CLIAMS:1. An article, comprising:
a substrate; and
a coating system disposed on the substrate, the coating system comprising:
a thermal barrier coating; and
a calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive material present in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition, wherein the CMAS-reactive material is of nominal composition AB4-xDxO7, wherein
A comprises strontium, calcium, or a combination thereof;
B comprises a rare earth element;
D comprises aluminum, gallium, or a combination thereof; and x is a number less than or equal to 2.
2. The article of claim 1, wherein x is equal to zero.
3. The article of claim 1, wherein x is equal to 2.
4. The article of claim 1, wherein B comprises samarium, gadolinium, yttrium, lanthanum, neodymium, europium, or combinations thereof.
5. The article of claim 1, wherein the CMAS-reactive material comprises a nominal composition of SrB2Al2O7.
6. The article of claim 1, wherein the CMAS-reactive material comprises a nominal composition of SrGd2Al2O7, SrGd4O7, CaGd4O7, SrLa4O7, or combinations thereof.

7. The article of claim 1, wherein the CMAS-reactive material is disposed as a protective overlaying coating on the thermal barrier coating.
8. The article of claim 7, wherein the protective overlaying coating comprises a porosity in a range from about 10 volume percent to about 20 volume percent of the protective overlaying coating.
9. The article of claim 7, wherein a thickness of the protective overlaying coating is less than 50 percent of the thickness of the coating system.
10. The article of claim 1, wherein the thermal barrier coating comprises surface-connected pores, and wherein the CMAS-reactive material is disposed within a plurality of the surface connected pores.
11. The article of claim 10, wherein the CMAS-reactive material is disposed in an amount in a range from about 20 volume percent to about 70 volume percent of the plurality of surface-connected pores.
12. The article of claim 10, wherein at least a portion of the surface-connected pores comprises a plurality of elongated surface-connected voids.
13. The article of claim 10, wherein the coating system further comprises a barrier material disposed within the surface-connected pores.
14. A turbine engine component comprising the article of claim 1.
15. An article, comprising:
a substrate; and
a coating system disposed on the substrate, the coating system comprising:
a thermal barrier coating comprising a plurality of elongated surface-connected voids;
a calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive material of nominal composition AB4-xDxO7 disposed within at least some of the voids in the plurality of surface-connected voids, wherein
A comprises strontium, calcium, or a combination thereof;
B comprises gadolinium, lanthanum, neodymium, or combinations thereof;
D comprises aluminum, gallium, or a combination thereof; and x is a number less than or equal to 2.
16. The article of claim 15, wherein SrGd2Al2O7 is disposed as the CMAS-reactive material.
17. A method of manufacturing an article, comprising:
forming a coating system by:
disposing a thermal barrier coating on a substrate; and
disposing a calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive material on the thermal barrier coating,
wherein the CMAS-reactive material is present in the coating system in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition, and the CMAS-reactive material is of nominal composition AB4-xDxO7, wherein
A comprises strontium, calcium, or a combination thereof;
B comprises a rare earth element;
D comprises aluminum, gallium, or a combination thereof; and x is a number less than or equal to 2.
18. The method of claim 17, wherein the method comprises disposing the CMAS-reactive material as a protective overlaying coating on the thermal barrier coating.

19. The method of claim 17, wherein the method comprises disposing the CMAS-reactive material within a plurality of surface-connected pores in the thermal barrier coating.
20. The method of claim 19, further comprising disposing a barrier material within the plurality of surface-connected pores prior to disposing the CMAS-reactive material.
,TagSPECI:BACKGROUND OF THE INVENTION
[0001] The invention relates generally to articles including thermal barrier coatings. More particularly, the invention relates to articles having a coating system including a thermal barrier coating and a (CMAS)-reactive material.
[0002] Thermal barrier coatings are typically used in articles that operate at or are exposed to high temperatures. Aviation turbines and land-based turbines, for example, may include one or more components protected by the thermal barrier coatings. Under normal conditions of operation, thermal-barrier coated components may be susceptible to various types of damage, including erosion, oxidation, and attack from environmental contaminants.
[0003] For turbine components, environmental contaminant compositions of particular concern are those containing oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. These oxides combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide systems (Ca--Mg--Al--SiO), hereafter referred to as "CMAS." At the high turbine operating temperatures, these environmental contaminants can adhere to the heated or hot thermal barrier coating surface, and thus cause damage to the thermal barrier coating. For example, CMAS can form compositions that are liquid or molten at the operating temperatures of the turbines. The molten CMAS composition can dissolve the thermal barrier coating, or can infiltrate its porous structure by infiltrating the pores, channels or other cavities in the coating. Upon cooling, the infiltrated CMAS composition solidifies and reduces the coating strain tolerance, thus initiating and propagating cracks that may cause delamination and spalling of the coating material. This may further result in partial or complete loss of the thermal protection provided to the underlying metal substrate of the part or component. Further, spallation of the thermal barrier coating may create hot spots in the metal substrate leading to premature component failure. Premature component failure can lead to unscheduled maintenance as well as parts replacement resulting in reduced performance, and increased operating and servicing costs.
[0004] Thus, there is a need for improved coating systems that provide protection to thermal barrier coatings from the adverse effects of environmental contaminants, when operated at or exposed to high temperatures. In particular, there is a need for improved coating systems that provide protection to thermal barrier coatings from the adverse effects of the deposited CMAS.
BRIEF DESCRIPTION OF THE INVENTION
[0005] One embodiment is directed to an article including a substrate and a coating system disposed on the substrate. The coating system includes a thermal barrier coating and a calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive material. The CMAS-reactive material is present in the coating system in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition. The CMAS-reactive material has a nominal composition of AB4-xDxO7, wherein A includes strontium, calcium, or a combination thereof, B includes a rare earth element, D includes aluminum, gallium, or a combination thereof, and x is a number less than or equal to 2.
[0006] Another embodiment is directed to an article including a substrate and a coating system disposed on the substrate. The coating system includes a thermal barrier coating and a calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive material. The thermal barrier coating has a plurality of elongated surface-connected voids, and the CMAS-reactive material is disposed within at least some of the voids of the plurality of elongated surface-connected voids. The CMAS-reactive material is present in the coating system in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition. The CMAS-reactive material has a nominal composition of AB4-xDxO7, wherein A includes strontium, calcium, or a combination thereof, B includes a rare earth element, D includes aluminum, gallium, or a combination thereof, and x is a number less than or equal to 2.
[0007] Another embodiment of the invention is directed to a method of manufacturing an article. The method includes forming a coating system by disposing a thermal barrier coating on a substrate, and disposing a calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive material on the thermal barrier coating. The CMAS-reactive material is present in the coating system in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition. The CMAS-reactive material has a nominal composition of AB4-xDxO7, wherein A includes strontium, calcium, or a combination thereof, B includes a rare earth element, D includes aluminum, gallium, or a combination thereof, and x is a number less than or equal to 2.
DRAWINGS
[0008] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout the drawings, wherein:
[0009] FIG. 1 illustrates a schematic of an article, in accordance with an embodiment of the invention;
[0010] FIG. 2 illustrates a schematic of an article, in accordance with an embodiment of the invention;
[0011] FIG. 3 illustrates a schematic of an article, in accordance with an embodiment of the invention;
[0012] FIG. 4 illustrates a schematic of an article, in accordance with an embodiment of the invention;
[0013] FIG. 5 illustrates a schematic of an article, in accordance with an embodiment of the invention;
[0014] FIG. 6 illustrates a schematic of an article, in accordance with an embodiment of the invention;
[0015] FIG. 7 shows the powder X-ray diffraction pattern for reaction products of CaGd4O7 with CMAS at 1400 °C, in accordance with an embodiment of the invention;
[0016] FIG. 8 shows the powder X-ray diffraction pattern for reaction products of SrGd2Al2O7 with CMAS at 1400 °C, in accordance with an embodiment of the invention;
[0017] FIG. 9 shows the powder X-ray diffraction pattern for reaction products of CaGd4O7 with CMAS at 1260 °C, in accordance with an embodiment of the invention;
[0018] FIG. 10 shows the powder X-ray diffraction pattern for reaction products of SrGd2Al2O7 with CMAS at 1260 °C, in accordance with an embodiment of the invention; and
[0019] FIG. 11 shows a differential scanning calorimetry (DSC) data for a reaction mixture of SrGd2Al2O7 with CMAS, in accordance with an embodiment of the invention.
DETAILED DESCRIPTION
[0020] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, and “substantially” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.
[0021] In the following specification and the claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the term “or” is not meant to be exclusive and refers to at least one of the referenced components being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise.
[0022] As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.
[0023] As used herein, the term “coating” refers to a material disposed on at least a portion of an underlying surface in a continuous or discontinuous manner. Further, the term “coating” does not necessarily mean a uniform thickness of the disposed material, and the disposed material may have a uniform or a variable thickness. The term “coating” may refer to a single layer of the coating material or may refer to a plurality of layers of the coating material. The coating material may be the same or different in the plurality of layers.
[0024] As used herein, the term “disposed on” refers to layers or coatings disposed directly in contact with each other or indirectly by having intervening layers there between, unless otherwise specifically indicated. The term “depositing on” refers to a method of laying down material in contact with an underlying or adjacent surface in a continuous or discontinuous manner. The term “adjacent” as used herein means that the two materials or coatings are disposed contiguously and are in direct contact with each other.
[0025] As mentioned earlier, thermal barrier coatings are susceptible to molten CMAS compositions at high turbine operating temperatures. The molten CMAS composition can dissolve the thermal barrier coating, or can infiltrate a porous structure of the thermal barrier coating by infiltrating the pores, channels or other cavities present in the coating. Upon cooling, the infiltrated CMAS composition solidifies and reduces the coating strain tolerance, thus initiating and propagating cracks that may cause delamination and spalling of the coating material. Previous methods to protect the thermal barrier coatings include use of CMAS-reactive or CMAS-resistant thermal barrier coating compositions. However, the previously known CMAS-reactive compositions may not provide the desired CMAS-reactivity.
[0026] Embodiments of the invention described herein address the noted shortcomings of the state of the art. Some embodiments present an article including a substrate and a coating system disposed on the substrate. The coating system includes a thermal barrier coating disposed on the substrate, and a CMAS-reactive material. The CMAS-reactive material has a nominal composition AB4-xDxO7, wherein A includes strontium, calcium, or a combination thereof, B includes a rare earth element, D includes aluminum, gallium, or a combination thereof, and x is a number less than or equal to 2. Material of this type has been found by the present inventors to exhibit attractive reactivity with CMAS materials under environmental conditions of interest for turbo-machinery applications.
[0027] As used herein, the term "thermal barrier coating" refers to a coating that includes a material capable of reducing heat flow to the underlying substrate of the article, that is, form a thermal barrier. In some embodiments, the thermal barrier coating includes a material having a melting point greater than about 1090 °C. In some embodiments, the thermal barrier coating includes a material having a melting point greater than about 1200 °C. In some embodiments, the thermal barrier coating includes a material having a melting point in a range from about 1200 °C to about 1930 °C.
[0028] In some embodiments, the thermal barrier coating includes a ceramic thermal barrier material. Suitable ceramic thermal barrier coating materials include various zirconias, in particular chemically stabilized zirconias (for example, metal oxides blended with zirconia), such as yttria-stabilized zirconias, ceria-stabilized zirconias, calcia-stabilized zirconias, scandia-stabilized zirconias, magnesia-stabilized zirconias, india-stabilized zirconias, ytterbia-stabilized zirconias, lanthana-stabilized zirconias, gadolinia-stabilized zirconias, as well as mixtures of such stabilized zirconias.
[0029] In certain embodiments, the thermal barrier coating includes yttria-stabilized zirconias. Suitable yttria-stabilized zirconias may include from about 1 wt% to about 20 wt% yttria (based on the combined weight of yttria and zirconia), and more typically from about 3 wt% to about 10 wt% yttria. An example yttria-stabilized zirconia thermal barrier coating includes about 7 wt % yttria and about 93 wt % zirconia. These chemically stabilized zirconias may further include one or more of a second metal (e.g., a lanthanide or actinide) oxide such as dysprosia, erbia, europia, gadolinia, neodymia, praseodymia, urania, and hafnia to further reduce thermal conductivity of the thermal barrier coating. In some embodiments, the thermal barrier coating may further include an additional metal oxide, such as, titania.
[0030] In some embodiments, suitable ceramic thermal barrier coating materials may include pyrochlores of general formula A2B2O7 where A is a metal having a valence of 3+ or 2+ (e.g., gadolinium, aluminum, cerium, lanthanum or yttrium) and B is a metal having a valence of 4+ or 5+ (e.g., hafnium, titanium, cerium or zirconium) where the sum of the A and B valences is 7. Representative materials of this type include gadolinium zirconate, lanthanum titanate, lanthanum zirconate, yttrium zirconate, lanthanum hafnate, cerium zirconate, aluminum cerate, cerium hafnate, aluminum hafnate, and lanthanum cerate.
[0031] The thermal barrier coating may include the ceramic thermal barrier coating material in an amount of up to 100 wt%. In some embodiments, the thermal barrier coating includes the ceramic thermal barrier coating material in an amount in a range from about 95 wt% to about 100 wt%; more particularly from about 98 wt% to about 100 wt%. The composition of the thermal barrier coating in terms of the type and amount of the ceramic thermal barrier coating materials may depend upon one or more factors, including the composition of the adjacent bond coat layer (if present), the coefficient of thermal expansion (CTE) characteristics desired for the thermal barrier coating, and the thermal barrier properties desired for the thermal barrier coating.
[0032] The thickness of the thermal barrier coating may depend upon the substrate or the component it is deposited on. In some embodiments, the thermal barrier coating has a thickness in a range from about 30% to about 90% of the total thickness of the coating system. In some embodiments, the thermal barrier coating has a thickness in a range from about 25 microns to about 2000 microns. In some embodiments, the thermal barrier coating has a thickness in a range of from about 25 microns to about 1500 microns. In some embodiments, the thermal barrier coating has a thickness in a range of from about 25 microns to about 1000 microns.
[0033] The article may further include a bond coating disposed between the substrate and the thermal barrier coating. The bond coating may be formed from a metallic oxidation-resistant material that protects the underlying substrate and enables the thermal barrier coating to more tenaciously adhere to substrate. Suitable materials for the bond coating include M1CrAlY alloy powders, where M1 represents a metal such as iron, nickel, platinum or cobalt. Non-limiting examples of suitable bond coat materials include metal aluminides such as nickel aluminide, platinum aluminide, or combinations thereof. The bond coating may have a thickness in the range of from about 25 microns to about 500 microns.
[0034] Thermal barrier coating may be disposed over the afore-mentioned bond coat or directly onto the substrate depending upon the desired application. The type of substrate may depend in on part on the turbine component. Non-limiting examples of suitable substrates include metals, metal alloys, or combinations thereof. In certain embodiments, the substrate includes an alloy of nickel, cobalt, iron, or combinations thereof. For example, the substrate may include a high temperature, heat-resistant alloy, e.g., a superalloy. Non-limiting examples of suitable high temperature nickel-based alloys include Inconel® alloys, Nimonic® alloys, Rene® alloys (e.g., Rene® 80 alloys, Rene® 95 alloys), Udimet® alloys, or combinations thereof.
[0035] Some embodiments of the present invention are directed to an article including a coating system disposed on the substrate or on the bond coat. The coating system described herein includes a thermal barrier coating and a CMAS-reactive material. In accordance with some embodiments of the invention, the CMAS-reactive material protects the thermal barrier coating by undergoing one or both of chemical and physical changes when in contact with a CMAS composition.
[0036] The term “CMAS” or “CMAS composition” as used herein refers to a contaminant composition including calcium, magnesium, aluminum and silicon. In some embodiments, the CMAS composition primarily includes a mixture of magnesium oxide, calcium oxide, aluminum oxide and silicon oxide. Non-limiting example of a suitable CMAS composition includes calcium oxide present in an amount in a range from about 1 wt% to about 60 wt% of the total CMAS composition; magnesium oxide present in an amount in a range from about 0 wt% to about 20 wt% of the total CMAS composition; aluminum oxide present in an amount in a range from about 10 wt% to about 30 wt% of the total CMAS composition; and silicon oxide present in an amount in a range from about 20 wt% to about 80 wt% of the total CMAS composition.
[0037] In some embodiments, other elements, such as nickel, iron, titanium and chromium, may also be present in the CMAS composition. In such instances, the additional elements may be present in a small amount, for example, less than about 10 weight percent of total amount of CMAS composition present. In some such instances, the CMAS composition may include about 29 wt % calcium oxide, about 7 wt % magnesium oxide, about 11 wt % aluminum oxide, and about 43 wt % silicon oxide. Further, the composition may include about 2 wt % nickel oxide, about 8 wt % iron oxide, and small amounts of titanium oxide and chromium oxide, such that the total weight % of these elements is less than 10 wt %. The CMAS composition may have a melting temperature less than about 1315°C (2399° F) in some embodiments, and less than about 1227 °C (2240 °F) in some other embodiments.
[0038] The particular compositional characteristics of the CMAS composition may depend on the source of the environmental contaminants and the reaction temperature. The CMAS composition is typically formed at operational temperatures of about 1000 °C. (1832 °F) or more. Sources of CMAS composition include, but are not limited to, sand, dirt, volcanic ash, fly ash, cement, runway dirt, fuel and air sources, oxidation and wear products from engine components, or combinations thereof.
[0039] As used herein, the term "CMAS-reactive material" refers to a material capable of reacting with a CMAS composition to form a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition. In some instances, the reaction product may form a glassy (typically thin) protective product that the CMAS deposits are either unable to adhere to, or are less able to adhere to.
[0040] In accordance with some embodiments of the invention, a suitable CMAS-reactive material has a nominal composition AB4-xDxO7, wherein A includes strontium, calcium, or a combination thereof, B includes a rare earth element, D includes aluminum, gallium, or a combination thereof, and x is a number less than or equal to 2. The term “nominal AB4-xDxO7 composition” as used herein means that some substitution of different elements at the crystal lattice A-sites, B-sites, D-sites, or O sites are encompassed by the composition as described. For example, some amount of fluorine, nitrogen, or other suitable anion may be substituted for the oxygen at the O site, and the resulting material is considered to be within the scope of the nominal composition, so long as the resultant material retains reactivity with CMAS-type materials. As used herein, the term “rare earth element” encompasses elements of the lanthanide series, yttrium, and scandium.
[0041] Without being bound by any theory, it is believed that some materials with nominal composition of AB4-xDxO7 crystallize, at ambient conditions, in a layered perovskite structure having DO6 octahedra, alternating with rocksalt blocks. The A and B cations in the AB4-xDxO7 structure may be distributed in the rock salt layer. In some embodiments, the CMAS-reactive materials disclosed herein have the layered perovskite-rock salt alternating structure. In some embodiments, the CMAS-reactive materials have a layered perovskite structure belonging to Ruddlesden-Popper subgroup.
[0042] A variety of elements may occupy A or B sites in the nominal composition AB4-xDxO7. For example, A may include an alkaline earth metal such as barium (Ba), strontium (Sr), calcium (Ca), magnesium (Mg), or any combinations of these. A may further include lanthanum (La), europium (Eu), or any combinations of these.
[0043] B includes a rare earth element, such as, for example, La, Eu, praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), lutetium (Lu), ytrrium (Y), scandium (Sc), indium (In), rhodium (Rh), or any combinations of these. Moreover, in some embodiments, the B component of the nominal composition AB4-xDxO7 may further include a transition metal element, exclusive of the rare earth elements. Examples of the transition metal element may include titanium (Ti), iron (Fe), vanadium (V), chromium (Cr), ruthenium (Ru) manganese (Mn), zirconium (Zr), hafnium (Hf), tungsten (W), molybdenum (Mo), thallium (Tl), or any combinations of these. In certain embodiments, the B site of AB4-xDxO7 composition is occupied by samarium, gadolinium, yttrium, erbium, lanthanum, neodymium, europium, or combinations of these elements along with any other suitable rare earth elements or transition metal elements.
[0044] In some embodiments, D may include aluminum (Al), gallium (Ga), or a combination of Al and Ga. The above-mentioned elements in the composition of AB4-xDxO7 can beneficially enhance one or more of the required performance characteristics such as CMAS reactivity, phase stability, coefficient of thermal expansion, and toughness. In some embodiments, one or more of the above-mentioned elements may additionally provide for lower thermal conductivity of the AB4-xDxO7 composition.
[0045] As noted, “x” is a number less than or equal to 2, including 0. Depending on the compositions and crystal structure, x may be an integer or a fraction. In some embodiments, x may be 0, resulting in a nominal composition of AB4O7. Without being bound by any theory, it is believed that some materials with nominal composition of AB4O7 crystallize, at ambient conditions, in a calcium diferrite type structure. In certain other embodiments, x may be 2, thereby resulting in an AB2D2O7 nominal composition. In embodiments wherein x is 1, aluminum, gallium or a combination of aluminum and gallium will replace only one of the rare earth element in the B site and therefore result in a nominal composition of AB3DO7.
[0046] Those skilled in the art will appreciate that substitution of various elements within the AB4-xDxO7 structure, such as those noted above, may be suitable so long as certain constraints such as charge compensation and lattice geometrical considerations can be met to maintain the crystal structure. For instance, where substitution of one or more cation sites (or filling one or more vacant sites) would result in a charge imbalance in the stoichiometric AB4-xDxO7, the composition of the oxide may shift to include slightly less or more oxygen to compensate for the apparent imbalance.
[0047] In certain embodiments, A in AB4-xDxO7 includes strontium. In some embodiments, A in AB4-xDxO7 includes calcium. In certain embodiments, a nominal composition of AB4-xDxO7 includes SrGd2Al2O7, SrGd4O7, CaGd4O7, SrLa4O7, or any combinations of these compositions.
[0048] In some embodiments, a nominal composition of AB4-xDxO7 includes SrB2Al2O7. In certain embodiments, a nominal composition of AB4-xDxO7 includes SrGd2Al2O7. Presence of aluminum or gallium in the D site of AB4-xDxO7 composition may aid in matching the thermal expansion coefficient to the substrate or to the thermal barrier coating, and hence may be beneficial for the overall thermal stability of the CMAS-reactive material in the coating system. SrGd2Al2O7 was experimentally found to be reactive with CMAS and form apatite phases, even at a temperature of about 1260 ºC. Thermal expansion and thermal conductivity of SrGd2Al2O7 were observed to be similar to that of 8YSZ at 1000 ºC.
[0049] In some embodiments, A site of AB4-xDxO7 includes calcium. Steam stability of calcium oxide was found to be greater than the steam stability of strontium oxide, in some embodiments. Therefore, presence of calcium in the A site of AB4-xDxO7 structure may be desirable to obtain a high-temperature stable compound, especially when working in environments containing steam. In some embodiments, a combination of strontium and calcium occupy the A site of the AB4-xDxO7 composition. In some embodiments calcium may be present in an amount in a range from about 5 atomic % to about 25 atomic % of the total A sites in the AB4-xDxO7 composition.
[0050] Without being bound by any theory it is believed that the materials with a disclosed nominal composition of AB4-xDxO7 are highly reactive with the CMAS composition and, therefore, may react with molten CMAS composition such that the kinetics of reaction of the AB4-xDxO7 material with the CMAS to form a stable, solid phase competes with the infiltration of molten CMAS into the pores of the thermal barrier coating. Accordingly, further penetration of the molten CMAS composition through the pores of the thermal barrier coatings may be avoided. Further, the AB4-xDxO7 materials may form substantially stable solid reaction products when in contact with the molten CMAS composition. Formation of the solid product phase may plug the vertical cracks in the thermal barrier coatings, and also increase the viscosity of the CMAS composition, thereby avoiding its infiltration, and thus extending the life of the thermal barrier coating layer.
[0051] This is in contrast to previously known thermal barrier coating systems, where the AB4-xDxO7-type materials are used as thermal barrier materials. Previously known AB4-xDxO7 materials are not known to be reactive with CMAS to form a resistant layer that can mitigate further infiltration of CMAS. Further, in the previously known thermal barrier coating systems, if an AB4-xDxO7 material is used as a thermal barrier coating, a part of the thermal barrier coating may be sacrificed for the CMAS mitigation, and the reaction product of CMAS and the TBC formed in the thermal barrier coating layer may unfavorably alter a chemical or structural configuration of the thermal barrier coating.
[0052] In some embodiments, the CMAS-reactive material may be disposed on the thermal barrier coating such that the CMAS-reactive material overlies the thermal barrier coating as a protective overlaying layer. In certain embodiments, the overlay CMAS-reactive material is disposed adjacent to the thermal barrier coating. In some embodiments, the CMAS-reactive material is disposed as the outermost layer (sometimes also referred to as “top coat layer”) in the article. In some other embodiments, the article may further include one or more additional layers disposed on the protective coating to form the top coat layer. Non-limiting examples of suitable top-coat layers include erosion resistant layers.
[0053] The protective overlaying layer disposed on the thermal barrier coating may be a dense layer or may have some porosity. In certain embodiments, the protective overlaying layer is a porous layer. A porosity of the protective overlaying layer may be in a range from about 5 volume percent to about 30 volume percent of the protective overlaying layer. In some embodiments, the protective overlaying layer includes a porosity in a range from about 10 volume percent to about 20 volume percent of the protective overlaying coating. The presence of a porous overlaying layer on a thermal barrier coating is contrary to conventional wisdom in the art, which typically envisions application of a continuous sealing layer for isolating the CMAS from the underlying thermal barrier coating. However, the present inventors have made the surprising discovery that a protective overlaying layer in accordance to certain embodiments described herein was effective in mitigating CMAS infiltration even when the porosity present in the protective overlaying layer was greater than 15 volume %.
[0054] In some embodiments, the protective overlaying layer has a thickness in a range from 5% to about 70% of the total thickness of the coating system. As described herein, when the protective overlaying layer of CMAS-reactive material is present adjacent to the thermal barrier coating, the thickness of the protective overlaying layer is measured from the top layer of the thermal barrier coating regardless of the CMAS-reactive material being present or absent in the pores, cracks, or voids of the thermal barrier coating. In some embodiments, the protective overlaying layer has a thickness that is less than about 50 % of the total thickness of the coating system. In some embodiments, the protective overlaying layer has a thickness in a range from about 50 microns to about 500 microns. In some embodiments, the layer has a thickness in a range from about 50 microns to about 100 microns.
[0055] In some embodiments, the protective overlaying layer has a thermal resistance that is lower than the thermal resistance of the underlying thermal barrier coating. In certain instances, the thermal resistance of the protective overlaying coating is less than a thermal resistance of a layer of same thickness formed by the typical materials that are used to form thermal barrier coatings. In certain embodiments, a normalized (for thickness) thermal resistance of the protective overlaying layer is less than the thermal resistance of 8% yttria doped zirconia thermal barrier coating.
[0056] FIG. 1 shows an article 100 including a substrate 110 and a coating system 120 disposed on the substrate 110. The coating system includes a thermal barrier coating 122 disposed on the substrate 110, and a protective overlaying layer 124 disposed on the thermal barrier coating 122. In the embodiment illustrated in FIG. 1, the protective overlaying layer 124 overlies and is disposed adjacent to the thermal barrier coating.
[0057] FIG. 2 illustrates another embodiment of the invention similar to FIG. 1, with the addition of a bond coating 130 disposed between the substrate 110 and the thermal barrier coating 122. In the embodiment illustrated in FIG. 2, the thermal barrier coating 122 overlies and is disposed adjacent to the bond coating 130.
[0058] FIG. 3 illustrates another embodiment of the invention similar to FIG. 2, with the addition of a top-coat layer 140 disposed on the protective overlaying layer 124. As mentioned previously, the top-coat layer 140 may include an erosion resistance material in some embodiments.
[0059] As noted earlier, the thermal barrier coatings typically include pores, channels, voids, or other cavities that may be infiltrated by molten environmental contaminants, such as, CMAS. Without being bound by any theory it is believed that the porous structure may be one of the factors that provides for strain tolerance by the thermal barrier coatings during thermal cycling. Further, the porous structure may additionally provide for stress reduction due to the differences between the coefficient of thermal expansion (CTE) of the coating and the CTE of the underlying bond coat layer/substrate.
[0060] Certain embodiments of present invention include thermal barrier coatings having surface-connected pores that incorporate the CMAS-reactive material such that the CMAS-reactive material is disposed within a plurality of the surface-connected pores. The CMAS-reactive material disposed within a plurality of the surface-connected pores preferentially reacts with molten CMAS that typically infiltrates the thermal barrier coating and ultimately degrades it, as described previously. In some embodiments, the CMAS-reactive material may be disposed within the plurality of surface-connected pores of the thermal barrier coating and also as a protective overlaying coating disposed on the thermal barrier coating.
[0061] As used herein, the term “surface-connected pore” refers to a pore that is open to the surface. The surface connected pore may be a single pore or a combination of a plurality of pores, wherein the pores are interconnected and eventually connected to the surface through one or more openings to the surface. Further, the “surface” as used herein in the context of “surface-connected pore” is the top surface of the referred layer. For example, a surface connected pore of a thermal barrier coating is the pore that is open to the top surface of the thermal barrier coating, regardless of presence or absence of a protective layer on top of the thermal barrier coating. A surface-connected pore of the coating system is a pore present in the top most layer of the coating system and open to the operating atmosphere.
[0062] The surface-connected pores of the thermal barrier coating may be the pores, channels, voids, or other cavities that were created by environmental damage, normal wear and tear of the thermal barrier coatings, or a result of the deposition processes, as disclosed earlier. In some instances, these pores, channels, or cavities may be created by environmental damage or the normal wear and tear during operation of the thermal barrier coatings. In some instances, the pores, channels, voids, or other cavities in the thermal barrier coating surface may result due to the deposition processes. For example, thermal barrier coatings that are deposited by (air) plasma spray techniques may result in a sponge-like porous structure of open pores in at least the surface of the coating. Similarly, thermal barrier coatings that are deposited by physical vapor deposition techniques, such as, for example, electron beam physical vapor deposition (EBPVD) may result in a porous structure including a series of columnar grooves, crevices or channels in at least the surface of the coating.
[0063] In some embodiments, a porosity of the surface of the thermal barrier coating is in a range from about 1 volume % to about 40 volume % of the surface, regardless of the depth of the surface-connected pores present in the thermal barrier coating. In some embodiments, a thermal barrier coating surface has a porosity in a range from about 10 volume % to about 30 volume % of the surface.
[0064] In certain embodiments, at least a portion of the surface-connected pores of the thermal barrier coating includes a plurality of elongated surface-connected voids. Electron beam physical vapor deposition (EBPVD) is a typical example of a process used for deposition of thermal barrier coating, which may result in the formation of elongated surface-connected voids. As used herein, the term “elongated surface-connected void” refers to a void that has an aspect ratio higher than 1, and is often oriented such that contaminants entering the void can be conducted into the cross-sectional thickness of the thermal barrier coating. In some embodiments, elongated surface-connected voids include substantially vertically oriented (from the perspective of a cross-sectional view) cracks, grain boundaries, or other microstructural features. In some embodiments, a CMAS-reactive material of nominal composition AB4-xDxO7 is disposed within at least some voids of the plurality of the elongated surface-connected voids. In some such instances, A of the nominal composition AB4-xDxO7 includes strontium and B includes samarium, gadolinium, yttrium, or any combinations of these elements.
[0065] FIG. 4 illustrates an article 200 in accordance with some embodiments of the present invention. Article 200 includes a coating system 220 disposed on a substrate 210 with optional bond coating 230 disposed between the coating system 220 and the substrate 210. The coating system 220 includes a thermal barrier coating 222 that has surface connected pores, such as, for example, elongated surface-connected voids 240 that allow access for environmental contaminants to the interior of thermal barrier coating 222. In some embodiments, a CMAS-reactive material 224 is disposed within at least some of the voids 240 of the thermal barrier coating 222. In certain embodiments, the CMAS-reactive material 224 is further disposed on the top of the thermal barrier coating 222, as shown in FIG. 4. The CMAS-reactive material 224 that is disposed on the top of the thermal barrier coating 222 may be a continuous layer, or may have pores that are connected (as shown by 242) to the surface connected voids 240 or disconnected (as shown by 244) from the surface-connected voids 240. In some embodiments, such as the illustrative embodiment shown in FIG. 4, CMAS-reactive material 224 decorates the surface of voids 240 in a discrete arrangement. In certain embodiments, the CMAS-reactive material 224 forms a continuous or substantially continuous structure within the voids 240.
[0066] Rare-earth elements are included in the CMAS-reactive material 224 in many of the various embodiments, as disclosed earlier. While several compounds including these elements may show desirable reactivity with CMAS, there may be certain deleterious effects attributable to the presence of these elements within thermal barrier coating 222. Notably, it has been discovered that where CMAS-reactive material 224 is disposed at or near the interface between thermal barrier coating 222 and bond coating 230, chemical interaction between the rare-earth-bearing CMAS-reactive material and an oxide formed by the bond coating at elevated temperature (known as a thermally-grown oxide or TGO)- can result in premature spallation of coating 220.
[0067] To mitigate this potential issue, some embodiments of the present invention employ a barrier material 250 (illustrated in FIG. 4) disposed to substantially separate CMAS-reactive material 224 from bond coating 230 or, if no bond coating is present, from the substrate 210. The barrier material 250 is present in at least a portion of the surface connected pores 240 preventing a contact of the CMAS-reactive material 224 with the bond coating 230 or substrate 210. In some embodiments, the barrier material 250 is interposed between the substrate 210 and the CMAS-reactive material 224 as illustrated in FIG. 4. Barrier material 250 substantially prevents chemical interaction between CMAS-reactive material 224 and a TGO disposed on substrate 210 or, if present, bond coating 230. In some embodiments, the barrier material 250 may be distributed throughout the pores, along with the CMAS-reactive material 224, as illustrated in FIG. 5.
[0068] In some embodiments, barrier material 250 includes aluminum oxide, cerium oxide, yttrium oxide, zirconium oxide, hafnium oxide, tantalum oxide, niobium oxide, titanium oxide, or combinations thereof. Some of these oxides may, where appropriate, include a sufficient amount of stabilizer (often a rare earth element) to reduce propensity of the oxide to undergo stress-generating phase transformations during heat-up and cool-down, but the amount of rare-earth stabilizer may be maintained sufficiently small to mitigate issues of reactivity with the aforementioned TGO. The upper limit of rare-earth stabilizer content depends on the particular element being used and the identity of the barrier material 250; for example, rare earth bearing aluminate garnets tend not to dissolve alumina, and thus may contain higher amounts of rare earth material without deleterious interaction with TGO. In some embodiments, barrier material 250 includes less than about 40 atomic per cent rare earth element content. In certain embodiments, barrier material includes less than about 10 atomic per cent rare earth element content, for example as found in the commonly used zirconia stabilized with 8 weight percent yttria (“8YSZ”). In some embodiments, the barrier material may not be as reactive with CMAS as CMAS-reactive material 224, and is typically separated from contact with CMAS by the CMAS-reactive material 224. In some embodiments, the function of barrier material is primarily to physically separate CMAS-reactive material 224 from bond coating 230 or substrate 210.
[0069] Coating system 220 is shown in FIG. 4 as a single layer, but in some embodiments, such as those illustrated in FIG. 6, coating system 220 may include a plurality of layers of thermal barrier coating 222. In the embodiment illustrated in FIG. 6, coating system 220 includes a first layer 252 of thermal barrier coating, and a second layer 254 of thermal barrier coating. First layer 252 is disposed between second layer 254 and substrate 210. The second layer 254 of thermal barrier coating includes CMAS-reactive material 224 and is resistant to CMAS infiltration. The first layer 252 of thermal barrier coating may include the barrier material.
[0070] In some embodiments, the article may further include one or more additional layers disposed on the CMAS-reactive material or on the thermal barrier coating to form the top coat layer (not shown in Figures). Non-limiting examples of suitable top-coat layers include erosion resistant layers. In certain other embodiments, the protective overlaying layer of the CMAS-reactive material is the outer most layer of the coating system that is exposed to the environment. In some embodiments, the protective overlaying layer of the CMAS-reactive material may further act as an erosion resistant layer.
[0071] As noted earlier, the CMAS-reactive material is present in the coating system in an effective amount to react with the CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition.
[0072] The term “effective amount” as used herein refers to an amount (for example, volume) of the CMAS-reactive material sufficient to effectively increase one or both of the melting temperature and viscosity of the reaction product formed.
[0073] The term “operating temperature” of the thermal barrier coating refers to the temperature that the thermal barrier coating is exposed to environment in the turbine. In some embodiments, the operating temperature of the thermal barrier coating refers to the surface temperature of the thermal barrier coating. The term “reaction product” as used herein refers to a product or a mixture of products formed by reacting the CMAS-reactive material with the CMAS composition. In certain embodiments, the reaction product may include a mixture of products. Accordingly the terms “reaction product” and “reaction product mixture” are used herein interchangeably. In some such instances, one or more products in the reaction product mixture may include new crystal phases that have a melting temperature greater than that of the CMAS composition. Further, in some such instances, the reaction product mixture may have a viscosity greater than that of the CMAS composition.
[0074] In some embodiments, it is desirable to have a higher melting point for the reaction product than the surface temperature of the thermal barrier coating, so that the reaction product may act as a solid barrier for the CMAS that is further deposited on the surface. In certain embodiments, the reaction product of CMAS and CMAS-reactive material has a melting temperature greater than about 1200 °C. In some embodiments, the reaction product has a melting temperature greater than about 1260 °C.
[0075] In some embodiments, the CMAS-reactive material is present in the coating system in an amount such that the melting temperature of the reaction product is increased at least to the surface temperature of the thermal barrier coating. In some embodiments, the CMAS-reactive material is present in the coating system in an amount such that the melting temperature of the reaction product increases by at least about 10 °C above the surface temperature of the thermal barrier coating during its operation. In some embodiments, the CMAS-reactive material is present in the coating system in an amount such that the melting temperature of the reaction product increases by about 40 °C to about 100 °C above the surface temperature of the thermal barrier coating during its operation. Thus, by way of an example, if the surface temperature of the thermal barrier coating during operation is about 1230 °C, then the CMAS-reactive material is present in amount such that the melting temperature of the reaction product increases to at least about 1240 °C.
[0076] In some embodiments, the CMAS-reactive material is present in the coating system in an amount such that the viscosity of the reaction product increases by at least about 10 centipoise above the viscosity of the CMAS composition, at the operating temperature of the thermal barrier coating. In some embodiments, the CMAS-reactive material is present in the coating system in an amount such that the viscosity of the reaction product increases by about 10 centipoise to about 1000000 centipoise above the viscosity of the CMAS composition, at the operating temperature of the thermal barrier coating.
[0077] In some embodiments, the CMAS-reactive material is present in the coating system in an amount in a range from about 0.5 volume percent to about 75 volume percent. In some embodiments, the CMAS-reactive material is present in the coating system in an amount in a range from about 10 volume percent to about 50 volume percent. In some embodiments, the CMAS-reactive material is present in the coating system in an amount in a range from about 10 volume percent to about 25 volume percent.
[0078] In some of the embodiments wherein the CMAS-reactive material is present as a protective overlaying layer, the protective overlaying layer consists essentially of the CMAS-reactive material. That is, in these embodiments, the protective overlaying layer includes the CMAS-reactive material in an amount of up to 100 volume % without any other intentional additives to the protective overlaying layer. In some embodiments, the protective overlaying layer consisting essentially of the CMAS-reactive material is sufficient to protect the thermal barrier coating, at least partially against depositing CMAS.
[0079] In some of the embodiments, wherein the coating system is in operation or has previously reacted with some CMAS, the protective overlaying layer consists essentially of the CMAS-reactive material and the reaction product of the CMAS and the CMAS-reactive material. That is, in these embodiments, the protective overlaying layer is essentially a mixture of the CMAS-reactive material and the reaction product, without any other intentional additives to the protective overlaying layer.
[0080] In some other embodiments wherein the CMAS-reactive material is present as a protective overlaying layer, the protective overlaying layer may further include additional materials (for example, a ceramic thermal barrier coating material) to render the protective overlaying layer more compatible (for example, CTE matching) with the thermal barrier coating. In some embodiments, the protective overlaying layer includes from about 20 volume % to about 100 volume % CMAS-reactive material and from about 0 volume % to about 80 volume % ceramic thermal barrier coating material. In some embodiments, the protective overlaying layer includes from about 40 volume % to about 60 volume t% CMAS-reactive material and from about 40 volume % to about 60 volume % ceramic thermal barrier coating material.
[0081] As mentioned previously, in some embodiments, the CMAs-reactive material may be disposed in the pores of the thermal barrier coating, regardless of a presence or absence of the protective overlaying layer. In some such instances, the coating system may have different effective amount of CMAS-reactive material in the surface-connected pores as compared to the amount present in the protective overlaying layer. In some embodiments, the effective amount of CMAS-reactive material present in the surface-connected pores of the coating system may depend in part on the specific volume of the reaction product formed between CMAS and the CMAS-reactive material, and on the structure and microstructure of the coating system.
[0082] In certain embodiments, wherein the CMAs-reactive material is disposed in the thermal barrier coating having surface-connected pores, at least about 5% by volume of the pores of the coating system is occupied by CMAS-reactive material. In some embodiments, the CMAS-reactive material occupies about 10 volume % to about 70 volume % of an overall porosity of the coating system. In some embodiments, the CMAS-reactive material is disposed in an amount in a range from about 20 volume percent to about 70 volume percent of the plurality of surface-connected pores. In some embodiments, the volume fraction of the porosity that is occupied by CMAS-reactive material is a function of the cross-sectional depth, with a comparatively higher concentrations of CMAS-reactive material at or near the thermal barrier coating surface trending to comparatively low concentrations as distance from the thermal barrier coating surface increases (that is, as distance away from substrate decreases). For instance, as an illustrative non-limiting example, an occupied (by CMAS-reactive material or a combination of CMAS-reactive material and the reaction product of CMAs and CMAs-reactive material) porosity is at least about 25% by volume at the surface of the thermal barrier coating , trending toward about 5% by volume at a point below the half-thickness of the thermal barrier coating.
[0083] The coating systems of the present invention may be useful in a wide variety of turbine components (e.g., turbine engine components) that are operated at, or exposed to, high temperatures. Non-limiting examples of suitable turbine engine components include turbine airfoils such as blades and vanes, turbine shrouds, turbine nozzles, buckets, combustor components such as liners and deflectors, heat shields, augmentor hardware of gas turbine engines, and the like. The coatings systems of the present invention may be disposed over a portion or over all of the metal substrate. For example, with regard to airfoils such as blades, the coating systems of the present invention are typically used to protect, cover or overlay portions of the metal substrate of the airfoil other than solely the tip thereof, for example, the thermal barrier coatings cover the leading and trailing edges and other surfaces of the airfoil.
[0084] In some embodiments, a turbine engine component is also presented. The turbine engine component includes a thermal barrier coating disposed on a superalloy substrate, and a CMAS-reactive material disposed on the thermal barrier coating. The CMAS-reactive material may be disposed on the thermal barrier coating as a protective overlaying layer, deposited within at least some of the surface-connected pores of the thermal barrier layer, or may be present within the pores and also as an overlaying layer. The CMAS-reactive material includes a nominal composition AB4-xDxO7, wherein A includes strontium, calcium, or a combination thereof, B includes a rare earth element, D includes aluminum, gallium, or a combination thereof, and x is a number less than or equal to 2, and the CMAS-reactive material is present in the protective overlaying layer in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition.
[0085] In some embodiments, a method of manufacturing an article is presented. The method includes disposing a thermal barrier coating on a substrate; and disposing a CMAS-reactive material on the thermal barrier coating. As mentioned earlier, the CMAS-reactive material is present in the coating system in an effective amount to react with a CMAS composition at an operating temperature of the thermal barrier coating, thereby forming a reaction product having one or both of melting temperature and viscosity greater than that of the CMAS composition, and the CMAS-reactive material has a nominal composition AB4-xDxO7, wherein A includes strontium, calcium, or a combination thereof, B includes a rare earth element, D includes aluminum, gallium, or a combination thereof, and x is a number less than or equal to 2.
[0086] The thermal barrier coating may be disposed or otherwise formed on a bond coating (if present) or on the substrate directly by any of a variety of conventional techniques, including vapor disposition, such as physical vapor deposition (PVD), electron beam physical vapor deposition (EBPVD); plasma spray, such as air plasma spray (APS), suspension plasma spray (SPS), and vacuum plasma spray (VPS); other thermal spray deposition methods such as high velocity oxy-fuel (HVOF) spray, detonation, or wire spray; chemical vapor deposition (CVD), sol-gel method, or combinations of two or more of the afore-mentioned techniques
[0087] The particular technique used for disposing, depositing or otherwise forming the thermal barrier coating may depend on one or more of the composition of the thermal barrier coating, the thickness, and the physical structure desired for the thermal barrier coating. In certain embodiments, the thermal barrier coating is disposed on the substrate using plasma spray techniques. Various types of plasma-spray techniques are well known to those skilled in the art, and may be utilized to dispose the thermal barrier coatings of the present invention.
[0088] In some embodiments, the thermal barrier coating may be disposed on the bond coating. In such instances, the bond coating may be applied, deposited or otherwise formed on the substrate by any of a variety of conventional techniques including, vapor disposition, such as physical vapor deposition (PVD), electron beam physical vapor deposition (EBPVD); plasma spray, such as air plasma spray (APS) and vacuum plasma spray (VPS); other thermal spray deposition methods such as high velocity oxy-fuel (HVOF) spray, detonation, or wire spray; chemical vapor deposition (CVD), sol-gel method, or combinations of two or more of the afore-mentioned techniques. In some embodiments, a plasma spray technique, such as that used for the thermal barrier coating, may be employed to dispose the bond coating on the substrate.
[0089] In some embodiments, the method of forming an article 100 (as illustrated in FIG.1, FIG. 2, and FIG. 3) further includes disposing the protective overlaying layer 124 on the thermal barrier coating 122. In embodiments wherein the protective overlaying layer 124 primarily includes of the CMAS-reactive material, the CMAS-reactive material may be applied, deposited or formed on the thermal barrier coating using one or more of the afore-mentioned techniques used to dispose the thermal barrier coating 122. In embodiments, wherein the protective overlaying layer 124 further includes a ceramic thermal barrier material, the CMAS-reactive material and the ceramic thermal barrier material may be co-deposited on the thermal barrier coating 122. In some embodiments, co-depositing may be achieved by blending, mixing or otherwise combining the CMAS-reactive material and the ceramic thermal barrier coating material together (for example, as powders) to provide a mixture that is then deposited onto the thermal barrier coating 122. In some embodiments, co-depositing may be achieved by separately depositing onto the thermal barrier coating 122 (e.g., as separate plasma spray streams) the respective CMAS-reactive material and ceramic thermal barrier coating material in a manner such that these materials blend, mix or otherwise combine together to form a mixture.
[0090] A method for making articles such as article 200 (as illustrated in FIG. 4, FIG. 5, and FIG. 6) may include disposing CMAS-reactive material 224 within surface connected voids 240 of thermal barrier coating 222 at an effective amount to substantially prevent incursion of CMAS materials into the voids 240. While other techniques have been described in which material is disposed on internal surfaces of ceramic coatings, for instance to prevent sintering of columnar microstructural features, the present technique according to some embodiments involves the disposition of an effective amount of CMAS-reactive material 250 to substantially prevent incursion of CMAS into voids in which the CMAS-reactive material 250 is disposed.
[0091] Disposing CMAS-reactive material typically involves infiltrating an existing thermal barrier coating 222 (as illustrated in FIG. 4, FIG. 5, and FIG. 6) with a vapor or liquid into the surface-connected voids 240 of the thermal barrier coating 222. In the case of a vapor infiltrant, CMAS-reactive material 224 may be formed by chemical interaction with the environment within voids 240 such as by reaction with material of thermal barrier coating 222. Liquid infiltrants, on the other hand, include one or more liquids such as water, or a carbon-bearing liquid such as an alcohol or acetone. In one embodiment involving a liquid infiltrant, the liquid includes a carrier fluid and a plurality of particles suspended within the carrier fluid. The particles may include the CMAS-reactive material 224 composition, or may be a chemical precursor to this composition, designed to further react during processing or during service to produce CMAS-reactive material 224. In an alternative embodiment, the liquid includes a solvent, with a solute dissolved in the solvent. The solute may be a precursor of CMAS-reactive material 224, such as a nitrate, sulfate, other salt, or other compound type, and the solvent is selected to appropriately accommodate the desired solute. More than one solute may be dissolved in the solvent. The solute may be reacted to form CMAS-reactive material 224, such as by heating to decompose the solute, or by reacting multiple solutes together, or by reacting one or more solute with the material of coating 220, or some combination of these.
[0092] The liquid infiltrant may be infiltrated into the surface-connected voids 240 using any appropriate technique. In some embodiments, the liquid is simply placed in contact with coating 220, such as by dipping or brushing, allowing capillary action to draw the liquid and CMAS-reactive material 224 (or precursor thereof) into the voids 240. Vacuum infiltration techniques are applied in some embodiments to further assist in driving liquid into coating 220. Other techniques such as electrophoretic deposition may be used in conjunction with a suspension to deposit particles of CMAS-reactive material 224 or a precursor of CMAS-reactive material 224 into voids 240. Use of electrophoretic deposition to deposit material within the voids of a ceramic coating is described by Hasz in US Patent number 7,780,832.
[0093] Where a liquid infiltrant is applied, whether the liquid is carrying a suspension of particles or has a solute dissolved in it, in some embodiments the method further includes volatilizing the liquid to form a residue that is disposed in voids 240. The residue may be a precursor to CMAS-reactive material 224, or it may be the CMAS-reactive material 224 composition itself. Volatilizing is typically done by heating the infiltrated coating to a temperature where the liquid is driven off at an acceptable rate. Often the heating rate (“ramp-rate”) to attain the desired temperature for volatilization is controlled to avoid building up undue pressure within the coating, which could damage the coating and/or could result in incomplete deposition of CMAS-reactive material 224 due to liquid being forced out by rapid bubble formation and escape.
[0094] As illustrated in FIG. 4, FIG. 5, and FIG. 6, in some embodiments, the method further includes interposing barrier material 250 between CMAS-reactive material 224 and substrate 210, for instance by disposing barrier material 250 within pores 240 where the substrate 210 and thermal barrier coating 222 meet. This disposition of barrier material 250 may be accomplished by any means, such as those described above for disposing CMAS-reactive material 224. In some embodiments, a suspension of particles comprising the desired barrier material composition, or combinations thereof, is infiltrated into the voids 240 of coating. The liquid portion of this suspension is volatilized, and then the barrier material 250 is disposed as described above. In alternative embodiments, a liquid solution of a precursor of the barrier material is infiltrated into the coating 220, the liquid is driven off, leaving a residue within the voids 240 that is later reacted to form the barrier material. This reaction can occur prior to, during, or after disposition of the CMAS-reactive material 224, depending on the desired processing and materials distribution.
[0095] Other techniques may be applied to provide barrier material 250 or to otherwise protect substrate 210 or bond coating 230 from interacting with CMAS-reactive material 224. In one embodiment, as illustrated in FIG. 6, a first layer 252 of thermal barrier coating is applied, such as for example, by electron-beam physical vapor deposition (EBPVD). This first layer 252 is infiltrated as described above to deposit barrier material 250 within voids (such as between columns). The infiltrated coating is then cleaned and coated with a second layer 254 of thermal barrier coating. This subsequent (second) layer 254 may be the same material as used in the first layer 252, or may be a different material, as noted above for the structure described in FIG. 6. The subsequent layer 254 is then infiltrated and processed to dispose CMAS-reactive material 224 within its voids 240. In another embodiment, the first layer 252 of coating is deposited using a deposition technique that provides a comparatively high density coating. This dense initial layer may have sufficient density to serve as a barrier between the CMAS-reactive material 224 and substrate 210 or bond coating 230. A subsequent technique or change in deposition parameters may then be applied to deposit the subsequent layer 254 of comparatively porous material over the initial (first) comparatively dense layer. This subsequent layer 254 is then subjected to infiltration and further processing to dispose CMAS-reactive material 224 within its voids 240.
[0096] The coating systems in accordance with some embodiments of the present invention are particularly useful in providing protection or mitigation against the adverse effects of environmental contaminant compositions for thermal barrier coatings used with metal substrates of newly manufactured articles. However, the coating systems of the embodiments presented herein are also useful in providing such protection or mitigation against the adverse effects of environmental contaminant compositions for refurbished worn or damaged thermal barrier coatings, or in providing thermal barrier coatings having such protection or mitigation for articles that did not originally have a thermal barrier coating.
[0097] According to embodiments of the invention, the thermal barrier coatings are provided with at least partial and up to complete protection and mitigation against the adverse effects of environmental contaminant compositions that can deposit on the surface of such coatings during normal turbine operation. In particular, the thermal barrier coatings of the present invention are provided with at least partial and up to complete protection or mitigation against the adverse effects of CMAS deposits on such coating surfaces. In addition to turbine engine parts and components, the coating system of the present invention may provide useful protection for metal substrates of other articles that operate at, or are exposed, to high temperatures, as well as to environmental contaminant compositions.
EXAMPLES
[0098] Synthesis of AB4-xDxO7 materials: CaGd4O7 and SrGd2Al2O7 were synthesized by firing stoichiometric mixtures at 1200 °C -1600 °C for a time duration of about 6-12 hours. All samples were checked for phase formation with a Rigaku X-ray diffractometer using CuKa radiation. Single phase materials were obtained for SrGd2Al2O7 and CaGd4O7 compositions as evidenced by powder X-ray diffraction (PXRD).
Example 1 Reaction of CMAS with CaGd4O7 and SrGd2Al2O7 at 1400 °C.
[0099] CaGd4O7 and SrGd2Al2O7 powders were separately mixed with CMAS composition at 1400 °C for 15 minutes. The ratio of AB4-xDxO7 material to the CMAS composition was 1:6 by volume. FIG. 7 shows the PXRD pattern for reaction product 260 of CaGd4O7 with CMAS at 1400 °C, along with reference pattern for CaGd4O7 262, and a reference pattern of an expected apatite phase 264 of the reaction product of CaGd4O7 with CMAS. FIG. 8 displays the PXRD pattern for reaction product 266 of SrGd2Al2O7 with CMAS 1400 °C, along with reference pattern for SrGd2Al2O7 268, and a reference pattern of an expected apatite phase 270 of the reaction product of SrGd2Al2O7 with CMAS. Substantial reaction of CaGd4O7 and SrGd2Al2O7 with CMAS was observed, forming crystalline apatite type major phases, with some amount of glassy phases at 1400 °C.
Example 2. Reaction of CMAS with CaGd4O7 and SrGd2Al2O7 at 1260 °C.
[0100] CaGd4O7 and SrGd2Al2O7 powders were separately mixed with CMAS composition at 1260 °C for 15 minutes and air-quenched, keeping the ratio of AB4-xDxO7 material to the CMAS composition as 1:6 by volume. FIG. 9 shows the PXRD pattern for reaction product 272 of CaGd4O7 with CMAS at 1260 °C, along with reference XRD pattern for CaGd4O7 262 and a reference product (calcium-gadolinium-silicate-oxide) XRD pattern 264. From the PXRD pattern, the products of CaGd4O7 with CMAS were found to have a substantial apatite phase with a very little amount of CaGd4O7 as displayed in FIG. 9. SrGd2Al2O7 reacted with CMAS at the temperature of 1260 °C forming a crystalline apatite major phase, and a minor glassy phase. FIG. 10 shows the PXRD pattern 274 for reaction product of SrGd2Al2O7 with CMAS at 1260 °C, along with reference pattern for SrGd2Al2O7 268, and a reference pattern of an expected apatite phase 270 of the reaction product of SrGd2Al2O7 with CMAS. Substantial presence of the apatite phase with a minor presence of SrGd2Al2O7 phase suggested that SrGd2Al2O7 had reacted with CMAS at 1260 °C. An extended dwell time at 1260 °C may result in complete reactions of CaGd4O7 and SrGd2Al2O7 with CMAS.
[0101] FIG. 11 shows a differential scanning calorimetry (DSC) data for a reaction mixture of SrGd2Al2O7 with CMAS in 1:1.1 volume ratio. Multiple exotherms were observed in the first heat up, illustrated by curve 280, indicating a reaction between the SrGd2Al2O7 and CMAS. A solidification step was not observed during the first cool down 282 indicating the absence of CMAS during cool down. Smooth curves observed in the second heat up 284 and second cool down 286 indicated completion of the reaction, and formation of a higher melting point reaction product in the first heat up.
[0102] The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, which are within the scope of the invention and the appended claims. Furthermore, all of the patents, patent applications, articles, and texts which are mentioned above are incorporated herein by reference.