Method for researching high-temperature structure and performance of boron nitride and alumina fiber based on special sintering method

Abstract

The invention belongs to the technical field of ceramic fibers. The invention provides a method for researching the high-temperature structure and performance of boron nitride and alumina fibers based on a special sintering method, which comprises the following steps: firstly, pretreating the boron nitride and alumina fibers, then sintering in a special sintering furnace, taking out after furnace cooling, and carrying out vacuum sintering to obtain the boron nitride and alumina fibers. The method comprises the following steps: systematically analyzing changes of microstructures of boron nitride and alumina fibers treated at different temperatures, researching a high-temperature evolution mechanism of the boron nitride and alumina fibers, respectively dispersing the boron nitride and alumina fibers in a brittle matrix, and then drying, curing and demolding to obtain the fiber toughened composite material. The breaking strength of the material is obtained by performing a three-point bending mechanical test on the material, so that the mechanical properties and the toughening effect of the boron nitride and alumina fiber are researched. The method provides a new thought for researching the high-temperature microstructure and mechanical properties of the boron nitride and aluminum oxide ceramic fibers, and can be used for analyzing and screening different ceramic fibers in the aerospace high-temperature environment.

Description

Technical Field

[0001] The invention relates to the technical field of ceramic fibers, and in particular to a method for studying the high-temperature structure and performance of boron nitride and alumina fibers based on a special sintering method. Background Art

[0002] The rapid development of the contemporary aviation field not only marks a significant improvement in mankind's ability to explore space, but also puts forward more stringent requirements on the performance of spacecraft. Among them, the thermal protection shield and engine insulation layer are key components to ensure the safe and efficient operation of spacecraft, and their material selection and design are particularly important. These components need to withstand the test of extreme high temperature environments for a long time. They must be able to effectively resist the impact of external high heat flow, and ensure that the temperature of the internal structure is maintained within a safe range to protect the precision instruments and personnel inside the spacecraft. Therefore, characteristics such as high temperature resistance, low thermal conductivity, light weight and high strength, and corrosion resistance have become important indicators for measuring the performance of these materials.

[0003] High-temperature ceramic fibers can meet the above requirements very well. Such materials are usually composed of high-temperature resistant inorganic substances such as Al2O3, SiO2, and BN. They can maintain structural stability at extremely high temperatures, effectively block heat transfer, and reduce overall weight, thereby improving the carrying efficiency of spacecraft. For example, the excellent high-temperature oxidation resistance and corrosion resistance of BN fibers make them an ideal fiber-reinforced material for aerospace components; Al2O3 fibers are light in weight, have a small thermal expansion coefficient, and have good thermal shock resistance. They can also be used to manufacture key parts such as aircraft engine components and satellite components, meeting the requirements of the aerospace field for lightweight and high-strength materials.

[0004] However, despite the many advantages of these high-temperature ceramic fibers, they often suffer from mass loss and phase transitions after continuous heating in a high-temperature environment. These changes not only weaken the thermal insulation properties of the material, but may also cause a decrease in structural strength, posing a potential threat to the safety of spacecraft, which makes it difficult to select fibers in different high-temperature environments. Therefore, it is urgent to provide a method that can systematically study the high-temperature structure and properties of boron nitride and alumina ceramic fibers. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a method for studying the high-temperature structure and performance of BN and Al2O3 fibers based on a special sintering method. The present invention innovatively proposes to use a special sintering method as a processing method for ceramic fibers, systematically study the changes in the fiber microstructure and its high-temperature evolution mechanism after sintering at different temperatures, analyze the phase change and grain growth of the fiber at high temperature, and innovatively use brittle materials as fiber toughening matrix, and evenly disperse the fibers in the matrix to obtain a fiber-toughened composite material. After complete solidification, the three-point bending method is used to test the bending resistance of the material, and the fracture toughness of the material is used to indirectly characterize the toughening effect of the fiber, thereby studying the mechanical properties and toughening effect of the ceramic fiber after high-temperature treatment. The method is simple to operate, and can intuitively characterize the residual toughening effect of BN and Al2O3 fibers after high-temperature treatment, which is conducive to repeated research and screening of high-temperature resistant ceramic fibers with excellent performance.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method, comprising the following steps:

[0008] Boron nitride and alumina fibers are first pretreated, then sintered in a special sintering furnace and taken out after the furnace is cooled. The changes in the microstructure of boron nitride and alumina fibers after treatment at different temperatures are systematically analyzed, and their high-temperature evolution mechanism is studied. Boron nitride and alumina fibers are respectively evenly dispersed in a brittle matrix and then dried and solidified. After demolding, a fiber-reinforced composite material is obtained. The fracture strength of the material is obtained by subjecting it to a three-point bending mechanical test, and then the mechanical properties and toughening effects of boron nitride and alumina fibers are studied.

[0009] Preferably, the pretreatment is to immerse the boron nitride and alumina fibers in a liquid solution;

[0010] The liquid solution is an acetone solution, an ethanol solution, water or an ethylene glycol solution, and the mass percentage concentration of the liquid solution is 50-75%;

[0011] The soaking time is 10 to 180 minutes.

[0012] Preferably, the special sintering furnace is first evacuated and then a protective gas is introduced, wherein the protective gas is one or more of nitrogen, argon, helium and neon.

[0013] Preferably, the sintering pressure is 0.5-10 kPa, the sintering temperature is 1300-1500° C., and the sintering time is 2-3 h.

[0014] Preferably, the heating rate to the sintering temperature is 6-8°C / min.

[0015] Preferably, the brittle matrix is ​​cement, calcium silicate, silicon boride or gypsum.

[0016] Preferably, the drying and curing temperature is 20 to 50° C., and the drying and curing time is 1 to 5 hours.

[0017] Preferably, when boron nitride and alumina fibers are dispersed in the brittle matrix respectively, the mass ratio of boron nitride to alumina fibers is 1:1.

[0018] Preferably, when the alumina fibers are dispersed in the brittle matrix, the mass ratio of the alumina fibers to the brittle matrix is ​​1:15 to 20.

[0019] The present invention also provides the application of the method for studying the high-temperature structure and performance of boron nitride and alumina fibers based on a special sintering method in the field of aerospace.

[0020] Compared with the prior art, the beneficial effects of the present invention include the following points:

[0021] 1) The method for studying the high-temperature structure and performance of BN and Al2O3 ceramic fibers in the present invention adopts a special sintering method, that is, heating and sintering under a protective gas atmosphere in a special sintering furnace, which can not only prevent the ceramic fibers from being oxidized under the action of the protective atmosphere, but also provide an ideal heating environment.

[0022] 2) The present invention utilizes a testing method for dispersing ceramic fibers in a brittle matrix, i.e., the fibers before and after sintering are uniformly dispersed in the brittle matrix, and are fully dried and solidified, so that the brittle matrix is ​​converted into a fiber-reinforced composite material, and the toughening effect of the fiber is then characterized by the fracture toughness of the material. This method is simple and convenient, can more intuitively detect the retention of the toughening effect of the fiber after high-temperature treatment, and has good adaptability.

[0023] 3) The present invention systematically studies the high-temperature structure and properties of BN and Al2O3 ceramic fibers, and innovatively uses brittle materials as the matrix of the load-bearing fibers. It can characterize the toughening effect retention of different fibers after high-temperature sintering in a more convenient and direct way, and further screen out high-temperature resistant ceramic fibers, which can be used in aerospace fields such as thermal protection covers and engine insulation layers of spacecraft. Moreover, its excellent high-temperature resistance, low thermal conductivity, lightweight and high-strength mechanical properties, and corrosion resistance can meet the needs of long-term exposure to extreme high-temperature environments, greatly improving the carrying efficiency of spacecraft. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0025] Figure 1 The SEM images of the cross-section of the boron nitride fiber and the gypsum cross-section of the boron nitride fiber before and after sintering in Example 1; wherein (a) and (b) are the cross-section SEM images of the boron nitride fiber after sintering at room temperature and 1400°C, respectively; (c) and (d) are the SEM images of the boron nitride fiber morphology of the gypsum cross-section dispersed with room temperature boron nitride fibers and boron nitride fibers after sintering at 1400°C, respectively;

[0026] Figure 2 Figure 2 is the SEM images of the alumina fiber cross section and the alumina fiber cross section of gypsum before and after sintering in Example 1; (a) and (b) are the SEM images of the cross section of the alumina fiber after sintering at room temperature and 1400°C, respectively; (c) and (d) are the SEM images of the alumina fiber morphology of the gypsum cross section dispersed with room temperature alumina fibers and alumina fibers after sintering at 1400°C, respectively. DETAILED DESCRIPTION

[0027] The present invention provides a method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method, comprising the following steps:

[0028] Boron nitride and alumina fibers are first pretreated, then sintered in a special sintering furnace and taken out after the furnace is cooled. The changes in the microstructure of boron nitride and alumina fibers after treatment at different temperatures are systematically analyzed, and their high-temperature evolution mechanism is studied. Boron nitride and alumina fibers are respectively evenly dispersed in a brittle matrix and then dried and solidified. After demolding, a fiber-reinforced composite material is obtained. The fracture strength of the material is obtained by subjecting it to a three-point bending mechanical test, and then the mechanical properties and toughening effects of boron nitride and alumina fibers are studied.

[0029] In the present invention, the pretreatment is preferably performed by soaking the boron nitride and alumina fibers in a liquid solution;

[0030] The liquid solution is preferably an acetone solution, an ethanol solution, water or an ethylene glycol solution, and the mass percentage concentration of the liquid solution is preferably 50-75%, more preferably 60-70%, and more preferably 65%;

[0031] The soaking time is preferably 10 to 180 min, more preferably 50 to 100 min, and even more preferably 80 min.

[0032] In the present invention, after the pretreatment is completed, the boron nitride and alumina fibers are preferably placed in a crucible; the crucible is preferably a graphite crucible, a porcelain crucible or a corundum crucible, and the bottom of the crucible is preferably coated with a layer of refractory powder, and the refractory powder is preferably boron nitride powder, silicon nitride powder, aluminum nitride powder or zirconium oxide powder.

[0033] The present invention coats the sinter-resistant powder on the bottom of the crucible, which can effectively prevent impurities in the crucible from penetrating into the sintering material at high temperature, and is conducive to demoulding of the fiber after sintering, while also protecting the crucible and reducing its wear and corrosion.

[0034] In the present invention, the pretreatment can not only soften the boron nitride and alumina fibers and remove impurities, but also reduce defects that may occur in the fibers during the sintering process, such as pores and cracks.

[0035] In the present invention, the special sintering furnace is preferably first evacuated and then a protective gas is introduced, and the protective gas is preferably one or more of nitrogen, argon, helium and neon.

[0036] The present invention utilizes a special sintering method as a processing method for boron nitride and alumina ceramic fibers, that is, sintering at a high temperature under a protective gas atmosphere in a sintering furnace, which can prevent oxidation under the action of the protective atmosphere and provide an ideal heating environment.

[0037] In the present invention, the sintering pressure is preferably 0.5 to 10 kPa, more preferably 3 to 7 kPa, and more preferably 4 to 5 kPa;

[0038] The sintering temperature is preferably 1300-1500°C, more preferably 1350-1450°C, and more preferably 1400°C;

[0039] The sintering time is preferably 2 to 3 hours, more preferably 2.4 to 2.6 hours, and even more preferably 2.5 hours.

[0040] In the present invention, the heating rate to the sintering temperature is preferably 6 to 8° C. / min, more preferably 6.5 to 7.5° C. / min, and even more preferably 7° C. / min.

[0041] In the present invention, the brittle matrix is ​​preferably cement, calcium silicate, silicon boride or gypsum.

[0042] In the present invention, the drying and curing temperature is preferably 20 to 50°C, more preferably 30 to 40°C, and more preferably 35°C;

[0043] The drying and curing time is preferably 1 to 5 hours, more preferably 2 to 4 hours, and even more preferably 3 hours.

[0044] In the present invention, when boron nitride and alumina fibers are dispersed in a brittle matrix respectively, the mass ratio of boron nitride to alumina fibers is preferably 1:1.

[0045] In the present invention, when the alumina fibers are dispersed in the brittle matrix, the mass ratio of the alumina fibers to the brittle matrix is ​​preferably 1:15 to 20, more preferably 1:17 to 19, and even more preferably 1:18.

[0046] The present invention also provides the application of the method for studying the high-temperature structure and performance of boron nitride and alumina fibers based on a special sintering method in the field of aerospace.

[0047] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0048] Example 1

[0049] Boron nitride fiber and alumina fiber were soaked in an ethanol solution with a mass percentage concentration of 75% for 1 hour, and then placed in a graphite crucible with a boron nitride powder coated on the bottom. Then, they were placed in a special sintering furnace. The furnace was evacuated and nitrogen was introduced. The pressure was maintained at 4 kPa, and the furnace was heated to 1400°C at a heating rate of 7°C / min. The fibers were sintered at this temperature for 2.5 hours. After the furnace was cooled to room temperature, the fibers were taken out.

[0050] By analyzing the microscopic morphology of boron nitride fibers and alumina fibers, the changes in the microstructures of the two fibers after treatment at different temperatures were systematically analyzed, and their high-temperature evolution mechanism was studied.

[0051] Equal amounts of boron nitride fibers before and after sintering were evenly dispersed in a gypsum matrix (the mass ratio of gypsum matrix to boron nitride fiber was 20:1), and then dried and cured at 30°C for 3 hours so that the gypsum matrix could be fully cured and tightly bonded to the boron nitride fibers. After demolding, microcracks were prefabricated in the middle of one side of the material to simulate the stress concentration that may be encountered in actual use. The bending stress-strain relationship curve of the gypsum was obtained using the three-point bending test method, which reflects the mechanical response characteristics of the material under bending load, and analyzes the retention rate of the mechanical properties and toughening effect of the boron nitride fiber after high-temperature sintering.

[0052] As above, the boron nitride fibers before and after sintering were replaced with equal amounts of alumina fibers before and after sintering, so as to analyze and obtain the retention rate of the mechanical properties and toughening effect of the alumina fibers after high-temperature sintering.

[0053] The SEM images of the boron nitride fiber cross section and the gypsum cross section before and after sintering in this embodiment are as follows: Figure 1As shown in the figure, (a) and (b) are cross-sectional SEM images of boron nitride fibers after sintering at room temperature and 1400°C, respectively. It can be seen that the boron nitride fibers after high-temperature treatment can be clearly observed to have stratification inside the fibers. This layered structure is beneficial for the fibers to resist deformation caused by external forces, so that the fibers still have high strength after high-temperature treatment, while maintaining a good toughening effect; (c) and (d) are SEM images of the morphology of boron nitride fibers on the cross-sectional surface of gypsum dispersed with room temperature boron nitride fibers and boron nitride fibers sintered at 1400°C, respectively. It can be observed that the boron nitride fibers sintered at 1400°C are mainly broken in the form of pull-out at the gypsum fracture section, and a large amount of deformation energy can be dissipated when the fibers are pulled out, which has a good toughening effect on the gypsum matrix.

[0054] The SEM images of the alumina fiber cross section and the gypsum cross section before and after sintering in this embodiment are as follows: Figure 2 As shown in the figure, (a) and (b) are the cross-sectional SEM images of alumina fibers after sintering at room temperature and 1400°C, respectively. It can be seen that the alumina fibers after high-temperature treatment can be obviously observed to have rapidly grown grains, and the fiber structure is no longer smooth and flat, but is composed of large grains. The bonding force between the grains is greatly weakened, and the fiber strength and toughening effect are also reduced accordingly; (c) and (d) are the SEM images of the alumina fiber morphology of the gypsum cross section dispersed with room temperature alumina fibers and alumina fibers sintered at 1400°C, respectively. It can be observed that the alumina fibers sintered at 1400°C are broken along the gypsum fracture section, and the toughening effect is almost completely lost.

[0055] Example 2

[0056] Same as Example 1, except that the pressure of the special sintering furnace is modified to 8 kPa.

[0057] Example 3

[0058] Same as Example 1, except that the heating rate is changed to 8°C / min.

[0059] Example 4

[0060] Same as Example 1, except that the drying and curing temperature is changed to 45°C.

[0061] Example 5

[0062] Same as Example 1, except that the drying and curing time is changed to 4 hours.

[0063] Example 6

[0064] Same as Example 1, except that the mass ratio of fiber to brittle matrix is ​​modified to 1:15.

[0065] Comparative Example 1

[0066] Same as Example 1, except that the boron nitride powder is replaced by an equal amount of titanium powder.

[0067] The results showed that titanium powder will undergo nitridation reaction at high temperature, and the generated titanium nitride will adhere to the fiber. Since titanium nitride has high hardness and high wear resistance, it will have a serious impact on the test of fiber performance.

[0068] Comparative Example 2

[0069] The same as Example 1, except that the special sintering furnace is changed to not evacuate the vacuum, but directly pass nitrogen.

[0070] The results showed that since the oxygen in the special sintering furnace could not be completely exhausted, the fiber was oxidized during the sintering process, affecting its performance.

[0071] Comparative Example 3

[0072] Same as Example 1, except that the drying and curing temperature is changed to 80°C.

[0073] The results showed that after the drying and curing temperature was changed to 80°C, due to the high temperature, the evaporation rate of water in the gypsum was too fast, the gypsum surface would dry and shrink quickly, while the internal water continued to evaporate, resulting in a lag in internal shrinkage. This uneven shrinkage inside and outside would produce stress differences and cause the gypsum to crack.

[0074] Comparative Example 4

[0075] Same as Example 1, except that the drying and curing time is changed to 0.5 h.

[0076] The results showed that due to the short drying time, the matrix could not be completely dried and still contained a large amount of moisture, resulting in a higher porosity of the gypsum and a greatly reduced strength.

[0077] Comparative Example 5

[0078] The same as Example 1, except that the mass ratio of the fiber to the brittle matrix is ​​modified to 1:30.

[0079] The results showed that after the mass ratio of fiber to brittle matrix was changed to 1:30, the toughening effect of fiber on gypsum was significantly reduced due to the low fiber content, resulting in poor mechanical properties of the gypsum matrix. There was no significant difference in the toughening effect of the obtained boron nitride fiber and alumina fiber on the matrix, which affected the analysis and testing.

[0080] Effect verification

[0081] The gypsum prepared in Examples 1 to 6 and Comparative Examples 1 to 5 was subjected to a three-point bending resistance test from a direction perpendicular to the gypsum plane. The samples were prepared into 40×4×3 mm strips according to the force direction, with a span of 30 mm and a loading speed of 0.5 mm / min to verify the toughening effect. The relevant effect data are shown in Tables 1 and 2.

[0082] Table 1 Bending stress peak value of boron nitride fiber toughened gypsum material

[0083]

[0084] It can be seen from Table 1 that when the gypsum prepared in Examples 1 to 6 is subjected to external pressure in a direction perpendicular to the gypsum plane, during the process of heating from room temperature to 1400°C, the fracture bending stress of the material does not decrease much, and the BN fiber still has high strength and good toughening effect, and can be applied to aerospace fields such as thermal protection covers and engine insulation layers of spacecraft.

[0085] Table 2 Peak bending stress of alumina fiber toughened gypsum materials

[0086]

[0087] It can be seen from Table 2 that when external pressure is applied to the gypsum prepared in Examples 1 to 6 in a direction perpendicular to the gypsum plane, the material fracture bending stress decreases significantly during the temperature rise from room temperature to 1400°C, indicating that although Al2O3 fiber has higher strength than BN fiber at room temperature, its fiber toughening effect retention rate is greatly reduced in a high temperature environment of 1400°C, and eventually the toughening potential is basically lost, which makes Al2O3 fiber unable to work at excessively high temperatures, which has a good reference significance for the screening of high-temperature aerospace materials.

[0088] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method, characterized in that: The following steps are involved: Boron nitride and alumina fibers are first pretreated, then sintered in a special sintering furnace and taken out after the furnace is cooled. The changes in the microstructure of boron nitride and alumina fibers after treatment at different temperatures are systematically analyzed, and their high-temperature evolution mechanism is studied. Boron nitride and alumina fibers are respectively dispersed in a brittle matrix and then dried and solidified. After demolding, a fiber-reinforced composite material is obtained. The fracture strength of the material is obtained by subjecting it to a three-point bending mechanical test, and then the mechanical properties and toughening effects of boron nitride and alumina fibers are studied.

2. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 1, characterized in that: The pretreatment is to immerse the boron nitride and alumina fibers in a liquid solution; The liquid solution is an acetone solution, an ethanol solution, water or an ethylene glycol solution, and the mass percentage concentration of the liquid solution is 50-75%; The soaking time is 10 to 180 minutes.

3. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 1 or 2, characterized in that: The special sintering furnace is first evacuated and then a protective gas is introduced, wherein the protective gas is one or more of nitrogen, argon, helium and neon.

4. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 1, characterized in that: The sintering pressure is 0.5-10 kPa, the sintering temperature is 1300-1500° C., and the sintering time is 2-3 hours.

5. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 4, characterized in that: The heating rate to the sintering temperature is 6 to 8°C / min.

6. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 1, characterized in that: The brittle matrix is ​​cement, calcium silicate, silicon boride or gypsum.

7. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 6, characterized in that: The drying and curing temperature is 20 to 50° C., and the drying and curing time is 1 to 5 hours.

8. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 6 or 7, characterized in that: When boron nitride and alumina fibers are dispersed in a brittle matrix respectively, the mass ratio of boron nitride to alumina is 1:

1.

9. The method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method according to claim 8, characterized in that: When the alumina fibers are dispersed in the brittle matrix, the mass ratio of the alumina fibers to the brittle matrix is ​​1:15-20.

10. Application of the method for studying the high temperature structure and performance of boron nitride and alumina fibers based on a special sintering method as described in any one of claims 1 to 9 in the field of aerospace.

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