Sintering method of large-size silicon carbide ceramic ball

Abstract

The invention provides a sintering method of a large-size silicon carbide ceramic ball, which belongs to the technical field of silicon carbide ceramic ball preparation, and specifically comprises the following steps: forming a green body ball, turning the green body ball, processing a sintering bottom support tool, loading the ball into a mold, sintering and post-processing. A special auxiliary tool is combined, the tool adopts a material formula and a pressing process which are the same as those of a green body ball, the green body ball and the auxiliary tool have the same shrinkage rate during sintering, the problems of deformation and poor roundness caused by high-temperature shrinkage in the sintering process of the green body ball can be effectively reduced, the roundness and uniformity of the prepared silicon carbide ceramic ball are remarkably improved, and the service life of the silicon carbide ceramic ball is prolonged. And the method is suitable for preparing industrial high-performance ceramic balls.

Description

Technical Field

[0001] The invention relates to the technical field of silicon carbide ceramic ball preparation, and in particular to a sintering method for large-size silicon carbide ceramic balls. Background Art

[0002] Silicon carbide ceramics have excellent properties such as high hardness, high strength, high temperature resistance, oxidation resistance, corrosion resistance, wear resistance, thermal shock resistance, and good chemical stability, making them an ideal material for preparing high-performance ceramic balls. Ceramic balls prepared using the excellent properties of silicon carbide ceramics have better performance than traditional metal balls and plastic balls, and are particularly suitable for harsh industrial and mining conditions such as strong corrosion, high speed, heavy load, and high temperature.

[0003] However, the strong bonding of silicon carbide ceramics makes it difficult to sinter. Even when sintered at a high temperature of 2100°C, the self-diffusion coefficient of C and Si is only 1.5×10 -10 and 1.5×10 -13 cm 2 / s, therefore, it is difficult to sinter SiC to a high density without adding sintering aids or applying high pressure. Generally, special sintering methods or the addition of sintering aids are required to achieve sintering densification. At present, the commonly used SiC ceramic sintering methods include reaction sintering, pressureless sintering, hot pressing sintering, hot isostatic pressing sintering and spark plasma sintering.

[0004] Reaction sintered silicon carbide has a low sintering temperature (generally between 1400-1800℃), a relatively simple process, and very little dimensional shrinkage during sintering. However, due to the presence of more free silicon (generally not less than 8wt.%), its hardness is low, and its wear resistance, high temperature resistance, and corrosion resistance are poor. Pressureless sintering includes pressureless solid phase sintering and pressureless liquid phase sintering. Pressureless solid phase sintering generally adds boron, carbon or their compounds as sintering aids, and the sintering temperature is high (generally between 2100-2300℃); pressureless liquid phase sintering generally uses metal oxides as sintering aids, such as alumina, yttrium oxide, etc., and the sintering temperature varies greatly depending on the type and content of the sintering aids used (generally between 1500-2100℃); but whether it is pressureless solid phase sintering or pressureless liquid phase sintering, there is a large size shrinkage (generally between 13-20%). If the appropriate method is not adopted, the sintered silicon carbide ceramic ball will be deformed, warped, or even cracked. Hot pressing sintering, hot isostatic pressing sintering and spark plasma sintering methods are all pressure sintering processes, which are suitable for the preparation of high-performance ceramic products, but the process is relatively complex, with low production efficiency and high production cost.

[0005] Silicon carbide ceramic balls prepared by pressureless solid phase sintering can meet the application requirements in most cases and are currently the most commonly used sintering method for silicon carbide ceramic balls. However, as discussed above, pressureless solid phase sintering of silicon carbide has a high sintering temperature and large sintering shrinkage, and is prone to deformation during sintering, especially when sintering larger ceramic balls (diameter greater than 80 mm). When the ceramic ball is placed directly on the supporting graphite plate, the graphite plate will not soften, shrink, or deform at the sintering temperature. However, since only the lower spherical crown of the ceramic ball is tangent to the graphite plate, the tangent point carries the entire mass of the ceramic ball, resulting in obvious deformation of the lower spherical crown under the action of gravity, and the shrinkage in the vertical direction is greater than the horizontal shrinkage, so the roundness of the ceramic ball deteriorates, and the larger the size of the ceramic ball, the more obvious this deformation and unevenness. Summary of the invention

[0006] The object of the present invention is to provide a sintering method for large-sized silicon carbide ceramic balls to solve the problems raised in the above background technology.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A sintering method for large-sized silicon carbide ceramic balls comprises the following steps:

[0009] (1) Green ball molding

[0010] The green balls are made by isostatic pressing process with silicon carbide ceramic granulation powder as raw material, and then dried at 50-80℃ for 1-5 hours to avoid cracking caused by rapid volatilization of water during sintering;

[0011] (2) Green ball turning

[0012] The green ball dried in step (1) is simply trimmed to remove excess corners, and the radius of the trimmed green ball is R;

[0013] (3) Processing of the firing support tooling

[0014] Using the same material formula and pressing process as those for preparing the green ball in step (1), pressing a round rod green body, and lathing the round rod into a cylinder, and processing a hemispherical ball hole on the upper cylindrical surface of the cylinder, the ball hole being used to support the green ball in step (2);

[0015] (4) Loading the ball into the mold

[0016] The green ball processed in step (2) is rolled in boron nitride powder so that a thin layer of boron nitride adheres to its surface to prevent it from sticking to the firing bottom support tooling during sintering; a thin layer of graphite fiber wool is padded in the semi-spherical hole of the firing bottom support tooling, and then the green ball adhered with boron nitride powder is placed in the firing bottom support tooling padded with graphite fiber wool;

[0017] (5) Sintering

[0018] The sintering base tooling containing the green ball in step (4) is placed in a vacuum sintering furnace and sintered at 2100-2200° C. for 1-7 hours to obtain a sintered body of silicon carbide ceramic balls;

[0019] (6) Post-processing

[0020] The silicon carbide ceramic ball sintered body is processed after grinding and polishing to obtain the silicon carbide ceramic ball of required specifications.

[0021] Preferably, in step (1), the pressure used in the isostatic pressing process is 180-250 MPa, and the holding time is 5-30 minutes.

[0022] Preferably, in step (1), the density of the green ball after drying is not less than 1.7 g / cm 3 .

[0023] Preferably, in step (3), the diameter of the cylinder is D, and the height is H, the value of D is not less than 2.15R, and the value of H is not less than 1.15R.

[0024] Preferably, the center of the spherical hole coincides with the center of the cylindrical surface of the cylinder, the radius of the spherical hole is R1, and the value of R1 is 1.01-1.05R.

[0025] The beneficial effects of the above technical solution of the present invention are as follows:

[0026] 1. The firing base support tooling and the green ball are made of the same material formula and pressing process, so they have the same shrinkage rate, and the shrinkage changes during the sintering process are synchronized, so it can be ensured that the green ball is always supported by the firing base support tooling, that is, the lower hemisphere of the green ball can be supported. The technical solution of the present invention solves the problem that in the traditional sintering process, when the green ball is directly placed on the firing graphite plate, only the lower spherical crown is tangent to the graphite plate during the sintering process, and the tangent point bears the entire mass of the silicon carbide ceramic ball, resulting in obvious deformation of the lower spherical crown under the action of gravity, and the shrinkage in the vertical direction is greater than the horizontal shrinkage, so the roundness of the ceramic ball deteriorates; at the same time, it also solves the problem that the hard support base support tooling cannot shrink synchronously with the ceramic ball, resulting in the separation of the ceramic ball and the support tooling during the sintering process; the present invention significantly improves the roundness and uniformity of the silicon carbide ceramic ball;

[0027] 2. Before sintering, the green ball is rolled in boron nitride powder to make a thin layer of boron nitride adhere to its surface, thus avoiding the scrapping of the ceramic ball caused by the adhesion between the silicon carbide ceramic ball and the support during the sintering process;

[0028] 3. Pad a thin layer of graphite fiber cotton in the semicircular hole of the firing support tooling, which can not only ensure that the firing support tooling and the silicon carbide ceramic ball green body fit well, but also play a buffering and flexible isolation role, avoiding hard-to-hard damage to ceramics, and can also effectively prevent the silicon carbide ceramic ball from sticking to the firing support. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] By reading the following detailed description with reference to the accompanying drawings, the above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily understood. In the accompanying drawings, several embodiments of the present invention are shown in an exemplary and non-limiting manner, and the same or corresponding reference numerals represent the same or corresponding parts, wherein:

[0030] Figure 1 This is a schematic diagram of the processing of the firing support tooling provided by the present invention. DETAILED DESCRIPTION

[0031] Example:

[0032] A sintering method for large-sized silicon carbide ceramic balls comprises the following steps:

[0033] (1) Green ball molding

[0034] The isostatic pressing process is adopted, with silicon carbide ceramic granulated powder as raw material, and the green ball is made at 180-250MPa, and the pressure is maintained for 5-30 minutes, and then the green ball is dried at 50-80℃ for 1-5 hours to avoid cracking caused by rapid volatilization of water during sintering. The density of the green ball after drying is not less than 1.7g / cm 3 .

[0035] The silicon carbide ceramic granulation powder and its formula used to prepare green balls are commonly used formulas in the industry.

[0036] (2) Green ball turning

[0037] The green ball dried in step (1) is simply trimmed to remove excess corners, and the radius of the trimmed green ball is R.

[0038] (3) Processing of the firing support tooling

[0039] The same material formula and pressing process as those used to prepare the green ball in step (1) are used to press a round rod green body, and the round rod is machined into a cylinder with a diameter of D and a height of H. The value of D is not less than 2.15R, and the value of H is not less than 1.15R. A hemispherical hole is machined on the upper cylindrical surface of the cylinder, and the hole is used to support the green ball in step (2). The center of the hole coincides with the center of the upper cylindrical surface of the cylinder. The radius of the hole is R1, and the value of R1 is 1.01-1.05R.

[0040] The schematic diagram of the processing of the bottom support tooling is as follows Figure 1 shown.

[0041] The firing base support fixture and the green ball are made of the same material formula and pressing process, so they have the same shrinkage rate, and the shrinkage changes of the firing base support fixture and the green ball during the sintering process are synchronized, thereby ensuring that the green ball is always supported by the firing base support fixture, that is, the lower hemisphere of the green ball can be supported, thus avoiding deformation of the green ball during the sintering process.

[0042] (4) Loading the ball into the mold

[0043] The green ball processed in step (2) is rolled in boron nitride powder so that a thin layer of boron nitride adheres to its surface to prevent it from sticking to the firing bottom support tooling during sintering; a thin layer of graphite fiber wool is padded in the semi-spherical hole of the firing bottom support tooling, and then the green ball adhered with boron nitride powder is placed in the firing bottom support tooling padded with graphite fiber wool.

[0044] Graphite fiber wool can not only ensure a good fit between the firing base and the green ball, but also play a role of buffering and flexible isolation; in addition, boron nitride and graphite fiber wool can also fill the gap between the green ball and the firing base to ensure that the green ball is evenly heated during the sintering process.

[0045] (5) Sintering

[0046] The firing support tooling containing the green ball in step (4) is placed in a vacuum sintering furnace and sintered at 2100-2200° C. for 1-7 hours to obtain a sintered body of silicon carbide ceramic balls.

[0047] (6) Post-processing

[0048] The silicon carbide ceramic ball sintered body is processed after grinding and polishing to obtain the silicon carbide ceramic ball of required specifications.

[0049] Embodiment 1:

[0050] A sintering method for large-sized silicon carbide ceramic balls comprises the following steps:

[0051] (1) Green ball molding

[0052] The raw material is the commonly used silicon carbide pressureless solid phase sintering granulation powder in the industry. The green ball is dried at 50℃ for 5 hours at 180MPa for 20 minutes. The density of the green ball after drying is 1.75g / cm 3 .

[0053] (2) Green ball turning

[0054] The green ball dried in step (1) is simply trimmed to remove excess corners, and the radius of the green ball after trimming is 100 mm.

[0055] (3) Processing of the firing support tooling

[0056] The same material formula and pressing process as those used in step (1) for preparing the green ball are used to press a round rod green body, and the round rod is machined into a cylinder with a diameter of 220 mm and a height of 120 mm; a hemispherical hole is machined on the upper cylindrical surface of the cylinder, the center of the hole coincides with the center of the upper cylindrical surface of the cylinder, and the radius of the hole is 101 mm.

[0057] (4) Loading the ball into the mold

[0058] The green ball processed in step (2) is rolled in boron nitride powder so that a thin layer of boron nitride adheres to its surface; a thin layer of graphite fiber wool is padded in the semi-spherical hole of the firing bottom support tooling, and the thickness of the fiber wool is 1 mm; and then the green ball adhered with boron nitride powder is placed in the firing bottom support tooling padded with graphite fiber wool.

[0059] (5) Sintering

[0060] The firing base tooling containing the green ball in step (4) is placed in a vacuum sintering furnace and sintered at 2120° C. for 4 hours to obtain a sintered silicon carbide ceramic ball. The diameter of the sintered silicon carbide ceramic ball is measured to be about 82 mm.

[0061] After testing, the maximum diameter of the sintered silicon carbide ceramic ball is 82.4mm, the minimum diameter is 82.0mm, and the relative error is 0.49%.

[0062] (6) Post-processing

[0063] The silicon carbide ceramic ball sintered body is processed after grinding and polishing to obtain the silicon carbide ceramic ball of required specifications.

[0064] After testing, the relative density of the obtained silicon carbide ceramic ball is 98.8%, and the Vickers microhardness is 26.6 GPa.

[0065] Embodiment 2:

[0066] A sintering method for large-sized silicon carbide ceramic balls comprises the following steps:

[0067] (1) Green ball molding

[0068] The raw material is the commonly used silicon carbide pressureless solid phase sintering granulation powder in the industry. The green ball is dried at 70℃ for 2 hours at 250MPa for 5 minutes. The density of the green ball after drying is 1.89g / cm 3 .

[0069] (2) Green ball turning

[0070] The green ball dried in step (1) is simply trimmed to remove excess corners, and the radius of the green ball after trimming is 150 mm.

[0071] (3) Processing of the firing support tooling

[0072] The same material formula and pressing process as those used in step (1) for preparing the green ball are used to press a round rod green body, and the round rod is machined into a cylinder with a diameter of 325 mm and a height of 175 mm; a hemispherical hole is machined on the upper cylindrical surface of the cylinder, the center of the hole coincides with the center of the upper cylindrical surface of the cylinder, and the radius of the hole is 152 mm.

[0073] (4) Loading the ball into the mold

[0074] The green ball processed in step (2) is rolled in boron nitride powder to make a thin layer of boron nitride adhere to its surface; a thin layer of graphite fiber wool is padded in the semi-spherical hole of the firing bottom support tooling, and the thickness of the fiber wool is 2 mm; then the green ball adhered with boron nitride powder is placed in the firing bottom support tooling padded with graphite fiber wool.

[0075] (5) Sintering

[0076] The firing base tooling containing the green ball in step (4) is placed in a vacuum sintering furnace and sintered at 2120° C. for 4 hours to obtain a sintered silicon carbide ceramic ball. The diameter of the sintered silicon carbide ceramic ball is measured to be about 123 mm.

[0077] After testing, the maximum diameter of the sintered silicon carbide ceramic ball is 123.3mm, the minimum diameter is 123.0mm, and the relative error is 0.24%.

[0078] (6) Post-processing

[0079] The silicon carbide ceramic ball sintered body is processed after grinding and polishing to obtain the silicon carbide ceramic ball of required specifications.

[0080] After testing, the relative density of the obtained silicon carbide ceramic ball is 99.2%, and the Vickers microhardness is 27.6 GPa.

[0081] Embodiment 3:

[0082] A sintering method for large-sized silicon carbide ceramic balls comprises the following steps:

[0083] (1) Green ball molding

[0084] The raw material is the commonly used silicon carbide pressureless solid phase sintering granulation powder in the industry. The green ball is dried at 50℃ for 5 hours at 180MPa for 20 minutes. The density of the green ball after drying is 1.75g / cm 3 .

[0085] (2) Green ball turning

[0086] The green ball dried in step (1) is simply trimmed to remove excess corners, and the radius of the green ball after trimming is 100 mm.

[0087] (3) Processing of the firing support tooling

[0088] The same material formula and pressing process as those used in step (1) for preparing the green ball are used to press a round rod green body, and the round rod is machined into a cylinder with a diameter of 220 mm and a height of 120 mm; a hemispherical hole is machined on the upper cylindrical surface of the cylinder, the center of the hole coincides with the center of the upper cylindrical surface of the cylinder, and the radius of the hole is 101 mm.

[0089] (4) Loading the ball into the mold

[0090] The green ball processed in step (2) is rolled in boron nitride powder so that a thin layer of boron nitride adheres to its surface; a thin layer of graphite fiber wool is padded in the semi-spherical hole of the firing bottom support tooling, and the thickness of the fiber wool is 1 mm; and then the green ball adhered with boron nitride powder is placed in the firing bottom support tooling padded with graphite fiber wool.

[0091] (5) Sintering

[0092] The firing base tooling containing the green ball in step (4) is placed in a vacuum sintering furnace and sintered at 2150° C. for 4 hours to obtain a sintered silicon carbide ceramic ball. The diameter of the sintered silicon carbide ceramic ball is measured to be about 81 mm.

[0093] After testing, the maximum diameter of the sintered silicon carbide ceramic ball is 81.4mm, the minimum diameter is 81.0mm, and the relative error is 0.49%.

[0094] (6) Post-processing

[0095] The silicon carbide ceramic ball sintered body is processed after grinding and polishing to obtain the silicon carbide ceramic ball of required specifications.

[0096] After testing, the relative density of the obtained silicon carbide ceramic ball is 98.9%, and the Vickers microhardness is 27.1 GPa.

[0097] Comparative Example 1:

[0098] The difference between this comparative example and Example 1 is that no firing support fixture is used, and the green ball is directly placed on the graphite plate for sintering.

[0099] After testing, the maximum diameter of the silicon carbide ceramic ball sintered body without post-processing after sintering is 83.8mm, the minimum diameter is 81.1mm, and the relative error is 3.33%; the relative density of the silicon carbide ceramic ball after post-processing is 98.7%, and the Vickers microhardness is 26.5GPa.

[0100] Comparative Example 2:

[0101] The difference between this comparative example and Example 1 is that no graphite fiber wool is padded in the semi-spherical hole of the firing support fixture, and the green ball with boron nitride powder adhered thereto is directly placed in the firing support fixture for sintering.

[0102] After sintering, it was found that the silicon carbide ceramic balls had point adhesion with some areas of the supporting base, making it difficult to remove.

[0103] After testing, the maximum diameter of the silicon carbide ceramic ball sintered body without post-processing after sintering is 83.1mm, the minimum diameter is 81.9mm, and the relative error is 1.47%; the relative density of the silicon carbide ceramic ball after post-processing is 98.8%, and the Vickers microhardness is 26.6GPa.

[0104] Result analysis:

[0105] 1. By comparing Example 1 with Example 2 and Example 3, it can be seen that in the scheme of the present invention, increasing the isostatic pressing pressure is beneficial to improving the roundness, density and microhardness; increasing the sintering temperature can improve the density and microhardness, and has little effect on the roundness.

[0106] 2. By comparing Example 1 with Comparative Example 1, it can be seen that the technical solution of the present invention significantly improves the roundness of the sintered body of the silicon carbide ceramic ball, which is beneficial to reducing the amount of post-processing.

[0107] 3. By comparing Example 1 with Comparative Example 2, it can be seen that when the graphite fiber wool is not padded in the firing base tooling, the roundness of the sintered ceramic ball deteriorates due to the poor fit between the firing base and the green silicon carbide ceramic ball, which increases the post-processing workload. In addition, due to the loss of buffering and flexible isolation, the probability of adhesion between the silicon carbide ceramic ball and the firing base increases, which reduces the yield rate of the ceramic ball.

[0108] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in the field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A sintering method for large-sized silicon carbide ceramic balls, characterized in that: The following steps are involved: (1) Green ball molding The green balls are made by isostatic pressing process with silicon carbide ceramic granulation powder as raw material, and then dried at 50-80℃ for 1-5 hours to avoid cracking caused by rapid volatilization of water during sintering; (2) Green ball turning The green ball dried in step (1) is simply trimmed to remove excess corners, and the radius of the trimmed green ball is R; (3) Processing of the firing support tooling Using the same material formula and pressing process as those for preparing the green ball in step (1), pressing a round rod green body, and lathing the round rod into a cylinder, and machining a hemispherical ball hole on the upper cylindrical surface of the cylinder, the ball hole being used to support the green ball in step (2); (4) Loading the ball into the mold The green ball processed in step (2) is rolled in boron nitride powder so that a thin layer of boron nitride adheres to its surface to prevent it from sticking to the firing bottom support tooling during sintering; a thin layer of graphite fiber wool is padded in the semi-spherical hole of the firing bottom support tooling, and then the green ball adhered with boron nitride powder is placed in the firing bottom support tooling padded with graphite fiber wool; (5) Sintering The sintering base tooling containing the green ball in step (4) is placed in a vacuum sintering furnace and sintered at 2100-2200° C. for 1-7 hours to obtain a sintered body of silicon carbide ceramic balls; (6) Post-processing The silicon carbide ceramic ball sintered body is processed after grinding and polishing to obtain the silicon carbide ceramic ball of required specifications.

2. The sintering method of a large-size silicon carbide ceramic ball according to claim 1, characterized in that: In step (1), the pressure used in the isostatic pressing process is 180-250 MPa, and the holding time is 5-30 minutes.

3. The sintering method of a large-size silicon carbide ceramic ball according to claim 1, characterized in that: In step (1), the density of the green ball after drying is not less than 1.7 g / cm 3 .

4. The sintering method of a large-size silicon carbide ceramic ball according to claim 1, characterized in that: In step (3), the diameter of the cylinder is D, and the height is H. The value of D is not less than 2.15R, and the value of H is not less than 1.15R.

5. The sintering method of a large-size silicon carbide ceramic ball according to claim 1, characterized in that: In step (3), the center of the spherical hole coincides with the center of the cylindrical surface of the cylinder, the radius of the spherical hole is R1, and the value of R1 is 1.01-1.05R.

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