Methods for the production and design of cooling channels and corresponding products

Abstract

Method for modifying the shape of a channel (20, 31, 42) in a metallic substrate (10, 33, 40) comprising the step of applying at least one metallic coating to selected sections of an inner surface of the channel (20, 31, 42) in order to change the heat transfer properties of the channel (20, 31, 42) during the passage of a coolant fluid through it; wherein the metallic coating is applied to the sections of the inner surface by a thermal spraying process or by a cold spraying process; and wherein at least one layer of the metallic coating is applied to selected sections of the inner channel surface in order to improve the heat transfer properties of the surface, and wherein at least one second layer of an oxidation-resistant coating is subsequently applied uniformly over the entire inner channel surface in order to impart oxidation resistance to the channel (20, 31, 42).

Description

BACKGROUND

[0001] The general subject matter of this invention relates to cooling channels for objects used at high temperatures, such as gas turbines. Some specific embodiments relate to methods for manufacturing and designing the cooling channels.

[0002] A gas turbine contains a compressor where the air is pressurized. The gas turbine also includes a burner where the pressurized air is mixed with fuel to produce hot combustion gases. In a typical design (e.g., for aircraft engines or stationary power systems), energy is extracted from the gases in a high-pressure turbine (HPT), which drives the compressor, and in a low-pressure turbine (LPT). The low-pressure turbine drives a fan in a turbofan aircraft engine application or drives an external shaft for marine and industrial applications.

[0003] The need to cool systems in gas turbines is critical because gas turbines typically operate in extremely hot environments. For example, gas turbine components are often exposed to hot gases with temperatures up to approximately 2093°C (3800°F) for aircraft applications and up to approximately 1482°C (2700°F) for stationary power generation applications. To cool the components exposed to these hot gases, these "hot gas path" components typically incorporate both internal convection cooling and external film cooling.

[0004] Many aspects of cooling circuits and cooling features in various hot gas path components have been described in engineering. For example, the combustion chamber contains radial outer and inner linings that require cooling during operation. Turbine guide vanes contain hollow guide vanes held between an outer and an inner band, which also require cooling. Turbine rotor blades are hollow and typically contain cooling circuits within them, with the blades surrounded by turbine casing rings that likewise require cooling. The hot combustion gases are discharged through an outlet, which may also be lined and appropriately cooled.

[0005] In all these exemplary gas turbine components, thin metal walls made of high-strength superalloy metals are typically used for improved durability while minimizing the need for cooling. Various cooling circuits and features are specifically designed for these individual components in their respective environments within the gas turbine. For example, a series of internal cooling passages or serpentine channels may be formed in a hot gas path component. A cooling fluid can be supplied to the serpentines from a collection chamber, and the cooling fluid can flow through the passages, cooling the substrate and coatings of the hot gas path components. While this type of cooling design can be effective in some cases, its use in other situations can lead to relatively low heat transfer rates and uneven temperature profiles of the components.

[0006] Microchannel cooling (as the feature is explained below) has the potential to significantly reduce cooling requirements by positioning the cooling system as close as possible to the hot zone, thus reducing the temperature difference between the hot and cold sides for a given heat transfer rate. However, current microchannel generation techniques typically require the use of a sacrificial filler to prevent the coating from depositing within the microchannels (which usually need to be created by machining). The sacrificial filler also carries the coating during application.

[0007] The specific path of a fluid through a cooling circuit is crucial to ensuring that the coolant effectively carries heat away from regions exposed to the highest temperatures experienced during the operation of a given turbine or other device. For example, a microchannel can be considered a three-dimensional region with various configurations, including both channel sidewalls and some type of bottom surface. Each surface of the microchannel can be exposed to a different temperature profile during machine operation, depending in part on the curvature, roughness, and overall design of the cooling channel. Supplying more coolant than necessary to an area with a low temperature profile would be inefficient and could deprive other high-temperature surfaces of sufficient coolant.This is particularly true when the amount of available cooling air is limited. In the case of a turbine, for example, cooling air often has to be drawn from the gas turbine compressor, and the gas turbine efficiency can suffer if too much cooling air is diverted for the cooling circuit.

[0008] Several techniques can be employed to influence the coolant flow characteristics within a cooling channel. For example, a selected cooling channel design could be achieved during the casting of the specific component in which the channel is located. Furthermore, machining techniques could be used to modify the depth and shape of an existing cooling channel. Examples of such methods include laser drilling, waterjet cutting, and electrical discharge machining (EDM).

[0009] Machining techniques can be effective in modifying the shape of a cooling channel to its advantage. However, there are disadvantages associated with such methods. For example, they can be time-consuming, which increases the cost of component manufacturing. Furthermore, special care must sometimes be taken to ensure that the machining processes do not adversely affect the integrity of the component.

[0010] US 7 722 325 B2 discloses an article, e.g. a gas turbine component, consisting of a substrate made of a refractory metal with cooling channels, wherein the inner surface of the cooling channels is equipped with a guide for directing a coolant made of a refractory metal.

[0011] DE 698 07 892 T2 discloses a gas turbine rotor blade with a substrate and at least one fluid flow channel, wherein the substrate is a nickel-based alloy and wherein a coating of MCrAlY is applied to the substrate by plasma spraying, which enters the opening of the fluid flow channel.

[0012] DE 10 2013 111 874 A1 discloses a component of a gas turbine system with undercut shaped cooling channels in the substrate of the component and a manufacturing process for it, wherein the inner surfaces of the cooling channels are coated by means of thermal spraying processes.

[0013] Further methods for modifying the shape of a cooling channel would be welcome in engineering. These new methods should be capable of modifying the channel in a way that improves its heat transfer properties. They should also be capable of efficient implementation and compatibility with other processes involved in the manufacture of the specific component. Furthermore, shaping methods that also improve other properties of the cooling channel, such as the oxidation resistance of the channel surfaces, would be of particular interest. BRIEF SUMMARY OF THE INVENTION

[0014] A method for modifying the shape of a channel in a metallic substrate is described. The method includes the step of applying at least one metallic coating to selected sections of an inner surface of the channel in order to modify the heat transfer properties of the channel during the passage of a cooling fluid through it. The metallic coating is applied to the sections of the inner surface by a thermal spraying process or by a cold spraying process. At least one layer of the metallic coating is applied to selected sections of the inner channel surface to improve the surface's heat transfer properties, and at least one second layer of an oxidation-resistant coating is subsequently applied uniformly over the entire inner channel surface to provide the channel with oxidation resistance.

[0015] The thermal spraying process can be a flame spraying process or a plasma spraying process.

[0016] The flame spraying process can be selected from the group that includes high-velocity flame spraying with oxygen as the oxidizing agent (HVOF) and high-velocity flame spraying with air as the oxidizing agent (HVAF).

[0017] Furthermore, the oxidation-resistant coating may have a metal aluminide composition or an MCrAlX composition, wherein “M” is iron, nickel, cobalt or combinations thereof; and “X” may be yttrium, tantalum, silicon, hafnium, titanium, zirconium, boron, carbon or combinations thereof.

[0018] In any of the aforementioned methods, the channel may be a microchannel comprising a section of a cooling circuit; and the metallic substrate may be a high-temperature component containing the cooling circuit.

[0019] The microchannel can terminate at an exit point on a surface of the substrate.

[0020] Alternatively or additionally, the microchannel can be undercut.

[0021] The undercut microchannel of the aforementioned method can contain a bottom surface and side walls that are essentially perpendicular or angled relative to the bottom surface.

[0022] Additionally, the coating can be applied to a section of the bottom surface of the undercut channel according to a pattern that increases the amount of coolant fluid that comes into contact with the side walls, while decreasing the amount of coolant fluid that comes into contact with the bottom surface, in order to enhance heat transfer away from the side walls.

[0023] In any method involving a microchannel terminating at an exit point on a substrate surface, the metallic coating can be applied to sections of the channel's inner surface in a non-uniform pattern, selected according to optimized heat transfer properties for the microchannel during the passage of a coolant fluid.

[0024] In the process of any of the aforementioned types, the substrate can be a section of a turbine component.

[0025] Another aspect of the invention relates to an object in the form of a high-temperature substrate. The object has at least one microchannel arranged within an inner region of the substrate as part of a cooling circuit, the microchannel terminating at an outlet on a surface of the substrate. The microchannel has an inner surface, with a metallic coating material applied to at least one section of the surface. The coating material is designed to improve the heat transfer properties through sections of the microchannel and to provide oxidation resistance to the inner surface of the microchannel. The metallic coating is applied to the sections of the inner surface by a thermal spraying process or by a cold spraying process.At least one layer of the metallic coating is applied to selected sections of the inner channel surface to improve the heat transfer properties of the surface, and at least one second layer of an oxidation-resistant coating is then applied uniformly over the entire inner channel surface to give the channel oxidation resistance.

[0026] In the aforementioned item, the coating material can impart oxidation resistance to essentially the entire inner surface of the microchannel.

[0027] The microchannel of any of the aforementioned objects can terminate at an exit point on the surface of the substrate.

[0028] Additionally or alternatively, the metallic material may have been applied using a cold spraying technique.

[0029] The object of any of the aforementioned types may be in the form of a gas turbine component. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 shows a schematic cross-section of a section of a cooling circuit with undercut cooling channels. Fig. Figure 2 shows an enlarged view of one of the undercut cooling channels. Fig. 1. Fig. Figure 3 shows a microscopic representation of a cross-section of an undercut channel formed on a superalloy substrate. Fig. Figure 4 shows another microscopic view of a cross-section of an undercut channel formed on a superalloy substrate. Fig. Figure 5 shows a schematic representation of a turbine system. DETAILED DESCRIPTION OF THE INVENTION

[0030] When introducing elements of different embodiments of the present invention, the articles "one," "an," "the," and "the aforementioned" mean that there is one or more of the elements, unless otherwise specified. The terms "have," "contain," and "have" are to be understood inclusively and mean that there may be additional elements besides those listed. As used herein, the term "and / or" includes any and all combinations of one or more of the related listed elements. Unless otherwise specified, the terms "arranged on," "applied to," or "arranged between" refer both to direct contact between layers, objects, and the like, and to indirect contact, e.g., with intervening layers.

[0031] Approximation terms, as used in the description and claims, may be applied to modify any quantitative representation that may permissibly vary without resulting in a change to the underlying function with which it may be related. Accordingly, a value modified by a term such as "approximately" is not limited to the exact value stated. In some cases, the approximation terms may correspond to the accuracy of an instrument used to measure the value.

[0032] Any substrate exposed to high temperatures and requiring cooling can be used for this invention. Examples include ceramics or metal-based materials. Non-limiting examples of metals or metal alloys that could form the substrate include steel, aluminum, titanium, refractory metals such as molybdenum, and superalloys such as those based on nickel, cobalt, or iron. The substrate can also be formed from a composite material, such as an intermetallic niobium silicide composite. Very often, the substrate is at least one wall or other surface of a gas turbine component, e.g., a gas turbine blade.

[0033] Fig. Figure 1 shows a schematic cross-section of a section of a cooling circuit within a substrate 10. The substrate 10 can represent a section of any high-temperature component 12, as described above, e.g., a turbine blade. The substrate 10 can include an outer surface 14 and an inner surface 16. (The outer surface can be covered with one or more top layers, as described below.) The inner surface 16 lies above a hollow interior 18, which itself communicates with an inner region of the specific component.

[0034] Each channel or groove 20 can be considered an outlet point for the specific cooling path. Furthermore, in this embodiment, each channel 20 includes a base 22 and side walls 24. However, the Fig. The shape of channel 20 shown is merely an example; many other shapes and channel sizes are possible.

[0035] While the term “channels” is used in this disclosure for the sake of simplicity, the preferred types of narrow channels for many types of end-use applications are also commonly referred to as “microchannels.” For industrial-sized power-generating turbine components, “narrow” or “micro” channel dimensions would include depths and widths in the range of about 0.25 mm to about 1.5 mm. For aircraft-sized turbine components, channel dimensions would include depths and widths in the range of about 0.1 mm to about 0.5 mm.

[0036] With further reference to Fig. Each channel 20 communicates with an access hole 26. The access holes 26 transfer a coolant flowing from and through the inner space 18, e.g., a collection chamber, which carries bypass air from a compressor. Further information about and illustrations of microchannels are provided in US 2012 / 0 114 912 A1, which is incorporated herein by reference. US 8,387,245 B2 is also instructive and is incorporated herein by reference. The patent specification describes various aspects of channel cooling circuits in which coolant access holes originating from inner regions of a part communicate with coolant channels that may terminate at outlets, e.g., "film outlet holes." It should be understood that the present invention can be applied to many different designs of cooling channels and cooling circuits.Furthermore, the channel design described herein can be implemented in many embodiments over the entire length of the channel or any section thereof, including the outlet point.

[0037] In this illustration, the channels 20 are formed as undercut openings. Experts in the field understand that this type of shape requires the base 22 of the channel 20 to be much wider than its upper opening 23. In some cases, the width of the base 22 can be approximately 3-4 times the width of the upper opening. There are several advantages to this type of shape. For example, it reduces the likelihood of excessive amounts of a coating material subsequently applied over the outer surface 14 being deposited in the channels 20, which would undesirably block the cooling passages.

[0038] Fig. Figure 2 shows in a schematic cross-section one of the undercut channels 20. Fig. Figure 1 (enlarged) shows the base / bottom surface 22, the top opening 23, and the side walls 24. (The bottom surface 22 and the side walls 24 can be collectively referred to as the "inner surface" of the channel 20.) In the case of a gas turbine operating at very high temperatures, some of the designed cooling configurations cause the side walls 24 to experience significantly higher thermal stress compared to the bottom surface 22. To counteract this "thermal imbalance," the side walls 24 would benefit from a greater exposure to a coolant fluid compared to the bottom surface 22.

[0039] Embodiments of the present invention are based on the discovery that the heat transfer properties of the channel 20 can be improved by using coatings applied to selected sections of the inner surface 28 of the channel 20. applied, could be balanced. As an example, controlled application (as simply shown by arrows 28 in Fig. (2 shown) of a coating material on at least a portion of the bottom surface 22 of the channel 20, to reduce the amount of coolant fluid flowing over the surface 22. This, in turn, results in a proportionally larger quantity and higher velocity of coolant fluid (arrows 30) flowing along all or a portion of the inner side walls 24 of the channel 20. In some cooling circuit configurations, the increased flow along the side walls 24 can result in a greater heat transfer rate from these surfaces, which can be highly desirable. (As those skilled in the art understand, different cooling circuit designs modified according to this invention can result in different pressure conditions across the cooling channels, which in turn can affect the coolant flow characteristics.)

[0040] The coating material can be applied to the inner surface 28 of the channel 20 using a number of techniques. For example, a thermal spraying process can be used. Examples include flame spraying and plasma spraying. In flame spraying, a stream of hot gas and coating powder is directed onto the substrate surface. The coating powder partially melts in the stream and is deposited on the substrate. The resulting coating typically exhibits low porosity and high bond strength. The shape of the coatings resulting from these techniques depends, in part, on the interaction of the sprayed material with the existing dimensions of the channel being modified.As further explained below, laboratory model tests conducted beforehand can generally predict the resulting heat transfer properties for the modified channel, thus serving as a guide or standard for additional coating applications. Flame spraying techniques are also highly adaptable for coating high-temperature components, such as turbine blades. Specific examples of flame spraying techniques include high-velocity oxygen-based flame spraying (HVOF) and high-velocity air-based flame spraying (HVAF).

[0041] In some preferred embodiments, a cold spray technique can be used to apply the coating material to the inner surface of the channel. Cold spray techniques are known in the prior art and described in various publications, such as "J. VILLAFUERTE: Recent trends in cold spray technology: Looking at the future. In: Surface Engineering, Vol. 26, 2010, No. 6, pp. 393-394. DOI:10.1179 / 026708410X12687356948715", which is incorporated herein by reference. Other publications include US 2013 / 0 002 010 A1 (Amancherla et al.) and US 2013 / 0 153 089 A1 (Ajdelsztajn et al.) and the parallel pending patent application US 2015 / 0 147 479 A1, all of which are incorporated herein by reference. In very general terms, "cold spraying" is a solid-state coating process, i.e., the particles do not melt during the process.Cold spraying uses a high-speed gas jet to accelerate metallic powder particles toward a substrate, causing the particles to deform plastically and compact upon impact. An advantage of using cold spraying in some specific embodiments of the invention is the ability to precisely apply a coating material with desired dimensions to specified areas of the channel.

[0042] As described in Ajdelsztajn et al. (as listed above), typical cold spraying techniques use a spray gun that carries a high-pressure gas, such as helium, nitrogen, or air, along with a base material, e.g., metals, alloys, or composites in powder form. The powder granules are introduced into a gas stream in the spray gun at high pressure and ejected from a suitable nozzle. The particles are accelerated to high speeds within the gas stream, potentially reaching supersonic speeds. The gas stream can be heated. Typically, the gases are heated to a temperature lower than the melting point of the particles to minimize oxidation during flight and phase changes in the applied material. In some embodiments, the carrier gas is maintained at a temperature ranging from about 20°C to about 1200°C.The velocity of the starting material during cold spraying is often in the range of approximately 500 m / s to approximately 1100 m / s.

[0043] As mentioned above, some of the spray systems described herein, such as cold spray systems, can be computer-controlled during the application process. For example, experts in the field understand that a spray system may contain or be connected to a multi-axis numerically controlled (CNC) unit or similar devices that operate on this principle. CNC systems themselves are well-established in engineering and allow the spray gun to move along a number of axes—X, Y, and Z—as well as around rotary axes. In this way, the desired height, width, and length of each coating feature within a channel can be precisely achieved. Since non-uniform patterns of the coating material may sometimes be required to modify the heat transfer properties of a channel in the most advantageous way, this precision in coating application can offer a significant advantage.

[0044] Fig. Figure 3 shows a microscopic view of a cross-section of a conventional undercut channel 31 formed from a nickel-based superalloy substrate 33. The channel 31 was initially produced on the substrate 33 by a machining technique, although various casting or other forming procedures could have been used. The channel 31 includes side walls 34 and a bottom surface 36. In this figure, a single protective coating 37 has been applied over the top surface of the substrate 33. As described below, the protective coating 37 is often a metallic coating, and several can be applied as a single coating, depending on the requirements for the specific component.

[0045] The in Fig. The undercut shape shown in Figure 3 provides several advantages for the outlet area of ​​the microchannels. One advantage is that the relatively narrow upper opening 38 of the channel 31 prevents or minimizes the introduction of the coating material into the channel 31, where it would otherwise block the coolant flow (unless removed). In this particular prior art figure, only a very small accumulation 39 of the coating material was deposited within the channel 31, and this is generally considered a particular processing advantage.

[0046] Fig. Figure 4 shows a microscopic view of a cross-section of an undercut channel 42 according to embodiments of the present invention. The channel 42 is produced on a superalloy substrate 40 and is, as shown in Fig. 3, initially produced by a machine processing technique. The channel 42 contains side walls 44 and a bottom surface 46. The channel 42 has been effectively "sealed" by applying the coating 48 to the surface of the substrate 40.

[0047] In the embodiment according to Fig. 4 A coating material was intentionally applied to the bottom surface 46 of the channel 42, forming a raised area or “heap” 50. The coating material for this feature was an MCrAlY-type coating (such as those described below) applied by a conventional HVOF process.

[0048] This feature 50 in Fig. 4 serves in part to direct cooling air away from the bottom surface 46 – an area where the presence of the coolant may not be critical. Instead, the coolant is effectively transferred to the side walls 44 of the channel 42 – areas where the presence of the coolant may often be more critical. Thus, heat can be transferred more effectively away from the side walls 44. In the case of a turbine, as described above, the overall turbine efficiency can be improved by reducing the amount of cooling air that needs to be drawn from the turbine compressor.

[0049] Furthermore, it should be noted that feature 50 could be configured more precisely than in the Fig. 4. The exemplary microscopic representation explained above demonstrates the use of CNC systems or similar tools and a cold spraying technique, as described above. In general, the shape of feature 50 can vary considerably and will, of course, depend largely on its location and end use for the coolant channel. In many cases, the most suitable shape can be determined based on modeling or other test procedures performed prior to modifying a coolant channel. As a very general example, laboratory model tests could be conducted on coolant flow through a number of sample channels with selected outlet shapes. The tests could measure coolant flow, heat transfer, and other properties to determine which channel shape is most effective for a desired level of thermal performance, such as heat transfer efficiency.Based on the test data, the techniques described above can be used to modify the shape of channel 42 as appropriate. Although feature 50 has been applied in a central area of ​​channel 42, there may also be situations in which application at another location, e.g., closer to one or the other of the side walls, is preferred.

[0050] In some embodiments, which may be very useful for certain applications, the coating applied to the interior of the channel could be an oxidation-resistant coating. Conventional oxidation-resistant coatings could be used, but specialty coatings used for high-temperature applications are often of particular interest. Examples of such coatings include metal aluminide compositions (e.g., nickel aluminide or platinum aluminide) and an MCrAlX composition. In the case of the MCrAlX coatings, "M" can be iron, nickel, cobalt, or combinations thereof, and X can be yttrium, Y, Ta, Si, Hf, Ti, Zr, B, C, or combinations thereof. Non-limiting examples of some of these materials can be found in various publications, such as US 6,234,755 B1, which is incorporated herein by reference.

[0051] It may sometimes be desirable to initially apply at least a relatively thin and uniform oxidation-resistant coating to most or all of the internal surface of the channels. The specific coating thickness (total) would depend on a number of factors, but in the case of turbine components, it would typically range from about 10 micrometers to about 50 micrometers. After this coating has been applied, additional coating material could then be applied over the first layer(s), but only to specific, selected areas of the internal surface, in order to influence heat transfer patterns, as explained above.

[0052] Alternatively, the application process can be reversed, i.e., a "shaping" application is first applied to selected sections of the inner surface, followed by a thin, uniform, oxidation-resistant coating over the entire surface. In either case, the application process for the top layer can also be used in some instances to advantageously influence the texture of the coating surface, e.g., its roughness properties, in order to further improve heat transfer characteristics.

[0053] The application of a uniform, oxidation-resistant coating to the interior of a channel has been carried out in the prior art. Typically, various vapor-phase techniques were effective in creating such a coating. However, these processes would not be effective in creating the precise, shaped coating features that can be obtained by embodiments of the present invention, which can often be described as "non-uniform" or "non-standard".

[0054] As mentioned above, at least one upper layer is applied over the upper part of the channels in order to seal them (“bridge”) and complete their formation. Layer 48 in Fig. Figure 4 is an example. The top layer can be produced from a suitable metallic material, such as MCrAlX (as described above), or it can be produced from another composition. Thermal spraying techniques could be used to apply the top layer, although other techniques, such as plasma-based processes, are also possible. The thickness of the top layer can vary and depends in part on factors such as the overall integrity required for the channel coating system. Furthermore, it is sometimes advantageous to apply one or more top layers at an angle relative to the substrate surface, as described in US 2012 / 0 114 912 A1, referenced previously.

[0055] In other embodiments, the top layer could be in the form of multiple layers. For example, a metallic layer could first be applied to seal the channels, followed by a ceramic layer. The ceramic layer could be a yttrium-stabilized zirconia material, serving as a thermal barrier coating (TBC) for the substrate. Experts in the field are familiar with coating processes for applying TBCs.

[0056] In some cases, a microchannel may already be formed, possessing an internal surface already shaped for a specific type and quantity of coolant flow. The microchannel may have been created by conventional machining techniques, as described above, or by the casting process used to produce the part. In this case, the process implementations described herein could be used to modify the shape if, for example, a different type of coolant flow were to be directed through the hot gas path component. In this way, modified heat transfer properties suitable for the new situation can be obtained.

[0057] As mentioned above, embodiments of this invention relate to channel features incorporated into sections of a high-temperature substrate. The substrate can take on a number of shapes and is sometimes a turbine, e.g., a gas turbine. Fig. Figure 5 shows a schematic representation of a typical gas turbine system 60. The system 60 can include one or more compressors 62, combustion chambers 64, turbines 66, and fuel nozzles 68. The compressor 62 and the turbine 66 can be connected by one or more shafts 70. The shaft can be a single shaft or several shaft segments connected together.

[0058] As mentioned above, the gas turbine system 60 can include a number of hot gas path components. Rotor blade assemblies (also referred to as blades or blade assemblies), guide vane assemblies (also referred to as guide vanes or guide vane assemblies), shroud or casing assemblies, transition pieces, retaining rings, and compressor outlet components are all typical hot gas path components. In fact, the hot gas path component can be any component that is at least partially exposed to a high-temperature gas flow. The patterned / modified microchannels of this invention can be efficiently incorporated into many of these hot gas path components, providing the associated advantages described herein.

[0059] Although only certain features of the invention have been illustrated and described herein, many modifications and alterations will be obvious to those skilled in the field. It should therefore be understood that the appended claims are intended to encompass all such modifications and alterations that fall within the true scope of the invention.

[0060] A method for modifying the shape of a channel in a metallic substrate is described. The method includes the step of applying at least one metallic coating to selected sections of an inner surface of the channel in order to modify the heat transfer properties of the channel during the passage of a coolant fluid through it. Corresponding objects containing the modified channels are also described, such as gas turbine components.

Claims

[1] Method for modifying the shape of a channel (20, 31, 42) in a metallic substrate (10, 33, 40) comprising the step of applying at least one metallic coating to selected sections of an inner surface of the channel (20, 31, 42) in order to change the heat transfer properties of the channel (20, 31, 42) during the passage of a coolant fluid through it; wherein the metallic coating is applied to the sections of the inner surface by a thermal spraying process or by a cold spraying process; and wherein at least one layer of the metallic coating is applied to selected sections of the inner channel surface in order to improve the heat transfer properties of the surface, and wherein at least one second layer of an oxidation-resistant coating is subsequently applied uniformly over the entire inner channel surface in order to impart oxidation resistance to the channel (20, 31, 42). [2] Method according to claim 1, wherein the thermal spraying method is a flame spraying method or a plasma spraying method; wherein the flame spraying method is preferably selected from the group which includes high-velocity flame spraying methods using oxygen as the oxidizing agent (HVOF) and high-velocity flame spraying methods using air as the oxidizing agent (HVAF). [3] Method according to claim 1 or 2, wherein the oxidation-resistant coating comprises a metal aluminide composition or an MCrAlX composition, wherein ‘M’ may be iron, nickel, cobalt or combinations thereof and X is yttrium, tantalum, silicon, hafnium, titanium, zirconium, boron, carbon or combinations thereof. [4] Method according to any one of the preceding claims, wherein the channel (20, 31, 42) is a microchannel (20, 31, 42) comprising a section of a cooling circuit; and wherein the metallic substrate (10, 33, 40) is a high-temperature component containing the cooling circuit. [5] Method according to claim 4, wherein the microchannel (20, 31, 42) terminates at an exit point on a surface of the substrate (10, 33, 40); and / or wherein the microchannel (20, 31, 42) is designed to be undercut. [6] Method according to claim 4, wherein the channel (20, 31, 42) is an undercut microchannel (20, 31, 42) comprising a bottom surface (22, 36, 46) and side walls (24, 34, 44) which are substantially perpendicular or angled relative to the bottom surface (22, 36, 46); wherein the coating is applied to a section of the bottom surface (22, 36, 46) of the undercut channel (20, 31, 42) according to a pattern which increases the amount of coolant fluid that comes into contact with the side walls (24, 34, 44) while decreasing the amount of coolant fluid that comes into contact with the bottom surface (22, 36, 46) in order to enhance heat transfer away from the side walls (24, 34, 44). [7] Method according to claim 5, wherein the metallic coating is applied to sections of the inner surface of the channel (20, 31, 42) in a non-uniform pattern selected according to optimized heat transfer properties for the microchannel during the passage of the coolant fluid. [8] An object in the form of a high-temperature substrate (10, 33, 40) comprising at least one microchannel (20, 31, 42) arranged within an inner region of the high-temperature substrate (10, 33, 40) as part of a cooling circuit, wherein the microchannel (20, 31, 42) includes an inner surface on which a metallic coating material is applied, wherein the coating material is designed in a manner that both improves the heat transfer properties through sections of the microchannel (20, 31, 42) and provides oxidation resistance to selected regions of the inner surface of the microchannel (20, 31, 42); wherein the metallic coating is applied to the sections of the inner surface by a thermal spraying process or by a cold spraying process; and wherein at least one layer of the metallic coating is applied to selected sections of the inner channel surface in order to improve the heat transfer properties of the surface, and wherein at least one second layer of an oxidation-resistant coating is subsequently applied uniformly over the entire inner channel surface in order to impart oxidation resistance to the microchannel (20, 31, 42). [9] Subject matter according to claim 8 in the form of a gas turbine component.

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