Patent Description:
Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight component materials have been proposed.

Ceramic matrix composites (CMCs) are a class of materials that consist of a reinforcing material surrounded by a ceramic matrix phase. Such materials, along with certain monolithic ceramics (i.e. ceramic materials without a reinforcing material), are currently being used for higher temperature applications. These ceramic materials are lightweight compared to superalloys yet can still provide strength and durability to the component made therefrom. Therefore, such materials are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as airfoils (e.g. turbines, and vanes), combustors, shrouds and other like components that would benefit from the lighter-weight and higher temperature capability these materials can offer.

CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment, which can rapidly oxidize silicon-containing CMCs and monolithic ceramics. Additionally, silicon oxide is not stable in high temperature steam, but rather, can be converted to volatile (gaseous) silicon hydroxide species. Thus, EBCs can help prevent dimensional changes in the ceramic component due to such oxidation and volatilization processes. Unfortunately, there can be some undesirable issues associated with standard, industrial coating processes such as plasma spray and vapor deposition (i.e. chemical vapor deposition, CVD, and electron beam physical vapor deposition, EBPVD) currently used to apply EBCs.

Accordingly, there remains a need for methods for environmental barrier coatings for ceramic component that are suitable for use in the high temperature steam environments present in gas turbine engines.

<CIT> relates to an electrode structure and to a plasma generating device using the electrode structure.

Metric equivalents of units: <NUM> mil = <NUM>, <NUM> mil = <NUM>, <NUM> mils = <NUM>, <NUM> mils = <NUM>, <NUM> mils = <NUM>; <NUM>µinch = <NUM>, <NUM>µinch = <NUM>, <NUM>µinch = <NUM>.

The invention herein generally relates to organic solvent based slurry compositions for making an environmental barrier coating according to the claim.

These and other features, aspects and advantages will become evident to those skilled in the art from the following disclosure.

While the specification concludes with claims particularly pointing out and distinctly claiming the invention, it is believed that the embodiments set forth herein will be better understood from the following description in conjunction with the accompanying figures, in which like reference numerals identify like elements.

Embodiments described herein generally relate to solvent based slurry compositions for making environmental barrier coatings and environmental barrier coatings comprising the same.

More specifically, the EBCs described herein comprise solvent based slurries having sintering aids, which can lower the sintering temperature, thereby promoting the formation of dense EBC layers that can act as a hermetic seal to protect the underlying component from corrosion from the gases generated during high temperature combustion without damaging the component through exposure to high sintering temperatures, as explained herein below.

The EBCs described herein may be suitable for use in conjunction with CMCs or monolithic ceramics. As used herein, "CMCs" refers to silicon-containing matrix and reinforcing materials. Some examples of CMCs acceptable for use herein can include, but should not be limited to, materials having a matrix and reinforcing fibers comprising silicon carbide, silicon nitride, and mixtures thereof. As used herein, "monolithic ceramics" refers to materials comprising silicon carbide, silicon nitride, and mixtures thereof. Herein, CMCs and monolithic ceramics are collectively referred to as "ceramics.

As used herein, the term "barrier coating(s)" can refer to environmental barrier coatings (EBCs). The barrier coatings herein may be suitable for use on "ceramic component," or simply "component" <NUM> found in high temperature environments (e.g. operating temperatures of above <NUM>°F (<NUM>)), such as those present in gas turbine engines. Examples of such ceramic components can include, for example, combustor components, turbine blades, shrouds, nozzles, heat shields, and vanes.

More specifically, EBC <NUM> may comprise a coating system including various combinations of the following: a bond coat layer <NUM>, an optional silica layer <NUM>, at least one transition layer <NUM>, an optional compliant layer <NUM>, an optional intermediate layer <NUM>, and an optional outer layer <NUM>, as shown generally in <FIG> and as set forth herein below.

Bond coat layer <NUM> may comprise silicon metal, silicide, or a combination thereof, and may generally have a thickness of from about <NUM> mils to about <NUM> mils. Due to the application method as described herein below, there may be some local regions where the silicon bond coat is missing, which can be acceptable. For example, in one embodiment, bond coat layer can cover about <NUM>% of the surface of the component, and in another embodiment, about <NUM>% or more of the surface area of the component.

As used herein "silicide" may include rare earth (Ln) silicides, chromium silicide (e.g. CrSi<NUM>), niobium silicide (e.g. NbSi<NUM>, NbSi<NUM>), molybdenum silicide (e.g. MoSi<NUM>, MoSi<NUM>), tantalum silicide (e.g.TaSi<NUM>, TaSi<NUM>), titanium silicide (e.g. TiSi<NUM>, TiSi<NUM>), tungsten silicide (e.g. WSi<NUM>, W<NUM>Si<NUM>), zirconium silicide (e.g. ZrSi<NUM>), hafnium silicide (e.g. HfSi<NUM>),.

As used herein, "rare earth" represented "(Ln)" refers to the rare earth elements of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and mixtures thereof.

Silica layer <NUM> can be amorphous or crystalline, and have an initial thickness of from about <NUM> mils to about <NUM> mils. However, the thickness of silica layer <NUM> can increase over time. Specifically, the silicon in bond coat layer <NUM> can oxidize slowly during the service life of the EBC to gradually increase the thickness of silica layer <NUM>. This oxidation of bond coat <NUM> can protect the underlying ceramic component from oxidation since the bond coat is oxidized rather than the ceramic component. Silica layer <NUM> can, in some embodiments, also be doped with a doping composition, as defined herein below, due to diffusion of the sintering aid into the silica layer.

Transition layer <NUM> may comprise a rare earth disilicate, a doped rare earth disilicate, or a doped rare earth disilicate containing secondary materials, as defined below. More specifically, transition layer <NUM> may include from about <NUM>% to about <NUM>% by volume of the transition layer of a primary transition material and up to about <NUM>% by volume of the transition layer of a secondary material, and in one embodiment from about <NUM>% to about <NUM>% by volume of the transition layer of the primary transition material and from about <NUM>% to about <NUM>% by volume of the transition layer of the secondary material. In another embodiment, transition layer <NUM> may comprise <NUM>% primary transition material wherein the primary transition material can be doped, as described below.

As used herein, "primary transition material" refers to a rare earth disilicate (Ln<NUM>Si<NUM>O<NUM>), or a doped rare earth disilicate. As used herein, "doped rare earth disilicate" refers to Ln<NUM>Si<NUM>O<NUM> doped with a "doping composition" selected from the group consisting of iron (Fe), aluminum (Al), titanium (Ti), gallium (Ga), nickel (Ni), boron (B), an alkali, an alkali-earth, and Lnb rare earths, as defined herein below. As used herein throughout, "secondary material" refers to a material comprising a doping composition (as defined previously), and specifically, can be selected from the group consisting of Fe<NUM>O<NUM>, iron silicates, rare earth iron oxides, Al<NUM>O<NUM>, mullite, rare earth aluminates, rare earth aluminosilicates, TiO<NUM>, rare earth titanates, Ga<NUM>O<NUM>, rare earth gallates, NiO, nickel silicates, rare earth nickel oxides, Lnb metals, Lnb<NUM>O<NUM>, Lnb<NUM>Si<NUM>O<NUM>, Lnb<NUM>SiO<NUM>, borosilicate glass, alkaline earth silicates, alkaline earth rare earth oxides, alkaline earth rare earth silicates, and mixtures thereof. Any doping composition present in the primary material should correspond to the doping composition contained in any secondary material present (e.g. Fe-doped Ln<NUM>Si<NUM>O<NUM> with Fe<NUM>O<NUM> secondary material; Ti-doped Ln<NUM>Si<NUM>O<NUM> with TiO<NUM> secondary material; or Ni-doped Ln<NUM>Si<NUM>O<NUM> with rare earth nickel oxide secondary material, for example).

Each transition layer <NUM> may have a thickness of from about <NUM> mils to about <NUM> mils, and may be made and applied to the underlying layer as set forth below. In one embodiment, there may be more than one transition layer present. In such instances, each transition layer may comprise the same or different combination of primary transition materials and secondary materials. Transition layer <NUM> may have a porosity level of from <NUM>% to about <NUM>% by volume of the transition layer, and in another embodiment, from about <NUM>% to about <NUM>% by volume of the transition layer.

Similarly, outer layer <NUM> may comprise a rare earth monosilicate, a doped rare earth monosilicate, or a doped rare earth monosilicate containing secondary material. More specifically, outer layer <NUM> can include from about <NUM>% to about <NUM>% by volume of the outer layer of a primary outer material and up to about <NUM>% by volume of the outer layer of the previously defined secondary material, and in one embodiment from about <NUM>% to about <NUM>% by volume of the outer layer of a primary outer material and from about <NUM>% to about <NUM>% by volume of the outer layer of the secondary material. In another embodiment, outer layer <NUM> may comprise <NUM>% primary outer material wherein the primary outer material can be doped as described below.

As used herein, "primary outer material" refers to a rare earth monosilicate, or a doped rare earth monosilicate. As used herein, "doped rare earth monosilicate" refers to Ln<NUM>SiO<NUM> doped with a doping composition, as defined previously. Outer layer <NUM> may have a thickness of from about <NUM> mils to about <NUM> mils, and may be made and applied to the underlying layer as set forth below. In one embodiment, outer layer <NUM> may have a porosity level of from <NUM>% to about <NUM>% by volume of the outer layer, and in another embodiment, from about <NUM>% to about <NUM>% by volume of the outer layer, and in another embodiment, from about <NUM>% to about <NUM>% by volume of the outer layer. In some embodiments, outer layer <NUM> can comprise cracks therein at a density of up to about <NUM> cracks/mm that can form during operation due to thermal expansion anisotropy.

In reference to the embodiments herein, "Lnb rare earth metal", or simply "Lnb" refers to a sub-set of rare-earth metals having a melting point below at least about <NUM> including lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, and ytterbium. In one embodiment, the sub-set can include only those rare earth elements having a melting point below about <NUM> including lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, and ytterbium. The Lnb rare earth metal can be utilized with SiC-SiC CMCs having an operation limit of about <NUM>.

As used herein throughout, "alkaline earth" can refer to magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). As used herein, "alkali" refers to lithium (Li), potassium (K), and sodium (Na). "Iron silicates" can include compounds such as Fe<NUM>SiO<NUM>, and glasses of rare earth iron silicates. "Rare earth iron oxides" can include compounds such as garnets (Ln<NUM>Fe<NUM>O<NUM>), monoclinic ferrites (Ln<NUM>Fe<NUM>O<NUM>), and perovskites (LnFeO<NUM>). "Rare-earth aluminates" can include compounds such as garnets (Ln<NUM>Al<NUM>O<NUM>), monoclinic aluminates (Ln<NUM>Al<NUM>O<NUM>), and perovskites (LnAlO<NUM>). "Rare earth aluminates" can include glassy materials comprised of about <NUM>-<NUM> wt% Ln<NUM>O<NUM>, about <NUM>-<NUM> wt% Al<NUM>O<NUM>, and about <NUM>-<NUM> wt% SiO<NUM>. "Rare-earth titanates" can include compounds such as Ln<NUM>Ti<NUM>O<NUM> (pyrochlore) and Ln<NUM>TiO<NUM>. "Rare-earth gallates" can include compounds such as garnets (Ln<NUM>Ga<NUM>O<NUM>), monoclinic gallates (Ln<NUM>Ga<NUM>O<NUM>), perovskites (LnGaO<NUM>), and Ln<NUM>GaO<NUM>. "Nickel silicates" can include compounds such as Ni<NUM>SiO<NUM>. "Borosilicate glass" can refer to any amorphous material containing up to about <NUM>% by weight boron oxide (B<NUM>O<NUM>), up to about <NUM>% alkali oxide selected from the group consisting of sodium (Na<NUM>O), potassium (K<NUM>O), lithium (Li<NUM>O), or any combinations of thereof, up to about <NUM>% alumina (Al<NUM>O<NUM>), and a balance of silica (SiO<NUM>). "Alkaline earth silicates" can include compounds such as Mg<NUM>SiO<NUM>, MgSiO<NUM>, Ca<NUM>SiO<NUM>, Ca<NUM>SiO<NUM>, Ca<NUM>Si<NUM>O<NUM>, CaSiO<NUM>, Ba<NUM>SiO<NUM>, BaSiO<NUM>, Ba<NUM>Si<NUM>O<NUM>, BaSi<NUM>O<NUM>, Sr<NUM>SiO<NUM>, and SrSiO<NUM>. "Alkali earth rare earth oxides" can include compounds such as BaLn<NUM>O<NUM>, Mg<NUM>Ln<NUM>O<NUM>, SrLn<NUM>O<NUM>, and Sr<NUM>Ln<NUM>O<NUM>. "Alkaline earth rare earth silicates" can include oxyapatite materials (i.e. Ae<NUM>Ln<NUM>Si<NUM>O<NUM>).

If present, compliant layer <NUM> may include from about <NUM>% to about <NUM>% by volume of the compliant layer of a primary compliant material and up to about <NUM>% by volume of the compliant layer of a secondary compliant material, and in one embodiment from about <NUM>% to about <NUM>% by volume of the compliant layer of a primary compliant material and from about <NUM>% to about <NUM>% by volume of the compliant layer of the secondary compliant material. In another embodiment, compliant layer <NUM> may comprise <NUM>% by volume of the compliant layer of a primary compliant material wherein the primary outer material may be doped with a rare earth element.

As used herein, "primary compliant material" refers to BSAS, or a rare earth doped BSAS, while "secondary compliant material" refers to Ln<NUM>O<NUM>, Ln<NUM>Si<NUM>O<NUM>, Ln<NUM>SiO<NUM>, Ln<NUM>Al<NUM>O<NUM>, Al<NUM>O<NUM>, mullite, and combinations thereof. Compliant layer <NUM> may have a thickness of from about <NUM> mils to about <NUM> mils, and may be made and applied as set forth below. In one embodiment, compliant layer <NUM> may have a porosity level of from <NUM>% to about <NUM>% by volume of the compliant layer, and in another embodiment, from about <NUM>% to about <NUM>% by volume of the compliant layer, and in another embodiment, from about <NUM>% to about <NUM>% by volume of the compliant layer.

Intermediate layer <NUM>, if present, can comprise the previously defined primary outer materials of rare earth monosilicate or doped rare earth monosilicate. Similar to the silica layer, intermediate layer <NUM> can form during the service life of the EBC. More specifically, high temperature steam penetrates the outer layer <NUM>, and as the steam reacts with the primary transition material of the transition layer to volatilize SiO<NUM>, intermediate layer <NUM> can form.

By way of example, and not limitation, the EBC systems described herein may include in one embodiment, component <NUM>, bond coat layer <NUM>, and transition layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, transition layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, transition layer <NUM>, compliant layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, transition layer <NUM>, compliant layer <NUM>, transition layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, and transition layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, transition layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, transition layer <NUM>, compliant layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, transition layer <NUM>, compliant layer <NUM>, transition layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, transition layer <NUM>, intermediate layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, transition layer <NUM>, intermediate layer <NUM>, and outer layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, transition layer <NUM>, intermediate layer <NUM> (which can form during operation), and outer layer <NUM>; and in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, transition layer <NUM>, compliant layer <NUM>, transition layer <NUM>, intermediate layer <NUM> (which can form during operation), and outer layer <NUM>. Such embodiments can be suitable for use in environments having a temperature up to about <NUM> (<NUM>°F).

Alternately, the EBC system may comprise component <NUM>, bond coat layer <NUM>, transition layer <NUM>, and compliant layer <NUM>; in another embodiment, component <NUM>, bond coat layer <NUM>, silica layer <NUM>, transition layer <NUM>, and compliant layer <NUM>. Such embodiments can be suitable for use in environments having a temperature of up to about <NUM> (<NUM>°F).

Those skilled in the art will understand that embodiments in addition to those set forth previously are also acceptable, and that not all of the layers need to be present initially, but rather, may form during engine operation.

The EBC can be made and applied in accordance with the description below.

Bond coat layer <NUM> may be applied by plasma spray processes, chemical vapor deposition processes, electron beam physical vapor deposition processes, dipping in molten silicon, sputtering processes, and other conventional application processes known to those skilled in the art.

As previously described, silica layer <NUM> can form during the service life of the EBC. Specifically, oxygen in the surrounding atmosphere can diffuse through any of the outer layer, compliant, and transition layer(s) present in the EBC and react with the silicon of bond coat layer <NUM> to form silica layer <NUM>. Alternately, silica layer <NUM> may be intentionally deposited by chemical vapor deposition, plasma spray, slurry deposition, or other conventional method.

Similar to silica layer <NUM>, intermediate layer <NUM> can also form during the service life of the EBC when high temperature steam reacts with transition layer <NUM>, as previously described.

The manufacturing and application processes for transition layer <NUM>, compliant layer <NUM> and outer layer <NUM> can consist of a solvent based slurry deposition cycle including sintering aids to lower the temperature needed to densify the layers. The slurry deposition cycle can generally include slurry formation, slurry application, drying, and sintering, with optional masking, leveling, sintering aid infiltration, mask removal, and binder burnout steps, as set forth below. Those skilled in the art will understand that slurries of varying compositions can be used to make EBC layers of varying composition and that multiple slurry deposition cycles can be used to build up the total thickness of a particular layer. Each layer can have the thickness set forth previously with the average thickness per slurry deposition cycle depending primarily on the slurry solids loading, sintering aid concentration, and number of dip, spray, or paint passes.

The slurries described in the embodiments herein can comprise various slurry components, but generally include an organic solvent, ceramic particles, sintering aid, and organic processing aids. The slurry comprises at least from <NUM> wt% solvent and at least from <NUM> wt% primary material. The slurry may also comprise from about <NUM> wt% to about <NUM> wt% of a dispersant, from about <NUM> wt% to about <NUM> wt % of a plasticizer, from about <NUM> wt% to about <NUM> wt% thickener, from about <NUM> wt% to about <NUM> wt% surfactant, and from about <NUM> wt% to about <NUM> wt % slurry sintering aid if there is one sintering aid, or alternately, from about <NUM> wt % to about <NUM> wt % slurry sintering aid if there are two sintering aids present, as described herein below.

More specifically, in such solvent based slurries, "organic solvent" refers to methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, dodecanol, acetone, methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK), toluene, ethylbenzene, propyl benzene, methoxybenzene, heptane, octane, nonane, decane, xylene, mineral spirits, naptha (such as VM&P naptha), tetrahydrofuran, ethers, and combinations thereof.

"Primary materials" may refer to Ln<NUM>Si<NUM>O<NUM>, Ln<NUM>SiO<NUM>, or BSAS depending on which layer is being made. In embodiments of the invention the primary material is a primary transition material and is a rare earth disilicate.

"Dispersant" can refer to polyacrylic acid, polyacrylic acid-polyethylene oxide copolymers, polymethacrylic acid, polyethylenimine, phosphate esters, menhaden fish oil, polyethylene oxide, polysilazane, and combinations thereof.

"Plasticizer" can refer to ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol glycerol, glycerin, polyethylene glycol, dibutyl phthalate, Bis(<NUM>-ethylhexyl) phthalate, Bis(n-butyl)phthalate, Butyl benzyl phthalate, Diisodecyl phthalate, Di-n-octyl phthalate, Diisooctyl phthalate, Diethyl phthalate, Diisobutyl phthalate, Di-n-hexyl phthalate, Di(propylene glycol) dibenzoate, Di(ethylene glycol) dibenzoate, tri(ethylene glycol) dibenzoate, and combinations thereof.

As used herein, "slurry sintering aid" can refer to sintering aid compositions suitable for inclusion in the slurry. There is from about <NUM> wt% to about <NUM> wt %, of a slurry sintering aid selected from iron oxide; carbonyl iron; iron metal,iron hydroxide, iron carbonate, iron oxalate, and solvent soluble iron salts,. Comparative examples may contain from about <NUM> wt% to about <NUM> wt %, and in some examples, not according to the invention, from about <NUM> wt% to about <NUM> wt %, of a slurry sintering aid selected from gallium oxide, aluminum oxide, nickel oxide, titanium oxide, boron oxide, and alkaline earth oxides; aluminum metal, boron, nickel metal, hydroxides including gallium hydroxide, aluminum hydroxide, nickel hydroxide, titanium hydroxide, alkaline earth hydroxides; carbonates including gallium carbonate, aluminum carbonate, nickel carbonate, boron carbonate, and alkaline earth carbonates; oxalates including gallium oxalate, aluminum oxalate, nickel oxalate, titanium oxalate; and "solvent soluble salts" including solvent soluble gallium salts, solvent soluble aluminum salts, solvent soluble nickel salts, solvent soluble titanium salts, solvent soluble boron salts, and solvent soluble alkaline earth salts. In the case of the compliant layer slurry, the "slurry sintering aid" may include rare earth nitrate, rare earth acetate, rare earth chloride, rare earth oxide, ammonium phosphate, phosphoric acid, polyvinyl phosphoric acid, and combination thereof.

In an alternate embodiment, the slurry can comprise from about <NUM> wt% to about <NUM> wt%, and in one embodiment from about <NUM> wt% to about <NUM> wt%, of an Lnb metal slurry sintering aid as defined previously herein, and from about <NUM> wt% to about <NUM> wt%, and in one embodiment from about <NUM> wt% to about <NUM> wt%, of a SiO<NUM> slurry sintering aid. In this embodiment, the Lnb and SiO<NUM> content can be held such that the mole ratio of Lnb to SiO<NUM> is about <NUM> to <NUM> for slurries containing rare earth disilicate primary transition material, and about <NUM> to <NUM> for slurries containing rare earth monosilicate primary outer material.

As used herein, "solvent-soluble iron salts" can include ethoxide, iron <NUM>,<NUM>-pentanedionate, and iron tetramethylheptanedionate; "solvent-soluble gallium salts" can include gallium <NUM>-hydroxyquinolinate, gallium <NUM>,<NUM>-pentanedionate, gallium ethoxide, gallium isopropoxide, and gallium <NUM>,<NUM>,<NUM>,<NUM>-tetramethylheptanedionate; "solvent-soluble aluminum salts" can include butoxide, aluminum di-s-butoxide ethylacetoacetate, aluminum diisopropoxide ethylacetoacetate, aluminum ethoxide, aluminum ethoxyethoxyethoxide, aluminum <NUM>,<NUM>-heptanedionate, aluminum isopropoxide, aluminum <NUM>-octadecenylacetoacetate diisopropoxide, aluminum <NUM>,<NUM>-pentanedionate, aluminum pentanedionate bis(ethylacetoacetate), aluminum <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl3,<NUM>-heptanedionate, and aluminum phenoxide; "solvent-soluble nickel salts" can include nickel <NUM>,<NUM>-pentanedionate, nickel <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-<NUM>-heptanedionate; "solvent-soluble titanium salts" can include titanium allylacetoacetatetriisopropoxide, titanium bis(triethanolamine)diisopropoxide, titanium butoxide, titanium di-n-butoxide bis(<NUM>-ethylhexanoate), titanium diisopropoxide(bis-<NUM>,<NUM>-pentanedionate), titanium diisopropoxide bis(tetramethylheptanedionate, titanium ethoxide, titanium diisopropoxide bis(ethylacetoacetate), titanium <NUM>-ethylhexoxide, titanium iodide triisopropoxide, titanium isobutoxide, titanium isopropoxide, titanium methacrylate triisopropoxide, titanium methacryloxyethylacetoacetate triisopropoxide, titanium methoxide, titanium methoxypropoxide, titanium methylphenoxide, titanium n-nonyloxide, titanium oxide bis(pentanedionate), titanium oxide bis(tetramethylheptanedionate), and titanium n-propoxide; "solvent-soluble boron salts" can include boron ethoxide, boron butoxide, boron isopropoxide, boron methoxide, boron methoxyethoxide, boron n-propoxide; and "solvent-soluble alkaline earth salts" can include calcium isopropoxide, calcium methoxyethoxide, calcium methoxide, calcium ethoxide, strontium isopropoxide, strontium methoxypropoxide, strontium <NUM>,<NUM>-pentanedionate, strontium <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>,<NUM>-heptanedionate, magnesium ethoxide, magnesium methoxide, magnesium methoxyethoxide, magnesium <NUM>,<NUM>-pentanedionate, magnesium n-propoxide, barium isopropoxide, barium methoxypropoxide, barium <NUM>,<NUM>-pentanedionate, barium <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>,<NUM>-heptanedionate.

"Thickener" refers to polyvinyl butyral, polyvinyl acetate, poly(isobutyl methacrylate), poly[(n-butyl methacrylate-co-isobutyl methacrylate), methyl methacrylate copolymers, ethyl methacrylate copolymers, poly methyl methacrylate, polyethyl methacrylate, polyvinylpyroline, ethyl cellulose, nitrocellulose, and other solvent soluble cellulose derivatives, and combinations thereof.

"Surfactant" refers to compositions selected from the group consisting of fluorocarbons, dimethylsilicones, and ethoxylated acetylenic diol chemistries (e.g. commercial surfactants in the Surfynol® series such as Surfynol® <NUM> and <NUM> (Air Products and Chemicals, Inc. )), and combinations thereof.

Also, as used herein, "organic processing aids" refers to any dispersants, plasticizers, thickeners, and surfactants present in the slurry. These organic processing aids are comprised primarily of carbon and other elements that volatilize during processing such that they are not present in the post-sintered coating.

The slurry can be formed by combining any or all of the previously described slurry components with mixing media in a container. The mixture can be mixed using conventional techniques known to those skilled in the art such as shaking with up to about a <NUM> inch (about <NUM>) diameter alumina or zirconia mixing media, ball milling using about a <NUM> inch to about a <NUM> inch (about <NUM> to about <NUM>) diameter alumina or zirconia mixing media, attritor milling using about a <NUM> to about a <NUM> diameter zirconia-based mixing media, planetary ball milling using from about a <NUM> to about a <NUM> diameter zirconia-based media, or mechanical mixing or stirring with simultaneous application of ultrasonic energy. The mixing media or ultrasonic energy can break apart any agglomerated ceramic particles in the slurry. Any mixing media present may then be removed by straining, for example.

If not added previously, thickener may be added to the slurry if desired and the resulting mixture may be agitated by such methods as mechanical stirring, rolling, blending, shaking, and other like methods until the thickener is fully dissolved, generally after about <NUM> to about <NUM> minutes.

Similarly, if not added previously, the addition of the sintering aids may follow along with mixing using the previously described methods until the sintering aids dissolve, which is about <NUM> to about <NUM> minutes.

Once all slurry components have been mixed, the slurry can be filtered through screens of varying mesh sizes to remove any impurities that may be present, such as after the initial mixing of the slurry or after use of the slurry to deposit coating layers. A <NUM> mesh screen, for example, can be used to filter out impurities having an average size of about <NUM> microns or greater.

After mixing and optional filtering, the slurry can be agitated indefinitely by slow rolling, slow mechanical mixing, or other like methods to avoid trapping air bubbles in the slurry. In one embodiment, the slurry may be refreshed by adding additional solvent to account for that which has evaporated during processing. Alternately, once mixed, the slurry can be set aside until needed for application. Those skilled in the art will understand that the previous embodiment sets forth one method for making the slurry compositions described herein, and that other methods are also acceptable, as set forth in the Examples below.

Optionally, masking can be applied to the ceramic component before the slurry is applied to prevent coating specific areas of the component. Masking may be carried out using conventional techniques known to those skilled in the art including, but not limited to, tapes, tooling, and paint-on adhesives.

Once all desired masking of the ceramic component is complete, the slurry can be applied to the component to produce a coated component. The slurry can be applied to the component (or on top of a previously applied layer) using any conventional slurry deposition method known to those skilled in the art, including but not limited to, dipping the component into a slurry bath, or painting, rolling, stamping, spraying, or pouring the slurry onto the component. Slurry application can be carried out manually or it may be automated.

Once the slurry has been applied to the component, and while the slurry is still wet, it may be leveled to remove excess slurry material. Leveling may be carried out using conventional techniques such as, but not limited to, spinning, rotating, slinging the component, dripping with or without applied vibration, or using a doctor blade, to remove excess slurry material. Similar to the slurry application, leveling can be conducted manually or it may be automated.

Next, the coated component can be dried. Drying may be carried out in an enclosed area having additional organic solvent present in secondary containers. This can help slow the drying process because the atmosphere can be saturated with organic solvent. Since this process utilizes an organic solvent, it is not strongly sensitive to humidity. Temperature variation can be used to control the drying rate, however, those skilled in the art will understand that that temperatures can be kept below the flash point of the organic solvent. Placing a coated component in a vacuum chamber and pulling a vacuum can also be used to accelerate drying.

After drying, any masking present may then be removed by peeling off tapes and adhesives, pyrolysis of tapes and adhesives, or removing multi-use tooling. Any rough edges remaining after masking removal may be scraped or cut away using a sharp or abrasive tool.

Next, burnout of the organic processing aids may be carried out by placing the dried component in an elevated temperature environment so that any residual solvent can be evaporated and the organic processing aids can be pyrolyzed. In one embodiment, burnout of the organic processing aids may be accomplished by heating the dried component at a rate of from about <NUM>/min to about <NUM>/min to a temperature of from about <NUM> to about <NUM> and holding the component at this temperature for from about <NUM> to about <NUM> hours. In another embodiment, the coated component may be heated at a rate of from about <NUM>/min to about <NUM>/min to a temperature of from about <NUM> to about <NUM> and holding the component at this temperature for from about <NUM> to about <NUM> hours. In another embodiment, the hold time can be eliminated by slowly ramping up to the target temperature without holding, followed by ramping up or down to another temperature at a different rate. In another embodiment, binder burnout can occur rapidly by placing the coated component into a furnace heated to a temperature of from about <NUM> to about <NUM>.

Binder burnout and sintering heat treatments may be carried out in an ambient air atmosphere, or in an inert gas atmosphere where the inert gas is selected from hydrogen, a noble gas such as helium, neon, argon, krypton, xenon, or mixtures thereof. In one embodiment, the inert gas atmosphere can be used in conjunction with Lnb and SiO<NUM> sintering aids so as not to convert the rare earth metal to an oxide before it melts. Maintaining the Lnb metal in a metal state can promote liquid phase sintering and subsequent reaction with the SiO<NUM>.

The dried component may then be sintered to produce a component comprising an environmental barrier coating. Sintering can serve to simultaneously densify and impart strength to the coating. Additionally, in the case of the transition and compliant layers of the EBC, sintering can impart a hermetic seal against high temperature steam present in the engine environment. In the case of the outer layer, sintering can provide a dense barrier against the infiltration of molten silicates, such as calcium-magnesium aluminosilicate (CMAS) that may be encountered as a result of particulate contamination (i.e. dirt and sand) in the engine environment. Sintering can be carried out using a conventional furnace, or by using such methods as microwave sintering, laser sintering, infrared sintering, and the like.

Sintering can be accomplished by heating the dried component at a rate of from about <NUM>/min to about <NUM>/min to a temperature of from about <NUM> to about <NUM> and holding the component at that temperature for from about <NUM> to about <NUM> hours. In another embodiment, sintering can be accomplished by heating the coated component at a rate of from about <NUM>/min to about <NUM>/min to a temperature of from about <NUM> to about <NUM> and holding the component at that temperature for from about <NUM> to about <NUM> hours. In another embodiment, sintering can occur rapidly by placing the coated component into a furnace heated to a temperature of from about <NUM> to about <NUM>.

In an alternate embodiment, all layers of the EBC can be applied, one on top of the other, before masking removal, organic processing aid burnout, and sintering are carried out. Those skilled in the art will understand that after application of each layer, the layer should be dried, or partially dried, before the application of the subsequent layer.

In another embodiment, the sintering aid does not need to be added directly to the transition or outer layer of the slurry to achieve the desired result. The sintering aid can be added to one layer of the EBC slurry, and during sintering, the sintering aid can diffuse throughout the EBC to the remaining layers. In another embodiment, a primary material slurry with no sintering aid can be densified by applying the layer, allowing it to dry, and then back infiltrating a sol-gel solution comprising a sintering aid prior to heat treatment as explained below.

Infiltration may allow for the densification of a thicker layer of EBC material at one time. Moreover, infiltration is a way to add more sintering aid after sintering if the coating is not as dense as desired. The sol-gel solution used for infiltration may be a solution of an organic solvent and a solvent soluble salt sintering aid, as defined previously, or a solution of water and a water soluble salt sintering aid.

As used herein "water soluble salt sintering aid" can refer to "water-soluble iron salts" such as iron nitrate and iron acetate; "water-soluble gallium salts" such as gallium nitrate and gallium acetate; "water-soluble aluminum salts" such as aluminum nitrate and aluminum acetate; "water-soluble nickel salts" such as nickel nitrate and nickel acetate; "water-soluble titanium salts" such as titanium chloride; "water-soluble boron salts" such as boric acid and ammonium borate; and "water-soluble alkaline earth salts" such as include Mg(NO<NUM>)<NUM>, Ca(NO<NUM>)<NUM>, Sr(NO<NUM>)<NUM>, Ba(NO<NUM>)<NUM>, MgC<NUM>H<NUM>O<NUM>, CaC<NUM>H<NUM>O<NUM>, SrC<NUM>H<NUM>O<NUM>, and BaC<NUM>H<NUM>O<NUM>.

As used herein, "sintering aid(s)" can refer to any of a "slurry sintering aid," a "solvent soluble sintering aid," or a "water soluble salt sintering aid," as defined previously. Without intending to be limited by theory, the inclusion of sintering aids to the EBC embodiments herein can increase the rate of diffusion of primary material such that surface area reduction (i.e. high surface area particles consolidating to form a dense coating) can occur at lower temperatures than it would have absent the sintering aid. As previously described, sintering at lower temperatures (i.e. about <NUM> or below) can not only result in a highly dense (i.e. greater than about <NUM>% for the transition layer, greater than about <NUM>% for the compliant layer, and greater than about <NUM>% for the outer layer) coating that can be less susceptible to the penetration of hot steam from the engine environment, but can also help prevent the degradation of the mechanical properties of the underlying component that could result from prolonged exposure to higher temperatures.

Sintering aids can act in a variety of ways depending on the amount of sintering aid included in the EBC and the time at which the coating is exposed to sintering temperatures. For example, in one embodiment, the sintering aid can dissolve completely into the primary material (i.e. primary transition, outer, or compliant, material) to "dope" the material. In another embodiment, if the amount of sintering aid that is soluble in the primary material is exceeded, the remaining insoluble portion of sintering aid can react with the primary material to form the secondary material (i.e. secondary transition, outer, or compliant material). In another embodiment, primary material and secondary material can be present as described previously, along with residual sintering aid.

In these latter two embodiments, when the secondary material is highly volatile in high temperature steam, such as but not limited to, alkali silicates, alkaline earth silicates, mullite, iron silicate, borosilicate glass, nickel silicate, and residual sintering aids of iron, aluminum, titanium, gallium, nickel, boron, alkali, and alkali-earth compounds, as long as the total volume of secondary material, plus porosity (plus residual sintering aid when present) in either of the intermediate layer or compliant layer (when present) of the EBC remains about <NUM>% by volume or less, the hermetic seal can be maintained. Alternately, in these latter two embodiments, when the secondary material is highly resistant to volatilization in high temperature steam, such as when the secondary material comprises a rare earth containing compound, such as but not limited to rare earth oxide, rare earth titanate, rare earth iron compound, rare earth gallate, rare earth aluminate, and rare earth aluminosilicate, the porosity in either of the intermediate or compliant layer (when present) of the EBC need remain about <NUM>% by volume or less to maintain the hermetic seal.

It should be noted that at low levels of sintering aid, the densified coating layer might not initially include any detectable secondary materials. In some embodiments, the secondary materials may never become detectable. In other embodiments, however, after hours of exposure to high temperature steam in the engine environment, the secondary materials can become detectable using techniques such as x-ray diffraction, electron microscopy, electron dispersive spectroscopy (EDS), and the like.

EBC embodiments described herein can offer a variety of benefits over current EBCs and manufacturing processes thereof. Specifically, as previously described, the inclusion of a sintering aid in the EBC embodiments herein can permit sintering at lower temperatures (i.e. about <NUM> or below). This can result in a highly dense (i.e. greater than about <NUM>% for the transition layer, and greater than about <NUM>% for each of the outer, and compliant, layers) coating that can be less susceptible to the penetration of hot steam from the engine environment, and can also help prevent the degradation of the mechanical properties of the underlying component that could result from prolonged exposure to higher temperatures. Also, the embodiments set forth herein can be made at less expense than current EBCs due to the use of the slurry deposition process, which is made possible by the incorporation of sintering aids into the various layers. Moreover, the present embodiments can provide for EBCs having a more uniform thickness than conventional techniques, such as plasma spraying, even when applying thin layers (<<NUM> mils). Additionally, the slurry deposition process can allow for the application of the EBCs to internal component passages as well as the ability to produce smooth surface finishes without an additional polishing step.

There can be occasions when the EBC develops small and/or narrow defects (e.g. about <NUM> microns to about <NUM> in diameter; or about <NUM> microns to about <NUM> in width) that need to be repaired. The following repair processes are applicable to the EBCs described herein and may be carried out after sintering of an individual EBC layer, or after sintering the entire applied EBC, as explained herein below.

In one embodiment, repairs may include remedying defects in one or more individual layers as the EBC is being applied using the methods described herein. In this embodiment, the repair can be carried out after sintering a given layer by applying a repair slurry comprising the same slurry materials used to make the layer having the defects. For example, if the transition layer develops a defect after sintering, the defect could be repaired using a "transition layer repair slurry" that comprises the same transition layer slurry materials used in the original application of the transition layer. In one embodiment, the repair slurry can comprise a higher solids loading of primary material ceramic particles than the original slurry layer as this can reduce shrinkage on drying and sintering of the repaired portion of the coating. In particular, the solids loading of primary material ceramic particles in the repair slurry can be greater than about <NUM>% to about <NUM>% by volume (as opposed to greater than about <NUM>% by volume in one embodiment of the original slurry, and from about <NUM>% to about <NUM>% by volume in another embodiment of the original slurry used to make the layer). The repair slurry may be applied using any conventional method including those described previously, and the resulting "repair(ed) coating" may then be processed as described previously herein before application of any subsequent layer of the EBC.

In an alternate embodiment, repairs may include fixing defects after application and sintering of the entire EBC. In this embodiment, the repair may be carried out on the EBC having defects using a transition layer repair slurry comprising the same materials present in the previously defined transition layer slurry (i.e. primary transition material, a sintering aid, and optionally secondary material). This particular repair slurry can seep into any defects present in the EBC and provide a hermetic seal to the repaired EBC coating after sintering. Again, the solids loading of the transition layer repair slurry may comprise upwards of about <NUM>% to <NUM>% by volume.

Additionally, repair processes may be used to reduce surface roughness of a plasma sprayed EBC having any composition. Specifically, if the surface roughness of a plasma sprayed EBC is unacceptable the coating can be smoothed over by applying either of the previously described transition layer slurry or outer layer slurry. When applied over the plasma sprayed EBC, the transition layer slurry or outer layer slurry can fill in any gaps, grooves, or uneven portions of the plasma sprayed coating and reduce the surface roughness to an acceptable degree. More specifically, depending on the thickness of the transition layer slurry or outer layer slurry, surface roughness of the plasma sprayed EBC can be reduced from greater than <NUM> micro inch Ra, to between <NUM> micro inch Ra and <NUM> micro inch Ra in one embodiment, and from between <NUM> micro inch Ra to <NUM> micro inch Ra in another embodiment. In one embodiment, the transition layer slurry or outer layer slurry can comprise a thickness of at least about <NUM> mils, and in another embodiment from about <NUM> mils to about <NUM> mils. The applied transition layer slurry can then be processed as described previously to produce a repaired EBC having an acceptable surface roughness. Additional slurry layers may be applied to the EBC if desired.

Such repair processes can provide the ability to repair localized defects, at varying points during the application or life of the coating, as opposed to stripping off and reapplying the entire coating. This, in turn, can result in a savings of time, labor, and materials.

Example <NUM>: A silicon bond coat was applied to a SiC-SiC CMC using a conventional air plasma spray process. Next, a primary transition material slurry was made by first mixing ytterbium disilicate (primary transition material), iron oxide nanoparticles (sintering aid), ethanol (solvent), and polyethylenimine (dispersant) in a plastic container, along with enough <NUM> inch (<NUM>) diameter, spherical zirconia media to line the bottom of container. This mixture was placed on a roller mill for <NUM> hours. After taking the container off of the roller mill, the zirconia media was removed and the slurry was filtered through a <NUM> mesh screen to remove any large particle agglomerates.

The resulting primary transition material slurry (Slurry A) consisted of <NUM>% ytterbium disilicate, <NUM>% iron oxide, <NUM>% polyethylenimine, and the balance ethanol (all percents by weight). The silicon-coated ceramic component was dipped into Slurry A, dried in ambient conditions, re-dipped into Slurry A, dried in ambient conditions, and heat-treated at <NUM>/minute to <NUM> to burn out any residual organic materials. Then, the temperature was increased at <NUM>/minute from <NUM> to <NUM> and held for <NUM> hours to form a transition layer comprising iron-doped ytterbium disilicate. The entire heat treatment was carried out in air. The heating environment resulted in the transition layer having a porosity of less than <NUM>% by volume. These dipping, drying, and heat treating processes were repeated <NUM> times to build thickness of the transition layer.

Next, a primary outer material slurry was made by first mixing ytterbium monosilicate (primary outer material), iron oxide nanoparticles (sintering aid), ethanol (solvent), and polyethylenimine (dispersant) in a plastic container, along with enough <NUM> inch (<NUM>) diameter, spherical zirconia media to line the bottom of container. This mixture was placed on a roller mill for <NUM> hours. After taking the container off of the roller mill, the zirconia media was removed and the slurry was filtered through a <NUM> mesh screen to remove any large particle agglomerates.

The resulting primary outer material slurry (Slurry B) consisted of <NUM>% ytterbium monosilicate, <NUM>% iron oxide, <NUM>% polyethylenimine, and the balance ethanol (all percents by weight). The silicon and transition layer-coated ceramic component was dipped into Slurry B, dried in ambient conditions, re-dipped into Slurry B, dried in ambient conditions, and heat-treated at <NUM>/minute to <NUM> to burn out any residual organic materials. Then, the temperature was increased at <NUM>/minute from <NUM> to <NUM> and held for <NUM> hours to densify the transition layer. After heat treatment, the layer comprised iron-doped yttrium monosilicate. The entire heat treatment was carried out in air. The heating environment resulted in the transition layer having a porosity of less than <NUM>% by volume. The dipping, drying, and heat treatments were repeated <NUM> times to build thickness of the outer layer.

shows a SEM micrograph of this coating microstructure with the air plasma spray silicon bond coat (<NUM>), a thin silica layer (<NUM> micrometers) (<NUM>), transition layer (<NUM>), and outer layer (<NUM>). The outer layer and transition layer appeared to consist only of iron-doped primary materials (iron-doped ytterbium monosilicate and iron-doped ytterbium disilicate, respectively), x-ray diffraction was not able to detect any secondary material present in the EBC. EDS analysis of the layers suggested that the iron oxide had dissolved into the primary materials.

Example <NUM>: A CMC (<NUM>) coated with the EBC of Example <NUM> was exposed to <NUM>°F (<NUM>) steam for <NUM> hours. <FIG> shows a SEM micrograph of this coating after high temperature steam exposure with silicon bond coat (<NUM>), a silica layer (<NUM>) that has grown to approximately <NUM> micrometer thickness, an iron-doped ytterbium disilicate transition layer (<NUM>), and a dense ytterbium monosilicate outer layer (<NUM>) with cracks. Some ytterbium disilicate has formed around the cracks in the monosilicate layer as an artifact of high gaseous silicon content in the static atmosphere of the steam test.

Example <NUM>: This is a comparative example not according to the claims. To demonstrate proof of principle, a primary transition layer was deposited on a silicon metal wafer using a slurry deposition process. A primary transition material slurry was made by first mixing ytterbium disilicate powder (primary transition material), gallium oxide powder (sintering aid), ethanol (solvent), and polyethylenimine (dispersant) in a plastic container, along with enough <NUM> inch (<NUM>) diameter, spherical zirconia media to line the bottom of container. This mixture was placed on a roller mill for <NUM> hours. After taking the container off of the roller mill, the zirconia media was removed and the slurry was filtered through a <NUM> mesh screen to remove any large particle agglomerates.

Claim 1:
An organic solvent based slurry composition for making an environmental barrier coating (<NUM>), the slurry composition comprising:
at least from <NUM> wt% of a solvent selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, dodecanol, acetone, methyl isobutyl ketone, methyl ethyl ketone, toluene, ethylbenzene, propyl benzene, methoxybenzene, heptane, octane, nonane, decane, xylene, mineral spirits, naptha, tetrahydrofuran, ethers, and mixtures thereof;
at least from <NUM> wt% of a primary material; wherein the primary material is a primary transition material and is a rare earth disilicate; and
from <NUM> wt% to <NUM> wt% of a slurry sintering aid,
wherein a sum of the solvent, the primary material and the slurry sintering aid is equal to or less than <NUM> wt% of the whole slurry composition,
wherein the slurry sintering aid is a composition that:
dissolves in the primary material at a sintering temperature of the primary material;
increases a rate of diffusion of the primary material during sintering such that surface area reduction occurs at lower temperatures than would occur in the absence of the slurry sintering aid; and
wherein the slurry sintering aid for use with the primary transition material is selected from the group consisting of iron oxide, carbonyl iron, iron metal, iron hydroxide, iron carbonate, iron oxalate, and solvent soluble iron salts.