Abstract:
Aluminide coatings or layers ( 14 ) for jet engine components ( 10 ) and a process for forming aluminide layers ( 14 ) that include additions of silicon and yttrium. A superalloy substrate ( 12 ) of the component ( 10 ) is initially coated with a layer of a silicon-containing material. The substrate ( 12 ) is then aluminided, for example by a chemical vapor deposition process, and is exposed to a yttrium-containing material during the aluminiding process to form the aluminide layer ( 14 ) containing silicon and yttrium. A ceramic thermal barrier layer ( 24 ) of yttria-stabilized zirconia may be optionally applied over the aluminide layer ( 14 ). Another optional zirconia layer ( 26 ) maybe provided between the aluminide layer ( 14 ) and the ceramic thermal barrier layer ( 24 ). The present invention provides a silicon- and yttrium-containing aluminide layer ( 14 ) having improved durability, either as a standalone environmental coating or as a bond coat for a subsequently-applied ceramic thermal barrier layer ( 24 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to formation of an intermetallic layer on a metal component and, more particularly, to formation of an intermetallic layer on an airflow surface of a jet engine metal component.  
       BACKGROUND OF THE INVENTION  
       [0002]     Intermetallic layers are often applied to a surface of a metal component for protecting the underlying metal substrate of the component and thereby extending its useful life during operation. For example, the aerospace industry coats many components having airflow surfaces in a jet engine, like turbine blades, vanes, and nozzle guides, with an aluminide layer to protect the underlying base metal from high temperature oxidation and corrosion.  
         [0003]     A ceramic thermal barrier coating may be applied over the aluminide layer to insulate the jet engine component from combustion and exhaust gases, permitting the combustion and exhaust gases from the engine to be hotter than would otherwise be possible with an aluminide layer alone. Increasing the temperature of the combustion and exhaust gases improves the efficiency of operation of the jet engine.  
         [0004]     However, such protective ceramic thermal barrier coatings may not adhere well directly to the superalloys commonly used to form jet engine components and, while in service, tend to spall.  
         [0005]     To improve adhesion and thereby provide resistance to spallation, a bond layer may be applied to the jet engine component before the ceramic thermal barrier coating is applied. Intermetallic aluminides, like platinum aluminide, are common examples of such bond coatings that have been in use for many years. However, platinum aluminides are expensive to produce, which contributes to increasing the cost of jet engine components and the cost of refurbishing used jet engine components.  
         [0006]     Accordingly, there is a need for an aluminide coating competitive in performance with platinum aluminide and less expensive to produce than platinum aluminide.  
       SUMMARY OF INVENTION  
       [0007]     In one embodiment of the present invention, a jet engine component consists essentially of a substrate of a nickel-based superalloy material and an aluminide layer including silicon and yttrium, in which the aluminide layer defines a working surface exposed to the environment when the jet engine component in service.  
         [0008]     In another embodiment of the invention, a jet engine component comprises an aluminide layer including silicon and yttrium and disposed on the substrate of a nickel-based superalloy, and a zirconia layer disposed on the aluminide layer. The jet engine component may further include a ceramic thermal barrier layer disposed on the zirconia layer.  
         [0009]     In another aspect of the invention, a deposition process comprises applying a silicon-containing material to at least a portion of a surface of a jet engine component formed of a superalloy and exposing the jet engine component with the silicon-containing material to a donor material including a metal to begin forming an aluminide layer including metal from the donor material. The deposition process further includes exposing the thickening aluminide layer to a yttrium-containing material.  
         [0010]     By virtue of the foregoing, there is provided an improved environmental coating, bond coat, and method of forming such coatings that include an aluminide layer containing minor concentrations of silicon and yttrium. The aluminide coating of the invention is competitive in performance with platinum aluminide and less expensive to produce than platinum aluminide.  
         [0011]     These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and description thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with a general description of the invention given above, and the detailed description of the embodiment given below, serve to explain the principles of the invention.  
         [0013]      FIG. 1  is a diagrammatic cross-sectional view of a coated jet engine component of the invention;  
         [0014]      FIG. 1A  is a diagrammatic cross-sectional view of a coated jet engine component similar to  FIG. 1 ;  
         [0015]      FIG. 2  is a diagrammatic view of the coated jet engine component of  FIG. 1  coated with a ceramic thermal barrier coating;  
         [0016]      FIG. 3  is a diagrammatic cross-sectional view of a coated jet engine component in accordance with another alternative embodiment of the invention;  
         [0017]      FIG. 3A  is a diagrammatic cross-sectional view of a coated jet engine component in accordance with yet another alternative embodiment of the invention;  
         [0018]      FIG. 4  is a schematic view showing jet engine components, such as that from  FIG. 1  or  FIG. 1A , in a deposition environment of a simple CVD deposition system for purposes of explaining the principles of the present invention; and  
         [0019]      FIG. 5  is a schematic view showing jet engine components, such as that from  FIGS. 3 and 3 A, in a deposition environment of a simple CVD deposition system similar to  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]     With reference to  FIG. 1 , a detailed view of a portion of a much larger jet engine component, generally indicated by reference numeral  10 , is shown. The jet engine component  10  includes a metallic substrate  12  and an aluminide layer  14  coating an original surface  16  of the substrate  12 . The metallic substrate  12  is made of any nickel-, cobalt-, or iron-based high temperature superalloy from which such jet engine components  10  are commonly made. For example, the substrate  12  may be the nickel-based superalloy Inconel 795 Mod5 A. The present invention is not intended to be limited to any particular jet engine component  10 , which may be a turbine blade, a vane, a nozzle guide, or any other part requiring protection from high temperature oxidation and corrosion while operating in a jet engine. The substrate  12  may be masked to define areas across which the aluminide layer  14  is absent.  
         [0021]     In this specific embodiment of the present invention, aluminide layer  14  operates as an environmental coating having a working surface  18  exposed to the atmosphere with the jet engine component  10  in service. The general composition of aluminide layer  14  is a chrome aluminide containing minor concentrations of silicon and a minor content of yttrium. The concentration of silicon in the aluminide layer  14  may be, for example, about 0.5 wt %. The concentration of yttrium in the aluminide layer  14  may be, for example, in a range of parts per million to less than about 0.5 wt %.  
         [0022]     Aluminide layer  14  may be formed by coating the substrate  12  with a layer of a silicon-containing material and placing it into a chemical vapor deposition environment suitable for forming an aluminide layer on jet engine component  10 . An exemplary procedure for coating jet engine components with a silicon-coating material prior to aluminiding is described in commonly-owned U.S. Pat. No. 6,605,161, issued on Aug. 12, 2003. After the growth of aluminide layer  14  is initiated, the deposition environment is modified to include a vapor of a yttrium-containing material. An exemplary method for introducing additional elements from a separate receptacle to a main reaction chamber defining the bulk of the chemical vapor deposition environment is described in commonly-owned U.S. application Ser. No. 10/613,620, entitled “Simple Chemical Vapor Deposition System and Methods for Depositing Multiple-metal Aluminide Coatings.” When the vapor of the yttrium-containing material is proximate to the jet engine component  10 , atoms of the yttrium-containing material are incorporated into the thickening aluminide layer  14 . Preferably, the exposure to the yttrium-containing material is limited to the latter 25% of the total deposition time for aluminide layer  14  and yttrium atoms diffuse from the deposition environment into aluminide layer  14  to provide a concentration gradient having a peak concentration near the working surface  18 . Alternatively, the yttrium may be distributed with a uniform concentration through the aluminide layer  14 . An additional post-deposition heat treatment may be required to diffuse the yttrium into aluminide layer  14 .  
         [0023]     The presence of silicon in the aluminide layer  14  permits a desired thickness of layer  14  to be formed in a reduced period of time as compared to a conventional deposition process. Alternatively, a thicker aluminide layer  14  may advantageously be formed where the cycle time is not substantially reduced with a pre-coated component  10  as compared to another component that was not pre-coated. Yttrium operates as a getter for the impurity or tramp element sulfur in the aluminide layer  14 , which originates from the donor material for forming the aluminide layer  14 . The gettering of sulfur by the yttrium is believed to reduce the likelihood that the aluminide layer  14  will spall.  
         [0024]     With reference to  FIG. 1A  in which like reference numerals refer to like features in  FIG. 1 , aluminide layer  14  may partially diffuse into the substrate  12  beneath the original surface  16  of the substrate  12 . The resulting aluminide layer  14  includes a diffusion region  20  that extends beneath the original surface  16  and an additive region  22  overlying the original surface  16  of substrate  12 . The outermost boundary of additive region  22  defines the working surface  18  of aluminide layer  14  when the jet engine component  10  is in service. Additive region  22  is an alloy that includes a relatively high concentration of the donor metal aluminum and a concentration of a metal, for example nickel, from substrate  12  outwardly diffusing from component  10 . By contrast, diffusion region  20  has a lower concentration of aluminum and a relatively high concentration of the metal of substrate  12 .  
         [0025]     With reference to  FIG. 2  in which like reference numerals refer to like features in  FIG. 1 , aluminide layer  14  may operate as a bond coat covered by a relatively thick ceramic thermal barrier coating or layer  24  of yttria stabilized zirconia (YSZ or Y 2 O 3 ). Such thermal barrier coatings and methods for the application thereof are familiar to those of ordinary skill in the art. The YSZ layer  24  may be applied to the jet engine component  10  by electron beam physical vapor deposition in a different deposition environment from the process forming aluminide layer  14 . When applied by this deposition technique, the YSZ layer  24  typically has a porous columnar microstructure with individual grains oriented substantially perpendicular to the original surface  16  of substrate  12 . Of course, the YSZ layer  24  may be omitted if not required when the jet engine component  10  is in service.  
         [0026]     With reference to  FIG. 3  in which like reference numerals refer to like features in  FIG. 1 , a thin layer  26  of zirconia is provided between the aluminide layer  14  and the YSZ layer  24 . The zirconia layer  26  operates to reduce the mismatch in atomic spacing between the aluminide layer  14  and the YSZ layer  24 . The zirconia layer  26  may be formed before YSZ layer  24  is applied, during application of YSZ layer  24 , or after YSZ layer  24  is formed by heating the jet engine component  10  in an oxidizing atmosphere at a suitable temperature. In one specific embodiment, zirconia layer  26  may be formed by depositing metallic zirconium on aluminide layer  14  and then heating jet engine component  10  in air at a temperature of about 1100° F. to about 1200° F. Alternatively, a metallic zirconium layer may be anodized to form the zirconia layer  26 . The zirconium layer for forming zirconia layer  26  may be provided from an external receptacle  80  to a deposition environment suitable for growing the aluminide layer  14 , as described below in the context of  FIG. 5 , or may be deposited in a different and distinct deposition environment from the aluminide layer  14 .  
         [0027]     As shown in  FIG. 3A , the layer of metallic zirconium used to form the zirconia layer  26  may be deposited under conditions of rapid deposition so that the morphology of the parent zirconium layer is rough, rather than smooth. The rough zirconium layer is then transformed into zirconia. This roughening increases the effective surface area available for bonding with the YSZ layer  24 , which operates to enhance the adhesion of the YSZ layer  24  to the aluminide layer  14 .  
         [0028]     With reference to  FIG. 4 , a CVD apparatus  40  suitable for use in the invention includes a main reaction chamber  42  enclosing an interior space  44  defining a deposition environment when purged of atmospheric gases, and evacuated. Inert gas, such as argon, is supplied from a gas supply  46  to the reaction chamber  42  through an inlet port  48  defined in the wall of chamber  42 . An exhaust port  50  defined in the wall of the reaction chamber  42  is coupled with a vacuum pump  52  capable of evacuating the reaction chamber  42  to a vacuum pressure. One or more jet engine components  10  are introduced into the reaction chamber  42  and are situated away from a source of extrinsic metal, as explained below.  
         [0029]     Positioned within the reaction chamber  42  is a mass or charge of a solid donor material  54 , a mass or charge of an activator material  56  and several jet engine components  10 . The jet engine components  10  are fabricated from a nickel-based superalloy material. Suitable solid donor materials  54  include alloys of chromium and aluminum, which are preferably low in sulfur content (&lt;3 ppm sulfur). One suitable donor material  54  is 44 wt % aluminum and balance chromium. Appropriate activator materials  56  suitable for use in the invention include, but are not limited to, aluminum fluoride, aluminum chloride, ammonium fluoride, ammonium bifluoride, and ammonium chloride. The reaction chamber  42  is heated to a temperature effective to cause vaporization of the activator material  56 , which promotes the release of a vapor phase reactant from the solid donor material  54 . This vapor contains an extrinsic metal, typically aluminum, that contributes a first extrinsic metal for incorporation into aluminide layer  14  ( FIG. 1 ) formed on component  10 , as diagrammatically indicated by arrows  58 . The first extrinsic metal is separate and distinct from the jet engine component  10 .  
         [0030]     With continued reference to  FIG. 4 , positioned outside the reaction chamber  42  is a receptacle  60  in which a second solid donor material  62  is provided. The solid donor material  62  furnishes a source of a second extrinsic metal separate and distinct from the jet engine component  10 . The second extrinsic metal combines with the first extrinsic metal supplied from donor material  54  to form the aluminide layer  14  on the jet engine component  10 . The receptacle  60  and a conduit  64  leading from the receptacle  60  to the reaction chamber  42  are heated with respective heaters  66 ,  68 .  
         [0031]     The second solid donor material  62  provided in receptacle  60  may be any solid yttrium-halogen Lewis acid, such as YCl 3 . The yttrium-halogen Lewis acid may be ACS grade or reagent grade chemical that is high in purity and substantially free of contaminants, such as sulfur. Upon heating, such yttrium-halogen Lewis acids convert from a dry solid form to a liquid form and, when the temperature of the receptacle  60  is further increased, convert from the liquid form to a vapor to provide the vapor phase reactant containing yttrium. The vapor phase reactant from solid donor material  62  is conveyed or transported through the conduit  64  to the main reaction chamber  42 , as diagrammatically indicated by arrows  70 . The rate at which the vapor phase reactant from solid donor material  62  is provided to the main reaction chamber  42  is regulated by controlling the temperature of the receptacle  60  with the power to heaters  66 ,  68 . Of course, the delivery vapor phase reactant from solid donor material  62  may be discontinued by sufficiently reducing the temperature of the receptacle  60  or with a valve (not shown) controlling flow in conduit  64 .  
         [0032]     In use and with continued reference to  FIG. 4 , a silicon-containing inoculant is applied to the original surface  16  of substrate  12 , preferably before jet engine component  10  is placed inside the main reaction chamber  42 . The inoculant is applied as a liquid and then dried to form a coating. Suitable liquid forms of the inoculant may be a mono-, bis- or tri-functional silane material provided in a solution. One particularly suitable silane solution is an organofunctional silane such as BTSE 1,2 bis(triethoxysilyl) ethane dissolved in a mixture of water, acetic acid and denatured alcohol with a silane concentration between about 1% and 10%. Innoculants, like the silane solution, may be applied liberally by a brush, as if being painted, by dipping, by spraying, or by any other suitable conventional application technique.  
         [0033]     The jet engine component  10  bearing the inoculant is then introduced into the main reaction chamber  42 , a charge of the first donor material  54 , and a charge of the activator material  56  are introduced into the reaction chamber  42 , and a charge of the solid yttrium-halogen Lewis acid is introduced as the second donor material  62  into the receptacle  60 . The receptacle  60  and the reaction chamber  42  are purged of atmospheric gases by repeatedly admitting an inert gas from inert gas supply  46  through inlet port  48  and evacuating through exhaust port  50  with vacuum pump  52 .  
         [0034]     The main reaction chamber  42  is heated to a temperature effective to release activator material  56 , which interacts with first donor material  54  to release the first vapor phase reactant including metal from material  54 . Aluminum present in the vapor phase reactant begins to form the silicon-containing aluminide layer  14  ( FIG. 1 ) on the jet engine component  10 . After the aluminide layer  14  begins to form, receptacle  60  is heated by heater  66  to a temperature effective to form a second vapor phase reactant from solid donor material  62 , which is provided as a yttrium-containing vapor to the reaction chamber  42  through heated conduit  64 . The yttrium is incorporated into the thickening aluminide layer  14 . Persons of ordinary skill in the art will recognize that additional steps, such as soaks and cleaning cycles, may be involved in the coating process. The jet engine components  10  are removed from the reaction chamber  42  and, optionally, the YSZ layer  24  may be applied by a different process.  
         [0035]     With reference to  FIG. 5  in which like reference numerals refer to like features in  FIG. 4 , another receptacle  71  may be positioned outside the reaction chamber  42 . Another solid donor material  72  provided in receptacle  71  furnishes a source of an extrinsic metal separate and distinct from the jet engine component  10  and separate and distinct from the yttrium-halogen Lewis acid comprising the second donor material  62  in receptacle  60 . Depending upon the deposition process, this extrinsic metal from the donor material  72  may combine with the first extrinsic metal supplied from donor material  54 , may combine with yttrium material supplied to the jet engine component  10  from the second donor material, or may deposit separately on the jet engine component  10 . The receptacle  71  and a conduit  74  leading from the receptacle  71  to the reaction chamber  42  are heated with respective heaters  76 ,  78  in order to release the vapor phase reactant from the donor material  72  and supply the vapor phase reactant to the main reaction chamber  42 .  
         [0036]     The solid donor material  72  provided in receptacle  71  may be any solid Lewis acid, such as AlCl 3 , CoCl 4 , CrCl 3 , CrF 3 , FeCl 3 , HfCl 3 , IrCl 3 , PtCl 4 , RhCl 3 , RuCl 3 , TiCl 4 , ZrCl 4 , and ZrF 4 . The Lewis acid may be ACS grade or reagent grade chemical that is high in purity and substantially free of contaminants, such as sulfur. Upon heating, such Lewis acids convert from a dry solid form to a liquid form and, when the temperature of the receptacle  71  is further increased, convert from the liquid form to a vapor to provide the vapor phase reactant containing the associated extrinsic metal. The vapor phase reactant from solid donor material  72  is conveyed or transported through the conduit  74  to the main reaction chamber  42 , as diagrammatically indicated by arrows  80 . The rate at which the vapor phase reactant from solid donor material  72  is provided to the main reaction chamber  42  is regulated by controlling the temperature of the receptacle  71  with variations in the power supplied to heaters  76 ,  78 . Of course, the delivery of the vapor phase reactant from solid donor material  72  may be discontinued by sufficiently reducing the temperature of the receptacle  71  to halt vaporization or with a valve (not shown) controlling flow through conduit  74 .  
         [0037]     The vapor phase reactants from receptacles  60  and  72  are typically provided separately to the main reaction chamber  42 , so that the extrinsic metals from solid donor materials  62 ,  72  are not co-deposited on jet engine component  10 . The separate control is achievable by, for example, lowering the temperature of each receptacle  60 ,  71 , as required, so that the corresponding vapor phase reactant is not produced and, hence, not supplied to the main reaction chamber  42 . In addition, the temperature of the main reaction chamber  42  may be controlled so that the vapor phase reactant from donor material  54  is controllably present or absent while one or both of the receptacles  60 ,  71  supplies the corresponding vapor phase reactant to the main reaction chamber  42 . These capabilities permit a vapor phase reactant of, for example, zirconium to be independently supplied from receptacle  71  to the main reaction chamber  42  and to, for example, deposit over the aluminide layer  14  ( FIG. 3 ) previously formed on component  10  by a deposition process inside the main reaction chamber  42 . Such a process may be used, as described above, for forming the zirconium layer that ultimately forms the zirconia layer  26  ( FIG. 3 ).  
         [0038]     While the present invention has been illustrated by the description of an embodiment thereof and specific examples, and while the embodiment has been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of applicant&#39;s general inventive concept.