Patent Publication Number: US-2006016191-A1

Title: Combined effusion and thick TBC cooling method

Description:
GOVERNMENT INTERESTS  
      The invention was made with Government support under contract with the US Army (DAAE07-02-3-0002). The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention generally relates to methods and apparatus for cooling components exposed to high temperatures, such as components of a gas turbine engine. More particularly, this invention relates to cooling methods and apparatus combining effusion cooling and thick thermal barrier coating (TBC).  
      Gas turbine engine components, such as combustors, turbine blades, vanes, nozzles and shrouds, are exposed to temperatures that can reduce the operating life of the components. Effusion cooling and TBCs have been used extensively to improve component life.  
      Effusion cooling comprises an array of effusion cooling holes through the component wall. A supply of cooling air is passed through the holes from the cooler surface of the component to the surface exposed to higher temperatures. The cooling air actively cools the component wall by convection taking place in the hole and film cooling after the cooling air is discharged. The cooling holes are typically formed by conventional drilling techniques such as electrical-discharge machining (EDM) and laser machining, or with complex casting techniques.  
      For additional thermal and/or environmental resistance, a TBC can be applied on the surface of the component that is exposed to higher temperatures. TBCs comprise ceramic thermal protective coatings, such as yttria stabilized zirconia, and are applied to the surface of the component to insulate the component from a high temperature source, such as a hot combustion effluent. When TBC application occurs after cooling hole formation, a significant amount of TBC can be deposited in the cooling holes. The TBC deposits in the cooling holes can detrimentally affect the service life of the component because the TBC can alter the shape and reduce the size of the cooling holes. Methods for removing TBC from the cooling holes and/or reducing the amount of TBC deposited into the cooling holes have been described.  
      In one method a masking material is positioned in the cooling holes prior to TBC application to prevent the TBC from entering the cooling holes. When the masking material is removed, chipping and cracking often occurs along the edge due to the high cohesive strength of the TBCs in a direction horizontal to the plane of the substrate. The force needed to remove the masking material can cause a portion of the TBC to be pulled off the coated section of the substrate. In the case of turbine engine components, the loss of a portion of coating material exposes the corresponding portion of the component to very high in-service temperatures. Additionally, chipping and cracking along the edge can serve as crack propagation sites for further degradation throughout the coating.  
      Another method comprises a water jet containing an abrasive media, such as particles with sharp corners and edges, for excess TBC removal. The erosion and abrasion caused by the abrasive particles in the water jet at pressures adequate to remove the TBC deposit also damages the cooling hole. Additionally, for some applications, the abrasive media cannot be reused and must be disposed of, which increases production costs. Another water jet method uses a very high-pressure water jet. The TBC accumulated in a cooling hole is removed by projecting the jet toward the uncoated surface of the hole, with the component itself serving as a mask to prevent the jet from eroding the coating. Although this method may reduce coating erosion, further improvements are still needed.  
      A method for reducing the TBC deposited in the cooling hole is disclosed in U.S. Pat. No. 6,620,457. In the described method, the TBC is applied in a direction such that the deposited TBC only partially blocks the holes. Unfortunately, following TBC deposition, this method also requires the holes to be cleaned by a water jet process.  
      A method that requires neither a water jet nor a masking material has been described in U.S. Pat. No. 5,941,686. The method comprises laser drilling the effusion holes such that the diameter of the holes is larger on the side on which the TBC is to be deposited. In one example, a combustor was provided with effusion holes having 0.02″ diameters on the “cold” side and 0.03″ diameters on the “hot” side. A metallic bond coat was applied to a thickness of about 0.004-0.006″. A TBC was deposited by plasma spray to a thickness of about 0.008-0.010″. Although the TBC deposited in the cooling holes in this example did not reduce fluid flow through the holes, this method may not be suitable for some applications. Using the same relative sizes for the “cold” side of about 0.02″ and the “hot” side of about 0.03″, a TBC coating of about 0.015″ did reduce fluid flow through the passage. For thick TBCs, further improvements are still needed.  
      Another cooling method combining effusion holes and TBC has been described in U.S. Pat. No. 6,573,474. In the disclosed method, the holes were drilled in a two-step process after the TBC was deposited. In the first step a counterbore was laser drilled to a depth that extended through the ceramic topcoat but not substantially into the workpiece. In the second step a smaller diameter hole was drilled through the workpiece. The two-step drilling process was found to reduce or avoid the formation of a recast bubble at the intersection of the TBC and substrate. In the disclosed example the TBC was deposited to a thickness between 0.009 and 0.014 inches, and the typical thickness for combustion liner TBC was described as between 0.003 and 0.010 inches. Unfortunately, thicker TBCs are desired for some applications.  
      When depositing a sufficiently thick TBC to thermally insulate such hot section components as combustor liners, cooling holes are often machined by EDM and laser drilling after deposition of the bond coat but prior to application of the TBC. After TBC application, a hole-cleaning step is necessary to remove the excess TBC. Although other methods have included cooling hole formation after TBC deposition, these methods are unsuitable when a thick TBC is desired. Laser drilling is prone to spalling the brittle ceramic TBC by cracking the interface between the component substrate and the ceramic. The spalling off severely reduces the sealing effectiveness and the insulative characteristics of the ceramic coating, causing component failure and expensive repairs. EDM cannot be used to form cooling holes in a component having a TBC because the ceramic is electrically nonconducting. Although cooling hole formation after TBC application may avoid excess TBC deposits, the described methods are unsuitable for some applications, especially for applications requiring thick TBC.  
      As can be seen, there is a need for improved combined effusion and TBC cooling methods and apparatus. Additionally, improved methods are needed wherein the TBC comprises a thick TBC, for example a TBC having a thickness greater than about 0.02 inches. Further, methods are needed wherein cooling hole masking and/or cleaning processes are unnecessary.  
     SUMMARY OF THE INVENTION  
      In one aspect of the present invention, a method of cooling comprises the steps of providing a substrate; depositing a thermal barrier coating to a thickness of at least about 0.020 inches onto the substrate to produce a coated material; and forming an effusion array through the coated material.  
      In another aspect of the present invention, a method of cooling a combustor comprises the steps of applying a bond coat to the combustor; depositing a thermal barrier coating to a thickness greater than about 0.020 inch onto the bond coat such that a segmentation microcracked coating is produced; and machining at least one effusion hole through the segmentation microcracked coating and the combustor.  
      In yet another aspect of the present invention, a method of cooling a combustor comprises the steps of applying a bond coat to the combustor; depositing a thermal barrier coating onto the bond coat such that a segmentation microcracked coating having a thickness between about 0.020 and about 0.050 inches is produced, the thermal barrier coating comprising a material selected from the group consisting of stabilized cubic zirconia, stabilized cubic hafnia, stabilized tetragonal zirconia, stabilized tetragonal hafnia, yttria-stabilized cubic zirconia, yttria-stabilized cubic hafnia, yttria-stabilized tetragonal zirconia, and yttria-stabilized tetragonal hafnia; and laser drilling at least one effusion hole through the segmentation microcracked coating and the combustor.  
      In still another aspect of the present invention, a method of forming an effusion hole comprises the steps of providing a substrate having a thermal barrier coating, the thermal barrier coating having a columnar crack structure and a thickness between about 0.020 and about 0.100 inches; and laser drilling at least one effusion hole through the substrate.  
      In a further aspect of the present invention, a method of cooling a substrate comprises the steps of depositing a thermal barrier coating on the substrate to a thickness of at least about 0.02 inches such that a coated material having a columnar crack structure is produced; and drilling at least one effusion hole through the coated material.  
      In still another aspect of the present invention, an apparatus for a gas turbine engine comprises a combustor having a segmentation microcracked thermal barrier coating and a plurality of effusion holes therethrough, the segmentation microcracked thermal barrier coating having a thickness between about 0.020 and about 0.100 inches.  
      These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow chart of a method for combined effusion and thick TBC cooling according to an embodiment of the present invention;  
       FIG. 2  is a perspective view of a combustor according to one embodiment of the present invention;  
       FIG. 3  is a close-up cross-sectional view of  FIG. 2 ;  
       FIG. 4  is a close-up view of  FIG. 3 ;  
       FIG. 5  is a cross-sectional view of a TBC coated substrate according to one embodiment of the present invention;  
       FIG. 6  is a boxplot of TBC-bond coating interface crack length (inch) vs laser pulse power setting (Joules) according to one embodiment of the present invention;  
       FIG. 7   a  is a cross-sectional view of an on-the-fly laser drilled TBC coated substrate according to one embodiment of the present invention;  
       FIG. 7   b  is a close-up cross-sectional view of  FIG. 7   a;    
       FIG. 8  is a boxplot of TBC-bond coating interface crack length vs laser defocus, which is the laser focus distance above the TBC surface, according to one embodiment of the present invention;  
       FIG. 9   a  is a cross-sectional view of stationary percussion laser drilled TBC coated substrate according to one embodiment of the present invention;  
       FIG. 9   b  is a close-up cross-sectional view of  FIG. 9   a;    
       FIG. 10  is a cross-sectional view of effusion holes drilled using a variety of pulses according to one embodiment of the present invention;  
       FIG. 11  is a close-up cross-sectional view of the hole produced by a series of 20 laser pulses in  FIG. 10 ; and  
       FIG. 12  is a close-up cross-sectional view of a hole drilled with 12 laser pulses using a 0.080″ defocus, a pulse power of 12 joules, and a pulse duration of 0.5 microsecond, according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
      The present invention generally provides combined effusion and thick TBC cooling methods and apparatus. The cooling methods and apparatus according to the present invention may find beneficial use in many industries including aerospace, automotive, and electricity generation. The present invention may be beneficial in applications including manufacturing and repair of aerospace and automotive components, such as turbine engines, combustors, nozzles, shrouds and vanes. This invention may be useful in any fluid cooled component application.  
      The present invention provides a combined effusion and thick TBC cooling method and apparatus. Unlike the prior art, a thick TBC may be deposited prior to effusion hole formation, making a step of removing TBC deposit from the effusion holes unnecessary. In prior art methods, cooling holes are machined by laser drilling after deposition of the bond coat but prior to application of the thick TBC because laser machining is prone to spalling the brittle ceramic TBC by cracking the interface between the component substrate and the ceramic. Unlike the prior art, the present invention provides a method comprising laser drilling the cooling holes into the substrate after a thick TBC has been deposited. The TBC of the present invention may be deposited such that the TBC has a columnar crack structure comprising a plurality of segmentation microcracks. The segmentation microcracks may reduce cracking and chipping of the TBC during the laser drilling process.  
      A method of the present invention is shown in  FIG. 1 . The method  20  may comprise a step  21  of providing a substrate, a step  22  of applying a bond coat, a step  23  of depositing a TBC to produce a TBC coated substrate, and a step  24  of laser drilling at least one effusion hole through the TBC coated substrate. In one embodiment of the present invention, the step  21  of providing a substrate  30  may comprise providing a combustor  40 , shown in  FIG. 2 . A bond coat  31  may be applied to the combustor  40 , better seen in  FIGS. 3-4 . A TBC  32  may be deposited onto the bond coat  31  and a plurality of effusion holes  34  may be laser drilled through the TBC coated substrate (coated material  35 ).  
      The substrate  30  of step  21 , as shown in  FIG. 5 , may comprise any component exposed to high temperatures. Useful components may include gas turbine engine components, for example combustors, vanes and shrouds. The substrate  30  may comprise a metal or a metal alloy, such as nickel based and cobalt based superalloys. Useful nickel based and cobalt based superalloy substrates may comprise sheet metal, equiaxed, DS (directionally solidified) and SC (single crystal) investment castings as well as other forms of these superalloys, such as forgings, pressed superalloy powder components, machined components, and other forms. Useful nickel based superalloys may include HA230™ (available from Haynes International), Rene&#39; alloy N5™ (available from General Electric), MarM247™ (available from Martin Marietta), PWA 1422™ (available from Pratt Whitney), PWA 1480™ (available from Pratt Whitney), PWA 1484™ (available from Pratt Whitney), Rene&#39; 80™ (available from General Electric), Rene&#39; 142™ (available from General Electric), SC 180™ (available from Honeywell) and others. Useful cobalt based superalloys may include HA188™ (available from Haynes International) and MarM509™ available from Martin Marietta and others.  
      The bond coat  31  of step  22  may be applied to the surface of the substrate  30  to improve TBC adhesion. The bond coat  31  may grade the thermal expansion mismatch between the TBC  32  and the substrate  30 . The bond coat  31  may comprise an additional metallic layer. The bond coat  31  may include oxidation-resistant coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, silicon, etc.), and diffusion coatings such as diffusion aluminides that contain aluminum intermetallics, for example NiAl and (Ni,Pt)Al. For nickel-based superalloy substrates useful bond coats  31  may include NiCrAlY and NiCoCrAlY. The composition of a useful bond coat  31  may depend on factors including the composition of the substrate  30 . The thickness of a useful bond coat  31  may depend on factors including the composition of the bond coat  31 , the application and the composition of the substrate  30 . For example, a bond coat  31  comprising NiCrAlY may be applied to a thickness between about 0.003 and about 0.008 inches on a substrate  30  comprising HA230™ (Haynes International) for a combustor application. The bond coat  31  may be applied by any known method, such as by plasma spray. For example, a bond coat comprising MCrAlY may be deposited by air plasma spray (APS), inert gas shrouded plasma spray, low pressure (vacuum) plasma spray (LPPS), or high velocity oxyfuel (HVOF) techniques. The bond coating may also be applied to the substrate by the electron beam evaporation-physical vapor deposition (EB-PVD) process. The bond coat  31  may be positioned between the substrate  30  and a TBC  32 .  
      If the TBC is applied by the electron beam evaporation-physical vapor deposition process, the bond coating may be either an MCrAlY or an intermetallic coating, such as a Pt-aluminide. Bond coatings for EB-PVD TBCs are disclosed U.S. Pat. No. 4,321,311 and U.S. Pat. No. 5,514,482, which are incorporated herein by reference.  
      The TBC  32  of step  23  may comprise a thermal-insulating ceramic material. The composition of a useful TBC  32  may comprise a stabilized zirconia, such as yttria-stabilized zirconia (YSZ). The TBC  32  may comprise one or more oxides. Useful oxides may include zirconia, hafnia, yttria, scandia, ytterbia, neodymia, samaria, gadolinia, magnesia, calcia, ceria, alumina, tantala and others. A useful TBC  32  may comprise zirconia stabilized with about 18% to about 22% by weight yttria. Another useful TBC  32  may comprise hafnia with about 18% to about 22% by weight yttria. Useful TBCs  32  may also include stabilized cubic zirconia, stabilized cubic hafnia, stabilized tetragonal zirconia, stabilized tetragonal hafnia, yttria-stabilized cubic zirconia, yttria-stabilized cubic hafnia, yttria-stabilized tetragonal zirconia, and yttria-stabilized tetragonal hafnia. Stabilizing oxides may comprise yttria, scandia, ytterbia, neodymia, samaria, gadolinia, magnesia, calcia, ceria, tantala, and other oxides to the compositional extent that they are soluble within the cubic or tetragonal phases of zirconia and hafnia. The concentration of the stabilizing oxide or oxides can be between the minimum solubility limit for full-stabilization of the tetragonal phase and the maximum solubility limit for full-stabilization of the cubic phase.  
      The composition of a useful TBC  32  may depend on factors including application. For example, the conductivity of a TBC  32  comprising 20% yttria stabilized zirconia may be about 60% of the conductivity of a TBC comprising 7% yttria stabilized zirconia. Useful TBCs  32  may be deposited to a thickness between about 0.020 and about 0.100 inches. For some applications, the TBC  32  may be deposited to a thickness between about 0.020 and about 0.050 inches. The TBC  32  may be deposited by plasma spray techniques such that the TBC  32  has a columnar crack structure.  
      The TBCs  32  may be deposited by known methods. A useful method for depositing the TBC  32  is disclosed in U.S. Pat. No. 5,073,433, which is incorporated herein by reference. A TBC  32  deposited by the &#39;433 method may provide a TBC  32  having a columnar crack structure. A columnar crack structure may comprise a TBC  32  having a plurality of segmentation microcracks  33 , as seen in  FIG. 5 . A segmentation microcrack  33 , as defined herein, is a crack in the coating if extended to contact the surface of the substrate will form an angle of from about 30° to about 0° with a line extended from a contact point normal to the surface of the substrate. As defined herein, a TBC having a columnar crack structure (or segmented columnar structure) is a TBC having at least about 20 segmentation microcracks per linear inch measured in a line parallel to the surface of the substrate and in a plane perpendicular to the substrate. A useful method of depositing the TBC  32  may provide a TBC  32  having a plurality of homogeneously dispersed segmentation microcracks  33  (segmentation microcracked TBC). TBCs  32  having segmentation microcracks  33  may have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural defects and pores at and between grain boundaries of the TBC microstructure. Any method of depositing the TBC  32  that provides a TBC  32  having a columnar crack structure may be useful with the present invention.  
      EB-PVD is another known method (U.S. Pat. No. 4,321,311 and U.S. Pat. No. 5,514,482) to deposit TBCs  32 . The EB-PVD process results in a thermal barrier coating with a finely-segmented columnar-grain ‘ceramic rug’ microstructure, which provides compliance for accommodating laser drilling strains. The microstructure of a TBC deposited by EB-PVD may comprise columnar grains with intercolumnar gaps.  
      The method  20  may comprise a step  24  of laser drilling at least one effusion hole  34  through the coated material  35 . The effusion holes  34  may be formed by known laser drilling methods. The step  24  may comprise stationary percussion laser drilling. The percussion method uses a series of laser energy pulses to drill the hole. The step  24  may comprise percussion on-the-fly laser drilling. A percussion on the fly method is particularly advantageous for economically drilling laser holes. The percussion on the fly method creates a line of percussion holes by rapidly moving the workpiece under the timed pulses of a laser. For example, when an annular combustion liner is rotated under a stationary laser&#39;s lens at a fixed speed, a line of 360 holes may be created by timing the laser pulses to occur after each degree of the liner&#39;s rotation. A useful method for forming the effusion holes  34  may comprise laser drilling through the TBC  32  coated substrate  30  (coated material  35 ) in a one step process.  
      Another useful drilling method is described in U.S. Pat. No. 6,573,474, which is incorporated herein by reference. The &#39;474 method is a two-step laser drilling process. The first step produces a counterbore to reduce the extent of the overhanging TBC  32  and the second step drills through the substrate  30 . The step  24  of laser drilling may provide a plurality of effusion holes  34  through the TBC  32  coated substrate  30 . The microstructure of the segmentation microcracked TBC  32  may reduce cracking and chipping of the TBC  32  during the step  24  of laser drilling. This may be because the strain-tolerant grain structure may be able to expand and contract without causing damaging stresses that lead to spallation.  
      The useful number and orientation of the effusion holes  34  may vary with application. The effusion holes  34  may be configured such that an airflow passing through an array of effusion holes  34  distributes a cooling film over the component surface. Due to mechanical limitations, the effusion holes  34  typically are drilled at an angle ranging from about 15° to about 90° relative to the surface. Computational fluid dynamic (CFD) analysis may be useful in determining the desired effusion array configuration for a particular application. The diameter of a useful effusion hole  34  may be between about 0.010 and about 0.050 inches. For some applications, the diameter of a useful effusion hole  34  may be between about 0.015 and about 0.025 inches.  
     EXAMPLE 1  
      A substrate comprising HA230™ (available from Haynes International) was formed into a 8″×12″ diameter cylinder. A bond coat comprising NiCrAlY, which had a nominal composition of 31 weight % Cr, 11 wt % Al, 0.5 wt % Y, and the balance Ni, was applied by plasma spray to a thickness of 0.0055 plus or minus 0.0025 inches. A TBC comprising 20 weight % yttria stabilized zirconia was deposited by Praxair Surface Technology, Inc. (Indianapolis, Ind.) to a thickness of 0.040 plus or minus 0.003 inches. The coated cylinder was laser drilled using conventional percussion on-the-fly laser drilling techniques. The TBC crack length vs laser pulse power setting (Joules) is shown in  FIG. 6 . For each power J setting, four holes were drilled. The holes each had a nominal diameter of 0.020 inch. The laser defocus, which is the distance of the lens focal point above the ceramic surface, was 0.08 inch. As can be seen, power setting 15.0 Joules resulted in TBC cracks ranging from about 0.00 to about 0.04 inch.  
       FIGS. 7   a  and  7   b  show cross-sectional views of the percussion on-the-fly laser drilled TBC coated substrate (15 J, defocus 0.08″). A TBC crack  36  (interface crack) about 0.03 inches in length can be seen. A TBC interface crack  36  is a crack in a direction parallel to the plane of the substrate  30 .  
     EXAMPLE 2  
      A substrate comprising HA230™ (available from Haynes International) was formed into a 8″×12″ diameter cylinder. A bond coat comprising NiCrAlY was applied by plasma spray to a thickness of 0.0055 plus or minus 0.0025 inches. A TBC comprising 20 weight % yttria stabilized zirconia was deposited by Praxair Surface Technology, Inc. to a thickness of 0.040 plus or minus 0.003 inches. The coated cylinder was laser drilled using conventional stationary percussion laser drilling techniques. The TBC interface crack length vs laser defocus relationship is shown in  FIG. 8 . Four holes were drilled for each laser defocus settings of 0.080, 0.125 and 0.250 inch. Three holes were drilled for the defocus setting of 0.500 inch. The holes each had a nominal diameter of 0.020 inch. As can be seen, laser defocus setting of 0.250 inch produced TBC interface cracks  36  ranging from about 0.005 to about 0.045 inches.  
       FIGS. 9   a  and  9   b  show cross-sectional views of the stationary percussion drilled TBC coated substrate (9.4 J, 0.25 defocus). A TBC interface crack  36  less than about 0.03 inches in length can be seen.  
     EXAMPLE 3  
      A substrate comprising HA230™ (available from Haynes International) was formed into a 8″×12″ diameter cylinder. A bond coat comprising NiCrAlY was applied by plasma spray to a thickness of 0.0055 plus or minus 0.0025 inches. A TBC comprising 20 weight % yttria stabilized zirconia was deposited by Praxair Surface Technology, Inc. to a thickness of 0.040 plus or minus 0.003 inches. The coated strip was laser drilled using conventional stationary percussion laser drilling techniques. The holes were drilled at laser pulse process settings of 9.4 J, 0.5 microsecond, 0.25″ defocus. A variety of pulses were used to drill four partial holes, shown in  FIG. 10 . These holes illustrate the propagation of percussion holes through the coating and initial penetration into the substrate. (These holes were not intended to penetrate the full thickness of the specimen.) The first hole  51  was formed using a series of  20  pulses, the second hole  52  was formed using 25 pulses, the third hole  53  was formed using 35 pulses, and the forth hole  54  was formed using 45 pulses. A close-up view of the forth hole  54  is shown in  FIG. 11 .  FIG. 12  shows a close-up cross-sectional view of an effusion hole drilled at 12 J, 0.5 μsec, 0.080″ defocus, and 12 pulses. As can be seen, TBC crack formation was reduced.  
      As can be appreciated by those skilled in the art, the present invention provides improved cooling methods and apparatus using effusion cooling and a thick TBC. Further, an improved method for providing an effusion hole array through a thick TBC coated substrate is provided.  
      It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.