Method for preparing boron carbide grinding medium based on pressureless sintering

Abstract

The invention discloses a method for preparing a boron carbide grinding medium based on pressureless sintering, and belongs to the technical field of pressureless sintering. The method comprises the following steps: mixing common micron-sized boron carbide powder with a doping aid, adding other additives and water, carrying out ball milling, carrying out spray granulation and cold press molding, and finally carrying out gradient pressureless sintering on the obtained blank to obtain the boron carbide grinding medium. According to the preparation method, the boron carbide dielectric material with the density reaching 96% or above is prepared by adopting a special doping aid in cooperation with gradient pressureless sintering, the limitation of traditional hot pressing sintering and pressureless sintering is well overcome, the product performance is improved while the cost is effectively reduced, and the preparation method is suitable for industrial large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of pressureless sintering technology, specifically relating to a method for preparing boron carbide grinding media based on pressureless sintering. Background Technology

[0002] Boron carbide (B4C) is an engineering ceramic material whose hardness is second only to diamond and cubic boron nitride. It also has a low density (approximately 2.52 g / cm³). 3 Boron carbide microspheres or grinding balls possess outstanding characteristics such as high hardness (Mohs hardness 9.5), strong wear resistance, good corrosion resistance, and strong neutron absorption capacity. As grinding media, boron carbide microspheres or grinding balls have significant advantages in high-end powder grinding fields (such as electronic ceramics, magnetic materials, special coatings, etc.), enabling high-purity, low-pollution fine grinding, and are especially suitable for processing high-value-added materials sensitive to impurities.

[0003] The preparation of boron carbide ceramics has long relied on hot pressing sintering technology. This method promotes particle contact diffusion and flow mass transfer through high temperature combined with unidirectional pressure, yielding high-performance products with near-theoretical density. However, hot pressing sintering equipment is complex, energy-intensive, and has high mold costs. Furthermore, uniaxial pressure application is limited to producing simple shapes (such as blocks and discs), making it difficult to directly form complex grinding media (such as microspheres and irregularly shaped grinding media). Compared to hot pressing sintering, pressureless sintering does not require pressure molds. In recent years, it has attracted widespread attention due to its simple equipment, lower cost, and suitability for large-scale production of complex-shaped parts, becoming an important research direction in the field of structural ceramics. However, traditional pressureless sintering processes rely on heat-driven diffusion. Due to the strong covalent bonds and low self-diffusion coefficient of boron carbide, densification is difficult, usually requiring the addition of large amounts of sintering aids, which easily introduces impurities, reduces density, and results in significantly lower hardness and strength of the finished product compared to hot-pressed products.

[0004] Therefore, it is of great significance to research and develop an improved method for preparing high-performance boron carbide grinding materials by pressureless sintering. Summary of the Invention

[0005] To address the problems mentioned in the background art, the present invention aims to provide a method for preparing boron carbide grinding media based on pressureless sintering. This invention employs special doping agents in conjunction with gradient pressureless sintering to prepare boron carbide grinding media materials with a density exceeding 96%, effectively overcoming the limitations of traditional hot pressing and pressureless sintering. This significantly reduces costs while improving product performance, making it suitable for large-scale industrial production.

[0006] To achieve the above objectives, the present invention specifically adopts the following technical solution: This invention provides a method for preparing boron carbide grinding media based on pressureless sintering, comprising the following steps: Step 1: Take boron carbide powder, add doping agent, binder, dispersant and defoamer, mix evenly, add water to obtain semi-dry mixed material; The doping agent is composed of silicon carbide, titanium diboride, lanthanum oxide, aluminum nitride, and needle coke. Step 2: The obtained semi-dry mixture is ball-milled and then spray-granulated and cold-pressed to obtain a green body; Step 3: Place the obtained blank in a pressureless furnace and sinter it under vacuum gradient at 1620-1750℃ to obtain boron carbide grinding media.

[0007] In a preferred embodiment, the boron carbide powder in step one is ordinary micron-sized with a particle size of 1-5 μm and a purity of 4N; the doping agents all have a particle size of 0.1-20 μm.

[0008] In a preferred embodiment, the mass ratio of boron carbide to doping agent in step one is 100:(4.2-11.5).

[0009] In a preferred embodiment, the mass ratio of silicon carbide, titanium diboride, lanthanum oxide, aluminum nitride, and needle coke in the doping agent is 10:(3.2-6.0):(0.3-0.7):1:(1.5-3.5).

[0010] This invention addresses the shortcomings of traditional pressureless sintered boron carbide in terms of density and other properties by further improving and optimizing the sintering aids: In the doped aid system of this invention, silicon carbide, as a second-phase reinforcing particle, can suppress the abnormal growth of boron carbide matrix grains during high-temperature sintering, which helps to obtain a uniform and fine microstructure, thereby improving the material's strength, hardness, and toughness; titanium diboride has a significantly different elastic modulus and coefficient of thermal expansion from the boron carbide matrix, which can generate residual stress at the two-phase interface. When crack propagation encounters this stress, it will deflect, thereby extending the crack path and consuming more energy. Lanthanum oxide can react with impurities in the raw materials or at grain boundaries to form a low-melting-point silicate or lanthanate glass phase, thereby purifying the grain boundaries and improving their strength. The resulting liquid phase can promote material migration and pore removal through the liquid phase sintering mechanism during the mid-sintering process, further driving densification. Aluminum nitride provides a rapid diffusion channel, promoting particle rearrangement, which not only lowers the sintering temperature but also further improves density and toughness. Unlike conventional carbon powder, needle coke is composed of highly oriented aromatic molecular layers. During sintering, this needle-like microcrystalline structure can not only transmit the sintering driving force, but its ordered layer structure also helps to induce the directional growth and close arrangement of boron carbide grains, thereby significantly improving density and hardness.

[0011] In a preferred embodiment, the binder in step one is polyvinyl alcohol, the dispersant is polycarboxylate, and the defoamer is ethylene oxide, with the addition amounts being 1.5%-4.5%, 0.2%-0.5%, and 0.1%-0.4% of the total mass of the boron carbide and doping agent mixed powder, respectively.

[0012] In a preferred embodiment, the ball-to-material ratio in step two is (3-10):1, the rotation speed is 100-400 rpm, and the time is 3-5 h.

[0013] In a preferred embodiment, the inlet temperature of the spray granulation in step two is 140-160℃, and the outlet temperature is 280-300℃.

[0014] In a preferred embodiment, the pressure for cold pressing in step two is 550-650 MPa, and the holding time is 3-5 min.

[0015] As a preferred embodiment, the vacuum gradient sintering in step three specifically involves: setting the vacuum level to -0.1±0.05 MPa; first, heating to 450-520℃ at a rate of 5±1℃ / min and holding for 30-45 min; then heating to 800-850℃ at a rate of 15±2℃ / min and holding for 45-90 min; next, heating to 1300-1420℃ at a rate of 5±1℃ / min and holding for 20-30 min; finally, heating to 1620-1750℃ at a rate of 2±1℃ / min and holding for 1.5-3 h; and then naturally cooling to room temperature in the furnace to obtain the final product. For the different raw material systems selected above, this invention employs a multi-stage pressureless sintering process with varying heating and holding rates. Excessive heating rates at different stages may lead to cracking of the green body, while insufficient holding time results in inadequate doping, affecting product performance. This effectively prevents cracking of the sintered ceramic material while further improving its density.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention uses special doping agents in conjunction with a gradient sintering process to solve the problem of low density in pressureless sintering of boron carbide (≤90% in traditional pressureless sintering process). The resulting product can achieve a density of over 96%, which is close to the level of hot pressing sintering.

[0017] 2. On the one hand, this invention uses ordinary micron-sized boron carbide powder as raw material to replace the high-purity nano boron carbide required by traditional hot pressing sintering. On the other hand, it reduces the maximum sintering temperature required by traditional pressureless sintering, effectively reducing costs and energy consumption from the perspectives of raw materials and processes.

[0018] 3. Adaptability to large-scale production: The wet mixing, granulation and continuous sintering processes of the present invention are easy to automate and are suitable for large-scale industrial production. Attached Figure Description

[0019] Figure 1 This is a scanning electron microscope (SEM) image of the boron carbide product prepared in Example 1 of the present invention.

[0020] Figure 2 This is an appearance diagram of the boron carbide product prepared in Example 1 of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with embodiments. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] Example 1 A method for preparing boron carbide grinding media based on pressureless sintering includes the following steps: 1. Weigh 100 parts by weight of boron carbide (particle size 1-5 μm, purity 4N grade) and add it to a ball mill. Separately, weigh silicon carbide (1-3 μm), titanium diboride (1-10 μm), lanthanum oxide (0.5-5 μm), aluminum nitride (0.1-2 μm), and needle coke (10-20 μm) in a mass ratio of 10:4.8:0.5:1:2.5 and mix them as doping aids (total weight 8 parts). Add the doping aids to the boron carbide, then add 3% polyvinyl alcohol binder, 0.35% polycarboxylate CE-64, and 0.25% ethylene oxide defoamer by mass of the two, and add 20 wt.% water to the mixture to obtain a semi-dry mixture.

[0024] 2. The obtained semi-dry mixed materials are ball-milled: the ball-to-material ratio is 5:1, the rotation speed is 250 rpm, and the time is 4 h; after ball milling, spray granulation is performed, with the inlet temperature set at 120℃ and the outlet temperature at 295℃; cold pressing is performed at 600 MPa and pressure held for 4 min to form a green body, which is then dried and debinded and placed in a special graphite mold sagger for later use.

[0025] 3. Transfer the graphite mold sagger into a pressureless furnace under argon vacuum (-0.1 MPa): First, heat to 490℃ at 5℃ / min and hold for 40 min; then heat to 820℃ at 15℃ / min and hold for 60 min; then heat to 1360℃ at 5℃ / min and hold for 25 min; finally heat to 1680℃ at 2℃ / min and hold for 2.4 h; allow the furnace to cool naturally to room temperature, and demold to obtain boron carbide grinding media.

[0026] Example 2 A method for preparing boron carbide grinding media based on pressureless sintering includes the following steps: 1. Weigh 100 parts by weight of boron carbide (particle size 1-5 μm, purity 4N grade) and add it to a ball mill. Separately, weigh silicon carbide (1-3 μm), titanium diboride (1-10 μm), lanthanum oxide (0.5-5 μm), aluminum nitride (0.1-2 μm), and needle coke (10-20 μm) in a mass ratio of 10:6:0.3:1:1.5 and mix them as doping aids (total weight 4.2 parts). Add the doping aids to the boron carbide, then add 1.5% polyvinyl alcohol binder, 0.2% polycarboxylate CE-64, and 0.1% ethylene oxide defoamer by the total mass of the two. Add 20 wt.% water to the mixture to obtain a semi-dry mixture.

[0027] 2. The obtained semi-dry mixed materials are ball-milled: the ball-to-material ratio is 3:1, the rotation speed is 400 rpm, and the time is 5 h; after ball milling, spray granulation is performed, with the inlet temperature set at 160℃ and the outlet temperature at 300℃; cold pressing is performed at 550 MPa and pressure held for 5 min to form a green body, which is then dried and degummed and placed in a special graphite mold sagger for later use.

[0028] 3. Transfer the graphite mold sagger into a pressureless furnace under argon vacuum (-0.05 MPa): First, heat to 450℃ at 5℃ / min and hold for 45 min; then heat to 800℃ at 15℃ / min and hold for 90 min; then heat to 1300℃ at 5℃ / min and hold for 30 min; finally heat to 1620℃ at 2℃ / min and hold for 3 h; allow the furnace to cool naturally to room temperature, and demold to obtain boron carbide grinding media.

[0029] Example 3 A method for preparing boron carbide grinding media based on pressureless sintering includes the following steps: 1. Weigh 100 parts by weight of boron carbide (particle size 1-5 μm, purity 4N grade) and add it to a ball mill. Separately, weigh silicon carbide (1-3 μm), titanium diboride (1-10 μm), lanthanum oxide (0.5-5 μm), aluminum nitride (0.1-2 μm), and needle coke (10-20 μm) in a mass ratio of 10:3.2:0.7:1:3.5 and mix them as doping aids (total weight 11.5 parts). Add the doping aids to the boron carbide, then add 4.5% polyvinyl alcohol binder, 0.5% polycarboxylate CE-64, and 0.4% ethylene oxide defoamer by the total mass of the two. Add 20 wt.% water to the mixture to obtain a semi-dry mixture.

[0030] 2. The obtained semi-dry mixed materials are ball-milled: the ball-to-material ratio is 10:1, the rotation speed is 100 rpm, and the time is 3 h; after ball milling, spray granulation is performed, with the inlet temperature set at 140℃ and the outlet temperature at 280℃; cold pressing is performed at 650 MPa and pressure held for 3 min to form a green body, which is then dried and debinded and placed in a special graphite mold sagger for later use.

[0031] 3. Transfer the graphite mold sagger into a pressureless furnace under argon vacuum (-0.15 MPa): First, heat to 520℃ at 5℃ / min and hold for 30 min; then heat to 850℃ at 15℃ / min and hold for 45 min; then heat to 1420℃ at 5℃ / min and hold for 20 min; finally heat to 1750℃ at 2℃ / min and hold for 1.5 h; allow the furnace to cool naturally to room temperature, and demold to obtain boron carbide grinding media.

[0032] Comparative Example 1 Referring to the steps and parameters of Embodiment 1 of the present invention, the only difference is that titanium diboride is not added to the doping agent.

[0033] Comparative Example 2 Referring to the steps and parameters of Embodiment 1 of the present invention, the only difference is that phenolic resin is used instead of needle coke in the doping agent.

[0034] Comparative Example 3 Referring to the steps and parameters of Embodiment 1 of the present invention, the only difference is that carbon black is used instead of needle coke in the doping agent.

[0035] Comparative Example 4 Referring to the steps and parameters of Embodiment 1 of the present invention, the only difference is that graphite powder is used instead of needle coke in the doping agent.

[0036] Comparative Example 5 The steps and parameters are the same as in Embodiment 1 of the present invention, except that aluminum nitride is not added to the doping agent.

[0037] Comparative Example 6 Referring to the step parameters of Embodiment 1 of the present invention, the only difference is that in step 3, the temperature is directly increased to 1680°C in one step at 5°C / min, and held for 3 hours for pressureless sintering.

[0038] The boron carbide samples prepared in the above embodiments and comparative examples were tested for performance (sintering density according to ISO18754, hardness according to ASTM C1327, elastic modulus according to ASTM E1876, fracture toughness according to ASTM C1421, and flexural strength according to GB / T 6569), and the results are shown in Table 1.

[0039] Table 1. Detection results of boron carbide samples

[0040] Based on the above test results, the boron carbide material prepared by the pressureless sintering process of this invention achieves a theoretical density of over 96%, approaching the level of hot-pressing sintering, with a hardness exceeding 2800 HV. It also exhibits good flexural strength and fracture toughness, demonstrating excellent overall performance. Comparative Examples 1 and 5, which doped without titanium diboride or aluminum nitride, had a relatively small impact on density, but both reduced hardness and fracture toughness to varying degrees. Comparative Examples 2-4 used different carbon sources such as phenolic resin, carbon black, or graphite as sintering aids, but none achieved the same performance improvement effect as the needle coke of this invention, especially with a significant deterioration in fracture toughness. Comparative Document 6, using a traditional sintering heating method, resulted in a substantial reduction in density and hardness. In summary, this invention effectively overcomes the limitations of traditional hot-pressing and pressureless sintering, effectively reducing costs while improving product performance, making it suitable for large-scale industrial production.

[0041] The embodiments described above are merely preferred embodiments of the present invention, and while the descriptions are specific and detailed, they are not intended to limit the present invention. It should be noted that various changes and modifications can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the concept and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing boron carbide grinding media based on pressureless sintering, characterized in that, Includes the following steps: Step 1: Take boron carbide powder, add doping agent, binder, dispersant and defoamer, mix evenly, add water to obtain semi-dry mixed material; The doping agent is composed of silicon carbide, titanium diboride, lanthanum oxide, aluminum nitride, and needle coke. Step 2: The obtained semi-dry mixture is ball-milled and then spray-granulated and cold-pressed to obtain a green body; Step 3: Place the obtained blank in a pressureless furnace and sinter it under vacuum gradient at 1620-1750℃ to obtain boron carbide grinding media.

2. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, The boron carbide powder mentioned in step one is ordinary micron-sized with a particle size of 1-5 μm and a purity of 4N grade; the doping agents all have a particle size of 0.1-20 μm.

3. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, In step one, the mass ratio of boron carbide to doping agent is 100:(4.2-11.5).

4. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, The mass ratio of silicon carbide, titanium diboride, lanthanum oxide, aluminum nitride, and needle coke in the doping agent is 10:(3.2-6.0):(0.3-0.7):1:(1.5-3.5).

5. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, The binder mentioned in step one is polyvinyl alcohol, the dispersant is polycarboxylate, and the defoamer is ethylene oxide. The addition amounts are 1.5%-4.5%, 0.2%-0.5%, and 0.1%-0.4% of the total mass of the mixed powder of boron carbide and dopant, respectively.

6. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, In step two, the ball-to-material ratio for ball milling is (3-10):1, the rotation speed is 100-400 rpm, and the time is 3-5 h.

7. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, The inlet temperature of the spray granulation process described in step two is 140-160℃, and the outlet temperature is 280-300℃.

8. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, The pressure for cold pressing in step two is 550-650 MPa, and the holding time is 3-5 min.

9. The method for preparing boron carbide grinding media based on pressureless sintering according to claim 1, characterized in that, The vacuum gradient sintering described in step three is as follows: the vacuum degree is set to -0.1±0.05 MPa. First, the temperature is increased to 450-520℃ at 5±1℃ / min and held for 30-45 min; then the temperature is increased to 800-850℃ at 15±2℃ / min and held for 45-90 min; then the temperature is increased to 1300-1420℃ at 5±1℃ / min and held for 20-30 min; finally, the temperature is increased to 1620-1750℃ at 2±1℃ / min and held for 1.5-3 h; the furnace is then allowed to cool naturally to room temperature to obtain the final product.

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