Abstract:
A method for forming a textured bond coat surface ( 48 ) for a thermal barrier coating system ( 44 ) of a gas turbine component ( 34 ). The method includes selectively melting portions of a layer of alloy particles ( 16 ) with a patterned energy beam ( 20 ) to form successive layers of alloy material ( 16′, 16 ″) until a desired surface geometric feature ( 26 ) is achieved. The energy beam pattern may be indexed between layers to form a protruding undercut ( 28 ) in the geometric feature. The patterned energy beam may be formed by directing laser energy from a diode laser ( 30 ) through a cartridge filter ( 32 ). Particles of a flux material ( 18 ) may be melted along with the alloy particles to form a protective layer of slag ( 22 ) over the melted and cooling alloy material.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of materials technology, and more particularly to a method for creating a textured surface in a bond coat of a thermal barrier coating system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ceramic thermal barrier coating systems are used on gas turbine engine hot gas path components to protect the underlying metal alloy substrate from combustion gas temperatures that exceed the safe operating temperature of the alloy. A typical thermal barrier coating system may include a bond coat, such as an MCrAlY material, deposited onto the substrate alloy and a ceramic topcoat, such as yttria stabilized zirconia, deposited onto the bond coat. It is known that strong adhesion between the layers of such systems is critical for proper functioning and long life of the coating system, and that a degree of surface roughness in the interface between the layers provides a beneficial mechanical interlock in that regard. 
         [0003]    Bond coat material is often deposited by a spray process, such as High Velocity Oxy-Fuel (HVOF) or Air Plasma Spray (APS). It is known to control spray parameters when depositing a bond coat layer in order to achieve a degree of surface roughness in the deposited coating. However, the degree of roughness and the shape of the surface features in the deposited coating that are created by controlling the spray parameters are limited. 
         [0004]    It is also known to texture the surface of a bond coat layer prior to the deposition of a ceramic insulating layer by using a material removal process, such as laser ablation, micromachining or photolithography, such as described in U.S. Pat. No. 5,723,078. As the firing temperatures of advance gas turbine engines continue to increase, further improvements in thermal barrier coating systems and methods of applying such coatings are desired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is explained in the following description in view of the drawings that show: 
           [0006]      FIGS. 1-5  illustrate sequential steps for depositing a coating onto a substrate with a surface texture including geometric features having protruding undercuts. 
           [0007]      FIG. 6  illustrates a layer of thermal barrier coating material deposited over a textured surface of a bond coat material. 
           [0008]      FIG. 7  illustrates a layer of bond coat material deposited over a textured surface of a superalloy component with an overlying layer of ceramic thermal barrier coating material. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    The present inventors have recognized that known bond coat texturing processes that rely upon material removal are inherently inefficient because unwanted material is first deposited and is then removed. The methods of the present invention cause material to be initially deposited in a way that creates the desired texture pattern. Furthermore, because the methods of the present invention create a textured surface by depositing a plurality of layers of material, a wide range of surface feature geometries, including undercuts, are made possible. 
         [0010]      FIG. 1  is a partial cross-sectional view of a gas turbine component  10  having a relatively smooth surface  12  to which it is desired to add a textured surface geometry. The surface  12  may be a surface of an existing bond coat material layer that is to be textured, or it may be the surface of a superalloy substrate over which a layer of bond coat material is to be applied. A layer of powder  14  is deposited onto the surface  12 . The layer of powder  14  includes particles of a metal alloy  16  and particles of a flux material  18 . The metal alloy  16  may be superalloy material for the embodiment where the surface of the superalloy substrate is to be textured, or it may be a bond coat material for the embodiment where the surface of a bond coat is to be textured. The flux material  18  is applied to provide cleansing and atmospheric protection functions during a subsequent melting step. Accordingly, the particles of metal alloy  16  and particles of flux material  18  may be mixed together and applied as a single layer in one embodiment, or mixed and applied simultaneously by directing a spray of metal alloy particles and a spray of flux material particles simultaneously toward the surface. Alternatively, the particles of metal alloy  16  may be applied to the surface  12  first and then covered by the particles of flux material  18  in another embodiment. 
         [0011]      FIG. 2  illustrates the layer of powder  14  being exposed to a pattern of energy  20  to melt selected regions of the layer of powder  14  to form a pattern of the metal alloy  16 ′ covered by slag  22  on the surface  12 . Upon cooling, the slag  22  and excess powder  14  are removed to reveal the component  10  with a textured surface  12 ′, as illustrated in  FIG. 3 . 
         [0012]    The steps of  FIGS. 1 and 2  may then be repeated, as illustrated in  FIG. 4  to add a further layer of metal alloy  16 ″. Note that the pattern of energy  20 ′ in  FIG. 4  is somewhat different than the pattern of energy  20  in  FIG. 2 , and as a result, the second layer of deposited metal alloy  16 ″ has a region  24  that is indexed from and somewhat cantilevered beyond the first layer of deposited metal alloy  16 ′. The heat input of the energy  20  is selected such that the region of melting does not extend to a full depth of the layer of powder  14  in the cantilevered region  24 . Such indexing of the pattern of energy  20  between layers can be repeated for additional layers as desired to create a desired final textured surface  12 ″ having geometric features  26  including protruding undercuts  28 , as illustrated in  FIG. 5 . One skilled in the art will appreciate that any surface geometric feature will provide a degree of mechanical interlocking with a subsequently applied overcoat layer (i.e. bond coat or thermal barrier coating), and that a feature  26  with a protruding undercut  28  may be especially beneficial in that regard. 
         [0013]      FIG. 6  illustrates the component  10  after the further deposit of a layer of material  31  over the surface  12 ″ having the layer of geometric features  26 . In this illustration, the surface  12 ″ is bond coat material and the layer of material  31  is a ceramic thermal barrier coating material wherein the protruding undercuts  28  function to mechanically anchor the ceramic material  31 . One will appreciate that the bond coat material may be deposited onto a superalloy substrate that is not illustrated in this view. 
         [0014]      FIG. 7  is a partial cross-sectional illustration of a gas turbine component  34  wherein a layer of bond coat material  36  is deposited over a textured surface  38  of a superalloy substrate  40  with an overlying layer of ceramic thermal barrier coating material  42  to form a thermal barrier coating system  44 . The surface  38  is textured by the addition of geometric features  46  that were deposited by the process described above. Note that the texturing of surface  38  is reflected in a more subtle but still effective texturing of the surface  48  of the bond coat material  36 . Advantageously, when the superalloy substrate  40  is a cast product and the superalloy material is one of the many difficult to weld materials that are commonly used for gas turbine engine components, the process described herein facilitates the joining of additional superalloy material to form the geometric features  46  with a greatly reduced risk of cracking and higher degree of geometric precision that could otherwise be achieved by using a welding additive process, as more fully described below. 
         [0015]    The layer of powder  14  may be one to several millimeters in thickness in some embodiments rather than the fraction of a millimeter typical with known selective laser melting and sintering processes. Typical powdered prior art flux materials have particle sizes ranging from 0.5-2 mm, for example. However, the powdered alloy material  16  may have a particle size range (mesh size range) of from 0.02-0.04 mm or 0.02-0.08 mm or other sub-range therein. This difference in mesh size range may work well in the embodiment where the materials constitute separate layers; however, in the embodiment where the particles are mixed together before being applied to the surface  12 , it may be advantageous for the powdered alloy material  38  and the powdered flux material  40  to have overlapping mesh size ranges, or to have the same mesh size range in order to facilitate mixing and feeding of the powders and to provide improved flux coverage during the melting process. 
         [0016]    The flux material  18  and resultant layer of slag  22  provide a number of functions that are beneficial during the melting process. First, they function to shield both the region of molten material and the solidified (but still hot) alloy material  16 ′ from the atmosphere as the material cools. The slag floats to the surface to separate the molten or hot metal from the atmosphere, and the flux may be formulated to produce a shielding gas in some embodiments, thereby avoiding or minimizing the use of expensive inert gas. Second, the slag  22  acts as a blanket that allows the solidified alloy material  16 ′ to cool slowly and evenly, thereby reducing residual stresses that can contribute to cracking. Third, the slag  22  helps to shape the pool of molten alloy  16 ′. Fourth, the flux material  18  provides a cleansing effect for removing trace impurities such as sulfur and phosphorous that contribute to cracking. Such cleansing includes de-oxidation of the metal alloy powder  16 . Because the flux powder  18  is in intimate contact with the alloy powder  16 , it is especially effective in accomplishing this function. Furthermore, the flux material  18  may provide energy absorption and trapping functions to more effectively convert the beam energy  20  into heat energy, thus facilitating a precise control of heat input, such as within 1-2%, and a resultant tight control of material temperature during the process. Finally, the flux may be formulated to compensate for loss of volatized elements during processing or to actively contribute elements to the deposit that are not otherwise provided by the alloy powder itself. 
         [0017]    The patterned energy beam  20  may be produced by a diode laser  30  having a generally rectangular cross-sectional shape, although other known types of energy beams may be used, such as electron beam, plasma beam, one or more circular laser beams, a scanned laser beam (scanned one, two or three dimensionally), an integrated laser beam, etc. The rectangular shape may be particularly advantageous for embodiments having a relatively large area to be textured. The broad area beam produced by a diode laser helps to reduce weld heat input, heat affected zone, dilution from the substrate and residual stresses, all of which reduce the tendency for the cracking effects normally associated with superalloy repair. The laser energy may be patterned by any known beam shaping optics, such as a cartridge filter  32  having pre-determine openings. The cartridge  32  used to deposit the first layer of material in  FIG. 2  is conveniently changed to a cartridge  32 ′ having a somewhat different pattern of openings for depositing the second layer in  FIG. 4 . Optical conditions and hardware optics used to generate a broad area laser exposure may include but are not limited to: defocusing of the laser beam; use of diode lasers that generate rectangular energy sources at focus; use of integrating optics such as segmented mirrors to generate rectangular energy sources at focus; scanning (rastering) of the laser beam in one or more dimensions; and the use of focusing optics of variable beam diameter (e.g. 0.5 mm at focus for fine detailed work varied to 2.0 mm at focus for less detailed work). The motion of the optics and/or substrate may be programmed as in a selective laser melting or sintering process to build a custom shape layer deposit. Advantages of this process over known laser melting or sintering processes include: high deposition rates and thick deposit in each processing layer; improved shielding that extends over the hot deposited metal without the need for inert gas; flux will enhance cleansing of the deposit of constituents that otherwise may lead to cracking; flux will enhance laser beam absorption and minimize reflection back to processing equipment; slag formation will shape and support the deposit, preserve heat and slow the cooling rate, thereby reducing residual stresses; flux may compensate for elemental losses or add alloying elements, and powder and flux pre-placement or feeding can efficiently be conducted selectively because the thickness of the deposit greatly reduces the time involved in total part building. 
         [0018]    Flux materials which could be used include commercially available fluxes such as those sold under the names Lincolnweld P2007, Bohler Soudokay NiCrW-412, ESAB OK 10.16 or 10.90, Special Metals NT100, Oerlikon OP76, Sandvik 50SW or SAS1. The flux particles may be ground to a desired smaller mesh size range before use. Any available structural alloy, superalloy or bond coat material that is appropriate for thermal barrier coating systems may be used. 
         [0019]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.