A silicon carbide composite and a method for its production

Abstract

The application belongs to the technical field of inorganic nonmetallic materials, and discloses a silicon carbide composite material and a preparation method thereof. The method is based on a biological template self-assembly structure, and a multi-level pore network composed of micron-level main channels, sub-micron-level branch channels and nano-level surface pores is constructed in a silicon carbide matrix through gradient impregnation, biomimetic mineralization and segmented sintering synergistic regulation. The pore connectivity rate is greater than or equal to 85%, the wall surface density is high, and the grain size is 0.3-2 microns. The method does not require complex equipment, and the process parameters are controllable. The method is suitable for batch preparation of silicon carbide composite materials with specific pore topology, has excellent high-temperature oxidation resistance and impact resistance, and can be widely used in the fields of aero-engine combustion chamber lining, nuclear reactor cladding materials, hypersonic aircraft thermal protection systems and high-performance brake discs.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of inorganic non-metallic materials, and relates to a silicon carbide composite material and a preparation method thereof. BACKGROUND

[0002] The silicon carbide composite material is widely applied to extreme service environments such as aerospace hot end components, nuclear energy structural materials and high-performance friction and wear systems due to excellent high-temperature stability, high hardness, good thermal conductivity and chemical inertness. The porous silicon carbide composite material becomes a research hotspot, and the core design idea is to realize stress dispersion, energy dissipation and crack path regulation by introducing a controllable pore structure.

[0003] In the prior art, the preparation of the porous silicon carbide material mainly depends on a physical pore-forming method (such as adding a pore-forming agent or a foaming method) or a chemical vapor deposition combined with a template sacrifice method. Although this kind of method can control the porosity to a certain extent, there are obvious limitations in the accurate control of the three-dimensional spatial distribution, connectivity and micro-morphology of the pores.

[0004] The traditional pore-forming process often leads to a wide distribution of pore sizes, isolated and random orientation of pore channels, and it is difficult to form an effective stress transmission network; such non-directional pore structure is prone to induce local stress concentration when bearing external load, which in turn accelerates the initiation and penetration of micro-cracks, thereby weakening the overall strength and toughness synergy effect of the material.

[0005] Under this background, the biological template method is concerned due to its unique self-assembly characteristics and natural multi-level ordered structure. Biological systems such as bacterial colonies, plant vascular bundles or cellulose networks in nature form highly optimized hierarchical pore configurations in the evolution process, such as honeycomb-like cavities, interconnected fiber networks and gradient pore size distribution, which not only endow the organism with excellent mechanical adaptability, but also provide an ideal template for the biomimetic construction of inorganic materials.

[0006] Existing explorations mainly stay at the level of structure replication, and the internal contradictions between the uniformity of precursor penetration, the thickness control of mineralization layer and sintering densification have not been systematically solved. If the pore connectivity is too high, the strength and stiffness of the material may be sacrificed; and if the structure fidelity is excessively pursued and the sintering temperature is limited, the weak crystal boundary combination will affect the high-temperature service performance. There is a lack of precise matching between the complex geometric shape of the biological template and the reaction kinetics of the inorganic precursor, so that the final material is difficult to balance between the functionality of the pores and the mechanical integrity. SUMMARY

[0007] To achieve the above-mentioned purposes, the present application provides a silicon carbide composite material and a preparation method thereof.

[0008] The silicon carbide composite material comprises a silicon carbide crystal phase skeleton and a multi-level pore network penetrating through the silicon carbide crystal phase skeleton; the multi-level pore network is composed of primary micron-level main channels, secondary sub-micron-level branch channels and tertiary nano-level surface pores; the primary main channels are arranged in a directional arrangement or a biomimetic honeycomb-like topological configuration, have a pore size range of 5-50 μm, and have a pore connectivity rate of greater than 85%; the secondary branch channels extend radially from the walls of the main channels, have a pore size range of 0.5-5 μm, and have a distribution density of 100-500 per square millimeter; the tertiary surface pores are uniformly distributed on the inner walls of the channels, have a pore size of less than 200 nm, and have a specific surface area of greater than 15 m 2 / g; the silicon carbide crystal phase skeleton has a grain size of 0.3-2 μm, complete grain boundary bonding, no glass phase residue, and a relative density of 65%-85%.

[0009] The preparation method comprises the following steps:

[0010] Step 1: Pretreatment of biological templates. Biological materials with natural hierarchical channel structures are selected as templates, and the biological materials are selected from Bacillus subtilis colony membranes, ramie fiber bundles or pine thin-walled tissue sections; the biological materials are ultrasonically cleaned in deionized water for 10 minutes to remove surface impurities, and then dried in a vacuum drying oven at 60°C for 24 hours; the dried biological templates are subjected to oxygen plasma treatment for 30 seconds to increase the surface hydroxyl density to 2.5 / nm 2 The above is to enhance the adsorption capacity of the subsequent precursor.

[0011] Step 2: Preparation of a precursor solution. Polycarbosilane is dissolved in a xylene solvent to prepare a solution with a mass concentration of 15%; nano-silicon carbide powder with an average particle size of 50 nm is added to the solution, and the addition amount is 8% of the mass of the polycarbosilane; zirconium acetylacetonate is added as a cross-linking promoter, and the molar concentration is 0.8% of the repeating units of the polycarbosilane; the mixed solution is magnetically stirred at 40°C for 4 hours, and then filtered through a 0.22 μm polytetrafluoroethylene filter membrane to obtain a homogeneous and stable precursor solution.

[0012] Step 3: Gradient impregnation and mineralization. The pretreated bio-template was completely immersed in the precursor solution, and the vacuum was applied for 15 min at a vacuum degree of -0.095 MPa to remove the air in the pores, and then the normal pressure was restored and the immersion was carried out for 2 h. After taking out, it was dried in a 80℃ air-drying oven for 1 h to complete the first immersion. The above-mentioned immersion-drying process was repeated for 3 times, and the addition amount of nano-silicon carbide powder in the precursor solution for each immersion was 8%, 12% and 15% respectively to realize the gradient thickening of the pore wall from outside to inside. After the last drying, the sample was placed in a water vapor saturated environment for 24 h at 40℃ and 95% relative humidity to promote the hydrolysis-polycondensation reaction of polycarbosilane on the surface of the bio-template fiber, forming an inorganic-organic hybrid layer with a thickness of 200-800 nm.

[0013] Step 4: Low-temperature cross-linking and curing. The mineralized sample was placed in a nitrogen atmosphere tube furnace, and the temperature was raised to 200℃ at a rate of 1℃ / min, and the temperature was kept for 2 h to make the polycarbosilane fully cross-linked. Then the temperature was continuously raised to 400℃ at a rate of 0.5℃ / min, and the temperature was kept for 4 h to complete the partial ceramic conversion of the organic component, and a carbon-containing intermediate was obtained.

[0014] Step 5: Step-by-step high-temperature sintering. The carbon-containing intermediate was transferred to a graphite crucible and placed in an induction sintering furnace in an argon protective atmosphere. First, the temperature was raised to 1000℃ at a rate of 3℃ / min, and the temperature was kept for 1 h to make the residual organic matter pyrolyze into amorphous carbon. Then the temperature was raised to 1600℃ at a rate of 2℃ / min, and the temperature was kept for 2 h to trigger the carbothermal reduction reaction: SiO2+3C→SiC+2CO↑, in which SiO2 comes from the ash of the bio-template and the by-product of the precursor oxidation. Finally, the temperature was raised to 1850℃ at a rate of 1℃ / min, and the temperature was kept for 1.5 h to promote the growth of silicon carbide grains and the densification of grain boundaries, while controlling the abnormal growth of grains. The argon flow was maintained at 5 L / min during the entire sintering process, and the pressure in the furnace was normal pressure.

[0015] Step 6: Post-processing and structure stabilization. After sintering, the sample was naturally cooled to room temperature. The obtained sample was heat-treated in air at 500℃ for 30 min to completely oxidize and remove the residual amorphous carbon, leaving only the silicon carbide crystal phase skeleton. Then it was ultrasonically cleaned in deionized water for 10 min, and dried to obtain the target silicon carbide composite material.

[0016] As a preferred embodiment of the present application, the biological template is a Bacillus subtilis colony membrane, which is obtained by culturing at 37℃ for 24 hours on LB solid medium, the colony diameter is 8-12mm, the surface presents regular hexagonal honeycomb microcavity structure, the cavity depth is 10-30μm, and adjacent cavities are connected by micropores with a diameter of 2-5μm; after the treatment by the method of the present application, the primary main channels of the obtained silicon carbide composite material inherit the honeycomb topology of the colony membrane, the pore size standard deviation is less than 3μm, and the channel wall thickness is 1.5-3μm.

[0017] As another preferred embodiment of the present application, the biological template is a degummed ramie fiber bundle, after alkali boiling degumming treatment, the fiber diameter is 20-50μm, and the surface of a single fiber has longitudinal grooves distributed along the axial direction, the groove width is 0.8-2μm, and the depth is 0.3-1μm; after the treatment by the method of the present application, the obtained silicon carbide composite material presents a fibrous network porous structure, the inter-fiber pores are primary main channels, the pore size is 20-40μm, the fiber surface grooves are converted into secondary branch channels, and stress buffer chambers are formed at the fiber intersection nodes, effectively inhibiting crack propagation in a single direction.

[0018] By controlling the addition proportion of nano-silicon carbide in the third immersion in step 3, the present application realizes the composition and density gradient of the channel wall surface: the outermost layer (the first immersion layer) has a lower silicon carbide content, which is beneficial to the interface combination with the biological template; the middle layer (the second immersion) has a moderate silicon carbide content, which provides structural support; the innermost layer (the third immersion) has the highest silicon carbide content, forming a dense inner wall to prevent the collapse of the channel in the sintering process. This gradient structure makes the material deflect the crack first at the dense inner wall when bearing external load, and then scatter and passivate the crack multiple times at the multi-level pore interfaces, finally dissipating a large amount of fracture energy.

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] The technical scheme of the present application solves the core problems of random pore structure, uncontrollable connectivity, loose wall surface and prominent contradiction between strength and toughness of the porous silicon carbide material in the prior art. Through the structure guiding effect of the biological template, the composition regulation of the precursor gradient impregnation, the interface strengthening of the biomimetic mineralization and the kinetics matching of the sectional sintering, the unification of the pore functionality and the mechanical integrity is realized. The primary main pore channel provides a lightweight and macro stress dispersion channel, the secondary branch pore channel induces crack branching and energy dissipation, and the tertiary surface pore enhances the interface friction damping effect, and the synergistic effect of the three significantly improves the damage tolerance of the material. At the same time, the wall densification design ensures the structural stability under high temperature service, and avoids the sharp drop in strength caused by excessive opening of the pores. The method described in the present application does not require complex equipment, and the process parameters are controllable, and is suitable for batch preparation of silicon carbide composite materials with specific pore topology, and can be widely applied in the fields of aero-engine combustion chamber lining, nuclear reactor cladding materials, hypersonic aircraft thermal protection system and high-performance brake disc, etc. DETAILED DESCRIPTION

[0021] The present application provides a silicon carbide composite material and a preparation method thereof, which fuses the self-assembly structure characteristics of the biological template, multi-stage precursor gradient impregnation, biomimetic mineralization interface strengthening and sectional sintering kinetics regulation, constructs a multi-level pore network with three-dimensional interconnectivity, pore size gradient distribution and wall densification in the silicon carbide matrix, thereby significantly improving the fracture toughness, impact resistance and crack deflection capability of the material on the basis of maintaining high thermal stability and chemical inertness.

[0022] The technical scheme of the present application will be described in detail below in combination with specific examples and comparative examples to ensure that those skilled in the art can fully understand and implement the present application.

[0023] Example 1: The biological template is Bacillus subtilis colony film; the nano-silicon carbide addition amount is 8% / 12% / 15% for three times of impregnation; the sectional sintering is 1000℃ / 1600℃ / 1850℃; the biomimetic mineralization is 24 hours; and the argon flow is 5L / min;

[0024] Preparation process: biological template pretreatment → precursor solution preparation → gradient impregnation-drying cycle → biomimetic mineralization → crosslinking and curing → sectional sintering → post-processing → finished product.

[0025] Example 2: The biological template is degummed ramie fiber bundle, and the rest of the formula and process are the same as those in Example 1;

[0026] Preparation process: the same as Example 1 (template pretreatment is changed to alkali boiling degumming).

[0027] Example 3: only two times of gradient impregnation (nano-silicon carbide addition amount is 8% / 12%), and the rest of the formula and process are the same as those in Example 1;

[0028] Preparation process: same as example 1 (reducing one impregnation cycle).

[0029] Example 4: four times gradient impregnation (nano-silicon carbide addition amount 8% / 12% / 15% / 18%), the rest of the formula and process are the same as example 1;

[0030] Preparation process: same as example 1 (increasing one impregnation cycle).

[0031] Example 5: the highest sintering temperature is 1800℃, the rest of the formula and process are the same as example 1;

[0032] Preparation process: same as example 1 (adjusting the highest sintering temperature).

[0033] Example 6: the highest sintering temperature is 1900℃, the rest of the formula and process are the same as example 1;

[0034] Preparation process: same as example 1 (adjusting the highest sintering temperature).

[0035] Example 7: biomimetic mineralization for 12 hours, the rest of the formula and process are the same as example 1;

[0036] Preparation process: same as example 1 (shortening the mineralization time).

[0037] Example 8: biomimetic mineralization for 36 hours, the rest of the formula and process are the same as example 1;

[0038] Preparation process: same as example 1 (extending the mineralization time).

[0039] Comparative example 1: no biological template, adding 15% starch as a pore-forming agent; no gradient impregnation and biomimetic mineralization; the rest of the formula and process are the same as example 1;

[0040] Preparation process: raw material mixing → molding → crosslinking and curing → staged sintering → post-processing → finished product.

[0041] Comparative example 2: no gradient impregnation, single impregnation of nano-silicon carbide addition amount 12%; no biomimetic mineralization; the rest of the formula and process are the same as example 1;

[0042] Preparation process: biological template pretreatment → single impregnation → drying → crosslinking and curing → staged sintering → post-processing → finished product.

[0043] Test method:

[0044] Mechanical property test: single edge notched beam method to determine the fracture toughness; universal material testing machine to determine the three-point bending strength; elastic modulus test.

[0045] Pore and structure test: mercury intrusion method to determine the pore connectivity and pore size distribution; specific surface area analyzer to determine the specific surface area; scanning electron microscope to observe the grain size and pore morphology.

[0046] High temperature stability test: 1400℃ air environment oxidation for 100 hours, mass loss rate was measured; surface integrity after oxidation was observed.

[0047] Test data comparison is shown in Table 1 and Table 2.

[0048] Table 1 Comparison table of fracture toughness, three-point bending strength, and pore connectivity rate

[0049] ;

[0050] Table 2 Comparison table of specific surface area, grain size, and mass loss rate after 1400℃ oxidation

[0051] ;

[0052] The fracture toughness of Examples 1-8 is ≥4.8 MPa·m 1 / 2 , and the pore connectivity rate is ≥85%, which is much better than the comparative examples; the pores of Comparative Example 1 are irregular, and the mechanical properties and stability of Comparative Example 2 are poor due to the single impregnation without gradient structure, which confirms that the core process is the key to high performance.

[0053] The fracture toughness and strength are simultaneously improved with the increase of impregnation times (Example 3→1→4); the wall densification is optimized with the extension of biomimetic mineralization time (Example 7→1→8); the grain growth and strength are balanced at a sintering temperature in the range of 1800-1900℃.

[0054] The examples have high toughness and high connectivity, which are suitable for lightweight high temperature scenes; the high temperature oxidation resistance is excellent, which meets the service in extreme environments; the process is compatible with existing sintering equipment, and is suitable for large-scale production.

[0055] Compared with the traditional pore forming method (Comparative Example 1), the fracture toughness of the examples is improved by 117%, and the bending strength is improved by 83%; compared with single impregnation (Comparative Example 2), the toughness is improved by 56%, and the pore connectivity rate is improved by 18%, which solves the industry problems of traditional porous silicon carbide, such as strength-toughness contradiction and irregular pores.

[0056] In summary, the composite material described in the application is guided by a biological template and cooperates with gradient impregnation, and different parameter combinations can achieve the cooperation of high toughness, high strength and high stability, which is suitable for harsh scenes such as aerospace and nuclear energy

[0057] The above is only a preferred embodiment of the application, and is not intended to limit the application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the application shall be included in the protection scope of the application.

Claims

1. A silicon carbide composite material, characterized by, The silicon carbide composite material comprises a silicon carbide crystalline phase skeleton and a multi-level pore network penetrating through the same; The multi-level pore network comprises primary micron-level main channels, secondary sub-micron-level branch channels and tertiary nano-level surface pores; The primary micron-level main channels are arranged in a directional arrangement or a biomimetic honeycomb-like topological configuration, have a pore size of 5-50 μm and a pore connectivity rate of greater than 85%; The secondary sub-micron-level branch channels extend radially from the walls of the main channels, have a pore size of 0.5-5 μm and a distribution density of 100-500 per square millimeter; The tertiary nano-level surface pores are uniformly distributed on the inner walls of the channels and have a pore size of less than 200 nm; The silicon carbide crystalline phase skeleton has a grain size of 0.3-2 μm, complete grain boundary bonding and no residual glass phase, and has a relative density of 65%-85%; The channel walls have an outward-to-inward composition and density gradient structure: the outermost layer has a silicon carbide volume fraction of 30%-40%, the middle layer has a silicon carbide volume fraction of 45%-55%, and the innermost layer has a silicon carbide volume fraction of 60%-70%.

2. The silicon carbide composite of claim 1, wherein, The primary micron-level main channels inherit the hexagonal honeycomb-like microcavity structure of the Bacillus subtilis colony membrane, have a pore size standard deviation of less than 3 μm, a channel wall thickness of 1.5-3 μm, and a wall surface density of greater than 92%.

3. The silicon carbide composite of claim 2, wherein, The adjacent cavities of the primary micron-level main channels are connected to each other through micropores with a diameter of 2-5 μm.

4. The silicon carbide composite of claim 1, wherein The primary micron-level main channels are formed by the gaps between degummed ramie fiber bundles, and have a pore size of 20-40 μm; the secondary sub-micron-level branch channels are converted from longitudinal grooves on the surface of the fibers, and have a groove width of 0.8-2 μm and a groove depth of 0.3-1 μm.

5. The silicon carbide composite of claim 4, wherein, Stress buffer cavities are formed at the fiber intersection nodes, and have a diameter of 10-20 μm.

6. A method of producing a silicon carbide composite material as claimed in any one of claims 1 to 5, characterized in that The method comprises the following steps: S10: pretreatment of the biological template, which is selected from Bacillus subtilis colony membrane, ramie fiber bundle or pine parenchyma section, is subjected to ultrasonic cleaning, vacuum drying and oxygen plasma treatment to make the surface hydroxyl density ≥2.5 / nm 2 ; S20: preparing a precursor solution, dissolving polycarbosilane in xylene to form a 15 wt% solution, adding nano-silicon carbide powder, the first addition amount being 8% of the mass of the polycarbosilane, and adding zirconium acetylacetonate with a molar concentration of 0.8% of the repeating units of the polycarbosilane, and stirring and filtering to obtain a homogeneous solution; S30: performing three gradient impregnation-drying cycles on the pretreated biological template, the addition amount of nano-silicon carbide in the precursor solution used in each cycle being 8%, 12% and 15% in turn, and performing biomimetic mineralization at 40°C and 95% relative humidity for 24 hours after the third drying to form an inorganic-organic hybrid layer with a thickness of 200 nm-800 nm; S40: under a nitrogen atmosphere, increasing the temperature to 200°C at a rate of 1°C / min and maintaining the temperature for 2 hours, then increasing the temperature to 400°C at a rate of 0.5°C / min and maintaining the temperature for 4 hours to complete crosslinking and curing; S50: under argon protection, performing staged sintering: increasing the temperature to 1000°C at a rate of 3°C / min and maintaining the temperature for 1 hour, then performing carbothermal reduction reaction by increasing the temperature to 1600°C at a rate of 2°C / min and maintaining the temperature for 2 hours, and finally realizing grain boundary densification by increasing the temperature to 1850°C at a rate of 1°C / min and maintaining the temperature for 1.5 hours; S60: removing residual carbon by heat treatment in air at 500°C for 30 minutes, then ultrasonic cleaning and drying to obtain the target silicon carbide composite material.

7. The preparation method according to claim 6, characterized in that, In the step S10, the radio frequency power of the oxygen plasma treatment is 100 W, the oxygen flow rate is 20 sccm, and the treatment time is 30 seconds.

8. The preparation method according to claim 6, characterized in that, In the step S30, each impregnation is vacuumed for 15 minutes under a vacuum degree of -0.095 MPa, followed by standing for 2 hours under normal pressure and air blowing drying for 1 hour at 80°C.

9. The preparation method according to claim 6, characterized in that, In the step S50, the argon flow rate is maintained at 5 L / min during the whole sintering process, and the pressure in the furnace is normal pressure.