Patent Application
20060211241
This invention broadly relates to a corrosion resistant metal silicate protective layer for a steam-resistant barrier coating overlaying a silicon-containing substrate. This invention further broadly relates to a process for forming this protective layer on the steam-resistant barrier coating overlaying this silicon-containing substrate.
Higher operating temperatures for gas turbine engines are continuously sought in order to improve their efficiency. Ceramic materials containing silicon, such as those comprising silicon carbide (SiC) as a matrix material and/or as a reinforcing material (e.g., as fibers) are currently being used for high temperature applications, such as gas turbine engines, heat exchangers, internal combustion engines, etc. These silicon-containing matrix/reinforcing materials are commonly referred to as ceramic matrix composites (CMCs). With regard to gas turbine engines, CMCs have been used in various turbine components, including combustors, airfoils, etc. However, as operating temperatures increase, the high temperature durability of such CMC materials must also correspondingly increase.
In normal gas turbine engine operating environments, substrates comprising these silicon-containing CMCs can recede and lose mass when exposed to high temperature, aqueous environments. For example, when exposed to a lean combustion environment of approximately 1 atmosphere pressure of water vapor at 1200° C., silicon carbide can exhibit weight loss and recession at a rate of approximately 6 mils (152 microns) per 1000 hrs. This weight loss and recession is believed to involve volatilization of the protective silica scale (formed by oxidation of the silicon carbide surface) by reaction with water vapor, as represented by the following equation:
SiO2+2H2O=Si(OH)4
The silica scale formed on the CMC substrate can provide an excellent diffusion barrier to prevent further diffusion of oxygen. Indeed, in some coating systems utilized to protect the underlying silicon carbide in the CMC substrate, this silica scale can be formed deliberately as a protective layer by preoxidation of the substrate. However, as described above, this silica scale layer can deteriorate in the presence of water or water vapor such as steam to form volatile species of silicon such as Si(OH)4. It is the loss of these volatile silicon species that cause recession and mass loss of the CMC substrate.
Various environmental barrier coating (EBC) systems have been suggested for protecting silicon-containing CMCs from oxidation at high temperatures and degradation in the presence of aqueous environments. These include EBCs comprising mullites (3Al2O3.2SiO2) disclosed in, for example, commonly-assigned U.S. Pat. No. 6,129,954 (Spitsberg et al), issued Oct. 10, 2000, and U.S. Pat. No. 5,869,146 (McCluskey et al), issued Feb. 9, 1999. However, mullite does not provide adequate protection in high aqueous temperature environments because mullite has, thermodynamically, significant silica activity due to the high concentration of SiO2 in mullite that volatilizes at high-temperatures in the presence of water or water vapor.
Other EBC systems suggested for protecting silicon-containing CMCs include those comprising barium strontium aluminosilicate (BSAS), with or without mullite, and with or without additional thermal barrier coatings such as those disclosed in, for example, commonly-assigned U.S. Pat. No. 5,985,470 (Spitsberg et al), issued Nov. 16, 1999; U.S. Pat. No. 6,444,335 (Wang et al), issued Sep. 3, 2002; U.S. Pat. No. 6,607,852 (Spitsberg et al), issued Aug. 19, 2003; and U.S. Pat. No. 6,410,148 (Eaton et al), issued Jun. 25, 2002. However, EBCs comprising BSAS may not be sufficiently resistant to other forms of environmental attack.
Accordingly, it would be desirable to be able to provide an environmental barrier coating for silicon-containing CMC substrates that: (a) provides protection against high temperature aqueous environments, such as steam; and (b) is resistant to environmental attack by other types of environmental contaminant compositions and corrosive agents.
An embodiment of this invention is broadly directed at an article comprising:
Another embodiment of this invention is broadly directed at a process comprising the following steps:
The article and method of this invention provide a number of advantages and benefits with regard to steam resistant-barrier coatings for silicon-containing substrates. The metal silicate protective layer provides corrosion resistance against other forms of environmental attack, especially that caused by environmental contaminant compositions and corrosive agents that can be formed from sulfates and/or chlorides of calcium, magnesium, sodium, etc., or mixtures thereof (e.g., from sea salt), as well as oxides of calcium, magnesium, etc., or mixtures thereof. The corrosion resistant metal silicate protective layer can also be formed on the outer barrier layer of the steam-resistant barrier coating by the use of conventional coating methods such as thermal spray techniques, physical vapor deposition techniques, etc.
As used herein, the term “steam resistant-barrier coating” refers to coatings that can provide a protective barrier for the underlying silicon-containing substrate against environmental attack caused by high temperature, aqueous environments (e.g., steam). The steam resistant-barrier coatings of this invention comprise an outer barrier layer consisting essentially of an alkaline earth silicate/aluminosilicate, plus one or more optional layers.
As used herein, the term “barrier layer consisting essentially of an alkaline earth silicate/aluminosilicate” refers to a barrier layer that is resistant to environmental attack caused by high temperature, aqueous environments (e.g., steam) and which consists essentially of an alkaline earth silicate/aluminosilicate, e.g., comprises at least about 90% of an alkaline earth silicate/aluminosilicate, typically at least about 95% of an alkaline earth silicate/aluminosilicate, and more typically at least about 99% of an alkaline earth silicate/aluminosilicate.
As used herein, the term “alkaline earth aluminate/aluminosilicate” refers to alkaline earth aluminate (also referred to as “AEAs”), alkaline earth aluminosilicates (also referred to as “AEASs”), or combinations thereof. These alkaline earth silicate/aluminosilicates typically comprise alkaline earth aluminates, alkaline earth aluminosilicates or combinations thereof, wherein the alkaline earth is barium, strontium, or more typically a mixture thereof. The AEASs typically comprise barium strontium aluminosilicates (also referred to as “BSASs”). Usually, the BSASs comprise from about 0.00 to about 1.00 moles BaO, from about 0.00 to about 1.00 moles SrO, about 1.00 moles Al2O3 and about 2.00 moles SiO2, wherein the combined moles of BaO and SrO is about 1.00 mole. Typically, the BSASs comprise from about 0.10 to about 0.90 moles (more typically from about 0.25 to about 0.75 moles) BaO, from about 0.10 to about 0.90 moles (more typically from about 0.25 to about 0.75 moles) SrO, about 1.00 moles Al2O3 and about 2.00 moles SiO2, wherein the combined moles of BaO and SrO is about 1.00 moles. One such BSAS comprises about 0.75 moles BaO, about 0.25 moles SrO, about 1.00 moles Al2O3 and about 2.00 moles SiO2. See U.S. Pat. No. 6,410,148 (Eaton et al.), issued Jun. 25, 2002, especially column 3, lines 6-25, the relevant portions of which are herein incorporated by reference.
As used herein, the term “corrosion resistant metal silicate protective layer” refers to protective layers of this invention that comprise a sufficient amount or level of a corrosion resistant metal silicate to protect the steam-resistant barrier coating, and especially the outer barrier layer consisting essentially of an alkaline earth silicate/aluminosilicate, against environmental attack caused by sulfates and/or chlorides of calcium, magnesium, sodium, etc., or mixtures thereof (e.g., from sea salt), or by oxides of calcium, magnesium, etc., or mixtures thereof, such as a contaminant composition comprising mixed calcium-magnesium-aluminum-silicon-oxide systems (Ca—Mg—Al—SiO), that are commonly referred to as “CMAS.” See, for example, U.S. Pat. No. 5,660,885 (Hasz et al), issued Aug. 26, 1997, which describes these CMAS environmental contaminant compositions. The corrosion resistant metal silicate protective layer can comprise at least about 50% of a corrosion resistant metal silicate, typically at least about 80% of a corrosion resistant metal silicate, and more typically 99% of a corrosion resistant metal silicate.
As used herein, the term “corrosion resistant metal silicate” refers to a metal silicate that is at least resistant to environmental attack caused by sulfates and/or chlorides of calcium, magnesium, sodium, etc., or mixtures thereof (e.g., from sea salt), or by oxides of calcium, magnesium, etc., or mixtures thereof (e.g., from CMAS). These metal silicates can also be resistant to environmental attack caused by other environmental contaminant compositions and corrosive agents, including high temperature, aqueous environments (e.g., steam), etc. Suitable corrosion resistant metal silicates for use herein can include yttrium silicates, scandium silicates, zirconium silicates, hafnium silicates, rare earth metal silicates such as lanthanum silicates, cerium silicates, praseodymium silicates, neodymium silicates, promethium silicates, samarium silicates, europium silicates, gadolinium silicates, terbium silicates, dysprosium silicates, holmium silicates, erbium silicates, thulium silicates, ytterbium silicates, lutetium silicates, etc., as well as various combinations of these metal silicates. The metal silicates can be in the form of a monosilicate, a disilicate, an orthosilicate, a metasilicate, a polysilicate, etc., or combinations thereof. Typically, the corrosion resistant metal silicate is a yttrium silicate, a scandium silicate, a lutetium silicate, a ytterbium silicate, a zirconium silicate, a hafnium silicate, or a combination thereof, and more typically a yttrium silicate or a lutetium silicate.
As used herein, the term “silicon-containing substrate” refers to any silicon-containing-substrate, including those comprising silicon-containing ceramic materials, silicon metal alloys (also referred to as “metal silicides”), or combinations of such silicon-containing ceramic materials and silicon metal alloys. The silicon-containing substrate can comprise a substantially continuous matrix of silicon-containing materials, can be a composite comprising a continuous matrix of silicon-containing materials reinforced with discrete elements such as fibers, particles, etc. dispersed, embedded, etc., in the continuous matrix, etc. The discrete elements such as fibers, particles, etc., can be formed from silicon-containing ceramic materials, or can be formed from other materials, e.g., carbon fibers. Such combinations of dispersed, embedded, etc., fibers, particles, etc., in a continuous matrix of silicon-containing ceramics are typically referred to as ceramic matrix composites or CMCs. Typical CMCs comprise a continuous silicon-containing ceramic matrix that is fiber reinforced, usually with silicon-based fibers. These reinforcing fibers typically include a coating material that fully covers the fiber surfaces to impart and maintain structural integrity of the composite material systems. Typical fiber coating materials include boron nitride, silicon nitride, silicon carbide, carbon, etc. Suitable silicon-containing ceramic materials include silicon carbide, silicon nitride, silicon carbide nitride, silicon oxynitride, silicon aluminum oxynitride, etc., or combinations thereof. Suitable silicon metal alloys useful as silicon-containing substrates include molybdenum-silicon alloys (molybdenum silicides), niobium-silicon alloys (niobium silicides), iron-silicon alloys (iron silicides), etc, or combinations thereof. Illustrative silicon-containing substrates suitable for use herein include silicon carbide coated silicon carbide fiber-reinforced silicon carbide particles and a silicon matrix, a carbon fiber-reinforced silicon carbide matrix, a silicon carbide fiber-reinforced silicon nitride matrix, etc.
As used herein, the term “thermal barrier coating” refers to those coatings that reduce heat flow to the corrosion resistant metal silicate protective layer, steam-resistant barrier coating, silicon-containing substrate, etc., of the article, i.e., form a thermal barrier, and which comprise ceramic materials have a melting point that is typically at least about 2600° F. (1426° C.), and more typically in the range of from about 3450° to about 4980° F. (from about 1900° to about 2750° C.). Suitable ceramic materials for thermal barrier coatings include, aluminum oxide (alumina), i.e., those compounds and compositions comprising Al2O3, including unhydrated and hydrated forms, various zirconias, in particular phase-stabilized zirconias (e.g., zirconia blended with various stabilizer metal oxides such as yttrium oxides), such as yttria-stabilized zirconias, ceria-stabilized zirconias, calcia-stabilized zirconias, scandia-stabilized zirconias, magnesia-stabilized zirconias, india-stabilized zirconias, ytterbia-stabilized zirconias, etc., as well as mixtures of such stabilized zirconias. See, for example, Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed., Vol. 24, pp. 882-883 (1984) for a description of suitable zirconias. Suitable yttria-stabilized zirconias can comprise from about 1 to about 20% yttria (based on the combined weight of yttria and zirconia), and more typically from about 3 to about 10% yttria. These phase-stabilized zirconias can further include one or more of a second metal (e.g., a lanthanide or actinide) oxide such as dysprosia, erbia, europia, gadolinia, neodymia, praseodymia, urania, and hafnia to further reduce thermal conductivity of the thermal barrier coating. See U.S. Pat. No. 6,025,078 (Rickerby et al), issued Feb. 15, 2000 and U.S. Pat. No. 6,333,118 (Alperine et al), issued Dec. 21, 2001, both of which are incorporated by reference. Suitable ceramic materials for thermal barrier coatings also include pyrochlores of general formula A2B2O7 where A is a metal having a valence of 3+ or 2+ (e.g., gadolinium, aluminum, cerium, lanthanum or yttrium) and B is a metal having a valence of 4+ or 5+ (e.g., hafnium, titanium, cerium or zirconium) where the sum of the A and B valences is 7. Representative materials of this type include gadolinium-zirconate, lanthanum titanate, lanthanum zirconate, yttrium zirconate, lanthanum hafnate, cerium zirconate, aluminum cerate, cerium hafnate, aluminum hafnate and lanthanum cerate. See U.S. Pat. No. 6,117,560 (Maloney), issued Sep. 12, 2000; U.S. Pat. No. 6,177,200 (Maloney), issued Jan. 23, 2001; U.S. Pat. No. 6,284,323 (Maloney), issued Sep. 4, 2001; U.S. Pat. No. 6,319,614 (Beele), issued Nov. 20, 2001; and U.S. Pat. No. 6,387,526 (Beele), issued May 14, 2002, all of which are incorporated by reference.
As used herein, the term “comprising” means various compositions, compounds, components, coatings, substrates, layers, steps, etc., can be conjointly employed in this invention. Accordingly, the term “comprising” encompasses the more restrictive terms “consisting essentially of” and “consisting of.”
All amounts, parts, ratios and percentages used herein are by weight unless otherwise specified.
This invention is based on the discovery that, while prior environmental barrier coating (EBC) systems can protect the underlying silicon-containing substrate of articles (e.g., gas turbine engine components) from certain types of environmental attack, primarily those caused high temperature aqueous environments (e.g., a hot steam environment), there are other environmental conditions that these prior EBC systems do not protect against or protect against poorly. For example, EBC systems comprising an outer barrier layer consisting essentially of an alkaline earth silicate/aluminosilicate such as barium strontium aluminosilicate (BSAS) that are protective against environmental attack caused by high temperature aqueous environments (e.g., steam) are still vulnerable to other forms of environmental attack. These other forms of environmental attack can be equally as detrimental as recession caused by hot aqueous (e.g., steam) environments.
Of particular concern are those environment attacks caused by sulfates and/or chlorides of calcium, magnesium, sodium, etc., or mixtures thereof (e.g., from sea salt), or by oxides of calcium, magnesium, etc., or mixtures thereof (e.g., from CMAS). At higher temperatures during engine operation, CMAS can adhere to the hot EBC surface comprising the outer BSAS layer, or in the case of sea salt, can be ingested into the engine with the air. It has been found that chemical and mechanical interactions can occur between CMAS and/or sea salt, and the outer BSAS layer in these EBC systems. In particular, these contaminant compositions have been found to interact with this outer BSAS layer so as to chemically alter the EBC, thus forming amorphous or glassy phases in the EBC when exposed to water vapor and creating open porous channels through the modified EBC, such that the protective capabilities of the EBC are compromised.
The corrosion resistant metal silicate protective layer of this invention solves these problems caused by other environmental contaminant compositions and corrosive agents such as sea salt and CMAS. In particular, the corrosion resistant metal silicate protective layer of this invention permits outer steam resistant-barrier layers that consist essentially of an alkaline earth aluminate/aluminosilicate such as BSAS to be included in barrier coatings overlaying silicon-containing substrates. Such outer steam resistant-barrier layers that consist essentially of an alkaline earth aluminate/aluminosilicate such as BSAS can be desirable for certain barrier coatings that comprise optional layers (e.g., mullite, combinations of mullite and BSAS, etc., with or without a silicon or silica scale underlayer) that are less resistant or not resistant at all to environmental attack caused by high temperature aqueous environments, but can provide other benefits to the barrier coating, including a better coefficient of thermal expansion (CTE) match with the underlying silicon-containing substrate. By improving the CTE match between the barrier coating and the underlying silicon-containing substrate, the barrier coating can be less prone to spallation problems.
The steam resistant-barrier coatings of this invention having a corrosion resistant metal silicate protective layer are useful with silicon-containing substrates used in a wide variety of turbine engine (e.g., gas turbine engine) parts and components operated at, or exposed to, high temperatures, especially higher temperatures that occur during normal engine operation. These turbine engine parts and components can include turbine airfoils such as turbine blades and vanes, turbine shrouds, turbine nozzles, combustor components such as liners, deflectors and their respective dome assemblies, augmentor hardware of gas turbine engines, etc. The steam resistant-barrier coatings of this invention having the corrosion resistant metal silicate protective layer are particularly useful for articles comprising silicon-containing substrates in the form of turbine blades and vanes, and especially the airfoil portions of such blades and vanes. However, while the following discussion of articles of this invention will be with reference to turbine blades and vanes, and especially the airfoil portions thereof, that comprise these blades and vanes, it should also be understood that the steam resistant-barrier coatings of this invention having the corrosion resistant metal silicate protective layer can be useful with other articles comprising silicon-containing substrates that require environmental barrier protection.
The various embodiments of this invention are further illustrated by reference to the drawings as described hereafter. Referring to the drawings,
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An alternative technique for forming TBCs 82 is by thermal spray. As used herein, the term “thermal spray” refers to any method for spraying, applying or otherwise depositing TBC 82 that involves heating and typically at least partial or complete thermal melting of the ceramic material and depositing of the heated/melted ceramic material, typically by entrainment in a heated gas stream, onto protective layer 74. Suitable thermal spray deposition techniques include plasma spray, such as air plasma spray (APS) and vacuum plasma spray (VPS), high velocity oxy-fuel (HVOF) spray, detonation spray, wire spray, etc., as well as combinations of these techniques. A particularly suitable thermal spray deposition technique for use herein is plasma spray. Suitable plasma spray techniques are well known to those skilled in the art. See, for example, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed., Vol. 15, page 255, and references noted therein, as well as U.S. Pat. No. 5,332,598 (Kawasaki et al), issued Jul. 26, 1994; U.S. Pat. No. 5,047,612 (Savkar et al) issued Sep. 10, 1991; and U.S. Pat. No. 4,741,286 (Itoh et al), issued May 3, 1998 (herein incorporated by reference) which describe various aspects of plasma spraying suitable for use herein, including apparatus for carrying out plasma spraying.
While specific embodiments of the this invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of this invention as defined in the appended claims.
This invention was made with Government support under Contract No. N00019-96-C-0176 awarded by the JSF Program Office. The Government may have certain rights to the invention.
