Abstract:
In accordance with an embodiment of the invention, an article is disclosed. The article comprises a gas turbine engine component substrate comprising a silicon material; and an environmental barrier coating overlying the substrate, wherein the environmental barrier coating comprises cerium oxide, and the cerium oxide reduces formation of silicate glass on the substrate upon exposure to corrodant sulfates.

Description:
GOVERNMENT RIGHTS 
       [0001]    The invention was made in part under contract number NAS3-01135 awarded by the Government (NASA). Accordingly, the Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to protecting materials comprising silicon from environmental corrosion, such as that experienced in the hostile thermal environment of a gas turbine engine. More particularly, the invention relates to an environmental barrier coating (EBC) system for use on silicon comprising substrates for providing protection against environmental corrosion. 
       BACKGROUND OF THE INVENTION 
       [0003]    Higher operating temperatures for gas turbine engines are continuously sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components within the engine must correspondingly increase. In this regard, materials comprising silicon, particularly those with silicon carbide (SiC) as a matrix material or a reinforcing material, are considered useful for high temperature applications, such as for combustor and other hot section components of gas turbine engines. 
         [0004]    However, some silicon substrates may recede and lose mass as a result of formation of volatile Si species, particularly Si(OH) x  and SiO when exposed to high temperature, aqueous environments, thus necessitating the use of a protective coating thereon. Accordingly, methods such as described in U.S. Pat. Nos. 5,985,470, 6,444,335, 6,410,148 and 6,759,151, the contents of each of which are incorporated by reference, have addressed shortcomings concerning the use of such silicon substrates by providing an environmental barrier coating (EBC) over the substrate. The EBCs inhibit formation of volatile silicon species, Si(OH) x  and SiO, thereby reducing recession and mass loss. A thermal barrier coating (TBC) typically comprising yttria stabilized zirconia may also be employed as an outer layer to the EBC depending upon the operating conditions employed. 
         [0005]    While the current state of the art EBCs, which are typically multi-layer EBCs, may be effective in preventing water vapor recession of the ceramic matrix composite (CMC) substrate, both BSAS, SiC and some rare earth silicates such as some silicates disclosed in U.S. Pat. No. 6,759,151 may be susceptible to silicate glass formation when in contact with sulfate salt deposits as operating conditions continue to increase. 
         [0006]    Accordingly, there is a need to reduce the silicate glass formation rate in materials comprising silicon, particularly in EBC materials. Embodiments of the present invention satisfy this need and others. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    In accordance with an embodiment of the invention, an article is disclosed. The article comprises a gas turbine engine component substrate comprising a silicon material; and an environmental barrier coating overlying the substrate, wherein the environmental barrier coating comprises cerium oxide, and the cerium oxide reduces formation of silicate glass on the substrate upon exposure to corrodant sulfates. 
         [0008]    In accordance with another embodiment of the invention, an article comprises a gas turbine engine component substrate comprising a silicon material. The article further comprises a bond layer comprising a silicon material and overlying the substrate; a first layer comprising mullite and overlying the bond layer; a second layer comprising an barium strontium aluminosilicate and overlying the first layer; and a third layer consisting of cerium oxide and overlying the second layer. The cerium oxide reduces formation of silicate glass on the substrate upon exposure to corrodant sulfates. 
         [0009]    In accordance with another embodiment of the invention, a gas turbine engine component is disclosed. The component comprises a substrate comprising a silicon material; and at least one layer on the substrate selected from the group consisting of: i) an environmental barrier coating comprising cerium oxide, and ii) a dual layer of barium strontium aluminosilicate and a layer consisting of cerium oxide overlying the barium strontium aluminosilicate; wherein the cerium oxide reduces formation of silicate glass on the substrate. 
         [0010]    In accordance with a further embodiment of the invention, a gas turbine engine component comprises a substrate comprising a silicon material. Cerium oxide is deposited directly on the substrate or admixed with the silicon material of the substrate, wherein the cerium oxide reduces formation of silicate glass on the substrate upon exposure to corrodant sulfates. 
         [0011]    In accordance with another embodiment, a method of reducing silicate glass formation on a gas turbine engine component is disclosed. The method comprises providing a substrate of the gas turbine engine component, wherein the substrate comprises a silicon material; and depositing cerium oxide directly on the substrate or admixing cerium oxide with the silicon material of the substrate. The cerium oxide reduces formation of silicon glass on the substrate upon exposure to corrodant sulfates. 
         [0012]    In accordance with another embodiment, a method of reducing silicate glass formation on a gas turbine engine component comprises providing a substrate of the gas turbine engine component, wherein the substrate comprises a silicon material. The method further comprises depositing a bond layer comprising a silicon material and overlying the substrate; depositing a first layer comprising mullite and overlying the bond layer; depositing a second layer comprising an environmental barrier coating overlying the first layer; and depositing a layer consisting of cerium oxide overlying second layer or admixing cerium oxide with barium strontium aluminosilicate. The cerium oxide reduces formation of silicon glass on the substrate upon exposure to corrodant sulfates. 
         [0013]    Other features and advantages will be apparent from the following more detailed description, taken in conjunction with the accompanying drawing, which illustrate by way of example the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0014]      FIG. 1  is a cross-sectional view of a gas turbine engine component formed of a material comprising Si and having an environmental barrier coating thereon, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Applicant has determined through testing that silicon comprising materials may adversely react with deposited sulfates. For example, this material may react with Na 2 SO 4  at temperatures such as about 1700° F. (927° C.) and higher, and MgSO 4 /CaSO 4  may react at temperatures such as about 2200° F. (1204° C.) and higher. This reaction causes corrosion and forms a silicate glass from the silicon comprising material. Deep pitting and CO evolution can degrade the performance of the silicon comprising material. Additionally, this may prevent EBCs from acting as optimal water recession barriers for silicon comprising substrates as silicate glass has a high recession rate. 
         [0016]    Embodiments of the present invention improve upon prior coated silicon comprising materials, particularly EBC systems used upon silicon comprising material substrates. These embodiments are generally applicable to components that operate within environments of relatively high temperatures, and are thus subjected to thermal cycling, stresses, oxidation, and corrosion. Examples of such components include, but are not limited to, combustor components, blades, shrouds, flaps, seals, vanes, and other components of gas turbine engines. 
         [0017]    Referring to  FIG. 1 , an EBC system  10  of a first embodiment is shown. The EBC system  10  includes an EBC  12 , and a surface region  16  or substrate of a component  18 . The component  18 , or at least the surface region (substrate)  16  of the component  18 , is formed of a silicon comprising material (particularly those for articles exposed to high temperatures), such as SiC/SiC ceramic matrix composites (CMC). However, embodiments of the invention are generally applicable to other materials comprising silicon. Examples of silicon comprising materials include, but are not limited to, those with a dispersion of silicon carbide, silicon carbide and/or silicon particles as a reinforcement material in a metallic or nonmetallic matrix. Also included are those having a silicon carbide, silicon aluminum oxynitride, silicon nitride and/or silicon comprising matrix, and particularly composite materials that employ silicon carbide, silicon nitride and/or silicon as both the reinforcement and matrix materials (e.g., SiC/SiC ceramic matrix composites (CMC)). Silicon comprising materials further include metal silicides including, but not limited to, molybdenum and niobium silicides. Thus, according to embodiments of the invention, the silicon comprising substrate  16  may be a silicon comprising ceramic material as, for example, silicon carbide, silicon nitride, silicon carbon nitride, silicon oxynitride and silicon aluminum oxynitride. In accordance with one embodiment, the silicon comprising substrate  16  comprises a silicon comprising matrix with reinforcing fibers, particles and the like and, more particularly, a fiber reinforced silicon based matrix. 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  16  for the article of embodiments of the invention include molybdenum-silicon alloys, niobium-silicon alloys, and other Si comprising alloys having a coefficient of thermal expansion compatible with the other layer(s) described herein. 
         [0018]    In accordance with one embodiment, the surface region or substrate  16  of the component  18  is protected by the multilayer EBC system  10  that includes the EBC  12  for providing environmental protection to the component  18 . Optionally, a top coat or conventional thermal barrier coating (not shown), as well as optional conventional intermediate layer(s) (not shown) may be provided on top of the EBC  12  for providing further thermal insulation to the underlying layers depending upon desired operational temperatures. 
         [0019]    The multi-layered EBC  12  of the embodiment shown in  FIG. 1 , preferably has four layers, as shown therein. These four layers may include a bond layer  22 , a first layer  24 , a second layer  26  and a third layer  28 . The bond layer  22  overlays the substrate  16  of the component  18  and preferably comprises silicon, such as at least one of silicon metal and silicon dioxide. This bond layer  22  is useful to improve oxidation resistance of the surface region  16  and enhance bonding between the first layer  24  and the surface region  16 , particularly if the surface region  16  comprises silicon carbide or silicon nitride. A suitable thickness for the bond layer  22  is about 12.5 to about 250 micrometers. Suitable materials for bond layer  22  also include those described in the afore-referenced U.S. Pat. No. 6,410,148. For example, bond layer  22  can include a silicon metal or a SiO 2  layer. 
         [0020]    The first layer  24  is located on the bond layer  22  and comprises mullite. This mullite comprising first layer  24  serves to adhere the second layer  26  to the surface region  16 , while also preventing interactions between the second layer  26  and the silicon comprising surface region  16  at elevated temperatures. The first layer  24  may also comprise BSAS for less demanding applications, e.g. temperatures below about 1300° C. The addition of BSAS to the layer  24  is also relatively compatible with the silicon comprising surface region  16  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/SiC CMC. Preferably, first layer  24  comprises mullite-barrium strontium aluminosilicate (BSAS) in an amount of between about 40 to 80 wt. % mullite and between about 20 to 60 wt. % BSAS. A suitable thickness range for the mullite comprising first layer  24  is about 25 to about 250 micrometers. 
         [0021]    The second layer  26  overlies the first mullite comprising layer  24  and typically comprises BSAS and may consist essentially of BSAS. This layer provides excellent environmental protection and thermal barrier properties due to its low thermal conductivity. Particularly, BSAS can serve as an environmental barrier to the underlying mullite comprising layer  24 , which would exhibit significant silica activity and volatilization if exposed to water vapor at high temperatures. Additionally, BSAS is physically compliant with a SiC comprising substrate, such as that suitable for surface region  16 , and is relatively compatible with the mullite comprising layer  24  and the silicon comprising surface region  16  in terms of CTE. A suitable thickness range for layer  26  is about 25 to about 500 micrometers, depending upon the particular application. 
         [0022]    The second layer  26  may alternatively or additionally comprise a rare earth silicate such as described in U.S. Pat. No. 6,759,151. For example, rare earth silicates, such as those described in U.S. Pat. No. 6,759,151 may be employed as in place of the BSAS described herein or admixed therewith. As a further example, rare earth silicates include, but are not limited to, RE 2 O 3 , SiO 2 , 2RE 2 O 3 .3SiO 2 , RE 2 O 3 .2SiO 2 , and combinations thereof, where RE is a rare earth element selected from the group consisting of Sc, Dy, Ho, Er, Tm, Yb, Lu, Eu, Gd, Tb and combinations thereof. 
         [0023]    In the embodiment shown in  FIG. 1 , the EBC  12  also comprises a cerium oxide (CeO 2 ) layer  28  (third layer  28 ) located on top of the second layer  26 . A suitable thickness range for this layer is between about 5 to about 75 micrometers, also depending upon the particular application. Alternatively, the cerium oxide may be admixed with the second layer  26 . For example, cerium oxide may be admixed in any suitable amounts such as in a 50:50-weight percentage with the constituents of the second layer  26 . 
         [0024]    In alternate embodiments of the invention, the EBC  12  may comprise the second layer  26  deposited directly on the substrate comprising silicon and the third layer  28  deposited on the second layer  26 . Alternatively, an admixed layer of the second layer and the third layer  28  may be deposited on the substrate comprising silicon. It should be noted that in these embodiments, bond layer  22  may also be employed and deposited directly on the substrate comprising silicon with the additional layers deposited on the bond layer  22 .
       Thus, in accordance with embodiments of the invention, a single layer, EBC of BSAS and CeO 2  admixed therewith, as described above, may be used to provide environmental protection to the underlying silicon comprising material. This embodiment is particularly useful for operating temperatures below about 3000° F. (1371° C.), Alternatively, a CeO 2  layer, as described above, may be deposited on top of the BSAS layer.       
 
         [0026]    The layers  22 ,  24 ,  26  and  28  can be individually deposited by air and vacuum plasma spraying (APS and VPS, respectively), though it is foreseeable that deposition could be performed by other techniques, such as chemical vapor deposition (CVD) and high velocity oxy fuel (HVOF). A heat treatment may also be performed after deposition of the individual layers to relieve stresses created during cooling from elevated deposition temperatures. 
         [0027]    Accordingly, Applicant has advantageously determined that deposition of this high melting point acidic compound, CeO 2 , will result in dissolution into the basic corrosion melt and increase the eutectic temperature and/or decrease the basicity of the solution. This will advantageously slow and/or stop the silicate glass formation on the surface of the silicon comprising material by raising the melting point or decreasing the basicity of the silicate glass of the corrodant mix. 
         [0028]    While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations and improvements therein may be made by those skilled in the art, and are within the scope of the invention.