WO2018160195A1

WO2018160195A1 - Protective oxide coating for a thermal barrier coating formed from particles having a metal oxide core and an oxidizable metal shell

Info

Publication number: WO2018160195A1
Application number: PCT/US2017/020708
Authority: WO
Inventors: Atin SHARMA; Anand A. Kulkarni; Ahmed Kamel
Original assignee: Siemens Aktiengesellschaft
Priority date: 2017-03-03
Filing date: 2017-03-03
Publication date: 2018-09-07

Abstract

There are provided processes and materials which extend the lifetime of a thermal barrier coating (14), and which reduce or eliminate the effects of molten contaminant attack (e.g., CMAS infiltration) to the thermal barrier coating (14). In particular, there are provided cored particles (20) comprising a metal oxide core (22) and an oxidizable metal shell (24) about the core (22), a component (10) including a layer (16) of the cored particles (20), and a process for forming a protective oxide coating (17) from the cored particles (20).

Description

PROTECTIVE OXIDE COATING FOR A THERMAL BARRIER COATING FORMED FROM PARTICLES HAVING A METAL OXIDE CORE AND AN

OXIDIZABLE METAL SHELL FIELD OF THE INVENTION

The present invention relates to high temperature coating materials, and more particularly to a protective oxide coating for a thermal barrier coating (TBC) and to processes for making the same, wherein the protective oxide coating is formed from particles having a metal oxide core and an oxidizable metal shell.

BACKGROUND OF THE INVENTION

Gas turbines comprise a casing or cylinder for housing a compressor section, a combustion section, and a turbine section. A supply of air is compressed in the compressor section and directed into the combustion section. The compressed air enters the combustion inlet and is mixed with fuel. The air/fuel mixture is then combusted to produce high temperature and high pressure gas. This working gas then travels past the combustor transition and into the turbine section of the turbine.

Generally, the turbine section comprises rows of vanes which direct the working gas to airfoil portions of turbine blades. The working gas travels through the turbine section, causing the turbine blades to rotate, thereby turning a rotor. The rotor is also attached to the compressor section, thereby turning a compressor and also an electrical generator for producing electricity. High efficiency of a combustion turbine is achieved by heating the gas flowing through the combustion section to as high a temperature as is practical. The hot gas, however, may degrade the various metal turbine components, such as the combustor, transition ducts, vanes, ring segments, and turbine blades that it passes when flowing through the turbine.

For this reason, strategies have been developed to protect turbine components from extreme temperatures, such as the development and selection of high temperature materials adapted to withstand these extreme temperatures and cooling strategies to keep the components adequately cooled during operation. One such strategy includes the deposition of a thermal barrier coating (TBC) onto a substrate surface of the component to reduce heat flow to the substrate, and hence reduce the exposure

temperature of the underlying substrate. TBCs thus must have high durability in a high temperature service environment. However, as engine operating temperatures increase, the chemical and mechanical interactions between the contaminant compositions and TBCs become more aggressive. For example, molten contaminant compositions can react with the TBCs or can infiltrate its pores and openings, thereby initiating and propagating cracks, and thereby causing delamination and loss of TBC material.

In particular, oxides of calcium, magnesium, aluminum, silicon, titanium, and mixtures thereof may combine to form contaminant

compositions referred to as CMAS (Ca-Mg-AI-SiO). These contaminant compositions may also combine with iron and nickel oxides in the engine to form low melting eutectics. In any case, these molten contaminant

compositions may infiltrate pores of the TBC and, upon cooling, the molten material may solidify. When this occurs, cracks may initiate and propagate in the TBC and the strain compliance of the TBC may be reduced, thereby increasing the risk of spallation and loss of the TBCs thermal protection properties.

A number of coating solutions have been proposed for protecting a TBC from CMAS-related damage or the like. Generally, these protective layers or coatings are described as being impermeable, sacrificial, or non- wetting to CMAS. Impermeable coatings are generally characterized as inhibiting infiltration of molten CMAS. Sacrificial coatings react with CMAS to increase the melting temperature and viscosity of CMAS, thereby inhibiting infiltration of the modified CMAS into an associated TBC. Non-wetting coatings reduce the attraction between the solid TBC and molten CMAS in contact therewith to reduce the infiltration of the CMAS into the TBC. For exemplary protective coating systems, see U.S. Patent Nos. 6,720,038;

6,627,323; 6,465,090; 5,914, 189; 5,871 ,820; 5,773, 141 ; and 5,660,885.

Depositing such CMAS-resistant coatings can be achieved by a number of known processes, such as chemical vapor deposition (CVD), electron beam physical vapor deposition (EBPVD), slurry coating, thermal spraying, and solution/suspension spraying amongst other methods. Ideal CMAS-resistant coatings should both penetrate into the open porosity within the TBC, as well as adhere as an outer layer on the TBC. This can be achieved by methods, such as CVD and solution/suspension spraying.

However, each method has drawbacks. For example, CVD deposition generally requires expensive specialized equipment and is typically limited to very low deposition rates. Similarly, solution/suspension spraying may be limited by identifying suitable precursor/solvent combinations and/or poor process efficiency due to low solids loading in suspensions.

U.S. Patent No. 7,807,231 discloses a process for applying a protective film on a TBC surface, which is designed to penetrate into the open porosity of the TBC, as well as adhere as an outer layer on the TBC. The protective film comprises aluminum or magnesium, the oxide of which resists infiltration of CMAS into the TBC. In addition, the protective film is applied so as to form a metal film on the TBC surface, and to infiltrate porosity within the TBC beneath its surface. The metal composition may then be converted to form an oxide film, with at least a portion of the oxide film forming a surface deposit on the RBC surface. However, when such metallic compositions, such as those including aluminum and magnesium, are applied on the TBC surface, a number of problems may be encountered.

For one, aluminum due to its sluggish oxidation kinetics requires either extensive heat treatment (e.g. , days) or heat treatment at high temperatures (e.g. , > 660° C) in order to convert to its oxide. Heating above the melting point of the metal, however, may lead to the formation of molten droplets in local areas that may significantly distort the uniformity of the protective over- layer. Furthermore, incomplete conversion of the metallic film into its respective oxide before the insertion of the coated component into an associated engine can lead to the oxidation of the metallic portion under an uncontrolled environment (e.g. , thermal exposure of the protective film during engine operation). This poses a potential risk of premature delamination of the film, thereby exposing the underlying TBC to the detrimental deposits. Similar issues can be expected with the use of magnesium (melting point ~650° C) alone or in combination of aluminum.

SUMMARY

In accordance with an aspect of the present invention, there are provided processes for forming a protective oxide coating on a thermal barrier coating (TBC) to reduce and/or prevent infiltration of contaminants, such as CMAS, to the TBC. The processes utilize a specially designed cored particle to form the protective oxide coating, wherein a core of the particle comprises a metal oxide (e.g. , a monoelemental metal oxide such as aluminum oxide) and the shell comprises a metallic material which can undergo oxidation to form a corresponding oxide.

In accordance with an aspect, the metal core of the particles - being already oxidized - may substantially reduce the processing necessary to provide the finished protective oxide coating, thereby saving time and expense. In addition, the large surface area to volume ratio of the metallic shell may significantly speed up the oxidation kinetics of metal shell to further reduce the processing time.

In accordance with yet another aspect, the metallic shells of the cored particles function as a binder, thereby providing strong interparticle adhesion within the protective oxide coating. This protects the integrity of the protective oxide coating over time. In addition, the cored particles may provide adhesion between the protective oxide coating and the underlying TBC, thereby further reducing the likelihood of spallation.

In accordance with yet another aspect, the cored particles with their metallic shell allow for increased deposition flexibility, thereby saving material costs, processing time, and all the while improving the adhesion to the underlying TBC as mentioned.

In accordance with still another aspect, the cored particles may be specially formulated to approach or match the coefficient of thermal expansion (CTE) of the underlying TBC. A difference in the CTE between the protective oxide coating and the TBC may cause thermal strains within the TBC system during thermal cycling, especially when the protective oxide coating is dense and/or thick. This can result in partial or complete spallation of the protective oxide coating, thereby reducing or losing the functionality of the protective oxide coating. Additionally, spallation of a strongly adhering protective oxide coating will often cause spallation of the underlying TBC layer, thereby leaving the underlying component undesirably exposed to a high temperature environment. In certain aspects, the composition of the cored particles may be tailored so as to minimize a CTE difference between the protective oxide coating and the TBC on which the protective oxide coating is deposited, thereby reducing thermal mismatches between the protective oxide coating and the TBC.

In accordance with still another aspect, there is provided a process for forming a protective oxide coating on a thermal barrier coating. The process comprises depositing a plurality of cored particles on the thermal barrier coating, wherein the cored particles comprise a metal oxide core and an oxidizable metal shell about the metal oxide core; and oxidizing the oxidizable metal shell of the cored particles to form the protective oxide coating on the thermal barrier coating.

Further, coating systems comprising the cored particles are provided herein. In accordance with an aspect of the present invention, there is provided a high temperature coating system comprising: a thermal barrier coating layer; and a cored particle layer on the thermal barrier coating layer. The cored particle layer comprises a plurality of cored particles, wherein the cored particles each comprise a metal oxide core and an oxidizable metal shell about the core. In accordance with another aspect, there is provided a component, the component comprising a substrate; a thermal barrier coating layer on the substrate; and a cored particle layer on the thermal barrier coating layer. The cored particle layer comprises a plurality of cored particles, wherein the cored particles comprise a metal oxide core and an oxidizable metal shell about the core. Upon application of a suitable amount of energy, the cored particle layer is converted to a protective oxide coating over the TBC.

In accordance with another aspect, there is provided a cored particle comprising a metal oxide core and an oxidizable metal shell about the core.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a cross-sectional view of gas turbine in accordance with an aspect of the present invention.

FIG. 2 illustrates an embodiment of a coating system in accordance with an aspect of the present invention.

FIG. 3 illustrates an embodiment of another coating system in accordance with an aspect of the present invention.

FIG. 4 illustrates an embodiment of yet another coating system in accordance with an aspect of the present invention.

FIG. 5 illustrates a cored particle having an oxide core and an oxidizable metal shell in accordance with an aspect of the present invention. DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, FIG. 1 shows, by way of example, a gas turbine engine 100 in the form of a longitudinal cross-section. In its interior, the gas turbine 100 has a rotor 103, which is mounted such that it rotates about an axis of rotation 102 and has a shaft, and is also known as a turbine rotor. An intake housing 104, a compressor 105, a combustion chamber 1 10, in particular an annular combustion chamber, with a plurality of coaxially arranged burners 107, a turbine section 108, and an exhaust casing 109 follow one another along the rotor 103. The combustion chamber 1 10 is in communication with a hot-gas duct 1 1 1 where, for example, there are four successive turbine stages 1 12.

Each turbine stage 1 12 is formed, for example, from a plurality of blades and guide vanes. As seen in the direction of flow of a working medium 1 13, a row 125 formed from rotor blades 120 follows a row 1 15 of guide vanes 130 in the hot gas duct 1 1 1 . The guide vanes 130 are secured to an inner housing 138 of a stator 143, whereas the rotor blades 120 of a row 125 are fitted to the rotor 103, for example, by a turbine disc 133. A generator or machine (not shown) may be coupled to the rotor 103.

In operation, the compressor 105 intakes air 135 through the intake housing 104 and compresses it. The compressed air, which is provided at the turbine-side end of the compressor 105, is passed to the burners 107 where it is mixed with a fuel. The mixture is then burned in the combustion chamber 1 10 to form the working medium 1 13. From there, the working medium 1 13 flows along the hot-gas duct 1 1 1 past the guide vanes 130 and the rotor blades 120. The working medium 1 13 expands at the rotor blades 120, transferring its momentum, so that the rotor blades 120 drive the rotor 103 and the rotor 103 drives the machine coupled to it.

Referring to FIG. 2, there is shown a partial cross-sectional view of a component 10, which may be any desired component, such as a gas turbine component described previously herein and shown in FIG. 1 . Thus, by way of example, the component 10 may comprise a component in a hot gas path of the turbine, such as a blade, a vane, a transition piece, or the like. It is understood, however, that the present invention is not so limited. In a particular embodiment, the component 10 comprises a turbine blade 120 or a (stationary) guide vane 130. As shown by the cross-section of FIG. 2, the component 10 includes a substrate 12 with a thermal barrier coating (TBC) 14 thereon, and a cored particle layer 16 on the TBC 14. In this embodiment, the TBC 14 and the cored particle layer 16 may be collectively referred to as a coating system 1 1 for the substrate 12. When subjected to an effective amount of energy, e.g. , a heat treatment process, the cored particle layer 16 is converted to a protective oxide coating 17 that reduces or eliminates infiltration of CMAS into the TBC 12 as shown in FIG. 3. As shown in FIG. 3, the TBC 14 is disposed over the substrate 12 while the protective oxide coating 17 is disposed over the TBC 14 such that the protective oxide coating 17 is rendered the outermost layer of the component 10. Accordingly, CMAS or other contaminants in a hostile (high temperature) environment will encounter the protective oxide coating 17 first.

The substrate 12 may be formed from any suitable material which would benefit from the TBC and protective oxide coating 17 as described herein. In certain embodiments, the substrate 12 comprises a superalloy material. The term "superalloy" is used herein as it is commonly used in the art to refer to a highly corrosion-resistant and oxidation-resistant alloy that exhibits excellent mechanical strength and resistance to creep even at high temperatures. Exemplary superalloys include, but are not limited to alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g. , IN 738, IN 792, IN 939), Rene alloys (e.g. Rene N5, Rene 41 , Rene 80, Rene 108, Rene 142, Rene 220), Haynes alloys, Mar M, CM 247, CM 247 LC, C263, 718, X-750, ECY 768, 262, X45, PWA 1483, CMSX (e.g. CMSX-4) single crystal alloys, GTD 1 1 1 , GTD 222, MGA 1400, MGA 2400, PSM 1 16, CMSX-8, CMSX-10, PWA 1484, IN 713C, Mar-M-200, PWA 1480, IN 100, IN 700, Udimet 600, Udimet 500, and titanium aluminide.

In still other embodiments, the substrate 12 may comprise a ceramic matrix composite (CMC) material as is known in the art. The CMC material may include a ceramic or a ceramic matrix material, each of which hosts a plurality of reinforcing fibers. In certain embodiments, the CMC material may be anisotropic, at least in the sense that it can have different strength characteristics in different directions. It is appreciated that various factors, including material selection and fiber orientation can affect the strength characteristics of a CMC material. In addition, the CMC material may comprise an oxide or a non-oxide CMC material. In an embodiment, the CMC material comprises an oxide-oxide CMC material as is known in the art.

The TBC 14 may comprise any suitable TBC material which provides a degree of thermal protection to the underlying substrate 12. In an

embodiment, the TBC material comprises a stabilized zirconia material as is known in the art, such as an yttria-stabilized zirconia (YSZ) material. In other embodiments, the zirconia may instead or partially be stabilized with other oxides, such as magnesia, ceria, scandia, or any other suitable oxide material. An exemplary YSZ material includes zirconium oxide (Zr02) with a predetermined concentration of yttrium oxide (Y2O3), pyrochlores, or the like. In still other embodiments, the TBC 14 may comprise a diffusion coating as is known in the art, such as a diffusion aluminide or a diffusion platinum aluminide coating. In certain embodiments, the TBC 14 may comprise a columnar microstructure, which may be provided via a physical vapor deposition (PVD) process such as electron beam PVD (EBPVD), or a non- columnar microstructure. Typically, the TBC 14 includes a degree of porosity, and thus is susceptible to spallation due to CMAS infiltration as was described above. The TBC 14 may also have any suitable thickness for the intended application. In an embodiment, for example, the TBC 14 has a thickness of from 50 to 500 micron, and in a particular embodiment from 75 to 250 micron, although the present invention is not so limited.

In certain embodiments, as shown in FIG. 4, the coating system 1 1 may further include a bond coat layer 18 between the TBC 14 and the substrate 12 in order to improve adhesion of the TBC 14 to the substrate 12, and to reduce the likelihood of oxidation of the underlying substrate 12. The cored particle layer 16 is again deposited over the TBC 14. Alternatively, the bond coat 18 between the TBC 14 and the substrate 12 may be omitted, and the TBC 14 may be applied directly onto a surface of the substrate 12 as was shown in FIGS. 2-3. The bond coat layer 18 may comprise any suitable material for its intended purpose. An exemplary bond coat layer 18 comprises an MCrAIY material, where M denotes nickel, cobalt, iron, or mixtures thereof, Cr denotes chromium, Al denotes aluminum, and Y denotes yttrium. Another exemplary bond coat 18 for use herein comprises alumina. The bond coat 18 may be applied to the substrate 12 by any known process, such as sputtering, plasma spray, or vapor deposition, e.g. , electron beam physical vapor deposition (EBPVD), or the like.

Referring again to FIG. 2, the cored particle layer 16 comprises a plurality of cored particles 20 deposited onto a surface of the TBC 14. As will be explained in further detail below, once deposited, the cored powder particles 20 may be subjected to a processing, e.g. , heat treatment, step in order to produce the (final) protective oxide coating 17. While fully oxidized particles could be deposited on the TBC 14, the present inventors have found that it is preferred to retain some metallic fraction in the film in as-deposited condition to allow better bonding among the particles, as well as with the underlying TBC 14 following processing, e.g. , heat treatment. The cored particle layer 16 may be deposited in any desired thickness. In an

embodiment, the cored particle layer 16 is applied with a thickness effective to provide a protective oxide coating 17 with a layer thickness of 50 micron or less, and in a particular embodiment from 10-25 micron following processing, e.g., heat treatment.

FIG. 5 illustrates a cross-section of an exemplary cored particle 20 of the layer 16, the cored particle 20 comprising an oxide core 22 and an oxidizable metal shell 24 about the core 22. In an embodiment, the oxide core 22 comprises a monoelemental metal oxide. For example, without limitation, the oxide core 22 may comprise one or more of aluminum oxide, hafnium oxide, magnesium oxide, scandium oxide, tin oxide, yttrium oxide, zirconium oxide, and combinations thereof.

The oxidizable metal shell 24 comprises any metal which can undergo oxidation to provide a stable metal oxide thereof. For example, in an embodiment, the oxidizable metal shell 24 may comprise aluminum, hafnium, magnesium, scandium, tin, yttrium, zirconium, or the like, or combinations thereof. In a particular embodiment, the shell 24 comprises an element which has a higher melting temperature in its oxide form vs. its elemental form. In this way, once oxidized, the shell 24 may "join" the oxide core 22 and form a relatively continuous material with the core 22 to provide the protective oxide coating 17. As noted above, due to the comparatively small amount of metal material in the shell 24 (by volume) and the fact the metal core 22 of the particles is already oxidized, the cored particles 20 may substantially reduce the processing necessary to provide the finished protective oxide coating 17, thereby saving time and expense. In addition, the large surface area to volume ratio of the metallic shell 24 may significantly speed up the oxidation kinetics of metal shell 24 to further reduce the processing time.

In a particular embodiment, the oxidizable metal shell 22 comprises at least one of aluminum and magnesium, each of which has a substantially higher melting temperature in their respective oxide forms relative to the elemental component alone. In certain embodiments, the shell 24 comprises a corresponding element to the oxide core 22. For example, in an

embodiment, the oxidizable metal shell 24 may comprise aluminum while the oxide core 22 comprises aluminum oxide. In this way and by way of example only, when the particles 20 are subjected to a treatment which at least partially melts the shell 24 and oxidizes the aluminum, the resulting material may comprise a continuous phase (coating 17) of aluminum oxide (shell and core), which will act to protect the TBC 14 from CAMS attack.

As mentioned above, in certain embodiments, the metal shell 24 of each cored particle 20 may function as a binder, thereby providing strong interparticle adhesion within the protective oxide coating. To explain, due to the low melting point of the metal and its slow oxidation rate, it is expected that there will be partial melting of the cored particles 20 before oxidation is complete. The molten metal, upon solidification, fuses the adjacent surfaces (of other particles and underlying TBC) together, thus acting as a binder. This binding effect protects the integrity of the protective oxide coating 17 over time. In addition, the cored particles 20 may provide improved adhesion between the protective oxide coating 17 and the underlying TBC 14, thereby further reducing the likelihood of spallation as a result of separation of the protective oxide coating 17 from the TBC 14.

To reiterate, one advantage of the cored particles 20 is that they will require significantly less material to oxidize relative to, for example, particles made solely from a non-oxide or monoelemental element (e.g. , aluminum or magnesium) as in US Patent No. 7,807,231 . Accordingly, in an embodiment, the oxidizable metal shell 24 may be sufficiently small in thickness about the core 22 such that only a fraction of the material of the particle 20 need be oxidized to provide the protective oxide coating 17 as described herein. In an embodiment, the core 22 has a diameter which is at least 2-1 Ox, and in some embodiments 3-4x, greater than a diameter of the shell 24.

Further, in certain embodiments, the oxide core 22 may have a diameter (thickness) of from about 4 to about 10 micron while the shell is provided on the oxide core 22 with a diameter (thickness) of about 1 to about 5 micron. In certain embodiments, the cored particle 20 has a particle size of from about 5 to about 15 micron. In an embodiment, the protective oxide coating 17 has a layer thickness of about 50 micron or less, and in a particular embodiment from about 10 to about 25 micron. While the above dimensions are provided as an illustration, it is also understood that the present invention is not so limited to the dimensions stated, and that the particles 20 and resulting protective oxide coating 17 may comprise any desired dimensions suitable for their intended purpose. When used with respect to numerical values, the term "about" may refer to an amount which is ± 5% of the stated value.

In accordance with another aspect, there is provided herein a process for protecting a thermal barrier coating on a substrate. In an embodiment, a component 10 is provided having a substrate 12, a bond coat 18 overlaying the substrate 12, and a TBC 14 overlaying the bond coat 18. In certain embodiments, the component 10 is a used or already manufactured article having the necessary layers (12, 18 (optional), and 14) on which the protective oxide coating 17 may be formed. When a bond coat 18 is utilized, the bond coat 18 can be applied over the substrate 12 by a suitable deposition process. Thereafter, the TBC 14 may be applied over the bond coat 18. Alternatively, the TBC 14 may be applied directly to the substrate 12. The deposition of the bond coat 18 (when present) and the TBC 14 may take place by any suitable deposition process, such as a plasma spray process, e.g. , an air plasma spray process; a physical vapor deposition (PVD) process, e.g. , an electron beam physical vapor deposition (EB-PVD) process; or any other suitable deposition technique. An EB-PVD process typically provides the TBC 14 with a columnar

microstructure having sub-micron sized gaps between adjacent columns of a TBC material as shown in U.S. Patent No. 5,562,998, the entirety of which is incorporated by reference herein. Such columnar microstructures may be particularly susceptible to CMAS attack.

Once the bond coat 18 (if present) and the TBC layer 14 have been applied to the substrate 12, the protective oxide coating 17 may now be formed over the TBC 14 from the plurality of cored particles 20 having a metal core 22 and an oxidizable metal shell 24. It is appreciated that the particles 20 may be prepared by any suitable process which completely or partially covers the oxide core 22 with the oxidizable metal shell 24. In an

embodiment, the particles 20 are prepared by a vapor deposition process, e.g. , chemical vapor deposition or a mechanical process; e.g. , a mechanical cladding process; or by any other suitable process.

In other embodiments, the particles 20 may be prepared by a

hydrometallurgical or a hydrochemical process (e.g. , autoclave leaching) process. By way of example, an aqueous slurry containing suitable

precursors of the shell material, as well as the core of the particles 20, may be prepared. The slurry may then be subjected to a temperature and/or pressure treatment under a reducing atmosphere. The precursor salts of the shell material are reduced to their metallic form and are deposited on the surfaces of the cores, thereby forming a thin shell around them. Alternatively, the cored particles 20 may be manufactured and provided from a suitable third party or commercial source.

In any case, once the cored particles 20 are prepared, the cored particles 20 may be applied to the TBC 14 by a suitable deposition process to form cored particle layer 16 (shown in FIG. 2). In accordance with an aspect, the deposition of the cored particles 20 is greatly simplified by the presence of the metal shell 24. Known coating processes require a chemical vapor deposition (CVD) process or the like, which require significant temperatures and expensive equipment yet provide low deposition rates. In contrast, the cored particles 20 may be deposited on the TBC 14 by a relatively low cost and low heat deposition method, such as by slurry/brush coating, spin coating (with a flat surface), or an additive manufacturing technique. In particular embodiments, the deposition process comprises an air-brush coating or a spray coating process. In certain embodiments, these processes can be performed at ambient temperature in contrast to thermal spraying which involves heat. In still other embodiments, however, the deposition may be done by a thermal spray process.

Thereafter, to improve adhesion and functionality of the protective oxide coating 17, the particles 20 may be subjected to an amount of energy effective to at least partially melt the outer metal shell 24 and oxidize the metal outer shell 24 of the particles 20, thereby forming the protective oxide coating 17 on the TBC (shown in FIG. 3) upon cooling. By "oxidize," it is understood that the processing step, such as heating, is effective to oxidize at least a majority of the metal material of the metal outer shell 24. In certain embodiments, substantially all to all (95-100 wt %) of the material of the metal shell 24 is oxidized, however, it is understood that the present invention is not so limited. The cooling may be done passively or actively, such as by blowers or the like for a suitable amount of time, e.g. 30 minutes to 3 hours, to form the protective oxide coating 17.

The application of energy for a duration of time and in an oxidizing environment (e.g. , in the presence of an oxidant such as air, oxygen-enriched air, or the like) to yield the desired results. In an embodiment, the particles 20 are subjected to heating at a temperature that is lower than a melting temperature of the outer metal shell 24. In particular embodiments, the heating temperature is within 100° C of a melting temperature of the material of the outer metal shell 24 yet remains below the melting temperature. In this way, the particles 20 do not completely melt, but the outer shell 24 of the particles 20 at least becomes sufficiently viscous to allow the partially melted particles 20 to cover the TBC 14 in desired locations, bind to each other, bind to the TBC 14, at least partially infiltrate the TBC 14, and cool to form the protective oxide coating 17 thereover.

In an embodiment, the particles 20 may be subjected to a heat treatment of from about 400° C to about 1000° C, and in a particular embodiment from 600° C to 800° C for a time period of from 10 minutes to 24 hours, and preferably from 10 minutes to 10 hours. When heating is utilized, the heating may be done isothermally or with a temperature gradient. The heating process may be a single step or a multistep process. In certain embodiments, the application of energy (e.g. , heat-treatment) to the cored particle layer 16 is done prior to inserting the subject component into an associated engine. In other embodiments, the subject component having the TBC 14 and cored particle layer 16 may be inserted into an the engine without a heat-treatment step, and thereafter subjected to heat energy during engine operation, thereby generating the protective oxide coating 17 with the desired properties and functionality. In still other embodiments, the processing may comprise oxidizing a portion of the metallic shell 24 prior to engine operation and oxidizing a remaining portion of the metallic shell 24 during engine operation.

In certain embodiments, prior to deposition of the cored particles 20 on the TBC 14, the TBC 14 or the component 10 (as a whole) may also be subjected to a heat treatment process to assist in providing the heat necessary for oxidation of the particles 20. In this way, when the cored particles 20 are deposited on the TBC 14, the substrate to which the particles 20 are applied is already warm or hot, and heat may be transferred from the substrate to the particles 20 to facilitate the oxidation process and reduce processing time.

In accordance with another aspect, the formed protective oxide coating overlays the TBC 14 and also infiltrates the pores of the TBC 14. This is particularly the case when the TBC 14 is in the form of a columnar layer (such as when deposited by EBPVD) where it is easier for the particles 20 (once subjected to heat or the like) to infiltrate the pores of the TBC 14. In addition, the infiltration of the TBC 14 by the particles is believed to provide improved anchoring of the protective oxide coating 17 to the TBC 14 upon cooling. Further, without wishing to be bound by theory, it is believed that the protective oxide coating 17 protects the underlying TBC layer 14 by reacting with molten CMAS and forming refractory phase(s) having a higher melting temperature than CMAS. In this way, the formation of molten CMAS is substantially reduced, and a result the infiltration of CMAS into the TBC 14 is also reduced by aspects of the present invention.

While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.

Claims

CLAIMS What we claim is:

1 . A high temperature coating system (1 1 ) comprising:

a thermal barrier coating layer (14); and

a cored particle layer (16) on the thermal barrier coating layer (14), the cored particle layer (16) comprising a plurality of cored particles (20), wherein the cored particles (20) comprise a metal oxide core (22) and an oxidizable metal shell (24) about the core (22).

2. The coating system (1 1 ) of claim 1 , wherein the metal oxide core (22) comprises a member from the group consisting of aluminum oxide, magnesium oxide, silicon dioxide, and combinations thereof.

3. The coating system (1 1 ) of claim 1 , wherein the oxidizable metal shell (24) comprises a member selected from the group consisting of aluminum, magnesium, silicon, and combinations thereof.

4. The coating system (1 1 ) of claim 1 , wherein the cored particles (20) comprise a particle size of from about 5 to about 15 micron.

5. A component (10) comprising:

a substrate (12);

a thermal barrier coating layer (14) on the substrate (12); and a cored particle layer (16) on the thermal barrier coating layer (14), the cored particle layer (16) comprising a plurality of cored particles (20), wherein the cored particles (20) comprise a metal oxide core (22) and an oxidizable metal shell (24) about the core (22).

6. The component (10) of claim 5, wherein the thermal barrier coating layer (14) comprises a stabilized zirconia material.

7. The component (10) of claim 5, wherein the substrate (12) comprises at least one of a ceramic matrix composite material or a superalloy material.

8. The component (10) of claim 5, wherein the metal oxide core

(22) comprises a member from the group consisting of aluminum oxide, magnesium oxide, silicon dioxide, and combinations thereof.

9. The component (10) of claim 5, wherein the oxidizable metal shell (24) comprises a member selected from the group consisting of aluminum, magnesium, silicon, and combination thereof.

10. The component (10) of claim 7, wherein the cored particles (20) comprise a particle size of from about 5 to about 15 micron.

1 1 . The component (10) of claim 5, wherein component (10) comprise a gas turbine rotor blade (120).

12. A cored particle (20) comprising a metal oxide core (22) and an oxidizable metal shell (24) about the core (22).

13. The cored particle (20) of claim 12, wherein the metal oxide core (22) comprises a member from the group consisting of aluminum oxide, magnesium oxide, and silicon dioxide.

14. The cored particle (20) of claim 12, wherein the oxidizable metal shell (24) comprises a member selected from the group consisting of aluminum, magnesium, silicon, and combinations thereof.

15. The cored particle (20) of claim 15, wherein the cored particle

(20) comprises a particle size of from about 5 to about 15 micron.

16. A process for forming a protective oxide coating (17) on a thermal barrier coating (14) comprising:

depositing a plurality of cored particles(20) on the thermal barrier coating (14), wherein the cored particles (20) comprise a metal oxide core (22) and an oxidizable metal shell (24) about the metal oxide core (22); and oxidizing at least one metal of the oxidizable metal shell (24) to form the protective oxide coating (17) on the thermal barrier coating (14).

17. The process of claim 16, wherein the oxidizing is done by subjecting the protective oxide coating (17) to heat treatment in the presence of oxygen, and wherein the heat treatment at least partially melts the shell (24) of the cored particles (20).

18. The process of claim 16, wherein the heat treatment comprises subjecting the plurality of cored particles (20) to a temperature within 100° C but below a melting temperature of the oxidizable metal shell (24).

19. The process of claim 16, wherein the cored particles (20) are formed by depositing the oxidizable metal shell (24) on the metal oxide core (22) by a process selected from the group consisting of a vapor deposition process, a hydrometallurgical process, a hydrochemical process, or via mechanical cladding.

20. The process of claim 16, wherein the protective oxide coating (17) is deposited on the thermal barrier coating (14) by a process selected from the group consisting of slurry coating, brush coating, spray coating, and additive manufacturing.

21 . The process of claim 16, wherein the metal oxide core (22) comprises a member from the group consisting of aluminum oxide, magnesium oxide, silicon dioxide, and combinations thereof.

22. The process of claim 16, wherein the oxidizable metal shell (24) comprises a member selected from the group consisting of aluminum, magnesium, and silicon.

23. The process of claim 16, wherein the protective oxide coating

(17) comprises a thickness of about 50 micron or less.

24. The process of claim 23, wherein the protective oxide coating (17) comprises a thickness of from about 10 micron to about 25 micron.