Method for manufacturing silicon nitride sintered body

Abstract

To provide a method for producing a silicon nitride sintered compact having the amount of solid dissolved oxygen controlled.SOLUTION: A method includes: a step of adding a sintering aid powder to a silicon nitride powder within a range of 1 pt.mass or more and 20 pts.mass or less; a mixing step of mixing a raw material powder using a ball mill; a forming step; a dewaxing step; and a sintering step. In the mixing step, the raw material powder is mixed by a ball mill using media having a diameter of 3 mm or more and 20 mm or less, at a rotational speed of 30 rpm or more and 500 rpm or less in a cylindrical rotating vessel, for a mixing time of 5 hours or longer and 40 hours or shorter. In a 20 μm×20 μm area of any cross section, an average amount of solid dissolved oxygen in silicon nitride crystal particles is 0.2 wt.% or more. In a 50 μmx50 μm area of any cross section, an average value of the long diameter of the silicon nitride crystal particles is 0.1 μm or more and 10 μm or less, and an average value of the aspect ratio of the silicon nitride crystal particles is 1.5 or more and 10 or less. The fracture toughness value is 6 MPa m1 / 2 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments described below generally relate to a method for producing a silicon nitride sintered body. [Background technology]

[0002] Silicon nitride sintered bodies are used in wear-resistant members, such as bearing balls, rollers, roll materials, compressor vanes, gas turbine blades, engine parts, and friction stir welding tool parts. Roll materials are used in rolling. Engine parts include cam rollers.

[0003] For example, Japanese Patent No. 5362758 (Patent Document 1) discloses a silicon nitride sintered body in which titanium nitride particles with an aspect ratio of 1.0 to 1.2 are dispersed. In Patent Document 1, the aspect ratio and particle size of the titanium nitride particles are controlled. In addition, Japanese Patent No. 6400478 (Patent Document 2) controls the area ratio of the grain boundary phase and the aspect ratio of the silicon nitride crystal particles. In Patent Document 2, an oxidized silicon nitride powder is used to control the area ratio of the grain boundary phase.

[0004] In Patent Document 1, the wear resistance test of bearing balls was conducted at a maximum contact stress of 5.9 GPa and a rotation speed of 1200 rpm. In Patent Document 2, the wear resistance test of bearing balls was conducted at a maximum contact pressure of 5.1 GPa and a rotation speed of 1200 rpm. The bearing balls described in Patent Documents 1 and 2 both exhibit excellent durability.

[0005] In recent years, bearings are required to rotate at high speeds. The loads that can be applied to bearings include radial loads, thrust loads, and moment loads. Radial loads are loads that are applied in a direction perpendicular to the rotating shaft (circumferential direction of the rotating shaft). Thrust loads are loads that are applied in a direction parallel to the rotating shaft (axial direction of the rotating shaft). Moment loads are loads that occur due to eccentricity of the rotating shaft. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5362758 [Patent Document 2] Patent No. 6400478 [Patent Document 3] Japanese Patent Publication No. 2022-71426 [Patent Document 4] International Publication No. 2016 / 047376 Summary of the Invention [Problem to be solved by the invention]

[0007] Increasing the rotational speed of a bearing increases the radial load, thrust load, and moment load. Conventional bearings using bearing balls made of silicon nitride sintered body sometimes experience a decrease in durability when rotating at high speeds. When a solution to this problem was investigated, it was found that the amount of dissolved oxygen in the silicon nitride crystal grains had an effect. The present invention is intended to address these problems and to provide a method for producing a silicon nitride sintered body in which the amount of dissolved oxygen is controlled. [Means for solving the problem]

[0008] A method for producing a silicon nitride sintered body according to an embodiment includes the steps of: adding 1 to 20 parts by mass of sintering aid powder to the silicon nitride powder, where the total amount of silicon nitride powder and sintering aid powder in the raw material powder is 100 parts by mass; mixing the raw material powder using a ball mill; molding the mixture; degreasing the resulting compact; and sintering the degreasing compact. In the mixing step, the raw material powder is mixed in the ball mill using media with a diameter of 3 to 20 mm, with a cylindrical rotating vessel containing the raw material powder rotating at a speed of 30 to 500 rpm for a mixing time of 5 to 40 hours. The resulting silicon nitride sintered body comprises silicon nitride crystal grains and a grain boundary phase. In a 20 μm × 20 μm area of ​​any cross section, the silicon nitride crystal grains have an average dissolved oxygen content of 0.2 wt% or more. In a 50 μm×50 μm region of any cross section, the silicon nitride crystal grains have an average major axis of 0.1 μm or more and 10 μm or less, and an average aspect ratio of 1.5 or more and 10 or less. The fracture toughness value is 6 MPa m 1 / 2 That's all. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a diagram showing an example of a cross-sectional structure of a silicon nitride sintered body according to an embodiment. [Figure 2] 1A and 1B are diagrams showing an example of a bearing ball according to an embodiment. [Figure 3] FIG. 2 is a diagram showing an example of a bearing according to an embodiment. [Figure 4] An example of the first plot. [Figure 5] An example of the second plot. [Figure 6] An example of the third plot. [Figure 7] FIG. 2 is a diagram showing an example of an XRD peak of a silicon nitride sintered body according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] A method for producing a silicon nitride sintered body according to an embodiment includes the steps of: adding 1 to 20 parts by mass of sintering aid powder to the silicon nitride powder, where the total amount of silicon nitride powder and sintering aid powder in the raw material powder is 100 parts by mass; mixing the raw material powder using a ball mill; molding the mixture; degreasing the resulting compact; and sintering the degreasing compact. In the mixing step, the raw material powder is mixed in the ball mill using media with a diameter of 3 to 20 mm, with a cylindrical rotating vessel containing the raw material powder rotating at a speed of 30 to 500 rpm for a mixing time of 5 to 40 hours. The resulting silicon nitride sintered body comprises silicon nitride crystal grains and a grain boundary phase. In a 20 μm × 20 μm area of ​​any cross section, the silicon nitride crystal grains have an average dissolved oxygen content of 0.2 wt% or more. In a 50 μm×50 μm region of any cross section, the silicon nitride crystal grains have an average major axis of 0.1 μm or more and 10 μm or less, and an average aspect ratio of 1.5 or more and 10 or less. The fracture toughness value is 6 MPa m 1 / 2 That's all.

[0011] FIG. 1 is a diagram showing an example of the cross-sectional structure of a silicon nitride sintered body according to an embodiment. In Fig. 1, reference numeral 1 denotes a silicon nitride sintered body, reference numeral 2 denotes silicon nitride crystal grains, and reference numeral 3 denotes a grain boundary phase. Fig. 1 is a schematic diagram showing an example of the cross-sectional structure of a silicon nitride sintered body. The silicon nitride sintered body 1 comprises silicon nitride crystal grains 2 and a grain boundary phase 3. The grain boundary phase 3 is distributed in the gaps between the silicon nitride crystal grains 2. The grain boundary phase 3 is formed by reaction with a sintering aid, which will be described later. The presence of the grain boundary phase 3 firmly bonds the silicon nitride crystal grains 2, making it possible to form a silicon nitride sintered body with high strength. The silicon nitride sintered body may also have pores (not shown).

[0012] The average long diameter of the silicon nitride crystal particles 2 is 0.1 μm or more and 10 μm or less. The average aspect ratio of the silicon nitride crystal particles 2 is 1.5 or more and 10 or less. Scanning electron microscope (SEM) photographs are used to measure the average long diameter and average aspect ratio. A measurement area is set on any cross section of the silicon nitride sintered body 1. The SEM photograph is taken at a magnification of 2000 times or more. The maximum diameter of each silicon nitride crystal particle is measured in the SEM photograph. The maximum diameter of the silicon nitride crystal particle shown in the SEM photograph is taken as the long diameter. The average value of the maximum diameters of the silicon nitride crystal particles shown in a 50 μm x 50 μm region in the measurement area is taken as the average long diameter.

[0013] The aspect ratio is measured using the major and minor axes. The maximum diameter mentioned above is taken as the major axis. For silicon nitride crystal particles 2 for which the major axis has been determined, the length of a line segment extending perpendicularly from the center point of the major axis is taken as the minor axis. The aspect ratio is calculated by dividing the major axis by the minor axis. The major axis / minor axis ratio is rounded to the nearest tenth. As with the average major axis, the average value of the individual silicon nitride crystal particles captured in a 50 μm × 50 μm area is taken as the average aspect ratio. The major and minor axes are measured using the portion of the silicon nitride crystal particle visible in the SEM photograph. For example, a silicon nitride crystal particle may overlap with other silicon nitride crystal particles, making the entire outline of that silicon nitride crystal particle invisible. In such cases, the major and minor axes of that silicon nitride crystal particle are measured using only the visible portion (the portion visible in the SEM photograph). Furthermore, for silicon nitride crystal particles whose outlines are interrupted at the edges of a 50 μm × 50 μm SEM image, the major and minor axes are measured using only the visible portion (the portion visible in the SEM image). If the outline of a silicon nitride crystal particle is difficult to see in the SEM image, the measurement area may be etched. Etching removes the surface layer and grain boundary phase of the silicon nitride crystal particle, making the outline of the silicon nitride crystal particle easier to see. Furthermore, if SiAlON crystal particles are present, they are counted as silicon nitride crystal particles. The etching rate of silicon nitride crystal particles differs from that of the grain boundary phase. We will explain this example using a case where the etching rate of silicon nitride crystal particles is faster than that of the grain boundary phase. In this case, a portion of each silicon nitride crystal particle 2 is removed to a greater extent than the grain boundary phase 3. As a result, the surface of the silicon nitride crystal particle 2 is located below the surface of the grain boundary phase 3. This allows the grain boundary phase 3 to have a three-dimensional wall-like appearance relative to the silicon nitride crystal grains 2, making it possible to easily distinguish the silicon nitride crystal grains 2 from the grain boundary phase 3 by contrast or the like.

[0014] The average major axis of the silicon nitride crystal particles 2 present in the 50 μm × 50 μm region is in the range of 0.1 μm to 10 μm. The average aspect ratio of the silicon nitride crystal particles 2 present in the 50 μm × 50 μm region is in the range of 1.5 to 10. Within this range, the durability of the silicon nitride sintered body 1 when rotated at high speed can be improved. The mechanical strength of the silicon nitride sintered body 1 can also be improved. If the average major axis value is less than 0.1 μm, the silicon nitride crystal particles 2 may be too small, resulting in reduced durability. If the average major axis value exceeds 10 μm, the mechanical strength of the silicon nitride sintered body 1 may be reduced. For this reason, the average major axis value is preferably in the range of 0.1 μm to 10 μm, and more preferably in the range of 0.5 μm to 8 μm.

[0015] If the average aspect ratio is less than 1.5, the number of elongated silicon nitride crystal grains will be small, which may reduce the mechanical strength of the silicon nitride sintered body 1. If the average aspect ratio exceeds 10, the gaps between the silicon nitride crystal grains 2 may become large. If the gaps between the silicon nitride crystal grains 2 become large, the grain boundary phase 3 will become large. A large grain boundary phase 3 may reduce the mechanical strength of the silicon nitride sintered body 1. For this reason, the average aspect ratio is preferably 1.5 to 10, and more preferably 2 to 10.

[0016] In any cross section, it is preferable that both silicon nitride crystal grains 2 with a major axis of less than 3 μm and silicon nitride crystal grains 2 with a major axis of 3 μm or more are present. By providing silicon nitride sintered body 1 with both small silicon nitride crystal grains 2 and large silicon nitride crystal grains 2, the small crystal grains can be present in the gaps between the large crystal grains. This can improve the durability and mechanical strength of silicon nitride sintered body 1.

[0017] In the silicon nitride sintered body 1, when the amount of dissolved oxygen in silicon nitride crystal grains 2 present in a 20 μm × 20 μm region of any cross section is measured, the average value is 0.2 wt% or more. TEM-EDS is used to measure the amount of dissolved oxygen in silicon nitride crystal grains 2. TEM stands for transmission electron microscope. EDS stands for energy dispersive X-ray spectrometer. The measurement method using TEM-EDS is also simply called EDS analysis. A method for measuring the amount of dissolved oxygen in silicon nitride crystal grains 2 using EDS analysis is disclosed in Japanese Patent Laid-Open Publication No. 2022-71426 (Patent Document 3).

[0018] An arbitrary cross section of the silicon nitride sintered body 1 is used as a sample for EDS analysis. A sample is taken from the arbitrary cross section by FIB (focused ion beam) processing or ion milling processing. The thickness of the sample is preferably in the range of 0.05 μm to 0.5 μm. To prevent surface oxidation of the sample, it is desirable to prepare and store the sample in a vacuum or in an inert gas atmosphere.

[0019] For EDS, a JEOL JED-2300T or an instrument with equivalent or better performance is used. For TEM, a JEOL JEM-200CX (accelerating voltage 200 kV) or an instrument with equivalent or better performance is used. For EDS analysis, the recommended conditions are an accelerating voltage of 200 kV, a probe current of 1.00 nA, and a spot diameter of 1 nm during analysis. The recommended analysis time is 30 seconds, and the sample tilt angles are X = 10°, Y = 0°. Although the measurement conditions exemplified may be changed, the above measurement conditions are used for the measurement to obtain the first plot diagram described below.

[0020] By using TEM-EDS, it is possible to select the silicon nitride crystal particles 2 as the analysis spot. There is a method using secondary ion mass spectrometry (SIMS) to measure dissolved oxygen. Measurement using SIMS method could only measure the amount of dissolved oxygen in large silicon nitride crystal particles 2. Furthermore, even with nano-SIMS method, which can reduce the irradiation diameter, it was difficult to recognize the image of the silicon nitride crystal particles 2. For this reason, measurement using SIMS method could not measure the amount of dissolved oxygen in small silicon nitride crystal particles 2.

[0021] Another method for measuring dissolved oxygen content is the total dissolution method. In this method, the grain boundary phase of a silicon nitride sintered body is dissolved and the silicon nitride crystal grains are extracted. The oxygen content of the extracted silicon nitride crystal grains is then measured. However, it is difficult to dissolve and remove all of the grain boundary phase, and the remaining grain boundary phase can result in reduced accuracy and reproducibility in measuring the dissolved oxygen content.

[0022] In the case of TEM-EDS, by setting the analysis spot diameter to 1 nm, it is possible to set the analysis spot only on the silicon nitride crystal particles 2. Furthermore, it becomes possible to measure the amount of dissolved oxygen in the silicon nitride crystal particles 2 regardless of the size of the silicon nitride crystal particles 2.

[0023] In EDS analysis, at least 10 analysis spots are selected from all silicon nitride crystal particles 2 present in a 20 μm × 20 μm area. The 10 analysis spots should be set on as many different silicon nitride crystal particles as possible. In other words, it is desirable to select 10 or more silicon nitride crystal particles 2 as analysis spots. The atomic ratios of silicon (Si), oxygen (O), and nitrogen (N) are measured using EDS analysis. Three or more locations with an Si count of 300,000 cps or greater are selected as measurement spots, totaling 10 or more locations. If there are fewer than three locations with an Si count of 300,000 cps or greater among the selected analysis spots, new analysis spots are selected until there are three or more locations with an Si count of 300,000 cps or greater. An Si count of 300,000 cps or greater indicates that the oxygen content can be measured without being affected by surface oxygen. Therefore, the dissolved oxygen content can be measured even if the sample surface is naturally oxidized.

[0024] Fig. 4 is an example of a first plot diagram. Fig. 5 is an example of a second plot diagram. Fig. 6 is an example of a third plot diagram. Figs. 4 to 6 show the results of measurements in Example 3, which will be described later. First, a first plot is created. In the first plot, the atomic ratio of oxygen element / silicon element is plotted against the Si count number. In the first plot, the horizontal axis shows the Si count number (cps) and the vertical axis shows the O / Si atomic ratio. Next, a second plot is created. In the second plot, the atomic ratio of nitrogen element / silicon element is plotted against the Si count number.

[0025] Next, the second plot is used to correct the oxygen / silicon atomic ratio in the first plot. This is because in silicon nitride sintered bodies, the X-ray absorption by oxygen (O), a light element, is greater than that by silicon (Si). Oxygen (O) and nitrogen (N) have similar absorption characteristics. Furthermore, the main phase of silicon nitride sintered bodies is Si3N4. Therefore, the theoretical value of the N / Si atomic ratio is 4 / 3. The first plot is corrected using the approximate data for the Si and N atomic ratio. In this correction method, the O / Si atomic ratio in the first plot is corrected using the N / Si atomic ratio at each measurement point in the second plot. In other words, the O / Si atomic ratio in the first plot is corrected using the difference between the N / Si atomic ratio at each measurement point and the theoretical value of 4 / 3 (= 1.33). For example, if the N / Si atomic ratio is 0.70, the correction coefficient is 1.9 (= 1.33 / 0.70). The correction value is calculated by multiplying the O / Si atomic ratio by the correction coefficient. Using this method, the O / Si atomic ratio in the first plot is corrected. The corrected first plot is used as the third plot.

[0026] Next, when the slope of the approximation line of three or more points on the third plot is shown as y=aX+b, it is -4×10 -8 ≦a≦4×10 -8 The convergence region is determined. In the third plot, the horizontal axis indicates the Si count number (cps) and the vertical axis indicates the corrected O / Si atomic ratio. The slope of the approximation line, y=aX+b, is where X is the horizontal axis, y is the vertical axis, a is the slope, and b is the point of contact with the vertical axis (y axis). The approximation function of the calculation software is used to calculate the approximation line. Microsoft Excel (registered trademark) can be used as the calculation software.

[0027] The convergence region where the slope a of the approximation line in the third plot is within the aforementioned range is the region where the influence of natural oxidation and grain boundary phases on the sample surface is minimized. If the measurement results are influenced by natural oxidation or grain boundary phases on the sample surface, the variation in the O / Si atomic ratio will also increase. For this reason, the slope a will not fall within the aforementioned range. If the slope a of the approximation line is -4×10 -8 Over 4×10 -8 The fact that the O / Si atomic ratio is within the range below indicates that the variation in the O / Si atomic ratio is reduced. Because the variation in the O / Si atomic ratio is reduced, it can be seen that the influence of natural oxidation and grain boundary phases is sufficiently reduced. Therefore, the value within the convergence region indicates the amount of dissolved oxygen.

[0028] After determining the convergence region, the average value of the O / Si atomic ratios from the largest to the third point in the convergence region is calculated. The average value of the O / Si atomic ratios from the largest to the third point in the Si count region is a region where the influence of natural oxidation or the second grain boundary phase is further reduced. The amount of dissolved oxygen is calculated using the calculated average value of the O / Si atomic ratio. In Figure 6 (third plot diagram), if the slope a of the approximation line of three or more points is -4 x 10 -8 Over 4×10 -8 The convergence region within the following range is the region where the Si count number is 350,000 or more.

[0029] Since the silicon nitride crystal grains are Si3N4, the amount of dissolved oxygen (wt%) can be calculated by (3 / 7) x (average value of O / Si atomic ratio). This is a method for calculating the amount of dissolved oxygen from the amount of oxygen corresponding to the amount of Si in the Si3N4 crystal grains. Also, as mentioned above, the slope a of the approximation line of three or more points is -4 x 10 -8 Over 4×10 -8The convergence region within the following range is determined from the measurement results of analysis spots where a Si count of 300,000 cps or more was obtained. Analysis spots with a high Si count are points where the effects of oxidation and grain boundary phases on the sample surface are minimized. EDS analysis cannot selectively measure only areas with a Si count of 300,000 cps or more. For this reason, it is effective to measure 10 or more analysis spots using EDS analysis, regardless of the count number. From the 10 or more analysis spots measured, analysis spots with a Si count of 300,000 cps or more are extracted. If there are fewer than three analysis spots with a Si count of 300,000 cps or more, EDS analysis is performed on new analysis spots until there are three or more analysis spots with a Si count of 300,000 cps or more.

[0030] The silicon nitride sintered body according to the embodiment has a dissolved oxygen content of 0.2 wt% or more as measured by the above method. The dissolved oxygen content measured by TEM-EDS is an average value corresponding to the number of measurement points. In other words, the average dissolved oxygen content of silicon nitride crystal grains present in a 20 μm × 20 μm region of any cross section is 0.2 wt% or more. The term "arbitrary cross-sectional structure" means that, regardless of the amount of dissolved oxygen measured for silicon nitride crystal grains present in a 20 μm × 20 μm region of any cross section, the average value is 0.2 wt% or more.

[0031] As described above, silicon nitride sintered bodies with controlled solute oxygen content in silicon nitride crystal grains exhibit improved durability under high-speed, high-load conditions. Furthermore, when the solute oxygen content is 0.2 wt% or higher, the thermal conductivity is 70 W / m·K or lower, and even 40 W / m·K or lower. Furthermore, it is preferable that the solute oxygen content of all silicon nitride crystal grains in the 20 μm × 20 μm region be within the range of 0.2 wt% to 1.5 wt%. As mentioned above, TEM-EDS methods allow for the analysis spot to be set only on silicon nitride crystal grains. Further performance improvement can be achieved by controlling the solute oxygen content of all silicon nitride crystal grains. When measuring the solute oxygen content of all silicon nitride crystal grains, an analysis spot is set on at least one location on each silicon nitride crystal grain in the 20 μm × 20 μm region. The analysis method is as described above. Setting an analysis spot on each silicon nitride crystal particle and finding that the dissolved oxygen content is 0.2 wt% or more and 1.5 wt% or less indicates that the dissolved oxygen content of each silicon nitride crystal particle is well controlled. Furthermore, if the dissolved oxygen content exceeds 1.5 wt%, the properties of the silicon nitride crystal particle may not be fully utilized. Therefore, the dissolved oxygen content should preferably be 0.2 wt% or more and 1.5 wt% or less, and even more preferably 0.3 wt% or more and 1.3 wt% or less.

[0032] By controlling the amount of dissolved oxygen in silicon nitride crystal grains, the durability of each silicon nitride crystal grain can be improved. Furthermore, the dissolved oxygen may be either substitutional or interstitial. It is preferable that at least a portion of the dissolved oxygen is substitutional. The substitutional type indicates that a portion of the elements constituting the crystal lattice is replaced by a solute element. In other words, the substitutional type is a state in which a portion of the crystal lattice of the silicon nitride crystal grain is replaced by oxygen. The presence of substitutional dissolved oxygen can prevent the silicon nitride crystal lattice from becoming distorted.

[0033] As will be described later, the silicon nitride sintered body according to the embodiment can be used for bearing balls. When the bearing ball slides, a load and frictional heat are generated. Even when a load and frictional heat are generated, distortion of the silicon nitride crystal lattice can be suppressed. Furthermore, when the silicon nitride crystal grains contain dissolved oxygen, the adhesion between the silicon nitride crystal grains and the grain boundary phase can be improved. The adhesion can be improved by forming part of the grain boundary phase through a reaction between the dissolved oxygen in the silicon nitride crystal grains and a sintering aid. This also improves durability when a load and frictional heat are generated.

[0034] In the 20 μm × 20 μm region, the difference between the dissolved oxygen content of silicon nitride crystal particles with a major axis of less than 3 μm and the dissolved oxygen content of silicon nitride crystal particles with a major axis of 3 μm or more is preferably 0.1 wt% or less. In other words, if the average dissolved oxygen content obtained by analyzing only silicon nitride crystal particles with a major axis of less than 3 μm in a 20 μm × 20 μm region is defined as dissolved oxygen content A, and the average dissolved oxygen content obtained by analyzing only silicon nitride crystal particles with a major axis of 3 μm or more is defined as dissolved oxygen content B, then |Solute oxygen content A - Solute oxygen content B| is preferably 0.1 wt% or less. The presence of both small and large silicon nitride crystal particles thus contributes to improved durability and mechanical strength.

[0035] The silicon nitride sintered body 1 preferably contains 1% by mass or more and 20% by mass or less of the grain boundary phase 3. The grain boundary phase 3 is formed by reactions between sintering aids or between the sintering aids and oxygen impurities on the surface of the silicon nitride powder. The grain boundary phase 3 has the effect of firmly bonding silicon nitride crystal grains together and suppressing the generation of pores. Controlling the amount of the grain boundary phase 3 can improve mechanical, electrical, and thermal properties. When the grain boundary phase 3 is less than 1% by mass, the proportion of the grain boundary phase 3 is low. When the grain boundary phase 3 is low, pores are likely to occur. Furthermore, when the grain boundary phase 3 exceeds 20% by mass, the generation of pores can be suppressed, but the mechanical strength is likely to decrease. For this reason, the content of the grain boundary phase 3 is preferably 1% by mass or more and 20% by mass or less, and more preferably 3% by mass or more and 15% by mass or less. Furthermore, by setting the porosity of the silicon nitride sintered body 1 to 2% or less and the pore size to 5 μm or less, the flexural strength of the silicon nitride sintered body 1 can be increased to 700 MPa or more, or even 900 MPa or more. Furthermore, an example of an electrical property is the effect of suppressing static charge. An example of a thermal property is the effect of suppressing thermal expansion. For example, when the silicon nitride sintered body 1 is used as a bearing ball, suppressing static charge can suppress the occurrence of electrolytic corrosion. Furthermore, suppressing thermal expansion can suppress changes in the gap between the inner ring and outer ring when the silicon nitride sintered body 1 is used as a bearing. The pore size is defined as the maximum diameter of the pores visible in a 50 μm x 50 μm SEM photograph. The total area of ​​the pores visible in the 50 μm x 50 μm measurement area is also calculated. This procedure is performed at three random locations, and the average of the total pore area is taken as the porosity (%). When the porosity of the silicon nitride sintered body 1 is 2% or less, the mass of the grain boundary phase 3 may be set to a value obtained by subtracting the total mass of the silicon nitride crystal grains 2 from 100% by mass.

[0036] The grain boundary phase 3 preferably contains one or more elements selected from rare earth elements, aluminum, magnesium, titanium, hafnium, tungsten, molybdenum, and silicon. Examples of rare earth elements include yttrium and lanthanoid elements. For example, rare earth elements include one or more elements selected from yttrium (Y), erbium (Er), ytterbium (Yb), and cerium (Ce). These elements are added as sintering aids. Examples of sintering aids that can be used include one or more elements selected from metal oxides, metal nitrides, metal carbides, and metal sulfides. Other components may be added as sintering aids as long as the content of the grain boundary phase 3 is within the range of 1% by mass to 20% by mass.

[0037] When an arbitrary cross section of silicon nitride sintered body 1 is analyzed by X-ray diffraction (XRD), the strongest peak intensity detected at 42.4±0.3° is defined as I 42.4° The strongest peak intensities detected at 27.1±0.3°, 33.6±0.3°, and 36.1±0.3° corresponding to the β-Si3N4 crystal are respectively defined as I 27.1° , I 33.6° , I 36.1° (I 42.4° ) / (I 27.1° +I 33.6° +I 36.1° ) is preferably 0.005 or more and 0.030 or less.

[0038] XRD analysis is performed using a BRUKER D8 ADVANCE or an instrument with equivalent or better performance. The surface polished to a surface roughness Ra of 1 μm or less is used as the measurement surface for XRD analysis. XRD analysis is performed under the following measurement conditions: Cu target (Cu-Kα), tube voltage 40 kV, tube current 40 mA, scan speed 2.0° / min, slit (RS) 0.15 mm, and scan range (2θ) 10°-60°. The strongest peak is the largest peak within the specified range. The strongest peak intensity is the diffraction intensity at the top of the largest peak. The peak position in XRD analysis is determined by the material and crystalline state of the crystalline phase. The peak ratio corresponds to the proportion of each crystalline phase present.

[0039] I 42.4° is a peak that does not appear in β-Si3N4 crystals. 42.4° is a peak based on the crystalline phase contained in the grain boundary phase. By including a crystalline phase in the grain boundary phase 3 around the silicon nitride crystal grains 2 with a controlled amount of dissolved oxygen, the grain boundaries are strengthened and durability can be improved. Therefore, 0.005≦(I 42.4° ) / (I 27.1° +I 33.6° +I 36.1° It is preferable that I )≦0.030. 42.4° can be controlled by having one or more elements selected from rare earth elements and aluminum present in the grain boundary phase 3. In other words, the presence of a rare earth element-aluminum-oxygen crystalline compound in the grain boundary phase 3 can control I 42.4° It is effective in controlling

[0040] FIG. 7 shows an example of XRD peaks of a silicon nitride sintered body according to an embodiment. Specifically, FIG. 7 shows the results of XRD analysis of a silicon nitride sintered body according to Example 3, which will be described later. FIG. 7 shows the results of only the scanning range of 20° to 50° extracted from the analysis results obtained by XRD analysis using the measurement conditions described above. In FIG. 7, the horizontal axis represents the diffraction angle (2θ), and the vertical axis represents the diffraction intensity. In the example shown, peaks P1 to P4 appear at 27.1±0.3°, 33.6±0.3°, 36.1±0.3°, and 42.4±0.3°, respectively. 27.1° , I 33.6° , I 36.1° , and I 42.4° are the intensities at the apexes of peaks P1 to P4, respectively. (I 42.4° ) / (I 27.1° +I 33.6° +I 36.1°) is 0.008, which is within the range of 0.005 to 0.030. In the example shown in FIG. 7, peaks P5 to P9 also appear. These peaks are due to the grain boundary phase 3 and the like, and can further improve the durability of the silicon nitride sintered body 1. There is no particular limitation on the presence or absence of peaks other than peaks P1 to P4 in the silicon nitride sintered body 1 according to the embodiment.

[0041] The silicon nitride sintered body 1 described above has high strength and excellent wear resistance. Its three-point bending strength can be increased to 700 MPa or more, and even 900 MPa or more. Its fracture toughness can be increased to 6 MPa m 1 / 2 or more, and even 7 MPa·m 1 / 2 It is possible to achieve a Vickers hardness of 1400 or more. The three-point bending strength can be measured using a method conforming to JIS-R-1601 (2008). JIS-R-1601 corresponds to ISO14704. Fracture toughness can be measured using the Niihara formula in accordance with the IF method of JIS-R-1607 (2015). JIS-R-1607 corresponds to ISO15732. Vickers hardness can be measured at HV1 with a test force of 9.807 N in accordance with JIS-R-1610 (2003). Wear resistance is durability at high speeds. JIS-R-1610 corresponds to ISO14705.

[0042] The silicon nitride sintered body 1 according to the embodiment can be used for a wear-resistant member, which is preferably one selected from a bearing ball, a roller, a roller, and a tool member for friction stir welding.

[0043] Fig. 2 is a diagram showing an example of a bearing ball, which is a type of wear-resistant member. Fig. 3 is a diagram showing an example of a bearing incorporating the bearing ball. In Figs. 2 and 3, reference numeral 4 denotes a bearing ball, reference numeral 5 denotes an inner ring, reference numeral 6 denotes an outer ring, and reference numeral 10 denotes a bearing. The bearing balls 4 are formed by processing the silicon nitride sintered body 1 into a spherical shape. The bearing 10 has a structure in which a plurality of bearing balls 4 are incorporated between the inner ring 5 and the outer ring 6. The number of bearing balls 4 used in the bearing 10 is arbitrary.

[0044] The bearing balls 4 are spherical. If necessary, the bearing balls 4 are polished to a surface roughness Ra of 0.1 μm or less. The surface roughness Ra of the bearing balls 4 is specified according to the grade in ASTM F2094 by the American Society for Testing and Materials. Therefore, the bearing balls 4 are polished to a surface roughness according to the grade. Even when the silicon nitride sintered body 1 is applied to a wear-resistant member other than a bearing ball, the surface is polished if necessary. In other words, the wear-resistant member according to the embodiment preferably has a polished surface with a surface roughness Ra of 0.1 μm or less, or even Ra 0.02 μm or less.

[0045] A roller is a cylindrical bearing ball. A roller is a cylindrical member. Rollers are used as conveyor rollers, rolling rollers, etc. The silicon nitride sintered body 1 can also be used as a friction stir welding tool member. A friction stir welding tool member is a member called a probe. For example, International Publication WO2016 / 047376 (Patent Document 4) shows a probe. Bearing balls, rollers, rollers, and friction stir welding tool members all have a sliding surface. These members are wear-resistant members that come into surface contact with a mating member.

[0046] In the bearing 10, a plurality of bearing balls 4 are arranged between an inner ring 5 and an outer ring 6. The bearing 10 is fixed to a rotating shaft (not shown). When the rotating shaft is rotated, a load is applied to the bearing 10. The loads applied to the bearing 10 include a radial load, a thrust load, and a moment load. A radial load is a load that is applied in a direction perpendicular to the rotating shaft (the circumferential direction of the rotating shaft). A thrust load is a load that is applied in a direction parallel to the rotating shaft (the axial direction of the rotating shaft). A moment load is a load that occurs due to eccentricity of the rotating shaft.

[0047] When the bearing 10 rotates, the bearing balls 4 slide against the inner ring 5 and outer ring 6 while making contact with them. In the bearing ball 4 according to the embodiment, the amount of dissolved oxygen in the silicon nitride crystal grains 2 is controlled. This contributes to the bearing ball 4's excellent contact durability. High-speed rotation increases radial and thrust loads. Bearing steel, such as SUJ2, may also be used for the inner ring 5 and outer ring 6. Conventionally, bearing steel has also been used for bearing balls. The bearing ball 4 according to the embodiment is made of silicon nitride. The specific gravity of silicon nitride is smaller than that of steel, reducing the aggressiveness of the bearing balls 4 toward the inner ring 5 and outer ring 6. This means that the wear of the sliding surfaces of the raceways due to the sliding of the bearing balls 4 can be suppressed. Wear on the sliding surfaces causes eccentricity of the rotating shaft. The reduced aggressiveness reduces eccentricity of the rotating shaft. Therefore, the embodiment also effectively suppresses an increase in moment load. For example, the higher the rotation speed, the greater the difference in centrifugal force. Furthermore, according to the embodiment, there is also an effect of suppressing temperature rise due to rotational friction.

[0048] Each load is affected by the weight, or volume, of the bearing ball. For small bearing balls with a diameter of 3 mm or less, an increase in rotational speed has little effect on the load. On the other hand, for bearing balls with a diameter of 5 / 16 inches (7.9375 mm) or more, the effect on the load becomes significant. The bearing 10 includes a plurality of bearing balls 4. According to the embodiment, the durability of the bearing balls 4 can be improved. Furthermore, aggressiveness toward mating members can be reduced. The aggressiveness of each bearing ball 4 toward mating members is reduced. Therefore, the durability of the bearing 10 using the bearing balls 4 according to the embodiment can be improved. Furthermore, bearings installed in automobiles, machine tools, and the like are used in vibrating environments. In vibrating environments, aggressiveness toward mating members increases. The bearing 10 using the bearing balls 4 according to the embodiment exhibits excellent durability even when used in a vibrating environment.

[0049] Next, a method for manufacturing the silicon nitride sintered body 1 according to the embodiment will be described. The method for manufacturing the silicon nitride sintered body 1 according to the embodiment is not particularly limited as long as it has the above-mentioned configuration. Here, a method for obtaining the silicon nitride sintered body 1 with a good yield will be described.

[0050] First, silicon nitride powder is prepared. The silicon nitride powder preferably has an average particle size of 2.5 μm or less and an impurity oxygen content of 2 mass% or less. If the impurity oxygen content exceeds 2 mass%, it may be difficult to control the amount of dissolved oxygen. It is also preferable to use silicon nitride powder with an alpha conversion rate of 90% or more. The alpha silicon nitride powder undergoes grain growth into beta silicon nitride crystal particles during the sintering process. The beta silicon nitride crystal particles tend to become elongated particles with an aspect ratio of 1.5 or more. The complex distribution of elongated particles can improve the mechanical properties of the silicon nitride sintered body 1.

[0051] Silicon nitride powders are produced by methods such as imide decomposition and direct nitridation. Silicon nitride powder produced by the imide decomposition method is sometimes called imide powder. Silicon nitride powder produced by the direct nitridation method is sometimes called direct nitride powder. Imide powder has a low amount of oxygen and a low amount of impurities within the particles. Direct nitride powder has a higher amount of oxygen and impurities within the particles than imide powder. Either imide powder or direct nitride powder can be used as long as the impurity oxygen content is 2 mass% or less.

[0052] Next, a sintering aid powder is prepared. The sintering aid powder is preferably a metal compound powder of one or more metals selected from rare earth elements, aluminum, magnesium, titanium, hafnium, tungsten, molybdenum, and silicon. The metal compound powder is one or more metals selected from metal oxides, metal nitrides, metal carbides, and metal sulfides.

[0053] The rare earth compound powder and the aluminum compound powder can react with each other to form the grain boundary phase 3. The rare earth element is preferably one or more selected from yttrium (Y), erbium (Er), ytterbium (Yb), and cerium (Ce). Adding the rare earth element as a rare earth oxide facilitates the reaction between the rare earth element and the aluminum compound. The aluminum compound is preferably one or two selected from aluminum oxide, aluminum nitride, and MgO·Al2O3 spinel. These can react with the rare earth oxide to form rare earth aluminum oxide, rare earth aluminum nitride, or rare earth aluminum oxynitride. Rare earth aluminum oxide, rare earth aluminum nitride, and rare earth aluminum oxynitride are collectively referred to as rare earth aluminum-based compounds. The rare earth aluminum-based compounds may contain other elements, such as magnesium, hafnium, and silicon.

[0054] One or more elements selected from titanium, hafnium, tungsten, molybdenum, and silicon have the effect of strengthening the grain boundary phase. For example, titanium oxide (TiO2) becomes titanium nitride (TiN) through a sintering process. Titanium nitride particles serve as strengthening particles that strengthen the grain boundary phase. In addition, molybdenum nitride, carbide, and sulfide also serve as strengthening particles. Silicon carbide also serves as strengthening particles. By dispersing strengthening particles in the grain boundary phase 3 made of a rare earth aluminum compound, the wear resistance can be further improved.

[0055] When the total amount of silicon nitride powder and sintering aid powder is 100 parts by mass, the amount of sintering aid powder is preferably in the range of 1 part by mass to 20 parts by mass. The grain boundary phase 3 is formed by a reaction between sintering aids or a reaction between silicon nitride and a sintering aid. The mass ratio of the grain boundary phase 3 can be controlled by controlling the amount of sintering aid powder added.

[0056] Next, the silicon nitride powder and sintering aid powder are mixed. To control the long diameter and aspect ratio of the silicon nitride crystal grains in the silicon nitride sintered body, uniform sinterability is necessary. To achieve this, the silicon nitride powder and sintering aid powder must be mixed uniformly. During the sintering process, the sintering aid reacts to form grain boundary phase 3. The growth reaction of silicon nitride crystal grains 2 progresses through this grain boundary phase 3. By uniformly mixing the silicon nitride powder and sintering aid powder, the reaction via grain boundary phase 3 can be made uniform. In other words, uniform sinterability refers to the uniformity of the reaction in which the sintering aid forms the grain boundary phase, and the uniformity of the grain growth of the silicon nitride crystal grains.

[0057] In addition, ball mills and bead mills are used in the mixing process. Silicon nitride powder and sintering aid powder often exist as agglomerated secondary particles. These secondary particles hinder uniform sintering. By uniformly mixing the secondary particles while crushing them into non-agglomerated primary particles, the uniformity of sintering can be improved.

[0058] In this mixing process involving crushing, it is preferable not to apply strong stress that would further destroy the primary particles. When the primary particles are destroyed, a fracture surface is formed in the silicon nitride powder. Because the fracture surface is an active surface, a surface different from the original reactivity of the primary particles is formed. This hinders uniform sintering. Therefore, it is effective to prevent the formation of fracture surfaces in the primary particles. By preventing the formation of fracture surfaces, the reaction between the silicon nitride powder and oxygen can be suppressed. This allows the amount of dissolved oxygen in the formed silicon nitride crystal particles to be controlled within the range of 0.2 wt% to 1.5 wt%.

[0059] Wet crushing using a solvent is suitable for crushing the secondary particles. A solvent with high wettability to the particle surface and low reactivity with the particles is used. This method reduces the stress required for crushing and prevents the destruction of primary particles. Organic solvents are suitable for mixing silicon nitride powder and sintering aid powder with crushing. Suitable organic solvents include alcohols, ketones, and aromatics. A mixture of two or more selected from alcohols, ketones, and aromatics may also be used. Alcohols are a general term for substances in which some of the hydrogen atoms in hydrocarbons are replaced with hydroxyl groups (OH groups). Ketones are substances represented by the formula RC(=O)-R'. R and R' are alkyl groups, etc. Aromatics are organic compounds containing a benzene ring. These organic solvents have high wettability with silicon nitride powder and sintering aid powder. Furthermore, they have low reactivity with the organic solvent. This allows the silicon nitride powder and sintering aid powder to be mixed uniformly.

[0060] When the crushing process is performed using a ball mill, the diameter of the media is preferably 20 mm or less, and even 12 mm or less. The media is ceramic balls. Ball milling is a method in which powder and media are placed in a cylindrical container and crushed while the cylindrical container is rotated. As mentioned above, the ball milling process using an organic solvent is a wet crushing and mixing process. By selecting an organic solvent suitable for the powder, the stress required for crushing can be reduced. In other words, secondary particles can be crushed using ceramic balls with a small diameter, which has the effect of reducing the energy with which the media impacts the powder. By reducing the energy with which the media impacts the powder, the formation of fracture surfaces on the primary particles can be suppressed. Note that the minimum diameter of the media is preferably 3 mm or more. If the media is too small, work efficiency may be reduced.

[0061] The time for wet disintegration and mixing using a ball mill is preferably within the range of 5 hours to 40 hours. If the wet disintegration and mixing time is less than 5 hours, the disintegration effect may be insufficient, and a large amount of secondary particles may remain. If the wet disintegration and mixing time is longer than 40 hours, there is a greater possibility that fracture surfaces may form on the primary particles. For this reason, the time for wet disintegration and mixing using a ball mill is preferably within the range of 5 hours to 40 hours, and more preferably within the range of 10 hours to 30 hours. Furthermore, in the ball mill process, the rotation speed of the cylindrical container is preferably within the range of 30 rpm to 500 rpm. More preferably, the rotation speed is within the range of 50 rpm to 180 rpm.

[0062] The fact that secondary particles have been crushed can be determined by examining the particle size distribution before and after crushing. As the secondary particles are crushed and the number of primary particles increases, the peak position of the particle size distribution (frequency distribution) shifts to smaller particle sizes. The peak of the particle size distribution also becomes sharper.

[0063] The suppression of the formation of fracture surfaces on the primary particles can be confirmed by measuring the amount of oxygen before and after crushing. The amount of oxygen before crushing is the amount of oxygen in the raw material powder. As long as the amount of oxygen in the raw material powder after crushing does not increase significantly compared to before crushing, there is no problem.

[0064] When the secondary particles are crushed into primary particles, a step of adsorbing a silane coupling agent onto the surface of the primary particles may be carried out. Adsorbing a silane coupling agent onto the surface of the primary particles improves dispersibility in the solvent and is effective for uniform mixing. Adsorbing a silane coupling agent forms an oxide film on the surface of the primary particles. This makes it possible to more precisely control the amount of oxygen on the surface of each primary particle. As a result, the uniformity of sintering can be improved.

[0065] Silicon nitride powder and sintering aid powder are mixed, and a solvent, binder, etc. are added to obtain a raw powder slurry. The obtained raw material slurry is then used to carry out a molding process. In the molding process, granulation is carried out as necessary to prepare a molded body. Examples of molding processes include die pressing and cold isostatic pressing (CIP). The molding pressure is preferably 100 MPa or more.

[0066] Next, a degreasing step is performed to degrease the compact. The degreasing step is preferably performed within a temperature range of 300°C to 700°C. The degreasing step is performed in the air or a non-oxidizing atmosphere. The atmosphere for the degreasing step is not particularly limited.

[0067] Next, a sintering step is carried out to sinter the degreased body. The sintering step is preferably carried out within a temperature range of 1600°C to 1900°C. The sintering step may be either atmospheric sintering or pressure sintering. The sintering step is preferably carried out in a non-oxidizing atmosphere. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere and an argon atmosphere.

[0068] During the sintering process, pressure is preferably applied from 1500°C while the temperature is rising to a range of 1600°C to 1900°C. At temperatures below 1500°C, atmospheric pressure (0.1 MPa) is used. At temperatures above 1500°C, the pressure is preferably 0.2 MPa or higher. Furthermore, it is desirable to control the heating rate from 1500°C to the sintering temperature within a range of 20°C / hr to 100°C / hr. Around 1500°C, α-type silicon nitride powder transforms to β-type, causing grain growth. During the β-type transformation, oxygen within the silicon nitride crystal particles is easily released. Applying pressure can prevent excessive oxygen release from the silicon nitride crystal particles. While there is no particular upper limit to the pressure, a pressure of 10 MPa or lower is preferred. Controlling the heating rate is also effective in homogenizing the degree of grain growth.

[0069] If necessary, the obtained sintered body is subjected to a hot isostatic pressing (HIP) process, which is preferably carried out at a temperature in the range of 1500° C. to 1900° C. In the HIP process, it is preferable to apply a pressure of 30 MPa or more in a non-oxidizing atmosphere.

[0070] The silicon nitride sintered body 1 according to the embodiment can be produced by the above steps. A wear-resistant member can be obtained by polishing the sliding surface of the silicon nitride sintered body 1. The surface roughness Ra of the sliding surface is preferably 1 μm or less.

[0071] (Example) (Examples 1 to 5, Comparative Examples 1 and 2) Silicon nitride powder with an average particle size of 2.5 μm or less, an impurity oxygen content of 2 mass% or less, and an alpha conversion rate of 90% or more was prepared. Next, a sintering aid was prepared. The components of the sintering aid, the proportion of silicon nitride powder, and the proportion of sintering aid are as shown in Table 1.

[0072] [Table 1]

[0073] Next, a mixing step was carried out using a ball mill to mix the silicon nitride powder and the sintering aid. In the mixing step using the ball mill, one or more organic solvents selected from alcohols, ketones, and aromatics were used. The media diameter, rotation speed, and mixing time are as shown in Table 2. In Examples 3 and 5, a silane coupling agent was added during the mixing step.

[0074] [Table 2]

[0075] A raw material slurry was prepared by a mixing process using a ball mill. Next, the obtained raw material slurry was spray granulated. The granulated powder was used to prepare a compact by die press molding. Furthermore, a compact was prepared by cold isostatic pressing. The molding pressure was set to 150 MPa.

[0076] Next, the compact was degreased. The degreasing process was carried out in a non-oxidizing atmosphere at 300°C or higher and 700°C or lower. The obtained degreased body was then subjected to a sintering process. In the sintering process, the pressure and temperature rise rate from 1500°C to the sintering temperature were as shown in Table 3. The sintering process was carried out in a non-oxidizing atmosphere.

[0077] [Table 3]

[0078] The silicon nitride sintered body thus obtained was subjected to HIP treatment at a temperature of 1600° C. to 1800° C. under a pressure of 30 MPa to 150 MPa.

[0079] The silicon nitride sintered bodies according to the examples and comparative examples were manufactured by the above sintering process. The amount of dissolved oxygen in the silicon nitride crystal grains contained in the silicon nitride sintered bodies, the average major axis length of the silicon nitride crystal grains, the average aspect ratio, and the XRD peak were measured.

[0080] The dissolved oxygen content of silicon nitride crystal particles was measured in a 20 μm × 20 μm region of any cross section. TEM-EDS was used to measure the dissolved oxygen content of each particle. The average value of the measured dissolved oxygen content was calculated. The TEM-EDS measurement conditions were as described above. The average dissolved oxygen content of silicon nitride crystal particles with a major axis of less than 3 μm was defined as (A), and the average dissolved oxygen content of silicon nitride crystal particles with a major axis of 3 μm or more was defined as (B), and the difference between them was calculated as |(A) - (B)|.

[0081] Furthermore, the measurement area for the average major axis length and average aspect ratio of silicon nitride crystal grains was a 50 μm × 50 μm region of any cross section. The average major axis length and average aspect ratio were measured using SEM photographs. The method using SEM photographs was as described above. Furthermore, the intensity ratio (I 42.4° ) / (I 27.1° +I 33.6° +I 36.1° The XRD analysis conditions were as described above.

[0082] 4 to 6 show the first, second, and third plots, respectively, obtained when measuring the silicon nitride sintered body according to Example 3. Also, FIG. 7 shows an example of the XRD peaks of the silicon nitride sintered body according to Example 3.

[0083] For each of Examples 1 to 5 and Comparative Examples 1 and 2, the average dissolved oxygen content (wt%), the range of each dissolved oxygen content, the average value (A), the average value (B), and the difference |(A)-(B)| are shown in Table 4. The average major axis, the average aspect ratio, the maximum major axis, and the peak intensity ratio are shown in Table 5.

[0084] [Table 4]

[0085] [Table 5]

[0086] As can be seen from Tables 4 and 5, the silicon nitride substrates according to the examples had a dissolved oxygen content of 0.2 wt% or more. In contrast, the dissolved oxygen content of Comparative Example 1 was low, less than 0.2 wt%. The dissolved oxygen content of Comparative Example 2 was also 0.2 wt% or more. However, the average major axis was outside the range of 0.1 μm or more and 10 μm or less, and the average aspect ratio was outside the range of 1.5 or more and 10 or less.

[0087] Next, the three-point bending strength, fracture toughness, and Vickers hardness of each silicon nitride sintered body were measured. Three-point bending strength was measured according to JIS-R-1601 (2008). Fracture toughness was measured using the Niihara formula in accordance with the IF method of JIS-R-1607 (2015). Vickers hardness was measured according to JIS-R-1610 (2003) at a test force of 9.807 N (HV1). The measurement results are shown in Table 6.

[0088] [Table 6]

[0089] In both the examples and comparative examples, the three-point bending strength was 700 MPa or more and the fracture toughness was 6 MPa m 1 / 2 The Vickers hardness HV1 was 1400 or more. Furthermore, when the thermal conductivity was measured, it was 40 W / m K or less in all cases.

[0090] Next, a wear resistance test was conducted. Bearing balls with a diameter of 3 / 8 inch (9.525 mm) were used as samples. The bearing balls were prepared by polishing them to a surface roughness of Ra 0.01 μm. A thrust-type rolling wear tester was used for the wear resistance test. A plate-shaped member made of bearing steel SUJ2 was prepared. The bearing balls were placed on the plate-shaped member, and the wear resistance test was conducted under the test conditions listed in Table 7.

[0091] [Table 7]

[0092] Each test was conducted with three bearing balls arranged. Nine bearing balls each according to the example and comparative example were tested. If no cracks or chips occurred on the surface of any of the bearing balls, the test result was rated as "OK." If cracks or chips occurred on the surface of even one bearing ball, the test result was rated as "NG." The results are shown in Table 8.

[0093] [Table 8]

[0094] Furthermore, a sample with a diameter of 1-3 / 16 inches (30.16 mm) was prepared and subjected to the same abrasion resistance test. The results are shown in Table 9.

[0095] [Table 9]

[0096] As can be seen from Tables 8 and 9, the bearing balls according to the Examples and Comparative Examples exhibited equivalent performance under Test Condition 1. Furthermore, the bearing balls according to the Examples also exhibited excellent properties under Test Condition 2. In particular, the bearing balls according to the Examples exhibited excellent durability, even for large balls such as 1-3 / 16 inches. In contrast, the performance of the bearing balls according to the Comparative Examples deteriorated under Test Condition 2. This demonstrates that controlling the amount of dissolved oxygen, etc., is an effective method for improving durability against load.

[0097] Embodiments of the present invention include the following configurations. (Appendix 1) A silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase, In an area of ​​20 μm×20 μm on any cross section, the average amount of dissolved oxygen in the silicon nitride crystal grains is 0.2 wt % or more, A silicon nitride sintered body, wherein in an area of ​​50 μm×50 μm in any cross section, the average major axis of the silicon nitride crystal grains is 0.1 μm or more and 10 μm or less, and the average aspect ratio of the silicon nitride crystal grains is 1.5 or more and 10 or less. (Appendix 2) 2. The silicon nitride sintered body according to claim 1, wherein the amount of dissolved oxygen in each silicon nitride crystal grain present in said 20 μm×20 μm region is 0.2 wt % or more and 1.5 wt % or less. (Appendix 3) 3. The silicon nitride sintered body according to claim 1, wherein, in the 20 μm × 20 μm region, the difference between the average dissolved oxygen content of the silicon nitride crystal grains whose major axis is less than 3 μm and the average dissolved oxygen content of the silicon nitride crystal grains whose major axis is 3 μm or more is 0.1 wt % or less. (Appendix 4) 4. The silicon nitride sintered body according to any one of claims 1 to 3, containing 1% by mass or more and 20% by mass or less of the grain boundary phase. (Appendix 5) 5. The silicon nitride sintered body according to any one of claims 1 to 4, wherein the silicon nitride crystal grains have a maximum major axis of 25 μm or less in a 300 μm×300 μm region of any cross section. (Appendix 6) When an arbitrary cross section is analyzed by XRD, the strongest peak intensity detected at 42.4±0.3° is defined as I 42.4° The strongest peak intensities detected at 27.1±0.3°, 33.6±0.3°, and 36.1±0.3° corresponding to the β-Si3N4 crystal were defined as I 27 .1°, I 33 .6°, I 36 .1°, (I 42 .4°) / (I 27 .1°+I 33 .6°+I 36 A silicon nitride sintered body described in any one of Appendix 1 to Appendix 5, in which the value of (.1°) is 0.005 or more and 0.030 or less. (Appendix 7) Fracture toughness value is 6 MPa m 1 / 2 The silicon nitride sintered body according to any one of Supplementary Notes 1 to 6, wherein: (Appendix 8) A wear-resistant member using the silicon nitride sintered body according to any one of claims 1 to 7. (Appendix 9) 9. The wear-resistant member according to claim 8, which is one selected from a bearing ball, a roller, a roller, and a tool member for friction stir welding.

[0098] 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. [Explanation of symbols]

[0099] 1...Silicon nitride sintered body 2...Silicon nitride crystal particles 3...Grain boundary phase 4...Bearing ball 5…Inner circle 6...Outer ring 10...Bearing

Claims

1. a step of adding the sintering aid powder to the silicon nitride powder in an amount of 1 part by mass to 20 parts by mass, where the total amount of the silicon nitride powder and the sintering aid powder in the raw material powder is 100 parts by mass; a mixing step of wet-disintegrating and mixing the raw material powder using a ball mill; a molding step of molding the mixture; a degreasing step of degreasing the compact; a sintering step of sintering the degreased compact; A method for producing a silicon nitride sintered body comprising: The silicon nitride powder has an alpha conversion rate of 90% or more and an impurity oxygen content of 2% by mass or less, In the mixing step, the raw material powder is mixed in the ball mill using media having a diameter of 3 mm or more and 20 mm or less, at a rotation speed of a cylindrical rotating container containing the raw material powder of 30 rpm or more and 500 rpm or less, and for a mixing time of 5 hours or more and 40 hours or less; In the sintering step, the sintering temperature is in the range of 1600°C or more and 1900°C or less, and during the temperature rise from 1500°C to the sintering temperature, the compact is pressurized in the range of 0.2 MPa or more and 10 MPa or less, and the temperature rise rate is controlled in the range of 20°C / hr or more and 100°C / hr or less, The obtained silicon nitride sintered body is a silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase, the average amount of dissolved oxygen in the silicon nitride crystal grains is 0.2 wt % or more and 1.5 wt % or less in a 20 μm × 20 μm region of any cross section, In a 50 μm × 50 μm region of any cross section, the silicon nitride crystal grains have an average major axis of 0.1 μm or more and 10 μm or less, and an average aspect ratio of 1.5 or more and 10 or less, Fracture toughness value is 6 MPa m １／２ This is the method for producing a silicon nitride sintered body.

2. 2. The method for producing a silicon nitride sintered body according to claim 1, wherein the amount of dissolved oxygen in each of the silicon nitride crystal grains present in the 20 μm×20 μm region of the obtained silicon nitride sintered body is 0.2 wt % or more and 1.5 wt % or less.

3. 3. The method for producing a silicon nitride sintered body according to claim 1, wherein the maximum value of the major axis of the silicon nitride crystal grains in the obtained silicon nitride sintered body in an area of ​​300 μm × 300 μm on any cross section is 25 μm or less.

4. 3. The method for producing a silicon nitride sintered body according to claim 1, further comprising a HIP step of subjecting the silicon nitride sintered body to hot isostatic pressing (HIP) at a temperature in the range of 1500°C to 1900°C after the sintering step.

5. 4. The method for producing a silicon nitride sintered body according to claim 3, further comprising a HIP step of subjecting the silicon nitride sintered body to hot isostatic pressing (HIP) at a temperature in the range of 1500°C to 1900°C after the sintering step.

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