Silicon nitride sintered body, mechanical parts using the same, and bearings

Abstract

To provide a silicon nitride sintered body having excellent mechanical characteristics and exhibiting an excellent product life when fabricated into a product, a machine component using the same, and a bearing.SOLUTION: The silicon nitride sintered body is of crystallinity of not lower than 75% and not higher than 90% obtained from a XRD diffraction pattern of a mirror polished cross section thereof using the following equation, with its amorphous phase including at least one species selected from the group consisting of Y, Ce, Nd and Eu, and neither an inclusion of a diameter of larger than 50 μm nor a pore of a diameter of larger than 50 μm being present in its surface layer. Crystallinity(%)=[peak area of crystalline phase / (peak area of crystalline phase+peak area of amorphous phase)]×100.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Since silicon nitride (Si3N4) sintered bodies have excellent mechanical properties, thermal conductivity, and electrical insulation properties, their applications to bearing members, engine parts, tool materials, heat dissipation substrate materials, etc. have been promoted. It is known that silicon nitride sintered bodies are manufactured using silicon nitride powder as a starting material. Since silicon nitride powder is difficult to sinter, a sintering aid is used together with the silicon nitride powder in order to manufacture a dense silicon nitride sintered body. As such sintering aids, generally, oxides of rare earth elements, aluminum oxide, magnesium oxide, silicon oxide, etc. are cited.

[0003] Since silicon nitride powder is expensive, when manufacturing a silicon nitride sintered body using silicon nitride powder, the price of the silicon nitride sintered body also tends to increase. Therefore, a manufacturing method that uses silicon powder (metallic silicon powder), which is lower in price than silicon nitride powder, as a starting material and produces a silicon nitride sintered body by reactive sintering has attracted attention (for example, Patent Documents 1 to 3). As such a manufacturing method, a method called the PS-RBSN (Post-Sintering of Reaction Bonded Silicon-Nitride) method is known. The PS-RBSN method includes a first step of nitriding a green compact formed of silicon powder by heat treatment at a temperature of, for example, around 1100°C to 1450°C in an environment containing nitrogen gas, and a second step of densifying the nitride obtained in the first step by heat treatment at a temperature of, for example, around 1600°C to 1950°C.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] For example, when manufacturing silicon nitride sintered bodies using the PS-RBSN method, if the silicon powder is not sufficiently nitrided, silicon will remain in the silicon nitride sintered body. This remaining silicon can cause a decrease in the mechanical properties of the silicon nitride sintered body, and as a result, silicon nitride sintered bodies manufactured using the PS-RBSN method may have inferior mechanical properties compared to silicon nitride sintered bodies manufactured using silicon nitride powder as the starting material. It has also been found that when silicon nitride sintered bodies are processed into mechanical parts such as rolling elements, the product life may be shorter.

[0006] The present invention aims to provide silicon nitride sintered bodies that have good mechanical properties and a good product life when processed into products, as well as mechanical parts and bearings using the same. [Means for solving the problem]

[0007] The silicon nitride sintered body of the present invention is characterized by having a crystallinity of 75% or more and 90% or less.

[0008] The silicon nitride sintered body described above is characterized by containing one or more elements selected from the group consisting of Y, Ce, Nd, and Eu in its amorphous phase.

[0009] The silicon nitride sintered body described above contains rare earth elements and aluminum elements, wherein the content of the rare earth elements is 6% to 13% by weight in terms of oxides relative to the total weight of the silicon nitride sintered body, and the content of the aluminum elements is 6% to 13% by weight in terms of oxides relative to the total weight of the silicon nitride sintered body.

[0010] The silicon nitride sintered body is characterized by having an inclusion (I) in the surface layer, which is within 2 mm of the surface, and the maximum diameter of the inclusion (I) being 50 μm or less.

[0011] The silicon nitride sintered body is characterized by having pores in the surface layer, which is within 2 mm of the surface, and the maximum diameter of these pores being 50 μm or less.

[0012] The mechanical component of the present invention is characterized by using the silicon nitride sintered body of the present invention. Furthermore, the mechanical component is characterized by being a rolling element.

[0013] The bearing of the present invention is characterized by using the above-mentioned rolling elements. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a silicon nitride sintered body having good mechanical properties and a good product life when processed into a product, as well as mechanical parts and bearings using the same. [Brief explanation of the drawing]

[0015] [Figure 1] This is a longitudinal cross-sectional view showing an example of a bearing of the present invention. [Figure 2] This is a longitudinal cross-sectional view showing another example of the bearing of the present invention. [Figure 3] This is a longitudinal cross-sectional view showing another example of the bearing of the present invention. [Modes for carrying out the invention]

[0016] Embodiments of the present invention will be described below. The silicon nitride sintered body of this embodiment (hereinafter also simply referred to as "sintered body") has a crystallinity of 75% or more and 90% or less. Here, the crystallinity of the silicon nitride sintered body refers to the ratio of the crystalline phase in all the components constituting the silicon nitride sintered body, and the above crystallinity is determined from the XRD diffraction pattern of the mirror-polished cross-section of the silicon nitride sintered body using the following formula. Crystallinity (%) = Peak area of crystalline material / (Peak area of crystalline material + Peak area of amorphous material) × 100

[0017] Specifically, the crystallinity of the silicon nitride sintered body is "Peak area of crystalline silicon nitride and other crystalline components in the XRD diffraction pattern" / "Sum of peak areas of all components constituting the sintered body in the XRD diffraction pattern". Here, the crystalline silicon nitride is crystalline silicon nitride having an α-type, β-type, or γ-type crystal structure. Also, all components constituting the sintered body are, for example, silicon, silicon nitride, components derived from a sintering aid that promotes sintering during heat treatment when producing silicon nitride, and the like.

[0018] When the silicon nitride sintered body is produced using silicon powder and a sintering aid, for example, by the PS-RBSN method (two-step sintering method), under predetermined conditions, the sintering aid becomes the main component of the amorphous phase in the sintered body. The PS-RBSN method refers to a two-step sintering method including a silicon nitriding step and a subsequent sintering step. For example, when a green compact composed of silicon powder and a sintering aid is heat-treated in a nitrogen atmosphere by the PS-RBSN method and the silicon is completely nitrided and all of the sintering aid is amorphous, the above crystallinity is "Peak area of crystalline silicon nitride in the XRD diffraction pattern" / "Sum of peak areas of all components constituting the sintered body in the XRD diffraction pattern". Also, when the silicon is completely nitrided and a part of the sintering aid is crystallized, the above crystallinity is "Peak area of crystalline silicon nitride and crystalline components derived from the sintering aid in the XRD diffraction pattern" / "Sum of peak areas of all components constituting the sintered body in the XRD diffraction pattern".

[0019] As the raw material of the silicon nitride sintered body, only silicon nitride powder may be used, or only silicon powder and nitrogen gas may be used, or silicon powder, a sintering aid, and nitrogen gas may be used. When producing a silicon nitride sintered body by the PS-RBSN method using a raw material powder containing silicon powder and a sintering aid, a glass phase (non-crystalline phase) is likely to exist in the system during sintering of the raw material powder, and the nitridation of silicon powder (metallic silicon powder) and subsequent sintering are likely to be promoted. As a result, the crystallinity of the sintered body is likely to be 75% or more and 90% or less, and the mechanical properties are improved. Note that as long as a silicon nitride sintered body can be obtained, any raw material may be used regardless of whether it is a powder or a gas for the raw material and manufacturing method of the silicon nitride sintered body, and the manufacturing method is not limited to the PS-RBSN method.

[0020] The crystallinity of the silicon nitride sintered body is 75% or more, preferably 78% or more, and may be 81% or more. The above crystallinity may be 90% or less, may be 87% or less, or may be 84% or less. When the crystallinity of the silicon nitride sintered body is within the above range, the mechanical properties are good, and a silicon nitride sintered body having a good product life when processed into a product is likely to be obtained.

[0021] The crystallinity of the silicon nitride sintered body is affected by the degree of nitridation of silicon and the influence of crystals derived from the sintering aid, and does not necessarily correlate with the amount of silicon nitride in the sintered body. For example, even if unnitridated silicon remains, the crystallinity of the sintered body may satisfy 75% or more and 90% or less due to the presence of crystals derived from the sintering aid.

[0022] The silicon nitride sintered body of the present embodiment may contain a rare earth element and / or an aluminum element. When the silicon nitride sintered body contains a rare earth element, the content of the rare earth element is preferably 6% by weight or more and 13% by weight or less in terms of oxide based on the total weight of the silicon nitride sintered body. When the silicon nitride sintered body contains an aluminum element, the content of the aluminum element is preferably 6% by weight or more and 13% by weight or less in terms of oxide based on the total weight of the silicon nitride sintered body.

[0023] Examples of rare earth elements include yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), neodymium (Nd), dysprosium (Dy), europium (Eu), and erbium (Er). Of these, yttrium (Y), cerium (Ce), neodymium (Nd), and europium (Eu) are preferred. In particular, the inclusion of cerium (Ce) is more preferable because it can further promote nitriding and improve manufacturing efficiency.

[0024] The silicon nitride sintered body preferably contains rare earth elements in its amorphous phase, and more preferably contains one or more selected from the group consisting of Y, Ce, Nd, and Eu. The silicon nitride sintered body may also contain rare earth elements in its crystalline phase.

[0025] The above content of rare earth elements is more preferably 6.5% by weight or more in terms of oxides relative to the total weight of the silicon nitride sintered body, but may also be 7.0% by weight or more, or 7.5% by weight or more. The above content of rare earth elements is more preferably 12.5% ​​by weight or less, but may also be 11.5% by weight or less, or 10.5% by weight or less. When the crystallinity of the silicon nitride sintered body is 75% or more and 90% or less, and the content of rare earth elements is within the above range, the majority of the crystalline phase in the sintered body tends to be silicon nitride. As a result, a silicon nitride sintered body with good mechanical properties, a good product life when processed into a product, and excellent energy efficiency during manufacturing is easily obtained.

[0026] Rare earth elements originate from sintering aids containing rare earth elements (usually oxides of rare earth elements) used in the production of silicon nitride sintered bodies, for example. By ensuring that the rare earth element content in the silicon nitride sintered body is within the above range, the nitriding reaction of the silicon powder (metallic silicon powder) used as the raw material is accelerated, thereby promoting subsequent sintering, when producing silicon nitride sintered bodies by the PS-RBSN method. The rare earth element content can be adjusted by the amount of sintering aids containing rare earth elements (e.g., oxides of rare earth elements) added to the raw material.

[0027] The above content of aluminum element is more preferably 6.5% by weight or more in terms of oxides relative to the total weight of the silicon nitride sintered body, but may be 7.0% by weight or more, or 7.5% by weight or more. The above content of aluminum element is more preferably 12.5% ​​by weight or less, but may be 11.5% by weight or less, or 10.5% by weight or less. When the silicon nitride sintered body contains rare earth elements and aluminum element, the content of aluminum element (in terms of oxides) may be within ±5% by weight of the content of rare earth elements (in terms of oxides), within ±2% by weight, within ±1% by weight, or may be the same as the content of rare earth elements. When the crystallinity of the silicon nitride sintered body is 75% or more and 90% or less, and the content of aluminum element is within the above range, the majority of the crystalline phase in the sintered body tends to be silicon nitride. As a result, a silicon nitride sintered body with good mechanical properties, a good product life when processed into a product, and excellent energy efficiency during manufacturing is easily obtained.

[0028] The aluminum element originates, for example, from sintering aids containing aluminum (usually aluminum oxide) used in the production of silicon nitride sintered bodies. By keeping the aluminum element content in the silicon nitride sintered body within the above range, sintering can be promoted when producing silicon nitride sintered bodies by the PS-RBSN method. The aluminum element content can be adjusted by the amount of sintering aids containing aluminum element (e.g., aluminum oxide) added to the raw materials.

[0029] The above-mentioned content of rare earth elements and aluminum elements can be determined using an X-ray fluorescence analyzer (XRF), energy-dispersive X-ray spectroscopy (EDX), or inductively coupled plasma (ICP) emission spectrometer. Specifically, the content of rare earth elements and aluminum elements in the silicon nitride sintered body can be determined using the above-mentioned analyzers, and then converted to rare earth element (RE) oxides (RE2O3 or REO2) and aluminum oxide (Al2O3). The elements of other components constituting the silicon nitride sintered body can also be analyzed using the above-mentioned analyzers, and the total weight of the silicon nitride sintered body can be calculated to determine the above-mentioned content of rare earth elements and aluminum elements. If the raw material powder used to manufacture the silicon nitride sintered body contains silicon (metallic silicon powder), and this silicon is converted to Si3N4 by nitriding, the weight of Si3N4 in the silicon nitride sintered body will be 1.67 times the weight of silicon. Therefore, by considering the weight change when silicon is nitrided, the content of rare earth element oxides and aluminum oxide can be calculated from the composition of the raw material powder.

[0030] The silicon nitride sintered body of this embodiment preferably has inclusions (I) in the surface layer, which is within 2 mm of the surface. Inclusions (I) contain components other than silicon nitride, and examples include inclusions (It) containing rare earth elements, aluminum elements, transition metal elements, and inclusions (Is) containing unnitrided silicon elements. Inclusions (It) preferably contain silicides of transition metal elements. Inclusions (Is) are, for example, aggregates of unnitrided silicon elements. Inclusions (I) preferably contain inclusions (It), and preferably do not contain inclusions (Is) or have a small proportion of inclusions (Is). Inclusions refer to those that are entirely present in the surface layer, which is within 2 mm of the surface of the silicon nitride sintered body.

[0031] Inclusions (It) originate from sintering aids used in the production of silicon nitride sintered bodies (e.g., oxides of rare earth elements, aluminum oxide, or oxides of transition metal elements). For example, silicides of transition metal elements are formed during the production of silicon nitride sintered bodies. For example, when producing silicon nitride sintered bodies by the PS-RBSN method, using sintering aids containing transition metal elements such as chromium oxide (Cr2O3) can accelerate the nitriding reaction of silicon powder and promote the growth of needle-shaped silicon nitride crystals. Therefore, the heat treatment time required to nitride silicon can be reduced, and the energy efficiency during the production of silicon nitride sintered bodies can be improved.

[0032] Inclusions (Is) can form, for example, when silicon nitride sintered bodies are manufactured by the PS-RBSN method, if the silicon powder (metallic silicon powder) used as the raw material is not sufficiently nitrided. If large diameter inclusions (Is) are present in the surface layer, or if the proportion of inclusions (Is) increases, the mechanical properties such as fracture toughness of the silicon nitride sintered body tend to decrease, and the product life after processing into a finished product tends to decrease. It is preferable to have few inclusions (Is) in the surface layer of the silicon nitride sintered body, and it is even preferable that they are absent.

[0033] The transition metal element is not particularly limited as long as it is an element contained in groups 3 through 11 of the IUPAC periodic table. Preferably, the transition metal element is one or more selected from the group consisting of Ti, Cr, and Mn, and more preferably includes Cr. By including Cr as the transition metal element, the fracture toughness of the silicon nitride sintered body can be further improved.

[0034] The maximum diameter of inclusions (I) present in the surface layer of the silicon nitride sintered body is not particularly limited. Specifically, the maximum diameter of inclusions (I) is preferably 50 μm or less. The maximum diameter of inclusions (I) may be 40 μm or less, 30 μm or less, 25 μm or less, and preferably 0.5 μm or more. The maximum diameter of inclusions (I) in the surface layer refers to the diameter of the largest inclusion (I) among the inclusions (I) present in the surface layer. By keeping the maximum diameter of inclusions (I) within the above range, it is easier to suppress the inclusions (I) from becoming a source of fracture, making it easier to obtain a silicon nitride sintered body with good fracture toughness. Furthermore, by keeping the maximum diameter of inclusions (I) within the above range, it is easier to suppress the detachment of inclusions from the silicon nitride sintered body and the formation of defects, making it easier to obtain a good product life when the silicon nitride sintered body is processed into products such as rolling elements for bearings. The maximum diameter of inclusions (I) can be adjusted, for example, by the degree of nitridation of the silicon powder raw material, the amount and / or particle size of sintering aids containing rare earth elements, aluminum elements, and transition metal elements added to the raw material, and the types of elements contained in the sintering aids. When the crystallinity of the silicon nitride sintered body is 75% or more and 90% or less, the amorphous phase accounts for 10% or more and 25% or less, and since the amorphous phase, which accounts for almost the entirety of the inclusions, is small in the sintered body, the maximum diameter of inclusions (I) tends to be 50 μm or less.

[0035] Furthermore, the silicon nitride sintered body of this embodiment preferably has voids in the surface layer, which is within 2 mm from the surface. Moreover, the maximum diameter of the voids is preferably 50 μm or less in the cross-section of the silicon nitride sintered body. The maximum diameter of the voids may be 40 μm or less, 30 μm or less, 25 μm or less, or there may be no voids at all. Having the maximum diameter of the voids within the above range makes it easier to obtain a good product life when the silicon nitride sintered body is processed into products such as rolling elements for bearings. Voids in the surface layer refer to those present in the surface layer, which is within 2 mm from the surface of the silicon nitride sintered body, and refer to those where the entire void is present in the surface layer. The maximum diameter of the voids in the surface layer refers to the diameter of the largest void among the voids present in the surface layer. The maximum diameter of the voids can be adjusted, for example, by adjusting the content of silicon nitride used as a raw material and / or the amount of sintering aid added when manufacturing a silicon nitride sintered body by the PS-RBSN method. When the crystallinity of a silicon nitride sintered body is between 75% and 90%, the amorphous phase is between 10% and 25%. As a result, the amorphous phase easily fills the spaces between the crystalline phases as a liquid phase during sintering, and the maximum diameter of the pores tends to be 50 μm or less.

[0036] The maximum diameter of inclusions (I) and voids are values ​​measured for inclusions (I) or voids that are entirely located in the surface layer of a cross-section of a test specimen prepared by the method described in the examples below. The maximum diameter of inclusions (I) and voids can be calculated by the method described in the examples below.

[0037] A particularly preferred form of the silicon nitride sintered body of this embodiment is a silicon nitride sintered body containing rare earth elements and aluminum elements, wherein the degree of crystallinity is 75% or more and 90% or less, and the amorphous phase contains one or more elements selected from the group consisting of Y, Ce, Nd, and Eu, the content of the rare earth elements is 6% to 13% by weight in terms of oxides relative to the total weight of the silicon nitride sintered body, and the content of the aluminum elements is 6% to 13% by weight in terms of oxides relative to the total weight of the silicon nitride sintered body. Furthermore, it is preferable that the surface layer, which is within 2 mm from the surface of the silicon nitride sintered body, has inclusions (I) and voids, the maximum diameter of the inclusions (I) is 50 μm or less, and the maximum diameter of the voids is 50 μm or less. In addition, the elements and numerical ranges mentioned above can be appropriately combined with this form.

[0038] The shape of the silicon nitride sintered body in this embodiment is not particularly limited and can be appropriately selected depending on the application, such as a spherical, cylindrical, conical, frustoconical, or rectangular parallelepiped shape, but a spherical shape is preferred. The size of the silicon nitride sintered body is also not particularly limited; for example, if it is spherical, the diameter can be 0.5 cm to 10 cm, and if it is cylindrical, the diameter of the base can be 0.5 cm to 15 cm and the height can be 3 cm to 20 cm.

[0039] The silicon nitride sintered body described above is preferably manufactured by, for example, the PS-RBSN method (two-stage sintering method). Specifically, it can be manufactured by the following first and second methods.

[0040] (First method) In the PS-RBSN method, granulation is often performed to improve the fluidity of the powder. The first method is a method for producing a silicon nitride sintered body having a crystallinity of 75% or more and 90% or less, and includes, for example, a granulation step of obtaining granulated powder using raw material powder containing silicon powder and a sintering aid, a molding step of forming the obtained granulated powder into a compact, a degreasing step, and a sintering step of sintering the degreasing compact.

[0041] In the granulation process, the raw material powder and binder components are mixed with water and / or an organic solvent (e.g., ethanol) to form a slurry, which is then spray-dried to obtain granulated powder. Organic binders are used as the binder components.

[0042] In the subsequent molding process, the granulated powder is molded into a predetermined shape to obtain a compacted powder. In the degreasing process, the obtained compacted powder is degreased by heating it in a nitrogen atmosphere at a temperature of 700°C to 1000°C.

[0043] The sintering process comprises a first step of nitriding the degreased compacted powder by heat treatment at a temperature of 1200°C to 1500°C in a nitrogen atmosphere, for example, and a second step of sintering the obtained nitrided material by heat treatment at a temperature of 1600°C to 1950°C (preferably 1600°C to 1900°C) in a nitrogen atmosphere.

[0044] (Second method) The second method is a method for producing a silicon nitride sintered body having a crystallinity of 75% or more and 90% or less, and includes, for example, a mixing step of dry-mixing raw material powder containing silicon powder and a sintering aid, a molding step of forming the mixed raw material powder into a compact, and a sintering step of sintering the compact. Unlike the first method, the second method is characterized in that all steps of the PS-RBSN method are performed dry-process. After the sintering step, polishing or other treatments may be performed on the silicon nitride sintered body as needed.

[0045] The mixing process involves dry mixing of the raw material powders without using water or organic solvents. It is also preferable to mix without using a binder component in this process. The particle size of the powder after mixing is not particularly limited, but it is preferable that D90 be between 10 μm and 100 μm. Furthermore, it is preferable that D50 be between 2 μm and 10 μm. By having D90 and / or D50 within the above ranges, a dense silicon nitride sintered body can be obtained while exhibiting good fluidity and moldability. Note that D50 and D90 are the cumulative 50% diameter and 90% diameter based on volume, respectively, and are obtained by methods such as laser diffraction scattering particle size distribution measurement.

[0046] In the subsequent molding process, the mixed powder is molded into a predetermined shape to obtain a compacted powder. The sintering process comprises a first step of nitriding the obtained compacted powder by heat treatment at a temperature of 1200°C to 1500°C in a nitrogen atmosphere, for example, and a second step of sintering the compacted powder by heat treatment at a temperature of 1600°C to 1950°C (preferably 1600°C to 1900°C) in a nitrogen atmosphere. From the viewpoint of improving manufacturing efficiency, it is preferable that the first step is not held at a temperature within the range of 1200°C to 1500°C for more than one hour. Specifically, it is preferable to nitrid the powder by raising the temperature from a temperature of about 1100°C to the sintering temperature of the second step at a predetermined heating rate. The heating rate is, for example, 2°C / min or more, may be 2.5°C / min or more, or 5°C / min or more. Also, the heating rate is, for example, 20°C / min or less, preferably 15°C / min or less.

[0047] The second method yields the following advantages compared to the first method. By performing the entire process dry using the PS-RBSN method, oxidation of silicon powder, for example, when using an aqueous solvent, can be prevented, and the environmental burden caused by organic solvents such as ethanol can be reduced. By using the PS-RBSN method to produce silicon nitride sintered bodies without using organic binders, shrinkage due to sintering can be reduced, and the dimensional accuracy of the sintered bodies can be improved. In the first method, since organic binders are used for granulation, a degreasing process is required afterward. However, since voids are created after the organic binder is removed during the degreasing process, there is a risk that shrinkage due to sintering will be correspondingly larger. Furthermore, reduced shrinkage allows for shorter polishing times in subsequent polishing processes.

[0048] Generally, to obtain a dense sintered body using conventional methods that utilize Si3N4 powder as a raw material, it is preferable to use fine Si3N4 powder (D50 of 1 μm or less). However, such fine powders have poor fluidity and moldability, so it is necessary to slurry the raw material powder and binder components with water or ethanol, and then obtain a granulated body by spray drying or similar methods. However, in the PS-RBSN method, the Si powder is refined by fracture due to volume expansion during the nitriding process, so it is not necessary to use fine powder like Si3N4 powder as a raw material to obtain a dense sintered body. Because the raw material powder is not fine, the fluidity and moldability necessary to obtain a molded body can be ensured even without granulated powder.

[0049] In the production of the silicon nitride sintered body described above, including the first and second methods, it is preferable to use a sintering aid containing rare earth elements, aluminum elements, and transition metal elements in the raw material powder, and more preferably to include oxides thereof. As a sintering aid containing rare earth elements, it is preferable to include any of Y2O3, CeO2, Nd2O3, and Eu2O3. As a sintering aid containing aluminum elements, it is preferable to include Al2O3. As a sintering aid containing transition metal elements, it is preferable to include any of Cr2O3, TiO2, MnO, and Fe2O3.

[0050] The raw material powder may contain silicon nitride powder and / or an organic binder in addition to silicon powder and a sintering aid, and may also contain a sintering aid containing elements other than rare earth elements, aluminum elements, and transition metal elements.

[0051] The silicon powder content in the raw material powder is preferably 65% ​​by weight or more, more preferably 67% by weight or more, even more preferably 69% by weight or more, may be 71% by weight or more, preferably 80% by weight or less, may be 78% by weight or less, or may be 76% by weight or less. The raw material powder may or may not contain silicon nitride powder.

[0052] The content of sintering aids containing rare earth elements (e.g., oxides of rare earth elements) in the raw material powder is preferably 10% by weight or more, more preferably 11% by weight or more, even more preferably 12% by weight or more, and may be 13% by weight or more, based on the total weight. The content of rare earth elements may be 17.5% by weight or less, 16.5% by weight or less, or 15.5% by weight or less. The content of sintering aids containing aluminum elements (e.g., aluminum oxide) in the raw material powder is preferably 10% by weight or more, more preferably 11% by weight or more, even more preferably 12% by weight or more, and may be 13% by weight or more, based on the total weight. The content of aluminum elements may be 17.5% by weight or less, 16.5% by weight or less, or 15.5% by weight or less. If the content of sintering aids in the raw material powder is low, it is difficult to obtain a dense silicon nitride sintered body, and if the content of sintering aids is high, the mechanical properties of the silicon nitride sintered body tend to deteriorate.

[0053] The average particle size of the silicon powder contained in the raw material powder can be, for example, 5 μm or less. If silicon nitride is included, its average particle size can be, for example, 0.5 μm or less. The average particle size of the sintering aid depends on the type of sintering aid, but is preferably 10 μm or less, may be 7 μm or less, may be 5 μm or less, may be 3 μm or less, may be 2 μm or less, may be 1 μm or less, or may be 0.4 μm or less. The average particle size is the cumulative 50% diameter based on volume and can be obtained by laser diffraction scattering particle size distribution measurement or the like.

[0054] One form of the second method described above is, for example, a method for producing a silicon nitride sintered body containing rare earth elements and aluminum elements, comprising a mixing step of dry-mixing silicon powder and raw material powder containing a sintering aid, a molding step of forming the mixed raw material powder into a compact, and a sintering step of sintering the compact, wherein the silicon powder is present in an amount of 65% by weight or more of the total raw material powder, and the crystallinity of the silicon nitride sintered body in the sintering step is 75% or more and 90% or less.

[0055] Furthermore, the above-mentioned form of the second method may have one or more of the following features (1) to (5). (1) The above mixing step is a step of mixing the above raw material powders without using a binder component. (2) The above sintering process includes a step of raising the temperature from a temperature in the range of 1000°C to 1200°C to the sintering temperature, without maintaining the predetermined temperature for more than one hour, and raising the temperature at a rate of 15°C / min or less. (3) The sintering temperature is in the range of 1600°C to 1900°C. (4) The sintering aid contains rare earth oxides and aluminum oxide, and the raw material powder contains 10% to 17.5% by weight of the rare earth oxides and 10% to 17.5% by weight of the aluminum oxides relative to the total amount of the raw material powder. (5) The above rare earth oxide includes one or more selected from the group consisting of Y2O3, CeO2, Nd2O3, and Eu2O3.

[0056] For example, by adding 10% to 17.5% by weight of rare earth oxides and 10% to 17.5% by weight of aluminum oxide to the raw material powder as sintering aids, the nitriding of silicon and subsequent sintering can be promoted (see (4) above). By promoting the nitriding of silicon, the long-term temperature maintenance at 1100°C to 1450°C in a nitrogen atmosphere, which is commonly performed, is eliminated, resulting in an energy-efficient method.

[0057] (Applications of silicon nitride sintered bodies) The applications of the silicon nitride sintered body of this embodiment are not particularly limited, but it is preferable to use it as a mechanical part due to its excellent mechanical properties. Mechanical parts are used, for example, in rolling parts and sliding parts. The mechanical part of the present invention is a part that uses the silicon nitride sintered body of the present invention as part or all of its components. Examples of mechanical parts include sliding members, bearing members, rolling mill rolls, compressor vanes, engine parts such as gas turbine blades, and cutting tools (tips). Examples of bearing members include raceways such as inner and outer rings, bearing rolling elements, and cages. The bearing of the present invention is a bearing that incorporates this mechanical part as part or all of its bearing components, and examples include rolling bearings, sliding bearings (such as spherical bushings), linear guide bearings, ball screws, and linear bearings. In particular, the bearing of the present invention is preferably a rolling bearing that uses the silicon nitride sintered body as the bearing rolling element.

[0058] An example of a bearing according to this embodiment will be described with reference to Figure 1. Figure 1 is a cross-sectional view of a deep groove ball bearing. The rolling bearing 1 has an inner ring 2 having an inner ring raceway surface 2a on its outer circumference and an outer ring 3 having an outer ring raceway surface 3a on its inner circumference, arranged concentrically, with a plurality of balls (rolling elements) 4 arranged between the inner ring raceway surface 2a and the outer ring raceway surface 3a. These balls 4 are formed from the silicon nitride sintered body described above. The balls 4 are held by a cage 5. In addition, the axial openings 8a and 8b at both ends of the inner and outer rings are sealed by a sealing member 6, and a grease composition 7 is sealed around at least the balls 4. The grease composition 7 interposed on the raceway surface with the balls 4 provides lubrication.

[0059] Another example of the bearing of this embodiment will be described with reference to Figure 2. Figure 2 is a cross-sectional view showing a ball screw. As shown in Figure 2, the ball screw has a plurality of balls 15 interposed between a screw groove 12 formed on the outer circumferential surface of a screw shaft 11, which is a guide member, and a screw groove 14 formed on the inner circumferential surface of a ball nut 13. The rotational power of the screw shaft 11 (or ball nut 13) is transmitted to the ball nut 13 (or screw shaft 11) via the balls 15, causing the ball nut 13 to move in the axial direction. In Figure 2, the balls 15 are formed from the silicon nitride sintered body described above, and the screw shaft 11 and ball nut 13 are formed from steel (for example, bearing steel or low carbon steel). In addition, a grease composition is sealed around the balls 15 between the screw shaft 11 and the ball nut 13 and sealed by a ball screw sealing member 16.

[0060] In the ball screw shown in Figure 2, the ball circulation method is not particularly limited, and any of the following circulation methods can be adopted: tube type, return tube (pipe) type, deflector type, end deflector type, end cap type, or ball type. In any circulation method, the circulation path greatly affects the smooth circulation of the balls.

[0061] Specifically, ball screws are used to convert the rotational motion of a motor into linear motion. For example, they can be suitably used in electric actuators, positioning devices, electric jacks, servo cylinders, electric servo presses, mechanical presses, electric brakes, transmissions, electric power steering systems, and electric injection molding machines.

[0062] In ball screws, wear resistance, toughness, and high load capacity are required. In recent years, due to miniaturization and other factors, there has been an increasing demand for performance that can withstand high loads, as well as the suppression of hydrogen embrittlement caused by hydrogen generated from lubricants due to slippage and high loads. In the example in Figure 2, the aforementioned silicon nitride sintered body is used as the ball, which makes it easier to meet these requirements and also results in excellent product life for the ball.

[0063] Furthermore, in the case of ball screws, if there is a large misalignment due to installation errors or other factors when installing the ball screw, twisting (i.e., a relative tilt between the screw shaft and the nut) may occur. When the moment caused by twisting acts on the ball screw, the load balance within the nut is disrupted, and areas with increased contact pressure may occur, potentially reducing the lifespan. In contrast, by using the aforementioned silicon nitride sintered body as the ball, the circulation performance of the ball can be improved, making it easier to suppress the reduction in lifespan.

[0064] Furthermore, other examples of bearings of this embodiment will be described with reference to Figure 3. Figure 3 is a cross-sectional view showing an example of a spherical plain bearing. As shown in Figure 3, the spherical plain bearing 21 consists of a combination of an inner ring 22 having a spherical outer surface 22b and a bearing hole 24 formed in its inner surface 22a through which a support shaft can be inserted, and an outer ring 23 having a concave surface 23a corresponding to the outer surface 22b. In the spherical plain bearing 21, at least one of the inner ring 22 and the outer ring 23 is formed of the silicon nitride sintered body described above. The material of the other member is not particularly limited and can be, for example, a metal such as aluminum alloy, stainless steel, or iron, or a synthetic resin, or a ceramic other than the silicon nitride sintered body described above.

[0065] Spherical plain bearings are self-aligning plain bearings with a spherical sliding surface that can withstand radial and bidirectional axial loads. Spherical plain bearings are suitable for oscillating and self-aligning motions and are used in joints of industrial and construction machinery. Both lubrication-free (see Figure 3) and lubrication-lubricated types can be used for spherical plain bearings; for example, in the lubrication-lubricated type, oil holes and grooves are provided in the inner and outer rings. Grease may also be applied to the sliding surface when installing spherical plain bearings. [Examples]

[0066] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to these examples.

[0067] <Examples 1-7, Comparative Examples 1-2> The raw material powders were prepared according to the mixing ratios shown in Table 2, and dry-mixed using silicon nitride balls as the media in a ball mill at a rotation speed of 200 rpm for 48 hours. As a result of the dry mixing, a mixed powder was obtained. The specifications of the materials used to obtain the mixed powder are shown in Table 1. All materials used were manufactured by Kojun Chemical Laboratory Co., Ltd. The average particle size is the volume-based cumulative 50% diameter measured by laser diffraction scattering particle size distribution analysis.

[0068] [Table 1]

[0069] [Table 2]

[0070] The obtained mixed powder was molded into a spherical compact with a diameter of 11 mm using a cold isobaric pressing method with a rubber mold. This compact was heated from 1550°C to 1800°C at a heating rate of 2.5°C / min, and held at 1800°C in a nitrogen atmosphere (pressure: 0.9 MPa) for 4 hours to obtain a silicon nitride sintered body.

[0071] Table 3 shows the composition ratios of each oxide in the obtained silicon nitride sintered body, calculated from the composition ratio of the raw material powder, assuming that all the silicon (metallic silicon) contained in the raw material powder is nitrided and the weight of silicon nitride is 1.67 times the weight of silicon.

[0072] [Table 3]

[0073] The obtained spherical silicon nitride sintered body was polished to a G5 grade according to JIS B 1563 to produce 3 / 8-inch (9.525 mm in diameter) spherical test specimens.

[0074] <Calculation of crystallinity> The crystallinity of the silicon nitride sintered body was determined using the following formula, based on the XRD diffraction pattern of the mirror-polished cross-section described above. Crystallinity (%) = Crystalline peak area / (Crystalline peak area + Amorphous peak area) × 100

[0075] <Measurement of the maximum diameter of inclusions (I) and voids> The test specimens obtained in the examples and comparative examples were cut along a cross-section passing through their center, and the cut surfaces were mirror-polished. The mirror-polished cut surfaces were photographed using a "VHX5000" camera manufactured by Keyence Corporation, and the captured images were analyzed using "WinRoof" software manufactured by Mitani Corporation to measure the maximum diameter of inclusions (I) and voids present in the surface layer, which corresponds to a region within 2 mm of the surface of the spherical test specimen. The diameters of inclusions (I) and voids were determined as the square root of the envelope area of ​​the inclusions (I) and voids (Diameter of inclusions (I) and voids = √(Envelope area of ​​inclusions (I) and voids)). Specimens without inclusions (I) with a diameter greater than 50 μm in the surface layer were evaluated as "A," and those with inclusions (I) with a diameter greater than 50 μm were evaluated as "B." Similarly, specimens without voids with a diameter greater than 50 μm in the surface layer were evaluated as "A," and those with voids with a diameter greater than 50 μm were evaluated as "B." For inclusions (I) and voids, only samples where the entirety of inclusions (I) and voids were present in the surface layer were included in the measurement. The results are shown in Table 4.

[0076] <Rolling fatigue test> Using the test specimens obtained in the examples and comparative examples, a rolling fatigue test was conducted using NTN Corporation's "6206" as the outer ring, inner ring, and cage of the bearing, at a rotational speed of 3000 rpm, a load of 1.5 GPa, and a test duration of 168 hours to evaluate the product life. The lubricant used was additive-free turbine oil "VG56" manufactured by JXTG Energy Corporation. Test specimens that did not delaminate within the test time were evaluated as "a," and those that delaminate were evaluated as "b." The results are shown in Table 4.

[0077] [Table 4]

[0078] The evaluation results showed that Examples 1-7, with crystallinity levels between 75% and 90%, all received an "a" rating in the rolling fatigue test, indicating excellent peel resistance. Furthermore, Examples 1-7 had a maximum inclusion diameter and maximum void diameter of "A," meaning there were no inclusions (I) larger than 50 μm in diameter in the surface layer of the test specimens, nor were there any voids larger than 50 μm in diameter. On the other hand, Comparative Example 1 (crystallinity level 91%) and Comparative Example 2 (crystallinity level 71%) experienced peeling in the rolling fatigue test. Comparative Example 2 also received an "A" rating for both the maximum inclusion diameter and maximum void diameter. Based on these results, it is considered that the silicon nitride sintered bodies of Examples 1-7 will have a good product life when processed into products.

[0079] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the embodiments described above, and all modifications within the meaning and scope of the claims are intended to be included. [Industrial applicability]

[0080] The silicon nitride sintered body of the present invention can be suitably used as a rolling element in bearings such as rolling bearings, linear motion guide bearings, ball screws, and linear motion bearings. [Explanation of Symbols]

[0081] 1 Rolling bearing 2 Inner ring 3 Outer ring 4 Rolling elements 5 Cage 6. Sealing member 7. Grease 8a, 8b opening 11 Screw shaft 12 screw grooves 13 Ball Nut 14 screw grooves 15 balls 16 Ball screw sealing component 21 Spherical plain bearings 22 Inner Ring 23 Outer ring 24 shaft bearing hole

Claims

1. A silicon nitride sintered body, The degree of crystallinity is 75% or higher and 90% or lower. The silicon nitride sintered body contains rare earth elements and aluminum elements. The content of the rare earth elements is 7.5% by weight or more and 13% by weight or less in terms of oxides, relative to the total weight of the silicon nitride sintered body. The content of the aluminum element is 7.5% by weight or more and 13% by weight or less in terms of oxides, relative to the total weight of the silicon nitride sintered body. The silicon nitride sintered body has an inclusion (I) in the surface layer, which is within 2 mm of the surface, and the inclusion (I) includes an inclusion (It) containing a silicide of a transition metal element. A silicon nitride sintered body characterized in that the maximum diameter of the inclusion (I) is 50 μm or less.

2. The silicon nitride sintered body according to claim 1, characterized in that the amorphous phase contains one or more selected from the group consisting of Y, Ce, Nd, and Eu.

3. The silicon nitride sintered body according to claim 1 or 2, characterized in that it has pores in the surface layer region within 2 mm from the surface of the silicon nitride sintered body, and the maximum diameter of the pores is 50 μm or less.

4. A mechanical part characterized by using a silicon nitride sintered body according to any one of claims 1 to 3.

5. A rolling element characterized by using a silicon nitride sintered body according to any one of Claims 1 to 3.

6. A bearing characterized by using the rolling elements described in claim 5.

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