Abstract:
A process for creating a near-surface cooling passage in an air-cooled turbomachine component. The process entails forming a channel in a surface of a surface region of the component so that the channel is open at the surface and fluidically connected to a first cooling passages within the component. A metallic layer is then deposited on the surface and over the channel without filling the channel. The metallic layer closes the channel at the surface of the surface region to define therewith a second cooling passage within the component that is fluidically connected to the first cooling passages. A coating system is then deposited on the metallic layer to define an outermost surface of the component. The second cooling passage is closer to the outermost surface of the component than the first cooling passages.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to components that operate at high temperatures, such as turbine airfoil components of turbomachinery. More particularly, this invention relates to a process of creating near-surface cooling channels in high-temperature components to promote the heat transfer characteristics of the components. 
         [0002]    Components of turbomachinery, such as buckets (blades), nozzles (vanes), and other hot gas path components of industrial and aircraft gas turbine engines, are typically formed of nickel, cobalt or iron-base superalloys with desirable mechanical and environmental properties for turbine operating temperatures and conditions. Because the efficiency of a turbomachine is dependent on its operating temperatures, there is a demand for components such as turbine buckets and nozzles to be capable of withstanding increasingly higher temperatures. As the maximum local temperature of a superalloy component approaches the melting temperature of the superalloy, forced air cooling becomes necessary. For this reason, airfoils of gas turbine buckets and nozzles often require complex cooling schemes in which air, typically bleed air, is forced through internal cooling passages within the airfoil and then discharged through cooling holes at the airfoil surface to transfer heat from the component. Cooling holes can also be configured so that cooling air serves to film cool the surrounding surface of the component. 
         [0003]    Buckets and nozzles formed by casting processes require cores to define the internal cooling passages. The cores and their potential for shifting during the casting process limits the extent to which a conventional casting process can locate a cooling passage in proximity to an exterior surface of the component. As a result, cooling passages are typically about 0.1 inch (about 2.5 millimeters) or more below a base metal surface of a cast turbine bucket or nozzle. However, the heat transfer efficiency could be significantly increased if the cooling passages could be placed closer to the surface than is currently possible. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    The present invention provides a process for creating one or more near-surface cooling passages in an air-cooled turbomachine component, notable but nonlimiting examples of which include buckets (blades), nozzles (vanes), shrouds, and other hot gas path components of gas turbines. 
         [0005]    According to a first aspect of the invention, the process entails forming a channel in a surface of a surface region of the component, so that the channel is open at the surface and fluidically connected to a first cooling passage within the component. A metallic layer is then deposited on the surface and over the channel without filling the channel. The metallic layer closes the channel at the surface of the surface region to define therewith a second cooling passage within the component that is fluidically connected to the first cooling passage and is closer to an outer surface of the metallic layer than the first cooling passage. A coating system is then deposited on the metallic layer to define an outermost surface of the component. The second cooling passage is closer to the outermost surface of the component than the first cooling passage. 
         [0006]    Another aspect of the invention is a component formed by a process comprising the steps described above. 
         [0007]    A technical effect of the invention is the ability to place a cooling passage within a cast component that is much closer to the component surface than cooling passages created with cores during the casting process. As a result, the invention has the capability of significantly increasing the heat transfer efficiency of a component, and particularly an air-cooled turbomachine component located in the hot gas path of a gas turbine engine. 
         [0008]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a high pressure turbine bucket of a type that can benefit from the present invention. 
           [0010]      FIG. 2  represents a partial cross-sectional view of a surface region of the bucket of  FIG. 1 , and depicts multiple channels defined in the surface of the surface region in accordance with an embodiment of this invention. 
           [0011]      FIG. 3  is a cross-sectional view representing a layer deposited over the channels of  FIG. 2 . 
           [0012]      FIG. 4  is a cross-sectional view representing an aluminized surface region in the layer of  FIG. 3 . 
           [0013]      FIG. 5  is a cross-sectional view representing a bond coat and thermal barrier coating deposited on the aluminized surface region in the layer of  FIG. 4 . 
           [0014]      FIG. 6  is a cross-sectional view representing a thermal barrier coating deposited directly on the aluminized surface region in the layer of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention is generally applicable to components that operate within environments characterized by relatively high temperatures, and particularly a component whose maximum surface temperature approaches the melting temperature of the material from which it is formed, necessitating the use of forced air cooling to reduce the component surface temperature. Notable examples of such components include the high and low pressure turbine buckets (blades), nozzles (vanes), shrouds, and other hot gas path components of a turbomachine, such as an industrial or aircraft gas turbine engine. 
         [0016]    A nonlimiting example of a turbine bucket  10  is represented in  FIG. 1 . The bucket  10  generally includes an airfoil  12  against which hot combustion gases are directed during operation of the gas turbine engine, and whose surface is therefore subjected to very high temperatures. The airfoil  12  is represented as configured to be anchored to a turbine disk (not shown) with a dovetail  14  formed on a root section of the bucket  10  that is separated from the airfoil  12  by a platform  16 . The airfoil  12  includes cooling holes  18  through which bleed air that enters the bucket  10  through its root section is forced to transfer heat from the bucket  10 . While the advantages of this invention will be described with reference to the bucket  10  shown in  FIG. 1 , the teachings of this invention are generally applicable to other hot gas path components of industrial and aircraft gas turbine engines, as well as a variety of other components that subjected to extreme temperatures. 
         [0017]      FIG. 2  represents an external surface region  22  of the bucket  10 , for example, a surface region of the airfoil  12  or platform  16  of the airfoil  12  in  FIG. 1 . The surface region  22  is typically the base material of the bucket  10 , for example, a nickel-, cobalt- or iron-based superalloy, notable but nonlimiting examples of which include nickel-based superalloys such as GTD-111® (General Electric Co.), GTD-444® (General Electric Co.), IN-738, René N4, René N5 and René 108. The bucket  10  may be formed as an equiaxed, directionally solidified (DS), or single crystal (SX) casting to withstand the high temperatures and stresses to which it is subjected within a gas turbine engine. Melting and casting processes suitable for producing the bucket  10  are well known and therefore will not be discussed here in any detail. 
         [0018]      FIG. 2  further represents multiple channels  23  that have been defined in the surface region  22  so that the channels  23  are open at the surface  24  of the region  22 . The channels  23  will subsequently define near-surface cooling passages ( FIGS. 5 and 6 ) within the bucket  10 , and therefore are desired to have a sufficient cross-sectional area to allow cooling air, such as compressor bleed air, to flow therethrough. For example, the channels  23  preferably have a width and depth (parallel and normal to the surface  24 , respectively) of up to about 0.1 inch (about 2.5 mm), with a typical range of about 0.01 to about 0.050 inch (about 0.25 to about 1.25 mm), though lesser and greater widths and depths are possible. Furthermore, the channels  23  preferably have a cross-sectional area of up to about 0.01 in 2  (about 6.5 mm 2 ), for example, about 0.0001 to about 0.0025 inch (about 0.065 to about 1.6 mm 2 ). The channels  23  are represented as having a rectangular cross-section, though it is foreseeable that cross-sectional shapes other than rectangular could be achieved for the channels  23 . However, a rectangular cross-section will be produced by various methods by which the channels  23  can be readily defined in the surface region  22 , for example, milling, wire EDM, milled EDM, waterjet trenching, and laser trenching. The channels  23  are represented as being formed in sets whose individual channels  23  are closer to each other than to channels  23  of adjacent sets. However, this configuration is not necessary, and other configurations are foreseeable. 
         [0019]    The channels  23  are formed in the surface  24  of the region  22  so as to be fluidically coupled to one or more cooling passages  28  (one of which is depicted in  FIGS. 2-6 ) that are located deeper beneath the surface  24 , as represented in  FIG. 2 . The cooling passage  28  receive cooling air, such as compressor bleed air, though one or more openings (not shown) in the root section of the bucket  10 , and then supply the cooling air to the channels  23  as well as the cooling holes  18 . As such, each cooling passage  28  preferably has a larger cross-sectional area than any of the channels  23 . The cooling passage  28  can be formed by conventional methods, for example, with cores employed in traditional casting methods used to cast the bucket  10 . The proximity of the cooling passage  28  relative to the cast surface  24  of the bucket  10  and eventually any outermost surface formed by a coating on the bucket  10  is limited by the ability to accurately place a core and maintain its position during the casting process, and in most cases will be about 0.1 inch (about 2.5 millimeters) or more from the casting surface  24 . 
         [0020]      FIG. 3  represents the result of applying a layer  30  over the casting surface  24  and its channels  23  to close the channels  23  at the surface  24 . The layer  30  can be applied over any portion of the bucket  10 , and particularly any external surfaces of the bucket  10 , though it is also possible to employing masking techniques so that the layer  30  is applied to just those surfaces of the bucket  10  in which the channels  23  are formed. As evident from  FIG. 3 , the channels  23  and layer  30  cooperate to define passages  26  that are internal to the bucket  10 . Because the channels  23  are separated from the surface  32  of the layer  30  by only the thickness of the layer  30 , the passages  26  are closer to the surface  32  of the layer  30  than the cooling passage  28  through which the passages  26  are fed cooling air. 
         [0021]    The layer  30  is preferably applied by a plating process to tightly adhere to the surface  24 . Notable plating techniques include electroplating and electroless plating, which are well known and therefore do not require any detailed discussion. To avoid plating material being deposited in the channels  23 ,  FIG. 3  further represents the channels  23  as being filled with a filler or masking material  34 . The masking material  34  is present during the deposition of the layer  30 , but is otherwise absent from the passages  26  prior to placing the bucket  10  in service. As such, the masking material  34  is preferably capable of being removed at some point after the layer  30  has been deposited, such as by melting the masking material  34 . Nonlimiting examples of suitable materials for this purpose include waxes, graphite, and other materials capable of filling the channels  23  and being plated over, while remaining removal by chemical or thermal treatments. As such, it is foreseeable that a variety of materials could be developed or otherwise identified for use as the masking material  34 . Plating methods are believed to be preferred processes for depositing the layer  30  in view of their relatively low processing temperatures that avoid prematurely melting the masking material  34 , the ability to plate surfaces of relatively complex shapes, the ability to accurately control the thickness of the deposited layer  30 , and the variety of materials that can be deposited by plating. However, it may be possible to adapt certain plasma spray techniques or brazing techniques to form the layer  30 . 
         [0022]    The composition of the layer  30  is preferably chemically and physically compatible with the material of the surface region  22 . As such, a particularly notable material for the layer  30  is nickel, a nickel-containing alloy, or a nickel-based alloy if the surface region  22  is formed of a nickel-based superalloy. For example, nickel can be deposited by a process by which particles of other elements can be dispersed in a nickel-based matrix. One such process is taught in U.S. Published Patent Application No. 2003/0211239, by which particles of chromium, aluminum, zirconium, hafnium, titanium, tantalum, silicon, calcium, iron, yttrium and/or gallium can be incorporated into a plated layer of nickel, cobalt and/or iron by a plating process. A desirable nickel-containing alloy that can be produced by a plating process is an MCrAlY-type coating, such as NiCoCrAlY. The thickness of the layer  30  affects the ability of cooling air flow through the passages  26  to cool the external surfaces of the bucket  10  subjected to the hot gas path. As such, the thickness of the layer  30  will typically be about 0.01 inch (about 250 micrometers) or less, though greater thicknesses are foreseeable. The thickness of the layer  30  will also affect the structural integrity of the surface region  22 , and as such a minimum thickness for the layer  30  will typically be about 0.005 inch (about 125 micrometers). While the composition of the layer  30  will determine its strength and thermal conductivity, it is believed that thicknesses in a range of about 0.005 to about 0.01 inch (about 125 to about 250 micrometers) will typically be suitable. 
         [0023]      FIG. 4  represents the result of removing the masking material  34  from the passages  26  and aluminizing the surface  32  of the layer  30  to form an aluminum-containing region  36  within the surface  32  of the layer  30 . The region  36  may be termed aluminum-rich, denoting that the region  36  contains a greater amount of aluminum (in atomic percent) than the substrate in which it is formed. The aluminizing process deposits aluminum and likely forms aluminides (aluminum intermetallics) on and beneath the surface  32  of the layer  30 . Various processes can be used to form the aluminum-containing region  36 , examples of which include those disclosed in U.S. Published Patent Application Nos. 2009/0214773 and 2009/0126833, though various other diffusion aluminide processes can be used similar to what is used to form diffusion aluminide bond coats and environmental coatings. 
         [0024]    The aluminizing of the surface  32  of the layer  30  is an optional but preferred step for several reasons relating to coating systems represented in  FIGS. 5 and 6 . In  FIG. 5 , a bond coat  38  is represented as having been deposited directly on the aluminum-containing region  36 , followed by a thermal barrier coating (TBC)  40  deposited on the bond coat  38 . In  FIG. 6 , a thermal barrier coating  42  is represented as having been deposited directly on the aluminum-containing region  36 , without an intervening bond coat. Typical but nonlimiting materials for the thermal barrier coatings  40  and  42  are ceramic materials, a notable example of which is zirconia partially or fully stabilized with yttria (YSZ) or another oxide such as magnesia, ceria, scandia and/or calcia, and optionally other oxides to reduce thermal conductivity. The thermal barrier coatings  40  and  42  are deposited to a thickness that is sufficient to provide a desired level of thermal protection for the underlying surface region  22  of the bucket  10 , generally on the order of about 75 to about 300 micrometers, though lesser and greater thicknesses are also possible. 
         [0025]    As is typical with TBC systems for components of gas turbine engines, the bond coat  38  is preferably an aluminum-containing composition, for example, an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, rare earth metals, and/or reactive metals), though the use of other bond coat compositions are also foreseeable. Aluminum-containing bond coats such as MCrAlX naturally develop an aluminum oxide (alumina) scale (not shown), which is capable of inhibiting oxidation of the surface it covers (such as the surface  32  of the layer  30 ), as well as capable of chemically bonding the thermal barrier coating  40  to the bond coat  38 . Particularly suitable MCrAlX coating materials typically contain about 5 weight percent or more of aluminum, though MCrAlX coatings containing less than 5 weight percent aluminum could also be used. The bond coat  38  typically has a thickness of about 12 to about 75 micrometers, though lesser and greater thicknesses are also possible. The bond coat  38  can be deposited by various processes, such as physical vapor deposition (PVD) processes and thermal spraying, with preferred processes believed to be thermal spray processes such as plasma spraying, HVOF (high velocity oxy-fuel) and wire arc spraying. 
         [0026]    If the layer  30  does not contain any aluminum, for example, a nickel or nickel alloy, the aluminum within the bond coat  38  is prone to diffuse into the layer  30 , depleting the aluminum content in the bond coat  38 . Eventually, the level of aluminum within the bond coat  38  could be sufficiently depleted to prevent further slow growth of the protective scale, allowing for the more rapid growth of nonprotective oxides and thereby reducing the ability of the bond coat  38  to provide oxidation resistance to the surface region  22  and adhere the thermal barrier coating  40 . Consequently, by creating the aluminum-containing region  36  within the surface  32  of the layer  30 , the chemical gradients that promote diffusion of aluminum from the bond coat  38  are reduced. 
         [0027]    In the embodiment of  FIG. 6 , the aluminum-containing region  36  replaces the bond coat  38  of  FIG. 5 , and an alumina scale that grows on the region  36  provides oxidation resistance and promotes the adhesion of the thermal barrier coating  42 . In this embodiment, the aluminum-containing region  36  is preferably deposited by a diffusion process to contain platinum aluminide (PtAl) intermetallics. 
         [0028]    The thermal barrier coatings  40  and  42  are represented in  FIGS. 5 and 6  as having different structures. The coating  40  represented in  FIG. 5  is deposited by a thermal spraying process, such as air plasma spraying (APS), by which softened particles deposit as “splats” on the deposition surface formed by the bond coat  38 , and result in the coating  40  having noncolumnar, irregular flattened grains and a degree of inhomogeneity and porosity. This category of thermal barrier coating includes coatings referred to as dense vertically cracked (DVC) TBCs, which are deposited by plasma spraying to have vertical microcracks to improve durability, as reported in U.S. Pat. Nos. 5,830,586, 5,897,921, 5,989,343 and 6,047,539. On the other hand, the coating  42  represented in  FIG. 6  is deposited by a PVD process, such as electron beam physical vapor deposition (EBPVD), which yields a columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Alternatively, the coating  42  of  FIG. 6  could be deposited as a thin film by a low pressure plasma spraying (LPPS) process, also known as vacuum plasma spraying (VPS). 
         [0029]    As a result of the process steps described above, the passages  26  defined by the channels  23  and layer  30  within the bucket  10  are near-surface cooling passages  26  that are closer to the outermost surface  44  of the bucket  10  (defined by one of the thermal barrier coatings  40  or  42 ) than the cooling passage  28  formed by conventional core methods during casting of the bucket  10 . Openings (not shown) may be formed in the passages  26  through which cooling air from the passage  28  is vented to the exterior of the bucket  10 , or the passages  26  may be fluidically connected to the cooling holes  18  present in the airfoil  12 . Because the distance between each passage  26  and the outermost surface  44  is determined by the layer  30 , bond coat  38  (if present), and thermal barrier coating  40  or  42 , and the combined thicknesses of these layers can be controlled by their respective deposition processes, the passages  26  can be two millimeters or less below the outermost surface  44  of the bucket  10 , more preferably about one millimeter or less below the outermost surface  44  for example, and can even be about 200 micrometers and less below the bucket&#39;s outermost surface  44 , each of which is significantly less than that possible with the conventional cooling passage  28  formed by a core using a traditional casting method. As such, the passages  26  are able to significantly increase the heat transfer efficiency of the bucket  10  in comparison to the cooling passage  28 . 
         [0030]    While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.