Thermoplastic preceramic composition for producing non-oxide ceramic objects of complex three-dimensional shape by melt deposition

Abstract

The present invention relates to the manufacture of ceramic objects of complex three-dimensional shape by means of additive manufacturing (FDM) or any other shaping method requiring a thermoplastic material. In particular, the invention relates to a novel thermoplastic preceramic composition, the thermoplastic properties of which are controlled by means of a polycarbosilane that has undergone a thermal or chemical crosslinking treatment to modify the extrusion temperature and glass transition temperature thereof. The invention also relates to a method for obtaining, shaping and ceramic transformation of said thermoplastic preceramic composition. The invention also relates to the ceramic objects obtained according to the invention.

Description

[0001]^^ Description Title: Thermoplastic preceramic composition for the production of non-oxide ceramic objects of complex three-dimensional shape by molten deposition. TECHNICAL FIELD OF THE INVENTION The present invention relates to the formulation, the molten shaping and the production of non-oxide ceramic objects of complex three-dimensional shape, as well as the field of thermofusible preceramic polymers and the molten shaping processes, and more specifically those associated with additive manufacturing. The invention finds applications in particular in the field of additive manufacturing of preceramic formulations, and in any molten shaping technique requiring the use of thermoplastic preceramic polymers. STATE OF THE ART At present, complex objects composed of non-oxide ceramics are produced by injection or pressing (hot or cold). For the latter process,A preform is generally developed and then machined and ground to achieve the desired dimensions and measurements. These processes allow for fairly high production rates but are absolutely not flexible and modular in terms of geometry. It will then be necessary to change injection or pressing molds, which are time-consuming operations. The wear of the latter, due to the great hardness of most non-oxide materials, is pronounced, resulting in a significant operating cost. The problem of consolidation, when the raw material used is "conventional", remains, whatever the process. The densification of a complex part by "natural" sintering (no external force can be applied due to the complexity and fragility of the object) is delicate and requires reaching very high temperatures (sometimes of the order of 2400°C). The necessary equipment is therefore very expensive and few manufacturers,entities or laboratories are equipped with them. A known solution is the LSI process involving the infiltration of a porous carbon preform by molten silicon (Si). The material thus obtained is of the SiSiC type. The problem of densification at very high temperatures is therefore resolved but ^ ^^ substituted by a significant development time and operational complexity as well as the presence of residual silicon. The use of the so-called preceramic polymer route provides a solution for the significant lowering of heat treatment temperatures. Preceramic polymers make it possible to prepare technical ceramics of the oxide type (oxycarbides and oxynitrides for example) and more particularly of the non-oxide type in very varied forms and in very broad compositional systems, many of which cannot be accessed by traditional methods such as powder metallurgy,vapor phase decomposition and physical methods (PVD, laser ablation). The implementation of the "preceramic polymers" route is justified due to a variety of advantages compared to traditional ceramic manufacturing techniques. Indeed, the synthesis and crosslinking of preceramic polymers are levers for adapting some of their characteristics such as their solubility, their fusibility and their rheological behavior (eg, their viscosity). Similarly,the addition of mineral fillers of the same composition as the ceramic derived from the polymer makes it possible to adjust these rheological properties. The modulation of these parameters offers a multitude of manufacturing and shaping possibilities making it possible to develop complex geometries. The maintenance of the shape of the post-heat treatment object that one seeks to obtain will also be optimized by the control of the chemistry of the polymers and the presence of mineral fillers possibly introduced initially into the polymer. For example, document EP 3825295 A2 relates to the modification of preceramic polymers for the manufacture of SiC ceramics and other types of ceramic polymers for applications requiring high structural and mechanical strength at temperatures below 1200°C. To this end, it is proposed to modify the skeleton of a polysilazane or a polycarbosilane with pendant modifiers chosen from silicon, boron, aluminum,a transition metal, a refractory metal, or a combination thereof. The modified preceramic polymer can be used to form a ceramic matrix composite with improved ceramic conversion rates and minimized mass losses. However, these ceramic matrix composites are shaped by conventional techniques such as injection and pressing, which are not very flexible or scalable. ^ ^^ The problem of reactive and scalable small-scale shaping can be solved by implementing an Additive Manufacturing process. Indeed, Additive Manufacturing processes dedicated to traditional ceramics (clay, porcelain) or high-performance oxides (Al2O3, ZrO2, etc.) and non-oxides SiC, Si3N4, etc. are expanding rapidly,and the development of new materials dedicated to Additive Manufacturing is considered the main growth factor of this very promising market in terms of the advantages of shaping pre-ceramic materials. For example,Fused Deposition Modeling (FDM) reduces equipment costs due to its simplicity of operation and also reduces the amount of material lost due to the possibility of recycling manufacturing scraps. This technology also limits post-processing operations such as grinding and polishing. These steps are nevertheless essential and depend on the resolution and the level of finish determined by the user within the limits of the process. The ceramic raw materials associated with the FDM process generally consist of mineral powder and one or more thermoplastic organic resins in the form of filaments or granules. Document CN107673763 A gives an example of a process for manufacturing a ceramic structure from a thermoplastic ceramic precursor shaped by FDM,and which is characterized by the use of a thermoplastic preceramic composition comprising a ceramic precursor, a thermoplastic resin, a filler, a crosslinking curing agent, a plasticizer and other modifiers fully mixed and in the form of powder particles which can be heated, melted, cooled and solidified repeatedly. However, the organic matter from the plasticizer or thermoplastic resin, as in the case of conventional processes, must be thermally removed (debinding). Generally, this step is essential and very time-consuming, as well as being able to generate numerous defects such as cracks or pores and can, in the worst case, lead to the collapse of the object. In addition,the loading rates (quantity of mineral powder) observed in the presence of organic additives are relatively low in the case of non-oxide materials. ^ ^^ The consolidation and sintering temperature of these materials is another problem to be solved when it is desired to reduce the sintering temperature, generally higher than 2000°C. The infiltration of prototypes with molten silicon is one of the most widespread solutions, but as already mentioned this technique is very restrictive in practice. Another solution consists of making sintering additions such as Boron, B4C, Carbon,Al2O3 or SiO2 can generally be introduced by powder metallurgy or by mixing by solid or liquid means. However, problems with stoichiometric homogeneity are common due to a non-homogeneous powder mixture. DISCLOSURE OF THE INVENTION The present invention aims to overcome the various drawbacks of the prior art cited above. In particular, the present invention provides a new thermoplastic and highly filled preceramic composition for the manufacture of three-dimensional ceramic objects of non-oxide types, and not requiring organic additives. The invention also relates to the shaping of the new composition by additive manufacturing, in particular by FDM, or any molten shaping process requiring a highly filled thermoplastic material as well as the consolidation of these objects at temperatures below 2000°C. More particularly, the invention relates to a thermoplastic preceramic composition,characterized in that it comprises: - a thermoplastic material chosen from a preceramic polymer crosslinked in a non-critical manner to increase its glass transition temperature and consequently its extrusion temperature, and being hot-melt and in solid or quasi-solid form at room temperature; - inorganic fillers chosen from the group comprising ceramics in the form of powders, whiskers or fibers and optionally comprising particles of metalloids or metals or mixtures thereof. This composition is very advantageous in that it uses a thermoplastic material composed exclusively of a preceramic polymer mixed with inorganic fillers, and therefore allows maximized ceramic yields while retaining optimal rheological properties for its shaping in the molten state, including by the FDM additive manufacturing process. ^ ^^ Advantageously,the composition of the invention is free of organic additives or organic resins. It therefore makes it possible to dispense with a debinding step during the ceramic transformation process, as well as a sintering step since the preceramic polymer acts as a binder. The composition of the invention is also called "highly filled" because it incorporates up to 50% by volume in inorganic fillers. More particularly, the composition of the invention is in the form of solid granules and comprising the following volume fractions: - 50 to 65% of thermoplastic material; - 35 to 50% of inorganic fillers comprising ceramics in the form of powders, whiskers or fibers, and optionally particles of metalloids or metals or mixtures thereof. By whiskers (also called "whiskers" in French),we mean thin and elongated single crystals. This optimal formulation of the composition of the invention is possible due to the adjustment of its rheological properties by means of the preceramic polymer chosen as thermoplastic material. Of course, this preceramic polymer is first of all hot-melt because the composition is intended to be shaped in its molten state. However, in order to optimize its shaping by FDM, this preceramic polymer must also be in solid or quasi-solid form at room temperature (i.e. around 25°C, preferably 20°C). The structure obtained with the composition of the invention will thus be sufficiently consolidated to retain its shape after its 3D printing, and this without an additional UV or thermal crosslinking step. The preceramic polymer chosen as thermoplastic material in the composition of the invention is therefore hot-melt and solid or quasi-solid at room temperature. However,Currently, there are few hot-melt preceramic polymers in solid or quasi-solid form at room temperature, offering rheological properties suitable for FDM and having optimal ceramic yields. In a preferred embodiment, the invention proposes to use a high-yield preceramic polymer with rheological properties adjusted by increasing its glass transition and extrusion temperature so that this polymer can be shaped, in particular by FDM-type processes, without collapse of the structure during production or successive heat treatments. In a preferred embodiment, the preceramic polymer is a non-critically crosslinked thermoplastic polycarbosilane having an extrusion temperature above 50°C, and preferably between 55 and 65°C, and / or having a glass transition temperature of at least 12°C,preferably at least 13°C, 14°C, 15°C or 16°C. This polycarbosilane is obtained from a thermal or chemical crosslinking treatment which has induced a non-critical and reversible crosslinking of the polymer, making it possible to raise its extrusion and glass transition temperature. By non-critical and reversible crosslinking is meant that the polymer has been partially crosslinked and retains its hot-melt character. In contrast, an irreversible critical crosslinking causes the hot-melt character to be lost. In another embodiment, the thermoplastic material is chosen from a polymethylaminoborazine (precursor of boron nitride (BN)), a polyalkylaminoalane (precursor of aluminum nitride), or a polyborosilazane (precursor of boron and silicon carbonitride), the latter being in solid form at room temperature. Of course,the extrusion temperature of the composition of the invention is also controlled by the addition of inorganic fillers. In a preferred embodiment, the composition has an extrusion temperature ^ to 70°C, and preferably between 75 and 120°C. The invention also proposes to control the particle size of inorganic fillers in order to optimize the rheology of the composition, as well as the homogeneity of the ceramic parts. In one embodiment, the composition is intended for its shaping by FDM, and the inorganic fillers have an average particle size ^ to 10 µm, or ^ to 5 µm, and preferably ^ to 1 µm. In another embodiment, the composition is shaped by an injection and / or pressing process and the inorganic fillers have an average particle size ^ to 100 µm, and preferably ^ to 10 µm. According to one embodiment of the composition of the invention,the inorganic fillers are chosen from: - ceramic fillers of the Si(B)C type and preferably having an atomic ratio between silicon and boron equivalent to Si / B=30, or - ceramic fillers of the Si(B)C type and having an atomic ratio between ^ ^^ silicon and boron equivalent to Si / B=30, and boron fillers when the generation of a boron carbide-reinforced composite is desired. The invention also relates to a process for obtaining the thermoplastic preceramic composition according to the invention, said process comprising the following steps carried out under an inert atmosphere: - a step of non-critical crosslinking of a thermoplastic fusible preceramic polymer by means of a chemical crosslinking agent or a heat treatment under conditions insufficient to induce critical crosslinking of said preceramic polymer,and wherein the step of said partial crosslinking of the preceramic polymer is induced to a degree making it possible to increase the extrusion temperature of said preceramic polymer while maintaining the fusibility of said preceramic polymer, and - a step of mixing said fusible and thermoplastic preceramic polymer with inorganic fillers and an organic solvent, said inorganic fillers being chosen from the group comprising ceramics in the form of powders, whiskers or fibers, and optionally comprising particles of metalloids or metals or mixtures thereof, and - a step of evaporating said solvent to obtain said thermoplastic preceramic composition. According to a desired embodiment, the crosslinking step may be carried out prior to the mixing step, or be carried out after the mixing step. For example,the non-critical crosslinking of the preceramic polymer is induced until: - increasing the extrusion temperature of said preceramic polymer to a temperature above 50°C, and preferably between 55 and 65°C, and / or - adjusting the extrusion temperature of the mixture of said preceramic polymer with the inorganic fillers so that an extrusion temperature of said mixture is ^at 70°C, and preferably between 75 and 120°C. In a preferred embodiment, the non-critical crosslinking step is carried out on a thermosetting polycarbosilane by means of a heat treatment at a temperature between 60 and 140°C and more precisely between 100°C and 130°C, and preferably for a duration greater than 30 minutes and more preferably ^ ^^ between 2 and 4 hours. For example, this polycarbosilane is StarPCS, TMSMP730 marketed by Starfire Systems. The method of the invention also comprises a step of planetary grinding of said inorganic fillers to have an average particle size: ^ to 100µm, or ^ to 10µm, or ^ to 1 µm and preferably carried out in an organic solvent and with tungsten carbide balls.The inorganic fillers may be commercially available ceramic fillers, or the method comprises a step of obtaining said inorganic fillers from the preceramic polymer particles in solid form and comprising the following actions; - planetary grinding of the preceramic polymer particles in solid form until an average particle size of ^ to 10 µm is obtained; and preferably with two grinding cycles, a first dry grinding cycle, and a second grinding cycle in an organic solvent with balls of diameter smaller than the diameter of the balls of the first grinding cycle, and - ceramization under an inert atmosphere of said polymer particles at a temperature below 2000°C and above 800°C to obtain said inorganic fillers.The invention also relates to a method for manufacturing a ceramic part, said method comprising: - a step of shaping the composition according to the invention by means of an additive manufacturing technique, or injection or pressing, and - a step of obtaining a preceramic part of predefined shape. In a preferred embodiment, said shaping step is carried out by additive manufacturing and by means of an FDM (fused deposition modeling) machine. The method also comprises a step of ceramizing said preceramic part under an inert atmosphere and at a temperature below 2000°C and preferably between 800 and 1600°C (chosen according to the fillers and the polymer used). At this step, the preceramic part is treated at said temperature with a 2-hour hold, and temperature rise and fall ramps of between 0.5°C and 5°C / min.^ ^^ Finally, the invention also relates to the ceramic parts obtained by FDM according to the preferred embodiment of the composition of the invention. In particular, a non-oxide ceramic part based on silicon carbide (amorphous or β) comprising boron heteroatoms, said ceramic part being derived from a thermoplastic preceramic composition, and having the following characteristics: - an essentially smooth surface free of cracks, and - at least one protruding shape on the surface of said part and having dimensions of the order of a millimeter, and / or different planes connected to each other by angles of less than 80°. In particular, said ceramic part is derived from a thermoplastic preceramic composition according to the invention and comprising inorganic fillers chosen from ceramic fillers of the Si(B)C type having an atomic ratio between silicon and boron equivalent to Si / B=30 and an average particle size ^ to 1 µm.BRIEF DESCRIPTION OF THE FIGURES Other advantages, aims and particular characteristics of the present invention will emerge from the following non-limiting description of at least one particular embodiment of the composition and methods which are the subject of the present invention, with reference to the appended drawings, in which: [Figure 1] shows the results of a ceramization test of the StarPCS polycarbosilane. TM SMP730 pyrolyzed at 1000°C. [Figure 2] shows the viscosity evolution of StarPCS polycarbosilane TMSMP730 as a function of temperature. [Figure 3] shows laser diffraction measurements on the particle size distribution (in number and volume) after grinding cycles 2 and 3 according to the invention. [Figure 4] illustrates non-oxide ceramic parts obtained from a thermoplastic preceramic composition according to the invention, and having been shaped by FDM: (a) photograph of the part pyrolyzed at 1000 °C under vacuum, (b) and (c) scanning electron microscopy (SEM) micrographs of the slice, (d) and (e) of the surface of the beads. [Figure 5] illustrates an impression of a honeycomb-shaped non-oxide ceramic part from the thermoplastic preceramic composition of the invention.^ ^^^ DETAILED DESCRIPTION OF THE INVENTION The present invention aims to produce non-oxide ceramic parts with three-dimensional and complex shapes, of maximum density and which can be consolidated at sintering temperatures below 2000°C. The invention relates in particular to obtaining ceramic parts made of silicon carbide and its derivatives (SiCN, SiCO, etc.), and which can contain a heteroatom (B, Al, etc.) and / or silicon nitride (single phase (Si3N4) and its derivatives (SiN2O2, SiCNO, etc.) or of a composite nature. In particular, the invention relates to a new thermoplastic preceramic composition and its shaping by additive manufacturing, or by any other molten shaping process requiring a thermoplastic material. The thermoplastic nature of the composition implies that the composition has, on the one hand, a hot-melt nature.A hot-melt material is a material that has the characteristic of melting or becoming fluid when subjected to heat. On the other hand, the plastic nature of the composition implies that when the composition is cooled, it returns to its solid state. This solid state is reversible and the material retains the ability to melt by re-application of heat up to its extrusion temperature. The composition of the invention is very advantageous in that it provides a composition whose thermoplastic nature is provided exclusively by the choice of a specific preceramic polymer and does not require additives or organic resins. The composition therefore makes it possible to dispense with a debinding step during the ceramic transformation process, as well as a sintering step since the preceramic polymer acts as a binder.It also makes it possible to increase the rate of ceramic fillers to limit the dimensional shrinkage of the material and has a maximized ceramic yield. The invention notably proposes a composition comprising: - a thermoplastic material chosen from a non-critically crosslinked preceramic polymer, hot-melt and in solid or quasi-solid form at room temperature; - inorganic fillers chosen from the group comprising ceramics (powders, whiskers or fibers) and optionally comprising particles of ^ ^^^ metalloids (B, Si, etc.) or metals (transition or not) or mixtures thereof. In a preferred embodiment, the composition of the invention is shaped by additive manufacturing, and in particular by FDM. Shaping by FDM requires a hot-melt material having a viscosity allowing the shape obtained to be retained at room temperature.It should be noted that the shape retention mechanism after extrusion does not involve UV irradiation during this printing step, but simply the cooling of the material, inducing a sudden increase in viscosity that can go as far as returning to the solid state. When a preceramic polymer is used as the sole thermoplastic material, it must be able to retain its shape after printing by FDM. However, commercially available fusible ceramic polymers in solid form are too soft at room temperature and most undergo too much volume shrinkage to allow them to be formed by FDM with satisfactory results and to maintain the extruded shape during subsequent heat treatments.In order to overcome these drawbacks, the invention proposes to optimize the extrusion and glass transition temperature of a preceramic polymer by means of a thermal or chemical crosslinking treatment and to add inorganic fillers to the polymer. It is these fillers – active and passive – (see later in the text) which will significantly reduce the volume shrinkage and allow the imposed shape to be maintained during extrusion. Advantageously, the crosslinking treatment of the fusible preceramic polymer makes it possible to modify the physical and chemical properties of the latter, and in particular: i) to shift the glass transition temperature of the polymer towards higher temperatures in order to allow the latter to be solid or quasi-solid at room temperature, as well as to increase its extrusion temperature, and ii) to stabilize the polymer during the forming process in the molten state.In the present invention, this polymer with adjusted rheological properties can be used as the only plastic material in the preceramic composition of the invention. This composition is suitable for shaping by FDM because it is ^ ^^^ capable of maintaining the given shape without the need for an additional step of UV radiation or thermal crosslinking. In one embodiment, the crosslinking treatment is chosen to increase the extrusion temperature of the preceramic polymer to a temperature above 50°C, and preferably between 55°C and 65°C and / or to increase its rigidity at room temperature, while maintaining its fusibility. For example, to increase its glass transition temperature to a minimum value between 12 and 16°C, this value being chosen according to the desired shaping process and the loading rate of the composition.The non-critical crosslinking treatment can be chosen from a heat treatment or a chemical treatment using a crosslinking agent. These chemical compounds, such as DiCumyl Peroxide (DCP) or metal complexes or metalloid elements, can be of different natures and used to chemically crosslink the preceramic polymer. The selection of this reaction agent is carried out according to the reactive groups present within the polymer chains. DCP is, for example, particularly well suited to polycarbosilanes. In a preferred embodiment, the crosslinking is carried out by carrying out a low-temperature heat treatment. The latter allows more precise control of the crosslinking of the preceramic polymer than by adding a crosslinking agent.In a preferred embodiment, the preceramic polymer is a polycarbosilane and the crosslinking treatment is a low-temperature heat treatment. By low-temperature treatment is meant a heat treatment at a temperature that does not induce critical crosslinking of the polymer. In particular, the polycarbosilane is a polycarbosilane having phenyl groups, such as StarPCS polycarbosilane. TMSMP730 marketed by Starfire Systems and its thermal crosslinking treatment is carried out at a temperature between 60°C and 140°C, and preferably between 100°C and 130°C. This preceramic polymer with adjusted rheological properties will be mixed in the composition with active and / or passive ceramic fillers (oxides or non-oxide) in order to obtain a preceramic composition allowing the production of objects of complex shape. ^ ^^^ In one embodiment, the invention proposes to prepare these inorganic fillers by the PDC route in order to work the molecular chemistry of these polymers, and in particular polycarbosilanes and polysilazanes, so as to allow the production of functional ceramic parts with optimized or even new properties. In particular, polycarbosilanes of formula [-R1R2Si-R3-]n (R1, R2 = H, or alkyl, alkene, aryl groups etc.; R3 = CH2 for example) result in SiC-based ceramics.Polysilazanes of formula [-R1R2Si- NR3-]n (R1, R2 = R3 = H, or alkyl, alkene, aryl groups etc.) form SiCxN4-x (0 ^x ^4) based ceramics including Si3N4 ceramics (the base unit cell is a tetrahedron centered on a silicon atom with four nitrogen atoms at the vertices; the nitrogen is on a trigonal site and is bonded to three silicon atoms, in the case where R1 = R2 = R3 = H) and the Si-CN system (the different polysubstituted tetrahedra (SiN4, SiCN3, SiC2N2, SiC3N, SiC4) describe this system). According to one embodiment, the invention provides a composition incorporating Si(B)C type ceramic fillers synthesized by the PDC route. For example, the preceramic polymer is AllylHydridoPolyCarboSilane (AHCPS), and the atomic ratio (Si / B) between silicon (Si) and boron (B) is 30, corresponding to the total hydroboration of the allylic functions.In another embodiment, the ceramic fillers synthesized according to the invention can be replaced by other ceramic particles (SiC, TiC, ZrC, Al2O3 etc.) or fibers (SiC, C, Al2O3, etc.), or by commercially available metalloid and metal particles (B, Ti, Zr etc.). These fillers can be divided into two main families: (i) passive fillers (SiC, TiC, ZrC, Al2O3, SiCf, Cf) which do not react with the polymer or with the gases released during pyrolysis and (ii) active fillers (Ti, B, Zr) which react with the gases released during pyrolysis, the atmosphere when it is reactive (ammonia) or with the material during the conversion of the polymer into ceramic to form carbide phases (TiC, B4C, ZrC) or even nitride (TiN, BN, ZrN). The list of anticipated charges described above is not exhaustive.In order to make the composition suitable for FDM, the invention proposes adjusting the particle size of ceramic fillers to have a particle size ^ to 1 µm, and suitable in particular for extrusion with nozzles with an opening diameter of 400 µm. ^ ^^^ In one embodiment, the ceramic fillers are obtained from a preceramic polymer and the invention proposes carrying out at least one cycle, and preferably two planetary milling cycles in order to reduce the preceramic particle size as much as possible. These milled preceramic particles are then ceramized at 1000°C under argon, and then milled to their final size by means of a third planetary milling cycle. The particle size can be modified to adapt to the requirements of the selected melt forming process by modifying the operating parameters of the milling step. A particle size scale of 0.1 to 100 µm can be considered.Advantageously, grinding in polymer form makes it possible to limit as much as possible the pollution of the ceramic powders by the grinding elements. These cycles also make it possible to effectively reduce agglomerates and to lower the particle size below 10 µm. Grinding in ceramic form will allow the reduction of the particle size around or below 1 µm. The size ^ to 1 µm is also very advantageous in that it makes it possible to obtain a perfectly homogeneous mixture with the thermoplastic material of the invention. These inorganic fillers are mixed with the thermoplastic material of the invention so as to obtain a homogeneous mixture in the form of granules. EXPERIMENTAL PART 1. Adjustment of physicochemical properties of the preceramic polymer by thermal crosslinking treatment In a preferred embodiment, the preceramic polymer used is the polycarbosilane StarPCS. TMSMP-730 marketed by Starfire Systems, which is a fusible and thermosetting polycarbosilane with a ceramic yield of around 65%. This StarPCS polymer TM SMP-730 differs from conventional commercial polycarbosilane (PCS) by the presence of aromatic groups in its formula and its production through a different synthesis route. Conventional polycarbosilane is synthesized from polydimethylsilane (PDMS) through a Kumada rearrangement (transformation of Si-Si bonds by Si-CH2-Si bridges), while StarPCS TM SMP-730 is believed to use a different but undisclosed synthesis route. StarPCS Polycarbosilane TMSMP-730 is said by the supplier to be solid at room temperature, however it remains malleable, and has an extrusion temperature between 30 and 40°C. Figure 1 shows a ceramization test carried out on this polymer as marketed, and pyrolyzed at 1000°C under argon. The image shows an extremely porous piece of ceramic made up of numerous bubbles. This result demonstrates that the polymer used as produced by the supplier is not suitable for 3D printing. Indeed, it is very likely that even with the addition of active or passive fillers the part collapses just after extrusion onto the receiving plate. In order to determine the temperature range to be used for the crosslinking treatment proposed by the invention, the viscosity of the polymer alone was first measured as a function of temperature at constant shear stress. Figure 2 shows the evolution of the viscosity as a function of temperature between 50 and 150°C.Indeed, below 50 °C, the polymer is too viscous to allow a measurement, and above 150 °C, it is estimated that the crosslinking of the polymer will already be too advanced to allow the conservation of its fusibility. The decrease in viscosity increases between 120 and 140 °C then a sudden increase in viscosity occurs at 140 °C. This sudden increase corresponds to the critical crosslinking of the polymer and therefore to its passage from the liquid state to the solid state. Different low temperature heat treatments were carried out in order to determine the effect of the heat treatment temperature on the extrusion temperature. These treatments are carried out according to the following protocol: The polymer is introduced into a three-necked flask in a glove box, then it is placed under stirring, heated to the temperature studied for 4 hours and connected to a bubbler to allow the escape of gases and avoid overpressure.In accordance with the viscosity measurement in temperature, 4 different temperatures were tested: 100 °C, 120 °C, 140 °C and 160 °C. At the end of the 4 hours, the balloon is cooled, separated from the rest of the assembly and placed for 30 minutes under dynamic vacuum in preparation for its introduction into a glove box where the polymer is recovered. Empirical extrusion tests were carried out on a test bench in air with a nozzle with a 400 µm opening diameter. Pressure is applied to the polymer and then it is heated. As soon as the extrusion of the material is observable, the corresponding temperature is noted. Table 1 below shows the effects of these treatments. The extrusion temperature is measured and as can be observed, ^ ^^^ the crosslinking treatment temperature increases the extrusion temperature. At a crosslinking temperature of 160°C, the polymer is no longer extrudable due to its increased crosslinking.The heat treatment temperature will therefore be chosen to be lower than 140°C and therefore avoid the start of critical crosslinking and loss of fusibility of the polymer. [Table 1] Treatment temperature Extrusion temperature Glass transition temperature (°C) (°C) (°C) No treatment 30-40 10 100 50 12.0 120 60 16.3 140 70 17.9 160 No extrusion 19.7 The table also shows an increase in the glass transition temperature as a function of the heat treatment temperature. The glass transition temperature of StarPCS. TM SMP-730 without treatment confirms that this polymer is not properly solid in its initial state. Indeed, its glass transition temperature of 10°C is lower than the average ambient working temperature of about 20°C. The non-critical crosslinking treatment of the invention makes it possible to increase the glass transition temperature of StarPCS TMSMP-730, and thus to solidify it further. In this case, the polymer will have to remain slightly soft at room temperature if we want to retain its hot-melt properties and be able to extrude it. We can therefore consider that the polymer is quasi-solid. For the purposes of the description, a preceramic polymer will be considered quasi-solid at room temperature (20°C) when its glass temperature is at least 12°C. Indeed, the inventors' experiments show that from a glass temperature of at least 12°C, the preceramic polymer can be used as the only thermoplastic material in a preceramic thermoplastic composition in a satisfactory manner. Preferably, the glass transition temperature of the preceramic polymer will be at least 13 to 15°C, and optimally at least 16°C to make it suitable for forming by FDM.^ ^^^ The inorganic fillers in the composition will solidify the composition sufficiently so that it can retain its shape after FDM printing, without an additional UV or thermal curing step. In other cases, and when the properties of the preceramic polymer allow it, the non-critical curing treatment should ideally be continued until the polymer is properly solid at room temperature. The glass transition temperature was determined using the standard method for determining the glass transition temperature by differential scanning calorimetry according to ASTM E1356-23. The sample is heated under an inert atmosphere and the heat exchanges are measured and compared to those of a reference. Thus, a physical transition such as the glass transition, and the associated temperature, can be studied using this technique.The analysis is carried out between -50 and 200 °C with a ramp of 10 °C / min under N2 On the other hand, it was also highlighted that the higher heat treatment temperatures tested make it possible to shift the start of mass loss to higher temperatures. This is a sought-after quality because it prevents gas releases associated with this mass loss at the temperatures used during polymer extrusion. The heat treatment temperature will therefore be chosen to increase the polymer extrusion temperature, but not induce irreversible critical crosslinking. In the case studied, the heat treatment temperature of 120 ° C will be preferred for a treatment duration of approximately 2 to 4 hours, but can be considered between 100 ° C and 135 ° C for a prolonged or shortened treatment duration, such as for durations between 30 min and 10 hours.Optionally, the non-critical crosslinking treatment can be carried out with higher temperatures for a very short time not inducing critical crosslinking of the preceramic polymer. 2. Preparation of a thermoplastic preceramic formulation 2.1 Preparation of Si(B)C type ceramic fillers according to the invention Si(B)C type ceramic fillers were synthesized from a polycarbosilane, in particular AllylHydridoPolyCarboSilane (AHCPS) marketed by the company Starfire Systems, under the name StarPCS. TMSMP10. The incorporation of boron into the polycarbosilane structure is done by a hydroboration reaction ^ ^^^ with borane dimethylsulfide (BDMS). Most of the chemical reagents used during the synthesis as well as the modified preceramic polymers are sensitive to air and humidity. In order to preserve them, all manipulations will be carried out under an inert atmosphere with a vacuum / argon ramp, i.e. under dynamic vacuum. In order to produce solid polymer particles, the atomic ratio between silicon (Si), provided by AHPCS, and boron (B), introduced by BDMS, is equivalent to Si / B=30. In this proportion, all the allylic functions of the polymer are hydroborated, this therefore corresponds to the maximum contribution of boron. Hydroboration of AHCPS The AHCPS polymer is first mixed with the solvent, here toluene, in a reaction flask. Borane dimethylsulfide is then introduced into a dropping funnel with solvent.After cooling the reaction flask to 0°C using an ice bath, the hydroboration agent is added dropwise to the polymer / solvent solution while stirring vigorously. To ensure a complete reaction between the boron and the allyl functions of the polymer, the reaction mixture is left stirring for 3 days at room temperature. The solvent is then extracted by placing the flask containing the reaction mixture under static vacuum, and gradually heating to 60°C until the majority of the solvent evaporates. The polymer in particulate form is recovered in a glove box after being placed under vacuum for 30 minutes. The latter will then be sealed in a sealed pillbox and placed in a freezer to freeze the rheology of the polymer and prevent its evolution over time.Grinding of polymeric and ceramic particles The size of the polymeric particles resulting from the hydroboration of AHCPS by BMDS is adjusted by means of the different planetary grinding cycles. The desired particle size is defined by the desired method of shaping the thermoplastic composition. In general, the invention proposes to carry out at least a first grinding of said polymeric particles in order to reduce their size as much as possible, and then to ceramize said particles at 1000 °C under argon. The ceramic particles thus obtained will be subjected to a final grinding step ^ ^^^ to achieve the desired particle size objective and to reduce the particle size distribution. According to a preferred embodiment, the particle size is optimized for shaping by additive manufacturing, and in particular by an FDM machine provided with a nozzle with a 400 µm opening.A particle size distribution equal to or less than one micron is therefore targeted in order to ensure extrusion with this nozzle size. To this end, the invention proposes three planetary grinding cycles: Cycle 1: First grinding of polymer particles: 32 x (5'-5') at 150 rpm with 20 mm diameter balls dry. Cycle 2: Second grinding of polymer particles: 25 x (1'-10') at 500 rpm with 1.6 mm diameter balls in toluene. Cycle 3: Grinding of ceramic particles: 25 x (1'-15') at 450 rpm with 1.6 mm diameter balls in ethanol. Advantageously, all grinding steps are carried out with tungsten carbide (WC) jars and balls in order to maximize the efficiency of this process, limit processing times and pollution. Control particle size measurements were carried out using a laser diffraction particle size distribution analysis technique.These measurements, presented in Figure 3 in the form of a volume and number distribution, were carried out after grinding cycles 2 and 3. Polymer grinding makes it possible to reduce the size of the agglomerates and then to go below 10 µm, however below a certain size, the grinding loses efficiency, particularly due to the lack of hardness of the polymer particles. Ceramic grinding cycle 3 is therefore very advantageous because it makes it possible to reduce the particle size below 1 µm (D50N = 0.760 µm D50V = 0.870 µm) as well as the size of the particle size distribution. 2.2 Protocol for the preparation of a thermoplastic preceramic composition. A thermoplastic preceramic composition according to the invention can be obtained according to the following simplified and non-limiting process: - 5 g of polymer are taken from a glove box under an argon atmosphere and placed in a three-necked flask.20 to 30 mL of toluene are then added in order to dissolve the polymer. Once the dissolution is complete, the flask is removed from the glove box; - 10 g of previously prepared SiBC ceramic fillers are then added to the mixture, which represents 66% by mass of ceramic fillers, or approximately 42% by volume; - the whole is placed under stirring, heated to the reflux temperature of toluene (110.6 °C) for 1 hour to bind the fillers to the polymer and connected to a bubbler to allow the escape of gases and avoid overpressure; - at the end of 1 hour, the flask is placed in an ultrasonic bath for 5 minutes to break up any agglomerates; - then, the flask is placed in an extraction system with a gradual rise to 60 °C, as presented previously, in order to extract the toluene. When boiling ends, the system is placed under dynamic vacuum for 30 min.- Finally, the three-necked flask is separated from the rest of the assembly and placed for 30 minutes under dynamic vacuum for heating to 120°C for 4 hours and connected to a bubbler to allow the gases to escape and avoid overpressure; - at the end of the 4 hours, the flask is cooled and placed for 30 minutes under dynamic vacuum for introduction into a glove box where the formulation is recovered in the form of granules. This same protocol can be used to prepare other compositions according to the invention, for example this same quantity of polymer can be mixed with 17% vol of SiBC powders and 25% vol of Boron particles. Similarly, the heat treatment of the polymer can be carried out prior to its mixing with the inorganic fillers. 3. Shaping of the thermoplastic preceramic composition by additive manufacturing The printing machine used is an FDM machine exposed to air.The thermoplastic preceramic composition, in the form of granules, is placed inside and heated to a temperature of about 110 °C (in the presence of SiBC ceramic fillers) or about 85 °C (in the presence of SiBC ceramic fillers mixed with Boron powders). The printing nozzle used is a 400 µm nozzle and woodpile and honeycomb structures are produced (Figs. 4 and 5). 4. Ceramic transformation The manufactured parts undergo a first heat treatment consisting of transforming the preceramic polymer into ceramic material. This treatment is carried out at 1000 °C, with a 2-hour hold and temperature rise and fall ramps of 0.5 °C / min. A dynamic flow of argon is set up to protect the part from temperature oxidation. Figure 4 shows the ceramized parts thus obtained.As observed in the figures, the ceramic parts do not present any notable defects, whether deformations due to partial melting of the polymer or cracks due to a too sudden or too significant departure of gaseous species. The dimensions of the part are preserved during pyrolysis due to the low proportion of preceramic polymer in the initial preparation as well as its ceramic yield of around 65%, which constitutes a major difference with commercial polymers or alternative robocasting techniques. The beads have a rough surface of a few micrometers (Figure 4 (e)), characteristic of the extrusion and pyrolysis of preceramic polymer, the carbonaceous elements condensing on the surface while the more volatile species escape in the form of gas. The volume of the beads is homogeneous, without porosity or bubbles, synonymous with controlled pyrolysis. ^.

Claims

^^^ Claims 1. Thermoplastic preceramic composition, characterized in that it comprises: - a thermoplastic material selected from a preceramic polymer crosslinked in a non-critical manner to increase its glass transition and extrusion temperature, and being hot-melt and in solid or quasi-solid form at room temperature; - inorganic fillers selected from the group comprising ceramics in the form of powders, whiskers or fibers, and optionally comprising particles of metalloids or metals, or mixtures thereof.

2. Composition according to claim 1, characterized in that the composition is suitable for forming by FDM, and the thermoplastic material of the composition consists of said preceramic polymer crosslinked in a non-critical manner, and is free or essentially free of organic additives or organic resins. 3.Composition according to one of the preceding claims, in the form of solid granules and comprising the following volume fractions: - 50 to 65% of thermoplastic material - 35 to 50% of inorganic fillers comprising ceramics in the form of powders, whiskers or fibers, and optionally particles of metalloids or metals or mixtures thereof.

4. Composition according to one of the preceding claims, in which the preceramic polymer is a polycarbosilane having undergone a thermal or chemical crosslinking treatment, and having an extrusion temperature greater than 50°C, and preferably between 55 and 65°C, and / or a glass transition temperature greater than 12°C and preferably of at least one value between 13 and 16°C.

5. Composition according to one of the preceding claims, said composition having an extrusion temperature ^ to 70°C, and preferably between 75 and 120°C. ^.^^^ 6. Composition according to one of the preceding claims, in which the inorganic fillers are chosen from ceramic fillers of the Si(B)C type, and preferably having an atomic ratio between silicon and boron equivalent to Si / B=30.

7. Composition according to one of the preceding claims, for the generation of a boron carbide reinforced composite, in which the inorganic fillers are chosen from: i) ceramic fillers of the Si(B)C type having an atomic ratio between silicon and boron equivalent to Si / B=30, and ii) boron fillers.

8. Composition according to one of the preceding claims, in which the inorganic fillers have an average particle size of ^ to 100 µm, and preferably ^ to 10 µm.

9. Composition according to one of the preceding claims, in which the inorganic fillers have an average particle size of ^ to 10 µm,and preferably ^ to 1 µm.

10. A method for obtaining a thermoplastic preceramic composition, said method comprising the following steps carried out under an inert atmosphere: - a step of non-critical crosslinking of a thermofusible preceramic polymer by means of a chemical crosslinking agent or a heat treatment under conditions insufficient to induce critical crosslinking of said preceramic polymer, and in which the non-critical crosslinking of the preceramic polymer is induced to a degree making it possible to increase the extrusion temperature of said preceramic polymer while retaining the fusibility of said preceramic polymer, and - a step of mixing said fusible preceramic polymer with inorganic fillers and an organic solvent, said inorganic fillers being chosen from the group comprising ceramics in the form of powders, whiskers or fibers,and optionally comprising particles of metalloids or metals or mixtures thereof, and - a step of evaporation of said solvent to obtain said thermoplastic preceramic composition. ^, ^^^ 11. The method of claim 10, wherein the non-critical crosslinking of the preceramic polymer is induced up to: - increasing the extrusion temperature of said preceramic polymer to a temperature above 50°C, and preferably between 55 and 65°C, and / or increasing the glass transition temperature to a temperature above 12°C, and preferably by at least a value between 13 and 16°C, and / or - adjusting the extrusion temperature of the mixture of said preceramic polymer with the inorganic fillers so that an extrusion temperature of said mixture is ^ at 70°C, and preferably between 75 and 120°C. 12.Process for obtaining a thermoplastic preceramic composition according to claim 10 or 11, said process comprising a step of planetary grinding of said inorganic fillers to have an average particle size: - ^ to 100µm, or ^ to 10µm, or ^ to 1 µm and preferably carried out in an organic solvent and with tungsten carbide balls. 13.Process for obtaining a thermoplastic preceramic composition according to claim 12, comprising a step of obtaining said inorganic fillers from the preceramic polymer particles in solid form and comprising the following actions; -planetary grinding of the preceramic polymer particles in solid form until an average particle size of 10 µm is obtained; and preferably with two grinding cycles, a first dry grinding cycle, and a second grinding cycle in an organic solvent with balls of diameter smaller than the diameter of the balls of the first grinding cycle, and -ceramization under an inert atmosphere of said polymer particles at a temperature below 2000°C and above 800°C to obtain said inorganic fillers. 14.Process for obtaining a thermoplastic preceramic composition according to one of claims 10 to 13, in which the non-critical crosslinking step is carried out on a thermosetting polycarbosilane by means of a ^. ^^^ heat treatment at a temperature between 60 and 140°C and more precisely between 100°C and 130°C, and preferably for a duration greater than 30 minutes and more precisely between 2 and 4 hours.

15. Method for manufacturing a ceramic part, said method comprising: - a step of shaping the composition according to one of claims 1 to 9, or the composition obtained by means of the method according to one of claims 10 to 14 by means of an additive manufacturing or injection or pressing technique, and - a step of obtaining a preceramic part of predefined shape.

16. Method according to claim 15, in which said shaping step is carried out by additive manufacturing and by means of an FDM machine. 17.

18. A method of manufacturing a ceramic part according to claim 17, wherein, in the ceramization step, the preceramic part is treated at said temperature with a plateau of 30 minutes to 4 hours, and preferably 1 to 2 hours, and temperature rise and fall ramps of between 0.5°C and 5°C / min. 19.Non-oxide ceramic part based on silicon carbide comprising boron heteroatoms according to one of claims 1 to 9, said ceramic part being derived from a thermoplastic preceramic composition, and having the following characteristics: - an essentially smooth surface free of cracks, and - at least one protruding shape on the surface of said part and having dimensions of the order of a millimeter, and / or different planes connected to each other by angles of less than 80°.

20. Ceramic part according to claim 19, and in which said preceramic composition comprises inorganic fillers chosen from ceramic fillers of the Si(B)C type having an atomic ratio between silicon and boron equivalent to Si / B=30 and an average particle size ^ to 1 µm. ^.

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