Abstract:
Selected areas of a component are covered with a maskant chamber during a coating process to protect the areas from the coating vapor. The covered areas are further protected by a flow of an inert gas in the maskant chamber.

Description:
BACKGROUND 
     The present invention relates to diffusion coating processes. In particular, the present invention relates to maskant free diffusion coating processes of selected areas. 
     Diffusion coating processes, such as diffusion aluminide coating processes, are used to apply protective coatings over metal components in a variety of industries. For example, turbine engine components are typically diffusion coated with aluminum based alloys to form aluminide coatings that protect the underlying engine components from oxidation and other environmental elements. During a diffusion coating process, it is desirable to selectively coat portions of a metal component, while allowing other portions to remain uncoated. 
     One technique for selectively coating a metal component involves the use of maskant film which is applied over a desired location. Another technique is to cover portions of the component with maskant powder that protects the covered portions from the coating vapor. 
     Following coating, the maskant needs to be removed from the component, typically by abrasive means, and the surface washed and dried. These steps add to the process time and substantially reduce the throughput of metal components. 
     SUMMARY 
     A method of applying a metal vapor coating to a selected area of a metal component while preventing the vapor from contacting other areas of the component is presented. The protected areas are encased in a masking chamber filled with flowing inert gas during vapor deposition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a gas turbine blade. 
         FIG. 2  is a flowchart of a prior art diffusion coating process. 
         FIG. 3  is a sketch of a prior art masking box used for coating a turbine blade airfoil and radially outward facing surface of a platform. 
         FIG. 4  is a schematic crosssection of a masking box of the invention. 
         FIG. 5  is a flowchart of the diffusion coating process of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 1 , turbine blade  10  includes airfoil  12 , serrated blade root  14  (used to attach the blade to a rotatable turbine disk), and platform  16  located between airfoil  12  and serrated root  14 . Platform  16  has a radially outward facing surface  19 , a radially inward facing surface  20 , side faces  17 , and end faces  18 . The region between underside  20  of blade platform  16  and root  14  is referred to as neck  15 . 
     The invention is used in conjunction with the application of a diffusion coating to airfoil portions  12  and radially outward facing portions  19  of platform  16 . Root  14 , neck  15 , radially inward facing underside portion  20  of platform  16 , side faces  17 , and end faces  18  are desirably kept free of coating. 
     In diffusion coating processes, aluminum based materials, chrome based materials, and silicon based materials are mixed with a halide activator and heated to form gaseous metal halide compounds which result in the deposition of the metal on the surface of the part to be coated. In an embodiment, suitable diffusion coating materials are aluminum based materials (e.g. aluminides). During heating, the aluminum based materials react with the halide activators to form gaseous metal halide compounds (e.g. aluminum halide compounds). Suitable temperatures are from about 1200° F. (about 650° C.) to about 2000° F. (about 1100° C.). The gaseous metal halide compounds decompose upon contact with the surfaces of the part, thereby depositing the diffusion coating on the surface of the part. The deposition of the diffusion coating correspondingly releases the halide activator to form additional gaseous metal halide compounds while the source of the diffusion coating material is still available. 
     Prior art masking techniques include gettering agents that decompose the gaseous metal halide compounds that deposit on the maskant and prevent the diffusion coating from forming on the underlying surface of the metal component. The maskant can be applied to the surface as a spray, paint, decal, and other techniques known in the art. One technique is described in commonly owned U.S. Pat. No. 7,763,326 and incorporated herein as reference. In another embodiment, portions of the part to be coated can be packed in a loose powder of the maskant. 
     An example of a prior art aluminide diffusion coating process wherein only airfoil  12  and radially outward facing portion  19  of platform  16  are diffusion coated is shown in  FIG. 2 . In this example, the process includes steps  30 ,  32 ,  34 ,  36 ,  38 ,  40 , and  42 . 
     To begin, turbine blade  10  is cleaned before coating using techniques well known in the art (Step  30 ). In the next step, the blade is inserted in masking box  50  shown in  FIG. 3  (Step  32 ). Masking box  50  comprises top  52  and bottom  54 . Top  52  has a cutout that closely matches the outline of platform  16  when blade  10  is inserted in top  52 . Side faces  17  and end faces  18  closely match side faces  56  and end faces  58  of top  52 . Masking box  50  is preferably formed of refractory ceramic or metal. Cooling passages at the bottom of root  14  (not shown) are then connected to an argon gas manifold (not shown) (Step  34 ). 
     Top  52  containing blade  10  and the argon connection is then filled with maskant powder and bottom  54  is attached (Step  36 ). Bottom  54  has an opening to accommodate the gas manifold. Examples of suitable commercially available gettering agents include those under the trade designation “M1 Maskant”, “M7 Maskant”, “M8 Maskant”, and “M10 Maskant” from APV Coatings, Akron, Ohio. As noted below, maskants are gettering materials that, when placed on a substrate exposed to gaseous metal halide compounds, decompose the metal halide compounds and are coated with aluminum thereby preventing the substrate from being coated. 
     Blade  10  and masking box  50  are then exposed to a gaseous metal halide compound (e.g. aluminide halide compound) formed by the decomposition of a mixture of an aluminum based powder and a halide activator powder in a closed furnace container or retort at a suitable elevated temperature to coat exposed blade  12  and top of platform  19  with aluminum based materials (Step  40 ). Suitable temperatures for initiating the reaction range from about 1200° F. (about 650° C.) to about 2000° F. (about 1100° C.). The aluminum based compound may be an aluminum intermetallic compound. Examples of suitable aluminum intermetallic compounds for use in the diffusion coating process include chromium-aluminum (CrAl) alloys, cobalt-aluminum (CoAl) alloys, chromium-cobalt-aluminum (CrCoAl) alloys, and combinations thereof. Examples of suitable concentrations of the aluminum based compound in the powder mixture range from about 1% by weight to about 40% by weight. 
     The halide activator is a compound capable of reacting with the aluminum based compound during the diffusion coating process. Examples of suitable halide activators for use in the diffusion coating process include aluminum fluoride (AlF 3 ), ammonium fluoride (NH 4 F), ammonium chloride (NH 4 Cl), and combinations thereof. Examples of suitable concentrations of the halide activator in the powder mixture range from about 1% by weight to about 50% by weight. 
     The powder mixture may also include inert materials such as aluminum oxide powder. The furnace container or retort may also contain one or more gases (e.g. H 2  and Argon) to obtain a desired pressure and reaction concentration during the diffusion coating process. 
     The elevated temperature initiates a reaction between the aluminum based compounds and the halide activators to form gaseous aluminum halide compounds which decompose at surfaces  12  and  19  of blade  10  to deposit aluminum in a molten state which then interdiffuses with surfaces  12  and  19  of blade  10 . The diffusion coating process continues until a desired coating thickness is reached, preferably between 25 microns and 125 microns. 
     Following diffusion coating, blade  10  and masking box  50  are cooled and blade  10  is removed from masking box  50  and detached from the argon manifold (Step  40 ). Any maskant powder remaining on the blade is then removed in a final cleaning process (Step  42 ). Preferably the powder is removed by an abrasive spray process followed by a water rinse. 
     An advantage of the present invention is the elimination of the use of maskant powder in the above-mentioned diffusion coating process. Rather than connecting an argon manifold to a cooling gas port in the bottom of blade  10  in masking box  50 , alternative masking box  60  was designed to allow an inert gas source to be directly connected to the bottom of masking box  60  as schematically shown in  FIG. 4 . The aluminide diffusion coating process of the invention is shown in  FIG. 5 . To begin with, turbine blade  10  is cleaned before coating using techniques well known in the art (Step  70 ). In the next step, the blade is inserted in masking box  60  as shown in  FIG. 4  (Step  72 ). An inert gas source is then connected to inlet port  62  such that the inert gas flows in the direction of arrow  64  and fills masking box  60  thereby protecting the portion of blade  10  inside masking box  60  from diffusion coating. Furthermore, the inert gas flows out through cooling channels in blade  10  thereby protecting the cooling channels from the diffusion coating. Masking box  60  and turbine blade  10  are then placed in a furnace or retort for diffusion coating (Step  76 ). The same diffusion coating process as described earlier is used in the process of the invention. As noted, inert gas prevents entry of any gaseous metal halide compounds into masking box  60  during a diffusion coating process of surfaces  12  and  19  of blade  10 . Argon, nitrogen, or hydrogen can be used as an inert gas. The inert gas, preferably argon, is maintained at a positive pressure to keep the gaseous aluminum halide coating vapors out of masking box  60  during coating. The positive pressure further assures that cooling circuitry in the blade is filled with inert gas during coating thereby preventing ingress of gaseous aluminum halide compounds in the cooling channels during coating. As a result, the use of M1 Maskant or other maskant powders has been eliminated in the diffusion coating process of the invention. In addition, the setup and cleaning procedures associated with other steps of the process have been eliminated as well thereby improving the throughput and efficiency of the coating process. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. 
     Following diffusion coating, blade  10  is removed from masking box  60  (Step  78 ). Since no maskant powder is used in the invention, the abrasive cleaning step of the prior art process is eliminated and the coated blade is simply washed (Step  80 ).