High-purity silicon nitride ceramic and preparation method thereof

Abstract

The invention belongs to the technical field of ceramic materials, and relates to high-purity silicon nitride ceramic and a preparation method thereof. According to the invention, magnesium oxide (MgO), barium oxide (BaO) and niobium oxide (Nb2O5) in a specific proportion are added, a MgO-BaO-Nb2O5 ternary liquid phase system is formed at a high temperature, and manganese oxide and vanadium oxide generate electricity price change at the high temperature, so that migration and diffusion among silicon nitride particles and activation of crystal lattices by the electricity price change are promoted, and the sintering density and the mechanical property are improved. A closed ball milling mode is adopted, ball milling is performed twice, multiple additives and solvents are added for the first time to ensure that powder is fully mixed, and maltose and a cross-linking agent are added for the second time to enhance the binding force between the powder and avoid caking.

Description

Technical Field

[0001] The invention belongs to the technical field of ceramic materials and relates to a high-purity silicon nitride ceramic and a preparation method thereof. Background Art

[0002] As a high-performance advanced ceramic material, silicon nitride ceramics have been widely used in aerospace, automotive industry, machinery manufacturing, energy and semiconductors due to their excellent mechanical properties, high temperature resistance, oxidation resistance and good chemical stability. However, despite the many advantages of silicon nitride ceramics, there are still some key technical challenges and limitations in the actual production and application process.

[0003] The production of silicon nitride ceramics usually requires high temperature and high pressure conditions, which makes its production cost significantly higher than that of traditional metal materials. This high cost limits the large-scale application of silicon nitride ceramics, especially in cost-sensitive industries. Although silicon nitride ceramics have high hardness, they have poor toughness and are prone to cracking or breaking under impact or stress. This brittleness limits the use of silicon nitride ceramics in applications that are subject to impact loads. Due to the extremely high hardness of silicon nitride ceramics, traditional machining methods are difficult to process them effectively. This means that there are greater technical challenges in manufacturing parts with complex shapes, increasing processing costs and cycles. Silicon nitride ceramics have a high coefficient of thermal expansion, which may affect its compatibility with other materials, especially in application scenarios where different materials need to be used in combination, which may cause component failure due to the mismatch of thermal expansion coefficients. Due to insufficient fracture toughness, silicon nitride ceramics are prone to cracks or even fractures when encountering impact or external forces. This feature limits the application of silicon nitride ceramics in fields that require high-toughness materials, such as high-acceleration rotating parts or parts subject to impact loads. Silicon nitride ceramics do not perform well under bending stress and may not be suitable for certain occasions that require materials to have excellent bending resistance, such as in mechanical structures that need to withstand bending stress. During the sintering process, silicon nitride ceramics may have tiny holes (sand holes) or cracks on the surface or inside. These defects not only affect the appearance quality of the product, but more importantly, they will significantly reduce the mechanical properties of the material, thereby affecting the reliability and life of the final product.

[0004] It can be seen that although silicon nitride ceramics have many excellent properties, their production and application still face many challenges such as cost, brittleness, processing difficulty, thermal expansion coefficient, fracture toughness, flexural strength and sintering defects. Therefore, the development of new production processes and technologies is crucial to overcome these limitations and expand the application range of silicon nitride ceramics. Summary of the invention

[0005] The purpose of the present invention is to solve the above problems in the prior art and to provide a silicon nitride having a content of ≥ 98.5% and a sintered density of ≥ 3.19 g / cm 3 , bending strength ≥ 800Mpa, relative density ≥ 99.0%, and high-purity silicon nitride ceramics with an apparent porosity close to 0.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A high-purity silicon nitride ceramic comprises the following raw materials in parts by mass: 90-120 parts of silicon nitride powder, 0.1-0.5 parts of magnesium oxide, 0.1-0.3 parts of barium oxide, 0.1-0.3 parts of niobium oxide, 0.03-0.08 parts of manganese oxide, 0.03-0.08 parts of vanadium oxide, 0.5-1.0 parts of a dispersant, 0.1-0.3 parts of a carbon source, and 1-5 parts of a cross-linking agent.

[0008] In the above-mentioned high-purity silicon nitride ceramic, the dispersant is a silane coupling agent.

[0009] In the above-mentioned high-purity silicon nitride ceramic, the carbon source is at least one of maltose and starch. In the preparation process of silicon nitride ceramics, maltose and starch are two commonly used carbon sources. When they are dissolved in ethanol, they can better promote the dispersion of the carbon source, thereby expanding the contact area between the carbon source and the silicon nitride particles, which is very beneficial for improving the sintering effect. By dissolving these carbon sources in ethanol, it can not only ensure that the carbon source is evenly distributed throughout the sintering process, but also promote the effective reduction of oxygen atoms on the surface of silicon nitride, thereby reducing the surface energy, improving the sintering activity, and finally obtaining a silicon nitride ceramic product with better performance.

[0010] In the above-mentioned high-purity silicon nitride ceramic, the crosslinking agent is at least two of polyvinyl pyrrolidone, oxidized polyethylene wax, and ethyl acrylate.

[0011] Preferably, the cross-linking agent is polyvinyl pyrrolidone, oxidized polyethylene wax, and ethyl acrylate in a mass ratio of 1:1:1. Oxidized polyethylene wax can effectively reduce the friction between particles during the pressing process, making the green body smoother during the molding process, and helping to improve the density and uniformity of the green body. Ethyl acrylate has the characteristics of fast drying speed and high hardness, which helps to prevent the slurry from forming a hard surface during vacuum drying. At the same time, it can also improve the hardness of the green body after pressing, ensuring the stability and mechanical strength of the green body in the subsequent processing process. Polyvinyl pyrrolidone has a good film-forming and shaping effect, which can ensure that the green body after molding maintains the desired shape and has high strength to prevent deformation during drying and sintering. The present invention uses polyvinyl pyrrolidone, oxidized polyethylene wax, and ethyl acrylate in a mass ratio of 1:1:1 as a cross-linking agent, which can not only improve the processing performance of silicon nitride ceramics, but also significantly improve the quality and performance of the final product.

[0012] The present invention also provides a method for preparing high-purity silicon nitride ceramics, the method comprising the following steps:

[0013] S1, prepare the above raw materials;

[0014] S2, adding ethanol to silicon nitride powder, and then sequentially adding magnesium oxide, barium oxide, niobium oxide, manganese oxide, vanadium oxide and a dispersant to perform ball milling treatment;

[0015] S3, then adding carbon source and cross-linking agent to continue ball milling;

[0016] S4, vacuum drying after ball milling, and then sieving;

[0017] S5. After screening, dry pressing is performed, vacuum degreasing is performed, and finally hot isostatic pressing and step-by-step sintering are performed to obtain high-purity silicon nitride ceramics.

[0018] In the above method for preparing high-purity silicon nitride ceramics, the ball milling treatment is performed using silicon nitride balls with a diameter of 10-15 mm, and the ball milling time is 20-30 hours.

[0019] In the above-mentioned method for preparing high-purity silicon nitride ceramics, the temperature during vacuum drying is 75-80° C. and the vacuum degree is minus 0.01-0.05 MPa.

[0020] In the above-mentioned method for preparing a high-purity silicon nitride ceramic, the screen in the screening process is 50-70 mesh. In the preparation process of silicon nitride ceramics, the particle size control of the powder is very important to ensure the quality and performance of the final product. The present invention controls the particle size to 50-70 mesh, which can not only improve the density of the green body, but also reduce defects in the processing process. If the particle size of the powder is too fine, it will cause cracks to form easily during the pressing process. This is because when fine particles are subjected to pressure, the interaction force between particles is strong, which can easily cause local stress concentration and then cracks. Therefore, too fine powder is not conducive to the integrity of the green body. If the particle size of the powder is too coarse, the gap between the particles is large during the pressing process, which can easily lead to more voids in the green body, thereby affecting the density of the final product. Ceramic materials with low density not only have low strength, but also their thermal shock resistance will be affected.

[0021] In the above-mentioned method for preparing high-purity silicon nitride ceramics, the dry pressing pressure is 120-180 MPa, and the density after molding is 1.70-1.90 g / cm 3 .

[0022] In the above-mentioned method for preparing high-purity silicon nitride ceramics, the vacuum degreasing temperature is 800-900°C, the time is 1-4h, and the vacuum degree is 5-10Kpa. The present invention controls the vacuum degreasing temperature at 800-900°C, which is conducive to timely discharge of volatile decomposed substances such as crosslinking agents and dispersants without retention to avoid reaction with the matrix, and can degrease cleanly to prevent the formation of closed pores.

[0023] In the above-mentioned preparation method of high-purity silicon nitride ceramics, hot isostatic pressing step-by-step sintering is specifically as follows: when the temperature rises to 1500-1520°C under vacuum conditions, nitrogen is then filled in and a pressure of 100-200MPa is maintained until the temperature rises to 1550-1600°C and then kept warm for 2-4h.

[0024] The present invention places the degreased green body in a hot isostatic pressing furnace, first evacuates the air in the furnace, and then starts heating and sintering. As the temperature gradually increases, the carbon source in the green body can react with the oxygen atoms on the surface of silicon nitride to reduce the oxygen atoms, thereby reducing the energy on the surface of the silicon nitride particles and activating the sintering process. When the temperature reaches 1450-1500°C, the pre-added oxides begin to melt, and magnesium oxide, barium oxide, and niobium oxide will form a MgO-BaO-Nb2O5 ternary liquid phase system. The presence of this liquid phase can significantly improve the migration ability and diffusion rate between silicon nitride particles, promote the shrinkage during ceramic sintering, and manganese oxide and vanadium oxide are variable valence compounds, and the electric valence can also change at high temperatures, activating the lattice and further promoting sintering.

[0025] When the temperature continues to rise to about 1500-1520℃, in order to prevent silicon nitride from decomposing at high temperatures, the vacuum valve needs to be closed, the vacuum pumping needs to be stopped, and high-purity nitrogen needs to be filled into the furnace. This allows the furnace to be filled with nitrogen and maintain a pressure of 100-200MPa until the temperature rises to the sintering temperature of 1550-1600℃, and is kept at this temperature for 2-4h. In this process, high-pressure nitrogen not only prevents the decomposition of silicon nitride, but also provides isostatic pressure, coordinates liquid phase migration and diffusion, and activates the variable valence lattice, which helps the ceramic material to continue to shrink and densify at high temperatures.

[0026] In the above-mentioned method for preparing high-purity silicon nitride ceramics, the silicon nitride content in the high-purity silicon nitride ceramics is ≥ 98.5%, and the sintered density is ≥ 3.19 g / cm 3 , flexural strength ≥800MPa, relative density ≥99.0%.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The present invention forms a ternary liquid phase system of MgO-BaO-Nb2O5 at high temperature by adding magnesium oxide (MgO), barium oxide (BaO) and niobium oxide (Nb2O5) in a specific proportion. Manganese oxide and vanadium oxide undergo a charge change at high temperature to promote migration and diffusion between silicon nitride particles and activate the lattice by charge change, thereby improving the sintering density and mechanical properties.

[0029] 2. The present invention adopts a closed ball milling method, through two ball millings, the first time adding a variety of additives and solvents to ensure that the powders are fully mixed, and the second time adding maltose and a cross-linking agent to enhance the binding force between the powders and avoid agglomeration.

[0030] 3. The present invention performs drying under vacuum conditions, accurately controls the temperature and vacuum degree, helps evaporate the solvent and promotes mild cross-linking of the cross-linking agent, and ensures good dispersion of the powder. Degreasing is also performed under vacuum conditions to effectively remove organic components; during the hot isostatic pressing sintering process, high-pressure nitrogen is used to prevent the decomposition of silicon nitride at high temperatures, and provides isostatic pressure to promote material densification. DETAILED DESCRIPTION

[0031] The following are specific embodiments of the present invention, which further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0032] Embodiment 1:

[0033] S1. Prepare the following raw materials: 100 parts of silicon nitride powder, 0.25 parts of magnesium oxide, 0.1 parts of barium oxide, 0.2 parts of niobium oxide, 0.06 parts of manganese oxide, 0.06 parts of vanadium oxide, 0.8 parts of dispersant, 0.2 parts of carbon source, and 3 parts of cross-linking agent;

[0034] The dispersant is a silane coupling agent, the carbon source is maltose, and the cross-linking agent is polyvinyl pyrrolidone, oxidized polyethylene wax, and ethyl acrylate in a mass ratio of 1:1:1;

[0035] S2. Using a closed ball milling method, add silicon nitride balls with a diameter of 13 mm in a ball mill in a ratio of 1:1, add silicon nitride powder, then add ethanol with a mass percentage of 80 wt% of the silicon nitride powder, and then add magnesium oxide, barium oxide, niobium oxide, manganese oxide, vanadium oxide and a dispersant in sequence and perform ball milling for 25 hours;

[0036] S3, then adding carbon source and cross-linking agent and continuing ball milling for 25h;

[0037] S4, after the ball milling is completed, vacuum drying is performed, the relative vacuum is controlled at -0.01Mpa, the temperature is controlled at 78°C, the ethanol is evaporated and recovered, and at the same time, the ethyl acrylate is lightly cross-linked, so that the final powder is dry and easy to disperse without agglomeration, and then crushed through a 60-mesh sieve;

[0038] S5, after sieving, dry pressing is carried out at 150Mpa and the density reaches 1.80g / cm 3 , then vacuum degreasing, with absolute vacuum of 8Kpa, temperature of 850℃ and heat preservation for 3h;

[0039] S6. Place the degreased green body in a hot isostatic pressing furnace. When the temperature rises to 1520°C under vacuum conditions, fill it with nitrogen and maintain a pressure of 150MPa until the temperature rises to 1580°C and keep it warm for 3h.

[0040] Embodiment 2:

[0041] S1. Prepare the following raw materials: 90 parts of silicon nitride powder, 0.1 parts of magnesium oxide, 0.1 parts of barium oxide, 0.1 parts of niobium oxide, 0.03 parts of manganese oxide, 0.03 parts of vanadium oxide, 0.5 parts of dispersant, 0.1 parts of carbon source, and 1 part of cross-linking agent;

[0042] The dispersant is a silane coupling agent, the carbon source is maltose, and the cross-linking agent is polyvinyl pyrrolidone, oxidized polyethylene wax, and ethyl acrylate in a mass ratio of 1:1:1;

[0043] S2. Using a closed ball milling method, add silicon nitride balls with a diameter of 13 mm in a ball mill in a ratio of 1:1, add silicon nitride powder, then add ethanol with a mass percentage of 80 wt% of the silicon nitride powder, and then add magnesium oxide, barium oxide, niobium oxide, manganese oxide, vanadium oxide and a dispersant in sequence and perform ball milling for 25 hours;

[0044] S3, then adding carbon source and cross-linking agent and continuing ball milling for 25h;

[0045] S4, after the ball milling is completed, vacuum drying is performed, the relative vacuum is controlled at -0.01Mpa, the temperature is controlled at 78°C, the ethanol is evaporated and recovered, and at the same time, the ethyl acrylate is lightly cross-linked, so that the final powder is dry and easy to disperse without agglomeration, and then crushed through a 60-mesh sieve;

[0046] S5, after sieving, dry pressing is carried out at 150Mpa and the density reaches 1.80g / cm 3 , then vacuum degreasing, with absolute vacuum of 8Kpa, temperature of 850℃ and heat preservation for 3h;

[0047] S6. Place the degreased green body in a hot isostatic pressing furnace. When the temperature rises to 1520°C under vacuum conditions, fill it with nitrogen and maintain a pressure of 150MPa until the temperature rises to 1580°C and keep it warm for 3h.

[0048] Embodiment 3:

[0049] S1. Prepare the following raw materials: 120 parts of silicon nitride powder, 0.5 parts of magnesium oxide, 0.3 parts of barium oxide, 0.3 parts of niobium oxide, 0.08 parts of manganese oxide, 0.08 parts of vanadium oxide, 1.0 parts of dispersant, 0.3 parts of carbon source, and 5 parts of cross-linking agent;

[0050] The dispersant is a silane coupling agent, the carbon source is maltose, and the cross-linking agent is polyvinyl pyrrolidone, oxidized polyethylene wax, and ethyl acrylate in a mass ratio of 1:1:1;

[0051] S2. Using a closed ball milling method, add silicon nitride balls with a diameter of 13 mm in a ball mill in a ratio of 1:1, add silicon nitride powder, then add ethanol with a mass percentage of 80 wt% of the silicon nitride powder, and then add magnesium oxide, barium oxide, niobium oxide, manganese oxide, vanadium oxide and a dispersant in sequence and perform ball milling for 25 hours;

[0052] S3, then adding carbon source and cross-linking agent and continuing ball milling for 25h;

[0053] S4, after the ball milling is completed, vacuum drying is performed, the relative vacuum is controlled at -0.01Mpa, the temperature is controlled at 78°C, the ethanol is evaporated and recovered, and at the same time, the ethyl acrylate is lightly cross-linked, so that the final powder is dry and easy to disperse without agglomeration, and then crushed through a 60-mesh sieve;

[0054] S5, after sieving, dry pressing is carried out at 150Mpa and the density reaches 1.80g / cm 3 , then vacuum degreasing, with absolute vacuum of 8Kpa, temperature of 850℃ and heat preservation for 3h;

[0055] S6. Place the degreased green body in a hot isostatic pressing furnace. When the temperature rises to 1520°C under vacuum conditions, fill it with nitrogen and maintain a pressure of 150MPa until the temperature rises to 1580°C and keep it warm for 3h.

[0056] Embodiment 4:

[0057] The only difference from Example 1 is that the density of the green blank after dry pressing in step S5 is 1.60 g / cm 3 .

[0058] Embodiment 5:

[0059] The only difference from Example 1 is that step S5 does not undergo vacuum degreasing.

[0060] Embodiment 6:

[0061] The only difference from Example 1 is that nitrogen is not charged in step S6.

[0062] Embodiment 7:

[0063] The only difference from Example 1 is that in step S6, the degreased green body is placed in a hot isostatic pressing furnace, the temperature is raised to 1520°C under nitrogen conditions, and the pressure is maintained at 150 MPa until the temperature reaches 1580°C and then kept warm for 3 hours.

[0064] Embodiment 8:

[0065] The only difference from Example 1 is that in step S6, the degreased green body is placed in a hot isostatic pressing furnace and the temperature is raised to 1580° C. under vacuum conditions.

[0066] Embodiment 9:

[0067] The only difference from Example 1 is that the crosslinking agent is only ethyl acrylate.

[0068] Comparative Example 1:

[0069] The only difference from Example 1 is that no barium oxide is added to the raw materials.

[0070] Comparative Example 2:

[0071] The only difference from Example 1 is that no niobium oxide is added to the raw material.

[0072] Comparative Example 3:

[0073] The only difference from Example 1 is that no vanadium oxide is added to the raw material.

[0074] Table 1: Performance test results of high-purity silicon nitride ceramics prepared in Examples 1-9 and Comparative Examples 1-3

[0075]

[0076] The parts of the embodiments herein that are not exhaustive of the midpoint values ​​of the technical scope claimed for protection by the present invention and the new technical solutions formed by equivalent replacement of single or multiple technical features in the technical solutions of the embodiments are also within the scope claimed for protection by the present invention; at the same time, in all the listed or unlisted embodiments of the scheme of the present invention, each parameter in the same embodiment merely represents an example of its technical solution (i.e., a feasible solution), and there is no strict coordination and limitation relationship between the parameters, wherein the parameters can be replaced with each other without violating the axioms and the claims of the present invention, unless otherwise stated.

[0077] The technical means disclosed in the scheme of the present invention are not limited to the technical means disclosed in the above technical means, but also include technical schemes composed of any combination of the above technical features. The above is a specific implementation 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, and these improvements and modifications are also regarded as the protection scope of the present invention.

[0078] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A high-purity silicon nitride ceramic, characterized in that: The silicon nitride ceramic comprises the following raw materials in parts by weight: 90-120 parts of silicon nitride powder, 0.1-0.5 parts of magnesium oxide, 0.1-0.3 parts of barium oxide, 0.1-0.3 parts of niobium oxide, 0.03-0.08 parts of manganese oxide, 0.03-0.08 parts of vanadium oxide, 0.5-1.0 parts of dispersant, 0.1-0.3 parts of carbon source, and 1-5 parts of cross-linking agent.

2. A high-purity silicon nitride ceramic according to claim 1, characterized in that: The carbon source is at least one of maltose and starch.

3. A high-purity silicon nitride ceramic according to claim 1, characterized in that: The crosslinking agent is at least two of polyvinyl pyrrolidone, oxidized polyethylene wax and ethyl acrylate.

4. A method for preparing high-purity silicon nitride ceramics as claimed in claim 1, characterized in that: The method comprises the following steps: S1. Prepare the raw materials according to claim 1; S2, adding ethanol to the silicon nitride powder, and then sequentially adding magnesium oxide, barium oxide, niobium oxide, manganese oxide, vanadium oxide and a dispersant to perform ball milling treatment; S3, then adding carbon source and cross-linking agent to continue ball milling; S4, vacuum drying after ball milling, and then sieving; S5. After screening, dry pressing is performed, vacuum degreasing is performed, and finally hot isostatic pressing and step-by-step sintering are performed to obtain high-purity silicon nitride ceramics.

5. A method for preparing high-purity silicon nitride ceramics according to claim 4, characterized in that: Ball milling treatment: ball milling is performed using silicon nitride balls with a diameter of 10-15 mm, and the ball milling time is 20-30 hours.

6. The method for preparing high-purity silicon nitride ceramics according to claim 4, characterized in that: The temperature during vacuum drying is 75-80°C and the vacuum degree is minus 0.01-0.05MPa.

7. A method for preparing high-purity silicon nitride ceramics according to claim 4, characterized in that: The dry pressing pressure is 120-180Mpa, and the density after molding is 1.70-1.90g / cm 3 .

8. The method for preparing high-purity silicon nitride ceramics according to claim 4, characterized in that: The vacuum degreasing temperature is 800-900℃, the time is 1-4h, and the vacuum degree is 5-10Kpa.

9. The method for preparing high-purity silicon nitride ceramics according to claim 4, characterized in that: The hot isostatic pressing step sintering is specifically as follows: when the temperature rises to 1500-1520°C under vacuum conditions, nitrogen is then filled in and a pressure of 100-200 MPa is maintained until the temperature rises to 1550-1600°C and then kept warm for 2-4 hours.

10. The method for preparing high-purity silicon nitride ceramics according to claim 4, characterized in that: The silicon nitride content in high-purity silicon nitride ceramics is ≥98.5%, and the sintered density is ≥3.19g / cm 3 , flexural strength ≥800MPa, relative density ≥99.0%.

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