Silicon nitride ceramic and pressureless sintering method and application thereof

Abstract

The application provides a silicon nitride ceramic and a pressureless sintering method and application thereof, and belongs to the technical field of ceramic materials. The application uses yttrium aluminum garnet and magnesium oxide as sintering aids, controls the sintering temperature and time of the yttrium aluminum garnet, reduces the formation of the subcrystal phase YAP, can provide more sintering power for the pressureless sintering process of the silicon nitride ceramic, can promote the sintering densification of the silicon nitride ceramic at a lower liquid phase formation temperature, and simultaneously adds neodymium oxide, so that the silicon nitride ceramic prepared by pressureless sintering simultaneously has excellent hardness, strength and fracture toughness.

Description

Technical Field

[0001] This invention belongs to the field of ceramic materials technology, specifically relating to a silicon nitride ceramic and its pressureless sintering method and application. Background Technology

[0002] Silicon nitride ceramics are widely used in various fields, such as bearings, high-speed cutting tools, electronic substrates, automotive engine parts, heat exchangers, and ceramic armor, due to their excellent mechanical properties, good resistance to oxidation, thermochemical corrosion, thermal shock resistance, and low coefficient of thermal expansion. However, the strong covalent bonds between Si and N atoms result in a low self-diffusion coefficient, making it difficult to densify silicon nitride ceramics. Consequently, silicon nitride ceramics have poor sinterability, and the sintering process places high demands on the sintering driving force. Conventional sintering methods include gas pressure sintering, spark plasma sintering, or hot isostatic pressing, but these are costly and cannot be mass-produced. Therefore, obtaining high-performance silicon nitride ceramics under pressureless sintering conditions has become a challenge in this field. Summary of the Invention

[0003] The purpose of this invention is to provide a silicon nitride ceramic, its pressureless sintering method, and its applications. The preparation method provided by this invention employs pressureless sintering, and the prepared silicon nitride ceramic exhibits excellent hardness, strength, and fracture toughness.

[0004] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a pressureless sintering method for silicon nitride ceramics, comprising the following steps: (1) Silicon nitride, sintering aid and neodymium oxide are mixed to obtain a mixed powder; The sintering aids include yttrium aluminum garnet and magnesium oxide; The preparation method of the yttrium aluminum garnet includes: mixing yttrium oxide and aluminum oxide, and sintering at 1350~1450℃ for 1.5~2.5h to obtain yttrium aluminum garnet; (2) The mixed powder obtained in step (1) is subjected to dry pressing and cold isostatic pressing in sequence to obtain a blank; (3) The blank obtained in step (2) is subjected to pressureless sintering to obtain silicon nitride ceramic.

[0005] Preferably, in step (1), the mass ratio of yttrium aluminum garnet to magnesium oxide is (4.5~6):(2.5~5).

[0006] Preferably, in step (1), the mass ratio of silicon nitride to sintering aid is (84~92):(8~10).

[0007] Preferably, in step (1), the mass ratio of silicon nitride to neodymium oxide is (84~92):(0.1~8).

[0008] Preferably, the particle size of the mixed powder in step (1) is ≤0.5μm.

[0009] Preferably, the pressure for dry pressing in step (2) is 6~8MPa, and the dry pressing time is 45~60s.

[0010] Preferably, the pressure of the cold isostatic pressing is 180~200MPa, and the time of the cold isostatic pressing is 60~90s.

[0011] Preferably, the pressureless sintering method in step (3) is as follows: heating from room temperature to 900-1000℃ at a heating rate of 4-6℃ / min, then heating to 1400-1420℃ at a heating rate of 2-4℃ / min and holding for 1.5-2.5h, then heating to 1600-1650℃ at a heating rate of 1-3℃ / min and holding for 1.5-2.5h.

[0012] The present invention also provides silicon nitride ceramics prepared by the pressureless sintering method described in the above technical solution.

[0013] This invention also provides the application of the silicon nitride ceramics described in the above technical solutions in the fields of engineering and defense.

[0014] This invention provides a pressureless sintering method for silicon nitride ceramics, comprising the following steps: (1) mixing silicon nitride, sintering aid and neodymium oxide to obtain a mixed powder; the sintering aid includes yttrium aluminum garnet and magnesium oxide; the preparation method of yttrium aluminum garnet includes: mixing yttrium oxide and aluminum oxide, and sintering at 1350~1450℃ for 1.5~2.5h to obtain yttrium aluminum garnet; (2) subjecting the mixed powder obtained in step (1) to dry pressing and cold isostatic pressing in sequence to obtain a green body; (3) subjecting the green body obtained in step (2) to pressureless sintering to obtain silicon nitride ceramics. This invention uses yttrium aluminum garnet (YA) and magnesium oxide as sintering aids and controls the sintering temperature and time of YA to reduce the formation of the secondary crystalline phase YAP (YAlO3). This provides more sintering kinetics for the pressureless sintering process of silicon nitride ceramics and promotes the densification of silicon nitride ceramics at a lower liquidus formation temperature. The addition of neodymium oxide further enhances the properties of the pressureless sintered silicon nitride ceramics, resulting in excellent hardness, strength, and fracture toughness. The results of the examples show that the silicon nitride ceramics prepared by this invention have a Vickers hardness ≥17.88 GPa, strength ≥719.5 MPa, and fracture toughness ≥8.96 MPa·m. 1 / 2 . Attached Figure Description

[0015] Figure 1 The images show the SEM images and particle size distribution of the mixed powders in steps (1) of Examples 1-3 and Comparative Example 1. Figure 2 SEM images of the surface and cross-section of silicon nitride ceramics prepared in Examples 1-3 and Comparative Example 1; Figure 3 The images show the XRD patterns of silicon nitride ceramics prepared in Examples 1-3 and Comparative Example 1. Figure 4 The graph shows the mechanical property test results of the silicon nitride ceramics prepared in Examples 1-3 and Comparative Example 1. Detailed Implementation

[0016] This invention provides a pressureless sintering method for silicon nitride ceramics, comprising the following steps: (1) Silicon nitride, sintering aid and neodymium oxide are mixed to obtain a mixed powder; (2) The mixed powder obtained in step (1) is subjected to dry pressing and cold isostatic pressing in sequence to obtain a blank; (3) The blank obtained in step (2) is subjected to pressureless sintering to obtain silicon nitride ceramic.

[0017] Unless otherwise specified, the present invention does not impose any special restrictions on the source of the raw materials, and commercially available products well known to those skilled in the art can be used.

[0018] This invention mixes silicon nitride, sintering aids and neodymium oxide to obtain a mixed powder.

[0019] In this invention, the sintering aids include yttrium aluminum garnet and magnesium oxide.

[0020] In this invention, the preparation method of yttrium aluminum garnet includes: mixing yttrium oxide (Y2O3) and aluminum oxide (Al2O3) and sintering at 1350~1450℃ for 1.5~2.5h to obtain yttrium aluminum garnet.

[0021] In one embodiment, the molar ratio of yttrium oxide to aluminum oxide is 3:5.

[0022] In one embodiment, the mixing is ball milling; the liquid medium for ball milling is anhydrous ethanol; the mass ratio of the anhydrous ethanol to the total mass of yttrium oxide and alumina is 1.5:1; the ball-to-material ratio for ball milling is (1~2):1; the rotation speed of the ball mill is 460 r / min; and the ball milling time is 24 h.

[0023] As one implementation method, after mixing, the mixed material is dried at 80°C for 24 hours.

[0024] In this invention, the sintering temperature is preferably 1400℃; the sintering time is preferably 2h; and the preferred method for raising the temperature to the sintering temperature is to raise the temperature from room temperature to 1000℃ at a heating rate of 5℃ / min, and then raise the temperature from 1000℃ to 1400℃ at a heating rate of 2℃ / min and hold the temperature for 2h.

[0025] As one implementation method, after sintering, the metal is cooled to room temperature in the furnace to obtain yttrium aluminum garnet.

[0026] This invention controls the sintering temperature and time of yttrium aluminum garnet to ensure that the main crystalline phase is yttrium aluminum garnet (YAG, Y3Al5O). 12 It also contains a small amount of secondary crystalline phase YAP (YAlO3). When the sintering temperature and time are too low, a large amount of secondary crystalline phase YAP will be formed. YAP is far less effective as a sintering aid than YAG. When the sintering temperature and time are too high, a distorted monoclinic phase Y4Al2O9 will be formed. The liquid phase formed during the silicon nitride sintering process is not conducive to the sintering process of silicon nitride ceramics.

[0027] In this invention, the preferred mass ratio of yttrium aluminum garnet to magnesium oxide is (4.5~6):(2.5~5). As one embodiment, the mass ratio of yttrium aluminum garnet to magnesium oxide may specifically be 5:3 or 5:5.

[0028] This invention uses yttrium aluminum garnet and magnesium oxide as sintering aids and controls their ratio to provide more sintering power for the pressureless sintering process of silicon nitride ceramics. It can promote the densification of silicon nitride ceramics at a lower liquid phase formation temperature and improve the various properties of silicon nitride ceramics.

[0029] In this invention, the preferred mass ratio of silicon nitride to sintering aid is (84~92):(8~10). As one embodiment, the mass ratio of silicon nitride to sintering aid can specifically be 84:8, 86:8, 88:8, 90:8, or 90:10. By controlling the mass ratio of silicon nitride to sintering aid within the above range, this invention can further improve the various properties of silicon nitride ceramics.

[0030] In this invention, the preferred mass ratio of silicon nitride to neodymium oxide is (84~92):(0.1~8). As one embodiment, the mass ratio of silicon nitride to neodymium oxide can specifically be 88:4, 86:6, or 84:8. By controlling the mass ratio of silicon nitride to neodymium oxide within the above range, this invention can further improve the various properties of silicon nitride ceramics.

[0031] In this invention, the mixing of silicon nitride, sintering aid, and neodymium oxide is preferably ball milling; the liquid medium for ball milling is preferably anhydrous ethanol; the mass ratio of the anhydrous ethanol to the total mass of silicon nitride, sintering aid, and neodymium oxide is preferably (1~1.8):1, more preferably 1.5:1; the grinding balls are preferably zirconia balls with a diameter of 3~8 mm; the ball-to-material ratio is preferably (1~2):1, more preferably 1.5:1; the ball milling speed is preferably 380~460 r / min, more preferably 420~460 r / min; and the ball milling time is preferably 20~24 h. This invention, by ball milling and controlling the ball milling parameters, enables more uniform mixing of the raw materials and yields a mixed powder with the desired particle size. As one embodiment, the ball milling is performed in a planetary ball mill.

[0032] After ball milling, the present invention preferably washes the ball mill jar three times with anhydrous ethanol, then mixes the washed product with the ball-milled product, and then sequentially dries, grinds, and sieves the mixed material to obtain a mixed powder. The present invention washes the ball mill jar to avoid raw material waste.

[0033] In this invention, the drying temperature is preferably 80~100℃; the drying time is preferably 20~24h. As one embodiment, the drying is carried out in a forced-air drying oven.

[0034] In one implementation, the grinding is performed using an agate mortar and pestle.

[0035] In one embodiment, the mesh size of the sieve used during sieving is 80 mesh.

[0036] In this invention, the particle size of the mixed powder is preferably ≤0.5μm. Controlling the particle size of the mixed powder in this invention can further improve the various properties of silicon nitride ceramics.

[0037] After obtaining the mixed powder, the present invention sequentially performs dry pressing and cold isostatic pressing on the mixed powder to obtain a green body.

[0038] In this invention, the dry pressing is preferably performed in a mold. As one embodiment, the diameter of the mold is preferably 20-30 mm; the mass of the mixed powder during dry pressing is preferably 5-6 g, more preferably 5.5 g.

[0039] In this invention, the pressure for dry pressing is preferably 6-8 MPa; the dry pressing time is preferably 45-60 s. As one embodiment, the pressure for dry pressing can be specifically 7 MPa; the dry pressing time can be specifically 50 s or 55 s.

[0040] In this invention, the preferred pressure for cold isostatic pressing is 180-200 MPa; the preferred time for cold isostatic pressing is 60-90 s. As one embodiment, the pressure for cold isostatic pressing can be specifically 190 MPa; the preferred time for cold isostatic pressing can be 70 s or 80 s.

[0041] This invention first performs dry pressing followed by cold isostatic pressing, and controls the pressing pressure and time to obtain the desired ideal shape of the blank and ensure that the blank will not crack or generate too many pores during the subsequent pressureless sintering process; if only dry pressing is performed, the sample will crack and have many pores during the sintering process, and densification cannot be achieved; if only cold isostatic pressing is performed, the powder cannot be prepared into the ideal shape.

[0042] After obtaining the green body, the present invention performs pressureless sintering on the green body to obtain silicon nitride ceramic.

[0043] In this invention, the preferred method of pressureless sintering is as follows: heating from room temperature to 900-1000°C at a heating rate of 4-6°C / min, then heating to 1400-1420°C at a heating rate of 2-4°C / min and holding at that temperature for 1.5-2.5 hours, and then heating to 1600-1650°C at a heating rate of 1-3°C / min and holding at that temperature for 1.5-2.5 hours.

[0044] In this invention, the pressureless sintering is preferably carried out under a nitrogen atmosphere; the green body is preferably covered with silicon nitride powder; the particle size of the silicon nitride powder is preferably the same as that of the silicon nitride powder in the mixed powder; the covering thickness of the silicon nitride powder is preferably 3-5 mm. This invention performs pressureless sintering under a nitrogen atmosphere and silicon nitride powder covering. The silicon nitride covering provides a nitrogen source, preventing silicon nitride decomposition due to insufficient nitrogen partial pressure during high-temperature sintering, while also ensuring more uniform heat transfer, preventing green body deformation, and absorbing impurities within the green body during sintering.

[0045] In one embodiment, the pressureless sintering is carried out in a boron nitride crucible. The boron nitride crucible has better thermal conductivity, which is more conducive to the sintering process.

[0046] After pressureless sintering is completed, the present invention preferably cools the pressureless sintered product to obtain silicon nitride ceramic. The present invention does not have any particular limitations on the cooling operation; cooling to room temperature using techniques well known to those skilled in the art is acceptable.

[0047] This invention uses yttrium aluminum garnet and magnesium oxide as sintering aids and controls the sintering temperature and time of yttrium aluminum garnet, while adding neodymium oxide, so that the silicon nitride ceramic prepared by pressureless sintering has excellent hardness, strength and fracture toughness.

[0048] The present invention also provides silicon nitride ceramics prepared by the pressureless sintering method described in the above technical solution.

[0049] The silicon nitride ceramics prepared by this invention possess excellent hardness, strength, and fracture toughness.

[0050] This invention also provides the application of the silicon nitride ceramics described in the above technical solutions in the fields of engineering and defense.

[0051] The present invention does not impose any special limitations on the operation of the application, and any technical solution known to those skilled in the art can be used.

[0052] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0053] The preparation method of yttrium aluminum garnet in the examples and comparative examples is as follows: Yttrium oxide powder and aluminum oxide powder are mixed in a molar ratio of 3:5 and added to a ball mill jar with a ball-to-powder ratio of 1.5:1. Anhydrous ethanol is used as the liquid medium, and the mass ratio of anhydrous ethanol to the total mass of yttrium oxide and aluminum oxide is 1.5:1. The mixture is ball-milled at 460 r / min for 24 h. The slurry after ball milling is dried at 80 °C for 24 h, then placed in an aluminum oxide crucible and placed in a muffle furnace. The temperature is increased from room temperature to 1000 °C at 5 °C / min, then increased to 1400 °C at 2 °C / min and held for 2 h. The mixture is then cooled to room temperature with the furnace to obtain yttrium aluminum garnet.

[0054] Example 1 A pressureless sintering method for silicon nitride ceramics: (1) Silicon nitride powder, sintering aid (yttrium aluminum garnet and magnesium oxide powder, with a mass ratio of yttrium aluminum garnet to magnesium oxide of 5:3) and neodymium oxide are mixed and ball-milled in a polytetrafluoroethylene ball mill jar. The liquid medium for ball milling is anhydrous ethanol (the mass ratio of anhydrous ethanol to the total mass of silicon nitride, sintering aid and neodymium oxide is 1.5:1). The grinding balls are zirconia balls with a diameter of 3~8 mm. The ball-to-material ratio is 1.5:1. The ball milling speed is 460 r / min and the time is 24 h. After ball milling, the ball mill jar is washed 3 times with anhydrous ethanol. Then the washed material is mixed with the ball-milled material and dried in a forced-air drying oven at 80℃ for 24 h. After grinding in an agate mortar, the mixture is passed through an 80-mesh sieve to obtain a mixed powder (particle size ≤0.5 μm). The mass ratio of silicon nitride to sintering aid is 88:8 and the mass ratio of silicon nitride to neodymium oxide is 88:4. (2) Place 5.5g of the mixed powder obtained in step (1) into a circular mold with a diameter of 30mm, dry press at 8MPa for 60s, and then cold isostatic press at 200MPa for 90s to obtain a blank. (3) Place the blank obtained in step (2) into a boron nitride crucible, cover it with 4 mm thick silicon nitride powder (particle size ≤ 0.5 μm), and heat it from room temperature to 1000℃ at 5℃ / min under a nitrogen atmosphere, then heat it to 1420℃ at 3℃ / min and hold it for 2 h, and finally heat it to 1620℃ at 2℃ / min and hold it for 2.5 h. After cooling, silicon nitride ceramic is obtained, denoted as SNNd4.

[0055] Example 2 The only difference from Example 1 is that the mass ratio of silicon nitride to sintering aid is 86:8, and the mass ratio of silicon nitride to neodymium oxide is 86:6. All other aspects are the same as in Example 1, and silicon nitride ceramic is obtained, denoted as SNNd6.

[0056] Example 3 The only difference from Example 1 is that the mass ratio of silicon nitride to sintering aid is 84:8, and the mass ratio of silicon nitride to neodymium oxide is 84:8. All other aspects are the same as in Example 1, and silicon nitride ceramic is obtained, denoted as SNNd8.

[0057] Comparative Example 1 A pressureless sintering method for silicon nitride ceramics: (1) Silicon nitride powder and sintering aid (yttrium aluminum garnet and magnesium oxide powder, with a mass ratio of yttrium aluminum garnet to magnesium oxide of 5:3) were mixed and ball-milled in a polytetrafluoroethylene ball mill jar. The liquid medium for ball milling was anhydrous ethanol (the mass ratio of anhydrous ethanol to the total mass of silicon nitride and sintering aid was 1.5:1). The grinding balls were zirconia balls with a diameter of 3~8 mm, the ball-to-material ratio was 1.5:1, the ball milling speed was 460 r / min, and the time was 24 h. After ball milling, the ball mill jar was washed 3 times with anhydrous ethanol. Then the washed material was mixed with the ball-milled material and dried in a forced-air drying oven at 80℃ for 24 h. After grinding in an agate mortar, the mixture was passed through an 80-mesh sieve to obtain a mixed powder (particle size ≤0.5 μm). The mass ratio of silicon nitride to sintering aid was 92:8. (2)~(3) Same as in Example 1, silicon nitride ceramics are obtained, denoted as SN.

[0058] Comparative Example 2 The only difference from Comparative Example 1 is that the mass ratio of silicon nitride to sintering aid is 90:10, while all other aspects are the same as Comparative Example 1, resulting in silicon nitride ceramics.

[0059] Comparative Example 3 The only difference from Example 2 is that neodymium oxide is replaced with erbium oxide; all other aspects are the same as in Example 2, resulting in silicon nitride ceramics.

[0060] Comparative Example 4 The only difference from Example 2 is that neodymium oxide is replaced with holmium oxide; all other aspects are the same as in Example 2, resulting in silicon nitride ceramic.

[0061] Comparative Example 5 The only difference from Example 2 is that neodymium oxide is replaced with cerium oxide; all other aspects are the same as in Example 2, resulting in silicon nitride ceramics.

[0062] Comparative Example 6 The only difference from Example 2 is that neodymium oxide is replaced with cobalt oxide; all other aspects are the same as in Example 2, resulting in silicon nitride ceramics.

[0063] SEM images and particle size distribution diagrams of the mixed powders in steps (1) of Examples 1-3 and Comparative Example 1 are shown below. Figure 1 As shown, SN represents Comparative Example 1, SNNd4 represents Example 1, SNNd6 represents Example 2, and SNNd8 represents Example 3. From Figure 1 As can be seen, the powder particle size of all embodiments and comparative examples is between 0.18 and 0.21 μm. The powder grains are short and blocky, with relatively uniform grain size and clearly visible grain boundaries. The smaller grain size and uniform distribution are beneficial to improving the uniformity of sintering, providing greater driving force during sintering, and promoting the densification of ceramics.

[0064] SEM images of the surface and cross-section of silicon nitride ceramics prepared in Examples 1-3 and Comparative Example 1 are shown below. Figure 2 As shown, (a) is a surface SEM image of Comparative Example 1, (b) is a surface SEM image of Example 1, (c) is a surface SEM image of Example 2, (d) is a surface SEM image of Example 3, (e) is a cross-sectional SEM image of Comparative Example 1, (f) is a cross-sectional SEM image of Example 1, (g) is a cross-sectional SEM image of Example 2, and (h) is a cross-sectional SEM image of Example 3. Figure 2 As can be seen from the scanning electron microscope images of the surface and cross-section of all samples, no large number of obvious pores were found. The grains were closely arranged, and the samples grew from the initial short blocky grains into long rods.

[0065] The XRD patterns of the silicon nitride ceramics prepared in Examples 1-3 and Comparative Example 1 are shown below. Figure 3 As shown. From Figure 3 As can be seen from the examples, the main crystalline phase is β-phase silicon nitride, and a small amount of untransformed α-phase silicon nitride also exists. Figure 3It was also shown that a small amount of Y2Si3O3N4 was generated during the sintering process, and the content of this phase decreased significantly with the increase of neodymium oxide content.

[0066] The mechanical property test results of the silicon nitride ceramics prepared in Examples 1-3 and Comparative Example 1 are shown in the figure below. Figure 4 As shown.

[0067] The hardness, strength, and fracture toughness of the silicon nitride ceramics prepared in Examples 1-3 and Comparative Examples 1-6 are shown in Table 1. The hardness of the silicon nitride ceramics was tested using a Vickers hardness tester (Wilson VH3300). Before testing, the ceramic samples were polished to eliminate surface roughness and ensure clear indentations during testing. A force of 1 kg was applied to the polished ceramic surface and held for 10 seconds. According to the national standard (GB / T 16534-2009), valid hardness data should be obtained from tests conducted under rhomboid indentations with nearly equal diagonals. To ensure accuracy, each sample was tested five times repeatedly, and the average value was taken to reduce error.

[0068] The fracture toughness of the samples was tested using the indentation method. According to the national standard (GB / T 37900-2019), a 10 kg load was applied to the sample surface and held for 10 seconds. The indentation fracture toughness value was calculated using the distance from the center of the Vickers indentation to the crack tip. To ensure data accuracy, the test was repeated five times, and the average value was taken to calculate the fracture toughness.

[0069] The bending strength of the sample was tested by performing a three-point bending test using a universal testing machine (Shanghai Xiangjie Instrument Technology Co., Ltd.). According to the national standard (GB / T 6569-2006), the sample was first cut into strips with a width and height of 4 mm and 2 mm respectively. The span was set to 20 mm and the loading speed was 0.5 mm / min. The average value of 5 specimens was taken to reduce the error.

[0070] Table 1. Hardness, strength, and fracture toughness of silicon nitride ceramics prepared in Examples 1-3 and Comparative Examples 1-6.

[0071] As can be seen from Table 1, the silicon nitride ceramics prepared by this invention have better hardness, strength and fracture toughness.

[0072] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A pressureless sintering method for silicon nitride ceramics, comprising the following steps: (1) Silicon nitride, sintering aid and neodymium oxide are mixed to obtain a mixed powder; The sintering aids include yttrium aluminum garnet and magnesium oxide; The preparation method of the yttrium aluminum garnet includes: mixing yttrium oxide and aluminum oxide, and sintering at 1350~1450℃ for 1.5~2.5h to obtain yttrium aluminum garnet; (2) The mixed powder obtained in step (1) is subjected to dry pressing and cold isostatic pressing in sequence to obtain a blank; (3) The blank obtained in step (2) is subjected to pressureless sintering to obtain silicon nitride ceramic.

2. The pressureless sintering method according to claim 1, characterized in that, In step (1), the mass ratio of yttrium aluminum garnet to magnesium oxide is (4.5~6):(2.5~5).

3. The pressureless sintering method according to claim 1, characterized in that, In step (1), the mass ratio of silicon nitride to sintering aid is (84~92):(8~10).

4. The pressureless sintering method according to claim 1, characterized in that, In step (1), the mass ratio of silicon nitride to neodymium oxide is (84~92):(0.1~8).

5. The pressureless sintering method according to claim 1, characterized in that, The particle size of the mixed powder in step (1) is ≤0.5μm.

6. The pressureless sintering method according to claim 1, characterized in that, In step (2), the pressure for dry pressing is 6~8MPa and the time for dry pressing is 45~60s.

7. The pressureless sintering method according to claim 1, characterized in that, In step (2), the pressure of cold isostatic pressing is 180~200MPa, and the time of cold isostatic pressing is 60~90s.

8. The pressureless sintering method according to claim 1, characterized in that, The pressureless sintering method in step (3) is as follows: the temperature is increased from room temperature to 900-1000℃ at a heating rate of 4-6℃ / min, then increased to 1400-1420℃ at a heating rate of 2-4℃ / min and held for 1.5-2.5h, and then increased to 1600-1650℃ at a heating rate of 1-3℃ / min and held for 1.5-2.5h.

9. Silicon nitride ceramics prepared by the pressureless sintering method according to any one of claims 1 to 8.

10. The application of the silicon nitride ceramic of claim 9 in engineering and defense fields.

Images