Silicon nitride sintered body, wear-resistant member using the same, and method for manufacturing silicon nitride sintered body

Abstract

This silicon nitride sintered body is characterized in that at least one black part having a major diameter of at least 10 μm is present in a visual field having a unit area of 5mm×5mm when any cross section of said silicon nitride sintered body is observed with a metallographic microscope. Moreover, the major diameter of the black part is preferably at most 500 μm. Further, it is preferable that two to ten black parts are present in the visual field having a unit area of 5mm×5mm. Furthermore, it is preferable that a Fe segregation part is present in the black part.

Description

[Technical Field]

[0001] The embodiments generally relate to a silicon nitride sintered body, a wear-resistant member using the same, and a method for manufacturing the silicon nitride sintered body. [Background technology]

[0002] Silicon nitride sintered bodies are used in wear-resistant components such as bearing balls and rollers. The grades of ceramic materials used in bearings are specified in ASTM F2094. ASTM stands for American Society for Testing and Materials. F2094 is also the Standard Specification for Silicon Nitride Bearing Balls. Ceramic material grades are divided into classes 1 to 3 based on three-point bending strength, Vickers hardness, fracture toughness, and other properties. Bearing balls are also graded according to the precision of their surface. Specifically, grades 2 to 48 are determined by the surface roughness Ra. Wear resistance is improved by flattening the surface.

[0003] Polishing is required to control the surface roughness of silicon nitride sintered bodies. Silicon nitride sintered bodies are difficult to process due to their high strength. For example, Japanese Patent No. 5944910 (Patent Document 1) discloses a silicon nitride sintered body in which the area ratio of the grain boundary phase is controlled. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5944910 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, both ease of processing and wear resistance are achieved by controlling the area ratio of the grain boundary phase and the machinability coefficient. However, there is still a need for further improvement. Furthermore, in Patent Document 1, the thixotropy index of the raw material powder is controlled, which does not necessarily result in good manufacturability.

[0006] The present invention is intended to address these problems and to provide a silicon nitride sintered body that is capable of achieving both workability and wear resistance. [Means for solving the problem]

[0007] The silicon nitride sintered body according to the embodiment is characterized in that when any cross section of the silicon nitride sintered body is observed with a metallurgical microscope, there is one or more black areas with a major axis of 10 μm or more within a field of view of a unit area of 5 mm x 5 mm. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram showing an example of a cross section of a silicon nitride sintered body according to an embodiment. [Figure 2] FIG. [Figure 3] FIG. 10 is a diagram showing an example of an Fe segregation region within a black region. [Figure 4] FIG. 10 is a diagram showing an example of a bearing ball. Embodiment

[0009] The silicon nitride sintered body according to the embodiment is characterized in that when any cross section of the silicon nitride sintered body is observed with a metallurgical microscope, there is one or more black areas with a major axis of 10 μm or more within a field of view of a unit area of 5 mm x 5 mm.

[0010] An example of a cross section of a silicon nitride sintered body according to an embodiment is shown in Figure 1. An example of a black portion is shown in Figure 2. In the figure, reference numeral 1 denotes the cross section of the silicon nitride sintered body, reference numeral 2 denotes the black portion, reference numeral 3 denotes the circumscribing circle of the black portion, and reference numeral 4 denotes the major axis of the black portion.

[0011] First, an arbitrary cross section of the silicon nitride sintered body is observed with a metallurgical microscope. The cross section to be observed is a polished surface with a surface roughness Ra of 1 μm or less.

[0012] An arbitrary cross section of the silicon nitride sintered body is observed using a dark-field magnified photograph of a metallurgical microscope. The metallurgical microscope used is an Olympus BX51M or one with equivalent performance. The digital camera used is a Panasonic Lumix DMC-G3 or one with equivalent performance. For example, images taken using the ring mode of a Keyence VK-X1000 laser microscope can also be used for analysis. The images taken with the metallurgical microscope are then photographed with a digital camera. The digital camera images are then analyzed using Image J or software with equivalent performance.

[0013] Dark-field images at 500x magnification are taken with a metallurgical microscope using a digital camera, and then binarized using Image J analysis. The threshold value used is a value obtained using the "mode method" or "discriminant analysis binarization method." Any threshold value that can distinguish black areas visible in dark-field images is acceptable. Areas determined to be black through this process are designated as black area 2. If the analysis software has a discriminant analysis binarization function, this method should be used. With discriminant analysis binarization, the threshold value is determined uniquely by the analysis software. This makes it easy to distinguish black areas. Magnified photographs taken using a dark-field metallurgical microscope are sometimes simply called enlarged photographs.

[0014] A circumscribing circle 3 is set on the black portion 2, and the diameter of the circumscribing circle 3 is set as the major axis 4 of the black portion 2. The circumscribing circle 3 is a perfect circle that touches the end of the black portion 2.

[0015] The silicon nitride sintered body according to the embodiment has at least one black area with a major axis of 10 μm or more present within a unit area of 5 mm × 5 mm in any cross section. This means that no matter which unit area of 5 mm × 5 mm is observed, a black area with a major axis of 10 μm or more is present. Note that a unit area of 5 mm × 5 mm is sometimes simply referred to as a "unit area." Furthermore, if it is not possible to observe the unit area within the positional field of view, it may be observed multiple times. For example, a bearing ball with a diameter of 5 mm or less should be observed multiple times. When multiple enlarged photographs are used, the minimum unit is 2 mm × 2 mm.

[0016] The presence of the black portion 2 improves workability. The black portion 2 indicates that changes in composition and structure have occurred within the silicon nitride sintered body. Silicon nitride sintered bodies achieve high strength by homogenizing the silicon nitride crystal grains and grain boundary phase. On the other hand, a homogenous structure may result in reduced workability. Here, by distributing specific elements under certain conditions, it is possible to improve workability without reducing the main properties. The silicon nitride sintered body according to the embodiment has the black portion 2, which allows for improved workability.

[0017] Furthermore, the major axis of the black portion is preferably 500 μm or less. If the major axis 4 of the black portion exceeds 500 μm, the processability is improved but the abrasion resistance may be reduced. Therefore, the major axis 4 of the black portion is preferably 10 μm or more and 500 μm or less, and more preferably 30 μm or more and 300 μm or less.

[0018] Furthermore, it is preferable that there are 2 to 10 black areas within a field of view of a unit area of 5 mm x 5 mm. If there is only one black area within a unit area, the effect of providing black areas is small. Furthermore, if there are more than 10 black areas within a unit area, abrasion resistance may decrease. For this reason, the number of black areas within a unit area is preferably 2 to 10, and even more preferably 2 to 6. Note that when counting the number of black areas within a unit area, only black areas whose outlines are visible in the enlarged photograph are counted. Black areas whose outlines are cut off at the edges of the enlarged photograph are not counted.

[0019] It is also preferable that Fe segregation regions exist within the black portion, and that the major axis of the Fe segregation regions is 1 μm or less.

[0020] An example of Fe segregation regions in a black portion is shown in Figure 3. In the figure, reference numeral 2 denotes a black portion, and reference numeral 5 denotes an Fe segregation region. Figure 3 shows an example in which four Fe segregation regions 5 are distributed in the black portion 2. In the embodiment, the distribution state is not limited to that shown in Figure 3.

[0021] The presence of Fe segregation areas 5 within the black portion 2 can be analyzed using an EPMA (electron probe microanalyzer) and a TEM (transmission electron microscope). To analyze the Fe segregation areas within the black portion 2, the cross section used to take the enlarged photograph described above is analyzed using an EPMA. If there is a portion in the black portion 2 where the quantitative value of Fe is 0.05 atom% or more, it is considered to be an Fe segregation area 5. Note that the quantitative analysis of Fe by EMPA may be semi-quantitative. The Fe segregation areas are also observed using a TEM to determine their major diameter.

[0022] The EMPA measurement conditions were an acceleration voltage of 20 kV and a probe current of 40 nA. The TEM observation conditions were an acceleration voltage of 200 kV and a STEM image magnification of 20,000 times or more. An imaginary circle was drawn around the Fe segregation area in the TEM image, and the diameter of that circle was taken as the major axis of the Fe segregation area.

[0023] The presence of Fe segregation 5 within the black portion 2 can further improve workability. Fe (iron) is also contained as an impurity in the silicon nitride powder used as the raw material. Conventionally, Fe segregation has been prevented to prevent the adverse effects of Fe as an impurity. However, it has been discovered that both workability and wear resistance can be achieved by setting the major axis of the Fe segregation to 1 μm or less. While the lower limit of the major axis of the Fe segregation is not particularly limited, it is preferably 0.05 μm (50 nm) or more. If the major axis of the Fe segregation is less than 0.05 μm, the effect of providing the Fe segregation may be insufficient. Therefore, the major axis of the Fe segregation is preferably 0.05 μm or more and 1 μm or less, and more preferably 0.1 μm or more and 0.8 μm or less.

[0024] It is also preferable that a plurality of Fe segregation regions exist within the black portion.It is also preferable that a plurality of Fe segregation regions exist within the black portion, and that the shortest distance between the Fe segregation regions is within the range of 1 μm to 20 μm.

[0025] The presence of multiple Fe segregation regions within the black region improves workability. The shortest distance between Fe segregation regions is the distance from one Fe segregation region to the closest Fe segregation region. By setting the shortest distance between Fe segregation regions within the range of 1 μm to 20 μm, workability can be improved.

[0026] It is also preferable that silicon (Si) is present in the region of the Fe segregation portion 5. When elemental analysis is performed by TEM-EDS, it is preferable that the region in the Fe segregation portion 5 where the detected amount of Fe is 10 atom % or more overlaps with the region where the detected amount of Si is 40 atom % or more. The detection of Si in the region of the Fe segregation portion 5 indicates the presence of an Fe-Si compound. By segregating Si as an Fe-Si compound, it is possible to suppress a decrease in wear resistance.

[0027] Furthermore, in the region of the Fe segregation portion 5, the detected amount of oxygen is preferably 10 atom% or less (including below the detection limit). A low amount of oxygen in the region of the Fe segregation portion 5 indicates a low amount of Fe-Si-O compounds present. A low amount of Fe-Si-O compounds present indicates a high proportion of Fe-Si compounds that do not contain oxygen. Oxygen-free Fe-Si compounds are compounds that do not react easily with rare earth elements. Oxygen-free Fe-Si compounds indicate a low proportion of compounds that form with rare earth elements. The low amount of compounds containing rare earth elements allows them to be distinguished from grain boundary phase components that contain rare earth elements.

[0028] Furthermore, it is preferable that the distribution amount of rare earth elements in the black portion 2 is small. Rare earth elements are components used as sintering aids. Rare earth element components react with other sintering aids to form grain boundary phases. By forming grain boundary phases containing rare earth element components, the strength of the silicon nitride sintered body is improved. By reducing the distribution amount of rare earth elements in the black portion 2, it is possible to further improve processability.

[0029] The distribution of rare earth elements inside and outside the black portion 2 can be analyzed by EPMA. A unit area of 5 mm x 5 mm of any cross section mentioned above is used as the measurement area. The average amount (atom %) of rare earth elements in the area outside the black portion 2 and the average amount (atom %) of rare earth elements in the area inside the black portion 2 are measured. It is preferable that the amount of rare earth elements inside the black portion 2 is 10 atom % or more lower than outside the black portion 2.

[0030] Furthermore, it is preferable that the variation in the distribution of the amount of rare earth elements in the grain boundary phase outside the black portion 2 is less than 10 atom %. The variation in the distribution of the amount of rare earth elements in the grain boundary phase outside the black portion 2 is measured using an EPMA at two locations with a unit area of 5 mm x 5 mm. The distribution amount of rare earth elements in the grain boundary phase outside the black portion 2 is measured. It is preferable that the difference in the distribution amount of rare earth elements is less than 10 atom %. This indicates that the composition of the black portion 2 and the other grain boundary phases is controlled to be slightly different.

[0031] The silicon nitride sintered body according to the embodiment has a three-point bending strength of 600 MPa or more and a fracture toughness of 5 MPa m 1 / 2 or more. In addition, the three-point bending strength can be set within the range of 600 MPa to 900 MPa. ASTM F2094 specifies the material class for bearing balls using silicon nitride sintered bodies. The silicon nitride sintered body according to the embodiment is suitable for providing those equivalent to material class 2 and class 3.

[0032] The three-point bending strength is measured in accordance with JIS-R-1601 (2008). The fracture toughness is calculated using the Niihara formula in accordance with the IF method of JIS-R-1607 (2015). JIS-R-1601 (2008) is based on ISO 14704. JIS-R-1607 (2015) is based on ISO 15732.

[0033] The silicon nitride sintered body described above is suitable for wear-resistant members. Examples of wear-resistant members include bearing balls. Bearing balls are rolling elements used in various bearings, such as ball bearings, roller bearings, and roll bearings. Wear-resistant members can also be applied to rollers and engine parts. Rollers can be used in various products, such as for rolling, conveying, and transferring. Engine parts can also be used in various products, such as check balls, cam rollers, wear pads, and plungers.

[0034] Furthermore, it is preferable that the surface roughness Ra of the sliding surface of the wear-resistant member is 1 μm or less. The wear-resistant member slides against a mating material. For example, a bearing has bearing balls arranged between an inner ring and an outer ring. The entire surface of the bearing ball serves as the sliding surface. Wear resistance is improved by reducing the surface roughness Ra of the sliding surface of the wear-resistant member. ASTM (American Society for Testing and Materials) F2094 sets the surface roughness Ra of a bearing ball at 0.013 μm or less.

[0035] Polishing is required to reduce the surface roughness Ra of the sliding surface to 1 μm or less. Examples of polishing include processes using grinding wheels (including abrasive grains). Examples of polishing processes using grinding wheels include lapping and polishing. Other examples of polishing include chemical polishing and electrolytic polishing.

[0036] Polishing is a process of grinding the surface of a silicon nitride sintered body. As shown in Patent Document 1, surface processing of a silicon nitride sintered body can be divided into rough processing and finish processing. Rough processing is classified into a brittle mode, and finish processing is classified into a ductile mode. The silicon nitride sintered body according to the embodiment can improve the workability of rough processing. Rough processing is performed using grindstones with different grit sizes. Grindstones (including abrasive grains) with grit sizes of approximately #80 to #1500 are used. By improving the workability, the processing time can be shortened.

[0037] The wear resistance is also good. For example, a rolling test can be conducted to test the wear resistance of bearing balls, in which the bearing balls are rolled on a plate made of bearing steel (SUJ2) under conditions of a maximum contact stress of 3.7 GPa and a rotation speed of 1200 rpm. This rolling test is conducted using a Strass-type bearing testing machine. Under these conditions, the method measures the time until defects such as cracks and breakage occur on the surface of the bearing ball. Bearing balls using silicon nitride sintered bodies according to the embodiments have durability of 400 hours, and even 600 hours, in the above rolling test.

[0038] As described above, the silicon nitride sintered body according to the embodiment corresponds to material class 2 to 3 of ASTM F2094. Although the material performance is in the intermediate class, it exhibits excellent durability in low-stress fields. In other words, it is suitable for use in environments with a maximum contact stress of 3.7 GPa or less.

[0039] Next, a method for producing the silicon nitride sintered body according to the embodiment will be described. The method for producing the silicon nitride sintered body according to the embodiment is not particularly limited as long as it has the above-mentioned configuration, but the following methods can be mentioned as methods for obtaining the silicon nitride sintered body with a good yield.

[0040] First, silicon nitride powder is prepared. The silicon nitride powder preferably has an Fe content of 20 wtppm or more. The Fe contained in the silicon nitride powder may be Fe as an impurity. Furthermore, when the Fe content as an impurity is less than 20 wtppm, Fe may be added. When Fe is added, it may be added as iron oxide.

[0041] By using silicon nitride powder containing Fe, it is possible to have Fe segregation in the black area. Furthermore, using silicon nitride powder containing Fe makes it easier to control the dispersion state of the Fe segregation. The upper limit of the Fe content of the silicon nitride powder is preferably 3000 wtppm or less. A content exceeding 3000 wtppm may lead to the formation of large structural defects in the sintered compact structure, potentially reducing wear resistance. Therefore, the Fe content of the silicon nitride powder is preferably 20 wtppm or more and 3000 wtppm or less, and more preferably 100 wtppm or more and 2000 wtppm or less.

[0042] The silicon nitride powder preferably has an average particle size of 2 μm or less, an oxygen content of 4 wt % or less, and an alpha conversion rate of 85% or more.

[0043] Next, a sintering aid powder is prepared. The sintering aid preferably contains a rare earth element in an amount of 1 wt% to 12 wt% in oxide equivalent. Examples of rare earth elements include yttrium (Y) and lanthanoid elements. The lanthanoid element is preferably one selected from cerium (Ce), neodymium (Nd), erbium (Er), and ytterbium (Yb). The rare earth element may be added as a rare earth element compound such as an oxide or oxynitride. When a rare earth element is represented as an R element, its oxide equivalent is R2O3. When Ce is used, its oxide equivalent is CeO2. When two or more rare earth elements are used, their total amount is preferably 1 wt% to 12 wt% in oxide equivalent.

[0044] The aluminum content is preferably 1 wt% or more and 10 wt% or less in terms of oxide. Aluminum (Al) may be added as an aluminum compound such as oxide, nitride, oxynitride, or composite oxide. Two or more types of oxide, nitride, oxynitride, or composite oxide may also be used. The oxide content is expressed in terms of Al2O3.

[0045] In addition, one or more elements selected from Group 4A elements, Group 5A elements, and Group 6A elements shown in the Japanese Periodic Table may be used in an amount of 1 wt% to 5 wt% in total, calculated as oxides. The notation of Group 4A elements, Group 5A elements, and Group 6A elements is based on the Japanese Periodic Table.

[0046] The 4A group elements are titanium (Ti), zirconium (Zr), and hafnium (Hf). The oxide equivalents are TiO2, ZrO2, and HfO2. The Group 5A elements are vanadium (V), niobium (Nb), and tantalum (Ta), and are converted to oxides V2O5, Nb2O3, and Ta2O3. The 6A group elements are chromium (Cr), molybdenum (Mo), and tungsten (W), which are converted to oxides of Cr2O3, MoO3, and WO3. The Group 4A elements, Group 5A elements, and Group 6A elements may be added as compounds such as oxides, carbides, nitrides, oxynitrides, etc. The Group 4A elements, Group 5A elements, and Group 6A elements are referred to as M elements.

[0047] Rare earth element compounds and aluminum compounds react during the sintering process to form grain boundary phases. The grain boundary phases become rare earth element-aluminum-oxygen compounds. Rare earth element-aluminum-oxygen compounds only need to contain rare earth elements, aluminum, and oxygen as constituent elements. Other constituent elements include silicon (Si) and nitrogen (N).

[0048] Furthermore, the M element (including M element compounds) is a component that strengthens the grain boundary phase. By adding the M element, the grain boundary phase can be strengthened. Strengthening the grain boundary phase can improve the strength of the silicon nitride sintered body. For this reason, the M element may be added in an amount appropriate for the desired strength. Another component that strengthens the grain boundary phase is silicon carbide (SiC). When silicon carbide is added, it is preferable that the amount is 1 wt% or more and 5 wt% or less. If the amount of M element added is less than 1 wt%, the effect of the addition may be insufficient. Furthermore, if the amount exceeds 5 wt%, the silicon nitride sintered body may be colored, making it difficult to observe whether or not there is a black portion.

[0049] A raw material mixing process is carried out using silicon nitride powder and sintering aid powder. It is preferable to use sintering aid powder with an average particle size of 2 μm or less. A pulverizer is preferably used in the raw material mixing process. Examples of pulverizers include a ball mill and a bead mill. The raw material mixing process may be either wet or dry.

[0050] In addition, the raw material mixing process is preferably wet mixing using water. Water reacts with silicon nitride powder to produce free Si. Free Si readily reacts with Fe. Fe-Si compounds readily form segregated Fe sites, promoting the formation of black areas. Wet mixing using water may also be used in mixing processes that include grinding. The water may be industrial water, tap water, or pure water, but pure water with reduced impurity concentrations such as Group 1A elements, Group 2A elements, and halogen elements is preferred. The grinding and mixing liquid is preferably adjusted to an alkaline state. Adjusting to an alkaline state means that the mixture of water and silicon nitride powder exhibits a pH value greater than 7. The zeta potential of the silicon nitride powder surface in the mixture of water and silicon nitride powder preferably exhibits a negative potential. The pH of the mixture of water, silicon nitride powder, and sintering aid powder may also be greater than 7. The Group 1A and Group 2A elements are based on the Japanese Periodic Table. Therefore, the Group 1A elements are hydrogen, etc. Also, Group 2A elements include magnesium.

[0051] The generation of free Si is thought to involve the following reaction: Si3N4+6H2O→3SiO2+4NH3 SiO2+2H2O→Si(OH)4

[0052] When wet mixing with water, adjusting the pH to alkaline makes it possible to make the zeta potential of the silicon nitride powder surface negative. By increasing the pH to 9 or higher, the negative potential can be increased. Furthermore, when the zeta potential of the silicon nitride powder surface is negative, cations tend to collect. This makes it easier for the cation Fe ions to collect on the silicon nitride powder surface. This makes it easier for FeSi compounds to form. Zeta potential is the potential of the sliding surface where liquid flow begins in the electric double layer formed around the powder. When the zeta potential is positive, anions collect. When the zeta potential is negative, cations collect. This property is used to make it easier for Fe ions to collect on the silicon nitride powder surface.

[0053] After the raw material mixing step, a binder is added to prepare a raw material slurry, which is preferably prepared using a pulverizer such as a ball mill.

[0054] Next, the obtained raw material slurry is used to granulate as necessary, and then subjected to a molding process to prepare a molded body. Examples of the molding process include die pressing and cold isostatic pressing (CIP). The molding pressure is preferably 100 MPa or more.

[0055] Next, a degreasing step is carried out to degrease the compact. The degreasing step is preferably carried out within a temperature range of 300°C to 700°C. The degreasing step is carried out in the air or a non-oxidizing atmosphere, and the atmosphere is not particularly limited.

[0056] Next, a sintering process is carried out to sinter the degreased body. The sintering process is preferably carried out within a temperature range of 1600°C to 1900°C. The sintering process may be either atmospheric sintering or pressure sintering. It is also preferable to carry out the process in a non-oxidizing atmosphere. Examples of non-oxidizing atmospheres include a nitrogen atmosphere and an argon atmosphere. It is also preferable to carry out a vacuum treatment in which the atmosphere is a vacuum atmosphere from the start of the sintering process until the temperature reaches 1450°C. The vacuum atmosphere is 100 Pa or less. A process of holding the material in a vacuum atmosphere of 1200°C to 1450°C for at least one hour is also carried out. Thereafter, atmospheric sintering or pressure sintering is carried out in a non-oxidizing atmosphere. It is believed that the vacuum treatment generates Si through the self-decomposition of silicon nitride, which easily bonds with the Fe element.

[0057] If necessary, the obtained sintered body is subjected to a hot isostatic pressing (HIP) process, which is preferably carried out at a temperature in the range of 1500°C to 1900°C. It is also preferable to apply a pressure of 30 MPa or more in a non-oxidizing atmosphere.

[0058] The silicon nitride sintered body according to the embodiment can be produced by the above process. Furthermore, by polishing the sliding surface of the silicon nitride sintered body, it can be made into a wear-resistant member. Furthermore, it is preferable that the sliding surface have a surface roughness Ra of 1 μm or less.

[0059] (Example) (Examples 1 to 9, Comparative Examples 1 and 2) The silicon nitride powders shown in Table 1 were prepared.

[0060] [Table 1] TIFF0007725462000001.tif33170

[0061] Next, the sintering aid powder was added and mixed as shown in Table 2. The amount of sintering aid components added was 100 wt% of the silicon nitride powder and sintering aid powder combined. The pH of the mixture of silicon nitride powder, sintering aid powder, and water was made alkaline, at pH 9 or higher. A ball mill was used in the mixing process. A binder was also added in the mixing process to prepare the raw material slurry.

[0062] [Table 2] TIFF0007725462000002.tif145170

[0063] The obtained raw material slurry was used for die press molding at a molding pressure of 100 MPa or more. Next, the obtained compact was subjected to a debinding process at 450°C in the atmosphere. Next, the obtained debinding body was sintered. The sintering process is shown in Table 3.

[0064] [Table 3] TIFF0007725462000003.tif98170

[0065] As shown in the table, Examples 1 to 9 and Comparative Example 2 were subjected to vacuum treatment. In the vacuum treatment, the temperature was raised from room temperature to the maximum temperature in a vacuum of 100 Pa or less. The maximum temperature was maintained for a predetermined time. Thereafter, atmospheric sintering was carried out in a nitrogen atmosphere. The obtained sintered body was subjected to HIP treatment to obtain a silicon nitride sintered body. Note that Example 8 was not subjected to HIP treatment. Furthermore, Comparative Example 1 was not subjected to vacuum treatment.

[0066] The silicon nitride sintered body thus obtained was subjected to measurements of the presence or absence of black portions, three-point bending strength, and fracture toughness. The presence or absence of black portions was determined by observing an arbitrary cross section with a metallurgical microscope under the same measurement conditions as described above.

[0067] The three-point bending strength was measured in accordance with JIS-R-1601, and the fracture toughness was determined by the Niihara formula in accordance with the IF method of JIS-R-1607. The results are shown in Tables 4 and 5.

[0068] [Table 4] TIFF0007725462000004.tif96170

[0069] [Table 5] TIFF0007725462000005.tif79170

[0070] As can be seen from Table 5, the silicon nitride sintered bodies according to the examples corresponded to ASTM F2094 Class 2 to 3. Comparative Example 1 corresponded to Class 1. The silicon nitride sintered bodies according to the examples and comparative examples were examined for workability and durability.

[0071] As a workability test, the processing time when producing bearing balls was measured. For rough processing, surface processing was performed using a polishing disk made of #80 diamond abrasive grains and a polishing disk made of #120 diamond abrasive grains. The mass of the sample was measured before polishing, and the mass was measured again after polishing for a certain period of time under a certain load. The mass change rate before and after polishing was investigated. The mass change rate during processing was shown as a ratio when the mass change rate of Comparative Example 1 was set to 100. A larger mass change rate value means that more polishing was performed than in Comparative Example 1 when polishing for the same period of time.

[0072] For finishing, surface processing was performed using free diamond abrasive grains. The surface roughness Ra before polishing was measured, and the surface roughness Ra after polishing for a certain period of time was measured. The rate of change in surface roughness before and after polishing (Ra change rate) was determined. The Ra change rate was expressed as a ratio when the Ra change rate of Comparative Example 1 was set to 100. A larger Ra change rate means that the surface roughness Ra can be made smaller than that of Comparative Example 1 when polishing for the same period of time, and indicates that it is easier to process the surface into a flat surface.

[0073] Each sintered body was processed into bearing balls (diameter 9.525 mm) with a surface roughness Ra of 0.01 μm, and durability tests were conducted on these. The durability tests involved rolling life tests in which the bearing balls were rolled on a bearing steel (SUJ2) plate under conditions of a maximum contact pressure of 3.7 GPa and a rotation speed of 1200 rpm, and measurements were taken using a thrust bearing tester. In this rolling life test, bearing balls that did not have defects such as surface cracks or chips after 400 and 600 hours were marked with an "O" and deemed good, while those that had defects were marked with an "X" and deemed unusable. The results are shown in Table 6.

[0074] [Table 6] TIFF0007725462000006.tif91170

[0075] As can be seen from the table, the silicon nitride sintered bodies according to the examples had improved workability. In particular, it was found that workability was improved in rough machining. Furthermore, good results were obtained for durability after 400 hours. On the other hand, problems were confirmed for Examples 8 to 10 after 600 hours. This was because Example 8 did not undergo HIP treatment. Furthermore, Example 9 was because the major axis of the black portion was outside the preferred range.

[0076] In addition, Examples 8 and 9 exhibited excellent durability up to 400 hours. In other words, they are fully usable in any field where the maximum contact pressure is 3.7 GPa or less and the guaranteed durability period is 400 hours or less.

[0077] The silicon nitride sintered body according to this embodiment can achieve both processability and wear resistance. In particular, the silicon nitride sintered body according to this embodiment can achieve both processability and wear resistance in silicon nitride sintered bodies corresponding to ASTM F2094 Class 2 to 3.

[0078] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. Modifications of these embodiments are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

Claims

1. When an arbitrary cross section of the silicon nitride sintered body is observed with a metallurgical microscope, there is one or more black portions having a major axis of 10 μm or more within a field of view of a unit area of 5 mm × 5 mm, the black portions being black portions obtained by binarizing a dark-field image taken with a metallurgical microscope at a magnification of 500 times using analysis software, The black portion contains segregated Fe portions having a major axis of 5 μm or less, the amount of Fe in the Fe segregation portion is 0.05 atom% or more, the average amount of rare earth elements in the grain boundary phase within the black portion is less than the average amount of rare earth elements in the grain boundary phase outside the black portion by 10 atom % or more; A silicon nitride sintered body characterized by:

2. 2. The silicon nitride sintered body according to claim 1, wherein the major axis of the black portion is 500 μm or less.

3. 3. The silicon nitride sintered body according to claim 1, wherein the number of said black portions within said field of view of unit area 5 mm x 5 mm is 2 or more and 10 or less.

4. 4. The silicon nitride sintered body according to claim 1, wherein a plurality of Fe segregation regions are present within the black region.

5. 5. The silicon nitride sintered body according to claim 1, wherein a plurality of Fe segregation regions are present within the black portion, and the shortest distance between the plurality of Fe segregation regions is within the range of 1 μm to 20 μm.

6. A wear-resistant member comprising the silicon nitride sintered body according to any one of claims 1 to 5.

7. 7. The wear-resistant member according to claim 6, wherein the surface roughness Ra is 1 μm or less.

8. 8. The wear-resistant member according to claim 6, wherein the wear-resistant member is a bearing ball.

9. The wear-resistant member according to claim 1, wherein an Fe-Si compound is present in the region of the Fe segregation portion.

10. A method for producing a silicon nitride sintered body, comprising a raw material mixing step, a molding step, and a sintering step, The raw material mixing step is a wet mixing step using silicon nitride powder having an Fe content as an impurity in the range of 20 wtppm to 3000 wtppm, a sintering aid powder containing a rare earth element, and water, and adjusting the mixed solution of the silicon nitride powder and water to an alkaline pH of more than 7, The sintering step comprises a step of holding the sintered body in a vacuum atmosphere at 1200°C or higher and 1450°C or lower for 1 hour or more, and a step of carrying out the sintering step at a temperature in the range of 1600°C or higher and 1900°C or lower.

11. 11. A method for producing a silicon nitride sintered body according to claim 10, characterized in that the zeta potential of the surface of the silicon nitride powder is made negative by adjusting the mixed solution of the silicon nitride powder and water to an alkaline pH of more than 7.

12. 12. The method for producing a silicon nitride sintered body according to claim 10, wherein the wet mixing is carried out to adjust the pH of the mixture to 9 or more.

13. 13. The method for producing a silicon nitride sintered body according to claim 10, further comprising a step of hot isostatically pressing the sintered body obtained by the sintering step at a temperature in the range of 1500°C to 1900°C.

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