Preparation method of low-temperature rapid densification silicon nitride ceramic based on aluminum oxide-yttrium oxide composite sintering aid

Abstract

The invention discloses a preparation method of low-temperature rapid densification silicon nitride ceramic based on an aluminum oxide-yttrium oxide composite sintering aid, and aims to solve the problems of high temperature, high energy consumption, difficult densification and the like in the existing Si3N4 ceramic preparation. The preparation method comprises the following steps: 1, mixing alpha-Si3N4 powder, Y2O3 powder and Al2O3 powder, and carrying out ball milling dispersion to obtain mixed powder; 2, carrying out rotary evaporation drying on the mixed powder under a vacuum condition; 3, carrying out heat preservation treatment under the conditions of 1600-1700 DEG C and 40-60 MPa through a spark plasma sintering process; and 4, demolding after cooling. Under the action of the Al2O3-Y2O3 composite sintering aid, phase transformation from alpha-Si3N4 to beta-Si3N4 is remarkably promoted, the relative density of the ceramic reaches 97%-99%, the fracture toughness of the ceramic is 5.21 MPa.m < 1 / 2 >, and the bending strength of the ceramic is 924.99 MPa.

Description

Technical Field

[0001] This invention belongs to the field of advanced ceramic material preparation, specifically relating to a method for preparing Si3N4 ceramics by using composite oxide sintering aids and spark plasma sintering (SPS) technology to achieve rapid densification at low temperatures. It is particularly suitable for obtaining Si3N4 structural ceramics with high density, high strength and high toughness at lower temperatures. Background Technology

[0002] Si3N4 ceramics are widely used in aerospace, machinery, energy, and biomedical fields due to their excellent mechanical properties, thermal stability, and biocompatibility. However, due to its strong covalent bond characteristics, the sintering and densification process of Si3N4 presents significant challenges, typically requiring high temperatures above 1800℃ and prolonged pressure holding to achieve sufficient densification. This not only results in high energy consumption but also easily leads to abnormal grain growth and degradation of mechanical properties.

[0003] In existing technologies, adding metal oxides or rare earth oxides as sintering aids can promote liquid-phase sintering and increase density to some extent, but it often reduces the bioinertness and chemical stability of the material. In addition, traditional hot pressing or gas pressure sintering processes are time-consuming, high-temperature, and inefficient, making it difficult to meet the modern manufacturing demand for low-energy-consumption, high-efficiency, and high-performance ceramic materials.

[0004] Therefore, developing a new method for preparing Si3N4 ceramics that can be rapidly densified at lower temperatures, produce uniform grains, and exhibit excellent performance has become a key research focus in this field. Summary of the Invention

[0005] This invention aims to overcome the problems of high temperature, high energy consumption and densification difficulties in the existing Si3N4 ceramic preparation, and provides a low-temperature rapid densification preparation method using Al2O3-Y2O3 composite sintering aid combined with SPS process to improve the density and mechanical properties of ceramics.

[0006] The present invention provides a method for preparing low-temperature rapid densification of silicon nitride ceramics based on an alumina-yttrium oxide (Al2O3-Y2O3) composite sintering aid, which is implemented according to the following steps:

[0007] Step 1: Mix 91-95 wt.% α-Si3N4 powder, 4-7 wt.% Y2O3 powder and 1-4 wt.% Al2O3 powder by mass percentage, and disperse by ball milling with anhydrous ethanol as the dispersion medium to obtain mixed powder.

[0008] Step 2: Dry the mixed powder under vacuum by rotary evaporation, and then sieve it to obtain the raw material to be sintered;

[0009] Step 3: Pour the raw material to be sintered into the mold, and then place it in the Spark Plasma Sintering (SPS) apparatus. Under vacuum conditions, perform heat treatment at 1600~1700℃ and 40~60 MPa to obtain a densified Si3N4 ceramic green body.

[0010] Step 4: After cooling to room temperature, demold to obtain dense silicon nitride ceramic (main phase is β-Si3N4).

[0011] This invention utilizes nano-scale Al2O3 and Y2O3 composite additives as sintering aids, and employs spark plasma sintering (SPS) at a low temperature of 1600℃~1700℃ for rapid sintering, achieving high densification of Si3N4 ceramics in just 600 s. Experiments show that the Al2O3-Y2O3 composite sintering aid significantly promotes the phase transformation from α-Si3N4 to β-Si3N4, resulting in a relative density of 97%~99% and a fracture toughness of 5.21 MPa·m. 1 / 2 The flexural strength is 924.99 MPa, and the compressive strength is as high as 3769.98 MPa. Compared with the traditional 1800℃ hot pressing sintering, this process significantly reduces temperature and energy consumption, and the resulting ceramic grains are fine and uniform, possessing excellent comprehensive mechanical properties and structural stability. This invention has the advantages of simple process, low energy consumption, and superior performance, and can be widely used in high-temperature structural materials, cutting tools, and biomedical fields.

[0012] Compared with traditional Si3N4 without sintering aids, the low-temperature rapid densification method for silicon nitride ceramics based on Al2O3-Y2O3 composite sintering aids of this invention has the following beneficial effects:

[0013] 1. Low-temperature rapid sintering: Near-full densification can be achieved at 1600℃, with a relative density of over 97%, resulting in significant energy savings;

[0014] 2. Uniform and refined microstructure: Al2O3-Y2O3 sintering aid forms a liquid phase, promotes the α→β phase transformation, and obtains slender β-Si3N4 grains;

[0015] 3. Significantly improved performance: compressive strength increased by 40 times, flexural strength increased by 30 times, and fracture toughness increased by 4 times;

[0016] 4. High controllability: Stable process parameters and good repeatability make it suitable for mass production;

[0017] 5. Wide range of applications: The obtained Si3N4 ceramics have high strength, high toughness and good biocompatibility, making them suitable for high-temperature structural components and medical implants. Attached Figure Description

[0018] Figure 1 The images show the mixing and dispersion morphology of silicon nitride and sintering aid powders at different ball milling speeds in the examples.

[0019] Figure 2 The following is a comparison of the macro- and micro-structures of Si3N4 under SPS sintering at 1600℃ / 50MPa with and without Al2O3-Y2O3 sintering aid in the examples.

[0020] Figure 3 The image shows the morphology of the α-Si3N4 to β-Si3N4 phase transformation after sintering with the addition of sintering aids in the example.

[0021] Figure 4 The microstructure and elemental distribution of silicon nitride after sintering at 1700℃ / 50MPa with added sintering aids are shown in the examples.

[0022] Figure 5 The image shows the microstructure and grain growth characteristics of silicon nitride after sintering at 1800℃ / 50MPa with added sintering aids in the example. Detailed Implementation

[0023] Specific Implementation Method 1: This implementation method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aids is carried out according to the following steps:

[0024] Step 1: Mix 91-95 wt.% α-Si3N4 powder, 4-7 wt.% Y2O3 powder and 1-4 wt.% Al2O3 powder by mass percentage, and disperse by ball milling with anhydrous ethanol as the dispersion medium to obtain mixed powder.

[0025] Step 2: Dry the mixed powder under vacuum by rotary evaporation, and then sieve it to obtain the raw material to be sintered;

[0026] Step 3: Pour the raw material to be sintered into the mold, and then place it in the Spark Plasma Sintering (SPS) apparatus. Under vacuum conditions, perform heat treatment at 1600~1700℃ and 40~60 MPa to obtain a densified Si3N4 ceramic green body.

[0027] Step 4: After cooling to room temperature, demold to obtain dense silicon nitride ceramic (main phase is β-Si3N4).

[0028] In this embodiment, the Al2O3-Y2O3 composite sintering aid forms a eutectic liquid phase during sintering, promoting particle rearrangement and diffusion, facilitating the phase transformation from α-Si3N4 to β-Si3N4, and improving grain boundary bonding strength. The resulting densified Si3N4 ceramic green body has a relative density ≥97%, with β-Si3N4 as the main crystalline phase. The grains in this phase exhibit long rod-like morphology and possess excellent densification, compressive strength, and fracture toughness. The compressive strength of the Si3N4 ceramic is 3769.98 ± 177.50 MPa, the flexural strength is 924.99 ± 26.86 MPa, and the fracture toughness is 5.21 ± 0.12 MPa·m. 1 / 2 .

[0029] The dense silicon nitride ceramics prepared in this embodiment are suitable for high-temperature structural components, cutting tools, and biomedical implants.

[0030] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the average particle size of the α-Si3N4 powder, Y2O3 powder, and Al2O3 powder in step one is 300~800 nm.

[0031] Specific Implementation Method 3: This implementation method differs from Specific Implementation Method 1 or 2 in that the ball milling speed is controlled at 300 rpm and the ball milling time is 8 hours in step 1.

[0032] Under these conditions, this embodiment can achieve uniform dispersion of Al2O3 and Y2O3 and avoid agglomeration.

[0033] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that in step one, 93~94 wt.% of α-Si3N4 powder, 4~6 wt.% of Y2O3 powder and 1.5~2.5 wt.% of Al2O3 powder are mixed by mass percentage.

[0034] Specific Implementation Method 5: This implementation method differs from Specific Implementation Method 4 in that in step one, 93 wt.% of α-Si3N4 powder, 5 wt.% of Y2O3 powder, and 2 wt.% of Al2O3 powder are mixed by mass percentage.

[0035] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the temperature for rotary drying in step two is 70°C.

[0036] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the raw material to be sintered is obtained after passing through a 100-mesh sieve in step two.

[0037] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that step three involves heat treatment at 1600~1700℃ and 50 MPa for 600~650 s.

[0038] In this embodiment, a density of 97% can be obtained at 1600°C. If the temperature is increased to 1700°C, the relative density reaches 99%. However, when the temperature exceeds 1800°C, abnormal grain growth and increased porosity will occur.

[0039] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One through Eight in that, during the plasma treatment process in step Three, the heating rate is controlled at 100℃·min. -1 .

[0040] In this embodiment, the cooling rate is controlled at 30°C·min. -1 .

[0041] Specific Implementation Method 10: This implementation method differs from Specific Implementation Methods 1 to 9 in that the density of the densified silicon nitride ceramic obtained in step 4 is greater than 97%.

[0042] Example 1: The preparation method of low-temperature rapid densification of silicon nitride ceramics based on Al2O3-Y2O3 composite sintering aid in this example is carried out according to the following steps:

[0043] Step 1: Mix 93 wt.% α-Si3N4 powder (α phase content 94.4%, purity ≥99.9%, average particle size 600 nm), 5 wt.% Y2O3 powder (purity ≥99.9%, average particle size 500 nm), and 2 wt.% Al2O3 powder (purity ≥99.9%, average particle size 500 nm) by mass percentage. Use anhydrous ethanol as the dispersion medium and Si3N4 balls as the grinding medium. Perform ball milling at 300 rpm for 8 hours to obtain the mixed powder.

[0044] Step 2: Dry the mixed powder under vacuum at 70℃ with a rotation speed of 35 rpm to prevent powder agglomeration, and then pass it through a 100-mesh sieve to obtain a uniform raw material to be sintered.

[0045] Step 3: Pour the raw material to be sintered into the mold, and then place it in the spark plasma sintering (SPS-211Lx, Japan) apparatus. Under vacuum, it is subjected to heat treatment at 1600℃ and 50 MPa for 600 s. During the plasma treatment, the heating rate is controlled at 100℃ / min and the cooling rate is controlled at 30℃ / min to obtain a dense Si3N4 ceramic green body with a dark gray color.

[0046] Step 4: After cooling to room temperature, demold to obtain dense silicon nitride ceramic (main phase is β-Si3N4).

[0047] The relative density, measured by the Archimedes method, is 97%. The compressive strength is 3769.98 MPa, the flexural strength is 924.99 MPa, and the fracture toughness is 5.21 MPa·m. 1 / 2 Compared with the sample without sintering aids (relative density 50%, flexural strength 32.33 MPa), the overall performance is significantly improved.

[0048] Example 2: This example differs from Example 1 in that the sintering temperatures in step three are set to 1600℃, 1700℃ and 1800℃ respectively, while the other process parameters of plasma treatment are the same as in Example 1.

[0049] In this embodiment, when the sintering temperatures were 1600℃, 1700℃, and 1800℃, the density of the prepared samples was 97%, 99%, and 98%, respectively. When the temperature increased from 1600℃ to 1700℃, the relative density increased from 97% to 99%, the flexural strength increased from 924.99 MPa to 950 MPa, and the fracture toughness increased from 5.21 MPa·m. 1 / 2 Increased to 5.35 MPa·m 1 / 2 When the temperature is further increased to 1800℃, the grains grow abnormally and the porosity increases, resulting in a slight decrease in mechanical properties.

[0050] Example 3: This example differs from Example 1 in that the ball milling speed is set to 100 rpm, 300 rpm, and 500 rpm in step one, and the time is 8 hours for each step. The ball-to-material ratio and drying conditions are the same as in Example 1.

[0051] Figure 1 The images show the mixing and dispersion morphology of Si3N4, Al2O3, and Y2O3 powders at different ball milling speeds. It can be seen that as the ball milling speed increases from 100 rpm to 300 rpm, powder agglomeration gradually decreases, and the additive distribution becomes more uniform. When the speed is further increased to 500 rpm, although the particle size decreases, the excessively high surface energy leads to re-agglomeration. In summary, 300 rpm is the optimal ball milling condition, ensuring uniform dispersion of the sintering aids and subsequent densification.

[0052] When the ball milling speed is 100 rpm, the powder agglomeration is obvious, and the density after sintering is only 89%. When the speed is 300 rpm, the powder distribution is uniform, and the relative density of the ceramic reaches 97%. When the speed is 500 rpm, the powder particle size decreases to 100–200 nm, and the excessively high surface energy leads to re-agglomeration. After sintering, the porosity increases, and the density decreases to 94%.

[0053] Comparative Example: This example differs from Example 1 in that pure Si3N4 powder is used in step one.

[0054] The comparative example sample had a relative density of only 50% and a microstructure resembling powder accumulation; the sample obtained in Example 1 had a relative density increased to 97% and a fracture toughness improved to 5.21 MPa·m. 1 / 2 SEM showed that the B group had dense and uniformly distributed grains, with the grain boundaries forming a glassy phase containing Y, Al, Si, and O.

[0055] The high-density Si3N4 ceramic prepared according to Example 1 was used as a sample with dimensions of 3 mm × 4 mm × 20 mm.

[0056] Cutting experiments with Ti alloy at 800℃ showed a 35% reduction in wear compared to conventional Si3N4, and a 20% reduction in surface roughness. Immersion in simulated physiological fluid for 30 days resulted in no corrosion products, indicating that the ceramic possesses excellent bioinertness.

[0057] Figure 2 The images show a comparison of the macro- and micro-structures of Si3N4 sintered at 1600℃ / 50 MPa SPS with and without the Al2O3-Y2O3 sintering aid. It can be seen that the sample without the aid has a rough surface, high porosity, and loose grain bonding; while the sample with the aid has a dense and smooth surface and tightly bonded grains. The Al2O3-Y2O3 composite aid significantly promotes liquid phase formation and particle rearrangement, achieving a highly dense structure at low temperatures.

[0058] Figure 3 The images show the morphology of the α-Si3N4 → β-Si3N4 phase transformation after the addition of sintering aids. It can be seen that α-Si3N4 exhibits an equiaxed grain morphology, while the β-Si3N4 formed after sintering presents as long columnar grains with tight grain boundary bonding. This transformation indicates that the composite aids promote the reconstruction and growth of the α→β phase during sintering, which is beneficial for crack deflection and toughness improvement.

[0059] Figure 4 The microstructure and elemental distribution of silicon nitride sintered at 1700 °C / 50 MPa with added sintering aids are shown in the image. Elemental mapping reveals that Y, Al, and O are uniformly distributed in the grain boundary regions, forming a Y–Al–Si–O eutectic liquid phase. The fracture morphology shows a clear sintering neck bonded to dense grains. This indicates that the liquid phase of the aid effectively promotes diffusion and grain boundary bonding, significantly improving density and mechanical properties.

[0060] Figure 5Microstructure and grain growth characteristics of silicon nitride sintered at 1800 °C / 50 MPa with sintering aids added. At high temperatures, grains grow significantly, accompanied by abnormal coarsening and pore formation, with some areas exhibiting a "tinydimple" characteristic. Excessively high sintering temperatures lead to excessive liquid phase and abnormal grain growth, resulting in a slight decrease in mechanical properties. The optimal sintering temperature range is 1600–1700 °C.

[0061] In summary, this invention achieves rapid densification of Si3N4 ceramics at low temperatures (1600℃) through the synergistic effect of Al2O3-Y2O3 composite sintering aid and spark plasma sintering technology. Results from various embodiments demonstrate that the process of this invention can stably yield ceramics with a relative density ≥97% and a fracture toughness ≥5 MPa·m. 1 / 2 The high-performance ceramics have a sintering temperature that is about 200°C lower than that of traditional methods. The process is simple and energy consumption is low, and they have significant potential for industrial applications.

Claims

1. A method for preparing low-temperature rapid densification of silicon nitride ceramics based on an alumina-yttrium oxide composite sintering aid, characterized in that... The method for preparing low-temperature rapid densification of silicon nitride ceramics is carried out according to the following steps: Step 1: Mix 91-95 wt.% α-Si3N4 powder, 4-7 wt.% Y2O3 powder and 1-4 wt.% Al2O3 powder by mass percentage, and disperse by ball milling with anhydrous ethanol as the dispersion medium to obtain mixed powder. Step 2: Dry the mixed powder under vacuum by rotary evaporation, and then sieve it to obtain the raw material to be sintered; Step 3: Pour the raw material to be sintered into the mold, and then place it in the spark plasma sintering device. Under vacuum, it is subjected to heat treatment at 1600~1700℃ and 40~60 MPa to obtain a densified Si3N4 ceramic green body. Step 4: After cooling to room temperature, demold to obtain dense silicon nitride ceramic.

2. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... In step one, the average particle size of α-Si3N4 powder, Y2O3 powder and Al2O3 powder is 300~800 nm.

3. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... In step one, the ball mill speed is controlled at 300 rpm and the ball milling time is 8 hours.

4. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... In step one, 93-94 wt.% of α-Si3N4 powder, 4-6 wt.% of Y2O3 powder and 1.5-2.5 wt.% of Al2O3 powder are mixed by mass percentage.

5. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 4, characterized in that... In step one, 93 wt.% of α-Si3N4 powder, 5 wt.% of Y2O3 powder and 2 wt.% of Al2O3 powder are mixed by mass percentage.

6. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... The temperature for rotary drying in step two is 70℃.

7. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... In step two, the raw material to be sintered is obtained after passing through a 100-mesh sieve.

8. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... In step three, the temperature is maintained at 1600~1700℃ and 50 MPa for 600~650 s.

9. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on an alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... In step three, the temperature rise rate is controlled at 100℃·min during the plasma treatment process. -1 .

10. The method for preparing low-temperature rapid densification of silicon nitride ceramics based on alumina-yttrium oxide composite sintering aid according to claim 1, characterized in that... The density of the densified silicon nitride ceramic obtained in step four is greater than 97%.

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