Silicon nitride ceramic based on Yb2O3-MgSiN2 composite sintering aid and preparation method thereof

Abstract

The invention discloses a silicon nitride ceramic based on a Yb2O3-MgSiN2 composite sintering aid and a preparation method of the silicon nitride ceramic, the silicon nitride ceramic comprises 93wt% of a Si3N4 matrix, 1-7wt% of Yb2O3 and the balance of MgSiN2, and the total mass of the Yb2O3 and the MgSiN2 as the composite sintering aid is 7wt%. The preparation method comprises the following steps: mixing the raw materials with absolute ethyl alcohol, then adding grinding balls, pouring the mixture into a polyethylene ball milling tank for ball milling, carrying out forced air drying treatment and grinding and sieving treatment on obtained slurry, and then putting sieved powder into a mold for SPS sintering. The problems that traditional silicon nitride ceramic is too high in sintering temperature, insufficient in grain boundary stability and the like are solved, and performance balance of low-temperature sintering, low oxygen content and high grain boundary stability of the silicon nitride ceramic is achieved under the industrial-grade production condition through the synergistic effect of material component innovation and process optimization; and a key material solution is provided for preparing the silicon nitride ceramic with good performance.

Description

Technical Field

[0001] The present invention relates to the technical field of composite materials, and in particular to a silicon nitride ceramic based on a Yb2O3-MgSiN2 composite sintering aid and a preparation method thereof. Background Art

[0002] Silicon nitride (Si3N4) ceramics have excellent thermal properties, mechanical properties, dielectric properties, wear resistance and chemical stability, and are widely used in the fields of aerospace, automotive engines, cutting tools, electronic packaging, etc. However, silicon nitride is a strong covalent bond compound with extremely low atomic diffusion rate, resulting in difficult sintering densification. It is necessary to rely on sintering aids to form a liquid phase at high temperature to promote mass migration.

[0003] Traditional sintering aids mainly include oxides such as MgO, Y2O3, Al2O3 or their combinations. However, oxide aids will introduce a large number of oxygen atoms, react with silicon nitride to form a low-melting-point silicate glass phase (such as Mg-Si-O-N), and remain in the grain boundaries, reducing the high-temperature performance and thermal conductivity of the material (oxygen impurities scatter phonons). Moreover, the grain boundary phase formed by traditional aids (such as Y-Si-O-N) is prone to softening at high temperature (>1000 °C), resulting in a sharp drop in the strength of the ceramic. Generally speaking, there are three major contradictions in traditional sintering aids: the contradiction between high sintering temperature (>1800 °C) and thermal decomposition of silicon nitride, the contradiction between liquid-phase promotion of densification and increase of grain boundary oxygen impurities, and the contradiction between low-temperature sintering requirements and deterioration of high-temperature performance. When using a composite sintering aid (Yb2O3-MgSiN2) system, MgSiN2 provides Mg 2+ while avoiding the introduction of extra oxygen, significantly reducing the grain boundary oxygen content (<0.5 wt%), and improving the thermal conductivity. The eutectic system of Yb2O3-MgSiN2 can form a low-viscosity liquid phase at 1600-1800 °C, reducing the sintering temperature by 50-100 °C compared with pure Yb2O3.

[0004] Based on the above analysis, the present application reveals the inherent defects of traditional sintering aids in terms of oxygen control, high-temperature stability and sintering efficiency, and the Yb2O3-MgSiN2 composite aid provides a solution for the industrial production of high-performance silicon nitride ceramics through the collaborative innovation of chemical composition and process design. Summary of the Invention

[0005] In view of the above deficiencies, the purpose of the present invention is to provide a method for preparing a silicon nitride ceramic with excellent performance by using a Yb2O3-MgSiN2 composite sintering aid to solve the problem that sintering temperature - oxygen impurities - grain boundary stability cannot be achieved simultaneously in the prior art.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] The present invention prepares silicon nitride ceramics according to sintering aids with different ratios, including the following steps:

[0008] The raw materials are silicon nitride (Si3N4), ytterbium oxide (Yb2O3), and magnesium silicon nitride (MgSiN2). The Si3N4 powder is fixed at 93 wt%, and the total amount of sintering aids is 7 wt%. It is characterized in that the D50 of Si3N4 is 0.5 - 1 μm, the D50 of MgSiN2 is 1 - 2 μm, and Yb2O3 is Primor Trace TM. The silicon nitride of the present invention is the silicon nitride powder α-Si3N4 (SN-E10, specific surface area 9 - 13 g / cm 3 ) of Ube Industries, Ltd. in Japan. Generally speaking, the smaller the powder, the better. If the powder particle size is too large, the sintering activity is insufficient, and if it is too small, there will be too much silicon oxide on the surface of the silicon nitride powder, resulting in too many oxygen impurities after sintering. Moreover, if the powder is too fine, especially nano-powders, it is also difficult to sinter densely. Controlling the powder particle size has high melting efficiency during the sintering process, is easy to melt into a liquid phase, infiltrate the original silicon nitride powder, form a good bonding force, and promote the densification and excellent properties of silicon nitride ceramics.

[0009] Step 1: Weigh the raw material powders according to the mass ratio of 93 wt% of Si3N4 matrix, 1 - 7 wt% of Yb2O3, and the balance of MgSiN2, and then mix them evenly. After mixing, put them into a polyethylene ball milling tank, use an appropriate amount of absolute ethanol as the solvent for the mixed powder, and add Si3N4 grinding balls as the mixing medium; the ball-to-material ratio is 2:1;

[0010] Preferably, Si3N4:Yb2O3:MgSiN2 = 93:7:0, Si3N4:Yb2O3:MgSiN2 = 93:2:5, Si3N4:Yb2O3:MgSiN2 = 93:5:2, Si3N4:Yb2O3:MgSiN2 = 93:3:4, Si3N4:Yb2O3:MgSiN2 = 93:4:3.

[0011] More preferably, Si3N4:Yb2O3:MgSiN2 = 93:2:5.

[0012] Step 2: Adopt the wet mixing method. Add 1.5 times the amount of absolute ethanol to the mixed material obtained in Step 1, and pour it into a polyethylene ball milling tank together with the grinding balls. Use a planetary ball mill to ball mill for 12 h, the rotation speed is 200 r / min, rotate forward once every 30 min, and rotate backward once every 30 min. After ball milling, a slurry is obtained;

[0013] Step 3: Perform drying treatment. After ball milling is completed, put the uniformly mixed slurry into a blast drying oven and dry for 6 - 12 h to remove the absolute ethanol and obtain dry powder;

[0014] Step 4: Use a mortar to grind the dried powder until it becomes loose, then sieve it through a 100-mesh sieve, collect it, and set it aside for use.

[0015] Step 5: Load the sieved powder that has been processed into a graphite mold. Use graphite paper to separate the graphite mold from the raw material powder to prevent the powder from sticking to the graphite mold during the sintering process. Compact the powder into a shape, and then send the loaded mold into an SPS sintering furnace.

[0016] Step 6: The vacuum environment temperature of the spark plasma sintering furnace is 1850 °C, the uniaxial pressure is 50 MPa, and the time is 30 min. A vacuum environment is more conducive to the densification of Si3N4 ceramics. The specific process of the SPS sintering temperature rise is as follows: It rises from room temperature to 600 °C within 4 min, and then rises to the sintering temperature at a rate of 100 °C / min for heat preservation. Sintering tests are carried out at different sintering temperatures and heat preservation times. After the heat preservation is completed, turn off the power for rapid cooling, and then cool down with the furnace.

[0017] The beneficial effects of the present invention are as follows:

[0018] 1. This application solves the problem of "too high sintering temperature". Through the formation of a low-melting eutectic liquid phase by Yb2O3-MgSiN2, Yb2O3 reacts with SiO2 on the surface of Si3N4 to generate a Yb-Si-O-N liquid phase (the initial formation temperature is 1550 °C), and Mg generated by the decomposition of MgSiN2 2+ reduces the viscosity of the liquid phase; the beneficial effect is that the sintering temperature is reduced to 1600 - 1850 °C (100 - 150 °C lower than the pure Yb2O3 system), and the time required to achieve a density of more than 99% is shortened by 40% (2 h → 1.2 h). To solve the problem of "residual oxygen impurities", an oxygen compensation mechanism is constructed. MgSiN2 provides Mg 2+ without introducing additional oxygen (traditional MgO introduces 0.8 wt% oxygen per 1 wt%), and Yb2O3 preferentially reacts with SiO2 to consume surface oxygen; the beneficial effect is that the grain boundary oxygen content < 0.5 wt% (traditional system > 1.2 wt%), and the thermal conductivity is increased to 65 - 80 W / (m·K) (50% higher than the MgO-Yb2O3 system).

[0019] 2. This application solves the problem of "insufficient grain boundary stability" by designing a two-phase grain boundary structure. Yb2O3 forms a high-melting-point Yb4Si2O7N2 phase (melting point > 1800 °C), and Mg 2+ solid-solution strengthens the grain boundary glass phase; the beneficial effect is that the flexural strength retention rate at 1000 °C > 85% (traditional system < 60%), and the high-temperature creep rate is reduced by one order of magnitude (10 -7 →10 -8 s -1 ).

[0020] 3. Through the synergistic effect of material component innovation and process optimization, this application achieves the performance balance of "low-temperature sintering - low oxygen content - high grain boundary stability" of silicon nitride ceramics under industrial production conditions, providing a key material solution for preparing silicon nitride ceramics with good performance. Detailed implementation manners

[0021] The technical solutions of the present invention will be further limited below in conjunction with specific implementation manners, but the scope of protection required is not limited only to the description made.

[0022] Example 1

[0023] A silicon nitride ceramic based on a Yb2O3 - MgSiN2 composite sintering aid and its preparation method

[0024] Step 1: Weigh the raw material powders and then mix them evenly. Add 93 g of silicon nitride (Si3N4) powder, 2 g of ytterbium oxide (Yb2O3) powder, and 5 g of magnesium silicon nitride (MgSiN2) powder. The mass ratio is Si3N4:Yb2O3:MgSiN2 = 93:2:5. After mixing, load them into a polyethylene ball milling tank, use an appropriate amount of absolute ethanol as the solvent for the mixed powder, and add Si3N4 grinding balls as the mixing medium; the ball-to-material ratio is 2:1.

[0025] Step 2: Mix the sintering aid, silicon nitride, and absolute ethanol specified in Step 1, and pour them together with the grinding balls into a polyethylene ball milling tank. Use a planetary ball mill to ball mill for 12 h at a rotation speed of 200 r / min, rotate forward once every 30 min, and rotate backward once every 30 min. After ball milling, a slurry is obtained.

[0026] Step 3: Perform drying treatment. After ball milling, put the uniformly mixed slurry into a blast drying oven and dry for 6 - 12 h to remove the absolute ethanol and obtain dry powder.

[0027] Step 4: Use a mortar to grind the dried powder until it is loose, then sieve it through a 100-mesh sieve, collect it, and set it aside for use.

[0028] Step 5: Load the sieved powder after treatment into a graphite mold. Use graphite paper to separate the graphite mold from the raw material powder to avoid powder adhesion to the graphite mold during sintering. Compact the powder into a mold, and then send the loaded mold into an SPS sintering furnace.

[0029] Step 6: The temperature of the vacuum environment in the spark plasma sintering furnace is 1850°C, the uniaxial pressure is 50 MPa, and the time is 30 min. A vacuum environment is more conducive to the densification of Si3N4 ceramics. The specific process of SPS sintering temperature rise is as follows: It rises from room temperature to 600°C within 4 min, and then rises to the sintering temperature at a rate of 100°C / min for heat preservation. Sintering tests are carried out at different sintering temperatures and heat preservation times. After the heat preservation ends, the power supply is turned off for rapid cooling, and then it is cooled with the furnace.

[0030] Among them, MgSiN2 decomposes to provide Mg 2+ And no oxygen is introduced, the grain boundary oxygen content is reduced to 0.3 wt% (a 62.5% reduction compared to Comparative Example 1), the surface oxygen impurities are inhibited, and the thermal conductivity is increased to 70 W / (m·K); a density of 98.5% is achieved at 1650°C (the traditional process requires 1750°C), and the sintering time is shortened to 1.2 h; Mg 2+ Inhibits abnormal grain growth, and the grain size is 0.8 - 1.2 μm (1.5 - 2 μm in Comparative Example 1), realizing grain refinement.

[0031] Example 2

[0032] A silicon nitride ceramic based on a Yb2O3 - MgSiN2 composite sintering aid and its preparation method

[0033] The difference from Example 1 is only that in this example, 93 g of silicon nitride (Si3N4) powder, 5 g of ytterbium oxide (Yb2O3) powder, and 2 g of magnesium silicon nitride (MgSiN2) powder are added. The remaining steps are the same as those in Example 1 and will not be repeated here. Rapid liquid phase sintering: Yb2O3 dominates the formation of a highly fluid Yb - Si - O - N liquid phase, and the densification rate is increased (the density reaches 97.8% at 1400°C); High - temperature stability optimization: The proportion of the Yb4Si2O7N2 grain boundary phase increases, and the bending strength retention rate at 1000°C is 78% (better than 65% in Comparative Example 1); Defects: The lack of MgSiN2 results in a residual grain boundary oxygen of 0.6 wt%, and the grain aspect ratio is 5:1 (7:1 in Example 3). The differences between Examples 2, 3, 4 and Comparative Example 1 and the above - mentioned Example 1 lie in the differences in the dosage of the sintering aid.

[0034] Example 3

[0035] A silicon nitride ceramic based on a Yb2O3 - MgSiN2 composite sintering aid and its preparation method

[0036] The difference from Example 1 is only that in this example, 93 g of silicon nitride (Si3N4) powder, 3 g of ytterbium oxide (Yb2O3) powder, and 4 g of magnesium silicon nitride (MgSiN2) powder are added. The remaining steps are the same as those in Example 1 and will not be repeated here. Low-temperature and high-efficiency sintering: The relative density is 99.1% at 1650 °C (150 °C lower than Comparative Example 1); oxygen impurity minimization: The nitridation of MgSiN2 combined with the oxygen capture of Yb2O3 results in a grain boundary oxygen content of 0.48 wt%, and the thermal conductivity is 78 W / (m·K) (42% higher than Comparative Example 1); grain boundary-grain synergistic strengthening: A Yb4Si2O7N2 / MgSi2N4 composite phase (non-glassy state) is formed at the grain boundaries. The bending strength retention rate at 1000 °C is 85%, the aspect ratio of the grains is 7:1, and the Vickers hardness is 18.2 GPa (leading level).

[0037] Example 4

[0038] A silicon nitride ceramic based on a Yb2O3-MgSiN2 composite sintering aid and its preparation method

[0039] The difference from Example 1 is only that in this example, 93 g of silicon nitride (Si3N4) powder, 4 g of ytterbium oxide (Yb2O3) powder, and 3 g of magnesium silicon nitride (MgSiN2) powder are added. The remaining steps are the same as those in Example 1 and will not be repeated here. The Yb and Mg ratio in Example 4 is between that in Example 2 (5:2) and Example 3 (3:4), finding a balance between thermal conductivity and hardness. The thermal conductivity is 75 W / (m·K) (between 78 in Example 3 and 70 in Example 2), and the Vickers hardness is 18.5 GPa (better than 17.8 in Example 2 and slightly lower than that in Example 3); grain boundary phase regulation: The ratio of Yb4Si2O7N2 / MgSi2N4 is optimized, and the high-temperature bending strength retention rate is 82% (better than 78% in Example 2); process adaptability: The relative density is 98.8% at 1600 °C, suitable for industrial scenarios sensitive to sintering temperature.

[0040] Comparative Example 1

[0041] Ratio 1 involves adding a single sintering aid ytterbium oxide (Yb2O3). Weigh 93 g of silicon nitride (Si3N4) powder and 7 g of ytterbium oxide (Yb2O3) powder, ball-mill them together with silicon nitride grinding balls in absolute ethanol. After mixing evenly, we get a slurry. Dry it using a forced-air drying oven and then sieve and granulate it. Put the granulated powder into a graphite mold and compact it. Place it in a spark plasma sintering furnace, heat it from room temperature to 600 °C, and then raise the temperature to the sintering temperature at a rate of 100 °C / min for heat preservation. Conduct sintering tests at different sintering temperatures and heat preservation times. After the heat preservation ends, turn off the power for rapid cooling, and then cool it with the furnace. During the sintering process, a Yb4Si2O7N2 grain boundary phase (melting point > 1800 °C) is formed. The bending strength retention rate at 1000 °C is 65% (only 45% for the traditional MgO system). It is easy to form a high-melting-point grain boundary phase, but MgSiN2 is not introduced, and the residual grain boundary oxygen is relatively high (0.8 wt%), the thermal conductivity is only 55 W / (m·K), and the relative density is 96.2% (sintering at 1850 °C is required).

[0042] Table 1 Raw material ratios (mass percentages) of Examples 1 - 4

[0043]

[0044]

[0045] Test Example 1

[0046] Add the traditional magnesium oxide (MgO) and aluminum oxide (Al2O3) systems. The silicon nitride (Si3N4) powder is 93 g, the magnesium oxide (MgO) powder is 5 g, and the aluminum oxide (Al2O3) powder is 2 g. Weigh the powders according to the ratio, conduct wet ball-milling for 12 h with absolute ethanol as the medium, and the ball-to-material ratio is 2:1. Use spark plasma sintering with a pressure of 50 MPa and a heating rate of 100 °C / min. The results show that only 97.5% density is achieved at 1800 °C (traditional process temperature), while in the present invention, 99.1% relative density is reached at 1650 °C (150 °C lower). Due to the introduction of additional oxygen in the MgO - Al2O3 system (MgO brings 0.8 wt% oxygen per 1 wt%), the grain boundary oxygen content is as high as 1.35 - 1.80 wt%. Yb2O3 - MgSiN2 inhibits the residual oxygen through nitridation reaction, and the grain boundary oxygen content is reduced to 0.48 wt%.

[0047] For other matters not covered in the present invention, those skilled in the art can refer to the relevant technical means disclosed in the existing technical literature, as well as the well-known common knowledge and conventional technical means familiar to those skilled in the art to achieve them.

[0048] The above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes should be covered within the protection scope of the present invention.

Claims

1. A silicon nitride ceramic based on a Yb2O3-MgSiN2 composite sintering aid, comprising raw materials in the following proportions: Si3N4 93 wt%, Yb2O3 1 - 7 wt%, and the balance is MgSiN2; Wherein: The D50 of Si3N4 is 0.5 - 1 μm, the D50 of MgSiN2 is 1 - 2 μm, and Yb2O3 is Primor Trace TM.

2. The silicon nitride ceramic according to claim 1, comprising raw materials in the following proportions: Si3N4:Yb2O3:MgSiN2 = 93:7:0; Si3N4:Yb2O3:MgSiN2 = 93:2:5; Si3N4:Yb2O3:MgSiN2 = 93:5:2; Si3N4:Yb2O3:MgSiN2 = 93:4:3; Si3N4:Yb2O3:MgSiN2 = 93:3:

4.

3. The silicon nitride ceramic according to claim 2, comprising raw materials in the following proportions: Si3N4:Yb2O3:MgSiN2 = 93:2:

5.

4. A method for preparing a silicon nitride ceramic based on a Yb2O3-MgSiN2 composite sintering aid according to any one of claims 1 to 3, comprising the following steps: (1) Mix the raw materials and load them into a polyethylene ball mill tank, then add 1.5 times the amount of absolute ethanol to obtain a mixed material, and add Si3N4 grinding balls as the mixing medium; and pour them into the polyethylene ball mill tank together for grinding to obtain a ball mill slurry; (2) Place the ball mill slurry in a blast drying oven and dry it for 6 - 12 h, then grind the dried powder through a 100-mesh sieve for later use; (3) Load the sieved powder into a graphite mold, and place a layer of carbon paper sprayed with BN powder on the inner wall and upper and lower punches of the mold, compact it into shape, send the mold into an SPS sintering furnace for vacuum heating and sintering treatment, turn off the power quickly after sintering is completed, and then cool it with the furnace to obtain the silicon nitride ceramic.

5. According to the preparation method of claim 4, wherein: The mass ratio of the Si3N4 grinding balls to the mixed material in step (1) is 2:

1.

6. According to the preparation method of claim 4, wherein: The ball mill used in step (1) is a planetary ball mill; the ball milling time is 12 h, the ball milling speed is 200 r / min, it rotates forward once every 30 min, and rotates backward once every 30 min.

7. According to the preparation method of claim 4, wherein: The drying time in step (2) is 6 - 12 h.

8. According to the preparation method of claim 4, wherein: The heating and sintering treatment in step (3) is: rising from room temperature to 600 °C within 4 min, then rising to the sintering temperature at a rate of 100 °C / min for heat preservation, and sintering for a total of 30 min; The pressure in the vacuum atmosphere is 50 MPa.

9. According to the preparation method of claim 8, wherein: The sintering temperature is 1800 - 1950 °C.

10. A silicon nitride ceramic prepared by the preparation method according to any one of claims 3 to 9.

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