Abstract:
There is described a Ceramic Powder, a Ceramic Layer and a Layer System with Pyrochlore Phase and Oxides. Besides a good thermal insulation property, thermal insulation layer systems must also have a long lifetime of the thermal insulation layer. A described layer system has a layer sequence of a metallic bonding layer, an inner ceramic layer and an outer ceramic layer, which are specially matched to one another.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of the provisional patent application filed on May 7, 2007, and assigned application No. 60/928,086, and of European Patent Office application No. 07009129 EP filed May 7, 2007, all of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to a ceramic powder, to a ceramic layer and to a layer system with pyrochlores and oxides. 
     BACKGROUND OF INVENTION 
     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. 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. 
     The metallic base body is protected against the aggressive hot gas flow by coatings. In this context, modern 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 are continually being made to achieve a higher performance of gas turbines by improving the coating system. 
     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 mechanical properties and good bonding to the substrate. 
     EP 0 992 603 A1 discloses a thermal insulation layer system of gadolinium oxide and zirconium oxide, which does not have a pyrochlore structure. 
     SUMMARY OF INVENTION 
     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 of the entire layer system. 
     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. 
     The object is achieved by a ceramic powder, a ceramic layer and a layer system as claimed in independent claims. 
     The dependent claims describe further advantageous measures, which may advantageously be combined in any desired way. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a layer system according to the invention, 
         FIG. 2  shows a list of superalloys, 
         FIG. 3  shows a perspective view of a turbine blade, 
         FIG. 4  shows a perspective view of a combustion chamber, 
         FIG. 5  shows a gas turbine. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The ceramic pyrochlore powder according to the invention of the general formula A 2 B 2 O 7  has as a further constituent an oxide C r O s  of a metal (O=oxygen; B=Hf, Zr, Ti, Sn; A=Gd, Sm, Nd, La, Y). The metal constituent of the secondary oxide is denoted here by C. 
     The composition of the ceramic powder will also be explained by way of example with the aid of the composition of the ceramic layer  13  ( FIG. 1 ). In general, departures from the stoichiometry of the general pyrochlore structure A 2 B 2 O 7  may always occur. 
     Pyrochlore structures in which A=gadolinium are preferably used, since good to very good thermal insulation properties are achieved in this case. Depending on the application, a hafnate or a zirconate will be used so that B=hafnium or zirconium. 
     Gadolinium hafnate or gadolinium zirconate will thus preferably be used. 
     Gadolinium hafnate as the powder comprises from 43 wt % to 50 wt %, preferably from 44.7 wt % to 47.7 wt % of gadolinium oxide, the remainder being hafnium oxide and optionally the secondary oxides, preferably only zirconium oxide, and the sintering aids. 
     Gadolinium zirconate as the powder comprises from 56 wt % to 63 wt %, preferably from 58 wt % to 61 wt % of gadolinium oxide, the remainder being zirconium oxide and optionally the secondary oxides, preferably only hafnium oxide, and sintering aids. 
     The ceramic layer  13  ( FIG. 1 ) or the ceramic powder comprises a pyrochlore phase of the general empirical formula A x B y O z  with x, y≈2, z≈7 and a secondary oxide C r O s  with r, s&gt;0. The secondary oxide C r O s  is in this case deliberately added to the powder and is thus significantly above the metrological detection limit of the secondary oxide, i.e. it has at least two times the value of the detection limit of the secondary oxide. 
     The secondary oxide has in particular a proportion of from 0.5 wt % to 10 wt %, more particularly a proportion of from 1 wt % to 10 wt %. The maximum proportion of the secondary oxide is preferably 8 wt %, in particular at most 6 wt % and more particularly between 5 wt % and 7 wt %. The maximum proportion of the secondary oxide is likewise preferably 3 wt %, in particular at most 2 wt % and more particularly between 1.5 wt % and 2.5 wt %. In particular, the ceramic powder consists of at least one pyrochlore phase and at least one secondary oxide. 
     For the secondary oxide, the oxide of B may be used (C=B) or not (C≠B). If C=B, then a high phase stability of the pyrochlore phase is ensured. If B≠C however, then an increase in the mechanical strength is achieved. 
     Hafnium oxide or zirconium oxide therefore preferably used, since they are particularly stable at high temperatures and they do not entail diffusion and therefore phase modification of the pyrochlore structure. 
     The ceramic layer  13  or the ceramic powder preferably comprises only one pyrochlore phase, so that thermal stresses do not occur between different phases when used with strongly alternating temperatures. 
     A mixture of only two pyrochlore phases may likewise be used, i.e. for example a powder mixture of Gd 2 Zr 2 O 7  and Gd 2 Hf 2 O 7 , in order to combine the improved thermal insulation properties of one pyrochlore phase with the better thermal expansion coefficients of the other pyrochlore phase. This is the case, in particular, for gadolinium zirconate and gadolinium hafnate. 
     The pyrochlore phase may likewise preferably be present as a mixed crystal, so that good mixing will have already taken place here or phase stability is provided. This is the case, in particular, for Gd 2 (Hf x Zr y )O 7  with x+y≈2. 
     The ceramic layer  13  or the ceramic powder preferably comprises only one secondary oxide. The secondary oxide may constitute hafnium oxide or zirconium oxide. Zirconium oxide is preferably used when a hafnate is employed as the pyrochlore phase. A zirconium oxide is preferably used when a hafnate is employed for the pyrochlore phase. 
     Two secondary oxides, in particular hafnium oxide and zirconium oxide, may likewise be used so that the mechanical properties are improved further. 
     The secondary oxides may in this case be present only as an oxide, so that there is a secondary phase here which leads to mechanical reinforcement, or they are present as a mixed crystal with one another or with the pyrochlore phase, so that the thermal conductivity can in this way be reduced further by the stresses thereby generated in the lattice. 
     In order to draw advantages from both presentation types of the secondary oxides, the secondary oxide or oxides may be present both as an oxide or as a mixed crystal in the pyrochlore phase. 
     Preferably, B≠C. 
     A pyrochlore powder of gadolinium zirconate, in particular Gd 2 Zr 2 O 7 , thus comprises hafnium oxide in particular with a proportion of from 1.5 wt % to 2.5 wt %, in particular 2 wt %. 
     Gadolinium hafnate, in particular Gd 2 Hf 2 O 7 , preferably comprises zirconium oxide in particular with a proportion of from 5 wt % to 7 wt %, in particular up to 6 wt %. 
     The pyrochlore or pyrochlores preferably have the following optional constituents as sintering aids: 
     Up to 0.05 wt % silicon oxide, 
     Up to 0.1 wt % calcium oxide, 
     Up to 0.1 wt % magnesium oxide, 
     Up to 0.1 wt % iron oxide, 
     Up to 0.1 wt % aluminum oxide and 
     Up to 0.08 wt % titanium oxide. 
     During the coating or during subsequent use at higher temperatures, these sintering aids lead to dense and stable layers. 
     No other sintering aids are preferably used. 
       FIG. 1  shows a layer system  1  according to the invention. 
     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 ). There is preferably a metallic bonding layer  7  directly on the substrate  4 , in particular of the NiCoCrAlX type, which preferably comprises (11-13) wt % cobalt, (20-22) wt % chromium (10.5-11.5) wt % aluminum, (0.3-0.5) wt % yttrium, (1.5-2.5) wt % rhenium and the remainder nickel, or which preferably comprises (24-26) wt % cobalt, (16-18) wt % chromium (9.5-11) wt % aluminum, (0.3-0.5) wt % yttrium, (1-1.8) wt % rhenium and the remainder nickel, and in particular consists thereof. 
     An aluminum oxide layer is preferably formed already on this metallic bonding layer  7  before further ceramic layers are applied, or such an aluminum oxide layer (TGO) is formed during operation. 
     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) or on the substrate  4 . Yttrium-stabilized zirconium oxide is preferably used, with 6 wt %-8 wt % of yttrium preferably being employed. Calcium oxide, cerium oxide and/or hafnium oxide may likewise be used to stabilize zirconium oxide. The zirconium oxide is preferably applied as a plasma-sprayed layer, although it may also preferably be applied as a columnar structure by means of electron beam deposition (EBPVD). 
     An outer ceramic layer  13  of the ceramic powder is applied on the stabilized zirconium oxide layer  10  or on the metallic bonding layer  7  or on the substrate. The layer  13  preferably constitutes the outermost layer, which is exposed directly to the hot gas. The layer  13  consists mainly of a pyrochlore phase, i.e. it comprises at least 90 wt % of the pyrochlore phase which preferably consists of either Gd 2 Hf 2 O 7  or Gd 2 Zr 2 O 7 . 
     The secondary oxides are distributed in the layer  13 , preferably homogeneously distributed. 
     The layer thickness of the inner layer  10  is preferably between 10% and 50% in particular between 10% and 40%, of the total layer thickness of the inner layer  10  plus the outer layer  13 . The inner ceramic layer  10  preferably has a thickness of from 100 μm to 200 μm, in particular 150 μm±10%. The total layer thickness of the inner layer  10  plus the outer layer  13  is preferably 300 μm or preferably 450 μm. The maximum total layer thickness is advantageously 600 μm or preferably at most 800 μm. The layer thickness of the inner layer  10  is preferably between 10% and 40% or between 10% and 30% of the total layer thickness. It is likewise advantageous for the layer thickness of the inner layer  10  to comprise from 10% to 20% of the total layer thickness. It is likewise preferable for the layer thickness of the inner layer  10  to be between 20% and 50% or between 20% and 40% of the total layer thickness. Advantageous results are likewise achieved if the contribution of the inner layer  10  to the total layer thickness is between 20% and 30%. The layer thickness of the inner layer  10  is preferably from 30% to 50% of the total layer thickness. It is likewise advantageous for the layer thickness of the inner layer  10  to comprise from 30% to 40% of the total layer thickness. It is likewise preferable for the layer thickness of the inner layer  10  to be between 40% and 50% of the total layer thickness. 
     For short-term use at high temperatures of the layer system, the outer layer  13  may preferably be configured to be thinner than the inner layer  10 , i.e. the layer thickness of the outer layer  13  is at most 40% of the total layer thickness of the inner layer  10  plus the outer layer  13 . 
     The layer system preferably consists of the substrate  4 , the metallic layer  7 , the inner ceramic layer  10  and the outer ceramic layer  13 , and optionally the TGO. 
       FIG. 3  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
     The turbomachine may be a gas turbine of an aircraft or of a power plant for electricity generation, a steam turbine or a compressor. 
     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 . As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
     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 zone  400 . The blade root  183  is configured, for example, as a hammerhead. Other configurations as a firtree or dovetail root are possible. The blade  120 ,  130  comprises a leading edge  409  and a trailing edge  412  for a medium which flows past the blade surface  406 . 
     In conventional blades  120 ,  130 , for example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade  120 ,  130 . 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. 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. 
     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. 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 workpiece, or is directionally solidified. 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 (polycrystalline) solidification in these methods, since nondirectional growth will necessarily form transverse and longitudinal grain boundaries which negate the beneficial properties of the directionally solidified or monocrystalline component. 
     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. 
     Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
     The blades  120 ,  130  may likewise have 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. 
     On the MCrAlX layer, there may furthermore be a ceramic thermal insulation layer  13  according to the invention. Rod-shaped grains are produced in the thermal insulation layer by suitable coating methods, for example electron beam deposition (EB-PVD). 
     Refurbishment means that components  120 ,  130  may need to have protective layers taken off (for example by sandblasting) after their use. The corrosion and/or oxidation layers or products are then removed. Optionally, cracks in the component  120 ,  130  are also repaired. The component  120 ,  130  is then recoated and the component  120  is used again. 
     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). 
       FIG. 4  shows a combustion chamber  110  of a gas turbine  100  ( FIG. 5 ). 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 . 
     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. Each heat shield element  155  made of an alloy 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). 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. 
     Refurbishment means that heat shield elements  155  may need to have protective layers taken off (for example by sandblasting) after their use. The corrosion and/or oxidation layers or products are then removed. Optionally, cracks in the heat shield element  155  are also repaired. The heat shield elements  155  are then recoated and the heat shield elements  155  are used again. 
     Owing to the high temperatures 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 film cooling holes (not shown) opening into the combustion chamber space  154 . 
       FIG. 5  shows a gas turbine  100  by way of example in a partial longitudinal section. The gas turbine  100  internally comprises a rotor  103 , which will also be referred to as the turbine rotor, mounted so as to rotate about a rotation axis  102  and having a shaft  101 . 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 . 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 . 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 . 
     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). 
     During operation of the gas turbine  100 , air  135  is taken in and compressed by the compressor  105  through the intake manifold  104 . 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. 
     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. In order to withstand the temperatures prevailing there, they may be cooled by means of a coolant. 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). 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 . 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. 
     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 on a fastening ring  140  of the stator  143 .