Patent Publication Number: US-2021188720-A1

Title: Environmental barrier coating

Description:
BACKGROUND 
     A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-energy exhaust gas flow. The high-energy exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines. 
     This disclosure relates to composite articles, such as those used in gas turbine engines. Components, such as gas turbine engine components, may be subjected to high temperatures, corrosive and oxidative conditions, and elevated stress levels. In order to improve the thermal and/or oxidative stability, the component may include a protective barrier coating. 
     SUMMARY 
     An article according to an exemplary embodiment of this disclosure, among other possible things includes a ceramic-based substrate and a barrier layer on the ceramic-based substrate. The barrier layer includes a matrix of SiO2 and a dispersion of silicon oxycarbide particles in the matrix. The silicon oxycarbide particles have Si, O, and C in a covalently bonded network. A dispersion of barium-magnesium alumino-silicate particles is in the matrix. The barium-magnesium alumino-silicate particles have an average maximum dimension that is between about 10-40% of an average maximum dimension of the silicon oxycarbide particles. 
     In a further example of the foregoing, the barrier layer includes, by volume, 1-30% of the barium-magnesium alumino-silicate particles. 
     In a further example of any of the foregoing, the barrier layer includes, by volume, 30-94% of the silicon oxycarbide particles. 
     In a further example of any of the foregoing, the barrier layer includes, by volume, 5-40% of the matrix of SiO 2 . 
     In a further example of any of the foregoing, the barrier layer includes, by volume, 1-30% of the barium-magnesium alumino-silicate particles, 5-40% of the matrix of SiO2, and a balance of the silicon oxycarbide particles. 
     In a further example of any of the foregoing, the barrier layer includes, by volume, 1-10% of the barium-magnesium alumino-silicate particles. 
     In a further example of any of the foregoing, the average maximum dimension of the barium-magnesium alumino-silicate particles is between about 5 and 30 micrometers. 
     In a further example of any of the foregoing, an average distance between adjacent barium-magnesium alumino-silicate particles is between about 60 and 200 micrometers. 
     In a further example of any of the foregoing, an average maximum dimension of the barium-magnesium alumino-silicate particles is between 8-15% of an average distance between adjacent barium-magnesium alumino-silicate particles. 
     In a further example of any of the foregoing, an average distance between adjacent barium-magnesium alumino-silicate particles is between about 60 and 200 micrometers. 
     In a further example of any of the foregoing, there is a distinct intermediate layer between the barrier layer and the ceramic-based substrate, the distinct intermediate layer including an intermediate layer matrix of SiO2 and a dispersion of intermediate layer silicon oxycarbide particles in the intermediate layer matrix. 
     In a further example of any of the foregoing, the matrix of SiO2 is continuous. 
     In a further example of any of the foregoing, the silicon oxycarbide particles have a composition SiOxMzCy, where M is at least one metal, x&lt;2, y&gt;0 and z&lt;1 and x and z are non-zero. 
     In a further example of any of the foregoing, the article includes a ceramic-based top coat on the barrier layer. 
     A composite material according to an exemplary embodiment of this disclosure, among other possible things includes a matrix of SiO2 and a dispersion of silicon oxycarbide particles in the matrix. The silicon oxycarbide particles have Si, O, and C in a covalently bonded network. A dispersion of barium-magnesium alumino-silicate particles is in the matrix. The barium-magnesium alumino-silicate particles have an average maximum dimension that is between about 10-40% of an average maximum dimension of the silicon oxycarbide particles. 
     In a further example of the foregoing, the average maximum dimension of the barium-magnesium alumino-silicate particles is between about 5 and 30 micrometers. 
     In a further example of any of the foregoing, an average distance between adjacent barium-magnesium alumino-silicate particles is between about 60 and 200 micrometers. 
     In a further example of any of the foregoing, the average maximum dimension of the barium-magnesium alumino-silicate particles is between 8-15% of the average distance between adjacent barium-magnesium alumino-silicate particles. 
     A method of applying a barrier layer to a substrate according to an exemplary embodiment of this disclosure, among other possible things includes mixing particles of barium-magnesium alumino-silicate, particles of SiO2, and silicon oxycarbide in a carrier fluid to form a slurry, where the barium-magnesium alumino-silicate particles have an average maximum dimension that is between about 10-40% of an average maximum dimension of the silicon oxycarbide particles. The slurry is applied to the substrate. The slurry is dried. The slurry is cured, such that the cross-linking occurs in the composite material. 
     In a further example of the foregoing, the barium-magnesium alumino-silicate particles are classified prior to the mixing. 
     Although the different examples have the specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
     These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an example gas turbine engine. 
         FIG. 2  illustrates an example article having a barrier layer of a composite material that includes barium-magnesium alumino-silicate particles. 
         FIG. 3  illustrates a network of silicon oxycarbide. 
         FIG. 4  illustrates another example article having a barrier layer of a composite material that includes barium-magnesium alumino-silicate particles. 
         FIG. 5  illustrates another example article having a barrier layer of a composite material that includes barium-magnesium alumino-silicate 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 housing  15  such as a fan case or nacelle, 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 through 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)]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). 
     The example gas turbine engine includes the fan section  22  that comprises in one non-limiting embodiment less than about 26 fan blades. In another non-limiting embodiment, the fan section  22  includes less than about 20 fan blades. Moreover, in one disclosed embodiment the low pressure turbine  46  includes no more than about 6 turbine rotors. In another non-limiting example embodiment the low pressure turbine  46  includes about 3 turbine rotors. A ratio between the number of fan blades and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine  46  provides the driving power to rotate the fan section  22  and therefore the relationship between the number of turbine rotors in the low pressure turbine  46  and the number of blades in the fan section  22  disclose an example gas turbine engine  20  with increased power transfer efficiency. 
       FIG. 2  schematically illustrates a representative portion of an example article  100  for the gas turbine engine  20  that includes a composite material  102  that is used as a barrier layer. The article  100  can be, for example, an airfoil in the compressor section  24  or turbine section  28 , a combustor liner panel in the combustor section  26 , a blade outer air seal, or other component that would benefit from the examples herein. In this example, the composite material  102  is used as an environmental barrier layer to protect an underlying substrate  104  from environmental conditions, as well as thermal conditions. As will be appreciated, the composite material  102  can be used as a stand-alone barrier layer, as an outermost/top coat with additional underlying layers, or in combination with other coating under- or over-layers, such as, but not limited to, ceramic-based topcoats. 
     The composite material  102  includes a matrix of silicon dioxide (SiO 2 )  106 , a dispersion of silicon oxycarbide particles (SiOC)  108  in the matrix  106 , and a dispersion of barium-magnesium alumino-silicate particles  110  (“BMAS particles  110 . The silicon oxycarbide particles  108  have silicon, oxygen, and carbon in a covalently bonded network, as shown in the example network  112  in  FIG. 3 . 
     The network  112  is amorphous and thus does not have long range crystalline structure. The illustrated network  112  is merely one example in which at least a portion of the silicon atoms are bonded to both 0 atoms and C atoms. As can be appreciated, the bonding of the network  112  will vary depending upon the atomic ratios of the Si, C, and O. In one example, the silicon oxycarbide particles  108  have a composition SiO x M z C y , where M is at least one metal, x&lt;2, y&gt;0, z&lt;1, and x and z are non-zero. The metal can include aluminum, boron, transition metals, refractory metals, rare earth metals, alkaline earth metals or combinations thereof. 
     In one example, the composite material  102  includes, by volume, 1-30% of the BMAS particles  110 . In a more particular example, the composite material  102  includes, by volume, 1-10% of BMAS particles. In a further example, the composite material  102  includes, by volume, 30-94% of the silicon oxycarbide particles  108 . In one further example, the composite material  102  includes, by volume, 5-40% of the matrix  26  of silicon dioxide. In a further example, the composite material  102  includes, by volume, 1-30% of the BMAS particles  110 , 5-40% of the matrix  106  of silicon dioxide, and a balance of the silicon oxycarbide particles  108 . 
     In one example, the silicon oxycarbide particles  108  have an average maximum dimension of 1-75 micrometers. An average maximum dimension of the BMAS particles  110  is less than the average maximum dimension of the silicon oxycarbide particles  108 . In a particular example, the BMAS particles  110  have an average maximum dimension that is between about 10% and 40% of the average maximum dimension of the silicon oxycarbide particles  108 . In a further example, the BMAS particles  110  have an average maximum dimension that is between about 5 and 30 micrometers. 
     The BMAS particles  110  are dispersed in the matrix  106 . For a given amount of BMAS particles  110  in the composite material  102 , the average maximum diameter of the BMAS particles  110  is proportional to an average distance d between adjacent BMAS particles  110 . The average distance d is defined as the distance between centerpoints of adjacent BMAS particles  110 . In some examples, the average distance d between adjacent BMAS particles  110  is between about 60 and 200 micrometers. In another example, the average maximum dimension of the BMAS particles  110  is about 8% and 15% of the average distance d. For example, the average distance d between adjacent BMAS particles  110  can be calculated from the cross section optical images of the composite material  102  using image processing technology, as would be known in the art. 
     The barrier layer protects the underlying substrate  104  from oxygen and moisture. For example, the substrate  104  can be a ceramic-based substrate, such as a silicon-containing ceramic material. One example is silicon carbide. The silicon oxycarbide particles  108  and the BMAS particles  110  of the barrier layer function as an oxygen and moisture diffusion barrier to limit the exposure of the underlying substrate  104  to oxygen and/or moisture from the surrounding environment. Without being bound by any particular theory, the BMAS particles  110  enhance oxidation and moisture protection by diffusing to the outer surface of the barrier layer opposite of the substrate  104  and forming a sealing layer that seals the underlying substrate  104  from oxygen/moisture exposure. Additionally, the cationic metal species of the BMAS particles  110  (barium, magnesium, and aluminum) can diffuse into the silicon oxycarbide particles  108  to enhance oxidation stability of the silicon oxycarbide material. Further, the diffusion behavior of the BMAS particles  110  may operate to seal any microcracks that could form in the barrier layer. Sealing the micro-cracks could prevent oxygen from infiltrating the barrier layer, which further enhances the oxidation resistance of the barrier layer. To this end, it has been discovered that selecting the average maximum dimension of the BMAS particles  110  and the average distance d between adjacent BMAS particles  110  (discussed above) is particularly important to improving the oxidation resistance of the barrier layer. In particular, it has been discovered that BMAS particles  110  having the average maximum dimension and average distance d between adjacent BMAS particles discussed above facilitates improving the oxidation resistance of the barrier layer. 
       FIG. 4  shows another example article  200  that includes the composite material  102  as a barrier layer arranged on the substrate  104 . In this example, the article  200  additionally includes a ceramic-based top coat  114  interfaced with the barrier layer. As an example, the ceramic-based top coat  114  can include one or more layers of an oxide-based material. The oxide-based material can be, for instance, halfnium-based oxides, yttrium-based oxides (such as hafnia, hafnium silicate, yttrium silicate, yttria stabilized zirconia or gadolinia stabilized zirconia), or combinations thereof, but is not limited to such oxides. 
       FIG. 5  illustrates another example article  300  that is somewhat similar to the article  200  shown in  FIG. 4  but includes a distinct intermediate layer  316  interposed between the barrier layer of the composite material  102  and the substrate  104 . In this example, the distinct intermediate layer  316  includes an intermediate layer matrix  318  of silicon dioxide and a dispersion of intermediate layer silicon oxycarbide particles  320  in the intermediate layer matrix  318 . The intermediate layer silicon oxycarbide particles  320  are similar to the silicon oxycarbide particles  108  in composition but, in this example, the intermediate layer silicon oxycarbide particles  320  have an average maximum dimension (D2) that is less than the average maximum dimension (D1) of the silicon oxycarbide particles  108 . The relatively small intermediate layer silicon oxycarbide particles  320  provide a relatively low roughness for enhanced bonding with the underlying substrate  104 . The larger silicon oxycarbide particles  108  of the barrier layer provide enhanced blocking of oxygen/moisture diffusion. Thus, in combination, the barrier layer and intermediate layer  316  provide good adhesion and good oxidation/moisture resistance. In one further example, D1 is 44-75 micrometers and D2 is 1-44 micrometers. 
     In one example, the intermediate layer  316  can include, by volume, 5-40% of the intermediate layer matrix  318  of silicon dioxide and a balance of the intermediate layer silicon oxycarbide particles  320 . In further examples, a portion of the BMAS particles  110  from the barrier layer can penetrate or diffuse into the intermediate layer  316 , during processing, during operation at high temperatures, or both. In a further example, a seal coat layer of SiO 2 , with or without BMAS particles, can be provided between the barrier layer and the intermediate layer  316  to provided adhesion and additional sealing. In further examples of any of the compositions disclosed herein, said compositions can include only the listed constituents. Additionally, in any of the examples disclosed herein, the matrix  106  and  318  can be continuous. The two-layer structure can also demonstrate good oxidation protection at 2000-2700° F. for 500 hours or longer as well as good adhesion with the ceramic-based top coat  114 . 
     The BMAS particles  110  can be prepared by crushing, milling, or another know method to achieve the desired average maximum dimension, as discussed above. In some examples, BMAS particles  110  can be classified to remove very small BMAS particles  110 , such as BMAS particles that have a maximum dimension less than about 1 micrometer. For instance, the BMAS particles  110  can be classified using an air classification method such as fluidized bed or air spinning. Very small BMAS particles  110  can reduce the mechanical robustness of the barrier layer, and generally do not contribute to the oxidation resistance of the barrier layer. 
     The BMAS particles  110  can be in the glass or crystalline phase during the preparation. In some examples, the BMAS particles  110  may be in the glass phase during the preparation, and change into the crystalline phase during application of the barrier layer to the substrate  104  and/or during use of the article  100 / 200 / 300  in a gas turbine engine  20 , for example. 
     The barrier layer and/or intermediate layer  316  can be fabricated using a slurry coating method. The appropriate slurries can be prepared by mixing components, such as silicon oxycarbide, barium-magnesium alumino-silicate, and powder of silicon dioxide or colloidal silica (Ludox) in a carrier fluid, such as water. The slurries can be mixed by agitation or ball milling and the resulting slurry can be painted, dipped, sprayed or otherwise deposited onto the underlying substrate  104 . The slurry can then be dried at room temperature or at an elevated temperature to remove the carrier fluid. In one example, the slurry is dried and cured at about 100-300° C. for about 5-60 minutes. During the heating, cross-linking of the colloidal silica occurs. The green coating can then be sintered at an elevated temperature in air for a selected amount of time. In one example, the sintering includes heating at 1500° C. or greater in an air environment for at least 1 hour. 
     The composite material  102  can be prepared using a slurry coating method. Slurries can be prepared by mixing components such as SiOC, BMAS, SiO 2  or Ludox (a source colloidal SiO 2 ) and water using agitation or ball milling. Various slurry coating methods such as painting, dipping and spraying can be used to coat ceramic matrix composite (CMC) substrates. Coatings formed from slurry are dried at room temperature and cured at 300° C. for about 5-60 minutes. During the heating, cross-linking of the colloidal silica occurs. This coating process can be repeated until all layers are coated. The bond coat is finally sintered at 1500° C. in air for at least 1 hour. 
     In one further example, a slurry of SiOC/SiO 2  75/25 vol % was prepared by mixing appropriate amounts of SiOC and Ludox colloidal silica. A small amount of water was added to adjust the viscosity. The slurry was further mixed by ball milling for at least 15 hours. A slurry of SiOC/BMAS/SiO 2  80/5/15 vol % was prepared likewise by mixing appropriate amounts of SiOC, BMAS and Ludox colloidal silica and ball milling for more than 15 hours. 
     An inner layer was applied on a cleaned CMC substrate  104  by painting. The coating was then dried at room temperature for about 5-60 minutes and heated in oven at between about 100-300° C. for about 5-60 minutes. During the heating, cross-linking of the colloidal silica occurs. An outer layer was applied in the same fashion as the inner layer with the exception that the outer layer was applied with two passes. In between the two passes, in one example, the specimen is submerged in Ludox colloidal silica solution, air dried at room temperature and heat treated at between about 100-300° C. for about 5-60 minutes to provide a silica sealing layer. After completion of the two layer bond coat, the specimen was sintered at 1500° C. for at least 1 hour. 
     Although the different examples are illustrated as having specific components, the examples of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from any of the embodiments in combination with features or components from any of the other embodiments. 
     The foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would understand that certain modifications could come within the scope of this disclosure. For these reasons, the following claims should be studied to determine the true scope and content of this disclosure.