METHOD FOR THE PRODUCTION OF PREPREGS FOR THE MANUFACTURING OF FIBER-REINFORCED CERAMIC COMPONENTS

Description

[0001] The present invention relates to a method for producing prepregs for the production of fiber-reinforced ceramic components, in which a slurry is produced, a fiber preform is impregnated with the produced slurry, and the fiber preform impregnated with the slurry is subjected to a freeze-drying process. According to the invention, the slurry contains inorganic solid particles, at least one water-soluble primary binder, at least one water-insoluble secondary binder, and / or at least one water-insoluble silicon source, and water. The inorganic solid particles form a gel network with the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source.In addition, the present invention also relates to a prepreg that can be produced by the method according to the invention for producing prepregs and to a method for producing fiber-reinforced ceramic components, in which a prepreg produced by the method according to the invention for producing prepregs is infiltrated with silicon melt.

[0002] Ceramic matrix composites (CMC) consist of reinforcing fibers and a ceramic matrix. Carbon fibers and ceramic fibers are used as the fibers, which can be either oxide or non-oxide. Gas infiltration (chemical vapor infiltration = CVI), liquid infiltration (polymer infiltration = PI), and melt infiltration (melt infiltration = MI) are used for the matrix structure. CMC components with ceramic fiber reinforcement can be used, among other things, in gas turbines at high temperatures > 1000 °C in the hot gas path.

[0003] For the production of CMC components, semi-finished textile products, so-called fiber preforms or prepregs, are used. These are composed of rovings, UD tapes, or, more often, 2D fabric layers. Semi-finished textile products with 2.5D to 3D reinforcement are also available on the market. In the latter case, the fiber layers are sewn, needled, or tufted together in the z-direction by individual threads (rovings). CMC components are typically constructed by stacking individual fabric layers, and the fiber orientations are adjusted by targeted fiber placement.

[0004] In addition, fiber coatings are used to protect the fibers from oxidation at high temperatures and to enable the fibers to slide under bending stress. The latter is a basic prerequisite for the damage-tolerant fracture behavior characteristic of CMCs, which is reflected in ductile fracture behavior with high elongation at break in the stress-strain diagram. For SiC fibers, protective layers made of pyrocarbon (PyC), boron nitride (BN), silicon carbide (SiC), and silicon nitride (Si 3 N 4 ) have proven effective. For adequate fiber protection in SiC ceramic fibers, double layers of BN / SiC or BN / Si 3 N 4 are usually used; these double layers are also resistant to air or combustion atmospheres (e.g., water vapor).

[0005] The primary matrix is ​​created by impregnating fiber preforms with preceramic resins or particle-containing slurries by doctoring, overpressure, or vacuum. Binders such as epoxy or phenolic resins are used for this purpose. After curing and thermal treatment by pyrolysis under protective gas, they form a carbon matrix. Alternatively, particle-laden slurries (suspensions) can be used, which contain, among other things, metal carbides or carbon in various grain distributions. After curing or drying the matrix-forming materials, a so-called prepreg is obtained, i.e., a semi-finished textile product pre-impregnated with matrix material.

[0006] The pyrolysis of carbon-forming resins or the evaporation of the solvent from the particle-containing slurry leaves a certain degree of open porosity in the prepreg. This can be infiltrated with molten metals, such as silicon or Si alloys, to create a dense, low-porosity CMC. The carbon component, i.e., pyrolyzed carbon-containing binder components and / or carbon particles, reacts to form SiC. In this way, a SiC / SiC material and component can be produced in which SiC fibers are embedded in a carbon-free mixed matrix of free silicon (excess in the interstices) and SiC. Ideally, high SiC matrix contents are achieved.

[0007] The manufacturing routes used to date and mentioned above have the disadvantage that the buildup of gas pressure and the volume shrinkage of the matrix during pyrolysis (resin systems) or drying (suspensions) create stresses in the prepreg, leading to cracks in the matrix. Although these cracks are filled with silicon during liquid siliconization, this always results in an unavoidable amount of unwanted, unreacted residual silicon in the finished composite, as well as a compromised mechanical stability.

[0008] Already known approaches to minimizing crack formation are presented below: EP 2 995 597 A1 proposes the production of a reinforced fiber preform by infiltrating a preform with a suspension of carbon nanotubes (CNTs) followed by freeze-drying. For this purpose, a fiber preform is first exposed to an aqueous suspension containing a binder (PVA, PVP, PEI, phenol, and others) and CNTs. Freezing causes the water volume to expand, and the CNT fibers are separated or distributed in the preform and bonded to the fibers, while the fibers in the fiber bundle are spread apart. After freezing using liquid nitrogen or dry ice, the prepreg is freeze-dried. The subsequent possible manufacturing steps for further processing of the stiff, CNT-stabilized fiber preform include slip casting, CVI, PI, and MI.Through thermal treatment using a boron- or SiO2-containing precursor, the CNTs are converted into SiC or boron nitride nanotubes. Cross-needling through the CNTs can improve the out-of-plane properties of the CMC. The disadvantage of this method is the necessity of an additional infiltration step for the construction of the actual matrix, for which no concept for crack-free construction is presented in this patent.

[0009] US 2013 / 0287941 A1 describes a process for producing a SiC-containing matrix that can be infiltrated by silicon melt-phase infiltration. The slurry forming the matrix consists of at least a carbon-containing binder (e.g., phenols, epoxy resins, polyesters) and SiC particles with a volume fraction of 45-80 vol. After thermal treatment, SiC particles, open porosity (> 20 vol%), and finely distributed carbon residues with 9.5-25 vol.% (binder content) are present in the matrix. The carbon residue of the binder (carbon char) forms a net-like structure that can be converted into SiC. The use of carbon particles such as soot particles in the matrix is ​​deliberately avoided, since in this structure the small pores are clogged by SiC formation during infiltration and thus the liquid silicon can no longer be sufficiently infiltrated into the interior of the sample volume.A second organic component is added in the form of a catalyst, an additive to improve the flow properties of the suspension, and / or pore-forming agent (e.g., PMMA, PVB). The latter also acts as a binder and improves impregnability with Si melts by providing open porosity. The described process uses organic resin systems to construct the matrix, which is why, in practice, this process is associated with material-related compliance with safety precautions, as well as significant costs for cleaning (again, solvents are required) and disposal. Furthermore, the viscosity of the resulting, highly filled resin systems and, accordingly, the quality of infiltration into the fiber interstices are not mentioned.

[0010] EP 3 000 797 A1 proposes the production of a CMC by infiltrating a fiber preform with an aqueous slurry containing a gelling agent (monomer) and particles of carbon and SiC (0 < 50 µm). Gelling (polymerization) takes place by raising the temperature to 30-80 °C. Polysaccharides and acrylates, which can be further stabilized by cross-linkers, are used as gelling agents. The gel is intended to hold the particles in the fiber preform. The gel content is approximately < 1 wt. %. Drying normally takes place at atmospheric pressure and a temperature of up to 150 °C. Since this process involves regular drying, this results in drying shrinkage of the matrix, which leads to drying cracks in the rigid fiber framework in the preforms.

[0011] DE 100 08 686 A1 describes a process for producing fiber-reinforced silicon carbide composites. The resulting composite exhibits high toughness, with bundles of reinforcing fibers densely covered with a glassy carbon derived from a resin to prevent any impairment of strength.

[0012] US 2017 / 183268 A1 is directed to a method for producing ceramic matrix composite articles. The method comprises impregnating an inorganic fiber preform with a slurry composition. The slurry composition contains a particulate material, a solvent, and a pregelling agent. Gelation of the pregelling agent in the slurry composition is initiated to immobilize the particles and obtain a gelled article, and substantially all of the solvent is removed from the gelled article to form a composite article green. The composite article green is then infiltrated with a molten infiltrant to form the composite article.

[0013] US 2018 / 155252 A1 relates to a method for producing a ceramic matrix composite. A fiber preform can be placed in a mold. An aqueous solution can be added to the fiber preform. The aqueous solution can contain water, carbon nanotubes, and a binder. The preform can be frozen. Freezing the preform can cause the water to expand and separate the fibers in the fiber preform. The carbon nanotubes can bond to the fibers. The preform can be freeze-dried to remove the water. The preform can then be processed using the standard CMC process.

[0014] DE 10 2005 037108 A1 describes a process for producing ceramic matrix composites using a water-based prepreg casting compound. The process involves applying a fiber coating to a fiber cable by chemical deposition from the gas or vapor phase; pulling the fiber cable through an aqueous casting compound composed of high- and low-temperature binders, silicon carbide powder, carbon black, and water to produce a prepreg tape; and winding the prepreg tape onto a drum.

[0015] Freeze-drying water-based slurry systems offers the opportunity to produce highly porous green matrices or prepregs, which can be converted into a dense, homogeneous ceramic matrix material through a post-infiltration or reactive infiltration step. However, prepregs processed in this way are extremely fragile after freeze-drying. However, for processing into geometrically complex components, a stable, flexible, and damage-tolerant matrix between the fiber layers is required to avoid handling errors such as microcracks, delaminations, particle or fiber redistribution, or particle loss. Near-net-shape infiltration of complex preform laminates with a slurry, including a subsequent freeze-drying step, is not practical, as the formation of ice crystals and thus the structure of the matrix depend in a complex way on factors such as the component geometry and / or the material properties.Wall thicknesses, as well as heat dissipation and freezing rates during freezing, depend on the material. Therefore, the matrix materials in the prepregs must be additionally stabilized.

[0016] Based on this, it is therefore the object of the present invention to provide a simple and environmentally friendly process for producing highly porous prepregs for the production of fiber-reinforced ceramic components, by means of which stable, flexible fiber preforms are obtained, which can then be further processed by infiltration in such a way that a dense, homogeneous ceramic matrix material is obtained, without the microstructure achieved during the production of the prepreg being disturbed by fiber or grain redistribution.

[0017] This object is achieved with respect to a method for producing prepregs for the production of fiber-reinforced ceramic components having the features of claim 1, with respect to a prepreg for the production of fiber-reinforced ceramic components having the features of claim 11, and with respect to a method for producing a fiber-reinforced ceramic component having the features of claim 12. The respective dependent patent claims represent advantageous developments.

[0018] According to the invention, a method for producing a prepreg is thus provided, in which a) a slip is produced which contains i) inorganic solid particles, ii) at least one water-soluble primary binder, iii) at least one water-insoluble secondary binder and / or at least one water-insoluble silicon source, and iv) water, wherein the inorganic solid particles form a gel network with the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source, b) a fiber preform is impregnated or infiltrated with the slip produced, and c) the fiber preform impregnated or infiltrated with the slip is subjected to a freeze-drying process.

[0019] In step a) of the process according to the invention, a slip containing components i) to iv) is first produced. In principle, any inorganic solid particles of the same type or of different types can be used as component i). Preference is given to using carbon-containing solid particles, such as carbon solid particles, metal carbide solid particles, or mixtures thereof. In addition to the carbon-containing solid particles, the slip can in this case also contain other solid particles, such as metal nitride solid particles and metal boride solid particles. Component ii) is a water-soluble primary binder. This is understood to mean a binder that is soluble in water, for example polyvinyl alcohol or polyethylene glycol. Component iii) is a water-insoluble secondary binder and / or a water-insoluble silicon source.A water-insoluble secondary binder is understood to be a binder that is insoluble in water, for example, a phenolic resin or an epoxy resin. A water-insoluble silicon source is understood to be a silicon-containing compound that is not soluble in water, for example, a silazane resin. Water is used as component iv). The slip produced in step a) preferably consists of components i) to iv).

[0020] Preferably, the slip produced in step a) contains, in addition to components i) to iv), at least one dispersing aid and / or at least one surfactant.

[0021] Thus, it is preferred that in step a) a slip is produced which i) inorganic solid particles, ii) at least one water-soluble primary binder, iii) at least one water-insoluble secondary binder and / or at least one water-insoluble silicon source, iv) water, and v) at least one dispersing aid and / or at least one surfactant, wherein the inorganic solid particles form a gel network with the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source.

[0022] Preferably, the slip produced in step a) consists of components i) to v).

[0023] By using at least one dispersing agent in the produced slip, the slip can be prevented from becoming agglomerated, which leads to improved functionality of the slip. By using at least one surfactant in the produced slip, demixing in the slip can be prevented, which also leads to improved functionality of the slip.

[0024] The at least one dispersing aid is preferably selected from the group consisting of amino alcohols, fatty acids, carboxylic acid derivatives, phosphates, sulfates, acrylates, polymers, (synthetic) polyelectrolytes with pigment-affinic groups (for steric and / or electrosteric stabilization), and mixtures thereof.

[0025] The at least one surfactant is preferably selected from the group consisting of ammonium lauryl sulfate, stearic acid, polyoxyethylene(12) tridecyl ether, PEG-18, and mixtures thereof. The at least one surfactant preferably has an HLB value of 3 to 18.

[0026] In principle, the components of the slip can be combined in any order. However, it is preferred if a suspension is first prepared which contains the solid particles; the water-soluble primary binder; preferably the optionally used dispersing agent; and the water, and then the water-insoluble secondary binder and / or the water-insoluble silicon source is added to the prepared suspension, with any surfactant preferably being added thereafter. Furthermore, it is preferred if a suspension is first prepared which contains the solid particles; the water-insoluble primary binder and / or the water-insoluble silicon source; preferably the optionally used dispersing agent; and the water, and then the water-soluble primary binder is added to the prepared suspension, with any surfactant preferably being added thereafter.The stability of the slip can be increased by adding the surfactant.

[0027] During the production of the slurry, the inorganic solid particles form a gel network with the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source. The gel network formation already occurs during the slurry production, and thus at least partially before the impregnation of the fiber preform with the slurry. As soon as the water-insoluble secondary binder comes into contact with the inorganic solid particles in the water, the inorganic solid particles are wetted by the water-insoluble secondary binder and cross-linked with each other. The slurry therefore has a yield point, but a shear-thinning flow behavior, which nevertheless enables the infiltration of fiber fabrics, thus also the infiltration of the fiber preform in step b). The gel network formation does not have to occur before the start of step b), i.e.The process may not only be completed before the impregnation of the fiber preform, but may also continue during step b) and after step b), i.e. during and after the impregnation of the fiber preform.

[0028] In step b), a fiber preform is impregnated or infiltrated with the slurry produced in step a). The slurry penetrates the fiber preform, resulting in a fiber preform impregnated or infiltrated with the slurry. The impregnation or infiltration of the fiber preform with the slurry can be carried out using one of the methods known to those skilled in the art. Due to the formation of the gel network, the slurry has a yield point, by which the slurry is at least partially fixed in the fiber preform. In this way, the leakage of the slurry and the sedimentation of the particles are hindered or prevented. Rovings or textiles such as ribbons that have a preferred fiber direction can be used as the fiber preform.

[0029] Preferably, the fiber preform is placed on a mold in step b), which can occur before, during, and / or after impregnation of the fiber preform with the prepared slip. Furthermore, it is preferred that the fiber preform be cut, perforated, punched, and / or stacked in any combination before, during, and / or after impregnation with the prepared slip.

[0030] Thus, it is preferred that in step b) a fiber preform is placed on a mold, impregnated with the produced slip and preferably cut, perforated, punched out, and / or stacked in any combination before or after this.

[0031] The fiber preforms can be deposited on any desired forming tool, such as a core, before, after, or between the impregnation step b), which may already have a near-net-shape. The preforms can also be cut and / or perforated and / or punched before, after, or between the deposition.

[0032] The core can be designed as a positive (convex) or negative (concave) shape and made of metal, ceramic, graphite, or carbon fiber reinforced carbon (CFC). Furthermore, the fiber preforms can be deposited on the forming tool with any fiber orientation and wall thickness. Unformed fiber preforms produce sheets that are deposited on a flat core. Fiber preforms formed by cores have radii of curvature.

[0033] In step c), the slurry-impregnated fiber preform obtained in step b) is finally subjected to a freeze-drying process. This removes the water from the slurry-impregnated fiber preform, resulting in a prepreg for the production of fiber-reinforced ceramic components. The freeze-drying process can be carried out over a certain period of time, e.g., over a period of 24 to 48 hours, at a pressure below atmospheric pressure, e.g., at 2 10 -3 < mbar to 3 10 -3 < mbar, and at temperatures below the freezing point of the slurry, e.g., at temperatures below -40 °C.

[0034] Freeze-drying water-based slip systems offers a simple, environmentally friendly, and shrinkage-free way to produce highly porous green matrices. These matrices can be converted into a dense, homogeneous ceramic matrix material through a post-infiltration step or reactive infiltration. In this case, the matrix remains crack-free because the water ice is sublimated without capillary forces, without stress or shrinkage, thus preventing volume loss. The resulting ice crystals can be controlled and their spatial expansion minimized by adjusting the cooling rate and adding antifreeze agents such as ethylene glycol or other organic additives. This prevents the formation of extensive dendritic pores in the matrix caused by ice crystals. This makes it possible to create a homogeneous, porous structure.

[0035] After the freeze-drying process, water-soluble binder systems largely remain in the voids between the particles, which is why their sole use cannot achieve a damage-tolerant matrix between the fibers. For this reason, the invention uses an organic binder, i.e., a water-insoluble secondary binder, for the production of freeze-dried prepregs with flexible matrix properties. This binder, after freeze-drying, is located in the contact points between the matrix particles. Alternatively, redistribution of the binder after freeze-drying is conceivable. The microstructure, in which the binder is located in contact with the inorganic particles, is important.

[0036] The slurry used according to the invention with components i) to iv) is a capillary suspension. This consists of a liquid phase (including a soluble binder) in which particles are dispersed, and a second liquid phase that is insoluble in the first phase. Capillary suspensions have the property that the insoluble phase accumulates between the particles and connects them to one another, forming a network. The second phase must have the property of ensuring a sufficiently strong bond between the matrix particles and the fibers even after the water has been removed; therefore, water-insoluble, liquid binder systems are suitable for this purpose. The second phase, which acts as a binder between the particles, ensures stability and, at the same time, flexibility of the anhydrous preform after freeze-drying. In this way, formable prepreg individual layers as well as flexible fiber preforms can be achieved.

[0037] Starting materials for the production of prepregs, such as SiC / SiC fiber prepregs, are aqueous slurries and fiber preforms. The slurries contain at least water as a suspension medium, solid particles of carbon and / or metal carbides / nitrides / borides, and a water-soluble primary binder. The particle sizes are preferably < 20 µm, particularly preferably < 1 µm, in order to achieve sufficiently high bundle infiltration with the suspension according to the invention. The viscosities of the slurries can ideally be set between 100 - 2000 mPas (shear rate 100 s -1< ). 1D-3D fiber preforms can be impregnated with the slurry. Rovings or textiles such as ribbons that have a preferred fiber direction are preferred. The slurries can penetrate such preforms homogeneously because there are no or only a few crossing points, thus avoiding shadowing.

[0038] As mentioned above, two different types of binder are necessary to produce a stable and flexible fiber preform. The first binder, the primary binder, is water-soluble, for example, polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinyl butyral (PVB), acrylates, and the like. Latex binders can also be used. A suspension can be created using this binder, water, and matrix particles. The second binder, the secondary binder, is water-insoluble, e.g., phenolic resin, furan resin, epoxy resin, polyester resin, or the like, and can be added subsequently. This creates a three-phase system. The individual particles are wetted by the non-water-soluble binder and thus form a gel network, preferably throughout the entire slurry volume. This creates a yield point in the slurry, which fixes the slurry, i.e.This prevents the slurry from leaking out of the preform and also prevents the particles from settling. This allows for a homogeneous matrix network to be achieved in the fiber preform.

[0039] The proportion of insoluble binder is preferably < 10 vol.%, particularly preferably < 1 vol.%. The secondary binder content also makes it possible to influence the pore size and pore distribution. The pore sizes increase with increasing secondary binder content, as network formation (through flocculation) progresses until agglomeration of the primary particles occurs. However, such agglomeration is undesirable, as it contributes to the inhomogeneity of the matrix and particle accumulations are not sufficiently infiltrated during Si melt-phase infiltration. By freeze-drying capillary suspensions, porosities of up to 60 vol.% and pore sizes of 0.1–100 µm can be achieved.

[0040] Alternatively, or in addition to the secondary binder, a water-insoluble silicon source, such as a silazane resin, can be added to the slurry. During freeze-drying, the Si precursor spreads over the pore surfaces. During heat treatment, it can react with existing carbon particles to form SiC, ensuring uniform infiltration of the Si melt.

[0041] In addition, common dispersing agents such as fatty acids or carboxylic acid derivatives, phosphates, sulfates, acrylates, synthetic polyelectrolytes with pigment-affine groups can be added to the slip for steric and / or electrosteric stabilization in order to improve the dispersion of the powder particles.

[0042] Furthermore, surfactants (preferably with an HLB value of 3 to 18) can be added to increase the stability of the suspension.

[0043] Since the invention uses water-based systems instead of resin systems, the cost of personal protective equipment, extraction systems, etc. is minimal in practical applications. The same applies to the cleaning and disposal of offcuts and slurry residues. Due to the predefined porosity in the prepregs (instead of a dense resin matrix), pyrolysis runs can be carried out faster and more efficiently. The porosity in the prepregs can be defined by the slurry formulation, the processing parameters, and the freeze-drying process, which offers advantages for subsequent liquid-phase or melt infiltration and for the development of a homogeneous matrix. Furthermore, freeze-drying is largely shrinkage-free, which prevents cracks and other defects in the prepregs.

[0044] In particular, by using the special slip with components i) to iv) and by using a freeze-drying process, the method according to the invention can thus produce a prepreg that has a homogeneous, fine, crack-free structure and is also very pliable or flexible (without flaking).

[0045] Consequently, the process according to the invention provides a simple and environmentally friendly process for producing highly porous prepregs for the production of fiber-reinforced ceramic components, by means of which stable, flexible fiber preforms are obtained, which can then be further processed by infiltration to obtain a dense, homogeneous ceramic matrix material without the microstructure achieved during the production of the prepreg being disturbed by fiber or grain redistribution.

[0046] A preferred variant of the process according to the invention is characterized in that the inorganic solid particles are selected from the group consisting of carbon solid particles, metal carbide solid particles, metal nitride solid particles, metal boride solid particles and mixtures thereof.

[0047] Preferably, the proportion of inorganic solid particles based on the total volume of the slip is 10 vol.% to 50 vol.%, particularly preferably 30 vol.% to 45 vol.%.

[0048] According to a further preferred variant of the process according to the invention, the at least one water-soluble primary binder is selected from the group consisting of polyvinyl alcohol, polyethylene glycol, polyvinyl butyral, acrylates, latex binders and mixtures thereof.

[0049] Preferably, the proportion of the at least one water-soluble primary binder based on the total weight of the solids content of the slip is 0.5 wt.% to 10 wt.%.

[0050] A further preferred variant of the process according to the invention is characterized in that the at least one water-insoluble secondary binder is selected from the group consisting of phenolic resins, furan resins, epoxy resins, polyester resins, and mixtures thereof, and / or the at least one water-insoluble silicon source is selected from the group consisting of silazane resins, siloxanes, silanes, and mixtures thereof.

[0051] Preferably, the proportion of the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source, based on the total volume of the slip, is less than 10 vol.%, preferably less than 5 vol.%. Furthermore, it is preferred that the proportion of the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source, based on the total weight of the solids content of the slip, is 0.1 wt.% to 10 wt.%.

[0052] Furthermore, it is preferred that the proportion of water, ie component iv), based on the total volume of the slip, is 30 vol.% to 80 vol.%, preferably 45 vol.% to 60 vol.%.

[0053] The slip preferably contains 10 vol.% to 50 vol.%, particularly preferably 30 vol.% to 45 vol.%, of the inorganic solid particles based on the total volume of the slip, and / or 0.5 wt.% to 10 wt.% of the at least one water-soluble primary binder based on the total weight of the solids portion of the slip, and / or 0.1 wt.% to 10 wt.% of the at least one water-insoluble secondary binder and / or of the at least one water-insoluble silicon source based on the total weight of the solids portion of the slip, and / or 30 vol.% to 80 vol.%, preferably 45 vol.% to 60 vol.%, of the water, ie component iv), based on the total volume of the slip.

[0054] A further preferred variant of the method according to the invention is characterized in that the fiber preform is a roving or a textile fabric, e.g., a ribbon, with a preferred fiber orientation. The slurries can penetrate such preforms homogeneously because there are no or only a few intersection points, thus avoiding shadowing.

[0055] According to a further preferred variant of the process according to the invention, in the preparation of the slip in step a), a suspension is first prepared which contains the solid particles, the water-soluble primary binder and the water, wherein the water-insoluble secondary binder and / or the water-insoluble silicon source is then added to the prepared suspension.

[0056] A further preferred variant of the method according to the invention is characterized in that, prior to impregnation of the fiber preform in step b), the fiber preform is subjected to a stabilization process in which accessible pore surfaces of the fiber preform are coated with a stabilizing binder by means of wet-chemical methods (e.g., wet-chemical coating) or vapor deposition. The stabilizing binder is preferably selected from the group consisting of (preferably vaporizable) metal precursors, e.g., silanes, silazanes, siloxanes, and mixtures thereof. In this way, the fiber fabric of the fiber preform can be stabilized even before infiltration with the slurry. This subsequently results in a more manageable, more stable prepreg.

[0057] According to a further preferred variant of the method according to the invention, after the freeze-drying process, the produced prepreg is subjected to a stabilization process in which the prepreg is impregnated with at least one stabilizer, e.g., paraffin wax. The impregnation is preferably carried out by spraying the at least one stabilizer onto the prepreg, by immersing the prepreg in the at least one stabilizer, or by doctoring the at least one stabilizer into the prepreg. Such treatment of the prepreg improves the stabilization and flexibility of the fiber preform. Further advantages of the method include: 1. Waxes have a water-repellent effect (hydrophobic) and therefore protect the porous fiber preform from reabsorption of water after freeze-drying; 2. The formability of wax-containing fiber preforms in pressing processes at very low temperatures is excellent, as they can be kneaded starting at approximately 20 °C; 3. The thermal behavior during pyrolysis is favorable, as no decomposition products remain (no residue), the melting point is very low at 40-70 °C, and the evaporation point is approximately 300 °C.

[0058] A further preferred variant of the method according to the invention is characterized in that, after the freeze-drying process, the produced prepreg is subjected to a stabilization process in which accessible pore surfaces of the prepreg are coated with a stabilizing binder by means of gas-phase deposition. The stabilizing binder is preferably selected from the group consisting of (preferably vaporizable) metal precursors, e.g., silanes, silazanes, siloxanes, and mixtures thereof. The monomeric gas particles can flow through the preforms and be deposited on the inner surface. In addition to mechanically stabilizing the prepreg, this has the advantage of improving wetting by the silicon or alloy melt during metal melt infiltration.In addition, this can also reduce the reactivity and reaction rate inside the component if the inorganic matrix components are carbon particles. Carbon particles convert to SiC more slowly in silicon contact if they are coated with a diffusion barrier.

[0059] A further preferred variant of the method according to the invention is characterized in that after the freeze-drying process, the produced prepreg is subjected to a stabilization process in which the water-insoluble secondary binder or the water-insoluble silicon source is cured in a thermal curing step.

[0060] It is preferred that after the freeze-drying process, the produced prepreg is subjected to a stabilization process, in which the prepreg is impregnated with at least one stabilizer, e.g. paraffin wax, wherein the impregnation is preferably carried out by spraying the at least one stabilizer onto the prepreg, by dipping the prepreg into the at least one stabilizer or by doctoring the at least one stabilizer into the prepreg, and / or in which accessible pore surfaces of the prepreg are coated with a stabilizing binder by means of gas phase deposition, wherein the stabilizing binder is preferably selected from the group consisting of (vaporizable) metal precursors, e.g. silanes, silazanes, siloxanes and mixtures thereof, and / or in which the water-insoluble secondary binder or the water-insoluble silicon source is cured in a thermal curing step.

[0061] Furthermore, it is preferred that after the freeze-drying process, the fiber preform is re-infiltrated with the slurry.

[0062] A further preferred variant of the method according to the invention is characterized in that, after the freeze-drying process (and preferably after optional stabilization), the produced prepreg(s) are cut, perforated, and / or punched (preferably on flat and curved surfaces) and deposited in layers and shaped into a desired form by pressing, autoclaving, and / or winding. The layer-by-layer deposition is preferably carried out by tape laying, since the deposition and cutting take place in a single process step, and the fiber preforms can be deposited at angles from 0° to 90°.

[0063] According to a further preferred variant of the inventive method, after the freeze-drying process, the prepreg is aged at elevated temperature, e.g., for 1 hour or longer at 140°C to 220°C in a drying cabinet. This allows the water-insoluble secondary binder, e.g., a phenolic resin or a correspondingly curable water-insoluble silicon source, to cure. The duration and temperature of the drying process can be varied depending on the desired degree of cure. At a lower degree of cure, the prepreg layers are subsequently easier to laminate.

[0064] The process according to the invention for producing prepregs is very particularly preferably a process for producing prepregs for the production of fiber-reinforced SiC ceramic components. In this case, the inorganic solid particles used as component i) are carbon-containing solid particles, which are preferably selected from the group consisting of carbon solid particles, metal carbide solid particles, metal nitride solid particles, metal boride solid particles, and mixtures thereof, wherein the carbon-containing solid particles are particularly preferably selected from the group consisting of carbon solid particles, metal carbide solid particles, and mixtures thereof. In addition to the carbon-containing solid particles, the slip can also contain further solid particles, such as metal nitride solid particles and metal boride solid particles.

[0065] The present invention also relates to a prepreg for the production of fiber-reinforced ceramic components, which can be produced by the process according to the invention for producing prepregs for the production of fiber-reinforced ceramic components or has been produced by this process.

[0066] The prepreg according to the invention differs from prepregs already known from the prior art in that it has a homogeneous, fine, crack-free structure and is also very flexible (without flaking). This is due to the freeze-drying process used in the production of the prepreg, as freeze-drying is virtually shrinkage-free.

[0067] The prepreg according to the invention is flexible to radii of up to ≤ 11 mm without flaking. The minimum bending radius can be determined by placing the prepreg at a 90° angle over various cylinder supports, starting with large radii and moving down to smaller radii. If flaking or dust formation is observed, the minimum bending radius has been exceeded. This flexibility enables the prepreg-based production of component contours with comparatively tight radii. The flexibility can be further increased by subsequent curing of the phenolic resin.

[0068] Furthermore, the present invention also relates to a process for producing fiber-reinforced ceramic components, in which a prepreg produced using the process according to the invention for producing prepregs for the production of fiber-reinforced ceramic components (or a green component joined and / or laminated therefrom) is infiltrated with silicon melt. When carbon-containing solid particles are used as inorganic solid particles, i.e., as component i), silicon carbide (SiC) is formed from the carbon and the infiltrated silicon. In this case, a dense, SiC-fiber-reinforced composite body is formed with a fine-scale SiSiC matrix structure with high strength and, if coated fibers are included.

[0069] The homogeneous, fine, crack-free structure of the produced prepreg is partially overshadowed by liquid-phase siliconization. In the competing process of repeated infiltration and pyrolysis of resins ("PIP" = polymer infiltration and pyrolysis, followed by liquid siliconization), shrinkage cracks develop after each pyrolysis process, typically resulting in a network heavily affected by microcracking, which is not the case with freeze-drying. Shadowing effects are also a problem during polymer infiltration, resulting in closed pores. During chemical vapor deposition of SiC for matrix construction, closed pores often remain in the material if certain areas become clogged. These closed pores also do not exist with the freeze-drying route if properly executed.

[0070] A preferred variant of the method according to the invention for producing fiber-reinforced ceramic components is characterized in that the prepreg is subjected to a pyrolysis process before infiltrating the prepreg with silicon melt.

[0071] However, due to the low binder content and the resulting very small change in shape, pyrolysis can be omitted, resulting in a three-stage process consisting of green production, freeze-drying and siliconization.

[0072] The process according to the invention for producing fiber-reinforced ceramic components is most preferably a process for producing fiber-reinforced SiC ceramic components. In this case, the inorganic solid particles used as component i) are carbon-containing solid particles, which are preferably selected from the group consisting of carbon solid particles, metal carbide solid particles, and mixtures thereof.

[0073] The fiber-reinforced ceramic component produced using the method according to the invention differs from fiber-reinforced ceramic components already known from the prior art in that, in addition to its pronounced damage-tolerant fracture behavior, it exhibits a homogeneous, fine, crack-free microstructure with high strength. This is due to the freeze-drying process used in the production of the ceramic component or prepreg, as freeze-drying is virtually shrinkage-free. By using and laminating or joining (optionally preformed) flexible prepregs, even complex, inaccessible geometries, such as those with tight radii, can be realized.

[0074] Furthermore, the use of the fiber-reinforced ceramic component produced by the method according to the invention as a CMC high-temperature component for gas turbines, for furnace systems, in high-temperature plant engineering, for waste incineration plants, in power plant and nuclear fusion technology, as engine components, e.g. as engine mounts, for solar receivers, and / or in aerospace engineering, e.g. in satellite technology, is described.

[0075] The present invention will be explained in more detail with reference to the following example and the figure, without limiting it to the specific embodiments and parameters shown here.

[0076] In Fig. 1 the process diagram of various process routes of exemplary embodiments of the method according to the invention is shown in the form of a flow chart.

[0077] The reference symbols used in the flowchart indicate process steps or points in time within the process and are explained below. 1 - The starting material is a fiber preform, preferably provided with a fiber coating (fiber roving, 2D / 2.5D / 3D fabric, tape, etc.). 2 - The fiber preform is impregnated with slurry and then laid down, or several preform units are stacked in this way. Conversely, the fiber preform can also be laid down first and then impregnated. The procedures can also be alternated or combined in other ways. 2 - Optionally, the impregnated preforms can be partially or completely cut and / or perforated and / or punched out. 3 - The fiber preform is impregnated with slurry and, if necessary, cut to size. 4 - The impregnated fiber preform is subjected to a freeze-drying process in which the water from the slurry is removed without shrinkage. This creates a stable fiber prepreg. 4'-Optionally, the freeze-dried impregnated fiber preform can be thermally cured with a crosslinkable, water-insoluble secondary binder ora cross-linkable, water-insoluble silicon source, by deposition of a stabilizing component via the gas phase, or by impregnation with a liquid stabilizer such as paraffin wax. 5 - This produces a stable, possibly additionally stabilized, flexible prepreg. 6 - A green component is created by lamination, winding, pressing, autoclaving and / or joining of the prepregs, or by any combination thereof. 6' - Optionally, the prepregs can be cut and / or perforated and / or punched out again, provided this has not already been done in step 2', or if the cutting takes place as part of the final processing. 7 - This produces a near-net-shape, laminated, possibly joined green component. 8 - The green component is infiltrated with a metal melt, preferably silicon melt, in a furnace process, resulting in a component made of dense, fiber-reinforced ceramic (e.g. SiC fiber-reinforced SiSiC with minimal residual silicon content).8' - Optionally, but preferably, the green component to be infiltrated is subjected to a defined pyrolysis process before melt infiltration to remove organic matter and deposit carbon. 9 - The component is made of fiber-reinforced ceramic, which is optionally finished. Example

[0078] 29.0 g of deionized water and 1.0 g of phenolic resin (PF0433, Hexion) are placed in a 75 ml plastic beaker for the Speedmixer VAC 1100.1 (Hauschild). 20.0 g of carbon powder (Luvomaxx MT N991, Lehmann&Voss&Co.) are then added and pre-homogenized with a spatula. 20.5 g of 3 mm zirconium oxide grinding beads are then added, and the mixture is treated in the Speedmixer for 1.5 minutes at 1750 rpm. 0.5 g of 20% PVA solution (Mowiol 4-88, Kuraray) and 1.50 g of glycerol are then added, and the mixture is treated again for 1.5 minutes at 1750 rpm.

[0079] The grinding beads are separated using a 1 mm sieve. Unidirectional carbon fiber tapes are cut into approximately 15 cm long strips and pre-attached to an aluminum carrier foil with adhesive tape at the ends. Using a rubber scraper, the slurry is spread into the UD tape so that the fibers are just covered. The adhesive tape is then carefully removed, another aluminum carrier foil is applied, the assembly is turned over, the previously underlying aluminum foil is removed, and the underside of the UD tape is evenly coated with the slurry. The infiltrated UD tape is placed on a flat surface in a freezer at -43 °C and frozen for at least 1 hour.

[0080] The tape is then removed, transferred to a freeze dryer, avoiding thawing, and freeze-dried for at least 24 hours at 2.8*10-3 mbar and -56°C. The freeze-dried prepreg is then slowly brought to room temperature and placed in a drying cabinet to cure the phenolic resin. Depending on the desired degree of cure, the prepreg is stored in the drying cabinet for at least 2 hours at 150-200°C. At a lower degree of cure, the prepreg layers are easier to laminate.

[0081] The resulting prepreg or laminated components can then be infiltrated with silicon melt to produce fiber-reinforced SiC ceramic components.

Claims

1. Method for producing prepregs for the production of fiber-reinforced ceramic components, in which a) a slurry is produced which contains i) inorganic solid particles, ii) at least one water-soluble primary binder, iii) at least one water-insoluble secondary binder and / or at least one water-insoluble silicon source, and iv) water, wherein the inorganic solid particles form a gel network with the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source, b) a fiber preform is impregnated with the produced slurry, and c) the fiber preform impregnated with the slurry is subjected to a freeze-drying process.

2. Method according to the preceding claim, characterized in that the inorganic solid particles are selected from the group consisting of carbon solid particles, metal carbide solid particles, metal nitride solid particles, metal boride solid particles, as well as mixtures thereof.

3. Method according to any one of the preceding claims, characterized in that the at least one water-soluble primary binder is selected from the group consisting of polyvinyl alcohol, polyethylene glycol, polyvinyl butyral, acrylates, latex binders, as well as mixtures thereof.

4. Method according to any one of the preceding claims, characterized in that the at least one water-insoluble secondary binder is selected from the group consisting of phenolic resins, furan resins, epoxy resins, polyester resins, as well as mixtures thereof, and / or the at least one water-insoluble silicon source is selected from the group consisting of silazane resins, siloxanes, silanes, as well as mixtures thereof.

5. Method according to any one of the preceding claims, characterized in that the proportion of the at least one water-insoluble secondary binder and / or the at least one water-insoluble silicon source is less than 10 vol.%, preferably less than 5 vol.%, based on the total volume of the slurry.

6. Method according to any one of the preceding claims, characterized in that the fiber preform is a roving or a textile fabric, e.g. a tape, which has a preferred fiber direction.

7. Method according to any one of the preceding claims, characterized in that during the production of the slurry in step a) firstly a suspension is produced which contains the solid particles, the water-soluble primary binder and the water, and then the water-insoluble secondary binder and / or the water-insoluble silicon source is added to the suspension produced.

8. Method according to any one of the preceding claims, characterized in that prior to the impregnation of the fiber preform in step b) the fiber preform is subjected to a stabilization process in which accessible pore surfaces of the fiber preform are coated with a stabilizing binder by means of wet-chemical coating or gas-phase deposition, wherein the stabilizing binder is preferably selected from the group consisting of (vaporizable) metal precursors, e.g. silanes, silazanes, siloxanes, as well as mixtures thereof.

9. Method according to any one of the preceding claims, characterized in that after the freeze-drying process the prepreg produced is subjected to a stabilization process - in which the prepreg is impregnated with at least one stabilizer, e.g. paraffin wax, wherein the impregnation is preferably carried out by spraying the at least one stabilizer onto the prepreg, by dipping the prepreg into the at least one stabilizer or by squeegeeing the at least one stabilizer into the prepreg, and / or - in which accessible pore surfaces of the prepreg are coated with a stabilizing binder by means of gas-phase deposition, the stabilizing binder preferably being selected from the group consisting of (vaporizable) metal precursors, e.g. silanes, silazanes, siloxanes, as well as mixtures thereof.

10. Method according to any one of the preceding claims, characterized in that after the freeze-drying process the produced prepreg is cut to size, deposited in layers and brought into the desired shape by pressing, autoclaving and / or winding, the layer-by-layer deposition preferably being carried out by means of tape layering.

11. Prepreg for the production of fiber-reinforced ceramic components, produced by a method according to any one of the preceding claims, wherein the prepreg is bendable without spalling to radii of up to ≤ 11 mm, which is determined by laying the prepreg at an angle of 90° over various cylinder supports, starting with large radii and moving to small radii.

12. Method for producing fiber-reinforced ceramic components, in which a prepreg according to claim 11 is infiltrated with silicon melt.

13. Method according to claim 12, characterized in that the prepreg is subjected to a pyrolysis process before the infiltration of the prepreg with silicon melt.