Silicon nitride sintered body and wear-resistant member using same

Abstract

In a silicon nitride sintered body having a plurality of silicon nitride crystal particles and a grain boundary phase according to an embodiment of the present invention, the plurality of silicon nitride crystal particles include specific silicon nitride crystal particles in each of which a first solid solution metal that is composed of at least one of tungsten and molybdenum and a second solid solution metal that is composed of aluminum form a solid solution with each other. With respect to the amount of the solid solution in the specific silicon nitride crystal particles among the silicon nitride crystal particles, it is preferable that (the number of atoms of the first solid solution metal) / (the number of atoms of silicon), which is the number of atoms of the first solid solution metal to the number of atoms of silicon, is 0.0001 to 0.01 inclusive, and (the number of atoms of the second solid solution metal) / (the number of atoms of silicon), which is the number of atoms of the second solid solution metal to the number of atoms of silicon, is 0.001 to 0.1 inclusive.

Description

Silicon nitride sintered body and wear-resistant member using the same

[0001] The embodiments described below generally relate to a silicon nitride sintered body and a wear-resistant member using the same.

[0002] Silicon nitride sintered bodies are used in wear-resistant members. Examples of wear-resistant members include bearing members, roll materials, compressor vanes, gas turbine blades, and engine parts. Examples of bearing members include bearing balls, inner rings of bearings, and outer rings of bearings. Examples of roll materials include those used in rolling mills and conveying. Examples of engine parts include cam rollers. In recent years, silicon nitride sintered bodies have been used in bearing balls as bearing members due to their excellent mechanical strength and wear resistance.

[0003] For example, Japanese Patent No. 5100201 (Patent Document 1) discloses a silicon nitride sintered body in which the aspect ratio and standard deviation of acicular silicon nitride crystal grains are controlled. Furthermore, Japanese Patent No. 6416088 (Patent Document 2) discloses a silicon nitride sintered body in which the variation in the area ratio of the grain boundary phase is suppressed. Patent Documents 1 and 2 disclose the uniform distribution of silicon nitride crystal grains and grain boundary phases. This improves the wear resistance of the silicon nitride sintered body.

[0004] Japanese Patent No. 5100201 Japanese Patent No. 6416088 JP 2001-335368 A International Publication No. 2020 / 121752

[0005] In recent years, silicon nitride sintered bodies have been used for bearing balls in electric vehicle motors. As a result, demand for bearing balls made of silicon nitride sintered bodies has grown significantly. However, silicon nitride sintered bodies are difficult to process. Therefore, in order to mass-produce bearing balls made of silicon nitride sintered bodies, further improvements in the processability of silicon nitride sintered bodies, i.e., shortening the processing time, have been required. It has been found that dissolving metal elements in silicon nitride crystal grains is an effective way to improve processability. For example, Japanese Patent Laid-Open Publication No. 2001-335368 (Patent Document 3) discloses a silicon nitride sintered body in which Al, Mg, and rare earth elements are dissolved in silicon nitride crystal grains. The silicon nitride sintered body in Patent Document 3 is intended for use in semiconductor device substrates, and therefore has improved thermal conductivity. However, although the processability was improved somewhat, the improvement was not necessarily satisfactory.

[0006] One of the problems to be solved by the embodiments is to provide a silicon nitride sintered body with improved processability and a wear-resistant member using the same.

[0007] In one embodiment, a silicon nitride sintered body having a plurality of silicon nitride crystal grains and a grain boundary phase includes specific silicon nitride crystal grains in which a first solid solution metal consisting of at least one of tungsten and molybdenum and a second solid solution metal consisting of aluminum are dissolved.

[0008] 1 is a schematic diagram showing an example of a cross-sectional structure of a silicon nitride sintered body according to an embodiment, an external view showing an example of a bearing ball based on the silicon nitride sintered body according to an embodiment, and an external view showing an example of a base sphere for a bearing ball made of the silicon nitride sintered body according to an embodiment. Embodiment

[0009] In one embodiment, a silicon nitride sintered body having a plurality of silicon nitride crystal grains and a grain boundary phase includes specific silicon nitride crystal grains in which a first solid solution metal consisting of at least one of tungsten and molybdenum and a second solid solution metal consisting of aluminum are dissolved.

[0010] FIG. 1 shows an example of the cross-sectional structure of a silicon nitride sintered body according to an embodiment. In the figure, reference numeral 1 denotes a silicon nitride sintered body, reference numeral 2 denotes silicon nitride crystal grains, and reference numeral 3 denotes a grain boundary phase. Reference numeral 21 denotes specific silicon nitride crystal grains 2 in which a first solid solution metal consisting of at least one of tungsten and molybdenum and a second solid solution metal consisting of aluminum are dissolved. Reference numeral 22 denotes silicon nitride crystal grains 2 that contain neither tungsten nor molybdenum (referred to herein as "other silicon nitride crystal grains"). Reference numeral 23 denotes silicon nitride crystal grains 2 obtained by removing the specific silicon nitride crystal grains 21 and the other silicon nitride crystal grains 22 (referred to herein as "remaining silicon nitride crystal grains"). Note that the presence of the other silicon nitride crystal grains 22 is not essential in the silicon nitride crystal grains 2, and the presence of the remaining silicon nitride crystal grains 23 is also not essential. For example, the other silicon nitride crystal grains 22 include those that contain neither tungsten nor molybdenum, and those that contain only aluminum, etc. The remaining silicon nitride crystal grains 23 include those that contain tungsten or molybdenum but do not contain aluminum.

[0011] The silicon nitride sintered body 1 has a structure in which a plurality of silicon nitride crystal grains 2 are intricately entangled. The silicon nitride sintered body 1 also has a structure in which the gaps between the plurality of silicon nitride crystal grains 2 are filled with a grain boundary phase 3.

[0012] The silicon nitride sintered body 1 comprises, as silicon nitride crystal grains 2, specific silicon nitride crystal grains 21 in which a first solid solution metal consisting of at least one of tungsten and molybdenum and a second solid solution metal consisting of aluminum are dissolved.

[0013] The solid solution metal elements are measured using TEM-EDS under the following conditions: acceleration voltage 200 kV, probe current 1.00 nA, spot diameter during analysis 1 nm, analysis time 30 seconds, and sample angle X = 10°, Y = 0°.

[0014] In any cross section of the silicon nitride sintered body 1, the silicon nitride crystal grain 2 is designated as the measurement area. An analysis spot is selected near the center of the silicon nitride crystal grain 2. If at least one of tungsten and molybdenum and aluminum are detected in one silicon nitride crystal grain 2 through this analysis, the silicon nitride crystal grain 2 is determined to be a specific silicon nitride crystal grain 21 in which the first solid solution metal and the second solid solution metal are solid-solved. Note that if tungsten or molybdenum or aluminum is not detected when measuring one silicon nitride crystal grain 2, the measurement cross section of the same silicon nitride crystal grain 2 may be changed and the measurement may be repeated. Furthermore, taking into consideration the size of the silicon nitride crystal grain 2, it is preferable to measure silicon nitride crystal grains 2 with a major axis length of 0.5 μm or more. This is because measuring silicon nitride crystal grains 2 with a length of less than 0.5 μm may be affected by the surrounding grain boundary phase 3.

[0015] Furthermore, for measuring the elements of the solid solution metal, FESEM-EDS may be used instead of TEM-EDS. FESEM stands for Field Emission Scanning Electron.

[0016] The fact that the first and second solute metals are in solid solution means that the combination of solute metal elements in the specific silicon nitride crystal particles 21 is one of "W + Al," "Mo + Al," or "W + Mo + Al." Tungsten and molybdenum are elements with higher electronegativity than silicon (Si). Generally, the electronegativity values ​​are Si = 1.90, N = 3.04, W = 2.36, Mo = 2.16, Al = 1.61, and O = 3.44.

[0017] The solid solutions of tungsten and molybdenum are thought to act within the silicon nitride crystal grains 2 with an interatomic bond strength different from that of Si-N bonds, improving the workability of the silicon nitride sintered body 1. The solid solution of aluminum is also effective in improving the hardness of the silicon nitride crystal grains.

[0018] With respect to the amount of the first solute metal dissolved in the specific silicon nitride crystal grains 21, the ratio of the number of atoms of the first solute metal to the number of silicon atoms (number of Si atoms), i.e., "number of atoms of the first solute metal / number of Si atoms," is defined as a first atomic ratio R1. It is preferable that the first atomic ratio R1 satisfies the following formula (1): 0.0001≦R1≦0.01 (1)

[0019] With respect to the amount of the second solute metal (Al) dissolved in the specific silicon nitride crystal grains 21, the ratio of the number of atoms of the second solute metal to the number of Si atoms, i.e., "number of atoms of the second solute metal / number of Si atoms," is defined as a second atomic ratio R2. It is preferable that the second atomic ratio R2 satisfies the following formula (2): 0.001≦R2≦0.1 (2)

[0020] When the first atomic ratio R1 satisfies the formula (1) and the second atomic ratio R2 satisfies the formula (2), the workability of the silicon nitride sintered body 1 is improved. If the first atomic ratio R1 is less than 0.0001, the effect of tungsten or molybdenum forming a solid solution may be insufficient. If the first atomic ratio R1 is greater than 0.01, the specific silicon nitride crystal grains 21 may become unstable. Note that when the specific silicon nitride crystal grains 21 form a solid solution of both tungsten and molybdenum, the first atomic ratio R1 is determined using the total number of atoms thereof.

[0021] If the second atomic ratio R2 is less than 0.001, the amount of the second solute metal may be insufficient, whereas if the second atomic ratio R2 exceeds 0.1, the amount of the second solute metal may be too great, making specific silicon nitride crystal particles 21 unstable.

[0022] It is more preferable that the first atomic ratio R1 satisfies the following formula (3): 0.0005≦R1≦0.002 (3) It is more preferable that the second atomic ratio R2 satisfies the following formula (4): 0.01≦R2≦0.04 (4)

[0023] In addition, the silicon nitride sintered body 1 may contain, as silicon nitride crystal grains 2, silicon nitride crystal grains in which only one element selected from tungsten, molybdenum, and aluminum is dissolved as a solid solution, as other silicon nitride crystal grains 22 or as the remaining silicon nitride crystal grains 23.

[0024] In addition, it is preferable that the percentage U1 [%] of the number of specific silicon nitride crystal grains 21 containing the first and second solid solution metals in a 20 μm × 20 μm measurement area in any cross section of the silicon nitride sintered body 1 satisfies the following formula (5): 10≦U1 [%]≦100 (5) where U1 is the percentage of the number of specific silicon nitride crystal grains 21 containing the first and second solid solution metals in a 20 μm × 20 μm measurement area.

[0025] The number of silicon nitride crystal grains 2 in a measurement area of ​​20 μm × 20 μm refers to the number of silicon nitride crystal grains 2 whose major axis length is 0.5 μm or more in a cross-sectional photograph. Ten or more silicon nitride crystal grains 2 are arbitrarily selected from the silicon nitride crystal grains 2 whose length is 0.5 μm or more, and the amounts of tungsten, molybdenum, and aluminum dissolved in each are measured. The number of specific silicon nitride crystal grains 21 that exhibit a solid solution of the first and second solid solution metals is counted.

[0026] The percentage U1 [%] of the number of specific silicon nitride crystal particles 21 can be calculated using the following formula (6): C is the number of silicon nitride crystal particles 2 with a major axis length of 0.5 μm or more, and C1 is the number of specific silicon nitride crystal particles 21 with a major axis length of 0.5 μm or more. U1 [%] = C1 / C × 100 (6)

[0027] Furthermore, if ten silicon nitride crystal particles 2 having a major axis length of 0.5 μm or more are not observed in a measurement area of ​​20 μm × 20 μm, a different measurement area should be used. The length of each silicon nitride crystal particle 2 is the longest diagonal of each silicon nitride crystal particle 2. The length is determined using the observation screen of a TEM or FESEM. Although the number of silicon nitride crystal particles 2 having a length of 0.5 μm or more in a measurement area of ​​20 μm × 20 μm was set to 10 or more, it is also possible to measure all silicon nitride crystal particles 2 having a length of 0.5 μm or more. The measurement target length was set to 0.5 μm or more in consideration of measurement accuracy, as described above. However, in order to obtain effects such as processability, it is preferable to have specific silicon nitride crystal particles 21 having a major axis length of 0.5 μm or more and containing tungsten or the like as a solid solution.

[0028] Silicon nitride crystal particles with metal dissolved therein are also called metal-dissolved silicon nitride crystal particles. A more effective effect can be achieved by ensuring that the ratio U1 of the number of specific silicon nitride crystal particles 21, which are part of the metal-dissolved silicon nitride crystal particles, to the number of silicon nitride crystal particles 2 in a measurement area of ​​20 μm×20 μm is 10% or more (see formula (5) above). It is more preferable that the ratio of the number of specific silicon nitride crystal particles 21 satisfies the following formula (7): 30≦U1 [%]≦100 (7)

[0029] Furthermore, regarding the amount of oxygen (O) dissolved in specific silicon nitride crystal grains 21, the number of oxygen atoms (number of O atoms) relative to the number of Si atoms, or "number of O atoms / number of Si atoms," is defined as a third atomic ratio R3. It is preferable that third atomic ratio R3 satisfy the following formula (8): 0.001≦R3≦1.20 (8)

[0030] This indicates the amount of oxygen dissolved in the specific silicon nitride crystal grains 21. The solid solution of oxygen in the specific silicon nitride crystal grains 21, together with the solid solution of aluminum, forms a sialon crystal structure, resulting in solid solution strengthening effects such as improved hardness. Oxygen and aluminum are primarily substitutional solid solutions, while tungsten and molybdenum are primarily interstitial solid solutions. By combining interstitial and substitutional solid solutions, even greater effects can be achieved. A substitutional solid solution is one in which part of the Si or N in the silicon nitride crystal lattice is replaced by O or Al. An interstitial solid solution is one in which W or Mo enters the gaps in the silicon nitride crystal lattice.

[0031] If the third atomic ratio R3 is less than 0.001, the amount of oxygen dissolved in the solid solution is small, which may result in insufficient effect. If the third atomic ratio R3 exceeds 1.20, the amount of oxygen is too large, which may cause specific silicon nitride crystal particles 21 to become unstable. Therefore, it is preferable that the third atomic ratio R3 satisfy the following formula (9): 0.05≦R3≦1.0 (9)

[0032] In any cross section of the silicon nitride sintered body 1, the ratio U2 [%] of the number of other silicon nitride crystal grains 22 to the number of silicon nitride crystal grains 2 in a measurement area of ​​20 μm×20 μm preferably satisfies the following formula (10). More preferably, it satisfies formula (11): 0≦U2≦30 (10) 0≦U2<10 (11)

[0033] The other silicon nitride crystal particles 22 refer to those in which neither W nor Mo is detected (including below the detection limit) by the above-mentioned TEM-EDS (whether or not Al is detected). Examples of the other silicon nitride crystal particles 22 include those that do not contain any solid solution metals, and those in which Al, O, or other metals (metals other than W and Mo) are solid-solved. The silicon nitride sintered body 1 according to the embodiment has improved properties due to the presence of the specific silicon nitride crystal particles 21. For this reason, it is preferable that the number of other silicon nitride crystal particles 22 is small.

[0034] Furthermore, Fe (iron) can be cited as a metal other than W and Mo that dissolves in specific silicon nitride crystal particle 21. When Fe is dissolved in specific silicon nitride crystal particle 21, it is preferable that the "fourth atomic number / Si atomic number" ratio R4, where "fourth atomic number / Si atomic number" is the fourth atomic number, which is the number of Fe atoms, in specific silicon nitride crystal particle 21, and that the fourth atomic number ratio R4 satisfies the following formula (12): 0.0001≦R4≦0.002 (12)

[0035] Furthermore, in any cross section of the silicon nitride sintered body 1, it is preferable that the proportion U3 [%] of the number of specific silicon nitride crystal grains 21 having dislocation defects relative to the total number of specific silicon nitride crystal grains 21 in a measurement area of ​​20 μm × 20 μm satisfies the following formula (13), in order to avoid deteriorating the wear resistance of the silicon nitride sintered body 1: 0≦U3 [%]≦10 (13) This indicates that few specific silicon nitride crystal grains 21 have dislocation defects.

[0036] Dislocation defects are crystal defects contained within a crystal. Crystal defects are also called lattice defects. Crystal defects are caused by disturbances in the atomic arrangement or impurities. Dislocation defects cause defects in a stable crystal structure. As mentioned above, the solid solutions of W, Mo, Al, and O in silicon nitride crystal particles can be interstitial or substitutional. The fact that there are few specific silicon nitride crystal particles 21 with dislocation defects indicates that the crystal structure is not destroyed even when the solute metal is present.

[0037] The presence or absence of dislocation defects can be analyzed by the method described in International Publication WO 2020 / 121752 (Patent Document 4). Specifically, this is determined by observing the dark field and bright field of a TEM observation image. Dislocation defects appear white in the dark field and appear inverted black in the bright field. In this way, the area where the pixel color is inverted when switching between the dark field and the bright field is considered to be a dislocation defect. The area where the pixel color is inverted black and white overlaps is considered to be a dislocation defect.

[0038] The presence of dislocation defects may reduce the wear resistance of the silicon nitride sintered body 1. For this reason, it is preferable that the ratio of the number of specific silicon nitride crystal grains 21 having dislocation defects to the total number of specific silicon nitride crystal grains 21 satisfies the above formula (13), and further the following formula (14): 0≦U3 [%]≦5 ... (14)

[0039] Furthermore, the proportion U4 [%] of the number of specific silicon nitride crystal grains 21 having dislocation defects to the total number of silicon nitride crystal grains 2 including other silicon nitride crystal grains 22 preferably satisfies the following formula (15): 0≦U4 [%]≦10 (15)

[0040] The silicon nitride crystal particles 2 preferably have an average major axis length of 0.5 μm to 10 μm and an average aspect ratio of 2 to 10. If the average major axis length is less than 0.5 μm, the silicon nitride crystal particles 2 may be too small, resulting in reduced durability. If the average major axis length exceeds 10 μm, the gaps between the silicon nitride crystal particles 2 may become large, resulting in reduced strength.

[0041] The major axis of the silicon nitride crystal grains 2 is measured using an SEM photograph. An SEM photograph is taken of an arbitrary cross section of the silicon nitride sintered body 1. The cross section is 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. The measurement area is 300 μm × 300 μm.

[0042] The longest diagonal line of silicon nitride crystal particles 2 seen in the SEM photograph is taken as the major axis. The 50 largest silicon nitride crystal particles 2 seen in the 300 μm × 300 μm measurement area are selected, and their average value is taken as the average length of the major axis. The minor axis is taken as a line drawn perpendicularly from the midpoint of the major axis of the selected silicon nitride crystal particles 2. The aspect ratio is taken as the major axis / minor axis, and the average value is taken as the average aspect ratio.

[0043] The grain boundary phase is mainly composed of a sintering aid, such as a rare earth compound, an aluminum compound, a tungsten compound, a molybdenum compound, a titanium compound, a hafnium compound, a zirconium compound, or silicon carbide.

[0044] In addition, it is preferable to use a rare earth compound and an aluminum compound as sintering aids, and then use at least one selected from a tungsten compound, a molybdenum compound, a titanium compound, a hafnium compound, a zirconium compound, and silicon carbide. The grain boundary phase can be strengthened by the presence of at least one selected from a tungsten compound, a molybdenum compound, a titanium compound, a hafnium compound, a zirconium compound, and silicon carbide as a crystalline compound in the grain boundary phase. This improves the wear resistance of the silicon nitride sintered body 1. Furthermore, silicon nitride sintered bodies 1 using rare earth compounds and aluminum compounds as sintering aids tend to have a thermal conductivity of 30 W / m·K or less.

[0045] The silicon nitride sintered body 1 as described above can have a three-point bending strength of 600 MPa or more, and even 900 MPa or more. Also, the fracture toughness value can be 6.0 MPa m １／２ or more, and even 7.0 MPa m １／２ Furthermore, the Vickers hardness can be set to 1400 or more, and further to 1500 or more.

[0046] Three-point bending strength is measured in accordance with JIS-R-1601 (2008). JIS-R-1601 corresponds to ISO 14704. Fracture toughness can be measured using the Niihara formula in accordance with the IF method of JIS-R-1607 (2015). JIS-R-1607 corresponds to ISO 15732. Vickers hardness can be measured in accordance with JIS-R-1610 (2003). JIS-R-1610 corresponds to ISO 14705.

[0047] The machinability coefficient Mc can be set to 0.12 or less. The machinability coefficient Mc is a function of the indentation load (Fn), Vickers hardness (Hv), and fracture toughness (K １ｃ ) is a coefficient that indicates machinability, and therefore workability. This is a relational expression for the lateral crack fracture model, and Mc indicates the amount of material removed by one abrasive grain. The larger the machinability coefficient Mc, the greater the amount that can be machined at one time.

[0048] The lateral crack fracture model is a model proposed by Evans and Marshall as a mechanism for removing material during grinding. In this model, the amount of material removed when one grinding grain passes through the material surface (delta V) is expressed as [Fn] in the relationship between the force Fn that presses the abrasive grain vertically into the material, the Vickers hardness (Hv), and the fracture toughness value (K1C). ９／８ / (K １ｃ １／２ ・Hv ５／８ ) ] where delta V is replaced with the machinability coefficient Mc.

[0049] Processing can be broadly divided into brittle mode and ductile mode. Brittle mode corresponds to rough processing, and ductile mode corresponds to finish processing. Since wear is considered to correspond to ductile mode, in order to satisfy the required performance of wear-resistant components, it is important to improve the workability in brittle mode without reducing the workability in ductile mode. One wear model is that a microscopic pre-crack occurs at the grain boundary, and its propagation leads to the destruction of the material surface, causing wear.

[0050] The parameter Sc.m, which indicates the severity of mechanical contact in the wear model, is expressed by the following equation (16) using the friction coefficient μ, the maximum Hertzian stress Pmax, the grain size d of the material, and the fracture toughness value K1c: Sc.m = [(1 + 10 μ) Pmax (d １／２ )] / K １ｃ …(16)

[0051] A large Sc.m parameter means that the material wear is large, and a small Sc.m parameter means that the material wear is small. １ｃ It can be seen that by increasing the machinability coefficient Mc, it is possible to suppress wear of the material. The silicon nitride sintered body 1 according to the embodiment can improve machinability even though the machinability coefficient Mc is small, at 0.12 or less.

[0052] The silicon nitride sintered body 1 according to the embodiment is also suitable for use as a wear-resistant member. The wear-resistant member preferably has a sliding surface with a surface roughness Ra of 0.1 μm or less. Examples of wear-resistant members include bearing members, roll materials, compressor vanes, gas turbine blades, and engine parts. Examples of bearing members include bearing balls, inner rings of bearings, and outer rings of bearings. Examples of roll materials include those for rolling and conveying. Examples of engine parts include cam rollers.

[0053] The wear-resistant member has a sliding surface that slides against a mating member. For example, a bearing ball is disposed between the inner and outer rings of a bearing. In the case of a bearing ball made of a ball-shaped silicon nitride sintered body 1, the entire spherical surface serves as the sliding surface. In the case of a roll made of a cylindrical silicon nitride sintered body 1, the roll surface serves as the sliding surface.

[0054] To improve the wear resistance of the sliding surface, it is effective to polish the surface to a surface roughness Ra of 0.1 μm or less. The silicon nitride sintered body 1 according to the embodiment has controlled amounts of dissolved metals such as W, Mo, and Al. This improves machinability. For example, the surface roughness of bearing balls is specified in ASTM F2094. Bearing balls are graded according to ASTM F2094, ISO 26602, or JIS R1669 depending on the application. They are polished to a surface roughness Ra according to that grade. Higher grades may be mirror-finished to a surface roughness Ra of 0.01 μm or less. The silicon nitride sintered body 1 according to the embodiment improves machinability while maintaining wear resistance. The polishing process to obtain a sliding surface with a surface roughness Ra of 0.1 μm or less can be carried out efficiently.

[0055] FIG. 2 shows an example of a bearing ball, and FIG. 3 shows an example of a base sphere for a bearing ball (hereinafter simply referred to as a "base sphere"). Reference numeral 5 denotes a bearing ball, 6 denotes a base sphere, 7 denotes a spherical portion of the base sphere 6, and 8 denotes a band-shaped portion of the base sphere 6. FIG. 3(A) shows the base sphere 6 as viewed in a direction perpendicular to a line connecting the two poles G1 and G2 of the band-shaped portion 8 (two vertices when the surface including the band-shaped portion 8 is the bottom), and FIG. 3(B) shows the base sphere 6 as viewed in a direction connecting the two poles G1 and G2 of the band-shaped portion 8. The base sphere 6 is polished to form a bearing ball 5. While FIG. 3 illustrates a case in which the base sphere 6 has a band-shaped portion 8 on the circumference of the spherical portion 7, the base sphere 6 may not have a band-shaped portion 8. In other words, the base sphere 6 is the one before being polished to form a bearing ball 5.

[0056] Next, a method for manufacturing the silicon nitride sintered body 1 according to the embodiment will be described. The method for manufacturing the silicon nitride sintered body 1 according to the embodiment is not particularly limited as long as it has the above-mentioned configuration, but the method for obtaining it with a good yield is as follows.

[0057] First, raw material powders are prepared. The raw material powders are silicon nitride powder and sintering aid powder. The silicon nitride powder preferably has an average particle size of 3 μm or less. It is also preferable that the oxygen content is 3 mass% or less and the alpha conversion rate is 90% or more. Alternatively, a mixture of silicon nitride powder and sialon powder may be used. In the case of sialon powder, Al is already dissolved in the sialon powder.

[0058] Examples of sintering aid powders include a rare earth element component powder, a first component powder corresponding to the first solid solution metal (at least one component powder selected from tungsten component powder (W component powder) and molybdenum component powder (Mo component powder)), and a second component powder corresponding to the second solid solution metal (aluminum component powder). For example, the sintering aid powder may be a rare earth element component powder, a tungsten component powder (or molybdenum component powder), and an aluminum component powder. The sintering aid powder preferably has an average particle size of 4 μm or less.

[0059] The rare earth element component powder may be a powder of oxide, nitride, or oxynitride of a rare earth element. Yttrium and lanthanoid elements are preferred as rare earth elements. The aluminum component powder, which is the second component powder, may be a powder of oxide, nitride, or oxynitride of aluminum. The first component powder may be a powder of oxide, nitride, carbide, sulfide, oxynitride, oxycarbide, or carbonitride. Of these, oxide or carbide is preferred. The oxide may be tungsten oxide (WO ３ ), molybdenum oxide (MoO ３ Examples of carbides include tungsten carbide (WC, W ２ C), molybdenum carbide (Mo ２ C) can be mentioned.

[0060] In particular, it is preferable to use an oxide powder as the first component powder. ３ The melting point of MoO is 1473°C. ３ The melting point of WC is 2870℃, and that of Mo is 795℃. ２The melting point of C is 2687°C. The melting point of the oxide is lower than the sintering temperature, which will be described later. By using an oxide having a melting point lower than the sintering temperature, the first solid solution metal (W and / or Mo) can be dissolved in the specific silicon nitride crystal particles 21.

[0061] Furthermore, to dissolve the first solid solution metal (W and / or Mo) in the silicon nitride crystal particles 21, it is also effective to use silicon nitride powder that has been coated with the first component powder in advance. Mechanical alloying is an effective method for producing silicon nitride powder coated with the first component powder. Mechanical alloying involves the coating process, such as fine grain refinement, which can enhance the reactivity between silicon nitride and the coating material. Therefore, the effect of promoting the solid solution of the coating element as the silicon nitride grains grow can be expected.

[0062] In addition, a method of mixing powders with different crushing conditions can be mentioned in the step of mixing raw material powders described later. When the total amount of the silicon nitride powder and the first solid solution metal powder is 100 parts by mass, it is preferable to preliminarily crush and mix a portion of 30 parts by mass or more but 95 parts by mass or less. In the preliminarily crushing and mixing treatment, the average particle size D of the mixed powder before the preliminarily crushing and mixing treatment is set to 100 parts by mass. ５０ It is preferable to carry out a process in which the average particle size D becomes 2 / 3 or less. It is preferable to mix the pre-crushed material with the non-pre-crushed material and then carry out a process of mixing the raw material powders. In the sintering process, the powder with a small particle size acts as a nucleus, and can promote the solid solution of the first solid solution metal (W, etc.) into the silicon nitride crystal particles 21. ５０ is the median diameter.

[0063] If necessary, a sintering aid other than those mentioned above may be added to the raw material powder, such as at least one selected from titanium component powder, hafnium component powder, zirconium component powder, cobalt component powder, iron component powder, and silicon carbide powder.

[0064] When "silicon nitride powder" + "rare earth element component powder" + "first component powder" + "second component powder (aluminum component powder)" + "other component powders" is taken as 100% by mass, it is preferable that the "rare earth element component powder" be 1% by mass or more and 13% by mass or less, the "first component powder" be 0.1% by mass or more and 7% by mass or less, the "second component powder" be 1% by mass or more and 10% by mass or less, the "other component powders" be 0% by mass or more and 8% by mass or less, and the remainder be "silicon nitride powder." Note that "first component powder" refers to W component powder, Mo component powder, or WMo component powder. Also, "other component powders" refers to those added as sintering aids. Organic binders and solvents are not counted.

[0065] Next, the raw material powder is mixed. The mixing process is performed using a disintegrating mixer such as a ball mill. The ball mill can disintegrate the raw material powder by optimizing the disintegrating media, solvent, etc. Disintegrating the raw material powder can suppress the remaining agglomerates. The ball mill may be either a wet type or a dry type. The mixing process may also be performed by adding an organic binder or a solvent to the raw material powder, if necessary.

[0066] Next, the raw materials that have been mixed are subjected to a molding process to obtain a silicon nitride molded body (hereinafter simply referred to as a "molded body"). The raw materials may be granulated before molding. The molding process can be carried out by mold molding, rolling granulation, cold isostatic pressing (CIP), doctor blade method, injection molding, or the like. When producing spheres, mold molding, rolling granulation, or CIP is preferably used. It is also effective to CIP the molded body obtained by mold molding or rolling granulation. CIP is a molding method that uses a liquid as the pressurizing medium. Because isotropic pressure is applied by the liquid, a molded body with a uniform density distribution can be obtained. The CIP pressure is preferably in the range of 80 MPa to 500 MPa.

[0067] The molded body is subjected to a drying process as necessary. The drying process has the effect of removing the solvent used in the wet mixing process. Examples of the drying process include natural drying and heat drying. Heat drying is preferably performed at a temperature in the range of 80°C or higher and 200°C or lower. If the temperature is lower than 80°C, the drying efficiency may decrease. If the temperature exceeds 200°C, uneven drying may occur. For this reason, the drying temperature is preferably in the range of 80°C or higher and 200°C or lower, and more preferably in the range of 100°C or higher and 160°C or lower.

[0068] Next, the compact is subjected to a degreasing process as necessary. By carrying out the degreasing process, the organic binder can be removed from the compact. The degreasing temperature is preferably in the range of 400°C or higher and 700°C or lower. The degreasing process can be carried out in air, a nitrogen atmosphere, or the like. The compact obtained by the degreasing process is sometimes called a degreased body.

[0069] Next, the compact, for example, the degreased body, is subjected to a sintering process. The sintering temperature in the sintering process is preferably in the range of 1600°C or higher and 2000°C or lower. Furthermore, the sintering process can be performed using atmospheric sintering, pressure sintering, hot isostatic pressing (HIP), or the like. Sintering can be performed in air, a non-oxidizing atmosphere, a reducing atmosphere, or a vacuum. Atmospheric pressure sintering refers to sintering under controlled conditions of 1 atmosphere (0.9 to 1.1 atm = 0.09 to 0.11 MPa). Pressure sintering refers to sintering under pressure higher than atmospheric pressure. Uniaxial pressure sintering is sometimes called hot pressing. Furthermore, HIP is a sintering method in which gas isostatic pressure is applied.

[0070] This reduces internal defects such as voids and cracks in the silicon nitride sintered body 1. The HIP pressure is preferably in the range of 10 MPa to 200 MPa. Alternatively, atmospheric pressure sintering, pressure sintering, and HIP may be combined. When used for a wear-resistant member, it is preferable to use either one or both of pressure sintering and HIP. Sintering under pressure can produce a silicon nitride sintered body 1 (e.g., a raw sphere 6) with few internal defects. As for the sintering time, the holding time at the sintering temperature is preferably 1 hour or more. In other words, the sintering temperature is a temperature that is held for 1 hour or more within the range of 1600°C to 2000°C.

[0071] In the sintering step, when the sintering temperature is higher than the melting point of the tungsten component powder or the molybdenum component powder, it is preferable to hold the sintered material at a temperature near the melting point of the tungsten component powder or the molybdenum component powder added as a sintering aid for 3 hours or more. Holding the sintered material at a temperature near the melting point of the tungsten component powder or the molybdenum component powder is called an intermediate holding step. The term "near the melting point" refers to a temperature within ±50°C of the melting point. For example, in the case of WO ３ Since the melting point of the powder is 1473°C, the holding temperature is within the range of 1473±50°C. ３ Since the melting point of is 795°C, the holding temperature is 795°C ± 50°C. The holding temperature in the intermediate holding step is called the intermediate holding temperature.

[0072] By holding the mixture at a temperature near the melting point of the tungsten component powder or molybdenum component powder added as a sintering aid, the first solid solution metal can be dissolved in the silicon nitride crystal particles 21. When both tungsten component powder and molybdenum component powder are added, it is preferable to hold the mixture at the same temperature for each of them. There is no upper limit to the holding time, but it is preferably 10 hours or less.

[0073] The aluminum component powder is aluminum oxide (Al ２ O ３It is preferable to use both aluminum oxide (AlN) powder and aluminum nitride (AlN) powder. Comparing aluminum oxide and aluminum nitride, aluminum nitride is more likely to dissolve in silicon nitride. This can promote the dissolution of aluminum in silicon nitride. Furthermore, the amount (mass) of aluminum oxide powder added is preferably equal to or less than the amount of aluminum nitride powder added. The more aluminum nitride there is, the more easily aluminum element dissolves. This is an effective method for increasing the number of silicon nitride crystal particles with dissolved aluminum.

[0074] The sintering step can produce a silicon nitride sintered body 1. A wear-resistant member is produced by polishing the sliding surface of the silicon nitride sintered body 1. For example, in the case of a ball-shaped silicon nitride sintered body 1 (e.g., a base ball 6) for producing a bearing ball, the entire surface becomes the sliding surface.

[0075] The surface roughness Ra of the sliding surface of the silicon nitride sintered body 1 is preferably 0.1 μm or less. By making the surface of the silicon nitride sintered body 1 substantially flat, the wear resistance of the silicon nitride sintered body 1 can be improved. For this reason, the surface roughness Ra of the sliding surface is preferably 0.1 μm or less, and more preferably 0.01 μm or less. The silicon nitride sintered body 1 according to the embodiment has excellent machinability, which improves its processability into wear-resistant members. In other words, the polishing time required to produce wear-resistant members from the silicon nitride sintered body 1 can be shortened.

[0076] Furthermore, it is possible to improve the durability of the grindstone used to polish the silicon nitride sintered body 1. Diamond grindstones are generally used in the polishing process of the silicon nitride sintered body 1. Improving the durability of the grindstone not only reduces costs but is also effective in improving the manufacturing efficiency (improving processability) of wear-resistant members. Therefore, it is possible to provide a silicon nitride sintered body 1 with improved processability.

[0077] Examples (Examples 1 to 5, Comparative Examples 1 and 2) Silicon nitride powder and sintering aid powder were prepared as raw material powders. The raw material powder for silicon nitride sintered body 1 according to Examples 1 to 5 and the raw material powder for silicon nitride sintered body according to Comparative Examples 1 and 2 were mixed under the conditions shown in Table 1. The mixing ratio is shown as silicon nitride powder + sintering aid powder = 100 mass%.

[0078]

[0079] Example 2 is Si ３ N ４ Powder and WO ３ The powder was treated by mechanical alloying. ３ N ４ Powder and MoO ３ The powder was treated by mechanical alloying.

[0080] The raw material powders were mixed using a ball mill. An organic binder and a solvent were also added to the raw material powders and mixed in the ball mill. The raw material powders after ball mill mixing were then granulated, molded, and subjected to CIP. The resulting ball-shaped compacts had band-like portions around the periphery of the spheres. These compacts were intended to obtain the base spheres 6 after sintering. The compacts were subjected to a debinding process. The debinding process was carried out in the range of 400 to 650°C. The resulting debound compacts were then subjected to a sintering process. The sintering process was carried out in two stages. The sintering conditions are as shown in Table 2.

[0081]

[0082] As shown in Table 2, Examples 1, 2, and 4 were held at 1500°C for 3 hours or more. Example 3 was held at 800°C for 3 hours or more. Example 5 was held at 800°C and 1500°C for 3 hours or more, respectively. Comparative Examples 1 and 2 did not undergo any intermediate holding steps. Through the above steps, silicon nitride sintered body 1 according to the example and a silicon nitride sintered body according to the comparative example were obtained.

[0083] The silicon nitride sintered body 1 obtained according to the example was examined for the presence or absence of solute metals and dislocation defects. The measurement conditions were as described above. Ten or more silicon nitride crystal grains 2 having a length of 0.5 μm or more were selected from the silicon nitride crystal grains 2 within a measurement area of ​​20 μm × 20 μm and measured. The atomic ratios R1 to R3 indicated the minimum to maximum values ​​among the specific silicon nitride crystal grains 21. The results are shown in Table 3. The measurement results for the silicon nitride sintered body according to the comparative example are also shown in Table 3.

[0084]

[0085] Referring to Table 3, in the examples, the first atomic ratio R1 ((W+Mo) / Si) and the second atomic ratio R2 were both within the preferred ranges of the formulas (1) and (2). In Example 5, both W and Mo were dissolved in the specific silicon nitride crystal particles 21. In Examples 1, 2, and 4, the third atomic ratio R3 was within the preferred range of the formula (8). In Examples 3 and 5, the third atomic ratio R3 in some of the specific silicon nitride crystal particles 21 was outside the preferred range of the formula (8).

[0086] Furthermore, referring to Table 3, it can be seen that in the examples, the proportion U1 [%] of the number of specific silicon nitride crystal particles 21 relative to the total number of silicon nitride crystal particles 2 satisfies both the formula (5) and the formula (7). The specific silicon nitride crystal particles 21 are solid-solubilized with a first solid solution metal (at least one of W and Mo) and a second solid solution metal (Al). Furthermore, although not shown in the table, it has been found in the examples that the proportion U2 [%] of the number of other silicon nitride crystal particles 22 relative to the total number of silicon nitride crystal particles 2 satisfies the formula (10).

[0087] It is noted that, referring to Table 3, in the Examples, the proportion U3 [%] of the number of specific silicon nitride crystal particles 21 having dislocation defects relative to all specific silicon nitride crystal particles 21 satisfies the above formula (13). Furthermore, although not shown in the Table, it is noted in the Examples that the proportion U4 [%] of the number of specific silicon nitride crystal particles 21 having dislocation defects relative to all silicon nitride crystal particles 2 satisfies the above formula (15).

[0088] In Comparative Examples 1 and 2, no solid solution of W or Mo was observed in the silicon nitride crystal particles. ３ It can be seen that simply adding powder of W does not result in W forming a solid solution in the silicon nitride crystal grains of the sintered spheres.

[0089] Next, the average major axis length and average aspect ratio were measured for the silicon nitride crystal particles 2 of the element spheres 6, which are silicon nitride sintered bodies 1 according to the example, and for the silicon nitride crystal particles of the element spheres, which are silicon nitride sintered bodies according to the comparative example. The three-point bending strength, fracture toughness, and Vickers hardness were also measured for the element spheres 6 according to the example and the comparative example. The measurement conditions were as described above. The results are shown in Table 4. The three-point bending strength was measured not using the element spheres 6, but using plate-shaped silicon nitride sintered bodies 1, which were manufactured under the same conditions as the element spheres 6.

[0090]

[0091] As can be seen from Table 4, the silicon nitride crystal particles 2 of the raw spheres 6 according to the example and the silicon nitride crystal particles of the raw spheres according to the comparative example had average major axis lengths in the range of 0.5 μm to 10 μm and average aspect ratios in the range of 2 to 10. There was also no significant difference in fracture toughness between the example and the comparative example. The three-point bending strength and Vickers hardness were also slightly higher in the example. The machinability coefficients of the example and the comparative example were both 0.12 or less, which was not a significant difference.

[0092] A blank ball 6 according to the example and a blank ball according to the comparative example were prepared. Both the blank ball 6 according to the example and the blank ball according to the comparative example were intended to obtain bearing balls of 3 / 8 inch (diameter 9.525 mm). Furthermore, both blank balls had a band-like portion on the circumference of the sphere.

[0093] Next, the base ball 6 according to the example and the base ball according to the comparative example were polished to a surface roughness Ra of 0.01 μm using a diamond grinding wheel. The replacement intervals of the diamond grinding wheels were compared between the example and the comparative example. The replacement intervals of the diamond grinding wheels of the example were shown as a ratio when the replacement interval of the diamond grinding wheel of comparative example 1 was set to 100. The larger the number, the longer the replacement interval of the grinding wheel and the better the durability of the grinding wheel.

[0094] The time required for polishing to a surface roughness Ra of 0.01 μm using a diamond grinding wheel was also measured. The polishing times of the examples are shown as a ratio when the polishing time of Comparative Example 1 is set to 100. The smaller the number, the shorter the polishing time. The results are shown in Table 5.

[0095]

[0096] As can be seen from Table 5, the polishing time for the examples was reduced by approximately 10 to 20%, and machinability was improved. This is due to improved machinability. In addition, the replacement interval for the grinding wheel was improved by approximately 10% for the examples. This improved the durability of the grinding wheel. Despite the low machinability coefficient of 0.12 or less, the machinability was improved. Furthermore, the bearing balls 5 obtained by polishing the base balls 6 had good durability.

[0097] In the examples and comparative examples, it was confirmed that the workability of the base sphere 6 (which can be polished to become a bearing ball), which is an example of the silicon nitride sintered body 1, was improved. However, this effect is not limited to the case where the silicon nitride sintered body 1 is the base sphere 6, and it is believed that this effect can also be obtained with silicon nitride sintered bodies 1 other than the base sphere 6 (which can be polished to become a wear-resistant member other than a bearing ball) as long as they contain the specific silicon nitride crystal particles 21.

[0098] According to at least one of the embodiments described above, it is possible to provide a silicon nitride sintered body 1 (e.g., base sphere 6) with improved processability and a wear-resistant member using the same (e.g., bearing ball 5). Furthermore, by controlling the proportion of the number of specific silicon nitride crystal grains 21 having dislocation defects, it is possible to provide a silicon nitride sintered body 1 with improved processability while maintaining wear resistance, and a wear-resistant member using the same.

[0099] 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. A silicon nitride sintered body comprising a plurality of silicon nitride crystal particles and a grain boundary phase, wherein the plurality of silicon nitride crystal particles include specific silicon nitride crystal particles in which at least one of tungsten and molybdenum as a first solid solution metal is solid-solved with aluminum as a second solid solution metal.

2. Regarding the solid-solution amount in the specific silicon nitride crystal particles, “the number of atoms of the first solid solution metal / the number of atoms of silicon” which is the number of atoms of the first solid solution metal with respect to the number of atoms of silicon is 0.0001 or more and 0.01 or less, and “the number of atoms of the second solid solution metal / the number of atoms of silicon” which is the number of atoms of the second solid solution metal with respect to the number of atoms of silicon is 0.001 or more and 0.1 or less. The silicon nitride sintered body according to claim 1, characterized in that.

3. In any cross-section of the silicon nitride sintered body, the ratio of the number of the specific silicon nitride crystal particles to the number of the plurality of silicon nitride crystal particles in a measurement area of 20 μm × 20 μm is in the range of 10% or more and 100% or less. The silicon nitride sintered body according to claim 1 or claim 2, characterized in that.

4. Regarding the solid-solution amount in the specific silicon nitride crystal particles, “the number of atoms of oxygen / the number of atoms of silicon” which is the number of atoms of oxygen with respect to the number of atoms of silicon is in the range of 0.001 or more and 1.20 or less. The silicon nitride sintered body according to claim 1 or claim 2, characterized in that.

5. Regarding the solid-solution amount in the specific silicon nitride crystal particles, “the number of atoms of oxygen / the number of atoms of silicon” which is the number of atoms of oxygen with respect to the number of atoms of silicon is in the range of 0.001 or more and 1.20 or less. The silicon nitride sintered body according to claim 3, characterized in that.

6. Regarding the silicon nitride crystal particles that do not contain either tungsten or molybdenum among the plurality of silicon nitride crystal particles as another silicon nitride crystal particle, in any cross-section of the silicon nitride sintered body, the ratio of the number of the other silicon nitride crystal particles to the number of the plurality of silicon nitride crystal particles in a measurement area of 20 μm × 20 μm is in the range of 0% or more and 30% or less. The silicon nitride sintered body according to claim 1 or claim 2, characterized in that.

7. In any cross-section of the silicon nitride sintered body, the ratio of the number of the specific silicon nitride crystal particles having a transition defect part to the total number of the specific silicon nitride crystal particles in a measurement area of 20 μm × 20 μm is in the range of 0% or more and 10% or less. The silicon nitride sintered body according to claim 1 or claim 2, characterized in that.

8. In any cross-section of the silicon nitride sintered body, the ratio of the number of the specific silicon nitride crystal particles having transition defect portions to the total number of the specific silicon nitride crystal particles in a measurement area of 20 μm × 20 μm is in the range of 0% or more and 10% or less, and the silicon nitride sintered body according to claim 5 is characterized by this.

9. The average length of the major axis of the plurality of silicon nitride crystal particles is 0.5 μm or more and 10 μm or less, and the average aspect ratio is in the range of 2 or more and 10 or less, and the silicon nitride sintered body according to claim 1 or claim 2 is characterized by this.

10. The average length of the major axis of the plurality of silicon nitride crystal particles is 0.5 μm or more and 10 μm or less, and the average aspect ratio is in the range of 2 or more and 10 or less, and the silicon nitride sintered body according to claim 3 is characterized by this.

11. The average length of the major axis of the plurality of silicon nitride crystal particles is 0.5 μm or more and 10 μm or less, and the average aspect ratio is in the range of 2 or more and 10 or less, and the silicon nitride sintered body according to claim 8 is characterized by this.

12. An abrasion-resistant member comprising the silicon nitride sintered body according to claim 1.

13. An abrasion-resistant member comprising the silicon nitride sintered body according to claim 3.

14. An abrasion-resistant member comprising the silicon nitride sintered body according to claim 10.

15. The abrasion-resistant member according to claim 13, characterized by having a sliding surface with a surface roughness of Ra 0.1 μm or less.

16. The abrasion-resistant member according to claim 14, characterized by having a sliding surface with a surface roughness of Ra 0.1 μm or less.

17. The abrasion-resistant member according to claim 15, characterized in that the abrasion-resistant member is a bearing ball.

18. The abrasion-resistant member according to claim 16, characterized in that the abrasion-resistant member is a bearing ball.

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