Abstract:
A method and apparatus for forming thermally grown alpha alumina oxide scale on a substrate is provided. The method includes the steps of: a) providing a heating chamber having a heat source and an oxidizing gas source selectively operable to provide a stream of oxidizing gas; b) providing at least one substrate disposed in the heating chamber, which substrate has a composition sufficient to permit formation of an alpha alumina scale on one or more surfaces; c) maintaining a vacuum in the heating chamber at a level that inhibits formation of one or more low temperature oxides on the one or more surfaces of the substrate; d) heating at least one of the one or more surfaces of the substrate to a predetermined temperature at or above 1800 degrees Fahrenheit; and e) directing the stream of oxidizing gas at a controlled rate toward one or more heated surfaces of the substrate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    This disclosure relates to Electron Beam Physical Vapor Deposited Thermal Barrier Coatings (EB-PVD TBC) and methods for applying the same to a substrate in general, and to such coatings and methods that utilize a thermally grown oxide for ceramic to metallic adhesion in particular. 
         [0003]    2. Background Information 
         [0004]    Thermal barrier coating (TBC) systems have been developed to fulfill the demands placed on current high-temperature Ni-base superalloys for gas turbine applications in both aero engine and land based gas turbines. TBC systems typically consist of a ceramic (e.g., yttria-stabilized zirconia) top layer that has low thermal conductivity, is chemically inert in combustion atmospheres, and that is reasonably compatible with Ni-base superalloys. The ceramic top layer is often applied by a deposition process such as Electron Beam Physical Vapor Deposition (EB-PVD). To ensure adequate bonding between the ceramic topcoat and the metallic substrate, it is common (but not required) to use a bond coat (e.g., NiCoCrAlY) disposed between the ceramic top coat and the metallic substrate. Ceramic adhesion to the bond coat depends on the formation of a thin, slow-growing oxide layer (also designated as TGO: thermally grown oxide) developing on the bond coat. 
         [0005]    TGOs grown from a NiCoCrAlY or similar bond coat in a vacuum (at about 10 0  to 10 −6  Torr) at temperatures less than 1800° F. will include certain oxides (e.g., eta phase alumina, and transition oxides, also referred to herein as “low temperature oxides”) that assume a voluminous, low integrity form that tend to have lower adhesion to the bond coat than other oxides. TBCs attached to these oxides will, therefore, be subject to these weaker bonds, and may be the basis for spallation. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    According to one aspect of the invention, a method for forming thermally grown alpha alumina oxide scale on a substrate is provided. The method includes the steps of: a) providing a heating chamber having a heat source and an oxidizing gas source selectively operable to provide a stream of oxidizing gas; b) providing at least one substrate (e.g., airfoil, turbine blade, stator vane, etc.) disposed in the heating chamber, which substrate has a composition sufficient to permit formation of an alpha alumina scale on one or more surfaces; c) maintaining a vacuum in the heating chamber at a level that inhibits formation of one or more low temperature oxides on the one or more surfaces of the substrate; d) heating at least one of the one or more surfaces of the substrate to a predetermined temperature at or above 1800 degrees Fahrenheit; and e) directing the stream of oxidizing gas at a controlled rate to the one or more heated surfaces of the substrate. 
         [0007]    According to another aspect of the invention, a method for conditioning a surface of a substrate prior to coating the surface is provided. The method includes the steps of: a) providing a coating chamber and a heating chamber, which heating chamber has a heat source; b) treating one or more surfaces of a substrate within the heating chamber by establishing a vacuum in the heating chamber, heating a surface of the substrate to a predetermined temperature, and directing a stream of oxidizing gas to the heated one or more surfaces of the substrate to form an oxide layer thereon; and c) coating the treated surface of the substrate in the coating chamber. 
         [0008]    According to still another aspect of the invention, a system for forming a thermally grown oxide on a surface of at least one substrate is provided. The system includes a heating chamber, a vacuum pump, a heat source, and an oxidizing gas inlet. The heating chamber has a target location for locating the substrate. The vacuum pump is connected to the heating chamber and is selectively operable to establish a vacuum within the heating chamber. The heat source is disposed within the heating chamber, and is operable to radiate heat energy to the target location. The oxidizing gas inlet is disposed within the heating chamber, and is positioned to direct oxidizing gas to the target location for forming an oxide layer on the surface of the substrate. 
         [0009]    The foregoing features of the invention will become more apparent in light of the following description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a side sectional diagrammatic illustration of one embodiment of a coating system for heating and coating a surface of at least one substrate. 
           [0011]      FIG. 2  is a top view diagrammatic illustration of one embodiment of an acceptor that is included in a heating chamber of the coating system in  FIG. 1 . 
           [0012]      FIG. 3  diagrammatically illustrates a process for treating the surface of the substrate in the heating chamber. 
           [0013]      FIG. 4  graphically illustrates formation growth rates of alumina scales on the surface of a substrate versus the surface temperature of the substrate. 
           [0014]      FIG. 5  is a flow chart illustrating an aspect of the present method. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Now referring to  FIG. 1 , a coating system  10  adapted to treat and coat a surface of at least one substrate  14  (e.g., a turbine blade airfoil for a gas turbine engine) is shown. The coating system  10  includes a plurality of successive vacuum chambers (e.g., a pre-heat chamber  16 , a coating chamber  18 ) connected together via one or more gate valves  20 ,  22 ,  24 . The coating system  10  further includes a transportation system  25  that directs the substrate  14  through the vacuum chambers  16 ,  18 . The vacuum chambers  16 ,  18  are connected to at least one vacuum pump  26  (e.g., a diffusion pump). In some embodiments, the coating system  10  may include additional vacuum chambers such as, but not limited to, a load-lock chamber, or a post-treatment chamber, or any combination thereof. 
         [0016]    The preheating chamber  16  is adapted to maintain a vacuum at or below approximately 10 −4  Torr (e.g., between approximately 10 −4  to 10 −6  Torr). The requisite vacuum may vary slightly depending upon the application at hand, thereby necessitating a preheating chamber adapted accordingly. The preheating chamber  16  has a target location  28  for locating the substrate  14  during a treatment/pre-treatment process, and houses a vacuum pump inlet  30  (hereinafter “vacuum inlet”), a radiant heat source  32  (hereinafter “heat source”), and at least one oxidizing gas inlet  34  (hereinafter “gas inlet”). The vacuum inlet  30  connects the diffusion pump to the preheating chamber  16 . The heat source  32  is adapted to heat the surface  12  of the substrate  14 . Surface  12  of the substrate  14  is aligned to receive the radiant heating from the heating source. The gas inlet  34  connects an oxidizing gas source  36  (hereinafter “gas source”) to the preheating chamber  16 . 
         [0017]    In the specific embodiment illustrated in  FIG. 1 , the heat source  32  includes one or more heating elements  38  and an acceptor plate  40  (hereinafter “acceptor”). The heating elements  38  and the acceptor  40  are aligned such that thermal heat energy (hereinafter “heat energy”) radiates from the heating elements  38  to the surface  12  of the substrate  14  through the acceptor  40 . Now referring to  FIGS. 1-2 , the acceptor  40  includes one or more flow apertures  42  that extend between first and second acceptor surfaces  44 ,  46  (e.g., top and bottom surfaces). Referring again to  FIG. 1 , each flow aperture  42  is configured to receive and orientate a respective one of the gas inlets  34  such that oxidizing gas injected therefrom is directed to the surface  12  of the substrate  14 . The present invention, however, is not limited to the aforesaid embodiment. For example, in an alternate embodiment, the heat source  32  can include a plurality of acceptors, where adjacent acceptors are spaced to receive and orientate at least one of the gas inlets. The acceptor  40  can be constructed from any suitable material such as, but not limited to, graphite or graphite composite. 
         [0018]    The coating chamber  18  is configured to deposit, for example, a ceramic (e.g., a TBC) coating on the surface of the substrate  14  by an EB-PVD process. EB-PVD coating chambers are well known in the art, and the present invention is not limited to any particular configuration thereof. Some examples of suitable EB-PVD coating chambers and processes are disclosed in U.S. Pat. No. 5,087,477 to Giggins, Jr. et al., and U.S. Publication No. 2008/0160171 (application Ser. No. 11/647,960) to Barabash et al., which are hereby incorporated by reference in their entirety. 
         [0019]    In the embodiment in  FIG. 1 , the transportation system  25  includes a sting shaft  48  operable to move a sting  50  (i.e., a substrate carriage device), and thus the substrate  14 , through the vacuum chambers  16 ,  18 . The sting  50  can be adapted to adjust/manipulate the spatial position (e.g., height, etc.) and/or orientation (e.g., pitch, roll, etc.) of the substrate  14  in the vacuum chambers  16 ,  18 . Such substrate transportation systems are well known in the art, and the present invention is not limited to any particular configuration thereof. For example, in alternate embodiments, the transportation system  25  includes a conveyor and a robotic manipulator disposed in each vacuum chamber  16 ,  18 . 
         [0020]    Referring to  FIG. 3 , during operation, a vacuum, below approximately 10 −4  Torr (e.g., between approximately 10 −4  to 10 −6  Torr), is established and maintained in the preheating chamber  16  via the vacuum inlet  30  and the diffusion pump  26 . The substrate  14  is directed, through a first gate valve  20 , into the preheating chamber  16 , and is positioned in the target location  28  via the sting  50  such that the surface  12  of the substrate  14  that is to be treated is aligned with (i.e., faces) the heat source  32  (e.g., the heating elements  38  and the acceptor  40 ). Under vacuum, gas  52  (e.g., oxidizing gases like oxygen or carbon dioxide) flows from a top region of the preheating chamber  16 , for example proximate the heat source  32 , creating a partial pressure adjacent surface  12 ; i.e., on the heated side of substrate  14 . 
         [0021]    The heat source  32  heats the surface  12  of the substrate  14  via thermal radiation to a temperature above approximately 1800° F. For most applications, an acceptable substrate surface temperature range is about 1800° F. to about 1950° F., and substrate surface temperatures above 1830° F. work particularly well. For example, in the embodiment in  FIGS. 1 and 3 , the heating elements  38  radiate heat energy  54  to the top surface  44  of the acceptor  40 . In the acceptor  40 , the heat energy  54  disperses therethrough and radiates, in a substantially even/uniform pattern, from its bottom surface  46  to the surface  12  of the substrate  14 . Referring to  FIG. 4 , as the surface temperature of the substrate  14  rises rapidly to approximately 1800° F., the surface  12  of the substrate  14  oxidizes, forming various oxides thereon such as theta phase alumina, nickel oxide, cobalt oxide, chromium oxide, etc. Low temperature (&lt;1800° F.) phases of alumina and metallic oxides like nickel oxide, cobalt oxide and chromium oxide are loosely adherent and create a low integrity link between the metallic and ceramic as compared to thermally grown alpha alumina scale. With sufficiently high vacuum and a very small amount of time during ramp up between 700 and 1800° F., the formation of theta phase alumina, and other metallic oxides like nickel oxide, cobalt oxide, chrome oxide, etc. will be relatively minor When the temperature of the surface  12  of substrate  14  rises above approximately 1800° F. (e.g., to or above approximately 1830° F.), the oxidization reaction primarily forms a layer of alpha alumina on the surface  12  of the substrate  14  (e.g., on the NiCoCrAlY bond coat). In addition, at least a portion of the previously formed theta phase alumina will be transformed into alpha alumina. 
         [0022]    Thus, for favorable adhesion of TBC ceramic on a bond coat (or on a substrate or other coating), a cohesive alpha alumina scale or layer (i.e., serves as a “metallic-ceramic bond”) is desirable. Other thermally grown oxides can adversely affect TBC ceramic adhesion. The surface temperature of the substrate  14  should be rapidly heated above 1800° F. (e.g., to or above approximately 1830° F.) to reduce the quantity of the undesirable theta phase alumina, and other undesirable metallic oxides, that may form on the surface  12  of the bond coated substrate  14  at temperatures below 1800° F. 
         [0023]    Referring again to  FIG. 3 , when the surface temperature of the substrate  14  has risen to or above approximately 1800° F. (e.g., to or above approximately 1830° F.), the gas source  36  injects, via each gas inlet  34 , a stream of oxidizing gas  56  into the preheating chamber  16  for impingement against the heated surface  12  of the substrate  14  creating conditions promoting alpha alumina formation. For example, in the embodiment in  FIGS. 1 and 3 , the oxidizing gas  56  is directed from the gas inlet  34  to the heated surface  12  of the substrate  14 . A controlled flow of oxidizing gas  56  provides oxygen (i.e., reactants) that directly influences the formation rate of alpha alumina on the heated surface  12  of the substrate  14 . The flow of oxidizing gas is provided only after the surface  12  temperature of the substrate  14  has increased above 1800° F. (e.g., to 1830° F.). As a result, the conditions promote the formation of desirable alpha alumina and decrease the potential for the formation of undesirable oxides like theta phase alumina on the surface  12  of the substrate  14 . 
         [0024]    To form the alpha alumina layer on a large, compound, and/or irregular surface, the substrate  14  can be re-orientated (e.g., rotated, shifted, etc.) such that each portion of the surface is successively aligned with (e.g., directly below) the heat source  32  For example, referring to  FIGS. 1 and 3 , side and bottom surfaces  58 ,  60  of the substrate  14  can be treated (i.e., heated) by rotating the substrate  14  about, for example, its longitudinal axis such that each respective surface  12 ,  58 ,  60  is aligned with and treated by the heat source  32 . In some embodiments, the rotational speed is controlled/regulated, via the sting  50 , such that a substantial portion of the surface of the substrate  14  that is aligned with the heat source  32  is maintained at or above approximately 1800° F. (e.g., at or above approximately 1830° F.). 
         [0025]    After the TGO is developed on the coating required surface of substrate  14  treated in the preheating chamber  16 , the substrate  14  is directed, via the sting  50 , from the preheating chamber  16  to the coating chamber  18  through a respective second gate valve  22 . In the coating chamber  18 , the surface  12  of the substrate  14  is coated with, for example, a ceramic (e.g., TBC, etc.). The coating can be applied using any suitable deposition process such as, but not limited to, electron beam physical vapor deposition. When the surface of the substrate  14  has been coated, the substrate  14  is directed, through a respective third gate valve  24 , out of the coating chamber  18  and the coating system  10 . The flow chart shown in  FIG. 5  summarizes the present process. 
         [0026]    While various embodiments of the present invention have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents.