Silicon nitride ceramic balls and methods of making the same

Abstract

The application discloses a kind of silicon nitride ceramic balls and preparation method thereof, its steps are: first with gamma-ammonia propyl triethoxysilane to alpha-silicon nitride powder is modified and is screened 0.1-0.2mm ball seed;With polyvinyl alcohol / carboxymethyl cellulose sodium aqueous solution as binder, rolling forming is obtained in sugar-coated machine silicon nitride ball blank;Again ball blank is placed in by nano silicon nitride and polyacrylic acid ethanol dispersion liquid is prepared by premixing-middle mixing-high speed dispersion enhanced mother liquor, 60-65 ℃ low speed stirring ≥8h complete enhancement;Subsequently 350-500W microwave drying 30-50min, 0.5MPa nitrogen atmosphere 1700-1750 ℃ pressure sintering 4h, finally polishing is obtained high density, high hardness, high wear resistance silicon nitride ceramic ball finished product.Innovatively proposed silicon nitride ball blank mother liquor enhancement technology, using nano silicon nitride sol as mother liquor to ball blank is maintained and enhanced, improve the density, uniformity and strength of ball blank, solve the difficulty that the strength and uniformity of rolling forming ball blank are poor.

Description

Technical Field

[0001] This invention relates to the field of silicon nitride ceramic ball technology, and in particular to a silicon nitride ceramic ball and its preparation method. Background Technology

[0002] Silicon nitride (Si3N4) ceramics, with their superior comprehensive properties such as high hardness, high strength, good wear resistance, chemical stability, and low coefficient of thermal expansion, demonstrate extremely high application value in many fields, especially in high-end bearings and precision grinding. In high-end bearing applications, high-quality silicon nitride ceramic balls can significantly improve bearing speed, accuracy, and service life, and can withstand extreme conditions such as high temperature, high speed, and strong corrosion. In the field of precision grinding, silicon nitride ceramic balls, due to their stable physicochemical properties, can achieve high-precision grinding of various materials, ensuring the surface quality and dimensional accuracy of the ground workpiece.

[0003] The preparation of silicon nitride ceramic spheres also conforms to the standard process of ceramic manufacturing, including mixing, molding, sintering, and processing. Compared with ordinary ceramic materials, the molding process of silicon nitride ceramic spheres, as a spherical material, is particularly important. Developing a method for molding spherical blanks with high efficiency, high density, and high precision has always been a research hotspot in this field. Currently, common ceramic sphere molding technologies mainly include dry pressing, isostatic pressing, vacuum casting, and roll forming. Patent CN113800919A involves a dry pressing method. Dry pressing is generally suitable for molding simple shapes, but it is difficult to directly mold spherical blanks with high sphericity. It usually requires a lot of machining, increasing equipment and manufacturing costs. Compared with dry pressing, roll forming, isostatic pressing, and vacuum casting can all directly prepare spherical blanks with good sphericity. However, these three molding methods also have their own advantages and disadvantages. Patent CN113135762A discloses a vacuum injection molding method, which produces spherical blanks with very high strength and excellent uniformity. However, due to the long cycles of slurry preparation, molding, curing, and demolding, high raw material costs, complex molds, long process flow, and high energy consumption, this method has low production efficiency and high production costs. Patent CN110076882A discloses an isostatic pressing method, which produces spherical blanks with strength close to that of the vacuum injection molding method and also with good uniformity. However, this method also faces the problems of low production efficiency and high production costs due to the long cycles of bagging, pressurizing, and unloading, expensive equipment, and consumable elastic molds. Patent CN111825435A relates to a rolling molding method, which can achieve continuous production. One machine can simultaneously mold tens of thousands of spheres. The equipment is simple, the mold cost is extremely low, the operation is simple, and the labor cost is low. By controlling the rolling time and the amount of material fed, spherical blanks of different diameters can be easily prepared, and large-sized spheres can be easily produced. Roll forming has a much lower production time and cost than vacuum injection molding and cold isostatic pressing. However, roll forming also faces the problem of a large number of pores caused by the bonding of powder layers during the molding process, resulting in insufficient strength and poor uniformity of the formed spherical blank. Summary of the Invention

[0004] This invention provides a method for preparing silicon nitride ceramic spheres based on roll forming. The method involves strengthening the silicon nitride sphere blank obtained by roll forming with mother liquor, thereby improving the strength and uniformity of the sphere blank and solving the problems of insufficient strength and poor uniformity of sphere blanks produced by existing roll forming.

[0005] Compared to conventional roll forming processes, this invention adds a mother liquor enhancement process to the preform. In this process, the roll-formed silicon nitride preform is placed in a nano-silicon nitride sol for aging and curing. Utilizing the numerous interconnected pores within the roll-formed silicon nitride preform, the sol spontaneously penetrates the preform under capillary force. The nano-sized silicon nitride particles in the sol effectively fill the pores of the preform, while also acting as "pinning" and "bridging" elements within the preform. This not only improves the density and uniformity of the preform but also enhances its strength.

[0006] The objective of this invention is achieved through the following technical solution: This invention provides a method for preparing silicon nitride ceramic spheres, the preparation process including the following steps: The modifier γ-aminopropyltriethoxysilane was mixed with α-silicon nitride powder and stirred for 5-10 minutes to obtain modified silicon nitride powder. The modified silicon nitride powder is screened in a sorting machine to obtain spherical particles with a diameter of 0.1-0.2 mm, and these particles are called spheres. Polyvinyl alcohol and sodium carboxymethyl cellulose are dispersed in pure water in a certain proportion to prepare an adhesive solution. The modified silicon nitride powder and the spheres are placed in a sugar coating machine for rolling molding. During the rolling process, the adhesive liquid is sprayed onto the surface of the spheres through a spraying device. After a period of rolling molding, silicon nitride sphere blanks can be obtained. The silicon nitride pellets are placed in a microwave drying oven and dried at a specific power. The dried silicon nitride pellets are placed in a pressure sintering furnace and sintered at a specific temperature for a period of time to obtain silicon nitride ceramic pellets. The sintered silicon nitride spheres are polished to obtain finished silicon nitride ceramic spheres.

[0007] Furthermore, it also includes reinforcement of the spheroid: Nano-silicon nitride powder and polyacrylic acid are dispersed in ethanol in a certain proportion, and then premixed, medium mixed and high-speed dispersed in sequence to obtain nano-silicon nitride reinforced mother liquor. The silicon nitride pellets are dispersed in a reinforcing mother liquor and stirred at a low speed at a specific temperature for a period of time to achieve the reinforcement of the silicon nitride pellets.

[0008] Furthermore, the mass ratio of the modifier to the α-silicon nitride powder is 0.01-0.03 wt%, the particle size of the α-silicon nitride powder is 0.5-0.8 μm, and the purity of the α-silicon nitride powder is 93%-96%.

[0009] Furthermore, the polyvinyl alcohol has a molecular weight of 146-186 kDa, the sodium carboxymethyl cellulose has a molecular weight of 250 kDa, the mass ratio of polyvinyl alcohol to sodium carboxymethyl cellulose is 1:1, and the total mass of polyvinyl alcohol and sodium carboxymethyl cellulose accounts for 5-10 wt% of the adhesive solution.

[0010] Furthermore, the sugar coating machine rotates at a speed of 20-30 rpm, the mass ratio of the ball to the modified silicon nitride powder is 1:8-1:10, and the adhesive spraying rate is 4-10 mL / min.

[0011] Furthermore, the nano-silicon nitride powder has a particle size of 20 nm, a purity of 99.9%, a molecular weight of 450 kDa for the polyacrylic acid, a mass fraction of 1-3 wt% for the nano-silicon nitride in the mother liquor, a mass fraction of 0.1-0.3 wt% for the polyacrylic acid in the mother liquor, a premixing speed and time of 300 rpm and 3-5 min, a medium mixing speed and time of 3000 rpm and 2-3 min, and a high-speed dispersion speed and time of 10000 rpm and 25-30 min, respectively.

[0012] Furthermore, the volume ratio of the silicon nitride pellet to the reinforcing mother liquor is 1:10, and the stirring parameters are: rotation speed of 50-100 rpm, temperature of 60-65℃, and time of not less than 8 hours.

[0013] Furthermore, the power of the microwave drying oven is 350-500W, and the drying time is 30-50 minutes.

[0014] Furthermore, the atmosphere for the gas pressure sintering is nitrogen, the sintering temperature is 1700-1750℃, the sintering time is 4h, and the sintering gas pressure is 0.5MPa.

[0015] On the other hand, the present invention provides a silicon nitride ceramic ball prepared by any of the above methods.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. An innovative mother liquor enhancement technology for silicon nitride spherical blanks is proposed, which uses nano-silicon nitride sol as the mother liquor to cure and enhance the spherical blanks, thereby improving the density, uniformity and strength of the spherical blanks and solving the problem of poor strength and uniformity of spherical blanks produced by roll forming. 2. Nano-silicon nitride sol is prepared using nano-silicon nitride, polyacrylic acid, and ethanol as raw materials. High-purity silicon nitride powder with a diameter of 20 nm is preferred, as its small particle size and high purity ensure that the nano-silicon nitride particles can penetrate the pores of the preform and improve its uniformity. Ethanol is preferred as the solvent because the binders polyvinyl alcohol and sodium carboxymethyl cellulose inside the preform are insoluble in ethanol, thus ethanol as the mother liquor solvent will not cause structural damage to the preform. Polyacrylic acid is optimized as the dispersant. Polyacrylic acid is soluble in ethanol and can help the nano-silicon nitride particles be stably dispersed in ethanol to form a sol. Simultaneously, polyacrylic acid can form strong hydrogen bonds with the polyvinyl alcohol in the preform, strengthening the bonding strength between the nano-silicon nitride particles and the preform. 3. After dispersing the billet in the reinforcing mother liquor, the "slow stirring combined with heating" method is used to accelerate the penetration rate of the mother liquor into the billet, effectively improving the penetration depth and uniformity of the mother liquor into the billet, thereby improving the reinforcing effect of the mother liquor on the billet. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The experimental flowchart for preparing silicon nitride ceramic spheres according to the present invention is shown; Figure 2 A photograph of the silicon nitride ceramic spheres prepared in Example 1 of the present invention is shown; Figure 3 The SEM image of the surface of the silicon nitride ceramic sphere prepared in Example 1 of the present invention is shown. Detailed Implementation

[0019] The present invention will be further described below with reference to embodiments.

[0020] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0021] All raw materials used in this invention are common in the field, and those skilled in the art can directly purchase them from the market or prepare the same / similar raw materials themselves.

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: A technique for preparing silicon nitride ceramic spheres based on roll forming, the steps of which are as follows: S1. Surface modification: The modifier γ-aminopropyltriethoxysilane and α-silicon nitride powder are mixed at a mass ratio of 0.02wt% and stirred for 10 min to obtain modified silicon nitride powder.

[0024] S2. Preparation of spheres: The modified spherical silicon nitride powder is screened in a sorting machine to obtain spherical particles with a diameter of 0.1-0.2 mm. These spherical particles are the spheres.

[0025] S3. Preparation of adhesive solution: Take equal mass of polyvinyl alcohol and sodium carboxymethyl cellulose, disperse polyvinyl alcohol and sodium carboxymethyl cellulose in pure water to prepare an adhesive solution with a solid content of 8wt%.

[0026] S4. Roll forming: The modified silicon nitride powder prepared in S1 and the spheres prepared in S2 are placed in a sugar coating machine at a mass ratio of 10:1 and rolled at a speed of 20 rpm. During the rolling process, the adhesive liquid prepared in S3 is sprayed onto the surface of the spheres at a spraying rate of 6 mL / min through a spraying device. After rolling forming, silicon nitride sphere blanks can be obtained.

[0027] S5. Preparation of the reinforcing mother liquor: 20nm diameter nano-silicon nitride powder and polyacrylic acid are dispersed in ethanol, and then subjected to premixing at 300rpm for 3min, neutral mixing at 3000rpm for 2min, and high-speed dispersion at 10000rpm for 25min to obtain the nano-silicon nitride reinforcing mother liquor. The mass fraction of nano-silicon nitride in the mother liquor is 1wt%, and the mass fraction of polyacrylic acid in the mother liquor is 0.1wt%.

[0028] S6. Mother liquor reinforcement: The silicon nitride pellets and the reinforcing mother liquor are mixed at a volume ratio of 1:10, and the mixture is stirred at 75 rpm for 8 hours at 60°C to achieve reinforcement of the silicon nitride pellets. The volume ratio of the silicon nitride pellets to the reinforcing mother liquor is 1:10.

[0029] S7. Drying: Place the preforms reinforced by the mother liquor into a microwave drying oven and dry them at 350W power for 50 minutes.

[0030] S8. Sintering: The dried silicon nitride spherical blanks are placed in a nitrogen atmosphere pressure sintering furnace and sintered at 1750℃ and 0.5MPa pressure for 4 hours to obtain silicon nitride ceramic spheres.

[0031] S9. Polishing: Polish the sintered silicon nitride spheres to obtain the finished silicon nitride ceramic spheres.

[0032] Example 2: A technology for preparing silicon nitride ceramic balls based on rolling molding. The process is basically the same as that in Example 1, except that the mass ratio in step S1 is changed to 0.03wt%, the rolling speed and spraying rate in step S4 are changed to 30rpm and 10mL / min, the mass fractions of nano-silicon nitride and polyacrylic acid in step S5 are changed to 3wt% and 0.3wt%, the temperature, stirring speed and time in step S6 are changed to 65℃, 100rpm and 12h, and the power and drying time in step S7 are changed to 500W and 30min.

[0033] Example 3: A technology for preparing silicon nitride ceramic balls based on roll forming. The process is basically the same as that in Example 2, except that the solid content of the binder solution in step S3 is changed to 10%, the mass fraction of polyacrylic acid in step S5 is changed to 0.1wt%, and the premixing, intermediate mixing and high-speed dispersion times in step S5 are changed to 5min, 3min and 30min, respectively.

[0034] Example 4: A technology for preparing silicon nitride ceramic balls based on roll forming. The process is basically the same as that in Example 1, except that the solid content of the binder solution in step S3 is changed to 5%, the mass fraction of polyacrylic acid in step S5 is changed to 0.3wt%, the premixing time in step S5 is changed to 5min, and the stirring speed in step S6 is changed to 100rpm.

[0035] Comparative Example 1: A technology for preparing silicon nitride ceramic balls based on rolling molding. The difference between Comparative Example 1 and Example 1 is that steps S5 and S6 are not included in Comparative Example 1. The shaped ball blank is directly dried, sintered and polished. The remaining steps are the same as in Example 1.

[0036] Comparative Example 2: A technology for preparing silicon nitride ceramic balls based on roll forming. The difference between Comparative Example 2 and Example 1 is that polyacrylic acid is not added when preparing the mother liquor in step S5. The remaining steps are the same as in Example 1.

[0037] Comparative Example 3: A technology for preparing silicon nitride ceramic balls based on rolling molding. The difference between Comparative Example 3 and Example 1 is that step S5 is replaced with 200nm silicon nitride powder, while the remaining steps are the same as in Example 1.

[0038] Comparative Example 4: A technology for preparing silicon nitride ceramic balls based on roll forming. The difference between Comparative Example 4 and Example 1 is that the mass fractions of nano-silicon nitride and polyacrylic acid in the mother liquor of step S5 are changed to 5 wt% and 0.5 wt%, respectively. The remaining steps are the same as in Example 1.

[0039] Comparative Example 5: A technology for preparing silicon nitride ceramic balls based on rolling molding. The difference between Comparative Example 5 and Example 1 is that the temperature of step S6 is changed to 25°C, while the other steps are the same as in Example 1.

[0040] Comparative Example 6: A technology for preparing silicon nitride ceramic balls based on rolling molding. The difference between Comparative Example 6 and Example 1 is that the stirring time in step S6 is changed to 4 hours, while the other steps are the same as in Example 1.

[0041] According to standard GB / T25995-2010, the density of the silicon nitride ceramic balls prepared in all examples and comparative examples was measured; according to standard GB / T16534-2009, the Vickers hardness of the silicon nitride ceramic balls prepared in all examples and comparative examples was measured; according to standard GB / T308-2002, the crushing strength of the silicon nitride ceramic balls prepared in all examples and comparative examples was tested; and according to industry standard T / IFP003-2020, the wear of the silicon nitride ceramic balls prepared in all examples and comparative examples was tested. The test results are shown in Table 1. Table 1 Test results of silicon nitride ceramic balls .

[0042] As can be seen from the test results in Table 1, the silicon nitride ceramic balls prepared in Examples 1 to 4 of this invention, after complete and effective mother liquor reinforcement, have sufficient nanoparticles filling the interior of the ceramic blank, resulting in high density, Vickers hardness, crushing strength, and low wear. In Comparative Example 1, the ceramic balls without mother liquor reinforcement have more internal pores, lower density, hardness, and strength, and higher wear. In Comparative Example 2, no polyacrylic acid dispersant was added to the mother liquor, so the nanoparticles in the mother liquor could not be effectively dispersed. Some nanoparticles agglomerated into large particles that could not enter the pores of the ceramic blank to enhance the density and strength of the ceramic balls. Therefore, its density, Vickers hardness, and strength decreased, and its wear increased. In Comparative Example 3, 200nm silicon nitride particles were used instead of 20nm silicon nitride particles. 200nm silicon nitride particles are already at the submicron level and cannot enter the nanopores inside the ceramic blank. Therefore, they can only achieve a surface coating effect on the ceramic blank, resulting in lower density, hardness, and strength, and higher wear. Comparative Example 4 used silicon nitride particles with higher solids content and polyacrylic acid to prepare the mother liquor. The excessively high solids content and viscosity caused some nanoparticles to agglomerate and reduced the fluidity of the mother liquor, which in turn reduced the reinforcing effect of the mother liquor. As a result, its density, hardness, and strength were lower than those of the example, and wear was slightly increased. Comparative Examples 5 and 6 reduced the temperature and stirring time during the mother liquor reinforcement process, which reduced the speed and time of the nano-silicon nitride sol penetrating into the spherical blank during the mother liquor reinforcement process, thus affecting the reinforcement effect. The density, strength, and hardness of the ceramic balls produced were lower, and wear was increased.

[0043] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing silicon nitride ceramic spheres, characterized in that, The preparation process includes the following steps: The modifier γ-aminopropyltriethoxysilane was mixed with α-silicon nitride powder and stirred for 5-10 minutes to obtain modified silicon nitride powder. The modified silicon nitride powder is screened in a sorting machine to obtain spherical particles with a diameter of 0.1-0.2 mm, and these particles are called spheres. Polyvinyl alcohol and sodium carboxymethyl cellulose are dispersed in pure water in a certain proportion to prepare an adhesive solution. The modified silicon nitride powder and the spheres are placed in a sugar coating machine for rolling molding. During the rolling process, the adhesive liquid is sprayed onto the surface of the spheres through a spraying device. After a period of rolling molding, silicon nitride sphere blanks can be obtained. The silicon nitride pellets are placed in a microwave drying oven and dried at a specific power. The dried silicon nitride pellets are placed in a pressure sintering furnace and sintered at a specific temperature for a period of time to obtain silicon nitride ceramic pellets. The sintered silicon nitride spheres are polished to obtain finished silicon nitride ceramic spheres.

2. The method for preparing silicon nitride ceramic spheres according to claim 1, characterized in that, This also includes reinforcement of the spheroid: Nano-silicon nitride powder and polyacrylic acid are dispersed in ethanol in a certain proportion, and then premixed, medium mixed and high-speed dispersed in sequence to obtain nano-silicon nitride reinforced mother liquor. The silicon nitride pellets are dispersed in a reinforcing mother liquor and stirred at a low speed at a specific temperature for a period of time to achieve the reinforcement of the silicon nitride pellets.

3. The method for preparing silicon nitride ceramic spheres according to claim 1, characterized in that, The mass ratio of the modifier to the α-silicon nitride powder is 0.01-0.03 wt%, the particle size of the α-silicon nitride powder is 0.5-0.8 μm, and the purity of the α-silicon nitride powder is 93%-96%.

4. The method for preparing silicon nitride ceramic spheres according to claim 1, characterized in that, The polyvinyl alcohol has a molecular weight of 146-186 kDa, the sodium carboxymethyl cellulose has a molecular weight of 250 kDa, the mass ratio of polyvinyl alcohol to sodium carboxymethyl cellulose is 1:1, and the total mass of polyvinyl alcohol and sodium carboxymethyl cellulose accounts for 5-10 wt% of the adhesive solution.

5. The method for preparing silicon nitride ceramic spheres according to claim 1, characterized in that, The sugar coating machine rotates at a speed of 20-30 rpm, the mass ratio of the ball to the modified silicon nitride powder is 1:8-1:10, and the adhesive spraying rate is 4-10 mL / min.

6. The method for preparing silicon nitride ceramic spheres according to claim 2, characterized in that, The nano-silicon nitride powder has a particle size of 20 nm and a purity of 99.9%. The polyacrylic acid has a molecular weight of 450 kDa. The nano-silicon nitride has a mass fraction of 1-3 wt% in the mother liquor, and the polyacrylic acid has a mass fraction of 0.1-0.3 wt% in the mother liquor. The premixing speed and time are 300 rpm and 3-5 min, respectively. The intermediate mixing speed and time are 3000 rpm and 2-3 min, respectively. The high-speed dispersion speed and time are 10000 rpm and 25-30 min, respectively.

7. The method for preparing silicon nitride ceramic spheres according to claim 2, characterized in that, The volume ratio of the silicon nitride pellet to the reinforcing mother liquor is 1:10, and the stirring parameters are: rotation speed of 50-100 rpm, temperature of 60-65℃, and time of not less than 8 hours.

8. The method for preparing silicon nitride ceramic spheres according to claim 2, characterized in that, The microwave drying oven has a power of 350-500W and a drying time of 30-50 minutes.

9. The method for preparing silicon nitride ceramic spheres according to claim 2, characterized in that, The atmosphere for the gas pressure sintering is nitrogen, the sintering temperature is 1700-1750℃, the sintering time is 4h, and the sintering gas pressure is 0.5MPa.

10. A silicon nitride ceramic ball, characterized in that, Prepared by any of the methods described in claims 1-9.

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