GB2320929A

GB2320929A - Electric arc spray process for applying a heat transfer enhancement metallic coating

Info

Publication number: GB2320929A
Application number: GB9726859A
Authority: GB
Inventors: Bhupendra Kumar Gupta; Thomas John Tomlinson; Gilbert Farmer; Mark Gerard Reitig
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1997-01-02
Filing date: 1997-12-19
Publication date: 1998-07-08
Anticipated expiration: 2017-12-19
Also published as: FR2757879B1; FR2757879A1; GB9726859D0; GB2320929B

Abstract

A method is disclosed for applying a highly rough thermally conductive metallic coating 1 to a substrate 2 by electric arc spraying an alloy in the form of a feed wire from a group of alloys consisting of Ni-base or Co-base materials. The electric arc spray conditions are selected to produce a sufficiently rough texture to the surface of the coating to enhance the cooling efficiency of the substrate material. The arc spray method provides a coating thickness of between 0.005-0.025 inches with an associated surface roughness average (Ra) of at least 500 microinches. The application of the thermally conductive metallic coating is followed by a heat treatment of the component to improve further the bonding between the coating and the substrate.

Description

ELECTRIC ARC SPRAY PROCESS FOR APPLYING A HEAT TRANSFER ENHANCEMENT METALLIC COATING This invention relates to a method for applying a highly rough thermally conductive coating to a substrate and an article formulated by the same. More specifically, the invention relates to an electric arc wire process to deposit a highly rough thermally conductive metallic coating to gas turbine engine components to enhance heat transfer.

The propulsive thrust of a gas turbine engine can be increased by allowing the internal operating temperatures to increase. Limitations in an engine component's material performance, however, prevent unchecked increases in the operating temperature of a gas turbine engine. To further increase the maximum temperature at which a component can operate, cooling air is applied to a component to reduce the local temperature of the component. The cooling effectiveness increases further if the component also incorporates a thermally conductive coating applied to the surface in contact with the cooling air. The thermally conductive coating increases the heat transfer coefficient of the component by increasing the surface area in contact with the cooling air. The increase in surface area directly increases the efficiency and effectiveness of the heat transfer between the component and the cooling air.

Various techniques are available to apply thermally conductive coatings to a substrate material. One such technique is thermal spraying.

The specific thermal spray process chosen depends on the type of coating being applied. Thermal spraying can be one of four types: flame spraying, plasma spraying, detonation spraying, and electric arc spraying. For applying thermally conductive metallic coatings such as used on gas turbine engine components, electric arc spraying is a preferred method. In addition to strong bond strength, electric arc spraying offers low cost, ease of application including large components, and lower overall capital investment compared to other spraying processes.

Electric arc spraying is a thermal spray process in which one or more wires of either similar or dissimilar materials are melted, atomized and the molten particles are propelled onto a prepared surface to build up a metallic coating. The thermal energy required to melt the wire is produced by an electric arc developed at the wire ends. A high velocity gas stream is used to atomize the molten metal in the arc and propel the fine droplets onto the surface to be coated.

Arc spray equipment may be used to apply different types of coatings including conductive coatings, corrosion protection coatings, wear resistant coatings, or resurfacing coatings.

The temperature of the arc may be up to tens of thousands of degrees Fahrenheit. Because of these temperatures the particles, when accelerated, impact and bond to the minute protrusions of a properly cleaned and roughened substrate, producing a high coating adhesion to the substrate and strong inner particle cohesive strength.

Optimizing coating material and coating topography improves the cooling efficiency of the component. Higher cooling efficiency benefits a gas turbine engine in several ways: lower component operating temperature; longer component life; and lighter-weight components. Industry-standard process conditions for electric arc spraying typically form a coating of uniform thickness. A uniform thickness coating limits the cooling efficiency of the conductive coating. A sufficiently rough and irregular surface coating would provide more surface area for the cooling air to contact, thereby increasing cooling efficiency of the coating. Accordingly, there is a need for improving the process conditions of electric arc spraying to provide a sufficiently rough surface coating to increase the cooling efficiency of the coating. The invention described in the following disclosure and claims is directed towards such an improvement.

The present invention is a method for applying a highly rough thermally conductive metallic coating to a substrate by the electric arc spraying of an alloy in the form of a feed wire from a group of alloys consisting of Ni-base or Co-base. A NiCrAlY feed wire is one example of the feed wire material used to coat the substrate.

The electric arc spray conditions are optimized to provide a sufficiently rough texture to the surface of the thermally conductive metallic coating. The disclosed process conditions will apply a rough coating to various substrates for different applications. However, the rough coating has special application to components used in gas turbine engines. For instance, the rough coating enhances the cooling efficiency of gas turbine engine parts such as combustor liners, exhaust liners, flaps and seals, and turbine frames. The arc spray method provides a coating thickness in the range of 0.005-0.025 inches with an associated surface roughness average(Ra) of at least 500 microinches or higher. The application of the thermally conductive metallic coating is followed by a heat treatment to the component to improve further the bonding between the coating and the substrate.

In order that the invention may be clearly understood, it will now be described in more detail with reference to the FIGURE of the accompanying drawing showing a schematic representation of the rough surface coating as applied to a substrate.

The disclosed invention applies a metallic coating to a substrate using an electric arc spray device. Under appropriate process conditions, the coating as applied is sufficiently rough to promote efficient cooling by air directed over the coating.

The embodiment of the present invention is especially designed for use with gas turbine engine components that are in fluid communication with hot gases on one side of the component and in fluid communication with cooling air on the other side.

The gas turbine engine components that have such characteristics include, but are not limited to, combustor liners, exhaust liners, flaps and seals, and turbine frames.

Referring now to the drawing, the Figure shows the thermally conductive metallic coating 1 with a thickness, t, applied to a representative gas turbine engine component 2. The engine component contemplated for this invention typically operates in an environment in which one side of the component operates in fluid communication with the hot engine gases 3 and the other side of the component operates in fluid communication with directed cooling air 4. The hot side of the component is protected from the hot gases by an applied thermal barrier coating (ThC)5.

The surface of the engine component to which the thermally conductive metallic coating 1 will be applied is typically roughened by grit blasting or some other roughening technique. One preferred technique uses 60-120 grit under a pressure of 60120 psi. This prepares the surface to more readily accept the molten droplets sprayed onto the surface from the electric arc spray device and increases the surface area of the coating 1 to promote adhesion. The thermally conductive metallic coating 1 is applied via an electric arc spraying device.

The electric arc spraying device is positioned between 2-8 inches from the component surface with 3-4 inches the preferred spraying distance. Two pieces of feed wire of a desired coating material, which in a particularly preferred embodiment are composed of an oxidation resistant Ni-base or Cobase material, are fed into the spraying device.

The feed wire material is fed into the electric arc spray device at a rate in the range of 3 pounds per hour to 9 pounds per hour, where a preferred rate is 5.5 pounds per hour. NiCrAlY is the preferred feed wire composition. The opposing tips of the two feed wires are positioned close to one another while an electric current between 100-500 amps, where the range of 150-250 amps is preferred, is applied to the feed wire sufficient to create an arc between the tips of the two feed wires.

Consequently, the feed wire material is heated to produce molten droplets. Positioned directly behind the arc and in a line with the arc is a nozzle through which an atomizing gas flows at high velocity. The atomizing gas is typically compressed air or an inert gas. The atomizing gas carries the molten droplets of feed wire material to the surface of the engine component.

As the molten droplets are being propelled by the atomizing gas, the engine component is translated relative to the stream of molten coating material so the coating is applied over a desired section of the component. Depending on the translation speed, a layer of coating between 0.001-0.007 inches thick is applied during each pass. Accordingly, approximately 1-10 passes are required to achieve a coating thickness, t, between 0.005-0.025 inches with an associated roughness average (Ra) of 500-1500 microinches.

The typical electric arc spraying parameters yield a relatively smooth coating of material on the substrate. The parameters disclosed in this invention yield a much rougher surface profile which means a larger surface area over which cooling air will pass. The increased surface roughness of the thermally conductive metallic coating increases the heat transfer coefficient (hc) between 40-60 relative to the hc of a smooth coating. The amount of roughness produced by the electric arc spraying process is dependent on the material of the feed wire, the electric current through the feed wire, and the spray distance. High electric current and optimized spray distance will lead to larger molten drops. The larger molten drops produce greater coating roughness.

The application of the thermally conductive metallic coating is followed by a heat treatment of the component to improve further the bonding between the coating and the substrate. The heat treatment is conducted at a temperature in the range of 1700 and 2000 degrees Fahrenheit for 2 to 8 hours, where a preferred temperature is 1975 degrees Fahrenheit applied for 4 hours.

Claims

1. A method for applying a metallic coating to a surface of a substrate by electric arc spraying comprising the steps of: supplying metallic material from a group of desired coating materials as a feed wire to an electric arc sprayer; applying an electrical current in the range of 100-500 amps to the said feed wire, thereby melting the end of said feed wire; flowing a stream of gas over said end of said feed wire to form droplets of said metallic material; projecting said droplets a distance of between 2 and 8 inches to deposit said droplets onto said surface of said substrate; allowing said droplets to solidify to form a metallic coating on said surface of said substrate with a predetermined coating thickness and roughness average; and applying a heat treatment to said coated surface.

2. The method of claim 1, wherein: said metallic material is selected from the group comprising Ni-base and Co-base materials.

3. The method of claim 2, wherein: said metallic material is NiCrAlY.

4. The method of claim 1, wherein: said roughness average of said metallic coating is 500-1500 microinches.

5. The method of claim 1, wherein: said heat treatment step comprises: subjecting said metallic coating and said substrate to a temperature in the range of 1700 to 2000 degrees Fahrenheit for 2 to 8 hours to improve the bonding between said metallic coating and said substrate.

6. The method of claim 1, wherein: the current applied to the said feed wire is in the range 150-250 amps.

7. The method of claim 1, wherein: the molten droplets are projected over a distance of 3-4 inches.

8. The method of claim I, wherein: said coating thickness is in a range of 0.0050.025 inches.

9. An article comprising: a generally elongated sheet with first and second opposed sides, a heat conductive layer with first and second opposed sides where said second side is bonded to said first side of said elongated sheet, and said conductive layer has a substantially roughened surface texture with a predetermined thickness and roughness average.

10. The article of claim 9, wherein: said heat conductive layer comprises material selected from the group consisting of Ni-base and Co-base materials.

11. The article of claim 10, wherein: said material for the heat conductive layer comprises NiCrAlY.

12. The article of claim 9, wherein: said thickness is between 0.005-0.025 inches and said roughness average is between 500-1500 microinches.