Silicon nitride sintered body, sliding member, and bearing

Abstract

The present invention provides a silicon nitride sintered body that can suppress damage during polishing and yield products with excellent mechanical properties, as well as a sliding member and a bearing using the same. [Solution] The silicon nitride sintered body is a silicon nitride sintered body containing rare earth elements and aluminum elements, wherein the maximum length of silicon nitride crystal grains in the region within 250 μm from the surface of the silicon nitride sintered body is 0.6 μm or more and less than 1.0 μm for the top 50%, and 1.0 μm or more and 1.5 μm or less for the top 20%, and the aspect ratio of the silicon nitride crystal grains is 1.2 or more and less than 2.0 for the top 50%.

Description

Technical Field

[0001] The present invention relates to a silicon nitride sintered body, a sliding member using the same, and a bearing.

Background Art

[0002] Silicon nitride (Si3N4) sintered bodies are excellent in mechanical properties and corrosion resistance over a wide temperature range from low to high temperatures, and are widely used in engine parts, cutting tools, sliding members, and the like. In particular, as sliding members, since it is possible to reduce the coefficient of friction in lubrication and also obtain excellent wear resistance, in recent years, the demand for bearings and the like using a silicon nitride sintered body as a rolling element has been increasing.

[0003] A silicon nitride sintered body is composed of innumerable silicon nitride crystal grains, and the size and shape of the crystal grains affect the mechanical properties. When the mechanical properties of the silicon nitride sintered body are low, for example, it may have an adverse effect on rolling fatigue properties and the like, and therefore control of the crystal grains is required. For example, in Patent Document 1, in a silicon nitride-based composite sintered body composed of β-type silicon nitride particles, oxynitride silicon particles, and a grain boundary phase, it is described that the oxynitride silicon particles are columnar particles having an aspect ratio of 4 or more. Conventionally, a technique of forming particles having a large aspect ratio in crystal grains to achieve high strength and high toughness is known.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in a silicon nitride sintered body, even if the silicon nitride crystal grains are small particles, the aspect ratio may become large, and in such a case, it is considered that it does not contribute to high strength, high toughness, etc.

[0006] Furthermore, while silicon nitride sintered bodies are polished on the surface after sintering to become the final product, there is room for investigation regarding the relationship between the microstructure of silicon nitride crystal grains and the polishing process. For example, even when the aspect ratio is large, the polishing process may not be satisfactory.

[0007] This invention has been made in view of these circumstances, and aims to provide a silicon nitride sintered body that can suppress damage during polishing and yield products with excellent mechanical properties, a sliding member using the same, and a bearing. [Means for solving the problem]

[0008] The silicon nitride sintered body of the present invention is a silicon nitride sintered body containing rare earth elements and aluminum elements, characterized in that the maximum length of silicon nitride crystal grains in a region within 250 μm from the surface of the silicon nitride sintered body is 0.6 μm or more for the top 50% and 1.0 μm or more for the top 20%.

[0009] Furthermore, the silicon nitride crystal grains are characterized in that the maximum length is 0.6 μm or more and less than 1.0 μm for the top 50%, and 1.0 μm or more and 1.5 μm or less for the top 20%.

[0010] The aspect ratio of the silicon nitride crystal grains described above is characterized by being 1.2 or more and less than 2.0 for the top 50%.

[0011] The total content of the above-mentioned rare earth elements and aluminum elements is characterized in that, in terms of oxides, it is 18% by weight or more and 26% by weight or less of the total weight of the silicon nitride sintered body.

[0012] The above-mentioned rare earth element is Ce, and the silicon nitride sintered body is characterized by not containing any transition metal elements.

[0013] The above rare earth element is Ce, and the above silicon nitride sintered body is characterized in that the content of the above aluminum element is greater than the content of Ce in terms of oxides.

[0014] The sliding member of the present invention is characterized by being a polished product of the silicon nitride sintered body of the present invention. Furthermore, the sliding member is characterized by being a rolling element for a bearing. The rolling element for the bearing has a maximum contact pressure of 3.6 GPa and a rotational speed of 3000 min⁻¹. -1 It is characterized by having a rolling life of 1000 hours or more when measured using a radial bearing testing machine under the specified conditions.

[0015] The bearing of the present invention is characterized by comprising the sliding member of the present invention as a bearing member. [Effects of the Invention]

[0016] The silicon nitride sintered body of the present invention contains rare earth elements and aluminum elements, and the maximum length of the crystal grains in the region within 250 μm from the surface of the silicon nitride sintered body is 0.6 μm or more for the top 50% and 1.0 μm or more for the top 20%. Because a large proportion of crystal grains with relatively large maximum lengths occupy the surface layer, damage during polishing of the surface layer can be suppressed, and a product with excellent mechanical properties can be obtained.

[0017] Since the sliding member of the present invention is a polished product of the silicon nitride sintered body of the present invention, damage during the polishing process of the silicon nitride sintered body can be suppressed, resulting in a sliding member with excellent mechanical properties. Since the sliding member of the present invention is a rolling element for a bearing, peeling caused by damage during polishing can be suppressed, contributing to a longer lifespan. Furthermore, since the bearing of the present invention is equipped with the sliding member of the present invention as a bearing component, it has excellent sliding characteristics and rolling life. [Brief explanation of the drawing]

[0018] [Figure 1] This is a longitudinal cross-sectional view showing an example of a bearing of the present invention. [Figure 2] This is an analytical image of silicon nitride crystal grains in a cross-section of a silicon nitride sintered body. [Figure 3] It is a graph showing the cumulative frequency of the maximum length of silicon nitride crystal grains in the silicon nitride sintered compacts of the examples and comparative examples. [Figure 4] It is a graph showing the cumulative frequency of the aspect ratio of silicon nitride crystal grains in the silicon nitride sintered compacts of the examples and comparative examples. [Figure 5] It is an observation image of the surface of the silicon nitride sintered compact of Example 1 after polishing. [Figure 6] It is an observation image of the surface of the silicon nitride sintered compact of Comparative Example 1 after polishing. [Figure 7] It is an explanatory diagram regarding the relationship between the aspect ratio and the grain size of crystal grains. [Figure 8] It is an observation image of the surface of the ceramic ball before and after the bearing life test. [Figure 9] It is a schematic diagram of the calculation method of the maximum length and aspect ratio of crystal grains.

Embodiments for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described. (Silicon Nitride Sintered Compact) The silicon nitride sintered compact of the present embodiment mainly consists of a crystalline phase of silicon nitride and contains an appropriate amount of a glass phase (non-crystalline phase) composed of a sintering aid containing rare earth elements and aluminum elements. The silicon nitride sintered compact targeted herein refers to one that has not been polished on the surface after sintering.

[0020] As described above, when the silicon nitride sintered compact is finally processed into a product, the surface layer is removed by polishing. The present inventors have found that, regarding the silicon nitride crystal grains present in the surface layer portion of the sintered compact, the ratio of the silicon nitride crystal grains having a maximum length of a predetermined value or more affects the subsequent polishing process more than the aspect ratio, and thus affects the mechanical properties of the product and the like. The present invention is based on such findings.

[0021] In the silicon nitride sintered body of the present invention, the values ​​for the top 50% and top 20% of the maximum length of silicon nitride crystal grains present in the region within 250 μm from the surface are specified. While the silicon nitride crystal grains in the silicon nitride sintered body are mainly β-phase, α-phase may also be present. The calculation of the maximum length of the crystal grains in the silicon nitride sintered body is described below.

[0022] First, a silicon nitride sintered body is cut, and a test specimen is prepared by mirror-polishing the cut surface. On the cut surface of the test specimen, an area within 250 μm from the surface is photographed with a microscope (e.g., a field emission scanning electron microscope (FE-SEM)) within a field of view of, for example, 130 μm × 130 μm, and the captured image is obtained. The obtained captured image is analyzed using image analysis software. The image magnification used for analysis is, for example, 1000x to 2000x.

[0023] In image analysis, the longest diagonal of each silicon nitride crystal grain in the image is measured and defined as its maximum length. This measurement is performed on a predetermined number of silicon nitride crystal grains. When the maximum lengths of the predetermined number of silicon nitride crystal grains are arranged in descending order, the maximum length that falls within the top 50% is defined as the top 50% value, and the maximum length that falls within the top 20% is defined as the top 20% value. In other words, the top 50% value corresponds to the cumulative frequency (0%-100%) of 50%, and the top 20% value corresponds to the cumulative frequency of 80%. It is preferable that the number of silicon nitride crystal grains used to calculate the maximum length described above be 500 or more, obtained by observing multiple field ranges.

[0024] The silicon nitride crystal grains subject to calculation are assumed to be entirely contained within a region of 250 μm from the surface. In this invention, the maximum length of the silicon nitride crystal grains is 0.6 μm or more for the top 50% and 1.0 μm or more for the top 20%. These numerical ranges can be adjusted by controlling the type and amount of sintering aid, sintering conditions (sintering temperature, sintering time, etc.), etc. For example, as the sintering temperature increases, the values ​​for the top 50% and top 20% of the maximum length of the silicon nitride crystal grains tend to increase. This is thought to be because, during sintering, when the silicon nitride crystal grains undergo a phase transition from the α phase to the β phase, β phase particles precipitate and grow due to thermal energy, constructing an anisotropic structure.

[0025] In this invention, the crystalline structure of the region within 250 μm from the surface is designed to have at least 50% silicon nitride grains with a maximum length of 0.6 μm or more, and at least 20% silicon nitride grains with a maximum length of 1.0 μm or more, thereby improving the mechanical properties in that region. As a result, the occurrence of defects when polishing that region can be suppressed, and the impact on the mechanical properties of the polished product can be minimized. As shown in the examples described later, the maximum length of the crystal grains was found to be more strongly correlated with the occurrence of damage during polishing than the aspect ratio, which was conventionally the target of crystal grain control.

[0026] In the silicon nitride sintered body described above, the maximum length of the silicon nitride crystal grains in the top 50% may be 0.65 μm or greater, or 0.7 μm or greater. The upper limit of the maximum length in the top 50% is not particularly limited, but may be, for example, less than 1.0 μm, 0.9 μm or less, or 0.8 μm or less. Furthermore, the maximum length of the top 20% of silicon nitride crystal grains may be 1.1 μm or greater, or 1.2 μm or greater. The upper limit of the maximum length in the top 20% is not particularly limited, but may be, for example, less than 1.5 μm, or 1.4 μm or less.

[0027] The ratio of the top 20% of maximum lengths to the top 50% of maximum lengths (top 20% of maximum lengths / top 50% of maximum lengths) is, for example, between 1.5 and 2.0.

[0028] In the silicon nitride sintered body described above, the aspect ratio may be further defined after defining the size of the silicon nitride crystal grains as described above. This aspect ratio is calculated using the image analysis described above. Specifically, for each silicon nitride crystal grain visible in the image, the longest diagonal is defined as the major axis L, and the shortest diagonal is defined as the minor axis S, and the aspect ratio is calculated as the ratio of the major axis L to the minor axis S (L / S).

[0029] The aspect ratio of silicon nitride crystal grains is defined as the top 50%, for example, between 1.2 and less than 2.0, or between 1.2 and less than 1.5. Furthermore, the aspect ratio of silicon nitride crystal grains is defined as the top 20%, for example, between 1.5 and less than 3.0, or between 1.5 and less than 2.0. These aspect ratios are defined as the top 50% of aspect ratios when a predetermined number of silicon nitride crystal grains are arranged in descending order of their aspect ratios, with the top 50% being the largest aspect ratio and the top 20% being the largest aspect ratio.

[0030] Furthermore, during the polishing process, it is not necessary to remove all silicon nitride crystal grains in the area in question. In the silicon nitride sintered body of the present invention, the machining allowance can be, for example, about 200 μm to 300 μm from the surface. By reducing the machining allowance, the finished product can be manufactured at a low cost while still satisfying requirements such as bearing function.

[0031] The silicon nitride sintered body of this embodiment contains rare earth elements and aluminum elements. Silicon powder (metallic silicon powder), which is less expensive than silicon nitride powder, can be used as the starting material for the silicon nitride sintered body. The silicon nitride sintered body can be manufactured by reaction sintering this powder (PS-RBSN (Post-Sintering of Reaction Bonded Silicon-Nitride) method).

[0032] Examples of rare earth elements include yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), neodymium (Nd), dysprosium (Dy), europium (Eu), and erbium (Er). These may be included individually or in groups of two or more. Of these, it is preferable to include one or more selected from the group consisting of Y, La, and Ce. For example, when silicon powder is used as a raw material, it is preferable to include Ce from the viewpoint of promoting nitriding.

[0033] The content of rare earth elements is, for example, 4% to 15% by weight in terms of oxides relative to the total weight of the silicon nitride sintered body. Having a rare earth element content within this range facilitates the densification of the silicon nitride sintered body. The above content of rare earth elements may be 5% or more by weight, 6.5% or more by weight, or 10% or more by weight relative to the total weight of the silicon nitride sintered body in terms of oxides. The above content of rare earth elements may be 14% or less by weight, or 12% or less by weight.

[0034] Furthermore, the aluminum content is, for example, 4% to 15% by weight in terms of oxides relative to the total weight of the silicon nitride sintered body. Having an aluminum content within this range makes it easier to densify the silicon nitride sintered body. The above aluminum content may be 5% or more by weight, 6.5% or more by weight, or 10% or more by weight, relative to the total weight of the silicon nitride sintered body in terms of oxides. The above aluminum content may be 14% or less by weight, or 12% or less by weight.

[0035] The aluminum content (in oxide equivalent) may be in a ratio of 1:3 to 3:1 relative to the rare earth element content (in oxide equivalent), or in a ratio of 1:2 to 2:1. Furthermore, from the viewpoint of sinterability, it is preferable that the aluminum content be higher than the rare earth element content. In addition, the aluminum content (in oxide equivalent) may be within ±5% by weight of the rare earth element content (in oxide equivalent), within ±2% by weight, within ±1% by weight, or may be the same as the rare earth element content.

[0036] By ensuring that the content of rare earth elements and aluminum elements in the silicon nitride sintered body is within the above range, it is possible to accelerate the nitriding reaction of the silicon powder raw material and subsequently accelerate sintering, for example, when producing the silicon nitride sintered body by the PS-RBSN method. The content of rare earth elements and aluminum elements can be adjusted by the amount of sintering aids containing rare earth elements (e.g., oxides of rare earth elements) and sintering aids containing aluminum elements (e.g., aluminum oxide) added to the raw material.

[0037] Furthermore, the total content of rare earth elements and aluminum elements is, for example, 8% to 26% by weight in terms of oxides relative to the total weight of the silicon nitride sintered body. For example, when silicon powder is used as a raw material, the total content may be 18% to 26% by weight, or 18% to 22% by weight.

[0038] The above-mentioned content of rare earth elements and aluminum elements can be determined using an X-ray fluorescence analyzer (XRF), energy-dispersive X-ray spectroscopy (EDX), or inductively coupled plasma (ICP) emission spectrometer. Specifically, the content of rare earth elements and aluminum elements in the silicon nitride sintered body can be determined using the above-mentioned analyzers, and then converted to rare earth element (RE) oxides (RE2O3 or REO2) and aluminum oxide (Al2O3). The elements of other components constituting the silicon nitride sintered body can also be analyzed using the above-mentioned analyzers, and the total weight of the silicon nitride sintered body can be calculated to determine the above-mentioned content of rare earth elements and aluminum elements. If the raw material powder used to manufacture the silicon nitride sintered body contains silicon (metallic silicon powder), and this silicon is converted to Si3N4 by nitriding, the weight of Si3N4 in the silicon nitride sintered body will be 1.67 times the weight of silicon. Therefore, by considering the weight change when silicon is nitrided, the content of rare earth element oxides and aluminum oxide can be calculated from the composition of the raw material powder.

[0039] Silicon nitride sintered bodies may contain components derived from sintering aids other than rare earth elements and aluminum elements. Specifically, they may contain at least one metallic element selected from titanium (Ti), tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Nb), iron (Fe), and chromium (Cr). These metallic elements are added, for example, in the raw material powder as elemental metals, oxides of metallic elements, or nitrides of metallic elements.

[0040] The content of the above-mentioned metal elements (total amount if there are two or more) is, for example, 0.1% to 5% by mass in terms of oxides, preferably 0.5% to 3% by mass, and may also be 1% to 3% by mass, relative to the total weight of the silicon nitride sintered body. This content can be calculated in the same way as the content of the rare earth elements and aluminum elements described above.

[0041] Furthermore, when silicon powder is used as a raw material, it is preferable that the silicon nitride sintered body does not contain transition metal elements. Transition metal elements are elements contained in groups 3 through 11 of the IUPAC periodic table, and include the aforementioned Ti, Fe, and Cr.

[0042] The silicon nitride sintered body of the present invention has excellent mechanical properties. For example, a silicon nitride sphere obtained by polishing the silicon nitride sintered body has a maximum contact pressure of 3.6 GPa and a rotational speed of 3000 min⁻¹. -1 It is preferable that the rolling life is 1000 hours or more when measured using a radial bearing testing machine under these conditions. This rolling life can be measured, for example, by the test described in the examples below.

[0043] The shape of the silicon nitride sintered body in this embodiment is not particularly limited and can be appropriately selected depending on the application, such as a spherical, cylindrical, conical, frustoconical, or rectangular parallelepiped shape, but a spherical shape is preferred. The size of the silicon nitride sintered body is also not particularly limited; for example, if it is spherical, the diameter can be 0.5 cm to 10 cm (it may also be 5 mm to 20 mm), and if it is cylindrical, the diameter of the base can be 0.5 cm to 15 cm and the height can be 3 cm to 20 cm.

[0044] (Manufacturing of silicon nitride sintered bodies) The silicon nitride sintered body described above may use silicon nitride powder as the starting material, but it is preferable to manufacture it using inexpensive silicon powder by the PS-RBSN method (two-stage sintering method). In that case, the method for manufacturing the silicon nitride sintered body includes a mixing step of mixing silicon powder and raw material powder containing a sintering aid, a molding step of forming the mixed raw material powder into a compact, and a sintering step of sintering the compact.

[0045] In the mixing process, for example, the raw material powders are mixed dry without using water or organic solvents. In this case, it is preferable to mix without using binder components. The particle size of the powder after mixing is not particularly limited, but it is preferable that D90 is 10 μm or more and 100 μm or less. It is also preferable that D50 is 2 μm or more and 10 μm or less. By having D90 and / or D50 within the above ranges, a dense silicon nitride sintered body can be obtained while exhibiting good fluidity and moldability. Note that D50 and D90 are the cumulative 50% diameter and cumulative 90% diameter based on volume, respectively, and are obtained by laser diffraction scattering particle size distribution measurement or the like.

[0046] Alternatively, wet granulation may be performed during the mixing process to obtain granulated powder as a mixture. In this case, the raw material powder and binder component are mixed with water and / or an organic solvent (e.g., ethanol) to form a slurry, which is then spray-dried to obtain granulated powder. An organic binder is used as the binder component, and it is added in an amount of, for example, 1% to 10% by weight relative to the total raw material powder.

[0047] In the molding process, the mixture obtained in the mixing process is molded into a predetermined shape to obtain a compacted powder by applying known molding methods such as the CIP method (cold isostatic pressing) or press molding. For example, in the case of a spherical shape, a spherical compacted powder may be formed, and then the compacted powder may be processed with a greening device to obtain a green sphere. A degreasing process may also be performed as needed.

[0048] In the sintering process, the obtained compacted powder is sintered by heat treatment in a nitrogen atmosphere at a predetermined temperature, for example, 1650°C to 1800°C (preferably 1700°C to 1800°C), for a predetermined time. The sintering time is set to, for example, 3 to 10 hours (preferably 5 to 8 hours). Various sintering methods can be applied, such as atmospheric pressure sintering, atmospheric pressure sintering, and pressure sintering (hot press). In atmospheric pressure sintering, for example, the pressure is set to 0.1 MPa to 10 MPa. Furthermore, primary and secondary sintering may be performed under different pressures during the sintering process.

[0049] In the production of the silicon nitride sintered body described above, the sintering aid used in the raw material powder may contain rare earth elements, aluminum elements, or transition metal elements. Among the rare earth elements, it is preferable to use one of Y2O3, La2O3, or CeO2 as the sintering aid. Among the aluminum elements, it is preferable to use Al2O3. Examples of sintering aids containing transition metal elements include Cr2O3, TiO2, MnO, and Fe2O3.

[0050] The raw material powder may contain silicon nitride powder and / or an organic binder in addition to silicon powder and a sintering aid, and may also contain a sintering aid containing elements other than rare earth elements, aluminum elements, and transition metal elements.

[0051] The silicon powder content in the raw material powder is preferably more than 60% by weight, more preferably 65% ​​by weight or more, and may be 70% by weight or more, relative to the total weight of silicon powder, silicon nitride powder, and sintering aid. It is also preferably less than 85% by weight, and more preferably 80% by weight or less. It is preferable that the raw material powder does not contain silicon nitride powder.

[0052] The amount of sintering aids containing rare earth elements (e.g., oxides of rare earth elements) in the raw material powder may be 10% by weight or more, 15% by weight or more, or 20% by weight or less, relative to the total weight. The amount of sintering aids containing aluminum elements (e.g., aluminum oxide) in the raw material powder may be 10% by weight or more, 15% by weight or more, or 20% by weight or less, relative to the total weight. If the amount of sintering aids in the raw material powder is low, it is difficult to obtain a dense silicon nitride sintered body, and if the amount of sintering aids is high, the mechanical properties of the silicon nitride sintered body tend to deteriorate.

[0053] The average particle size of silicon powder contained in the raw material powder can be, for example, 5 μm or less. If silicon nitride powder is included, its average particle size can be, for example, 0.5 μm or less. The average particle size of the sintering aid depends on the type of sintering aid, but is preferably 10 μm or less, may be 7 μm or less, may be 5 μm or less, may be 3 μm or less, may be 2 μm or less, may be 1 μm or less, or may be 0.4 μm or less. The average particle size is the cumulative 50% diameter based on volume and can be obtained by laser diffraction scattering particle size distribution measurement or the like.

[0054] When performing the PS-RBSN method, the rare earth oxide is preferably CeO2, and it is preferable that the raw material powder contains 10% to 20% by weight (more preferably 10% to 16% by weight) of CeO2 relative to the total raw material powder. It is also preferable that the raw material powder contains 10% to 20% by weight (more preferably 16% to 20% by weight) of Al2O3 relative to the total raw material powder. Furthermore, it is preferable that the raw material powder contains a total of 20% to 35% by weight (more preferably 25% to 35% by weight) of the rare earth oxide and aluminum oxide relative to the total raw material powder. It is also preferable that the raw material powder does not contain transition metal oxides. In addition, the above numerical ranges and the like can be appropriately combined with the above method.

[0055] (Applications of silicon nitride sintered bodies) The silicon nitride sintered body of this embodiment is polished to meet predetermined product specifications to obtain a silicon nitride sintered body product. The application of this product is not particularly limited, but it is preferable to use it as a sliding member because of its excellent mechanical properties. Sliding members are used, for example, in rolling parts and sliding parts. The sliding member of the present invention is a component that uses a polished product of the silicon nitride sintered body of the present invention as part or all of its components. Examples of sliding members include bearing members, rolling mill rolls, and compressor vanes. Examples of bearing members include raceways such as inner and outer rings, bearing rolling elements, and cages. The bearing of the present invention is a bearing that incorporates this sliding member as part or all of its bearing components, and examples include rolling bearings, sliding bearings (such as spherical bushings), linear guide bearings, ball screws, and linear bearings. In particular, the bearing of the present invention is preferably a rolling bearing that uses the polished product of the silicon nitride sintered body described above as bearing rolling elements.

[0056] An example of a bearing according to this embodiment will be described with reference to Figure 1. Figure 1 is a cross-sectional view of a deep groove ball bearing. The rolling bearing 1 has an inner ring 2 having an inner ring raceway surface 2a on its outer circumference and an outer ring 3 having an outer ring raceway surface 3a on its inner circumference, arranged concentrically, with a plurality of balls (rolling elements) 4 arranged between the inner ring raceway surface 2a and the outer ring raceway surface 3a. These balls 4 are polished products of the silicon nitride sintered body described above. The balls 4 are held by a cage 5. In addition, the axial openings 8a and 8b at both ends of the inner and outer rings are sealed by a sealing member 6, and a grease composition 7 is sealed around at least the balls 4. The grease composition 7 interposed on the raceway surface with the balls 4 provides lubrication.

[0057] Furthermore, the type of rolling bearing is not limited to deep groove ball bearings; angular contact ball bearings, thrust ball bearings, etc., may also be used. In the case of roller bearings, polished silicon nitride sintered bodies can also be used as rollers. [Examples]

[0058] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to these examples.

[0059] Table 1 shows the composition of the raw material powders used in the following test examples. Al2O3 and CeO2 were added to the silicon base powder in the ratios shown in Table 1. This powder was mixed in a ball mill at 200 rpm for 48 hours using silicon nitride balls as the media. Using each powder, spherical compacts with a diameter of 11 mm were prepared by cold isostatic pressing using a rubber mold. Each compact was sintered under the sintering conditions shown in Table 1 in a nitrogen atmosphere pressure of 0.9 MPa to obtain silicon nitride sintered bodies. The heating rate from 1500°C to each sintering temperature was 2.5 °C / min.

[0060] [Table 1]

[0061] Table 2 shows the composition ratios of each oxide in the obtained silicon nitride sintered body, calculated from the composition ratio of the raw material powder, assuming that all the silicon (metallic silicon) contained in the raw material powder is nitrided and the weight of silicon nitride is 1.67 times the weight of silicon.

[0062] [Table 2]

[0063] <Quantification of microstructure> 1. Maximum grain length The silicon nitride sintered bodies of Examples 1-5 and Comparative Examples 1-3 were cut through their centers, and the cut surfaces were mirror-polished. The mirror-polished cut surfaces were photographed at approximately five locations within 250 μm of the surface, in the circumferential direction of the rolling element cross-section (e.g., at 0°, 72°, 144°, 216°, and 288°), using a Carl Zeiss Microscopy Co., Ltd. "FE-SEM MERLIN". In the case of roller rolling elements, the bodies were cut at a cross-section half the length in the axial direction, and similarly, approximately five locations within 250 μm of the surface, in the circumferential direction of the rolling element cross-section (e.g., at 0°, 72°, 144°, 216°, and 288°). The captured images were analyzed using a Mitani Corporation "WinROOF" as shown in Figure 2, and the maximum length of each test piece was calculated. Field of view: 25 μm × 25 μm Quantity: 1500-3000 pieces Method for calculating maximum length: Maximum length portion of the particle (see Figure 9(a))

[0064] Figure 3 shows the analysis results for the maximum grain length. Of the cumulative frequency (0%-100%) for the maximum length, Figure 3(a) shows a magnified view of approximately 50%, and Figure 3(b) shows a magnified view of approximately 80%.

[0065] From Figure 3(a), the maximum lengths at a cumulative frequency of 50% were, in ascending order, Comparative Example 2, Comparative Example 1, Comparative Example 3, Example 1, Example 3, Example 5, Example 4, and Example 2. From Figure 3(b), the maximum lengths at a cumulative frequency of 80% were, in ascending order, Comparative Example 2, Comparative Example 1, Comparative Example 3, Example 1, Example 3, Example 5, Example 2, and Example 4. In relation to the sintering conditions, there was a tendency for the maximum length to increase with increasing sintering temperature.

[0066] 2. Aspect ratio of crystal grains The images captured as described in 1. above were analyzed using "WinROOF" manufactured by Mitani Corporation, as shown in Figure 2, and the aspect ratio of each test specimen was calculated. Field of view: 25 μm × 25 μm Quantity: 1500-3000 pieces Aspect ratio calculation method: The ratio of the long side to the short side of the particle (see Figure 9(b)).

[0067] Figure 4 shows the analysis results of the aspect ratio of the crystal grains. Of the cumulative frequency (0%-100%) of the aspect ratio, Figure 4(a) shows a magnified view of approximately 50%, and Figure 4(b) shows a magnified view of approximately 80%.

[0068] As shown in Figure 4(a), the aspect ratios at a cumulative frequency of 50% were, in ascending order, Example 1, Example 3, Comparative Example 1, Example 2, Example 5, Comparative Example 3, Comparative Example 2, and Example 4. As shown in Figure 4(b), the aspect ratios at a cumulative frequency of 80% were, in ascending order, Example 1, Comparative Example 1, Comparative Example 3, Comparative Example 2, Example 2, Example 3, Example 5, and Example 4. The relative sizes (order) of each test example between the cumulative frequency of 50% and 80% varied compared to the maximum length. Furthermore, no particular relationship was observed between the aspect ratio and the sintering conditions.

[0069] <Presence or absence of damage due to polishing process> Silicon nitride sintered bodies from Examples 1-5 and Comparative Examples 1-3 were polished under the same polishing conditions (machining allowance: 0.2-0.25 mm on each side) to produce 3 / 8-inch (diameter 9.525 mm) silicon nitride spheres (ceramic balls). These ceramic balls are finished spheres and correspond to the sliding members in the present invention. The surface condition of the polished ceramic balls was checked using a KEYENCE VHX-5000 to evaluate whether or not there was any damage. The results are shown in Table 3. Figure 5 shows the observation images (bright-field and dark-field) of Example 1, and Figure 6 shows the observation image of Comparative Example 1.

[0070] As shown in Table 3 and Figures 5-6, in Comparative Example 1 (and similarly in Comparative Examples 2-3), damage due to polishing occurred throughout the entire sphere, whereas in Example 1 (and similarly in Examples 2-5), no damage due to polishing was observed. As shown in Table 3, the specification of the maximum length of silicon nitride crystal grains was highly correlated with the presence or absence of damage due to polishing. Furthermore, in Comparative Example 1, although the top 50% met the requirement of 0.6 μm or longer, the top 20% did not meet the requirement of 1.0 μm or longer, suggesting the importance of meeting both requirements. In other words, it can be said that a certain proportion of crystal grains with a maximum length of 1.0 μm or longer is necessary. Thus, a higher proportion of crystal grains with a large maximum length leads to suppression of damage during polishing.

[0071] On the other hand, the aspect ratio of silicon nitride crystal grains showed little correlation with the presence or absence of damage during polishing. This is because the aspect ratio is the ratio of the length to the width of the silicon nitride crystal grain, and it is possible for large and small crystal grains to have the same aspect ratio regardless of their size. Therefore, simply defining the aspect ratio is insufficient, and in relation to damage that occurs during polishing, the maximum length of the crystal grain is particularly effective (see Figure 7).

[0072] <Bearing life test> To confirm the rolling fatigue characteristics of each ceramic ball obtained by the above polishing process, bearing life tests were conducted. Using each ceramic ball, and with NTN Corporation's "6206" as the outer ring, inner ring, and cage, bearing life (rolling life) tests were performed under the following conditions. The termination time for the bearing life test was set at 1000 hours, and exceeding 1000 hours was considered a pass. The results are shown in Table 3. In addition, for Example 1 and Comparative Example 1, observation images of the surface condition of the ceramic balls before and after the test are shown in Figure 8. Load (kN): Fr = 13.72 (6.86 kN / brg) Maximum contact pressure (GPa): Inner ring-to-ball: 3.5, Outer ring-to-ball: 3.6 Rotational speed (min -1 ):3000 Lubricant: JX Energy Corporation's additive-free turbine oil VG56 Lubricating oil supply temperature (℃): 50 Fueling method: Clean oil circulation

[0073] [Table 3]

[0074] As shown in Table 3, the finished spheres using silicon nitride sintered bodies from Examples 1 to 5, in which the maximum grain length was 0.6 μm or more in the top 50% and 1.0 μm or more in the top 20%, exhibited good rolling fatigue characteristics. In Examples 1 to 5, the process was terminated because it exceeded the 1000-hour termination time. On the other hand, in Comparative Examples 1 to 3, delamination occurred before the 1000-hour termination time (see Figure 8). This is thought to be because damage caused by the polishing process served as the starting point for delamination.

[0075] Thus, in the above embodiment, a bearing rolling element that satisfies the bearing function was obtained by controlling the maximum length of the crystal grains, which affects polishing and rolling fatigue life.

[0076] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the embodiments described above, and all modifications within the meaning and scope of the claims are intended to be included. [Industrial applicability]

[0077] The silicon nitride sintered body of the present invention can suppress damage during polishing and yields a product with excellent mechanical properties, ultimately providing a silicon nitride product with superior mechanical and sliding properties. Polished silicon nitride sintered bodies can be suitably used as rolling elements in bearings such as rolling bearings, linear guide bearings, ball screws, and linear bearings. [Explanation of Symbols]

[0078] 1 Rolling bearing 2 Inner ring 3 Outer ring 4 Rolling elements 5 Cage 6. Sealing member 7. Grease 8a, 8b opening

Claims

1. A silicon nitride sintered body containing rare earth elements and aluminum elements, A silicon nitride sintered body characterized in that the maximum length of silicon nitride crystal grains in a region within 250 μm from the surface of the silicon nitride sintered body is 0.6 μm or more for the top 50% and 1.0 μm or more for the top 20%.

2. The silicon nitride sintered body according to claim 1, characterized in that the maximum length of the silicon nitride crystal grains is 0.6 μm or more and less than 1.0 μm for the top 50%, and 1.0 μm or more and 1.5 μm or less for the top 20%.

3. The silicon nitride sintered body according to claim 2, characterized in that the aspect ratio of the silicon nitride crystal grains is 1.2 or more and less than 2.0 for the top 50%.

4. The silicon nitride sintered body according to claim 1 or 2, characterized in that the total content of the rare earth element and the aluminum element is 18% by weight or more and 26% by weight or less in terms of oxides relative to the total weight of the silicon nitride sintered body.

5. The silicon nitride sintered body according to claim 4, characterized in that the rare earth element is Ce, and the silicon nitride sintered body does not contain a transition metal element.

6. The silicon nitride sintered body according to claim 4, characterized in that the rare earth element is Ce, and the content of the aluminum element in the silicon nitride sintered body is greater than the content of Ce in terms of oxides.

7. A sliding member characterized by being a polished product of a silicon nitride sintered body according to claim 1 or claim 2.

8. The sliding member according to claim 7, characterized in that the sliding member is a rolling element for a bearing.

9. The rolling elements for the bearing have a maximum contact pressure of 3.6 GPa and a rotational speed of 3000 min. －１ The sliding member according to claim 8, characterized in that it has a rolling life of 1,000 hours or more when measured using a radial bearing testing machine under the specified conditions.

10. A bearing characterized by using the bearing rolling elements described in claim 8.

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