Abstract:
There is described a ceramic powder, ceramic layer and layer system having gadolinium/mixed crystal pyrochlore phases and oxides. Besides a good thermal insulation property, thermal insulation layer systems must also have a long lifetime of the thermal insulation layer. The layer system according to the invention has an outer ceramic layer, which comprises a mixed crystal of gadolinium zirconate and gadolinium hafnate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of the provisional patent application filed on May 07, 2007, and assigned application Ser. No. 60/928,085, and the European Patent Office application No. 07009113.7 EP filed May 07, 2007, all of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a ceramic powder, to a ceramic layer and to a layer system having pyrochlores and oxides. 
       BACKGROUND OF INVENTION 
       [0003]    Such a layer system has a substrate comprising a metal alloy based on nickel or cobalt. Such products are used especially as a component of a gas turbine, in particular as gas turbine blades or heat shields. The components are exposed to a hot gas flow of aggressive combustion gases. They must therefore be able to withstand heavy thermal loads. It is furthermore necessary for these components to be oxidation- and corrosion-resistant. Especially moving components, for example gas turbine blades, but also static components, are furthermore subject to mechanical requirements. The power and efficiency of a gas turbine, in which there are components exposable to hot gas, increase with a rising operating temperature. 
         [0004]    In order to achieve a high efficiency and a high power, those gas turbine components which are particularly exposed to high temperatures are coated with a ceramic material. This acts as a thermal insulation layer between the hot gas flow and the metallic substrate. 
         [0005]    The metallic base body is protected against the aggressive hot gas flow by coatings. In this context, modem components usually comprise a plurality of coatings which respectively fulfill specific functions. The system is therefore a multilayer system. Since the power and efficiency of gas turbines increase with a rising operating temperature, attempts have continually been made to achieve a higher performance of gas turbines by improving the coating system. 
         [0006]    EP 0 944 746 B1 discloses the use of pyrochlores as a thermal insulation layer. The use of a material as a thermal insulation layer, however, requires not only good thermal insulation properties but also good bonding to the substrate. 
         [0007]    EP 0 992 603 A1 discloses a thermal insulation layer system of gadolinium oxide and zirconium oxide, which is not intended to have a pyrochlore structure. 
       SUMMARY OF INVENTION 
       [0008]    It is therefore an object of the invention to provide a ceramic powder, a ceramic layer and a layer system having good thermal insulation properties and good bonding to the substrate and therefore a long lifetime. 
         [0009]    The object is achieved by a ceramic powder, a ceramic layer and a layer system as claimed in independent claims. 
         [0010]    In one aspect the invention is based on the discovery that in order to achieve a long lifetime, the entire system must be considered as a whole and individual layers or some layers together should not be considered and optimized separately from one another. 
         [0011]    Further advantageous measures, which may advantageously be combined in any desired way, are mentioned in the dependent claims. 
         [0012]    In an embodyment the layer system consists of an outer ceramic layer, which comprises a mixed crystal of gadolinium zirconate and gadolinium hafniate that has particularly good thermal properties (expansion coefficient adapted to the substrate, low thermal conduction coefficient) and harmonizes very well with an interlayer and the substrate of the component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    Exemplary embodiments of the invention will be explained in more detail below with reference to the drawings, in which: 
           [0014]      FIG. 1  shows a layer system according to the invention, 
           [0015]      FIG. 2  shows compositions of superalloys, 
           [0016]      FIG. 3  shows a gas turbine, 
           [0017]      FIG. 4  shows a turbine blade in perspective, and 
           [0018]      FIG. 5  shows a combustion chamber in perspective. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0019]    The composition of the ceramic powder is also explained with the aid of the composition of the ceramic layer  13  ( FIG. 1 ). 
         [0020]    In general, there may always be deviations from the stoichiometry of the pyrochlore structure A 2 B 2 O 7 . 
         [0021]    A pyrochlore structure has the empirical formula Gd v (Zr x Hf y ) O z , where v≅2, x+y≅2 and z≅7. Deviations from this stoichiometric composition for v, x, y and z may arise as a result of defects or minor, deliberate or inadvertent dopings. 
         [0022]    The ceramic powder further comprises secondary oxides of zirconium and/or hafnium oxide, in particular with a proportion of from 0.5 wt % to 10 wt %, in particular from 1 wt % to 10 wt %. The secondary oxide is deliberately added to the powder, is therefore well above the measuring detection limit of the secondary oxide, that is to say comprises at least twice the value of the detection limit of the secondary oxide. 
         [0023]    The maximum proportion of the secondary oxide or the secondary oxides is preferably 8 wt %, 
         [0024]    in particular at most 6 wt % and most particularly between 5 wt % and 7 wt %. 
         [0025]    This preferably applies to zirconium oxide. 
         [0026]    The maximum proportion of the secondary oxide is similarly preferably 3 wt %, in particular at most 2 wt % and most particularly between 1.5 wt % and 2.5 wt %. 
         [0027]    In particular, the proportions apply to hafnium oxide. 
         [0028]    In particular, the ceramic powder consists of at least one pyrochlore phase and at least one secondary oxide. 
         [0029]    Depending on the application, the secondary oxide may only consist of hafnium oxide, to achieve better thermal insulating properties, or the secondary oxide is only formed by zirconium oxide, to achieve better adaptability of the expansion coefficients to the underlying layers or to the substrate. 
         [0030]    A combination of the advantageous properties of hafnium oxide and zirconium oxide can also be achieved by the use of both secondary oxides. 
         [0031]    Secondary oxides are preferably present as an oxide. 
         [0032]    The ceramic powder can preferably have in each case optionally sintering aids up to 0.05 wt % of silicon oxide, up to 0.1 wt % of calcium oxide, up to 1 wt % of calcium oxide, up to 0.1 wt % of magnesium oxide, up to 0.1 wt % of iron oxide, up to 0.1 wt % of aluminum oxide and up to 0.08 wt % of titanium oxide. 
         [0033]    The sintering aids help the layer to hold together after application and/or during use at higher temperatures. The ceramic powder preferably consists of a pyrochlore phase, secondary oxides and optionally the sintering aids. 
         [0034]    Preferably, two secondary oxides are present, in particular hafnium oxide and zirconium oxide. 
         [0035]    Gadolinium hafnate comprises from 43 wt % to 50 wt %, preferably from 44.7 wt % to 47.7 wt % of gadolinium oxide as powder, the remainder being hafnium oxide and optionally the secondary oxides, preferably zirconium oxide, and the sintering aids. Gadolinium zirconate comprises from 56 wt % to 63 wt %, preferably from 58 wt % to 61 wt % of gadolinium oxide as powder, the remainder being zirconium oxide and optionally the secondary oxides, preferably hafnium oxide, and sintering aids. As the mixed crystal, these proportions are mixed according to the ratio of Hf and Zr. 
         [0036]    The outer ceramic layer  13  preferably comprises 
         [0000]      Gd v (Hf x Zr y )O z , with v, x+y≈2, z≈7. 
         [0037]    The outer ceramic layer  13  preferably also comprises 
         [0000]      Gd v (Hf x Zr y )O 7 , with x+y≈2, v≈2. 
         [0038]    The outer ceramic layer  13  preferably comprises 
         [0000]      Gd 2 (Hf x Zr y )O z , with x+y≈2, z≈7. 
         [0039]    The pyrochlore structure of the ceramic powder preferably consists of Gd v  (Hf x Zr y ) O z , in particular with v≈2, x+y≈2 and z≈7. 
         [0040]    Various mixing ratios y:x of zirconium and hafnium may in this case be used. 
         [0041]    A larger proportion of zirconium is preferably used. Mixing ratios of 10:90, 20:80, 30:70 or 40:60 are likewise preferably used for hafnium to zirconium. It is furthermore advantageous to use mixing ratios of 50:50, 60:40, 70:30, 80:20 or 90:10 for hafnium to zirconium. 
         [0042]    Therefore, the figures given for the ratio of hafnium to zirconium advantageously apply to the ratios of x to y (Hf:Zr=80:20 corresponds to y:x=1.6:0.4). 
         [0043]      FIG. 1  shows a layer system according to the invention. 
         [0044]    The composition of the ceramic powder is explained by way of example using the ceramic layer  13 . 
         [0045]    The layer system  1  comprises a metallic substrate  4  which, in particular for components at high temperatures, consists of a nickel- or cobalt-based superalloy ( FIG. 2 ). 
         [0046]    There is preferably a metallic bonding layer  7  of MCrAlX type preferably directly on the substrate  4 , in particular of the NiCoCrAlX type, which preferably comprises either 
         [0047]    (11-13) wt % cobalt, (20-22) wt % chromium (10.5-11.5) wt % aluminum, (0.3-0.5) wt % yttrium (=x), (1.5-2.5) wt % rhenium and the remainder nickel, or preferably of 
         [0048]    (24-26) wt % cobalt, (16-18) wt % chromium (9.5-11) wt % aluminum, (0.3-0.5) wt % yttrium (=x), (1-1.8) wt % rhenium and the remainder nickel. 
         [0049]    The NiCoCrAl bonding layer  7  particularly consists of one of these two compositions. 
         [0050]    An aluminum oxide layer is preferably already formed on this metallic bonding layer  7  before further ceramic layers are applied, or such an aluminum oxide layer (TGO) is formed during operation. 
         [0051]    There is preferably an inner ceramic layer  10 , preferably a fully or partially stabilized zirconium oxide layer, on the metallic bonding layer  7  or on the aluminum oxide layer (not shown). 
         [0052]    Yttrium-stabilized zirconium oxide, preferably with 6 wt % -8 wt % of yttrium, is preferably used. 
         [0053]    Calcium oxide, cerium oxide or hafnium oxide may likewise be used to stabilize zirconium oxide. 
         [0054]    The zirconium oxide is preferably applied as a plasma-sprayed layer, it may also preferably be applied as a columnar structure by means of electron beam deposition (EBPVD). 
         [0055]    An outer ceramic layer  13  which according to the invention comprises a mixed crystal of gadolinium, hafnium and zirconium with a pyrochlore structure, that is to say is produced from the ceramic powder described above, is applied on the TGO, on the bonding layer  7  or on the inner layer  10 . The ceramic layer  13  is preferably produced exclusively from the ceramic powder. The layer  13  is preferably the outermost layer which is exposed to the hot gas. 
         [0056]    The secondary oxides are distributed, in particular homogeneously distributed, in the ceramic layer  13 . 
         [0057]    The layer  13  may have been produced from a powder which provides the proportions of the aforementioned composition. Similarly, the mixed crystals may also be produced during the coating process or by a heat treatment after the coating process. 
         [0058]    The layer thickness of the inner layer  10  is preferably between 10% and 50% of the total layer thickness D of the inner layer  10  plus the outer layer  13 . 
         [0059]    The layer thickness of the inner layer  10  is preferably between 10% and 40% or between 10% and 30% of the total layer thickness. 
         [0060]    It is likewise advantageous for the layer thickness of the inner layer  10  to be from 10% to 20% of the total layer thickness. 
         [0061]    It is likewise preferable for the layer thickness of the inner layer  10  to be to be between 20% and 50% or between 20% and 40% of the total layer thickness. 
         [0062]    Advantageous results are likewise achieved if the contribution of the inner layer  10  to the total layer thickness is between 20% and 30%. 
         [0063]    The layer thickness of the inner layer  10  is preferably from 30% to 50% of the total layer thickness. 
         [0064]    It is likewise advantageous for the layer thickness of the inner layer  10  to comprise from 30% to 40% of the total layer thickness. 
         [0065]    It is likewise preferable for the layer thickness of the inner layer  10  to be between 40% and 50% of the total layer thickness. 
         [0066]    Although the pyrochlore phase has better heat insulating properties than the ZrO 2  layer, the ZrO 2  layer may be made the same thickness as the pyrochlore phase. 
         [0067]    For short-term use with high temperatures of the layer system, the outer layer  13  may be configured to be thinner than the inner layer  10 , i.e. the layer thickness of the outer layer  13  is between 10% and 40% of the total layer thickness of the inner layer  10  plus the outer layer  13 . 
         [0068]    The inner ceramic layer  10  preferably has a thickness of from 100 μm to 200 μm, in particular 150 μm±10%. 
         [0069]    The total layer thickness D of the inner layer  10  plus the outer layer  13  is preferably 300 μm of preferably 450 μm. The maximum total layer thickness is advantageously 800 μm or advantageously at most 600 μm. 
         [0070]      FIG. 3  shows a gas turbine  100  by way of example in a partial longitudinal section. 
         [0071]    The gas turbine  100  internally comprises a rotor  103  mounted so as to rotate about a rotation axis  102  and having a shaft  101 , which will also be referred to as the turbine rotor. 
         [0072]    Successively along the rotor  103 , there are an intake manifold  104 , a compressor  105 , an e.g. toroidal combustion chamber  110 , in particular a ring combustion chamber, having a plurality of burners  107  arranged coaxially, a turbine  108  and the exhaust manifold  109 . 
         [0073]    The ring combustion chamber  110  communicates with an e.g. annular hot gas channel  111 . There, for example, four successively connected turbine stages  112  form the turbine  108 . 
         [0074]    Each turbine stage  112  is formed for example by two blade rings. As seen in the flow direction of a working medium  113 , a guide vane row  115  is followed in the hot gas channel  111  by a row  125  formed by rotor blades  120 . 
         [0075]    The guide vanes  130  are fastened on an inner housing  138  of a stator  143  while the rotor blades  120  of a row  125  are fastened on the rotor  103 , for example by means of a turbine disk  133 . Coupled to the rotor  103 , there is a generator or a work engine (not shown). 
         [0076]    During operation of the gas turbine  100 , air  135  is taken in by the compressor  105  through the intake manifold  104  and compressed. The compressed air provided at the turbine-side end of the compressor  105  is delivered to the burners  107  and mixed there with a fuel. The mixture is then burnt to form the working medium  113  in the combustion chamber  110 . From there, the working medium  113  flows along the hot gas channel  111  past the guide vanes  130  and the rotor blades  120 . At the rotor blades  120 , the working medium  113  expands by imparting momentum, so that the rotor blades  120  drive the rotor  103  and the work engine coupled to it. 
         [0077]    During operation of the gas turbine  100 , the components exposed to the hot working medium  113  experience thermal loads. Apart from the heat shield elements lining the ring combustion chamber  110 , the guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the flow direction of the working medium  113 , are heated the most. 
         [0078]    In order to withstand the temperatures prevailing there, they may be cooled by means of a coolant. 
         [0079]    Substrates of the components may likewise comprise a directional structure, i.e. they are monocrystalline (SX structure) or comprise only longitudinally directed grains (DS structure). 
         [0080]    Iron-, nickel- or cobalt-based superalloys are for example used as material for the components, in particular for the turbine blades  120 ,  130  and components of the combustion chamber  110 . 
         [0081]    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; these documents form part of the disclosure with respect to the chemical composition of the alloys. 
         [0082]    The guide vanes  130  comprise a guide vane root (not shown here) facing the inner housing  138  of the turbine  108 , and a guide vane head lying opposite the guide vane root. The guide vane head faces the rotor  103  and is fixed to a fastening ring  140  of the stator  143 . 
         [0083]      FIG. 4  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along the longitudinal axis  121 . 
         [0084]    The turbomachine may be a gas turbine of an aircraft or of a power plant for electricity generation, a steam turbine or a compressor. 
         [0085]    The blade  120 ,  130  comprises, successively along the longitudinal axis  121 , a fastening zone  400 , a blade platform  403  adjacent thereto as well as a blade surface  406  and a blade tip  415 . 
         [0086]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0087]    A blade root  183  which is used to fasten the rotor blades  120 ,  130  on a shaft or a disk (not shown) is formed in the fastening region  400 . 
         [0088]    The blade root  183  is configured, for example, as a hammerhead. Other configurations as a firtree or dovetail root are possible. 
         [0089]    The blade  120 ,  130  comprises a leading edge  409  and a trailing edge  412  for a medium which flows past the blade surface  406 . 
         [0090]    In conventional blades  120 ,  130 , for example, solid metallic materials are used in all regions  400 ,  403 ,  406  of the blade  120 ,  130 . 
         [0091]    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; these documents form part of the disclosure with respect to the chemical composition of the alloy. 
         [0092]    The blades  120 ,  130  may in this case be manufactured by a casting method, also by means of directional solidification, by a forging method, by a machining method or combinations thereof. 
         [0093]    Workpieces with a monocrystalline structure or structures are used as components for machines which are exposed to heavy mechanical, thermal and/or chemical loads during operation. 
         [0094]    Such monocrystalline workpieces are manufactured, for example, by directional solidification from the melts. These are casting methods in which the liquid metal alloy is solidified to form a monocrystalline structure, i.e. to form the monocrystalline workpieces, or is directionally solidified. 
         [0095]    Dendritic crystals are in this case aligned along the heat flux and form either a rod crystalline grain structure (columnar, i.e. grains which extend over the entire length of the workpiece and in this case, according to general terminology usage, are referred to as directionally solidified) or a monocrystalline structure, i.e. the entire workpiece consists of a single crystal. It is necessary to avoid the transition to globulitic (monocrystalline) solidification in this method, since nondirectional growth will necessarily form transverse and longitudinal grain boundaries which negate the beneficial properties of the directionally solidified or monocrystalline component. 
         [0096]    When directionally solidified structures are referred to in general, this is intended to mean both single crystals which have no grain boundaries or at most small-angle grain boundaries, and also rod crystal structures which, although they do have grain boundaries extending in the longitudinal direction, do not have any transverse grain boundaries. These latter crystalline structures are also referred to as directionally solidified structures. 
         [0097]    Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents form part of the disclosure with respect to the solidifying process. 
         [0098]    The blades  120 ,  130  may likewise represent coatings against corrosion or oxidation, for example (MCrAlX; M is at least one element from the group ion (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, 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 which are intended to form part of this disclosure with respect to the chemical composition of the alloy. 
         [0099]    The density is preferably 95% of the theoretical density. 
         [0100]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an interlayer or as the outermost layer). 
         [0101]    On the MCrAlX layer, there may be furthermore a thermal insulation layer which is preferably the outermost layer and consists of the layer system  1  according to the invention. 
         [0102]    The thermal insulation layer covers the entire MCrAlX layer. 
         [0103]    Rod-shaped grains are produced in the thermal insulation layer by suitable coating methods, for example electron beam deposition (EBPVD). 
         [0104]    Other coating methods may also be envisaged, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal insulation layer may have porous, micro-cracked or macro-cracked grains for better thermal shock resistance. The thermal insulation layer is thus preferably more porous than the MCrAlX layer. 
         [0105]    The blade  120 ,  130  may be designed to be a hollow or solid. If the blade  120 ,  130  is intended to be cooled, it will be hollow and optionally also comprise film cooling holes  418  (indicated by dashes). 
         [0106]      FIG. 5  shows a combustion chamber  110  of the gas turbine  100 . The combustion chamber  110  is designed for example as a so-called ring combustion chamber in which a multiplicity of burners  107 , which produce flames  156  and are arranged in the circumferential direction around a rotation axis  102 , open into a common combustion chamber space  154 . To this end, the combustion chamber  110  as a whole is designed as an annular structure which is positioned around the rotation axis  102 . 
         [0107]    In order to achieve a comparatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M, i.e. about 1000° C. to 1600° C. In order to permit a comparatively long operating time even under these operating parameters which are unfavorable for the materials, the combustion chamber wall  153  is provided with an inner lining formed by heat shield elements  155  on its side facing the working medium M. 
         [0108]    Owing to the high temperature inside the combustion chamber  110 , a cooling system may also be provided for the heat shield elements  155  or for their retaining elements. The heat shield elements  155  are then hollow, for example, and optionally also have cooling holes (not shown) opening into the combustion chamber space  154 . 
         [0109]    Each heat shield element  155  is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) on the working medium side or is made of refractory material (solid ceramic blocks). 
         [0110]    These protective layers may be similar to the turbine blades, i.e. for example MCrAlX means: M is at least one element from the group ion (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, 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 which are intended to form part of this disclosure with respect to the chemical composition of the alloy. 
         [0111]    On the MCrAlX layer, there may furthermore be a ceramic thermal insulation layer and consists of the layer system  1  according to the invention. 
         [0112]    Rod-shaped grains are produced in the thermal insulation layer by suitable coating methods, for example electron beam deposition (EBPVD). 
         [0113]    Other coating methods may also be envisaged, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal insulation layer may have porous, micro-cracked or macro-cracked grains for better thermal shock resistance. 
         [0114]    Refurbishment means that turbine blades  120 ,  130 , heat shield elements  155  may need to have protective layers taken off (for example by sandblasting) after their use. Corrosion and/or oxidation layers or products are then removed. Optionally, cracks in the turbine blade  120 ,  130  or the heat shield element  155  are also repaired. The turbine blade  120 ,  130 , heat shield element  155  is then recoated and the turbine blade  120 ,  130 , heat shield element  155  is used again.