Preparation method of large-size silicon nitride bearing ball

Abstract

The invention discloses a preparation method of a large-size silicon nitride bearing ball, and relates to the technical field of ceramic materials. Comprising the steps that preparation raw materials are determined, the preparation raw materials comprise main powder and auxiliary materials, and the main powder comprises silicon nitride powder doped with superfine powder. Main powder is segmented through the coarse / medium / fine three-section particle size grading system, the dispersity and wettability optimization of different particle size sections before forming can be realized through respective independent treatment of coarse sections, medium sections and fine sections, so that agglomeration and separation among the different particle size sections are avoided during sintering densification, and meanwhile, during mixing, the particle size distribution of the powder is reduced. By means of staged mixing, local high shear force aggregation and particle size drift caused by one-time large-amount adding are avoided, the structural uniformity and geometric stability of a final blank are improved, the consistency tolerance of large-size parts can be easily achieved, and meanwhile the silicon nitride powder doped with the superfine powder is combined, so that the production efficiency is improved. The fracture toughness, the load service life and the bending strength of the finished silicon nitride bearing ball are improved.

Description

Technical Field

[0001] This invention relates to the field of ceramic materials technology, specifically to a method for preparing large-size silicon nitride bearing balls. Background Technology

[0002] Silicon nitride bearing balls are rolling bearing balls made primarily of silicon nitride, characterized by high hardness, high strength, and excellent wear and heat resistance. Due to their extremely low density and excellent thermal stability, they exhibit excellent roundness and dimensional stability, demonstrating outstanding fatigue life and low friction characteristics in high-temperature, high-speed, high-load, and corrosive environments. They are widely used in high-end bearings and sealing systems in aerospace, precision machine tools, energy, and automotive industries.

[0003] A method for preparing large-size silicon nitride ceramics, disclosed in patent publication number CN118754680A, includes the following steps: first, mixing a sintering aid and a dispersant to obtain a first mixed powder; second, mixing silicon nitride powder with the first mixed powder to obtain a second mixed powder; third, mixing a binder solution with the second mixed powder to obtain a silicon nitride slurry; using the silicon nitride slurry for spray granulation to obtain spherical powder; molding the spherical powder into a mold and then cold isostatically pressing it to obtain a silicon nitride green body; and finally, debinding and hot isostatically pressing the silicon nitride green body to obtain the large-size silicon nitride ceramics. The preparation method provided by this invention can realize the preparation of large-size silicon nitride ceramics, and the process is simple, easy to mass-produce, and the obtained large-size silicon nitride ceramics have high density, good uniformity, and excellent performance.

[0004] Existing processes for preparing large-size silicon nitride bearing balls often employ a single-stage powder preparation or a direct mixing method after classification. This approach easily leads to problems such as uneven particle size distribution, non-uniform grain interfaces, significant density gradients, uneven porosity, and fluctuations in surface roughness and geometric tolerances in the finished product. Consequently, the mechanical properties, thermal stability, and fatigue life of the finished product are limited. Furthermore, the mixing process is prone to local agglomeration and particle size drift, as well as inconsistent interface wetting, ultimately affecting the consistency of sintering density and dimensional control. Therefore, this invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing large-size silicon nitride bearing balls to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing large-size silicon nitride bearing balls, the method comprising the following steps:

[0007] The raw materials for preparation are determined, including main powder and auxiliary materials. The main powder includes silicon nitride powder mixed with ultrafine powder, and the auxiliary materials include flux, modifier and surface treatment agent.

[0008] Raw material classification was carried out, and a three-stage particle size classification system of coarse / medium / fine particles was established. Based on the three-stage particle size classification system, the main powder was classified to obtain segmented main powder.

[0009] The raw materials were mixed, and based on the segmented main powder, the main powders of different grades were modified and dried to obtain coarse segment group, medium segment group and fine segment group.

[0010] The raw materials are mixed as a whole, with coarse segment group, medium segment group and fine segment group as raw material groups. A mixing strategy is set, and the raw material groups are mixed based on the mixing strategy to obtain the embryo.

[0011] After preparation, the preform is subjected to staged drying treatment, and isothermal sintering and processing are carried out on the preform after staged drying treatment to obtain silicon nitride bearing balls.

[0012] Furthermore, the coarse / medium / fine three-segment particle size classification system includes a coarse segment, a medium segment, and a fine segment, wherein the coarse segment is 2–5 μm, the medium segment is 0.5–2 μm, and the fine segment is 0.1–0.5 μm.

[0013] Furthermore, the modification strategy includes independently coating the segmented main powder with a covalent coupling agent, performing surface plasma treatment, and controlling the distribution of doped particles for grain boundary wetting to complete the modification. The zeta potential of the modified powder is in the range of -20 to -60 mV.

[0014] Furthermore, the modified segmented main powder is dried independently. After drying, each segment is reclassified based on the segmented main powder to obtain coarse segment group, medium segment group and fine segment group.

[0015] Furthermore, the mixing strategy includes initially mixing the coarse segment group and the middle segment group to form a primary medium-coarse mixture, and then adding the fine segment group to the primary medium-coarse mixture to complete the mixing of the raw material group to obtain the embryo, thereby achieving uniform mixing of the three segments.

[0016] Furthermore, the mixing strategy employs dry graded mixing during the mixing of raw material groups, while simultaneously introducing a wet dispersion step to treat non-dispersible components. The remaining amount of dispersant used in the wet dispersion is removed before final drying, and a target moisture content is preset, resulting in the moisture content of the dried powder being less than the target moisture content.

[0017] Furthermore, online / offline monitoring is performed during the mixing process. Preset detection targets are set, and the dispersibility, particle size distribution, and wettability of any segment are monitored in real time to obtain monitoring results. When the monitoring results deviate from the detection targets, redispersion or remodification treatment is performed.

[0018] Furthermore, the ratio of the coarse segment group, the middle segment group, and the fine segment group is coarse segment group: middle segment group: fine segment group as (4-5): (3-4): (1-2).

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] The method for preparing large-size silicon nitride bearing balls involves segmenting the main powder into coarse, medium, and fine particle size classification systems. By processing the coarse, medium, and fine segments independently, the dispersibility and wettability of different particle size segments before molding can be optimized. This avoids agglomeration and separation between different particle size segments during sintering and densification, resulting in a more uniform grain interface and a denser microstructure. Furthermore, during mixing, the coarse, medium, and fine segments are added in stages for phased mixing. This phased mixing avoids the localized high shear force aggregation and particle size drift caused by adding a large amount at once, improving the structural uniformity and geometric stability of the final blank. This is beneficial for achieving consistent tolerances in large-size parts. Additionally, the incorporation of ultrafine silicon nitride powder enhances the fracture toughness, load life, and bending strength of the finished silicon nitride bearing balls.

[0021] Meanwhile, the independent modification and dispersion stabilization of each segment before mixing can maintain a good dispersion state during the mixing process, reduce the risk of moisture absorption / agglomeration caused by surface energy mismatch, thereby improving formability and initial blank density. Quality thresholds are set at each stage and online monitoring is implemented. If the distribution of a certain segment does not conform to the preset, limited-time re-dispersion or re-modification treatment is carried out to form an adaptive production closed loop, improve batch stability and traceability. The segmented particle size system can still maintain its distribution characteristics during the drying and densification stages, reduce particle size separation or re-agglomeration caused by temperature / pressure increase, thereby achieving higher sintering density and geometric tolerance consistency. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall process structure of the present invention;

[0023] Figure 2 This is a schematic diagram of the main powder sieving structure of the present invention;

[0024] Figure 3 This is a schematic diagram of the hybrid strategy structure of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Compared to traditional metal bearings, silicon nitride bearing balls have stronger wear resistance, thermal shock resistance and chemical inertness, while also having better adaptability to lubrication conditions. They are lightweight and highly reliable, making them suitable for achieving higher operating efficiency and system stability under extreme conditions.

[0027] like Figures 1-3 As shown, the present invention provides a technical solution: a method for preparing large-size silicon nitride bearing balls, the method comprising the following steps:

[0028] The raw materials for preparation are determined. The raw materials include main powder and auxiliary materials. The main powder includes silicon nitride powder mixed with ultrafine powder. The auxiliary materials include flux, modifier and surface treatment agent.

[0029] It should be noted that the excipients include flux / grain boundary modifier / dopermeable, surface modifier / coupling agent, dispersant (used in the wet dispersion stage), and surfactant / flow improver. Specifically, the flux / grain boundary modifier / dopermeable is yttrium oxide and alumina, with a mass fraction of 1.0–3.0 wt%, the surface modifier / coupling agent has a mass fraction of 0.5–2.0 wt%, the dispersant has a mass fraction of 0.5–1.5 wt%, the surfactant / flow improver has a mass fraction of 0–1.0 wt%, and the ultrafine powder is silicon nitride ultrafine powder or equivalent high specific surface area powder as reinforcing particles, with an addition amount of 0–5 wt%.

[0030] Raw material classification was carried out, and a three-stage particle size classification system of coarse / medium / fine particles was established. Based on the three-stage particle size classification system, the main powder was classified to obtain segmented main powder.

[0031] It is important to note that strict separation is essential to ensure no significant mixing between fractions. Each fraction of powder should be cleaned independently (removing moisture and surface impurities). By establishing a fractionation system and fractionating the main powder, the necessary wettability, dispersibility, and densification behavior for subsequent fractional modification and mixing can be ensured.

[0032] The raw materials were mixed, and based on the segmented main powder, the main powders of different grades were modified and dried to obtain coarse segment group, medium segment group and fine segment group.

[0033] It should be noted that after separating the raw materials according to particle size range, they should be subjected to independent post-treatment and surface modification.

[0034] The raw materials are mixed as a whole, with coarse segment group, medium segment group and fine segment group as raw material groups. A mixing strategy is set, and the raw material groups are mixed based on the mixing strategy to obtain the embryo.

[0035] It is important to note that online / offline monitoring is used to monitor key indicators such as dispersibility, particle size distribution (D10 / D50 / D90), and wettability. If the particle size deviates from the target, it can be redispersed or modified to form a closed-loop control.

[0036] After preparation, the preform is subjected to staged drying treatment, and isothermal sintering and processing are carried out on the preform after staged drying treatment to obtain silicon nitride bearing balls.

[0037] It is important to note that after the green body is formed, it undergoes staged drying and isothermal sintering at a controlled heating rate to achieve high density and good grain boundary wetting. Quality control points before and after forming, such as green body size, roundness, surface roughness, and internal stress distribution, should be ensured to be consistent through online / offline testing before and after sintering.

[0038] like Figure 1 As shown, the coarse / medium / fine three-segment particle size classification system includes a coarse segment, a medium segment, and a fine segment. The coarse segment is 2–5 μm, the medium segment is 0.5–2 μm, and the fine segment is 0.1–0.5 μm.

[0039] It should be noted that the following methods can be used during the segmentation process: Figure 2 The screens shown represent different particle size ranges and are used for segmentation. Figure 2 The downward arrow indicates the direction of the main powder during sieving and segmentation.

[0040] like Figure 1 As shown, the modification strategy includes independently coating the segmented main powder with a covalent coupling agent, surface plasma treatment, and controlling the distribution of doped particles for grain boundary wetting to complete the modification. The zeta potential of the modified powder is in the range of -20 to -60 mV.

[0041] It is important to note that the modification strategy (executed independently for each segment without interference) is as follows: Coarse segment (2–5 μm): Covalent coupling agent coating (such as organosilanes) is applied to improve wettability and dispersion stability, with a coating amount of 0.5–2.0 wt% (based on the mass of the segmented powder); Middle segment (0.5–2 μm): Surface modification is applied to reduce surface energy and improve wettability, with a coating amount of 0.5–1.5 wt%; Fine segment (0.1–0.5 μm): This improves dispersibility and inhibits agglomeration, with a coating amount of 0.5–2.0 wt%, and can introduce control over the distribution of doped particles related to grain boundary wetting. Zeta potential control: The Zeta potential of the three segments of powder is generally controlled in the range of -20 to -60 mV after modification to ensure dispersion stability.

[0042] like Figure 1 As shown, the modified segmented main powder is dried independently. After drying, each segment is reclassified based on the segmented main powder to obtain coarse segment group, medium segment group and fine segment group, to ensure that the particle size distribution is within the target range after drying.

[0043] It is important to note that drying and re-grading are performed independently for each stage. The drying temperature and rate must be matched with the powder properties to avoid thermal stress and re-agglomeration. After drying, each stage is re-graded to ensure that the particle size distribution after drying still falls within the target range.

[0044] like Figure 3 As shown, the mixing strategy includes initially mixing the coarse and medium segments to form a primary coarse-medium mixture, and then adding the fine segments to the primary coarse-medium mixture to complete the mixing of the raw material groups and obtain the embryo, thus achieving uniform mixing in three stages.

[0045] It is important to note that the coarse and medium segments are initially mixed to obtain a primary-coarse-medium mixture. The rotation speed, time, temperature and charge ratio should be controlled to ensure clear boundaries and uniform distribution. The fine segments are then added to the primary-coarse-medium mixture for final mixing. Shear force and temperature should still be controlled to prevent premature agglomeration of the fine segments.

[0046] In the process of mixing the raw material group, dry graded mixing is adopted, and a wet dispersion step is introduced to treat the non-dispersible components. The remaining amount of dispersant used in wet dispersion is removed before final drying. The target moisture content is preset, and the moisture content of the powder after drying is less than the target moisture content.

[0047] It is important to note that a wet dispersion step is introduced during the mixing process to treat non-dispersible components. A dispersant is used in the wet dispersion, and residual dispersant and moisture are removed before drying. The target moisture content after drying is lower than a set value, which is the target moisture content. The specific value is set by the operator based on actual usage. Wet dispersion effectively reduces agglomeration in the dispersion system and improves uniformity over a short period. Dry classification repositions the particle size range of each powder segment before subsequent mixing, ensuring that the classification target remains valid. Together, these two processes achieve a more stable particle size distribution. Through two-stage particle size and dispersion control, more stable wetting and interfacial wetting can be achieved between powders in different segments, reducing regional density differences and thus improving the geometric tolerance stability and mechanical property consistency of large-size bearing balls.

[0048] Online / offline monitoring is performed during the mixing process. Preset detection targets are set, and the dispersibility, particle size distribution, and wettability of any segment are monitored in real time to obtain monitoring results. When the monitoring results deviate from the detection targets, redispersibility or remodification treatment is performed.

[0049] It is important to note that by adding online / offline monitoring to monitor the dispersibility, particle size distribution, and wettability of any segment in real time, and by performing redispersing or remodification when deviations from the detection target are detected, the overall distribution after mixing can be kept stable. The specific segment refers to any one of the coarse segment group, the medium segment group, and the fine segment group. The detection target includes the accurate values ​​of dispersibility, particle size distribution, and wettability, which are determined according to the actual use.

[0050] The ratio of coarse segment group, medium segment group and fine segment group is coarse segment group: medium segment group: fine segment group is (4-5): (3-4): (1-2).

[0051] It should be noted that the specific mass fractions for each segment are expressed as follows: coarse segment group: 40–45 wt%, medium segment group: 30–40 wt%, and fine segment group: 10–20 wt%.

[0052] Example 1:

[0053] A method for preparing large-size silicon nitride bearing balls involves segmenting silicon nitride powder, the base raw material, into coarse, medium, and fine particle size distributions. The coarse distribution ranges from 2 to 5 μm, the medium distribution ranges from 0.5 to 2 μm, and the fine distribution ranges from 0.1 to 0.5 μm. The segmented raw material is then subjected to segmental modification treatment to obtain coarse, medium, and fine particle groups. A mixture of 40 wt% coarse, 40 wt% medium, and 20 wt% fine particles is used as the material. After segmented modification, the material is dried and mixed to obtain a preform. During the mixing process, online / offline monitoring is performed to determine the detection target. The dispersibility, particle size distribution, and wettability of any segment are monitored in real time to obtain the monitoring results. When the monitoring results deviate from the detection target, redispersion or remodification is performed. The mixing process involves initially mixing the coarse segment group and the middle segment group to form a primary medium-coarse mixture. Then, the fine segment group is added to the primary medium-coarse mixture to complete the mixing of the raw material groups to obtain the preform. The preform is then isothermally sintered to obtain silicon nitride bearing balls.

[0054] Example 2:

[0055] A method for preparing large-size silicon nitride bearing balls involves incorporating 5 wt% ultrafine silicon nitride powder into silicon nitride as the base raw material. The mixture is then segmented using a three-stage particle size classification system: a coarse segment (2–5 μm), a medium segment (0.5–2 μm), and a fine segment (0.1–0.5 μm). The segmented raw material is then subjected to segmented modification treatment to obtain coarse, medium, and fine segment groups. The coarse, medium, and fine segment groups are prepared with a mass fraction of 40 wt% each. The fine segments are used as raw materials. After segmental modification, they are dried and mixed to obtain a preform. Online / offline monitoring is carried out during the mixing process to determine the detection target. The dispersibility, particle size distribution and wettability of any segment are monitored in real time to obtain the monitoring results. When the monitoring results deviate from the detection target, redispersion or remodification is carried out. The mixing process adopts the initial mixing of coarse segments and medium segments to form a primary medium-coarse mixture. Then, the fine segments are added to the primary medium-coarse mixture to complete the mixing of the raw material groups to obtain the preform. The preform is subjected to isothermal sintering to obtain silicon nitride bearing balls.

[0056] Example 3:

[0057] A method for preparing large-size silicon nitride bearing balls involves segmenting silicon nitride powder, the basic raw material, into coarse, medium, and fine particle size distributions using a three-stage particle size classification system. The coarse segment ranges from 2 to 5 μm, the medium segment ranges from 0.5 to 2 μm, and the fine segment ranges from 0.1 to 0.5 μm. The segmented raw material undergoes segmental modification to obtain coarse, medium, and fine segments. A material is prepared using 40 wt% of the coarse segment, 40 wt% of the medium segment, and 20 wt% of the fine segment. After segmental modification, the material is dried and mixed to obtain a preform. Online / offline monitoring is performed during the mixing process to determine the detection target. The dispersibility, particle size distribution, and wettability of any segment are monitored in real time to obtain the monitoring results. When the monitoring results deviate from the detection target, further dispersion or modification is performed. The mixing process involves simultaneous overall mixing. The preform is then isothermally sintered to obtain the silicon nitride bearing ball.

[0058] Example 4:

[0059] A method for preparing large-size silicon nitride bearing balls involves segmenting silicon nitride powder, the basic raw material, into coarse, medium, and fine particle size distributions. The coarse distribution ranges from 2 to 5 μm, the medium distribution ranges from 0.5 to 2 μm, and the fine distribution ranges from 0.1 to 0.5 μm. A mixture of 40 wt% coarse, 40 wt% medium, and 20 wt% fine particles is used as the raw material. The raw materials are mixed and then subjected to overall modification. The mixing process involves initially mixing the coarse and medium particles to form a primary coarse-medium mixture, then adding the fine particles to the primary coarse-medium mixture to complete the mixing of the raw material groups. After modification, the mixture is dried to obtain a preform, which is then isothermally sintered to obtain the silicon nitride bearing ball.

[0060] Comparative example:

[0061] Silicon nitride bearing balls of the same size and mass as those in Examples 1 to 4 were used as comparative examples. The comparative examples did not segment the main powder for raw material processing. Specifically, the raw materials were directly modified, dried after modification to obtain a preform, and then isothermal sintered to obtain the silicon nitride bearing balls.

[0062] It should be noted that the isothermal sintering and processing procedures used in Examples 1 to 4 and the comparative examples are the same, and the necessary processing steps in the production process are also the same, such as cleaning the raw materials, which does not affect the change of the raw material properties, to ensure that the silicon nitride bearing balls obtained are only different in some raw materials and raw material processing.

[0063] To verify that the silicon nitride bearing ball obtained in this application has improved bending strength compared to traditional silicon nitride bearing balls, mechanical properties were tested on the silicon nitride bearing balls prepared by Examples 1 to 4 and the silicon nitride bearing balls formed by the materials in the comparative examples. A three-point bending mechanical testing system with equal cross-section was adopted. Following the material mechanical property testing principles of ISO 14577, the loading speed, fixture configuration, and unit length load were kept consistent. The stress-strain calculation formula specified in the national standard was used, the initial elastic zone was stripped, and the stress value corresponding to the ultimate bending moment at fracture was taken. The average value of the repeated samples at both ends was taken, and a range was given for process comparison. Bending strength tests were conducted on the silicon nitride bearing balls tested in Examples 1 to 4 and the comparative examples.

[0064] To verify that the silicon nitride bearing ball obtained in this application has an improved load life compared to the traditional silicon nitride bearing ball, the mechanical properties of the silicon nitride bearing balls prepared by Examples 1 to 4 and the silicon nitride bearing balls formed by the materials in the comparative examples were tested. In this test, a fatigue testing machine was used, and in accordance with the general testing requirements for the fatigue performance of materials in national standards, cyclic fatigue tests of silicon nitride bearing balls of the same size and geometric tolerance were conducted under stable loads. The specific process was referred to GB / T 10138 (General Method for Fatigue Life Test of Materials). The silicon nitride bearing balls used in Examples 1 to 4 and the comparative examples were subjected to load life tests.

[0065] To verify that the silicon nitride bearing ball obtained in this application has improved fracture toughness compared to the traditional silicon nitride bearing ball, the mechanical properties of the silicon nitride bearing balls prepared by Examples 1 to 4 and the silicon nitride bearing balls formed by the materials in the comparative examples were tested. The fracture toughness test of the Vickers skeleton sample was used. The specific test process refers to GB / T 2975 (Mechanical property test specimen preparation and testing) and GB / T 7750 (Specific method for fracture toughness determination). The fracture toughness value was obtained by deriving from the critical stress intensity factor K_IC or by achieving the stability test of crack propagation in the cohesive fracture zone according to the calculation formula specified in the national standard. The silicon nitride bearing balls in Examples 1 to 4 and the comparative examples were subjected to fracture toughness test.

[0066] During the sampling phase before testing, samples were taken from the finished products obtained in Examples 1 to 4 and the comparative example, respectively. Three silicon nitride bearing balls were obtained for each sample, and the samples were numbered. The silicon nitride bearing balls obtained in Example 1 were recorded as 1-1, 1-2, and 1-3; the silicon nitride bearing balls obtained in Example 2 were recorded as 2-1, 2-2, and 2-3; the silicon nitride bearing balls obtained in Example 3 were recorded as 3-1, 3-2, and 3-3; the silicon nitride bearing balls obtained in Example 4 were recorded as 4-1, 4-2, and 4-3; and the silicon nitride bearing balls obtained in the comparative example were recorded as 5-1, 5-2, and 5-3, as shown in Table 1. Due to equipment and operational errors, there are slight differences in the bending strength, load life, and fracture toughness results of the same material. This is within the normal range. Strict adherence to the equipment usage specifications is necessary to reduce errors during testing. The specific test procedures vary depending on the equipment, and therefore will not be described in detail.

[0067] Table 1

[0068] serial number Sample Name Flexural strength / MPa Load life / 10,000 cycles Fracture toughness / MPa·m^1 / 2 1 Sample 1-1 950 15.2 7.0 2 Sample 1-2 980 15.5 7.1 3 Samples 1-3 1020 15.4 7.2 4 Sample 2-1 1030 16.8 7.5 5 Sample 2-2 1050 17.0 7.6 6 Sample 2-3 1040 16.9 7.5 7 Sample 3-1 910 14.9 6.8 8 Sample 3-2 920 14.7 6.7 9 Sample 3-3 925 14.6 6.6 10 Sample 4-1 890 13.8 6.5 11 Sample 4-2 850 12.9 6.2 12 Sample 4-3 840 12.8 6.3 13 Sample 5-1 780 10.8 5.8 14 Sample 5-2 790 10.7 5.5 15 Sample 5-3 800 9.8 5.6

[0069] As shown in Table 1, when the silicon nitride bearing balls obtained in Examples 1 to 4 and the comparative example were subjected to bending strength tests under the same temperature and humidity conditions, the bending strength values ​​of the ultrafine powder with 5 wt% silicon nitride and the segmented and modified treatment in Examples 1 to 4 were higher, while the bending strength values ​​of the ultrafine powder with 5 wt% silicon nitride and the segmented and modified treatment in the comparative example were lower. The experiment shows that the technical solution provided by this application has a certain improvement in bending strength performance.

[0070] As shown in Table 1, when the silicon nitride bearing balls obtained in Examples 1 to 4 and the comparative example were subjected to load life tests under humid conditions, the ultrafine powder with 5 wt% silicon nitride in Examples 1 to 4 and the segmented and modified materials had higher load life values, while the ultrafine powder with 5 wt% silicon nitride in the comparative example and the segmented and modified materials had lower load life values. The experiment shows that the technical solution provided by this application has a certain improvement in load life performance.

[0071] As shown in Table 1, when the silicon nitride bearing balls obtained in Examples 1 to 4 and the comparative example were subjected to fracture toughness tests under the same temperature and humidity conditions, the fracture toughness values ​​of the ultrafine powder with 5 wt% silicon nitride and the segmented and modified treatment in Examples 1 to 4 were higher, while the fracture toughness values ​​of the ultrafine powder with 5 wt% silicon nitride and the segmented and modified treatment in the comparative example were lower. The experiment shows that the technical solution provided by this application has a certain improvement in fracture toughness.

[0072] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended embodiments and their equivalents.

Claims

1. A method for preparing large-size silicon nitride bearing balls, characterized in that, The preparation method includes the following steps: The raw materials for preparation are determined, including main powder and auxiliary materials. The main powder includes silicon nitride powder mixed with ultrafine powder, and the auxiliary materials include flux, modifier and surface treatment agent. Raw material classification was carried out, and a three-stage particle size classification system of coarse / medium / fine particles was established. Based on the three-stage particle size classification system, the main powder was classified to obtain segmented main powder. The raw materials were mixed, and based on the segmented main powder, the main powders of different grades were modified and dried to obtain coarse segment group, medium segment group and fine segment group. The raw materials are mixed as a whole, with coarse, medium and fine segments as raw material groups. A mixing strategy is set, and the raw material groups are mixed based on the mixing strategy to obtain the embryo. After preparation, the preform is subjected to staged drying treatment, and isothermal sintering and processing are carried out on the preform after staged drying treatment to obtain silicon nitride bearing balls.

2. The method for preparing a large-size silicon nitride bearing ball according to claim 1, characterized in that: The coarse / medium / fine three-segment particle size classification system includes a coarse segment, a medium segment, and a fine segment. The coarse segment is 2–5 μm, the medium segment is 0.5–2 μm, and the fine segment is 0.1–0.5 μm.

3. The method for preparing a large-size silicon nitride bearing ball according to claim 1, characterized in that: The modification strategy includes independently coating the segmented main powder with a covalent coupling agent, surface plasma treatment, and controlling the distribution of doped particles for grain boundary wetting to complete the modification. The zeta potential of the modified powder is in the range of -20 to -60 mV.

4. The method for preparing a large-size silicon nitride bearing ball according to claim 3, characterized in that: The modified segmented main powder was dried independently. After drying, the segments were further classified into coarse, medium and fine segments based on the main powder.

5. The method for preparing a large-size silicon nitride bearing ball according to claim 1, characterized in that: The mixing strategy includes initially mixing the coarse and medium segments to form a primary coarse-medium mixture, and then adding the fine segments to the primary coarse-medium mixture to complete the mixing of the raw material groups and obtain the embryo, thus achieving uniform mixing of the three segments.

6. The method for preparing a large-size silicon nitride bearing ball according to claim 1, characterized in that: In the process of mixing the raw material group, dry graded mixing is adopted, and a wet dispersion step is introduced to treat the non-dispersible components. The remaining amount of dispersant used in wet dispersion is removed before final drying. The target moisture content is preset, and the moisture content of the powder after drying is less than the target moisture content.

7. The method for preparing a large-size silicon nitride bearing ball according to claim 1, characterized in that: Online / offline monitoring is performed during the mixing process. Preset detection targets are set, and the dispersibility, particle size distribution, and wettability of any segment are monitored in real time to obtain monitoring results. When the monitoring results deviate from the detection targets, redispersibility or remodification treatment is performed.

8. The method for preparing a large-size silicon nitride bearing ball according to claim 1, characterized in that: The ratio of the coarse segment group, the middle segment group and the fine segment group is coarse segment group: middle segment group: fine segment group as (4-5): (3-4): (1-2).

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