Patent Publication Number: US-2016222803-A1

Title: Method of simultaneously applying three different diffusion aluminide coatings to a single part

Description:
BACKGROUND 
     The present invention relates to methods for coating metal components, such as aerospace components. In particular, the present invention relates to methods for forming diffusion aluminide coatings on external surfaces and internal passageways that provide corrosion and oxidation resistance. 
     A gas turbine engine typically consists of an inlet, a compressor, a combustor, a turbine and an exhaust duct. The compressor draws in ambient air and increases its temperature and pressure. Fuel is added to the compressed air in the combustor where it is burned to raise gas temperature, thereby imparting energy to the gas stream. To increase gas turbine engine efficiency, it is desirable to increase the temperature of the gas entering the turbine. This requires the first stage turbine vanes and rotor blades to be able to withstand the thermal and oxidation conditions of the high temperature combustion gas during the course of operation. 
     To protect the first stage turbine vanes and rotor blades from extreme conditions, such components typically include coatings (e.g. aluminide coatings) that provide oxidation and corrosion resistance. Different aluminide coatings are applied from different solid and vapor sources using separate operations for each coating type. 
     SUMMARY 
     A method of simultaneously applying three different diffusion aluminide coatings to a metal part with an external surface and internal passageways includes first placing the part in an enclosed retort. Areas of the part are then coated with aluminum containing slurry for a first diffusion aluminide coating. Interior walls of the retort are then coated with a vapor source slurry for a second vapor phase aluminum coating on the surface of the part. Areas on the surface of the part not requiring coating are coated with masking materials. A closed chamber inside the retort containing source material for a third vapor phase diffusion aluminide coating for the internal passageways is then attached to the internal passageways. The retort is then placed in a furnace under a protective atmosphere and subjected to diffusion heat treat to coat the external surfaces and internal passageways with the three different aluminide coatings. 
     In an embodiment, a metal part having first, second and third outside surface areas and internal passageways has the first and second outside surface areas and internal passageway surface coated with different diffusion aluminide coatings during a single diffusion heat treatment. The first outside surface area is coated with a first aluminide coating. The second outside surface area is coated with a second aluminide coating. The third outside surface area is protected from coating with a masking material and the internal passageways are coated with a third diffusion aluminide coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a turbine blade with internal passageways. 
         FIG. 2  is a schematic of a coating apparatus of the invention. 
         FIG. 3  is a description of the coating process of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of a blade, In this embodiment, a turbine blade. In other embodiments, blade  10  may be a vane, nozzle, or other gas path component known in the art. Turbine blade  10  is comprised of airfoil  12 , root  14  (used to attach the blade to a rotatable turbine disc) and platform  16  located between the airfoil and root. Blade  10  may have internal passageways  17  through which engine air is directed to cool the blade during operation in the hot gas path of a turbine engine. Cooling holes  18  may be connected to the internal passageways wherein the cooling air emerges as a gas film to further cool the blade during operation. Turbine blades and vanes particularly in high pressure turbines (HPT) and low pressure turbines (LPT) require coatings to protect the components from the oxidative and erosive environment of the gas path. In an embodiment, blade  10  may be a Ni base, Co base, Fe base superalloy or mixtures thereof. It should be understood that the invention is not limited to blades and vanes and that other turbine components are included. 
     One family of coatings comprises aluminum containing aluminide coatings wherein the intermetallic phase beta—NiAl forms on the surface of a component. NiAl is highly oxidation resistant and also acts as a supplier of aluminum atoms to form a protective aluminum oxide layer on the surface of the component. The aluminum coatings typically contain Cr, Ti, Ni, and/or Si. The alloying elements control the activity of aluminum during coating. Low activity coatings form via the outward diffusion of solutes from the underlying component into an aluminum containing layer deposited on the component to form a coating. More specifically, Ni diffuses out to an aluminum rich deposited layer to form NiAl during a diffusion anneal. Outward type coatings are characterized by a two zone microstructure known to those in the art. High activity coatings form via inward diffusion of aluminum into the component to form NiAl during the diffusion anneal. Both high and low activity aluminide coatings rely on a carrier such as a complex aluminum halide or an alkali or alkaline earth metal to transport the aluminum to the substrate in the form of a gas wherein upon contact the carrier gas reacts to deposit aluminum on the surface. This process is discussed in U.S. Pat. No. 4,132,816 to Benden et al. Inward type diffusion aluminide coatings may be deposited directly from a slurry containing an aluminum alloy source and a binder or from a slurry containing an aluminum alloy source, a halide activator and a binder. Inward type coatings have a three zone microstructure known to those in the art. Outward type diffusion aluminide coatings may be formed by vapor phase aluminiding (VPA) wherein the aluminiding vapor source may be a slurry containing an aluminum alloy source, a halide activator and a binder, or a concentration of vapor source material in the form of particles or pellets containing an aluminide alloy and a halide activator. In both cases, the slurry and particles are composed of an aluminum alloy source of aluminum, a halide activator that forms an aluminum halide gas or a complex aluminum halide gas and, in the case of the slurry, a binder, typically an organic solution. 
     Both inward type and outward type aluminide coatings may be formed from a slurry applied directly to a part and subsequently dried. The thickness of an aluminide coating as known in the art depends on the slurry composition, thickness, diffusion temperature, and time. High activity coatings such as PWA 73 and PWA 273, as described in U.S. Pat. No. 5,366,765 to Milaniak, form inward type coatings. An aluminum alloy source may be an intermetallic compound such as Co 2 Al 5  and the activator may be ammonium chloride. Outward type diffusion aluminide coatings are a low activity coating such as PWA 545 as described in U.S. Pat. Nos. 6,022,362 and 6,045,863 to Olsen et al. The aluminum source may be pure aluminum and the activator may be aluminum trifluoride. 
     Coating separate areas of the surface of a component with different diffusion aluminide coatings with different thicknesses and microstructures usually involve multiple thermal cycles. If two or more areas on a single component could be coated with different diffusion aluminide coatings in a single operation, significant time, cost and energy savings would be accomplished. In the present invention, three separate areas of a turbine component including outside surface areas and interior passageways are coated with three different diffusion aluminide coatings in a single operation. 
     A schematic illustrating the method of the present invention is shown in  FIG. 2 . Apparatus  20  comprises turbine blade  10  positioned in retort  22 . Retort  22  comprises retort wall  24  and lid  26  creating a closed chamber for applying the diffusion aluminide coatings of the invention to blade  10 . 
     With reference to  FIG. 2 , coating process  50  is shown in  FIG. 3 . The surfaces of blade  10  are first prepared for coating by mechanical and chemical cleaning procedures known in the art. (Step  52 ). 
     A first diffusion aluminide coating source slurry is then directly applied to certain areas of blade  10  such as area  36  in  FIG. 2  (Step  54 ). The inner walls of cylinder  28  on support  30 , surrounding blade  10  in retort  22  are then coated with second diffusion aluminide vapor source slurry  34  (Step  56 ). Vapor from vapor source  34  on the inside wall of cylinder  28  will coat area  37  on blade  12 . Outside areas of blade  10  not requiring diffusion aluminide coating are masked with masking material (Step  58 ). In  FIG. 2 , root  14  of blade  10  is covered with masking material  38 . As shown in  FIG. 2 , blade  10  is positioned such that internal passageways  17  of blade  10  are attached to closed chamber  32 . Closed internal chamber  32  contains receptacle  40  containing diffusion aluminide vapor source material  42  for coating the internal passageway surfaces (Step  60 ). An inert carrying gas entering closed internal chamber  32  as indicated by arrow  44  transports the aluminizing vapor through port  46  in support  30  into the internal passageways  17  of blade  10 . 
     Blade  10  is then placed in retort  22  as shown in  FIG. 2 . Apparatus  20  is then placed in a diffusion heat treat furnace under a protective atmosphere. (Step  69 ). In one embodiment, apparatus  20  is heated to about 1800° F. to about 2100° F. for from about 2 hours to about 10 hours in an argon atmosphere and removed from the furnace and cooled under argon. (Step  66 ). 
     During the diffusion heat treatment, area  36  on blade  10  will be coated with an aluminide coating by the slurry directly applied to that area. In an embodiment, area  36  may be coated with an inward type aluminide coating. Outside areas  37  not coated with a slurry or a maskant (area  38 ) will be coated with vapor emanating from vapor source slurry  34  coated on cylinder wall  28 . In an embodiment, these areas may be coated with an outward type low activity aluminide coating. At the same time, carrier gas  44  entering closed chamber  32  carries aluminizing vapor from vapor source material  42  into internal passageways  17  of blade  10  through port  46  in support  30 . In an embodiment, the diffusion aluminide coating on the internal passageways may be an outward type coating. 
     Following the diffusion anneal, the retort is removed from the furnace and the component is removed from the retort (Step  68 ). 
     DISCUSSION OF POSSIBLE EMBODIMENTS 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A method of simultaneous application of three different diffusion aluminide coatings to a metal part with an external surface and internal passageways may include: placing the part in a retort; applying an aluminum containing source slurry for a first diffusion aluminide coating to a first area of the external surface of the part; coating the interior walls of the retort with a vapor phase coating source slurry for a second vapor phase aluminide coating on a second surface of the part; masking areas not requiring diffusion aluminide coating with masking material; and attaching a closed chamber inside the retort containing source material for a third vapor phase diffusion aluminide coating to the internal passageways of the part. 
     The method of the preceding paragraph may optionally include, additionally and/or alternatively any, one or more of the following features, configurations and/or additional components: 
     The retort is placed in a furnace under a protective atmosphere and a diffusion heat treat may be performed to coat the external surface and internal passageways of the part with the three diffusion aluminide coatings. 
     The aluminide coatings may include outward type and inward type coatings. 
     The diffusion heat treat may be between about 1800° F. and about 2100° F. 
     The diffusion heat treat may be for a period of between about 2 hours and about 10 hours. 
     The first diffusion aluminide coating may be an inward type coating. 
     The second vapor phase diffusion aluminide coating may be an outward type coating. 
     The third vapor phase diffusion aluminide coating may be an outward type coating. 
     The part may be a turbine component. 
     The component may be a blade or a vane. 
     The metal may be a nickel-based, cobalt-based, iron-based superalloy or mixtures thereof. 
     A metal part with first and second outside surface areas and internal passageways wherein the first and second outside surface areas and the internal passageways are ready to be coated with different diffusion aluminide coatings formed during a single diffusion heat treatment may include: a first outside surface area coated with an inward type diffusion aluminide coating; a second outside surface area coated with an outward type diffusion aluminide coating; and internal passageways coated with an outward type diffusion aluminide coating. 
     The metal part of the preceding paragraph may optionally include, additionally and/or alternatively any, one or more of the following features, configurations and/or additional components: 
     The part may be in a closed retort. 
     Areas not requiring aluminide coating may be masked with masking material. 
     The inward type diffusion aluminide coating on the first outside area may be formed from a coating source slurry directly applied to the first outside surface area. 
     The outward type diffusion aluminide coatings on the internal passageways may be formed from vapor supplied from a vapor source slurry material in a closed internal chamber in the closed retort. 
     The diffusion heat treat may be between about 1800° F. and about 2100° F. The diffusion heat treat may be for a period of between about 2 hours and about 10 hours. 
     The part may be a turbine component. 
     The metal may be a nickel-based, cobalt-based, iron-based superalloy or mixtures thereof. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.