Patent Publication Number: US-2021162624-A1

Title: Repair process using networked ceramic nanofibers

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present disclosure is a divisional application of U.S. patent application Ser. No. 16/270,660 filed Feb. 8, 2019. 
    
    
     BACKGROUND 
     Gas turbine engine components in the core gaspath may be subject to temperatures in excess of the melting temperature of the component substrate. Cooling features and barrier coatings are used to protect the substrate from these extreme temperatures. Barrier coatings are typically formed of ceramic materials, such as yttria stabilized zirconia or gadolinium zirconate. A thermally grown oxide layer is provided on the substrate as a bond coat to enhance bonding of the barrier layer on the substrate. 
     SUMMARY 
     A repair process according to an example of the present disclosure includes providing an article that has a substrate and a ceramic barrier coating disposed on the substrate. The ceramic barrier coating has a damaged region, and the damaged region is abraded to provide a dimple in the ceramic barrier coating. A remaining region of the ceramic barrier coating adjacent the dimple remains intact. A patch of networked ceramic nanofibers is then deposited in the dimple. 
     In a further embodiment of any of the foregoing embodiments, the ceramic barrier coating has a porous columnar microstructure and the networked ceramic nanofibers extend into pores of the porous columnar microstructure in the dimple. 
     In a further embodiment of any of the foregoing embodiments, the abrading includes spraying the damaged region with an abrasive media. 
     In a further embodiment of any of the foregoing embodiments, the abrasive media includes dry ice. 
     In a further embodiment of any of the foregoing embodiments, the damaged region is a spalled or worn region. 
     In a further embodiment of any of the foregoing embodiments, the depositing includes blow-spinning and sintering. 
     In a further embodiment of any of the foregoing embodiments, the sintering includes heating using an energy beam. 
     A further embodiment of any of the foregoing embodiments includes polishing the patch to be flat with the remaining region of the ceramic barrier coating adjacent the patch. 
     In a further embodiment of any of the foregoing embodiments, the ceramic nanofibers include zirconium oxide. 
     In a further embodiment of any of the foregoing embodiments, the ceramic nanofibers are selected from the group consisting of yttria stabilized zirconia, gadolinia zirconate, and combinations thereof, and the ceramic barrier coating is selected from the group consisting of yttria stabilized zirconia, gadolinia zirconate, and combinations thereof. 
     In a further embodiment of any of the foregoing embodiments, the ceramic barrier coating has a thickness t1 taken at the remaining portion adjacent the damaged region, and the patch has a thickness t2 that is less than the thickness t1. 
     A repair process according to an example of the present disclosure includes providing an airfoil that has a substrate and a ceramic barrier coating disposed on the substrate. The ceramic barrier coating has a damaged region, and the damaged region is abraded to provide a dimple in the ceramic barrier coating. A remaining region of the ceramic barrier coating adjacent the dimple remains intact. A patch of networked ceramic nanofibers is then deposited in the dimple. 
     In a further embodiment of any of the foregoing embodiments, the adbrading includes spraying the damaged region with an abrasive media. 
     In a further embodiment of any of the foregoing embodiments, the abrasive media includes dry ice. 
     In a further embodiment of any of the foregoing embodiments, the damaged region is a spalled or worn region. 
     In a further embodiment of any of the foregoing embodiments, the depositing includes blow-spinning and sintering. 
     In a further embodiment of any of the foregoing embodiments, the sintering includes heating using an energy beam. 
     A further embodiment of any of the foregoing embodiments includes polishing the patch to be flat with the remaining region of the ceramic barrier coating adjacent the patch. 
     An article according to an example of the present disclosure includes a substrate and a ceramic barrier coating disposed on the substrate. The ceramic barrier coating includes a dimple, and there is a patch of networked ceramic nanofibers disposed in the dimple. 
     In a further embodiment of any of the foregoing embodiments, the ceramic nanofibers are selected from the group consisting of yttria stabilized zirconia, gadolinia zirconate, and combinations thereof, and the ceramic barrier coating is selected from the group consisting of yttria stabilized zirconia, gadolinia zirconate, and combinations thereof. 
    
    
     
       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 example article subject to repair. 
         FIG. 3  illustrates a view of a representative portion of the article of  FIG. 2  during a repair process involving abrading the article. 
         FIG. 4  illustrates the article of  FIG. 3  during the repair process after abrading the article. 
         FIG. 5  illustrates the article of  FIG. 4  during the repair process involving deposition of a patch of networked ceramic nanofibers. 
         FIG. 6  illustrates a magnified view of networked ceramic nanofibers. 
         FIG. 7  illustrates the article of  FIG. 5  during the repair process involving polishing the patch of networked ceramic nanofibers. 
     
    
    
     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 . In this example, the article  60  is a rotatable turbine blade of the engine  20  (see also  FIG. 1 ). It is to be understood that, although the examples herein may be described with reference to the turbine blade, this disclosure is also applicable to static turbine vanes, other types of airfoils, seals, combustors, or other gas turbine engine components. 
     The turbine blade generally includes a platform  62 , an airfoil section  64  that extends from the platform  62 , and a root  66 . The article  60  is subjected during use to extreme temperatures in the engine  20 . The article  60  includes a coating system to protect against the high temperatures and environmental effects that might otherwise damage the article  60 . The coating system may include a ceramic barrier coating. Such ceramic barrier coatings can be subject to damage during use, such as damage from spalling, erosion, or wear. From time to time and inasmuch as feasible, such articles are repaired in order to extend use. 
       FIGS. 3, 4, and 5  depict an example repair process for a representative portion of the article  60 . For a turbine blade, the portion may be on the platform  62 , airfoil section  64 , or root  66 . As will also be appreciated, the article  60  may have numerous portions which are repaired in a similar manner as described below. 
     The article  60  is formed of a substrate  68  and a coating system  70 . Most typically, the substrate  68  will be formed of a superalloy, such as a nickel- or cobalt-based alloy. Alternatively, the substrate  68  may be formed of a ceramic or ceramic composite material. 
     The coating system  70  includes a ceramic barrier coating  72  disposed on the substrate  68 . An optional bond coat  74  that has a thermally grown oxide region  74   a  is disposed between the ceramic barrier coating  72  and the substrate  68 . The bond coat  74  may be MCrAlY, where M is nickel, iron or cobalt, Cr is chromium, Al is aluminum, and Y is yttrium. A portion of the bond coat  74  may oxidize to form the thermally grown oxide region  74   a , which facilitates bonding of the ceramic barrier coating  72 . 
     As an example, the ceramic barrier coating  72  is formed primarily of zirconium oxide. For instance, the zirconium oxide may be a stabilized or partially stabilized zirconia, such as yttrium stabilized zirconia or gadolinia stabilized zirconia, or a zirconate that is doped with a rare earth stabilizer, such as yttria or gadolinia. The ceramic barrier coating  72  may, for example, be deposited by plasma spray or physical vapor deposition, which generally result in a porous structure. 
     In the illustrated example, the ceramic barrier coating  72  has a columnar microstructure, represented schematically by microstructural columns  72   a . Such a columnar microstructure is a result of fabrication by electron beam physical vapor deposition. The columns  72   a  are substantially perpendicular to the bond coat  74  and substrate  68 . There are pores  72   b  defined by the gaps between the columns  72   a . Such a columnar microstructure facilitates coating durability. 
     The repair process begins with the providing of the article  60  for repair, where the ceramic barrier coating  72  has a damaged region  72   c . The “providing” may include identifying that the article  60  is in need of repair. In this regard, known inspection techniques may be used to detect and assess damage. More typically however, the article for repair will have already been identified and the “providing” may refer to the selection of the article to begin the repair process. 
     The damaged region  72   c  may be a region of the ceramic barrier coating  72  which has a physical deformity or imperfection, especially to a point that is unacceptable for the given article  60 . Most typically, the damaged region  72   c  will be the result of physical phenomena that are incurred during use of the article  60  in its intended operating environment. As an example, the phenomena may be related to thermal-mechanical stresses that cause spallation, impact events that cause erosion, rubbing events that cause wear, or combinations of these. In these regards, the physical deformity may be a spalled, eroded, and/or worn portion of the ceramic barrier coating  72 . Alternatively, damage may be incurred prior to use or outside of use, such as during handling and transport of the article  60 . In the illustrated example, the physical deformity is a depression  76  in the ceramic barrier coating  72 . In another alternative, the physical deformity may be an imperfection, such as a crack or void, that results from original fabrication of the ceramic barrier coating  72 . For instance, a fabrication imperfection that renders the article  60  unacceptable for its intended use may be repaired using the disclosed repair process to render the article acceptable for its intended use. 
     The next step in the repair process includes abrading the damaged region  72   c . The depression  76  may contain debris or other undesirable substances which could be detrimental to the ceramic barrier coating  72  or hinder the remainder of the repair process. In this respect, the abrading facilitates removal of much or all of the debris, as well as possibly small portions of the ceramic barrier coating  72  in the vicinity of the damaged region  72   c . The abrading thereby produces a fresh, clean surface in the ceramic barrier coating  72 . 
     As an example, the abrading may include spraying an abrasive media  78  from a spray nozzle  80  onto the damaged region  72   c . For instance, the abrasive media  78  may be carried in a pressurized process gas. The abrasive media  78  strikes the damaged region  72   c  and thereby removes the debris and possibly portion of the ceramic barrier coating  72 . The pressure of the process gas may be controlled in order to control removal of the debris and ceramic barrier coating  72 . In one example, the abrasive media  78  includes particles of dry ice, which is solid carbon dioxide. The dry ice facilitates clean removal because it is substantially non-reactive at normal pressure and temperature (NTP) with the ceramic barrier coating  72  and thus does not leave residue or stains. Additionally, the dry ice rapidly evaporates and thus does not leave a mess. 
       FIG. 4  depicts the article  60  after the abrading. The abrading has removed debris and a portion of the ceramic barrier coating  72  such that a dimple  82  remains in the ceramic barrier coating  72 . A remaining region of the ceramic barrier coating, a representative portion of which is shown at  84 , adjacent the dimple  82  remains intact after the abrading step. For instance, “intact” may refer to this region as being unchanged in physical character before and after the abrading. In this example, the dimple  82  is only modestly larger than the damaged region  72   c  and does not extend entirely through the thickness of the ceramic barrier coating  72 . 
     The next step in the repair process, depicted in  FIG. 5 , is the deposition of a patch  86  into the dimple  82 . The patch  86  is composed of networked ceramic nanofibers  88 .  FIG. 6  illustrates a magnified view of the networked ceramic nanofibers  88 . The nanofibers  88  are elongated, randomly oriented filaments that have a maximum diameter of 1 nanometer to 500 nanometers. More typically, the diameter will be 1 nanometer to 250 nanometers, 1 nanometer to 100 nanometers, or 1 nanometer to 50 nanometers. The filaments are non-linear and curve or turn such that the filaments are intertwined to form a tangled porous network. As used herein, “networked” refers to the intertwining of the fibers or filaments. Where the filaments contact each other, they may be bonded together as a result of the process used to form the patch  86 . 
     The nanofibers  88  are formed of a ceramic, such as an oxide. In one example, the ceramic is zirconium oxide. For instance, the zirconium oxide may be a stabilized or partially stabilized zirconia, such as yttrium stabilized zirconia or gadolinia stabilized zirconia, or a zirconate that is doped with a rare earth stabilizer, such as yttria or gadolinia. 
     The patch  86  of networked ceramic nanofibers  88  fills the dimple  82  and seals the pores  72   b  of the ceramic barrier coating  72 . The nanofibers  88  may also mechanically interlock with the surface roughness in the dimple  82  that results from the abrading, thereby providing good bonding between the patch  86  and the ceramic barrier coating  72 . In that regard, the pressure of the process gas may also be controlled to produce a desirable level of roughness for mechanical interaction with the nanofibers  88 . 
     The pores  72   b  in the ceramic barrier coating  72  may be prone to infiltration of debris and other material during use of the article  60  that could damage the ceramic barrier coating  72 , bond coat  74 , or underlying substrate  68 . In particular, since the pores  72   b  are also substantially perpendicular to the bond coat  74  and substrate  68 , they can provide a direct path of infiltration for CMAS (calcium-magnesium-aluminosilicate) and foreign material. In this regard, the networked ceramic nanofibers  88  seal the pores  72   b  in the dimple  82 . As an example, the networked ceramic nanofibers  88  may infiltrate partially into the pores  72   b  during deposition, thereby enhancing sealing. 
     Although the patch  86  of networked ceramic nanofibers  88  is itself porous, the networked ceramic nanofibers  88  provide a sponge-like structure of smaller pores that provides superior thermal insulation. Therefore, the patch  86  of networked ceramic nanofibers  88 , even if formed of the same composition as the ceramic barrier coating  72 , provides thermal sealing of the ceramic barrier coating  72 . As an example based on zirconia, the patch  86  of networked ceramic nanofibers  88  may have a thermal conductivity of approximately 0.027 Watts per meter-Kelvin. 
     Additionally, the filaments of the networked ceramic nanofibers  88  are also flexible and strain tolerant. The flexibility of the filaments may further facilitate entrapment of foreign particles, debris, or materials, as well as act as “bumper” to absorb impact of particles and debris. The patch  86  of networked ceramic nanofibers  88  thereby provides thermal and physical sealing/protection. 
     It is further noted that networked ceramic nanofibers are not known for being produced on barrier coatings as a patch. Rather, networked ceramic nanofibers have been produced in a screen-like cage structure. As a result, use of a patch of networked ceramic nanofibers has not been suggested in combination with a ceramic barrier coating, nor have the thermal and physical sealing benefits of a patch of ceramic nanofibers been realized for protection with a ceramic barrier coating. 
     The patch  86  of networked ceramic nanofibers  88  may be deposited directly into the dimple  82 . For example, the patch  76  of networked ceramic nanofibers  88  may be deposited by a blow-spinning process. Blow-spinning involves spraying a precursor solution through an inner nozzle while flowing a process gas from an outer concentric nozzle such that the precursor when sprayed elongates into ultra-thin filaments. The filaments deposit in the dimple  82  and, after further processing, are converted into the ceramic nanofibers. The precursor solution includes binders and salts of the constituents that will form the ceramic, such as zirconium, oxygen, and any dopants. An example binder includes polyvinylpyrrolidone, and example salts may include aqueous oxynitrate, nitrate, nitrite, or chloride salts of zirconium and the selected dopants, zirconyl chloride, or metal organics such as zirconium isobutoxide or isopropoxide in a solvent. The amounts of the constituents may be controlled in order to control the final composition of the ceramic nanofibers. After spinning, the filaments are then thermally treated to remove binders, etc. and sinter the ceramic. The thermal treatment may include heating the filaments to temperatures from 800° C. to 1000° C., for example. It is during the thermal treatment that the filaments may diffuse and thereby bond together where they are in contact. The thermal treatment may also cause diffusion at points of contact between the nanofibers  88  and the ceramic barrier coating  72 , thereby providing additional bonding. 
     In one further example, the thermal treatment may be conducted using an energy beam, such as a laser. In particular, the energy beam can be aimed to impinge on the patch  86 , with minimal or no impingement on the remaining regions  84  of the ceramic barrier coating  72  adjacent the patch  86 . The effects of the heating can thus be confined to the patch  86 , while avoiding the expenditure of time and energy to heat the entire article  60 . 
     After deposition, the patch  86  may project from the ceramic barrier coating  72 , which generally has a continuous, smooth outer surface. If desired, as depicted in  FIG. 7 , the patch  86  can be polished such that it is flush with the remaining region  84  of the ceramic barrier coating  72  adjacent the patch  86 , as represented at  90 . 
     At the remaining region  84 , the ceramic barrier coating  72  may define a thickness t1. The patch  86  may also define a maximum thickness t2, wherein the thickness t2 is less than the thickness t1. The thickness t2 being less than the thickness t1 is a representation that the patch  86  is not as thick as the ceramic barrier coating  72 . That is, the patch  86  most typically will be a relatively small piece of material that mends a relatively small region of the coating  72 . In particular, the ceramic barrier coating  72  thus has a locally thin portion at the dimple  82 . This locally thin portion, but for the patch  86 , may otherwise be prone to CMAS or foreign substance infiltration. 
     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.