Methods for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite

Abstract

A method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite is disclosed, the method comprising obtaining a fibre-reinforced polymer matrix composite, FRPMC, comprising fibres embedded within a polymer matrix, and ablating a first surface of the polymer matrix to expose a portion of the fibres at the first surface. The method then further comprises depositing metal onto the ablated first surface to embed the exposed portion of the fibres within a metal matrix to form an integrated metallic surface layer. A second method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite is also discloses, the method comprising obtaining a metallised fibre surfacing veil and at least partially embedding the metallised fibre surfacing veil into a first surface of a FRPMC comprising fibres embedded within a polymer matrix.

Description

[0001] Methods for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite

[0002] Field of the invention

[0003] The present disclosure relates to a fibre-reinforced polymer matrix composite comprising an integrated metallic surface layer, and methods for forming the integrated metallic surface layer on a fibre-reinforced polymer matrix composite, in particular for forming a graded interface between a metallic layer and a fibre-reinforced polymer matrix composite.

[0004] Background

[0005] Fibre-reinforced polymer matrix composites (FRPMCs) are commonly used in a variety of applications. However, FRPMCs often have poor resistance to surface damage, including erosion and degradation.

[0006] The aerospace industry is one example where FRPMCs, such as carbon fibre polymer composites, may be used, for example for aerodynamic structures. Another example is in wind turbine construction. However, materials for both applications need to withstand corrosion, abrasion, weather, UV radiation, erosion, and temperature variations in the most extreme conditions.

[0007] As such, there is a need to provide FRPMCs with improved surface properties.

[0008] Whilst surface coatings are commonly used to improve surface properties of materials, a poor interface between the FRPMC and surface coating can cause the surface coating to fail and delaminate from the composite. As a result, surface layers or coatings often require strong adhesive or mechanical fastenings, such as bolts, to reduce delamination. However, points of mechanical fastening and / or adhesive are also prone to material failure.

[0009] As such, there is a need to provide fibre-reinforced polymer matrix composites with enhanced surface properties, and methods of manufacture for such.

[0010] Summary of the invention

[0011] Aspects of the invention are as set out in the independent claims and optional features are set out in the dependent claims. Aspects of the invention may be provided in conjunction with each other and features of one aspect may be applied to other aspects.

[0012] In a first aspect of the invention, there is provided a method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite. The method comprises obtaining a fibre-reinforced polymer matrix composite (FRPMC), wherein the FRPMC comprises fibres embedded within a polymer matrix. The method then comprises ablating a first surface of the polymer matrix to expose a portion of the fibres at the first surface, and depositing metal onto the ablated first surface to embed the exposed portion of the fibres within a metal matrix.

[0013] Depositing metal onto the ablated first surface may advantageously enhance the surface properties of the composite, for example wherein the metallic surface layer may improve erosion and / or corrosion performance of the composite.

[0014] By embedding fibres spanning the polymer matrix and the metal matrix, this method may also advantageously obtain a graded interface between the deposited metal surface layer and the polymer matrix of the FRPMC. The graded interface may advantageously provide enhanced adhesion between the metallic surface layer and FRPMC relative to conventional surface coatings with non-graded interfaces.

[0015] The integrated and / or graded interface may improve adhesion performance and reduce failure between the metallic surface layer and FRPMC by at least one of:

[0016] • reducing the mismatch between the coefficient of thermal expansion of the surface coating material (e.g. the metallic surface material) and the FRPMC substrate;

[0017] • reducing the mismatch between the Young’s modulus of the surface coating material and the FRPMC substrate;

[0018] • increasing the surface area for adhesion of the metallic surface layer by exposing fibres; and

[0019] • encapsulating the fibres in metal to enable adhesion of further metallic layers.

[0020] The graded interface therefore improves functionality of FRPMC materials by enhancing wear and erosion resistance, increasing durability, and extending the use conditions to more demanding environments, such as but not limited to higher and lower temperature environments, environments experiencing rapid temperature changes, and / or environments experiencing sand, water, and / or ice impact challenges.

[0021] In some examples, the fibres of the FRPMC may be carbon (graphite), glass, aramid, metal, natural fibres, or any other fibre. The polymer matrix may be a thermoset polymer or resin, thermoplastic polymer or resin, or any other polymer or resin.

[0022] Ablating the first surface of the polymer matrix may be achieved by, but is not limited to, at least one of thermal ablation, chemical ablation, laser ablation, plasma ablation, removal with solvents, or other forms of ablation.

[0023] Ablating the surface of the polymer matrix may comprise ablating the surface of the polymer matrix to a pre-determined depth. For example, the pre-determined depth of ablation may be, but is not limited to, between 10 and 100 microns. In some embodiments, the pre-determined depth of ablation may be approximately 25 microns.

[0024] The method may further comprise obtaining a fibre surfacing veil and embedding the fibre surfacing veil into a first surface of the polymer matrix prior to ablating the first surface. Ablating the first surface of the polymer matrix may then be configured to expose a portion of fibres from the fibre surfacing veil at the first surface. Providing the fibre surfacing veil may be advantageous as it can be added to a surface of a prefabricated FRPMC, and thereby reduce ablation of the prefabricated FRPMC itself, rather only ablating the polymer matrix which at least partially embeds the fibre surfacing veil to adhere it to the prefabricated FRPMC. A fibre surfacing veil may also be included in the surface of a FRPMC at the time of manufacture to create are more even surface.

[0025] Preferably, the fibre surfacing veil is a metallised fibre surfacing veil. This may be advantageous because the metallised fibre surfacing veil may assist in providing the graded interface to provide enhanced adhesion between the metallic surface layer and FRPMC. Incorporation of a metallised veil may also improve the consistency of metal deposition onto the exposed fibres. In some examples, obtaining a metallised fibre surfacing veil comprises obtaining a fibre surfacing veil, and metallising the fibre surfacing veil by metal deposition. The metallised fibre surfacing veil may then be embedded into a first surface of the polymer matrix, as described above.

[0026] The fibre surfacing veil may be a carbon fibre surfacing veil. However, the skilled person will understand that the fibre surfacing veil is not limited to a carbon fibre surfacing veil and may, alternatively or in addition, comprise otherfibres such as glass, aramid, metal, natural fibres, or any other fibre.

[0027] Embedding the metallised fibre surfacing veil into the first surface of the polymer matrix may comprise curing the metallised fibre surfacing veil into a first surface of the polymer matrix. The curing process parameters may be defined by the properties of the polymer matrix of the FRPMC. For example, the polymer matrix may comprise a thermoplastic polymer. Alternatively, the polymer matrix may comprise a thermoset polymer.

[0028] Curing the metallised fibre surfacing veil into the first surface of the polymer matrix may additionally comprises applying pressure to the metallised fibre surfacing veil and the polymer matrix. This may advantageously improve adhesion between the polymer matrix and the metallised fibre surfacing veil, for example by promoting penetration of the polymer matrix, such as a resin, within the metallised fibre surfacing veil. However, the skilled person will understand that this is not essential and may depend on the composite system and its normal curing process.

[0029] The skilled person will understand that, in some examples, the obtained FRPMC may already comprise a metallised fibre surfacing veil embedded into a first surface of the polymer matrix, for example wherein ablating the first surface of the polymer matrix may be configured to expose a portion of fibres from the metallised fibre surfacing veil at the first surface.

[0030] The method may further comprise depositing a second metal layer onto the metal matrix, wherein the second metal layer is configured to provide desired surface properties. Metal may be deposited on the ablated surface by at least one of (i) electroless deposition, and (ii) electrodeposition. In some examples, depositing metal on the ablated surface comprises a first electroless deposition process, and a second electrolytic deposition process, wherein the electrolytic deposition process is performed after the first electroless deposition process. Metal deposition by electroless deposition may be advantageous to achieve better penetration of metal within the exposed portion of fibres and resulting fibre bundles. By contrast, metal deposition by electrodeposition may be advantageous to deposit and build metal layers onto the outer surface of the exposed portion of fibres. Thus, providing a first electroless deposition process and a second electrolytic deposition process may achieve a synergistic effect to create a metal gradient across the integrated metallic surface layer, for example wherein metal penetrates the exposed portion of fibres and the metal volume fraction increases towards the outer surface of the integrated metallic surface layer.

[0031] Surface processing steps may also be carried out to improve adhesion of the deposited metal to the fibres and the polymer matrix. Examples of suitable surface processing steps may include, but are not limited to, at least one of surface roughening, grit blasting, laser ablation, chemical etching (for example using permanganates), and plasma ablation.

[0032] Metal may occupy up to a 90 % volume fraction of the integrated metallic surface layer on the FRPMC, for example wherein at least a 10 % volume fraction of the integrated metallic surface layer on the FRPMC may comprise fibres and / or polymer matrix. In some examples, metal may occupy from a 15 % volume fraction of the integrated metallic surface layer, for example wherein metal may occupy from a 15 % to a 90% volume fraction of the integrated metallic surface layer on the FRPMC. This may be advantageous as too little metal integrated with the fibres may not create sufficient reinforcement, however too much metal integration (i.e. not enough fibres) may create a material that is too stiff and may reduce the adhesion performance of the metallic surface layer.

[0033] The portion of exposed fibres at the ablated first surface may occupy from a 12.5 % volume fraction of the integrated metallic surface layer. For example, the exposed fibres may result from a fibre surfacing veil, wherein the fibre surfacing veil comprises at least a 12.5 % volume fraction of fibres, such as carbon fibres, within the fibre surfacing veil. Optionally, the fibres, such as carbon fibres, may occupy approximately a 20 % volume fraction of the integrated metallic surface layer and / or fibre surfacing veil.

[0034] In some examples, the portion of metallised fibres may occupy from a 35 % volume fraction of the integrated metallic surface layer. The volume fraction of metallised fibres may be defined as the volume fraction of fibres and metal. For example, the volume fraction of metallised fibres may comprise the volume fraction of exposed fibres at the ablated first surface and the volume fraction of the metal matrix deposited onto the fibres. Purely for illustration, a 35 % volume fraction of metallised fibres may comprise a 20 % of volume fraction of fibres, such as carbon fibres, and a 15 % volume fraction of metal deposited onto the fibres. Alternatively, the volume fraction of metallised fibres may comprise the volume fraction of the obtained metallised fibre surfacing veil exposed at the first surface, and optionally the volume fraction of the metal matrix embedding the metallised fibre surfacing veil. In some examples, the portion of metallised fibres may occupy at least a 50 % volume fraction of the integrated metallic surface layer. Optionally, the portion of metallised fibres may occupy an average volume fraction of approximately a 50 % of the integrated metallic surface layer.

[0035] The volume fraction of metal may increase towards the outer surface of the integrated metallic surface layer of the FRPMC, for example such that the volume fraction of metal is configured to be graded across the integrated metallic surface layer.

[0036] In embodiments comprising depositing a second metal layer onto the metal matrix, the second metal layer may comprise a volume fraction of 100 % metal. In some examples, the volume fraction of metal is configured to provide a gradient of metal volume fraction from up to 100 % metal on the outer surface to the bulk polymer matrix on the inside of the FRPMC.

[0037] The volume fraction of fibres may be constant across the integrated metallic surface layer. However, a variable volume fraction of metal matrix may provide a gradient of metal, for example wherein the volume fraction of metal is greater towards the outer surface of the integrated metallic surface layer, and the volume fraction of polymer matrix increases towards the bulk polymer matrix on the inside of the FRPMC. In another aspect of the invention, there is provided an alternative method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite. The method comprises obtaining a metallised fibre surfacing veil and at least partially embedding the metallised fibre surfacing veil into a first surface of a fibre-reinforced polymer matrix composite, FRPMC, wherein the FRPMC comprises fibres embedded within a polymer matrix. The at least partially embedded metallised fibre surfacing veil and FRPMC form a stack.

[0038] As with the preceding aspect, the metallised fibre surfacing veil may advantageously assist in providing the integrated interface to provide enhanced adhesion between a metallic surface layer and the FRPMC. Incorporation of a metallised veil may also improve the consistency of metal deposition onto the exposed fibres.

[0039] As such, the method may further comprise depositing metal onto an outer surface of the stack, the outer surface comprising the at least partially embedded metallised fibre surfacing veil.

[0040] Depositing metal onto the outer surface of the stack may be configured to form a metallic surface layer, wherein the metallic surface layer is configured to provide desired surface properties.

[0041] The metallised fibre surfacing veil may be partially exposed at the outer surface of the stack. As such, depositing metal onto the outer surface of the stack may be configured to embed the exposed portion of the metallised fibre surfacing veil within a metal matrix.

[0042] Surface processing steps may also be carried out to improve adhesion of the deposited metal to the outer surface of the stack. Examples of suitable surface processing steps may include, but are not limited to, at least one of surface roughening, grit blasting, laser ablation, chemical etching (for example using permanganates), and plasma ablation.

[0043] Metal may be deposited onto the outer surface of the stack by at least one of (i) electroless deposition, and (ii) electrodeposition. In some examples, depositing metal on the ablated surface comprises a first electroless deposition process, and a second electrolytic deposition process, wherein the electrolytic deposition process is performed after the first electroless deposition process. Metal deposition by electroless deposition may be advantageous to achieve better penetration of metal within the exposed portion of fibres and resulting fibre bundles than electrodeposition. By contrast, metal deposition by electrodeposition may be advantageous to deposit and build metal layers onto the outer surface of the exposed portion of fibres. Thus, providing a first electroless deposition process and a second electrolytic deposition process may achieve a synergistic effect to create a metal gradient across the integrated metallic surface layer, for example wherein metal penetrates the exposed portion of fibres, and the metal volume fraction increases towards the outer surface of the integrated metallic surface layer.

[0044] In some examples, at least partially embedding the metallised fibre surfacing veil into the first surface of the FRPMC comprises curing the metallised fibre surfacing veil at least partially into the first surface of the FRPMC polymer matrix. The curing process parameters may be defined by the properties of the polymer matrix of the FRPMC. For example, the polymer matrix may comprise a thermoplastic polymer. Alternatively, the polymer matrix may comprise a thermoset polymer.

[0045] Curing the metallised fibre surfacing veil into the first surface of the polymer matrix may additionally comprises applying pressure to the metallised fibre surfacing veil and the polymer matrix. This may advantageously improve adhesion between the polymer matrix and the metallised fibre surfacing veil.

[0046] Obtaining the metallised fibre surfacing veil may comprises obtaining a fibre surfacing veil, and metallising the fibre surfacing veil by metal deposition.

[0047] The fibre surfacing veil may be a carbon fibre surfacing veil. However, the skilled person will understand that the fibre surfacing veil is not limited to a carbon fibre surfacing veil and may, alternatively or in addition, comprise otherfibres such as glass, aramid, metal, natural fibres, or any other fibre.

[0048] The method may further comprise ablating an outer surface of the stack to expose a portion of the embedded metallised fibre surfacing veil from the FRPMC, and depositing metal onto the outer surface of the stack to embed the exposed portion of the embedded metallised fibre surfacing veil within a metal matrix.

[0049] The metallised fibres within the metallised fibre surfacing veil may occupy up to a 90 % volume fraction within the metallised fibre surfacing veil. In some examples, the metallised fibres within the metallised fibre surfacing veil may occupy from a 35 % volume fraction within the metallised fibre surfacing veil. For example, the metallised fibres within the metallised fibre surfacing veil occupy at least a 50 % volume fraction within the metallised fibre surfacing veil. Preferably, the portion of metallised fibres may occupy an average volume fraction of approximately 50 % of the integrated metallic surface layer.

[0050] The volume fraction of metallised fibres within the metallised fibre surfacing veil may be defined as the volume fraction of fibres and metal. Purely for illustration, a 35 % volume fraction of metallised fibres may comprise a 20 % of volume fraction of fibres, such as carbon fibres, and a 15 % volume fraction of metal deposited onto the fibres. The remaining volume fraction within the obtained metallised fibre surfacing veil may be empty space or air. The empty volume fraction within the metallised fibre surfacing veil may then be saturated by (i) polymer matrix to at least partially embed the metallised fibre surfacing veil into the FRPMC, and / or (ii) metal matrix during metal deposition on the stack surface.

[0051] In some examples, the fibres, such as carbon fibres, may occupy from a 12.5 % volume fraction of the fibre surfacing veil. Optionally, the fibres, such as carbon fibres, may occupy approximately a 20 % volume fraction of the fibre surfacing veil.

[0052] The volume fraction of metal may increase towards the outer surface of the stack, for example such that the volume fraction of metal is configured to be graded across the integrated metallic surface layer.

[0053] In embodiments comprising depositing a second metal layer onto the metal matrix, the second metal layer may comprise a volume fraction of metal of 100 %. In some examples, the volume fraction of metal is configured to provide a gradient of metal volume fraction from 100 % metal on the outer surface to the bulk polymer matrix on the inside of the FRPMC.

[0054] The volume fraction of fibres from the fibre surfacing veil may be constant across the integrated metallic surface layer. However, the volume fraction of metal matrix may provide a gradient of metal volume fraction, for example wherein the volume fraction of metal is greater towards the outer surface of the stack and the volume fraction of polymer matrix increases towards the bulk polymer matrix on the inside of the FRPMC.

[0055] In another aspect of the invention, there is provided a fibre-reinforced polymer matrix composite, FRPMC, comprising a metallic surface layer, obtained by the method of any of the preceding aspects of the invention.

[0056] In another aspect of the invention, there is provided a fibre-reinforced polymer matrix composite, FRPMC, comprising a polymer matrix comprising a first portion of embedded fibres, and a second layer comprising a second portion of fibres, wherein the second portion of fibres are partially embedded within the polymer matrix and partially embedded within a metal matrix. As such, the second layer is configured to form an integrated metallic layer on a surface of the polymer matrix.

[0057] The second portion of fibres partially embedded within the polymer matrix and partially embedded within a metal matrix may advantageously provide an integrated interface between the metal matrix and the polymer matrix of the FRPMC. The integrated interface may advantageously provide enhanced adhesion, not least by increasing the surface area for adhesion of the metallic matrix to the second portion of fibres. In addition, encapsulating the fibres in the metal matrix may advantageously enable deposition of further metallic layers onto the metal matrix, having improved adhesion by virtue of the metal-metal interface.

[0058] The fibre-reinforced polymer matrix composite, FRPMC, may further comprise a third metallic layer. The third metallic layer may be arranged adjacent to the metal matrix of the second layer. The third metallic layer may be configured to provide desired surface properties. Depositing a third metallic layer onto the metal matrix may advantageously provide enhanced adhesion of the third metallic layer due to a similar metallic interface of between the third metallic layer and metal matrix.

[0059] The second layer may be configured to provide a graded interface between the polymer matrix and the third metallic layer. The graded interface may advantageously provide enhanced adhesion between the third metallic layer and the polymer matrix of the FRPMC, for example relative to conventional surface coatings with non-graded interfaces.

[0060] Optionally, the second layer may comprise a fibre surfacing veil, such that the second portion of fibres are provided by the fibre surfacing veil. For example, the second layer may comprise a metallised fibre surfacing veil.

[0061] Metal may occupy up to approximately a 90 % volume fraction of the second layer. The volume fraction of metal may increase towards the outer surface of the second layer, for example such that the volume fraction of metal is configured to be graded across the integrated metallic surface layer.

[0062] In embodiments comprising a third metal layer, the third metal layer may comprise a volume fraction of metal of 100 %. In some examples, the volume fraction of metal in the second layer is configured to provide a gradient of metal volume fraction from up to 100 % metal in the third layer to the bulk polymer matrix of the FRPMC.

[0063] In some examples, the portion of metallised fibres may occupy from a 35 % volume fraction of the second layer. The volume fraction of metallised fibres may be defined as the volume fraction of the second portion of fibres and metal. For example, the volume fraction of metallised fibres may comprise the volume fraction of the second portion of fibres in the second layer and the volume fraction of the metal matrix deposited onto the fibres. Purely for illustration, a 35 % volume fraction of metallised fibres may comprise a 20 % of volume fraction of fibres, such as carbon fibres, and a 15 % volume fraction of metal deposited onto the fibres. Alternatively, the volume fraction of metallised fibres may comprise the volume fraction of the obtained metallised fibre surfacing veil exposed at the first surface, and optionally the volume fraction of the metal matrix embedding the metallised fibre surfacing veil. In some examples, the portion of metallised fibres may occupy at least a 50 % volume fraction of the second layer. Optionally, the portion of metallised fibres may occupy an average volume fraction of approximately 50 % of the integrated metallic surface layer.

[0064] In some examples, the second portion fibres, such as carbon fibres, may occupy from a 12.5 % volume fraction of the second layer. Optionally, the fibres, such as carbon fibres, may occupy at least a 20 % volume fraction of the fibre surfacing veil. For example, the second portion of fibres may occupy at least a 50 % volume fraction of the second layer. Optionally, the second portion of fibres may occupy approximately a 50 % volume fraction of the integrated metallic surface layer.

[0065] The volume fraction of the second portion of fibres may be constant across the integrated metallic surface layer. However, the volume fraction of metal matrix may provide a gradient of metal volume fraction across the second layer, for example wherein the volume fraction of metal is greater towards the outer surface of the second layer, and the volume fraction of polymer matrix in the second layer increases towards the bulk polymer matrix of the FRPMC.

[0066] Drawings

[0067] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0068] Fig. 1 shows a flow diagram of an example method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite.

[0069] Fig. 2 shows a schematic illustrating an example method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite, for example such as the method of Fig. 1.

[0070] Fig. 3 shows a flow diagram of another example method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite.

[0071] Fig. 4 shows a schematic illustrating an example method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite, for example such as the method of Fig. 3.

[0072] Fig. 5 shows a plan view of a metallised fibre veil, for example for use with the method of Figs. 3 and 4, and optionally Figs. 1 and 2.

[0073] Fig. 6 shows a cross-section through a metallised veil integrated with fibre-reinforced polymer matrix composite (FRPMC).

[0074] Specific description

[0075] Embodiments of the claims relate to a fibre-reinforced polymer matrix composite comprising an integrated metallic surface layer, and methods for forming the integrated metallic surface layer on a fibre-reinforced polymer matrix composite, in particular for forming a graded interface between a metallic layer and a fibre-reinforced polymer matrix composite.

[0076] Fig. 1 shows a flow diagram 100 of an example method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite. Fig. 2 shows a schematic illustrating an embodiment of the example method of Fig. 1 for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite.

[0077] Firstly, the method comprises obtaining a fibre-reinforced polymer matrix composite 200 (FRPMC) (102). As shown in Fig. 2, the FRPMC 200 comprises a plurality of fibres 202 embedded within a polymer matrix 204. In this example, the fibres 202 are carbon fibres, however the skilled person will understand that any other suitable reinforcing fibre may be used.

[0078] Optionally, the FRPMC 200 may further comprise a fibre surfacing veil 206 arranged at a first surface 208 of the FRPMC 200 and at least partially embedded in the polymer matrix 204. A veil 206 may be included in the surface 208 of the FRPMC 200 to create are more even surface finish. In this example, the fibres within the fibre surfacing veil may occupy approximately a 50 % volume fraction of the fibre surfacing veil, however the skilled person will understand that other volume fractions may be used. The method 100 then comprises ablating the polymer matrix 204 at the first surface 208 of the FRPMC 200 (104). Ablating the polymer matrix 204 at the first surface 208 is configured to expose a portion of FRPMC fibres to a certain depth (known as the depth of ablation). In embodiments where the FRPMC 200 comprises a fibre surfacing veil 206 arranged at the first surface 208, for example as shown in Fig. 2, ablating the polymer matrix 204 at the first surface 208 is configured to expose a portion of fibres from the fibre surfacing veil 206. However, the skilled person will understand that in embodiments wherein the FRPMC 200 does not comprise a fibre surfacing veil 206, ablating the polymer matrix 204 at the first surface 208 is configured to expose a portion of the fibres 202 arranged at the first surface 208.

[0079] The ablation step 104 is configured to selectively remove the polymer matrix 204, whilst preserving the fibres 206 (and / or fibres 202). This may be achieved by thermal or chemical ablation of the polymer matrix, including but not limited to laser treatment, plasma ablation, removal with solvents, or other forms of ablation. Purely by way of example, international patent application WO2019091873A1 “SURFACE PREPARATION” discloses an example method of plasma ablation to remove a surface portion of matrix by plasma ablation so as to reveal a new surface with at least a portion of a plurality of the fibres exposed thereon.

[0080] In this example, the depth of ablation is approximately 25 pm, however the skilled person will understand that other ablation depths may be used, for example but not limited to between 10 and 100 pm.

[0081] The method 100 then comprises depositing metal onto the ablated first surface 208B to embed the exposed portion of the fibres within a metal matrix 210 (104). In this example, metal is deposited by electroless deposition, electrodeposition, or both, to create a metal matrix 210 around the revealed fibres 206.

[0082] Surface processing steps, such as but not limited to at least one of surface roughening, grit blasting, laser ablation, chemical etching (for example using permanganates), and plasma ablation, may also be carried out to improve adhesion of the metal between both the fibres 206 (or 202) and the polymer matrix 204, depending on the substrate materials. The amount of metal deposited on the exposed fibres 206 (or 202) at the ablated first surface 208B of the FRPMC 200 influences the properties of the resulting graded interface that forms. Preferably, the limit on the volume occupied by metal is up to 90 % volume fraction of the integrated metallic surface layer 212. For example, the metal matrix may occupy approximately a 50 % volume fraction of the newly formed surface layer 212.

[0083] Optionally, the method 100 comprises further deposition of metal layer 214 onto the metal matrix 210. This may be advantageous to provide desired surface properties. For example, the method may comprise depositing a nickel layer onto the metal matrix 210 for a hard, erosion resistant surface. However, the skilled person will understand that other metal layers may be used. The metal of metal layer 214 may be the same or different to the metal of the metal matrix 210.

[0084] Fig. 3 shows a flow diagram 300 of another example method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite. Fig. 4 shows a schematic illustrating an embodiment of the example method of Fig. 3 for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite.

[0085] The method 300 comprises obtaining a metallised fibre surfacing veil 404 (302). The metallised fibres within the metallised fibre surfacing veil 404 may occupy from a 35 % volume fraction up to a 90 % volume fraction within the metallised fibre surfacing veil 404. Preferably, the metallised fibres within the metallised fibre surfacing veil 404 occupy approximately a 50 % volume fraction within the metallised fibre surfacing veil 404.

[0086] As shown in Fig. 4, obtaining the metallised fibre surfacing veil 404 may comprise first obtaining a fibre surfacing veil 402, and then metallising the fibre surfacing veil 402 by metal deposition.

[0087] Purely for illustration, an obtained carbon fibre surfacing veil 402 may comprise carbon fibres approximately 6 micrometres in diameter, wherein the carbon fibres occupy approximately a 20 % volume fraction of the metallised fibre surfacing veil 404. Metallising the carbon fibre surfacing veil may comprise depositing 2 to 3 micrometres of metal to this to give each metallised fibre an approximate diameter of 10 to12 micrometres. However, the skilled person will understand that this is merely one example and other fibre surfacing veils, and metallisation processing parameters may be used.

[0088] A plan view of an example metallised fibre surfacing veil 404 is shown in Fig. 5. Fig. 5 shows metallised carbon fibres with metal penetrating through the veil structure.

[0089] The veil 404 has a sheet-like structure, similar to paper, comprising a random orientation of fibres in a first direction, as shown in Fig. 5, and a more ordered orientation of fibres on a second perpendicular direction.

[0090] Fibre surfacing veils may be specified by their mass per area, for example in grams per square metre (gsm). In this example, a 20 gsm carbon fibre veil may be used and metallised to achieve a 50 % volume fraction of metallised fibres. However, the skilled person will understand that other metallised fibre surfacing veils may also be used, such as but not limited to Kevlar, glass, aramid fibres etc, and / or fibre surfacing veils having a different mass per area. Preferably, the fibre of the fibre surfacing veil may be the as the same fibres within the underlying composite, FRPMC. This may be advantageous for property matching between the veil and the composite, such as but not limited to matched thermal expansion coefficients, however the skilled person will understand that this is not essential.

[0091] In this example, the metallised fibre surfacing veil 404 is about 40 micrometres in thickness.

[0092] The method 300 then comprises at least partially embedding the metallised fibre surfacing veil 404 into a first surface of a fibre-reinforced polymer matrix composite (FRPMC) 200 (304). In the example shown in Fig. 4, a polymer composite structure, comprising a plurality of fibres 202 and a polymer matrix, is cured onto, and hence into, the metallised fibre surfacing veil 404 by curing the polymer matrix 204. The curing process is determined by the normal curing process of the composite system, for example based on the polymer matrix. The polymer matrix may comprise, but is not limited to, a thermoplastic polymer, or thermoset polymer. The polymer matrix may be, but is not limited to, a resin. In some examples, pressure may be applied during curing and / or curing may be undertaken in a vacuum or low-pressure environment. This may be advantageous to ensure penetration of the polymer matrix, such as a resin, within the composite and into the metallised fibre surfacing veil 404. However, the skilled person will understand that this is not essential and may depend on the composite system and its normal curing process.

[0093] As shown in Fig. 4, the method may further comprise ablating the first surface of the FRPMC stack 200 comprising the metallised veil 404 to remove polymer matrix from the outer surface and expose a portion of the metallised veil 404 to a defined ablation depth, for example as described in relation to step 104 of the method of Figs. 1 and 2. However, the skilled person will understand that the ablation step may not be necessary in embodiments wherein the metallised fibre surfacing veil 404 is only partially embedded into the first surface of the FRPMC 200, i.e. wherein the metallised fibre surfacing veil 404 is already partially exposed at the outer surface of the FRPMC stack 200.

[0094] The method 300 may then comprise depositing metal onto the outer surface of FRPMC stack 200 comprising the partially exposed metallised fibre surfacing veil 404 (306). The metal deposition (306) is configured to embed the exposed portion of the metallised fibre surfacing veil 404 within a metal matrix 210, for example as described in relation to method step 106 of method 100 shown in Figs. 1 and 2. This may increase the volume fraction of metal at the outer surface.

[0095] Fig. 6 shows an example cross-section of a fibre-reinforced polymer matrix composite (FRPMC) comprising an integrated metallised veil and metallic surface layer, for example resulting from any of the method described herein. Fig 6 shows a first layer 602 comprising a polymer matrix 204 and a first portion of fibres 202 embedded in the polymer matrix 204. A second layer 604, adjacent to the first layer 602, comprises a second portion of fibres from a metallised veil 404. The composite of the first layer 602 has been cured in situ onto the metallised veil 404, as discussed in relation to the method 300 of Figs. 3 and 4. The second portion of fibres 404 are partially embedded within the polymer matrix 204 of the first layer 602 and partially embedded within a metal matrix 210. As such, the second layer 604 is configured to form an integrated metallic layer within a surface of the polymer matrix 204.

[0096] Optionally, the method 300 comprises further deposition of metal layer 214 onto the metal matrix 210. This may be advantageous to provide desired surface properties. For example, the method may comprise depositing a nickel layer onto the metal matrix 210 for a hard, erosion resistant surface. However, the skilled person will understand that other metal layers may be used. The metal of metal layer 214 may be the same or different to the metal of the metal matrix 210.

[0097] Preferably, the second layer 604 is configured to provide a graded interface between a metallic surface layer and the polymer matrix 204.

[0098] The superior adhesion between a composite and an integrated metallic surface layer, in accordance with the present invention described herein, may be demonstrated by the adhesion test data below.

[0099] The adhesion test data was obtained by conforming to testing standard ASTM D4541 for testing the pull-off strength of coatings. The adhesion test data shows a normalised improvement in adhesive strength of 3.5 times for an embedded metallised veil in the surface of a carbon fibre composite, compared to electroformed metal layer on a carbon fibre composite with no surface integration. It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed, or replaced as described herein and as set out in the claims. In the context of the present disclosure other examples and variations of the apparatus and methods described herein will be apparent to a person of skill in the art.

Claims

CLAIMS:

1. A method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite, the method comprising: obtaining a fibre-reinforced polymer matrix composite, FRPMC, wherein the FRPMC comprises fibres embedded within a polymer matrix; ablating a first surface of the polymer matrix to expose a portion of the fibres at the first surface; and depositing metal onto the ablated first surface to embed the exposed portion of the fibres within a metal matrix.

2. The method of any preceding claim wherein ablating the surface of the polymer matrix comprises ablating the surface of the polymer matrix to a pre-determined depth.

3. The method of claim 2 wherein the pre-determined depth of ablation is between 10 and 100 microns, for example wherein the pre-determined depth of ablation is approximately 25 microns.

4. The method of any preceding claim, comprising: obtaining a metallised fibre surfacing veil; and embedding the metallised fibre surfacing veil into a first surface of the polymer matrix; wherein ablating the first surface of the polymer matrix is configured to expose a portion of fibres from the metallised fibre surfacing veil at the first surface.

5. The method of claim 4 wherein obtaining a metallised fibre surfacing veil comprises: obtaining a fibre surfacing veil; and metallising the fibre surfacing veil by metal deposition.

6. The method of any claims 4 to 5 wherein embedding the metallised fibre surfacing veil into the first surface of the polymer matrix comprises curing the metallised fibre surfacing veil into a first surface of the polymer matrix.

7. The method of any preceding claim further comprising depositing a second metal layer onto the metal matrix, wherein the second metal layer is configured to provide desired surface properties.

8. The method of any preceding claim wherein metal is deposited on the ablated surface by at least one of (i) electroless deposition, and (ii) electrodeposition, optionally wherein depositing metal on the ablated surface comprises: a first electroless deposition process; and a second electrolytic deposition process, wherein the electrolytic deposition process is performed after the first electroless deposition process.

9. The method of any preceding claim wherein depositing metal onto the ablated first surface to embed the exposed portion of the fibres within a metal matrix forms an integrated metallic surface layer, and wherein metal occupies up to a 90 % volume fraction of the integrated metallic surface layer , optionally wherein metallised fibres occupy from a 35 % volume fraction of the integrated metallic surface layer, the volume fraction of metallised fibres being defined as the volume fraction of the portion of exposed fibres and metal, optionally wherein the portion of metallised fibres occupy approximately a 50 % volume fraction of the integrated metallic surface layer.

10. A method for forming an integrated metallic surface layer on a fibre-reinforced polymer matrix composite, the method comprising: obtaining a metallised fibre surfacing veil; and at least partially embedding the metallised fibre surfacing veil into a first surface of a fibre-reinforced polymer matrix composite, FRPMC, wherein the FRPMC comprises fibres embedded within a polymer matrix; wherein the at least partially embedded metallised fibre surfacing veil and FRPMC form a stack.11 . The method of claim 10 further comprising depositing metal onto an outer surface of the stack, the outer surface comprising the at least partially embedded metallised fibre surfacing veil.

12. The method of claim 11 wherein the metallised fibre surfacing veil is partially exposed at the outer surface of the stack, and wherein depositing metal onto the outer surface of the stack is configured to embed the exposed portion of the metallised fibre surfacing veil within a metal matrix.

13. The method of any of claims 11 to 12 wherein metal is deposited onto the outer surface of the stack by at least one of (i) electroless deposition, and (ii) electrodeposition.

14. The method of any of claims 11 to 13 wherein depositing metal onto the outer surface of the stack is configured to form a metallic surface layer, wherein the metallic surface layer is configured to provide desired surface properties.

15. The method of any of claims 10 to 14 wherein at least partially embedding the metallised fibre surfacing veil into the first surface of the FRPMC comprises curing the metallised fibre surfacing veil at least partially into the first surface of the FRPMC polymer matrix.

16. The method of any of claims 10 to 15 wherein obtaining a metallised fibre surfacing veil comprises: obtaining a fibre surfacing veil; and metallising the fibre surfacing veil by metal deposition.

17. The method of any claims 13 to 16 further comprising: ablating an outer surface of the stack to expose a portion of the embedded metallised fibre surfacing veil from the FRPMC; and depositing metal onto an outer surface of the stack to embed the exposed portion of the embedded metallised fibre surfacing veil within a metal matrix.

18. The method of any claims 10 to 17 wherein metallised fibres within the metallised fibre surfacing veil occupy up to a 90 % volume fraction within the metallised fibre surfacing veil, optionally wherein the metallised fibres within the metallised fibre surfacing veil occupy from a 35 % to 90 % volume fraction within the metallised fibre surfacing veil, for example wherein metallised fibres within the metallised fibre surfacing veil occupyapproximately a 50 % volume fraction within the metallised fibre surfacing veil.

19. A fibre-reinforced polymer matrix composite, FRPMC, comprising a metallic surface layer, obtained by the method of any preceding claim.

20. A fibre-reinforced polymer matrix composite, FRPMC, comprising: a polymer matrix comprising a first portion of embedded fibres; and a second layer comprising a second portion of fibres, the second portion of fibres being partially embedded within the polymer matrix and partially embedded within a metal matrix; wherein the second layer is configured to form an integrated metallic layer on a surface of the polymer matrix.

21. The fibre-reinforced polymer matrix composite, FRPMC, of claim 20, further comprising a third metallic layer, the third metallic layer being arranged adjacent to the metal matrix of the second layer, and wherein the third metallic layer is configured to provide desired surface properties.

22. The fibre-reinforced polymer matrix composite, FRPMC, of claim 21 , wherein the second layer is configured to provide a graded interface between the polymer matrix and the third metallic layer.

23. The fibre-reinforced polymer matrix composite, FRPMC, of any claims 20 to 22, wherein the second layer comprises a fibre surfacing veil, such that the second portion of fibres are provided by the fibre surfacing veil.

24. The fibre-reinforced polymer matrix composite, FRPMC, of any claims 20 to 23, wherein metal occupies up to approximately a 90 % volume fraction of the second layer, optionally wherein metallised fibres occupy from approximately a 35 % volume fraction of the second layer, wherein the volume fraction of metallised fibres comprises the volume fraction of (i) the second portion of fibres and (ii) metal, optionally wherein metallised fibres occupy at least a 50 % volume fraction of the second layer.

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