Silicon nitride sintered body, mechanical part, and bearing

Abstract

To provide a silicon nitride sintered body capable of suppressing damage during polishing and obtaining a product excellent in mechanical properties, and to provide a mechanical part using the silicon nitride sintered body, and a bearing.SOLUTION: A silicon nitride sintered body comprises a rare earth element and an aluminum element, wherein: a content of the rare earth element is 4 wt.% or more and 15 wt.% or less in terms of oxide conversion with respect to the total weight of the silicon nitride sintered body; a content of the aluminum element is 4 wt.% or more and 15 wt.% or less in terms of oxide conversion with respect to the total weight of the silicon nitride sintered body; and √areamax of snowflakes present in a region within 500 μm or less from a surface of the silicon nitride sintered body is 30 μm or more and 200 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Silicon nitride (Si3N4) sintered bodies have excellent mechanical properties and corrosion resistance over a wide temperature range, from low to high, and are widely used in engine parts, cutting tools, sliding components, etc. In particular, as sliding components, they can reduce the coefficient of friction when lubricated and also provide excellent wear resistance, so in recent years there has been increasing demand for bearings and other products that use silicon nitride sintered bodies as rolling elements.

[0003] When silicon nitride sintered bodies are used as structural materials for rolling elements and the like, defects such as scratches, cracks, and pores can become the starting point of fracture, so measures are taken to eliminate such defects as much as possible. For example, Patent Document 1 describes that after sintering, the resulting silicon nitride sintered body is further subjected to hot isostatic pressing (HIP) treatment at temperatures of 1600°C to 1850°C in a non-oxidizing atmosphere of 300 atmospheres or more. This is said to further reduce the effect of pores in the sintered body, which can be the starting point of fatigue fracture.

[0004] In general, pores present near the surface of a sintered body are removed by polishing when the sintered body is finally processed into a product. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4795588 Summary of the Invention [Problem to be solved by the invention]

[0006] Various countermeasures against defects such as pores have been studied. However, since submicron defects can also be the starting point of fracture, their management is also important. One such defect is an aggregate of minute defects called snowflakes (white spots). Snowflakes mainly occur in the surface layer of sintered compacts.

[0007] In Patent Document 1, the reduction of pores in the sintered body is considered, but snowflakes are not considered. Furthermore, in Patent Document 1, the sintered body is finally polished, but depending on the size and density of the snowflakes present in the surface layer, this may have an adverse effect on the polishing process and the mechanical properties of the final product.

[0008] On the other hand, from the viewpoint of quality assurance, it is possible to increase the processing allowance (depth from the surface to be removed by processing) on ​​the surface of the sintered body, but in that case, there is a risk that the product will become expensive.

[0009] The present invention has been made in view of the above circumstances, and aims to provide a silicon nitride sintered body that can suppress damage during polishing and that can yield products with excellent mechanical properties, as well as a mechanical part and a bearing that use the same. [Means for solving the problem]

[0010] The silicon nitride sintered body of the present invention is a silicon nitride sintered body containing a rare earth element and an aluminum element, wherein the content of the rare earth element is 4% by weight or more and 15% by weight or less in terms of oxide relative to the total weight of the silicon nitride sintered body, and the content of the aluminum element is 4% by weight or more and 15% by weight or less in terms of oxide relative to the total weight of the silicon nitride sintered body, and the √area of ​​snowflakes present in a region within 500 μm from the surface of the silicon nitride sintered body is max is 30 μm or more and 200 μm or less.

[0011] Snowflakes are aggregates of minute defects, and as will be shown in the examples below, they can be observed as white spots by dark-field microscopic observation of the cross section of a sintered body. Snowflakes are believed to be formed by sink marks in the glass phase, which is made of sintering aids containing rare earth elements and aluminum elements. The √area of ​​the snowflakes max is preferably 50 μm or more.

[0012] The silicon nitride sintered body is characterized by containing at least one metal element selected from Ti, W, Mo, Ta, Nb, Fe and Cr.

[0013] The total content of the rare earth element and the aluminum element is 18% by weight or more and 26% by weight or less in terms of oxides relative to the total weight of the silicon nitride sintered body.

[0014] The rare earth element is Ce, and the silicon nitride sintered body is characterized in that it does not contain any transition metal elements.

[0015] The silicon nitride sintered body is characterized in that the crushing strength of a 3 / 8-inch ball obtained by polishing the silicon nitride sintered body is 20 kN or more.

[0016] The mechanical component of the present invention is a polished product of the silicon nitride sintered body of the present invention. The mechanical component is also characterized by being a rolling element for a bearing. The rolling element for a bearing has a maximum contact pressure of 3.6 GPa and a rotational speed of 3000 min -1 When the rolling life is measured using a radial bearing tester under the above conditions, the rolling life is characterized by being 1000 hours or more.

[0017] The bearing of the present invention is characterized by including the mechanical component of the present invention as a bearing member. [Effects of the Invention]

[0018] The silicon nitride sintered body of the present invention has a rare earth element and aluminum element content of 4% by weight or more and 15% by weight or less in terms of oxide, based on the total weight of the silicon nitride sintered body, and the √area of ​​snowflakes present in a region within 500 μm from the surface of the silicon nitride sintered body is max Since the particle size is 200 μm or less, the sintering aid contained in the silicon nitride sintered body is able to densify, while the size of the snowflakes formed in the surface layer is limited, which suppresses damage (cracks, etc.) during the polishing process, resulting in a product with excellent mechanical properties.

[0019] The mechanical component of the present invention is a polished product of the silicon nitride sintered body of the present invention, and therefore damage to the silicon nitride sintered body during polishing can be suppressed, resulting in a mechanical component with excellent mechanical properties. The mechanical component of the present invention is a rolling element for a bearing, and therefore peeling caused by damage during polishing can be suppressed, contributing to a longer life. Furthermore, the bearing of the present invention includes the mechanical component of the present invention as a bearing member, and therefore has excellent sliding properties, rolling life, etc. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a vertical cross-sectional view showing an example of a bearing of the present invention. [Figure 2] 1 shows cross-sectional images of silicon nitride sintered bodies of Examples and Comparative Examples. [Figure 3] 1 is an enlarged cross-sectional image of silicon nitride sintered bodies of Examples and Comparative Examples. [Figure 4] FIG. 1 is a diagram showing an outline of a snowflake evaluation method. [Figure 5] This is an observation image of the surface of the finished ball. [Figure 6] FIG. 1 is a diagram showing an outline of a crushing test. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment of the present invention will be described. (Silicon nitride sintered body) The silicon nitride sintered body of this embodiment is mainly composed of a crystalline silicon nitride phase and contains an appropriate amount of a glass phase (amorphous phase) made of a sintering aid containing a rare earth element and an aluminum element, etc. The silicon nitride sintered body in question is one whose surface has not been polished after sintering.

[0022] As described above, when a silicon nitride sintered body is finally processed into a product, its surface layer is removed by polishing. Meanwhile, micro-defects known as snowflakes are known to occur in silicon nitride sintered bodies. The present inventors have discovered that the size of snowflakes present in the surface layer of a sintered body affects the subsequent polishing process, and ultimately the mechanical properties of the product. The present invention is based on this finding.

[0023] In the silicon nitride sintered body of the present invention, the √area of ​​snowflakes present in the region within 500 μm from the surface max This √area max The calculation of will be explained below.

[0024] The cut surface of the silicon nitride sintered body is mirror-polished and the specimen is observed under a microscope (e.g., scanning electron microscope (SEM)) in a dark field to determine the reference area S0 (mm 2 ) image is acquired. The acquired image is then binarized using image analysis software, and the envelope area of ​​the snowflakes observed as white spots is analyzed. The largest of the acquired envelope areas is taken as the maximum snowflake envelope area within the reference area S0, and the square root of this is the √area max This measurement is repeated n times by changing the inspection area.

[0025] n measured √areas max Arrange them in ascending order, and then add √area max,j (j=1 to n) (see formula (1) below).

number

[0026] For each j (j = 1 to n), the cumulative distribution function F is expressed by the following formula (2): j (%) and the normalized variable y j Calculate.

number

number

[0027] On the extreme value probability sheet, the horizontal axis is √area max The above results are plotted to obtain the extreme value distribution (note that the vertical axis of the extreme value probability paper is F or y). The approximate line using the least squares method is extrapolated to the extreme value distribution to obtain a and b expressed in the following formula (4). Here, y is the normalized variable expressed in the following formula (5), T is the recurrence period expressed in the following formula (6), and V is the volume (mm 3 ), V0 is the reference volume (mm 3 ), h is the measured √area expressed by the following equation (8) max is the average value (mm).

number

number

number

number

number

[0028] Check that the plot points between 10 and 85% of the F scale, which is the vertical axis of the extreme value probability paper, fall on an approximate straight line. This confirms that the obtained extreme value distribution follows a double exponential distribution. Substitute the volume V (predicted volume) of the estimation target area into the above formula (8), and the point where the recurrence period T and the obtained extreme value distribution intersect is the square root of the maximum snowflake envelope area estimated to be contained within the predicted volume √area max is.

[0029] Snowflakes are scattered within a region of 500 μm from the surface of a silicon nitride sintered body, but the snowflakes that are the subject of the calculation are assumed to be entirely within a region of 500 μm from the surface. max is 200 μm or less, and may be 180 μm or less, 160 μm or less, or 120 μm or less. Note that snowflakes may exist within a region within 500 μm from the surface, and √area max may be 30 μm or more, 50 μm or more, or 80 μm or more.

[0030] Snowflakes are formed by, for example, sink marks in the glass phase, and as shown in the examples, in silicon nitride sintered bodies having a predetermined composition, their size can be adjusted by controlling the cooling rate of the sintered body immediately after sintering. For example, the √area of ​​snowflakes max When silicon nitride sintered bodies having a size exceeding 200 μm are polished, defects such as cracks are likely to occur starting from the relatively coarse snowflakes, which may result in defects remaining in the final product and may lead to a deterioration in the mechanical properties of the product.

[0031] Also, the snowflake's √area maxBy keeping the thickness below a certain level, damage during polishing and the impact on mechanical properties such as rolling fatigue characteristics can be suppressed. Therefore, it is not necessary to remove all of these snowflakes, and in the silicon nitride sintered body of the present invention, the processing allowance can be set to, for example, about 200 μm to 300 μm from the surface. By suppressing the processing allowance in this way, the finished product can be manufactured inexpensively and still satisfy bearing functions, etc.

[0032] The silicon nitride sintered body of this embodiment contains a rare earth element and an aluminum element. In the silicon nitride sintered body, the content of the rare earth element is preferably 4 to 15 wt % in terms of oxide, based on the total weight of the silicon nitride sintered body, and the content of the aluminum element is preferably 4 to 15 wt % in terms of oxide, based on the total weight of the silicon nitride sintered body. By having the contents of the rare earth element and the aluminum element within these ranges, the silicon nitride sintered body can be easily densified.

[0033] In addition, silicon powder (metallic silicon powder), which is cheaper than silicon nitride powder, is used as the starting material, and silicon nitride sintered body is manufactured by reactive sintering (PS-RBSN (Post-Sintering of Reaction When the nitriding is carried out by the Bonded Silicon-Nitride method, the nitriding reaction is promoted and the silicon nitride sintered body is easily densified.

[0034] Examples of rare earth elements include yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), neodymium (Nd), dysprosium (Dy), europium (Eu), and erbium (Er). These elements may be contained alone or in combination of two or more. Among these, it is preferable to contain one or more elements selected from the group consisting of Y, La, and Ce. For example, when silicon powder is used as the raw material, it is preferable to contain Ce in order to promote nitridation.

[0035] The content of the rare earth element may be 5% by weight or more, 6.5% by weight or more, or 10% by weight or more, calculated as oxide, relative to the total weight of the silicon nitride sintered body. The content of the rare earth element may be 15% by weight or less, 14% by weight or less, or 12% by weight or less.

[0036] The aluminum content may be 5% by weight or more, 6.5% by weight or more, or 10% by weight or more, calculated as oxide, relative to the total weight of the silicon nitride sintered body. The aluminum content may be 15% by weight or less, 14% by weight or less, or 12% by weight or less.

[0037] The aluminum element content (in terms of oxide) relative to the rare earth element content (in terms of oxide) may be contained in a ratio of 1:3 to 3:1, or may be contained in a ratio of 1:2 to 2:1. The aluminum element content (in terms of oxide) may be within ±5% by weight, ±2% by weight, or ±1% by weight of the rare earth element content (in terms of oxide), or may be the same as the rare earth element content.

[0038] By keeping the rare earth and aluminum contents in the silicon nitride sintered body within the above ranges, for example, when producing a silicon nitride sintered body by the PS-RBSN method, the nitriding reaction of the raw material silicon powder can be promoted, and subsequent sintering can be promoted. The rare earth and aluminum contents can be adjusted by the amount of sintering aid containing rare earth elements (e.g., rare earth oxides) and sintering aid containing aluminum elements (e.g., aluminum oxide) added to the raw materials.

[0039] The total content of rare earth elements and aluminum elements is, for example, 8 to 26% by weight, calculated as oxides, relative to the total weight of the silicon nitride sintered body. For example, when silicon powder is used as the raw material, the total content may be 18 to 26% by weight, or 18 to 22% by weight.

[0040] 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.

[0041] The silicon nitride sintered body may contain components derived from sintering aids other than rare earth elements and aluminum. Specifically, it may contain at least one metal element selected from titanium (Ti), tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Nb), iron (Fe), and chromium (Cr). These metal elements are added to the raw material powder, for example, as simple metals, oxides of metal elements, or nitrides of metal elements.

[0042] The content of the above metal elements (the total amount when two or more elements are present) is, for example, 0.1 to 5 mass %, preferably 0.5 to 3 mass %, and may be 1 to 3 mass % in terms of oxide, relative to the total weight of the silicon nitride sintered body. This content can be calculated in the same manner as the content of the rare earth element and aluminum element described above.

[0043] Furthermore, when silicon powder is used as the raw material, the silicon nitride sintered body preferably does not contain transition metal elements, which are elements contained in groups 3 to 11 of the IUPAC periodic table, such as Ti, Fe, and Cr.

[0044] The silicon nitride sintered body of the present invention has excellent mechanical properties, and preferably has a crushing strength of 20 kN or more when a 3 / 8-inch ball obtained by polishing the silicon nitride sintered body is used. The crushing strength is, for example, 30 kN or less. The crushing strength can be measured, for example, by the two-ball crushing test described in the Examples below.

[0045] 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.

[0046] (Production of sintered silicon nitride) The silicon nitride sintered body described above may be produced using silicon nitride powder as a starting material, but is preferably produced using inexpensive silicon powder by the PS-RBSN method (two-stage sintering method). In this case, the method for producing the silicon nitride sintered body includes a mixing step of mixing raw material powder containing silicon powder and a sintering aid, a molding step of molding the mixed raw material powder into a green compact, a sintering step of sintering the green compact, and a cooling step of cooling the sintered body immediately after sintering.

[0047] In the mixing step, for example, the raw material powders are mixed dry without using water or an organic solvent. In this case, it is preferable to mix without using a binder component. The particle size of the powder after mixing is not particularly limited, but it is preferable that D90 is 10 μm or more and 100 μm or less. It is also preferable that D50 is 2 μm or more and 10 μm or less. By having D90 and / or D50 within the above range, it is possible to obtain a dense silicon nitride sintered body while exhibiting good fluidity and moldability. Note that D50 and D90 are the cumulative 50% diameter and cumulative 90% diameter on a volume basis, respectively, and are obtained by laser diffraction / scattering particle size distribution measurement, etc.

[0048] Alternatively, wet granulation may be performed in the mixing step to obtain a granulated powder as a mixture. In this case, 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 a granulated powder. An organic binder or the like is used as the binder component, and is added in an amount of, for example, 1% to 10% by weight based on the total raw material powder.

[0049] In the molding step, the mixture obtained in the mixing step is molded into a predetermined shape using a known molding method such as cold isostatic pressing (CIP) or press molding to obtain a green compact. For example, in the case of a spherical shape, a spherical green compact may be formed and then processed in a green processing device to obtain a green sphere. In addition, a debinding step may be performed as necessary.

[0050] In the sintering step, the obtained green compact is sintered by heat treatment in a nitrogen atmosphere, for example, at 1600°C to 1800°C (preferably 1750°C to 1800°C), by holding it for a predetermined time. The sintering time is set, for example, to 3 hours to 10 hours. Applicable sintering methods include atmospheric pressure sintering, atmospheric pressure sintering, and pressure sintering (hot pressing). In atmospheric pressure sintering, the pressure is set, for example, to 0.1 MPa to 10 MPa. Furthermore, in the sintering step, primary sintering and secondary sintering may be performed under different pressures.

[0051] In the production of the silicon nitride sintered body, the sintering aid used in the raw material powder can contain rare earth elements, aluminum elements, and transition metal elements. Sintering aids containing rare earth elements are preferably Y2O3, La2O3, or CeO2. Sintering aids containing aluminum elements preferably contain Al2O3. Sintering aids containing transition metal elements include Cr2O3, TiO2, MnO, and Fe2O3.

[0052] 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.

[0053] The content of silicon powder in the raw material powder is preferably more than 60 wt%, more preferably 65 wt% or more, and may be 70 wt% or more, based on the total weight of silicon powder, silicon nitride powder, and sintering aid. It is also preferably less than 85 wt%, more preferably 80 wt% or less. It is preferable that the raw material powder does not contain silicon nitride powder.

[0054] 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 may be 10 wt % or more, 15 wt % or more, or 20 wt % or less, based on the total weight. The content of the sintering aid containing an aluminum element (e.g., aluminum oxide) contained in the raw material powder may be 10 wt % or more, 15 wt % or more, or 20 wt % or less, based on the total weight. If the content of the sintering aid contained in the raw material powder is low, it is difficult to obtain a dense silicon nitride sintered body, and if the content of the sintering aid is high, the mechanical properties of the silicon nitride sintered body are likely to deteriorate.

[0055] The average particle size of the silicon powder contained in the raw material powder can be, for example, 5 μm or less. When silicon nitride powder is contained, its average particle size 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 preferably 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. The average particle size is the cumulative 50% diameter on a volume basis and can be obtained by laser diffraction / scattering particle size distribution measurement or the like.

[0056] The method for producing a silicon nitride sintered body of this embodiment includes a mixing step of mixing a raw material powder containing silicon powder and a sintering aid containing a rare earth oxide and aluminum oxide, a molding step of molding the mixed raw material powder into a green compact, a sintering step of sintering the green compact, and a cooling step of cooling the sintered body immediately after sintering. The sintering step is preferably a heat treatment step in which the sintered body is held in a nitrogen atmosphere at a sintering temperature of 1600°C to 1800°C for a predetermined time.

[0057] In addition, the cooling step is preferably carried out at a cooling rate of 10°C / min or more for the sintered body immediately after sintering. The cooling rate may be 12°C / min or more and 30°C / min or less, or 12°C / min or more and 20°C / min or less. Normally, the furnace is naturally cooled after sintering, but in this cooling step, intentionally cooling relatively rapidly makes it easier to control the size of the snowflakes, as shown in the examples, and as a result, the √area max The above cooling rate can be achieved by, for example, controlling the cooling and heating by circulating cooling water. It is believed that increasing the cooling rate to achieve relatively rapid cooling makes it difficult for the glass phase to crystallize, and suppresses snowflakes that occur due to volumetric shrinkage caused by crystallization, resulting in smaller snowflakes.

[0058] In the present invention, it is preferable not to perform HIP treatment on the sintered body obtained after the sintering step. This HIP treatment is performed, for example, by holding the body at a temperature of 1500°C to 1700°C for a predetermined time under a gas pressure of 100 MPa or more. By cooling at a predetermined cooling rate in the cooling step, the size of the snowflakes can be controlled to be small without performing HIP treatment, and the increase in costs associated with HIP treatment can be avoided.

[0059] Furthermore, when the PS-RBSN method is performed, the rare earth oxide is preferably CeO2. Furthermore, the raw material powder preferably contains the rare earth oxide and the aluminum oxide in total in an amount of 20% by weight or more and 35% by weight or less (more preferably 25% by weight or more and 35% by weight or less) based on the entire raw material powder. Furthermore, the raw material powder preferably does not contain a transition metal oxide. Furthermore, the above-mentioned numerical ranges can be appropriately combined with the above method.

[0060] (Applications of sintered silicon nitride) The silicon nitride sintered body of this embodiment is polished to meet predetermined product specifications to obtain a silicon nitride sintered body product. While the application of this product is not particularly limited, due to its excellent mechanical properties, it is preferably used as a mechanical part. Mechanical parts are used, for example, in rolling or sliding parts. The mechanical part of the present invention is a part that uses the polished silicon nitride sintered body of the present invention as part or all of its structure. Examples of mechanical parts include sliding members, bearing members, rolling roll materials, compressor vanes, gas turbine blades, and other engine parts, as well as cutting tools (chips). Examples of bearing members include raceways such as inner and outer rings, rolling elements for bearings, and cages. The bearing of the present invention is a bearing that includes this mechanical part as part or all of its bearing member, such as a rolling bearing, a sliding bearing (e.g., a spherical bushing), a linear guide bearing, a ball screw, and a linear bearing. In particular, the bearing of the present invention is preferably a rolling bearing that uses the polished silicon nitride sintered body as the rolling element.

[0061] An example of a bearing according to 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, with a plurality of balls (rolling elements) 4 arranged between inner ring raceway surface 2a and outer ring raceway surface 3a. These balls 4 are polished products of the silicon nitride sintered body described above. Balls 4 are held in cage 5. Openings 8a, 8b at both axial ends of the inner and outer rings are sealed by sealing members 6, and a grease composition 7 is enclosed around at least balls 4. Grease composition 7 is present on the raceway surfaces of balls 4 to provide lubrication.

[0062] The type of rolling bearing is not limited to a deep groove ball bearing, but may be an angular contact ball bearing, a thrust ball bearing, or the like. In the case of a roller bearing, the above-mentioned polished silicon nitride sintered body may be applied to the rollers. [Example]

[0063] 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.

[0064] The raw material powders used in the following test examples are shown in Table 1. Silicon nitride powder was used as the raw material in Examples 1 and 2 and Comparative Examples 1 and 2. On the other hand, silicon powder was used as the raw material in Examples 3 and 4 and Comparative Examples 3 and 4, and silicon nitride sintered bodies were produced by the PS-RBSN method.

[0065] [Example 1] A specified amount of organic binder was added to the raw material powder with the compounding ratio shown in Table 1, and mixed using a ball mill or other device. After press molding at a molding pressure of 30 MPa, a green body was produced using the CIP method at 200 MPa. The resulting green body was degreased in a degreasing furnace and then pressure sintered at 1750°C for 4 hours in a nitrogen atmosphere at 0.9 MPa. The sintered body was cooled to 1000°C at a rate of 15°C / min to obtain a spherical silicon nitride sintered body.

[0066] [Example 2] A specified amount of organic binder was added to the raw material powders with the compounding ratio shown in Table 1, and after mixing using a ball mill or similar, a compact was produced using the CIP method at 200 MPa. The resulting compact was degreased in a degreasing furnace and then pressure sintered at 1750°C for 4 hours in a nitrogen atmosphere at 0.9 MPa. The sintered compact was cooled to 1000°C at a rate of 15°C / min to obtain a spherical silicon nitride sintered compact.

[0067] [Examples 3 to 4] The raw material powders with the compounding ratios shown in Table 1 were press-molded at a molding pressure of 30 MPa, and then a compact was produced using the CIP method at 200 MPa. The resulting compact was pressure-sintered at 1750°C for 10 hours in a nitrogen atmosphere at 0.9 MPa. The sintered compact was cooled to 1000°C at a rate of 15°C / min to obtain a spherical silicon nitride sintered compact.

[0068] [Comparative Example 1] A specified amount of organic binder was added to the raw material powder with the compounding ratio shown in Table 1, and mixed using a ball mill or other device. After press molding at a molding pressure of 30 MPa, a green body was produced using the CIP method at 200 MPa. The resulting green body was degreased in a degreasing furnace and then pressure sintered at 1750°C for 4 hours in a nitrogen atmosphere at 0.9 MPa. The sintered body was cooled to 1000°C at a rate of 5°C / min to obtain a spherical silicon nitride sintered body.

[0069] Comparative Example 2 A specified amount of organic binder was added to the raw material powders with the compounding ratio shown in Table 1, and after mixing using a ball mill or similar, a compact was produced using the CIP method at 200 MPa. The resulting compact was degreased in a degreasing furnace and then pressure sintered at 1750°C for 4 hours in a nitrogen atmosphere at 0.9 MPa. The sintered compact was then cooled to 1000°C at a rate of 5°C / min to obtain a spherical silicon nitride sintered compact.

[0070] [Comparative Examples 3 to 4] The raw material powders with the compounding ratios shown in Table 1 were press-molded at a molding pressure of 30 MPa, and then a compact was produced using the CIP method at 200 MPa. The resulting compact was pressure-sintered at 1750°C for 10 hours in a nitrogen atmosphere at 0.9 MPa. The sintered compact was cooled to 1000°C at a rate of 5°C / min to obtain a spherical silicon nitride sintered compact.

[0071] [Table 1]

[0072] For Examples 3 and 4 and Comparative Examples 3 and 4, the composition ratios of each oxide in the obtained silicon nitride sintered bodies were calculated from the composition ratios of the raw material powders, assuming that all of the silicon (metallic silicon) contained in the raw material powders was nitrided and that the weight of silicon nitride was 1.67 times the weight of silicon. These values ​​are shown in Table 2.

[0073] [Table 2]

[0074] <Cross-section observation of silicon nitride sintered body> The silicon nitride sintered bodies of Examples 1 to 4 and Comparative Examples 1 to 4 were cut and cross-sectional observation was performed using an SEM. As representative examples, cross-sectional images of Examples 1, 4, and Comparative Example 3 are shown in Figure 2. For each example, bright-field and dark-field images are shown. Note that snowflakes cannot be observed in bright-field, but can be observed in dark-field.

[0075] 2, the presence of snowflakes was hardly observed in the surface layer within 500 μm from the surface for the silicon nitride sintered bodies of Examples 1 and 4. On the other hand, the presence of snowflakes, which are white spots, was observed in Comparative Example 3.

[0076] Figure 3 shows images of the surface layer at a higher magnification. The left column shows images corresponding to the Examples, and the right column shows images corresponding to the Comparative Examples. The images in the lower row are even higher magnifications. As shown in Figure 3, snowflakes are present in the surface layer in the Examples, but their size is smaller than that of the Comparative Examples. As such, it is believed that snowflakes are generated by the cooling rate immediately after sintering, and that their size changes depending on the rate of cooling. Furthermore, as will be described later, the snowflakes present in the Examples did not affect the polishing process or bearing life tests using polished balls.

[0077] <Quantitative evaluation of snowflakes in silicon nitride sintered bodies> For the silicon nitride sintered bodies of Examples 1 to 4 and Comparative Examples 1 to 4, the snowflakes were quantitatively evaluated by the √area max Image processing was performed to calculate the σ. The image analysis software WinRoof2021 was used for image processing. An outline of the image analysis method is shown in Figure 4. The observation field at 500x magnification was 0.5 mm vertical x 0.688 mm horizontal, and images of 8 locations x 5 spheres were acquired.

[0078] Snowflake √area max was calculated by the method using the above-mentioned formulas (1) to (8). In this example, the reference area S0 was set to 0.334 mm 2 , the number of inspections n is 40, and the predicted volume V is 3456 mm 3 The evaluation results are shown in Table 3.

[0079] <Surface condition of silicon nitride balls after polishing> The silicon nitride sintered bodies of Examples 1 to 4 and Comparative Examples 1 to 4 were polished (removal allowance: 0.2 to 0.25 mm) to produce 3 / 8-inch (9.525 mm diameter) ceramic balls. These ceramic balls are finished spheres and correspond to the mechanical parts of the present invention. The surface condition of the polished ceramic balls was confirmed using a field emission scanning electron microscope (FE-SEM). A representative example is shown in Figure 5. While almost no cracks were observed in Examples 1 and 3, multiple cracks occurred in Comparative Example 3. This is thought to be due to defects originating from relatively coarse snowflakes during the polishing process.

[0080] <Crushing test> A two-ball crushing test was conducted using each ceramic ball obtained by the above-mentioned polishing process. The crushing test conformed to JIS B 1501. As shown in Figure 6, the testing machine had a fixed jig 9 and a movable jig 10, which was moved up and down by a crosshead 11. Conical depressions were formed in each of the fixed jig 9 and the movable jig 10, and two test balls 12 were set between these depressions. The stroke speed of the crosshead 11 was 1.0 × 10 mm / min. The load at which the test balls 12 were crushed was measured. A crushing strength of 20 kN or more was considered to be acceptable. The results are also shown in Table 3.

[0081] <Bearing life test> A bearing life test was conducted to confirm the rolling contact fatigue properties of each ceramic ball obtained by the above polishing process. Using each ceramic ball, bearing outer ring, bearing inner ring, and cage "6206" manufactured by NTN Corporation, a bearing life (rolling life) test was conducted under the following conditions. The cutoff time for the bearing life test was set at 1000 hours, and a test exceeding 1000 hours was considered a pass. The results are also shown in Table 3. Load (kN): Fr=13.72 (6.86kN / brg) Maximum contact pressure (GPa): Inner ring-ball: 3.5, Outer ring-ball: 3.6 Rotation speed (min -1 ):3000 Lubricant: JX Energy Corporation additive-free turbine oil VG56 Lubricating oil supply temperature (℃): 50 Oil supply method: Clean oil circulation

[0082] [Table 3]

[0083] As shown in Table 3, the √area of ​​the snowflake max In the case of the finished balls using the silicon nitride sintered bodies of Examples 1 to 4 with a diameter of 200 μm or less, the crushing strength and rolling fatigue properties were good. Among Examples 1 to 4, no difference in mechanical properties was observed due to differences in manufacturing methods such as the PS-RBSN method. However, Examples 3 and 4, which contained a relatively large amount of sintering aid, showed a higher √area of ​​snowflakes than Examples 1 and 2. max This resulted in significant results.

[0084] In this way, by controlling the size of snowflakes, which affect the polishing process and rolling fatigue life, it was possible to obtain a bearing rolling element that is inexpensive and has satisfactory bearing function.

[0085] 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]

[0086] The silicon nitride sintered body of the present invention can be prevented from being damaged during polishing and can be produced as a product with excellent mechanical properties, ultimately providing a silicon nitride product with excellent mechanical properties and sliding properties. Polished products of the silicon nitride sintered body can be suitably used as rolling elements in bearings such as rolling bearings, linear guide bearings, ball screws, and linear bearings. [Explanation of symbols]

[0087] 1. Rolling bearings 2. Inner circle 3 outer ring 4 rolling elements 5 retainer 6 seal member 7 grease 8a, 8b openings 9 fixing jig 10 movable jig 11 cross head 12 test ball

Claims

1. A silicon nitride sintered body containing a rare earth element and an aluminum element, the content of the rare earth element is 4% by weight or more and 15% by weight or less in terms of oxide relative to the total weight of the silicon nitride sintered body, and the content of the aluminum element is 4% by weight or more and 15% by weight or less in terms of oxide relative to the total weight of the silicon nitride sintered body, The √area of ​​snowflakes present in the region within 500 μm from the surface of the silicon nitride sintered body ｍａｘ A silicon nitride sintered body characterized in that the particle size is 30 μm or more and 200 μm or less.

2. 2. The silicon nitride sintered body according to claim 1, wherein the silicon nitride sintered body contains at least one metal element selected from the group consisting of Ti, W, Mo, Ta, Nb, Fe and Cr.

3. 2. The silicon nitride sintered body according to claim 1, wherein the total content of the rare earth element and the aluminum element is 18% by weight or more and 26% by weight or less, calculated as oxides, based on the total weight of the silicon nitride sintered body.

4. 4. The silicon nitride sintered body according to claim 3, wherein said rare earth element is Ce, and said silicon nitride sintered body does not contain any transition metal element.

5. 3. The silicon nitride sintered body according to claim 1, wherein the crushing strength of a polished 3 / 8-inch ball of said silicon nitride sintered body is 20 kN or more.

6. 3. A machine part which is a polished product of the silicon nitride sintered body according to claim 1 or 2.

7. 7. The mechanical part according to claim 6, wherein the mechanical part is a rolling element for a bearing.

8. The rolling elements for the bearing have a maximum contact pressure of 3.6 GPa and a rotation speed of 3000 min －１ 8. The machine part according to claim 7, wherein the rolling life is 1000 hours or more when measured under the above conditions using a radial bearing tester.

9. A bearing using the rolling element for bearing according to claim 7.

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