Abstract:
A metallic article for high temperature applications such as a turbine engine component is protected by a thermal barrier coating system on the article&#39;s metallic substrate. The thermal barrier coating system includes a bond coat layer of aluminum containing alloy on the metal substrate, an alumina layer on the bond coat layer and a ceramic thermal barrier layer on the alumina layer. The bond coat layer is doped with elemental barium that enhances the creep resistance of the alumina layer, thus, minimizing spallation of the ceramic thermal barrier layer.

Description:
GOVERNMENTAL INTEREST 
     This invention was made with government support under contract FA9550-07-1-0063 awarded by USAR/AFOSR. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This disclosure is generally related to a thermal barrier coating applied to the surface of a superalloy article such as a gas turbine engine turbine blade, and to a method of applying the thermal barrier coating. 
     BACKGROUND 
     Superalloys of nickel, cobalt or nickel-iron base alloying element are often used in extreme heat and corrosive environments such as the turbine blades and vanes of a gas turbine engine. To protect the superalloy components from the heat, oxidation and corrosion effects of the impinging hot gas stream, the superalloy components are protected by thermal barrier coating (TBC) systems. A typical TBC system has a three-layer structure where an outer coat of ceramic layer provides the thermal protection. The ceramic layer is typically a yttria-stabilized zirconia (YSZ). A thin metallic layer or bond coat layer is applied under the ceramic layer to provide adhesion between the ceramic layer and the superalloy substrate. The metallic bond coat layer is generally aluminum based alloy such as nickel aluminide, cobalt aluminide or platinum aluminide. Subsequently, a layer of aluminum oxide scale is thermally grown at the interface between the metallic bond coat layer and the ceramic layer. The metallic bond coat layer serves as an aluminum reservoir for the formation of the adherent aluminum oxide scale layer. This thermally grown aluminum oxide scale protects the superalloy substrate from oxidative corrosion. Oxygen readily diffuses through the YSZ ceramic layer and the aluminum oxide resists the oxidizing effects of the hot combustion gas stream. 
     Unfortunately, the coefficient of thermal expansion (CTE) of alumina is considerably lower than that of the underlying superalloy metal substrate. Upon thermal cycling of the superalloy components, the CTE mismatch causes stress to accumulate in the growing oxide layer. Once the thickness of the oxide reaches a critical value (around 10 μm), the stresses become so large that they must be alleviated by either creep or plastic deformation, which leads to spalling of the coating layers and failure of the TBC system. Thus, slowing the oxide growth and/or increasing its creep resistance are ways to extend the life of the TBC system. 
     SUMMARY 
     According to an embodiment of the present disclosure, a thermal barrier coating system for coating a metallic substrate is disclosed wherein a significant improvement in spalling resistance for the ceramic layer is achieved by doping the bond coat layer to include elemental barium. The thermal barrier coating system comprises a bond coat layer on the metallic substrate, the bond coat layer comprising an aluminum containing alloy and a doping material that comprises elemental barium. An aluminum oxide layer is provided on the bond coat layer and a ceramic thermal barrier layer is provided on the aluminum oxide layer. 
     According to another embodiment, a metallic article comprises a metallic substrate, the metallic substrate comprising a superalloy, and a thermal barrier coating system on the metallic substrate. The thermal barrier coating system comprises a bond coat layer on the metallic substrate. The bond coat comprises an aluminum containing alloy and a doping material comprising elemental barium in the amount of about 0.01 to about 5.0% by weight. An aluminum oxide layer is provided on the bond coat and a ceramic thermal barrier layer is provided on the aluminum oxide layer. 
     According to another implementation of the present disclosure, a method of applying a thermal barrier coating on a metallic substrate comprises forming a bond coating on the metallic substrate, the bond coat comprising an aluminum containing alloy and a doping material comprising elemental barium, applying a ceramic thermal barrier layer to the bond coating; and thermally growing an aluminum oxide layer between the bond coat and the ceramic thermal barrier layer. 
     The addition of elemental barium in the bond coat layer substantially improves the creep resistance property of the aluminum oxide layer by diffusing into the aluminum oxide layer and segregating to the alumina grain boundaries. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . is a schematic illustration of a cross-sectional view of a metallic article having a thermal barrier coating according to the present disclosure. 
     
    
    
     The drawing is schematic and the structures rendered therein are not intended to be in scale. The embodiments of this disclosure are described below with reference to the above drawing. 
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration is a cross-sectional view of a portion of a metallic article showing the thermal barrier coating (TBC) system  20  provided on a surface of a metallic substrate portion  10  of the metallic article according to an embodiment of the present disclosure. The metallic article may comprise a superalloy of nickel, cobalt or nickel-iron base. The TBC system  20  comprises a bond coat layer  22  on the surface of the superalloy substrate  10 , a thermally grown oxide layer  24  on the bond coat layer  22  and a ceramic thermal barrier coat layer  26  on the oxide layer  24 . 
     The bond coat layer  22  may be comprised of an aluminum containing alloy having an MCrAlY or MAlY compositions, where “M” may be selected from the group consisting of iron, cobalt, nickel, platinum and mixtures thereof. The “Y” may be one or more of yttrium, hafnium, lanthanum, cerium and scandium. 
     According to an embodiment, the bond coat layer  22  contains a small amount of elemental barium in the amount of about 0.01 to about 5.0% by weight. We have found that the doping of the metallic bond coat layer with elemental barium increases the activation energy of grain boundary sliding in alumina surprisingly more than any of the conventionally used transition metal dopants. This means that larger stresses are needed for grain boundary sliding so the rate of grain boundary sliding and, in turn, the creep rate decreases. A decrease in the creep rate strengthens the alumina layer  24  and extends its time to spallation. 
     The bond coat layer  22  may be applied or otherwise formed on the metal substrate  10  by any of a variety of conventional techniques. For example, the bond coat layer  22  may be applied by a physical vapor deposition, electron beam deposition, plasma spray, or other thermal spray deposition methods such as high velocity oxy-fuel spray, chemical vapor deposition, or a combination of such techniques. Typically, the deposited bond coat layer  22  has a thickness of about 1 to about 19.5 mils. 
     The ceramic thermal barrier coat layer  26  may be applied or otherwise formed on the alumina layer  24  by any of a variety of conventional techniques, such as those used for the bond coat layer  22 . For example, the thermal barrier coat layer  26  may be applied using a physical vapor deposition, electron beam deposition, plasma spray, or other thermal spray deposition methods such as high velocity oxy-fuel spray, chemical vapor deposition, or a combination of such techniques. The thickness of the thermal barrier coat layer  26  is typically from about 1 to about 100 mils (from about 25.4 to about 2540 microns) and will depend upon a variety of factors, including the operational environmental condition of the metal article  10  that is involved. 
     The ceramic thermal barrier coat layer  26  may be comprised of those materials that are capable of reducing heat flow to the underlying superalloy metal substrate  10 . These materials usually have a melting point of at least about 2000° F. typically at least about 2200° F., and more typically in the range of from about 2200° F. to about 3500° F. Suitable materials for the ceramic thermal barrier coat layer  26  include various zirconias, chemically stabilized zirconias (i.e., various metal oxides such as yttrium oxides blended with zirconia), such as yttria-stabilized zirconias, ceria-stabilized zirconias, calcia-stabilized zirconias, scandia-stabilized zirconias, magnesia-stabilized zirconias, india-stabilized zirconias, ytterbia-stabilized zirconias as well as mixtures of such stabilized zirconias and some incidental impurities. The ceria, india, magnesia, scandia, yttria or ytterbia is added to the zirconia to stabilize the zirconia in the tetragonal/cubic crystal structure. 
     During manufacture of the superalloy article, after the thermal barrier coat layer  26  is formed on the bond coat layer  22 , the system is thermally cycled which results in a thin layer of alumina forming between the bond coat and the thermal barrier coat layer  26 . The alumina layer  24  may comprise alumina and may also include other oxides. The elemental barium from the bulk bond coat layer  22  then diffuses into the alumina layer  24  and segregates to alumina grain boundaries. Conventionally, transition metals such as yttrium, hafnium, lanthanum, cerium and scandium are added to the bond coat layer  22  to strengthen the alumina layer  24  and lower its growth rate. The conventional transition metal dopants also tend to inhibit the diffusion creep of the alumina layer because the oxide growth and diffusion creep occur by mechanistically similar processes. 
     However, the inventors have found that the presence of elemental barium in the alumina grain boundaries significantly enhances the overall creep resistance of the alumina layer  24  well beyond the enhancement achieved by the conventional doping materials. The inventors have found that the presence of barium in the alumina grain boundary increases the grain boundary sliding activation energy in alumina substantially more than any of the conventional transition metal dopants. The inventors believe that because creep occurs through a combination of both mechanisms: 1) diffusion and 2) grain boundary sliding, the increase in the grain boundary sliding activation energy resulting from the presence of elemental barium substantially improves the overall creep resistance of the alumina layer. 
     The TBC system  20  of the present disclosure may be useful with a variety of turbine engine parts and components that are formed from metal substrates comprising metals, metal alloys, including superalloys that are used in operational conditions exposing the components to high temperatures that occur during normal turbine engine operation. 
     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention. 
     One or more features from any embodiment may be combined with one or more features of any other embodiment without departing from the scope of the invention. A recitation of “a”, “an” or “the” in the above description is intended to mean “one or more” unless specifically indicated to the contrary.