Abstract:
Coating systems according to the prior art, wherein a ceramic layer is applied to a metallic layer of the coating system, the connection between metal and ceramic often being poor. 
     A coating system ( 20 ) according to the invention has a porous layer ( 4 ) in which a ceramic ( 7 ) is at least partly disposed, so that the connection between ceramic ( 7 ) and the metal of the porous layer ( 4 ) is improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP02/13752, filed Dec. 4, 2002 and claims the benefit thereof. The International Application claims the benefits of European application No. 02000874.4 flied Jan. 15, 2002, both applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to a coating system with a ceramic component in the coating layer. 
     BACKGROUND OF INVENTION 
     Coating systems consist of a substrate and at least one coating layer overlying said substrate. In gas turbine construction, for example, substrates must be protected from excessively high temperatures and/or corrosive attack. This protection can be provided by depositing metal with a honeycomb structure on the substrate, said structure being filled with a ceramic and said ceramic essentially performing the protective function. The metal of the honeycomb structure is used for mechanical stabilization of the ceramic. However, the mechanical connection of the ceramic and the internal surfaces of the honeycomb structure is not good, causing the ceramic to continually peel off. 
     U.S. Pat. No. 5,634,189 describes a system having a porous internal structure formed by spherical elements of various diameters, said porous inner core being surrounded by a non-porous outer shell. The outer shell is not used for protection. The porous core is used for filling the cavity in order to achieve a degree of mechanical stability, the thickness of the porous core, however, being less than that of the shell in order to save weight. 
     U.S. Pat. No. 5,720,597 shows a gas turbine blade, at least part of the interior of which has a foam section. 
     U.S. Pat. No. 6,299,935 discloses a method for producing a coating wherein a suspension consisting of foam and a metallic powder is deposited on the surface of a substrate. 
     However, all the known systems or methods have the disadvantage that the mechanical connection between metal and ceramic is inadequate. 
     SUMMARY OF INVENTION 
     The object of the present invention is therefore to demonstrate a coating system which improves the mechanical strength between metal and ceramic. 
     This object is achieved according to the invention by a coating system consisting of a substrate on which a porous layer having a porosity of at least 30 percent by volume is deposited, a ceramic being partially incorporated in the porous layer in the form of a coating or as ceramic particles. In comparison to a flat-faced contact surface, the honeycomb structure with the metallic surface and the ceramic deposited thereon provides many small curved surfaces which improve the mechanical bond between metal and ceramic by increasing the surface area and mechanical adhesion. 
     It is advantageous to use an open pore structure because this improves the penetration depth of ceramic into the porous layer so that the adhesion of the layer is increased still further. 
     The ceramic applied to the porous layer and at least partially incorporated in same can also constitute a mixture of various ceramic materials in order to selectively adjust required characteristics. 
     The porous layer can be filled at least in certain areas with ceramic in such a way that it is virtually non-porous in these areas, so that a virtually non-porous ceramic layer is achieved in the porous coating layer in order to exploit the advantages of the ceramic in respect of heat resistance. 
     For example, an additional protective ceramic layer of the type known from heat insulation layers of gas turbine blades can be deposited on the ceramic in the porous layer or over the porous layer in order to protect the ceramic in the porous layer from oxidation by providing an additional coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are explained in greater detail in  FIGS. 1 to 7 : 
         FIGS. 1   a, b  show a substrate with a porous layer and a ceramic in the porous layer, 
         FIG. 2   a  shows a substrate with a porous layer, said porous layer being coated on its inner surfaces with a ceramic layer ( FIG. 2   b ), 
         FIG. 3  shows a substrate with a porous layer which has been rendered virtually non-porous by the ceramic, 
         FIG. 4  shows another exemplary embodiment of the invention, 
         FIG. 5  shows a typical example of a coating system of this kind, 
         FIGS. 6   a, b  show manufacturing operations for producing a coating system according to the invention, 
         FIGS. 7 ,  8 ,  9   a, b  show further exemplary embodiments for producing a coating system according to the invention, and 
         FIG. 10   a, b  show further exemplary embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1   a  shows a substrate  1 , e.g. made of metal, specifically a superalloy, for a gas turbine blade onto which a porous layer  4  has been deposited. The porous layer  4  can be made of metal or ceramic. 
     The porous structure is schematically represented by the line-strokes which are intended to represent the individual walls surrounding the pores in the porous layer  4 . The porosity is at least thirty (30) percent by volume. An open pore structure is particularly advantageous, i.e. there are connection paths from the outer surface of the porous layer  4  to its underside which faces the substrate  1 , as is known, for example, from use in filter systems. A ceramic can be incorporated particularly well into the porous layer. 
       FIG. 1   b  shows a coating system according to the invention wherein a ceramic  7  is present in the porous layer  4 . The ceramic  7  can consist of a single ceramic material or a mixture of various ceramic materials or take the form of ceramic particles. Metallic additions or coatings are likewise possible. 
       FIG. 2   a  shows another exemplary embodiment of a coating system  20  according to the invention wherein no individual ceramic particles  7  are identifiable ( FIG. 2   a ) because the pore walls  13  of the porous layer  4  have been coated with a ceramic layer  16  ( FIG. 2   b ). Thus, for example, the inner surfaces of the pores of the porous layer  4  are completely covered with a ceramic  16 . 
       FIG. 3  shows another exemplary embodiment of a coating system  20  implemented according to the invention. Onto the substrate  1  is deposited a porous layer  4  whose pores are filled with the ceramic  7  to produce a non-porous layer. 
       FIG. 4  shows that at least one additional intermediate layer  10  can be present between the substrate  1  and the porous layer  4 . 
       FIG. 5  shows another typical application of the coating system  20  according to the invention. The coating system  20  forms part of a gas turbine casing  23  which encloses e.g. turbine blades  26  installed downstream of a burner in a gas turbine. The rotational axis of the turbine blade  26  is indicated with  29 . The coating system  20  according to the invention forms a seal between gas turbine casing  23  and turbine blade  26  and replaces the honeycomb structure described above. Other typical applications may be found for gas turbine blades and heat shielding elements. 
       FIGS. 6   a, b  show the manufacturing steps for producing a coating system according to the invention  20 . The substrate  1  is interconnected with a prefabricated porous component  4  by means of a joining technique ( FIG. 6   b ). This can be performed e.g. by welding, diffusion welding or diffusion soldering. Other joining techniques are possible. 
       FIG. 7  shows another method for producing a coating system  20  according to the invention. Onto the substrate  1  a suspension  32  is deposited which is converted into a porous layer  4  during treatment at a temperature T. This can take place in the known manner in that the suspension  32  contains a metal powder and an activator which is gasified during heat treatment and foams the suspension containing the metal, the metal particles then being e.g. sintered together at elevated temperature to form the porous layer  4 , and a good connection with the substrate  1  simultaneously taking place. Other manufacturing methods for producing porous, specifically foam-like structures can be used here, such as precision casting, for example. 
       FIG. 8  shows another exemplary embodiment for producing a coating system  20  according to the invention. This can be performed, for example, by first casting the material for the substrate  1  in one casting process and then continuously casting a metal or an alloy having a porous structure or a mixture of metal and ceramic to produce, on the substrate  1 , a porous metallic layer  4  possibly tightly filled with ceramic. A substrate  1  and a porous layer  4  can also be formed from a blank  38  by means of an intermediate treatment. 
     In order to definitively produce the coating system  20  according to the invention, it is often still necessary to incorporate the ceramic  7  into the porous layer  4 . This can be performed by a coating device  35  ( FIG. 9   a ) by means of plasma spraying, for example, so that a ceramic coating  16  is produced in the porous layer  4 . The coating process can be continued in such a way that not only the walls  13  of the porous layer are coated, but the pore structure is also at least partially closed in order to achieve a non-porous layer. 
     A ceramic suspension with ceramic particles can also be incorporated, more specifically injected, into the porous layer  4  by a spray nozzle  35  ( FIG. 9   a ). In a subsequent process step the carrier medium of the suspension is vaporized so that the ceramic particles  7  are left behind and combine with the metallic walls  13  of the porous layer  4  after a heat treatment. 
     The porous layer  4  can also be completely filled with the ceramic  7  only in an upper area  11  ( FIG. 9   b ). 
     The porous layer  4  is advantageously filled with a ceramic  7  exhibiting good mechanical properties at elevated temperatures and serving as a thermal barrier. However, in order to protect this ceramic and also the metallic walls of the porous layer  4  from oxidation and/or corrosion and/or heat, yet another protective ceramic layer  41  can be applied to the porous layer  4  ( FIG. 10   a ) or over the ceramic particles  7  or the ceramic layer  16  within the porous layer  7  ( FIG. 10   b ).