Silicon nitride sintered compact, machine part using the same, and bearing

Abstract

To provide a silicon nitride sintered compact having both processability and good mechanical properties, a machine part using the same, and a bearing.SOLUTION: A silicon nitride sintered compact comprises a sintering auxiliary component having a rare earth element and an aluminum element. In the silicon nitride sintered compact, the surface layer contains a higher amount of the sintering auxiliary component than the central part, the ratio of the sintering auxiliary component in the surface layer to that in the central part is 1.10 or more and less than 1.40, a rich layer having a high content of the sintering auxiliary component is formed in the surface layer, and the depth of the rich layer from the surface of the silicon nitride sintered compact is 0.12 mm or more and less than 0.19 mm.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a silicon nitride sintered body, and a machine part and a bearing using the same. [Background technology]

[0002] Silicon nitride (Si3N4) sintered bodies have excellent mechanical properties, thermal conductivity, and electrical insulation, and therefore are being applied to bearing components, engine parts, tool materials, and heat dissipation substrate materials. It is known that silicon nitride sintered bodies are manufactured using silicon nitride powder as a starting material. Since silicon nitride powder is difficult to sinter, sintering aids are used together with silicon nitride powder to manufacture densified silicon nitride sintered bodies. Such sintering aids generally include oxides of rare earth elements, aluminum oxide, magnesium oxide, silicon oxide, and the like.

[0003] Since silicon nitride powder is expensive, the price of silicon nitride sintered bodies tends to rise when silicon nitride powder is used to manufacture the silicon nitride sintered bodies. Therefore, a manufacturing method that uses silicon powder (metallic silicon powder), which is less expensive than silicon nitride powder, as a starting material and produces silicon nitride sintered bodies by reactive sintering has been attracting attention. One such manufacturing method is known as the PS-RBSN (Post-Sintering of Reaction Bonded Silicon-Nitride) method.

[0004] In addition, while silicon nitride sintered bodies have excellent mechanical properties, they are difficult to process due to their extremely high hardness. This results in a long processing time and high costs. For example, a technology has been proposed that can improve processability by adjusting the concentration of the processing liquid (Patent Document 1), but it is difficult to control the concentration and achieve stable processing. On the other hand, lowering the hardness to improve processability also reduces the wear resistance, so that using silicon nitride sintered bodies as bearing members, etc., may lead to a shortened product life. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-036695 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above circumstances, and has an object to provide a silicon nitride sintered body that combines workability with good mechanical properties, and a machine part and a bearing that use the same. [Means for solving the problem]

[0007] The silicon nitride sintered body of the present invention is a silicon nitride sintered body containing a sintering aid component including a rare earth element and an aluminum element, characterized in that the surface layer contains more of the sintering aid component than the central portion, and the ratio of the sintering aid component of the surface layer to the central portion (also simply referred to as the component ratio) is 1.10 or more and less than 1.40. In the present invention, the surface layer refers to the region of the silicon nitride sintered body from the surface to a depth of 0.25 mm, and the central portion refers to the region other than the surface layer.

[0008] The present invention is characterized in that a rich layer containing a large amount of the sintering aid component is formed on the surface layer, and the rich layer has a depth from the surface of the silicon nitride sintered body of 0.12 mm or more and less than 0.19 mm.

[0009] The content of the rare earth element is 6% by weight or more and 15% by weight or less, calculated as an oxide, based on the total weight of the silicon nitride sintered body, and the content of the aluminum element is 6% by weight or more and 15% by weight or less, calculated as an oxide, based on the total weight of the silicon nitride sintered body.

[0010] The total content of the rare earth element and the aluminum element is 18% by weight or more and 22% by weight or less, calculated as oxide, based on the total weight of the silicon nitride sintered body.

[0011] The rare earth element is Ce, and the silicon nitride sintered body is characterized in that it does not contain any transition metal element.

[0012] The silicon nitride sintered body has a surface layer formed with a rich layer containing a large amount of the sintering aid component, the rich layer having a depth from the surface of the silicon nitride sintered body of 0.12 mm or more and less than 0.19 mm, the rare earth element content is 6% by weight to 15% by weight, calculated as an oxide, relative to the total weight of the silicon nitride sintered body, the aluminum element content is 6% by weight to 15% by weight, calculated as an oxide, relative to the total weight of the silicon nitride sintered body, the sum of the rare earth element and the aluminum element content is 18% by weight to 22% by weight, calculated as an oxide, relative to the total weight of the silicon nitride sintered body, the rare earth element is Ce, and the silicon nitride sintered body does not contain a transition metal element.

[0013] The machine part of the present invention is a polished product of the silicon nitride sintered body of the present invention. The machine part is a rolling element.

[0014] The bearing of the present invention is characterized by using the above-mentioned rolling elements. Effect of the Invention

[0015] In the silicon nitride sintered body of the present invention, the surface layer contains more sintering aid components than the center portion, and the ratio of the sintering aid components in the surface layer to the center portion is 1.10 or more and less than 1.40, so that, for example, a layer rich in sintering aid components is formed in the surface layer, and both processability and good mechanical properties can be achieved. As a result, when processed into products such as bearing members, a good product life can be obtained. [Brief description of the drawings]

[0016] [Figure 1] 1 is a vertical sectional view showing an example of a bearing of the present invention. [Diagram 2] FIG. 4 is a vertical sectional view showing another example of a bearing of the present invention. [Diagram 3]FIG. 4 is a vertical sectional view showing another example of a bearing of the present invention. [Figure 4] The images show the surface layer and the central part that were the measurement targets in the examples. [Diagram 5] FIG. 1 is a diagram showing an outline of a crushing test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described. (Sintered silicon nitride) The silicon nitride sintered body of this embodiment is mainly composed of a crystalline phase of silicon nitride and contains an appropriate amount of a glass phase (amorphous phase) made of a sintering aid containing a rare earth element and an aluminum element. Note that the silicon nitride sintered body to be treated refers to one whose surface has not been polished after sintering.

[0018] In the silicon nitride sintered body of this embodiment, for example, the surface layer of the silicon nitride sintered body contains more sintering aid components than the center portion. This silicon nitride sintered body is obtained by sintering a green compact obtained by pressure molding of raw material powder containing sintering aids. It is considered that the sintering aid becomes liquid during sintering and is pushed out to the surface layer side, resulting in the uneven distribution of the sintering aid components.

[0019] In particular, this embodiment is characterized in that the ratio of the sintering aid component in the surface layer to the center portion is 1.10 or more and less than 1.40. By making the component ratio 1.10 or more and allowing the sintering aid component to be present in a large amount in the surface layer, processability such as polishing is improved. In addition, by making the component ratio less than 1.40, it is easier to ensure the mechanical properties of the finished product after processing. This component ratio can be calculated by the method of the examples described later. The component ratio is preferably 1.20 or more, and may be 1.25 or more. In addition, from the viewpoint of mechanical properties after processing, it may be 1.35 or less.

[0020] A rich layer containing more sintering aid components than other parts is formed on the surface layer of the silicon nitride sintered body. The rich layer is formed so as to cover the entire surface of the silicon sintered body. The rich layer contains more sintering aid components than the center part, is formed in a layer shape with a thickness from the surface, and can be observed by a scanning electron microscope. The depth of the rich layer from the surface of the silicon nitride sintered body is not particularly limited, but if it becomes too thick, it may be difficult to remove by polishing and may tend to remain on the surface of the finished product. This may lead to a decrease in strength and a decrease in bearing characteristics. Therefore, the depth of the rich layer is preferably less than 0.25 mm from the surface of the silicon nitride sintered body, more preferably less than 0.19 mm, and may be less than 0.16 mm. In addition, from the viewpoint of processability, the depth of the rich layer is preferably 0.10 mm or more from the surface, more preferably 0.12 mm or more. The depth of this rich layer can be calculated by the method of the examples described later.

[0021] In addition, in the manufacturing of the finished product, the rich layer of the silicon nitride sintered body is basically removed by polishing. By using the above-mentioned silicon nitride sintered body, for example, the processing rate is increased, so that the processing time can be shortened and the wear of the grinding wheel can be suppressed. As a result, the finished product can be manufactured at low cost.

[0022] The silicon nitride sintered body of this embodiment contains a rare earth element and an aluminum element. In the silicon nitride sintered body, the content of the rare earth element is preferably 6% by weight or more and 15% by weight or less in terms of oxide relative to the total weight of the silicon nitride sintered body, and the content of the aluminum element is preferably 6% by weight or more and 15% by weight or less in terms of oxide relative to the total weight of the silicon nitride sintered body.

[0023] Examples of rare earth elements include yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), neodymium (Nd), dysprosium (Dy), europium (Eu), erbium (Er), etc. Among these, it is preferable to contain one or more elements selected from the group consisting of Y, Ce, and La, and it is more preferable to contain Ce because it can promote nitridation even at a relatively low sintering temperature.

[0024] The content of the rare earth element is more preferably 6.5% by weight or more, and may be 7% by weight or more, calculated as oxide, based on the total weight of the silicon nitride sintered body.The content of the rare earth element is more preferably 14% by weight or less, and may be 12% by weight or less.

[0025] The aluminum content is more preferably 6.5% by weight or more, and may be 7% by weight or more, calculated as oxide, based on the total weight of the silicon nitride sintered body.The aluminum content is more preferably 14% by weight or less, and may be 12% by weight or less.

[0026] The aluminum element content (in terms of oxide) may be contained in a ratio of 1:3 to 3:1 relative to the rare earth element content (in terms of oxide), or may be contained in a ratio of 1:2 to 2:1. The aluminum element content (in terms of oxide) may be within ±5% by weight, ±2% by weight, or ±1% by weight of the rare earth element content (in terms of oxide), or may be the same as the rare earth element content.

[0027] By having the rare earth element and aluminum element contents in the silicon nitride sintered body within the above ranges, for example, when manufacturing a silicon nitride sintered body by the PS-RBSN method, the nitriding reaction of the raw material silicon powder (metallic silicon powder) can be promoted, and the subsequent sintering can be promoted, making it easier to set the component ratio of the sintering aid in the surface layer and the center of the silicon nitride sintered body and the depth of the rich layer within the desired range. The rare earth element and aluminum element contents can be adjusted by the amount of sintering aid containing a rare earth element (e.g., oxide of a rare earth element) and sintering aid containing an aluminum element (e.g., aluminum oxide) added to the raw material.

[0028] The total content of rare earth elements and aluminum elements is, for example, 15% by weight or more and 26% by weight or less, and preferably 18% by weight or more and 22% by weight or less, calculated as oxide, relative to the total weight of the silicon nitride sintered body.

[0029] The above contents of rare earth elements and aluminum elements may be determined using an X-ray fluorescence analyzer (XRF), an energy dispersive X-ray analyzer (EDX), or an inductively coupled plasma (ICP) emission analyzer. Specifically, the contents of rare earth elements and aluminum elements in the silicon nitride sintered body may be determined using the above analyzer, and converted into oxides of rare earth elements (RE) (RE2O3 or REO2) and aluminum oxide (Al2O3). The elements of other components constituting the silicon nitride sintered body may also be analyzed using the above analyzer, and the total weight of the silicon nitride sintered body may be calculated to determine the above contents of rare earth elements and aluminum elements. When silicon (metal silicon powder) is contained in the raw material powder used to manufacture the silicon nitride sintered body and the silicon is nitrided to become Si3N4, the weight of Si3N4 in the silicon nitride sintered body is 1.67 times the weight of silicon. Therefore, taking into account the weight change when silicon is nitrided, the contents of the oxides of rare earth elements and aluminum oxide can be calculated from the composition of the raw material powder.

[0030] The silicon nitride sintered body preferably does not contain a transition metal element, which is an element included in Groups 3 to 11 of the IUPAC periodic table, such as Fe, Ti, Cr, and Mn.

[0031] The shape of the silicon nitride sintered body of this embodiment is not particularly limited, and may be appropriately selected depending on the application from spherical, cylindrical, conical, truncated conical, rectangular, etc., but is preferably spherical. The size of the silicon nitride sintered body is also not particularly limited, and for example, if it is spherical, the diameter can be 0.5 cm to 10 cm, and if it is cylindrical, the bottom diameter can be 0.5 cm to 15 cm and the height can be 3 cm to 20 cm.

[0032] (Production of sintered silicon nitride) The silicon nitride sintered body is preferably manufactured by the PS-RBSN method (two-stage sintering method). The method for manufacturing the silicon nitride sintered body includes a mixing step of mixing raw material powder containing silicon powder and a sintering aid, a molding step of molding the mixed raw material powder into a green compact, and a sintering step of sintering the green compact.

[0033] In the mixing step, for example, the raw material powder is mixed in a dry manner without using water and an organic solvent. In this case, it is preferable to mix without using a binder component. 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 range, it is possible to obtain a dense silicon nitride sintered body while exhibiting good fluidity and moldability. Note that D50 and D90 are the cumulative 50% diameter and cumulative 90% diameter on a volume basis, respectively, and are obtained by laser diffraction scattering type particle size distribution measurement, etc.

[0034] In addition, wet granulation may be performed in the mixing step to obtain granulated powder as a mixture. In this case, the raw material powder and the binder component are mixed with water and / or an organic solvent (e.g., ethanol) to form a slurry, which is then spray-dried and granulated to obtain granulated powder. An organic binder is used as the binder component, and is added in an amount of, for example, 1% by weight to 10% by weight based on the total raw material powder.

[0035] In the molding step, the mixture obtained in the mixing step is molded into a predetermined shape by applying a known molding method such as a cold isostatic pressing (CIP) method or press molding to obtain a green compact. For example, in the case of a spherical shape, a spherical green compact may be formed and then the green compact may be processed by a green processing device to obtain a green sphere.

[0036] In the sintering step, the obtained green compact is sintered by heat treatment in a nitrogen atmosphere, for example, at 1600°C to 1800°C (preferably 1600°C to 1700°C, more preferably 1600°C to 1650°C) for a predetermined time. The sintering time is set, for example, for 3 hours to 10 hours. Applicable sintering methods include normal pressure sintering, atmospheric pressure sintering, and pressure sintering (hot press). In atmospheric pressure sintering, the pressure is set, for example, to 0.1 MPa to 10 MPa. In the sintering step, the primary sintering and secondary sintering may be performed under different pressures.

[0037] In the manufacture of the silicon nitride sintered body, the sintering aid used in the raw material powder may contain rare earth elements, aluminum elements, and transition metal elements. As the sintering aid containing rare earth elements, it is preferable to use any one of Y2O3, CeO2, and La2O3, and it is more preferable to use CeO2. As the sintering aid containing aluminum elements, it is preferable to use Al2O3. As the sintering aid containing transition metal elements, Cr2O3, TiO2, MnO, Fe2O3, etc. may be mentioned.

[0038] The raw material powder may contain, in addition to silicon powder and a sintering aid, silicon nitride powder and / or an organic binder, and may contain a sintering aid containing an element other than a rare earth element, an aluminum element, and a transition metal element.

[0039] The content of silicon powder 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, based on 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.

[0040] The content of the sintering aid containing a rare earth element (e.g., oxide of a rare earth element) contained in the raw material powder is preferably 10% by weight or more, may be 15% by weight or more, or may be 20% by weight or less, based on the total weight. The content of the sintering aid containing an aluminum element (e.g., aluminum oxide) contained in the raw material powder is preferably 10% by weight or more, may be 15% by weight or more, or may be 20% by weight or less, based on the total weight. If the content of the sintering aid contained in the raw material powder is low, it is difficult to obtain a dense silicon nitride sintered body, and if the content of the sintering aid is high, the mechanical properties of the silicon nitride sintered body are likely to deteriorate.

[0041] The average particle size of the silicon powder contained in the raw material powder can be, for example, 5 μm or less. When silicon nitride is contained, the average particle size can be, for example, 0.5 μm or less. The average particle size of the sintering aid is preferably 10 μm or less, and may be 7 μm or less, 5 μm or less, 3 μm or less, 2 μm or less, 1 μm or less, or 0.4 μm or less, depending on the type of sintering aid. The average particle size is the cumulative 50% diameter based on volume, and is obtained by laser diffraction scattering type particle size distribution measurement or the like.

[0042] The method for producing the silicon nitride sintered body of this embodiment includes a mixing step of mixing silicon powder with a raw material powder containing a sintering aid containing a rare earth oxide and aluminum oxide, a molding step of molding the mixed raw material powder into a green compact, and a sintering step of sintering the green compact, and the sintering step is preferably a step of performing a heat treatment by holding the mixture in a nitrogen atmosphere at a sintering temperature of 1600°C to 1700°C for a predetermined time. For example, by using a predetermined amount of sintering aid and sintering at a relatively low sintering temperature (for example, 1600°C to 1700°C), the ratio of the sintering aid component described later can be easily set within a desired range.

[0043] The rare earth oxide is preferably CeO2. The raw material powder preferably contains the rare earth oxide and the aluminum oxide in total in an amount of 20% by weight or more and 35% by weight or less (more preferably 25% by weight or more and 35% by weight or less) based on the entire raw material powder. The raw material powder preferably does not contain a transition metal oxide. The above-mentioned numerical ranges can be appropriately combined with the above method.

[0044] (Applications of sintered silicon nitride) The silicon nitride sintered body of the present embodiment is polished to meet a predetermined product standard to obtain a silicon nitride sintered body product. The use of the product is not particularly limited, but it is preferably used as a machine part because of its excellent mechanical properties. The machine part is used, for example, in a rolling part or a sliding part. The machine part of the present invention is a part using the polished product of the silicon nitride sintered body of the present invention as a part or all of its configuration. Examples of the machine part include sliding members, bearing members, rolling roll materials, compressor vanes, engine parts such as gas turbine blades, cutting tools (chips), etc. Examples of the bearing member include raceways such as inner and outer rings, rolling elements for bearings, and retainers. The bearing of the present invention is a bearing that includes this machine part as a part or all of its bearing member, and examples of the bearing include rolling bearings, sliding bearings (spherical bushes, etc.), linear guide bearings, ball screws, linear bearings, etc. In particular, the bearing of the present invention is preferably a rolling bearing using the polished product of the silicon nitride sintered body as the rolling elements for bearings.

[0045] An example of the bearing of this embodiment will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view of a deep groove ball bearing. In the rolling bearing 1, an inner ring 2 having an inner ring raceway surface 2a on its outer circumferential surface and an outer ring 3 having an outer ring raceway surface 3a on its inner circumferential 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 polished products of the above-mentioned silicon nitride sintered body. The balls 4 are held by a cage 5. In addition, openings 8a, 8b at both axial ends of the inner and outer rings are sealed by a seal member 6, and a grease composition 7 is enclosed at least around the balls 4. The grease composition 7 is interposed between the raceway surface of the balls 4 to lubricate them. The bearing type of the rolling bearing is not limited to a deep groove ball bearing, and may be an angular ball bearing, a thrust ball bearing, or the like.

[0046] Another example of the bearing of this embodiment will be described with reference to FIG. 2. FIG. 2 is a cross-sectional view showing a ball screw. As shown in FIG. 2, the ball screw has a screw groove 12 formed on the outer peripheral surface of a screw shaft 11, which is a guide member, and a screw groove 14 formed on the inner peripheral surface of a ball nut 13, with a plurality of balls 15 interposed between them. The rotational power of the screw shaft 11 (or the ball nut 13) is transmitted to the ball nut 13 (or the screw shaft 11) via the balls 15, and the ball nut 13 is moved in the axial direction. In FIG. 2, the balls 15 are the above-mentioned polished products of the silicon nitride sintered body, and the screw shaft 11 and the ball nut 13 are made of steel (for example, bearing steel, low carbon steel, etc.). In addition, a grease composition is filled around the balls 15 between the screw shaft 11 and the ball nut 13, and is sealed by a seal member 16 for the ball screw.

[0047] In the ball screw shown in Fig. 2, the ball circulation system is not particularly limited, and any of the following circulation systems can be used: tube type, return tube (pipe) type, deflector type, end deflector type, end cap type, top type, etc. In any circulation system, the circulation path has a large effect on the smooth circulation of the balls.

[0048] Specifically, the ball screw is used to convert the rotational motion of a motor into linear motion, and can be suitably used in, for example, electric actuators, positioning devices, electric jacks, servo cylinders, electric servo presses, mechanical presses, electric brake devices, transmissions, electric power steering devices, electric injection molding machines, and the like.

[0049] Here, ball screws are required to have wear resistance, toughness, high load capacity, etc. In recent years, with the trend towards miniaturization, there is a growing demand for performance that can withstand high loads, as well as the need to suppress hydrogen embrittlement caused by hydrogen generated from lubricants due to slippage or high loads. In the example of Figure 2, the balls used are the polished silicon nitride sintered bodies described above, which makes it easy to meet these requirements and also provides a long product life for the balls.

[0050] Furthermore, in a ball screw, if there is a large misalignment due to an installation error or the like when installing the ball screw, there is a risk of twisting (i.e., a relative inclination between the screw shaft and the nut). If a moment due to twisting acts on the ball screw, the load balance in the nut is lost, and there are places where the contact surface pressure increases partially, which may shorten the lifespan. In response to this, by using the above-mentioned polished silicon nitride sintered body as the balls, the circulation performance of the balls can be improved, making it easier to suppress a decrease in lifespan.

[0051] Further, another example of the bearing of this embodiment will be described with reference to FIG. 3. FIG. 3 is a cross-sectional view showing an example of a spherical plain bearing. As shown in FIG. 3, the spherical plain bearing 21 is a combination of an inner ring 22 having a spherical outer peripheral surface 22b and a bearing hole 24 formed in an inner peripheral surface 22a through which a support shaft can be inserted, and an outer ring 23 having a concave surface 23a corresponding to the outer peripheral surface 22b. In the spherical plain bearing 21, at least one of the inner ring 22 and the outer ring 23 is formed of a polished product of the silicon nitride sintered body described above. The material of the other member is not particularly limited, and can be, for example, made of metal such as aluminum alloy, stainless steel, or steel, made of synthetic resin, or made of ceramics other than the silicon nitride sintered body described above.

[0052] A spherical plain bearing is a self-aligning plain bearing with a spherical sliding surface that can withstand radial loads and axial loads in both directions. Spherical plain bearings are suitable for oscillating and aligning motions, and are used in the joints of industrial and construction machinery. Spherical plain bearings can be either oil-free (see Figure 3) or oil-lubricated; for example, in the case of an oil-lubricated type, oil holes and oil grooves are provided in the inner and outer rings. When installing a spherical plain bearing, grease may be applied to the sliding surfaces. EXAMPLES

[0053] 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.

[0054] [Examples 1 to 5, Comparative Examples 1 to 6] Raw powders were prepared in the ratios shown in Table 1 and dry mixed in a ball mill. They were then press molded at a molding pressure of 30 MPa and molded into a green compact by cold isostatic pressing (200 MPa). The green compact obtained was held at 1650°C for 10 hours in a nitrogen atmosphere (pressure: 0.9 MPa) and sintered under pressure to obtain a silicon nitride sintered body.

[0055] [Example 6] Raw powders were prepared with the compounding ratio shown in Table 1, and a specified amount of organic binder was added and wet mixed in a ball mill. After that, the mixture was press molded at a molding pressure of 30 MPa, and then molded into a green compact by cold isostatic pressing (200 MPa). The obtained green compact was degreased in a degreasing furnace, and then held at 1750°C for 4 hours in a nitrogen atmosphere and sintered at atmospheric pressure. Furthermore, the atmospheric sintered compact was subjected to HIP treatment at 1700°C for 1 hour under nitrogen pressure of 100 MPa.

[0056] [Table 1]

[0057] Regarding the composition ratio of each oxide in the obtained silicon nitride sintered body, the values ​​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, are shown in Table 2.

[0058] [Table 2]

[0059] The silicon nitride sintered body produced under each condition was used to evaluate the processing characteristics and various physical properties, and the strength and rolling fatigue characteristics were evaluated using polished silicon nitride sintered body (machining allowance 0.2 to 0.25 mm). The evaluation results are summarized in Table 3.

[0060] <Calculation of processing rate> In accordance with JIS B 1563, the ball was polished to G5 to prepare a 3 / 8 inch (diameter 9.525 mm) spherical test piece. In the above polishing, a #200 diamond surface plate was used to polish the ball until its diameter reached 9.625 mm. During this process, the ball dimensions were measured every hour in accordance with JIS B 1501, and the processing rate μm / h was calculated. In this test, a processing rate of 10 μm / h or more was considered to have passed.

[0061] <Calculation of assistant ratio and assistant-rich layer depth> The silicon nitride sintered body (before polishing) prepared above was cut at a cross section passing through its center, and the cut surface was mirror-polished. The mirror-polished cut surface was subjected to a component analysis of the surface layer and the center using a scanning electron microscope manufactured by Hitachi. The image of the cut surface and the measurement points are shown in Figure 4(a). The surface layer to be measured was a region within 0.25 mm from the surface, and the center was a specified region including the center of the cross section. The component analysis was performed by EDX analysis at a 1000x field of view using a scanning electron microscope for both the surface layer and the center. Specifically, EDX analysis was performed in the surface layer (region from the surface to a depth of 0.25 mm) toward the center at multiple points, and the average value of the measured values ​​was used as the value of the surface layer. The measurement points for the component analysis of the surface layer were 8 or less points on the silicon nitride sintered body. The center was also measured with the same number of fields of view, and the average value was used as the value of the center. Specifically, the center was scanned in a specified region including the center of the cross section, which corresponds to the scanning range of the surface layer. The amount of auxiliary components detected in the surface layer (the total amount of CeO2 (or La2O3) and Al2O3) was calculated from the analysis results, assuming that the amount of auxiliary components detected in the center (the total amount of CeO2 (or La2O3) and Al2O3) was 1.

[0062] The depth of the rich layer was measured by performing element mapping by EDX analysis at a 500x field of view, scanning from the surface in the depth direction, and calculating the range where the amount of detected auxiliary components (the total amount of detected CeO2 (or La2O3) and Al2O3) was high from the obtained mapping image as the rich layer. Specifically, line analysis was performed in a region from the surface to 0.5 mm (e.g., two 500x fields of view), and the depth of the rich layer was determined as the end point of the rich layer, where the detection intensity became weak (e.g., below a threshold value (20% from the detection intensity at the surface)).

[0063] Figure 4(b) shows the color chart of an electron probe microanalyzer (EPMA), and Figure 4(c) shows an SEM image. These figures also show that a large amount of auxiliary components are present in the surface layer.

[0064] <Crushing test> A crushing test was conducted as a strength test. A two-ball crushing test was conducted using the above test piece. The crushing test was in accordance with JIS B 1501. As shown in FIG. 5, the test machine has a fixed jig 31 and a movable jig 32, and the movable jig 32 moves up and down by a crosshead 33. The fixed jig 31 and the movable jig 32 each have a conical depression formed therein, and two test balls 34 were set between these depressions. The stroke speed of the crosshead 33 was 1.0×10 mm / min. The load at which the test balls 34 were crushed was measured. A crushing strength of 20 kN or more was deemed to pass.

[0065] <Rolling fatigue test> A rolling fatigue test was conducted to evaluate the bearing characteristics. Using the above test specimens and NTN Corporation's "6206" bearing outer ring, inner ring, and cage, a rolling fatigue test was conducted at a rotation speed of 3000 rpm, a load of 1.5 GPa, and a test time of 168 hours to evaluate the product life. The lubricant used was JXTG Nippon Oil & Energy Corporation's additive-free turbine oil "VG56." Test specimens that did not peel off within the test time were deemed to have passed.

[0066] [Table 3]

[0067] As shown in Table 3, the processing rate was significantly improved when the ratio of the auxiliary agent in the surface layer to the center portion was 1.10 or more. In particular, the silicon nitride sintered bodies (Examples 1 to 5) produced by the PS-RBSN method had processing rates of 20 μm / h or more, and showed superior processability compared to Example 6. Furthermore, when the ratio of the auxiliary agent was less than 1.40, good results were obtained in the crushing strength and rolling fatigue tests.

[0068] On the other hand, the crushing strength decreased in Comparative Examples 4 to 6, in which the ratio of sintering aids was 1.40 or more. One of the reasons for this is thought to be that the total content of the sintering aids used in the raw materials was relatively high at 40% by weight or more, resulting in a large amount of glass phase formed after sintering. In addition, the deeper the layer rich in sintering aids, the more difficult it is to remove by polishing and the more likely it is to remain on the surface of the finished ball, which may have led to a decrease in strength and bearing characteristics.

[0069] In the rolling fatigue test, the test pieces did not peel off within the test time in Examples 1 to 6. On the other hand, peeling due to voids occurred in Comparative Examples 1 to 3. Furthermore, minute peeling occurred in Comparative Examples 5 and 6, which led to the test being stopped due to abnormal vibration.

[0070] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is defined by the claims rather than the above-described embodiments, and is intended to include the equivalent meaning of the claims and all modifications within the scope of the claims. [Industrial Applicability]

[0071] The silicon nitride sintered body of the present invention can be suitably used as a rolling element of bearings such as rolling bearings, linear guide bearings, ball screws, and linear bearings. [Explanation of symbols]

[0072] 1. Rolling bearings 2. Inner Circle 3 Outer ring 4 Rolling elements 5 Cage 6 Sealing material 7 Grease 8a, 8b opening 11 Screw shaft 12 Thread groove 13 Ball nut 14 Thread groove 15 Ball 16 Ball screw seal material 21 Spherical plain bearing 22 Inner Circle 23 Outer Ring 24 Bearing hole 31 Fixture 32 Movable jig 33 Crosshead 34 Test ball

Claims

1. A silicon nitride sintered body containing a sintering aid component including a rare earth element and an aluminum element, A silicon nitride sintered body, characterized in that the surface layer contains a larger amount of the sintering aid component than the central portion, and the ratio of the sintering aid component of the surface layer to the central portion is 1.10 or more and less than 1.

40.

2. 2. The silicon nitride sintered body according to claim 1, characterized in that a rich layer containing a large amount of the sintering aid component is formed on the surface layer, and the depth of the rich layer from the surface of the silicon nitride sintered body is 0.12 mm or more and less than 0.19 mm.

3. 3. The silicon nitride sintered body according to claim 1, wherein the content of the rare earth element is, in terms of oxide, 6 to 15% by weight, both inclusive, of the total weight of the silicon nitride sintered body, and the content of the aluminum element is, in terms of oxide, 6 to 15% by weight, both inclusive, of the total weight of the silicon nitride sintered body.

4. 4. The silicon nitride sintered body according to claim 3, characterized in that the total content of said rare earth element and said aluminum element is 18% by weight or more and 22% by weight or less, calculated as oxide, based on the total weight of said silicon nitride sintered body.

5. 3. The silicon nitride sintered body according to claim 1, wherein the rare earth element is Ce, and the silicon nitride sintered body does not contain any transition metal element.

6. a rich layer containing a large amount of the sintering aid component is formed on the surface layer, and the rich layer has a depth from the surface of the silicon nitride sintered body of 0.12 mm or more and less than 0.19 mm; the content of the rare earth element is, in terms of oxide, 6% by weight or more and 15% by weight or less, based on the total weight of the silicon nitride sintered body, the content of the aluminum element is, in terms of oxide, 6% by weight or more and 15% by weight or less, based on the total weight of the silicon nitride sintered body, and the total content of the rare earth element and the aluminum element is, in terms of oxide, 18% by weight or more and 22% by weight or less, based on the total weight of the silicon nitride sintered body; 2. The silicon nitride sintered body according to claim 1, wherein the rare earth element is Ce, and the silicon nitride sintered body does not contain any transition metal element.

7. 3. A machine part which is a polished product of the silicon nitride sintered body according to claim 1 or 2.

8. 8. The mechanical part according to claim 7, wherein the mechanical part is a rolling element.

9. A bearing using the rolling element according to claim 8.

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