Abstract:
NiCoCrAl layers used as anticorrosive layers characterized by additional corrosion stability enhancing agents that substantially improve the anticorrosive properties are provided. Corrosion stability is not only determined by the composition and the percentage of the main alloy elements of nickel, cobalt, chromium and aluminium, but also by the addition of corrosion stability enhancing agents, such yttrium, cerium, tantalum, niobium, silicon, titanium, zirconium, and hafnium.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2007/058429, filed Aug. 15, 2007 and claims the benefit thereof. The International Application claims the benefits of European application No. 06024450.6 EP filed Nov. 24, 2006, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a metallic layer as claimed in the claims and to a metallic layer system as claimed in the claims. 
       BACKGROUND OF INVENTION 
       [0003]    Metallic layers are frequently used for bonding ceramic layers to a metallic substrate and/or as an anti-corrosion/anti-oxidation coating. 
         [0004]    The formation of an oxide layer on the metal layer is crucial to the bonding of the ceramic layer and to the corrosion and oxidation behavior. 
         [0005]    The oxide layer has to be dense and solid such that no oxidizing or corrosive elements, or as few oxidizing or corrosive elements as possible, can diffuse through the dense oxide layer to the metallic substrate, and sufficient strength is required such that the oxide layer does not flake off and any ceramic layer which may be present on the latter can likewise remain bonded thereto. 
       SUMMARY OF INVENTION 
       [0006]    Therefore, it is an object of the invention to overcome the problem mentioned above. 
         [0007]    This object is achieved by means of an NiCoCrAl layer as claimed in claim  1  or a metallic layer system as claimed in claim  20 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the figures: 
           [0009]      FIG. 1  shows a layer system according to the invention, comprising a metallic layer, 
           [0010]      FIG. 2  shows a gas turbine, 
           [0011]      FIG. 3  is a perspective view of a turbine blade or vane, and 
           [0012]      FIG. 4  is a perspective view of a combustion chamber. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0013]    The metallic NiCoCrAl layer comprises at least 1% by weight, in particular at most 5% by weight, of cerium (Ce), tantalum (Ta), niobium (Nb), silicon (Si), titanium (Ti), zirconium (Zr), hafnium (Hf) or RE (rare earth element). 
         [0014]    The rare earth element is, in particular, yttrium (Y). The NiCoCrAl layer preferably comprises, in addition to the rare earth elements, at least 0.5% by weight, in particular 1% by weight, of cerium (Ce), tantalum (Ta), niobium (Nb), silicon (Si), titanium (Ti), zirconium (Zr) or hafnium (Hf). 
         [0015]      FIG. 1  shows a layer system  1  comprising a metallic layer  11 . 
         [0016]    The metallic layer  11  is applied to a substrate  4  which, particularly in the case of components of a gas turbine  100  ( FIG. 2 ), consists of nickel-base or cobalt-base superalloys. 
         [0017]    The metallic layer  11  may be used as an overlay layer (not shown) or as a bonding layer, such that in this case an outer ceramic layer  13  is present on the metallic layer  11 . 
         [0018]    The metallic layer  11  may comprise one layer (layer  11 =inner layer  7 , as described below) or two layers (inner layer  7  and outer layer  10 ). 
         [0019]    An oxide layer (TGO) is formed on the surface  15  of the metallic layer  11  during operation or as a result of pre-oxidation. 
         [0020]    The metallic layer  11  preferably comprises two layers and comprises an inner metallic layer  7  and an outer metallic layer  10  (NiCoCrAl layer); according to the invention, the outer metallic layer  10  comprises at least one of the elements cerium (Ce), tantalum (Ta), niobium (Nb), silicon (Si), titanium (Ti), zirconium (Zr), hafnium (Hf) or RE (rare earth element) as corrosion resistance enhancers. 
         [0021]    It is possible to use one, two, three or four of these elements in the outer metallic layer  10 , the minimum difference with respect to the rare earth element content (RE) preferably being 0.5% by weight. 
         [0022]    The total content of the corrosion resistance enhancers is at least 1% by weight. At least yttrium is used as the rare earth element (RE). With preference, only yttrium is used as the rare earth element (RE). 
         [0023]    As well as the addition of the rare earth element, the NiCoCrAl layer comprises at least 0.5% by weight, in particular 1% by weight, of the elements cerium, tantalum, niobium, silicon, titanium, zirconium and/or hafnium. 
         [0024]    The silicon, zirconium, cerium and/or hafnium contents are preferably 0.5% by weight, in particular ≧1% by weight. 
         [0025]    The maximum content of the corrosion resistance enhancers is 5% by weight, in particular 2.5% by weight. 
         [0026]    The outer metallic layer  10  is preferably thinner than the inner metallic layer  7 . This is preferably &lt;100 μm. 
         [0027]    The corrosion resistance enhancers (Si, Zr, Hf, Ce, Y, Ti, Nb, Ta) may be present in a metallic layer having the following composition (in % by weight):
   1. Co-(27-29)Ni-(23-25)Cr-(9-11)Al-(0.5-0.7)Y, in particular Co-28Ni-24Cr-10Al-0.6Y,   2. Ni-(11-13)Co-(20-22)Cr-(10-12)Al-(0.3-0.5)Y-(1.5-2.5)Re, in particular Ni-12Co-21Cr-11Al-0.4Y-2Re,   3. Ni-(24-26)Co-(16-18)Cr-(9-11)Al-(0.3-0.)Y-(1.0-2.5)Re, in particular Ni-25Co-17Cr-10Al-0.4Y-1.5Re,   4. Ni-(27-29)Cr-(7-9)Al-(0.5-0.7)Y-(0.06-0.8)Si, in particular Co-30Ni-28Cr-8Al-0.6Y-0.7Si.   
 
         [0032]    Preference is given to the following combinations of the corrosion resistance enhancers:
       Y/Si   Y/Zr   Y/Ce   Y/Al   Y/Si/Zr   Y/Si/Ce   Y/Si/Hf   Y/Zr/Ce   Y/Zr/Hf   Y/Ce/Hf   Y/Si/Zr/Ce   Y/Si/Zr/Hf   Y/Si/Ce/Hf   Y/Zr/Ce/Hf.       
 
         [0047]    Further examples of alloys to which the elements silicon, zirconium, cerium, hafnium or yttrium are preferably added are firstly a system of β-NiAl containing chromium and/or cobalt admixtures, in which the β-NiAl phase is not destroyed, or an alloy which comprises only the γ-Ni phase. 
         [0048]    This NiCoCrAl layer may preferably be used in a metallic layer system. Preference is given to using the following composition for the inner layer  7 :
   Co-(27-29)Ni-(23-25)Cr-(9-11)Al-(0.5-0.7)Y, in particular Co-28Ni-24Cr-10Al-0.6Y,   2. Ni-(11-13)Co-(20-22)Cr-(10-12)Al-(0.3-0.5)Y-(1.5-2.5)Re, in particular Ni-12Co-21Cr-11Al-0.4Y-2Re,   3. Ni-(24-26)Co-(16-18)Cr-(9-11)Al-(0.3-0.)Y-(1.0-2.5)Re, in particular Ni-25Co-17Cr-10Al-0.4Y-1.5Re,   4. Ni-(27-29)Cr-(7-9)Al-(0.5-0.7)Y-(0.06-0.8)Si, in particular Co-30Ni-28Cr-8Al-0.6Y-0.7Si.   
 
         [0053]    The inner layer  7  preferably comprises a composition from these four examples. It preferably consists of one of the four compositions. 
         [0054]    The four alloy compositions mentioned above may likewise be used for the outer layer  10 , but they comprise the corrosion resistance enhancers mentioned above as additional elements. 
         [0055]    The inner layer  7  preferably does not comprise any corrosion resistance enhancers or comprises only yttrium as corrosion resistance enhancer. 
         [0056]    The total content of the corrosion resistance enhancers in the inner layer  7  is preferably lower than in the outer layer  10 . 
         [0057]      FIG. 2  shows by way of example a partial longitudinal section through a gas turbine  100 . 
         [0058]    In its 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. 
         [0059]    An intake casing  104 , a compressor  105 , a for example toric combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust gas casing  109  follow one another along the rotor  103 . 
         [0060]    The annular combustion chamber  110  is in communication with a for example annular hot gas duct  111 . There, by way of example, four successive turbine stages  112  form the turbine  108 . 
         [0061]    Each turbine stage  112  is formed for example from two blade rings. As seen in the direction of flow of a working medium  113 , a guide vane row  115  is followed in the hot gas duct  111  by a row  125  formed from rotor blades  120 . 
         [0062]    The guide vanes  130  are secured to an inner casing  138  of a stator  143 , whereas the rotor blades  120  belonging to a row  125  are arranged on the rotor  103 , for example by means of a turbine disk  133 . 
         [0063]    A generator (not shown) is coupled to the rotor  103 . 
         [0064]    While the gas turbine  100  is operating, air  135  is drawn in through the intake casing  104  and compressed by the compressor  105 . The compressed air provided at the turbine end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mixture is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot gas duct  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. 
         [0065]    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 elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0066]    To be able to withstand the temperatures which prevail there, they can be cooled by means of a coolant. 
         [0067]    Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal fatal (SX structure) or have only longitudinally oriented grains (DS structure). 
         [0068]    By way of example, iron-base, nickel-base or cobalt-base 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 . 
         [0069]    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; these documents foun part of the disclosure with regard to the chemical composition of the alloys. 
         [0070]    The guide vane  130  has a guide vane root (not shown here) facing the inner casing  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 . 
         [0071]      FIG. 3  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0072]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0073]    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 , a main blade or vane part  406  and a blade or vane tip  415 . 
         [0074]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0075]    A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . 
         [0076]    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. 
         [0077]    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 . 
         [0078]    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 . 
         [0079]    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; these documents form part of the disclosure with regard to the chemical composition of the alloy. 
         [0080]    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. 
         [0081]    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. 
         [0082]    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 fault the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
         [0083]    In this case, dendritic crystals are oriented along the direction of heat flow and fowl 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 foiins transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
         [0084]    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). 
         [0085]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents form part of the disclosure with regard to the solidification process. 
         [0086]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation, e.g. MCrAIX according to the invention (M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one of the rare earth elements, or hafnium (Hf)). 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, which are intended to form part of this disclosure with regard to the chemical composition of the alloy. 
         [0087]    The density is preferably 95% of the theoretical density. 
         [0088]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAIX layer (as an interlayer or as the outermost layer). 
         [0089]    It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists 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, to be present on the MCrAlX. 
         [0090]    The thermal barrier coating covers the entire MCrAIX layer. 
         [0091]    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). 
         [0092]    Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have grains that are porous and/or include micro-cracks or macro-cracks in order to improve the resistance to thermal shocks. Therefore, the thermal barrier coating is preferably more porous than the MCrAlX layer. 
         [0093]    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). 
         [0094]      FIG. 4  shows a combustion chamber  110  of the gas turbine  100 . The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107 , which generate flames  156  and are arranged circumferentially around an axis of rotation  102 , open out into a common combustion chamber space  154 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0095]    To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . 
         [0096]    A cooling system may also be provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . The heat shield elements  155  are then for example hollow and may also have cooling holes (not shown) which open out into the combustion chamber space  154 . 
         [0097]    On the working medium side, each heat shield element  155  made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks). 
         [0098]    These protective layers may be similar to the turbine blades or vanes, i.e. for example MCrAlX, in particular according to the invention: M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one of the rare earth elements, or hafnium (Hf). 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, which are intended to form part of this disclosure with regard to the chemical composition of the alloy. 
         [0099]    A for example ceramic theimal 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, may also be present on the MCrAIX. 
         [0100]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0101]    Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have grains that are porous and/or include micro-cracks or macro-cracks in order to improve the resistance to thermal shocks. 
         [0102]    Refurbishment means that after they have been used, protective layers may have to be removed from turbine blades or vanes  120 ,  130 , heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the turbine blade or vane  120 ,  130  or the heat shield element  155  are also repaired. This is followed by recoating of the turbine blades or vanes  120 ,  130 , heat shield elements  155 , after which the turbine blades or vanes  120 ,  130  or the heat shield elements  155  can be reused.