Abstract:
Layered systems, which are used at high temperatures, often degrade rapidly when a layer has been lost, leading to the damage or loss of the component that consists of said layered system. A described layered system comprises at least one cooling safety orifice, which is e.g. covered by an intermediate layer and an outer layer. The cooling safety orifice opens if the layers are damaged, in such a way that the layered system is additionally cooled by a coolant that flows through the cooling safety orifice.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2005/054842, filed Sep. 27, 2005 and claims the benefit thereof. The International Application claims the benefits of European application No. 04029079.3 EP filed Dec. 8, 2004, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a layer system and a method to produce a layer system. 
       BACKGROUND OF INVENTION 
       [0003]    Components which are used at high temperatures often have protective layers protecting the component against corrosion or excessive introduction of heat. 
         [0004]    However, the layer may flake off or degrade while the component is in use, with the result that the desired protection function can no longer be provided. This leads to damage to the substrate and sometimes to component failure. In the event of damage to the substrate, it may no longer be possible to refurbish the component, i.e. to recoat it with a new layer. 
         [0005]    EP 1 318 273 A2, EP 1 375 825 A1 and U.S. Pat. No. 6,039,537 disclose a turbine blade or vane in which a film cooling hole is closed by a single layer. EP 1 318 273 A2 also describes the presence of a plug in the film cooling hole even after coating. 
       SUMMARY OF INVENTION 
       [0006]    Therefore, an object of the invention is to provide a layer system, a use of a layer system and processes for producing a layer system which overcomes the problem. 
         [0007]    The object is achieved by the layer system as claimed in a first independent claim. The layer system has at least one safety cooling hole, which is closedA processes for producing a layer system is claimed in a further independent claim. 
         [0008]    When the layer flakes off in the region of a safety cooling hole, this cooling hole is opened and the layer system is cooled in this region, so that there is little if any damage under the operating conditions to which the layer system is exposed. 
         [0009]    The subclaims list further advantageous measures, which can advantageously be combined with one another in any desired way. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the drawing, in exert and diagrammatic form: 
           [0011]      FIGS. 1 to 5  show exemplary embodiments of a layer system, 
           [0012]      FIG. 6  diagrammatically depicts the way in which the safety cooling holes operate, 
           [0013]      FIGS. 7 to 11  show process steps involved in the production of a layer system according to the invention, 
           [0014]      FIG. 12  shows a turbine blade or vane, 
           [0015]      FIG. 13  shows a combustion chamber, and 
           [0016]      FIG. 14  shows a gas turbine. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0017]    The layer system  1  may be a component of a turbine, in particular a turbine blade or vane  120 ,  130  ( FIG. 12 ) or a heat shield element  155  ( FIG. 13 ) of a gas turbine  100  ( FIG. 14 ) or steam turbine. 
         [0018]    The layer system  1  ( FIGS. 1 to 5 ) has a substrate  4 , which in the case of components used at high temperatures, in particular turbine components  120 ,  130 ,  155 , consists of a nickel-, cobalt- or iron-base superalloy. 
         [0019]      FIG. 1  shows an example of a layer system  1  according to the invention: 
         [0020]    In the substrate  4  there is at least one safety cooling hole  13  which, for example, extends as far as an outer surface  25  of the substrate  4  which is closest to an external medium, for example a hot gas  28 . However, the safety cooling holes  13  are deliberately covered by an outer layer  10 , for example a MCrAlX layer, i.e. the openings  37  in the safety cooling hole  13  are closed by this outer layer  10 , with the result that cooling medium from a cooling reservoir  34  does not penetrate to the outside  28  through a safety cooling hole  13 . 
         [0021]    It is also possible for at least one further intermediate layer  7  to be present beneath the layer  10 , so that the safety cooling holes  13  are covered for example by two layers  7  (MCrAlX),  10  (ceramic thermal barrier coating) ( FIG. 2 ). 
         [0022]    However, the intermediate layer  7  may also be a ceramic layer (e.g. aluminum oxide). 
         [0023]    In these exemplary embodiments ( FIGS. 1 ,  2 ), all the safety cooling holes  13  are covered. 
         [0024]    The safety cooling holes  13  can be closed by a layer  7 ,  10  or by a pin  31  ( FIG. 3 ) which extends into the layer  7 ,  10  and/or into the substrate  4 . This pin  31  may be solid and/or fitted in or is produced for example by a slurry which is introduced into the cooling hole and hardened (sintered). 
         [0025]      FIG. 4  shows a further exemplary embodiment of a layer system  1  according to the invention. 
         [0026]    In this case, the layer system  1  has an intermediate layer  7  and a further, outer layer  10 . The intermediate layer  7  is, for example, a MCrAlX layer, on which for example a ceramic thermal barrier coating  10  is arranged. 
         [0027]    The safety cooling holes  13 , which are covered only by an outer layer  10 , in this case extend within the substrate  4  and through the intermediate layer  7 , i.e. they adjoin an interface  32  between layer  7  and layer  10 . It is also possible for the covered safety cooling holes  13  to extend only as far as the surface  25  of the substrate  4  ( FIG. 5 ). 
         [0028]    The layer system  1  ( FIGS. 4 ,  5 ) may but need not necessarily have at least one cooling hole  16  that is open, i.e. constitutes a passage hole, through which, in normal operation, a cooling medium, for example cooling air from a cooling reservoir, flows. 
         [0029]      FIG. 6  shows the way in which the covered safety cooling holes  13  operate. 
         [0030]    A hot medium  28  is adjacent to the outer surface of the outer layer  10 . 
         [0031]    The layer  10  flakes off in a locally limited fashion as a result of the impingement of foreign particles (foreign object damage, FOD) or as a result of degradation of the layers  7 ,  10  covering the safety cooling hole, in this case the layer  10 , after which the safety cooling hole  13  is then opened or a pin  31  ( FIG. 3 ) can no longer maintain its closure action. Since the safety cooling hole  13  is likewise adjacent to a reservoir  34  of a cooling medium, the cooling medium then flows through the safety cooling hole  13 , which now constitutes a cooling hole  16 . 
         [0032]    The covered safety cooling holes  13  are arranged at locations where, when the component  1  is in operation, no additional or further film cooling is required on account of the presence of the thermal barrier coating  10 . By contrast, local disruption of the ceramic thermal barrier coating  10  would lead to increased, unacceptable rises in temperature of the substrate  4 . 
         [0033]    However, this is prevented by the additional cooling of the opened safety cooling hole  13 , with the result that the substrate  4  is not subject to any unacceptable level of damage and/or can be reused. 
         [0034]    The consumption of cooling medium, which is slightly increased a as result of safety cooling holes  13  being opened up and leads to a slight reduction in efficiency, is more than compensated for by the advantage of the expensive substrate  4  not being damaged. 
         [0035]      FIG. 7  shows, by way of example, how a layer system  1  according to the invention can be produced. 
         [0036]    Holes (passage holes), which are intended to form safety cooling holes  13  and optionally further cooling holes  16  which are to be used during normal operation of the substrate  4 , are introduced into the substrate  4 . 
         [0037]    Depending on the coating process, a hole may (but need not) be filled with a masking agent  19  which prevents the material of the layer  10  from penetrating into and blocking the hole. 
         [0038]    The masking agent  19  only has to withstand the coating temperatures used to coat the layer  10 , and thereafter can be removed, for example by evaporation or leaching. If appropriate, a further layer is also applied. 
         [0039]      FIG. 8  shows another possible option for producing the layer system  1  according to the invention. 
         [0040]    The layer system  1  has safety cooling holes  13 , which are covered, and at least one cooling hole  16 , which remains open. 
         [0041]    A masking agent  19 , on or over which the material of the layer  7 ,  10  that is to be applied can be deposited, is introduced into the holes for the safety cooling holes  13  that are to be produced. 
         [0042]    By contrast, by way of example a nonstick masking agent  22 , on which the material of the coating  7 ,  10  is not deposited (right-hand part of  FIG. 8 ), is introduced into a hole  16  that is to remain open, thereby forming a cooling hole  16 . 
         [0043]      FIG. 9  shows a further variant for producing a layer system  1  according to the invention. 
         [0044]    Here, by way of example, the substrate  4  with the coating  7  were produced by a nonstick masking agent  22  being introduced into all the safety cooling holes  13  and cooling holes  16  that are to be produced, or by the substrate  4  being coated with the layer  7  and then holes being introduced into the layer  7  and into the substrate  4  in the region of the safety cooling holes  13  and cooling holes  16  that are to be produced. 
         [0045]    Thereafter, the safety cooling holes  13  that are to be produced are filled with a masking agent  19  ( FIG. 9 ), whereas the cooling hole  16  that is to be produced is not filled with a masking agent or is filled with a nonstick masking agent  22 . 
         [0046]    During the application of the outer layer  10 , the holes are covered and, in the desired way, form safety cooling holes  13 , while a hole remains open as a cooling hole  16 . 
         [0047]    It is also possible for the cooling holes  16  that remain open to be covered and to be opened up again by removal of material, for example by laser beams or electron beams, above the cooling holes  16  that are to remain open. 
         [0048]      FIG. 10  shows another possible form of a layer system  1  according to the invention. 
         [0049]    In this case, in a first process step, a substrate  4  is coated with a layer  7 , with passage holes  14  then being produced through the layer  7  and the substrate  4 . It is also possible for passage holes  14  to be produced in the substrate  4  before it is coated with the layer  7 , in which case no material is applied above the passage holes  14  during the coating of the layer  7 , with the result that the passage hole  14  extends through the layer  7  even after the substrate  4  has been coated with the layer  7 . 
         [0050]    In a further process step, a further layer, for example an outer layer  10 , which then covers the passage holes  14 , is applied; in this case, by way of example, passage holes  14  were filled with masking agent  14 , or alternatively a gas flows out of the passage holes during the coating operation, in order thereby to form safety cooling holes  13 . 
         [0051]    In particular when using the EB-PVD process, there is no need to use any masking agent. 
         [0052]    In one of the last process steps, the continuous film cooling hole  16  is then introduced into the layer  7 ,  10  and the substrate  4 . 
         [0053]    The process can be correspondingly employed if only a single outer layer  10  is to be present on a substrate  4 , i.e. passage holes  14  are produced in the substrate  4  and the single layer  10  is applied, covering the passage holes  14  in the substrate  4 , after which at least one passage hole  14  is introduced into the single outer layer and the substrate  4 . 
         [0054]      FIG. 11  shows another possible form of a layer system  1  according to the invention. 
         [0055]    In this case, in a first process step passage holes  14  are produced in the substrate  4  (not shown). 
         [0056]    In a further process step, a layer  7  is applied to the substrate  4 , covering the passage holes  14 , so as to form safety cooling holes  13 . In a second step, the layer  10 , for example an outer ceramic layer, is applied. 
         [0057]    In a final step, the layer  7 ,  10  and the substrate  4  are provided with a continuous film cooling hole  16 . The process has the advantage that in this case the outer ceramic layer is arranged on the bonding layer  7 , resulting in secure bonding of the outer ceramic layer. 
         [0058]      FIG. 12  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0059]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0060]    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 . 
         [0061]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0062]    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 . 
         [0063]    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. 
         [0064]    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 . 
         [0065]    In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
         [0066]    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 are incorporated by reference herein in their entirety. 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. 
         [0067]    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. 
         [0068]    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. 
         [0069]    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. 
         [0070]    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). 
         [0071]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents are incorporated by reference herein in their entirety. 
         [0072]    The blades or vanes  120 ,  130  may likewise have coatings (as part of the layer system according to the invention) protecting against corrosion or oxidation (e.g. 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)). 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 incorporated by reference herein in their entirety. 
         [0073]    It is also possible for a thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 4 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX. 
         [0074]    Columnar grains are produced in the thermal barrier coating by means of suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0075]    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. 
         [0076]    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) and safety cooling holes  13 . 
         [0077]      FIG. 13  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  107  arranged circumferentially around the axis of rotation  102  open out into a common combustion chamber space. For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0078]    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 . 
         [0079]    On the working medium side, each heat shield element  155  is equipped with a particularly heat-resistant protective layer (for example as part of a layer system according to the invention) or is made from material that is able to withstand high temperatures. 
         [0080]    These may be solid ceramic bricks or alloys with MCrAlX and/or ceramic coatings. 
         [0081]    The materials of the combustion chamber wall and their coatings may be similar to the turbine blades or vanes. 
         [0082]    On account of the high temperatures in the interior of the combustion chamber  110 , it is also possible for a cooling system to be provided for the heat shield elements  155  and/or their holding elements, or for the arrangement with the heat shield elements  155  and/or for the heat shield elements  155  themselves to have, for example, cooling holes  16  and safety cooling holes  13 . 
         [0083]      FIG. 14  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0084]    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. 
         [0085]    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 . 
         [0086]    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 . 
         [0087]    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 . 
         [0088]    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 . 
         [0089]    A generator (not shown) is coupled to the rotor  103 . 
         [0090]    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. 
         [0091]    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. 
         [0092]    To be able to withstand the temperatures which prevail there, they have to be cooled by means of a coolant. 
         [0093]    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). 
         [0094]    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 . 
         [0095]    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 are incorporated by reference herein in their entirety. 
         [0096]    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). 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, these documents are incorporated by reference herein in their entirety. 
         [0097]    A thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 —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). 
         [0098]    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 .