Silicon nitride sintered body, machine parts and bearings

Abstract

To provide a silicon nitride sintered compact with excellent mechanical character and sliding character, mechanical components and a shaft bearing.SOLUTION: A rolling element 4 is a silicon nitride sintered compact including a silicon nitride particle, a rare earth element and an aluminum element, and when a range of a crystal particle diameter of β-type Si3 N4 particle of the silicon nitride sintered compact is the particle orientation 15-180° and the sum of areas of the crystal particle diameters of an upper-level size is 30% to the area of the total crystal particle diameter, it satisfies at least either the crystal particle diameter of the β-type Si3 N4 particle is 1 μm-4 μm as an equivalent circle diameter or an aspect ratio of the β-type Si3 N4 particle is 3-6.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a silicon nitride sintered body, a machine part, and a bearing having excellent mechanical properties and sliding properties. [Background technology]

[0002] Silicon nitride (Si3N4) sintered bodies have excellent mechanical properties and corrosion resistance over a wide temperature range, from low to high, and are widely used in engine parts, cutting tools, sliding components, etc. In particular, as sliding components, they can reduce the coefficient of friction when lubricated and also provide excellent wear resistance, so in recent years there has been increasing demand for bearings and other products that use silicon nitride sintered bodies as rolling elements.

[0003] For bearing components such as rolling elements that use silicon nitride sintered bodies, it has been proposed to improve mechanical strength, wear resistance represented by rolling life, and the like by controlling the sintered body composition (type and amount of sintering aid), control of crystal grain size, control of the morphology of each aid component in the sintered body, and control of the manufacturing process. For example, Patent Document 1 describes a ceramic composite material in which a Ti compound with a long-to-short axis ratio (aspect ratio) of 2 or more is dispersed in the range of 1 to 50 mass % in a silicon nitride matrix with an average particle size of 10 μm or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-122563 Summary of the Invention [Problem to be solved by the invention]

[0005] The technology described in Patent Document 1 is effective in improving the strength and toughness of the sintered body, but when TiN whiskers with large shape anisotropy (for example, an aspect ratio of 15) are present on the sliding surface, they form thorn-like protrusions that may become the starting point of fracture or may increase the aggressiveness toward the mating material, ultimately reducing the rolling life.

[0006] The present invention has been made in view of the above circumstances, and has as its object to provide a silicon nitride sintered body having excellent mechanical properties and sliding properties, and a mechanical part and a bearing using this silicon nitride sintered body. [Means for solving the problem]

[0007] The silicon nitride sintered body of the present invention is a silicon nitride sintered body containing silicon nitride particles, a rare earth element, and an aluminum element, and is characterized by satisfying at least one of the following: the crystal grain size of the β-type Si3N4 particles of the silicon nitride sintered body is 1 μm to 4 μm in equivalent circle diameter, and the aspect ratio of the β-type Si3N4 particles is 3 to 6, within a range in which the sum of the areas of the crystal grains of the larger sizes in the crystal orientation range of 15° to 180° is 30% of the area of ​​the total crystal grain size.

[0008] The silicon nitride sintered body is characterized by containing at least one metal element selected from Ti, Zr, Hf, W, Mo, Ta, Nb, Fe and Cr.

[0009] The silicon nitride sintered body is characterized in that it contains Fe, and the content of Fe is less than 0.5 mass % relative to the total mass of the silicon nitride sintered body.

[0010] When an image of the cross section of the silicon nitride sintered body taken at 100x magnification is analyzed, the proportion of the area of ​​snowflakes in the image is 7% or less of the total cross-sectional area in the image.

[0011] The silicon nitride sintered body is characterized in that the crushing strength of a 3 / 8-inch sphere made of the silicon nitride sintered body is 20 kN or more.

[0012] The silicon nitride sintered body of the present invention is a silicon nitride sintered body containing silicon nitride particles, a rare earth element, and an aluminum element, and is characterized in that when an image of a cross section of the silicon nitride sintered body taken at 100x magnification is analyzed, the proportion of the area of ​​snowflakes in the image is 7% or less of the total cross-sectional area in the image.

[0013] A machine component of the present invention is characterized by comprising the silicon nitride sintered body of the present invention. Also, a bearing of the present invention is characterized by comprising the machine component of the present invention as a bearing member.

[0014] The mechanical component is a rolling element for a bearing. The rolling element for a bearing has a maximum contact pressure of 3.6 GPa and a rotational speed of 3000 min -1 When the rolling life is measured using a radial bearing tester under the above conditions, the rolling life is 600 hours or more. [Effects of the Invention]

[0015] The silicon nitride sintered body of the present invention contains silicon nitride particles, a rare earth element, and an aluminum element, and satisfies at least one of the following: the crystal grain size of the β-type Si3N4 particles is 1 μm to 4 μm in equivalent circle diameter, and the aspect ratio of the β-type Si3N4 particles is 3 to 6, within a range in which the sum of the areas of the larger crystal grain sizes in the crystal orientation range of 15° to 180° is 30% of the area of ​​the total crystal grain size; and the silicon nitride sintered body exhibits excellent crushing strength and has excellent mechanical properties and sliding properties.

[0016] The silicon nitride sintered body of the present invention contains silicon nitride particles, a rare earth element, and an aluminum element. When an image of the cross section of the silicon nitride sintered body taken at 100x magnification is analyzed, the area ratio of snowflakes (aggregates of microscopic defects) in the image is 7% or less of the total cross-sectional area in the image. This means that the occurrence of peeling initiated by snowflakes can be suppressed, resulting in a silicon nitride sintered body with excellent mechanical properties and sliding properties.

[0017] The mechanical part of the present invention includes the silicon nitride sintered body of the present invention, and therefore can be suitably used, for example, as a rolling element for a bearing, whereby defect-induced spalling can be suppressed and the bearing of the present invention has excellent sliding characteristics, rolling life, etc. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram illustrating an outline of the EBSD method. [Figure 2] FIG. 1 is a diagram showing the application of a silicon nitride sintered body of the present invention to a rolling bearing. [Figure 3] 1 is a cross-sectional image of a ceramic ball of Example 1. [Figure 4] 1 is a cross-sectional image of a ceramic ball of Example 18. [Figure 5] 1 is a cross-sectional image of a ceramic ball of Comparative Example 1. [Figure 6] FIG. 1 is a diagram showing an example of a crystal orientation map image obtained by EBSD measurement. [Figure 7] FIG. 1 is a diagram showing an outline of a crushing test. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described. (Silicon nitride sintered body) The silicon nitride sintered body of the present invention contains β-type silicon nitride particles as the main component and at least a rare earth element and aluminum element. The rare earth element is derived from a rare earth-containing sintering aid used in the production of the silicon nitride sintered body. The aluminum element is derived from an aluminum-containing sintering aid used in the production of the silicon nitride sintered body. The rare earth element and aluminum element form a grain boundary phase consisting of, for example, a Si-RE-Al-ON compound (RE is a rare earth element), which densifies the silicon nitride sintered body.

[0020] The rare earth element contained in the silicon nitride sintered body is not particularly limited, and examples thereof include yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), neodymium (Nd), dysprosium (Dy), europium (Eu), and erbium (Er). These elements may be contained alone or in combination of two or more. Among these, yttrium (Y), lanthanum (La), and erbium (Er) are preferred from the viewpoint of controlling the crystal grain size. Examples of sintering aids containing rare earth elements include oxides of rare earth elements and nitrides of rare earth elements.

[0021] The content of the rare earth element is, for example, 2 to 20 mass% in terms of oxide, relative to the total weight of the silicon nitride sintered body. By keeping the content of the rare earth element within this range, the silicon nitride sintered body can be easily densified, and good mechanical strength can be easily obtained by suppressing the amount of grain boundary phase. The content of the rare earth element is preferably 2 to 15 mass%, but may also be 4 to 10 mass%, or 4 to 6 mass%.

[0022] The content of aluminum element is, for example, 0.5% by mass to 10% by mass, calculated as oxide, relative to the total weight of the silicon nitride sintered body. When the content of aluminum element is within this range, the silicon nitride sintered body is easily densified, and good mechanical strength is easily obtained by suppressing the amount of grain boundary phase. The content of aluminum element is preferably 0.5% by mass to 6% by mass, and may be 2% by mass to 6% by mass. As the sintering aid containing aluminum element, for example, aluminum oxide, aluminum nitride, etc. are used.

[0023] In the silicon nitride sintered body, the rare earth element content (in terms of oxide) and the aluminum element content (in terms of oxide) may be the same, or the rare earth element content (in terms of oxide) may be greater, or the aluminum element content (in terms of oxide) may be greater. The total amount of the rare earth element content (in terms of oxide) and the aluminum element content (in terms of oxide) is, for example, 2 to 18 mass%, preferably 4 to 15 mass%, and more preferably 9 to 13 mass%, based on the total weight of the silicon nitride sintered body.

[0024] The above content of rare earth elements can be calculated as the amount of sintering aid added relative to the total amount of raw material powder when an oxide of the rare earth element is used as the sintering aid, and the above content of aluminum element can be calculated as the amount of sintering aid added relative to the total amount of raw material powder when aluminum oxide (Al2O3) is used as the sintering aid. The contents of rare earth elements and aluminum elements can also be measured using an X-ray fluorescence analyzer (XRF), an energy dispersive X-ray analyzer (EDX), an inductively coupled plasma (ICP) optical emission analyzer, etc. Specifically, the contents of rare earth elements and aluminum elements in the silicon nitride sintered body can be determined using the above analyzers, and then calculated as oxides of the rare earth elements (RE) (RE2O3 or REO2) and aluminum oxide (Al2O3).

[0025] The silicon nitride sintered body may contain components derived from the sintering aid other than the rare earth element and aluminum element.

[0026] The silicon nitride sintered body may further contain at least one metal element selected from titanium (Ti), zirconium (Zr), hafnium (Hf), tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Nb), iron (Fe), and chromium (Cr). These metal elements are added to the raw material powder, for example, as simple metals, oxides of metal elements, or nitrides of metal elements.

[0027] The content of the above metal elements (the total amount when two or more elements are present) is, for example, 0.1 to 5 mass %, preferably 0.5 to 3 mass %, and may be 1 to 3 mass % in terms of oxide, relative to the total weight of the silicon nitride sintered body. This content can be calculated in the same manner as the content of the rare earth element and aluminum element described above.

[0028] The silicon nitride sintered body may contain iron (Fe) as the metal element. The iron content is, for example, 0.05 mass% or more, or may be 0.1 mass% or more, based on the total mass of the silicon nitride sintered body. The inclusion of iron can improve fracture toughness and make it easier to suppress crack propagation. On the other hand, a high iron content makes iron particles and the like more likely to be exposed on the sphere surface, and the exposed iron particles may be desulfurized, leading to peeling and the like. From this perspective, the iron content is preferably less than 0.5 mass% based on the total mass of the silicon nitride sintered body. The iron is added to the raw material powder, for example, as iron powder or iron oxide (Fe2O3).

[0029] Next, we will explain the shape characteristics of β-type silicon nitride particles in silicon nitride sintered bodies. Silicon nitride particles exist in α-phase and β-phase, and when the α-phase changes to β-phase during sintering, β-phase particles precipitate and grow, creating an anisotropic shape structure.

[0030] One embodiment of the silicon nitride sintered body of the present invention is characterized in that at least one of the crystal grain size and aspect ratio of β-type Si3N4 particles within a predetermined measurement range is set to a predetermined numerical range. The crystal grain size and aspect ratio are calculated by electron backscatter diffraction (EBSD) analysis. EBSD is a method of analyzing crystal orientation based on an EBSD pattern obtained by EBSD using, for example, a field emission scanning electron microscope (FE-SEM). The magnification of the FE-SEM image used for analysis is, for example, 1000x to 2000x.

[0031] Figure 1 shows a schematic diagram illustrating the measurement principle of the EBSD method. As shown in Figure 1, a test piece (sintered silicon nitride) is placed inside a scanning electron microscope. An electron beam is irradiated onto any surface of the test piece, and the diffraction pattern of the reflected electron beam is acquired by a detector. The test piece is positioned so that the angle between the test piece and a plane perpendicular to the optical axis of the electron beam incident on the test piece is appropriate. The EBSD pattern is acquired using the detector's fluorescent screen, and orientation information is mapped by scanning the electron beam over the surface of the test piece. This allows the crystal orientation of a specific localized region of a crystalline material to be determined. Note that while Figure 1 shows analysis using reflection EBSD, transmission EBSD can also be used for analysis.

[0032] The above grain size and aspect ratio are calculated from the range where the sum of the areas of the upper grain sizes is 30% of the total grain size area, assuming that the grain size of β-type Si3N4 particles with the same crystal orientation in the range of 15° to 180° crystal orientation is one grain size. This range satisfies a reliability index (CI value) of 0.1 or more.

[0033] The silicon nitride sintered body of the above-mentioned form satisfies at least one of the following: the crystal grain size of the β-type Si3N4 particles in the above-mentioned range is 1 μm to 4 μm in equivalent circle diameter, and the aspect ratio of the β-type Si3N4 particles in the above-mentioned range is 3 to 6. As a result, as shown in the examples described later, good crushing strength can be exhibited and excellent mechanical properties can be obtained. Furthermore, by specifying the crystal grain size and aspect ratio in this way, the generation of voids can be suppressed. The crystal grain size is preferably 1 μm to 3 μm, and the aspect ratio is preferably 3 to 5.

[0034] The crystal grain size of each β-type Si3N4 particle is calculated by the following formula (1).

number

[0035] The "area when a group of crystals with the same crystal orientation is considered as one" in the above formula (1) was calculated by software.

[0036] The aspect ratio is the ratio (L / S) of the long diameter L to the short diameter S of the β-Si3N4 particles in the above range. Specifically, the long diameter L and short diameter S of each β-silicon nitride particle are measured, and each aspect ratio is calculated, and then the average of these is calculated. By aligning the shape of the β-silicon nitride particles to a predetermined aspect ratio, the density of the silicon nitride sintered body can be increased.

[0037] In the silicon nitride sintered body of the above-mentioned form, when an image of an arbitrary cross section taken at 100x magnification is analyzed, the snowflake area in the image preferably accounts for 7% or less of the total cross-sectional area in the image. This snowflake area ratio is more preferably 5% or less, and even more preferably 3% or less. Since snowflakes are an aggregate of microscopic defects and affect wear resistance, durability, etc., a low area ratio is preferable.

[0038] As will be shown in the Examples below, snowflakes can be observed as white spots in cross-sectional dark-field SEM observations. However, they cannot be observed in bright-field SEM observations. The snowflake area ratio can be calculated by binarizing dark-field SEM images using image analysis software.

[0039] The silicon nitride sintered body of the present invention preferably has no pores of 50 μm or larger in the surface layer (for example, from the surface to a depth of 500 μm), more preferably no pores of 30 μm or larger, and even more preferably no pores of 10 μm or larger. If pores exist in the surface layer, they may serve as starting points for peeling. The pores can be confirmed by SEM observation of the cut surface of the silicon nitride sintered body that has been cut and mirror-polished. The pore diameter can be calculated, for example, as the square root of the pore envelope area (pore diameter = √(pore envelope area)).

[0040] The silicon nitride sintered body of the present invention has excellent mechanical properties, and for example, the crushing strength of a 3 / 8-inch ball made of the silicon nitride sintered body is preferably 20 kN or more. The crushing strength is, for example, 30 kN or less. The crushing strength can be measured, for example, by the two-ball crushing test described in the Examples below.

[0041] Another embodiment of the silicon nitride sintered body of the present invention is characterized in that, when an image of an arbitrary cross section taken at 100x magnification is analyzed, the snowflake area ratio in the image is 7% or less of the total cross-sectional area in the image. In this other embodiment, the snowflake area ratio is preferably 5% or less, and more preferably 3% or less.

[0042] The other forms may also have the shape characteristics and physical properties of the silicon nitride sintered body described above.

[0043] The production of the silicon nitride sintered body of the present invention will be described below.

[0044] The silicon nitride sintered body of the present invention is produced mainly through a mixing step in which raw material powders containing silicon nitride powder and a sintering aid are mixed, a molding step in which a molded body is obtained from the resulting mixed powder, and a sintering step in which the molded body is sintered.

[0045] In the mixing step, a predetermined amount of binder component (e.g., organic binder) is added to the raw material powder and mixed in a ball mill or the like to obtain a molding powder. In the molding step, a molded body of a desired shape is obtained using the molding powder by applying a known molding method such as CIP (cold isostatic pressing) molding or press molding. After the molding step, a debinding step may be performed as necessary. The debinding step is performed, for example, by heating the molded body at a predetermined temperature in a debinding furnace.

[0046] In the sintering step, the compact is sintered by heat treatment at a temperature of 1600°C to 1950°C (preferably 1600°C to 1900°C) in an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere. The sintering time is set, for example, to 3 hours to 10 hours. Applicable sintering methods include atmospheric pressure sintering, atmospheric pressure sintering, and pressure sintering (hot pressing). In atmospheric pressure sintering, the pressure is set, for example, to 0.1 MPa to 10 MPa. Furthermore, in the sintering step, primary sintering and secondary sintering may be performed under different pressures.

[0047] After the sintering step, the resulting sintered body may be subjected to HIP (hot isostatic pressing) treatment, which is carried out by, for example, holding the body at a temperature of 1500°C to 1700°C under a gas pressure of 100 MPa or more for a predetermined period of time.

[0048] In the production of silicon nitride sintered bodies, the content of silicon nitride powder used in the raw material powder is preferably 70% to 97% by mass, more preferably 80% to 97% by mass, and may be 85% to 92% by mass, based on the total weight of the raw material powder (excluding the binder component used during mixing). The average particle size of the silicon nitride powder can be, for example, 0.5 μm or less.

[0049] The sintering aids used in the raw material powder are preferably oxides of rare earth elements and aluminum elements. Examples of rare earth elements that can be used include Y2O3, La2O3, CeO2, Sm2O3, Nd2O3, Dy2O3, Eu2O3, and Er2O3. These may be used alone or in combination of two or more.

[0050] The raw material powder may contain materials other than silicon nitride powder, rare earth element-containing sintering aids, and aluminum element-containing sintering aids. For example, a metal compound containing at least one metal element selected from titanium (Ti), zirconium (Zr), hafnium (Hf), tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Nb), iron (Fe), and chromium (Cr) is preferably added. This metal compound may be added as, for example, a simple metal, an oxide of the metal element, or a nitride of the metal element. For example, TiO2, Fe2O3, Cr2O3, etc. may be added as a sintering aid containing a transition metal element.

[0051] The average particle size of the sintering aid varies depending on the type of sintering aid, but is usually 10 μm or less, and may be 5 μm or less, 3 μm or less, 1 μm or less, or 0.4 μm or less.

[0052] The shape of the silicon nitride sintered body of the present invention is not particularly limited, and may be selected appropriately depending on the application, such as a spherical shape, a cylindrical shape, a conical shape, a truncated conical shape, or a rectangular parallelepiped shape.

[0053] (Applications of sintered silicon nitride) The silicon nitride sintered body of the present invention has excellent mechanical properties and sliding characteristics, and is therefore preferably used as a mechanical part for use in a rolling or sliding part. The mechanical part of the present invention is a part that uses the silicon nitride sintered body of the present invention as part or all of its configuration. Examples of mechanical parts include sliding members, bearing members, roll materials for rolling mills, compressor vanes, gas turbine blades, and other engine parts. Examples of bearing members include raceways such as inner and outer rings, rolling elements for bearings, and cages. The bearing of the present invention is a bearing that includes the mechanical part as part or all of its bearing member, and examples include rolling bearings, sliding bearings, linear guide bearings, ball screws, and linear bearings. In particular, the bearing of the present invention is preferably a rolling bearing that uses the silicon nitride sintered body as the rolling elements for bearings, due to its excellent rolling life.

[0054] FIG. 2 shows an example of an application of the silicon nitride sintered body. FIG. 2 is a cross-sectional view of a deep groove ball bearing. In rolling bearing 1, an inner ring 2 having an inner ring raceway surface 2a on its outer peripheral surface and an outer ring 3 having an outer ring raceway surface 3a on its inner peripheral surface are concentrically arranged, and a plurality of balls (rolling elements) 4 are arranged between the inner ring raceway surface 2a and the outer ring raceway surface 3a. These balls 4 are formed of the silicon nitride sintered body described above. The balls 4 are held in place by a cage 5. Openings 8a, 8b at both axial ends of the inner and outer rings are sealed by sealing members 6, and a grease composition 7 is enclosed around at least the balls 4. The grease composition 7 is interposed between the raceway surface of the balls 4 to provide lubrication. While FIG. 2 shows the silicon nitride sintered body applied to the balls, it can also be applied to the rollers in a roller bearing. [Example]

[0055] <Examples 1 to 10, Examples 18 to 22, Comparative Examples 1 and 2> A predetermined amount of organic binder was added to the raw material powders with the compounding ratios shown in Table 1, and the mixture was mixed using a ball mill or the like, after which a compact was produced by the CIP method. The resulting compact was degreased in a degreasing furnace and then sintered in a nitrogen atmosphere (atmospheric pressure) at 1750°C for 4 hours. The resulting sintered compact was then subjected to HIP treatment in a nitrogen atmosphere (pressure: 100 MPa) at 1700°C for 1 hour to obtain a silicon nitride sintered compact.

[0056] <Examples 11 to 17, Examples 23 and 24, Comparative Example 3> A specified amount of organic binder was added to the raw material powder with the compounding ratio shown in Table 1, and mixed using a ball mill or other device. After press molding at a molding pressure of 30 MPa, a compact was produced using the CIP method. After primary sintering under conditions of 1650°C for 3 hours in a nitrogen atmosphere (pressure: 0.1 MPa), secondary sintering under conditions of 1650°C for 3 hours in a nitrogen atmosphere (pressure: 8 MPa) yielded a silicon nitride sintered body.

[0057] The silicon nitride sintered body prepared above was polished to prepare 3 / 8 inch (9.525 mm diameter) ceramic balls, and various physical properties and rolling contact fatigue characteristics were evaluated.

[0058] [Table 1]

[0059] <Cross-section observation> Each ceramic ball in the examples and comparative examples was cut and its cross section was observed. As representative examples, cross-sectional images of Example 1 (see FIG. 3), Example 18 (see FIG. 4), and Comparative Example 1 (see FIG. 5) are shown. Each figure (a) shows a bright-field image, and each figure (b) shows a dark-field image.

[0060] Example 1 shown in FIG. 3 has no voids in the surface layer and almost no snowflakes, resulting in a dense silicon nitride sintered body. Similar results were obtained in Examples 2 to 17. On the other hand, Example 18 shown in FIG. 4 has no voids in the surface layer, but many snowflakes were observed due to the relatively large amount of sintering aid. Similar results were obtained in Examples 19 to 21 and 23 to 24. Furthermore, in Comparative Example 1, in which a relatively small amount of sintering aid was added, voids were observed in the surface layer. Similar voids were also observed in Comparative Examples 2 and 3.

[0061] <Measurement of crystal grain size and aspect ratio> Measurements were performed using the EBSD method on each ceramic ball in the examples and comparative examples. A field emission scanning electron microscope (FE-SEM) was used for the measurements, and the crystal orientation of the surface (observation field) of each ceramic ball was analyzed using a test specimen, as shown in Figure 1. Figure 6 shows an example of a crystal orientation map image of the observation field. This map image is color-coded according to the crystal orientation of the β-type Si3N4 particles. Using this crystal orientation map image, a group of grains with the same crystal orientation in the range of 15° to 180° was considered as one crystal grain size, and the crystal grain size and aspect ratio were calculated within a range in which the sum of the areas of the upper-sized crystal grains was 30% of the area of ​​the total crystal grain size. Within this range, the reliability index (CI value) in the analysis software was 0.1 or higher.

[0062] Specifically, image analysis was performed on the above range to determine the area of ​​each particle with the same crystal orientation, and the diameter of a circle equal to each area (circle equivalent diameter) was calculated using the above formula (1). The average value of these was then used as the crystal grain size (average crystal grain size). Image analysis was also performed on the above range to determine the long diameter (L) and short diameter (S) of each particle with the same crystal orientation, and each aspect ratio (L / S) was calculated from these. The average value of these was then used as the aspect ratio (average aspect ratio). The measurements were performed three times at different analysis locations on the surface of each ceramic ball, and the crystal grain size and aspect ratio were calculated as the average values ​​(n=3). The magnification of the FE-SEM images used in this study was 2000x. The results are shown in Table 2.

[0063] <Snowflake area ratio measurement> Image processing was performed on one field of a cross-section image of each ceramic ball of the Examples and Comparative Examples to measure the snowflake area ratio. The image processing was performed using image analysis software WinRoof2013. Images acquired at 100x magnification were binarized, and the snowflake area ratio was calculated using the following formula (2). Snowflake area ratio [%] = snowflake area ÷ total cross-sectional area × 100 (2) The "total cross-sectional area" in the above formula (2) is the total cross-sectional area of ​​the ceramic balls in the target field of view, and includes the area of ​​the snowflakes. The results are shown in Table 2.

[0064] <Crushing test> A two-ball crushing test was conducted using each ceramic ball of the examples and comparative examples. The crushing test was conducted in accordance with JIS B 1501. As shown in Figure 7, the testing machine had a fixed jig 9 and a movable jig 10, and the movable jig 10 was moved up and down by a crosshead 11. Conical depressions were formed in the fixed jig 9 and the movable jig 10, and two test balls 12 were set between these depressions. The stroke speed of the crosshead 11 was 1.0 x 10 mm / min. The load at which the test balls 12 were crushed was measured. The results are shown in Table 2.

[0065] <Rolling fatigue test> To confirm the rolling contact fatigue properties of each ceramic ball in the examples and comparative examples, each ceramic ball was incorporated into an NTN Corporation "Deep Groove Ball Bearing 6206" to prepare a rolling bearing, and a bearing life (rolling life) test was conducted under the following conditions. The cutoff time for the bearing life test was 600 hours. The results are shown in Table 2. Load (kN): Fr=13.72 (6.86kN / brg) Maximum contact pressure (GPa): Inner ring-ball: 3.5, Outer ring-ball: 3.6 Rotation speed (min -1 ):3000 Lubricant: JX Energy Corporation additive-free turbine oil VG56 Lubricating oil supply temperature (℃): 50 Oil supply method: Clean oil circulation

[0066] [Table 2]

[0067] (Grain size and aspect ratio) As shown in Table 2, the crystal grain size of Comparative Examples 1 to 3 was smaller than that of Examples 1 to 24, being less than 1 μm. This is thought to be because the amount of sintering aid added in Comparative Examples 1 to 3 was relatively small, resulting in poor densification by sintering. This is also thought to be the reason for the formation of voids in the cross-sectional observation (see FIG. 5). In Examples 1 to 24, the crystal grain size of the β-type Si3N4 particles was 1 μm to 4 μm and the aspect ratio was 3 to 6. Furthermore, the crystal grain size and aspect ratio tended to increase as the amount of sintering aid added increased.

[0068] (Regarding snowflake area ratio) There was a certain degree of correlation between the amount of sintering aid added and the snowflake area ratio. In Examples 18 to 21 and 23 to 24, which added relatively large amounts of sintering aid, the snowflake area ratio exceeded 7% (specifically, 7.11% to 8.45%). In the other examples, the snowflake area ratio was 7% or less.

[0069] (Crushing strength) The crushing strength was 20 kN or more in Examples 1 to 24, whereas the crushing strength was low, less than 20 kN, in Comparative Examples 1 to 3. This result is thought to be influenced by the voids confirmed by cross-sectional observation.

[0070] (Rolling contact fatigue characteristics) In Examples 1 to 17, the test was terminated because it exceeded the 600-hour cutoff time. On the other hand, in Examples 18 to 24 and Comparative Examples 1 to 3, spalling occurred at 600 hours or less. In Comparative Examples 1 to 3, spalling occurred at 200 hours or less, which is thought to be because spalling originated from voids. In Examples 18 to 21 and 23 to 24, spalling is thought to have originated from snowflakes. In Example 22, iron powder was added to improve fracture toughness, but the amount added was relatively large, which made the iron powder more likely to be exposed on the ball surface, and the exposed iron powder was desulfurized, leading to spalling.

[0071] From the above results, from the viewpoint of crushing strength, Examples 1 to 24, in which the crystal grain size of the β-type silicon nitride particles in the specified measurement range was 1 μm to 4 μm and the aspect ratio was 3 to 6, showed good results. Among these, Examples 1 to 17, in which the area ratio of snowflakes was 7% or less, also showed excellent results in terms of rolling contact fatigue properties. Thus, according to the present invention, a silicon nitride sintered body with good mechanical properties and sliding properties is obtained. [Industrial Applicability]

[0072] The silicon nitride sintered body of the present invention has excellent mechanical properties and sliding characteristics, and can be widely used as a machine part that requires high mechanical properties. For example, when it is applied to a rolling element of a bearing, it can suppress flaking caused by defects, leading to a longer life. [Explanation of symbols]

[0073] 1. Rolling bearings 2. Inner circle 3 outer ring 4 rolling elements 5 Cage 6 Sealing material 7. Grease 8a, 8b opening 9 Fixture 10 Movable jig 11 Crosshead 12 Test ball

Claims

1. A silicon nitride sintered body containing silicon nitride particles, a rare earth element, and an aluminum element, β-type Si of the silicon nitride sintered body ３ N ４ In the measurement of particles by the electron backscatter diffraction (EBSD) method, a group of particles with the same crystal orientation in the range of 15° to 180° is considered as one crystal grain, and the sum of the areas of the individual crystal grains of the upper size is within 30% of the area of ​​the total crystal grains. ３ N ４ The crystal grain size of the particles is 1 μm to 4 μm in equivalent circle diameter, and β-type Si ３ N ４ The particle aspect ratio is 3 to 5, the content of the rare earth element is 4% by mass to 10% by mass, calculated as an oxide, relative to the total weight of the silicon nitride sintered body, and the content of the aluminum element is 2% by mass to 6% by mass, calculated as an oxide, relative to the total weight of the silicon nitride sintered body; the silicon nitride sintered body contains at least one metal element selected from Ti, Zr, Hf, W, Mo, Ta, Nb, and Cr, and the content of the metal element is 0.5 mass% or more and 3 mass% or less in terms of oxide, relative to the total mass of the silicon nitride sintered body; the silicon nitride sintered body contains Fe, and the content of the Fe is 0.1 mass% or more and less than 0.5 mass% relative to the total mass of the silicon nitride sintered body; A silicon nitride sintered body, the remainder of which is silicon nitride.

2. A silicon nitride sintered body according to claim 1, characterized in that when an image of a cross section of the silicon nitride sintered body taken at 100x magnification is analyzed, the proportion of the area of ​​snowflakes in the image is 7% or less of the total cross-sectional area in the image.

3. 3. The silicon nitride sintered body according to claim 1, wherein a 3 / 8-inch sphere made of said silicon nitride sintered body has a crushing strength of 20 kN or more.

4. A machine part comprising the silicon nitride sintered body according to any one of claims 1 to 3.

5. 5. The mechanical part according to claim 4, wherein the mechanical part is a rolling element for a bearing.

6. The rolling elements for the bearing have a maximum contact pressure of 3.6 GPa and a rotation speed of 3000 min －１ 6. The machine part according to claim 5, wherein the rolling life is 600 hours or more when measured under the above conditions using a radial bearing tester.

7. A bearing comprising the mechanical component according to any one of claims 4 to 6 as a bearing member.

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