Patent Publication Number: US-2011059321-A1

Title: Method of repairing a thermal barrier coating and repaired coating formed thereby

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine. More particularly, this invention is directed to a method for repairing thermal barrier coatings that have suffered localized spallation. 
     Hot section components of aircraft and industrial (power generation) gas turbine engines are often protected by a thermal barrier coating (TBC), which reduces the temperature of the underlying component substrate and thereby prolongs the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying, and physical vapor deposition (PVD) such as electron beam physical vapor deposition (EBPVD). Air plasma spraying (APS) is often preferred over other deposition processes due to relatively low equipment costs and ease of application and masking. 
     Analysis has shown that YSZ TBC&#39;s deposited by APS are about 20% to 70% transparent to thermal radiation (wavelengths of about 780 nm to about 1 mm) when deposited at typical thicknesses of about 250 to 500 micrometers. As a result, the thermal protection provided by such TBC&#39;s is compromised by their infrared (IR) transmissivity in environments that have high thermal radiation loads, such as within the combustor section of a gas turbine. In response, other materials have been proposed for insulating combustion section hardware and other hardware subject to similar operating conditions. Notable examples include materials containing alumina and silica that are non-transparent to IR wavelengths of particular concern in the combustor, for example, wavelengths of about 0.3 to about 6 micrometers. These coating materials also have the advantage of having lower mass and excellent insulating properties. Particular examples of TBC&#39;s formed of these coating materials involve the use of a mixture containing alumina powder particles and a silica precursor, which are applied to the surface to be protected and heated to thermally decompose the precursor to form a silica matrix in which the powder particles are dispersed. The mixture can be preformed as a tape that is applied to the surface or can be sprayed onto the surface. As examples, see commonly-assigned U.S. Pat. Nos. 6,165,600, 6,177,186, 6,210,791, 6,465,090, and 6,827,969. 
     To be effective, TBC systems must have low thermal conductivity, strongly adhere to the component, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between the ceramic materials of the TBC and the substrates they protect, which are typically metallic superalloys. To promote adhesion and extend the service life of a TBC system, an oxidation-resistant bond coat is often employed. Bond coats are typically in the form of overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, rare earth elements, and/or reactive elements), or diffusion aluminide coatings. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (Al 2 O 3 ) layer or scale that adheres the TBC to the bond coat. 
     The service life of a TBC system is typically limited by a spallation event driven by bond coat oxidation, increased interfacial stresses, and the resulting thermal fatigue. Though significant advances have been made, there is the inevitable requirement to repair components whose thermal barrier coatings have spalled. Though spallation typically occurs in localized regions or patches, the conventional repair method has been to completely remove the thermal barrier coating, restore or repair the bond layer surface as necessary, and then recoat the entire component. As an alternative, U.S. Pat. No. 5,723,078 to Nagaraj et al. teach a process for selectively repairing a spalled region of a TBC using a plasma spray technique. 
     In the case of large power generation turbines, completely halting power generation for an extended period in order to remove components whose TBC&#39;s have suffered only localized spallation is not economically desirable. As a result, components identified as having spalled TBC are often analyzed to determine whether the spallation has occurred in a high stress area, and a judgment is then made as to the risk of damage to the turbine due to the reduced thermal protection of the component, which if excessive can lead to catastrophic failure of the component. If the decision is to repair the TBC, the spalled component is removed and new TBC material is deposited by plasma spraying on the spalled surface region. Such repair processes have found wide use for the repair of YSZ TBC&#39;s. However, there is an ongoing need for new repair materials and techniques, including those particularly adapted to repair the aforementioned alumina-silica TBC materials. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides a coating composition and repair method suitable for repairing a TBC on a component, and particularly TBC materials based on alumina-silica compositions. 
     According to a first aspect of the invention, the method includes preparing a coating composition comprising solid ceramic particles, hollow ceramic particles, and a silica precursor binder, applying the coating composition on an exposed surface area of the component, for example, exposed by localized spallation, and then reacting the binder to yield a repair coating that covers the surface area of the component. The resulting repair coating comprises the solid ceramic particles and the hollow ceramic particles in a silica matrix formed by thermally decomposing the binder. Also encompassed by the invention is the coating composition and the repaired TBC. 
     Coating compositions and the resulting repair coatings described above are compatible with alumina-silica based TBC materials, and the hollow ceramic particles provide the additional benefit of reducing density and enhancing the insulative and erosion-resistant properties of the repair coating. Preferred materials for the solid and hollow ceramic particles are non-transparent to IR wavelengths of particular concern in the combustor section of a turbine engine, for example, wavelengths of about 0.3 to about 6 micrometers. As such, preferred materials for the repair coating do not degrade the thermal reflectivity of the TBC being repaired. 
     In view of the above, it can also be appreciated that the method of this invention does not require the TBC to be completely removed, and does not require removal of the component in order to repair its TBC. The method also does not require a high temperature treatment, as the silica precursor binder may be initially cured to enable the repair coating to exhibit sufficient strength to withstand engine operation, during which time the precursor binder is gradually converted to form the silica matrix. As a result, minimal downtime is necessary to complete the repair and resume operation of the turbine engine. In the case of large power generation turbines, the cost is avoided of completely halting power generation for an extended period in order to remove, repair and then reinstall a component that has suffered only localized spallation. 
     The method of this invention can be used to repair ceramic coatings on a wide variety of components exposed to thermal loads, including but not limited to TBCs formed of materials other than alumina-silica compositions and applied to hot section components of aircraft and industrial (power generation) gas turbine engines. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional representation of a component surface protected by a thermal barrier coating that has suffered localized spallation. 
         FIGS. 2 and 3  are cross-sectional representations of the component surface of  FIG. 1  during the repair of the thermal barrier coating in accordance with a preferred embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to components protected by thermal barrier coatings for operation within environments characterized by relatively high temperatures, and are therefore subjected to severe thermal stresses, cycling, and radiation loads. Notable examples of such components include the high and low pressure turbine nozzles and blades, shrouds, combustor liners, secondary seals, and augmentor hardware of gas turbine engines for use in aircraft and industrial applications. The present invention is particularly directed to thermal barrier coatings (TBCs) that exhibit thermal insulating properties to conduction and thermal radiation. The advantages of this invention will be described as particularly applicable to combustor components of turbine engines, though the invention is generally applicable to any components in which thermal barrier-type coatings as described herein may be used to thermally insulate a component from its environment. 
     Represented in  FIG. 1  is a surface region of a component  10  with a thermal barrier coating (TBC) system  12  having an exposed region  20 , for example, as a result of localized spallation. The TBC system  12  is shown as being composed of a bond coat  14  on the surface of the component  10 , and a ceramic layer (TBC)  16  deposited on the bond coat  14  as the thermal barrier coating. As is the situation with high temperature components of gas turbine engines, the component  10  may be formed of a nickel, cobalt or iron-base superalloy. The bond coat  14  is preferably formed of a metallic oxidation-resistant material, so as to protect the underlying component  10  from oxidation and enable the ceramic layer  16  to more tenaciously adhere to the component  10 . Suitable materials for the bond coat  14  include, but are not limited to, MCrAlX overlay coatings and diffusion aluminide coatings. Following deposition of the bond coat  14 , an oxide scale  18  forms on the surface of the bond coat  14  at elevated temperatures. The oxide scale  18  provides a surface to which the ceramic layer  16  more tenaciously adheres, thereby promoting the spallation resistance of the ceramic layer  16 . 
     The ceramic layer  16  may be formed of a variety of materials, including YSZ materials widely employed as TBC materials, though results of the repair method of this invention will depend in part on the extent to which the thermal expansion properties of the ceramic layer  16  are compatible with the repair materials discussed below. In a preferred embodiment of the invention, the ceramic layer  16  is an alumina/silica-based material (alumina, silica, and/or aluminosilicates are the predominant constituents) that is non-transparent to IR wavelengths of particular concern in the combustion sections of turbine engines. Particular examples of such materials include the aforementioned U.S. Pat. Nos. 6,165,600, 6,177,186, 6,210,791, 6,465,090, and 6,827,969, whose contents regarding coating compositions and coating processes are referenced here. The ceramic layer  16  may have been formed from tapes applied to the surface of the component  10 , or a composition sprayed onto the component surface, or another suitable deposition process. Preferred examples of these coating materials contain alumina powder particles, optionally additional ceramic particles, and a silica precursor, which are mixed and applied to the component surface and then heated to thermally decompose the precursor to form a silica matrix in which the powder particles are dispersed. The coating material is deposited to a thickness that is sufficient so that the resulting ceramic layer  16  is capable of providing the required thermal protection for the component  10 . 
     If located within the combustion section of a turbine engine, surfaces of the component  10  are subjected to hot combustion gases during operation of the engine and are therefore subjected to severe attack by oxidation, corrosion and erosion. Accordingly, the component  10  must remain protected from its hostile operating environment by the TBC system  12 . Loss of the ceramic layer  16  due to spallation leads to premature and often rapid deterioration of the component  10 . As such, the localized spalled region  20  of the ceramic layer  16  represented in  FIG. 1  must be repaired or the component  10  scrapped. A preferred TBC repair process of this invention is represented in  FIGS. 2 and 3 . Each of the following steps performed in the repair of the component  10  can be performed while the component  10  remains installed in the turbine engine, thereby completely avoiding the requirement to remove and later reinstall the component. 
     The repair process begins with cleaning the surface  22  exposed by the localized spalled region  20  so as to remove loose oxides and contaminants such as grease, oils and soot without damaging the undamaged ceramic layer  16 . The oxide scale  18  as well as any well-adhered remnants of the ceramic layer  16  exposed by the spalled region  20  may remain to promote adhesion of a coating composition  24  deposited in the spalled region  20 , as represented by  FIG. 2 . According to the invention, the coating composition  24  is a mixture containing one or more powders of solid (non-hollow) ceramic particles  26 , one or more powders of hollow ceramic particles (microballoons)  28 , and a silica precursor that when sufficiently heated forms a ceramic repair coating  30  shown in  FIG. 3 . In addition, the coating composition  24  may contain other filler materials, including glass compositions. 
     The solid particles  26  are preferably alumina, though the use of other ceramic materials or mixtures thereof is also foreseeable. Preferred materials are refractory oxides that are non-transparent to IR wavelengths, including alumina, magnesia (MgO), titania (TiO 2 ), and calcia (CaO). Additionally, YSZ particles may be included as part of the solid particles  26  as scattering sites for wavelengths of about 0.9 about 2.5 micrometers. Suitable particle sizes for the powder particles  26  and other solid constituents of the coating composition  24  are generally in a range of about 0.01 to about 100 micrometers, more preferably about 0.1 to about 25 micrometers. Depending on the particular application and desired construction of the coating  30 , preferred alumina powders for the solid particles  26  may include A-14 (an unground calcined alumina powder; particle size range of about 3 to 5.5 micrometers) available from Almatis, A-16SG (a super-ground thermally reactive alumina powder; particle size range of about 0.3 to 0.5 micrometer) also available from Almatis, and SM8 (particle size range of about 0.10 to 0.6 micrometer) available from Baikowski International Corp. 
     Depending on the particular application and desired construction of the coating  30 , the hollow ceramic particles  28  may be alumina, another ceramic material, or a mixture of one or more ceramic materials, with preferred materials being those non-transparent to IR wavelengths, such as alumina and silicates including aluminosilicates. Though shown throughout the coating composition  24  and repair coating  30  in  FIGS. 2 and 3 , the hollow particles  26  may be limited to certain regions of the composition  24  and coating  30 , for example, their innermost, outermost, and/or intermediate regions. According to a preferred aspect of the invention, an important role played by the hollow ceramic particles  28  is to increase the insulation properties of the repair coating  30  without diminishing the overall non-transparency of the alumina-silica ceramic layer  16  to IR wavelengths of particular concern in the combustion sections of turbine engines. Other potential benefits include improved erosion resistance of the repair coating  30  with minimal or no adverse increase in weight. The low density of the hollow powder particles  28  compared to the solid powder particles  26  allows for greater thicknesses of the repair coating  30  without exceeding the weight of a similar coating composition containing only the solid particles  26 , or a repair coatings  30  of similar thicknesses but lower weight. Preferred materials for the hollow particles  28  include aluminosilicates having a particle density of less than 0.40 g/cm 3 . Various sources of hollow ceramic powders exist, including Sphere One, Inc. (Extendospheres®), 3M® (Zeeospheres), Sphere Services Inc. (cenospheres), and Trelleborg Emerson &amp; Cuming, Inc. (Eccospheres®). Preferred hollow ceramic particles  28  include aluminosilicates with a particle size range of about 0.05 to about 200 micrometers, such as Extendospheres® SL (about 10 to about 150 micrometers) available from Sphere One, Inc., and Zeeospheres (D50 of about 18 micrometers) available from 3M. 
     The silica precursor serves as a binder in the coating composition  24 . Preferred silica precursors are believed to be silicone-based compositions such as polymethyl siloxane, though it is foreseeable that other silicon sources and ceramic precursors could be used, such as TEOS (tetra-ethyl-ortho-silicate) or possibly a colloidal silicon source. Particularly suitable precursors include methylsesquisiloxane mixtures of the polysiloxane family available from sources such as Apollo Plastics Corporation (for example, SR350 and SR355) and Dow Chemical Corporation, and a polyvinyl butyral available under the name B-79 from Monsanto Co. 
     The remaining constituents for the composition  24  are preferably organics, primarily a carrier liquid or solvent and optionally surfactants, dispersants, and/or additional binders/plasticizers capable of adhering the powder particles  26  and  28  together to yield a composition  24  that can be applied to the surface  22 . Depending on the types and amounts of the additional ingredients, the composition  24  may be formulated and processed as solid but pliable tapes that can be individually applied to the surface  22 , or a more pliable and malleable material that can be applied as a putty or paste. A suitable carrier liquid/solvent is an anhydrous alcohol such as ethanol, denatured alcohol and isopropyl alcohol, though acetone, trichloroethylene, and others compatible with silicone materials could be used. Suitable plasticizers include dibutyl pthalate (DBP) and polyvinyl butyral (for example, the aforementioned B-79). If the composition  24  is to be used in the form of a tape, a sufficient fraction of binders and plasticizers should be present to allow the tape to be applied and chemically or mechanically bonded to the surface  22  with the use of heat and pressure. Surfactants can also be used to achieve a suitably tacky consistency that enables the composition  24 , particularly those prepared as a tape, to adhere to the surface  22  exposed by the spalled region  20 . Suitable surfactants include an alkyl organic phosphate ester acid surfactant commercially available as PS21A from Whitco Chemical. Another surfactant that can be used is available under the name Merpol A from Stephan. 
     The fraction of organics used in the composition  24  may also depend on whether the repair process is intended to produce a coating  30  whose structural properties vary through its thickness, in which case multiple layers of the composition  24  with different compositions are applied to the surface  22 . For example, it may be desirable that the innermost layer or region of the applied coating material  24  contains sufficient binders/plasticizers to produce submicron voids and yield a desirable porosity level that increases the thermal insulation capability of the coating  30 , while the fraction of organics used in the outermost layer or any intermediate layers is preferably lower to minimize porosity to promote abrasion and infiltration resistance. 
     Whether formulated as a paste or tape, it may be desirable to formulate certain regions of the coating  30  to have additional enhanced properties. For example, the outermost surface region of the coating  30  may incorporate IR-reflective or IR-absorbing particles, as well as other constituents such as erosion- and/or corrosion-resistant materials. Another example is to formulate the outermost surface region of the coating  30  to achieve a smoother surface finish that promotes the aerodynamics of the component  10 . In this case, it may be desirable to apply as the outermost layer a coating composition  24  that uses finer solid and/or hollow particles  26  and  28 . For example, the interior of the coating  30  may be formed by a composition  24  that contains an alumina powder material such as A14, while the exterior of the coating  30  may be formed by a composition  24  that contains a finer alumina powder material such as A16SG, and may completely omit the hollow particles  28 . 
     Approximate broad and preferred ranges are stated in weight percents in Tables I and II below for individual constituents for coating compositions  24  in the form of pastes and tapes, respectively. 
     
       
         
           
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                 CONSTITUENTS 
                 BROAD 
                 PREFERRED 
                 EXAMPLE 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Solvent 
                  10-60% 
                   15-45% 
                 29.3% 
               
               
                 Solid Powder Particles 
                 5-55 
                 12-35 
                 22.0 
               
               
                 Hollow Powder Particles 
                 5-45 
                 10-35 
                 30.2 
               
               
                 Silica Precursor (binder) 
                 6-40 
                 10-25 
                 18.5 
               
               
                   
               
            
           
         
       
     
     For the quantities indicated for the Example in Table I, the solvent is preferably denatured alcohol or acetone, the solid powder particles are preferably A16SG or A14 alumina, and the silica precursor is preferably SR350. 
     
       
         
           
               
               
               
               
             
               
                 TABLE II 
               
               
                   
               
               
                 CONSTITUENTS 
                 BROAD 
                 PREFERRED 
                 EXAMPLE 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Solvent 
                  10-60% 
                   12-40% 
                 21.5% 
               
               
                 Solid Powder Particles 
                 5-55 
                 15-40 
                 28.4 
               
               
                 Hollow Powder Particles 
                 5-45 
                 10-40 
                 33.3 
               
               
                 Silica Precursor (binder) 
                 3-40 
                  5-20 
                 8.5 
               
               
                 Other binders 
                 2-20 
                  3-10 
                 4.0 
               
               
                 Plasticizer(s) 
                 1-10 
                 1-5 
                 2.0 
               
               
                 Surfactant(s) 
                 0-9  
                 1-5 
                 2.6 
               
               
                   
               
            
           
         
       
     
     For the quantities indicated for the Example in Table II, the solvent is preferably denatured alcohol or acetone, the solid powder particles are preferably A16SG alumina, the silica precursor is preferably SR355, the additional binder is preferably B-79, the plasticizer is preferably DBP, and the surfactant is preferably PS21A. The solvent is evaporated from the tape compositions of Table II prior to application of the tape to the component surface  22  and sintering to form the repair coating  30 . 
     The choice of silica precursor in Tables I and II is due in part to their differing silica yields. The SR350 binder indicated in Table I yields silica in an amount of about 60 to about 75 weight percent of the original amount of SR350 binder present in the coating composition  24 , whereas a like amount of the SR355 binder indicated in Table II yielding silica in lower amounts of about 30 to about 40 weight percent of the original amount of SR355 binder present in the tape coating composition  24 . 
     A suitable process for forming coating compositions  24  of this invention as a paste involves combining the above-noted constituents of Table I to achieve a suitable paste-like consistency, after which the composition  24  can be applied to fill the spalled region  20  in any suitable manner, such as with a trowel. Depending on its composition, the binder of the paste composition  24  may react at room temperature, or its reaction accelerated by heating such as with a heat lamp, torch, or other heat source until the strength of the resulting repair coating  30  has reached a required level for operation in the turbine engine. A suitable cure treatment is about sixteen hours at room temperature to cure the preferred silicone binders, though cure times can be significantly reduced at elevated temperatures. Conformance of the paste composition  24  to the spalled region  20  and curing of the binder can be promoted by using a thermal treatment that includes pressing the applied composition  24  with a heated iron. Thereafter, post processing operations can be performed to prepare the component  10  for use. 
     During operation of the turbine engine, the repair coating  30  continues to react, associated with an increase in the strength and other mechanical properties of the coating  30 . The preferred silicone binders initially cure by polymerization to form a silicone matrix whose strength is sufficient for engine operation. With extended use at high temperatures, the silicone thermally decomposes to silica, forming a silica matrix in which the ceramic particles  26  and  28  are dispersed. 
     A suitable process for forming coating compositions  24  of this invention as a tape involves casting the one or more tapes on a tetrafluoroethylene (i.e., TEFLON®) sheet. Compositions within the ranges defined in Table II are applied to the TEFLON sheet and then dried for a duration sufficient to evaporate the solvent. The dried tapes are then removed from the TEFLON sheet and transferred to the component surface to be protected by the repair coating  30 . If a coating  30  with multiple layers of different properties is desired, a tape formulated to produce an innermost layer or region of the coating  30  may be applied first, followed by one or more tapes to form the outermost and any intermediate layers or regions of the coating  30 . Alternatively, a single multilayer tape may be cast that contains the desired different coating compositions  24 , such that only a single tape application is required. An advantage with using a single multilayer tape is that the relative amounts of the solid particles  26 , hollow particles  28 , and binder can be varied to achieve different porosity levels within the coating  30 , for example, greater porosity near the surface  22  of the component  10  and lower porosity near the outer surface of the coating  30 . As noted above, the smoothness of the surface of the repair coating  30  can also be improved by appropriately selecting the relative sizes of the solid and hollow particles  26  and  28 , and the relative amounts of the solid and hollow particles  26  and  28  and binder. 
     Following tape application, pressure is preferably applied to the outer surface of the tape(s) through the use of a caul plate, rubber form, or other suitable means in order to produce the desired final surface finish and geometry for the coating  30 . If the component  10  has been removed, a vacuum bag can be used in conjunction with an autoclave to apply the heat and pressure required to chemically or mechanically bond the tape(s) to the component  10 . The unsintered tape or tapes can then be sintered by operating the engine or an additional thermal treatment to consolidate and set the tape(s). In either case, sintering is performed at a temperature that will not adversely affect the desired properties for the component  10 , but above the temperatures at which the binders and plasticizers will burn off and the ceramic particles  26  and  28  will sinter. 
     While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.