Methods for the production of green bodies

Abstract

In this process, particles of a ceramic material and / or carbon particles are coated with a thermoplastic polymer such that at least 30% of each particle's surface is covered with a layer of the thermoplastic polymer. The particles are then successively built up layer by layer into a powder bed and distributed within each layer. After the application of a layer, the surface is locally irradiated with a laser beam, at least partially melting the thermoplastic polymer. This creates a metallurgical bond between the adjacent and superimposed particles and the thermoplastic polymer, resulting in a three-dimensional semi-finished product.After the semi-finished product is removed from the powder bed, a thermal treatment is carried out in which the thermoplastic material is cross-linked to form an infusible plastic and subsequently converted into carbon in an inert atmosphere with outgassing of volatile components, so that the resulting carbon-bound porous body can be converted into a reaction-bound ceramic by means of a reaction bond.

Description

[0001] The invention relates to a powder bed-based additive manufacturing process for the production of green bodies which can be converted into ceramic components by reaction bonding.

[0002] The invention enables the production of complex green bodies from ceramic, especially carbide ceramic, powders and / or carbon powders, particularly using the Powder Bed Fusion - Laser Beam (PBF-LB) process. These can then be conventionally reaction-bonded by pyrolysis followed by siliconization. The result is ceramic components or composite components that can be used, for example, in the manufacturing or chemical industries or in the energy sector.

[0003] Ceramic components are typically manufactured by shaping and subsequent thermal treatment, which may consist of debinding and sintering. Examples of materials used in classic solid-state sintering include sintered aluminum oxide and sintered silicon carbide. It is also possible to produce ceramic components through reaction bonding (reaction sintering or reaction infiltration) of ceramic powders or powders of elements or element compounds. Silicon-infiltrated silicon carbide is an example of this. Ceramic components can also be produced by bonding ceramic powders with a liquefiable, often dissimilar, bonding agent (e.g., liquid-phase sintering of silicon carbide with dissimilar sintering aids consisting of yttrium oxide and aluminum oxide or aluminum nitride). Glass-bonded silicon carbide is an example of this.These various sintering processes have in common that the ceramic powders must be bonded together to form a green body prior to thermal treatment. Bonding ceramic powders into a defined green body, which can then be transformed into a ceramic component through thermal treatment, presents a technical challenge.

[0004] There are numerous ways to bind particles into green bodies. A green body, in this context, refers to a shaped component that has not yet undergone final heat treatment and usually consists of particles and binders. These binders are typically organic compounds.

[0005] One method of shaping is classic compression molding. Here, a granulate is typically produced from several particles agglomerated together by a binder. These granules generally have diameters of a few hundred micrometers. The goal is for these granules to exhibit good flowability and compactability. In a press, the granules are compacted together to form green bodies. During this process, the granules break apart and ideally form a homogeneous, denser green body structure.

[0006] Another possibility is casting. Here, slips – consisting essentially of binder, water, and powder – are poured into porous molds. The binder is typically organic as well. The porous mold draws the water out of the slip, and a defined, stable shard (i.e., green body) forms on the surface of the mold.

[0007] It is also possible to produce green bodice molds by injection molding, extrusion, casting with curable resins, and many other methods. All these methods have in common that a negative mold or shaping die (e.g., extrusion) must always be used.

[0008] It is also possible to produce ceramic green bodies using additive manufacturing processes before they are debound and sintered or reaction-bonded.

[0009] In Fused Filament Fabrication (FFF), a filament is produced from particles combined with a thermoplastic polymer. The polymer is melted in a nozzle by heat and then deposited under pressure in defined paths to create a three-dimensional green body.

[0010] In binder jetting (BJT), particles embedded in a powder bed are bound by a pressurized fluid introduced via a printhead. This pressurized fluid typically consists of an organic binder and a solvent.

[0011] Vat Photo Polymerisation (VPP) is used to create shapes from a photosensitive suspension consisting of particles and a photoactive organic binder, which hardens through exposure to light.

[0012] Powder Bed Fusion - Laser Beam (PBF-LB) is another additive manufacturing process. In PBF-LB technology, the machine build chamber is filled stepwise with powdered material, similar to BJT. In each layer, the powder is locally melted or partially melted using a laser. The three-dimensional workpiece is then produced layer by layer. By precisely tracing paths with a laser beam, the material is melted in a locally defined manner. The powder particles bond together, forming a cohesive unit up to the previously applied layer. This process is repeated to build the component step by step, ensuring high detail accuracy and material stability.

[0013] Methods for producing SiSiC already exist, e.g., through PBF-LB.

[0014] It is known to process a silicon + SiC mixture using PBF-LB to obtain three-dimensional bodies. Silicon acts as the binder in this process. Due to the high temperatures required to liquefy silicon, this technique must be carried out with high laser power and under inert conditions, otherwise oxide formation will occur. The resulting molded parts have high porosity and require post-infiltration with phenolic resin and subsequent silicification. This technology is therefore very complex.

[0015] Manufacturing processes are also described in the literature in which SiC powder is mixed with carbon and phenolic resin, dried, and then milled together. This process produces resin-ceramic powder mixtures that can be solidified via PBF-LB. In PBF-LB, the resin melts and bonds the solid particles together. A disadvantage is that the milling process leads to a separation of the ceramic particles and the polymer. This results in a broad particle size distribution consisting of fine plastic particles and SiC particles. Such systems have the disadvantage of being prone to layer defects due to their lower flowability in the powder bed and exhibit a relatively low bulk density. Consequently, the quality of the printed components is often suboptimal. Green bodies produced in this way exhibit comparatively high shrinkage during pyrolysis.Due to the poor density of the green material in the powder bed, such components have a very high Si content after reaction bonding.

[0016] A similar process is used, employing phenolic resin powders mixed with ultrashort carbon fibers and formed via PBF-LB. This is followed by pyrolysis and silicification, during which the carbon fibers react completely to form cubic SiC. However, this method also suffers from poor green density and a lack of detail due to the presence of fibers during powder bed buildup.

[0017] EP 2 998 282 B1 describes a process in which a thermoplastic binder and SiC, both in powder form, are used and solidified via PBF-LB. PA (nylon) is proposed as the binder. Due to the low carbon yield, the additively formed body must be re-infiltrated with a carbon source (PF resin or sugar) in the green state.

[0018] According to the process described in DE 198 09 657 B4, a thermoplastic binder and SiC powder are used and soldered via PBF-LB. Post-infiltration is mandatory with the type of binder polymers used.

[0019] According to current technology, the production of green bodies generally involves the use of powder mixtures consisting of solids and organic binders. Due to differing densities, these powder mixtures can segregate during handling (pouring and doctor blade application). This results in an inhomogeneous distribution of the binders within the powder bed. Consequently, this leads to low strengths in both the green and pyrolyzed states. The low strengths and low densities in the green body necessitate post-infiltration after shaping or pyrolysis of the components with a resin or polysaccharide as a carbon donor. Furthermore, the thermal conductivity of these powder mixtures is suboptimal in the laser beam. Only small contact areas exist between the ceramic particles and the polymeric binder in particulate form. Melting of the binder takes longer when it is located in the shadow of the laser beam, obscured by particles.This necessitates working with higher energy densities per unit area (higher laser power, longer dwell times, or smaller distances between laser paths). The inhomogeneous melting of the binder and the high energy input of the laser beam make inerting the powder bed essential. Furthermore, the higher energy densities make precise manufacturing of green bodies less likely, as they result in a larger heat-affected zone, similar to welding. This can lead to unplanned "thickening" of the components.

[0020] The use of powder-based binders results in a greater distance between individual particles in the powder bed, which implies a lower green density. During the melting of the binders, viscous flow and changes in shape or length can occur. This can lead to significant shrinkage perpendicular to the layer orientation during the melting process. This compaction in a preferred direction, which occurs during the PBF-LB process, can lead to mechanical stresses during subsequent high-temperature processes (pyrolysis and, if applicable, silicification), resulting in component distortion or cracking.

[0021] The use of powder-based binders results in relatively low densities during the doctor blade application process, as the typically fine powders of the binder reduce flowability. This leads to doctor blade defects being incorporated into the powder bed within a layer. Consequently, layer displacements can occur within the volume, with the underlying layer, fused with new powder, shifting or tearing away due to its low flowability.

[0022] It is therefore an object of the invention to provide possibilities for the production of green bodies by means of a powder bed-based additive manufacturing process, which are formed with particles, with which green bodies with increased strength and green density can be produced without a mandatory subsequent organic infiltration, and in which particles are arranged more homogeneously.

[0023] According to the invention, this problem is solved by a method having the features of claim 1. Advantageous embodiments and further developments of the method can be realized with features specified in dependent claims.

[0024] In the inventive process for producing green bodies using a powder bed-based additive manufacturing process with which reaction-bonded ceramic components can be produced, particles made of a ceramic, in particular a carbide ceramic material and / or of carbon are coated with a coating of a thermoplastic polymer such that the particles are coated with a layer of the thermoplastic polymer to at least 30% of their surface area.

[0025] The coated particles are successively formed layer by layer into a powder bed and distributed in each layer, as is already the case with powder bed-based additive manufacturing processes.

[0026] After each layer has been applied, the surface of the uppermost layer is locally irradiated with a laser beam, and the locally introduced thermal energy melts the thermoplastic polymer at least partially, resulting in a material-bonded connection between adjacent and superimposed particles and the thermoplastic polymer, thus creating a three-dimensional semi-finished product.

[0027] The laser beam used for this purpose should be focused, operated, and its focal spot / laser spot moved at a feed rate sufficient to at least partially melt the thermoplastic polymer. The energy input during the additive manufacturing of the green bodies should be low enough to prevent thermal decomposition, oxidation, or other transformation of the polymer components. Furthermore, the energy input should not lead to increased oxidation of the surfaces of the ceramics and carbons used.

[0028] A relative density of at least 30% should be maintained for the bed of coated powders according to ISO 697. After solidification using the laser beam, a relative density of at least 30% should be achieved.

[0029] Following shaping and removal of the green body from the powder bed, a thermal treatment is carried out in which the thermoplastic binder at least partially cross-links and transitions into a non-melting state (thermoset). This largely prevents component distortion during the subsequent pyrolysis under inert conditions, in which the polymeric binder is converted to carbon at temperatures between 150°C and 1000°C, releasing volatile compounds. Such carbon-bonded semi-finished products can then be transformed into a ceramic material through reaction bonding with liquid silicon or a liquid alloy containing silicon as its main component. At least some of the carbon formed during pyrolysis also reacts to form a carbide, which increases the density and strength of the resulting reaction-bonded ceramic components.

[0030] It is also possible to produce mixtures of carbide ceramics and carbon particles and coat their surfaces with the polymer. Furthermore, it is also possible to coat only carbon particles with the polymer. The carbon particles can be predominantly amorphous (e.g., glassy carbon), predominantly hexagonal (e.g., graphite or hard graphite), or have a predominantly face-centered cubic crystal structure (e.g., diamond). Mixtures of these carbon particles are also possible. The processing of the carbon-containing powders is identical to that of carbide ceramic powders. In contrast to carbide ceramic powders, at least some of the carbon powder reacts with the inflowing silicon to form silicon carbide during the reaction bonding process. Since the pure density of the carbon powders used is above 0.96 g / cm³ 3This reaction is associated with a reduction in the size of the pores and thus a reduction in the free silicon content in the component structure after the reaction bond, which improves the mechanical and chemical properties of the resulting ceramic material.

[0031] The particles should be coated with thermoplastic material such that the proportion of the thermoplastic polymer can be between 5 volume%, preferably between 10 volume% and 80 volume%, with respect to the carbide ceramic material and / or carbon.

[0032] SiC, B4C, ZrC, HfC, WC, TiC can preferably be used as the carbide ceramic material.

[0033] At least 80% of the particles should be coated with thermoplastic polymer to at least 30% of their surface area, and a maximum of 20% of the particles should remain coated with less than 30% of their surface area.

[0034] Particles with a medium particle size d can be used. 50 It can be used in the range of 10 µm - 200 µm. Thermoplastic polymer should be applied to the surfaces of particles with a maximum average coating thickness of 10 µm.

[0035] The coating of particles can be carried out with a solution of the respective thermoplastic polymer or by melting or vapor deposition of the polymer, preferably in a mixer or with a fluidized bed process or a spray process.

[0036] To maintain optimal processing properties, the particles should predominantly remain as individually coated particles after the coating process. Increased agglomeration of several particles together to form a granular structure impairs processing properties, especially in a doctor blade application. If necessary, the powders should be sieved after the coating process to separate or break up agglomerates.

[0037] Suitable thermoplastic polymers include phenolic resins, furan resins, lignins, and polysaccharides. Novolacs are particularly well-suited. These specific phenolic resins are synthesized in a formaldehyde-phenol ratio of less than 1:1 by acidic condensation of the starting materials. By mixing them with hexamethylenetetramine (HMTA) at a concentration of 5–10 wt% (typically 8 wt%), thermoplastic novolacs can be crosslinked into infusible thermosets upon application of heat.

[0038] A key advantage of the invention lies in applying a thin polymer layer to the ceramic powder particles and / or carbon powder particles. Due to the polymer coating, virtually no unbound, loose polymer-ceramic powder mixture or carbon powder remains in the additively processable powder. The polymer partially or completely encapsulates the surfaces of the ceramic or carbon particles, thus forming a metallurgical bond.

[0039] Coating the ceramic, especially carbide or carbon-based, particles results in less splintery and improved roundness. This enhances the flowability and dredging properties of such prepared powders in the powder bed. The bulk density and density of the powders also increase, as, unlike in the prior art, very few, or ideally no, loose polymer powder particles are present between the ceramic or carbon powder particles. The result is that highly homogeneous and dense powder layers can be achieved in the individual layers of the powder bed process, surpassing the state of the art.

[0040] The partial temperature increase in the laser focal spot causes the polymer layer surrounding the particles to melt, at least partially, and bond with the polymer layer or the surface of adjacent particles. Using laser energy input, bonds between individual particles in the doctored powder of the respective layers can thus be formed in a locally defined manner, controlled by time and temperature. This makes it possible to bond the coated particles in a powder bed (poured, rolled, or doctored) together by selective heating, thereby producing three-dimensionally shaped green bodies or semi-finished products for sintering.

[0041] Since ceramic and carbon-based powders possess a very high thermal conductivity compared to polymers, and only a thin polymer film surrounds the ceramic or carbon-based particles, the thermoplastic polymer present on the surfaces melts very well and homogeneously during the PBF-LB process. In contrast, prior art processes, which largely utilize inorganic and organic powder mixtures, typically involve pure powder contact between the polymer particles and the ceramic or carbon-based particles. The very small contact areas often result in poor melting of the polymer particles. Consequently, prior art laser processes must be operated with higher laser energy densities to ensure at least partial melting of the polymer particles.This necessitates inerting the atmosphere in the powder bed, as is known from the prior art. The polymer-coated particles according to the invention can also be melted and metallurgically bonded in an air atmosphere using very low laser energy densities.

[0042] Since, in the prior art, the polymer particles also act as spacers between the solid particles, a high degree of shrinkage in the depth direction occurs during their melting process. This occurs to a significantly lesser extent in the polymer-coated particles according to the invention.

[0043] The thermoplastic polymer used should be at least partially meltable by the application of thermal energy. The polymer can also be soluble in a solvent, such as ethanol, to form the coatings with such a solution. Simultaneously, it should exhibit the property of crosslinking under the influence of temperature, which can be achieved, for example, by adding a suitable polymer-specific hardener. In this process, the polymer largely loses its thermoplastic character and reacts in such a way that a predominantly infusible polymer is formed. The key factors influencing the crosslinking reaction into an infusible polymer are temperature and time.

[0044] The polymer or its solution should exhibit good wetting properties on the particles to be coated (ceramic or carbon). The wetting angle should be less than 60°.

[0045] It is advantageous if the ceramic and / or carbon particles used have a spherical particle shape. This increases the homogeneity of the polymer coating during the coating process, leads to better flow and doctor blade behavior, and increases the density of the powder bed.

[0046] The polymer coatings on the particle surfaces should cover at least 30%, preferably at least 50%, of the respective particle surfaces and be metallurgically bonded there. The thickness of the polymer layers should be a maximum of 10 µm, and the polymer content should be approximately 10 to 80 volume% relative to the ceramic or carbon. At least 80% of the particles should be at least partially coated with the thermoplastic polymer on their surfaces, and a maximum of 20% should remain uncoated. A bulk density > 45% of the theoretical density should be achievable during layer formation in the powder bed.

[0047] The proportion of particles consisting of free thermoplastic binder, which can form during processing through abrasion or flaking, should be kept as low as possible to avoid impairing the processability of the powders. A maximum of 20% of the binder should be free, and at least 80% should be bonded to the ceramic and / or carbon particles in the form of the powder coating.

[0048] The strong, defined, and uniform bond between the polymer and the ceramic or carbon particles has the advantage of resulting in a homogeneous green body in PBF-LB. If the polymer is predominantly in powder form, as is often described in the prior art, segregation effects inevitably occur during the doctor blade process due to the different particle sizes and densities, leading to inhomogeneities of the ceramic particles in the green body. This has previously resulted in rather poor surface finishes, lower green densities, and an inhomogeneous microstructure. The thermal conductivity into the polymer, which is present as a thin layer around the ceramic or carbon powder particles, is very good and very homogeneous. The polymer therefore melts very well and uniformly, even when very low laser powers and residence times are used. Consequently, inerting the powder bed is not necessary in a process according to the invention.

[0049] The coating with thermoplastic polymer can be very thin, so that shrinkage during the manufacturing process is lower compared to the state of the art.

[0050] The achievable bulk and tapped density of the powders is very high. Consequently, the green density of the produced green bodies is also very high. The strength in the pyrolyzed state is sufficient for handling, so that post-infiltration in the green or brown state, as always described in the prior art, is not strictly necessary.

[0051] No inert atmosphere is required during the laser process. Post-infiltration is not mandatory, which offers a significant cost advantage. The green bodies are sufficiently solid after pyrolysis to be handled. If post-infiltration is desired (though not mandatory), it is performed only after the pyrolysis of the semi-finished product.

[0052] The reaction bond can be formed with silicon or an alloy containing silicon as the main alloying element. Preferably, transition metals that form stable carbides (transition metals of groups 4-5 and 6) should be used as alloying elements. In the reaction bond, silicon reacts with at least a portion of the polymeric binder converted to carbon during pyrolysis to form silicon carbide.

[0053] The length shrinkage of the entire pyrolysis and reaction bonding process is less than 4%, in relation to the additively manufactured green part.

[0054] The following examples should further illustrate the possibilities of shaping: Example 1: To produce a complex green body from SiC by selective melting using laser radiation and conversion into a reaction-bonded ceramic composite

[0055] Silicon carbide powder (mean particle diameter d 50 The particles (measured by laser diffractometry at 53 µm) are placed in an inerted Eirich mixer. A novolak solution dissolved in ethanol, to which hexamethylenetetramine (HMTA) is added as a hardener at a rate of 8 wt%, is introduced stepwise through a nozzle during the stirring process. The mass ratio of novolak (including HMTA) to SiC is 1:13.3. The density of the novolak is 1.4 g / cm³. 3 The density of SiC is 3.2 g / cm³. 3The volume ratios in the mixture are therefore 14.67 vol% phenolic resin to 85.33 vol% silicon carbide. After mixing and evaporation of the solvent, a thin layer of the polymer is present on the surfaces of the silicon carbide particles. A sieving process then takes place, in which any agglomerations of the particles are broken up or sieved away, and any exposed fine polymer particles are separated. The polymer layer thickness on the powders averages 1–2 µm. The surface of each silicon carbide particle is 80% coated with the polymer. Microscopic examination reveals that less than 5% of the particles do not have at least a partially present polymer layer. The powder mixture prepared in this way is selectively and locally bonded layer by layer in a typical laser-based powder bed fusion laser beam printer for polymers. The laser energy input is 681 mJ / mm². 3The hatch spacing was 0.25 mm, the laser beam feed rate was 135 mm / s, and the layer thickness of each powder bed was 100 µm. The laser beam heats the particles, causing the thermoplastic polymer coating to melt, at least partially, and the adjacent particles to bond together. The density of the resulting green bodies is 1.3 g / cm³. 3Following the selective melting process of the polymer-coated powder particles, the resulting semi-finished products can be removed from the powder bed. Subsequently, a tempering process cross-links the organic binder to such an extent that it is no longer meltable. This is followed by pyrolysis and, in this example, silicification. After the reaction bonding, a silicon-infiltrated silicon carbide body is formed. The 4-point flexural strength of the material produced in this way is 200 MPa, and the Weibull modulus is 16. If, after the pyrolysis of the semi-finished product, it undergoes further infiltration with phenolic resin, followed by another pyrolysis and subsequent silicification, the Weibull strength increases to 270 MPa, while the Weibull modulus remains at 16.

[0056] Example 2: Production of a complex green body from graphite and SiC by selective melting using a laser and conversion into a reaction-bonded ceramic.

[0057] Silicon carbide powder (mean particle diameter d 50 70 µm) and graphite powder particles (mean particle diameter d 50 The two components (70 µm measured by laser diffractometry) are mixed together in a tumbling mixer at a ratio of 70 wt% to 30 wt%. A novolac containing hexamethylenetetramine (HMTA) as a hardener component at a ratio of 8 wt% relative to the polymer is dissolved in ethanol at a mass ratio of 1:10. The mass ratio of the polymeric binder to the ceramic powder mixture is 1:9.

[0058] The density of novolac is 1.4 g / cm³. 3 The density of SiC is 3.2 g / cm³. 3 The density of carbon is 2.4 g / cm³. 3The volume ratios in the mixture are therefore 18.76 vol% phenolic resin to 81.24 vol% of the powder mixture.

[0059] The solution is then added to the ceramic powder mixture in an Eirich mixer and mixed in an inert atmosphere. During the mixing process, the solvent evaporates due to evacuation and heating of the suspension. After this drying process, the powder particles have a pronounced polymer coating. The binder layer thickness on the powders on the particle surfaces is less than 2 µm. Approximately 80% of the surface area of ​​both the SiC and graphite particles is covered by the polymer. Less than 5% of the particles lack at least a partial polymer coating. After sieving, in which any particle agglomerations are broken up or removed, the coated particles are selectively bonded layer by layer in a typical laser-based Powder Bed Fusion printer for polymers. The laser energy input was 600 mJ / mm². 3The hatch spacing is 0.25, the laser spot feed rate is 120 mm / s, and the powder bed thickness is 100 µm. The laser beam heats the particles, the thermoplastic coating melts briefly, at least partially, and the adjacent particles bond together. The resulting green body density is 1.33 g / cm³. 3 .

[0060] Following the selective melting process of the polymer-coated powders, the semi-finished products formed in this way can be removed from the powder bed. Subsequently, the organic binder is cross-linked by a downstream tempering process until it is no longer meltable. This is followed by pyrolysis and silicification. The result is a silicon-infiltrated silicon carbide with embedded graphite particles. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 2 998 282 B1

[0017] DE 198 09 657 B4

[0018]