Abstract:
Embodiments of the invention relate generally to turbine blades and, more particularly, to the formation of cooling channels on a surface of a turbine blade and turbine blades including such cooling channels. In one embodiment, the invention provides a method of forming a cooling channel along a surface of a turbine blade, the method comprising: applying a first mask material to a first portion of a surface of a turbine blade; forming a first barrier layer atop the first mask material and atop a second portion of the surface of the turbine blade; removing the first mask material and the barrier layer atop the first mask material to expose the first portion of the surface of the turbine blade; and etching the first portion of the surface of the turbine blade to form a cooling channel along the surface of the turbine blade.

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate generally to turbine blades and, more particularly, to the formation of cooling channels on a surface of a turbine blade and turbine blades including such cooling channels. 
     Turbine blades employed in high-temperature applications are typically a nickel-based super alloy and covered with a metallic bond coat and a ceramic thermal barrier coating. Embodiments of the invention facilitate improved cooling of a turbine blade, as compared to known configurations and methods of forming cooling channels. In turn, this enables use of the turbine blade in hot gas paths having a higher temperature, the use of a thinner thermal barrier coating, and a reduced cost, as compared to the use of nickel alloys. In some cases, cooling passages within the turbine blade may be simplified, since more of the active cooling of the turbine blade occurs at the blade surface. In addition, all cooling channels may be fabricated simultaneously, which reduces expense as compared to known methods of cooling channel formation, such as by water jet or electro-discharge machining. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, the invention provides a method of forming a cooling channel along a surface of a turbine blade, the method comprising: applying a first mask material to a first portion of a surface of a turbine blade; forming a first barrier layer atop the first mask material and atop a second portion of the surface of the turbine blade; removing the first mask material and the barrier layer atop the first mask material to expose the first portion of the surface of the turbine blade; and etching the first portion of the surface of the turbine blade to form a cooling channel along the surface of the turbine blade. 
     In another embodiment, the invention provides a method of coating a turbine blade, the method comprising: aluminizing a metal layer of the turbine blade surface; converting the aluminized metal layer to an aluminide layer; and removing aluminum from the aluminide layer, forming a porous metal layer. 
     In still another embodiment, the invention provides a turbine blade comprising: a nickel-based superalloy airfoil; an oxidized porous metal layer on a surface of the airfoil; and a thermal barrier coating over the oxidized porous material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
         FIG. 1  shows a perspective view of a turbine blade according to an embodiment of the invention. 
         FIG. 2  shows a flow diagram and cross-sectional side views of a method according to an embodiment of the invention. 
         FIG. 3  shows a flow diagram and cross-sectional side views of a method according to another embodiment of the invention. 
         FIGS. 4 and 5  show schematic top views of cooling channels formed according to embodiments of the invention. 
         FIG. 6  shows a cross-sectional side view of a step of a method according to an embodiment of the invention. 
         FIGS. 7-9  show schematic top views of cooling channels formed according to embodiments of the invention. 
         FIG. 10  shows a flow diagram of a method according to another embodiment of the invention. 
     
    
    
     It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a cross-sectional side view of a portion of a turbine blade  1  according to an embodiment of the invention. Turbine blade  1  includes a leading surface  8  and a trailing surface  10 . A plurality of cooling channels  20  have been formed along trailing surface  10  according to one method of the invention. A bond coat layer  70  and thermal barrier coating layer  72  are formed atop trailing surface  10  and cover the plurality of cooling channels  20 . Although cooling channels  20  are shown only along trailing surface  10  in  FIG. 1 , it should be appreciated that cooling channels may similarly be placed along leading surface  8  rather than or in addition to trailing surface  10 . 
       FIG. 2  shows a flow diagram and accompanying cross-sectional side views of a method according to one embodiment of the invention. At S 1 , a first mask material  30  is deposited atop a surface  10  of a turbine blade. Mask materials suitable for use according to embodiments of the invention include, for example, photoresist or a polymer material. First mask material  30  may be deposited using a number of methods or techniques, including, for example, dipping, spraying, or vapor deposition. The particular method or technique employed will depend, at least in part, on first mask material  30 . First mask material  30  may be discretely deposited or may be deposited across a larger area and then patterned. As shown in  FIG. 2 , first mask material  30  covers a first portion  12  of surface  10 , leaving a second portion  14  exposed. First portion  12  includes an area or areas of surface  10  in which cooling channels are to be formed. Second portion  14  includes areas of surface  10  in which cooling channels are not to be formed and may comprise some or all of surface  10  other than first portion  12 . One skilled in the art will recognize, of course, that materials and deposition techniques other than those disclosed may be employed. 
     At S 2 , a first barrier layer  40  is formed atop surface  10 , covering both first mask material  30  and second portion  14  of surface  10 . First barrier layer  40  may include, for example, Titanium oxynitride, TiO 2 , TaO 2 , TiN, SiO 2 , and high melting point oxides, such as aluminum oxide. First barrier layer  40  may be formed using any number of methods or techniques, including, for example, chemical vapor deposition, sputtering, or reactive sputtering. The particular method or technique employed will depend, at least in part, on first barrier layer  40 . At S 3 , first mask material  30  is removed, along with the portion of barrier layer  40  atop first mask material  30 , exposing first portion  12  of surface  10 . First portion  12  may then be etched at S 4  to form cooling channel  20  in surface  10 . Etching first portion  12  may include any number of methods or techniques, including, for example, liquid chemical etching and reactive ion etching. 
     In some embodiments of the invention, cooling channels  20  may be further processed to form overhanging structures above the cooling channels  20 . This effectively reduces an opening to the cooling channel  20 , which may be desirable in some circumstances.  FIG. 3  shows a flow diagram and accompanying cross-sectional side views of a method of forming such overhanging structures. At S 5 , cooling channel  20  is filled with a second mask material  32 . Second mask material  32  may be the same as first mask material  30  ( FIG. 2 ) or may be a different mask material. Similarly, second mask material  32  may be deposited using the same method or technique as first mask material  30  or by a different method or technique. 
     At S 6 , a high-temperature metal layer  50  is deposited, formed, or applied atop second mask material  32  and first barrier layer  40 . High-temperature metal layer  50  may include, for example, a nickel-based super alloy or a refractory metal and may be deposited, formed, or applied using any number of methods or techniques, such as vapor deposition, sputtering, or electrochemical deposition. 
     A third mask material  34  and second barrier layer  42  are then deposited or formed atop high-temperature metal layer  50  at S 7 . As can be seen in  FIG. 3 , third mask material  34  is deposited such that, in at least one dimension, its width is less than that of cooling channel  20 . The deposition or forming of third mask material  34  and second barrier layer  42  are similar to the deposition or forming of first mask material  30  and first barrier layer  40  in  FIG. 2 . Third mask material  34  may be the same as first mask material  30  or second mask material  32  or may be a different mask material and may be deposited using the same or a different method or technique. Similarly, second barrier layer  42  may be the same as first barrier layer  40  or may be a different mask material and may be deposited using the same or a different method or technique. 
     At S 8 , third mask material  34  and the portion of second barrier layer  42  atop third mask material  34  are removed, similar to the removal of first mask material  40  and a portion of first barrier layer  40  at S 3  of  FIG. 2 . At S 9 , high-temperature metal layer  50  is etched where exposed by the removal of third mask material  34  and second barrier layer  42 , forming an opening  22  through which second mask material  32  is removed from cooling channel  20 . As can be seen in  FIG. 3 , the smaller dimension of third mask material  34 , as compared to cooling channel  20 , results in overhangs  60 ,  62  of high-temperature metal layer  50  and second barrier layer  42  above cooling channel  20 . 
       FIG. 4  shows a top view of a cooling channel  20  according to one embodiment of the invention. For ease of illustration and explanation, only second barrier layer  42  is shown. One skilled in the art will recognize, however, that high-temperature metal layer  50  lies below second barrier layer  42 . In  FIG. 4 , overhangs  60 ,  62  reside adjacent opening  22  and over a portion of cooling channel  20 . Other configurations are possible. In  FIG. 5 , for example, overhang  60  is continuous around a substantially square opening  22 . 
       FIG. 6  shows a cross-sectional view of another embodiment of the invention. Here, opening  122  is offset and substantially flush with a wall  121  of cooling channel  120 . As such, a single overhang  160  is formed above cooling channel  120 .  FIGS. 7-9  show top views of various arrangements of opening  122  relative to cooling channel  120  according to such an embodiment. 
     In  FIGS. 3-9 , opening  22 ,  122  is shown as being substantially square- or rectangular-shaped. This is neither necessary nor essential, however, and openings formed according to the various embodiments of the invention may have any number of two-dimensional shapes. 
     In any of the embodiments of the invention, once surface  10 ,  110  is etched to form cooling channel  20 ,  120 , a metallic bond coat, such as MCrAlY, may be applied in a manner that is sufficient to cover first barrier layer  40  or second barrier layer  140 , as well as to cover the surfaces of, but not fill, cooling channel  20 ,  120 . Similarly, in any of the embodiments of the invention, the cooling channel  20 ,  120  formed may be joined to a source of cooling fluid, such as air or steam, for example, within the turbine blade  1  ( FIG. 1 ). For example, once cooling channel  20 ,  120  is formed, a passage may be formed, such as by drilling, from a bottom surface of the cooling channel  20 ,  120  through to a source of cooling air in the center of the turbine blade. 
     In some embodiments of the invention, high-temperature metal layer  50 ,  150  includes a porous metal layer. Use of such a porous metal layer reduces stress in a thermal barrier coating (TBC) applied to the turbine blade during later processing steps, since it is more compliant than either the turbine blade itself or the TBC. Porous metal layers also reduces the thermal diffusivity, as compared to a similar non-porous metal layers. This increases the temperature drop between the hot gas and the turbine blade. 
       FIG. 10  shows a flow diagram of a method of forming a porous metal layer on a turbine blade according to an embodiment. At S 10 , a metal layer, for example,  42  in  FIG. 3 , is aluminized. This may be achieved using any number of methods or techniques, including, for example, dipping the metal layer in an aluminum bath, spray depositing aluminum onto the metal layer, or vapor depositing aluminum onto the metal layer. 
     At S 11 , the aluminized metal layer is converted to an aluminide layer. Typically, this is achieved by heating the aluminized metal layer to a temperature between about 660° C. and about 1200° C. in the absence of oxygen. 
     At S 12 , aluminum is removed from the aluminide layer to form a porous metal layer. The aluminum may be removed using any number of methods or techniques, but is typically removed by applying a caustic solution to the aluminide layer. Where the metal layer was a nickel alloy, the porous metal layer thus formed comprises a porous nickel alloy layer. 
     A number of additional processes may be carried out on the porous metal layer. For example, at S 13 , the porous metal layer may optionally be passivated by oxidation. This may be desirable, for example, where the metal layer will be exposed to high temperatures, since the high surface area of the porous metal layer is likely to be pyrophoric. Oxidizing the porous metal layer may be achieved by, for example, heating in air around 400 C. 
     At S 14 , a bond coat and/or thermal barrier coating may optionally be applied to the porous metal layer formed at S 12  or the oxidized porous metal layer formed at S 13 . 
     As described herein, the porous metal layer is formed from high-temperature metal layer  50 ,  150 , although other metal layers may similarly be made porous to provide increased compliance. For example, the nickel-based superalloy of the turbine blade itself may be made porous using the method described above or a similar method. In addition, the turbine blade may be coated with a layer of a nickel-based heat resistant alloy which is then made porous using the method described above or a similar method. 
     In any case, additional layers may be deposited atop the porous metal layer to complete the finishing of the turbine blade. For example, in some embodiments of the invention, a turbine blade comprises a nickel-based superalloy airfoil, an oxidized porous metal layer on a surface of the airfoil, a bond coat, and a thermal barrier coating over the oxidized porous material. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any related or incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.