Alpha-phase silicon nitride and preparation method and application thereof

Abstract

The invention discloses alpha-phase silicon nitride as well as a preparation method and application thereof. The crystal structure of the alpha-phase silicon nitride belongs to a hexagonal system, and the crystal grains are equiaxed spheres with isotropic characteristics; the grain size of the crystal grains is 100-500 nm, the hardness is 21-28 GPa, the fracture toughness is 6-9 MPa.m < 1 / 2 >, and the bending strength is 1000-1400 MPa. The obtained high-performance silicon nitride ceramic material has higher density, hardness, fracture toughness and strength. The performance index of the silicon nitride ceramic structural member is remarkably superior to that of a commercial silicon nitride ceramic structural member, the highest hardness can reach 28 GPa, the highest fracture toughness can reach 9 MPa.m < 1 / 2 >, and the highest bending strength can reach 1400 MPa. In addition, the preparation method is simple, parameters are easy to control, and the method is suitable for industrial production.

Description

Technical Field

[0001] The invention belongs to the field of high-performance structural ceramics, and in particular relates to alpha-phase silicon nitride and a preparation method and application thereof. Background Art

[0002] Silicon nitride ceramics are covalently bonded ceramic materials with low density (3.20 g / cm³), high toughness, and high chemical and thermal stability. Due to its excellent properties, silicon nitride ceramics have become one of the structural ceramic materials with high application value and are widely used in cutting tools, semiconductor packaging, metallurgy, nuclear industry seals, and aerospace components. However, due to its covalent bond characteristics, the preparation of silicon nitride ceramics usually requires harsh sintering conditions, which limits the possibility of optimizing its microstructure and improving its performance.

[0003] As a covalently bonded ceramic material, a liquid phase sintering method with the introduction of sintering aids is often used when sintering silicon nitride. Suitable sintering aids can reduce the sintering temperature of silicon nitride, help achieve ceramic densification during the preparation process, and avoid the decomposition of silicon nitride at high temperatures and abnormal grain growth. Silicon nitride has better strength and toughness due to its stable β-phase long columnar grains at high temperatures. These long columnar grains can effectively prevent crack propagation in the matrix through bridging and interlocking structures. Therefore, obtaining grains with high aspect ratios has become an important strategy to improve the strength and toughness of silicon nitride. However, the formation of β-phase silicon nitride needs to be carried out at a higher temperature (>1400 ℃), which easily leads to coarsening of silicon nitride grains, thereby reducing the aspect ratio of the grains, and ultimately leading to a decrease in the strength and toughness of silicon nitride. Although β-phase silicon nitride can improve the toughness of the material, it reduces the hardness of the matrix. Therefore, it has become an effective method to improve the hardness, strength and toughness of the material by adjusting the ratio of different phases. Summary of the invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide an α-phase silicon nitride and a preparation method and application thereof. The equiaxed spherical α-phase silicon nitride prepared by the present invention has a uniform microstructure, a high green body density, a low product density, and high hardness, bending strength and toughness.

[0005] The purpose of the present invention is achieved through the following technical solutions: An α-phase silicon nitride, whose crystal structure belongs to the hexagonal system, and whose grains are equiaxed spherical with isotropic characteristics; whose grain size is 100-500 nm, whose hardness is 21-28 GPa, and whose fracture toughness is 6-9 MPa·m 1 / 2 , the flexural strength is 1000~1400 MPa.

[0006] The preparation method of the α-phase silicon nitride comprises the following steps: (1) Pre-ball-milling α-Si3N4 powder to obtain slurry; preparing a sintering aid dispersion; mixing the slurry and the sintering aid dispersion and then ball-milling again; drying and deoxidizing the slurry obtained after ball milling, and sieving to obtain α-Si3N4-Al2O3-Y2O3 mixed powder; (2) placing the α-Si3N4-Al2O3-Y2O3 mixed powder in an oscillating pressure furnace for vacuum pre-sintering to obtain a pre-sintered green body; (3) In a non-oxygen atmosphere, axial pressure is applied to the pre-sintered body and spark plasma sintering is performed, and then the temperature is reduced to prepare α-phase silicon nitride.

[0007] Preferably, in step (1), the specific operation of pre-ball milling the α-Si3N4 powder includes: using anhydrous ethanol as a ball milling aid and silicon nitride balls as a ball milling medium to ball mill the α-Si3N4 powder for 1 to 4 hours.

[0008] Preferably, in step (1), the specific operation of preparing the sintering aid dispersion includes: dispersing the sintering aid in anhydrous ethanol to obtain a dispersion.

[0009] Preferably, in step (1), the sintering aid in the sintering aid dispersion is Al2O3 and Y2O3.

[0010] Preferably, the mass ratio of α-Si3N4 powder, Al2O3 and Y2O3 is (84-98): (1-8): (1-8).

[0011] Preferably, in step (1), the slurry and the sintering aid dispersion are stirred under ultrasonic conditions for 0.5 to 2 h to mix them evenly, and then ball milled again.

[0012] Preferably, in step (1), the drying method is rotary evaporation.

[0013] Preferably, in step (1), the specific operation of deoxygenation includes: placing the dried sample in a tube furnace, and introducing a hydrogen atmosphere at 600° C. to remove oxygen in the powder.

[0014] Preferably, in step (1), the purity of α-Si3N4 powder is greater than or equal to 90%, and the purity of sintering aids Al2O3 and Y2O3 is greater than or equal to 90%.

[0015] Preferably, in step (1), the particle size of the α-Si3N4 powder is 10 to 900 nm, more preferably 10 to 200 nm.

[0016] Preferably, in step (2), the vacuum pre-sintering condition parameters are: constant pressure of 30-80 MPa, oscillation pressure of 2-8 MPa, oscillation frequency of 1-5 Hz, pre-sintering temperature of 600-800 °C, and holding time of 1-2 h. The density of the green body after pre-sintering is 60%-75%.

[0017] Preferably, in step (3), the axial pressure is 80 to 500 MPa.

[0018] Preferably, in step (3), the spark plasma sintering conditions are as follows: sintering temperature is 1100-1500°C, heating rate is 5-500°C / min, and holding time is 2-180 min.

[0019] Preferably, in step (3), the non-oxygen-containing atmosphere is a vacuum condition or a protective gas atmosphere; 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; the pressure of the protective gas atmosphere is greater than or equal to 0.01 MPa and less than or equal to 10 MPa.

[0020] The α-phase silicon nitride is used in the preparation of cutting tools, corrosion-resistant components and wear-resistant parts.

[0021] Compared with the prior art, the beneficial effects of the present invention include: First, the present invention avoids the problem of uneven distribution of additives caused by agglomeration of nanopowders by mixing ethanol solutions of different powders under an ultrasonic environment, effectively improves the condition that silicon nitride is difficult to sinter to a dense state at low temperatures, and makes it possible to prepare silicon nitride ceramic blocks with high density, high strength and high toughness.

[0022] Secondly, the present invention adopts oscillating pressure sintering to keep the grains from growing in the early stage of solid phase sintering, and promotes the rearrangement of powder particles by coupling oscillating pressure, so as to promote more uniform mixing of the additive and silicon nitride particles.

[0023] Third, the present invention inhibits the transformation of silicon nitride into anisotropic columnar crystals through a quasi-plastic high-pressure, low-temperature and high-efficiency densification mechanism and a liquid-phase sintering mechanism, reduces the microstructure, and allows the α phase to grow finely along the equiaxial direction to obtain fine equiaxial spherical grains, thereby significantly improving the hardness and fracture toughness of silicon nitride ceramics, and ultimately preparing a silicon nitride ceramic block with high density, high hardness, high fracture toughness and high strength.

[0024] Fourth, the technical solution of the present invention overcomes technical prejudice: (1) In the present invention, nano silicon nitride powder is used as raw material. Since nano powder has a large specific surface area and a high surface energy, it is conducive to the sintering and densification of silicon nitride ceramics, and provides a new way to prepare high-density, high-performance silicon nitride ceramic blocks. (2) The preparation method of the present invention is easy to implement, and the sintering equipment used are all general equipment available on the market. The equipment models are not unique and can be replaced by equipment of the same type as needed. For example, the spark plasma rapid sintering furnace used in the present invention is a third-generation product produced by Shanghai Chenhua Company; the high-strength mold is a carbon fiber mold provided by a domestic high-temperature material equipment manufacturing company.

[0025] Fifth, the present invention fills the technical gaps at home and abroad: (1) The grains of the high-strength and tough silicon nitride ceramics produced by the present invention are all equiaxed. The equiaxed grains not only increase the hardness of the material, but also the nano-spherical grains are arranged in chain clusters in the matrix, which can prevent the expansion of cracks like columnar crystals. (2) Compared with silicon nitride ceramics prepared by prior art, the high-performance silicon nitride ceramic material obtained by the present invention has higher density, hardness, fracture toughness and strength. Its performance indicators are significantly better than those of commercial silicon nitride ceramic structural parts, with a hardness of up to 28 GPa and a fracture toughness of up to 9 MPa·m 1 / 2 , the bending strength can reach up to 1400 MPa. Equiaxed spherical grain silicon nitride ceramic blocks have broad application prospects in cutting tools, corrosion-resistant components, wear-resistant parts and various precision-made ceramic structural parts. (3) The preparation method of the present invention is simple, the parameters are easy to control, and it is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a scanning electron microscope image of the α-Si3N4 powder used in Example 1.

[0027] Figure 2 This is the X-ray diffraction pattern of the α-phase silicon nitride prepared in Example 1.

[0028] Figure 3 This is a scanning electron microscope morphology image of the α-phase silicon nitride prepared in Example 1.

[0029] Figure 4 This is the Vickers indentation diagram of the α-phase silicon nitride prepared in Example 1.

[0030] Figure 5 This is a scanning electron microscope morphology image of the α-phase silicon nitride prepared in Example 2.

[0031] Figure 6 This is a scanning electron microscope morphology image of the α-phase silicon nitride prepared in Comparative Example 1. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0033] The mold used for the α-phase silicon nitride ceramics prepared by the present invention can be a conventional SPS sintering mold type in the prior art, or various improved mold shapes. The mold material of the spherical grain high-strength and tough silicon nitride ceramics prepared by the present invention under higher axial pressure can be adjusted according to the required pressure. For example, high-strength graphite, carbon fiber and other materials can be used to prepare molds that can withstand greater pressure. These molds can be processed by themselves or commissioned to mold manufacturers for processing and customization.

[0034] Example 1 A method for preparing α-phase silicon nitride, comprising the following steps: (1) α-Si3N4 powder (produced by UBE, Japan, with a purity of more than 95% and an average particle size of 200 nm) was used as the matrix raw material, and Al2O3 and Y2O3 powders (produced by Shanghai McLean Biochemical Technology Co., Ltd., with a purity of 99.9% and a particle size of 30 nm) were used as sintering aids. The mass percentages of α-Si3N4 powder, Al2O3 powder, and Y2O3 powder were 90%, 5%, and 5%, respectively. Using ethanol as solvent, α-Si3N4 powder was pre-milled with mixed silicon nitride balls with diameters of 5 mm and 3 mm (each accounting for 50 wt%) as ball milling media, with a ball-to-material mass ratio of 5:1 and a pre-milling time of 2 h to obtain a slurry; Al2O3 and Y2O3 powders were uniformly dispersed in anhydrous ethanol (the mass ratio of anhydrous ethanol to the total of Al2O3 and Y2O3 powders was 10:1) to obtain a sintering aid dispersion; the slurry and the sintering aid dispersion were mixed and stirred under ultrasonic conditions for 1 h, and then the mixture was planetarily ball milled again for 4 h. The ball-milled sample was placed in a tube furnace at 600 ℃ after rotary evaporation, and a gas containing 5% hydrogen was introduced for deoxygenation. The deoxygenated powder was screened through a 200-mesh sieve to obtain an α-Si3N4-Al2O3-Y2O3 mixed powder; (2) The α-Si3N4-Al2O3-Y2O3 mixed powder was vacuum pre-sintered in an oscillating pressure furnace. The vacuum pre-sintering conditions were: constant pressure of 50 MPa, oscillation pressure of 5 MPa, oscillation frequency of 2 Hz, pre-sintering temperature of 650 °C, and holding time of 1 h. The density of the green body after pre-sintering was 68%; (3) The pre-sintered green body is placed in an SPS high-strength mold, which is a carbon fiber mold, comprising a sleeve, an upper pressure head and a lower pressure head, wherein the sleeve diameter is 12 mm, and graphite paper is placed between the mold and the powder to facilitate demolding. Under the conditions of an axial pressure of 400 MPa and a nitrogen pressure 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, and the α-phase silicon nitride is obtained after cooling.

[0035] After sampling and testing, the relative density of the Si3N4 ceramic prepared in Example 1 is 99.9% (relative to the theoretical density of 3.20 g / cm 3 The average diameter of equiaxed grains was 220 nm, the hardness of Si3N4 ceramics was 28 GPa, and the fracture toughness was 9 MPa·m 1 / 2 , the flexural strength is 1400 MPa.

[0036] Example 2 A method for preparing α-phase silicon nitride, comprising the following steps: (1) α-Si3N4 powder (produced by UBE, Japan, with a purity of more than 95% and an average particle size of 200 nm) was used as the matrix raw material, and Al2O3 and Y2O3 powders (produced by Shanghai McLean Biochemical Technology Co., Ltd., with a purity of 99.9% and a particle size of 30 nm) were used as sintering aids. The mass percentages of α-Si3N4 powder, Al2O3 powder, and Y2O3 powder were 92%, 5%, and 3%, respectively. Using ethanol as solvent, α-Si3N4 powder was pre-milled with mixed silicon nitride balls with diameters of 5 mm and 3 mm (each accounting for 50 wt%) as ball milling media, with a ball-to-material mass ratio of 5:1 and a pre-milling time of 2 h to obtain a slurry; Al2O3 and Y2O3 powders were uniformly dispersed in anhydrous ethanol (the mass ratio of anhydrous ethanol to the total of Al2O3 and Y2O3 powders was 10:1) to obtain a sintering aid dispersion; the slurry and the sintering aid dispersion were mixed and stirred under ultrasonic conditions for 1 h, and then the mixture was planetarily ball milled again for 4 h. The ball-milled sample was placed in a tube furnace at 600 ℃ after rotary evaporation, and a gas containing 5% hydrogen was introduced for deoxygenation. The deoxygenated powder was screened through a 200-mesh sieve to obtain an α-Si3N4-Al2O3-Y2O3 mixed powder; (2) The α-Si3N4-Al2O3-Y2O3 mixed powder was vacuum pre-sintered in an oscillating pressure furnace. The vacuum pre-sintering conditions were: constant pressure of 50 MPa, oscillation pressure of 5 MPa, oscillation frequency of 2 Hz, pre-sintering temperature of 650 °C, and holding time of 1 h. The density of the green body after pre-sintering was 68%; (3) The pre-sintered green body is placed in an SPS high-strength mold, which is a carbon fiber mold, comprising a sleeve, an upper pressure head and a lower pressure head, wherein the sleeve diameter is 12 mm, and graphite paper is placed between the mold and the powder to facilitate demolding. Under the conditions of an axial pressure of 250 MPa and a nitrogen pressure 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, and the α-phase silicon nitride is obtained after cooling.

[0037] After sampling and testing, the relative density of the Si3N4 ceramic prepared in Example 2 is 99.8% (relative to the theoretical density of 3.20 g / cm 3 The average diameter of equiaxed grains was 220 nm, the hardness of Si3N4 ceramics was 27 GPa, and the fracture toughness was 7 MPa·m 1 / 2 , the flexural strength is 1268 MPa.

[0038] Example 3 A method for preparing α-phase silicon nitride, comprising the following steps: (1) α-Si3N4 powder (produced by UBE, Japan, with a purity of more than 95% and an average particle size of 200 nm) was used as the matrix raw material, and Al2O3 and Y2O3 powders (produced by Shanghai McLean Biochemical Technology Co., Ltd., with a purity of 99.9% and a particle size of 30 nm) were used as sintering aids. The mass percentages of α-Si3N4 powder, Al2O3 powder, and Y2O3 powder were 92%, 5%, and 3%, respectively. Using ethanol as solvent, α-Si3N4 powder was pre-milled with mixed silicon nitride balls with diameters of 5 mm and 3 mm (each accounting for 50 wt%) as ball milling media, with a ball-to-material mass ratio of 5:1 and a pre-milling time of 2 h to obtain a slurry; Al2O3 and Y2O3 powders were uniformly dispersed in anhydrous ethanol (the mass ratio of anhydrous ethanol to the total of Al2O3 and Y2O3 powders was 10:1) to obtain a sintering aid dispersion; the slurry and the sintering aid dispersion were mixed and stirred under ultrasonic conditions for 1 h, and then the mixture was planetarily ball milled again for 4 h. The ball-milled sample was placed in a tube furnace at 600 ℃ after rotary evaporation, and a gas containing 5% hydrogen was introduced for deoxygenation. The deoxygenated powder was screened through a 200-mesh sieve to obtain an α-Si3N4-Al2O3-Y2O3 mixed powder; (2) The α-Si3N4-Al2O3-Y2O3 mixed powder is vacuum pre-sintered in an oscillating pressure furnace. The vacuum pre-sintering condition parameters are: constant pressure of 50 MPa, oscillation pressure of 5 MPa, oscillation frequency of 2 Hz, pre-sintering temperature of 800 °C, and holding time of 1 h. The density of the green body after pre-sintering is 75%; (3) The pre-sintered green body is placed in an SPS high-strength mold, which is a carbon fiber mold, comprising a sleeve, an upper pressure head and a lower pressure head, wherein the sleeve diameter is 12 mm, and graphite paper is placed between the mold and the powder for easy demoulding. Under the conditions of an axial pressure of 80 MPa and a nitrogen pressure 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, and the α-phase silicon nitride is obtained after cooling.

[0039] After sampling and testing, the relative density of the Si3N4 ceramic prepared in Example 3 is 99.1% (relative to the theoretical density of 3.20 g / cm 3 The hardness of Si3N4 ceramics is 25 GPa, and the fracture toughness is 6 MPa·m 1 / 2 , the flexural strength is 1080 MPa.

[0040] Comparative Example 1 Comparative Example 1 provides a method for preparing α-phase silicon nitride. Compared with Example 2, the only difference is that in step (2), the vacuum pre-sintering condition parameters are: constant pressure 50 MPa, pre-sintering temperature 650 ° C, and holding time 1 h. The density of the pre-sintered green body obtained in Comparative Example 1 is 58%.

[0041] After sampling and testing, the relative density of the Si3N4 ceramic prepared in Comparative Example 1 is 97.5% (relative to the theoretical density of 3.20 g / cm 3 The hardness of Si3N4 ceramics is 21 GPa, and the fracture toughness is 4 MPa·m 1 / 2 , the flexural strength is 850 MPa.

[0042] Figure 2 is the X-ray diffraction pattern of α-phase silicon nitride obtained in Example 1. Figure 2 It can be seen that the prepared product is indeed α-phase Si3N4.

[0043] Figure 3 The scanning electron microscope morphology of the α-phase silicon nitride prepared in Example 1 is shown in FIG. Figure 3 It can be seen that the black spherical grains in the photo are α-Si3N4, and the white grain boundaries distributed at the edges of the grains are the eutectic phase composed of Al2O3 and Y2O3.

[0044] Figure 6 The scanning electron microscope morphology of the α-phase silicon nitride prepared in comparative example 1. Compared with example 1, we can find that due to the lack of rearrangement of the powder particles by the oscillating pressure, the microstructure and grain boundary phase distribution are uneven, and the hardness and fracture toughness of the product are significantly reduced.

[0045] The specific implementation of the present invention described above does not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. An alpha-phase silicon nitride, It is characterized in that Its crystal structure belongs to the hexagonal system, and the grains are equiaxed spherical with isotropic characteristics; the grain size is 100-500 nm, the hardness is 21-28 GPa, and the fracture toughness is 6-9 MPa·m 1 / 2 , the flexural strength is 1000~1400 MPa.

2. The method for preparing the α-phase silicon nitride according to claim 1, It is characterized in that The steps include: (1) α-Si 3 N 4 The powder is pre-ball-milled to obtain a slurry; a sintering aid dispersion is prepared; the slurry and the sintering aid dispersion are mixed and ball-milled again; the slurry obtained after ball milling is dried, deoxidized, and sieved to obtain α-Si 3 N 4 -Al 2 O 3 -Y 2 O 3 Mixed powder; (2) The α-Si 3 N 4 -Al 2 O 3 -Y 2 O 3 The mixed powder is placed in an oscillating pressure furnace for vacuum pre-sintering to obtain a pre-sintered green body; (3) Under a non-oxygen-containing atmosphere, applying axial pressure to the pre-sintered green body and performing spark plasma sintering, and then cooling the green body to obtain the α-phase silicon nitride.

3. The method for preparing α-phase silicon nitride according to claim 2, It is characterized in that Step (1) the α-Si 3 N 4 The specific operation of pre-milling the powder includes: using anhydrous ethanol as a ball-milling aid and silicon nitride balls as a ball-milling medium to grind α-Si 3 N 4 The powder was ball milled for 1-4 h.

4. The method for preparing α-phase silicon nitride according to claim 2, It is characterized in that The specific operation of preparing the sintering aid dispersion in step (1) includes: dispersing the sintering aid in anhydrous ethanol to obtain a dispersion; The sintering aid in the sintering aid dispersion in step (1) is Al 2 O 3 and Y 2 O 3 ; Step (1) the α-Si 3 N 4 The particle size of the powder is 10 to 900 nm.

5. The method for preparing α-phase silicon nitride according to claim 4, It is characterized in that The α-Si 3 N 4 Powder, Al 2 O 3 and Y 2 O 3 The mass ratio is (84~98):(1~8):(1~8).

6. The method for preparing α-phase silicon nitride according to claim 2, It is characterized in that In step (1), the slurry and the sintering aid dispersion are stirred under ultrasonic conditions for 0.5 to 2 h to mix them evenly, and then ball milled again.

7. The method for preparing α-phase silicon nitride according to any one of claims 2 to 5, It is characterized in that The specific operation of deoxygenation in step (1) includes: placing the dried sample in a tube furnace, and introducing a hydrogen atmosphere at 600° C. to remove oxygen in the powder.

8. The method for preparing α-phase silicon nitride according to claim 2, It is characterized in that The vacuum pre-sintering conditions in step (2) are as follows: constant pressure of 30-80 MPa, oscillation pressure of 2-8 MPa, oscillation frequency of 1-5 Hz, pre-sintering temperature of 600-800 °C, and holding time of 1-2 h.

9. The method for preparing α-phase silicon nitride according to claim 2, It is characterized in that The axial pressure in step (3) is 80 to 500 MPa; The spark plasma sintering conditions in step (3) are as follows: sintering temperature is 1100-1500°C, heating rate is 5-500°C / min, and holding time is 2-180 min; The non-oxygen-containing atmosphere in step (3) is a vacuum condition or a protective gas atmosphere; the protective gas atmosphere is one or more of nitrogen, helium, argon and neon.

10. Use of the α-phase silicon nitride according to claim 1 in preparing cutting tools, corrosion-resistant components and wear-resistant parts.

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