Abstract:
Conventionally, cavities and cracks are filled with a solder metal which forms brittle phases with a subsequently applied coating, which have a negative effect on the mechanical properties. According to the invention, the components which form brittle phases are removed from the solder metal. The above is achieved, whereby a second material is applied which reacts with said component and which is removed again with the brittle phases, before the coating.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. Ser. No. 10/574,722 filed on Apr. 6, 2006. This application is the US National Stage of International Application No. PCT/EP2004/010349, filed Sep. 15, 2004 and claims the benefit thereof. The International Application claims the benefits of European application No. EP03022634.4 filed Oct. 6, 2003. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a process for producing a layer system in accordance with the claims, and to a component as claimed in the claims. 
       BACKGROUND OF THE INVENTION 
       [0003]    Turbine blades and vanes as well as other components for high-temperature applications, after prolonged use, in many cases have cracks or regions in which corrosion has occurred, in which case these cracks or regions have to be removed during refurbishment of the components, with the result that a recess is in each case foitned there. 
         [0004]    The cracks or the recess are then filled with a solder. The multicomponent solder contains, inter alia, agents for reducing the melting point (e.g. boron) or other constituents which, as tests carried out in the context of the present invention have demonstrated, form brittle phases (e.g. chromium boride) with a final coating (e.g. corrosion-resistant layer, heat-resistant layer) which is to be applied. 
         [0005]    Therefore, layer systems of this type have poor mechanical properties, in particular at high temperatures. The function of the coating of protecting against oxidation and corrosion is also reduced. 
       SUMMARY OF THE INVENTION 
       [0006]    Therefore, it is an object of the invention to provide a process and a component which improves these properties of the layer system with an underlying filled recess. 
         [0007]    The invention is based on the discovery that a component of the solder reacts in an undesirable way with a coating that is subsequently applied. 
         [0008]    The object is achieved by the process as claimed in the claims, in that in an intermediate step a second material is applied, which for example reacts with the first material, for example a solder, and extracts the undesirable constituents of the first material by virtue of at least in part forming compounds therewith. 
         [0009]    The second material is then removed again together with the undesirable constituents that have been taken out of the first material, i.e. is sacrificed. Therefore, the boundary surface of the first material adjoining the coating no longer contains any constituents which can react in an undesirable way with the material of a coating that is yet to be applied. 
         [0010]    However, the second material may also simply cover the first material and create a distance between it and the layer that is yet to be applied, preventing or significantly reducing a reaction of constituents of the first material with the coating. The coating of the second material therefore constitutes a diffusion barrier or threshold. 
         [0011]    The object is also achieved by the component as claimed in the claims. 
         [0012]    Further advantageous measures are listed in the subclaims. 
         [0013]    The measures listed in the subclaims can be combined with one another in advantageous ways. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    An exemplary embodiment is illustrated in the figures, in which: 
           [0015]      FIG. 1  shows a layer system according to the prior art, 
           [0016]      FIGS. 2 to 6  show process steps of the process according to the invention, 
           [0017]      FIG. 7  shows a gas turbine, 
           [0018]      FIG. 8  shows a combustion chamber. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  shows a layer system  1  which at least comprises a substrate  4  and at least one layer  7 . 
         [0020]    The layer system  1  is, for example, a component of a steam turbine or gas turbine  100  (blade or vane  120 ,  130 , combustion chamber lining  155 , etc.). 
         [0021]    Therefore, the substrate  4  is, for example, a nickel-base or cobalt-base superalloy, i.e. is metallic. The substrate  4  may also be ceramic. 
         [0022]    The coating  7  is ceramic or metallic and consists, for example, of an MCrAlX alloy. M stands for at least one element selected from the group consisting of Fe (iron), Co (cobalt) or Ni (nickel). X stands for yttrium and/or at least one rare earth element. 
         [0023]    The substrate  4  has a recess  10  (crack, recess, milled-out portion), for example caused by the long operating use of a component or by the way in which it is produced. Correct coating of the substrate  4  is not yet possible in this state. 
         [0024]    The recess  10  is pre-cleaned if necessary (removal of oxides) and then filled with a first multicomponent material  13 . This is, for example, a solder  13 . Solders  13  contain components which react with the coating  7  at elevated temperatures, for example as agents for reducing the melting point. Agents for reducing the melting point are often required to provide the solder with a low viscosity and to enable it to penetrate into and completely fill a narrow crack. By way of example, the solder  13  contains boron (B) or boron-containing compounds, which with the chromium (Cr) of the MCrAlX layer  7  form chromium boride phases  16 , which are brittle and adversely affect the mechanical properties and/or corrosion-prevention properties. 
         [0025]    These undesirable phases  16  are formed when at least one component (agent for reducing the melting point) of the first multicomponent material  13  reacts with the coating  7  during a subsequent heat treatment or during use in operation. In the process, boron diffuses into the coating  7  and/or chromium diffuses out of the coating  7  into the recess  10  containing the material  13 . 
         [0026]      FIG. 2  shows a first step of the process according to the invention. 
         [0027]    The recess  10  has already been filled with the solder  13  and if appropriate joined to the substrate  4  by a soldering heat treatment. In this case, the agent for reducing the melting point in the material  13  is still desirable in order to effect bonding of the material  13  to the substrate  4  in the recess  10 . 
         [0028]    In the region of the recess  10 , a second material  22  is applied to a surface  19  of the substrate  4 , forming a local coating (in this context, the term local means that the surface area of  22  (=cross-sectional area of recess  10 ) is smaller than (&lt;20%) the surface area of the coating  7 ). The thickness of this layer of the material  22  is, for example, thinner than the coating  7  that is yet to be applied. The second material  22  is, for example, chromium, a chromium-containing compound or alloy or an alloy of other metals. Other materials are conceivable. The second material  22  can be applied using pastes, slurries, tapes, plasma spraying, etc. which have at least a high content of the second material  22 . 
         [0029]    By means of a removal heat treatment, in particular a separate diffusion heat treatment at high temperatures, but alternatively for example even simultaneously with the above soldering heat treatment, during which the substrate  4  is heated together with the first material  13  and the second material  22 , phases (compounds)  16  ( FIG. 3 ) are formed, for example in the form of precipitations, with the result that at least one undesirable component, which forms undesirable phases  16  with the coating  7  that is yet to be applied ( FIG. 5 ), is withdrawn from the first material  13 . 
         [0030]    These phases are, for example, chromium borides which form with the boron which is still unbonded following the soldering heat treatment. Interstitially dissolved boron (a lattice) in the material  22  is also conceivable. 
         [0031]    The phases  16  can form in the material  22  and/or in the material  13  in the recess  10 . Therefore, the material  22  is, for example, a constituent of the coating  7 , for example chromium of the MCrAlX coating. However, it is also possible to select a material which does not contain any elements or constituents of the coating  7 . It merely has to be able to react with the at least one undesirable component of the first material  13  which would otherwise react in an undesirable way with the coating  7 . 
         [0032]    A coating can be carried out in this state ( FIG. 6 ), because the second material  22  securely bonds the undesirable components of the first material  13  in the form of compounds, with the result that little if any reaction with the material of the coating  7  can then take place. The coating comprising the material  22  also constitutes a diffusion barrier or threshold to undesirable components which are still diffusing. 
         [0033]    The second material  22  and/or material  13  comprising the brittle phases  16  may, however, also be removed ( FIG. 4 ), in particular by grinding. This is followed by the application of the coating  7  ( FIG. 5 ). Even subsequent heat treatments of the substrate  4  do not lead to the formation of any brittle phases in the coating  7 , since the first material  13 , at least in the vicinity of the surface  19 , now contains little if any undesirable components which react with the material of the coating  7  to form undesirable compounds. 
         [0034]    The component which is produced in this way may be a newly produced component or a used component. The process is employed in particular in the repair soldering of components (refurbishment). 
         [0035]    This involves coating removal beforehand. The defects (cracks) are repaired using the process and if appropriate coated again, in particular with an MCrAlX followed by a ceramic thermal barrier coating. 
         [0036]      FIG. 7  shows a partial longitudinal section through a gas turbine  100 . 
         [0037]    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. 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 . 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 . 
         [0038]    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 . 
         [0039]    The guide vanes  130  are secured to the stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  by means of a turbine disk  133 . A generator (not shown) is coupled to the rotor  103 . 
         [0040]    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. 
         [0041]    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. To be able to withstand the temperatures which prevail there, they are cooled by means of a coolant. It is also possible for the blades or vanes  120 ,  130  to have coatings which protect against corrosion (MCrAlX; M=Fe, Co, Ni, X=Y, rare earths) and heat (thermal barrier coating, for example ZrO 2 , Y 2 O 4 —ZrO 2 ). 
         [0042]    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 . 
         [0043]      FIG. 8  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 . 
         [0044]    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. Moreover, on account of the high temperatures in the interior of the combustion chamber  110 , a cooling system is provided for the heat shield elements  155  and/or for their holding elements. 
         [0045]    The combustion chamber  110  is designed in particular to detect losses of the heat shield elements  155 . For this purpose, a number of temperature sensors  158  are positioned between the combustion chamber wall  153  and the heat shield elements  155 .