Manufacturing method for high toughness and high strength silicon nitride ball

Abstract

5. The method for producing a silicon nitride ball according to claim 4, wherein the boron nitride is wet-coated on the outer surface of the isostatically pressed silicon ball in a slurry form. [Solution] The present invention provides a method for producing silicon nitride balls, which can significantly improve the production yield of silicon nitride balls, enable control of the reaction with carbon during sintering, and enable the production of silicon nitride balls with high toughness and high strength.

Description

[Technical field]

[0001] The present invention relates to a method for producing silicon nitride balls with high toughness and high strength, and more particularly to a method for producing silicon nitride balls that can significantly improve the production yield of silicon nitride balls, can control the reaction with carbon during sintering, and can produce silicon nitride balls with high toughness and high strength. [Background technology]

[0002] Ceramic materials have low density and coefficient of thermal expansion, and are excellent in hardness and heat resistance, and can be used for various machine parts.

[0003] Because of these characteristics, ceramics are widely used as a bearing ball material in harsh operating conditions.

[0004] Currently, bearing balls used in acidic and alkaline conditions are made of ZrO 2 system, Si 3 N 4 Most of the balls for high temperature and high speed bearings are made of Si, which has a relatively low specific gravity. 3 N 4 System and Al 2 O 3 The system is mainly used.

[0005] ZrO, a common corrosion-resistant bearing ball material 2 As the sintering temperature rises, a phase transformation occurs between three types of crystal structure (monoclinic → tetragonal → cubic), and when cooled after sintering, a phase transformation occurs from tetragonal to monoclinic. At this time, a volume expansion of about 4% occurs, and the test piece is damaged, so sintering alone is difficult.

[0006] Therefore, in order to improve the sinterability, ZrO 2 Y as a stabilizer 2 O 3 , CEO 2 In this case, one of ZrO and MgO is added. 2 To Y 2O 3 When CeO is added, the mechanical properties are excellent but the corrosion resistance is poor, and it is mainly used for general wear-resistant products such as cutters. 2 When Al or MgO is added, the corrosion resistance is excellent, but the mechanical properties are poor due to excessive grain growth. 2 O 3 In the case of , it is an economical material with excellent hardness and corrosion resistance, but its strength and wear resistance are comparable to other ceramic materials such as Si 3 N 4 and ZrO 2 Therefore, there is a problem that it is difficult to apply to bearing balls. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for producing silicon nitride balls, which can produce silicon nitride balls with high toughness and high strength that can be used as bearing balls.

[0008] Another object of the present invention is to provide a method for producing silicon nitride balls that can significantly improve the production yield. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the present invention provides a method for producing silicon nitride balls, comprising the steps of: preparing silicon powder; adding a solvent and an organic binder to the silicon powder to form a slurry; thermally spray drying the slurry to produce granules having a predetermined particle size; press-molding the granules to produce silicon balls; positioning the silicon balls in each hole of a soft porous foam; putting the porous foam in which the silicon balls are arranged into a vinyl bag and packing it by evacuating the bag; putting the packed vinyl bag into an isostatic press to perform isostatic molding; obtaining the isostatically molded silicon balls and then degreasing them; and nitriding and sintering the degreased silicon balls to produce silicon nitride balls.

[0010] In a preferred embodiment, the porous foam is a PE foam, each pore is circular or polygonal and open through the top and bottom surfaces of the porous foam, and the maximum width of each pore is greater than the diameter of the silicon ball in which it is located.

[0011] In a preferred embodiment, the porous foam is a PE foam, each of the pores is circular or polygonal, open only to the upper surface of the porous foam, and a silicon ball is seated and positioned inside each pore.

[0012] In a preferred embodiment, the method further includes coating the exterior surface of the isostatically pressed silicon ball with boron nitride after degreasing to control reaction with carbon inside the furnace when the silicon ball is degreasing and sintering.

[0013] In a preferred embodiment, the boron nitride is wet coated onto the exterior surface of the isostatically pressed silicon balls in slurry form. Effect of the Invention

[0014] The present invention has the following excellent effects.

[0015] According to the method for producing silicon nitride balls of the present invention, a plurality of silicon balls can be placed on a porous foam, which is then packed and isostatically pressed, thereby offering the advantage of significantly improving the production yield of silicon nitride balls.

[0016] Furthermore, according to the method for producing silicon nitride balls of the present invention, by coating the outer surface of the isostatically pressed silicon ball with boron nitride, it is possible to control the reaction with carbon during degreasing and sintering, which has the advantage of producing silicon nitride balls with high toughness and strength. [Brief description of the drawings]

[0017] [Figure 1] 1 is a flow chart of a method for manufacturing a silicon nitride ball according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing an example of a porous foam used during isostatic molding in a method for manufacturing a silicon nitride ball according to one embodiment of the present invention. [Diagram 3] FIG. 2 is a diagram showing a silicon ball isostatically pressed in a method for manufacturing a silicon nitride ball according to one embodiment of the present invention. [Figure 4] 2A and 2B are diagrams showing a silicon ball coated with boron nitride in a method for manufacturing a silicon nitride ball according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the technical configuration of the present invention will be described in detail with reference to the preferred embodiments shown in the accompanying drawings.

[0019] However, the present invention is not limited to these embodiments and may be embodied in other embodiments.Like reference numbers refer to like elements throughout the specification.

[0020] FIG. 1 is a flow chart of a method for producing a high toughness and high strength silicon nitride ball according to one embodiment of the present invention.

[0021] Referring to FIG. 1, a method for producing a high toughness and high strength silicon nitride ball according to one embodiment of the present invention is a method for producing a silicon nitride ball that is inserted into a machine or mechanical device, such as a bearing, to reduce contact friction.

[0022] First, silicon powder is prepared (S1000).

[0023] The silicon powder is made by crushing metal silicon scraps to control impurities and particle size.

[0024] Furthermore, the smaller the particle size of the silicon powder, the better, but for mixing with sintering additives and for final densification, silicon powder with an average particle size of 1 μm to 10 μm can be used.

[0025] Next, a solvent and an organic binder are added to the silicon powder to form a slurry (S2000).

[0026] Furthermore, there are no particular limitations on the types of the solvent and organic binder, but it is desirable to use one or more selected from ethanol, methanol, isopropanol, distilled water, and acetone as the solvent, and a polyvinyl butyral (PVB)-based binder as the organic binder.

[0027] Next, the slurry is thermally spray dried to produce granules having a predetermined particle size (S3000).

[0028] In addition, to prevent the granules obtained by thermal spray drying from having a hollow interior or becoming doughnut-shaped granules, the internal temperature of the chamber can be fixed at 120°C, the external temperature of the chamber can be fixed at 90°C, and the atomizer speed can be set to 8,000 rpm to 15,000 rpm.

[0029] The granules thus produced have an average particle size of 60 μm to 120 μm, preferably 80 μm to 120 μm.

[0030] Next, the granules are press-molded to produce silicon balls 100 (S4000).

[0031] The molding pressure of the press molding is set to 20 kg / cm in order to apply a uniform pressure when performing the isostatic molding described below, and to facilitate handling after molding. 3 ~50kg / cm 3 It is desirable to do so.

[0032] Next, the silicon balls 100 are placed in each of the holes 210 of the soft porous foam 200 (S5000).

[0033] In addition, the porous foam 200 is a PE foam having a plurality of holes 210 spaced apart from one another, and the holes 210 may be circular as shown in FIG. 2(b) or polygonal as shown in FIG. 2(a).

[0034] Furthermore, the holes 210 may be drilled in a form penetrating the upper and lower surfaces of the porous foam 200, or may be dug in the form of grooves that are closed on the lower surface and open only on the upper surface.

[0035] Furthermore, the maximum width of the hole 210 must be greater than the diameter of the silicon ball 100 .

[0036] This is to minimize contact with the PE foam when the silicon ball 100 is placed in the hole 210 .

[0037] However, the size must be adjusted so that only one silicon ball 100 is positioned in one hole 210 .

[0038] That is, the porous foam 200 serves as a jig on which a number of silicon balls can be placed.

[0039] In addition, in order to position the silicon balls 100 in each hole 210 of the porous foam 200, a method is used in which a plurality of silicon balls 100 are placed on the porous foam 200 and shaken left and right using a vibrator so that the silicon balls 100 are positioned so as to hang on each hole 210.

[0040] Moreover, the thickness of the porous foam 200 is preferably smaller than the diameter of the silicon balls 100 , and is desirably 20% to 50% of the diameter of the silicon balls 100 .

[0041] Next, the porous foam 200 on which the silicon balls 100 are placed is placed in a vinyl bag 300, and the bag is evacuated and packed (S6000).

[0042] Furthermore, any material or shape of vinyl bag can be used for the vinyl bag 300 as long as it can withstand the pressure during isostatic molding described below.

[0043] Meanwhile, the applicant previously placed one silicon ball in one rubber bag, sealed it, and molded it using an isostatic press, which resulted in a low yield. However, by using the porous foam 200 to pack multiple silicon balls at once and then performing isostatic press molding, the applicant was able to significantly improve the yield.

[0044] Next, the packed vinyl bag 310 is placed in an isostatic press and isostatically molded (S7000).

[0045] It is also preferable that the pressure applied during the isostatic molding be maintained at 200 atm to 400 atm for a period of time of about 30 to 60 minutes.

[0046] Next, as shown in FIG. 3, the isostatically pressed silicon ball 320 is taken out of the vinyl bag and then subjected to degreasing (S9000) and sintering (S9100) processes to complete the manufacture of high-toughness and high-strength silicon nitride balls.

[0047] The degreasing is a process in which the silicon powder is not oxidized and organic matter contained in the silicon powder is removed, and is carried out in an oxidizing atmosphere for 2 to 10 hours at a heating rate of 0.11 to 1°C / min up to 450°C. Residual carbon that is not completely removed is degreased in a nitrogen atmosphere at 900°C for 1 to 10 hours.

[0048] The sintering is a process of nitriding silicon balls while sintering using a gas pressure sintering (GPS) method, where the temperature is increased at a rate of 0.1°C / min to 2°C / min up to 1400°C, the nitrogen pressure is 1.0 atm to 1.8 atm, and the maintenance time is 0.5 to 10 hours. For the final sintering, the temperature is increased at a rate of 1°C to 5°C up to 1900°C, and the nitrogen pressure is maintained at 9 atm for 1 to 10 hours.

[0049] On the other hand, after the isostatic molding, a step (S8000) of coating the silicon balls with boron nitride before the degreasing step can be added.

[0050] FIG. 4 shows the silicon ball coated with boron nitride.

[0051] The boron nitride is prepared by adding a solvent to boron nitride powder to form a slurry, and then wet-coating the silicon nitride balls with the boron nitride slurry.

[0052] It is preferable to use water as the solvent.

[0053] The reason for using water as the solvent is that a polyvinyl butyral (PVB)-based binder is used as the organic binder during thermal spray drying. If an organic solvent other than water is used, the polyvinyl butyral (PVB)-based binder on the surface of the silicon ball will dissolve and react with the boron nitride, making it difficult to densify the material during sintering.

[0054] Also, in the boron nitride slurry, the water content is 50% to 98% by weight of the boron nitride powder.If the weight ratio is low, the viscosity of the slurry increases, the coating thickness becomes too thick, and the thermal conductivity during sintering decreases, which can affect the strength of the ball by changing the surface and internal structure.

[0055] In addition, the thickness of the boron nitride coating should be 5 μm to 20 μm to suppress the reaction at the interface between the silicon balls during sintering and to control the grain growth toward the surface.

[0056] Next, the silicon ball coated with the boron nitride slurry is dried, and then the degreasing process (S9000) is carried out, followed by sintering (S9100) to complete the manufacture of the high toughness and high strength silicon nitride ball.

[0057] Also, it is desirable that the moisture content of the dried silicon balls is within 5%. The moisture that evaporates during the sintering process under high gas pressure can oxidize the graphite heating element inside the equipment and the graphite parts inside the chamber, and the oxidized parts can damage the equipment and products.

[0058] In this way, when the boron nitride coated silicon balls are degreased and sintered, if the reaction with carbon in the furnace is not controlled, the carbon will penetrate the silicon balls and be synthesized into silicon carbide, making it difficult to manufacture silicon nitride balls with high toughness and strength. Therefore, it is important to control the carbon well to prevent the formation of silicon carbide.

[0059] Although the present invention has been described in detail above by way of preferred embodiments, it is not limited to the above embodiments, and various changes and modifications can be made by those having ordinary skill in the art to which the present invention pertains without departing from the spirit of the present invention. [Explanation of symbols]

[0060] 100 Silicon nitride ball 200 Porous Foam 210 holes 300 plastic bags 310 Packed plastic bags 320 Isostatically pressed silicon balls 400 Boron nitride coated silicon ball

Claims

1. Providing a silicon powder; adding a solvent and an organic binder to the silicon powder to form a slurry; thermally spray drying the slurry to produce granules having a predetermined particle size; pressing the granules to produce silicon balls; placing said silicon balls into each of the pores of the flexible porous foam; A step of placing the porous foam having the plurality of silicon balls disposed therein into a plastic bag, evacuating the bag, and packing the bag; placing the filled plastic bag in an isostatic press for isostatic molding; degreasing the isostatically pressed silicon balls obtained; A step of sintering the degreased silicon ball while nitriding the silicon ball to produce a silicon nitride ball; and coating the outer surface of the isostatically pressed silicon ball with boron nitride after degreasing, and controlling the reaction with carbon inside a furnace when the silicon ball is degreased and sintered.

2. The method for producing silicon nitride balls as described in claim 1, characterized in that the porous foam is PE foam, each hole is circular or polygonal and open through the upper and lower surfaces of the porous foam, and the maximum width of each hole is larger than the diameter of the silicon ball in which it is located.

3. 2. The method for manufacturing silicon nitride balls as described in claim 1, characterized in that the porous foam is PE foam, each hole is circular or polygonal, open only to the top surface of the porous foam, and a silicon ball is seated and positioned inside each hole.

4. 4. The method for producing a silicon nitride ball according to claim 1, wherein the boron nitride is wet-coated in a slurry form on the outer surface of the isostatically pressed silicon ball.

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