(12) United States Patent
`Eaton, Jr. et al.
`
`US006387456B1
`US 6,387,456 B1
`May 14, 2002
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`(54) SILICON BASED SUBSTRATE WITH
`ENVIRONMENTAL/THERMAL BARRIER
`LAYER
`
`(75) Inventors: Harry Edwin Eaton, Jr., Woodstock;
`William Patrick Allen, Portland, both
`of CT (US); Nathan S. Jacobson,
`Westlake, OH (US); Narottam P.
`Bansal, North Olmsted, OH (US);
`Elizabeth J. Opila, Lakewood, OH
`(US); James L. Smialek, Strongsville,
`OH (US); Kang N. Lee, Westlake, OH
`(US); Irene T. Spitsberg, Loveland,
`OH (US); Hongyu Wang; Peter 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); The United States of America as
`represented by the Administrator of
`the National Aeronautics and Space
`Administration, Washington, DC (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`USC 154(b) by 0 days.
`
`( * ) Notice:
`
`(21) Appl. No.: 09/655,123
`Sep. 5, 2000
`(22) Filed:
`
`Related US. Application Data
`
`(62) Division of application No. 09/292,349, ?led on Apr. 15,
`1999.
`
`(51) Int. Cl? ............................ .. czsc 4/06; c23c 4/10
`(52)
`427/452; 427/450; 427/453
`(58) Field of Search ............................... .. 427/450, 452,
`427/453
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,332,619 A * 7/1994 Lacoste et al. ........... .. 428/245
`5,985,470 A * 11/1999 Spitsberg et a1. ......... .. 428/689
`
`* cited by examiner
`
`Primary Examiner—Deborah Jones
`Assistant Examiner—Jennifer McNeil
`(74) Attorney, Agent, or Firm—Bachman & LaPointe, PC.
`(57)
`ABSTRACT
`
`A barrier layer for a silicon containing substrate Which
`inhibits the formation of gaseous species of silicon When
`exposed to a high temperature aqueous environment com
`prises a barium-strontium alumino silicate.
`
`28 Claims, 4 Drawing Sheets
`
`GE-1006.001
`
`

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`U.S. Patent
`
`May 14, 2002
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`Sheet 1 0f 4
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`US 6,387,456 B1
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`0 haw-aw
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`T
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`Number of Cycles
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`Fig. 1
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`GE-1006.002
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`U.S. Patent
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`May 14, 2002
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`Sheet 2 0f 4
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`US 6,387,456 B1
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`22-.
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`4%
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`~
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`GE-1006.003
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`U.S. Patent
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`May 14, 2002
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`Sheet 3 0f 4
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`US 6,387,456 B1
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`GE-1006.004
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`U.S. Patent
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`May 14, 2002
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`Sheet 4 0f 4
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`US 6,387,456 B1
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`EBC Wt Change Data
`1200C
`4O “"I"“I""I“"I""I""I"“I""I
`98-17D(M+B/B reproc)
`
`Wt.Change(mg) ll: 0
`
`IIIIIIIIIII
`
`Illlllllllllllll
`
`[TU] 98—17C (M/B reproc)
`[SQ] 98-178 (M +B/B as-rec)
`
`98-17A (Ml)
`
`Fig. 4
`
`_80 llllllllIlllllllllllllllllllllllllllllll
`O
`25
`5O
`75 100 125 150 175 200
`Exposure Time (hrs)
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`GE-1006.005
`
`

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`US 6,387,456 B1
`
`1
`SILICON BASED SUBSTRATE WITH
`ENVIRONMENTAL/THERMAL BARRIER
`LAYER
`
`This is a Division of application Ser. No. 09/292,349,
`?led Apr. 15, 1999 allowed.
`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 12000 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
`
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`preferably a ceramic or metal alloy containing silicon. The
`external barrier layer is characteriZed by a coefficient of
`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 barium alumino
`silicate and, preferably, a barium-alkaline earth aluminosili
`cate Wherein the alkaline earth metal is ideally strontium. 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 interme
`diate 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;
`FIG. 2 is a photomicrograph through a sample of the
`barrier layer of the present invention on a silicon carbide
`substrate;
`FIG. 3 is a photomicrograph of a sample of the barrier
`layer of the present invention applied to an intermediate
`layer on a silicon carbide substrate; and
`FIG. 4 demonstrates the effect of three specimens of the
`barrier layer of the present invention on Weight loss in high
`temperature, aqueous environments.
`
`DETAILED DESCRIPTION
`
`The present invention relates to an article comprising a
`silicon containing 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 environment. 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 environmental barrier layer, the barrier layer also func
`tions as a thermal barrier layer and thus the present invention
`broadly encompasses the use of environmental/thermal bar
`rier 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 containing 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 carbon nitride, silicon oxynitride and silicon
`aluminum oxynitride. In accordance With a particular
`embodiment of the present invention, the silicon containing
`ceramic substrate comprises a silicon containing matrix With
`reinforcing 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 par
`ticle and silicon matrix, a carbon ?ber-reinforced silicon
`
`GE-1006.006
`
`

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`US 6,387,456 B1
`
`10
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`3
`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,
`and other Si containing alloys having a coef?cient of thermal
`expansion compatable With the barrier layer of the present
`invention.
`Barrier layers particularly useful in the article of the
`present invention include alkaline earth metal aluminosili
`cates. In accordance With a preferred embodiment, barium
`aluminosilicates, barium-alkaline earth metal aluminosili
`cates and, particularly, barium-strontium aluminosilicates
`are preferred. In a particular embodiment, the barrier layer
`comprises from about 0.00 to 1.00 mole BaO, from about
`0.00 to 1.00 mole of an oxide of a second alkaline earth
`metal, preferably SrO, about 1.00 mole A1203 and about
`2.00 mole SiO2, Wherein the total of the BaO and the second
`alkaline earth metal or SrO is about 1.00 mole. The preferred
`barrier layer of the present invention comprises from about
`0.10 mole to about 0.9 mole, preferably 0.25 to about 0.75
`mole BaO, 0.1 mole to about 0.9 mole, preferably 0.25 to
`about 0.75 SrO, 1.00 mole A1203 and about 2.00 mole SiO2
`Wherein the BaO and SrO total is about 1.00 mole. A
`particularly suitable barrier layer for use on silicon contain
`ing ceramic compositions in the article of the present
`invention comprises about 0.75 mole BaO, about 0.25 mole
`SrO, about 1.00 mole A1203, and about 2.00 mole 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 coef?cient 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 barium-strontium aluminosilicate barrier
`layer of the present invention, it is necessary to develop a
`stable crystallographic structure in the barrier layer of at
`least 50% by volume celsian in order to insure both struc
`tural integrity of the barrier layer and the desired thermal
`compatibility With respect to expansion coefficient betWeen
`the silicon containing substrate and the barrier layer. The
`crystallographic structure of the barium-strontium alumino
`silicate barrier layer is obtained as a result of preferred
`processing application and heat treating processing steps as
`Will be described hereinbeloW.
`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 based 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
`
`4
`diate layer; hoWever, mullite by itself tends to be cracked as
`the result of thermal spray 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
`containing 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
`betWeen about 870° C. to 1200° C. in order to help equili
`brate the as-sprayed, splat quenched, microstructure and to
`provide a means to manage stresses Which control delami
`nation. When the article being prepared in accordance With
`the method of the present invention is a silicon containing
`ceramic With a barium strontium aluminosilicate barrier
`layer, it is preferred that the barium strontium aluminosili
`cate barrier layer have celsian crystallographic structure in
`an amount of at least 50% by volume in the barrier layer. The
`formation of the celsian crystallographic structure insures
`compatibility betWeen the coef?cient of thermal expansion
`of the silicon containing ceramic and the barium strontium
`aluminosilicate barrier layer as described above.
`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 be hard enough to
`remove the undesired contamination but not as hard as the
`substrate material to prevent erosive removal of the sub
`strate. Further, the particles must be small to prevent impact
`damage to the substrate. When processing an article com
`prising a silicon carbide ceramic composite 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 intermediate 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.
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`US 6,387,456 B1
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`5
`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, to promote bonding between the sprayed powder
`particles and between the layers and the substrate, and to
`develop the celsian phase in the BSAS barrier layer which
`is essentially amorphous in the as sprayed condition. 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
`
`FIG. 1 shows a comparison of a hot pressed bulk speci
`men of BSAS, of composition 0.75 BaO-0.25 SrO~Al2O3~2
`SiOZin comparison to silicon carbide thermal cycle tested
`250 cycles between room temperature and 1200° C. in a
`simulated combustion environment of high steam, lean fuel
`conditions. The results show 8 mg/cm2 weight loss for the
`silicon carbide while the BSAS gains a very small amount
`of weight ~0.4 mg/cm2. The results show that silicon carbide
`is not stable to this environment and that the BSAS system
`is much more stable.
`
`EXAMPLE 2
`
`FIG. 2 is a cross section of a 4 mil thick BSAS of
`composition 0.75 BaO-0.25 SrO~Al2O3~2 SiO2 coating on
`SiC composite. The BSAS was thermal sprayed onto the
`silicon carbide composite using the following parameters:
`
`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
`
`30kw
`2.5—3"
`
`25kw
`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. As can be seen from FIG. 2, the invention
`results in an excellent barrier layer structure.
`
`EXAMPLE 3
`
`FIG. 3 is a cross sectional view of a BSAS of composition
`0.75 BaO-0.25 SrO-Al2O3 ~2 SiO2 barrier layer on a mullite/
`BSAS intermediate layer of 4:1 mils thickness on a silicon
`layer on silicon carbide composite. The coating was fabri
`cated using the following parameters.
`
`Plasma torch
`Nozzle
`Anode
`Powder port
`Primary gas
`Secondary gas
`Substrate temp.
`Carrier gas
`Powder feed
`
`Metco 3M
`GH
`std.
`Metco #2
`Ar@80 Metco gage
`H2@8 Metco gage
`8500 C.
`Ar@37 Metco gage
`15 to 25 gpm
`Interface
`Intermed. Surface
`
`Power
`Stand-off
`
`25 kw
`4"
`
`30kw
`2.5—3"
`
`25kw
`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. As can be seen from FIG. 3, the invention
`results in an excellent barrier layer structure.
`
`EXAMPLE 4
`
`Coatings of the BSAS (barium strontium aluminosilicate)
`based barrier layer coating system were fabricated onto
`silicon carbide composite substrates having an intermediate
`layer as indicated below and were exposed along with an
`uncoated silicon carbide composite substrate (98-17A) to
`high pressure, combustion environment, burner rig testing
`similar to conditions that occur in gas turbines engines. The
`BSAS coatings on all samples had the following composi
`tion: 0.75 BaO-0.25 SrO-Al2O3 ~2 SiO2. The BSAS coatings
`were 411 mils in thickness and the intermediate layers were
`also 411 mils in thickness and were applied by thermal
`spraying as indicated below. These coatings consisted of
`variations of BSAS on mullite (98-17C) and BSAS on
`mullite plus BSAS (98-17B and 98-17D). Testing occurred
`using conditions of 1200° C. test temperature, 200 hours
`exposure time, a fuel to air ratio of 0.053, and 6 atm
`pressure. After 200 hours exposure the uncoated substrate
`exhibited approximately 65 mg weight loss compared with
`weight gain of the coated coupons demonstrating that the
`coatings protected the substrate based on weight change
`data. No bond coat was used for these specimens. Thermal
`spray parameters were:
`
`Parameter
`
`Setting
`
`Plasma torch
`Nozzle
`Anode
`Powder port
`Primary gas
`Secondary gas
`Substrate temp.
`Carrier gas
`Powder feed
`
`Power
`Stand-off
`
`Electro-plasma 03CA
`03CA-27
`03CA-167
`ext. 90°
`Ar@14.4 SLM
`He@9.8 SLM
`1050—1250° C.
`Ar@3—6 SLM
`20 gpm
`Intermed. Surface
`
`45kW
`4"
`
`45kW
`4"
`
`The results are shown in FIG. 4 below which shows the
`effectiveness of the barrier layer of the present invention.
`The coefficient of thermal expansion (CTE) was measured
`for BSAS system of 0.75 BaO-0.25 SrO~Al2O3~2 SiO2
`having different celsian contents. Celsian content was deter
`mined by x-ray analysis of hot pressed coupons of BSAS.
`The celsian content affected the CTE as seen below.
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`

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`US 6,387,456 B1
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`% Celsian Content
`
`5
`25
`95
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`CTE
`
`8.1
`7.4
`5.2
`
`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. A method for preparing an article comprising the steps
`of:
`providing a substrate comprising silicon; and
`applying a gaseous species of Si inhibiting barrier layer to
`the substrate Wherein the barrier layer has a crystallo
`graphic structure Which is at least 5.0% by volume
`celsian and inhibits the formation of a gaseous species
`of Si When the article is exposed to a high temperature,
`aqueous environment and Wherein the coefficient of
`thermal expansion of the barrier layer is Within 13.0
`ppm/° C. the coefficient of thermal expansion of the
`substrate.
`2. A method according to claim 1 Wherein the coefficient
`of thermal expansion of the barrier layer is Within 10.5
`ppm/° C. the coefficient of thermal expansion of the sub
`strate.
`3. A method according to claim 1 further including the
`step of grit blasting the substrate prior to applying the barrier
`layer.
`4. A method according to claim 3 including grit blast With
`alumina particles having a particle siZe of 230 microns.
`5. A method according to claim 4 including grit blasting
`at a velocity of about 150/m/sec to about 200/m/sec.
`6. A method according to claim 1 including applying the
`barrier layer by thermal spraying.
`7. A method according to claim 6 including thermal
`spraying While holding the substrate at a temperature of
`betWeen about 870° C. to 1200C.
`8. A method according to claim 1 including the step of
`preoxidiZing the substrate to form a layer of SiO2 prior to
`applying the barrier layer.
`9. Amethod according to claim 8 Wherein the preoxidiZ
`ing comprises heating the substrate at a temperature of
`betWeen about 800° C. to 1200° C. for about 15 minutes to
`100 hours.
`10. A method according to claim 8 Wherein the coefficient
`of thermal expansion of the barrier layer is Within 10.5
`ppm/° C. the coefficient of thermal expansion of the sub
`strate.
`11. A method according to claim 1 including the step of,
`after applying the barrier layer, heat treating the article at a
`temperature of about 1250° C. for about 24 hours.
`12. Amethod according to claim 1 including heat treating
`at a temperature of about 12500° C. for about 24 hours.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`13. A method for preparing an article comprising the steps
`of:
`providing a substrate comprising silicon;
`grit blasting the substrate With particles having a particle
`size 230 microns and applying a gaseous species of Si
`inhibiting barrier layer to the substrate Wherein the
`barrier layer inhibits the formation of a gaseous species
`of Si When the article is exposed to a high temperature,
`aqueous environment.
`14. A method for preparing an article according to claim
`13 Wherein the particles are alumina particles.
`15. Amethod according to claim 13 or claim 14 including
`grit blasting at a velocity of about 100/m/sec to about
`200/m/sec.
`16. A method according to claim 1 or 13 Wherein the
`barrier layer comprises from about 0.00 to 1.00 moles BaO,
`from about 0.00 to 1.00 mole SrO, about 1.0 mole A1203 and
`about 2.00 mole SiO2, Wherein the total of BaO and SrO is
`about 1.00 mole.
`17. A method according to claim 1 or Wherein the barrier
`layer consists essentially of from about 0.00 to 1.00 mole
`BaO, from about 0.00 to 1.00 mole of an oxide of a second
`alkaline earth metal, about 1.00 mole A1203 and about 2.00
`mole SiO2, Wherein BaO plus the other alkaline earth metal
`oxide total 1 mole.
`18. A method according to claim 1 or 13 Wherein the
`barrier layer comprises from about 0.10 mole to about 0.90
`mole BaO and from about 0.10 mole to about 0.90 mold
`SrO.
`19. A method according to claim 1 or 13 Wherein the
`barrier layer comprises from about 0.25 mole to about 0.75
`mole BaO and from about 0.25 mole to about 0.75 mole SrO.
`20. A method according to claim 1 or 13 Wherein the
`barrier layer comprises about 0.75 mole BaO and about 0.25
`mole SrO.
`21. A method for preparing an article comprising the steps
`of:
`providing a substrate comprising silicon;
`oxidiZing the substrate to form a layer of SiO2;
`applying a gaseous species of Si inhibiting barrier layer to
`the oxidiZed substrate Wherein the barrier layer inhibits
`the formation of gaseous species of Si When the article
`is exposed to a high temperature, aqueous environment.
`22. A method according to claim 21 Wherein the preoxi
`diZing comprises heating the substrate at a temperature of
`betWeen about 800° C. to 1200° C. for about 15 minutes to
`100 hours.
`23. A method according to claim 21 Wherein the coeffi
`cient of thermal expansion of the barrier layer is Within 13.0
`ppm/° C. the coefficient of thermal expansion of the sub
`strate.
`24. A method according to claim 21 Wherein the coeffi
`cient of thermal expansion of the barrier layer is Within 10.5
`ppm/° C. the coefficient of thermal expansion of the sub
`strate.
`25. A method according to claim 21 Wherein the barrier
`layer has a crystallographic structure Which is at least 50%
`by volume celsian.
`26. A method for preparing an article comprising the steps
`of:
`
`GE-1006.009
`
`

`
`US 6,387,456 B1
`
`providing a substrate comprising silicon;
`applying a gaseous species of Si inhibiting barrier layer to
`the substrate Wherein the barrier layer inhibits the
`formation of gaseous species of Si When the article is
`exposed to a high temperature, aqueous environment;
`and
`heat treating the article at a temperature of about 1250° C.
`for about 24 hours.
`27. A method for preparing an article comprising the steps
`of:
`providing a substrate comprising silicon; and
`applying a gaseous species of Si inhibiting barrier layer to
`the substrate Wherein the barrier layer has a crystallo
`graphic structure Which is at least 5.0% by volume
`celsian and consists essentially of from about 0.00 to
`1.00 mole BaO, from about 0.00 to 1.00 mole of an
`oXide of a second alkaline earth metal, about 1.00 mole
`A1203 and about 2.00 mole SiO2, Wherein BaO plus the
`other alkaline earth metal oXide total 1 mole and
`inhibits the formation of gaseous species of Si When the
`article is eXposed to a high temperature, aqueous envi
`ronment.
`
`10
`
`15
`
`20
`
`10
`28. A method for preparing an article comprising the steps
`of:
`providing a substrate comprising silicon; and
`applying a gaseous species of Si inhibiting barrier layer to
`the substrate Wherein the barrier layer is selected from
`the group consisting of (a) barium aluminosilicate, (b)
`barium strontium aluminosilicate, (c) from about 0.00
`to 1.00 moles BaO, from about 0.00 to 1.00 mole SrO,
`about 1.0 mole A1203 and about 2.00 mole SiO2
`Wherein the total of BaO and SrO is about 1.00 mole
`and (d) from about 0.00 to 1.00 mole BaO, from about
`0.00 to 1.00 mole of an oXide of a second alkaline earth
`metal, about 1.00 mole A1203 and about 2.00 mole
`SiO2, Wherein BaO plus the other alkaline earth metal
`oXide total 1 mole Wherein the barrier layer has a
`crystallographic structure Which is at least 50% by
`volume celsian and inhibits the formation of a gaseous
`species of Si When the article is eXposed to a high
`temperature, aqueous environment.
`
`*
`
`*
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`*
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`*
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`*
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`GE-1006.010