Silicon nitride ceramic bearing ball production process

Abstract

The invention relates to the technical field of bearing ball manufacturing, in particular to a silicon nitride ceramic bearing ball production process which comprises the following steps: pretreating raw materials, mixing silicon nitride powder with a sintering aid, adding absolute ethyl alcohol, and ball-milling into slurry; carrying out spray granulation, and controlling the temperature to obtain granulated particles with specific particle sizes; performing cold isostatic pressing, pressurizing and maintaining pressure to obtain a biscuit; degreasing treatment is conducted, and heating is conducted in the air atmosphere to remove impurities; sintering, heating and preserving heat in a nitrogen atmosphere, and then cooling; roughly grinding, and controlling the roundness by a diamond grinding wheel; fine-mesh grinding wheels are used for improving the precision; ultra-fine grinding is performed, and the finish degree is increased through grinding paste; cleaning and removing residual grinding materials; and detecting, and packaging after qualification. The bearing ball produced by the process is superhard, wear-resistant, high-temperature-resistant and electrically insulated, adapts to special working conditions such as high temperature and high speed, and meets the requirements of multiple fields.

Description

Technical Field

[0001] This invention relates to the field of bearing ball manufacturing technology, specifically a silicon nitride ceramic bearing ball production process. Background Technology

[0002] Silicon nitride ceramic bearing balls, as a typical representative of advanced ceramic products, are in high demand in high-end fields such as new energy vehicles, aerospace, and wind power due to their excellent properties such as ultra-hardness, wear resistance, high temperature resistance, and electrical insulation. Especially in the electric drive system of new energy vehicles, traditional steel bearing balls have become a key bottleneck restricting the upgrading of motors to higher speeds and higher voltages due to defects such as energy consumption caused by high density, insufficient dimensional stability caused by large coefficient of thermal expansion, and inability to resist electro-corrosion.

[0003] The existing silicon nitride ceramic bearing ball manufacturing process has three major technical challenges: First, the raw material mixing uniformity is insufficient, and functional components such as graphene and nano-additives easily agglomerate, leading to fluctuations in the material's mechanical properties and making it difficult to consistently exceed 9 MPa·m in fracture toughness. 1 / 2 Secondly, uncontrolled grain growth during sintering reduces the material's impact resistance. Thirdly, the contradiction between precision machining efficiency and accuracy is prominent; surface microcracks are easily generated during rough grinding and fine grinding, affecting the fatigue life of bearing balls at high speeds.

[0004] During the forming stage, conventional cold isostatic pressing has poor mold compatibility, resulting in uneven density distribution of the green blank and easy deformation during subsequent sintering. The debinding process uses traditional hot air heating, which has a slow heating rate and is prone to generating thermal stress, causing the green blank cracking rate to exceed 10%. Insufficient atmosphere control during the sintering process, with fluctuations in nitrogen purity and flow rate, can lead to a decrease in material density and high porosity, failing to meet the stringent reliability requirements of new energy vehicle bearings.

[0005] The technical shortcomings in the processing stage are particularly evident: traditional grinding wheels lack an auxiliary energy field, resulting in low processing efficiency and a surface roughness that can only reach Ra≤0.1μm; the cleaning process relies on chemical solvents, which easily leave impurities, affecting the self-lubricating performance of the bearing balls. In addition, existing processes do not fully cover all aspects of product testing, especially lacking dynamic performance evaluation, making it difficult to guarantee its long-term stability under extreme operating conditions.

[0006] With the inclusion of high-performance silicon nitride ceramic materials in the national key new materials application demonstration catalog, upgrading their production processes has become an urgent need for the industry. Addressing the shortcomings of existing processes in material uniformity, sintering controllability, and processing precision, developing a production process for high-density, fine-grained, and high-precision silicon nitride ceramic bearing balls is key to breaking through the bottleneck of domestic production of high-end bearings. Summary of the Invention

[0007] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a manufacturing process for silicon nitride ceramic bearing balls.

[0008] (II) Technical Solution A process for manufacturing silicon nitride ceramic bearing balls includes the following steps: S1, Preparation of nanocomposite powder: Silicon nitride powder, graphene quantum dots, and sintering aid were mixed at a ratio of 94:0.5:5.5; anhydrous ethanol was added, with a solid-liquid ratio of 1:2.5; the mixture was ball-milled using a planetary ball mill to obtain a composite slurry; the milling media were zirconium oxide beads, with a ball-to-material ratio of 6:1; the sintering aid was a Y2O3-ZrO2 composite agent, with a mass ratio of 2:1. S2, Gradient spray granulation: The slurry is sprayed and granulated through a dual-fluid nozzle; the inlet air temperature is controlled in stages, namely 200℃, 230℃, and 210℃; the outlet air temperature is 85-95℃, and the turntable speed is 3000-3500rpm; gradient structure granulated particles are obtained. S3, warm isostatic pressing: Granulated particles are loaded into a polytetrafluoroethylene mold, and the inner wall of the mold is coated with boron nitride release agent; the mold is held under pressure at 120-150℃ and 250-300MPa for 4-6 minutes to form a green blank. S4, Microwave-assisted degreasing: The raw blank is placed in a microwave degreasing oven; the temperature is raised to 650-700℃ in an air atmosphere, the microwave power is 600-800W, and the temperature is maintained for 2.5-3.5h; S5, sintering: The degreased green blank is directly heated to 1700~1800℃ in a nitrogen atmosphere and held for 3-5 hours; S6, Ultrasonic Assisted Rough Grinding: Uses a 150-200 mesh diamond grinding wheel, combined with ultrasonic vibration for rough grinding; feed speed 6-9 mm / min, cooling system uses oil mist lubrication, cutting fluid to compressed air ratio 1:20; S7, Magnetorheological Grinding: Grinding is performed using a mixture of 800-1200 mesh diamond abrasive and magnetorheological fluid; magnetic field strength 0.3-0.5T, workpiece rotation speed 60-70rpm; S8, Chemical Mechanical Ultra-precision Grinding: Uses colloidal silica grinding slurry, particle size 50-80nm; grinding disc is made of tin alloy, pressure 0.08-0.12MPa, rotation speed 120-160rpm; S9, Plasma Cleaning: Plasma cleaning is carried out in an Ar / O2 mixed gas with a volume ratio of 9:1; plasma power is 200-300W, and cleaning time is 10-15 minutes. S10, full-item testing: laser diameter gauge measurement deviation ≤0.001mm; micro-CT detection internal porosity ≤0.1%; mechanical properties meet Vickers hardness ≥15GPa and fracture toughness >9MPa·m. 1 / 2 .

[0009] Preferably, the method also includes surface modification of graphene quantum dots in S1: dispersing graphene quantum dots in an ethanol solution of 3-5 wt% silane coupling agent KH570, refluxing at 70°C for 2 h; centrifuging and drying, and then mixing with silicon nitride powder.

[0010] Preferably, the process also includes pretreatment of the S3 medium-temperature isostatic pressing mold: the inner wall of the mold is sandblasted with diamond abrasive particles of 50-80μm, and after roughening, a 0.5-1μm thick boron nitride coating is sprayed on it; plasma spraying is used with a power of 15kW.

[0011] Preferably, the sintering atmosphere control in S5 is also included: the nitrogen flow rate is controlled throughout the entire process from the start of heating to the end of holding. The initial nitrogen flow rate is set to 5L / min during the heating stage. When the temperature reaches 1750℃, the nitrogen flow rate is adjusted to 10L / min and maintained at this flow rate until the holding process is completed.

[0012] Preferably, it also includes the grinding wheel dressing in S6 for ultrasonic-assisted rough grinding: every 500 workpieces are ground, a diamond dressing pen with a grit size of 300 mesh is used to dress the grinding wheel; the dressing feed rate is 0.01 mm / time.

[0013] Preferably, the parameters for magnetorheological fine grinding in S7 are also optimized: the magnetorheological fluid is composed of carbonyl iron powder, base oil and diamond abrasive; it is stirred at 3000 rpm for 10 min before use, and 5% new magnetorheological fluid is added every 30 min during the fine grinding process.

[0014] Preferably, it also includes a slurry circulation system for chemical mechanical ultrafine grinding in S8: the slurry is circulated through a 0.2μm filter, the pH value is checked every hour and maintained at 9.0-10.0; the surface scratches caused by slurry failure are avoided by adding ammonia water to adjust the pH.

[0015] Preferably, it also includes low-temperature annealing after S9: the bearing balls after plasma cleaning are held at 600-700℃ for 2 hours in a nitrogen atmosphere; the cooling rate is 2℃ / min.

[0016] Preferably, it also includes hot isostatic pressing treatment between S5 and S6: the sintered green body is held at a nitrogen pressure of 180-220 MPa and a temperature of 1650-1700℃ for 3-4 hours.

[0017] Preferably, it also includes dynamic performance testing in S10: a 100-hour fatigue test at 20,000 rpm; after the test, there is no peeling on the surface, and the roundness change is ≤0.05μm, which meets the requirements for high-speed use.

[0018] (iii) Beneficial technical effects Compared with existing technologies, the beneficial effects of this invention are: 1. In the nanocomposite powder preparation stage, graphene quantum dots were introduced and surface modified to solve the problem of functional phase agglomeration. Combined with the efficient mixing of planetary ball milling, the material composition was more uniformly distributed, laying the foundation for the subsequent sintering to form a fine-grained structure. The sintering process, through precise control of the heating rate and nitrogen atmosphere, effectively suppressed abnormal grain growth. The resulting fine-grained structure significantly improved the fracture toughness and bending strength of the bearing balls, enabling them to better adapt to the dynamic load requirements under high speeds.

[0019] 2. Warm isostatic pressing combined with mold pretreatment technology improves the uniformity of the green blank density and reduces subsequent sintering deformation; microwave-assisted debinding utilizes its characteristic of heating the entire material simultaneously to accelerate the debinding rate while reducing thermal stress, significantly reducing the risk of green blank cracking. Hot isostatic pressing further improves material density and reduces internal porosity, resulting in superior reliability of the bearing balls under special conditions such as high temperature and strong corrosion.

[0020] 3. Ultrasonic-assisted rough grinding improves grinding efficiency and reduces processing time through energy field assistance; the combined process of magnetorheological fine grinding and chemical mechanical ultra-precision grinding achieves higher surface finish while avoiding micro-cracks caused by traditional grinding, thus improving product fatigue life. Plasma cleaning replaces chemical solvent cleaning, thoroughly removing residual impurities from the surface, ensuring the self-lubricating performance of bearing balls, and meeting the low friction and low noise requirements of new energy vehicle motors.

[0021] 4. Comprehensive testing covers dynamic performance evaluation, ensuring products meet the requirements of high-speed and high-voltage operating conditions, and are compatible with 800V platforms and SiC inverter systems in new energy vehicles, fundamentally solving the problem of electro-corrosion. Optimization of process parameters improves production stability and reduces scrap rates. Combined with the advantages of material properties, this provides technical support for the large-scale application of silicon nitride ceramic bearing balls, contributing to the domestic substitution of high-end bearings. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the production process of silicon nitride ceramic bearing balls disclosed in this invention; Figure 2 This is a histogram comparing the density and compaction of the embodiments and comparative examples; Figure 3 This is a graph comparing the Vickers hardness and fracture toughness of the examples and comparative examples; Figure 4 This is a radar comparison chart created by standardizing the dimensions of the performance comparison data of the examples and comparative examples. Detailed Implementation

[0023] according to Figures 1 to 4 The specific embodiments of the present invention are as follows: A process for manufacturing silicon nitride ceramic bearing balls includes the following steps: S1, Preparation of nanocomposite powder: Silicon nitride powder, graphene quantum dots, and sintering aid were mixed at a ratio of 94:0.5:5.5; anhydrous ethanol was added, with a solid-liquid ratio of 1:2.5; the mixture was ball-milled using a planetary ball mill to obtain a composite slurry; the milling media were zirconium oxide beads, with a ball-to-material ratio of 6:1; the sintering aid was a Y2O3-ZrO2 composite agent, with a mass ratio of 2:1. S2, Gradient spray granulation: The slurry is sprayed and granulated through a dual-fluid nozzle; the inlet air temperature is controlled in stages, namely 200℃, 230℃, and 210℃; the outlet air temperature is 85-95℃, and the turntable speed is 3000-3500rpm; gradient structure granulated particles are obtained. S3, warm isostatic pressing: Granulated particles are loaded into a polytetrafluoroethylene mold, and the inner wall of the mold is coated with boron nitride release agent; the mold is held under pressure at 120-150℃ and 250-300MPa for 4-6 minutes to form a green blank. S4, Microwave-assisted degreasing: The raw blank is placed in a microwave degreasing oven; the temperature is raised to 650-700℃ in an air atmosphere, the microwave power is 600-800W, and the temperature is maintained for 2.5-3.5h; S5, sintering: The degreased green blank is directly heated to 1700~1800℃ in a nitrogen atmosphere and held for 3-5 hours; S6, Ultrasonic Assisted Rough Grinding: Uses a 150-200 mesh diamond grinding wheel, combined with ultrasonic vibration for rough grinding; feed speed 6-9 mm / min, cooling system uses oil mist lubrication, cutting fluid to compressed air ratio 1:20; S7, Magnetorheological Grinding: Grinding is performed using a mixture of 800-1200 mesh diamond abrasive and magnetorheological fluid; magnetic field strength 0.3-0.5T, workpiece rotation speed 60-70rpm; S8, Chemical Mechanical Ultra-precision Grinding: Uses colloidal silica grinding slurry, particle size 50-80nm; grinding disc is made of tin alloy, pressure 0.08-0.12MPa, rotation speed 120-160rpm; S9, Plasma Cleaning: Plasma cleaning is carried out in an Ar / O2 mixed gas with a volume ratio of 9:1; plasma power is 200-300W, and cleaning time is 10-15 minutes. S10, full-item testing: laser diameter gauge measurement deviation ≤0.001mm; micro-CT detection internal porosity ≤0.1%; mechanical properties meet Vickers hardness ≥15GPa and fracture toughness >9MPa·m. 1 / 2 .

[0024] The present invention also includes surface modification of graphene quantum dots in S1: dispersing graphene quantum dots in an ethanol solution of 3-5 wt% silane coupling agent KH570, refluxing at 70°C for 2 h; centrifuging and drying, and then mixing with silicon nitride powder.

[0025] This invention also includes the pretreatment of the S3 medium-temperature isostatic pressing mold: the inner wall of the mold is sandblasted with diamond abrasive particles of 50-80μm, and after roughening, a 0.5-1μm thick boron nitride coating is sprayed; plasma spraying is used with a power of 15kW.

[0026] The present invention also includes atmosphere control for sintering in S5: nitrogen flow rate is controlled throughout the entire process from the start of heating to the end of holding. The initial nitrogen flow rate is set to 5 L / min during the heating stage. When the temperature rises to 1750°C, the nitrogen flow rate is adjusted to 10 L / min and maintained at this flow rate until the holding process is completed.

[0027] The present invention also includes the grinding wheel dressing in S6 for ultrasonic-assisted rough grinding: every 500 workpieces are ground, a diamond dressing pen with a grit size of 300 mesh is used to dress the grinding wheel; the dressing feed rate is 0.01 mm / time.

[0028] This invention also includes parameter optimization for magnetorheological fine grinding in S7: the magnetorheological fluid is composed of carbonyl iron powder, base oil, and diamond abrasive; it is stirred at 3000 rpm for 10 min before use, and 5% new magnetorheological fluid is added every 30 min during the fine grinding process.

[0029] The present invention also includes a slurry circulation system for chemical mechanical ultrafine grinding in S8: the slurry is circulated through a 0.2μm filter, the pH value is detected once per hour and maintained at 9.0-10.0; the surface scratches caused by slurry failure are avoided by adding ammonia water to adjust the pH value.

[0030] This invention also includes low-temperature annealing after S9: the bearing balls after plasma cleaning are kept at 600-700℃ for 2 hours in a nitrogen atmosphere; the cooling rate is 2℃ / min.

[0031] This invention also includes hot isostatic pressing treatment between S5 and S6: the sintered green body is kept at a nitrogen pressure of 180-220 MPa and a temperature of 1650-1700℃ for 3-4 hours.

[0032] The present invention also includes dynamic performance testing in S10: a 100-hour fatigue test is performed at a speed of 20,000 rpm; after the test, there is no peeling on the surface and the roundness change is ≤0.05μm, which meets the requirements for high-speed use.

[0033] Example 1 Raw material preparation: silicon nitride powder (purity 99.6%, particle size 0.8μm), Y2O3-ZrO2 composite sintering aid (mass ratio 2:1), graphene quantum dots (particle size 4nm), anhydrous ethanol, and zirconium oxide beads (diameter 5mm). Weigh the silicon nitride powder, graphene quantum dots, and sintering aid according to a mass ratio of 94:0.5:5.5.

[0034] Surface modification of graphene quantum dots: The weighed graphene quantum dots were dispersed in an ethanol solution of 4wt% silane coupling agent KH570 and refluxed at 70℃ for 2h. After the reaction was completed, the precipitate was collected and dried to obtain the surface-modified graphene quantum dots.

[0035] Preparation of nanocomposite powder: Surface-modified graphene quantum dots, silicon nitride powder, and sintering aids were added to a planetary ball mill, along with anhydrous ethanol. The solid-liquid ratio was 1:2.5, the ball-to-particle ratio was 6:1, and the milling speed was 450 rpm for 30 hours. Sampling and testing showed that the slurry D50 was 0.9 μm, and the particles were uniformly dispersed without agglomeration.

[0036] Gradient spray granulation: The slurry enters the granulator through a dual-fluid nozzle. The inlet air temperature is set sequentially to 200℃, 230℃, and 210℃, the outlet air temperature is 90℃, and the rotary table speed is 3200 rpm. The collected granules have a particle size of 90-110 μm and a loose packing density of 1.4 g / cm³. 3 .

[0037] Warm isostatic pressing: The inner wall of the PTFE mold is sandblasted with 60μm diamond abrasive, followed by plasma spraying of a 0.8μm thick boron nitride coating at a power of 15kW. Granulated particles are then loaded and pressed at 130℃ and 280MPa for 5 minutes, with a pressurization rate of 50MPa / min. The green blank density is 2.3g / cm³. 3 Strength 16MPa.

[0038] Microwave-assisted degreasing: The raw blank was placed in a microwave degreasing oven and heated to 680°C in an air atmosphere at a rate of 8°C / min, with a microwave power of 700W, and held at that temperature for 3 hours. The weight loss after degreasing was 0.25%, and there were no cracks.

[0039] Sintering: The defatted green blank was placed in a nitrogen furnace with nitrogen purity of 99.999% and pressure of 0.3 MPa; the temperature was directly increased to 1780℃ at a rate of 15℃ / min and held for 4 hours, with the nitrogen flow rate maintained at 10 L / min; the cooling rate was 5℃ / min, and the density after sintering was 3.24 g / cm³. 3 The grain size is 1.5 μm.

[0040] Hot isostatic pressing: The sintered billet was held at 1680℃ under nitrogen pressure of 200MPa for 3.5h, resulting in a density of 99.8%.

[0041] Ultrasonic-assisted rough grinding: 180-grit diamond grinding wheel, 20kHz ultrasonic wave, 400W power, feed rate 7mm / min, oil mist lubrication, cutting fluid to air ratio 1:20. After grinding 500 workpieces, a 300-grit diamond dressing pen is used for finishing, with a feed rate of 0.01mm / cycle. Roundness after rough grinding is 7μm.

[0042] Magnetorheological finishing: The magnetorheological fluid consists of 50wt% carbonyl iron powder, 45wt% base oil, and 5wt% 1000-mesh diamond abrasive. Stir at 3000 rpm for 10 minutes before use. Magnetic field strength: 0.4T; workpiece rotation speed: 65 rpm; replenish with 5% fresh fluid every 30 minutes. Roundness after finishing: 1.8μm; Ra=0.07μm.

[0043] Chemical mechanical ultrafine grinding: colloidal silica slurry, 60nm particle size, tin alloy grinding disc, pressure 0.1MPa, rotation speed 140rpm. The slurry was circulated through a 0.2μm filter, and the pH was measured hourly and maintained at 9.5, adjusted with ammonia. After ultrafine grinding, the roundness was 0.25μm, and Ra=0.008μm.

[0044] Plasma cleaning: Ar / O2 mixed gas, volume ratio 9:1, power 250W, cleaning for 12 minutes, contact angle 8°.

[0045] Low-temperature annealing: Hold at 650℃ for 2 hours in a nitrogen atmosphere, with a cooling rate of 2℃ / min, and a residual stress of 45MPa.

[0046] Full-item testing: Laser diameter gauge deviation 0.0008mm, microscopic CT porosity 0.08%. Vickers hardness 15.2 GPa, fracture toughness 9.3 MPa·m. 1 / 2 After a 100-hour fatigue test at 20,000 rpm, no surface spalling was observed, and the roundness change was 0.04 μm.

[0047] Example 2 Raw material preparation: silicon nitride powder (purity 99.7%, particle size 1.0μm), Y2O3-ZrO2 composite sintering aid (mass ratio 2:1), graphene quantum dots (particle size 3nm). Weigh the silicon nitride powder, graphene quantum dots, and sintering aid according to a mass ratio of 94:0.5:5.5. Also prepare anhydrous ethanol and zirconium oxide beads (5mm in diameter).

[0048] Surface modification of graphene quantum dots: The weighed graphene quantum dots were dispersed in an ethanol solution of 3.5 wt% silane coupling agent KH570 and refluxed at 70 °C for 2 h. After the reaction was completed, the precipitate was collected by centrifugation and dried to obtain the surface-modified graphene quantum dots.

[0049] Preparation of nanocomposite powder: Surface-modified graphene quantum dots, silicon nitride powder, and sintering aids were added to a planetary ball mill. Anhydrous ethanol was also added. The solid-liquid ratio was 1:2.5, the ball-to-particle ratio was 6:1, and the milling speed was 480 rpm for 31 hours. Sampling and testing showed that the slurry D50 was 0.95 μm, and the particles were uniformly dispersed without agglomeration.

[0050] Gradient spray granulation: The slurry enters the granulator through a dual-fluid nozzle. The inlet air temperature is set sequentially to 200℃, 230℃, and 210℃, the outlet air temperature is 92℃, and the rotary table speed is 3300 rpm. The collected granules have a particle size of 95-115μm and a loose packing density of 1.45g / cm³. 3 .

[0051] Warm isostatic pressing: The inner wall of the PTFE mold was sandblasted with 60μm diamond abrasive, followed by plasma spraying of a 0.9μm thick boron nitride coating at a power of 15kW. Granulated pellets were then loaded and pressed at 140℃ and 290MPa for 5.5 min, with a pressurization rate of 50MPa / min. The green blank density was 2.35g / cm³. 3 Strength 17MPa.

[0052] Microwave-assisted degreasing: The raw blank was placed in a microwave degreasing oven and heated to 690°C in an air atmosphere at a rate of 8°C / min, with a microwave power of 750W, and held at that temperature for 3.2 hours. The weight loss after degreasing was 0.23%, and there were no cracks.

[0053] Sintering: The defatted green blank was placed in a nitrogen furnace with nitrogen purity of 99.999% and pressure of 0.35 MPa; the temperature was directly increased to 1790℃ at a rate of 15℃ / min and held for 4.5 hours with a nitrogen flow rate of 10 L / min; the cooling rate was 5℃ / min, and the density after sintering was 3.25 g / cm³. 3 The grain size is 1.6 μm.

[0054] Hot isostatic pressing: The sintered billet was held at 1690℃ under nitrogen pressure of 210MPa for 3.8h, resulting in a density of 99.85%.

[0055] Ultrasonic-assisted rough grinding: 180-grit diamond grinding wheel, 20kHz ultrasonic wave, 400W power, feed rate 8mm / min, oil mist lubrication, cutting fluid to air ratio 1:20. After grinding 500 workpieces, a 300-grit diamond dressing pen is used for finishing, with a feed rate of 0.01mm / cycle. Roundness after rough grinding is 6.5μm.

[0056] Magnetorheological finishing: The magnetorheological fluid consists of 50wt% carbonyl iron powder, 45wt% base oil, and 5wt% 1000-mesh diamond abrasive. Stir at 3000 rpm for 10 minutes before use. Magnetic field strength: 0.45T; workpiece rotation speed: 65 rpm; replenish with 5% fresh fluid every 30 minutes. After finishing, the roundness is 1.7μm, and Ra=0.06μm.

[0057] Chemical mechanical ultrafine grinding: colloidal silica slurry, 60nm particle size, tin alloy grinding disc, pressure 0.1MPa, rotation speed 140rpm. The slurry was circulated through a 0.2μm filter, and the pH was measured hourly and maintained at 9.7, adjusted with ammonia. After ultrafine grinding, the roundness was 0.23μm, and Ra=0.007μm.

[0058] Plasma cleaning: Ar / O2 mixed gas, volume ratio 9:1, power 250W, cleaning for 12 minutes, contact angle 7°.

[0059] Low-temperature annealing: Hold at 650℃ for 2 hours in a nitrogen atmosphere, with a cooling rate of 2℃ / min, resulting in a residual stress of 42MPa.

[0060] Full-item testing: Laser diameter gauge deviation 0.0007mm; Microscopic CT porosity 0.07%; Vickers hardness 15.5 GPa; Fracture toughness 9.5 MPa·m. 1 / 2 After a 100-hour fatigue test at 20,000 rpm, no surface spalling was observed, and the roundness change was 0.03 μm.

[0061] Example 3 Raw material preparation: silicon nitride powder (purity 99.8%, particle size 1.2μm), Y2O3-ZrO2 composite sintering aid (mass ratio 2:1), graphene quantum dots (particle size 5nm). Weigh the silicon nitride powder, graphene quantum dots, and sintering aid according to a mass ratio of 94:0.5:5.5. Also prepare anhydrous ethanol and zirconium oxide beads (5mm in diameter).

[0062] Surface modification of graphene quantum dots: The weighed graphene quantum dots were dispersed in an ethanol solution of 5wt% silane coupling agent KH570 and refluxed at 70℃ for 2h. After the reaction was completed, the precipitate was collected and dried to obtain the surface-modified graphene quantum dots.

[0063] Preparation of nanocomposite powder: Surface-modified graphene quantum dots, silicon nitride powder, and sintering aids were added to a planetary ball mill, along with anhydrous ethanol. The solid-liquid ratio was 1:2.5, the ball-to-particle ratio was 6:1, and the milling speed was 500 rpm for 32 hours. Sampling and testing showed that the slurry D50 was 1.0 μm, and the particles were uniformly dispersed without agglomeration.

[0064] Gradient spray granulation: The slurry enters the granulator through a dual-fluid nozzle. The inlet air temperature is set sequentially to 200℃, 230℃, and 210℃, the outlet air temperature is 95℃, and the rotary table speed is 3500 rpm. The collected granules have a particle size of 100-120μm and a loose packing density of 1.5g / cm³. 3 .

[0065] Warm isostatic pressing: The inner wall of the PTFE mold is sandblasted with 60μm diamond abrasive, followed by plasma spraying of a 1μm thick boron nitride coating at a power of 15kW. Granulated particles are then loaded, and the pressure is maintained at 150℃ and 300MPa for 6 minutes, with a pressurization rate of 50MPa / min. The green body density is 2.4g / cm³. 3 Strength 18MPa.

[0066] Microwave-assisted degreasing: The raw blank was placed in a microwave degreasing oven and heated to 700°C in an air atmosphere at a rate of 8°C / min, with a microwave power of 800W, and held at that temperature for 3.5 hours. The weight loss after degreasing was 0.2%, and there were no cracks.

[0067] Sintering: The defatted green blank was placed in a nitrogen furnace with nitrogen purity of 99.999% and pressure of 0.4 MPa; the temperature was directly increased to 1800℃ at a rate of 15℃ / min and held for 5 hours, with a nitrogen flow rate maintained at 10 L / min; the cooling rate was 5℃ / min, and the density after sintering was 3.26 g / cm³. 3 The grain size is 1.8 μm.

[0068] Hot isostatic pressing: The sintered billet was held at 1700℃ under nitrogen pressure of 220MPa for 4 hours, resulting in a density of 99.9%.

[0069] Ultrasonic-assisted rough grinding: 180-grit diamond grinding wheel, 20kHz ultrasonic wave, 400W power, feed rate 9mm / min, oil mist lubrication, cutting fluid to air ratio 1:20. After grinding 500 workpieces, a 300-grit diamond dressing pen is used for finishing, with a feed rate of 0.01mm / cycle. Roundness after rough grinding is 6μm.

[0070] Magnetorheological finishing: The magnetorheological fluid consists of 50wt% carbonyl iron powder, 45wt% base oil, and 5wt% 1000-mesh diamond abrasive. Stir at 3000 rpm for 10 minutes before use. Magnetic field strength: 0.5T; workpiece rotation speed: 65 rpm; replenish with 5% fresh fluid every 30 minutes. After finishing, the roundness is 1.5μm, and Ra=0.05μm.

[0071] Chemical mechanical ultrafine grinding: colloidal silica slurry, 60nm particle size, tin alloy grinding disc, pressure 0.1MPa, rotation speed 140rpm. The slurry was circulated through a 0.2μm filter, and the pH was measured hourly and maintained at 9.8, adjusted with ammonia. After ultrafine grinding, the roundness was 0.2μm, and Ra=0.006μm.

[0072] Plasma cleaning: Ar / O2 mixed gas, volume ratio 9:1, power 250W, cleaning for 12 minutes, contact angle 6°.

[0073] Low-temperature annealing: Hold at 650℃ for 2 hours in a nitrogen atmosphere, with a cooling rate of 2℃ / min, and a residual stress of 40MPa.

[0074] Full-item testing: Laser diameter gauge deviation 0.0006mm, microscopic CT porosity 0.06%. Vickers hardness 15.8 GPa, fracture toughness 9.8 MPa·m. 1 / 2 After a 100-hour fatigue test at 20,000 rpm, no surface spalling was observed, and the roundness change was 0.02 μm.

[0075] Comparative Example Raw material preparation: silicon nitride powder (purity 99.5%, particle size 1.5μm), single Y2O3 sintering aid, anhydrous ethanol, and zirconium oxide beads (5mm in diameter). Weigh the silicon nitride powder and sintering aid at a mass ratio of 95:5.

[0076] Powder preparation: The raw materials were added to a ball mill, anhydrous ethanol was added, the solid-liquid ratio was 1:2, the ball-to-material ratio was 5:1, the speed was 300 rpm, and the milling was carried out for 24 hours. The slurry D50 was 1.8 μm.

[0077] Spray granulation: inlet air temperature 200℃, outlet air temperature 80℃, rotary table speed 2500rpm, granulated particle size 150-200μm, loose packing density 1.1g / cm³ 3 .

[0078] Cold isostatic pressing: Rubber mold, pressure held at 200MPa for 3 minutes, green blank density 1.9g / cm³ 3 Strength 10MPa.

[0079] Traditional degreasing: The temperature is increased to 600℃ in air at a rate of 5℃ / min and held for 4 hours. The weight loss rate is 0.8%, and cracks appear in 10% of the green blanks.

[0080] Sintering: Nitrogen purity 99.99%, pressure 0.1 MPa, heating to 1700℃ at 10℃ / min and holding for 3 hours, cooling rate 10℃ / min, density after sintering 3.1 g / cm³ 3 The grain size is 4μm.

[0081] Conventional rough grinding: 120-grit diamond grinding wheel, no ultrasonic assistance, feed speed 5mm / min, water cooling, roundness after rough grinding 15μm.

[0082] Standard fine grinding: 800-grit grinding wheel, non-magnetic rheological aid, roundness after fine grinding is 5μm, Ra=0.2μm.

[0083] Cleaning: Ultrasonic cleaning with deionized water, residual abrasive is obvious.

[0084] Test results: Diameter deviation 0.003 mm, porosity 0.8%, Vickers hardness 13 GPa, fracture toughness 7 MPa·m 1 / 2 After a 100-hour fatigue test at 20,000 rpm, surface spalling occurred, and the roundness changed by 0.5 μm.

[0085] The performance comparison between the examples and the comparative examples is shown in the table below: Table 1 project Example 1 Example 2 Example 3 Comparative Example <![CDATA[Density (g / cm 3 ).]]> 3.24 3.25 3.26 3.1 Density (%) 99.8 99.85 99.9 95 Vickers hardness (GPa) 15.2 15.5 15.8 13 <![CDATA[Fracture toughness (MPa·m 1 / 2 )]]> 9.3 9.5 9.8 7 Final roundness (μm) 0.25 0.23 0.2 5 Surface roughness Ra (μm) 0.008 0.007 0.006 0.2 The fatigue performance of the examples and comparative examples is compared in the table below: Table 2 Test conditions Example 1 Performance Example 2 Performance Example 3 Performance Comparison of proportions 20000rpm / 100h No surface peeling, roundness variation of 0.04μm. No surface peeling, roundness variation of 0.03μm No surface peeling, roundness variation of 0.02μm. Surface peeling, roundness change of 0.5μm Residual stress (MPa) 45 42 40 80 Porosity (%) 0.08 0.07 0.06 0.8 As can be seen from the two tables above, the embodiments, through innovative processes such as nanocomposite powder design, gradient granulation, and warm isostatic pressing, combined with sintering and hot isostatic pressing treatment, significantly improved the material density and mechanical properties, solving the problems of low density and poor toughness in the comparative embodiment. Precision machining technologies such as ultrasonic-assisted grinding and magnetorheological grinding greatly improved the dimensional accuracy and surface quality of the bearing balls. Combined with plasma cleaning and low-temperature annealing, residual stress was effectively reduced, resulting in excellent performance in high-speed fatigue testing and fully meeting the application requirements of high-end fields such as new energy vehicles.

[0086] 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 claims and their equivalents.

Claims

1. A manufacturing process for silicon nitride ceramic bearing balls, characterized in that, Includes the following steps: S1, Preparation of nanocomposite powder: Silicon nitride powder, graphene quantum dots, and sintering aid were mixed at a ratio of 94:0.5:5.5; anhydrous ethanol was added, with a solid-liquid ratio of 1:2.5; the mixture was ball-milled using a planetary ball mill to obtain a composite slurry; the milling media were zirconium oxide beads; the sintering aid was a Y2O3-ZrO2 composite agent with a mass ratio of 2:

1. S2, Gradient spray granulation: The slurry is sprayed and granulated through a dual-fluid nozzle; the inlet air temperature is controlled in stages, namely 200℃, 230℃, and 210℃; the outlet air temperature is 85-95℃, and the turntable speed is 3000-3500rpm; gradient structure granulated particles are obtained. S3, warm isostatic pressing: Granulated particles are loaded into a polytetrafluoroethylene mold, and the inner wall of the mold is coated with boron nitride release agent; the mold is held under pressure at 120-150℃ and 250-300MPa for 4-6 minutes to form a green blank. S4, Microwave-assisted degreasing: The raw blank is placed in a microwave degreasing oven; the temperature is raised to 650-700℃ in an air atmosphere, the microwave power is 600-800W, and the temperature is maintained for 2.5-3.5h; S5, sintering: The degreased green blank is directly heated to 1700~1800℃ in a nitrogen atmosphere and held for 3-5 hours; S6, Ultrasonic Assisted Rough Grinding: Uses a 150-200 mesh diamond grinding wheel, combined with ultrasonic vibration for rough grinding; feed speed 6-9 mm / min, cooling system uses oil mist lubrication, cutting fluid to compressed air ratio 1:20; S7, Magnetorheological Grinding: Grinding is performed using a mixture of 800-1200 mesh diamond abrasive and magnetorheological fluid; magnetic field strength 0.3-0.5T, workpiece rotation speed 60-70rpm; S8, Chemical Mechanical Ultra-precision Grinding: Uses colloidal silica grinding slurry, particle size 50-80nm; grinding disc is made of tin alloy, pressure 0.08-0.12MPa, rotation speed 120-160rpm; S9, Plasma Cleaning: Plasma cleaning is carried out in an Ar / O2 mixed gas with a volume ratio of 9:1; plasma power is 200-300W, and cleaning time is 10-15 minutes. S10, full-item testing: laser diameter gauge measurement deviation ≤0.001mm; micro-CT detection internal porosity ≤0.1%; mechanical properties meet Vickers hardness ≥15GPa and fracture toughness >9MPa·m. 1 / 2 .

2. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes surface modification of graphene quantum dots in S1: dispersing graphene quantum dots in an ethanol solution of 3-5wt% silane coupling agent KH570, refluxing at 70℃ for 2h; centrifuging and drying, and then mixing with silicon nitride powder.

3. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes the pretreatment of the S3 medium-temperature isostatic pressing mold: the inner wall of the mold is sandblasted with diamond abrasive particles of 50-80μm, and after roughening, a 0.5-1μm thick boron nitride coating is sprayed; plasma spraying is used with a power of 15kW.

4. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes the atmosphere control for sintering in S5: the nitrogen flow rate is controlled throughout the entire process from the start of heating to the end of holding. The initial nitrogen flow rate is set to 5L / min during the heating stage, and the nitrogen flow rate is adjusted to 10L / min when the temperature reaches 1750℃.

5. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes the grinding wheel dressing in S6 ultrasonic-assisted rough grinding: every 500 workpieces are ground, a diamond dressing pen with a grit size of 300 is used to dress the grinding wheel; the dressing feed rate is 0.01 mm / time.

6. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes parameter optimization for magnetorheological grinding in S7: the magnetorheological fluid is composed of carbonyl iron powder, base oil and diamond abrasive; it is stirred at 3000 rpm for 10 min before use, and 5% new magnetorheological fluid is added every 30 min during the grinding process.

7. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes the S8's chemical mechanical ultrafine grinding slurry circulation system: the grinding slurry is circulated through a 0.2μm filter, and the pH value is checked every hour to maintain it at 9.0-10.0; by adding ammonia water to adjust, surface scratches caused by grinding slurry failure are avoided.

8. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes low-temperature annealing after S9: the bearing balls after plasma cleaning are held at 600-700℃ for 2 hours in a nitrogen atmosphere; the cooling rate is 2℃ / min.

9. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes hot isostatic pressing between S5 and S6: after sintering, the green body is held at a nitrogen pressure of 180-220MPa and a temperature of 1650-1700℃ for 3-4 hours.

10. The silicon nitride ceramic bearing ball manufacturing process according to claim 1, characterized in that, It also includes dynamic performance testing in S10: a 100-hour fatigue test at 20,000 rpm; after the test, there was no peeling on the surface, and the roundness change was ≤0.05μm, meeting the requirements for high-speed use.

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