Silicon nitride-based fiber monolith ceramic and method of making the same

Abstract

The application belongs to the field of ceramic materials, and particularly relates to a silicon nitride-based fiber monolithic ceramic and a preparation method thereof. The application combines wet spinning technology and a preparation process of silicon nitride ceramic, the diameter of the spinning dope extrusion process is controllable, easy to process, and has certain strength and toughness, sodium alginate solution is used as a binder solution, and solidification can be achieved at room temperature, and the environmental condition requirement is simple. By wrapping boron nitride interface on the surface of the silicon nitride fiber precursor after spinning, when subjected to external force, the crack is more inclined to expand at the boron nitride interface, a large amount of fracture energy is absorbed, and the material fracture toughness is improved. In the case of maintaining the strength of the silicon nitride-based fiber monolithic ceramic, the material is endowed with excellent toughness, the bending strength and fracture toughness reach 500-650 MPa and 10-16 MPa.m 1 / 2 , respectively.

Description

Technical Field

[0001] This invention belongs to the field of ceramic materials, and particularly relates to a silicon nitride-based fiber monolithic ceramic and its preparation method. Background Technology

[0002] Silicon nitride ceramics are a typical nitride ceramic material, a functional and structurally integrated ceramic material. They possess not only functional properties such as good dielectric properties, uniform broadband transmittance, high temperature resistance, wear resistance, rain erosion resistance, and good thermal shock resistance, but also have wide applications in petrochemical, metallurgical, mechanical, biomedical, and aerospace fields. Especially with the rapid development of the aerospace field in recent years, silicon nitride ceramics have become one of the hot research topics in aerospace materials. However, existing silicon nitride ceramics often exhibit brittleness and poor fracture toughness, which greatly limits their applications. Summary of the Invention

[0003] To address the aforementioned problems in the prior art, this invention provides a silicon nitride-based fiber monolithic ceramic and its preparation method, which can improve the fracture toughness while maintaining the strength of the ceramic material.

[0004] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0005] In a first aspect, this application provides a method for preparing a silicon nitride-based fiber monolithic ceramic, comprising the following steps:

[0006] S1: Add silicon nitride and sintering aid to a solvent and ball mill, then dry to obtain ceramic powder;

[0007] S2: Add ceramic powder to gel solution and ball mill to obtain spinning solution. Extrude the spinning solution through a spinning nozzle, solidify and dry to obtain silicon nitride fiber precursor.

[0008] S3: Disperse flake hexagonal boron nitride powder and nano alumina powder in water and ball mill to obtain an interface layer slurry; immerse the silicon nitride fiber precursor in the interface layer slurry and dry to obtain interface layer modified silicon nitride fiber.

[0009] S4: The interface-modified silicon nitride fibers are arranged in the mold and held under pressure at 10~20 MPa for 10~20 min to obtain the ceramic pre-pressed blank;

[0010] S5: After drying and debinding the ceramic pre-pressed blank, hot pressing sintering is carried out. The hot pressing sintering is specifically as follows: the debinded ceramic pre-pressed blank is placed in a sintering furnace, heated to 1800~2000℃, and pressurized to 25~30MPa. Then, it is maintained at this pressure and temperature for 1~2 hours to obtain the silicon nitride-based fiber monolithic ceramic.

[0011] Optionally, during hot pressing sintering, the temperature is first raised to a first temperature, and then the load is gradually applied to 25~30 MPa while the temperature is simultaneously raised to 1800~2000℃; the first temperature is 1200~1600℃, and the heating rate during the process is 2~15℃ / min.

[0012] Optionally, the mass fraction of boron nitride in the interface layer slurry in S3 is 6~10%, the mass fraction of alumina is 1.5~2.5%, and the impregnation time is 5~10 min.

[0013] Optionally, the sheet-like hexagonal boron nitride has a particle size of 100~300 nm, and the nano-alumina powder has a particle size of 5-30 nm.

[0014] Optionally, the thickness of the interface layer on the modified silicon nitride fiber is 5~15μm.

[0015] Optionally, the gel solution in S2 is a 3-5% sodium alginate solution, wherein the solvent of the sodium alginate solution is water or ethanol, and the diameter of the spinning nozzle used is 0.4-1 mm.

[0016] Optionally, the sintering aids mentioned in S1 include alumina and yttrium oxide, and the ceramic powder includes the following raw materials in parts by weight: 90-95 parts silicon nitride, 2-4 parts alumina, and 3-6 parts yttrium oxide.

[0017] Optionally, the coagulation method is to extrude the spinning solution into a calcium chloride solution at room temperature.

[0018] Optionally, the arrangement in S4 can be either unidirectional or cross-arranged.

[0019] Optionally, the adhesive removal conditions are as follows: the temperature is increased to 500-600℃ at a rate of 1-5℃ / min, and then maintained for 1-2 hours.

[0020] Optionally, during hot pressing sintering, the temperature is first raised to 1200 ℃ at a heating rate of 15 ℃ / min, then raised to 1600 ℃ at a heating rate of 10 ℃ / min; finally, pressure is gradually applied while maintaining a heating rate of 10 ℃ / min to raise the temperature to 1800 ℃. When the temperature reaches 1800 ℃, the pressure is raised to 30 MPa, and this temperature and pressure are maintained for 1 h.

[0021] Optionally, the drying conditions in S5 are: natural drying for 10 to 14 days.

[0022] On the other hand, this application also provides a silicon nitride-based fiber monolithic ceramic, characterized in that it is obtained by the above-described preparation method.

[0023] Optionally, the diameter of the silicon nitride cells in the silicon nitride-based fiber monolithic ceramic is 500~700μm.

[0024] Compared with the prior art, this application has at least the following beneficial effects:

[0025] The present invention uses a 3-5% sodium alginate solution as the gel solution and water or ethanol as the solvent for the sodium alginate solution. The spinning solution obtained by mixing ceramic powder is easier to extrude through the spinning nozzle than other gel solutions such as polyethylene glycol butyral, resulting in smooth and convenient extrusion.

[0026] The calcium chloride solution used in this invention is the corresponding coagulation bath solution. The spinning solution is extruded into the calcium chloride solution at room temperature, which makes it easy for the silicon nitride ceramic powder and sintering aid powder extruded from the spinning nozzle to quickly solidify into fibers, thereby improving the spinning efficiency. At the same time, the resulting fiber precursor has high strength, which facilitates subsequent rapid pressing and molding.

[0027] This invention integrates wet spinning technology with the preparation process of silicon nitride ceramics, thereby endowing the material with excellent toughness while maintaining the strength of silicon nitride-based fiber monolithic ceramics. The flexural strength and fracture toughness reach 500–650 MPa and 10–16 MPa·m, respectively. 1 / 2 .

[0028] By wrapping a boron nitride interface on the surface of the silicon nitride fiber precursor after spinning, cracks tend to propagate at the boron nitride interface when subjected to external force, absorbing a large amount of fracture energy, which is beneficial to improving the fracture toughness of the material.

[0029] The extrusion process of the spinning solution offers controllable diameter, ease of processing, and provides a certain level of strength and toughness, thus improving the strength and toughness of the fiber itself. Using sodium alginate solution as the binder allows for curing at room temperature, simplifying environmental requirements. Attached Figure Description

[0030] Figure 1 To assess the stability of the boron nitride interface layer slurry under ball milling time in Embodiment 1 of this invention;

[0031] Figure 2 The image shown is a scanning electron microscope image of the silicon nitride fiber precursor in Embodiment 1 of the present invention.

[0032] Figure 3 The tensile strength of silicon nitride fiber precursors with different diameters in Example 2 of the present invention;

[0033] Figure 4 This is the microstructure of the silicon nitride-based fiber monolithic ceramic in Example 3 of the present invention. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings:

[0035] Unless otherwise specified, the experimental methods used in the embodiments of this invention are all conventional methods.

[0036] All reagents and materials used in this example can be purchased routinely. The quantitative experiments involved in the examples were all repeated at least three times, and the results were averaged.

[0037] Raw material source: Silicon nitride with a particle size of 0.5μm.

[0038] Example 1

[0039] A method for preparing silicon nitride-based fiber monolithic ceramics includes the following steps:

[0040] (1) Preparation of silicon nitride fiber precursor: First, 95 g of silicon nitride, 2 g of alumina and 3 g of yttrium oxide were wet-mixed using anhydrous ethanol as the solvent. After mixing silicon nitride, alumina, yttrium oxide and anhydrous ethanol, the mixture was ball-milled for 24 h at a ball-to-material ratio of 1:2 and a rotation speed of 250 r / min. Then, the wet-mixed material was dried using a rotary evaporator to obtain a uniformly mixed ceramic powder.

[0041] (2) Dissolve 5 g of sodium alginate in 95 g of deionized water and mechanically stir for 2 h to ensure complete dissolution, obtaining a gel solution. Add ceramic powder to the gel solution at a mass ratio of 1:1 and ball mill for 24 h to obtain a uniformly mixed spinning solution. Use a 0.51 mm spinning nozzle to extrude the spinning solution into a coagulation bath. At room temperature, the silicon nitride fiber precursor can be rapidly formed in the coagulation bath and then drawn to obtain the precursor. Its scanning electron microscope image is shown below. Figure 2 It can be observed that the fiber surface is smooth and without pores. The coagulation bath is a saturated calcium chloride solution, and the diameter of the silicon nitride fiber precursor is 0.55 mm. Silicon nitride fiber precursors of different diameters were prepared by changing the diameter of the spinning nozzle.

[0042] (3) Interface modification: 6 g of 300 nm plate-shaped hexagonal boron nitride and 1.5 g of 30 nm alumina powder were mixed with 92.5 g of deionized water. The boron nitride interface layer slurry was mixed evenly using a ball mill for 3 h to obtain the interface layer slurry. The silicon nitride fiber precursor was impregnated in the interface layer slurry for 20 min, and dried after impregnation to obtain interface-modified silicon nitride fibers. The thickness of the interface layer on the silicon nitride fibers was 14.73 μm.

[0043] The stability of the boron nitride interface slurry obtained by ball milling for different times was tested during the experiment. The stability of the boron nitride interface slurry was characterized by the ratio of the liquid level height (H) of the boron nitride interface slurry after standing for 20 min to the initial liquid level height (H0) of the slurry before standing. The test results are as follows: Figure 1 As shown in the figure, it can be seen that the liquid level of the interface layer slurry obtained after ball milling for 3 hours is closest to the initial liquid level, and the stability is the best.

[0044] (4) Pre-compression molding: The interface-modified silicon nitride fibers are cut into short fibers with a length of 50 mm and then arranged in the same direction in a graphite mold. Pre-compression is performed at room temperature using a hydraulic press at a pressure of 20 MPa. After holding at this pressure for 10 min, a ceramic pre-compressed blank is obtained.

[0045] (5) The ceramic pre-pressed blank is naturally dried at room temperature for 10 days until the quality of the ceramic pre-pressed blank no longer changes, that is, the drying process is completed;

[0046] The dried ceramic pre-pressed blank was debinded in a box-type resistance furnace, and the temperature was raised to 200 ℃ at a heating rate of 1 ℃ / min, and then raised to 500 ℃ at a heating rate of 2 ℃ / min. After holding at this temperature for 1 h, it was naturally cooled to room temperature.

[0047] After debinding, the green body was placed in a hot-pressing sintering furnace for hot-pressing sintering: the temperature was raised to 1200 ℃ at a heating rate of 15 ℃ / min, and the heating rate was 10 ℃ / min within the temperature range of 1200~1600 ℃. When the temperature rose from 1600 ℃ to 1800 ℃, the pressure was gradually increased to 30 MPa. Finally, it was held at 1800 ℃ and 30 MPa for 1 h, and the resulting silicon nitride-based fiber monolithic ceramic had a flexural strength of 556.71 MPa and a fracture toughness of 15.37 MPa·m, respectively. 1 / 2 .

[0048] Example 2

[0049] A method for preparing silicon nitride-based fiber monolithic ceramics includes the following steps:

[0050] (1) Preparation of silicon nitride fiber precursor: First, 92 g of silicon nitride, 5 g of yttrium oxide, 3 g of alumina and an appropriate amount of anhydrous ethanol were mixed together for wet mixing. The wet mixing process involved placing the raw materials and anhydrous ethanol into a ball mill jar and ball milling for 24 h at a ball-to-material ratio of 1:2 and a rotation speed of 250 r / min. The wet-mixed powder was then dried using a rotary evaporator to obtain a uniformly mixed ceramic powder.

[0051] (2) 5 g of sodium alginate was mixed with 95 g of deionized water and mechanically stirred for 2 h until the sodium alginate was completely dissolved in the deionized water to obtain a gel solution. 100 g of the gel solution was mixed with 100 g of ceramic powder and ball-milled for 24 h to obtain a spinning solution. The spinning solution was extruded into a coagulation bath through a 0.58 mm spinning nozzle at room temperature. The coagulation bath was a saturated calcium chloride solution. The spinning solution could be formed within 20 s after entering the coagulation bath through the spinning nozzle. After traction, a silicon nitride fiber precursor with a diameter of 0.64 mm was obtained. By changing the diameter of the spinning nozzle, silicon nitride fiber precursors of different diameters could be prepared. The tensile strength of silicon nitride fiber precursors of different diameters is shown in [reference needed]. Figure 3 It can be seen that the fiber precursor prepared by this invention has high strength, which facilitates subsequent arrangement and molding, as well as sintering after molding; and when the fiber diameter reaches a certain value, the strength decreases with further increase in fiber diameter. When the fiber diameter is large, the defects introduced during the preparation process increase, resulting in a decrease in fiber strength.

[0052] (3) Interface modification: 8 g of 200 nm flake-shaped hexagonal boron nitride powder, 2 g of 10 nm alumina powder, and 90 g of deionized water were weighed and mixed. The interface layer solution was then ball-milled for 3 h to obtain a uniformly mixed interface layer slurry. The silicon nitride fiber precursor was immersed in the interface layer slurry for 10 min, removed, and dried to obtain interface-modified silicon nitride fibers with an interface layer thickness of 12.15 μm.

[0053] (4) Pre-compression molding: The interface-modified silicon nitride fibers are first cut into short fibers with a length of 50 mm, and then placed into a graphite mold in a 0° / 90° cross arrangement. The fibers are pre-compressed using a hydraulic press at a pressure of 15 MPa and held at this pressure for 10 min to obtain the ceramic pre-compressed blank.

[0054] (5) Allow the ceramic pre-pressed green body to dry naturally at room temperature for 14 days, until the quality of the ceramic green body no longer changes, i.e., the drying of the ceramic green body is complete;

[0055] The dried ceramic pre-pressed green body was debinded in a box-type resistance furnace. First, the temperature was raised to 150 ℃ at a heating rate of 2 ℃ / min, then to 250 ℃ at a heating rate of 5 ℃ / min, and finally to 550 ℃ at a heating rate of 5 ℃ / min. The body was held at this temperature for 2 h, and then cooled to room temperature with the furnace after the holding period.

[0056] The debinding preform was hot-pressed in a hot-pressing sintering furnace: first, the temperature was raised to 800 °C at a heating rate of 15 °C / min, then to 1200 °C at 10 °C / min, and finally to 1800 °C at 5 °C / min. Throughout the heating process, the pressure was gradually increased to 30 MPa, and the temperature was maintained at 1800 °C and 30 MPa for 1.5 h. This yielded silicon nitride-based fiber monolithic ceramic with a flexural strength of 607.34 MPa and a fracture toughness of 14.21 MPa·m. 1 / 2 .

[0057] Example 3

[0058] A method for preparing silicon nitride-based fiber monolithic ceramics includes the following steps:

[0059] (1) Preparation of silicon nitride fiber precursor: 92.5 g silicon nitride, 2.5 g alumina, 5 g yttrium oxide and anhydrous ethanol were mixed and ball-milled for 48 h to obtain wet mixed powder, wherein the ball-to-material ratio was 1:2 and the rotation speed was 250 r / min. Then the wet mixed material was dried using a rotary evaporator to obtain ceramic powder.

[0060] (2) Dissolve 3 g of sodium alginate in 97 g of anhydrous ethanol and mechanically stir for 3 h to obtain a gel solution. Mix 100 g of the gel solution with 100 g of ceramic powder and continue ball milling for 24 h to obtain a spinning solution. Prepare silicon nitride fibers using wet spinning technology. Extrude the silicon nitride fiber precursor into a saturated calcium chloride coagulation bath at room temperature through a 0.84 mm spinning nozzle and coagulate in the coagulation bath for 3 h. The diameter of the silicon nitride fiber precursor is 1.0 mm.

[0061] (3) Interface modification: 10 g of 100 nm flake-shaped hexagonal boron nitride powder and 2.5 g of 30 nm alumina powder were dispersed in 87.5 g of deionized water and ball-milled for 3 h to obtain an interface layer slurry. The silicon nitride fiber precursor was impregnated in the interface layer slurry for 10 min and dried to obtain interface-modified silicon nitride fiber; the thickness of the BN interface layer was 10.26 μm.

[0062] (4) Pre-compression molding: After drying the interface-modified silicon nitride fibers, pre-compression is performed using a hydraulic press. First, the above fibers are cut into short fibers with a length of 50 mm, and then arranged unidirectionally in the mold for pre-compression. The pre-compression pressure is 20 MPa, and the pressure is maintained for 10 min to obtain the ceramic pre-compressed blank.

[0063] (5) Dry the ceramic pre-pressed blank at room temperature for 14 days until the quality of the ceramic blank no longer changes, i.e., the drying is complete;

[0064] The dried blank was debonded in a box-type resistance furnace, and the box-type resistance furnace was heated to 600 °C at a heating rate of 5 °C / min and held for 1 h.

[0065] The debinding preform was then hot-pressed in a hot-pressing sintering furnace. The temperature was increased to 1200 °C at a heating rate of 15 °C / min. After reaching 1200 °C, pressure was applied, and the temperature was simultaneously increased to 1800 °C at a heating rate of 15 °C / min, at which point the pressure reached 30 MPa. The preform was held at 1800 °C and 30 MPa for 2 hours to obtain silicon nitride-based fiber monolithic ceramic, with a flexural strength of 631.18 MPa and a fracture toughness of 11.37 MPa·m. 1 / 2 .

[0066] Table 1 Performance parameters of silicon nitride-based fiber monolithic ceramics

[0067] BN interface layer thickness (μm) Bending strength (MPa) <![CDATA[Fracture toughness (MPa·m 1 / 2 ).]]> Example 1 14.73 556.71 15.37 Example 2 12.15 607.34 14.21 Example 3 10.26 631.18 11.37

[0068] As shown in the table above, the silicon nitride-based fiber monolithic ceramic obtained in Example 1 exhibits the highest fracture toughness, while the silicon nitride-based fiber monolithic ceramic obtained in Example 3 exhibits the highest flexural strength. This is related to the thickness of the BN interface layer; as the thickness of the BN interface layer increases, its flexural strength decreases while its fracture toughness increases.

[0069] Figure 4 The microstructure of the silicon nitride-based fiber monolithic ceramic in Example 3 is shown. Figure 4 Figure a shows the cross-sectional morphology, revealing that the black phase is silicon nitride cells, and the gray phase is the boron nitride interface layer. The silicon nitride cells exhibit a hexagonal structure, which is highly beneficial to the mechanical properties of the ceramic. Figure 4 b is Figure 4 Enlarged view of the rectangle in section a, along Figure 4 Scanning in the direction of the blue arrow in b yields... Figure 4 c. It can be observed that when the BN element content is the highest, the Si element content is the lowest, indicating that the intermediate interface is BN.

[0070] The above are merely preferred embodiments of the present invention, and the present invention is not limited to the contents of these embodiments. For those skilled in the art, various changes and modifications can be made within the scope of the technical solutions of the present invention, and any changes and modifications made are within the protection scope of the present invention.

Claims

1. A method for producing a silicon nitride-based fiber monolith ceramic, characterized by, The method comprises the following steps: S1: adding silicon nitride and sintering aids into a solvent for ball milling, and then drying to obtain a ceramic powder; S2: ball milling the ceramic powder into a gel solution to obtain a spinning dope, extruding the spinning dope through a spinning nozzle, drying after coagulation to obtain a silicon nitride fiber precursor; S3: dispersing flaky hexagonal boron nitride powder and nano-aluminum oxide powder in water, ball milling to obtain an interface layer slurry; immersing the silicon nitride fiber precursor into the interface layer slurry, and drying to obtain an interface layer modified silicon nitride fiber; S4: arranging the interface layer modified silicon nitride fiber in a mold, and keeping pressure for 10-20 min under 10-20 MPa to obtain a ceramic preform; S5: drying and degassing the ceramic preform, and then performing hot-pressing sintering, wherein the hot-pressing sintering is specifically as follows: placing the degassed ceramic preform into a sintering furnace, heating to 1800-2000 ℃, applying pressure of 25-30 MPa, and then keeping the pressure and temperature for 1-2 h to obtain the silicon nitride-based fiber monolithic ceramic; The gel solution in S2 is a 3-5% sodium alginate solution, the solvent of the sodium alginate solution is water or ethanol, and the diameter of the spinning nozzle used is 0.4-1 mm; The sintering aids in S1 include aluminum oxide and yttrium oxide, and the ceramic powder comprises the following raw materials by mass fraction: 90-95 parts of silicon nitride, 2-4 parts of aluminum oxide, and 3-6 parts of yttrium oxide; The coagulation method is as follows: extruding the spinning dope into a calcium chloride solution at room temperature.

2. The method for preparing silicon nitride-based fiber monolithic ceramics according to claim 1, characterized in that, In the hot-pressing sintering, the temperature is first increased to a first temperature, then a load is gradually applied to 25-30 MPa while the temperature is simultaneously increased to 1800-2000 ℃; the first temperature is 1200-1600 ℃, and the temperature increasing speed is 2-15 ℃ / min.

3. The method for preparing silicon nitride-based fiber monolithic ceramics according to claim 1, characterized in that, The mass fraction of boron nitride in the interface layer slurry in S3 is 6-10%, and the mass fraction of aluminum oxide is 1.5-2.5%; the immersing time is 5-10 min.

4. The method of making a silicon nitride-based fiber monolith ceramic according to claim 1, wherein The arrangement mode in S4 is unidirectional arrangement or cross arrangement.

5. The method of making a silicon nitride-based fiber monolith ceramic according to claim 1, wherein The degassing condition is specifically as follows: increasing the temperature to 500-600 ℃ at a speed of 1-5 ℃ / min, and then keeping the temperature for 1-2 h.

6. The method of making a silicon nitride-based fiber monolith ceramic according to claim 2, wherein In the hot-pressing sintering, the temperature is first increased to 1200 ℃ at a speed of 15 ℃ / min, then increased to 1600 ℃ at a speed of 10 ℃ / min; finally, the pressure is gradually increased while the temperature is kept increasing at a speed of 10 ℃ / min to 1800 ℃, when the temperature reaches 1800 ℃, the pressure is increased to 30 MPa, and the temperature and pressure are kept for 1 h.

7. A silicon nitride-based fiber monolith ceramic, characterized by, The silicon nitride-based fiber monolithic ceramic is obtained by the preparation method in any one of claims 1-6.

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