A method for producing a silicon nitride ceramic having a core-rim structure

Abstract

The application belongs to the technical field of non-oxide-based ceramic materials, and more particularly relates to a preparation method of silicon nitride ceramic with a core-edge structure. A mixture of alpha-Si3N4 and Mg2Si is used as raw material, and a Si3N4 ceramic with a radial gradient distribution of alpha and beta phases is prepared by discharge plasma sintering under a large axial pressure. The content of the edge alpha-Si3N4 phase is 60-80 wt%, the balance is beta-Si3N4 phase, the hardness is 21-28 GPa, and the fracture toughness is 3-5 MPa·m 1 / 2 ; the content of the center alpha-Si3N4 ceramic phase is 20-40 wt%, the balance is beta-Si3N4 phase, the hardness is 16-21 GPa, and the fracture toughness is 5-10 MPa·m 1 / 2 .

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of non-oxide based ceramic materials, and more particularly relates to a preparation method of silicon nitride ceramic with core-edge structure. BACKGROUND

[0002] The layered ceramic material is a balanced combination of the engineering properties of each layer, and obtains the comprehensive performance that a single layer cannot achieve. However, the differences in physical properties such as the thermal expansion coefficient and Young's modulus of each layer will cause unevenly distributed micro internal stress, and further lead to the reduction of the mechanical strength of the ceramic on the macroscopic. In order to avoid the stress concentration on the interface caused by thermal mismatch, it is necessary to prepare a silicon nitride ceramic with continuously changing microstructure and disappearing internal interface.

[0003] At present, there are many studies on Si3N4 gradient ceramic, but most of them are focused on the structural design of the sintering mold, and the silicon nitride ceramic is prepared by the method of gradient accumulation of powder and two-step sintering. For example, Du et al. realized the temperature gradient between samples during sintering by changing the relative position of the punch and the mold. The lower punch is shortened and completely inserted into the mold, and the upper punch remains unchanged to form an asymmetric arrangement of upper and lower punches, which increases the axial temperature gradient and obtains a silicon nitride ceramic with microstructure or composition gradient. However, the obtained silicon nitride ceramic has an asymmetric gradient structure. Yu et al. sintered a silicon nitride ceramic with a tough center and a hard edge by hot pressing and spark plasma sintering. First, the inner layer of silicon nitride is sintered by hot pressing, and then the powder is laid on the outer layer, and the inner layer of silicon nitride grains is grown and the crystal form is transformed by SPS, and the outer layer of silicon nitride powder is sintered, forming a silicon nitride ceramic with obvious structural differences on both sides of the interface. However, when the temperature changes, the mismatch of the thermal expansion coefficient will cause stress concentration on the interface, affecting the performance of the material.

[0004] Through the above analysis, the defects of the existing technology are:

[0005] (1) The two-step sintering method for preparing silicon nitride ceramic has a long sintering process, and there is an obvious interface inside, which is easy to cause stress concentration on the interface and lead to structural instability.

[0006] (2) Changing the configuration of the SPS sintering mold or introducing a large current will not form a symmetric silicon nitride gradient. Long holding time and high direct current field will also increase energy consumption. SUMMARY

[0007] In view of the defects of the prior art, the purpose of the present application is to provide a preparation method of silicon nitride ceramic with core-edge structure, so as to solve the technical problems of long process flow, interface stress leading to structural instability, or inability to form a symmetric gradient in the preparation of Si3N4 gradient ceramic in the prior art.

[0008] To achieve the above object, the present application provides a preparation method of silicon nitride ceramic with core-edge structure, comprising the following steps:

[0009] (1) mixing α-Si3N4 powder and sintering aid Mg2Si by wet ball milling, drying and sieving after ball milling to obtain Si3N4-Mg2Si mixed powder;

[0010] (2) under non-oxygen-containing atmosphere, discharging plasma sintering the Si3N4-Mg2Si mixed powder under higher axial pressure to obtain silicon nitride ceramic with core-edge structure; the axial pressure is 100-500 MPa.

[0011] Preferably, the mass ratio of α-Si3N4 powder to sintering aid Mg2Si in step (1) is (85-99):(1-15).

[0012] Preferably, the purity of α-Si3N4 powder in step (1) is greater than or equal to 95%, and the purity of sintering aid Mg2Si is greater than or equal to 95%.

[0013] Preferably, the wet ball milling in step (1) uses silicon nitride balls as milling balls and anhydrous ethanol as ball milling aid, and the ball milling time is 2-100 h.

[0014] Preferably, the particle size of α-Si3N4 powder in step (1) is 0.1-100 μm, further preferably 0.1-10 μm, and more further preferably 0.2-2 μm.

[0015] Preferably, the sintering temperature of discharging plasma sintering in step (2) is 1200-1800 ℃, the heating rate is 5-200 ℃ / min, and the holding time is 2-60 min.

[0016] Preferably, the non-oxygen-containing atmosphere is vacuum condition or protective gas atmosphere; the protective gas atmosphere is one or more of nitrogen, helium, argon and neon; wherein the absolute pressure of the vacuum condition is less than or equal to 50 Pa; the pressure of the protective gas atmosphere is greater than or equal to 0.01 MPa and less than or equal to 10 MPa.

[0017] Preferably, the silicon nitride ceramic with core-edge structure comprises α-Si3N4 phase and β-Si3N4 phase, and the content of α-Si3N4 phase and β-Si3N4 phase is gradiently distributed along the radial direction, wherein the content of α-Si3N4 phase in the edge of the silicon nitride ceramic is 60-80 wt%, and the balance is β-Si3N4 phase; the content of α-Si3N4 phase in the center of the silicon nitride ceramic is 20-40 wt%, and the balance is β-Si3N4 phase.

[0018] The hardness of the silicon nitride ceramic edge is 21-28 GPa, and the fracture toughness of the silicon nitride ceramic edge is 3-5 MPa·m 1 / 2 The hardness of the silicon nitride ceramic center is 16-21 GPa, and the fracture toughness of the silicon nitride ceramic center is 5-10 MPa·m 1 / 2 The hardness of the silicon nitride ceramic edge is greater than that of the silicon nitride ceramic center, and the fracture toughness of the silicon nitride ceramic edge is less than that of the silicon nitride ceramic center.

[0019] The present application uses a mixture of alpha-Si3N4 powder and Mg2Si as raw material, and obtains a Si3N4 ceramic with a gradient distribution of alpha and beta phase contents along the radial direction by one-step spark plasma sintering. The content of the edge alpha-Si3N4 phase is 60-80wt%, the hardness is 21-28 GPa, and the fracture toughness is 3-5 MPa·m 1 / 2 The content of the center alpha-Si3N4 ceramic phase is 20-40wt%, the hardness is 16-21 GPa, and the fracture toughness is 5-10 MPa·m 1 / 2 Overall, compared with the prior art, the above technical scheme of the present application has the following beneficial effects:

[0020] First, compared with the traditional two-step hot pressing or hot pressing combined with spark plasma sintering to prepare silicon nitride gradient ceramic, the present application uses spark plasma sintering to obtain gradient structure silicon nitride ceramic in one step. Two-step hot pressing and hot pressing combined with spark plasma sintering are both first sintering the inner layer to achieve partial phase transformation. Then stack the powder on the outside, and perform the second sintering. The inner layer will induce the generation of bimodal structure of silicon nitride grains due to longer holding time and generated beta seeds. The grains in the outer layer are mainly fine equiaxed due to shorter holding time and the competition mechanism of grain growth. The sintered silicon nitride ceramic lacks a gradually transitional interface layer, and there is a sudden change in microstructure and performance on both sides of the interface. In the case of thermal shock and high load, stress concentration will cause microcrack propagation, leading to cracking of the ceramic. At the same time, the two-step sintering method requires long holding time and complex process flow, which greatly reduces the production efficiency. The one-step spark plasma sintering under high pressure adopted by the present application promotes the contribution of grain dynamic sliding to sintering densification, reduces the influence of dissolution and precipitation mechanism on densification. The introduction of electric current produces an electromechanical force that enhances the wettability of the liquid phase and promotes grain rearrangement. The holding time is shortened, and the diffusion mechanism dominates the grain growth in the late stage of liquid phase sintering of the particles in contact with each other to form a skeleton. The overgrowth of grains is avoided, which causes the deterioration of mechanical properties such as bending strength. The prepared in-situ gradient silicon nitride ceramic has high hardness and strong wear resistance.

[0021] Second, different from the commonly used rare earth oxide sintering aids, the application uses non-oxide sintering aid Mg2Si. Magnesium silicide reacts with the SiO2 layer on the surface of silicon nitride during sintering to form SiO(g), reducing the oxygen content in the liquid phase and forming a nitrogen-rich and oxygen-deficient liquid phase. Lattice oxygen is one of the main defects affecting the performance of silicon nitride. Reducing the oxygen in the liquid phase can avoid the introduction of silicon vacancies into the lattice, causing lattice distortion and affecting the physical and chemical properties of silicon nitride. At high temperatures, magnesium silicide will also react with nitrogen to form silicon nitride, reducing the content of the glass phase after cooling and inhibiting phenomena such as grain boundary embrittlement and intergranular fracture.

[0022] Third, the technical solution of the application overcomes the technical bias: (1) The existing technology for the preparation of gradient ceramics of silicon nitride has been devoted to changing the mold structure. Due to the insulating properties of silicon nitride, the joule heating effect mainly occurs at the mold wall and the pressure head. Therefore, by changing the relative position of the pressure head, an asymmetric arrangement is formed, thereby forming an axial temperature gradient. SPS sintering of silicon nitride is a heat-activated process, so the speed of temperature rise of different parts of the powder and local overheating form differences in phase transition and grain growth in different regions. However, this gradient is asymmetric. This method is essentially similar to applying a large direct current field and increasing the Peltier effect on the current flowing into and out of the end, both of which utilize temperature gradients to affect phase transition and grain growth. Unlike the above-mentioned prior art, the application uses one-step spark plasma sintering to obtain silicon nitride ceramics with a symmetric gradient structure, which expands the application of the material. The two-step sintering process of the prior art may be able to obtain an asymmetric gradient silicon nitride ceramic structure, but it is difficult to obtain the central symmetric structure of the gradient silicon nitride ceramic of the application.(2) In the process of sintering silicon nitride by SPS, the edge temperature will be higher than the center, with a temperature difference of about 40-60℃. The edge will preferentially densify and phase transition, and the densified edge will inhibit the shrinkage of the center. Therefore, it is difficult to densify the center of the prepared sample, and there are a large number of pores. However, the SPS sintering of the application to prepare silicon nitride ceramics with a core-edge structure does not have specific requirements for the mold structure and shape, as long as it can withstand high pressure. The application is sintered at a high axial pressure of 100-500 MPa, which affects the viscosity of the liquid phase, leading to uneven distribution of the liquid phase. The liquid phase segregation effect will affect the activation energy of the grain surface, controlling the reconstitution phase transition of α-β. Under the precondition of obtaining a dense sample, the phase transition amount of the center is higher than that of the edge, forming a unique structure of hard edge and tough center.

[0023] Fourthly, the present application fills the technical blank at home and abroad: the existing technology of spark plasma sintering can prepare gradient silicon nitride ceramic under low pressure. The contact between the powder and the mold wall is loose under low pressure (<50 MPa), the stress on the edge part of the punch is smaller than that on the center part, which leads to higher temperature and pressure in the center, which may be beneficial to the phase transition in the center, so as to obtain radially gradient distributed silicon nitride ceramic. However, this radial gradient is uncontrollable, and the gradient change range is very small. The present application is prepared under high pressure of 100-500 MPa, and Mg2Si is used as sintering aid to obtain obvious gradient structure of silicon nitride, wide range of mechanical property change, and layering and gradual change phenomenon in color, which is beneficial to the commercial application of silicon nitride ceramic. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The surface color morphology photo of the silicon nitride ceramic with radially symmetric gradient morphology and microstructure prepared in Example 1.

[0025] Figure 2 The phase content distribution diagram of the silicon nitride ceramic with radially symmetric gradient morphology and microstructure prepared in Example 1.

[0026] Figure 3 The Vickers hardness diagram of the silicon nitride ceramic with radially symmetric gradient morphology and microstructure prepared in Example 1.

[0027] Figure 4 The fracture toughness diagram of the silicon nitride ceramic with radially symmetric gradient morphology and microstructure prepared in Example 1.

[0028] Figure 5 The Vickers hardness and fracture toughness distribution diagram of the ceramic samples prepared in Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the present application.

[0030] The present application provides a preparation method of silicon nitride ceramic with core-edge structure. α-Si3N4 powder and sintering aid Mg2Si are mixed to obtain Si3N4-Mg2Si mixed powder, and then the Si3N4-Mg2Si mixed powder is prepared by spark plasma sintering under high pressure. In some embodiments, the method specifically comprises the following steps:

[0031] S1: mixing the α-Si3N4 powder and sintering aid Mg2Si, using anhydrous ethanol as a ball milling aid, using Si3N4 balls as a ball milling medium, ball milling mixing, and drying to obtain a mixed powder of Si3N4-Mg2Si;

[0032] S2: placing the Si3N4-Mg2Si mixed powder into a mold, under a non-oxygen-containing atmosphere, at an axial pressure of 100 MPa-500 MPa, heating at a rate of 5 ℃-200 ℃ / min to 1200-1800 ℃, and maintaining the temperature for 2-60 min, and obtaining a silicon nitride ceramic with a radially symmetric gradient morphology and microstructure by spark plasma sintering.

[0033] In some embodiments, the mass ratio of Si3N4:Mg2Si in step S1 is (85-99):(1-15). The purity of the α-Si3N4 powder (SN-E10) is ≥95%, and the purity of the Mg2Si powder is ≥95%. The planetary ball milling mixing time in step S1 is 2-100 h, preferably 2-20 h.

[0034] During sintering, appropriately reducing the powder particle size can reduce the sintering temperature and the amount of sintering aid to some extent. In some embodiments, the particle size of the α-Si3N4 powder is 0.1-100 μm, preferably 0.1-10 μm, and more preferably 0.2-2 μm.

[0035] In some embodiments, the non-oxygen-containing atmosphere is a vacuum condition or a protective gas atmosphere, and the protective gas atmosphere is one or more of nitrogen, helium, argon, and neon. The absolute pressure of the vacuum condition is less than or equal to 50 Pa, and the pressure of the protective gas atmosphere is greater than or equal to 0.01 MPa and less than or equal to 10 MPa. Sintering in a vacuum or a protective gas atmosphere can prevent the silicon nitride ceramic from being oxidized, and the pressure of the protective gas can be appropriately increased when sintering at a higher temperature to prevent the silicon nitride ceramic from decomposing.

[0036] The silicon nitride ceramic with a core-edge structure prepared by the present application can use a mold of the type of a conventional SPS sintering mold in the prior art, or various improved mold shapes. The present application prepares a silicon nitride ceramic with a core-edge structure at a high axial pressure, and the material of the mold can be adjusted according to the required pressure, such as using high-strength graphite, carbon fiber, etc. to prepare a mold that can withstand a large pressure, which can be processed by oneself or ordered from a mold manufacturer.

[0037] The Si3N4 ceramic center prepared by the method has higher edge fracture toughness and greater edge hardness than the center, and has a clear core-edge structure. In the core-edge structure, the "core" refers to the central part or the vicinity of the central part of the Si3N4 ceramic, and the "edge" refers to the edge part or the vicinity of the edge part of the Si3N4 ceramic. The "core" and the "edge" are relative expressions.

[0038] The Si3N4 ceramic with the core-edge structure prepared by the method comprises α-Si3N4 phase and β-Si3N4 phase, and the contents of the α-Si3N4 phase and the β-Si3N4 phase are gradiently distributed along the radial direction. The content of the α-Si3N4 phase in the edge of the Si3N4 ceramic is 60-80 wt%, and the balance is the β-Si3N4 phase. The content of the α-Si3N4 phase in the center of the Si3N4 ceramic is 20-40 wt%, and the balance is the β-Si3N4 phase. The hardness of the edge of the Si3N4 ceramic is 21-28 GPa, the fracture toughness of the edge of the Si3N4 ceramic is 3-5 MPa·m 1 / 2 1, the hardness of the center of the Si3N4 ceramic is 16-21 GPa, and the fracture toughness of the center of the Si3N4 ceramic is 5-10 MPa·m 1 / 2 1. The hardness of the edge of the Si3N4 ceramic is greater than the hardness of the center of the Si3N4 ceramic, and the fracture toughness of the edge of the Si3N4 ceramic is less than the fracture toughness of the center of the Si3N4 ceramic.

[0039] In the embodiment, the core-edge structure Si3N4 ceramic prepared is characterized in mechanical properties (including fracture toughness and hardness), and shows a clear gradient distribution and central symmetry.

[0040] The following is an example:

[0041] Example 1

[0042] A Si3N4 ceramic with a core-edge structure and a preparation method thereof are provided, and the specific method is as follows:

[0043] (1) α-Si3N4 powder (produced by Japan Ube, purity greater than 95%, median particle size 0.5 μm) is used as a base raw material, and Mg2Si powder (purity 99.5%, average particle size 10 μm) is used as a sintering aid. According to the mass fraction of α-Si3N4 powder 95% and the mass fraction of Mg2Si powder 5%, ethanol is used as a solvent, Si3N4 balls (5 mm and 3 mm mixed Si3N4 balls, each accounting for 50 wt%) are used as a ball milling medium, and the ball-to-material mass ratio is 5:1. The mixture is mixed on a planetary ball mill for 8 h. After drying, the Si3N4-Mg2Si uniform powder is obtained by passing through a 200-mesh screen.

[0044] (2) The Si3N4-Mg2Si mixed powder was placed in an SPS mold made of carbon fiber, which included a sleeve, an upper pressure head, and a lower pressure head. The sleeve had a diameter of 12 mm. Graphite paper was placed between the mold and the powder to facilitate demolding. Under an axial pressure of 500 MPa and a nitrogen pressure of 0.1 MPa, the temperature was increased to 1500 °C at a rate of 100 °C / min and held for 15 min. After cooling, the powder was removed.

[0045] Sampling tests showed that the Si3N4 gradient ceramic prepared in this example had a relative density of 99.8% (relative to the theoretical density of 3.18 g / cm³). 3 The Si3N4 gradient ceramic has the following composition (percentage): edge α-Si3N4 content 80 wt%, β-Si3N4 content 20 wt%; center α-Si3N4 content 20 wt%, β-Si3N4 phase content 80 wt%; the hardness of the Si3N4 gradient ceramic edge is 26.3 GPa, and the fracture toughness of the Si3N4 gradient ceramic edge is 3.4 MPa·m. 1 / 2 The hardness of the Si3N4 gradient ceramic center is 16.9 GPa, and the fracture toughness of the Si3N4 gradient ceramic center is 9.5 MPa·m. 1 / 2 .

[0046] Figure 1 The image shows the surface color morphology of a silicon nitride ceramic with radially symmetrical gradient morphology and microstructure obtained in Example 1. Figure 1 As can be seen, the center of the sample is reddish-brown and the edge is bluish-gray, showing obvious differences in color distribution; Figure 2 This is a phase content distribution diagram of silicon nitride ceramics with radially symmetrical gradient morphology and microstructure prepared in Example 1. From... Figure 2 As can be seen from the data, the α phase content in the center is lower than that at the edge, but the β phase content is higher than that at the edge, thus the edge exhibits higher hardness; Figure 3 and Figure 4 The Vickers hardness and fracture toughness diagrams are for the silicon nitride ceramic with radially symmetrical gradient morphology and microstructure prepared in Example 1. Figure 3 and Figure 4 As can be seen, hardness and toughness show a negative correlation and exhibit central symmetry.

[0047] The silicon nitride ceramic prepared in this embodiment exhibits a clear gradient distribution in terms of mechanical properties. This gradient change in mechanical properties reflects, to some extent, the phase content of its α-phase and β-phase silicon nitride and the size of its grains. It is determined by the combined distribution of its two phases and the grain size distribution. In the experiment, the ceramic was also subjected to electron microscopy scanning test, which revealed that the silicon nitride ceramic has large grains (β phase) in the center and small grains at the edge.

[0048] Example 2

[0049] A silicon nitride ceramic with core-mantle structure and a preparation method thereof, the specific method is as follows:

[0050] (1) The α-Si3N4 powder and Mg2Si powder are the same as in Example 1. The α-Si3N4 powder is used as the base raw material, and the median particle size of the Si3N4 powder is 0.5 μm. The Mg2Si powder is used as a sintering aid, and the average particle size is 10 μm. According to the mass fraction of 95% of Si3N4 powder and 5% of Mg2Si powder, ethanol is used as the solvent, Si3N4 balls (a mixture of Si3N4 balls with a diameter of 5 mm and 3 mm, each accounting for 50 wt%) are used as the ball milling medium, and the ball-to-material mass ratio is 5:1. The Si3N4-Mg2Si uniform powder is obtained by mixing for 8 h on a planetary ball mill, drying, and then passing through a 200-mesh screen.

[0051] (2) The Si3N4-Mg2Si mixed powder is placed in an SPS mold, the mold is a carbon fiber material mold, including a sleeve, an upper press head, and a lower press head, wherein the sleeve has a diameter of 12 mm. Graphite paper is added between the mold and the powder to facilitate demolding. Under the conditions of an axial pressure of 200 MPa and nitrogen of 0.1 MPa, the temperature is raised to 1500 °C at a rate of 100 °C / min, and the temperature is maintained for 15 min.

[0052] The relative density of the Si3N4 gradient ceramic prepared in this example is 99.6%. The content of the edge α-Si3N4 phase is 72 wt%, and the content of the β-Si3N4 phase is 28 wt%. The content of the central α-Si3N4 phase is 30 wt%, and the content of the β-Si3N4 phase is 70 wt%. The hardness of the edge of the Si3N4 gradient ceramic is 24.4 GPa, and the fracture toughness of the edge is 4.5 MPa·m 1 / 2 ; the hardness of the center of the Si3N4 gradient ceramic is 18.5 GPa, and the fracture toughness of the center of the Si3N4 gradient ceramic is 8.5 MPa·m 1 / 2 ; the center and the edge have obvious color differentiation, which is a gradient silicon nitride ceramic with core-mantle structure.

[0053] Example 3

[0054] A silicon nitride ceramic with core-mantle structure and a preparation method thereof, the specific method is as follows:

[0055] (1) The α-Si3N4 powder and Mg2Si powder are the same as in Example 1. The Si3N4 powder is used as the base raw material, and the median particle size of the Si3N4 powder is 0.5 μm. The Mg2Si powder is used as the sintering aid, and the average particle size is 10 μm. According to the mass fraction of 95% of the Si3N4 powder and 5% of the Mg2Si powder, the Si3N4-Mg2Si uniform powder is obtained by mixing in a planetary ball mill for 10 h with ethanol as the solvent and Si3N4 balls (mixed Si3N4 balls with a diameter of 5 mm and 3 mm, each accounting for 50 wt%) as the ball milling medium, and the ball-to-material mass ratio is 5:1. After drying, the Si3N4-Mg2Si uniform powder is sieved through a 200-mesh screen.

[0056] (2) The Si3N4-Mg2Si mixed powder is placed in an SPS mold, and the mold is a carbon fiber material mold, including a sleeve, an upper press head, and a lower press head. The sleeve has a diameter of 12 mm, and graphite paper is added between the mold and the powder to facilitate demolding. The temperature is raised to 1500 °C at a rate of 100 °C / min under the conditions of an axial pressure of 100 MPa and nitrogen of 0.1 MPa, and the temperature is maintained for 15 min.

[0057] The relative density of the Si3N4 gradient ceramic prepared in this example is 99.3%. The content of the edge α-Si3N4 phase is 62 wt%, and the content of the β-Si3N4 phase is 38 wt%. The content of the center α-Si3N4 phase is 35 wt%, and the content of the β-Si3N4 phase is 65 wt%. The hardness of the edge of the Si3N4 gradient ceramic is 23.1 GPa, and the fracture toughness of the edge is 5.5 MPa·m 1 / 2 The hardness of the center of the Si3N4 gradient ceramic is 19.2 GPa, and the fracture toughness of the center is 8.1 MPa·m 1 / 2 The center and the edge have obvious color differentiation, and the Si3N4 gradient ceramic has a core-edge structure.

[0058] Comparative Example 1

[0059] The other conditions are the same as in Example 1, except that the axial pressure in step (2) is 30 MPa.

[0060] The relative density of the Si3N4 ceramic prepared in this example is 90.2%, and the center and the edge do not have obvious color differentiation. No in-situ gradient formation of silicon nitride is found.

[0061] Comparative Example 2

[0062] The other conditions are the same as in Example 1, except that the axial pressure in step (2) is 10 MPa.

[0063] The relative density of the Si3N4 ceramic prepared in this example is 80.2%, and the center and the edge do not have obvious color differentiation. No in-situ gradient formation of silicon nitride is found.

[0064] The samples prepared in Comparative Example 1 and Comparative Example 2 were sampled from the center to the outside to test their Vickers hardness and fracture toughness, and the test results are shown in Table 1. Figure 5 It can be seen that from the center to the outside, the mechanical properties are relatively uniform, and there is no obvious gradient regularity distribution of the mechanical properties, indicating that there is indeed no in-situ gradient formation.

[0065] Comparative Example 3

[0066] Other than Example 1, the difference is that in step (1), MgO and Y2O3 powder are used as sintering aids.

[0067] The relative density of the Si3N4 ceramic prepared in this example is 99.6%, and there is no obvious color distinction between the center and the edge, and no in-situ gradient formation of silicon nitride is found.

[0068] Those skilled in the art will readily understand that the above description is only preferred embodiments of the present application and is not intended to limit the present application, and any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for producing a silicon nitride ceramic having a core-rim structure, characterized by, The method comprises the following steps: (1) mixing α-Si3N4 powder and sintering aid Mg2Si by wet ball milling, drying and sieving the mixture after ball milling to obtain Si3N4-Mg2Si mixed powder; the mass ratio of the α-Si3N4 powder and the sintering aid Mg2Si is (85-99):(1-15); (2) under a non-oxygen-containing atmosphere, subjecting the Si3N4-Mg2Si mixed powder to one-step spark plasma sintering under a high axial pressure to obtain a silicon nitride ceramic with a core-edge structure; the axial pressure is 100 MPa-500 MPa; the sintering temperature of the spark plasma sintering is 1200-1800 ℃, the heating rate is 5-200 ℃ / min, and the holding time is 2-60 min.

2. The production method according to claim 1, wherein In step (1), the purity of the α-Si3N4 powder is greater than or equal to 95%, and the purity of the sintering aid Mg2Si is greater than or equal to 95%.

3. The production method according to claim 1, wherein In step (1), the ball milling is performed for 2-100 h using silicon nitride balls as the grinding balls and anhydrous ethanol as the ball milling aid.

4. The production method according to claim 1, wherein In step (1), the particle size of the α-Si3N4 powder is 0.1-100 μm.

5. The production method according to claim 1, wherein In step (1), the particle size of the α-Si3N4 powder is 0.1-10 μm.

6. The production method according to claim 1, wherein The non-oxygen-containing atmosphere is a vacuum condition or a protective atmosphere; the protective atmosphere is one or more of nitrogen, helium, argon and neon; wherein the absolute pressure of the vacuum condition is less than or equal to 50 Pa; the pressure of the protective atmosphere is greater than or equal to 0.01 MPa and less than or equal to 10 MPa.

7. The production method according to claim 1, wherein The silicon nitride ceramic with a core-edge structure comprises α-Si3N4 phase and β-Si3N4 phase, and the contents of the α-Si3N4 phase and the β-Si3N4 phase are gradiently distributed along the radial direction, wherein the content of α-Si3N4 at the edge of the silicon nitride ceramic is 60-80 wt%, and the balance is β-Si3N4 phase; the content of α-Si3N4 at the center of the silicon nitride ceramic is 20-40 wt%, and the balance is β-Si3N4 phase; The silicon nitride ceramic with a core-edge structure comprises α-Si3N4 phase and β-Si3N4 phase, and the contents of the α-Si3N4 phase and the β-Si3N4 phase are gradiently distributed along the radial direction, wherein the content of α-Si3N4 at the edge of the silicon nitride ceramic is 60-80 wt%, and the balance is β-Si3N4 phase; the content of α-Si3N4 at the center of the silicon nitride ceramic is 20-40 wt%, and the balance is β-Si3N4 phase; the hardness of the silicon nitride ceramic edge is 21-28 GPa, the fracture toughness of the silicon nitride ceramic edge is 3-5 MPa·m 1 / 2 ; the hardness of the silicon nitride ceramic center is 16-21 GPa, the fracture toughness of the silicon nitride ceramic center is 5-10 MPa·m 1 / 2 ; and the hardness of the silicon nitride ceramic edge is greater than the hardness of the silicon nitride ceramic center, and the fracture toughness of the silicon nitride ceramic edge is less than the fracture toughness of the silicon nitride ceramic center.

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