Patent Description:
The components of gas turbine engines operate in severe environments. For example, the high-pressure turbine airfoils exposed to hot gases in commercial aeronautical engines typically experience surface temperatures of about <NUM>. Components of high-temperature mechanical systems may include a superalloy substrate or other type metallic substrate. In many examples, the substrates may be coated with one or more coatings to modify properties of the surface of the substrate. For example, metallic substrates may be coated with a thermal barrier coating to reduce heat transfer from the external environment to the substrate, and/or an environmental barrier coating to reduce exposure of the substrate to environmental species, such as oxygen, water vapor, or Calcia-Magnesia-Alumina Silicate (CMAS) containing materials, and/or an abradable coating to improve a seal. <CIT> describes, in some examples, a system that includes a blade track or blade shroud and a gas turbine blade that includes a blade tip. The blade track or blade shroud may include a substrate and an abradable layer formed over the substrate. The abradable layer may include at least one of zirconia or hafnia; ytterbia; samaria; and at least one of lutetia, scandia, ceria, gadolinia, neodymia, or europia. The abradable layer may include a porosity between about <NUM> vol. % and about <NUM> vol. The blade track or blade shroud and the gas turbine blade may be configured so the blade tip contacts a portion of the abradable layer during rotation of the gas turbine blade, and the abradable layer may be configured to be abraded by the contact by the blade tip. <CIT> describes, in some examples, a component includes a substrate and a noncontinuous abradable coating on the substrate. The abradable coating includes a first portion defining a first plurality of coating blocks, a second portion defining a second plurality of coating blocks, and a blade rub portion extending between the first portion and the second portion and defining a third plurality of coating blocks. At least one of the first plurality of coating blocks or the second plurality of coating blocks is different than the third plurality of coating blocks in at least one coating block parameter. <CIT> describes a turbine article includes a substrate with a geometric surface having a multiple of divots recessed into the substrate, and a ceramic topcoat disposed over the geometric surface, the topcoat including at least a first layer having a first hardness and a second layer having a second hardness, the first hardness different than the second hardness.

In general, the present disclosure describes example articles including CMAS resistant abradable coating systems on metallic substrates, such as superalloy substrates, and techniques and systems for manufacturing the same. The CMAS resistant abradable coating systems may including a plurality of abradable layers in an alternating arrangement with a plurality of CMAS resistant layers. In various examples, the layered CMAS resistant abradable coatings may allow for a component defining a seal segment in a gas turbine engine that exhibits a relatively high level of CMAS resistance at varying blade cut depths, while also abrading in a manner that allows for a beneficial seal for the component relative to the tip of an opposing part to enhance the power and efficiency of the gas turbine engine.

According to a first aspect of the present invention, there is provided an article including a metallic substrate; and an abradable coating on the metallic substrate, the abradable coating comprising a plurality of abradable layers in an alternating arrangement with a plurality of CMAS resistant layers, wherein the plurality of CMAS resistant layers are configured to react with molten CMAS to form a stable phase.

According to a second aspect of the present invention, there is provided an abradable system including a metallic substrate; an abradable coating on the metallic substrate, the abradable coating comprising a plurality of abradable layers in an alternating arrangement with a plurality of CMAS resistant layers, wherein the plurality of CMAS resistant layers are configured to react with molten CMAS to form a stable phase; and an opposing abrasive element acting on the abradable coating.

In another unclaimed aspect, the present disclosure is directed to a method comprising forming an abradable coting on a metallic substrate, the abradable coating comprising a plurality of abradable layers in an alternating arrangement with a plurality of CMAS resistant layers, wherein the plurality of CMAS resistant layers are configured to react with molten CMAS to form a stable phase.

Like symbols in the drawings indicate like elements.

In the context of gas turbine engines, increasing demands for greater operating efficiency (e.g., fuel efficiency) has led to the operation of gas turbine engines at higher temperatures. In some examples, substrates, such as metallic substrates, of high-temperature mechanical systems are coated with a TBC system to provide thermal protection as well as environmental protection, in some instances, for the underlying substrate material(s) in a high temperature environment. In some examples, the topcoat for a TBC system may include a Yttria-stabilized zirconia (YSZ)-based TBC composition (e.g. 7YSZ) for vane and blade components of an engine and a YSZ-based TBC composition with a Mg-Spinel abradable coating for seal segments. The YSZ-based TBC composition may be applied via electron beam-physical vapor deposition (EB-PVD) or air plasma spraying (APS) while the Mg-Spinel may be applied via APS.

However, such coatings may be deleteriously attacked from CMAS due to the higher operating conditions of the engine and flying in regions with greater concentrations of CMAS on the ground and in the air. For example, increased operating temperatures, may lead to increased damage due to the presence of CMAS deposits within the high temperature environments. The presence of CMAS deposits in the high temperature environments of a gas turbine engine may result from the ingestion of siliceous minerals (e.g., airborne dust, sand, volcanic dust and ashes, fly ash, runway debris, concrete dust, fuel residue, and the like) in the intake gas drawn through the gas turbine engine. For example, when siliceous debris such as sand and dust is ingested into the engine, it can melt and deposit on coated hot section components (e.g. seal segments, vanes and blades). These partial or fully molten deposits are commonly referred to as calcium magnesium aluminosilicates ("CMAS") because the primary oxide constituents are CaO, MgO, Al¬2O3 and SiO2. Once molten CMAS has deposited on the surface of TBCs and/or abradable coatings, the CMAS may dissolve, react and/or infiltrate the coating system which leads to coating recession and/or spallation. Therefore, it may be preferable that coating systems including TBC and/or abradable coatings possess sufficient CMAS resistance to meet coating life requirements.

For gas turbine seal segments, it is desirable to have a layer of abradable coating, such as a Mg-Spinel that turbine blade tips cut into during operation. Due to airborne CMAS, the abradable coating should also have high CMAS resistance. It has been found that certain materials such as gadolinium zirconate (GdZO), higher yttria stabilized zirconia (e.g., greater than <NUM> wt% yttria), and yttrium aluminate garnets (YAG) may have relatively high CMAS resistance and, thus, it may be preferable to have one or more layers of such materials on the flow side path of an abradable coating. GdZO or other CMAS tolerant coatings may, however, have a relatively low fracture toughness, thereby making such coatings ideal for an abradable coating material.

Due to manufacturing tolerances and relative thermal/mechanical deformation of the blades and seal segments, the blade tip cut depth can vary from one seal segment to another, and from one turbine engine to another. For example, the designed nominal blade cut-depth can be <NUM>, but the deepest blade cut-depth could be as high as <NUM>. Under such cut conditions, it may be desirable to have a CMAS resistant abradable coating that can accommodate a variable cut depth.

In accordance with examples of the disclosure, a CMAS resistant abradable coating system including a is described. The CMAS resistant abradable coating may be applied on a metallic substrate, e.g., a super alloy substrate. The CMAS resistant abradable coating systems includes a plurality of abradable layers in an alternating arrangement with a plurality of CMAS resistant layers. In various examples, the layered CMAS resistant abradable coatings may allow for a component defining a seal segment in a gas turbine engine that exhibits a relatively high level of CMAS resistance at varying blade cut depths, while also abrading in a manner that allows for a beneficial seal for the component relative to the tip of an opposing part to enhance the power and efficiency of the gas turbine engine.

In some examples, the abradable coating systems according to the present disclosure may allow high-temperature components having metallic substrates, such as metallic substrate-based seal segments, to more safely operate in relatively higher temperature, steamy, or dusty environments, and may provide better coating strength, better resistance to oxidation, water vapor, and CMAS attack, or combinations thereof. In addition to the CMAS resistance, the coating system may also be abradable, e.g., by an opposing blade tip that rotates during operation, to provide a seal in the seal segment to enhance the power and efficiency of the gas turbine engine.

The abradable layers of the coating system may be configured to abrade in the manner described herein but may have relatively low CMAS resistance (e.g., as a result of the layers exhibiting a relatively high porosity or other void volume. Conversely, in some examples, the CMAS resistant layers of the coating system may react with CMAS to form stable products that slow the reaction/infiltration/penetration rate of CMAS. Additionally, or alternatively, the multiple interfaces formed between the alternating layers may prevent or slow down the propagation of thermal shock cracks (e.g., that originate at the interface of the coating system and underlying substrate) through the coating system, to eliminate or reduce pathways through the coating that CMAS may infiltrate. Thus, the coating system may resist or arrest CMAS infiltration through the coating system and restrict CMAS effects to only an outer region of the coating system, thus more effectively protecting the underlying metallic substrate.

<FIG> illustrates a perspective diagram of an example article <NUM> that may be used in a high-temperature mechanical system. As described above, article <NUM> includes a substrate <NUM>, optional bond coat <NUM> deposited on substrate <NUM>, and CMAS resistant abradable coating <NUM> deposited on bond coat <NUM> and substrate <NUM>. The respective individual discrete layers of CMAS resistant abradable coating <NUM> are not shown in <FIG>.

Article <NUM> may be a high-temperature component, for example, an industrial, automotive, or aeronautical component. In some examples, article <NUM> may be a component of a high-temperature mechanical system, such as, for example, a gas turbine engine or the like. In some examples, article <NUM> may form a portion of a flow path structure, a seal segment, a blade track, blade shroud, an airfoil, a blade, a vane, a combustion chamber liner, or another portion of a gas turbine engine article <NUM>. While the description herein may be directed to a gas turbine blade track or shroud, it will be understood that the disclosure is not limited to such examples. Rather, CMAS resistant abradable coating <NUM> may be deposited over any article which requires or may benefit from an abradable coating with a relatively high level of CMAS resistance. For example, coating system <NUM> may be deposited on a cylinder of an internal combustion engine, an industrial pump, a housing or internal seal ring of an air compressor, or an electric power turbine.

While operating article <NUM> in high-temperature environments, a rotating component (e.g., blade tip <NUM>) may abrade coating system <NUM> to cut track <NUM> in coating system <NUM>. The thickness of coating system <NUM> may be selected such that track <NUM> does not penetrate all the way through coating system <NUM> into bond coat <NUM> and/or substrate <NUM>. In the case of a turbine, as the turbine blade rotates, tip <NUM> of the turbine blade contacts coating system <NUM> and wears away a portion of coating system <NUM> to form track <NUM> in the coating system corresponding to the path of the turbine blade. The intimate fit between the blade tip and coating system <NUM> provides a seal that can reduce the clearance gap between the rotating component and an inner wall of the track or shroud, which can reduce leakage around a tip of the rotating part to enhance the power and efficiency of the gas turbine engine.

<FIG> is a conceptual cross-sectional view of article <NUM> including a substrate <NUM>. Substrate <NUM> may be a metallic substrate such as a superalloy substrate. As shown, article <NUM> may include an optional bond coat <NUM>, optional thermal barrier coating layer <NUM>, and CMAS resistant abradable coating <NUM> on substrate <NUM>.

Substrate <NUM> may include a material suitable for use in a high-temperature environment. In some examples, substrate <NUM> includes a superalloy including, for example, an alloy based on Ni, Co, Ni/Fe, or the like. In examples in which substrate <NUM> includes a superalloy material, substrate <NUM> may also include one or more additives such as titanium (Ti), cobalt (Co), or aluminum (Al), which may improve the mechanical properties of substrate <NUM> including, for example, toughness, hardness, temperature stability, corrosion resistance, oxidation resistance, or the like.

In the example of <FIG>, an optional bond coating <NUM> is formed on all or a part of a surface <NUM> of substrate <NUM>, e.g., to promote adherence or retention of overlaying layers on the substrate <NUM> (e.g., optional TBC layer <NUM> and/or coating system <NUM>. As used herein, "formed on" and "on" mean a layer or coating that is formed on top of another layer or coating, and encompasses both a first layer or coating formed immediately adjacent a second layer or coating and a first layer or coating formed on top of a second layer or coating with one or more intermediate layers or coatings present between the first and second layers or coatings. In contrast, "formed directly on" and "directly on" denote a layer or coating that is formed immediately adjacent another layer or coating, e.g., there are no intermediate layers or coatings.

In some examples, as shown in <FIG>, bond coat <NUM> may be directly on the substrate <NUM>. In other examples, one or more coatings or layers of coatings may be between bond coat <NUM> and substrate <NUM>.

Bond coat <NUM> may be between optional TBC layer <NUM>/CMAS resistant abradable coating system <NUM> and substrate <NUM>, and may increase the adhesion of optional TBC layer <NUM>/CMAS resistant abradable coating system <NUM> to substrate <NUM>. Bond coat <NUM> may be in direct contact with substrate <NUM> and TBC layer <NUM>. In cases in which optional TBC layer <NUM> is not present, bond coat <NUM> may be in direct contact with CMAS resistant abradable coating system <NUM>. In some examples, bond coat <NUM> has a thickness of approximately <NUM> microns to approximately <NUM> microns, although other thicknesses are contemplated.

Bond coat <NUM> may include any suitable material configured to improve adhesion between substrate <NUM> and TBC layer <NUM>/CMAS resistant abradable coating system <NUM>. In some examples, bond coat <NUM> may include an alloy, such as an MCrAlY alloy (where M is Ni, Co, or NiCo), a β-NiAl nickel aluminide alloy (either unmodified or modified by Pt, Cr, Hf, Zr, Y, Si, and combinations thereof), a γ-Ni + γ'-Ni<NUM>Al nickel aluminide alloy (either unmodified or modified by Pt, Cr, Hf, Zr, Y, Si, and combination thereof), or the like. The composition of bond coat <NUM> may be selected based on the chemical composition and/or phase constitution of substrate <NUM> and the overlying layer (e.g., TBC layer <NUM>/CMAS resistant abradable coating system <NUM> of <FIG>). For example, if substrate <NUM> includes a superalloy with a γ-Ni + γ'-Ni<NUM>Al phase constitution, bond coat <NUM> may include a γ-Ni + γ'-Ni<NUM>Al phase constitution to better match the coefficient of thermal expansion of the superalloy substrate <NUM>. In turn, the mechanical stability (e.g., adhesion) of bond coat <NUM> to substrate <NUM> may be increased.

In some cases, bond coat <NUM> may include a single layer or multiple layers having substantially the same or different compositions. In some examples, bond coat <NUM> includes multiple layers to provide multiple functions of bond coat <NUM>, such as, for example, adhesion of substrate <NUM> to an overlying layer (e.g., TBC layer <NUM>/CMAS resistant abradable coating system <NUM> of <FIG>), chemical compatibility of bond coat <NUM> with each of substrate <NUM> and the overlying layer, a better coefficient of thermal expansion match of adjacent layers, or the like.

Bond coat <NUM> may be applied on substrate <NUM> using, for example, thermal spraying, e.g., air plasma spraying, high velocity oxy-fuel (HVOF) spraying, low vapor plasma spraying, suspension plasma spraying; physical vapor deposition (PVD), e.g., electron beam physical vapor deposition (EB-PVD), directed vapor deposition (DVD), cathodic arc deposition; chemical vapor deposition (CVD); slurry process deposition; sol-gel process deposition; electrophoretic deposition; or the like.

As shown in <FIG>, article <NUM> may include optional thermal barrier coating (TBC) layer <NUM> on the optional bond coating <NUM>, or on all or a portion of the surface <NUM> of the substrate <NUM>. TBC layer <NUM> may provide thermal cycling resistance, a low thermal conductivity, erosion resistance, combinations thereof, or the like. In some examples, TBC layer <NUM> may include zirconia or hafnia stabilized with one or more metal oxides, such as one or more rare earth oxides, alumina, silica, titania, alkali metal oxides, alkali earth metal oxides, or the like. For example, TBC layer <NUM> may include yttria-stabilized zirconia (ZrO<NUM>) or yttria-stabilized hafnia, or zirconia or hafnia mixed with an oxide of one or more of lutetium (Lu), ytterbium (Yb), thulium (Tm), erbium (Er), holmium (Ho), dysprosium (Dy), gadolinium (Gd), terbium (Tb), europium (Eu), samarium (Sm), promethium (Pm), neodymium (Nd), praseodymium (Pr), cerium (Ce), lanthanum (La), yttrium (Y), or scandium (Sc).

As one example, TBC layer <NUM> may include a rare earth oxide-stabilized zirconia or hafnia layer including a base oxide of zirconia or hafnia and at least one rare-earth oxide, such as, for example, at least one oxide of lutetium (Lu), ytterbium (Yb), thulium (Tm), erbium (Er), holmium (Ho), dysprosium (Dy), gadolinium (Gd), terbium (Tb), europium (Eu), samarium (Sm), promethium (Pm), neodymium (Nd), praseodymium (Pr), cerium (Ce), lanthanum (La), yttrium (Y), or scandium (Sc). In some such examples, TBC layer <NUM> may include predominately (e.g., the main component or a majority) the base oxide including zirconia or hafnia mixed with a minority amount of the at least one rare-earth oxide. In one example, TBC layer <NUM> may have a composition substantially the same or similar to that described below for abradable layers <NUM>.

TBC layer <NUM> may have any suitable thickness. While TBC layer <NUM> is shown at a single layer, in other examples, article <NUM> may include multiple TBC layers having the same or different compositions.

Article <NUM> include CMAS resistant abradable coating system <NUM>. In the example of <FIG>, coating system <NUM> is on a surface <NUM> of TBC layer <NUM>. In other examples, coating system <NUM> may be on bond coat <NUM> or directly on surface <NUM> of substrate <NUM>.

In one example, coating system <NUM> may be on a flowpath surface above the moving metal blade tips in a turbine engine can reduce over tip leakage and improve efficiencies. For example, as described above, a rotating part may abrade a portion of a fixed abradable coating applied on an adjacent stationary part to form a seal having a very close tolerance. In an example, which is not intended to be limiting, abradable coatings are used in aircraft jet engines in the compressor and turbine sections where a minimal clearance is needed between the blade tips and a casing. For example, an abradable coating can be used to minimize the clearance between blade tips and an inner wall of an opposed track or shroud, which can reduce leakage or guide leakage flow of a working fluid, such as steam or air, across the blade tips, and thereby enhance turbine efficiency.

CMAS resistant abradable coating system18 includes a plurality of CMAS resistant layers <NUM> in an alternating arrangement with a plurality of abradable layers <NUM>. For example, as shown in <FIG>, coating system <NUM> includes a repeating pattern of a single discrete CMAS resistant layer and single discrete abradable layer <NUM> with a CMAS resistant layer <NUM> being nearest substrate <NUM> and also another CMAS resistant layer <NUM> forming the topcoat of coating system <NUM>. In other examples, an abradable layer <NUM> may be nearest substrate <NUM> and/or may form the topcoat of coating system <NUM>. In some examples, coating system <NUM> may include at least two abradable layers <NUM> and at least two CMAS resistant layers <NUM>. The total number of layers of coating system <NUM> may be from four layers to <NUM> layers, such as <NUM> layers to <NUM> layers. In the example of <FIG>, CMAS resistant layers <NUM> and abradable layers <NUM> are directly on each other in the alternating arrangement, e.g., rather than having another intervening layer in between a respective CMAS resistant layer <NUM> and a respective abradable layer <NUM> at one or more locations in the alternating layer stack.

Each of plurality of abradable layers <NUM> may be configured to abrade, e.g., when brought into contact with an opposing surface such as a blade tip <NUM> (<FIG>). In some examples, abradable layers <NUM> may have a different composition, microstructure and/or porosity compared to that of CMAS resistant layers <NUM> so that abradable layers <NUM> are more abradable than CMAS resistant layers <NUM> but less CMAS resistant than CMAS resistant layers <NUM>. For example, one or more (e.g., substantially all) of abradable layers <NUM> may have a greater porosity or other void volume than one or more (e.g., substantially all) CMAS resistant layers <NUM>. The greater porosity or other void volume of abradable layers <NUM> may result in abradable layers <NUM> being more abradable than CMAS resistant layers <NUM> but less resistant to CMAS infiltration through layers <NUM> compared to layers <NUM>. As another example, one or more (e.g., substantially all) of abradable layers <NUM> may have a composition that is different than the composition of one or more (e.g., substantially all) of CMAS resistant layers <NUM>, where the composition of CMAS resistant layers <NUM> is configured to react with molten CMAS to form a stable phase while the composition of abradable layers <NUM> does not react in such a manner.

Abradable layers <NUM> may have any suitable composition. In some example, abradable layers <NUM> may comprise, consist or consist essentially of a magnesium (Mg)-spinel (MgAl<NUM>O<NUM>). A non-stochiometric spinel may be employed, such as, a spinel with about <NUM> wt% alumina and about <NUM> wt% magnesia, for abradable layer <NUM>. Other example compositions may include a zirconia/rare earth mixture, alumina, ytterbium and/or other ceramic material. Each respective layer of the plurality of abradable layers <NUM> in the alternating arrangement may have the same or substantially similar composition or, in other examples, the composition of one or more the respective layers <NUM> may be different than the composition of other respective layers <NUM> that make up the total number of abradable layers <NUM>.

Abradable layers <NUM> may have any suitable microstructure. In some examples, abradable layer <NUM> may be relatively porous, e.g., with a porosity of about <NUM>% or greater or a fugitive phase of about <NUM>% or greater when formed and that is subsequently burned off. Example fugitive materials may include polyester, HBn, or the like. Each respective layer of the plurality of abradable layers <NUM> in the alternating arrangement may have the same microstructure or, in other examples, the microstructure of one or more the respective layers <NUM> may be different than the microstructure of other respective layers <NUM> that make up the total number of abradable layers <NUM>.

Abradable layers <NUM> may have any suitable porosity or other void volume. The porosity or other void volume may allow for abradable layers <NUM> to be abraded in the manner described herein. As described above, in some examples, the porosity or other void volume of abradable layers <NUM> may be greater than CMAS resistant layers <NUM>. Abradable layers <NUM> may include a porosity of, e.g., greater than about <NUM> volume percent (vol. %), such as about <NUM> vol. % to about <NUM> vol. % or about <NUM> vol. % to about <NUM> vol. %, where porosity is measured as a percentage of pore volume divided by total volume of abradable layers <NUM> (either on a individual layer basis or overall layer basis). In each case, the porosity of layers <NUM> may be measured using mercury porosimetry, optical microscopy or Archimedean method. In some examples, abradable layers <NUM> may include a void volume of, e.g., greater than about <NUM> volume percent (vol. %), such as about <NUM> vol. % to about <NUM> vol. % or about <NUM> vol. % to about <NUM> vol. %, where the void volume is the volume of void space over total volume. Each respective layer of the plurality of abradable layers <NUM> in the alternating arrangement may have the same or substantially similar void volume or, in other examples, the void volume of one or more the respective layers <NUM> may be different than the void volume of other respective layers <NUM> that make up the total number of abradable layers <NUM>.

CMAS resistant layers <NUM> may be less abradable than abradable layers <NUM> but may be more resistant to CMAS, e.g., to molten CMAS that may infiltrate coating system <NUM>. In some examples, the composition of CMAS resistant layers <NUM> may be configured to react with molten CMAS to form stable products that slow the reaction/infiltration/penetration rate of CMAS, e.g., through the respective CMAS resistant layers <NUM>. For example, the reaction product may be an apatite phase.

In some examples, CMAS resistant layers <NUM> may comprise, consist or consist essentially of GdZO, yttria stabilized zirconia, yttrium aluminate garnet(s) (YAG), YAlO - Yttrium Aluminate Garnets YAG, or some combination thereof. As one example, CMAS resistant layers <NUM> may comprise, consist or consist essentially of yttria stabilized zirconia with at least about <NUM> wt% yttria, such as about <NUM> wt% to about <NUM> wt% yttria, e.g., with a remainder being zirconia. As another example, CMAS resistant layers <NUM> may comprise, consist or consist essentially of Gd doped zirconia, with at least about <NUM> wt% Gd, such as about <NUM> wt% to about <NUM> wt% Gd, e.g., with a remainder being zirconia. The reaction product between GdZO and CMAS may be primarily an apatite phase based on Gd<NUM>Ca<NUM>(SiO<NUM>)6O<NUM>. For YSZ, CMAS may penetrates the open structure of the coating as soon as melting occurs, whereupon the original 8YSZ dissolves in the CMAS and reprecipitates with different morphology and composition that depends on the local melt chemistry. YAG may be less reactive to CMAS, and may form (CaAl<NUM>Si<NUM>O<NUM>(Anorthite) and/or Ca<NUM>Y6O(SiO<NUM>)<NUM> (Calcium Yttrium Oxide Silicate).

Each respective layer of the plurality of CMAS resistant layers <NUM> in the alternating arrangement may have the same or substantially similar composition or, in other examples, the composition of one or more the respective layers <NUM> may be different than the composition of other respective layers <NUM> that make up the total number of CMAS resistant layers <NUM>.

CMAS resistant layers <NUM> may have any suitable microstructure. In some examples, CMAS resistant layers <NUM> may be relatively dense (e.g., denser than the abradable layer) with a porosity of, e.g., less than about <NUM>%, or less than about <NUM>%. The microstructure may be the same or different than abradable layers <NUM>. Each respective layer of the plurality of CMAS resistant layers <NUM> in the alternating arrangement may have the same microstructure or, in other examples, the microstructure of one or more the respective layers <NUM> may be different than the microstructure of other respective layers <NUM> that make up the total number of CMAS resistant layers <NUM>.

CMAS resistant layers <NUM> may have any suitable porosity or other void volume. The porosity or other void volume of CMAS resistant layers <NUM> may contribute to the overall CMAS resistance of the layers <NUM>. In some examples, CMAS resistant layers <NUM> may have a porosity or other void volume that is less than that of abradable layers <NUM>. CMAS resistant layers <NUM> may include a porosity of, e.g., less than about <NUM> vol. %, such as less than about <NUM> vol. %, or less than about <NUM> vol. % or less than about <NUM> vol. %, where porosity is measured as a percentage of pore volume divided by total volume of CMAS resistant layers <NUM> (either on a individual layer basis or overall layer basis). In each case, the porosity of layers <NUM> may be measured using mercury porosimetry, optical microscopy or Archimedean method. In some examples, CMAS resistant layers <NUM> may include a void volume of, e.g., less than about <NUM> vol. %, such as less than about <NUM> vol. %, or less than about <NUM> vol. % or less than about <NUM> vol. %, where the void volume is the volume of void space over total volume. Each respective layer of the plurality of CMAS resistant layers <NUM> in the alternating arrangement may have the same or substantially similar void volume or, in other examples, the void volume of one or more the respective layers <NUM> may be different than the void volume of other respective layers <NUM> that make up the total number of CMAS resistant layers <NUM>.

In some examples, the alternating layer arrangement of coating system <NUM> may improve the CMAS resistance. For example, the multiple interfaces formed between the alternating layers may prevent or slow down the propagation of thermal shock cracks (e.g., that originate at the interface of coating system <NUM> and underlying layer <NUM>, <NUM> or substrate <NUM>) through the coating system, to eliminate or reduce pathways through coating system <NUM> that CMAS may infiltrate. Thus, in addition to or as an alternatively to the relatively low porosity of layers <NUM> and/or reaction of layers <NUM> with CMAS to form a stable phase, the reduced crack propagation of coating system <NUM> may resist or arrest CMAS infiltration through the coating system and restrict CMAS effects to only an outer region of the coating system, thus more effectively protecting the underlying metallic substrate <NUM>.

In some examples, the alternating coating structure described herein may help to manage the thermal stress due to thermal expansion mismatch between the dense GdZo layers (or other CMAS resistant layers) and the porous Mg Spinel layers (or other abradable layers). For example, since the CTE mismatch between the CMAS resistant layers and abradable layers may be relatively large, it may be better to have the CTE mismatch over a number of alternating layers/interfaces rather than only have two layers (one abradable and one CMAS resistant layer) in the coating system. In some examples, the CTE of Mg-Spinel is about <NUM>-<NUM> ppm/C, while that of GDZ may be about 11ppm/C.

While the layers <NUM>, <NUM> may be substantially planar, some or all of the layers <NUM>, <NUM> may be made partially or completely non-planar as needed to conform to a shape of a CMC article. For example, one or more of the layers <NUM>, <NUM> may deviate from planarity, and have any suitable shape or follow any suitable contour, such as planar, undulating, zig-zag, corrugated, or curved, or combinations thereof.

The respective discrete CMAS resistant layers <NUM> and respective discrete abradable layers <NUM> may have any suitable thickness. In some examples, the thickness of individual discrete CMAS resistant layers <NUM> may be about <NUM> (<NUM> inches) to about <NUM> (<NUM> inches). In some examples, the thickness of individual discrete abradable layers <NUM> may be about <NUM> (<NUM> inches) to about <NUM> (<NUM> inches). In some examples, the individual CMAS resistant layers <NUM> and individual abradable layers <NUM> may have the same thickness as each other or the thickness may differ. In some examples, each of the individual CMAS resistant layers <NUM> may have the same thickness throughout coating system <NUM> or it may vary between respective layers. In some examples, each of the individual abradable layers <NUM> may have the same thickness throughout coating system <NUM> or it may vary between respective layers. The overall total thickness of the abradable coating <NUM> may be about <NUM> (<NUM> inches) to about <NUM> (<NUM> inches), such as about <NUM> (<NUM> inches) to about <NUM> (<NUM> inches). In some examples, CMAS resistant layers <NUM> may have a layer thickness of about <NUM> (<NUM> mils) and the abradable layers <NUM> have a layer thickness of about <NUM> (<NUM> mils). In some examples, CMAS resistant layers <NUM> may have the minimum thickness required to provide continuous coverage over the underlying layer. The overall total thickness of the abradable coating <NUM> may be about <NUM> mils. Other values are contemplated.

In some examples, CMAS resistant layers <NUM> may be relatively thin to allow for a transfer of force from a mechanical interaction with, e.g., blade tip <NUM>, to an underlying porous abradable layer <NUM> across the CMAS resistant layer <NUM>. Such a force may allow for a respective CMAS resistant layer <NUM> to be "knocked off" or otherwise remove from coating system <NUM> when interacting with blade tip <NUM>.

In various examples, the number of discrete layers <NUM>, <NUM> in the coating system <NUM>, as well as the arrangement of layers <NUM>, <NUM>, needed in a particular application may be determined by the penetration or rub depth of a blade or other rotating element contacting the coating system <NUM>. For example, referring now to <FIG>, article <NUM> includes metallic substrate <NUM>, optional bond coat <NUM>, and optional TBC layer <NUM>. CMAS resistant abradable coating <NUM> resides on TBC layer <NUM>, and includes an alternating arrangement of discrete CMAS resistant layers <NUM> and abradable layers <NUM>. Substrate <NUM>, bond coat <NUM>, TBC layer <NUM>, and coating system <NUM> may be substantially the same or similar to substrate <NUM>, bond coat <NUM>, TBC layer <NUM>, and coating system <NUM> of <FIG> and <FIG>.

As shown schematically in <FIG>, a rotating element <NUM> (such as blade tip <NUM> of <FIG>) makes a cut in the abradable coating system <NUM> with a maximum cut depth d. The CMAS resistant layer <NUM> closest to substrate <NUM> is positioned at a depth deeper than the maximum cut depth d of the rotating element <NUM>, so that there is at least one undisturbed CMAS resistant layer <NUM> after the deepest cut made by the rotating element <NUM>. The continuous and undisturbed CMAS resistant layer <NUM> may help to maintain CMAS resistance even after the deepest cut made by the rotating element <NUM> is made, e.g., during operation of a high temperature gas turbine engine.

In another example shown schematically in <FIG>, article <NUM> includes metallic substrate <NUM>, optional bond coat <NUM>, and TBC layer <NUM>. CMAS resistant abradable coating system <NUM> resides on TBC layer <NUM>, and includes an alternating arrangement of discrete CMAS resistant layers <NUM> and abradable layers <NUM>, with a CMAS resistant layer <NUM> nearest CMC substrate <NUM> as well as another CMAS resistant layer <NUM> as the topcoat of coating system <NUM>. Substrate <NUM>, bond coat <NUM>, TBC layer <NUM>, and coating system <NUM> may be substantially the same or similar to substrate <NUM>, bond coat <NUM>, TBC layer <NUM>, and coating system <NUM> of <FIG> and <FIG>.

As shown schematically in <FIG>, a rotating element <NUM> makes a cut in coating system <NUM> with a maximum cut depth d. As in the example of <FIG>, the CMAS resistant layer <NUM> that is closest to the CMC substrate <NUM> is positioned at a depth deeper than the maximum cut depth d of the rotating element <NUM>, so that there is at least one undisturbed CMAS resistant layer <NUM> after the deepest cut made by the rotating element <NUM>. As in <FIG>, the continuous and undisturbed CMAS resistant layer <NUM> may help to maintain CMAS resistance. In the case of the relatively shallow blade cut depth of <FIG>, the alternating layered structure of the coating system <NUM> is configured such that only a single CMAS resistant layer <NUM> and a single abradable layer <NUM> are abraded, e.g., during operation of a high temperature gas turbine engine. The alternating layer arrangement of coating system <NUM> may help manage the thermal stress due to thermal expansion mismatch between the relatively dense CMAS resistant layer <NUM> and relatively porous abradable layers <NUM>, e.g., which may be particularly beneficial in examples such as that shown in <FIG> where the cut depth d into coating system <NUM> is relatively shallow.

Referring again to <FIG>, the layers of article <NUM> may be formed by wide variety of techniques. For example, the optional bond coat <NUM> may be formed on metallic substrate <NUM> by vapor deposition, spraying, and the like. TBC layer <NUM> and the layers <NUM>, <NUM> of the abradable coating system <NUM> may be formed by one or more of vapor deposition, slurry deposition, electrophoretic deposition, or thermal spraying, for example, air plasma spray, low pressure plasma spray, suspension plasma spray, or high velocity oxy-fuel (HVOF) spraying. One or more parameters of the deposition process may be tailored to achieve layers having the properties (e.g., composition, microstructure, void volume) described herein.

In some examples, the alternating layer arrangement of coating system <NUM> may be achieved by switching between two powder feeds and/or spray parameter that are suitable for each of CMAS resistant layers <NUM> and abradable layers <NUM> during a thermal spray process.

In some examples, the porosity or other void volume of CMAS resistant layers <NUM> and abradable layers <NUM> may be controlled by the use of coating material additives and/or processing techniques to create the desired void volume. For example, to form abradable layers <NUM> of coating system <NUM> in <FIG>, a fugitive material that melts or burns at the use temperatures of the component (e.g., a blade track) may be incorporated into the coating material that forms abradable layers <NUM>. The fugitive material may include, for example, graphite, hexagonal boron nitride, or a polymer such as a polyester, and may be incorporated into the coating material prior to deposition of the coating material on substrate <NUM> to form abradable layers <NUM>. The fugitive material then may be melted or burned off in a post-formation heat treatment, or during operation of the gas turbine engine, to form pores in layers <NUM>.

In another example for forming the various layers on substrate <NUM>, a slurry may be deposited using painting, dip coating, spraying, or the like, followed by drying and sintering. The slurry particles may include the desired composition of the final coating, or may include precursors that react during the sintering process to form the coating with a desired composition and other properties.

Claim 1:
An abradable system comprising:
a metallic substrate (<NUM>, <NUM>, <NUM>);
an abradable coating (<NUM>, <NUM>, <NUM>) on the metallic substrate (<NUM>, <NUM>, <NUM>), the abradable coating (<NUM>, <NUM>, <NUM>) comprising:
a plurality of abradable layers (<NUM>, <NUM>, <NUM>) including a first abradable layer (<NUM>, <NUM>, <NUM>) and a second abradable layer (<NUM>, <NUM>, <NUM>), and
a plurality of CMAS resistant layers (<NUM>, <NUM>, <NUM>) including a first CMAS resistant layer (<NUM>, <NUM>, <NUM>) and a second CMAS resistant layer (<NUM>, <NUM>, <NUM>),
wherein the plurality of abradable layers (<NUM>, <NUM>, <NUM>) are in an alternating arrangement with the plurality of CMAS resistant layers (<NUM>, <NUM>, <NUM>) such that the abradable coating (<NUM>, <NUM>, <NUM>) alternates between the first CMAS resistant layer (<NUM>, <NUM>, <NUM>), the first abradable layer (<NUM>, <NUM>, <NUM>), the second CMAS resistant layer (<NUM>, <NUM>, <NUM>), and the second abradable layer (<NUM>, <NUM>, <NUM>), in that order, on the metallic substrate (<NUM>, <NUM>, <NUM>), wherein the plurality of CMAS resistant layers (<NUM>, <NUM>, <NUM>) are configured to react with molten CMAS to form a phase which slows the infiltration of the molten CMAS through the plurality of CMAS resistant layers (<NUM>, <NUM>, <NUM>); and
an opposing abrasive element (<NUM>, <NUM>, <NUM>) configured to abrade the abradable coating (<NUM>, <NUM>, <NUM>) to a maximum penetration depth (d) in the abradable coating (<NUM>, <NUM>, <NUM>) that includes abrading at least the first abradable layer (<NUM>, <NUM>, <NUM>), the second CMAS resistant layer (<NUM>, <NUM>, <NUM>), and the second abradable layer (<NUM>, <NUM>, <NUM>), and wherein the abradable coating (<NUM>, <NUM>, <NUM>) comprises at least one respective CMAS resistant layer (<NUM>, <NUM>, <NUM>) of the plurality of CMAS resistant layers (<NUM>, <NUM>, <NUM>) between the maximum penetration depth (d) and the metallic substrate (<NUM>, <NUM>, <NUM>).