Rare-earth oxide coatings for refractory alloys, methods of manufacturethereof and articles comprising the same

Abstract

Disclosed herein is a thermal barrier coating composition comprising: (M1(1-a-b-c-d . . . n)M2aM3bM4c, MNn)O3(1), where the subscript “a” represents the molar fraction of element M2, “b” represents the molar fraction of element M3, “c” represents the molar fraction of element M4, and “n” represents molar fraction of element MN (in the metal oxide of formula (1)); and where the sum of a+b+c+d+..,+n is less than 1; where M1 represents a base element derived from a metal oxide that has a density of less than or equal to 7.0 grams per cubic centimeter, while M2, M3, M4, . . . , MN each represent a different element derived from the plurality of metal oxides each of which have a density greater than 7.0 g / cm3.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 445,683 filed on Feb. 14, 2023, the entire contents of which are hereby incorporated by reference.GOVERNMENTAL SUPPORT

[0002] This invention was made with government support under Grant Nos. DE-AR0001425 and DE-AR0002337-1530, awarded by the Department of Energy. The government has certain rights in the invention.BACKGROUND

[0003] Brayton cycle aero-turbine engines display an increase in efficiency with an increase in temperature. Nickel-based or cobalt-based superalloy turbine and stator blades with thermal barrier coatings (TBCs) disposed thereon have enabled high temperature efficient aero-turbine engines. However, the superalloy / TBC combination has reached its upper use temperature limit. Yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) TBCs have been engineered to be a low intrinsic thermal conductivity coating material by introducing porosity through microstructural during the manufacturing process. When this low intrinsic thermal conductivity coating material is used as a coating on a back surface of a cooled metallic component, it facilitates the achievement of significant temperature drops from the hot gas surface to the underlying metallic bond coat, thus reducing metallic surface temperatures.

[0004] However, further significant improvements in the superalloy / TBC systems are unlikely. It is therefore desirable to have improved thermal barrier coatings that may be used on superalloys that can withstand higher temperatures than nickel-based or cobalt-based superalloys.SUMMARY

[0005] Disclosed herein is a thermal barrier coating composition comprising: (M1(1-a-b-c-d . . . n)M2aM3bM4c..MNn)2O3(1), where the subscript “a” represents the molar fraction of element M2, “b” represents the molar fraction of element M3, “c” represents the molar fraction of element M4, and “n” represents molar fraction of element MN (in the metal oxide of formula (1)); and where the sum of a+b+c+d+ . . . +n is less than 1; where M1 represents a base element derived from a metal oxide that has a density of less than or equal to 7.0 grams per cubic centimeter, while M2, M3, M4, . . . , MN each represent a different element derived from the plurality of metal oxides each of which have a density greater than 7.0 g / cm3.

[0006] Disclosed herein too is method of disposing a thermal barrier coating on a turbine airfoil, the method comprising blending metal oxides in a blender to form a composition having the formula (1) (M1(1-a-b-c-d . . . n)M2aM3bM4c..MNn)2O3(1), where the subscript “a” represents the molar fraction of element M2, “b” represents the molar fraction of element M3, “c” represents the molar fraction of element M4, and “n” represents molar fraction of element MN (in the metal oxide of formula (1)); and where the sum of a+b+c+d+ . . . +n is less than 1; where M1 represents a base element derived from a metal oxide that has a density of less than or equal to 7.0 grams per cubic centimeter, while M2, M3, M4, . . . , MN each represent a different element derived from the plurality of metal oxides each of which have a density greater than 7.0 g / cm3; suspending the metal oxide in a solvent to form a suspension; disposing the suspension on an airfoil; and heating the suspension to dry the composition.BRIEF DESCRIPTION OF THE FIGURES

[0007] FIG. 1A is a graph of temperature versus ionic radius for polymorphs of rare earth oxides (RE2O3) showing that rare earth oxides with small rare earth cations form cubic phases, whereas monoclinic and hexagonal phases are formed rare earth oxides with larger rare earth cations;

[0008] FIG. 1B is a graph of temperature versus site fraction solubility limit of the Y2O3 crystal occupied by another rare earth cation;

[0009] FIG. 2 shows a cross-sectional micrograph and energy dispersive spectroscopy elemental maps of a five component rare earth metal oxide thermal barrier coating material after reaction with CMAS at 1500° C., for 100 hours in air showing formation of a protective barrier layer of reaction product; and

[0010] FIGS. 3A and 3B are graphs depicting that both migration barriers for O2− (FIG. 3A) and RE3+ (FIG. 3B) in the rare earth metal oxide thermal barrier coating decrease with increasing effective ionic radius.DETAILED DESCRIPTIONDefinitions

[0011] CMAS resistance refers to the ability of materials, particularly coatings and ceramics, to withstand attack from calcium-magnesium-aluminum-silicate (CMAS) deposits at high temperatures. CMAS is a type of volcanic ash or other sand, dirt, mineral debris that can be ingested into aircraft engines during flight. When exposed to the high temperatures within the engine, CMAS melts and forms molten deposits on engine components. These deposits can adhere to surfaces and cause erosion, blockages, and other damage, leading to degradation of engine performance and potentially compromising safety.

[0012] A high-entropy rare earth oxide refers to a class of materials that are characterized by having multiple elements of varied atomic sizes in roughly equiatomic proportions. These materials exhibit unique properties compared to traditional single-component or binary oxide materials. The idea behind high-entropy materials is to create materials with a high degree of disorder in their atomic arrangement, which can lead to unique and potentially beneficial properties.

[0013] Rare earth oxides are compounds composed of oxides of the lanthanide series elements. When these elements are combined with other elements in equiatomic proportions, they can form high-entropy oxides. High-entropy oxides are part of a broader class of materials known as high-entropy alloys or multi-principal element alloys (MPEAs). The rare earth elements are a group of chemical elements in the periodic table, consisting of the lanthanide series (elements 57-71) and scandium (Sc) and yttrium (Y). For purposes of this application, yttrium and scandium are considered to be rare earth elements due to their similar chemical properties and their historical association with the group.

[0014] Disclosed herein are thermal barrier coatings that have a low thermal conductivity of less than 3.5 watts per meter Kelvin (W / m-K) and that comprise a plurality of rare earth metal oxides having rare earth cations of different sizes that are alloyed to create a material that preferably comprises a cubic crystalline structure. The use of cations of different sizes may increase phonon scattering thereby reducing the thermal conductivity of the thermal barrier coating. The cubic crystalline phase facilitates a reduction in stresses due to anisotropic thermal expansion upon cooling.

[0015] The thermal barrier coating is formed from at least two or more rare earth metal oxides, three or more rare earth metal oxides, four or more rare earth metal oxides and preferably five or more rare earth metal oxides. In one embodiment, the thermal barrier coating comprises a plurality of rare earth metals (hereinafter elements) M2, M3, M4, M5, . . . MN in their oxide forms, where N is an integer that represents the number of different metal oxides present in the composition. For example, if two rare earth metal oxides were used in the thermal barrier composition, then N=2 and the two rare earth elements (in the rare earth metal oxide) would be M1 and M2. If three rare earth metal oxides were used in the thermal barrier composition, then N=3 and the three rare earth elements (in the rare earth metal oxide) would be M1, M2 and M3. The term “metal” as used in reference to M1, M2, M3, M4, M5, . . . MN in formula (1) refers to metallic elements present in metal oxides M1xOy, M2xOy, M3xOy, M4xOy, . . . , MNxOy respectively, where x is an integer from 2 to 4 and y is an integer from 3 to 7, where x and y are independent of each other.

[0016] The composition for the thermal barrier coating may be represented by the formula shown in equation (1) (M1(1-a-b-c-d . . . n)M2aM3bM4c..MNn)2O3(1), where the subscript “a” represents the molar fraction of element M2 (in the metal oxide of formula (1)), “b” represents the molar fraction of element M3 (in the metal oxide of formula (1)), “c” represents the molar fraction of element M4 (in the metal oxide of formula (1)), and “n” represents molar fraction of element MN (in the metal oxide of formula (1)); and where the sum of a+b+c+d+ . . . +n is less than 1. In formula N is an integer that ranges from 1 to 17 (the total number of rare earth metals in the periodic table), preferably 2 to 8, and more preferably 3, 4, 5, 6 or 7.

[0017] In formula (1), M1 represents the base element from a metal oxide that has a density of less than or equal to 7.0 grams per cubic centimeter while M2, M3, M4, . . . , MN each represent a different element from the plurality of metal oxides present in the thermal barrier coating. Each of elements M2, M3, M4, . . . , MN are derived from metal oxides that have a density greater than 7.0 g / cm3. The density of various rare earth metal oxides is shown in the Table 1 below.TABLE 1Rare Earth Metal OxideApproximate Density (g / cm3)Yttrium oxide (Y2O3)Density approximately 5.01 g / cm3Scandium oxide (Sc2O3)Density approximately 3.86 g / cm3Cerium(IV) oxide (CeO2)Density approximately 7.13 g / cm3Praseodymium(III) oxide (Pr2O3)Density approximately 6.88 g / cm3Neodymium(III) oxide (Nd2O3)Density approximately 7.24 g / cm3Samarium(III) oxide (Sm2O3)Density approximately 8.347 g / cm3Europium(III) oxide (Eu2O3)Density approximately 7.42 g / cm3Gadolinium(III) oxide (Gd2O3)Density approximately 7.41 g / cm3Terbium(III) oxide (Tb4O7)Density approximately 7.3 g / cm3Dysprosium(III) oxide (Dy2O3)Density approximately 7.81 g / cm3Holmium(III) oxide (Ho2O3)Density approximately 8.41 g / cm3Erbium(III) oxide (Er2O3)Density approximately 8.64 g / cm3Thulium(III) oxide (Tm2O3)Density approximately 8.5 g / cm3Ytterbium(III) oxide (Yb2O3)Density approximately 9.17 g / cm3Lutetium(III) oxide (Lu2O3)Density approximately 9.41 g / cm3

[0018] In an embodiment, base element M1 is derived from a rare earth metal oxide preferably present in an amount of 15 to 80 (a mole fraction of 0.15 to 0.8), preferably 30 to 70 (a mole fraction of 0.3 to 0.7), and preferably 40 to 60 mole percent (a mole fraction of 0.4 to 0.6) based on the total number of moles of M1, M2, M3, M4, . . . , MN in the thermal barrier coating. In an embodiment, the mole fraction of M1=the mole fraction of M2=the mole fraction of M3=the mole fraction of M4= . . . =the mole fraction of MN.

[0019] In an embodiment, the base element M1 is present in a larger amount (on a weight percent basis) than each of the other elements M2, M3, M4, . . . , MN present in the rare earth metal oxide thermal barrier coating. In another embodiment, the base metal oxide is present in a larger amount (on a weight percent basis) than the sum of the other metal oxides present in the thermal barrier coating.

[0020] The base element M1 is preferably derived a rare earth metal oxide M1xOy having a density of less than 7.0 g / cm3, preferably less than 6.0 g / cm3. In an embodiment, the base metal oxide M1xOy is derived from yttria (Y2O3), scandia (Sc2O3), praseodymium(III) oxide (Pr2O3), or a combination thereof. The use of a rare metal oxide having a low density is desirable because it minimizes the weight of the thermal barrier coating. A preferred base metal oxide for use in the thermal barrier coating is yttria (Y2O3).

[0021] The other base elements M2, M3, M4, . . . , MN may be derived from rare earth metal oxides M2xOy, M3xOy, M4xOy, . . . , MNxOy different from M1xOy. M2, M3, M4 . . . , MN are each chemically different from one another and are each chemically different from M1. From equation (1), it may be surmised that the amount of M2 in the thermal barrier coating is represented by mole fraction “a”, while the amount of M3 in the thermal barrier coating is represented by mole fraction “b” moles, the amount of M4 in the thermal barrier coating is represented by mole fraction “c” moles, and so on, and the amount of MN in the thermal barrier coating is represented by mole fraction “n” moles, where a, b, c, . . . , n can each vary from 0.0 to less than 0.5, preferably 0.1 to 0.4, and preferably 0.15 to 0.25 and where the sum of a+b+c+ . . . +n are not equal to 0.

[0022] In an embodiment, at least one of a+b is greater than 0 to less than 0.5; a+b+c is greater than 0 to less than 0.5, where only one of a, b or c can be zero; or a+b+c+ . . . +n is greater than 0 to less than 0.5, where no more than 2 terms of a, b, c, . . . , n can simultaneously be zero. In an embodiment, a can be equal to b, which can be equal to c, and so on, which can be equal to n. In other words, the mole fractions a=b=c= , . . . =n. In another a, b, c, . . . , n each have different non-zero values from each other. In an exemplary embodiment, the mole fraction of each of the base elements M1, M2, M3, M4, . . . , MN can be equal to each other.

[0023] In an embodiment, M2, M3, M4, . . . , MN are chemically different from each other and may be selected from the group of rare earth metals represented by at least one of lanthanum, cerium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or a combination thereof. Preferred metal oxides that may be used for deriving M2, M3, M4, . . . , MN include Yb, Er, Eu, Ho, or a combination thereof.

[0024] In an embodiment, while the base element M1 is derived from a less dense metal oxide to reduce the weight and cost of the thermal barrier coating, the other elements M2, M3, M4, . . . , MN may be selected to optimize other desirable properties as detailed below.

[0025] In an embodiment, at least one of M2, M3, M4, . . . , MN is neodymium. Neodymium imparts resistance against CMAS to the thermal barrier coating. In another embodiment, it is desirable to have a difference in cationic radius between some of the elements used in the thermal barrier coating. Cationic radii for different rare earth cations are provided in Table 2 below. In an embodiment, it is desirable to have a difference in cationic radius of at least 5%, preferably at least 10% between any two of elements M2, M3, M4, . . . , MN present in the thermal barrier coating.TABLE 2Cationic radius in picometersRare earth element(pm)Lanthanum103 pmCerium101 pmPraseodymium 99 pmNeodymium 98 pmSamarium 96 pmEuropium 95 pmGadolinium 93 pmTerbium 92 pmDysprosium 91 pmHolmium 90 pmErbium 89 pmThulium 88 pmYtterbium 86 pmLutetium 86 pm

[0026] A large difference in radii between two cations used in the thermal barrier coating facilitates a dissolution resistance in the alloy. For example, as the rare earth cation size increases, the solubility in the cubic Y2O3 decreases. In addition, the thermal conductivity can be varied by varying the cationic radii of at least one element used in the thermal barrier coating. FIG. 1A is a graph of temperature versus ionic radius for polymorphs of rare earth oxides (RE2O3) showing that rare earth oxides with small rare earth cations form cubic phases, whereas monoclinic and hexagonal phases are formed rare earth oxides with larger rare earth cations.

[0027] It is desirable for the thermal barrier coatings to comprise cubic crystalline structures. The cubic crystalline structures (when comprised of a plurality of rare earth metal oxides) of these thermal barrier coatings have a lower thermal conductivity when compared with other comparative unit crystal structures as well as when compared with traditional cubic crystalline structures (that do not comprise a plurality of rare earth metal oxides). The cubic crystalline structure when comprised of a plurality of rare earth metal oxides endows the thermal barrier coating with an unexpected low thermal conductivity as well as substantial CMAS resistance.

[0028] For example, when yttria is used as the base metal oxide, Y2O3 is corundum-like structured and crystallizes in the cubic 1a3 space group. There are two inequivalent Y3+ sites. In the first Y3+ site, Y3+ is bonded to six equivalent O2− atoms to form a mixture of distorted edge and corner-sharing YO6 octahedra. The corner-sharing octahedral tilt angles range from 54-56°. There are a spread of Y—O bond distances ranging from 2.25-2.33 Å. In the second Y3+ site, Y3+ is bonded to six equivalent O2− atoms to form a mixture of edge and corner-sharing YO6 octahedra. The corner-sharing octahedral tilt angles are 56°. All Y—O bond lengths are 2.28 Å. O2− is bonded to four Y3+ atoms to form a mixture of distorted edge and corner-sharing OY4 trigonal pyramids.

[0029] While conventional cubic crystalline structures typically tend to exhibit higher thermal conductivity compared with non-cubic crystalline structures, the disclosed thermal barrier coatings have an unexpected lower thermal conductivity because of the combination of atoms of different masses and different sizes in the rare earth alloy. Without being limited to theory, the reduction in thermal conductivity (while comprising cubic crystals) may be the result of several different reasons. In an embodiment, the rare earth alloys may comprise some disordered or highly defective crystal structures that disrupt the phonon movement and hinder thermal conductivity. These defects can act as scattering centers for phonons, reducing the overall thermal conductivity of the rare earth alloys. In another embodiment, the rare earth alloy may comprise cubic compounds that have inherently low thermal conductivity due to their electronic structure or phonon scattering mechanisms. In yet another embodiment, the cubic crystals may exhibit strong phonon-phonon interactions that reduce thermal conductivity. This can occur due to factors such as anharmonic lattice vibrations, which can scatter phonons and hinder their propagation, leading to lower overall thermal conductivity. In an embodiment, it is desirable for the rare earth metal oxide thermal barrier composition to have additional scattering mechanisms beyond phonon-phonon and phonon-mass impurity scattering alone.

[0030] FIG. 1B is a graph of temperature versus site fraction of the crystal occupied by a rare earth cation. The graph depicts the solubility of various rare earth oxides in Y2O3. As the rare earth cation size increases, the solubility in the cubic Y2O3 decreases. The solubility limits are determined from the phase boundaries between cubic Y2O3 with other phases. Nevertheless, significant solubility of even La2O3, La being the largest cation, exists allowing selection of an assemblage of rare earth (RE) cations to optimize sometimes competing properties.

[0031] In an exemplary embodiment, it may be desirable to have M2 be neodymium, M3 and / or M4 respectively be one of ytterbium, erbium, gadolinium, terbium, dysprosium, holmium, thulium, lutetium, or a combination thereof, while MN is lanthanum because of its large sized cationic radius. In a preferred embodiment, it is desirable to have one cation comprise neodymium and another cation comprise lanthanum.

[0032] The thermal barrier coating may be applied to a substrate comprising high temperature alloys that are used in turbine blades and rotors as a protective coating. The thermal barrier coating may have its coefficient of thermal expansion adjusted by varying the composition as detailed above. In an embodiment, the coefficient of thermal expansion may be adjusted to match that of the substrate upon which the thermal barrier coating is disposed on. In an embodiment, the coefficient of thermal expansion of the thermal barrier coating can be adjusted to match that of the substrate such that no excessive stress generation occurs during thermal cycling.

[0033] In an embodiment, the thermal barrier coating comprising rare earth metal oxides disclosed herein may be used in a multilayer coating with other commonly used thermal barrier coatings such as yttria-stabilized zirconia (YSZ), rare earth zirconates (REZ), strontium-containing TBCs (e.g., strontium zirconate (SrZrO3)), plasma-sprayed MCrAlY bond coats comprising MCrAlY (where M=Ni, Co, or a combination of both), or a combination thereof.

[0034] The other thermal barrier coatings (e.g., YSZ, REZ, plasma sprayed bond coats, and the like) may be disposed directly on the substrate (e.g., the turbine airfoil such as a blade or rotor) as a first layer and the rare earth metal oxide thermal barrier coating may be disposed as a second layer on the YSZ, REZ, and so on. In another embodiment, the rare earth metal oxide thermal barrier coating may be disposed directly on the substrate (e.g., the turbine blade or rotor) as a first layer with the other thermal barrier coating (e.g., YSZ, REZ, plasma sprayed bond coats, and the like) being disposed as a second layer directly on the rare earth metal oxide thermal barrier coating.

[0035] The thermal barrier coating has a thermal conductivity of less than 3.5 W / m-K, preferably less than 3.3 W / m-K, preferably less than 3.2 W / m-K and more preferably less than 3.0 W / m-K.

[0036] The thermal barrier coating comprising the rare earth metal oxides described herein has a thickness of 50 to 1000 micrometers. In a preferred embodiment, the thermal barrier coating comprising the rare earth metal oxides has a thickness of 100 to 400 micrometers.

[0037] The thermal barrier coating may be disposed on high temperature alloys used in turbine components such as blades and rotors that have higher operational temperatures than conventional nickel-based or cobalt-based superalloys. An exemplary high temperature alloy that has a higher operational temperatures than conventional nickel-based or cobalt-based superalloys is a niobium based alloy.

[0038] Higher temperature niobium-based alloys are a class of materials specifically engineered to withstand extreme heat environments, typically found in aerospace, energy, and other high-temperature applications. Niobium, a transition metal, exhibits excellent high-temperature properties, such as high melting point, poor oxidation resistance, and good mechanical strength. When alloyed with other elements, niobium-based alloys can achieve even greater performance in extreme conditions. Niobium itself has a high melting point of approximately 2,468° C. (4,474° F.), making it suitable for use in environments with extremely high temperatures. Niobium-based alloys exhibit poor resistance to oxidation at elevated temperatures necessitating an environmental barrier coating. Niobium-based alloys are often designed to have high creep resistance, allowing them to withstand mechanical loads at high temperatures without significant deformation. These alloys maintain their mechanical strength and structural integrity even at elevated temperatures, which is useful for applications where components are subjected to high mechanical loads in hot environments.

[0039] Niobium-based alloys are typically alloyed with other elements such as tantalum, tungsten, molybdenum, and sometimes other refractory metals or elements. These alloying elements can enhance various properties of the alloy, including high-temperature strength, creep resistance, and oxidation resistance. Examples of such high temperature niobium-based alloys include niobium-titanium (Nb—Ti), niobium-zirconium (Nb—Zr), niobium-hafnium (Nb—Hf), niobium-tungsten (Nb—W), niobium-molybdenum (Nb—Mo), niobium-tantalum (Nb—Ta), niobium-chromium (Nb—Cr), or the like, or a combination thereof. An example of a commercially available niobium-based alloy is C-103.

[0040] In one embodiment, the rare earth metal oxide thermal barrier coating may be manufactured by i) atmospheric plasma spraying, (APS); or via a ii) hybrid-slurry coating (HSC) manufacturing route. Both of these are detailed below.

[0041] In the application of a rare earth metal oxide coating to an airfoil (e.g., a turbine blade or a rotor) a suspension or slurry of the metal oxide particles is first manufactured. The suspension is typically used for spraying a coating of the thermal barrier coating on the airfoil surface, while the slurry may be used for dip coating, painting, doctor blading, and the like.

[0042] In preparing the suspension or slurry, the requisite metal oxide particles in the correct stoichiometric ratios are mixed together and ground to form an intimately mixed powder. The mixing of the metal oxides may be conducted in a variety of mixers where shear mixing forces are applied to the metal oxide particles to comminute and blend them. The mixing may be conducted in a dry mixing process (one where no liquids are used) or a wet mixing process (where a liquid is present with the rare metal oxides during the mixing process).

[0043] In a wet mixing process, a liquid may be used to facilitate comminution. The liquid is preferably one that may be subsequently used in the atmospheric plasma spraying or in the hybrid-slurry coating. The liquid is preferably one that is environmentally friendly and not toxic. However, liquids that contain volatile organic compounds may also be used.

[0044] The liquids may be polar solvents, non-polar solvents, or a combination thereof. In an embodiment, a co-solvent system comprising a polar and a non-polar solvent may be used as the liquid in the suspension or slurry. Examples of liquids that may be used during the comminution process include water, propylene carbonate, ethylene carbonate, butyrolactone, acetonitrile, benzonitrile, nitromethane, nitrobenzene, sulfolane, dimethylformamide, N-methylpyrrolidone, methanol, acetonitrile, nitromethane, ethanol, propanol, isopropanol, butanol, benzene, toluene, methylene chloride, carbon tetrachloride, hexane, diethyl ether, tetrahydrofuran or the like, or a combination thereof. In an embodiment, the liquid may be water, ethanol or a combination thereof.

[0045] The liquid may be present in an amount of 20 to 90 wt %, preferably 30 to 80 wt %, based on a total weight of the liquid, a dispersant and the metal oxide particles.

[0046] In an embodiment, the suspension or slurry is an electrosteric dispersion that involves a combination of both electrostatic and steric stabilization mechanisms to prevent the aggregation or flocculation of colloidal particles in a dispersed phase. The pH of the suspension or slurry is adjusted to be greater than or equal to 7, preferably greater than or equal to 8, more preferably greater than or equal to 9, and preferably greater than or equal to 9.4.

[0047] The mixing is preferably conducted in a Waring blender, a ball mill, a Henschel mixer, an attritor mill, or a combination thereof. The particles of the various metal oxides in the intimately mixed powder typically have particle sizes of 0.1 to 100 micrometers, preferably 5 to 50 micrometers, and more preferably 10 to 30 micrometers.

[0048] In an embodiment, the suspension and / or slurry may comprise a dispersant. In an embodiment, the dispersant may be a surfactant. Dispersants facilitate compatibility between the metal oxide particles and the liquids. An optimized dispersion that creates a better deflocculated ceramic slurry will result in a high-quality ceramic final product with fewer defects. Slip-casting of ceramics uses concentrated dispersions and the use of a dispersant facilitates the production of uniform coatings with very few defects. Suitable dispersants include ionic surfactants, amphoteric surfactants, zwitterionic surfactants, or a combination thereof. In an embodiment, the dispersant is a carbonic acid based polyelectrolyte DOLAPIX CE 64.

[0049] The dispersant may be used in amounts of 0.1 to 5 wt %, preferably 0.5 to 2 wt %, based on a total weight of the metal oxides, the solvent and the dispersant.

[0050] The various metal oxides (that are desired for use in the rare earth metal oxide thermal barrier coating) are present in the suspension or slurry in an amount of 5 to 85 wt %, preferably 15 to 75 wt %, based on a total weight of the various metal oxides, dispersants and liquids. The suspension may typically contain a smaller amount of the metal oxides relative to the liquid, while the slurry may contain a larger amount of the metal oxides relative to the liquid.

[0051] Other additives such as antioxidants, antiozonants, thermal stabilizers, dyes, colorants, electrically conducting fillers, mold release agents, and the like may be added to the mixture prior to plasma spraying or hybrid-slurry coating.

[0052] Atmospheric plasma spraying involves the generation of a high-temperature plasma jet by ionizing a feedstock gas (typically argon) using a DC or RF arc discharge. The plasma jet, with temperatures reaching up to 15,000° C., accelerates and heats powder particles of the rare earth metal oxides, melting them and propelling them toward the substrate.

[0053] The atmospheric plasma spraying system comprises a plasma torch, a powder feeder, a substrate holder, a gas supply system and a control system. The plasma torch generates and maintains the high-temperature plasma jet. It typically comprises a cathode and an anode, between which the arc discharge occurs, and a nozzle through which the plasma jet is expelled.

[0054] The powder feeder delivers the feedstock powder (a combination of the rare earth metal oxides in a stoichiometric ratio to produce the thermal barrier coating of the equation (1)) into the plasma jet at controlled rates. The powder is injected axially or radially into the plasma plume. The substrate to be coated is mounted on a holder that can be manipulated to achieve the desired coating thickness and uniformity. Argon or other inert gases are supplied to the plasma torch to create and sustain the plasma jet. The entire process is typically controlled by a computerized system that regulates parameters such as gas flow rates, powder feed rates, torch positioning, and spraying parameters.

[0055] The substrate surface is cleaned and prepared to remove contaminants and ensure proper adhesion of the coating. The feedstock powder is injected into the plasma jet, where it is rapidly heated and accelerated. The powder particles undergo partial or complete melting and are accelerated toward the substrate by the high-velocity plasma jet. Upon impact with the substrate, the molten particles flatten and solidify, forming a coating layer. Multiple passes may be required to achieve the desired coating thickness. The deposited coating rapidly cools and solidifies upon contact with the substrate, forming a dense and adherent layer.

[0056] The hybrid-slurry coating method comprises suspending metal oxide particles in a solvent with surface chemistry additives (e.g., surfactants, polymeric binders, or a combination thereof) to control interparticle forces. These additives provide different stabilization mechanisms to guarantee good particle dispersion (no particle agglomeration) and avoid sedimentation. These mechanisms are based on creating repulsion forces between particles (electric-double layer repulsion, steric or electrosteric repulsion) that are larger than the attractive forces (van der Waals) between particles. Suspensions behave as a liquid, even at high solid concentrations, with a controlled rheological behavior, until a consolidation mechanism transforms them from a liquid to a solid coating. The hybrid slurry coating approach incorporates additional consolidation mechanisms to drying paired with a tailored suspension formulation that permits higher solid concentration for the rare earth metal oxides.

[0057] In an embodiment, the hybrid slurry coating and the atmospheric plasma spraying can use a suspension having a similar composition to dispose a thermal barrier coating on a substrate such as, for example, an airfoil.

[0058] A preferred suspension comprises the metal oxides in an amount of 25 to 35 wt %, 0.5 to 1.5 wt % of the dispersant with the remainder being water. The weight percents are calculated based on the total weight of the metal oxide, dispersant and water. The pH of the suspension is adjusted to be approximately 9.4 and the dispersion is an electrosteric dispersion.

[0059] The advantages from this method of forming a thermal barrier coating are i) creation of a “binder” network that will “hold in place” the particulates in the coating; ii) higher particle packing that will translate into higher strength, more homogeneity and less defects; iii) minimization of cracking, delamination or defects that typically arise from the capillary pressures associated with drying and removal of solvent, especially when the thickness of the coating increases; iv) shorter consolidation times, v) suitability for coating repairs in complex shapes and difficult-to-access sections of the samples (non-line-of-sight) and; vi) possibility of developing mixed phases, multi-layer, porous and or compositionally graded coatings for higher thermal stability and control of the fracture toughness.

[0060] The amount of organic material to be used in the formation of the hybrid-slurry coating is between 0.5 to 10 wt. % with respect to the overall amount of rare earth metal oxide powder. This amount of organic material avoids shrinkage during their removal via heat treatment. The green coatings are then sintered to create the final material.

[0061] Concentrated aqueous suspensions from 30 to 50 volume percent solids of the rare earth metal oxide equimolar powder may be blended with a dispersant to form a low viscosity coating solution that displayed shear thinning behavior, indicating good particle stability and homogeneity in suspension. The suspensions may be slip cast into self-standing pellets and sintered at a temperature of 1400 to 1800° C. for 1 to 5 hours, rendering a sintered density of 97%, confirming that the solid content translates to a good green particle packing and sintered density.

[0062] Alternatively, the suspensions are deposited on the substrates using a simple paint spray gun (20 oz. HVLP Gravity Feed Air Spray Gun, Harbor Freight), connected to an air compressor. Niobium-based alloy coupons are placed horizontally in a metallic mesh, and coated on one face, then dried, turned over and then coated on the other side. The coupons with the coatings disposed thereon may then be subjected to sintered at a temperature of 1400 to 1800° C. for 1 to 5 hours.

[0063] The rare earth metal oxide thermal barrier coatings disclosed herein are exemplified by the following non-limiting examples.Example 1

[0064] This example was conducted to demonstrate the thermal conductivity of the rare earth metal oxide thermal barrier coating and the variation in thermal conductivity with different cationic sizes.

[0065] Thermal conductivity is a key design consideration for thermal / environmental barrier coatings (T / EBC). Lower thermal conductivity materials are desirable to prevent thermomechanical and thermochemical-driven failure mechanisms. Fortunately, high entropy and entropy stabilized oxides have exhibited remarkably low thermal conductivities due to increased phonon scattering mechanisms driven by the ability for these classes of oxides to support a wide array of cations, thus resulting in a relatively large distribution of different types of elements in the ceramic. This variety of masses and atomic radii leads to phonon scattering based on mass differences and local volume changes that can result in ultralow thermal conductivities of high entropy and entropy stabilized oxides, exceeding even those of their respective amorphous phases and theoretical minimum limits. The key to these ceramics achieving ultralow thermal conductivities is introducing additional phonon scattering mechanisms beyond just mass-different scattering alone. Table 3 shows thermal conductivities for the compositions detailed below.TABLE 3Thermal ConductivityThermal barrier compositionCrystal Structure(Wm−1K−1)Y2O3cubic9.99 ± 0.92Nd2O3hexagonal3.82 ± 0.50(Y—Yb—Er)2O3cubic3.31 ± 0.39(Y—Yb—Er—Ho—Nd)2O3cubic3.16 ± 0.37(Y0.28Yb0.28Er0.28La0.16)2O3cubic + monoclinic2.71 ± 0.35

[0066] The thermal conductivity of an equimolar rare earth metal oxide thermal barrier composition (Y—Yb—Er—Ho—Nd)2O3 is shown in Table 3, along with thermal conductivities of two parent rare earth-oxides of these composition (Y2O3 and Nd2O3), a three cation rare earth solid solutions ((Y—Yb—Er)2O3), and a four cation rare earth composition with an additional La2O3-rich phase that has separated from the cubic phase. First, even the thermal conductivities of the Y2O3 sample is lower than that of a single crystal. The three cation solid solution ((Y—Yb—Er)2O3) and the cubic rare earth metal oxide composition have similar thermal conductivities only slightly lower than that of the Nd2O3 sample. However, Nd2O3 has an anisotropic hexagonal crystal structure while the others all have isotropic cubic crystal structures. (Y0.28Yb0.28Er0.28La0.16)2O3, which has a La2O3 content above the solubility limit and forms two phases has a reduced thermal conductivity compared with all other samples, including the cubic rare earth composition. This is attributed to the influence of the second phase. This demonstrates that the key to reducing the thermal conductivity in a rare earth metal oxide composition is introducing additional scattering mechanisms beyond phonon-phonon and phonon-mass impurity scattering alone.Example 2

[0067] This example was conducted to determine thermochemical stability in thermochemical environments. It is desirable for environmental barrier coatings to be able to withstand degradation by high temperature combustion gases, specifically high-velocity water vapor—a product of combustion, as well as molten deposits resulting from siliceous debris ingested into engines. Rare earth metal oxides are a class of oxides that are relatively impervious to reaction with high-temperature steam. A photomicrograph (not shown here) shows surface faceting and insignificant recession of a five component rare earth metal oxide material (Y,Yb,Er,Eu,Ho)2O3 after exposure to the extreme conditions of 1500° C., 100 hours, 260 m / s steam. For comparison, Al2O3 undergoes a 55 micrometer recession due to Al(OH)3(g) formation at 1450° C. after 24 hours under the same high-velocity steam conditions.

[0068] Rare earth metal oxide thermal barrier coatings are also comparatively stable to dissolution in molten siliceous debris (CaO·MgO·Al2·O3·SiO2-CMAS) relative to rare earth silicates (RE2Si2O7) environmental barrier coatings (EBCs) for SiC-based composite substrates. FIG. 2 shows a cross-sectional micrograph and energy dispersive spectroscopy elemental maps of a five component rare earth metal oxide thermal barrier coating material after reaction with CMAS at 1500° C., for 100 hours in air showing formation of a protective barrier layer of reaction product composed of Ca-stabilized apatite Ca2RE8(SiO4)6O2 and cuspidine (CaRE)4(Al,Si)2O9 the mineral name for Ca and Si-substituted YAM. Here, the selection of rare earth metal oxide components in the thermal barrier coating composition are useful to control the barrier product phase. Large rare earth cations, such as Nd, are strongly favored in the apatite phase.Example 3

[0069] This example is conducted to determine oxygen diffusion through the thermal barrier coating disclosed herein. The prime function of a rare earth metal oxide thermal barrier coating is to limit oxidant access from the combustion gases to the underlying refractory alloy. The lack of oxide formed at the rare earth metal oxide thermal barrier coating / alloy interface for a rare earth metal oxide coating densified by spark plasma sintering around a C-103 button suggests that the rare earth metal oxide thermal barrier coating compositions have intrinsically low oxygen diffusivity, sufficient to protect refractory alloys.

[0070] This is in agreement with the oxygen diffusion coefficients of 10−11 cm2 / s reported for Y2O3 at 1400° C. Rare earth cation diffusion is also important for densification of coatings deposited by atmospheric plasma spray or slurry processing. To examine the rates of diffusions, the migration barriers of O2− anion and rare earth (RE3+) cations in cubic rare earth metal oxide thermal barrier coatings are calculated using DFT (density functional theory) in combination with the climbing image nudged elastic band method. DFT is used to describe the electronic density of a system rather than the wave functions of individual electrons. In this theory, the ground state energy of a system is expressed as a functional of the electron density. This means that the total energy of the system is determined solely by the electron density and not by the individual electron wave functions, which significantly reduces the computational complexity compared to methods that explicitly calculate wave functions.

[0071] The magnetic states of the rare earth ions cause difficulties in structural relaxations with DFT. As shown in FIGS. 3A and 3B, both the migration barriers for O2− (FIG. 3A) and RE3+ (FIG. 3B) in the rare earth metal oxide thermal barrier coating with lanthanides generally decrease with increasing effective ionic radius. Therefore, rare earth cations with smaller radii should be used to mitigate oxygen diffusion, while rare earth cations with larger radii should be prioritized for better densification.Example 4

[0072] This example is conducted to demonstrate manufacturing of a rare earth metal oxide thermal barrier coating by atmospheric plasma spraying (APS) as well as by hybrid-slurry coating (HSC). The manufacturing technologies selected in this study require very distinct type of particle size distributions: spherical larger granules, for APS, and individual particles below 1 μm size for HSC, with narrow size distributions. The 3-component compositions were sourced from Oerlikon-Metco (Westbury, NY), with the same chemical purity but in two distinctive size ranges, suitable for each of the manufacturing routes, in large batches of 5-10 kg for each composition. Each composition was confirmed by XRD analysis of the raw materials to be single phase cubic. The powder for APS has a Dv50 of 34 μm, and the powder for HSC has a particle size distribution with a Dv50 between 0.8-1 μm, Dv10~0.5 μm, and Dv90~3-4 μm.

[0073] In addition, to compare the effect of the processing on microstructure and performance, the geometry of the Nb-base C-103 alloy (Nb10Hf1Ti0.7Zr) (wt. %)) substrate coupons were selected to be discs of ~25 mm diameter and 3.1 mm thicknesses. The coupons were sand-blasted prior depositing any coating for any of the processing routes selected. The homogeneous coverage of the edges was identified as one of the biggest challenges for both routes.Atmospheric Plasma Spraying (APS) Thermal Barrier Coating

[0074] Thick coatings (100-400 μm thick) of rare earth oxide blends containing yttrium, erbium, and ytterbium were successfully applied to C-103 alloy substrates via atmospheric plasma spray. These coatings were applied directly to grit-blasted surfaces with no intermediate bondcoat layer. Process conditions including standoff distance, raster speed and step size, plasma power, and powder flow rate were tuned to maximize calculated coating density. Density was estimated from the measured coating mass divided by volume (determined by the product of coating thickness and surface area). C-103 specimens were cross-sectioned and polished for optical microscopy and scanning electron microscopy.

[0075] A C-103 disk coated at 22 kW, 50 g / min mass flow, and 300 mm / s raster speed was examined using a microscope along with its cross-section. Good coverage is indicated by the macro-uniformity of coating thickness on the faces, corners, and sides. Average coating thicknesses measured at ~400 μm or 170 μm were produced. Higher-magnification light optical and scanning electron microscopy images of a corner show good substrate adhesion, and uniformly-distributed porosity within the coating. The SEM image of the same area, however, reveals cracks within the coating, including some near the substrate interface. This microstructure is representative of that observed on the faces of the specimen.Hybrid Slurry Coating

[0076] Concentrated aqueous suspensions from 30 to 50 vol % solids of the rare earth metal oxide equimolar powder were prepared with 1 wt % dispersant respect to powder (electrosteric mechanism) and pH 10, that showed low viscosity and shear thinning behavior, indicating good particle stability and homogeneity in suspension. The liquid used for the suspension is water. Shear thinning behavior is desirable to ensure flow when the suspension is applied to the substrate to create the coating, and when the shear decreases, the viscosity increases to allow for the coating to stay in place and proceed towards consolidation. The suspensions were slip cast into self-standing pellets and sintered at 1600° C. / 2.5 hours, rendering a sintered density of 97%, confirming that the solid content translates to a good green particle packing and sintered density.

[0077] The suspensions are deposited on the substrates using a simple paint spray gun (20 oz. HVLP Gravity Feed Air Spray Gun, Harbor Freight), connected to an air compressor. The coupons are placed horizontally in a metallic mesh, and coated in one face, then dried, turn over and then coated on the other side. The number of passes, flow, distance of spraying and layout of the coupons have been studied and optimized, producing successful coatings in green with coating thicknesses between 100 and 250 μm.

[0078] While the invention has been described with reference to some embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A thermal barrier coating composition comprising:(M1(i-a-b-c-d . . . n)M2aM3bM4c..MNn)2O3 (1), where the subscript “a” represents the molar fraction of element M2, “b” represents the molar fraction of element M3, “c” represents the molar fraction of element M4, and “n” represents molar fraction of element MN (in the metal oxide of formula (1)); and where the sum of a+b+c+d+ . . . +n is less than 1; where M1 represents a base element derived from a metal oxide that has a density of less than or equal to 7.0 grams per cubic centimeter, while M2, M3, M4, . . . , MN each represent a different element derived from the plurality of metal oxides each of which have a density greater than 7.0 g / cm3.

2. The composition of claim 1, where element M1 is derived from a rare earth metal oxide and is present in a mole fractional amount of 0.15 to 0.8, based on the sum of moles of M1, M2, M3, M4, . . . , MN in the thermal barrier coating.

3. The composition of claim 2, where the mole fraction of M1=the mole fraction of M2=the mole fraction of M3=the mole fraction of M4==the mole fraction of MN.

4. The composition of claim 1, where the base element M1 is present in a larger amount on a weight percent basis than each of elements M2, M3. M4, . . . , MN present in the thermal barrier coating composition.

5. The composition of claim 1, where the base element M1 is present in a larger amount on a weight percent basis than a sum of elements M2, M3, M4, . . . , MN present in the thermal barrier coating composition.

6. The composition of claim 1, where the base element M1 is derived from yttria (Y2O3), scandia (Sc2O3), praseodymium(II) oxide (Pr2O3), or a combination thereof.

7. The composition of claim 1, where the base element M1 is derived from yttria (Y2O3).

8. The composition of claim 1, where a, b, c, . . . , n can each vary from 0.0 to less than 0.5 and where the sum of a+b+c+ . . . +n are not equal to 0; and where no more than 2 terms of a, b, c, . . . , n can simultaneously be zero.

9. The composition of claim 1, where A) a+b is greater than 0 to less than 0.5; B) a+b+c is greater than 0 to less than 0.5, where only one of a, b or c can be zero; orC) a+b+c+ , . . . ,+n is greater than 0 to less than 0.5, and where no more than 2 terms of a, b, c, . . . , n can simultaneously be zero.

10. The composition of claim 1, where elements M2, M3, M4, . . . , MN are selected from the group of rare earth metals consisting of lanthanum, cerium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or a combination thereof.

11. The composition of claim 1, where at least one of element M2. M3, M4, . . . , MN is neodymium.

12. The composition of claim 1, where there is a difference in cationic radius of at least 5%, between any two of elements M2, M3. M4, . . . , MN present in the thermal barrier coating composition.

13. A coating derived from the composition of claim 1.

14. The coating of claim 13, where the coating is disposed on a turbine airfoil.

15. The coating of claim 14, where the turbine airfoil is a niobium-based superalloy.

16. A method of disposing a thermal barrier coating on a turbine airfoil, the method comprising:blending metal oxides in a blender to form a composition having the formula (1)(M1(1-a-b-c-d, . . . n)M2aM3bM4c..MNn)2O3(1), where the subscript “a” represents the molar fraction of element M2, “b” represents the molar fraction of element M3, “c” represents the molar fraction of element M4, and “n” represents molar fraction of element MN (in the metal oxide of formula (1)); and where the sum of a+b+c+d+ . . . +n is less than 1; where M1 represents a base element derived from a metal oxide that has a density of less than or equal to 7.0 grams per cubic centimeter, while M2, M3, M4, . . . , MN each represent a different element derived from the plurality of metal oxides each of which have a density greater than 7.0 g / cm3;suspending the metal oxide in a solvent to form a suspension;disposing the suspension on an airfoil; andheating the suspension to dry the composition.

17. The method of claim 16, wherein the disposing the suspension on the airfoil is accomplished via plasma spraying.

18. The method of claim 16, wherein the disposing the suspension on the airfoil is accomplished via dip coating, doctor blading, painting, slip coating or spin-coating.

19. The method of claim 16, where the heating is conducted at a temperature of 1200 to 1800° C.

20. The method of claim 16, where the mole fraction of M1=the mole fraction of M2=the mole fraction of M3=the mole fraction of M4= . . . =the mole fraction of MN.

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