Patent Publication Number: US-10760443-B2

Title: Turbine abradable air seal system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. patent application Ser. No. 15/026,755 filed Apr. 1, 2016 which is a National Stage application of PCT/US2014/056051 filed on Sep. 17, 2014 which claims the benefit of and priority to U.S. Provisional Patent Application No. 61/885,774 filed Oct. 2, 2013, all of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to blade assemblies, and more particularly to air seal systems for a rotor blade assembly of a gas turbine engine, for example. 
     2. Description of Related Art 
     Air seals are formed between various components of gas turbine engines, such as between rotating turbine blades and the inner surface of a turbine casing. In this regard, some air seals are provided as an abradable air seal that incorporates an abradable material affixed to the inner surface of the casing. The abradable material is contacted and abraded by the rotating blade tips of the turbine blades during operation. 
     Other air seals are provided as wear resistant seals. Wear resistant seals also employ materials affixed to the inner surface of the casing. Such materials, however, are selected for erosion resistance, oxidation resistance and/or thermal protection, for example. 
     Selection of materials for forming such an air seal typically involves a compromise between resistance to wear by erosion, abradability by blade tips, spallation resistance and environment durability. Exemplary materials are set forth in U.S. Pat. Nos. 4,936,745 and 5,780,171, which are incorporated by reference herein in their entireties. Additionally, each of U.S. Pat. Nos. 6,284,323; 7,662,489; 8,343,587; 8,506,243; 8,535,783; and U.S. Patent Application Publication Nos. 2008/0138658 and 2010/0098865 is incorporated by reference herein in its entirety. Notably, materials with higher abradability tend to suffer from excess erosion, while less abradable materials tend to be susceptible to spallation. Erosion, spallation, and the like can reduce a seal&#39;s effectiveness, which can have a negative impact on thrust specific fuel consumption (TSFC). 
     Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for air seal systems that allow for improved lifetime thrust specific fuel consumption. There also remains a need in the art for such systems that are easy to make and use. The present disclosure provides a solution for these problems. 
     SUMMARY OF THE INVENTION 
     An air seal system for a rotor blade assembly of a gas turbine engine includes a substrate having a bond coat thereon. An erosion resistant thermal barrier coating (E-TBC) layer is disposed inboard of the bond coat. An abradable layer is disposed inboard of the erosion resistant thermal barrier coating (E-TBC) layer. 
     In certain embodiments, the bond coat is deposited onto the prepared substrate. The bond coat layer can be between 0.5 to 18 mils (1.27×10 −5  to 4.572×10 −4  meters) thick. In certain embodiments, the bond coat is metallic having a thickness of 1.27×10 −4  to 2.286×10 −4  meters (5 to 9 mils). The substrate layer can be metallic. An optional ceramic interlayer can be disposed between the bond coat and the erosion resistant thermal barrier coating (E-TBC) layer. In certain embodiments, the interlayer can be deposited inboard of the bond coat at an elevated substrate temperature. The ceramic interlayer can be 2.54×10 −6  to 7.62×10 −5  meters (0.1 to 3.0 mils) thick. For example, the ceramic interlayer can be of 7 weight % yttria stabilized zirconia (7YSZ) at less than or equal to 6% porosity. 
     In certain embodiments, the erosion resistant thermal barrier coating (E-TBC) layer can be deposited inboard of the ceramic interlayer. The erosion resistant thermal barrier coating (E-TBC) layer can be 2.54×10-5 to 3.81×10-4 meters (1.0 to 15 mils) thick. In certain embodiments, the abradable layer can be a porous layer having a thickness of 5.0 to 50 mils (1.27×10 −4  to 1.27×10 −3  meters). For example, 23% porous 7YSZ, 8-15% porous 20-60 wt % GdZrOx, or any other suitable material can be used for the abradable layer. 
     A transition layer can be formed between each of the ceramic layers, e.g., between the optional ceramic interlayer and the thermal barrier coating as well as between the erosion resistant thermal barrier coating (E-TBC) layer and the abradable layer. The transition layers can have a mixture of properties of the adjacent layers. The transition layers can each have a thickness of 0 to 2.54×10 −4  meters (0 to 10 mils). 
     A method for forming an outer air seal system for a rotor blade assembly of a gas turbine engine includes applying an erosion resistant thermal barrier coating (E-TBC) layer inboard of a substrate. The method also includes applying an abradable layer inboard of the erosion resistant thermal barrier coating (E-TBC) layer. 
     A bond coat can be applied onto the substrate prior to applying the E-TBC layer. A ceramic interlayer can be applied onto the bond coat, and the E-TBC layer can be applied onto the ceramic interlayer. In certain embodiments, an abradable layer is added inboard of the erosion resistant thermal barrier coating (E-TBC) layer. The step of applying can include grit blast preparation of the substrate, HVOF (high velocity oxygen fuel) spraying of a metallic bond coat on to the substrate, diffusion heat treating the bond coat and substrate at 1975° F. in a protective atmosphere, air plasma spraying a ceramic interlayer while the part is held at an elevated temperature, air plasma spraying an erosion resistant thermal barrier coating, and then air plasma spraying an abradable layer. 
     These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
         FIG. 1  is a schematic diagram depicting an exemplary embodiment of a gas turbine engine; 
         FIG. 2  is a schematic diagram depicting a portion of a representative blade tip in proximity to the casing of  FIG. 1 , showing detail of the air seal system formed therebetween; 
         FIG. 3  is a schematic diagram illustrating exemplary portions of the air seal system of  FIG. 2 , showing the layers which abrade when the turbine engine is in use; and 
         FIG. 4  is a flow chart illustrating an exemplary embodiment of a method for forming the air seal system of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an exemplary embodiment of a blade outer seal system in accordance with the disclosure is shown in  FIG. 1  and is designated generally by reference character  200 . Other embodiments of the air seal system in accordance with the disclosure, or aspects thereof, are provided in  FIGS. 2 and 3 , as will be described. 
       FIG. 1  is a schematic diagram depicting an exemplary embodiment of a gas turbine engine. As shown in  FIG. 1 , engine  100  is configured as a turbofan that incorporates a fan  102 , a compressor section  104 , a combustion section  106 , and a turbine section  108 . Notably, the turbine section is defined, at least in part by a turbine casing  109 , which forms a portion of the casing of the engine. An air seal  200  (e.g., an abradable air seal) is formed between an inner surface of the turbine casing and a rotating blade (e.g., blade  111 ) of the turbine. 
     As shown in  FIG. 2 , air seal  200  is an outer air seal located between the rotating blades  111  of the turbine (a portion of one of which is depicted in  FIG. 2 ) and an inner surface  120  of turbine casing  109 . In this embodiment, several layers of material are applied to the inner surface of the casing to form the air seal  200 . These layers include a metallic substrate  210  having a bond coat  212  thereon. The bond coat  212  is metallic having a thickness of 1.27×10 −4  to 2.286×10 −4  meters (5 to 9 mils), although it is also contemplated that the bond coat thickness can be between 0.5 to 18 mils (1.27×10 −5  to 4.572×10 −4  meters) thick. 
     A ceramic interlayer  214  is disposed inboard of the bond coat  212 . The ceramic interlayer  214  is deposited onto the inboard surface of the bond coat  212  at an elevated substrate temperature and is 2.54×10 −6  to 2.54×10 −5  meters (0.1 to 1.0 mils) thick, but it is also contemplated that the interlayer  214  can be between 2.54×10 −6  to 7.62×10 −5  meters (0.1 to 3.0 mils) thick. The interlayer  214  provides a tough interface with the bond coat  212  within which stresses are dissipated and a reduced stress interface with the erosion resistant thermal barrier coating E-TBC layer  216  (described below) results, thus improving spallation resistance. Interlayer  214  can be of 7YSZ at less than or equal to 6% porosity, for example, or of any other suitable material. 
     An erosion resistant thermal barrier coating (E-TBC) layer  216  is applied onto the inboard surface of the ceramic interlayer  214 . The erosion resistant thermal barrier coating (E-TBC) layer  216  provides erosion and spallation resistance and may be deposited at a high power with low relative spray torch to part surface speed. The erosion resistant thermal barrier coating (E-TBC) layer  216  can be 3.0 to 15 mils (7.62×10-5 to 3.81×10-4 meters) thick. It is also contemplated that the thickness can be as low as 1.0 mils (2.54×10-5 meters) and as high as 18 mils (4.572×10-4 meters), or any other suitable thickness. 
     An abradable layer  220  is disposed onto the inboard surface of the erosion resistant thermal barrier coating (E-TBC) layer  216 . The abradable layer  220  is a porous thermal barrier coating having a balance between erosion resistance and the ability to be cut by abrasive blade tips. For example, 23% porous 7YSZ or 8-15% porous 20-60 wt % GdZrOx can be used for the abradable layer  220 . It is also contemplated that any other suitable ceramic abradable material can be used, for example ceramics filled with a softer phase such as hexagonal boron nitride (hBN). The abradable layer  220  can be 5.0 to 50 mils (1.27×10-4 to 1.27×10-3 meters) thick. 
     A transition layer  218  is formed between the erosion resistant thermal barrier coating (E-TBC) layer  216  and the abradable layer  220 . The transition layer  218  has a mixture of properties of the erosion resistant thermal barrier coating (E-TBC) layer  216  and the abradable layer  220 . The transition layer  218  has a thickness between 0 to 2.54×10 −4  meters (0 to 10 mils). It should be noted that transition layers can be formed between any of the layers, and that the transition layers do not necessarily add to the overall thickness, but may be considered to be part of the inner most adjacent layer with an engine frame of reference, for example. The transition layers can each be a 5 mil transition from the inner adjacent layer to the next adjacent layer. It is contemplated that the transition layers can be between 0 to 2.54×10 −4  meters (0 to 10 mils) thick. For example, one transition layer can be a 5 mil transition from the interlayer to the erosion resistant thermal barrier coating (TBC) layer  216 . So, for example, in an application with a 1 mil interlayer and 15 mil erosion resistant thermal barrier coating (E-TBC) layer could be considered to be 1 mil interlayer, 5 mil transition, and a 10 mil erosion resistant thermal barrier coating (E-TBC). 
     Referring to  FIG. 3 , the erosion resistant thermal barrier coating (E-TBC) layer  218  and the abradable layer  220  are shown depicting an exemplary rub path  230  created by the rotating blades  111  of the turbine engine. As shown, the abradable layer  220  is allowed to be removed by the rotating blades. The erosion resistant thermal barrier coating (E-TBC) layer  216  contributes very little to the volume of material that the abrasive tips need to cut because the arc length of the blade path intersecting with the E-TBC layer is relatively short given the shallow depth of cut into this layer. 
     An embodiment of a method  240  for forming an air seal system is shown in the flow chart of  FIG. 4 . An exemplary preparation of the substrate, e.g., substrate  210 , can include grit blasting the substrate. The substrate can be flat as depicted in  FIG. 2 , or can include geometric surface features as described for example in U.S. Pat. No. 8,506,243, which is incorporated by reference herein in its entirety. The bond coat, e.g., bond coat  212 , is an oxidation resistant alloy which may be applied (at block  242 ) to the substrate by any of a number of processes. The bond coat alloy is known generically as MCrAlY and may be supplied in powder form. The powder may be deposited by any of a number of processes, however one that produces a dense (e.g. less than 5% porosity), low oxide (e.g., less than 2% oxygen) coating is suitable, for example. It is also contemplated that cathodic-arc or cat-arc that can also be used, e.g., using an MCrAlY ingot and can produce lower oxide and better oxidation resistance. A process that operates with particle velocity greater than 1000 ft/s (304.8 m/s) is suitable, such as in high velocity plasma spray, high velocity oxygen fuel, high velocity air fuel, cold spray and warm spray processes, for example. The bond coat thickness may be 8 to 12 mils (2.032×10 −4  to 3.048×10 −4  meters). The coating and part, e.g. bond coat  212  and substrate  210 , may then be diffusion heat treated to further improve bonding and oxidation resistance. For example, PWA 1386 powder, available from Sulzer Metco (all Sulzer Metco products described herein are available from Sulzer Metco (US) Inc. of Westbury, N.Y.), is deposited by high velocity oxygen fuel (HVOF) spraying and is then diffusion heat treated at 1975° F. (1079° C.) for 1 hour, e.g., in a protective atmosphere. 
     In a suitable setup for deposition of the above described ceramic layers, a plurality of bond coated substrates are loaded into a hollow cylindrical fixture such that the bond coated surfaces face the inner diameter of the cylindrical fixture. An auxiliary heat source, such as gas burners, is positioned around the fixture and a means for monitoring and controlling the part temperature is employed. This may include a thermocouple and temperature controller for regulating gas flow to the gas burners. A plasma spray torch is positioned in the interior of the cylindrical fixture for depositing the layers. In another exemplary setup, the parts are insulated and the plasma torch provides the heat. Variable air blower pressure may be employed to limit and control part temperature in this configuration. 
     The parts can then be preheated to the desired process temperature and then the ceramic interlayer, e.g., ceramic interlayer  214 , is applied at block  244 . After a preheat time of approximately 10 minutes and the parts are at the temperature set point, which is in one example 1200° F. (648.9° C.), the coating process is begun. Air plasma spraying can be used while the part is held at the elevated temperature. As an example, a Sulzer Metco 9 MB torch is operated at 60 kilowatts with 100 scfh (standard cubic feet per hour) (2.83 standard cubic meters per hour) of nitrogen and 25 scfh (0.708 standard cubic meters per hour) of hydrogen gas flow. A suitable powder is a yttria partially stabilized zirconia (yttria partially stabilized zirconia herein refers to a composition of about 12 weight percent or less yttria stabilized zirconia). However, a composition of between about 6 weight percent and about 20 weight percent yttria stabilized zirconia may be used. In certain applications, a suitable range between about 7 weight percent and about 12 weight percent yttria stabilized zirconia may be chosen based on material strength. Similarly, other zirconia based compositions can be used, such as ceria stabilized zirconia, magnesia stabilized zirconia, calcia stabilized zirconia, and mixtures thereof may be substituted for the yttria stabilized zirconia. An example of a suitable powder is Sulzer Metco 204B NS of ZrO 2  8Y 2 O 3  composition. This can be fed through a #2 powder port at 20 g/minute with 12 scfh (0.340 cubic meters per hour) of nitrogen carrier gas. The parts are arranged on a 30 inch (76.2 cm) diameter fixture which is rotated at 10 rpm, for example, which can be adapted for other sizes of fixtures. The torch can be traversed back and forth in front of the parts at a 2.75 inch (6.99 cm) stand-off distance and 3 inches (7.62 cm) per minute traverse speed. 
     The erosion resistant thermal barrier coating (E-TBC) layer, e.g., erosion resistant thermal barrier coating (E-TBC) layer  216 , is applied onto the ceramic interlayer, e.g., interlayer  214 , at block  246 . Again, air plasma spraying can be used. The process may begin while the parts are still at elevated temperature or may be conducted as a separate spray event after the parts have cooled. The powder may be the same as that used for the ceramic interlayer or may be switched to one of different particle morphology or composition. Suitable compositions include those suitable for the ceramic interlay with the additional option of gadolinia stabilized zirconia and other low conductivity ceramic materials. Suitable powers for the erosion resistant thermal barrier coating (E-TBC) layer include compositions disclosed in European Patent No. EP0992603, which is incorporated by reference herein in its entirety. In an exemplary process, Sulzer Metco 204B NS powder continues spraying onto the parts as described above without interruption, while the process parameters are gradually changed to those for the erosion resistant thermal barrier coating (E-TBC) layer. This produces a graded zone, or transition layer, which in this example is 3 to 5 mils (7.62×10 −5  to 1.27×10 −4  meters) thick. Including the graded zone, the erosion resistant thermal barrier coating (E-TBC) layer is 12 to 18 mils (3.048×10 −4  to 4.572×10 −4  meters) thick. While depositing the graded zone, the exemplary process increases rpm to 80, stand-off to 4 inches (0.1016 meters), traverse rate to 12 inches (0.3048 meters) per minute, powder feed rate to 50 g/min (grams per minute) and reduces torch power to 35 kW with hydrogen flow rate reduced to 18 scfh (0.510 cubic meters per hour) and carrier gas flow rate to 10 scfh (0.283 cubic meters per hour). Auxiliary heating is also stopped, allowing the part temperature to passively drop as the process progresses. This process can be continued at these conditions until the target thickness is achieved for the erosion resistant thermal barrier coating (E-TBC) layer. 
     The abradable top layer, e.g., abradable layer  220 , is then applied onto the erosion resistant thermal barrier coating (E-TBC) layer, e.g., using air plasma spraying, at block  248 . This may begin while the parts are still at elevated temperature following the erosion resistant thermal barrier coating (E-TBC) layer application or may be conducted as a separate spray event after the parts have cooled. The powder may be the same as that used for the erosion resistant thermal barrier coating (E-TBC) layer, or may be switched to one of different particle morphology or composition. In an exemplary process, parameters are gradually adjusted while the first 5 mils (1.27×10 −4  meters) of abradable layer is being applied. 40 Wt % gadolinia stabilized zirconia powder is used at a rate of 40 g/minute and injected into the plasma stream with a carrier gas flow rate of 11 scfh (0.311 cubic meters per hour). Torch stand-off distance is increased to 5 inches (12.7 cm) and traverse rate to 20 inches (50.8 cm) per minute. Fixture speed is increased to 100 rpm. Coating application is continued until the final desired coating thickness is reached. In this example that is 40 mils (1.016×10 −3  meters) of abradable top layer, which includes 5 mils (1.27×10 −4  meters) of graded transition much as described above. While an exemplary composition for the abradable top layer has been described above, any other suitable composition can be used, for example, the composition can be 8 Wt % yttria stabilized zirconia blended with approximately 6 Wt % of methylmethacrylate particles (SM2602 from Sulzer Metco). 
     While described above in the exemplary context of including a bond coat on the substrate, it is possible to use the ceramic layers on a special base metal that does not require a bond coat. However it is to be expected that in typical high temperature applications the bond coat will be required. 
     While certain layers are referred to herein as being thermally insulative, e.g., thermal barrier coatings, and others are referred to herein as being abradable, it is to be understood that all the ceramic layers can be considered to be insulating, and all may be considered to be abradable. Moreover, the erosion resistant aspect, e.g., of the erosion resistant thermal barrier coating (E-TBC), is indicative of the presence of a layer that has lower erosion resistance such as the abradable layer, since all the ceramic layers can be considered to have some erosion resistance. For example, abradable layer  220  may be erosion resistant like erosion resistant thermal barrier coating (E-TBC) layer, or may be an abradable or low conductivity layer having higher porosity, less strong bonding between spray particles, and/or a composition containing a higher concentration of stabilizing elements such as Y and Gd. So, for example, a coating system in accordance with this disclosure can include a ceramic coating with arbitrary top layer(s) and a toughened interlayer to help prevent spallation. In another aspect, a coating system in accordance with this disclosure can include an abradable coating with a toughened interlayer to help prevent spallation and an optional erosion resistant middle layer. 
     While described in the exemplary context of turbine abradable layers, it is also contemplated that systems as disclosed herein can be used as a thermal barrier coating (TBC). i.e. in applications without rub. In such applications, the erosion resistant thermal barrier coating (E-TBC) layer provides some minimum required durable thermal barrier function, while the inboard layer, e.g., the abradable layer, provides additional thermal insulation in a less durable form. 
     The methods and systems of the present disclosure, as described above and shown in the drawings, provide for an air seal system with superior properties, and may include an optional toughened ceramic interlayer with the erosion resistant thermal barrier layer for improved lifetime thrust specific fuel consumption. While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the spirit and scope of the subject disclosure.