Abstract:
A plasma spray nozzle is provided. Owing to their high degree of wear, previous plasma spray nozzles were not suitable for the coating of components for which long coating times were necessary. The coating times may be reduced considerably by the triple injection of powder into the inner channel through the plasma spray nozzle.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 09013864.5 EP filed Nov. 4, 2009, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a plasma spray nozzle, wherein the powder is injected. 
       BACKGROUND OF INVENTION 
       [0003]    In order to increase the efficiency of the turbine, it is necessary to facilitate high temperatures at the turbine intake. This is achieved by applying a metallic and ceramic coating onto the turbine blade, the thickness of this coating being up to 800 micrometers. 
         [0004]    The process has to date proven very inefficient because the coating operation lasts more than 70 minutes. The reason is that such long coating times cause the spray spot to vary because of wear to the nozzle, so that the spraying result varies over time. This is undesirable. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to resolve the aforementioned problem. 
         [0006]    The object is achieved by a plasma spray nozzle as claimed in the claims. 
         [0007]    Further advantageous measures are listed in the dependent claims, and these may be combined in a variety of ways in order to achieve further advantages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIGS. 1 ,  4 ,  5  show plasma spray nozzles in longitudinal section, 
           [0009]      FIGS. 2 ,  3 ,  6  show plasma spray nozzles in cross section, and 
           [0010]      FIG. 7  shows a turbine blade. 
       
    
    
       [0011]    The description and the figures only represent exemplary embodiments of the invention. 
       DETAILED DESCRIPTION OF INVENTION 
       [0012]      FIG. 1  shows a plasma spray nozzle  1  in longitudinal section. 
         [0013]    The plasma spray nozzle  1  has, on its inside, an elongate inner channel  4  with a longitudinal axis  22 , in which  4  a plasma is generated and into which  4  powder is injected through at least one hole  7 . 
         [0014]    The inner channel  4  is formed so that it is longer than the divergent region  16 , and in particular comprises 60%, more particularly 75%, of the total length. 
         [0015]    There is a divergent part  16  at the end  19  of the plasma spray nozzle  1 , so that the inner cross section of the inner channel  4  increases toward the exit or end  19 . 
         [0016]    The outer diameter of the end  28  of the nozzle  1 , which lies opposite the divergent part  16 , is preferably more than the outer diameter at the end  19  of the divergent region  16 . This means that the mass per axial length is greater at the end  28 . 
         [0017]    The powder injection is carried out internally, i.e. before the divergent region  16 . It may take place through one hole  7  ( FIG. 3 ) or through several holes  7 ′,  7 ″,  7 ′″ ( FIG. 2 ). 
         [0018]    The distance between the at least one hole  7 ,  7 ′,  7 ″,  7 ′″ and the end  19  of the nozzle  1  is preferably at least 60%, in particular at least 70%, more particularly 80% of the total length L of the nozzle  1 . 
         [0019]    At the start of the divergent part  16 , there is preferably a shoulder  25  ( FIG. 1 ,  4 ) which guides the electric arc of the plasma toward the inner channel  4 . 
         [0020]    The shoulder  25  constitutes a non-constant or discontinuous transition  25  to the divergent region  16 . 
         [0021]    There is preferably an edge at the transition  25  from the inner channel  4  with a constant cross section to the divergent region  16 . 
         [0022]    The shoulder  25  preferably extends perpendicularly to the longitudinal axis  22  of the inner channel  4 . 
         [0023]    It is also possible for there to be no shoulder  25  ( FIG. 5 ). 
         [0024]    Cooling fins  10  are preferably provided externally along the flow direction for the plasma spray nozzle  1 , that is to say parallel to the longitudinal axis  22  of the nozzle  1  or of the channel  4  ( FIG. 4 ). 
         [0025]    The outer diameter of these  10  may exceed the outer diameter at the end  19  of the divergent region  16 . 
         [0026]    A sealing ring  13  is preferably arranged between the cooling fins  10  ( FIG. 4 ). 
         [0027]      FIG. 2  shows another exemplary embodiment. 
         [0028]    The powder is delivered into the channel  4  of the plasma spray nozzle  1  not through one, but in particular through two holes, particularly through three holes  7 ,  7 ′,  7 ″, which are preferably distributed uniformly around the circumference of the inner channel  4 . 
         [0029]    Owing to this arrangement of triple injection, the injection of the powder can be controlled accurately in relation to the jet, and the pass spacing, i.e. the spacing between runs over the component to be coated, can be at least doubled, the spray spot being kept constant in the same position so that the coating time is reduced significantly. Except for the inner channel  4  and the powder injection holes  7 ,  7 ′,  7 ″,  7 ′″, the nozzle  1  is formed solidly. 
         [0030]    The at least one hole  7  has a taper  8  at the end, i.e. close to where it enters the inner channel  4 , in order to inject into the plasma jet in a controlled way. 
         [0031]      FIG. 7  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0032]    The turbomachine may be a gas turbine of an aircraft or of a power plant for electricity generation, a steam turbine or a compressor. 
         [0033]    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 . 
         [0034]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0035]    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 . 
         [0036]    The blade root  183  is configured, for example, as a hammerhead. Other configurations such as a firtree or dovetail root are possible. 
         [0037]    The blade  120 ,  130  comprises a leading edge  409  and a trailing edge  412  for a medium which flows past the blade surface  406 . 
         [0038]    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 . 
         [0039]    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. 
         [0040]    The blade  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. 
         [0041]    Workpieces with a single-crystal structure or single-crystal structures are used as components for machines which are exposed to heavy mechanical, thermal and/or chemical loads during operation. 
         [0042]    Such single-crystal 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 single-crystal structure, i.e. to form the single-crystal workpiece, or is directionally solidified. 
         [0043]    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 single-crystal 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 single-crystal component. 
         [0044]    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. 
         [0045]    Such methods are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
         [0046]    The blades  120 ,  130  may also have coatings against corrosion or oxidation, for example MCrAlX (M is at least one element from the group 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)). 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. 
         [0047]    The density is preferably 95% of the theoretical density. 
         [0048]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX coating (as an interlayer or as the outermost coat). 
         [0049]    The coating composition preferably comprises Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. Besides these cobalt-based protective coatings, it is also preferable to use nickel-based protective coatings 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. 
         [0050]    On the MCrAlX, there may furthermore be a thermal barrier coating, which is preferably the outermost coat and consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. it is not stabilized or is partially or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0051]    The thermal barrier coating covers the entire MCrAlX coating. 
         [0052]    Rod-shaped grains are produced in the thermal barrier coating by suitable coating methods, for example electron beam deposition (EB-PVD). 
         [0053]    Other coating methods may be envisaged, for example atmospheric plasma spraying (APS), LPPS, VPS or CDV. The thermal barrier coating may comprise porous, micro- or macro-cracked grains for better thermal shock resistance. The thermal barrier coating is thus preferably more porous than the MCrAlX coating. 
         [0054]    Refurbishment means that components  120 ,  130  may need to be stripped of protective coatings (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 ,  130  is used again. 
         [0055]    The blade  120 ,  130  may be designed to be 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).