(12) United States Patent
`Eaton, Jr. et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,296,941 B1
`Oct. 2, 2001
`
`US006296941B1
`
`(54) SILICON BASED SUBSTRATE WITH
`YTTRIUM SILICATE
`ENVIRONMENTAL/THERMAL BARRIER
`LAYER
`
`(75) Inventors: Harry Edwin Eaton, Jr., Woodstock;
`William Patrick Allen, Portland, both
`of CT (US); Nathan S. Jacobson;
`Kang N_ Lee both of Westlake OH
`_
`-
`’
`-
`’
`gUHS)(Ulgl;?giilzgilog’l?iglikewo0d’
`Strongsville, OH (US); Hongyu Wang;
`Péter Joel Meschter, both of
`Niskayuna, NY (US); Krishan Lal
`Luthra, Schenectady, NY (US)
`
`(73) Assignees: General Electric Company, NY (US);
`United Technologies Corporation, DE
`(US); NASA, Washington, DC (US)
`
`(58) Field of Search ................................... .. 428/325, 446,
`428/450, 469, 472, 472.2, 701, 702, 621,
`629, 632, 633, 698, 364, 384
`
`56
`(
`)
`
`References Cited
`
`[)5 PATENT DOCUMENTS
`
`4,713,302 * 12/1987 Komatsu ............................ .. 428/698
`4,816,349
`3/1989 Matsui et a1. .
`5,308,806 * 5/1994 Maloney et a1. ..................... .. 501/95
`5’871’820
`2/1999 HaSZ et al' '
`FOREIGN PATENT DOCUMENTS
`
`0 707 188
`
`4/1996 (EP) -
`
`* Cited by eXarniner
`
`Primary Examiner—Deborah Jones
`
`_
`
`_
`
`_
`
`_
`
`_
`
`_
`
`Assistant Examiner—Bryant Young
`
`( * ) Nonce'
`
`igltkggftigoeirgngsghgrmsgligfeff?égr?gg
`U.S.C. 154(b) by 0 days.
`(21) App1_ No; 09/292,348
`_
`_
`(22) Flled'
`Apr‘ 15’ 1999
`(51) Int. c1.7 ...................................................... .. B32B 9/00
`(52) US. Cl. ........................ .. 428/446; 428/450; 428/472;
`428/698
`
`(74) Attorney, Agent, or Firm—Bachman & LaPointe, P.C.
`(57)
`ABSTRACT
`Ahhka'rrierh layer for a silicon containing subs'tlr'ate Wh?Ch
`1n 1 its t e ormation o gaseous species 0 s11con W en
`exposed to a high temperature aqueous environment com
`prises a yttrium Silicate
`
`29 Claims, 2 Drawing Sheets
`
`1
`
`UTC 2019
`General Electric v. United Technologies
`IPR2016-01289
`
`

`

`U.S. Patent
`
`0a. 2, 2001
`
`Sheet 1 of2
`
`US 6,296,941 B1
`
`
`
`
`
`ANEQEWEV @9650 $22
`
`2.2%.“,
`
`—E1—- YS/Y-25O cycles
`—A—- YS/Y-96 cycles
`"'0- SiC
`
`100
`150
`200
`Number of Cycles
`
`250
`
`300
`
`FIG.
`
`1
`
`2
`
`

`

`U.S. Patent
`U.S. Patent
`
`0a. 2, 2001
`Oct. 2, 2001
`
`Sheet 2 of2
`Sheet 2 0f2
`
`US 6,296,941 B1
`US 6,296,941 B1
`
`3
`
`

`

`US 6,296,941 B1
`
`1
`SILICON BASED SUBSTRATE WITH
`YTTRIUM SILICATE
`ENVIRONMENTAL/THERMAL BARRIER
`LAYER
`
`The invention described herein Was made in the perfor
`mance of Work under NASA Contract No. NAS3-26385, and
`is subject to the provisions of Section 305 of the National
`Aeronautics and Space Act of 1958, as amended (42 U.S.C.
`2457).
`
`BACKGROUND OF THE INVENTION
`The present invention relates to an article comprising a
`substrate containing silicon and a barrier layer Which func
`tions as a protective environmental/thermal barrier coating
`and, more particularly, a barrier layer Which inhibits the
`formation of gaseous species of Si, particularly Si(OH)x
`When the article is exposed to a high temperature, aqueous
`(Water and/or steam) environment.
`Ceramic materials containing silicon and metal alloys
`containing silicon have been proposed for structures used in
`high temperature applications as, for example, gas turbine
`engines, heat exchangers, internal combustion engines, and
`the like. Aparticular useful application for these materials is
`for use in gas turbine engines Which operate at high tem
`peratures in aqueous environments. It has been found that
`these silicon containing substrates can recede and lose mass
`as a result of a formation volatile Si species, particularly
`Si(OH)x and SiO When exposed to high temperature, aque
`ous environments. For example, silicon carbide When
`exposed to a lean fuel environment of approximately 1 ATM
`pressure of Water vapor at 1200° C. Will exhibit Weight loss
`and recession at a rate of approximately 6 mils per 1000 hrs.
`It is believed that the process involves oxidation of the
`silicon carbide to form silica on the surface of the silicon
`carbide folloWed by reaction of the silica With steam to form
`volatile species of silicon such as Si(OH)x. Naturally it
`Would be highly desirable to provide a external barrier
`coating for silicon containing substrates Which Would inhibit
`the formation of volatile silicon species, Si(OH)x and SiO,
`and thereby reduce recession and mass loss.
`Accordingly, it is the principle object of the present
`invention to provide an article comprising a silicon contain
`ing substrate With a barrier layer Which inhibits the forma
`tion of gaseous species of Si, particularly Si(OH)x, When the
`article is exposed to a high temperature, aqueous environ
`ment.
`A second objective of this invention is to provide an
`article comprising a substrate With a barrier layer providing
`thermal protection, such layer closely matching the thermal
`expansion of the substrate.
`It is a further object of the present invention to provide a
`method for producing an article as aforesaid.
`
`SUMMARY OF THE INVENTION
`The present invention relates to an article comprising a
`silicon containing substrate having a barrier layer on the
`substrate, Wherein the barrier layer functions to both inhibit
`the formation of undesirable gaseous species of silicon When
`the article is exposed to a high temperature, aqueous envi
`ronment and to provide thermal protection. By high tem
`peratures is meant the temperature at Which the Si in the
`substrate forms Si(OH)x and/or SiO in an aqueous environ
`ment. By aqueous environment is meant a Water and/or
`steam environment. The silicon containing composite is
`preferably a ceramic or metal alloy containing silicon. The
`external barrier layer is characteriZed by a coefficient of
`
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`thermal expansion Which is Within plus or minus 3.0 ppm
`per degree centigrade of the coefficient of expansion of the
`silicon containing substrate. The preferred barrier layer in
`accordance With the present invention is a yttrium silicate
`barrier layer. In a preferred embodiment of the present
`invention the article can include one or more intermediate
`layers betWeen the silicon based substrate and the barrier
`layer. The intermediate layer(s) serve(s) to provide enhanced
`adherence betWeen the barrier layer and the substrate and/or
`to prevent reactions betWeen the barrier layer and the
`substrate.
`The invention further relates to a method for producing an
`article comprising a silicon containing substrate and a bar
`rier layer Which inhibits the formation of gaseous species of
`silicon and/or provides thermal protection When the article is
`exposed to a high temperature, aqueous environment as
`de?ned above.
`Further objects and advantages of the present invention
`Will appear hereinbeloW from the folloWing detailed descrip
`tion.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a graph shoWing the stability of the barrier layer
`of the present invention With respect to recession and mass
`loss; and
`FIG. 2 is a photomicrograph through a sample of the
`barrier layer of the present invention on a silicon carbide
`substrate.
`
`DETAILED DESCRIPTION
`
`The present invention relates to an article comprising a
`silicon substrate and a barrier layer, Wherein the barrier layer
`inhibits the formation of gaseous species of silicon When the
`article is exposed to a high temperature, aqueous environ
`ment. The invention also relates to a method for producing
`the aforesaid article. In addition, it should be appreciated
`that While the barrier is particularly directed to an environ
`mental barrier layer, the barrier layer also functions as a
`thermal barrier layer and thus the present invention broadly
`encompasses the use of environmental/thermal barrier layers
`on silicon containing substrates and on substrates having
`comparable thermal expansion coefficients.
`According to the present invention, the silicon containing
`substrate may be a silicon ceramic substrate or a silicon
`containing metal alloy. In a preferred embodiment, the
`silicon containing substrate is a silicon containing ceramic
`material as, for example, silicon carbide, silicon nitride,
`silicon oxynitride and silicon aluminum oxynitride. In
`accordance With a particular embodiment of the present
`invention, the silicon containing ceramic substrate com
`prises a silicon containing matrix With reinforcing materials
`such as ?bers, particles and the like and, more particularly,
`a silicon based matrix Which is ?ber-reinforced. Particularly
`suitable ceramic substrates are a silicon carbide coated
`silicon carbide ?ber-reinforced silicon carbide particle and
`silicon matrix, a carbon ?ber-reinforced silicon carbide
`matrix and a silicon carbide ?ber-reinforced silicon nitride
`matrix. Particularly useful silicon-metal alloys for use as
`substrates for the article of the present invention include
`molybdenum-silicon alloys, niobium-silicon alloys, iron
`silicon alloys, and aluminum-silicon alloys.
`Barrier layers particularly useful in the article of the
`present invention include yttrium silicates. In particular, the
`barrier layer comprises YZO3 in an amount of betWeen about
`66% by Weight to about 78% by Weight, balance SiO2,
`
`4
`
`

`

`US 6,296,941 B1
`
`3
`preferably between about 75% by Weight to about 76% by
`Weight Y2O3, balance SiO2. A particularly suitable barrier
`layer for use on silicon containing ceramic compositions in
`the article of the present invention comprises about 75.45%
`by Weight YZO3 and 24.55% by Weight SiO2.
`It is an important feature of the present invention to
`maintain compatibility betWeen the coefficient of thermal
`expansion of the silicon containing substrate and the barrier
`layer. In accordance With the present invention it has been
`found that the coefficient of thermal expansion of the barrier
`layer should be Within 13.0 ppm per degrees centigrade,
`preferably 10.5 ppm per degrees centigrade, of the coeffi
`cient of thermal expansion of the silicon containing sub
`strate. When using a silicon containing ceramic substrate
`such as a silicon carbide or a silicon nitride matrix With or
`Without reinforcing ?bers as described above in combination
`With the preferred yttrium silicate barrier layer of the present
`invention, the desired thermal compatibility With respect to
`expansion coefficient betWeen the silicon containing sub
`strate and the barrier layer should be 12.00 ppm per degrees
`centigrade.
`The barrier layer should be present in the article at a
`thickness of greater than or equal to about 0.5 mils (0.0005
`inch), preferably betWeen about 3 to about 30 mils and
`ideally betWeen about 3 to about 5 mils. The barrier layer
`may be applied to the silicon containing substrate by any
`suitable manner knoWn in the art, hoWever, it is preferable
`that the barrier layer be applied by thermal spraying as Will
`be described hereinbeloW.
`In a further embodiment of the article of the present
`invention, an intermediate layer can be provided betWeen
`the silicon containing substrate and the barrier layer. The
`intermediate layer(s) serve(s) to provide enhanced adhesion
`betWeen the barrier layer and the substrate and/or to prevent
`reactions betWeen the barrier layer and the substrate. The
`intermediate layer consists of, for example, SiO2, mullite,
`mullite-barium strontium aluminosilicate, mullite-yttrium
`silicate, mullite-calcium aluminosilicate, and silicon metal.
`Mullite has been found to be a particularly useful interme
`diate layer; hoWever, mullite by itself tends to be cracked as
`the result of thermal spraying fabrication processing.
`Accordingly, it is preferred that the barrier layer comprises
`mullite-barium strontium aluminosilicate, mullite-yttrium
`silicate, or mullite-calcium aluminosilicate in an amount of
`betWeen about 40 to 80 Wt. % mullite and betWeen about 20
`to 60 Wt. % barium strontium aluminosilicate or yttrium
`silicate or calcium aluminosilicate. The thickness of the
`intermediate layer is typical to those described above With
`regard to the barrier layer and the intermediate layer may
`likeWise be disposed in any manner knoWn in the prior art,
`hoWever, preferably by thermal spraying as described here
`inbeloW.
`In addition to the intermediate layer, a bond layer may be
`provided betWeen the silicon containing substrate and the
`intermediate layer. A suitable bond layer includes silicon
`metal in a thickness of 3 to 6 mils. Alternatively, the silicon
`based substrate may be pre-oxidiZed to provide a SiO2 bond
`layer prior to application of the intermediate layer.
`The method of the present invention comprises providing
`a silicon containing substrate and applying a barrier layer
`Wherein the barrier layer inhibits the formation of gaseous
`species of silicon When the article is exposed to a high
`temperature, aqueous environment. In accordance With the
`present invention it is preferred that the barrier layer be
`applied by thermal spraying. It has been found that the
`barrier layer should be thermal sprayed at a temperature of
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`betWeen about 800° C. to 1200° C. in order to help equili
`brate as-sprayed, splat quenched, microstructure and to
`provide a means to manage stresses Which control delami
`nation.
`
`The silicon containing substrate should be cleaned prior to
`application of the barrier layer to remove substrate fabrica
`tion contamination. It is preferred that the silicon based
`substrate be subjected to a grit blasting step prior to appli
`cation of the barrier layer. The grit blasting step must be
`carried out carefully in order to avoid damage to the surface
`of the silicon-containing substrate such as silicon carbide
`?ber reinforced composite. It has been found that the par
`ticles used for the grit blasting should not be as hard as the
`substrate material to prevent erosive removal of the substrate
`and the particles must be small to prevent impact damage to
`the substrate. When processing an article comprising a
`silicon carbide ceramic substrate, it has been found that the
`grit blasting should be carried out With A1203 particles,
`preferably of a particle siZe of 230 microns and, preferably,
`at a velocity of about 150 to 200 m/sec. In addition to the
`foregoing, it may be particularly useful to preoxidiZe the
`silicon based substrate prior to application of the interme
`diate and/or barrier layer in order to improve adherence. It
`has been found that bond layers of betWeen 100 nanometers
`to 2000 nanometers are preferred. SiO2 bond layers of the
`desired thickness can be achieved by preoxidiZing the
`silicon-carbide substrate at a temperature of betWeen 800°
`C. to 1200° C. for about 15 minutes to 100 hours.
`The silicon bond layer may be applied directly to the grit
`blasted surface by thermal spraying at approximately 870°
`C. to a thickness of 3 to 6 mils.
`
`Intermediate layers may be applied betWeen the substrate
`and/or bond layer and the barrier layer or betWeen the bond
`layer and barrier layer by thermal spraying in the same
`manner described above With respect to the barrier layer. As
`noted above, the preferred intermediate layers include
`mullite, mullite-barium strontium aluminosilicate, mullite
`yttrium silicate, and mullite-calcium aluminosilicate.
`After application of the desired layers to the silicon-based
`substrate material, the article is subjected to a heat treatment
`step in order to provide stress relief to the thermal sprayed
`structure, and to promote bonding betWeen the sprayed
`poWder particles and betWeen the layers and the substrate.
`The heat treatment step is carried out at a temperature of
`about 1250° C. for about 24 hours.
`
`The advantages of the article of the present invention Will
`become clear from consideration of the folloWing examples.
`
`EXAMPLE 1
`
`0.9/1.1 mole ratio yttrium silicate specimens Were fabri
`cated via hot pressing in Argon at 1400° C. for up to 250
`thermal cycles in comparison to silicon carbide. The results
`shoW that the silicon carbide looses up to 8 mg/cm2 Weight
`during the testing While the yttrium silicate does not. See
`FIG. 1.
`
`EXAMPLE 2
`
`FIG. 2 is a cross section of a 4 mil thick 0.9/1.1 mole ratio
`yttrium silicate of composition 09/ 1.1 mole ratio of yttria to
`silica coating on 4 mils of mullite on SiC composite. The
`yttrium silicate and mullite Were thermal sprayed onto the
`silicon carbide composite using the folloWing parameters:
`
`5
`
`

`

`US 6,296,941 B1
`
`5
`
`Parameter
`
`Setting
`
`plasma torch
`nozzle
`anode
`powder port
`primary gas
`secondary gas
`substrate temp.
`carrier gas
`powder feed
`
`power
`stand-off
`
`Metco 3M
`GH
`std.
`metco #2
`Ar @ 80 Metco gage
`H2 @ 8 Metco gage
`8500 C.
`Ar @ 37 Metco gage
`15 to 25 gpm
`intermed.
`
`surface
`
`30 kw
`2.5-3H
`
`25 kw
`5 "
`
`Prior to coating the substrate was cleaned by grit blasting
`with 27 micron alumina particles at an impact velocity of
`150 to 200 mps. The yttrium silicate powder was obtained
`from Novel Technologies, Cayuga, New York as a free
`?owing —200+400 mesh powder.
`This invention may be embodied in other forms or carried
`out in other ways without departing from the spirit or
`essential characteristics thereof. The present embodiment is
`therefore to be considered as in all respects illustrative and
`not restrictive, the scope of the invention being indicated by
`the appended claims, and all changes which come within the
`meaning and range of equivalency are intended to be
`embraced therein.
`What is claimed is:
`1. An article comprising:
`a substrate comprising silicon; and
`a yttrium containing gaseous species of Si formation
`inhibiting barrier layer, wherein the barrier layer inhib
`its the formation of gaseous species of Si when the
`article is exposed to a high temperature, aqueous envi
`ronment.
`2. An article according to claim 1 wherein the substrate is
`selected from the group consisting of silicon containing
`ceramic and metal alloys containing silicon.
`3. An article according to claim 2 wherein the substrate is
`a silicon containing ceramic selected from the group con
`sisting of silicon carbide, silicon nitride, silicon oxynitride
`and silicon aluminum oxynitride.
`4. An article according to claim 2 wherein the substrate is
`a composite comprising a silicon based matrix and a rein
`forcing particle.
`5. An article according to claim 4 wherein said substrate
`is selected from the group consisting of silicon carbide
`?ber-reinforced silicon carbide matrix, carbon ?ber
`reinforced silicon carbide matrix and silicon carbide ?ber
`reinforced silicon nitride.
`6. An article according to claim 2 wherein said substrate
`is a silicon containing metal alloy selected from the group
`consisting of molybdenum-silicon alloys, niobium silicon
`alloys, iron-silicon alloys, and iron-nickel-silicon based
`alloys.
`7. An article according to claim 1 wherein the barrier layer
`comprises yttrium oxide.
`
`6
`8. An article according to claim 1 wherein the barrier layer
`comprises a yttrium silicate.
`9. An article according to claim 1 wherein the barrier layer
`comprises from about 66% by weight to about 78% by
`weight Y2O3, balance essentially SiO2.
`10. An article according to claim 1 wherein the barrier
`layer comprises from about 75% by weight to about 76% by
`weight Y2O3, balance SiO2.
`11. An article according to claim 1 wherein the coef?cient
`of thermal expansion of the barrier layer is within 13.0
`ppm/° C. the coef?cient of thermal expansion of the sub
`strate.
`12. An article according to claim 1 wherein the coef?cient
`of thermal expansion of the barrier layer is within 10.5
`ppm/° C. the coef?cient of thermal expansion of the sub
`strate.
`13. An article according to claim 1 wherein the barrier
`layer has a thickness of 20.5 mils (0.0005 inch).
`14. An article according to claim 1 including an interme
`diate layer between the substrate and the barrier layer.
`15. An article according to claim 14 wherein said inter
`mediate layer is selected from the group consisting of SiO2,
`mullite, mullite-barium strontium aluminosilicate, mullite
`yttrium silicate, mullite-calcium aluminosilicate, and silicon
`metal.
`16. An article according to claim 14 wherein said inter
`mediate layer is selected from the group consisting of
`mullite, barium strontium aluminosilicate, mullite-yttrium
`silicate, calcium aluminosilicate and mixtures thereof.
`17. An article according to claim 14 wherein said inter
`mediate layer comprises mullite.
`18. An article according to claim 14 wherein said inter
`mediate layer comprises from about 40 to 80 wt. % mullite
`and from about 60 to 20 wt. % barium strontium alumino
`silicate.
`19. An article according to claim 14 wherein said inter
`mediate layer comprises from about 40 to 80 wt. % mullite
`and from about 60 to 20 wt. % yttrium silicate.
`20. An article according to claim 14 wherein said inter
`mediate layer comprises from about 40 to 80 wt. % mullite
`and from about 60 to 20 wt. % calcium aluminosilicate.
`21. An article according to claim 14 including a bond
`layer between the substrate and the intermediate layer.
`22. An article according to claim 14 wherein the bond
`layer is silicon metal or SiO2.
`23. An article according to claim 14 wherein the inter
`mediate layer has a thickness of 20.5 mils (0.0005 inch).
`24. An article according to claim 9 wherein the barrier
`layer has a thickness of between about 3 to 30 mils.
`25. An article according to claim 9 wherein the barrier
`layer has a thickness of between about 3 to 5 mils.
`26. An article according to claim 14 wherein the inter
`mediate layer has a thickness of 3 to 30 mils.
`27. An article according to claim 14 wherein the inter
`mediate layer has a thickness of 3 to 5 mils.
`28. An article according to claim 21 wherein the bond
`layer has a thickness of between about 3 to 6 mils.
`29. An article according to claim 1 wherein the barrier
`layer has a thickness of between 3—5 mils.
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