`Spitsberg et al.
`
`US005985470A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,985,470
`Nov. 16, 1999
`
`[54] THERMAL/ENVIRONMENTAL BARRIER
`COATING SYSTEM FOR SILICON_BASED
`MATERIALS
`
`.... .. 427/452
`2/1995 Lee et al.
`5,391,404
`.... .. 428/698
`5,683,825 11/1997 Bruce et al.
`5,759,932
`6/1998 Sangeeta et a1. ....................... .. 501/85
`
`[75] Inventors: Irene T. Sp1tsberg', Cincinnati, Ohio;
`Hongyu Wang’ Nlskayuna’ N'Y'
`
`_
`
`_
`
`_
`
`_
`
`_
`
`Primary Examiner—Timothy Speer
`
`Assistant Examiner_Bryant Young
`Attorney, Agent, or Firm—AndreW C. Hess; David L.
`
`[73] Assignee: General Electric Company, Cincinnati,
`Ohio
`
`Narclso
`[57]
`
`ABSTRACT
`
`[21] AppL NO‘: 09/039,477
`
`[22]
`
`Filed;
`
`M31; 16, 1998
`
`6
`Int. Cl. .................................................... .. B3213 17/06
`[51]
`[52] U-S- Cl- ------------------------ ~- 428/689; 428/698; 428/701;
`428/702; 428/469; 428/472
`[58] Field of Search ................................... .. 428/408, 432,
`428/457, 469, 472, 688, 689, 698, 699,
`701, 702; 501/88, 8, 32, 89
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`A coating system for a substrate containing a silicon-based
`material, such as silicon carbide-containing ceramic matrix
`materials containing silicon carbide and used to form articles
`exposed to high temperatures, including the hostile thermal
`environment of a gas turbine engine_ The Coating System
`includes a layer of barium strontium aluminosilicate (BSAS)
`as a bond coat for a thermal-insulating top coat. As a bond
`coat, the BSAS layer serves to adhere the top coat to a
`SiC-containing substrate. The BSAS bond coat exhibits
`suf?cient environmental resistance such that, if the top coat
`should spall, the BSAS bond coat continues to provide a
`level of environmental protection to the underlying SiC
`Contammg Substrate"
`
`5,214,004
`
`5/1993 Bansal ....................................... .. 501/8
`
`20 Claims, 1 Drawing Sheet
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`1 0
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`x
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`GE-1013.001
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`
`
`U.S. Patent
`
`Nov. 16, 1999
`
`5,985,470
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`10x
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`FIG.
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`1
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`GE-1013.002
`
`
`
`1
`THERMAL/ENVIRONMENTAL BARRIER
`COATING SYSTEM FOR SILICON-BASED
`MATERIALS
`
`This invention Was made With Government support
`under Agreement No. NAS3-26385 awarded by NASA. The
`Government has certain rights in the invention.
`
`FIELD OF THE INVENTION
`
`This invention relates to coating systems suitable for
`protecting components exposed to high-temperature
`environments, such as the hostile thermal environment of a
`gas turbine engine. More particularly, this invention is
`directed to a thermal/environmental barrier coating system
`for a substrate formed of a material containing silicon.
`
`10
`
`15
`
`BACKGROUND OF THE INVENTION
`
`Higher operating temperatures for gas turbine engines are
`continuously sought in order to increase their ef?ciency.
`HoWever, as operating temperatures increase, the high tem
`perature durability of the components of the engine must
`correspondingly increase. Signi?cant advances in high tem
`perature capabilities have been achieved through formula
`tion of iron, nickel and cobalt-base superalloys. HoWever,
`components formed from superalloys must be provided With
`some form of thermal and/or environmental protection in
`order to exhibit adequate service lives in certain sections of
`a gas turbine engine, such as the turbine, combustor and
`augmentor. Acommon solution is to thermally insulate such
`components in order to minimize their service temperatures.
`For this purpose, thermal barrier coatings (TBC) formed on
`the exposed surfaces of high temperature components have
`found Wide use. For superalloy components, oxidation
`resistant aluminum-based intermetallic diffusion coatings
`such as platinum aluminide, and oxidation-resistant
`aluminum-containing overlay coatings such as MCrAlY
`(Where M is iron, cobalt and/or nickel), are Widely used as
`environmental coatings. These coating materials are also
`used to form bond coats to adhere a TBC, Which is typically
`a metal oxide such as Zirconia (ZrO2) that is partially or fully
`stabiliZed by yttria (Y2O3), magnesia (MgO) or other oxides.
`While superalloys have found Wide use for components
`throughout gas turbine engines, alternative materials have
`been proposed. Materials containing silicon, particularly
`those With silicon carbide (SiC) as a matrix material or a
`reinforcing material, are currently being considered for high
`temperature applications, such as combustor and other hot
`section components of gas turbine engines. In many
`applications, a protective coating over the Si-containing
`material is bene?cial. For example, protection With a suit
`able thermal-insulating layer reduces the operating tempera
`ture and thermal gradient through the material. Additionally,
`such coatings can provide environmental protection by
`inhibiting the major mechanism for degradation of silicon
`carbide in a corrosive environment, namely, the formation of
`volatile silicon monoxide (SiO) and silicon hydroxide (Si
`(OH)4) products. On this basis, besides loW thermal
`conductivity, a critical requirement of a thermal barrier
`coating system for a SiC-containing material is loW activity
`of silica (SiO2) in its composition. Other important proper
`ties for the coating material include a coef?cient of thermal
`expansion (CTE) compatible With the SiC-containing
`material, loW permeability for oxidants, and chemical com
`patibility With SiC and silica scale. Consequently, the coat
`ing essentially has a dual function, serving as a thermal
`barrier and simultaneously providing protection from the
`
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`5,985,470
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`environment. A coating system having this dual function
`may be termed a thermal/environmental barrier coating
`(TBC/EBC) system.
`While various coating systems have been investigated,
`each has exhibited shortcomings relating to the above-noted
`requirements and properties for compatibility With a
`Si-containing material. For example, an yttria-stabiliZed
`Zirconia (YSZ) coating serving as a thermal barrier layer
`exhibits excellent environmental resistance by itself, since it
`does not contain silica in its composition. HoWever, YSZ
`exhibits high permeability to oxygen and other oxidants. In
`addition, YSZ cannot be adhered directly to silicon carbide
`because of a CTE mismatch. As a result, mullite
`(3Al2O3.2SiO2) has been proposed as a bond coat betWeen
`a SiC-containing substrate material and a ceramic TBC in
`order to compensate for differences in CTE. HoWever,
`mullite exhibits signi?cant silica activity and volatiliZation
`at high-temperature exposures to a Water vapor-containing
`environment. This can especially be the case if the YSZ TBC
`is deposited by electron beam physical vapor deposition
`(EBPVD) techniques, and consequently has a columnar
`grain structure that is permeable to oxidant species.
`Accordingly, there is a need for an improved TBC/EBC
`system for Si-based materials.
`
`SUMMARY OF THE INVENTION
`The present invention generally provides a coating system
`for a Si-based material, such as those used to form articles
`exposed to high temperatures, including the hostile thermal
`environment of a gas turbine engine. Examples of such
`materials include those With a dispersion of silicon carbide
`particles as a reinforcement material in a metallic or non
`metallic matrix, as Well as those having a silicon carbide
`matrix, and particularly composite materials that employ
`silicon carbide as both the reinforcement and matrix mate
`rials (Si/SiC composites).
`The invention is a thermal/environmental barrier coating
`(TBC/EBC) system that includes a coating of barium stron
`tium aluminosilicate (BSAS) as a bond coat for a thermal
`insulating layer. In a preferred embodiment, the coating
`consists essentially of barium strontium aluminosilicate. As
`a bond coat, the BSAS coating serves to adhere a top coat,
`such as a ceramic coating, to a SiC-containing substrate.
`Suitable materials for the top coat include Zirconia partially
`or fully stabiliZed With yttria (YSZ), and yttrium silicate.
`According to this invention, a bond coat formed of BSAS
`is able to provide environmental protection to a SiC
`containing substrate as a result of being chemically and
`physically compatible With SiC. In particular, BSAS is
`chemically compatible in terms of loW silica activity and
`loW reactivity With silica, and physically compatible in
`terms of having a CTE close to that of SiC. Furthermore,
`diffusivity of oxygen in BSAS is loW, thereby inhibiting the
`groWth of an interfacial silica layer at the surface of a
`Si-based substrate exposed an oxidiZing atmosphere, such as
`that found in the turbine, combustor and augmentor sections
`of a gas turbine engine. BSAS also exhibits loW alumina
`activity, Which inhibits the formation of deleterious alumi
`nosilicates at the interface betWeen the coating and substrate.
`Advantageously, a BSAS bond coat exhibits suf?cient envi
`ronmental resistance such that, if a top coat adhered to the
`substrate With the bond coat should spall, the BSAS bond
`coat Will continue to provide a level of environmental
`protection to the underlying SiC-containing substrate.
`Other objects and advantages of this invention Will be
`better appreciated from the folloWing detailed description.
`
`GE-1013.003
`
`
`
`3
`DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a cross-sectional vieW of a gas turbine engine
`component having a thermal/environmental barrier coating
`system in accordance With this invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The present invention is generally applicable to compo
`nents that operate Within environments characterized by
`relatively high temperatures, and are therefore subjected to
`severe thermal cycling and stresses, oxidation, and corro
`sion. Notable examples of such components include com
`bustor components, high pressure turbine vanes, and other
`hot section components of gas turbine engines. A surface
`region 12 of a hot section component 10 is represented in
`FIG. 1 for purposes of illustrating the invention. The com
`ponent 10 or at least the surface region 12 of the component
`10 is formed of a SiC/SiC composite material, though the
`invention is believed to be generally applicable to any
`material containing a silicon-based material in any form.
`As shoWn in FIG. 1, the surface region 12 is protected by
`a thermal/environmental barrier coating (TBC/EBC) system
`14 that includes a bond coat 16 and a top coat 18. The
`coating system 14 provides environmental protection to the
`underlying surface region 12 as Well as reduces the operat
`ing temperature and thermal gradient through the bond coat
`16 and the component 10, thereby enabling the component
`10 to survive Within higher-temperature environments than
`otherWise possible. Suitable materials for the top coat 18
`include Zirconia partially or fully stabiliZed With yttria
`(YSZ), e.g., about 7 Weight percent yttria, and yttrium
`silicate, though it is foreseeable that other ceramic materials
`could be used.
`The major mechanism for degradation of silicon carbide
`in a corrosive environment is the formation of volatile
`silicon monoxide (SiO) and silicon hydroxide (Si(OH)4)
`products. The diffusivity of oxidants in materials suitable for
`the top coat 18 is generally very high. Therefore, in order to
`protect the SiC-containing surface region 12, the bond coat
`16 must exhibit loW diffusivity to oxidants, e.g., oxygen and
`Water vapor, to inhibit oxidation of the silicon carbon Within
`the surface region 12, While also being suf?ciently chemi
`cally and physically compatible With the surface region 12
`to remain adherent to the region 12 under severe thermal
`conditions. According to this invention, the bond coat 16
`ful?lls these requirements if formed of barium strontium
`aluminosilicate (BSAS). In a preferred embodiment, the
`coating consists essentially of barium strontium alumino
`silicate. A suitable thickness for the BSAS bond coat 16 is
`about 25 to about 500 pm, though greater and lesser thick
`nesses could foreseeably be used.
`As indicated above, the BSAS bond coat 16 of this
`invention provides adhesion of the top coat 18 and is
`physically compliant With a SiC-containing substrate.
`Simultaneously, the BSAS bond coat 16 serves as an envi
`ronmental barrier, since it exhibits loW silica activity and
`loW diffusivity to oxidants, thereby inhibiting the groWth of
`an interfacial silica layer at the surface region 12 When the
`component 10 is exposed to the oxidiZing environment of a
`gas turbine engine. BSAS is also physically compatible With
`the SiC-containing surface region 12 in terms of having a
`CTE of about 5.27 ppm/°C., as compared to a CTE of about
`4.9 ppm/°C. for SiC ceramic matrix composites (CMC).
`Finally, BSAS exhibits loW alumina activity, Which inhibits
`the formation of aluminosilicates that Would create an
`undesirable CTE mismatch at the interface betWeen the bond
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`5,985,470
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`4
`coat 16 and surface region 12. As a result, the TBC/EBC
`system 14 of this invention is able to thermally and envi
`ronmentally protect the SiC-containing surface region 12
`over numerous thermal cycles and at elevated temperatures
`as a result of being chemically and physically compatible
`With SiC. Advantageously, the BSAS bond coat 16 of this
`invention exhibits suf?cient environmental resistance such
`that, if the top coat 18 Were to spall, the bond coat 16
`continues to provide a level of environmental protection to
`the underlying SiC-containing surface region 12.
`As With prior art bond coats and environmental coatings,
`the BSAS bond coat 16 of this invention can be deposited by
`air and vacuum plasma spraying (APS and VPS,
`respectively), though it is foreseeable that deposition could
`be performed by other knoWn techniques, such as physical
`vapor deposition (PVD) and high velocity oxy-fuel (HVOF).
`The top coat 18 can be deposited on the bond coat by knoWn
`techniques, including plasma spraying and PVD techniques.
`Thereafter, a heat treatment may be performed after depo
`sition of the bond coat 16 and/or top coat 18 to relieve
`residual stresses created during cooling from elevated depo
`sition temperatures.
`While our invention has been described in terms of a
`preferred embodiment, it is apparent that other forms could
`be adopted by one skilled in the art. In particular, the bene?ts
`of the thermal/environmental barrier coating system of this
`invention can be realiZed for various substrate materials
`containing silicon or a silicon-based material. Accordingly,
`the scope of our invention is to be limited only by the
`folloWing claims.
`What is claimed is:
`1. An article comprising:
`a substrate comprising silicon;
`a bond coat on the substrate, the bond coat comprising
`barium strontium aluminosilicate; and
`a top coat on the bond coat.
`2. An article as recited in claim 1, Wherein the bond coat
`consists essentially of barium strontium aluminosilicate.
`3. An article as recited in claim 1, Wherein the bond coat
`consists of barium strontium aluminosilicate.
`4. An article as recited in claim 1, Wherein the top coat is
`a ceramic material.
`5. An article as recited in claim 1, Wherein the top coat is
`formed of a material selected from the group consisting of
`yttria stabiliZed Zirconia and yttrium silicate.
`6. An article as recited in claim 1, Wherein the substrate
`is formed of silicon carbide particles dispersed in a matrix
`material.
`7. An article as recited in claim 1, Wherein the substrate
`is formed of a composite having a silicon carbide matrix.
`8. An article as recited in claim 1, Wherein the substrate
`is formed of a composite having silicon carbide reinforce
`ment in a silicon carbide matrix.
`9. An article as recited in claim 1, Wherein the bond coat
`has a thickness of about 25 to about 500 micrometers.
`10. An article as recited in claim 1, Wherein the article is
`a component of a gas turbine engine.
`11. An article comprising:
`a substrate containing silicon carbide;
`a bond coat on the substrate, the bond coat consisting of
`barium strontium aluminosilicate; and
`a thermal-insulating layer overlying the bond coat.
`12. An article as recited in claim 11, Wherein the thermal
`insulating layer is selected from the group consisting of
`yttria stabiliZed Zirconia and yttrium silicate.
`13. An article as recited in claim 11, Wherein the substrate
`is formed of silicon carbide particles dispersed in a matrix
`material.
`
`GE-1013.004
`
`
`
`5,985,470
`
`6
`5
`14. An article as recited in claim 11, Wherein the substrate
`a bond coat on the substrate, the bond coat consisting of
`barium strontium aluminosilicate; and
`is formed of a composite having a silicon carbide matrix.
`a yttria stabiliZed Zirconia layer overlying the bond coat.
`15. An article as recited in claim 11, Wherein the substrate
`19. Acomponent as recited in claim 18, Wherein the bond
`is formed of a composite having silicon carbide reinforce
`5 coat has a thickness of about 25 to about 500 micrometers.
`ment in a silicon carbide matrix.
`20. A component as recited in claim 18, Wherein the
`16. An article as recited in claim 11, Wherein the bond coat
`substrate is formed of a material selected from the group
`has a thickness of about 25 to about 500 micrometers.
`consisting of metal matrix composites reinforced With sili
`17. An article as recited in claim 1, Wherein the article is
`con carbide, composites having a silicon carbide matrix, and
`a component of a gas turbine engine.
`silicon carbide matrix composites reinforced With silicon
`18. A gas turbine engine component having a coating 10 Carbide_
`system on a surface thereof, the component comprising:
`a substrate comprising a silicon;
`
`GE-1013.005