Silicon nitride sintered body and wear-resistant member using same

Abstract

A silicon nitride sintered body according to an embodiment of the present invention contains, in a measurement region of 20 μm × 20 μm in an arbitrary cross section of a silicon nitride sintered body comprising a plurality of silicon nitride crystal particles and a grain boundary phase: (A) 80-99% first silicon nitride crystal particles relative to the total number of the plurality of silicon nitride crystal particles, the first silicon nitride crystal particles being silicon nitride crystal particles having an O / Si atom ratio of 0.001-0.030 from among the plurality of silicon nitride crystal particles; and (B) 1-20% second silicon nitride crystal particles relative to the total number of the plurality of silicon nitride crystal particles, the second silicon nitride crystal particles being silicon nitride crystal particles having an O / Si atom ratio of greater than 0.030 to 0.200 from among the plurality of silicon nitride crystal particles.

Description

Silicon nitride sintered body and wear-resistant member using same

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

[0002] Silicon nitride sintered bodies are used as structural materials due to their excellent mechanical properties, and their use as wear-resistant members is a major example. Wear-resistant members made of silicon nitride sintered bodies are used in a variety of fields, including machine tools, electronic devices, automobiles, aircraft, and wind power generation. For example, wear-resistant members are used in bearing balls, rollers, various roll materials for rolling mills, compressor vanes, gas turbine blades, cam rollers, and other engine parts, as well as friction stir welding tool members.

[0003] The mechanical properties of silicon nitride sintered bodies, such as hardness and fracture toughness, generally provide excellent durability and reliability in wear-resistant component applications, but make them difficult to process during manufacturing. Therefore, there is a demand for silicon nitride sintered bodies that have both excellent mechanical properties and excellent processability.

[0004] In response to such demands, for example, Japanese Patent No. 5002155 (Patent Document 1) proposes a wear-resistant member made of a silicon nitride sintered body, which achieves both workability and mechanical properties by controlling the composition of the sintered body and the manufacturing process.

[0005] However, the compatibility between workability and mechanical properties in conventional sintered silicon nitride is not necessarily satisfactory, and further improvement is required.

[0006] Patent No. 5002155

[0007] The problem to be solved by the embodiments is to provide a silicon nitride sintered body having excellent processability and mechanical properties, and a wear-resistant member using the same.

[0008] The silicon nitride sintered body according to the embodiment contains, in a 20 μm × 20 μm measurement region in any cross section of a silicon nitride sintered body comprising a plurality of silicon nitride crystal grains and a grain boundary phase, (A) first silicon nitride crystal grains having an O / Si atomic ratio of 0.001 or more and 0.030 or less, in a number ratio of 80% to 99% relative to the total number of silicon nitride crystal grains, and (B) second silicon nitride crystal grains having an O / Si atomic ratio of more than 0.030 and 0.200, in a number ratio of 1% to 20% relative to the total number of silicon nitride crystal grains.

[0009] 1 is a schematic diagram showing an example of a cross-sectional structure of a silicon nitride sintered body according to an embodiment, an external view showing an example of a bearing ball made of a silicon nitride sintered body according to an embodiment, and an external view showing an example of a base sphere for a bearing ball made of a silicon nitride sintered body according to an embodiment. Embodiment

[0010] The silicon nitride sintered body according to the embodiment contains, in a 20 μm × 20 μm measurement region in any cross section of a silicon nitride sintered body comprising a plurality of silicon nitride crystal grains and a grain boundary phase, (A) first silicon nitride crystal grains having an O / Si atomic ratio of 0.001 or more and 0.030 or less, in a number ratio of 80% to 99% of the plurality of silicon nitride crystal grains relative to the entire plurality of silicon nitride crystal grains, and (B) second silicon nitride crystal grains having an O / Si atomic ratio of more than 0.030 and 0.200, in a number ratio of 1% to 20% of the plurality of silicon nitride crystal grains relative to the entire plurality of silicon nitride crystal grains.

[0011] Hereinafter, embodiments will be described with reference to the drawings. FIG. 1 is a cross-sectional schematic diagram of a silicon nitride sintered body 1 according to an embodiment. In the figure, reference numeral 1 denotes a silicon nitride sintered body, reference numeral 2 denotes silicon nitride crystal grains, and reference numeral 3 denotes a grain boundary phase. Reference numeral 21 denotes first silicon nitride crystal grains 2 having an atomic ratio of oxygen atoms O to silicon atoms Si (O / Si atomic ratio) of 0.001 or more and 0.030 or less. Reference numeral 22 denotes second silicon nitride crystal grains 2 (O-rich silicon nitride crystal grains 2) having an O / Si atomic ratio of more than 0.030 and less than 0.200. Reference numeral 23 denotes third silicon nitride crystal grains 2 having an O / Si atomic ratio of less than 0.001. Reference numeral 24 denotes fourth silicon nitride crystal grains 2 (even more O-rich silicon nitride crystal grains 2) having an O / Si atomic ratio of more than 0.200. In the silicon nitride crystal grains 2, the presence of the third silicon nitride crystal grains 23 is not essential, and the presence of the fourth silicon nitride crystal grains 24 is also not essential. Unless otherwise specified, the following description will be given assuming that the third silicon nitride crystal grains 23 and the fourth silicon nitride crystal grains 24 are not present in the silicon nitride crystal grains 2. The silicon nitride sintered body 1 has silicon nitride crystal grains 2 and a grain boundary phase 3. The silicon nitride crystal grains 2 refer to a crystal layer containing silicon nitride as a main component, and a solid solution thereof.

[0012] The silicon nitride sintered body 1 contains, as silicon nitride crystal grains 2, first silicon nitride crystal grains 21 and second silicon nitride crystal grains 22 in a 20 μm × 20 μm region (referred to as the "measurement region" in this specification) in any cross section, as measured by a method described below. The silicon nitride sintered body 1 contains first silicon nitride crystal grains 21 having an O / Si atomic ratio of 0.001 or more and 0.030 or less, with a number ratio R1 of 80% to 99% relative to the total silicon nitride crystal grains 2 (the following formula (1)). On the other hand, the silicon nitride crystal grains 2 contain second silicon nitride crystal grains 22 having an O / Si atomic ratio of more than 0.030 and 0.200 or less, with a number ratio R2 of 1% to 20% relative to the total silicon nitride crystal grains 2 (the following formula (2)). Preferably, the number ratio R2 is 4% to 20%. 80%≦R1≦99% (1) 1%≦R2≦20% (2)

[0013] The O / Si atomic ratio and other atomic ratios described below refer to values ​​obtained by the measurement methods described below. The sum of the number ratio R1 of first silicon nitride crystal particles 21 and the number ratio R2 of second silicon nitride crystal particles 22 does not exceed 100% by definition.

[0014] In the measurement region, the number ratio R3 of third silicon nitride crystal particles 23 having an O / Si atomic ratio of less than 0.001 to the total number of silicon nitride crystal particles 2 is preferably 0% or more and 1% or less (the following formula (3)). More preferably, the number ratio R3 of third silicon nitride crystal particles 23 is 0%. Note that R3 of 0% includes values ​​below the detection limit. 0%≦R3≦1% (3)

[0015] In the measurement region, the proportion R4 of fourth silicon nitride crystal grains 24 having an O / Si atomic ratio exceeding 0.200 to the total silicon nitride crystal grains 2 is preferably 0% or more and 1% or less (the following formula (4)). More preferably, the proportion R4 of fourth silicon nitride crystal grains 24 is 0%. 0%≦R4≦1% (4)

[0016] In other words, the sum of the number ratios R1, R2 of the first silicon nitride crystal grains 21 and the second silicon nitride crystal grains 22 is preferably 98% or more and 100% or less, and more preferably 100%.

[0017] The number ratios R1 and R2 of the first silicon nitride crystal grains 21 and the second silicon nitride crystal grains 22 are controlled by appropriately promoting the amount of dissolved oxygen and its variation based on the raw material composition, manufacturing conditions, etc., which will be described later. The same applies to the preferred solid solution states of Al (aluminum), W (tungsten), and Fe (iron), which will be described later.

[0018] In silicon nitride sintered body 1, when the oxygen solid solution of silicon nitride crystal grains 2 is controlled so as to satisfy the above formulas (1) to (4) (particularly formulas (1) and (2)), the second silicon nitride crystal grains 22 with a large amount of dissolved oxygen are scattered in the sintered body structure, resulting in partial structural strengthening. The reason for this partial structural strengthening is that silicon nitride crystal grains 2 with a moderately large amount of dissolved oxygen, i.e., second silicon nitride crystal grains 22 (e.g., sialon, as described below), have improved mechanical properties, including hardness, compared to first silicon nitride crystal grains 21. Furthermore, the second silicon nitride crystal grains 22 with a large amount of dissolved oxygen have an oxygen content close to that of the grain boundary phase, which is thought to be a factor in strengthening the bond with the grain boundary phase. This partial structural strengthening improves the balance between strength and toughness of silicon nitride sintered body 1. Furthermore, stress propagation within the material of silicon nitride sintered body 1 is controlled, thereby suppressing crack progression.

[0019] The reason why the first silicon nitride crystal grains 21 and the second silicon nitride crystal grains 22 are separated by the O / Si atomic ratio is that the larger the O / Si atomic ratio, the closer the grains become to sialon crystals.

[0020] In the silicon nitride sintered body 1, when the silicon nitride crystal grains 2 satisfy the above formulas (1) to (4) (particularly the above formulas (1) and (2)), the following effects can be obtained, for example.

[0021] First, the processing efficiency of the silicon nitride sintered body 1 is improved. Generally, in the manufacturing process of the silicon nitride sintered body 1, processing such as polishing is performed after sintering in order to process it into a desired shape and remove scratches and cracks that occur near the surface. In this processing, increasing the processing speed increases damage to the silicon nitride sintered body 1 and tools, so the processing speed is limited. However, when the silicon nitride sintered body 1 satisfies the above formulas (1) to (4), damage during this processing is reduced, making it possible to increase the processing speed. In particular, the silicon nitride sintered body 1 according to the embodiment has excellent resistance to grain shedding in the brittle mode, and therefore has excellent machinability.

[0022] The polishing of silicon nitride sintered body 1 utilizes brittle mode and ductile mode. Polishing in brittle mode turns chips into powder. Polishing in ductile mode turns chips into larger lumps than powder. Polishing in ductile mode allows a large amount to be removed at once. In brittle mode, the amount of material removed is small, so a flat surface can be obtained. In the case of silicon nitride sintered body 1 according to the embodiment, it is resistant to grain shedding (hard to shed grains) during polishing in brittle mode, so the amount of material removed can be reduced and a flat surface can be obtained in a shorter time.

[0023] Second, it improves the wear resistance of the silicon nitride sintered body 1. Third, it reduces the amount of machining required during processing because it suppresses the progression of cracks from the surface to the interior of the silicon nitride sintered body 1. Fourth, it improves yield because defects caused by external damage are less likely to occur during the manufacturing process of the silicon nitride sintered body 1.

[0024] A silicon nitride sintered body 1 in which the silicon nitride crystal grains 2 satisfy the above formulas (1) to (4) (particularly the above formulas (1) and (2)) has an excellent balance between strength and toughness. On the other hand, if the number proportion R1 of the first silicon nitride crystal grains 21 in the measurement region is less than 80%, the silicon nitride sintered body 1 may not have desirable mechanical properties. Furthermore, if the number proportion R1 of the first silicon nitride crystal grains 21 is less than 80%, the workability of the silicon nitride sintered body 1 decreases.

[0025] Furthermore, if the number ratio R1 of the first silicon nitride crystal grains 21 in the silicon nitride sintered body 1 in the measurement region exceeds 99% (i.e., if the number ratio R2 of the second silicon nitride crystal grains 22 is 1% or less), there is a risk that partial structural strengthening in the sintered body structure will be insufficient and the desired characteristics will not be obtained.

[0026] In addition to the ranges indicated by the above formulas (1) to (4), it is more preferable that the first silicon nitride crystal grains 21 of the silicon nitride sintered body 1 do not include any grains having an O / Si atomic ratio of less than 0.010 in the measurement region. That is, the first silicon nitride crystal grains 21 preferably have an O / Si atomic ratio of 0.010 or more and 0.030 or less. If the first silicon nitride crystal grains 21 of the silicon nitride sintered body 1 include grains having an O / Si atomic ratio of less than 0.010, the difference in hardness between the first silicon nitride crystal grains 21 and the second silicon nitride crystal grains 22 will be large, which may result in warping and a decrease in the structural strength. On the other hand, if the first silicon nitride crystal grains 21 do not include any grains having an O / Si atomic ratio of less than 0.010, the silicon nitride sintered body 1 can be considered to be uniformly sintered.

[0027] In the measurement region of the silicon nitride sintered body 1, silicon nitride crystal grains 2 preferably contain 10% or more silicon nitride crystal grains 2 having an atomic ratio of aluminum atoms Al to silicon atoms Si (Al / Si atomic ratio) of 0.001 or more and 0.010 or less, as expressed as a number ratio S1 relative to all silicon nitride crystal grains 2 (the following formula (5)): 10%≦S1 (5)

[0028] Additionally, the silicon nitride crystal grains 2 of the silicon nitride sintered body 1 preferably have an Al / Si atomic ratio of 0.001 or more and 0.100 or less in the measurement region. More preferably, the silicon nitride crystal grains 2 of the silicon nitride sintered body 1 preferably contain 10% or more silicon nitride crystal grains 2 having an Al / Si atomic ratio of 0.010 or more and 0.100 or less (silicon nitride crystal grains 2 with a relatively high Al content) in terms of number fraction S2 (the following formula (6)). The upper limit of the number fraction of silicon nitride crystal grains 2 having an Al / Si atomic ratio of 0.01 or more and 0.1 or less is 100% or less. 10%≦S2 (6)

[0029] Furthermore, it is more preferable that the silicon nitride crystal grains 2 have an Al / Si atomic ratio of 0.030 or more (i.e., silicon nitride crystal grains 2 with a relatively high Al content). Silicon nitride sintered body 1 having silicon nitride crystal grains 2 in which Al is dissolved at an atomic ratio of 0.001 to 0.100 exhibits excellent mechanical properties. Furthermore, silicon nitride crystal grains in which aluminum is dissolved in addition to oxygen undergo the formation of sialon. The formation of sialon allows more oxygen atoms and other sintering aid components to dissolve while maintaining the stability of the crystals within the grains. The dissolution of aluminum in addition to oxygen strengthens the silicon nitride crystal grains 2 as described above. Furthermore, the sintering aid components remaining as a glass phase in the grain boundary phase 3 are reduced, thereby strengthening the grain boundary phase 3. Therefore, when the solute oxygen and solute Al in silicon nitride sintered body 1 satisfy the above conditions, the aforementioned effects based on localized structural strengthening can be achieved.

[0030] The silicon nitride crystal grains 2 preferably have an Al / Si atomic ratio of 0.001 or more and 0.100 or less. In this case, the silicon nitride crystal grains 2 preferably include silicon nitride crystal grains 2 having an Al / Si atomic ratio of 0.030 or more. The number proportion S3 of silicon nitride crystal grains 2 having an Al / Si atomic ratio of 0.030 or more is preferably in the range of 2% or more and 20% or less (the following formula (7)). 2%≦S3≦20 (7)

[0031] Additionally, the silicon nitride sintered body 1 preferably contains 10% or more silicon nitride crystal grains 2 in which W is dissolved and the W / Si atomic ratio is less than 0.002 in the measurement region, as expressed by the total number ratio T1 relative to the total silicon nitride crystal grains 2 (the following formula (8)). The W / Si atomic ratio is preferably greater than 0. Furthermore, the W / Si atomic ratio is preferably 0.0001 or greater. 10%≦T1 (8)

[0032] A silicon nitride sintered body 1 having silicon nitride crystal grains 2 containing solute W, whose number ratio T1 satisfies the above formula (8), has excellent mechanical properties and thermal characteristics. Therefore, when the silicon nitride sintered body 1 satisfies the above formula (8), it is possible to obtain effects such as improved mechanical properties of the silicon nitride sintered body 1, including strength at high temperatures.

[0033] Additionally, the silicon nitride sintered body 1 preferably contains 10% or more silicon nitride crystal grains 2 in which Fe is dissolved, with an Fe / Si atomic ratio of less than 0.002 in the measurement region, as expressed as a percentage U1 of the number of silicon nitride crystal grains 2 relative to the total silicon nitride crystal grains 2 (the following formula (9)). The Fe / Si atomic ratio is preferably greater than 0. Furthermore, the Fe / Si atomic ratio is preferably 0.0001 or greater. 10%≦U1 (9)

[0034] A silicon nitride sintered body 1 having silicon nitride crystal grains 2 containing solute Fe, whose number ratio U1 satisfies the above formula (9), has excellent mechanical properties. Therefore, when the silicon nitride sintered body 1 satisfies the above formula (9), effects such as improved mechanical properties of the silicon nitride sintered body 1 can be obtained.

[0035] A description will be given of a method for measuring the cross section of the silicon nitride sintered body 1. The values ​​relating to the above atomic ratios in the measurement region of the silicon nitride sintered body 1 are preferably measured by the following method using energy dispersive X-ray spectroscopy (EDS).

[0036] Prior to measurement, a measurement sample of the silicon nitride sintered body 1 is prepared by focused ion beam processing (FIB) or ion milling. The sample is preferably sufficiently thin to prevent overlapping of the sintered body structure during measurement. Therefore, the thickness of the sample is preferably 0.05 μm or more and 0.5 μm or less.

[0037] Next, the sample is analyzed using a transmission electron microscope (TEM) or scanning electron microscope (SEM) equipped with an EDS device. The analysis is performed focusing on the ratio of count numbers as described below, with the aim of performing a qualitative analysis that is not affected by conditions. Therefore, the measurement conditions are not particularly limited, but for example, measurements can be performed using a TEM-EDS under the following conditions: acceleration voltage 200 kV, irradiation current 1.00 nA, spot diameter during analysis 1 nm, analysis time 30 seconds, and sample tilt angles X = 10°, Y = 0°. Note that a JEM-2100F is used for the TEM, and a JED-2300T (dry-type silicon drift detector (SDD, detection area 100 mm)) is used for the EDS. ２ ) or one with equivalent performance is used.

[0038] A specific method for EDS measurement will be described. A 20 μm × 20 μm region of the sample is defined as the measurement location. A plurality of silicon nitride crystal grains 2 contained in the measurement location are then analyzed, and the EDS spectrum of each silicon nitride crystal grain 2 is measured. This measurement is performed so that the electron spot does not extend beyond the cross section of the silicon nitride crystal grain 2.

[0039] The number of silicon nitride crystal particles 2 to be analyzed is preferably 100 or more. If the number of silicon nitride crystal particles 2 to be analyzed is less than 100, there is a risk that the measurement results will not be reliable due to variations in the analysis results. If the number of silicon nitride crystal particles 2 contained in the first measurement region is less than 100, a new second measurement region that does not overlap with the first measurement region may be determined, and only the second measurement region may be measured. There is no particular upper limit on the number of silicon nitride crystal particles 2 to be analyzed, but it is preferably 200 or less, for example, in consideration of measurement efficiency.

[0040] In the above-described measurement method, it is not necessary to measure all silicon nitride crystal particles 2 within the measurement region. Specific silicon nitride crystal particles that meet certain conditions (e.g., size) may be extracted from all silicon nitride crystal particles 2 within the measurement region, and the number ratio measured from the specific silicon nitride crystal particles may be regarded as the number ratio measured from all silicon nitride crystal particles 2 within the measurement region.

[0041] Next, elements are identified in the EDS spectrum of each silicon nitride crystal particle 2, and the count number for each element is determined. Here, quantitative correction is not performed on the element count numbers. If the Si count number for a given silicon nitride crystal particle 2 is less than 3,000, the surface of that silicon nitride crystal particle 2 may be oxidized, potentially preventing proper measurement, and therefore that silicon nitride crystal particle 2 is excluded from the analysis target. As a result of this exclusion, the number of silicon nitride crystal particles 2 to be analyzed will be less than 100, and other silicon nitride crystal particles 2 included in the measurement region are then added as new analysis targets.

[0042] The O / Si atomic ratio of each silicon nitride crystal grain 2 is determined by dividing the oxygen count by the Si count in the individual silicon nitride crystal grain 2. The proportions R1 to R4 of the number of silicon nitride crystal grains 2 within the corresponding range of O / Si atomic ratio contained in the measurement region of the silicon nitride sintered body 1 are determined.

[0043] Similarly, the atomic ratio of each element to Si in each silicon nitride crystal particle 2 is determined by dividing the counts of Al, W, and Fe by the counts of Si. This also determines the number proportions S1, S2, T1, and U1 of silicon nitride crystal particles 2 whose Al / Si atomic ratio, W / Si atomic ratio, and Fe / Si atomic ratio are within the corresponding ranges in the measurement region of silicon nitride sintered body 1. When O, Al, W, or Fe is detected by the EDS-based analysis method, it is determined that each element is present in solid solution within silicon nitride crystal particle 2.

[0044] FIG. 2 shows a bearing ball (an example of a wear-resistant member) made of silicon nitride sintered body 1, and FIG. 3 shows an example of a base sphere for a bearing ball (hereinafter simply referred to as a "base sphere") made of silicon nitride sintered body 1. Reference numeral 5 denotes a bearing ball, reference numeral 6 denotes a base sphere, reference numeral 7 denotes a spherical portion of base sphere 6, and reference numeral 8 denotes a band-shaped portion of base sphere 6. FIG. 3(A) shows base sphere 6 as viewed in a direction perpendicular to a line connecting the two poles G1 and G2 of band-shaped portion 8 (two vertices when the surface including band-shaped portion 8 is the bottom), and FIG. 3(B) shows base sphere 6 as viewed in a direction connecting the two poles G1 and G2 of band-shaped portion 8. The base sphere 6 is polished to form a bearing ball 5. While FIG. 3 illustrates a case in which base sphere 6 has band-shaped portion 8 on the circumference of spherical portion 7, the base sphere 6 may not have band-shaped portion 8. In other words, the base sphere 6 is the one before being polished to form a bearing ball 5.

[0045] Next, a description will be given of a method for manufacturing the silicon nitride sintered body 1 of the embodiment. The manufacturing process of the silicon nitride sintered body 1 of the embodiment is not particularly limited as long as it has the above formulas (1) to (4) (particularly the above formulas (1) and (2)), but it can be manufactured by, for example, the following method.

[0046] First, silicon nitride powder is prepared as the main raw material. The silicon nitride powder preferably has an average particle size of 2.5 μm or less and contains α-type silicon nitride at a rate of 90 mass % or more.

[0047] When the average particle size of the silicon nitride powder is 2.5 μm or less, it is possible to refine the silicon nitride crystal grains 2 and improve the mechanical strength in the resulting silicon nitride sintered body 1. When the proportion of α-phase silicon nitride in the silicon nitride powder is 90 mass % or more, when the α-phase silicon nitride grains grow to β-phase silicon nitride in the sintering step, a sintered body structure is formed in which silicon nitride crystal grains with large aspect ratios are intertwined in a complex manner, and the mechanical properties of the resulting silicon nitride sintered body 1 can be improved.

[0048] The silicon nitride powder may also contain W or Fe as impurities in order to dissolve W or Fe in the silicon nitride crystal grains 2 in the resulting silicon nitride sintered body 1 .

[0049] Next, a sintering aid is prepared. The sintering aid preferably contains a compound of a rare earth element from the viewpoint of improving sinterability, and more preferably contains Y (yttrium) as the rare earth element. Furthermore, the sintering aid preferably contains an Al compound from the viewpoints of improving sinterability, forming a solid solution of Al, and forming a sialon. Furthermore, the sintering aid may contain one or more compounds selected from compounds of transition metals such as Ti, Hf, Zr, W, Mo, Ta, Nb, and Cr, and compounds of Si, for the purpose of strengthening the properties of the sintered body. When using a rare earth element or an Al compound as a sintering aid, the amount of these added is not particularly limited, but a total of 2% by mass or more and 20% by mass or less in terms of oxides relative to the raw material powder is preferred from the viewpoints of improving sinterability and strengthening the properties of the sintered body.

[0050] It is more preferable that one or more of the compounds selected as the sintering aids are oxides. By using oxides as sintering aids, the sinterability can be further improved. In addition, the solid solution of oxygen into two silicon nitride crystal grains can be promoted. In particular, rare earth oxides, Al ２ O ３ It is more preferable to contain one or more of the above from the viewpoints of improving sinterability and strengthening the properties of the sintered body.

[0051] Also, preferably, AlN and Al are used as sintering aids. ２ O ３ It is more preferable that the amount of AlN added and the amount of Al are in a range from the viewpoints of improving sinterability, solid solution of Al, and formation of sialon. ２ O ３ When the amount of AlN added is compared by weight with the amount of AlN added, it is more preferable that the amount of AlN added is larger. ２ O ３ This is because AlN is more likely to promote solid solution of Al in silicon nitride crystal grains 2 than AlN.

[0052] Sialon may be used as the sintering aid. Addition of sialon can increase the amount and variability of oxygen dissolved in the silicon nitride crystal grains 2. The sintering aid may contain W and Fe as impurities to dissolve W and Fe in the silicon nitride crystal grains 2 in the resulting silicon nitride sintered body 1.

[0053] The silicon nitride powder and various sintering aids may be mixed in part or in whole by mechanical alloying before the mixing step described below. Mechanical alloying is a method of producing an alloy by repeatedly pressurizing and spreading the mechanically alloyed powder and the grinding medium by placing one or more types of mechanically alloyed powder in a ball mill together with a grinding medium such as ceramics and rotating the ball mill. By subjecting the silicon nitride powder and sintering aids to mechanical alloying in advance, it is possible to promote the solid solution of sintering aid components, including oxygen, in the silicon nitride crystal particles 2 in the resulting silicon nitride sintered body 1. Furthermore, by adjusting the proportion of the raw material powder that is mechanically alloyed, it is possible to control the variation in the amount of sintering aid components dissolved in the solid solution.

[0054] Next, the raw materials are crushed and mixed. First, a bead mill, ball mill, or the like is used to crush and mix the silicon nitride powder and sintering aid. In the sintering step described below, it is preferable to perform sintering in a temperature range where α-sialon exists. Therefore, it is preferable to sufficiently crush the raw material powder in this step so that sintering at a low temperature is possible. Next, additives such as binders are added to the raw material mixture and mixed. The additives are mixed into the raw material mixture using a ball mill, or the like, while crushing and granulating as necessary.

[0055] The mixture is then subjected to a molding process to obtain a silicon nitride molded body (hereinafter simply referred to as "molded body"). The raw material mixture can be molded by die pressing, cold isostatic pressing (CIP), rolling granulation, or the like. Furthermore, to uniformly increase the density of the molded body, CIP may be performed on a molded body obtained by die pressing or other methods. When molding by die pressing or CIP, the molding pressure is preferably 80 MPa or more. If the molding pressure is less than 80 MPa, the molded body may not be sufficiently densified.

[0056] Next, the CIP body is subjected to a degreasing process to obtain a degreased body as a compact. The degreasing of the compact is preferably carried out at a temperature of 400 to 700°C. The atmosphere during degreasing is not particularly limited, but it is preferable to carry out the degreasing in a non-oxidizing atmosphere, for example. Furthermore, if necessary, a drying process may be carried out at a temperature of 80 to 200°C prior to the degreasing process.

[0057] Next, the compact, e.g., the degreased body, is subjected to a sintering process. The atmosphere for the sintering process is not particularly limited, but it is preferably performed in a non-oxidizing atmosphere. The compact is preferably sintered at a temperature of 1650 to 1800°C. Sintering temperatures below 1650°C may result in insufficient grain growth of the silicon nitride crystal grains, while temperatures above 1800°C may result in dissociation of the sialon, resulting in insufficient variation in the amount of dissolved oxygen. Sintering is preferably performed at a temperature of 1700 to 1800°C. The sintering time for the compact is not particularly limited, but to prevent coarsening of the silicon nitride crystal grains 2, a sintering time of 3 hours or more but less than 8 hours is preferred, and a sintering time of 3.5 hours or more but less than 7 hours is even more preferred. The compact may be sintered under a pressure of 0.5 MPa or more to densify the sintered body.

[0058] Furthermore, in the sintering process for the compact, a temperature holding step is preferably carried out during heating at a temperature of 1500 to 1700°C for 3 hours or more as a pre-sintering step. This temperature range is the temperature range in which silicon nitride crystal particles 2 grow, and holding the temperature under this condition can promote the formation of sialon. It is more preferable that the holding temperature be 1550 to 1650°C. Furthermore, the upper limit of the holding time for the temperature holding step is preferably 12 hours or less.

[0059] Furthermore, after the sintering step, the sintered body may be subjected to hot isostatic pressing (HIP). For example, by performing HIP under a pressure of 30 MPa or more at a temperature of 1650 to 1800°C, the sintered body can be further densified. If the temperature during HIP is 1800°C or higher, dissociation of the sialon may occur, resulting in insufficient variation in the amount of dissolved oxygen. It is more preferable to perform HIP at a temperature of 1700 to 1800°C.

[0060] By the above-described manufacturing method, the silicon nitride sintered body 1 according to the embodiment can be obtained. For example, a base sphere 6 made of the ball-shaped silicon nitride sintered body 1 can be obtained.

[0061] A wear-resistant member can be obtained by polishing the base sphere 6 produced in this manner. For example, a base sphere 6 made of a ball-shaped silicon nitride sintered body 1 can be polished to a surface roughness Ra of 0.01 μm or less to obtain a bearing ball 5 as a wear-resistant member. When the silicon nitride sintered body 1 is to be used as a wear-resistant member other than the bearing ball 5, it is preferable to polish the silicon nitride sintered body 1 so that the surface roughness Ra is 0.1 μm or less.

[0062] As raw material powders for the bearing ball 5 according to each example and the bearing ball according to the comparative example, silicon nitride powder and sintering aid powder were prepared in the mass ratios [mass %] shown in Table 1. In example 1, sialon (SiAlON) was added in advance. In examples 2 and 3, 50% of the various powders were mixed in advance by mechanical alloying, and the obtained powder was added again to the raw material powder. In examples 4 and 5, 30% of the various powders were mixed in advance by mechanical alloying, and the obtained powder was added again to the raw material powder. A ball mill was used for the mechanical alloying.

[0063]

[0064] Next, the raw material powders were mixed in a ball mill. An organic binder and a solvent were then added to the resulting mixed powder, and further ball mill mixing was performed. The resulting mixture was then granulated, molded, and subsequently subjected to CIP treatment to obtain a CIP body as a ball-shaped compact. The CIP body had a ring-shaped band portion around the periphery of the sphere. The band portion of the CIP body corresponded to the band portion 8 of the base sphere 6. The resulting CIP body was then degreased to obtain a degreased body as a compact. The degreasing process was performed in a temperature range of 400 to 650°C. The resulting degreased body was then sintered and HIP-treated under the conditions shown in Table 2. In Examples 1 to 5, a temperature holding step was performed during the sintering process, with a holding temperature of 1600°C during heating and a holding time of 3 to 12 hours.

[0065]

[0066] Through the above steps, ball-shaped silicon nitride sintered bodies 1 according to each example and ball-shaped silicon nitride sintered bodies according to a comparative example were obtained. A base sphere 6 made of the ball-shaped silicon nitride sintered body 1 and a base sphere made of a ball-shaped silicon nitride sintered body were produced as samples for cross-sectional observation and evaluation of mechanical properties and processability. The base sphere 6 according to the example and the base sphere according to the comparative example are sintered bodies having a band-shaped portion, and are used to obtain 3 / 8-inch (diameter 9.525 mm) bearing balls.

[0067] Additionally, samples for three-point bending strength, fracture toughness, and Vickers hardness tests were prepared based on JIS_R1601, JIS_R1607, and JIS_R1610. JIS_R1601 corresponds to ISO14704, JIS_R1607 corresponds to ISO15732, and JIS_R1610 corresponds to ISO14705.

[0068] Next, the measurement area of ​​each example and each comparative example was measured using TEM-EDS, and the O / Si atomic ratio and Al / Si atomic ratio were determined for 100 silicon nitride crystal particles present within the measurement area. The TEM-EDS measurement was performed under the following conditions: acceleration voltage 200 kV, probe current 1.00 nA, analysis spot diameter 1 nm, analysis time 30 seconds, and sample tilt angles X = 10°, Y = 0°.

[0069] The number ratio R1 of first silicon nitride crystal particles 21 having an O / Si atomic ratio of 0.001 or more and 0.030 or less, and the number ratio R1 of second silicon nitride crystal particles 22 having an O / Si atomic ratio of more than 0.030 and 0.200 or less in the measurement region are shown in Table 3. In addition, the maximum and minimum values ​​of the Al / Si atomic ratio are shown in Table 3.

[0070] In all of the examples, the elementary spheres 6 made of silicon nitride sintered body 1 had a number ratio R1 of 80% to 99% of first silicon nitride crystal particles 21 having an O / Si atomic ratio of 0.001 to 0.030 within the measurement area, satisfying the above formula (1). In all of the examples, the elementary spheres 6 made of silicon nitride sintered body 1 had a number ratio R2 of 1% to 20% of second silicon nitride crystal particles 22 having an O / Si atomic ratio of more than 0.030 to 0.200 within the measurement area, satisfying the above formula (2). In addition, in all of the examples, there were no silicon nitride crystal particles with an O / Si atomic ratio less than 0.001, and no silicon nitride crystal particles with an O / Si atomic ratio exceeding 0.200.

[0071] In the example, in the element 6 made of silicon nitride sintered body 1, the silicon nitride crystal particles 2 all had an Al / Si atomic ratio of 0.001 or more and 0.100 or less within the measurement region. Furthermore, the number proportion S1 of silicon nitride crystal particles 2 having an Al / Si atomic ratio of 0.001 or more and 0.010 or less was 10% or more, satisfying the above formula (5). In the example, in the element 6 made of silicon nitride sintered body 1, the silicon nitride crystal particles having an Al / Si atomic ratio of 0.001 or more and 0.100 or less included silicon nitride crystal particles 2 having an Al / Si atomic ratio of 0.030 or more. The number proportion S3 of silicon nitride crystal particles 2 having an Al / Si atomic ratio of 0.030 or more relative to the total silicon nitride crystal particles 2 was 2% or more and 20% or less, satisfying the above formula (7).

[0072] On the other hand, in all of the silicon nitride sintered bodies of the comparative examples, the number ratio R1 of silicon nitride crystal grains having an O / Si atomic ratio of 0.001 to 0.030 was 100%. Also, in all of the silicon nitride sintered bodies of the comparative examples, the maximum value of the Al / Si atomic ratio of the silicon nitride crystal grains was less than 0.03.

[0073] Next, the ball-shaped silicon nitride sintered body 1 according to each example (base ball 6) and the ball-shaped silicon nitride sintered body according to the comparative example were processed into 3 / 8-inch bearing balls, and their processability was evaluated. When processing the base balls into bearing balls, a brittle processing step and a ductile processing step are performed. With regard to these brittle processing and ductile processing steps, the time required to process the silicon nitride sintered body according to each example and comparative example was measured.

[0074] In Examples 1 to 5 and Comparative Example 1, Comparative Example 2 was used as the standard, and those in which the processing time was equivalent to that of Comparative Example 2 (0.8 to 1.0 times) were rated as "STD" workability, while those in which the processing time was significantly shorter than that of Comparative Example 2 (less than 0.8 times) were rated as "UP" workability, with the results shown in Table 3. Additionally, the ratio of the usage time until replacement of the diamond grinding wheel used in the brittle processing step was determined. Comparative Example 2 was used as the standard (100), and the results are shown in Table 4.

[0075] The blank balls 6 of Examples 1 and 2 exhibited improved workability in both brittle processing and ductile processing compared to the comparative example. The blank balls 6 of Examples 1 and 2 also exhibited a larger Al / Si atomic ratio (minimum value - maximum value) and a relatively large amount of solute Al compared to the blank balls 6 of Examples 3 to 5. Furthermore, the blank balls 6 of Examples 3 to 5 exhibited improved workability in brittle processing compared to the comparative example. Furthermore, the blank balls 6 of the examples all exhibited improved usable grinding wheel life.

[0076] Next, the three-point bending strength, fracture toughness, and Vickers hardness of the blank spheres 6 according to each example and the blank spheres according to the comparative examples were measured in accordance with JIS R1601, JIS R1607, and JIS R1610. The three-point bending strength was measured not using blank spheres, but using plate-shaped silicon nitride sintered bodies manufactured under the same conditions as the blank spheres. The results are shown in Table 4.

[0077] As shown in Figure 4, all of the blank balls 6 of the Examples exhibited desirable mechanical properties. That is, in the Examples, the three-point bending strength was 700 MPa or greater, the toughness was 6 or greater, and the hardness was 1400 or greater. Furthermore, all of the blank balls 6 of the Examples had mechanical properties roughly equivalent to or superior to those of the blank balls of the Comparative Examples. In particular, the three-point bending strength and Vickers strength of all of the blank balls 6 of the Examples were in more favorable ranges than those of the blank balls of the Comparative Examples.

[0078]

[0079]

[0080] In the examples and comparative examples, improvements in the mechanical properties and processability of the base spheres 6 (which are polished to become bearing balls 5) made of ball-shaped silicon nitride sintered bodies 1 were confirmed. However, this effect is not limited to the base spheres 6, and it is believed that it can also be obtained with silicon nitride sintered bodies 1 other than the base spheres 6 that satisfy the above formulas (1) and (2).

[0081] According to at least one of the embodiments described above, it is possible to provide a silicon nitride sintered body 1 and a wear-resistant member using the same (e.g., bearing ball 5) that have excellent processability and mechanical properties. Furthermore, by controlling the amount of dissolved Al in addition to the amount of dissolved O in the silicon nitride crystal grains 2, it is possible to further improve the processability (particularly brittle processing) of the silicon nitride sintered body 1 and a wear-resistant member using the same.

[0082] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

Claims

1. A silicon nitride sintered body comprising a plurality of silicon nitride crystal grains and a grain boundary phase, in a 20 μm x 20 μm measurement region in any cross section of which the plurality of silicon nitride crystal grains contain first silicon nitride crystal grains having an O / Si atomic ratio of 0.001 or more and 0.030 or less, in a number ratio of 80% to 99% of the plurality of silicon nitride crystal grains as a whole, and the plurality of silicon nitride crystal grains contain second silicon nitride crystal grains having an O / Si atomic ratio of more than 0.030 and 0.200 or less, in a number ratio of 1% to 20% of the plurality of silicon nitride crystal grains as a whole.

2. The silicon nitride sintered body according to claim 1, wherein in the measurement region, silicon nitride crystal grains having an Al / Si atomic ratio of 0.001 or more and 0.010 or less account for 10% or more of the total number of silicon nitride crystal grains.

3. The silicon nitride sintered body according to claim 2, wherein the first silicon nitride crystal grains have an O / Si atomic ratio of 0.010 or more and 0.030 or less.

4. The silicon nitride sintered body according to claim 1, wherein the Al / Si atomic ratio of the plurality of silicon nitride crystal grains in the measurement region is 0.001 or more and 0.100 or less.

5. The silicon nitride sintered body according to claim 2, wherein the Al / Si atomic ratio of the plurality of silicon nitride crystal grains in the measurement region is 0.001 or more and 0.100 or less.

6. The silicon nitride sintered body according to claim 1, wherein the plurality of silicon nitride crystal grains in the measurement region include silicon nitride crystal grains having an Al / Si atomic ratio of 0.030 or more.

7. The silicon nitride sintered body according to claim 2, wherein the plurality of silicon nitride crystal grains in the measurement region include silicon nitride crystal grains having an Al / Si atomic ratio of 0.030 or more.

8. The silicon nitride sintered body according to claim 5, wherein the plurality of silicon nitride crystal grains in the measurement region include silicon nitride crystal grains having an Al / Si atomic ratio of 0.030 or more.

9. A silicon nitride sintered body according to any one of claims 1 to 8, wherein the measurement region contains 10% or more of the plurality of silicon nitride crystal grains in a solid solution with W in a W / Si atomic ratio range of 0.002 or less, in terms of number ratio relative to the entire plurality of silicon nitride crystal grains.

10. A silicon nitride sintered body according to any one of claims 1 to 8, wherein in the measurement region, silicon nitride crystal grains which dissolve Fe in a solid solution with an Fe / Si atomic ratio of 0.002 or less account for 10% or more of the total number of silicon nitride crystal grains.

11. The silicon nitride sintered body according to claim 9, wherein the measurement region contains 10% or more of the silicon nitride crystal grains that are solid-solubilized with Fe in an Fe / Si atomic ratio range of 0.002 or less, in terms of number ratio relative to the entire silicon nitride crystal grains.

12. A wear-resistant member made of the silicon nitride sintered body according to any one of claims 1 to 8.

13. A wear-resistant member made of the silicon nitride sintered body according to claim 9.

14. A wear-resistant member made of the silicon nitride sintered body according to claim 10.

15. A wear-resistant member made of the silicon nitride sintered body according to claim 11.

16. The wear-resistant member according to claim 12, having a surface roughness Ra of 0.1 μm or less.

17. The wear-resistant member according to claim 13, having a surface roughness Ra of 0.1 μm or less.

18. The wear-resistant member according to claim 14, having a surface roughness Ra of 0.1 μm or less.

19. The wear-resistant member according to claim 15, having a surface roughness Ra of 0.1 μm or less.

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