Silicon nitride powder and method for producing same

Abstract

The present disclosure provides a silicon nitride powder having a D97 of 2.0 μm or less and a (D90 - D10) / D50 of 1.70 or less, where D10 is the particle diameter when an integrated value from the smallest particle diameter reaches 10% of the whole in a cumulative distribution of volume-based particle diameters measured by a particle size distribution measurement device using a laser diffraction / scattering method, D50 is the particle diameter when an integrated value from the smallest particle diameter reaches 50% of the whole, D90 is the particle diameter when an integrated value from the smallest particle diameter reaches 90% of the whole, and D97 is the particle diameter when an integrated value from the smallest particle diameter reaches 97% of the whole.

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] When silicon nitride powder is sintered, the silicon nitride particles dissolve in the melt of the sintering aid, and then the silicon nitride reprecipitates and undergoes grain growth. During this process, a phase transformation to the thermally more stable β phase occurs. The silicon nitride grain growth accompanying this phase transformation causes the silicon nitride powder to sinter, forming a silicon nitride sintered body. Further improvements in mechanical properties are required for silicon nitride sintered bodies. Furthermore, when multiple sintered bodies are produced using the same lot of silicon nitride powder, variations in quality (e.g., bending strength) can occur between the multiple sintered bodies. The Weibull coefficient is known as an index for measuring the bending strength of multiple nitride sintered bodies and evaluating the degree of variation. For example, a large Weibull coefficient value indicates less variation in bending strength between silicon nitride sintered bodies.

[0005] The present disclosure aims to provide a silicon nitride powder from which a sintered body having a large Weibull modulus and excellent bending strength can be prepared, and also to provide a method for producing such a silicon nitride powder at lower cost.

[0006] One aspect of the present disclosure provides the following silicon nitride powder:

[0007] [1] In a cumulative distribution of particle diameters on a volume basis measured with a particle size distribution analyzer using a laser diffraction / scattering method, when the particle diameter when the integrated value from small particle diameters reaches 10% of the total is defined as D10, the particle diameter when the integrated value from small particle diameters reaches 50% of the total is defined as D50, the particle diameter when the integrated value from small particle diameters reaches 90% of the total is defined as D90, and the particle diameter when the integrated value from small particle diameters reaches 97% of the total is defined as D97, a silicon nitride powder in which D97 is 2.0 μm or less, and (D90-D10) / D50 is 1.70 or less.

[0008] The silicon nitride powder [1] above has a D97 of 2.0 μm or less and a (D90-D10) / D50 of 1.70 or less. Therefore, the particle diameter of the constituent particles of the silicon nitride powder is relatively small and the particle size uniformity is high. When such a silicon nitride powder is used as a sintered body raw material, silicon nitride dissolves in the liquid phase of the sintering aid, and the environment during reprecipitation is homogenized, allowing the growth of silicon nitride particles having a thermally stable β phase. In other words, when the silicon nitride powder is used as a sintered body raw material, the generation of coarse particles in the prepared sintered body is suppressed, and excellent bending strength can be exhibited. Furthermore, since the silicon nitride powder described above has high uniformity of the powder itself, when multiple sintered bodies are prepared, variation in bending strength between the sintered bodies is suppressed.

[0009] In addition, when conventional silicon nitride powders contain silicon nitride having a thermally stable β-phase, the silicon nitride having the β-phase remains unmelted and preferentially acts as a nucleus for crystal growth, resulting in large variations in the particle size of the silicon nitride in the sintered body and the inclusion of coarse particles. The presence of coarse particles in the sintered body causes stress concentration in the coarse particles, reducing the bending strength of the sintered body. Furthermore, the powder has low homogeneity, which leads to variations in quality when multiple sintered bodies are produced. In contrast, the silicon nitride powder according to the present disclosure has D97 and (D90-D10) / D50 values ​​within predetermined ranges, thereby further suppressing the generation of coarse particles in the sintered body as described above. Even when multiple sintered bodies are produced, the variations in bending strength between the individual sintered bodies are suppressed, and the Weibull coefficient can be increased.

[0010] The silicon nitride powder of [1] above may be any one of the following [2] to [4].

[0011] [2] The silicon nitride powder according to [1], having a gelatinization rate of 96.0 mass% or less. [3] The silicon nitride powder according to [1] or [2], having an oxygen content of 0.60 to 1.60 mass%. [4] The silicon nitride powder according to [1] or [2], having a BET specific surface area of ​​12.0 m ２ [5] The silicon nitride powder according to any one of [1] to [4], wherein the oxygen content / (BET specific surface area×D10) is 0.30 to 0.80.

[0012] The silicon nitride powder of [2] above has a gelatinization rate of 96.0 mass % or less. By using silicon nitride powder with a low gelatinization rate, it is possible to reduce production costs.

[0013] The silicon nitride powder [3] has an oxygen content of 0.60 to 1.60 mass%. Silicon nitride powder with this oxygen content can improve reactivity during sintering while suppressing the formation of embrittled phases in the sintered body due to oxygen, thereby suppressing embrittlement of the sintered body itself. Therefore, the bending strength of the sintered body can be further improved.

[0014] The silicon nitride powder [4] has a BET specific surface area of ​​12.0 m２ / g or more. Silicon nitride powder with a large BET specific surface area has a larger contact area with the liquid phase of the sintering aid during sintering, making it more easily dissolved in the sintering aid. This allows for a more uniform growth field for silicon nitride particles having the β phase. Such silicon nitride powder has a more uniform particle size, which further improves the Weibull coefficient, which relates to the bending strength of the sintered body.

[0015] The silicon nitride powder of [5] above has an oxygen content / (BET specific surface area x D10) ratio of 0.30 to 0.80. By ensuring that the silicon nitride powder's oxygen content / (BET specific surface area x D10) ratio is within the above range, the silicon nitride powder is composed of smaller particles, and when this powder is used as a sintered body raw material, the generation of coarse particles in the sintered body can be further suppressed. Furthermore, the effect of promoting the sintering reaction based on the oxygen content of the silicon nitride particles and the contact area with the liquid phase of the sintering aid, and the effect of suppressing the generation of an embrittlement phase based on adjustment of the oxygen content of the silicon nitride particles can be achieved at a higher level. As a result, the bending strength of sintered bodies obtained from the above silicon nitride powder can be further improved.

[0016] One aspect of the present disclosure provides the following method for producing a silicon nitride powder.

[0017] [6] A method for producing silicon nitride powder, comprising: adjusting the particle size of a raw material silicon nitride powder so that, in a cumulative distribution of particle sizes on a volume basis measured with a particle size distribution analyzer using a laser diffraction / scattering method, D97 is 2.0 μm or less and (D90-D10) / D50 is 1.70 or less, where D10 is the particle size when the integrated value from small particle sizes reaches 10% of the total, D50 is the particle size when the integrated value from small particle sizes reaches 50% of the total, D90 is the particle size when the integrated value from small particle sizes reaches 90% of the total, and D97 is the particle size when the integrated value from small particle sizes reaches 97% of the total.

[0018] The method for producing silicon nitride powder described in [6] above is relatively simple, as it involves adjusting the particle size of the raw silicon nitride powder to produce the desired silicon nitride powder. Therefore, silicon nitride powder can be produced inexpensively compared to the production costs of silicon nitride powder with a high alpha content as a raw material for sintered bodies that exhibit excellent bending strength. Furthermore, the particle size of the silicon nitride powder obtained by the production method described in [6] above is adjusted so that the D97 is 2.0 μm or less and the (D90-D10) / D50 is 1.70 or less, making it possible to produce sintered bodies with high particle size uniformity and high bending strength. Furthermore, by using the silicon nitride powder obtained by the above production method, it is also possible to produce multiple sintered bodies with high Weibull coefficients and excellent bending strength.

[0019] The method for producing silicon nitride powder described above in [6] may be the following method described in [7] or [8].

[0020] [7] The method for producing a silicon nitride powder according to [6], wherein the particle size adjusting step uses an air classifier to classify the raw material silicon nitride powder at a swirling air velocity of 170 to 300 m / s to adjust the particle size. [8] The method for producing a silicon nitride powder according to [6] or [7], wherein the raw material silicon nitride powder has a gelatinization rate of 96.0 mass% or less.

[0021] The method for producing silicon nitride powder described above in [7] 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.

[0022] According to the present disclosure, it is possible to provide a silicon nitride powder from which a sintered body having a large Weibull modulus and excellent bending strength can be prepared. According to the present disclosure, it is also possible to provide a production method capable of producing the above-mentioned silicon nitride powder more inexpensively.

[0023] 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. FIG. 2 is a scanning electron microscope photograph showing a cross section of a sintered body produced using the silicon nitride powder of Example 4.

[0024] 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.

[0025] <Silicon nitride powder> Silicon nitride powder (Si ３ N ４ In the cumulative distribution of volumetric particle sizes measured with a particle size distribution analyzer using a laser diffraction / scattering method, the particle size when the cumulative value from the small particle size reaches 10% of the total is defined as D10, the particle size when the cumulative value from the small particle size reaches 50% of the total is defined as D50, the particle size when the cumulative value from the small particle size reaches 90% of the total is defined as D90, and the particle size when the cumulative value from the small particle size reaches 97% of the total is defined as D97, where D97 is 2.0 μm or less and (D90-D10) / D50 is 1.70 or less. By using such silicon nitride powder, a sintered body having a large Weibull modulus and excellent bending strength can be obtained.

[0026] 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 D97 is the particle diameter at which the integrated value from the smallest particle diameter reaches 97% of the total, D97 is 2.0 μm or less. The D97 may be, for example, 1.80 μm or less, 1.60 μm or less, 1.40 μm or less, 1.30 μm or less, or 1.25 μm or less. Silicon nitride powders having a D97 in this range have sufficiently small particle diameters. Even when silicon nitride powders having a D97 in this range contain a large amount of β-phase, which is likely to form crystal nuclei during sintering, the silicon nitride particles having each β-phase are themselves small, so that even when they form crystal nuclei, the particle diameter difference between the newly precipitated silicon nitride particles having β-phase is not significant, thereby further suppressing the generation of coarse particles during sintering. Therefore, the generation of coarse particles in the sintered body can be further suppressed. Such sintered bodies have even higher bending strength.

[0027] D97 may be, for example, 0.80 μm or more, 0.90 μm or more, 1.00 μm or more, 1.10 μm or more, 1.15 μm or more, 1.20 μm or more, or 1.21 μm or more. The range of D97 may be, for example, 0.80 to 2.0 μm, 0.80 to 1.80 μm, 0.80 to 1.60 μm, 0.80 to 1.40 μm, 0.80 to 1.30 μm, or 0.80 to 1.25 μm.

[0028] In the cumulative distribution of volumetric particle diameters measured using a particle size distribution analyzer using a laser diffraction / scattering method, the particle diameter when the cumulative value from the smallest particle diameter reaches 10% of the total is defined as D10, the particle diameter when the cumulative value from the smallest particle diameter reaches 50% of the total is defined as D50, and the particle diameter when the cumulative value from the smallest particle diameter reaches 90% of the total is defined as D90. (D90-D10) / D50 is 1.70 or less, and may be 1.60 or less, or even 1.50 or less. The value of (D90-D10) / D50 is called the span value and is an index representing the degree of spread of the particle size distribution. The smaller the span value, the shorter the difference in particle diameter between D90 and D10, and the sharper the particle size distribution. A sharp particle size distribution results in a powder with high particle size uniformity due to the small variation in particle diameter of the silicon nitride powder. Therefore, when multiple sintered bodies are produced using silicon nitride powder whose (D90-D10) / D50 is within the above range, the variation in bending strength of each sintered body can be further reduced. This also makes it possible to further increase the Weibull coefficient for bending strength of the sintered body. Furthermore, sintering can be progressed more uniformly, and the generation of coarse particles in some parts of the sintered body can be further suppressed. Therefore, the bending strength of the sintered body can be further improved.

[0029] 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 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 further improves the bending strength of the resulting sintered body. The range of the (D90-D10) / D50 value of the sintered body may be, for example, 1.30 to 1.70, or 1.40 to 1.70.

[0030] D90 may be 1.50 μm or less, 1.30 μm or less, or 1.10 μm or less. Silicon nitride powders with an upper D90 value in this range are composed of smaller particles, and when the powder is used as a sintered body raw material, the generation of coarse particles in the sintered body can be further suppressed, resulting in a sintered body with higher bending strength. Furthermore, D90 may be, for example, 0.30 μm or more, 0.40 μm or more, 0.50 μm or more, 0.60 μm or more, 0.70 μm or more, 0.80 μm or more, or 0.83 μm or more. By having the lower D90 value in this range, excessive increases in the oxygen content due to smaller particle size can be prevented, the generation of embrittled phases during sintered body production can be further suppressed, and the bending strength of the sintered body can be further improved. Furthermore, the productivity of classification is improved, making it possible to produce silicon nitride powder more inexpensively, and stabilizing production conditions such as the firing temperature during production of the sintered body. The D90 range may be, for example, 0.30 to 1.50 μm, 0.50 to 1.30 μm, or 0.60 to 1.10 μm.

[0031] D50 may be, for example, 0.70 μm or less, 0.65 μm or less, 0.60 μm or less, or 0.58 μm or less. Silicon nitride powders with an upper D50 value in this range are composed of smaller particles, and when used as a sintered body raw material, the generation of coarse particles in the sintered body can be further suppressed, resulting in a sintered body with higher bending strength. Furthermore, D50 may be, for example, 0.20 μm or more, 0.30 μm or more, 0.40 μm or more, 0.45 μm or more, or 0.47 μm or more. By setting the lower limit of D50 in this range, excessive increases in oxygen content due to smaller particle size can be prevented, the generation of embrittled phases during sintered body production can be further suppressed, and the bending strength of the sintered body can be further improved. Furthermore, classification productivity can be improved, silicon nitride powder can be produced more inexpensively, and manufacturing conditions such as the firing temperature during sintered body production can be stabilized. The D50 range may be, for example, 0.20 to 0.70 μm, 0.30 to 0.65 μm, 0.30 to 0.60 μm, or 0.30 to 0.58 μm.

[0032] D10 may be, for example, 0.40 μm or less, 0.30 μm or less, or 0.20 μm or less. Silicon nitride powders with an upper D10 value in this range are composed of smaller particles, and when used as a sintered body raw material, the generation of coarse particles in the sintered body can be further suppressed, resulting in a sintered body with higher bending strength. Furthermore, D10 may be, for example, 0.02 μm or more, 0.05 μm or more, 0.10 μm or more, 0.12 μm or more, 0.14 μm or more, or 0.16 μm or more. By setting the lower limit of D10 in this range, excessive increases in oxygen content due to smaller particle size can be prevented, the generation of embrittled phases during sintered body production can be further suppressed, and the bending strength of the sintered body can be further improved. Furthermore, classification productivity can be improved, silicon nitride powder can be produced more inexpensively, and manufacturing conditions such as the firing temperature during sintered body production can be stabilized. The range of D10 may be, for example, 0.05 to 0.30 μm, 0.10 to 0.20 μm, or 0.12 to 0.20 μm.

[0033] 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.

[0034] Since the silicon nitride powder according to the present disclosure has adjusted values ​​of D97 and (D90-D10) / D50, it is possible to prepare a sintered body that can exhibit excellent bending strength even if the alpha conversion rate is not so high. ３ N ４ α-Si relative to the total amount ３ N ４The α-phase ratio (mass proportion) may be 96.0% by mass or less, 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.

[0035] The alpha-phase ratio of silicon nitride powder can be adjusted by the conditions for calcining the silicon nitride powder. Silicon nitride powder with a high alpha-phase ratio can be obtained by chemical synthesis using imide pyrolysis or by high-temperature vapor phase synthesis, in which idiomorphous particles are formed by crystal growth at high temperatures. These production methods require chemical synthesis, which generates a large amount of by-products, and are therefore costly. On the other hand, silicon nitride powder with a low alpha-phase ratio can be obtained by a nitridation reaction in which metal silicon is directly nitrided, which allows for simple production while suppressing cost increases. 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.

[0036] The BET specific surface area of ​​the silicon nitride powder is, for example, 12.0 m 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.

[0037] The BET specific surface area is, for example, 20.0 m ２ / g or less, and ２ / g or less, and ２ / g or less, and 15.0m ２ / g or less. There is no technical upper limit to the BET specific surface area, but generally, as the BET specific surface area increases, the oxygen content also increases. Therefore, as the BET specific surface area increases, the oxygen content increases excessively, which generates an embrittlement phase during the preparation of a sintered body, resulting in a decrease in strength. In other words, a silicon nitride powder having an upper limit value of the BET specific surface area in this range can maintain the oxygen content within an appropriate range. Therefore, the bending strength of the resulting sintered body can be further improved. The range of the BET specific surface area is, for example, 12.0 to 20.0 m ２ / g, 13.0-20.0 m ２ / g, or 14.0 to 20.0 m ２ / g.

[0038] 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 single-point BET 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."

[0039] The oxygen content of the silicon nitride powder may be, for example, 0.60 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.

[0040] The oxygen content may be, for example, 0.70% by mass or more, 0.72% by mass or more, or 0.74% 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.

[0041] The oxygen content may be, for example, 1.40% by mass or less, 1.30% by mass or less, 1.20% by mass or less, 1.10% by mass or less, 1.00% by mass or less, 0.90% by mass or less, or 0.80% by mass or less. Silicon nitride powder with such an oxygen content can further suppress the formation of embrittled phases due to excess oxygen during sintering and further improve the bending strength of the sintered body. The oxygen content may be, for example, 0.70 to 1.60% by mass, 0.72 to 1.60% by mass, 0.74 to 1.60% by mass, 0.74 to 1.40% by mass, 0.74 to 1.20% by mass, 0.74 to 1.00% by mass, or 0.74 to 0.90% by mass.

[0042] The BET specific surface area of ​​the silicon nitride powder is 12.0 m ２ / g or more, and the oxygen content may be 0.60 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 12.0 to 20.0 m, the BET specific surface area of ​​the silicon nitride powder may be 12.0 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 both the effect of promoting the sintering reaction based on the amount of oxygen contained in the silicon nitride particles and the contact area between the sintering aid and the liquid phase, 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, at a higher level.

[0043] The oxygen content / (BET specific surface area x D10) ratio may be 0.30 to 0.80. When the oxygen content / (BET specific surface area x D10) ratio of the silicon nitride powder is within the above-mentioned range, the silicon nitride powder is composed of smaller particles, and when the powder is used as a raw material for a sintered body, the generation of coarse particles in the sintered body can be further suppressed. Furthermore, the effect of promoting the sintering reaction based on the oxygen content of the silicon nitride particles and the contact area with the liquid phase of the sintering aid, and the effect of suppressing the generation of an embrittlement phase based on adjustment of the oxygen content of the silicon nitride particles can be achieved at a higher level. As a result, the bending strength of the sintered body obtained from the above silicon nitride powder can be further improved.

[0044] If the oxygen content / (BET specific surface area x D10) is 0.80 or more, the particle size becomes small, which leads to an excessive increase in the oxygen content, which significantly increases the formation of embrittlement phases during the preparation of the sintered body, and the bending strength of the sintered body may decrease.If the oxygen content / (BET specific surface area x D10) is 0.30 or less, coarse particles are formed in the sintered body, which may decrease the bending strength of the sintered body.

[0045] <Method for Producing Silicon Nitride Powder> A method for producing silicon nitride powder according to one embodiment includes adjusting the particle size of a raw silicon nitride powder so that, in the cumulative distribution of volumetric particle sizes measured using a particle size distribution analyzer using a laser diffraction / scattering method, D10 is the particle size when the cumulative value from the small particle size reaches 10% of the total, D50 is the particle size when the cumulative value from the small particle size reaches 50% of the total, D90 is the particle size when the cumulative value from the small particle size reaches 90% of the total, and D97 is the particle size when the cumulative value from the small particle size reaches 97% of the total. This method includes adjusting the particle size of the raw silicon nitride powder to produce the desired silicon nitride powder. Therefore, silicon nitride powder can be produced more inexpensively than silicon nitride powder with a high alpha content as a raw material for sintered compacts, which exhibits excellent bending strength.

[0046] In the above-mentioned method for producing silicon nitride powder, the starting silicon nitride powder may have a gelatinization rate of 96.0 mass% or less. If such starting silicon nitride powder is available, it may be used as is, or a starting silicon nitride powder having a gelatinization rate of 96.0 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 starting silicon nitride powder before the step of adjusting the particle size. The step of preparing the starting silicon nitride powder may include at least one step selected from the group consisting of a mixing step of mixing silicon-containing starting 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 starting silicon nitride powder may be obtained through the mixing step, the nitriding step, and the post-treatment step.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] The alpha phase ratio of the raw silicon nitride powder may be 96.0% by mass or less, 95.0% by mass or less, or 94.0% by mass or less. Silicon nitride powders having an alpha phase ratio within the above-described 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 having an alpha phase ratio within the above-described range are used, silicon nitride powders from which sintered bodies having excellent bending strength can be produced can be provided. The alpha phase ratio may be 89.0% by mass or more, or 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.

[0052] In the particle size adjusting step, the silicon nitride powder obtained is adjusted so that the D97 is 2.0 μm or less and (D90−D10) / D50 is 1.70 or less. Examples of particle size adjustment methods include adjusting particle size by mixing powders of different particle sizes, adjusting particle size by pulverization, and adjusting particle size by classification.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] The particle size adjustment step can also employ a method of dry classification to obtain silicon nitride powder so that D97 is 2.0 μm or less and (D90-D10) / D50 is 1.70 or less. For example, if the silicon nitride powder contains agglomerated particles (also called secondary particles) formed by agglomeration of multiple primary particles of silicon nitride, 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 primary particles of silicon nitride with large particle diameters.

[0058] 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 and secondary air. For example, the "EVX-1" manufactured by Nippon Pneumatic Mfg. Co., Ltd. may be used as such an air classifier.

[0059] The operating conditions of an air classifier can be adjusted to control the swirling air velocity, for example, by adjusting the classification air volume and the louver opening of the air guide vanes. 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 170 to 300 m / s, it is possible to produce silicon nitride powder having a D97 of 2.0 μm or less and a (D90-D10) / D50 value of 1.70 or less. When such silicon nitride powder is sintered, it has high bending strength and the Weibull modulus of the silicon nitride sintered body can be increased.

[0060] 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 m３ / min. The louver opening of the air guide vane may be 5 mm or less and 2 mm or more, for example, in the range of 2 to 3 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° or 80 to 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 170 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.

[0061] From the viewpoint of accelerating the classification of silicon nitride powder and further reducing D97 to obtain silicon nitride powder that becomes a sintered body with a large Weibull coefficient, the swirling air velocity may be 170 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 170 to 300 m / s, 220 to 300 m / s, or 220 to 250 m / s. The swirling air velocity is expressed as V, where V is the flow velocity (m / s), 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.).

[0062] The D97 may be adjusted to, for example, 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, the range of 0.80 to 2.0 μm, 0.80 to 1.80 μm, 0.80 to 1.60 μm, or 0.80 to 1.40 μm.

[0063] In the particle size adjustment step, D90 may be adjusted to, for example, 1.50 μm or less, 1.30 μm or less, or 1.10 μm or less. Silicon nitride powders having an upper limit of D90 in this 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, 0.30 μm or more, 0.40 μm or more, or 0.50 μm or more. D90 may be adjusted to, for example, a range of 0.30 to 1.50 μm, 0.50 to 1.30 μm, or 0.50 to 1.10 μm.

[0064] In the particle size adjustment step, D50 may be adjusted to, for example, 0.70 μm or less, 0.65 μm or less, or 0.60 μm or less. Silicon nitride powders with an upper D50 value 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, D50 may be adjusted to, for example, 0.20 μm or more, or 0.30 μm or more. D50 may be adjusted to, for example, a range of 0.20 to 0.70 μm, 0.30 to 0.65 μm, or 0.30 to 0.60 μm.

[0065] In the particle size adjustment step, D10 may be adjusted to, for example, 0.40 μm or less, 0.30 μm or less, or 0.20 μm or less. Silicon nitride powders with an upper D10 value in this range have smaller particle diameters, 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, 0.02 μm or more, 0.05 μm or more, or 0.10 μm or more. The D10 range may be adjusted to, for example, 0.05 to 0.30 μm or 0.10 to 0.20 μm.

[0066] 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, 1.60 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.

[0067] 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, for example, to the range of 1.30 to 1.70, or 1.40 to 1.70.

[0068] The silicon nitride powder of this embodiment can be produced by the above steps. However, the above-mentioned production method is an example and is not limited to this. Since the silicon nitride powder of this embodiment has adjusted D97 and (D90-D10) / D50 values, it can be suitably used as a raw material for sintered bodies with high bending strength.

[0069] 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 %.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] The bending strength of the sintered body can be 920 MPa or more, 930 MPa or more, 950 MPa or more, 960 MPa or more, 970 MPa or more, or 980 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, 950 to 1100 MPa, 960 to 1100 MPa, 970 to 1100 MPa, or 980 to 1100 MPa.

[0074] The fracture toughness of the sintered body is 4.5 (MPa / m ２ ) or more, 4.6 (MPa / m ２ ) or more, 4.7 (MPa / m ２ ) or more, or 4.8 (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 ), 4.6 to 6.0 (MPa / m ２ ), 4.7 to 6.0 (MPa / m ２ ), or 4.8 to 6.0 (MPa / m ２ ) may be.

[0075] The Vickers hardness of the sintered body can be 1.400 HV or more, 1.420 HV or more, 1.430 HV or more, or 1.440 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, 1.430 to 1.600 HV, or 1.440 to 1.600 HV.

[0076] 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 may 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 may also be 25.0 or less. The range of the Weibull coefficient may be, for example, 10.0 to 25.0, 12.0 to 25.0, 13.0 to 25.0, 15.0 to 25.0, 17.0 to 25.0, or 20.0 to 25.0.

[0077] 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.

[0078] Although several embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments.

[0079] 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.

[0080] (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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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 %.

[0085] 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 %.

[0086] 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 %.

[0087] 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 %.

[0088] 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 %.

[0089] 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

[0090] 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

[0091] 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

[0092] 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

[0093] Example 5 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

[0094] Example 6 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

[0095] [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 and comparative example. alpha phase ratio (mass %) = (Ia102 + Ia210) / (Ia102 + Ia210 + Ib101 + Ib210) × 100

[0096] [Measurement of Particle Size Distribution] The particle size distribution of silicon nitride powder was measured using a laser diffraction / scattering method. 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. This was mixed with 2 mL of a 20% aqueous solution of sodium hexametaphosphate and 200 g of water as dispersants. This container was placed in a Sharp Corporation ultrasonic disperser so that the entire dispersion was immersed, and ultrasonic dispersion was performed for 1 minute. The particle size distribution measurement described above was performed using the ultrasonically dispersed sample. D10, D50, D90, and D97 were determined for each example and comparative example. Furthermore, (D90-D10) / D50 was calculated from the obtained results. The results are shown in Table 1.

[0097] [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 example and comparative example are shown in Table 1.

[0098] [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 example and comparative example are shown in Table 1.

[0099] <Preparation of Sintered Body> 91 parts by mass of silicon nitride powder of each Example and Comparative Example, Y of an average particle size of 1.5 μm ２ O ３ 5 parts by mass of powder and Al having an average particle size of 1.2 μm ２ O ３ Four 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.

[0100] [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 produced 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 and comparative example. The results are shown in Table 2.

[0101] [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.

[0102] [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.

[0103] [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 and 2. 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 a portion of the cross section of the sintered body of Example 4. Comparing Figures 1 and 2, it was found that the sintered body of Figure 1 had more coarse particles than the sintered body of Figure 2.

[0104]

[0105]

[0106] Comparison of the Examples and Comparative Examples confirmed that the sintered body obtained by using silicon nitride powder as a raw material, whose D97 and (D90-D10) / D50 values ​​are within the ranges specified in the present disclosure, tends to have higher bending strength than the sintered body obtained by using the silicon nitride powder of the Comparative Examples as a raw material. Furthermore, it was confirmed that when multiple silicon nitride sintered bodies were produced using the silicon nitride powder of the Examples, the variation in bending strength was small, and the Weibull coefficient related to bending strength increased.

[0107] According to the present disclosure, it is possible to provide a silicon nitride powder from which a sintered body having a large Weibull modulus and excellent bending strength can be prepared. According to the present disclosure, it is also possible to provide a production method capable of producing the above-mentioned silicon nitride powder more inexpensively.

Claims

1. A silicon nitride powder in which, in the cumulative distribution of volumetric particle diameters measured with a particle size distribution analyzer using a laser diffraction / scattering method, the particle diameter when the cumulative value from small particle diameters reaches 10% of the total is defined as D10, the particle diameter when the cumulative value from small particle diameters reaches 50% of the total is defined as D50, the particle diameter when the cumulative value from small particle diameters reaches 90% of the total is defined as D90, and the particle diameter when the cumulative value from small particle diameters reaches 97% of the total is defined as D97, D97 is 2.0 μm or less, and (D90-D10) / D50 is 1.70 or less.

2. The silicon nitride powder according to claim 1, wherein the gelatinization rate is 96.0 mass% or less.

3. The silicon nitride powder according to claim 1 or 2, having an oxygen content of 0.60 to 1.60 mass %.

4. BET specific surface area is 12.0 m ２ 3. The silicon nitride powder according to claim 1, wherein the SiO 2 content is 1 / g or more.

5. The silicon nitride powder according to claim 1 or 2, wherein the oxygen content / (BET specific surface area×D10) is 0.30 to 0.

80.

6. A method for producing silicon nitride powder, comprising a step of adjusting the particle size of raw material silicon nitride powder so that, in the cumulative distribution of volumetric particle sizes measured with a particle size distribution analyzer using a laser diffraction / scattering method, D97 is 2.0 μm or less and (D90-D10) / D50 is 1.70 or less, where D10 is the particle size when the cumulative value from small particle sizes reaches 10% of the total, D50 is the particle size when the cumulative value from small particle sizes reaches 50% of the total, D90 is the particle size when the cumulative value from small particle sizes reaches 90% of the total, and D97 is the particle size when the cumulative value from small particle sizes reaches 97% of the total.

7. A method for producing silicon nitride powder according to claim 6, wherein the particle size adjusting step uses an air classifier to classify the raw silicon nitride powder at a swirling air velocity of 170 to 300 m / s to adjust the particle size.

8. A method for producing silicon nitride powder according to claim 6 or 7, wherein the gelatinization rate of the raw silicon nitride powder is 96.0 mass% or less.

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