Patent Description:
The components of high-temperature mechanical systems, such as, for example, gas-turbine engines, must operate in severe environments. For example, the high-pressure turbine blades, vanes, blade tracks and blade shrouds exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about <NUM>, with short-term peaks as high as <NUM>.

Components of high-temperature mechanical systems may include a Ni- or Co-based superalloy substrate or a ceramic-based substrate, such as a ceramic matrix composite. The substrate can be coated with a thermal barrier coating (TBC), an environmental barrier coating (EBC), or both to reduce surface temperatures. The TBC, EBC, or both may allow use of the component at higher temperatures, which may improve efficiency of the high-temperature mechanical system.

In addition to raising the inlet temperature, gas turbine power and efficiency also may be improved by reducing the clearance between a gas turbine blade and a surrounding blade track or blade shroud. One method of reducing the clearance between blade and track or shroud includes coating the blade track or blade shroud with an abradable coating. As the turbine blade rotates, the tip of the turbine blade may contact the abradable coating and wear away a portion of the coating to form a groove in the abradable coating corresponding to the path of the turbine blade. The intimate fit between the blade and abradable coating provides a seal, which may reduce or eliminate leakage of gas around the blade tip and increase the efficiency of the gas turbine engine by up to <NUM>% in some cases.

<CIT> discloses a thermal barrier coating enabling confirmation of the degree of wear. The thermal barrier coating includes a ceramics layer comprising partially stabilized zirconia. The ceramics layer comprises a surface layer, a second layer and a third layer which are a plurality of divided layers having different color between adjacent layers in a thickness direction.

<CIT> discloses a thermal barrier coating wherein multilayers of (I) a ZrO<NUM> layer containing a luminous activator and stabilized by Y<NUM>O<NUM>, CaO, MgO or a mixture thereof and (II) a ZrO<NUM> layer containing a luminous activator different from that used in (I) and stabilized by Y<NUM>O<NUM>, CaO, MgO or a mixture thereof, or a ZrO<NUM> layer not containing a luminous activator and stablized by Y<NUM>O<NUM> are stacked, and the layers at least under an uppermost layer opposite to a base member to be protected have a total thickness sufficient for exhibiting a thermal barrier effect. The disclosed arrangement is to provide a thermal barrier coating which allows non-destructive testing to determine whether it has a thickness sufficient for a thermal barrier effect.

<CIT> discloses a corrosion resistant member to be exposed to a halogen-containing gas atmosphere or a halogen-containing gas plasma atmosphere, comprising a substrate and a plurality of layers deposited thereon including a layer of rare earth fluoride providing the outermost surface and a layer of rare earth oxide having a porosity of less than <NUM>% underlying the rare earth fluoride layer.

<CIT> discloses a coated silicon carbide-type fiber reinforced ceramic composite material having an intermediate layer and a topcoat layer sequentially arranged on a base material. The intermediate layer contains a mixture comprising aluminosilicate-type glass ceramic and rare-earth silicate. The topcoat layer contains rare-earth silicate. The base material is silicon carbide-type fiber reinforced ceramic composite material.

<CIT> discloses a methods for allowing inspection of components having a barrier coating such as thermal or environmental barrier coatings on turbine components. The method comprises providing a component and applying a barrier coating having at least one layer to the component where the layers of the barrier coating include a taggant. The taggant may comprise a rare earth metal or a compound thereof. The taggant changes the color of the coating. Each layer of the coating may have a different taggant added thereto.

<CIT> discloses a method of depositing abradable coating on an engine component wherein the engine component is formed of ceramic matrix composite (CMC), and wherein one or more layers, including at least one environmental barrier coating, may be disposed on the outer layer of the CMC. An outermost layer of the structure may further comprise a porous abradable layer that is disposed on the environmental barrier coating and provides a breakable structure which inhibits blade damage. The abradable layer may be gel-cast on the component and sintered or may be direct written by extrusion process and subsequently sintered.

<NPL> describes alternative methods for determination of composition and porosity in abradable materials.

Viewed from a first aspect, there is provided a system as defined in claim <NUM> of the appended claims.

Viewed from a second aspect, there is provided a method as defined in claim <NUM> of the appended claims.

The disclosure describes coatings that include at least one feature that facilitates visual determination of a thickness of the coating. The coating includes a plurality of microspheres disposed at a predetermined depth of the coating. The plurality of microspheres defines a distinct visual characteristic. By inspecting the coating and viewing at least one of the microspheres, the thickness of the coating may be estimated. The plurality of microspheres is embedded in a matrix material, and the distinct visual characteristic of the microspheres is different than the visual characteristic of the matrix material.

Also disclosed is that the at least one feature may include at least one distinct layer in the coating system that includes a distinct visual characteristic, such as a color of the distinct layer. Similar to the microspheres, the at least one distinct layer may be disposed at a predetermined depth of the coating. By inspecting the coating and viewing the distinct layer, the remaining thickness of the coating may be estimated.

In order to minimize losses in gas turbine efficiency, proper clearance should be maintained between a gas turbine blade and a surrounding track or shroud during the entire life of the component. The abradable coating described herein includes at least one feature that may provide a simple visual indicator detectable during an on-wing inspection. This visual indicator may provide a simple and accurate indication of the ability of the abradable coating to provide proper sealing with the gas turbine blade, resulting in the system meeting efficiency targets.

The disclosure describes a turbine blade track or a turbine blade shroud, including a substrate and a coating on the substrate. The coating includes at least one abradable layer. The at least one abradable layer includes a matrix material and a plurality of microspheres located within the at least one abradable layer at a predetermined depth from an outer surface of the coating. The plurality of microspheres defines a visual characteristic distinct from the matrix material. The plurality of microspheres includes at least one rare earth silicate, at least one rare earth oxide, or at least one rare earth silicate and at least one rare earth oxide.

There is further disclosed (but not claimed) an article including a substrate and a coating on the substrate. The coating may include at least one abradable layer, a neodymium silicate layer, an erbium silicate layer, and a gadolinium silicate layer. The neodymium silicate layer, the erbium silicate layer, and the gadolinium silicate layer may be adjacent to each other within the coating.

The disclosure describes a method including forming a coating on a substrate of a turbine blade track or a turbine blade shroud. The coating includes at least one abradable layer. The at least one abradable layer includes a matrix material and a plurality of microspheres located within the at least one abradable layer at a predetermined depth from an outer surface of the coating. The plurality of microspheres defines a visual characteristic distinct from the matrix material. The plurality of microspheres includes at least one rare earth silicate, at least one rare earth oxide, or at least one rare earth silicate and at least one rare earth oxide.

There is further disclosed (but not claimed) a method including forming a coating on a substrate. The coating may include at least one abradable layer, a neodymium silicate layer, an erbium silicate layer, and a gadolinium silicate layer. The neodymium silicate layer, the erbium silicate layer, and the gadolinium silicate layer may be adjacent to each other within the coating.

The disclosure also describes (but does not claim) a method including visually inspecting an abradable coating, wherein the coating comprises at least one abradable layer, and wherein the at least one abradable layer comprises a matrix material and a plurality of microspheres located within the at least one abradable layer at a predetermined depth from an outer surface of the coating. The plurality of microspheres defines a visual characteristic distinct from the matrix material. The plurality of microspheres may include at least one rare earth silicate, at least one rare earth oxide, or at least one rare earth silicate and at least one rare earth oxide. The method also may include determining a wear level of the abradable coating based at least in part on observation of at least one of the plurality of microspheres.

The present disclosure describes an abradable coating that includes at least one feature that facilitates visual determination of a thickness of the abradable coating. The abradable coating is formed on a component of a high temperature mechanical system, namely a gas turbine blade track or blade shroud. During operation of the gas turbine engine, the blade track or blade shroud is exposed to gases. Rubbing of a gas turbine engine blade against at least a portion of the abradable coating, erosion by the gases flowing past the abradable coating and contact with debris in the gases, or both, may remove portions of the abradable coating (e.g., cause the coating to wear). The at least one feature may facilitate visual determination of a remaining thickness of the abradable coating. In some examples, the visual inspection may be performed at room temperature using visible wavelengths, UV wavelengths, or both. Additionally or alternatively, X-ray inspection or ultrasonic techniques may be used to detect a depth of the at least one feature below an outer surface of the abradable coating.

In some examples, the abradable coating may include a plurality of layers (e.g., at least two abradable layers). At least one abradable layer of the plurality of abradable layers may include the at least one feature. In some examples, the abradable coating may be part of a coating system that includes at least one other layer. For example, the coating system may include at least one of an environmental barrier coating (EBC) layer or a bond layer.

In some examples, the at least one feature that facilitates visual determination of a thickness of the abradable coating includes a plurality of microspheres that possess a distinctive visual characteristic under visual or UV-assisted inspection. In some examples, the abradable coating includes an abradable layer comprising a matrix material and the plurality of microspheres that define a visual characteristic distinct from the matrix material. In some examples, the matrix material includes at least one of a rare earth silicate, a stabilized zirconium oxide, a mullite, or barium-strontium-aluminum silicate (BSAS).

The plurality of microspheres includes at least one rare earth silicate, at least one rare earth oxide, or at least one rare earth silicate and at least one rare earth oxide. The rare earth silicate may include at least one of yttrium monosilicate (Y<NUM>SiO<NUM>), yttrium disilicate (Y<NUM>Si<NUM>O<NUM>), ytterbium monosilicate (Yb<NUM>SiO<NUM>), ytterbium disilicate (Yb<NUM>Si<NUM>O<NUM>), erbium monosilicate (Er<NUM>SiO<NUM>), erbium disilicate (Er<NUM>Si<NUM>O<NUM>), neodymium monosilicate (Nd<NUM>SiO<NUM>), neodymium disilicate (Nd<NUM>Si<NUM>O<NUM>), gadolinium monosilicate (Gd<NUM>SiO<NUM>), or gadolinium disilicate (Gd<NUM>Si<NUM>O<NUM>). The rare earth oxide may include at least one of yttrium oxide (Y<NUM>O<NUM>), ytterbium oxide (Yb<NUM>C<NUM>), erbium oxide (ErO<NUM>), neodymium oxide (Nd<NUM>C<NUM>), or gadolinium oxide (Gd<NUM>C<NUM>). In some examples, the at least one rare earth silicate, the at least one rare earth oxide, or the at least one rare earth silicate and rare earth oxide is present in the microspheres in a glass phase. In other examples, the at least one rare earth silicate, the at least one rare earth oxide, or the at least one rare earth silicate and the at least one rare earth oxide is present in the microspheres in a crystalline or semi-crystalline phase. The microspheres may be substantially solid or may be hollow.

In some examples, the plurality of microspheres may include a ceramic oxide and the at least one rare earth silicate, the at least one rare earth oxide, or the at least one rare earth silicate and at least one rare earth oxide. For example, the plurality of microspheres may include aluminum oxide doped with yttrium oxide, yttrium monosilicate, or yttrium disilicate; or may include yttrium-aluminum-garnet (YAG) doped with ytterbium oxide, ytterbium monosilicate, or ytterbium disilicate.

In some examples, at least some microspheres of the plurality of microspheres may include between about <NUM> atomic percent (at. %) and about <NUM> at. % of at least one element selected from the Lanthanide series of the periodic table, excluding ytterbium, e.g., 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), or lutetium (Lu).

In some examples, a further feature that facilitates visual determination of a thickness of the abradable coating may include a plurality of layers that possess respective distinct visual characteristics under visual or UV-assisted inspection. For example, the abradable coating may include a neodymium silicate layer, an erbium silicate layer, and a gadolinium silicate layer. Each of these three layers may appear differently (e.g., have different colors) under visual inspection.

By incorporating the at least one feature in the form of a plurality of microspheres that facilitates visual determination of a thickness of the abradable coating at a predetermined depth within the thickness of the abradable coating, the at least one feature may be used to determine a remaining thickness of the abradable coating. For example, the thickness of the abradable coating between the at least one feature and the immediately underlying layer (e.g., EBC, bond layer, or substrate) may be known. Thus, when the at least one feature is exposed at a surface of the abradable coating, the remaining thickness of the abradable coating at that location may be determined to be the thickness between the at least one feature and the immediately underlying layer. In some examples, this may facilitate at least one of determination of a remaining life of the abradable coating, determination of a size, depth, or both of damage to the abradable coating, and removal and repair of the abradable coating to maintain clearances between the abradable coating and a blade tip below a predetermined level to maintain a desired efficiency level of the gas turbine engine.

Although the description describes abradable coatings, in other examples (not claimed), the subject matter described herein may be applied to coatings of other types, such as abrasive coatings, thermal barrier coatings, environmental barrier coatings, self-lubricating coatings, or the like. The layer structure and compositions may differ for these different applications, but layers including a plurality of microspheres in a matrix material or a plurality of layers with distinct visual characteristics may also be used in these other coating systems.

<FIG> is a conceptual diagram illustrating a cross-sectional view of an example article <NUM> that includes a substrate <NUM> coated with an abradable coating <NUM>. In the example illustrated in <FIG>, abradable coating <NUM> includes a plurality of microspheres <NUM> and a matrix material <NUM> and is on a surface <NUM> of substrate <NUM>.

Article <NUM> is a turbine blade track or a turbine blade shroud component of a high temperature mechanical system, namely a gas turbine engine or the like.

Substrate <NUM> may include a metal alloy that includes silicon, a ceramic, or a ceramics matrix composite (CMC). In examples in which substrate <NUM> includes a ceramic, the ceramic may be substantially homogeneous. In some examples, a substrate <NUM> that includes a ceramic includes, for example, a Si-containing ceramic, such SiO<NUM>, silicon carbide (SiC) or silicon nitride (Si<NUM>N<NUM>); Al<NUM>O<NUM>; aluminosilicate (e.g., Al<NUM>SiO<NUM>); or the like. In other examples, substrate <NUM> includes a metal alloy that includes Si, such as a molybdenum-silicon alloy (e.g., MoSi<NUM>) or a niobium-silicon alloy (e.g., NbSi<NUM>).

In examples in which substrate <NUM> includes a CMC, substrate <NUM> includes a matrix material and a reinforcement material. The matrix material includes a ceramic material, such as, for example, SiC, Si<NUM>N<NUM>, Al<NUM>O<NUM>, aluminosilicate, SiO<NUM>, or the like. The CMC further includes a continuous or discontinuous reinforcement material. For example, the reinforcement material may include discontinuous whiskers, platelets, or particulates. As other examples, the reinforcement material may include a continuous monofilament or multifilament weave.

Article <NUM> further includes abradable coating <NUM>, which in the example of <FIG> is directly on surface <NUM> of substrate <NUM>. Abradable coating <NUM> includes a plurality of microspheres <NUM> and a matrix material <NUM>. Matrix material <NUM> may include at least one of a rare earth silicate, a stabilized zirconium oxide, mullite, or BSAS. In some examples, matrix material <NUM> includes at least one rare earth silicate.

The plurality of microspheres <NUM> possesses a visual characteristic that, under visual or UV-assisted inspection, is distinct from a visual characteristic of matrix material <NUM>. For example, the plurality of microspheres includes microspheres including at least one rare earth silicate, at least one rare earth oxide, or at least one rare earth silicate and at least one rare earth oxide, and may include a composition different from the composition of matrix material <NUM>. In some examples, the at least one rare earth silicate, the at least one rare earth oxide, or the at least one rare earth silicate and the at least one rare earth oxide is present in the microspheres in a glass phase. In other examples, the at least one rare earth silicate, the at least one rare earth oxide, or the at least one rare earth silicate and the at least one rare earth oxide is present in the microspheres in a crystalline or semi-crystalline phase.

The rare earth silicate may include at least one of yttrium monosilicate (Y<NUM>SiO<NUM>), yttrium disilicate (Y<NUM>Si<NUM>O<NUM>), ytterbium monosilicate (Yb<NUM>SiO<NUM>), ytterbium disilicate (Yb<NUM>Si<NUM>O<NUM>), erbium monosilicate (Er<NUM>SiO<NUM>), erbium disilicate (Er<NUM>Si<NUM>O<NUM>), neodymium monosilicate (Nd<NUM>SiO<NUM>), neodymium disilicate (Nd<NUM>Si<NUM>O<NUM>), gadolinium monosilicate (Gd<NUM>SiO<NUM>), or gadolinium disilicate (Gd<NUM>Si<NUM>O<NUM>). The rare earth oxide may include at least one of yttrium oxide (Y<NUM>O<NUM>), ytterbium oxide (Yb<NUM>C<NUM>), erbium oxide (ErO<NUM>), neodymium oxide (Nd<NUM>O<NUM>), or gadolinium oxide (Gd<NUM>C<NUM>). The plurality of microspheres <NUM> may be substantially solid or may be hollow.

In some examples, the plurality of microspheres <NUM> may fluoresce when exposed to UV radiation. For example, the plurality of microspheres <NUM> may include at least one of yttrium monosilicate (Y<NUM>SiO<NUM>), yttrium disilicate (Y<NUM>Si<NUM>O<NUM>), ytterbium monosilicate (Yb<NUM>SiO<NUM>), or ytterbium disilicate (Yb<NUM>Si<NUM>O<NUM>), all of which may fluoresce when exposed to UV radiation. In some examples, the plurality of microspheres <NUM> possesses a visual characteristic observable in the visible wavelengths that is different from a visual characteristic of matrix material <NUM>. For example, the color of the plurality of microspheres <NUM> may be different than the color of matrix materials. For example, the plurality of microspheres <NUM> may include at least one of erbium monosilicate (Er<NUM>SiO<NUM>), erbium disilicate (Er<NUM>Si<NUM>O<NUM>), neodymium monosilicate (Nd<NUM>SiO<NUM>), neodymium disilicate (Nd<NUM>Si<NUM>O<NUM>), gadolinium monosilicate (Gd<NUM>SiO<NUM>), or gadolinium disilicate (Gd<NUM>Si<NUM>O<NUM>).

In some examples, the plurality of microspheres <NUM> may include at least one dopant. The at least one dopant may include at least one element from the Lanthanide series of the periodic table (excluding ytterbium). For example, the dopant may include at least one of 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), or lutetium (Lu). In examples in which the plurality of microspheres <NUM> include at least one dopant, the plurality of microspheres <NUM> may include between about <NUM> atomic percent (at. %) and about <NUM> at. % of the at least one dopant.

In some examples, the plurality of microspheres <NUM> may include a mean diameter, median diameter, or the like, that is less than a thickness of the layer in which the plurality of microspheres <NUM> are located. For example, the plurality of microspheres <NUM> may have a mean diameter, median diameter, or the like, that is less than the thickness of abradable coating <NUM> (measured in the y-axis direction of <FIG>, where orthogonal x-y-z axes are shown for ease of description only). In some examples, the plurality of microspheres <NUM> may include a mean diameter, median diameter, or the like, that is less than <NUM> micrometers (e.g., between <NUM> micrometer and <NUM> micrometer), or less than about <NUM> micrometers (e.g., between <NUM> micrometer and <NUM> micrometer). In some examples, at least some microspheres of plurality of microspheres <NUM> may have a diameter less than a mean or median diameter of pores in abradable coating <NUM>.

The plurality of microspheres <NUM> is located at a predetermined depth from an outer surface <NUM> of abradable coating <NUM>. For example, the predetermined depth may be a substantially consistent (e.g., consistent or nearly consistent) depth from surface <NUM> of substrate <NUM>. When at least some of the plurality of microspheres <NUM> are exposed at a surface of abradable coating <NUM> due to removal of a portion of abradable coating <NUM>, the remaining thickness of abradable coating <NUM> at that location may be determined by observing the microspheres <NUM> and knowing the distance (e.g., in the y-axis direction of <FIG>) between the plurality of microspheres <NUM> and surface <NUM> of substrate <NUM>. In some examples, this may facilitate at least one of determination of a remaining life of abradable coating <NUM>, determination of a size, depth, or both of damage to abradable coating <NUM>, and removal and repair of abradable coating <NUM> to maintain clearances between abradable coating <NUM> and substrate <NUM> below a predetermined level to maintain a desired efficiency level of the gas turbine engine.

In some examples, abradable coating <NUM> defines a thickness, measured in a direction substantially normal to surface <NUM> of substrate <NUM> (e.g., measured in the y-axis direction of <FIG>), between about <NUM> millimeters (about <NUM> inch) and about <NUM> (about <NUM> inch). The thickness of abradable coating <NUM> may be selected to be sufficient to allow abrasion of a portion of abradable coating <NUM> by a gas turbine engine blade without the blade contacting substrate <NUM>.

Abradable coating <NUM> is formed as a porous structure, which may facilitate abrasion of a portion of abradable coating <NUM> upon contact with a gas turbine engine blade or another moving component with which abradable coating <NUM> forms a seal. The abradable coating <NUM> includes porosity between about <NUM> vol. % and about <NUM> vol. In some examples, abradable coating <NUM> includes a porosity between about <NUM> vol. % and about <NUM> vol. %, or a porosity of about <NUM> vol. Porosity of abradable coating <NUM> may be defined as a volume of pores or cracks in abradable coating <NUM> divided by a total volume of abradable coating <NUM> (including both the volume of material in abradable coating <NUM> and the volume of pores/cracks in abradable coating <NUM>). In some examples, the porosity of abradable coating <NUM> may be controlled to vary throughout at least a portion of abradable coating <NUM>. For example, the porosity of abradable coating <NUM> may be controlled to be lower in a portion of abradable coating <NUM> closer to surface <NUM> of substrate <NUM> and greater in a portion of abradable coating <NUM> further from surface <NUM> of substrate <NUM>.

Abradable coating <NUM> may be formed over or formed directly on substrate <NUM> using, for example, a thermal spraying technique, such as, for example, air plasma spraying, as described in further detail with reference to <FIG>.

Abradable coating <NUM> including matrix material <NUM> and plurality of microspheres <NUM> facilitates visual inspection of a thickness of abradable coating <NUM> by including the plurality of microspheres <NUM> at a predetermined depth within the thickness of abradable coating <NUM>. In some examples, this visual inspection may occur at room temperature. The plurality of microspheres <NUM> possesses a visual characteristic under visual or UV-assisted inspection (e.g., color or fluorescence) that is distinct from a visual characteristic of matrix material <NUM>. In some examples, this may facilitate at least one of determination of a remaining life of the abradable coating <NUM>, determination of a size, depth, or both of damage to abradable coating <NUM>, or removal and repair of the abradable coating to maintain clearances between the abradable coating <NUM> and a blade tip below a predetermined level to maintain a desired efficiency level of the gas turbine engine.

In some examples, instead of including a single layer, an abradable coating may include a plurality of layers, and at least one of the layers may be an abradable layer including a matrix material and a plurality of microspheres. <FIG> is a conceptual cross-sectional diagram illustrating another example article <NUM> that includes an example abradable coating including a plurality of microspheres. In the example of <FIG>, article <NUM> that includes a substrate <NUM>, an optional bond layer <NUM> on a surface <NUM> of substrate <NUM>, an optional environmental barrier coating (EBC) <NUM> on bond layer <NUM>, a first abradable layer <NUM> on EBC <NUM>, a second abradable layer <NUM> on first abradable layer <NUM>, and a third abradable layer <NUM> on second abradable layer <NUM>. In the example illustrated in <FIG>, second abradable layer <NUM> includes a plurality of microspheres <NUM> and a matrix material <NUM>.

Article <NUM> may be similar to or substantially the same as article <NUM> of <FIG>, as described above. Article <NUM> is a turbine blade track or a turbine blade shroud component of a high temperature mechanical system, namely a gas turbine engine.

Substrate <NUM> may be similar to or substantially the same as substrate <NUM> of <FIG>, as described above. For example, substrate <NUM> may include a metal alloy that includes silicon, a ceramic, or a CMC.

As shown in <FIG>, article <NUM> may include a bond layer <NUM> on substrate <NUM>. Although not illustrated in <FIG>, in other embodiments, article <NUM> may not include a bond layer <NUM>. Bond layer <NUM> may improve adhesion between substrate <NUM> and the layer overlying bond layer <NUM> (e.g., second abradable layer <NUM> in <FIG>). Bond layer <NUM> may include any useful material that improves adhesion between substrate <NUM> and an overlying layer. For example, bond layer <NUM> may include silicon. Regardless of the composition of bond layer <NUM>, bond layer <NUM> may have a thickness of between about <NUM> micrometers (µm, about <NUM> inch) and about <NUM> (about <NUM> inch). Bond layer <NUM> may be formed on substrate <NUM> using, for example, plasma spraying, physical vapor deposition (PVD), electron beam physical vapor deposition (EB-PVD), directed vapor deposition (DVD), chemical vapor deposition (CVD), cathodic arc deposition slurry process deposition, sol-gel process deposition, electrophoretic deposition, or the like.

In some examples, article <NUM> does not include bond layer <NUM>. For example, optional EBC <NUM> may be formed directly on substrate <NUM>. Article <NUM> may not include bond layer <NUM> when the layer on substrate <NUM> and substrate <NUM> are sufficiently chemically and/or mechanically compatible. For example, in examples where EBC <NUM> and substrate <NUM> adhere sufficiently strongly to each other, article <NUM> may not include bond layer <NUM>. Additionally, in examples where the coefficients of thermal expansion of substrate <NUM> and EBC <NUM> are sufficiently similar, article <NUM> may not include bond layer <NUM>.

EBC <NUM> is on bond layer <NUM> and is optional. EBC <NUM> may reduce or substantially prevent attack of bond layer <NUM> and/or substrate <NUM> by chemical species present in the environment in which article <NUM> is utilized, e.g., in the intake gas or exhaust gas of a gas turbine engine. For example, EBC <NUM> may include a material that is resistant to oxidation or water vapor attack. EBC <NUM> may include, for example, at least one of mullite; a glass ceramic such as barium strontium aluminosilicate (BaO-SrO-Al<NUM>O<NUM>-2SiO<NUM>; BSAS), calcium aluminosilicate (CaAl<NUM>Si<NUM>Os; CAS), cordierite (magnesium aluminosilicate), and lithium aluminosilicate; or a rare earth silicates (silicates of Lu, Yb, Tm, Er, Ho, Dy, Tb, Gd, Eu, Sm, Pm, Nd, Pr, Ce, La, Y, or Sc). The rare earth silicate may be a rare earth mono-silicate (RE<NUM>SiO<NUM>, where RE stands for "rare earth") or a rare earth di-silicate (RE<NUM>Si<NUM>O<NUM>, where RE stands for "rare earth"). In some examples, EBC <NUM> is formed as a substantially non-porous layer, while in other examples, EBC <NUM> is formed as a layer that includes a plurality of cracks or pores. In some examples, EBC <NUM> may define a thickness, measured in a direction substantially normal to surface <NUM> of substrate <NUM>, between <NUM> (about <NUM> inch) and <NUM> (about <NUM> inch). EBC <NUM> may be formed using, for example, CVD; PVD, including EB-PVD and DVD; plasma spraying or another thermal spraying process; or the like.

First abradable layer <NUM> is on optional EBC <NUM>. When article <NUM> does not include EBC <NUM>, first abradable layer <NUM> may be on bond layer <NUM> or substrate <NUM> (when both EBC <NUM> and bond layer <NUM> are omitted). First abradable layer <NUM> may include a composition similar to or substantially the same (e.g., the same or nearly the same) as matrix material <NUM> of <FIG>. For example, first abradable layer <NUM> may include at least one of a rare earth silicate, a stabilized zirconium oxide, mullite, or BSAS. First abradable layer <NUM> defines a thickness, measured in a direction substantially normal to surface <NUM> of substrate <NUM>, between about <NUM> (about <NUM> inch) and about <NUM> (about <NUM> inch). First abradable layer <NUM> may be porous. In some examples, first abradable layer <NUM> includes porosity between about <NUM> vol. % and about <NUM> vol. In other examples, first abradable layer <NUM> includes porosity between about <NUM> vol. % and about <NUM> vol. %, or about <NUM> vol. In some examples, the porosity of first abradable layer <NUM> may be controlled to vary throughout at least a portion of first abradable layer <NUM>. For example, the porosity of abradable layer may be controlled to be lower in a portion of first abradable layer <NUM> closer to a surface of substrate <NUM> and greater in a portion of abradable layer further from the surface of substrate <NUM>.

Second abradable layer <NUM> is on first abradable layer <NUM>. Second abradable layer <NUM> may be similar to or substantially the same (e.g., the same or nearly the same) as abradable coating <NUM> of <FIG>. For example, the composition of second abradable layer <NUM> may include plurality of microspheres <NUM> and matrix material <NUM>. Plurality of microspheres <NUM> may be similar to or substantially the same (e.g., the same or nearly the same) as plurality of microspheres <NUM> of <FIG>. Similarly, matrix material <NUM> may be similar to or substantially the same (e.g., the same or nearly the same) as matrix material <NUM> of <FIG>. Matrix material <NUM> may be same or different than the material in first abradable layer <NUM>.

Second abradable layer <NUM> defines a thickness, measured in a direction substantially normal to surface <NUM> of substrate <NUM>, between about <NUM> (about <NUM> inch) to about <NUM> (about <NUM> inch). Second abradable layer <NUM> may be formed as a porous structure. In some examples, second abradable layer <NUM> includes porosity between about <NUM> vol. % and about <NUM> vol. In other examples, second abradable layer <NUM> includes porosity between about <NUM> vol. % and about <NUM> vol. %, or about <NUM> vol. Similar to first abradable layer <NUM>, in some examples, the porosity of second abradable layer <NUM> may be controlled to vary throughout the thickness of second abradable layer <NUM>.

Third abradable layer <NUM> is on second abradable layer <NUM>. In some examples, the composition of third abradable layer <NUM> may be similar to or substantially the same as first abradable layer <NUM>, matrix material <NUM> in second abradable layer <NUM>, or both. In some examples, the composition of third abradable layer <NUM> may be different than the composition of at least one of first abradable layer <NUM> and matrix material <NUM>. For example, third abradable layer <NUM> may include at least one of a rare earth silicate, a stabilized zirconium oxide, mullite, or BSAS. Similar to first abradable layer <NUM> and second abradable layer <NUM>, third abradable layer <NUM> may define a thickness between about <NUM> (about <NUM> inch) to about <NUM> (about <NUM> inch). Third abradable layer <NUM> also may be porous, and may have a porosity between about <NUM> vol. % and about <NUM> vol. %, between about <NUM> vol. % and about <NUM> vol. %, or about <NUM> vol.

Second abradable layer <NUM>, and, more particularly, the plurality of microspheres <NUM>, may be positioned within coating <NUM> at a predetermined depth from outer surface <NUM> of coating <NUM>. In some examples, the predetermined depth may be determined based at least in part on a clearance between a blade tip and substrate <NUM>.

Second abradable layer <NUM> may facilitate visual determination of a thickness of second abradable layer <NUM> by including the plurality of microspheres <NUM> at a predetermined depth within the thickness of the coating <NUM>. In some examples, the visual inspection of coating <NUM> may occur at room temperature. The plurality of microspheres <NUM> possesses a visual characteristic under visual or UV-assisted inspection (e.g., color or fluorescence) that is distinct from a visual characteristic of matrix material <NUM>. In some examples, this may facilitate at least one of determination of a remaining life of coating <NUM>, determination of a size, depth, or both of damage to coating <NUM>, or removal and repair of the abradable coating to maintain clearances between coating <NUM> and a blade tip below a predetermined level to maintain a desired efficiency level of the gas turbine engine.

In some examples, rather than including a single type of microspheres, a coating may include multiple types of microspheres in a single layer or in multiple layers of a coating. <FIG> is a conceptual diagram illustrating a cross-sectional view of an example article <NUM> that includes a substrate <NUM>, a bond layer <NUM>, an EBC <NUM>, a first abradable layer <NUM>, a second abradable layer <NUM>, and a third abradable layer <NUM>. In the example illustrated in <FIG>, second abradable layer <NUM> includes a first plurality of microspheres <NUM>, a second plurality of microsphere <NUM>, and a matrix material <NUM>.

Article <NUM> of <FIG> may be similar to or substantially the same as article <NUM> illustrated in and described with respect to <FIG>, aside from the differences described herein. Unlike article <NUM> in <FIG>, article <NUM> of <FIG> includes a second abradable layer <NUM> that includes first plurality of microspheres <NUM>, second plurality of microspheres <NUM>, and matrix material <NUM>.

First plurality of microspheres <NUM> and second plurality of microspheres <NUM> may be similar to or substantially the same as plurality of microspheres <NUM> of <FIG>. For example, first plurality of microspheres <NUM> and second plurality of microspheres <NUM> include at least one rare earth silicate, at least one rare earth oxide, or at least one rare earth silicate and at least one rare earth oxide. In some examples, the composition of first plurality of microspheres <NUM> may be different than the composition of second plurality of microspheres <NUM>. For example, first plurality of microspheres <NUM> may include a first rare earth silicate, and second plurality of microspheres <NUM> may include a second, different rare earth silicate. First plurality of microspheres <NUM> may be located at a first predetermined depth from an outer surface <NUM> of coating <NUM>. As shown in <FIG>, second plurality of microspheres <NUM> may be located at a second predetermined depth from outer surface <NUM>. The first and second predetermined depth are different from each other. First plurality of microspheres <NUM> possesses a visual characteristic under visual or UV-assisted inspection that is distinct from a visual characteristic of matrix material <NUM> and a visual characteristic of second plurality of microspheres <NUM>.

Coating <NUM> illustrated in <FIG>, which includes first and second plurality of microspheres <NUM>, <NUM> facilitates visual determination of a thickness of second abradable layer <NUM> at two predetermined depths within the thickness of coating <NUM>. As described above, the plurality of microspheres may be used to determine a remaining thickness of coating <NUM>. A coating including microspheres located at multiple, different depths from outer surface <NUM> may facilitate more precise determination of the remaining thickness of coating <NUM>. Although <FIG> illustrates an example in which second abradable layer <NUM> includes two types of microspheres <NUM> and <NUM>, in other examples, second abradable layer <NUM> may include more than two types of microspheres. For example, second abradable layer <NUM> may include a plurality of types of microspheres or at least two types of microspheres.

In the example illustrated in <FIG>, first plurality of microspheres <NUM> and second plurality of microspheres <NUM> are disposed within the same layer of coating <NUM> (second abradable layer <NUM>). In other examples, first plurality of microspheres <NUM> and second plurality of microspheres <NUM> may be disposed in different layers of a coating. <FIG> is a conceptual cross-sectional diagram illustrating another example article <NUM> that includes an example coating <NUM> including a plurality of microspheres. Article <NUM> includes a substrate <NUM>, a bond layer <NUM> on substrate <NUM>, an EBC <NUM> on bond layer <NUM>, a first abradable layer <NUM> on EBC <NUM>, a second abradable layer <NUM> on first abradable layer <NUM>, a third abradable layer <NUM> on second abradable layer <NUM>, and a fourth abradable layer <NUM> on third abradable layer <NUM>. Article <NUM> of <FIG> may be similar to or substantially the same as article <NUM> illustrated in and described with respect to <FIG>, aside from the differences described herein. In the example illustrated in <FIG>, second abradable coating <NUM> includes a first plurality of microspheres <NUM> and a matrix material <NUM>, and third abradable layer <NUM> includes a second plurality of microspheres <NUM> and a matrix material <NUM>.

First plurality of microspheres <NUM> and second plurality of microspheres <NUM> may be similar to or substantially the same as plurality of microspheres <NUM> of <FIG>. For example, first plurality of microspheres <NUM> and second plurality of microspheres <NUM> may include at least one rare earth silicate. In some examples, the composition of first plurality of microspheres <NUM> may be different than the composition of second plurality of microspheres <NUM>. For example, first plurality of microspheres <NUM> may include a first rare earth silicate, and second plurality of microspheres <NUM> may include a second, different rare earth silicate. First plurality of microspheres <NUM> may be located in second abradable layer <NUM> at a first predetermined depth from an outer surface <NUM> of coating <NUM>. As shown in <FIG>, second plurality of microspheres <NUM> may be located in third abradable layer <NUM> at a second predetermined depth from outer surface <NUM>. The first and second predetermined depths are different from each other. First plurality of microspheres <NUM> possesses a visual characteristic under visual or UV-assisted inspection that is distinct from a visual characteristic of matrix material <NUM> and second plurality of microspheres <NUM> possesses a visual characteristic that is distinct from a visual characteristic of matrix material <NUM>. In some examples, the visual characteristic of first plurality of microspheres <NUM> is different from the visual characteristic of second plurality of microspheres <NUM>.

Coating <NUM> illustrated in <FIG>, which includes first and second plurality of microspheres <NUM> and <NUM> facilitates visual determination of a thickness of coating <NUM> at two predetermined depths within the thickness of coating <NUM>. As described above, the plurality of microspheres may be used to determine a remaining thickness of the coating <NUM>. A coating <NUM> including microspheres located at multiple, different depths from outer surface <NUM> may facilitate more precise determination of the remaining thickness of coating <NUM>. Although <FIG> illustrates an example in which coating <NUM> includes two abradable layers <NUM> and <NUM> including a respective plurality of microspheres <NUM> and <NUM>, in other examples, coating <NUM> may include more than two layers including microspheres. For example, coating <NUM> may include a plurality of layers including microspheres or at least two layers including microspheres.

As described above, in some examples, the layers including a plurality of microspheres may be formed using air plasma spraying (APS). <FIG> are conceptual diagrams illustrating example techniques for forming an abradable coating <NUM> including a plurality of microspheres <NUM> using air plasma spraying. In the examples illustrated in <FIG>, an article <NUM> includes an abradable coating <NUM> on surface <NUM> of substrate <NUM>. Abradable coating <NUM> includes a plurality of microspheres <NUM> and a matrix material <NUM>. The APS technique may utilize an APS gun <NUM> to spray plurality of microspheres <NUM> and matrix material <NUM> onto surface <NUM> of substrate <NUM>.

In the example illustrated in <FIG>, plurality of microspheres <NUM> and matrix material <NUM> in powder form may be mixed upstream of APS gun <NUM>, then sprayed onto abradable coating <NUM>. In some examples, the plurality of microspheres <NUM> may be deformed during the APS technique when passed through the APS gun <NUM>, e.g., due to softening or melting of the plurality of microspheres <NUM>. Hence, in some examples, rather than the plurality of microspheres <NUM> being mixed with matrix material <NUM> upstream of APS gun <NUM>, the plurality of microspheres <NUM> and matrix material <NUM> may be mixed downstream of APS gun <NUM>, as shown in <FIG>.

<FIG> is a flow diagram illustrating an example technique for forming an example abradable coating including a plurality of microspheres. The technique of <FIG> will be described with respect to article <NUM> of <FIG>. However, the technique of <FIG> may be used to form other articles, and article <NUM> of <FIG> may be formed using other techniques. In the example illustrated in <FIG>, the technique optionally includes forming bond layer <NUM> on substrate <NUM> (<NUM>). Bond layer <NUM> may be formed on substrate <NUM> using plasma spraying; PVD, such as EB-PVD or DVD; CVD; cathodic arc deposition; slurry deposition; sol-gel deposition; electrophoretic deposition; or the like. In some examples, article <NUM> does not include bond layer <NUM>, and the technique of <FIG> does not include forming bond layer <NUM> on substrate <NUM> (<NUM>).

The technique also optionally includes forming EBC <NUM> on optional bond layer <NUM> (or substrate <NUM> if bond layer <NUM> is not present) (<NUM>). EBC <NUM> may be formed using, for example, CVD; PVD, including EB-PVD and DVD; plasma spraying or another thermal spraying process; or the like. In some examples, article <NUM> does not include EBC <NUM>, and the technique of <FIG> does not include forming EBC <NUM> on bond layer <NUM> (or substrate <NUM> if bond layer <NUM> is not present) (<NUM>).

The technique of <FIG> also includes forming an optional first abradable layer <NUM> on EBC <NUM> (<NUM>). As both EBC <NUM> and bond layer <NUM> are optional, first abradable layer <NUM> also may be formed on bond layer <NUM> or substrate <NUM>. When present in coating <NUM>, first abradable layer <NUM> may be formed using, for example, a thermal spraying technique, such as air plasma spraying or the like.

The technique of <FIG> also includes forming second abradable layer <NUM> on optional first abradable layer <NUM> (<NUM>). As first abradable layer <NUM>, EBC <NUM> and bond layer <NUM> are optional, second abradable layer <NUM> also may be formed on EBC <NUM>, bond layer <NUM>, or substrate <NUM>. In some examples, second abradable layer <NUM> may be formed using air plasma spraying, such as one of the technique illustrated with respect to <FIG>. In other examples, plurality of microspheres <NUM> may be applied directly to the surface of the underlying layer (e.g., the surface of first abradable layer <NUM>). The plurality of microspheres <NUM> may be deposited using slurry deposition or application of an adhesive loaded with the plurality of microspheres <NUM>. After application of the plurality of microspheres, matrix material <NUM> is applied, e.g., using or slurry deposition, to form second abradable layerr28.

The technique of <FIG> further optionally includes forming third abradable layer <NUM> on second abradable layer <NUM> (<NUM>). When present in coating <NUM>, third abradable layer <NUM> may be formed using, for example, a thermal spraying technique, such as air plasma spraying or the like.

In some examples (not within the scope of the claims), rather than including a plurality of microspheres that have a visual characteristic distinct from a visual characteristic of surrounding matrix material, a coating may include a plurality of layers within the coating that each have distinct visual characteristics compared to surrounding layers. <FIG> is a conceptual cross-sectional diagram illustrating another example article that includes an example coating <NUM> including a neodymium silicate layer, an erbium silicate layer, and a gadolinium silicate layer.

Article <NUM> of <FIG> may be similar to or substantially the same as article <NUM> illustrated in and described with respect to <FIG>, aside from the differences described herein. For example, substrate <NUM>, bond layer <NUM>, EBC <NUM>, and first abradable layer <NUM>, may be similar to or substantially the same as the corresponding structures described with respect to <FIG>. Second abradable layer <NUM> may be similar to or substantially the same as third abradable layer <NUM> in <FIG>. Unlike article <NUM> in <FIG>, article <NUM> of <FIG> includes at least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM>.

At least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM> are between first abradable layer <NUM> and second abradable layer <NUM>. Although <FIG> illustrates at least one gadolinium silicate layer <NUM> on at least one erbium silicate layer <NUM> and at least one erbium silicate layer <NUM> on at least one neodymium silicate layer <NUM>, in other examples, the order of these layers may be changed into any order. Additionally or alternatively, coating <NUM> may include more than one of at least one neodymium silicate layer <NUM>, more than one of at least one erbium silicate layer <NUM>, or more than one of at least one gadolinium silicate layer <NUM>. In some examples, coating <NUM> may include multiple sets of at least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM>. In other examples, coating <NUM> may include a different numbers of the at least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM> (e.g., more neodymium silicate layers <NUM> than erbium silicate layers <NUM> and gadolinium silicate layers <NUM>, or the like).

At least one neodymium silicate layer <NUM> may include neodymium monosilicate or neodymium disilicate. At least one erbium silicate layer <NUM> may include erbium monosilicate or erbium disilicate. At least one gadolinium silicate layer <NUM> may include gadolinium monosilicate, or gadolinium disilicate. In some examples, the erbium monosilicate, erbium disilicate, neodymium monosilicate, neodymium disilicate, gadolinium monosilicate, or gadolinium disilicate may be doped with between about <NUM> at. % and about <NUM> at. % of an element selected from the Lanthanide series of the periodic table, excluding ytterbium.

At least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM> may be located at respective a predetermined depths from an outer surface <NUM> of coating <NUM>. In some examples, at least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM> each defines a thickness, measured in a direction substantially normal to surface <NUM> of substrate <NUM>, of between about <NUM> (about <NUM> inch) and about <NUM> (about <NUM> inch). The thicknesses of at least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM> may be the same or may be different. The positions and thicknesses of at least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM> may be determined from clearance requirements between the blade and seal segment (or blade track), such that the layers <NUM>, <NUM>, and <NUM> are located at a depth from surface <NUM> that approximate corresponds to an depth to which coating <NUM> is abraded during use. At least one neodymium silicate layer <NUM>, at least one erbium silicate layer <NUM>, and at least one gadolinium silicate layer <NUM> may be applied by APS or by physical vapor deposition (e.g., EB-PVD, DVD, or the like).

Each of layers <NUM>, <NUM>, and <NUM> possesses a distinct visual characteristic (e.g., color) that is different than a visual characteristic of an adjacent layer. Layers <NUM>, <NUM>, and <NUM> may facilitate visual inspection of a thickness of coating <NUM> by including the plurality of layers <NUM>, <NUM>, and <NUM> at a predetermined depth within the thickness of coating <NUM>. In some examples, this visual inspection may occur at room temperature. Coating <NUM>, which includes a plurality of layers <NUM>, <NUM>, and <NUM> (e.g., at least three) located at multiple, different depths from outer surface <NUM>, may facilitate more precise determination of the remaining thickness of coating <NUM>. In some examples, this may facilitate at least one of determination of a remaining life of the coating <NUM>, determination of a size, depth, or both of damage to coating <NUM>, or removal and repair of coating <NUM> to maintain clearances between coating <NUM> and a blade tip below a predetermined level to maintain a desired efficiency level of the gas turbine engine.

Claim 1:
A system comprising:
a turbine blade track or a turbine blade shroud (<NUM>) comprising a substrate (<NUM>); and
a coating on the substrate (<NUM>), wherein the coating comprises at least one abradable layer (<NUM>), wherein the at least one abradable layer (<NUM>) is porous, wherein the at least one abradable layer (<NUM>) comprises a matrix material (<NUM>) and a plurality of microspheres (<NUM>) located within the at least one abradable layer (<NUM>) at a predetermined depth from an outer surface (<NUM>) of the coating, wherein the plurality of microspheres (<NUM>) define a visual characteristic distinct from the matrix material (<NUM>), and wherein the plurality of microspheres (<NUM>) comprise at least one rare earth oxide, at least one rare earth silicate, or at least one rare earth oxide and at least one rare earth silicate; and
a gas turbine blade, wherein the gas turbine blade is configured to contact a portion of the at least one abradable layer (<NUM>) on the substrate (<NUM>) during rotation of the gas turbine blade, and wherein the at least one abradable layer (<NUM>) is configured to be abraded to at least the predetermined depth by the contact by the gas turbine blade.