Patent Publication Number: US-2023160311-A1

Title: Coating fabrication method for producing engineered microstructure of silicate-resistant barrier coating

Description:
BACKGROUND 
     Components in a gas turbine engine often include barrier coatings to protect the underlying component from the effects of the severe operating environment. Barrier coatings are available in numerous varieties, which can include thermal barrier coatings and environmental barrier coatings. Thermal barrier coatings are typically designed for maximizing thermal insulation of a component from the surrounding high-temperature environment. Environmental barrier coatings are typically designed for maximizing resistance of infiltration or attack by the environment. 
     SUMMARY 
     A gas turbine engine article according to an example of the present disclosure includes a substrate, and a silicate-resistant barrier coating disposed on the substrate. The silicate-resistant barrier coating has an engineered microstructure including a refractory matrix formed of grains, and calcium aluminosilicate additive (CAS additive) dispersed in grain boundaries between the grains. 
     In a further embodiment of any of the foregoing embodiments, the refractory matrix is selected from zirconia, hafnia, hafnium silicate, zirconium silicate, rare earth silicates, rare earth oxides, mullite, silica, aluminum oxide, or combinations thereof, and the CAS additive includes SiO 2 , AlO 1.5 , and CaO. 
     In a further embodiment of any of the foregoing embodiments, the silicate resistant barrier coating includes, by mol. %, 0.1 to 30 of the CAS additive. 
     In a further embodiment of any of the foregoing embodiments, the silicate resistant barrier coating includes, by mol. %, 3 to 8 of the CAS additive. 
     In a further embodiment of any of the foregoing embodiments, the CAS additive includes, by mol. %, 30 to 90 of SiO 2 , 5 to 20 of AlO 1.5 , and 10 to 50 of CaO. 
     In a further embodiment of any of the foregoing embodiments, each said engineered granule is an aggregate of at least one refractory matrix region and at least one calcium aluminosilicate additive region (CAS additive region) attached with the at least one refractory matrix region. 
     In engineered granules according to an example of the present disclosure, each said engineered granule is an aggregate of: at least one refractory matrix region and at least one calcium aluminosilicate additive region (CAS additive region) attached with the at least one refractory matrix region. 
     In a further embodiment of any of the foregoing embodiments, each said granule is selected from a mixed granule, a core/shell granule, or combinations thereof. The mixed granule has a plurality of the refractory matrix regions and a plurality of the CAS additive regions attached with the plurality of refractory matrix regions, and the core/shell granule in which the at least one refractory matrix region is a coarse core particle and the at least one CAS additive regions is selected from a plurality of fine shell particles attached on the coarse core particle and a shell coating on the coarse core particle. 
     In a further embodiment of any of the foregoing embodiments, the at least one refractory matrix region is selected from zirconia, hafnia, hafnium silicate, zirconium silicate, rare earth silicates, rare earth oxides, mullite, silica, aluminum oxide, and combinations thereof, and the at least one CAS additive region includes SiO 2 , AlO 1.5 , and CaO. 
     In a further embodiment of any of the foregoing embodiments, the at least one CAS additive region is attached with the at least one refractory matrix region by sintering, by electrostatic force, or by a binder. 
     In a further embodiment of any of the foregoing embodiments, the at least one refractory matrix region has a size of greater than 10 micrometers, and the at least one CAS additive region has a size of less than 5 micrometers. 
     In a further embodiment of any of the foregoing embodiments, the at least one refractory matrix region has a size of less than 5 micrometers, and the at least one CAS additive region has a size of less than 5 micrometers. 
     In a further embodiment of any of the foregoing embodiments, each said engineered granule additionally includes at least one auxiliary refractory matrix region attached with the at least one refractory matrix region and the at least one CAS additive region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG.  1    illustrates an example gas turbine engine. 
         FIG.  2    illustrates an article of the engine and representative section of the article. 
         FIG.  3    illustrates another example representative section. 
         FIG.  4    illustrates a graph that shows the chemical activity of constituents of an EBC and two CAS Eutectic compositions. 
         FIG.  5 A  illustrates a graph showing a range of activities of CaO as a function of temperature for the two lowest melting eutectic compositions in the CaO—AlO 1.5 —SiO 2  system. 
         FIG.  5 B  illustrates a graph showing a range of activities of AlO 1.5  as a function of temperature for the two lowest melting eutectic compositions in the CaO—AlO 1.5 —SiO 2  system. 
         FIG.  5 C  illustrates a graph showing a range of activities of SiO 2  as a function of temperature for the two lowest melting eutectic compositions in the CaO—AlO 1.5 —SiO 2  system. 
         FIG.  6    illustrates a method of fabricating a silicate-resistant barrier coating. 
         FIG.  7    illustrates an engineered microstructure of a silicate-resistant barrier coating. 
         FIG.  8    illustrates a core/shell granule for the method. 
         FIG.  9    illustrates another example of a core/shell granule having a shell coating. 
         FIG.  10    illustrates a mixed granule for the method. 
         FIG.  11    illustrates another example of a mixed granule with auxiliary refractory matrix particles. 
         FIG.  11    illustrates a variation with a granule and separate auxiliary refractory matrix particles. 
         FIG.  13    illustrates a sintered attachment between particles. 
         FIG.  14    illustrates an electrostatic attachment between particles. 
         FIG.  15    illustrates a binder attachment between particles. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . The fan section  22  drives air along a bypass flow path B in a bypass duct defined within a nacelle  15 , and also drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
     The exemplary engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided, and the location of bearing systems  38  may be varied as appropriate to the application. 
     The low speed spool  30  generally includes an inner shaft  40  that interconnects, a first (or low) pressure compressor  44  and a first (or low) pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a speed change mechanism, which in exemplary gas turbine engine  20  is illustrated as a geared architecture  48  to drive a fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a second (or high) pressure compressor  52  and a second (or high) pressure turbine  54 . A combustor  56  is arranged in exemplary gas turbine  20  between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  57  of the engine static structure  36  may be arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  57  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
     The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  57  includes airfoils  59  which are in the core airflow path C. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. It will be appreciated that each of the positions of the fan section  22 , compressor section  24 , combustor section  26 , turbine section  28 , and fan drive gear system  48  may be varied. For example, gear system  48  may be located aft of the low pressure compressor, or aft of the combustor section  26  or even aft of turbine section  28 , and fan  42  may be positioned forward or aft of the location of gear system  48 . 
     The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about five 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1 and less than about 5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
     A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and 35,000 ft (10,668 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (¢TSFC)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram °R)/(518.7°R)]{circumflex over ( )}0.5. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 meters/second). 
       FIG.  2    illustrates an example article  60 , with a representative section of the article  60  shown in the encircled inset. In the illustrated example, the article  60  is a turbine vane (see also  FIG.  1   ). It is to be understood, however, that the article  60  is not limited to vanes and that although a vane is shown the examples herein are also applicable to blades, outer air seals, or other engine components, particularly those that are exposed to combustion gases. 
     The article  60  includes a substrate  62  and a coating system  64  disposed on the substrate  62 . For example, the substrate  62  is formed of a ceramic, such as a silicon-containing ceramic. The ceramic may be a monolithic ceramic or a ceramic matrix composite (“CMC”). One example CMC is SiC/SiC in which SiC fibers (designated at  62   a  in  FIG.  2   ) are disposed within a SiC matrix (designated at  62   b  in  FIG.  2   ). For example, the substrate  62  is generally a self-supporting structure that defines the geometry of the article  60  or a portion thereof, as opposed to a conformal coating. 
     Engine components may be exposed to relatively severe temperatures and environmental conditions during operation. Such conditions can reduce the durability of ceramics, such as silicon-containing ceramics and silicon carbide. In particular, high velocity, high pressure water vapor in the combustion gases reacts with silicon-containing ceramics such as silicon carbide to form volatile species leading to recession of the substrate. As such, environmental barrier coatings (EBCs) are utilized to combat recession of silicon-containing ceramics. However, silicate-containing deposits, such as calcium-magnesium-aluminosilicate (“CMAS”), from dirt/debris can deposit on engine component surfaces. The silicate-containing deposits can be molten at times, and this viscous liquid can undesirably react with and wick into an environmental barrier coating and ultimately cause sintering, loss of compliance, and spallation. In this regard, as will be described further below, the coating system  64  serves as an environmental barrier coating that is designed to protect the underlying substrate  62  from steam recession and, in particular, resist infiltration of the silicate-containing deposits. 
     The coating system  64  at least includes a silicate-resistant barrier coating  66  (“coating  66 ”). As used herein, a “coating” refers to a continuous, relatively thin, substantially uniform thickness layer. The coating system  64  may be a single layer configuration with only the coating  66  but more typically will be a multi-layer configuration and include an intermediate bond coating  68  that is located between the coating  66  and the substrate  62 . In a multi-layer configuration the coating  66  is a topcoat, i.e., the outermost, exposed coating. 
     The bond coating  68  may be a single layer or a multi-layer configuration and, in addition to bonding the coating  66  to the substrate  62 , may be composed to serve as a thermal barrier, volatilization barrier, oxidation barrier, and/or mechanical stabilizer. Although not limited, in one example the bond coating  68  is composed of a silicon-containing matrix, such as a silica (SiO 2 ) matrix. In a further example, the bond coating  68  additionally includes silicon oxycarbide particles dispersed in the silicon-containing matrix. In yet a further example, the bond coating  68  further includes barium-magnesium aluminosilicate particles dispersed in the silicon-containing matrix. 
     The coating  66  resists infiltration of silicate-containing deposits, such as CMAS, and is composed of a refractory matrix  66   a  and a calcium aluminosilicate additive  66   b  (“CAS additive  66   b ”) dispersed in the refractory matrix  66   a . The CAS additive  66   b  may reside at grain boundaries in the refractory matrix  66   a . For example, the refractory matrix is selected from zirconia (ZrO 2 ), hafnia (HfO 2 ), hafnium silicate, zirconium silicate, rare earth silicates, rare earth oxides, mullite, silica (SiO 2 ), aluminum oxide, or combinations thereof. Rare earth elements include cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium (Y). The zirconia may be a stabilized zirconia that has a rare earth oxide stabilizer, such as yttria (e.g., Y 2 O 3 —ZrO 2 ) or alkaline earth oxide stabilizer, such as calcia (e.g. CaO—ZrO 2 ). The rare earth silicate may be a rare earth monosilicate (ReSiO 5 ), a rare earth disilicate (Re 2 Si 2 O 7 ), or combinations thereof. The rare earth oxides may be of the form Re 2 O 3 , ReO 2 , or other stoichiometry corresponding to the base rare earth element oxidation state. The hafnium silicate may be Hf 0.5 Si 0.5 O 2 . The zirconium silicate may be Zr 0.5 Si 0.5 O 2 . 
     In one example, the CAS additive  66   b  includes, by mol. %, 30 to 90 of SiO 2 . The units of mol. % used herein refer to mole percentages of single cations of the given compound. In another example, the CAS additive  66   b  includes, by mol. %, 5 to 30 of AlO 1.5 . In another example, the CAS additive  66   b  includes, by mol. %, 10 to 50 of CaO. In an additional example, the CAS additive  66   b  includes, by mol. %, 30 to 90 of SiO 2 , 5 to 30 of AlO 1.5 , and 10 to 50 of CaO. As will be described below in the processing methodology, one or more of the constituents may be incorporated via a binder, such as colloidal silica. 
     In another example, where X 1 , X 2 , and X 3  are variables that sum to 100, the CAS additive  66   b  includes X 1  mol. % of SiO 2 , X 2  mol. % of CaO, and X 3  mol. % of AlO 1.5 , where X 1  is greater than X 2  and X 1  is greater than X 3 . In a further example, X 2  is greater than X 1 . In an additional example, the combined amount of X 1  and X 3  is at least 50 and is no more than 90. In yet a further example, the combined amount of X 1  and X 3  is at least 70 and is no more than 90. In an additional example, the combined amount of X 2  and X 3  is less than X 1 . 
     Table 1 below demonstrates additional example compositions of the CAS additive  66   b , designated as CAS-A, CAS-B, CAS-C, CAS-D, and CAS-E. In further examples, the mol. % of each constituent of CAS-A, CAS-B, CAS-C, CAS-D, and CAS-E given in Table 1 varies by +/−5. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 CAS Additive Compositions 
               
            
           
           
               
               
               
            
               
                   
                 Constituent 
                 mol. % 
               
               
                   
               
               
                 CAS-A 
                 CaO 
                 15 
               
               
                   
                 AlO 1.5   
                 10 
               
               
                   
                 SiO 2   
                 75 
               
               
                 CAS-B 
                 CaO 
                 26 
               
               
                   
                 AlO 1.5   
                 11 
               
               
                   
                 SiO 2   
                 63 
               
               
                 CAS-C 
                 CaO 
                 27 
               
               
                   
                 AlO 1.5   
                 13 
               
               
                   
                 SiO 2   
                 60 
               
               
                 CAS-D 
                 CaO 
                 43 
               
               
                   
                 AlO 1.5   
                 14 
               
               
                   
                 SiO 2   
                 43 
               
               
                 CAS-E 
                 CaO 
                 28 
               
               
                   
                 AlO 1.5   
                 27 
               
               
                   
                 SiO 2   
                 45 
               
               
                   
               
            
           
         
       
     
     Further examples of the CAS additive  66   b  additionally include magnesium, barium, or strontium, such as magnesium oxide (MgO), barium oxide (BaO), or strontium oxide (SrO). The following examples are based on MgO, but may also apply to BaO, SrO, or mixtures of two or more of MgO, BaO, and SrO. For example, where X 1 , X 2 , X 3 , and X 4  are variables that sum to 100, the CAS additive  66   b  includes X 1  mol. % of SiO 2 , X 2  mol. % of CaO, X 3  mol. % of AlO 1.5 , and X 4  mol. % of MgO, where X 4  is less than X 2 . In another example, X 4  is also less than X 3 . In a further example, X 4  is also less than X 1 . In additional examples, X 4  is less than 10. 
     Table 2 below demonstrates additional example compositions of the CAS additive  66   b  that include MgO (i.e. CMAS), designated as CMAS-A, CMAS-B, CMAS-C, CMAS-D, and CMAS-E. In further examples, the mol. % of each constituent given in Table 2 varies by +/−5, except that the MgO varies by +/−2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 CAS Additive Compositions With Magnesium 
               
            
           
           
               
               
               
            
               
                   
                 Constituent 
                 Mol. % 
               
               
                   
               
            
           
           
               
               
               
            
               
                 CMAS-A 
                 CaO 
                 14 
               
               
                   
                 AlO 1.5   
                 10 
               
               
                   
                 SiO 2   
                 71 
               
               
                   
                 MgO 
                 5 
               
               
                 CMAS-B 
                 CaO 
                 25 
               
               
                   
                 AlO 1.5   
                 10 
               
               
                   
                 SiO 2   
                 60 
               
               
                   
                 MgO 
                 5 
               
               
                 CMAS-C 
                 CaO 
                 26 
               
               
                   
                 AlO 1.5   
                 11 
               
               
                   
                 SiO 2   
                 58 
               
               
                   
                 MgO 
                 5 
               
               
                 CMAS-D 
                 CaO 
                 38 
               
               
                   
                 AlO 1.5   
                 14 
               
               
                   
                 SiO 2   
                 43 
               
               
                   
                 MgO 
                 5 
               
               
                 CMAS-E 
                 CaO 
                 27 
               
               
                   
                 AlO 1.5   
                 26 
               
               
                   
                 SiO 2   
                 42 
               
               
                   
                 MgO 
                 5 
               
               
                   
               
            
           
         
       
     
     The coating  66  generally includes a minority amount (substantially less than 50 mol. %) of the CAS additive  66   b , with the remainder being the refractory matrix  66   a . For example, the coating  66  includes, by mol. %, 0.1 to 30 of the CAS additive  66   b . In a more particular example, the coating  66  includes 3 to 8 of the CAS additive  66   b . Table 3 below demonstrates additional example compositions of the coating  66 , designated as Composition-1, Composition-2, and Composition-3. For example, the CAS additive listed for each of Composition-1, Composition-2, and Composition-3 can be the compositions of CAS-A, CAS-B, CAS-C, or CAS-D above. In further examples, the mol. % of each constituent of Composition-1, Composition-2, and Composition-3 given in Table 3 varies by +/−3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Silicate-Resistant Barrier Coating Compositions 
               
            
           
           
               
               
               
            
               
                   
                 Constituent 
                 mol % 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Composition-1 
                 HfO 2   
                 0 
               
               
                   
                 Hf 0.5 Si 0.5 O 2   
                 95 
               
               
                   
                 CAS Additive 
                 5 
               
               
                 Composition-2 
                 HfO 2   
                 75 
               
               
                   
                 Hf 0.5 Si 0.5 O 2   
                 20 
               
               
                   
                 CAS Additive 
                 5 
               
               
                 Composition-3 
                 HfO 2   
                 95 
               
               
                   
                 Hf 0.5 Si 0.5 O 2   
                 0 
               
               
                   
                 CAS Additive 
                 5 
               
               
                   
               
            
           
         
       
     
       FIG.  3    illustrates another example coating system  164 . In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate corresponding modified elements. The coating system  164  at least includes a silicate-resistant barrier coating  166  (“coating  166 ”). Like the coating system  64 , the coating system  164  may be a single layer configuration or a multi-layer configuration with the bond coating  68 . 
     There is also a silicate-containing deposit  70  (“deposit  70 ”) on, and in contact with, the coating  166 . The deposit  70  is non-native to the coating system  164 . The term “non-native” refers to the source of the deposit  70  being from other than the coating system  164 . For instance, the coating system  164  is an engineered system, whereas the deposit  70  is non-engineered and may occur from the environment of the article  60  during operation or testing (e.g., from dirt/debris carried in the core gas path and combustion gases) or from artificial placement of the deposit  70  for testing/evaluation purposes. For example, the deposit  70  is CMAS. 
     The coating  166  has a composition that approaches chemical equilibrium with the deposit  70  with respect to the chemical activity of the constituent within the coating  166  and deposit  70 . The chemical activity, represents the thermodynamic potential of the each constituent within a mixture with respect to the thermodynamic potential of the pure constituent. Activity is a dimensionless quantity that is the ratio of the vapor pressure in equilibrium with a pure material, to the vapor pressure in equilibrium with a mixture. Without being bound by a particular theory, the thermodynamic driving force for a species within a substance to migrate into an adjacent substance is proportional to the difference in chemical activity between the two substances. In terms of the coating  166  being equilibrated with the deposit  70 , this means that the chemical activity of the constituent in deposit  70  are not substantially different from the chemical activity of the constituent of the coating  166 . As a result of being in a state of near equilibrium there is minimal thermodynamic driving force for the deposit  70  to react with or diffuse into the coating  166 . In contrast, if the coating  166  were not equilibrated, there would be a thermodynamic driving force for the deposit  70  to react with or diffuse into the coating  166 . 
     In an example, the deposit  70  is CMAS and the coating  166  is composed of the refractory matrix  66   a  and the CAS additive  66   b  dispersed in the refractory matrix  66   a . For instance, the compositions of the refractory matrix  66   a  and the CAS additive  66   b  are selected from the compositions as disclosed above to be equilibrated with the CMAS. 
     In further examples, the composition or expected composition of the deposit  70  may vary. For instance, the engine may be exposed to different compositions of dirt/debris, depending on which region of the Earth the engine is operating in. A target composition of the deposit  70  can be identified, such as by estimation from known data, by experiment from collection of samples from engines in service, and/or by literature. Once a target composition of the deposit  70  is identified, a composition of the coating  166  that is equilibrated with the target composition of the deposit  70  can be selected from among the compositions disclosed herein. The following working example demonstrates further aspects of such an approach. 
     As an example, the chemical activity of coatings  166  and deposits  70  are calculated (e.g., using the thermodynamic database FactSage). To represent the chemistry of typical deposits  70 , two compositions were chosen that correspond to the two lowest congruent melting points in the calcia-alumina-silica system and are labeled CAS Eutectic-1 and CAS Eutectic-2 in Table 4 below. A coating  166  with an “equilibrated chemistry” is denoted EBC-1 and two coatings with non-equilibrated chemistry are denoted EBC-2 and EBC-3. The graph in  FIG.  4    shows the activity of the constituent of EBC-1 are similar to CAS Eutectic-1 and CAS Eutectic-2. Conversely, EBC-2 and EBC-3 do not have the similarity in chemical activity due to the absence of CaO, AlO 1.5 . 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 CAS and EBC compositions 
               
            
           
           
               
               
               
            
               
                   
                 Constituent 
                 mol. % 
               
               
                   
               
            
           
           
               
               
               
            
               
                 EBC1 
                 ZrO2 
                 75 
               
               
                   
                 Zr0.5Si0.5O2 
                 20 
               
               
                   
                 CAS-C 
                 5 
               
               
                 EBC-2 
                 Zr0.5Si0.5O2 
                 100 
               
               
                 EBC-3 
                 ZrO2 
                 100 
               
               
                 CAS 
                 CaO 
                 26 
               
               
                 Eutectic-1 
                 AlO1.5 
                 15 
               
               
                   
                 SiO2 
                 59 
               
               
                 CAS 
                 CaO 
                 39 
               
               
                 Eutectic-2 
                 AlO1.5 
                 20 
               
               
                   
                 SiO2 
                 41 
               
               
                   
               
            
           
         
       
     
     The equilibrium or near equilibrium between the coating  166  and the deposit  70  can be further described in terms of the activity of the constituents in the coating  166  falling within the range of activities of one or more constituents in the deposit  70 . For example, for constituents of interest, such as CaO, AlO 1.5 , and SiO 2 , a range of activities in deposits are identified, and then the constituents present with in the coating  166  are selected to be within that range. As an example, the three primary constituents of engine deposits are calcia, alumina, and silica. From these, a range is established based on the lowest melting eutectic compositions for the constituents of interest. This is represented below in the graphs in  FIGS.  5 A,  5 B, and  5 C  for, respectively, calcia, alumina, and silica.  FIG.  5 A  is the activity of CaO,  FIG.  5 B  is the activity of AlO 1.5 , and  FIG.  5 C  is the activity of SiO 2 , each as a function of temperature for the two lowest melting eutectic compositions in the CaO—AlO 1.5 —SiO 2  system. The shaded regions represent the target range of CAS activities in the coating  166 . The lowest eutectic has a composition, by mol. %, of 25.8 CaO, 15.2 Al 2 O 5 , and 59.0 SiO 2 . The second lowest eutectice has a composition, by mol. %, of 39.3 CaO, 20.1 Al 2 O 5 , and 40.6 SiO 2 ). The “band” of activity between the two eutectics thus becomes the target activity and the corresponding constituent, calcia, alumina, or silica, is selected within the coating  166  in an amount that corresponds to an activity that falls within that activity band. In further examples, if aluminum diffusion into a coating  166  does not have as deleterious an effect as calcium would, the chemical potential of calcium in the coating  166  can be designed to be equilibrated with calcium in the deposit  70 . That is, the chemical potentials of one or more constituents in the coating  166  is designed to be equilibrated with one or more target species in the deposit  70 . 
     The coating systems  64 / 164  disclosed herein may be fabricated using techniques such as, but not limited to, spray drying, sintering, plasma spraying, slurry deposition, and combinations of various techniques. 
       FIG.  6    illustrates a coating fabrication method  72  that can be used to produce the coating  66 / 166  with an engineered microstructure. The method  72  generally includes steps  72   a  and  72   b . At step  72   a  engineered granules are provided. The engineered granules will be discussed in further detail below, but each engineered granule is an aggregate of at least one refractory matrix region and at least one calcium aluminosilicate additive region (CAS additive region) attached with the at least one refractory matrix region. Ultimately, the refractory matrix region(s) of the granules will form at least a portion of the refractory matrix  66   a  in the coating  66 / 166  and the CAS additive region(s) of the granules will form the CAS additive  66   b  in the coating  66 / 166 . 
     At step  72   b  the engineered granules are thermally consolidated on the substrate  62  to form the coating  66 / 166  with the engineered microstructure. In the thermal consolidation the refractory matrix region(s) from the engineered granules form grains of the refractory matrix  66   a  and the CAS additive region(s) from the engineered granules form the CAS additives  66   b  which are dispersed between the refractory matrix grains. The term “thermal consolidation” or variations thereof refers to a process by which the constituents of a coating are joined together to form the coating. In one example, the thermal consolidation is by a thermal spray process. The method  72  may further include a heat treatment of the as-deposited coating. For instance, the heat treatment may facilitate transport of the CAS additive  66   b  to the grain boundaries described below, as well as into any porosity of the coating. 
     The following is a non-limiting working example of a thermal spray process that may be used. The engineered granules are deposited onto a substrate that is located about 4 inches from a plasma spray gun nozzle. The granules are fed through a thermal spray system at a rate of 2-3 lbs/hr using an argon carrier gas. The granules are introduced into a 42 kW argon/hydrogen plasma and deposited onto the substrate at a surface speed of 30 ft/min. The granules sufficiently soften or melt to form the coating  66 / 166 . 
     In another example, the thermal consolidation is by a slurry coating and sintering process. For instance, the engineered granules are first provided to the substrate via an aqueous or non-aqueous slurry containing about 10 to 50 vol % of the granules. The slurry may contain an organic binder, inorganic binder, dispersants or other modifiers such as plasticizers. The slurry is coated on the substrate by one of slip casting, spin coating, slurry spraying, tape casting, aerosol jet spraying or other suitable method. The slurry coating is dried and thermally treated to a temperature sufficient to form the coating  66 / 166 . The thermal treatment may comprise firing in a high temperature furnace and may include a binder burnout step, a pre-sintering step. The furnace thermal treatment may be conducted in air, inert, or reactive atmospheres, or vacuum, at temperatures up to 1500° C. or higher, depending on the thermal stability of the substrate. 
       FIG.  7    illustrates a representative portion of an example coating  266  produced in accordance with the method  72 . The coating  266  has an engineered microstructure  74  that includes grains  74   a  that make up the refractory matrix  66   a . The grains  74   a  border each other at grain boundaries  76 , and the CAS additives  66   b  are dispersed in the grain boundaries  76  between the grains  74   a . A grain  74   a  may be a singular grain with a definitive boundary or a region of multiple grains (i.e., sub-grains) with a definitive perimeter boundary delimited by the boundaries of the sub-grains. For instance, a grain  74   a  may be produced from a “splat” or region of multiple “splats” of molten or partially molten material deposited during thermal spray. 
     As used herein the term “engineered” refers to a deliberate or manipulated arrangement of structures relative to each other, as opposed to a random arrangement of the structures. For an engineered granule, the structures are compositional regions that are in a deliberate or manipulated arrangement relative to each other. For an engineered microstructure, the structures are compositional regions, grains, phases, and/or constituents that are in a deliberate or manipulated arrangement relative to each other. Here, the engineered microstructure  74  results from the engineered granules, i.e., the granules are structured to control the microstructure. 
     The engineered microstructure  74  facilitates resistance to infiltration of the deposits  70 . Deposits may wick into a coating via grain boundaries or porosity in the coating. In a thermal spray process, when using blends of conventional, loose feedstock particles of different compositions, the particles (particularly relatively small particles in relatively small amounts) can agglomerate or segregate. This is especially true for blends of particles of differing sizes, shapes, and densities. The agglomerates or segregated particles prevent a good dispersing and, as a result, the different compositions of the particles are not well dispersed in the final coating. In particular, this may debit performance of a silicate-resistant barrier coating, wherein additive particles that may be intended to hinder infiltration though grain boundaries end up as relatively large isolated agglomerate regions in the coating, leaving the grain boundaries substantially open for infiltration by the deposits. In contrast, through use of engineered granules to produce the engineered microstructure  74 , the CAS additive  66   b  is well-dispersed through the grain boundaries  76 , thereby enhancing the blocking effect of the CAS additive  66   b.    
     The engineered granules used in the method  72  may be of one or more different configuration types, each of which is an aggregate of at least one refractory matrix region and at least one CAS additive region attached with the at least one refractory matrix region. The aforementioned regions may be particles of either the refractory matrix material or the CAS additive material, coatings of either the refractory matrix material or the CAS additive material, or regions that are produced from films, suspensions, or the like. 
       FIG.  8    illustrates an example first type of engineered granule  78 , which is a core/shell granule. The core/shell granule  78  in this example is formed from a coarse core particle  78   a  that is a refractory matrix particle. A refractory matrix particle is a particle that is composed of one or more constituents that will make up the refractory matrix  66   a . There are a plurality of fine shell particles  78   b  attached on the coarse core particle  78   a . The fine shell particles  78   b  are CAS additive particles. A CAS additive particle is a particle that is composed of one or more constituents that will make up the CAS additive  66   b , although most typically the CAS additive particle will be composed of all of the constituents of the CAS additive  66   b.    
     The coarse core particle  78   a  is generally substantially larger in particle size than the fine shell particle  78   b  so that the coarse core particle  78   a  can serve as a support for many of the fine shell particles  78   b . As an example, the coarse core particle  78   a  has a particle size of greater than 10 micrometers, and the shell particle  78   b  has a size of less than 5 micrometers, such as 1-2 micrometers. In further examples, the coarse core particle  78   a  has a maximum particle size of no greater than 150 micrometers and minimum particle size of no less than 45 micrometers. In another embodiment, the coarse core particle  78   a  has a particle size of 15 micrometers to 45 micrometers. A useful size range may also be described by mesh sizes, in which coarse core particles  78   a  have a size of −100 mesh/+325 mesh. The resulting core/shell granule  78  thus has a granule size that is somewhat larger than the particle size of the coarse core particle  78   a . Such a granule size may facilitate the thermal spray process, as sizes under 10 micrometers, such as under 5 micrometers, tend to challenge the flow of starting materials in the thermal spray process. Particle size may be measured using ASTM B822: Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering. 
     The fine shell particles  78   b  are attached on the outer surface of the coarse core particle  78   a . During thermal processing, such as thermal spraying, the attachment of the fine shell particles  78   b  on the coarse core particle  78   a  limits the fine shell particles  78   b  from agglomerating and segregating to thereby maintain the dispersion of the fine shell particles  78   b . As a result, the fine shell particles  78   b  remain dispersed between the coarse core particles  78   a  as they are deposited into the substrate  62  such that in the consolidated state the CAS additive  66   b  formed by the fine shell particles  78   b  resides in the grain boundaries  76  between the grains  74   a  formed by the coarse core particles  78   b  in the engineered microstructure  74 . 
     The core/shell granules  78  can be provided for the method  72  as pre-fabricated core/shell granules  78 . Alternatively, the core/shell granules  78  can be provided for the method  72  by preparing the core/shell granules  78  from starting powders of the coarse core particles  78   a  and fine shell particles  78   b . For example, the fine shell particles  78   b  can be attached on the coarse core particles  78   a  using a granulating technique such as mechanical mixing, acoustic mixing, spray drying or wet chemical or vapor deposition techniques. As shown, the granules  78  may then be used in a thermal spray to deposit the coating  266 , which corresponds to step  72   b  of the method  72 . 
     The following are non-limiting examples that can be used to attach the fine shell particles  78   b  on the coarse core particles  78   a . Acoustic mixing techniques may be utilized whereby dry blends of coarse particles  78   a  and fine shell particles  78   b  are provided in a suitable container and placed in an acoustic mixer. The mixer is operated at a frequency of 60 Hz, generating a mixing force of up to 100 times the gravitation force (e.g. 100 g&#39;s), with mixing times typically on the order of a few minutes. The fine shell particles  78   b  are bound to the coarse particles  78   a  via electrostatic forces. Small amounts of liquid, including an organic or inorganic binder, may optionally be used to further facilitate bonding of the fine shell particles  78   b  to the coarse particles  78   a.    
       FIG.  9    illustrates a variation of a core/shell granule  178  that is somewhat similar to the granule  78 . In this example, rather than the fine shell particles  78   b  (or alternatively in addition to the fine shell particles  78   b ), the shell is provided by a shell coating  178   b . The shell coating  178   b  may be applied or deposited onto the coarse core particle  78   a  by vapor deposition and results in a continuous solid shell surrounding the coarse core particle  78   a . The techniques for applying the shell coating  178   b  onto the coarse core particles  78   a  are not particularly limited, but may include fluidized bed chemical vapor deposition or atomic layer deposition. Additionally, the shell coating  178   b  may be composed of all of the constituents of the CAS additive  66   b . However, in alternative examples, the shell coating  178   b  may be multi-layered and include individual layers of one or more of the constituents of the CAS additive  66   b , such as a layer of CaO, a layer of Al2O3, a layer SiO2, or layers of combinations of two of these constituents mixed together. 
       FIG.  10    illustrates an example second type of engineered granule  80 , which is a mixed granule. As shown, the mixed granule  80  is formed from a plurality of refractory matrix particles  80   a  and a plurality of CAS additive particles  80   b  attached with the refractory matrix particles  80   a . The CAS additive particles  80   b  may be composed of one or more constituents that will make up the CAS additive  66   b . For instance, a single CAS additive particle  80   b  may contain all of the constituents that will make up the CAS additive  66   b . In another example, the CAS additive particles  80   b  are individual constituents of the CAS additive  66   b . For instance, one CAS additive particle  80   b  is CaO and another CAS additive particle is SiO 2 . In further examples, the individual constituents may be in the form of nitrates and carbonates of calcium, aluminum, and/or silica. As an example, the refractory matrix particles  80   a  and the CAS additive particles  80   b  are substantially similar in size, to facilitate good mixing and inter-dispersion. For instance, the refractory matrix particles  80   a  and the CAS additive particles  80   b  are both less than 10 micrometers, such as less than 5 micrometers or in the range of 0.5-5 micrometers. 
     The refractory matrix particles  80   a  and the CAS additive particles  80   b  are attached together in the granules  80 . During thermal processing, such as thermal spraying, the attachment between the refractory matrix particles  80   a  and the CAS additive particles  80   b  limits the CAS additive particles  80   b  from agglomerating or segregating to thereby maintain the dispersion of the CAS additive particles  80   b . As a result, the CAS additive particles  80   b  remain dispersed among or local to the refractory matrix particles  80   a  as they are deposited into the substrate  62  such that in the consolidated state the CAS additive  66   b  formed by the CAS additive particles  80   b  resides in the grain boundaries  76  between the grains  74   a  formed by the refractory matrix particles  80   a  in the engineered microstructure  74 . 
     Like the core/shell granules  78 , the mixed granules  80  can be pre-fabricated for the method  72  or prepared from starting powders. For example, the mixed granules  80  can be prepared using a spray drying technique in which starting powders of the refractory matrix particles  80   a  and the CAS additive particles  80   b  are mixed in a slurry with a carrier fluid and then spray dried. Optionally, the spray drying may be followed by a sintering step to further consolidate the granules  80  prior to the thermal consolidation at step  72   b  of the method  72 . 
     The following is a non-limiting example that can be used to attach the refractory matrix particles  80   a  and the CAS additive particles  80   b  to form the granules  80 . The refractory matrix particles  80   a  and CAS additive particles  80   b  may be formulated in an aqueous or non-aqueous slurry containing on the order of 10 to 50 vol % of the granules  80 . The slurry may contain an organic binder, inorganic binder, dispersants or other modifiers such as plasticizers to facilitate the appropriate slurry viscosity for formation of the granules  80 . The slurry is atomized in a spray drier using, for example, a pressure or rotary atomizer, with atomization pressures and rotation speeds selected to achieve the desired granule size distribution. Slurry feed rates and spray drying temperatures also influence granule formation, with typical gas inlet temperatures being between about 200-400° C. and outlet temperatures less than 160° C. The various size fractions of granules  80  may be collected in a chamber or cyclone collection unit, with the coarser granules  80  collected in the former and finer granules  80  collected in the latter. 
     The refractory matrix particles  80   a  and the CAS additive particles  80   b  can also be attached to form the granules  80  using a colloidal dispersion or solution technique. For example, colloidal silica, solutions of calcium nitrate and aluminum nitrates, or any metal nitrate solutions that contain the same metal as the refractory matrix may be used. Examples include, but are not limited to, hafnium nitrate, zirconium nitrate, rare earth metal nitrates, or combination thereof. 
       FIG.  11    illustrates another example second type of engineered granule  180 . In this example, the mixed granule  180  is formed from a plurality of refractory matrix particles  180   a , a plurality of auxiliary refractory matrix particles  180   c , and the plurality of CAS additive particles  80   b  attached with the particles  180   a / 180   c . The refractory matrix particles  180   a  are composed of one or more constituents of the refractory matrix  66   a  and the auxiliary refractory matrix particles  180   c  are composed or one or more constituents of the refractory matrix  66   a  that differ in composition from those in the particles  180   a . For example, the particles  180   a  are composed of HfO 2  and the particles  180   c  are composed of SiO 2 . It is to be understood that the compositions of the particles  180   a / 180   c  may be selected from any of the constituents disclosed herein, such as zirconia (ZrO 2 ), hafnia (HfO 2 ), hafnium silicate, zirconium silicate, rare earth silicates, rare earth oxides, mullite, silica (SiO 2 ), aluminum oxide, or combinations thereof. 
       FIG.  12    demonstrates a variation that can be used in the thermal spray process. In the prior examples, the granules may contain all of the constituents of the coating  266 . In the illustrated example, however, at least one constituent of the refractory matrix  66   a  is excluded from the granule  80 . The excluded constituent is provided as separate, loose auxiliary refractory matrix particles  280   c . The granules  80  and the auxiliary refractory matrix particles  280   c  are then co-sprayed in the thermal spray process, such as by premixing the granules  80  and the auxiliary refractory matrix particles  280   c  or by mixing the granules  80  and the auxiliary refractory matrix particles  280   c  in the thermal spray stream. The composition of the auxiliary refractory matrix particles  280   c  may be one or more of the constituents of the refractory matrix  66   a , such as zirconia (ZrO 2 ), hafnia (HfO 2 ), hafnium silicate, zirconium silicate, rare earth silicates, rare earth oxides, mullite, silica (SiO 2 ), aluminum oxide, or combinations thereof. 
     The use of the separate auxiliary refractory matrix particles  280   c  may also permit an additional parameter of control in forming the coating  266 . For instance, the amount of the granules  80  and/or amount of the auxiliary refractory matrix particles  280   c  provided during the thermal spraying may be adjusted in order to adjust the relative amounts of the constituents that are deposited. By adjusting the amounts during thermal spraying, a graded composition of the coating  266  is produced. Further, additional auxiliary refractory matrix particles of one or more other constituents can be used to provide graded compositions of multiple constituents. Thus, the amounts of the zirconia (ZrO 2 ), hafnia (HfO 2 ), hafnium silicate, zirconium silicate, rare earth silicates, rare earth oxides, mullite, silica (SiO 2 ), aluminum oxide, or combinations thereof may be varied in the coating  266  to provide a gradual grading in the through-thickness or along the plane of the coating  266 . 
     The nature of the attachment between particles in the afore-described granules can also be varied and manipulated. For example, the attachment may be by sintering, by electrostatic force, or by a binder.  FIG.  13    illustrates a sintered attachment  82  between particles  82   a / 82   b , which may be any of the afore-mentioned particles. In the sintered attachment  82 , the particles  82   a / 82   b  coalesce and densify, thereby intimately bonding the particles  82   a / 82   b  together. As an example, after forming granules, the granules may be heated to a sintering temperature in air, vacuum, or an inert atmosphere that is substantially non-reactive with the particles  82   a / 82   b , wherein the particles  82   a / 82   b  diffuse and coalesce. 
       FIG.  14    illustrates an electrostatic attachment  182  between particles  182   a / 182   b , which may be any of the aforementioned particles. 
       FIG.  15    illustrates a binder attachment  282  between particles  282   a / 282   b , which may be any of the afore-mentioned particles. The binder attachment  282  includes a binder  284  that bonds or adheres to the particles  282   a / 282   b  together. For example, the binder  284  is an organic binder that eventually burns off, such as during thermal spraying. Additionally or alternatively, the binder  284  is inorganic, such as colloidal silica. In this case, rather than burning off, the binder may incorporate into the coating  266 . For example, an inorganic binder is selected to be compatible with one or more constituents of the coating  266 . For instance, for a coating  266  that includes silica, the colloidal silica is compatible because it incorporates with the silica already present in the coating  266 . The binder  284  may include other types of oxides or other inorganic compounds such as nitrates, phosphates, or carbonates of calcium, aluminum, hafnium, zirconium, rare earth metals. If a carbonate, the carbonate would decompose to oxides under the high temperatures during thermal spraying. 
     The examples disclosed herein may be used to tailor the performance of the silicate-resistant barrier coatings. For instance, there are a range of compositions disclosed that may be used to tailor the composition of the silicate-resistant barrier coating to achieve a desired performance goal for silicate-resistance, as well as other goals, such as thermal expansion. As also discussed, the range of compositions may also be used to tailor the silicate-resistant barrier coating to be equilibrated with a target deposit composition. And finally, the exemplary methods of fabricating the silicate-resistant barrier coating may be used to further enhance silicate-resistance via the engineered microstructure. The examples thus enable newfound flexibility in designing the silicate-resistant barrier coating to meet specific challenges. 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.