Method for manufacturing a sliding member made of a silicon nitride sintered body

Abstract

To provide a manufacturing method for a slide member made from a silicon nitride sintered body, capable of manufacturing a slide member showing stable durability even if a load is changed.SOLUTION: A manufacturing method related to one embodiment includes: a cracking step of obtaining a raw material blend by cracking and blending a silicon nitride powder, a sintering aid powder, and a binder; a molding step of molding the raw material blend; a degreasing step of obtaining a degreased body by degreasing the molded body; a sintering step of obtaining a silicon nitride sintered body by sintering the degreased body; a step of performing HIP treatment to the silicon nitride sintered body; and a processing step of processing the silicon nitride sintered body into a slide member. The sintering step is performed in a non-oxidative atmosphere and at a sintering temperature of 1650-2000°C. In the sintering step, changes in pressure due to a gas generated from the degreased body is 0.3 MPa or under. In any given 50-μm2 observation region of a specific cross section or surface, the slide member displays a ratio of 0-10% for silicon nitride crystal grains with internal dislocation defects out of any 50 silicon nitride crystal grains with their entire contours visible.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The embodiment relates to a method for manufacturing a sliding member made of a silicon nitride sintered body.

Background Art

[0002] Wear-resistant sliding members made of silicon nitride sintered bodies are used in various fields such as various roll materials for rolling, engine parts such as cam rollers, bearing members, compressor vanes, gas turbine blades, friction stir welding tool members, etc. These are used as members that slide against mating members. The silicon nitride sintered body is required to have strength and wear resistance. For example, Japanese Patent No. 5380277 (Patent Document 1) discloses a silicon nitride sintered body in which the silicon nitride crystal particle size and void size are controlled. The silicon nitride sintered body of Patent Document 1 can improve both strength and wear resistance. As a result, in Patent Document 1, a rolling life of 600 hours or more is obtained. In recent years, electric vehicles have become popular. For motors mounted on electric vehicles, inverter drive has become the mainstream. In the inverter drive method, the rotational speed of the motor can be changed by changing the frequency of the power source that drives the motor. That is, in the inverter drive method, the rotational speed of the motor can be changed. Inverter-driven motors are spreading in various fields such as electric vehicles and industrial equipment. By adopting inverter drive, the rotational speed of the motor could be changed from 1000 rpm to about 15000 rpm at the fastest.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As described above, inverter drive is a driving method capable of changing the rotational speed of a motor. In Patent Document 1, the durability test of bearing balls was conducted under the conditions of a maximum contact pressure of 5.9 MPa and a rotational speed of 1200 rpm. Usually, the durability test is conducted with a constant rotational speed. With the spread of inverter drive, driving the motor while changing the rotational speed is increasing. When the rotational speed changes, the load applied to the bearing balls and bearings becomes non-constant. In conventional silicon nitride sintered bearing balls, variations in durability occurred in the usage environment where the load changes. Also, problems such as the occurrence of electrolytic corrosion occurred. The present invention is for addressing such problems, and an object thereof is to provide a method for manufacturing a silicon nitride sintered sliding member capable of manufacturing a sliding member that exhibits stable durability even when the load changes.

Means for Solving the Problems

[0005] The method for manufacturing a silicon nitride sintered sliding member according to an embodiment includes a pulverizing step of obtaining a raw material mixture by pulverizing and mixing silicon nitride powder, sintering aid powder, and a binder using a pulverizer, a molding step of molding the raw material mixture, a degreasing step of degreasing the molded body to obtain a degreased body, a sintering step of sintering the degreased body to obtain a silicon nitride sintered body, a step of performing hot isostatic pressing (HIP) treatment on the silicon nitride sintered body, and a processing step of processing the silicon nitride sintered body into a sliding member. The sintering step is performed in a non-oxidizing atmosphere at a sintering temperature of 1650 °C or higher and 2000 °C or lower. In the sintering step, the change in pressure due to the gas generated from the degreased body is 0.3 MPa or less. In the sliding member, in an observation region of 50 μm × 50 μm of an arbitrary cross-section or surface, the ratio of the number of silicon nitride crystal particles having a dislocation defect portion inside among any 50 silicon nitride crystal particles whose entire contour can be seen is 0% or more and 10% or less.

Effects of the Invention

[0006] The sliding member manufactured by the method for manufacturing a silicon nitride sintered body sliding member according to the embodiment has a reduced ratio of silicon nitride crystal particles having dislocation defect portions. Thereby, excellent durability can be obtained even when the sliding conditions change such as a change in the rotational speed. Also, the occurrence of electrolytic corrosion can be suppressed.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0008] The sliding member manufactured by the method for manufacturing a silicon nitride sintered body sliding member according to the embodiment has a reduced ratio of silicon nitride crystal particles having dislocation defect portions. Thereby, excellent durability can be obtained even when the sliding conditions change such as a change in the rotational speed. Also, the occurrence of electrolytic corrosion can be suppressed.

[0009] The silicon nitride sintered body has silicon nitride crystal particles and a grain boundary phase. The grain boundary phase is mainly composed of components of a sintering aid. The grain boundary phase is formed by the reaction of the sintering aid in the sintering process. The reaction occurs between the sintering aids, between the sintering aid and silicon nitride, or between the sintering aid and impurity oxygen. The sliding member according to the embodiment is characterized in that, in an observation region of 50 μm × 50 μm of an arbitrary cross section or surface, the ratio of the number of the silicon nitride crystal particles having dislocation defect portions inside among any 50 of the silicon nitride crystal particles whose entire contour can be seen is 0% or more and 10% or less.

[0010] For the observation of dislocation defects, an arbitrary cross section or surface of the silicon nitride sintered body is used. First, process any cross-section or surface of the silicon nitride sintered body by ion milling or FIB (focused ion beam) processing so that the surface roughness Ra becomes 1 μm or less. The processed cross-section or surface shall be the evaluation surface. Next, observe the evaluation surface with a transmission electron microscope (TEM). The magnification during evaluation by TEM shall be set to 10,000 times or more. The area of the evaluation surface shall be set to 50 μm × 50 μm. When an area of 50 μm × 50 μm cannot be observed in a single field of view, the evaluation surface may be divided into multiple parts for observation. For example, observation may be performed with a field of view of 10 μm × 10 μm or less. In the evaluation, after observing one observation area (50 μm × 50 μm), observe another area more than 1000 μm away from that area. That is, observe the area of 50 μm × 50 μm at two or more locations, and calculate the ratio of the number of silicon nitride crystal particles (defective particles) having dislocation defect parts. The presence or absence of dislocation defect parts is determined by observing the dark field and bright field of the TEM observation image. The dislocation defect part appears white in the dark field and appears black and inverted in the bright field. In this way, the part where the color of the pixel is inverted when switching between the dark field and the bright field is defined as the dislocation defect part.

[0011] Figure 1 is a conceptual diagram illustrating a silicon nitride crystal particle having a dislocation defect part. In Figure 1, 1 is a silicon nitride crystal particle, and 2 is a dislocation defect part. As shown in Figure 1, in the silicon nitride sintered body of the sliding member according to the embodiment, a dislocation defect part 2 may exist inside the silicon nitride crystal particle 1. If there is a dislocation defect part in the silicon nitride crystal particle, the durability when the sliding conditions change decreases. The dislocation defect part is a crystal defect contained in the crystal. Crystal defects are also called lattice defects. Crystal defects occur due to disorder in the atomic arrangement or impurities. The dislocation defect part causes defects in the stable crystal structure. For example, the rotational speed of a motor using inverter drive varies in the range of about 0 rpm to 15,000 rpm. A rotational speed of 0 rpm means the motor is in a stopped state. The motor driven by the inverter can be driven while changing the rotational speed to about 50 to 15,000 rpm. Along with this, the load on the bearing balls used in the motor bearings also changes. It has been found that when a strong load such as that on a bearing ball is applied to a silicon nitride sintered body, the dislocation defect portions affect the reduction in durability. The silicon nitride sintered body is a material with high strength and high wear resistance. As in Patent Document 1, there is no problem when the surface of the sliding member slides against the mating member at a constant rotational speed. However, it has been found that when the sliding member slides while the way the load is applied changes, it affects the long-term life of the sliding member. The vibration frequency of the motor changes depending on the rotational speed. That is, when the rotational speed changes, the vibration frequency also changes. The vibration of the motor leads to the vibration of the bearings. The bearings resonate at a specific vibration frequency. Resonance is a phenomenon in which when vibrations equal to the natural vibration frequency are applied to the vibrating body from the outside, the amplitude of the vibration increases. Resonance occurs when the vibration frequency of the bearings is close to the natural vibration frequency of the bearings. Under resonance, the load on the rolling elements (bearing balls) increases. In a motor driven by an inverter, the vibration frequency changes. When passing through the vibration frequency band that causes resonance of the bearings during the change of the vibration frequency, the load on the rolling elements increases. Thus, when the rotational speed changes, the sliding conditions change. As described above, the dislocation defect portions are crystal defects. When the load applied to the silicon nitride sintered body is small, no problem occurs, but when the load applied to the silicon nitride sintered body is large, an influence appears. This is because the way of receiving stress is different between the silicon nitride sintered body having dislocation defect portions and the silicon nitride sintered body not having dislocation defect portions. In the silicon nitride sintered body of the sliding member according to the embodiment, in an arbitrary minute region of 50 μm × 50 μm, the ratio of the number of silicon nitride crystal particles having dislocation defect portions is 0% or more and 10% or less. The fact that the ratio in an arbitrary observation region of 50 μm × 50 μm is 0% or more and 10% or less indicates that, no matter which 50 μm × 50 μm region is observed, the ratio of the number is 0% or more and 10% or less.

[0012] When the ratio of the number of silicon nitride crystal particles having dislocation defect portions exceeds 10%, electrical erosion is likely to occur. When a bearing is arranged near a device driven by an inverter or a device that generates high frequency, an electric current flows into the bearing due to the influence of electromagnetic noise. The phenomenon in which the raceway surface of the bearing is damaged by this electric current is called electrical erosion. Although the inside of the bearing is insulated by grease or the like, when the electric current exceeds a certain amount, a discharge phenomenon occurs. The dislocation defect portions of the silicon nitride crystal particles are crystal defects. Since a potential difference is likely to occur in the crystal defect portions, it is considered that they are likely to become discharge paths. When the ratio of the number of silicon nitride crystal particles having dislocation defect portions exceeds 10%, the dislocation defect portions in the silicon nitride sintered body become discharge paths, and as a result, electrical erosion is likely to occur on the raceway surface of the bearing.

[0013] Generally, grease is filled inside the bearing (between the inner ring and the outer ring). The grease can improve the lubricity, heat resistance, water resistance, etc. of the bearing. When a motor is driven by an inverter, a voltage is generated on the rotating shaft. Electrical erosion occurs when the grease inside the bearing breaks down due to this voltage. The silicon nitride sintered body is an insulator, and its volume resistivity is 1 × 10 14 Ω·cm or more at room temperature. When an electric field is applied to an insulator, it polarizes. In an inverter drive, the rotation speed can be changed by changing the frequency. Therefore, in an inverter, an alternating electric field is generated. The dislocation defect portions have defects. Therefore, an electric field difference due to the polarization phenomenon is likely to occur. In the sliding member according to the embodiment, since there are few dislocation defect portions, the generation of the electric field difference can be suppressed. Therefore, in the silicon nitride crystal particles present in the observation region of 50 μm × 50 μm, the ratio of the number of silicon nitride crystal particles having dislocation defect portions inside is preferably 0% or more and 10% or less, more preferably 0% or more and 3% or less. Most preferably, the ratio is 0%. That is, by having no silicon nitride crystal particles having dislocation defect portions, the effects of improving durability and suppressing electrolytic corrosion can be enhanced. Note that two or more regions separated from each other by 1000 μm or more are used as observation targets. Also, in a TEM photograph showing at least a part of the observation region of 50 μm × 50 μm, silicon nitride crystal particles whose entire contours are not shown are not used for calculating the ratio of the number. For example, silicon nitride crystal particles whose contours are cut off at the edge of the photograph are not used for calculating the ratio of the number. Also, when 50 silicon nitride crystal particles with all their contours shown can be confirmed, the ratio of the number of silicon nitride crystal particles having dislocation defect portions among the 50 silicon nitride crystal particles is determined. That is, observation is continued until 50 silicon nitride crystal particles with all their contours shown can be confirmed. When 50 silicon nitride crystal particles with all their contours shown cannot be observed in one observation region of 50 μm × 50 μm, 50 silicon nitride crystal particles with all their contours shown are observed in another observation region of 50 μm × 50 μm. When more than 50 silicon nitride crystal particles are shown in the observation region of 50 μm × 50 μm, any 50 silicon nitride crystal particles are selected. Also, the magnification for TEM observation of individual silicon nitride crystal particles is 10000 times. When one silicon nitride crystal particle does not fit into one image, it may be photographed in multiple parts. In the silicon nitride sintered body of the sliding member according to the embodiment, the ratio of the number of silicon nitride crystal particles having dislocation defect portions among any 50 silicon nitride crystal particles is 0% or more and 10% or less. This indicates that when more than 50 silicon nitride crystal particles are shown in the observation region of 50 μm × 50 μm, the ratio of the number of silicon nitride crystal particles having dislocation defect portions is 0% or more and 10% or less regardless of which 50 silicon nitride crystal particles are selected.

[0014] Further, in the dislocation defect portion, components other than silicon, oxygen, and nitrogen are 1 μm 2It is preferably not in the form of the above-mentioned agglomerates. Also, in the dislocation defect portion, it is preferable that components other than silicon, oxygen, and nitrogen are not detected at 10 mol% or more. The components other than silicon, oxygen, and nitrogen are the components constituting the grain boundary phase. The grain boundary phase is mainly composed of a sintering aid. For this reason, the components other than silicon, oxygen, and nitrogen correspond to the metal components of the sintering aid. For example, when yttrium oxide (Y2O3) is used as the sintering aid, the component other than silicon, oxygen, and nitrogen is yttrium (Y). Also, when the component other than silicon, oxygen, and nitrogen is 2 not in the form of an agglomerate of 1 μm or more, it means that in the dislocation defect portion, the metal component constituting the grain boundary phase is 2 less than 1 μm (including 0 μm). 2 Also, even when a plurality of sintering aids are used, it is preferable that the component other than silicon, oxygen, and nitrogen is not in the form of an agglomerate of 1 μm or more. This indicates that the sintering aid component does not become the nucleus of the dislocation defect portion. 2 Also, the fact that components other than silicon, oxygen, and nitrogen are not detected at 10 mol% or more indicates that in the dislocation defect portion, the metal component of the sintering aid is less than 10 mol% (including 0 mol%). For example, when yttrium oxide (Y2O3) is used as the sintering aid, it indicates that yttrium (Y) is less than 10 mol% (including 0 mol%) in the dislocation defect portion. Also, when a plurality of sintering aids are used, it is preferable that the total of the metal components of the sintering aids is less than 10 mol%. This indicates that the sintering aid component does not become the nucleus of the dislocation defect portion. Also, the fact that components other than silicon, oxygen, and nitrogen are not detected at 10 mol% or more indicates that in the dislocation defect portion, the metal component of the sintering aid is less than 10 mol% (including 0 mol%). For example, when yttrium oxide (Y2O3) is used as the sintering aid, it indicates that yttrium (Y) is less than 10 mol% (including 0 mol%) in the dislocation defect portion. Also, when a plurality of sintering aids are used, it is preferable that the total of the metal components of the sintering aids is less than 10 mol%. This indicates that the sintering aid component does not become the nucleus of the dislocation defect portion. Also, in the dislocation defect portion, it is preferable that the metal component constituting the grain boundary phase is 2 less than 1 μm (including 0 μm) 2 and less than 10 mol% (including 0 mol%).

[0015] Since the sintering aid component does not become the nucleus of the dislocation defect portion, the durability against electrolytic corrosion is improved. Further, when a discharge phenomenon leading to electrolytic corrosion occurs, heat is generated. Since the sintering aid component forms a low melting point compound, internal destruction of the silicon nitride crystal particles is likely to occur. Therefore, it is preferable that the sintering aid component does not become the nucleus of the dislocation defect portion. Analysis of the size and concentration of components other than silicon, oxygen, and nitrogen in the dislocation defect portion is performed by EDX (energy dispersive X-ray analysis) or WDS (wavelength dispersive X-ray analysis). Analysis may also be performed in combination with EPMA (electron probe microanalyzer).

[0016] In addition, the ratio of the number of silicon nitride crystal particles having a dislocation defect portion and having an occupied area ratio of the dislocation defect portion of 5% or less to the number of silicon nitride crystal particles having a dislocation defect portion is preferably 70% or more. As described above, if the number of silicon nitride crystal particles having a dislocation defect portion is small, it does not cause electrolytic corrosion. On the other hand, if there is a large dislocation defect portion in one silicon nitride crystal particle, the mechanical durability may decrease. Therefore, the occupied area ratio of the dislocation defect portion in one silicon nitride crystal particle is preferably 5% or less. Further, when the ratio of the number of silicon nitride crystal particles having a dislocation defect portion and having an occupied area ratio of the dislocation defect portion of 5% or less to the number of silicon nitride crystal particles having a dislocation defect portion is 70% or more, the effects of improving durability and suppressing electrolytic corrosion can be enhanced. This ratio is also determined in the observation region of 50 μm × 50 μm. That is, first, in an observation region of 50 μm × 50 μm, observe any 50 silicon nitride crystal particles in which all the contours are imaged. Next, check each of the observed 50 silicon nitride crystal particles for the presence of dislocation defect sites. Calculate the ratio of the number of silicon nitride crystal particles having dislocation defect sites among the observed 50 silicon nitride crystal particles. It is preferable that this ratio is 0% or more and 10% or less. Subsequently, when there are silicon nitride crystal particles having dislocation defect sites, calculate the occupancy area ratio of the dislocation defect sites for each of these silicon nitride crystal particles. Calculate the ratio of the number of silicon nitride crystal particles in which the occupancy area ratio of the dislocation defect sites is 5% or less to the number of silicon nitride crystal particles having dislocation defect sites. It is preferable that this ratio is 70% or more. When there are no 50 silicon nitride crystal particles in which all the contours can be seen in one observation region of 50 μm × 50 μm, search for 50 silicon nitride crystal particles in which all the contours can be seen in another observation region of 50 μm × 50 μm. After measuring the ratio of the number of silicon nitride crystal particles having dislocation defect sites, the occupancy area ratio of the dislocation defect sites, etc. in a certain observation region of 50 μm × 50 μm, observe another observation region of 50 μm × 50 μm that is 1000 μm or more away from that observation region. In the silicon nitride sintered body of the sliding member according to the embodiment, in any observation region of 50 μm × 50 μm of any cross-section, the above-mentioned ratio of the number of silicon nitride crystal particles having dislocation defect sites is 0% or more and 10% or less. Also, the above-mentioned ratio of the number of silicon nitride crystal particles in which the occupancy area ratio of the dislocation defect sites is 5% or less is 70% or more. In other words, even in a minute region of 50 μm × 50 μm, the size of the dislocation defect sites of the silicon nitride crystal particles is controlled.

[0017] In addition, for the measurement of the occupied area ratio of dislocation defect portions in one silicon nitride crystal particle, the dark-field image of the aforementioned TEM photograph is used. In the dark-field image, the dislocation defect portions are observed as white. In one silicon nitride crystal particle observed in the dark-field image, the sum of the area of the region that appears white and the area of the region that appears black is defined as the area of the silicon nitride crystal particle. The area of the region that appears white in the dark-field image is defined as the area of the dislocation defect portion. (Area of dislocation defect portion / Area of silicon nitride crystal particle) × 100 (%) is defined as the occupied area ratio of the dislocation defect portion. Also, for the measurement of this occupied area ratio, image processing software is used. As the image analysis software, Image-j or software having a resolution equal to or higher than that is used. In addition, the ratio of the number of silicon nitride crystal particles with an occupied area ratio of dislocation defect portions of 5% or less is calculated by photographing 50 silicon nitride crystal particles whose entire contours are captured in an observation region of 50 μm × 50 μm.

[0018] In addition, in an observation region of 300 μm × 300 μm of an arbitrary cross-section of the silicon nitride sintered body, the major axis of the silicon nitride crystal particles is preferably 25 μm or less. Also, in an observation region of 300 μm × 300 μm of an arbitrary cross-section of the silicon nitride sintered body, the average of the major axes of the silicon nitride crystal particles is preferably in the range of 1 μm or more and 10 μm or less. Also, in a unit area of 300 μm × 300 μm of an arbitrary cross-section of the silicon nitride sintered body, the area of each grain boundary phase is 9 μm 2 or less, which is preferable. The area of the grain boundary phase means the area of the region surrounded by a plurality of silicon nitride crystal particles. The fact that the major axis of the silicon nitride crystal particles is 25 μm or less indicates that there are no silicon nitride crystal particles with a major axis exceeding 25 μm. That is, it indicates a state without large coarse grains. As described above, by reducing the dislocation defect portions, the durability and the corrosion resistance characteristics are improved. By not having large coarse grains, the variation in the partial mechanical strength can be suppressed. Therefore, the major axis of the silicon nitride crystal particles is preferably 25 μm or less, and more preferably 15 μm or less. Also, the average major axis of the silicon nitride crystal particles is preferably in the range of 1 μm or more and 10 μm or less. If the average major axis is less than 1 μm, the silicon nitride crystal particles are too small and the durability may decrease. Also, if the average major axis exceeds 10 μm, the gaps between the silicon nitride crystal particles become large and the strength may decrease. Also, in the observation region of 300 μm × 300 μm of an arbitrary cross-section of the silicon nitride sintered body, the area of each grain boundary phase is 9 μm 2 or less is preferable. If the area of the grain boundary phase exceeds 9 μm 2 it may cause variations in insulation. The grain boundary phase and the silicon nitride crystal particles have different insulating properties. Therefore, if the grain boundary phase is too large, the effect of suppressing the dislocation defect portions of the silicon nitride crystal particles becomes small. Therefore, the area of each grain boundary phase is 9 μm 2 or less, more preferably 5 μm 2 or less. Most preferably, the area of each grain boundary phase is 1 μm 2 or less.

[0019] The major axis of the silicon nitride crystal particles and the area of the grain boundary phase are measured using SEM photographs. Take an SEM photograph of an arbitrary cross-section of the silicon nitride substrate. The cross-section should be a polished surface with a surface roughness Ra of 1 μm or less. The magnification of the SEM photograph is set to 1000 times or more. The recommended magnification of the SEM photograph is 4000 times. When it is not possible to photograph a 300 μm × 300 μm area in one field of view, the 300 μm × 300 μm area may be divided into multiple areas for photographing. When the length in one direction of the cross-section is less than 300 μm, after making the length of the observation region in that one direction as long as possible, observe an observation region of 90000 μm 2 The major axis of the silicon nitride crystal particles is measured based on the observation results in the 90000 μm 2 observation region. As shown in Fig. 2, the major axis is the length of the longest line segment among the line segments obtained by connecting any two points on the outer edge of a single silicon nitride crystal particle. In Fig. 2, 3 indicates the major axis of the silicon nitride crystal particle 1. Also, the average of the major axes of the individual silicon nitride crystal particles shown in the SEM photograph of the observation region of 300 μm × 300 μm is measured. Silicon nitride crystal particles whose contours are cut off at the edge of the SEM photograph and not entirely shown are excluded from the count. The major axis is determined using only the silicon nitride crystal particles in which the longest line segment can be observed in the SEM photograph. In the SEM photograph, the silicon nitride crystal particles and the grain boundary phase can be distinguished by the difference in contrast. The silicon nitride crystal particles appear dark gray, and the grain boundary phase appears light gray. The area of the grain boundary phase can be determined by image analysis of the SEM photograph. Specifically, the binarized image is analyzed using image analysis software. In the binary image, the silicon nitride crystal particles are represented in black, and the grain boundary phase is represented in white. The area is determined by taking the area of the region surrounded by three or more silicon nitride crystal particles as the grain boundary phase. That is, the area is determined excluding the grain boundary phase existing between two silicon nitride crystal particles. Also, as the image analysis software, Image-j or one having a resolution equal to or higher than it is used.

[0020] By having the above configuration, a sliding member excellent in durability and corrosion resistance can be provided. Also, as the silicon nitride sintered body, it can have a three-point bending strength of 900 MPa or more and a fracture toughness of 5.5 MPa·m 1 / 2 or more. That is, while maintaining the strength as a material, the durability and corrosion resistance can be further improved. The measurement of the three-point bending strength is performed according to JIS-R-1601. The measurement of the fracture toughness is determined by the Shinohara formula according to the IF method of JIS-R-1607. For JIS-R-1601, ISO 14704 can be referred to. For JIS-R-1607, ISO 15732 can be referred to. Further, the content of the sintering aid is preferably 20% by mass or less in terms of oxide conversion. When the content of the sintering aid exceeds 20% by mass, the proportion of the grain boundary phase increases. When the grain boundary phase increases, the three-point bending strength or the fracture toughness value may decrease. Also, the effect of reducing the proportion of silicon nitride crystal particles having dislocation defect portions decreases.

[0021] That is, the silicon nitride sintered body preferably contains 20% by mass or less of additive components other than silicon nitride, and the additive components preferably contain three or more elements selected from Y, lanthanoid elements, Al, Mg, Si, Ti, Hf, Mo, and C.

[0022] That is, the silicon nitride sintered body contains 20% by mass or less of additive components. The additive components mean components other than silicon nitride. In the silicon nitride sintered body, the additive components other than silicon nitride refer to the sintering aid components. The sintering aid components constitute the grain boundary phase. When the additive components are excessively large exceeding 20% by mass, the grain boundary phase becomes excessive. The silicon nitride sintered body has a structure in which elongated β-silicon nitride crystal particles are intricately intertwined. When the amount of the sintering aid component increases, a portion where the silicon nitride crystal particles cannot form an intricately intertwined structure is formed, which is not desirable. By having more of the intricately intertwined structure, the three-point bending strength and the fracture toughness value can be improved.

[0023] Also, the amount of the additive components is preferably 3% by mass or more and 15% by mass or less. When the additive components are less than 3% by mass, the grain boundary phase may become too small and the density of the silicon nitride sintered body may decrease. The grain boundary phase is a component that fills the gaps between the silicon nitride crystal particles. Therefore, by defining the additive components to be 3% by mass or more, it becomes easy to form the relative density of the sintered body to be 95% or more. Also, by defining the additive components to be 5% by mass or more, it becomes easy to form the relative density of the sintered body to be 98% or more.

[0024] In addition, the silicon nitride sintered body preferably contains three or more elements selected from Y, lanthanoid elements, Al, Mg, Si, Ti, Hf, Mo, and C as additive components. As long as the silicon nitride sintered body contains Y (yttrium), lanthanoid elements, Al (aluminum), Mg (magnesium), Si (silicon), Ti (titanium), Hf (hafnium), Mo (molybdenum), and C (carbon) as constituent elements of the additive components, their forms of existence are not limited. For example, forms such as oxides (including complex oxides), nitrides (including complex nitrides), oxynitrides (including complex oxynitrides), carbides (including complex carbides), etc. can be mentioned. Further, as the lanthanoid element, one selected from Yb (ytterbium), Er (erbium), Lu (lutetium), and Ce (cerium) is preferable.

[0025] As described later, when added as a sintering aid in the manufacturing process, oxides (including complex oxides), nitrides (including complex nitrides), and carbides (complex carbides) are preferable. When the Y element is used, yttrium oxide (Y2O3) is preferable. Further, as the lanthanoid element, one selected from ytterbium oxide (Yb2O3), erbium oxide (Er2O3), lutetium oxide (Lu2O3), and cerium oxide (CeO2) is preferable. The Y element and the lanthanoid element can promote the formation of elongated columnar β-silicon nitride crystal particles.

[0026] When the Al element is used, aluminum oxide (Al2O3), aluminum nitride (AlN), MgO·Al2O3 spinel, or mullite (Al2O3 - SiO2) is preferable. When the Mg element is used, magnesium oxide (MgO), MgO·Al2O3 spinel, or talc (MgO - SiO2) is preferable. When the Si element is used, silicon oxide (SiO2) or silicon carbide (SiC) is preferable. In addition, the Al element, Mg element, and Si element have the effect of lowering the sintering temperature and improving the sinterability.

[0027] When Ti element is used, titanium oxide (TiO2) or titanium nitride (TiN) is preferred. When Hf element is used, hafnium oxide (HfO2) is preferred. When Mo element is used, molybdenum oxide (MoO2) or molybdenum carbide (Mo2C) is preferred. Regarding the C element, it is preferably added as silicon carbide (SiC), titanium carbide (TiC), or titanium carbonitride (TiCN). The Ti element, Hf element, Mo element, and C element function as components that strengthen the grain boundary phase. Thereby, the silicon nitride sintered body can be made to have high strength or high toughness.

[0028] By combining and adding two or more of these additive components, a grain boundary phase including three or more elements selected from Y, lanthanoid elements, Al, Mg, Si, Ti, Hf, Mo, and C can be formed. Also, as a combination of sintering aids added in the manufacturing process, the following combinations are preferred.

[0029] The first combination contains 2 - 8 mass% of Y2O3, 1 - 6 mass% of Al2O3, 1 - 6 mass% of AlN, and 0.5 - 4 mass% of TiO2. Although Al2O3 and AlN are added, the components added to the silicon nitride sintered body are counted as one type of Al element. Therefore, according to the first combination, three types of Y, Al, and Ti are added.

[0030] The second combination contains 2 - 8 mass% of Y2O3, 0.1 - 3 mass% of Al2O3, 1 - 6 mass% of AlN, 0.5 - 3 mass% of HfO2, and 0.1 - 3 mass% of Mo2C. According to the second combination, five types of Y, Al, Hf, Mo, and C are added. The third combination contains 2 - 8 mass% of Y2O3, 1 - 5 mass% of Al2O3, 1 - 6 mass% of AlN, 0.5 - 3 mass% of HfO2, 0.1 - 3 mass% of Mo2C, and 1 - 6 mass% of SiC. According to the third combination, six types of Y, Al, Hf, Mo, C, and Si are added. The fourth combination contains 0.2 to 3% by mass of Y2O3, 0.5 to 5% by mass of MgO·Al2O3 spinel, 2 to 6% by mass of AlN, 0.5 to 3% by mass of HfO2, and 0.1 to 3% by mass of Mo2C. According to the fourth combination, six types of elements, namely Y, Mg, Al, Hf, Mo, and C, are added. In the first to fourth combinations, an oxide of a lanthanoid element may be used instead of Y2O3. In the first to fourth combinations, the total content of the sintering aid components is 15% by mass or less.

[0031] Among the above first to fourth combinations, the first combination is preferred. In the first combination, Al2O3 and AlN are used in combination as the Al element. Thereby, it becomes easier to form a Y-Al-Si-O-N compound in the grain boundary phase. The Y-Al-Si-O-N compound is a reaction product of Y2O3, Al2O3, AlN, and silicon nitride (Si3N4). That is, the reaction between the sintering aid powder and the silicon nitride powder can be promoted by the sintering process. Further, by adding AlN, the impurity oxygen in the silicon nitride powder can be utilized for the formation of the Y-Al-Si-O-N compound. Thereby, the formation of dislocation defect portions in the silicon nitride crystal particles can be suppressed. Also, TiO2 becomes TiN (titanium nitride) by the sintering process. The TiN particles function as a component for strengthening the grain boundary phase. Also, the abnormal grain growth of the long diameter of the silicon nitride crystal particles can be suppressed. Thereby, a dense sintered body can be obtained. Therefore, according to the first combination, the three-point bending strength can be 1000 MPa or more, and the fracture toughness value can be 6.5 MPa·m 1 / 2 or more.

[0032] In the second to fourth combinations, HfO2 is added. HfO2 can react with Y2O3 to form an Hf-Al-Y-O-based compound. This acts as a low melting point liquid phase, leading to an improvement in sinterability. Therefore, the formation of dislocation defect portions in the silicon nitride crystal particles can be suppressed. Also, Mo2C or SiC functions as a component for strengthening the grain boundary phase. Thus, according to the second to fourth combinations, a silicon nitride sintered body having a three-point bending strength of 900 MPa or more and a fracture toughness value of 5.5 MPa·m 1 / 2 or more can be obtained. Note that the average value of the three-point bending strength of the first combination is about 100 MPa or more higher than that of the second to fourth combinations. According to the first combination, TiO2 changes to TiN (titanium nitride) particles. Therefore, fine dispersion of 1 μm or less is also possible. TiO2, Mo2C, and SiC are dispersed in a particulate form in the grain boundary phase. Since TiO2 undergoes a change to TiN (titanium nitride) particles, it is easily dispersed as fine particles in the grain boundary phase in the silicon nitride sintered body. Thereby, the size of the grain boundary phase can be reduced and the grain boundary phase can be strengthened. According to the first combination, the area of the grain boundary phase can be made 5 μm 2 or less, and further 1 μm 2 or less. Depending on the use environment of the sliding member, a silicon nitride sintered body having a desired strength and fracture toughness value can be used. Also, the sliding surface of the silicon nitride sintered body preferably has a surface roughness Ra of 1 μm or less.

[0033] A sliding member provided with a silicon nitride sintered body as described above exhibits excellent durability. Also, the sliding member has corrosion resistance. Examples of the sliding member include a bearing member, a roll member, a compressor member, a pump member, an engine member, and a member for a friction stir welding apparatus. A bearing includes a combination of rolling elements and raceways. The rolling elements are spherical or cylindrical in shape. Here, the rolling elements are called bearing balls. The spherical shape is a ball, and the cylindrical shape is a cylinder. Also, a bearing using spherical rolling elements is called a ball bearing. A bearing using cylindrical rolling elements is called a roller bearing. Roller bearings also include needle bearings, tapered roller bearings, and spherical roller bearings. Also, the raceways include an outer ring and an inner ring. Examples of the roll member include rollers for rolling and rollers for feed components of electronic devices. Examples of the compressor member or the pump member include vanes. Here, the compressor is distinguished as a device that increases pressure, and the pump is distinguished as a device that decreases pressure. Examples of the engine member include cam rollers, cylinders, pistons, check balls, etc. Examples of the member for a friction stir welding device include a tool member for a friction stir welding device. Since the sliding member according to the embodiment has excellent durability, it can be applied to various fields as described above. Regarding the sliding member in the above fields, at least one of the sliding member and the mating member slides relative to the other. For example, in a bearing, the position of the rolling element in the bearing does not change. However, a part of the surface of the rolling element contacts a part of the surface of the mating member, and a part of the surface of the rolling element slides relative to the mating member. The sliding member according to the embodiment has excellent durability due to few dislocation defect parts. In particular, the sliding member according to the embodiment is suitable for a surface sliding member. It is also preferably used in a use environment where the sliding conditions change. An example of such a sliding member is a bearing mounted on an inverter-driven motor.

[0034] JIS-B-1518 defines the measurement methods for the dynamic rated load and rated life of rolling bearings. The rated life calculation formula in JIS-B-1518 uses the rated load. Thus, the life of a general bearing has been measured by load and rotational speed. Therefore, the consideration for a use environment where the rotational speed changes has not necessarily been sufficient. For JIS-B-1518, ISO 281 can be referred to. Here, the torque of an inverter-driven motor will be described. Let the synchronous speed of the motor be Ns (r / min), the rated rotational speed (r / min) of the motor be N, the slip (%) be s, the power supply frequency (Hz) be f, and the number of poles of the motor be p. The synchronous speed Ns of the motor is expressed as Ns = (2f / p) × 60. The rotor generates torque at a rotational speed slightly lower than the synchronous speed Ns. A load corresponding to this torque is applied to the bearing. The rated rotational speed N (r / min) of the motor is given by N = Ns(1 - s) = (120f / p)(1 - s). The slip s (%) is given by s = ((Ns - N) / Ns) × 100. As can be seen from these equations, to change the rotational speed of a motor driven by an inverter, it is effective to change the frequency of the power supply. Note that the unit r / min is the same as rpm. For measuring the durability of a bearing in a usage environment where the sliding conditions change, a method of measuring the sliding noise is effective. JIS-B-1548 defines a method for measuring the noise level of rolling bearings. As described above, in a motor driven by an inverter, the torque changes. The change in torque leads to a change in the stress on the surface of the bearing ball. If there is damage on the surface of the bearing ball, a change occurs in the sliding noise. Therefore, the change in the sliding noise is effective as a method for measuring the durability of the bearing ball. The sliding member according to the embodiment has few dislocation defect parts. Therefore, even when the sliding member according to the embodiment is used in an application where the sliding conditions change, the occurrence of damage or electrolytic corrosion of the sliding member can be suppressed, and it has excellent sliding characteristics. For example, the motor can be driven by an inverter while changing the rotational speed of the motor within a range of about 50 to 15,000 rpm. Even under usage conditions where the rotational speed changes by 1,000 rpm or more, the sliding member according to the embodiment exhibits excellent durability.

[0035] Next, a method for manufacturing the sliding member according to the embodiment will be described. The manufacturing method of the sliding member is not particularly limited as long as it has the above configuration. Here, the following examples are given as a method for obtaining the sliding member according to the embodiment with good yield. First, raw material powders are prepared. The raw material powders include silicon nitride powder and sintering aid powder. The sintering aids shown above are preferably used. The sintering aid preferably satisfies any one of the first combination to the fourth combination. Also, for the sintering aid, each metal simple substance is converted to an oxide so that the total amount is 20% by mass or less. The lower limit value of the addition amount of the sintering aid is preferably 2% by mass or more.

[0036] Regarding the silicon nitride powder, it is preferable that the α-phase conversion rate is 80% by mass or more, the average particle size is 0.4 to 2.5 μm, and the impurity oxygen content is 2% by mass or less. The impurity oxygen content is preferably 2% by mass or less, more preferably 1.0% by mass or less. Even more preferably, the impurity oxygen content is 0.1 to 0.8% by mass. If the impurity oxygen content is excessively more than 2% by mass, the reaction between the impurity oxygen and the sintering aid may occur, and there is a possibility that the grain boundary phase is formed more than necessary. Also, the average particle size D 50 of the sintering aid powder is preferably 1.0 μm or less, more preferably 0.4 μm or less. In the process of grain growth of the silicon nitride crystal particles, it is effective to form a state in which the sintering aid component is easily coordinated on the surface of the silicon nitride crystal particles. Therefore, as the sintering aid powder, a fine powder form is preferable.

[0037] The above raw material powders are mixed, and a binder is further added to prepare a raw material mixture. The raw material powder is a mixed powder of silicon nitride powder and sintering aid powder. The raw material mixture obtained by adding a binder to the mixed powder becomes a slurry. In the sintering process, in order to control the grain growth of the silicon nitride crystal particles, it is preferable that the mixed powder is uniformly mixed. In the mixing process, pulverization and mixing are performed using a pulverizer such as a ball mill. At this time, if the pulverization stress is too large, dislocation defect portions are formed in the silicon nitride powder, and dislocation defect portions tend to remain in the silicon nitride particles after sintering. Therefore, in the mixing process, under a soft stress such that dislocation defect portions are not formed in the silicon nitride powder, the average particle size D 50 is preferably pulverized to be 1 μm or less. As the mixing process under soft stress, a method of taking 20 hours or more to make the average particle size D 50 be 1 μm or less can be mentioned. Also, in order to perform the mixing process under soft stress, examples include reducing the rotation speed of a pulverizer such as a ball mill, reducing the amount of media, and using light media. The media is put into the pulverizer together with the mixed powder. By using the media, the mixed powder can be efficiently pulverized. Therefore, the media is also called pulverization media. By pulverizing under soft stress, it is possible to achieve both pulverization efficiency and defect suppression. The rotation speed of the pulverizer is preferably 20% or more and 40% or less of the critical rotation speed. For example, the critical rotation speed of a ball mill pulverizer is given by the theoretical formula Nc = 42.3·D -1 / 2 where Nc is the critical rotation speed (rpm) and D is the inner diameter of the pot (m). This theoretical formula defines the critical state as the state where the gravitational force and the centrifugal force on the balls are balanced at the apex of the mill cylinder. The rotation speed of a general ball mill is said to be optimally about 55 - 85% of the critical rotation speed. By setting the rotation speed to 40% or less of the critical rotation speed, the mixing process can be carried out under soft stress. Also, even if the rotation speed is less than 20% of the critical rotation speed, the stress is soft, but the mixing time becomes too long and the mass productivity decreases. Therefore, the rotation speed of the pulverizer is preferably 40% or less of the critical rotation speed, particularly 20% or more and 40% or less. Also, the amount of media is preferably 10% by mass or less with respect to 100% by mass of the mixed powder of silicon nitride powder and sintering aid powder. When using light media, it is preferable that the media has a specific gravity of 4 times or less with respect to the specific gravity of the slurry. By reducing the amount of media or the specific gravity, soft stress can be realized. The above-mentioned "taking 20 hours or more", "setting to 40% or less of the critical rotation speed", "reducing the amount of media", and "using media with a small specific gravity" may be carried out in combination respectively.

[0038] Next, a molding process for molding the raw material mixture is performed. As the molding method of the raw material mixture, a die pressing method, a cold isostatic pressing (CIP) method, a sheet molding method, etc. can be applied. Examples of the sheet molding method include a doctor blade method and a roll molding method. These molding methods may be combined. If necessary, the raw material mixture may be mixed with a solvent such as toluene, ethanol, butanol, etc. If necessary, the raw material mixture may be mixed with an organic binder. Examples of the organic binder include butyl methacrylate, polyvinyl butyral, polymethyl methacrylate, etc. Also, when the total amount of the raw material mixture (silicon nitride powder and sintering aid powder) is 100% by mass, the addition amount of the organic binder is preferably 3 - 17% by mass. If the addition amount of the organic binder is less than 3% by mass, the amount of the binder is too small and it becomes difficult to maintain the shape of the molded body. Further, when the addition amount of the organic binder exceeds 17% by mass, the voids in the molded body (the molded body after the debinding treatment) become large after the debinding step, and a dense sintered body cannot be obtained. Next, a debinding step of the molded body is performed. In the debinding step, heating is performed at a temperature of 500 to 800°C for 1 to 4 hours in a non-oxidizing atmosphere to debind most of the organic binder previously added. Examples of the non-oxidizing atmosphere include a nitrogen gas atmosphere and an argon gas atmosphere. If necessary, treatment may be performed in an oxidizing atmosphere such as an air atmosphere to control the amount of organic matter remaining in the debound body. Next, the debound body (the molded body that has undergone the debinding treatment) is sintered furnace It is housed in a sintering furnace and sintered in a non-oxidizing atmosphere in the sintering furnace. The temperature in the sintering step is preferably in the range of 1650°C or higher and 2000°C or lower. As the non-oxidizing atmosphere, a nitrogen gas atmosphere or a reducing atmosphere containing nitrogen gas is preferable. Further, the pressure inside the sintering furnace is preferably a pressurized atmosphere.

[0039] When the debound body is sintered at a low temperature of less than 1650°C, the grain growth of the silicon nitride crystal particles is not sufficient, and it is difficult to obtain a dense sintered body. On the other hand, when the debound body is sintered at a temperature higher than 2000°C, there is a risk of decomposition into Si and N2 when the atmospheric pressure inside the furnace is low. Therefore, the sintering temperature is preferably controlled within the above range. The sintering time is preferably in the range of 3 hours or more and 12 hours or less.

[0040] In the sintering step, it is preferable to set the heating rate in the range of 1300°C or higher and 1500°C or lower to 50°C / h or less. In this temperature range, a liquid phase mainly composed of a sintering aid is generated. By controlling the heating rate, diffusion to the surface of the silicon nitride crystal particles can be promoted. It is preferable to set the heating rate from 1500°C to the sintering temperature to 50°C / h or less. By setting the heating rate to 50°C / h or less, uniform grain growth can be promoted, the generation of coarse grains can be suppressed, and the formation of dislocation defect portions in the silicon nitride crystal particles can be suppressed. Furthermore, it is preferable that the amount of change in pressure is 0.3 MPa or less. By suppressing the amount of change in pressure, it is also possible to suppress the formation of dislocation defect portions in the silicon nitride crystal particles. The change in pressure affects the grain growth of the silicon nitride crystal particles. Whether the sintering process is carried out at normal pressure or under pressure, it is preferable that the amount of change in pressure is 0.3 MPa or less, and further preferably 0.1 MPa or less. In order to control the amount of change in pressure, it is effective to control the gas pressure in the sintering atmosphere. During the sintering process of the silicon nitride sintered body, gas is likely to be generated. In the sintering process, the binder in the green body, the sintering aid, and the impurity oxygen in the silicon nitride powder become gas components. Due to the generated gas components, the pressure of the sintering atmosphere changes. That is, even in the case of normal pressure sintering without applying pressure, the pressure of the sintering atmosphere may change depending on the generated gas components. Therefore, in order to prevent the atmospheric pressure from changing during the sintering process, it is effective to control such as removing the generated gas components.

[0041] In this way, by controlling the heating rate or suppressing the pressure change, abnormal grain growth of the silicon nitride crystal particles can be suppressed. As a result, in the observation region of 300 μm × 300 μm of an arbitrary cross-section of the silicon nitride sintered body, the maximum value of the major axis of the silicon nitride crystal particles can be controlled to 25 μm or less. Also, in the observation region of 300 μm × 300 μm of an arbitrary cross-section of the silicon nitride sintered body, the average of the major axis of the silicon nitride crystal particles can be controlled within the range of 1 μm or more and 10 μm or less. Also, since abnormal grain growth can be suppressed, in the observation region of 300 μm × 300 μm of an arbitrary cross-section of the silicon nitride sintered body, the area of each grain boundary phase is 9 μm 2 or less, and further preferably 5 μm 2 or less can be controlled. Also, after the sintering process, it is preferable to perform HIP (Hot Isostatic Pressing) treatment on the sintered body. Here, the process of sintering the aforementioned degreased body is referred to as the first sintering process, and the process of performing HIP treatment on the sintered body is referred to as the second sintering process. The HIP treatment is preferably carried out within a temperature range of 1600°C or higher and 1900°C or lower, and a pressure range of 80 MPa or higher and 200 MPa or lower. By performing the HIP treatment, pores in the sintered body can be reduced. As a result, a dense sintered body can be obtained. If the pressure is less than 80 MPa, the effect of applying the pressure is insufficient. Also, if the pressure exceeds 200 MPa and is too high, the load on the manufacturing equipment may increase.

[0042] The heat treatment temperature of the second sintering process is preferably lower than the heat treatment temperature of the first sintering process. By lowering the heat treatment temperature of the second sintering process, grain growth of silicon nitride crystal particles can be suppressed. Also, according to the above manufacturing method, the formation of dislocation defect portions is suppressed in the first sintering process. Therefore, even when HIP treatment with pressure application is performed, an increase in dislocation defect portions can be suppressed. In other words, it is important to prevent the formation of dislocation defect portions in silicon nitride crystal particles in the first sintering process. Further, the obtained silicon nitride sintered body is processed into a sliding member. Examples of the processing include surface polishing, cutting, machining, electrical discharge machining, etc. In surface polishing, the portion that becomes the sliding surface of the silicon nitride sintered body is processed so that the surface roughness Ra is 1 μm or less. By setting the surface roughness Ra of the sliding surface to 1 μm or less, and further 0.2 μm or less, the durability of the sliding member is improved. Also, the aggressiveness to the mating member can be reduced, and thus the durability is improved from this point as well.

[0043] An example of a bearing is shown in FIG. 3. In FIG. 3, 10 is a bearing, 11 is a bearing ball, 12 is an inner ring, and 13 is an outer ring. The bearing 10 has a structure in which bearing balls 11 are arranged between the inner ring 12 and the outer ring 13. For example, a sliding member according to an embodiment is used for bearing balls (rolling elements) 11. Bearing steel SUJ2 is used for raceway rings (inner ring 12 and outer ring 13). These members constitute a bearing 10. The entire surface of the spherical bearing ball serves as a sliding surface. Therefore, the entire surface of the sphere is polished. By reducing the surface roughness Ra of the bearing ball, the aggressiveness to the raceway ring can be reduced. That is, it is possible to suppress the sliding surface of the raceway ring from being scraped as the bearing ball slides. For this reason, the durability of the entire bearing can be improved.

[0044] Fig. 4 shows a motor using a bearing and a driving device using the motor. In Fig. 4, 20 is a motor, 10 is a bearing, 14 is a rotating shaft, 15 is a rotor, 16 is a stator, and 17 is a case. Also, 30 is a driving device and 25 is a control unit. The motor 20 according to the embodiment has the bearing 10 according to the embodiment. The driving device 30 according to the embodiment has the motor 20 and the control unit 25 according to the embodiment. The bearings are provided around one end and the other end of the rotating shaft. As the rotating shaft rotates, the inner ring of the bearing rotates and slides on the surface of the bearing ball. Also, the bearing ball rotates and the surface of the bearing ball slides on the outer ring. A rotor is further provided around the rotating shaft. The rotor is located between the bearings. A stator is provided around the rotor. The bearings, a part of the rotating shaft, the rotor, and the stator are housed inside the case. The stator is fixed to the case. The rotor is fixed to the rotating shaft, and the rotating shaft and the rotor rotate inside the stator. The control unit is electrically connected to the stator. The control unit supplies an alternating current to the stator. Thereby, the rotating shaft of the motor is inverter-driven. Also, by changing the frequency of the alternating current, the rotation speed of the motor can be changed. The motor and the driving device according to the embodiment have bearings with improved durability. Thereby, the life of the motor and the driving device can be extended and the reliability can be improved. Also, the rotational speed of the motor corresponds to the rotational speeds of the bearing, the rotating shaft, and the rotor. By applying the sliding member according to the embodiment to the bearing balls, the reliability of motors and drive devices with a bearing rotational speed of 50 rpm or more can be improved. Even when the motors and drive devices are configured such that the rotational speed of the bearing changes by 1000 rpm or more, the reliability of the motors and drive devices can be improved.

[0045] (Example) (Examples 1 to 7, Comparative Example 1) A mixed raw material powder obtained by mixing silicon nitride powder and a sintering aid powder was prepared. Next, the mixed raw material powder was pulverized and mixed to prepare mixed raw material powders 1 to 6. Note that the pulverization and mixing were performed using a ball mill. Regarding the mixed raw material powders 1 to 5, the pulverization process until the average particle size D 50 became 1 μm or less was carried out over 20 hours or more. Regarding the mixed raw material powder 6, the pulverization process was carried out in a short time of 10 hours. Also, the first combination was used for the mixed raw material powders 1 to 2 and the mixed raw material powder 6. The second combination was used for the mixed raw material powder 3. The third combination was used for the mixed raw material powder 4. The fourth combination was used for the mixed raw material powder 5. The results are shown in Table 1.

[0046]

Table 1

[0047] Next, 5 to 10 wt% of a binder was added to the mixed raw material powder to prepare a mixed raw material paste. Using the mixed raw material paste, die forming was performed. The formed body was subjected to a debinding process at 500 to 800 °C for 1 to 4 hours to prepare a debound body. Next, the first sintering process shown in Table 2 was carried out in a nitrogen atmosphere. A pressure of 0.1 MPa indicates normal pressure.

[0048]

Table 2

[0049] After the first sintering process was completed, the obtained sintered body was cooled to room temperature. Thereafter, a second sintering process was carried out. The second sintering process was HIP treatment. The HIP treatment conditions were as shown in Table 3.

[0050]

Table 3

[0051] The obtained silicon nitride sintered body was subjected to surface polishing so that the surface roughness Ra was 0.01 μm or less. Further, a silicon nitride sintered body having a sample size defined by JIS was produced, and the three-point bending strength and fracture toughness value were measured. Also, a silicon nitride sintered bearing ball having a diameter of 9.525 mm (3 / 8 inch) was produced, and the wear resistance was tested.

[0052] Next, with respect to the examples and comparative examples, the dislocation defect portion, the major axis length of the silicon nitride crystal particles, the area of the grain boundary phase, the three-point bending strength, and the fracture toughness value were measured. In the measurement of the dislocation defect portion, an arbitrarily selected cross-section processed by ion milling to have a surface roughness Ra of 1 μm or less was used as the evaluation surface. The evaluation surface was observed by TEM. In TEM, a region of 50 μm × 50 μm on the evaluation surface was observed and photographed. Also, another region 1000 μm away from one observation region was observed and photographed. That is, the dislocation defect portion was measured for a total of two regions. The magnification of the TEM photograph was set to 10,000 times, and bright-field images and dark-field images were photographed. The region that appears white in the dark-field image was defined as the dislocation defect portion. By comparing the bright-field image and the dark-field image, the presence or absence and the occupied area ratio of the dislocation defect portion in one silicon nitride crystal particle were determined. Note that silicon nitride crystal particles that were cut off at the end of the TEM photograph (those in which the entire contour of the silicon nitride crystal particle was not shown) were excluded from the count. Also, the occupied area ratio of the dislocation defect portion was measured by binarizing the dark-field image with image processing software and obtaining the area ratio between the white region and the black region. Also, the element that is the core of the dislocation defect portion was analyzed. The dislocation defect portion was analyzed using EDX. As a result, in the dislocation defect portion, elements other than silicon, oxygen, and nitrogen were 1 μm 2It was measured whether it was in the form of the above-mentioned agglomerates and whether it was detected at 10 mol% or more.

[0053] Also, in the measurement of the major axis of the silicon nitride crystal particles, an arbitrary cross-section was observed by SEM. The magnification of the SEM photograph was set at 3000 times, and a region of 300 μm × 300 μm was observed and photographed. For one silicon nitride crystal particle shown in the SEM photograph, the distance between two points on the outermost edge that was the longest was taken as the major axis. In the SEM photograph, the average of the major axes of the silicon nitride crystal particles shown in the observation region of 300 μm × 300 μm was determined. The longest major axis was determined as the maximum value of the major axis. Also, among the grain boundary phases shown in the SEM photograph, the largest area was determined. The three-point bending strength was carried out in accordance with JIS-R-1601. The fracture toughness was carried out in accordance with the IF method of JIS-R-1607 and determined using the formula of Shinohara. The results are shown in Tables 4 and 5.

[0054]

Table 4

[0055]

Table 5

[0056] As can be seen from the table, in the examples, the proportion of dislocation defect parts was small. Also, excellent values were obtained for the three-point bending strength and the fracture toughness value. Next, a durability test as a bearing ball was carried out. Sixteen bearing balls were taken as a set and incorporated into raceway rings (inner ring and outer ring) to fabricate bearings. The durability and anti-corrosion characteristics of each bearing were examined. The durability test was carried out by continuously driving the bearing under the condition that one set consists of "50 rpm for 1 hour" → "raising the speed from 50 rpm to 1000 rpm over 1 hour" → "1000 rpm for 1 hour" → "raising the speed from 1000 rpm to 10000 rpm over 1 hour" → "10000 rpm for 1 hour" → "lowering the speed from 10000 rpm to 50 rpm over 1 hour". The change rate of the sliding noise for continuous driving of 400 hours or 800 hours with respect to the sliding noise of continuous driving of 100 hours was determined. An increase in sliding noise of 10% or less was rated as the best (◎), an increase exceeding 10% and less than 20% was rated as good (○), and an increase exceeding 20% was rated as bad (×). This test examined the change in the sliding state due to the breakage of the bearing balls or the seizure of the raceway rings. The measurement of the sliding noise was carried out in accordance with JIS - B - 1548. In addition, the anti - electrolytic corrosion property was examined by the presence or absence of electrolytic corrosion after 800 hours. The results are shown in Table 6.

[0057]

Table 6

[0058] As described above, the bearing balls according to the examples were excellent in durability and anti - electrolytic corrosion property. In particular, Examples 1 to 4 in which the sintering aid was the first combination were excellent in both durability and anti - electrolytic corrosion property. On the other hand, in Comparative Example 1, the durability test was equivalent up to about 400 hours, but decreased after 800 hours. Also, electrolytic corrosion occurred. It was found that the presence of a predetermined amount of dislocation defect portions can affect the sliding characteristics in a usage environment where the rotational speed changes.

[0059] As described above, several embodiments of the present invention have been illustrated. However, these embodiments are shown by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, changes, etc. can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and its equivalent scope. In addition, the above-described embodiments can be implemented in combination with each other.

Explanation of Reference Numerals

[0060] 1... Silicon nitride crystal particles 2... Dislocation defect part 3... Major axis of silicon nitride crystal particles 10... Bearing 11... Bearing ball 12... Inner ring 13... Outer ring 14... Rotating shaft 15... Rotor 16... Stator 17... Case 20... Motor 25... Control unit 30... Driving device

Claims

1. A step of obtaining a raw material mixture by crushing and mixing silicon nitride powder, sintering aid powder, and a binder using a crusher; A molding step of molding the raw material mixture; A debinding step of debinding the molded body at 500 to 800 °C to obtain a debound body; A sintering step of housing the debound body in a sintering furnace and heat-treating the debound body in the sintering furnace to obtain a silicon nitride sintered body; A step of performing a hot isostatic pressing (HIP) treatment on the silicon nitride sintered body; A processing step of processing the silicon nitride sintered body into a sliding member; comprising; In the sintering step, the debound body is sintered at a sintering temperature of 1650 °C or higher and 2000 °C or lower in a non-oxidizing atmosphere, In the sintering step, by removing the gas generated from the debound body, the change in pressure in the sintering furnace is suppressed to 0.3 MPa or less, The sliding member has a ratio of the number of silicon nitride crystal particles having a dislocation defect portion inside to the total number of any 50 silicon nitride crystal particles whose entire contour can be seen in an observation region of 50 μm × 50 μm with a surface roughness Ra of 1 μm or less by ion milling of an arbitrary cross-section or surface, in the range of 0% or more and 10% or less. A method for manufacturing a sliding member made of a silicon nitride sintered body.

2. In the sintering step, the heating rate in the range of 1300 °C or higher and 1500 °C or lower is 50 °C / h or less, and the heating rate from 1500 °C to the sintering temperature is 50 °C / h or less. The method for manufacturing a sliding member made of a silicon nitride sintered body according to Claim 1.

3. The crushing step is performed over 20 hours or more so that the average particle size D of the raw material mixture ５０ is 1 μm or less. The method for manufacturing a sliding member made of a silicon nitride sintered body according to claim 1 or claim 2.

4. In the HIP treatment, the temperature is in the range of 1600 °C or higher and 1900 °C or lower, and the pressure is in the range of 80 MPa or higher and 200 MPa or lower. The method for manufacturing a sliding member made of a silicon nitride sintered body according to any one of Claims 1 to 3.

5. In the processing step, the sliding surface of the sliding member is surface-treated so that the surface roughness Ra is 1 μm or less. The method for manufacturing a sliding member made of a silicon nitride sintered body according to any one of Claims 1 to 4.

6. On an arbitrary cross-section or surface, the ratio of the number of silicon nitride crystal particles having a dislocation defect portion with an occupied area ratio of the dislocation defect portion to the number of the silicon nitride crystal particles having the dislocation defect portion being 5% or less is 70% or more. The method for manufacturing a sliding member made of a silicon nitride sintered body according to any one of Claims 1 to 5.

7. The sliding member is a bearing ball. The method for manufacturing a sliding member made of a silicon nitride sintered body according to any one of Claims 1 to 6.

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