Silicon nitride sintered body, wear resistant member using the same, and method for producing silicon nitride sintered body

Abstract

To provide a steel sheet which is extremely excellent in surface workability, in which the difference in workability between the surface and the inside is suppressed, and in which the working grade and dimensional variation upon mass production working are suppressed.SOLUTION: A silicon nitride sintered body includes silicon nitride crystal grains and a grain boundary phase, wherein a relationship between a mean grain size dA and a mean aspect ratio rA of the silicon nitride crystal grains in a first region and a mean grain size dB and a mean aspect ratio rB of the silicon nitride crystal grains in a second region inside the first region satisfies a formula of 0.8 ≤ dA / dB ≤ 1.20.8 ≤ rA / rB ≤ 1.2, where D is a thickness of the silicon nitride sintered body before surface processing. 01D.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments relate to a silicon nitride sintered body, a wear-resistant member using the same, and a method for manufacturing the silicon nitride sintered body. [Background technology]

[0002] Ceramic sintered bodies, whose main component is silicon nitride (Si3N4), exhibit excellent heat resistance and a small coefficient of thermal expansion, giving them excellent thermal shock resistance and other properties, and as such are increasingly being used as high-temperature structural materials to replace conventional heat-resistant alloys in engine parts, steel-making machine parts, etc. Furthermore, because of their excellent wear resistance, efforts are underway to put them to practical use as rolling components and cutting tools.

[0003] Because silicon nitride is difficult to sinter, achieving uniform sintering is difficult, and various methods have been employed. Patent Document 1 describes a method of sintering silicon nitride by embedding it in a powder mixture of silicon nitride and silica (SiO2), etc., thereby increasing the partial pressure of the surrounding SiO2 gas and eliminating weight loss, resulting in a uniform sintered body. Patent Document 2 describes a method of sintering a silicon nitride substrate by coating it with a powder mixture of silicon nitride, sintering aids, etc., and suppressing evaporation of the sintering aid from the interface, thereby resulting in a uniform sintered body. Patent Document 3 describes a method of controlling the ratio of α and β phases using spark plasma sintering to obtain a uniform sintered body. Patent Document 4 describes a method of sintering silicon nitride and aluminum oxide (Al2O3) in a heat-treated carbonaceous container, resulting in a uniform sintered body. Patent Document 5 describes a method of using granulated powder to which moisture has been added after drying, and controlling the cooling rate during sintering to obtain a uniform sintered body. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-53376 [Patent Document 2] Japanese Patent Application Publication No. 9-77560 [Patent Document 3] Japanese Patent Application Publication No. 9-157031 [Patent Document 4] Japanese Patent Application Publication No. 9-235165 [Patent Document 5] Patent No. 251206 Summary of the Invention [Problem to be solved by the invention]

[0005] Silicon nitride sintered compacts are used in a variety of wear-resistant components, including engine parts, machine parts, bearing balls, cutting tools, etc. Silicon nitride sintered compacts are far more durable than metallic components such as bearing steel (SUJ2), and have therefore achieved long-term reliability in a variety of wear-resistant components, including bearing balls. This has also enabled them to be maintenance-free for long periods of time.

[0006] In recent years, ceramics with excellent properties have come to be used in large bearings for large generators, wind power generators, aircraft engines, and other applications. These large components require stricter quality characteristics than ever before, placing greater stress on the silicon nitride components used. However, as ceramic components become larger, unevenness tends to occur during sintering, and homogeneity is not always sufficient. For this reason, for example, when manufacturing silicon nitride bearing balls, the surface must be polished, but differences in the microstructure between the surface and the interior can result in differences in the amount of polishing required. [Means for solving the problem]

[0007] The silicon nitride sintered body according to the embodiment is intended to solve these problems and is a silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase, characterized in that, when the width before surface processing is D, the average particle size dA and average aspect ratio rA of the silicon nitride crystal grains in a first region from the outermost surface to a depth of 0 to 0.01D satisfy the following relationship with the average particle size dB and average aspect ratio rB of the silicon nitride crystal grains in a second region inside the first region: When the silicon nitride sintered body has a spherical or cylindrical shape, the width is the diameter of the sphere or the diameter of the circle of the cylinder. 0.8≦dA / dB≦1.2 0.8≦rA / rB≦1.2 [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram showing an example of a bearing ball as a wear-resistant member using a silicon nitride sintered body according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an example of a cross section of a silicon nitride sintered body according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] The silicon nitride sintered body according to the embodiment, the wear-resistant member using the same, and the method for producing the silicon nitride sintered body will be described in detail below.

[0010] Fig. 1 is a diagram showing an example of a bearing ball as a wear-resistant member using a silicon nitride sintered body according to an embodiment, and Fig. 2 is a diagram showing an example of a cross section of the silicon nitride sintered body according to an embodiment.

[0011] FIG. 1 shows a bearing ball as a wear-resistant member using a silicon nitride sintered body according to an embodiment. In FIGS. 1 and 2, reference numeral 1 denotes a bearing ball (sliding member), reference numeral 2 denotes a sliding surface, reference numeral 3 denotes a cross section of the silicon nitride sintered body, and reference numeral 4 denotes the surface of the sintered body. Note that the wear-resistant member using the silicon nitride sintered body is not limited to the bearing ball 1, and may be an engine part, a machine part, a bearing ball, a cutting tool, or the like. The wear-resistant member (or the silicon nitride sintered body) has a shape including a circular arc. For example, the wear-resistant member (or the silicon nitride sintered body) has a spherical shape or a cylindrical shape with circles as the top and bottom surfaces. The sphere includes a circular arc shape in a cross section including its center. The cylinder includes a circular arc shape in a cross section parallel to the top surface (or bottom surface). Here, the term "sphere" includes both a perfect sphere (sphericity = 0) and a non-perfect sphere within the tolerance range in manufacturing a perfect sphere (for example, 0 < sphericity ≦ 0.45 μm), the term "cylinder" includes both a perfect cylinder and a non-cylinder within the tolerance range in manufacturing a cylinder, and the term "circle" includes both a perfect circle and a non-circle within the tolerance range in manufacturing a perfect circle. Unless otherwise specified, the following description will be given of the case where the wear-resistant member (or silicon nitride sintered body) has a spherical shape.

[0012] It is preferable that the width, i.e., the diameter, of the sphere and the top and bottom circles of the cylinder be 70 mm or less. If the diameter of the wear-resistant member exceeds 70 mm, the larger the member, the more likely it is that unevenness will occur during sintering, and homogeneity will not necessarily be sufficient. More preferably, the diameter of the sphere and the top and bottom circles of the cylinder is 60 mm or less. Furthermore, it is more effective for the wear-resistant member (or silicon nitride sintered body), i.e., the sphere and the top and bottom circles of the cylinder, to be large, for example, with a diameter of 8 mm or more. This is because the wear-resistant member will meet strict quality characteristics required to withstand heavy loads.

[0013] The silicon nitride sintered body according to the embodiment has silicon nitride crystal grains and a grain boundary phase. When the width of the silicon nitride sintered body before surface processing is D, the relationship between the average particle size dA and average aspect ratio rA of the silicon nitride crystal grains in a first region extending from the outermost surface to a depth of 0 to 0.01D and the average particle size dB and average aspect ratio rB of the silicon nitride crystal grains in a second region located inside the first region satisfies the following formula: When the silicon nitride sintered body has a spherical or cylindrical shape, the width is the diameter of the sphere or the diameter of the circle of the cylinder. 0.8≦dA / dB≦1.2 0.8≦rA / rB≦1.2

[0014] More preferably, the relationship between the average particle size dA and the average aspect ratio rA and the average particle size dB and the average aspect ratio rB further satisfies the following formula: 0.8≦dA / dB≦0.97, 1.01≦dA / dB≦1.2 0.8≦rA / rB≦0.95, 1.05≦rA / rB≦1.2 A sintered body with a dA / dB of approximately 1 or an rA / rB of approximately 1 is ideal from the standpoint of uniformity, but its production requires time and effort and is costly.

[0015] The silicon nitride crystal grains that make up silicon nitride sintered bodies grow into needle-like shapes during sintering, achieving high strength and toughness. The shape of the needle-like crystals can be expressed by the grain size and aspect ratio (the ratio of the long side to the short side of a rectangle). During the sintering process of silicon nitride, grain growth occurs so that the grain boundaries (spaces) are filled, increasing the grain size and aspect ratio. As the grain size increases, the grain boundaries (spaces) are filled, increasing strength, but if the grain size becomes too large, gaps (defects) occur between the silicon nitride crystal grains, reducing strength. The aspect ratio increases as the grains grow, and the needle-like crystals become intricately intertwined, improving strength.

[0016] When comparing the crystal grains near the surface and inside of silicon nitride sintered bodies, the grain size near the surface can be larger and the aspect ratio smaller. This is because the surface crystal grains become more spherical due to the application of heat from the outside during sintering and the gas generated inside the sintered body. Particles with large grain size and small aspect ratios are less entangled with surrounding grains and have weaker strength due to the defects around them, so they are preferentially shed during polishing and become the starting point for processing.

[0017] Conversely, near the surface, the grain size may be small and the aspect ratio may be large. This is because the needle-shaped crystals grow long and thin depending on the sintering speed and the state of the raw materials and additives. These elongated crystal grains become strongly entangled with surrounding grains, making them difficult to remove during polishing. If the grain size and aspect ratio differ between the surface and the interior, the amount of processing during polishing will differ. In order to eliminate the processing difference of the entire silicon nitride sintered body during polishing, it is important to make the state of the crystal grains on the surface and inside similar, and it is effective to make the grain size and aspect ratio of the crystal grains on the surface and inside similar.

[0018] The average particle diameter dA of silicon nitride crystal grains in the first region extending from the outermost surface to a depth of 0 to 0.01D is compared with the average particle diameter dB of silicon nitride crystal grains in the second region extending inward from the first region, and the relationship 0.8≦dA / dB≦1.2 is established. For example, if the silicon nitride sintered body is spherical, the average particle diameters dA and dB are calculated based on the silicon nitride crystal grains present in a unit area of ​​20 μm × 20 μm in each of the two-dimensional first and second regions in a circular cross section including the center of the silicon nitride sintered body (i.e., a cross section including the diameter). This is because if dA / dB is less than 0.8, the surface crystal grains become too small and are difficult to shed, potentially resulting in uneven processing. On the other hand, if dA / dB is greater than 1.2, the surface crystal grains become too large and shed, potentially resulting in an increased number of processing starting points and thus in uneven processing. The closer the average particle size ratio is to 1.0, the less likely the crystal grains will fall off, resulting in an ideal distribution of crystal grains. Therefore, a more preferable range is 0.9≦dA / dB≦1.1.

[0019] The average aspect ratio rA of silicon nitride crystal grains in the first region extending from the outermost surface to a depth of 0 to 0.01D is compared with the average aspect ratio rB of silicon nitride crystal grains in the second region extending inward from the first region, and is set to 0.8 ≦ rA / rB ≦ 1.2. For example, if the silicon nitride sintered body is spherical, the average aspect ratios rA and rB are determined based on the silicon nitride crystal grains present in the two-dimensional first and second regions within a unit area of ​​20 μm × 20 μm in a circular cross section including the center of the silicon nitride sintered body. This is because if rA / rB is less than 0.8, the acicular crystal grains on the surface become too short, resulting in shedding and an increased number of processing starting points, resulting in processing irregularities. Furthermore, if rA / rB is greater than 1.2, the acicular crystal grains on the surface become strongly entangled, making them difficult to process, resulting in processing irregularities. The closer the aspect ratio is to 1.0, the less likely the grains will fall off, resulting in an ideal distribution of acicular crystal grains. Therefore, a more preferable range is 0.9≦rA / rB≦1.1.

[0020] It is preferable that silicon nitride crystal grains having both dA and dB of 1.1 μm or more account for 40% or more in each region, because to prevent shedding, it is necessary to have many silicon nitride crystal grains that have undergone sufficient grain growth to a size that is unlikely to shedding.

[0021] Furthermore, the ratio pA of the total amount of elements other than Si and N to silicon nitride crystal grains in the first region (0 to 0.01D depth) from the outermost surface to the silicon nitride crystal grains is compared with the ratio pB of the total amount of elements other than Si and N to silicon nitride crystal grains in the second region (inner than the first region). For example, in a cross section including the center of a silicon nitride sintered body, two-dimensional quantitative elemental analysis per unit area of ​​the first and second regions can determine the detected elements other than Si and N. This is because when pA / pB is less than 0.8, the sintering aid components are dispersed from the surface, resulting in less sintering aid components at the surface compared to the interior. This leads to grain shedding due to defects (voids), increasing the number of processing initiation points and resulting in processing irregularities. Furthermore, when pA / pB is greater than 1.2, the high amount of sintering aid components at the surface leads to the formation of a large grain boundary phase between the crystal grains. Because the grain boundary phase is more brittle than the silicon nitride crystal grains, it can act as a fracture initiation point, causing grain shedding and resulting in processing irregularities.

[0022] The closer the ratio of the total value of detected elements other than Si and N to the silicon nitride crystal grains is to 1.0, the less likely the grains will fall off, and the more ideal the sintering aid distribution will be. Therefore, the more preferable range is 0.9≦pA / pB≦1.1.

[0023] The average particle size and aspect ratio of silicon nitride crystal particles are measured as follows. First, a cross section including the center of a sphere or a circular cross section parallel to the top (or bottom) surface of a cylinder is obtained. This cross section is then mirror-polished to a surface roughness Ra of 1 μm or less. If the diameter of the circular cross section is D, a first region from 0 to 0.01D from the outermost surface and a second region inside the first region are photographed using a scanning electron microscope (SEM) so that a 20 μm x 20 μm area can be observed. The particle sizes of 50 silicon nitride crystal particles in each region are measured in descending order, and the average value is calculated. The reason for using the average value calculated from the 50 largest particles as the average value for the observation surface is to prevent the calculation from being influenced infinitely by small particles, which can lead to variations in the average value.

[0024] The aspect ratio is calculated by finding the length of the long side and the short side of the silicon nitride particles present in each region whose particle size has been measured as described above, and then dividing the long side by the short side to obtain the aspect ratio. The average value of these aspect ratios is then calculated. The measurement methods for the total value of the quantitative analysis of elements detected other than Si and N in the silicon nitride cross section and the quantitative analysis of silicon nitride crystal particles are as follows.

[0025] The mirror-finished cross section prepared using the average particle size and aspect ratio measurement method is quantitatively analyzed for silicon nitride and added sintering aids using an electron probe microanalyzer (EPMA). However, if a silicon compound is added as a sintering aid, it is difficult to distinguish it from silicon nitride, so it is excluded from the sintering aids that are subject to quantitative analysis.

[0026] Materials that are added as sintering aids to react during the sintering process to form grain boundary phases include Group 2 elements, Group 4 elements, Group 5 elements, Group 6 elements, Group 13 elements, Group 14 elements, and rare earth elements.

[0027] When adding a Group 2 element, it is preferable to select from Be (beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium), or Ra (radium), or if possible, one or more of Be, Mg, Ca, and Sr. When adding a Group 4 element, it is preferable to select from Ti (titanium), Zr (zirconium), or Hf (hafnium). When adding a Group 5 element, it is preferable to select from V (vanadium), Nb (niobium), or Ta (tantalum). When adding a Group 6 element, it is preferable to select from Cr (chromium), Mo (molybdenum), or W (tungsten). When adding a Group 13 element, it is preferable to select from B (boron) or Al (aluminum). When adding a Group 14 element, it is preferable to select from C (carbon) or Si (silicon). When adding a Group 2 element component, a Group 4 element component, a Group 5 element component, a Group 6 element component, a Group 13 element component, or a Group 14 element component as a sintering aid, it is desirable to add it as one of oxides, carbides, and nitrides.

[0028] Furthermore, when adding a rare earth element, it is desirable to select one or more of Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium). When a rare earth element is added during sintering of silicon nitride, the sinterability improves and the aspect ratio of the silicon nitride crystal grains improves, resulting in a sintered body with excellent strength and wear resistance.

[0029] The silicon nitride sintered body according to the embodiment is not particularly limited to a specific manufacturing method as long as it has the above-described configuration, but the following methods can be mentioned as efficient methods for obtaining it.

[0030] First, silicon nitride powder is prepared. The silicon nitride powder preferably has an oxygen content of 1 to 4 wt%, contains 85 wt% or more of α-phase silicon nitride, and has an average particle size of 0.8 μm or less. A high oxygen content allows for a homogeneous grain boundary phase, so by growing the α-phase silicon nitride powder into β-phase silicon nitride crystal grains during the sintering process, a homogeneous silicon nitride sintered body with excellent wear resistance can be obtained.

[0031] In the silicon nitride sintered body of the present invention, the surface layer and the inner surface are controlled to be homogeneous. To achieve this control, it is effective to control the dispersion of the sintering aid. To control the dispersion of the sintering aid, it is effective to control the amount added and to uniformly disperse it with the silicon nitride powder.

[0032] The amount of sintering aid added is preferably 2.0 to 6.0 wt% of one or more of Group 2 elements, Group 4 elements, Group 5 elements, Group 6 elements, Group 13 elements, Group 14 elements, and rare earth elements. The average particle size of the sintering aid powder is preferably 1.8 μm or less. Sintering aids come in the form of oxides, carbides, nitrides, etc., but the amount of oxide added is preferably 3.0 wt% or less. This is because adding an excessive amount of oxide sintering aid to raw materials with a high oxygen content increases the overall oxygen content, resulting in an excess of grain boundary phase.

[0033] To uniformly disperse silicon nitride powder and sintering aid powder, it is effective to disperse the target particles in micro-size. A crushing and mixing process using a bead mill, ball mill, or pot mill is effective, but a bead mill is preferable for efficient production.

[0034] By constantly stirring or vibrating the raw material compounds during the crushing and mixing process or after the process is completed, it is possible to prevent silicon nitride powder particles, sintering aid powder particles, and silicon nitride powder and sintering aid powder particles from bonding together to form secondary particles.By keeping most of the silicon nitride powder and sintering aid powder as primary particles, uniform dispersion can be achieved.

[0035] Next, an organic additive is added to the raw material mixture, which is a mixture of silicon nitride powder and sintering additive powder. The raw material mixture and organic additive are mixed using a bead mill, ball mill, or other mill, but a bead mill is preferred for efficient production. The slurry containing the organic additive is granulated using a spray dryer or other mill, and the resulting granulated powder is molded into the desired shape. The molding process is carried out using a mold press or cold isostatic pressing (CIP), for example. The molding pressure is preferably 200 MPa or higher. The size of the molded body, in the form of a spherical sintered body, is preferably 70 mm in diameter or less. This is because a sintered body with a diameter exceeding 70 mm is prone to uneven sintering, impairing uniformity between the surface and interior.

[0036] The compact obtained in the compacting process is debound. The debounding process is preferably carried out at a temperature in the range of 400 to 800°C. The debounding process is carried out in air or a non-oxidizing atmosphere, but it is preferable to carry out oxidation treatment at the maximum debounding temperature. Furthermore, if the sintered compact has a diameter of 40 mm or more, the temperature is raised to 300 to 600°C in a non-oxidizing atmosphere, and then the furnace is cooled to 300 to 400°C, after which the air is replaced and the temperature is raised again to the maximum debounding temperature. This controls the volatilization rate of the organic additive and prevents damage to the side surfaces of the spheres or cylinders due to sudden gas volatilization.

[0037] Next, the degreased body obtained in the degreasing step is sintered at a temperature in the range of 1600 to 1900°C. If the sintering temperature is below 1600°C, the grain growth of silicon nitride crystal grains may be insufficient. In other words, the reaction from α-phase silicon nitride to β-phase silicon nitride may be insufficient, and a dense sintered body structure may not be obtained. In this case, the reliability of the silicon nitride sintered body as a material may be reduced. If the sintering temperature exceeds 1900°C, the silicon nitride crystal grains may grow too much, which may reduce workability. The sintering step may be performed by either atmospheric sintering or pressure sintering. The sintering step is preferably performed in a non-oxidizing atmosphere. Examples of non-oxidizing atmospheres include a nitrogen atmosphere and an argon atmosphere. Furthermore, it is preferable to use a constant amount of atmospheric gas to exhaust gases generated from the sintered body during sintering out of the furnace.

[0038] After the sintering step, it is preferable to perform a hot isostatic pressing (HIP) treatment at 10 MPa or more in a non-oxidizing atmosphere. Examples of non-oxidizing atmospheres include a nitrogen atmosphere and an argon atmosphere. The HIP treatment temperature is preferably in the range of 1500 to 1900°C. By performing the HIP treatment, it is possible to eliminate pores in the silicon nitride sintered body. If the HIP treatment pressure is less than 10 MPa, such an effect cannot be sufficiently obtained.

[0039] The silicon nitride sintered body thus produced is polished where necessary to produce a wear-resistant member, preferably using diamond abrasive grains.

[0040] Example 1 The silicon nitride powder used had an average particle size of 0.8 μm, an alpha conversion rate of 92%, and an impurity oxygen content of 0.8 wt%. Auxiliary powder was added so that Si was 1.0 wt%, Y was 2.5 wt%, and Al was 1.0 wt% when the total amount of silicon nitride powder and sintering aid was 100 wt%, and the mixture was crushed and mixed in a bead mill for 50 hours to produce a raw material mixture.

[0041] A resin binder was mixed with the resulting raw material mixture in a bead mill to produce a slurry. The resulting slurry was dried and sprayed in a spray dryer while constantly stirring to produce a granulated powder. The granulated powder was press-molded at a molding pressure of 150 MPa. A mold that would result in a diameter of 60 mm after sintering was used for press molding, and a spherical pressed compact was obtained. The resulting compact was subjected to a debinding process in a nitrogen atmosphere at 700°C for 1 hour. In the debinding process, an oxidation treatment was performed by introducing air at the maximum debinding temperature. The resulting debound compact was subjected to atmospheric sintering in a nitrogen atmosphere at 1800°C for 4 hours. The nitrogen gas flow rate at the maximum sintering temperature for atmospheric sintering was set to 30 L / min. The inner volume of the sintering furnace used for sintering was approximately 0.9 m 3 The resulting sintered body was subjected to HIP treatment at 1600°C, 20 MPa, and 2 hours.

[0042] A circular cross section of a spherical silicon nitride sintered body was cut and mirror-polished, and then enlarged (SEM) photographs were taken at approximately 0.3 mm (0.005D) and 1.8 mm (0.03D) from the surface. A unit area of ​​20 μm x 20 μm was set from the enlarged photograph, and 50 particles were selected in descending order of particle size. The average particle size and aspect ratio of each was calculated. The average particle size (dA) at 0.3 mm from the surface was 1.16 μm, and the aspect ratio (rA) was 2.0. The average particle size (dB) at 1.8 mm from the surface was 1.05 μm, and the aspect ratio (rB) was 2.1. Therefore, the dA / dB ratio was 1.10, and the rA / rB ratio was 0.95. Next, the percentage of particles with an average particle size (dA and dB) of 1.1 μm or more was measured from each enlarged photograph, and the percentage at a position 0.3 mm from the surface was 49%, and the percentage at a position 1.8 mm from the surface was 47%.

[0043] Furthermore, quantitative analysis of Si, Al, and Y was performed using EPMA at the same locations as those observed with SEM. The ratio (pA) calculated by dividing the sum of the quantitative analysis values ​​of Al and Y, elements other than Si and N detected at 0.3 mm from the surface, by the quantitative analysis value of Si was 0.037. Similarly, the ratio (pB) at a position 1.8 mm from the surface was 0.036. Therefore, pA / pB was 1.03.

[0044] Sintered bodies manufactured under the same conditions were roughly processed to remove surface protrusions, and then polished using a polishing machine for 10 hours under medium-finishing conditions (3 μm abrasive grains) and for 4 hours under finishing conditions (0.25 μm abrasive grains).The diameter variation (difference between maximum and minimum values), sphericity, and surface roughness (Ra) of the completed spheres were measured in an arbitrary circumferential direction, and the results were 0.28 μm, 0.24 μm, and 0.027 μm, respectively.

[0045] Next, the hardness (HV) and three-point bending strength (σf) of each silicon nitride sintered body were measured, and the hardness was 1480 and the bending strength was 880 MPa. The samples (silicon nitride sintered body) for three-point bending strength measurement were processed to a size of 3 mm x 4 mm x 50 mm and measured according to the method of JIS-R-1601.

[0046] (Examples 1 to 6, Comparative Examples 1 to 4) Test pieces of silicon nitride sintered bodies were prepared under different manufacturing conditions based on Example 1. Table 1 shows the type and amount of sintering aid, the method of crushing and mixing the aid (mixing time), the method of mixing the organic aid (mixing time), the degreasing conditions (degreasing temperature and whether or not oxidation treatment was performed), and the sintering conditions (sintering temperature, sintering time, gas flow rate) for Examples (1-6) and Comparative Examples (1-4). In the Comparative Example, stirring was not performed until spray drying using a spray dryer. All other conditions were the same as in Example 1. The amount of sintering aid added is the ratio when the total amount of silicon nitride powder and sintering aid is 100 wt%.

[0047] [Table 1]

[0048] Table 2 shows the average particle diameter dA of silicon nitride crystal particles in any circular cross section within the first region from the outermost surface to a depth of 0 to 0.01D, the average particle diameter dB in the second region inside the first region, the ratio of dA to dB (dA / dB), the average aspect ratio rA of silicon nitride crystal particles in any circular cross section within the first region, the average aspect ratio rB in the second region, and the ratio of rA to rB (rA / rB) for Examples 1 to 6 and Comparative Examples 1 to 4. The diameters of the silicon nitride sintered bodies described in Examples 1 to 6 and Comparative Examples 1 to 4 were 8 mm or more and 70 mm or less.

[0049] [Table 2]

[0050] Table 3 shows the percentage of the area (%) of the region in which the average particle diameter dA of silicon nitride crystal particles in any circular cross section within the first region from the outermost surface to a depth of 0 to 0.01D is 1.1 μm or more, the percentage of the area (%) of the region in the second region inside the first region where the average particle diameter dB is 1.1 μm or more, the ratio pA of the total value of elements other than Si and N detected per unit area in any circular cross section within the first region by elemental quantitative analysis to silicon nitride crystal particles, the ratio pB of the total value of elements other than Si and N detected in the second region to silicon nitride crystal particles, and the ratio of pA to pB (pA / pB).

[0051] [Table 3]

[0052] Table 4 shows the diameter variation (difference between maximum and minimum values), sphericity, surface roughness (Ra), hardness (HV), and three-point bending strength (σf) for the finished spheres in any circumferential direction set in Examples 1 to 6 and Comparative Examples 1 to 4.

[0053] [Table 4]

[0054] The silicon nitride sintered bodies according to the examples and comparative examples all have high hardness values ​​of 1400 or more and three-point bending strength values ​​of 760 MPa or more. The silicon nitride sintered bodies according to Examples 1 to 6 all had a diameter variation of 0.5 μm or less, a sphericity of 0.45 μm or less, and a surface roughness (Ra) of 0.04 μm or less.

[0055] In contrast, in Comparative Examples 1 to 4, the diameter variation was 0.71 to 1.01 μm, the sphericity was 0.76 to 1.10 μm, and the surface roughness (Ra) was 0.05 to 0.97 μm, all of which were larger than those in the Examples, despite the same processing conditions.

[0056] From these experimental results, it can be said that the examples have excellent surface processability, suppress the difference in processability between the surface and the interior, and suppress the processing quality and dimensional variation during mass-production processing.

[0057] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

Claims

1. A method for producing a spherical silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase, comprising: a step of mixing an organic solvent with a raw material mixture obtained by mixing silicon nitride powder with a sintering aid to obtain a slurry; granulating the slurry using a spray dryer while stirring to obtain a granulated powder; forming the granulated powder into a green body, and debinding and sintering the green body to obtain the silicon nitride sintered body having the spherical shape, When sintering the degreased compact, the flow rate of gas at the maximum temperature is 3% or more of the internal volume of the sintering furnace per minute; The diameter of the sphere before surface treatment is 8 mm or more and 60 mm or less, When the diameter of the sphere before surface processing is D, the relationship between the average particle size dA and the average aspect ratio rA of the silicon nitride crystal particles in a first region from the outermost surface to a depth of 0 to 0.01D and the average particle size dB and the average aspect ratio rB of the silicon nitride crystal particles in a second region inside the first region is as follows: 0.8≦dA / dB≦1.2 0.8≦rA / rB≦1.2 A method for producing a silicon nitride sintered body, characterized in that the above formula is satisfied.

2. 2. The method for producing a silicon nitride sintered body according to claim 1, wherein both the average particle diameter dA and the average particle diameter dB of the silicon nitride sintered body are 1.1 μm or more.

3. 3. The method for producing a silicon nitride sintered body according to claim 1, wherein the silicon nitride crystal grains are present in an amount of 40% or more in both the first region and the second region.

4. The relationship between the ratio pA of the total amount of elements other than Si and N to the silicon nitride crystal grains in the first region and the ratio pB of the total amount of elements other than Si and N to the silicon nitride crystal grains in the second region is 0.8≦pA / pB≦1.2 4. The method for producing a silicon nitride sintered body according to claim 1, wherein the following is satisfied:

5. 5. The method for producing a silicon nitride sintered body according to claim 4, wherein the detected elements other than Si and N are determined by quantitative elemental analysis per unit area.

6. The relationship between the average particle diameter dA and the average aspect ratio rA, and the average particle diameter dB and the average aspect ratio rB is further 0.8≦dA / dB≦0.97, 1.01≦dA / dB≦1.2 0.8≦rA / rB≦0.95, 1.05≦rA / rB≦1.2 6. The method for producing a silicon nitride sintered body according to claim 1, wherein the following formula is satisfied:

7. 7. The method for producing a silicon nitride sintered body according to claim 1, wherein the average particle diameter dA, the average aspect ratio rA, the average particle diameter dB, and the average aspect ratio rB are determined based on the silicon nitride crystal grains present in a unit area of ​​20 μm × 20 μm in each of the first region and the second region.

8. 8. The method for producing a silicon nitride sintered body according to claim 1, further comprising a step of constantly stirring or vibrating the raw material compounds after the crushing and mixing steps are completed.

9. 9. A method for producing a silicon nitride sintered body according to claim 1, wherein the degreasing step comprises heating the compact in a non-oxidizing atmosphere, cooling the compact, and then heating the compact in the air.

10. 10. A method for producing a silicon nitride sintered body according to claim 1, further comprising a molding step of molding a granulated powder obtained by granulating a raw material mixture obtained by mixing silicon nitride powder and sintering aid powder at a pressure of 200 MPa or more.

11. 11. A method for producing a wear-resistant member, further comprising the step of polishing the silicon nitride sintered body according to claim 1.

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