Patent Publication Number: US-2010129673-A1

Title: Reinforced oxide coatings

Description:
This application claims priority from U.S. Provisional Application Ser. No. 61/117,770 filed Nov. 25, 2008, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to oxide coatings. 
     BACKGROUND 
     The components of high-temperature mechanical systems, such as, for example, gas turbine engines, must operate in severe environments. For example, the high-pressure turbine blades and vanes exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about 1000° C., with short-term peaks as high as 1100° C. 
     Typical components of high-temperature mechanical systems include a Ni or Co-based superalloy substrate. The substrate can be coated with a thermal barrier coating (TBC) to reduce surface temperatures. The thermal barrier coating may include a thermally insulative ceramic topcoat, and is bonded to the substrate by an underlying metallic bond coat. 
     The TBC, usually applied either by air plasma spraying or electron beam physical vapor deposition, is most often a layer of yttria-stabilized zirconia (YSZ) with a thickness of about 100-500 μm. The properties of YSZ include low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion. The YSZ TBC is also typically made “strain tolerant” and the thermal conductivity further lowered by depositing a structure that contains numerous pores and/or pathways. 
     Economic and environmental concerns, i.e., the desire for improved efficiency and reduced emissions, continue to drive the development of advanced gas turbine engines with higher inlet temperatures. In some cases, this may lead to the replacement of the superalloy substrate with a silicon-based ceramic or ceramic matrix composite (CMC) substrate. Silicon-based ceramics or CMCs possess excellent high temperature mechanical, physical and chemical properties, which allow gas turbine engines to operate at higher temperatures than gas-turbine engines having superalloy components. 
     However, silicon-based ceramics and CMCs suffer from rapid recession in combustion environments due to the volatilization of silica by water vapor. Thus, silicon-based ceramic and CMC substrates may be coated by an environmental barrier coating (EBC), which protects the substrate from water vapor attack. Additionally, the EBC may thermally insulate the substrate, and an additional TBC may be provided over the EBC to further thermally insulate the silicon-based ceramic or CMC substrate. 
     SUMMARY 
     While a TBC or EBC may provide beneficial thermal and/or environmental protection for the substrate on which the TBC or EBC is deposited, TBCs and EBCs generally possess relatively low fracture toughness. Accordingly, a TBC or EBC easily may be damaged by, for example, debris that impacts the TBC or EBC during operation of a gas turbine engine. This may reduce the service life of an article coated with a TBC or EBC. 
     In general, the present disclosure is directed to a reinforced coating including an oxide matrix and a reinforcement within the oxide matrix. The reinforced coating may be a TBC, EBC, or bond coat, and may be deposited over a substrate. The reinforcement may improve the fracture toughness of the reinforced coating compared to a coating including the oxide matrix and no reinforcement. In some embodiments, other layers may be deposited over the substrate in addition to the reinforced coating, including, for example, one or more additional oxide layers and/or a bond coat. 
     In one aspect, the disclosure is directed to a reinforced coating including an oxide matrix and a reinforcement within the oxide matrix. The reinforcement may include at least one of SiC and Si 3 N 4 . 
     In another aspect, the disclosure is directed to an article including a substrate and a reinforced coating deposited over the substrate. The reinforced coating includes an oxide matrix and a reinforcement within the oxide matrix, and the reinforcement comprises at least one of SiC and Si 3 N 4 . 
     In another aspect, the disclosure is directed to a method that includes depositing over a substrate a coating composition including an oxide matrix and a reinforcement within the oxide matrix. The reinforcement may include at least one of SiC and Si 3 N 4 . 
     A TBC formed as a reinforced coating of the present disclosure may provide improved fracture toughness while still providing desirable thermal insulation for the substrate over which the TBC is deposited. Similarly, an EBC formed as a reinforced coating as described in the present disclosure may provide improved fracture toughness while still providing desirable environmental protection for the substrate over which the EBC is deposited. Increased fracture toughness may result in an increased service life for the article that is coated with the reinforced coating. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example article including a reinforced coating deposited over a substrate. 
         FIG. 2  is a conceptual diagram of another example article including a reinforced coating deposited over a substrate. 
         FIG. 3  is a conceptual diagram of another example article including a reinforced coating deposited over a substrate. 
         FIG. 4  is a cross-sectional view illustrating an example article including a reinforced environmental barrier coating deposited over a substrate and a reinforced thermal barrier coating deposited over the reinforced environmental barrier coating. 
         FIG. 5  is a cross-sectional view illustrating an example article including a reinforced coating deposited over a bond coat, which is deposited over a substrate. 
         FIG. 6  is a cross-sectional view illustrating an example article including a first oxide layer deposited over a reinforced coating, which is deposited over a second oxide layer, which is deposited over a substrate. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a conceptual diagram that illustrates an exemplary article  100  used in a high-temperature mechanical system. Article  100  includes a reinforced coating  104  deposited over a substrate  102 . In some embodiments, as illustrated in  FIG. 1 , reinforced coating  104  may be deposited directly on substrate  102 . As used herein, “deposited over” is defined as a layer that is deposited on top of another layer, and encompasses both a first layer deposited immediately adjacent a second layer and a first layer deposited on top of a second layer with one or more intermediate layer present between the first and second layers. In contrast, “deposited directly on” denotes a layer that is deposited immediately adjacent another layer, i.e., there are no intermediate layers. 
     Substrate  102  may be a component of a high temperature mechanical system, such as, for example, a gas turbine engine or the like. In some embodiments, substrate  102  may include a superalloy, such as a superalloy based on Ni, Co, Ni/Fe, and the like. A substrate  102  including a superalloy may include other additive elements to alter its mechanical properties, such as toughness, hardness, temperature stability, corrosion resistance, oxidation resistance, and the like, as is well known in the art. Any useful superalloy may be utilized for substrate  102 , including, for example, those available from Martin-Marietta Corp., Bethesda, Md., under the trade designation MAR-M247; those available from Cannon-Muskegon Corp., Muskegon, Mich., under the trade designation CMSX-4 and CMXS-10; and the like. 
     In other embodiments, substrate  102  may include a ceramic or ceramic matrix composite (CMC). A substrate  102  including a ceramic or CMC may include any useful ceramic material, including, for example, silicon carbide, silicon nitride, alumina, silica, and the like. The CMC may further include any desired filler material, and the filler material may include a continuous reinforcement or a discontinuous reinforcement. For example, the filler material may include discontinuous whiskers, platelets, or particulates. As another example, the filler material may include a continuous monofilament or multifilament weave. 
     The filler composition, shape, size, and the like may be selected to provide the desired properties to the CMC. For example, in some embodiments, the filler material may be chosen to increase the toughness of a brittle ceramic matrix. In other embodiments, the filler may be chosen to provide a desired property to the CMC, such as thermal conductivity, electrical conductivity, thermal expansion, hardness, or the like. 
     In some embodiments, the filler composition may be the same as the ceramic matrix material. For example, a silicon carbide matrix may surround silicon carbide whiskers. In other embodiments, the filler material may comprise a different composition than the ceramic matrix, such as aluminum silicate fibers in an alumina matrix, or the like. One preferred CMC includes silicon carbide continuous fibers embedded in a silicon carbide matrix. 
     Some example ceramics and CMCs which may be used for substrate  102  include ceramics containing Si, such as SiC and Si 3 N 4 ; composites of SiC or Si 3 N 4  and silicon oxynitride or silicon aluminum oxynitride; metal alloys that include Si, such as a molybdenum-silicon alloy (e.g., MoSi 2 ) or niobium-silicon alloys (e.g., NbSi 2 ); and oxide-oxide ceramics, such as an alumina or aluminosilicate matrix with a NEXTEL™ Ceramic Oxide Fiber 720 (available from 3M Co., St. Paul, Minn.). 
     As described above, a substrate  102  based on a superalloy may be coated with a TBC to provide thermal insulation for substrate  102  and reduce the temperatures experienced by substrate  102 , while a substrate  102  including a ceramic or CMC may be coated with an EBC to protect the substrate  102  from water vapor attack. In addition, in some embodiments, a substrate  102  including a ceramic or CMC may have a TBC deposited over or deposited directly on the EBC to provide further thermal insulation for the substrate  100 . 
     A TBC may be deposited as a porous structure, which may increase the stress tolerance and decrease the effective thermal conductivity of the TBC. For example, the pores in the TBC may reduce the thermal conductivity by reducing the area through which heat is conducted and by providing a large refractive index difference between the pores and the material from which the TBC is formed, which can reduce heat transfer by radiation. In contrast, an EBC may be deposited with little or substantially no porosity, as the EBC is designed to reduce or eliminate exposure of substrate  102  to water vapor or other vapors present in the environment in which article  100  is used. TBCs and EBCs may be deposited as monolithic materials; e.g., they may not include domains of a second material within a matrix of a first material. 
     While the TBC and EBC provide effective thermal insulation and environmental protection of substrate  102 , respectively, each may possess relatively low fracture toughness, at least in part because TBCs and EBCs are deposited as monolithic materials. This may result in a coating that is relatively easy to damage, and may lead to a reduced service life of an article  100  including a TBC and/or EBC. 
     In order to improve fracture toughness of a TBC or EBC, the TBC or EBC may be deposited over substrate  102  as a reinforced coating  104 . In the example illustrated in  FIG. 1 , reinforced coating  104  is deposited directly on substrate  102 , while in other embodiments, such as those illustrated in  FIGS. 5 and 6 , a reinforced coating may be deposited over substrate  102  with intermediate layers present between the reinforced coating and substrate  102 . Reinforced coating  104  may include a reinforcement  108  within an oxide matrix  106 . Oxide matrix  106  may include any TBC or EBC material, such as, for example, alumina, zirconia, hafnia, rare earth oxide-stabilized zirconia, rare earth oxide-stabilized hafnia, a rare earth silicate, a glass ceramic, mullite, or the like, and may include combinations of two or more TBC or EBC materials. 
     The rare earth oxide-stabilized zirconia and rare earth oxide-stabilized hafnia may include oxides of rare earth elements, including, for example, oxides of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof. 
     The rare earth silicate may be formed by mixtures of one or more rare earth oxide with silica (SiO 2 ) in approximately a 1:1 or 1:2 rare earth oxide:silica ratio. For example, the rare earth silicate may include a rare earth monosilicate, represented as RE 2 SiO 5 , where RE is a rare earth element, a rare earth disilicate, represented as RE 2 Si 2 O 7 , where RE is a rare earth element, or both. In some embodiments, the rare earth silicate may also include a rare earth oxide or silica (e.g., in mixtures including ratios of rare earth oxide:silica other than 1:1 or 1:2). 
     The glass ceramic may include, for example, barium strontium alumina silicate (BaO x —SrO 1−x —Al 2 O 3 —2SiO 2 ; BSAS), barium alumina silicate (BaO—Al 2 O 3 —2SiO 2 ; BAS), strontium alumina silicate (SrO—Al 2 O 3 —2SiO 2 ; SAS), calcium alumina silicate (CaO—Al 2 O 3 —2SiO 2 ; CAS), magnesium alumina silicate (2MgO—2Al 2 O 3 —5SiO 2 ; MAS) and lithium alumina silicate (Li 2 O—Al 2 O 3 —2SiO 2 ; LAS); and the like. 
     In some embodiments, reinforcement  108  may include at least one of SiC and Si 3 N 4 . Reinforcement  108  may be formed as particles, whiskers, platelets, chopped fibers or the like, and may be selected to provide the desired properties to reinforced coating  102 . Reinforcement  108  may generally range in size up to a thickness L of reinforced coating  104 , as illustrated in  FIG. 1 . In other embodiments, as illustrated in  FIG. 2 , an article  200  may include a reinforced coating  204 , which may include a reinforcement  208  that is much smaller that the thickness L of the reinforced coating  204 . In some embodiments, as illustrated in  FIG. 3 , an article may include a reinforced coating  304  that includes a mixture of larger reinforcement  108  and smaller reinforcement  208 . Together, the size and shape of the reinforcement  108 ,  208  (hereafter “reinforcement  108 ”) may contribute to properties of reinforced coating  104 . In some embodiments, reinforcement  108  may have a size up to about 500 microns, preferably about 20 microns to about 200 microns, and more preferably about 50 microns to about 150 microns. 
     The shape and/or size of reinforcement  108  may influence the properties of reinforced layer  104 , such as, for example, an effective thermal conductivity of reinforced layer  104 . For instance, a larger reinforcement  108  may reduce porosity of porous reinforced layer  104 , which may increase the effective thermal conductivity of the layer  104 . In this example, it may be desirable to utilize smaller reinforcement particles, whiskers, platelets, chopped fibers or the like, while maintaining a size of reinforcement  108  that provides increased fracture toughness. As another example, in embodiments in which reinforced coating  104  is applied to a turbine blade tip, reinforcement  108  may be selected to have an angular shape, which may improve abrasiveness of reinforced coating  104 . As a further example, in embodiments in which reinforced coating  104  is to provide impact resistance, reinforcement  108  may be selected to have a large aspect ratio (e.g., a size in at least one dimension that is significantly greater than a size in another dimension). 
     Reinforced coating  104  may be deposited over substrate  102  using, for example, plasma spraying, physical vapor deposition, cathodic arc deposition, chemical vapor deposition, slurry dip coating, sol-gel coating, electrophoretic deposition, or the like. 
     When depositing reinforced coating  104  using plasma spraying, oxide matrix  106  may comprise a material that can be melted. For example, SiC and Si 3 N 4  do not melt at temperatures used for plasma spraying. Thus, to deposit reinforced coating  104 , the reinforcement  108  may be encapsulated in a meltable oxide matrix, which results in a plasma-sprayable coating  104 . 
     In some embodiments, it may be important to provide a reinforcement  108  that is well mixed in oxide matrix  106  to provide improved properties, such as impact resistance or the like. The process by which the reinforcement  108  is mixed into oxide matrix  106  may influence the exfoliation of the reinforcement  108 , and sol-gel, slurry or spray coating processes may provide greater exfoliation of the reinforcement  108 . 
     In some embodiments, as illustrated in  FIG. 4 , an article  400  may include a reinforced EBC  404  deposited over or deposited directly on substrate  102  and a reinforced TBC  406  deposited over or deposited directly on EBC  404 . As described above, a substrate  102  including a ceramic or CMC may have an EBC deposited over substrate  102  to provide protection from water vapor attack and some thermal insulation, and a TBC deposited over the EBC to provide further thermal insulation. In some embodiments, as illustrated in  FIG. 4 , the EBC may be a reinforced EBC  404  and the TBC may be a reinforced TBC  406 . In other embodiments, only one of the EBC or the TBC may be a reinforced coating. 
     Reinforced EBC  404  may include an oxide matrix (e.g, oxide matrix  106 ) and a reinforcement (e.g., reinforcement  108 ). The oxide matrix may include, for example, rare earth silicates; mullite; alumina; glass ceramics such as BSAS, BAS, SAS, CAS, MAS and LAS; and the like. The reinforcement in reinforced EBC  404  may include SiC, Si 3 N 4 , and the like, and may be present as particles, whiskers, platelets, chopped fibers or the like. 
     Reinforced TBC  406  also may include an oxide matrix and a reinforcement. The oxide matrix may include, for example, zirconia, hafnia, a rare earth oxide-stabilized zirconia, a rare earth oxide-stabilized hafnia, or the like. The reinforcement in reinforced TBC  406  may include SiC, Si 3 N 4 , and the like, and may be present as particles, whiskers, platelets, chopped fibers or the like. 
     In some embodiments, a reinforced coating may be deposited over or deposited directly on a bond coat. For example, as illustrated in  FIG. 5 , reinforced coating  504  may be deposited directly on bond coat  506 . Bond coat  506  may improve adhesion between reinforced coating  504  and substrate  102 . In some embodiments, substrate  102  may include a superalloy, and bond coat  506  may include an alloy, such as a conventional MCrAlY alloy (where M is Ni, Co, or NiCo), a β-NiAl nickel aluminide alloy (either unmodified or modified by Pt, Cr, Hf, Zr, Y, Si, and combinations thereof), a γ-Ni+γ′-Ni 3 Al nickel aluminide alloy (either unmodified or modified by Pt, Cr, Hf, Zr, Y, Si, and combinations thereof), or the like. 
     In other embodiments, bond coat  506  may include ceramics or other materials that are compatible with a substrate  102  including a ceramic or CMC. For example, bond coat  506  may include mullite (aluminum silicate, Al 6 Si 2 O 13 ), silica, silicides, silicon, or the like. Bond coat  506  may further include other ceramics, such as rare earth silicates including lutetium silicates (Lu: Lutetium), ytterbium silicates (Yb: Ytterbium), thulium silicates (Tm: Thulium), erbium silicates (Er: Erbium), holmium silicates (Ho: Holmium), dysprosium silicates (Dy: Dysprosium), terbium silicates (Tb: Terbium), gadolinium silicates (Gd: Gadolinium), europium silicates (Eu: Europium), samarium silicate (Sm: Samarium), promethium silicates (Pm: Promethium), neodymium silicates (Nd: Neodymium), praseodymium silicates (Pr: Praseodymium), cerium silicates (Ce: Cerium), lanthanum silicates (La: Lanthanum), yttrium silicates (Y: Yttrium), scandium silicates (Sc: Scandium), or the like. Some preferred compositions of a bond coat  506  for depositing over a CMC substrate  102  include silicon, mullite and ytterbium silicate. 
     Bond coat  506  may be selected based on a number of considerations, including the chemical composition and phase constitution of reinforced coating  504  and substrate  102 . For example, when substrate  102  includes a superalloy with γ-Ni+y′-Ni 3 Al phase constitution, bond coat  506  preferably includes a γ-Ni+γ′-Ni 3 Al phase constitution to better match the coefficient of thermal expansion of substrate  102 , and therefore increase the mechanical stability (adhesion) of bond coat  506  to substrate  102 . Alternatively, when substrate  102  comprises a CMC, bond coat  506  preferably comprises silicon and/or a ceramic, such as, for example, mullite or a rare earth silicate. 
     In some embodiments, bond coat  506  may include multiple layers. For example, in some embodiments where the substrate  102  is a CMC including silicon carbide, a bond coat  506  including a first layer of silicon may be deposited on substrate  102 , followed by the deposition of a second layer including mullite (aluminum silicate, 3Al 2 O 3 —2SiO 2 ) or a rare earth silicate. The bond coat  506  including multiple layers may be desirable because the silicon layer provides bonding while the ceramic bond coat provides a gradual transition of thermal expansion and prevents water vapor from reaching the silicon layer. 
     In other embodiments, such as article  100  illustrated in  FIG. 1 , the article  100  may not include a bond coat  506 , and reinforced coating  104  may be deposited directly on substrate  102 . Bond coat  506  may not be required or desired when reinforced coating  104  and substrate  102  are chemically and/or mechanically compatible. For example, in embodiments in which reinforced coating  104  and substrate  102  adhere sufficiently strongly to each other, a bond coat  506  may not be necessary. Additionally, in embodiments where the coefficients of thermal expansion of substrate  102  and reinforced coating  104  are sufficiently similar, a bond coat  506  may not be necessary. 
       FIG. 6  illustrates a cross-sectional view of an article  600  including a first oxide layer  608  and a second oxide layer  610 , in addition to reinforced coating  604 , deposited over substrate  102 . First oxide layer  608  and second oxide layer  610  may provide additional properties, such as, for example, calcia-magnesia-alumina-silicate (CMAS) resistance, additional thermal insulation, additional environmental protection, prevention of reaction between substrate  102  and reinforced coating  604 , or the like. 
     As illustrated in  FIG. 6 , first oxide layer  608  may be deposited directly on substrate  102 , reinforced coating  604  may be deposited directly on first oxide layer  608 , and second oxide layer  610  may be deposited directly on reinforced coating  604 . However, in other embodiments, article  600  may include only one of first oxide layer  608  and second oxide layer  610 . Further, in some embodiments, article  600  may include a bond coat  506  deposited over substrate  102  in combination with one or both of first oxide layer  608  and second oxide layer  610 . For example, article  600  may include a bond coat  506  deposited over substrate  102 , first oxide layer  608  deposited over bond coat  506  and reinforced coating  604  deposited over first oxide layer  608 . As another example, article  600  may include reinforced coating  604  deposited over bond coat  506  and second oxide layer  610  deposited over reinforced coating  604 . 
     First oxide layer  608  may include, for example, alumina, zirconia and a rare earth oxide, hafnia and a rare earth oxide, a rare earth silicate, a rare earth silicate and silicon, a glass ceramic such as BAS, BSAS, SAS, CAS, MAS or LAS, or mullite. 
     In some embodiments, first oxide layer  608  may prevent chemical reaction between reinforced coating  604  and substrate  102  or reinforced coating  604  and bond coat  506 . In some embodiments, first oxide layer  608  may provide additional environmental protection for substrate  102 , such as, for example, when reinforced coating  604  is a TBC deposited over substrate  102  including a ceramic or CMC. 
     Second oxide layer  610  may include rare earth oxide stabilized zirconia, rare earth oxide stabilized hafnia, rare earth silicates, glass ceramics (including, for example, BAS, BSAS, SAS, CAS, MAS and LAS), and the like. Second oxide layer  610  may provide environmental or thermal protection for substrate  102 . 
     In some embodiments, second oxide layer  610  may be a sacrificial layer which provides protection for reinforced layer  604  from CMAS. For example, as described in further detail in U.S. patent application Ser. No. 12/016,649, entitled “CMAS-Resistant Thermal Barrier Coatings,” to Lee, which is incorporated herein by reference in its entirety, a CMAS-resistant layer including a rare earth oxide and essentially free of zirconia and hafnia may be deposited over a TBC or an EBC to provide CMAS-degradation resistance. 
     Each of first and second oxide layers  608  and  610  may be deposited over substrate  102  using a variety of coating techniques, including, for example, plasma spraying, physical vapor deposition, cathodic arc deposition, chemical vapor deposition, slurry dip coating, sol-gel coating, electrophoretic deposition, or the like. First and second oxide layer  608  and  610  may be deposited over substrate  102  using the same coating technique, or may be deposited over substrate using different coating techniques. 
     In some embodiments, at least one of first and second oxide layers  608  and  610  may also include a reinforcement. For example, second oxide layer  610  may be a sacrificial layer including a rare earth oxide and a SiC or Si 3 N 4  reinforcement. In addition, article  600  may include further oxide layers in addition to first and second oxide layers  608  and  610 . The additional oxide layers may further provide or tailor the properties of the coating including first and second oxide layer  608  and  610 , reinforced coating  604  and any additional oxide layers. 
     While the present disclosure has been primarily directed to TBCs or EBCs including an oxide matrix and a reinforcement within the oxide matrix, the invention is not so limited. For example, an oxide matrix and reinforcement within the oxide matrix may be used in other layers of a coating, such as, for example a bond coat. 
     Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.