Method for producing granules, method for producing nitride ceramic sintered compact, and granules

Abstract

Provided is a method for producing granules comprising granulation of a slurry containing a nitride ceramic powder, a sintering agent, an aqueous solvent and an organic additive by spray freeze granulation drying.

Description

Granule manufacturing method, nitride ceramic sintered body manufacturing method, and granule

[0001] The present disclosure relates to a method for producing granules, a method for producing a nitride ceramic sintered body, and the granules.

[0002] Because of their excellent mechanical strength and wear resistance, nitride ceramic sintered bodies such as silicon nitride are used for wear-resistant members, gas turbine blades, engine parts, etc. In particular, bearing balls used in rolling bearing members require high wear resistance, and therefore nitride ceramic sintered bodies are preferably used.

[0003] The nitride ceramic sintered body used for bearing balls is generally obtained by granulating a slurry containing raw materials using a spray dryer to obtain granules, press-molding these granules into the shape of bearing balls, and then sintering them (see, for example, Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2008-222469

[0005] However, the sintered body obtained by the above-mentioned manufacturing method is not homogeneous and may have internal defects. Furthermore, from the viewpoint of reducing environmental impact and eliminating the need for explosion-proof equipment, it is desirable to use an aqueous solvent in the manufacturing method of granules. Therefore, the present disclosure aims to provide a manufacturing method of granules, a manufacturing method of nitride ceramic sintered body, and granules that can suppress the occurrence of internal defects in nitride ceramic sintered bodies even when an aqueous solvent is used.

[0006] Specific means for achieving the above object are as follows. <1> A method for producing granules, comprising granulating a slurry containing a nitride ceramic powder, a sintering aid, an aqueous solvent, and an organic additive by spray-freeze granulation and drying. <2> A method for producing granules according to <1>, wherein the nitride ceramic powder contains at least one selected from the group consisting of silicon nitride powder and SiAlON powder. <3> A method for producing granules according to <1> or <2>, wherein the organic additive contains at least one selected from the group consisting of a dispersant, a binder, a lubricant, an antifoaming agent, and a plasticizer. <4> A method for producing granules according to <3>, wherein the dispersant contains at least one selected from the group consisting of a pH adjuster, a polycarboxylic acid compound, an ammonium salt of condensed naphthalenesulfonic acid, a salt of condensed naphthalenesulfonic acid, a polyacrylic acid amide compound, and an alkylsulfonic acid compound. <5> A method for producing granules according to <3> or <4>, wherein the content of the dispersant in the slurry is 0.05 to 3 mass% relative to the content of the nitride ceramic powder. <6> The method for producing granules according to any one of <3> to <5>, wherein the binder comprises at least one selected from the group consisting of paraffin wax, carnauba wax, microcrystalline wax, polyvinyl alcohol, cellulose derivatives, alginic acid, polyethylene glycol, polyvinylpyrrolidone, acrylic resins, vinyl acetate, styrene resins, and polysaccharides. <7> The method for producing granules according to any one of <3> to <6>, wherein the content of the binder in the slurry is 0.5 to 6 mass% relative to the content of the nitride ceramic powder. <8> The average particle size D of the nitride ceramic powder ５０<9> A method for producing granules according to any one of <1> to <7>, wherein the average particle size is 0.1 to 1.5 μm. <9> A method for producing granules according to any one of <1> to <8>, wherein the spray freeze granulation drying method is carried out in this order, followed by spray freezing and vacuum drying, and, after the vacuum drying, further drying is carried out at 35 to 150°C under atmospheric pressure to reduce the amount of residual solvent to 2 mass% or less. <10> A method for producing granules according to any one of <1> to <9>, wherein the granules are raw materials for producing bearing balls. <11> A method for producing granules for producing bearing balls, comprising granulating a slurry containing a nitride ceramic powder by spray freeze granulation drying. <12> A method for producing a nitride ceramic sintered body, comprising press-molding the granules obtained by the method according to any one of <1> to <11> to produce a molded body, and sintering the molded body. <13> A method for producing a nitride ceramic sintered body according to <12>, wherein the nitride ceramic sintered body is a base sphere for a bearing ball. <14> Granules comprising a nitride ceramic, a sintering aid, and an organic additive, and having a granule strength of 0.01 to 0.50 MPa. <15> The granules according to <14>, wherein the nitride ceramic comprises at least one selected from the group consisting of silicon nitride and SiAlON. <16> The granules according to <14> or <15>, having a fluidity index of 60 or more. <17> The granules according to any one of <14> to <16>, which are used as a raw material for producing bearing balls.

[0007] According to the present disclosure, there are provided a method for producing granules that can suppress the occurrence of defects inside a nitride ceramic sintered body even when an aqueous solvent is used, a method for producing a nitride ceramic sintered body, and the granules.

[0008] 1 is an optical microscope photograph of a cross section of granule D1 obtained in Example 1. FIG. 2 is an infrared microscope photograph of a cross section of molded body E1 obtained in Example 1, taken by immersion transillumination. FIG. 3 is an optical microscope photograph of a cross section of secondary sintered body G1 obtained in Example 1, taken by incident-light bright field observation. FIG. 4 is an optical microscope photograph of a cross section of granule D7 obtained in Example 7. FIG. 5 is an infrared microscope photograph of a cross section of molded body E7 obtained in Example 7, taken by immersion transillumination. FIG. 6 is an optical microscope photograph of a cross section of secondary sintered body G7 obtained in Example 7, taken by incident-light bright field observation.

[0009] Embodiments of the present disclosure are described in detail below. However, the present disclosure is not limited to the following embodiments. In this disclosure, the term "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the upper and lower limits. Unless otherwise specified, "to" is used in the following disclosure with the same meaning. In numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In this disclosure, unless otherwise specified, units used to indicate physical property values ​​apply to the entire numerical range. For example, "0.16 to 0.24 MPa" is synonymous with "0.16 MPa to 0.24 MPa." In this disclosure, unless otherwise specified, the measurement environment for each physical property is a temperature of 23 to 25°C and a humidity of 40 to 50%. In this disclosure, the term "process" includes not only a process that is independent of other processes, but also a process that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved.

[0010] In this disclosure, the average particle size D ５０ is the particle diameter (D) at which the cumulative volume from the small diameter side reaches 50% in the volume-based particle size distribution curve obtained by measurement using a laser diffraction / scattering method. ５０ In this disclosure, D １０ and D ９０ is D ５０ These are particle sizes at which the cumulative volume from the small diameter side reaches 10% and 90% in the same volume-based particle size distribution curve as above. １０ , D ５０ , D ９０ In the present disclosure, the average particle size D ｇ５０ is the particle diameter (D) at which the cumulative volume from the small diameter side reaches 50% in the volume-based particle size distribution curve obtained by measurement using an image particle size distribution analyzer. ｇ５０ In this disclosure, D ｇ１０ , Dｇ６０ and D ｇ９０ is D ｇ５０ These are the particle sizes at which the cumulative volume from the small diameter side in a particle size distribution curve similar to that shown in FIG. 1 becomes 10%, 60%, and 90%, respectively. ｇ１０ , D ｇ６０ , D ｇ５０ , D ｇ９０ When the notation "amount of particle size distribution" is used, it indicates a value obtained by measurement using an image particle size distribution measuring device.

[0011] <Method for producing granules> In the method for producing granules disclosed herein, a slurry containing a nitride ceramic powder, a sintering aid, an aqueous solvent, and an organic additive is granulated by spray freeze granulation and drying. In this disclosure, the aqueous solvent refers to water or a mixed solvent of water and a water-miscible solvent. A solvent that is not compatible with the aqueous solvent may also be used. The use of granules obtained by this production method produces a nitride ceramic sintered body in which the occurrence of internal defects is suppressed. The reason for this is unclear, but is presumed to be as follows.

[0012] When observing the inside of a nitride ceramic sintered body obtained by a conventional manufacturing method, traces due to the shape of the granules are found, which are thought to be caused by internal defects of the granules. It is presumed that even when the granules are pressure-molded, the granules do not become integrated, and the outer peripheries of the granules remain as boundary traces. Furthermore, if the granules are not spherical and have depressions, it is presumed that even when the granules are pressure-molded, the depressions will easily become voids or low-density areas and remain as defects. Therefore, in this disclosure, a spray-freeze granulation drying method is applied instead of the spray-drying method, which is a conventional method for manufacturing granules.

[0013] In the spray-drying method, droplets formed by spraying a slurry containing raw materials are dried with hot air. As the solvent evaporates, the droplets shrink, increasing the packing density of the resulting individual granules. Here, the packing density of individual granules refers to the packing density of the granules themselves. Note that this is different from the bulk density of the granule "layer," which indicates the packing density of a large number of granules. Hereafter, "packing density" refers to the individual granules, and "bulk density" refers to the granule layer. Furthermore, evaporation is more likely to occur on the outside of the droplets, resulting in a high packing density of individual granules. Furthermore, as the solvent migrates from the inside to the outside of the droplets, the components are dragged outward, forming a hard shell on the outside and a hollow center. In particular, when water is used as the solvent in the spray-drying method, smooth evaporation of water from the droplets is hindered, resulting in granules with internal defects, such as rupture of parts of the hard shell, which can connect to the hollow interior, resulting in depressed spherical shapes. When these irregularly shaped granules are used to form a compact and then used to produce a sintered nitride ceramic body, the resulting sintered nitride ceramic body is prone to internal defects. Furthermore, the movement of the solvent from the inside of the droplets to the outside also drags the binder and other components outward, resulting in the binder and other components easily precipitating on the surface of the granules. Thus, the spray-drying method tends to result in uneven distribution of components within the granules and variations in the packing density of individual granules, which increases the packing density of individual granules and results in hard granules.

[0014] In contrast, spray freeze granulation drying involves spraying a slurry containing raw materials and rapidly cooling it to produce frozen granules, which are then vacuum-dried at a temperature below the freezing point of the solvent to obtain granules. When the frozen granules are vacuum-dried, the solvent contained in the frozen granules sublimes from the frozen state to a gas without passing through a liquid state, making the transformation from frozen granules to granules less likely to cause shrinkage. Therefore, the granules obtained by spray freeze granulation drying have a reduced packing density, resulting in soft granules and a more uniform internal structure. The same effect can be achieved using an aqueous solvent as the solvent. When such soft granules with a uniform packing density are pressure-molded, the granules easily integrate with each other, resulting in a homogeneous molded body. Sintering this molded body reduces the occurrence of internal defects.

[0015] The nitride ceramic powder used as a raw material preferably contains at least one selected from the group consisting of silicon nitride powder and SiAlON powder, for example, αSi ３ N ４ , βSi ３ N ４ , M-αSiAlON, and βSiAlON. M-αSiAlON refers to αSiAlON in which a metal atom M (M=Li, Mg, Ca, Y, La, etc.) exists within the crystal lattice. The nitride ceramic powder may be used alone or in combination of two or more types.

[0016] Average particle size D of nitride ceramic powder ５０ is preferably 0.1 to 1.5 μm, more preferably 0.3 to 1.0 μm, and even more preferably 0.4 to 0.8 μm.

[0017] The sintering aid is preferably a compound containing at least one element selected from the group consisting of Al and rare earth elements, and specifically, Al ２ O ３ , AlN, MgAl ２ O ４ , Y ２ O ３ , Yb ２ O ３ , Er ２ O ３ , Sc ２ O ３ etc. Average particle size D of the sintering aid ５０ From the viewpoint of sinterability, the thickness is preferably 0.05 to 1.5 μm, more preferably 0.05 to 1.0 μm, and even more preferably 0.05 to 0.5 μm.

[0018] The content of the sintering aid in the slurry is preferably 1 to 20 mass % relative to the content of the nitride ceramic powder, more preferably 3 to 18 mass %, and even more preferably 5 to 15 mass %. The ratio of the total volume of the nitride ceramic powder and the sintering aid to the total volume of the slurry is preferably 25 to 50 volume %, and more preferably 35 to 50 volume %.

[0019] In the present disclosure, the aqueous solvent preferably contains at least one of water and tert-butyl alcohol, with water being more preferred. The solvent may contain a solvent other than the aqueous solvent (a non-aqueous solvent), such as ethyl alcohol, propyl alcohol, toluene, xylene, or cyclohexane. The content of the aqueous solvent relative to the total amount of solvent in the slurry is preferably 50% by mass or more, more preferably 80% by mass or more, and may even be 100% by mass. When the content of the aqueous solvent relative to the total amount of solvent in the slurry is 50% by mass or more, the slurry is easily frozen immediately after spraying, making granulation easier, and the resulting frozen granules tend to have less distortion in shape and a uniform structure. Therefore, when the content of the aqueous solvent relative to the total amount of solvent in the slurry is 50% by mass or more, a dense molded body is easily obtained, the relative density of the sintered body is high, and the number of defects within the sintered body tends to be reduced.

[0020] The organic additive preferably includes at least one selected from the group consisting of a dispersant, a binder, a lubricant, an antifoaming agent, and a plasticizer, and more preferably includes a dispersant, a binder, and a lubricant.

[0021] Adding an antifoaming agent to the slurry can remove bubbles that occur during stirring of the slurry. The antifoaming agent preferably contains at least one selected from the group consisting of silicone, vegetable oil, and organic polymer. Note that silicone here has an organic group, so it is considered an organic additive in this disclosure. These compounds used as antifoaming agents are preferably diluted with water and used as an aqueous solution.

[0022] Adding a plasticizer to the slurry can impart flexibility to the granules. The plasticizer preferably contains at least one selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polyglycerin. These compounds used as plasticizers may be dissolved in water and used as an aqueous solution.

[0023] The dispersant preferably contains at least one selected from the group consisting of a pH adjuster, a polycarboxylic acid compound such as an ammonium polycarboxylate, an ammonium salt of condensed naphthalenesulfonic acid, a salt of condensed naphthalenesulfonic acid, a polyacrylic acid amide compound, and an alkylsulfonic acid compound. These compounds used as dispersants may be dissolved in water to form an aqueous solution. The pH adjuster preferably contains at least one selected from the group consisting of aqueous ammonia, tetraethylammonium hydroxide (TEAH), tetramethylammonium hydroxide (TMAH), quaternary ammonium hydroxide compounds, and organic amines. These compounds used as pH adjusters may be dissolved in water to form an aqueous solution. The content of the dispersant in the slurry is preferably 0.05 to 3 mass%, more preferably 0.1 to 1.5 mass%, and even more preferably 0.3 to 1.0 mass%, relative to the content of the nitride ceramic powder.

[0024] From the viewpoints of molded body strength and degreasing ability, the binder preferably contains at least one selected from the group consisting of paraffin wax, carnauba wax, microcrystalline wax, polyvinyl alcohol, cellulose derivatives, alginic acid, polyethylene glycol, polyvinylpyrrolidone, acrylic resins, vinyl acetate, styrene-based resins, and polysaccharides, and more preferably contains at least one selected from the group consisting of paraffin wax, polyvinyl alcohol, and acrylic resins. Examples of cellulose derivatives include methyl cellulose, carboxymethyl cellulose, and hydroxyethyl cellulose. The binder can also be dissolved or dispersed in water and used as an aqueous solution or aqueous dispersion (including emulsion form). The binder content in the slurry is preferably 0.5 to 6 mass % and more preferably 1 to 5 mass % relative to the content of the nitride ceramic powder.

[0025] Examples of lubricants include fatty acids and mineral oils. Specifically, at least one selected from the group consisting of fatty acids and fatty acid amides having 8 or more carbon atoms is preferred. At least one selected from the group consisting of stearic acid, caprylic acid, lauric acid, palmitic acid, araginic acid, oleic acid, stearamide, oleamide, erucamide, methylene bisstearamide, and ethylene bisstearamide is more preferred, and at least one selected from the group consisting of stearic acid and stearamide is even more preferred. The lubricant can also be dissolved or dispersed in water and used as an aqueous solution, aqueous dispersion, or a mixed solvent of an aqueous solution and an aqueous dispersion. The content of the lubricant in the slurry is preferably 0.01 to 2 mass%, more preferably 0.1 to 1 mass%, relative to the content of the nitride ceramic powder.

[0026] The content of the organic additive in the slurry is preferably 1 to 20 mass %, more preferably 2 to 15 mass %, and even more preferably 3 to 10 mass %, based on the total content of the nitride ceramic powder and the sintering aid.

[0027] Furthermore, the slurry may contain metal powder, metal carbide, metal oxide raw material, etc. for the purpose of coloring the sintered body or controlling the particle size. Examples of metal elements added for the purpose of coloring include W, Mo, Ti, etc.

[0028] From the viewpoint of suppressing settling of solids, the slurry viscosity is preferably 100 mPa·s or more, more preferably 500 mPa·s or more, even more preferably 1000 mPa·s or more, and particularly preferably 2300 mPa·s or more. From the viewpoint of sprayability, the slurry viscosity is preferably 6000 mPa·s or less, more preferably 5000 mPa·s or less, and even more preferably 4000 mPa·s or less. The shear rate when evaluating the slurry viscosity is 1 to 10 s －１In the granule manufacturing method of the present disclosure, when the slurry viscosity is 100 mPa·s or more, settling of solids in the slurry is easily suppressed. Furthermore, when the slurry viscosity is 6000 mPa·s or less, the slurry is easily sprayed, the shape of the frozen granules is less distorted, and a uniform structure is easily obtained. Therefore, when the slurry viscosity is in the range of 100 to 6000 mPa·s, a dense compact is easily obtained, the relative density of the sintered body is high, and the number of defects inside the sintered body tends to be reduced.

[0029] In the present disclosure, the solid content refers to the nitride ceramic powder and the sintering aid. When metal powder, metal carbide, metal oxide raw material, etc. are contained for the purpose of controlling the coloring or crystal grain size of the sintered body, these are also included in the solid content.

[0030] From the viewpoint of increasing granule strength, the solids concentration of the slurry is preferably 22 vol% or more, more preferably 26 vol% or more, even more preferably 30 vol% or more, particularly preferably 34 vol% or more, and extremely preferably 38 vol% or more. From the viewpoint of sprayability, the solids concentration of the slurry is preferably less than 50 vol%, more preferably 46 vol% or less, and even more preferably 42 vol% or less. The solids concentration of the slurry is calculated from the specific gravities of the nitride ceramic powder, sintering aid, aqueous solvent, organic additive, and further metal powder, metal carbide, metal oxide raw material, etc.

[0031] The method for preparing the slurry is not particularly limited, and the slurry may be prepared by mixing using a ball mill, bead mill, roll mill, homomixer, ultramixer, disperser mixer, attritor, jet mill, homogenizer, or a penetration or collision type high-pressure dispersion device. Among these, preparation using a ball mill or bead mill is preferred. When using a ball mill, it is preferable to use a ball mill container and balls made of the same material as the nitride ceramic raw material in order to prevent the incorporation of other components. The balls are removed before granulation by the spray freeze granulation drying method. If necessary, operations such as filtration may be performed to remove coarse particles from the slurry.

[0032] The obtained slurry is granulated by spray-freeze granulation drying. In the spray-freeze granulation drying method, spray freezing, vacuum drying, and drying under atmospheric pressure are carried out in this order. Spray freezing may be either a wet cooling method or a dry cooling method. In the wet cooling method, the slurry is sprayed into a cooling liquid such as liquid nitrogen. In the dry cooling method, the slurry is sprayed into a cooled gas. The gas may be cooled by circulating liquid nitrogen or the like around a cooling granulation chamber filled with the gas. The gas filled in the cooling granulation chamber may be air or an inert gas such as nitrogen or argon.

[0033] The temperature of the cooling liquid or cooling granulation chamber is maintained below the freezing point, which is the temperature at which the solvent contained in the slurry freezes. From the viewpoint of reliable freezing, the cooling temperature is preferably −15°C or lower, more preferably −30°C or lower, and even more preferably −40°C or lower. Furthermore, from the viewpoint of reducing the load on the spraying device, the cooling temperature is preferably −85°C or higher, more preferably −60°C or higher, and even more preferably −50°C or higher. When the cooling temperature for spray freezing is −15°C or lower, freezing is facilitated immediately after spraying, making granulation easier. Therefore, the resulting frozen granules tend to have less distortion in shape and a uniform structure. Therefore, when the cooling temperature is −15°C or lower in spray freezing, a dense compact is easily obtained, the relative density of the sintered body is increased, and the number of defects inside the sintered body tends to be reduced. Furthermore, when the cooling temperature is −85°C or higher, the load on the spraying device is reduced.

[0034] Spraying is performed by discharging the slurry from a nozzle, rotary atomizer, or the like. The size and particle size distribution of the frozen granules can be adjusted by the size and method of the holes in the discharge section. From the viewpoint of obtaining frozen granules with a narrow particle size distribution, a rotary disk atomizer method is preferred. The size of the frozen granules is adjusted appropriately depending on the size of the target granules, and is preferably set to be approximately the same as the size of the target granules. Below, the first drying (vacuum drying) and the second drying (atmospheric pressure drying) will be explained separately.

[0035] (First drying) The obtained frozen granules are vacuum-dried. Vacuum drying sublimes and removes the solvent in the frozen granules. The degree of vacuum in vacuum drying may be 610 Pa or less, preferably 100 Pa or less, more preferably 50 Pa or less, and even more preferably 20 Pa or less. The lower limit of the vacuum degree may be 1 Pa. In the granule manufacturing method of the present disclosure, when the degree of vacuum in the first drying is 610 Pa or less, the ice crystals in the frozen granules are prevented from starting to melt, liquefaction is suppressed, and the frozen granules tend to have a uniform structure with less distortion in shape. Therefore, when the degree of vacuum in the first drying is 610 Pa or less, a dense molded body is more likely to be obtained, the relative density of the sintered body is increased, and the number of defects inside the sintered body tends to be reduced.

[0036] The temperature in the vacuum drying is preferably −60 to 50°C, more preferably −50 to 25°C, and even more preferably −40 to 10°C. The temperature may be changed in multiple stages. The time for vacuum drying is preferably 12 hours or more, more preferably 24 hours or more, and even more preferably 36 hours or more. In the granule production method of the present disclosure, when the temperature in the first drying is 50°C or less, the ice crystals in the frozen granules are prevented from starting to melt, liquefaction is suppressed, and distortion of the shape of the frozen granules is reduced. When the time for the first drying is 12 hours or more, particularly when the temperature in the first drying is 50°C or less and the time for the first drying is 12 hours or more, ice crystals are less likely to remain in the frozen granules, liquefaction in subsequent steps is suppressed, and the shape of the frozen granules is less likely to be distorted and the structure is more likely to be uniform. Therefore, in the first drying, when the temperature is 50°C or less and the time for vacuum drying is 12 hours or more, a particularly dense molded body is likely to be obtained, the relative density of the sintered body is high, and the number of defects inside the sintered body tends to be reduced. Also, when the temperature in the first drying is -60°C or higher, the load on the freeze-drying apparatus is reduced.

[0037] (Second Drying) After the first drying, drying may be performed under atmospheric pressure to further reduce the amount of residual solvent in the frozen granules. The temperature for drying under atmospheric pressure is preferably 35 to 150°C, more preferably 45 to 130°C, even more preferably 55 to 120°C, and particularly preferably 65 to 110°C. When the temperature for the second drying is 35°C or higher, the amount of residual solvent in the granules is reduced, which inhibits excessive solvent leaching during the production of a compact, and tends to suppress the occurrence of cracks within the compact. Therefore, when the temperature for the second drying is 35°C or higher, a dense compact is more likely to be obtained, the relative density of the sintered body is increased, and the number of defects within the sintered body tends to be reduced. Furthermore, when the temperature for the second drying is 150°C or lower, thermal decomposition of the binder is inhibited, making it easier to maintain the granule shape. Furthermore, when the temperature for the second drying is 150°C or lower, granule collapse during pressure molding is inhibited, granule rearrangement is facilitated.

[0038] The second drying step preferably reduces the residual solvent content in the granules to 2% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less. When the residual solvent content of the granules is 2.0% by mass or less, excessive moisture on the granule surface is prevented, and adhesion between granules due to the surface tension of water and interactions with the binder is suppressed, tending to maintain the fluidity of the granules. Furthermore, when the residual solvent content of the granules is 2% by mass or less, excess solvent is prevented from seeping out during the production of a compact, tending to suppress the occurrence of cracks within the compact. Therefore, when the residual solvent content of the granules is 2% by mass or less, a dense compact is more likely to be obtained, the relative density of the sintered body is increased, and the number of pores in the sintered body tends to be reduced. Pores are internal defects observed as voids of 10 μm or more in the sintered body. Dense refers to a sintered body with a relative density of 99.0% or more. Pores are a type of internal defect in a sintered body.

[0039] The amount of residual solvent in granules in the present disclosure can be measured by the loss on drying method or the Karl Fischer method, for example, when the solvent is water. For example, when the solvent includes a liquid other than water, it can be measured by the loss on drying method. In the present disclosure, the residual solvent amount refers to a value measured with reference to JIS K0068:2001. Specifically, the residual solvent amount is the mass of the solvent contained in the granule divided by the mass of the granule containing the solvent, and is expressed as a percentage. (Mass of solvent contained in the granule) / (Mass of the granule containing the solvent) × 100 Formula (A) In the loss on drying method, the amount is measured using a heated dryer moisture meter (e.g., MB45 halogen moisture meter, Ohaus). In the Karl Fischer titration method (coulometric titration method), the amount is measured using a Karl Fischer moisture meter (e.g., MKC-710, Kyoto Electronics Manufacturing Co., Ltd.) at a heating temperature of 120°C.

[0040] The second drying can be performed using an apparatus such as a thermostatic bath, an oven, an electric furnace, etc. The drying time can be appropriately set depending on the amount of remaining solvent in the granules.

[0041] The method for producing granules of the present disclosure may further include other processes, such as sizing and classification of the obtained granules.

[0042] Average particle size of granules D ｇ５０ is preferably 40 μm or more, more preferably 60 μm or more, and even more preferably 80 μm or more. ｇ５０ is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 125 μm or less.

[0043] The granules obtained by the manufacturing method of the present disclosure are suitable for use as a raw material for manufacturing bearing balls.

[0044] In another embodiment of the method for producing granules according to the present disclosure, a slurry containing a nitride ceramic powder is granulated by a spray freeze granulation drying method. The nitride ceramic powder and the spray freeze granulation drying method in this embodiment are the same as those described above.

[0045] <Granules> The granules of the present disclosure contain nitride ceramics, a sintering aid, and an organic additive, and have a granule strength of 0.01 to 0.50 MPa or less. The granules of the present disclosure may be obtained by the granule manufacturing method of the present disclosure. In the spray-freeze granulation drying method, shrinkage is less likely to occur when the solvent evaporates from the frozen granules to form granules, and an increase in density is suppressed, resulting in granules with lower granule strength than conventional methods. Granules with a granule strength of 0.50 MPa or less are more likely to deform and integrate with each other when press-molded to obtain a molded body, resulting in the disappearance of interfaces between granules and the production of a homogeneous molded body. Furthermore, when the granule strength is 0.01 MPa or more, the granules are less likely to crumble during transportation, vibration, etc., which reduces molding irregularities and helps to suppress the generation of dust.

[0046] The granule strength of the granules is 0.50 MPa or less, preferably 0.40 MPa or less, and more preferably 0.30 MPa or less. From the viewpoint of handleability, the granule strength of the granules is 0.01 MPa or more, preferably 0.05 MPa or more, and more preferably 0.10 MPa or more. The granule strength is measured by measuring the crushing test force P using a micro-compression tester (for example, Shimadzu Corporation's "MCT series"), and calculating the granule strength using the following formula 1. The crushing test force P is the average value of 10 granules. σ = 2.8 × P / (π × d ２ ) Equation 1 In equation 1, σ is granule strength (MPa), P is crushing test force (N), and d is granule particle size (mm).

[0047] The nitride ceramics, sintering aids, and organic additives in the granules have the same meanings as the nitride ceramics, sintering aids, and organic additives described in the method for producing the granules, respectively.

[0048] The respective contents of sintering aids, dispersants, binders, lubricants, antifoaming agents, and plasticizers relative to the content of nitride ceramic powder in the granules are the same as the respective contents of sintering aids, dispersants, binders, lubricants, antifoaming agents, and plasticizers relative to the content of nitride ceramic powder in the slurry. The components of the slurry after removing the solvent can be considered to be the components of the granules. The content of organic additives relative to the total content of nitride ceramic powder and sintering aids in the granules is the same as the content of organic additives relative to the total content of nitride ceramic powder and sintering aids in the slurry.

[0049] The amount of residual solvent in the granules is preferably 2% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less. The method for measuring the amount of residual solvent in the granules is the same as the method for measuring the amount of residual solvent in the granules in the above-mentioned production method.

[0050] Average particle size of granules D ｇ５０ is the same range as that described in the method for producing granules.

[0051] The angle of repose of the granules is preferably 40° or less, more preferably 35° or less, even more preferably 30° or less, and particularly preferably 25° or less. When the angle of repose of the granules is within the above range, the flowability of the granules is good and the uniformity of filling into a mold used for pressure molding is excellent. There is no particular restriction on the lower limit of the angle of repose of the granules, and it may be 15° or more.

[0052] The degree of compression of the granules is preferably 30% or less, more preferably 20% or less, even more preferably 15% or less, and particularly preferably 12% or less. When the degree of compression of the granules is within the above range, the granules tend to be integrated when compressed. There is no particular lower limit to the degree of compression, and the degree of compression may be 4% or more.

[0053] The spatula angle of the granules is preferably 60° or less, more preferably 50° or less, even more preferably 40° or less, and particularly preferably 30° or less. When the spatula angle of the granules is within the above range, the flowability of the granules is good and the uniformity of filling into a mold used for pressure molding is excellent. There is no particular restriction on the lower limit of the spatula angle, and it may be 10° or more.

[0054] The angle of repose, compressibility and spatula angle of the granules are measured using a powder tester (for example, PT-X, Hosokawa Micron Corporation).

[0055] The angle of repose is measured three times, and the average value of the three measurements is used as the angle of repose. 200 to 300 g of granules to be measured are used for measuring the angle of repose.

[0056] The compressibility is a value measured and calculated by the following method: 200 to 300 g of the granules to be measured are filled into a predetermined jig, and the loose bulk density ρ of the granule layer is calculated from the mass and volume of the granules when the jig is leveled. Ａ Next, after adding an appropriate amount of granules to be measured and performing tapping 180 times, the packed bulk density ρ of the granule layer is calculated from the mass and volume of the granules in the leveled state. Ｐ The compressibility C (%) is calculated using Equation 2: C = (ρ Ｐ -ρ Ａ ) / ρ Ｐ ×100 formula 2

[0057] The spatula angle is a value measured by the following method. A spatula (flat plate) is inserted horizontally into a pile of 200 to 300 g of granules to be measured, and then the spatula is lifted vertically. At this time, the angle between the slope of the pile of granules to be measured formed on the spatula and the flat surface of the spatula (horizontal plane) is measured (before impact). After a certain amount of impact is further applied, the angle between the slope of the pile and the horizontal plane is measured (after impact). The average value of the angles before and after impact is taken as the spatula angle.

[0058] The uniformity of the granules is preferably not more than 6, more preferably not more than 5, even more preferably not more than 4, and particularly preferably not more than 2. When the uniformity of the granules is within the above range, the granules tend to be integrated when compacted under pressure.

[0059] The uniformity of the granules is measured using an image analysis particle size distribution analyzer (e.g., Morphologi G3, Malvern Panalytical). The particle diameter D is the particle diameter at which the cumulative volume from the small diameter side reaches 60% in the obtained volume-based particle size distribution curve. ｇ６０ and the particle diameter D when the cumulative volume from the small diameter side is 10% ｇ１０Using these, the uniformity U is calculated by the following formula 3: U=D ｇ６０ / D ｇ１０ Formula 3

[0060] The fluidity index of the granules is preferably 60 or more, more preferably 70 or more, even more preferably 75 or more, and particularly preferably 80 or more. When the fluidity index of the granules is within the above range, the fluidity of the granules is good, and the uniformity of filling into a mold used for pressure molding is excellent. The fluidity index of the granules in the present disclosure is the sum of the angle of repose index, compressibility index, uniformity index, and spatula angle index. Each index is assigned according to Table 1 below (Carr's fluidity index table). For example, if the angle of repose is 27°, the angle of repose index is 24.

[0061]

[0062] The granules of the present disclosure may be a raw material for manufacturing bearing balls.

[0063] <Method for producing a nitride ceramic sintered body> In the method for producing a nitride ceramic sintered body of the present disclosure, the granules obtained by the method for producing granules of the present disclosure are pressure-molded to produce a compact, and the compact is then sintered.

[0064] The molding is performed by filling the granules into a desired mold. The shape and size of the mold are not particularly limited. For example, when a nitride ceramic sintered body is used as a base sphere for a bearing ball, a mold of a shape and size suitable for the base sphere for a bearing ball is used. Known molding methods such as uniaxial pressing, die pressing, doctor blade pressing, and rubber pressing can be applied as the pressure molding method. The molded body may be further compressed by cold isostatic pressing (CIP) or the like. Because the granules of the present disclosure have low granule strength and are soft, the effects of CIP are easily obtained. The pressure conditions for CIP are preferably 100 to 200 MPa, more preferably 120 to 185 MPa, and even more preferably 140 to 170 MPa. The holding time at the maximum pressure is preferably 1 to 15 minutes, more preferably 1 to 10 minutes, and even more preferably 1 to 5 minutes. The pressure increase time to the maximum pressure is preferably 1 to 10 minutes, more preferably 1 to 5 minutes, and even more preferably 1 to 3 minutes. When the maximum pressure is 100 to 200 MPa, the pressure required to crush the granules is obtained and the load on the equipment is low. Therefore, when the maximum pressure is within the range of 100 to 200 MPa, a dense compact is easily obtained, the relative density of the sintered body is high, and the number of pores in the sintered body tends to be reduced. When the holding time at the maximum pressure is 1 to 15 minutes, the pressure required to crush the granules is transmitted to the entire compact and the load on the equipment is low. Therefore, when the holding time at the maximum pressure is within the range of 1 to 15 minutes, a dense compact is easily obtained, the relative density of the sintered body is high, and the number of pores in the sintered body tends to be reduced. When the pressure rise time to the maximum pressure is 1 to 10 minutes, cracks and breaks due to pressure rise are suppressed and the load on the equipment is low. Therefore, when the pressure rise time to the maximum pressure is within the range of 1 to 10 minutes, a dense compact is easily obtained, the relative density of the sintered body is high, and the number of pores in the sintered body tends to be reduced.

[0065] The compact may be degreased before firing. Degreasing may be performed in either a non-oxidizing or oxidizing atmosphere. When degreasing is performed in a non-oxidizing atmosphere, a temperature of 550 to 800°C is preferred, and when degreasing is performed in an oxidizing atmosphere such as air, a temperature of 400 to 650°C is preferred. The heating time at these temperatures is preferably 1 to 2 hours. As mentioned above, the preferred range of the degreasing temperature differs between a non-oxidizing atmosphere and an oxidizing atmosphere such as air. When the degreasing temperature is 550°C or higher in a non-oxidizing atmosphere, the amount of carbon remaining due to organic substances such as binders tends to decrease, and the number of pores in the sintered body tends to decrease. When the degreasing temperature is 800°C or lower in a non-oxidizing atmosphere, excessive sintering is suppressed, making degreasing easier and tending to reduce the amount of carbon remaining. When the degreasing temperature is 400°C or higher in an oxidizing atmosphere, the amount of carbon remaining due to organic substances such as binders tends to decrease, and the number of pores in the sintered body tends to decrease. When the debinding temperature is 650°C or lower in an oxidizing atmosphere, oxidation of the nitride ceramic powder is suppressed and the number of internal defects in the sintered body tends to decrease. Therefore, when the debinding temperature is within the range of 550 to 800°C in a non-oxidizing atmosphere, or within the range of 400 to 650°C in an oxidizing atmosphere, the relative density of the sintered body tends to increase and the number of pores in the sintered body tends to decrease.

[0066] The degreased molded body is preferably heated under vacuum to minimize oxygen, with the degree of vacuum being preferably 200 Pa or less, more preferably 100 Pa or less, and even more preferably 10 Pa or less. When the degree of vacuum is 200 Pa or less, oxygen is easily removed. The heating temperature under vacuum is preferably 900 to 1200°C, and the holding time at this heating temperature is preferably 1 to 10 hours.

[0067] After heating under vacuum, it is preferable to sinter it in an inert gas atmosphere such as nitrogen gas or argon gas. Either atmospheric pressure sintering or pressure sintering may be used, and the sintering temperature is preferably 1600 to 1850°C. If the sintering temperature is 1600°C or higher, the sintered body will be sufficiently densified, the defect rate will be low, the mechanical strength will be further improved, and when it is made into a bearing ball, the rolling life will be improved. If the sintering temperature is 1850°C or lower, it will be easier to obtain a sintered body with the desired composition.

[0068] Examples of pressure sintering methods include atmospheric pressure sintering and hot pressing. The pressure in atmospheric pressure sintering is preferably 2 to 100 atmospheres, more preferably 3 to 50 atmospheres, and even more preferably 3 to 10 atmospheres. In the granule manufacturing method disclosed herein, when the pressure of atmospheric pressure sintering is 2 atmospheres or higher, the primary sintered body is sufficiently sintered, and the number of pores in the secondary sintered body tends to be reduced. Furthermore, when the pressure of atmospheric pressure sintering is 100 atmospheres or lower, the difference with the pressure in the hot isostatic pressing process is ensured, and the number of pores in the secondary sintered body tends to be reduced. Therefore, when the pressure of atmospheric pressure sintering is within the range of 2 to 100 atmospheres, the relative density of the sintered body tends to be high and the number of pores in the sintered body tends to be reduced. Note that atmospheric pressure sintering is atmospheric pressure sintering performed before hot isostatic pressing (HIP) processing. The pressure of this atmosphere is higher than atmospheric pressure and lower than the pressure in the HIP processing.

[0069] When sintering is performed by atmospheric pressure sintering, hot pressing, or the like, it is preferable to subject the resulting silicon nitride sintered body to hot isostatic pressing (HIP) treatment at a temperature of 1550 to 1850°C in a non-oxidizing atmosphere of 300 atmospheres or more. In this disclosure, the sintered body before HIP treatment is referred to as the "primary sintered body," and the sintered body after HIP treatment is referred to as the "secondary sintered body." Furthermore, in this disclosure, the term "sintered body" includes both the primary sintered body and the secondary sintered body. In the granule manufacturing method of the present disclosure, when the HIP treatment temperature is 1550°C or higher, sintering is sufficient and the number of pores in the secondary sintered body tends to be reduced. When the HIP treatment temperature is 1850°C or lower, the desired crystal structure is more likely to be obtained, and deformation of the sintered body tends to be suppressed. Therefore, when the HIP treatment temperature is within the range of 1550 to 1850°C, the relative density of the sintered body tends to be high and the number of pores in the sintered body tends to be reduced.

[0070] The gas pressure of the non-oxidizing atmosphere in the HIP treatment is preferably 300 atmospheres or more, more preferably 1000 atmospheres or more, from the viewpoint of densifying the secondary sintered body. From the viewpoint of the load on the apparatus, the upper limit is preferably 2000 atmospheres or less. By performing the HIP treatment, defects that serve as starting points for fatigue fracture in the silicon nitride sintered body can be reduced, and when the silicon nitride sintered body is made into a bearing ball, the sliding properties and rolling life properties are further improved.

[0071] The nitride ceramic sintered body is suitable for use as a base sphere for a bearing ball.

[0072] The present embodiment will be described in more detail below using examples, but the present embodiment is not limited thereto. Examples 1 to 6 are examples, and Example 7 is a comparative example.

[0073] (Average particle size D of nitride ceramic powder and sintering aid ５０ Measurement of average particle size D of nitride ceramic powder and sintering aid ５０ was measured using a laser diffraction particle size distribution analyzer (LA-950V2, Horiba, Ltd.). The solvent used during measurement was water, and an appropriate amount of dispersant was added as needed, followed by ultrasonic dispersion treatment before measurement.

[0074] (Average particle size D of solids in the slurry ５０ Measurement of the average particle size D of the solids in the slurry ５０ was measured using a laser diffraction particle size distribution analyzer (LA-950V2, Horiba, Ltd.) after diluting the slurry to a predetermined concentration required by the analyzer and then subjecting it to ultrasonic dispersion treatment.

[0075] (Measurement of Slurry Viscosity) The slurry viscosity was measured using a rheometer (MCR302, Anton Paar Japan Co., Ltd.). The slurry viscosity was measured at 25°C at a shear rate of 2.2 s using a SUS cone-shaped plate with a diameter of 40 mm and an angle of 4°. －１ was measured.

[0076] (Measurement of residual solvent amount in granules) The residual solvent amount in granules was measured by loss on drying method or Karl Fischer method. In the loss on drying method, it was measured using a heated dryer moisture meter (MB45 halogen moisture meter, Ohaus). In the Karl Fischer titration method (coulometric titration method), it was measured using a Karl Fischer moisture meter (MKC-710, Kyoto Electronics Manufacturing Co., Ltd.).

[0077] (Measurement of Granule Strength) Granule strength was measured using a microcompression tester (MCT series) manufactured by Shimadzu Corporation. Granules were dispersed on a glass slide so as not to overlap, and when viewed from above, granules whose dimensions in both the X and Y directions were within ±10% of the average granule particle size (for example, 90 to 110 μm when the average granule particle size was 100 μm) were selected. Twelve granules from the same lot were measured, and the average value of the 10 granules excluding the minimum and maximum values ​​was adopted as the granule strength.

[0078] (Average particle size of granules D ｇ１０ , D ｇ５０ and D ｇ９０ Measurement of average particle size D of granules ｇ１０ , D ｇ５０ and D ｇ９０ The measurement was carried out using an image analysis particle size distribution measuring device (Morphologi G3, Malvern Panalytical).

[0079] (Measurement of Granule Uniformity) The uniformity of granules was measured using an image analysis particle size distribution measuring device (Morphologi G3, Malvern Panalytical). ｇ１０ and D ｇ６０ The uniformity and uniformity index were calculated.

[0080] (Measurement of Angle of Repose, Compressibility, and Spatula Angle of Granules) The angle of repose, compressibility, and spatula angle of granules were measured using a powder tester (PT-X, Hosokawa Micron Corporation). From the obtained angle of repose, compressibility, and spatula angle, the angle of repose index, compressibility index, and spatula angle index were also calculated.

[0081] (Measurement of intragranular defects) The prepared granules were filled into an alumina pipe (inner diameter 13 mm, outer diameter 15 mm, height 5 mm) placed on a glass slide, and a thermosetting resin was poured into it and cured by heat treatment at 120°C for 5 hours. The alumina pipe was then polished to expose the cross section of the granule, which was then observed under an optical microscope (DM6, Leica Microsystems). For the observation, granules of average size and average structure were selected. From the image of the granule cross section, image analysis software was used to determine the intragranular defects G, where P is the area of ​​the depression in the observed granule and F is the area of ​​the powder-filled portion. Ｄ The percentage (%) was calculated using the following formula 4. The depressions in the granules are internal defects that appear gray when observed under an optical microscope. The powder filling areas are areas that appear white when observed under an optical microscope. G Ｄ =P / F×100 Formula 4

[0082] [Example 1] (Preparation of Slurry) Silicon nitride (Si ３ N ４ ): yttrium oxide (Y ２ O ３ ): Aluminum oxide (Al ２ O ３ ) was added to silicon nitride (Si) so that the mass ratio was 9:0.5:0.5. ３ N ４ , Denka Co., Ltd., SN-9FWS) powder 1800.0 g, and yttrium oxide (Y ２ O ３ 100.0 g of aluminum oxide (α-Al ２ O ３ 100.0 g of powder (ALO14PB, 4N, Kojundo Chemical Laboratory Co., Ltd.), 625.6 g of ion-exchanged water, 49.8 g of dispersant (ammonium polycarboxylate, Y-181, Chukyo Yushi Co., Ltd., nonvolatile content 20% by weight), 42.0 g of pH adjuster (TEAH, concentration 35% by weight), and 3500 g of silicon nitride balls with a diameter of 10 mm were placed in a 5-liter high-density polyethylene (HDPE) container with a diameter of 175 mm. The container was then capped and subjected to ball milling at a rotation speed of 60 rpm for 48 hours. This yielded 2717.4 g of primary slurry A1.

[0083] To the obtained primary slurry A1, 301.5 g of binder (paraffin wax, Chukyo Yushi Co., Ltd., Y-773, non-volatile content 20% by mass), 111.1 g of lubricant (fatty acid, Chukyo Yushi Co., Ltd., Cellosol 920, non-volatile content 18% by mass), 42.0 g of pH adjuster (TEAH, concentration 35% by mass), and 4.9 g of ion-exchanged water were added, and the mixture was ball milled at a rotation speed of 30 rpm for 4 hours. This yielded 3176.9 g of secondary slurry B1.

[0084] The solid concentration of the secondary slurry B1 (silicon nitride, yttrium oxide, and aluminum oxide) was about 35% by volume, and the average particle size D ５０ The particle size was 0.7 μm, the slurry viscosity was 3800 mPa·s (25° C.), and the pH was 10.8 (25° C.) The secondary slurry B1 was prepared multiple times under the same conditions as necessary.

[0085] (Preparation of Granules) Granules were prepared using a freeze granulator (FG) (Pris Co., Ltd., CS220). First, all silicon nitride balls were removed from the secondary slurry B1, and multiple batches of secondary slurry B1 were placed in one container and slowly stirred using a stirring blade to avoid foaming. Spray freezing was carried out under the following conditions: slurry liquid supply rate 9570 g / h, disk rotation speed 6000 rpm, inlet temperature -40 ° C, outlet temperature -37 ° C, to obtain 8870 g of frozen granules C1.

[0086] Next, moisture was removed from the frozen granules C1 using a vacuum dryer. The frozen granules C1 were dried in the vacuum dryer at a vacuum of 10 Pa, a drying temperature of −10° C. for 72 hours, then at a vacuum of 10 Pa, a drying temperature of 40° C. for 12 hours, and further dried under atmospheric pressure at a drying temperature of 80° C. for 12 hours to obtain 5,748 g of granules D1.

[0087] (Evaluation of Granules) Granule strength of Granule D1 was 0.08 MPa. ｇ１０ is 53.6 μm, D ｇ５０ is 85.1 μm, D ｇ９０The particle diameter was 141.0 μm. Granule D1 had an angle of repose of 22.7°, a compressibility of 9.7%, a uniformity of 1.6, a spatula angle of 24.1°, and a fluidity index of 98. The amount of residual solvent in granule D1 was 0.5% by mass. When the cross section of granule D1 was observed with an optical microscope, it was found to have a spherical, solid structure as shown in Figure 1. The intragranular defects of granule D1 were approximately 0%. Therefore, it was found that granule D1 had a solid structure with no intragranular defects, low granule strength, and excellent fluidity.

[0088] (Preparation of Molded Body) 10 g of the obtained granules D1 was weighed out and filled into a rectangular SUS mold of 20 × 40 mm, and subjected to uniaxial pressure molding under a molding load of 200 kg / cm. ２ The green compact obtained by uniaxial pressing was vacuum-packed and subjected to cold isostatic pressing (CIP) at a pressure of 150 MPa to obtain a green compact E1.

[0089] (Evaluation of the Compacted Body) The structure of compact E1 was evaluated by infrared microscopy using the immersion transillumination method. However, because compact E1 is prone to collapse when directly immersed in liquid, necks were formed at the contact points between primary particles in the compact for evaluation. After compact E1 was immersed in liquid with necks formed in at least a portion, it was observed using an infrared microscope. Compact E1 was heat-treated in nitrogen at 1200°C for 1 hour to form necks in at least a portion of the compact. The dimensions were the same before and after the heat treatment, and there was no shrinkage due to the heat treatment. Therefore, it can be inferred that the structure of compact E1 was the same before and after the heat treatment. Figure 2 shows an infrared microscopic photograph of the cross section of compact E1 with necks formed, taken using the immersion transillumination method. No granular marks or noticeable pores were observed, and the structure was homogeneous. Hereinafter, all compacts used in (Evaluation of the Compacted Body) were heat-treated at 1200°C for 1 hour as described above, "to form necks in at least a portion of the compact." All the compacts used in the (production of secondary sintered bodies) below are compacts that have not been heat-treated at 1200°C for 1 hour.

[0090] (Preparation of Secondary Sintered Body) The compact E1 was degreased by heat treatment at 600°C for 2 hours in air, and then sintered using a pressure sintering furnace to obtain a primary sintered body F1. The sintering conditions were 1750°C for 5 hours in a nitrogen atmosphere at 5 atmospheres. The primary sintered body F1 was then subjected to hot isostatic pressing (HIP) treatment to obtain a secondary sintered body G1. The HIP conditions were 1650°C for 1 hour in a nitrogen atmosphere at 1000 atmospheres. The bulk density of the obtained secondary sintered body G1 was measured using the Archimedes method, and it was confirmed to be 99% or more of the theoretical density (relative density). The theoretical density of silicon nitride including sintering aids is 3.27 g / cm ３ It was decided.

[0091] (Evaluation of secondary sintered body) Fig. 3 shows an optical microscope photograph of the cross section of the secondary sintered body G1 observed under bright field illumination. No pores of 10 µm or more were observed in the secondary sintered body G1. Therefore, it was found that a sintered body with reduced pores could be obtained from granules D1.

[0092] [Example 2] (Slurry Preparation) Granules D2 were obtained in the same manner as in Example 1, except that the amount of ion-exchanged water was adjusted so that the solids concentration of the slurry was about 35% to about 25% by volume, and no pH adjuster was added. Specifically, 100.5 g of binder (Y-773, Chukyo Yushi Co., Ltd.), 55.6 g of lubricant (Cellosol 920, Chukyo Yushi Co., Ltd.), and 542.4 g of ion-exchanged water were added to primary slurry A2 prepared in the same manner as in Example 1, except that 1085.2 g of ion-exchanged water and 0 g of pH adjuster were used, and ball mill treatment was carried out at a rotation speed of 30 rpm and a rotation time of 4 hours. As a result, 3833.4 g of secondary slurry B2 with a solids concentration of about 25% by volume was obtained. The average particle size D of the solids of secondary slurry B2 was ５０ The particle size was 0.7 μm, the slurry viscosity was 2600 mPa·s (25° C.), and the pH was 9.3 (25° C.) Secondary slurry B2 was prepared multiple times under the same conditions.

[0093] (Preparation of Granules) Granules D2 were prepared in the same manner as in Example 1, except that secondary slurry B1 was replaced with secondary slurry B2, using a freeze granulator (Pris Co., Ltd., CS220).

[0094] (Evaluation of Granules) Granule strength of Granule D2 was 0.01 MPa. ｇ１０ is 62.0 μm, D ｇ５０ is 109.6 μm, D ｇ９０ The particle diameter was 171.3 μm. Granule D2 had an angle of repose of 36.0°, a compressibility of 27.3%, a uniformity of 2.4, a spatula angle of 58.0°, and a fluidity index of 70.5. The amount of residual solvent in granule D2 was 0.5% by mass. When the cross section of granule D2 was observed with an optical microscope, it was found to have a spherical shape and a solid structure. The intragranular defects of granule D2 were approximately 0%. Therefore, it was found that granule D2 had a solid structure with no intragranular defects, low granule strength, and excellent fluidity.

[0095] (Preparation of compact E2 and secondary sintered body G2) Compact E2 and secondary sintered body G2 were prepared in the same manner as in Example 1, except that granules D1 were replaced with granules D2. Compact E2 and secondary sintered body G2 were observed in the same manner as in Example 1, and no pores of 10 μm or more were found in secondary sintered body G2. Therefore, it was found that a sintered body with reduced pores could be obtained from granules D2.

[0096] [Example 3] (Slurry Preparation) Granules D3 were obtained in the same manner as in Example 1, except that the amount of ion-exchanged water was adjusted so that the solids concentration of the slurry was about 35% to about 30% by volume, and no pH adjuster was added. Specifically, 301.5 g of binder (Y-773, Chukyo Yushi Co., Ltd.), 111.1 g of lubricant (Cellosol 920, Chukyo Yushi Co., Ltd.), and 96.6 g of ion-exchanged water were added to primary slurry A3 prepared in the same manner as in Example 1, except that 866.9 g of ion-exchanged water and 0 g of pH adjuster were used, and ball mill treatment was carried out at a rotation speed of 30 rpm and a rotation time of 4 hours. As a result, 3425.9 g of secondary slurry B3 with a solids concentration of about 30% by volume was obtained. The average particle size D of the solids of secondary slurry B2 ５０ The particle size was 0.7 μm, the slurry viscosity was 3200 mPa·s (25° C.), and the pH was 9.5 (25° C.) Secondary slurry B3 was prepared multiple times under the same conditions.

[0097] (Preparation of Granules) Granules D3 were prepared in the same manner as in Example 1, except that secondary slurry B1 was replaced with secondary slurry B3, using a freeze granulator (Pris Co., Ltd., CS220).

[0098] (Evaluation of Granules) Granule strength of Granule D3 was 0.05 MPa. ｇ１０ is 69.2 μm, D ｇ５０ is 126.2 μm, D ｇ９０ The particle diameter was 167.2 μm. Granule D3 had an angle of repose of 24.1°, a compressibility of 12.5%, a uniformity of 1.7, a spatula angle of 23.6°, and a fluidity index of 96. The residual solvent content of granule D3 was 0.5% by mass. When the cross section of granule D3 was observed with an optical microscope, it was found to have a spherical shape and a solid structure. The intragranular defects of granule D3 were approximately 0%. Therefore, it was found that granule D3 had a solid structure with no intragranular defects, low granule strength, and excellent fluidity.

[0099] (Preparation of compact E3 and secondary sintered body G3) Compact E3 and secondary sintered body G3 were prepared in the same manner as in Example 1, except that granules D1 were replaced with granules D3. Compact E3 and secondary sintered body G3 were observed in the same manner as in Example 1, and no pores of 10 μm or more were found in secondary sintered body G3. Therefore, it was found that a sintered body with reduced pores could be obtained from granules D3.

[0100] [Example 4] (Slurry Preparation) Granules D4 were obtained in the same manner as in Example 1, except that the amount of ion-exchanged water was adjusted so that the solids concentration of the slurry was about 35% to about 40% by volume, the type and amount of binder and lubricant were changed, and no pH adjuster was added in the preparation of the secondary slurry. Specifically, 153.8 g of binder (paraffin wax, P-222, Chukyo Yushi Co., Ltd., non-volatile content 39% by mass), 80.0 g of lubricant (fatty acid amide, L-271, Chukyo Yushi Co., Ltd., non-volatile content 25% by mass), and 20.8 g of ion-exchanged water were added to primary slurry A4 prepared in the same manner as in Example 1, except that the amount of ion-exchanged water was changed to 570.3 g, and ball mill treatment was carried out at a rotation speed of 30 rpm for a rotation time of 4 hours. This resulted in 2916.7 g of secondary slurry B4 with a solids concentration of about 40% by volume. The average particle size D of the solids of secondary slurry B4 ５０ The particle size was 0.7 μm, the slurry viscosity was 4800 mPa·s (25° C.), and the pH was 10.5 (25° C.) Secondary slurry B4 was prepared multiple times under the same conditions.

[0101] (Preparation of Granules) Granules D4 were prepared in the same manner as in Example 1, except that secondary slurry B1 was replaced with secondary slurry B4, using a freeze granulator (Pris Co., Ltd., CS220).

[0102] (Evaluation of Granules) Granule strength of Granule D4 was 0.25 MPa. ｇ１０ is 72.9 μm, D ｇ５０ is 110.0 μm, D ｇ９０ The particle diameter was 174.1 μm. Granule D4 had an angle of repose of 26.0°, a compressibility of 9.7%, a uniformity of 1.8, a spatula angle of 20.8°, and a fluidity index of 97. The amount of residual solvent in granule D4 was 0.5% by mass. When the cross section of granule D4 was observed with an optical microscope, it was found to have a spherical shape and a solid structure. The intragranular defects of granule D4 were approximately 0%. Therefore, it was found that granule D4 had a solid structure with no intragranular defects, low granule strength, and excellent fluidity.

[0103] (Preparation of compact E4 and secondary sintered body G4) Compact E4 and secondary sintered body G4 were prepared in the same manner as in Example 1, except that granules D1 were replaced with granules D4. Compact E4 and secondary sintered body G4 were observed in the same manner as in Example 1, and no pores of 10 μm or more were found in secondary sintered body G4. Therefore, it was found that a sintered body with reduced pores could be obtained from granules D4.

[0104] [Example 5] (Slurry Preparation) Granules D5 were obtained in the same manner as in Example 1, except that the amount of ion-exchanged water was adjusted so that the solids concentration of the slurry was about 35% by volume to about 25% by volume, and no pH adjuster was added. Specifically, 1085.2 g of ion-exchanged water and 0 g of pH adjuster were used to prepare primary slurry A5 in the same manner as in Example 1, and 301.5 g of binder (Y-773, Chukyo Yushi Co., Ltd.), 111.1 g of lubricant (Cellosol 920, Chukyo Yushi Co., Ltd.), and 285.8 g of ion-exchanged water were added, and the mixture was subjected to ball milling at a rotation speed of 30 rpm for 4 hours. This resulted in 3833.4 g of secondary slurry B5 with a solids concentration of about 25% by volume. The average particle size D of the solids of secondary slurry B5 was ５０ The particle size was 0.7 μm, the slurry viscosity was 2600 mPa·s (25° C.), and the pH was 9.3 (25° C.) Secondary slurry B5 was prepared multiple times under the same conditions.

[0105] Granules D5 were produced using a freeze granulator (Pris Co., Ltd., CS30) in Example 1, except that secondary slurry B1 was replaced with secondary slurry B5. A two-fluid nozzle was used as the spray method, and spray freezing was carried out under conditions of a slurry liquid supply rate of 5,510 g / h, an inlet temperature of −57° C., and an outlet temperature of −89° C., to obtain 10,380 g of frozen granules C5, which were then dried in the same manner as in Example 1 to obtain 5,336 g of granules D5.

[0106] (Evaluation of Granules) Granule strength of Granule D5 was 0.05 MPa. ｇ１０ is 19.0 μm, D ｇ５０ is 51.7 μm, D ｇ９０The particle diameter was 154.6 μm. Granule D5 had an angle of repose of 38.1°, a compressibility of 25.7%, a uniformity of 3.8, a spatula angle of 47.2°, and a fluidity index of 72. The amount of residual solvent in granule D5 was 0.5% by mass. When the cross section of granule D5 was observed with an optical microscope, it was found to have a spherical shape and a solid structure. The intragranular defects of granule D5 were approximately 0%. Therefore, it was found that granule D5 had a solid structure with no intragranular defects, low granule strength, and excellent fluidity.

[0107] (Preparation of compact E5 and secondary sintered body G5) Compact E5 and secondary sintered body G5 were prepared in the same manner as in Example 1, except that granules D1 were replaced with granules D5. Compact E5 and secondary sintered body G5 were observed in the same manner as in Example 1, and no pores of 10 μm or more were found in secondary sintered body G5. Therefore, it was found that a sintered body with reduced pores could be obtained from granules D5.

[0108] [Example 6] (Slurry Preparation) Granules D6 were obtained by the following method. Instead of silicon nitride, yttrium oxide, and aluminum oxide, sialon (SiAlON, Combustion Synthesis Co., Ltd., Ca-a-SiAlON) powder was used. 2000.0 g of sialon, 1395.4 g of ion-exchanged water, 49.8 g of dispersant (ammonium polycarboxylate, Chukyo Yushi Co., Ltd., D305, nonvolatile content 40% by weight), 22.4 g of pH adjuster (TEAH, 35% by weight), and 3500 g of 10 mm diameter silicon nitride balls were placed in a 175 mm diameter, 5 L high-density polyethylene (HDPE) container. The container was then capped and subjected to ball milling at a rotation speed of 60 rpm for 48 hours. This produced 3467.5 g of primary slurry A6.

[0109] To the obtained primary slurry A6, 301.5 g of binder (Y-773, Chukyo Yushi Co., Ltd.), 111.1 g of lubricant (Cellosol 920, Chukyo Yushi Co., Ltd.), and 6.7 g of ion-exchanged water were added, and the mixture was ball milled at a rotation speed of 30 rpm for 4 hours, thereby obtaining 3,886.8 g of secondary slurry B6 with a solids concentration of approximately 25% by volume.

[0110] Average particle size D of solid content of secondary slurry B6５０ The particle size was 0.5 μm, the slurry viscosity was 3000 mPa·s (25° C.), and the pH was 9.0 (25° C.) Secondary slurry B6 was prepared multiple times under the same conditions.

[0111] (Preparation of Granules) Granules D6 were prepared in the same manner as in Example 1, except that secondary slurry B1 was replaced with secondary slurry B6, using a freeze granulator (Pris Co., Ltd., CS220).

[0112] (Evaluation of Granules) Granule strength of Granule D6 was 0.08 MPa. ｇ１０ is 76.1 μm, D ｇ５０ is 133.2 μm, D ｇ９０ The particle diameter was 181.0 μm. Granule D6 had an angle of repose of 32.5°, a compressibility of 14.7%, a uniformity of 1.8, a spatula angle of 34.9°, and a fluidity index of 85. The amount of residual solvent in granule D6 was 0.5% by mass. When the cross section of granule D6 was observed with an optical microscope, it was found to have a spherical shape and a solid structure. The intragranular defects of granule D6 were approximately 0%. Therefore, it was found that granule D6 had a solid structure with no intragranular defects, low granule strength, and excellent fluidity.

[0113] (Preparation of compact E6 and secondary sintered body G6) Compact E6 and secondary sintered body G6 were prepared in the same manner as in Example 1, except that granules D1 were replaced with granules D6. Compact E6 and secondary sintered body G6 were observed in the same manner as in Example 1, and no pores of 10 μm or more were found in secondary sintered body G6. Therefore, it was found that a sintered body with reduced pores could be obtained from granules D6.

[0114] [Example 7] (Slurry Preparation) Granules D7 were obtained in the same manner as in Example 3, except that the amount of ion-exchanged water was adjusted so that the solid content concentration of the slurry was about 30% to about 25% by volume, and a spray dryer was used instead of a freeze granulator for the spray granulation method. Specifically, 301.5 g of binder (Y-773, Chukyo Yushi Co., Ltd.), 111.1 g of lubricant (Cellosol 920, Chukyo Yushi Co., Ltd.), and 285.8 g of ion-exchanged water were added to primary slurry A7 prepared in the same manner as in Example 3, except that the amount of ion-exchanged water was 1085.2 g, and ball mill treatment was carried out at a rotation speed of 30 rpm for a rotation time of 4 hours. As a result, 3833.4 g of secondary slurry B7 with a solid content concentration of about 30% by volume was obtained. The average particle size D of the solid content of secondary slurry B7 was ５０ The particle size was 0.7 μm, the slurry viscosity was 2600 mPa·s (25° C.), and the pH was 9.3 (25° C.) Secondary slurry B7 was prepared multiple times under the same conditions.

[0115] (Granule Preparation) Granules were prepared using a spray dryer (SD) (Pris Co., Ltd., P260). First, all silicon nitride balls were removed from secondary slurry B7, and multiple batches of secondary slurry B7 were placed in one container and slowly stirred using a stirring blade to avoid foaming. Spray granulation was carried out under the following conditions: slurry liquid supply rate 16,000 g / h, disk rotation speed 6,000 rpm, inlet temperature 180 ° C, outlet temperature 112 ° C, to obtain 7,650 g of granules D7. Note that the granules C7 obtained using the spray dryer contained almost no residual solvent, so the drying step was omitted.

[0116] (Evaluation of Granules) Granule strength of Granule D7 was 0.31 MPa. ｇ１０ is 42.9 μm, D ｇ５０ is 67.0 μm, D ｇ９０The particle diameter was 98.7 μm. Granule D7 had an angle of repose of 23.1°, a compressibility of 10.9%, a uniformity of 1.7, a spatula angle of 22.8°, and a fluidity index of 97. The amount of residual solvent in granule D7 was 0.5% by mass. When the cross section of granule D7 was observed with an optical microscope, it was found to have a spherical shape but with depressions and a hollow structure, as shown in FIG. 4. The intragranular defects of granule D7 were approximately 12.2%. Therefore, it was found that granule D7 has a hollow structure with many intragranular defects, low but high granule strength, and excellent fluidity.

[0117] (Preparation of Compacted Body E7 and Secondary Sintered Body G7) Compacted body E7 and secondary sintered body G7 were prepared in the same manner as in Example 1, except that granules D1 were replaced with granules D7. Compacted body E7 and secondary sintered body G7 were observed in the same manner as in Example 1. Figure 5 shows an infrared microscope photograph of the cross section of compacted body E7 taken using immersion transillumination. Figure 6 shows an optical microscope photograph of the cross section of secondary sintered body G7 taken using incident light bright field observation. In this photograph alone, there were five pores of 10 μm or larger, with pore sizes ranging from 14 to 26 μm. Secondary sintered body G7 had five or more pores of 10 μm or larger. Therefore, it was found that a pore-free sintered body could not be obtained from granules D7.

[0118] The results of Examples 1 to 7 are summarized in Table 2. In Table 2, FG indicates a freeze granulator and SD indicates a spray dryer. The evaluation criteria are as follows:

[0119] <Granule strength> A: 0.10 MPa or more and less than 0.30 MPa B: 0.01 MPa or more and less than 0.10 MPa C: 0.30 MPa or more and less than 0.50 MPa D: Less than 0.01 MPa or more than 0.50 MPa

[0120] <Flowability> Evaluation was made based on the flowability index. A: 80 or more, B: 70 or more and less than 80, C: 60 or more and less than 70, D: less than 60

[0121] <Intragranular defect>: Intragranular defect G Ｄ A: less than 2%, B: 2% or more and less than 5%, C: 5% or more and less than 10%, D: 10% or more.

[0122] <Number of defects inside the sintered body> 10 mm cross section of the sintered body ２The number of pores having a size of 10 μm or more was counted.

[0123]

[0124] The above results show that the granules of Examples 1 to 6, obtained by granulating a slurry containing nitride ceramic powder, sintering aid, aqueous solvent, and organic additives using the spray freeze granulation drying method, significantly reduced the number of internal defects in the resulting sintered body. The reason for this is presumably because the granules of Examples 1 to 6 had a solid structure with no visible intragranular defects, low granule strength, and excellent fluidity. In contrast, the granules of Example 7, obtained by granulation using a spray dryer, had internal defects in the resulting sintered body. The reason for this is presumably because the granules of Example 7 had a hollow structure with many intragranular defects.

[0125] The disclosure of Japanese Patent Application No. 2024-22213 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards in this disclosure are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A method for producing granules in which a slurry containing nitride ceramic powder, a sintering aid, an aqueous solvent, and organic additives is granulated by a spray-freeze granulation drying method.

2. The method for producing granules according to claim 1, wherein the nitride ceramic powder comprises at least one selected from the group consisting of silicon nitride powder and SiAlON powder.

3. The method for producing granules according to claim 1 or 2, wherein the organic additive comprises at least one selected from the group consisting of a dispersant, a binder, a lubricant, an antifoaming agent and a plasticizer.

4. The method for producing granules described in claim 3, wherein the dispersant comprises at least one selected from the group consisting of a pH adjuster, a polycarboxylic acid compound, an ammonium salt of condensed naphthalenesulfonic acid, a salt of condensed naphthalenesulfonic acid, a polyacrylic acid amide compound, and an alkylsulfonic acid compound.

5. The method for producing granules according to claim 3, wherein the content of the dispersant in the slurry is 0.05 to 3 mass % relative to the content of the nitride ceramic powder.

6. The method for producing granules described in claim 3, wherein the binder comprises at least one selected from the group consisting of paraffin wax, carnauba wax, microcrystalline wax, polyvinyl alcohol, cellulose derivatives, alginic acid, polyethylene glycol, polyvinylpyrrolidone, acrylic resins, vinyl acetate, styrene resins, and polysaccharides.

7. The method for producing granules according to claim 3, wherein the content of the binder in the slurry is 0.5 to 6 mass % relative to the content of the nitride ceramic powder.

8. Average particle size D of the nitride ceramic powder ５０ The method for producing granules according to claim 1 or 2, wherein the particle size is 0.1 to 1.5 μm.

9. The method for producing granules according to claim 1 or 2, wherein the spray-freeze granulation drying method is carried out in the order of spray-freezing and vacuum drying, and after the vacuum drying, further drying is carried out at atmospheric pressure at 35 to 150°C to reduce the amount of residual solvent to 2% by mass or less.

10. The method for producing granules according to claim 1 or 2, wherein the granules are raw materials for producing bearing balls.

11. A method for producing granules for bearing balls, in which a slurry containing nitride ceramic powder is granulated by spray-freeze granulation drying.

12. A method for producing a nitride ceramic sintered body, comprising press-molding granules obtained by the method of claim 1 or 11 to produce a molded body, and sintering the molded body.

13. The method for producing a nitride ceramic sintered body according to claim 12, wherein the nitride ceramic sintered body is a base sphere for a bearing ball.

14. Granules containing nitride ceramics, sintering aids and organic additives, having a granule strength of 0.01 to 0.50 MPa.

15. Granules according to claim 14, wherein said nitride ceramic comprises at least one selected from the group consisting of silicon nitride and SiAlON.

16. Granules according to claim 14 or 15, having a flowability index of 60 or more.

17. Granules according to claim 14 or 15, used as a raw material for manufacturing bearing balls.

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