Abstract:
A method of applying an additive to an abrasive coated blade tip comprises applying the additive to the abrasive coated blade tip and wicking the additive into the abrasive coated blade tip, wherein the additive is configured to prevent adhesion of an organic component of an abradable seal onto the abrasive coated blade tip.

Description:
BACKGROUND 
       [0001]    This disclosure relates to abrasive tips for rotatable blades. Abradable seals or coatings (rub coatings) can be used to protect moving parts from damage during rub interaction while providing a small clearance. Such seals are used in turbomachines to interface with abrasive tips of a rotating blade stage. 
         [0002]    The abrasive tips include coatings to enhance performance and limit heat generation and heat transfer to the blade. The abrasive tips, upon rubbing the abradable seal, remove portions of the abradable seal. The portions of the abradable seal can transfer to the abrasive tips and adhere to the abrasive tips. Carbonized portions of the abradable seal that adhere to the abrasive tips can build up. The portions of the carbon based materials that build up between the grit particles of the abrasive tip (i.e. clogging) can cause frictional heating during rub events. The frictional heating can affect the polyurethane erosion resistant coating on the gas path surfaces of the blade. Thus, the very problem that the abrasive blade tip is designed to prevent, instead occurs through a different mechanism. Pending U.S. non-provisional application no. U.S. 2015/0233255 is incorporated herein by reference. 
         [0003]    What is needed is an abrasive tip that resists clogging due to build-up of carbonized rub debris. 
       SUMMARY 
       [0004]    In accordance with the present disclosure, there is provided a method of applying an additive to an abrasive coated blade tip comprising; applying the additive to the abrasive coated blade tip; and wicking the additive into the abrasive coated blade tip, wherein the additive is configured to prevent adhesion of an organic component of an abradable seal onto the abrasive coated blade tip. 
         [0005]    In another and alternative embodiment, the method further comprises heating the abrasive coated blade tip. 
         [0006]    In another and alternative embodiment, the heating comprises heating the abrasive coated blade tip to a temperature above the melting point of the additive. 
         [0007]    In another and alternative embodiment, the method further comprises removing excess additive from the abrasive coated blade. 
         [0008]    In another and alternative embodiment, wicking the additive includes at least one of a natural wicking mechanism of a molten form of the additive drawing the molten additive material into pores of the abrasive coated blade tip, and pressure infiltration with the additive into the pores. 
         [0009]    In another and alternative embodiment, the additive is selected from the group consisting of metal stearate and stearates or palmitates of zinc, aluminum, barium, calcium and strontium. 
         [0010]    In another and alternative embodiment, the abrasive coated blade tip comprises a thermally sprayed aluminum and abrasive composite coating having a porosity to a level of about 5-10% by volume and the porosity being open to a surface of the abrasive coated blade tip. 
         [0011]    In another and alternative embodiment, applying the additive comprises brushing the additive on the abrasive coated blade tip. 
         [0012]    In accordance with the present disclosure, there is provided an abrasive blade tip comprises an additive infiltrated into an abrasive blade tip having a porous structure to a level of about 5-10% by volume open to a surface of the abrasive blade tip, the additive configured to prevent adhesion of an organic component of an abradable seal onto the abrasive blade tip. 
         [0013]    In another and alternative embodiment, the additive is selected from the group consisting of metal stearate and stearates or palmitates of zinc, aluminum, barium, calcium and strontium. 
         [0014]    In another and alternative embodiment, the abrasive blade tip comprises a metal matrix, the additive and hard particles dispersed through the metal matrix. 
         [0015]    In another and alternative embodiment, the additive is configured to prevent clogging of the spaces between the hard particles that protrude from the abrasive blade tip. 
         [0016]    In another and alternative embodiment, the additive is configured to incorporate into rub debris comprising constituents of an organic matrix composite of the abradable seal. 
         [0017]    In another and alternative embodiment, the additive is configured to lubricate the organic component responsive to a rub event with the abrasive blade tip. 
         [0018]    In another and alternative embodiment, the additive is dispersed throughout the abrasive blade tip. 
         [0019]    Other details of the abrasive blade tip are set forth in the following detailed description and the accompanying drawing wherein like reference numerals depict like elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  illustrates an example gas turbine engine. 
           [0021]      FIG. 2  illustrates an isolated view of the fan section of the gas turbine engine of  FIG. 1 . 
           [0022]      FIG. 3  illustrates an abrasive tip interfacing with an abradable seal. 
           [0023]      FIG. 4  illustrates a cross-section of an abrasive tip. 
           [0024]      FIG. 5  illustrates an exemplary process of applying an additive to an abrasive tip. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      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 . Alternative engines might include an augmenter section (not shown) among other systems or features. The fan section  22  drives air along a bypass flow path B in a bypass duct defined within a fan case  15 , and into a core flow path C to the compressor section  24  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. 
         [0026]    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. 
         [0027]    The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , 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 the 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  is 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. 
         [0028]    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 combustor section  26  or even aft of turbine section  28 , and fan section  22  may be positioned forward or aft of the location of gear system  48 . 
         [0029]    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. 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. 
         [0030]    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. The flight condition of 0.8 Mach and 35,000 ft, 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 operating 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. 
         [0031]      FIG. 2  illustrates an isolated view of the fan section  22  of the engine  20 . The fan  42  includes a rotor  60  that has a plurality of circumferentially-spaced blades  62 . Each blade  62  includes an airfoil section  64  that extends between leading and trailing edges  66 / 68 , first and second opposed sides  70 / 72  that each joins the leading and trailing edges  66 / 68 , and an inner end  74  and a free tip end  76 . Each blade includes an abrasive tip  78  at the free tip end  76 . 
         [0032]    The fan case  15  is annular in shape and circumscribes the blades  62 . The fan section  22  is designed such that the abrasive tips  78  of the blades  62  rub against the fan case  15  during rotation. In this regard, the fan case  15  includes an abradable seal  80  mounted on a radially inner side of the fan case  15 . 
         [0033]    When two components are in rubbing contact, at least one of the components may wear. The term “abradable” refers to the one of the two components that wears, while the other component is “abrasive” and does not wear or wears less. Thus, when the abrasive tips  78  of the blades  62  rub against the seal  80 , the seal  80  will be worn whereas the abrasive tips  78  will not wear or will wear less than the seal  80 . The word “abrasive” thus also implies that there is or can be contact with an abradable component. 
         [0034]      FIG. 3  shows a cutaway view of a representative portion of the airfoil section  64  of one of the blades  62  and a portion of the abradable seal  80 . The airfoil section  64  is formed of a metal-based material with a polymeric overcoat  62   a  on the surfaces thereof. For example, the polymeric overcoat  62   a  serves to protect the underlying airfoil section  64  from erosion due to foreign particulate ingested into the engine  20 . In one example, the metal-based material of the airfoil section  64  is an aluminum alloy. 
         [0035]    The polymeric coating  62   a  can be a polyurethane-based coating, an epoxy-based coating, or a silicone rubber-based coating, but is not limited to these types of polymeric coatings or materials. The polymeric coating  62   a  can cover the first and second sides  70 / 72  of the blades  62  and can span the entire lateral surface of the blade  62  between the leading and trailing edges  66 / 68 . 
         [0036]    Friction between a blade tip and a surrounding case generates heat. The heat can be conducted into the case, into the blade, or both. However, in particular for metal blades and polymeric-based cases, the metal of the blade is generally a better thermal conductor than the polymer of the case, and a majority of the heat thus can conduct into the blade. While this may normally not present any detriments for a plain metal blade, the heat conduction can be detrimental to a metal blade that has a polymeric overcoat because the heat can cause delamination of the polymeric overcoat and thus compromise the erosion protection. In this regard, the abrasive tip  78  has a composition selected with respect to heat-induced delamination of the polymeric overcoat  62   a  from frictional heat generated during rubbing between the abrasive tip  78  and the abradable seal  80 . That is, the respective compositions of the abradable seal  80  and the abrasive tip  78  are complimentarily selected with respect to frictional heat generated and heat-induced delamination of the polymeric overcoat  62   a.  For example, the compositions are selected with regard to a blade temperature at which the polymeric overcoat  62   a  does not delaminate nor has defined delamination durability over an extended period of time, such as the life of the engine  20 . 
         [0037]    The abradable seal  80  is formed of a polymeric-based material, such as a polymer matrix composite. In one further example, the polymer matrix composite includes an epoxy matrix and a silica-containing filler dispersed through the matrix. In a further example, the silica-containing filler is or includes hollow glass microspheres. An example is 3M™ Scotch-Weld™ Structural Void Filling Compound EC-3555. 
         [0038]      FIG. 4  illustrates a cross-section of representative portion of a further example of the abrasive tip  78 . In this example, the abrasive tip  78  includes a metal matrix  90 , hard particles  92 , and an additive  94  dispersed through the metal matrix  90 . In one further example, the metal matrix  90  and the metal-based material of the airfoil section  64  are compositionally composed of the same predominant metal, which can promote strong adhesion if the abrasive tip  78  interfaces with the metal-based material (i.e., the abrasive tip  78  is in direct contact with the metal-based material, as depicted in  FIG. 4 ). As an example, the metal can be aluminum. 
         [0039]    Incorporated into the abrasive tip  78  is the additive, release agent or lubricant or simply additive  94 . The additive  94  is infiltrated into the metal matrix composite of the abrasive tip  78 . The additive  94  can comprise a metal stearate. In exemplary embodiments, the additive  94  can comprise stearates or palmitates of zinc, aluminum, barium, calcium, strontium and the like. The additive  94  can be a suspension, melt or solution of the calcium stearate or other metal stearate. In an example, the calcium stearate can be composed of a pretty fine material at about 10 micron nominal, such as, “SAK-CS-PPT sized to −120+325 mesh” (75 micron D50, 45 to 125 microns). The material SAK-CS-PPT can be sourced from Silver Fern Chemical, located at 2226 Queen Anne Ave N, Seattle, Wash. 98109, USA. 
         [0040]    The additive  94  is compatible with plastic materials and other solid binders such as PVA (poly vinyl alcohol) and acrylic latex emulsion (Duramax B1001, Rohm and Haas Co., Philadelphia, Pa.). Compounding the additive  94  with binders and processing them into powder form may facilitate the deposition of the additive  94 . This can make application more efficient. Compounding with a polymer such as epoxy or polymethylmethacrylate (Lucite) may also promote the longevity of the additive  94  under the environmental conditions to which it is exposed in service. 
         [0041]    In an exemplary embodiment, the thermally sprayed aluminum and abrasive composite coating is porous to a level of about 5-10% by volume. This porosity is open to a surface  98  of the abrasive coated blade tip  78 . The porous coating can be infiltrated by simple wicking mechanism or pressure infiltration with the additive  94 . The additive  94  can be in the form of a melt, suspension or solution of calcium stearate or other metal stearate. An optional binder component  96  can also be employed with the additive  94 . The binder can improve the durability of the additive  94 . 
         [0042]    An exemplary embodiment of the application of the additive  94  shown at  FIG. 5 , can include providing the porous abrasive tip  100 . The abrasive coating can be heated to a temperature above the melting point of the additive  94 , at step  110 . The temperature can be, for example, above the melting point of calcium stearate. In alternative embodiments, the temperature can be above the melting point of other additive compositions. The process includes at  112 , applying the additive  94  to the heated abrasive coating. One technique of applying the additive can include brushing the additive on. The next step  114  includes wicking the additive into the porous abrasive coating. The additive has become a molten material after being applied on the heated abrasive coating. Natural wicking action draws the molten material into the porosity of the coating. After the abrasive coating has cooled, the excess additive can be removed from the abrasive coating at  116 . An exemplary technique of removing the excess additive can include wiping off the excess additive with a solvent wetted cloth. 
         [0043]    The additive  94  becomes incorporated into the rub debris upon a rub event. The additive  94  within the rub debris prevents the organic compounds of the abradable seal  80  from adhering to the abrasive blade tip  78 . Since the components of the abradable seal  80  are prevented from adhering to the abrasive tip  78 , then clogging of the abrasive tip  78  between the cutting surfaces can be prevented. 
         [0044]    In one further example, the metal matrix  90  is a eutectic aluminum-silicon alloy having a composition, by atomic weight, of 88% aluminum and 12% silicon. The eutectic composition provides high hardness and strength to enhance holding the hard particles  92  in the metal matrix  90 . The eutectic composition also has good high temperature properties and retains strength at high temperatures rather than softening. 
         [0045]    In one further example, the metal matrix  90  is, or predominantly includes, aluminum, and the hard particles  92  are, or predominantly include, alumina (Al 2 O 3 ). In an additional example, the hard particles  92  are, or predominantly include, zirconia (ZrO 2 ). In yet another example, the hard particles  92  are, or predominantly include, alumina and zirconia. It is to be understood that the hard particles  92  are not limited to alumina and zirconia, and other oxides, nitrides, carbides, oxycarbides, oxynitrides, diamond and combinations thereof can be used selectively, with respect to heat-induced delamination of the polymeric overcoat  62   a  from frictional heat generated during rubbing between the abrasive tip  78  and the abradable seal  80 . Examples of the abrasive material can be NorZon NV manufactured by Saint-Gobain with a 220 grit. The aluminum can be an Oerlikon Metco 54NS powder. 
         [0046]    The abrasive tip  78  can have a thickness in a thickness range of 0.025-1.3 millimeters, and the hard particles  92  can have an average maximum dimension in a particle size range of 10-200 micrometers. The hard particles  92  may protrude from the metal matrix  90  or be completely covered by the metal matrix. 
         [0047]    In one further example a polymer matrix filled with hollow glass microspheres for the abradable seal  80  is complimentary with a metal matrix  90  of aluminum and hard particles  92  of alumina, zirconia, or both in the abrasive tip  78 , with respect to frictional heat generated and heat-induced delamination of the polymeric overcoat  62   a.  That is, the frictional heat generated between the abradable seal  80  and the abrasive tip  78  cause a blade  62  temperature at which the example polymer of the polymeric overcoat  62   a  does not delaminate, or at least meets delamination durability over an extended period of time, such as the life of the engine  20 . 
         [0048]    In the illustrated example in  FIG. 4 , the hard particles  92  are faceted and thus have angled facets  92   a.  The angled facets  92   a  provide relatively sharp corners that facilitate efficient “cutting” through the abradable seal  80  with low cutting forces, which lowers frictions and, in turn, contributes to lowering the amount of heat generated. In one example, the hard particles  92  are DURALUM ATZ II that has approximately 40% tetragonal zirconia with titania evenly distributed throughout the individual alumina grains. 
         [0049]    The additive  94  helps to prevent clogging of the spaces between the hard particles  92  that protrude, as the hard particles are exposed to the upper surface of the metal matrix  90 . 
         [0050]    The benefit of the additive  94  in the abrasive tip  78  is to prevent thermal damage to the polyurethane erosion resistant coating  62   a  on the blades. This is accomplished by preserving the cutting ability of the abrasive tips  78 . The present composite abrasive coating with “anti-clogging” constituent will expand the range of rub conditions under which effective abrasive cutting will take place and thereby significantly reduce the possibility of thermal damage to the erosion resistant coating at the blade tips. 
         [0051]    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. 
         [0052]    There has been provided an abrasive blade tips resistant to clogging and methods of applying an additive to the abrasive blade tip. While the abrasive blade tip 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.