Preparation method of silicon nitride composite structural member

Abstract

The invention discloses a preparation method of a silicon nitride composite structural part, and relates to the technical field of thermal protection parts of overspeed aircrafts. According to the method, the functional gradient principle is adopted, ZrO2 is doped into one layer to achieve the high temperature resistance of the material, the high-temperature resistance of the material is further improved by adopting a porous technology, the physical strength of the whole component is compensated for by adopting a high-toughness and high-strength technology for the second layer, and the ZrO2 is subjected to conversion from a monoclinic phase to a tetragonal phase and then to a cubic phase in the heating process. The phase change process can absorb a large amount of energy along with the change of the volume, so that the toughness of the material is improved, the outer temperature-resistant layer is prepared, the characteristic of the strength of the silicon nitride material is exerted, and the inner force-bearing layer is prepared.

Description

Technical Field

[0001] The invention relates to the technical field of thermal protection parts for hypersonic vehicles, and in particular to a method for preparing a silicon nitride composite structural part. Background Art

[0002] Super-speed aircraft are the inevitable result of the continuous pursuit of higher flight speeds and better performance in the field of aviation technology. As the flight speed increases, the thermodynamic, mechanical and environmental challenges faced by aircraft are becoming increasingly severe, which directly promotes the demand for new high-performance materials. During high-speed flight, the surface of the aircraft will generate extremely high temperatures due to air compression and friction. Conventional materials are difficult to withstand such extreme conditions. Therefore, it is necessary to develop materials with excellent high-temperature resistance to ensure the stability and integrity of the aircraft structure at high temperatures.

[0003] The emergence of functionally gradient materials provides a new idea for solving the different requirements of material performance in different parts of aircraft. Through continuously changing composition or structural design, this material has good heat resistance on one side and excellent mechanical properties on the other side, thus better adapting to the complex and changeable working environment of high-speed aircraft.

[0004] Silicon nitride is a covalent crystal with a hardness second only to diamond and far exceeding other traditional materials. This high hardness allows silicon nitride to withstand greater loads and stresses. Silicon nitride can remain stable at temperatures up to 1900°C, which is very important for aircraft because many traditional materials lose their strength and stability at these temperatures. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a silicon nitride composite structural part. The method adopts the functional gradient principle, wherein a first layer is doped with ZrO2 to achieve high temperature resistance of the material and porous technology is adopted, and a second layer adopts high toughness and high strength technology.

[0006] ZrO2 has three crystal forms: monoclinic, tetragonal and cubic. During heating, zirconium oxide undergoes a transformation from monoclinic phase to tetragonal phase and then to cubic phase. This phase change process is accompanied by a change in volume, which can absorb a large amount of energy, thereby improving the toughness of the material. By adding the stabilizer yttrium oxide, the phase change process of zirconium oxide can be controlled so that it exists in the tetragonal phase at room temperature and undergoes phase change toughening after heating.

[0007] A method for preparing a silicon nitride composite structural part, the steps of which are: S1. Put Si3N4 powder, sintering additive 1 and ZrO2 into a ball mill for ball milling until the particle size of all particles is less than 0.5 μm, thereby completing the preparation of a layer of silicon nitride powder; S2, putting Si3N4 powder and sintering additive 2 into a ball mill for ball milling, and ball milling until the particle size of all particles is less than 0.5μm, thereby completing the preparation of the two-layer silicon nitride powder; S3, preparing a molding mold, the molding mold can be used for powder compression molding, the molding mold is divided into a one-layer segment mold and a two-layer segment mold, the one-layer segment mold is used to mold a layer of silicon nitride blank, the two-layer segment mold is used to mold a two-layer silicon nitride blank, the inner and outer walls of the one-layer segment mold can be movably installed with a needle group mold, and the two-layer segment mold is partially connected to the cavity of the one-layer segment mold; S4, the needle group mold is installed in the first-layer segment mold, and a layer of silicon nitride powder is added to fill the first-layer segment mold, and compacted into a layer of silicon nitride blank; the second-layer segment mold is fixed to the first-layer segment mold, and a second layer of silicon nitride powder is added to fill the second-layer segment mold, and after compaction, a complete blank is pressed out; S5. Take out the complete green body and perform hot pressing sintering. In a nitrogen atmosphere of 1 atm, heat up to 1800°C at a rate of 80°C / min, pre-sinter and keep warm for 5 minutes, and the pre-sintering pressure is 5MPa. After the pre-sintering, cool down to 1500°C at a rate of 50°C / min, sinter and keep warm for 25 minutes, and the sintering pressure is 5-10MPa, so as to obtain a single-layer silicon nitride structural part with relatively high strength, and obtain a high-strength two-layer silicon nitride structural part.

[0008] Furthermore, in step S1, the sintering additive 1 comprises AlN, Y2O3, MgO, Li2O, La2O3, Mg(OH)2·4MgCO3·6H2O, Al(OH)3, and ZrO2; the mass fraction ratio of Si3N4 powder, AlN, Y2O3, MgO, Li2O, La2O3, Mg(OH)2·4MgCO3·6H2O, Al(OH)3:ZrO2 is (82-86):(0.1-0.5):(0.1-0.5):(5-9):(0.1-0.5):(0.1-0.5):(4-6):(1-3):(4-7).

[0009] Furthermore, in step S2, the sintering additive 2 comprises AlN, Y2O3, MgO, Li2O, and La2O3. The mass fraction ratio of the Si3N4 powder, AlN, Y2O3, MgO, Li2O, and La2O3 is (93-96): (0.5-1.0): (0.1-0.5): (3-6): (0.1-0.5): (0.1-0.5). Furthermore, when the green body is taken out for hot pressing and sintering in step S5, the needle cluster-shaped mold described in step S4 is attached to a layer of silicon nitride green body and hot pressed and sintered simultaneously.

[0010] Preferably, the melting point of the material used for the needle cluster mold is lower than 1200 degrees Celsius.

[0011] The structural parts made using this technology have a single-layer silicon nitride structural part, that is, the outer layer of the structural part is a porous structure. The outer layer has a higher resistance to high temperatures and can remain in an environment of 2200 degrees Celsius for 10 minutes without decomposing. If it is necessary to further increase the flight speed of the high-speed aircraft or extend the flight time of the high-speed aircraft, the thickness of the outer layer of the structural part can be increased. The two-layer silicon nitride structural part, that is, the inner layer of the structural part is a high-strength structure, which provides mechanical support for the overall component. DETAILED DESCRIPTION

[0012] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0013] (1) Prepare Si3N4 powder, AlN, Y2O3, MgO, Li2O, La2O3, Mg(OH)2·4MgCO3·6H2O, Al(OH)3:ZrO2 in a mass fraction ratio of 83:0.3:0.3:5:0.2:0.2:4:2:5. Put them into a ball mill for ball milling until the particle size of all particles is less than 0.5μm, thus completing the preparation of silicon nitride powder required for the outer layer structure.

[0014] (2) Prepare Si3N4 powder, AlN, Y2O3, MgO, Li2O, and La2O3 in a mass fraction ratio of 95:0.8:0.4:3:0.4:0.4. Put them into a ball mill for ball milling until the particle size of all particles is less than 0.5 μm, thus completing the preparation of silicon nitride powder required for the inner layer structure.

[0015] The silicon nitride powder prepared in step (1) is placed in a mold and compression molded. Then, the silicon nitride powder prepared in step (2) is placed in the mold and compression molded to complete the preparation of a two-layer silicon nitride blank. Six samples are prepared and marked as group A samples A1, A2, A3, A4, A5, and A6.

[0016] The silicon nitride powder prepared in step (1) was added into the molding die and compression molding was performed to complete the preparation of a single outer layer silicon nitride blank. Six samples were prepared and marked as group B samples B1, B2, B3, B4, B5, and B6.

[0017] The silicon nitride powder prepared in step (2) was added into the molding die and compression molding was performed to complete the preparation of a single inner layer silicon nitride blank. Six samples were prepared and labeled as samples C1, C2, C3, C4, C5, and C6.

[0018] All the above-mentioned green bodies were taken out and hot-pressed sintered. In a nitrogen atmosphere of 1 atm, the temperature was raised to 1800°C at a rate of 80°C / min for pre-sintering and kept warm for 5 minutes. The pre-sintering pressure was 5 MPa. After the pre-sintering, the temperature was lowered to 1500°C at a rate of 50°C / min for sintering and kept warm for 25 minutes. The sintering pressure was 6 MPa, and 18 green body test blocks were obtained.

[0019] Perform performance tests on the test blocks.

[0020] The hardness is measured with reference to the standard GB / T16534-2009 "Test method for room temperature hardness of fine ceramics"; the fracture toughness is measured with reference to the standard GB / T23806-2009 "Test method for fracture toughness of fine ceramics: single-sided pre-cracked beam method"; the fire resistance is tested by burning with an acetylene torch at a center flame of 9.3 cm.

[0021] The performance of 18 samples in three groups were tested, and a conventional silicon nitride sample purchased from outside was taken as a control group. The test results are shown in Table 1: Table 1

[0022] Referring to the above Group A data, it can be concluded that the physical properties of the junction are low and the fire resistance is average.

[0023] Referring to the above data of Group B, it can be concluded that the physical properties of Group B silicon nitride materials are lower, but their fire resistance is significantly higher than that of Group A and Group C.

[0024] Referring to the above data of Group C, it can be concluded that the silicon nitride materials of Group C have good physical properties and average fire resistance.

Claims

1. A method for preparing a silicon nitride composite structural part, characterized in that The steps are: S1. Put Si3N4 powder, sintering additive 1 and ZrO2 into a ball mill for ball milling until the particle size of all particles is less than 0.5 μm, thereby completing the preparation of a layer of silicon nitride powder; S2, putting Si3N4 powder and sintering additive 2 into a ball mill for ball milling, and ball milling until the particle size of all particles is less than 0.5 μm, thereby completing the preparation of the two-layer silicon nitride powder; S3, preparing a molding mold, the molding mold can be used for powder compression molding, the molding mold is divided into a one-layer segment mold and a two-layer segment mold, the one-layer segment mold is used to mold a layer of silicon nitride blank, the two-layer segment mold is used to mold a two-layer silicon nitride blank, the inner and outer walls of the one-layer segment mold can be movably installed with a needle group mold, and the two-layer segment mold is partially connected to the cavity of the one-layer segment mold; S4, the needle group mold is installed in the first-layer segment mold, and a layer of silicon nitride powder is added to fill the first-layer segment mold, and compacted into a layer of silicon nitride blank; the second-layer segment mold is fixed to the first-layer segment mold, and a second layer of silicon nitride powder is added to fill the second-layer segment mold, and after compaction, a complete blank is pressed out; S5. Take out the complete green body and perform hot pressing sintering. In a nitrogen atmosphere of 1 atm, heat up to 1800°C at a rate of 80°C / min, pre-sinter and keep warm for 5 minutes, and the pre-sintering pressure is 5MPa. After the pre-sintering, cool down to 1500°C at a rate of 50°C / min, sinter and keep warm for 25 minutes, and the sintering pressure is 5-10MPa, so as to obtain a single-layer silicon nitride structural part with relatively high strength, and obtain a high-strength two-layer silicon nitride structural part.

2. The method for preparing a silicon nitride composite structural member according to claim 1, characterized in that: In step S1, the sintering additive 1 comprises AlN, Y2O3, MgO, Li2O, La2O3, Mg(OH)2·4MgCO3·6H2O, Al(OH)3, and ZrO2; the mass fraction ratio of Si3N4 powder, AlN, Y2O3, MgO, Li2O, La2O3, Mg(OH)2·4MgCO3·6H2O, Al(OH)3:ZrO2 is (82-86): (0.1-0.5): (0.1-0.5): (5-9): (0.1-0.5): (0.1-0.5): (4-6): (1-3): (4-7).

3. The method for preparing a silicon nitride composite structural member according to claim 1, characterized in that: In step S2, the sintering additive 2 comprises AlN, Y2O3, MgO, Li2O, and La2O3. The mass fraction ratio of the Si3N4 powder, AlN, Y2O3, MgO, Li2O, and La2O3 is (93-96): (0.5-1.0): (0.1-0.5): (3-6): (0.1-0.5): (0.1-0.5).

4. The method for preparing a silicon nitride composite structural member according to claim 1, characterized in that: When the green body is taken out for hot pressing and sintering in step S5, the needle cluster mold described in step S4 is attached to a layer of silicon nitride green body and hot pressed and sintered simultaneously.

5. The method for preparing a silicon nitride composite structural member according to claim 1, characterized in that: The melting point of the material used for the needle cluster mold in step S4 is lower than 1200 degrees Celsius.

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