Patent Publication Number: US-2016237831-A1

Title: Abrasive blade tip with improved wear at high interaction rate

Description:
BACKGROUND 
     The present disclosure is directed to abrasive blade tip coating. More particularly, a modified tip that lowers the blade tip material in strategic locations and replaces the removed tip material with a composite structure including a matrix and grit particles of the abrasive blade tip coating. 
     Gas turbine engines and other turbomachines have rows of rotating blades contained within a generally cylindrical case. As the blades rotate, their tips move in close proximity to the case. To maximize engine operating efficiency, the leakage of the gas or other working fluid around the blade tips should be minimized. This may be achieved by blade and sealing systems in which the blade tips rub against a seal attached to the interior of the engine case. Generally, the blade tip is made to be harder and more abrasive than the seal; thus, the blade tips will cut into the seal during those portions of the engine operating cycle when they come into contact with each other. 
     During the operation of a gas turbine engine, it is desired to maintain minimum clearance between the tips of the turbine blades and the corresponding seals. A large gap results in decreased efficiency of the turbine, due to the escape of high-energy gases. Conversely, friction between the blades and seals causes excessive component wear and wastes energy. Since aircraft turbines experience cyclic mechanical and thermal load variations during operation their geometry varies during the different stages of the operating cycle. Active clearance control and abrasive blade tips are currently used to establish and maintain optimum clearance during operation. Ideally, those tips should retain their cutting action over many operating cycles compensating for any progressive changes in turbine geometry. 
     During certain engine operating conditions engines have shown very high radial interaction rate (˜40″/s) that cause rapid depletion of the abrasive grit portions of the abrasive blade tip coating when rubbed against the air seals. 
     The unwanted rubbing of exposed blade tips, such as Ti blade material, results from the depletion of the abrasive blade tip coating. This results in Ti blade material contact with the abradable material of the air seal. The Ti blade material contact with the abradable where the abrasive has been depleted can create Ti sparking which has the potential to cause unwanted ignition within the gas turbine engine. To remedy the unwanted contact of the blade tip on the abradable seal material at the worst case location, the R3 seal is being pre-trenched by 25 mils to prevent tip contact with the abradable. The pre-trenching opens clearance, and thus reduces efficiency and operability. 
     An abrasive tip is needed that provides a higher wear ratio with abradable outer air seal material. There is a need to provide a blade tip system that can provide better protection of the Ti blade tip material from contact with the abradable at the high interaction rates associated with certain off-normal engine operating conditions, such as, a bird strike and surge. 
     SUMMARY 
     In accordance with the present disclosure, there is provided a modified blade tip with abrasive coating comprising a blade having a tip with a center between corners proximate a leading edge and a trailing edge, the blade having a top surface, the top surface being modified proximate said corners. The abrasive coating is bonded to the tip at the top surface; wherein the abrasive coating is thicker proximate the corners than at the center of the blade tip. 
     In another and alternative embodiment, the abrasive coating further comprises a plurality of grit particles dispersed over said top surface of the blade tip. A matrix material is bonded to the top surface. The matrix material envelops and bonds to and partially surrounds the grit particles, wherein the grit particles extend above the matrix material relative to the top surface. 
     In another and alternative embodiment, the matrix material comprises a matrix formed from MCrAlY, wherein M is Ni or Co. 
     In another and alternative embodiment, the blade tip is reduced in thickness proximate to the corners. 
     In another and alternative embodiment, an adhesion layer is coupled to the top surface, wherein the adhesion layer is configured to adhere the grit particles to the top surface. 
     In another and alternative embodiment, the adhesion layer comprises the same material as the matrix material. 
     In another and alternative embodiment, a bond layer is bonded to the top surface. 
     In another and alternative embodiment, a turbine engine component comprises an airfoil portion having a tip; the tip comprises reduced corners proximate a leading edge and a trailing edge of the airfoil. A composite abrasive coating is bonded to the tip. The composite abrasive coating comprises an adhesion layer bonded to the tip. A layer of grit particles is bonded to the adhesion layer in a matrix material surrounding an unexposed portion of the grit particles. The matrix material is coupled to the adhesion layer; wherein the composite abrasive coating is thicker proximate the corners. 
     In another and alternative embodiment, the coating includes a thicker region at locations most likely to rub an air seal. 
     In another and alternative embodiment, the tip is reduced to a depth greater than a rub depth. 
     In another and alternative embodiment, the composite abrasive coating comprises a thickness at the corners of about 30 mils. 
     In another and alternative embodiment, the composite abrasive coating includes a taper from the corners inwardly at about a 30 degree angle resulting in a thickness of about 6 mils proximate a center of the airfoil. 
     In another and alternative embodiment, the turbine engine component is a blade. 
     In another and alternative embodiment, a process for coating a turbine engine blade with an abrasive comprises reducing the thickness of a tip of the blade proximate at least one corner of the tip; applying an adhesion layer onto the tip of the blade; adhering a plurality of grit particles to the adhesion layer, wherein narrow spaces are formed between the grit particles; filling the narrow spaces between the grit particles with a matrix material; surrounding each of the first grit particles with the matrix material exposing a portion of the first grit particles above the matrix material and the second grit particles; wherein the grit particles and the matrix material have a thickness greater than the reduced thickness of the tip proximate the at least one corner. 
     In another and alternative embodiment, the tip is reduced to a depth greater than a rub depth. 
     In another and alternative embodiment, the matrix material and the grit particles are applied as a thicker region at locations most likely to rub an air seal. 
     In another and alternative embodiment, the process further comprises tapering a thickness of the tip reduction from the corners inwardly at about a 30 degree angle resulting in a thickness of the grit particles and matrix coating of about 30 mils proximate the at least one corner and a thickness of about 6 mils proximate a center of the blade. 
     In another and alternative embodiment, the process further comprises applying a base layer to the blade tip prior to applying the adhesion layer. 
     Other details of the abrasive blade tip coating are set forth in the following detailed description and the accompanying drawing wherein like reference numerals depict like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of abrasive composite coating applied to a modified tip of a turbine engine component; 
         FIG. 2  is a schematic representation of abrasive composite coating applied to a modified tip of a turbine engine component; and 
         FIG. 3  is a schematic cross-sectional view of the exemplary abrasive blade tip coating. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1  there is illustrated a turbine engine component  10 , such as a compressor blade or vane. The blade  10  has an airfoil portion  12  with a tip  14 . The tip  14  has an abrasive coating  16  applied to it. The abrasive coating  16 , comprises a composite material that includes an abrasive particulate/grit or simply grit  18 , such as cubic boron nitride (CBN), coated Silicon carbide (SiC), or another hard ceramic phase. In an exemplary embodiment, the grit  18  can comprise, zirconia, aluminum diboride, aluminum nitride, aluminum nitride-cabon, or diamond. The grit  18  can be sized as a coarse grit. In an exemplary embodiment the grit  18  can be sized from about 70 to about 150 microns. The grit  18  is embedded in a plating layer matrix composite  20 . The matrix  20  comprises a suitable oxidation-resistant alloy matrix. In an exemplary embedment the plating layer comprises a matrix formed from MCrAlY, the M standing for either Ni or Co or both. In an exemplary embodiment, the matrix  20  can comprise pure nickel, copper, copper alloy, cobalt, cobalt alloy, or chrome alloy. 
     The tip  14  has been modified at the tip corner  22 . The blade tip  14  material is cut back at each tip corner  22  that most likely impacts the abradable seal material (not shown). The abrasive coating  16  is added at the locations that the tip  14  corner  22  has been reduced. The modification creates a thicker region of abrasive coating  16  at the locations most likely to rub. The modified tip  14  also locates the tip  14  material of the blade  10  farther away from the locations most likely to rub. The resulting blade tip  14  with thickened abrasive coating  16  is particularly well suited for rubbing metal as well as ceramic air seals (not shown). 
     The turbine engine component/blade  10  may be formed from a titanium-based alloy or a nickel-based alloy. In an exemplary embodiment, the blade  10  includes a (Ti) titanium-based alloy. The blade  10  includes a leading edge  24  and a trailing edge  26  opposite the leading edge  24 . The blade  14  also includes a suction side  28  of the airfoil  12  and a pressure side  30  opposite the suction side  28  of the airfoil  12 . 
     Referring to  FIG. 2  and  FIG. 3  an exemplary abrasive coating  16  on a modified blade tip  14  is shown. The abrasive coating  16  includes the grit particles  18  interspersed throughout the matrix  20 . The abrasive coating  16  is applied in a thicker layer proximate the corners  22  of the tip  14 . In an exemplary embodiment, the corners  22  of the leading edge  24  and trailing edge  26  are cut back based on a predetermined rub depth  32 . In an exemplary embodiment, the rub depth can be 0.030 inches. Thus, the thickness  34  of the abrasive coating  16  at the corners  22  can be about 30 mils. The thickness  34  of the coating  16  can then be tapered from the corners  22  inwardly at about a 30 degree angle resulting in a thickness of about 6 mils near a center  36  of the airfoil  12 . In another exemplary embodiment, the tip  14  can be cut back in a profile similar to a “hip roof” with corners cut and edges along the tip cut between the corners. 
     In an exemplary embodiment, the grit particles  18  range in size from about 0.04 to about 0.15 millimeters (mm) nominally. Grit  18  particle sizes can range up to about 0.15 mm nominally. An exemplary embodiment can include grit  18  from a DURALUM ATZ II R brand from Washington Mills, of dense fused alumina-zirconia refractor grain. 
     The abrasive coating  16  can include a base layer  38  bonded to the blade tip  14 . The base layer  38  can be applied directly to the tip  14  to improve adhesion of the coating  16 . The base layer  24  can be optionally applied. With the addition of the base layer  38  the abrasive coating  16  has a thickness  35 . 
     An adhesion layer  40  comprising the plating material utilized in the matrix  20  can be applied to the base layer  38  or can be coated directly to a top surface of the blade tip  14 . The adhesion layer  40  prepares the surface of the tip  14  for the first grit  18  to adhere to during application of the first grit  18 . The adhesion layer  40  can comprise the same basic material as the matrix  20  or other beneficial materials that bind the grit  18  to the blade tip  14  or alternatively the base layer  38 . In an exemplary embodiment the adhesion layer  40  comprises a Ni alloy matrix material. In alternative embodiments, the adhesion layer  40  can comprise, pure nickel, copper or copper alloy, cobalt or cobalt alloy, or a chrome alloy. 
     In an exemplary embodiment, the abrasive coating  16  can be applied thicker near the corners  22  similar to a wedge shape  42  as shown in cross-section at  FIG. 3 . The remainder of the coating layer  16  can then be applied to level off the coating  16  to form the final blade  10  shape. In another exemplary embodiment, the abrasive coating  16  can be applies thinner than the base layer  38  and the base layer  38  being thicker and vice versa. 
     The exemplary abrasive coating  16  includes a portion of each grit particle  18  projecting outward above the surface of the matrix material  20 , thereby enabling favorable rubbing interaction with metal or ceramic seals during engine operation. The unexposed portion of the grit particles  18  are surrounded by matrix material  20 . The grit particles  18  can be spaced apart from each other with a minimal distance of separation and arranged uniformly spaced apart. The matrix material  20 , as well as grit particles  18  can be securely bonded to the blade tip  14 . 
     The current disclosure includes an improvement to previously known abrasive coating. The modified blade tip and additional coating adds thickness to the Ni matrix abrasive tip. The improvement provides more Ni/abrasive thickness so that more abradable can be cut at a given wear ratio. The improvement further separates the Ti blade material from the abradable seal at existing build clearance so that the blade can tolerate more wear before the Ti blade material is rubbed directly by the abradable seal material. The modified blade tip and thicker coating system adds Ni thickness to base layer under the abrasive or in an alternative embodiment utilizes a thicker layer of abrasive. In an exemplary embodiment that free corners of the abrasive have been shown to wear preferentially and that there is a benefit to applying the added thickness. In alternative embodiments the coating is applied only to the leading and trailing tip corners. Operability and efficiency can be maintained by using existing build clearances while the contact of Ti blade material to abradable seal material can be prevented. Thus, a reduction in the likelihood of creating an ignition source in the gas turbine engine is achieved. The improved blade tip can also improve resistance to unwanted wear in both the radial incursion as well as any possible axial incursion due to the twisting motion of the blade. 
     There has been provided a modified blade tip with thickened coating. While the abrasive blade tip and coating has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.