Abstract:
A turbine engine system comprising a turbine engine component, such as a fan includes a fan casing and a fan blade rotatable within the fan casing. An abradable seal coupled to the fan casing proximate the fan blade. The abradable seal comprising an abradable composite layer including a MAX phase solid and a polymer based abradable composition.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of provisional application Ser. No 61/993,033, filed May 14, 2014. 
     
    
     BACKGROUND 
       [0002]    The present disclosure is directed to the use of MAX phase solids mixed with polymers to form an outer air seal of a turbine engine, for enhanced thermal conductivity and improved abradability to protect the blade tips from overheating. 
         [0003]    Fan blade technology uses abradable coatings for fuel burn reduction and increases in temperature capability. In some fans, a coating is applied to the airfoil portion of the fan. Abradable systems can be susceptible to efficiency losses and premature failure due to high rub forces, rough coating surfaces, local heat generation, high coating temperature, coating spallation, and durability issues in certain environments. There are several requirements to mitigate issues associated with abradable coatings, such as improved abradable damage tolerance and toughness, improved thermal cycling and durability, and reduced frictional forces and low coefficient of friction, self-lubrication and low energy of cut, high abradable thermal conductivity and higher erosion resistance and desired wear ratio. 
         [0004]    The abradable seal is typically positioned between a stationary component on the opposite side of a rotating component. For example, the stationary component may be an outer engine casing or a shroud and the rotating component may be a blade tip, a sealing ring, a knife-edge seal, and the like. In operation, the blade initially engages the abradable seal and rubs or cuts into the abradable seal. The abradable seal helps ensure that the blade tip does not contact the outer casing, it is the abradable material of the seal that is removed, rather than the blade tip. The abradable seal prevents damage to components of gas turbine engines during rubs thus allows operation at reduced clearances between the stationary component and the rotating component. Minimizing clearance between the abradable seal and the rotating component also reduces leakage over the blade tip, resulting in increased efficiency and power output. 
         [0005]    Current aluminum fan blades are polyurethane coated to enhance erosion resistance. The aluminum blade tips rub against polymer based abradable outer air seal coatings. The heat generated by friction during a rub event conducts into the airfoil introducing a temperature rise which in turn may result in the polyurethane coating blistering. High surface temperatures and heating of the blades occur due to low thermal conductivity of the polymer based abradable outer air seal coating. There is a need for improved capability and increased thermal conductivity of abradable outer air seal coatings in order to mitigate temperature rise in such airfoils during rub events and prevent spallation and blistering of polyurethane coatings from the blades. 
       SUMMARY 
       [0006]    In accordance with the present disclosure, there is provided a turbine engine system comprising a turbine engine fan section. The fan section including a fan casing and a fan blade rotatable within the fan casing. An abradable seal is coupled to the fan casing between the fan casing and the fan blade. The abradable seal comprises an abradable composite layer including MAX phase solids and a polymer based abradable composition. 
         [0007]    In an exemplary embodiment the MAX phase solids include a composite having MAX phases and a metal matrix. The metal matrix is at least one of a low, medium, and high melting point metal or metal alloy. 
         [0008]    In an exemplary embodiment the MAX phase solids are defined by the formula M n+1 AX n  where M is an early transition metal element, A is an A-group element, X is C or N, and n=1 to 3. 
         [0009]    In an exemplary embodiment a coating layer deposited between the fan casing and the abradable seal. 
         [0010]    In an exemplary embodiment the abradable composite layer includes the MAX phase solids from about 1% to about 66% by volume and the polymer based abradable composition being the remainder. 
         [0011]    In an exemplary embodiment the abradable composite layer includes the MAX phase solids and the polymer based abradable material and a hollow micro sphere material. 
         [0012]    In another exemplary embodiment a turbine engine abradable outer air seal comprises a casing substrate. An abradable composite layer is coupled to the substrate and the abradable composite layer comprises MAX phase solids and a polymer based abradable composition. 
         [0013]    In an exemplary embodiment the turbine engine abradable outer air seal comprises a fan abradable outer air seal. 
         [0014]    In an exemplary embodiment the abradable composite layer includes the MAX phase solids of between about 1 wt. % and about 10 wt. % and the polymer based abradable composition being from about 90 wt. % to about 99 wt. % 
         [0015]    In an exemplary embodiment a bond coat is coupled between the substrate and the abradable composite layer. 
         [0016]    In another exemplary embodiment a process for manufacturing a turbine engine abradable outer air seal component comprises the steps of combining MAX phase solids and a polymer based abradable material into an abradable composition. The process includes applying the abradable composition to a fan casing substrate in the form of an outer air seal. 
         [0017]    In an exemplary embodiment the combining step comprises mixing the MAX phase solids of between about 1 wt. % and about 10 wt. % and the polymer based abradable material from about 90 wt. % to about 99 wt. % into the abradable composition. 
         [0018]    In an exemplary embodiment further comprises coupling a bond coating to the substrate between the substrate and the abradable composition. 
         [0019]    In an exemplary embodiment the process includes combining a filler with the MAX phase solids and the polymer based abradable material into the abradable composition. 
         [0020]    The disclosure includes the use of MAX phases into silicone rubber abradable or other polymer-based compositions. Incorporation of the MAX phases with high metallic-like thermal conductivity will enhance thermal conductivity of the abradable seal coatings. 
         [0021]    The disclosure provides for increased thermal conductivity of abradable outer air seal coatings and has the potential to mitigate temperature rise in the aluminum airfoils during rub events and prevent spallation and blistering of polyurethane coatings from the current fan blade aluminum material. 
         [0022]    Other details of the MAX phase composites for turbine engine component fan air seals are set forth in the following detailed description and the accompanying drawing wherein like reference numerals depict like elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic representation of an exemplary turbine engine. 
           [0024]      FIG. 2  is a side view of an abradable seal positioned proximate a fan blade. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring now to  FIG. 1  illustrates selected portions of an exemplary gas turbine engine  10 , such as a gas turbine engine  10  used for propulsion. In this example, the gas turbine engine  10  is circumferentially disposed about an engine centerline  12 . The engine  10  may include a fan  14 , a compressor  16 , a combustion section  18 , and a turbine section  20  that includes rotating turbine blades  22  and static turbine vanes  24 . The fan  14  can include a fan blade  26  that rotates within the fan casing  28 . A fan air flow  30  passes through the fan casing  28  over the fan blade(s)  26 . It is to be understood that other types of engines may also benefit from the examples disclosed herein, such as engines that include multiple fan sections or engines having other types of compressors, combustors, and turbines. 
         [0026]      FIG. 2  shows a side view of an abradable seal  32  positioned proximate fan blade  26  of a gas turbine engine. The abradable seal  32  improves the efficiency of the gas turbine engine by increasing the aerodynamic efficiency and stability of the gas turbine engine. This is accomplished in part by using a more thermally stable coating material. In addition, the abradable seal  32  has low interaction energy when abraded. The abradability of a material may be measured by the amount of energy required for the fan blade  26  to wear down the abradable seal  32 . The abradable seal  32  also reduces damage to the fan blade  26  as well as components located downstream due to its brittle fracture mode below temperatures of approximately 1200 degrees Centigrade by turning to dust. 
         [0027]    The abradable seal  32  includes a substrate  34  proximate the fan casing  28  and a coating  36 . The substrate  34  provides a base for the coating  36 , which faces the fan blade  26 . In an exemplary embodiment, the substrate  34  may be formed of metal, ceramic, or composite material. The coating  36  may be a two layer system with a bond coat  38  and an abradable composite layer  40 . The abradable composite layer  40  is formed by a MAX phase solid reinforcement  42  and a silicone rubber or other polymer-based composition matrix material  44 . The bond coat  38  is used when additional adhesion is needed between the substrate  34  and the abradable composite layer  40 . 
         [0028]    The abradable composite layer  40  of coating  36  may be applied as a dense single phase layer, a porous single phase layer, or a composite on substrate  34  and bond coat  38 . MAX phase solids  42  have a layered structure at an atomic scale, and exhibits both metallic and ceramic properties, making it both durable and abradable. 
         [0029]    Examples of suitable MAX phase solids include, but are not limited to: Ti 2 AlC and/or Ti 3 SiC 2 . The MAX phase solid is a high modulus and high damage tolerant material. The MAX phase solid may be in the form of a powder or particles having a crystalline nanolaminated structure. The MAX phase solid typically has the formula M n+1 AX n , where M is a transition metal, A is an A-group element, X is carbon (C), nitrogen (N) or both, and n=1-3. 
         [0030]    Examples of particularly suitable materials include, but are not limited to: M n+1 AX n  where M is an early transition metal element, A is an A-group element, X is C or N, and n=1 to 3. Numerous MAX phase solids are disclosed and described in detail in “Microstructure and mechanical properties of porous Ti 3 SiC 2 ”, published online on Jul. 14, 2005, by Z. M. Sun, A. Murugaiah, T. Zhen, A. Zhou, and M. W. Barsoum; “Mechanical Properties of MAX Phases” published in 2004 by Encyclopedia of Materials Science and Technology, Eds. by Buschow, Cahn, Flemings, Kramer, Mahajan, and Veyssiere, Elsevier Science; and “The MAX Phases: Unique New Carbide and Nitride Materials”, published in July-August 2001, by Michel W. Barsoum and Tamer El-Raghy, and are incorporated by reference herein. 
         [0031]    The atomic layers within the MAX phase solids  42  are layers of hard, strong, high modulus carbides and nitrides. The atoms are also arranged in layers so that they form very weak crystallographic planes. Thus, both high modulus strong planes and very weak planes are present in the MAX phase solids  42 . This results in kink band forming tendencies, which gives it both ceramic and metallic properties. When MAX phase solids  42  deform, there is slip between the atomic planes of the molecules, forming kink bands. The MAX phases provide toughness similar to a metal, making the composite material capable of withstanding damage while the high modulus and high hardness of the MAX phases make the composite capable of withstanding fine particle erosion. At the same time, the slip planes have low strength such that the MAX phase solid particles are machinable using a sharp cutting point or abrasive. 
         [0032]    The MAX phase solids  42  are dispersed within a polymer matrix to form a composite material. The matrix may be any polymer suitable for use in the intended environment. Suitable polymers include polyester, epoxy, polyimide, polyurethane and silicone among others. It is desirable that the polymer is a thermoset or cross linked polymer so that wear debris from rub is liberated in the form of dust. Thermoplastics may be undesirable due to their tendency to soften and form undesirable buildup on mating components. 
         [0033]    In addition to the primary constituents of Max phase particles and polymer matrix, it may be desirable to add a low density filler to reduce the density of an abradable seal. The ratio of MAX to polymer matrix can remain constant in an exemplary embodiment. The MAX phase solids and polymer matrix mixture can be diluted with the filler in an exemplary embodiment. Fillers particularly well suited for this application are glass and polymer micro-balloons. Considerations for their selection include operating temperature, permitted heat load into the blade and desired abradable bulk properties. Glass micro-balloons will result in higher bulk modulus of the abradable and higher heat generation during rub than polymer micro-balloons. An exemplary glass micro-balloon can include Sphericel® glass micro spheres of 39 micron mean size and density of 0.34 g/cc, produced by Potters Industries, LLC of Valley Forge, Pa. An exemplary polymer micro-balloons can include DUALITE® E065-135D 0.065 g/cc with a 135 micron mean particle size from Henkle of Greenville, S.C. 
         [0034]    In an exemplary embodiment, the MAX phase solids reinforcement  42  preferably constitutes between approximately 1% and approximately 66% of the abradable composite layer  40  by volume. The polymer matrix material  44  constitutes between approximately 34% and approximately 99% by volume of the abradable composite layer  40 . A composition particularly well suited for fan application is 75 volume % epoxy with 25 volume % of 3 micron Ti 2 AlC MAX phase. The density of this mixture is then reduced by mixing at 50 volume % with DUALITE® E065-135D polymer hollow micro spheres. 
         [0035]    The abradable composite layer  40  of abradable seal  32  may be applied to substrate  34  and bond coat  38  by any suitable method known in the art. Examples of suitable methods include injection molding, spraying, manual application with a trowel and adhesive bonding of a composite preform. In an exemplary embodiment, abradable composite layer  40  is applied onto bond coat  38  of MAX phase reinforcement material  42  and polymer based abradable material  44  to a thickness of between approximately 1 millimeter and approximately 10 millimeters. In an exemplary embodiment, reinforcement material  42  is applied to bond coat  38  by spraying. In another exemplary embodiment, MAX phase reinforcement material  42  and rubber abradable material  44  are blended to create a mixture that is fed through a single port of an applicator device. In another exemplary embodiment, composite powder particles containing both MAX phase reinforcement material  42  and rubber abradable material  44  make up the feedstock. 
         [0036]    Due to its metallic characteristics, such as toughness and ductility, abradable seal  32  may be placed in harsh environments without eroding. In an exemplary embodiment, the abradable seal composite layer  40  is positioned on substrate  34 , proximate fan casing  28 , of a gas turbine engine proximate the fan blade  26  tips. The abradable seal  32  is positioned between the fan casing  28  and the rotating fan blade  26  tips and functions to help control the clearance between fan casing  28  and fan blade  26 . The fan casing  28  may serve directly as substrate  34  for coating  36 , and thus be an integral part of the abradable seal  32 . The fan casing  28  and abradable seal  32  are stationary relative to the engine with moving blades  26 . The fan blade  26  tips operate with a small clearance to the abradable blade outer air seal surface, and typically do not come into direct contact with abradable seal  32 . However, due to thermal events such as expansion or contraction, or changing loads such as g-loads or maneuver loads, the position of fan casing  28  can occasionally shift relative to the fan blade tips. 
         [0037]    While the abradable seal  32  exhibits desirable metallic/rubber characteristics, abradable seal  32  also exhibits desirable ceramic characteristics. Thus, when fan blade  26  tips do contact the abradable seal  32 , damage to the fan blade  26  tips are either minimized or prevented. Because matrix material  42  has brittle, ceramic properties, coating  36  is easily abraded from substrate  34 , allowing fan blade  26  tips to contact with the abradable seal  32  without damaging the fan blade  26  tips. This is beneficial because repairing or replacing fan blades is more costly and time-consuming than replacing the abradable seal  32 . In addition, damage to the blade tips and casing are prevented by the low rub forces, low heat generation, and lack of coating smearing and galling. Due to the MAX phase solids reinforcement material  42  excellent thermal conduction properties, the abradable seal  32  has superior heat transfer properties. Thus, the thermal energy dissipation can allow for lower temperatures at the abradable seal  32  and the fan  26 . The polymer coatings on the fan blade  26  are protected from excessive heat and the resultant thermal damage caused by the excessive heat of the fan blade  26 . 
         [0038]    The abradable seal is positioned in a gas turbine engine proximate the fan blade and includes a substrate and a coating composite applied on a top surface of the substrate. By using the MAX phase solids reinforcement material with the polymer based abradable material rather than completely being composed of polymer based material, the thermal conductivity of the abradable seal is increased. This is due to the low high thermal conductivity of the MAX phase solids. Approximately 1-10 wt. % MAX phase ternary carbide and/or nitride is blended into silicone rubber abradable or other polymer-based compositions in order to enhance thermal conductivity. The high thermal conductivity of MAX phase particles (at ˜ 40  W/m-K) will provide increase in thermal conductivity of the abradable coatings even at low concentrations. The MAX phases also offer ultrahigh damping capability combined with excellent mechanical properties, improved toughness, high damage tolerance, high thermal stability, and superb machinability, among others. Therefore, the MAX phases will also reinforce the polymer based materials and significantly increase damping capacity of the rubber or polymers used in the abradable coatings. 
         [0039]    Additional benefits are provided by blending MAX phase ternary carbide and/or nitride into silicone rubber abradable or other polymer-based compositions. The rub temperature and blade tip temperature will be lower due to the increased thermal conductivity of the enhanced abradable material. The lower temperatures will help to prevent damage to the polyurethane erosion resistant coating on the aluminum airfoils of the fan blades. The durability of the outer air seal abradable coatings will be increased due to the significant toughening characteristics of the MAX phase ternary carbides and nitrides. The rubbed surface finish shall be improved with the addition of MAX phases due to the increased stiffness and damping capability of the composite outer air seal. The layered structure of the MAX phase itself will allow the seal to wear away during rub interaction more readily than other hard phase fillers like glass micro balloons despite the high modulus of the material. 
         [0040]    There has been provided a MAX phase with abradable polymer based composite for turbine engine seals. While the MAX phase and polymer based abradable composite 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.