Abstract:
A thermal coating includes a substrate, a first coating layer, and a second coating layer. The substrate is selected from the group consisting of superalloys and ceramic matrix composites. The first coating layer comprises an alumina powder, a silica binder, and at least one additive selected from either a first group or a second group. The second coating layer comprises at least one of zinc titanate or cerium oxide. A method for applying a thermal coating system includes spraying a bond coat mixture onto a substrate using a liquid electrostatic sprayer. The bond coat mixture comprises an alumina powder, a silica binder, and at least one additive selected from either a first group or a second group. The method further includes applying a top coat mixture onto the bond coat mixture, wherein the top coat mixture comprises at least one of zinc titanate or cerium oxide.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to thermal barrier coatings applied to protect components in high temperature environments. In particular, the present invention describes a composition and method for applying a thermal coating system. 
     BACKGROUND OF THE INVENTION 
     Systems located or operated in high temperature environments often include thermal barrier coatings (TBCs) on components to reflect heat and prevent the components from absorbing heat. For example, jet engines and gas turbines include combustors and turbines designed to operate in very demanding high temperature and pressure environments. As a result, many components, such as combustor liners, turbine blades, turbine casings, and rotors routinely operate in high temperature environments that approach or exceed the melting temperature of the constituent elements included in the components. A TBC applied to the surface of these components allows the components to operate at increasingly higher temperatures and/or with increased intervals between maintenance cycles. 
     The underlying components are typically designed to operate for extended periods in the structurally demanding high temperature and/or pressure environments. Superalloys such as Rene 80, Rene N4, and other nickel-based superalloys are commonly used in the underlying components. These superalloys may contain, by weight percent, 10 to 80 percent nickel, 5 to 22 percent chromium, up to 10 percent molybdenum, up to 5.5 percent titanium, up to 6.5 percent aluminum, up to 3 percent columbium, up to 9 percent tantalum, up to 15 percent tungsten, up to 2 percent hafnium, up to 1 percent rhenium, up to 1.5 percent vanadium, up to 40 percent cobalt, and up to 6 percent iron. 
     Ceramic matrix composites (CMCs) may also be selected for use in the underlying components Examples of commonly used CMCs include zirconia-based ceramics, alumina-based ceramics, magnesia-based ceramics, and ceramic composites such as alumina-silica (GE Gen 4), or a refractory material with, for example, silicon carbide, silicon nitride, alumina, silica, and/or calcia. 
     A suitable TBC applied to the underlying component should include one or more of the following characteristics: low emissivity or high reflectance for heat, particularly infrared heat having a wavelength of 0.5 to 60 micrometers; a smooth finish; and good adhesion to the underlying component. For example, thermal barrier coatings known in the art include metal oxides, such as zirconia (ZrO 2 ), partially or fully stabilized by yttria (Y 2 O 3 ), magnesia (MgO), or other noble metal oxides. The selected TBC may be deposited by conventional methods using air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD), which yields a strain-tolerant columnar grain structure. The selected TBC may also be applied using a combination of any of the preceding methods to form a tape which is subsequently transferred for application to the underlying substrate, as described, for example, in U.S. Pat. No. 6,165,600, assigned to the same assignee as the present invention. 
     The thermal barrier coatings described above have coefficients of thermal expansion that are significantly lower than the coefficients of thermal expansion of the underlying components. As a result, cyclic thermal stresses incident to repetitive heating and cooling of the system components disrupts the adhesion between the TBC and the underlying substrate, leading to spalling of the coating system. 
     A bond coat may be used between the TBC and the underlying substrate to improve the adhesion between the TBC and the underlying substrate. The bond coat may be formed from an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant alloy such as MCrAlY (where M is iron, cobalt and/or nickel). Aluminide coatings are distinguished from MCrAlY coatings, in that the former are intermetallics, while the latter are metallic solid solutions. U.S. Pat. No. 6,210,791, assigned to the same assignee as the present invention, describes one such bond coat applied between the TBC and the underlying substrate that substantially improves adhesion between the TBC and the underlying substrate. The bond described therein is an alumina and silica mixture in an alcohol solvent. 
     The thermal barrier coatings, with our without a bond coat to improve adhesion, typically require some type of post-application drying or heating at 500 to 2000 degrees Fahrenheit to sinter and/or stabilize the coating system. The application and post-application curing produces volatile organic compounds (VOCs) which may exceed current environmental, health, and safety limits for VOC emissions. To reduce VOC emissions during the application and post-application curing, the thickness of the TBC and/or bond coat may be reduced. However, the thinner TBC and/or bond coat results in a corresponding decrease in the thermal reflection of the thermal barrier. 
     Therefore, the need exists for an improved thermal coating system to protect system components from excessive heat. Ideally, the thermal coating system will have low emissivity or high reflectance for heat, particularly infrared heat having a wavelength of 0.5 to 60 micrometers. In addition, the thermal coating system should be able to be easily applied so as to produce a smooth finish surface that adheres to the underlying substrate component without producing excessive VOCs during the application or post-application curing. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     One embodiment of the present invention is a thermal coating system. The thermal coating system includes a substrate, a first coating layer applied to the substrate, and a second coating layer applied to the first coating layer. The substrate is selected from the group consisting of superalloys and ceramic matrix composites. The first coating layer comprises an alumina powder, a silica binder, and at least one additive selected from either a first group or a second group. The first group consists of toluene, zylene, cellosolve acetate, EE acetate, and mineral spirits. The second group consists of methyl ethyl ketone, methyl isobutyl ketone, lacquer thinner, and acetone. The second coating layer comprises at least one of zinc titanate or cerium oxide. 
     Another embodiment of the present invention is a method for applying a thermal coating system. The method includes applying a first charge to a bond coat mixture, wherein the bond coat mixture comprises an alumina powder, a silica binder, and at least one additive selected from either a first group or a second group. The first group consists of toluene, zylene, cellosolve acetate, EE acetate, and mineral spirits. The second group consists of methyl ethyl ketone, methyl isobutyl ketone, lacquer thinner, and acetone. The method further includes applying a second charge to a substrate, wherein the second charge has an opposite polarity of the first charge, and spraying the bond coat mixture onto the substrate. The method also includes applying a top coat mixture onto the bond coat mixture, wherein the top coat mixture comprises at least one of zinc titanate or cerium oxide. 
     A further embodiment of the present invention is a method for applying a thermal coating system that includes spraying a bond coat mixture onto a substrate using a liquid electrostatic sprayer. The bond coat mixture comprises an alumina powder, a silica binder, and at least one additive selected from either a first group or a second group. The first group consists of toluene, zylene, cellosolve acetate, EE acetate, and mineral spirits. The second group consists of methyl ethyl ketone, methyl isobutyl ketone, lacquer thinner, and acetone. The method further includes applying a top coat mixture onto the bond coat mixture, wherein the top coat mixture comprises at least one of zinc titanate or cerium oxide. 
     Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which: 
         FIG. 1  provides a cross-sectional view of one embodiment of a coating system within the scope of the present invention; 
         FIG. 2  illustrates liquid electrostatic spraying of a coating system within the scope of the present invention; and 
         FIG. 3  is a graph of the reflective performance of one embodiment of a thermal barrier coating system within the scope of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. 
     Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
       FIG. 1  shows a cross-sectional view of a thermal coating system  10  applied to a substrate  12  according to one embodiment of the present invention. In this particular embodiment, the thermal coating system  10  includes first and second coating layers referred to respectively as a bond coat mixture  14  and a top coat mixture  16 . 
     The substrate  12  may be any material composition suitable for use in a high temperature environment. For example, superalloys and ceramic matrix composites as previously described are frequently selected for use in high temperature environments because of their suitable strength, ductility, and other physical characteristics. 
     The first layer or bond coat mixture  14  is applied to the substrate  12  and provides tight adhesion between the substrate  12  and any additional layers. The bond coat mixture  14  may be a modification of the bond coat described in U.S. Pat. No. 6,210,791, the entirety of which is herein incorporated by reference for all purposes. As described therein, the bond coat mixture  14  may be metallic, non-metallic, or a combination thereof, depending on the underlying substrate, and may include alumina powder, such as aluminum oxide, with a silica binder. An evaporable solvent, typically ethanol or isopropyl alcohol, is added to the bond coat mixture  14  to achieve the desired consistency. A suitable thickness for the bond coat mixture  14  may be approximately 0.5 to 8 mils (0.0005-0.008 inches), depending on the method of application and design needs. 
     To reduce the amount of VOCs generated during the application and drying, the bond coat mixture  14  may be applied using liquid electrostatic spraying (LES) techniques. In LES applications, an electrical charge is applied to the material being deposited, and a ground or opposite electrical charge is applied to the substrate. The charged material is then sprayed onto the substrate, and the polar attraction between the charged material and the substrate results in an increased deposit efficiency of the material onto the substrate with significantly less overspray and waste. The increased deposit efficiency produces a more uniform coverage, allowing the application of thinner layers of the material to the substrate to provide the same or better performance. As a result, LES applications provide significant cost savings of materials compared to conventional application techniques. In addition, the thinner application of the materials results in lower VOC emissions during both the application and the subsequent curing. 
     The electrical conductivity of the bond coat mixture  14  may need to be adjusted to obtain a desired particle size that allows the use of LES and improves the deposit efficiency. Additives such as toluene, zylene, cellosolve acetate, EE acetate, and mineral spirits may be added to the bond coat mixture  14  to make the mixture less electrically conductive and prevent agglomeration of the bond coat mixture  14  during spraying. Conversely, additives such as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), lacquer thinner, and acetone may be added to the bond coat mixture  14  to make the mixture more electrically conductive. 
       FIG. 2  illustrates an application of the bond coat mixture  14  using LES. A powder spray gun  18 , such as a Nordson Kinetix air spray system sold by Nordson Corporation, Westlake, Ohio, includes a nozzle  20  with an electrode  22 . An opposite charge or ground  24  is applied to a substrate  26 . As the spray gun  18  propels the bond coat mixture  14  through the nozzle  20 , the electrode  22  applies an electrical charge to the bond coat mixture  14 . The charged bond coat mixture  14  flows to the oppositely charged or grounded substrate  26  where the polar attraction between the charged bond coat mixture  14  and substrate  26  deposits the bond coat mixture  14  uniformly on a surface  28  of the substrate  26 . The magnitude of the electrical potential between the charged bond coat mixture  14  and the oppositely charged substrate  26  may be adjusted to increase or decrease the deposition rate on the substrate surface  28 , depending on the desired thickness of the application. 
     The use of nano-sized particles as the constituent elements in the bond coat mixture  14  further improves the benefits of LES. For example, LES application of nano-sized particles having an average diameter of less than approximately 500 nanometers may readily achieve uniform thicknesses of the bond coat mixture  14  as low as approximately 0.5 mils (0.0005 inches). A thinner application of the bond coat mixture  14  produces several benefits. For example, a thinner bond coat mixture  14  will have a correspondingly smaller change in temperature across the bond coat mixture  14 , resulting in better adhesion to the substrate  26 . In addition, the nano-sized particles will produce a more tightly packed and dense layer that increases the resistance of the bond coat mixture  14  to erosion. 
     Referring back to  FIG. 1 , the thinner application of the bond coat mixture  14  using LES may not adequately cover all imperfections  30  in the surface  32  of the substrate  12 . As a result, an additional undercoat layer (not shown) may be included between the bond coat mixture  14  and the substrate  12 . The undercoat layer may comprise the same bond coat as previously described in U.S. Pat. No. 6,210,791. That is, the undercoat layer may comprise an alcohol mixture of alumina powder, such as aluminum oxide, with a silica binder. The undercoat layer may be applied using conventional application techniques known in the art. For example, the undercoat layer may be applied as a slurry spray, using air plasma spraying (APS), low pressure plasma spraying (LPPS), or physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD). If needed, the undercoat layer is applied to a thickness of approximately 1 to 8 mils (0.001-0.008 inches) to fill in any imperfections  30  in the surface  32  of the substrate  12 . For applications in which the substrate surface  32  is sufficiently smooth, the undercoat layer may be reduced in thickness or omitted entirely. 
     The top coat mixture  16  is located on top of the bond coat mixture  14 . The combination of the bond coat mixture  14  and top coat mixture  16  provides the desirable smooth, wear, and reflective characteristics of the thermal coating system  10 . Specifically, a smooth outermost surface of the thermal coating system  10  promotes improved aerodynamics across the surface which may be important in various applications. The surface roughness of the top coat mixture  16  is preferably less than approximately 60 micrometers Ra and potentially less than 20 micrometers Ra. In addition, the bond coat mixture  14  tightly adheres the top coat mixture  16  to the substrate  12  to resist wear or spalling even after numerous thermal cycles. Lastly, the top coat mixture  16  possesses the desired reflectance characteristics, particularly for infrared heat having a wavelength between 0.5 and 60 micrometers, to protect the substrate  12  from heat in a high temperature environment. 
     The top coat mixture  16  may be comprised of zinc titanate or cerium oxide to provide the desired heat reflectance characteristics of the thermal coating system  10 . Suitable substitutes that may also provide the desired heat reflectance characteristics include barium titanate, yttrium oxide, dysprosium oxide, erbium oxide, europium, lanthanum oxide, lutetium oxide, thorium oxide, tungsten oxide, barium stannate, and barium tungstate, many of which may be supplied by Nano-Tek Technologies, Ltd. 
     The top coat mixture  16  may be applied using conventional application techniques known in the art. For example, the top coat mixture  16  may be wetted and layered on top of the bond coat mixture  14  as a slurry spray, using air plasma spraying (APS), low pressure plasma spraying (LPPS), or physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD). The thickness of the top coat mixture  16  depends on the desired heat reflectance and application method and typically ranges from approximately 1 to 10 mils (0.001-0.010 inches). 
     In particular embodiments of the present invention, the thermal coating system  10  may be applied to the substrate  12  using a tape process as described in U.S. Pat. No. 6,165,600 and assigned to the same assignee as the present invention. In this process, compositions of the bond coat mixture  14  and/or top coat mixture  16  and/or optional undercoat layer as described above may be cast on a tetrafluoroethylene sheet. After the solvent evaporates from the compositions, the dried compositions are removed from the tetrafluoroethylene sheet and transferred to the substrate  12  to form the thermal coating system  10 . Pressure may then be applied to the thermal coating system  10  to mechanically bond the thermal coating system  10  to the substrate  12 . 
     Regardless of the application method, whether directly onto the substrate  12  or using the tape process as previously described, the thermal coating system  10  may be heated or cured after application to the substrate  12 . An autoclave, oven, or similar device may be used to heat the thermal coating system  10  at a temperature of between 500 and 2,000 degrees Fahrenheit. The heat removes the binders and remaining solvent and sinters the thermal coating system layers  14 ,  16 . The sintering forms both chemical and mechanical bonds both in the thermal coating system layers  14 ,  16  and with the substrate  12 . Alumina in the substrate  12  mixes with the molten bond coat mixture  14  and raises the melting point of the thermal coating system  10 . The melting point of the resulting thermal coating system  10  may thus be increased from approximately 1,500 degrees Fahrenheit to approximately 1,950 degrees Fahrenheit or higher, depending upon the actual composition of the substrate  12 . The increased melting point of the thermal coating system  10  allows the substrate to be exposed to higher temperatures, which, for jet engine and gas turbine applications, typically produces increased thermodynamic efficiency. 
     The duration of the heating varies from approximately 30 minutes to several hours, depending on the composition of the substrate  12 , bond coat mixture  14 , and top coat mixture  16 . For example,  FIG. 3  provides a graph of the reflective performance of one embodiment of a thermal barrier thermal coating system within the scope of the present invention. In this embodiment, the bond coat mixture  14  was cast on a tetrafluoroethylene sheet and transferred as a tape to the substrate  12 . The top coat mixture  16  was then sprayed onto the bond coat mixture  14 , and the combination was then heated at 1,650 degrees Fahrenheit for approximately 1 hour. The graph provided in  FIG. 3  shows the resulting reflectance values for heat applied at various angles to the substrate. 
     It should be appreciated by those skilled in the art that modifications and variations can be made to the embodiments of the invention set forth herein without departing from the scope and spirit of the invention as set forth in the appended claims and their equivalents.