Abstract:
The invention relates to an anti-wear coating, specifically for components which are subject to erosion under mechanical loading, in particular for gas turbine components, said coating comprising at least two different individual layers which preferably alternate with one another multiply and are applied to a surface of a component which is to be coated. The individual layers comprise a ceramic main layer ( 45, 46, 47, 48 ) and a quasi-ductile, non-metallic intermediate layer ( 41, 42, 43, 44 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an antiwear coating especially for components which are subject to erosion under mechanical stress, in particular for gas turbine components, which consists of at least two different individual layers which have been applied in a multiply alternating manner with one another to a surface to be coated of a component. 
     2. Discussion of Background Information 
     Gas turbine components are provided with an antiwear layer for protection against wear, especially erosion and corrosion. This antiwear layer consists of a plurality of individual layers composed of different materials, as is known from the document DE 10 2004 001 392.6. Here, a metallic layer is firstly applied to a component in order to make good bonding of the antiwear layer to the metallic substrate material possible. This is followed by a metal alloy and a gradated metal-ceramic material. This multilayer system is concluded by a ceramic layer. This multilayer system can also be deposited a plurality of times on top of the first multilayer system, always commencing with a metallic layer to achieve better bonding to the metallic substrate material and ending with a ceramic layer on the surface. In addition, a bonding layer can be inserted between the first multilayer system and the component. 
     In general, such multilayer systems based on this principle are made up of hard (main) and soft (intermediate) layers. The main layers have a high erosion resistance and the intermediate layers have a high ductility. As a result, cracks which form in the case of overloading in the multilayer structure are stopped in the ductile intermediate layers by blunting of the crack tips. 
     To prevent erosion, structuring the hard ceramic layers of a multilayer system is known from the document DE 10 2006 001 864.8. Such ceramic layers are segmented in the vertical direction in a columnar manner in order to prevent detachment of relatively large regions of the layer during particle erosion attack. Here, the columnar segmentation is in the form of columns or stems or fibers. The interfaces between the columns of the layers segmented in a columnar fashion prevent the growth of microcracks in the direction parallel to the surface which can be caused during erosive stress. However, it is a disadvantage that cracks in the vertical direction can be propagated unhindered along the interfaces. When a component is stressed, these interfaces between columns act as micronotches or initial microcracks. In the case of severe overstressing, the ductile intermediate layers can no longer stop the arriving microcracks and the latter grow into the substrate material. The microcrack formed under tensile stress can propagate far into the substrate material and lead to premature failure of the component. This has the substantial disadvantage that the life of a component is considerably reduced. 
     It is therefore an object of the invention to provide an antiwear layer which firstly increases the life of a component and secondly prevents microcrack formation. 
     SUMMARY OF THE INVENTION 
     The object of the invention is achieved by an antiwear coating as set forth in the independent claim(s). Further advantageous embodiments of the invention are specified in the dependent claims. 
     The invention relates to an antiwear coating which is especially suitable for components which are subject to erosion under mechanical stress, in particular for gas turbine components, and comprises at least two different individual layers which have preferably been applied in an alternating manner (in the case of more than two layers) to a surface to be coated of a component. however, in contrast to the known antiwear layers, the individual layers in the antiwear coating of the invention are formed firstly by a known ceramic main layer and secondly by a pseudoductile, non-metallic intermediate layer. the pseudoductile, non-metallic intermediate layer is, as will be shown below, configured in such a way that energy is withdrawn from cracks which grow in the direction of the substrate material by crack branching in the pseudoductile, non-metallic intermediate layer, so that crack growth can be slowed or stopped. 
     A corresponding antiwear coating can likewise be configured as a multilayer coating, with the pseudoductile non-metallic intermediate layer and the ceramic main layer which has a brittle and hard property profile being able to be arranged alternately a number of times above one another. 
     In particular, the pseudoductile, non-metallic intermediate layer may be arranged directly on the component to be coated, while a hard, ceramic main layer may be arranged at the surface of the antiwear coating. 
     In an embodiment of the invention, the pseudoductile, non-metallic intermediate layer may comprise a material having weak bonds, preferably materials having a sheet structure or a hexagonal lattice structure which make easy shearing-off of the material possible or comprise easily activatable sliding planes, with the sheet planes or basal planes of the material or the easily activatable sliding planes being arranged parallel to the surface of the component. Thus, arsenic and antimony, for example, are suitable materials since they have a sheet structure, and also, for example, graphite, molybdenum disulfide and/or hexagonal boron nitride since they have a hexagonal lattice structure. The low adhesion between the basal planes or the easy sliding-off of adjacent planes in these materials results in crack deflection, so that the crack spreads out between the basal planes or the planes which can readily slide relative to one another. Since the materials are applied in such a way that the basal planes or the planes which can readily slide relative to one another are aligned parallel to the surface to be coated, crack growth in the direction of the substrate material is avoided. 
     In a further embodiment of the invention, the pseudoductile, non-metallic intermediate layer may have a multilayer system which has ceramic layers in addition to layers having relatively weak bonding (sliding layers). Materials combinations such as C/TiC, C/SiC, C/ZrC, hexagonal BN/cubic BN and/or hexagonal BN/SiC are suitable for this purpose. 
     In a further embodiment of the invention, the sublayers in the multilayer system of the pseudoductile, non-metallic intermediate layer may have weak interfaces with one another. 
     In a further embodiment of the invention, layers having relatively weak bonds (sliding layers) in the multilayer system may have no or minor chemical reactions with ceramic layers in the multilayer system. 
     In a further embodiment of the invention, layers having relatively weak bonding (sliding layers) and ceramic layers in the multilayer system may have a low surface roughness. This low surface roughness ensures weak mechanical intermeshing between the individual layers. 
     In a further embodiment of the invention, the pseudoductile, non-metallic intermediate layer may comprise a ceramic material and/or further hard material compounds having deliberately introduced pores. Here, the ceramic material may comprise chromium nitride, titanium nitride and/or compounds therefrom, in particular with further elements such as aluminum or silicon, so that, for example, chromium aluminum nitride or titanium aluminum nitride or chromium silicon nitride or titanium silicon nitride is present. 
     In a further embodiment of the invention, the pseudoductile, non-metallic intermediate layer may comprise a ceramic and/or hard material compounds having deliberately introduced microcracks which run parallel to the surface. The material may once again comprise chromium nitride, titanium nitride and/or compounds therefrom, in particular with further elements such as aluminum or silicon, and/or further known hard material compounds having a nitride or carbide basis. 
     In a further embodiment of the invention, the pseudoductile, non-metallic intermediate layer may comprise a ceramic material and/or a hard material compound having deliberately introduced foreign phases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is illustrated below with the aid of an example and reference to the accompanying drawings. The drawings show, purely schematically, 
         FIG. 1  crack branching in an intermediate layer according to the invention; 
         FIG. 2  a sheet structure or basal planes in the hexagonal lattice structure of graphite; 
         FIG. 3  a structure of the intermediate layer according to the invention as multilayer system; 
         FIG. 4  a depiction of the intermediate layer according to the invention having pores which stop the cracks; 
         FIG. 5  a depiction of the intermediate layer according to the invention having microcracks parallel to the surface or to the substrate material; 
         FIG. 6  a depiction of the intermediate layer according to the invention having deliberately introduced foreign phases. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a substrate material  40  and a multilayer system  39  applied thereto. 
     Influencing of the properties (e.g. mechanical strength) of the substrate material  40  by cracks which propagate under stress from the multilayer system  39  into the substrate material  40  can be prevented by a specific structure of the individual layers of the multilayer system  39 . The multilayer system  39  has a first intermediate layer  41 , a hard ceramic main layer  45 , a second intermediate layer  42 , a ceramic main layer  46 , a third intermediate layer  43 , a third ceramic main layer  47 , a fourth intermediate layer  44  and a fourth ceramic main layer  48 . The hard ceramic main layers make it possible for the crack to be propagated directly in the direction of the substrate material  40 . The pseudoductile, non-metallic intermediate layers  41 ,  42 ,  43 ,  44  according to the invention in the multilayer system  39  prevent the crack  50  from growing further in the direction of the substrate material  40  and leading to premature damage to the component. Energy is withdrawn from the crack  50  by crack branching in the intermediate layers  41 ,  42 ,  43 ,  44  and the ceramic multilayer system  39  is thereby given pseudoductile behavior. 
     Materials having a sheet structure are suitable for the intermediate layers  41 ,  42 ,  43 ,  44 . Such a sheet structure is displayed by arsenic and antimony. In addition, hexagonal modifications of carbon can also be used. Thus, a hexagonal lattice structure of graphite can be seen in  FIG. 2 . The strength in the planes of the sheets (basal planes  51 ) is, due to atom bonds, greater than perpendicular thereto. However, weak secondary valence forces  52  bring about low adhesion between the basal planes. The basal planes  51  should therefore be oriented parallel to the surfaces of the substrate material. Cracks which go out from the ceramic main layers  45 ,  46 ,  47 ,  48  in the multilayer system  39  will then grow preferentially along the weak bond  52  (between the basal planes  51 ) of the intermediate layer  41 ,  42 ,  43 ,  44  according to the invention. This enables crack deflection and splitting up into many smaller cracks to be achieved, which leads to stopping of the crack. 
       FIG. 3  schematically shows an intermediate layer  41 ,  42 ,  43 ,  44  according to the invention as multilayer system  57 . The structure of the multilayer  57  is selected so that either layers having relatively weak bonding  55  and/or weak interfaces  58  are present between the sublayers  55 ,  56  of the multilayer  57 . The deflection of vertical cracks  50  which go out from the ceramic main layers  45 ,  46 ,  47 ,  48  occurs either at the weak interfaces  58  of the multilayer according to the invention or in sublayers  55  having weak bonds. Weak interfaces  58  can be produced by using suitable material pairs which do not undergo a chemical reaction. A low surface roughness of the individual layers ensures weak mechanical intermeshing and thus also low adhesion. 
       FIG. 4  schematically shows an embodiment of the intermediate layer  41 ,  42 ,  43 ,  44  according to the invention having pores  59 . The deliberate introduction of pores  59  into the intermediate layer  41 ,  42 ,  43 ,  44  results in cracks which go out from the ceramic main layer  45 ,  46 ,  47 ,  48  of the multilayer system  39  altering the direction of propagation, branching and not growing through to the substrate material  40 . The change in the direction of propagation is brought about by the pores which are joined to one another only via weak material bridges. 
       FIG. 5  schematically shows an embodiment of the intermediate layer  41 ,  42 ,  43 ,  44  according to the invention having microcracks parallel to the surface or to the substrate material. Microcracks  60  have the same effects as pores  59 . However, they have to be oriented parallel to the surface of the substrate material in order to stop cracks which go out from the ceramic main layer  45 ,  46 ,  47 ,  48  of the multilayer system  39 . 
       FIG. 6  shows an intermediate layer  41 ,  42 ,  43 ,  44  having deliberately introduced foreign phases  61 . These foreign phases  61  result in cracks which go out from the ceramic main layer  45 ,  46 ,  47 ,  48  of the multilayer system  39  changing the direction of propagation, branching and not growing through to the substrate material  40 . Here, the cracks are deflected either at the weak interface to the foreign phase  62  or by preferential propagation into the foreign phase. 
     It should be stated that the embodiments in  FIG. 4 , in  FIG. 5  and in  FIG. 6  can be combined with one another. Thus, an intermediate layer  41 ,  42 ,  43 ,  44  can comprise pores, microcracks parallel to the surface or to the substrate material and/or foreign phases.