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

Abstract

In the silicon nitride sintered body of the embodiment having a plurality of silicon nitride crystal grains and a grain boundary phase, the plurality of silicon nitride crystal grains include a specific silicon nitride crystal grain in which at least one of tungsten and molybdenum and aluminum are solid-solved. In the silicon nitride crystal grain, with respect to the solid-solution amount to the specific silicon nitride crystal grain, the atomic number of the first solid-solution metal with respect to the atomic number of silicon, "atomic number of first solid-solution metal / atomic number of silicon", is preferably 0.0001 or more and 0.01 or less, and the atomic number of the second solid-solution metal with respect to the atomic number of silicon, "atomic number of second solid-solution metal / atomic number of silicon", is preferably 0.001 or more and 0.1 or less.

Description

Technical Field

[0001] The embodiments described later generally relate to silicon nitride sintered bodies and wear-resistant components using them. Background Technology

[0002] Silicon nitride sintered bodies are used in wear-resistant components. Examples of wear-resistant components include bearing components, rollers, compressor blades, gas turbine blades, and engine components. Examples of bearing components include bearing balls, bearing inner rings, and bearing outer rings. Additionally, examples of rollers include rolling and conveying rollers. Examples of engine components include cam rolls. In recent years, silicon nitride sintered bodies have demonstrated excellent mechanical strength and wear resistance, thus they are used for bearing balls as bearing components.

[0003] For example, Japanese Patent No. 5100201 (Patent Document 1) discloses a silicon nitride sintered body that controls the aspect ratio and standard deviation of needle-like silicon nitride grains. Furthermore, Japanese Patent No. 6416088 (Patent Document 2) discloses a silicon nitride sintered body that suppresses deviations in the area ratio of grain boundary phases. Patent Documents 1 and 2 achieve a uniform distribution of silicon nitride grains and grain boundary phases. This improves the wear resistance of the silicon nitride sintered body.

[0004] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 5100201 Patent Document 2: Japanese Patent No. 6416088 Patent Document 3: Japanese Patent Application Publication No. 2001-335368 Patent Document 4: International Publication No. 2020 / 121752 Summary of the Invention

[0005] The problem that the invention aims to solve In recent years, silicon nitride sintered bodies have been used in bearing balls for electric motors in electric vehicles. Consequently, the demand for silicon nitride sintered body bearing balls has increased significantly. On the other hand, silicon nitride sintered bodies are difficult to process. Therefore, in order to mass-produce silicon nitride sintered body bearing balls, further improvements in the processability of silicon nitride sintered bodies are required, i.e., a reduction in processing time. To improve processability, it is effective to dissolve metal elements in the silicon nitride grains. For example, Japanese Patent Application Publication No. 2001-335368 (Patent Document 3) discloses a silicon nitride sintered body in which Al, Mg, and rare earth elements are dissolved in the silicon nitride grains. The silicon nitride sintered body of Patent Document 3 is used as a substrate for semiconductor devices, thus achieving improved thermal conductivity. However, although the processability is slightly improved, it may not be to a satisfactory degree.

[0006] One of the challenges to be addressed by the implementation method is to provide a silicon nitride sintered body with improved processability and a wear-resistant component using the silicon nitride sintered body.

[0007] Methods for solving problems In the embodiment of the silicon nitride sintered body having a plurality of silicon nitride grains and grain boundary phases, the plurality of silicon nitride grains include specific silicon nitride grains having a first solid solution metal composed of at least one of tungsten and molybdenum and a second solid solution metal composed of aluminum dissolved in it. Attached Figure Description

[0008] Figure 1 This is a schematic diagram illustrating an example of the cross-sectional microstructure of a silicon nitride sintered body according to an embodiment.

[0009] Figure 2 This is an external view showing an example of a bearing ball based on a silicon nitride sintered body according to an embodiment.

[0010] Figure 3 This is an external view showing an example of a raw material ball for bearings formed from a silicon nitride sintered body according to the embodiment. Detailed Implementation

[0011] In the embodiment of the silicon nitride sintered body having a plurality of silicon nitride grains and grain boundary phases, the plurality of silicon nitride grains include specific silicon nitride grains having a first solid solution metal composed of at least one of tungsten and molybdenum and a second solid solution metal composed of aluminum dissolved in it.

[0012] Figure 1 An example of the cross-sectional microstructure of the silicon nitride sintered body according to the embodiment is shown in the figure. In the figure, symbol 1 represents the silicon nitride sintered body, symbol 2 represents the silicon nitride grain, and symbol 3 represents the grain boundary phase. Symbol 21 is a specific silicon nitride grain 2 containing a first solid solution metal composed of at least one of tungsten and molybdenum and a second solid solution metal composed of aluminum. Symbol 22 is a silicon nitride grain 2 that does not contain either tungsten or molybdenum (referred to as "other silicon nitride grains" in this specification).

[0013] Symbol 23 refers to the silicon nitride grain remaining after removing the specific silicon nitride grain 21 and other silicon nitride grains 22 from the silicon nitride grain 2 (referred to as "remaining silicon nitride grain" in this specification). It should be noted that the presence of the other silicon nitride grains 22 in the silicon nitride grain 2 is not essential, nor is the presence of the remaining silicon nitride grain 23. For example, other silicon nitride grains 22 may include grains that do not contain both tungsten and molybdenum, or grains that contain only aluminum. Furthermore, the remaining silicon nitride grain 23 may include silicon nitride grains containing tungsten or molybdenum but not aluminum.

[0014] The silicon nitride sintered body 1 has a structure formed by the complex entanglement of multiple silicon nitride grains 2. In addition, the silicon nitride sintered body 1 has a structure in which the grain boundary phase 3 fills the gaps between the multiple silicon nitride grains 2.

[0015] The silicon nitride sintered body 1 has specific silicon nitride grains 21 as silicon nitride grains 2, which are solid-dissolved with a first solid-dissolved metal composed of at least one of tungsten and molybdenum and a second solid-dissolved metal composed of aluminum.

[0016] The determination of solid-solution metal elements was performed using TEM-EDS. The TEM-EDS conditions were set as follows: accelerating voltage 200 kV, irradiation current 1.00 nA, spot diameter 1 nm, analysis time 30 seconds, and sample angles X=10° and Y=0°.

[0017] In any cross-section of the silicon nitride sintered body 1, the silicon nitride grain 2 is used as the measurement area. An analysis point is selected near the center of the silicon nitride grain 2. Through this analysis, if at least one of tungsten and molybdenum and aluminum are detected in a silicon nitride grain 2, it is determined that the silicon nitride grain 2 is a specific silicon nitride grain 21 with a first solid solution metal and a second solid solution metal. It should be noted that if tungsten or molybdenum, or aluminum, is not detected when measuring a silicon nitride grain 2, the measurement cross-section of the same silicon nitride grain 2 can be changed for re-measurement. In addition, considering the size of the silicon nitride grain 2, it is preferable to use silicon nitride grains 2 with a major axis length of 0.5 μm or more as the measurement object. This is because if silicon nitride grains 2 with a length less than 0.5 μm are used as the measurement object, they may be affected by the surrounding grain boundary phase 3.

[0018] In addition, FESEM-EDS can be used instead of TEM-EDS in the determination of solid-solid metal elements. FESEM stands for Field Emission Scanning Electron.

[0019] The so-called first and second solid solution metals refer to the combination of solid solution metal elements in a specific silicon nitride grain 21 that is any 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.

[0020] It is believed that the solid solutions of tungsten and molybdenum, possessing interatomic bonding strengths different from those of Si-N bonds, act within the silicon nitride grains 2, thus improving the processability of the silicon nitride sintered body 1. Furthermore, the solid solution of aluminum is effective in increasing the hardness of the silicon nitride grains.

[0021] Regarding the amount of the first solid solution metal dissolved in a specific silicon nitride grain 21, the first atomic ratio R1 is defined as "the number of atoms of the first solid solution metal relative to the number of atoms of silicon (Si atoms)". The first atomic ratio R1 preferably satisfies the following formula (1).

[0022] 0.0001≤R1≤0.01 …(1) Regarding the amount of the second solid solution metal (Al) dissolved in a specific silicon nitride grain 21, the "number of atoms of the second solid solution metal / number of atoms of Si" is set as the second atomic number ratio R2. The second atomic number ratio R2 preferably satisfies the following formula (2).

[0023] 0.001≤R²≤0.1 …(2) If the first atomic number ratio R1 satisfies equation (1) and the second atomic number ratio R2 satisfies equation (2), the processability of the silicon nitride sintered body 1 is improved. If the first atomic number ratio R1 is less than 0.0001, the effect of tungsten or molybdenum solid solution may be insufficient. In addition, if the first atomic number ratio R1 exceeds 0.01, a particular silicon nitride grain 21 may become unstable. Furthermore, in the case where both tungsten and molybdenum are solidly dissolved in a particular silicon nitride grain 21, the first atomic number ratio R1 is calculated using their combined atomic number.

[0024] If the second atomic number ratio R2 is less than 0.001, the amount of the second solid solution metal may be insufficient. In addition, if the second atomic number ratio R2 exceeds 0.1, the amount of the second solid solution metal becomes excessive, and the specific silicon nitride grain 21 may become unstable.

[0025] Furthermore, the first atomic number ratio R1 is more preferably satisfied by the following equation (3).

[0026] 0.0005≤R1≤0.002 …(3) Furthermore, the second atomic number ratio R2 is further preferably satisfied by the following equation (4).

[0027] 0.01≤R²≤0.04 …(4) In addition, in the silicon nitride sintered body 1, as silicon nitride grains 2, there may also be silicon nitride grains 22 with only one of tungsten, molybdenum and aluminum dissolved in solid solution, or the remaining silicon nitride grains 23.

[0028] Furthermore, in any cross section of the silicon nitride sintered body 1, the ratio U1[%] of the number of specific silicon nitride grains 21 with the first solid solution metal and the second solid solution metal, relative to the total number of silicon nitride grains 2 in the measured area of ​​20μm×20μm, preferably satisfies the following formula (5). Here, "any cross section" can be any cross section in the silicon nitride sintered body 1 as long as it is a cross section with a size of "20μm×20μm" that can be cut.

[0029] 10≤U1[%]≤100 …(5) The number of silicon nitride grains 2 in a 20μm × 20μm area is determined by counting the number of silicon nitride grains 2 with a major axis length of 0.5μm or more in the cross-sectional photograph. Any 10 or more silicon nitride grains 2 with a length of 0.5μm or more are selected, and the solid solution content of tungsten, molybdenum, and aluminum is measured for each. The number of specific silicon nitride grains 21 exhibiting solid solutions of the first and second solid solution metals is counted.

[0030] The percentage of specific silicon nitride grains 21, U1[%], is determined by the following formula (6). The number of silicon nitride grains 2 with a major diameter of 0.5 μm or more is set as C, and the number of specific silicon nitride grains 21 with a diameter of 0.5 μm or more is set as C1.

[0031] U1[%]=C1 / C×100 …(6) Furthermore, if no more than 10 silicon nitride grains 2 with a major diameter length of 0.5 μm or more are observed in a measurement area of ​​20 μm × 20 μm, other measurement areas are used. The length of the silicon nitride grain 2 refers to the longest diagonal of each silicon nitride grain 2. The length is determined by observation in a TEM or FESEM. While it is possible to set 10 or more silicon nitride grains 2 with a length of 0.5 μm or more in the 20 μm × 20 μm measurement area, all silicon nitride grains 2 with a length of 0.5 μm or more can also be measured. Additionally, as mentioned above, setting the measurement target to have a length of 0.5 μm or more considers measurement accuracy; however, for the sake of processability and other effects, specific silicon nitride grains 21 with a major diameter length of 0.5 μm or more and containing tungsten or the like can also be used.

[0032] Sometimes, silicon nitride grains with metal solution are also referred to as metal-solid-solid silicon nitride grains. Better results can be obtained by ensuring that the proportion U1 of specific silicon nitride grains 21, which are part of the metal-solid-solid silicon nitride grains, relative to silicon nitride grains 2 in a measured area of ​​20 μm × 20 μm is 10% or more (as described in equation (5) above). The proportion of specific silicon nitride grains 21 is further preferably satisfied by the following equation (7).

[0033] 30≤U1[%]≤100 …(7) Furthermore, regarding the amount of acid (O) dissolved in a specific silicon nitride grain 21, the third atomic ratio R3 is defined as "number of O atoms / number of Si atoms" relative to the number of oxygen atoms. The third atomic ratio R3 preferably satisfies the following formula (8).

[0034] 0.001≤R3≤1.20 …(8) This indicates the amount of dissolved oxygen in a specific silicon nitride grain 21. The solid solution of oxygen into the specific silicon nitride grain 21, together with the solid solution of aluminum, forms a silane crystal structure, resulting in solid solution strengthening effects such as increased hardness. Oxygen and aluminum are mainly substitutional solid solutions, while tungsten and molybdenum are mainly intrusive solid solutions. By combining intrusive and substitutional solid solutions, a more effective effect can be obtained. Substitutional solid solutions refer to solid solutions in which a portion of the Si or N in the silicon nitride lattice is replaced by O or Al. In addition, intrusive solid solutions refer to solid solutions in which W or Mo enters the interstitial spaces of the silicon nitride lattice.

[0035] When the third atomic number ratio R3 is less than 0.001, the oxygen solid solution content is low, and therefore there is a possibility of insufficient effect. In addition, if the third atomic number ratio R3 exceeds 1.20, the oxygen content is too high, and the specific silicon nitride grain 21 may become unstable. Therefore, the third atomic number ratio R3 preferably satisfies the following equation (9).

[0036] 0.05≤R3≤1.0 …(9) Furthermore, in any cross section of the silicon nitride sintered body 1, the proportion U2[%] of the number of other silicon nitride grains 22 relative to the number of silicon nitride grains 2 in the measured area of ​​20μm×20μm preferably satisfies the following formula (10). More preferably, it is the case of formula (11).

[0037] 0≤U2≤30 …(10) 0≤U2<10 …(11) Other silicon nitride grains 22 refer to silicon nitride grains in which neither W nor Mo is detected (including those below the detection limit) in the aforementioned TEM-EDS (regardless of whether Al is detected or not). Examples of other silicon nitride grains 22 include silicon nitride grains that originally have no dissolved metal, and silicon nitride grains that have dissolved Al, O, or other metals (metals other than W and Mo). The silicon nitride sintered body 1 of the embodiment improves its properties by the presence of specific silicon nitride grains 21. Therefore, it is preferable to have fewer other silicon nitride grains 22.

[0038] In addition, Fe (iron) can be cited as a solid solution metal other than W and Mo in the specific silicon nitride grain 21. In the case of solid solution Fe, in the specific silicon nitride grain 21 in which Fe is solid solution, the "fourth atomic number / Si atomic number" is set as the fourth atomic number ratio R4, which is the fourth atomic number of Fe. The fourth atomic number ratio R4 preferably satisfies the following formula (12).

[0039] 0.0001≤R4≤0.002 …(12) Furthermore, in any cross section of the silicon nitride sintered body 1, the proportion U3[%] of the number of specific silicon nitride grains 21 having dislocation defects relative to the total number of specific silicon nitride grains 21 in the measured area of ​​20μm×20μm preferably satisfies the following formula (13). This is to prevent the wear resistance of the silicon nitride sintered body 1 from deteriorating.

[0040] 0≤U3[%]≤10 …(13) This means that there are fewer grains with dislocation defects in a specific silicon nitride grain 21.

[0041] Dislocation defects are crystal defects contained within a crystal. Crystal defects are also called lattice defects. Crystal defects are caused by disordered atomic arrangement or impurities. Dislocation defects introduce defects into a stable crystal structure. As mentioned above, the solid solutions of W, Mo, Al, and O into silicon nitride grains can be intrusive or substitutional. A specific silicon nitride grain 21 with dislocation defects indicates that the crystal structure has not been destroyed even in the presence of solid solution metals.

[0042] The presence or absence of dislocation defects can be analyzed using the method described in International Publication No. WO2020 / 121752 (Patent Document 4). Specifically, it is determined by observing the dark field and bright field of a TEM image. Dislocation defects appear white in the dark field and are reversed to appear black in the bright field. Thus, the part of the pixel whose color is reversed when switching between the dark and bright fields is defined as a dislocation defect. The overlapping part of the black-and-white color reversal of the pixel is defined as a dislocation defect.

[0043] If dislocation defects are present, the wear resistance of the silicon nitride sintered body 1 may be reduced. Therefore, the ratio of the number of specific silicon nitride grains 21 with dislocation defects to the total number of specific silicon nitride grains 21 preferably satisfies the above formula (13), and more preferably satisfies the following formula (14).

[0044] 0≤U3[%]≤5 …(14) Furthermore, the proportion U4[%] of the specific silicon nitride grains 21 having dislocation defects is preferably satisfied by the following formula (15), relative to the total number of silicon nitride grains 2 that also include other silicon nitride grains 22.

[0045] 0≤U4[%]≤10 …(15) Furthermore, the average length of the major diameter of the silicon nitride grain 2 is preferably 0.5 μm or more and 10 μm or less, and the average aspect ratio is preferably in the range of 2 or more and 10 or less. When the average major diameter is less than 0.5 μm, the silicon nitride grain 2 is too small, and its durability may decrease. On the other hand, when the average major diameter exceeds 10 μm, the gaps between the silicon nitride grains 2 become larger, and their strength may decrease.

[0046] The major and minor axes of silicon nitride grains 2 were determined using SEM images. SEM images of any cross-section of the silicon nitride sintered body 1 were taken. The cross-section was prepared with a polished surface having a surface roughness Ra of less than 1 μm. The SEM image magnification was set to 1000x or higher. A magnification of 4000x is recommended. The measurement area was 300 μm × 300 μm.

[0047] The longest diagonal of the silicon nitride grain 2 captured in the SEM image is defined as the major axis. From the silicon nitride grains 2 captured in a measurement area of ​​300 μm × 300 μm, 50 grains are selected in descending order of size, and their average value is taken as the average length of the major axis. Additionally, a line drawn perpendicularly from the midpoint of the major axis of the selected silicon nitride grains 2 is defined as the minor axis. Major axis / minor axis = aspect ratio, and its average value is taken as the average aspect ratio.

[0048] In addition, the grain boundary phase is mainly composed of sintering aids. Examples of sintering aids include rare earth compounds, aluminum compounds, tungsten compounds, molybdenum compounds, titanium compounds, hafnium compounds, zirconium compounds, and silicon carbide.

[0049] Furthermore, in addition to using rare earth compounds and aluminum compounds as sintering aids, it is preferable to use at least one selected from tungsten compounds, molybdenum compounds, titanium compounds, hafnium compounds, zirconium compounds, and silicon carbide. By having at least one selected from tungsten compounds, molybdenum compounds, titanium compounds, hafnium compounds, zirconium compounds, and silicon carbide as a crystalline compound in the grain boundary phase, the grain boundary phase can be strengthened. This improves the wear resistance of the silicon nitride sintered body 1. Furthermore, the thermal conductivity of the silicon nitride sintered body 1 using rare earth compounds and aluminum compounds as sintering aids tends to be below 30 W / m·K.

[0050] The silicon nitride sintered body 1 described above can achieve a three-point flexural strength of 600 MPa or more, and further, 900 MPa or more. Furthermore, it can achieve a fracture toughness of 6.0 MPa·m. 1 / 2 The above is further specified as 7.0 MPa·m 1 / 2 That's all. Additionally, the Vickers hardness can be set to 1400 or higher, and further to 1500 or higher.

[0051] Three-point bending strength was determined according to JIS-R-1601 (2008). JIS-R-1601 corresponds to ISO 14704. Fracture toughness was determined using the new formula according to the IF method of JIS-R-1607 (2015). JIS-R-1607 corresponds to ISO 15732. Vickers hardness was determined according to JIS-R-1610 (2003). JIS-R-1610 corresponds to ISO 14705.

[0052] Furthermore, it enables a machinability factor of 0.12 or less. The machinability factor Mc represents the machinability factor achieved by using indentation load (Fn), Vickers hardness (Hv), and fracture toughness value (K). 1c The machinability coefficient (Mc) is a coefficient related to the machinability and consequently, the machinability of a single abrasive grain. This is the relationship in the transverse cracking failure model, where Mc represents the amount of material removed by one abrasive grain. A larger machinability coefficient (Mc) means a larger amount of material can be machined in a single pass.

[0053] The so-called transverse cracking failure model, proposed by Evans and Marshall, is a material removal mechanism during grinding. In this model, the amount of material removed (δV) by a single abrasive grain passing through the material surface is expressed as a relationship between the force Fn pressing the abrasive grain in a direction perpendicular to the material, Vickers hardness (Hv), and fracture toughness (K1C), and [Fn...]. 9 / 8 / (K 1c 1 / 2 ·Hv 5 / 8 The value of δV is proportional to the value of Mc. Here, δV is replaced by the machinability factor Mc.

[0054] Machining can be broadly categorized into brittle and ductile modes. Brittle modes correspond to rough machining, while ductile modes correspond to finish machining. Wear is considered equivalent to the ductile mode; therefore, to meet the performance requirements of wear-resistant components, it is important to improve the machinability of the brittle mode without compromising the machinability of the ductile mode. Furthermore, as one wear model, the following mechanism is considered: the formation of micro-pre-cracks at grain boundaries, which propagate and cause surface damage, resulting in wear.

[0055] The parameter Sc.m, which represents the severity of mechanical contact in the wear model, is expressed by the friction coefficient μ, the maximum Hertzian stress Pmax, the grain size d of the material, and the fracture toughness value K1c, as shown in Equation (16).

[0056] Sc.m=[(1+10·μ)·Pmax·(d 1 / 2 )] / K 1c …(16) A large parameter Sc.m indicates greater material wear, while a small parameter Sc.m indicates less material wear. Therefore, reducing the grain size d or increasing the fracture toughness K can help mitigate this wear. 1c This can suppress material wear. Although the machinability coefficient Mc of the silicon nitride sintered body 1 of the embodiment is as small as 0.12 or less, it can improve machinability.

[0057] Furthermore, the silicon nitride sintered body 1 of the embodiment is suitable for wear-resistant components. Additionally, the wear-resistant component preferably has a sliding surface with a surface roughness Ra of 0.1 μm or less. Examples of wear-resistant components include bearing components, rollers, compressor blades, gas turbine blades, and engine components. Examples of bearing components include bearing balls, inner rings of bearings, and outer rings of bearings. Examples of rollers include rolling mills and conveying rollers. Examples of engine components include cam rollers.

[0058] Wear-resistant components have sliding surfaces that slide against the target component. For example, bearing balls are disposed between the inner and outer rings of a bearing. The entire surface of a bearing ball, which is composed of a spherical silicon nitride sintered body 1, serves as a sliding surface. Additionally, the roller surface of a roller, which is composed of a cylindrical silicon nitride sintered body 1, serves as a sliding surface.

[0059] To improve the wear resistance of the sliding surface, grinding to a surface roughness Ra of less than 0.1 μm is effective. The silicon nitride sintered body 1 of the embodiment controls the solid solution metals such as W, Mo, and Al. This improves machinability. For example, the surface roughness of bearing balls is specified by ASTM_F2094. Depending on the application, bearing balls may adopt a grade based on ASTM_F2094, ISO_26602, or JIS_R1669. Grinding is performed to a surface roughness Ra based on that grade. As the grade increases, a mirror finish with a surface roughness Ra of less than 0.01 μm is sometimes also performed. In the silicon nitride sintered body 1 of the embodiment, machinability is improved while maintaining wear resistance. The grinding process for obtaining a sliding surface with a surface roughness Ra of less than 0.1 μm can be performed efficiently.

[0060] Figure 2 Showing the bearing balls, Figure 3 An example of a raw ball for bearing balls (hereinafter referred to as "raw ball") is shown. Symbol 5 is a bearing ball, symbol 6 is a raw ball, symbol 7 is the spherical surface of raw ball 6, and symbol 8 is the strip-shaped portion of raw ball 6. Figure 3 (A) indicates the raw material ball 6 as viewed along a direction orthogonal to the straight line G1 and G2 connecting the two poles of the strip 8 (the two vertices with the surface containing the strip 8 as the bottom). Figure 3(B) shows the raw material ball 6 as viewed along the directions G1 and G2 of the two poles of the connecting strip 8. The raw material ball 6 is ground to form the bearing ball 5. Furthermore, in Figure 3 In the example shown, the raw material ball 6 has a strip-shaped portion 8 on the circumference of the spherical surface 7, but it may also be without the strip-shaped portion 8. In other words, the material before the bearing ball 5 is ground is called the raw material ball 6.

[0061] Next, the manufacturing method of the silicon nitride sintered body 1 according to the embodiment will be described. The manufacturing method of the silicon nitride sintered body 1 according to the embodiment is not particularly limited as long as it has the above-described structure, but a method for obtaining a good yield is described below.

[0062] First, prepare the raw material powder. The raw material powder consists of silicon nitride powder and sintering aid powder. The silicon nitride powder preferably has an average particle size of 3 μm or less. Additionally, it preferably has an oxygen content of 3% by mass or less and an α-saturation rate of 90% or more. Alternatively, a material in which silon powder is mixed with silicon nitride powder can also be used. If silon powder is used, it becomes a powder with Al pre-dissolved in it.

[0063] Examples of sintering aid powders include rare earth element powders, first component powders corresponding to the first solid solution metal (at least one of tungsten component powder (W component powder) and molybdenum component powder (Mo component powder), and second component powders corresponding to the second solid solution metal (aluminum component powder). For example, rare earth element powders, tungsten component powders (or molybdenum component powders), and aluminum component powders are used as sintering aid powders. Furthermore, the average particle size of the sintering aid powder is preferably 4 μm or less.

[0064] Examples of rare earth element powders include oxides, nitrides, and oxynitrides of rare earth elements. Yttrium and lanthanides are preferred rare earth elements. Examples of aluminum powders (secondary component powder) include oxides, nitrides, and oxynitrides of aluminum. Examples of oxide, nitride, carbide, sulfide, oxynitride, carbon oxide, and carbonitride powders are also possible primary component powders. Oxides or carbides are preferred. Examples of oxides include tungsten oxide (WO3) and molybdenum oxide (MoO3). Examples of carbides include tungsten carbide (WC, W2C) and molybdenum carbide (Mo2C).

[0065] Oxide powders are particularly preferred as the first component powder. WO3 has a melting point of 1473°C, MoO3 has a melting point of 795°C, WC has a melting point of 2870°C, and Mo2C has a melting point of 2687°C. The melting points of oxides are lower than the sintering temperatures described later. By using oxides with melting points lower than the sintering temperatures, the first solid solution metal (W and / or Mo) can be dissolved in specific silicon nitride grains 21.

[0066] Furthermore, using silicon nitride powder pre-coated with the first component powder is also effective in dissolving the first solid solution metal (W and / or Mo) into the silicon nitride grains 21. Mechanical alloying is effective for producing silicon nitride powder coated with the first component powder. In mechanical alloying, the coating is performed along with material miniaturization, thus improving the reactivity between silicon nitride and the coating material. Therefore, as the silicon nitride grains grow, it is expected that the effect of promoting the solid solution of the coating element can be achieved.

[0067] Furthermore, in the process of mixing raw material powders described later, a method of mixing powders with different crushing conditions can be cited. When the total amount of silicon nitride powder and first solid solution metal powder is set to 100 parts by mass, it is preferable to pre-crush and mix a portion of 30 parts by mass or more and 95 parts by mass or less. As a pre-crushing and mixing treatment, it is preferable that the average particle size D of the mixed powder before the pre-crushing and mixing treatment is... 50 The processing is less than 2 / 3. It is preferable to perform a process of mixing the pre-crushed powder with the un-crushed powder to form the raw material powder. In the sintering process, the smaller particle size of the powder forms nuclei, which promotes the solid solution of the first solid solution metal (W, etc.) into the silicon nitride grains 21. Average particle size D 50 This refers to the median particle size.

[0068] Additionally, sintering aids other than those mentioned above may be added to the raw material powder as needed. Examples of sintering aids other than those mentioned above include at least one selected from titanium powder, hafnium powder, zirconium powder, cobalt powder, iron powder, and silicon carbide powder.

[0069] When the total weight of "silicon nitride powder" + "rare earth element powder" + "first component powder" + "second component powder (aluminum component powder)" + "other component powders" is set to 100% by mass, it is preferable that "rare earth element powder" is 1% to 13% by mass or less, "first component powder" is 0.1% to 7% by mass or less, "second component powder" is 1% to 10% by mass or less, "other component powders" is 0% to 8% by mass or less, and the remainder is "silicon nitride powder". Furthermore, "first component powder" refers to W component powder, Mo component powder, or WMo component powder. Additionally, "other component powders" refers to powders added as sintering aids. Organic binders and solvents are not included in the count.

[0070] Next, the process of mixing the raw material powder is carried out. This mixing process is performed using a crushing mixer such as a ball mill. The ball mill can crush the raw material powder through optimized crushing media and solvents. By crushing the raw material powder, the residue of agglomerates can be suppressed. Furthermore, the ball mill can be either a wet or dry type. Additionally, organic binders and solvents can be added to the raw material powder as needed during the mixing process.

[0071] Next, the raw material that has undergone the mixing process is used in a molding process to obtain a silicon nitride molded body (hereinafter referred to as "molded body"). The raw material can also be granulated before molding. The molding process can be performed by die forming, roll granulation, cold isostatic pressing (CIP), scraper molding, injection molding, etc. When producing spheres, die forming, roll granulation, and CIP are preferred. In addition, CIP is also effective for molded bodies obtained by die forming or roll granulation. CIP is a molding method using a pressurized medium as a liquid. Since it is isotropically pressurized based on a liquid, a molded body with a uniform density distribution can be obtained. In addition, the CIP pressure is preferably in the range of 80 MPa or more and 500 MPa or less.

[0072] A drying process is performed on the molded body as needed. The drying process removes the solvent that was present during wet mixing in the mixing process. Examples of drying processes include natural drying and heat drying. Heat drying is preferably performed at a temperature between 80°C and 200°C. Below 80°C, drying efficiency may decrease. Furthermore, above 200°C, uneven drying may occur. Therefore, the drying temperature is preferably between 80°C and 200°C, and more preferably between 100°C and 160°C.

[0073] Next, a degreasing process is performed on the molded body as needed. This degreasing process removes the organic binder from the molded body. The degreasing temperature is preferably in the range of 400°C to 700°C. Examples of degreasing processes include atmospheric conditions and nitrogen atmospheres. The molded body obtained through this degreasing process is sometimes referred to as a degreased body.

[0074] Next, the shaped body, such as a degreased body, undergoes a sintering process. The sintering temperature is preferably in the range of 1600°C or higher and 2000°C or lower. Furthermore, the sintering process can use atmospheric pressure sintering, pressure sintering, hot isostatic pressing (HIP), etc. Examples of sintering processes include atmospheric pressure, non-oxidizing atmosphere, reducing atmosphere, and vacuum. Atmospheric pressure sintering refers to sintering under 1 atm pressure (0.9~1.1 atm = 0.09~0.11 MPa). Pressure sintering involves applying pressure higher than atmospheric pressure. Uniaxial pressure sintering is sometimes also referred to as hot pressing. HIP is a sintering method that uses gas for isotropic pressure application.

[0075] This reduces internal defects such as voids and cracks within the silicon nitride sintered body 1. The pressure used for HIP is preferably in the range of 10 MPa or higher and 200 MPa or lower. Alternatively, atmospheric pressure sintering, pressure sintering, and HIP can be combined. Furthermore, in the case of wear-resistant components, either or both of pressure sintering and HIP are preferred. By applying pressure during sintering, a silicon nitride sintered body 1 with fewer internal defects (e.g., raw material sphere 6) can be obtained. Regarding the sintering time, the holding time at the sintering temperature is preferably 1 hour or more. In other words, the sintering temperature refers to a temperature held for 1 hour or more in the range of 1600°C or higher and 2000°C or lower.

[0076] Furthermore, in the sintering process, when the sintering temperature is higher than the melting point of the tungsten or molybdenum powder, it is preferable to hold the powder near its melting point (as a sintering aid) for at least 3 hours. This holding process near the melting point of the tungsten or molybdenum powder is called the intermediate holding process. "Near the melting point" means within ±50°C of the melting point. For example, since the melting point of WO3 powder is 1473°C, the holding temperature is within the range of 1473 ± 50°C. Similarly, since the melting point of MoO3 is 795°C, the holding temperature is 795°C ± 50°C. The holding temperature in this intermediate holding process is called the intermediate holding temperature.

[0077] By holding the powder near the melting point of the tungsten or molybdenum component, which is added as a sintering aid, the first solid solution metal can be dissolved within the silicon nitride grains 21. Furthermore, when both tungsten and molybdenum component powders are added, it is preferable to hold them separately. There is no particular upper limit to the holding time, but it is preferably 10 hours or less.

[0078] Furthermore, both alumina (Al₂O₃) powder and aluminum nitride (AlN) powder are preferably used for the aluminum component. Comparing alumina and aluminum nitride, aluminum nitride facilitates the solid solution of aluminum into silicon nitride. This promotes the solid solution of Al into silicon nitride. Additionally, the amount (by mass) of alumina powder added is preferably less than or equal to the amount of aluminum nitride powder added. The more aluminum nitride added, the easier it is to induce the solid solution of aluminum. This is an effective method for increasing the grain size of silicon nitride with dissolved Al.

[0079] A silicon nitride sintered body 1 can be obtained through a sintering process. The sliding surface of the silicon nitride sintered body 1 is ground to produce a wear-resistant component. For example, in a spherical silicon nitride sintered body 1 (e.g., raw material ball 6) used to obtain bearing balls, the entire surface becomes a sliding surface.

[0080] 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. Therefore, 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 of the embodiment has excellent machinability, thus improving the machinability of wear-resistant parts. That is, the grinding time for manufacturing wear-resistant parts from the silicon nitride sintered body 1 can be shortened.

[0081] Furthermore, the durability of the grinding wheel used for grinding the silicon nitride sintered body 1 can be improved. Diamond grinding wheels are typically used in the grinding process of the silicon nitride sintered body 1. Improving the durability of the grinding wheel not only reduces costs but also improves the manufacturing efficiency (machinability) of wear-resistant parts. Therefore, it is possible to provide a silicon nitride sintered body 1 with improved machinability.

[0082] (Example) (Examples 1-5, Comparative Examples 1-2) Silicon nitride powder and sintering aid powder were prepared as raw material powders. The raw material powders of silicon nitride sintered bodies 1 of Examples 1-5 and the raw material powders of silicon nitride sintered bodies of Comparative Examples 1-2 were mixed under the conditions shown in Table 1. The mixing ratio is expressed as silicon nitride powder + sintering aid powder = 100% by mass.

[0083] Table 1

[0084] Example 2 uses materials treated with Si3N4 powder and WO3 powder by mechanical alloying. Example 3 uses materials treated with Si3N4 powder and MoO3 powder by mechanical alloying.

[0085] The raw material powder is mixed using a ball mill. Additionally, organic binders and solvents are added to the raw material powder before ball milling.

[0086] Then, the raw material powder mixed in the ball mill is granulated, molded, and CIP (Cell-In-Place). The resulting spherical molded body has a strip-shaped portion around its circumference. This molded body is used to obtain raw material balls 6 after sintering.

[0087] A degreasing process is performed on the molded body. The degreasing process is carried out in the range of 400~650℃. The resulting molded body, i.e., the degreased body, is then subjected to a sintering process. The sintering process is carried out in two stages. The sintering conditions are shown in Table 2.

[0088] Table 2

[0089] As shown in Table 2, Examples 1, 2, and 4 were held at 1500°C for more than 3 hours. Example 3 was held at 800°C for more than 3 hours. Example 5 was held at 800°C and 1500°C for more than 3 hours each. Comparative Examples 1 and 2 did not undergo an intermediate holding process. Through the above processes, silicon nitride sintered bodies 1 of the Examples and silicon nitride sintered bodies of the Comparative Examples were obtained.

[0090] For the silicon nitride sintered body 1 obtained in the example, the presence or absence of solid solution metal and dislocation defects was investigated. The measurement conditions were as described above. Ten or more silicon nitride grains 2 with a length of 0.5 μm or more were selected from the silicon nitride grains 2 in a measurement area of ​​20 μm × 20 μm for measurement. The atomic ratios R1 to R3 showed minimum to maximum values ​​in the specific silicon nitride grains 21. The results are shown in Table 3. The measurement results of the silicon nitride sintered body of the comparative example are also shown in Table 3.

[0091] Table 3

[0092] Referring to Table 3, in the embodiments, the first atomic number ratio R1(W+Mo) / Si and the second atomic number ratio R2 are both within the preferred range of the above formulas (1) and (2). Furthermore, in Embodiment 5, both W and Mo are dissolved in a specific silicon nitride grain 21. Additionally, in Embodiments 1, 2, and 4, the third atomic number ratio R3 is within the preferred range of the above formula (8). In Embodiments 3 and 5, in a portion of the specific silicon nitride grains 21, the third atomic number ratio R3 deviates from the preferred range of the above formula (8).

[0093] Furthermore, referring to Table 3, it can be seen that in the embodiments, the proportion U1 [%] of the number of specific silicon nitride grains 21 relative to the total number of silicon nitride grains 2 satisfies both equation (5) and equation (7) above. The specific silicon nitride grain 21 is a grain with a first solid solution metal (at least one of W and Mo) and a second solid solution metal (Al) dissolved in it. In addition, although not shown in the table, it can be seen that in the embodiments, the proportion U2 [%] of the number of other silicon nitride grains 22 relative to the total number of silicon nitride grains 2 satisfies equation (10) above.

[0094] Furthermore, referring to Table 3, it can be seen that the ratio U3 [%] of the number of specific silicon nitride grains 21 with dislocation defects to the total number of specific silicon nitride grains 21 in the embodiment satisfies the above formula (13). Additionally, although not shown in the table, it can be seen that the ratio U4 [%] of the number of specific silicon nitride grains 21 with dislocation defects to the total number of silicon nitride grains 2 in the embodiment satisfies the above formula (15).

[0095] In Comparative Examples 1 and 2, no solid solution of W or Mo was observed in the silicon nitride grains. In particular, when only WO3 powder was added as a sintering aid powder, as in Comparative Example 2 in Table 1, it was found that W did not dissolve in the silicon nitride grains of the sintered raw material spheres.

[0096] Next, the average length-to-diameter ratio and average aspect ratio were measured for the silicon nitride grains 2 of the raw material spheres 6 of the silicon nitride sintered body 1 used in the example and the silicon nitride grains of the raw material spheres of the silicon nitride sintered body used in the comparative example. Additionally, the three-point bending strength, fracture toughness, and Vickers hardness were also measured for the raw material spheres 6 of the example and the raw material spheres of the comparative example. The measurement conditions were as described above. The results are shown in Table 4. Furthermore, regarding the three-point bending strength, it was measured using a plate-shaped silicon nitride sintered body 1 manufactured under the same conditions as the raw material spheres 6, instead of the raw material spheres 6.

[0097] [Table 4]

[0098] As shown in Table 4, in the silicon nitride grains 2 of the raw material sphere 6 in the example and the silicon nitride grains of the raw material sphere in the comparative example, the average length of the major axis is 0.5 μm or more and 10 μm or less, and the average aspect ratio is 2 or more and 10 or less. Furthermore, there is no significant difference in fracture toughness between the example and the comparative example. Additionally, the three-point bending strength and Vickers hardness are slightly higher in the example. Furthermore, the machinability coefficients of the example and the comparative example are 0.12 or less, showing no significant difference.

[0099] Raw material balls 6 from the embodiment and raw material balls from the comparative example were prepared. Both raw material balls 6 from the embodiment and raw material balls from the comparative example were used to obtain 3 / 8-inch (9.525 mm in diameter) bearing balls. In addition, both raw material balls have a strip-shaped portion on the circumference of the ball.

[0100] Next, the raw material balls 6 of the examples and the raw material balls of the comparative examples were ground using diamond grinding wheels to a surface roughness Ra of 0.01 μm. In the examples and comparative examples, the replacement frequency of the diamond grinding wheels was compared. The replacement frequency of the diamond grinding wheels in the examples is expressed as a ratio to the replacement frequency of the diamond grinding wheel in Comparative Example 1 being set to 100. A larger number indicates a longer replacement frequency and better wheel durability.

[0101] In addition, the time required to grind to a surface roughness Ra of 0.01 μm using a diamond grinding wheel was measured. The grinding time of the examples is expressed as a ratio to the grinding time of Comparative Example 1 when it is set to 100. The smaller the number, the shorter the grinding time. The results are shown in Table 5.

[0102] Table 5

[0103] As shown in Table 5, the grinding time in the embodiment was shortened by approximately 10-20%, and the machinability was improved. This is because the machinability was improved. Furthermore, the wheel replacement interval in the embodiment was improved by approximately 10%. Consequently, the durability of the grinding wheel was improved. Even with a machinability coefficient as low as 0.12 or below, the machinability was improved. In addition, the bearing balls 5 obtained by grinding the raw material balls 6 exhibited good durability.

[0104] It should be noted that, in the embodiments and comparative examples, an improvement in the processability of the silicon nitride sintered body 1, namely the raw material ball 6 (which is ground to become a bearing ball), can be confirmed. However, it is believed that this effect is not limited to the case where the silicon nitride sintered body 1 is the raw material ball 6. As long as it has specific silicon nitride grains 21, even silicon nitride sintered bodies 1 other than the raw material ball 6 (wear-resistant parts other than bearing balls that are ground to become bearing balls) can be obtained.

[0105] According to at least one embodiment described above, it is possible to provide a silicon nitride sintered body 1 with improved processability (e.g., raw material ball 6) and a wear-resistant component using the silicon nitride sintered body 1 (e.g., bearing ball 5). Furthermore, by controlling the proportion of specific silicon nitride 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 component using the silicon nitride sintered body 1.

[0106] The above embodiments of the present invention have been illustrated, but these embodiments are provided as examples and are not intended to limit the scope of the invention. These new embodiments can be implemented in various other ways, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These variations of the embodiments are included in the scope and spirit of the invention, and are included within the scope of the invention as described in the claims and its equivalents. Furthermore, the above embodiments can be combined with each other for implementation.

Claims

1. A silicon nitride sintered body, comprising a plurality of silicon nitride grains and grain boundary phases, characterized in that, The plurality of silicon nitride grains comprise specific silicon nitride grains having a first solid solution metal composed of at least one of tungsten and molybdenum and a second solid solution metal composed of aluminum.

2. The silicon nitride sintered body according to claim 1, characterized in that, Regarding the amount of solid solution in the specific silicon nitride grain, the ratio of the number of atoms of the first solid solution metal to the number of atoms of silicon, "number of atoms of the first solid solution metal / number of atoms of silicon", is 0.0001 or more and 0.01 or less, and The ratio of the number of atoms of the second solid solution metal to the number of atoms of silicon, "number of atoms of the second solid solution metal / number of atoms of silicon", is 0.001 or more and 0.1 or less.

3. The silicon nitride sintered body according to claim 1 or 2, characterized in that, In any cross section of the silicon nitride sintered body, the proportion of the specific silicon nitride grains relative to the number of the plurality of silicon nitride grains in the measured area of ​​20μm×20μm is in the range of 10% or more and 100% or less.

4. The silicon nitride sintered body according to claim 1 or 2, characterized in that, Regarding the amount of solid solution in the specific silicon nitride grain, the ratio of oxygen atoms to silicon atoms, "oxygen atoms / silicon atoms", is in the range of 0.001 or more and 1.20 or less.

5. The silicon nitride sintered body according to claim 3, characterized in that, Regarding the amount of solid solution in the specific silicon nitride grain, the ratio of oxygen atoms to silicon atoms, "oxygen atoms / silicon atoms", is in the range of 0.001 or more and 1.20 or less.

6. The silicon nitride sintered body according to claim 1 or 2, characterized in that, The silicon nitride grains that do not contain either tungsten or molybdenum are designated as other silicon nitride grains. In any cross section of the silicon nitride sintered body, the ratio of the number of the other silicon nitride grains in a 20μm×20μm area to the total number of silicon nitride grains is in the range of 0% to 30%.

7. The silicon nitride sintered body according to claim 1 or 2, characterized in that, In any cross section of the silicon nitride sintered body, the proportion of the specific silicon nitride grains having dislocation defects relative to the total number of the specific silicon nitride grains in a measured area of ​​20 μm × 20 μm is in the range of 0% to 10%.

8. The silicon nitride sintered body according to claim 5, characterized in that, In any cross section of the silicon nitride sintered body, the proportion of the specific silicon nitride grains having dislocation defects relative to the total number of the specific silicon nitride grains in a measured area of ​​20 μm × 20 μm is in the range of 0% to 10%.

9. The silicon nitride sintered body according to claim 1 or 2, characterized in that, The average length of the major diameter of the plurality of silicon nitride grains is 0.5 μm or more and 10 μm or less, and the average aspect ratio is 2 or more and 10 or less.

10. The silicon nitride sintered body according to claim 3, characterized in that, The average length of the major diameter of the plurality of silicon nitride grains is 0.5 μm or more and 10 μm or less, and the average aspect ratio is 2 or more and 10 or less.

11. The silicon nitride sintered body according to claim 8, characterized in that, The average length of the major diameter of the plurality of silicon nitride grains is 0.5 μm or more and 10 μm or less, and the average aspect ratio is 2 or more and 10 or less.

12. A wear-resistant component, characterized in that, It is formed from the silicon nitride sintered body as described in claim 1.

13. A wear-resistant component, characterized in that, It is formed from the silicon nitride sintered body as described in claim 3.

14. A wear-resistant component, characterized in that, It is formed from the silicon nitride sintered body as described in claim 10.

15. The wear-resistant component according to claim 13, characterized in that, It has a sliding surface with a surface roughness of less than Ra0.1μm.

16. The wear-resistant component according to claim 14, characterized in that, It has a sliding surface with a surface roughness of less than Ra0.1μm.

17. The wear-resistant component according to claim 15, characterized in that, The wear-resistant component is the bearing ball.

18. The wear-resistant component according to claim 16, characterized in that, The wear-resistant component is the bearing ball.

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