Abstract:
A coating suitable for use as a bondcoat for a thermal barrier coating system includes about 5 to about 10 weight percent of aluminum (Al), about 10 to about 18 weight percent of cobalt (Co), about 4 to about 8 weight percent of chromium (Cr), about 0 to about 1 weight percent of hafnium (Hf), about 0 to about 1 weight percent of silicon (Si), about 0 to about 1 percent of yttrium (Y), about 1.5 to about 2.5 weight percent of molybdenum (Mo), about 2 to about 4 weight percent of rhenium (Re), about 5 to about 10 weight percent of tantalum (Ta), about 5 to about 8 weight percent of tungsten (W), about 0 to about 1 weight percent of zirconium (Zr), and a remainder of nickel (Ni).

Description:
BACKGROUND 
       [0001]    The present invention relates generally to a thermal barrier coating system for a component that is exposed to high temperatures, such as a gas turbine engine component (e.g., blades, vanes, etc.). More particularly, the present invention relates to a coating including a low aluminum content, where the coating is suitable for use as a bondcoat in a thermal barrier coating system. 
         [0002]    A gas turbine engine component (“component”), such as a blade tip, blade trailing edge, blade platform, vane trailing edge, or vane platform, is typically exposed to a high temperature and high stress environment. The high temperature environment may be especially problematic with a superalloy component. Namely, the high temperatures may cause the superalloy to oxidize, which then decreases the life of the component. In order to extend the life of the component, a thermal barrier coating system (TBC system) may be applied to the entire superalloy component or selective surfaces, such as surfaces of the superalloy component that are exposed to the high temperatures and other harsh operating conditions. A TBC system reduces the temperature of the underlying material (also generally called the “substrate”) and helps inhibit oxidation, corrosion, erosion, and other environmental damage to the substrate. Desirable properties of a TBC system include low thermal conductivity and strong adherence to the underlying substrate. 
         [0003]    The TBC system typically includes a metallic bondcoat and a ceramic topcoat (i.e., a thermal barrier coating or TBC topcoat). The bondcoat is applied to the substrate and aids the growth of a thermally grown oxide (TGO) layer, which is typically aluminum oxide (Al 2 O 3  or “alumina”). Specifically, prior to or during deposition of the TBC topcoat on the bondcoat, the exposed surface of the bondcoat can be oxidized to form the alumina TGO layer. The TGO forms a strong bond to both the topcoat and the bondcoat, and as a result, the TGO layer helps the TBC topcoat adhere to the bondcoat. The bond between the TGO and the topcoat is typically stronger than the bond that would form directly between the TBC topcoat and the bondcoat. The TGO also acts as an oxidation resistant layer, or an “oxidation barrier”, to help protect the underlying substrate from damage due to oxidation. 
         [0004]    During use in a gas turbine engine, the TGO thickens as aluminum diffuses into the TGO from the bondcoat and oxygen diffuses into the TGO from the combustion products and cooling air in the turbine gas path, reacting to form more alumina TGO. As the TGO thickness increases, it carries a proportionally larger share of any stresses that arise in the TBC system. Eventually, this share of the stress exceeds the strength of the TGO, leading to its failure. Once the TGO fails, the TBC topcoat spalls from the bondcoat because there is little to no TGO to provide adhesion. 
         [0005]    Most bondcoats are designed to maintain slow growth of the TGO and to insure that the TGO consists of pure alumina. If the amount of aluminum that diffuses from the bondcoat to the TGO is insufficient to sustain growth of pure alumina, spinel oxide may form. Spinels grow quickly because they do not act as oxidation barriers. A TGO containing spinels has a significantly shorter life than a pure alumina TGO because spinels are not as strong as pure alumina. 
         [0006]    The TBC topcoat may consist of a zirconia material that includes yttria as a stabilizing material. The primary role of the TBC topcoat is to provide insulation, thereby reducing the temperature of the bondcoat and the substrate. Thus, TBC ceramic topcoats are designed to have low thermal conductivity. There are various techniques of applying the TBC topcoat on the component, including air plasma spraying, vapor deposition and thermal spraying methods such as a high velocity oxy-fuel method. 
         [0007]    The bondcoat is important to the life of the TBC system. If the bondcoat fails, the TBC topcoat may spall, after which the TBC system quickly deteriorates. TBC topcoat spallation exposes the bondcoat to the high-temperature, oxidizing environment of the turbine gas path, as well as to any corrosive species that may be in the gas path arising from impurities in the fuel and ingested fine particulates or contaminants. The higher temperatures and any deposits of contaminants accelerate the oxidation or corrosion of the bondcoat, eventually consuming the bondcoat. Once the bondcoat is consumed, the harsh environment attacks the underlying substrate. A deteriorated TBC system is undesirable because it may shorten the life of the component, and at the very least, requires the component to be taken out of service in order for the TBC system to be repaired. Whenever possible, the TBC system is removed and replaced prior to its complete failure to ensure no degradation of the underlying substrate. 
         [0008]    In addition to its role in protecting the substrate from oxidation and corrosion, it is preferable that the bondcoat adheres well to the substrate with minimal reaction and interdiffusion. 
       BRIEF SUMMARY 
       [0009]    The present invention is a bondcoat suitable for use with a superalloy gas turbine engine component. In one embodiment, the bondcoat includes about 5 to about 10 weight percent of aluminum (Al), about 10 to about 18 weight percent of cobalt (Co), about 4 to about 8 weight percent of chromium (Cr), about 0 to about 1 weight percent of hafnium (Hf), about 0 to about 1 weight percent of silicon (Si), about 0 to about 1 percent of yttrium (Y), about 1.5 to about 2.5 weight percent of molybdenum (Mo), about 2 to about 4 weight percent of rhenium (Re), about 5 to about 10 weight percent of tantalum (Ta), about 5 to about 8 weight percent of tungsten (W), about 0 to about 1 weight percent of zirconium (Zr), and a remainder of nickel (Ni). 
         [0010]    In another embodiment, the bondcoat includes about 5 to about 10 weight percent of Al, about 10 to about 18 weight percent of Co, about 4 to about 8 weight percent of Cr, about 0 to about 1 weight percent of Hf, about 0 to about 1 weight percent of Si, about 0 to about 1 percent of Y, about 1.5 to about 2.5 weight percent of Mo, about 2 to about 4 weight percent of Re, about 5 to about 10 weight percent of Ta, about 5 to about 8 weight percent of W, about 0 to about 1 weight percent of zirconium Zr, about 10 to about 40 weight percent of platinum (Pt) or other noble metals, such as palladium (Pd) or iridium (Ir), and a remainder of nickel (Ni). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a turbine blade. 
           [0012]      FIG. 2  is a cross-sectional view of the turbine blade of  FIG. 1 , where a section has been taken at line  2 - 2  (shown in  FIG. 1 ), and shows a TBC system overlying the body of the turbine blade. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The present invention is a coating that is suitable for use as a bondcoat in a thermal barrier coating system. In one embodiment, the composition of the inventive coating includes about 5 to about 10 weight percent of Al, about 10 to about 18 weight percent of Co, about 4 to about 8 weight percent of Cr, about 0 to about 1 weight percent of Hf, about 0 to about 1 weight percent of Si, about 0 to about 1 percent of Y, about 1.5 to about 2.5 weight percent of Mo, about 2 to about 4 weight percent of Re, about 5 to about 10 weight percent of Ta, about 5 to about 8 weight percent of W, about 0 to about 1 weight percent of Zr, and a balance (or “remainder”) of Ni. In another embodiment, about 10 to about 40 weight of Pt or other noble metals, such as Pd, Ir or combinations thereof, are substituted for some or all of the balance of Ni. The exact composition of the bondcoat may be modified within the broad ranges in order to form a bondcoat that exhibits desirable mechanical and chemical properties that are compatible with the underlying substrate. 
         [0014]    The bondcoat of the present invention may be applied (or “deposited”) to a suitable substrate, including a superalloy gas turbine engine component, in order to protect the substrate from oxidation, corrosion, erosion, and other adverse effects attributable to a harsh operating environment. The inventive bondcoat is particularly suitable for use with a component formed of a single crystal superalloy that is commonly designated a “fourth generation single crystal superalloy.” The fourth generation single crystal superalloy contains high levels of ruthenium and rhenium, and exhibits enhanced creep strength compared to other single crystal alloys. An example of a fourth generation single crystal superalloy is described in U.S. Pat. No. 6,007,645, entitled, “Advanced High Strength, Highly Oxidation Resistant Single Crystal Superalloy Composition Having Low Cr Content.” 
         [0015]    One preferred composition of the bondcoat when used in conjunction with the fourth generation single crystal superalloy substrate described in U.S. Pat. No. 6,007,645 is: about 7.5 weight percent of Al, about 14 weight percent of Co, about 6 weight percent of Cr, about 0.5 weight percent of Hf, about 0.5 weight percent of Si, about 0.5 percent of Y, about 2.0 weight percent of Mo, about 3 weight percent of Re, about 7.5 weight percent of Ta, about 6.5 weight percent of W, about 0.5 weight percent of Zr, and about 51.5 weight percent of Ni. Another preferred composition is about 7.5 weight percent of Al, about 14 weight percent of Co, about 6 weight percent of Cr, about 0.5 weight percent of Hf, about 0.5 weight percent of Si, about 0.5 percent of Y, about 2.0 weight percent of Mo, about 3 weight percent of Re, about 7.5 weight percent of Ta, about 6.5 weight percent of W, about 0.5 weight percent of Zr, about 25 weight percent of Pt (or Pd or Ir), and about 26.5 weight percent of Ni. 
         [0016]    The inventive bondcoat provides an oxidation resistant coating for a TBC system. After cycling a bondcoat having a composition in the ranges disclosed above in a burner rig at about 1148.89° C. (about 2100° F.), it was found that there was no evidence of bondcoat failure by oxidation or cracking after more than 100 cycles and more than 100 hours. 
         [0017]    It has been found that when many existing TBC system bondcoats are applied to a single crystal superalloy substrate, such as the fourth generation single crystal superalloy substrate, aluminum diffuses from the bondcoat to the underlying substrate because the bondcoat typically has a higher aluminum content than the superalloy substrate. In addition, nickel and cobalt diffuse from the substrate into the bondcoat. It is desirable to minimize this interdiffusion in order to maximize the aluminum available for the stable and slow growth of a pure alumina TGO. Furthermore, when the single crystal superalloy substrate contains high concentrations of refractory metals such as rhenium (Re), the aluminum, nickel, and cobalt interdiffusion can result in the formation of deleterious topographically close packed (TCP) phases (i.e., phase instability) near the bondcoat-substrate interface. The phase instability adversely affects the mechanical properties of the superalloy, and essentially weakens the superalloy substrate. The deleterious phases act as sites for crack initiation because these phases typically form weak, high-angle grain boundaries with the superalloy substrate. If the degree of phase instability is extensive enough to consume a fair percentage of the substrate forming the component, the integrity of the component may be compromised. 
         [0018]    The present invention is a bondcoat that includes a lower aluminum content than conventional bondcoats, while at the same time exhibits adequate oxidation resistance for a high-pressure turbine engine environment. The lower content of aluminum in the bondcoat contributes to a lower amount of diffusion of aluminum from the bondcoat to a superalloy substrate. As a result, the bondcoat of the present invention is more compatible with a fourth generation single crystal superalloy substrate, as well as other alloy substrates having high concentrations of refractory metals, than many existing bondcoats having a higher aluminum content. The possibility that the fourth generation single crystal superalloy substrate will form a TCP phase is decreased with a bondcoat of the present invention. 
         [0019]    The aluminum content in a bondcoat aids the formation of an alumina TGO layer. As mentioned in the Background section, this TGO layer serves at least two purposes. First, the oxide layer acts as a barrier to oxidation of the underlying substrate (e.g., the fourth generation single crystal superalloy). Second, the TBC topcoat is inclined to chemically bond to the oxide layer. In a sense, the oxide layer acts as an adhesive to bond the TBC topcoat to the bondcoat. Despite a lower aluminum content than existing-bondcoats, the bondcoat of the present invention contains a sufficient amount of aluminum to aid the growth of an oxide layer that is thick enough to act as an oxidation barrier. The aluminum in combination with the yttrium content of the inventive bondcoat aids the slow growth of a spinel-free oxide layer with good adhesion to both the TBC topcoat and to the bondcoat. 
         [0020]    It has been found that the bondcoat of the present invention exhibits a coefficient of thermal expansion (CTE) of about 6.4 microns/inch/° F. This value is very similar to the CTE of a state-of-the-art superalloy, including a fourth generation superalloy. The propensity for thermal cycle induced cracking or failure of the bondcoat in a TBC system increases with an increase in the difference of CTE values between the bondcoat and the substrate on which it is applied. For this reason, it is preferable to minimize the difference in the CTE values, and even more preferable to have the same CTE value for the bondcoat and the substrate. 
         [0021]    In addition to being used as a bondcoat in a thermal barrier coating system, a bondcoat of the present invention may also be a stand alone environmental coating, such as an oxidation and corrosion resistant coating. 
         [0022]      FIG. 1  is a perspective view of turbine blade  10  of a gas turbine engine. Turbine blade  10  includes platform  12  and body  14 . Body  14  of turbine blade  10  is formed of a fourth generation single crystal superalloy in accordance with the composition described in U.S. Pat. No. 6,007,645, where the specific composition exhibits a CTE of about 6.4 microns/inch/° F. Turbine blade  10  is exposed to high temperatures and high pressures during operation of the gas turbine engine. In order to extend the life of turbine blade  10  and protect turbine blade  10  from high stress operating conditions and the resulting oxidation and corrosion, TBC system  16  (shown in  FIG. 2 ) is applied over body  14  of turbine blade  10 . 
         [0023]    The exact placement of TBC system  16  depends upon many factors, including the type of turbine blade  10  employed and the areas of turbine blade  10  that are exposed to the most stressful conditions. For example, in alternate embodiments, TBC system  16  may be applied over a part of the outer surface of body  14  rather than over the entire outer surface of body  14 . Furthermore, because bondcoat  18  may also be a stand-alone environmental coating, body  14  may be fully covered with bondcoat  18  and only partly covered with ceramic layer  20 . If turbine blade  10  includes cooling holes leading from internal cooling passages to the outer surface of body  14 , TBC system  16  may also be applied to the surface of the cooling holes. 
         [0024]    Turbine blade  10  is shown as an example of a gas turbine engine component that requires the use of a TBC system. However, the bondcoat of the present invention may be used with any suitable component for which it is desirable to protect against its operating environment, including other components in a gas turbine engine. 
         [0025]      FIG. 2  is a sectional view of turbine blade  10 , where a section is taken at line  2 - 2  in  FIG. 1 . TBC system  16  is applied to an exterior surface of body  14 . TBC system  16  includes bondcoat  18  and ceramic layer  20 . Bondcoat  18  overlays and bonds to body  14 , while ceramic layer  20  overlays bondcoat  18 . In the embodiment shown in  FIG. 2 , bondcoat  18  is applied to body  14  in a thickness range of about 0.0127 millimeters (about 0.5 mils) to about 0.254 millimeters (about 10 mils). Ceramic layer  20  may be any thermal barrier coating (or “topcoat”) that is suitable for use on alumina-forming bondcoats and/or alloys, such as, but not limited to, zirconia stabilized with yttria (Y 2 O 3 ), gadolinia (Gd 3 O 3 ), ceria (CeO 2 ), scandia (Sc 2 O 3 ), or other oxides. Ceramic layer  20  is deposited in a thickness that is sufficient enough to provide the required thermal protection for bondcoat  18 . 
         [0026]    Bondcoat  18  consists essentially of about 5 to about 10 weight percent of Al, about 10 to about 18 weight percent of Co, about 4 to about 8 weight percent of Cr, about 0 to about 1 weight percent of Hf, about 0 to about 1 weight percent of Si, about 0 to about 1 percent of Y, about 1.5 to about 2.5 weight percent of Mo, about 2 to about 4 weight percent of Re, about 5 to about 10 weight percent of Ta, about 5 to about 8 weight percent of W, about 0 to about 1 weight percent of Zr, and a balance of Ni. 
         [0027]    In another embodiment, bondcoat  18  consists essentially of about 5 to about 10 weight percent of Al, about 10 to about 18 weight percent of Co, about 4 to about 8 weight percent of Cr, about 0 to about 1 weight percent of Hf, about 0 to about 1 weight percent of Si, about 0 to about 1 percent of Y, about 1.5 to about 2.5 weight percent of Mo, about 2 to about 4 weight percent of Re, about 5 to about 10 weight percent of Ta, about 5 to about 8 weight percent of W, about 0 to about 1 weight percent of Zr, about 10 to about 40 weight percent of Pt, and a balance Ni. Alternatively, other noble metals such as Pd or Ir, or combinations thereof, can be substituted for Pt. 
         [0028]    Bondcoat  18  includes a lower level of aluminum as compared to conventional bondcoats, and as a result, less aluminum diffuses from bondcoat  18  to body  14 . This helps to minimize the formation of a TCP phase in body  14  that is attributable to the diffusion of aluminum from bondcoat  18  to body  14 . As previously stated, body  14  is formed of superalloy that exhibits a CTE of about 6.4 microns/inch/° F. Bondcoat  18  also exhibits a CTE of about 6.4 microns/inch/° F. Because the CTE values for bondcoat  18  and body  14  are similar, the propensity for thermal cycle induced cracking or failure of bondcoat  18  is decreased. 
         [0029]    Bondcoat  18  in accordance with the present invention may be applied to body  14  (or another substrate in alternate embodiments) with any suitable technique, including thermal spray processes (e.g., plasma spray deposition or high velocity oxyfuel (HVOF) deposition), physical vapor deposition (e.g., cathodic arc deposition), or chemical vapor deposition. In one embodiment, a vacuum plasma spray deposition method is used to apply bondcoat  18  to body  14 . As known in the art, vacuum plasma spray deposition is a thermal spray process that is carried out in a vacuum chamber. A plasma spray torch, which applies a powdered form of the coating onto a substrate, typically operates in a low-pressure environment of an inert gas, such as Argon. The plasma spray torch is typically manipulated by a control mechanism that allows the torch to move at least in one direction relative to the substrate. 
         [0030]    In one method of applying bondcoat  18  to body  14 , a vacuum plasma spray process is used, where the process utilizes a deposition chamber pressure ranging from about 13.33 kilopascals (kPa) (about 100 Torr) to about 40 kPa (about 300 Torr), a plasma torch current ranging from about 400 amperes (A) to about 900 A, and a plasma torch voltage ranging from about 50 volts (V) to about 85 V. The plasma spray torch is controlled by a 3 to 5 axis robot, which positions the torch about 15.24 centimeters (about 6 inches) to about 50.8 centimeters (about 20 inches) from body  14 . During the deposition process, body  14  exhibits a temperature in a range of about 815.56° C. (about 1500° F.) to about 982.22° C. (about 1800° F.). In alternate embodiments, a vacuum plasma spray method including different parameters may be used. 
         [0031]    In another embodiment, a cathodic arc deposition method, which is a type of physical vapor deposition, is used to apply bondcoat  18  to body  14 . As known in the art, in a cathodic arc deposition method, a source material and a substrate to be coated are disposed in an evacuated deposition chamber. In the cathodic arc deposition embodiment, a deposition chamber pressure ranges from about 0.133 kPa (about 1 Torr) to about 13.33 kPa (about 100 Torr). Argon gas, flowing at a rate from about 100 standard cubic centimeters per minute (sccm) to about 500 sccm, maintains the chamber pressure in the desired range. Arc currents ranging from about 300 A to about 650 A are established and a negative bias from about 30 V to about 100 V is applied to body  14 . In alternate embodiments, a cathodic arc deposition method including different parameters may be used. 
         [0032]    The terminology used herein is for the purpose of description, not limitation. Although the present invention has been described with reference to a preferred embodiment, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.