Abstract:
One aspect of the disclosure involves a rub material comprising a polymeric matrix and polymer micro-balloon filler in the matrix. In one or more embodiments of any of the foregoing embodiments, the matrix comprises a silicone. In one or more embodiments of any of the foregoing embodiments, the rub material is at least I.0 mm thick. In one or more embodiments of any of the foregoing embodiments, the silicone is selected from the group consisting of dimethyl- and fluoro-silicone rubbers and their copolymers. In one or more embodiments of any of the foregoing embodiments, the micro-balloons at least locally have a concentration of 5-50% by volume.

Description:
BACKGROUND 
       [0001]    The disclosure relates to blade rub coatings. More particularly, the disclosure relates to abradable coatings for turbomachines such as gas turbine engines. 
         [0002]    Abradable coatings (rub coatings) protect moving parts from damage during rub interaction and wear to establish a mating surface to the moving parts with smallest possible clearance. The coatings are used in turbomachines to interface with the tips of a rotating blade stage, tips of cantilevered vanes and knife edge seals. 
         [0003]    In an exemplary turbomachine such as a gas turbine engine, more particularly, a turbofan engine, coatings may be used to interface with the blade tips of fan blade stages, compressor blade stages, and turbine blade stages. Because temperature generally increases through the fan and compressor and is yet much higher in the turbine, different blade materials, surrounding case materials, and coating materials may be desired at different locations along the engine. 
         [0004]    With relatively low temperatures in the fan and compressor sections, relatively low temperature materials may be used for their blades and the surrounding cases (at least through upstream (lower pressure) portions of the compressor). The exemplary blade materials in such lower temperature stages may be aluminum alloy, titanium alloy, carbon fiber or other composite, combinations thereof, and the like. Similarly, relatively lower temperature case materials may be provided. Particularly because the case material is not subject to the centrifugal loading that blades are, even lower temperature capability materials may be used (e.g., aramid or other fiber composites) in the case than in the blades. 
         [0005]    It is known to use a coating along the inboard or inner diameter (ID) surface of the case component to interface with the blade tips. Such coatings serve to protect blade tips from damage during rub contact between the blades and case. When the blade tips are protected from damage during rub, clearance between the blades and case ID can be set closer and tighter operating clearance can be achieved. 
         [0006]    To limit blade damage, the adjacent surfaces of the surrounding shroud may be formed by an abradable rub coating. Examples of abradable rub coatings are found in U.S. Pat. Nos. 3,575,427, 6,334,617, and 8,020,875. One exemplary baseline coating comprises a silicone matrix with glass micro-balloon filler. Without the glass filler, the elastic properties of the abradable coating result in vibrational resonances and non-uniform rub response. The glass increases the effective modulus of the coating so as to reduce deformation associated with aerodynamic forces and resonances. 
       SUMMARY 
       [0007]    One aspect of the disclosure involves a rub material comprising a polymeric matrix and polymer micro-baloon filler in the matrix. 
         [0008]    In one or more embodiments of any of the foregoing embodiments, the matrix comprises a silicone. 
         [0009]    In one or more embodiments of any of the foregoing embodiments, the rub material is at least 1.0 mm thick. 
         [0010]    In one or more embodiments of any of the foregoing embodiments, the silicone is selected from the group consisting of dimethyl- and fluoro-silicone rubbers and their copolymers. 
         [0011]    In one or more embodiments of any of the foregoing embodiments, the micro-balloons at least locally have a concentration of 5-50% by volume. 
         [0012]    In one or more embodiments of any of the foregoing embodiments, over a depth of at least 1 mm, the micro-balloons have said concentration of 5-50% by volume. 
         [0013]    In one or more embodiments of any of the foregoing embodiments, over a depth of at least 1 mm, the micro-balloons have said concentration of 20-33% by volume. 
         [0014]    In one or more embodiments of any of the foregoing embodiments, the micro-balloons have diameters of 10-80 micrometer. 
         [0015]    In one or more embodiments of any of the foregoing embodiments, the micro-balloons have diameters of 20-45 micrometer. 
         [0016]    In one or more embodiments of any of the foregoing embodiments, the material is injection molded in situ to a substrate or separately injection molded and bonded to the substrate. 
         [0017]    In one or more embodiments of any of the foregoing embodiments, the material of further comprises up to 50% by volume additional filler. 
         [0018]    In one or more embodiments of any of the foregoing embodiments, the material of further comprises up to 50% by volume additional filler selected from the group consisting of polymeric particulate filler, polymeric fiber filler, glass micro-balloons and combinations thereof. 
         [0019]    Another aspect of the disclosure is a turbomachine comprising: a plurality of blades, each blade having a tip, the blades mounted for rotation about an axis; a case surrounding the plurality of blades and having: a substrate; and a coating of the blade rub material on the substrate facing the blade tips. 
         [0020]    In one or more embodiments of any of the foregoing embodiments, the blades comprise an aluminum based substrate and a polyurethane coating. 
         [0021]    In one or more embodiments of any of the foregoing embodiments, the turbomachine is a gas turbine engine wherein the plurality of blades are a fan blade stage. 
         [0022]    In one or more embodiments of any of the foregoing embodiments, the blades comprise an aluminum alloy or a titanium alloy. 
         [0023]    Another aspect of the disclosure involves a blade rub segment comprising: a substrate; and a coating of the rub material of any foregoing embodiment on the substrate. 
         [0024]    In one or more embodiments of any of the foregoing embodiments, the substrate has a transversely concave first surface and an opposite second surface and a plurality of mounting features. 
         [0025]    In one or more embodiments of any of the foregoing embodiments, the coating has a transversely concave first surface and an opposite second surface secured to the substrate. 
         [0026]    Another aspect of the disclosure involves a method for manufacturing the material of any of the foregoing embodiments, the method comprising: dispersing the micro-balloons in uncured polymer for the matrix material by mixing. 
         [0027]    In one or more embodiments of any of the foregoing embodiments, the micro-balloons are first dispersed in a solvent. 
         [0028]    In one or more embodiments of any of the foregoing embodiments, the method further comprises injection molding of the uncured polymer and dispersed micro-balloons. 
         [0029]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a partially schematic half-sectional view of a turbofan engine. 
           [0031]      FIG. 2  is an enlarged transverse cutaway view of a fan blade tip region of the engine of  FIG. 1  taken along line  2 - 2  and showing a first rub coating. 
           [0032]      FIG. 3  is a view of a blade rub segment. 
           [0033]      FIG. 4  is longitudinal sectional view of a fan blade tip region of an alternative engine. 
           [0034]      FIG. 5  is an alternative enlarged transverse cutaway view of a fan blade tip region of the engine of  FIG. 1  taken along line  2 - 2  and showing a second rub coating. 
       
    
    
       [0035]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0036]    With a polyurethane coated airfoil, heat from rub friction conducts into the airfoil causing a temperature rise which, in turn, causes the coating to blister. 
         [0037]    Thermal conductivity of the abradable coating material, size of individual filler particles/bodies, and temperature capability of filler material influence heating of the airfoil. If the matrix or filler material has low thermal conductivity, it will poorly conduct away generated heat and airfoil temperatures would be higher than with an otherwise similar coating of greater thermal conductivity. The temperature capability of the coating material is defined as the temperature at which the material decomposes, melts, or exhibits a substantial drop in wear resistance when rubbed by a blade. 
         [0038]    In the baseline coating (glass micro-balloon-filled silicone), because of machining of the coating during manufacture, or because of rub or erosion, the glass filler stands slightly proud of the polymer matrix. This results in preferential contact of the glass with the airfoil tips. Being of higher temperature capability than the polymer matrix, the glass filler undesirably allows the surface temperature during rub to reach a relatively higher temperature than if the polymer matrix were being rubbed. The extent of heating may approach the lower of the melting points of the glass and the mating aluminum blade. Not only does the glass filler produce a higher rub temperature, the hard phase (glass) particles increase the coating roughness after rub, reducing aerodynamic efficiency. 
         [0039]    Use of thermoset polymer particles or micro-balloons (at least partially in place of baseline glass micro-balloons) may create a composite with similar bulk properties to the baseline material. However, it will limit the maximum temperature at the rub contact to the maximum capability of the polymers. This will result in lower blade tip surface temperature, less heat input into the blade and lower temperature at the interface between blade substrate and blade coating. 
         [0040]    Abradable surface wear occurs during sliding contact of the blade tip against the abradable material. During contact the surfaces increase in temperature due to frictional heating. As the surfaces heat up during a rub interaction, the rub forces are limited as the matrix and filler materials approach their temperature capability. This allows both the filler and matrix to rub off during contact and allows the rub surface to adopt a profile that conforms to the shape of the blade path. With a polymer filler in the silicone matrix, the surface temperatures are thus limited to the higher of the temperature capability of the matrix and the filler. This temperature is in contrast to the temperature during rub with low conductivity glass and mineral-based fillers of the prior art with which filler surface temperature may significantly exceed the capability of the matrix. The maximum rub temperature is thus limited by the temperature capability of the matrix and polymer filler. 
         [0041]    Thus, replacement of glass filler with polymer filler reduces the temperature capability of the composite and reduces the maximum rub temperature. Lower temperature capability of the filler thus helps to limit the maximum rub temperature by allowing the filler to wear away at a reduced temperature compared to glass micro-balloons. 
         [0042]    Reduced temperature capability of the filler material reduces rub temperature. Reduced rub temperature helps keep the blade tips cooler by reducing heat flux into the blades during rub. Reduced blade tip temperature improves the durability of urethane erosion-resistant coating on the airfoils. 
         [0043]      FIG. 1  shows a gas turbine engine  20  having an engine case  22  surrounding a centerline or central longitudinal axis  500 . An exemplary gas turbine engine is a turbofan engine having a fan section  24  including a fan  26  within a fan case  28 . The exemplary engine includes an inlet  30  at an upstream end of the fan case receiving an inlet flow along an inlet flowpath  520 . The fan  26  has one or more stages  32  of fan blades. Downstream of the fan blades, the flowpath  520  splits into an inboard portion  522  being a core flowpath and passing through a core of the engine and an outboard portion  524  being a bypass flowpath exiting an outlet  34  of the fan case. 
         [0044]    The core flowpath  522  proceeds downstream to an engine outlet  36  through one or more compressor sections, a combustor, and one or more turbine sections. The exemplary engine has two axial compressor sections and two axial turbine sections, although other configurations are equally applicable. From upstream to downstream there is a low pressure compressor section (LPC)  40 , a high pressure compressor section (HPC)  42 , a combustor section  44 , a high pressure turbine section (HPT)  46 , and a low pressure turbine section (LPT)  48 . Each of the LPC, HPC, HPT, and LPT comprises one or more stages of blades which may be interspersed with one or more stages of stator vanes. 
         [0045]    In the exemplary engine, the blade stages of the LPC and LPT are part of a low pressure spool mounted for rotation about the axis  500 . The exemplary low pressure spool includes a shaft (low pressure shaft)  50  which couples the blade stages of the LPT to those of the LPC and allows the LPT to drive rotation of the LPC. In the exemplary engine, the shaft  50  also drives the fan. In the exemplary implementation, the fan is driven via a transmission (not shown, e.g., a fan gear drive system such as an epicyclic transmission) to allow the fan to rotate at a lower speed than the low pressure shaft. 
         [0046]    The exemplary engine further includes a high pressure shaft  52  mounted for rotation about the axis  500  and coupling the blade stages of the HPT to those of the HPC to allow the HPT to drive rotation of the HPC. In the combustor  44 , fuel is introduced to compressed air from the HPC and combusted to produce a high pressure gas which, in turn, is expanded in the turbine sections to extract energy and drive rotation of the respective turbine sections and their associated compressor sections (to provide the compressed air to the combustor) and fan. 
         [0047]      FIG. 2  shows a cutaway blade  100  showing a blade substrate (e.g., an aluminum alloy)  102  and a polymeric coating  104  (e.g., a polyurethane-based coating) on the substrate. The exemplary coating is along pressure and suction sides and spans the entire lateral surface of the blade between the leading edge and trailing edge. The exemplary coating, however, is not on the blade tip  106 . If originally applied to the tip, the coating may have been essentially worn off during rub. Circumferential movement in a direction  530  is schematically shown. 
         [0048]      FIG. 2  also shows an overall structure of the fan case facing the blade. This may include, in at least one example, a structural case  120 . It may also include a multi-layer liner assembly  122 . An inboard layer of the liner assembly may be formed by a rub material  124 . The exemplary rub material  124  has an inboard/inner diameter (ID) surface  126  facing the blade tips and positioned to potentially rub with such tips during transient or other conditions. 
         [0049]    As is discussed further below, the rub material  124  comprises a polymeric matrix material  128  and a polymeric filler  130 . The exemplary rub material may be formed as a coating on an ID surface  132  of a substrate  134  of the liner assembly. An exemplary substrate  134  is titanium alloy AMS  4911 . The rub material is shown as having an overall thickness T R . Exemplary T R  is 1-10 mm, more particularly, 3-6 mm. 
         [0050]    The matrix material may initially be an uncured rubber such as a liquid silicone rubber (e.g., family including liquid dimethyl- and fluoro-silicone rubbers and their copolymers). The silicone rubber is processed as a liquid and cross-links with time and temperature to form a thermoset solid material. Exemplary matrix material is RTV630 of Momentive Performance Materials Inc., Columbus Ohio. 
         [0051]    Exemplary polymeric filler is present in the rub material at an overall concentration of about 25% by volume, more broadly 20-33% or 5-50%. This may be an overall average or a local average (e.g., if concentrated in a depth zone (e.g., at least 1 mm or at least 2 mm) near the surface). 
         [0052]    Exemplary filler is polymer micro-balloons to reduce coating density and improve abradability. Exemplary micro-balloons are acrylonitrile such as Duolite E030 acrylonitrile micro-balloons produced by Henkel Corporation, West Haven, Conn. Exemplary balloon diameter is 20-45 micrometer (more broadly, 5-150 micrometer or 10-80 micrometer). 
         [0053]    The coating may be manufactured by mechanical mixing of the micro-balloons with the uncured silicone (e.g., in an extruder). The coating may be applied by injection molding (e.g., directly to the substrate or separately followed by adhesive bonding to the substrate). Alternative material may be formed in sheets (e.g., by extrusion) and then cut and bonded for installation. 
         [0054]      FIG. 3  shows a rub strip segment  160  including a segment  161  of the substrate  134  and a segment of the rub material  124 . The exemplary segment  160  is dimensioned for use in a fan case  150  ( FIG. 4 , e.g., itself of a two-piece circumferential segmentation) of a low-bypass military-style turbofan engine. An exemplary  4 - 20  (more narrowly  6 - 12 ) such segments  160  may be assembled to form an annulus circumscribing the engine axis. The substrate segment may be formed as a metallic carrier having a main body with a forward edge  162 , a rear edge  164 , and first and second circumferential edges  166  and  168 . Mounting features  170  (e.g., hooks such as shown or studs, clips, sockets or the like) may be positioned at leading and/or trailing ends or the outer diameter (OD) surface  172  of the body for mounting to complementary features (e.g., rails  152 ) of the case  150 . The ID surface  132  is the ID surface of the body and is transversely concave to match blade sweep. In alternate implementations of such a low-bypass engine, the coating may be molded directly to the fan case substrate (e.g. of a horizontally-split case). 
         [0055]    The segment of rub material may have a planform generally coextensive with the planform of the substrate main body. For example, the rub material segment  180  may have a forward edge  182 , a rear edge  184 , and first and second circumferential edges  186  and  188 , each slightly proud of the corresponding substrate main body features. This allows the substrate segments to be spaced slightly apart from each other while allowing snug mating of the rub material segments with each other. It also has an outer diameter (OD) surface  190  along and secured to the ID surface  132 . 
         [0056]    In an original use situation, a virgin substrate may be manufactured such as by machining (if metallic) or by molding or composite lay-up or other technique, whereafter the rub material may be applied. 
         [0057]    After molding, the rub material may be machined, if not molded directly to size, to establish desired dimensions of the surface adjacent to the airfoils and to establish desired clearances at engine build. The rub material may be machined on the individual segments, split case halves, assembled in a fixture as a full circle or in an engine sub-assembly (he last being known as an “assembly grind”). Exemplary machining of the abradable material is by grinding, single point turning, or milling. 
         [0058]    In remanufacture situations, after at least partial consumption of the rub material, the remaining rub material may be removed (e.g., mechanically, chemically, and/or thermally). Exemplary mechanical processes include any of the manufacturing machining processes or preferably by water jet strip if the substrate is metallic and can survive the water jet process. A new body of rub material may then be overmolded to the cleaned substrate. 
         [0059]    The potential benefits from reducing the temperature capability of the filler used in a silicone rubber abradable coating are not limited to coating consisting of rubber-polymer microballoon composites. Thus, the composites may contain other components (e.g., fillers and reinforcements). The benefit of reducing the maximum temperature at the rub interface also exists in fiber-reinforced and solid particle-filled abradables. Solid particle polymer fillers such as polyethyle (PE), polymethylmethacrylate (PMMA), acrylonitrile copolymer, etc. have lower temperature capability than glass and mineral fillers and result in lower blade tip temperature. Among such fillers are those disclosed in U.S. Pat. No. 6,334,617 mentioned above and the disclosure of which is incorporated herein by reference in its entirety as if set forth at length. Additionally, fiber reinforcements such as carbon nanotubes and polyester may be added to further modify the mechanical properties of the matrix. Exemplary concentrations of polymeric particulate and fiber fillers are in the range of up to 50% by volume overall, more particularly up to 20% or up to 33% or 5-33% or 10-20% by volume, whereas the carbon nanotube fiber reinforcement concentration is in the range of 1-33% by volume in the matrix (e.g., as first blended into the matrix prior to adding micro-balloons) or more ideally 5-15% by volume in the matrix. Exemplary combined concentration of the polymeric micro-balloons and other filler is up to 60% by volume, more narrowly, up to 50% or 20-40%. 
         [0060]    One particular example of a beneficial abradable composition uses chopped polyester fibers  140  ( FIG. 5 ) to reduce coating density and improve tear resistance and visio-elastic damping properties to keep the material stable during rub abradability. Exemplary concentration of such fibers is ˜5% by weight overall or ˜8% by weight relative to the matrix. 
         [0061]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied in the redesign of a baseline engine configuration or for the remanufacturing of a baseline engine, details of the baseline will influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.