Silicon nitride sintered body and production method therefor

Abstract

To provide a silicon nitride sintered body that comprises predefined hardness and is hard to be broken, and a production method therefor.SOLUTION: A silicon nitride sintered body is the silicon nitride sintered body that contains an α silicon nitride where a diffraction angle 2θ in an X-ray diffraction chart has peaks in the vicinity of 20.5°, 30.9°, 34.4°and 35.3°, and a β silicon nitride where a diffraction angle 2θ in an X-ray diffraction chart has peaks in the vicinity of 27.0°, 33.6°and 36.0°, and diffraction intensity of a peak at each diffraction angle of the α silicon nitride is equal to each other, and the diffraction intensity of a peak at each diffraction angle of the α silicon nitride is smaller than diffraction intensity of the β silicon nitride.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a silicon nitride sintered body and a method for producing the same.

Background Art

[0002] Silicon nitride (Si3N4) ceramic is a material excellent in thermal conductivity and mechanical strength and is applied to various structural members. As a method for producing silicon nitride ceramic, for example, the following method is known. A kneaded body obtained by adding yttrium oxide and aluminum oxide as sintering aids to silicon nitride powder is prepared. Next, using the kneaded body, a molded body is produced by pressure molding. Next, the molded body is sintered and densified to obtain silicon nitride ceramic.

[0003] Patent Document 1 describes silicon nitride ceramics having a high relative density and being densified. When the constituent phases of the molded body are identified by an X-ray diffraction chart (measurement range 15° to 60°), the respective diffraction intensities of α-silicon nitride having peaks around 20°, 31°, 34°, and 35° are substantially equal, and the respective diffraction intensities of β-silicon nitride having peaks around 26°, 34°, and 35° are substantially equal (see the examples in FIG. 3 of Patent Document 1).

[0004] Further, Patent Document 2 describes a silicon nitride-based cutting tool excellent in strength characteristics. In this silicon nitride-based cutting tool, the precipitation ratio of crystals of α-silicon nitride and β'-sialon (including β-silicon nitride) is 0% < α-silicon nitride ≤ 30%, 70% ≤ β'-silicon nitride < 100% in terms of the diffraction intensity ratio by X-ray diffraction, and when the precipitation ratio of α-silicon nitride exceeds 30% and shifts toward the high α-silicon nitride side, the effect of the β'-sialon columnar crystal structure decreases, the effect of compositeization in the crystal phase does not sufficiently appear, and the effect of improving strength is not sufficient.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] The silicon nitride ceramics described in Patent Document 1 have a ratio of α-silicon nitride / β-silicon nitride greater than 1 (referring to the example in FIG. 1 of Patent Document 1, the ratio is about 1.3). In silicon nitride ceramics, the strength (hardness) increases as the ratio of β-silicon nitride to α-silicon nitride increases. However, since the hardness and the toughness value are in an inverse proportional relationship, if the hardness is increased without reason, the toughness value will decrease and it will be prone to cracking.

[0007] The present invention aims to solve the above problems, and an object thereof is to provide a silicon nitride sintered body having a predetermined hardness and being difficult to crack and a method for manufacturing the same. [Means for Solving the Problems]

[0008] A silicon nitride sintered body comprising α-silicon nitride having peaks in the vicinity of diffraction angles 2θ of 20.5°, 30.9°, 34.4° and 35.3° in an X-ray diffraction chart, and β-silicon nitride having peaks in the vicinity of diffraction angles 2θ of 27.0°, 33.6° and 36.0° in the X-ray diffraction chart, wherein the diffraction intensities of the peaks at each diffraction angle of the α-silicon nitride are equal, and the diffraction intensity of the peaks at each diffraction angle of the α-silicon nitride is smaller than the diffraction intensity of the β-silicon nitride. The ratio of the sum of the diffraction intensities of the peaks at each diffraction angle of the α-silicon nitride to the sum of the diffraction intensities of the peaks at each diffraction angle of the β-silicon nitride is preferably less than 1. Furthermore, a method for manufacturing a silicon nitride sintered body containing α-silicon nitride and β-silicon nitride, comprising: a granulation step of mixing 94 to 98% by mass of α-sialon powder, 1 to 3% by mass of silicon oxide powder, 1 to 3% by mass of yttrium oxide powder, and a binder to granulate the granulated powder; a molding step of pressure-molding the granulated powder to form a molded body; a firing step of firing the molded body to form a fired body; and an isostatic pressing step of isostatically pressing the fired body. The firing step may include a primary firing step of firing the molded body at 400 to 600°C for 16 hours in a non-nitrogen atmosphere, and a secondary firing step of firing the molded body fired in the primary firing step at 1650 to 1800°C for 2 hours in a nitrogen atmosphere. Also, in the isostatic pressing step, the fired body may be pressed at 50 to 200 MPa in a nitrogen atmosphere. Furthermore, the isostatic pressing step may be hot isostatic pressing at 1700°C for 2 hours.

Advantages of the Invention

[0009] According to the silicon nitride sintered body and its manufacturing method of the present invention, it is possible to provide a silicon nitride sintered body having a predetermined hardness and being difficult to crack and its manufacturing method.

Brief Description of the Drawings

[0010]

Figure 1

Embodiments for Carrying Out the Invention

[0011] Hereinafter, the silicon nitride sintered body and its manufacturing method of the present invention will be described. First, based on an example of manufacturing a spherical rolling element made of a silicon nitride sintered body, the manufacturing method of the silicon nitride sintered body according to an embodiment of the present invention will be described.

[0012] (Granulation Step) First, as a granulation process, a raw material liquid is prepared by mixing a nitride ceramic powder, SiO2 (silicon oxide) powder, an auxiliary agent, a binder, and water in a ball mill. Here, the nitride ceramics are α-sialon (manufactured by Combustion Synthesis Co., Ltd.) with an average particle size of 0.5 μm, and the proportion in the raw material liquid is 94 to 98% by mass. Also, the SiO2 powder is SiO2 (silicon oxide) powder with an average particle size of 1 μm, and the proportion in the raw material liquid is 1 to 3% by mass. The auxiliary agent is Y2O3 (yttrium oxide) powder with an average particle size of 0.8 μm, and the proportion in the raw material liquid is 1 to 3% by mass. The binder is an acrylic binder. In the raw material liquid, the binder and water are blended in a proportion excluding the nitride ceramic powder, SiO2 powder, and auxiliary agent. Then, the raw material liquid prepared by mixing these materials is spray-dried by a spray dryer to produce silicon nitride particles of a predetermined size. The size of the silicon nitride particles is, for example, 40 μm.

[0013] (Forming process) Next, as a forming process, a molded body is produced by press-forming the silicon nitride particles produced in the granulation process. In this embodiment, the molded body is substantially spherical with a diameter of 1 mm.

[0014] (Firing process) Next, the above-mentioned substantially spherical silicon nitride sintered body is fired. The firing is performed twice, a first firing and a second firing. The first firing is to heat the molded body in a non-nitrogen atmosphere at 400 to 600 °C for about 16 hours. In this firing, a debinding process to remove the binder in the molded body is performed. Subsequently, the molded body fired in the first firing is subjected to a second firing. The second firing is performed by holding the molded body in a nitrogen gas atmosphere at 1650 to 1800 °C for 2 hours.

[0015] (Isostatic pressing process) Next, the molded body fired in the previous process is subjected to an isostatic pressing treatment at, for example, 50 to 200 MPa in a nitrogen atmosphere. In this embodiment, this process is performed by hot isostatic pressing in which isostatic pressing is performed under high-temperature conditions, and the temperature condition and time condition are 1700 °C and 2 hours. By this process, a sintered body with a uniform density throughout is obtained.

[0016] Here, α-silicon nitride is known to become unstable in the high-temperature range and transform into β-silicon nitride. In the primary and secondary firing processes and the isostatic pressing process, the compact is exposed to a high-temperature environment. However, the α-silicon nitride contained in the nitride ceramic powder reacts with other components as the sintering progresses, and SiO2 contained in the silicon nitride generates a glass phase. Also, a part of Al and O in Al2O3 contained in the nitride ceramic powder dissolves in the silicon nitride in the nitride ceramic powder to form β-silicon nitride grains, and densification proceeds by the dissolution-precipitation reaction at the solid / liquid interface and diffusion within the grain boundary phase. The solid phase precipitated at this time is the β-type, which is stable at high temperatures. Therefore, by the primary and secondary firing processes and the isostatic pressing process, the transformation of the α-phase contained in the silicon nitride in the compact into the β-phase can be adjusted, and a sintered body with high toughness can be obtained while maintaining the hardness of the resulting sintered body. In the production of nitride sintered bodies, the hardness and toughness values are particularly greatly affected by the temperature conditions of the isostatic pressing process, and the hardness and toughness values can be adjusted by appropriately adjusting the temperature conditions of the isostatic pressing process.

[0017] (Polishing process) Finally, as a polishing process, the sintered body obtained in the previous process is polished using a known polishing device, and the surface of the further polished sintered body is lapped, thereby completing a silicon nitride sintered body with excellent roundness used for rolling elements. The diameter of the rolling element obtained in this process is, for example, 0.6 mm.

[0018] Next, the silicon nitride sintered body of the present invention will be described. The silicon nitride sintered body according to an embodiment of the present invention is manufactured by the aforementioned manufacturing method. FIG. 1 is a diagram showing an X-ray diffraction chart of the silicon nitride sintered body according to an embodiment of the present invention.

[0019] In FIG. 1, the horizontal axis represents the diffraction angle, the vertical axis represents the diffraction intensity, and the diffraction angle, diffraction intensity, and identified substance at the peak that appears in the range where the diffraction angle 2θ is 20.0° to 37.0° are as follows. (1) Diffraction angle: 20.5°, diffraction intensity: 3,065 cps, identified substance: α-silicon nitride with plane index (1, 0, 1) (2) Diffraction angle: 30.9°, diffraction intensity: 2,927 cps, identified substance: α-silicon nitride with plane index (2, 0, 1) (3) Diffraction angle: 34.4°, diffraction intensity approximately 2,826 cps, identified substance: α-silicon nitride with plane index (1, 0, 2) (4) Diffraction angle: 35.2°, diffraction intensity: 2,981 cps, identified substance: α-silicon nitride with plane index (2, 1, 0) (5) Diffraction angle: 27.0°, diffraction intensity: 4,631 cps, identified substance: β-silicon nitride with plane index (2, 0, 0) (6) Diffraction angle: 33.6°, diffraction intensity: 4,489 cps, identified substance: β-silicon nitride with plane index (1, 0, 1) (7) Diffraction angle: 36.0°, diffraction intensity: 3,692 cps, identified substance: β-silicon nitride with plane index (2, 1, 0)

[0020] X-ray diffraction of the silicon nitride sintered body was performed as follows. First, a silicon nitride sintered body with a diameter of 0.6 mm was embedded in resin, and the internal surface of the silicon nitride sintered body was exposed by polishing this resin using a polishing device or the like. Preferably, a cross-section passing through the center of the silicon nitride sintered body was exposed. The roughness (Ra) of the exposed internal surface of the silicon nitride sintered body is, for example, 5 μm. Next, an X-ray diffraction chart was obtained by measuring the exposed internal surface of the silicon nitride sintered body using an X-ray diffractometer (in this example, D2 PHASER manufactured by Bruker). The obtained X-ray diffraction chart was used to identify the crystal phase of the detected peak by comparing it with, for example, the database of the X-ray diffractometer.

[0021] When the silicon nitride sintered body according to an embodiment of the present invention is identified by an X-ray diffraction chart (measurement range: 20° to 37°) obtained by X-ray diffraction, peaks are present at diffraction angles 2θ of 20.5°, 30.9°, 34.4°, and 35.2°. These peaks are the peaks of α-silicon nitride, and the diffraction intensity of this α-silicon nitride is approximately 2,800 to 3,100 cps at each diffraction angle and is equivalent at each diffraction angle. The sum of the diffraction intensities at each diffraction angle is approximately 11,800 cps. Here, considering the measurement error of the diffraction intensity ratio, etc., when the difference in each diffraction intensity is within 500 cps, it is determined that the diffraction intensities are equivalent. Also, regarding the diffraction angle, an error of about ±0.05° / 2θ may be expected due to the mechanical accuracy of the X-ray diffractometer, deficiencies in sample adjustment, deficiencies in the selection of measurement conditions, etc. When the expression "in the vicinity of a predetermined diffraction angle" is used, it means including the error range in the predetermined diffraction angle.

[0022] In addition, the silicon nitride sintered body also has peaks at diffraction angles 2θ of 27.0°, 33.6°, and 36.0°. These peaks are the peaks of β-silicon nitride, and the diffraction intensity of this β-silicon nitride is approximately 3,700 to 4,600 cps at each diffraction angle. The sum of the diffraction intensities at each diffraction angle is approximately 12,800 cps.

[0023] Here, the diffraction intensity of the peak at each diffraction angle of α-silicon nitride in the X-ray diffraction chart is smaller than the diffraction intensity of the peak at each diffraction angle of β-silicon nitride. Also, the ratio of the sum of the diffraction intensities of α-silicon nitride to the sum of the diffraction intensities of β-silicon nitride (sum of the diffraction intensities of α-silicon nitride / sum of the diffraction intensities of β-silicon nitride) is 0.92, which is smaller than 1. This ratio represents the ratio of α-silicon nitride to β-silicon nitride contained in the silicon nitride sintered body. Since this ratio is smaller than 1, the silicon nitride sintered body has the hardness of the silicon nitride sintered body, high toughness, and is difficult to crack. When the silicon nitride sintered body is used as a rolling element, it is preferable that the ratio of the sum of the diffraction intensities of α-silicon nitride to the sum of the diffraction intensities of β-silicon nitride is 0.8 or more and less than 1. When the ratio is smaller than 0.8, the hardness is not sufficient, and when it is 1 or more, the toughness is low and it is easy to crack. The silicon nitride sintered body of this embodiment has a Vickers hardness of 1526 HV and a fracture toughness value of 6.0 MPa·m1 / 2 In the case of a rolling element, when the Vickers hardness is 1500 HV or more and the fracture toughness value is 5.5 MPa·m 1 / 2 or more, it becomes a rolling element with excellent wear resistance and low crack susceptibility.

[0024] As described above, the silicon nitride sintered body and its manufacturing method of the present invention have been described based on the embodiments. However, the scope of the present invention is not limited to the above-described embodiments and can be arbitrarily changed within the scope of the technical idea of the present invention. For example, although the silicon nitride sintered body has a spherical shape, it is not limited to the spherical shape and may have a rectangular shape.

Claims

1. A silicon nitride sintered body, comprising α-silicon nitride having peaks in the vicinity of diffraction angles 2θ of 20.5°, 30.9°, 34.4° and 35.3° in an X-ray diffraction chart, and β-silicon nitride having peaks in the vicinity of diffraction angles 2θ of 27.0°, 33.6° and 36.0° in the X-ray diffraction chart, wherein the diffraction intensities of the peaks at each diffraction angle of the α-silicon nitride are equal, and the diffraction intensity of the peaks at each diffraction angle of the α-silicon nitride is smaller than the diffraction intensity of the β-silicon nitride, characterized in that it is a silicon nitride sintered body.

2. The silicon nitride sintered body according to claim 1, characterized in that the ratio of the sum of the diffraction intensities of the peaks at each diffraction angle of the α-silicon nitride to the sum of the diffraction intensities of the peaks at each diffraction angle of the β-silicon nitride is less than 1.

3. A method for manufacturing a silicon nitride sintered body containing α-silicon nitride and β-silicon nitride, a granulation step of mixing 94 to 98% by mass of α-sialon powder, 1 to 3% by mass of silicon oxide powder, 1 to 3% by mass of yttrium oxide powder, and a binder to granulate a granulated powder; a molding step of pressure-molding the granulated powder to form a molded body; a firing step of firing the molded body to form a fired body; an isostatic pressing step of isostatically pressing the fired body; characterized in that it comprises the above steps, and is a method for manufacturing a silicon nitride sintered body.

4. The method for manufacturing a silicon nitride sintered body according to claim 3, characterized in that the firing step includes a primary firing step of firing the molded body at 400 to 600 °C for 16 hours in a non-nitrogen atmosphere, and a secondary firing step of firing the molded body fired in the primary firing step at 1650 to 1800 °C for 2 hours in a nitrogen atmosphere.

5. The method for manufacturing a silicon nitride sintered body according to claim 4, characterized in that the isostatic pressing step presses the fired body at 50 to 200 MPa in a nitrogen atmosphere.

6. The method for manufacturing a silicon nitride sintered body according to claim 5, characterized in that the isostatic pressing step is hot isostatic pressing at 1700 °C for 2 hours.

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