Patent Description:
A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-speed exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section may include low and high pressure compressors, and the turbine section may also include low and high pressure turbines.

Airfoils in the turbine section are typically formed of a superalloy and may include thermal barrier coatings to extend temperature capability and lifetime. Ceramic matrix composite ("CMC") materials are also being considered for airfoils. Among other attractive properties, CMCs have high temperature resistance. Despite this attribute, however, there are unique challenges to implementing CMCs in airfoils.

<CIT> discloses a method for producing a vane.

<CIT> discloses an airfoil with a dual-profile leading end.

<CIT> discloses a turbine vane with high temperature capable skins.

<CIT> discloses a cooled gas turbine blade.

An aspect of the present invention includes an airfoil according to claim <NUM>.

Further embodiments are according to the dependent claims <NUM>-<NUM>.

The engine <NUM> includes a compressor section <NUM> and a turbine section <NUM> interconnected by a shaft <NUM>. A combustor <NUM> is arranged between the compressor and turbine sections <NUM>/<NUM>. The turbine section <NUM> includes first and second turbines <NUM>/<NUM>, which correspond to high and low pressure turbines, respectively. A generator <NUM> is rotationally driven by a shaft coupled to the low pressure turbine <NUM>, or power turbine. The generator <NUM> provides electricity to a power grid <NUM>. It should be understood that the illustrated engine <NUM> is highly schematic, and may vary from the configuration illustrated. Moreover, the examples herein are not limited to industrial turbines and may be used in propulsion gas turbine engines.

<FIG> illustrates an isolated view of an airfoil <NUM> that demonstrates several aspects of the present disclosure, and <FIG> illustrates a sectioned view of the airfoil <NUM>. As will be appreciated, the airfoil <NUM> is from the turbine section <NUM> of the engine <NUM>. Although the airfoil <NUM> is depicted as a stationary turbine vane that has radially inner and outer endwalls, it is to be understood that the airfoil <NUM> may alternatively be a rotating blade. As used herein, directional terms such as "axial," "radial," and the like are taken with regard to the rotational axis of the engine <NUM>, which is generally coaxial with the shaft <NUM>.

The airfoil <NUM> is generally comprised of an airfoil section <NUM> that defines a leading edge 60a, a trailing edge 60b, a first or pressure side 60c, and a second or suction side 60d. The terminology "first" and "second" as used herein is to differentiate that there are two architecturally distinct components or features. It is to be further understood that the terms "first" and "second" are interchangeable in the embodiments herein in that a first component or feature could alternatively be termed as a second component or feature, and vice versa.

Referring to <FIG>, the airfoil section <NUM> is formed of an airfoil wall <NUM> that surrounds a core cavity <NUM>. In this example, the airfoil section <NUM> includes a single cavity, although multiple cavities that are divided by ribs or other structures are also contemplated. The core cavity <NUM> is connected to a cooling air source, such as bleed air from the compressor <NUM>, which provides relatively cool air into the core cavity <NUM> for cooling the airfoil <NUM>.

The airfoil wall <NUM> includes an exterior monolithic ceramic shell <NUM> (hereafter "shell <NUM>") and an interior ceramic matrix composite liner <NUM> (hereafter "liner <NUM>") that lines, and is bonded to, the interior surfaces of the shell <NUM>. Generally the shell <NUM> facilitates providing good high temperature resistance and stability against oxidation, corrosion, erosion (recession), and compressive strength, and the liner <NUM> facilitates providing good tensile strength, and creep resistance. The combined shell and liner provide superior impact resistance.

The shell <NUM> may be, but is not limited to, silicon carbide (SiC), silicon nitride (Si<NUM>N<NUM>), alumina (Al<NUM>O<NUM>), silicon aluminum oxynitride (SiAlON), nitride bonded silicon carbide (NBSC), aluminum nitride (AlN), silicon oxynitride (Si<NUM>N<NUM>O), hafnia (HfO<NUM>), zirconia (ZrO<NUM>), or other oxides, carbides, or nitrides, and particulate composites thereof. In some examples, the shell <NUM> has a thickness from about <NUM> millimeters to about <NUM> millimeters.

The ceramic matrix composite (CMC) of the liner <NUM> includes bundles of fibers called tows in the form of yarns that can be woven into plies or laid out on unidirectional tape and disposed in a ceramic matrix. The fibers within the CMC layers include fiber bundles woven into plies, like cloth, which are assembled into a fiber-reinforced preform which is later infiltrated with the ceramic matrix. Most typically, the fibers are ceramic fibers that are provided as a fiber network, such as woven plies, fibrous mats, and the like. The fibers may be, but are not limited to, non-oxide fibers such as SiC fibers or oxide fibers such as aluminosilicate fibers. The fibers may also be coated with boron nitride (BN) or other interface material to prevent bonding with the matrix.

The ceramic matrix of the liner <NUM> may be, but is not limited to, amorphous compounds of silicon, carbon, nitrogen, oxygen, boron, or other light elements. Example compounds include SiC, Al<NUM>O<NUM>, Si<NUM>N<NUM>, boron nitride (BN), SiAlON, AIN, magnesium aluminum silicate (MAS), lithium aluminum silicate, barium aluminum silicate (BAS), barium magnesium aluminum silicate (BMAS), and combinations thereof. Those skilled in the art will recognize that other matrices, including metalloids such as silicon or alloys thereof, could be employed.

As shown in the view of the leading edge region in <FIG>, the liner <NUM> includes four fiber plies 66a/66b/66c/66d, although additional fiber plies or fewer plies may alternatively be used. In the example depicted, the ply 66a is an innermost ply, the ply 66d is an outermost ply, the ply 66b is an inner intermediate ply, and the ply 66c is an outer intermediate ply. The plies 66a/66b/66c/66d are bonded together via the matrix of the CMC material.

<FIG> illustrates an isolated view of a portion of the liner <NUM> and plies 66a/66b/66c/66d. The ply 66d forms tabs <NUM> that are raised from the underlying ply 66c so as to define a slot <NUM> that allows for cooling air to pass between the tabs <NUM> and the underlying ply 66c. The tabs <NUM> in this example are formed from the single ply 66d and are thus one ply thick. The tabs <NUM> may alternatively be formed from two plies 66c/66d or more than two plies for additional strength.

Each tab <NUM> is generally axially elongated and includes a base 68a that projects outwards, away from the underlying ply 66c and a body portion 68b that projects off of the base portion 68a. The body portion 68b extends along the underlying ply 66c such that the slot <NUM> has a relatively uniform width at the body portion 68b. For reasons which will be evident below, a tip portion 68c projects off of the body portion 68b in a direction away from the underlying ply 66c. The tabs <NUM> are generally spaced apart from one another and are arranged in radial rows (R), as depicted in <FIG> excludes a portion of the shell <NUM> so that the tabs <NUM> are visible). In the illustrated example, there are two rows (R) of tabs <NUM>.

Turning again to <FIG>, the shell <NUM> is made up of multiple distinct shell pieces, including a first shell piece <NUM>, a second shell piece <NUM>, a third shell piece <NUM>, and a fourth shell piece <NUM>. In this example, the first and second shell pieces <NUM>/<NUM> form the leading edge region, while the third shell piece <NUM> forms the pressure side and the fourth shell piece <NUM> forms the suction side. The multiple pieces may facilitate alleviating mechanical stresses and the thermal stress between different regions of the airfoil <NUM>. Moreover, the shell pieces <NUM>/<NUM>/<NUM>/<NUM> are of relatively simple construction in that they do not contain any orifices, which facilitates reductions in thermal gradients and stress concentrations for enhanced damage tolerance.

The first shell piece <NUM> and the second shell piece <NUM> are of similar construction to each other and while the first shell piece <NUM> is described below, the second shell piece <NUM> should be understood to include the same features. The first shell piece <NUM> is formed of a body portion 72a and a tapered flange 72b that projects (axially) off of the body portion 72a. The body portion 72a is bonded to the outermost ply 66d at interface (I), as depicted in <FIG>. In comparison to the tapered flange 72b, the body portion 72a is relatively thick. In the illustrated example, the body portion 72a and the flange 72b are radially elongated. The body portion 72a defines a side edge 72c that has an exterior sloped lip 72d.

The second shell piece <NUM> is located adjacent the first shell piece <NUM>. Together, the side edges 72c of the shell pieces <NUM>/<NUM> form sides of a trench <NUM>, with the outermost ply 66d of the liner <NUM> forming a floor of the trench <NUM>. In that regard, the floor includes a plurality of trench orifices 80a through the liner <NUM> that connect the cavity <NUM> and the trench <NUM>. At least a portion of the trench orifices 80a are sloped radially outwards, as indicated at 80b (<FIG>). In the illustrated example, the trench <NUM> is located at the leading edge 60a of the airfoil section <NUM>, however, it is to be understood that the trench <NUM> could alternatively or additionally be located on the pressure side, the suction side, or both.

The third shell piece <NUM> and the fourth shell piece <NUM> are of similar construction to each other and while the third shell piece <NUM> is described below, the fourth shell piece <NUM> is understood include the same features. The third shell piece <NUM> is formed of a body portion 76a and a tapered flange 76b that projects (axially) off of the body portion 76a. In comparison to the tapered flange 76b, the body portion 76a is relatively thick.

As best shown in <FIG>, the flange 76b of the third shell piece <NUM> is disposed in the slot <NUM> under the tabs <NUM>. The first shell piece <NUM> overlies the outer ply 66d such that the flange 72b overlaps the tabs <NUM>. The flanges 72b/76b thus overlap each other, with the tabs <NUM> situated there between to structurally support the flange 72b of the first shell piece <NUM> as well as to allow cooling air to pass between the shell pieces. In this regard, the third shell piece <NUM> is an underlapping shell piece and the first shell piece <NUM> is an overlapping shell piece. Similarly, the fourth shell piece <NUM> is an underlapping shell piece and the second shell piece <NUM> is an overlapping shell piece. As shown in a sectioned view in <FIG>, the flange 72b of the first shell piece <NUM> is bonded to the top surface of the tabs <NUM> at interfaces (I1) and the flange 76b of the third shell piece <NUM> is bonded to the inside surface of the tabs <NUM> at interfaces (I2).

Channels <NUM> are bound by adjacent tabs <NUM> that serve as the channel sides and the flanges 72b/76b that serve as the respective channel top and bottom. As can be seen in <FIG>, there are channel orifices 82a through the liner <NUM> that connect the channels <NUM> and the cavity <NUM>. The channel orifices 82a serve as inlets for cooling air to flow into the channels <NUM>. The channels <NUM> are axially elongated and generally extend from the channel orifices 82a to a channel outlet section 82b. The outlet section 82b is sloped relative to the surface of the third shell piece <NUM>. The tip portions 68c of the tabs <NUM> abut and are bonded with the sloped surface portion of the third shell piece <NUM> adjacent the outlet section 82b. The slope of the outlet section 82b is generally shallow in order to discharge a film of cooling air along the outer surface of the third shell piece <NUM>. In the example shown, the tabs <NUM> extend axially and are straight-sided, which forms the channels <NUM> to be axial and straight-sided. In another example, the tabs <NUM> taper in width along the body portion 68b, resulting in the channels <NUM> widening with further distance from the orifices 82a in order to diffuse cooling air flow. Additionally or alternatively, the tabs <NUM> are tapered in thickness, such that the channels <NUM> are also set back from the edge of the flanges 72b. In further examples, the tabs <NUM> can also have aerodynamic contours to facilitate cooling air flow and/or minimize mixing losses when the air is discharged for film cooling.

During operation of the engine <NUM> cooling air is provided into the core cavity <NUM>. The cooling air flows through the trench orifices 80a and cools the side edges 72c of the shell pieces <NUM>/<NUM>. The sloped lip 72d of the side edge 72c permits the cooling air to leak out of the trench <NUM> as a cooling film along the outer surface of the shell piece <NUM>. The cooling air also flows through the channel orifices 82a into the channels <NUM>. The cooling air flowing in the channels <NUM> cools the flanges 72b/76b and then may also be discharged as a cooling film along the outside of the shell piece <NUM>.

The fabrication of the shell pieces <NUM>/<NUM>/<NUM>/<NUM> is not particularly limited and may be produced by slip casting, isostatic pressing and green machining, injection molding, or additive manufacturing, followed by densification. Densification techniques include, but are not limited to, sintering, hot isostatic pressing (HIP), sinter-HIPing, silicon infiltration and reaction bonding, and reaction bonding in combination with other techniques listed herein.

The fabrication of the liner <NUM> also is not particularly limited and may be produced using a fiber-reinforced preform and then infiltrating the preform with a ceramic matrix material or precursor to the ceramic matrix material. The infiltration may be conducted by any of a variety of methods, including but not limited to, chemical vapor infiltration (CVI), polymer infiltration and pyrolysis (PIP), transfer molding, and melt infiltration (MI). The shell <NUM> may be prefabricated using known ceramic processing techniques. The preform may be constructed in the desired geometry in the shell <NUM>, in contact with the interior surfaces of the shell <NUM>. For example, fiber plies are built-up to construct the walls of the liner <NUM>. Upon densification of the ceramic matrix of the liner <NUM>, due to the contact between the preform and the monolithic ceramic of the shell <NUM>, the ceramic matrix material strongly bonds with the monolithic ceramic. Such bonding facilitates mechanical strength in the airfoil <NUM> as well as heat transfer through joined interfaces between the shell <NUM> and the liner <NUM>. Additionally, since the shell <NUM> and liner <NUM> are bonded together at high temperature during firing and curing, having multiple shell pieces can allow for relief of residual stresses generated during the manufacturing process.

Alternatively, the liner <NUM> may be prefabricated and densified prior to bonding with the shell <NUM> and then assembled into the shell <NUM>. A brazing material, such as but not limited to elemental silicon, may be provided at the interfaces that are to be joined. Upon heating to an appropriate brazing temperature for the selected brazing material, the brazing material diffuses and bonds the shell <NUM> and liner <NUM> together.

Claim 1:
An airfoil (<NUM>) for a gas turbine engine (<NUM>), the airfoil (<NUM>) comprising:
an airfoil section (<NUM>) having an airfoil wall (<NUM>) surrounding a cavity (<NUM>), the airfoil wall (<NUM>) including an exterior monolithic ceramic shell (<NUM>) and an interior ceramic matrix composite (CMC) liner (<NUM>) bonded with the exterior monolithic ceramic shell (<NUM>), the exterior monolithic ceramic shell (<NUM>) including a first shell piece (<NUM>) having a first side edge (72c) and a second shell piece (<NUM>) having a second side edge, the first and second side edges (72c) defining sides of an elongated trench (<NUM>) and the CMC liner (<NUM>) defining a floor of the trench (<NUM>), the floor including a plurality of orifices (80a) through the CMC liner (<NUM>) connecting the cavity (<NUM>) and the trench (<NUM>).