Abstract:
The turbine parts, when they are used, form oxide layers which by the undesirable rapid growth thereof generate the damage of the parts substrate. The inventive method consists in depleting the part in an element in such a way that the oxide layer is reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2005/055331, filed Oct. 18, 2005 and claims the benefit thereof. The International Application claims the benefits of European application No. 04031008.8 filed Dec. 30, 2004, both of the applications are incorporated by reference herein in their entirety 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a process for producing a component in which an oxide layer can form, and to a component. 
       BACKGROUND OF THE INVENTION 
       [0003]    Components which are used at high temperatures often form an oxide layer, during which process “metallic” atoms diffuse out of the component to the component/metal oxide interface and this metal element is depleted in the component in the region below the oxide layer. 
         [0004]    Gas turbine components which are exposed to hot gases are often protected by thermal barrier coatings on MCrAlX bonding layers. The bonding of the thermal barrier coating to the MCrAlX layer is substantially produced by an aluminum oxide layer between the MCrAlX layer and the thermal barrier coating. However, the aluminum oxide layer (thermally grown oxide—TGO) grows during operation, until ultimately the bonding fails. 
         [0005]    The MCrAlX layers are often provided with a platinum plating, which as a diffusion barrier is supposed to slow down the growth of the TGO. One problem is that this process is expensive, since pure platinum is used and an additional coating process increases costs. A further problem is that during operation the platinum diffuses into the component substrate. However, during refurbishment of the component this leads to a change in the known refurbishment processes, since the platinum increases the attack of acid on the substrate. 
       SUMMARY OF INVENTION 
       [0006]    Therefore, it is an object of the invention to provide a process for treating a component resulting in slower growth of an oxide layer on a component. 
         [0007]    Furthermore, it is an object of the invention to provide a component in which an oxide layer is formed more slowly. 
         [0008]    Further advantageous measures, which can advantageously be combined with one another in any desired way, are listed in the process and component subclaims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    In the drawing: 
           [0010]      FIGS. 1 ,  2 ,  3  show a component that has been treated by means of the process according to the invention, 
           [0011]      FIGS. 4 ,  5  show components according to the invention, 
           [0012]      FIG. 6  shows a turbine blade or vane, 
           [0013]      FIG. 7  shows a combustion chamber element, and 
           [0014]      FIG. 8  shows a gas turbine. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0015]      FIG. 1  shows an example of a component  1  which is treated by means of the process according to the invention. 
         [0016]    The component  1  comprises, for example, a metallic substrate  4 . The substrate  4  may be an elemental metal or an alloy. Of course, secondary phases may also be present. 
         [0017]    In the case of components for turbines, such as for example turbine blades or vanes  120 ,  130  ( FIG. 6 ), heat shield elements  155  ( FIG. 7 ) of gas turbines  100  ( FIG. 8 ) or steam turbines, the substrate  4  is an iron-base, cobalt-base or nickel-base superalloy. 
         [0018]    By way of example, a corrosion-resistant layer or bonding layer  7  is present on the substrate  4 . In the case of turbine components, this layer generally consists of an alloy of the MCrAlX type. 
         [0019]    The corrosion-resistant layer  7  may also have been aluminized and/or chromized in the region of its outer surface  22 , with the result that in the protective layer  7  a surface region  13  has formed with a higher concentration of aluminum and/or chromium than the other, underlying region of the corrosion-resistant layer  7 . 
         [0020]      FIG. 2  shows an example of another component  1  which can be treated by means of the process according to the invention. 
         [0021]    In this case, the component  1  does not have a corrosion-resistant layer and has for example been aluminized and/or chromized in the region of its outer surface  25 , resulting in the formation of a surface region  10  which has a higher concentration of aluminum and/or chromium than the remainder of the substrate  4  (for example made from a superalloy). 
         [0022]    An oxide layer Me1 y Me2 z O x , and/or a mixture of Me1O x  and Me2O x  of at least one element Me1, Me2 forms on the substrate  4  or on the corrosion-resistant layer  7 ; the at least one element Me1, Me2 is, for example, a metal oxide-forming element (“metallic element”), in particular aluminum. In the case of aluminum, aluminum oxide Al 2 O 3  is formed (Me1=Al,  y =2, z=0, x=3). 
         [0023]    It is also possible for oxides of a plurality of elements Me1, Me2 to form, for example Cr 2 O 3  and Al 2 O 3  or mixed oxides, such as La—Al—O. 
         [0024]    According to the invention, the corrosion-resistant layer  7 , the aluminized corrosion-resistant layer  7 , the substrate  4  or the aluminized substrate  4  is subjected to a treatment in which at least one element Me1, Me2 which forms an oxide layer  28  ( FIG. 3 ) is depleted in the metal or the alloy. This leads to the formation of vacancies or locally depleted regions  16  in the crystal lattice. 
         [0025]    There is no or scarcely any removal of material from a layer region at the surface  22 ,  25 . Rather, it is merely regions that are depleted of this element Me1, Me2 which are formed in the corrosion-resistant layer  7  or the substrate  4  (not shown). 
         [0026]    By way of example for an aluminized and/or chromized corrosion-resistant layer  7  or a substrate  4 , the following applies: the layer region in which the element aluminum and/or chromium has been depleted is, however, preferably thinner than the layer region  10 ,  13  which has been aluminized or chromized. 
         [0027]    In the case of the MCrAlX layers or a superalloy, the depleted elements are either aluminum or chromium or chromium and aluminum, which were depleted in the substrate  4  or the layer  7 . 
         [0028]    Prior to use, the surface  22 ,  25  treated in this way can be slightly pre-oxidized and forms the oxide layer  28 , in which case a ceramic thermal barrier coating  19  (indicated by dashed lines on the right-hand side of  FIG. 3 ) can then be applied. In use, i.e. at high temperatures T, the oxide layer  28  continues to grow on the corrosion-resistant layer  7 . 
         [0029]    However, the ceramic thermal barrier coating  19  may also be applied direct, without pre-oxidation, to the corrosion-resistant layer  7  or to the substrate  4  having the zones which have been depleted of the at least one element Me1, Me2. In this case, the oxide layer  28  forms during operation between the corrosion-resistant layer  7  and the ceramic thermal barrier coating  19 . 
         [0030]    According to the invention, the depletion of the at least one element Me1, Me2 in the substrate  4  or the corrosion-resistant layer  7  is carried out, for example, by a treatment involving contacting with a treatment fluid, i.e. by means of one or more acids or bases or base mixtures, an electrolyte treatment (i.e. with the application of an electric voltage) or by exposing the component  1  to an environment containing at least one halogen, in particular fluorine or chlorine, or at least one halide, as is known from the fluoride ion cleaning process. As a result of this depletion, the TGO grows very much more slowly, in particular by up to about 20 μm per service cycle less than is known from the prior art, resulting in a gain of several thousand operating hours of the component  1  at high temperatures. The surface  22 ,  25  of the corrosion-resistant layer  7  or the substrate  4  can preferably be roughened by the treatment, resulting in the formation of fissures  31  ( FIG. 4 ), giving better bonding of the ceramic thermal barrier coating  19 , in particular a plasma-sprayed thermal barrier coating (APS: atmospheric plasma spraying, VPS: vacuum plasma spraying, LPPS: low-pressure plasma spraying). 
         [0031]    Examples of corrosion-resistant layers  7  that can be used include those whose chemical compositions are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1, the chemical compositions of which corrosion-resistant layers are intended to form part of the subject matter of the present disclosure. 
         [0032]      FIG. 5  shows an enlarged illustration of the corrosion-resistant layer  7  or of the substrate  4 . 
         [0033]    The corrosion-resistant layer  7  comprises, for example, individual grains  37  (granular or columnar) which have grain boundaries  34  with one another. 
         [0034]    According to the invention, at least in certain locations an oxide layer, prepared in particular using the process according to the invention, has formed on the grain boundaries  34 . 
         [0035]    An acid, base or halogen attack takes place in particular into the grain boundaries  34 , since the latter constitute weak points compared to the grain, so that an attack can preferentially take place at these grain boundaries. Then, during the coating with a ceramic thermal barrier coating  19  or during a pre-oxidation process carried out in the corrosion-resistant layer  7 , a thin oxide film is formed within the grain boundaries  34  in the region of the surface  22 ,  29 , reducing the diffusion of the at least one element Me1, Me2, in particular aluminum, along the grain boundaries  34 , since the diffusion preferentially takes place along the grain boundaries  34 , and the at least one element Me1, Me2 has a lower diffusion coefficient in the oxide of the grain boundaries  34 . 
         [0036]    Used corrosion-resistant layers  7  are completely removed for refurbishment, after which they are generally treated with an acid for 4-8 hours at an elevated temperature of 50° C.-80° C. 
         [0037]    By contrast, the treatment times with an acid, a base, an electrolyte or a halogen/halide when using the process according to the invention are considerably shorter. They are reduced to 50%, in particular 25% of these standard treatment times and last at most 1 hour (h), in particular ½ h. 
         [0038]    The treatment temperatures may remain the same but tend to be at the lower end of the temperature ranges used to remove corrosion-resistant layers. 
         [0039]    The treatment time in a hydrochloric acid, for example with a concentration of 30%, is 30 minutes at 50° C. In the case of an electrolytic treatment, the concentration of the hydrochloric acid can be reduced, for example, to 5%, and the treatment time is also shorter, for example at 10 minutes. Depending on the size of the component, voltages of from 0.1 to 0.34 volts are applied. 
         [0040]      FIG. 6  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0041]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0042]    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 . 
         [0043]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0044]    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 . 
         [0045]    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. 
         [0046]    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 . 
         [0047]    In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
         [0048]    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. 
         [0049]    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. 
         [0050]    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. 
         [0051]    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. 
         [0052]    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). 
         [0053]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0054]    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. 
         [0055]    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). 
         [0056]    To protect against corrosion, the blade or vane  120 ,  130  has, for example, corresponding, generally metallic coatings (MCrAlX), which can be treated by means of the process according to the invention, and to protect against heat it generally also has a ceramic coating. 
         [0057]      FIG. 7  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  102  arranged circumferentially around the turbine shaft  103  open out into a common combustion chamber space. For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the turbine shaft  103 . 
         [0058]    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 . 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. On account of the high temperatures inside the combustion chamber  110 , a cooling system is additionally provided for the heat shield elements  155  or for their holding elements. 
         [0059]    The materials of the combustion chamber wall and their coatings may be similar to those of the turbine blades or vanes. 
         [0060]      FIG. 8  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0061]    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. 
         [0062]    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 . 
         [0063]    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 . 
         [0064]    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 . 
         [0065]    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 . 
         [0066]    A generator (not shown) is coupled to the rotor  103 . 
         [0067]    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. 
         [0068]    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. 
         [0069]    To be able to withstand the temperatures which prevail there, they have to be cooled by means of a coolant. 
         [0070]    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). 
         [0071]    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 . 
         [0072]    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. 
         [0073]    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 [Hf]), which can be treated using the process according to the invention. 
         [0074]    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. 
         [0075]    A thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 4 —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). 
         [0076]    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 .