Abstract:
Components with improved erosion resistance are disclosed. A surface of the component or a substrate of the component is modified by coating the substrate with an elastomer layer. The elastomer layer is then modified by embedding hard particles onto an outer side of the elastomer layer. The hard particles exhibit higher fractured toughness providing enhanced erosion protection. The elastic properties of the elastomer experience little reduction because the surface embedded particles are located only at the outer side or outer surface of the elastomer layer. Therefore, the bond between the inner side of the elastomer layer and the substrate or component surface is not interfered with and the potential for electro-chemical corrosion and poor adhesion are not increased by the presence of the hard particles as the hard particles are located away from the inner face between the elastomer layer and the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional application under 35 USC §121 of U.S. patent application Ser. No. 13/216,954 filed on Aug. 24, 2011. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to erosion resistant coatings for substrates and methods of applying such erosion resistant coatings to substrates. 
       BACKGROUND 
       [0003]    The durability of helicopter rotor blades is dependent to a large extent on the erosion of the component by friction or by impact of finely divided solid or liquid particles. There is no way to avoid this friction or particulate impact during use of these components and therefore some means is needed to protect the components against erosion. 
         [0004]    For example, the air through which the helicopter rotor blade rotates may contain particulate matter, such as sand. The size of the sand particles typically ranges from about 0.1 to 2000 microns and more typically from about 20 to 30 microns in diameter. If the air contains sand, the sand impinges upon the rotor blades as they rotate, thereby causing abrasion to the blades, or at least to portions thereof. Unless the blades are adequately protected, such repetitive abrasive contact can eventually cause the blades to erode. 
         [0005]    The potential for erosion also exists if the rotor blades circulate through air containing water droplets. The size of water droplets ranges from about 1000 to 4000 microns and is typically about 2000 microns in diameter. Although the size of the water droplets is typically greater than the size of sand, under high velocity conditions, water droplets may behave similar to sand, thereby causing erosion to the rotating rotor blades. 
         [0006]    Moreover, the combination of rain and sand can exacerbate the amount of abrasion and/or erosion. As a result, when translating a component through air comprising both rain and sand, the potential for erosion further increases. 
         [0007]    The potential for erosion is also a function of the force at which the particulate matter impacts the rotor blade. Specifically, as the impact force increases, so does the potential for erosion. The force at which the particulate matter impacts the rotor blade is dependent upon the geometric shapes of both the rotor blade and the impacting particle and their relative velocities. For example, the leading edge of a rotor blade is the portion of the blade that first cleaves through the air. Therefore, the leading edge is the portion of the blade most susceptible to erosion caused by the abrasive contact of particulate matter. 
         [0008]    The amount of erosion to the rotor blade is also a function of the velocity at which the blade impacts the particulate matter or vice versa. In other words, the potential for erosion increases as the speed of the blade increases. For example, because a rotor blade typically rotates around a central axis, the velocity of the rotor blade, relative to the air, differs along the leading edge of the blade. More specifically, the velocity at a point on a blade is equal to the product of the distance from the center rotational axis and the rotational velocity. As the distance from the rotational axis along the leading edge increases, so does the rotational velocity. The outboard tip of the rotor blade is the furthest from the rotational axis. Therefore, the potential for erosion is greatest at the outboard tip of the leading edge of the rotor blade. 
         [0009]    Various techniques have been attempted to minimize the amount of erosion to the leading edge of rotor blades. One technique includes adhesively bonding an appropriately shaped piece of ductile metal onto the leading edge of the blade, such that the ductile metal is an integral part of the blade. The ductile metal leading edge is typically constructed of nickel, which provides increased wear resistance. The extended exposure of the nickel to the impinging particulate matter, however, causes the ductile metal leading edge to erode. The eroded nickel must, therefore, be replaced. Because the ductile metal leading edge is adhesively bonded to the blade, replacing the ductile metal leading edge requires a certain amount of time and skill, which is not typically available in the field. 
         [0010]    Repairs that are performed in the field are referred to as “field level” repairs because such repairs require an acceptable amount of time and a minimal amount of skill to complete. Repairs requiring an extended amount of time and a heightened skill level occur back at the aircraft depot and are referred to as “depot” repairs. Depot repairs are undesirable because depot repairs increase the amount of time that the aircraft is unavailable in comparison to a field level repair. Because the replacement of the ductile metal leading edge is considered a depot repair, bonding ductile metal onto the leading edge of a rotor blade is an undesirable technique for minimizing erosion. 
         [0011]    One type of “field level” repair technique for improving a rotor blade&#39;s wear resistance includes applying an elastomeric material to the leading edge of the blade. Typically, the elastomeric material is applied to the leading edge as a tape. As the tape becomes worn, it can quickly and easily be removed, and a new layer of tape can be applied. Unfortunately, the elastomeric tape must be replaced more frequently than a nickel leading edge and the ability of the elastomer to resist erosion caused by the combined rain and sand is less than that of nickel. Specifically, the elastomeric tape fails to adequately absorb the impact energy of the particulate matter. Without adequate absorption capabilities, the elastomer fails to dissipate the impact energy, thereby allowing the particulate matter to erode the elastomer. Without frequent replacement of the elastomeric tape, the leading edge of the rotor blade remains unprotected. 
         [0012]    In an attempt to supplement the deficiencies of elastomeric tape, current designs include particles disposed in the elastomer layer. The particles are mixed in with the elastomer material before it is cured onto the substrate. Unfortunately, the particles at the interface of the elastomer layer and the substrate can cause poor bonding to the substrate and/or electrochemical corrosion problems such as a galvanic coupling. Further, the embedded particles can adversely affect the elastic properties of the elastomer coating thereby reducing the ability of the elastomer coating to absorb energy from particles in the air, such as sand, water droplets and other debris. 
       SUMMARY OF THE DISCLOSURE 
       [0013]    To address the above problems, an improved component is disclosed which comprises a substrate comprising an outer surface. The outer surface is at least partially covered by an elastomer layer. The elastomer layer has an inner side that is bonded to the outer surface of the substrate and an outer side that is at least partially embedded with a plurality of particles. 
         [0014]    Another improved component is disclosed which comprises a substrate having an outer surface. An elastomer layer is affixed to the outer surface of the substrate. The elastomer layer is fabricated from an elastomer that is selected from the group consisting of a polyurethane, a polyurea, a silicone and a fluoropolymer. The elastomer layer has an outer side disposed opposite the elastomer layer from the substrate and an inner side that is affixed to the outer surface of the substrate. The outer side of the elastomer layer is embedded with particles and the inner side of the elastomer layer is free of particles. The particles are fabricated from a material selected from the group consisting of alumina, silicon carbide, silicon nitride, boron carbide, tungsten carbide, steel alloys, nickel alloys, diamond, chromium carbide, mullite, zirconia, yttria stabilized zirconia, magnesium stabilized zirconia and combinations thereof. 
         [0015]    A method for improving the erosion resistance of a substrate is also disclosed. The method comprises coating the substrate with at least one elastomer layer. The at least one elastomer layer includes an inner side that engages the substrate and an outer side disposed opposite the at least one elastomer layer from the inner side. The method further includes partially curing the at least one elastomer layer. The method also further includes applying particles to the partially-cured at least one elastomer layer so the particles embed into the outer side of the elastomer layer but do not pass through the elastomer layer to the inner side of the elastomer layer. The embedded particles can be used to control the surface energy of the exposed layer as measured by contact angle using standard methods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a side sectional view of a substrate coated with an elastomer layer that includes an outer side embedded with particles (surface embedded particles or SEPs). 
           [0017]      FIG. 2  compares, graphically, the mass of the erodent that engages the elastomer layer (x-axis) and the mass of the elastomer layer removed by the erodent (y-axis) for both an untreated elastomer layer and an elastomer layer treated with particles as illustrated in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring to  FIG. 1 , a substrate  10  is illustrated that includes an outer surface  12  that is at least partially covered by an elastomer layer  14 . The elastomer layer  14  includes an inner side  16  that covers, is bonded to, affixed to or engaged with the outer surface  12  of the substrate  10 . The elastomer layer  14  also includes an outer side  18  that is at least partially covered with a plurality of particles  20 . An additional embodiment is also disclosed in  FIG. 1  which includes dual elastomer layers  114 ,  214 . In this example, the lower elastomer layer  214  can be applied to the outer surface  12  of the substrate  10  and may be allowed to cure. Then, the upper layer  114  is prepared and, optionally, the particles  20  may be mixed with the elastomer of the upper layer  114  and applied to the lower layer  214  thereby providing a dual layer  114 ,  214  structure wherein the outer layer  114  includes the plurality of hard particles but which cannot migrate to the inner layer  214  as the inner layer  214  is cured or at least substantially cured by the time the outer layer  114  is applied onto the inner  214 . The properties of the layers  14 ,  114  and  214  can be selected to provide a preferred combination of bonding, erosion protection and thermal and environmental resistance to the coated substrate  10 . 
         [0019]    Other techniques for applying the particles  20  to the elastomer layer  14  include partially-curing the elastomer layer  14  and spraying or pressing the particles  20  onto the outer side  18  of the partially-cured elastomer layer. The particles  20  may also be strategically placed on the outer side  18  of the elastomer layer  14  by using a screen or mesh  22  and pressing or spraying or otherwise delivering the particles  20  through the screen or mesh  22  onto the outer side  18  of the partially-cured elastomer layer  14 . 
         [0020]    Depending on the type of elastomer and desired properties, the particles  20  may be fabricated from materials selected from the group consisting of alumina, silicon carbide, silicon nitride, boron carbide, tungsten carbide, steel alloys, nickel alloys, diamond, chromium carbide, mullite, zirconia, yttria stabilized zirconia, magnesium stabilized zirconia and combinations thereof. If the particles  20  and the elastomer layer  14  are not inherently compatible, it may be necessary to add a known coupling agent to the elastomer in order to combine the elastomer and the particles. For that matter, the elastomer layer  14  should be selected so that it is compatible with the substrate  10  and, when cured onto the substrate  10 , the elastomer layer  14  should be mechanically and/or chemically bonded to the substrate  10 . 
         [0021]    It is preferable that the elastomer has a strain to failure of at least 20% and a tensile strength of at least 1,000 PSI. Even more preferable, the elastomer may have a strain to failure of at least 100% and a tensile strength of at least 3,000 PSI. It is still more preferable that the elastomer has a strain to failure of at least 1000% and a tensile strength of at least 5,000 PSI. Elastomers such as polyurethanes, polyureas, silicones or silicone rubbers and fluoropolymers can satisfy these requirements. Other suitable elastomers may include, but are not limited to natural rubber, polyurethanes, chlorosulfonated polyethylene, chlorinated polyethylene and ethylene-propylene copolymers and terpolymers. 
         [0022]    One example of a possible polyurethane is a product manufactured by AIR PRODUCTS under the trade name Airthane®. Examples of potentially suitable fluoropolymers include those manufactured by Dupont Dow Elastomers under the tradenames Viton® and Kalrez®. E.I. Dupont de Nemours Company also manufactures Teflon® fluoropolymer. Another example of a possible fluoropolymer includes that which is manufactured by the Minnesota, Mining &amp; Manufacturing Company (3M) under the tradenames Fluorel®. Further examples of potential elastomers include Engage® polyolefin, Ascium® and Hypalon® chlorinated polyethylenes, and Tyrin® chlorinated polyethylene, all manufactured by Dupont Dow Elastomers. Another example of a possible polymer is a fluorinated polymer, such as polychlorotrifluoroethylene manufactured by 3M under the tradename Kel-F®. Examples of a potential silicone include NuSil R-2180, fluorosilicone and polydimethylsiloxane. 
         [0023]    It is preferable that the hard particles  20  have a diameter ranging from about 5 microns to about 3000 microns. It is even more preferable that the hard particles  20  have a diameter ranging from about 100 microns to about 1000 microns. It is especially preferable that the hard particles  20  have a diameter ranging from about 500 microns to about 800 microns. It is also preferable that the hard particles  20  have an aspect ratio ranging from about 1 to about 20. It is even more preferable that the hard particles  20  have an aspect ratio ranging from about 1 to about 10, and it is especially preferable that the hard particles  20  have an aspect ratio ranging from about 1 to about 5. It is also preferable that the hard particles  20  have favorable mechanical and chemical properties, such as high hardness, abrasion resistance, high modulus of stiffness, high compressive strength, water resistance, and thermal stability. It is also preferable that the hard particles  20  account for from about 1% to about 50% of the volume of the elastomer/particle layer  14 . It is even more preferable that the hard particles  20  account for from about 1% to about 25% of the volume of the elastomer/particle layer  14 , and it is especially preferable that the hard particles  20  account for from about 5% to about 20% of such volume. 
         [0024]    Depending upon the type of elastomer and the desired properties, the hard particles may comprise a material from the group consisting of alumina, silicon carbide, silicon nitride, boron carbide, tungsten carbide, steel alloys, nickel alloys, diamond, chromium carbide, mullite, zirconia, yttria-stabilized zirconia, magnesium-stabilized zirconia and combinations thereof. It is desired to include particles having a hardness generally higher than that of incoming erodent particles such as sand. 
         [0025]    The elastomer serves as a matrix for the hard particles, which add the desired physical properties to the elastomer, thereby increasing the toughness and/or stiffness of the elastomeric matrix. Incorporating hard particles into the elastomeric matrix allows an additional pathway for the elastomer to dissipate impact energy over a larger relative volume because the sizes of the particulate matter (sand or water) impacting the elastomer are significantly less than the size of the hard particles within the elastomeric matrix. It is anticipated that the smaller erodent particle will impact the reinforcement particle within the elastomer or with the elastomer itself. If the reinforcement particle is impacted, the force will be transferred into both the reinforcement particle and the elastomeric matrix. The elastomer layer(s) is, thereby, less susceptible to erosion than a pure elastomer. The particle-embedded elastomer layer  14 , therefore, will typically have a longer useful life compared to a pure elastomer layer. The elastomer layer  14  will also have a longer life expectancy than an elastomeric matrix reinforced with conventional reinforcing particles because the disclosed elastomer layer  14  with hard particles  20  embedded on its outer side  18  can absorb a particulate matter&#39;s impact energy over a significantly greater volume. Applying the particle-coated or particle-embedded elastomer layer  14  onto a substrate such as an airfoil, rotor or fan, especially a leading edge, reduces the energy density absorbed by the elastomer layer  14 , thereby reducing its potential for eroding which, in turn, increases the component&#39;s erosion resistance. 
         [0026]    The particles  20  may be partially or totally embedded in the elastomer layers  14 ,  114  as illustrated in  FIG. 1 , but are most effective when particles are disposed partially on or just below the outer side  18  of elastomer layers  14 ,  114 . The elastomer layers  14 ,  114  may have particles deposited to a depth ranging from about 1% to about 99% of the overall thickness of the layer or layers  14 ,  114 . Other suitable particle depth ranges can range from about 1% to about 50% or from about 5% to about 10%. At least about 1% of the outer layer  14 ,  114  thickness would have embedded particles with an upper limit of about 99% of the outer thickness of the layers  14 ,  114  would have embedded particles. One preferred range is from about 5% to about 10% of the outer layer thickness. 
         [0027]    Turning to  FIG. 2 , simulated data for an untreated elastomer layer and a treated elastomer layer  14  is presented graphically. The line  30  represents an untreated elastomer layer that is being bombarded with an erodent in the form of sand particles and/or water particles. The initial dip in the curve is evidence of a weight gain as the erodent particles are embedded into the untreated elastomer layer. However, as the untreated elastomer layer erodes, it loses weight as indicated by the upswing of the curve  30 . The curve  32  represents a treated elastomer layer  14  like that shown in  FIG. 1 . The treated elastomer layer  14  also gains weight as it is initially bombarded with particles, but at an initially slower rate than the untreated elastomer, and then slowly begins to lose weight as it erodes. The reader will note the substantial offset between the curves  30  and  32  thereby establishing that the treated elastomer layer  14  will take longer to erode than the untreated elastomer layer represented by the curve  30 . 
       INDUSTRIAL APPLICABILITY 
       [0028]    Various components that are susceptible to erosion by sand or dirt particles and/or water particles may be provided with a superior erosion-resistant coating in the form of an elastomer layer  14  with hard particles  20  embedded into or onto the outer side  18  of the elastomer layer  14 . The inner side  16  of the elastomer layer  14  that engages the substrate  10  is free of particles and therefore the bond between the inner side  16  and the outer surface  12  of the substrate  10  will not be interfered with by the hard particles  20 . Further, the risk of causing electro-chemical corrosion and/or adhesion problems at the interface between the inner sides  16  of the elastomer layer  14  and the upper surface  12  of the substrate  10  is avoided by keeping the hard particles  20  away from this interface. 
         [0029]    The modified elastomer layer  14  as shown in  FIG. 1  or the dual layer  114 ,  214  with the modified outer layer  114  can be applied to a variety of components including, but not limited to a propeller, a helicopter rotor blade, an airfoil, a compressor blade of a gas turbine, a fan blade of a gas turbine and a rotor blade of a pump, a rotor blade of a compressor, a fan blade of a heating-ventilation-air conditioning unit (HVAC) and other components as will be apparent to those skilled in the art.