A Low-Temperature Glass Brazing Method for SiC Ceramics Based on Surface Mullite Treatment

Abstract

This invention relates to the field of SiC ceramic joining technology for nuclear energy, specifically a low-temperature glass brazing method for SiC ceramics based on surface mullite modification. This method uses aluminum sulfate and sodium sulfate double salt as raw materials to modify the surface of SiC ceramics by mullite modification in an air atmosphere at 750℃-900℃. Then, using a B2O3-SiO2-ZnO glass brazing filler metal, low-temperature brazing of SiC ceramics is achieved at 700℃ in an air atmosphere without pressure. This invention features a simple process, requires no vacuum or protective atmosphere, has a low joining temperature, and the mullite layer improves the wettability of the brazing filler metal and reduces interfacial stress. The joint is defect-free and has high bonding strength, making it suitable for efficient and reliable joining of SiC ceramic components for nuclear applications.

Description

Technical Field

[0001] This invention relates to the field of SiC ceramic bonding technology for nuclear energy, and more specifically to a low-temperature glass brazing method for SiC ceramics based on surface mullite treatment. Background Technology

[0002] As is well known, SiC ceramics possess low density, high strength, excellent thermal stability, high thermal conductivity, low inductive activity, and excellent radiation resistance, making them a promising core-structure material. However, due to the intrinsic brittleness and poor machinability of SiC ceramics, it is difficult to form them in one step during manufacturing. Typically, joining techniques are used to manufacture components of the required shape and size.

[0003] Currently, the main methods for joining SiC ceramics and their composites include metal brazing, solid-state diffusion, polymer precursor bonding, and glass brazing. Metal brazing filler metals have poor corrosion resistance, which cannot meet the application requirements of nuclear service environments. Solid-state diffusion and polymer precursor bonding both require high-temperature and high-pressure bonding conditions, making the processes complex and significantly limiting the size and shape of the joints. Glass brazing for SiC ceramics does not require pressure, but the bonding temperature exceeds 1200℃, which will significantly impact the composition and properties of the base materials for SiC ceramics and their composites that may be used in future nuclear applications. The reason for this is that the covalent bonds in SiC are extremely stable, making it difficult to achieve good wetting and reliable bonding directly under low-temperature and pressureless conditions. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a simple method for low-temperature glass brazing of SiC ceramics based on surface mullite modification. This method involves modifying the SiC surface in air to generate a mullite layer in situ, and then using a low-temperature glass brazing filler metal to achieve reliable bonding of SiC ceramics under pressureless conditions.

[0005] The technical solution adopted by this invention to solve its technical problem is: A low-temperature glass brazing method for SiC ceramics based on surface mullite alteration, characterized by the following steps: First, two SiC ceramics are selected as substrates. The surfaces of the two SiC ceramics to be soldered are ground, polished and cleaned to obtain a clean and flat surface to be soldered. 2. Weigh analytically pure Al2(SO4)3•18H2O and Na2SO4 into a mortar in a certain proportion, grind them thoroughly until they are mixed evenly to obtain a double salt. 3. The two polished SiC ceramics were embedded in the complex salt obtained in step 2 and placed in a muffle furnace for sintering reaction. After the reaction was completed, the samples were cooled to room temperature in the furnace and taken out. They were then washed with water in an ultrasonic water bath until the unreacted molten salt was washed away. The samples were then dried in an oven to obtain two mullite-treated SiC ceramics. 4. Assemble a "sandwich" structure of the workpiece to be welded by sequentially assembling one mullitized SiC ceramic, glass brazing filler metal, and another mullitized SiC ceramic. 5. Place the workpieces to be welded obtained in step 4 into a muffle furnace for brazing. The brazing temperature is 700℃ and the holding time is 60min. Then cool down to room temperature to obtain an all-ceramic SiC joint.

[0006] The mass ratio of the double salt in step two of this invention is Al2(SO4)3·18H2O:Na2SO4=9:10.

[0007] In step three of this invention, the sintering temperature is 750℃-900℃ and the sintering time is 60min.

[0008] Steps three and five of this invention are both performed in an air atmosphere, without the need for a vacuum or additional protective gas.

[0009] The glass brazing filler metal used in step four of this invention is a B2O3-SiO2-ZnO glass.

[0010] The heating method in step five of this invention includes first heating to 300°C at a heating rate of 15°C / min and holding for 10 min, then heating to 700°C at a heating rate of 10°C / min and holding for 30 min; then cooling down to 200°C at a cooling rate of 5°C / min and then cooling to room temperature with the furnace.

[0011] The two SiC ceramics selected in step one of this invention are of different sizes to facilitate subsequent shear force detection.

[0012] Compared with existing SiC ceramics and their composite joining technologies, this invention has the following significant advantages: 1. Significantly reduced connection temperature: This invention can achieve reliable connection under pressureless conditions at approximately 700°C, which is far lower than the 1200°C or more required for traditional glass brazing. This breakthrough avoids irreversible damage to the microstructure, mechanical properties, and radiation resistance of SiC ceramic matrix material caused by high temperature, and preserves the intrinsic advantages of SiC as a core structure material.

[0013] 2. Operation in air atmosphere, extremely simple process: The entire surface modification and bonding process is carried out in air atmosphere, without the need for vacuum equipment or inert gas protection, which significantly reduces the requirements for special equipment and manufacturing costs, and is conducive to engineering promotion.

[0014] 3. Pressureless connection, adaptable to complex shapes: The present invention adopts a pressureless connection process, which is not limited by the shape, size and number of joints of the components. It is especially suitable for the assembly of large-sized, thin-walled or internally complex SiC ceramic components, and overcomes the dependence on pressure equipment for solid-phase diffusion and other methods.

[0015] 4. The in-situ mullite layer plays multiple core roles: By constructing a mullite layer in situ on the SiC surface, on the one hand, the wettability of the low-temperature glass solder is significantly improved, solving the common problem that covalent SiC is difficult to wet directly at low temperatures; on the other hand, mullite and SiC have similar coefficients of thermal expansion, which can effectively alleviate the residual stress at the joint interface and improve the mechanical reliability and high-temperature service stability of the joint.

[0016] 5. Excellent nuclear environment compatibility: The glass brazing filler metal and its reaction products exhibit excellent corrosion resistance, low inductive activity, and radiation resistance, meeting the stringent service requirements of nuclear reactors. Compared to traditional metal brazing, it avoids the risk of filler metal corrosion failure.

[0017] 6. High versatility and can be extended to other ceramic systems: This method is not only applicable to SiC ceramics and their composites, but can also be further extended to the joining of other non-oxide ceramics, providing a general technical approach for low-temperature pressureless joining of various advanced ceramics, and has good cross-material promotion value.

[0018] 7. The present invention has the advantages of simple method, ability to modify SiC surface in air, in-situ generation of mullite layer, and then use low temperature glass brazing filler metal to achieve reliable connection of SiC ceramic under pressureless conditions. Attached Figure Description

[0019] Figure 1 The image shows the XRD pattern of SiC ceramics produced by mullite processing in Example 1.

[0020] Figure 2 The images show the surface morphology of SiC ceramics at different mullite sintering temperatures in Examples 1-4. (a) is the surface morphology of SiC ceramic in Example 1, (b) is the surface morphology of SiC ceramic in Example 2, (c) is the surface morphology of SiC ceramic in Example 3, and (d) is the surface morphology of SiC ceramic in Example 4.

[0021] Figure 3 The images show the SiC joint interface morphology obtained in Examples 1-4, where (a) is the SiC joint interface morphology of Example 1, (b) is the SiC joint interface morphology of Example 2, (c) is the SiC joint interface morphology of Example 3, and (d) is the SiC joint interface morphology of Example 4.

[0022] Figure 4 This is a diagram of the interface structure morphology for Comparative Example 5.

[0023] Figure 5 The image shows a comparison of the shear strength of the SiC joints obtained in Examples 1-4.

[0024] Figure 6 The diagrams show the fracture locations of the SiC joints obtained in Comparative Example 5 and Example 2, where (a) is the fracture location diagram of the SiC joint in Comparative Example 5 and (b) is the fracture location diagram of the SiC joint in Example 2. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings: As shown in the attached figure, a low-temperature glass brazing method for SiC ceramics based on surface mullite alteration is characterized by the following steps: First, two SiC ceramics are selected as substrates. The surfaces of the two SiC ceramics to be soldered are ground, polished and cleaned to obtain a clean and flat surface to be soldered. 2. Weigh analytically pure Al2(SO4)3•18H2O and Na2SO4 into a mortar in a certain proportion, grind them thoroughly until they are mixed evenly to obtain a double salt. 3. The two polished SiC ceramics were embedded in the complex salt obtained in step 2 and placed in a muffle furnace for sintering reaction. After the reaction was completed, the samples were cooled to room temperature in the furnace and taken out. They were then washed with water in an ultrasonic water bath until the unreacted molten salt was washed away. The samples were then dried in an oven to obtain two mullite-treated SiC ceramics. 4. Assemble a "sandwich" structure of the workpiece to be welded by sequentially assembling one mullitized SiC ceramic, glass brazing filler metal, and another mullitized SiC ceramic. 5. Place the workpieces to be welded obtained in step 4 into a muffle furnace for brazing. The brazing temperature is 700℃ and the holding time is 60min. Then cool down to room temperature to obtain an all-ceramic SiC joint.

[0026] Furthermore, the mass ratio of the double salt in step two is Al2(SO4)3·18H2O:Na2SO4=9:10.

[0027] Furthermore, in step three, the sintering temperature is 750℃-900℃, and the sintering time is 60 minutes.

[0028] Furthermore, steps three and five are both performed in an air atmosphere, without the need for a vacuum or additional protective gas.

[0029] Furthermore, the glass brazing filler metal used in step four is a B2O3-SiO2-ZnO glass.

[0030] Furthermore, the heating method in step five includes first heating to 300°C at a heating rate of 15°C / min and holding for 10 min, then heating to 700°C at a heating rate of 10°C / min and holding for 30 min; subsequently cooling to 200°C at a cooling rate of 5°C / min and then cooling to room temperature with the furnace.

[0031] Furthermore, the two SiC ceramics selected in step one are of different sizes to facilitate subsequent shear force detection.

[0032] Compared with existing SiC ceramics and their composite joining technologies, this invention has the following significant advantages: 1. Significantly reduced connection temperature: This invention can achieve reliable connection under pressureless conditions at approximately 700°C, which is far lower than the 1200°C or more required for traditional glass brazing. This breakthrough avoids irreversible damage to the microstructure, mechanical properties, and radiation resistance of SiC ceramic matrix material caused by high temperature, and preserves the intrinsic advantages of SiC as a core structure material.

[0033] 2. Operation in air atmosphere, extremely simple process: The entire surface modification and bonding process is carried out in air atmosphere, without the need for vacuum equipment or inert gas protection, which significantly reduces the requirements for special equipment and manufacturing costs, and is conducive to engineering promotion.

[0034] 3. Pressureless connection, adaptable to complex shapes: The present invention adopts a pressureless connection process, which is not limited by the shape, size and number of joints of the components. It is especially suitable for the assembly of large-sized, thin-walled or internally complex SiC ceramic components, and overcomes the dependence on pressure equipment for solid-phase diffusion and other methods.

[0035] 4. The in-situ mullite layer plays multiple core roles: By constructing a mullite layer in situ on the SiC surface, on the one hand, the wettability of the low-temperature glass solder is significantly improved, solving the common problem that covalent SiC is difficult to wet directly at low temperatures; on the other hand, mullite and SiC have similar coefficients of thermal expansion, which can effectively alleviate the residual stress at the joint interface and improve the mechanical reliability and high-temperature service stability of the joint.

[0036] 5. Excellent nuclear environment compatibility: The glass brazing filler metal and its reaction products exhibit excellent corrosion resistance, low inductive activity, and radiation resistance, meeting the stringent service requirements of nuclear reactors. Compared to traditional metal brazing, it avoids the risk of filler metal corrosion failure.

[0037] 6. High versatility and can be extended to other ceramic systems: This method is not only applicable to SiC ceramics and their composites, but can also be further extended to the joining of other non-oxide ceramics, providing a general technical approach for low-temperature pressureless joining of various advanced ceramics, and has good cross-material promotion value.

[0038] 7. The present invention has the advantages of simple method, ability to modify SiC surface in air, in-situ generation of mullite layer, and then use low temperature glass brazing filler metal to achieve reliable connection of SiC ceramic under pressureless conditions.

[0039] Example 1: Step 1: Select SiC ceramics (α-SiC, 6H type) with dimensions of 5mm×5mm×5mm and 15mm×7mm×5mm as substrates. Grind the surfaces to be soldered on both substrates with a 2000# sanding disc, then polish them with diamond polishing paste, and finally ultrasonically clean them in acetone for 15 minutes to remove surface oil and impurities, obtaining a clean and flat surface to be soldered with Ra≈0.05μm. Step 2: Weigh analytical grade Al2(SO4)3•18H2O and Na2SO4 into a mortar at a mass ratio of 9:10, grind them thoroughly until they are evenly mixed, and obtain the double salt; Step 3: Spread the complex salt powder obtained in Step 2 evenly in a corundum crucible, completely embedding the SiC ceramic substrate cleaned in Step 1 within the complex salt powder. Place the crucible in a muffle furnace and heat to 900℃ at a rate of 10℃ / min under air atmosphere, holding for 60 min to carry out the sintering reaction. After the reaction, cool to room temperature with the furnace and remove the sample. Place the sample in an ultrasonic water bath and repeatedly wash it with deionized water until the unreacted molten salt is removed, then dry it in an oven to obtain SiC ceramic with an in-situ mullite layer on the surface (referred to as mullite-treated SiC ceramic). Step 4: Select commercially available B2O3-SiO2-ZnO series glass brazing filler powder, and assemble the small-sized SiC ceramics, glass brazing filler powder, and large-sized SiC ceramics from mulliteralized material into a "sandwich" structure for the workpiece to be welded in sequence. The specific operation is as follows: spread the glass brazing filler powder evenly on the surface of the large-sized SiC ceramic to be welded, then stack the small-sized SiC ceramics, and place a lightweight alumina pressure block on top (only to maintain contact and not to apply additional welding pressure) to obtain the workpiece to be welded. Step 5: Place the parts to be brazed in a muffle furnace and braze them in an air atmosphere. The heating program is as follows: raise the temperature from room temperature to 300℃ at a heating rate of 15℃ / min and hold for 10min; then raise the temperature to 700℃ at a heating rate of 10℃ / min and hold for 30min; then lower the temperature to 200℃ at a cooling rate of 5℃ / min and finally cool it to room temperature with the furnace. Remove the sample to obtain the SiC ceramic joint.

[0040] Example 2: The difference between this embodiment and embodiment 1 is that the sintering temperature in step 3 is 850°C, while the other steps are the same as in embodiment 1.

[0041] Example 3: The difference between this embodiment and embodiment 1 is that the sintering temperature in step 3 is 800℃, while the other steps are the same as in embodiment 1.

[0042] Example 4: The difference between this embodiment and embodiment 1 is that the sintering temperature in step 3 is 750°C, while the other steps are the same as in embodiment 1.

[0043] Comparative Example 5: The difference between this comparative example and Example 2 is that the mullite surface modification treatment is not performed in step three, while the other steps are the same as in Example 2.

[0044] Figure 1 The XRD pattern of the mullite-modified SiC ceramic obtained in Example 1 is shown. The XRD pattern is an X-ray diffraction pattern, which shows that a large amount of mullite has formed on the SiC surface.

[0045] Figure 2 The surface morphology of the mullite-treated SiC ceramics obtained in Examples 1-4 is shown. As the temperature increases, the density of the surface mullite layer gradually increases; at the same time, the microstructure of mullite also undergoes significant evolution. The samples at 750–800℃ are mainly composed of amorphous particles with a loose structure; above 850℃, needle-like mullite whiskers begin to form, and the density continues to increase with increasing temperature; at 900℃, the whiskers grow fully and bridge each other, resulting in a high-density continuous structure.

[0046] Figure 3 The images show the microstructure of the SiC joint interface obtained in Examples 1-4. It is evident that there are no defects such as cracks or pores at the joint interface, and a good metallurgical bond is achieved between the ceramic and the brazing seam. For comparison, Figure 4 For the interface morphology of Comparative Example 5, there are obvious macroscopic cracks at the interface between the ceramic and the brazing seam, the joint connection quality is poor, and no effective connection is formed.

[0047] Figure 5 The shear strength of the SiC joints obtained in Examples 1-4 is shown. With increasing sintering temperature, the shear strength of the joints first increases and then decreases. At a sintering temperature of 850℃, the shear strength of the joint can reach 42 MPa.

[0048] Figure 6 The fracture locations of the SiC joints obtained in Comparative Example 5 and Example 2 are shown. The SiC joints without mullite surface modification mainly fractured at the brazing seam / SiC interface, and... Figure 4The interface structure results were consistent; the SiC joint treated with mullite surface modification cracked at the center of the brazing seam and gradually deflected into the ceramic interior, indicating that the presence of mullite effectively enhanced the metallurgical bond between the brazing seam and SiC, and improved the load-bearing capacity of the joint.

[0049] The above-described method uses aluminum sulfate and sodium sulfate double salt as raw materials to perform surface mullite modification on SiC ceramics in an air atmosphere at 750℃-900℃. Then, a B2O3-SiO2-ZnO glass brazing filler metal is used to achieve low-temperature brazing of SiC ceramics at 700℃ in an air atmosphere without pressure. This invention features a simple process, requires no vacuum or protective atmosphere, has a low connection temperature, and the mullite layer improves the wettability of the brazing filler metal and reduces interfacial stress. The joint is defect-free and has high bonding strength, making it suitable for efficient and reliable connections of nuclear-grade SiC ceramic components.

[0050] While the disclosure is as stated above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the protection scope of this invention.

Claims

1. A low-temperature glass brazing method for SiC ceramics based on surface mullite formation, characterized in that... The steps of the method are as follows: First, two SiC ceramics are selected as substrates. The surfaces of the two SiC ceramics to be soldered are ground, polished and cleaned to obtain a clean and flat surface to be soldered.

2. Weigh analytically pure Al2(SO4)3•18H2O and Na2SO4 into a mortar in a certain proportion, grind them thoroughly until they are mixed evenly to obtain a double salt.

3. The two polished SiC ceramics were embedded in the complex salt obtained in step 2 and placed in a muffle furnace for sintering reaction. After the reaction was completed, the samples were cooled to room temperature in the furnace and taken out. They were then washed with water in an ultrasonic water bath until the unreacted molten salt was washed away. The samples were then dried in an oven to obtain two mullite-treated SiC ceramics.

4. Assemble a "sandwich" structure of the workpiece to be welded by sequentially assembling one mullitized SiC ceramic, glass brazing filler metal, and another mullitized SiC ceramic.

5. Place the workpieces to be welded obtained in step 4 into a muffle furnace for brazing. The brazing temperature is 700℃ and the holding time is 60min. Then cool down to room temperature to obtain an all-ceramic SiC joint.

2. The low-temperature glass brazing method for SiC ceramics based on surface mullite formation according to claim 1, characterized in that... The mass ratio of the double salt in step two is Al2(SO4)3·18H2O:Na2SO4=9:

10.

3. The low-temperature glass brazing method for SiC ceramics based on surface mullite formation according to claim 1, characterized in that... In step three, the sintering temperature is 750℃-900℃ and the sintering time is 60 minutes.

4. The low-temperature glass brazing method for SiC ceramics based on surface mullite formation according to claim 1, characterized in that... Steps three and five are both performed in an air atmosphere, without the need for a vacuum or additional protective gas.

5. The low-temperature glass brazing method for SiC ceramics based on surface mullite formation according to claim 1, characterized in that... The glass brazing filler metal used in step four is a B2O3-SiO2-ZnO glass.

6. The low-temperature glass brazing method for SiC ceramics based on surface mullite formation according to claim 1, characterized in that... The heating method in step five includes first heating to 300°C at a heating rate of 15°C / min and holding for 10 min, then heating to 700°C at a heating rate of 10°C / min and holding for 30 min; then cooling down to 200°C at a cooling rate of 5°C / min and then cooling to room temperature with the furnace.

7. The low-temperature glass brazing method for SiC ceramics based on surface mullite formation according to claim 1, characterized in that... The two SiC ceramics selected in step one are of different sizes.

Images