Abstract:
A two-ply heat-insulating ceramic layer is provided with a highly porous crackfree lower ply and an outermost heat-insulating ply with vertical cracks in order to ensure both a high heat insulation as well as a high erosion resistance. In one aspect is a layer system having a two-ply, outermost ceramic layer, which has a lower ceramic layer and an outermost ceramic layer. The lower ceramic layer has a porosity of at least 5%, in particular at least 8%, very particularly preferably at least 10%, and barely any or no vertical cracks, in particular no vertical cracks running right through, and the outermost ceramic layer has a layer thickness of not more than 40%, in particular not more than 20%, very particularly preferably not more than 10% of the layer thickness of the lower ceramic layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/EP2014/059738, having a filing date of May 13, 2014, based off of EP Application No. 13189688.8 having a filing date of Oct. 22, 2013, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The following relates to a ceramic layer which has a two-ply structure, with different microstructures being present in the layers. 
       BACKGROUND 
       [0003]    Ceramic layers are used, in particular, as thermal barrier layers in turbine blades and have a porosity. 
         [0004]    Vertically segmented thermal barrier layers in which cracks are formed during coating by means of a subsequent treatment are likewise known. 
         [0005]    However, there is the problem that when the porosity is increased to achieve greater thermal insulation, the erosion resistance of a thermal barrier layer, which is generally plasma-sprayed, is reduced. 
       SUMMARY 
       [0006]    The advantages are good thermal insulation and good erosion resistance. 
     
    
     
       BRIEF DESCRIPTION 
         [0007]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0008]      FIG. 1  is a working example of a layer system in accordance with embodiments of the invention; 
           [0009]      FIG. 2  is a working example of a layer system in accordance with embodiments of the invention; 
           [0010]      FIG. 3  is a working example of a layer system in accordance with embodiments of the invention; 
           [0011]      FIG. 4  is a working example of a layer system in accordance with embodiments of the invention; 
           [0012]      FIG. 5  a list of superalloys in accordance with embodiments of the invention; and 
           [0013]      FIG. 6  a turbine blade in accordance with embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The description and the figures represent only working examples of embodiments of the invention. 
         [0015]      FIG. 1  and  FIGS. 2 to 4  in each case show a layer system  1 ′,  1 ″, . . . which has at least one metallic substrate  4 . 
         [0016]    The metallic substrate  4  comprises, in particular, a cobalt- or nickel-based superalloy, in particular as shown in  FIG. 5 . 
         [0017]    A metallic bonding layer  7  has preferably been applied to the substrate  4  ( FIGS. 1-4 ), very particularly preferably directly to the substrate  4 . 
         [0018]    This metallic bonding layer  7  preferably comprises an alloy of the NiCoCrAl (X) type, on the surface of which a protective aluminum oxide layer (not shown) is formed during further coating or during operation (TGO). 
         [0019]    A lower ceramic layer  10 ′ ( FIG. 1 ) of a two-ply, outermost ceramic thermal barrier layer  15 ′ is applied to the substrate  4  or the metallic bonding layer  7 . 
         [0020]    The porosity is preferably reported in percent by volume. 
         [0021]    An APS process is preferably used for the lower ceramic layer  10 ′ as per  FIG. 1  and the lower ceramic layer  10 ′ of the two-ply, outermost ceramic thermal barrier layer  15 ′ preferably has a porosity of (12+/−4) %. 
         [0022]    The lower ceramic layer  10 ′ preferably has a layer thickness of up to 1 mm. 
         [0023]    The minimum thickness of the lower ceramic layer  10 ′ is at least 100 μm, very particularly preferably at least 150 μm ( FIGS. 1-4 ). 
         [0024]    The outermost, ceramic layer  13  in  FIGS. 1 to 4  has a layer which is dense compared to the lower layer  10 ′, . . . ,  10   IV  of the two-ply ceramic thermal barrier layers  15 ′,  15 ″, . . . and through which cracks run vertically, i.e. the porosity is preferably &lt;8%. 
         [0025]    The minimum layer thickness of the outermost ceramic layer  13  is 30 μm, in particular at least 50 μm ( FIGS. 1-4 ). 
         [0026]    The maximum layer thickness of the outermost ceramic layer  13  is not more than 500 μm, in particular not more than 300 μm ( FIGS. 1-4 ). 
         [0027]    The porosity of the segmented layers like that of the outermost ceramic layer  13  here corresponds to that from the known art. 
         [0028]      FIG. 2  shows a further working example having a layer system  1 ″. 
         [0029]    In contrast to  FIG. 1 , the lower layer  10 ″ of the ceramic thermal barrier layer  15 ″ has a porosity of (15+/−4)%. 
         [0030]    The lower ceramic layer  10 ″ in  FIG. 2  can likewise preferably have a layer thickness of up to 1.5 mm, in particular from &gt;1 mm to 1.5 mm, and then have a porosity of (20+/−5)%. 
         [0031]    The minimum layer thickness of the outermost, ceramic layer  13  is 30 μm, in particular at least 50 μm. 
         [0032]    The porosity of the lower ceramic layer  10 ″ in  FIG. 2  can likewise preferably be increased further to (25+/−5)% and layer thicknesses of &gt;1.5 mm are then preferably produced. 
         [0033]    The minimum layer thickness of the outermost, ceramic layer  13  is 30 μm, in particular at least 50 μm. 
         [0034]      FIG. 3  shows a further working example of a layer system  1 ′″ according to embodiments of the invention. 
         [0035]    The lower ceramic layer  10 ′″ of the thermal barrier layer  15 ′″ has a porosity of preferably greater than 15% and has been produced by means of an APS process. However, the pores have been produced by spraying a ceramic powder, preferably by means of polymers. 
         [0036]    This gives a characteristic microstructure of the pores. 
         [0037]    The lower ceramic layer  10 ′″ can preferably have a layer thickness of a plurality of millimeters, in particular ≧2 mm. 
         [0038]    The minimum layer thickness of the outermost, ceramic layer  13  is 30 μm, in particular at least 50 μm. 
         [0039]      FIG. 4  shows a further layer system  10   IV  according to embodiments of the invention. 
         [0040]    The lower ceramic layer  10   IV  of the two-ply, ceramic thermal barrier layer  15   IV  has been produced by the suspension plasma spraying (SPS) process and has a ductile columnar structure having a certain porosity of 4% and cracks up to &lt;8%. 
         [0041]    The outermost layer  13  in  FIG. 4  is configured with the minimum layer thickness and structure and maximum layer thickness in  FIGS. 1-3 . 
         [0042]    Possible materials for the outermost, ceramic thermal barrier layers  15 ′, . . .  15   IV  are yttrium oxide, partially stabilized zirconium oxide or thermal barrier layers composed of fully stabilized zirconium oxide. 
         [0043]    It is likewise possible to use pyrochlores such as gadolinium zirconate, gadolinium hafnate, lanthanum zirconate, gadolinium zirconate. 
         [0044]    Here, the materials for the lower, ceramic layer  10 ′,  10 ″, . . . and the outermost layer  13  can be varied as a function of use conditions and production possibilities. 
         [0045]    The two-ply outermost ceramic layer  15  is preferably the outermost layer of the layer system  1 ′,  1 ″, . . . . 
         [0046]      FIG. 6  shows a perspective view of a rotor blade  120  or guide blade  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0047]    The turbomachine can be a gas turbine of an aircraft or of a power station for generating electricity, a steam turbine or a compressor. 
         [0048]    The blade  120 ,  130  has, in succession along the longitudinal axis  121 , a fastening region  400 , a blade platform  403  adjoining this and also a blade leaf  406  and a blade tip  415 . As guide blade  130 , the blade  130  can have a further platform (not shown) at its blade tip  415 . 
         [0049]    In the fastening region  400 , there is a blade foot  183  which serves for fastening the rotor blades  120 ,  130  to a shaft or a disk (not shown). 
         [0050]    The blade foot  183  is, for example, configured as a hammer head. Other configurations as Christmas tree foot or swallowtail foot are possible. 
         [0051]    The blade  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium flowing past the blade leaf  406 . 
         [0052]    In the case of conventional blades  120 ,  130 , all regions  400 ,  403 ,  406  of the blade  120 ,  130  are, for example, made of massive metallic materials, in particular superalloys. Such superalloys 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. 
         [0053]    The blade  120 ,  130  can have been made by a casting process, including by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0054]    Workpieces having a monocrystalline structure or structures are used as components for machines which are subjected to high mechanical, thermal and/or chemical stresses during operation. 
         [0055]    The manufacture of such monocrystalline workpieces is carried out, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form a monocrystalline structure, i.e. the monocrystalline workpiece, or directionally. 
         [0056]    Here, dendritic crystals are aligned along the heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and here referred to, in keeping with general language usage, as directionally solidified) or a monocrystalline structure, i.e. the entire workpiece consists of a single crystal. In these processes, the transition to globulitic (polycrystalline) solidification has to be avoided since transverse and longitudinal grain boundaries are necessarily formed by nondirectional growth and these nullify the good properties of the directionally solidified or monocrystalline component. 
         [0057]    If general reference is made to directionally solidified microstructures, this encompasses both single crystals which have no grain boundaries or at most low-angle grain boundaries and also columnar crystal structures which do have grain boundaries running in the longitudinal direction but no transverse grain boundaries. These crystalline structures mentioned second are also referred to as directionally solidified microstructures. 
         [0058]    Such processes are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0059]    The blades  120 ,  130  can likewise have coatings to protect against corrosion or oxidation, e.g. (MCrAlX; M is at least one element from the group iron (Fe), cobalt (Co), nickel (Ni), X is an active element and is yttrium (Y) and/or silicon and/or at least one element of the rare earths, or hafnium (Hf)). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0060]    The density is preferably 95% of the theoretical density. 
         [0061]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) forms on the MCrAlX layer (as intermediate layer or as outermost layer). 
         [0062]    The layer composition preferably comprises Co-30Ni-28Cr-8Al-0, 6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. Apart from these cobalt-based protective coatings, preference is also given to using nickel-based protective layers such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re. 
         [0063]    A thermal barrier layer can be additionally present on the MCrAlX and is preferably the outermost layer and consists, for example, of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0064]    The thermal barrier layer covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier layer by suitable treatment processes, e.g. electron beam vaporization (EB-PVD). 
         [0065]    Other coating processes are conceivable, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier layer can have grains which are porous, have microcracks or have macrocracks for better thermal shock resistance. The thermal barrier layer is thus preferably more porous than the MCrAlX layer. 
         [0066]    Refurbishment means that components  120 ,  130  have to be freed of any protective layers (e.g. by sand blasting) after they have been used. This is followed by removal of the corrosion and/or oxidation layers or products. Cracks in the component  120 ,  130  are optionally also repaired. This is followed by recoating of the component  120 ,  130  and renewed use of the component  120 ,  130 . 
         [0067]    The blade  120 ,  130  can be hollow or solid. When the blade  120 ,  130  is to be cooled, it is hollow and optionally has film cooling holes  418  (indicated by dashes). 
         [0068]    Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
         [0069]    For the sake of clarity, it is to be understood that the use of ‘a’ or ‘an’ throughout this application does not exclude a plurality, and ‘comprising’ does not exclude other steps or elements.