Patent Publication Number: US-2007116883-A1

Title: Process for forming thermal barrier coating resistant to infiltration

Description:
BACKGROUND OF THE INVENTION  
      This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a protective coating for a thermal barrier coating on a gas turbine engine component, in which the protective coating is resistant to infiltration by contaminants present in the operating environment of a gas turbine engine.  
      Hot section components of 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) techniques. Plasma spraying processes such as air plasma spraying (APS) yield noncolumnar coatings characterized by a degree of inhomogeneity and porosity, and have the advantages of relatively low equipment costs and ease of application. TBC&#39;s employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures with a degree of porosity can be produced using other atomic and molecular vapor processes.  
      To be effective, a TBC must 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 (CTE) between ceramic materials and the substrates they protect, which are typically superalloys, though ceramic matrix composite (CMC) materials are also used. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or a diffusion aluminide coating. 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. In addition to the CTE mismatch between a ceramic TBC and a metallic substrate, spallation can be promoted as a result of the TBC being contaminated with compounds found within a gas turbine engine during its operation. Notable contaminants include such oxides as calcia, magnesia, alumina and silica, which when present together at elevated temperatures form a compound referred to herein as CMAS. CMAS has a relatively low melting temperature of about 1225° C. (and possibly lower, depending on its exact composition), such that during engine operation the CMAS melts and infiltrates the porosity within the cooler subsurface regions of the TBC, where it resolidifies. As a result, during thermal cycling TBC spallation is likely to occur from the infiltrated solid CMAS interfering with the strain-tolerant nature of columnar TBC and the CTE mismatch between CMAS and the TBC material, particularly TBC deposited by PVD and APS due to the ability of the molten CMAS to penetrate their columnar and porous grain structures, respectively. Another detriment of CMAS is that the bond coat and substrate underlying the TBC are susceptible to corrosion attack by alkali deposits associated with the infiltration of CMAS.  
      Various studies have been performed to find coating materials that are resistant to infiltration by CMAS. Notable examples are commonly-assigned U.S. Pat. Nos. 5,660,885, 5,773,141, 5,871,820 and 5,914,189 to Hasz et al., which disclose three types of coatings to protect a TBC from CMAS-related damage. These protective coatings are generally described as being impermeable, sacrificial, or non-wetting to CMAS. Impermeable coatings are defined as inhibiting infiltration of molten CMAS, and include silica, tantala, scandia, alumina, hafnia, zirconia, calcium zirconate, spinels, carbides, nitrides, silicides, and noble metals such as platinum. Sacrificial coatings are said to react with CMAS to increase the melting temperature or the viscosity of CMAS, thereby inhibiting infiltration. Suitable sacrificial coating materials include silica, scandia, alumina, calcium zirconate, spinels, magnesia, calcia, and chromia. As its name implies, a non-wetting coating reduces the attraction between the solid TBC and the liquid (e.g., molten CMAS) in contact with it. Suitable non-wetting materials include silica, hafnia, zirconia, beryllium oxide, lanthana, carbides, nitrides, silicides, and noble metals such as platinum. According to the Hasz et al. patents, an impermeable coating or a sacrificial coating can be deposited directly on the TBC, and may be followed by a layer of an impermeable coating (if a sacrificial coating was deposited first), a sacrificial coating (if the impermeable coating was deposited first), or a non-wetting coating. If used, the non-wetting coating is the outermost coating of the protective coating system.  
      Other coating systems resistant to CMAS have been proposed, including those disclosed in commonly-assigned U.S. Pat. Nos. 6,465,090, 6,627,323, and 6,720,038. With each of these, alumina is a noted candidate as being an effective sacrificial additive or coating, in other words, reducing the impact of CMAS infiltration by reacting with CMAS (being sacrificially consumed) to raise the melting point and viscosity of CMAS. A number of approaches have been considered for applying alumina and other materials capable of inhibiting CMAS infiltration (hereinafter, CMAS inhibitors), including those disclosed by the above-identified commonly-assigned patents. Certain approaches are more effective at placing a CMAS inhibitor into the open porosity within the TBC, while others such as EB-PVD deposition, slurry top coats, and laser glazing tend to be more effective at depositing the CMAS inhibitor as a discrete outer layer on the TBC. In the case of alumina, the approach has generally been to provide alumina in the form of an additive layer overlying the TBC, rather than as a co-deposited additive within the TBC, since solid alumina and zirconia are essentially immiscible and the mechanism by which alumina provides CMAS protection is through sacrificial consumption. Nonetheless, it is desirable to have at least some alumina deposited in the open porosity of a TBC to maintain a level of CMAS protection in the event the alumina layer is breached or lost through spallation, erosion, and/or consumption.  
      Chemical vapor deposition (CVD) processes have been shown to be capable of being optimized for either higher deposition rates that primarily deposit alumina as a discrete additive layer on the outer TBC surface, or lower deposition rates that promote infiltration of a relatively small amount of alumina into the open porosity of a TBC. Spallation tests with CMAS contamination have indicated that TBC&#39;s protected with either approach exhibit similar CMAS resistance, even though those primarily infiltrated with alumina have much lower alumina contents. However, the CVD deposition of alumina with good penetration into the porosity of a TBC generally requires expensive specialized equipment and is typically limited to very low deposition rates.  
      Another approach capable of infiltrating a TBC with a CMAS inhibitor is liquid infiltration with a precursor of the inhibitor. To be successful, the precursor and any solvents, carriers, etc., used therewith must not damage the TBC, other layers of the TBC system, or the substrate protected by the TBC system. Other key requirements for a successful liquid infiltration approach include achieving an adequate degree of infiltration and depositing an effective quantity of alumina. To promote the latter, the precursor should contain a relatively high level of aluminum that can be converted to yield a known or predictable amount of alumina. Some known alumina precursors and their conversion efficiencies include aluminum chloride (0.237), aluminum bromide (0.128), aluminum acetate (0.161), aluminum nitrate (0.052), and aluminum sulfate (0.033). However, these sulfate and halide compounds are known to attack bond coat and superalloy materials typically present in TBC applications, and aqueous solutions of these compounds exhibit poor wettability to TBC materials. For those precursors requiring a solvent or carrier, another important consideration is the solubility of the precursor in its carrier since a precursor with a high conversion efficiency will not be effective if only a small loading of the precursor can be placed into solution.  
      As indicated above, the degree of infiltration is associated with the ability of the system to wet and flow into the very small pores found in TBC&#39;s produced by such methods as PVD and plasma spraying. The precursor-containing liquid being infiltrated must be able to wet the TBC surface and quickly flow into its small pores. These characteristics are associated with the surface tension and viscosity of the liquid. Excessively high surface tensions and viscosities will result in a CMAS inhibitor located primarily on the TBC surface where it is susceptible to erosion and spallation loss.  
      In view of the above, while various approaches are known for depositing alumina and other CMAS inhibitors, there is an ongoing need for deposition methods capable of depositing an effective amount of a CMAS inhibitor on and/or within a TBC that will optimize the ability of the inhibitor to prevent damage from CMAS infiltration.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention generally provides a process for protecting a thermal barrier coating (TBC) on a component used in a high-temperature environment, such as the hot section of a gas turbine engine. The invention is particularly directed to a process by which a CMAS inhibitor is applied so as to form a protective deposit on the surface of the TBC as well as infiltrate porosity within the TBC, thereby providing the benefits of an additive portion overlying the TBC and available for sacrificial consumption as well as an internal portion within the TBC to maintain a level of CMAS protection in the event the additive portion is breached or lost through spallation, erosion, and/or consumption.  
      The process of this invention generally entails applying to a surface of the TBC a liquid containing at least one alumina precursor chosen from the group consisting of long chain aluminum alkoxides, aluminum beta-diketonates, aluminum carboxylates, and aluminum alkyls. The liquid is applied so as to form a liquid film on the TBC surface, and has viscosity and wetting properties that cause the liquid to infiltrate porosity within the TBC beneath its surface. The TBC is then heated to convert the alumina precursor to alumina. A first portion of the alumina forms a surface deposit on the TBC surface, while a second portion of the alumina forms an internal deposit within the porosity of the TBC.  
      In view of the above, the process of this invention produces a protective deposit capable of increasing the temperature capability of a TBC by reducing the vulnerability of the TBC to spallation and the underlying substrate to corrosion from CMAS contamination. As a result of the type of precursor used and the process by which the precursor is applied, the protective deposit can be formed so as to not only cover the surface of the TBC, but also extend protection into subsurface regions of the TBC where resistance to CMAS is also important for long-term resistance to CMAS contamination.  
      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 perspective view of a high pressure turbine blade.  
       FIG. 2  is a cross-sectional view of a surface region of the blade of  FIG. 1 , and shows a protective deposit on a TBC in accordance with an embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will be described in reference to a high pressure turbine blade  10  shown in  FIG. 1 , though the invention is applicable to a variety of components that operate within a thermally and chemically hostile environment. The blade  10  generally includes an airfoil  12  against which hot combustion gases are directed during operation of the gas turbine engine, and whose surfaces are therefore subjected to severe attack by oxidation, hot corrosion and erosion as well as contamination by CMAS. The airfoil  12  is anchored to a turbine disk (not shown) with a dovetail  14  formed on a root section  16  of the blade  10 . Cooling holes  18  are present in the airfoil  12  through which bleed air is forced to transfer heat from the blade  10 .  
      The surface of the airfoil  12  is protected by a TBC system  20 , represented in  FIG. 2  as including a metallic bond coat  24  that overlies the surface of a substrate  22 , the latter of which is typically the base material of the blade  10  and preferably formed of a superalloy, such as a nickel, cobalt, or iron-base superalloy. As widely practiced with TBC systems for components of gas turbine engines, the bond coat  24  is preferably an aluminum-rich composition, such as an overlay coating of an MCrAlX alloy or a diffusion coating such as a diffusion aluminide or a diffusion platinum aluminide, all of which are well-known in the art. Aluminum-rich bond coats develop an aluminum oxide (alumina) scale  28 , which grows as a result of oxidation of the bond coat  24 . The alumina scale  28  chemically bonds a TBC  26 , formed of a thermal-insulating material, to the bond coat  24  and substrate  22 . The TBC  26  of  FIG. 2  is represented as having a strain-tolerant microstructure of columnar grains. As known in the art, such columnar microstructures can be achieved by depositing the TBC  26  using a physical vapor deposition (PVD) technique, such as EBPVD. The invention is also applicable to noncolumnar TBC deposited by such methods as plasma spraying, including air plasma spraying (APS). A TBC of this type is in the form of molten “splats,” resulting in a microstructure characterized by irregular flattened (and therefore noncolumnar) grains and a degree of inhomogeneity and porosity.  
      As with prior art TBC&#39;s, the TBC  26  of this invention is intended to be deposited to a thickness that is sufficient to provide the required thermal protection for the underlying substrate  22  and blade  10 . A suitable thickness is generally on the order of about 75 to about 300 micrometers. A preferred material for the TBC  26  is yttria-stabilized zirconia (YSZ), a preferred composition being about 3 to about 8 weight percent yttria (3-8% YSZ), though other ceramic materials could be used, such as nonstabilized zirconia, or zirconia partially or fully stabilized by magnesia, ceria, scandia or other oxides.  
      Of particular interest to the present invention is the susceptibility of TBC materials, including YSZ, to attack by CMAS. As discussed previously, CMAS is a relatively low melting compound that when molten is able to infiltrate columnar and noncolumnar TBC&#39;s, and subsequently resolidify to promote spallation during thermal cycling. To address this concern, the TBC  26  in  FIG. 2  is shown as being provided with a protective deposit  30  of this invention. As a result of being on the outermost surface of the blade  10 , the protective deposit  30  serves as a barrier to CMAS infiltration of the underlying TBC  26 . The protective deposit  30  is shown in  FIG. 2  as comprising an additive portion that overlies the surface  32  of the TBC  26  so as to be available for sacrificial reaction with CMAS, and further comprises an internal infiltrated portion that extends into porosity within the TBC  26  so as to maintain a level of CMAS protection in the event the additive portion is breached or lost through spallation, erosion, and/or consumption. In the case of the columnar TBC  26  schematically represented in  FIG. 2 , such porosity is represented in part as being defined by gaps  34  between individual columns of the TBC  26 . However, porosity is also likely to be present within the columns, for example, in the surfaces of individual columns if the TBC  26  were deposited by EB-PVD to have a feather-like grain structure as known in the art.  
      As represented in  FIG. 2 , the additive portion of the protective deposit  30  may form a discontinuous layer on the outer surface  32  of the TBC  26 . As such, a suitable amount of the protective deposit  30  for protecting the TBC  26  is believed to be best quantified by weight per unit TBC surface area. For example, a suitable amount of protective deposit  30  is about 1 to 10 mg/cm 2  of surface area for an EBPVD TBC having a thickness of about three to ten mils (about 75 to about 250 micrometers), with a more preferred amount for such a coating being about 1.5 to 6 mg/cm 2 . The degree to which the internal portion of the protective deposit  30  occupies the gaps  34  between TBC grains will depend in part on the particular composition used to form the protective deposit  30 , as discussed in greater detail below, and particularly on the structure of the TBC  26 , with more open porosity receiving (and needing) greater amounts of the internal deposit.  
      According to a preferred aspect of the invention, the protective deposit  30  contains alumina, more preferably is predominantly alumina, and may consist entirely of alumina, though other compounds could be used such as the sacrificial coating materials disclosed in the above-noted patents to Hasz et al., whose contents relating to such sacrificial coating materials are incorporated herein by reference. The alumina content of the protective deposit  30  is sacrificially consumed by reacting with molten CMAS that deposits on the deposit  30  and possibly infiltrates the gaps  34  of the TBC  26 , and in doing so forms one or more refractory phases with higher melting temperatures than CMAS. In effect, the alumina content of the molten CMAS is increased, yielding a modified CMAS with a higher melting and/or greater viscosity. As a result, the reaction product of CMAS and the alumina content of the protective deposit  30  more slowly infiltrates the TBC  26  and tends to resolidify before sufficient infiltration has occurred to cause spallation.  
      According to the invention, the protective deposit  30  is formed by applying to the TBC surface  32  a coating liquid containing an alumina precursor, more particularly one or more metallo-organic (organometallic) compounds that contain aluminum, and preferably one or more long chain aluminum alkoxides (Al(OR) 3 ), aluminum carboxylates (Al(RCOO) 3 ), aluminum beta-diketonates (Al(R 2 C 3 O 2 ) 3 ), and aluminum alkyls (AlR 3 ), where R is an alkyl or aryl organic fragment. Most preferred of these are aluminum isopropoxide (Al(OC 3 H 7 ) 3 ) and aluminum s-butoxide (Al(OC 4  H 9 ) 3 ). These precursors are believed to have adequate alumina conversion capability and are non-corrosive to the TBC system  20  (e.g., yttria-stabilized zirconia of the TBC  26 , aluminum and aluminides of the bond coat  24 , and alumina of the scale  28 ) or the underlying superalloy substrate  22 . Long chain aluminum alkoxides, beta-diketonates, alkyls, and carboxylates such as aluminum isopropoxide, aluminum s-butoxide, aluminum methoxide, aluminum ethoxide, and aluminum acetylacetonate, and particularly aluminum isopropoxide and aluminum s-butoxide, further have the advantage of low melting points (about 128 to 132° C. for aluminum isopropoxide and below room temperature for aluminum s-butoxide), allowing a coating liquid consisting entirely of the precursor to be used. However, the preferred precursors are also highly soluble in organic solvents. By dissolving the precursors in a suitable solvent, improved wettabilty and reduced viscosity result, thereby promoting the infiltration of the intra-columnar gaps  34  of the TBC  26 . Particularly suitable solvents are believed to be those with a polarity equal to or less than that of acetone, with preferred solvents believed to be acetone, xylene, hexane, toluene, methyl ethyl ketone (MEK), and furan.  
      The coating liquid may optionally contain a suspension of fine alumina particles. To promote infiltration of the liquid into the porosity (e.g., gaps  34 ) of the TBC  26 , the alumina particles are preferably limited to a mean diameter of less than one micrometer and do not constitute more than 20 volume percent of the liquid, with a suitable volume content believed to be in a range of about 5 to about 10 percent.  
      Application of the coating liquid to the TBC  26  can be by dipping or spraying, though other application techniques are also possible. Once deposited, the coating liquid forms a liquid film that both overlies the TBC surface  32  as well as penetrates the TBC  26  through the open porosity within the TBC  26 , such as the gaps  34  between columns. The film is optionally dried to evaporate excess moisture from the liquid for the purpose permitting handling, after which the component  10  is heated to convert the precursor to alumina. In the case of the preferred aluminum isopropoxide and aluminum s-butoxide precursors, suitable conversion temperatures are in a range of about 300 to about 1100° C. The application and heating steps may be repeated multiple times to achieve the targeted weight gain per unit area of the TBC surface  32 . As an aid to increase the infiltration efficiency, a vacuum or pressure infiltration technique may be used, and/or the coating liquid and/or component  10  can be heated to reduce the viscosity of the applied liquid.  
      There are various opportunities for depositing the protective deposit  30  of this invention. For example, the deposit  30  can be applied to newly manufactured components that have not been exposed to service. Alternatively, the deposit  30  can be applied to a component that has seen service and whose TBC must be cleaned and rejuvenated before being returned to the field. In the latter case, applying the deposit  30  to the TBC can significantly extend the life of the component beyond that otherwise possible if the TBC was not protected by the deposit  30 . In addition, the deposit  30  may be deposited on only those surfaces of a component that are particularly susceptible to damage from CMAS infiltration. In the case of the blade  10  shown in  FIG. 1 , of particular interest is often the concave (pressure) surface of the airfoil  12 , which is significantly more susceptible to attack than the convex (suction) surface as a result of aerodynamic considerations. The deposit  30  can be selectively deposited on the concave surface of the airfoil  12 , thus minimizing the additional weight and cost of the deposit  30 . For this purpose, preferred deposition techniques include spraying the coating liquid. While the concave surface of the airfoil  12  may be of particular interest, circumstances may exist where other surface areas of the blade  10  are of concern, such as the leading edge of the airfoil  12  or the region of the convex surface of the airfoil  12  near the leading edge.  
      In an investigation leading to the present invention, nickel-base superalloy specimens having a columnar 7% YSZ TBC deposited by EB-PVD on a PtAl diffusion bond coat were prepared. Some of these specimens were set aside as control samples, while other (experimental) specimens were dipped in a solution of aluminum s-butoxide and xylene at a volume ratio of 85/15. After drying, the experimental specimens were heated to about 700° C. for a duration of about 120 minutes, during which time the xylene evaporated and the aluminum s-butoxide was converted to alumina. The dipping, drying, and heating process was then repeated for the experimental specimens, resulting in a weight gain of about 2.5 mg/cm 2  per specimen. About 33 mg of a synthetically-prepared CMAS composition was then applied to an approximately 2.5 cm 2  surface area of each control and experimental specimen, after which all specimens underwent one-hour cycles between room temperature and about 1230° C. until spallation of the TBC occurred. The average life for the experimental specimens was about 2.4 times that of the untreated control samples. SEM analysis of the experimental specimens confirmed that alumina had infiltrated the columnar gaps of the TBC.  
      In another investigation, specimens essentially identical to that of the previous investigation underwent essentially identical processing and testing, with the exception that a solution of aluminum s-butoxide and xylene at a volume ratio of 95/5 was used as the infiltrant, and the experimental specimens were dipped four times in the solution, resulting in a weight gain of about 4 mg/cm 2  per specimen. The average life for the experimental specimens was about 4 times that of the untreated control samples of the previous investigation. Similar investigations were then performed with acetone, hexane, and MEK as the solvent for aluminum s-butoxide, with similar results.  
      In a third investigation, specimens essentially identical to that of the previous investigations were infiltrated with a solution of aluminum isopropoxide and xylene at a volume ratio of 50/50, to which about 10% by volume of submicron alumina particles were added. After air drying, the airfoil was heated to about 700° C. and held for a duration of about 120 minutes, during which time the xylene evaporated and the aluminum isopropoxide was converted to alumina. The infiltration and bake cycle was repeated for a total of two infiltration/bake cycles, resulting in a weight gain of about 1.5 mg/cm 2  per specimen. The average life for the experimental specimens was about 1.7 times that of the untreated control samples of the first investigation.  
      While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art, such as by substituting other TBC, bond coat, and substrate materials, or by utilizing other or additional methods to deposit and process the protective deposit. Accordingly, the scope of the invention is to be limited only by the following claims.