Patent Publication Number: US-2016245110-A1

Title: Hard phaseless metallic coating for compressor blade tip

Description:
BACKGROUND 
     Gas turbine engines include one or more compressors for compressing air prior to combustion. One or more rows of compressor blades extend radially outward from a hub or rotor and towards an inner surface of the compressor casing. During operation, the blades rotate about a central axis to direct the flow of fluids and to compress fluids (e.g., air) within the compressor. A seal is formed between the radially outermost end of a compressor blade, the blade tip, and the inner surface of the casing. In some compressors, the blade tip is coated with an abrasive material and the inner surface of the casing is coated with an abradable material to form a seal. During rotation, the abrasive material on the blade tip rubs against and abrades the abradable material on the casing to form a tight seal between the blade tip and the casing. Maintaining a tight air seal between the blades and compressor casing is needed for optimum gas turbine engine operation. 
     SUMMARY 
     A blade includes an airfoil having pressure and suction side walls extending in a spanwise direction from a blade root to a blade tip and a hard phaseless metallic coating located at the blade tip. 
     An assembly includes a casing having an inner diameter surface, an abradable coating located on a portion of the inner diameter surface, a blade having a blade tip configured to rotate within the casing where the blade tip and the inner diameter surface of the casing form a seal, and a hard phaseless metallic coating located on the blade tip. 
     A method of making a blade includes forming a blade having a tip and depositing a hard phaseless metallic coating on the blade tip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section view of a gas turbine engine. 
         FIG. 2  is a cross section view of a compressor illustrating a blade and compressor casing. 
         FIG. 3  is a cross section view of a compressor blade having a hard phaseless metallic coating. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes a hard phaseless metallic coating for use on blade tips. The metallic coating strengthens the blade tip so that the tip is stronger than the corresponding abradable material on the compressor casing adjacent the blade tip at all compressor operating temperatures. This reduces wear on the blade tip and increases the operational lifetime of blades having the metallic coating. 
       FIG. 1  illustrates a cross-sectional view of gas turbine engine  20 . Gas turbine engine  20  includes fan  22  with bypass duct  24  oriented about a turbine core having compressor section  26 , combustor section  28 , and turbine section  30 , which are arranged in flow series along an axial direction with an upstream inlet  32  and downstream exhaust  34 . 
     Turbine section  30  includes high-pressure turbine (HPT) section  36  and low-pressure turbine (LPT) section  38 . Turbine sections  36  and  38  each have a number of alternating turbine blades  40  and turbine vanes  42 . Turbine vanes  42  are circumferentially oriented with respect to one another, and collectively form a full, annular vane ring about turbine centerline axis C L  of gas turbine engine  20 . HPT section  36  of turbine  30  is coupled to compressor section  26  via shaft  44 , forming the high pressure spool. LPT section  38  is coupled to fan  22  via shaft  46 , forming the low pressure spool. Shaft  46  is coaxially mounted within shaft  44 , about turbine centerline axis C L . 
     Fan  22  is typically mounted to a fan disk or other rotating member, which is driven by shaft  46 . As shown in  FIG. 1 , for example, fan  22  is forward-mounted in engine cowling  48 , upstream of bypass duct  24  and compressor section  26 , with spinner  50  covering the fan disk to improve aerodynamic performance. Alternatively, fan  22  is aft-mounted in a downstream location, and the coupling configuration varies. Furthermore, while  FIG. 1  illustrates a particular two-spool high-bypass turbofan embodiment of gas turbine engine  20 , this example is merely illustrative. In other embodiments, gas turbine engine  20  is configured either as a low-bypass turbofan or a high-bypass turbofan, as described above, and the number of spools and fan position vary. 
     In operation of gas turbine engine  20 , airflow F enters via upstream inlet  32  and divides into bypass flow F B  and core flow F C  downstream of fan  22 . Bypass flow F B  passes through bypass duct  24  and generates thrust; core flow F c  passes along the gas path through compressor section  26 , combustor section  28  and turbine section  30 . 
     Compressor section  26  includes low pressure compressor  52  and high pressure compressor  54 , which together compress incoming air for combustor section  28  where it is mixed with fuel and ignited to produce hot combustion gas. The combustion gas exits combustor section  28  and enters HPT section  36  of turbine section  30 , driving shaft  44  and thereby compressor section  26 . Partially expanded combustion gas transitions from HPT section  36  to LPT section  38 , driving fan  22  via shaft  46 . Exhaust gas exits gas turbine engine  20  via downstream exhaust  34 . 
     The thermodynamic efficiency of gas turbine engine  20  is strongly tied to the overall pressure ratio, as defined between the compressed air pressure entering combustor section  28  and the delivery pressure at upstream inlet  32 . In general, higher pressure ratios offer increased greater specific thrust, and may result in higher peak gas path temperatures, particularly downstream of combustor section  28 , including HPT section  36 . 
       FIG. 2  illustrates a cross section view of a compressor showing blade  60  and compressor casing  62 . Blade  60  rotates within compressor casing  62  to compress air from fan  22  before it is delivered to combustor section  28 . In some embodiments, blade  60  is part of an integrally bladed rotor. Blade tip  64  is located at a radially outward portion of blade  60 . Abradable material  66  is located at a radially inward portion of casing  62 . In order to operate at peak efficiency, blade  60  and casing  62  must form a tight seal. One way to seal blade  60  and casing  62  is to form or coat blade tip  64  with an abrasive (abrasive material  68 ) and position abradable material  66  along inner surface  68  of casing  62  directly across from blade tip  64 . As blade  60  rotates, temperatures within the compressor increase, blade  60  and casing  62  expand, and blade tip  64  (with abrasive material  68 ) and abradable material  66  rub against one another. As the two components rub, a portion of abradable material  66  is worn away. Just enough abradable material  66  is removed to accommodate thermal growth and relative movement so that a tight seal is formed between blade tip  64  and casing  22 . 
     Abradable material  66  can be “soft” and have a porosity that makes it abradable with respect to bare metal blades. Alternatively, abradable material  66  can be generally “hard” and have a higher density and smoother surface and be gas impermeable. “Hard” abradables typically yield improved efficiency when compared to “soft” abradables. In some embodiments, abradable material  66  is a CoNiCrAlY or NiCoCrAlY alloy, where Co is cobalt, Ni is nickel, Cr is chromium, Al is aluminum and Y is yttrium. According to the prior art, blade tip  64  is generally constructed from abrasive material  68  or abrasive material  68  is applied to blade tip  64 . In some examples, blade  60  is a metallic blade and abrasive material  68  applied to blade tip  64  is a ceramic. Abrasive material  68  is generally stronger than abradable material  66  so that blade tip  64  cuts into abradable material  66  to form the seal. However, wear occurs on both abradable material  66  and abrasive material  68 . Depending on composition, blade tip  64  can soften at high temperatures and experience increased wear. 
     The present disclosure describes a blade tip, and a method for forming such a blade tip, that experiences decreased wear even when paired with “hard” abradable materials. According to the present disclosure, a hard phaseless metallic coating is applied to blade tip  64  in place of a conventional abrasive material. A “hard phaseless metal” is a metal with minimal hard abrasive phase. Hard phaseless metals according to the present disclosure contain no phases having a Mohs scale of hardness greater than 7 at a concentration more than 5% by volume. In some embodiments, the hard phaseless metals contain no phases having a Mohs scale of hardness greater than 7 at a concentration more than 1% by volume. The hard phaseless metals can be single-phase metals or metals having multiple phases. Single-phase metallic coatings are homogeneous and have a single solid phase microstructure (as opposed to a heterogeneous composition having a mixture of phases). Single-phase metallic coatings are generally free of precipitates and do not undergo phase changes with changes in temperature, avoiding the possible detrimental effects of the associated volume changes. Multiple-phase metals can include alloys having two or more crystal structures (phases). Alloys can contain carbide, nitride and intermetallic phases. 
       FIG. 3  illustrates a view of compressor blade tip  64 A with hard phaseless metallic coating  70 . The composition of hard phaseless metallic coating  70  is selected so that blade tip  64  is stronger than abradable material  66  on inner surface  68  of compressor casing  62  at all compressor operating temperatures up to the melting point of abradable material  66  or hard phaseless metallic coating  70 . In one embodiment, hard phaseless metallic coating  70  is pure molybdenum. In another embodiment, hard phaseless metallic coating  70  is molybdenum that is at least 99.5% pure. Molybdenum has a Mohs hardness of about 5.5. In another embodiment, hard phaseless metallic coating  70  is a nickel-chromium alloy that is at least 80% chromium by weight. In yet another embodiment, hard phaseless metallic coating  70  is an alloy or metal that has a higher melting temperature than nickel. 
     Hard phaseless metallic coating  70  can be applied to blade tip  64  a number of different ways. These include, but are not limited to: plating, thermal spray, cold spray, weld overlay, laser powder cladding, direct metal laser sintering, other additive methods and combinations thereof. Hard phaseless metallic coating  70  can be applied to blade tip  64  so that it is relatively dense. In some embodiments, hard phaseless metallic coating  70  has a relative density of 80% or greater. Hard phaseless metallic coating  70  can also have a low concentration of oxides. In some embodiments, hard phaseless metallic coating  70  is less than 10% oxide. 
     Hard phaseless metallic coating  70  can be deposited onto blade tip  64  at varying thicknesses. In some embodiments, hard phaseless metallic coating  70  is applied to blade tip  64  so that it has a thickness between about 0.002 millimeters (0.0001 inches) and about 0.05 millimeters (0.002 inches). In other embodiments, hard phaseless metallic coating  70  is applied to blade tip  64  so that it has a thickness between about 0.13 millimeters (0.005 inches) and about 0.76 millimeters (0.030 inches). In some large engine applications, hard phaseless metallic coating can have a thickness up to about 1.27 millimeters (0.050 inches). Higher thicknesses can help prevent blade base metal contact at the leading and trailing edges during deep interactions that have an axial motion component. 
     Hard phaseless metallic coatings  70  according to the present disclosure have higher melting temperatures than typical Inconel 718 blade alloys. As a result, hard phaseless metallic coatings  70  retain greater strength than the blade material as rub contact surface temperatures exceed 649° C. (1200° F.). By placing hard phaseless metallic coating  70  on blade tip  64 , the amount of blade wear is reduced. As noted above, hard phaseless metallic coatings  70  is selected so that blade tip  64  is stronger than abradable material  66  on inner surface  68  of compressor casing  62  at all compressor operating temperatures up to the melting point of abradable material  66  or hard phaseless metallic coating  70 . Instead of having increased blade wear at high temperatures, hard phaseless metallic coating  70  keeps the level of wear to blade tip  64  low even at high temperatures. The composition, density and thickness of hard phaseless metallic coating  70  can be selected to provide a desired wear ratio between abradable material  66  and blade tip  64 . For example, a wear ratio of blade tip  64  (with hard phaseless metallic coating  70 ) to abradable material  66  of about 1:50 can be provided by varying the density and composition of hard phaseless metallic coating  70 . 
     Hard phaseless metallic coating  70  also offers advantages over state of the art ceramic and abrasive coatings. Blade tips  64  with hard phaseless metallic coating  70  have reduced fatigue debits when compared to blade tips having ceramic coatings. Abrasive coatings also have hard particles extending from the surface of the blade tips requiring larger tip clearance. Increasing the tip clearance reduces the quality of the seal between the blade tip and compressor casing and reduces overall efficiency. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A blade can include an airfoil having pressure and suction side walls extending in a spanwise direction from a blade root to a blade tip and a hard phaseless metallic coating located at the blade tip. 
     The blade of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A further embodiment of the foregoing blade can include that the hard phaseless metallic coating comprises at least 99.5% molybdenum. 
     A further embodiment of any of the foregoing blades can include the hard phaseless metallic coating comprises a nickel-chromium alloy having at least 80% chromium. 
     A further embodiment of any of the foregoing blades can include the hard phaseless metallic coating has a relative density of at least 80%. 
     A further embodiment of any of the foregoing blades can include the hard phaseless metallic coating has an oxygen concentration of less than 10% by weight. 
     A further embodiment of any of the foregoing blades can include the hard phaseless metallic coating has a thickness between about 0.002 millimeters (0.0001 inches) and about 1.27 millimeters (0.050 inches). 
     A further embodiment of any of the foregoing blades can include that the hard phaseless metallic coating contains no more than 5% by volume of a phase having a Mohs hardness greater than 7. 
     An assembly can include a casing having an inner diameter surface, an abradable coating located on a portion of the inner diameter surface, a blade having a blade tip configured to rotate within the casing where the blade tip and the inner diameter surface of the casing form a seal, and a hard phaseless metallic coating located on the blade tip. 
     The assembly of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A further embodiment of the foregoing assembly can include that the hard phaseless metallic coating has a strength greater than that of the abradable coating at all temperatures up to a melting point of one of the hard phaseless metallic coating and the abradable coating. 
     A further embodiment of any of the foregoing assemblies can include that the hard phaseless metallic coating comprises at least 99.5% molybdenum. 
     A further embodiment of any of the foregoing assemblies can include that the hard phaseless metallic coating comprises a nickel-chromium alloy having at least 80% chromium. 
     A further embodiment of any of the foregoing assemblies can include that the abradable coating is a CoNiCrAlY alloy. 
     A further embodiment of any of the foregoing assemblies can include that the hard phaseless metallic coating has a relative density of at least 80%. 
     A further embodiment of any of the foregoing assemblies can include that the hard phaseless metallic coating has an oxygen concentration of less than 10% by weight. 
     A further embodiment of any of the foregoing assemblies can include that the hard phaseless metallic coating has a thickness between about 0.002 millimeters (0.0001 inches) and about 1.27 millimeters (0.050 inches). 
     A further embodiment of any of the foregoing assemblies can include that the hard phaseless metallic coating contains no more than 5% by volume of a phase having a Mohs hardness greater than 7. 
     A method of making a blade can include forming a blade having a tip and depositing a hard phaseless metallic coating on the blade tip. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A further embodiment of the foregoing method can include that the step of depositing the hard phaseless metallic coating on the blade tip uses a technique selected from the group consisting of plating, thermal spray, cold spray, weld overlay, laser powder cladding, direct metal laser sintering and combinations thereof. 
     A further embodiment of any of the foregoing methods can include that the hard phaseless metallic coating contains no more than 5% by volume of a phase having a Mohs hardness greater than 7. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.