Abstract:
Protective layers of the MCrAlX type according to the prior art are often provided with a platinum layer to prevent diffusion of elements out of the base material into the MCrAlX. The MCrAlX alloy according to the invention includes halogens (F, Cl, Br, I), which prevent this diffusion, in particular of titanium.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority of the European application No. 05003583 EP filed Feb. 18, 2005, which is incorporated by reference herein in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to an alloy of MCrAlX type as claimed in the claims, to a protective layer as claimed in the claims and to processes for producing the protective layer as claimed in the claims.  
       BACKGROUND OF THE INVENTION  
       [0003]     Thermal barrier coating systems are mostly used on nickel-base or cobalt-base systems as base material, in which case, to improve the mechanical properties, the proportion of the γ′ phase is increased by additions of aluminum and titanium. However, since titanium has a very high diffusion coefficient at the temperatures of use, titanium diffuses from the base material into a bonding layer of the MCrAlX type in the layer system, where it is incorporated in the thermally grown aluminum oxide layer (TGO), so that titanium spinels, which have very unfavorable effects on the bonding of a ceramic layer above, are formed on the bonding layer.  
         [0004]     The MCrAlX bonding layer is often coated with platinum in order to prevent this diffusion, but the costs of this are very high.  
         [0005]     EP 0 489 659 B1 discloses a process for treating metals in which metallic halides are applied as a layer.  
         [0006]     Therefore, it is an object of the invention to overcome the abovementioned problem.  
       SUMMARY OF THE INVENTION  
       [0007]     The object is achieved by the MCrAlX alloy as claimed in the claims, the protective layer as claimed in the claims and production processes as claimed in the claims.  
         [0008]     The subclaims list further advantageous measures which can be advantageously combined with one another in any desired way.  
         [0009]     The invention consists in introducing elements of at least one halogen (fluorine [F], chlorine [Cl], bromine [Br], iodine [I]) into a layer of an MCrAlX alloy. On account of the low vapor pressure, these halogens inter alia trap the titanium atoms, so that they are immobilized in the metal lattice and scarcely any titanium atoms are able to reach the surface of a layer of an MCrAlX alloy and form spinels there.  
         [0010]     In addition, the aluminum activity is also increased in such a manner that a homogenous α-aluminum oxide layer is formed. The application therefore also leads to a clear increase in the oxidation prevention action of layers based on MCrAlX. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     In the drawing:  
         [0012]      FIG. 1  shows a layer system,  
         [0013]      FIG. 2  shows a turbine blade or vane,  
         [0014]      FIG. 3  shows a combustion chamber,  
         [0015]      FIG. 4  shows a gas turbine, and  
         [0016]      FIG. 5  shows a steam turbine. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  shows a layer system  1 .  
         [0018]     The layer system  1  comprises a substrate  4 , a protective layer  7  as bonding layer and/or to protect against corrosion, based on the MCrAlX alloy according to the invention.  
         [0019]     In addition, a ceramic layer  10  may but does not have to be arranged on the protective layer  7 , serving as an additional thermal barrier.  
         [0020]     Layer systems  1  of this type are used, for example, for components of turbines, for example of steam or gas turbines  100  ( FIG. 4 ), on turbine blades or vanes  120 ,  130  ( FIG. 2 ), heat shield elements  155  ( FIG. 3 ), steam inflow regions  333  ( FIG. 5 ).  
         [0021]     In this context, M stands for at least one element selected from the group consisting of iron (Fe) and/or nickel (Ni) and/or cobalt (Co).  
         [0022]     X stands at least for yttrium (Y), silicon (Si), hafnium (Hf) or at least one other element from the group of the rare earths.  
         [0023]     Further constituents in the MCrAlX alloy, which serve to improve mechanical and/or corrosive properties, are possible.  
         [0024]     Examples of an MCrAlX alloy are described in U.S. Pat. No. 5,401,307, U.S. Pat. No. 5,582,635, U.S. Pat. No. 5,599,385, EP 0 486 489, WO 91/02108, U.S. Pat. No. 5,154,885, U.S. Pat. No. 5,273,712, U.S. Pat. No. 5,268,238, EP 0 412 397, EP 0 786 017, WO 96/12049, U.S. Pat. No. 5,993,980, the chemical composition of which in each case forms part of the present disclosure.  
         [0025]     The MCrAlX layer consists, for example, of  
         [0000]     25-40 wt %, in particular 29-31 wt % nickel,  
         [0000]     27-32 wt %, in particular 27-29 wt % chromium,  
         [0000]     7-9 wt %, in particular 7-8 wt % aluminum,  
         [0000]     0.3-1 wt %, in particular 0.5-0.7 wt % X, in particular Y,  
         [0000]     0.3-2 wt %, in particular 0.3-0.7 wt % silicon,  
         [0000]     if appropriate with an addition of rhenium (Re): max: 3 wt % and a minimum cobalt content of 5 wt %, which also forms the remainder.  
         [0026]     It is also possible for the halogens to be present in a rhenium-containing MCrAlX alloy.  
         [0027]     A further advantageous MCrAlX alloy consists of  
         [0000]     20-50 wt %, in particular 20-22 wt % chromium,  
         [0000]     0-15 wt %, in particular 10.5-11.5 wt % aluminum,  
         [0000]     0.3-2 wt %, in particular 0.3-0.5 wt % X, in particular Y,  
         [0000]     1-20 wt %, in particular 1.5-2.5 wt % rhenium,  
         [0000]     optional addition of silicon: max. 2 wt %, in particular 11-13 wt % cobalt.  
         [0028]     The remainder can consist of nickel. The remainder can also consist of cobalt, or the alloy may be based on nickel/cobalt.  
         [0029]     A further MCrAlX alloy consists of  
         [0000]     0-30 wt %, in particular 24-26 wt % cobalt,  
         [0000]     15-26 wt %, in particular 16-18 wt % chromium,  
         [0000]     9-15 wt %, in particular 9.5-11 wt % aluminum,  
         [0000]     0.3-2 wt %, in particular 0.3-0.5 wt % X, in particular Y,  
         [0000]     1-15 wt %, in particular 1.0-1.8 wt % rhenium,  
         [0000]     and optional addition of silicon (max: 2 wt %) and remainder nickel.  
         [0030]     According to the invention, halogens or halides are introduced into this protective layer  7  at least in a subregion of the protective layer  7 , in particular close to the substrate  4 . The concentration of the at least one halogen can therefore, for example, have a gradient.  
         [0031]     The halogens can be introduced in various ways. 
        1. Dip process with an MCrAlX layer in halogen-salt-containing solutions at room temperature followed by heat treatment (preferably 700° C.-1000° C.);     2. Dip process in halogen-containing organic liquids followed by heat treatment, preferably at temperatures of 700° C.-1000° C.;     3. Application of solid halogen-containing salts to an MCrAlX layer, for example by a powder pack process with subsequent heat treatment (preferably 700° C.-1000° C.);     4. Cooling of a warm MCrAlX layer in halogen-containing gas atmospheres;     5. Ion implantation of halogen ions into an MCrAlX layer.        
 
         [0037]     It is also possible, for example, to use an installation which carries out what is known as the fluoride ion cleaning (FIC) process in order to introduce halogens into the MCrAlX alloy.  
         [0038]     Further possible ways of introduction are possible.  
         [0039]     A solution anneal (preferably 4 h at 1160° C.) and/or a precipitation heat treatment (preferably 24 h at 840° C.), depending on the material of the substrate  4 , can be carried out with the MCrAlX layer  7  with the halogens.  
         [0040]     It is also possible for halides, i.e. compounds of halogens and a further element (for example AlF 3 , AlCl 3 ), to form in the protective layer  7  or to be produced directly during the production of the MCrAlX alloy or to be admixed with the MCrAlX protective layer  7  before the latter is applied to the base material that has to be protected.  
         [0041]     The amount of halogens in the alloy as an element or as a constituent of a halide is, for example, at least 100 ppm, preferably at least 200 ppm or 300 ppm, and is for example at most 500 ppm, in particular at most 1000 ppm or 5000 ppm.  
         [0042]      FIG. 2  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine  100 , which extends along a longitudinal axis  121 .  
         [0043]     The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor.  
         [0044]     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 .  
         [0045]     As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 .  
         [0046]     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 .  
         [0047]     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.  
         [0048]     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 .  
         [0049]     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 .  
         [0050]     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. 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.  
         [0051]     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.  
         [0052]     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.  
         [0053]     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.  
         [0054]     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).  
         [0055]     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.  
         [0056]     The blades or vanes  120 ,  130  may likewise have protective layers  7  according to the invention protecting against corrosion or oxidation (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 (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 the present disclosure.  
         [0057]     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 completely stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX.  
         [0058]     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).  
         [0059]     Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused.  
         [0060]     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).  
         [0061]      FIG. 3  shows a combustion chamber  110  of a gas turbine. The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107  arranged circumferentially around the axis of rotation  102  open out into a common combustion chamber space. For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 .  
         [0062]     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 .  
         [0063]     On the working medium side, each heat shield element  155  is equipped with a particularly heat-resistant protective layer or is made from material that is able to withstand high temperatures. These may be solid ceramic bricks or alloys with MCrAlX and/or ceramic coatings.  
         [0064]     The materials of the combustion chamber wall and their coatings may be similar to the turbine blades or vanes  120 ,  130 .  
         [0065]     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 .  
         [0066]      FIG. 4  shows, by way of example, a partial longitudinal section through a gas turbine  100 .  
         [0067]     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  and is also referred to as the turbine rotor.  
         [0068]     An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber  106 , with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 .  
         [0069]     The annular combustion chamber  106  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 .  
         [0070]     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 .  
         [0071]     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 .  
         [0072]     A generator (not shown) is coupled to the rotor  103 .  
         [0073]     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.  
         [0074]     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  106 , are subject to the highest thermal stresses.  
         [0075]     To be able to withstand the temperatures which prevail there, they have to be cooled by means of a coolant.  
         [0076]     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).  
         [0077]     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 .  
         [0078]     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.  
         [0079]     The blades or vanes  120 ,  130  may also have coatings which protect against corrosion (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). 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 the present disclosure.  
         [0080]     A thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or completely stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).  
         [0081]     The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is 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 .  
         [0082]      FIG. 5  illustrates, by way of example, a steam turbine  300 ,  303  with a turbine shaft  309  extending along an axis of rotation  306 .  
         [0083]     The steam turbine has a high-pressure part-turbine  300  and an intermediate-pressure part-turbine  303 , each with an inner casing  312  and an outer casing  315  surrounding it.  
         [0084]     The high-pressure part-turbine  300  is, for example, of pot-type design.  
         [0085]     The intermediate-pressure part-turbine  303  is of two-flow design.  
         [0086]     It is also possible for the intermediate-pressure part-turbine  303  to be of single-flow design.  
         [0087]     Along the axis of rotation  306 , a bearing  318  is arranged between the high-pressure part-turbine  300  and the intermediate-pressure part-turbine  303 , the turbine shaft  309  having a bearing region  321  in the bearing  318 . The turbine shaft  309  is mounted on a further bearing  324  next to the high-pressure part-turbine  300 . In the region of this bearing  324 , the high-pressure part-turbine  300  has a shaft seal  345 . The turbine shaft  309  is sealed with respect to the outer casing  315  of the intermediate-pressure part-turbine  303  by two further shaft seals  345 . Between a high-pressure steam inflow region  348  and a steam outlet region  351 , the turbine shaft  309  in the high-pressure part-turbine  300  has the high-pressure rotor blading  354 ,  357 . This high-pressure rotor blading  354 ,  357 , together with the associated rotor blades (not shown in more detail), constitutes a first blading region  360 . The intermediate-pressure part-turbine  303  has a central steam inflow region  333 . Assigned to the steam inflow region  333 , the turbine shaft  309  has a radially symmetrical shaft shield  363 , a cover plate, on the one hand for dividing the flow of steam between the two flows of the intermediate-pressure part-turbine  303  and also for preventing direct contact between the hot steam and the turbine shaft  309 . In the intermediate-pressure part-turbine  303 , the turbine shaft  309  has a second blading region  366  comprising the intermediate-pressure rotor blades  354 ,  342 . The hot steam flowing through the second blading region  366  flows out of the intermediate-pressure part-turbine  303  from an outflow connection piece  369  to a low-pressure part-turbine (not shown) which is connected downstream in terms of flow.