Abstract:
Processes and systems for forming a coating system on a component. The process of forming the coating system on the component includes placing an apparatus in a location that promotes coating particles in flight to be redirected towards a surface on the component. The surface is obstructed by portions of the component limiting line-of-sight from a source of the coating particles to the surface. The coating particles are then deposited onto the surface of the component. The coating particles initially travel in a direction of initial particle travel and are redirected by the apparatus towards the surface on the component at a direction of final particle travel relative to the surface. The line-of-site from the source of the coating particles is at an angle of less than 30 degrees relative to the surface of the component and the direction of final particle travel is at an angle of 30 degrees or more relative to the surface of the component.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/670,171, filed Jul. 11, 2012, the contents of which are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under Contract No. N00019-04-C-0093 awarded by U.S. Government (Department of Defense, Air Force). The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention generally relates to coating systems and processes for their deposition. More particularly, this invention relates to a process and system for forming a coating on a component by redirecting coating particles during a spray deposition process. 
         [0004]    Various coating processes have been developed to deposit metallic and ceramic coating materials capable of surviving and remaining adherent in chemically and thermally hostile environments such as those of a gas turbine. Examples include thermal spraying, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Thermal spraying processes are line-of-sight processes. In the thermal spray process a stream of plasma containing metallic or ceramic particles exits a spray nozzle (“gun”) at a high velocity and high temperature in the direction of an article on whose surface the particles are deposited. The intention of the coating is to protect the article with a coating that shows complete coverage over the surface and has a consistent microstructure. Typically, the stream of particles travels line-of-sight to deposit on the surface of the article. 
         [0005]    The line-of-sight accessibility of articles can be a major limitation in the design of gas turbine engine components. To illustrate,  FIG. 1  represents ceramic or metallic coating particles  16  being deposited on seal teeth  12  of a gas turbine component  10 . The coating particles  16  are schematically represented as being deposited on the seal teeth  12  by a nozzle  14  of a thermal spraying device. Due to the limited line-of-sight of the nozzle  14  to the component  10 , the coating particles  16  may be unable to uniformly coat the seal teeth  12 . A seal tooth  12  that has been coated by a process similar to what is represented in  FIG. 1  is shown in  FIG. 2 . The resulting coating is not uniform, and shows areas on the surface of the seal tooth  12  with almost no coating. Generally, with thermal spray processes line-of-sight access to a surface to be coated must be at an angle of at least 30 degrees relative to the surface to obtain a coating with conforming microstructure along with complete coverage over the surface. Anything less than a 30 degrees access angle will likely result in a coating structure that is nonconforming to specifications and has intermittent coating coverage, such as shown in  FIG. 2 . 
         [0006]    Even coatings sprayed at an access angle of approximately 30 degrees may have marginally acceptable coatings requiring significant amounts of rework. Further, with restricted line-of-sight accessibility, the robustness of the coating quality is reduced and may not be repeatable. Both of these issues introduce a significant amount of variation into the thermal spray process. 
         [0007]    Presently, in instances where the direct-line-of-sight access is restricted to less than 30 degrees, engineers must resort to other processes to deposit the coating or must design around a nonconforming coating with intermittent coverage. Other potential processes include plating the surface of the component  10 . In some instances, depending on the risk, the surface of a component  10  may be uncoated. Historically, components have also been designed to account for line-of-sight limitations of coating deposition processes to achieve increased spray access angles, though potentially at the expense of weight or performance. 
         [0008]    Accordingly, there is a need for a spray process capable of depositing a ceramic or metallic coating on a component in situations where the line-of-site access angle to the surface to be coated is less than 30 degrees. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    The present invention provides processes and systems for forming a coating on a component when the line-of-site access angle to a surface of the component to be coated is less than 30 degrees. 
         [0010]    According to a first aspect of the invention, a process of forming a coating system on a component includes placing an apparatus in a location that promotes coating particles in flight to be redirected towards a surface on the component. The surface is obstructed by portions of the component limiting line-of-sight from a source of the coating particles to the surface. The coating particles are then deposited onto the surface of the component. The coating particles initially travel in a direction of initial particle travel and are redirected by the apparatus towards the surface on the component at a direction of final particle travel relative to the surface. The direction of initial particle travel forms an angle relative the surface on the component that is different than the angle formed by the direction of final particle travel relative to the surface. 
         [0011]    According to a second aspect of the invention, a system includes means for depositing coating particles onto a surface of a component. The surface is obstructed by portions of the component limiting line-of-sight from a source of the coating particles to the surface. The depositing means causes the coating particles to travel in a direction of initial particle travel relative to the surface of the component. The system includes means for causing the coating particles to be redirected in flight towards the surface on the component from the direction of initial particle travel to a direction of final particle travel relative to the surface. The direction of initial particle travel forms an angle relative the surface on the component that is different than the angle formed by the direction of final particle travel relative to the surface. 
         [0012]    A technical effect of the invention is the ability to spray coat a surface in the event that the line-of-site access angle to the surface is less than 30 degrees. In particular, it is believed that by using an apparatus to redirect the coating particles towards the surface on the component to be coated, a uniform coating may be deposited on the surface despite the low line-of-site access angle. 
         [0013]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  represents a conventional thermal spraying process wherein coating particles are being deposited onto seal teeth of a component. 
           [0015]      FIG. 2  shows a micrograph of a seal tooth formed on a component coated by a conventional thermal spraying process similar to that shown in  FIG. 1 . 
           [0016]      FIGS. 3 and 4  represent a thermal spraying process wherein coating particles are redirected with ramps prior to being deposited onto seal teeth of a component in accordance with an embodiment of the present invention. 
           [0017]      FIG. 5  shows a micrograph of a seal tooth of a component on which a coating has been deposited by a thermal spraying process in accordance with an embodiment of the present invention. 
           [0018]      FIG. 6  represents a thermal spraying process wherein coating particles are redirected with ramps secured to the thermal spraying device prior to being deposited onto seal teeth of a component in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present invention is generally applicable to components that may be coated by a spraying process wherein the design of the components provides a line-of-site access angle to the surface to be coated of less than 30 degrees. Notable examples of such components include gas turbine engine components, such as the gas turbine component  10  of  FIG. 1  comprising seal teeth  12 . Although the invention will be described hereinafter in reference to the gas turbine component  10 , it will be appreciated that this is exemplary and that the invention has application to other components. Coatings formed by the invention may be comprised of any suitable material such as, but not limited to, ceramics, metallics, cermets, and carbides. 
         [0020]      FIGS. 3 and 4  represent a component  10  of the type shown in  FIG. 1  undergoing a thermal spray process in accordance with an embodiment of the present invention. As such,  FIGS. 3 and 4  represent a seal tooth  12  of the component  10  as being thermal sprayed with coating particles  16 , for example, ceramic or metallic particles deposited on surfaces  13  of the tooth  12 .  FIGS. 3 and 4  further represent one or more ramps  18  positioned to redirect the coating particles  16  after they have been propelled from one or more nozzles  14  to impinge the ramps  18  and then travel across surfaces of the ramps  18  towards the surfaces  13  of the seal tooth  12 . From  FIG. 4 , it should be appreciated that one or more ramps  18  can be used in combination with one or more nozzles  14  to optimize the trajectory or trajectories of the coating particles  16  and/or enable simultaneous coating of one or more surfaces of an article, including oppositely-disposed surfaces of the article. 
         [0021]    After leaving one of the nozzles  14  at an initial direction of particle travel relative to a targeted surface of the tooth  12 , the coating particles  16  impact and then slide along a surface  19  of a corresponding one of the ramps  18 , enabling the coating particles  16  to be re-vectored at a more favorable access angle  30  (that is, at least 30 degrees) for line-of-sight deposition onto the targeted surface  13  of the tooth  12 . The ramps  18  can be mounted directly to the component  10 , as represented in  FIGS. 3 and 4 , or mounted to the spray device or the nozzle  14  itself.  FIG. 6  represents the ramps  18  as being secured to the spray device by connectors  36 . The ramps  18  are preferably adapted to be located and secured to the component  10  by aligning and attaching the ramps  18  on well-defined features of the component  10 , for example, bolt holes, rabbets, mounting flanges, or under blade platforms, allowing for uniformity and consistency in the microstructure of the deposited coating and ease of installation. The ramps  18  may further provide masking of other features of the component  10  where a coating is undesirable. When the coating particles  16  arrive at the surface  13  of the seal tooth  12 , the coating particles  16  directly impinge the surface  13  while traveling in a final direction of particle travel at an access angle  30  of at least 30 degrees relative to the surface  13 , though the actual line-of-sight angle  28  between the nozzle  14  and surface  13  being coated may have been less than 30 degrees. In order for the coating particles  16  to be effectively re-vectored, the initial direction of particle travel leaving the nozzle  14  should form an impact angle  32  of not less than 10 degrees with the surface  19  of the ramp  18 . Preferably, the impact angle  32  is between about 10 degrees and about 20 degrees, and most preferably, between about 10 degrees and about 15 degrees. It will be appreciated that due to the spray pattern of the trajectory of the coating particles  16 , the terms “direction” and “angle” are in reference to a “nominal” direction of particle travel, e.g., the central axis of the flow pattern. Preferably, the access angle  30  is as close to 90 degrees as possible in order to provide a suitable coating on the surface  13 . 
         [0022]    Each ramp  18  defines the surface  19  whose shape or contour serves to redirect the coating particles  16  towards a surface of the tooth  12  to be coated.  FIGS. 3 and 4  represent each ramp  18  as comprising a substrate  20 , and further represent each substrate  20  as preferably having a surface material or coating  22  that defines its respective ramp surface  19 . The coating  22  is preferably adapted to promote sliding of the coating particles  16  as they travel across the surface  19  of the ramp  18  as well as survive the temperature of the plasma spray process. For this purpose, the coating  22  may be, for example, an elastomeric (rubberized) or ceramic material applied to the substrate  20 . Although the surface  19  of the ramps  18  are represented as being flat, it is foreseeable that the surface  19  could be curved or cupped, that is, higher on the edges and lower in the center of the ramp  18 , to promote coating particles  16  to remain on the ramp  18  during redirection. In addition, the ramps  18  could be a fully contained contoured tube-like structure through which the coating particles  16  travel towards the surface  13  of the tooth  12 . Any number of ramps  18  may be used in the spraying process and the surfaces  19  of the ramps  18  may have any shape or size suitable for redirecting the coating particles  16  in a desired manner. Other parameters such as the distance between the ramp  18  and the surface  13  depend on the particular component to be coated. 
         [0023]    Further optimization of the process can be achieved with modifications to conventional spray parameters for applications where the line-of-sight is at least 30 degrees. Other modifications may include alternative types of nozzles  14 , the use of coating particles  16  having a particular size distribution range, alternative types of materials for the coatings  22  on the ramps  18 , and the amount of contact surface  19  of the ramp  18 . Actual modifications to conventional spray parameters depend on the shape, size, and line-of-sight access angle  28  to the particular surface  13  to be coated in any given application. All such optimizations and modifications are within the scope of the invention. 
         [0024]    In investigations leading to the present invention, seal teeth  12  were thermal spray coated first with a metallic (NiAl) bond coat and then with a ceramic (alumina; Al 2 O 3 ) top coat. Over one hundred trials were performed in order to investigate this process. Several parameters were investigated, such as the particle size and composition of the coating particles  16 , gun type, nozzle type, gases used, shape and size of ramps  18 , number of ramps  18 , etc. A suitable particle size and distribution were found to be between about 400 to about 200 mesh (about 35 to about 75 micrometers) with no more than about five percent of the particles being larger than 200 mesh (about 75 micrometers) and no more than about fifteen percent of the particles being smaller than 400 mesh (about 35 micrometers). 
         [0025]    A particularly suitable embodiment was determined to be essentially the configuration and process schematically represented in  FIGS. 3 and 4 . As represented, a first ramp  18  has a lower portion whose surface  19  is flat (planar) and angled towards a surface  13  of a seal tooth  12  to be coated.  FIG. 4  depicts the use of a second ramp  18  whose surface  19  is arcuate and curved towards the opposite surface  13  of the same seal tooth  12 . The planar shape of the first ramp  18  was found to be particularly effective at coating a surface  13  of a seal tooth  12  that is facing an adjacent seal tooth  12 . The ramp  18  was found to fully coat the surface  13  of the seal tooth  12  without interference from the adjacent surface. The curved shape of the second ramp  18  was found to be more effective at coating a surface  13  of a seal tooth  12  that was immediately facing an adjacent surface of the component  10 . The additional unoccupied area (access area) around the surface  13  of the seal tooth  12  allowed for the use of the second ramp  18  that provided a more even coating. Consequently, it will be appreciated that, as an alternative to the represented arrangement, two planar ramps  18  or two curved ramps  18  can be used depending on the available access area and adjacent objects in the vicinity of the surface  13  to be coated. In order to provide adequate redirection of the coating particles  16  along the surface  19  of the ramp  18 , the coating particles  16  preferably travel a distance of at least about 0.5 inch (about 12.5 millimeters) along the surface  19  of the ramp  18  prior to impacting the surface  13 . 
         [0026]    In  FIG. 4 , each of the seal teeth  12  to be coated is individually sprayed utilizing the two ramps  18  as shown so that the oppositely disposed surfaces  13  of an individual tooth  12  are simultaneously coated. Although  FIG. 4  represents only one seal tooth  12  being coated at any given time, it is foreseeable that the ramps  18  could be arranged to allow multiple seal teeth  12  to be coated at once. For example, multiple ramps  18  could be attached wherein each set of ramps  18  are located in a position to coat a separate seal tooth  12 . A coated seal tooth  12  resulting from a trial performed by this process is shown in  FIG. 5 . Metallographic evaluation of the seal tooth  12  confirmed complete coverage with a uniform coating microstructure. To date, this process has been successfully applied to rotor abrasive seal teeth for turbofan engines, though the technology is believed to be applicable to substantially any thermal spray coating. 
         [0027]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the ramps  18  could differ from that shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.