High-toughness silicon nitride ceramic and low-temperature preparation method and application thereof

Abstract

The application belongs to the technical field of ceramic material production, and discloses a high-toughness silicon nitride ceramic and a low-temperature preparation method and application thereof. The method comprises the following steps: mixing alpha-Si3N4 powder, Re2O3 powder, MgO powder, HfN powder and anhydrous ethanol through ball milling, drying, pre-pressing in a graphite mold, placing the graphite mold in a discharge plasma sintering furnace, keeping vacuum state, heating to 800-1000 DEG C, introducing nitrogen, heating to 1600-1700 DEG C under 1atm nitrogen atmosphere, applying 30-50 MPa axial pressure, keeping for 5-15 min, then cooling to 600-800 DEG C, and finally naturally cooling to room temperature to obtain the high-toughness silicon nitride ceramic. The application realizes toughening by HfN as a second phase to promote silicon nitride phase change at low temperature, and realizes low-temperature preparation of the high-toughness silicon nitride ceramic. The silicon nitride ceramic can be used in the fields of cutting tools and engine parts.

Description

Technical Field

[0001] This invention belongs to the field of ceramic material production technology, and specifically relates to a high-toughness silicon nitride ceramic and its low-temperature preparation method and application. Background Technology

[0002] Silicon nitride (Si3N4) is considered an ideal material for key components of next-generation high-performance engines (such as turbine rotors and bearing rings) and advanced cutting tools due to its superior overall properties. The high hardness, low density, and excellent thermal stability of silicon nitride ensure the dimensional stability and durability of components under high-speed and high-temperature conditions. Among all its properties, high fracture toughness is crucial for ensuring the structural integrity of engine components under extreme mechanical loads and thermal shocks, and for preventing chipping of cutting tools during unstable machining processes such as intermittent cutting. It directly determines the service life and reliability of components and tools.

[0003] However, existing high-performance products heavily rely on high-temperature sintering technology exceeding 1800℃, which suffers from high energy consumption, high cost, and abnormal grain growth. For example, Chinese patent application CN113480319A discloses a method for obtaining high-performance silicon nitride ceramics with a flexural strength of up to 830MPa at 1800℃~2000℃, but the process cost is high. To reduce energy consumption and cost, existing research attempts to lower the sintering temperature by optimizing the composite sintering aid system. However, lowering the temperature often leads to new technical contradictions: in the low-temperature range below 1700℃, the sintering driving force is significantly insufficient. This results in poor densification of the ceramic body, and residual micropores severely degrade its thermal conductivity and electrical insulation strength. On the other hand, and more importantly, the low temperature greatly inhibits the transformation kinetics of silicon nitride from the equiaxed α phase to the rod-shaped β phase and the full growth of β grains. This microstructure composed of interlocking rod-shaped β grains is the fundamental source of the high toughness of silicon nitride ceramics. Therefore, silicon nitride ceramics prepared by existing low-temperature processes typically have a fracture toughness that is difficult to exceed 8 MPa·m. 1 / 2 It cannot meet the stringent reliability requirements of high-end applications.

[0004] Therefore, it is crucial to develop new methods for preparing silicon nitride ceramics that combine low-temperature processing and high toughness, which has become a key technical bottleneck that needs to be addressed to promote the large-scale application of silicon nitride ceramics in engine components and cutting tools. Summary of the Invention

[0005] In order to overcome the shortcomings and disadvantages of the existing technology, the primary objective of this invention is to provide a low-temperature preparation method for high-toughness silicon nitride ceramics.

[0006] Another objective of this invention is to provide a high-toughness silicon nitride ceramic prepared by the above-described low-temperature preparation method; the ceramic has excellent mechanical properties.

[0007] Another object of the present invention is to provide applications of the above-mentioned high-toughness silicon nitride ceramics.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A low-temperature preparation method for high-toughness silicon nitride ceramics includes the following steps: α-Si3N4 powder, Re2O3 powder, MgO powder, HfN powder, and anhydrous ethanol are ball-milled and mixed, dried, and then pre-pressed into a graphite mold. The graphite mold is then placed in a spark plasma sintering furnace, and nitrogen gas is introduced at 800–1000°C under vacuum. Under a nitrogen atmosphere of 1 atm, the temperature is raised to 1600–1700°C while applying an axial pressure of 30–50 MPa, and held for 5–15 minutes. The temperature is then lowered to 600–800°C, and finally allowed to cool naturally to room temperature to obtain high-toughness silicon nitride ceramics. The amount of HfN powder used is 10–20% of the total volume of α-Si3N4 powder, MgO powder, and Re2O3 powder.

[0010] The resulting silicon nitride ceramics have a relative density of over 97%, a flexural strength of 660–800 MPa, and a fracture toughness of 7.5–9 MPa·m. 1 / 2 .

[0011] The α-Si3N4 powder has a purity of ≥98wt% and a particle size of 0.3~1µm; the MgO powder has a purity of ≥99.8wt% and a particle size of 50~100nm; the Re2O3 powder contains Re as La, Lu, Ce, Sm, Yb or Ho, and has a purity of ≥99.8wt% and a particle size of 50~100nm; the HfN powder has a purity of ≥99.9wt% and a particle size of 1~3µm.

[0012] The volume ratio of the α-Si3N4 powder, MgO powder and Re2O3 powder is (90~95):(2~5):(3~5).

[0013] The above-mentioned low-temperature preparation method for high-toughness silicon nitride ceramics specifically includes the following steps:

[0014] S1. Mix and ball-mill α-Si3N4 powder, MgO powder, Re2O3 powder, HfN powder and anhydrous ethanol, then dry by rotary evaporation, and sieve after complete drying to obtain ceramic powder;

[0015] S2. Place the ceramic powder obtained in step S1 into a graphite mold in a spark plasma sintering furnace and pre-press it using a manual hydraulic press.

[0016] S3. Next, the graphite mold of the spark plasma sintering furnace is placed into the spark plasma sintering furnace, and the temperature is raised to 800-1000°C under vacuum. Nitrogen gas is introduced, and the temperature is raised to 1600-1700°C under a nitrogen atmosphere of 1 atm. At the same time, an axial pressure of 30-50 MPa is applied and the temperature is held for 5-15 minutes. Then the temperature is lowered to 600-800°C, and finally the high-toughness silicon nitride ceramic is obtained by natural cooling to room temperature.

[0017] The ball milling speed in step S1 is 250~300 r / min, and the ball milling time is 18~24 h; the rotary evaporation drying temperature is 45~55℃; the sieving is done through a 100~120 mesh sieve.

[0018] The pre-compression pressure in step S2 is 20~30MPa, and the pressure holding time is 2~5min.

[0019] The heating rate in step S3 is 100~150℃ / min, and the cooling rate is 100℃ / min.

[0020] A high-toughness silicon nitride ceramic prepared by the above-described low-temperature preparation method.

[0021] The above-mentioned high-toughness silicon nitride ceramics are used in cutting tools or engine components.

[0022] The present invention has the following advantages and effects compared with the prior art:

[0023] (1) The present invention creatively selects HfN as the second reinforcing phase and adds it to the silicon nitride matrix. At low temperature, it can significantly promote the phase transformation of equiaxed α-Si3N4 to long rod-shaped β-Si3N4. The generated long rod-shaped β-Si3N4 intertwines with each other to form a skeleton structure and improves fracture toughness. At the same time, it optimizes the interfacial energy during the sintering process and creates favorable conditions for the anisotropic growth (growth along the c-axis) of β-Si3N4 grains.

[0024] (2) The present invention significantly reduces the sintering temperature of high-performance silicon nitride ceramics from the traditional 1800℃ or above to 1600~1700℃; this low-temperature process not only greatly reduces energy consumption and equipment requirements, but also avoids the problem of abnormal grain growth that may be caused by high temperature.

[0025] (3) The silicon nitride ceramic obtained by this invention can achieve high density (>97%) at a relatively low sintering temperature and exhibits excellent properties: flexural strength (660~800MPa) and fracture toughness (7.5~9 MPa·m). 1 / 2 ). Attached Figure Description

[0026] Figure 1 This is a microstructure diagram of the silicon nitride ceramic prepared in Example 1 of the present invention.

[0027] Figure 2 This is a microstructure diagram of the silicon nitride ceramic prepared in Comparative Example 1 of this invention.

[0028] Figure 3 This is a schematic diagram of the low-temperature promotion of silicon nitride phase transformation by the second reinforcing phase HfN of the present invention. It can be seen that: when α-silicon nitride and sintering aids (MgO and Re2O3) are used as raw materials for low-temperature sintering, the sintered product obtained without the addition of the second reinforcing phase HfN is mainly composed of equiaxed α-silicon nitride, with only a small amount of high-toughness long rod-shaped β-silicon nitride. However, when the second reinforcing phase HfN is added, a large amount of long rod-shaped β-silicon nitride is generated in the sintered product.

[0029] Figure 4 This is a microstructure diagram of the silicon nitride ceramic prepared in Comparative Example 3 of this invention.

[0030] Figure 5 This is a microstructure diagram of the silicon nitride ceramic prepared in Comparative Example 4 of this invention.

[0031] Figure 6 This is a microstructure diagram of the silicon nitride ceramic prepared in Comparative Example 5 of this invention. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments, but these should not be construed as limiting the invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.

[0033] The α-Si3N4 powder used in the following examples has a purity of ≥98wt% and a particle size of 0.3~1µm; MgO powder has a purity of ≥99.8wt% and a particle size of 50~100nm; Re2O3 powder, wherein Re is La, Lu, Ce, Sm, Yb or Ho, has a purity of ≥99.8wt% and a particle size of 50~100nm; and HfN powder has a purity of ≥99.9wt% and a particle size of 1~3µm.

[0034] Example 1

[0035] S1. α-Si3N4 powder, Yb2O3 powder, MgO powder, HfN powder, and anhydrous ethanol were mixed and ball-milled at 300 r / min for 24 h, followed by rotary evaporation drying at 50 °C. After complete drying, the mixture was passed through a 100-mesh sieve to obtain ceramic powder. The volume ratio of α-Si3N4 powder, MgO powder, and Yb2O3 powder was 95:2:3, and the amount of HfN powder was 15% of the total volume of α-Si3N4 powder, MgO powder, and Yb2O3 powder.

[0036] S2. Place the ceramic powder obtained in step S1 into a graphite mold in a spark plasma sintering furnace, apply an axial pressure of 20MPa using a manual hydraulic press and hold the pressure for 5 minutes to perform pre-pressing molding;

[0037] S3. Next, the graphite mold of the spark plasma sintering furnace is placed in the spark plasma sintering furnace and heated to 800°C at a rate of 150°C / min while maintaining a vacuum. Nitrogen gas is introduced and heated to 1650°C at a rate of 100°C / min under a nitrogen atmosphere of 1 atm, while applying an axial pressure of 30 MPa. The temperature is held for 10 min, and then cooled to 600°C at a rate of 100°C / min. Finally, it is allowed to cool naturally to room temperature to obtain high-toughness silicon nitride ceramic.

[0038] The microstructure of the silicon nitride ceramic prepared in this embodiment is as follows: Figure 1 As shown, the α→β-Si3N4 phase transformation is extensive, with a large number of long rod-shaped β-Si3N4 particles. Its relative density is 98.8%, its flexural strength is 789 MPa, and its fracture toughness is 8.1 MPa·m. 1 / 2 The hardness is 17.8 GPa, indicating that this silicon nitride ceramic possesses excellent mechanical properties.

[0039] Example 2

[0040] The other steps are the same as in Example 1, except that step S3 is different. Specifically, the graphite mold of the spark plasma sintering furnace is then placed into the spark plasma sintering furnace and heated to 800°C at a rate of 150°C / min while maintaining a vacuum. Nitrogen gas is introduced and heated to 1700°C at a rate of 100°C / min under a nitrogen atmosphere of 1 atm, while applying an axial pressure of 30 MPa. The temperature is held for 10 min, and then cooled to 600°C at a rate of 100°C / min. Finally, the temperature is allowed to cool naturally to room temperature to obtain a high-toughness silicon nitride ceramic.

[0041] The silicon nitride ceramic prepared in this embodiment has a relative density of 98.7%, a flexural strength of 738 MPa, and a fracture toughness of 7.9 MPa·m. 1 / 2 Its hardness is 16.9 GPa.

[0042] Example 3

[0043] The other steps are the same as in Example 1, except that in step S1, the amount of HfN powder used is 10% of the total volume of α-Si3N4 powder, MgO powder and Yb2O3 powder.

[0044] The silicon nitride ceramic prepared in this embodiment has a relative density of 97.2%, a flexural strength of 671 MPa, and a fracture toughness of 7.5 MPa·m. 1 / 2 Its hardness is 17.2 GPa.

[0045] Example 4

[0046] The other steps are the same as in Example 1, except that in step S1, the amount of HfN powder used is 20% of the total volume of α-Si3N4 powder, MgO powder and Yb2O3 powder.

[0047] The silicon nitride ceramic prepared in this embodiment has a relative density of 98.1%, a flexural strength of 698 MPa, and a fracture toughness of 7.9 MPa·m. 1 / 2 Its hardness is 17 GPa.

[0048] Examples 1-4 demonstrate that the silicon nitride ceramics prepared according to this invention have a relative density of 97-99%, flexural strength (660-800 MPa), and fracture toughness (7.5-9 MPa·m). 1 / 2 The hardness (16~18 GPa) indicates that the silicon nitride ceramics prepared by the method of the present invention have excellent mechanical properties.

[0049] Comparative Example 1

[0050] The other steps are the same as in Example 1, except that HfN powder is not added in step S1.

[0051] The microstructure of the silicon nitride ceramic prepared in this comparative example is as follows: Figure 2 As shown, in conjunction with a comparison with Example 1, and a schematic diagram of the effect of adding HfN on promoting the silicon nitride phase transition at low temperature (e.g.) Figure 3 As shown in the figure, this comparative example is dominated by low-toughness equiaxed α-Si3N4, with only a small amount of high-toughness long rod-shaped β-Si3N4. This leads to a decrease in the flexural strength and fracture toughness of the Si3N4 in this comparative example, while the presence of more α-Si3N4 increases its hardness. Its relative density is 97.2%, flexural strength is 652 MPa, and fracture toughness is 7.1 MPa·m. 1 / 2 Its hardness is 20.1 GPa.

[0052] Comparative Example 2

[0053] The other steps are the same as in Example 1, except that HfN powder is not added in step S1, and step S3 is also different. Step S3 is as follows: The graphite mold of the spark plasma sintering furnace is then placed in the spark plasma sintering furnace, and the temperature is raised to 800°C at a rate of 150°C / min while maintaining a vacuum. Nitrogen gas is introduced, and the temperature is raised to 1850°C at a rate of 100°C / min under a nitrogen atmosphere of 1 atm, while applying an axial pressure of 30 MPa. The temperature is held for 10 min, and then the temperature is lowered to 600°C at a rate of 100°C / min. Finally, the temperature is allowed to cool naturally to room temperature to obtain silicon nitride ceramic.

[0054] In this comparative example, the silicon nitride ceramic exhibited excessive growth of long rod-shaped β-Si3N4 due to increased temperature, resulting in grain crowding. This led to a decrease in the flexural strength and fracture toughness of the Si3N4 in this comparative example. Its relative density was 98.8%, flexural strength was 622 MPa, and fracture toughness was 6.9 MPa·m. 1 / 2 Its hardness is 17.8 GPa.

[0055] Comparative Example 3

[0056] The other steps are the same as in Example 1, except that the HfN powder added in step S1 is replaced with TiN powder (purity above 99.9wt%, particle size 0.7~2µm), and the amount of TiN powder added is still 15% of the total volume of α-Si3N4 powder, MgO powder and Yb2O3 powder.

[0057] The microstructure of the silicon nitride ceramic prepared in this comparative example is as follows: Figure 4 As shown, the addition of TiN did not promote the α→β-Si3N4 phase transformation as HfN did. The prepared silicon nitride ceramics were mainly composed of low-toughness, fine, equiaxed α-Si3N4, with less high-toughness, long rod-shaped β-Si3N4. This resulted in a decrease in the flexural strength and fracture toughness of the Si3N4 in this comparative example, while the abundance of α-Si3N4 increased its hardness. Its relative density was 98.2%, flexural strength was 408 MPa, and fracture toughness was 5.1 MPa·m. 1 / 2 Its hardness is 20.1 GPa.

[0058] Comparative Example 4

[0059] The other steps are the same as in Example 1, except that the amount of HfN powder used in step S1 is 5% of the total volume of α-Si3N4 powder, MgO powder and Yb2O3 powder.

[0060] The microstructure of the silicon nitride ceramic prepared in this comparative example is as follows: Figure 5As shown, the addition of a small amount of HfN did not significantly promote the α→β-Si3N4 phase transformation, resulting in the dominance of low-toughness, fine, equiaxed α-Si3N4. This led to a decrease in the flexural strength and fracture toughness of the Si3N4 in this comparative example, while the presence of more α-Si3N4 increased its hardness. Its relative density was 99.3%, its flexural strength was 567 MPa, and its fracture toughness was 6.4 MPa·m. 1 / 2 Its hardness is 20.6 GPa.

[0061] Comparative Example 5

[0062] The other steps are the same as in Example 1, except that the amount of HfN powder used in step S1 is 35% of the total volume of α-Si3N4 powder, MgO powder and Yb2O3 powder.

[0063] The microstructure of the silicon nitride ceramic prepared in this comparative example is as follows: Figure 6 As shown, the high content of HfN inhibits the diffusion of the intergranular liquid phase in Si3N4 ceramics, thereby limiting the α→β-Si3N4 phase transformation and grain growth of Si3N4 grains. This leads to a decrease in the fracture toughness of the Si3N4 in this comparative example. Its relative density is 98.8%, its flexural strength is 671 MPa, and its fracture toughness is 7.2 MPa·m. 1 / 2 Its hardness is 17.3 GPa.

[0064] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A low-temperature preparation method for high-toughness silicon nitride ceramics, characterized in that... The process includes the following steps: α-Si3N4 powder, Yb2O3 powder, MgO powder, HfN powder, and anhydrous ethanol are ball-milled and mixed. After drying, the mixture is pre-pressed into a graphite mold. The graphite mold is then placed in a spark plasma sintering furnace. Under vacuum, the temperature is raised to 800–1000°C, and nitrogen gas is introduced. Under a nitrogen atmosphere of 1 atm, the temperature is raised to 1600–1700°C while applying an axial pressure of 30–50 MPa. The temperature is held for 5–15 minutes, then lowered to 600–800°C, and finally allowed to cool naturally to room temperature to obtain high-toughness silicon nitride ceramics. The resulting silicon nitride ceramics have a relative density of over 97%, a flexural strength of 660–800 MPa, and a fracture toughness of 7.5–9 MPa·m. 1 / 2 The amount of HfN powder used is 10-20% of the total volume of α-Si3N4 powder, MgO powder and Yb2O3 powder; the volume ratio of α-Si3N4 powder, MgO powder and Yb2O3 powder is (90-95):(2-5):(3-5); the heating rate is 100-150℃ / min, and the cooling rate is 100℃ / min.

2. The low-temperature preparation method of high-toughness silicon nitride ceramic according to claim 1, characterized in that: The α-Si3N4 powder has a purity of ≥98wt% and a particle size of 0.3~1µm; the MgO powder has a purity of ≥99.8wt% and a particle size of 50~100nm; the Yb2O3 powder has a purity of ≥99.8wt% and a particle size of 50~100nm; and the HfN powder has a purity of ≥99.9wt% and a particle size of 1~3µm.

3. The low-temperature preparation method of high-toughness silicon nitride ceramic according to claim 1, characterized in that... The specific steps include the following: S1. Mix and ball-mill α-Si3N4 powder, MgO powder, Yb2O3 powder, HfN powder and anhydrous ethanol, then dry by rotary evaporation, and sieve after complete drying to obtain ceramic powder; S2. Place the ceramic powder obtained in step S1 into a graphite mold in a spark plasma sintering furnace and pre-press it using a manual hydraulic press. S3. Next, the graphite mold of the spark plasma sintering furnace is placed into the spark plasma sintering furnace, and the temperature is raised to 800-1000°C under vacuum. Nitrogen gas is introduced, and the temperature is raised to 1600-1700°C under a nitrogen atmosphere of 1 atm. At the same time, an axial pressure of 30-50 MPa is applied and the temperature is held for 5-15 minutes. Then the temperature is lowered to 600-800°C, and finally the high-toughness silicon nitride ceramic is obtained by natural cooling to room temperature.

4. The low-temperature preparation method of high-toughness silicon nitride ceramic according to claim 3, characterized in that: The ball milling speed in step S1 is 250~300 r / min, and the ball milling time is 18~24 h; the rotary evaporation drying temperature is 45~55℃; and the sieving is done through a 100~120 mesh sieve.

5. The low-temperature preparation method of high-toughness silicon nitride ceramic according to claim 3, characterized in that: The pre-compression pressure in step S2 is 20~30MPa, and the pressure holding time is 2~5min.

6. A high-toughness silicon nitride ceramic prepared by the low-temperature preparation method according to any one of claims 1 to 5.

7. The application of the high-toughness silicon nitride ceramic according to claim 6 in the field of cutting tools or engine components.

Images