Silicon nitride ceramic ball with high fracture toughness and preparation method thereof

Abstract

The invention discloses a high-fracture-toughness silicon nitride ceramic ball and a preparation method thereof, and belongs to the field of high-performance structural ceramic materials. The ceramic ball is prepared by taking high-purity alpha-SiN powder as a matrix and adopting a composite sintering aid system composed of an oxygen-free rare earth compound, aluminum nitride and beta-SiN seed crystal through an air pressure sintering process controlled by program pressure. Under the synergistic effect of the auxiliary system and a specific process, 'bimodal distribution 'beta-SiN columnar crystals and high-melting-point and high-toughness intergranular phases are generated in situ in the material, so that the fracture toughness of the material is remarkably improved on the premise of not sacrificing hardness and strength. The fracture toughness of the silicon nitride ceramic ball is up to 8.5-11.0 MPa.m / , the bending strength is 750-1050 MPa, and the silicon nitride ceramic ball is particularly suitable for precision bearings under high-speed and high-load working conditions, such as a wind power generation main shaft, an aerospace engine main shaft and the like.

Description

Technical Field

[0001] This invention relates to special ceramic materials and their preparation methods, and particularly to a silicon nitride ceramic ball with high fracture toughness and its rare earth composite sintering aid system. Background Technology

[0002] Silicon nitride ceramics are widely recognized as an ideal material for manufacturing high-speed precision bearing balls due to their excellent properties such as low density, high hardness, good wear resistance, high temperature resistance, and corrosion resistance. However, the inherent brittleness, sensitivity to defects, and low fracture toughness of ceramic materials are the main reasons for their sudden failure under high impact and high load conditions, which seriously restricts their application in high-end equipment.

[0003] To improve the fracture toughness of silicon nitride ceramics, the industry commonly employs the addition of sintering aids (such as MgO, Y2O3, Al2O3, etc.) to promote liquid-phase sintering densification and uses process control to grow long columnar β-Si3N4 grains, utilizing mechanisms such as crack deflection, bridging, and grain pull-out to achieve toughening. However, traditional oxide aids introduce oxygen impurities, forming a low-melting-point intergranular glassy phase that softens at high temperatures, leading to a decrease in the ceramic's high-temperature strength and creep resistance. Simultaneously, excessive glassy phase can also reduce the material's thermal conductivity.

[0004] Existing technologies, such as CN112250395A, disclose a method for preparing silicon nitride ceramics using a Y2O3-MgO-Si3N4 composite additive, but their fracture toughness is typically below 8.0 MPa·m¹ / ². Other studies have used rare earth oxides such as Yb2O3 and Er2O3, which can increase the melting point of the intergranular phase, but their improvement in fracture toughness is limited, and it is difficult to simultaneously achieve high strength and thermal conductivity.

[0005] Therefore, developing a novel sintering aid system and supporting process to prepare silicon nitride ceramic spheres with ultra-high fracture toughness, high strength and good high-temperature performance has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a silicon nitride ceramic ball with significantly improved fracture toughness.

[0007] Another object of the present invention is to provide a method for preparing the ceramic ball.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: The high fracture toughness silicon nitride ceramic spheres of the present invention are made from high-purity α-Si3N4 powder and an innovative composite sintering aid system. This composite sintering aid system consists of oxygen-free rare earth compounds, aluminum nitride, and β-Si3N4 seed crystals.

[0009] Oxygen-free rare earth compounds: Yttrium fluoride and / or ytterbium fluoride are preferred. They can form high-melting-point rare earth oxynitride intergranular phases during sintering, reducing the formation of low-melting-point glassy phases and improving high-temperature performance. At the same time, the introduction of fluoride ions helps to activate the particle surface in the early stage of sintering, promoting mass transfer and densification.

[0010] The core role of oxygen-free rare earth compounds: Taking YF3 as an example, the Y³⁺ produced during its decomposition during sintering reacts with SiO2 on the surface of Si3N4 to generate a Y-Si-ON liquid phase. This liquid phase has low viscosity and good wettability, greatly promoting material transport and densification in the early stages of sintering. More importantly, this liquid phase eventually crystallizes into high-melting-point, high-toughness intergranular phases such as Y2Si3O3N4 and Y5(SiO4)3N, rather than an amorphous glassy phase. The introduction of fluoride ions (F⁻) preferentially reacts with oxygen on the powder surface, playing a role in "cleaning" the particle surface and reducing the diffusion activation energy.

[0011] Aluminum nitride: As a mineralizer and solid solution forming component, it can dissolve in the Si3N4 lattice to form the SiAlON phase, which regulates the growth kinetics of grains and helps to form finer and more uniform columnar crystals.

[0012] β-Si3N4 seed crystals: As nucleation centers, they guide the β-Si3N4 grains to grow heterogeneously, effectively breaking the grain size distribution and forming an ideal "bimodal structure" (i.e., the coexistence of fine equiaxed crystals and coarse columnar crystals). The coarse columnar crystals are the main carriers of crack deflection and bridging, and are key to high fracture toughness.

[0013] The ceramic spheres are prepared using a programmed pressure-controlled gas pressure sintering process. In the first stage of sintering (1450-1550 °C), a relatively low pressure (0.5-1.0 MPa) is applied. During this stage, particle rearrangement and the initiation of the α→β phase transformation mainly occur. The low pressure facilitates the expulsion of pores and the formation of a uniform microstructure. In the second stage of sintering (1750-1850 °C), a high pressure (2-8 MPa) is applied. This high pressure effectively suppresses the high-temperature decomposition of Si3N4, promotes the full growth and densification of grains, and thus optimizes the morphology and distribution of columnar crystals.

[0014] Optionally, hot isostatic pressing after gas pressure sintering can further eliminate residual closed pores, bring the material closer to its theoretical density, and promote the crystallization of intergranular phases, thereby comprehensively improving mechanical properties.

[0015] The beneficial effects of this invention are: 1. Ultra-high fracture toughness: The bimodal grain structure formed by “seed guidance + composition regulation” can efficiently induce toughening mechanisms such as crack deflection, bridging and pull-out, so that the fracture toughness reaches the industry-leading level of 8.5-11.0 MPa·m¹ / ².

[0016] 2. Excellent comprehensive performance: While achieving high toughness, it maintains high strength (flexural strength 750-1050MPa) and high hardness (HRA 92-94).

[0017] 3. Excellent high-temperature performance and thermal conductivity: The oxygen-free additive system reduces the content of intergranular glass phase and oxygen impurities, and increases the melting point of intergranular phase, so that the ceramic balls can maintain excellent mechanical properties at high temperatures, while having high thermal conductivity (≥70 W / m·K).

[0018] 4. Stable process, suitable for industrialization: The sintering process adopted can be realized in a conventional gas pressure sintering furnace without special equipment. It has a wide process window, good repeatability, and is suitable for large-scale production. Detailed Implementation

[0019] The present invention will be further illustrated by the following embodiments, but the scope of protection of the present invention is not limited thereto. Example

[0020] 1. Raw material ratio (mass fraction): α-Si3N4 powder (purity 99.9%, d50=0.8μm): 90%; Composite sintering aid: 8.5% (yttrium fluoride: 5.0%, aluminum nitride: 2.0%, β-Si3N4 seed crystals (aspect ratio ~8): 1.5%); Molding aid (polyvinyl butyral): 1.5% 2. Preparation process: (1) Place the above raw materials, anhydrous ethanol, and Si3N4 grinding balls in a nylon ball milling jar and mill for 8 hours.

[0021] (2) The slurry is spray-dried to obtain granulated powder.

[0022] (3) Fill the granulated powder into a spherical mold and cold isostatically press it at 200 MPa to obtain a green sphere.

[0023] (4) Place the green pellets into an atmosphere pressure sintering furnace and sinter them under nitrogen according to the following procedure: raise the temperature to 1500 ℃ at 8 ℃ / min, apply a pressure of 0.8 MPa, and hold for 1 hour; then raise the temperature to 1800 ℃ at 5 ℃ / min, increase the pressure to 5 MPa, and hold for 3 hours.

[0024] (5) Cool with the furnace to obtain silicon nitride ceramic balls A. Example

[0025] Based on Example 1, hot isostatic pressing (HIP) was performed after sintering: the sintered ceramic balls were treated at 1820 °C and 180 MPa pressure for 1 hour under an argon atmosphere. Silicon nitride ceramic balls B were obtained.

[0026] Comparative Example The raw materials were basically the same as in Example 1, but the composite sintering aid was replaced with an equal amount of a traditional mixture of Y2O3 (5%) and Al2O3 (3.5%), and the programmed pressure control was removed. Sintering was carried out at 1800 °C and a constant pressure of 2 MPa for 3 hours. Silicon nitride ceramic spheres C were obtained.

[0027] Performance testing The ceramic spheres prepared above were cut and polished to make standard test specimens, and their mechanical properties were tested. The results are shown in the table below: sample Fracture toughness (MPa·m¹ / ²) Flexural strength (MPa) Vickers hardness (GPa) Relative density (%) Ceramic ball A 9.5 890 15.2 99.5 Ceramic ball B 10.8 980 15.5 99.9 Ceramic ball C 7.2 810 14.8 98.8 As shown in the table above, ceramic spheres A and B prepared using the composite sintering aid system and programmed pressure sintering process provided by this invention exhibit significantly higher fracture toughness and flexural strength than comparative example C, which uses traditional aids and processes. In particular, ceramic sphere B, after hot isostatic pressing treatment, achieves optimal performance, fully demonstrating the advanced nature and effectiveness of the technical solution of this invention.

[0028] The silicon nitride ceramic balls of this invention can be widely used in: High-end equipment manufacturing: such as high-speed CNC machine tool spindles, high-power wind turbine generator gearboxes and spindle bearings.

[0029] Aerospace: such as aircraft engine spindles and UAV turbocharger bearings.

[0030] Military and defense applications: such as bearings in the transmission systems of tanks and armored vehicles, or as components of high-performance bulletproof ceramics.

[0031] Other applications: corrosion-resistant bearings in the chemical industry, magnetic levitation bearings in medical devices, etc.

[0032] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A silicon nitride ceramic sphere with high fracture toughness, characterized in that, Its raw material formula, by weight percentage, includes: a. Matrix material: 85-92% α-Si3N4 powder with a purity ≥ 99.5% and an average particle size of 0.5-1.0 μm; b. Composite sintering aid: 5-12%, which is composed of oxygen-free rare earth compounds, aluminum nitride and β-Si3N4 seed crystals; c. Molding aids: Balance.

2. The high fracture toughness silicon nitride ceramic ball according to claim 1, characterized in that, In the composite sintering aid, the mass ratio of oxygen-free rare earth compound, aluminum nitride and β-Si3N4 seed crystal is (3-6):(1-3):(0.5-2).

3. The high fracture toughness silicon nitride ceramic ball according to claim 2, characterized in that, The oxygen-free rare earth compound is yttrium fluoride and / or ytterbium fluoride.

4. The high fracture toughness silicon nitride ceramic ball according to claim 1, characterized in that, The β-Si3N4 seed crystal is a rod-shaped single crystal with an aspect ratio greater than 5, prepared by a self-propagating high-temperature synthesis method, and its addition amount is 0.5%-2% of the total raw material mass.

5. A method for preparing silicon nitride ceramic spheres with high fracture toughness as described in any one of claims 1-4, characterized in that, Includes the following steps: a. Mixing: α-Si3N4 powder, composite sintering aid and molding aid are ball-milled in anhydrous ethanol medium for 4-12 hours to obtain a uniform slurry; b. Drying and granulation: The slurry is spray-dried to obtain spherical granules with good flowability; c. Molding: The granulated powder is placed in a spherical mold and cold isostatically pressed at a pressure of 150-250 MPa to obtain a green sphere; d. Gas pressure sintering: Place the green pellets in a pressure sintering furnace and sinter under a flowing nitrogen atmosphere according to the following procedure: First stage: Increase the temperature to 1450-1550 ℃ at 5-10 ℃ / min and apply a nitrogen pressure of 0.5-1.0 MPa, and hold at the temperature and pressure for 1-2 hours; Second stage: Continue to increase the temperature to 1750-1850 ℃ at 3-5 ℃ / min and increase the nitrogen pressure to 2-8 MPa, and hold at the temperature and pressure for 2-4 hours; e. Cooling: Cool to room temperature with the furnace to obtain dense silicon nitride ceramic spheres.

6. The method for making high fracture toughness silicon nitride ceramic spheres according to claim 5, characterized in that, After sintering in step d, hot isostatic pressing is performed under the following conditions: 1750-1850 ℃ in an argon atmosphere, 150-200 MPa, and holding for 0.5-1.5 hours.

7. The application of a high fracture toughness silicon nitride ceramic ball as described in any one of claims 1-4 in the manufacture of high-speed precision machine tool spindle bearings, high-power wind turbine generator spindle bearings, aerospace vehicle bearings, or high-performance bulletproof armor components.