Patent Publication Number: US-2020277694-A1

Title: Thermal spray deposited environmental barrier coating

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 62/812,524, filed Mar. 1, 2019, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to techniques for forming environmental barrier coatings using thermal spray deposition. 
     BACKGROUND 
     Ceramic or ceramic matrix composite (CMC) materials may be useful in a variety of contexts where mechanical and thermal properties are important. For example, components of high temperature mechanical systems, such as gas turbine engines, may be made from ceramic or CMC materials. Ceramic or CMC materials may be resistant to high temperatures, but some ceramic or CMC materials may react with some elements and compounds present in the operating environment of high temperature mechanical systems, such as water vapor. Reaction with water vapor may result in the recession of the ceramic or CMC material. These reactions may damage the ceramic or CMC material and reduce mechanical properties of the ceramic or CMC material, which may reduce the useful lifetime of the component. Thus, in some examples, a ceramic or CMC material may be coated with an environmental barrier coating, which may reduce exposure of the substrate to elements and compounds present in the operating environment of high temperature mechanical systems. 
     SUMMARY 
     In some examples, the disclosure describes a method that comprises depositing an environmental barrier coating (EBC) on a substrate via a thermal spray apparatus to form an as-deposited EBC; heat treating the as-deposited EBC at or above a first temperature for first period of time following the deposition of the as-deposited EBC on the substrate; and cooling the as-deposited EBC to a second temperature following the heat treatment at a controlled rate over a second period of time to form a heat-treated EBC on the substrate, wherein the first temperature, the first period of time, the controlled rate, and the second period of time are selected to increase a weight percent of crystalline phase in the heat-treated EBC compared to the as-deposited EBC 
     In some examples, the disclosure describes a system comprising a thermal spray device configured to deposit an environmental barrier coating (EBC) on a substrate to form an as-deposited EBC; a furnace configured to heat the as-deposited EBC following deposition of the as-deposited EBC by the thermal spray device; and a computing device configured to control the thermal spray device to deposit the EBC on the substrate to form the as-deposited EBC, control the heat treatment of the as-deposited EBC at or above a first temperature for a first period of time following the deposition of the as-deposited EBC on the substrate, and control the cooling of the as-deposited EBC to a second temperature following the heat treatment at a controlled rate over a second period of time, wherein the first temperature, the first period of time, the controlled rate, and the second period of time are selected to increase a weight percent of crystalline phase in the heat-treated EBC compared to the as-deposited EBC. 
     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 DRAWINGS 
         FIG. 1  is a conceptual and schematic diagram illustrating an example system for forming an EBC on a substrate in accordance with an example of the disclosure. 
         FIG. 2  is a conceptual block diagram illustrating an example thermal spray device. 
         FIG. 3  is a flow diagram illustrating an example technique for forming EBC on a substrate. 
         FIG. 4  is a flow diagram illustrating another example technique for forming EBC on a substrate. 
         FIG. 5  is a conceptual and schematic diagram illustrating an example article including an EBC on a substrate. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure describes systems and techniques for forming an environmental barrier coating (EBC) system using thermal spray deposition, such as air plasma spraying. The EBC coating system may be deposited on a substrate, such as, CMC substrates, that serves as components of jet engines or other high temperature systems. Thermal spray systems may be used in a wide variety of industrial applications to coat such substrates with EBC systems to modify or improve the properties of underlying substrate or component as a whole. Thermal spray systems may use heat generated electrically, by plasma, or by combustion to heat material injected in a plume, so that molten or softened material propelled by the plume contact the surface of the target. Upon impact, the molten or softened material adheres to the target surface, resulting in a coating. 
     EBC systems may be an important contributor to the success of CMCs in a high temperature system. For example, the coatings may be configured to protect against oxidation, water vapor recession, and other deleterious reactions from damaging the structural CMC, e.g., during operation of the high temperature system. In some examples, an EBC system may contain a multilayered structure including a silicon bond layer and a rare-earth disilicate layer. The layers of the EBC system may be deposited using a thermal spraying process, such as, air plasma spraying, which may produce an amorphous structure within the coating, e.g., due to the high cooling rates/quenching of the particles upon impact with a substrate. The resulting amorphous structure may change to a crystalline structure over time when subjected to higher temperatures, e.g., during operation of a high temperature system. An uncontrolled transition from amorphous to crystalline structure over time may also result in volumetric changes and, thus, internal stresses in the layer(s) (e.g., a rare-earth disilicate layer). In particular, in some examples, as the EBC structure changes from amorphous to crystalline, there may be shrinkage in the overall area. This may cause a build-up in stress on the EBC as well as the silicon bond coat. Eventually, the build-up in stress reaches a threshold and causes a crack to pop to relieve the stress state. 
     In accordance with examples of the disclosure, systems and techniques are described that include controlling the substrate and/or coating temperatures before, during and/or after deposition of an EBC system, e.g., to increase or otherwise control the amount of crystalline phase in the EBC system. The crystalline phase of an EBC system may be controlled to reduce internal stresses during operation of a coated component due to the amorphous phase to crystalline phase transition. In some examples, following deposition of one or more layers of an EBC system, the coated substrate (EBC system and underlying substrate) may be heat treated at a relatively high temperature for a selected duration of time. Following the heat treatment, the coated substrate may be cooled at a controlled rate, e.g., such that the coated substrate (e.g., the EBC system on the underlying substrate) cools at a desired rate and/or for a desired duration of time to a prescribed temperature. Some examples systems and techniques of the disclosure include controlling the post-deposition heating and cooling rate of a coated substrate through a selected temperature range during which an amorphous phase would otherwise form if cooled at too high a rate, e.g., between 800 degrees Celsius (C) and 1100 degrees C. 
     Controlling the post-deposition temperature, cooling rate, and/or time may allow for a transition from amorphous to crystalline phase in one or more layers of the deposited coating system, e.g., in a manner that minimizes or otherwise reduces the internal stresses in the layer(s) of the EBC system, e.g., that would otherwise be present during heating of the EBC system during operation of a jet engine including the coated component. For example, thermal sprayed rare earth silicates may effectively quench in an amorphous phase during rapid solidification on a cold substrate that is below the amorphous-crystalline transition temperature. Upon heating the coating past amorphous-crystalline transition temperature, two events may occur: 1) transformation from amorphous to crystalline atom structure, and 2) viscous flow of the amorphous coating prior to the phase transformation (may not occur if heating rate is too rapid). The combination of these events may act to resolve the residual stress. In some example, the goal may be to have fully crystalline coatings (e.g., the one or more layers of the EBC system being substantially all crystalline phase with minimum, relatively low, or trace amounts of amorphous phase. In some examples, the one or more layers of an EBC system may have about 95 wt % crystallinity post-heat treatment (e.g., including a controlled cooling phase). 
     In some examples, when the one or more layers of an EBC system is sprayed onto a cold substrate, the coating locks in an amorphous microstructure. When the amorphous structure is heated, the coating transitions to a crystalline (lower energy state) microstructure. During this phase change, the overall volume decreases, causing a build-up of residual stress. If this stress is significant, it will crack the EBC and/or substrate. By controlling the post-deposition heat treatment, cooling rate, and/or temperature of deposition, the rate at which the stresses form may be controlled and/or the residual stress may be relaxed out. In some examples, the heat treatment temperature and/or cooling rate of a deposited coating may be controlled to obtain a relaxed EBC system, e.g., prior to employing a coated substrate in operation as part of a high temperature gas turbine engine. 
     In some examples, systems and techniques of the disclosure include movement of a substrate from a spraying position within the air plasma spray system or other thermal spray system to a furnace with temperature control following deposition of one or more layers of an EBC coating system on the substrate. Such a transfer may employ the use of robotic systems and fixtures for substrate holding and manipulation, thus allowing for fast transition of a substrate from spray position to the furnace upon completion of the spray process. In some examples, this transition may take place relatively quickly following the coating of the substrate and prior to the component cooling below a threshold temperature, e.g., below 800 degrees C. or some other pre-heated temperature in examples in which substrate pre-heating has taken place. The coated substrate temperature may then be maintained in the furnace for a duration that allows for the transition of the coating to a more highly crystalline state, e.g., depending on furnace configuration, fixturing size, overall thermal load, and/or the like. 
     In some examples, systems and techniques of the disclosure include pre-heating of a substrate to a prescribed temperature prior to coating in a furnace (or other suitable heating system). In such instances, a robotic system may withdraw the substrate from the furnace after being heated to the prescribed temperature to the thermal spray system fixturing apparatus, upon which the coating is applied and maintained at elevated temperature. The coated substrate may be then re-introduced to the furnace (or moved to another furnace or other suitable heating system) for further controlled heat treatment, e.g., to enhance microstructure, crystallinity, and/or residual stress. In other examples, systems and techniques of this disclosure may include a post-coating heat treatment that is effective in controlling the crystalline structure of deposited layers even in the absence of pre-heating of a substrate and/or in-situ heating of the substrate during the deposition process. 
       FIG. 1  is a conceptual and schematic diagram illustrating an example system  10  for depositing an EBC system on a substrate using a thermal spray process that includes controlled heating before and/or after the deposition of the EBC system. As shown, system  10  includes thermal spray device  12 , pre-heat/post-heat furnace  14  (referred to also as “furnace  14 ”), and robotic transfer device  16 . Although furnace  14  is shown as a single furnace in  FIG. 1 , in some examples, system  10  may include more than one furnace, e.g., one furnace for the pre-deposition heat treatment and another furnace for post-deposition heat treatment. 
     Thermal spray device  12  may be configured to deposit one or more layers of a coating system on a substrate to form a coated article, such as article  66  in  FIG. 5  which includes EBC system  68  on substrate  24 , using a thermal spray process. Example thermal spray processes may include suspension plasma spray, low pressure plasma spraying, plasma spray physical vapor deposition, and air plasma spraying. As will be described below, in one example, thermal spray device  12  may be configured to deposit the one or more layers of a coating system using a plasma spray process, such as an air plasma spray process. In an air plasma spray process, the plasma is sprayed in an air environment, e.g., as compared to a spraying in a vacuum or an inert gas (e.g., argon) environment. Air plasma spraying may be amenable to automation for the application of coatings onto complex surfaces. The deposition rates may be very economical compared to other processes such as HVOF. For ease of description, the operation of system  10  will primarily be described herein with regard to article  66  of  FIG. 5  although other articles formed using system  10  are contemplated. 
     Furnace  14  may be configured to heat and/or maintain article  66  at a relatively high temperature following the deposition of layer(s) of a coating system using thermal spray device  12 , e.g., to perform a post-deposition heat treatment on the coated substrate. Furnace  14  may also be configured to cool down article  66  following the post-deposition heat treatment at a controlled rate over a duration of time to a reduced temperature. Furnace  14  may also be configured to heat and/or maintain substrate  24  at a relatively high temperature following the deposition of layer(s) of a coating system using thermal spray device  12 , e.g., to perform a pre-deposition heat treatment of substrate  24 . Furnace  14  may include an internal cavity sized and otherwise configured to contain substrate  14 , either before or after being coated with EBC system  68 . Any suitable type of furnace  14  may be used that is capable of functioning as described in this disclosure. Furnace  14  may be an air furnace or a box furnace. In one example, a box furnace may be used with a controllable heat source. In some examples, furnace  14  may include one or more suitable heat sources such as moly-disilicide and/or carbide heating elements, although other types of heat sources are contemplated. In one example, a conveyor-belt furnace may be employed. 
     Robotic transfer device  16  may be configured to robotically transfer substrate  24  between furnace  14  and thermal spray device  12 , as desired before and/or after the deposition of EBC system  68  vie thermal spray device  12 . 
     Computing device  18  may be configured as a control device that controls thermal spray device  12 , furnace  14 , and/or robotic transfer device  16  to operate in the manner described herein. For example, computing device  18  may be configured to control the temperature, including heating and cooling rates, of furnace  14 , e.g., during pre-deposition and/or post-deposition heat treatment of substrate  24  and article  66 , respectively. Computing device  18  may be configured to control robotic transfer device  16  to control the transfer of substrate  24  and article  66  between thermal spray device  12  and furnace  14 . Computing device  18  may be communicatively coupled to at least one of thermal spray device  12 , furnace  14 , and/or robotic transfer device  16  using respective communication connections. Such connections may be wireless and/or wired connections. While computing device  18  is shown as a single device, in other examples, computing device  18  may be more than one computing device, such as, e.g., where each of furnace  14 , thermal spray device  12  and robotic transfer device  16  are controlled by different computing devices. 
     Computing device  18  may include, for example, a desktop computer, a laptop computer, a workstation, a server, a mainframe, a cloud computing system, or the like. Computing device  18  may include or may be one or more processors or processing circuitry, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor” and “processing circuitry” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some examples, the functionality of computing device  18  may be provided within dedicated hardware and/or software modules. 
     In one example, system  10  may be configured to form an article such as article  66  shown in  FIG. 5 , which includes EBC system  68  deposited on substrate  24 . For example, system  10  may be configured to deposit one or more layers of EBC system  66  on substrate  24  using thermal spray device  12 , e.g., by air plasma spraying or other thermal spray deposition process. Following the deposition of EBC system  66  on substrate  24  by thermal spray device  12 , article  66  may be moved to furnace  14  (e.g., via robotic transfer device  16 ) for a post deposition heat treatment. As will be described further below, the post-deposition heat treatment in furnace  14  may be controlled by computing device  18  so that article  66  is at an elevated temperature (e.g., a temperature at or above the crystallization temperature of EBC system  68 ) for a desired duration of time. Computing device  18  may also control the cooling of article  66  such that article  66  is cooled, e.g., at a specific rate to a lower temperature over a duration of time, as compared to only removing article  66  from furnace  14  so that it cool based on the ambient temperature of the surrounding environment. In some examples, the post-deposition heat treatment and/or controlled cool down in furnace  14  may provide for an increase in the amount of crystalline phase to amorphous phase in EBC system  68 , e.g., as compared to an article in which the EBC system is deposited by thermal spray device  12  without such heat treatment and/or control cool down. In some examples, system  10  may form article  66  using such a technique with an optional pre-heating of substrate  24  within furnace  14 , where substrate  24  is heated to an elevated temperature of a desired duration of time before being transferred to thermal spray device  12  for the deposition of EBC system  68 . 
       FIG. 2  is a block diagram illustrating the example thermal spray system  12  of  FIG. 1 . In the example of  FIG. 2 , thermal spray system  12  includes components such as enclosure  20  and a thermal spray gun  22 . Enclosure  20  encloses some components of thermal spray system  12 , including, for example, thermal spray gun  22 . In some examples, enclosure  20  substantially completely surrounds thermal spray gun  22  and encloses an atmosphere. The atmosphere may include, for example, air, an inert atmosphere, a vacuum, or the like. In some examples, the atmosphere may be selected based on the type (e.g., composition) of coating being applied using thermal spray system  12 . Enclosure  20  also encloses a spray target  24 . 
     Spray target  24  include a substrate to be coated using thermal spray system  12 . In some examples, spray target  24  may include, for example, a substrate on which a bond coat, a primer coat, a hard coat, a wear-resistant coating, a thermal barrier coating, an EBC system, or the like is to be deposited. Spray target  24  may include a substrate or body of any regular or irregular shape, geometry or configuration. In some examples, spray target  24  may include metal, plastic, glass, or the like. Spray target  24  may be a component used in any one or more mechanical systems, including, for example, a high temperature mechanical system such as a gas turbine engine. 
     Thermal spray gun  22  is coupled to a gas feed line  26  via gas inlet port  134 , is coupled to a spray material feed line  30  via material inlet port  32 , and includes or is coupled to an energy source  124 . Gas feed line  26  provides a gas flow to gas inlet port  134  of thermal spray gun  22 . Depending upon the type of thermal spray process being performed, the gas flow may be a carrier gas for the coating material, may be a fuel that is ignited to at least partially melt the coating material, or both. Gas feed line  26  may be coupled to a gas source (not shown) that is external to enclosure  20 . 
     Thermal spray gun  22  also includes a material inlet port  32 , which is coupled to spray material feed line  30 . Material feed line  30  may be coupled to a material source (not shown) that is located external to enclosure  20 . Coating material may be fed through material feed line  30  in powder form, and may mix with gas from gas feed line  26  within thermal spray gun  22 . The composition of the coating material may be based upon the composition of the coating to be deposited on spray target  24 , and may include, for example, a metal, an alloy, a ceramic, or the like. 
     Thermal spray gun  22  also includes energy source  34 . Energy source  34  provides energy to at least partially melt the coating material from coating material provided through material inlet port  32 . In some examples, energy source  34  includes a plasma electrode, which may energize gas provided through gas feed line  26  to form a plasma. In other examples, energy source  34  includes an electrode that ignites gas provided through gas feed line  26 . 
     As shown in  FIG. 2 , an exit flow stream  38  exits outlet  36  of thermal spray gun  22 . In some examples, outlet  36  includes a spray gun nozzle. Exit flow stream  38  may include at least partially melted coating material carried by a carrier gas. Outlet  36  may be configured and positioned to direct the at least partially melted coating material at spray target  24 . 
     Computing device  18  may be configured to control operation of one or more components of thermal spray system  12  automatically or under control of a user. For example, computing device  18  may be configured to control operation of thermal spray gun  22 , gas feed line  26  (and the source of gas to gas feed line  26 ), material feed line  30  (and the source of material to material feed line  30 ), and the like. For example, computing device  18  may be configured to control at least one of a temperature, a pressure, a mass flow rate, a volumetric flow rate, a molecular flow rate, a molar flow rate, a composition or a concentration, of a flow stream flowing through thermal spray system  12 , for instance, of gas flowing through gas feed line  26 , or of exit flow stream  38 , or of material flowing through material feed line  30 . 
     In some examples, thermal spray device may include a stage or other component configured to selectively position and restrain substrate  24  in place during formation of coating  66 . In some examples, the stage or other component is movable relative to thermal spray gun  22 . For example, in this manner, substrate  24  may be translatable and/or rotatable along at least one axis to position substrate  24  relative to plasma spray gun  22 . Similarly, in some examples, plasma spray gun  22  may be movable relative to substrate  24  to position plasma spray device  20  relative to substrate  24 . 
     In some examples, the temperature within enclosure  20  may be controlled by computing device  18 . For example, computing device  18  may elevate the temperature in enclosure  20  above room temperature during the thermal deposition of EBC system  68 . In other examples, enclosure  20  is not heated but substrate  24  may be pre-heated in furnace  14  and/or the backside of substrate  24  (surface of substrate opposite the deposition surface) may be heated only (e.g., via a furnace or other heating device) during the deposition process. 
     In some examples, computing device  18  may employ one or more temperature sensors to monitor the temperature of enclosure  20  to use as feedback to control the temperature of enclosure  20 , substrate  24 , and/or EBC system  68 . In some examples, a temperature sensor may directly monitor the temperature of the deposition surface of substrate  24  and/or EBC system  68  to use as a feedback to control the temperature of enclosure  20 , substrate  24 , and/or EBC system  68 . Computing device  18  may control the temperature to maintain a surface temperature of substrate  24  conducive to the production of crystalline coatings. In some examples, computing device  18  may control the temperature of substrate  24  to be about 800 degrees Celsius to about 1100 degrees Celsius, such as, about 850 degree Celsius or greater. In some examples, the method of control may be a line of site, non-contact surface measurement, e.g., given that the part may be in motion while coating. 
       FIG. 5  is a conceptual schematic diagram illustrating article  66  that may be formed using system  10  of  FIG. 1 . In some examples, article  66  may include a component of a gas turbine engine. For example, article  66  may include a part that forms a portion of a flow path structure, a seal segment, a blade track, an airfoil, a blade, a vane, a combustion chamber liner, or another portion of a gas turbine engine. 
     As described above, article  66  includes EBC system  68  formed on substrate  24 . EBC system  68  may be a single layer or multi-layer coating, where each layer has substantially the same or different compositions. 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. 
     Substrate  24  may include a material suitable for use in a high-temperature environment. In some examples, substrate  24  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  24  includes a ceramic, the ceramic may be substantially homogeneous. 
     In examples in which substrate  24  includes a CMC, substrate  24  may include a matrix material and a reinforcement material. The matrix material may include, for example, silicon metal or a ceramic material, such as silicon carbide (SiC), silicon nitride (Si 3 N 4 ), an aluminosilicate, silica (SiO 2 ), a transition metal carbide or silicide (e.g., WC, Mo 2 C, TiC, MoSi 2 , NbSi 2 , TiSi 2 ), or another ceramic material. The CMC may further include a continuous or discontinuous reinforcement material. 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 reinforcement material may include carbon (C), silicon carbide (SiC), silicon nitride (Si 3 N 4 ), an aluminosilicate, silica (SiO 2 ), a transition metal carbide or silicide (e.g. WC, Mo 2 C, TiC, MoSi 2 , NbSi 2 , TiSi 2 ), or the like. 
     Substrate  12  may be manufactured using one or more techniques including, for example, chemical vapor deposition (CVD), chemical vapor infiltration (CVI), polymer impregnation and pyrolysis (PIP), slurry infiltration, melt infiltration (MI), combinations thereof, or other techniques. 
     EBC system  68  may help protect underlying substrate  24  from chemical species present in the environment in which article  66  is used, such as, e.g., water vapor, calcia-magnesia-alumina-silicate (CMAS; a contaminant that may be present in intake gases of gas turbine engines), or the like. Similarly, the EBC system may also be CMAS resistant, e.g., the EBC system itself may be resistant to damage caused by CMAS. Similarly, EBC system  66  may also be CMAS resistant, e.g., the EBC system itself may be resistant to damage caused by CMAS. Additionally, in some examples, EBC system  68  may also protect substrate  24  and provide for other functions besides that of an EBC, e.g., by functioning as a thermal barrier coating (TBC), abradable coating, erosion resistant coating, and/or the like. 
     Although not directly shown in  FIG. 5 , in some examples, article  66  may include a bond coat between EBC system  68  and substrate  24 , e.g., where the bond layer is directly on substrate  24  and EBC system  68  is directly on the bond layer. The bond layer may increase the adhesion between substrate  24  and EBC system  68 . In some examples, the bond coat has a thickness of approximately 25 microns to approximately 250 microns, although other thicknesses are contemplated. In examples in which substrate  24  includes a ceramic or CMC, the bond coat may include a ceramic or another material that is compatible with the material from which substrate  12  is formed. For example, the bond coat may include mullite (aluminum silicate, Al 6 Si 2 O 13 ), silicon metal or alloy, silica, a silicide, or the like. The bond coat may further include other elements, such as a rare earth silicate including a silicate 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), and/or scandium (Sc). 
     EBC system  68  may include one or more EBC layers, which may be configured to help protect substrate  24  against deleterious environmental species, such as CMAS and/or water vapor. The layer(s) of EBC system  68  may include at least one of a rare-earth oxide, a rare-earth silicate, an aluminosilicate, or an alkaline earth aluminosilicate. For example, the layer(s) of EBC system  68  may include mullite, barium strontium aluminosilicate (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 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). EBC system  68  may be any suitable thickness. For example, EBC system  68  may be about 0.005 inches (about 127 micrometers) to about 0.100 inches (about 2540 micrometers). Other thicknesses are contemplated. 
     In some examples, the layer(s) of EBC system  68  additionally and optionally may include at least one additive, such as at least one of silica, a rare earth oxide, alumina, an aluminosilicate, an alkali metal oxide, an alkaline earth metal oxide, an alkali metal aluminosilicate, an alkaline earth aluminosilicate, TiO 2 , Ta 2 O 5 , HfSiO 4 , or the like. The additive may be added to the EBC to modify one or more desired properties of the EBC. For example, the additive components may increase or decrease the reaction rate of the EBC with calcia-magnesia-alumina-silicate (CMAS; a contaminant that may be present in intake gases of gas turbine engines), may modify the viscosity of the reaction product from the reaction of CMAS and constituent(s) of the EBC, may increase adhesion of the EBC to the bond coat, may increase the chemical stability of the EBC, or the like. 
       FIG. 3  is a flow diagram illustrating an example technique for forming a coating that includes an environmental barrier coating on a substrate using a thermal spray process. The technique of  FIG. 3  will be described with respect to system  10  of  FIG. 1  and article  66  of  FIG. 5  for ease of description only. A person having ordinary skill in the art will recognize and appreciate that the technique of  FIG. 3  may be implemented using systems other than system  10  of  FIG. 1 , may be used to form articles other than article  68  of  FIG. 5 , or both. 
     As shown in  FIG. 3 , system  10  may optionally perform a pre-deposition heat treatment on substrate  24  within furnace  14  prior to EBC system  68  being deposited on substrate  24  ( 40 ). For example, substrate  24  may be placed within furnace  14 , e.g., by robotic transfer device  16  under the control of computing device  18 . Furnace  14  may be heated to a desired pre-heat temperature under control of computing device  18  before or after substrate  24  is inserted in furnace  14 . The pre-heat temperature of furnace  14  may be selected such that the temperate of substrate  24  is elevated above the ambient temperature of the external environment and/or the ambient temperature within thermal spray device  12 . In some examples, the pre-heat temperature of furnace  14  is at least about 800 degrees Celsius (C), such as, about 850 degrees C. to about 1100 degrees C., about 900 degrees C. to about 1100 degrees C., or about 850 degrees C. to about 1400 degrees C. Substrate  24  may be held within furnace  24  for a sufficient amount of time to elevate the temperature of substrate  24  to the pre-heat temperature of furnace  14  and/or to a temperature of at least about 800 degrees Celsius (C), such as, about 850 degrees C. to about 1100 degrees C., about 900 degrees C. to about 1100 degrees C., or about 850 degrees C. to about 1400 degrees C. Other values than those listed above are contemplated. 
     Once substrate  24  is optionally pre-heated to the desired pre-deposition temperature, robotic transfer device, under the control of computing device  18 , may transfer substrate  24  to the desired spray position within thermal spray device  12  ( 42 ). Once in the desired spray position, the one or more layers of EBC system  68  may be deposited on substrate  24  by thermal spraying (e.g., air plasma spraying) using thermal spray device  12  ( 44 ). As described above, thermal spray device  12  may deposit the one or more layers of EBC system  68  under the control of computing device  18 . In some examples, the temperature within thermal spray device  12  is elevated. 
     In some examples, substrate  24  may have a temperature of about 800 to about 1100 degrees C., such as, about 850 degrees C. or greater or about 850 degrees C. to about 1400 degrees C. when the material of EBC system  68  is first deposited by thermal spray device  12 . In cases in which substrate  24  is pre-heated in furnace  14 , the transfer time of substrate  24  between furnace  14  and initial thermal spraying may be relatively short to prevent substantially cooling of substrate  24  from that of the pre-heating temperature. 
     When EBC system  68  is deposited, the layer(s) of system  68  may have a relatively high amorphous phase concentration, e.g., due to the high cooling rates/quenching of the particles upon impact with substrate  24 . For example, the layer(s) of EBC system  68  may have an amorphous phase of at least about 85 wt %. Conversely, the layer(s) of EBC system  68  may have a crystalline phase of less than about 15 wt %. As noted above, without a post-deposition heat treatment, the amorphous phase may change to a crystalline structure over time when subjected to higher temperatures, e.g., during operation of a jet engine. An uncontrolled transition from amorphous to crystalline structure with time may also result in volumetric changes and, thus, internal stresses in the layer(s). 
     In accordance with examples of the disclosure, following deposition of EBC system  68  on substrate  24 , article  66  may be transferred to furnace  14  by robotic transfer device for a post-deposition heat treatment ( 46 ). In some examples, the post-deposition heat treatment may take place before or after article  66  cools to room temperature following deposition. The post-deposition heat treatment temperature and duration within furnace  14  may be controlled by computing device  18  and may be selected to increase the crystalline phase concentration of EBC system  68  on substrate  24 . For example, furnace  14  may be at a treatment temperature of at or above the crystalline temperature of the layer(s) of EBC system  68 . In some examples, furnace  14  may be at a treatment temperature of at least about 850 degrees C., such as, e.g., about 850 degrees C. to about 1400 degrees C., about 900 degrees C. to about 1400 degrees C., about 850 degrees C. to about 1100 degrees C., about 1000 degrees C. to about 1200 degrees C., or about 900 degrees C. to about 1100 degrees C., and less than about 1400 degrees C. Computing device  18  may control furnace  18  to hold a substantially constant heat treatment temperature within furnace or a heat treatment temperature that varies within a prescribed range over a selected period of time. 
     Article  66  may be held within furnace  14  at the heat treatment temperature such that EBC system  68  reaches a temperature at or above the crystalline phase temperature of the one or more layers of EBC system  68 . Article  66  may be held within furnace  14  at the heat treatment temperature such that EBC system  68  reaches a temperature at or above the temperature of the one or more layers of EBC system  68  at which the amorphous phase transitions to a crystalline phase. In some examples, depending on the composition of the layer(s), the layer(s) of EBC system  68  may have a temperature of at least about 850 degrees C., such as, e.g., about 850 degrees C. to about 1400 degrees C., about 900 degrees C. to about 1400 degrees C., about 850 degrees C. to about 1100 degrees C., about 1000 degrees C. to about 1200 degrees C., or about 900 degrees C. to about 1100 degrees C., and less than about 1400 degrees C. during the post-deposition heat treatment. Article  66  may be held within furnace  14  for heat treatment for a suitable amount of time to provide for a desired amount of crystalline phase in EBC system  68 . Values other than that described above are contemplated. 
     Following the post-deposition cooling, article  66  may undergo a controlled cooling within furnace  14  ( 48 ) from that of the heat treatment temperature. For example, computing device  18  may control the rate of cooling of furnace  18  over a particle period of time such that article  66  cools at a controlled rate over the period of time, as compared to simply removing article  66  from furnace  14  and or simply turning off furnace  18  while article  66  is inside. By controlling the cooling of article  66  for a period of time following the heat treatment, the amount of crystalline phase may be further tailored, e.g., by not cooling EBC system  68  too fast at room temperature. 
     In some examples, computing device  18  may control the cooling of article  68  such that the temperature of EBC layer  68  cools at a rate of about 5 degrees C./minute or less. In some examples, the cooling of article  66  is controlled until the temperature of EBC system  68  is at or below about 500 degrees C. 
     In some examples, the heat treatment and/or controlled cooling of article  66  within furnace  14  may be selected to increase the crystalline phase concentration and/or decrease the amorphous phase concentration within EBC system  68  compared to that of the amorphous and crystalline phase content of EBC system  68  following deposition by thermal spray device  14  but before the heat treatment and/or controlled cooling In some examples, the heat treatment and/or cooling of article  66  within furnace  14  may be selected to increase the crystalline phase concentration and/or decrease the amorphous phase concentration within EBC system  68  compared to that of the amorphous and crystalline phase content of EBC system  68  following deposition by thermal spray device  14  but without any post-deposition heat treatment and/or controlled cooling. In some examples, increasing the crystalline phase content of the layer(s) of EBC system  68  may reduce or eliminate the undesired issues that may arise from amorphous phase being present in EBC system  68 , e.g., as described above. 
     In some examples, EBC system  68  may have an amorphous phase of less than about 50 wt % following the heat treatment describe above. In some examples, EBC system  68  may have a crystalline phase of greater than about 50 wt %, such as, e.g., greater than about 60 wt %, greater than about 70 wt %, greater than about 80 wt %, greater than about 90 wt %, greater than about 95 wt %, greater than about 96 wt %, less than about 96 wt %, less than 100 wt %, about 50 wt % and less than 100 wt %, or substantially all crystalline phase following the heat treatment describe above. In some examples, the remainder of EBC system  68  may be amorphous phase. In some examples, the amorphous phase content of the layer(s) of EBC system  68  may be decreased compared to an article such as article  66  that does not undergo the described post-deposition heat treatment. In some examples, the crystalline phase content of the layer(s) of EBC system  68  may be increased by compared to an article such as article  66  that does not undergo the described post-deposition heat treatment. Other values are contemplated. 
     In some examples, EBC system  68  may have an amorphous phase of less than the as deposited coating following the heat treatment and controlled cooling describe above. In some examples, EBC system  68  may have a crystalline phase of greater than about 50 wt %, such as, e.g., greater than about 60 wt %, greater than about 70 wt %, greater than about 80 wt %, greater than about 90 wt %, greater than about 95 wt %, greater than about 96 wt %, less than about 96 wt %, less than 100 wt %, about 50 wt % and less than 100 wt %, or substantially all crystalline phase following the heat treatment and controlled cooling describe above. In some examples, the amorphous phase content of the layer(s) of EBC system  68  may be decreased compared to an article such as article  66  that does not undergo the described post-deposition heat treatment. In some examples, the crystalline phase content of the layer(s) of EBC system  68  may be increased compared to an article such as article  66  that does not undergo the described post-deposition heat treatment. Other values are contemplated. 
     In some examples, in order to achieve and control desired temperature(s), system  10  may be configured to monitor the temperature of EBC system  68 , substrate  24 , and/or furnace  14  using one or more suitable temperature sensors (e.g., thermocouples) located to accurately measure temperature (e.g., in substantially real-time) during the described techniques. In some examples, such components may be thermocoupled during process development trials to confirm that the desired heating/cooling rates are as expected, with the measured temperatures in that particular furnace zone used for control afterwards. 
       FIG. 4  is a flow diagram illustrating another example technique for forming a coating that includes an environmental barrier coating on a substrate using a thermal spray process. The technique of  FIG. 4  will be described with respect to system  10  of  FIG. 1  and article  66  of  FIG. 5  for ease of description only. A person having ordinary skill in the art will recognize and appreciate that the technique of  FIG. 4  may be implemented using systems other than system  10  of  FIG. 1 , may be used to form articles other than article  68  of  FIG. 5 , or both. 
     As shown in  FIG. 4 , substrate  24  may be pre-heated in furnace  14  to a temperature of at least about 850 degrees C. ( 50 ). Computing device  18  may start thermal spray gun  22  and a stabilizer feeder of thermal spray device  12  ( 52 ), e.g., while or after substrate  24  is being heated ( 50 ). The pre-heated substrate  24  may then be transferred from furnace  14  to the spray position within thermal spray device  12  by robotic transfer device  16  under the control of computing device  18  ( 54 ). Computing device  18  may then control plasma spray device  12  to deposit the one or more layers of EBC system  68  of article  66  on substrate via plasma spray coating while maintaining a substrate temperature of at least about 850 degrees C. ( 56 ). Once EBC system  68  is deposited on substrate  24 , article  66  is transferred from the spray position within thermal spray device  12  back to furnace  14  by robotic transfer device  16  under the control of computing device  18  for post-deposition heat treatment ( 58 ). Article  60  may be maintained at a temperature of at least about 850 degrees C. during the heat treatment for a desired period of time ( 60 ). Following the heat treatment, article  66  may be cooled in furnace  14  at a controlled rate (e.g., about 5 degrees C./minute or less) to a cooled temperature at or below about 500 degrees C. ( 62 ), at which time article  66  may be cooled in an uncontrolled fashion (e.g., outside furnace  14 ) to room temperature (e.g., about 23 degrees C.) ( 64 ). The technique of  FIG. 4  may be configured to form one or more layers of EBC system  68  (e.g., a rare-earth disilicate EBC). 
     The techniques described in this disclosure may be implemented, at least in part, in hardware, software, firmware, or any combination thereof. For example, various aspects of the described techniques may be implemented within one or more processors, including one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. A control unit including hardware may also perform one or more of the techniques of this disclosure. 
     Such hardware, software, and firmware may be implemented within the same device or within separate devices to support the various techniques described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware, firmware, or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware, firmware, or software components, or integrated within common or separate hardware, firmware, or software components. 
     The techniques described in this disclosure may also be embodied or encoded in a computer system-readable medium, such as a computer system-readable storage medium, containing instructions. Instructions embedded or encoded in a computer system-readable medium, including a computer system-readable storage medium, may cause one or more programmable processors, or other processors, to implement one or more of the techniques described herein, such as when instructions included or encoded in the computer system-readable medium are executed by the one or more processors. Computer system readable storage media may include random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, a hard disk, a compact disc ROM (CD-ROM), a floppy disk, a cassette, magnetic media, optical media, or other computer system readable media. In some examples, an article of manufacture may comprise one or more computer system-readable storage media. 
     Various examples have been described. These and other examples are within the scope of the following claims.