Method for toughening silicon nitride ceramic by coating silicon nitride whisker through coprecipitation
Through the preparation process of combining co-precipitation-coated silicon nitride whiskers with air pressure sintering, the problem of uneven distribution of whiskers, powders and additives in silicon nitride ceramics is solved, and the preparation of silicon nitride ceramics with high density, high strength and high toughness is achieved, improving the overall performance of the material.
Patent Information
- Application Number
- CN202510513355.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-23
- Publication Date
- 2025-07-08
AI Technical Summary
In the prior art, the uneven distribution of whiskers, α-Si3N4 powder and sintering additives of silicon nitride ceramics and the damage to the whisker morphology leads to a decrease in material strength and toughness, limiting its application in the fields of lightweight, high strength and high temperature resistance.
The preparation process of combining co-precipitation coated silicon nitride whiskers with air pressure sintering is adopted. A sintering additive precursor is formed on the whiskers and powder surfaces through chemical co-precipitation reaction, and an oxide sintering additive is calcined at high temperature to ensure uniform distribution of the additives and avoid damage to the whisker morphology.
It improves the intergranular interface bonding strength, enhances the mechanical properties and toughness of silicon nitride ceramics, maintains the original morphology of the whiskers, and improves the density and mechanical properties of the material.
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Figure CN120271362A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of ceramic material preparation, and particularly relates to a method for toughening silicon nitride ceramics by co-precipitation coating of silicon nitride whiskers. Background Art
[0002] Si3N4 belongs to a strong covalent bond ceramic material, and has excellent properties such as light weight, high hardness, high strength, insulation, wear resistance, high temperature resistance, and excellent thermal shock resistance. It has been deeply studied and widely used in industries such as energy, chemical industry, machinery, electronic information, national defense, aerospace, etc. Continuously pursuing by researchers is to improve the toughness of silicon nitride ceramic materials to further expand their application scope.
[0003] The commonly used toughening method for silicon nitride ceramics is whisker toughening. Pre-adding β-Si3N4 whiskers as seeds to induce the polymorphic transformation of α-Si3N4 on the surface of β-Si3N4 whiskers to form silicon nitride ceramics with a bimodal microstructure. Using α-Si3N4 powder, pre-added β-Si3N4 whiskers and sintering aids as raw materials, the most commonly used method for preparing whisker-toughened silicon nitride ceramics at present is to ball-mill and mix the three evenly and then perform high-temperature sintering. At high temperature, the sintering aids react with SiO2 on the surface of α-Si3N4 powder and pre-added β-Si3N4 whiskers to form a high-temperature resistant grain boundary phase to enhance the intergranular bonding of silicon nitride ceramics and improve their mechanical properties. The uniform distribution of sintering aids with α-Si3N4 powder and pre-added β-Si3N4 whiskers is the key to forming a strongly bonded microstructure. However, rod-shaped whiskers are extremely easy to interlace and distribute to produce gaps of uneven sizes. During the ball-milling and mixing process, the sintering aids fill the positions of the uneven-sized gaps, resulting in uneven distribution of the aids and whiskers, causing uneven distribution of silicon nitride grains and intergranular phases in the ceramic sintered body, and being extremely easy to generate defects under external force, leading to a reduction in the strength and toughness of the material. Moreover, the ball-milling and mixing method will damage the rod-shaped morphology of silicon nitride whiskers, greatly reducing the whisker toughening effect. At present, there is no effective solution to the problems of uniform dispersion of whiskers, α-Si3N4 powder particles and sintering aids and damage to the whisker morphology. This problem will restrict the further development and application of silicon nitride in the fields of light weight, high strength, high temperature resistance, etc. Summary of the Invention
[0004] Aiming at the problems existing in the current silicon nitride ceramic preparation technology, such as uneven distribution of whiskers, α-Si3N4 powder and sintering aids and damage to the whisker morphology, the purpose of the present invention is to provide a preparation method for toughening silicon nitride ceramics with silicon nitride whiskers, which has a simple process, low cost, and can promote the uniform distribution of whiskers, α-Si3N4 powder and sintering aids while keeping the whisker morphology unchanged. A preparation process combining co-precipitation coating of silicon nitride whiskers and gas pressure sintering is designed, and a high-density, high-strength and high-toughness silicon nitride ceramic material is prepared.
[0005] The object of the present invention can be achieved by the following technical solutions:
[0006] A method for toughening silicon nitride ceramics by co-precipitation coating of silicon nitride whiskers, comprising the following steps:
[0007] (1) Refining the particle size of α-Si3N4 powder: Weigh silicon nitride grinding balls, α-Si3N4 powder, and measure absolute ethanol. Put the three into a high-energy ball milling tank filled with nitrogen protective gas, place it in a high-energy ball mill, set the rotation speed and milling time, grind and refine the α-Si3N4 powder and mix the three evenly, dry and sieve.
[0008] (2) Dispersing β-Si3N4 whiskers: Weigh β-Si3N4 whiskers according to the proportion and add them to a beaker containing absolute ethanol solvent, and ultrasonically disperse to obtain a β-Si3N4 whisker suspension.
[0009] (3) Preparing a precipitant solution: Weigh the precipitant and add it to a beaker containing absolute ethanol to prepare a precipitant solution. After stirring and mixing the precipitant solution in the beaker evenly, pour it into a burette for standby.
[0010] (4) Preparing co-precipitation-coated α-Si3N4 powder and β-Si3N4 whiskers: Weigh the corresponding mass of metal nitrates according to the mass ratio and place them in a beaker containing absolute ethanol, and ultrasonically stir until completely dissolved. Under ultrasonic stirring conditions, add the α-Si3N4 powder with refined particle size obtained in step (1) and the β-Si3N4 whisker suspension obtained in step (2). Seal the obtained slurry and continue ultrasonic stirring to form a uniform suspension. Slowly drip the precipitant solution in step (3) into the suspension until the pH value is appropriate, then stop dripping and let it stand for several hours. After all the precipitation, wash it repeatedly with deionized water by centrifugation for more than three times to obtain a mixed powder; put the mixed powder into a muffle furnace for heat preservation to obtain β-Si3N4 whiskers and α-Si3N4 powder coated with an oxide sintering aid.
[0011] (5) Sintering silicon nitride ceramics: Place the β-Si3N4 whiskers and α-Si3N4 powder coated with an oxide sintering aid obtained in step (4) in a mold and perform cold isostatic pressing. The formed silicon nitride ceramic green body is placed in a gas pressure sintering furnace, heated under nitrogen protection, and kept warm. After the heat preservation is completed, it is cooled to room temperature with the furnace, and the silicon nitride ceramic is taken out, cut, surface polished, and then subjected to performance testing.
[0012] Preferably, in step (1), the rotation speed is 600 - 1400 rpm, and the milling time is 20 - 60 min; the drying and sieving are carried out in a vacuum environment, and the mesh number of the sieve is in the range of 400 - 600 meshes.
[0013] Preferably, in step (2), the weighing is carried out according to a ratio, and the ratio is that 1 g of β-Si3N4 whiskers are dispersed in 100-300 ml of anhydrous ethanol solution; the ultrasonic power is 50-200 W, and the ultrasonic time is 30-120 min.
[0014] Preferably, in step (3), the precipitant solution is an ammonia ethanol solution or a sodium hydroxide ethanol solution. The molar concentration of the ammonia ethanol solution is 4-11 mol / L, and the molar concentration of the sodium hydroxide ethanol solution is 0.5-1 mol / L.
[0015] Preferably, in step (4), the mass ratio of α-Si3N4 powder, β-Si3N4 whiskers and metal nitrate is the molar ratio of: (75-92.5 wt%):(2.5-10 wt%):(5-15 wt%); the metal nitrate includes but is not limited to one or more of Al(NO3)·9H2O, Y(NO3)3·6H2O, Lu(NO3)3·4H2O, Gd(NO3)3·6H2O, Er(NO3)3·5H2O; the slow dropping is 1-2 drops / s; the appropriate pH value is pH 8-10; the standing for several hours is standing for 1-3 h; the holding temperature is 450-700 °C, and the holding time is 0.5-3 h.
[0016] Preferably, in step (5), the nitrogen protection pressure is 0.5-2 MPa, the holding temperature is 1800-1900 °C, and the holding time is 2-4 h.
[0017] Advantages of the present invention:
[0018] 1. Improve the interfacial bonding strength between grains: The present invention uses a chemical co-precipitation reaction to form a sintering aid precursor on the surfaces of silicon nitride whiskers and α-Si3N4 powder, and performs high-temperature calcination on the sintering aid precursor to form an oxide sintering aid; the particle size of the oxide sintering aid after chemical precipitation and high-temperature calcination is more refined, which is conducive to improving the uniformity of the distribution of the sintering aid, promoting the reaction between the oxide sintering aid and silicon nitride grains to generate an intergranular phase, enhancing the interfacial bonding strength between the intergranular phase and silicon nitride grains, and improving the mechanical properties of silicon nitride ceramics.
[0019] 2. Enhance the toughness of silicon nitride ceramics: Compared with the traditional method of ball-milling and mixing α-Si3N4 powder, silicon nitride whiskers and a sintering aid, the present invention uses a method of chemical precipitation and high-temperature calcination on the surface of silicon nitride whiskers to introduce silicon nitride whiskers, effectively avoiding damage to the morphology of silicon nitride whiskers, retaining the original length and diameter of silicon nitride whiskers, and facilitating the toughening effect of rod-shaped whiskers.
[0020] Of course, it is not necessary for any product implementing the present invention to achieve all the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] To clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for describing the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.
[0022] Figure 1 XRD diffraction pattern of the silicon nitride ceramic sample in Example 1;
[0023] Figure 2 Microscopic tissue structure diagram of the silicon nitride ceramic sample in Example 1;
[0024] Figure 3 Microscopic tissue structure diagram of the silicon nitride ceramic sample in Example 2;
[0025] Figure 4 Microscopic tissue structure diagram of the silicon nitride ceramic sample in Example 3;
[0026] Figure 5 Microscopic tissue structure diagram of the silicon nitride ceramic sample in Example 4;
[0027] Figure 6 Microscopic tissue structure diagram of the silicon nitride ceramic sample in Example 5. DETAILED DESCRIPTION OF THE EMBODIMENTS
[0028] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.
[0029] Example 1
[0030] Step 1: Weigh 90 g of α-Si3N4 powder and 200 g of silicon nitride grinding balls, measure 1 L of anhydrous ethanol, and place the three in a high-energy ball mill tank filled with nitrogen. Place it in a high-energy ball mill and grind for 45 min at a rotation speed of 800 rpm. After grinding, separate the ceramic slurry and place it in a vacuum drying oven for drying for 24 h. After complete drying, sieve it with a 400-mesh sieve to obtain α-Si3N4 powder with refined particle size;
[0031] Step 2: Weigh 5 g of β-Si3N4 whiskers and add them to a beaker containing 500 ml of anhydrous ethanol solution. Place the beaker in an ultrasonic device for dispersion. Set the ultrasonic power to 100 W and the ultrasonic time to 60 min to obtain a β-Si3N4 whisker suspension;
[0032] Step 3: Weigh 40 g of NaOH and add it to a beaker containing 1 L of anhydrous ethanol. Stir evenly to prepare a 1 mol / L sodium hydroxide ethanol solution, and then pour the sodium hydroxide ethanol solution into a burette;
[0033] Step 4: Weigh 10 g of Lu(NO3)3·6H2O and add it to a beaker containing 500 mL of anhydrous ethanol. Stir ultrasonically until completely dissolved. Under ultrasonic stirring conditions, add 85 g of the α-Si3N4 powder obtained in Step 1 and the β-Si3N4 whisker suspension obtained in Step 2. Seal the resulting slurry and continue ultrasonic stirring to form a uniform suspension. Drop the sodium hydroxide ethanol solution in Step 3 into the suspension at a rate of 2 drops / s until the pH value reaches 8, then stop dropping and let it stand for 1 hour. After all the precipitation, wash it repeatedly with deionized water by centrifugation for more than three times to obtain a mixed powder. Put the mixed powder into a muffle furnace and keep it at 600 °C for 2 h to obtain β-Si3N4 whiskers and α-Si3N4 powder coated with Lu2O3 sintering aid;
[0034] Step 5: Place the β-Si3N4 whiskers and α-Si3N4 powder coated with Lu2O3 sintering aid obtained in Step 4 in a mold and perform cold isostatic pressing. Load the formed silicon nitride ceramic green body into a gas pressure sintering furnace. Under the protection of 0.5 MPa nitrogen, heat it to 1800 °C at a rate of 10 °C / min and keep it at this temperature for 2 h. After heat preservation, cool it to room temperature with the furnace. Take out the silicon nitride ceramic, cut it and polish its surface, and then perform performance testing.
[0035] Example 2
[0036] Step 1: Weigh 90 g of α-Si3N4 powder and 200 g of silicon nitride grinding balls, measure 1 L of anhydrous ethanol, put the three into a high-energy ball milling tank filled with nitrogen, place it in a high-energy ball mill, grind it at a speed of 1200 rpm for 30 min. After grinding, separate the ceramic slurry and place it in a vacuum drying oven for drying for 24 h. After complete drying, sieve it with a 600-mesh sieve to obtain α-Si3N4 powder with refined particle size;
[0037] Step 2: Weigh 2.5 g of β-Si3N4 whiskers and add them to a beaker containing 1000 ml of anhydrous ethanol solution. Place the beaker in an ultrasonic device for dispersion. Set the ultrasonic power to 200 W and the ultrasonic time to 30 min to obtain a β-Si3N4 whisker suspension;
[0038] Step 3: Measure 500 ml of ammonia water solution with a concentration of 11 mol / L and add it to a beaker containing 1 L of absolute ethanol, and stir evenly to obtain an ammonia water-ethanol solution with a concentration of 5.5 mol / L. Then, transfer the evenly mixed precipitant solution into a burette.
[0039] Step 4: Weigh 10 g of Lu(NO3)3·6H2O and add it to a beaker containing 500 mL of absolute ethanol, and ultrasonically stir until it is completely dissolved. Under the condition of ultrasonic stirring, add 85 g of the α-Si3N4 powder obtained in Step 1 and the β-Si3N4 whisker suspension obtained in Step 2. Seal the resulting slurry and continue ultrasonic stirring to form a uniform suspension. Then, slowly drip the ammonia water-ethanol solution in Step 3 into the suspension at a rate of 2 drops per second until the pH value reaches 8, then stop dripping and let it stand for 1 hour. After all the precipitation, wash it repeatedly with deionized water by centrifugation for more than three times to obtain a mixed powder. Put the mixed powder into a muffle furnace and keep it at 600 °C for 2 h to obtain β-Si3N4 whiskers and α-Si3N4 powder coated with Lu2O3 sintering aid.
[0040] Step 5: Place the β-Si3N4 whiskers and α-Si3N4 powder coated with Lu2O3 sintering aid obtained in Step 4 into a mold and perform cold isostatic pressing. Place the formed silicon nitride ceramic green body in a gas pressure sintering furnace, heat it to 1800 °C at a rate of 10 °C / min under the protection of 0.5 MPa nitrogen and keep it at this temperature for 2 h. After heat preservation, cool it to room temperature with the furnace. Take out the silicon nitride ceramic, cut it and polish its surface, and then conduct performance tests.
[0041] The difference between Example 2 and Example 1 is that, compared with Example 1, in Step 1 of Example 2, the ball milling speed of α-Si3N4 by high-energy ball milling increases, the ball milling time shortens, and the mesh number of the sieve increases; in Step 2, the volume of the absolute ethanol solvent for ultrasonic dispersion of β-Si3N4 whiskers doubles, the ultrasonic power of the ultrasonic device doubles, and the ultrasonic time reduces by 50%; in Step 2, the mass of β-Si3N4 whiskers reduces by 50%; in Step 3, the sodium hydroxide ethanol solution is replaced with ammonia water-ethanol solution as the precipitant. The reaction between the ammonia water-ethanol solution and Lu(NO3)3·6H2O is milder, and the particle size of the formed Lu(OH)3 precipitate is more uniform, which is beneficial to controlling the formation of the final calcined product Lu2O3 sintering aid.
[0042] Example 3
[0043] Step 1: Weigh 90 g of α-Si3N4 powder and 200 g of silicon nitride grinding balls, measure 1 L of absolute ethanol, put the three into a high-energy ball milling tank filled with nitrogen, place it in a high-energy ball mill, grind it at a speed of 1200 rpm for 30 min, separate the ceramic slurry after grinding and place it in a vacuum drying oven to dry for 24 h. After complete drying, sieve it with a 600-mesh sieve to obtain α-Si3N4 powder with refined particle size.
[0044] Step 2: Weigh 2.5 g of β-Si3N4 whiskers and add them to a beaker containing 1000 ml of anhydrous ethanol solution. Place the beaker in an ultrasonic device for dispersion. Set the ultrasonic power to 200 W and the ultrasonic time to 30 min to obtain a β-Si3N4 whisker suspension;
[0045] Step 3: Measure 500 ml of ammonia ethanol solution with a concentration of 11 mol / L and pour it into a burette;
[0046] Step 4: Weigh 10 g of Lu(NO3)3·6H2O and add it to a beaker containing 500 mL of anhydrous ethanol. Ultrasonically stir until completely dissolved. Under ultrasonic stirring conditions, add 87.5 g of the α-Si3N4 powder obtained in Step 1 and the β-Si3N4 whisker suspension obtained in Step 2; Seal the resulting slurry and continue ultrasonic stirring to form a uniform suspension. Drop the ammonia ethanol solution in Step 3 into the suspension at a rate of 1 drop / s until the pH value reaches 8, then stop dropping and let it stand for 3 hours. After all the precipitation, wash it repeatedly with deionized water by centrifugation more than three times to obtain a mixed powder. Put the mixed powder into a muffle furnace and keep it at 600 °C for 2 h to obtain β-Si3N4 whiskers and α-Si3N4 powder coated with Lu2O3 sintering aid;
[0047] Step 5: Place the β-Si3N4 whiskers and α-Si3N4 powder coated with Lu2O3 sintering aid obtained in Step 4 in a mold and perform cold isostatic pressing; Load the formed silicon nitride ceramic green body into a gas pressure sintering furnace. Under the protection of 0.5 MPa nitrogen, heat it to 1900 °C at a rate of 10 °C / min and keep it at this temperature for 2 h. After heat preservation, cool it to room temperature with the furnace. Take out the silicon nitride ceramic, cut it and polish its surface, and then conduct performance testing.
[0048] On the basis of Example 2, in Step 3 of Example 3, the concentration of the ammonia ethanol solution is increased from 5.5 mol / L to 11 mol / L, doubling the concentration. The dropping rate of the ammonia precipitant is slowed down by half, and the precipitation standing time is increased by 3 times; In Step 5, the final sintering temperature is further optimized from 1800 °C to 1900 °C.
[0049] Example 4
[0050] Step 1: Weigh 90 g of α-Si3N4 powder and 200 g of silicon nitride grinding balls, measure 1 L of anhydrous ethanol, put the three into a high-energy ball milling tank filled with nitrogen, place it in a high-energy ball mill, grind it at a speed of 1200 rpm for 30 min, separate the ceramic slurry after grinding and dry it in a vacuum drying oven for 24 h. After complete drying, sieve it with a 600-mesh sieve to obtain α-Si3N4 powder with refined particle size;
[0051] Step 2: Weigh 2.5 g of β-Si3N4 whiskers and add them to a beaker containing 1000 ml of anhydrous ethanol solution. Place the beaker in an ultrasonic device for dispersion. Set the ultrasonic power to 200 W and the ultrasonic time to 30 min to obtain a β-Si3N4 whisker suspension;
[0052] Step 3: Measure 500 ml of ammonia ethanol solution with a concentration of 11 mol / L and pour it into a burette;
[0053] Step 4: Weigh 10 g of Y(NO3)3·6H2O and add it to a beaker containing 500 mL of anhydrous ethanol. Stir ultrasonically until completely dissolved. Under ultrasonic stirring conditions, add 87.5 g of the α-Si3N4 powder obtained in Step 1 and the β-Si3N4 whisker suspension obtained in Step 2. Seal the resulting slurry and continue ultrasonic stirring to form a uniform suspension. Drop the ammonia ethanol solution in Step 3 into the suspension at a rate of 1 drop / s until the pH value reaches 8, then stop dropping and let it stand for 1 hour. After all the precipitation, wash it repeatedly with deionized water by centrifugation more than three times to obtain a mixed powder. Place the mixed powder in a muffle furnace and keep it at 600 °C for 2 h to obtain β-Si3N4 whiskers and α-Si3N4 powder coated with Y2O3 sintering aids;
[0054] Step 5: Place the β-Si3N4 whiskers and α-Si3N4 powder coated with Y2O3 sintering aids obtained in Step 4 in a mold and perform cold isostatic pressing. Place the formed silicon nitride ceramic green body in a gas pressure sintering furnace. Under the protection of 0.5 MPa nitrogen, heat it to 1900 °C at a rate of 10 °C / min and keep it at this temperature for 2 h. After heat preservation, cool it to room temperature with the furnace. Take out the silicon nitride ceramic, cut it, and polish its surface, and then perform performance testing.
[0055] Example 5
[0056] Step 1: Weigh 90 g of α-Si3N4 powder and 200 g of silicon nitride grinding balls. Measure 1 L of anhydrous ethanol. Put the three into a high-energy ball milling tank filled with nitrogen, place it in a high-energy ball mill, and grind it at a rotation speed of 1200 rpm for 30 min. After grinding, separate the ceramic slurry and place it in a vacuum drying oven for drying for 24 h. After complete drying, sieve it with a 600-mesh sieve to obtain α-Si3N4 powder with refined particle size;
[0057] Step 2: Weigh 5 g of β-Si3N4 whiskers and add them to a beaker containing 1000 ml of anhydrous ethanol solution. Place the beaker in an ultrasonic device for dispersion. Set the ultrasonic power to 200 W and the ultrasonic time to 30 min to obtain a β-Si3N4 whisker suspension;
[0058] Step 3: Measure 500 ml of ammonia ethanol solution with a concentration of 11 mol / L and pour it into a burette;
[0059] Step 4: Weigh 10 g of Al2(NO3)3·9H2O and Y(NO3)3·6H2O and add them to a beaker containing 500 mL of absolute ethanol, and ultrasonically stir until completely dissolved. Under the condition of ultrasonic stirring, add 85 g of the α-Si3N4 powder obtained in Step 1 and the β-Si3N4 whisker suspension obtained in Step 2. Seal the resulting slurry and continue ultrasonic stirring to form a uniform suspension. Drop the ammonia ethanol solution in Step 3 into the suspension at a rate of 1 drop / s until the pH value reaches 8, then stop dropping and let it stand for 1 hour. After all the precipitation, wash it repeatedly with deionized water by centrifugation for more than three times to obtain a mixed powder. Put the mixed powder into a muffle furnace and keep it at 600 °C for 2 h to obtain β-Si3N4 whiskers and α-Si3N4 powder coated with Al2O3 and Y2O3 composite sintering aids;
[0060] Step 5: Place the β-Si3N4 whiskers and α-Si3N4 powder coated with Al2O3 and Y2O3 composite sintering aids obtained in Step 4 in a mold and perform cold isostatic pressing. Place the formed silicon nitride ceramic green body in a gas pressure sintering furnace, heat it to 1900 °C at a rate of 10 °C / min under the protection of 0.5 MPa nitrogen and keep it for 2 h. After heat preservation, cool it to room temperature with the furnace, take out the silicon nitride ceramic, cut it and polish its surface, and then conduct performance tests.
[0061] Compared with Example 3, in Example 4, Y(NO3)3·6H2O nitrate is used to obtain Y2O3 sintering aids through coprecipitation and high-temperature calcination, and in Example 5, Al2(NO3)3·9H2O and Y(NO3)3·6H2O nitrates are used to obtain Al2O3 and Y2O3 composite sintering aids.
[0062] Cut, grind, and polish the silicon nitride ceramic samples prepared in Examples 1-5, and measure their performance parameters such as density, flexural strength, and fracture toughness respectively.
[0063] Table 1 Mechanical property data of the silicon nitride ceramic samples corresponding to Examples 1-5
[0064]
[0065] From the XRD diffraction pattern of the silicon nitride ceramic sample of Example 1 shown in the appendix Figure 1 it can be concluded that the silicon nitride ceramic is composed of the main phase of β-Si3N4 and a small amount of intergranular phases of Lu2SiO5 and Lu4Si2O7N2, indicating that all α-Si3N4 has been transformed into β-Si3N4. The added nitrate generates lutetium oxide sintering aids through precipitation reaction and high-temperature calcination, and lutetium oxide reacts with β-Si3N4 whiskers, α-Si3N4, and SiO2 on their surfaces to generate intergranular phases of Lu2SiO5 and Lu4Si2O7N2. Appendix Figure 2The microstructural diagram of the silicon nitride ceramic sample of Example 1 shown indicates that the microstructure of the silicon nitride ceramic consists of long rod-shaped grains and short rod-shaped grains, and the rare earth intergranular phase is uniformly distributed between the silicon nitride grains. Further, from the mechanical property data of the silicon nitride ceramic samples corresponding to Examples 1-5 in Table 1, it can be seen that the silicon nitride ceramic samples prepared by the present invention all have high density, high flexural strength and fracture toughness.
[0066] Compared with Example 1, in Example 2, the high-energy ball milling process conditions of α-Si3N4 were optimized. The ball milling speed was increased to promote powder refinement, and while reducing the grinding time, the powder particle size was also reduced; the volume and ultrasonic power of the ultrasonic dispersion of β-Si3N4 whiskers in absolute ethanol solvent were increased, significantly shortening the ultrasonic time and reducing the agglomeration and damage of the whiskers due to excessive ultrasonic treatment; the content of β-Si3N4 whiskers was reduced, which helped to improve the mechanical strength of the silicon nitride ceramic; the sodium hydroxide ethanol solution was replaced with ammonia water as the precipitant. The reaction between ammonia water and Lu(NO3)3·6H2O was milder, and the particle size of the precipitated Lu(OH)3 was more uniform, which was beneficial to controlling the formation of the final calcined product Lu2O3 sintering aid. Attached Figure 3 It shows that the pores in the microstructure of the silicon nitride ceramic corresponding to Example 2 are reduced. Further, Table 1 shows that the density, flexural strength and fracture toughness of the silicon nitride ceramic in Example 2 are all better than those of the silicon nitride ceramic obtained in Example 1.
[0067] On the basis of Example 2, in Example 3, the concentration of ammonia water was increased, the dropping rate of the ammonia water precipitant was slowed down, and the precipitation standing time was increased, further promoting the formation of the sintering aid precursor; the final sintering temperature was further optimized from 1800 °C to 1900 °C. Attached Figure 4 It shows the microstructure of the silicon nitride ceramic obtained in Example 3. The silicon nitride grains are closely combined, indicating that the ceramic has high density; the rare earth nitride oxide intergranular phase is uniformly distributed at the grain junctions, and the long rod-shaped β-Si3N4 whisker grains are distributed in the matrix formed by β-Si3N4 grains with smaller particle sizes, indicating that the full growth of silicon nitride grains is promoted through condition optimization, which helps to improve the strength and toughness of the silicon nitride ceramic. Table 1 also shows that the density, flexural strength and fracture toughness of the silicon nitride ceramic are all significantly improved.
[0068] Compared with Example 3, in Example 4, Y2O3 sintering aid was obtained by co-precipitation and high-temperature calcination of Y(NO3)3·6H2O nitrate, and in Example 5, Al2O3 and Y2O3 composite sintering aid was obtained by using Al2(NO3)3·9H2O and Y(NO3)3·6H2O nitrates. The composite sintering aid helps to reduce the formation temperature of the sintering liquid phase and is beneficial to increasing the density of the silicon nitride ceramic and improving the mechanical properties of the silicon nitride ceramic under the condition of the same sintering temperature. Attached Figure 5 And attached Figure 6The silicon nitride grains and the intergranular phase are evenly distributed and tightly bonded. Further, Table 1 shows that compared with Example 3 and Example 4, the mechanical properties of the silicon nitride ceramic sample in Example 5 are more excellent.
[0069] By using the powder refinement, whisker dispersion, types and concentrations of precipitating agents, and conditions and parameters during the sintering process of silicon nitride ceramics in Examples 1-5, high-strength and high-toughness silicon nitride ceramic samples can be obtained, indicating that the method provided by the present invention has broad applicability.
[0070] The above content is only an example and illustration of the concept of the present invention. Those skilled in the art of this technology can make various modifications or supplements to the described specific embodiments or use similar methods for substitution, as long as they do not deviate from the concept of the invention or exceed the scope defined by this claim book, they should fall within the protection scope of the present invention.
Claims
1. A method for toughening silicon nitride ceramics by co-precipitation coating of silicon nitride whiskers, characterized in that: It includes the following steps: (1) Refine the particle size of α-Si3N4 powder: Weigh silicon nitride grinding balls and α-Si3N4 powder, measure absolute ethanol, and put the three into a high-energy ball milling jar filled with nitrogen protective gas. Place it in a high-energy ball mill, set the rotation speed and milling time, grind and refine the α-Si3N4 powder and mix the three evenly, dry and sieve; (2) Disperse β-Si3N4 whiskers: Weigh β-Si3N4 whiskers according to the proportion and add them into a beaker containing absolute ethanol solvent, and ultrasonically disperse to obtain a β-Si3N4 whisker suspension; (3) Prepare a precipitant solution: Weigh a precipitant and add it into a beaker containing absolute ethanol to prepare a precipitant solution. Stir and mix the precipitant solution in the beaker evenly and then put it into a burette for standby; (4) Prepare co-precipitation-coated α-Si3N4 powder and β-Si3N4 whiskers: Weigh the corresponding mass of metal nitrates according to the mass ratio and put them into a beaker containing absolute ethanol, and ultrasonically stir until completely dissolved. Under the condition of ultrasonic stirring, add the α-Si3N4 powder with refined particle size obtained in step (1) and the β-Si3N4 whisker suspension obtained in step (2). Seal the obtained slurry and continue ultrasonic stirring to form a uniform suspension. Slowly drip the precipitant solution in step (3) into the suspension until the pH value is appropriate, then stop dripping and let it stand for several hours. After all the precipitation, wash it repeatedly with deionized water by centrifugation for more than three times to obtain a mixed powder; Put the mixed powder into a muffle furnace for heat preservation to obtain β-Si3N4 whiskers and α-Si3N4 powder coated with an oxide sintering aid; (5) Sinter silicon nitride ceramics: Place the β-Si3N4 whiskers and α-Si3N4 powder coated with an oxide sintering aid obtained in step (4) in a mold and perform cold isostatic pressing. The formed silicon nitride ceramic green body is placed in a gas pressure sintering furnace, heated under nitrogen protection, and heat-preserved. After the heat preservation is completed, it is cooled to room temperature with the furnace. Take out the silicon nitride ceramic, cut it, polish its surface, and then perform performance testing.
2. The method for toughening silicon nitride ceramics by coprecipitation coating of silicon nitride whiskers according to claim 1, wherein: In step (1), the rotation speed is 600 - 1400 rpm, and the milling time is 20 - 60 min; The drying and sieving are carried out in a vacuum environment, and the mesh number of the sieve is in the range of 400 - 600 meshes.
3. A method for toughening silicon nitride ceramics by coprecipitation coating of silicon nitride whiskers according to claim 1, characterized in that: In step (2), the weighing according to the proportion means that 1 g of β-Si3N4 whiskers are dispersed with 100 - 300 ml of absolute ethanol solution; The ultrasonic power is 50 - 200 W, and the ultrasonic time is 30 - 120 min.
4. A method for toughening silicon nitride ceramics by co-precipitation coating of silicon nitride whiskers according to claim 1, characterized in that: In step (3), the precipitant solution is an ammonia ethanol solution or a sodium hydroxide ethanol solution. The molar concentration of the ammonia ethanol solution is 4 - 11 mol / L, and the molar concentration of the sodium hydroxide ethanol solution is 0.5 - 1 mol / L.
5. A method for toughening silicon nitride ceramics by co-precipitation coating of silicon nitride whiskers according to claim 1, characterized in that: In step (4), the mass ratio, which is the molar ratio of α-Si3N4 powder, β-Si3N4 whiskers and metal nitrate, is: (75-92.5 wt%):(2.5-10 wt%):(5-15 wt%); the metal nitrate includes but is not limited to one or more of Al(NO3)·9H2O, Y(NO3)3·6H2O, Lu(NO3)3·4H2O, Gd(NO3)3·6H2O, Er(NO3)3·5H2O; the slow dropping is at a rate of 1-2 drops / s; the appropriate pH value is 8-10; the standing for several hours is standing for 1-3 h; the heat preservation temperature is 450-700 °C, and the heat preservation time is 0.5-3 h.
6. A method for toughening silicon nitride ceramics by coprecipitation coating of silicon nitride whiskers according to claim 1, characterized in that: In step (5), the nitrogen protection pressure is 0.5-2 MPa, the heat preservation temperature is 1800-1900 °C, and the heat preservation time is 2-4 h.