Method for coating for electrical conductivity and part having the coating

Abstract

A metal part (112) made from a base metal and a coating system (500) thereon is characterized in that the coating system includes a conductive layer (502) on the base metal and a resin-based layer (504) containing conductive pigments (508) on the conductive layer. The conductive pigments (508) form an electrically conductive 3D network in the resin, which is randomly distributed in the resin. Aerospace fasteners are also provided having a coating system of a phenolic resin-based coating containing a nickel flash on the base metal and nickel fibers on the nickel flash. Additionally, methods for coating metal parts are disclosed. The coating system (500) can be applied to metal parts including aerospace fasteners, such as pins, bolts, collars, nuts and nut plates, and washers, as well as studs, latches, helicopter rotors, and landing gear structures.

Description

[Technical Field]

[0001] The present disclosure relates to surface treatments for aerospace parts, such as fasteners and other components, that provide a low coefficient of friction, are useful in high-interference assemblies, promote electrical conductivity in electrical grounding and electrical bonding applications, while also providing protection from galvanic corrosion in composite and / or metal joints, paint adhesion, and resisting chemicals and fluids.

[0002] The coatings can be applied for use on several different base metals and metal combinations. The coatings are particularly applicable to coating titanium and titanium alloys, stainless steels, and superalloys. A particular application relates to titanium fasteners commonly used in aluminum and / or carbon fiber reinforced polymer (CFRP) structures, such as aircraft. By way of example, the coatings are useful for protecting either or both of the titanium fasteners and the aluminum and / or CFRP structures of aircraft. [Background technology]

[0003] Related technologies It is common practice to construct aluminum or aluminum alloy structures, such as aircraft structures, using high-strength fasteners made of titanium or titanium alloys, such as Ti-6Al-4V. In other instances, fasteners made of stainless steel, such as A286, and nickel-chromium-based superalloys, such as Inconel® 718, have been used. Additionally, carbon fiber reinforced polymers (CFRP) have been used in place of aluminum-based structures. In some cases, Al-CFRP hybrid structures have been used.

[0004] Various coatings have been used for aerospace fasteners, primarily for protection from corrosion. Below is a list of these coatings and their drawbacks:

[0005] Ion vapor deposition (IVD) aluminum coatings: Rely on chromate for corrosion protection, are not suitable for interference fit applications, do not promote paint adhesion, and are not compatible with CFRP structures in terms of dissimilar metal contact.

[0006] Aluminum-pigmented resin-based coatings: Poor electrical conductivity and do not meet electrical bonding or electrical grounding requirements.

[0007] Cadmium: Relies on chromates for corrosion protection and is not compatible with CFRP structures.

[0008] Sulfuric Acid Anodizing (SAA) on Titanium: Not recommended for aluminum structures, not suitable for interference fit applications, and does not promote paint adhesion.

[0009] There remains a need for aerospace component coatings that are chromate-free, have low electrical resistivity for electrical grounding and electrical bonding applications, and comply with environmental agency regulations or requirements. Summary of the Invention [Means for solving the problem]

[0010] overview Briefly and generally, the present invention provides a chromate-free, electrically conductive coating system for application to aircraft fasteners and other aerospace components and surfaces. Other aerospace components that may benefit from the teachings herein include, but are not limited to, fasteners such as pins, bolts, collars, nuts and nut plates, washers, and studs. For example, non-fastener applications such as latches, helicopter rotors, and landing gear structures may also benefit from the teachings herein. The coated aircraft components, such as fasteners, are compatible with aluminum and CFRP structures.

[0011] Thus, one aspect of the present invention provides a coating system for a component made from a base metal, the coating system including a conductive layer on the base metal and a resin-based layer including a conductive pigment superimposed on the conductive layer.

[0012] In one example, a coating system for a metal part made from a base metal includes a metal flash layer on at least a portion of the metal of the part, such as, for example, on a titanium aerospace part, and a resin-based layer on the metal flash layer. In another example, the coating system includes a nickel flash layer between the base metal and the resin-based layer, such as, for example, on a titanium part. In a further example, the coating system includes a metal flash layer on the metal part, such as, for example, on a titanium aerospace part, and metal fibers in the resin-based layer over the metal flash layer. In another example, the coating system includes a metal flash layer on the metal part and a resin-based layer over the metal flash layer that includes a conductive pigment. In one example of a metal flash layer on a metal part and a resin-based layer over the metal flash, the metals used for the metal flash layer and the conductive pigment include the same metal, and in one example, the metal is nickel.

[0013] In a further example of a coating system for a metal part made from a base metal, the coating system includes a metal flash layer on at least a portion of the metal of the part and a pigmented resin-based layer on the metal flash layer. In another example, a coating system for a metal part made from a base metal includes a metal flash layer on at least a portion of the metal surface of the part, such as on a titanium aerospace fastener, and a resin-based layer having a metal pigment on the metal flash layer. In one example of such a coating system having a metal flash layer and a metal pigment in the resin-based layer, the two metals are the same, and in another example, the two metals are nickel. In a further example of such a coating system having a metal flash layer and a metal pigment in the resin-based layer, the metal pigment is nickel fiber, and in a further example of the metal pigment, the metal pigment is present in a solution at a concentration of between about 5% and about 15% by weight before drying, and in another example, about 11% by weight. In another example of such a coating system having a metal flash layer and a metal pigment in the resin-based layer, the metal pigment is formed from metal fibers or filaments that extend in a direction other than perpendicular to the surface of the metal aerospace part, and in a further example, the metal pigment is aggregated and randomly distributed in the resin-based layer. In any of the foregoing examples of a metal flash layer on a metal part and a resin-based layer that includes an electrically conductive component across the metal flash, in one example, the metal in the resin-based layer is randomly distributed in an electrically conductive three-dimensional network.

[0014] In another aspect, a part made from a base metal includes thereon a coating system, including any of the coating systems described herein, The coating system includes a conductive layer on the base metal and a resin-based layer including a conductive pigment on the conductive layer.

[0015] In yet another aspect, a method is provided for coating a component made from a base metal with a coating system, the method comprising any of the coating systems described herein. The method includes the steps of providing a metal component, depositing a conductive layer on a surface of the metal component, depositing a liquid mixture comprising an electrically conductive pigment dispersed in a resin onto the conductive layer, and drying the liquid mixture such that the conductive pigment forms an electrically conductive 3D network in the resin, and such network is randomly distributed in the resin.

[0016] The aspect of the present invention directed to coating systems containing organic resins differs from cadmium, sulfuric acid anodized, and pure aluminum vapor deposition coatings because it is a pure metal deposit and does not contain any organic resins. The present invention provides improved electrical conductivity properties compared to aluminum pigmented resin-based coatings.

[0017] The coating system of the present invention is chromate-free and electrically conductive enough to meet some electrical grounding and electrical bonding applications.The coating system of the present invention is compatible with all-metal structures, CFRP structures, or a combination of both (so-called hybrid structures).The coating system of the present invention can be used on metal fasteners for interference fit applications.

[0018] These and other aspects and advantages of the present invention will become apparent from the following detailed description and accompanying drawings, which illustrate, by way of example, the features of the invention. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a side view of a partially exploded structure in schematic form including a fastener assembly including a fastener according to an embodiment consistent with principles described herein.

[0020] [Figure 2] FIG. 2 is a side elevational view of the stud of the fastener assembly of FIG.

[0021] [Figure 3] 3 is a longitudinal cross-sectional view of a sleeve that can be substituted as part of the stud and fastener assembly of FIG. 1 according to an embodiment consistent with principles described herein.

[0022] [Figure 4] FIG. 4 is a detailed view of a portion of the stud and sleeve of FIG.

[0023] [Figure 5] FIG. 5 is a further detailed view of the fastener stud of FIGS.

[0024] [Figure 6A] 6A-6D are images of high resolution scans of components of embodiments described herein. [Figure 6B] 6A-6D are images of high resolution scans of components of embodiments described herein. [Figure 6C] 6A-6D are images of high resolution scans of components of embodiments described herein. [Figure 6D] 6A-6D are images of high resolution scans of components of embodiments described herein.

[0025] [Figure 7] FIG. 7 is a flow diagram of a method for coating an aerospace part according to an embodiment consistent with principles described herein.

[0026] [Figure 8] FIG. 8 is a plot of resistivity as a function of pigment concentration on coordinates of resistivity (milliohms) and concentration (weight percent) for fasteners having a nickel flash layer and a nickel-pigmented resin layer according to an embodiment consistent with principles described herein.

[0027] [Figure 9]FIG. 9 is an interval plot of resistivity (in milliohms) for various coatings, including a comparative coating of the present invention, according to an embodiment consistent with principles described herein. DETAILED DESCRIPTION OF THE INVENTION

[0028] Detailed Description As used herein, the articles "a" and "an" are intended to have their ordinary meaning in the art of the present invention, i.e., "one or more." For example, "an aerospace part" means one or more aerospace parts, and thus, "the aerospace part" means "aerospace part(s)" herein. Additionally, any reference herein to "top," "bottom," "upper," "lower," "upper," "lower," "front," "rear," "first," "second," "left," or "right" is not intended to be limiting. The terms "about" or "approximately," when used herein, generally refer to variations in the amount of a component due to the tolerance of the device used to produce that value, such as layer thickness, or, in the case of a composition, minor measurement errors, or may mean plus or minus 10% unless otherwise clearly specified. Furthermore, the term "substantially," as used herein, means a majority, or nearly all, or all, or an amount in the range of about 51% to about 100%. Additionally, the examples herein are intended to be merely illustrative and are presented for discussion purposes, and are in no way limiting.

[0029] During aircraft design and construction, OEMs can select fastener surface treatments that fulfill several technical functions. Some of these functions may include low coefficient of friction properties, such as to allow high-interference interference-fit fasteners to be inserted with less force and minimize galling; corrosion prevention; promotion of electrical conductivity; promotion of exterior paint adhesion; and / or resistance to chemicals and fluids. Currently, no solution provides that combination of properties without the use of chromates and is compatible with metal or full CFRP (carbon fiber reinforced polymer) or CFRP / metal hybrid structures.

[0030] Past metal coating deposits have used chromates for corrosion protection. Chromates are known carcinogens, mutagens, and are toxic to the reproductive system, raising environmental and health concerns. Alternative coating systems may be based on organic resins that are inherently electrically insulating, but the electrical conductivity of the resin can be increased by the addition of metallic pigments. However, organic coatings are limited in the amount of conductive elements that can be added due to formulation instability or detrimental effects on other properties, such as friction. There are no known industrially pigmented organic resin systems that are electrically conductive and capable of meeting the electrical bonding and electrical grounding applications intended for aerospace fasteners.

[0031] According to embodiments of the principles described herein, the solution disclosed herein is a coating system for an aerospace part that combines a conductive layer with a resin-based layer containing a conductive pigment on the conductive layer to meet electrical grounding and electrical bonding applications, as well as an aerospace part coated with such a coating system. Note that this does not imply that only the first layer is electrically conductive and the second layer is not. While both layers are electrically conductive, they are not necessarily equally conductive, and together they conspire to provide a desired electrical resistivity compatible with electrical grounding and / or electrical bonding.

[0032] The aerospace part may be any of the aerospace parts listed above, including, for example, a fastener. The aerospace part may comprise a metal selected from the group consisting of titanium or a titanium alloy, such as Ti-6Al-4V, a stainless steel, such as A286, and a nickel-chromium based superalloy, such as Inconel® 718.

[0033] Examples of fasteners suitable for use in practicing embodiments herein are illustrated in Figures 1 to 5. As shown in Figure 1, fastener assembly 100 includes fastener 110.

[0034] By way of example, the coating construction can be implemented using fasteners 110 of a type commonly used in aircraft structure, such as a typical threaded bolt or stud 112 and threaded nut 114 used in combination, having, for example, an external coating system 500 as described herein. (Details of the coating system are shown schematically in FIG. 5 , with it being understood that one or more of the illustrated fastener elements have some or all of their surfaces coated as described herein, but that the coating(s) are not visible in FIGS. 1-4 for ease of illustration.) With reference to FIGS. 1 and 2 , bolt 112 includes a shank 116 and a head 118 at one end of the shank and a threaded portion 120 at the other end of the shank. All of the bolts are made from solid metal, which may be of the type previously mentioned in embodiments of the present invention, and all or a portion of the entire surface of the bolt and nut may be coated with coating system 500 as described herein. Coating system 500 can also provide a lubricating effect to reduce galling effects between the respective threads of bolt 112 and nut 114. As further shown in Figure 1, fastener 100 can be used to fasten together two components of an aircraft structure, such as first component 124 and second component 126 having aligned openings.

[0035] An assembly 100 of a stud or bolt 112 and a mating part 114, such as that illustrated in FIG. 1, or a fastener having a pulling stem and collar (not shown), is a common fastener assembly for some applications. In another alternative fastener assembly, the stud 112 can pass axially through a sleeve 122 (FIGS. 3-4), which is configured to fit into aligned openings in structures 124 and 126 with only a clearance fit, for example, while the stud 112 is configured with the sleeve for an interference fit. Such an arrangement, with a nut 114 or other fastening part (not shown), can be suitable for use in composite structures, or composite and metal structures to be fastened together. In the arrangement illustrated in FIGS. 3-4, the head 118 of the stud 112 fits into a flared end 128 of the sleeve 122, providing a flush mounting, although other stud and sleeve arrangements can alternatively be used. The sleeve material and sleeve geometry relative to the stud may be conventional, except that the coatings described herein may be used on one or more of the surfaces of the sleeve.

[0036] All exterior and interior surfaces or one or more parts of one or more fastener components or metal components, including those described herein, can be coated with a coating system 500 that helps provide one or more of the benefits described herein, including, but not limited to, electrical conductivity, corrosion resistance, or reduced galvanic interaction between materials. The components illustrated in Figures 1-4 can be understood to include coatings described herein on one or more surfaces, although the representation of the coatings has been omitted from those figures for clarity. An example coating system is illustrated schematically in Figure 5, with the representation of the coating system elements enlarged for clarity. Characteristics of exemplary coating systems, including the chemical composition, thickness, and other physical characteristics of the coating systems and their components, can be understood from the discussion herein. Figure 5, an enlarged view of bolt 112, illustrates an example coating system having two layers 502, 504 applied to the exterior surface of bolt 112. In one example, the first layer 502, also referred to as the conductive layer, is a highly conductive metal layer applied to a metal surface of a metal part, in this embodiment of the invention, one or more metal surfaces of the stud 112, nut 114, and / or sleeve 122, and the outer surface of the bolt as illustrated in FIG. 5 . Typically, the conductive layer is a material different from the material of the base metal part. "Highly conductive," as used herein, means having an electrical resistivity of less than or equal to 1 mΩ. The second layer 504 of the system of FIG. 5, also referred to as the resin-based layer, is a resin composition containing conductive pigment 508. In one arrangement, the first layer 502 comprises a nickel flash layer, and the second layer 504 comprises a resin composition 506 containing conductive pigment 508, e.g., nickel fibers. These layers are shown schematically in FIG. 5 and are discussed more fully below. Although the resin composition 506 is illustrated schematically in FIG. 5 as forming a layer including conductive pigments 508, it should be understood that the resin composition identified by reference numeral 506 illustrated in FIG. 5 refers not only to the resin material but also to any other components in addition to the conductive pigments 508 that make up the resin-based layer 504.Such other components may be any of the components commonly used in binder formulations, such as corrosion inhibitors, lubricants such as polytetrafluoroethylene (PTFE), and plasticizers such as terephthalates, other polymers, etc.

[0037] The conductive layer 502 can be applied as a nickel flash or strike. The nickel flash layer can be applied by electrodeposition, although other forms of application or deposition, such as electroless, brushing, pasting, dipping, spraying, printing, PVD (physical vapor deposition), CVD (chemical vapor deposition), or IVD (ion vapor deposition), can also be used. Other metals can be used in place of nickel, including, but not limited to, copper, zinc, tin, silver, lead, tin / lead, gold, and platinum. By way of example, the nickel flash can be electrodeposited in a nickel sulfamate solution according to SAE AMS-QQ-N-290 standard.

[0038] Resin composition 506 is the binder, commonly called the vehicle, and is the actual film-forming component of second layer 504. Resin composition 506 provides adhesion, binds pigments 508 together, and influences properties such as potential gloss potential, durability, flexibility, and toughness.

[0039] The binder for the resin-based layer 504 may be a phenolic resin, which is a thermosetting resin and provides chemical resistance. Phenolic resins are desirable for fasteners because they are very hard and have durable chemical resistance (which is useful due to frequent exposure to hydraulic fluids, oils, etc.). Phenolic resins also have high abrasion resistance, which is desirable for many fastener applications, especially interference fit applications. In some embodiments, phenol-formaldehyde resins may be used.

[0040] The resin composition may contain additional components, such as a second binder, which may be thermoplastic, and / or other additives commonly used in binder formulations, such as corrosion inhibitors, lubricants such as polytetrafluoroethylene (PTFE), and plasticizers such as phthalates. In many applications, the resin composition may also contain polytetrafluoroethylene as a non-metallic pigment. PTFE serves to reduce the coefficient of friction. Other polymers that can be added to perform one or more of these functions include, but are not limited to, PEEK (polyether ether ketone), polyimide, PPS (polyphenylene sulfide), nylon and other polyamides, acetal (polyoxymethylene, POM), and polyesters, as well as variations thereof.

[0041] The conductive pigment 508 may be any material with low resistivity, including metals such as nickel, copper, silver, or aluminum, or non-metals such as molybdenum disulfide or graphene. To minimize galvanic corrosion, the conductive layer 502 and the conductive pigment 508 may be the same metal. It has been found that nickel provides more desirable electrical conductivity than other resin additives, such as silver and graphene, and that nickel fibers provide more reliable electrical conductivity than nickel platelets and nickel spheres. Fortunately, nickel, a low-resistivity metal, is an excellent combination because it retards self-corrosion and galvanic corrosion.

[0042] In some embodiments, the ratio of nickel pigment to the resin composition in the liquid state, also referred to as the liquid mixture, may range from about 5 weight percent (wt%) to less than 15 wt% of the liquid mixture before application to the metal part. In some embodiments, the maximum concentration of nickel pigment may be about 11 wt%, with about 9 wt% being preferred. When polytetrafluoroethylene is used, PTFE may range from about 1 wt% to about 10 wt% of the liquid mixture. In one embodiment, PTFE may be approximately 2 wt% of the liquid mixture. PTFE, nickel, and other components, such as corrosion inhibitors, may contribute to the CPVC (critical pigment volume concentration, see below) because they are not sintered into the coating during polymerization (the temperature is not high enough to contribute to PTFE solubility). In addition to the nickel concentration, the amount of PTFE and / or other components, if added, contributes to the CPVC.

[0043] In some embodiments, the nickel pigment concentration ranges from about 9 wt% to about 11 wt% of the liquid mixture. Below about 10 wt%, the electrical conductivity typically is not as high as may be desirable in some applications. On the other hand, in the range of about 5 wt% to about 10 wt%, the electrical resistivity is still below 10 mΩ, which may be acceptable for some applications.

[0044] At concentrations between about 14 wt% and about 15 wt% nickel pigment in the liquid mixture, the resin and pigment mixture may exceed formulation stability (known as the critical pigment volume concentration, or CPVC). The CPVC is the point at which there is enough binder present to fill the voids between the pigment particles; beyond this point, there is no longer enough binder to fill the voids. Therefore, the CPVC can limit the preferred upper limit of pigment concentration in the liquid mixture.

[0045] The conductive pigment 508 is advantageously randomly shaped nickel fibers, such as 525 Conductive Nickel Powder, commercially available from Novamet Specialty Products Corp. (Lebanon, Tenn.). Analysis of the nickel fiber-pigmented resin-based layer 504 showed that the nickel fibers exhibited disordered, three-dimensional clumps or aggregates of nickel fibers, as illustrated in FIG. 6. FIG. 6A shows energy dispersive X-ray spectroscopy (EDS) from a scanning electron microscope of a cross section of a fastener 600 having only a resin-based layer 604 pigmented with nickel fibers 602, where the nickel fibers are 22.9 wt% of the resin composition, and FIG. 6B shows a resin-based layer 604 pigmented with nickel fibers, where the nickel fibers are 30.8 wt% of the resin composition, demonstrating the disorder and clumps or aggregates. (New reference numbers have been applied to these images when referring to specific elements, which have the same characteristics as elements described herein.) These images show a portion of a titanium fastener 606 extending to the surface represented by dashed line 608, representing the variable surface shown at the present invention's magnification. The titanium surface 608 is directly coated with a resin-based layer 604 having a thickness represented by dashed line 610, representing the variable thickness extending over the titanium surface at the present invention's magnification. Material 612 outside of the resin-based layer 604 is a support for the sample, carrying the phenolic resin.

[0046] Figure 6C also shows EDS from a scanning electron microscope of a cross section of a fastener 620 having a nickel flash layer 614 deposited on the bare metal surface of the fastener. The nickel fiber-pigmented resin-based layer 604, where the nickel fibers are 30.8 wt% of the resin composition, shows a more continuous flash layer compared to the pigment distribution. The nickel flash is more evenly distributed along the bare metal base surface 608 than the nickel pigment fibers, indicating the randomness and clumps or aggregates of the nickel pigment. Figure 6D shows EDS from a scanning electron microscope of a top view of a bare fastener 630 containing only the nickel fiber-pigmented resin-based coating 604, where the nickel fibers are 22.9 wt% of the resin composition. In this scan, a portion of the fastener surface is shown through the resin-based coating 604. This figure also demonstrates the randomness and clumps or aggregates of the nickel fibers 602 in the resin-based layer 604. The nickel pigment 508 has a branch-like structure, which can be dispersed substantially randomly in the resin composition 506. The branched-like structure provides beneficial 3D aggregates that are randomly distributed in the resin composition, some of which are in contact with the conductive layer and some of which extend perpendicular to the conductive layer 502, for example, forming an electrically conductive 3D network structure.

[0047] Nickel was the best-performing material tested. It has good corrosion resistance and electrical conductivity. Nickel is a ferromagnetic material, which can be useful in some applications. Nickel pigments also showed a high degree of compatibility in liquid mixtures, such as solvent-based systems. As important as nickel is as an element, its fibrous and / or filamentous morphology allowed for acceptable coating thicknesses (5 μm to 20 μm). Nickel is highly resistant to oxidation, which allows it to maintain electrical conductivity over time, even in salt spray. These morphologies allow for penetration (network formation) and promote current flow at a percentage of nickel in the resin composition (once dried) that is low enough to be below CPVC, while still promoting electrical conductivity for electrical bonding and grounding.

[0048] The fibrous and / or filamentous nickel pigments 508 can become entangled in three dimensions when dispersed in the resin composition 506. The disordered spatial orientation and entanglement of the pigments is beneficial and useful for electrical conductivity, which is desirable for performance. When combined with nickel flash, the nickel fibers enable a pigmented coating below CPVC while still meeting electrical bonding and electrical grounding requirements.

[0049] The nickel fibers 508 may have an average length of about 20 μm, with a length ranging from about 1.4 μm to about 88 μm. The nickel fibers 508 may have an average diameter of about 2 μm, with a diameter ranging from about 0.5 μm to about 10 μm.

[0050] The total thickness of the conductive layer 502 and resin-based layer 504 combination may range from about 5 μm to about 20 μm, with the conductive layer having a thickness of less than 2.5 μm and the resin-based layer 504 having a thickness that makes up the remainder. This total thickness range is considered an offset of the coating dimensions allowed for fasteners. The thickness ranges of the conductive layer 502 and resin-based layer 504 for other aerospace parts, such as fasteners, may have somewhat different thicknesses consistent with conventional thicknesses for that use, depending on their specific use with such aerospace parts.

[0051] According to an embodiment of the principles described herein, a method is provided for coating an aerospace part 110 with a coating system 500. Figure 7 shows a flow chart of the method 700, which includes providing an aerospace part 705. The aerospace part 110 may be any of the aerospace parts listed above, including, for example, fasteners such as pins, bolts, collars, nuts and nut plates, washers, and studs. The aerospace part 110 may be formed from a base metal selected from the group consisting of titanium or a titanium alloy, such as Ti-6Al-4V, a stainless steel, such as A286, and a nickel-chromium-based superalloy.

[0052] The method 700 further includes a step 710 of depositing a conductive layer 502 on the surface of the aerospace component 100. The surface may be bare metal, an anodized surface, or a metal having a machined, blasted, or otherwise mechanically prepared surface. In one embodiment, the conductive layer 502 is a nickel flash layer deposited on the surface of the aerospace component. The nickel flash 502 may be electrodeposited on the surface of the aerospace component.

[0053] The method 700 further includes a step 715 of depositing a liquid mixture including a resin and an electrically conductive pigment 508 onto the conductive layer 502 to form a resin-based layer 504 containing the conductive pigment.

[0054] The conductive pigment 508 is suspended in a resin dissolved in a volatile solvent, giving the mixture a liquid consistency but providing rapid drying after application. For example, the pigment 508 can be milled into the resin according to conventional milling techniques.

[0055] The liquid mixture can be thoroughly and uniformly mixed in a solvent according to conventional paint mixing techniques. The solvent can be a low molecular weight alkyl alcohol, such as methyl, ethyl, propyl, or isopropyl alcohol, or a similar solvent, such as methyl ethyl ketone, or a range of volatile petroleum distillates, such as xylene or toluene, or a mixture of two or more of these solvents. The amount of solvent used can be sufficient to provide the desired fluidity, depending in part on whether the paint is applied by spraying, dipping, or brushing.

[0056] In some embodiments, the liquid mixture can be applied by spraying, although either dipping or brushing can be used instead. Because the solvent is volatile, it dries and solidifies quickly. After application, the aerospace component can be dried (720), for example, by heating, to evaporate the solvent and form a solid resin-based layer 504 with 3D aggregates of nickel fibers 508 randomly distributed in the resin composition 506. Heating can be performed according to conventional methods at a temperature and for a time sufficient to achieve the desired results, for example, by the desired crosslinking. The temperature can be between approximately 150°C and 205°C for the desired time, which can be, for example, approximately 1 hour.

[0057] The thickness of the coating system 500 once solidified on the fastener is advantageously between about 5 μm and about 20 μm. This thickness control is particularly desirable for threaded fasteners to ensure proper thread fit, and is also desirable for aircraft-quality, interference or non-interference fasteners. Interference-fit fasteners are generally fabricated to have a diameter slightly larger than the diameter of the hole in the structural member into which they are to be fastened. Forcing such fastener parts into their intended holes typically causes wear on the coated surface of the fastener part and can damage the surface of the hole and surrounding workpiece structure into which the fastener part is driven. It has been discovered that the coating system 500 applied in accordance with these inventions can lubricate the fastener part 110, avoiding coating degradation and helping to maintain adhesion of the coating to the fastener part. For example, such a coating system 500 can sustain a mounting force of 34,000 N, meeting the requirements of the EN 4473 standard.

[0058] To achieve spatially random dispersion of the conductive pigment 508 in the liquid mixture, the conductive pigment 508 can first be dosed with a surfactant, such as Anti-Terra-U, a wetting and dispersing additive that is a solution of a salt of an unsaturated polyamine amide and a low molecular weight acidic polyester available from BYK (Austin, TX), and / or EFKA 7500, a modified polyether that improves dispersion or suspension stability and is available from BASF (Florham Park, NJ). The conductive pigment 508 with surfactant, as described above, can then be dosed with the resin under mechanical shear, e.g., milling.

[0059] Because titanium is highly susceptible to hydrogen and oxygen uptake, the prior art recommends not electroplating titanium fasteners for aerospace applications. This can easily cause titanium to embrittle or lead to stress cracking. Electroplating is typically performed using chlorine-based hydrochloric acid, which is prohibited in aerospace applications. Outside of aerospace, titanium is electroplated to add solderability, improve corrosion resistance in high-temperature acidic environments, add lubricity, and improve anti-galling properties. Nevertheless, the conductive layer 502 used herein, which can utilize electroplating, is clearly short enough in duration that hydrogen and oxygen uptake is minimized or even eliminated.

[0060] Some materials, such as titanium, stainless steel, and nickel-based alloys, are naturally passivating (when an oxide-free surface comes into contact with air or water, a thin oxide film forms on the surface), making electroplating adhesion impossible without overcoming this tenacious oxide film. The plating industry has developed methods for flashing such metals, including materials such as nickel or copper, to overcome the passivating oxide layer and allow subsequent adhesion of the final electroplated layer, such as gold.

[0061] The use of a nickel flash as a base for the resin-based layer 504 is counterintuitive. Nickel flashes are typically intended to overcome passivation films to allow subsequent electroplating adhesion, and are not known to be performed prior to placing a resin-based layer on a metal-based component, such as a fastener made from titanium, stainless steel, or a nickel alloy. The nickel flash does not promote adhesion of the resin-based layer and serves no functional purpose prior to coating. Coatings are typically intended to be coated directly onto the fastener substrate, and if adhesion promotion is required, this is typically accomplished by using chemical or mechanical surface preparation, such as sandblasting or a base coat such as a primer, rather than a nickel flash.

[0062] The resin-based layer 504 may be formed on a titanium fastener. Titanium itself is a metal and provides high conductivity. The conductive layer 502, also made of metal, is highly electrically conductive. To achieve the electrical conductivity of a metallic coating, the resin-based layer typically could not match this performance without suppressing the coating's CPVC. However, when the conductive layer 502 is bonded to titanium, the combination of both layers 502 and 504 unexpectedly increases electrical conductivity, thereby reducing the electrical resistivity of the coating system 500 to a level much lower than that of the resin-based layer. The conductive layer 502 and the resin-based layer 504 are both finishes, yet combining them to achieve enhanced electrical conductivity on a metal substrate is counterintuitive. The net effect is unexpected: electrical bonding and electrical grounding with desirable high conductivity (low resistivity) while maintaining the protective effects of the resin's fuel and chemical resistance, protection from galvanic corrosion, allowing for material compatibility, and maintaining low friction to aid in high-interference installation and torque-tension.

[0063] The coating system 500 can be applied to the entire surface of the metal part, or to only a small portion. For example, if the part is a fastener assembly 100, the top surface of the head 118 can be bare, while the entire surface of the shank 116 and the threaded portion 118 can be coated with the coating system 500. In another example, the head 118 and the adjacent annular portion of the shank 116 can be bare, with the remaining surface of the shank 116 and the threaded portion 118 coated with the coating system 500. In another example, the shank 116 of the bolt and / or the inner and / or outer surfaces of the sleeve 122 can be coated in two, three, four, or more longitudinal stripes, or one, two, or more annular bands, or any other shape.

[0064] The base metal portion covered by coating system 500 may be at least 50 percent of the total surface of the part, or at least 60 percent of the total surface of the part, or at least 70 percent of the total surface of the part, or at least 80 percent of the total surface of the part.

[0065] The coating system 500 can also be coated with a low-adhesion, third, lubricious layer, which reduces the coefficient of friction of the fastener assembly during installation into a hole in the structure. For example, this third lubricious layer can be a grease, cetyl alcohol, or coating as described in U.S. Patent Application No. 16,742,274, published as US2020 / 0149566 (incorporated by reference). It has been found that such low-adhesion lubricious layers do not impair the electrical conductivity of the coating system 500, as they are at least partially peeled off during interference installation of the fastener.

[0066] The third lubricating layer can be applied to the entire surface of the metal part, or only a small portion of the surface, and can be applied to portions not covered by coating system 500, the entire surface coated with coating system 500, or a portion of the surface coated with coating system 500. [Example]

[0067] Example 1 Several titanium-based fasteners 112 were coated with the coating system 500. The fasteners were made of Ti-6Al-4V alloy. The first layer 502 was a nickel flash layer, which was formed by electroplating nickel onto the titanium using a nickel sulfamate process in accordance with SAE AMS-QQ-N-290 standard.

[0068] After depositing the nickel flash layer on the fastener, the nickel-coated surface was then abrasive blasted to promote mechanical adhesion of the resin-based layer 504. The second layer 504 was a phenol-formaldehyde resin composition 506 in which nickel pigment 508 was dispersed. The phenol-formaldehyde resin composition also contained common components such as solvents, PTFE, corrosion inhibitors, and plasticizers. The resin composition was then sprayed directly onto the abrasive-blasted nickel flash layer. The aerospace part was then heated at 204°C for 1 hour to crosslink the coating. The nickel pigment was nickel fiber 508 with an average length of 20 μm and an average diameter of 2 μm. Various concentrations of nickel fiber were used in the resin composition, as listed in Table I below. It should be noted that while the concentrations of nickel fiber in the resin composition discussed in the examples herein were based on the resin composition in its liquid (wet) state prior to polymerization, Table 1 below also lists the equivalent dry concentration by weight for purposes of considering the final product that can be used in fastener assemblies such as those illustrated in Figures 1-5.

[0069] The electrical resistivity of each coated fastener 112 was measured. The fasteners were mounted to an aluminum test specimen with a gap between them, allowing the countersunk holes in the fastener head and the test specimen to fit snugly against each other. The top of the fastener head was peeled away to create a path to the fastener, where a Kelvin probe was contacted. An electrical connector was connected to the test specimen. The resistivity was measured using an ATEQ AX6000 four-terminal milliohm meter. The results are listed in Table I and plotted in Figure 8. Column 2 is the nickel pigment concentration in the liquid mixture, column 3 is the nickel pigment concentration in the resin composition once the liquid mixture solvent had evaporated, and column 4 is the electrical resistivity of each fastener. [Table 1]

[0070] Pigment network formation began at about 6.9 wt% nickel pigment concentration in the liquid mixture. Best performance was achieved at about 11 wt% nickel pigment, where the electrical resistivity dropped to less than 1 milliohm. Although about 13 wt% nickel pigment was also below 1 milliohm, this liquid formulation exceeded what was considered desirable due to exceeding the critical pigment volume concentration.

[0071] It should be noted that the nickel pigment at a concentration of about 4.7 wt % had an electrical resistivity of 7.1 mΩ, which is still less than 10 mΩ and may be acceptable in certain applications. Example 2

[0072] A comparison was made between the fastener 112 having the aforementioned two-layer coating system 500 and other coated and uncoated fasteners. The results are listed in Table II and plotted in Figure 9. Example 2C in Table II is a coating system within the scope of the present invention. [Table 2]

[0073] For electrical bonding, the general requirement is less than 10 mΩ, and for electrical grounding, the requirement is less than 1 mΩ. The all-pure metal platings 2E and 2G do not meet either requirement, while the resin-based coating 2A is in the 35-55 mΩ range. The nickel-based resin layer 2B, containing only 11 wt% nickel fibers (concentration in the liquid mixture), averages approximately 16 mΩ. A nickel-based resin layer containing approximately 11 wt% nickel fibers 508 (concentration in the liquid mixture) combined with nickel flash 502 (Examples 2C1 and 2C2, the coating system 500 disclosed and claimed herein) drops below the 10 mΩ threshold and has an electrical resistivity similar to that of pure metal plating in the electrical grounding range. Surprisingly, a thinner nickel flash 502 layer reduces the overall electrical resistivity of the coating system 500 to less than 0.5 mΩ (Example 2C2). The SAA-treated 2D and IVD-coated 2G were considered less desirable as electrical bond and electrical ground coatings for the reasons previously described. Bare titanium 2F was unacceptable because it could lead to galvanic corrosion in aluminum structures. Nickel flash only 2E was also unacceptable for the same reasons.

[0074] Although the coating systems described herein have been described with particular reference to use as coating systems for fasteners, the coating systems are not limited to fasteners and can be applied generally to other surfaces, particularly aerospace components where low electrical conductivity, and corrosion protection and lubrication are desirable, such as titanium and titanium alloys, high-temperature tool steels, or other parts made from alloy steels and superalloys. Similarly, coatings do not always need to be applied as very thin as would typically be applied to fasteners; thicker coatings can be used in other applications.

[0075] It will be apparent from the foregoing that, while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not intended to be limited except as by the appended claims. The present invention provides, for example, the following items. (Item 1) 1. A metal part comprising a base metal and a coating system thereon, said coating system comprising: a conductive layer on the base metal; and a resin-based layer on the conductive layer wherein the resin-based layer comprises a conductive pigment that forms an electrically conductive 3D network in the resin-based layer, and such network is randomly distributed in the resin-based layer. Metal parts. (Item 2) Item 10. The metal part of item 1, wherein the coating system is applied to only a portion of the base metal of the metal part. (Item 3) 3. The metal part according to claim 1 or 2, comprising a further lubricating layer. (Item 4) 4. The metal part according to any one of the preceding items 1 to 3, wherein the base metal is selected from the group consisting of Ti-6Al-4V, A286, and nickel-chromium-based superalloys. (Item 5) 5. The metal part according to any one of the above items 1 to 4, which is a fastener. (Item 6) 6. The metal part according to any one of items 1 to 5 above, wherein the conductive layer is a nickel flash layer. (Item 7) 7. The metal part according to any one of items 1 to 6, wherein the conductive pigment in the resin-based layer contains nickel fibers, and the resin-based layer contains a phenolic resin. (Item 8) 8. The metal part according to item 7, wherein the nickel fibers have a length in the range of about 1.4 μm to about 88 μm and a diameter in the range of about 0.5 μm to about 10 μm. (Item 9) 8. The metal part according to item 7, wherein the resin-based layer has a concentration of the nickel fibers in the range of about 10 wt% to less than 35 wt%. (Item 10) 10. The metal part according to any one of the preceding items 1 to 9, wherein the coating system has a total thickness in the range of about 5 μm to about 20 μm, the conductive layer has a thickness of less than or equal to 3 μm, and the resin-based layer has a thickness that constitutes the remainder. (Item 11) 1. A method for coating a metal part comprising a base metal, comprising: providing the metal part; depositing a conductive layer on a surface of the metal component; depositing a liquid mixture containing an electrically conductive pigment dispersed in a resin onto the conductive layer; drying the liquid mixture to form a resin-based layer, such that the conductive pigments form an electrically conductive 3D network in the resin-based layer, such network being randomly distributed in the resin-based layer; A method comprising: (Item 12) Item 12. The method of item 11, wherein the conductive layer is a nickel flash deposited on the surface of the metal part. (Item 13) Item 13. The method of claim 12, wherein the nickel flash is electrodeposited onto the surface of the metal component. (Item 14) 14. The method according to any one of the preceding items 11 to 13, wherein the liquid mixture is deposited on the conductive layer by spraying. (Item 15) 15. The method according to any one of the preceding items 11 to 14, wherein the pigment is nickel fiber, and the concentration of the nickel fiber in the liquid mixture is within the range of about 5 wt% to less than 15 wt%.

Claims

1. 1. A metal part comprising a base metal and a coating system thereon, said coating system comprising: a conductive layer on the surface of the base metal; and a resin-based layer on the conductive layer the resin-based layer comprises an electrically conductive pigment dispersed in the resin and forming an electrically conductive 3D network structure in the resin-based layer, the network structure being randomly distributed in the resin-based layer. Metal parts.

2. The metal component of claim 1 , wherein the coating system is applied to only a portion of the base metal of the metal component.

3. 3. The metal part of claim 1 or claim 2, comprising a further lubricating layer.

4. The metal part according to any one of the preceding claims 1 to 3, wherein the base metal is selected from the group consisting of Ti-6Al-4V, A286, and nickel-chromium based superalloys.

5. The metal part according to any one of claims 1 to 4, which is a fastener.

6. The metal part according to any one of claims 1 to 5, wherein the conductive layer is a nickel flash layer.

7. The metal part according to any one of claims 1 to 6, wherein the electrically conductive pigment of the resin-based layer comprises nickel fibers, and the resin-based layer comprises a phenolic resin.

8. The metal part of claim 7, wherein the nickel fibers have a length in the range of 1.4 μm to 88 μm and a diameter in the range of 0.5 μm to 10 μm.

9. The metal part of claim 7, wherein the resin-based layer has a concentration of the nickel fibers in the range of 10 wt% to less than 35 wt%.

10. 10. The metal part according to any one of the preceding claims, wherein the coating system has a total thickness in the range of 5 μm to 20 μm, the conductive layer has a thickness of 1 μm to 3 μm, and the resin-based layer has a thickness that makes up the remainder.

11. 1. A method for coating a metal part comprising a base metal, comprising: providing the metal part; depositing a conductive layer on a surface of the metal component; depositing a liquid mixture containing an electrically conductive pigment dispersed in a resin onto the conductive layer; drying the liquid mixture to form a resin-based layer, such that the electrically conductive pigments form an electrically conductive 3D network in the resin-based layer, such network being randomly distributed in the resin-based layer; A method comprising:

12. The method of claim 11 , wherein the conductive layer is a nickel flash deposited on the surface of the metal component.

13. The method of claim 12 wherein the nickel flash is electrodeposited onto the surface of the metal component.

14. The method according to any one of the preceding claims 11 to 13, wherein the liquid mixture is deposited on the conductive layer by spraying.

15. 15. The method according to any one of claims 11 to 14, wherein the electrically conductive pigment is nickel fiber, and the concentration of nickel fiber in the liquid mixture is in the range of 5 wt% to less than 15 wt%.

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