zr9Od. Specifically, in certain embodiments, the composition has the formula CaY8Zr9O31. In some other embodiments, composition has the formula CaxGdyZrzOd having non-zero amounts of calcium, gadolinium, zirconium, and oxygen and gadolinium is present in the composition in an amount that is equal to or greater than 28 atomic percent. An example for this composition is (CaxGd(1-x))2Zr2Od. Specifically, in certain embodiments, the composition has the molecular formula (Ca0.15Gd0.85)2Zr2Od. An X-ray diffraction of this composition confirmed the formation of a cubic fluorite structure. In some embodiments, the orthorhombic structure is present in the material along with the cubic fluorite structure. In some example embodiments, the orthorhombic structure is formed by a composition of formula II comprising calcium zirconate, magnesium zirconate, or a combination of calcium zirconate and magnesium zirconate. In certain embodiments, wherein the rare earth metal is present as a solid solution in the orthorhombic structure of formula II, one or more of calcium zirconate: rare earth solid solution and magnesium zirconate: rare earth solid solution may be present.
[0040] In some specific embodiments of the disclosure, an article 10 having a nickel-based superalloy substrate 14 is disclosed. A bond coating 16 disposed on the nickel-based superalloy substrate, and a thermal barrier coating 20 is disposed on the bond coating 16. The thermal barrier coating 20 comprises an inner ceramic layer 22 and an outer ceramic layer 24. The outer ceramic layer 24 comprises a material comprising a composition of formula (Ca, Mg)x(Y, Gd)yZrzOd, wherein x>0, y>0, z>0, d >0, and [y / (x+y+z)] = 0.28. The said composition has a cubic fluorite structure.
[0041] Some non-limiting example compositions having calcium as the alkaline earth metal and yttrium as the rare earth metal that may be used as the composition of formula I in the material of the outer ceramic layer 24 are disclosed in Table 1. The formula denoting the atomic percent of each cation and the corresponding weight% of the oxides of the constituents are included.
Table 1. Example compositions of formula I of the material of the outer ceramic layer
weight %
Formula CaO Y2O3 ZrO2
Ca2.00Y38.08Zr59.92O178.96 0.95 36.45 62.59
Ca4.01Y36.07Zr59.92O177.96 1.92 34.87 63.21
Ca8.02Y32.06Zr59.92O175.95 3.93 31.61 64.47
Ca12.02Y28.06Zr59.92O173.95 6.01 28.22 65.77
Ca2Y43Zr55O176.5 0.96 41.34 57.71
Ca4Y41Zr55O175.5 1.93 39.80 58.27
Ca6Y39Zr55O174.5 2.92 38.23 58.84
Ca2.78Y47.22Zr50.00O173.61 1.34 45.77 52.89
Ca5.56Y44.44Zr50.00O172.22 2.71 43.67 53.62
Ca11.11Y38.89Zr50.00O169.44 5.58 39.29 55.13
Ca2.86Y54.29Zr42.85O169.99 1.38 52.98 45.63
Ca5.72Y51.44Zr42.85O168.57 2.81 50.91 46.28
Ca11.43Y45.72Zr42.85O165.71 5.78 46.58 47.63
Ca17.15Y40.01Zr42.85O162.85 8.94 41.99 49.08

[0042] Table 2 compares compositional densities of some of the compositions of table 1, along with the compositional densities of corresponding rare earth zirconate systems without the substitution of calcium.
Table 2. Compositional density of rare earth zirconates and calcium substituted rare earth zirconates
Formula Geometric (Compositional) density
Y40.08Zr59.92O179.96 (38YSZ)
5.53
Ca2.00Y38.08Zr59.92O178.96
5.51
Ca4.01Y36.07Zr59.92O177.96
5.47
Ca8.02Y32.06Zr59.92O175.95
5.32
Ca12.02Y28.06Zr59.92O173.95
5.28
Y57.15Zr42.85O171.42 (55YSZ)
5.36
Ca2.86Y54.29Zr42.85O169.99
5.22
Ca5.72Y51.44Zr42.85O168.57
5.19
Ca11.43Y45.72Zr42.85O165.71
5.13
Ca17.15Y40.01Zr42.85O162.85
5.07

[0043] As shown in Table 2, for both 38 YSZ and 55 YSZ, the compositional density decreases as the amount of calcium substitution increases. This indicates that the compositional density decreases with the increasing amount of alkaline earth metal substitution for the rare earth metal in the zirconates containing high content of rare earth element.
[0044] Table 3 illustrates the thermal conductivity data of some of the compositions of formula I of Table 1 measured at about 58 K LMP (Larsen Miller Parameter). Thermal conductivity data of 8YSZ and 20 YSZ samples are also given for comparison.
Table 3. Thermal Conductivity (TC) data of rare earth zirconates and calcium substituted rare earth zirconates.
Formula TC at 900°C (W/m-K) TC at 1100°C (W/m-K)
Y8.67Zr91.33O179.96 (8YSZ)
2.15
Y21.44Zr78.56O179.96 (20YSZ)
1.89 1.88
Ca4.01Y36.07Zr59.92O177.96 1.75 1.54
Ca8.02Y32.06Zr59.92O175.95 1.73 1.55
Ca5.56Y44.44Zr50.00O172.22 1.56 1.61
Ca2.86Y54.29Zr42.85O169.99 1.51 1.38
Ca11.43Y45.72Zr42.85O165.71 1.6 1.49

[0045] It can be observed that the thermal conductivities of calcium containing high yttrium zirconates are significantly lower than the thermal conductivities of 8YSZ and 20YSZ. FIG. 3 shows a schematic representation of average thermal conductivity data of some other compositions in the CaO-Y2O3-ZrO2 space. The average thermal conductivity data for each of the data points were calculated by measuring thermal conductivity at both 1000°C and at 1100°C and then taking the average of both the measured values. The axes represent mole percent substitution of CaO and Y2O3 in a ZrO2 compound. Weight percent of Y2O3 for certain compositions were further indicated in the horizontal axis for some data points. The gray scale indicates the observed thermal conductivity span. The grey scale of individual circles corresponds to the observed thermal conductivity, a darker data point indicating an increased thermal conductivity (as shown in the legend). The sizes of the circles indicate the number of data points that are observed for a given composition. Thus, a bigger size of the circle indicates having more repeat data for that particular point.
[0046] FIG. 4 is an illustration of a measurement of thermal expansion data of a representative composition CaY8Zr9O31 during heating and cooling. It is seen that the coefficient of thermal expansion (CTE) of the composition at 1000°C is about 10.3 ppm/°C, which is similar to the CTE of 6-9 weight% YSZ (~9.5-11ppm/°C) that may be used as the inner ceramic layer 22 and the typical super alloys that may be used as the substrate 14 (~13-14 ppm/°C) for depositing TBC. The observed CTE is ideal for a composition that can be used as the TBC on the superalloy substrate, so that the formed TBCs will not be subjected to severe spallation due to CTE mismatch. The observed CTE of the representative composition, and the compositions of formula I may not match the CTE of typical silicon-based substrates and bond coatings that may be used for an environmental barrier coating (EBC). CTE of typical silicon-based substrates that are used for EBC is in a range of about 4-6 ppm/°C) and the CTE of typical bond coats that may be used in EBC is in a range of about 3-8 ppm/°C).
[0047] In some embodiments, the outer ceramic layer 24 also differs from the inner ceramic layer 22 in terms of its density (thereby porosity) and thickness. In particular, the inner ceramic layer 22 may be deposited in a manner that achieves a relatively porous macrostructure, characterized by a porosity level of about 10 to about 25 volume percent. In some embodiments, the porosity of the inner ceramic layer 22 is in a range from about 10 to about 20 volume percent of the inner ceramic layer 22. In contrast, the outer ceramic layer 24 is deposited in a manner that achieves a less porous macrostructure than the inner ceramic layer 22. The outer ceramic layer 24 may have a porosity level of about 3 to about 15 volume percent. In some embodiments, the porosity of the outer ceramic layer 24 is in a range from about 5 to about 10 volume percent. The relatively higher structural density of the outer ceramic layer 24 is desirable to have a high toughness and erosion resistance. The desired porosity levels in the layers 22 and 24 may be achieved by depositing these layers by thermal spraying technique, thereby avoiding columnar voids. Non-limiting examples of thermal spraying techniques include air plasma spraying (APS), vacuum plasma spraying (VPS) and low pressure plasma spraying (LPPS). Thermal spraying involves propelling melted or at least heat-softened particles of a heat fusible material (e.g., metal, ceramic) against a surface, where the particles are quenched and bond to the surface to produce a coating. As such, the inner and outer TBC layers 22 and 24 are deposited in the form of molten “splats,” resulting in a microstructure characterized by horizontal porosity resulting from the presence of the splats (flattened grains).
[0048] The outer ceramic layer 24 may also differ from the inner ceramic layer 22 in terms of its thickness. A controlled relative thickness of the TBC layers 22 and 24 enhances spallation resistance of the TBC 20. In some embodiments, the thickness ratio of the outer ceramic layer 24 to the inner ceramic layer 22 is less than one. In some embodiments, the thickness ratio of the outer ceramic layer 24 to the inner ceramic layer 22 is less than 0.5. The individual thicknesses of the TBC layers can be varied to achieve the desired ratio for increasing stability and performance of the TBC 20. For example, the inner ceramic layer 22 may have a thickness of about 50 micrometers up to about 500 micrometers, for example, a thickness of about 250 micrometers; and the outer ceramic layer 24 may have a thickness of about 25 micrometers up to about 250 micrometers, for example, a thickness of about 125 micrometers.
[0049] The characteristics of the outer ceramic layer 24, specifically with a higher content of rare earth metal along with the alkaline metal substitution, enables the TBC 20 to exhibit lower thermal conductivity without compromising on the phase stability and thermal expansion characteristics. Alkaline earth substitution in the rare earth sites also considerably decreases the cost of the TBC 20. Controlling the porosity and thickness of the inner and outer ceramic layers may further enhance thermal cycling life of the TBC 20. As such, the TBC 20 is particularly well suited for protecting hot section components of gas turbine engines at low cost, and can enable such components to operate for longer durations and/or at higher temperatures.
[0050] While only certain features of the disclosure have been illustrated, and described herein, many modifications and changes will occur to those skilled in the art that fall within the true spirit of this disclosure. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes that fall within the true spirit of this disclosure.