High-performance carbon nanotube reinforced silicon nitride composite ceramic ball and preparation method thereof

Abstract

The invention discloses a high-performance carbon nanotube reinforced silicon nitride composite ceramic ball and a preparation method thereof. The method comprises the following steps: firstly, carrying out surface functionalization on carbon nanotubes through acidification, and then realizing highly uniform dispersion of the carbon nanotubes in SiN powder in combination with high-energy ultrasound and planetary ball milling under the protection of inert gas; then carrying out spray granulation, mould pressing and cold isostatic pressing to obtain a high-density biscuit; carrying out degreasing, air pressure sintering and hot isostatic pressing post-treatment to obtain an almost fully-dense composite material; and finally, the high-precision ceramic ball is obtained through precise grinding. According to the preparation method disclosed by the invention, through a combined sintering process of chemical-physical-mechanical synergistic dispersion and air pressure sintering-hot isostatic pressing, the problems that the carbon nanotubes are easy to agglomerate and the interface is combined and controlled are effectively solved, and the prepared composite ceramic ball has high toughness (greater than or equal to 7.5 MPa.m / ), high reliability (the Weibull modulus is greater than or equal to 15) and long fatigue life; the method is suitable for the high-end equipment manufacturing field.

Description

Technical Field

[0001] Silicon nitride (Si3N4) composite ceramic balls and their preparation method. These ceramic balls possess high toughness, high hardness, high wear resistance, and long fatigue life, and can be widely used in high-end equipment manufacturing fields such as high-speed precision bearings, spindles, aerospace, and new energy vehicles. Background Technology

[0002] Silicon nitride (Si3N4) ceramics, due to their low density, high hardness, corrosion resistance, high temperature resistance, and self-lubricating properties, have become an ideal material for manufacturing high-performance rolling elements, replacing traditional bearing steel. However, intrinsic brittleness is a fatal weakness of ceramic materials. The unavoidable microscopic defects (such as pores and impurities) within traditional silicon nitride ceramics can easily become crack initiations under cyclic and impact loads, leading to catastrophic brittle fracture of components under low stress, making it difficult to meet the reliability requirements of extreme working conditions.

[0003] To overcome the brittleness of silicon nitride ceramics, researchers have attempted to introduce various toughening phases. Carbon nanotubes (CNTs), with their extremely high elastic modulus (approximately 1 TPa) and tensile strength (approximately 50-200 GPa), are ideal toughening materials for ceramics. Theoretical studies have shown that introducing CNTs into a silicon nitride matrix can effectively dissipate crack propagation energy through mechanisms such as bridging, pull-out, and crack deflection, significantly improving the fracture toughness and thermal shock resistance of the composite material.

[0004] However, fully translating the superior properties of carbon nanotubes into performance enhancements in macroscopic composite materials faces two major technical bottlenecks: Dispersion Challenges: Carbon nanotubes possess a huge specific surface area and strong van der Waals forces, making them prone to agglomeration in ceramic powders, forming stress concentration points that become defects in the material. This is particularly problematic when fabricating spherical components with extremely high isotropic requirements; uneven dispersion leads to uneven internal stress distribution, significantly reducing rolling contact fatigue life.

[0005] Interface control challenges: The interfacial bonding strength between carbon nanotubes and the ceramic matrix is ​​crucial. If the bonding is too weak, load transfer is ineffective, and the carbon nanotubes function as pores; if the bonding is too strong, the pull-out effect of the carbon nanotubes is suppressed, weakening the main toughening mechanism. Existing technologies (such as CN112125557A) typically employ simple mechanical ball milling for mixing, resulting in limited dispersion. Furthermore, prolonged ball milling can introduce impurities or cause mechanical damage to the carbon nanotube structure, reducing its reinforcing efficiency. In addition, achieving high densification of the composite material while preventing carbon nanotube degradation at high temperatures and controlling its interfacial reaction during sintering is another key challenge. Conventional pressureless sintering or single gas-pressure sintering struggles to achieve a suitable interfacial bonding strength while maintaining high density.

[0006] Therefore, developing a method to prepare silicon nitride composite ceramic spheres that can achieve uniform dispersion of carbon nanotubes, precise control of interfacial bonding, and ultimately obtain high density, high toughness, and high reliability is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a carbon nanotube-reinforced silicon nitride composite ceramic ball with uniform carbon nanotube dispersion, controllable interfacial bonding, excellent comprehensive mechanical properties, and in particular, significantly improved fracture toughness and rolling contact fatigue life, as well as its preparation method.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing high-performance carbon nanotube-reinforced silicon nitride composite ceramic spheres, characterized by comprising the following steps: a) Raw material preparation: Provide high-purity, high-α-phase silicon nitride (Si3N4) powder with an average particle size of 0.5~1.0 μm; provide surface-functionalized multi-walled carbon nanotubes with a diameter of 10~30 nm and a length of 10~50 μm; provide sintering aids, including yttrium oxide (Y2O3) and alumina (Al2O3); provide binders, plasticizers and solvents; b) Carbon nanotube pre-dispersion: The surface-functionalized carbon nanotubes are dispersed in part of the solvent at a ratio of 0.5~3.0 wt% of the total mass of ceramic powder, and then subjected to high-energy ultrasonic treatment at a power of 500~1000W for 1~2 hours to form a stable carbon nanotube dispersion. c) Preparation of composite powder: The silicon nitride powder, sintering aid and carbon nanotube dispersion are mixed and planetary ball milled for 12 to 24 hours at a ball-to-powder ratio of 3:1 to 5:1 under inert gas protection. Then the mixture is dried and sieved to obtain CNTs / Si3N4 composite powder with uniform composition. d) Molding: Add a binder to the composite powder, and obtain spherical granulated powder by spray granulation. Then, it is subjected to molding and cold isostatic pressing in sequence. The pressure of cold isostatic pressing is 200~300 MPa to form a high-density, highly uniform green spherical blank. e) Sintering: The green spherical blank is heated to 500-600°C in air at a heating rate of 0.5-1°C / min and held for 1-2 hours for degreasing treatment. Then, under a nitrogen or argon protective atmosphere, it is heated to 1750-1850°C at a heating rate of 5-8°C / min and held for 1-3 hours for gas pressure sintering. f) Post-treatment: The sintered ceramic balls are subjected to hot isostatic pressing under an argon atmosphere at 1700~1800℃ and 150~200 MPa for 0.5~1 hour. g) Finishing: After hot isostatic pressing, the ceramic balls are roughed, fined and ultra-fine ground with diamond grinding paste to make the surface roughness Ra ≤ 0.02 μm and the dimensional accuracy reach G3 or above.

[0009] Preferably, in step a), the surface functionalization treatment of the carbon nanotubes is achieved by ultrasonic refluxing at 60-80°C for 2-4 hours in a mixed acid of concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 1:2 to 1:3, so that the surface is rich in carboxyl or hydroxyl functional groups.

[0010] Preferably, in step a), the adhesive is polyvinyl butyral (PVB), the plasticizer is dibutyl phthalate (DBP), and the solvent is a mixture of anhydrous ethanol and xylene.

[0011] Preferably, in step c), the rotational speed of the planetary ball mill is 300~450 rpm.

[0012] Secondly, the present invention provides a high-performance carbon nanotube-reinforced silicon nitride composite ceramic sphere prepared by the above method, characterized in that its relative density is ≥99%, Vickers hardness is ≥15.5 GPa, fracture toughness is ≥7.5 MPa·m¹ / ², and Weibull modulus is not less than 15.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention specifically addresses the unique technical challenges of spherical components: It recognizes that for ceramic spheres requiring extremely high isotropy, the uniform dispersion of carbon nanotubes is crucial for ensuring consistent mechanical properties and long fatigue life. The adopted three-step synergistic dispersion strategy—"chemical functionalization (introducing functional groups) → high-energy ultrasound (de-agglomeration) → planetary ball milling (mechanical dispersion and recombination)"—is a highly targeted and effective solution that surpasses simple mechanical mixing in existing technologies, fundamentally solving the agglomeration problem of carbon nanotubes in ceramic powders.

[0014] Precise and controllable interfacial bonding was achieved: functional groups were introduced onto the surface of carbon nanotubes through acidification treatment, which not only improved dispersibility but, more importantly, significantly optimized their chemical compatibility and wettability with the Si3N4 matrix. This enabled the formation of a strong and tough interfacial bond during subsequent sintering. This interface can effectively transfer loads and allow carbon nanotubes to pull out and bridge during crack propagation, fully stimulating their toughening potential, thereby achieving a significant improvement in fracture toughness of over 20%.

[0015] Near-perfect densification is achieved through a combined sintering process: This invention innovatively employs a combined sintering process of "gas pressure sintering + hot isostatic pressing". Gas pressure sintering promotes densification through liquid-phase sintering under high temperature and gas pressure, and effectively inhibits the decomposition of carbon nanotubes at high temperatures; the subsequent hot isostatic pressing treatment applies isotropic pressure to the material under high temperature and high pressure, which can effectively close the closed pores remaining inside the material after gas pressure sintering, and almost completely eliminate microscopic defects. This combined process ensures that the final product achieves near-complete densification (relative density ≥ 99%), and significantly improves the material's reliability (Weibull modulus ≥ 15) and fatigue life.

[0016] The final product obtained is a high-performance and reliable composite ceramic ball. It not only performs well in individual indicators such as hardness and toughness, but more importantly, its comprehensive performance and reliability have achieved a qualitative leap, which can meet the stringent requirements of high-end fields such as high-speed precision bearings for material performance consistency and long service life. Attached Figure Description

[0017] Figure 1 This is a process flow diagram of the preparation method of the present invention.

[0018] Figure 2 This is a photograph of the carbon nanotube-reinforced silicon nitride composite ceramic spherical preform prepared in Example 1.

[0019] Figure 3 The XRD patterns are those of Example 1 and Comparative Example 1. Detailed Implementation

[0020] The present invention will be further described in detail below through specific embodiments, but the scope of protection of the present invention is not limited thereto. Example

[0021] Raw material ratio: 92 wt% α-Si3N4 powder (average particle size 0.8 μm, purity >99.9%), 5 wt% Y2O3 (purity >99.99%), 3 wt% Al2O3 (purity >99.99%), and 1.0 wt% multi-walled carbon nanotubes (MWCNTs) of the total powder mass.

[0022] Functionalization and predisposition of carbon nanotubes: MWCNTs were placed in a mixed acid solution with a ratio of V(concentrated HNO3):V(concentrated H2SO4) = 1:3 and refluxed by sonication at 70°C for 3 hours. After cooling, the MWCNTs were repeatedly washed with deionized water until neutral and dried. The functionalized MWCNTs were then dispersed in anhydrous ethanol and subjected to high-energy ultrasonic dispersion at 800W for 1.5 hours to form a stable dispersion.

[0023] Preparation of composite powder: Si3N4, Y2O3, and Al2O3 powders were mixed with the above CNTs dispersion and placed in a planetary ball mill. Silicon nitride balls were used as grinding balls at a ball-to-powder ratio of 4:1. The mixture was ball-milled at 400 rpm for 24 hours under argon protection. The slurry was then vacuum-dried at 80°C and passed through a 200-mesh sieve to obtain CNTs / Si3N4 composite powder.

[0024] Molding: Add 5 wt% PVB binder (dissolved in ethanol / xylene mixed solvent) solution to the composite powder, mix evenly, and then spray granulate to obtain spherical granulated powder. Pre-form the granulated powder by molding at 150 MPa pressure, and then perform cold isostatic pressing at 250 MPa pressure for 5 minutes to obtain the green spherical preform.

[0025] Sintering: The green spherical blanks are placed in an air atmosphere furnace and heated to 550°C at a rate of 1°C / min and held for 2 hours for degreasing. The degreased blanks are then placed in a pneumatic sintering furnace, first evacuated to 10⁻³ Pa, heated to 1150°C at a rate of 8°C / min, then filled with high-purity nitrogen to a pressure of 0.5 MPa, and then heated to 1800°C at a rate of 7°C / min and held for 2 hours.

[0026] Post-treatment: The sintered ceramic balls were subjected to hot isostatic pressing at 1750℃ and 180 MPa for 1 hour under an argon atmosphere.

[0027] Finishing: After double-end grinding, a special ceramic ball grinding machine is used with diamond grinding paste for rough grinding, fine grinding and ultra-fine grinding, finally obtaining Φ9.525 mm (3 / 8 inch) ceramic balls with G5 grade precision and surface roughness Ra=0.015 μm.

[0028] Performance testing: The ceramic ball has a relative density of 99.7%, a Vickers hardness of 16.8 GPa, a fracture toughness of 7.9 MPa·m¹ / ², and a Weibull modulus of 22. Example

[0029] The amount of carbon nanotubes added was adjusted to 2.0 wt% of the total powder mass, and the remaining steps were exactly the same as in Example 1.

[0030] Performance testing: The fracture toughness of the obtained ceramic spheres was further improved to 8.5 MPa·m¹ / ², the Vickers hardness was 16.2 GPa, the relative density was 99.3%, and the Weibull modulus was 18. Example

[0031] The amount of carbon nanotubes added was adjusted to 0.5 wt% of the total powder mass, the planetary ball milling time was shortened to 15 hours, the cold isostatic pressing pressure was adjusted to 200 MPa, the gas pressure sintering temperature was adjusted to 1780℃, and the remaining steps were similar to those in Example 1.

[0032] Performance testing: The obtained ceramic spheres have a relative density of 99.8%, a Vickers hardness of 17.0 GPa, a fracture toughness of 7.5 MPa·m¹ / ², and a Weibull modulus of 24.

[0033] Comparative Example 1 Except for the absence of carbon nanotubes, the raw materials, preparation steps and process parameters are exactly the same as in Example 1.

[0034] Performance testing: The obtained pure silicon nitride ceramic spheres have a relative density of 99.8%, a Vickers hardness of 17.2 GPa, a fracture toughness of 6.5 MPa·m¹ / ², and a Weibull modulus of 25.

[0035] Comparative Example 2 The same raw material ratio as in Example 1 was used, but the acidification and functionalization step of carbon nanotubes was omitted. The original carbon nanotubes were directly dispersed by high-energy ultrasound and ball milling. The remaining steps were the same as in Example 1.

[0036] Performance testing: The relative density of the obtained ceramic spheres is 98.5%, and the fracture toughness is 6.8 MPa·m¹ / ².

[0037] Comparative Example 3 The same raw material ratio and dispersion steps as in Example 1 were used, but the sintering process only involved gas pressure sintering, omitting the subsequent hot isostatic pressing treatment.

[0038] Performance testing: The relative density of the obtained ceramic spheres is 98.9%, and the fracture toughness is 7.2 MPa·m¹ / ².

[0039] Comparative analysis of experimental data: As can be seen from the comparison between Example 1 and Comparative Example 1, the fracture toughness of the CNTs-reinforced ceramic spheres prepared by the method of the present invention is significantly improved by about 21.5%.

[0040] As can be seen from the comparison between Example 1 and Comparative Example 2, the surface functionalization treatment of carbon nanotubes is crucial for achieving uniform dispersion and obtaining strong interfacial bonding.

[0041] As can be seen from the comparison between Example 1 and Comparative Example 3, the post-treatment of hot isostatic pressing plays an irreplaceable role in achieving near-full density (>99%), eliminating internal defects, and maximizing material properties.

Claims

1. A method for preparing high-performance carbon nanotube-reinforced silicon nitride composite ceramic spheres, characterized in that, Includes the following steps: a) Raw material preparation: Provide silicon nitride powder, surface-functionalized carbon nanotubes, sintering aids, binders and solvents; b) Carbon nanotube pre-dispersion: Surface-functionalized carbon nanotubes are dispersed in a solvent and subjected to high-energy ultrasonic treatment to form a stable carbon nanotube dispersion. c) Preparation of composite powder: The silicon nitride powder, sintering aid and carbon nanotube dispersion are mixed, ball-milled, dried and sieved to obtain uniform CNTs / Si3N4 composite powder; d) Molding: A binder is added to the composite powder, and granulation is performed to obtain spherical granulated powder. Then, the powder is subjected to molding and cold isostatic pressing to form a green spherical blank. e) Sintering: The green spherical blank is first degreased and then sintered under pressure in a protective atmosphere; f) Post-treatment: Hot isostatic pressing is applied to the sintered ceramic balls; g) Finishing: Grinding and polishing the hot isostatically pressed ceramic balls to obtain high-precision finished ceramic balls.

2. The method according to claim 1, characterized in that, In step a), the carbon nanotubes are multi-walled carbon nanotubes, which undergo surface functionalization treatment by ultrasonic reflux of concentrated acid mixture, and the surface contains carboxyl or hydroxyl functional groups; the amount of carbon nanotubes added is 0.5~3.0 wt% of the total mass of silicon nitride powder and sintering aid.

3. The method according to claim 2, characterized in that, The concentrated acid mixture is a mixture of concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 1:2 to 1:

3. The ultrasonic reflux treatment is performed at a temperature of 60 to 80°C for 2 to 4 hours.

4. The method according to claim 1, characterized in that, In step b), the power of the high-energy ultrasonic treatment is 500~1000W, and the treatment time is 1~2 hours.

5. The method according to claim 1, characterized in that, In step c), the ball milling is a planetary ball milling performed under inert gas protection, with a milling time of 12 to 24 hours, a ball-to-material ratio of 3:1 to 5:1, and a rotation speed of 300 to 450 rpm.

6. The method according to claim 1, characterized in that, In step d), the granulation is spray granulation; the molding pressure is 100~200 MPa; and the cold isostatic pressing pressure is 200~300 MPa.

7. The method according to claim 1, characterized in that, In step e), the degreasing process is carried out in an air atmosphere, with the temperature increased to 500-600°C at a heating rate of 0.5-1°C / min and held for 1-2 hours; the gas pressure sintering is carried out in a nitrogen or argon protective atmosphere, with the temperature increased to 1750-1850°C at a heating rate of 5-8°C / min and held for 1-3 hours.

8. The method according to claim 1, characterized in that, In step f), the hot isostatic pressing treatment is carried out under an argon atmosphere at a temperature of 1700~1800℃ and a pressure of 150~200 MPa for 0.5~1 hour.

9. The method according to claim 1, characterized in that, In step g), the finishing process includes roughing, fine grinding, and ultra-fine grinding using diamond polishing paste to make the surface roughness Ra of the ceramic ball ≤ 0.02 μm and the dimensional accuracy reach G3 level or above.

10. A high-performance carbon nanotube-reinforced silicon nitride composite ceramic sphere prepared by the method according to any one of claims 1-9, characterized in that, Its relative density is ≥99%, Vickers hardness is ≥15.5 GPa, fracture toughness is ≥7.5 MPa·m¹ / ², and its Weibull modulus is not less than 15. It is used in high-speed precision bearings, spindles, aerospace or new energy vehicle fields.

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