Rolling elements and bearings

Abstract

To provide a silicon nitride sintered body having good mechanical properties, especially good fracture toughness and a good product life when processed into a product and to provide a rolling element and a bearing using the same.SOLUTION: There is provided a rolling element 4 of a rolling bearing 1 comprising a silicon nitride sintered body containing a rare earth element and an aluminum element, wherein the content of the rare earth element is 6 wt.% or more and 13 wt.% or less in terms of oxide based on the total weight of the silicon nitride sintered body, the content of the aluminum element is 6 wt.% or more and 13 wt.% or less in terms of oxide based on the total weight of the silicon nitride sintered body, the silicon nitride sintered body has vacancies in the surface layer part, which is an area within 2 mm from the surface of the silicon nitride sintered body and the maximum diameter of the vacancies is 50 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a silicon nitride sintered body, and a rolling element and a bearing using the same. [Background technology]

[0002] Silicon nitride (Si3N4) sintered bodies have excellent mechanical properties, thermal conductivity, and electrical insulation, and are therefore being increasingly applied to bearing components, engine parts, tool materials, and heat dissipation substrate materials. It is known that silicon nitride sintered bodies are produced using silicon nitride powder as a starting material. Because silicon nitride powder is difficult to sinter, sintering aids are used together with the silicon nitride powder to produce densified silicon nitride sintered bodies. Common examples of such sintering aids include oxides of rare earth elements, aluminum oxide, magnesium oxide, and silicon oxide. However, the use of materials containing transition metal elements as sintering aids has also been investigated to improve the mechanical properties of silicon nitride sintered bodies (e.g., Patent Documents 1 and 2).

[0003] Because silicon nitride powder is expensive, using silicon nitride powder to produce a silicon nitride sintered body tends to increase the price of the silicon nitride sintered body. Therefore, a manufacturing method using silicon powder (metallic silicon powder), which is less expensive than silicon nitride powder, as a starting material and then reactively sintering it to produce a silicon nitride sintered body has attracted attention (e.g., Patent Documents 3 to 5). One such manufacturing method is known as the PS-RBSN (Post-Sintering of Reaction Bonded Silicon-Nitride) method. The PS-RBSN method includes a first step in which a compact formed from silicon powder is nitrided by heat treatment at a temperature of, for example, about 1100°C to 1450°C in a nitrogen gas-containing environment, and a second step in which the nitride obtained in the first step is densified by heat treatment at a temperature of, for example, about 1600°C to 1950°C. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-234120 [Patent Document 2] International Publication No. 2015 / 099148 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-149328 [Patent Document 4] Japanese Patent Application Laid-Open No. 2008-247716 [Patent Document 5] Japanese Patent Application Laid-Open No. 2013-49595 Summary of the Invention [Problem to be solved by the invention]

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

[0006] An object of the present invention is to provide a silicon nitride sintered body that has good mechanical properties, particularly fracture toughness, and that has a good product life when processed into products, as well as rolling elements and bearings that use the same. [Means for solving the problem]

[0007] The rolling element of the present invention is a rolling element for a bearing made of a silicon nitride sintered body containing a rare earth element and an aluminum element, wherein the content of the rare earth element is 6 to 13 wt % in terms of oxide relative to the total weight of the silicon nitride sintered body, and the content of the aluminum element is 6 to 13 wt % in terms of oxide relative to the total weight of the silicon nitride sintered body, and the silicon nitride sintered body has pores in a surface layer portion that is a region within 2 mm from the surface, and the maximum diameter of the pores is 50 μm or less.

[0008] In a rolling fatigue test using the above rolling element as a test piece, the surface of the rolling element does not peel off when rotated for 168 hours at a rotation speed of 3000 rpm and a load of 1.5 GPa in the presence of lubricating oil.

[0009] The rolling elements are balls.

[0010] The bearing of the present invention is characterized by using the rolling element of the present invention. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a silicon nitride sintered body that has good fracture toughness and has a good product life when processed into products, as well as a rolling element and a bearing that use the same. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a vertical cross-sectional view showing an example of a bearing of the present invention. [Figure 2] 1 is a perspective view of an electric vertical take-off and landing aircraft on which a bearing according to the present invention is mounted. [Figure 3] FIG. 1 is a partial cross-sectional view of a motor in a drive unit of an electric vertical take-off and landing aircraft. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described. (Silicon nitride sintered body) The silicon nitride sintered body of this embodiment contains a rare earth element and an aluminum element. The content of the rare earth element in the silicon nitride sintered body is 6 to 13 wt % in terms of oxide, based on the total weight of the silicon nitride sintered body, and the content of the aluminum element in terms of oxide, based on the total weight of the silicon nitride sintered body, is 6 to 13 wt % in terms of oxide, based on the total weight of the silicon nitride sintered body.

[0014] Examples of rare earth elements include yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), neodymium (Nd), dysprosium (Dy), europium (Eu), and erbium (Er). Among these, yttrium (Y), cerium (Ce), neodymium (Nd), and europium (Eu) are preferred. In particular, it is more preferred to include cerium (Ce) because it can further promote nitriding and improve production efficiency.

[0015] The rare earth element content is 6% by weight or more, preferably 6.5% by weight or more, and may be 7% by weight or more. The rare earth element content is 13% by weight or less, and may be 12% by weight or less, or may be 11% by weight or less. When the rare earth element content is within the above range, it is easy to obtain a silicon nitride sintered body that has good fracture toughness and a good product life when processed into a product.

[0016] The rare earth element originates, for example, from a rare earth-containing sintering aid (usually an oxide of the rare earth element) used in the production of the silicon nitride sintered body. When the rare earth element content in the silicon nitride sintered body is within the above range, the nitriding reaction of the raw material silicon powder (metallic silicon powder) can be promoted, facilitating the subsequent sintering when producing the silicon nitride sintered body using the PS-RBSN method. The PS-RBSN method is a two-stage sintering method that includes a silicon nitriding step and a subsequent sintering step. The rare earth element content can be adjusted by the amount of rare earth-containing sintering aid (e.g., an oxide of the rare earth element) added to the raw material.

[0017] The aluminum element content is 6% by weight or more, preferably 6.5% by weight or more, and may be 7% by weight or more. The aluminum element content is 13% by weight or less, may be 12% by weight or less, or may be 11% by weight or less. The aluminum element content (oxide equivalent) may be within ±5% by weight, ±2% by weight, or ±1% by weight of the rare earth element content (oxide equivalent), or may be the same as the rare earth element content. When the aluminum element content is within the above range, it is easy to obtain a silicon nitride sintered body that has good fracture toughness and a good product life when processed into a product.

[0018] The aluminum element originates, for example, from an aluminum-containing sintering aid (usually aluminum oxide) used during the production of the silicon nitride sintered body. By ensuring that the aluminum element content in the silicon nitride sintered body is within the above range, sintering can be promoted when producing the silicon nitride sintered body by the PS-RBSN method. The aluminum element content can be adjusted by the amount of aluminum-containing sintering aid (e.g., aluminum oxide) added to the raw material.

[0019] The rare earth element and aluminum element contents can be determined using an X-ray fluorescence analyzer (XRF), energy dispersive X-ray analyzer (EDX), or inductively coupled plasma (ICP) optical emission analyzer. Specifically, the contents of rare earth elements and aluminum elements in the silicon nitride sintered body can be determined using the analyzer and then converted into rare earth element (RE) oxides (RE2O3 or REO2) and aluminum oxide (Al2O3). The other constituent elements of the silicon nitride sintered body can also be analyzed using the analyzer, and the total weight of the silicon nitride sintered body can be calculated to determine the rare earth element and aluminum element contents. If the raw material powder used to produce the silicon nitride sintered body contains silicon (metallic silicon powder) and the silicon is nitrided to form Si3N4, the weight of Si3N4 in the silicon nitride sintered body is 1.67 times the weight of the silicon. Therefore, taking into account the weight change when silicon is nitrided, the contents of rare earth element oxides and aluminum oxide can be calculated from the composition of the raw material powder.

[0020] The silicon nitride sintered body of this embodiment preferably has inclusions (I) in the surface layer, which is a region within 2 mm from the surface. The inclusions (I) contain components other than silicon nitride, such as inclusions (It) containing transition metal elements and inclusions (Is) containing non-nitrided silicon elements. The inclusions (It) are preferably silicides of transition metal elements. The inclusions (Is) are, for example, aggregates of non-nitrided silicon elements. The inclusions (I) preferably contain inclusions (It), and preferably do not contain inclusions (Is) or have a small presence rate. The inclusions refer to inclusions that are present entirely in the surface layer, which is a region within 2 mm from the surface of the silicon nitride sintered body.

[0021] The inclusions (It) originate, for example, from sintering aids (usually oxides of transition metal elements) containing transition metal elements used in the production of silicon nitride sintered bodies. For example, silicides of transition metal elements are formed during the production of silicon nitride sintered bodies. When producing silicon nitride sintered bodies using the PS-RBSN method, the use of sintering aids containing transition metal elements can promote the nitridation reaction of silicon powder and the growth of needle-like silicon nitride crystals. This reduces the heat treatment time required to nitride silicon, improving energy efficiency during the production of silicon nitride sintered bodies.

[0022] On the other hand, when silicon nitride powder is included in the raw materials used to produce sintered silicon nitride, mixing it with a sintering aid (oxide of a transition metal element) containing a transition metal element such as chromium oxide (Cr2O3) can cause the sintering aid to oxidize the silicon nitride powder, resulting in a deviation in the composition of the raw materials and preventing good sintering. In contrast, when producing sintered silicon nitride using the PS-RBSN method, silicon powder is used primarily as the raw material, and the content of silicon nitride powder in the raw materials can be reduced, making it less likely to suffer from the above-mentioned problems and allowing for the production of dense sintered silicon nitride.

[0023] Inclusions (Is) can form when silicon nitride sintered bodies are produced using the PS-RBSN method if the raw material silicon powder (metallic silicon powder) 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 of the silicon nitride sintered body, such as fracture toughness, tend to decrease, and the product lifespan when processed into products is likely to be shortened. It is preferable for there to be few inclusions (Is) in the surface layer of silicon nitride sintered bodies, and it is even more preferable for there to be no inclusions (Is).

[0024] The transition metal element is not particularly limited as long as it is an element contained in Groups 3 to 11 of the IUPAC periodic table. The transition metal element is preferably one or more elements selected from the group consisting of Ti, Cr, and Mn, and more preferably contains Cr. By containing Cr as a transition metal element, the fracture toughness of the silicon nitride sintered body can be further improved.

[0025] In the silicon nitride sintered body, the content of the transition metal element, calculated as oxide, is preferably 0.1% by weight or more, more preferably 0.3% by weight or more, and may be 0.5% by weight or more, and is usually 5% by weight or less, may be 3% by weight or less, more preferably 2% by weight or less, and may be 1% by weight or less, based on the total weight of the silicon nitride sintered body. The content of the transition metal element can be determined in the same manner as the method for determining the content of the rare earth element and aluminum element.

[0026] The maximum diameter of the inclusions (I) present in the surface layer portion of the silicon nitride sintered body is not particularly limited. Specifically, the maximum diameter of the inclusions (I) is 50 μm or less, may be 40 μm or less, may be 30 μm or less, may be 25 μm or less, and is usually 0.5 μm or more. The maximum diameter of the inclusions (I) in the surface layer portion refers to the diameter of the largest inclusion (I) among the inclusions (I) present in the surface layer portion. When the maximum diameter of the inclusions (I) is within the above range, the inclusions (I) are more likely to be prevented from becoming a fracture source, making it easier to obtain a silicon nitride sintered body with good fracture toughness. Furthermore, when the maximum diameter of the inclusions (I) is within the above range, it is easier to prevent the inclusions from falling out of the silicon nitride sintered body and becoming defects, making it easier to obtain a good product life when the silicon nitride sintered body is processed into products such as rolling elements of bearings. The maximum diameter of the inclusions (I) can be adjusted, for example, by the degree of nitriding of the silicon powder as the raw material, the amount and / or particle size of the sintering aid containing a transition metal element added to the raw material, and the type of the transition metal element.

[0027] In the cross section of the silicon nitride sintered body, the ratio of the total cross-sectional area of ​​the inclusions (I) to the total cross-sectional area of ​​the surface layer portion ([total cross-sectional area of ​​inclusions (I) / total cross-sectional area of ​​surface layer portion] × 100) is preferably 0.05% or more, and may be 0.1% or more, 0.15% or more, 0.3% or more, or 0.6% or more. This ratio is usually 7.0% or less, may be 3.0% or less, 2.0% or less, or may be 1.5% or less. The above ratio of inclusions (I) is the ratio of the total cross-sectional area of ​​all inclusions present in the surface layer portion to the total cross-sectional area of ​​the surface layer portion. When this ratio is within the above range, it is easy to obtain a silicon nitride sintered body that has good fracture toughness and a good product life when processed into a product. On the other hand, if this ratio is too large, the inclusions will join together and fall off, which is likely to have an adverse effect on the results of bearing life tests. The above proportion of inclusions (I) can be adjusted, for example, by the degree of nitriding of the raw material silicon powder, the amount and / or particle size of the sintering aid containing a transition metal element added to the raw material, and the type of the transition metal element.

[0028] Furthermore, the silicon nitride sintered body of this embodiment preferably has pores in the surface layer, which is a region within 2 mm from the surface. Furthermore, the maximum diameter of the pores is preferably 50 μm or less in the cross section of the silicon nitride sintered body. The maximum diameter of the pores may be 40 μm or less, 30 μm or less, or 25 μm or less, or the silicon nitride sintered body may not have pores. Having the maximum diameter of the pores within the above range facilitates achieving a good product life when processed into products such as rolling elements of bearings. The pores in the surface layer refer to those present in the surface layer, which is a region within 2 mm from the surface of the silicon nitride sintered body, and refer to pores present entirely in the surface layer. The maximum diameter of the pores in the surface layer refers to the diameter of the largest pore among the pores present in the surface layer. For example, when producing a silicon nitride sintered body by the PS-RBSN method, the maximum diameter of the pores can be adjusted by adjusting the content of silicon nitride used as a raw material and / or the amount of sintering aid added.

[0029] The maximum diameter of the inclusions (I), the proportion of the inclusions (I), and the maximum diameter of the pores are values ​​measured for inclusions (I) or pores that are entirely present in the surface layer portion in a cross section of a test piece prepared by the method described in the Examples below. The maximum diameter of the inclusions (I), the proportion of the inclusions (I), and the maximum diameter of the pores can be calculated by the method described in the Examples below.

[0030] The silicon nitride sintered body of this embodiment is mainly produced by the PS-RBSN method, as described below. Silicon nitride sintered bodies produced by the PS-RBSN method have a smaller shrinkage rate than sintered bodies made from silicon nitride powder as raw material, because the relative density of the green compact increases once the green compact is nitrided. The shrinkage rate is calculated using the following formula: The "dimensions" in the formula refer to the dimensions of corresponding parts of the green compact and the silicon nitride sintered body. For example, if both are spherical, their respective diameters can be used. Shrinkage rate [%] = [{(size of green compact) - (size of sintered silicon nitride)} / size of green compact] x 100 The shrinkage rate of the silicon nitride sintered body of this embodiment is not particularly limited, but from the viewpoint of the dimensional accuracy of the sintered body, it is preferably 15% or less, may be 14% or less, or may be 13% or less, and may be, for example, 7% or more, may be 8% or more, or may be 10% or more.

[0031] Furthermore, the silicon nitride sintered body of this embodiment contains rare earth elements in an amount of 6 to 13 wt % (oxide equivalent) relative to the total weight of the silicon nitride sintered body, and aluminum in an amount of 6 to 13 wt % (oxide equivalent) relative to the total weight of the silicon nitride sintered body. By using a corresponding amount of sintering aids, for example, containing rare earth elements and aluminum, during production, the nitriding reaction can be sufficiently promoted even when silicon powder is used as the raw material. As a result, good fracture toughness can be obtained. The fracture toughness (based on JIS R 1607) is, for example, 3 MPa m 1 / 2 and above 4 MPa m 1 / 2 More than 5 MPa m is preferable. 1 / 2 More preferably, the fracture toughness is, for example, 8 MPa m1 / 2 The following is the result.

[0032] A particularly preferred embodiment of the silicon nitride sintered body of this embodiment is a silicon nitride sintered body containing a rare earth element and an aluminum element, further comprising inclusions (I) and voids in a surface layer region within 2 mm of the surface of the silicon nitride sintered body, wherein the rare earth element content is 6 to 13 wt% (as calculated as oxide) of the total weight of the silicon nitride sintered body, and the aluminum content is 6 to 13 wt% (as calculated as oxide) of the total weight of the silicon nitride sintered body, the inclusions (I) present in the surface layer region have a maximum diameter of 50 μm or less, the ratio of the total cross-sectional area of ​​the inclusions (I) to the total cross-sectional area of ​​the surface layer region in the cross section of the silicon nitride sintered body is 0.1% or more, and the voids present in the surface layer region have a maximum diameter of 50 μm or less. Furthermore, the above-mentioned elements and numerical ranges can be appropriately combined with this embodiment.

[0033] The shape of the silicon nitride sintered body of this embodiment is not particularly limited, and may be selected appropriately depending on the application from spherical, cylindrical, conical, truncated conical, rectangular parallelepiped, etc., but spherical is preferable. The size of the silicon nitride sintered body is also not particularly limited, and for example, if it is spherical, the diameter can be 0.5 cm to 10 cm, and if it is cylindrical, the bottom diameter can be 0.5 cm to 15 cm and the height can be 3 cm to 20 cm.

[0034] The silicon nitride sintered body is preferably produced by the PS-RBSN method (two-stage sintering method). Specifically, it can be produced by the following first and second methods.

[0035] (First method) In the PS-RBSN method, granulation is often performed to improve the powder's fluidity. The first method is a method for producing a silicon nitride sintered body containing rare earth elements and aluminum elements, and includes, for example, a granulation step in which a raw material powder containing silicon powder and a sintering aid is used to obtain a granulated powder, a molding step in which the obtained granulated powder is molded into a green compact, a debinding step, and a sintering step in which the debound green compact is sintered. After the sintering step, the silicon nitride sintered body may be polished, if necessary.

[0036] In the granulation process, the raw material powder and binder component are mixed with water and / or an organic solvent (e.g., ethanol) to form a slurry, which is then spray-dried and granulated to obtain granulated powder. An organic binder is used as the binder component, and is added in an amount of, for example, 1% to 10% by weight based on the total weight of the raw material powder.

[0037] In the subsequent molding step, the granulated powder is molded into a predetermined shape to obtain a green compact. In the debinding step, the obtained green compact is heated at a temperature of 700°C to 1000°C in a nitrogen atmosphere to be debound.

[0038] The sintering process includes a first step in which the degreased compact is nitrided by heat treatment at a temperature of 1200°C to 1500°C in a nitrogen atmosphere, for example, and a second step in which the resulting nitride is sintered by heat treatment at a temperature of 1600°C to 1950°C (preferably 1600°C to 1900°C) in a nitrogen atmosphere, for example. In the first step, in order to completely nitride the silicon, it is preferable to maintain the temperature at 1200°C to 1500°C (preferably 1300°C to 1500°C) for a long period of time (for example, one hour or more). In this specification, "maintaining the temperature" means maintaining the temperature for a certain period of time. The temperature rise rate when transitioning from the first step to the second step is, for example, 2°C / min or more, optionally 2.5°C / min or more, or even 5°C / min or more. The temperature rise rate is, for example, 20°C / min or less, preferably 15°C / min or less.

[0039] As will be shown in the examples below, nitriding can be promoted by adjusting the amount and / or particle size of the sintering aid and the type of rare earth element. As a result, the temperature maintenance in the first step can be omitted. Also, the temperature rise rate during the transition from the first step to the second step can be increased. This allows for a reduction in manufacturing time and improved energy efficiency during manufacturing.

[0040] (Second method) The second method is a method for producing a silicon nitride sintered body containing rare earth elements and aluminum elements, and includes, for example, a mixing step of dry-mixing silicon powder and raw material powder containing a sintering aid, a molding step of molding the mixed raw material powder into a powder compact, and a sintering step of sintering the powder compact. Unlike the first method, the second method is characterized in that all steps of the PS-RBSN method are performed dry. After the sintering step, the silicon nitride sintered body may be polished, if necessary.

[0041] The mixing step is a dry mixing step in which the raw material powders are mixed without using water or an organic solvent. It is also preferable to perform mixing without using a binder component in this step. The particle size of the powder after mixing is not particularly limited, but D90 is preferably 10 μm to 100 μm, more preferably 10 μm to 50 μm, and even more preferably 10 μm to 20 μm. Furthermore, D50 is preferably 2 μm to 10 μm, more preferably 3 μm to 9 μm, and even more preferably 4 μm to 8 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 cumulative 90% diameter, respectively, based on volume, and can be obtained by laser diffraction / scattering particle size distribution measurement, etc.

[0042] In the subsequent molding step, the mixed powder is molded into a predetermined shape to obtain a green compact. The sintering step includes a first step of nitriding the obtained green compact by heat treatment, for example, in a nitrogen atmosphere at a temperature of 1200°C to 1500°C, and a second step of sintering the green compact by heat treatment, for example, in a nitrogen atmosphere at a temperature of 1600°C to 1950°C (preferably 1600°C to 1900°C). From the viewpoint of improving manufacturing efficiency, it is preferable that the first step does not maintain the temperature within the range of 1200°C to 1500°C for more than one hour. Specifically, it is preferable to nitride the green compact by increasing the temperature from, for example, 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, or alternatively, 2.5°C / min or more, or even 5°C / min or more. The heating rate is, for example, 20°C / min or less, and preferably 15°C / min or less.

[0043] The second method has the following advantages over the first method. By carrying out all processes in the PS-RBSN method in a dry manner, it is possible to prevent oxidation of silicon powder, which occurs when using a water solvent, and also to reduce the environmental burden caused by organic solvents such as ethanol. By using the PS-RBSN method to produce silicon nitride sintered bodies without using organic binders, it is possible to reduce shrinkage due to sintering and improve the dimensional precision of the sintered bodies. In the first method, because organic binders are used for granulation, a debinding process is required afterwards, but voids are created after the organic binder is removed during the debinding process, which can increase shrinkage due to sintering. Furthermore, the reduced shrinkage can shorten the polishing time in the subsequent polishing step. By using the PS-RBSN method to produce silicon nitride sintered bodies without using binder components, the debinding process can be omitted, and the generation of greenhouse gases such as CO2 that can be generated by the decomposition of binder components during the debinding process can be prevented, thereby reducing the environmental impact.

[0044] Generally, to obtain a dense sintered body using conventional methods that use Si3N4 powder as a raw material, it is necessary to use fine Si3N4 powder (D50 of 1 μm or less). Because such fine powders have poor fluidity and moldability, the raw powder and binder components must be slurried with water or ethanol, and then spray-dried to obtain granules. However, with the PS-RBSN method, the Si powder is refined by fracture due to volume expansion during the nitriding process, so there is no need to use a powder as fine as Si3N4 powder as a raw material to obtain a dense sintered body. Because the raw powder is not fine, the fluidity and moldability required to obtain a compact can be ensured even without granulated powder.

[0045] In the production of the silicon nitride sintered body described above, including the first and second methods, the sintering aid used in the raw material powder preferably contains a rare earth element, aluminum element, and transition metal element, and more preferably contains an oxide of these elements. The rare earth element-containing sintering aid preferably contains one of Y2O3, CeO2, Nd2O3, and Eu2O3. The transition metal element-containing sintering aid preferably contains one of Cr2O3, TiO2, MnO, and Fe2O3, more preferably one of Cr2O3, TiO2, and MnO, and even more preferably Cr2O3.

[0046] The raw material powder may contain, in addition to silicon powder and a sintering aid, silicon nitride powder and / or an organic binder, and may also contain a sintering aid containing an element other than a rare earth element, an aluminum element, and a transition metal element.

[0047] The content of silicon powder contained in the raw material powder is preferably 45% by weight or more, more preferably 50% by weight or more, and even more preferably 55% by weight or more, based on the total weight of silicon powder, silicon nitride powder, and sintering aid, and may be 60% by weight or more, but is usually 90% by weight or less, may be 85% by weight or less, or may be 80% by weight or less. The content of silicon nitride powder contained in the raw material powder is usually 30% by weight or less, preferably 25% by weight or less, more preferably 20% by weight or less, may be 15% by weight or less, based on the total weight, and may not contain silicon nitride powder.

[0048] The content of the sintering aid containing a rare earth element (e.g., an oxide of a rare earth element) contained in the raw material powder is 7 wt% or more, preferably 9 wt% or more, more preferably 9.5 wt% or more, and may be 10 wt% or more, based on the total weight. The content of the rare earth element is 17 wt% or less, optionally 15 wt% or less, and may be 14.5 wt% or less. The content of the sintering aid containing an aluminum element (e.g., aluminum oxide) contained in the raw material powder is 5 wt% or more, preferably 9 wt% or more, more preferably 9.5 wt% or more, and may be 10 wt% or more, based on the total weight. The content of the aluminum element is 17 wt% or less, optionally 15 wt% or less, and may be 14.5 wt% or less. The content of the sintering aid containing a transition metal element (e.g., an oxide of a transition metal element) contained in the raw material powder is typically 0.1 wt% or more, preferably 0.5 wt% or more, and typically 5 wt% or less, more preferably 3 wt% or less, based on the total weight. If the content of sintering aid in the raw material powder is low, it is difficult to obtain a dense silicon nitride sintered body, and if the content of sintering aid is high, the mechanical properties such as fracture toughness of the silicon nitride sintered body tend to deteriorate.

[0049] The average particle size of the silicon powder contained in the raw material powder can be, for example, 5 μm or less. The average particle size of the silicon nitride can be, for example, 0.5 μm or less. The average particle size of the sintering aid, although depending on the type of sintering aid, is usually 10 μm or less, and may be 7 μm or less, 5 μm or less, 3 μm or less, 2 μm or less, 1 μm or less, or 0.4 μm or less.

[0050] One form of the second method described above is a method for producing a silicon nitride sintered body containing a rare earth element and an aluminum element, which includes a mixing step of dry-mixing silicon powder and a raw material powder containing a sintering aid, a molding step of molding the mixed raw material powder into a green compact, and a sintering step of sintering the green compact, wherein the silicon powder accounts for 45 wt % or more of the total raw material powder.

[0051] Furthermore, the above-described aspect of the second method may have one or more of the following features (1) to (7). (1) The mixing step is a step of mixing the raw material powders without using a binder component. (2) The sintering step includes a step of increasing the temperature from a temperature in the range of 1000°C to 1200°C to the sintering temperature at a rate of 15°C / min or less without maintaining the predetermined temperature for one hour or more. (3) The sintering temperature is in the range of 1600°C to 1900°C. (4) The sintering aid contains a rare earth oxide and aluminum oxide, and the raw material powder contains 9.5% by weight or more and 17% by weight or less of the rare earth oxide and 9.5% by weight or more and 17% by weight or less of the aluminum oxide, based on the total weight of the raw material powder. (5) The rare earth oxide includes at least one selected from the group consisting of Y2O3, CeO2, Nd2O3, and Eu2O3. (6) The sintering aid contains a transition metal compound, and the raw material powder contains the transition metal compound in an amount of 0.1% by weight to 5% by weight based on the total weight of the raw material powder. (7) The transition metal element includes at least one element selected from the group consisting of Ti, Cr, and Mn.

[0052] For example, adding 9.5 to 17 wt % of rare earth oxides and 9.5 to 17 wt % of aluminum oxide as sintering aids to the raw material powder can promote the nitridation of silicon and the subsequent sintering (see (4) above). Furthermore, adding 0.1 to 5 wt % of a transition metal compound as a sintering aid can promote the nitridation of silicon (see (6) above). Promoting the nitridation of silicon eliminates the need for long-term temperature maintenance at 1100 to 1450°C in a nitrogen atmosphere, as is commonly done, resulting in a method with excellent energy efficiency.

[0053] (Applications of sintered silicon nitride) The applications of the silicon nitride sintered body of this embodiment are not particularly limited, but because of its excellent mechanical properties and thermal conductivity, it can be used for bearing members, rolling roll materials, compressor vanes, gas turbine blades, engine parts, etc. As bearing members, it can be used for, for example, rolling bearings, linear guide bearings, ball screws, linear bearings, etc., and is particularly suitable for use as rolling elements in bearings.

[0054] The bearing of this embodiment will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view of a deep groove ball bearing. In rolling bearing 1, an inner ring 2 having an inner ring raceway surface 2a on its outer peripheral surface and an outer ring 3 having an outer ring raceway surface 3a on its inner peripheral surface are concentrically arranged, and a plurality of balls (rolling elements) 4 are arranged between inner ring raceway surface 2a and outer ring raceway surface 3a. These balls 4 are formed from the above-mentioned silicon nitride sintered body. Balls 4 are held in cage 5. Openings 8a, 8b at both axial ends of the inner and outer rings are sealed by seal members 6, and a grease composition 7 is enclosed at least around balls 4. Grease composition 7 is present on the raceway surfaces of balls 4 to provide lubrication.

[0055] (Bearing applications) The applications of the bearing of this embodiment are not particularly limited, but by using rolling elements made of sintered silicon nitride, it functions as an insulating bearing, making it suitable for use in structures where current may flow inside the bearing during use. For example, it can be used in applications such as main motors for railway vehicles, general-purpose motors, and generators. It can also be used in flying cars, which have recently attracted attention as an alternative means of transportation to automobiles. Flying cars are expected to solve various social problems and are expected to be used in a variety of situations, including intra-regional travel, inter-regional travel, tourism and leisure, emergency medical care, and disaster relief.

[0056] Vertical take-off and landing aircraft (VTOL) are attracting attention as flying cars. VTOLs can ascend and descend vertically between the sky and takeoff and landing sites, eliminating the need for runways and offering superior convenience. In particular, in recent years, due to societal demands for reducing CO2 emissions, electric vertical take-off and landing aircraft (eVTOL), which fly using batteries and motors, have become the mainstream of development.

[0057] An electric vertical take-off and landing aircraft equipped with a bearing of the present invention will be described with reference to Figure 2. The electric vertical take-off and landing aircraft 11 shown in Figure 2 is a multicopter having a main body 12 located in the center of the aircraft and four drive units 13 arranged in the front, rear, left and right directions. The drive units 13 are devices that generate lift and thrust for the electric vertical take-off and landing aircraft 11, and the electric vertical take-off and landing aircraft 11 flies when driven by the drive units 13. The electric vertical take-off and landing aircraft 11 may have multiple drive units 13, and is not limited to four.

[0058] The main body 12 has a living space large enough to accommodate a crew member (for example, one to two people). This living space is provided with an operating system for determining the direction of travel and altitude, and instruments for indicating altitude, speed, flight position, etc. Four arms 12a extend from the main body 12, and a drive unit 13 is provided at the tip of each arm 12a. In FIG. 2, a circular ring portion that covers the rotating periphery of the rotor 14 is integrally provided on each arm 12a to protect the rotor 14. In addition, a skid 12b that supports the aircraft during landing is provided below the main body 12.

[0059] The drive unit 13 has rotors 14 and a motor 15 that rotates the rotors 14. In the drive unit 13, a pair of rotors 14 are provided on both axial sides of the motor 15. Each rotor 14 has two blades that extend radially outward.

[0060] The main body 12 is provided with a battery (not shown) and a control device (not shown). The control device is also called a flight controller. The electric vertical take-off and landing aircraft 11 is controlled by the control device, for example, as follows: The control device outputs a command to change the rotation speed to the motor 15, which should adjust lift based on the difference between the current attitude and the target attitude. Based on this command, an amplifier provided in the motor 15 adjusts the amount of power sent from the battery to the motor 15, and the rotation speed of the motor 15 (and the rotor 14) is changed. Furthermore, the rotation speed of the motors 15 is adjusted simultaneously for multiple motors 15, and the attitude of the aircraft is determined thereby.

[0061] Figure 3 shows a partial cross section of the motor in the drive unit. In Figure 3, the above-mentioned rotor is attached to one end (upper side of the figure) of the rotating shaft 17 of the motor 15, and a rotor is attached to the other end (lower side of the figure). The rotor is disposed opposite a stator fixed to the housing and is rotatable relative to the stator. Note that the motor 15 can be configured as an outer rotor brushless motor or an inner rotor brushless motor.

[0062] In FIG. 3, the motor 15 includes a housing (device housing) 16, a rotor (not shown), a stator (not shown), an amplifier (not shown), and two rolling bearings (deep groove ball bearings) 21, 21. The housing 16 has an outer cylinder 16a and an inner cylinder 16b, with a coolant flow path 16c provided between them. By flowing a coolant through this flow path 16c, excessive temperature rise can be prevented. The rolling bearings 21, 21 rotatably support the rotating shaft 17 within the inner cylinder 16b. In FIG. 3, the balls 24 of the rolling bearing 21 are formed from the above-mentioned silicon nitride sintered body. The rolling bearing 21 corresponds to the bearing of the present invention.

[0063] In the rolling bearing 21, the outer diameter shape of the outer ring 23 is substantially the same as the shape of the fitting portion on the inner periphery of the housing, and it is fitted directly into the housing 16 without an intervening bearing housing or the like. An inner ring spacer 18 and an outer ring spacer 19 are inserted between the rolling bearings 21, 21, and a preload is applied. The outer ring spacer 19 is provided with nozzle members 20, 20 for spraying lubricating oil to cool and lubricate the rolling bearings 21, 21. The nozzle member 20 has an internal lubricating oil flow path that guides air oil supplied from an external lubricating oil supply device (not shown) into the bearing space.

[0064] In electric vertical take-off and landing aircraft, motors have a higher capacity than those used in drones, which means that the drive current is larger and the voltage (shaft voltage) generated on the motor's rotating shaft is expected to increase. This raises concerns about the occurrence of electrolytic corrosion, but by applying bearings equipped with rolling elements made of the aforementioned sintered silicon nitride, electrolytic corrosion caused by current flow can be effectively prevented while maintaining a long product life. This reduces the occurrence of bearing abnormalities and leads to safe flight of electric vertical take-off and landing aircraft. Furthermore, using rolling elements made of sintered silicon nitride also enables the bearing weight to be reduced compared to rolling elements made of iron-based materials, making it particularly suitable for bearings in electric vertical take-off and landing aircraft, which require lightweight construction.

[0065] The bearing configuration in the drive unit is not limited to the configuration in Fig. 3. In Fig. 3, the rotating shaft of the motor and the rotating shaft of the impeller are the same rotating shaft, but the rotating shaft of the motor and the rotating shaft of the impeller may be connected via a transmission mechanism. In this case, the rolling bearing supporting the rotating shaft in the drive unit may be the rolling bearing supporting the rotating shaft of the motor, or may be the rolling bearing supporting the rotating shaft of the impeller. [Example]

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

[0067] [Test Example 1] Raw material powders were prepared according to the compounding ratios shown in Table 2, to which 3% by weight of an organic binder was added. Silicon nitride balls were used as the media, and ethanol was used as the solvent. The mixture was mixed in a ball mill at 200 rpm for 48 hours. The mixed slurry was then dried and granulated by spray drying to obtain granulated powder. The specifications of the materials used to obtain the granulated powder are shown in Table 1.

[0068] [Table 1]

[0069] <Examples 1 to 23, Example 27, Comparative Examples 1 and 2> The granulated powder obtained above was molded into a spherical powder compact with a diameter of 11 mm using cold isostatic pressing with a rubber mold. The powder compact was degreased in a nitrogen atmosphere at 800°C for 48 hours, then heated to 1400°C at a heating rate of 2.5°C / min and held at 1400°C for 4 hours in a nitrogen atmosphere (pressure: 0.9 MPa) to nitride it. The nitrided powder compact was then heated to 1550°C-1950°C at a heating rate of 2.5°C / min-20°C / min and held at that sintering temperature for 4 hours in a nitrogen atmosphere (pressure: 0.9 MPa) to obtain a silicon nitride sintered body.

[0070] <Examples 24 to 26> The granulated powder obtained above was molded into a spherical powder compact with a diameter of 11 mm by cold isostatic pressing using a rubber mold. The powder compact was degreased in a nitrogen atmosphere at 800°C for 48 hours, then heated to 1800°C at a heating rate of 20°C / min and held at the sintering temperature of 1800°C for 4 hours in a nitrogen atmosphere (pressure: 0.9 MPa) to obtain a silicon nitride sintered body. In Examples 24 to 26, the step of nitriding at 1400°C for 4 hours (temperature holding) was omitted.

[0071] The dimensions of the green compacts and the silicon nitride sintered bodies obtained in the examples and comparative examples were measured with a micrometer, and the shrinkage ratio was calculated using the following formula. The shrinkage ratio is shown in Table 4 together with other measurement results. Shrinkage rate [%] = [(diameter of green compact) - (diameter of sintered silicon nitride)} / diameter of green compact] x 100

[0072] The composition ratios of each oxide in the obtained silicon nitride sintered body were calculated from the composition ratios of the raw material powder, assuming that all of the silicon (metallic silicon) contained in the raw material powder was nitrided and that the weight of silicon nitride was 1.67 times the weight of silicon. These values ​​are shown in Table 3.

[0073] The obtained spherical silicon nitride sintered body was subjected to ball polishing to G5 in accordance with JIS B 1563 to prepare spherical test pieces of 3 / 8 inch (diameter 9.525 mm).

[0074] [Table 2]

[0075] [Table 3]

[0076] <Measurement of the maximum diameter and area ratio of inclusions (I), and measurement of the maximum diameter of voids> The test pieces obtained in the examples and comparative examples were cut at a cross section passing through their centers, and the cut surfaces were mirror-polished. The mirror-polished cut surfaces were photographed using a Keyence VHX5000. The photographed images were analyzed using WinRoof software from Mitani Shoji Co., Ltd. to measure the maximum diameter of inclusions (I) and voids present in the surface layer, which corresponds to a region within 2 mm from the surface of the spherical test piece. The diameters of the inclusions (I) and voids were calculated as the square root of the envelope area of ​​the inclusions (I) and voids (diameter of inclusions (I) and voids = √(enveloping area of ​​inclusions (I) and voids)). Test pieces that did not have inclusions (I) with a diameter greater than 50 μm in the surface layer were evaluated as "A," and those that did have inclusions (I) were evaluated as "B." Test pieces that did not have voids with a diameter greater than 50 μm in the surface layer were evaluated as "A," and those that did have them were evaluated as "B." Test pieces that did not have voids with a diameter greater than 50 μm in the surface layer were evaluated as "A," and those that did have them were evaluated as "B." The inclusions (I) and voids were measured when the entire inclusions (I) and voids were present in the surface layer. The ratio of the total cross-sectional area of ​​the inclusions (I) to the total cross-sectional area of ​​the surface layer was calculated (ratio of the total cross-sectional area of ​​inclusions (I) = enveloping area of ​​inclusions (I) ÷ total cross-sectional area of ​​the surface layer × 100). The results are shown in Table 4.

[0077] <Evaluation of fracture toughness> The test pieces obtained in the examples and comparative examples were cut at a cross section passing through the center thereof, the cut surface was mirror-polished, and the fracture toughness value was measured in accordance with JIS R 1607.

[0078] <Crushing strength measurement> The test pieces obtained in the examples and comparative examples were subjected to a two-ball crushing test in accordance with JIS B 1501.

[0079] <Rolling fatigue test> Using the test specimens obtained in the examples and comparative examples, and NTN Corporation's "6206" bearing outer ring, inner ring, and cage, rolling fatigue tests were conducted at a rotation speed of 3000 rpm, a load of 1.5 GPa, and a test time of 168 hours to evaluate product life. The lubricant used was JXTG Nippon Oil & Energy Corporation's additive-free turbine oil "VG56." Test specimens that did not peel within the test time were rated "a," and those that peeled were rated "b." The results are shown in Table 4.

[0080] [Table 4]

[0081] <Analysis of Inclusions (I)> The type and content of elements in the inclusions (I) contained in the surface layer were measured by EDX analysis using a scanning electron microscope (S300, manufactured by Hitachi, Ltd.) on the cut surface of the test piece obtained in Example 6. The inclusions (I) contained chromium silicide, and the elements contained in the inclusions (I) and their contents were 56 wt% chromium (Cr) and 44 wt% silicon (Si).

[0082] [Test Example 2] In Test Example 2, granulated powders were obtained by dry mixing, except for Example 27. First, the raw material powders shown in Table 1 above were prepared in the blending ratios shown in Table 2 above.

[0083] <Examples 1 to 26, Comparative Examples 1 and 2> Using silicon nitride balls as media, the powders were dry mixed in a ball mill at 200 rpm for 48 hours. The resulting mixed powder was then molded into a spherical powder compact with a diameter of 11 mm using cold isostatic pressing with a rubber mold. The powder compact was heated from room temperature to 1550-1950°C at a heating rate of 2.5-20°C / min (Table 2), and held at that sintering temperature for 4 hours in a nitrogen atmosphere (pressure: 0.9 MPa), yielding a silicon nitride sintered body.

[0084] Example 27 An organic binder was added to the raw powder at 3 wt. % of the total raw powder, and the mixture was mixed in a ball mill using silicon nitride balls as the medium and ethanol as the solvent at 200 rpm for 48 hours. The mixed slurry was spray-dried to obtain granulated powder. The resulting granulated powder was then molded into spherical powder compacts with a diameter of 11 mm using cold isostatic pressing with a rubber mold. The compacts were degreased in a nitrogen atmosphere at 800°C for 48 hours, then heated to 1800°C at a heating rate of 2.5°C / min, and held at 1800°C for 4 hours in a nitrogen atmosphere (pressure: 0.9 MPa) to obtain silicon nitride sintered compacts.

[0085] The dimensions of the green compacts obtained in the examples and comparative examples and the dimensions of the silicon nitride sintered bodies were measured with a micrometer, and the shrinkage rates were calculated in the same manner as in Test Example 1. The results are shown in Table 5.

[0086] The obtained spherical silicon nitride sintered body was subjected to ball polishing to G5 in accordance with JIS B 1563 to prepare spherical test pieces of 3 / 8 inch (diameter 9.525 mm).

[0087] Using the test specimens obtained in the examples and comparative examples, the maximum diameter and area ratio of the inclusions (I), the maximum diameter of the pores, the fracture toughness, the crushing strength, and the rolling fatigue test were measured in the same manner as in Test Example 1. The results are shown in Table 5.

[0088] [Table 5]

[0089] Next, a comparison was made between test pieces prepared by wet granulation and test pieces prepared by dry blending. Example 24, which gave good results in both cases, was used as the test piece (see Tables 4 and 5).

[0090] <Measurement of maximum pore diameter> Using each test piece of Example 24, the maximum diameter of pores present in the surface layer was measured in the same manner as in Test Example 1 above. From the results in Tables 4 and 5 above, it can be seen that no pores with a diameter of more than 50 μm were present in the surface layer of each test piece. This time, an additional evaluation was conducted to determine whether or not pores with a diameter of 10 μm or more were present. The results are shown in Table 6.

[0091] <Rolling fatigue test> Using each test piece of Example 24, a rolling fatigue test was conducted under higher load conditions than in Test Example 1. The test conditions were the same as those in Test Example 1, except that the applied load was changed to 3.5 GPa and the test time was changed to 630 hours. The test pieces were evaluated for the presence or absence of peeling during the test time. The results are shown in Table 6.

[0092] [Table 6]

[0093] As shown in Table 6, Example 24 (wet) had voids with a diameter of 10 μm or more but less than 50 μm, whereas Example 24 (dry) had no voids with a diameter of 10 μm or more. Furthermore, Example 24 (dry) did not exhibit peeling during the rolling fatigue test under high load conditions. Compared to dry mixing, wet granulation using an organic binder or the like makes the granulated powder harder, making it less likely to be crushed by pressure, and it is thought that gaps are more likely to remain at the interface between the granulated powder particles in the compact. Depending on the usage pattern, this may result in defects in the sintered compact. In the rolling fatigue test of Example 24 (wet), the sintered compact was used as a rolling element under high surface pressure, and as a result, shedding occurred along the defects at the interface, which is thought to have led to a shortened lifespan.

[0094] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims rather than the above-described embodiments, and it is intended to include any modifications within the scope of the claims and meanings equivalent to the claims. [Industrial Applicability]

[0095] The silicon nitride sintered body of the present invention can be suitably used as a rolling element in bearings such as rolling bearings, linear guide bearings, ball screws, and linear bearings. [Explanation of symbols]

[0096] 1. Rolling bearings 2. Inner circle 3 outer ring 4 rolling elements 5 Cage 6 Sealing material 7. Grease 8a, 8b opening 11 Electric vertical take-off and landing aircraft 12 Main body 13 Drive unit 14 Rotor 15 Motor 16 Housing 17 Rotation axis 18 Inner ring spacer 19 Outer ring spacer 20 Nozzle member 21 Rolling bearings 22 Inner Circle 23 Outer ring 24 balls

Claims

1. A rolling element of a bearing, comprising a silicon nitride sintered body containing a rare earth element and an aluminum element, the content of the rare earth element is 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; the content of the aluminum element is 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; the silicon nitride sintered body has pores in a surface layer portion that is a region within 2 mm from the surface, the pores having a maximum diameter of 50 μm or less; The silicon nitride sintered body has a fracture toughness (in accordance with JIS R 1607) of 5 MPa·m 1 / 2 or more, A rolling element characterized in that, in a rolling fatigue test using the rolling element as a test piece, the surface of the rolling element does not peel off when rotated for 168 hours at a rotation speed of 3000 rpm and a load of 1.5 GPa in the presence of lubricating oil.

2. 2. The rolling element according to claim 1, wherein the rolling element is a ball.

3. A bearing using the rolling element according to claim 1 or 2.

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