Patent Publication Number: US-2023139765-A1

Title: Reactive thermal barrier coating

Description:
BACKGROUND 
     The present disclosure relates generally to coatings and more particularly to reactive thermal barrier coatings (TBCs). 
     TBCs are conventionally applied to components of gas turbine engines exposed to high temperatures and/or environmental contaminants to protect and extend the life of the components. TBCs can help inhibit oxidation, corrosion, erosion, and other environmental damage to the underlying substrate. During operation, TBCs can be susceptible to damage by environmental dust and debris, including calcium-magnesium-alumino-silicate (CMAS) contaminants. At elevated temperatures, CMAS can melt and infiltrate the porous TBC, which can reduce the strain tolerance of the TBC and promote spallation. The degradation of TBCs is becoming a growing concern as operating temperatures are increased to improve engine efficiency. New and advanced TBCs with improved durability at increasing temperatures are needed. 
     SUMMARY 
     In accordance with the present disclosure, there is provided a calcium-magnesium-alumino-silicate (CMAS)-reactive thermal barrier coating comprising a ceramic coating; and a CMAS-reactive overlay coating, wherein the CMAS-reactive overlay coating conforms to a surface of the ceramic coating and comprises a compound that forms a stable high melting point crystalline precipitate when reacted with molten CMAS at a rate that is competitive with CMAS infiltration kinetics into the thermal barrier coating; wherein the CMAS-reactive overlay coating comprises a material selected from a group consisting of a non-rare earth oxide and a mixed non-rare earth oxide; and wherein the ceramic coating phase is chemically stable with the CMAS-reactive overlay coating. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the non-rare earth oxide is selected from the group consisting of aluminum oxide, iron oxide and titania dioxide. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the ceramic coating comprises a plurality of vertically-oriented gaps and wherein the CMAS-reactive overlay coating extends into the gaps and is deposited on inner walls of the ceramic coating, and wherein the plurality of vertically-oriented gaps remain open. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the CMAS-reactive overlay is deposited in pores open to the vertically-oriented gaps. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the thickness of the CMAS-reactive overlay ranges from 10 to 500 nanometers. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the CMAS-reactive overlay extends into the vertically-oriented gaps to a depth of at least one-third of a thickness of the ceramic coating from an outer surface of the ceramic coating. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the CMAS-reactive overlay extends into the vertically-oriented gaps to a depth of at least one-half of a thickness of the ceramic coating from an outer surface of the ceramic coating. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the CMAS-reactive overlay has a material composition that will react with molten CMAS. 
     In accordance with the present disclosure, there is provided a gas turbine engine substrate with a calcium-magnesium-alumino-silicate (CMAS)-reactive thermal barrier coating comprising a substrate with a ceramic coating; a CMAS-reactive overlay coating, wherein the CMAS-reactive overlay coating conforms to a surface of the ceramic coating and comprises a compound that forms a stable high melting point crystalline precipitate when reacted with molten CMAS at a rate that is competitive with CMAS infiltration kinetics into the thermal barrier coating; wherein the CMAS-reactive overlay coating comprises a material selected from a group consisting of a non-rare earth oxide and a mixed non-rare earth oxide; and wherein the ceramic coating phase is chemically stable with the CMAS-reactive overlay coating. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the non-rare earth oxide is selected from the group consisting of aluminum oxide, iron oxide and titania dioxide. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the CMAS-reactive overlay has a material composition that will react with molten CMAS. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the thickness of the CMAS-reactive overlay ranges from 10 to 500 nanometers. 
     In accordance with the present disclosure, there is provided a process for forming a CMAS-reactive thermal barrier coating, the process comprising depositing a ceramic coating on a substrate; and; depositing a CMAS-reactive overlay on the ceramic coating, wherein the CMAS-reactive overlay coating conforms to a surface of the ceramic coating and comprises a compound that forms a stable high melting point crystalline precipitate when reacted with molten CMAS at a rate that is competitive with CMAS infiltration kinetics into the thermal barrier coating; wherein the CMAS-reactive overlay coating comprises a material selected from a group consisting of a non-rare earth oxide and a mixed non-rare earth oxide wherein the ceramic coating phase is chemically stable with the CMAS-reactive overlay coating. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the CMAS-reactive overlay is deposited using atomic layer deposition. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the ceramic coating is applied by electron beam-physical vapor deposition. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the non-rare earth oxide is selected from the group consisting of aluminum oxide, iron oxide and titania dioxide. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the process further comprising reacting the CMAS-reactive overlay material composition with molten CMAS. 
     A further embodiment of any of the foregoing embodiments may additionally and/or alternatively include the process further comprising forming the thickness of the CMAS-reactive overlay to range from 10 to 500 nanometers. 
     The present summary is provided only by way of example, and not limitation. Other aspects of the present disclosure will be appreciated in view of the entirety of the present disclosure, including the entire text, claims and accompanying figures. 
     Other details of the reactive thermal barrier coating are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic cross-sectional view of one embodiment of a coating. 
         FIG.  2    is a flowchart of a method for applying a coating to a component according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Thermal barrier coatings (TBCs) are conventionally applied to components of gas turbine engines exposed to high temperatures and/or environmental contaminants to protect and extend the life of the components. Conventional TBCs are porous, compliant structures with high strain tolerance. The strain tolerance can be reduced when TBC surface temperatures exceed the melting point (approximately 2200 degrees Fahrenheit) of ingested calcium magnesium-alumino-silicate (CMAS) materials—a temperature that is commonly exceeded on the TBC surface in modern gas turbine engines. Due to its low viscosity, the molten CMAS material can penetrate the TBC, thereby densifying and reducing the compliance of the TBC. The current state-of-the art CMAS-resistant TBC material is gadolinium zirconate (GDZ). The gadolinium in the GDZ reacts with CMAS to precipitate an apatite material. The reaction occurs quickly allowing the apatite reaction product to seal the remainder of the TBC from direct contact with the molten CMAS. Time and temperature increase the rate of attack and therefore improvements beyond GDZ are needed for future higher surface temperature TBC uses. 
     The rate limiting step in the GDZ-CMAS reaction is the dissolution rate of gadolinium from the GDZ in a sufficient concentration to react with CMAS. The dissolution time translates directly to the dissolution depth within the TBC layer. The present disclosure is directed to a TBC with a concentrated non-rare earth overlay, which provides a CMAS reactive species, including but not limited to iron and titanium, available for reaction with molten CMAS in high concentrations at the surface of the TBC. The increased concentration of CMAS-reactive species at the surface of the TBC reduces the dissolution depth and thereby time required for the molten CMAS to be sufficiently concentrated with reactive species to form silicates with a low crystallization temperature including anorthite. As discussed further herein, the reactive overlay material can be any material in which the crystalline phases formed by the incorporation of Ti, Al, or Fe (for example) would be something other than apatite, but would still be sufficient to seal the TBC from infiltration, including but not limited to non-rare-earth oxides. The reactive overlay is applied to conform to a surface of the TBC including surfaces of internal walls of the TBC formed by columnar gaps open to the TBC surface. 
       FIG.  1    is a schematic cross-sectional view of a CMAS-resistant TBC system  10  applied to substrate  12 . CMAS-resistant TBC system  10  can include bond coat  14 , ceramic top coat  16 , and CMAS-reactive overlay  18 . Substrate  12  can be a nickel- or cobalt-based superalloy, refractory alloy, or ceramic matrix composite (CMC), as contemplated for use in the manufacture of components for a hot section of a gas turbine engine (e.g., combustor panels and turbine blades, blade outer air seals, and stationary vanes). Bond coat  14  can protect substrate  12  from high-temperature oxidation and corrosion and improve adhesion of ceramic top coat  16 . Bond coat  14  can be a diffusion or overlay bond coat comprising, for example, an aluminide or MCrAlY (where M can be iron, cobalt, nickel, or a mixture thereof) applied to substrate  12 . In some embodiments, a surface of substrate  12  and/or bond coat  14  can be roughened, such as by grit blasting or peening, to improve adhesion of TBC ceramic top coat  16 . 
     Ceramic top coat  16  can include or consist of partially or fully chemically stabilized structures that are thermodynamically stable with non-rare earth oxides, such as aluminum oxide, iron oxide or titania dioxide of CMAS-reactive overlay  18 . Fully stabilized structures will have a reduced interaction potential with CMAS-reactive overlay  18 . Another option can be to utilize a phase-stable diffusion barrier (like aluminum oxide over YSZ) to prevent interdiffusion. The reduced interaction potential allows a high concentration of the non-rare earth oxide to remain available in the CMAS-reactive overlay to rapidly dissolve into the molten CMAS and precipitate out stable high melting point phases capable of suppressing further CMAS ingress. 
     Ceramic top coat  16  can be deposited on bond coat  14  using any suitable manner to provide a desired microstructure and thickness. Microstructures that provide high strain tolerance can be preferred for high temperature applications. Strain tolerance is generally improved with through-thickness cracks or separations that accommodate thermal expansion.  FIG.  1    illustrates a ceramic top coat applied using electron beam physical vapor deposition (EB-PVD), which produces columnar grains  20  that grow vertically from the surface of substrate  12  with featherlike features. The columnar grains can be separated vertically-oriented gaps  22  over a partial or full vertical thickness of the TBC and can be weakly bonded to each other to accommodate thermal stress. 
     In other embodiments, ceramic top coat  16  can be applied using suspension or solution precursor plasma spray techniques (SPS and SPPS). SPS TBCs have been developed as an alternative to TBCs deposited via EB-PVD. SPS techniques can produce coatings having columnar structures similar to those produced by EB-PVD. Unlike EB-PVD coatings, SPS coatings can have porous columns, which are formed by randomly oriented particle splats. The columns are generally separated by large through-thickness vertically-oriented inter-columnar gaps. The columnar structures can also have horizontally-oriented inter-pass porosity bands or cracks formed between material deposition passes, which can connect vertically-oriented gaps. The generally large vertically-oriented inter-columnar gaps and inter-pass porosity or cracks can accommodate ingress of molten CMAS through the thickness of the ceramic top coat and into the material matrix, substantially compromising the mechanical stability of the ceramic top coat. Alternative dense, vertically cracked TBCs can also be formed with SPS techniques as well as air plasma spray. Dense, vertically cracked coatings can have a more lamellar-appearing structure with through-thickness cracks formed to relieve stress. While cracks can be narrow, parallel facing surfaces extending through a full or nearly full thickness of the coating can provide a substantially straight pathway for molten CMAS infiltration. As used herein, “vertically-oriented” refers to gaps oriented in a direction extending from an inner surface of the coating or a substrate to an outer surface of the coating and is not limited to gaps extending transverse to the substrate. The orientation of vertically-oriented gaps can extend at angles between 45° and 90° from the substrate depending on the deposition method. As used throughout the present Specification and Claims, the term “vertically-oriented gaps” encompasses both “inter-columnar gaps,” referring to the spaces formed between columnar structures, and vertically-oriented “cracks” formed between columnar structures or in a dense vertically cracked coating. 
     The present disclosure is not limited to TBCs with columnar microstructures. The disclosed CMAS-reactive overlay can also be applied to dense or porous ceramic top coats without vertical gaps and applied by any known method, although the use of the disclosed CMAS-reactive overlay is particularly beneficial for coatings susceptible to molten CMAS ingress. 
     Ceramic top coat  16  can have a single layer or multiple layers of material (not shown) and can include layers of differing chemical composition. For example, ceramic top coat  16  can include an inner yttrium stabilized zirconia (YSZ) layer deposited on bond coat  14  and an outer GDZ layer deposited on the YSZ layer. YSZ has superior thermochemical compatibility with bond coat  14  and high durability, which makes YSZ desirable for TBC applications. Ceramic top coat  16  can have a thickness ranging from 100 to hundreds of microns. For an EB-PVD ceramic top coat, spacing between adjacent vertically-oriented gaps  22  can be smaller than 1 micron. Vertically-oriented gaps  22  can extend from an outer surface of ceramic top coat  16  to a depth in ceramic top coat  16  equal to a partial or full thickness of ceramic top coat  16 . Vertically-oriented gaps in dense vertically cracked coatings can be significantly larger than those formed by EB-PVD. Vertically-oriented gaps between columnar microstructures formed using conventional SPS techniques can be significantly larger than those formed in dense vertically cracked coatings, however, advancements in SPS have provided for significantly reduced gap widths including gap widths of less than 5 microns. 
     CMAS-reactive overlay  18  can be a CMAS-reactive compound that forms a stable high melting point crystalline precipitate reaction product at a rate that is competitive with molten CMAS infiltration kinetics. As used herein, a precipitation rate that is competitive with molten CMAS infiltration kinetics refers to a rate of reaction that is substantially equal to or greater than a rate at which the molten CMAS moves through the coating. As such, the CMAS-reactive overlay  18  reacts with the molten CMAS to form a high melting point crystalline precipitate that can seal vertically-oriented gaps and open porosity and suppress further ingress of molten CMAS. CMAS-reactive compounds capable of suppressing ingress of molten CMAS can include non-rare earth oxides (for example, Al 2 O 2 , Fe 2 O 2 , TiO 2 ), mixed non-rare earth oxides, (for example, Al2Fe2O6 or Al2TiO5). The viscosity of CMAS is reduced with increased temperature and therefore consideration of the operation temperature should be considered in selection of the CMAS-reactive material. CMAS-reactive overlay  18  can be a non-rare earth oxide (NREO). As used herein, the term “non-rare earth” refers to elements that are not in the lanthanide or actinide series. Molten CMAS can react with the NREO to form crystalline silicates that incorporate the glass modifier elements (like Fe3+, Al3+ and Ti4+) which can consume the molten CMAS and solidify. Certain crystalline silicates can form at a rate greater than the infiltration of molten CMAS within vertically-oriented gaps  22  and can thereby seal vertically-oriented gaps  22  and suppress further infiltration of molten CMAS into ceramic top coat  16 . 
     CMAS-reactive overlay  18  can be applied using atomic layer deposition (ALD), which can form a thin conforming layer of CMAS-reactive material on the surfaces of ceramic top coat  16  and extending into open pores. As illustrated in  FIG.  1   , CMAS-reactive overlay  18  can conform to the surfaces of the featherlike columnar grains and extend between individual barb-like structures along the length of the columnar grains open to the vertically-oriented gaps between adjacent columnar grains. The use of ALD can provide a uniform surface coverage that allows the CMAS-reactive overlay material to move deeper into inner surfaces of the structure. The thin layer of CMAS-reactive overlay  18  can provide a sufficient concentration of the reactive rare earth element without closing gaps  22  between adjacent columnar grains. As such, the TBC strain tolerance can be maintained with the deposition of CMAS-reactive overlay  18 . 
     The thickness of CMAS-reactive overlay can vary depending on the size of vertically-oriented gaps  22 . For the EB-PVD coating illustrated in  FIG.  1   , having an average gap width of approximately 1 micron, CMAS-reactive overlay  18  can have a thickness generally ranging from 100 to 500 nanometers. In some embodiments, and particularly for coatings with very narrow gaps  22 , CMAS-reactive overlay  18  can be less than 100 nanometers and as low as 10 nanometers. CMAS-reactive overlay  18  can be applied with a thickness sufficient to limit molten CMAS ingress to a small fraction of the coating thickness. The thickness of CMAS-reactive overlay  18  can be greater for ceramic top coats having larger vertically-oriented gaps (e.g., ceramic top coats applied by plasma spray techniques). CMAS-reactive overlay  18  can extend a partial to nearly full thickness of ceramic top coat  16 . To avoid interaction with bond coat  14  and/or substrate  12 , it may be desirable to maintain separation between CMAS-reactive overlay  18  and bond coat  14 . In some embodiments, CMAS-reactive overlay can extend one-third of the thickness of ceramic top coat  16  from the outer surface. Preferably, CMAS-reactive overlay  18  can extend half of the thickness of ceramic top coat  16 . 
     While ALD is a preferred deposition method for the disclosed EB-PVD coating, given the capability of ALD to form a thin conforming coating layer that can extend deep into open pores, other chemical vapor deposition (CVD) or coating methods may be suitable for some TBC systems. The deposition method must be capable of providing sufficient reacting species near the surface of the coating, and including surfaces separated by vertically-oriented gaps, to compete with a CMAS infiltration rate and thereby provide sufficient protection from CMAS ingress. To maintain strain tolerance, the deposition method should not fill vertically-oriented gaps. 
       FIG.  2    illustrates a flowchart of method  30  for applying CMAS-reactive TBC system  10  to a component according to one disclosed non-limiting embodiment. It should be appreciated that alternative and additional steps may be provided without departing from the teaching herein. 
     Metallic bond coat  14  is deposited on substrate  12  in step  32 . Metallic bond coat  14  provides environmental protection for substrate  12  against oxidation, corrosion and improved adhesion along with a thermally grown oxide for ceramic top coat  16 . Metallic bond coat  14  can be an MCrAlY material applied via low pressure plasma spray, however, other bond coat materials and deposition methods may be used. Metallic bond coat  14  can generally have a thickness ranging from 50 to several hundred microns. 
     Ceramic top coat  16  can be deposited on metallic bond coat  14  in step  34 . In some embodiments, the surface of bond coat  14  can first be prepared for ceramic top coat deposition (e.g., contaminants can be removed and the surface can be grit blasted to provide a desired surface roughness). Ceramic top coat  16  can be applied using any of a variety of deposition techniques used to provide a desired microstructure (e.g., desired porosity and vertically-oriented gaps). Preferably, ceramic top coat  16  can be deposited in a manner that provides vertically-oriented gaps to provide needed strain tolerance during operation. EB-PVD can provide very narrow vertically-oriented gaps (approximate 1 micron), which can provide high strain tolerance and which can be easily sealed near an upper surface of the coating to limit CMAS-ingress to a small fraction of the coating thickness. SPS can also be used to form coatings with columnar microstructures and vertically-oriented gaps. SPS as well as air plasma spraying techniques can be used to form dense vertically-cracked coatings. In some embodiments, ceramic top coat  16  can be substantially free of vertically-oriented gaps. 
     Ceramic top coat  16  can have a thickness ranging from 100 to several hundred microns depending on the application. Ceramic top coat  16  can be fully stabilized structures that are thermodynamically stable with non-rare earth oxides of the CMAS-reactive overlay  18 . In some embodiments, ceramic top coat  16  can have layers of differing chemical compositions. For example, ceramic top coat  16  can have a 7YSZ inner layer and fully stabilized RE zirconate composition outer layer. 
     CMAS-reactive overlay  18  is applied to ceramic top coat  16  in step  36 . In some embodiments, additional surface treatment of ceramic top coat  16  can be provided to prepare ceramic top coat  16 . For example, thermal exposure can be used to burn off deleterious organics. CMAS-reactive overlay  18  can be a NREO capable of forming a high melting point crystalline precipitate upon reaction with molten CMAS. 
     CMAS-reactive overlay  18  can be applied via ALD to form a thin conforming layer over surfaces of ceramic top coat  16 , including deep into open pores of the microstructure. In other embodiments, CMAS-reactive overlay  18  can be applied by other CVD or physical deposition methods (e.g., dipping). CMAS-reactive overlay  18  is applied to provide a sufficient concentration of CMAS reactive species to be able to suppress CMAS ingress. CMAS-reactive overlay  18  is applied with a thickness that can maintain vertically-oriented gaps while also being capable of limiting molten CMAS ingress to a small fraction of the coating thickness. For an EB-PVD ceramic top coat, CMAS-reactive overlay  18  can have a thickness of 500 nanometers or less. The thickness can increase for ceramic top coats with larger gap sizes. 
     The concentrated non-rare earth overlay applied to the ceramic TBC in a thin conforming layer can suppress molten CMAS ingress during operation while maintaining strain tolerance of the TBC. The increased concentration of CMAS-reactive species at the surface of the TBC reduces the dissolution depth and thereby time required for the molten CMAS to be sufficiently concentrated with reactive species to form a high melting point crystalline precipitate, which seals the TBC from further molten CMAS infiltration. 
     Any relative terms or terms of degree used herein, such as “substantially”, “essentially”, “generally”, “approximately” and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, transient alignment or shape variations induced by thermal, rotational or vibrational operational conditions, and the like. Moreover, any relative terms or terms of degree used herein should be interpreted to encompass a range that expressly includes the designated quality, characteristic, parameter or value, without variation, as if no qualifying relative term or term of degree were utilized in the given disclosure or recitation. 
     A technical advantage of the disclosed reactive thermal barrier coating includes the crystalline phases formed by the incorporation of Ti, Al, or Fe (for example) would be something other than apatite, but would still be sufficient to seal the TBC from infiltration. 
     Another technical advantage of the disclosed reactive thermal barrier coating includes the addition of the concentrated oxide outer layer over the entirety of the EBPVD column (or possibly APS splat structure). This concentrated layer will therefore require less dissolution and therefore less time before the CMAS melt is sufficiently concentrated for crystallization. 
     Another technical advantage of the disclosed reactive thermal barrier coating includes a layer that conforms not only to the outer surface of the coating, but down the column gaps and preferentially into the feather structure of the column edges to maximize surface area for the dissolution process. 
     Another technical advantage of the disclosed reactive thermal barrier coating includes faster sealing layer formation during exposure to CMAS, which reduces the thickness of the high modulus CMAS-stiffened layer and reduces the stress in the TBC as a function of time. 
     Another technical advantage of the disclosed reactive thermal barrier coating includes a coating that is expected to yield an improvement in CMAS life for GDZ EB-PVD TBCs and opens up the possibility of using a high toughness ceramic system (such as YSZ) by providing a separate source for the crystallization. 
     There has been provided a reactive thermal barrier coating. While the reactive thermal barrier coating has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.