Patent Publication Number: US-2021188723-A1

Title: Tape casting coating for ceramic matrix composite

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 62/951,662, entitled “TAPE CASTING COATING FOR CERAMIC MATRIX COMPOSITE,” and filed on Dec. 20, 2019, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to tape cast coatings for ceramic matrix composites. 
     BACKGROUND 
     Substrates including a ceramic matrix composite (CMC) may be useful in a variety of contexts where mechanical and thermal properties are important, such as, for example, components in high-temperature mechanical systems, including gas turbine engines. Some substrates including a CMC may be coated with an overlying layer to reduce exposure of the substrate including a CMC to radiant heat or elements and compounds present in the operating environment of high-temperature mechanical systems. 
     SUMMARY 
     In some examples, the disclosure describes a method including forming a braze tape defining at least one layer extending in a plane. The at least one layer includes a first segment including a first coating material and a second segment including a second coating material. A portion of the second segment in the plane is adjacent to a portion of the first segment in the plane. The method also includes positioning the braze tape on a surface of a substrate. The plane of the layer of the braze tape is parallel to the surface of the substrate. The method also includes heating the braze tape to melt a constituent of at least one of the first coating material and the second coating material to form a densified coating on the surface of the substrate. 
     In some examples, the disclosure describes a component including a substrate and a coating system on a surface of the substrate. The substrate includes a ceramic matrix composite (CMC). The coating system includes at least one layer extending in a plane adjacent to the surface of the substrate. The at least one layer includes a first segment extending in the plane and including a first coating material, and a second segment extending in the plane and including a second coating material. A portion of the second segment is adjacent to a portion of the first segment. 
     In some examples, the disclosure describes a multilayer braze tape including a first layer and a second layer disposed on the first layer and extending in a plane. The second layer includes a first segment comprising a first coating material and extending in the plane and a second segment comprising a second coating material and extending in the plane. A portion of the second segment is adjacent to a portion of the first segment. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example component that includes a substrate and a coating system on substrate, the coating system including a plurality of layers. 
         FIG. 2  is a conceptual diagram illustrating an example component that includes a substrate and a coating system on substrate, the coating system including a plurality of layers and a plurality of segments. 
         FIG. 3  is a conceptual diagram illustrating an example tape casting system. 
         FIG. 4  is a conceptual diagram illustrating an example system for assembling a multilayer tape from multiple tapes. 
         FIG. 5  is a flow diagram illustrating an example technique for applying a tape cast coating onto a component. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure describes articles including, and techniques for forming, tape cast coatings for ceramic matrix composite (CMC) components. An example tape cast coating may include a plurality of layers (e.g., layers normal to a surface of the CMC component) and/or a plurality of adjacent segments (e.g., within a plane parallel to the surface of the CMC component). Each layer of the plurality of layers and/or each segment of the plurality of adjacent segments may include a selected microstructure or chemistry. The selected microstructures and/or chemistries may be selected to improve a functionality at a selected positions on the CMC component. In some examples, a tape cast coating may be formed using at least one of slurry casting, tape casting, or gel casting. Each layer of the plurality of layers or each segment of the plurality of adjacent segments may be separately cast or cast with other layers or segments. Multiple cast layers or segments may be assembled to form the tape cast coating for positioning and sintering on a CMC component. The tape cast coating may be used, for example, to form a coating on CMC components or portions of CMC components that cannot be coated by other techniques (e.g., shadowed regions that cannot be coated by, for example, air plasma spray), and/or to repair damaged coatings. 
     Typically, coating systems for CMC components are applied using thermal spray processes, such as air plasma spray (APS). An example coating system may include, for example, a silicon-based bond coat, a rare earth silicate-based environmental barrier coating (EBC), and an abradable topcoat. Application of a coating system using APS has disadvantages including, but not limited to, preferential volatilization of elements/compounds, amorphous microstructure of as-deposited coating, lack of control of the coating microstructure, and limited ability of APS, as a line-of-sight technique, to coat components having shadowed regions or other complex geometries. Preferential volatilization of elements/compounds during spraying may result in the as-deposited coating chemistry being different than that of the coating feedstock powder. For example, silica in an ytterbium disilicate EBC may preferentially volatilize during spraying and result in a coating having various ytterbium monosilicate contents. As-deposited coatings that are amorphous may require a heat treatment to crystallize the coating. In some examples, the crystallization of the coating may result in significant cracking due to the volume change associated with the amorphous-to-crystalline transformation. For example, heat treatment of amorphous mullite may result in significant cracking, e.g., cracking to an extent that affects the chemical resistance or mechanical integrity of the coating. A “splat” microstructure may result from APS. Even with post-deposition processing, it may be difficult to control the coating microstructure including, for example, phase distribution, porosity, grain size, or the like. 
     The described techniques of forming coatings using a tape cast overcome these disadvantages of APS processes. For example, the chemistry of a tape cast coating does not change during processing. Hence, the desired coating chemistry can be controlled. Additionally or alternatively, a tape cast coating is not amorphous during processing. As such, coatings made from materials such as mullite can be produced with reduced cracking compared to amorphous materials. Additionally or alternatively, the microstructure of a tape cast coating may be tuned by controlling the chemistry, phase distribution, and/or grain size of the starting powders, controlling the solid loading in the slurry, and/or controlling the sintering conditions. Additionally or alternatively, the tape could be applied to components with complex geometries, such as shadowed regions, and/or be used to repair coatings that are damaged in service. 
       FIG. 1  is a conceptual diagram illustrating an example component  10  that includes a substrate  12  and a coating system  14  on substrate  12 . Coating system  14  includes a plurality of overlying layers  16 A- 16 D (collectively “layers  16 ”). In some examples, coating system  14  may include an optional bond coat  18 . Component  10  may include a mechanical component operating at relatively high conditions of temperature, pressure, or stress, for example, a component of a turbine, a compressor, or a pump. In some examples, component  10  includes a gas turbine engine component, for example, an aeronautical, marine, or land-based gas turbine engine. Component  10  may include, for example, a blade, a blade track or blade shroud (or segment of a blade track or blade shroud) that circumferentially surrounds a rotating component, for example, a rotating blade  26 . 
     Substrate  12  may include a material suitable for use in a high-temperature environment. In some examples, substrate  12  may include a ceramic or a ceramic matrix composite (CMC). Suitable ceramic materials may include, for example, a silicon-containing ceramic, such as silica (SiO 2 ) and/or silicon carbide (SiC); silicon nitride (Si 3 N 4 ); alumina (Al 2 O 3 ); an aluminosilicate; a transition metal carbide (e.g., WC, Mo 2 C, TiC); a silicide (e.g., MoSi 2 , NbSi 2 , TiSi 2 ); combinations thereof; or the like. In some examples in which substrate  12  includes a ceramic, the ceramic may be substantially homogeneous. In examples in which substrate  12  includes a CMC, substrate  12  may include a matrix material and a reinforcement material. The matrix material and reinforcement materials may include, for example, any of the ceramics described herein. The reinforcement material may be continuous or discontinuous. For example, the reinforcement material may include discontinuous whiskers, platelets, fibers, or particulates. Additionally, or alternatively, the reinforcement material may include a continuous monofilament or multifilament two-dimensional or three-dimensional weave, braid, fabric, or the like. In some examples, the CMC includes an SiC matrix material (alone or with residual Si metal) and an SiC reinforcement material. 
     In some examples, substrate  12  includes a superalloy including, for example, an alloy based on Ni, Co, Ni/Fe, or the like. In examples in which substrate  12  includes a superalloy material, substrate  12  may also include one or more additives for improving the mechanical properties of substrate  12  including, for example, toughness, hardness, temperature stability, corrosion resistance, oxidation resistance, or the like. For example, the one or more additives may include titanium (Ti), cobalt (Co), or aluminum (Al). 
     In examples in which component  10  includes a turbine blade track, substrate  12  may define a leading edge  22  and a trailing edge  24 . In some examples, leading edge  22  and trailing edge  24  may be substantially parallel to each other. In other examples, leading edge  22  and trailing edge  24  may not be substantially parallel to each other. In some cases, a first axis extending between leading edge  22  and trailing edge  24  may be in a substantially axial direction of a gas turbine engine including component  10  (e.g., parallel to the axis extending from the intake to the exhaust of the gas turbine engine). Thus, in some such cases, leading edge  22  and trailing edge  24  may be perpendicular or substantially perpendicular to the axial direction of the gas turbine engine including component  10 . 
     Component  10  includes coating system  14  on a surface  13  of substrate  12 . Coating system  14  may extend from leading edge  22  to trailing edge  24  of substrate  12 . In some examples, coating system  14  may include a plurality of layers  16 A- 16 D (collectively, “layers  16 ”) and optional bond coat  18 . In some examples, one or more of layers  16  may be formed on substrate  12  by the tape casting techniques described herein. 
     In some examples, bond coat  18  is disposed directly on an entirety of, or a portion of, surface  13  with no intermediate layers between substrate  12  and bond coat  18 . In other examples, bond coat  18  may not be disposed directly on surface  13 , i.e., one or more additional intermediate layers may be disposed between substrate  12  and bond coat  18 , such as, for example, one or more impurity barrier layer. An impurity barrier layer may be configured to, for example, reduce migration of elements or compounds from substrate  12  to bond coat  18  or layers  16 . 
     Bond coat  18  may include a composition that provides adherence between substrate  12  and a layer formed on bond coat  18 , such as layer  16 A. In some examples, the adherence provided by bond coat  18  between substrate  12  and layers  16  may be greater than the adherence between substrate  12  and layers  16 , without bond coat  18 . 
     In some examples, bond coat  18  may include a composition that may be stable at temperatures above 1350° C. and/or above about 1410° C. In this way, bond coat  18  may allow use of article  10  at temperatures which lead to temperatures of bond coat  18  above 1350° C. and/or above about 1410° C. In some examples, article  10  may be used in an environment in which ambient temperature is greater than the temperature at which bond coat  18  is thermally stable, e.g., because bond coat  18  may be coated with at least one layer, such as layers  16 , that provides thermal insulation to bond coat  18  and reduces the temperature experienced by bond coat  18  compared to the ambient temperature or the surface temperature of the layer(s) formed on bond coat  18 , e.g., layers  16 . 
     Bond coat  18  may include silicon metal (e.g., elemental silicon; Si), a silicon-containing alloy, a silicon-containing ceramic, or a silicon-containing compound. In some examples, bond coat  18  may include a ceramic-based material including, but not limited to, rare earth monosilicate, rare earth disilicate, mullite, mullite blended with silicon, hafnon, hafnon blended with silicon, ytterbium disilicate, ytterbium disilicate blended with silicon, scandium disilicate, yttrium monosilicate and scandium monosilicate, combinations thereof, or the like. In some examples, the presence of Si in bond coat  18  may promote adherence between bond coat  18  and substrate  12 , such as, for example, when substrate  12  includes silicon metal or a silicon-containing alloy or compound. In some examples, the presence of Si in bond coat  18  may reduce oxygen transport to substrate  12 . 
     Bond coat  18  may optionally include at least one additive. The optional at least one additive may include, for example, at least one of a metal oxide, SiC, a melting point depressant, an oxidation enhancer, a transition metal carbide, a transition metal boride, or a transition metal nitride. SiC may affect the properties of bond coat  18 . For example, SiC particles may modify oxidation resistance of bond coat  18 , modify chemical resistance of bond coat  18 , influence the coefficient of thermal expansion (CTE) of bond coat  18 , or the like. In some examples, bond coat  18  may include between about 1 vol. % and about 40 vol. % SiC, such as between about 1 vol. % and about 20 vol. % SiC, or between about 5 vol. % and about 40 vol. % SiC, or between about 5 vol. % and about 20 vol. % SiC. 
     In examples in which bond coat  18  includes a melting point depressant, the melting point depressant may include a metal or alloy, such as at least one of zirconium metal, yttrium metal, titanium metal, aluminum metal, chromium metal, niobium metal, tantalum metal, or a rare earth metal. Rare earth metals may include Lu (lutetium), Yb (ytterbium), Tm (thulium), Er (erbium), Ho (holmium), Dy (dysprosium), Tb (terbium), Gd (gadolinium), Eu (europium), Sm (samarium), Pm (promethium), Nd (neodymium), Pr (praseodymium), Ce (cerium), La (lanthanum), Y (yttrium), or Sc (scandium). In some examples in which bond coat  18  includes a melting point depressant, bond coat  18  may include greater than 0 wt. % and less than about 30 wt. % of the melting point depressant, such as greater than 0 wt. % and less than about 10 wt. % of the melting point depressant. The melting point depressant may reduce a melting point of a bond coat precursor of bond coat  18  that is formed as part of the technique for forming bond coat  18 . This may allow melting of the bond coat precursor at lower temperatures, which may reduce a chance that the melting of the bond coat precursor to form bond coat  18  damages substrate  12  or impurity barrier layer  14 . 
     In examples in which bond coat  18  includes an oxidation enhancer, the oxidation enhancer may include at least one of molybdenum, hafnium, or ytterbium. In some examples in which bond coat  18  includes an oxidation enhancer, bond coat  18  may include greater than 0 wt. % and less than about 10 wt. % of the oxidation enhancer. The oxidation enhancer may facilitate formation of a stable oxide scale on a surface of bond coat  18 , which may increase adhesion between bond coat  18  and layers  16 , reduce diffusion of elements through bond coat  18 , or both. 
     Bond coat  18  additionally or alternatively may include at least one of a transition metal carbide, a transition metal boride, or a transition metal nitride. Bond coat  18  may include silicon and at least one transition metal carbide; silicon and at least one transition metal boride; silicon and at least one transition metal nitride; silicon, at least one transition metal carbide, and at least one transition metal boride; silicon, at least one transition metal carbide, and at least one transition metal nitride; silicon, at least one transition metal boride, and at least one transition metal nitride; or silicon, at least one transition metal carbide, at least one transition metal boride, and at least one transition metal nitride. The transition metal may include, for example, Cr, Mo, Nb, W, Ti, Ta, Hf, or Zr. The at least one transition metal carbide may include at least one of Cr 3 C 2 , Cr 7 C 3 , Cr 23 C 6 , Mo 2 C, NbC, WC, TaC, HfC, or ZrC. The at least one transition metal boride may include at least one of TaB, TaB 2 , TiB 2 , ZrB 2 , HfB, or Hfb s . The at least one transition metal nitride may include at least one of TiN, ZrN, HfN, Mo2N, or TaN. 
     In some examples, bond coat  18  may include between about 40 volume percent (vol. %) and about 99 vol. % silicon and a balance of the at least one of a transition metal carbide, a transition metal nitride, or a transition metal boride. In some examples, bond coat  18  may include between about 1 vol. % and about 30 vol. %, or between about 5 vol. % and about 20 vol. % of the at least one of a transition metal carbide, a transition metal boride, or a transition metal nitride, and a balance silicon metal and any additional constituents. The particular composition ranges may vary based on the CTE of the at least one of a transition metal carbide, a transition metal boride, or a transition metal nitride. 
     Transition metal carbides, transition metal borides, and transition metal nitrides may have a different CTE than silicon metal. For example, transition metal carbides and transition metal borides may have CTEs between about 5 ppm/° C. and about 8 ppm/° C., and transition metal nitrides may have CTEs of about 9 ppm/° C. By mixing silicon and a transition metal carbide, a transition metal boride, or transition metal nitride, the CTE of bond coat  18  may be increased to more closely match the CTE of substrate  12 , the CTE of impurity barrier layer  14 , the CTE of layers  16 , or any combination thereof. This may reduce stress at the interfaces between bond coat  18  and adjacent layers during thermal cycling of article  10 . 
     Additionally or alternatively, the addition of the at least one of the transition metal carbide, the transition metal boride, or the transition metal nitride may improve oxidation resistance of bond coat  18  compared to a bond layer including only silicon. For example, the at least one of the transition metal carbide, the transition metal boride, or the transition metal nitride may be incorporated into a thermally grown silicon oxide on a surface  24  of bond coat  18 , which may improve adherence of the thermally grown silicon oxide to bond coat  18 , decrease oxygen diffusivity through the thermally grown silicon oxide (which reduces the rate of oxidation of the remaining bond layer), or both. 
     Bond coat  18  may define any suitable thickness, measured in a direction substantially normal to surface  13  of substrate  12 . In some examples, bond coat  18  defines a thickness of between about 0.5 mils (about 12.7 micrometers) and about 40 mils (about 1016 micrometers), such as between about 1 mils (about 25.4 micrometers) and about 10 mils (about 254 micrometers). 
     Bond coat  18  may be formed on substrate  12  using the tape casting techniques described herein. In examples in which bond coat  18  includes mullite, bond coat  18  formed by the tape casting techniques described herein may include reduced cracking compared to other bond coats formed using APS. In examples in which bond coat  18  includes hafnon and silicon, bond coat  18  formed by the tape casting techniques described herein may include improved oxidation resistance compared to other bond coats, e.g., formed by APS, having splat microstructures and/or a nonuniform distribution of the silicon, which may oxidize and result in spallation after heat cycling in an oxidizing environment. 
     In other examples, bond coat  18  may be formed on substrate  12  using, for example, thermal spraying, e.g., air plasma spraying (APS), high velocity oxy-fuel (HVOF) spraying, low vapor 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. 1 , bond coat  18  defines a surface  19  on which layers  16  are disposed. Layers  16  may include, for example, one or more of an environmental barrier coating (EBC), an abradable layer, a thermal barrier coating (TBC), a calcia-magnesia-aluminosilicate (CMAS)-resistant layer, or the like. In some examples, a single layer of layers  16  may perform two or more of these functions. For example, an EBC may provide environmental protection, thermal protection, CMAS-resistance, and the like to substrate  12 . As another example, an abradable layer may provide wear protection, impact protection, and the like to substrate  12 . As illustrated in  FIG. 1 , component  10  includes a plurality of layers  16 , such as at least one EBC layer, at least one abradable layer, at least one TBC layer, at least one CMAS-resistant layer, or combinations thereof. In some examples, one or more layers of layers  16  may include one or more of silicon, silica, mullite, barium strontium aluminosilicate, a rare earth oxide, a rare earth monosilicate, a rare earth disilicate, a mixture of a rare earth monosilicate and a rare earth disilicate, a mixture of a plurality of rare earth silicates, a mixture of a rare earth oxide and one or more rare earth silicates (e.g., a rare earth monosilicate and/or a rare earth disilicate), scandium oxide, scandium monosilicate, scandium disilicate, yttrium oxide, yttrium monosilicate, yttrium disilicate, hafnia, hafnon, zircon, mixtures thereof, or the like. 
     Layers  16  may be formed on surface  19  of bond coat  18  using the tape casting techniques describe herein. In other examples, one or more layers of layers  16  may be formed on surface  19  using, for example, thermal spraying, e.g., APS, HVOF spraying, low vapor plasma spraying; PVD, including EB-PVD, DVD, and cathodic arc deposition; CVD; slurry process deposition; sol-gel process deposition; electrophoretic deposition; or the like. 
     An EBC layer may include at least one of a rare earth oxide, a rare earth silicate, an aluminosilicate, an alkaline earth aluminosilicate, or metal oxides. In some examples, an EBC layer may include one or more dopants, such as CaO, MgO, Al 2 O 3 , Fe 2 O 3 , Fe 3 O 4 , RE 2 O 3  (where RE is a rare earth element), Y 2 O 3 , Sc 2 O 3 , Ta 2 O 5 , HfO 2 , ZrO, SrO, LiREO 2 , or RE 3 Al 5 O 12  (e.g., YbAG or YAG). For example, an EBC layer may include mullite, BSAS, barium aluminosilicate (BAS), strontium aluminosilicate (SAS), at least one rare earth oxide, at least one rare earth monosilicate (RE 2 SiO 5 , where RE is a rare earth element), at least one rare earth disilicate (RE 2 Si 2 O 7 , where RE is a rare earth element), or combinations thereof. The rare earth element in the at least one rare earth oxide, the at least one rare earth monosilicate, or the at least one rare earth disilicate may include at least one of Lu, Yb, Tm, Er, Ho, Dy, Tb, Gd, Eu, Sm, Pm, Nd, Pr, Ce, La, Y, or Sc. In some examples, the one or more dopants may act as sintering aids to densify the EBC layer thereby increasing its hermeticity and/or decreasing the porosity compared to the porosity of a similar EBC deposited without the sintering aids. 
     In examples in which the EBC layer includes a mixture of a rare earth monosilicate and a rare earth disilicate or a mixture of a plurality of rare earth silicates, the mixture may include up to three RE cations with varying molar concentrations of the RE cation (e.g. (Yb 0.5 , Lu 0.5 ) 2 SiO 5 , (Yb 0.5 , Lu 0.5 ) 2 Si 2 O 7 ). 
     In some examples, an EBC layer may include at least one rare earth oxide and alumina, at least one rare earth oxide and silica, or at least one rare earth oxide, silica, and alumina. In some examples, an EBC layer may include an additive in addition to the primary constituents of the EBC layer. For example, the additive may include at least one of TiO 2 , Ta 2 O 5 , HfSiO 4 , an alkali metal oxide, or an alkali earth metal oxide. The additive may be added to the EBC layer to modify one or more desired properties of the EBC layer. For example, the additive components may increase or decrease the reaction rate of the EBC layer with CMAS, may modify the viscosity of the reaction product from the reaction of CMAS and the EBC layer, may increase adhesion of the EBC layer to bond coat  18 , may increase or decrease the chemical stability of the EBC layer, or the like. 
     In some examples, the EBC layer may be substantially free (e.g., free or nearly free) of hafnia and/or zirconia. Zirconia and hafnia may be susceptible to chemical attack by CMAS, so an EBC layer substantially free of hafnia and/or zirconia may be more resistant to CMAS attack than an EBC layer that includes zirconia and/or hafnia. 
     Regardless of the composition of the EBC layer, in some examples, the EBC layer may have a dense microstructure, a porous microstructure, a columnar microstructure, or a combination of at least two of dense, porous, or columnar microstructures. A dense microstructure may be more effective in preventing the infiltration of CMAS and other environmental contaminants, while a porous or columnar microstructure may be more strain tolerant during thermal cycling. In some examples, an EBC layer with a dense microstructure may have a porosity of less than about 10 vol. %, such as less than about 8 vol. %, less than about 5 vol. %, or less than about 2 vol. %, where porosity is measured as a percentage of pore volume divided by total volume of the EBC layer. In some examples, an EBC layer with a porous microstructure may have a porosity of more than about 10 vol. %, such as more than about 15 vol. %, more than 20 vol. %, or more than about 30 vol. %, where porosity is measured as a percentage of pore volume divided by total volume of the EBC layer. 
     As described above, the EBC layer may be used as a single layer  16  or may be used in combination with at least one other layer, such as an abradable layer or TBC layer. 
     Additionally or alternatively, layers  16  may include an abradable layer. Abradability may include a disposition to break into relatively small pieces when exposed to a sufficient physical force. Abradability may be influenced by the material characteristics of the abradable layer, such as fracture toughness and fracture mechanism (e.g., brittle fracture), as well as the porosity of the abradable layer. Thermal shock resistance and high temperature capability may be important for use in a gas turbine engine, in which the abradable layer is exposed to wide temperature variations from high operating temperatures to low environmental temperatures when the gas turbine engine is not operating. In addition to at least some of the above properties, the abradable layer may possess other properties. 
     The abradable layer may include any suitable material. For example, the abradable layer may include at least one of a rare earth oxide, a rare earth silicate, an aluminosilicate, or an alkaline earth aluminosilicate. For example, an abradable layer may include mullite, BSAS, BAS, SAS, at least one rare earth oxide, at least one rare earth monosilicate, at least one rare earth disilicate, or combinations thereof. In some examples, the abradable layer may include any of the compositions described herein with respect to the EBC layer. 
     The abradable layer may be relatively porous. For example, the abradable layer may have a porosity between about 10 vol. % and about 50 vol. %, such as between about 15 vol. % and about 35 vol. %, or about 20 vol. %. Porosity of the abradable layer may reduce a thermal conductivity of the abradable layer and/or may affect the abradability of the abradable layer. Porosity of the abradable layer is defined herein as a volume of pores or cracks in the abradable layer divided by a total volume of the abradable layer (including both the volume of material in the abradable layer and the volume of pores/cracks in the abradable layer). 
     The abradable layer may be formed using, for example, a tape casting technique. Porosity of the abradable layer may be controlled by the use of coating material additives and/or processing techniques, such as by controlling heat treatment and/or material infiltration during tape casting, to create the desired porosity. In some examples, substantially closed pores may be desired. 
     For example, a coating material additive 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 the abradable layer. The coating material additive 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 over substrate  12  to form the abradable layer. The coating material additive 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 the abradable layer. The post-deposition heat-treatment may be performed at up to about 1600° C. 
     The porosity of the abradable layer can also be created and/or controlled during formation of a tape. For example, forming the tape may include application of a slurry of the coating material onto a carrier film using the tape cast machine, followed by drying and sintering or partial sintering. During sintering, the coating material may shrink. Shrinkage may result in crack formation. To reduce crack formation, the coating material may be partially sintered. The remaining porosity and/or cracks may be infiltrated with the same material or a different material. Partially sintering and infiltrating may be repeated until a selected porosity or resistance to cracking is achieved. For example, the additional infiltration and partial sintering steps may be performed several times to achieve a dense uncracked coating. In some examples, using a different material for infiltration may produce a composite coating structure with enhanced mechanical properties and environmental resistance. An example composite structure formed by partial sintering and infiltration may include ytterbium disilicate, ytterbium monosilicate, and/or hafnon with silicon. 
     As described above, the abradable layer may be used as a single layer  16  or may be used in combination with at least one other layer, such as an EBC layer or TBC layer. 
     Layers  16  additionally or alternatively may include a TBC layer. The TBC may have a low thermal conductivity (i.e., both/either an intrinsic thermal conductivity of the material(s) that forms the TBC and/or an effective thermal conductivity of the TBC as constructed) to provide thermal insulation to substrate  12 , bond coat  18 , and/or layers  16 . Heat is transferred through the TBC through conduction and radiation. The inclusion of rare earth oxides such as ytterbia, samaria, lutetia, scandia, ceria, gadolinia, neodymia, europia, yttria-stabilized zirconia (YSZ), zirconia stabilized by a single or multiple rare earth oxides, hafnia stabilized by a single or multiple rare earth oxides, zirconia-rare earth oxide compounds, such as RE 2 Zr 2 O 7  (where RE is a rare earth element), hafnia-rare earth oxide compounds, such as RE 2 Hf 2 O 7  (where RE is a rare earth element), and the like as dopants may help decrease the thermal conductivity (by conduction) of the TBC. 
     As described above, the TBC layer may be used as a single layer  16  or may be used in combination with at least one other layer, such as an EBC layer or an abradable layer. 
     In some examples, coating system  14 , e.g., layers  16  and/or bond coat  18 , may conform to a three-dimensional geometry of component  10 . In some examples, conforming to the three-dimensional geometry of component  10  may improve aerodynamics of component  10  and/or improve bonding of layers  16  and/or bond coat  18  to substrate  12 . 
     As discussed above, coating system  14 , e.g., layers  16  and/or bond coat  18 , may be formed using slurry casting, tape casting, or gel casting. For example, a composition of layers  16  may be selected to enable partial sintering, sintering, or brazing of layers  16 . Specific chemistries, slurry solid loadings, and/or sintering and infiltration conditions may be selected to achieve a coating system  14  with desired phase fractions and porosities. For example, as illustrated in  FIG. 1 , each of layers  16  may include a unique microstructure, a unique chemistry, and/or a unique slurry solid loading, and/or be formed using a unique sintering condition. 
     In some examples, layers  16  may each include a silicon-containing braze material. The silicon-containing braze material may include, for example, silicon metal, a silicon alloy, or silicon metal and an alloying element. In some examples, the silicon alloy may include silicon metal alloyed with transition metals, transition metal carbides, transition metal borides, transition metal silicides, or mixtures thereof. Similarly, the alloying element may include at least one of titanium, boron, carbon, or the like. The alloying element may modify the melting temperature of silicon, modify the viscosity or wetting characteristics of the melted alloy compared to molten silicon, or the like. The silicon metal, the silicon alloy, or the silicon metal and the alloying element may be present in the silicon-containing braze material as a particulate. 
     In some examples, layers  16  also may include a binder, which may assist in maintaining the silicon-containing braze material in the respective layers  16 . In some examples, the binder may include a carbon-yielding organic binder system, for example, furan derived binders. The carbon-yielding organic binder system may be formed, for example, during slurry casting, tape casting, or gel casting of the respective layers  16 . 
     In some examples, at least one of layers  16  may additionally include graphite, carbon black, diamond, or the like. The graphite, carbon black, diamond or the like may react with the silicon (e.g., silicon metal) to form silicon carbide. 
     In examples in which layers  16  include an EBC and an abradable coating, the composition of each layer of layers  16  may include selected ratios of ytterbium disilicate and ytterbium monosilicate. In some examples, the selected ratios of ytterbium disilicate and ytterbium monosilicate may improve water vapor and/or calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide (“CMAS”) resistance, and/or improve porosity for improved compliance and abradability. 
     In some examples, coating system  14 , e.g., one or more of layers  16  and/or bond coat  18 , may be formed by a casting technique, such as tape casting, slurry casting, or gel casting the layers of coating system  14 . For example, each of the layers of coating system  14  may be separately casted, then joined to form coating system  14 . As another example, a first layer (e.g., layer  16 A) may be cast, a second layer (e.g. layer  16 B) may be cast on the first layer, a third layer (e.g., layer  16 C) may be cast on the second layer, and a fourth layer (e.g., layer  16 D) may be cast on the third layer. Some example techniques for forming coating system  14  will be described below in further detail. 
     Although described above as being formed using tape casting, in some examples, one or more layers of coating system  14  may be formed by other techniques, such as, for example, thermal spraying, APS, chemical vapor deposition (CVD), or the like, with other layers applied using the tape casting techniques described herein. For example, bond coat  18  may be formed by tape casting and one or more of layers  16  may be formed by APS. As another example, bond coat  18  may be formed by APS and one or more layers  16  may be formed by tape casting an EBC. As another example, an impermeable barrier layer may be formed by CVD followed by a tape cast bond coat  18  and EBC layers  16 , followed by an APS abradable layer. 
     In some examples, adjacent segments of a tape may be selected to control a microstructure or chemistry, which may be positioned on the CMC component to improve a functionality at a selected portion of the CMC component, such as a selected portion of a surface of the CMC component.  FIG. 2  is a conceptual diagram illustrating an example component  40  that includes a substrate  42  and a coating system  44  on substrate  42 . Component  40  may be the same as or substantially similar to component  10  discussed above in reference to  FIG. 1 , expect for the differences described herein. 
     For example, coating system  44  includes a plurality of layers  46 A,  46 B, and  46 C (collectively “layers  46 ”) and a plurality of adjacent segments  50 A,  50 B, and  50 C (collectively, “segments  50 ”). Layers  46  are arranged normal to surface  43  of substrate  42 . Segments  50  are within a plane  51  parallel to surface  43  of substrate  42 . Each layer of layers  46  and/or each segment of segments  50  may include a selected microstructure or chemistry. For example, layers  46  and segments  50  may include any of the coating materials discussed above in reference to  FIG. 1 . Additionally, segments  50  may be applied using the tape casting techniques discussed above. For example, a tape including one or more segments of segments  50  may be prepared, positioned on component  10 , and sintered to define a densified coating of coating system  44 . 
     In some examples, the selected microstructures and/or chemistries may be selected to improve a functionality at one or more selected positions on component  10 . As one example, segments  50  may define a non-continuous abradable portion of coating system  44  of a gas turbine engine shroud that includes a first portion (e.g., segment  50 A), a second portion (e.g., segment  50 C), and a blade rub portion (e.g., segment  50 B). Blade rub portion  50 B may extend between first portion  52  and second portion  54 , and may be configured to be abraded, e.g., by the tips of blades of a gas turbine engine, in order to form a relatively tight seal between component  40  and the blades. An abradability, as discussed above in reference to  FIG. 1 , of blade rub portion  50 B may include a disposition to break into relatively small pieces, granules, or powder, when exposed to a sufficient physical force. Abradability may be influenced by the material characteristics of the material forming blade rub portion  50 B of coating system  44 , such as fracture toughness and fracture mechanism (e.g., brittle fracture) and/or the porosity of the blade rub portion  50 B. In this way, a portion of coating system  44  over a region of component  10  may be controlled by application of different segments  50  via casting to improve mechanical and/or chemical properties of the portion of coating system  44 . 
     In some examples, coating system  14  and/or  44  may be formed using one or more casting techniques. For example, each layer of coating system  14  or  44  may be formed using gel casting, slurry casting, or tape casting. 
       FIG. 3  is a conceptual diagram illustrating an example tape casting system  70 . Tape casting system  70  includes a first roller  72   a  and a second roller  72   b  (collectively, “rollers  72 ”). A belt  74  is carried by and rotates about the rollers  72 . In some examples, belt  74  may include a nonstick material that has low adhesion to the tape  86  formed using tape casting system  70 , such that tape  86  may easily separate from belt  74  without damaging the tape  86 . In other examples, belt  74  may carry a release liner on which the tape  86  is formed. The release liner may be formed of any film that has relatively low adhesion to the material from which the tape is formed  86 . 
     Tape casting system  70  also includes a reservoir  76 , which contains a slurry  80 . One wall  78  of reservoir  76  is raised from belt  74  and defines a gap, which sets the thickness of the slurry layer  82  formed on belt  74 . 
     Slurry  80  may include components or precursors of tape  84  disposed in a solvent. For example, the slurry may include particles, a pre-gellant material, an optional gelation initiator or promoter, optional additives, and a solvent. The composition of the particles may depend on, for example, whether the tape  84  being formed includes a silicon-containing braze material or other coating material described above in reference to  FIG. 1 . In examples in which tape  84  includes a silicon-containing braze material, the particles may include, for example, silicon metal, a silicon alloy, or silicon metal and an alloying element. 
     The pre-gellant material may include any material that can be processed to form a gel-like network distribute and retain the particles within tape  84  as the tape  84  is subsequently processed. In this application, the term gel refers to a viscous, jelly-like colloid including a disperse phase of the particles. 
     In some examples, the pre-gellant material includes a polysaccharide such as, for example, methyl cellulose, carboxymethyl cellulose, hydroxypropyl methyl cellulose, gellan gum, xanthan gum, agarose, carrageenan, and mixtures and combinations thereof. In some examples, the slurry composition may optionally further include a gelation initiator or promoter such as a monovalent or a divalent salt. 
     In some examples, the pre-gellant material includes one or more gelation monomers which, when polymerized, form a gel. In various examples, the monomeric pre-gellant material may include, but are not limited to, acrylamides, acrylates, vinyls, allyls, and mixtures and combinations thereof. The gelation monomers may optionally include one, two, or more functional groups such as, for example, (meth)acryl, acrylamido, vinyl, allyl, and the like. 
     In some examples, the slurry can include an optional polymerization initiator to aid gelation of the pre-gellant material. The polymerization initiator may vary widely depending on the selected monomeric pre-gellant material, and in various example examples may include a peroxide, a persulfate, a perchlorate, an amine, an azo compound, and mixtures and combinations thereof. 
     The slurry also includes a solvent selected to disperse or dissolve the monomeric pre-gellant material and the optional polymerization initiator. In various examples, the solvent is aqueous (includes a major amount of water), or is water. Other solvents that can be used in the slurry include, but are not limited to, alcohols. In some examples, the slurry may optionally include less than about 10 weight percent (wt. %) of additives such as, for example, dispersants, binders, surfactants, pH adjustors, and the like. 
     In other examples, the slurry may include particles, a binder, and a solvent. For example, the slurry can include between about 40 vol. % and about 60 vol. % of particles, between about 10 vol. % and about 30 vol. % binder, optionally, up to about 40 vol. % additives, and between about 10 vol. % and about 20 vol. % of a solvent or mixture of solvents. As described above, in some examples, the binder may include an organic binder system. In some examples, the slurry additionally may include graphite, carbon black, diamond, or the like. 
     In operation, as rollers  72  rotate, belt  74  is moved in a clockwise direction under reservoir  76  and receives the slurry  80 . The gap between wall  78  and belt  74  defines the thickness of slurry layer  82  on belt. Slurry layer  82  is carried by belt  74  through a furnace  84 , which dries slurry layer  82  by removing the solvent from slurry layer  82 . In some examples, heat from furnace  84  also may facilitate the gelation reaction in slurry layer  82 . Exiting from furnace  84  is a tape  86 . Due to the presence of the binder or gel, tape  86  may be at least somewhat flexible. In some examples, tape  86  may be rolled on a roller for storage and/or transport. 
     In some examples, instead of forming tape  86  using a single stage including a reservoir  76  and furnace  84 , a tape casting system may form tape  86  using multiple stages, each stage including a respective reservoir and furnace. Additionally, or alternatively, each stage may include a plurality of reservoirs  76  positioned substantially adjacent to each other, each respective reservoir configured to dispense a respective slurry onto belt  74  (e.g., as adjacent segments  50 ), either simultaneously or nearly simultaneously in accordance with common tape casting techniques. Each stage may deposit one or more layers or segments onto belt  74  (or previously deposited layers or segments). Thus, in a single process, multiple layers may be sequentially formed, with each layer being dried before the next layer is formed. In this way, a single tape casting system may be used to form a coating system  14  or  44 . 
     In other examples, a tape may be formed using gel casting or slurry casting. In both gel casting and slurry casting, a slurry may be formed. The slurry in gel casting may include any of the components described above (e.g., any of the pre-gellant materials). Similarly, the slurry in slurry casting may include particles, a binder, a solvent, and optionally, additives. The slurry may be deposited in a mold, which may define the shape of the tape. The slurry then may be dried to remove the solvent, and, in the case of gel casting, gelled, either during or after the slurry is dried. 
     In some examples, regardless if slurry, gel, or tape casting is used, each layer may be formed separately (e.g., as shown in  FIG. 3 ). The resulting layers then may be combined to form a coating system  14  or  44 .  FIG. 4  is a conceptual diagram illustrating an example system  90  for assembling a multilayer tape  104  from multiple tapes  98 ,  100 , and  102 . As shown in  FIG. 4 , each tape  98 ,  100 , and  102  is initially carried by a respective roller  92 ,  94 , and  96 . Each of tapes  98 ,  100 , and  102  may include substantially similar or unique compositions, such as any of the coating compositions discussed above in reference to  FIG. 1 . 
     The tapes  98 ,  100 , and  102  are unwound from rollers  92 ,  94 , and  96 , and pass through a gap between a first set of forming rollers  106 . The first set of forming rollers  106  may exert a pressure against tapes  98 ,  100 , and  102 . The tapes  98 ,  100 , and  102  also may pass through a second set of forming rollers  106 , which also may exert a pressure against tapes  98 ,  100 , and  102 . In some examples, the temperature of system  90 , or at least the portion of system  90  near first set of forming rollers  106  and second set of forming rollers  108 , may be maintained above the brittle to ductal transition temperature for a constituent of the tape, such as, for example, silicon (e.g., silicon metal) or the silicon alloy. The first set of forming rollers  106  and the second set of forming rollers  108  thus may press tapes  98 ,  100 , and  102  together to form multilayer tape  104 . In some examples, multilayer tape  104  may be sufficiently flexible to be rolled on roller  110  without damage to multilayer tape  104 . 
     In some examples, multilayer tape  104  may be subsequently cut or otherwise formed into different sizes or shapes, e.g., shapes substantially conforming to the geometry of the component (e.g., component  10 ) on which the piece of multilayer tape  104  is to be used (e.g., as coating system  14 ). In some examples, a multilayer tape  104  may not be sufficiently flexible  104  to be manipulated into different shapes, and, instead, may be formed in a mold corresponding to the shape of the joint in which the piece of multilayer tape is to be used. 
       FIG. 5  is a flow diagram illustrating an example technique for applying a tape cast coating onto a component. The technique of  FIG. 5  will be described with reference to the component  40  of  FIG. 2  and tape casting system  70  of  FIG. 4  for ease of description, although the technique may be used to form other components (e.g., component  10  of  FIG. 1 ) or with other systems. Additionally, other techniques may be used to form components  10  and  40 . 
     The technique of  FIG. 5  includes forming a tape (e.g., pre-sintered coating system  14 ) defining at least one layer  46  that includes a first segment  50 A and a second segment  50 B ( 112 ). The first segment  50 A includes a first coating material, such as any of the coating materials discussed above in reference to  FIG. 1 . The second segment  50 B includes a second coating material, such as any of the coating materials discussed above in reference to  FIG. 1 , that is difference from the first coating material. First segment  50 A and second segment  50 B may be disposed substantially in the same plane, e.g., within tolerances of common casting techniques. At least a portion of the second segment  50 B is directly adjacent to at least a portion of the first segment  50 A. In some examples, layer  16  may include at least one layer comprising a silicon-containing braze material. 
     In some examples, forming the tape may include applying, by a tape casting system  70 , a first slurry containing the first coating material to a carrier film  74 . The technique also may include applying, by the tape casting system  70 , a second slurry containing the second braze material to the carrier film  74  adjacent to the first slurry. The technique may include, before or after applying the second slurry, drying the first slurry. The technique may include, before, after, or simultaneously with drying the first slurry, drying the second slurry for form the tape. 
     The technique illustrated in  FIG. 5  also includes positioning the tape on a surface  43  of a substrate  42  ( 114 ). In some examples, positioning the tape may include positioning the tape on a portion of substrate  12  that would be shadowed in a thermal spray process. In some examples, positioning the tape may include positioning the tape on a portion of substrate  12  to improve a mechanical and/or chemical property of component  10 . 
     In some examples, forming the tape ( 114 ) may include at least partially sintering at least one of the first coating material or the second coating material. The technique then may include infiltrating cracks in the partially sintered first or second coating material with a third coating material. The third coating material may be the same as the first or second coating material, or a different material. The technique also may include sintering the infiltrated tape to at least partially fill a porosity or cracks in the tape. 
     The technique illustrate in  FIG. 5  also includes heating the tape to sinter a constituent of at least one of the first coating material and the second coating material to form a densified coating on surface  43  of substrate  42  ( 116 ). In some examples, heating the tape may include heating the tape and/or substrate  10  in a box furnace. In some examples, heating the tape may include heating the tape using a hot isostatic press. Heating the tape may include heating the tape and/or substrate  10  to a temperature between about 1200° C. and about 1600° C., such as between about 1327° C. and about 1427° C. In some examples, molten silicon-containing material may flow into or between adjacent layers  46  and/or segments  50 . In some examples, molten silicon-containing material may react with carbon, e.g., from an organic binder system in layers  46  and/or segments  50  to form silicon carbide. 
     In some examples, the technique may include applying pressure to compress coating system  44  onto surface  43  of substrate  44 . For example, a clamp, a press, or similar device may be used to apply pressure to coating system  44  during heating ( 116 ), which may transmit the force to coating system  44  and compress coating system  44 . 
     Various examples have been described. These and other examples are within the scope of the following claims.