Fiber composite material and methods for its production

Abstract

Method for producing a fiber composite material, comprising: (i) Providing a granulate comprising polymer and fibers, (ii) Manufacturing a preform from the granules, wherein the preform is additively manufactured, (iii) Pyrolysis of the preform to a porous preform using a laser, and (iv) Infiltrating the porous preform with a matrix or matrix-forming material.

Description

[0001] The invention relates to a method for producing a fiber-reinforced composite material. Furthermore, the invention relates to a fiber-reinforced composite material comprising a metal or ceramic matrix and fibers and optionally particles distributed within the material, wherein the fibers are isotropically distributed.

[0002] Fiber-reinforced composites generally consist of a ceramic or metallic matrix and reinforcing fibers embedded within the matrix. Thus, they are a composite of matrix and fiber, with the fibers enclosed by the matrix. Through the mutual interaction of the two components, fiber-reinforced composites exhibit superior properties compared to either component alone. Fiber-reinforced composites can be divided into ceramic matrix composites (CMCs) and metal matrix composites (MMCs).

[0003] Ceramic fiber-reinforced composites have a ceramic matrix in which fibers are embedded, meaning the matrix is ​​reinforced by fibers, thus providing a fiber-reinforced ceramic, composite ceramic, or fiber ceramic. Metallic fiber-reinforced composites have a metallic matrix in which fibers are embedded, such as reinforced aluminum, titanium, magnesium, and copper alloys. The matrix is ​​present as elemental metal, in the form of an alloy, or as the corresponding metal carbide. Therefore, metal carbide fiber-reinforced composites are also considered metallic fiber-reinforced composites in this context. Carbon or silicon carbide fibers can be used.

[0004] Fiber-reinforced composites are usually abbreviated in the form "reinforcement phase / matrix phase". For example, "C / C" stands for carbon fiber-reinforced carbon and "C / SiC" for carbon fiber-reinforced silicon carbide. Other examples of fiber-reinforced composites are SiC / SiC and Al₂O₃ / Al₂O₃.

[0005] In the production of fiber-reinforced composites, blocks of a polymer and fibers are typically prepared and pressed into raw blocks using press tools. The pressing process generally results in fiber alignment and thus an anisotropic material. Ingots with isotropic properties can be produced by isostatic pressing, which is complex and only feasible with simple geometries. The raw blocks are then pyrolyzed, converting the polymer to carbon and volatile gas. This produces an open-pore fiber-reinforced material. This material must then be shaped into a porous preform with the desired form using a forming process, such as milling or sawing, which results in material loss. The porous preform can then be coated with a material, such as...liquid silicon or metal, are infiltrated to provide a fiber-reinforced metal carbide or silicon carbide composite material.

[0006] The infiltration of molten metals into open-porous structures is usually only carried out under high pressures and high temperatures (so-called pressure infiltration) due to low capillarity, wetting, and reactivity. For example, the production of fiber-reinforced metals (MMC) is generally performed using pressure infiltration processes. For the production of copper MMC, pressures of approximately 20 MPa and temperatures of approximately 850°C are typically applied in various pressing processes. In all the described processes, the applied pressure during the infiltration process can affect the preform or the fiber network within the material structure.

[0007] Materials can be manufactured using additive, or generative, manufacturing processes. These processes, also known as rapid prototyping, allow for the quick and cost-effective production of models, samples, prototypes, tools, and end products. This manufacturing process utilizes chemical and / or physical processes to create materials that are either shapeless (liquids, gels / pastes, powders, etc.) or have a neutral shape (ribbon, wire, or sheet-like). The three-dimensional printing of the material can be categorized into powder bed fusion, free-space fusion, liquid material fusion, and other layer-by-layer processes. Particularly noteworthy powder bed fusion processes include selective laser melting (SLM), selective laser sintering (SLS), selective heat sintering (SHS), binder jetting, and electron beam melting (EBM).

[0008] Moon et al. fabricated carbon preforms from glassy carbon powder, which they reactively infiltrated with silicon, thus obtaining a SiC / C material as a ceramic composite [1]. Rambo et al. infiltrated an additively manufactured carbon preform with a TiCu alloy. Through reactive, pressureless infiltration, TiC / Ti-Cu was produced as a metallic fiber composite [2]. Shahzad et al. embedded submicrometer-scale aluminum oxide particles in polyamide by first dissolving the components and then obtaining the granules via precipitation. Using SLS, it could be further processed into preforms, whereby the polyamide liquefied and cavities formed in the preform. These resulted in a maximum density of 50% of the theoretical density being achieved after the sintering process [3]. Weber et al.CFRP preforms were manufactured from short fiber-reinforced compounds which were injection-molded into preforms and subsequently ceramicized via the LSI process [4]. Further prior art is disclosed in DE 10 2004 043 985 A1, US 2016 / 0175929 A1 and WO 2015 / 112889 A1.

[0009] The invention is directed to provide a method for producing a fiber-reinforced composite material that exhibits porosity and isotropically distributed fibers, whereby the method enables material savings. Furthermore, the invention is directed to provide a fiber-reinforced composite material that is easy to manufacture and exhibits high thermal conductivity and stability (strength and stiffness).

[0010] This problem is solved by a method according to claim 1 and a fiber composite material according to claim 10.

[0011] A process for manufacturing a fiber-reinforced composite material includes: (i) Providing a granulate comprising polymer and fibers, (ii) Manufacturing a preform from the granules, (iii) Pyrolysis of the prebody to a porous prebody, and (iv) Infiltrating the porous preform with a matrix or matrix-forming material.

[0012] In step (i), a granulate comprising polymer and fibers is prepared. This granulate can be produced, for example, by blending a polymer (e.g., an uncured phenolic resin) with fibers (e.g., carbon fibers), heating the mixture until it is liquid or viscous (e.g., to 180–200°C), and applying it to a surface using a spraying process. The polymer-fiber mixture collects in droplet form on the surface and can be collected as granules after cooling. The granule diameter, among other things, can be adjusted by modifying parameters (pressure, temperature, etc.) during the spraying process.

[0013] The fibers in such granules can be isotropically distributed (i.e., uniformly or homogeneously distributed in all directions). Compared to an anisotropic distribution, such as that which occurs when pressing raw blocks, this has the advantage in the final product that the material properties, especially thermal conductivity and stability, are improved.

[0014] In step (ii), a preform is produced from the granules. Due to the use of granules, the preform can be manufactured to near-final dimensions. This is achieved using an additive manufacturing process. The preform is built up layer by layer from granules using a physical melting or sintering process. The layer-by-layer construction of the preform can, for example, be carried out in thicknesses of 100 to 2000 µm.

[0015] Since the preform can be manufactured close to the final shape, the raw material requirement is low, which can contribute to resource conservation and to improving the CO2 balance and economic efficiency of the process.

[0016] The fibers in the preform can be isotropically distributed. As described above, a granulate can be used in which the fibers are already isotropically distributed. This isotropic distribution then continues in the preform. Furthermore, for example, individual areas of the granulate can be targeted and heated using a laser, thus also promoting an isotropic distribution of the fibers.

[0017] In step (iii), the resulting preform is pyrolyzed to form a porous preform (or a porous semi-finished product). During the pyrolysis in step (iii), the conversion of the plastic or polymer matrix into carbon and volatile gases creates an open-pore, fiber-reinforced matrix as the porous preform. Due to the pyrolysis of the polymer matrix to a carbon matrix, carbon particles may also be present, which, for example, in the case of silicon infiltration, are converted to SiC particles through carbide formation. This leads to so-called hybrid reinforcement, i.e., a combination of particle and fiber reinforcement. Since particle and fiber reinforcement exhibit different reinforcement mechanisms, they can complement each other, thus achieving a further increase in strength.

[0018] The previously provided isotropic distribution of fibers can be transferred in this step to the porous prebody, so that the porous prebody has isotropically distributed fibers.

[0019] Steps ii) and iii) can be combined to enable the layer-by-layer construction of an already pyrolyzed precursor. This is referred to below as in-situ pyrolysis.

[0020] In step (iv), the porous preforms are then infiltrated with a matrix or matrix-forming material. Matrix or matrix-forming materials are preferably those that create a wettable intermediate layer and enable infiltration. A wettable intermediate layer may be necessary, for example, if the contact angle of the infiltrating fluid with the substrate is greater than 90°, in order to reduce the contact angle accordingly.

[0021] Carbide-forming materials are preferred, as they generate a wettable intermediate layer of the carbide-forming material, thus enabling spontaneous, reactive infiltration. The use of additives that enable a wettable intermediate layer, such as potassium hexafluorotitanate (K₂TiF₆), is also possible.

[0022] Potassium hexafluorotitanate can be used, for example, to create an intermediate layer in the case of infiltration with aluminum. Since potassium hexafluorotitanate melts before aluminum, the molten salt infiltrates the porous preform first and forms a wettable TiC layer on the capillary walls. Furthermore, the salt prevents oxidation of the aluminum melt. This can be necessary because the oxide layer on the surface of the aluminum melt can prevent the melt from penetrating the preform. Therefore, using potassium hexafluorotitanate as a melting agent can promote spontaneous infiltration of the preform.

[0023] Carbide-forming materials include, for example, semimetals and metals. The semimetal is preferably silicon. Metals are preferably selected from aluminum, titanium, magnesium, copper, or alloys of these metals. Preferably, molten metals or other fluids, such as silicon, are infiltrated for infiltration, resulting in CMC (ceramic matrix composite) or MMC (metallic matrix composite) fiber-reinforced composites. Preferred infiltration methods are drop impregnation, wick impregnation, and immersion impregnation. Infiltration can be carried out under an oxidic atmosphere, an inert atmosphere, or under vacuum. Preferably, infiltration takes place in an inert atmosphere, such as a nitrogen or argon atmosphere, or under vacuum.

[0024] The previously provided isotropic distribution of fibers can be transferred to the fiber composite material in this step, so that the fiber composite material has isotropically distributed fibers.

[0025] Thus, the process for manufacturing a fiber composite material can produce fiber composite materials in which the fibers are isotropically distributed in the matrix.

[0026] Furthermore, as described above, the preforms can be manufactured close to their final shape thanks to the use of granules, thus eliminating the need for unnecessary materials. This leads to a reduction in raw material requirements, resource conservation, and consequently, an improved CO2 balance and the economic viability of the process.

[0027] Preferably, in step (i), the polymer comprises a thermoset. Thermosets (or thermomers) are polymers that, after curing by heating or other means, can no longer be deformed. They can contain hard, amorphous, insoluble polymers. The polymers are cross-linked via covalent bonds, which causes slight softening upon heating. The precursors (prepolymers) are synthetic resins that are still meltable or soluble—and often contain fillers and colorants—and can be cast or hot-pressed. The prepolymers are tri- or multifunctional compounds that polymerize with the aid of hardeners and possibly catalysts, or by high temperatures, resulting in a tightly cross-linked structure.

[0028] An example of such thermosets are phenolic resins, which are produced by curing based on phenolic resins. Besides phenol, compounds such as 3-cresol, 3,5-xylenol, or resorcinol can also be used. Acid-catalyzed electrophilic substitution of phenol forms precursors that, depending on the amount of formaldehyde used, bear one to three hydroxymethyl groups (-CH₂-OH). Hydrochloric acid or oxalic acid are among the acidic catalysts employed. With an excess of formaldehyde and under basic conditions, compounds with up to three hydroxymethyl groups can form. Resins are formed by the catalyzed polycondensation of these phenol derivatives. Depending on the desired result, the precondensates are then treated with acidic or basic condensing agents. In an acidic environment, oligomers linked via methylene groups (-CH₂-) are formed from the phenyl alcohols by condensation; these are known as novolacs.Novolacs are semi-liquid or still meltable. They are stable over time, meaning they are not self-curing. When combined with formaldehyde donors such as hexamethylenetetramine, novolacs cure at temperatures above 120 °C to form infusible, thermosetting materials. With basic condensing agents, however, they form meltable resins soluble in many solvents, known as resols. Resols tend to self-cure through further condensation and form the intermediate resitol. Heating these pre-condensates, with the further elimination of water, yields cross-linked polymers, the infusible and insoluble final stage, resit. After a resin has cured, tightly cross-linked polymers are formed because the phenol groups are linked to each other by up to three methylene groups. Thermosets are further characterized by high hardness, tensile strength, and flame retardancy.

[0029] Thermosets have the advantage over thermoplastics that they do not degrade at higher temperatures (e.g., during additive manufacturing or pyrolysis). Therefore, porous structures cannot be produced with thermoplastics through pyrolysis. Furthermore, the shape of thermoplastics is difficult to control and leads to significant shrinkage during pyrolysis.

[0030] Specifically, in step (i) the thermoset can already be cross-linked in the granules, or in step (ii) the uncured thermoset can be cross-linked in the granules during the production of the preform. During the subsequent or in-situ pyrolysis, this ensures that the thermoset, unlike a thermoplastic, does not melt and thus retains the shape of the preform.

[0031] Furthermore, the polymer is preferably selected to provide a high carbon yield during pyrolysis. This is the case, for example, for phenolic resins, which are characterized by a high carbon-to-oxygen / hydrogen ratio. A high carbon yield ensures a correspondingly high SiC content during reactive infiltration, e.g., with silicon, and minimizes free silicon as an undesirable by-phase.

[0032] Carbon fibers or silicon carbide fibers are preferably used as fibers. Carbon fibers are particularly preferred. Carbon fibers are fibers made from carbon-containing starting materials that are converted into graphite-like carbon through chemical reactions adapted to the raw material. A distinction is made between isotropic and anisotropic types: Isotropic fibers have only low strength and are of lesser technical importance, while anisotropic fibers exhibit high strength and stiffness with simultaneously low elongation at break in the axial direction. This isotropy should not be confused with an isotropic distribution of the fibers in the matrix.

[0033] Preferably, the granules (or the individual grains of the granules) have a diameter of 20 µm to 1000 µm. The average diameter of the granules also lies within this range. As described above, the diameter can be adjusted, for example, by changing the parameters during granule production using a spraying process. The granule size distribution can be measured, for example, using laser diffraction, light microscopy and image analysis, or light scattering. With a diameter below 20 µm, the granules tend to agglomerate. This can lead to clumping, making the granules difficult to use. With a diameter above 1000 µm, the granule resolution may be too low to produce small molded parts. Therefore, a diameter of 20 µm to 1000 µm is preferred, preferably 20 µm to 500 µm, more preferably 50 µm to 500 µm, and even more preferably 50 µm to 200 µm.

[0034] Preferably, the fibers have a length of 50 µm to 1000 µm or 100 µm to 1000 µm. Short fibers with a length of 0.1 mm to 1 mm are particularly preferred. The length of the fibers should not be less than the diameter of the granules. Therefore, a length of 20 µm to 1000 µm, preferably 20 µm to 500 µm, more preferably 50 µm to 500 µm, and even more preferably 50 µm to 200 µm is selected for the fibers.

[0035] Preferably, the fiber content in the granules is 1% to 50%, more preferably 5% to 50%, even more preferably 10% to 50%, even more preferably 20% to 50%, and most preferably 30% to 40%, based on the volume of the granules. The fiber content in the granules is adjusted according to the desired properties, such as the hardness of the fiber-reinforced composite material.

[0036] The granules may also contain an additive. Examples of such additives are carbon black, silicon carbide (SiC), silicon dioxide (Si), or graphite. By adding these additives, the mechanical and thermal properties of the material can be specifically tailored.

[0037] Preferably, in step (ii), the preform is additively manufactured. Preferred additive manufacturing processes include powder bed fusion, free-space fusion, liquid material fusion, and other layer-by-layer processes. Particularly noteworthy powder bed processes include selective laser melting (SLM), selective laser sintering (SLS), selective heat sintering (SHS), binder jetting, and electron beam melting (EBM).

[0038] The preform in step (ii) is preferably produced using a powder bed process. Selective laser sintering (SLS) or selective laser melting (SLM) are particularly preferred. In such a process, a physical hardening / melting process is carried out.

[0039] Selective laser sintering (SLS) is an additive layer manufacturing process: the preform is built up layer by layer. The effect of the laser beams allows for the creation of any three-dimensional geometry. The process can be used as a rapid manufacturing or rapid tooling method for the fast production of preforms. A CO2 laser, an Nd:YAG laser, or a fiber laser can be used. In this case, granules are used as the powder material. The powder can be applied to a surface in a layer thickness of, for example, 10 to 200 µm. The layers are sintered or melted into the powder bed step by step by controlling the laser beam according to the layer contour of the component. The surface is then lowered slightly, and a new layer is applied. The powder is supplied by raising a powder platform or as a reservoir in the doctor blade.The processing is carried out layer by layer in a vertical direction, making it possible to create undercut contours. The energy supplied by the laser is absorbed by the granules, resulting in localized sintering of the granule particles and a reduction in the overall surface area. The granules are preferably supplied as spheres, as the process places very high demands on the material's properties, such as its flowability.

[0040] In selective laser melting, the granules to be processed are applied to a surface in a thin layer. The granules can be completely remelted locally using laser radiation and, after solidification, form a solid layer of material. The surface is then lowered by the thickness of one layer, and more granules are applied. This cycle is repeated until all layers have been remelted. Layer thicknesses between 10 µm and 500 µm can be used to build the preform. To prevent oxygen contamination of the preform, the process can be carried out under a protective gas atmosphere of argon or nitrogen. Components manufactured by selective laser melting are characterized by high component densities (> 99%). This ensures that the mechanical properties of the additively manufactured component largely correspond to those of the base material.

[0041] Compared to conventional processes (e.g., casting), laser melting is distinguished by the fact that tools or molds can be eliminated (moldless manufacturing), thereby reducing time to market. Another advantage is the high degree of geometric freedom, which enables component shapes that are impossible or extremely difficult to produce using mold-based processes. Furthermore, storage costs can be reduced because specific components do not need to be kept in stock but can be additively manufactured on demand.

[0042] The above-mentioned methods are known to those skilled in the art and are not described in further detail here. Equipment that can be used for such a method is provided, for example, by the company EOS.

[0043] In step (iii), pyrolysis is carried out using a laser. This allows individual areas of the preform to be targeted, so that gases are generated and escape from the material only at these points. Compared to a process in which a raw block is pyrolyzed as a semi-finished product, this has the advantage that lower pressures are generated within the material and cracking does not occur. Pyrolysis is particularly preferably carried out with the same device used for selective laser sintering or selective laser melting in step (ii). The temperatures during pyrolysis are, for example, between 900 and 1000°C, with pyrolysis taking place in an inert atmosphere, e.g., under nitrogen or argon. One advantage of a laser is that short pulses can be applied to the material, resulting in only local and temporary heating.

[0044] After pyrolysis, the porous precursor can have a mean pore diameter of 10 µm to 1000 µm. More preferably, the mean pore diameter is 10 µm to 800 µm, even more preferably 20 µm to 600 µm, even more preferably 50 µm to 400 µm, and most preferably 100 µm to 200 µm.

[0045] The pyrolysis in step (iii) can be carried out after the preparation of the precursor in step (ii), but it is preferred that the pyrolysis of step (iii) is carried out in situ during the preparation of the precursor in step (ii). This saves a process step and allows the precursor to be prepared and pyrolyzed simultaneously, so that only local heating occurs and any gases produced can easily escape.

[0046] The process essentially consists of only two steps: additive manufacturing and in-situ pyrolysis of the preform from granules, and infiltration of the porous preform with the matrix material. By performing in-situ pyrolysis during the additive manufacturing process, for example with SLS (selective laser sintering) or SLM (selective laser melting), the process can eliminate a further heat treatment step, thus further improving the CO2 balance, as well as shortening production times and increasing cost-effectiveness.

[0047] The above method has the further advantage that a low heating rate is required during the pyrolysis heating process. Especially with large raw blocks, the material would have to be heated slowly to ensure that the entire polymer does not pyrolyze at once. In such a case, so much gas would be produced that the pressure within the material would increase, potentially leading to material failure. In contrast, when the combined additive manufacturing process with in-situ pyrolysis is used, a low heating rate is not necessary, which can result in a shorter process time and improved cost-effectiveness.

[0048] Preferably, in step (iv), the porous preform is reactively infiltrated with silicon or a metal, preferably selected from aluminum, titanium, magnesium, copper, or alloys of these metals. The infiltration can be carried out as a spontaneous, pressureless infiltration, for example, by impregnating the porous preform in liquid silicon or liquid metal. Preferred infiltration methods are drop impregnation, wick impregnation, and immersion impregnation. The infiltration can be carried out under an oxidic atmosphere, an inert atmosphere, or under vacuum (at low pressure, such as approximately 10–100 mbar). Preferably, the infiltration takes place in an inert atmosphere, such as a nitrogen or argon atmosphere. Alternatively, gases can be used, employing a carrier gas that carries the material to be infiltrated into the pores. Infiltration with liquid silicon or liquid metal is preferred.In particular, reactive infiltration of the porous preform with liquid silicon is preferred.

[0049] To achieve or improve wettability between the porous precursor and the infiltrating liquid semimetal / metal, fluxes can be used. These can simultaneously serve as oxidation protection for the melt intended for infiltration of the precursor. Such fluxes are additives, particularly salts of the material to be infiltrated, which create a wettable layer during infiltration.

[0050] Preferably, the fibers are isotropically distributed in the granules from step (i), in the preform from step (ii), in the porous preform from step (iii), and / or in the infiltrated porous preform from step (iv). As described above, the fibers are preferably isotropically distributed in the fiber-reinforced composite, which enhances its properties, particularly thermal conductivity, strength, and stiffness. The fibers can already be isotropically distributed in the granules from step (i). Alternatively or additionally, the granules can be distributed such that the preform formed from them in step (ii) has isotropically distributed fibers. The isotropic distribution of the fibers can be transferred to the porous preform formed during the pyrolysis of step (iii). Finally, the isotropic distribution of the fibers can be transferred to the fiber-reinforced composite during infiltration in step (iv).

[0051] Preferably, the fibers are coated with a coating material. This can serve to protect them during infiltration, from environmental influences, or to achieve a specific fiber-matrix bond. Coating the fibers during compound production or granulation is also possible. Fiber coating allows for the optimization of fiber-matrix interactions. Due to their fiber structure, the surfaces of carbon fibers exhibit largely chemically inert behavior. They also have low surface energies. Surface treatment can optimize the adhesion between the fiber and the surrounding matrix. Furthermore, premature breakage of the composites, for example due to brittle fracture, can be prevented by the coating. A coating also serves as protection against chemical attack.Coating fibers can also offer the advantage of increasing their isotropic properties, which further enhances the composite strength. Fibers are preferably coated with pyrolytic carbon, boron nitride, titanium nitride, titanium carbide, or silicon carbide.

[0052] Preferably, the fiber-reinforced composite produced by the process described above is a ceramic fiber-reinforced composite or a metallic fiber-reinforced composite. As already described above, ceramic fiber-reinforced composites are those that have a ceramic matrix. C / SiC and SiC / SiC fiber-reinforced composites are preferred. Metallic fiber-reinforced composites are those that have a metallic matrix or a metal carbide matrix.

[0053] In a preferred embodiment, the method for producing a fiber composite material comprises: (i) Providing a granulate comprising polymer and fibers, (ii) Fabricating a preform from the granules and in-situ pyrolyzing the preform to a porous preform, and (iii) Infiltrating the porous preform with a matrix or matrix-forming material.

[0054] In a further preferred embodiment, the method for producing a fiber composite material comprises: (i) Providing a granulate comprising thermoset plastic and carbon fibers or silicon carbide fibers, (ii) Producing a preform from the granules using an additive manufacturing process and in-situ pyrolyzing the preform to a porous preform, and (iii) Infiltrating the porous preform with silicon or a metal selected from aluminium, titanium, magnesium, copper or alloys of these metals.

[0055] A fiber-reinforced composite material comprises a metal or ceramic matrix in which fibers are embedded and isotropically distributed within the material. Due to this isotropic distribution, the material exhibits isotropic material properties.

[0056] It is noted that the open porosity is required during the manufacturing process described above for infiltration with the matrix material. The finished fiber-reinforced composite with the metal or ceramic matrix can be essentially or completely pore-free after infiltration. This can lead to a further increase in strength and gas tightness.

[0057] Due to the pyrolysis of the polymer matrix to a carbon matrix, carbon particles may also be present. These can be converted to SiC particles through carbide formation, for example, in the case of silicon infiltration. This leads to so-called hybrid reinforcement, i.e., a combination of particle and fiber reinforcement. Therefore, in addition to fibers, particles, especially carbon particles, could be distributed throughout the material. Since particle and fiber reinforcement exhibit different reinforcement mechanisms, they can complement each other, thus achieving a further increase in strength.

[0058] A preferred ceramic matrix comprises silicon carbide. A preferred metal matrix comprises aluminum or titanium. The fibers are preferably selected from carbon or silicon carbide fibers. Therefore, the fiber composite material particularly preferably comprises a SiC matrix and carbon fibers, i.e., it is a C / SiC fiber composite material.

[0059] The fiber-reinforced composite material may also exhibit porosity. The fiber-reinforced composite material preferably has a mean pore diameter of 10 µm to 1000 µm. More preferably, the mean pore diameter is 10 µm to 800 µm, even more preferably 20 µm to 600 µm, even more preferably 50 µm to 400 µm, and most preferably 100 µm to 200 µm.

[0060] Preferably, the fibers have a length of 50 µm to 1000 µm or 100 µm to 1000 µm. Short fibers with a length of 0.1 mm to 1 mm are particularly preferred. A length of 20 µm to 1000 µm, preferably 20 µm to 500 µm, more preferably 50 µm to 500 µm, even more preferably 50 µm to 200 µm, and most preferably 100 µm to 200 µm is preferred.

[0061] Preferably, the fiber content in the fiber-reinforced composite is 1% to 50%, more preferably 5% to 50%, even more preferably 10% to 50%, even more preferably 20% to 50%, and most preferably 30% to 40%, based on the weight of the fiber-reinforced composite. The fiber content in the fiber-reinforced composite is adjusted according to the desired properties, such as the hardness of the fiber-reinforced composite.

[0062] A fiber composite material can be manufactured using the method described above.

[0063] When manufacturing a raw block using a press tool, large blocks are produced which must be shaped after pyrolysis. Especially for lightweight applications where thin-walled structures are required, this can mean that up to 90% of the material has to be removed. Furthermore, reinforcement with long fibers is often not feasible, and reinforcement with short fibers is more efficient, for example, in components with fittings.

[0064] The present invention provides a method in which granules are used to additively manufacture the preform, for example, by selective laser sintering or selective laser melting. By using the granules, the preforms can be directly shaped into the desired form without the need for a complex shaping process of a large raw block, such as milling or sawing. Since no material needs to be removed and the excess granules can be reused, significant material savings are also achieved.

[0065] When manufacturing a raw block using a pressing tool, the fibers are typically arranged spatially relative to each other. Fiber orientation can only be reduced through complex, isostatic pressing.

[0066] The present invention provides a method in which granules are used. Each granule has its own fiber distribution, resulting in a preform with homogeneously distributed, or isotropically distributed, fibers. Due to the isotropic distribution of the fibers, the fiber composite material exhibits isotropic material properties.

[0067] The process and the fiber composite material can be further improved by using a thermoset as the polymer in the granules, ensuring the fibers are isotropic and coating the fibers.

[0068] Fiber-reinforced composites exhibit isotropic thermal, mechanical, tribological, and electrical properties. These properties can be easily and precisely tailored through additives. In contrast to fiber-reinforced composites with anisotropic properties, components made from this material can be more easily designed and manufactured to accommodate load collectives with a wide variety of load directions.

[0069] Further advantages of the invention include material savings, lower energy consumption, higher material quality (isotropy, homogeneity, reproducibility), reduction of component mass through more effective geometries, individually adapted material properties through variation of powder proportions, production of very complex component structures, arbitrary freeform surfaces, undercuts, hollow structures, and production of components from graded material using 3D printing on base structures. Example of a manufacturing process

[0070] In one example, a phenolic resin is mixed with 20% carbon fibers. A spraying process produces granules with an average diameter of approximately 50–100 µm. These granules are then transformed into a preform with the desired shape using selective laser melting with an EOS device. Simultaneously, the preform is pyrolyzed in situ during selective laser melting at approximately 900–1000°C in a nitrogen atmosphere. Finally, the resulting porous preform is immersed in liquid silicon to reactively infiltrate it. This produces a porous C / SiC fiber composite in which the carbon fibers are isotropically distributed. literature [1] Moon J, Caballero AC, Hozer L, Chiang YM, Cima MJ. Fabrication of functionally graded reaction infiltrated SiC-Si composite by three-dimensional printing (3DPTM) process. Materials Science and Engineering: A. 2001;298(1-2):110-9. [2] Rambo CR, Travitzky N, Zimmermann K, Greil P. Synthesis of TiC / Ti-Cu composites by pressureless reactive infiltration of TiCu alloy into carbon preforms fabricated by 3D-printing. Materials Letters. 2005;59(8-9):1028-31. [3] Shahzad K, Deckers J, Boury S, Neirinck B, Kruth J-P, Vleugels J. Preparation and indirect selective laser sintering of alumina / PA microspheres. Ceramics International. 2012;38(2):1241-7. [4] Weber S, Sommer F, Kern F, Gadow R, Voggenreiter H, Koch D. Thermoplastic Forming as an Alternative Shaping Process for Near-Net-Shape Production of C-SiSiC Ceramics via Liquid Silicon Infiltration (LSI) Process. Journal of Ceramic Science and Technology. 2014;5(3):223-36.

Claims

[1] A method for producing a fiber composite material, comprising: (i) providing a granulate comprising polymer and fibers, (ii) producing a preform from the granulate, the preform being manufactured additively, (iii) pyrolyzing the precursor to a porous precursor by means of a laser, and (iv) Infiltrating the porous preform with a matrix or matrix-forming material. [2] The method of claim 1, wherein in step (i) the polymer comprises a thermoset and the fibers comprise carbon fibers or silicon carbide fibers. [3] A process according to any one of the preceding claims, wherein in step (i) the granules have a diameter of 20 µm to 1000 µm, the fibers have a length of 20 µm to 1000 µm and the fiber content in the granules is 1 to 50%, based on the weight of the granules. [4] Method according to one of the preceding claims, wherein in step (ii) the preform is produced by means of a powder bed process, preferably selected from selective laser sintering or selective laser melting. [5] A process according to any one of the preceding claims, wherein the pyrolysis of step (iii) is carried out in-situ with the production of the preform of step (ii). [6] Method according to one of the preceding claims, wherein in step (iv) a reactive infiltration of the porous preform with a semimetal, preferably selected from silicon, or a metal, preferably selected from aluminum, titanium, magnesium, copper or alloys of these metals, is carried out. [7] Method according to one of the preceding claims, wherein the fibers are distributed isotropically in the granulate of step (i), in the preform of step (ii), in the porous preform of step (iii) and / or in the infiltrated porous preform of step (iv). [8] Method according to one of the preceding claims, wherein the fibers are coated with a coating material, preferably selected from pyrolytic carbon, boron nitride, titanium nitride, titanium carbide or silicon carbide. [9] Method according to one of the preceding claims, wherein the fiber composite material is a ceramic fiber composite material or a metallic fiber composite material. [10] Fiber composite material produced according to one of claims 1 to 9. [11] Fiber composite material according to claim 10, comprising - a metal or ceramic matrix, - fibers embedded in the matrix, whereby the fibers are distributed isotropically in the material. [12] Fiber composite material according to one of claims 10 to 11, wherein the matrix comprises a silicon carbide, titanium carbide, or aluminum carbide matrix. [13] Fiber composite material according to one of claims 10 to 12, wherein the fibers have a length of 20 µm to 1000 µm and the fiber content in the fiber composite material is 1 to 50%, based on the weight of the fiber composite material.