Abstract:
A method for coating a part includes applying a ceramic mask to a first surface portion of the part. A coating is applied to a second surface portion of the part and at least partially contacting the mask. The mask is destructively removed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation application of Ser. No. 11/267,965, filed Nov. 7, 2005, and entitled COATING METHODS AND APPARATUS USING PRE-FORMED CERAMIC MASK, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to protective coatings. More particularly, the invention relates to the application of protective coatings to metallic substrates. 
         [0003]    In the aerospace industry, it is known to apply protective coatings to metallic substrates. Exemplary coatings may be configured to provide one or more of wear protection, thermal protection, and chemical protection. Exemplary substrate materials are nickel-, cobalt-, and/or iron-based superalloys. Coating materials may vary by application. Exemplary wear coatings include carbide-based coatings and MCrAlY coatings. Such coatings may be applied as single layers or in multiple layers. MCrAlY coatings are also known for use as bond coat layers in multilayer thermal barrier coatings (e.g., having a ceramic top coat). Exemplary application techniques include high temperature spray methods such as high velocity oxy-fuel (HVOF) deposition. 
         [0004]    It is often desirable to mask areas of the substrate during application of the coating so that the masked portions of the substrate remain uncoated. As applied, the coating often bridges between the substrate and adjacent portions of the mask. To withstand the temperatures of deposition, exemplary masks are metallic. Adhesion between the coating and the mask may, upon mask removal, produce chipping, delamination, or other damage of the coating along the substrate. 
         [0005]    U.S. Pat. No. 6,592,948 of Fusaro, Jr. et al. identifies a multi-part/multi-step masking system for thermal barrier coatings. That exemplary system utilizes the combination of a metal foil sheet and a durable tape. 
       SUMMARY OF THE INVENTION 
       [0006]    A method for coating a part includes applying a ceramic mask to a first surface portion of the part. A coating is applied to a second surface portion of the part and at least partially contacting the mask. The mask is destructively removed. 
         [0007]    In various implementations, the mask may be held to the part by clamping. The applying may include inserting into a compartment. The mask removal may include shattering. The coating application may involve HVOF coating. The method may be used to replace or restore an initial worn or otherwise damaged coating. 
         [0008]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a sectional view of a compartment in a coated substrate. 
           [0010]      FIG. 2  is an enlarged view of a portion of the compartment of  FIG. 1 . 
           [0011]      FIG. 3  is a view of the portion of  FIG. 2  during application of a coating material. 
       
    
    
       [0012]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows a part  20  including a coated substrate  22 . An exemplary substrate may be of a nickel-, cobalt-, and/or iron-based superalloy. The exemplary part  20  may be an aerospace component. Exemplary components may be of gas turbine engines (e.g., blades, vanes, seals, structural components, shafts, disks, and the like). The exemplary part  20  is shown as having a compartment  24 , one portion of which is to be coated (e.g., using conventional or other HVOF apparatus) and another portion of which is to remain uncoated. Nevertheless, the present teachings may be applicable to other coated and uncoated portions of other parts and to other basic coating apparatus/methods. 
         [0014]    The exemplary compartment  24  may extend along a central axis  500  inward from an exterior surface  26  of the substrate. The exemplary compartment  24  may be a blind counterbored compartment having an inboard bore  28  having a base  30 . An outboard counterbore  32  may extend to the surface  26 . The counterbore  32  may be separated from the bore  28  by an annular shoulder surface  34 . 
         [0015]      FIG. 2  shows the bore  28  as having a lateral/side surface  40  at a radius R 1  from the axis  500 . The counterbore  32  may have a lateral/side surface  42  at a radius R 2 . Along a portion  44  the surface  42  may be uncoated. A coating  46  is formed on the surface  42  along a portion  48  outboard of the portion  44 . The coating  46  may have a radially inboard surface  50  at a radius R 3  from the axis  500 . The coating  46  may have a longitudinally inboard terminal/end surface  52 . 
         [0016]    To apply the coating  46 , it may be desirable to mask the shoulder surface  34  and the surface  42  along the inboard portion  44 . 
         [0017]    A mask  60  may be inserted into the compartment (e.g., in a direction  502 ). The exemplary mask  60  may be formed as an annular disk having a body extending from a radially inboard surface  62  at a central aperture to a radially outboard surface  64  at a perimeter. The mask has first and second faces  66  and  68  with a thickness T therebetween. The mask  60  may be placed in the compartment so that an inboard portion  70  extends over the bore  28 . Along an outboard portion  72 , the first face  66  may contact the shoulder surface  34 . The surface  64  may be in close facing or contacting relationship to the surface  42  along the inboard portion  44  of the counterbore. 
         [0018]    The mask may be retained in engagement with the substrate. Exemplary engagement is by non-adhesive mechanical means. Exemplary mechanical means include a clamp  400  engaging the inboard portion  70  and biasing the mask in the direction  502 . The exemplary clamp  400  may be a portion of a fixture supporting the substrate during coating application and/or additional steps. With the mask engaged, the coating  80  may be built-up (e.g., by HVOF spray). Exemplary coatings are carbide-based (e.g., consisting in majority weight part of one or more of WC, CrC, and NiCrC), NiCrCo, MCrAlY, NiAl, or other nickel alloy coatings. Exemplary HVOF spray apparatus and methods may involve substrate temperatures of 300-500° F. 
         [0019]    The as-sprayed coating  80  has a first portion  82  principally along the surface  42  and a bridging portion  84  protruding radially inward along the mask second face  68 . The as-applied coating has a compartment-facing surface  86  and a surface  88  along the mask. After coating application, it may be desirable to machine the surface  86  down to the diameter R 3 . During machining, vibration and/or direct contact of the machining tool with the mask  60  may tend to destroy the mask, preferably by shattering. By shattering the mask (distinguished from pulling off a metal mask), the chances of coating delamination from the substrate may be greatly reduced. The mask is advantageously sufficiently brittle to facilitate the shattering. The machining may occur with the clamp  400  engaged or disengaged. 
         [0020]    Exemplary mask materials include AlOx-based ceramics. The mask may be manufactured by molding a so-called green ceramic in the shape of the area to be masked and then firing the green ceramic to harden. Exemplary mask thicknesses are in excess of 0.001 inch, more narrowly at least 0.10 inch (e.g., 0.10-5 inches, or more). 
         [0021]    In addition to use of the mask  60  during an original coating, the mask may be used during a recoating or restoration. For example, wear or other damage to the surface  50  may bring such surface out of a desired specification (e.g., by expanding average R 3  or variations in R 3  beyond specified amounts). In such a recoating/restoration, the coating  46  may be fully or partially removed (e.g., by grit blasting). Thereafter, the mask  60  may be put in place and clamped. Additional coating may be applied to achieve similar results as described above. 
         [0022]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the methods may be used with existing parts and coating systems. In such situations, details of the existing parts and/or systems may influence details of any particular implementation. The methods may be used to apply coatings to parts or specific parts or types of parts where metallic or other dissimilar masks had previously been used. Additionally, the methods may be used in yet new situations. Accordingly, other embodiments are within the scope of the following claims.