Patent Description:
Ceramic matrix composites generally include a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications, such as in gas turbines, are silicon-based composites, which include silicon carbide (SiC) as the matrix and/or reinforcement material.

Different processing methods have been employed in forming CMCs. For example, one approach includes melt infiltration (MI), which employs a molten silicon to infiltrate into a fiber-containing perform. CMCs formed by prepreg MI are generally fully dense, e.g., having generally zero, or less than <NUM> percent by volume, residual porosity. This very low porosity gives the composite desirable mechanical properties, such as a high proportional limit strength and interlaminar tensile and shear strengths, high thermal conductivity and good oxidation resistance. However, the matrices of MI composites contain a free silicon phase (i.e., elemental silicon or silicon alloy) that limits the use temperature of the system to below that of the melting point of the silicon or silicon alloy, or about <NUM> degrees Celsius to <NUM> degrees Celsius (<NUM> degrees Fahrenheit to <NUM> degrees Fahrenheit). Moreover the free silicon phase causes the MI SiC matrix to have relatively poor creep resistance.

Another approach for forming CMCs is chemical vapor infiltration (CVI). CVI is a process whereby a matrix material is infiltrated into a fibrous preform by the use of reactive gases at elevated temperature to form the fiber-reinforced composite. Generally, limitations introduced by having reactants diffuse into the preform and by-product gases diffusing out of the perform result in relatively high residual porosity of between about <NUM> percent and about <NUM> percent in the composite. In particular, typically in forming CMCs using CVI, the inner portion of the composite formed by CVI typically has a higher porosity than the porosity of the outer portion of the composite. The presence of this porosity degrades the in-plane and through-thickness mechanical strength, thermal conductivity, and oxidation resistance of the CVI CMC relative to MI CMCs. However, CVI composite matrices typically have no free silicon phase, and thus have better creep resistance than MI matrices and the potential to operate at temperatures above <NUM> degrees Celsius (<NUM> degrees Fahrenheit). <CIT> relates to a ceramic matrix composite article including a melt infiltration ceramic matrix composite substrate comprising a ceramic fiber reinforcement material in a ceramic matrix material having a free silicon proportion, and a chemical vapor infiltration ceramic matrix composite outer layer comprising a ceramic fiber reinforcement material in a ceramic matrix material having essentially no free silicon proportion disposed on an outer surface of at least a portion of the substrate.

However, there is a need for further ceramic matrix composites (CMC), and more particularly, to articles and methods for forming ceramic matrix composite articles.

The present invention is defined in the accompanying claims and relates to methods for forming a ceramic matrix composite article. The ceramic matrix composite articles of the present disclosure represent articles that can be formed by the method according to the invention.

In one exemplary embodiment of the present disclosure a ceramic matrix composite article is provided. The ceramic matrix composite article includes a chemical vapor infiltration ceramic matrix composite base portion including ceramic fiber reinforcement material in a ceramic matrix material having between <NUM>% and <NUM>% free silicon. The ceramic matrix composite article further includes a melt infiltration ceramic matrix composite covering portion including a ceramic fiber reinforcement material in a ceramic matrix material having a greater percentage of free silicon than the chemical vapor infiltration ceramic matrix composite base portion.

In certain exemplary embodiments the chemical vapor infiltration ceramic matrix composite base portion has substantially <NUM>% free silicon.

In certain exemplary embodiments the melt infiltration ceramic matrix composite covering portion substantially completely surrounds at least a portion of the chemical vapor infiltration ceramic matrix composite base portion.

In certain exemplary embodiments the article is configured for use in a gas turbine engine.

For example, in certain exemplary embodiments the article is a nozzle, wherein the melt infiltration ceramic matrix composite covering portion includes a first melt infiltration ceramic matrix composite covering portion forming a radially inner band of the nozzle and a second melt infiltration ceramic matrix composite covering portion forming a radially outer band of the nozzle, and wherein the chemical vapor infiltration ceramic matrix composite base portion forms an airfoil section of the nozzle.

For example, in certain exemplary embodiments the article is a shroud, wherein the chemical vapor infiltration ceramic matrix composite base portion is exposed to a core air flowpath defined by the gas turbine engine when installed in the gas turbine engine.

For example, in certain exemplary embodiments the article is a liner, wherein the liner includes a hot side configured to be exposed to a core air flowpath defined by the gas turbine engine when installed in the gas turbine engine and an opposite cold side, and wherein the chemical vapor infiltration ceramic matrix composite base portion forms the hot side and the melt infiltration ceramic matrix composite covering portion forms the cold side.

For example, in certain exemplary embodiments the article is an airfoil, wherein the airfoil includes a first section and a separately formed second section, wherein each of the first section and the second section each include a chemical vapor infiltration ceramic matrix composite base portion and a melt infiltration ceramic matrix composite covering portion, and wherein the melt infiltration ceramic matrix composite covering portions of the first section and the second section are substantially completely enclosed within the chemical vapor infiltration ceramic matrix composite base portion when the first section and the second section of the airfoil are joined.

For example, in certain exemplary embodiments the chemical vapor infiltration ceramic matrix composite base portion is configured to be at least in part exposed to a core air flowpath defined by the gas turbine engine when installed in the gas turbine engine.

In certain exemplary embodiments the chemical vapor infiltration ceramic matrix composite base portion is formed separately from the melt infiltration ceramic matrix composite covering portion such that substantially all of a surface of the chemical vapor infiltration ceramic matrix composite base portion is exposed to one or more reactive gasses during formation.

For example, in certain exemplary embodiments the melt infiltration ceramic matrix composite covering portion is formed on the chemical vapor infiltration ceramic matrix composite base portion after the chemical vapor infiltration ceramic matrix composite base portion is formed.

In certain exemplary embodiments the chemical vapor infiltration ceramic matrix composite base portion defines a porosity between about five percent and about thirty percent, and wherein the melt infiltration ceramic matrix composite covering portion defines a porosity less than the porosity of the chemical vapor infiltration ceramic matrix composite base portion.

For example, in certain exemplary embodiments the melt infiltration ceramic matrix composite covering portion defines a porosity less than about three percent.

A method for forming a ceramic matrix composite article is provided according to claim <NUM>. The method includes forming a chemical vapor infiltration ceramic matrix composite base portion, wherein forming the chemical vapor infiltration ceramic matrix composite base portion includes exposing substantially all of a surface of the chemical vapor infiltration ceramic matrix composite base portion to one or more reactive gasses; and providing a melt infiltration ceramic matrix composite portion on a portion of the outer surface of the chemical vapor infiltration ceramic matrix composite base portion after forming the chemical vapor infiltration ceramic matrix composite base portion.

In certain exemplary aspects providing the melt infiltration ceramic matrix composite portion on the portion of the outer surface of the chemical vapor infiltration ceramic matrix composite base portion includes forming the melt infiltration ceramic matrix composite portion on the portion of the outer surface of the chemical vapor infiltration ceramic matrix composite base portion.

For example, in certain exemplary aspects forming the melt infiltration ceramic matrix composite covering portion on the chemical vapor infiltration ceramic matrix composite base portion includes laying up one or more layers of prepreg on the portion of the outer surface of the chemical vapor infiltration ceramic matrix composite base portion and performing a melt infiltration of the one or more layers of prepreg.

In certain exemplary aspects exposing substantially all of the surface of the chemical vapor infiltration ceramic matrix composite base portion to one or more reactive gasses includes forming ceramic fiber reinforcement material in a ceramic matrix material of the chemical vapor infiltration ceramic matrix composite base portion having between <NUM>% and <NUM>% free silicon, and wherein providing the melt infiltration ceramic matrix composite portion includes forming the melt infiltration ceramic matrix composite portion to include ceramic fiber reinforcement material in a ceramic matrix material having a greater percentage of free silicon than the chemical vapor infiltration ceramic matrix composite base portion.

In certain exemplary aspects the chemical vapor infiltration ceramic matrix composite base portion has substantially <NUM>% free silicon.

In certain exemplary aspects the chemical vapor infiltration ceramic matrix composite base portion is configured to be at least in part exposed to a core air flowpath defined by the gas turbine engine when installed in the gas turbine engine.

In certain exemplary aspects providing the melt infiltration ceramic matrix composite portion on the portion of the outer surface of the chemical vapor infiltration ceramic matrix composite base portion includes affixing the melt infiltration ceramic matrix composite portion onto the portion of the outer surface of the chemical vapor infiltration ceramic matrix composite base portion.

<FIG> represent CMC articles that can be formed by the method of the present invention.

For example, the approximating language may refer to being within a ten percent margin.

Generally, the present disclosure is directed to a ceramic matrix composite (CMC) articles having generally good mechanical properties such as tensile and compressive strength, and increased temperature capability. For example, a CMC article may includes a CMC base portion and a CMC covering portion or layer. The CMC base portion and the CMC covering portion or layer have different properties allowing tailoring of the CMC article to result in a CMC article having generally good mechanical properties with, e.g., increased creep resistance (resistance to deformation or change in shape over time due to stress), and increased temperature capability. The technique of the present disclosure results in both the CMC base portion and the CMC covering portion being CMCs having a reinforcing material, and thus both the CMC base portion and the CMC covering portion offering mechanical properties such as tensile and compressive strength. In addition, the CMC covering portion may also offer better mechanical properties and the CMC base portion may offer increased temperature capability to the CMC article. Such a technique of the present disclosure may be advantageous in CMC components where stresses are high and where creep is typically a problem, or where high temperatures are experienced. The CMC article may be configured such that the CMC base portion and the CMC surface portion more efficiently handle these obstacles. For example, forming the CMC covering portion using melt infiltration may result in a more fully dense CMC covering portion, which provides improved oxidation resistance to the overall laminate by reducing ingress of oxygen containing gasses to the underlying, and more porous, CMC base portion (which is formed using a chemical vapor infiltration). Such a CMC covering portion will also offer superior interlaminar strength (both interlaminar tensile strength and interlaminar shear strength). Further, forming the CMC base portion using chemical vapor infiltration results in a base portion having superior creep resistance and higher temperature capability.

Referring now to the drawings, wherein identical numerals indicate the same elements throughout the figures, <FIG> provides a CMC article <NUM> having a CMC base portion <NUM> and a CMC covering portion <NUM> in accordance with aspects of the present disclosure. As described in greater detail below, CMC base portion <NUM> includes a ceramic fiber reinforcement material in a ceramic matrix material including no free silicon content or proportion.

By contrast, CMC covering portion <NUM> includes a ceramic fiber reinforcement material in a ceramic matrix material having a free silicon content or proportion (e.g., the amount of elemental silicon or silicon alloy relative to the base portion as a whole). Herein the term free silicon refers to the presence of elemental silicon or silicon alloy where silicon makes up greater than about <NUM> atomic percent of the alloy. CMC covering portion <NUM> may be a silicon-rich silicon carbide portion having, for example, free silicon (e.g., at least about <NUM> percent, <NUM> percent, <NUM> percent, <NUM> percent, <NUM> percent, or greater free silicon by volume of elemental silicon or silicon alloy phase). CMC covering portion <NUM> includes a ceramic fiber reinforcement material in a ceramic matrix material disposed on a surface of at least a portion of the CMC base portion <NUM>.

CMC covering portion <NUM> may have generally full density, or none or little porosity (e.g., about <NUM> percent, less than <NUM> percent, between about <NUM> and less than <NUM> percent). By contrast, CMC base portion <NUM> may contain silicon carbide with generally no or zero free silicon content, or slightly carbon rich silicon carbide. CMC covering portion <NUM> may be formed by a first process and CMC base portion <NUM> may be formed by a second process different from the first process. For example, the CMC covering portion <NUM> may be formed by using a melt infiltration process, and the CMC base portion <NUM> may be formed using a chemical vapor infiltration process. CMC covering portion <NUM> may have improved mechanical properties over base portion <NUM> and may result in CMC article <NUM> having an overall mechanical strength greater than an overall mechanical strength of a CMC article not having CMC covering portion <NUM>. CMC base portion <NUM> which has no free elemental silicon or silicon alloy may withstand higher temperature (e.g., higher than the melting point of silicon) compared to CMC covering portion <NUM> (which may include free silicon) and may result in CMC article <NUM> that can withstand temperatures greater than that of a CMC article not having CMC base portion <NUM>.

With reference to <FIG>, CMC article <NUM> (<FIG>) includes initially forming CMC base portion <NUM>. A surface region of CMC base portion <NUM> may include multiple laminae <NUM>, each derived from an individual prepreg that includes unidirectionally-aligned tows <NUM> impregnated with a ceramic matrix precursor. As a result, each lamina <NUM> contains unidirectionally-aligned fibers <NUM> encased in a ceramic matrix <NUM> formed by conversion of the ceramic matrix precursor during firing and chemical vapor infiltration.

For example, CMC base portion <NUM> may be fabricated from multiple layers of "prepreg," often in the form of a tape-like structure, comprising the reinforcement material of the desired CMC impregnated with a precursor of the CMC matrix material. The prepreg may undergo processing (including firing) to convert the precursor to the desired ceramic. The prepregs may be continuous fiber reinforced ceramic composite (CFCC) materials and may include a two-dimensional fiber array comprising a single layer of unidirectionally-aligned tows impregnated with a matrix precursor to create a generally two-dimensional laminate. Multiple plies of the resulting prepregs are stacked and debulked to form a laminate preform, a process referred to as "lay-up. " The prepregs are typically arranged so that tows of the prepreg layers are oriented transverse (e.g., perpendicular) or at an angle to each other, providing greater strength in the laminar plane of the preform (corresponding to the principal (load-bearing) directions of the final CMC component).

Following lay-up, the laminate preform may undergo debulking and curing while subjected to applied pressure and an elevated temperature, such as in an autoclave or localized application of pressure and heat. In the case of chemical vapor infiltration (CVI), the debulked and cured preform undergoes additional processing. First, the prepreg layers/ preform may be heated in vacuum or in an inert atmosphere in order to decompose the organic binders, at least one of which pyrolyzes during this heat treatment to form a ceramic char, and produces a porous layer for chemical vapor infiltration. Further heating, either as part of the same heat cycle as the binder burn-out step or in an independent subsequent heating step, the layer is chemical vapor infiltrated, such as with a gaseous source of silicon carbide supplied externally. Appropriate reactant gases and processing conditions for performance of the CVI process are well known in the art. The gaseous source of silicon carbide infiltrates into the porosity, reacts on the internal surfaces of the porous base portion to deposit SiC with no free Si metal.

The CMC base portion <NUM> is completely formed first. More specifically, in at least certain exemplary embodiments, the CMC base portion <NUM> is laid up and taken through the chemical vapor infiltration process before adding the CMC covering portion <NUM>. Such a process allows for substantially all of a surface of the base portion <NUM> to be exposed to the reactant gases during the chemical vapor infiltration process, resulting in a more quickly and completely formed base portion <NUM>.

With reference now to <FIG>, forming the CMC article <NUM> includes forming the covering portion <NUM> on the initially formed CMC base portion <NUM>. For example, a lamina <NUM> may be derived from an individual prepreg that includes unidirectionally-aligned tows <NUM> impregnated with a ceramic matrix precursor. Lamina <NUM> contains unidirectionally-aligned fibers <NUM> encased in a ceramic matrix <NUM> formed by conversion of the ceramic matrix precursor during firing and melt infiltration (MI).

For example, CMC covering portion <NUM> may be fabricated from a layer of "prepreg," often in the form of a sheet-like structure, comprising the reinforcement material of the desired CMC impregnated with a precursor of the CMC matrix material. The prepreg undergoes processing (including firing) to convert the precursor to the desired ceramic. The prepreg may be continuous fiber reinforced ceramic composite (CFCC) materials and may include a two-dimensional fiber array comprising a single layer of unidirectionally-aligned tows impregnated with a matrix precursor to create a generally two-dimensional laminate. Alternately the prepreg may comprise layers with woven fibers. A ply of the prepreg may be disposed on CMC base portion <NUM>. The prepreg can be arranged so that tows of the prepreg layer are oriented parallel, transverse (e.g., perpendicular) or at an angle to the tows of the outermost layer of the CMC base portion.

Accordingly, it will be appreciated that for the exemplary embodiment depicted, the first layer of the covering portion <NUM> may be applied directly to a surface of the base portion <NUM>. However, as the covering portion <NUM> may be processed using a melt infiltration process, in at least certain exemplary embodiments, the article <NUM> may further include a barrier layer between the base portion <NUM> and the first layer of the covering portion <NUM>.

Referring still to <FIG>, the prepreg layers/ preform may undergo curing while subjected to applied pressure and an elevated temperature, such as in an autoclave or localized application of pressure and heat. In the case of melt-infiltrated (MI), the cured preform undergoes additional processing. First, the preform may be heated in vacuum or in an inert atmosphere in order to decompose the organic binders, at least one of which pyrolyzes during this heat treatment to form a carbon char, and produces a porous preform for melt infiltration. Further heating, either as part of the same heat cycle as the binder burn-out step or in an independent subsequent heating step, the preform is melt infiltrated, such as with molten silicon supplied externally. The molten silicon infiltrates into the porosity, reacts with the carbon constituent of the matrix to form silicon carbide, and fills the porosity to yield the desired CMC covering portion <NUM>.

Notably, for the embodiment of <FIG>, the covering portion <NUM> is depicted including a single layer of CMC processed/formed using melt infiltration. It should be appreciated, however, that in other exemplary embodiments, the covering portion <NUM> may instead include any suitable number of layers of CMC processed/formed using melt infiltration to result in a CMC article <NUM> having a desire geometry.

For example, <FIG> illustrates a CMC article <NUM> having a CMC base portion <NUM> and a CMC covering portion <NUM> in accordance with another aspect of the present disclosure. CMC base portion <NUM> may include essentially no free silicon proportion or content, and CMC covering portion <NUM> may include a ceramic fiber reinforcement material in a ceramic matrix material including a free silicon proportion or content and disposed on a surface of at least a portion of the CMC base portion <NUM>.

More specifically, for the embodiment depicted the CMC covering portion <NUM> may have generally full density, or none or little porosity (e.g., about <NUM> percent, less than <NUM> percent, or between about <NUM> and less than <NUM> percent). CMC covering portion <NUM> may be a silicon-rich silicon carbide covering portion having, for example, free silicon (e.g., at least about <NUM> percent, <NUM> percent, <NUM> percent, <NUM> percent, <NUM> percent, or greater free silicon by volume of elemental silicon or silicon alloy phase). By contrast, CMC base portion <NUM> may be generally pure silicon carbide, generally silicon carbide with no or zero free silicon content, or slightly carbon rich silicon carbide. CMC covering portion <NUM> may be formed by a first process and CMC base portion <NUM> may be formed by a second process different from the first process. For example, the CMC covering portion <NUM> may be formed using a melt infiltration process, and the CMC base portion <NUM> may be formed using a chemical vapor infiltration process. CMC covering portion <NUM> may therefore have an increased mechanical strength as compared to the CMC base portion <NUM>, which may result in CMC article <NUM> having an overall mechanical strength greater than an overall mechanical strength of a CMC article not having covering portion <NUM>. CMC base portion <NUM> which may have no free elemental silicon or silicon alloy may withstand higher temperatures (e.g., higher than the melting point of silicon) compared to CMC covering portion <NUM> (which may include free silicon) and may result in CMC article <NUM> that can withstand higher temperatures than that of a CMC article not having CMC base portion <NUM>.

CMC article <NUM> includes initially forming CMC base portion <NUM> in a similar manner as noted above in connection with forming base portion <NUM> (<FIG>). With reference still to <FIG>, CMC article <NUM> includes forming covering portion <NUM> on initially formed CMC base portion <NUM>. For example, covering portion <NUM> may include a plurality of laminae <NUM>, each derived from an individual prepreg that includes unidirectionally-aligned tows impregnated with a ceramic matrix precursor. Each lamina <NUM> may contain unidirectionally-aligned fibers or woven fibers encased in a ceramic matrix formed by conversion of the ceramic matrix precursor during firing and melt infiltration (MI).

For example, similar to the embodiments discussed above, CMC covering portion <NUM> may be fabricated from a plurality of layers of "prepreg," often in the form of a tape-like structure, comprising the reinforcement material of the desired CMC impregnated with a precursor of the CMC matrix material. The prepreg undergoes processing (including firing) to convert the precursor to the desired ceramic. The prepregs may be continuous fiber reinforced ceramic composite (CFCC) materials and may include a two-dimensional fiber array comprising a single layer of unidirectionally-aligned tows impregnated with a matrix precursor to create a generally two-dimensional laminate. Alternately the prepreg may comprise layers with woven fibers. The plurality of plies of the resulting prepregs are stacked and debulked. The prepregs are typically arranged so that tows of the prepreg layers are oriented parallel to, transverse (e.g., perpendicular) to or at an angle to the tows of the outermost layer of the CMC base portion.

The plurality of layers may typically undergo debulking and curing while subjected to applied pressure and an elevated temperature, such as in an autoclave or localized application of pressure and heat. In the case of infiltration (MI), the cured preform undergoes additional processing. First, the plurality of layers disposed on the CMC base portion may be heated in vacuum or in an inert atmosphere in order to decompose the organic binders, at least one of which pyrolyzes during this heat treatment to form a carbon char, and produces a porous preform for melt infiltration. Further heating, either as part of the same heat cycle as the binder burn-out step or in an independent subsequent heating step, the preform is melt infiltrated, such as with molten silicon supplied externally. The molten silicon infiltrates into the porosity, reacts with the carbon constituent of the matrix to form silicon carbide, and fills the porosity to yield the desired CMC covering portion <NUM>.

An alternate embodiment would be to place the CVI composite base portion into a mold with a layer, or layers, of fiber plies occupying the space between the outer surface of the CVI composite base portion and inner surface of the mold, and the structure subjected to MI. The mold material would be compatible with the MI process.

In the above embodiments, a material for the tows may be SiC fibers. An example of a material suitable for the tows is HI-NICALON® from Nippon Carbon Co. A suitable range for the diameters of the fibers is about two to about twenty micrometers, though fibers with larger and smaller diameters are also within the scope of this disclosure. The fibers may be preferably coated with materials to impart certain desired properties to the CMC base portion and/or CMC covering portion, such as a carbon or boron nitride interface layer (not shown). The fibers in the covering portion may be coated prior to forming into the prepreg tapes and application to the CVI CMC base portion, or the fiber coatings may be applied during the initial part of the MI process. Those skilled in the art will appreciate that the teachings of this disclosure are also applicable to other CMC material combinations, and that such combinations are within the scope of this disclosure.

As described above, the CMC base portion formed by a CVI process having generally no free silicon phase may result in the CMC base portion having greater creep resistance and temperature capability than the CMC covering portion formed by MI and having generally full density, or none or little porosity such as about <NUM> percent, less than <NUM> percent, or between about <NUM> and less than <NUM> percent. In addition, the CMC covering portion such as formed by a silicon melt infiltration may result in a silicon-rich silicon carbide covering portion having, for example, <NUM> percent, <NUM> percent, <NUM> percent, <NUM> percent, <NUM> percent, or greater free silicon by volume of elemental silicon or silicon alloy phase. The CMC base portion may comprise generally pure silicon carbide, e.g., about <NUM> to <NUM> ratio of silicon to carbon, or slightly carbon rich such as a ratio of <NUM> silicon to <NUM> carbon. The thickness of the plies or unidirectional tape for forming the CMC article may be about <NUM> mils (<NUM> inch) to about <NUM> mils (<NUM> inch). The CMC article may be formed having a single ply or layer of reinforcement fibers, a plurality of plies or layers of reinforcement fibers, or multiple plies or layers of reinforcement fibers for forming the CMC covering portion. For example, the CMC article of the present disclosure may comprise a CMC base portion formed from about eight plies or layers of reinforcement fibers and CVI, and an covering portion formed from one or two plies or layers of reinforcement fibers and melt infiltration so that the covering portion may be about <NUM> percent to about <NUM> percent of the thickness of the CMC article. In other embodiments of the ceramic matrix composite article, the covering portion may be about <NUM> percent to about <NUM> percent of the thickness of the ceramic matrix composite article. In other embodiments, the CMC article may have about <NUM> to about <NUM> plies. It will be appreciated that other configurations of the number of plies and thickness of the CMC base portion relative to the CMC covering portion are also possible.

While the CMC article may be formed from unidirectional prepreg tapes, it will be appreciated that woven prepreg tape may be employed to form the CMC base portion and/or the CMC covering portion. The aligned fibers in the unidirectional prepreg tape may result in fewer pores than that resulting from a prepreg woven fiber fabric. In addition, one or more additional layers or coatings may be formed on the CMC covering portion of the CMC article. For example, in some embodiments, an environmental barrier coating (EBC) may be formed on the covering portion.

It will be appreciated that one or more the exemplary CMC articles described above with reference to <FIG> may be configured for use in a gas turbine engine. For example, in certain exemplary embodiments of the present disclosure, the CMC article may be configured for use within an aeronautical gas turbine engine (such as a turbofan engine, turboprop engine, turboshaft engine, turbojet engine, etc.), a power generation gas turbine engine, or an aeroderivative gas turbine engine. However, in other embodiments, the CMC articles of the present disclosure may be utilized with any other suitable machine.

For example, referring briefly to <FIG>, a simplified, schematic view is provided of a gas turbine engine <NUM> which may include a CMC article formed in accordance with one or more exemplary aspects of the present disclosure. It will be appreciated, however, that the exemplary gas turbine engine <NUM> described with reference to <FIG> is by way of example only, and in other embodiments the gas turbine engine may have any suitable configuration. The exemplary gas turbine engine <NUM> defines an axial direction A (extending parallel to a longitudinal centerline <NUM> provided for reference), and a radial direction R. In general, the gas turbine engine <NUM> includes a fan section <NUM> and a turbomachine <NUM> disposed downstream from the fan section <NUM>. The exemplary turbomachine <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>. The compressor section, combustion section <NUM>, and turbine section together define a core air flowpath <NUM>. A first, high pressure (HP) shaft or spool <NUM> drivingly connects the HP turbine <NUM> to the HP compressor <NUM>. A second, 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 fan <NUM> having a plurality of fan blades <NUM> coupled to a disk <NUM> in a spaced apart manner. The disk <NUM> is covered by rotatable front hub <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 turbomachine <NUM>. As is depicted, the fan blades <NUM>, disk <NUM>, and front hub <NUM> are together rotatable about the longitudinal axis <NUM> directly by the LP spool <NUM>.

It will be appreciated that during operation of the gas turbine engine <NUM>, certain components may be exposed relatively high temperatures, and therefore it may be beneficial to form one or more of such components of a CMC material. For example, within the combustion section <NUM>, a combustor is provided having combustor liners <NUM>, and more specifically, an inner combustor liner in an outer combustor liner. Additionally, within the turbine section of the turbomachine <NUM>, the turbomachine <NUM> includes one or more liners <NUM> defining a portion of the core air flowpath <NUM> therethrough. Although depicted between the HP turbine <NUM> and LP turbine <NUM>, in other embodiments the liner(s) <NUM> may be located at any other suitable location along the core air flowpath <NUM>. Also, the HP turbine <NUM> and LP turbine <NUM> each include a plurality of turbine airfoils, which may be configured as part of rotor blades <NUM> coupled to the HP spool <NUM> or the LP spool <NUM>, or as part of stator vanes <NUM> coupled to casing <NUM>. Moreover, within the HP turbine <NUM> and LP turbine <NUM>, the turbomachine <NUM> further includes one or more shrouds <NUM> positioned at radially outer ends of the plurality of rotor blades <NUM> to form a seal with such rotor blades <NUM>. As will be discussed below, one or more of these components, as well as one or more other components, may be formed in a manner similar to the CMC articles discussed above with reference to <FIG>.

More specifically, for example, referring now generally to <FIG>, various ceramic matrix composite articles <NUM> in accordance with certain embodiments of the present disclosure are provided, e.g., as may be incorporated into the exemplary gas turbine engine of <FIG>. Each of the ceramic matrix composite articles <NUM> depicted in <FIG> include a CVI ceramic matrix composite base portion <NUM> and an MI ceramic matrix composite covering portion <NUM>. The MI ceramic matrix composite covering portions <NUM> of each of the respective ceramic matrix composite articles <NUM> are attached to, or formed on, the CVI ceramic matrix composite base portion <NUM>.

For example, referring particularly to <FIG>, a side, cross-sectional view is provided of a CMC article <NUM> in accordance with an embodiment of the present disclosure as may be incorporated in a gas turbine engine, such as the exemplary gas turbine <NUM> engine of <FIG>. More specifically, for the embodiment depicted, the article <NUM> is configured as a nozzle generally including an airfoil section <NUM>, a radially inner band <NUM>, and a radially outer band <NUM>. The CVI base portion <NUM> for the embodiment depicted substantially completely forms the airflow section <NUM>. Moreover, the CMC article <NUM> includes a first MI covering portion 254A and a second MI covering portion 254B. The first MI covering portion 254A forms the inner band <NUM>, and the second MI covering portion 254B forms the outer band <NUM>. Accordingly, the CVI base portion <NUM> may be formed first, and the MI covering portions 254A, 254B may be subsequently formed around, or on, at least a portion of the previously formed CVI base portion <NUM>.

Notably, it will be appreciated that the gas turbine engine into which the nozzle is incorporated will define a core air flowpath (e.g., a flowpath through a compressor section, combustion section, and turbine section). When incorporated into such a gas turbine engine, at least a portion of the CVI base portion <NUM> of the nozzle <NUM> will be exposed to the core air flowpath. More particularly, substantially all of the airfoil section <NUM> of the nozzle will be exposed to the core air flowpath, and given the greater temperature resistance of the CVI base portion <NUM>, as compared to the MI covering portions 254A, 254B, the resulting nozzle may be capable of withstanding higher temperatures within the gas turbine engine. By contrast, the MI covering portions 254A, 254B will not be as exposed to the core air flowpath, but will be tasked with supporting the CVI base portion <NUM>, and given the improved mechanical properties as compared to the CVI base portion <NUM>, the resulting nozzle may be better capable of handling the forces. It will be appreciated, that as used herein, a portion of a component being "exposed to the core air flowpath" refers to such portion of the component generally being exposed to the environment of the core air flowpath, and is meant to include a portion of the component having one or more of a bond coating, environmental barrier coating, or the like applied to a surface thereof. Accordingly, if an airfoil section includes a bond coating and/or an environmental barrier coating it is still "exposed to the core air flowpath.

Referring now to <FIG>, another embodiment of a ceramic matrix composite article <NUM> in accordance with an exemplary embodiment of the present disclosure is provided. More particularly, <FIG> depicted a shroud as may be incorporated into a gas turbine engine, such as the exemplary gas turbine <NUM> engine of <FIG>. For example, the shroud may be positioned within a turbine section of the gas turbine engine. The shroud generally includes a CVI base portion <NUM> and an MI covering portion <NUM>. The CVI base portion <NUM> may be exposed to a core air flowpath of the gas turbine engine when installed in the gas turbine engine. Accordingly, the CVI base portion <NUM> may form a hot side of the shroud, while the MI covering portion <NUM> may form an opposite, cold side of the shroud. Such a configuration may allow for the shroud to withstand higher temperatures from within the gas turbine engine.

Similarly, referring now to <FIG>, yet another embodiment of a ceramic matrix composite article <NUM> in accordance with an exemplary embodiment of the present disclosure is provided. Specifically, <FIG> depicts a liner as may be incorporated into a gas turbine engine, such as the exemplary gas turbine engine <NUM> of <FIG>. For example, the liner may be configured as a liner within a turbine section of the gas turbine engine, a liner of a combustor within a combustion section of the gas turbine engine, etc. The liner generally includes a CVI base portion <NUM> and an MI covering portion <NUM>. The CVI base portion <NUM> may be exposed to a core air flowpath within the gas turbine engine when installed in the gas turbine engine. Accordingly, the CVI base portion <NUM> may form a hot side of the liner, while the MI covering portion <NUM> may form an opposite, cold side of the liner. Such a configuration may allow for the liner to withstand higher temperatures from within the gas turbine engine.

Moreover, referring now to <FIG>, still another exemplary embodiment of the ceramic matrix composite article <NUM> in accordance with an exemplary embodiment of the present disclosure is provided. Particularly, <FIG> depict an airfoil as may be incorporated into a gas turbine engine, such as the exemplary gas turbine engine <NUM> of <FIG>. By contrast with the airfoil section <NUM> of the nozzle described above with reference to <FIG>, the exemplary airfoil of <FIG>is formed of at least two parts. More specifically, for the embodiment depicted, the airfoil is formed of a first section <NUM> and a second section <NUM>, the first section <NUM> and second section <NUM> being joined together to form the airfoil. <FIG> shows the first section <NUM> being separated from the second section <NUM> (i.e., after the first and second sections <NUM>, <NUM> have been formed, but before the first and second sections <NUM>, <NUM> have been joined together), and <FIG> shows the first and second sections <NUM>, <NUM> joined together to form the airfoil.

Each of the first section <NUM> and second section <NUM> include a CVI ceramic matrix composite base portion <NUM> and an MI ceramic matrix composite covering portion <NUM>. Notably, when the first section <NUM> and the second section <NUM> are joined together, the MI ceramic matrix composite covering portions <NUM> are substantially completely enclosed within the CVI ceramic matrix composite base portions <NUM>. Accordingly, once fully assembled and installed within a gas turbine engine, the MI ceramic matrix composite covering portions <NUM> are not exposed to a core air flowpath of the gas turbine engine, and instead, only the CVI ceramic matrix composite base portions <NUM> of the first section <NUM> and second section <NUM> are exposed to the core air flowpath of the gas turbine engine. Such a configuration may allow for the airfoil to withstand higher temperatures from within the gas turbine engine. It will be appreciated that although for the embodiment depicted the airfoil includes a first section <NUM> and a second section <NUM>, in other exemplary embodiments the airfoil may be formed of any other suitable number of distinct sections. Additionally, it will be appreciated that distinct sections of the airfoil may be joined together using any suitable method.

With each of the embodiments of the ceramic matrix composite article <NUM> described above with reference to <FIG>, it will be appreciated that the CVI ceramic matrix composite base portion <NUM> is formed first, with the MI ceramic matrix composite covering portion <NUM> formed thereafter on the CVI ceramic matrix composite base portion <NUM>. Such may allow for the CVI ceramic matrix composite base portion <NUM> to be formed more completely and with a reduced porosity, as substantially all of a surface of the CVI ceramic matrix composite base portion <NUM> may be exposed to the one or more reactive gases used to form the CVI ceramic matrix composite base portion <NUM>. By reducing the porosity of the CVI ceramic matrix composite base portion <NUM> in such a manner, the CVI ceramic matrix composite base portion <NUM> may exhibit improved properties.

Referring now to <FIG>, a method <NUM> for forming a ceramic matrix composite article in accordance with an embodiment according to the invention is provided. The method <NUM> may be utilized to form one or more of the exemplary CMC articles described above with reference to <FIG>.

As is depicted, the method <NUM> of <FIG> includes at (<NUM>) forming a chemical vapor infiltration (CVI) ceramic matrix composite base portion. Forming the CVI ceramic matrix composite base portion at (<NUM>) includes at (<NUM>) exposing substantially all of a surface of the CVI ceramic matrix composite base portion to one or more reactive gases. The surface of the CVI CMC base portion may refer to an entirety of the surface of the CVI CMC base portion.

Moreover, the method <NUM> of <FIG> includes at (<NUM>) providing a melt infiltration (MI) ceramic matrix composite portion on a portion of the surface of the CVI ceramic matrix composite base portion after forming the CVI ceramic matrix composite base portion at (<NUM>). For example, for the exemplary aspect of the method <NUM> depicted, providing the MI ceramic matrix composite portion on the CVI ceramic matrix composite base portion at (<NUM>) includes at (<NUM>) forming the MI ceramic matrix composite portion on the CVI ceramic matrix composite base portion. Further, forming the MI ceramic matrix composite portion on the CVI ceramic matrix composite base portion at (<NUM>) includes at (<NUM>) laying up one or more layers of pre-peg on the portion of the surface of the CVI ceramic matrix composite base portion and at (<NUM>) performing a melt infiltration of the one or more layers of prepreg.

Notably, exposing substantially all of the outer surface of the CVI ceramic matrix composite base portion to one or more reactive gases at (<NUM>) includes at (<NUM>) forming the CVI ceramic matrix composite base portion to include ceramic matrix material having a ceramic fiber reinforcement material having between <NUM> percent and <NUM> percent free silicon. Additionally, forming the MI ceramic matrix composite portion at (<NUM>) further includes at (<NUM>) forming the MI ceramic matrix composite portion to include ceramic matrix material having ceramic fiber reinforcement material with a greater percent of free silicon and the CVI string matrix composite base portion.

Claim 1:
A method (<NUM>) for forming a ceramic matrix composite article comprising:
forming (<NUM>) a chemical vapor infiltration ceramic matrix composite base portion, wherein forming (<NUM>) the chemical vapor infiltration ceramic matrix composite base portion comprises exposing (<NUM>) substantially all of a surface of the chemical vapor infiltration ceramic matrix composite base portion to one or more reactive gasses, wherein exposing (<NUM>) substantially all of the surface of the chemical vapor infiltration ceramic matrix composite base portion to one or more reactive gasses comprises forming (<NUM>) ceramic fiber reinforcement material in a ceramic matrix material of the chemical vapor infiltration ceramic matrix composite base portion having between <NUM>% and <NUM>% free silicon; and
providing (<NUM>) a melt infiltration ceramic matrix composite portion on a portion of an outer surface of the chemical vapor infiltration ceramic matrix composite base portion after forming (<NUM>) the chemical vapor infiltration ceramic matrix composite base portion, providing (<NUM>) the melt infiltration ceramic matrix composite portion comprises forming (<NUM>) the melt infiltration ceramic matrix composite portion to include ceramic fiber reinforcement material in a ceramic matrix material having a greater percentage of free silicon than the ceramic matrix material of the chemical vapor infiltration ceramic matrix composite base portion.