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. Still, with many hot gas path components constructed from superalloys, 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, in particular silicon carbide (SiC) fiber reinforced SiC and SiC-Si matrix composites, so called SiC/SiC composites. 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 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 nitrous oxide gas. The carbonaceous (i.e., CO, CO<NUM>) or nitrous (i.e., NO, NO<NUM>, etc.) oxide gases cannot escape out through the dense EBC and thus, blisters form, which can cause spallation of the EBC. 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.

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, at <NUM>. Above these melting temperatures, the silicon bond coat may delaminate from the underlying substrate, effectively removing the bond coat and the EBC thereon. Recently, high temperature EBCs have been contemplated that utilize a bond coat containing silicon particles as an oxygen getter.

However, these high temperature EBCs have shown a weakness in intermediate temperature ranges. Due to oxidation expansion of the getter phase, the matrix phase experiences tensile stress that cannot be accommodated at lower temperatures, leading to micro-cracking and fast oxidation that invalidates the EBC. As such, it is desirable to have improved bond coats in the EBC to achieve a higher operational temperature limit for the EBC while remaining effective in lower and intermediate temperatures.

<CIT> relates to an environmental barrier coating with a mullite bondcoat that includes an oxygen getter phase.

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figs. , in which:.

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, "RE" refers to a rare earth element or a mixture of rare earth elements. More specifically, the " RE" 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.

As used herein, "alumina" refers to an aluminum oxide in the form of Al<NUM>O<NUM>.

As used herein, "silica" refers to a silicon oxide in the form of SiO<NUM>.

As used herein, the term "mullite" generally refers to a mineral containing alumina and silica. That is, mullite is a chemical compound of alumina and silica with an alumina (Al<NUM>O<NUM>) and silica (SiO<NUM>) ratio of <NUM> to <NUM> (e.g., within <NUM> mole % of <NUM> to <NUM> of alumina to silica). However, a ratio of <NUM> to <NUM> has also been reported as mullite (e.g., within <NUM> mole % of <NUM> to <NUM> of alumina to silica).

As used herein, the term "substantially free" means no more than an insignificant trace amount present and encompasses completely free (e.g., <NUM> molar % up to <NUM> molar %).

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.

A coated component is generally provided that includes a bond coat containing a matrix phase having a plurality of discrete particles dispersed therein, along with methods of its formation and use. The discrete particles are generally coarse so as to have a large enough size such that the oxidation reaction thereof occurs relatively slowly through the bond coat's thickness. Without wishing to be bound by any particular theory, it is believed that the oxidation occurs on only a thin surface layer of the relatively coarse particles, effectively slowing the oxidation of the discrete particles compared to finer particles. As such, the presence of the relatively coarse particles may create a stress state that is more manageable compared to finer particles, thus leading to less cracking tendency. Additionally, oxidation of the relatively coarse particles may create a diffuse reaction zone within the thickness of the bond coat, instead of a sharp reaction front that may be seen with smaller particles. In addition, the discrete particles are subject to oxidation or evaporation during the manufacturing process of the coating. Coarse particles provide the advantage of slowing down such degradation and thus may help maintain a desired proportion of particles in the bond coat.

The bond coat containing the matrix phase and including the discrete particles is, in one particular embodiment, generally positioned between the surface of the substrate and an environmental barrier coating (EBC) thereon. Referring to <FIG>, an exemplary coated component <NUM> is shown formed from a substrate <NUM> having a surface <NUM> with a coating system <NUM> thereon. Generally, the coating system <NUM> includes a bond coat <NUM> on the surface <NUM> of the substrate, and an EBC <NUM> on the bond coat <NUM>. In the embodiment shown, the bond coat <NUM> is directly on the surface <NUM> without any layer therebetween.

In the exemplary embodiment of <FIG>, the bond coat <NUM> is shown having a matrix phase <NUM> with discrete particles <NUM> dispersed therein. In the embodiment shown, the matrix phase <NUM> forms a continuous phase of the bond coat <NUM>. Additionally, in the embodiment shown, the matrix phase <NUM> spans the thickness of the bond coat <NUM> and is bonded directly to the surface <NUM> of the substrate <NUM> and to an inner surface <NUM> of the EBC <NUM>.

Generally, the plurality of discrete particles comprises an oxygen getter configured to react with oxygen during use of the component, so as to inhibit oxygen from reaching the underlying surface <NUM> of the substrate <NUM>. In one embodiment, the oxygen getter comprises silicon, such as elemental silicon. For example, the oxygen getter may consist essentially of silicon (at the time of the bond coat's formation) such that at least <NUM>% by weight of the oxygen getter is silicon (prior to exposure to any oxygen), such as at least <NUM>% by weight of the oxygen getter.

In one particular embodiment, the bond coat <NUM> may be formed from elemental silicon (as the discrete particles <NUM>) contained within the matrix phase <NUM>. As explained in greater detail below, the elemental silicon within the mullite may melt during operation of the coated component, while remaining contained within the matrix phase and while keeping the functions of the bond coat <NUM>. Such functions of the bond coat <NUM> may include, but are not limited to, bonding the substrate to the EBC thereon and gettering of oxygen without releasing gas to prevent oxidation of the underlying substrate that would otherwise result in a gaseous by-product. Thus, liquid discrete particles may be utilized within the bond coat <NUM> during operation of the coated component (e.g., within a gas turbine engine). Since the bond coat <NUM> continues to function above the melting point of the discrete particles, the coated component can be operated at temperatures above the melting point of the discrete particles.

Although silicon does not oxidize to form a gas like CO from SiC, it does form silicon hydroxides gaseous species when in contact with water vapor. However, the partial pressures of the silicon hydroxides gaseous species are sufficiently low so that they do not significantly form gas bubbles. Moreover, these partial pressures inhibit recession unless there are interconnected pores to the outside gas surface. The density of the mullite matrix and the hermeticity of the upper layers of the EBC may be controlled to minimize the formation of silicon hydroxide.

As stated above, the plurality of discrete particle has a relatively large size (i.e., coarse particles) such that an oxidation reaction of the oxygen getter occurs relatively slowly through the bond coat's thickness. However, if the discrete particles <NUM> are too large in size and/or too much in content, the discrete particles may form a continuous phase in use at temperatures where the oxygen getter liquifies (e.g., above <NUM> when comprising silicon). Too much continuous phase (formed by the liquified too large discrete particles) could lead to spallation of the coating.

The plurality of discrete particle has <NUM>% of its volume or greater (e.g., <NUM>% of its volume or greater) formed from particles having an average size of <NUM> to <NUM>. In one embodiment, the plurality of discrete particle has <NUM>% of its volume or greater (e.g., <NUM>% of its volume or greater) formed from particles having an average size of <NUM> to <NUM>. In one particular embodiment, the plurality of discrete particle has <NUM>% of its volume or greater (e.g., <NUM>% of its volume or greater) formed from particles having an average size of <NUM> to <NUM>. Thus, without wishing to be bound by any particular theory, it is believed that the presence of these relatively large particles slows the oxidation reaction to create a diffuse reaction zone, leading to reduced stress due to expansion and inhibiting cracking.

No matter the configuration of the bond coat <NUM>, the discrete particles <NUM> are contained, upon melting of the oxygen getter, within the matrix phase <NUM> between the surface <NUM> of the substrate <NUM> and the inner surface <NUM> of the environmental barrier coating <NUM>. That is, the matrix phase <NUM> may form a <NUM>-dimensional network that spans the thickness of the bond coat <NUM> and is bonded to the surface <NUM> of the substrate <NUM> and to the inner surface <NUM> of the environmental barrier coating <NUM>. As such, the matrix phase <NUM> works with the surface <NUM> of the substrate <NUM> and the environmental barrier coating <NUM> to contain the melted discrete particles therein while keeping the integrity of the bond coat <NUM> without delamination from the surface <NUM> of the substrate <NUM>.

Thus, the matrix phase <NUM> is included in the bond coat <NUM> in an amount to provide structural integrity to the bond coat <NUM> while the discrete particles <NUM> are melted at operating temperatures, such as above the melting point of elemental silicon (i.e., <NUM>) when the oxygen getter is silicon, while incorporating sufficient amounts of the oxygen getter therein. The matrix phase <NUM> also functions to limit the diffusion of oxidants, namely oxygen or water vapor, to reach the discrete particles <NUM>. The bond coat <NUM> includes <NUM>% to <NUM>% by volume of the matrix phase comprising mullite, such as <NUM>% to <NUM>% by volume matrix phase comprising mullite, such as <NUM>% to <NUM>% by volume matrix phase comprising mullite.

The discrete particles <NUM> are included in the bond coat <NUM> in an amount sufficient to serve as an oxygen getter to inhibit oxygen from reaching the underlying substrate <NUM>. In one particular embodiment, the discrete particles <NUM> may be formed from silicon metal (i.e., elemental silicon), a silicon alloy (e.g., a silicon eutectic alloy), a silicide, or mixtures thereof. As such, the discrete particles <NUM> may melt when the bond coat <NUM> reaches temperatures of <NUM> to <NUM>, depending on the composition of the discrete particles <NUM>. In some embodiments, the matrix phase <NUM> comprises mullite and has a melting point of <NUM> to <NUM>. Under some circumstances, the temperature of the bond coat <NUM> can be above the melting point of the discrete particles <NUM> but below the melting point of the matrix phase <NUM>, so that the discrete particles <NUM> become molten. For example, the discrete particles <NUM> may have at a melting temperature of <NUM> (i.e., the melting point of elemental silicon) to <NUM>. In particular embodiments, the discrete particles <NUM> may be formed from a silicon material that is molten at a bond coat temperature of <NUM>, <NUM>, <NUM>, and/or <NUM>.

For example, the bond coat <NUM> may, in certain embodiments, include <NUM>% to <NUM>% by volume of the discrete particles <NUM>, such as <NUM>% to <NUM>% by volume of the discrete particles <NUM> (e.g., <NUM>% to <NUM>% by volume of the discrete particles <NUM>). In particular embodiments, for example, the discrete particles <NUM> may include <NUM>% to <NUM>% by volume of elemental silicon, such as <NUM>% to <NUM>% by weight of elemental silicon (e.g., <NUM>% to <NUM>% by volume of elemental silicon). Elemental silicon has a melting point of <NUM>. As used herein, "elemental silicon" refers to silicon without any alloying materials present, outside of incidental impurities.

In certain embodiments, a silicide having a melting point of <NUM> or less (e.g., <NUM> to <NUM>) may also be in the discrete particles <NUM>. Determining the melting point of a particular silicide may be easily achieved using Si phase diagrams.

In particular embodiments, the oxygen getter within the discrete particles may have minimal thermal expansion coefficient mismatch with the substrate and mullite (e.g., no more than <NUM>-<NUM> ppm per °C) to avoid matrix cracks. However, those skilled in the art know that a larger expansion coefficient mismatch can be accommodated by reducing the volume fraction of the getter phase. In another particular embodiment, the oxygen getter should have minimal volume increase on oxidation (e.g., no more than <NUM>%, preferably no more than <NUM>%, and more preferably a volume reduction on oxidation rather than a volume expansion) to reduce the stresses in the mullite layer and its cracking. Many embodiments of the discrete particles are possible that satisfy the requirements for the expansion coefficient mismatch and the volume change on oxidation. In one embodiment, the oxygen getter comprises elemental silicon, and may be pure elemental silicon. In another embodiment, it comprises a silicon alloy and/or a silicide. In the embodiment where the discrete particles includes elemental silicon-, then the bond coat may be referred to as a "mullite/Si bond coat. " Silicon oxidation causes a volume expansion of <NUM>% to <NUM>% when it forms amorphous silica and a volume expansion of <NUM>% when it forms crystalline silica. However, the oxidation product is invariably amorphous first, which can then become crystalline with time.

Other embodiments of oxygen getters may include, but are not limited to, nickel, cobalt, chromium, or mixtures thereof. These oxygen getters may also be used in particular embodiments with silicon, a silicon alloy, and/or a silicide. For example, nickel has a much higher expansion mismatch (almost <NUM> ppm per °C) with the substrate and mullite. Therefore, the maximum volume fraction of nickel that can be tolerated would be lower than that of silicon which has a mismatch of less than <NUM> ppm per °C. However, nickel has a volume expansion on oxidation of <NUM>% compared to <NUM>% to <NUM>% for silicon converting to amorphous silica. Chromium, on the other hand, has a lower expansion mismatch with the substrate and mullite than does nickel. It also has a higher melting temperature (<NUM>) compared to silicon (<NUM>) and nickel (<NUM>).

The matrix phase <NUM> may comprise an oxide material, particularly oxide materials having a melting temperature that is greater than the melting temperature of the oxygen getter. Additionally, the matrix phase <NUM> may comprise an oxide material that is generally unreactive with the oxygen getter of the discrete particles <NUM>, even at elevated operating temperatures. In one particular embodiment, the matrix phase <NUM> is formed from crystallized mullite having a melting temperature that is greater than the melting temperature of the oxygen getter. In particular embodiments, the mullite has a melting temperature that is about <NUM> to <NUM> and is generally unreactive with the silicon material of the discrete particles <NUM> (e.g., elemental silicon).

In one embodiment, the matrix phase <NUM> may include mullite having an excess of alumina, up to <NUM> mole % of excess alumina. For example, the matrix phase <NUM> may include mullite formed from alumina and silica in a stoichiometric ratio of <NUM> to <NUM> up to <NUM> to <NUM> or in a stoichiometric ratio of <NUM> to <NUM> up to <NUM> to <NUM>. In another embodiment, the matrix phase <NUM> may include mullite containing an excess of silica.

Mullite generally has a relatively slow diffusion rate for oxygen at all temperatures of interest, even up to <NUM> (e.g., <NUM> to <NUM>). At temperatures over <NUM>, it is believed that the only other crystalline oxide that has lower oxygen diffusion rate than mullite is alumina, which has a very high expansion coefficient compared to the substrate and cannot be deposited as dense coatings without spallation. Although mullite has a coefficient of thermal expansion ("CTE") that is similar to that of a SiC CMC substrate <NUM>, the CTE of mullite is not an exact match to SiC. The slight mismatch of CTE of mullite and SiC could lead to problems related to thermal expansion, such as cracking and/or delamination, if the bond coat <NUM> is too thick. For example, it is believed that a bond coat <NUM> having a thickness of <NUM> mils (i.e., <NUM>) would lead to problems related to the CTE mismatch after repeated exposure to the operating temperatures. On the other hand, it is believed that a bond coat <NUM> having a maximum thickness of <NUM> mils or less, such as <NUM> mil to <NUM> mils (i.e., <NUM> or less, such as <NUM> to <NUM>), would survive such operating temperatures without significant problems from the CTE mismatch. In one particular embodiment, the bond coat <NUM> has a maximum thickness of <NUM> mils, such as <NUM> mils to <NUM> mils (i.e., <NUM>, such as <NUM> to <NUM>).

The substrate <NUM> is formed from a ceramic matrix composite ("CMC") material comprising silicon carbide As used herein, ceramic-matrix-composite or "CMC" refers to a class of materials that include a reinforcing material (e.g., reinforcing fibers) surrounded by a ceramic matrix phase. Generally, the reinforcing fibers provide structural integrity to the ceramic matrix. Some examples of matrix materials of CMCs can include, but are not limited to, non-oxide silicon-based materials (e.g., silicon carbide, silicon nitride, or mixtures thereof), oxide ceramics (e.g., silicon oxycarbides, silicon oxynitrides, aluminum oxide (Al<NUM>O<NUM>), silicon dioxide (SiO<NUM>), aluminosilicates, or mixtures thereof), or mixtures thereof. Optionally, ceramic particles (e.g., oxides of Si, Al, Zr, Y, and combinations thereof) and inorganic fillers (e.g., pyrophyllite, wollastonite, mica, talc, kyanite, and montmorillonite) may also be included within the CMC matrix.

Some examples of reinforcing fibers of CMCs can include, but are not limited to, non-oxide silicon-based materials (e.g., silicon carbide, silicon nitride, or mixtures thereof), non-oxide carbon-based materials (e.g., carbon), oxide ceramics (e.g., silicon oxycarbides, silicon oxynitrides, aluminum oxide (Al<NUM>O<NUM>), silicon dioxide (SiO<NUM>), aluminosilicates such as mullite, or mixtures thereof), or mixtures thereof.

Generally, particular CMCs may be referred to as their combination of type of fiber/type of matrix. For example, C/SiC for carbon-fiber-reinforced silicon carbide; SiC/SiC for silicon carbide-fiber-reinforced silicon carbide, SiC/SiN for silicon carbide fiber-reinforced silicon nitride; SiC/SiC-SiN for silicon carbide fiber-reinforced silicon carbide/silicon nitride matrix mixture, etc. In other examples, the CMCs may include 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.

In certain embodiments, the reinforcing fibers may be bundled and/or coated prior to inclusion within the matrix. For example, bundles of the fibers may be formed as a reinforced tape, such as a unidirectional reinforced tape. A plurality of the tapes may be laid up together to form a preform component. The bundles of fibers may be impregnated with a slurry composition prior to forming the preform or after formation of the preform. The preform may then undergo thermal processing, such as a cure or burn-out to yield a high char residue in the preform, and subsequent chemical processing, such as melt-infiltration with silicon, to arrive at a component formed of a CMC material having a desired chemical composition.

Such materials, along with certain monolithic ceramics (i.e., ceramic materials without a reinforcing material), are particularly suitable for higher temperature applications. Additionally, 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.

As stated above, the bond coat <NUM> may be used in conjunction with an EBC <NUM> to form a coated component <NUM> with an increased operating temperature compared to that using only a silicon bond coat (without the matrix phase). The EBC <NUM> may include any combination of one or more layers formed from materials selected from typical EBC or thermal barrier coating ("TBC") layer chemistries, including but not limited to rare earth silicates (e.g., mono-silicates and di-silicates), aluminosilicates (e.g., mullite, barium strontium aluminosilicate (BSAS), rare earth aluminosilicates, etc.), hafnia, zirconia, stabilized hafnia, stabilized zirconia, rare earth hafnates, rare earth zirconates, rare earth gallium oxide, etc. The EBC may include a hafnia layer, an alumina layer, or both. Alternatively or additionally, the EBC may include a rare earth disilicate layer, a rare earth monosilicate layer, or both.

As used herein, environmental-barrier-coating or "EBC" refers to a coating system comprising one or more layers of ceramic materials, each of which provides specific or multi-functional protections to the underlying CMC. EBCs generally include a plurality of layers, such as rare earth silicate coatings (e.g., rare earth disilicates such as slurry or APS-deposited yttrium ytterbium disilicate (YbYDS)), alkaline earth aluminosilicates (e.g., comprising barium-strontium-aluminum silicate (BSAS), such as having a range of BaO, SrO, Al<NUM>O<NUM>, and/or SiO<NUM> compositions), hermetic layers (e.g., a rare earth disilicate), and/or outer coatings (e.g., comprising a rare earth monosilicate, such as slurry or APS-deposited yttrium monosilicate (YMS)). One or more layers may be doped as desired, and the EBC may also be coated with an abradable coating.

The EBC <NUM> may be formed from a plurality of individual layers <NUM>, as shown in <FIG>. In the embodiments shown, EBC <NUM> includes a hermetic layer <NUM> positioned in directly on the bond coat <NUM> so as to encase the discrete particles <NUM>, upon melting, within the bond coat <NUM>. In one embodiment, this hermetic layer is of mullite, up to <NUM> mil thick, such as preferably <NUM> mil to <NUM> mil thick (e.g., <NUM> mil to <NUM> mil thick). Since the discrete particles <NUM> are reactive with oxygen to form silicon oxide, there is minimal gaseous oxide produced (e.g., a carbon oxide) upon exposure of the component <NUM> to oxygen at operating temperatures. Thus, there is no need for a gas escape layer through the bond coat <NUM>, and the hermetic layer may be included within the EBC <NUM>. It is even desirable to have a hermetic layer to prevent the ingress of water vapor to the bond coat. In one embodiment, the hermetic layer <NUM> may be positioned directly on the bond coat <NUM>, but may also be positioned elsewhere within the EBC <NUM>.

The coated component <NUM> is particularly suitable for use as a component 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 particular, the coated component <NUM> may be a CMC component positioned within a hot gas flow path of the gas turbine such that the coating system <NUM> forms an environmental barrier for the underlying substrate <NUM> to protect the component <NUM> within the gas turbine when exposed to the hot gas flow path. In certain embodiments, the bond coat <NUM> is configured such that the coated component <NUM> is exposed to operating temperatures of <NUM> to <NUM>.

<FIG> is a schematic cross-sectional view of a gas turbine engine in accordance with an exemplary embodiment of the present disclosure. More particularly, for the embodiment of <FIG>, the gas turbine engine is a high-bypass turbofan jet engine <NUM>, referred to herein as "turbofan engine <NUM>. " As shown in <FIG>, the turbofan engine <NUM> defines an axial direction A (extending parallel to a longitudinal axis <NUM> provided for reference) and a radial direction R. In general, the turbofan engine <NUM> includes a fan section <NUM> and a core turbine engine <NUM> disposed downstream from the fan section <NUM>. Although described below with reference to a turbofan engine <NUM>, the present disclosure is applicable to turbomachinery in general, including turbojet, turboprop and turboshaft gas turbine engines, including industrial and marine gas turbine engines and auxiliary power units. It is also applicable to other high temperature applications that contain water vapor in the gas phase, such as those arising from combustion of hydrocarbon fuels.

The exemplary core turbine engine <NUM> depicted generally includes a substantially tubular outer casing <NUM> that defines an annular inlet <NUM>. The outer casing <NUM> encases, in serial flow relationship, a compressor section including a booster or low pressure (LP) compressor <NUM> and a high pressure (HP) compressor <NUM>; a combustion section <NUM>; a turbine section including a high pressure (HP) turbine <NUM> and a low pressure (LP) turbine <NUM>; and a jet exhaust nozzle section <NUM>. A high pressure (HP) shaft or spool <NUM> drivingly connects the HP turbine <NUM> to the HP compressor <NUM>. A low pressure (LP) shaft or spool <NUM> drivingly connects the LP turbine <NUM> to the LP compressor <NUM>.

For the embodiment depicted, the fan section <NUM> includes a variable pitch fan <NUM> having a plurality of fan blades <NUM> coupled to a disk <NUM> in a spaced apart manner. As depicted, the fan blades <NUM> extend outwardly from disk <NUM> generally along the radial direction R. Each fan blade <NUM> is rotatable relative to the disk <NUM> about a pitch axis P by virtue of the fan blades <NUM> being operatively coupled to a suitable actuation member <NUM> configured to collectively vary the pitch of the fan blades <NUM> in unison. The fan blades <NUM>, disk <NUM>, and actuation member <NUM> are together rotatable about the longitudinal axis <NUM> by LP shaft <NUM> across an optional power gear box <NUM>. The power gear box <NUM> includes a plurality of gears for stepping down the rotational speed of the LP shaft <NUM> to a more efficient rotational fan speed.

Referring still to the exemplary embodiment of <FIG>, the disk <NUM> is covered by rotatable front nacelle <NUM> aerodynamically contoured to promote an airflow through the plurality of fan blades <NUM>. Additionally, the exemplary fan section <NUM> includes an annular fan casing or outer nacelle <NUM> that circumferentially surrounds the fan <NUM> and/or at least a portion of the core turbine engine <NUM>. It should be appreciated that the nacelle <NUM> may be configured to be supported relative to the core turbine engine <NUM> by a plurality of circumferentially-spaced outlet guide vanes <NUM>. Moreover, a downstream section <NUM> of the nacelle <NUM> may extend over an outer portion of the core turbine engine <NUM> so as to define a bypass airflow passage <NUM> therebetween.

During operation of the turbofan engine <NUM>, a volume of air <NUM> enters the turbofan engine <NUM> through an associated inlet <NUM> of the nacelle <NUM> and/or fan section <NUM>. As the volume of air <NUM> passes across the fan blades <NUM>, a first portion of the air <NUM> as indicated by arrows <NUM> is directed or routed into the bypass airflow passage <NUM> and a second portion of the air <NUM> as indicated by arrow <NUM> is directed or routed into the LP compressor <NUM>. The ratio between the first portion of air <NUM> and the second portion of air <NUM> is commonly known as a bypass ratio. The pressure of the second portion of air <NUM> is then increased as it is routed through the high pressure (HP) compressor <NUM> and into the combustion section <NUM>, where it is mixed with fuel and burned to provide combustion gases <NUM>.

The combustion gases <NUM> are routed through the HP turbine <NUM> where a portion of thermal and/or kinetic energy from the combustion gases <NUM> is extracted via sequential stages of HP turbine stator vanes <NUM> that are coupled to the outer casing <NUM> and HP turbine rotor blades <NUM> that are coupled to the HP shaft or spool <NUM>, thus causing the HP shaft or spool <NUM> to rotate, thereby supporting operation of the HP compressor <NUM>. The combustion gases <NUM> are then routed through the LP turbine <NUM> where a second portion of thermal and kinetic energy is extracted from the combustion gases <NUM> via sequential stages of LP turbine stator vanes <NUM> that are coupled to the outer casing <NUM> and LP turbine rotor blades <NUM> that are coupled to the LP shaft or spool <NUM>, thus causing the LP shaft or spool <NUM> to rotate, thereby supporting operation of the LP compressor <NUM> and/or rotation of the fan <NUM>.

The combustion gases <NUM> are subsequently routed through the jet exhaust nozzle section <NUM> of the core turbine engine <NUM> to provide propulsive thrust. Simultaneously, the pressure of the first portion of air <NUM> is substantially increased as the first portion of air <NUM> is routed through the bypass airflow passage <NUM> before it is exhausted from a fan nozzle exhaust section <NUM> of the turbofan engine <NUM>, also providing propulsive thrust. The HP turbine <NUM>, the LP turbine <NUM>, and the jet exhaust nozzle section <NUM> at least partially define a hot gas path <NUM> for routing the combustion gases <NUM> through the core turbine engine <NUM>.

Methods are also generally provided for coating a ceramic component. For example, <FIG> shows a diagram of an exemplary method <NUM> of forming a coating system on a surface of a substrate. At <NUM>, a bond coat is formed on the surface of the substrate to include discrete particles contained within a matrix phase, such as described above with respect to bond coat <NUM>. In one embodiment, the bond coat is formed by air plasma spray. In another embodiment, it is formed by suspension plasma spray where a liquid suspension of the desired chemistry is used for air plasma spray. In still another embodiment, the coating is formed by low pressure plasma spray. In yet another embodiment one or more of the coated layers may be formed by a slurry coating process followed by sintering of the layers. Different coating layers may be formed by one or more of these processes.

At <NUM>, an environmental barrier coating (EBC) is formed on the bond coat. As described above, the discrete particles, when melted, are contained within matrix phase between the surface of the substrate and an inner surface of the environmental barrier coating.

An exemplary bond coat material was prepared using relatively coarse particles dispersed within a continuous mullite phase, and a comparative bond coat material was prepared using relatively fine particles dispersed within a continuous mullite phase.

The exemplary bond coat material was formed using mullite and elemental silicon particles having an average size of <NUM>. In the resulting exemplary bond coat material, the elemental silicon particles formed about <NUM>% by volume of the exemplary bond coat material. The exemplary bond coat material was formed by spark plasma sintering (SPS) of a powder mixture of mullite and silicon, followed by hot-pressing into a pellet. This bond coat material served as an exemplary bond coat material, even though it is not a coating.

The comparative bond coat material was formed using mullite and elemental silicon particles having an average size of <NUM>-<NUM>, under the same method as the exemplary bond coat material. In the resulting comparative bond coat material, the elemental silicon particles formed about <NUM>% by volume of the comparative bond coat material.

The exemplary bond coat material and the comparative bond coat material were then tested for oxidation rates by heating to <NUM> °F <NUM>) for <NUM> hours in an atmosphere containing <NUM>% by volume water and <NUM>% by volume oxygen.

<FIG> shows the exemplary bond coat <NUM> after the oxidation test, and <FIG> shows the comparative bond coat <NUM> after the oxidation test. As clearly shown, the exemplary coating of <FIG> has a diffuse oxidation front <NUM> that has penetrated about <NUM> of the thickness of the bond coat <NUM>, with the coarse discrete particles <NUM> remaining in the matrix <NUM> in the bond coat below the diffuse oxidation front <NUM>. Conversely, the comparative coating of <FIG> has a sharp oxidation front <NUM> that has penetrated about <NUM> of the thickness of the bond coat <NUM>, with the fine discrete particles <NUM> remaining in the matrix <NUM> in the bond coat <NUM> below the sharp oxidation front <NUM>.

These results clearly show the exemplary bond coat <NUM> of <FIG> (with its larger discrete particles <NUM> of Si) had a slower oxygen diffusion rate than the comparative bond coat <NUM> of <FIG> (with its smaller discrete particles <NUM> of Si).

It is noted that the apparent mixture of coarse and smaller discrete particles in the exemplary bond coat <NUM> is believed to be a result of the pressing method used to make the exemplary bond coat <NUM>. If formed into a bond coat via a slurry or other deposition technique, it is believed that the resulting bond coat would have much less (if any) of the smaller particles such that the coarse particles are a majority of the discrete phases (e.g., <NUM>% of its volume or greater formed from the coarse particles, such as <NUM>% of its volume or greater).

Claim 1:
A coated component (<NUM>) comprising:
a ceramic matrix composite substrate (<NUM>) comprising silicon carbide and having a surface (<NUM>);
a bondcoat (<NUM>) on the surface (<NUM>) of the substrate (<NUM>), wherein the bondcoat (<NUM>) comprises a plurality of discrete particles (<NUM>) dispersed within a matrix phase (<NUM>), wherein the matrix phase (<NUM>) comprises mullite and defines <NUM>% to <NUM>% by volume of the bondcoat (<NUM>), and wherein the plurality of discrete particles (<NUM>) comprises an oxygen getter and has <NUM>% of its volume or greater formed of particles having an average size of <NUM> to <NUM>; and
an environmental barrier coating (<NUM>) on the bondcoat (<NUM>).