A method for regulating the pore structure of a ceramic green body by direct writing

Abstract

The application provides a method for regulating a pore structure of a ceramic green body obtained through direct writing molding, comprising the following steps: firstly, using a pre-configured ceramic slurry as raw material, a ceramic wet green body is obtained through DIW printing molding; then, the ceramic wet green body is sequentially subjected to drying, solidification and high-temperature pyrolysis to obtain a porous ceramic green body, the density of the green body is 0.5 g / cm 3 to 1.4 g / cm 3 ; then, the ceramic green body is immersed in an impregnation liquid, and impregnation is carried out under vacuum conditions, and after multiple impregnation, cross-linking solidification and pyrolysis, a densified ceramic green body is obtained, the density of the ceramic green body is 0.8 g / cm 3 to 2.0 g / cm 3 . The application realizes regulation of the pore structure of the green body, the inter-beam through holes and the intra-beam pores of the green body are simultaneously filled after treatment, the prepared green body has a higher densification degree, compared with the green body without densification treatment, the residual silicon content of the SiC ceramic material obtained after a reaction infiltration densification process is greatly reduced, and the mechanical properties are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of composite material forming, in particular, to a method for regulating the pore structure of a ceramic green body obtained by direct writing forming. BACKGROUND

[0002] SiC-based ceramic material is a kind of composite material taking SiC as matrix and taking particles, fibers or whiskers as reinforcing phase, which has the performance advantages of high strength and high modulus, high thermal conductivity, low thermal expansion coefficient, corrosion resistance, wear resistance and thermal shock resistance. It has been widely used in aerospace, chemical industry, semiconductor and nuclear power fields. With the increasing requirements of performance of materials in various application fields, the demands for lightweight design of components, complex structure forming and low-cost preparation are becoming more and more prominent. Traditional forming and preparation processes usually require a long process cycle, and it is difficult to obtain components with complex shape, which greatly limits the application and development of SiC-based ceramic materials. Therefore, developing new forming and preparation technology has become an important research direction in the field of SiC-based ceramics.

[0003] 3D printing technology is a new material forming and preparation technology that has developed gradually in recent years. It provides a new way for the preparation of complex structure ceramic products based on the principle of layer-by-layer forming from bottom to top, and has great development potential in shortening the process cycle and reducing production cost. DIW is a 3D printing technology suitable for ceramic material forming, and its forming principle is as follows: taking ceramic slurry as raw material, under the action of pressure, the slurry is extruded from the nozzle, and the nozzle moves according to the designed path, the bundle-shaped slurry extruded from the nozzle is stacked layer by layer and the target structure is obtained, and finally the ceramic product is obtained after high temperature treatment. The ceramic components obtained by the above process are usually low in densification degree, and the components must be densified to obtain ceramic materials with higher performance. RMI sintering technology is a commonly used densification method, which principle is that Si is infiltrated into the green body at high temperature and high vacuum degree, and reacts with C in the green body to generate SiC to achieve the purpose of sintering. This sintering technology has the advantages of short process cycle, low production cost and high densification degree of SiC-based ceramic products.

[0004] The ceramic green body prepared by DIW forming technology usually has high porosity. In order to obtain good printing precision and forming effect, it is usually required that the ceramic slurry has the ability to maintain its own shape. In the printing process, the bundle-shaped slurry is stacked layer by layer, and a certain size and regular arrangement of through holes are reserved between the bundles. In addition, a large number of pores will also be formed in the bundle during the subsequent treatment process as the solvent in the slurry volatilizes. The pore structure of the green body directly affects the RMI sintering effect, and the high porosity greatly increases the difficulty of densification of the green body, which further affects the performance of the final SiC ceramic material. Therefore, it is urgent to develop a densification method to regulate the pore structure of the green body.

[0005] The SiC ceramic material with certain three-dimensional structure can be obtained by using the DIW combined with RMI process in the prior art, and has certain mechanical strength. However, there are still obvious deficiencies based on the process characteristics, especially the high residual silicon content in the final material. The reason is mainly that the green body porosity is high, and the SiC matrix generated in the RMI process cannot fully fill the pores, resulting in a large amount of residual silicon remaining in the material. SUMMARY

[0006] To solve the problems existing in the prior art, the application provides a method for regulating the pore structure of a ceramic green body obtained by direct writing, which uses a mixed solution of an organic precursor and graphite powder as an impregnating solution to greatly improve the filling efficiency of the process for the inter-beam through holes while ensuring the filling of the intra-beam pores. The application realizes the regulation of the pore structure of the green body, and the inter-beam through holes and the intra-beam pores of the green body are simultaneously filled after treatment, so that the green body has a higher degree of densification, and the residual silicon content of the SiC ceramic material obtained after the RMI densification process is greatly reduced, and the mechanical properties are improved.

[0007] To achieve the above purpose, the application provides a method for regulating the pore structure of a ceramic green body obtained by direct writing, comprising the following steps:

[0008] Step 1) Using silicon carbide powder, carbon black, phenolic resin, chopped carbon fiber and a first solvent as raw materials to prepare ceramic slurry meeting the requirements of direct writing;

[0009] Step 2) Printing the ceramic slurry prepared in step 1) through a DIW device to obtain a ceramic wet green body with a target three-dimensional structure, and then performing high-temperature treatment on the ceramic wet green body to obtain a porous ceramic green body; the density of the porous ceramic green body is 0.5 g / cm 3 ~ 1.4 g / cm 3 ;

[0010] Step 3) Preparing an impregnating solution using an organic precursor, a second solvent and graphite powder as raw materials;

[0011] Step 4) Densifying the porous ceramic green body: first, completely immerse the porous ceramic green body prepared in step 2) in the impregnating solution prepared in step 3), then perform impregnation under vacuum, and finally perform solidification treatment and high-temperature pyrolysis treatment; one impregnation, solidification and pyrolysis is one complete process cycle, and the densified ceramic green body is obtained after 2-5 times of the above process cycle; the density of the densified ceramic green body is 0.8 g / cm 3 ~ 2.0 g / cm 3 ;

[0012] Step 5) The densified ceramic body prepared in step 4) is densified by using the RMI process to obtain the SiC-based ceramic material.

[0013] Further, in step 1), the percentage of the amount of each raw material is as follows: 10-20 vol% of silicon carbide powder, 8-18 vol% of carbon black, 4-15 vol% of phenolic resin, 0-10 vol% of chopped carbon fiber, and 40-65 vol% of the first solvent. The dispersion method can be ball milling or mechanical stirring.

[0014] Further, in step 1), the first solvent used for preparing the ceramic slurry is any one of deionized water, ethylene glycol, and n-hexane.

[0015] Further, in step 2), the specific treatment process for high-temperature treatment of the ceramic wet body is as follows: the ceramic wet body is placed in an oven for drying and curing, the oven temperature is set to 150-220℃, and the holding time is 1-2h; then the dried and cured body is placed in a vacuum furnace, heated to 800-1200℃, and held for 1-2h.

[0016] Further, in step 3), the one-time impregnation, curing, and pyrolysis are a complete process cycle, and the specific process cycle steps are as follows: first, the porous ceramic body prepared in step 2) is placed in an open container, the impregnation liquid prepared in step 3) is added to the container until the upper surface of the body is completely submerged; then the open container is placed in a sealed container, vacuum is continuously drawn, and after 12-24h of standing, the body is taken out and placed in an oven for curing; then the cured body is placed in a high-temperature pyrolysis furnace for high-temperature pyrolysis, and the curing and pyrolysis temperatures and corresponding holding times are set according to the type of organic precursor.

[0017] Further, in step 3), the organic precursor is phenolic resin, furan resin, or polycarbosilane, and the corresponding second solvent is ethanol, ethylene glycol, or xylene, respectively. The impregnation liquid is mixed and prepared by mechanical stirring or other methods. To accelerate the dispersion of graphite powder and obtain good suspension effect, the mixing is preferably performed by vacuum stirring.

[0018] Further, in step 3), when the organic precursor is phenolic resin or furan resin, the mass ratio of the organic precursor to the corresponding second solvent is 1:2 to 2:1, the curing and pyrolysis temperatures are 150-220℃ and 800-1200℃, respectively, and the holding time is 1-2h; when the organic precursor is polycarbosilane, the mass ratio of the organic precursor to the corresponding second solvent is 3:7 to 7:3, the curing and pyrolysis temperatures are 120-180℃ and 1000-1400℃, respectively, and the holding time is 1-2h. The viscosity of the mixed liquid is moderate, and the graphite powder has good dispersion effect in the mixed liquid.

[0019] Further, in step 3), the particle size of the graphite powder is 10-80 μm, and the addition amount is 5-30 wt% of the sum of the mass of the organic precursor and the solvent.

[0020] Further, the graphite powder is a flaky graphite powder, and the flaky graphite powder is more stable in dispersion in the precursor solution.

[0021] Further, in step 5), the densified ceramic green body is subjected to densification treatment by using the RMI process, specifically, the densified ceramic green body is placed in a sintering furnace for high-temperature sintering, the sintering temperature is 1300-1600 ℃, and the holding time is 30-120 min.

[0022] The present application has the following beneficial effects:

[0023] 1. The present application provides a method for obtaining a ceramic green body pore structure regulation by direct writing molding, which uses a mixed solution of an organic precursor and graphite powder as an impregnation liquid to densify the green body and simultaneously fill the inter-beam through holes and the intrabeam pores. The organic precursor is converted into a ceramic matrix by high-temperature pyrolysis, accompanied by a certain volume shrinkage. In order to achieve the desired densification effect, multiple process cycles are often required. In addition, during the solidification process, a large amount of small molecule gas is generated by the volatilization of the impregnation liquid, which easily leads to the escape of the impregnation liquid in the inter-beam through holes, greatly reducing the filling efficiency. In the present application, graphite powder is added to the organic precursor solution. During the vacuum impregnation process, the graphite powder is impregnated into the inter-beam through holes along with the precursor solution, effectively alleviating the problem of impregnation liquid escape during solidification, and greatly improving the filling efficiency of the through holes. At the same time, the organic precursor impregnated into the intrabeam is converted into a ceramic matrix by high-temperature pyrolysis, and the intrabeam porosity is reduced. Finally, after RMI sintering, the graphite powder located in the inter-beam through holes reacts with liquid silicon to form a SiC matrix as a carbon source, effectively reducing the distribution of silicon-rich, and the content of residual silicon in the intrabeam is simultaneously reduced, improving the mechanical properties of the material. The composite material prepared by the present application is expected to be applied in aerospace, ship components, etc.

[0024] 2. The application provides a method for regulating the pore structure of a ceramic green body obtained by direct writing molding, which uses a mixed solution of an organic precursor and graphite powder as an impregnating solution to realize the synchronous filling of the inter-bundle through holes and the intra-bundle pores of the porous ceramic green body. The selection of graphite powder is very important. When the particle size of the graphite powder is too small, the graphite powder at the inter-bundle through holes is easy to escape with the precursor during the solidification process, and the filling efficiency of the through holes is poor; when the particle size of the graphite powder is too large, the stability of the mixed solution of the graphite powder and the precursor is poor, and the mixed solution is easy to separate during the standing process. At the same time, when the amount of graphite powder is too low, the amount of graphite powder entering the inter-bundle through holes of the green body during the impregnation process is limited, and it is difficult to achieve effective filling effect; when the amount of graphite powder is too high, the impregnating solution is too viscous and has poor fluidity. The viscosity of the impregnating solution is moderate, the graphite powder can maintain a good suspended state for a long time, and the inter-bundle through holes can be efficiently filled during the impregnation process; at the same time, the organic precursor in the impregnating solution can fill the intra-bundle small pores. In addition, the process is simple to operate and has high filling efficiency, and ideal sintering effect can be obtained after only 2 to 5 process cycles. When the number of process cycles is too high, the performance improvement is not obvious, and the cost increases.

[0025] 3. The SiC-based ceramic material prepared by the regulating method has a significantly reduced content of residual silicon in the intra-bundle and enriched silicon in the inter-bundle. Compared with silicon carbide ceramic phase, the large distribution of silicon phase will lead to poor mechanical strength, high-temperature oxidation resistance and wear resistance of the material. The SiC-based ceramic material prepared by the application has better comprehensive performance.

[0026] In addition to the objects, features, and advantages described above, the application has other objects, features, and advantages. The application will be further described in detail below with reference to the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0027] The drawings that form a part of this application are intended to provide a further understanding of the application, and the illustrative embodiments of the application and their description serve to explain the application without imposing undue limitation on the application. In the drawings:

[0028] Figure 1 The pore size distribution of the 0th, 1st, 2nd and 3rd green bodies prepared in Comparative Examples 1-2 and Example 1-2 is shown in FIG. 1;

[0029] Figure 2 The fracture morphologies of the green bodies and corresponding SiC-based ceramic materials prepared in Example 1 and Comparative Example 1 are shown in FIG. 2; wherein, Figure 2 (a) is the 0th green body, (b) is the 3rd green body, (c) is the 0th ceramic sample, and (d) is the 3rd ceramic sample;

[0030] Figure 3 The fracture morphologies of the 4th green body and the corresponding SiC-based ceramic material are shown in FIG. 3; wherein, Figure 3Middle (a) is a 4# green body, and (b) is a 4# ceramic sample. DETAILED DESCRIPTION

[0031] The embodiments of the present application are described in detail below with reference to the accompanying drawings, but the present application can be implemented in various different ways as defined and covered by the claims.

[0032] The commonly used means for densifying the green body at present includes chemical vapor deposition (CVI) and precursor impregnation and pyrolysis (PIP). Among them, the CVI process is to use gaseous precursor as raw material, under certain conditions, through complex chemical reaction and diffusion process, to generate solid product in the internal pores of the green body, the densification rate of this method is slow, the equipment structure is complex, and the process cost is high; the PIP process is to use organic precursor solution as raw material, under certain conditions, the raw material is impregnated into the internal pores of the green body, and then through the crosslinking, solidification and pyrolysis process, the organic precursor generates solid product and fills the pores, the filling efficiency of the pores is improved, but the traditional organic precursor solution is used as the impregnation liquid, a large amount of small molecules is discharged during the crosslinking and solidification process, and the un-solidified precursor is easy to be discharged from the through hole, so the filling efficiency of the through hole is poor.

[0033] The present application provides a method for controlling the pore structure of a ceramic green body obtained by direct writing molding, comprising: first, using a pre-configured ceramic slurry as raw material, a ceramic wet green body is obtained by DIW printing molding; then the wet green body is sequentially dried, solidified and high-temperature pyrolyzed to obtain a porous ceramic green body, the density of the green body is 0.5g / cm 3 to 1.4g / cm 3 ; then the ceramic green body is immersed in an impregnation liquid, and impregnated under vacuum conditions, and after multiple impregnation, crosslinking and solidification and pyrolysis, a densified ceramic green body is obtained, the density is 0.8g / cm 3 to 2.0g / cm 3 . The present application realizes the control of the pore structure of the green body, the through hole between the bundles and the intrabundle pores of the green body are simultaneously filled after treatment, the densification degree of the prepared green body is higher, compared with the green body without densification treatment, the residual silicon content of the SiC ceramic material obtained after the reaction infiltration densification process is greatly reduced, and the mechanical properties are improved.

[0034] The present application is explained and described below in conjunction with specific embodiments.

[0035] Example 1:

[0036] SiC powder, carbon black, chopped carbon fiber as the main raw material, phenolic resin as the binder (for improving the rheological properties), polyethylene glycol as the dispersant, the percentage of each raw material: SiC powder is 10-20vol%, carbon black is 8-18vol%, phenolic resin is 4-15vol%, chopped carbon fiber is 0-10vol%, solvent is 40-65vol%. The ceramic slurry is prepared by dispersing in ethylene glycol by ball milling for 6h, and the ceramic slurry prepared meets the requirements of direct writing forming, and the solid content of the obtained ceramic slurry is 45vol%. The final SiC-based ceramic material is prepared from the ceramic slurry by the following steps:

[0037] 1. The ceramic slurry is injected into the DIW equipment barrel, the nozzle diameter is 0.9mm, and the wet body with the size of 45mm×22.4mm×6.4mm is obtained by printing forming.

[0038] 2. The wet body obtained by printing is placed in an oven for drying, and the curing is completed at a temperature of 200℃ for 2 hours, then it is placed in a vacuum high-temperature furnace, heated to 1000℃ under the protection of inert atmosphere, and kept for 1 hour to complete the pyrolysis process, and the density of the obtained porous ceramic body is 1.03g / cm 3 .

[0039] 3. Polycarbosilane and dimethylbenzene are weighed according to the mass ratio of 1:1, mixed by mechanical stirring to prepare a solution, and graphite powder with a particle size of 20μm is weighed according to 20wt% of the mass of the solution, and the impregnating liquid is prepared by mixing with vacuum stirring.

[0040] 4. The porous ceramic body is placed in a suitable size container, the impregnating liquid is added until the body is completely submerged, the container is placed in a sealed container, and vacuum is continuously applied for 120min; the body is taken out and placed in an oven, the temperature is set to 200℃, the heating rate is 1℃ / min, and the holding time is 120min; then the preform is placed in a vacuum high-temperature furnace and pyrolyzed under the protection of argon atmosphere, the pyrolysis temperature is 1000℃, and the pyrolysis time is 60min. Repeat the above steps to prepare the No. 3 body after 3 cycles of vacuum impregnation and pyrolysis, and the density is 1.58g / cm 3 .

[0041] 5. The No. 3 body is placed in a sintering furnace, and the corresponding four groups of SiC-based ceramic materials are prepared by RMI sintering, which are named as No. 3 sample, and the densities are 2.85g / cm 3 .

[0042] Example 2:

[0043] SiC powder, carbon black, chopped carbon fiber as the main raw material, phenolic resin as the binder (for improving the rheological properties), polyethylene glycol as the dispersant, the percentage of each raw material: SiC powder is 10-20vol%, carbon black is 8-18vol%, phenolic resin is 4-15vol%, chopped carbon fiber is 0-10vol%, solvent is 40-65vol%. The ceramic slurry is prepared by dispersing in ethylene glycol by ball milling for 6h, and the ceramic slurry prepared meets the requirements of direct writing forming, and the solid content of the obtained ceramic slurry is 45vol%. The final SiC-based ceramic material is prepared from the ceramic slurry by the following steps:

[0044] 1. The ceramic slurry is injected into the DIW equipment barrel, the nozzle diameter is 0.9mm, and the wet blank with the size of 45mm×22.4mm×6.4mm is obtained by printing forming.

[0045] 2. The wet blank obtained by printing is placed in an oven for drying, and the curing is completed at a temperature of 200℃ for 2 hours, then it is placed in a vacuum high-temperature furnace, heated to 1000℃ under the protection of inert atmosphere, and kept for 1 hour to complete the pyrolysis process, and the density of the obtained porous ceramic blank is 1.03g / cm 3 .

[0046] 3. Polycarbosilane and dimethylbenzene are weighed according to the mass ratio of 1:1, mixed by mechanical stirring to prepare a solution, and graphite powder with a particle size of 10μm is weighed according to 20wt% of the mass of the solution, and the impregnating liquid is prepared by vacuum stirring.

[0047] 4. The porous ceramic blank is placed in a suitable size container, the impregnating liquid is added until the blank is completely submerged, the container is placed in a sealed container, and vacuum is continuously applied for 120min; the blank is taken out and placed in an oven, the temperature is set to 200℃, the heating rate is 1℃ / min, and the holding time is 120min; then the preform is placed in a vacuum high-temperature furnace and pyrolyzed under the protection of argon atmosphere, the pyrolysis temperature is 1000℃, and the pyrolysis time is 60min. Repeat the above steps, and the No. 2 blank with a density of 1.56g / cm 3 .

[0048] 5. The No. 2 blank is placed in a sintering furnace, and the corresponding four groups of SiC-based ceramic materials are prepared by RMI sintering, named as No. 2 sample, and the densities are 2.83g / cm 3 .

[0049] Example 3:

[0050] SiC powder, carbon black, chopped carbon fiber as the main raw material, phenolic resin as the binder (for improving the rheological properties), polyethylene glycol as the dispersant, the percentage of each raw material: SiC powder is 10-20vol%, carbon black is 8-18vol%, phenolic resin is 4-15vol%, chopped carbon fiber is 0-10vol%, solvent is 40-65vol%. Adopt the way of ball milling in glycol to prepare ceramic slurry, ball milling time is 6h, prepare the ceramic slurry that meets the requirements of direct writing forming, the solid content of the obtained ceramic slurry is 45vol%, the ceramic slurry is used to prepare the final SiC-based ceramic material through the following steps:

[0051] 1. The ceramic slurry is injected into the DIW equipment barrel, the nozzle diameter is 0.9mm, and the wet blank with the size of 45mm×22.4mm×6.4mm is obtained by printing forming.

[0052] 2. The wet blank obtained by printing is placed in an oven for drying, and the curing is completed at a temperature of 200℃ for 2 hours, then it is placed in a vacuum high-temperature furnace, heated to 1000℃ under the protection of inert atmosphere, and kept for 1 hour to complete the cracking process, the density of the obtained porous ceramic blank is 1.03g / cm 3 .

[0053] 3. Polycarbosilane and dimethylbenzene are weighed according to the mass ratio of 1:1, mixed by mechanical stirring to prepare a solution, and graphite powder with a particle size of 80μm is weighed according to 30wt% of the mass of the solution, and the impregnating liquid is prepared by mixing the graphite powder with the solution by vacuum stirring.

[0054] 4. The porous ceramic blank is placed in a suitable size container, the impregnating liquid is added until the blank is completely submerged, the container is placed in a sealed container, and vacuum is continuously applied for 120min; the blank is taken out and placed in an oven, the temperature is set to 200℃, the heating rate is 1℃ / min, and the holding time is 120min; then the preform is placed in a vacuum high-temperature furnace and cracked under the protection of argon atmosphere, the cracking temperature is 1000℃, and the cracking time is 60min. Repeat the above steps, and the No. 5 blank with a density of 1.60g / cm 3 .

[0055] 5. The No. 5 blank is placed in a sintering furnace, and the corresponding four groups of SiC-based ceramic materials are prepared by RMI sintering, named as No. 5 sample, with a density of 2.86g / cm 3 .

[0056] Comparative Example 1: (without impregnating liquid treatment)

[0057] SiC powder, carbon black, chopped carbon fiber as the main raw material, phenolic resin as the binder (for improving the rheological properties), polyethylene glycol as dispersant, the percentage of each raw material is: SiC powder is 10-20vol%, carbon black is 8-18vol%, phenolic resin is 4-15vol%, chopped carbon fiber is 0-10vol%, solvent is 40-65vol%. Adopt the way of ball milling in glycol to prepare ceramic slurry, ball milling time is 6h, to prepare ceramic slurry that meets the requirements of direct writing forming, the solid content of the obtained ceramic slurry is 45vol%, the ceramic slurry is used to prepare the final SiC-based ceramic material by the following steps:

[0058] 1. The ceramic slurry is injected into the DIW equipment barrel, the nozzle aperture is 0.9mm, and the wet body with the size of 45mm×22.4mm×6.4mm is obtained by printing forming.

[0059] 2. The wet body obtained by printing is placed in an oven for drying, and the curing is completed at a temperature of 200℃ for 2 hours, then it is placed in a vacuum high-temperature furnace, heated to 1000℃ under the protection of inert atmosphere, and kept for 1 hour to complete the pyrolysis process, the obtained porous ceramic body is named as No. 0 body, and the density is 1.03g / cm 3 .

[0060] 3. The No. 0 body is placed in a sintering furnace, and the corresponding four groups of SiC-based ceramic materials are prepared by RMI sintering, which are named as No. 0 sample, and the density is 2.75g / cm 3 .

[0061] Comparative example 2: (the impregnation liquid does not contain graphite powder, 3 times of circulation process)

[0062] SiC powder, carbon black, chopped carbon fiber as the main raw material, phenolic resin as the binder (for improving the rheological properties), polyethylene glycol as dispersant, the percentage of each raw material is: SiC powder is 10-20vol%, carbon black is 8-18vol%, phenolic resin is 4-15vol%, chopped carbon fiber is 0-10vol%, solvent is 40-65vol%. Adopt the way of ball milling in glycol to prepare ceramic slurry, ball milling time is 6h, to prepare ceramic slurry that meets the requirements of direct writing forming, the solid content of the obtained ceramic slurry is 45vol%, the ceramic slurry is used to prepare the final SiC-based ceramic material by the following steps:

[0063] 1. The ceramic slurry is injected into the DIW equipment barrel, the nozzle aperture is 0.9mm, and the wet body with the size of 45mm×22.4mm×6.4mm is obtained by printing forming.

[0064] 2. The printed wet green body is dried in an oven and cured at a temperature of 200 °C for 2 hours, then placed in a vacuum high-temperature furnace and heated to 1000 °C under the protection of an inert atmosphere for 1 hour to complete the pyrolysis process. The density of the obtained porous ceramic body is 1.03 g / cm3. 3 .

[0065] 3. Polycarbosilane and xylene are weighed in a mass ratio of 1:1 and mixed by mechanical stirring to obtain a solution.

[0066] 4. The porous ceramic body is placed in a suitable size holding device using a vacuum impregnation process, and the impregnation liquid is added until the body is completely submerged. The holding device is placed in a sealed container and vacuum is continuously applied for 120 min. The body is removed and placed in an oven, set to a temperature of 200 °C, with a heating rate of 1 °C / min and a holding time of 120 min. The preform is then placed in a vacuum high-temperature furnace and pyrolyzed under the protection of an argon atmosphere at a pyrolysis temperature of 1000 °C for 60 min. Repeat the above steps to obtain No. 4 body by 3 cycles of vacuum impregnation and pyrolysis, with a density of 1.45 g / cm3. 3 .

[0067] 5. The No. 4 body is placed in a sintering furnace and sintered by RMI to obtain four groups of SiC-based ceramic materials, designated as No. 4 sample, with a density of 2.81 g / cm3. 3 .

[0068] Comparative Example 3: (The impregnation liquid contains graphite powder, but only one cycle process)

[0069] Silicon carbide powder, carbon black, and chopped carbon fiber are used as main raw materials, phenolic resin is used as a binder (to improve rheological properties), and polyethylene glycol is used as a dispersant. The percentage of each raw material is as follows: silicon carbide powder 10-20 vol%, carbon black 8-18 vol%, phenolic resin 4-15 vol%, chopped carbon fiber 0-10 vol%, and solvent 40-65 vol%. The ceramic slurry is prepared by ball milling in ethylene glycol for 6 h to meet the requirements of direct writing forming. The obtained ceramic slurry has a solid content of 45 vol%. The final SiC-based ceramic material is prepared by the following steps:

[0070] 1. The ceramic slurry is injected into the DIW equipment barrel, and the nozzle diameter is 0.9 mm. The wet green body with a size of 45 mm x 22.4 mm x 6.4 mm is obtained by printing.

[0071] 2. The printed wet green body is dried in an oven and cured at a temperature of 200℃ for 2 hours, then placed in a vacuum high-temperature furnace and heated to 1000℃ under an inert atmosphere for 1 hour to complete the pyrolysis process, and the density of the obtained porous ceramic body is 1.03 g / cm3. 3 .

[0072] 3. Polycarbosilane and xylene are weighed in a mass ratio of 1:1, mixed by mechanical stirring to obtain a solution, and graphite powder with a particle size of 20 μm is weighed at 20 wt% of the mass of the solution, and the solution is mixed by vacuum stirring to obtain an impregnating solution.

[0073] 4. The porous ceramic body is placed in a suitable size container, the impregnating solution is added until the body is completely submerged, the container is placed in a sealed container, and vacuum is continuously applied for 120 min; the body is taken out and placed in an oven, the temperature is set to 200℃, the heating rate is 1℃ / min, and the holding time is 120 min; then the preform is placed in a vacuum high-temperature furnace and pyrolyzed under an argon atmosphere at a pyrolysis temperature of 1000℃ for 60 min. The body obtained by one vacuum impregnation and pyrolysis process using the mixed solution with graphite powder as the impregnating solution is named No. 1 body, and the density is 1.35 g / cm3. 3 .

[0074] 5. The No. 1 body is placed in a sintering furnace and subjected to RMI sintering to obtain four groups of SiC-based ceramic materials, which are named No. 1 sample, and the densities are 2.82 g / cm3, 2.85 g / cm3, 2.88 g / cm3, and 2.91 g / cm3, respectively. 3 .

[0075] Figure 1 The pore size distribution of the No. 0, No. 1, No. 2, and No. 3 bodies obtained by using the comparative example 1-2 and example 1-2 by mercury intrusion test is shown in FIG. 1. The No. 0 body has two sizes of pores, of which the pores with a pore size of 100-200 nm are intrabundle pores, and the pores with a pore size of 100-300 microns are interbundle through holes. With the increase of the number of PIP process cycles, the volume content of the two sizes of pores decreases significantly, indicating that after the body is treated by the impregnating solution described in the application, the intrabundle pores and the interbundle through holes are simultaneously filled.

[0076] Figure 2 are the fracture morphologies of the bodies and the corresponding SiC-based ceramic materials obtained by example 1 and comparative example 1 of the application; wherein, Figure 2 (a) is the No. 0 body, (b) is the No. 3 body, (c) is the No. 0 ceramic sample, and (d) is the No. 3 ceramic sample. From Figure 2As can be seen, there are through-holes with a scale of hundreds of micrometers between the bundles of blank 0. After PIP treatment, the through-holes between the bundles of blank 3 are filled with the ceramic matrix formed by the cracking of graphite powder and polycarbosilane. The through-holes of SiC-based ceramic materials without PIP treatment have a large amount of silicon enrichment. However, for ceramic materials prepared after three PIP treatments, the graphite powder distributed in the through-holes reacts with silicon during the sintering process to form a SiC matrix. This SiC matrix, together with the SiC matrix formed by the cracking of organic precursors, fills the through-holes, and the silicon enrichment content is significantly reduced.

[0077] Figure 3 The fracture morphology of blank No. 4 and the corresponding SiC-based ceramic material is shown; among them, Figure 3 (a) shows green body No. 4, and (b) shows ceramic sample No. 4. From... Figure 3 As can be seen, only a small amount of SiC matrix is ​​generated on the inner wall of the through hole of blank No. 4. The impregnation solution without graphite powder has poor efficiency for the through hole of blank, and the through hole area of ​​the corresponding SiC-based ceramic material still has a lot of enriched silicon.

[0078] Samples 0-5 were treated with a mixed solution of hydrofluoric acid and concentrated nitric acid, and the calculated residual silicon contents were 34.81%, 22.56%, 20.72%, 17.17%, 28.52%, and 17.11 vol%, respectively. The bending properties of the SiC-based ceramic materials were tested using a three-point bending test. The test specimen dimensions were 40 mm × 4 mm × 3 mm, the span was 30 mm, and the loading speed was 0.5 mm / min. The measured bending strengths of samples 0-5 were 285, 291, 309, 326, 289, and 329 MPa, respectively, and the bending moduli were 187, 258, 282, 306, 218, and 309 GPa, respectively. It can be observed that using a traditional organic precursor solution as the impregnation liquid resulted in poor filling of the through-pores in the green body, a small decrease in the residual silicon content of the SiC-based ceramic materials, and no significant improvement in bending properties. After PIP treatment of the green body using the impregnation solution described in this invention, the residual silicon content of the obtained SiC-based ceramic material is significantly reduced, and the bending performance is significantly improved. Compared with silicon carbide, silicon has poor mechanical properties, and the presence of a large amount of residual silicon seriously affects the mechanical properties of the final material. By densifying the green body, the residual silicon content of the final material is reduced. Compared with the control sample, when the green body is treated with the precursor solution containing graphite powder as described in this invention, the inter-beam through-pores and intra-beam pores are filled simultaneously, and the reduction in residual silicon content of the final material is greater with the increase of process cycles. Compared with traditional impregnation solutions, the improvement in the mechanical properties of the final material obtained by treating the green body with the impregnation solution described in this invention is more significant.

[0079] The application provides a method for regulating the pore structure of a ceramic green body obtained by direct writing molding, which uses a mixed solution of an organic precursor and graphite powder as an impregnating solution to realize synchronous filling of the inter-bundle through holes and the intra-bundle pores of the porous ceramic green body.

[0080] In summary, the application provides a method for regulating the pore structure of a ceramic green body obtained by direct writing molding, which uses a mixed solution of an organic precursor and graphite powder as an impregnating solution to densify the green body and synchronously fill the inter-bundle through holes and the intra-bundle pores. The organic precursor is converted into a ceramic matrix by pyrolysis, which is accompanied by volume shrinkage. To achieve ideal densification, multiple process cycles are often required. In addition, during the solidification process, a large amount of small molecule gas is generated by the volatilization of the impregnating solution, which easily leads to the escape of the impregnating solution from the inter-bundle through holes, greatly reducing the filling efficiency. The application adds graphite powder to the organic precursor solution. During the vacuum impregnation process, the graphite powder is impregnated into the inter-bundle through holes of the green body along with the precursor solution, effectively alleviating the problem of impregnating solution escape during solidification and greatly improving the filling efficiency of the through holes. At the same time, the organic precursor impregnated into the intra-bundle is converted into a ceramic matrix by pyrolysis, reducing the intra-bundle porosity. Finally, after RMI sintering, the graphite powder in the inter-bundle through holes acts as a carbon source and reacts with liquid silicon to form a SiC matrix, effectively reducing the distribution of silicon-rich, synchronously reducing the content of residual silicon in the intra-bundle, and improving the mechanical properties of the material. The composite material prepared by the application is expected to be applied in aerospace, shipbuilding components.

[0081] In addition, the SiC-based ceramic material prepared by the regulating method of the application has a significantly reduced content of residual silicon in the intra-bundle and silicon-rich in the inter-bundle. Compared with silicon carbide ceramic, the large distribution of silicon phase will result in poor mechanical strength, high-temperature oxidation resistance and wear resistance of the material. The SiC-based ceramic material prepared by the application has better comprehensive performance.

[0082] The above merely provides the preferred embodiments of the present application, and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the principles and technical scope of the present application shall fall into the scope of the present application.

Claims

1. A method for controlling the pore structure of ceramic green bodies obtained by direct writing molding, characterized in that, Includes the following steps: Step 1) Prepare a ceramic slurry that meets the requirements of direct writing molding using silicon carbide powder, carbon black, phenolic resin, chopped carbon fiber, and a first solvent as raw materials; wherein, the percentage of each raw material is as follows: silicon carbide powder 10-20 vol%, carbon black 8-18 vol%, phenolic resin 4-15 vol%, chopped carbon fiber 0-10 vol%, and the first solvent 40-65 vol%. Step 2) The ceramic slurry obtained in Step 1) is printed using a DIW (Digital-to-Wood) machine to obtain a ceramic green body with the target three-dimensional structure. The ceramic green body is then subjected to high-temperature treatment to obtain a porous ceramic body. The density of the porous ceramic body is 0.5 g / cm³. 3 ~1.4g / cm 3 The specific process for high-temperature treatment of ceramic wet blanks is as follows: the ceramic wet blanks are placed in an oven for drying and curing, the oven temperature is set to 150-220℃, and the holding time is 1-2 hours; then the dried and cured blanks are placed in a vacuum furnace, heated to 800-1200℃, and held for 1-2 hours. Step 3) Prepare an impregnation solution using an organic precursor, a second solvent, and graphite powder as raw materials; wherein the organic precursor is polycarbosilane, the second solvent is xylene, and the mass ratio of the two is 3:7 to 7:3; the particle size of the graphite powder is 10-80 μm, and the amount added is 5-30 wt% of the sum of the mass of the organic precursor and the solvent. Step 4) Densification treatment of the porous ceramic green body: First, the porous ceramic green body obtained in step 2) is completely submerged in the impregnation solution prepared in step 3), then impregnated under vacuum conditions, and finally cured and pyrolyzed at high temperature; one impregnation, curing and pyrolysis constitutes a complete process cycle, and the densified ceramic green body is obtained after 2 to 5 of the above process cycles; the density of the densified ceramic green body is 0.8 g / cm³. 3 ~2.0g / cm 3 The curing and pyrolysis temperatures are 120-180℃ and 1000-1400℃, respectively, and the holding time is 1-2 hours for both. Step 5) The densified ceramic blank obtained in step 4) is densified using the RMI process to obtain SiC-based ceramic material; wherein, the densification process specifically involves placing the densified ceramic blank in a sintering furnace for high-temperature sintering at a temperature of 1300-1600℃ and a holding time of 30-120min.

2. The method for controlling the pore structure of ceramic green bodies obtained by direct writing molding according to claim 1, characterized in that, In step 1), the first solvent used to prepare the ceramic slurry is any one of deionized water, ethylene glycol, or n-hexane.

3. The method for controlling the pore structure of ceramic green bodies obtained by direct writing molding according to claim 1, characterized in that, In step 3), one impregnation, curing, and pyrolysis constitute a complete process cycle. The specific process cycle steps are as follows: First, place the porous ceramic green body obtained in step 2) into an open container, add the impregnation liquid prepared in step 3) to the container until the upper surface of the green body is completely submerged; then place the open container into a sealed container, continuously evacuate the vacuum, and let it stand for 12-24 hours before taking out the green body and placing it in an oven for curing; then place the cured green body in a high-temperature pyrolysis furnace for high-temperature pyrolysis. The curing and pyrolysis temperatures and corresponding holding times are set according to the type of organic precursor.

4. The method for controlling the pore structure of ceramic green bodies obtained by direct writing molding according to claim 1, characterized in that, The graphite powder is a lamellar graphite powder.

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