Abstract:
A soft lithography template or stamp is made by casting a polydimethysiloxane (PDMS) or other suitable elastomeric precursor onto a master pattern. The master pattern may be formed utilizing known micro-fabrication techniques. The PDMS template includes an inverse copy of the micro-structures on the master pattern, and can be placed into a mold used to prepare a carbon-fiber reinforced polymer composite part or other polymer molding systems where a matrix material passes through a fluid state during the cure process. The liquid resin material flows into the structures on the surface of the PDMS template and hardens during the curing cycle. After the part is released from the mold, the PDMS template can be peeled from the surface of the part to reveal the free standing micro structures which are a replica of the master pattern.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION(S) 
       [0001]    This patent application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/777,748, filed on Mar. 12, 2013, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The invention described herein was made in the performance of work under a NASA contract and by employees of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. §202, the contractor elected not to retain title. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Known soft lithography techniques utilize a soft polymeric mold or template made from a material such as polydimethylsiloxane (PDMS). The mold is cast using a master that comprises a hard material. The master is fabricated using photolithography, e-beam, micro-machining or other suitable process. The mold or template is an exact structural inverse of the master surface. The molds can be used to transfer the master pattern to various surfaces. 
         [0004]    Various types of micro-topographical surface patterns or features have been developed. A known type of surface includes moderate to high aspect ratio micro-structures that allow for reduced interactions of particles and fluids with the surface. A reduced contact area reduces the energy that would otherwise be required to remove contamination from the surface. In the case of superhydrophobic surfaces, fluids are suspended over air that is trapped between micro-structures on the surface in a Cassie-Baxter state. Abhesive and superhydrophobic surfaces help protect a part from contamination and fouling. 
         [0005]    Various types of micro surface structures have also been developed to reduce drag in aerodynamic and hydrodynamic applications. An example of a naturally-occurring drag reducing surface structure can be found on the skin of a shark, which helps the sharks swim more efficiently. 
         [0006]    Other surfaces have been developed to improve adhesion between two parts in an adhesively bonded joints. Surface roughness may be created by mechanical abrasion such as sand blasting or sanding. However, such techniques may not provide the desired degree of control of the surface roughness, and may introduce contamination into the material that can be difficult to remove. Furthermore, if a composite material is blasted or sanded, removal of the matrix resin from the surface may expose the reinforcing fibers, which creates a point of ingress for degenerative environmental components such as water and oxygen. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    One aspect of the present invention is a method of forming a surface in a composite material having at least a curable matrix and a fiber reinforcement. The method includes forming a flexible template having a template surface that has at least a plurality of surface features. The surface features can be inverses of micro-structures to be formed in the surface of an object. The object can be any physical or tangible thing, such as for example, a part, a component, a piece, a portion, a segment, a section, a fragment, a tool, a die, a sheet, a patch, a layer, and/or a design, and so on. In some embodiments, the inverses of micro-structures can have a specifically defined shape that can be uniform or non-uniform. In some embodiments, the inverses of micro-structures can cover any portion of the template surface or, in the alternative, the entire template surface. 
         [0008]    The flexible template is positioned in a mold tool such that it conforms to the surface of the mold. In some embodiments, the mold tool has a non-planar surface. In an embodiment, the flexible template is positioned in a mold tool having a curved surface, and the flexible template flexes to conform to the curved surface of the mold. In some embodiments, the flexible template flexes by bending, moving, deforming, distorting, and/or changing shape. Next, at least a portion of the template surface is covered with a composite. The composite includes at least a matrix material and a fiber reinforcement. When the composite material is applied to the flexible template, the matrix material is in a flowable, malleable, and/or deformable state. Pressure is applied to the composite material while the matrix material is in a flowable, malleable, or deformable state to cause at least some of the matrix material to enter and/or flow into the surface features of the template surface. The matrix material is solidified to form a composite object having an object surface defining micro-structures that are inverses of the surface features of the template surface. Solidifying the matrix material includes hardening, becoming a solid form, and curing. Once the matrix material is in a solid or cured form, the object formed from the composite material is disengaged from the flexible template to expose the object surface. 
         [0009]    Another aspect of the present invention is a method of forming a surface having at least a plurality of predefined microscopic features. The method includes forming a flexible template having a plurality of microscopic cavities on the template surface. The flexible template is flexed or deformed by positioning the flexible template in contact with a non-planar surface. The method includes causing a material, such as a polymer or polymer composite, to flow into at least a portion of the cavities while the flexible template is in contact with the non-planar surface. The material can be solidified or cured while it is in contact with the non-planar surface. The material is disengaged from the template to reveal a surface having at least a plurality of protrusions formed by the cavities. The material may be in a liquid or flowable state at the time it enters at least a portion of the cavities, and the material may be cured prior to disengaging the material from the template. The material may comprise at least a polymer material forming a matrix of a fiber reinforced composite material that is cured utilizing heat. The flexible template may be formed from an elastomeric material that is brought into contact with a master surface while the elastomeric material is in a liquid or flowable form, and curing the elastomeric material while it is in contact with the master surface. The master surface may be formed utilizing an etching process. The non-planar surface may comprise a curved mold surface that is positioned in a curing device, a pressure and/or temperature vessel, or the like. Examples of devices/vessels into which the curved mold can be placed include an autoclave, a heated press, a heated vacuum press, or the like. Any suitable means of applying a load to the mold to achieve the desired results can be used. In some embodiments, the load applied to the mold is pressure. In some embodiments, heat is applied to the mold in addition to the load. 
         [0010]    These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]      FIG. 1  is a partially schematic cross sectional view of a composite part positioned in a mold in an autoclave; 
           [0012]      FIG. 2  is a fragmentary, enlarged view of a portion of the composite part and mold of  FIG. 1 ; 
           [0013]      FIG. 3  is an isometric view of a master surface formed in a polyimide film, wherein the image was formed utilizing an optical profilometer; 
           [0014]      FIG. 4  is a plan view of the master surface of  FIG. 3 ; 
           [0015]      FIG. 5  is an isometric view of a flexible template formed from the master surface of  FIGS. 3 and 4 , wherein the image is formed utilizing an interferometric microscope; 
           [0016]      FIG. 6  is a plan view of the flexible template of  FIG. 5 ; 
           [0017]      FIG. 7  is an isometric view of the surface of a part formed utilizing the flexible template of  FIGS. 5 and 6 , wherein the image is formed utilizing an interferometric microscope; 
           [0018]      FIG. 8  is a plan view of the surface of  FIG. 7 ; 
           [0019]      FIG. 9  is an isometric view of a surface topography according to another aspect of the present invention; and 
           [0020]      FIG. 10  is a plan view of the surface topography of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0022]    With reference to  FIGS. 1 and 2 , the present invention involves forming a flexible template  10  that may be positioned on or in a mold tool  12  having a mold surface  14 . The mold surface  14  may include a convexly curved portion  16  and/or a portion  18  having a concave curvature. It will be understood that the mold surface  14  may also comprise a more complex curvature such as a “saddle surface” (e.g. a hyperbolic paraboloid). The template  10  is preferably made of a flexible material such as a cast polydimethylsiloxane (PDMS) or other suitable elastomeric material. As discussed in more detail below, a surface  20  of template  10  includes a plurality of micro-structures that are the inverse of micro-structures formed in a surface  22  of a part  24 . The part  24  may comprise a composite material, such as a carbon fiber reinforced structure having a thermosetting polymer matrix. 
         [0023]    Referring again to  FIG. 1 , the template  10  may comprise a one piece member, or it may comprise a plurality of pieces  10 A- 10 D that are positioned directly adjacent one another on the mold surface  14 . In the illustrated example, the part  24  is formed from a prepreg carbon fiber material. The layers  26 A- 26 D of uncured prepreg material are laid on the mold surface  24  in the orientations required for a particular application. It will be understood that the number of layers  26  of the carbon fiber material utilized will vary depending upon the requirements of a particular application. In the illustrated example, the layers  26 A- 26 D of composite part  24  are positioned inside an impermeable envelope  28 , and the layers  26 A- 26 D are subject to a vacuum resulting from vacuum pump or device  32  which is operably connected to the envelope  28  by a vacuum line  34 . The mold tool  12  and part  24  (i.e. layers  26 A- 26 D) may be positioned in an autoclave  30  to cure the matrix material of the layers  26 . As known in the art, the autoclave  30  may be utilized to provide heat and pressure that cures a thermosetting polymer material forming the matrix of the layers  26 A- 26 D. This forms a rigid composite part having a shape that generally conforms to the shape of the mold surface  14 . 
         [0024]    The present invention generally involves forming a master part  38  ( FIGS. 3 and 4 ) having a master surface  40  having a plurality of surface features such as protrusions  42 . A template  10  ( FIGS. 5 and 6 ) is then cast from master part  38  utilizing an elastomeric material such as a PDMS material. The template  10  includes a surface  20  having a plurality of features such as openings or cavities  52  that are an inverse of the master surface  40  and surface features  42  of master part  38 . A part  24  is then formed utilizing a high pressure and/or heat process such as the autoclave process described above in connection with  FIGS. 1 and 2  to form a part  24  having a surface  22  ( FIGS. 7 and 8 ) that may include a plurality of micro-structures such as protrusions  54 . 
         [0025]    Referring again to  FIGS. 3 and 4 , a master part  38  is first formed utilizing standard micro-fabrication techniques. In the illustrated example, the master part  38  is formed from a KAPTON® (polyimide) film having a master surface  40  including a plurality of surface features such as protrusions  42  that are formed using an etching process. However, the master part  38  may be fabricated from a wide range of materials utilizing various known processes. For example, the protrusions  42  and/or other surface features may be formed by a laser writing or electron beam writing process. The master part  38  may be formed from virtually any material having the required characteristics for a particular application. The master surface  40  is formed such that it includes a plurality of surface features corresponding to the surface features that are to be formed in the final part  24  ( FIGS. 7 and 8 ). In the illustrated example, the protrusions  42  have a generally square cross-sectional shape with sides having a dimension “D” of about 20 microns. It will be understood that protrusions  42  or other such features may have a uniform cross-sectional shape and size, or the protrusions may have a non-uniform or tapered configuration whereby the bases of the protrusions  42  have a greater cross-section area than the end portions. Also, in the illustrated example, the protrusions  42  have a height “H” of about 20 microns. In the illustrated example, the protrusions  42  are equally and/or evenly spaced apart in a square grid pattern with spacing of “X” (center-to-center) spacing between protrusions  42  of about 60 microns. 
         [0026]    The master surface  40  is not limited to the arrangement shown in  FIGS. 3 and 4 , and the surface  22  of the final part  24  is not limited to the configuration of  FIGS. 7 and 8 . Rather, the master surface  40  and surface  22  may be configured as required to provide a particular surface property. For example, the surfaces  22  and  40  may comprise superhydrophobic surfaces having a moderate to high aspect ratio. The aspect ratio is the ratio of the height of the micro-structures (e.g. height “H” in  FIG. 3 ) divided by the diameter or transverse dimension (e.g. “D” in  FIG. 3 ). In general, moderate to high aspect ratio micro-structures (e.g. aspect ratios greater than about 1.0) allow for reduced interactions of particles and fluids with a surface. A reduced contact area reduces the energy needed to remove contamination from the surface. In the case of superhydrophobic surfaces, fluids may be suspended over air trapped between the micro-structures (e.g. protrusions  42 ) in a Cassie-Baxter state. Abhesive (non-stick) and superhydrophobic surfaces help protect a part from contamination and fouling. The surface  22  ( FIGS. 7 and 8 ) of the part  24  may comprise an outer surface of a wing or other aerodynamic structure. The size, configuration, spacing, and other geometric features such as protrusions  54  may be utilized to form superhydrophobic surfaces that act to repel water, prevent water damage, and also prevent accumulation of foreign matter such as dirt, bug splatter, and ice on aircraft wings and other structures. 
         [0027]    Still further, the protrusions  42 - 54  may be configured to reduce skin drag if surface  22  of part  24  comprises an aerodynamic surface (e.g. an outer wing surface) or a hydrodynamic surface (e.g. an outer surface of a boat hull or submarine). The protrusions  42 / 54  may comprise riblets, pyramids or other such structures (not shown) that reduce skin drag. Micro-structures of the type that reduce aerodynamic and/or hydrodynamic drag are generally known in the art. Examples of such structures are disclosed in “Effects of Riblets on Skin Friction and Heat Transfer in High-Speed Turbulent Boundary Layers,” Lian Duan and Meelan M. Choudhari, 50 th  AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Jan. 9-12, 2012, Nashville, Tenn., “Riblets as a Viscous Drag Reduction Technique,” Michael J. Walsh, AIAA Journal, Vol. 21, No. 4, April 1983 and “Delaying Transition to Turbulence by a Passive Mechanism” Jens H. M. Fransson, Alessandro Talamelli, Luca Brandt, and Carlo Cossu, PRL 96, 064501 (2006), the entire contents of each being incorporated herein by reference. 
         [0028]    Furthermore, the master surface  40  of master part  38  ( FIGS. 3 and 4 ) may be configured to produce a part surface  22  ( FIGS. 7 and 8 ) having improved adhesion properties for an adhesive bonded joint. In general, an average surface roughness in the range of about 1.0 microns to about 2.0 microns provides significantly improved adhesion compared to a smooth surface. As used herein, the definition of surface roughness is the arithmetic average deviation of the average line profile. For purposes of providing improved adhesion, the master surface  40  and surface  22  of part  24  may have an average roughness in the range of about 100 nm to about 100 microns. 
         [0029]    As discussed above, a master part  38  ( FIGS. 3 and 4 ) having a master surface  40  is fabricated utilizing a suitable known micro-fabrication technique. A template  10  ( FIGS. 5 and 6 ) is then formed from a PDMS or other suitable elastomeric precursor. In the illustrated example, liquid PDMS material is poured onto the master surface  40  of master part  38 , and the material is then cured. The template  10  is then peeled away from the master part  38 . As shown in  FIGS. 5 and 6 , the template  10  includes features that are the inverse of the master surface  40  and the surface  22  of the part  24 . In the illustrated example, the openings or cavities  52  formed in template  10  have an opening size “D” that is substantially similar to the dimension “D” ( FIG. 3 ) of the protrusions  42 . Similarly, the openings  52  may have a depth that is substantially the same as the height “H” of protrusions  42 . As discussed above, the protrusions  42  of master part  38  have a generally square cross-sectional shape. The openings or cavities  52  of template  10  also have a generally square cross-sectional shape. However, because the PDMS material of template  10  does not exactly match the geometry of surface  40  of master part  38 , the openings or cavities  52  may have a shape that is somewhat rounded relative to the square cross-sectional shape of the protrusions  42 . It will be understood that the protrusions may have virtually any cross-sectional shape, height, spacing, and other geometric features as required to provide the desired surface characteristics. 
         [0030]    As discussed above, the template  10  is positioned on a tool surface  14  with surface  20  of template  10  facing upwardly. The layers  26 A- 26 D of prepreg carbon fiber composite material are then positioned on surface  20  of template  10 , and the uncured layers  26  are positioned in an autoclave  30  or other suitable device. 
         [0031]    As known in the art, the layers  26  may be heated to lower the viscosity of the thermosetting polymer matrix material of the prepreg layers  26 . As pressure is applied to surface  56  ( FIGS. 1 and 2 ) of an outer layer  26 D, the matrix material flows into the openings or cavities  52  of template  10 . As the temperature is increased, the matrix material cures, thereby forming a surface  22  ( FIGS. 7 and 8 ) having a plurality of protrusions  54  or other such features. The amount of pressure applied to surface  56  and the temperatures utilized in the autoclave  30  will vary as required for a particular application. 
         [0032]    If the layers  26 A- 26 D comprise prepreg carbon fiber, thermosetting polymer matrix material of layers  26  may have sufficiently low viscosity to flow into openings or cavities  52  at a temperature in the range of about 65° F. to about 700° F., more specifically from about 65° F. to about 350° F., and even more specifically from about 150° F. to about 300° F. In some embodiments the thermosetting polymer matrix flows at a temperature of about 150° F. The matrix material may cure/soldify, for example, at temperatures of about 200° F. to about 400° F., more specifically at temperatures of about 250° F. to about 350° F., even more specifically at temperatures of about 300° F. to about 350° F. In some embodiments the thermosetting polymer matrix cures/solidifies at a temperature of about 350° F. 
         [0033]    In general, pressures in the range of about 100 psi to about 200 psi may be applied to surface  56  to cause the thermosetting polymer matrix material to flow into the cavities or openings  52  of template  10 . The temperature within the autoclave  30  may be held at a flow temperature (e.g. about 65° F. to about 700° F.) for a period of time at an elevated pressure (e.g. about 100 to about 200 psi) for a period of time (e.g. about 30 to about 60 minutes) to ensure that the matrix material flows into cavities  52 . The temperature can then be raised to a cure temperature (e.g. about 200° F. to about 400° F.). Alternatively, the temperature within the curing device and/or the pressure/temperature vessel, such as the autoclave  30 , may be gradually increased at a relatively slow rate. For example, the temperature can be gradually increased at a rate of about 2° C. per minute to about 10° C. per minute (about 3° F. per minute to about 18° F. per minute), specifically at a rate of about 5° C. per minute to about 10° C. per minute (about 9° F. per minute to about 18° F. per minute) while pressure is applied to the surface  56  to thereby ensure that the polymer matrix material is in a flowable state for a period of time that is sufficient to permit the matrix material to flow into the apertures or openings  52  of template  10 . 
         [0034]    Because the template  10  is made from a relatively thin layer of elastomeric material, the template  10  curves and conforms to curved portions  16  and  18  ( FIG. 1 ) of mold surface  14 . This enables forming of parts  24  having a curved outer profile  8 . The outer profile  8  may comprise an aerodynamic surface of an aircraft fuselage, wing, or other structure. 
         [0035]    After the part  24  is cured, the part  24  is released from the mold  12 , and the template  10  is peeled from the surface  22  of part  12  to reveal the freestanding micro-structures (e.g. protrusions  54 ) which are substantially a replica of the master pattern (e.g. master surface  40 ). If templates  10  are formed from a PDMS material, the templates typically have a low stick surface that permits removal of templates  10  from surface  22 . However, a mold release agent may be utilized if required. 
         [0036]    In general, the templates  10  can be re-used indefinitely. Before loading the template  10  into a mold  12  the template  10  is inspected for damage and/or debris. Debris is removed from the template  10  with a solvent rinse to the extent possible. Although damaged templates  10  cannot normally be repaired, a new copy of the master pattern or part  38  can be made. 
         [0037]    As discussed above, the surface topography of part surface  22  may vary as required for a particular application. Accordingly, it will be understood that the protrusions  54  are merely an example of one possible surface topography. In general, the surface  22  may include a wide range of micro-structures or features as required to produce a desired surface characteristic. Also, in the example described above, the part  24  comprises a composite part made from layers  26  of prepreg carbon fiber material. However, it will be understood that other materials and processes may also be utilized according to other aspects of the present invention. For example, the part  24  may be fabricated from a polymer material that does not include a fiber reinforcement. Still further, the part  24  may be fabricated from materials other than thermosetting polymers. For example, the part  24  may be formed from a thermoplastic polymer material. In this case, a sheet of thermoplastic material may be positioned on a mold surface  14 , and the material may be heated to lower the viscosity of the thermoplastic polymer. Pressure may then be applied to the polymer material to thereby cause the surface of the material to form a surface that substantially conforms to the surface of template  10 . 
         [0038]    With further reference to  FIGS. 9 and 10 , a master part  38 A according to another aspect of the present invention includes a master surface  40  comprising a plurality of raised ridges  60  forming channels  62  therebetween. In this example, master part  38 A comprises an epoxy-based SU-8 photoresist. The master part  38 A can be utilized to form a part  24 A having a surface  22 A that is substantially identical to the master surface  40 A. The master surface  40 A may be formed in a suitable material utilizing a standard micro-fabrication technique such as laser writing or electron beam writing, and a template  10  may be formed from PDMS or the like utilizing the master surface  40 A. A template  10  formed utilizing master surface  40 A will generally have a surface (not shown) that is an inverse of the micro-structures shown in  FIGS. 9 and 10 . A template  10  can then be utilized to form a part  22 A having surface  24 A as described in more detail above in connection with  FIGS. 1 and 2 . 
         [0039]    The ridges  60  and channels  62  of  FIGS. 9 and 10  may have tapered surfaces to form a dovetail structure. In this case, a master template having a plurality of channels  62  in the form of dovetails is formed. A template  10  is then formed from PDMS or other suitable material, and utilized in a molding process to form a part having a dovetail surface. A pair of parts having interlocking dovetail surfaces can be formed in this way to provide a mechanical bond between the parts to hold the joint together in the event that a chemical bond is too weak (e.g. due to contamination or corrosion in the joint). 
         [0040]    All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference. 
         [0041]    All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. Each range disclosed herein constitutes a disclosure of any point or sub-range lying within the disclosed range. 
         [0042]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. As also used herein, the term “combinations thereof” includes combinations having at least one of the associated listed items, wherein the combination can further include additional, like non-listed items. Further, the terms “first,” “second,” and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity). 
         [0043]    Reference throughout the specification to “another embodiment”, “an embodiment”, “some embodiments”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and can or cannot be present in other embodiments. In addition, it is to be understood that the described elements can be combined in any suitable manner in the various embodiments and are not limited to the specific combination in which they are discussed. 
         [0044]    It is to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present invention, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.