Method of removing EBC from substrate and repairing coated part

Abstract

A method of removing a bond coating and an environmental barrier coating from a coated component is provided. The method may include contacting the coated component with an etchant comprising phosphoric acid, 5% to 70% by volume hydrogen fluoride, and a solvent. Hydrogen fluoride and phosphoric acid react with components in the bond coat. A bond coating is located within the coated component between the silicon-containing substrate and the environmental barrier coating, the bond coating comprising mullite.

Description

Technical Field

[0001] The present disclosure relates to methods of removing environmental barrier coatings (EBCs) from substrates. Background Art

[0002] Silicon-based materials are used in high-temperature components of gas turbine engines, such as airfoils (e.g., blades and vanes), combustor liners, and shrouds. Silicon-based materials can include silicon-based monolithic ceramics, intermetallic materials, and composites. For example, silicon-based ceramic matrix composites (CMCs) can include silicon-containing fibers reinforced with a silicon-containing matrix phase. Summary of the Invention

[0003] A method for removing a coating from a coated component, the method comprising:

[0004] contacting the coated component with an etching solution, wherein the etching solution comprises phosphoric acid, 5% to 70% by volume hydrogen fluoride, and a solvent, wherein the coated component comprises a coating on a silicon-containing substrate, the coating comprising a bond coat and an environmental barrier coating, wherein the bond coat comprises mullite, and the phosphoric acid and hydrogen fluoride react with the coating to weaken chemical bonds therein, thereby forming a chemically weakened coating, and

[0005] The chemically weakened coating is removed from the silicon-containing substrate.

[0006] The method of the preceding clause, wherein the bond coat comprises at least 50 wt% mullite.

[0007] The method of any of the preceding clauses, wherein the bond coat comprises at least 75 wt.% mullite. The method of any of the preceding clauses, wherein the environmental barrier coating is disposed directly on the bond coat prior to contacting the coated component with the etching solution.

[0008] A method according to any of the preceding clauses, wherein the coated component comprises substantially no thermally grown oxide layer.

[0009] A method according to any of the preceding clauses, wherein contacting the coated component with the etching liquid comprises immersing the coated component in the etching liquid.

[0010] A method according to any of the preceding clauses, wherein the etching solution comprises 25% to 49% by volume of hydrogen fluoride.

[0011] A method according to any of the preceding clauses, wherein the etching solution comprises 10% to 60% by volume of phosphoric acid.

[0012] A method according to any of the preceding clauses, wherein the etching solution comprises 30% to 40% by volume phosphoric acid.

[0013] A method according to any of the preceding clauses, wherein the solvent comprises water.

[0014] A method according to any of the preceding clauses, wherein the etching solution consists essentially of hydrogen fluoride, phosphoric acid and water.

[0015] The method according to any of the preceding clauses, wherein the etching solution further comprises hydrogen chloride, nitric acid, fluosilicic acid, hydrogen peroxide or a mixture thereof.

[0016] A method according to any of the preceding clauses, wherein the etching solution further comprises a wetting agent.

[0017] The method according to any of the preceding clauses, wherein the processing temperature of the etching liquid during the contacting period is 20°C to 60°C.

[0018] The method of any of the preceding clauses, removing the chemically weakened coating from the silicon-containing substrate comprises:

[0019] The coated component is contacted with a mechanical etchant to remove the bond coat and the environmental barrier coat, thereby exposing the surface of the silicon-containing substrate.

[0020] The method of any of the preceding clauses, wherein contacting the coated component with the mechanical etchant comprises mechanically blasting the environmental barrier coating and the bond coating with a plurality of particles to remove the environmental barrier coating and the bond coating.

[0021] A method for repairing a coated component, the method comprising:

[0022] forming an intermediate component according to the method of any of the preceding clauses by removing the bond coat and the environmental barrier coating from the coated component to expose the silicon-containing substrate; and

[0023] A replacement bond coat is formed on the silicon-containing substrate.

[0024] The method of any of the preceding clauses, wherein the replacement bond coat comprises mullite.

[0025] A method according to any of the preceding clauses, wherein the method further comprises:

[0026] An alternative environmental barrier coating is formed over the alternative bond coating.

[0027] A method according to any of the preceding clauses, wherein the replacement environmental barrier coating is formed directly on the bond coat without a thermally grown oxide layer therebetween. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] A full and enabling disclosure of the present invention, including the best mode thereof, is set forth in the specification to one skilled in the art, with reference to the accompanying drawings, in which:

[0029] Figure 1 is a schematic cross-sectional view of an exemplary coated component comprising a substrate coated with a silicon-containing bond coating, a thermal barrier coating, and an EBC;

[0030] Figure 2 FIG. 1 shows a method of immersing a substrate in an etching solution according to one embodiment of the method described herein. Figure 1 An exemplary coated component of

[0031] Figure 3A Shown Figure 1 Schematic diagram of a cross-section of an exemplary coated component after removal from an etching solution, showing that the silicon-containing bond coat remains on the substrate while the thermal barrier coating and EBC are removed;

[0032] Figure 3B Shown Figure 3A A schematic cross-sectional view of an alternative removal process for a remaining silicon-containing bond coat on a component in FIG.

[0033] Figure 4A Shown in Figure 3A Schematic cross-sectional view of an exemplary repair assembly with an alternative environmental barrier coating applied over the remaining silicon-containing bond coat;

[0034] Figure 4B shows a schematic cross-sectional view of an exemplary repair assembly having an alternative environmental barrier coating applied over an alternative silicon-containing bond coating;

[0035] Figure 5A A flow chart illustrating an exemplary method of removing an EBC from an exemplary coated component comprising a silicon-containing bond coating, a thermal barrier coating, and an EBC;

[0036] Figure 5B A flow chart illustrating an exemplary method of removing an EBC from an exemplary coated component comprising a silicon-containing bond coating, a thermal barrier coating, and an EBC;

[0037] Figure 6 is a schematic cross-sectional view of an exemplary coated component including a substrate coated with a mullite-containing bond coat and an EBC;

[0038] Figure 7An embodiment of the method described herein is shown immersed in an etching solution. Figure 6 An exemplary coated component of

[0039] Figure 8 A schematic cross-sectional view illustrating an alternative method for removing the remaining mullite-containing bond coat after contact with an etching solution;

[0040] Figure 9 An exemplary repair assembly is shown with an alternative environmental barrier coating applied over the remaining silicon-containing bond coat;

[0041] Figure 10 A method of removing EBC from an exemplary coated component including a mullite-containing bond coat and a thermal barrier coating is shown.

[0042] Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present disclosure. DETAILED DESCRIPTION

[0043] definition

[0044] As used herein, the terms “first,” “second,” and “third” are used interchangeably to distinguish one component from another, and are not intended to indicate the position or importance of each component.

[0045] Common chemical abbreviations are used in this disclosure to discuss chemical elements, such as those commonly found in the periodic table. For example, hydrogen is represented by the common chemical abbreviation H; helium is represented by the common chemical abbreviation He; and so on. As used herein, "RE" refers to a rare earth element or a mixture of rare earth elements. More specifically, "RE" refers to the rare earth elements scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), or mixtures thereof.

[0046] As used herein, "alumina" refers to aluminum oxide having the chemical formula Al2O3.

[0047] As used herein, "silicon dioxide" refers to silicon oxide with the chemical formula SiO2. In contrast, "elemental silicon" refers to silicon without any alloying materials other than incidental impurities. This is sometimes referred to in the art as "silicon metal." The melting point of elemental silicon is approximately 1414°C.

[0048] As used herein, the term "mullite" generally refers to a mineral containing aluminum oxide and silicon dioxide. That is, mullite is a compound of aluminum oxide and silicon dioxide in which the ratio of aluminum oxide (Al2O3) to silicon dioxide (SiO2) is approximately 3:2 (e.g., a ratio of aluminum oxide to silicon dioxide of 3:2 within a range of ±10 mol%). However, mullite has also been reported to have a ratio of approximately 2:1 (e.g., a ratio of aluminum oxide to silicon dioxide of 2:1 within a range of ±10 mol%).

[0049] As used herein, the term "substantially free" is understood to mean completely free of the ingredient or containing trace amounts of the ingredient. "Trace amount" refers to a quantitative level of a chemical ingredient that is barely detectable and does not contribute to the functional or aesthetic properties of the subject composition. The term "substantially free" also includes completely free.

[0050] In this disclosure, when a layer is described as being "on" or "over" another layer or substrate, it should be understood that the layers can be directly in contact with each other or contain another layer or feature between the layers, unless explicitly stated otherwise. Therefore, these terms only describe the relative positions of the layers and do not necessarily mean "over...", as the relative position of above or below depends on the orientation of the device with respect to the viewer.

[0051] As used herein, ceramic matrix composites or "CMCs" refer to a class of materials that include reinforcement materials (e.g., reinforcing fibers) that are surrounded by a ceramic matrix phase. Typically, the reinforcing fibers provide structural integrity to the ceramic matrix. Some examples of matrix materials for CMCs may include, but are not limited to, non-oxide silicon-based materials (e.g., silicon carbide, silicon nitride, or mixtures thereof), oxide ceramics (e.g., silicon oxycarbide, silicon oxynitride, aluminum oxide (Al2O3), silicon dioxide (SiO2), aluminosilicates, or mixtures thereof), or mixtures thereof. Optionally, ceramic particles (e.g., oxides of Si, Al, Zr, Y, and combinations thereof) and inorganic fillers (e.g., pyrophyllite, wollastonite, mica, talc, kyanite, and montmorillonite) may also be included in the CMC matrix.

[0052] Some examples of CMC reinforcing fibers may include, but are not limited to, non-oxide silicon-based materials (e.g., silicon carbide, silicon nitride, or mixtures thereof), non-oxide carbon-based materials (e.g., carbon), oxide ceramics (e.g., silicon oxycarbide, silicon oxynitride, aluminum oxide (Al2O3), silicon dioxide (SiO2), aluminosilicates such as mullite, or mixtures thereof), or mixtures thereof.

[0053] Often, a specific CMC can be referred to by its fiber type / matrix type combination. For example, C / SiC represents carbon fiber reinforced silicon carbide; SiC / SiC represents silicon carbide fiber reinforced silicon carbide; SiC / SiN represents silicon carbide fiber reinforced silicon nitride; SiC / SiC-SiN represents silicon carbide fiber reinforced silicon carbide / silicon nitride matrix mixture, and so on. In other examples, a CMC can be composed of a matrix and reinforcing fibers, which include oxide-based materials such as aluminum oxide (Al2O3), silicon dioxide (SiO2), aluminosilicates, and mixtures thereof. Aluminosilicates can include crystalline materials such as mullite (3Al2O3·2SiO2), as well as glassy aluminosilicates.

[0054] In certain embodiments, the reinforcing fibers may be bundled and / or coated prior to being added to the matrix. For example, the fiber bundles may be formed into reinforcement tapes, such as unidirectional reinforcement tapes. Multiple tapes may be stacked together to form a preform. The fiber bundles may be impregnated with the slurry composition before or after forming the preform. The preform may then be thermally treated, such as by curing or burning to produce a high carbon residue in the preform, followed by chemical treatment, such as by melt infiltration with silicon, to produce a component formed from a CMC material having a desired chemical composition.

[0055] These materials, along with certain monolithic ceramics (i.e., ceramic materials without reinforcements), are particularly well-suited for high-temperature applications. Furthermore, these ceramic materials are lightweight compared to superalloys while still providing strength and durability to components made from them. Consequently, these materials are currently being considered for use in many gas turbine components within the high-temperature sections of gas turbine engines, such as airfoil assemblies (e.g., turbine blades and guide vanes), combustors, shrouds, and other similar components that would benefit from the lightweight and high-temperature performance offered by these materials.

[0056] As used herein, an environmental barrier coating or "EBC" refers to a coating system composed of one or more layers of ceramic materials, each of which provides specific or multifunctional protection to the underlying CMC. An EBC typically includes multiple layers, such as a rare earth silicate coating (e.g., a rare earth disilicate, such as yttrium ytterbium disilicate (YbYDS) deposited as a slurry or APS), an alkaline earth aluminosilicate (e.g., including barium strontium aluminosilicate (BSAS), such as a composition having varying ranges of BaO, SrO, Al2O3, SiO2, or a combination thereof), a sealing layer (e.g., a rare earth disilicate), an overcoat (e.g., including a rare earth monosilicate, such as yttrium monosilicate (YMS) deposited as a slurry or APS), or a combination thereof. One or more layers can be doped as desired, and the EBC can also be coated with an abradable coating.

[0057] As used herein, the term "defect" refers to a portion of the protective layer, substrate, or both that is exposed to the environment due to damage.

[0058] The present embodiments of the present disclosure will now be described in detail, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numbers and letters to refer to features in the drawings. The same or similar numbers in the drawings and the description are used to refer to the same or similar parts of the present disclosure.

[0059] Although silicon-containing substrates exhibit desirable high-temperature properties, such substrates can be rapidly degraded in combustion environments. For example, silicon-containing substrates readily volatilize when exposed to reactive species such as water vapor at high temperatures. In such cases, coatings are used to protect the silicon-containing substrate. The surface of a silicon-containing substrate (e.g., CMC) may have multiple protective coatings, such as a silicon bond coat, a disilicate EBC, a monosilicate EBC, or a combination thereof. These protective layers help prevent the silicon-containing substrate from degrading in corrosive aqueous environments by inhibiting the intrusion of water vapor and the subsequent formation of volatile products such as silicon hydroxides (e.g., Si(OH)4). Multiple additional layers (e.g., abradable layers) may also be deposited on the EBC to provide specific functionality to the CMC component. Thus, protective layers can enhance the high-temperature environmental stability of the silicon-containing substrate. Other desirable properties of an EBC include thermal expansion compatibility with the silicon-containing substrate, low permeability to oxidants, low thermal conductivity, and chemical compatibility with thermally grown silicon-based oxides.

[0060] During use, one or more of these protective layers may become damaged, such as by defects. If the EBC exhibits localized flaking or pinhole defects, the underlying substrate may suffer material loss due to volatilization and subsequent surface loss caused by water vapor. If left unchecked, this material loss can reduce the component's load-bearing capacity, disrupt airflow, or even develop into through-the-thickness holes, adversely affecting the machine's operational performance and durability.

[0061] In order to repair a worn or damaged environmental barrier coating, a method is needed to remove any existing environmental barrier coating to create a new surface on the substrate for forming a replacement bond coat, a replacement environmental barrier coating, or both.

[0062] A method for removing an environmental barrier coating from a coated component is provided. The method includes contacting the coated component with an etching solution comprising hydrogen fluoride and a solvent. The etching solution can be further tailored based on the chemical properties of the layers between the coated component substrate and the environmental barrier coating, such as the chemical composition of the intervening bond coat (and thermally grown oxide layer, if present).

[0063] according to Figure 1 , which shows an exemplary coated component 100 comprising a silicon-containing substrate 102 having a surface 103 with a coating system 106 thereon. In one particular embodiment, the silicon-containing substrate 102 is made of a CMC material. Similarly, according to Figure 6, which shows an exemplary coated component 100 comprising a silicon-containing substrate 102 having a surface 103 with a coating system 106 thereon. In one particular embodiment, the silicon-containing substrate 102 is made of a CMC material.

[0064] Typically, the coating system 106 includes a bond coat 104 located on a substrate surface 103. In the embodiment shown, the bond coat 104 is located directly on the surface 103 without any layers therebetween. The EBC 108 is located above the bond coat 104. In one embodiment, the bond coat 104 comprises silicon (e.g., elemental silicon), a silicon-based material (e.g., silicide), mullite, or a combination thereof. Typically, the bond coat 104 is relatively thin, for example, having a thickness of 25 micrometers (μm) to 275 μm, for example, 25 μm to 150 μm (e.g., 25 μm to 100 μm).

[0065] The EBC 108 may include a plurality of individual EBC layers 109, each of which is formed from a material selected from typical EBC or thermal barrier coating ("TBC") layer chemistries, including, but not limited to, rare earth silicates (e.g., monosilicates and disilicates), aluminosilicates (e.g., mullite, barium strontium aluminosilicate (BSAS), rare earth aluminosilicates, etc.), hafnium oxide, zirconium oxide, stabilized hafnium oxide, stabilized zirconium oxide, rare earth hafnates, rare earth zirconates, rare earth gallium oxide, etc. For example, an individual EBC layer 109 may include hafnium oxide (e.g., a hafnium oxide layer), aluminum oxide (e.g., an aluminum oxide layer), or both. Alternatively or additionally, the EBC may include one or more rare earth silicate layers (e.g., a rare earth disilicate layer, a rare earth monosilicate layer, or both). In a particular embodiment, the EBC 108 may include a hermetic layer.

[0066] As described above, the method for removing EBC 108 can be tailored to the composition of the bond coat 104 (and the thermally grown oxide layer, if present). Suitable methods for removing EBC 108 are provided in the following description, tailored to the specific chemical composition of the coating system 106 (e.g., the type of bond coat 104).

[0067] Example

[0068] I. Coated Parts with Silicone-Containing Bond Coatings

[0069] exist Figure 1In an embodiment, as a non-limiting example, the bonding coating 104 includes a silicon-containing material, such as elemental silicon, a silicide, or a combination thereof. For example, the bonding coating 104 may include at least 75% by weight of elemental silicon. It may include at least 95% by weight of elemental silicon. In one embodiment, the bonding coating 104 may consist essentially of elemental silicon and therefore substantially does not include any other material. In other embodiments, the bonding coating 104 may include at least 75% by weight of elemental silicon (e.g., at least 95% by weight of elemental silicon), with the remainder being another bonding coating material (e.g., a silicide).

[0070] When the bond coat 104 comprises a silicon-containing material, such as Figure 1 As shown, a thermally grown oxide ("TGO") layer 110 is present on a surface 105 of the bond coating 104. Thus, the bond coating 104 and the TGO layer 110 are positioned between the silicon-containing substrate 102 and the EBC 108. The TGO layer 110 may form on the surface 105 of the bond coating 104 when the silicon in the bond coating 104 oxidizes due to exposure to air. For example, the TGO layer 110 may be a silicon oxide layer (sometimes referred to as "silicon oxide flakes" or "silicon dioxide flakes") formed when the coated component 100 is exposed to oxygen (e.g., during manufacturing and / or use).

[0071] When the TGO layer 110 is present, the coated component 100 may be contacted with an etching solution 112, such as Figure 2 As shown, the etching solution 112 comprises 5% to 70% by volume of hydrogen fluoride and a solvent. In a specific embodiment, the etching solution 112 comprises 25% to 49% by volume of hydrogen fluoride, thereby forming a commercial grade hydrogen fluoride solution that is easily accessible. Typically, hydrogen fluoride reacts with the silicon oxide in the TGO layer 110. This reaction weakens the chemical bonds in the TGO layer 110, causing the TGO layer 110 and any coating (i.e., the EBC 108 in the illustrated embodiment) to be removed from the surface 105 of the bonding coating 104. In contrast, hydrogen fluoride does not react with the underlying bonding coating 104 or substrate 102, i.e., the bonding coating 104 is not removed from the surface 103 of the substrate 102.

[0072] In one specific embodiment, the solvent in the etching solution 112 is water. For example, the etching solution 112 may consist essentially of hydrogen fluoride in water, and thus, may be free of any other components except for unavoidable trace components. Alternatively, the etching solution 112 may further include another acid (e.g., hydrogen chloride, nitric acid, hydrosilicic acid (H2SiF6), hydrogen peroxide, or mixtures thereof), a wetting agent, etc.

[0073] For example, up to 5% by volume (e.g., 0.1 to 5% by volume) of the second acid may be present in the etching solution 112. In one embodiment, the second acid may be nitric acid added to HF, depending on the type of silicon-containing substrate 102, because nitric acid will also etch and remove any elemental Si in the silicon-containing substrate 102. For example, a silicon-containing substrate 102 formed from a chemical vapor infiltration (CVI) CMC or a polymer infiltration pyrolysis (PIP) CMC does not substantially contain any elemental Si. Therefore, nitric acid can be used to remove the bond coat 104 on a CVICMC silicon-containing substrate 102. In an alternative embodiment, when elemental Si is present in the silicon-containing substrate 102 (e.g., a melt infiltration (MI) CMC), the etching solution 112 may be free of nitric acid to avoid etching elemental Si in the silicon-containing substrate 102.

[0074] When included, the wetting agent may contain fluorine, such as quaternary ammonium perfluoroalkanesulfonates, perfluoroalkane carboxylates, alkoxylated products of perfluoroalkanesulfonamides, etc. The wetting agent may be present in an amount up to 5% by volume (eg, 0.1 to 5% by volume).

[0075] The coated component 100 may be contacted with an etching solution 112 having a processing temperature of 20° C. to 60° C. so as to remain in a liquid state during the chemical etching of the TGO layer 110. Figure 2 As shown, the coated component 100 may be immersed in an etching solution 112 .

[0076] like Figure 3A As shown, from the etching solution 112 ( Figure 2 ) is removed to form the intermediate component 115. As a non-limiting example, the etching solution 112 can be rinsed with a rinse solution (e.g., water, alcohol, acetone, etc.), and the intermediate component 115 can be heated to a drying temperature (e.g., 30° C. to 80° C.) under vacuum or flowing inert gas to volatilize the rinse solution, or rinsing and heating can be performed simultaneously. The intermediate component 115 has an exposed surface 105 of the bonding coating 104 located on the silicon-containing substrate 102, wherein the TGO layer 110 and the EBC 108 ( Figure 1 ) has been removed therefrom. The intermediate component 115 can then be converted into a repair component by forming a new replacement EBC on the exposed surface 105 of the bond coating 104. In one embodiment, Figure 3A The exposed surface 105 of the mid-bond coating 104 may form a surrogate TGO layer when exposed to oxygen (air).

[0077] according to Figure 4A , which shows a repair assembly 120, wherein the Figure 3AThe replacement EBC 122 is formed on the intermediate part 115. As shown in the figure, the replacement EBC 122 includes a plurality of separate layers 123. The replacement EBC 122 and the separate layers 123 can be combined with the original EBC 108 ( Figure 1 ) may be similar or different in composition, thickness, layering sequence, or other variables. The alternative EBC 122 may be formed according to any suitable method.

[0078] Figure 5A A method for removing a coating component (e.g. Figure 1 The method 150 generally includes contacting the coated component with an etching solution (as shown in FIG. Figure 2 As shown), to remove the EBC and expose the bond coat. Thus, step 152 may form Figure 3A Optionally, in step 154, a replacement EBC (e.g., Figure 4A substituted EBC 122) to form a repair assembly (e.g., Figure 4A The repair assembly 120 shown in FIG.

[0079] In another embodiment, in the middle part 115 ( Figure 3A ) before forming the replacement layer on the intermediate component 115, the adhesive coating 104 is removed from the intermediate component 115. Figure 3B , which shows the process of removing the bond coat 104. More specifically, Figure 3B A mechanical etchant 130 is shown in contact with the intermediate component 115 to remove the bonding coating 104, thereby exposing the surface 103 of the silicon-containing substrate 102. As a non-limiting example, the coating 104 can be mechanically sandblasted using a plurality of particles 132 to remove the bonding coating 104. As shown, a plurality of particles 132 can be sprayed by a spray gun 134, which moves on the surface 105 of the bonding coating 104, with sufficient force to direct the plurality of particles 132 to the surface to remove the bonding coating 104. In some embodiments, the plurality of particles 132 can be gravel particles, glass particles, metal particles, or mixtures thereof. Therefore, after removing the bonding coating 104, the surface 103 of the silicon-containing substrate 102 may be exposed.

[0080] After removing the bond coating 104, a repaired assembly can be formed by forming a new replacement bond coating and a new replacement EBC on the exposed surface 103 of the silicon-containing substrate 102. Figure 4B , which shows a repair assembly 120' having an alternative bond coat 136 and an alternative EBC 122. Figure 4BIn the illustrated embodiment, the replacement bond coating 136 is positioned directly on the surface 103 of the silicon-containing substrate 102 , while the replacement EBC 122 is positioned directly on the surface 137 of the replacement bond coating 136 .

[0081] The replacement bond coating 136 may be formed according to any suitable method. As desired, the replacement bond coating 136 may be different from the original bond coating 104 ( Figure 1 ) are similar or different. For example, the alternative bond coating 136 may be formed of a silicon-containing material (e.g., elemental silicon, silicide, etc.), mullite, or a combination thereof. In one embodiment, the alternative bond coating 136 may include mullite (e.g., at least 50 wt. % mullite, such as at least 75 wt. % mullite) to increase Figure 1 In such an embodiment, the repair assembly 120' is substantially free of a TGO layer between the replacement bond coating 136 and the replacement EBC 122.

[0082] As mentioned above about Figure 4A Discussion, Figure 4B The replacement EBC 122 and the layers 123 in the repair assembly 120' can be connected to the original EBC 108 ( Figure 1 ) or may be the same or different, or may differ in composition, thickness, layering sequence, or other variables as desired. The alternative EBC 122 may be formed on the surface 137 of the alternative bond coating 136 according to any suitable method.

[0083] Figure 5B Shows that from Figure 1 The method 156 for removing EBC from a coated component is shown. The method 156 generally includes contacting the coated component with an etching solution in step 158 to remove the EBC and expose the bond coat, such as Figure 2 Therefore, step 158 can form Figure 3A The intermediate component 115. Optionally, in step 160, as Figure 3B As shown, the bonding coating can be removed to expose the surface of the silicon-containing substrate. Optionally, in step 162, a replacement bonding coating is formed on the exposed surface of the silicon-containing substrate, such as Figure 4B The alternative bond coating 136 is shown. In step 164, an alternative EBC may be formed on the alternative bond coating, such as Figure 4B substituted EBC 122 as shown in , to form a repair assembly, such as Figure 4B The repair assembly 120' is shown in FIG.

[0084] II. Coated Parts with Mullite-Containing Bond Coats

[0085] exist Figure 6In one embodiment, the coating system 106 includes a bond coat 104 comprising mullite. For example, the bond coat 104 may comprise at least 50% by weight mullite (e.g., at least 75% by weight mullite). In one embodiment, the bond coat 104 may consist essentially of mullite, thereby being substantially free of any other material. In other embodiments, the bond coat 104 may comprise at least 50% by weight mullite (e.g., at least 75% by weight mullite), with the remainder being another bond coat material (e.g., a silicon-containing material as described above).

[0086] When the bond coating 104 comprises mullite, the coated component 100 may be substantially free of any TGO layer on the surface 105 of the bond coating 104. Figure 6 As shown, the EBC 108 may be positioned directly on the surface 105 of the bond coating 104 .

[0087] because Figure 6 The coated component 100 in the embodiment of the present invention does not substantially contain any TGO layer, so the etching solution requires additional components to weaken the chemical bond between the bond coat 104 and the EBC 108. Figure 7 , the coating component 100 can be contacted with an etching solution 112, which includes phosphoric acid and 3% to 70% by volume of hydrogen fluoride in a solvent. In a specific embodiment, the etching solution 112 includes 5% to 49% by volume (e.g., 25% to 49% by volume) of hydrogen fluoride, thereby forming a commercial grade hydrogen fluoride solution that is easily obtained. In addition to hydrogen fluoride, the etching solution 112 also includes phosphoric acid to help weaken the chemical bonds in the bonding coating 104, EBC 108, or both. For example, the etching solution 112 may include an aqueous solution of phosphoric acid of 10% to 60% by volume, such as an aqueous solution of phosphoric acid of 30% to 40% by volume. A possibly relevant example is to use an etching solution containing about 37% by volume of phosphoric acid, which can be combined with an aqueous solution of hydrogen fluoride containing 10% by volume to etch mullite. The etching rate can be controlled by selecting the concentration of hydrogen fluoride and phosphoric acid, because a lower concentration in the solvent can slow down the etching rate, while a higher concentration can increase the etching rate.

[0088] Typically, hydrogen fluoride and phosphoric acid react with the mullite in bond coat 104, the material of at least one layer of EBC 108, or both. This reaction weakens the chemical bonds in bond coat 104, EBC 108, or both, resulting in removal of bond coat 104 or allowing for easier removal in subsequent etching. In contrast, hydrogen fluoride and phosphoric acid do not react with the underlying silicon-containing substrate 102.

[0089] In one specific embodiment, the solvent in the etching solution 112 is water. For example, the etching solution 112 may consist primarily of hydrogen fluoride and phosphoric acid in water, thereby being free of other components other than unavoidable trace components. Alternatively, the etching solution 112 may further include another acid (e.g., hydrogen chloride, nitric acid, fluorosilicic acid (H2SiF6), hydrogen peroxide, or mixtures thereof), a wetting agent, etc.

[0090] For example, up to 5% by volume (e.g., 0.1 to 5% by volume) of the third acid may be present in the etching solution 112. In one embodiment, the third acid may be nitric acid added to HF, depending on the type of silicon-containing substrate 102, because the nitric acid will also etch and remove any elemental Si in the silicon-containing substrate 102. For example, a silicon-containing substrate 102 formed from chemical vapor infiltration (CVI) CMC or polymer infiltration pyrolysis (PIP) CMC does not substantially contain any elemental Si. Therefore, nitric acid can be used to remove the bond coat 104 on a CVICMC silicon-containing substrate 102. In an alternative embodiment, when elemental Si is present in the silicon-containing substrate 102 (e.g., melt infiltration (MI) CMC), the etching solution 112 may not contain nitric acid to avoid etching the elemental Si in the silicon-containing substrate 102.

[0091] Additionally or alternatively, when a wetting agent is included, the wetting agent may contain fluorine, such as quaternary ammonium perfluoroalkanesulfonates, perfluoroalkane carboxylates, alkoxylated products of perfluoroalkanesulfonamides, etc. The wetting agent may be present in an amount up to 5% by volume (e.g., 0.1 to 5% by volume).

[0092] The coated component 100 may be contacted with an etching solution 112 having a processing temperature of 20° C. to 60° C. to maintain a liquid state during chemical etching. Figure 7 As shown, the coated component 100 may be immersed in an etching solution 112 .

[0093] After removing from the etching solution, Figure 8 As shown, the resulting intermediate component 115' includes a chemically weakened bond coat 104' and a chemically weakened EBC 108'. Figure 8 As shown, before forming the replacement layer on the intermediate component 115, the intermediate component 115' ( Figure 8 ) removes the chemically weakened bond coat 104' and the chemically weakened EBC 108' (together forming the chemically weakened coating 106') and then forms a replacement layer thereon. Figure 8, wherein shown is a mechanical etchant 130 contacting with an intermediate component 115' to remove a chemically weakened bond coating 104' and a chemically weakened EBC 108', to remove the chemically weakened bond coating 104' and expose the surface 103 of the silicon-containing substrate 102. For example, the chemically weakened bond coating 104' can be removed using a plurality of particles 132 by mechanical sandblasting. As shown, a plurality of particles 132 are sprayed by a spray gun 134, which moves above the chemically weakened bond coating 104' and the chemically weakened EBC 108', with sufficient force, a plurality of particles 132 are directed onto its surface to remove the chemically weakened bond coating 104' and the chemically weakened EBC 108'. In some embodiments, a plurality of particles 132 can be gravel particles, glass particles, metal particles or a mixture thereof. Therefore, after removing the chemically weakened bond coating 104' and the chemically weakened EBC 108', the surface 103 of the silicon-containing substrate 102 can be exposed. The intermediate component 115 ′ can then be converted into a repaired assembly by forming a new replacement bond coating and a new replacement EBC on the exposed surface 103 of the silicon-containing substrate 102 .

[0094] according to Figure 9 , which shows a repair assembly 120' having the bond coat 104' and the EBC 108' ( Figure 8 ) after that, Figure 8 The alternative bonding coating 136' and the alternative EBC 122' are formed on the intermediate member 115'. Figure 9 In the illustrated embodiment, the replacement bond coating 136 ′ is positioned directly on the surface 103 of the silicon-containing substrate 102 , and the replacement EBC 122 ′ is positioned directly on a surface 137 ′ of the replacement bond coating 136 ′.

[0095] The replacement bond coating 136' may be formed according to any suitable method. The replacement bond coating 136' may be formed in a manner similar to the original bond coating 104' ( Figure 6 ) or may be the same or different, or may differ in composition, thickness, or other variables as desired. For example, the alternative bond coating 136' may be formed from a silicon-containing material (e.g., elemental silicon, silicide, etc.), mullite, or a combination thereof. In one embodiment, the alternative bond coating 136' may include mullite (e.g., at least 50% by weight mullite, such as at least 75% by weight mullite). In such an embodiment, the repair assembly 120" may be substantially free of a TGO layer between the alternative bond coating 136' and the alternative EBC 122'.

[0096] Figure 9 The replacement EBC 122' and the various layers 123' in the repair assembly 120" can be used in conjunction with the original EBC 108 ( Figure 6) or may be the same or different, or may differ in composition, thickness, layering sequence, or other variables as desired. The replacement EBC 122' may be formed on the surface 137' of the replacement bond coating 136' according to any suitable method.

[0097] Figure 10 A method for removing a coating component (e.g. Figure 6 The method 170 of removing the bond coating and EBC on the surface of the substrate 170 generally includes contacting the coated component with an etching solution (such as Figure 7 As shown), chemically weakening the bond coat and EBC on the substrate. Thus, step 172 may form Figure 8 Optionally, in step 174, the weakened bond coat and the weakened EBC may be removed to expose the surface of the silicon-containing substrate (e.g., Figure 8 In step 176, a replacement bonding coating may optionally be formed on the exposed surface of the silicon-containing substrate, such as Figure 9 The alternative bond coating 136' is shown. In step 178, an alternative EBC may be formed on the alternative bond coating, such as Figure 9 The alternative EBC 122' shown in , to form a repair assembly, e.g. Figure 9 The repair assembly 120" is shown in FIG.

[0098] In any of the above embodiments, after removing the EBC 108, the resulting intermediate component can be refurbished for use as a repair assembly. Such repair assemblies are particularly suitable for use as components in high temperature environments, such as components in gas turbine engines, such as combustor components, turbine blades, shrouds, nozzles, heat shields, and guide vanes. Specifically, the repair assembly can be a repair CMC assembly located within the hot gas flow path of the gas turbine, such that the repair coating system forms a repair EBC for the underlying substrate to protect the repair assembly within the gas turbine when exposed to the hot gas flow path. In some embodiments, the repair bond coating is configured such that the repair assembly is exposed to operating temperatures of about 1475°C to about 1650°C, and the repair bond coating is substantially unaffected by these operating temperatures. Therefore, the repair bond coating can withstand operating temperatures of about 1475°C to about 1650°C.

[0099] Further aspects of the present disclosure are provided by the subject matter of the following clauses:

[0100] A method for removing an environmental barrier coating from a coated component, the coated component comprising a silicon-containing substrate, a bond coating located on the silicon-containing substrate and comprising a silicon-containing material, an environmental barrier coating located on the bond coating, and a thermally grown oxide layer located between the bond coating and the environmental barrier coating. The method comprises contacting the coated component with an etching solution comprising 5% to 70% by volume of hydrogen fluoride and a solvent, wherein the hydrogen fluoride reacts with silicon oxide in the thermally grown oxide layer, thereby removing the thermally grown oxide layer and the environmental barrier coating from the bond coating.

[0101] A method according to any preceding clause, wherein the bond coat comprises elemental silicon.

[0102] A method according to any preceding clause, wherein the bond coat comprises at least 75 wt% elemental silicon.

[0103] A method according to any preceding clause, wherein contacting the coated component with the etching liquid comprises immersing the coated component in the etching liquid.

[0104] A method according to any preceding clause, wherein the etching solution comprises 25% to 49% by volume of hydrogen fluoride.

[0105] A method according to any preceding clause, wherein the solvent comprises water.

[0106] A method according to any preceding clause, wherein the etching solution consists essentially of hydrogen fluoride and water.

[0107] The method according to any of the preceding clauses, wherein the etching solution further comprises hydrogen chloride, nitric acid, fluorosilicic acid, hydrogen peroxide or a mixture thereof.

[0108] A method according to any of the preceding clauses, wherein the etching solution further comprises a wetting agent.

[0109] A method according to any preceding clause, wherein the coated component is contacted with an etching solution having a processing temperature of 20°C to 60°C.

[0110] A method according to any of the preceding clauses, wherein the method further comprises removing the bond coat to expose the surface of the silicon-containing substrate after contacting the coated component with the etching solution.

[0111] The method of any preceding clause, wherein removing the bond coating from the substrate comprises contacting the coated component with a mechanical etchant to remove the bond coating and expose the surface of the silicon-containing substrate.

[0112] The method of any preceding clause, wherein contacting the coated component with a mechanical etchant comprises mechanically blasting the bond coating with a plurality of particles to remove the bond coating.

[0113] A method according to any preceding clause, wherein the plurality of particles comprises grit particles, glass particles, metal particles or mixtures thereof.

[0114] A method of repairing a coated component, the method comprising: forming an intermediate component by removing the environmental barrier coating from the coated component to expose the bond coating and define the exposed bond coating according to any of the preceding clauses; and forming a replacement environmental barrier coating on the intermediate component.

[0115] A method according to any preceding clause, wherein the replacement environmental barrier coating is formed directly on the exposed bond coat.

[0116] The method of any of the preceding clauses further comprising, after exposing the bond coating, removing the bond coating to expose a surface of the silicon-containing substrate, thereby defining an exposed silicon-containing substrate; and forming a replacement bond coating on the exposed silicon-containing substrate, wherein the replacement environmental barrier coating is formed on the replacement bond coating.

[0117] A method as in any preceding clause, wherein removing the bond coat comprises contacting the intermediate component with a mechanical etchant.

[0118] The method of any preceding clause, wherein contacting the intermediate component with the mechanical etchant comprises mechanically blasting the bond coat with a plurality of particles.

[0119] A method according to any preceding clause, wherein the replacement bond coat comprises mullite.

[0120] A method for removing a coating from a coated component, the method comprising: contacting the coated component with an etching solution, wherein the etching solution comprises phosphoric acid, 5% to 70% by volume of hydrogen fluoride, and a solvent, wherein the coated component comprises a coating on a silicon-containing substrate, the coating comprising a bond coat and an environmental barrier coating, wherein the bond coat comprises mullite, and wherein the phosphoric acid and hydrogen fluoride react with the coating to weaken chemical bonds therein, thereby forming a chemically weakened coating, and removing the chemically weakened coating from the silicon-containing substrate.

[0121] A method according to any preceding clause, wherein the bond coat comprises at least 50 wt% mullite.

[0122] A method according to any preceding clause, wherein the bond coat comprises at least 75 wt% mullite.

[0123] A method according to any preceding clause, wherein the environmental barrier coating is located directly on the bond coating prior to contacting the coated component with the etching solution.

[0124] A method according to any preceding clause, wherein the coated component comprises substantially no thermally grown oxide layer.

[0125] A method according to any preceding clause, wherein contacting the coated component with an etching liquid comprises immersing the coated component in the etching liquid.

[0126] A method according to any preceding clause, wherein the etching solution comprises 25% to 49% by volume of hydrogen fluoride.

[0127] A method according to any preceding clause, wherein the etching solution comprises 10% to 60% by volume of phosphoric acid.

[0128] A method according to any preceding clause, wherein the etching solution comprises 30% to 40% by volume of phosphoric acid.

[0129] A method according to any preceding clause, wherein the solvent comprises water.

[0130] A method according to any preceding clause, wherein the etching solution consists essentially of hydrogen fluoride, phosphoric acid and water.

[0131] The method according to any of the preceding clauses, wherein the etching solution further comprises hydrogen chloride, nitric acid, fluorosilicic acid, hydrogen peroxide or a mixture thereof.

[0132] A method according to any of the preceding clauses, wherein the etching solution further comprises a wetting agent.

[0133] The method according to any of the preceding clauses, wherein the processing temperature of the etching solution during the contacting period is 20°C to 60°C.

[0134] The method of any preceding clause, wherein removing the chemically weakened coating from the silicon-containing substrate comprises contacting the coated component with a mechanical etchant to remove the bond coat and the environmental barrier coating, thereby exposing the surface of the silicon-containing substrate.

[0135] The method of any preceding clause, wherein contacting the coated component with the mechanical etchant comprises mechanically blasting the environmental barrier coating and the bond coating with a plurality of particles to remove the environmental barrier coating and the bond coating.

[0136] A method of repairing a coated component, the method comprising forming an intermediate component by removing the bond coat and the environmental barrier coating from the coated component to expose the silicon-containing substrate according to the method of claim 1; and forming a replacement bond coat on the silicon-containing substrate.

[0137] A method according to any preceding clause, wherein the replacement bond coat comprises mullite.

[0138] A method according to any preceding clause, further comprising forming a replacement environmental barrier coating on the replacement bond coating.

[0139] A method according to any preceding clause, wherein the replacement environmental barrier coating is formed directly on the bond coat without an intervening thermally grown oxide layer.

[0140] This description uses examples to disclose the disclosure, including the best mode, and to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

1. A method for removing a coating from a coated component, the method comprising: contacting the coated component with an etching solution, wherein the etching solution comprises phosphoric acid, 5% to 70% by volume hydrogen fluoride, and a solvent, wherein the coated component comprises a coating on a silicon-containing substrate, the coating comprising a bond coat and an environmental barrier coating, wherein the bond coat comprises mullite, and the phosphoric acid and hydrogen fluoride react with the coating to weaken chemical bonds therein, thereby forming a chemically weakened coating, and The chemically weakened coating is removed from the silicon-containing substrate.

2. The method according to claim 1, wherein The bond coat comprises at least 50 wt. % mullite.

3. The method according to claim 1, wherein The bond coat comprises at least 75 wt. % mullite.

4. The method according to claim 1, wherein The environmental barrier coating is located directly on the bond coating prior to contacting the coated component with the etching solution.

5. The method according to claim 1, wherein The coated component does not substantially include a thermally grown oxide layer.

6. The method according to claim 1, wherein Contacting the coating component with the etching liquid includes immersing the coating component in the etching liquid.

7. The method according to claim 1, wherein The etching solution contains 25 volume % to 49 volume % of hydrogen fluoride.

8. The method according to claim 1, wherein The etching solution contains 10 volume % to 60 volume % of phosphoric acid.

9. The method according to claim 1, wherein The etching solution contains 30 volume % to 40 volume % of phosphoric acid.

10. The method according to claim 1, wherein The solvent comprises water.

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