Abstract:
On account of their form of coating, layer systems according to the prior art often only have a low level of attachment to the substrate. The layer may then become detached in the event of high mechanical loads being applied to the components. The layer system according to the invention has separately produced anchoring means which are more strongly attached to the substrate than the attachment of the layer to the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority of the European application No. 03022540.3 EP filed Oct. 2, 2003, which is incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The invention relates to a layer system, and to processes for producing a layer system. 
   BACKGROUND OF THE INVENTION 
   Nowadays, components which are to be exposed to high temperatures are generally provided with protective layers. These may be metallic corrosion-resistant layers (MCrAlX layers) or ceramic thermal barrier coatings, as well as layer systems comprising metallic corrosion-resistant layers and ceramic thermal barrier coatings. Plasma-enhanced powder-spraying processes are used as coating processes for these coatings, on account of their relatively favorable economics. Layers of this type are attached to the substrate by mechanical interlocking and subsequent diffusion heat treatment. In some cases, the layer may become detached in operation in regions which are subject to high levels of loading or at unfavorable locations on the component, i.e. at locations which are subject to high mechanical loads. Flaking of the layer during operation causes damage to the base material, thereby significantly reducing the service life of the component. 
   Therefore, it is an object of the invention to provide a layer system and a process for producing a layer system with better attachment of a protective layer to a substrate and/or of layers to one another. 
   SUMMARY OF THE INVENTION 
   The object is achieved by a layer system and by a process for producing a layer system 
   The layer system according to the invention has separately produced anchoring means which have a very strong attachment to the substrate or to a layer arranged beneath them on the substrate and are attached to the substrate or the other layer in a different way than the layer. 
   The stronger attachment of the anchoring means compared to the existing layer bonding (e.g. mechanical interlocking provided by surface roughness) is effected, for example, by melt-metallurgy bonding, which is produced in a separate process. Therefore, it is also possible to use the inexpensive and economical plasma-spraying process in order to apply the layer. 
   Further advantageous measures are listed in the subclaims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The measures listed in the subclaims can advantageously be combined with one another. In the drawing: 
       FIG. 1  shows a layer system according to the prior art, 
       FIGS. 2 ,  6 ,  7 ,  8  show layer systems designed in accordance with the invention, 
       FIG. 3  shows a perspective plan view of a layer system configured in accordance with the invention, 
       FIG. 4  shows steps involved in a process according to the invention, 
       FIG. 5  shows steps involved in another process according to the invention, 
       FIG. 9  shows a gas turbine, and 
       FIG. 10  shows a combustion chamber. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a layer system according to the prior art. The layer system has a substrate  4 . The substrate  4  may be metallic or ceramic and in the case of gas turbine components is produced in particular from an iron-, nickel- or cobalt-based superalloy. 
   At least one layer  7 ,  9  (two layers in  FIGS. 6 ,  7 ,  8 ) is present on the substrate  4 . This may be a metallic and/or ceramic layer  7 ,  9 . 
   For turbine blades or vanes  120 ,  130  ( FIG. 9 ), by way of example, a metallic corrosion-resistant layer  7  ( FIGS. 6 ,  7 ,  8 ) of type MCrAlX is applied to the substrate  4 , and then in addition an outer thermal barrier coating  9 , for example a ceramic thermal barrier coating  9  ( FIGS. 6 ,  7 ,  8 ), is also applied. 
   The interlayer  7  is attached to the substrate  4 , or the layers  7 ,  9  are attached to one another, purely by mechanical interlocking (surface roughness) to the underlying surface, followed by a diffusion heat treatment, in accordance with the prior art. 
     FIG. 2 , which proceeds from  FIG. 1 , shows a layer system  1  according to the invention. Anchoring means  10 ,  13  are present on the surface  5  of the substrate  4 . The anchoring means  10 ,  13  have a form of attachment to the surface  5  which results in an increased attachment force (more accurately: force per unit contact area) to the surface  5  compared to the form of attachment of the interlayer  7  to the surface  5 . 
   The anchoring means  10 ,  13  are attached to the substrate  4 , by way of example, by melt metallurgy by means of a suitably executed laser welding process. It is also conceivable for the layer  7  to be applied at defined locations by laser cladding (laser powder coating), so as to form anchoring means  10 ,  13 . The anchoring means  10 ,  13  may also be cast on or produced integrally during casting of the substrate  4 . The anchoring means  10 ,  13  form bonding bridges for the layer  7 ,  9  surrounding the anchoring means  10 ,  13 . The anchoring means  10  may extend from the surface  5  of the substrate  4  to the outer surface  8  of the interlayer  7 , or alternatively the anchoring means  13  may be covered by the layer  7 , so that the anchoring means  13  do not extend all the way to the surface  8  of the layer  7 , i.e. are arranged so as to end within the layer  7 ,  9 . In this case, the anchoring means  13  extend at least 10%, 20%, 30%, 40% of the thickness of the layer  7 ,  9  into the layer  7 ,  9 . 
   The anchoring means  10 ,  13  are, for example, only present locally, i.e. in a spatially delimited manner ( FIG. 3 ) on the substrate  4  or the layer  7 , specifically wherever the mechanical loading is highest. This is, for example, the region of the leading edge of a turbine blade or vane  120 ,  130 . The remainder of the blade or vane would then not have any anchoring means. 
     FIG. 3  shows a plan view of a surface  8  of a layer  7 . In this illustration, the anchoring means  13 , which do not extend all the way to the surface  8  of the layer  7 , are indicated by dashed lines. The anchoring means  10 ,  13  may have various geometries on the surface  5 , for example circles, stitch seams (i.e. elongate and crossing one another), wavy shapes, parallel paths and combinations thereof. 
     FIG. 6  shows a further layer system  1  formed in accordance with the invention. 
   The layer system  1  comprises a substrate  4  and two layers  7 ,  9 . 
   The interlayer  7  is, for example, a metallic MCrAlX layer, and the outer layer  9  is, for example, a ceramic thermal barrier coating  9  on the interlayer  7 . 
   Anchoring means  10 ,  13  are present both in the interlayer  7  and in the outer layer  9 . 
   However, the interlayer  7  does not have to have anchoring means  10 ,  13  in the sense of the present invention ( FIG. 8 ). Equally, the anchoring means may be present only in the interlayer  7  ( FIG. 7 ). 
   The anchoring means  10 ,  13  in the layers  7 ,  9  may extend from the surface  5 ,  8  of the substrate  4  or the interlayer  7  to the outer surface  8 ,  16  of the layer  7 ,  9  or may be covered by the layers  7 ,  9 , so that the anchoring means  13  do not extend all the way to the surface  8 ,  16  of the layers  7 ,  9 . 
   The anchoring means  10 ,  13  in the interlayer  7  improve the attachment of the interlayer  7  to the substrate  4 . The material of the anchoring means  10  in the layer  7  may, for example, also be selected in such a way as to produce improved bonding of the outer layer  9  to the anchoring means  10  ( FIG. 7 ). By way of example, it is possible for the material of the anchoring means  10  in the interlayer  7  to be ceramic, so that the ceramic thermal barrier coating  9  can be more successfully joined to the anchoring means  10 , which extend as far as the surface  8  of the interlayer  7 , or the anchoring means  10  serve as a growth nucleus, in particular for epitaxial growth, when the interlayer  7  is being coated with the ceramic material of the outer layer  9 . 
   The material composition of the anchoring means  10 ,  13  in the layers  7 ,  9  is selected appropriately according to the particular requirements. 
   The anchoring means  10 ,  13  are present in particular in highly thermally and/or mechanically loaded regions. 
   The layer system  1  is, for example, a component of a gas turbine  100  ( FIG. 9 ) (or aircraft turbine) or a steam turbine. Components of the turbines which are subject to high thermal loads have a layer system of this type, for example turbine blades or vanes  120 ,  130 , linings  155  of a combustion chamber  110  and further parts of housing which are located along the flow path of a hot steam or hot gas. 
   The layer system  1  may be applied to a newly produced component and to components which have been refurbished after use. In this case, degraded layers are first removed from the components, any cracks are repaired and the substrate  4  is then recoated. 
     FIG. 7  shows a further exemplary embodiment of a layer system  1  according to the invention. In this layer system  1 , the anchoring means  10 ,  13  are present only in the interlayer  7 . The outer layer  9  is present on the interlayer  7 . A contact surface of the anchoring means  10  at the surface  8  improves the bonding of the outer layer  9  compared to a comparable contact surface with the interlayer  7 . This is achieved, for example, by virtue of the fact that the contact surfaces of the anchoring means  10  at the surface  8  form nucleus points for, for example, epitaxial growth of an outer layer  9  on the interlayer  7 . Even without an interlayer  7  ( FIGS. 4 ,  5 , right), an improved layer system  1  is achieved by virtue of the fact that the anchoring means  10 ,  13  lead to improved attachment of the outer layer  9  to the substrate  4 . 
     FIG. 8  shows a further exemplary embodiment of a layer system  1  according to the invention. In this exemplary embodiment, the anchoring means  10 ,  13  are present only in the outer layer  9 , i.e. they are present on the interlayer  7  and lead to improved attachment of the outer layer  9  to the underlying interlayer  7 . The anchoring means  10 ,  13  are then bonded to the surface  8  of the interlayer  7 . 
     FIG. 4  shows, by way of example, steps involved in a process according to the invention for producing a layer system  1 . In a first step, the at least one layer  7 ,  9  is applied in a known way to the substrate  4  or to a layer which is already present on the substrate. 
   The layer  7 ,  9  is treated with a laser  16  or an electron beam gun  16 , which emits a corresponding laser or electron beam  19 . This form of treatment causes the material of the layer  7 ,  9  to be locally transformed, for example melted, all the way down to the surface  5 ,  8  of the substrate  4  or the interlayer  7 , resulting in melt-metallurgy attachment of material from the layer  7 ,  9  to the substrate  4  or a layer which has already been applied thereto. This process produces anchoring means  10  which extend from the surface  5 ,  8  to the surface  8 ,  16  of the layer  7 ,  9 . 
   The anchoring means  10  are, for example, columnar in form, and may also be designed with a concave or convex curvature ( FIG. 7 ). 
     FIG. 5  shows a further example of a process according to the invention. 
   In a first step, first of all the anchoring means  10 ,  13  are applied to the substrate  4  or the layer  7 , i.e. are produced separately. This can be effected in various ways, such as for example by a suitably executed laser welding process or laser cladding. The anchoring means  10 ,  13  have a very strong, in particular melt-metallurgy attachment to the surface  5 ,  8  of the substrate  4  or of the interlayer  7 . 
   However, the anchoring means  10 ,  13  may also have been produced during the production of the substrate  4 , for example by means of a casting process. 
   In a subsequent process, the layer  7 ,  9  is applied, with the anchoring means  10 ,  13  being surrounded by the material of the layer  7 ,  9  and forming bonding bridges for the layer  7 ,  9 . 
   The material of the anchoring means  10 ,  13  may be the same as the material of the layer  7 ,  9 , the same as the material of the substrate  4  or the same as the material of a following layer, or may also have a different material composition. The material of the anchoring means  10 ,  13  in the layer  7  does not necessarily have to be identical to the material of the substrate  4 . 
     FIG. 9  shows a longitudinal part-section through a gas turbine  100 . In its interior, the gas turbine  100  has a rotor  103  which is mounted rotatably 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 torus-like combustion chamber  110 , in particular an annular combustion chamber  106 , having 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 an, for example, annular hot-gas duct  111 , where, by way of example, four turbine stages  112 , connected in series, form the turbine  108 . Each turbine stage  112  is formed from two bladed rings. As seen in the direction of flow of a working medium  113 , a row  125  formed from rotor blades  120  follows a row  115  of guide vanes in the hot-gas duct  111 . 
   The guide vanes  130  are secured to the stator  143 , whereas the rotor blades  120  belonging to a row  125  are arranged on the rotor  103  by means of a turbine wheel  133 . A generator or a machine (not shown) is coupled to the rotor  103 . 
   While the gas turbine  100  is operating, air  135  is sucked in through the intake housing  104  and compressed by the compressor  105 . 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 mixture is then burnt in the combustion chamber  110 , so as to form 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  expands at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter drives the machine coupled to it. 
   The components which are exposed to the hot working medium  113  are subject to thermal loads while the gas turbine  100  is operating. 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 , as well as the heat shield bricks lining the annular combustion chamber  106 , are subjected to the highest thermal loads. To be able to withstand the prevailing temperatures, these components are cooled by means of a coolant. It is also possible for the blades and vanes  120 ,  130  to have coatings protecting 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 ). 
   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 on the opposite side 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 . 
     FIG. 10  shows a combustion chamber  110  of a 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  102 , arranged around the turbine shaft  103  in the circumferential direction, open out into a common combustion chamber space. For this purpose, the combustion chamber  110  as a whole is configured as an annular structure positioned around the turbine shaft  103 . 
   To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium  113  of approximately 1000° C. to 1600° C. To allow a relatively long operating time to be achieved even under these operating parameters which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side facing the working medium  113 , 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 which 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 the holding elements hereof.