Patent Description:
Embodiments of the present invention generally relate to environmental barrier coatings for ceramic components, along with methods of making the same.

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. Still, with many hot gas path components constructed from super alloys, thermal barrier coatings (TBCs) can be utilized to insulate the components and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface, thus limiting the thermal exposure of the structural component.

While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight substrate materials have been proposed, such as ceramic matrix composite (CMC) materials. 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.

Silicon carbide and silicon nitride ceramics undergo oxidation in dry, high temperature environments. This oxidation produces a passive, silicon oxide scale on the surface of the material. In moist, high temperature environments containing water vapor, such as a turbine engine, both oxidation and recession occurs due to the formation of a passive silicon oxide scale and subsequent conversion of the silicon oxide to gaseous silicon hydroxide. To prevent recession in moist, high temperature environments, environmental barrier coatings (EBC's) are deposited onto silicon carbide and silicon nitride materials.

Currently, EBC materials are made out of oxides such as mullite, celsian-phase barium strontium aluminosilicate (BSAS), and most recently rare earth silicate compounds. These materials seal out water vapor, preventing it from reaching the silicon oxide scale on the silicon carbide or silicon nitride surface, thereby preventing recession. Such materials cannot prevent oxygen penetration, however, which results in oxidation of the underlying substrate. Oxidation of the substrate yields a passive silicon oxide scale, along with the release of carbonaceous or nitrogen based gases (e.g., nitrous oxide gas) for silicon carbide- and silicon nitride-based substrates, respectively. The carbonaceous (i.e., CO, CO<NUM>) or nitrogen-based gases (i.e., N<NUM>, NO, NO<NUM>, etc.) oxide gases cannot escape out through the dense EBC and thus, blisters form. The use of a silicon bond coat has been the solution to this blistering problem to date. The silicon bond coat provides a layer that oxidizes (forming a passive silicon oxide layer beneath the EBC) without liberating a gaseous by-product. <CIT>, <CIT> and <CIT> disclose barrier layers which are attached to a substrate via a bond coat. However, the presence of a silicon bond coat limits the upper temperature of operation for the EBC because the melting point of silicon metal is relatively low compared to the oxides.

As such, it is desirable to eliminate the use of a silicon bond coat in the EBC to achieve a higher operational temperature limit for the EBC.

According to one aspect of the invention, a coated substrate is generally provided, as defined in claim <NUM>. The silicon-containing glass phase is a continuous phase within the barrier layer (e.g., a breathable grain boundary of the barrier layer).

In one particular embodiment, the refractory material phase includes a rare earth silicate material having a rare earth component at a first atomic percent, while the silicon-containing glass phase comprises the rare earth component at a second atomic percent that is less than the first atomic percent.

According to another aspect of the invention, a method is also generally provided, as defined in claim <NUM>.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:.

In the present disclosure, when a layer is being described as "on" or "over" another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers, unless expressly stated to the contrary. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean "on top of" since the relative position above or below depends upon the orientation of the device to the viewer.

Chemical elements are discussed in the present disclosure using their common chemical abbreviation, such as commonly found on a periodic table of elements. For example, hydrogen is represented by its common chemical abbreviation H; helium is represented by its common chemical abbreviation He; and so forth. As used herein, "Ln" refers to a rare earth element or a mixture of rare earth elements. More specifically, the "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), or mixtures thereof (or the appropriate salt thereof). As used herein, the term "alkaline earth metals" refers to beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), occupying Group IIA (<NUM>) of the periodic table (or the appropriate salt thereof).

Environmental barrier coatings are generally provided on the surface of the substrate, with the environmental barrier coating including a barrier layer having a breathable silicon-containing glass phase. The barrier layer includes a refractory material phase and a silicon-containing glass phase, with the refractory material phase including a rare earth silicate material. The silicon-containing glass phase is "breathable" in the sense that they increase the transport (permeability and/or diffusion rate) of carbonaceous, nitrogen, or nitrous gas byproducts through the EBC layer, which can lower any gaseous pressure within the EBC to help inhibit blistering and/or delamination of the EBC. As such, the silicon-containing glass phase is generally an amorphous, non-crystalline material with creep behavior that allows gas bubbles to transport therethrough, as explained in greater detail below.

The presence of such breathable silicon-containing glass phases eliminates any need for a silicon bond coat, allowing higher temperature operation of the EBC. That is, in one particular embodiment, the EBC barrier coating can be directly on the surface of the substrate (i.e., without any intermediate bond coat). The resulting EBC allows for the use of advanced CMC/EBC material systems that can operate at CMC surface temperatures exceeding about <NUM> °F (about <NUM>) and up to about <NUM> °F (about <NUM>), and possibly exceeding about <NUM> °F (about <NUM>) for short durations with EBC surface temperatures up to about <NUM> °F (about <NUM>).

The barrier layer with breathable silicon-containing glass phase is made by over-doping rare earth silicate materials such that a secondary breathable silicon-containing glass phase emerges in the precursor powder as a glass. It is noted that, at a lesser doping level, only single phase, doped rare earth silicate would be present. The powder mix is then applied to form the coating with doped rare earth silicate plus continuous, breathable silicon-containing glass phase. Another way to make the barrier layer with breathable silicon-containing glass phase, not according to the invention, is by mixing a glass powder of breathable silicon-containing glass phase composition with a powder of refractory material suitable for environmental barrier coating applications such as rare earth silicate, mullite, BSAS, zirconia, hafnia, stabilized (tetragonal or cubic) zirconia, stabilized (tetragonal or cubic) hafnia, zircon (ZrSiO<NUM>), hafnon (HfSiO<NUM>), rare earth zirconate, rare earth hafnate, rare earth gallate, etc. Yet another way to make the barrier layer with breathable silicon-containing glass phase, not according to the invention, is by depositing the glass composition directly on the substrate as a coating layer, then applying a porous layer of material suitable for environmental barrier coating applications on top of the glass layer.

In one embodiment, the breathable silicon-containing glass phase yields enhanced permeability of carbonaceous and/or nitrogen based gases (e.g., nitrogen, and/or nitrous gases) because these gases are released to build up pressure beneath the coating layer as the substrate is oxidized. Once the pressure is built-up to a sufficient level, the gas is forced through the secondary breathable silicon-containing glass phase. In such case, the secondary breathable silicon-containing glass phase behaves like a viscous phase that gas bubbles may transport through due to the build in pressure. The coating layer remains largely hermetic to water vapor on the outside of the coating, however, because the external pressure of the water vapor is too low for it to be forced the other direction through the breathable silicon-containing glass phase. In such case, materials that decrease the viscosity of the secondary breathable silicon-containing glass phase may improve its functionality (e.g. boron oxide). In contrast, elements that increase the viscosity of the secondary breathable silicon-containing glass phase may destroy its functionality. Such materials are those that would raise the viscosity of the glass (e.g. oxides of aluminum, niobium, and tantalum).

In another embodiment, the breathable silicon-containing glass phase may be a glass that exhibits high solubility for carbonaceous, nitrogen, and/or nitrous gases. In such case, materials in the glass phase that result in a glass of high permeability of carbonaceous gas may include those with cations that tend to form more stable carbonates than hydroxides (such as those described for rare earth silicate doping and over-doping). Materials with cations that form stable hydroxides but do not form carbonates (e.g., aluminum) may be a poor choice to be a part of the secondary breathable silicon-containing glass phase glass composition.

Over-doping refers to doping the rare earth silicate material in excess at the rare earth site with a dopant that includes an alkali metal cation, Cu<NUM>+, a noble metal cation, an alkaline earth metal cation, Cu<NUM>+, Ni<NUM>+, Mn<NUM>+, Zn<NUM>+, Sn<NUM>+, or a mixture thereof) to form the secondary breathable silicon-containing glass phase. Additionally, the dopant has a greater atomic percentage in the resulting silicon-containing glass phase than in the rare earth silicate material of the refractory material phase.

Doped rare earth silicate may also enhance the net breathability of a layer comprised of both continuous, breathable silicon-containing glass phase and the refractory material phase including a rare earth silicate material (e.g., a doped rare earth silicate). As stated, doping is accomplished chemically via substitution on the rare earth "site" of the compound with any element with oxidation state of +<NUM> or +<NUM>. Alkaline earth elements, with +<NUM> oxidation state, are particularly useful for this task. Partial substitution of the +<NUM> oxidation state rare earth with a lower oxidation state element results in oxygen vacancies in the material. This network of oxygen vacancies, combined with open-nature of the rare earth silicate monoclinic structure, may enhance the diffusion of carbonaceous, nitrogen, and/or nitrous gas through the two-phase layer.

In each case described above, the barrier layer is a dense layer that allows for venting of the carbonaceous and nitrogen-based gases while also preventing steam from the combustion environment from reaching the substrate. The coating may be hermetic to steam (no steam gets to the substrate), or it may only allow a low flow rate of steam to the substrate via a slow molecular diffusion process. It is also possible that the flow of carbonaceous and/or nitrogen-based gases out through the layer may disrupt or slow transport of water vapor from going through the EBC to attack the underlying substrate. In certain embodiments, the barrier layer has a porosity that is about <NUM>% or less.

In the approach where rare earth silicate materials are over-doped to form a two phase mixture (breathable silicon-containing glass phase plus refractory rare earth silicate), ytterbium, thulium, and lutetium silicate rare earths or combinations thereof are particularly useful, and perhaps preferred, as compared to all of the other rare earths. All of the other rare earths, when over-doped, particularly with alkaline earths, tend to form gas impermeable phases instead of gas permeable glass. For example, alkaline earth substitution on the ytterbium site of Yb<NUM>Si<NUM>O<NUM> can produce a two phase mixture of gas permeable glass phase and a refractory phase of doped Yb<NUM>Si<NUM>O<NUM>, however, alkaline earth substitution on the yttrium site of Y<NUM>Si<NUM>O<NUM> produces a two phase mixture of gas impermeable apatite and doped Y<NUM>Si<NUM>O<NUM>. However, this does not exclude the possibility of forming a gas permeable glass phase combined with rare earth silicates other than ytterbium, thulium, or lutetium silicates.

The refractory material phase of the barrier layer includes a rare earth silicate material (e.g., a monosilicate compound, a disilicate compound, or a mixture thereof). In such an embodiment, the silicon-containing glass phase may comprise a rare earth silicate glass (e.g., a doped rare earth silicate glass, such as Ca-Yb-silicate glass).

In one embodiment, a monosilicate compound is combined with a breathable silicon-containing glass phase. For example, the monosilicate compound is, in one embodiment, Ln<NUM>SiO<NUM> and/or Ln<NUM>SiO<NUM> doped at the Ln site with a dopant (e.g., an alkali metal cation, Cu<NUM>+, Au<NUM>+, Ag<NUM>+, a noble metal cation, an alkaline earth metal cation, Cu<NUM>+, Ni<NUM>+, Fe<NUM>+, Mn<NUM>+, Zn<NUM>+, Sn<NUM>+, or a mixture thereof). In such an embodiment, the refractory material phase can include a monosilicate compound as the rare earth silicate material, with the monosilicate compound having the formula:.

Ln<NUM>-x-y(D<NUM>+)x(D<NUM>+)ySi<NUM>-z(D<NUM>+)zO<NUM>-δ     (Formula <NUM>).

In another embodiment, a disilicate compound is combined with a breathable silicon-containing glass phase. For example, the disilicate compound is, in one embodiment, Ln<NUM>Si<NUM>O<NUM> and/or Ln<NUM>Si<NUM>O<NUM> doped at the Ln site with a dopant (e.g., an alkali metal cation, Cu<NUM>+, Au<NUM>+, Ag<NUM>+, a noble metal cation, an alkaline earth metal cation, Cu<NUM>+, Ni<NUM>+, Fe<NUM>+, Mn<NUM>+, Zn<NUM>+, Sn<NUM>+, or a mixture thereof). In such an embodiment, the refractory material phase can include a disilicate compound as the rare earth silicate material, with the disilicate compound having the formula:.

In one embodiment, <NUM> < (x + y + z) ≤ <NUM>, such as <NUM> < (x + y + z) ≤ <NUM>. In one particular embodiment, <NUM> < (x + y) ≤ <NUM>, such as <NUM> < (x + y) ≤ <NUM>. One exemplary monosilicate is Yb<NUM>-xCaxSiO<NUM>-δ where <NUM> ≤ x ≤ <NUM> and <NUM> < δ ≤ <NUM>.

In one embodiment, <NUM> < (x + y + z) ≤ <NUM>, such as <NUM> < (x + y + z) ≤ <NUM>. In one particular embodiment, <NUM> < (x + y) ≤ <NUM>, such as <NUM> < (x + y) ≤ <NUM>. One exemplary disilicate is Yb<NUM>-xCaxSi<NUM>O<NUM>-δ where <NUM> ≤ x ≤ <NUM> and <NUM> < δ ≤ <NUM>.

As stated, this doped silicate layer (e.g., doped monosilicate compound and/or doped disilicate compound) is combined with a permeable secondary phase formed by an excess of doping that results in the formation of the secondary phase that is continuous and breathable.

In another embodiment, the breathable silicon-containing glass phase is directly on top of the substrate and a single or plurality of combinations of porous Ln<NUM>Si<NUM>O<NUM> and Ln<NUM>SiO<NUM> layers are on the breathable silicon-containing glass phase layer. In one embodiment, the doped disilicate can also be combined with a permeable secondary phase.

Many of the compounds of Formula <NUM> or <NUM> have monoclinic rare earth silicates that may have a thermal expansion nearly equivalent to a SiC CMC or a higher thermal expansion than a SiC CMC. If higher than a SiC, however, vertically cracked layers comprised these materials can still offer some resistance to high T steam, particularly if there are additional layers underneath that are crack free to act as a hermetic layer.

In addition, many of the compounds of Formula <NUM> or <NUM> offer some protection from molten dust, particularly dirt or sand comprised of alkaline earth aluminosilicates. For additional protection, a porous or vertically cracked layer of rare earth silicate with rare earth of Y, Gd, Nd, Er, and Sm or combinations thereof can be used on top of the barrier layer. A plurality of these layers can also be used. These can form protective apatite layers when they react with molten dust (i.e., once the apatite forms due to reaction with molten dust, the remaining molten dust has difficulty penetrating the apatite layer).

Such a coating can be included as a stand-along coating layer (with or without additional outer layers present) on a substrate, or as a first layer of an EBC system directly on a substrate.

<FIG> shows a coating component <NUM> formed from a substrate <NUM> defining a surface <NUM> having an environmental barrier coating (EBC) <NUM> thereon. The EBC <NUM> includes a barrier layer <NUM> described above that includes a breathable silicon-containing glass phase <NUM> therein (e.g., having the compound of Formula <NUM> and/or <NUM> above). In the embodiment shown, the breathable silicon-containing glass phase <NUM> is directly on the surface <NUM> of the substrate <NUM> (i.e., without any bond coating present). In this embodiment, the breathable silicon-containing glass phase <NUM> is a continuous phase within the barrier layer <NUM>, such as a grain boundary phase. For example, the continuous, breathable silicon-containing glass phase may define as little as about <NUM>% of the volume of the barrier layer or as much as about <NUM>% of the volume of the barrier layer. However, in other embodiments, not according to the invention, the breathable silicon-containing glass phase <NUM> may be a discontinuous phase (e.g., a plurality of dispersed glass phases) within the barrier layer <NUM>. For example, the discontinuous, breathable silicon-containing glass phase may define as little as about <NUM>% of the volume of the barrier layer or as much as about <NUM>% of the volume of the barrier layer.

Although shown as forming only a portion of the barrier layer <NUM>, the breathable silicon-containing glass phase <NUM> can form and make-up substantially all of the barrier layer <NUM>. For example, the barrier layer <NUM> can have a thickness of about <NUM> or less (e.g., about <NUM> to about <NUM>), with the breathable silicon-containing glass phase <NUM> defining about <NUM>% to less than <NUM>% of the thickness of the barrier layer <NUM>. In such an embodiment, the breathable silicon-containing glass phase <NUM> can define about <NUM>% to about <NUM>% of the total volume of the barrier layer <NUM> (e.g., about <NUM>% to about <NUM>% of the total volume). For example, the breathable silicon-containing glass phase <NUM> can have a thickness of about <NUM> or less (e.g., about <NUM> to about <NUM>). For additional thickness on the breathable silicon-containing glass phase <NUM>, the barrier layer <NUM> can include a refractory material phase that includes a plurality of microcracks extending from the breathable silicon-containing glass phase <NUM> to the opposite surface of the barrier layer <NUM> to provide breathable, porous thickness. Such a porous material can serve to protect from harmful particles (e.g., CMAS) and/or heat to define a protection layer for the underlying breathable silicon-containing glass phase <NUM>.

The substrate <NUM> (e.g., a CMC component) may oxidize, particularly on its surface <NUM>, over time. During this oxidation, gas is released in the form of CO, CO<NUM>, N2, NO, NO<NUM>, or a mixture thereof. As it oxidizes, a SiO/SiO<NUM> layer (sometimes referred to as "silicon oxide scale" or "silica scale") is formed on the surface <NUM> and into the substrate <NUM>. Typically, it is desired that this silica scale remain amorphous, as its crystallization could lead to shedding of the overlying coatings from the substrate <NUM>. However, without wishing to be bound by any particular theory, it is believed that if the silica scale crystalizes and forms cracks in the surface <NUM>, the breathable silicon-containing glass phase <NUM> of the barrier layer <NUM> may migrate into the cracks to fill them and prevent coating to shed.

Furthermore, the silica scale may partially dissolve into the breathable silicon-containing glass phase. This dissolution strengthens the chemical bond with the barrier layer <NUM>. Also, this may raise the silicon content in the silicon containing glass phase without changing the functionality of the layer.

As stated above, the substrate <NUM> is formed from a CMC material (e.g., a silicon based, non-oxide ceramic matrix composite). As used herein, "CMCs" refers to silicon-containing, or oxide-oxide, matrix and reinforcing materials. Some examples of CMCs acceptable for use herein can include, but are not limited to, materials having a matrix and reinforcing fibers comprising non-oxide silicon-based materials such as silicon carbide, silicon nitride, silicon oxycarbides, silicon oxynitrides, and mixtures thereof. Examples include, but are not limited to, CMCs with silicon carbide matrix and silicon carbide fiber; silicon nitride matrix and silicon carbide fiber; and silicon carbide/silicon nitride matrix mixture and silicon carbide fiber. Furthermore, CMCs can have a matrix and reinforcing fibers comprised of oxide ceramics. These oxide-oxide composites are described below.

Specifically, the oxide-oxide CMCs may be comprised of a matrix and reinforcing fibers comprising oxide-based materials such as aluminum oxide (Al<NUM>O<NUM>), silicon dioxide (SiO<NUM>), aluminosilicates, and mixtures thereof. Aluminosilicates can include crystalline materials such as mullite (3Al<NUM>O<NUM> 2SiO<NUM>), as well as glassy aluminosilicates. The coating system described herein can be used to protect oxide-oxide CMC's from high temperature steam recession. However, one particular advantage of this system is for silicon-based non-oxide CMCs that evolve gas upon oxidation. In such systems, a silicon bond coat is typically used so that it oxidizes instead of the underlying substrate. In this manner, gas is not released from the silicon bond coat as it is oxidized, and it thereby promotes a stable foundation for refractory steam hermetic and steam recession resistant layers that are deposited on the bond coat. Unfortunately, silicon bond coats melt at temperatures of about <NUM> or less (depending on the purity of the silicon). The gas breathable technology described herein allows for a coating to be applied directly to silicon-based non oxide CMCs and monolithic ceramics. By eliminating the bond coat, the CMC and coating can be taken to higher temperature without a bond coat that melts.

As used herein, "monolithic ceramics" refers to materials without fiber reinforcement. Herein, CMCs and monolithic ceramics are collectively referred to as "ceramics.

The EBC <NUM> is particularly suitable for use on ceramic substrate components found in high temperature environments, such as those present in gas turbine engines, for example, combustor components, turbine blades, shrouds, nozzles, heat shields, and vanes. In the embodiment shown in <FIG>, at least one outer coating <NUM> is shown on the barrier layer <NUM> to define an external surface <NUM>. The outer coating <NUM> can be any number of layers of EBC materials, TBC materials, or combinations thereof. Although shown as a single coating in <FIG>, the outer coating <NUM> can be formed from multiple layers of materials selected from typical EBC or TBC layer chemistries such as rare earth silicates (mono- and di-silicates), mullite, BSAS, hafnia, zirconia, stabilized hafnia, stabilized zirconia, rare earth hafnates, rare earth zirconates, rare earth gallates.

As stated, the coated substrate <NUM> can be utilized as a turbine component for a gas turbine. In particular, the turbine component can be a CMC component positioned within a hot gas flow path of the gas turbine such that the coating forms an environmental barrier coating on the component to protect the component within the gas turbine when exposed to the hot gas flow path.

<FIG> illustrates a cross-sectional view of one embodiment of a gas turbine engine <NUM> that may be utilized within an aircraft in accordance with aspects of the present subject matter, with the engine <NUM> being shown having a longitudinal or axial centerline axis <NUM> extending therethrough for reference purposes. In general, the engine <NUM> may include a core gas turbine engine <NUM> and a fan section <NUM> positioned upstream thereof. The core engine <NUM> may generally include a substantially tubular outer casing <NUM> that defines an annular inlet <NUM>. In addition, the outer casing <NUM> may further enclose and support a booster compressor <NUM> for increasing the pressure of the air that enters the core engine <NUM> to a first pressure level. A high pressure, multi-stage, axial-flow compressor <NUM> may then receive the pressurized air from the booster compressor <NUM> and further increase the pressure of such air. The pressurized air exiting the high-pressure compressor <NUM> may then flow to a combustor <NUM> within which fuel is injected into the flow of pressurized air, with the resulting mixture being combusted within the combustor <NUM>. The high energy combustion products are directed from the combustor <NUM> along the hot gas path of the engine <NUM> to a first (high pressure) turbine <NUM> for driving the high pressure compressor <NUM> via a first (high pressure) drive shaft <NUM>, and then to a second (low pressure) turbine <NUM> for driving the booster compressor <NUM> and fan section <NUM> via a second (low pressure) drive shaft <NUM> that is generally coaxial with first drive shaft <NUM>. After driving each of turbines <NUM> and <NUM>, the combustion products may be expelled from the core engine <NUM> via an exhaust nozzle <NUM> to provide propulsive jet thrust.

It should be appreciated that each turbine <NUM>, <NUM> may generally include one or more turbine stages, with each stage including a turbine nozzle (not shown in <FIG>) and a downstream turbine rotor (not shown in <FIG>). As will be described below, the turbine nozzle may include a plurality of vanes disposed in an annular array about the centerline axis <NUM> of the engine <NUM> for turning or otherwise directing the flow of combustion products through the turbine stage towards a corresponding annular array of rotor blades forming part of the turbine rotor. As is generally understood, the rotor blades may be coupled to a rotor disk of the turbine rotor, which is, in turn, rotationally coupled to the turbine's drive shaft (e.g., drive shaft <NUM> or <NUM>).

Additionally, as shown in <FIG>, the fan section <NUM> of the engine <NUM> may generally include a rotatable, axial-flow fan rotor <NUM> that configured to be surrounded by an annular fan casing <NUM>. In particular embodiments, the (LP) drive shaft <NUM> may be connected directly to the fan rotor <NUM> such as in a direct-drive configuration. In alternative configurations, the (LP) drive shaft <NUM> may be connected to the fan rotor <NUM> via a speed reduction device <NUM> such as a reduction gear gearbox in an indirect-drive or geared-drive configuration. Such speed reduction devices may be included between any suitable shafts / spools within engine <NUM> as desired or required.

It should be appreciated by those of ordinary skill in the art that the fan casing <NUM> may be configured to be supported relative to the core engine <NUM> by a plurality of substantially radially-extending, circumferentially-spaced outlet guide vanes <NUM>. As such, the fan casing <NUM> may enclose the fan rotor <NUM> and its corresponding fan rotor blades <NUM>. Moreover, a downstream section <NUM> of the fan casing <NUM> may extend over an outer portion of the core engine <NUM> so as to define a secondary, or by-pass, airflow conduit <NUM> that provides additional propulsive jet thrust.

During operation of the engine <NUM>, it should be appreciated that an initial air flow (indicated by arrow <NUM>) may enter the engine <NUM> through an associated inlet <NUM> of the fan casing <NUM>. The air flow <NUM> then passes through the fan blades <NUM> and splits into a first compressed air flow (indicated by arrow <NUM>) that moves through conduit <NUM> and a second compressed air flow (indicated by arrow <NUM>) which enters the booster compressor <NUM>. The pressure of the second compressed air flow <NUM> is then increased and enters the high pressure compressor <NUM> (as indicated by arrow <NUM>). After mixing with fuel and being combusted within the combustor <NUM>, the combustion products <NUM> exit the combustor <NUM> and flow through the first turbine <NUM>. Thereafter, the combustion products <NUM> flow through the second turbine <NUM> and exit the exhaust nozzle <NUM> to provide thrust for the engine <NUM>. During operation, hot gases of combustion <NUM> may flow in an axial direction from a combustion zone of the combustor <NUM> into the annular, first stage turbine nozzle of the turbine <NUM>.

Methods of forming a barrier layer that includes a breathable silicon-containing glass phase and a refractory material phase.

In one embodiment, the barrier layer is formed by first making input powders. For example, the precursors of dopant (e.g., a dopant oxide such as CaO), SiO<NUM>, and Ln<NUM>O<NUM> (e.g., Yb<NUM>O<NUM>) can be put into a suspension, then dried, and heat treated to form a powder mixture of glass and doped silicate material (e.g., doped Ln<NUM>Si<NUM>O<NUM>, such as Ca-doped Yb<NUM>Si<NUM>O<NUM>). Finally, the powder mixture can be milled to the appropriate particle size of input powder for a slurry process. The slurry is then formed from the input powders by combining the input powder (i.e., the powder mixture of glass and doped silicate material) with a solvent and slurry processing aids such as dispersants and binders. The slurry is then mixed (e.g., by rolling on a roller mill along with some <NUM> inch zirconia media). The substrate can be dip coated into the slurry to form a coating layer, and dried. The coating layer can then be heat treated the coating layer at a temperature between about <NUM> and about <NUM> to form a coating that has less than <NUM>% porosity. The resulting coating layer (i.e., the barrier layer) is a mixture of the doped silicate material and the breathable silicon-containing glass phase.

In another embodiment, the barrier layer is formed by first making a slurry with input powders of undoped silicate material (e.g., Ln<NUM>Si<NUM>O<NUM>, such as Yb<NUM>Si<NUM>O<NUM>), CaO, SiO<NUM>, and Ln<NUM>O<NUM> (e.g., Yb<NUM>O<NUM>) by combining the input powders with a solvent and slurry processing aids, such as dispersants and binders. The slurry is then mixed (e.g., by rolling on a roller mill along with some <NUM> inch zirconia media). The substrate can be dip coated into the slurry to form a coating layer, and dried. The coating layer can then be heat treated the coating layer at a temperature between about <NUM> and about <NUM> to form a coating that has less than <NUM>% porosity. The resulting coating layer (i.e., the barrier layer) is a mixture of the doped silicate material and the breathable silicon-containing glass phase.

In another embodiment, the barrier layer is formed by first making a slurry with input powders of undoped silicate material (e.g., Ln<NUM>Si<NUM>O<NUM>, such as Yb<NUM>Si<NUM>O<NUM>) and rare earth silicate glass (e.g., Ca-Si-Ln-O glass) by combining the input powders with a solvent and slurry processing aids, such as dispersants and binders. The slurry is then mixed (e.g., by rolling on a roller mill along with some <NUM> inch zirconia media). The substrate can be dip coated into the slurry to form a coating layer, and dried. The coating layer can then be heat treated the coating layer at a temperature between about <NUM> and about <NUM> to form a coating that has less than <NUM>% porosity. The resulting coating layer (i.e., the barrier layer) is a mixture of the doped silicate material and the breathable silicon-containing glass phase.

In another embodiment, the barrier layer is formed by first mixing input powders of undoped silicate material (e.g., Ln<NUM>Si<NUM>O<NUM>, such as Yb<NUM>Si<NUM>O<NUM>) and rare earth silicate glass (e.g., Ca-Si-Ln-O glass). The input powder can then be plasma sprayed onto the substrate and heat treated to form a mixture of doped-Ln<NUM>Si<NUM>O<NUM> (e.g., Ca-doped Yb<NUM>Si<NUM>O<NUM>) along with a breathable silicon-containing glass phase that is dense (e.g., having a porosity that is about <NUM>% or less).

In another embodiment, the barrier layer is formed by first dispersing a mixture of input powders. For example, the precursors of dopant (e.g., a dopant oxide such as CaO), SiO<NUM>, and Ln<NUM>O<NUM> (e.g., Yb<NUM>O<NUM>) can be put into a suspension, then dried, and heat treated to form a powder mixture of glass and doped silicate material (e.g., doped Ln<NUM>Si<NUM>O<NUM>, such as Ca-doped Yb<NUM>Si<NUM>O<NUM>). Finally, the powder mixture can be spray dried to the appropriate particle size of input powder for an air plasma spray process. The input powder is plasma sprayed onto the substrate and heat treated to form a mixture of doped silicate material (e.g., doped Ln<NUM>Si<NUM>O<NUM>, such as Ca-doped Yb<NUM>Si<NUM>O<NUM>) along with a breathable silicon-containing glass phase that is dense (less than <NUM>% porosity).

One advantage of taking the approach of over-doping the precursor powder, and then applying the powder(s) to the substrate is that that mixture is more thermodynamically stable (i.e., the glass and doped rare earth silicate in the powder mixture react less with one another when you get to the stage of applying the powder).

No matter its method of formation, additional layers may be applied on the barrier layer, such as layers of Ln<NUM>Si<NUM>O<NUM> (e.g., Yb<NUM>Si<NUM>O<NUM>) and/or Ln<NUM>SiO<NUM> (e.g., Y<NUM>SiO<NUM>), to protect from temperature, temperature gradient, or CMAS. Such layers would, in particular embodiments, be porous, vertically cracked, or have columnar grains to allow for escape of any carbonaceous gases (e.g., CO, CO<NUM>) or nitrogen-based gases (e.g., N<NUM>, NO, NO<NUM>, etc.).

While the invention has been described in terms of one or more particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. It is to be understood that the use of "comprising" in conjunction with the coating compositions described herein specifically discloses and includes the embodiments wherein the coating compositions "consist essentially of" the named components (i.e., contain the named components and no other components that significantly adversely affect the basic and novel features disclosed), and embodiments wherein the coating compositions "consist of" the named components (i.e., contain only the named components except for contaminants which are naturally and inevitably present in each of the named components).

Claim 1:
A coated substrate, comprising:
a substrate defining a surface; and
an environmental barrier coating on the surface of the substrate, wherein the environmental barrier coating comprises a barrier layer having a refractory material phase and a silicon-containing glass phase, wherein the silicon-containing glass phase is a continuous phase, wherein the barrier layer comprises Ln<NUM>SiO<NUM> where Ln is a rare earth element or a mixture of rare earth elements doped at the Ln site with a dopant, Ln<NUM>Si<NUM>O<NUM> where Ln is a rare earth element or a mixture of rare earth elements doped at the Ln site with the dopant, or a mixture thereof; and wherein the dopant comprises an alkali metal cation, Cu<NUM>+, a noble metal cation, an alkaline earth metal cation, Cu<NUM>+, Ni<NUM>+, Mn<NUM>+, Zn<NUM>+, Sn<NUM>+, or a mixture thereof, and further wherein the dopant has a greater atomic percentage in the silicon-containing glass phase than in the rare earth silicate material of the refractory material phase.