Sintered body, bearing ball, and bearing

Abstract

To provide a sintered compact which has excellent strength and in which the generation of snowflakes is suppressed, and to provide a bearing ball and a bearing using the same.SOLUTION: A sintered body comprising: β SiAlON as a main phase; and a dispersed phase comprising carbon or a carbide of at least one selected from the group consisting of Ti, Zr, and W, wherein an average particle size of the dispersed phase is 0.05 to 0.50 μm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a sintered body, a bearing ball, and a bearing. [Background technology]

[0002] Silicon nitride sintered bodies have excellent mechanical strength and wear resistance, and are therefore used in wear-resistant components, gas turbine blades, engine parts, etc. Silicon nitride sintered bodies used in bearing components, which require particularly high wear resistance, are produced by, for example, adding YO, AlO, etc. as sintering aids to silicon nitride as a raw material, and then firing the resulting material to form a liquid phase, thereby increasing the density and strength of the sintered body. To obtain such dense sintered bodies, sintering aids containing rare earth elements such as YO are used.

[0003] Another method is to add AlN as a sintering aid to form a silicon nitride solid solution called SiAlON, which improves wear resistance. However, because AlN has low water resistance, this method requires the use of an organic solvent instead of water to prepare the slurry and granulate it, which increases costs and produces a small amount of SiAlON.

[0004] Therefore, it has been proposed to use β-SiAlON synthesized by combustion synthesis as a raw material and sinter it. For example, Patent Document 1 discloses a silicon nitride sintered body obtained by adding aluminum oxide and yttrium oxide as sintering aids to β-SiAlON powder synthesized by combustion synthesis and then performing a single-stage sintering process, and the maximum pore size, contact area ratio, and average grain size of the crystal grains are within specific ranges. Patent Document 1 states that the resulting silicon nitride sintered body has small grain size and is almost free of defects that could serve as fracture bases, and is produced by a single heat treatment. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-12985 Summary of the Invention [Problem to be solved by the invention]

[0006] Since sintered bodies are used for applications such as bearing balls, there is a demand for improved strength. Furthermore, there is a need to suppress the occurrence of white spot defects called snowflakes in silicon nitride sintered bodies. Snowflakes appear as irregular white spots near the periphery of sintered bodies and can be confirmed in dark-field (DF) images using an optical microscope. Therefore, an object of one embodiment of the present disclosure is to provide a sintered body that is excellent in strength and suppresses the generation of snowflakes, a bearing ball, and a bearing using the same. [Means for solving the problem]

[0007] The specific means for achieving the above object are as follows: <1> A sintered body having a βSiAlON as a main phase and containing a dispersed phase containing carbon or at least one carbide selected from the group consisting of Ti, Zr and W, wherein the average particle size of the dispersed phase is 0.03 to 0.50 μm. <2> The proportion of the dispersed phase when measured with a scanning electron microscope is 0.1 to 3.0 area %. <1> The sintered body according to claim 1. <3> The dispersed phase comprises carbon or TiC. <1> or <2> The sintered body according to claim 1. <4> The dispersed phase contains at least one carbide selected from the group consisting of Ti, Zr, and W. <1> or <2> The sintered body according to claim 1. <5> The base ball for bearing balls, <1> ~ <4> The sintered body according to any one of the above items. <6> <1> ~ <5> A bearing ball comprising a mirror-finished sintered body according to any one of claims 1 to 4. <7> <6> A bearing comprising the bearing ball according to claim 1. <8> It is for electric vehicles, <7> The bearing described in [Effects of the Invention]

[0008] According to one embodiment of the present disclosure, a sintered body having excellent strength and suppressed generation of snowflakes, a bearing ball, and a bearing using the same are provided. [Brief explanation of the drawings]

[0009] [Figure 1] (A) and (B) are images (dark field) of the cross section of the sintered body observed with an optical microscope at 10x magnification, and are diagrams for explaining snowflakes. [Figure 2] 1 is an example of an XRD measurement spectrum of a sintered body. [Figure 3] (A) and (B) are examples of Raman spectroscopy spectra of the sintered body. [Figure 4] (A) and (B) are images of a cross section of a sintered body that has been mirror-finished and observed with a scanning electron microscope at 25,000 magnifications, and are used to explain the solid solution state. [Figure 5] FIG. 2 is a schematic diagram of a molded product having protrusions when a raw material composition is molded into a spherical shape using a die. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described in detail, but the present disclosure is not limited to the following embodiments. In this disclosure, the use of "to" to indicate a range of values ​​means that the values ​​before and after it are included as the lower and upper limits. Unless otherwise specified, "to" will be used in the following disclosure in the same sense. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. When describing embodiments with reference to the drawings in this disclosure, the configuration of the embodiment is not limited to the configuration shown in the drawings. Furthermore, the sizes of components in each drawing are conceptual, and the relative size relationships between components are not limited to these.

[0011] In this disclosure, the major axis of a snowflake is determined by observation and measurement under a dark field (DF) of an optical microscope. Specifically, the cross section of the sintered body is polished to a mirror finish, and an arbitrary unit area of ​​5 × 5 mm on the polished cross section is magnified 10 to 20 times under a dark field (DF) to confirm the location of defects. The major axis of the snowflake is measured by magnifying the defects 100 to 200 times. The longest diameter of a single snowflake is measured. Repeat the above magnification, observation, and measurement procedures to check whether there are snowflakes with a major axis of 50 μm or more or 25 μm or more in a unit area of ​​5 mm × 5 mm. If the sample size is less than 5 mm × 5 mm, observe multiple samples so that the total unit area is 5 mm × 5 mm. "Snowflakes" are white spot-like defects that can be confirmed, for example, in dark-field (DF) images taken with an optical microscope, as shown in Figures 1(A) and 1(B). In the sintered compact shown in Figure 1(B), snowflakes appear as amorphous white spots near the periphery of the sintered compact. Snowflakes are observed at any location on a sample whose cross section has been mirror-finished, and in the surface region within 250 μm inward from the outer surface, which corresponds to the periphery of the sintered compact.

[0012] In the present disclosure, the three-point bending strength is measured in accordance with JIS R 1601:2008, by preparing a test piece of 3 mm × 4 mm × 40 mm, under the conditions of a span (distance between supports) of 30 mm and a load application rate of 0.5 mm / min. The three-point bending strength value is the average value when 10 test pieces are measured.

[0013] In the present disclosure, fracture toughness is measured by the indentation method (IF method) specified in JIS R 1607: 1995 and calculated by the formula of Niihara et al. For a spherical sintered body, measurements are taken at 10 points in total, including 5 arbitrary points near the center and 5 arbitrary points near the periphery, and the average value is used as the fracture toughness value.

[0014] In the present disclosure, measurements by X-ray diffraction (XRD) are performed using an X-ray diffractometer (for example, Rigaku Corporation's "Smart lab," detector "D / teXUltra," and X-ray analysis software "PDXL2") under the following conditions. Output: 45kV-200mA, Scanning range: 10°-60°, Optical system: Concentration method, Incident Soller slit: 5°, Length limiting slit: 10mm, Receiving slit 1: 8mm, Receiving slit 2: 13mm, Receiving parallel slit: 5.0°, Attenuator open, Scanning speed: 5° / min, Step width: 0.005°

[0015] In this disclosure, the main phase, liquid phase, and dispersed phase can be identified by observing the plasma-etched measurement sample in a backscattered electron image using a scanning electron microscope (SEM). During plasma etching, the etching rates of the main phase and the liquid phase differ, resulting in the removal of more of either the main phase or the liquid phase. As a result, the main phase and the liquid phase can be identified in the backscattered electron image using the SEM. In the SEM, the liquid phase appears brighter than the main phase, and the dispersed phase appears even brighter than the liquid phase. In the present disclosure, confirmation that the dispersed phase contains carbon or at least one carbide selected from the group consisting of Ti, Zr, and W is carried out by combining a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDS) with Raman spectroscopy.

[0016] In this disclosure, the average particle size of the dispersed phase is measured by observing the cross section of the sintered body using SEM-EDS under conditions such as an acceleration voltage of 15 kV and a probe current of 10 μA. The cross section to be observed is electrically conductively coated, assuming that EDS analysis will be performed. The conductive coating is preferably made of a material other than carbon, such as gold or platinum. The cross section of the sintered body is mirror-polished and then observed at a magnification of 2000x. The obtained images are analyzed using image processing software (e.g., ImageJ). The particle size of each dispersed phase is defined as the diameter (equivalent circle diameter) of each particle calculated using the image processing software, assuming that the area of ​​each particle is a circle. Three fields of view are observed using the SEM at 2000x magnification: one field including the center of the cross section, one field including the periphery, where the area ratio of the sintered body in the field of view is 90% or more, and another field including the periphery diagonally extending from the first two fields of view, where the area ratio of the sintered body in the field of view is 90% or more. The diameters of the dispersed phase are analyzed, and the arithmetic mean value is taken as the average particle size.

[0017] In the present disclosure, the proportion (area %) of the dispersed phase in the sintered body is calculated by observing the cross section of the sintered body under the same observation conditions and pretreatment as above with an SEM at 2000x magnification in the above three fields of view, and calculating the area area occupancy of the dispersed phase.

[0018] In the present disclosure, elemental analysis of the sintered body is carried out using an electron probe microanalyzer (EPMA) (for example, "JXA-8500F" manufactured by JEOL Ltd.) and a standard sample under the following conditions: The surface of the measurement sample is carbon-coated (30 nm, 10 flashes) before being subjected to analysis. Acceleration voltage: 15 keV, probe current: 30 nA, beam diameter: 30 μm

[0019] In the present disclosure, the contents of Mg, Ca, and Y in the liquid phase of the sintered body are determined by energy dispersive X-ray spectroscopy (EDS) using an EDS analyzer (for example, Noran system 6 manufactured by Thermo Fisher Scientific, detector: Ultradry manufactured by Thermo Fisher Scientific) under the following conditions. The line analysis thickness is 1, the number of points is 100, and the point acquisition interval is 50 nm. The acquired net count is converted into a moving average of three points, including one before and one after. The converted data is normalized so that the maximum value is 1 and the minimum value is 0. The point where Si and O intersect is defined as the phase boundary, and measurements are taken at five points within 150 nm of the boundary toward the liquid phase, and the average value of the obtained values ​​is calculated. In the present disclosure, the contents of Mg, Ca, and Y in the matrix phase of the sintered body are determined in the same manner as the contents of Mg, Ca, and Y in the liquid phase, except that the average values ​​obtained at points within 150 nm from the boundary toward the matrix are calculated.

[0020] In the present disclosure, Raman spectroscopy is performed using a Raman spectrometer (for example, LabRAM HR Evolution manufactured by Horiba, Ltd.) under the following conditions. Measurement conditions: exposure time 1 second, 10 times accumulation, 100% neutral density filter (power 30 mW), 15 μm confocal hole, laser wavelength: 532 nm, 1200 grating lines, 100x objective lens (NA = 0.6)

[0021] <Sintered body> The sintered body of the present disclosure has a βSiAlON as a main phase and a dispersed phase containing carbon or at least one carbide selected from the group consisting of Ti, Zr, and W, and the dispersed phase has an average particle size of 0.05 to 0.50 μm. Hereinafter, the at least one carbide selected from the group consisting of Ti, Zr, and W is also referred to as a specific carbide. The sintered body having the above-described structure has excellent fracture toughness and suppresses the generation of snow flakes. The reason for this is not clear, but is presumed to be as follows.

[0022] The dispersed phase is thought to have the function of increasing wettability or lubrication at the boundary between the main phase and the liquid phase, and the presence of a dispersed phase with an average particle size of 0.05 μm or more is thought to suppress the generation of snowflakes. In particular, it has been experimentally found that the generation of snowflakes is effectively suppressed when the dispersed phase contains carbon or a specific carbon. Furthermore, by setting the average particle size of the dispersed phase to 0.50 μm or less, it is believed that carbon or specific carbides are prevented from becoming fracture origins, thereby increasing strength.

[0023] Furthermore, since snowflakes have a lower density than the surrounding area, sintered bodies containing snowflakes require hot isostatic pressing (HIP) to fill defects, which increases the manufacturing cost. Furthermore, since such defects tend to occur near the surface of the sintered body, grinding work near the surface is required, which increases the manufacturing cost.

[0024] The sintered body of the present disclosure has βSiAlON as its main phase. SiAlON has a structure in which O partially substitutes for N in silicon nitride (Si3N4), and Al partially substitutes for Si. SiAlON exists as αSiAlON, which exhibits the α phase, and βSiAlON, which exhibits the β phase. αSiAlON is considered to have a structure in which a metal atom M (M = Li, Mg, Ca, Y, La, etc.) is present inside the crystal lattice. βSiAlON does not have a metal atom M inside the crystal lattice.

[0025] In the present disclosure, when the β fraction of SiAlON is 50% or more, it is considered that "βSiAlON is the main phase." The β fraction of SiAlON is preferably 60% or more. The β fraction is 100% or less, preferably 95% or less, preferably 90% or less, preferably 85% or less, preferably 80% or less, preferably 75% or less, and preferably 70% or less.

[0026] The β ratio is the ratio of the β phase to the total amount of the α phase and the β phase, calculated from the heights of the peaks corresponding to the α phase (210), α phase (201), β phase (101), and β phase (120) in an X-ray diffraction spectrum. The ratio of the α phase to the total amount of the α phase and the β phase: α phase / (α phase + β phase) × 100 is also called the α ratio (%).

[0027] In this disclosure, the main peaks representing the α phase in the X-ray diffraction spectrum are those corresponding to the α phase (210) and α phase (201), and the main peaks representing the β phase are those corresponding to the β phase (101) and β phase (120). In the X-ray diffraction spectrum, the peak corresponding to the α phase (210) appears at 2θ = 30.5 to 32°, the peak corresponding to the α phase (201) appears at 2θ = 35 to 36°, the peak corresponding to the β phase (101) appears at 2θ = 33 to 34°, and the peak corresponding to the β phase (120) appears at 2θ = 36 to 37°. An example of an X-ray diffraction spectrum is shown in Figure 2.

[0028] In the present disclosure, the β rate is calculated using the following formula. β rate = (β(101)+β(120)) / (α(210)+α(201)+β(101)+β(120))×100 In the above formula, α(201) is the maximum peak height in the 2θ range of 30.5 to 32°, α(210) is the maximum peak height in the 2θ range of 35 to 36°, β(101) is the maximum peak height in the 2θ range of 33 to 34°, and β(120) is the maximum peak height in the 2θ range of 36 to 37°. The baseline of the XRD profile is the line connecting the points where no peaks exist in the diffraction pattern between 30 and 40°. Absence of a peak means that the intensity is below the noise level. Noise can be automatically determined using the X-ray analysis software "PDXL2." The intensity of each peak is the length of the line segment drawn perpendicular to the peak top to the point where it intersects with the baseline.

[0029] SiAlON contains the elements Si, Al, O, and N. The Al content in the sintered body is preferably 3.0 mass% or more, preferably 3.5 mass% or more, preferably 4.0 mass% or more, preferably 4.5 mass% or more, preferably 5.0 mass% or more, preferably 5.5 mass% or more, and preferably 6.0 mass%. From the viewpoint of improving fracture toughness, the Al content in the sintered body is preferably 15 mass% or less, preferably 14 mass% or less, preferably 12 mass% or less, preferably 11 mass% or less, preferably 10 mass% or less, preferably 9.0 mass% or less, preferably 8.5 mass% or less, preferably 8.0 mass% or less, preferably 7.5 mass% or less, and preferably 7.0 mass% or less.

[0030] The O content in the sintered body is preferably 3.0 mass% or more, preferably 3.5 mass% or more, preferably 4.0 mass% or more, preferably 4.5 mass% or more, preferably 5.0 mass% or more, preferably 5.5 mass% or more, and preferably 6.0 mass%. From the viewpoint of improving fracture toughness, the O content in the sintered body is preferably 15% by mass or less, preferably 14% by mass or less, preferably 12% by mass or less, preferably 11% by mass or less, preferably 10% by mass or less, preferably 9.0% by mass or less, preferably 8.5% by mass or less, preferably 8.0% by mass or less, preferably 7.5% by mass or less, and preferably 7.0% by mass or less.

[0031] The Si content in the sintered body is preferably 30 mass % or more, preferably 40 mass % or more, preferably 42 mass % or more, preferably 43 mass % or more, preferably 44 mass % or more, preferably 45 mass % or more. The Si content in the sintered body is preferably 60% by mass or less, preferably 58% by mass or less, preferably 56% by mass or less, preferably 54% by mass or less, preferably 52% by mass or less, preferably 50% by mass or less, preferably 49% by mass or less, preferably 48% by mass or less, preferably 47% by mass or less, and preferably 46% by mass or less.

[0032] The N content in the sintered body is preferably 25 mass % or more, preferably 27 mass % or more, preferably 29 mass % or more, preferably 31 mass % or more, preferably 33 mass % or more, preferably 34 mass %. The N content in the sintered body is preferably 45% by mass or less, preferably 43% by mass or less, preferably 42% by mass or less, preferably 41% by mass or less, preferably 40% by mass or less, preferably 39% by mass or less, preferably 38% by mass or less, preferably 37% by mass or less, preferably 36% by mass or less, and preferably 35% by mass or less.

[0033] The sintered body preferably contains at least one of Mg, Ca, and Y in an amount of 1 mass % or more. The sintered body may or may not contain Mg. When the sintered body contains Mg, the Mg content in the sintered body is preferably 0.1% by mass, preferably 0.5% by mass or more, preferably 1.0% by mass or more, preferably 1.5% by mass or more, preferably 2.0% by mass or more, and preferably 2.5% by mass or more. The Mg content in the sintered body is preferably 6.0% by mass or less, preferably 5.0% by mass or less, preferably 4.0% by mass or less, and preferably 3.0% by mass or less.

[0034] The sintered body may or may not contain Ca, but preferably contains Ca from the viewpoint of promoting grain boundary sintering. When the sintered body contains Ca, the Ca content in the sintered body is preferably 1.0 mass% or more, more preferably 1.5 mass% or more, even more preferably 2.0 mass% or more, and preferably 2.5 mass% or more. Furthermore, from the viewpoint of strength, the Ca content in the sintered body is preferably 10 mass% or less, preferably 9.0 mass% or less, preferably 8.0 mass% or less, preferably 7.0 mass% or less, preferably 6.0 mass% or less, preferably 5.0 mass% or less, preferably 4.0 mass% or less, and preferably 3.0 mass% or less. The sintered body of the present disclosure preferably contains 1.0 mass% or more of Ca.

[0035] The sintered body may or may not contain Y. When the sintered body contains Y, the Y content in the sintered body may be 1.0 mass% or more, 1.5 mass% or more, 2.0 mass% or more, or 2.5 mass% or more. The Y content in the sintered body may be 10 mass% or less, 9.0 mass% or less, 8.0 mass% or less, 7.0 mass% or less, 6.0 mass% or less, 5.0 mass% or less, 4.0 mass% or less, or 3.0 mass% or less.

[0036] The sintered body preferably has a total content of Mg, Ca, and Y of 1.0% by mass or more, but may also be 1.5% by mass or more, 2.0% by mass or more, 2.5% by mass or more, or 3.0% by mass or more. Also, the total content of Mg, Ca, and Y may be 10% by mass or less, 9.0% by mass or less, 8.0% by mass or less, 7.0% by mass or less, 6.0% by mass or less, 5.0% by mass or less, 4.0% by mass or less, or 3.5% by mass or less.

[0037] The sintered body of the present disclosure has a Raman spectrum of 177±2 cm -1 (177cm -1 (also called "nearby"), 192±5cm -1 (192cm -1 (also called "nearby"), 219±3cm -1 (219cm -1 It is preferable that a peak corresponding to the β phase exists in the region around 177 cm -1 Around 192cm -1 The nearby peaks are preferably broad. 3(A) and 3(B) show an example of the Raman spectrum of the sintered body. The sintered body in Figure 3(A) has a 177 cm -1 Around 192cm -1 Near 219cm -1These peaks correspond to the β phase of the sintered body. In the Raman spectrum shown in Figure 3(A), -1 Around 192cm -1 The peak around this point is broad, indicating that the sintered body is in a solid solution state. This suggests that a large amount of Al and O (oxygen) are dissolved in solid solution, resulting in low crystallinity. In contrast, the sintered body in Figure 3(B) has a 180cm -1 , 200cm -1 , 223cm -1 In the Raman spectrum shown in Figure 3(B), these peaks are sharp, indicating that the sintered body is not in a solid solution state.

[0038] 177cm in Raman spectroscopy -1 Around 192cm -1 The peaks in the vicinity of 177-197 cm are broad. -1 The minimum peak intensity at 170-190 cm -1 This is confirmed from the ratio of the maximum peak intensities at 1.0 to 3.0. If this ratio is 1.0 to 3.0, it can be considered to be a solid solution. This ratio is preferably 1.0 or more, preferably 1.1 or more, preferably 1.2 or more, preferably 1.3 or more, preferably 1.4 or more, and preferably 1.5 or more. This ratio is preferably 4 or less, preferably 3.5 or less, preferably 3.0 or less, preferably 2.5 or less, preferably 2.0 or less, preferably 1.9 or less, and preferably 1.8 or less.

[0039] The Raman spectrum shown in Figure 3(A) is from 170 to 190 cm -1 The peak with a peak top at 177 to 197 cm -1 Since the ratio of the peak to the minimum value is 1.26, it can be said that the peak is broad, indicating that the solid solution is in a state. In contrast, the sintered body shown in Figure 3(B) has a surface area of ​​170 to 190 cm -1 The peak with a peak top at 177 to 197 cm -1Since the difference between the minimum value and the peak is more than 3.0, it can be said that the peak is sharp, indicating that it is not a solid solution.

[0040] As the sintered body, a solid solution of SiAlON is preferred. In the present disclosure, whether or not the sintered body is in a solid solution state is confirmed by Raman spectroscopy as described above, but it may also be confirmed additionally by a scanning electron microscope (SEM, for example, IM4000plus manufactured by Hitachi High-Technologies Corporation). For example, as shown in FIG. 4, the sintered body represented by (B) is confirmed to have a clear separation between the main phase and the liquid phase, while the sintered body represented by (A) is confirmed to have an incomplete separation between the main phase and the liquid phase and to be in a solid solution state. When the sintered body is in a solid solution state, the liquid phase region with low mechanical strength is reduced, thereby improving the mechanical strength. Furthermore, a small amount of liquid phase tends to further suppress the generation of snowflakes.

[0041] The sintered body of the present disclosure includes a dispersed phase. The dispersed phase may be contained in a liquid phase. The dispersed phase contains carbon or at least one specific carbide selected from the group consisting of Ti, Zr, and W. From the viewpoint of ease of dispersion of a compound serving as a carbon source or specific carbon source in a slurry for obtaining a silicon nitride material as a raw material, the dispersed phase preferably contains carbon or TiC. Furthermore, from the viewpoint of improving fracture toughness, the dispersed phase preferably contains at least one specific carbide selected from the group consisting of Ti, Zr, and W. Furthermore, from the viewpoint of ease of detecting surface defects (ease of sorting defective products by visual inspection), the dispersed phase preferably contains at least one specific carbide selected from the group consisting of Ti, Zr, and W.

[0042] The average particle size of the dispersed phase is 0.03 to 0.50 μm, preferably 0.04 to 0.48 μm, and more preferably 0.05 to 0.45 μm. From the viewpoint of increasing fracture toughness, the average particle size of the dispersed phase is preferably 0.03 to 0.35 μm, more preferably 0.04 to 0.30 μm, even more preferably 0.05 to 0.20 μm, and particularly preferably 0.05 to 0.15 μm. From the viewpoint of increasing both fracture toughness and strength, 0.05 to 0.0.9 μm is preferred.

[0043] In the sintered body of the present disclosure, the proportion of the dispersed phase as measured by scanning electron microscope (SEM) is preferably 0.1 to 3.0 area%, more preferably 0.14 to 2.0 area%, more preferably 0.2 to 1.0 area%, and even more preferably 0.3 to 0.6 area%. When the proportion of the dispersed phase is 0.1 volume% or more, the generation of snowflakes tends to be further suppressed, and when it is 3.0 area% or less, the grain growth of βSiAlON is less likely to be inhibited, and the fracture toughness value tends to be further improved.

[0044] The sintered body of the present disclosure is less likely to produce snowflakes with a major axis of 50 μm or more, and it is preferable that when an area of ​​5 × 5 mm is observed under a dark field optical microscope at 10 to 200 magnifications, no snowflakes with a major axis of 25 μm or more are observed. The maximum major axis of the snowflakes of the sintered body is preferably 50 μm or less, preferably 40 μm or less, preferably 30 μm or less, preferably 20 μm or less, preferably 15 μm or less, preferably 12 μm or less, preferably 10 μm or less.

[0045] Generally, snowflakes tend to form on sintered bodies, especially near the surface. However, the sintered body of the present disclosure is less susceptible to snowflakes even near the surface, so the amount of surface cutting (grinding allowance) required to remove snowflakes is reduced, thereby suppressing manufacturing costs. When the sintered body is observed in a region within 250 μm from the surface (also referred to as the outer periphery) under an optical microscope in a dark field at 10 to 200 magnifications, it is preferable that no snowflakes with a major axis of 50 μm or more are present in the magnified image. The maximum major axis of the snowflakes in the outer periphery of the sintered body is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less.

[0046] The sintered body of the present disclosure preferably does not have a sinter-induced alteration layer in a region within 250 μm from the surface (also referred to as the outer periphery). The sinter-induced alteration layer is defined as a region within 250 μm from the surface of the sintered body before surface processing, in which snowflakes with a major axis of 50 μm or more exist when observed in a dark field at 10 to 200 magnification using an optical microscope.

[0047] The three-point bending strength of the sintered body is preferably 700 MPa or more, preferably 750 MPa or more, preferably 800 MPa or more, and preferably 850 MPa or more. The upper limit of the three-point bending strength is not particularly limited, but may be 1200 MPa or less, 1100 MPa or less, 1050 MPa or less, 1000 MPa or less, 980 MPa or less, or 930 MPa or less.

[0048] The fracture toughness of the sintered body is 5.0 MPa m 1 / 2 More than 5.2 MPa m is preferable. 1 / 2 More than 5.4 MPa m is preferable. 1 / 2 More than 5.6 MPa m is preferable. 1 / 2 More than 5.8 MPa m is preferable. 1 / 2 More than 6.0 MPa m is preferable. 1 / 2 More than 6.3 MPa m is preferable. 1 / 2 More than 6.5 MPa m is preferable. 1 / 2 More than 7.0 MPa m is preferable. 1 / 2 The above is preferable. There is no particular upper limit to the fracture toughness value, but it is set to 10.0 MPa m 1 / 2 It may be less than 9.0 MPa m 1 / 2 It may be less than 8.0 MPa m 1 / 2 It may be 7.5 or less.

[0049] <Application> The sintered body of the present disclosure is suitable for use as a wear-resistant member, and may be used, for example, as a base sphere for a bearing ball.

[0050] <Method of manufacturing sintered body> The method for producing the sintered body of the present disclosure is not particularly limited as long as it can produce a sintered body having a main phase of βSiAlON and a dispersed phase containing carbon or at least one carbide selected from the group consisting of Ti, Zr, and W, and having an average particle size of the dispersed phase of 0.05 to 0.50 μm.

[0051] One example of a method for producing a sintered body involves preparing a raw material composition containing a silicon nitride material as a raw material, granulating, molding, pressing, degreasing, and firing the raw material composition. Examples of additives added to the raw material composition include sintering aids, binders, solvents, and sintering accelerators. The carbon source of the carbon or specific carbide contained in the dispersed phase may be an organic substance such as a binder. Furthermore, the Ti, Zr, or W contained in the dispersed phase may be derived from a compound containing Ti, Zr, or W added as a sintering accelerator. In the method for producing a sintered body, processes other than the firing step may be omitted as appropriate, and processes other than those mentioned above, such as classification, may be added as appropriate.

[0052] Examples of silicon nitride raw materials include αSi3N4, βSi3N4, M-αSiAlON, and βSiAlON. From the viewpoint of sinterability, αSi3N4 and M-αSiAlON are preferred. βSiAlON and M-αSiAlON are preferred because of their low or no hydrolysis and the ability to use oxygen-containing solvents such as water. M-αSiAlON is preferred as a silicon nitride raw material because it transforms from granular α-phase crystals to acicular β-phase crystals during liquid-phase sintering, improving fracture toughness. Furthermore, M-αSiAlON is preferred as a silicon nitride raw material because it can omit the use of sintering aids and produces a solid-solution sintered body, resulting in a homogeneous, dense sintered body. M-αSiAlON is αSiAlON with metal atoms M (M = Li, Mg, Ca, Y, La, etc.) present within the crystal lattice. During liquid-phase sintering, the metal elements form a liquid phase with Si, Al, O, etc., resulting in a homogeneous, dense sintered body. The silicon nitride material as a raw material may be used alone or in combination of two or more kinds.

[0053] M-αSiAlON may be a commercially available product or may be manufactured. M-αSiAlON can be obtained by firing a mixture of αSi3N4, AlN, Al2O3, and metal M or a compound containing metal M. Alternatively, M-αSiAlON can be synthesized by combustion synthesis using raw material powders in which predetermined elements are mixed in predetermined ratios.

[0054] The metal atom M in M-αSiAlON may function as a sintering aid during sintering, and therefore the content of the metal atom M in M-αSiAlON is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, more preferably 1.5 mass% or more, more preferably 2.0 mass% or more, and more preferably 2.5 mass%. In addition, from the viewpoint of suppressing the precipitation of crystalline phases other than M-αSiAlON, the content of metal atom M in M-αSiAlON is preferably 9.0 mass%, preferably 8.0 mass% or less, preferably 7.0 mass% or less, preferably 6.0 mass% or less, preferably 5.5 mass% or less, preferably 5 mass% or less, preferably 4.5 mass% or less, and preferably 4.0 mass% or less.

[0055] The more α-phase SiAlON there is in M-αSiAlON, the better. α-phase SiAlON has superior sinterability compared to β-phase SiAlON. The α fraction of M-αSiAlON is preferably 81% or more, preferably 82% or more, preferably 83% or more, preferably 84% or more, preferably 85% or more, preferably 86% or more, preferably 87% or more, preferably 88% or more, preferably 89% or more, preferably 90% or more, preferably 91% or more, preferably 92% or more, preferably 93% or more, preferably 94% or more, preferably 95% or more, preferably 96% or more, preferably 97% or more, preferably 98% or more, preferably 99% or more, and preferably a single phase (100%).

[0056] The α rate is calculated using the following formula. α rate = (α(210)+α(201)) / (α(210)+α(201)+β(101)+β(120))×100 In the above formula, α(201), α(210), β(101), and β(120) are synonymous with α(201), α(210), β(101), and β(120), respectively, in the β ratio.

[0057] Examples of sintering aids include compounds containing Li, Mg, Ca, Y, La, etc., and specific examples include Al2O3, Y2O3, AlN, and MgAl2O4. The binder may be an organic binder. Examples of solvents include water, alcohol, and hydrocarbons. Conventional silicon nitride sintered bodies use α-Si3N4 as the raw material, but if a solvent containing oxygen atoms, such as water, is used as the solvent, oxygen atoms may enter the Si3N4, affecting the properties of the final silicon nitride sintered body. Therefore, when manufacturing conventional silicon nitride sintered bodies, it is preferable to select the solvent appropriately. On the other hand, since SiAlON already contains oxygen atoms as a constituent element, using water when using SiAlON as the raw material has little effect on the properties of the final product, thereby widening the options for the manufacturing process.

[0058] When a sintering aid is used, from the viewpoint of obtaining the effect of the addition of the sintering aid, the amount of the sintering aid added, in oxide equivalent, is preferably 0.5 mass% or more, preferably 1 mass% or more, preferably 2 mass% or more, and preferably 3 mass% or more, relative to the silicon nitride material. Also, from the viewpoint of further increasing the mechanical strength, the amount of the sintering accelerator added, in oxide equivalent, is preferably 10 mass% or less, preferably 8 mass% or less, preferably 7 mass% or less, and preferably 6 mass% or less, relative to the silicon nitride material.

[0059] Examples of sintering accelerators include compounds containing Ti, Hf, Zr, W, Mo, Nb, Cr, etc., including oxides, carbides, nitrides, silicides, and borides of these elements. The elements in the sintering accelerator may enhance the dispersibility in the crystalline structure, thereby improving the mechanical strength of the sintered body. To ensure that the dispersed phase is uniformly distributed and that the average particle size of the dispersed phase is 0.50 μm or less, the sintering accelerator is preferably added as a water-soluble organic metal, such as a water-soluble metal salt or an aqueous metal chelate. When the dispersed phase contains a specific carbide, it is preferable to use a compound containing Ti, W, or Zr as the sintering accelerator. Compounds containing Ti, W, or Zr also function as light-blocking agents, coloring the sintered body black and imparting opacity. Furthermore, blackening the sintered body makes it easier to obtain information about defects, especially in the surface layer, during visual inspection. Examples of compounds containing Ti, W, or Zr include oxycarboxylic acid chelate titanium, alkanolamine chelate titanium, titanium tetrachloride, titanyl sulfate, sodium tungstate, tungsten chloride, ammonium zirconium carbonate, zirconium chloride, and zirconium acetate.

[0060] When a sintering accelerator is used, from the viewpoint of obtaining the effect of adding the sintering accelerator, the amount of the sintering accelerator added, in terms of oxide, is preferably 0.1 mass% or more, preferably 0.2 mass% or more, preferably 0.3 mass% or more, preferably 0.4 mass% or more, preferably 0.5 mass% or more, based on the silicon nitride material. Also, from the viewpoint of further increasing the mechanical strength, the amount of the sintering accelerator added, in terms of oxide, is preferably 5 mass% or less, preferably 3 mass% or less, preferably 2 mass% or less, preferably 1 mass% or less, based on the silicon nitride material.

[0061] The raw material composition may be molded into a desired shape before firing for sintering. Known molding methods such as uniaxial pressing, mold pressing, doctor blade pressing, rubber pressing, and cold isostatic pressing (CIP) can be used. The molded product may be further compressed by CIP or the like. The molded product may then be degreased before firing.

[0062] When molding into a spherical shape using upper and lower molds, the molds are often designed so that there is some slack between them when they are closed, and in this case, the molded product obtained has protrusions due to the slack. For example, when a hemispherical upper mold and a hemispherical lower mold are used, a molded product such as that shown in Figure 5 is obtained. To remove these protrusions, it is preferable to grind them before firing, and if the molded product is subjected to CIP treatment, it is preferable to grind them after CIP treatment and before firing. If degreasing is performed before firing, it is preferable to grind the protrusions before degreasing.

[0063] When stripping is performed before firing, the height of the strip-shaped convex portions is preferably 500 μm or less, preferably 40 μm or less, preferably 30 μm or less, preferably 20 μm or less, preferably 10 μm or less, and most preferably 0 μm.

[0064] Alternatively, the mixture may be granulated before molding, and the granulated mixture may be used for molding. The granulation method is not particularly limited, and examples thereof include spray drying.

[0065] Degreasing may be carried out in either a non-oxidizing or oxidizing atmosphere. When degreasing is carried out in a non-oxidizing atmosphere, the temperature is preferably 550 to 800° C., and when degreasing is carried out in an oxidizing atmosphere such as air, the temperature is preferably 420 to 650° C. The heating time at these temperatures is preferably 1 to 2 hours. By controlling the heating temperature and holding time, it is possible to control the amount of carbon remaining from the added binder component and to prevent the dispersed phase from being localized in the sintered body or molded product. From this viewpoint, the carbon content before firing is preferably 500 to 2000 ppm by mass, and more preferably 800 to 1500 ppm by mass.

[0066] The silicon nitride material or molded product is preferably heated under reduced pressure, preferably in a vacuum of 0.01 Pa or less. The heating temperature in vacuum is preferably 800 to 1500°C, and the holding time at this heating temperature is preferably 1 to 10 hours.

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

[0068] The α phase undergoes a phase transition to the β phase due to oxygen present on the surface during sintering. During the phase transition of M-αSiAlON to βSiAlON, metal atoms M incorporated within the lattice migrate from the lattice to the grain boundaries outside the lattice, forming a solid solution between the crystal grains. Therefore, sintered bodies of M-αSiAlON are uniform and dense, as shown in Figure 4(A), for example. Carbon components remaining during debinding easily disperse homogeneously in the liquid phase or at the interface between the liquid phase and βSiAlON (main phase), providing lubrication for the liquid phase to the main phase, improving sinterability and suppressing snowflake formation, thereby improving toughness. Furthermore, these carbon components combine with Ti, W, or Zr added as sintering accelerators to form metal carbides, further improving the toughness of the sintered body. While the above description explains that Ti, W, or Zr are added as sintering accelerators, this is not a limitation.

[0069] After sintering, the resulting sintered body is preferably subjected to hot isostatic pressing (HIP) treatment in a non-oxidizing atmosphere of 300 atmospheres or more at a temperature of 1600°C to 1850°C. By subjecting the sintered body to hot isostatic pressing (HIP) treatment, defects that can be the starting point of fatigue fracture can be reduced, and when the sintered body is made into a bearing ball, the sliding properties and rolling life properties are further improved.

[0070] <Bearing ball> The bearing ball of the present disclosure is made of a mirror-finished sintered body of the present disclosure. The bearing ball can be obtained by subjecting the sintered body of the present disclosure to a mirror-finishing process. Any mirror-finishing process can be used as long as the arithmetic mean surface roughness Ra can be 0.5 μm or less. The arithmetic mean surface roughness Ra of the bearing ball is preferably 0.5 μm or less, preferably 0.4 μm or less, preferably 0.3 μm or less, preferably 0.2 μm or less, preferably 0.1 μm or less, preferably 0.08 μm or less, preferably 0.06 μm or less, preferably 0.05 μm or less. The smaller the arithmetic mean surface roughness Ra of the bearing ball, the better, and it may be 0 μm.

[0071] The surface layer may be cut prior to mirror finishing, but since the sintered body of the present disclosure has fewer defects such as snowflakes and pores in the surface layer compared to conventional sintered bodies, cutting of the surface layer may be omitted. Alternatively, the amount of cutting of the surface layer may be reduced.

[0072] <Bearings> The bearing of the present disclosure includes the bearing ball of the present disclosure. The bearing ball of the present disclosure uses a sintered body with few defects or a sintered body with good processability, and is therefore suitable as a bearing for electric vehicles. [Example]

[0073] The present invention will be described below using examples, but the present invention is not limited thereto. Examples 1 to 7 are working examples, and Examples 8 to 10 are comparative examples.

[0074] The raw materials and sintering accelerators listed in Table 1 were prepared. The raw material used was Ca-αSiAlON powder synthesized by combustion synthesis. The following sintering accelerators were used. ·Oxycarboxylic acid chelate titanium (organic titanium) Sodium tungstate (W salt) Ammonium zirconium carbonate (Zr salt) Titanium oxide (TiO 2、 average particle size 2μm)

[0075] In Examples 1, 2, and 9, a solvent and an organic binder were added to Ca-αSiAlON powder and mixed for 48 hours to prepare a slurry. In Example 8, a solvent was added to Ca-αSiAlON powder, and a slurry was prepared without adding an organic binder. In Examples 3, 5, and 6, a sintering accelerator was added to the Ca-αSiAlON powder so that the amount was 1.0 mass % in terms of the oxide of each metal, and then a solvent and an organic binder were added and mixed for 48 hours to prepare a slurry. In Examples 4 and 10, a sintering accelerator was added to the Ca-αSiAlON powder so that the amount was 3.0 mass % in terms of the oxide of each metal, and then a solvent and an organic binder were added and mixed for 48 hours to prepare a slurry. In Example 7, a sintering accelerator was added to Ca-αSiAlON powder so that the amount was 5.0 mass % in terms of the oxide of each metal, and then a solvent and an organic binder were added and mixed for 48 hours to prepare a slurry. The organic binders used were 1: polycarboxylic acid compound, 2: paraffin wax, and 3: fatty acid, and the solvent used was water.

[0076] The resulting slurry was spray-dried to obtain a granulated powder. The obtained granulated powder was placed in a mold and press-molded under molding pressure into a rectangular parallelepiped of 60 mm x 50 mm x 10 mm thick or a sphere of 13.5 mm diameter, and then subjected to CIP treatment.

[0077] The obtained molded products were degreased in an air atmosphere by heating at 600°C for 1 hour for Examples 1 to 8 and 10, and at 400°C for 1 hour for Example 9. -2 The temperature was raised from room temperature under a vacuum of 1000°C or less and held at 1000°C for 2 hours, and then sintered at 1750°C for 5 hours under a nitrogen gas atmosphere of 0.6 MPa to obtain a sintered body.

[0078] Furthermore, the obtained sintered body was subjected to a hot isostatic pressing (HIP) treatment in which it was heated at 1650°C to 1800°C for 1 hour under a pressure of 100 MPa in a nitrogen gas atmosphere.

[0079] The components and β ratio of the main phase, as well as the components, average particle size, and area ratio of the dispersed phase of the obtained sintered body were confirmed and measured by the methods described above. In Table 1, "-" indicates that the values ​​were below the detection limit. For the X-ray diffraction measurement, Rigaku Corporation's "Smart lab" was used, Rigaku Corporation's "D / teXUltra" was used as the detector, and Rigaku Corporation's "PDXL2" was used as the X-ray analysis software. For elemental analysis of the sintered body, an electron probe microanalyzer (EPMA) "JXA-8500F" manufactured by JEOL Ltd. was used, along with a standard sample manufactured by JEOL Ltd. For elemental analysis of the powder, a ZSX Primus II manufactured by Rigaku Corporation was used. The Raman spectroscopy was measured using LabRAM HR Evolution manufactured by Horiba Ltd. Scanning electron microscopy (SEM) was performed using a Hitachi High-Technologies SU6600 under the following conditions: acceleration voltage: 6 kV, probe current: Medium, emission current: 29 μA, extraction voltage: 1.70 kV, detector conditions: backscattered electrons, suppressor voltage: 300 V, WD: 15 mm, and C-coat: approximately 24 nm. The EDS analyzer used was a Noran system 6 manufactured by Thermo Fisher Scientific, with a detector: Thermo Fisher Scientific Ultradry, map resolution: 512 × 384, map pixel size: 0.01 μm, magnification: 25,000x, kernel size: 5 × 5, minimum valid intensity: high, filter fit type: high precision, quantitative peak separation method: standardless filter method, and correction method: probe (Phi-Rho-Z).

[0080] The sintered bodies thus obtained were evaluated for fracture toughness, strength (three-point bending strength), and snowflake properties. The results are shown in Table 1.

[0081] The fracture toughness was evaluated according to the following criteria. A: 6.0 MPa m 1 / 2 End B: 5.5 MPa m 1 / 2 More than 6.0MPa m 1 / 2 less than C: 5.0 MPa m 1 / 2 More than 5.5MPa m 1 / 2 less than D: 5.0 MPa·m 1 / 2 less than

[0082] The strength was evaluated based on the three-point bending strength according to the following criteria. A:900MPa or more B: 700 MPa or more and less than 900 MPa C: Less than 700MPa

[0083] The presence or absence of snowflakes with a major axis of 25 μm or more was confirmed by the method described above, and the snowflakes were evaluated according to the following criteria. A: None B: Yes

[0084] The appearance color tone was evaluated by visually comparing the measurement sample with a standard color chart prepared in accordance with JIS-Z8721: 1993. The measurement sample was mirror-polished to an arithmetic mean roughness Ra of 0.5 μm or less, and the polished surface was evaluated (visually). [Table 1]

[0085] The results in Table 1 show that the sintered bodies of Examples 1 to 7, which have a βSiAlON main phase, a dispersed phase containing carbon or at least one carbide selected from the group consisting of Ti, Zr, and W, and have an average particle size of 0.05 to 0.50 μm, have superior strength and suppress the generation of snowflakes compared to the sintered bodies of Examples 8 to 10. In particular, the sintered bodies of Examples 3 to 6, in which the dispersed phase contains at least one carbide selected from the group consisting of Ti, Zr, and W, are also excellent in fracture toughness. The sintered body of Example 7 contains at least one carbide selected from the group consisting of Ti, Zr, and W, but the area ratio of the dispersed phase exceeds 3.0 area %. For this reason, it is presumed that the columnar grain growth of βSiAlON is less inhibited in the sintered bodies of Examples 3 to 6 than in the sintered body of Example 7, and therefore the fracture toughness is superior.

[0086] On the other hand, in the sintered body of Example 8, no dispersed phase was observed, and snowflakes were generated. Furthermore, the sintered bodies of Examples 9 and 10 had large average particle sizes of the dispersed phase, 2.41 μm and 3.43 μm, respectively, and were inferior in strength. Furthermore, the sintered bodies of Examples 3 to 7 had a black appearance color tone, and therefore, defect information, particularly on the surface layer, was easily obtained during appearance inspection. [Industrial Applicability]

[0087] The sintered body of the present disclosure is useful as a wear-resistant member, and can be suitably used in particular as a bearing ball and a bearing member.

Claims

1. A sintered body having a βSiAlON as a main phase and a dispersed phase containing carbon or at least one carbide selected from the group consisting of Ti, Zr and W, wherein the average particle size of the dispersed phase is 0.03 to 0.50 μm.

2. 2. The sintered body according to claim 1, wherein the proportion of the dispersed phase is 0.1 to 3.0 area % when measured with a scanning electron microscope.

3. The sintered body according to claim 1 or 2, wherein the dispersed phase contains carbon or TiC.

4. 3. The sintered body according to claim 1, wherein the dispersed phase contains at least one carbide selected from the group consisting of Ti, Zr, and W.

5. 3. The sintered body according to claim 1, which is a base sphere for a bearing ball.

6. A bearing ball made of the mirror-finished sintered body according to claim 1 or 2.

7. A bearing comprising the bearing ball according to claim 6.

8. 8. The bearing of claim 7 for use in an electric vehicle.

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