Gradient silicon nitride bearing ball and preparation method thereof

Abstract

The invention discloses a gradient silicon nitride bearing ball and a preparation method thereof, relates to the technical field of ceramic product manufacturing, and solves the problems that in the prior art, the internal toughness is insufficient, residual stress is generated, the performance is influenced, and the service life is limited. According to the method, the gradient silicon nitride bearing ball is prepared by treating the green body through twice cold isostatic pressing, so that the defects of microcracks, air holes and the like on a micro-level of the green body caused in a press molding process are avoided, and the green body can have enough mechanical strength in a degreasing stage through the first low-pressure cold isostatic pressing; the density and activity of the green body can be improved through second-time high-pressure cold isostatic pressing forming, no residual air holes and no additive residues exist in the high-density green body during sintering, the inner layer of the bearing ball has high toughness, and the surface of the bearing ball has high hardness, high strength and a self-lubricating function.

Description

A gradient silicon nitride bearing ball and its preparation method Technical Field

[0001] This invention relates to the field of ceramic product manufacturing technology, specifically to a gradient silicon nitride bearing ball and its preparation method. Background Technology

[0002] Silicon nitride (Si3N4) is an important structural ceramic material. It has high hardness, inherent lubricity, and wear resistance. As an atomic crystal, it is resistant to oxidation at high temperatures. Silicon nitride-based ceramic bearing balls are mature rolling element materials used in high-end fields such as aerospace, automotive, and precision machinery. However, unlike steel, ceramic materials cannot naturally form a surface gradient structure through carburizing or quenching. A controllable gradient structure must be achieved through artificial design during the forming and sintering stages.

[0003] The current methods for preparing silicon nitride-based ceramic bearing balls generally have a structure similar to that described in patent application number "CN202510294982.8", which discloses a method for preparing silicon nitride bearing balls with internal stress. High α-phase Si3N4 powder is the main material for the silicon nitride bearing ball, Sb powder is the main material providing internal stress, AlN is used to further improve the wear resistance of the material, Y2O3 is used to improve the density of the silicon nitride bearing ball, and pores are visible under a 0.5 μm electron microscope after adding Y2O3. MgO is used to promote the transformation of the α-phase Si3N4 powder to the β-phase, and Li2O and La2O3 form a high-temperature liquid phase to promote the densification of the bearing ball. Alternatively, as disclosed in patent application number "CN202510173677.3", a high-density silicon nitride bearing ball and its preparation method are described, in which high α-phase Si3N4 powder is the main material of silicon nitride bearing ball, AlN is used to further improve the wear resistance of the material, Y2O3 is used to improve the density of silicon nitride bearing ball, and pores are visible under 0.5μm electron microscopy after adding Y2O3, MgO is used to promote the transformation of α-phase Si3N4 powder to β-phase, and Li2O and La2O3 form a high-temperature liquid phase to promote the densification of bearing ball.

[0004] Currently, the publicly available silicon nitride ceramic bearing ball preparation technologies at home and abroad mainly focus on material composition modification and sintering densification control. For example, patent CN201510992624.0 discloses a "preparation method of Si3N4 gradient ceramic ball material with hard surface and tough core". The method uses Si3N4 powder as the matrix material, with β-Si3N4 seed crystals and Al2O3-Re2O3 as sintering aids in the core, and AlN and Al2O3-Re2O3 as sintering aids in the surface layer. Si3N4, β-Si3N4 seed crystals and Al2O3-Re2O3, as well as Si3N4, AlN and Al2O3-Re2O3, are mixed and dried according to their respective mass fraction ratios to obtain a mixed powder with a core of Si3N4-β-Si3N4 seed crystals-Al2O3-Re2O3 and a surface layer of Si3N4-AlN-Al2O3-Re2O3. The mixed powder is then used to obtain a gradient ball blank through a molding device, and the gradient ball blank is sintered under air pressure to obtain a high-performance Si3N4 gradient ceramic ball material with a hard surface and tough core. Although a certain degree of gradient functionalization has been achieved, there are still defects such as a single gradient control method and high sintering stress between different layers.

[0005] In summary, most existing silicon nitride bearing balls use a pure Si3N4 material system. Although they have high hardness and wear resistance, they lack internal toughness and are prone to crack propagation and brittle failure under high load, impact, or thermal shock conditions, thus limiting their service life. Furthermore, the high-pressure one-time molding process results in uneven powder distribution, especially in spray-granulated powders where agglomeration or particle size differences exist. This leads to density gradients and residual stress during compaction, affecting the performance of subsequent sintered samples. At the same time, high-pressure molding results in a high density of the green body, making it difficult for internal organic additives to be fully expelled during degreasing. This can easily lead to defects such as gas retention, pore expansion, and even cracking, affecting subsequent sintering densification and strength.

[0006] Therefore, this invention proposes a gradient silicon nitride bearing ball and its preparation method to solve the above-mentioned problems. Summary of the Invention

[0007] The purpose of this invention is to provide a gradient silicon nitride bearing ball and its preparation method, which solves the problem of insufficient internal toughness in the prior art, which generates residual stress that affects performance and leads to limited service life.

[0008] The technical solution adopted by this invention to solve its technical problem is:

[0009] A method for preparing a gradient silicon nitride bearing ball, the bearing ball comprising a surface layer, a middle layer, and an inner layer; the steps include:

[0010] S1. Prepare inner layer powder, middle layer powder and surface layer powder separately using a ball mill;

[0011] S2. The billet is subjected to gradient forming and then dried;

[0012] S3. The billet is subjected to low-pressure cold isostatic pressing at 50-100 MPa for 60-180 seconds.

[0013] S4. Degrease the green body in a N2 atmosphere at 550 ℃;

[0014] S5. The degreased blank is subjected to high-pressure cold isostatic pressing at a pressure of 250-300 MPa for 120-600 seconds.

[0015] S6. The blank is sintered under normal pressure to obtain a gradient silicon nitride bearing ball blank.

[0016] S7. Machining the gradient silicon nitride bearing ball blank to obtain the final product;

[0017] The inner layer composition includes Si3N4 matrix powder, β-Si3N4 seed crystals, and sintering aids;

[0018] The surface layer composition includes Si3N4 matrix powder, β-Sialon powder, nano-reinforcing phase, and solid lubricating phase;

[0019] The intermediate layer is located between the inner layer and the surface layer, and its composition includes a gradient combination of surface layer components and inner layer components, as well as a nanoscale interfacial bonding phase.

[0020] Furthermore, in step S1:

[0021] The surface powder is controlled to have a D50 particle size of 20-80 μm by spray granulation.

[0022] The inner layer powder is controlled to have a D50 particle size of 40-150 μm by spray granulation.

[0023] Further, the gradient forming of the blank in step S2 specifically includes:

[0024] S2.1 The inner layer powder is pre-pressed into spheres in a fully flexible cold isostatic pressing mold;

[0025] S2.2 The intermediate layer powder is uniformly adhered to the outer surface of the sphere by wet coating with slurry;

[0026] S2.3 The surface powder is coated with the intermediate layer in a semi-wet state to form a green body;

[0027] S2.4. Wait for the green body to dry.

[0028] Furthermore, the fully flexible cold isostatic pressing mold includes a polyurethane mold and a low-friction liner. The low-friction liner is detachably connected to the inner surface of the polyurethane mold. The low-friction liner is a polytetrafluoroethylene (PTFE) composite film with a thickness of 0.25-0.6 mm.

[0029] A gradient silicon nitride bearing ball, wherein the surface layer comprises, by weight percentage, 40-65% Si3N4 matrix powder, 15-30% β-Sialon powder, 0.5-3% nano-reinforcing phase, and 3-5% solid lubricating phase; the inner layer comprises, by weight percentage, 80-85% Si3N4 matrix powder, 0-10% β-Si3N4 seed crystals, and 5-15% sintering aid; and the intermediate layer comprises, by weight percentage, a gradient combination of surface powder and inner powder in a 6:4 ratio, and further comprises 0.5-5% nanoscale interfacial bonding phase.

[0030] Furthermore, the inner layer has a thickness of 10.5-12.7 mm; the outer layer has a thickness of 3-5 mm; and the middle layer has a thickness of 1.5-3.0 mm.

[0031] Further, the nano-reinforcing phase is one or more of elemental W, TiN, or Fe2O3. The solid lubricating phase is one or more of BN, graphite / graphene, Ag microphase, or CaF2. The sintering aid is one or more of MgO, Al2O3, or Y2O3. The nanoscale interfacial bonding phase is one or more of TiN, TiC, or nano-SiC.

[0032] In summary, the beneficial effects of this invention compared to the prior art are as follows:

[0033] 1. This invention employs a two-stage cold isostatic pressing process to prepare gradient silicon nitride bearing balls, avoiding micro-cracks and pores in the blank caused by the press forming process. The first low-pressure cold isostatic pressing ensures that the blank has sufficient mechanical strength during the degreasing stage, while the second high-pressure cold isostatic pressing improves the density and activity of the blank. The high-density blank has no residual pores or additive residues during sintering.

[0034] 2. The bearing ball of the present invention achieves a continuous gradient interface by controlling the particle size, additives and molding pressure of each layer of powder. The inner layer has high toughness and low cost; the middle layer is a transition layer that can adjust thermal expansion and stress matching; the surface layer has high hardness, high strength and self-lubricating function.

[0035] 3. This invention adopts a fully flexible cold isostatic pressing mold structure. The inner surface of the polyurethane mold is provided with a replaceable low-friction liner, which can ensure the smooth removal of the sample during the molding and demolding process. The low-friction liner is a PTFE composite film with a thickness of 0.25-0.6 mm, which has excellent pressure resistance and chemical stability and will not fall off or contaminate the sample during high pressure. The liner is fixed to the inside of the mold by application, which can ensure uniform pressure in all directions while effectively reducing demolding resistance, improving the integrity of the blank surface, and extending the service life of the mold. Attached Figure Description

[0036] Figure 1 is a process flow diagram for the preparation of gradient silicon nitride bearing balls according to the present invention;

[0037] Figure 2 is a process flow diagram of atmospheric pressure sintering according to the present invention;

[0038] Figure 3 is a schematic diagram of the fully flexible cold isostatic pressing mold structure of the present invention;

[0039] Figure 4 shows the mechanical property data in the specification of this invention;

[0040] Figure 5 shows the SEM data from the specification of this invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, 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.

[0042] In this application, the terms "upper," "inner," "outer," "middle," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing this application and its embodiments, and are not intended to limit the indicated device, element, or component to having a specific orientation, or to be constructed and operated in a specific orientation.

[0043] This invention provides a method for preparing gradient silicon nitride bearing balls, comprising the following steps:

[0044] First, inner layer powder, middle layer powder and surface layer powder are prepared separately using a ball mill; the surface layer powder is spray granulated to control its D50 particle size to be 20-80 μm; the inner layer powder is spray granulated to control its D50 particle size to be 40-150 μm.

[0045] The green body is subjected to gradient forming. The inner layer powder is pre-pressed into spheres in a fully flexible cold isostatic pressing mold. The middle layer powder is wet-coated and uniformly attached to the outer surface of the spheres by a slurry. The surface layer powder is dry-coated in the semi-wet state of the middle layer to form the green body. The green body is then left to dry.

[0046] The blank is subjected to low-pressure cold isostatic pressing at 50-100 MPa for 60-180 seconds to ensure that the green blank has a certain strength and to prevent peeling or deformation during degreasing.

[0047] The green body is degreased in a nitrogen atmosphere at 550 °C to remove internal organic matter.

[0048] The degreased preform is subjected to high-pressure cold isostatic pressing at a pressure of 250-300 MPa for a holding time of 120-600 seconds to improve product density and interfacial bonding.

[0049] The blank was sintered under normal pressure according to the process shown in Figure 2 to obtain a gradient silicon nitride bearing ball blank.

[0050] The gradient silicon nitride bearing ball blank is machined to obtain the final product;

[0051] The fully flexible cold isostatic pressing mold includes a polyurethane mold and a low-friction liner. The low-friction liner is detachably connected to the inner surface of the polyurethane mold. The low-friction liner is a polytetrafluoroethylene (PTFE) composite film with a thickness of 0.25-0.6 mm.

[0052] In this invention, the gradient silicon nitride bearing ball comprises a surface layer, a middle layer, and an inner layer. The inner layer consists of Si3N4 matrix powder, β-Si3N4 seed crystals, and sintering aids. The inner layer composition, by weight percentage, includes 80-85% Si3N4 matrix powder, 0-10% β-Si3N4 seed crystals, and 5-15% sintering aids. The inner layer thickness is 10.5-12.7 mm. The sintering aids are one or more of MgO, Al2O3, or Y2O3. Consequently, the inner layer exhibits high toughness and low cost.

[0053] The surface layer composition includes Si3N4 matrix powder, β-Sialon powder, nano-reinforcing phase, and solid lubricating phase; by weight percentage, the surface layer composition includes 40-65% Si3N4 matrix powder, 15-30% β-Sialon powder, 0.5-3% nano-reinforcing phase, and 3-5% solid lubricating phase; the surface layer thickness is 3-5 mm; the nano-reinforcing phase is one or more of elemental W, TiN, or Fe2O3; the solid lubricating phase is one or more of BN, graphite / graphene, Ag microphase, or CaF2. Therefore, the surface layer possesses high hardness, high strength, and self-lubricating properties.

[0054] The intermediate layer, located between the inner and outer layers, comprises a gradient combination of surface and inner layer components, as well as a nanoscale interfacial binder phase. By weight percentage, the intermediate layer consists of a gradient combination of surface and inner layer powders in a 6:4 ratio, and 0.5-5% of the nanoscale interfacial binder phase. The intermediate layer thickness is 1.5-3.0 mm. The nanoscale interfacial binder phase is one or more of TiN, TiC, or nano-SiC. Furthermore, by controlling the particle size, additives, and molding pressure, a continuous gradient interface is achieved between each powder layer.

[0055] Example 1:

[0056] (1) The surface powder consisted of 60% Si3N4 matrix powder, 22% β-Sialon powder, 2% TiN nano-reinforcing phase, 4% BN and 1% graphene as solid lubricating phase by weight percentage. The above powder was mixed with dispersant and binder, and ball-milled with 3 mm zirconia grinding balls using deionized water as medium to prepare a slurry with a solid content of 55%. Then, it was spray-granulated to control the D50 particle size of the granulated powder to be 50 μm.

[0057] The inner layer powder consists of 83% Si3N4 matrix powder, 5% β-Si3N4 seed crystals, and 12% sintering aid (composed of Y2O3 and Al2O3 in a 5:1 ratio) by weight percentage. The powder is then mixed with a binder to prepare a slurry with a solid content of 65%. The slurry is ball-milled using 9 mm grinding media and then spray-granulated to control the D50 particle size of the granulated powder to be 100 μm.

[0058] The intermediate layer powder is made by taking the above-prepared surface powder (60 wt%) and inner powder (40 wt%), and adding an additional 2 wt% of nano-TiC as an interface binder phase, and mixing them to prepare a low-viscosity, high-solids slurry for later use.

[0059] (2) The inner layer powder is filled into a special fully flexible cold isostatic pressing mold and pre-pressed into a spherical blank with a diameter of about 29 mm under a pressure of 80 MPa.

[0060] (3) Immerse the inner spherical blank into the prepared intermediate layer slurry, and control it by lifting to make a thin layer of slurry evenly adhere to its surface.

[0061] (4) While the intermediate layer slurry is still in a semi-wet state, the sphere is transferred to the fluidized bed so that the surface powder is evenly bonded and coated on the surface of the sphere to form a complete blank.

[0062] (5) Dry the coated blank slowly in an 80 ℃ oven for 12 hours.

[0063] (6) The dried complete billet is subjected to the first cold isostatic pressing at 80 MPa pressure and held for 120 seconds to obtain a gradient billet with preliminary strength and interlayer bonding force.

[0064] (7) Place the blank after one-time molding into a degreasing furnace, and in a flowing N2 atmosphere, heat it to 550 ℃ at a heating rate of 1 ℃ / min and keep it at that temperature for 2 hours to completely remove the organic additives in the blank.

[0065] (8) The degreased green body is subjected to a second cold isostatic pressing under a high pressure of 280 MPa and held for 300 seconds, which greatly improves the density of the green body and the bonding strength of the interlayer interface.

[0066] (9) The preform after secondary densification is placed in a high-temperature sintering furnace and sintered under normal pressure according to the sintering curve shown in Figure 2 under nitrogen atmosphere protection. The maximum sintering temperature is 1600 ℃.

[0067] (10) After sintering, a dense gradient silicon nitride bearing ball blank is obtained. Finally, through machining methods such as fine grinding and polishing, a gradient silicon nitride bearing ball with a final size of 30 mm and a surface finish of G5 is obtained.

[0068] Example 2:

[0069] (1) The surface powder consists of 55% Si3N4 matrix powder, 25% β-Sialon powder, 1.5% Fe2O3 nano-reinforcing phase, 3.5% CaF2 and 1% Ag micro-phase as solid lubricating phase by weight percentage. After mixing and granulation, the D50 particle size is controlled to be 30 μm.

[0070] The inner layer powder consists of 85% Si3N4 matrix powder, 2% β-Si3N4 seed crystals, and 13% sintering aid (MgO) by weight percentage. After mixing and granulation, the D50 particle size is controlled to be 80 μm.

[0071] The intermediate layer powder was prepared by mixing the surface and inner layer powder components in a 6:4 ratio, and adding 1 wt% nano-SiC as an interfacial binder.

[0072] (2) The inner layer powder is pre-pressed into spheres with a diameter of about 19 mm under a pressure of 70 MPa.

[0073] (3) The intermediate layer slurry is uniformly coated onto the surface of the inner sphere using a spin coating method.

[0074] (4) Apply surface powder to the semi-wet intermediate layer through a precisely controlled powder rolling process.

[0075] (5) Dry the coated blank slowly in an 80 ℃ oven for 12 hours.

[0076] (6) After the billet is dried, it is subjected to the first cold isostatic pressing at 60 MPa and held for 180 seconds to ensure that the layers are fully bonded under low pressure.

[0077] (7) Place the blank after one-time molding into a degreasing furnace and degrease it at 1 ℃ / min under a flowing N2 atmosphere.

[0078] The heating rate is increased to 550 ℃ and held for 2 hours to completely remove organic additives from the green body.

[0079] (8) The secondary cold isostatic pressing pressure is selected as 260 MPa, and the pressure is held for 400 seconds to ensure that smaller spheres have the same densification effect.

[0080] (9) The sintering process is the same as in Example 1, but the holding time is adjusted according to the size.

[0081] (10) Finally, a 20 mm gradient silicon nitride bearing ball is obtained.

[0082] The two embodiments described above achieve the production of gradient silicon nitride bearing balls of different specifications through different powder formulations, and can also adjust key parameters within the publicly available scope according to product requirements.

[0083] The double cold isostatic pressing and single degreasing process not only ensures smooth degreasing but also improves the density and activity of the green body. The high-density green body has no residual pores or additive residues during sintering.

Claims

1. A method for preparing gradient silicon nitride bearing balls, characterized in that, The bearing ball comprises a surface layer, a middle layer, and an inner layer; the steps are as follows: S1, preparing inner layer powder, middle layer powder, and surface layer powder separately using a ball mill; S2, performing gradient molding on the green body and drying it; S3, performing low-pressure cold isostatic pressing on the green body at 50-100 MPa, holding the pressure for 60-180 seconds; S4, degreasing the green body in a N2 atmosphere at 550 °C; S5, pressing the degreased green body at a pressure of 250-300... High-pressure cold isostatic pressing is performed at MPa for 120-600 seconds; S6, the blank is sintered at normal pressure to obtain a gradient silicon nitride bearing ball blank; S7, the gradient silicon nitride bearing ball blank is machined to obtain the final product; the inner layer composition includes Si3N4 matrix powder, β-Si3N4 seed crystals and sintering aids; the surface layer composition includes Si3N4 matrix powder, β-Sialon powder, nano-reinforcing phase and solid lubricating phase; the intermediate layer is located between the inner layer and the surface layer, and its composition includes a gradient combination of the surface layer composition and the inner layer composition, and also includes a nano-scale interfacial bonding phase.

2. The method for preparing a gradient silicon nitride bearing ball according to claim 1, characterized in that, In step S1: the surface powder is spray-granulated to control its D50 particle size to be 20-80 μm; the inner powder is spray-granulated to control its D50 particle size to be 40-150 μm.

3. The method for preparing a gradient silicon nitride bearing ball according to claim 1, characterized in that, The gradient forming of the blank in step S2 specifically includes: S2.1, the inner layer powder is pre-pressed into a sphere in a fully flexible cold isostatic pressing mold; S2.2, the middle layer powder is wet-coated and uniformly attached to the outer surface of the sphere by a slurry; S2.3, the surface layer powder is dry-coated in the semi-wet state of the middle layer to form a blank; S2.4, wait for the blank to dry.

4. A fully flexible cold isostatic pressing mold for use in the method of claim 3, characterized in that, The fully flexible cold isostatic pressing mold includes a polyurethane mold and a low-friction liner. The low-friction liner is detachably connected to the inner surface of the polyurethane mold. The low-friction liner is a polytetrafluoroethylene (PTFE) composite film with a thickness of 0.25-0.6 mm.

5. A gradient silicon nitride bearing ball prepared by the method according to any one of claims 1-3, characterized in that, The surface layer composition, by weight percentage, includes 40-65% Si3N4 matrix powder, 15-30% β-Sialon powder, 0.5-3% nano-reinforcing phase, and 3-5% solid lubricating phase; the inner layer composition, by weight percentage, includes 80-85% Si3N4 matrix powder, 0-10% β-Si3N4 seed crystals, and 5-15% sintering aids; the intermediate layer composition, by weight percentage, includes surface powder and inner layer powder in a gradient combination at a ratio of 6:4, and also includes 0.5-5% nanoscale interfacial bonding phase.

6. The gradient silicon nitride bearing ball according to claim 5, characterized in that, The inner layer has a thickness of 10.5-12.7 mm; the outer layer has a thickness of 3-5 mm; and the middle layer has a thickness of 1.5-3.0 mm.

7. The gradient silicon nitride bearing ball according to claim 6, characterized in that, The nano-reinforcing phase is one or more of elemental W, TiN, or Fe2O3.

8. The gradient silicon nitride bearing ball according to claim 7, characterized in that, The solid lubricating phase is one or more of BN, graphite / graphene, Ag microphase, or CaF2.

9. The gradient silicon nitride bearing ball according to claim 8, characterized in that, The sintering aid is one or more of MgO, Al2O3, or Y2O3.

10. The gradient silicon nitride bearing ball according to claim 9, characterized in that, The nanoscale interfacial bonding phase is one or more of TiN, TiC, or nano-SiC.

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