Silicon based substrate with yttrium silicate environmental/thermal barrier layer and method for preparing an article therefrom

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 comprises a yttrium silicate.

Description: The present invention relates to an article comprising a substrate containing silicon and a barrier layer which functions 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. A particular useful application for these materials is for use in gas turbine engines which operate at high temperatures 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, aqueous 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. US 4 816 349 A shows a silicon nitride sintered member which comprises a silicon nitride sintered body and a zirconia layer coated onto the surface of the silicon nitride sintered body. The zirconia layer is preferably coated onto the surface of the silicon nitride sintered body through plasma spraying. The zirconia-coated silicon nitride sintered member has both high strength at high temperatures and excellent heat insulating property. US 5,560,993 discloses a lanthanoid silicide-coated silicon carbide material whereof the surface is coated with a silicide, this silicide being a reaction product of an oxide of a lanthanoid rare earth element or yttrium with silicon carbide, or a reaction product of a compound oxide of a lanthanoid rare earth element or yttrium and silicon with silicon carbide and a lanthanoid silicide- coated silicon carbide as above whereof the surface is further coated with an oxide of a lanthanoid rare earth element or yttrium, or with a compound oxide of a lanthanoid rare earth element or yttrium and silicon. Accordingly, it is the principle object of the present invention to provide an article comprising a silicon containing substrate with 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 environment. 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. The problems are solved by the teaching according to the independent claims. Particular developments are given in the dependent claims. The present invention relates to an article comprising silicons and a barrier layer comprising a yttrium silicate, wherein the barrier layer inhibits the formation of undesirable gaseous species of silicon when the article is exposed to a high temperature, aqueous environment; the article includes one or more intermediate layers between the silicon based substrate and the barrier layer. The intermediate layer is selected from the group consisting of SiO 2 , mullite, mullite-barium strontium aluminosilicate, mullite-yttrium silicate, mullite-calcium aluminosilicate, silicon metal, barium strontium aluminosilicate, calcium aluminosilicate and mixtures thereof. By high temperatures is meant the temperature at which the Si in the substrate forms Si(OH) x and/or SiO in an aqueous environment. By aqueous environment is meant a water and/or steam environment. The silicon containing substrate is 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 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 said intermediate layer may comprise from 40 to 80 wt.% mullite and from 60 to 20 wt.% barium strontium aluminosilicate or yttrium silicate or calcium aluminosilicate. The intermediate layer has a thickness of ≥ 0,0127 mm (0.5 mils or 0.0005 inch), preferably a thickness of 0,0762 mm to 0,762 mm (3 to 30 mils) or especially a thickness of 0,0762 mm to 0,127 mm (3 to 5 mils). Within the frame of the invention is a bond layer between the substrate and the intermediate layer. This bond layer may be a silicon metal or SiO 2 . This bond layer has a thickness of between 0,0762 mm to 0,1524 mm (3 to 6 mils). Preferably the aforesaid.barrier layer has a thickness of between 0,0762 mm to 0,762 mm (3 to 30 mils), 30 especially a thickness of between 0,0762 mm to 0,127 mm (3 to 5 mils). The invention further relates to a method for producing an article comprising a silicon containing substrate and a barrier 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 defined above. The method for preparing the article comprises the steps of: providing a substrate comprising silicon; and applying a yttrium silicate containing 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 wherein is formed an intermediate layer between the substrate and the barrier layer. The coefficient of thermal expansion of the barrier layer may be within ± 3.0 ppm/°C the coefficient of thermal expansion of the substrate or within ± 0.5 ppm/°C. Within the frame of the invention is the step of grit blasting the substrate prior to applying the barrier layer. This step may include grit blast with alumina particles having a particle size of ≤ 30 microns, preferably grit blasting at a velocity of between about 150/m/sec to 200/m/sec. The barrier layer may be applied by thermal spraying. This thermal spraying may include heat treatment of between 800°C to 1200°C. Another step is to preoxidize the substrate to form a layer of SiO 2 prior to applying the barrier layer. The preoxidizing may comprise heating the substrate at a temperature of between about 800°C to 1200°C for about 15 minutes to 100 hours. After applying the barrier layer, article should be heat treated. The heat treating comes to a temperature of about 1250°C for about 24 hours. Eventually the barrier layer comprises from 66% by weight to 78% by weight Y 2 O 3 , balance essentially SiO 2 especially from 75% by weight to 76% by weight Y 2 O 3 , balance SiO 2 . Further advantages, characteristics and details of the invention are apparent from the following detailed description of preferred embodiments of the invention with reference to the attached drawing. Figure 1 is a graph showing the stability of the barrier layer of the present invention with respect to recession and mass loss; and Figure 2 is a photomicrograph through a sample of the barrier layer of the present invention on a silicon carbide substrate. 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 and wherein an intermediate layer is formed between the substrate and the barrier layer. 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 functions as a thermal barrier layer and thus the present invention broadly encompasses the use of environmental/thermal barrier layers on silicon containing substrates. 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 comprises a silicon containing matrix with reinforcing materials such as fibers, particles and the like and, more particularly, a silicon based matrix which is fiber-reinforced. Particularly suitable ceramic substrates are a silicon carbide coated silicon carbide fiber-reinforced silicon carbide particle and silicon matrix, a carbon fiber-reinforced silicon carbide matrix and a silicon carbide fiber-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 iron-nickel-silicon alloys. The barrier layer in the article of the present invention includes yttrium silicate. In particular, the barrier layer comprises Y 2 O 3 in an amount of between about 66% by weight to about 78% by weight, balance SiO 2 , preferably between about 75% by weight to about 76% by weight Y 2 O 3 , balance SiO 2 . 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 Y 2 O 3 and 24.55% by weight SiO 2 . 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 ± 3.0 ppm per degrees centigrade, preferably ± 0.5 ppm per degrees centigrade, of the coefficient of thermal expansion of the silicon containing substrate. When using a silicon containing ceramic substrate such as a silicon carbide or a silicon nitride matrix with or without reinforcing fibers as described above in combination with the yttrium silicate barrier layer of the present invention, the desired thermal compatibility with respect to expansion coefficient between the silicon containing substrate and the barrier layer should be ± 2.00 ppm per degrees centigrade. The barrier layer should be present in the article at a thickness of greater than or equal to 0,0127 mm (0.5 mils or 0.0005 inch), preferably between 0,0762 mm to 0,762 mm (3 to 30 mils) and ideally between 0,0762 mm to 0,127 mm (3 to 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 the article of the present invention, an intermediate layer is 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 SiO 2 , mullite, mullite-barium strontium aluminosilicate, mullite-yttrium silicate, mullite-calcium aluminosilicate, and silicon metal. Mullite has been found to be a particularly useful intermediate layer; however, mullite by itself tends to be cracked as the result of thermal spraying fabrication processing. Accordingly, it is preferred that the intermediate layer comprises mullite-barium strontium aluminosilicate, mullite-yttrium silicate, or mullite-calcium aluminosilicate in an amount of between 40 to 80 wt.% mullite and between 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 hereinbelow. 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 0,0762 mm to 0,1524 mm (3 to 6 mils). Alternatively, the silicon based substrate may be pre-oxidized to provide a SiO 2 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 800°C to 1200°C in order to help equilibrate as-sprayed, splat quenched, microstructure and to provide a means to manage stresses which control delamination. The silicon containing substrate should be cleaned prior to application of the barrier layer to remove substrate fabrication contamination. It is preferred that the silicon based substrate be subjected to a grit blasting step prior to application 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 fiber reinforced composite. It has been found that the particles 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 Al 2 O 3 particles, preferably of a particle size of ≤ 30 microns and, preferably, at a velocity of 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. SiO 2 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 0,0762 mm to 0,1524 mm (3 to 6 mils). The 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 fabricated 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/cm 2 weight during the testing while the yttrium silicate does not. See Figure 1. Example 2 Figure 2 is a cross section of a 4 mil thick 0.9/1.1 mole ratio yttrium silicate of composition 0.9/1.1 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: Parameter Setting plasma torch Metco 3M nozzle GH anode std. powder port metco #2 primary gas Ar @80 Metco gage secondary gas H2 @-8 Metco gage substrate temp. 850°C carrier gas Ar @ 37 Metco gage powder feed 15 to 25 gpm intermed. surface power 30kw 25 kw stand-off 2.5 - 3" 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 flowing -200 + 400 mesh powder.

Claims: An article comprising: a substrate comprising silicon; and a barrier layer, comprising a yttrium silicate wherein the barrier layer inhibits the formation of gaseous species of Si when the article is exposed to a high temperature, aqueous environment, wherein an intermediate layer is formed between the substrate and the barrier layer, wherein said intermediate layer is selected from the group consisting of SiO 2 , mullite, mullite-barium strontium aluminosilicate, mullite-yttrium silicate, mullite-calcium aluminosilicate, silicon metal, barium strontium aluminosilicate, calcium aluminosilicate and mixtures thereof. An article according to claim 1 wherein the substrate is selected from the group consisting of silicon containing ceramic and metal alloys containing silicon. An article according to claim 2 wherein the substrate is a silicon containing ceramic selected from the group consisting of silicon carbide, silicon nitride, silicon oxynitride and silicon aluminum oxynitride. An article according to claim 2 wherein the substrate is a composite comprising a silicon based matrix and a reinforcing particle, wherein preferably said substrate is selected from the group consisting of silicon carbide fiber-reinforced silicon carbide matrix, carbon fiber-reinforced silicon carbide matrix and silicon carbide fiber-reinforced silicon nitride. 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. An article according to one of the claims 1 to 5, wherein the barrier layer comprises from 66% by weight to 78% by weight Y 2 O 3 , balance essentially SiO 2 , preferably wherein the barrier layer comprises from 75% by weight to 76% by weight Y 2 O 3 , balance SiO 2 . An article according to one of the claims 1 to 6 wherein the coefficient of thermal expansion of the barrier layer is within ± 3.0 ppm/°C the coefficient of thermal expansion of the substrate or wherein the coefficient of thermal expansion of the barrier layer is within ± 0.5 ppm/°C the coefficient of thermal expansion of the substrate. An article according to one of the claims 1 to 7 wherein the barrier layer has a thickness of 0,0127 mm (0.0005 inch), preferably of 0,0762 mm to 0,127 mm (3-5 mils). An article according to one of the claims 1 to 8, wherein said intermediate layer comprises from 40 to 80 wt.% mullite and from 60 to 20 wt.% barium strontium aluminosilicate or wherein said intermediate layer comprises from 40 to 80 wt.% mullite and from about 60 to 20 wt.% yttrium silicate. An article according to one of the claims 1 to 8, wherein said intermediate layer comprises from 40 to 80 wt.% mullite and from 60 to 20 wt.% calcium aluminosilicate. An article according to one of the claims 1 to 10 wherein the intermediate layer has a thickness of ≥ 0,0127 mm (0.0005 inch). An article according to one of the claims 1 to 11 wherein the intermediate layer has a thickness of 0,0762 mm to 0,762 mm (3 to 30 mils), wherein preferably the intermediate layer has a thickness of 0,0762 mm to 0,127 mm (3 to 5 mils). An article according to one of the claims 1 to 12. including a bond layer between the substrate and the intermediate layer. An article according to claim 13 wherein the bond layer is silicon metal or SiO 2 . An article according to claim 13 or 14 wherein the bond layer has a thickness of between 0,0762 mm to 0,1524 mm (3 to 6 mils). A method for preparing an article comprising the steps of: providing a substrate comprising silicon; and applying a yttrium silicate containing 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 wherein an intermediate layer is formed between the substrate and the barrier layer, wherein said intermediate layer is selected from the group consisting of SiO 2 , mullite, mullite-barium strontium aluminosilicate, mullite-yttrium silicate, mullite-calcium aluminosilicate, silicon metal, barium strontium aluminosilicate, calcium aluminosilicate and mixtures thereof. A method according to claim 16, wherein the coefficient of thermal expansion of the barrier layer is within ± 3.0 ppm/°C the coefficient of thermal expansion of the substrate or wherein the coefficient of thermal expansion of the barrier layer is within ± 0.5 ppm/°C the coefficient of thermal expansion of the substrate. A method according to claim 16 or 17, further including the step of grit blasting the substrate prior to applying the barrier layer. A method according to one of the claim 16 to 18 including grit blast with alumina particles having a particle size of ≤ 30 microns, including preferably grit blasting at a velocity of between 150/m/sec to 200/m/sec. A method according to one of the claims 16 to 19 including applying the barrier layer by thermal spraying, including preferably thermal spraying at a temperature of between about 800°C to 1200°C. A method according to one of the claims 16 to 20 including the step of preoxidizing the substrate to form a layer of SiO 2 prior to applying the barrier layer. A method according to one of the claims 16 to 21 wherein the preoxidizing comprises heating the substrate at a temperature of between 800°C to 1200°C for 15 minutes to 100 hours. A method according to one of the claims 16 to 22 including the step of, after applying the barrier layer, heat treating the article. A method according to one of the claims 16 to 23 including heat treating at a temperature of 1250°C for 24 hours. A method according to one of the claims 16 to 24 wherein the barrier layer comprises from about 66% by weight to about 78% by weight Y 2 O 3 , balance essentially SiO 2 . A method according to one of the claims 16 to 25, wherein the barrier layer comprises from about 75% by weight to about 76% by weight Y 2 O 3 , balance SiO 2 .

Inventors: ALLEN WILLIAM P, EATON HARRY E, JACOBSON NATHAN S, LEE KANG N, LUTHRA KRISHAN L, MESCHTER PETER J, OPILA ELIZABETH J, SMIALEK JAMES L, WANG HONGYU, ALLEN, WILLIAM P., EATON, HARRY E., JACOBSON, NATHAN S., LEE, KANG N., LUTHRA, KRISHAN L., MESCHTER, PETER J., OPILA, ELIZABETH J., SMIALEK, JAMES L., WANG, HONGYU
Applicants: GEN ELECTRIC, NASA, UNITED TECHNOLOGIES CORP, GENERAL ELECTRIC COMPANY, THE UNITED STATES OF AMERICA, REPRESENTED BY THE ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (NASA), UNITED TECHNOLOGIES CORPORATION
Application year: 2000
Main classification: C04B41
IPC codes: B32B15, C04B41, B32B27, B05D7, C23C4, B05D3, C23C30, C23C8, B32B18