Patent Publication Number: US-2011076414-A1

Title: Process for Applying a Bonding Primer Layer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2009/055044, filed Apr. 27, 2009 and claims the benefit thereof. The International Application claims the benefits of European application No. 08009773.6 filed May 29, 2008 and European application No. 08009771.0 filed May 29, 2008, all of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a process for applying a bonding primer layer for a protective ceramic coating on a component surface by high-velocity flame spraying (HVOF), in which process a coating material in the form of at least one metal alloy powder is at least partially melted and is emitted as a particle stream onto the component surface at a high velocity, wherein the coating material has two powder fractions having a fine grain size and a coarse grain size. 
     BACKGROUND OF THE INVENTION 
     Components used in hot and aggressive environments have to be protected from said harmful influences in order to increase the service life thereof. By way of example, turbine blades or vanes of gas turbines are equipped with coating systems consisting of a bonding primer layer which is applied directly to the surface of the turbine blade or vane and, in turn, bears a ceramic thermal barrier coating. By way of example, the ceramic-containing thermal barrier coatings may contain zirconium oxides (ZrO 2 ) partially or fully stabilized by yttrium oxide (Y 2 O 3 ), magnesium oxide (MgO) or another oxide. The ceramic coating is typically deposited by air plasma spraying (APS), vacuum plasma spraying (VPS), low-pressure plasma spraying (LPPS) or physical vapor deposition (PVD). In this context, preference is given to air plasma spraying (APS) rather than other deposition processes owing to the low apparatus costs and the simplicity of application and masking. 
     The bonding primer layers are typically formed from an oxidation-resistant alloy, such as for example MCrAlY, where M represents at least one of the elements from the group consisting of iron, cobalt and nickel and the letter Y is yttrium or a further equivalent element from the group consisting of scandium and the rare earth elements. The object of the bonding primer layer is firstly to afford protection against corrosion and/or oxidation and secondly to ensure a strong bond between the thermal barrier coating and the component to be coated. In this type of coating system, it is therefore particularly important for the bonding primer layer to have a high surface roughness, since only then is it possible to ensure sufficient interlocking between the bonding primer layer and the thermal barrier coating. 
     The bonding primer layer can be applied to the turbine blade or vane by high-velocity flame spraying (HVOF). For this purpose, MCrAlY particles are introduced with a carrier gas into a burner, which burns the supplied fuel and oxygen at a high temperature. The MCrAlY particles are at least partially melted in the burner flame thereby formed and are then emitted as a particle stream onto the component surface at a high velocity. The problem associated with such bonding primer layers deposited by HVOF techniques is that they are very sensitive to the particle size distribution of the powder owing to the relatively low spraying temperature of the HVOF process. Accordingly, the parameters of the HVOF process are typically set such that powders having a very narrow particle size distribution range are used. 
     In order to produce a bonding primer layer using the HVOF process, it is typically necessary to use a coarse powder in order to obtain an adequate surface roughness. Since relatively coarse particles cannot typically be melted completely given suitable HVOF parameters, HVOF bond coats frequently exhibit a relatively high porosity and a poor bond between sprayed particles. 
     In order to counter this problem, DE 698 28 732 T2 discloses a process of the type mentioned in the introduction, in which use is made of a coating material comprising a powder fraction having a fine grain size and a powder fraction having a coarse grain size. In this process, the surface roughness of the bonding primer layer is determined by the particles of the relatively coarse powder, which are melted incompletely during the deposition. The particles of the relatively fine powder melt completely and sufficiently fill the interstices between the particles of the relatively coarse powder to obtain a high density. The relatively fine powder also contributes to the microsurface roughness of the bonding primer layer. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to further develop the process of the type mentioned in the introduction in such a manner as to obtain an optimum surface roughness. 
     According to the invention, this object is achieved in the case of a process of the generic type by virtue of the fact that the coating material consists of an agglomerated and sintered powder. 
     This can be produced in a manner known per se from a metal melt. For this purpose, a “globular” powder, from which the required grain size is obtained by various screening steps etc., is produced from the melt by means of inert gas and/or vacuum atomization. The powder is then mixed homogeneously with a binder with the desired grain size ratios and then brought together by a spray drying process to form agglomerates. During spraying, the fine fraction will clog the HVOF nozzles owing to sintering, and therefore the relatively coarse particles can be used. The different grain sizes have the effect that the fine particles readily melt and compact, whereas the coarse particles are embedded in the fine particles and provide the desired roughness. 
     It has been found that the surface roughness can be optimized by using such an agglomerated and sintered material. 
     According to one embodiment of the invention, the powder fraction having a fine grain size is 60 to 80% by volume, in particular 65 to 75% by volume and preferably about 70% by volume. According to the invention, it has been realized that a high powder fraction having a fine grain size leads to good results and that, in particular, a very good surface roughness can be obtained if the fraction of fine powder is about 70% and accordingly the fraction of the coarse powder is about 30%. 
     In a manner known per se, the coating material may be a metal alloy from the group consisting of NiAl, MCrAlY, MCrAl, aluminum-containing intermetallic materials, chromium-containing intermetallic materials and combinations thereof. These materials have proved to be thoroughly suitable as bonding primer layers. In this case, it is preferable to use MCrAlY, since this material can be applied very readily by high-velocity flame spraying (HVOF). 
     In a further embodiment of the invention, the component surface is coated initially with a layer of a metal alloy powder having a fine grain size, and then a top layer made of the coating material having the powder fractions having different grain sizes is applied to the bottom layer thus formed. In this case, particles of the coating material which have a smaller mean diameter than the particles of the coating material can be used for the bottom layer. By way of example, it is possible to produce the bottom layer from the powder having a fine grain size of the top layer. This effectively provides a dense bottom layer which can be formed, in particular, from MCrAlY. 
     It has proved to be advantageous if the coarse powder fraction has a particle size distribution of 45 to 75 μm, in particular of 22 to 63 μm. Tests have shown that the fine powder fraction should advantageously have a particle size distribution of 11 to 44 μm, in particular of 16 to 44 μm. Alternatively, the fine powder fraction may also have a particle size distribution of 22 to 53 μm. 
     According to a preferred embodiment of the invention, at least 90% of the particles of the fine powder fraction are smaller than the particles of the coarse powder fraction. The powder fractions can be combined before the spraying or mixed during the spraying process to form a powder mixture. In this case, the powder fractions expediently have an identical composition, although they may also consist of different materials. The coating material and, in particular, the top layer may also consist of an agglomerated and sintered powder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With respect to further advantageous refinements of the invention, reference is made to the dependent claims and to the description, which follows, of an exemplary embodiment with reference to the drawing, in which: 
         FIG. 1  schematically shows the application of a bottom layer of a bonding primer layer to a turbine blade or vane, and 
         FIG. 2  schematically shows the application of a top layer to the bottom layer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  schematically show a process according to the invention for applying a bonding primer layer to the surface of a turbine blade or vane  2 . In this case, the bonding primer layer consists of a bottom layer  3 , which is applied directly to the turbine blade or vane  2 , and a top layer  7 , which covers the bottom layer  3 . Since the bonding primer layer is produced by high-velocity flame spraying (HVOF), MCrAlY is used as the coating material. In this case, the coating material consists of a powder blend having two powder fractions with different mean grain sizes. Specifically, the coating material is present in the form of an agglomerated and sintered powder. This can be produced in a manner known per se from a metal melt. For this purpose, a “globular” powder, from which the required grain size is obtained by various screening steps etc., is produced from the melt by means of inert gas and/or vacuum atomization. The powder is then mixed homogeneously with a binder with the desired grain size ratios and then brought together by a spray drying process to form agglomerates. During spraying, the fine fraction will clog the HVOF nozzles owing to sintering, and therefore the relatively coarse particles can be used. The different grain sizes have the effect that the fine particles readily melt and compact, whereas the coarse particles are embedded in the fine particles and provide the desired roughness. 
     The sintered coating material consists of MCrAlY powder having a fine grain size to an extent of 60 to 80% by volume, in particular 65 to 75% by volume and preferably about 70% by volume, and consists of MCrAlY powder having a coarse grain size to the remaining extent. 
     Here, the grain size of the coarse powder fraction is between 45 and 75 μm, in particular between 22 and 63 μm, and the grain size of the fine powder fraction is 11 to 44 μm, in particular 16 to 44 μm. The fine powder of the top layer  7  is used as the material for the bottom layer  3 . 
     In the course of the coating, the bottom layer  3  is initially applied to the surface of the turbine blade or vane  2  by HVOF. For this purpose, coating particles of MCrAlY are supplied to a burner  4  in a carrier gas. At the same time, a fuel and oxygen are fed into the burner  4 . The fuel and the oxygen are mixed and burnt in the burner. The coating particles in the carrier gas are injected into the flame  5  thereby produced as a particle stream  6  at a high velocity. The coating particles at least partially melt as they pass through the flame  5  and then impinge on the surface of the turbine blade or vane  2 , where they remain adhering. 
     The particle stream  6  is guided over the surface in order to form the bottom layer  3 . During the application of the bottom layer  3 , the particle stream  6  is oriented such that it includes an angle a of 90° with the surface of the turbine blade or vane  2 . This has the effect that the bottom layer  3  obtained has a relatively low surface roughness. 
     The top layer  7  is then applied to the bottom layer  3 . For this purpose, the coating material having the fractions having different grain sizes is supplied to the burner  4 . The coating particles are at least partially melted in the flame  5  (in the manner described above) and are emitted as a particle stream  6  in the direction of the surface of the turbine blade or vane  2  at a high velocity. There, they impinge on the bottom layer  3  and form the top layer  7  thereon as the particle stream  6  is moved over the bottom layer  3 . 
     During this operation, the fine coating particles are readily melted. However, the energy of the flame is not sufficient to also completely melt the coarse particles, and therefore these are embedded in the molten, liquid material as solid particles. As a result, the fine powder fraction leads to a good bond between the top layer  7  and the bottom layer  3  and a high density of the top layer  7 , whereas the coarse powder fraction is responsible for the desired surface roughness which is required to fix a thermal barrier coating, for example a ceramic APS thermal barrier coating, to the bonding primer layer. 
     According to the invention, it has been realized that an optimum surface roughness can be obtained if the fraction of the fine powder is about 70% and accordingly the fraction of the coarse powder is about 30%.