Abstract:
A method and apparatus for applying a vapor deposition coating onto a substrate with a non line of sight or limited line of sight is disclosed. A vapor stream is provided in a chamber that is below atmospheric pressure. The vapor stream is impinged with a working gas that provides a flow that transports the vapor stream. The flow of the working gas is modified with a physical object that directs the flow to achieve a desired coating on the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority as a divisional application under 35 U.S.C. §121 of earlier filed application Ser. No. 13/083,727 entitled “GUIDED NON-LINE OF SIGHT COATING” by Michael d. Greenberg, et al. and filed Apr. 11, 2011, which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention provides a method and an apparatus for applying a coating to a surface using a directed vapor deposition (DVD) approach, and more particularly applying a directed coating to the line of sight region as well as non-line of sight and limited line of sight regions of a substrate. 
         [0003]    The application of a coating to a substrate is required in a wide variety of engineering applications, including thermal or environmental protection, improved wear resistance, altered optical or electronic properties, decorative, biocompatibility, etc. In each of these cases, the ability to deposit compositionally controlled coatings efficiently, uniformly, at a high rate, with a high part throughput, and in a cost effective manner is highly desired 
         [0004]    As stated above, the ability to uniformly deposit ceramic or metallic coatings onto substrates is desirable for a number of applications. They include the deposition of metal on fibers to create metal matrix composites, deposition of coatings having low shear resistance and good thermochemical stability on the fibers used in ceramic matrix composites and the deposition of metals on sacrificial fiber templates to create hollow fibers. More generally, vapor deposition approaches which allow the creation of conformal coatings on a variety of non-planar substrates is also of interest. For example, the deposition of reaction inhibiting coatings onto the surfaces of a multi-airfoil vane segment for a gas turbine engine. 
         [0005]    Several options for creating coatings of this type exist. These include chemical vapor deposition (CVD), electroplating processes and physical vapor deposition (PVD) approaches (such as electron beam evaporation or inverted cylindrical magnetron sputtering). However, despite the many needs, the advancement of processing approaches for these applications above are limited by several factors; namely, the inability to uniformly coat such substrates without sophisticated substrate translation and rotation capabilities, the inability to deposit metal, alloys and ceramics with the same process, the inability to create a coating with the desired microstructure, and low deposition rates which often limit high volume throughputs. 
         [0006]    In CVD, uniform coating thicknesses are readily produced in some cases. However, the deposition rates can be low and the process often requires the use of toxic (and expensive) precursor materials. The deposition of multicomponent alloys can also be challenging. Electroplating can provide uniform coating over the surface of complex shaped parts. Although useful for depositing elemental layers, this process is less suitable for the creation of alloy or ceramic coatings. 
         [0007]    In PVD approaches vapor atoms are created in high vacuum and deposited onto a substrate. One method to created vapor atoms is sputtering. A wide variety of materials can be deposited, but deposition rates are low. The high vacuums employed in these techniques result in few collisions with the background gas resulting in “line-of-sight” coating. Thus, substrate manipulation and/or shadowing is required to achieve acceptable coating uniformity on non-planar surfaces. Higher deposition rates require more energetic/higher density plasma sputtering (e.g. magnetrons). 
         [0008]    Atomic vapor can be more rapidly created using electron beam evaporation approaches. However, the materials utilization efficiency (MUE) of electron beam physical vapor deposition is often low. When a relatively long source to substrate distances is required, the deposition efficiency can be low and the deposition rate limited. The high vacuum environments required for the creation of electron beam also lead to line of sight coating. 
         [0009]    Electron beam-physical vapor deposition (EB-PVD) is a widely used method for the high-rate production of atomic and molecular vapor (metal or ceramic) for vapor deposition of a coating. During EB-PVD, vapor is transported to a substrate under high vacuum conditions where it condenses on surfaces that are in the line-of-sight of the vapor cloud source. This requires the use of complicated translation and rotation systems and shadowing to deposit a uniform coating onto complex or non linear structures that contain areas not in line of sight of the vapor stream. Even with known methods and equipment, EB-PVD processes often fail to create uniform coating thicknesses on difficult to coat locations of a substrate, i.e., non-line of sight and limited line of sight areas. 
         [0010]    Electron beam-physical vapor deposition (EB-PVD) of metal and ceramic coatings can be quite costly to apply due to high equipment cost, low deposition efficiencies and relatively low deposition rates. The high equipment costs of EB-PVD are a result of the high vacuum environment, which is necessary during deposition, the high cost of high power electron beam guns, and the sophisticated component manipulation systems needed to achieve uniform coating on non-planar substrates. The operating pressure defines the vacuum pump requirements with lower pressures generally needing more expensive pumps. The low deposition rate and low materials utilization efficiency (MUE) of EB-PVD is related to the distribution of vapor cloud as it leaves the evaporated source. When relatively long source-to-substrate distances are required, the deposition efficiency is dramatically reduced. 
         [0011]    Low deposition efficiencies result from cloud spreading beyond the periphery of the substrate and non line-of-sight deposition. One approach to reduce the spread of the cloud exploits entrainment of the vapor on a controllable inert (e.g. helium or argon) carrier gas flow. Such an approach is used in electron beam-directed vapor deposition (EB-DVD). In this approach, the combination of a continuously operating electron beam gun (modified to function in a low vacuum environment) and an inert carrier gas jet. In this system, the vapor plume is intersected with a rarefied trans- or supersonic inert gas jet, to entrain the evaporated cloud in a non-reacting gas flow and transport it to a substrate under low vacuum conditions. Deposition of the atomistic cloud then occurs by gas phase scattering from the streamlines of the flow and is deposited onto the substrate at high rates and with high materials utilization efficiency. However, this process may still result in unacceptable non-uniform deposition of coatings, especially when comparing line of sight to non line of sight areas. In addition, current systems of this type require the use of large amounts of gas, which results in substantial costs associated with the procurement of the gas as well as costs associated with the pumping capacity. 
       SUMMARY 
       [0012]    Disclosed is a method for applying at least one coating on a substrate. The substrate is placed into a chamber with at least one evaporant source, which is impinged with an electron beam in the chamber to generate an evaporated vapor cloud. A one carrier gas stream is presented into the chamber to disperse the evaporated vapor cloud. The evaporated vapor cloud being moved by the carrier gas stream is deflected, wherein the deflecting is done with at least one physical article that disrupts a flow of the at least one carrier gas stream. The evaporated vapor cloud at least partially coats at the at least one substrate. 
         [0013]    In another embodiment, an apparatus for applying at least one coating on at least one substrate is disclosed. The apparatus has a chamber operating pressure below atmospheric pressure, with the substrate present in said chamber, an evaporant source disposed in the chamber, an electron beam generator that impinges the at least one said evaporant source with an electron beam in the chamber to generate an evaporated vapor cloud, a carrier gas stream source for introducing a carrier gas into the chamber that transports the evaporated vapor cloud to coat the at least one substrate, and at least one physical flow disrupting object that modifies a flow of the carrier gas to direct the deposition of the evaporated vapor cloud in a desired pattern on the substrate. 
         [0014]    In yet another embodiment, a method for applying a vapor deposition coating onto a substrate with a non line of sight or limited line of sight is disclosed. A vapor stream is provided in a chamber that is below atmospheric pressure. The vapor stream is impinged with a working gas that provides a flow that transports the vapor stream. The flow of the working gas is modified with a physical object that directs the flow to achieve a desired coating on the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of a prior art multi-airfoil vane segment of a gas turbine engine. 
           [0016]      FIG. 2  is a schematic of a vapor deposition system. 
           [0017]      FIG. 3  is a schematic of a workpiece and vapor cloud modifiers. 
           [0018]      FIG. 4  is a cross-sectional plan view of workpieces and vapor cloud modifiers. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a perspective view of a prior art multi-airfoil vane segment  10  of a gas turbine engine. Multi-airfoil vane segment  10  contains a plurality of airfoils  12  between inner shroud  14  and outer shroud  16 . The surfaces of airfoils  12  and inner surfaces  18  and  20  of inner and outer shroud  14  and  16 , respectively, are covered at least partially with a thermal barrier coating (TBC). The TBC protects multi-airfoil vane segment  10  from the hot gases from the turbine engine, and coating the multi-airfoil vane segment  10  increases the life of the part by preventing failure due to oxidation and thermal mechanical fatigue. 
         [0020]    The geometry of multi-airfoil vane segment  10  creates some obstacles for manufacturing, including the application of the TBC, as airfoils  12  and shrouds  14  and  16  prevent direct line of sight of the entire part during TBC application. This is especially apparent when utilizing electron beam physical vapor deposition (EB-PVD), The geometry results in a non-line of sight area  13  of multi-airfoil vane segment  10 . 
         [0021]    The non-line of sight coating system described herein provides improved resistance to oxidation and thermal mechanical fatigue by comprehensively applying a thermal barrier coating to a workpiece. The workpiece is positioned within a chamber that is maintained at a low pressure by a vacuum pumping system. A fixture positions the workpiece adjacent a vapor source and a gas source. The vapor source introduces a vapor cloud into the housing toward the line of sight regions of the part. A gas is introduced into the housing by a plurality of nozzles. As the vapor cloud and the gas interact, particle-to-particle collisions cause randomization of the vapor cloud and push the vapor cloud toward the workpiece. Thus, the coating system coats the line of sight regions of the workpiece in the path of the flow of the inert gas. However, non-line of sight area  13  does not obtain the same coverage as the line of sight areas, which are in the direct flow paths of the vapor cloud and inert gas mixture. Non-line of sight area  13  includes the area between airfoils  12  and inner and outer shrouds  14  and  16 . Although some of the vapor cloud will contact the non-line of sight areas, the thickness of the resultant coating will not be the same as the thickness of the coating applied to line of sight areas. 
         [0022]      FIG. 2  illustrates a schematic of a vapor deposition system  22  for applying TBC to multi-airfoil vane segment  10 . The apparatus illustrated allows for non-line of sight and limited line of sight application of TBC to multi-airfoil vane segment  10 . Vapor deposition system  22  includes chamber  24 , energetic source  26  to produce beam  28 , evaporate material  30 , workpiece  32  supported by fixture system  34 , vapor cloud modifier  36 , vacuum system  38 , and fluid generator  40  with nozzles  42 . Vapor deposition system  22  may be utilized to apply one or more coatings, including multilayer coatings, of ceramic base or metallic base, or a combination thereof. 
         [0023]    Vapor deposition system  22  vaporizes evaporate material  30  with beam  28  of energetic source  26 , such as a continuous high voltage and medium power axial e-beam gun. In alternate embodiments, energetic source  26  with beam  28  is an electron beam, laser source, heat source, ion bombardment source, or highly focused incoherent light source. The vaporized material will coat workpiece  32 , which acts as a substrate. 
         [0024]    Evaporate material  30  is any material used to create a desire coating, such as a TBC. Evaporate material  30  may be one or more barrier layers applied to workpiece  32 , and may be any of the barrier layers from the group consisting of yttrium monosilicate, yttrium disilicate, rare-earth silicates, alkaline-earth aluminosilicates such as barium-strontium-aluminosilicate, niobium oxide, tantalum oxide, zirconium oxide, hafnium oxide, yttrium oxide, titanium oxide, mullite, rare-earth oxides, and mixtures and compounds thereof, and a variety of metal sources including aluminum, chromium, tantalum, niobium, silicon, molybdenum, hafnium, titanium, zirconium, platinum, palladium, gold, or mixtures and alloys thereof. These coatings form over the substrate of workpiece  32 , which may be manufactured from a variety of metal, metal alloys, or composites. 
         [0025]    The vaporization of evaporate material  30  takes place within chamber  24 . In one embodiment, chamber  24  is a hermetic housing that partially or completely encloses energetic source  26  and beam  28 , evaporate material  30 , workpiece  32  supported by fixturing system  34 , and vapor cloud modifier  36 . Chamber  24  may be constructed from metals, polymers or glass, and contain a transparent section to view the process taking place therein. Vacuum system  38  is connected to chamber  24 , and maintains the pressure within chamber  24  below atmospheric pressure. In one embodiment, vacuum system  38  is capable of maintaining the pressure within chamber  24  at any point between 1×10̂−8 kPa and 101 kPa. 
         [0026]    Chamber  24  is also connected to fluid generator  40 . Fluid generator  40  provides a stream of fluid through nozzles  42  that mixes with and directs the vapor created from evaporate material  30 . As the fluid contacts the vapor, the fluid causes particle-to-particle interaction and increases randomization within the vapor cloud, as well as directing the vapor cloud movement within chamber  24 . Due to the vacuum created by vacuum system  38 , the fluid from fluid generator is introduced into chamber  24  at a relatively low pressure. Nozzles  42  may introduce the fluid at any number of points in chamber  24  to create a desired flow and movement of the vapor created from evaporate material. The fluid is a carrier gas. In one embodiment, the fluid from fluid generator  40  is an inert gas, such as a noble gas. In alternate embodiments, the fluid is a gas such as helium, argon, oxygen, nitrogen, hydrocarbons, silanes, or a combination of any of the aforementioned gases. 
         [0027]    Workpiece  32  is present within chamber  24  and held by fixture system  34 . Fixture system  34  may include a series of clamps, screws, and similar structures to secure the workpiece  32  within chamber  24 . In one embodiment, fixture system  34  is connected to a movable system that enables shift of the workpiece in any independent direction. Alternately, fixture system  34  may contain a rotatable shaft. In these embodiments, the translation motion from the fixture system  34  allows for exposing different surfaces of workpiece  32  to control the thickness of the application of a coating. Additionally, fixture system  34  can be used to keep a constant distance between workpiece  32  and the source of the vapor cloud applying the coating. Fixture system  34  is used to assure all line of sight areas are evenly coated through translation of workpiece  32  during the application process. Despite such a design of fixture system  32 , some part geometries result in non-line of sight or limited line of sight areas, such as the multi-airfoil vane segment  10 , that are not able to be moved into the line of sight of the vapor cloud produced by vapor deposition system  22 . 
         [0028]    Vapor cloud modifier  36  is also present in chamber  24 . Vapor cloud modifier is a physical object that disrupts the flow of the vapor cloud, either by inhibiting flow or modifying and redirecting the flow of the carrier gas generated by fluid generator  40  with nozzles  42 . The modification of the flow of the carrier gas affects the deposition of the coating on the substrate of workpiece  32 . In some embodiments, vapor cloud modifier  36  may be used to thicken the coating on a non line of sight or limited line of sight area of workpiece  32 , while in alternate embodiments vapor cloud modifier  36  is used to reduce the coating coverage on a line of sight area of workpiece  32 . 
         [0029]      FIG. 3  is a top plan schematic of workpiece  32  and vapor cloud modifier  36  in the path of vapor cloud  44 . Vapor cloud  44  is comprised of evaporated material  30  and carrier gas from fluid generator  40 . In this embodiment, vapor cloud modifier  36  contains flow modifiers  46   a  and  46   b  and flow inhibitor  48 . 
         [0030]    Flow modifiers  46   a  and  46   b  are generally airfoil shaped. Flow modifier  46   a  will direct the flow of vapor cloud  44  to the gap between airfoils  12   a  and  12   b,  and towards flow modifier  46   b,  as represented by  44   a.  Flow modifier  44   b  again redirects the flow, represented by  44   b  and  44   c,  to enhance the coating of inner side  50  of airfoil  12   a.  As some of the flow of vapor cloud  44  continues through the gap created by airfoils  12   a  and  12   b,  the flow will contact flow inhibitor  48 . Flow inhibitor  48  will impede the flow and redirect it back towards the outer side  52  of airfoil  12   b.  Thus, the application of the coating being deposited by vapor cloud  44  will be more uniformly dispersed on workpiece  32 . In an alternate embodiment, the flow modifiers are minors of  46   a  and  46   b,  and the flow is directed towards the outer side of airfoil  12   b.  Although illustrated as a cross section, the flow modifiers  46   a  and  46   b  are three dimensional, and may contain features that modify the flow to enhance or prohibit the coating of the shrouds  14  and  16 . 
         [0031]      FIG. 4  is a cross-sectional plan view of workpieces  32  and vapor cloud modifiers  36  in the flow path of vapor cloud  44 . In this embodiment, three airfoil shaped workpieces  32  are to be coated by vapor cloud  44 . Vapor cloud modifiers  36  in the form of flow inhibitors  48  and flow modifiers  46   a  and  46   c.  In this embodiment, flow inhibitors  48  are cylindrical pegs placed adjacent the leading edges of the airfoils of workpieces  32 . Flow inhibitors  48  will block and redirect the flow of vapor cloud  44 , thus reducing the amount of coating deposited on the leading edge of the airfoils of workpieces  32 . Flow modifiers  46   a  are shaped like airfoils, and in one embodiment have the same dimensions as the dimensions of workpieces  32 . Flow modifiers  46   c  are illustrated as containing alternate dimensions and shapes, which may be of any dimension so long as the dimension adjacent workpieces  32  provides the desired flow for creating the coating on workpieces  32  from vapor cloud  44 . The remaining vapor cloud  44  will pass workpieces  32 , flow inhibitors  48 , and flow modifiers  46  and be exhausted through openings  58 . 
         [0032]    Vapor cloud modifiers  36  (including flow modifiers  46  and flow inhibitors  48 ) may be of any dimension so long as the resultant flow of the carrier gas that creates vapor cloud  44  is disrupted to achieve the desire result. In one embodiment, vapor cloud modifiers  36  are constructed from the same material as the substrate of workpiece  32 . In alternate embodiments, various metals, alloys, composites, or any combination thereof is utilized in construction of the vapor cloud modifiers  36 . 
         [0033]    Vapor cloud modifiers  36  may be located throughout chamber  24 . In one embodiment, vapor cloud modifiers are located on fixture system  34 . As fixture system  34  is moved, vapor cloud modifiers  36  also move in the same manner. In an alternate embodiment, vapor cloud modifiers  36  move independent of the motion of fixture system  34 . Vapor cloud modifiers  36  may be independent of fixture system  34 . Again, vapor cloud modifiers  36  that are separate from fixture system  34  may move in concert with fixture system, or move independent from fixture system  34 . Vapor cloud modifiers may be constructed to be removed when not in used or needed, thus being present only for a portion of the application of one or more coatings. In all embodiments, vapor cloud modifiers  36  are removable from chamber  24  for replacement as build of coating on the vapor cloud modifiers may alter intended flow properties of vapor cloud  44 . 
         [0034]    The presence of vapor cloud modifiers  36  in chamber  24  may also be utilized to change process parameters in the coating process. In one embodiment, vapor cloud modifiers allow for an increase in the flow of carrier gas to further improve the coating of non line of sight areas. 
         [0035]    Alternately, vapor cloud modifiers  36  contain an additional property that affects the flow of the carrier gas, and thus vapor cloud  44 , aside from the geometry and dimensions of the objects. In one embodiment, vapor cloud modifiers  36  are electrically biased to attract or repel more of an ionized vapor cloud. In alternate embodiments, vapor cloud modifiers may be magnetized, ionized, heated, or cooled. Additionally, vapor cloud modifiers  36  may contain apertures that allow for the venting of or introduction of the carrier gas to modify the flow of the vapor cloud. All of these embodiments may affect the coating process parameters in chamber  24 , and one or more of the embodiments may be utilized for vapor cloud modifiers. 
         [0036]    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.