Abstract:
A metallic coating or alloy is provided, which is nickel based, and includes at least γ and γ′ phases. The metallic coating or the alloy further includes tantalum (Ta) in the range of between 4 wt % to 7.5 wt %. The metallic coating or the alloy also includes cobalt (Co) in the range between 11 wt %-14.5 wt %.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Stage of International Application No. PCT/EP2011/069513 filed Nov. 7, 2011 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the U.S. application Ser. No. 12/953,520 filed Nov. 24, 2010, the entire contents of which is hereby incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a metallic bondcoat with phases of γ and γ′ a component. 
       BACKGROUND OF INVENTION 
       [0003]    Components for the hot gas path in gas turbines are made from Ni- or Co based materials. These materials are optimized for strength and are not able to withstand oxidation and/or corrosion attack at higher temperatures. Therefore, these kinds of materials must be protected against oxidation by MCrAlY-coatings which can be used as bondcoats for thermal barrier coating (TBC) systems as well. In TBS systems, the MCrAlY coating is needed against hot gas attack on one side and on the other side this coating is needed to adhere the TBC to the substrate Improving such systems against oxidation will lead to increased bondcoats service temperatures with increased life properties. 
         [0004]    To protect the materials against hot corrosion/oxidation, MCrAlY overlay coatings are coated mainly by low pressure plasma spraying (LPPS), air plasma spraying (APS), electron beam physical vapor deposition (EBPVD), cold spray (CS) or high velocity oxy-fuel (HVOF) process. The MCrAlY coating is based on nickel and/or cobalt, chromium, aluminum, silicon, rhenium and rare earth elements like yttrium. With increasing bondcoat temperatures, these coatings can fail which can lead to spallation of the thermal barrier coating. Therefore, with increasing service temperatures, improved coatings are needed to withstand the oxidation attack. Additionally this kind of coatings should have acceptable thermo-mechanical properties. These requests can only be achieved by an optimized composition of the bond coat. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore the aim of the invention to solve the above mentioned problem. 
         [0006]    The problem is solved by the features of the independent claim(s). 
         [0007]    In the dependent claims further amendments are disclosed which can be arbitrarily combined with each other to yield further advantages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    It shows 
           [0009]      FIG. 1  a turbine blade, 
           [0010]      FIG. 2  a gas turbine and 
           [0011]      FIG. 3  a list of superalloys. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0012]    The figures and the description are only embodiments of the invention. 
         [0013]      FIG. 1  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0014]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0015]    The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406  as well as a blade or vane tip  415 . 
         [0016]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0017]    A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or disk (not shown), is formed in the securing region  400 . 
         [0018]    The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. 
         [0019]    The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the main blade or vane part  406 . 
         [0020]    In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
         [0021]    Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0022]    The blade or vane  120 ,  130  may in this case be produced by a casting process, also by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0023]    Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
         [0024]    Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
         [0025]    In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
         [0026]    Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). 
         [0027]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0028]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation, e.g. MCrAlX (M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and represents yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (HO). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0029]    The density is preferably 95% of the theoretical density. 
         [0030]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) forms on the MCrAlX layer (as an intermediate layer or an outermost layer). 
         [0031]    It is also possible for a thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 -ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide and/or one or more of rare earth element (lanthanum, gadolinium, yttrium, etc.), which is preferably the outermost layer, to be present on the MCrAlX. 
         [0032]    The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by means of suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0033]    Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS, solution precursor plasma spray (SPPS) or CVD. The thermal barrier coating may include porous grains which have microcracks or macrocracks for improving its resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0034]    The blade or vane  120 ,  130  may be hollow or solid in form. If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes  418  (indicated by dashed lines). 
         [0035]      FIG. 4  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0036]    In the interior, the gas turbine  100  has a rotor  103  which is mounted such that it can rotate about an axis of rotation  102 , has a shaft  101  and is also referred to as the turbine rotor. 
         [0037]    An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
         [0038]    The annular combustion chamber  110  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . 
         [0039]    Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . 
         [0040]    The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . 
         [0041]    A generator (not shown) is coupled to the rotor  103 . 
         [0042]    While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it. 
         [0043]    While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield bricks which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0044]    To be able to withstand the temperatures which prevail there, they can be cooled by means of a coolant. 
         [0045]    Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). 
         [0046]    By way of example, iron-based, nickel-based or cobalt-based superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . 
         [0047]    Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0048]    The guide vane  130  has a guide vane root (not shown here) facing the inner housing  138  of the turbine  108  and a guide vane head at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 . 
         [0049]    A new modified coating was developed which fulfils the requirements described above. This coating has a good long term life, acceptable mechanical properties and improved oxidation resistance. This is based on the presence of tantalum (Ta) in a nickel based alloy but preferably without rhenium (Re). Tantalum (Ta) stabilizes the formation of a three phase system (γ′/γ/β) with a high γ′/γ transition temperature. This will reduce the local stresses as well because tantalum (Ta) will stabilize the high transition temperatures of γ′ which is higher than the bondcoat service temperature of a bond coat of this alloy. 
         [0050]    Therefore there is preferably no need for hafnium (Hf) or silicon (Si) or zirconium (Zr) or platinum (Pt) or any melting depressant (like boron B) in the coating. 
         [0051]    Very good results show the following elemental composition for getting the proposed 3-phase-system with increased γ′ transition temperatures: Ni—13Co—15.8Cr—11Al—6Ta. 
         [0052]    A composition (Ni—25Co—17Cr—10Al—1.5Re—Y) which contains rhenium (Re) instead of tantalum (Ta) has a lower γ′/γ transition temperature because no tantalum is added. 
         [0053]    The bondcoat is preferably a nickel (Ni) based super alloy with addition of cobalt (Co), chromium (Cr), aluminum (Al) and optionally yttrium (Y) which is preferably consisting of these elements. 
         [0054]    Very preferably it is a MCrAlY alloy, with M═Ni, Co. 
         [0055]    Preferably the alloy contains no molybdenum (Mo), and/or no tungsten (W) and/or columbium (Nb). 
         [0056]    The substrate of the component comprises a nickel-based or cobalt-based superalloy especially one of  FIG. 3 .