Claims:1. An article comprising:

a coating disposed on a substrate, the coating comprising a plurality of surface-connected voids; and

a protective agent disposed within at least some of the voids of the coating;

wherein the protective agent comprises an oxide, the oxide comprising an alkaline earth element and a rare-earth element.

2. The article of claim 1, wherein the protective agent is present in the coating in an effective concentration to substantially prevent incursion by nominal CMAS into voids in which the protective agent is disposed.

3. The article of claim 1, wherein the alkaline earth element comprises calcium, strontium, or a combination comprising one or more of the foregoing.

4. The article of claim 1, wherein the alkaline earth element comprises strontium.

5. The article of claim 1, wherein the rare earth element comprises lanthanum, neodymium, erbium, cerium, gadolinium, or a combination comprising any of the foregoing.

6. The article of claim 1, wherein the rare earth element comprises gadolinium.

7. The article of claim 1, wherein the oxide further comprises aluminum.

8. The article of claim 1, wherein the oxide further comprises tantalum, niobium, or a combination comprising any of the foregoing.

9. The article of claim 1, wherein the coating comprises a plurality of layers.

10. The article of claim 9, wherein the coating comprises a first layer comprising a first material and a second layer comprising a second material, the first layer disposed between the second layer and the substrate,
wherein the second material is more resistant to infiltration by nominal CMAS relative to 8 weight percent yttria-stabilized zirconia at a temperature of 1300 degrees Celsius.

11. The article of claim 10, wherein the second material comprises yttria-stabilized zirconia, with a yttria content greater than about 38 weight percent.

12. The article of claim 11, wherein the yttria content is at least about 55 weight percent.

13. The article of claim 1, further comprising a barrier agent interposed between the substrate and the protective agent.

14. The article of claim 13, wherein the barrier agent comprises less than 34 atomic per cent rare earth elements.

15. An article comprising:

a coating disposed on a substrate, the coating comprising a plurality of surface-connected voids; and

a protective agent disposed within at least some of the voids of the coating;

wherein the protective agent comprises an oxide, the oxide comprising strontium and gadolinium.

16. A method, comprising:
disposing a protective agent within a plurality of surface-connected voids of a coating;
wherein the protective agent comprises an oxide, the oxide comprising an alkaline earth element and a rare-earth element.

17. The method of claim 16, wherein disposing comprises infiltrating the voids with a liquid.

18. The method of claim 17, wherein the liquid comprises a carrier fluid and a plurality of particles suspended within the carrier fluid.

19. The method of claim 17, wherein the liquid comprises a solvent and a solute dissolved in the solvent.

20. The method of claim 19, wherein the solute comprises a precursor of the oxide.

21. The method of claim 20, further comprising reacting the precursor to form the oxide.

22. The method of claim 17, further comprising volatilizing the liquid to form a residue disposed in the voids.

23. The method of claim 16, further comprising interposing a barrier agent between the protective agent and a substrate upon which the coating resides.
24. The method of claim 16, wherein the alkaline earth element comprises strontium.

25. The method of claim 16, wherein the rare-earth element comprises gadolinium.

26. The method of claim 16, wherein the oxide further comprises aluminum.

27. The method of claim 16, wherein the oxide further comprises tantalum, niobium, or a combination comprising any of the foregoing.
, Description:BACKGROUND
[0001] This disclosure generally relates to articles employing thermally protective coatings. More particularly, this disclosure relates to articles employing coatings that are resistant to degradation due to high-temperature interactions with dust materials.
[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, 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; dirt, ash, and dust ingested by gas turbine engines, for instance, are often made up of such compounds. These oxides often combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide systems (Ca-Mg-Al-Si-O), hereafter referred to as "CMAS." At the high turbine operating temperatures, these environmental contaminants can adhere to the hot thermal barrier coating surface, and 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 fill its porous structure by infiltrating the pores, channels, cracks, 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, and methods for making such coatings, that provide protection from the adverse effects of deposited CMAS.
BRIEF DESCRIPTION
[0005] Embodiments of the present invention are provided to meet this and other needs. One embodiment is an article. The article comprises a coating disposed on a substrate. The coating comprises a plurality of surface-connected voids. The article further includes a protective agent disposed within at least some of the voids of the coating; the protective agent comprises an oxide, the oxide comprising an alkaline earth element and a rare-earth element.
[0006] Another embodiment is a method. The method includes disposing a protective agent within a plurality of surface-connected voids of a coating, wherein the protective agent comprises an oxide, the oxide comprising an alkaline earth element and a rare-earth element.
DRAWINGS
[0007] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawing in which like characters represent like parts, wherein:
[0008] Figure 1 is a schematic cross-sectional view of one embodiment of the present invention; and
[0009] Figure 2 is a schematic cross-sectional view of another embodiment of the present invention.
DETAILED DESCRIPTION
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] Embodiments of the present invention incorporate a coating that includes one or more substances, referred to herein collectively as “protective agents,” disposed within spaces in the coating, that is, surface-connected voids such as cracks, pores, voids, and the like, through which molten CMAS typically infiltrates the coating and ultimately degrades it as described previously. A protective agent is designed to be highly reactive to CMAS-type material, such that, at typical temperatures where CMAS is encountered in liquid form, the protective agent rapidly reacts with the CMAS to form a solid reaction product that itself is thermally and chemically stable in the presence of liquid CMAS, forming a solid-phase barrier against further CMAS ingress.
[0015] To perform the function described above, a “protective agent” includes a substance that is reactive with CMAS material. More particularly, a substance is considered suitable as a substance for use in the protective agent as described herein if the substance has the characteristic property, that is, the capability, of chemically reacting with a nominal CMAS liquid composition at atmospheric pressure to form a solid, crystalline product. The solid crystalline product has a higher melting temperature than the nominal CMAS composition so that it remains as a solid barrier to liquid infiltration.
[0016] For the purposes of this description, the term “nominal CMAS” refers to the following composition, with all percentages in mole percent: 41.6% silica (SiO2), 29.3% calcia (CaO), 12.5% alumina (AlO1.5), 9.1% magnesia (MgO), 6.0% iron oxide (FeO1.5), and 1.5% nickel oxide (NiO). It will be appreciated that the nominal CMAS composition given in this definition represents a reference composition to define a benchmark for the a substance’s CMAS reactivity in a way that can be compared to the CMAS reactivity of other substances; use of this reference composition does not limit in any way the actual composition of ingested material that becomes deposited on the coating during operation which, of course, will vary widely in service. If a given substance is capable of reacting with molten CMAS having the above nominal composition, thereby forming a reaction product that has a melting point higher than about 1200 degrees Celsius, then the substance may be useful in the protective agent as described herein.
[0017] Referring to Figure 1, an article 100 in accordance with one embodiment of the present invention includes a coating 110 disposed on a substrate 120. The coating 110 includes surface-connected voids 130 such as cracks and porosity that allow access for environmental contaminants to the interior of coating 110. Typically the voids 130 of highest interest are elongated, that is, they have an aspect ratio higher than 1, and are often oriented such that contaminants entering the void 130 can be conducted into the cross-sectional thickness of the coating 110. In some embodiments, voids 130 include substantially vertically oriented (from the perspective of a cross-sectional view as in Figure 1) cracks and/or boundaries of grains or other microstructural features. These voids 130 may be present due to inherent characteristics of deposition processes used to deposit the coating 110; some voids 130 may also form after deposition due to normal wear and tear during operation.
[0018] Coating 110, such as a thermal barrier coating, may be applied by any technique suitable for a given application. Coatings that are deposited by air plasma spray techniques, for instance, may result in a sponge-like porous structure of open pores in at least the surface of the coating. Under certain deposition conditions, well developed, vertically oriented (relative to the plane of the substrate/coating interface) cracks are also formed by plasma spraying thermal barrier coating materials. Similarly, thermal barrier coatings that are deposited by physical- or chemical- vapor deposition techniques may result in a structure including a series of columnar grooves, crevices or channels in at least the surface of the coating. A porous structure, especially (though not exclusively) a structure incorporating vertically oriented and/or columnar features as noted above, may be one of the factors that provides for strain tolerance by the thermal barrier coatings during thermal cycling. Further, the porous structure may 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.
[0019] An optional bond coat 140 is disposed between coating 110 and substrate 120 in some embodiments. Bond coat 140 provides functionality—adhesion promotion and oxidation resistance, for example—similar to what such coatings generally provide in conventional applications. In some embodiments, bond coat 140 comprises an aluminide, such as nickel aluminide or platinum aluminide, or a MCrAlY-type coating well known in the art. These bond coats may be especially useful when applied to a metallic substrate 120, such as a superalloy. In other embodiments, bond coat 140 comprises a silicide compound or elemental silicon, which are often associated with ceramic-based substrates, such as silicon carbide-reinforced silicon carbide ceramic matrix composites (CMC’s). These coatings 140 may be applied using any of various coating techniques known in the art, such as plasma spray, thermal spray, chemical vapor deposition, or physical vapor deposition.
[0020] Article 100 may be any component that is subject to service in a high-temperature environment, such as a component of a gas turbine assembly. Examples of such components include, but are not limited to, components that include turbine airfoils such as blades and vanes, and combustion components such as liners and transition pieces. Substrate 120, then, may be any material suitable for use in such applications; examples include nickel-base superalloys, cobalt-base superalloys, and ceramic matrix composites, to name a few.
[0021] Coating 110 generally includes a ceramic thermal barrier material. Suitable ceramic thermal barrier coating materials include various types of oxides, such as hafnium oxide (“hafnia”) or zirconium oxide (“zirconia”), in particular stabilized hafnia or stabilized zirconia, and blends including one or both of these. Examples of stabilized zirconia include, without limitation, yttria-stabilized zirconia, ceria-stabilized zirconia, calcia-stabilized zirconia, scandia-stabilized zirconia, magnesia-stabilized zirconia, india-stabilized zirconia, ytterbia-stabilized zirconia, lanthana-stabilized zirconia, gadolinia-stabilized zirconia, as well as mixtures of such stabilized zirconia. Similar stabilized hafnia compositions are known in the art and suitable for use in embodiments described herein.
[0022] In certain embodiments, coating 110 includes yttria-stabilized zirconia. Suitable yttria-stabilized zirconia may include from about 1 weight percent to about 20 weight percent yttria (based on the combined weight of yttria and zirconia), and more typically from about 3 weight percent to about 10 weight percent yttria, although in certain cases even higher yttria content is desirable (see below). An example yttria-stabilized zirconia thermal barrier coating includes about 7% yttria and about 93% zirconia. These types of zirconia 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 material. In some embodiments, the thermal barrier coating material may further include an additional metal oxide, such as, titania.
[0023] Suitable ceramic thermal barrier coating materials may also include pyrochlores of general formula A2B2O7 where A is a metal having a valence of 3+ or 2+ (e.g., gadolinium, aluminum, cerium, lanthanum or yttrium) and B is a metal having a valence of 4+ or 5+ (e.g., hafnium, titanium, cerium or zirconium) where the sum of the A and B valences is 7. Representative materials of this type include gadolinium zirconate, lanthanum titanate, lanthanum zirconate, yttrium zirconate, lanthanum hafnate, cerium hafnate, and lanthanum cerate.
[0024] Coating 110 may include the ceramic thermal barrier coating material in an amount of up to 100 weight percent. In some embodiments, the coating 110 includes the ceramic thermal barrier coating material in a range from about 95 weight percent to about 100 weight percent and more particularly from about 98 weight percent to about 100 weight percent. The selected composition of coating 110 may depend upon one or more factors, including the composition of the optional, adjacent bond coat layer 140 (if present), the coefficient of thermal expansion (CTE) characteristics desired for coating 110, and the thermal barrier properties desired for coating 110.
[0025] The thickness of coating 110 may depend upon the substrate or the component it is deposited on. In some embodiments, coating 110 has a thickness in a range of from about 25 microns to about 2000 microns. In some embodiments, coating 110 has a thickness in a range of from about 25 microns to about 1500 microns. In some embodiments, the thickness is in a range of from about 25 microns to about 1000 microns.
[0026] Referring again to Figure 1, a protective agent 150 is disposed within at least some of the voids 130 of coating 110. Protective agent 150 has the characteristics described above with respect to the reactivity of its component substance(s) with CMAS. In some embodiments, such as the illustrative embodiment shown in Figure 1, agent 150 decorates the surface of voids 130 in a discrete arrangement, while in other embodiments agent forms a continuous or substantially continuous structure within voids 130. 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 involves the disposition of an effective concentration of agent 150 to substantially prevent incursion of CMAS, such as the nominal CMAS composition used as a benchmark, above, into voids in which agent 150 is disposed. This effective concentration of protective agent 150 will depend in large part on the specific volume of the reaction product formed between CMAS and the agent 150, and on the microstructure of coating 110. For instance, where the width of voids 130 is comparatively large, or where the volume fraction of voids is comparatively high, the amount of agent 150 needed to be effective in stopping incursion of molten CMAS will be higher than in instances where void width and/or void volume fraction is comparatively low. In certain embodiments, at least about 25% by volume of the void fraction (porosity) of coating 110 is occupied by agent 150. In some embodiments, the volume fraction of the porosity that is occupied by agent 150 is a function of the cross-sectional depth, with a comparatively higher concentrations of agent at or near the coating surface trending to comparatively low concentrations as distance from the coating surface increases (that is, as distance away from substrate 120 decreases). For instance, as an illustrative, non-limiting example, the occupied porosity is at least about 25% by volume at the surface of coating 110, trending toward about 5% by volume at a point below the half-thickness of the coating 110.
[0027] As noted previously, protective agent 150 includes one or more substance that is highly reactive with liquid CMAS, and forms a reaction product that can serve as a barrier to prevent further incursion of liquid CMAS into voids 130, thereby helping to maintain strain tolerance of coating 110. Typically, the protective agent comprises an oxide. In accordance with embodiments described herein, the oxide generally comprises an alkaline earth element and a rare-earth element.
[0028] An “alkaline earth element” as that term is used herein means any of the elements beryllium, magnesium, calcium, strontium, barium, and radium, occupying Group IIA (2) of the periodic table of the elements. Strontium in particular has demonstrated a remarkable capability to promote fast reaction with CMAS, and it is expected that other alkaline earth elements, such as calcium, may also behave similarly to strontium. Thus, in some embodiments, the alkaline earth element of the protective agent oxide includes strontium, and in certain embodiments the oxide includes strontium, calcium, or a combination that includes one or both of these.
[0029] As used herein, the terms “rare-earth” and “rare-earth element” are used interchangeably, and encompass elements of the lanthanide series, yttrium, and scandium. For example, in some embodiments, the oxide includes lanthanum, neodymium, erbium, cerium, gadolinium, or combinations including any one or more of these. Gadolinium in particular has been observed to promote the formation of an apatite phase when exposed to liquid CMAS; the apatite phase has a comparatively high melting point, and serves to inhibit CMAS infiltration, and so in certain embodiments the rare earth element of the oxide includes gadolinium. It is expected that other rare earth elements may promote similar behavior as that observed for gadolinium.
[0030] The oxide of the protective agent described herein thus includes an alkaline earth element and a rare-earth element, and in certain embodiments the oxide further includes additional elements. For example, as described in India Patent Application No. 3695/CHE/2015, filed 18 July 2015, oxides 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, have been shown to react with liquid CMAS to form solid phases such as an apatite-type phase. Moreover, in US Patent Application 14/576,760, oxides that include tantalum and/or niobium in addition to alkaline earth and rare-earth components may show desirable reactivity with molten CMAS. Thus, to take advantage of the potential benefits of such additions, in certain embodiments the oxide described herein further includes aluminum, tantalum, niobium, or a combination of these.
[0031] The composition of the protective agent described herein is based upon multiple observations made by the inventors about the mechanism of protection afforded by the materials regarding liquid CMAS infiltration. For example, strontium-bearing compounds such as SrGd2O4 and SrGd2Al2O7 appear to allow strontium to migrate into CMAS, where it appears to react with the CMAS to form phases such as gehlenite-type phases with higher melting temperatures than the CMAS. This result suggests that any oxide that includes strontium, and/or perhaps other alkaline earth elements such as calcium, may provide protection from liquid CMAS via this mechanism, provided the alkaline earth element is able to migrate to the CMAS as described above. In particular, where the protective agent includes both alkaline earth and rare-earth components, the agent may provide a synergistic protection effect whereby the alkaline earth protects via one mechanism, such as for example the formation of gehlenite-type phase, while the rare-earth component protects via a different, possibly (but not necessarily) independent mechanism, such as the formation of apatite-type phase. Other additions such as the aforementioned aluminum, tantalum, niobium, and the like, may participate in either or both of these mechanisms, or may react with CMAS in a separate mechanism. These additions may, additionally or alternatively, serve to provide phase stability or other desirable properties for the protective agent and/or its reaction products with molten CMAS.
[0032] While compounds such as SrGd2Al2O7 have been disclosed as being effective protective agents, the discovery of the protection mechanisms operating for this and similar compounds suggests that other oxide compositions that include, for example, strontium (and/or other alkaline earth) oxide, aluminum (and/or tantalum and/or niobium) oxide, and gadolinium (and/or other rare earth) oxide, combined in other proportions than, for instance, the 1:1:1 molar ratio required to form SrGd2Al2O7, may be effective. Such combinations may result in formation of a protective agent comprising multiple oxide phases, rather than just a single phase, depending on, for instance, the chemical equilibrium behavior of the component oxides and the manner in which they are processed. Thus, in some embodiments the protective agent includes a combination of two or more oxide phases, where, in the aggregate, the protective agent comprises oxygen, a rare-earth element, and an alkaline earth element. Aluminum, niobium, and/or tantalum may also be present in one or more oxides, consistent with embodiments described above. Examples include a protective agent that includes SrGd2Al2O7 and a second oxide, such as a rare-earth aluminate.
[0033] As is evident from the above description, rare-earth elements are included in the protective agent 150 in various embodiments. 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 coating 110. Notably, where protective agent 150 is disposed at or near the interface between coating 110 and bond coat 140, chemical interaction between the rare-earth-bearing protective agent and an oxide formed by the bond coat at elevated temperature—known as a thermally-grown oxide, or TGO—can result in premature spalling of coating 110.
[0034] To mitigate this potential issue, some embodiments of the present invention employ a barrier agent 160 disposed to substantially separate protective agent 150 from bond coat 140 or, if no bond coat is present, from substrate 120. Thus the barrier agent 160 is interposed between substrate 120 and protective agent 150. Barrier agent 160 may act as a physical barrier; alternatively or additionally, it may serve as a “getter” for rare earth species, combining with these species before, or instead of, the TGO. Regardless of the mechanism, barrier agent 160 substantially prevents chemical interaction between protective agent 150 and a TGO disposed on substrate 120 or, if present, bond coat 140. In some embodiments, barrier agent 160 comprises 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 should be 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 agent 160; 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 agent 160 comprises less than about 40 atomic per cent rare earth element content. In certain embodiments barrier agent comprises less than about 10 atomic per cent rare earth element content, for example as found in the commonly used zirconia stabilized with 8 mole percent yttria (“8YSZ”). The barrier agent material need not be as reactive with CMAS as protective agent 150, because it is typically separated from contact with CMAS by protective agent; the function of barrier agent, then is primarily to physically separate protective agent 150 from bond coat 140 and/or substrate 120.
[0035] Coating 110 is shown in Figure 1 as a single layer, but in some embodiments, such as that illustrated in Figure 2, coating 110 comprises a plurality of layers 220. In the embodiment illustrated in Figure 2, coating 110 includes a first layer 222, comprising a first material, and a second layer 224 comprising a second material. First layer 222 is disposed between second layer 224 and substrate 120. The second material is resistant to CMAS infiltration. A material is considered “resistant to CMAS infiltration” in this context if it is more resistant, relative to 8 mole percent yttria stabilized zirconia (“8YSZ” as noted above), to infiltration by liquid CMAS having the nominal CMAS composition described previously herein at a temperature of 1300 degrees Celsius. Similarly to what has been noted previously, it will be appreciated that the 1300 degree Celsius temperature and the nominal CMAS composition given in this definition represent a reference temperature and a reference composition to define a benchmark for the material’s CMAS resistance in a way that can be compared to the CMAS resistance of 8YSZ; use of these reference values does not limit in any way the actual temperature at which article 100 may operate or the actual composition of ingested material that becomes deposited on the coating during operation, both of which, of course, will vary widely in service.
[0036] Many different materials have been described in the art as providing enhanced CMAS protection relative to yttria-stabilized zirconia and other standard TBC materials, and any of these materials may be considered for use in second layer 222 described above. In one embodiment, the second material includes an oxide. Oxides that include one or more transition metal elements, rare-earth elements, silicon, and/or indium have been described in the art as being resistant to CMAS. In one embodiment, the oxide includes zirconium, hafnium, titanium, or combinations thereof. Zirconia, hafnia, and/or titania materials stabilized with one or more rare-earth elements have been described in the art of CMAS-resistant coatings. Examples of such materials include coatings containing gadolinia and zirconia, such as gadolinia-stabilized zirconia; and coatings containing mixtures of gadolinia and hafnia. Examples of other potentially suitable oxide materials include pyrochlores, such as lanthanum zirconate; garnets, such as those described in US Patent Number 7,722,959; and oxyapatites, such as those described in US Patent Number 7,722,959. Sodium-containing oxides, such as sodium oxide, sodium silicate, and sodium titanate, are other examples of CMAS resistant oxide materials.
[0037] In one particular example, the second material includes yttria-stabilized zirconia (YSZ) having higher yttria content (relative to the overall YSZ content) than typical 8YSZ. Generally, the yttria content in this example is greater than 38 weight percent, and in specific embodiments the yttria content is at least about 55 weight percent. Coatings as described herein using YSZ with yttria content greater than 38 weight percent were superior in CMAS resistance to coatings made with lower-yttria YSZ materials.
[0038] Protective agent 150 is distributed within surface-connected coating voids 130 as described previously, and in the context of embodiments involving multiple layers 220 (Figure 2), the protective agent 150 is, in some embodiments, disposed in just a portion of the layers 220, such as in second layer 224 only, or first layer 222 only; while in certain embodiments, such as the example illustrated in Figure 2, protective agent 150 is disposed in all layers 220. Barrier agent 160 similarly may be disposed in one or more layers 220; in embodiments such as that illustrated in Figure 2, barrier agent 160 is disposed in first layer 222 to protect bond coat 140 and/or substrate 120 from interaction with protective agent 150.
[0039] One potential advantage of the technique described herein is that it allows the use of an outermost layer, such as layer 222, that has desirable wear, erosion, thermal, or other properties, while disposing CMAS resistant material (i.e., protective agent 150) in the places where it is most needed. Many of the materials that are highly reactive with CMAS lack suitable levels of mechanical properties or other properties to be desirable choices for a bulk coating that is exposed to the ambient service environment of, for instance, a gas turbine. On the other hand, zirconia stabilized with 7%-9% yttria by weight is a very attractive material for use in thermal barrier coatings because of its advantageous thermal and mechanical properties, but its resistance to CMAS is not particularly high. The life of such a coating material may be enhanced by applying protective agent 150 to vulnerable areas (such as the surface-connected voids 130 as noted herein) without resorting to the use of a bulk topcoat made of a CMAS-resistant material, which may itself be more vulnerable than YSZ to erosion or other degradation mechanism.
[0040] A method for making articles such as article 100 includes disposing protective agent 150 within surface connected voids 130 of coating 110 at an effective concentration to substantially prevent incursion of CMAS materials, such as nominal CMAS noted above, into voids 130 in which agent 150 is disposed.
[0041] As noted previously, coating 110 may be disposed on substrate 120 by any of several different coating techniques, such as plasma spray techniques (for example, air plasma spray using dry or liquid feedstock materials), chemical vapor deposition, physical vapor deposition (for example, electron-beam physical vapor deposition or evaporation), slurry deposition, sol-gel techniques, and other coating methods. In some embodiments, a bond coat 140 is disposed between coating 110 and substrate 120.
[0042] Disposing protective agent typically involves infiltrating an existing coating 110 with a vapor or liquid into the surface-connected voids 130 of the coating 110. In the case of a vapor infiltrant, protective agent 150 may be formed by chemical interaction with the environment within voids 130 such as by reaction with material of coating 110. 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 comprise the protective agent 150 composition, or may be a chemical precursor to this composition, designed to further react during processing or during service to produce protective agent 150. In an alternative embodiment, the liquid includes a solvent, with a solute dissolved in the solvent. The solute may be a precursor of protective agent 150, 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 agent 150, 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 110, or some combination of these.
[0043] The liquid infiltrant is infiltrated into the surface-connected voids 130 using any appropriate technique. In some embodiments, the liquid is simply placed in contact with coating 110, such as by dipping or brushing, allowing capillary action to draw the liquid and agent 150 (or precursor thereof) into the voids 130. Vacuum infiltration techniques are applied in some embodiments to further assist in driving liquid into coating 110. Other techniques such as electrophoretic deposition may be used in conjunction with a suspension to deposit particles of agent or a precursor of agent 150 into voids 130. 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.
[0044] 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 130. The residue may be a precursor to agent 150, or it may be the agent 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 protective agent due to liquid being forced out by rapid bubble formation and escape.
[0045] In some embodiments, the method further includes interposing barrier agent 160 between protective agent 150 and substrate 120, for instance by disposing barrier agent 150 within pores 130 where bond coat 140 and coating 110 meet. This disposition of barrier agent may be accomplished by any means, such as those described above for disposing protective agent 150. In some embodiments, a suspension of particles comprising the desired barrier agent composition, such as aluminum oxide, cerium oxide, zirconium oxide, hafnium oxide, tantalum oxide, niobium oxide, titanium oxide, or combinations thereof, is infiltrated into the voids 130 of coating. The liquid portion of this suspension is volatilized, and then the protective agent is disposed as described above. In alternative embodiments, a liquid solution of a precursor of the barrier agent is infiltrated into coating 110, the liquid is driven off, leaving a residue within the voids 130 that is later reacted to form the barrier agent. This reaction can occur prior to, during, or after disposition of the protective agent, depending on the desired processing and materials distribution.
[0046] Other techniques may be applied to provide barrier agent 160 or to otherwise protect substrate 120 or bond coat 140 from interacting with protective agent 150. In one embodiment, an initial layer of coating 110 is applied, such as by electron-beam physical vapor deposition (EBPVD). This initial layer is infiltrated as described above to deposit barrier agent 160 within voids (such as between columns) of this initial layer of coating 110. The infiltrated layer is then cleaned and coated with a subsequent layer of coating 110. This subsequent layer may be the same material as used in the initial layer, or may be a different material, as noted above for the structure described in Figure 2. The subsequent layer is then infiltrated and processed to dispose protective agent 150 within its voids 130. In another embodiment, an initial layer 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 protective agent 150 and substrate 120 or bond coat 140. A subsequent technique or change in deposition parameters may then be applied to deposit a subsequent layer of comparatively porous material over the initial comparatively dense layer. This subsequent layer is then subjected to infiltration and further processing to dispose protective agent 150 within its voids 130.
EXAMPLES
[0047] The following examples are presented to further illustrate non-limiting embodiments of the present invention.
[0048] As a general example, in one embodiment an article 100 includes a coating 110 disposed on a substrate 120. The coating 110 includes surface-connected voids 130, such as cracks and porosity. A protective agent 150 is disposed within at least some of voids 130. To take advantage of certain desirable aspects noted above, the protective agent includes an oxide that includes strontium and gadolinium. One non-limiting example of such an oxide is Sr2GdTaO6, for which the following specific example illustrates a synthesis path and resulting behavior in the presence of CMAS.
[0049] Pellets of Sr2GdTaO6 were synthesized by mixing SrCO3, Gd2O3 and Ta2O5 in stoichiometric amounts, dry-milled for more than 8 hours, heated up to 1100 degrees Celsius for 18 hours, wet-milled in isopropyl alcohol for more than 8 hours, and sieved to a particle size distribution of -200 mesh after the isopropyl alcohol evaporated. The resulting powder was pressed into pellets. CMAS tape with nominal density of 8 mg/cm2 was applied to the pellets, and the pellets were then heated to 1260 degrees Celsius for 15 minutes before cooling to room temperature. An exposed pellet was then mounted for cross-sectional scanning electron microscopy and energy dispersive x-ray spectroscopy according to standard industry practices. A strontium-depleted zone was clearly seen in the Sr2GdTaO6 material that was adjacent to the CMAS, and the CMAS was found to be strontium- rich, indicating that the strontium indeed desirably migrated into the CMAS layer to form reaction product. This behavior observed for the bulk material suggests that Sr2GdTaO6 exhibits characteristics desirable for use as a protective agent as described herein.
[0050] While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.