Silicon nitride powder and method for producing same

Abstract

The present disclosure provides a silicon nitride powder in which the alpha-phase percentage is 96.0 mass % or less, and D97 is 2.25 μm or less, wherein D97 is the particle diameter when the integrated value from the small particle diameter reaches 97% of the total in a cumulative distribution of the volume-based particle diameters measured by a particle size distribution measurement device using a laser diffraction / scattering method.

Description

Silicon nitride powder and its manufacturing method

[0001] The present disclosure relates to silicon nitride powders and methods for making the same.

[0002] Silicon nitride is a material with excellent strength, hardness, toughness, heat resistance, corrosion resistance, and thermal shock resistance, and is therefore used in various industrial parts such as die-casting machines and melting furnaces, as well as automotive parts. Furthermore, because silicon nitride also has excellent mechanical properties at high temperatures, its use in gas turbine parts, which require high-temperature strength and high-temperature creep properties, is being considered. Patent Document 1 describes a silicon nitride sintered body characterized by a thermal conductivity of 100 to 300 W / (m·K) at room temperature and a three-point bending strength of 600 to 1500 MPa at room temperature.

[0003] Japanese Patent Application Laid-Open No. 2004-262756

[0004] Further improvements in the mechanical properties of silicon nitride sintered bodies are required. Because silicon nitride sintered bodies are obtained by sintering silicon nitride powder, it is believed that the properties of the silicon nitride powder affect the bending strength of the silicon nitride sintered body.

[0005] During sintering of silicon nitride powder, silicon nitride particles dissolve in a sintering aid melt, then reprecipitate and undergo grain growth. During this process, a phase transformation to the thermally more stable β phase occurs. Grain growth of silicon nitride accompanies this phase transformation, resulting in sintering of the silicon nitride powder to form a sintered body. Using silicon nitride with a high proportion of the thermally unstable α phase can improve the properties (e.g., bending strength) of the resulting sintered body. However, silicon nitride with a high α phase ratio (the proportion of α phase relative to the total mass of silicon nitride) can be obtained by chemical synthesis using imide pyrolysis or high-temperature vapor phase synthesis, which forms euhedral particles by high-temperature crystal growth. These manufacturing methods tend to produce a large amount of by-products and require corrosion protection for the equipment, resulting in low productivity and expensive silicon nitride powders and sintered silicon nitride bodies. Therefore, it would be useful to provide a silicon nitride powder with a low α phase ratio that can produce sintered bodies with excellent bending strength.

[0006] The direct nitridation method, which uses metal silicon as the starting material, is an exothermic reaction accompanied by rapid heat generation, making it difficult to control the reaction rate. As a result, the silicon nitride obtained by the direct nitridation method has a lower alpha phase ratio than silicon nitride obtained by chemical synthesis and high-temperature vapor phase processes. On the other hand, the direct nitridation method does not produce by-products, so the yield of silicon nitride is high and productivity is good. However, because it contains the beta phase, there is room for improvement in the properties of the sintered body when the silicon nitride powder produced by the direct nitridation method is used as a raw material for the sintered body.

[0007] The present disclosure aims to provide a silicon nitride powder having a low gelatinization rate (a gelatinization rate of 96.0 mass% or less) from which a sintered body having excellent bending strength can be prepared. Another object of the present disclosure is to provide a method for producing such silicon nitride at lower cost.

[0008] One aspect of the present disclosure provides the following silicon nitride powder:

[0009] [1] A silicon nitride powder having an gelatinization rate of 96.0% by mass or less, and in a cumulative distribution of volumetric particle diameters measured with a particle size distribution analyzer using a laser diffraction / scattering method, when D97 is the particle diameter at which the integrated value from small particle diameters reaches 97% of the total, D97 is 2.25 μm or less.

[0010] The silicon nitride powder of [1] above has a gelatinization rate of 96.0% by mass or less, and the D97 value is 2.25 μm or less. In the case of silicon nitride powder with a low gelatinization rate, silicon nitride having a relatively thermally stable β-phase remains unmelted during the sintered body production process, and grain growth proceeds preferentially as nuclei for crystal growth, resulting in the inclusion of coarse particles in the sintered body. If the sintered body has a non-uniform structure, such as containing coarse particles, stress concentration occurs when external force is applied, and bending strength tends to be low. In the case of silicon nitride powder with a high gelatinization rate, the particle size of the raw material is not a major problem because it is easily melted during the sintered body production process, but it can become a problem if the gelatinization rate is reduced. On the other hand, the silicon nitride powder [1] above has a small particle size adjusted so that D97 is equal to or less than a predetermined value, and even when a sintered body is produced by a conventional method, the silicon nitride can be sufficiently melted, and even if the silicon nitride having a β phase starts to grow without melting, it is possible to suppress the difference in particle size between the silicon nitride particles and the new silicon nitride particles generated by reprecipitation. Therefore, the obtained sintered body has a reduced proportion of coarse silicon nitride particles, which reduces stress concentration when an external force is applied, and can exhibit excellent bending strength.

[0011] The silicon nitride powder of [1] above may be the following [2] or [3].

[0012] [2] The silicon nitride powder according to [1], wherein, in the cumulative distribution, the particle diameter when the cumulative value from the small particle diameter reaches 10% of the total is defined as D10, the particle diameter when the cumulative value from the small particle diameter reaches 50% of the total is defined as D50, and the particle diameter when the cumulative value from the small particle diameter reaches 90% of the total is defined as D90, the ratio (D90-D10) / D50 being 1.70 or less. [3] The silicon nitride powder according to [1] or [2], wherein the silicon nitride powder has an oxygen content of 0.65 to 1.60 mass%.

[0013] The silicon nitride powder of [2] above has a (D90-D10) / D50 ratio of 1.70 or less. Such silicon nitride powder has smaller particle size variations, and when this silicon nitride powder is used as a sintered body raw material, it is possible to suppress variations in bending strength between the resulting sintered bodies. In other words, when multiple sintered bodies are produced using the silicon nitride powder, it is possible to increase the Weibull coefficient of bending strength. This allows for the stable production of sintered bodies with excellent bending strength.

[0014] The silicon nitride powder [3] has an oxygen content of 0.65 to 1.60 mass %. Silicon nitride powder with this oxygen content can improve reactivity during sintering while suppressing embrittlement of the sintered body due to oxygen. Therefore, the bending strength of the sintered body can be further improved.

[0015] One aspect of the present disclosure provides the following method for producing a silicon nitride powder.

[0016] [4] A method for producing silicon nitride powder, comprising a step of adjusting the particle size of a raw material silicon nitride powder having an alpha conversion rate of 96.0% by mass or less, so that D97 is 2.25 μm or less, where D97 is the particle size at which the integrated value from small particle sizes reaches 97% of the total in the cumulative distribution of volumetric particle sizes measured with a particle size distribution analyzer using a laser diffraction / scattering method.

[0017] The method for producing silicon nitride powder described in [4] above is relatively simple, as it uses silicon nitride powder with a low degree of gelatinization as a raw material and adjusts the particle size to produce the desired silicon nitride powder. Therefore, silicon nitride powder can be produced at a lower cost than the cost of producing silicon nitride powder with a high degree of gelatinization as a raw material for sintered bodies that exhibit excellent bending strength. Furthermore, the particle size of the silicon nitride powder obtained by the method described in [4] above is adjusted so that the D97 is 2.25 μm or less, making it possible to produce sintered bodies with high particle size uniformity and high bending strength.

[0018] The method for producing silicon nitride powder described above in [4] may be the following method described in [5] or [6].

[0019] [5] The method for producing a silicon nitride powder according to [4], wherein the particle size adjusting step adjusts the particle size by classifying the raw silicon nitride powder using an air classifier at a swirling air velocity of 100 to 300 m / s. [6] The method for producing a silicon nitride powder according to [4] or [5], wherein the particle size adjusting step adjusts the particle size so that (D90 - D10) / D50 is 1.70 or less, where D10 is the particle size when an integrated value from small particle diameters reaches 10% of the total, D50 is the particle size when an integrated value from small particle diameters reaches 50% of the total, and D90 is the particle size when an integrated value from small particle diameters reaches 90% of the total.

[0020] The method for producing silicon nitride powder described above in [5] adjusts the particle size of the silicon nitride powder by classification under predetermined conditions using an air classifier. By adjusting the particle size of the silicon nitride powder through such classification, the desired silicon nitride powder can be produced more easily than, for example, mixing powders of different particle sizes or adjusting the particle size by pulverization.

[0021] According to the present disclosure, it is possible to provide a silicon nitride powder having a low gelatinization rate, which can be used to prepare a sintered body having excellent bending strength. According to the present disclosure, it is also possible to provide a production method for producing the above-mentioned silicon nitride at a lower cost.

[0022] 1 is a scanning electron microscope photograph showing a cross section of a sintered body produced using the silicon nitride powder of Comparative Example 4. 2 is a scanning electron microscope photograph showing a cross section of a sintered body produced using the silicon nitride powder of Example 4. 3 is a scanning electron microscope photograph showing a cross section of a sintered body produced using the silicon nitride powder of Example 8.

[0023] Hereinafter, embodiments of the present disclosure will be described. However, the following embodiments are merely examples for explaining the present disclosure and are not intended to limit the present disclosure to the following content. The upper or lower limit of a numerical range specified in this disclosure may be replaced with any value shown in the examples. Furthermore, the upper and lower limits individually stated may be arbitrarily combined. The symbol "to" used in a numerical range indicates a numerical range that includes the upper and lower limit values. For example, "X to Y" indicates a numerical range "greater than or equal to X and less than or equal to Y." Unless otherwise specified, the materials or components exemplified in this disclosure can be used alone or in combination of two or more types.

[0024] <Silicon nitride powder> Silicon nitride powder (Si ３ N ４ The silicon nitride powder has a gelatinization rate of 96.0% by mass or less, and when D97 is defined as the particle diameter at which the integrated value from the smallest particle diameter reaches 97% of the total in the cumulative volumetric particle diameter distribution measured with a particle size distribution analyzer using a laser diffraction / scattering method, D97 is 2.25 μm or less. By using such a silicon nitride powder, a sintered body excellent in bending strength can be obtained.

[0025] Alpha conversion rate of silicon nitride powder (Si ３ N ４ α-Si relative to the total amount ３ N ４ The α-phase ratio (mass proportion) is 96.0% by mass or less, but may be, for example, 95.0% by mass or less, or 94.0% by mass or less. By keeping the α-phase ratio within this range, the proportion of the β-phase in the silicon nitride powder is further increased, resulting in excellent productivity. The α-phase ratio may be, for example, 89.0% by mass or more, or 90.0% by mass or more. By keeping the α-phase ratio within this range, a sintered body with excellent bending strength can be obtained. The α-phase ratio may be, for example, 89.0 to 96.0% by mass, 89.0 to 95.0% by mass, 89.0 to 94.0% by mass, 90.0 to 96.0% by mass, 90.0 to 95.0% by mass, or 90.0 to 94.0% by mass.

[0026] The alpha-phase ratio of silicon nitride powder can be adjusted by the conditions for firing the silicon nitride powder. Obtaining silicon nitride powder with a high alpha-phase ratio requires chemical synthesis, which generates a large amount of by-products, and is therefore costly. On the other hand, silicon nitride powder with a low alpha-phase ratio can be obtained by a nitriding reaction in which metal silicon is directly nitrided, which prevents cost increases and allows for simple production. The alpha-phase ratio of silicon nitride powder can be determined by the diffraction line intensity of X-ray diffraction. Specifically, the alpha-phase ratio of silicon nitride powder is determined by the method described in the Examples.

[0027] In the cumulative distribution of volumetric particle diameters of silicon nitride powder measured with a particle size distribution analyzer using a laser diffraction / scattering method, when the particle diameter at which the integrated value from the smallest particle diameter reaches 97% of the total is defined as D97, D97 is 2.25 μm or less. The D97 may be, for example, 2.00 μm or less, 1.80 μm or less, 1.60 μm or less, or 1.40 μm or less. Silicon nitride powders having a D97 in this range have sufficiently small particle diameters. Silicon nitride powders having a D97 in this range can further suppress the generation of coarse particles during sintering, even when they contain a large amount of β phase, which easily becomes a crystal nucleus during sintering. Therefore, the generation of coarse particles in the sintered body can be further suppressed. Such sintered bodies have even higher bending strength.

[0028] D97 may be, for example, 0.80 μm or more, 0.90 μm or more, or 1.00 μm or more. The range of D97 may be, for example, 0.80 to 2.25 μm, 0.80 to 2.00 μm, 0.80 to 1.80 μm, 0.80 to 1.60 μm, or 0.80 to 1.40 μm.

[0029] In the cumulative distribution of volumetric particle diameters of silicon nitride powder measured using a particle size distribution analyzer using a laser diffraction / scattering method, the particle diameter at which the cumulative value from the smallest particle diameter reaches 90% of the total is defined as D90. D90 may be, for example, 2.00 μm or less, 1.50 μm or less, or 1.30 μm or less. Silicon nitride powders having an upper D90 value in this range are composed of smaller particles, which further suppresses the generation of coarse particles in sintered bodies obtained using the powder as a raw material, resulting in sintered bodies with higher bending strength. Furthermore, D90 may be, for example, 0.60 μm or more, 0.80 μm or more, or 1.20 μm or more. Having a lower D90 value in this range improves classification productivity, enables silicon nitride powder to be produced more inexpensively, and stabilizes production conditions such as the firing temperature during sintered body production. The D90 range may be, for example, 0.60 to 2.00 μm, 0.60 to 1.50 μm, or 0.60 to 1.30 μm.

[0030] In the cumulative distribution of volumetric particle diameters of silicon nitride powder measured using a particle size distribution analyzer using a laser diffraction / scattering method, the particle diameter at which the cumulative value from the smallest particle diameter reaches 50% of the total is defined as D50. D50 may be, for example, 1.00 μm or less, 0.90 μm or less, 0.80 μm or less, or 0.70 μm or less. Silicon nitride powders with an upper D50 value in this range are composed of smaller particles, which further suppresses the generation of coarse particles in sintered bodies obtained using the powder as a raw material, resulting in sintered bodies with higher bending strength. Furthermore, D50 may be, for example, 0.30 μm or more, or 0.40 μm or more. Having a lower D50 value in this range improves classification productivity, enables silicon nitride powder to be produced more inexpensively, and stabilizes production conditions such as the firing temperature during sintered body production. The D50 range may be, for example, 0.30 to 1.00 μm, 0.30 to 0.90 μm, 0.30 to 0.80 μm, or 0.30 to 0.70 μm.

[0031] In the cumulative particle size distribution of silicon nitride powder measured by a particle size distribution analyzer using a laser diffraction / scattering method, the particle size D10 is defined as the particle size at which the cumulative value from the smallest particle size reaches 10% of the total. D10 may be, for example, 0.60 μm or less, 0.50 μm or less, or 0.40 μm or less. Silicon nitride powders with an upper D10 value in this range are composed of smaller particles, which further suppresses the generation of coarse particles in sintered bodies obtained using the powder as a raw material, resulting in sintered bodies with higher bending strength. Furthermore, D10 may be, for example, 0.05 μm or more or 0.10 μm or more. Having a lower D10 value in this range improves classification productivity, enables silicon nitride powder to be produced more inexpensively, and stabilizes production conditions such as the firing temperature during sintered body production. The range of D10 may be, for example, 0.05 to 0.60 μm, 0.05 to 0.50 μm, or 0.05 to 0.40 μm.

[0032] The (D90-D10) / D50 value of the silicon nitride powder, calculated from the D90, D50, and D10 values, may be, for example, 1.70 or less, 1.60 or less, or 1.50 or less. The (D90-D10) / D50 value, called the span value, is an index representing the sharpness of the particle size distribution. The smaller the span value, the shorter the difference in particle size between D90 and D10, and the closer the value is to the median D50, resulting in a sharper particle size distribution. A sharp particle size distribution reduces the variation in particle size of the silicon nitride powder, resulting in a powder with even higher particle size uniformity. Therefore, when multiple sintered bodies are produced using such silicon nitride powder, the variation in bending strength of each sintered body can be reduced. This allows the Weibull coefficient of the sintered body to be increased.

[0033] The value of (D90-D10) / D50 may be, for example, 1.30 or greater, or 1.40 or greater. Silicon nitride powders with a lower limit of (D90-D10) / D50 in this range have smaller particle size variations. Therefore, during the production of a sintered body, the silicon nitride powder dissolves in the liquid phase of the sintering aid and the environment during reprecipitation is further homogenized, making it possible to more sufficiently suppress the particle size variations of growing β-phase particles. This further improves the bending strength of the resulting sintered body. The range of the (D90-D10) / D50 value of the silicon nitride powder may be, for example, 1.30 to 1.70, or 1.40 to 1.70.

[0034] The laser diffraction / scattering method can be measured in accordance with the method described in JIS R 1629:1997 "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction / scattering method." For the measurement, a laser diffraction / scattering particle size distribution measuring device (manufactured by Microtrac Bell, product name: MT-3300EX) or the like can be used.

[0035] The BET specific surface area of ​​the silicon nitride powder is, for example, 9.0 m ２ / g or more, and 10.0m ２ / g or more, and ２ / g or more, and ２ / g or more. Silicon nitride powders with a lower limit of the BET specific surface area within this range have a large contact surface with the liquid phase of the sintering aid during sintering, and tend to dissolve more easily. Therefore, a large BET specific surface area can make the growth field of silicon nitride particles having the β phase more uniform.

[0036] The BET specific surface area is, for example, 20.0 m ２ / g or less, and 15.0m ２ / g or less, and ２ / g or less. Silicon nitride powders with an upper limit of the BET specific surface area in this range do not have an excessively wide growth field for silicon nitride particles, allowing the auxiliary agent to be dissolved at more uniform intervals, and allowing the silicon nitride powder particles to grow more uniformly. This makes it possible to further improve the bending strength of the resulting sintered body. The range of the BET specific surface area is, for example, 9.0 to 20.0 m ２ / g, 10.0-20.0 m ２ / g, 11.0-20.0 m ２ / g, or 12.0 to 20.0 m ２ / g.

[0037] The BET specific surface area of ​​the silicon nitride powder may be adjusted, for example, by changing the grinding conditions during production of the silicon nitride powder. The BET specific surface area in the present disclosure is a value measured by the BET single-point method using nitrogen gas in accordance with the method described in JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method."

[0038] The oxygen content of the silicon nitride powder may be, for example, 0.65 to 1.60 mass%. The oxygen content in this disclosure refers to the total amount of oxygen. Silicon nitride powder with such an oxygen content contains a sufficient amount of oxygen to promote sintering, while further suppressing the formation of embrittlement phases due to excess oxygen. Therefore, sintered bodies using silicon nitride powder with such an oxygen content have even higher bending strength. The oxygen content can be measured using a commercially available oxygen / nitrogen analyzer.

[0039] The oxygen content may be, for example, 0.70% by mass or more, or 0.80% by mass or more. Silicon nitride powder with such an oxygen content contains sufficient oxygen, which further promotes the sintering reaction and further improves the bending strength of the sintered body.

[0040] The oxygen content may be, for example, 1.50% by mass or less, or 1.30% by mass or less. Silicon nitride powder with such an oxygen content can further suppress the formation of embrittlement phases due to excess oxygen during sintering, and can further improve the bending strength of the sintered body. The oxygen content may be, for example, in the range of 0.70 to 1.60% by mass, 0.80 to 1.60% by mass, 0.65 to 1.50% by mass, 0.70 to 1.50% by mass, 0.80 to 1.50% by mass, 0.65 to 1.30% by mass, 0.70 to 1.30% by mass, or 0.80 to 1.30% by mass.

[0041] The BET specific surface area of ​​the silicon nitride powder is 9.0 m ２ / g or more, and the oxygen content may be 0.65 mass% or more. When the lower limits of both the BET specific surface area and the oxygen content are within the above ranges, a large amount of oxygen can be adsorbed on the surface of the silicon nitride particles, which further promotes the sintering reaction and allows the production of sintered bodies with higher bending strength. In addition, when the BET specific surface area of ​​the silicon nitride powder is 9.0 to 20.0 m, the BET specific surface area of ​​the silicon nitride powder may be 9.0 to 20.0 m. ２ When the BET specific surface area and oxygen content of the silicon nitride powder are within the above-mentioned ranges, it is possible to achieve at a higher level both the effect of promoting the sintering reaction based on the amount of oxygen contained in the silicon nitride particles and the contact area of ​​the sintering aid with the melt, and the effect of suppressing the formation of an embrittlement phase based on the adjustment of the amount of oxygen contained in the silicon nitride particles.

[0042] <Method for Producing Silicon Nitride Powder> A method for producing silicon nitride powder according to one embodiment includes a step of adjusting the particle size of a raw silicon nitride powder having a gelatinization rate of 96.0% by mass or less, where D97 is the particle size at which the cumulative particle size distribution measured by a particle size distribution analyzer using a laser diffraction / scattering method is 97% of the total, based on the volumetric particle size cumulative distribution. This method uses silicon nitride powder with a relatively low gelatinization rate as a raw material and adjusts the particle size to produce the desired silicon nitride powder. Therefore, silicon nitride powder can be produced at a lower cost than the cost of producing silicon nitride powder with a high gelatinization rate as a raw material for sintered compacts that exhibit excellent bending strength.

[0043] In the above-mentioned method for producing silicon nitride powder, if a raw material silicon nitride powder having a gelatinization rate of 96.0% by mass or less is available, it may be used as is, or a raw material silicon nitride powder having a gelatinization rate of 96.0% by mass or less may be separately prepared and used. In other words, the method for producing silicon nitride powder may include a step of preparing the raw material silicon nitride powder before the step of adjusting the particle size. The step of preparing the raw material silicon nitride powder may include at least one step selected from the group consisting of a mixing step of mixing silicon-containing raw materials to obtain a mixture, a nitriding step of firing the mixture to obtain a nitride, and a post-treatment step of treating the nitride with hydrofluoric acid. The raw material silicon nitride powder may be obtained through the mixing step, the nitriding step, and the post-treatment step.

[0044] In the mixing step, raw silicon powder and fluoride are mixed. The fluoride mixed with the silicon powder functions as a nitriding aid to promote nitriding. Examples of fluorides include fluorides of elements such as Li, Na, K, Mg, Ca, Sr, and Ba. The fluoride content may be 0.5 to 1.5 parts by mass per 100 parts by mass of silicon powder. If fluoride is not added, nitriding is not promoted and unreacted metallic silicon remains, resulting in a decrease in the purity of the raw silicon nitride powder and a decrease in productivity. The purity of the silicon powder mixed with the fluoride may be, for example, 99.0 to 99.9% by mass. Silicon powder with such a purity can be obtained by treating with a mixed acid containing hydrofluoric acid (hydrofluoric acid) and hydrochloric acid.

[0045] In the nitriding step, a raw material containing silicon powder and a fluoride is fired in a mixed atmosphere containing nitrogen and at least one selected from the group consisting of hydrogen and ammonia to obtain a nitride. The total content of hydrogen and ammonia in the mixed atmosphere may be, for example, 10 to 40% by volume based on the entire mixed atmosphere. The firing temperature may be, for example, 1100 to 1450°C, or 1200 to 1400°C. The firing time may be, for example, 30 to 100 hours.

[0046] In the post-treatment step, the nitride is mixed with hydrofluoric acid having a hydrogen fluoride concentration of, for example, 1.0 to 4.0 mass % and treated. For example, the nitride may be dispersed in hydrofluoric acid for treatment. The hydrogen fluoride concentration in the hydrofluoric acid may be, for example, 1.3 to 2.3 mass %. The temperature of the hydrofluoric acid in the post-treatment step is, for example, 40 to 80°C. The time for immersing the nitride in hydrofluoric acid is, for example, 1 to 10 hours.

[0047] The raw silicon nitride powder preparation process described above allows the adjustment of the alpha-conversion rate and the preparation of the raw silicon nitride powder. This production method corresponds to the so-called direct nitridation method, and allows the production of silicon nitride powder at a lower production cost than the imide method. Although the raw silicon nitride powder produced by the direct nitridation method has a low alpha-conversion rate and contains a certain amount of impurities, it can be suitably used for the production of silicon nitride powder as a sintering raw material according to the present disclosure, which can provide a silicon nitride sintered body having high bending strength while keeping production costs low.

[0048] The alpha phase ratio of the raw silicon nitride powder is 96.0% by mass or less, and may be 95.0% by mass or less, or 94.0% by mass or less. Silicon nitride powders with an alpha phase ratio within the above-mentioned range have low production costs and excellent productivity, and can be easily obtained or prepared. According to the method for producing silicon nitride powder according to the present disclosure, even when raw material powders with an alpha phase ratio within the above-mentioned range are used, silicon nitride powders from which sintered bodies with excellent bending strength can be produced can be provided. The alpha phase ratio may be 89.0% by mass or more, or may be 90.0% by mass or more. The alpha phase ratio of the raw silicon nitride powder may be in the range of, for example, 89.0 to 96.0% by mass, 89.0 to 95.0% by mass, 89.0 to 94.0% by mass, 90.0 to 96.0% by mass, 90.0 to 95.0% by mass, or 90.0 to 94.0% by mass.

[0049] In the particle size adjustment step, the silicon nitride powder obtained is adjusted so that the D97 is 2.25 μm or less. Examples of particle size adjustment methods include mixing powders of different particle sizes, pulverizing, and classifying. When mixing powders of different particle sizes, the raw silicon nitride powder that can be used for mixing is limited to those with a gelatinization rate of 96.0 mass% or less, as described above.

[0050] The particle size adjustment by pulverization may be performed by dry or wet pulverizing the raw silicon nitride powder (hereinafter also referred to as the pulverization step). The pulverization step may be performed in multiple stages, such as coarse pulverization and fine pulverization. For example, the pulverization step may include two steps: a ball mill pulverization step and a vibration mill pulverization step. For example, a ball mill, a bead mill, an attritor mill, or the like may be used in the pulverization step.

[0051] The filling rate of the balls in the container in the grinding step may be, for example, 30 to 70% by volume. The lower limit of the filling rate of the balls in the container may be, for example, 50% by volume or 60% by volume, based on the volume of the container. The upper limit of the filling rate of the balls in the container may be, for example, 65% by volume, based on the volume of the container.

[0052] The time for the pulverization treatment (pulverization time) in the pulverization step may be, for example, 5 to 15 hours, or 8 to 12 hours, which allows the silicon nitride powder to be sufficiently fine.

[0053] In the pulverization step, for example, the pulverized product obtained in the ball mill pulverization step may be further pulverized in a vibration mill pulverization step. The ball filling rate in the container in the vibration mill pulverization step may be, for example, 50 to 80 volume %, or 60 to 75 volume %. The pulverization time (pulverization time) in the vibration mill pulverization step may be 8 to 20 hours, or 12 to 17 hours. This allows the raw silicon nitride powder to be sufficiently fine, adjusts the D97, and more easily obtains the silicon nitride powder specified in the present application.

[0054] The particle size adjustment step can also employ a method of dry classification to obtain silicon nitride powder with a D97 of 2.25 μm or less. For example, when agglomerated particles (also called secondary particles) formed by agglomeration of multiple silicon nitride primary particles are present, this increases the particle size of the silicon nitride powder as a whole. Therefore, the particle size distribution of the silicon nitride powder can be adjusted by eliminating at least a portion of these agglomerated particles, and the particle size distribution of the silicon nitride powder can also be adjusted by eliminating a portion of the silicon nitride primary particles with large particle diameters.

[0055] Dry classification may be performed by sieving or by using an air classifier. The air classifier may be a swirling air classifier that uses primary air and secondary air. For example, the "MP-150" manufactured by Nippon Pneumatic Mfg. Co., Ltd. may be used as such an air classifier.

[0056] The operating conditions of an air classifier can be adjusted, for example, by adjusting the classification air volume and the louver opening of the air guide vanes to control the swirling air velocity. Here, the classification air volume refers to the amount of air required to generate a swirling airflow within the air classifier. Primary air introduced into the air classifier becomes a swirling flow as it passes through the air guide vanes. By introducing raw silicon nitride powder into the swirling flow, a large centrifugal force can be applied to coarse particles, allowing for easy classification. Furthermore, by introducing air (secondary air) compressed more than the primary air into the swirling flow, the accuracy of classification can be further improved. Thus, for example, by adjusting the classification air volume and louver opening of the air classifier and adjusting the swirling air velocity to 100 to 300 m / s, silicon nitride powder can be produced with a D97 of 2.25 μm or less. Such silicon nitride powder exhibits high bending strength when sintered.

[0057] An example of the operating conditions for the air classifier is as follows: Classification air volume: 1 to 4 m ３ / min, and 1 to 3 m ３ / min. The louver opening of the air guide vane may be 12 mm or less and 2 mm or more, for example, in the range of 2 to 8 mm. The louver opening refers to the distance between the centers of the louvers. The louver opening can also be expressed by the angle of the louvers. In this case, the angle may be 70°, 80°, or 90°. The louver opening indicates the size of the inlet for compressed air (secondary air). When the distance between the centers of the louvers is small or the angle of the louvers is large, the inlet becomes smaller, increasing the pressure and the airflow rotation speed. The secondary air pressure may also be 0.2 to 1.0 MPa or 0.4 to 0.8 MPa. By classifying under these conditions, the swirling airflow velocity can be adjusted to 100 to 300 m / s, and agglomerated particles that are not sufficiently pulverized in the pulverization process can be removed as coarse particles with high precision. Furthermore, the desired silicon nitride powder can be produced more easily than by mixing powders of different particle sizes or adjusting particle size by pulverization.

[0058] From the viewpoint of accelerating the classification of silicon nitride powder and further reducing D97 to obtain silicon nitride powder that will become a sintered body with a large Weibull coefficient, the swirling air velocity may be 150 m / s or more, 200 m / s or more, 220 m / s or more, or 230 m / s or more. Furthermore, from the viewpoint of improving the yield after classification and increasing productivity, the swirling air velocity may be 300 m / s or less, or 250 m / s or less. The range of the swirling air velocity may be 150 to 300 m / s or 220 to 250 m / s. Regarding the swirling air velocity, V is the flow velocity (m / s), and Q is the flow rate (m ３ / s), and A is the cross-sectional area of ​​the swirling field (m ２ ) and can be calculated from the equation V = Q / A. The swirling air velocity can also be measured using, for example, a thermal airflow transducer (product name: TA10 ZG2d, manufactured by Centronic Co., Ltd.).

[0059] The D97 may be adjusted to, for example, 2.00 μm or less, 1.80 μm or less, 1.60 μm or less, or 1.40 μm or less. Silicon nitride powder with a D97 adjusted to this range further suppresses the generation of coarse particles after sintering, allowing for the production of sintered bodies with higher bending strength. The D97 may be adjusted to, for example, 0.80 μm or more, 0.90 μm or more, or 1.00 μm or more. The D97 may be adjusted to, for example, 0.80 to 2.25 μm, 0.80 to 2.00 μm, 0.80 to 1.80 μm, 0.80 to 1.60 μm, or 0.80 to 1.40 μm.

[0060] In the particle size adjustment step, D90 may be adjusted to, for example, 2.00 μm or less, 1.50 μm or less, or 1.30 μm or less. Silicon nitride powders having an upper D90 value within such a range have a smaller particle size, which further suppresses the generation of coarse particles in the sintered body, thereby enabling the production of sintered bodies with higher bending strength. Furthermore, D90 may be adjusted to, for example, 0.60 μm or more, 0.80 μm or more, or 1.20 μm or more. D90 may be adjusted to, for example, a range of 0.60 to 2.00 μm, 0.60 to 1.50 μm, or 0.60 to 1.30 μm.

[0061] In the particle size adjustment step, D50 may be adjusted to, for example, 1.00 μm or less, 0.90 μm or less, 0.80 μm or less, or 0.70 μm or less. Silicon nitride powders having an upper D50 value within such a range have a smaller particle size, which further suppresses the generation of coarse particles in the sintered body, resulting in a sintered body with even higher bending strength. Furthermore, D50 may be adjusted to, for example, 0.30 μm or more, or 0.40 μm or more. D50 may be adjusted to, for example, a range of 0.30 to 1.00 μm, 0.30 to 0.90 μm, 0.30 to 0.80 μm, or 0.30 to 0.70 μm.

[0062] In the particle size adjustment step, D10 may be adjusted to, for example, 0.60 μm or less, 0.50 μm or less, or 0.40 μm or less. Silicon nitride powders with an upper limit of D10 in this range have a smaller particle size, which further suppresses the generation of coarse particles in the sintered body, resulting in a sintered body with even higher bending strength. Furthermore, D10 may be adjusted to, for example, 0.05 μm or more, or 0.10 μm or more. D10 may be adjusted to, for example, a range of 0.05 to 0.60 μm, 0.05 to 0.50 μm, or 0.05 to 0.40 μm.

[0063] In the particle size adjustment step, the value of (D90-D10) / D50 of the silicon nitride powder, calculated from the D90, D50, and D10, may be adjusted to, for example, 1.70 or less, 1.60 or less, or 1.50 or less. When multiple sintered bodies are produced using such silicon nitride powder, the variation in bending strength of each sintered body can be reduced. In other words, the Weibull modulus of the sintered body can be increased.

[0064] The value of (D90-D10) / D50 may be adjusted to, for example, 1.30 or greater, or 1.40 or greater. Silicon nitride powders with a (D90-D10) / D50 value in this range have smaller particle size variations. Therefore, during the production of a sintered body, the environment in which the silicon nitride powder dissolves in the liquid phase of the sintering aid and reprecipitates is more homogenized, and the particle size variations of the growing β-phase particles can be more sufficiently suppressed. This allows for further improvement in the bending strength of the resulting sintered body. The value of (D90-D10) / D50 of the sintered body may be adjusted to, for example, 1.30 to 1.700, or 1.40 to 1.70.

[0065] The silicon nitride powder of this embodiment can be produced by the above steps. However, the above-mentioned production method is only an example and is not limited thereto. Since the silicon nitride powder of this embodiment has reduced coarse particles, it can be suitably used as a raw material for sintered bodies having high bending strength.

[0066] When producing a sintered body using silicon nitride powder as a raw material, the sintering raw material containing the silicon nitride powder is molded and sintered. The sintering raw material may contain an oxide-based sintering aid in addition to the silicon nitride powder. Examples of the oxide-based sintering aid include Y ２ O ３ , MgO and Al ２ O ３ The content of the oxide-based sintering aid in the sintering raw material may be, for example, 3 to 10 mass %.

[0067] The above-mentioned sintering raw materials are pressed at a molding pressure of, for example, 3.0 to 200 MPa to obtain a green body. The green body may be produced by uniaxial pressing or by CIP. In some cases, the process may include two steps: uniaxial pressing and CIP. Alternatively, the green body may be fired while being molded by hot pressing. Firing the green body may include two steps: a primary firing step and a secondary firing step, both of which are carried out in an inert gas atmosphere such as nitrogen gas or argon gas. The pressure in the primary firing step may be, for example, 0.7 to 1 MPa. The firing temperature may be, for example, 1700 to 1900°C or 1750 to 1850°C. The firing time at the firing temperature may be, for example, 0.5 to 20 hours or 1.5 to 16 hours. The heating rate to the firing temperature may be, for example, 1.0 to 10.0°C / min.

[0068] The pressure in the secondary firing step in which the sintered body obtained in the primary firing step is fired may be, for example, 70 MPa or more, preferably 100 MPa or more. The firing temperature may be, for example, 1650 to 1850°C, or 1700 to 1800°C. The firing time at the firing temperature may be, for example, 0.5 to 5 hours, or 1 to 2 hours. The rate of temperature rise up to the firing temperature may be, for example, 1.0 to 10.0°C / hour.

[0069] The resulting sintered body has a fine structure with excellent uniformity due to reduced coarse particles. It also has a sufficiently dense structure, resulting in excellent bending strength. Furthermore, the reduced variation in particle size reduces the variation in the properties of the silicon nitride sintered body.

[0070] The bending strength of the sintered body can be 920 MPa or more, 930 MPa or more, or 950 MPa or more. Such silicon nitride sintered bodies have excellent strength and can be suitably used as parts for various industries. The bending strength of the sintered body is a three-point bending strength and can be measured using a commercially available flexural strength tester in accordance with JIS R 1601:2008. The bending strength of the sintered body may be 1100 MPa or less. The bending strength of the sintered body may be in the range of, for example, 920 to 1100 MPa, 930 to 1100 MPa, or 950 to 1100 MPa.

[0071] The fracture toughness of the sintered body is 4.5 (MPa / m ２ ) or more, 4.8 (MPa / m ２ ) or more, or 5.2 (MPa / m ２ ) or more. Such silicon nitride sintered bodies have excellent strength and can be suitably used as parts for various industries. The fracture toughness value of the sintered bodies can be measured in accordance with JIS R 1607:2015. The fracture toughness value of the sintered bodies can be 6.0 (MPa / m ２ The fracture toughness value of the sintered body may be in the range of, for example, 4.5 to 6.0 (MPa / m ２ ), 4.8 to 6.0 (MPa / m ２ ), or 5.2 to 6.0 (MPa / m ２ ) may be.

[0072] The Vickers hardness of the sintered body can be 1.400 HV or more, 1.420 HV or more, or 1.430 HV or more. Such silicon nitride sintered bodies have excellent wear resistance and can be suitably used as parts for various industries. The Vickers hardness of the sintered body can be measured in accordance with JIS R 1610:2003. The Vickers hardness of the sintered body may be 1.600 HV or less. The Vickers hardness of the sintered body may be in the range of, for example, 1.400 to 1.600 HV, 1.420 to 1.600 HV, or 1.430 to 1.600 HV.

[0073] The silicon nitride sintered body has a highly uniform microstructure, and therefore the above-mentioned distribution of bending strength is sufficiently suppressed. The silicon nitride sintered body exhibits a relatively large Weibull coefficient in a Weibull statistical analysis of its bending strength. The Weibull coefficient of the silicon nitride sintered body for bending strength can be, for example, 10.0 or more, 12.0 or more, 13.0 or more, 15.0 or more, 17.0 or more, or 20.0 or more. The Weibull coefficient can also be 25.0 or less. The range of the Weibull coefficient can be, for example, 10.0 to 25.0, 12.0 to 25.0, 13.0 to 25.0, 17.0 to 25.0, or 20.0 to 25.0.

[0074] Weibull statistics are used to evaluate the distribution of bending strength. When a Weibull plot is created for a silicon nitride sintered body, with the fracture probability F(σ) on the vertical axis and the bending strength σ (strength at fracture, transverse strength) on the horizontal axis, the slope m is the Weibull coefficient. A large Weibull coefficient means that the distribution of bending strength is narrow and close to a normal distribution. The fracture probability F(σ) in a Weibull plot is given by the following formula (1): F(σ) = 1 - exp [- (σ / η) m] (1) In the above formula (1), η is a fitting parameter.

[0075] Although several embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments.

[0076] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0077] (Comparative Example 1) <Preparation of Raw Silicon Nitride Powder> A raw powder was prepared by blending 1 part by mass of fluorite with 100 parts by mass of silicon powder. That is, the raw powder contained 1 part by mass of fluoride (fluorite) with 100 parts by mass of silicon powder. An alumina container having a main body with a recess and a lid was prepared. The recess was filled with raw powder. The filling shape of the raw powder was a rectangular parallelepiped, and the filling height was 45 mm. The recess of the main body was covered with the lid, and the raw powder was placed in the alumina container. The raw powder placed in the container was fired using the following procedure.

[0078] The container containing the raw material powder was placed in an electric furnace and fired under the following temperature conditions. The temperature was increased from 20°C to 1150°C at a rate of 5°C / min. After holding at 1150°C for 8 hours, the temperature was increased to 1450°C at a rate of 0.15°C / min. After holding at 1450°C for 4 hours, the material was allowed to cool naturally to room temperature. The atmosphere in the electric furnace was nitrogen gas. The time from the start of holding at 1150°C to the end of holding at 1450°C was 45 hours.

[0079] The resulting ingot was coarsely crushed and then wet-pulverized for 8 hours in an attritor mill with a ball filling rate of 70% by volume, followed by drying in a nitrogen atmosphere.

[0080] The pulverized product obtained by wet pulverization was immersed in hydrofluoric acid (hydrogen fluoride concentration: 1.6% by mass) at 70°C for 4 hours for acid treatment. The pulverized product was then removed from the hydrofluoric acid, washed with water, and dried under a nitrogen atmosphere. In this way, a raw material silicon nitride powder (powder a) was obtained.

[0081] Comparative Example 2 A raw material silicon nitride powder (powder b) of Comparative Example 2 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.8 mass %.

[0082] Comparative Example 3 A raw material silicon nitride powder (powder c) of Comparative Example 3 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.9 mass %.

[0083] Comparative Example 4 A raw material silicon nitride powder (powder d) of Comparative Example 4 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.7 mass %.

[0084] Comparative Example 5 A raw material silicon nitride powder (powder e) of Comparative Example 5 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.4 mass %.

[0085] Comparative Example 6 A raw material silicon nitride powder (powder f) of Comparative Example 6 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 2.0 mass %.

[0086] Example 1 Classification of Raw Silicon Nitride Powder The silicon nitride powder of Comparative Example 1 (powder a) was used as the raw silicon nitride powder and classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1 m ３ / min Louver opening in main air guide vane: 2 mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0087] Example 2 The silicon nitride powder (powder b) of Comparative Example 2 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1 m ３ / min Louver opening in main air guide vane: 2 mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0088] Example 3 The silicon nitride powder (powder c) of Comparative Example 3 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1 m ３ / min Louver opening in main air guide vane: 2 mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0089] Example 4 The silicon nitride powder (powder d) of Comparative Example 4 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1 m ３ / min Louver opening in main air guide vane: 2 mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0090] Example 5 The silicon nitride powder (powder a) of Comparative Example 1 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: MP-150) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.3 m ３ / min Louver opening in main air guide vane: 3 mm Secondary air pressure: 0.6 MPa Swirling air velocity: 160 m / s

[0091] Example 6 The silicon nitride powder (powder b) of Comparative Example 2 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: MP-150) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.3 m ３ / min Louver opening in main air guide vane: 3 mm Secondary air pressure: 0.6 MPa Swirling air velocity: 160 m / s

[0092] Example 7 The silicon nitride powder (powder c) of Comparative Example 3 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: MP-150) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.3 m ３ / min Louver opening in main air guide vane: 3 mm Secondary air pressure: 0.6 MPa Swirling air velocity: 160 m / s

[0093] Example 8 The silicon nitride powder (powder d) of Comparative Example 4 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: MP-150) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.3 m ３ / min Louver opening in main air guide vane: 3 mm Secondary air pressure: 0.6 MPa Swirling air velocity: 160 m / s

[0094] Example 9 The silicon nitride powder (powder e) of Comparative Example 5 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nisshin Engineering Inc., product name: AC-20) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.0 m ３ / min Louver opening angle in main air guide vane: 90° Secondary air pressure: 0.7 MPa Swirling air velocity: 200 m / s

[0095] Example 10 The silicon nitride powder (powder e) of Comparative Example 5 was used as a raw silicon nitride powder and was classified using an air classifier (manufactured by Nisshin Engineering Inc., product name: AC-20) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.4 m ３ / min Louver opening angle in main air guide vane: 90° Secondary air pressure: 0.8 MPa Swirling air velocity: 300 m / s

[0096] (Reference Example) High-purity silicon nitride powder "SN-E10" (manufactured by UBE Co., Ltd.), which is silicon nitride powder with a high alpha conversion rate, was purchased and used as the silicon nitride powder of the Reference Example.

[0097] [Measurement of alpha phase ratio] The alpha phase ratio of silicon nitride powder was measured as follows. X-ray diffraction of silicon nitride powder was performed using CuKα radiation using an X-ray diffractometer (manufactured by Rigaku, device name: Ultima IV). The alpha phase was represented by the diffraction line intensity Ia102 of the (102) plane and the diffraction line intensity Ia210 of the (210) plane. The beta phase was represented by the diffraction line intensity Ib101 of the (101) plane and the diffraction line intensity Ib210 of the (210) plane. Using these diffraction line intensities, the alpha phase ratio was calculated according to the following formula. Table 1 shows the measurement results for each example, comparative example, and reference example. alpha phase ratio (mass %) = (Ia102 + Ia210) / (Ia102 + Ia210 + Ib101 + Ib210) × 100

[0098] [Measurement of Particle Size Distribution] The particle size distribution of silicon nitride powder was measured by laser diffraction / scattering. The measurement was performed in accordance with the method described in JIS R 1629:1997, "Method for Measuring Particle Size Distribution of Fine Ceramics Raw Materials by Laser Diffraction / Scattering." For particle size distribution measurement, 60 mg of silicon nitride powder was weighed into a 500 mL container. A 20% aqueous solution of sodium hexametaphosphate (2 mL) and water (200 g) were added as dispersants. This container was placed in an ultrasonic disperser manufactured by Sharp Corporation so that the entire dispersion was immersed, and ultrasonic dispersion was performed for 1 minute. The above-described particle size distribution measurement was performed using the sample after ultrasonic dispersion. D10, D50, D90, and D97 were determined for each Example, Comparative Example, and Reference Example. Furthermore, (D90-D10) / D50 was calculated from the obtained results. The results are shown in Table 1.

[0099] [Measurement of BET specific surface area] The BET specific surface area of ​​the silicon nitride powder was measured by the single-point BET method using nitrogen gas in accordance with JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method." The measurement results for each of the Examples, Comparative Examples, and Reference Examples are shown in Table 1.

[0100] [Measurement of oxygen content] The oxygen content of the silicon nitride powder was determined as the total amount of oxygen. The oxygen content was measured using an oxygen / nitrogen analyzer (manufactured by Horiba, Ltd., device name: EMGA-920). Specifically, the silicon nitride powder was heated from 20°C to 2000°C at a temperature increase rate of 8°C / s in a helium atmosphere, and the amount of oxygen released was quantified to determine the oxygen content (mass%) of the entire silicon nitride powder. The measurement results for each of the Examples, Comparative Examples, and Reference Example are shown in Table 1.

[0101] <Preparation of Sintered Body> 91 parts by mass of silicon nitride powder of each Example, Comparative Example, and Reference Example, Y 1.5 μm average particle size ２ O ３ 5 parts by mass of powder and Al having an average particle size of 1.2 μm ２ O ３Four parts by mass of the powders were blended and wet-mixed in methanol for 4 hours. The dried mixed powder was then molded under a pressure of 10 MPa and then further molded by CIP at a pressure of 100 MPa. The resulting molded body was placed in a carbon crucible together with a packed powder consisting of a mixed powder of silicon nitride powder and BN powder. It was heated to 900 ° C at 10 ° C / min under vacuum, then nitrogen was introduced and heated to 1400 ° C at 5 ° C / min under a nitrogen atmosphere. It was maintained at 1400 ° C for 2 hours and then heated at 5 ° C / min to the sintering temperature of 1800 ° C. It was maintained at 1800 ° C for 2 hours, cooled to 1000 ° C at 5 ° C / min, and then naturally cooled to produce a primary silicon nitride sintered body. The obtained sintered body was further pressurized to 100 MPa in a nitrogen atmosphere, heated to 1700°C at a temperature increase rate of 5°C / hour, and then maintained at the firing temperature of 1700°C for 1 hour, thereby carrying out a secondary firing step to produce a silicon nitride sintered body.

[0102] [Measurement of bending strength of sintered body and calculation of Weibull modulus] The bending strength was a three-point bending strength, and was measured in accordance with JIS R 1601:2008 using a commercially available flexural strength tester (manufactured by Shimadzu Corporation, device name: AG-2000). The measurement was performed at room temperature (20°C). The results are shown in Table 2. Twenty sintered bodies were prepared for each silicon nitride powder in each example, and the Weibull modulus was calculated based on the distribution of their bending strengths. Similar measurements were performed for each example, comparative example, and reference example. The results are shown in Table 2.

[0103] [Measurement of fracture toughness value of sintered body] Fracture toughness (K ＩＣ ) is a value measured by the IF method in accordance with JIS R1607:2015 using a commercially available measuring device (manufactured by Matsuzawa Corporation, device name: Via-F). The results are shown in Table 2.

[0104] [Measurement of Vickers Hardness of Sintered Body] The Vickers hardness was measured in accordance with JIS R 1610: 2003 using an electric Vickers hardness tester (manufactured by Matsuzawa Co., Ltd., trade name: Via-F). The results are shown in Table 2.

[0105] [Observation of Sintered Body Cross Section] A portion of the cross section of the produced sintered body was observed with a scanning electron microscope. The results are shown in Figures 1, 2, and 3. Figure 1 is a scanning electron microscope photograph showing a portion of the cross section of a sintered body prepared using the silicon nitride powder of Comparative Example 4. Similarly, Figure 2 shows the cross section of the sintered body of Example 4, and Figure 3 shows the cross section of the sintered body of Example 8. Comparing Figures 1, 2, and 3, it was found that the sintered body of Figure 1 had the most coarse particles, while the sintered body of Figure 2 had the finest particles.

[0106]

[0107]

[0108] It was confirmed that the sintered body obtained using the silicon nitride powder of the Reference Example with a high alpha-phase ratio as a raw material exhibits excellent bending strength. On the other hand, a comparison of the Examples and Comparative Examples with a low alpha-phase ratio confirmed that the sintered body obtained using the silicon nitride powder obtained by classifying the silicon nitride powder of the Comparative Example to reduce D97 as a raw material tends to have higher bending strength than the sintered body obtained using the silicon nitride powder of the Comparative Example before particle size adjustment as a raw material. Furthermore, it was confirmed that by reducing the value of (D90 - D10) / D50, the Weibull coefficient of bending strength is improved and sintered bodies with small bending strength variation can be stably obtained. From the above results, it was confirmed that a sintered body with high bending strength can be prepared using a silicon nitride powder with a low alpha-phase ratio, and that such silicon nitride powder can be easily produced.

[0109] According to the present disclosure, it is possible to provide a silicon nitride powder having a low gelatinization rate, which can be used to prepare a sintered body having excellent bending strength. According to the present disclosure, it is also possible to provide a production method for producing the above-mentioned silicon nitride at a lower cost.

Claims

1. A silicon nitride powder having an alpha conversion rate of 96.0% by mass or less, and in the cumulative distribution of volumetric particle diameters measured using a particle size distribution analyzer using a laser diffraction / scattering method, when the particle diameter D97 is defined as the particle diameter at which the integrated value from small particle diameters reaches 97% of the total, D97 is 2.25 μm or less.

2. The silicon nitride powder according to claim 1, wherein, in said cumulative distribution, the particle diameter when the integrated value from the small particle diameters reaches 10% of the total is defined as D10, the particle diameter when the integrated value from the small particle diameters reaches 50% of the total is defined as D50, and the particle diameter when the integrated value from the small particle diameters reaches 90% of the total is defined as D90, and the ratio (D90-D10) / D50 is 1.70 or less.

3. The silicon nitride powder according to claim 1 or 2, having an oxygen content of 0.65 to 1.60 mass %.

4. A method for producing silicon nitride powder, comprising a step of adjusting the particle size of raw material silicon nitride powder having an alpha conversion rate of 96.0% by mass or less, so that D97 is 2.25 μm or less, where D97 is the particle size at which the cumulative value from small particle sizes reaches 97% of the total in the cumulative distribution of volumetric particle sizes measured with a particle size distribution measuring device using a laser diffraction / scattering method.

5. A method for producing silicon nitride powder as described in claim 4, wherein the particle size adjusting step uses an air classifier to classify the raw silicon nitride powder at a swirling air velocity of 100 to 300 m / s to adjust the particle size.

6. A method for producing silicon nitride powder according to claim 4 or 5, wherein the particle size adjusting step adjusts the particle size so that (D90 - D10) / D50 is 1.70 or less, where D10 is the particle size when the cumulative value from small particle diameters reaches 10% of the total, D50 is the particle size when the cumulative value from small particle diameters reaches 50% of the total, and D90 is the particle size when the cumulative value from small particle diameters reaches 90% of the total.

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