Method for preparing SiC-based nanocrystalline multiphase ceramic material based on polycarbosilane precursor or derivative thereof

Abstract

The invention provides a method for preparing a SiC-based nanocrystalline multiphase ceramic material from a liquid silicon-carbon precursor or a derivative thereof, and belongs to the technical field of ceramic material preparation. Comprising the following steps: mixing liquid polycarbosilane or a derivative thereof, a reinforcement and a sintering aid, heating under an oil bath condition, and continuously stirring to form a gel-state mixture; the obtained gel-state mixture is subjected to high-temperature curing treatment and then subjected to thermal cracking in an inert atmosphere, and amorphous SiC-based ceramic composite powder is obtained; and sintering the obtained amorphous SiC-based ceramic composite powder at a high temperature for a short time to obtain the material. According to the preparation method disclosed by the invention, through a combination method of precursor gelation compounding and rapid sintering in-situ crystallization coating, the problems that a reinforcement is difficult to uniformly disperse and impurities are easy to introduce to influence the densification effect in a traditional mixing method such as ball milling and the like are effectively solved, and nanocrystallization, densification and matrix-reinforcement phase interface strengthening of the material are synchronously realized; the high-strength and high-toughness SiC-based nanocrystalline composite ceramic is prepared.

Description

Technical Field

[0001] This invention belongs to the field of ceramic material preparation technology, specifically relating to a method for preparing SiC-based nanocrystalline multiphase ceramic materials from liquid silicon-carbon precursors or their derivatives. Background Technology

[0002] Silicon carbide (SiC) ceramics have broad application prospects in aerospace, nuclear energy, machinery manufacturing, and electronic devices due to their high melting point, high hardness, high thermal conductivity, and excellent high-temperature resistance, corrosion resistance, and oxidation resistance. However, the inherent brittleness and high densification difficulty of traditional SiC ceramics limit their further application in structural-functional integration and extreme environments.

[0003] To improve the mechanical properties of SiC ceramics, reinforcement and toughening are currently achieved mainly through two approaches: one is grain nanofiberization, which improves the strength and toughness of the material by refining the grain size; the other is the introduction of reinforcements, such as whiskers, chopped fibers, or nanoparticles as a second phase, which enhances fracture toughness through mechanisms such as bridging, pull-out, and crack deflection. However, both approaches face numerous technical challenges in actual fabrication.

[0004] In terms of grain nanostructuring, traditional methods typically use commercially available SiC nanopowder as a raw material for sintering. However, this approach has the following prominent problems:

[0005] (1) The preparation of SiC nanopowder is difficult, the process is complex and the cost is high. There are few high-quality nanopowder products available on the market, which seriously restricts the large-scale preparation of nanocrystalline SiC ceramics.

[0006] (2) Due to the extremely high specific surface area and surface energy of nanopowders, rapid grain growth is very likely to occur during high-temperature sintering, making it difficult to maintain the nanocrystalline structure. Even with low-temperature sintering or the addition of inhibitors, it is still difficult to effectively suppress grain coarsening, and the resulting material often loses the microstructural advantages of nanocrystals, resulting in limited improvement in mechanical properties.

[0007] Therefore, how to prepare and maintain the nanocrystalline structure in a low-cost and simple manner is the key technical bottleneck facing the nanostructuring of SiC ceramics.

[0008] In introducing reinforcements, researchers typically prepare composite materials by adding whiskers or fibers to the SiC matrix. Current main preparation methods include chemical vapor infiltration (CVI), polymer infiltration and pyrolysis (PIP), reactive melt infiltration (RMI), and powder ball milling and sintering. However, these methods generally suffer from the following problems:

[0009] (1) Uneven mixing: When mixing whiskers or short-cut fibers with SiC powder using the traditional ball milling method, the whiskers are prone to entanglement and agglomeration due to their large aspect ratio and high specific surface area, making it difficult to achieve uniform dispersion in the matrix.

[0010] (2) Poor interfacial bonding: The reinforcing phase and the matrix often rely on physical contact or mechanical interlocking, lacking chemical bonding or strong interfacial bonding, resulting in low interfacial bonding strength, poor load transfer efficiency, and difficulty in fully exerting the toughening effect of the reinforcement.

[0011] (3) Complex process and long cycle: For example, CVI and PIP processes require repeated impregnation and pyrolysis processes, resulting in long production cycles and high costs;

[0012] (4) Many material defects: The composite material prepared by RMI method often has residual free silicon, which is less stable at high temperature than pure SiC, affecting its high temperature mechanical properties and oxidation resistance.

[0013] (5) High sintering difficulty: Conventional powder sintering process is high temperature and long time, which not only easily leads to matrix grain growth, but also introduces interfacial stress and microcracks due to the mismatch of the thermal expansion coefficients of the reinforcing phase and the matrix.

[0014] (6) Secondary pollution and reinforcing phase breakage: Traditional preparation methods typically employ a two-step process: first preparing SiC powder, then mixing the reinforcing phase and sintering aid through ball milling. During the ball milling process, the high-speed collision and friction between the milling media (such as ZrO2 milling balls) and the powder inevitably introduces impurities from the milling media. Simultaneously, the reaction of oxygen in the air with the powder surface leads to a significant increase in oxygen content. These impurities (such as ZrO2 particles and oxides) severely hinder the densification and sintering of the material, resulting in a low density and high porosity in the final product. Furthermore, the shearing and impact forces during the ball milling process cause the reinforcing phase, such as whiskers and chopped fibers, to break and fracture, reducing their aspect ratio and causing them to lose their original bridging and pull-out toughening effects. More importantly, the timing of the mixing of the reinforcing phase and sintering aid directly affects the densification process of the material: if they are mixed in after powdering, it is difficult to achieve uniform distribution at the microscale, and the pollution problem caused by ball milling becomes more prominent.

[0015] Therefore, there is an urgent need for a preparation technology that can simultaneously achieve uniform dispersion of the reinforcing phase, strong interfacial bonding, and nanocrystallization of the matrix. Summary of the Invention

[0016] In view of this, this invention addresses two major technical challenges in the preparation of existing SiC-based multiphase ceramic materials by providing a method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors, including a dual innovative strategy:

[0017] In terms of nanocrystallization, this invention adopts the technical route of "amorphous precursor + rapid in-situ crystallization": amorphous SiC-based composite powder is obtained by low-temperature pyrolysis (950°C) of polycarbosilane precursor in an inert atmosphere, avoiding the problems of difficult and costly preparation of commercial nanopowders; then, the short-time high-temperature characteristics of rapid sintering technology are used to rapidly crystallize amorphous SiC into nanoscale grains (<200 nm) in the matrix, effectively suppressing the abnormal growth of nanocrystals in traditional sintering, thereby stabilizing the nanocrystalline microstructure.

[0018] Regarding the reinforcement composite, this invention adopts a technical route of "direct composite of liquid-phase precursors + in-situ coating": utilizing the fluidity and wettability of liquid polycarbosilane, uniform coating and dispersion of whiskers, chopped fibers or nanoparticles are achieved during the gelation stage, fundamentally avoiding the agglomeration problem in traditional ball milling; more importantly, during the rapid sintering process, the amorphous SiC coated on the surface of the reinforcement crystallizes in situ simultaneously with the matrix, forming a continuous and dense nanocrystalline SiC coating layer on the surface of the reinforcement, constructing an in-situ bonding interface between the "reinforcement-matrix", which significantly improves the interfacial bonding strength and load transfer capability.

[0019] This method can solve the two major technical challenges mentioned above in a single process, and obtain SiC-based nanocrystalline composite materials with high density, nanocrystalline structure, uniform distribution of reinforcement, good interfacial bonding and excellent mechanical properties.

[0020] To achieve the above objectives, the present invention provides a method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors, comprising the following steps:

[0021] a) Mixing and gelling treatment

[0022] Liquid polycarbosilane or its derivatives, reinforcement and sintering aid are mixed in a certain proportion, heated and continuously stirred under oil bath conditions to cause the polycarbosilane or its derivatives to undergo a gelation reaction, forming a gel mixture in which the reinforcement is uniformly dispersed.

[0023] Key technical points: The fluidity of the liquid precursor ensures complete wetting and uniform coating of the reinforcement; the three-dimensional network structure formed by the gelation reaction fixes the spatial distribution of the reinforcement, avoiding sedimentation or segregation caused by gravity in subsequent processing. Furthermore, this step involves simultaneously adding the sintering aid and the reinforcement to the liquid precursor during the gelation stage, forming a ternary integrated composite system of "reinforcement-aid-precursor." This avoids contamination from impurities such as ZrO2 introduced by the ball milling media in the traditional "curing and pyrolysis followed by ball milling and mixing of aids" process, as well as the breakage damage to the reinforcement caused by the ball milling shear force, ensuring high purity and uniform microstructure of the material from the source.

[0024] b) Solidification and pyrolysis treatment

[0025] The obtained gel mixture was subjected to high-temperature curing to achieve crosslinking stabilization of polycarbosilane or its derivatives; then thermal pyrolysis was carried out in an inert atmosphere to obtain amorphous SiC-based ceramic composite powder.

[0026] Key technical points: The pyrolysis temperature is controlled at 800~1100°C, which is lower than the crystallization temperature of SiC (about 1300°C) to ensure that completely amorphous SiC is obtained, laying the foundation for subsequent in-situ crystallization; the amorphous SiC is uniformly coated on the surface of the reinforcement to form a composite structure of "reinforcement-amorphous SiC coating layer".

[0027] c) Rapid sintering

[0028] The obtained amorphous SiC-based ceramic composite powder is placed in a rapid sintering equipment and sintered at a short time and high temperature to allow the amorphous SiC to crystallize in situ and form an in situ crystalline coating layer on the surface of the reinforcement, thereby obtaining a SiC-based nanocrystalline multiphase ceramic material with uniformly distributed reinforcement.

[0029] Synergistic effect of dual in-situ mechanisms:

[0030] (1) Matrix nanocrystallization: Amorphous SiC rapidly crystallizes in situ under rapid sintering conditions of short time (300-600 s) and high temperature (1600-1800°C). Due to the extremely short time, the grains do not have enough time to grow sufficiently, forming a nanoscale grain structure of <200 nm.

[0031] (2) In-situ interfacial coating: Amorphous SiC coated on the surface of the reinforcement crystallizes synchronously with the matrix, forming a continuous and dense nanocrystalline SiC coating layer on the surface of the reinforcement, realizing in-situ chemical bonding between the reinforcement and the matrix, and significantly improving the interfacial bonding strength. Finally, a SiC-based multiphase ceramic material with uniform distribution of reinforcement in the matrix, dense structure, and nanocrystalline structure is obtained.

[0032] Preferably, in step a), the liquid polycarbosilane or its derivative is selected from at least one of polycarbosilane, liquid polycarbosilane, polyvinylcarbosilane, allyl hydrogen polycarbosilane, and hydrogen polycarbosilane.

[0033] Preferably, in step a), the reinforcing body is selected from at least one of silicon carbide whiskers, chopped silicon carbide fibers, carbon fibers, silicon nitride whiskers, metal carbide nanoparticles, metal boride nanoparticles, and metal nitride nanoparticles; preferably, the metal carbide nanoparticles are selected from at least one of HfC, TiC, ZrC, and WC, the metal boride nanoparticles are selected from at least one of HfB2, TiB2, ZrB2, and CrB2, and the metal nitride nanoparticles are selected from at least one of AlN, BN, TiN, ZrN, and HfN.

[0034] Preferably, in step a), based on a total raw material mass of 100 wt%, the amount of the reinforcing agent added is 5-30 wt% of the total raw material mass, more preferably 10-20 wt%. The raw material includes liquid polycarbosilane or its derivatives, the reinforcing agent, and sintering aids. The above-mentioned addition amount can ensure the uniform distribution of the reinforcing agent in the SiC matrix and the interfacial bonding effect.

[0035] Preferably, in step a), the sintering aid is selected from liquid-phase sintering aids, solid-phase sintering aids, and combinations of the above two aids. Specifically, the liquid-phase sintering aid is selected from at least two combinations of Al2O3, Y2O3, MgO, CaO, La2O3, CeO2, Nd2O3, Sm2O3, and Yb2O3; the solid-phase sintering aid is selected from at least one of B4C, AlN, and BN; and the composite sintering aid is a combination of a liquid-phase sintering aid and a solid-phase aid.

[0036] The preferred method is to use a combination of Al2O3 and Y2O3 as additives, with a mass ratio of (5~7):(3~5), preferably 6:4. Other commonly used combinations of additives include Al2O3-Y2O3-MgO (mass ratio 6:3:1), Al2O3-La2O3 (mass ratio 7:3), Y2O3-MgO (mass ratio 7:3), etc.

[0037] Preferably, in step a), the amount of sintering aid added is 10-15 wt% of the total mass of the raw materials, based on the total mass of the raw materials being 100%.

[0038] Preferably, in step a), the oil bath heating temperature is 140–180°C, and the duration is 5–10 min. This temperature can promote the gelation reaction of polycarbosilane and obtain a stable gel-state composite system.

[0039] Preferably, in step b), the high-temperature curing is carried out at 200–260°C for 30–120 min, preferably at 250°C for 60 min, in order to achieve full cross-linking and structural stabilization of the polycarbosilane.

[0040] Preferably, in step b), the thermal pyrolysis is carried out under an inert atmosphere (such as argon) at 800–1100°C for 60–180 min, preferably at 950°C for 120 min, to obtain amorphous SiC-based ceramic composite powder. This temperature is lower than the crystallization temperature of SiC, ensuring that the pyrolysis product is completely amorphous.

[0041] Preferably, in step c), the sintering process is selected from one of spark plasma sintering (SPS), electric field-assisted pressure sintering, and Joule heating. Spark plasma sintering is preferred, with a sintering temperature of 1600–1800°C, more preferably 1650–1750°C; a sintering time of 300–600 s, more preferably 360 s; and an applied axial pressure of 50–200 MPa, more preferably 150 MPa.

[0042] By precisely controlling the above process parameters, rapid in-situ crystallization and densification of amorphous SiC can be achieved, while an in-situ crystalline coating layer is formed on the surface of the reinforcement, resulting in SiC-based nanocrystalline multiphase ceramic materials with nanoscale grain size, uniform reinforcement distribution, and strong interfacial bonding.

[0043] The present invention also provides a SiC-based nanocrystalline multiphase ceramic material prepared by the method, which has a relative density ≥97%, a micro Vickers hardness ≥21 GPa, and a fracture toughness ≥2.1 MPa·m^(1 / 2).

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] I. Innovative Advantages in Nanocrystallization

[0046] 1. Low-cost nanocrystal preparation route:

[0047] Amorphous SiC is generated by low-temperature pyrolysis of polycarbosilane or its derivative precursors, avoiding the problems of high difficulty, high cost and limited market supply in the preparation of commercial SiC nanopowder, and significantly reducing raw material costs and process complexity.

[0048] 2. The nanocrystalline structure remains stable:

[0049] Through the dual mechanism of "amorphous precursor + rapid sintering in-situ crystallization", amorphous SiC is rapidly crystallized into <200 nm nanoscale grains at a high temperature of 300-600 s for a short time. This effectively suppresses the abnormal growth of nanocrystals in traditional long-time sintering, stabilizes the nanocrystal microstructure, and thus fully utilizes the grain refinement and grain boundary strengthening effects of nanocrystals.

[0050] 3. Synergistic effect of grain refinement and densification:

[0051] The short-time high-temperature characteristics of the rapid sintering process achieve both high material densification (relative density ≥97%) and preservation of nanoscale grain size, resulting in the optimal combination of high density and nanocrystalline structure. Furthermore, the mechanical properties of the material are significantly improved through the nanocrystalline strengthening effect.

[0052] II. Innovative Advantages in Reinforced Composites

[0053] 4. Excellent dispersibility of the reinforcing agent:

[0054] By utilizing the fluidity and gelation properties of liquid polycarbosilane, uniform coating and dispersion of the reinforcement can be achieved during the mixing stage, avoiding the agglomeration problem of traditional ball milling and achieving a truly uniform distribution of the reinforcement in the matrix.

[0055] 5. Strong interfacial bonding and synergistic toughening effect:

[0056] During the rapid sintering process at short, high temperatures, the amorphous SiC coating on the surface of the reinforcement crystallizes simultaneously with the matrix in situ, forming a continuous and dense nanocrystalline SiC coating layer. This constructs an integrated interface structure of "reinforcement-in-situ coating layer-matrix". This in-situ coating layer has the same chemical composition and crystal structure as the matrix, achieving true interfacial chemical bonding rather than the simple physical contact of traditional physical mixing methods, significantly improving the interfacial bonding strength. This excellent interfacial bonding performance effectively enhances load transfer capability and fully leverages the toughening mechanisms of the reinforcement, such as bridging and crack deflection.

[0057] 6. Prevent secondary pollution and mechanical damage:

[0058] This invention employs a one-step process that simultaneously mixes the liquid-phase precursor, reinforcement, and sintering aid. All components are composited during the gelation stage, forming an integrated gel composite system of "reinforcement-aid-precursor," completely avoiding the traditional "powdered ball milling mixing" process. Traditional methods inevitably introduce impurities (such as ZrO2) and oxygen contamination from the ball milling media during mixing, severely hindering the densification of SiC and resulting in low material density and high porosity. Simultaneously, the shear and impact forces during ball milling cause breakage and damage to the reinforcement, such as whiskers and chopped fibers, reducing their aspect ratio and eliminating their original bridging and pull-out toughening effects. The liquid-phase coating process of this invention eliminates secondary contamination and mechanical damage introduced by ball milling at the source, maintaining the original morphology and structural integrity of the reinforcement. It also achieves a highly uniform distribution of the reinforcement and sintering aid at the microscale, providing technical assurance for high material density, high purity, and the efficient toughening effect of the reinforcement.

[0059] III. Technological and Overall Performance Advantages

[0060] 7. Simple and efficient process:

[0061] Compared with traditional preparation methods of continuous fiber multiphase ceramics such as CVI and PIP, this invention only uses one mixing and one sintering, with fewer process steps, no need for multiple infiltration and pyrolysis, short production cycle and high repeatability; compared with the RMI method, the system of this invention has no residual free silicon and better high-temperature stability; compared with the powder sintering method for preparing multiphase ceramics, this invention avoids secondary pollution and mechanical damage to the reinforcement caused by the ball milling mixing process, and at the same time achieves uniform dispersion of the reinforcement through liquid phase gel coating, ensuring material microstructure uniformity and performance stability.

[0062] 8. Excellent overall performance:

[0063] The obtained SiC-based nanocrystalline multiphase ceramic material has a dense structure (relative density ≥97%), uniform reinforcement distribution, and stable nanocrystalline structure (grain size <200 nm). It has high hardness (micro Vickers hardness ≥21 GPa), high toughness, and excellent high-temperature mechanical properties, and can be widely used in aerospace, nuclear energy equipment, and high-temperature structural components.

[0064] In summary, this invention, through the dual innovative mechanisms of "in-situ crystallization of amorphous precursors" and "in-situ coating of liquid-phase precursors," simultaneously solves two major technical challenges—the nanocrystallization and reinforcement compositing of SiC ceramics—in a single process flow. It achieves high densification, nanocrystalline structure, reinforcement homogenization, and strong interfacial bonding in SiC-based multiphase ceramic materials, providing a simple, efficient, and low-cost new technical approach for the preparation of high-performance SiC-based structural ceramics. Attached Figure Description

[0065] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0066] Figure 1 The theoretical molecular formula of the liquid polycarbosilane precursor used in the examples and comparative examples is given.

[0067] Figure 2 Viscosity-temperature rheological profiles of the liquid polycarbosilane precursors used in the examples and comparative examples.

[0068] Figure 3 Differential thermal-thermogravimetric (DTA-TG) analysis graphs of the liquid polycarbosilane precursors used in the examples and comparative examples.

[0069] Figure 4 This is a SEM image of the silicon carbide whiskers used in Example 2.

[0070] Figure 5 This is a powder SEM image of the SiC-based nanocrystalline multiphase ceramic material prepared in Example 2.

[0071] Figure 6 The XRD pattern of the polycarbosilane powder obtained by sequentially performing gelation reaction and solidification pyrolysis on liquid polycarbosilane is shown.

[0072] Figure 7 The XRD patterns of the sample in Example 1 before and after SPS sintering are shown in the comparison diagram.

[0073] Figure 8 The image shows a SEM image of the fracture surface of the SiCw / SiC nanocrystalline composite ceramic material with 10 wt% silicon carbide whiskers prepared in Example 2.

[0074] Figure 9 The image shows the SEM image of the nanocrystals of the nanocrystalline SiC multiphase ceramic material prepared in Example 1.

[0075] Figure 10 The energy spectrum of the product of Comparative Example 2 is shown below.

[0076] Figure 11 This is a flowchart of a method for preparing SiC-based nanocrystalline multiphase ceramic materials from liquid silicon-carbon precursors or their derivatives according to the present invention. Detailed Implementation

[0077] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0078] Example 1

[0079] The preparation of SiC nanocrystalline ceramics (without reinforcement) using polycarbosilane precursors includes the following steps:

[0080] a) Determination of key process parameters

[0081] Figure 1 The theoretical molecular formula of the liquid polycarbosilane precursor used in the examples and comparative examples is given. First, based on the viscosity-temperature rheological curves of the liquid polycarbosilane precursors used, such as… Figure 2 As shown, the crosslinking and gelation temperature for the transformation from a linear polymer to a three-dimensional network polymer under thermal action was determined. The preferred temperature was 165°C, and the gelation time was 5 min. At this temperature, a stable gel composite system can be obtained, avoiding uneven distribution of the reinforcement due to sedimentation caused by gravity during subsequent processing.

[0082] Select the combination of liquid phase sintering aids Al2O3 and Y2O3, and then according to Figure 3 The differential thermal-thermogravimetric (DTA-TG) curves of the liquid polycarbosilane precursor shown indicate that the residual weight of the polycarbosilane precursor used is approximately 87.37%, which is used to determine the mass of sintering aids to be added to ensure the final composite powder contains m a-SiC :m Al2O3 :m Y2O3 =90:6:4. Where a-SiC is amorphous SiC obtained after the reaction of liquid polycarbosilane precursor.

[0083] Then according to Figure 3 Differential thermal analysis curves were used to determine the optimal curing temperature and optimal pyrolysis temperature of the polycarbosilane precursor used. The figure shows that it has two endothermic peaks at approximately 250°C and 950°C, which correspond to its curing temperature and pyrolysis temperature, respectively.

[0084] Finally, the SPS sintering parameters were selected from the range of common process parameters, with the preferred sintering temperature being 1700°C, the sintering time being 360 s, and the axial pressure being 150 MPa.

[0085] b) Mixing and gelling treatment

[0086] The mass of each raw material added is determined based on the residual weight of the polycarbosilane precursor. The polycarbosilane is then stirred and heated in an oil bath at 160°C to 165°C to promote gelation and obtain a stable gel composite system.

[0087] c) Solidification and pyrolysis

[0088] Based on the curing and pyrolysis temperatures determined by differential thermal analysis, the gel composite mixture obtained in step b) was subjected to curing and pyrolysis treatments. The curing process was carried out at 250°C for 60 min. The pyrolysis process was conducted under a nitrogen atmosphere at 950°C for 120 min. The pyrolysis temperature was controlled below the SiC crystallization temperature to generate amorphous SiC.

[0089] d) Discharge plasma sintering

[0090] Based on common SPS sintering process parameters, the amorphous SiC in-situ coated whisker composite material powder obtained in step c) was sintered at 1700°C for 360 s with an axial pressure of 150 MPa, and finally a nanocrystalline SiC multiphase ceramic material with uniformly distributed whiskers was obtained.

[0091] The relative density was 97.71% as measured by Archimedes' displacement method; the micro Vickers hardness was 22.3 GPa; and the fracture toughness was 2.14 MPa·m^1 / 2.

[0092] Example 2

[0093] The process flow for preparing SiCw / SiC nanocrystalline composite ceramic materials using polycarbosilane precursors and silicon carbide whiskers is shown in the figure below. Figure 11 As shown, it includes the following steps:

[0094] a) Using liquid polycarbosilane as a precursor and silicon carbide whiskers (600 nm in diameter and 10-50 µm in length) as reinforcement, combined with liquid-phase sintering aids Al2O3 and Y2O3, according to m a-SiC :m SiCw :m Al2O3 :m Y2O3 The ingredients were mixed in a ratio of 80:10:6:4. a-SiC is amorphous SiC, and SiCw is silicon carbide whiskers. The mass of each ingredient was determined based on the residual weight of the polycarbosilane precursor. The mixture was stirred and heated in an oil bath at 165°C to promote gelation of the polycarbosilane for 5 minutes. Consistent with Example 1, a stable gel composite system was obtained.

[0095] b) Solidification and pyrolysis

[0096] The gel composite mixture obtained in step a) was subjected to curing and pyrolysis treatment, and the treatment steps and parameters were consistent with those in Example 1.

[0097] c. Discharge plasma sintering

[0098] According to common SPS sintering process parameters, the amorphous SiC-based ceramic composite powder obtained in step b) was sintered at 1700°C for 360 s with an axial pressure of 150 MPa, consistent with that in Example 1, and finally nanocrystalline SiC multiphase ceramic material was obtained.

[0099] The relative density was 98.15% as measured by Archimedes' displacement method; the micro Vickers hardness was 23.5 GPa; and the fracture toughness was 2.1 MPa·m^1 / 2.

[0100] Example 3

[0101] The preparation of HfC / SiC nanocrystalline composite ceramic materials using polycarbosilane precursors and HfC nanoparticles includes the following steps:

[0102] a) Using liquid polycarbosilane as a precursor, HfC nanoparticles (100-200 nm in diameter) as a reinforcement, and combining liquid-phase sintering aids Al2O3 and Y2O3, according to m a-SiC :m HfC :m Al2O3 :m Y2O3 The ingredients were mixed in a ratio of 70:20:6:4. The mass of each ingredient was determined based on the residual weight of the polycarbosilane precursor. The mixture was stirred and heated in an oil bath at 165°C to promote gelation of the polycarbosilane for 5 minutes. Consistent with Example 1, a stable gel composite system was obtained.

[0103] b) Solidification and pyrolysis

[0104] The gel composite mixture obtained in step a) was subjected to curing and pyrolysis treatment, and the treatment steps and parameters were consistent with those in Example 1.

[0105] c. Discharge plasma sintering

[0106] According to common SPS sintering process parameters, the amorphous SiC-based ceramic composite powder obtained in step b) was sintered at 1700°C for 360 s with an axial pressure of 150 MPa, consistent with that in Example 1, and finally nanocrystalline SiC multiphase ceramic material was obtained.

[0107] The relative density was 97.60% as measured by Archimedes' displacement method; the micro Vickers hardness was 23.8 GPa; and the fracture toughness was 3.66 MPa·m^1 / 2.

[0108] Table 1 compares the performance test results of the composite materials obtained by adding HfC reinforcement in two different proportions in Example 3. It shows that when the HfC content is 20 wt%, the composite material achieves the optimal balance between density, hardness, and toughness, with a hardness of 23.8 GPa and a toughness as high as 3.66 MPa·m^1 / 2. However, when the HfC content increases to 30 wt%, the relative density decreases to 96.86%, and the toughness also drops significantly to 2.88 MPa·m^1 / 2. This indicates that excessively high reinforcement content hinders the sintering and densification of the material and may introduce stress concentration due to excessively small particle spacing, thus leading to a decrease in toughness. This optimization result provides a useful reference for the design of different reinforcement ratios.

[0109] Table 1 Properties of composite materials with different HfC reinforcement contents

[0110]

[0111] Comparative Example 1

[0112] SiC nanocrystalline ceramics (without reinforcement) were prepared by secondary ball milling and mixing of cured and pyrolyzed polycarbosilane precursor powder with additives, including the following steps:

[0113] a) Mixing and gelation

[0114] The liquid polycarbosilane precursor was stirred and heated in an oil bath at 165°C to promote gelation of the polycarbosilane for 5 minutes. Consistent with Example 1, a stable gel system was obtained.

[0115] b) Solidification and pyrolysis

[0116] The gel system obtained in step a) was subjected to curing and pyrolysis treatment, and the treatment regime was consistent with that in Example 1.

[0117] c) Ball milling mixing

[0118] Using a combination of liquid-phase sintering aids Al2O3 and Y2O3, according to m a-SiC :m Al2O3 :m Y2O3 The ratio of the solidified and pyrolyzed polycarbosilane precursor powder to the sintering aid was 90:6:4. The mixture was ball-milled at 300 rpm for 12 hours.

[0119] d) Discharge plasma sintering

[0120] Based on common SPS sintering process parameters, the amorphous SiC powder obtained in step c) was sintered at 1700°C for 360 s with an axial pressure of 150 MPa, consistent with that in Example 1, and finally nanocrystalline SiC ceramics were obtained.

[0121] The relative density was measured to be 95.26% using Archimedes' displacement method.

[0122] Comparative Example 2

[0123] HfC / SiC nanocrystalline multiphase ceramic materials were prepared by secondary ball milling and mixing of cured and pyrolyzed polycarbosilane precursor powder with reinforcement and additives, including the following steps:

[0124] a) Mixing and gelation

[0125] The liquid polycarbosilane precursor was stirred and heated in an oil bath at 165°C to promote gelation of the polycarbosilane for 5 minutes. Consistent with Example 1, a stable gel system was obtained.

[0126] b) Solidification and pyrolysis

[0127] The gel system obtained in step a) was subjected to curing and pyrolysis treatment, and the treatment regime was consistent with that in Example 1.

[0128] c) Ball milling mixing

[0129] HfC nanoparticles (100-200 nm in diameter) were used as reinforcements, combined with liquid-phase sintering aids Al2O3 and Y2O3, according to m a-SiC :m HfC :m Al2O3 :m Y2O3 The ratio of polycarbosilane precursor powder after curing and pyrolysis was 70:20:6:4. The powder was mixed with HfC reinforcement and sintering aid by ball milling at 300 rpm for 12 hours.

[0130] d) Discharge plasma sintering

[0131] Based on common SPS sintering process parameters, the amorphous SiC powder obtained in step c) was sintered at 1700°C for 360 s with an axial pressure of 150 MPa, consistent with that in Example 1, and finally nanocrystalline SiC ceramics were obtained.

[0132] The relative density was measured to be 85.96% using Archimedes' displacement method.

[0133] Through the above examples and comparative examples, the effect of changing the timing of adding the reinforcement and sintering aid on the density of the composite material was studied, and the results are shown in Table 2. "One-step composite" refers to the method of this invention, in which the liquid polycarbosilane precursor is mixed with other raw materials, followed by gelation treatment, curing pyrolysis, and sintering. "Secondary ball milling" refers to first gelling the liquid polycarbosilane precursor, then adding the aid and ball milling the mixture, and finally sintering.

[0134] Specifically, neither Example 1 nor Comparative Example 1 contained any reinforcing material; the difference lay in the timing of the mixing of the sintering aids, allowing for comparison. Example 3 and Comparative Example 2 contained the same types and proportions of reinforcing materials, differing only in the timing of the mixing of the sintering aids, also allowing for comparison.

[0135] Table 2. Effect of the timing of addition of reinforcements and sintering aids on densification.

[0136]

[0137] The densification performance comparison in Table 2 shows that the gelation coating of liquid-phase precursors has significant advantages over traditional ball milling and mixing. Specifically, comparing the data from Example 1 and Comparative Example 1 (without reinforcement), Example 1, using the process route of "direct mixing of liquid-phase precursors with sintering aids," obtained nanocrystalline SiC ceramics with a relative density of 97.71%, while Comparative Example 1, using the traditional process route of "ball milling and mixing with sintering aids after curing and pyrolysis," obtained nanocrystalline SiC ceramics with a relative density of only 95.26%, a decrease in density of 2.45%.

[0138] This difference is even more pronounced in composite systems containing reinforcements. Comparing the data from Example 3 (20wt% HfC - one-step composite) and Comparative Example 2 (20wt% HfC - two-stage ball milling), it can be seen that Example 3, using the gelation coating process provided by this invention, achieved a relative density of 97.60% for the multiphase ceramic material. In contrast, Comparative Example 2, using a traditional ball milling mixing process, achieved a relative density of only 85.96% for the multiphase ceramic material. Compared to Example 3, the density of Comparative Example 2 decreased by as much as 11.64%, which fully demonstrates that the traditional ball milling mixing method severely hinders the sintering densification of materials in reinforced composite systems.

[0139] Furthermore, Examples 2 and 3 used silicon carbide whiskers (one-dimensional fibrous reinforcement) and HfC nanoparticles (zero-dimensional particulate reinforcement), respectively, as reinforcements, fully verifying the broad applicability of the method of the present invention. Both types of reinforcements achieved high density (≥97.60%) and excellent comprehensive mechanical properties under the process of the present invention, fully demonstrating that the dual innovative mechanism of "liquid-phase precursor gelation coating + in-situ crystallization" of the present invention does not depend on the morphology and size characteristics of the reinforcement, and has good applicability to various reinforcements such as whiskers, chopped fibers, and nanoparticles. This lays a solid foundation for the application of the present invention in a wider range of reinforcement systems.

[0140] The polycarbosilane powder obtained after curing and pyrolysis was ball-milled with ZrO2 beads, and the change in oxygen content before and after ball milling was tested. The results are shown in Table 3.

[0141] Table 3. Oxygen content of polycarbosilane powder before and after ball milling

[0142]

[0143] Table 3 shows that the ball milling process significantly increased the oxygen content in amorphous SiC powder from 1.55 wt% to 2.97 wt%, an increase of 91.6%. This indicates that the traditional ball milling mixing method affects the densification process of ceramics during sintering.

[0144] Figure 10 The image shows the energy dispersive spectroscopy (EDS) spectrum of the product from Comparative Example 2, with the left image being an electron microscope (EM) image and the right image showing the elemental distribution. EDS analysis further confirmed that the ball milling process introduced a large amount of Zr impurities, which were unevenly distributed. These oxygen and Zr impurities hindered the densification sintering of SiC during the sintering process, resulting in a significant decrease in the density of the product from Comparative Example 2. In contrast, the liquid-phase precursor gelation process of this invention completely avoids the ball milling mixing step, fundamentally eliminating secondary contamination and impurity introduction, and creating favorable conditions for subsequent sintering and densification.

[0145] The following test results verify that the reinforcement in the SiC-based nanocrystalline multiphase ceramic material obtained by this invention can be uniformly dispersed. Figure 4 The image shows a SEM image of the silicon carbide whiskers used in Example 2, with a diameter of 600 nm and a length of 10-50 µm. Figure 5 This is a powder SEM image of the SiC-based nanocrystalline multiphase ceramic material prepared using the gelation coating process of the present invention in Example 2. It can be clearly seen that the amorphous SiC generated after solidification and pyrolysis is uniformly coated on the surface of the silicon carbide whisker reinforcement. The whiskers exhibit a highly uniform dispersion in the matrix, without obvious agglomeration or entanglement. This is due to the excellent fluidity and wettability of liquid polycarbosilane, which achieves complete coating and spatial fixation of the reinforcement during the gelation stage, avoiding the entanglement and agglomeration problems caused by the large aspect ratio and high specific surface area of ​​the whiskers in traditional ball milling mixing.

[0146] The following test results verify that the SiC-based nanocrystalline multiphase ceramic material obtained by this invention has a greater interfacial bonding strength. Figure 8 The fracture morphology of the 10 wt% SiCw / SiC nanocrystalline multiphase ceramic material prepared in Example 2 is shown. It can be clearly observed from the figure that the silicon carbide whiskers fracture synchronously with the matrix rather than being pulled out, which is direct evidence of high interfacial bonding strength. This phenomenon stems from the core innovative mechanism of this invention: during rapid sintering, the amorphous SiC coated on the whisker surface crystallizes synchronously with the matrix in situ, forming a continuous and dense nanocrystalline SiC coating layer on the surface of the reinforcement, achieving in-situ chemical bonding between the reinforcement and the matrix rather than simple physical contact.

[0147] Figure 7The XRD patterns of the sample in Example 1 before and after SPS sintering show that the sample exhibits amorphous bulging peaks before sintering and sharp diffraction peaks after sintering, indicating that the amorphous SiC undergoes in-situ crystallization during sintering to form 3C-SiC. This in-situ crystallization forms an interface layer with the same chemical composition (SiC) and crystal structure (3C-SiC) as the matrix, creating an integrated interface structure of "reinforcement-in-situ coating-matrix". There are no significant abrupt changes in composition or crystal structure mismatch at the interface, resulting in extremely high interfacial bonding strength and significantly improved load transfer efficiency, fully leveraging the toughening mechanisms of reinforcement such as bridging, pull-out, and crack deflection.

[0148] The following test results verify that the present invention achieves nanocrystallization through in-situ crystallization of amorphous precursors. Figure 6 The XRD pattern of the polycarbosilane powder obtained by using only liquid polycarbosilane as raw material without the addition of reinforcing agents and sintering aids, followed by a gelation reaction and curing pyrolysis, is shown. The gelation reaction and curing pyrolysis parameters are the same as in Example 1, and it can be seen that the pyrolysis product is amorphous SiC.

[0149] Figure 9 The grain morphology of the SiC nanocrystalline ceramic material prepared in Example 1 is shown. These nanocrystals were obtained from liquid polycarbosilane through gelation, solidification pyrolysis, and sintering. The figures clearly show that the grain size range of the nanocrystalline ceramic is <200 nm, successfully achieving a nanoscale grain structure. This achievement benefits from the dual mechanism of "amorphous precursor + rapid in-situ crystallization" in this invention.

[0150] First, amorphous SiC was obtained by pyrolyzing a polycarbosilane precursor at a low temperature of 950°C. Figure 6 XRD confirmed the amorphous structure, avoiding the problems of high difficulty and cost in preparing commercial nanopowders. Subsequently, utilizing the short-time high-temperature (sintering at 1700°C for 360 s) characteristics of SPS, the amorphous SiC was rapidly crystallized in situ into nanoscale grains. Figure 7 XRD analysis showed sharp 3C-SiC diffraction peaks after sintering. Due to the extremely short sintering time, the grains did not have enough time to grow sufficiently, effectively suppressing the abnormal coarsening of nanocrystals in traditional long-time sintering and stabilizing the nanocrystalline microstructure.

[0151] In summary, this invention, through a dual innovative mechanism of "liquid-phase precursor gelation coating + amorphous in-situ crystallization," has the following significant advantages compared to traditional ball milling mixing processes:

[0152] (1) Significant improvement in densification performance: In a system containing 20 wt% reinforcement, the relative density of the sample prepared by the method of the present invention is 97.60%, which is 11.64% higher than that of the traditional ball milling method (85.96%), which fully demonstrates the outstanding advantages of the gelation coating process in avoiding secondary pollution and promoting sintering densification.

[0153] (2) High interface bonding strength: The integrated interface structure of "reinforcement-coating layer-matrix" formed by the in-situ crystallization coating mechanism realizes the chemical bonding of the interface. The reinforcement and the matrix break simultaneously rather than being pulled out, giving full play to the role of the reinforcement in improving the performance of multiphase ceramics.

[0154] (3) Stable nanocrystalline structure: The rapid in-situ crystallization process of amorphous precursor successfully prepared nanocrystalline SiC ceramics with a grain size of <200 nm, which effectively suppressed abnormal grain growth during sintering and provided a fine grain strengthening effect for the material.

[0155] (4) The method has strong universality: Whether it is a one-dimensional whisker-like reinforcement (SiC whiskers) or a zero-dimensional particulate reinforcement (HfC nanoparticles), high density and excellent mechanical properties can be achieved by the method of the present invention, which proves the wide applicability of the method in various reinforcement systems.

[0156] (5) Excellent overall performance: The composite material with 20 wt% HfC as reinforcement achieved an excellent combination of properties, including a relative density of 97.60%, a hardness of 23.8 GPa, and a toughness of 3.66 MPa·m^1 / 2. Compared with the material without reinforcement (Example 1: density 97.71%, hardness 22.3 GPa, toughness 2.14 MPa·m^1 / 2), the hardness increased by 6.7% and the toughness increased by 71.0%, which fully demonstrates the significant advantages of the method of the present invention in preparing high-performance SiC-based nanocrystalline multiphase ceramic materials.

[0157] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors or their derivatives, characterized in that, Includes the following steps: a) Mix liquid polycarbosilane or its derivatives, reinforcement and sintering aid, heat and stir continuously in an oil bath to form a gel mixture; b) The obtained gel-state mixture was subjected to high-temperature curing treatment, followed by thermal decomposition in an inert atmosphere to obtain amorphous SiC-based ceramic composite powder; c) The obtained amorphous SiC-based ceramic composite powder is sintered at a short-time high temperature to obtain the SiC-based nanocrystalline multiphase ceramic material.

2. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 1, characterized in that, In step a), the liquid polycarbosilane or its derivative is selected from at least one of polycarbosilane, liquid polycarbosilane, polyvinyl carbosilane, allyl hydrogen polycarbosilane, and hydrogen polycarbosilane; And / or, in step a), the reinforcing body is selected from at least one of silicon carbide whiskers, chopped silicon carbide fibers, carbon fibers, silicon nitride whiskers, metal carbide nanoparticles, metal boride nanoparticles, and metal nitride nanoparticles; preferably, the metal carbide nanoparticles are selected from at least one of HfC, TiC, ZrC, and WC, the metal boride nanoparticles are selected from at least one of HfB2, TiB2, ZrB2, and CrB2, and the metal nitride nanoparticles are selected from at least one of AlN, BN, TiN, ZrN, and HfN.

3. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 1, characterized in that, In step a), based on a total raw material mass of 100 wt%, the amount of the reinforcing agent added is 5 to 30 wt% of the total raw material mass, preferably 10 to 20 wt%.

4. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 1, characterized in that, In step a), the sintering aid is selected from liquid phase sintering aids, solid phase sintering aids, and combinations of the above two aids; Wherein, the liquid phase sintering aid is selected from at least two combinations of Al2O3, Y2O3, MgO, CaO, La2O3, CeO2, Nd2O3, Sm2O3, and Yb2O3; the solid phase sintering aid is selected from at least one of B4C, AlN, and BN; and the composite sintering aid is a combination of the liquid phase sintering aid and the solid phase aid. The preferred additive is a combination of Al2O3 and Y2O3 in a mass ratio of (5~7):(3~5), more preferably 6:

4.

5. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 1, characterized in that, In step a), the amount of sintering aid added is 5 to 15 wt% of the total mass of the raw materials, based on the total mass of the raw materials being 100%.

6. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 1, characterized in that, In step a), the oil bath heating temperature is 140–180°C, and the duration is 5–10 min.

7. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 1, characterized in that, In step b), high-temperature curing is carried out at 200–260°C for 30–120 min. And / or, in step b), the thermal decomposition is carried out under an inert atmosphere at 800–1100°C for 60–180 min.

8. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 1, characterized in that, In step c), the sintering process is selected from one of spark plasma sintering (SPS), electric field assisted pressure sintering, and Joule heating.

9. The method for preparing SiC-based nanocrystalline multiphase ceramic materials based on polycarbosilane precursors according to claim 8, characterized in that, The sintering process uses discharge plasma sintering, with a sintering temperature of 1600–1800°C, a sintering time of 300–600 s, and an applied axial pressure of 50–200 MPa.

10. The SiC-based nanocrystalline multiphase ceramic material prepared by the method according to any one of claims 1 to 9, characterized in that, The composite material has a relative density ≥97%, a micro Vickers hardness ≥21 GPa, and a fracture toughness ≥2.1 MPa·m^(1 / 2).

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