Patent Description:
It is desirable to ensure that the bulk of the products of combustion pass over turbine blades on the turbine rotor. As such, it is known to provide blade outer air seals radially outwardly of the blades.

Blade outer air seals have been proposed made of ceramic matrix composite fiber layers.

<CIT> discloses a blade outer air seal according to the preamble of claim <NUM>.

<CIT> discloses a segmented thermally insulating coating, <CIT> discloses a turbine component thermal barrier coating with a vertically aligned, engineered surface and multifurcated groove features, and <CIT> discloses a geometrically segmented coating on contoured surfaces.

According to a first aspect, there is provided a blade outer air seal as set forth in claim <NUM>.

According to another aspect, there is provided a method as set forth in claim <NUM>.

Embodiments of the disclosure can be found in the dependent claims.

<FIG> schematically illustrates a portion of the turbine section <NUM>. The turbine section <NUM> includes alternating series of rotating blades <NUM> and stationary vanes <NUM> that extend into the core flow path C of the gas turbine engine <NUM>. Turbine blades <NUM> rotate and extract energy from the hot combustion gases that are communicated along the core flow path C of the gas turbine engine <NUM>. The turbine vanes <NUM>, which generally do not rotate, guide the airflow and prepare it for the next set of blades <NUM>. As is known, it is desirable to pass the bulk of products of combustion downstream of the combustor section <NUM> across the turbine blades <NUM>. Thus, a blade outer air seal ("BOAS") <NUM> is positioned slightly radially outwardly of the outer tip of the blades <NUM>. It should be understood that the turbine section <NUM> could be utilized in other gas turbine engines, and even gas turbine engines not having a fan section at all. The BOAS <NUM> is made up of a plurality of BOAS seal segments <NUM> arranged circumferentially about the axis of rotation A.

<FIG> illustrates an exemplary BOAS seal segment <NUM>. Each seal segment <NUM> is a body that defines radially inner and outer sides R1, R2, respectively, first and second circumferential sides C1, C2, respectively, and first and second axial sides A1, A2, respectively. The radially inner side R1 faces in a direction toward the engine central axis A. The radially inner side R1 is thus the hotwall or gas path side of the seal segment <NUM> that bounds a portion of the core flow path C. The first axial side A1 faces in a forward direction toward the front of the engine <NUM> (i.e., toward the fan <NUM>), and the second axial side A2 faces in an aft direction toward the rear of the engine <NUM> (i.e., toward the exhaust end).

As shown in <FIG> and with continuing reference to <FIG>, the BOAS seal segment <NUM> has attachment members <NUM> and <NUM> and a central web <NUM>. The attachment members <NUM>, <NUM> secure the BOAS seal segment <NUM> to an engine structure, such as the engine static structure <NUM>. Although a particular BOAS seal segment <NUM> is shown, this disclosure may apply to other BOAS or attachment configurations.

In the illustrated embodiment, the BOAS seal segment <NUM> is formed of a ceramic matrix composite ("CMC") material. The BOAS seal segment <NUM> is formed of a plurality of CMC laminates <NUM>. The laminates may be silicon carbide fibers, formed into a woven fabric in each layer. The fibers may be coated by a boron nitride.

In one embodiment, the seal segment <NUM> may have a central reinforcement laminate <NUM> including a plurality of layers. An overwrap <NUM> also includes a plurality of layers or laminates, and spans a central web <NUM> which is defined axially between radially extending attachment members <NUM> and <NUM>, and may extend axially outwardly of one or both attachment members <NUM>, <NUM>. In the illustrated embodiment, attachment members <NUM>, <NUM> extend radially from the central web and have holes <NUM> for attachment to the engine structure <NUM>. In other embodiments, the attachment members <NUM>, <NUM> may be hooks, for example. Other BOAS seal segments <NUM> and/or structures for attachment of the BOAS seal segment <NUM> to the engine structure <NUM> may be contemplated within the scope of this disclosure. In other embodiments, the BOAS seal segment <NUM> may be formed from a metallic material or monolithic ceramic, for example.

The seal segment <NUM> includes a cavity layer or honeycomb layer <NUM> and a geometrically segmented abradable coating ("GSAC") <NUM> on the radially inner side R1. An abradable coating on a BOAS seal segment <NUM> contacts tips of the turbine blades <NUM> such that the blades <NUM> abrade the coating upon operation of the engine <NUM>. This provides a minimum clearance between the BOAS seal segment <NUM> and the tip of the blade <NUM>. Over time, internal stresses can develop in the coating that may make the coating vulnerable to erosion and spalling. When the coating is spalling, small fragments or chips wear off the BOAS. In some known coatings, the BOAS needs to be replaced after a period of use. The use of a GSAC can help reduce this spalling. A GSAC forms segmentation cracks within the coating at corners of the underlying component upon sintering. These segmentation cracks provide locations to accommodate the strain associated with internal stresses. That is, the energy associated with the internal stresses is maintained at a lower level due to the cracks such that there is less energy available for causing delamination cracking and spallation. In one embodiment, the GSAC is a ceramic material.

The cavity layer or honeycomb layer <NUM> has a plurality of cavities <NUM> that facilitate the cracks in the GSAC <NUM> for spallation reduction. In an example embodiment, the cavities <NUM> are all the same size and shape. In an embodiment, the cavities <NUM> are evenly spaced from one another and span the entire honeycomb layer <NUM>. The GSAC <NUM> fills each of the cavities <NUM>, and has an additional thickness radially inward of the honeycomb layer <NUM>. In one example, the GSAC <NUM> provides a generally smooth radially inner surface for the BOAS seal segment <NUM>. That is, the GSAC <NUM> provides the hot wall surface immediately adjacent the tips of the turbine blades <NUM>.

<FIG> and <FIG> show method steps of forming a BOAS seal segment <NUM>. The BOAS seal segment <NUM> is formed of a ceramic matrix composite ("CMC") material. First, a seal body <NUM> is formed, such as from a plurality of laminates. In some embodiments, the seal body <NUM> is formed from a central reinforcement laminate <NUM> including a plurality of layers and an overwrap <NUM>, which also includes a plurality of laminates. Next, a CMC honeycomb layer <NUM> is added to the radially inner side R1. The CMC honeycomb is 3D woven and produced in sheets. A CMC honeycomb sheet is then cut to size and applied to the radially inner side R1 of the seal body <NUM> to form the honeycomb layer <NUM>, as shown in <FIG>. The seal body <NUM> and CMC honeycomb layer <NUM> are then densified. The CMC honeycomb layer <NUM> is densified onto the gaspath location on the BOAS seal segment <NUM>.

A densification process is utilized to increase the density of the laminate material after forming the seal body <NUM>. This makes the laminates more stiff than their free woven fiber state. During densification, materials, such as a silicon carbide matrix material, are injected into spaces between the fibers in the woven layers. Spaces <NUM> and <NUM> or the honeycomb layer <NUM> may be filled with loose fibers during densification, for example.

This densification process may be utilized to provide <NUM>% of the desired densification, or only some percentage. As an example, the densification may be utilized to form between <NUM> and <NUM>% of a desired densification. One hundred percent densification may be defined as the layers being completely saturated with the matrix and about the fibers. One hundred percent densification may be defined as the theoretical upper limit of layers being completely saturated with the matrix and about the fibers, such that no additional material may be deposited. In practice, <NUM>% may be difficult to achieve. This method of densifying the honeycomb layer <NUM> with the BOAS seal segment <NUM> allows the honeycomb layer <NUM> to act as an additional structural layer of the BOAS hotwall and no additional machining steps are necessary to create the cavities <NUM>.

After the seal body <NUM> and CMC honeycomb layer <NUM> have been densified, the abradable coating <NUM> is applied. In some embodiments, the coating <NUM> is applied after all machining operations are completed on the seal segment <NUM>. Individual cavities <NUM> of the honeycomb layer <NUM> are filled with the abradable coating, and additional abradable coating thickness <NUM> is applied on the radially inner side R1. The honeycomb layer <NUM> provides a base for the abradable coating to have GSAC properties.

Known methods of making a BOAS with a GSAC include drilling divots into the BOAS gaspath surface before applying the GSAC. The present disclosure eliminates the drilling step, by using a woven honeycomb sheet to form the honeycomb layer <NUM>. Once the cavities <NUM> are filled with GSAC <NUM>, additional GSAC thickness provides further environmental and thermal protection to the BOAS seal segment <NUM>.

The honeycomb layer <NUM> is initially formed as a continuous sheet. <FIG> illustrate exemplary CMC honeycomb sheets <NUM>. As shown in <FIG>, the honeycomb sheet <NUM> has a plurality of hexagonal cavities <NUM>. The sheet <NUM> may be a silicon carbide sheet. In some examples, the sheet <NUM> is a carbon fiber reinforced silicon carbide composite. The sheet <NUM> may be 3D woven with the honeycomb structure, and may be adapted for chemical vapor infiltration. In other embodiments, the sheet <NUM> may be woven as a solid CMC sheet, with cavities <NUM> drilled in.

In some examples, each cavity <NUM> may have a width <NUM> of between about <NUM> and <NUM> inches (<NUM>-<NUM>). In a further example, each cavity <NUM> has a width <NUM> of about <NUM> inches (<NUM>). The sheet <NUM> may have a solidity of between about <NUM>% and about <NUM>%. In some examples, the sheet <NUM> has about <NUM>% solidity. This means the about <NUM>% of the area is solid structure, while <NUM>% is cavities <NUM>. However, greater or smaller solidities may be used. The hexagonal cavities <NUM> in the honeycomb sheet <NUM> may provide better spallation-mitigation properties than traditional machined circular divots, as there are additional faces for bonding of the GSAC <NUM>.

<FIG> shows an alternative CMC sheet <NUM>. In this example, the sheet <NUM> has a plurality of round cavities <NUM>. In some embodiments, the CMC sheet <NUM> is a full continuous CMC sheet with cavities <NUM> drilled in. Circular cavities <NUM> may be easier to manufacture than hexagonal cavities, while still providing faults for segmentation of the GSAC <NUM>. Other shaped cavities may also be used, such as rectangular or triangular, for example.

The honeycomb sheet is lightweight and structural, while providing cavities for insulation, cooling, or void space. The honeycomb structure may provide internal, film and/or perspiration cooling. The honeycomb sheet <NUM> provides a base for cracks to propagate with a lower risk of coating spallation. The honeycomb structure and GSAC coating may also provide a lower CMC bulk temperature.

Claim 1:
A blade outer air seal (<NUM>) for a gas turbine engine, the blade outer air seal (<NUM>) comprising:
a blade outer air seal body (<NUM>) having a radially inner side (R1) and a radially outer side (R2), wherein the radially inner side (R1) has a cavity layer (<NUM>) having a plurality of cavities (<NUM>; <NUM>) and an abradable coating (<NUM>) over the cavity layer (<NUM>),
characterised in that:
the abradable coating (<NUM>) is a geometrically segmented abradable coating (<NUM>) formed from a ceramic material; and
the cavity layer (<NUM>) is a ceramic matrix composite honeycomb layer (<NUM>) produced from a woven honeycomb sheet.