Abstract:
Masking layers for components according to the prior art react with the base material of the component and/or are difficult to remove again. The component according to the invention has a masking layer which can very easily be removed following coating of the components, since on the one hand the bonding between the masking layer and the base material of the component is poor, or the masking layer can easily be removed through penetration of a liquid.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is the US National Stage of International Application No. PCT/EP03/03283, filed Mar. 28, 2003 and claims the benefit thereof. The International Application claims the benefits of European application No. 02008045.3 EP filed Apr. 10, 2002, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to a coated component having a masking layer.  
       BACKGROUND OF THE INVENTION  
       [0003]     Components, such as for example turbine blades and vanes, in particular for gas turbines, are coated in particular in the main blade region, since they are exposed to high thermal loads.  
         [0004]     Lower temperatures prevail in the base or securing region of the turbine blade or vane, and consequently there is no need for a coating in the form of a thermal barrier coating there. Ceramic coatings are even undesirable in this region, since the base has to be fitted accurately into a metallic disk.  
         [0005]     Masks in accordance with the prior art which are intended to prevent coating are often difficult to remove again, since the material of the mask bonds well to the base material of the turbine blade or vane or there is an undesired diffusion of elements out of the masking layer into the base material of the turbine blade or vane.  
       SUMMARY OF THE INVENTION  
       [0006]     Therefore, it is an object of the present invention to provide a masking layer which, following desired coating of the turbine blade or vane, can easily be removed again in the undesired regions without the base material or the geometry of the turbine blade or vane being affected in the masked region.  
         [0007]     The object is achieved by a turbine blade or vane as described in the claims. A ceramic is applied direct to the base material of the turbine blade or vane.  
         [0008]     Thermal barrier coatings which are applied to a turbine blade or vane in the main blade region generally have intermediate layers between a substrate, i.e. the base material of the turbine blade or vane, and the thermal barrier coating, such as for example what are known as bonding layers, for example metallic MCrAlY, or diffusion barriers.  
         [0009]     These intermediate layers are dispensed with at the masking in order to prevent good bonding of the masking layer. The masking layer is formed in particular from ceramic, since the brittle ceramic can be removed by simple processes, such as for example sand blasting, dry ice blasting. The material for the ceramic is selected in such a way that there is little or no diffusion from the ceramic into the substrate.  
         [0010]     The object of the invention is also achieved by a turbine blade or vane as described in the claims. The masking layer reacts with the material of the material that is to be applied and is therefore easy to remove.  
         [0011]     Further advantageous configurations of the component according to the invention as described in the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     In the drawing:  
         [0013]      FIG. 1  shows a turbine blade or vane in accordance with the prior art,  
         [0014]      FIG. 2  shows process steps involved in the production of a coating in accordance with the prior art,  
         [0015]      FIG. 3  shows a masking layer of a turbine blade or vane according to the invention,  
         [0016]      FIG. 4  shows a further masking layer of a turbine blade or vane according to the invention,  
         [0017]      FIGS. 5, 6  show a masking layer which reacts with material of layers which are to be applied, and  FIGS. 7, 8  show how the masking layer can easily be removed again following a reaction. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Identical reference numerals have the same meaning throughout the various figures.  
         [0019]      FIG. 1  shows a perspective view of a turbine blade or vane  1 , in particular a rotor blade for a gas turbine, which extends along a longitudinal axis  4 . In succession along the longitudinal axis  4 , the turbine blade  1  has a securing region  7 , an adjoining blade platform  10  and a main blade region  13 .  
         [0020]     The securing region  7  is designed as a blade root  16  which is used to secure the turbine blade  1  to a shaft (not shown in more detail) of a turbine machine (likewise not shown in more detail). The blade root  16  is designed, for example, in the form of a hammerhead. Other configurations, for example as a fir-tree root or dovetail root are also possible.  
         [0021]     In conventional turbine blades  1 , solid metallic materials, in particular nickel- or cobalt-base superalloys, are used in all regions of the turbine blade. The turbine blade may in this case be produced by a casting process, by a forging process, by a milling process or by combinations of the above.  
         [0022]     In particular the securing region  7  is made from metal, since this region is clamped in an accurately fitting manner into a corresponding shape in a disk. Brittle ceramic coatings would flake off and alter the geometry in the securing region.  
         [0023]     The main blade region  13  is coated, for example, with a thermal barrier coating, it being possible for further layers, such as for example bonding layers (MCrAlY layers), to be arranged between the base material of the turbine blade  1 .  
         [0024]     A component according to the invention in the form of a turbine blade or vane  1  may be a guide vane or rotor blade of any desired turbine, in particular a steam or gas turbine.  
         [0025]      FIG. 2  shows what happens when the surface of the blade  1  does not have a masking layer  25  ( FIG. 3 ). The material  22  of an interlayer  19  (MCrAlY), which has been applied in any desired form to a surface of the turbine blade  1 , for example by plasma spraying, by PVD or CVD or by dipping in a liquid metal or application of powder, so as to form the interlayer  19 , leads to a reaction of the material  22  with the turbine blade  1  and to good bonding of the interlayer  19  to the base material of the turbine blade  1 .  
         [0026]     When the interlayer  19  is to be removed again, for example because it is undesired in the securing region  7 , it therefore presents considerable problems, since the geometry of the securing region  7  changes as a result of partial removal of the base material of the substrate  40 .  
         [0027]      FIG. 3  shows a component according to the invention in the form of a turbine blade  1  with a masking layer  25 . By way of example, first of all a first functional layer  28  is applied to the turbine blade  1 . This first functional layer is, for example, a polycarbosilane layer with a thickness in the nanometer range, which crosslinks at 200° C. in air, allowing good bonding to the base material  40  of the turbine blade  1 .  
         [0028]     By way of example, a gradient layer  31  is applied to the first functional layer  28 , the material used for the gradient layer  31  being a mixture of polysiloxane and a metal-ceramic and/or metal.  
         [0029]     The gradient layer  31  may, for example, be applied in the form of a slip with layer thicknesses of 10-30 μm and can likewise be crosslinked at approximately 200° C. in air.  
         [0030]     A further powder, in particular of the composition MCrAlY, where M stands for Fe, Co, Ni, is added as a metallic filler to this material of the gradient layer  31 , since such a filler, on account of its expansion coefficient, is used as an interlayer (bonding layer) between base material  40  and ceramic thermal barrier coating.  
         [0031]     A reactive layer  34 , consisting, for example, of a pure carbon precursor, is applied to the gradient layer. The crosslinking within the reactive layer  34  takes place at 180° C. in air.  
         [0032]     The crosslinked layers  28 ,  31 ,  34  are converted into a ceramic by what is known as the pyrolysis process as a result of a heat treatment at 1000° C. under an argon atmosphere. On account of the change in density of the organometallic precursor, such as for example the polysiloxane, having a density of 1 g/cm 3 , to a silicon oxicarbide phase (SiOC) with a density of approx. 2.3 g/cm 3 , a 10-30 μm thick, dense and crack-free coating is not possible. Therefore, metallic or ceramic fillers are added to the polymer, for example in a proportion of 30-50% by volume, in order to deliberately control the phase transformation of the polymer and the crack formation which are taking place and to minimize or eliminate the thermomechanical stresses caused by different coefficients of thermal expansion at the interface between metal (turbine blade  1 ) and masking layer  25 .  
         [0033]     The required thermal stability of the masking layer is provided by the thermal phase transformation of the polycarbosilane into the corresponding high-temperature-resistant SiOC or graphite phase.  
         [0034]     During the coating process, a material  22  is applied to the main blade region  13  of the turbine blade  1  and to the masking layer  25 . The material  22  reacts with the reactive layer  34  to form a reaction layer  43 , i.e. to form a material which is able to withstand high temperatures but, for example, is soluble in water, i.e. can easily be removed.  
         [0035]     The material  22  is, for example, aluminum, which is applied to the turbine blade  1  in order to form an aluminide layer. An aluminide layer of this type can be applied by plasma spraying or processes as described in EP patent 0 525 545 B1 and EP patent 0 861 919 B1.  
         [0036]     In the case of aluminum, the carbon of the reactive layer  34  reacts with aluminum to form A 1   4 C 3 . If the main blade region  13  is completely coated, the entire blade, in particular the securing region  7 , can be introduced into water, with the result that the water-soluble reaction layer  43  which has reacted with the material  22  is dissolved.  
         [0037]     The underlying layers  28 ,  31  can easily be removed by dry ice blasting, and consequently the removal processes do not cause any change in the geometry in the securing region  7 . Aluminum is applied to a turbine blade  1  during refurbishment for example, i.e. inter alia during removal of used MCrAlY layers.  
         [0038]     As an alternative to the three-layer structure shown here by way of example, the masking layer  25  may also be a gradient layer which has a graduated structure, i.e. on the substrate  40  the composition is selected in such a way as to allow good bonding, and at the outer surface the composition is such that it reacts with the material  22  of layers which are yet to be applied.  
         [0039]      FIG. 4  shows a component according to the invention in the form of a turbine blade  1  having a masking layer  25 .  
         [0040]     A ceramic layer  37 , which forms the masking layer  25 , is applied direct to the, for example, metallic turbine blade  1 .  
         [0041]     This ceramic layer may, for example, be an oxide ceramic which is matched to the coefficient of thermal expansion of the substrate.  
         [0042]     There are no further layers, in particular no bonding layers, between the ceramic layer  37  and the metallic substrate  40  of the turbine blade  1 , and consequently the ceramic layer  25 ,  37  can be removed by gentle introduction of energy, such as for example sand blasting or dry ice blasting. The dense ceramic layer  37  also forms a diffusion barrier during a process of coating the turbine blade  1  with other layers, such as for example bonding layers or thermal barrier coatings.  
         [0043]     The masking layer  25  may also only react with the material  22  of layers which are yet to be applied, for example to form a brittle layer  43 , for example a ceramic layer  37 . The ceramic layer  37  may also form only after a further heat treatment (pyrolysis), by way of example.  
         [0044]     Brittle layers  43  of this type can be removed by simple processes, such as thermal shock processes or sand blasting or dry ice blasting, i.e. by blasting processes which introduce energy but do not have an abrasive action.  
         [0045]     It is particularly advantageous if the masking layer  25  reacts with the material  22  of layers which are to be applied to form a water-soluble layer  43 .  
         [0046]     Further layers may in this case be present beneath the top layer of the masking layer, i.e. the masking layer  25  may be a multilayer structure. In this case, it is possible for a joining layer to be applied direct to the substrate  40  of the coated component and for a gradient layer also to be applied, allowing matching to coefficients of thermal expansion, so that the masking layer  25  remains crack-free even during masking, and consequently it is impossible for any material to reach the substrate  40  of the component which is to be coated.  
         [0047]      FIG. 5  shows a turbine blade  1  having a substrate  40  to which a masking layer  25  has been applied. The material of the masking layer does not react and diffuse with the material of the substrate  40  at the elevated temperatures of the coating process.  
         [0048]     During the coating process, material  22  comes into contact with the masking layer  25  and reacts with the latter. The reaction may also take place in a subsequent heat treatment, if the reaction temperature is higher than the substrate temperature during the coating operation. The reaction layer  43  which is formed in this way ( FIG. 6 ) can easily be removed again following the process of coating the turbine blade, since it is, for example, brittle or water-soluble. The material  22  therefore also comes into contact with the unmasked regions of the substrate  40  of the turbine blade  1  and forms a desired coating  55  ( FIG. 6 ).  
         [0049]      FIG. 7  shows a water bath  46  into which a turbine blade having a water-soluble layer  43  has been introduced. Its water solubility allows the layer  43  to be removed easily, so that after the turbine blade  1  has been taken out of the water bath an uncoated part and a desired coated part  55  of the turbine blade  1  are present. The reaction layer  43  may also be removed by water blasting, in which case once again a small amount of energy is introduced.  
         [0050]     The, for example, brittle reaction layer  43  may also be removed by the introduction of energy from a blasting gun  49  (ultrasound, dry ice blaster, sand blaster) ( FIG. 8 ).