Abstract:
Thermal barrier coating systems according to the prior art are exposed to mechanical loads, and in particular ceramic layers tend to form cracks or to flake off on account of their brittleness. A thermal barrier coating system ( 19 ) according to the invention includes a second phase ( 34 ), which mechanically reinforces the matrix ( 31 ) of the layer ( 25, 28 ).

Description:
[0001]    This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/EP03/03358 which has an International filing date of Mar. 31, 2003, which designated the United States of America and which claims priority on European Patent Application number EP 02008046.1 filed Apr. 10, 2002, the entire contents of which are hereby incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention generally relates to a thermal barrier coating system which includes a substrate and layers applied thereto, and generally relates to processes for producing such a system.  
         BACKGROUND OF THE INVENTION  
         [0003]    EP 1 029 101 B1 has disclosed a thermal barrier coating in which the ceramic outer layer has the material composition La—Al—Gd—O or Sr—Ca—Zr—O.  
           [0004]    U.S. Pat. No. 5,545,484 includes a layer system in which the substrate is mechanically reinforced by fibers.  
           [0005]    The thermal barrier coatings are substantially functional layers and are intended to protect the substrate, for example from excessive heat. The substrate has a sufficiently high mechanical strength. The thermal barrier coatings are likewise exposed to thermal stresses or mechanical stresses and may fail as a result of flaking, because cracks are formed.  
         SUMMARY OF THE INVENTION  
         [0006]    Therefore, it is an object of an embodiment of the invention to provide a thermal barrier coating system and processes for its production, in which the layers are better able to withstand thermal and mechanical stresses.  
           [0007]    An object may be achieved by a thermal barrier coating system. In this system, a second phase is introduced into the at least one layer, improving the mechanical properties of the layer. Further advantageous improvements to the thermal barrier coating system improve the mechanical strength (bending strength, compressive strength, etc.) and fracture toughness (K 1   c ).  
           [0008]    The second phase may be present in the form of fibers or particles. Suitable fibers are conventional carbon fibers or fibers of another material or whiskers.  
           [0009]    The particles which form the second phase may include the same elements as the matrix material of the layer or alternatively may be selected from a different material system.  
           [0010]    If the particles are elongate, i.e. have a high length to cross-sectional area ratio (high aspect ratio), the layer or the matrix material of the layer is advantageously reinforced by crack diversion and/or crack branching.  
           [0011]    The layer which is to be reinforced which is selected may be a ceramic or metallic layer of the layers which have been applied to the substrate. In particular, this layer is the thermal barrier coating.  
           [0012]    An object of producing a thermal barrier coating system which is reinforced by a second phase may be achieved by the processes.  
           [0013]    The matrix material of the layer is applied by known coating processes, such as plasma spraying, CVD (chemical vapor deposition) or PVD (physical vapor deposition), the second phase being introduced into the matrix material of the layer via a separate source. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Exemplary embodiments are shown in the figures which follow, in which:  
         [0015]    [0015]FIG. 1 shows a turbine blade or vane,  
         [0016]    [0016]FIG. 2 shows a thermal barrier coating system,  
         [0017]    [0017]FIG. 3 diagrammatically depicts the microstructure of a layer of the thermal barrier coating system according to an embodiment of the invention, and  
         [0018]    [0018]FIG. 4 diagrammatically depicts a coating device allowing the processes according to an embodiment of the invention to be carried out, and  
         [0019]    [0019]FIGS. 5, 6 show further exemplary embodiments of the thermal barrier coating according to an embodiment of the invention.  
         [0020]    Identical reference symbols have the same meaning throughout the various figures. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 shows a perspective view of a rotor blade which extends along a longitudinal axis  4 . In succession along the longitudinal axis  4 , the rotor blade  1  has a securing region  7 , an adjoining blade platform  10  and a main blade region  13 .  
         [0022]    A blade root  16 , which is used to secure the rotor blade  1  to a shaft (not shown) of a turbomachine (likewise not shown), is formed in the securing region  7 . The blade root  16  is designed, for example, as a hammerhead. Other configurations are possible.  
         [0023]    In conventional rotor blades, solid metallic materials are used in all the regions  7 ,  10 ,  13  and  16 . The rotor blade may in this case be produced by a casting process, by a forging process, by a milling process or by combinations thereof.  
         [0024]    Particularly in the main blade region  13 , which is exposed to relatively high temperatures, for example from a gas turbine, at least one layer, in particular an oxidation/corrosion-resistant layer and thermal barrier coating, is applied, forming part of a thermal barrier coating system.  
         [0025]    [0025]FIG. 2 shows a thermal barrier coating system  19  according to AN embodiment of the invention of this type. The thermal barrier coating system  19  has a substrate  22 , for example a nickel-based or cobalt-based superalloy, to which, by way of example, at least one intermediate layer  25  is applied.  
         [0026]    The intermediate layer  25  may include a plurality of layers which serve, for example, as a diffusion barrier, as a bonding layer (MCrAlY, M═Fe, Co, Ni), a corrosion-resistant layer or as an anchoring layer. By way of example, a ceramic thermal barrier coating  28  is applied to the substrate  22  or to the at least one intermediate layer  25 . The at least one intermediate layer  25  and the thermal barrier coating  28  are exposed to mechanical loads.  
         [0027]    [0027]FIG. 3 diagrammatically depicts a microstructure of the intermediate layer  25  or of the thermal barrier coating  28 . The layer  25 ,  28  according to an embodiment of the invention includes a matrix  31  and a second phase  34 . The matrix  31  includes grains which may have various forms (morphologies). If the matrix  31  is produced, for example, by plasma spraying, the grains of the matrix material  31  are in platelet form. During the plasma spraying, the second phase  34  may, for example, be mixed with the matrix material  31  and applied jointly through a torch nozzle (FIG. 4).  
         [0028]    The second phase  34  may be another phase of the matrix material  31 , but may also be a fiber or a whisker. The second phase  34  has an object of improving the mechanical strength of the matrix  31 . This can be ensured by fiber reinforcement or particle reinforcement using the known reinforcement mechanisms. Particularly if the second phase  34  is elongate in form, the mechanical strength of the matrix  31  is particularly improved.  
         [0029]    The materials which are disclosed in EP 1 029 101 B1, i.e. the systems La 1−x  Gd x  AlO 3  or Ca 1−x  Sr x  ZrO2 3 , may be suitable as material  31  for the thermal barrier coating  28 . EP 1 029 101 B1, the entire contents of which are incorporated herein by reference, is in particular to form part of the disclosure of the present application. The elements Gd and La may be replaced by other lanthanides.  
         [0030]    In particular, the second phase  34  having the composition LaAl 11 O 18  (βaluminate) is used as second phase  34  for reinforcing the matrix  31  for the matrix material  31  Gd—La—Al—O.  
         [0031]    The layer  25 ,  28  can be produced by various coating processes, such as for example plasma spraying, CVD processes or PVD processes. In the case of plasma spraying, the second phase  34  can be mixed with the matrix material  31  and applied to the substrate  22  or the intermediate layer  25  through the torch nozzle.  
         [0032]    However, for certain second phases  34 , the situation may arise wherein the second phase is not phase-stable at the high temperatures of the plasma spraying. With the CVD or PVD processes, it may also be the case that only one material composition, namely that of the matrix  31 , can be applied using this process. In this case, the second phase is introduced into the layer  25 ,  28  via a separate auxiliary nozzle  40  (FIG. 4).  
         [0033]    [0033]FIG. 4 diagrammatically depicts a coating device which allows a second phase and a matrix material  31  to be applied. By way of a main nozzle  37 , which is, for example, a torch nozzle of a plasma coating installation, or a substrate  37  of a PVD installation, the matrix material  31  is applied to the substrate  22  or the at least one intermediate layer  25 .  
         [0034]    The second phase  34  is mixed into the particle stream of the matrix material  31  by way of a separate source, for example an auxiliary nozzle  40 . Thus, a matrix  31  with a second phase which amounts to less than 50% by volume is formed.  
         [0035]    [0035]FIG. 5 shows a coating of a thermal barrier coating system according to the invention. The coating  28  is in at least two-layer form, including two layers  46  which are not reinforced and one layer  43  which is reinforced by a second phase. The first layer  46  rests, for example, on the substrate  22  or the layer  25 . This is followed by the layer  43  and, as outer layer, the unreinforced layer  46 .  
         [0036]    Cracks cannot grow through the layer  43  or can only do so with difficulty, and this prevents the coating  28  from becoming detached from the substrate  22 .  
         [0037]    [0037]FIG. 6 shows a coating of a thermal barrier coating system according to the invention. The coating  28  is in two-layer form, including a layer  46  which is not reinforced and a layer  43  which is reinforced by a second phase. The first layer  43  rests, for example, on the substrate  22  or the layer  25 . This is followed by the layer  43  as the outer layer, or vice versa.  
         [0038]    Cracks cannot grow through the layer  43  or can only do so with difficulty, this prevents the coating  28  from becoming detached from the substrate  22 .  
         [0039]    List of reference symbols:  
         [0040]    [0040] 1  Turbine blade or vane  
         [0041]    [0041] 4  Longitudinal axis  
         [0042]    [0042] 7  Securing region  
         [0043]    [0043] 10  Blade platform  
         [0044]    [0044] 13  Main blade region  
         [0045]    [0045] 16  Blade root  
         [0046]    [0046] 19  Thermal barrier coating system  
         [0047]    [0047] 22  Substrate  
         [0048]    [0048] 25  Intermediate layer  
         [0049]    [0049] 28  Thermal barrier coating  
         [0050]    [0050] 31  Matrix  
         [0051]    [0051] 34  Second phases  
         [0052]    [0052] 37  Main nozzle  
         [0053]    [0053] 40  Auxiliary nozzle  
         [0054]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.