Abstract:
A spray nozzle for atmospheric plasma spraying is provided. The nozzle includes an attachment at an axial end of the spray nozzle from which a protective gas may be discharged in the outflow direction. By means of a plasma spray nozzle that enables atmospheric plasma spraying using protective gas, it is also possible to deposit oxidation-sensitive metal coatings in atmosphere.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2010/060051, filed Jul. 13, 2010 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 10000895.2 EP filed Jan. 28, 2010. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a process for atmospheric plasma spraying, to an apparatus for coating and to a component. 
       BACKGROUND OF INVENTION 
       [0003]    Atmospheric plasma spraying is a cost-effective alternative of plasma spraying since in this case it is possible to dispense with a vacuum installation. This is not possible with every powder, however. In the case of other coating processes, specific properties of the metallic layer are often not achieved. 
         [0004]    In order to increase the efficiency of a turbine, the turbine inlet temperature of the gas has to be increased. So that the turbine blades or vanes do not suffer any damage at these high temperatures of &gt;800° C., a metallic coating as protection against oxidation and an adhesion promoting layer are applied, and a ceramic coating for thermal insulation is applied thereto. So that the ceramic coating bonds to the adhesion promoting layer, a very rough surface is required. At present, this adhesion promoting layer is usually applied by vacuum processes for spraying technology, which are very complex and expensive. Furthermore, they lack the flexibility to also use coating materials other than MCrAlY for adhesion promoting layers. For these reasons, a start has therefore been made presently to replace the vacuum processes with other processes. One of these processes is high velocity flame spraying (HVOF). For technological reasons, it is very difficult to produce the required rough coating by way of an HVOF process. Particularly in the case of flat coating angles, i.e. &lt;90° to the surface, a sufficiently rough surface cannot be produced. Coating by means of atmospheric plasma spraying is not possible since the MCrAlY alloy oxidizes under the action of the atmospheric oxygen. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to solve the abovementioned problem. 
         [0006]    The object is achieved by a plasma spray nozzle as claimed in the claims, by a process as claimed in the claims, by an apparatus as claimed in the claims and by a component as claimed in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The dependent claims list further advantageous measures which can be combined with one another, as desired, in order to obtain further advantages. 
           [0008]      FIG. 1  shows an attachment for a plasma spray nozzle, 
           [0009]      FIGS. 2 and 3  show different attachments for the plasma spray nozzle, 
           [0010]      FIG. 4  shows a perspective view of a gas turbine, 
           [0011]      FIG. 5  shows a perspective view of a turbine blade or vane, 
           [0012]      FIG. 6  shows a perspective view of a combustion chamber, and 
           [0013]      FIG. 7  shows a list of superalloys. 
       
    
    
       [0014]    The description and the figures represent merely exemplary embodiments of the invention. 
         [0015]      FIG. 1  shows a spray nozzle  1 . 
       DETAILED DESCRIPTION OF INVENTION 
       [0016]    The spray nozzle  1  has a conventional nozzle  4  known from the prior art relating to plasma spray nozzles (APS, . . . ) and an attachment  19 . Parallel to a longitudinal direction  26  of an inner channel  22  of the nozzle  4 , at least partially molten coating material heated by a plasma flows from the nozzle  4  in an outflow direction  25 . The plasma is produced in the inner channel  22  of the nozzle  4 . 
         [0017]    The nozzle  4  is only modified to the effect that an attachment  19  can be fastened to it. The attachment  19  extends the inner channel of the nozzle  4 . A protective gas  28  flows out through holes  13 ,  13 ′,  13 ″ on the end face  31  of the attachment  19 , . . . , which preferably have a nozzle-like form, (also see  FIGS. 2 and 3 ) and produces a desired geometry of a protective gas shroud around the outflowing coating material. The protective gas  28  can also flow out of slots  14 ′,  14 ″ arranged in a circle ( FIG. 3 ). It is preferable for at least two, in particular four, slots  14 ′,  14 ″, . . . to be present. 
         [0018]    The protective gas  28  can preferably be argon, helium, nitrogen or a mixture thereof. 
         [0019]    The holes  13 ,  13 ′,  13 ″, . . . and/or slots  14 ′,  14 ″, . . . are oriented in the longitudinal direction  26  in such a way that the protective gas  28  flows out in an outflow direction  25 , the outflow direction  25  running parallel to the longitudinal direction  26 . 
         [0020]    The end face  31  of the attachment  19  on the nozzle  4  is preferably provided with holes  13 ′,  13 ″ arranged in a circle ( FIG. 2 ). 
         [0021]    The holes  13 ′,  13 ″, . . . and/or the slots  14 ′,  14 ″, . . . are preferably distributed uniformly in the radial circumferential direction over the end face  31 . 
         [0022]    It is preferable for some of the protective gas  28  to also flow through at least one opening  16  into the part of the inner channel  22  of the attachment  19 . This serves for cooling the attachment  19 . 
         [0023]    A powder feed  7  is also present and is preferably arranged upstream of the attachment  19 . 
         [0024]    The powder feed  7  can also be present at any other location on the nozzle  4 . 
         [0025]    The attachment  19  preferably has an outer fixed shell, such that only a few discrete holes  13 ,  13 ′, . . . or slots  14 ′,  14 ″, . . . are present. 
         [0026]    Similarly, the extension of the channel  22  in the region of the attachment is formed by a fixed inner shell of the attachment. 
         [0027]    The attachment  19  is preferably not made of a porous solid material. 
         [0028]    In an appropriate coating apparatus, cost-effective coating can be carried out by means of the HVOF process. However, in order to effect coating in the case of specific roughnesses or at an angle of up to 45° to the coating surface, an APS (atmospheric plasma spraying) nozzle which has an appropriate attachment  19  as per  FIG. 1  has to be used. Both coating options HVOF, APS are now preferably implemented in one apparatus. 
         [0029]    A rougher coating is applied using an APS burner to an existing coating, which has been applied by means of an HVOF process. After the HVOF coating, the HVOF nozzle is removed and an APS nozzle  1  is installed in the same apparatus. 
         [0030]    In this case, an attachment  19  is mounted on an APS burner (nozzle  4 ). A protective gas  28 , e.g. nitrogen, is conducted through said attachment  19 . Said protective gas at the same time also cools the attachment  19 . The, preferably metallic, coating material heated by the plasma flows through the inside of the attachment  19 . 
         [0031]    It is also possible for the entire layer to be produced with the attachment  19 . 
         [0032]    The coating material is at least partially melted in the plasma jet and is applied to a substrate. The protective gas  28  is conducted through the attachment  19  in such a manner that, after the molten particles leave the spray nozzle  1 , a protective gas shroud forms around the particle jet. 
         [0033]    This is particularly important in the case of metallic coating material, which would oxidize excessively during plasma spraying but, by contrast, would not oxidize to such an extent during HVOF. 
         [0034]    This shroud prevents oxidation of the particles. Since the particle velocity during APS is significantly lower than during HVOF, the particles remain adhering to the substrate surface more effectively. This makes it possible to effect coating at an angle of up to 45° to the surface. The greater roughness, as compared with HVOF, is always present in this process. 
         [0035]    The configuration of the attachment  19  makes it possible to influence the protective gas shroud. Various geometries and arrangements of the discharge holes  13 ,  13 ′,  13 ″ or slots  14 ′,  14 ″,  14 , . . . in turn influence the formation and the geometry of the protective gas shroud. 
         [0036]    For the widest variety of applications, it is merely necessary to exchange the attachment  19 . It is therefore possible to test and assess the widest variety of attachment configurations  19  and therefore protective gas shroud configurations with a nozzle  4 . If the protective gas shroud has to be more or less twisted for application reasons, only the geometry of the protective gas discharge holes is adapted. 
         [0037]    In the case of turbine blades or vanes  120 ,  130  with a complicated geometry and with poor accessibility to the regions to be coated, this type of coating is a good and simple solution. Expensive low-pressure and vacuum installations become superfluous, since the same installations as for the thermal barrier coating can be used. Compared to layers sprayed by HVOF, the layers which thus arise have a significantly higher roughness and a better layer morphology at sites which are difficult to reach. Owing to the variability of the easy-to-exchange attachment  19 , every application can be covered. The base body  4  remains on the plasma burner, as a result of which complex assembly and disassembly are no longer required. 
         [0038]      FIG. 4  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0039]    In the interior, the gas turbine  100  has a rotor  103  with a shaft which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. 
         [0040]    An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
         [0041]    The annular combustion chamber  110  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 . 
         [0042]    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 . 
         [0043]    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 . 
         [0044]    A generator (not shown) is coupled to the rotor  103 . 
         [0045]    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. 
         [0046]    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 elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0047]    To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. 
         [0048]    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). 
         [0049]    By way of example, iron-based, nickel-based or cobalt-based 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 . 
         [0050]    Superalloys of this type are known, for example, from EP 1 204 776 B 1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0051]    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 . 
         [0052]      FIG. 5  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0053]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0054]    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  and a blade or vane tip  415 . 
         [0055]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0056]    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 . 
         [0057]    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. 
         [0058]    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 . 
         [0059]    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 . 
         [0060]    Superalloys of this type are known, for example, from EP 1 204 776 B 1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0061]    The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0062]    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. 
         [0063]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0064]    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  and a blade or vane tip  415 . 
         [0065]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0066]    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 . 
         [0067]    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. 
         [0068]    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 . 
         [0069]    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 . 
         [0070]    Superalloys of this type are known, for example, from EP 1 204 776 B 1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. 
         [0071]    The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0072]    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. 
         [0073]    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. 
         [0074]    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. 
         [0075]    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). 
         [0076]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0077]    The blades or vanes  120 ,  130  may likewise have coatings 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 stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (HO). 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. 
         [0078]    The density is preferably 95% of the theoretical density. 
         [0079]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer). 
         [0080]    The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use 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. 
         [0081]    It is also possible for a thermal barrier coating, which is preferably the outermost layer, to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0082]    The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0083]    Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0084]    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). 
         [0085]      FIG. 6  shows a combustion chamber  110  of the gas turbine  100 . 
         [0086]    The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107 , which generate flames  156 , arranged circumferentially around an axis of rotation  102  open out into a common combustion chamber space  154 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0087]    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 . 
         [0088]    Moreover, a cooling system may be provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . The heat shield elements  155  are then, for example, hollow and may also have cooling holes (not shown) opening out into the combustion chamber space  154 . 
         [0089]    On the working medium side, each heat shield element  155  made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks). 
         [0090]    These protective layers may be similar to the turbine blades or vanes, i.e. for example 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 stands for 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. 
         [0091]    It is also possible for a, for example ceramic, thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0092]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0093]    Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. 
         [0094]    Refurbishment means that after they have been used, protective layers may have to be removed from turbine blades or vanes  120 ,  130  or heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the turbine blade or vane  120 ,  130  or the heat shield element  155  are also repaired. This is followed by recoating of the turbine blades or vanes  120 ,  130  or heat shield elements  155 , after which the turbine blades or vanes  120 ,  130  or the heat shield elements  155  can be reused.