Abstract:
A coating system and method for forming the coating system on an article designed for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The method employs a nitrided zone in the surface of the superalloy substrate to inhibit the formation of deleterious topologically-close packed (TCP) phases in the substrate when protected by an aluminum-rich coating and optionally a thermal insulating ceramic layer. Superalloys of particular interest are those containing significant levels of TCP phase-forming elements, such as tungsten, rhenium, tantalum, molybdenum and chromium.

Description:
This invention relates to protective coating systems 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 method of inhibiting the formation of deleterious topologically-close packed (TCP) phases in a superalloy protected by an aluminum-rich coating by nitriding the superalloy surface before depositing the aluminum-rich coating. 
     BACKGROUND OF THE INVENTION 
     Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating ceramic topcoat, typically referred to as the TBC. 
     To be effective, TBC systems must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. TBC systems capable of satisfying the above requirements typically employ a bond coat formed of an oxidation-resistant aluminum-containing alloy such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or an oxidation-resistant diffusion coating, such as diffusion aluminide coatings that contain aluminum intermetallics. These same compositions are often used alone as environmental coatings for superalloy components that do not require the added thermal protection of a TBC. 
     When bond coats and environmental coatings of the type described above are applied, a zone of chemical mixing occurs to some degree between the coating and the superalloy substrate. This zone is typically referred to as a diffusion zone (DZ), and results from the interdiffusion between the coating and substrate. For many alloys, it is typical to see topologically close-packed (TCP) phases in the diffusion zone after high temperature exposures, e.g., near or above about 900° C. The incidence of a moderate amount of TCP phases beneath the coating is typically not detrimental. However, certain high strength superalloys contain significant amounts of refractory elements, such as tungsten, rhenium, tantalum, molybdenum and chromium, which are all components of TCP phases. If these elements, and particularly rhenium, are present in sufficient amounts or combinations, a particularly detrimental type of diffusion zone containing deleterious TCP phases can form after coating. This instability was first seen beneath the diffusion zone of an aluminide coating, and has been termed a secondary reaction zone (SRZ). SRZ and/or its boundaries readily crack under stress and remove useful load-bearing area through its growth into the superalloy substrate. 
     Commonly-assigned U.S. Pat. No. 5,334,263 to Schaeffer teaches a method of inhibiting the formation of deleterious TCP phases in a superalloy protected by an aluminum-rich coating by carburizing the superalloy surface before depositing the aluminum-rich coating. According to Schaeffer, carbon can be diffused into a superalloy substrate to tie up certain TCP phase-forming refractory elements, and to serve as a barrier between the subsequently-deposited aluminide coating and the superalloy substrate to prevent interaction between the two. However, a limitation to this approach is that only those refractory elements that will form a stable carbide are affected. As a result, an element such as aluminum, which is not a carbide former but important in the occurrence of TCP formation, is not affected by carburization. However, aluminum content is critically related to alloy stability. Increasing the aluminum content in an alloy increases its gamma prime amount. Since refractory elements such as molybdenum, chromium, rhenium and tungsten do not normally partition to the gamma prime phase, their concentration in the remaining gamma phase is increased, producing a higher electron vacancy number and an increased propensity to form detrimental TCP phases. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention generally provides a coating system and method for forming the coating system on an article designed for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The method is particularly directed to inhibiting the formation of deleterious topologically-close packed (TCP) phases in a superalloy protected by an aluminum-rich coating and optionally a thermal insulating ceramic layer. Superalloys of particular interest are those containing significant levels of TCP phase-forming elements, such as tungsten, rhenium, tantalum, molybdenum and chromium. 
     According to this invention, the formation of deleterious TCP phases in the near-surface region of a superalloy substrate can be inhibited by nitriding the substrate prior to depositing the aluminum-rich coating. Within the nitrided surface region, nitrides of the elements of concern will be present, such as aluminum, tantalum and chromium. Titanium, boron, zirconium and niobium nitrides may also be formed if these elements are present in the base alloy. These nitrides are not detrimental to the physical, mechanical and environmental properties of the superalloy if limited to about 10 volume percent within the near-surface region and 20 micrometers in size. 
     Following nitriding, an aluminum-rich coating can be deposited on the nitrided surface region, yielding an aluminum-rich diffusion zone that extends into the nitrided surface region from the aluminum-rich coating. In a preferred embodiment, the diffusion zone extends into but not beyond the nitrided surface region. During deposition of the coating, less stable nitrides can dissolve and be replaced with more stable nitrides, such that the formation of aluminum nitride continues. 
     According to the above, appropriately nitriding the surface of a superalloy component serves to form stable nitrides that tie up more TCP phase-forming elements present in the near-surface region of the superalloy than possible by carburizing. By reducing the incidence of detrimental TCP phases, the service life of a superalloy component can be considerably improved, particularly if the superalloy contains relatively high levels, e.g., 5 weight percent or more, of detrimental TCP phase-forming elements such as of aluminum, rhenium, tungsten and/or tantalum. 
     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 the blade of FIG. 1 along line  2 — 2 , and shows a thermal barrier coating system on a nitrided surface region of the blade in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally applicable to components that operate within environments characterized by relatively high temperatures, and are therefore subjected to severe thermal stresses and thermal cycling. Notable examples of such components include the high and low pressure turbine nozzles and blades, shrouds, combustor liners and augmentor hardware of gas turbine engines. An example of a high pressure turbine blade  10  is shown in FIG.  1 . The blade  10  generally includes an airfoil  12  against which hot combustion gases are directed during operation of the gas turbine engine, and whose surface is therefore subjected to severe attack by oxidation, corrosion and erosion. 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 passages  18  are present in the airfoil  12  through which bleed air is forced to transfer heat from the blade  10 . While the advantages of this invention will be described with reference to the high pressure turbine blade  10  shown in FIG. 1, the teachings of this invention are generally applicable to any component on which an environmental coating may be used to protect the component from its environment. 
     Represented in FIG. 2 is a thermal barrier coating (TBC) system  20  of a type known in the art. As shown, the coating system  20  includes a bond coat  24  overlying a superalloy substrate  22 , which is typically the base material of the blade  10 . The bond coat  24  is shown as adhering a thermal-insulating ceramic layer  26 , or TBC, to the substrate  22 . Suitable materials for the substrate  22  (and therefore the blade  10 ) include equiaxed, directionally-solidified and single-crystal nickel-base superalloys, with the invention being particularly advantageous for single-crystal superalloys that contain one or more refractory metals. A notable example is a single-crystal nickel-base superalloy known as Rene N6 disclosed in U.S. Pat. No. 5,455,120. This superalloy nominally contains, in weight percent, about 4.2% chromium, about 1.4% molybdenum, about 5.75% tungsten, about 5.4% rhenium, and about 7.2% tantalum, in addition to various other important alloying constituents. 
     As is typical with TBC systems for components of gas turbine engines, the bond coat  24  is an aluminum-rich composition, such as a diffusion aluminide, a platinum aluminide, or an MCrAlX alloy of a type known in the art. Aluminum-rich bond coats of this type naturally develop an aluminum oxide (alumina) scale  28 , which can be more rapidly grown by forced oxidation of the bond coat  24 . The ceramic layer  26  is chemically bonded to the bond coat  24  with the oxide scale  28 . Notably, if the bond coat  24  is a diffusion aluminide, a diffusion zone  34  exists beneath the bond coat  24  within the substrate  22 . This diffusion zone (DZ)  34  typically extends about 25 to 50 micrometers into the substrate  22 , and is comprised of various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate  22 . Accordingly, the diffusion zone  34  contains constituents of the substrate  22  and the bond coat  24 , and is therefore susceptible to the formation of a secondary reaction zone (SRZ) that contains detrimental TCP phases. If the bond coat  24  is an MCrAlX overlay coating, there is a much shallower diffusion zone  34 , corresponding to a somewhat reduced but still detrimental susceptibility to the formation of deleterious SRZ constituents. 
     As shown, the ceramic layer  26  has a strain-tolerant columnar grain structure achieved by depositing the ceramic layer  26  using physical vapor deposition techniques known in the art, though air plasma spray techniques can also be used. A preferred material for the ceramic layer  26  is an yttria-stabilized zirconia (YSZ), a preferred composition being about 6 to about 8 weight percent yttria, though other ceramic materials could be used, such as yttria, nonstabilized zirconia, or zirconia stabilized by magnesia, ceria, scandia or other oxides. The ceramic layer  26  is deposited to a thickness that is sufficient to provide the required thermal protection for the underlying substrate  22  and blade  10 , generally on the order of about 75 to about 300 micrometers. 
     While described in reference to the TBC system  20  shown in FIG. 2, the present invention is also applicable to environmental coatings that do not include a ceramic TBC (i.e., ceramic layer  26 ), but instead are limited to an oxidation-resistant coating (e.g., a diffusion aluminide or MCrAlX coating essentially identical to the bond coat  24 ) and an oxide scale  28 . 
     According to this invention, the coating system  20  is deposited on a nitrided zone  32  in the surface of the substrate  22 , i.e., beneath the interface of the bond coat  24  with the substrate  22 . The nitrided zone  32  contains nitride precipitates  30  that serve to tie up TCP phase-forming elements such as tungsten, tantalum, chromium, niobium and titanium (when present in the substrate alloy) and TCP phase-promoting elements such as aluminum, in the superalloy substrate  22 . In so doing, the nitrided zone  32  inhibits the formation in the substrate  22  of an SRZ containing deleterious TCP-phases, which typically form in the diffusion zone  34  at temperatures near or above about 950° C. (for SRZ). Though the kinetics for nitriding are not as favorable as that for carburization, nitriding provides the capability of tying up more SRZ-forming elements than the carburizing approach taught by U.S. Pat. No. 5,334,263 to Schaeffer, whose teachings are incorporated herein by reference. The nitride precipitates  30  preferably constitute about two to about ten volume percent of the nitrided zone  32 , which preferably extends below the diffusion zone  34  of the bond coat  24 . Depending on the depth of the diffusion zone  34 , the depth of the nitrided zone  32  may be as little as about ten micrometers, but is preferably not deeper than about one hundred micrometers in order to avoid significantly affecting the mechanical properties of the substrate  22 . A suitable depth for the nitrided zone  32  is believed to be about twenty-five to seventy-five micrometers below the surface of the superalloy substrate  22 . Because nitrogen has a limited solubility of about 2 ppm in nickel and nickel alloys, the overall nitrogen content within the nitrided zone  32  is far above that conventionally present in a nickel-base superalloy. This nitrogen supersaturation of the substrate  22  results in the formation of nitride intermetallic phases. 
     According to the present invention, the nitrided zone  32  shown in FIG. 2 can further contain carbide precipitates  38  in accordance with Schaeffer. In addition or as an alternative to carburizing, the nitrided zone  32  can be overcoated with a diffusion barrier layer  36  to inhibit interdiffusion between the bond coat  24  and substrate  22 . 
     The nitrided zone  32  of this invention can be formed by several alternate processes, including the use of ammonia or nitrogen-based atmospheres. The resulting nitride dispersion is a strong function of process temperature, time and the substrate alloy. Ammonia-hydrogen mixtures (e.g., containing about 10 to 15 volume percent ammonia) provide a suitable nitriding medium at temperatures from about 700° C. to about 900° C. Nitrogen-hydrogen-helium mixtures (about 50, 5, and 45 volume percent, respectively) can be successfully used from 800° C. to 1050° C. Prior to nitriding, the substrate  22  is cleaned by a chemical, vacuum or controlled abrasive procedure to remove contaminants and surface oxides. After nitriding, conventional processing can be performed to form the bond coat  24 , oxide scale  28  and ceramic layer  26  of the coating system  20 . 
     While our 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. Accordingly, the scope of our invention is to be limited only by the following claims.