Abstract:
A protective overlay coating for articles used in hostile thermal environments, and more particularly a predominantly beta-phase NiAl intermetallic overlay coating for use as an environmental coating or as a bond coat for a thermal barrier coating deposited on the overlay coating. The overlay coating has inner and outer regions, with the inner region containing more chromium than the outer region. The lower chromium content of the outer region promotes the oxidation resistance of the overlay coating, while the higher chromium content of the inner region promotes the hot corrosion resistance of the coating interior. Under hot corrosion conditions, hot corrosion may attack the outer region, but further hot corrosion attack will substantially cease once the relatively high-chromium inner region of the overlay coating is encountered.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention generally relates to coatings of the type used to protect components subjected to oxidation and hot corrosion in high temperature environments, such as the hostile environment of a gas turbine engine. More particularly, this invention is directed to an overlay coating of predominantly beta-phase NiAl (βNiAl), in which the chemistry of the coating varies to promote oxidation resistance at its outer region and hot corrosion resistance within an inner region of the coating. 
   2. Description of the Related Art 
   Components within the turbine, combustor and augmentor sections of gas turbine engines are susceptible to oxidation and hot corrosion attack, in addition to high temperatures that can decrease their mechanical properties. Consequently, these components are often protected by an environmental coating alone or in combination with an outer thermal barrier coating (TBC), which in the latter case is termed a TBC system. Ceramic materials such as zirconia (ZrO 2 ) partially or fully stabilized by yttria (Y 2 O 3 ), magnesia (MgO) or other oxides, are widely used as TBC materials. 
   Various metallic coating systems have been used as environmental coatings for gas turbine engine components, the most widely used being diffusion coatings such as diffusion aluminides and platinum aluminides (PtAl). Diffusion aluminide coatings are formed by reacting the surface of a component with an aluminum-containing vapor to deposit aluminum and form various aluminide intermetallics that are the products of aluminum and elements of the substrate material. Diffusion aluminide coatings formed in a nickel-base superalloy substrate contain such environmentally-resistant intermetallic phases as beta NiAl and gamma prime (y′) Ni 3 Al. By incorporating platinum, the coating further includes PtAl intermetallic phases, usually PtAl and PtAl 2 , and platinum in solution in the NiAl intermetallic phases. 
   Another widely used coating system is an overlay coating known as MCrAlX, where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or another rare earth or reactive element. MCrAlX overlay coatings are typically deposited by physical vapor deposition (PVD), such as electron beam PVD (EBPVD) or sputtering, or by plasma spraying. MCrAlX overlay coatings differ from diffusion aluminide coatings as a result of the elements transferred to the substrate surface and the processes by which they are deposited, which can result in only limited diffusion into the substrate. If deposited on a nickel-base superalloy substrate, an MCrAlX coating will comprise a metallic solid solution that contains both gamma prime and beta nickel aluminide phases. 
   Used in combination with TBC, a diffusion aluminide or MCrAlX overlay coating serves as a bond coat to adhere the TBC to the underlying substrate. The aluminum content of these bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the TBC to the bond coat. 
   More recently, overlay coatings of predominantly beta-phase nickel aluminide intermetallic have been proposed as environmental and bond coat materials. The NiAl beta phase exists for nickel-aluminum compositions of about 30 to about 60 atomic percent aluminum, the balance of the nickel-aluminum composition being nickel. Notable examples of beta-phase NiAl coating materials include commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., which discloses a NiAl overlay bond coat optionally containing one or more active elements, such as yttrium, cerium, zirconium or hafnium, and commonly-assigned U.S. Pat. No. 6,291,084 to Darolia et al., which discloses a NiAl overlay coating material containing chromium and zirconium. Commonly-assigned U.S. Pat. Nos. 6,153,313 and 6,255,001 to Rigney et al. and Darolia, respectively, also disclose beta-phase NiAl bond coat and environmental coating materials. The beta-phase NiAl alloy disclosed by Rigney et al. contains chromium, hafnium and/or titanium, and optionally tantalum, silicon, gallium, zirconium, calcium, iron and/or yttrium, while Darolia&#39;s beta-phase NiAl alloy contains zirconium. 
   The beta-phase NiAl alloys of Nagaraj, Darolia et al., Rigney et al., and Darolia have been shown to improve the adhesion of a ceramic TBC layer, thereby inhibiting spallation of the TBC and increasing the service life of the TBC system. The alloys also exhibit good oxidation and hot corrosion resistance. However, a tradeoff appears to exist between oxidation and hot corrosion resistance. Therefore, further improvements are still desirable. 
   SUMMARY OF INVENTION 
   The present invention generally provides a protective overlay coating for articles used in hostile thermal environments, such as turbine, combustor and augmentor components of a gas turbine engine. The invention is particularly directed to a predominantly beta-phase NiAl intermetallic overlay coating for use as an environmental coating or as a bond coat for a thermal barrier coating (TBC) deposited on the overlay coating. An example of a suitable overlay coating contains nickel, aluminum, chromium, and a reactive element such as zirconium. 
   According to the invention, the overlay coating comprises inner and outer regions, with the inner region containing more chromium than the outer region and also preferably less aluminum than the outer region. As a result of their different compositions, the outer region promotes the oxidation resistance of the overlay coating while the inner region promotes the hot corrosion resistance of the interior of the overlay coating. For those surface regions of the overlay coating subjected to relatively high temperatures requiring optimum oxidation resistance, the outer region of the overlay coating provides a desirable level of oxidation protection. In comparison, at cooler regions of the overlay coating where damage from hot corrosion is more likely, hot corrosion may attack the outer region (containing lower amounts of chromium), but further hot corrosion attack will substantially cease once the relatively high-chromium inner region of the overlay coating is encountered. 
   In view of the above, the present invention provides an overlay coating that is suitable for use as a bond coat or an environmental coating, and which can be applied as a single coating on all exposed surfaces of a component to provide a balance of oxidation and hot corrosion resistance. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF 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 the blade in accordance with an embodiment of this invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts a high pressure turbine blade  10  that includes an airfoil  12  against which hot combustion gases are directed during operation of the gas turbine engine in which the blade  10  is installed. As such, the surface of the airfoil is subjected to severe attack by oxidation, hot corrosion, etc. 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 . 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 a coating system may be used to protect the component from its environment. 
   Represented in  FIG. 2  is a TBC system  20  in accordance with an embodiment of the invention. As shown, the coating system  20  includes a ceramic layer  26  bonded to the blade substrate  22  with an overlay coating  24 , which therefor serves as a bond coat to the ceramic layer  26 . The substrate  22  (blade  10 ) is preferably a high-temperature material, such as an iron, nickel or cobalt-base superalloy. To attain the strain-tolerant columnar grain structure  30  represented in  FIG. 2 , the ceramic layer  26  is preferably deposited by physical vapor deposition (PVD), though other plasma spray deposition techniques could be used. A preferred material for the ceramic layer  26  is an yttria-stabilized zirconia (YSZ), with a suitable composition being about 3 to about 20 weight percent yttria, though other ceramic materials could be used, such as yttria, nonstabilized zirconia, or zirconia stabilized by ceria (CeO 2 ), scandia (Sc 2 O 3 ) 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 100 to about 300 micrometers. As with prior art TBC systems, the overlay coating  24  contains sufficient aluminum so that its surface oxidizes to form an adherent oxide layer (scale)  28  to which the ceramic layer  26  chemically bonds. 
   While shown in combination with the ceramic layer  26  to yield a TBC system  20 , for applications in which a thermal barrier is not required the ceramic coating  26  can be omitted so that the overlay coating  24  serves as an environmental coating, with the oxide scale  28  acting as a protective barrier to oxidation. As such, the overlay coating  24  is suitable as a bond coat for the ceramic layer  24  as well as an environmental coating. 
   According to the invention, the overlay coating  24  is predominantly of the beta NiAl phase (beta-NiAl) with certain alloying additions. To attain the beta-NiAl inter-metallic phase, the overlay coating  24  has an aluminum content of about 30 to 60 atomic percent. According to this invention, the overlay coating  24  also contains chromium, with the chromium content in the coating  24  being higher within an inner region  32  of the coating  24  and lower within an outer region  34  of the coating  24 , the latter of which preferably defines the outer surface of the coating  24 . According to a preferred aspect of the invention, the aluminum content also varies within the coating  24 , with the aluminum content being higher in the outer region  34  than in the inner region  32 . As such, the overlay coating  24  may be termed a dual alloy coating, with a relatively high-aluminum, low-chromium outer region  34  and a relatively low-aluminum, high-chromium inner layer  32 . The inner and outer regions  32  and  34  may be formed as discrete layers, or be the result of a gradual change in the composition of the coating  24 . For example, the chromium content of the overlay coating  24  can gradually increase from the coating surface toward the underlying substrate  22 . 
   The intent of the dual alloy overlay coating  24  of this invention is to provide a single protective coating that can be deposited on a component (e.g., the blade  10 ) having surface regions that are particularly prone to oxidation as a result of being subjected to relatively high temperatures (e.g., above about 1100° C.), while other regions of its surface are more prone to hot corrosion as a result of being subjected to lower temperatures (e.g., below about 950° C.). By appropriately minimizing the chromium content of the outer region  34 , such as levels of 5 weight percent or less, oxidation resistance is enhanced for those regions of the blade  10  that are prone to oxidation, particularly if the outer region  34  is enriched with aluminum. On the other hand, within those regions of the blade  10  prone to hot corrosion, hot corrosion may proceed through the outer region  34  as a result of its relatively lower chromium content but will then stop when the high-chromium inner region  32  of the coating  24  is encountered. 
   A suitable chromium content for the outer region  34  of the coating  24  is about 1 to 5 weight percent (about 0.8 to 3.9 atomic percent), preferably about 2 weight percent, while a chromium content of 5 to 20 weight percent (about 4 to 19 atomic percent), preferably about 10 weight percent, is desired for the inner region  32  of the coating  24 . The compositions of the NiAl intermetallic within both the inner and outer regions  32  and  34  are preferably alloyed to contain a reactive element, with preferred compositions based on NiAlCrZr. A suitable composition for the inner region  32  is, by weight, about 20% to 30% aluminum, about 5% to 20% chromium, about 0.2 to 1.5% zirconium, and the balance nickel and incidental impurities. A suitable composition for the outer region  34  is, by weight, about 20% to 30% aluminum, about 1% to 5% chromium, about 0.2 to 1.5% zirconium, and the balance nickel and incidental impurities. In a preferred embodiment in which the outer region  34  has a higher aluminum than the inner region  32 , it is foreseeable that the aluminum content of the inner region  32  could be less than 18 weight percent, in which case a suitable minimum aluminum content for the outer region  34  is at least 18 weight percent. 
   The NiAl overlay coating  24  is preferably deposited in a single coating cycle using a PVD process such as sputtering, ion plasma, cathodic arc, or melting and evaporation with an electron beam, laser or other higher energy source. It is foreseeable that other deposition techniques could be used, such as thermal spraying of powders including air plasma spraying (APS) and low pressure plasma spraying (LPPS) techniques. The inner region  32  is deposited using a coating source (e.g., ingot if deposited by a melting and evaporation technique; powder if deposited by a spraying technique) having a relatively higher chromium content than the coating source for the outer region  34 . Precise control of when the inner region  32  ends and the outer region  34  begins is not believed to be necessary. To protect the underlying substrate  22  and provide an adequate supply of aluminum for formation of the protective oxide scale  28 , a suitable thickness for each region  32  and  34  of the overlay coating  24  is about 25 micrometers for a total thickness of about 50 micrometers, though thicknesses of about 15 to about 100 micrometers are believed to be acceptable for each region. Preferably, deposition of the overlay coating  24  results in virtually no diffusion between the overlay coating  24  and substrate  22 . During subsequent heat treatment to relieve residual stresses generated during the deposition process, a very thin diffusion zone, typically not more than about five micrometers, may develop. A suitable heat treatment is two to four hours at about 1800° F. to 2100° F. (about 980° C. to about 1150° C.) in a vacuum or an inert atmosphere such as argon. 
   While the invention has been described in terms of a preferred embodiment, it is apparent that modifications could be adopted by one skilled in the art. For example, based on investigations reported in U.S. Pat. No. 6,153,313, it is believed that the overlay coating of this invention could be modified to further contain one or more of hafnium, yttrium, titanium, tantalum and silicon, as well as possible additions of platinum, rhenium and/or ruthenium. Accordingly, the scope of the invention is to be limited only by the following claims.