JP3173251B2 - Method for producing silicon nitride sintered body and turbo rotor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a method for manufacturing a silicon nitride sintered body and a turbo rotor .
The silicon nitride sintered body obtained by the present invention is useful, for example, for a piston pin, a valve, a turbo rotor, and the like of an automobile engine.

[0002]

2. Description of the Related Art Sintered silicon nitride has attracted attention as a ceramic for high-temperature, high-strength materials such as engine parts for automobiles and has been actively researched and developed. However, the silicon nitride sintered body is a ceramic even though it has high strength, and has a drawback of low fracture toughness. In order to increase the toughness, for example, composite formation by fiber reinforcement has been attempted. However, it takes time and effort to prepare and mix fibers, and there is a problem in terms of cost. In addition, the mixing of a reinforcing material other than silicon nitride, such as fiber, may reduce the original effect of the silicon nitride sintered body. Further, molding and sintering of a component having a complicated shape such as a turbo rotor is very difficult.

The applicant of the present invention has converted α-Si ₃ N ₄ into β-Si ₃ N ₄ by sintering, and has an aspect ratio of 1
A silicon nitride sintered body having improved toughness by the action of β-Si ₃ N ₄ which is a long columnar crystal of 0 or more is disclosed (Japanese Patent Application Laid-Open No. 63-3 / 1988).
No. 10771).

[0004]

In the above-mentioned prior invention, the silicon nitride sintered body is made to have high toughness by containing 5 to 50% by volume of β-Si ₃ N ₄ columnar crystals grown to a size of 50 μm or more. Has been granted. However, there was a problem that the strength hardly changed even when the size and content of the β-Si ₃ N ₄ columnar crystal were changed, and the strength was rather lowered when the content was 30% by volume or more.

The present invention has been made in view of such circumstances, and has as its object to improve both the strength and toughness of a silicon nitride sintered body.

[0006]

[Means for Solving the Problems]

[0007] The turbo rotor of the present invention has a substantially β-S
i ₃ N ₄ consists columnar crystal has an orientation unit which hub rear and exit side blade root portion to β-Si ₃ N ₄ columnar crystals are oriented one-dimensionally in a predetermined direction, the alignment portion β-Si ₃ N ₄ columnar crystals of 20
It is characterized in that at least the volume% is one-dimensionally oriented in a certain direction from the outer peripheral side toward the axial direction.

If the ratio of the volume of the one-dimensionally oriented β-Si ₃ N ₄ columnar crystal to the total β-Si ₃ N ₄ columnar crystal volume (hereinafter referred to as the orientation ratio) in the oriented portion is less than 20% by volume, the strength is increased. And little improvement in toughness. In order to measure the orientation ratio, for example, the orientation portion is cut and polished on a plane parallel to the one-dimensional orientation direction, and after chemical etching, SEM
It can be measured by taking a photograph and measuring the area ratio of columnar crystals having an aspect ratio of 10 or more.

The method for producing a silicon nitride sintered body having high strength and high toughness according to the present invention comprises preparing a silicon nitride sintered body containing 20% by volume or more of α-Si ₃ N ₄ and having a relative density of 98% or more. And heat-treating the silicon nitride sintered body at a temperature of 1500 ° C. or more while giving a temperature gradient to β-Si by α-β transition.
_A first heat treatment step of one-dimensionally orienting and depositing ₃ N ₄ columnar crystals in a fixed direction, and holding the silicon nitride sintered body after the first heat treatment step at 1500 ° C. or higher to form the precipitated β-Si ₃ N ₄ columnar crystals. And a second heat treatment step of growing grains.

There are two crystal phases, α and β, in Si ₃ N _4, and α-Si ₃ N ₄ is transformed into a stable β phase at a high temperature by heat treatment. The β phase tends to grow into hexagonal columnar crystals. Therefore, assuming that α-Si ₃ N ₄ in the silicon nitride sintered body before the first heat treatment step is less than 20% by volume,
The remaining Si ₃ N ₄ components are β-Si ₃ N ₄ columnar crystals,
Because it will have been randomly oriented, α-Si ₃ N ₄ in the first heat treatment step are oriented one-dimensional beta-Si ₃
Even if it transforms into N ₄ columnar crystals, the amount becomes less than 20% by volume, and the orientation ratio becomes less than 20% by volume.

If the relative density of the silicon nitride sintered body before the first heat treatment step is less than 98%, sintering (densification) occurs during the first heat treatment step. In the present invention, since a temperature gradient is given during the heat treatment, when the relative density is less than 98%, stress is generated by sintering, and the sintered body is warped or cracked. It is particularly desirable that the relative density of the silicon nitride sintered body before the first heat treatment step is 99% or more.

In the first heat treatment step, it is preferable to use a microwave heating device as a heating means. With ordinary heating means, a temperature gradient easily occurs inside and on the surface of the sintered body,
This is because it is easy to orient the β-Si ₃ N ₄ columnar crystal in a desired direction, and the microwave generates heat from the material itself, so that there is no temperature distribution between the inside and the surface. Furthermore, according to the microwave, a local temperature gradient can be easily provided, and the alignment direction can be easily varied for each local area.

The heat treatment temperature of the first heat treatment step is 150
Is less than 0 ℃ and than was because 1500 ° C. or more does not cause alpha-beta transition, when the heat treatment in a nitrogen atmosphere at atmospheric pressure are 1800 ° C. or less, which does not decompose the Si ₃ N _4.
If the temperature of the second heat treatment step is lower than 1500 ° C., the oriented β-Si ₃ N ₄ columnar crystal cannot be grown to a desired size. In the case where heat treatment is locally performed in the first heat treatment step, β-Si of untransformed α-Si ₃ N _4
3 N ₄ transition to columnar crystals is difficult. In order to prevent the decomposition as described above, and if the temperature is too high, the columnar crystals become thick and the strength is reduced. Therefore, the heating temperature during grain growth is desirably 1800 ° C. or less.

[0014]

In the turbo rotor of the present invention, the β-Si ₃ N ₄ columnar crystals are one-dimensionally oriented in a certain direction, and have an oriented portion having an orientation ratio of 20% by volume or more. Therefore, in the orientation part, β-
The compressive / tensile strength of the Si ₃ N ₄ columnar crystal in the direction parallel to the orientation direction is remarkably improved, and the crack propagation resistance in the direction perpendicular to the orientation direction, that is, the fracture toughness, is remarkably improved.

Further, in the method for producing a silicon nitride sintered body of the present invention, a heat treatment is performed by giving a temperature gradient. This gives α
The phase is transformed into the β phase via the liquid phase, and the hexagonal columnar β-Si ₃
Although N ₄ columnar crystals are precipitated, the precipitation direction is regulated by the temperature gradient, and the β-Si ₃ N ₄ columnar crystals are surely one-dimensionally oriented parallel to the direction of the temperature gradient. Then, by growing β-Si ₃ N ₄ columnar crystals which are oriented in a certain direction and precipitated,
A silicon nitride sintered body having a high strength and high toughness oriented portion is obtained.

[0016]

The present invention will be specifically described below with reference to examples. (Examples 1 to 8) <Molding Step> Si ₃ N ₄ powder (average particle size 0.2 μm, α
Conversion rate of about 100%), Y ₂ O ₃ powder (average particle size 0.3 μm, purity 99.9%) as a sintering aid and MgAl ₂
O ₄ powder (average particle size 0.3 μm, purity 99.9%) was uniformly mixed in a Si ₃ N ₄ ball mill in ethanol with the composition shown in Table 1, and then dried to obtain a mixed powder. . It was pressed under a pressure of 200 kgf / cm ² . The molded body is packed in a thin rubber bag, vacuum sealed and 3,000 kgf / CIP.
Pressing was performed at a pressure of cm ² to form a molded body of 5 × 6 × 50 mm. <Firing step> The above-mentioned molded body was sintered in a furnace in a nitrogen atmosphere under the conditions shown in Table 1. The rate of temperature rise was 5 ° C./minute, and the pressure was 1 atm until the maximum temperature was reached. After reaching the maximum temperature, the pressure was increased to the maximum pressure at a pressure increase rate of 15 atm / minute. The holding time at the highest temperature and the highest pressure is 4 hours.

The dimensions of the obtained sintered body are about 4 × 4.8 × 4
0 mm, and the relative density and α-Si ₃ N ₄ fraction (α conversion) are shown in Table 1. In addition, the relative density was determined by the n-butanol substitution method, and the ratio of gelatinization was determined by the X-ray diffraction method. <First heat treatment step> The above sintered body was gyrotron (28
GHz, 15 kW high-frequency high-output oscillation tube), and heat-treated in a nitrogen atmosphere at 1 atm. At this time, the microwave is provided with an output distribution such that the output becomes smaller from one end 10 to the other end 11 along the longitudinal direction of the sintered body 1 as shown in FIG. Was set to 1600 ° C. Then, the output of the microwave was changed with time, and the other end 11 was adjusted to 1600 ° C. after 10 minutes as shown in FIG. <Second heat treatment step> Thereafter, heat treatment was performed for 4 hours under the temperature and pressure conditions shown in Table 1 in the same furnace as the above-mentioned baking step. <Test> X-ray diffraction analysis was performed on the obtained sintered body,
Table 2 shows the α conversion and β-Si ₃ N ₄ columnar crystal orientation ratio.
The β-Si ₃ N ₄ columnar crystals were generally oriented parallel to the longitudinal direction. That is, the sintered bodies of Examples 1 to 9 are all oriented portions 12. Table 2 shows the results of measuring the four-point bending strength at room temperature after processing according to JIS R-1601 and the results of measuring the fracture toughness after processing according to JIS R-1607. (Comparative Example 1) "Heating other than microwave"<FiringStep> The compacts formed in the same manner as in Examples 1 to 8 were sintered in the same manner as in Examples 1 to 8 under the conditions shown in Table 1. The dimensions of the obtained sintered body were about 4 × 4.8 × 40 mm, and the relative densities and α conversions are shown in Table 1. <First heat treatment step> As shown in a schematic diagram in FIG. 2, a temperature distribution was provided in the longitudinal direction of the sintered body 1 under a nitrogen atmosphere of 1 atm using carbon heaters 2a to 2d. The temperature of the heater 2a on the one end 10 side is set to 1600 ° C., and then the temperature is raised to 1600 ° C. in the order of the second heater 2b and the third heater 2c.
After 10 minutes, the temperature of the heater 2d on the other end 11 side was set to 1600 ° C. <Second heat treatment step> Thereafter, heat treatment was performed for 4 hours in the same manner as in Examples 1 to 8 under the temperature and pressure conditions shown in Table 1. <Test> X-ray diffraction analysis was performed on the obtained sintered body,
Table 2 shows the α conversion and β-Si ₃ N ₄ columnar crystal orientation ratio.
Table 2 shows the results of measuring the four-point bending strength at room temperature after processing according to JIS R-1601 and the results of measuring the fracture toughness after processing according to JIS R-1607. (Comparative Example 2) "Conventional Method"<FiringStep> The compacts formed in the same manner as in Examples 1 to 8 were sintered at 1600 ° C in a furnace in a nitrogen atmosphere. The rate of temperature rise was 5 ° C./minute, and 1 atm until reaching 1600 ° C. After the temperature reached 1600 ° C., the pressure was increased to 2,000 at a pressure of 15 atm / minute. 16
The holding time at 00 ° C. and 2000 atm is 4 hours.

The size of the obtained sintered body is about 4 × 4.8 × 4
0 mm, and the relative density and the ratio of pregelatinization are shown in Table 1. <Test> X-ray diffraction analysis was performed on the obtained sintered body,
Table 2 shows the α conversion and β-Si ₃ N ₄ columnar crystal orientation ratio.
Table 2 shows the results of measuring the four-point bending strength at room temperature after processing according to JIS R-1601 and the results of measuring the fracture toughness after processing according to JIS R-1607. (Comparative Examples 3 and 4) "Limitations of upper limit and lower limit"<Firingstep> A molded body formed in the same manner as in Examples 1 to 8 was sintered as in Examples 1 to 8 under the conditions shown in Table 1. did. The dimensions of the obtained sintered body were about 4 × 4.8 × 40 mm, and the relative densities and α conversions are shown in Table 1. <First heat treatment step> When heat treatment was performed in exactly the same manner as in Examples 1 to 8, warpage occurred in the sintered body of Comparative Example 3, and thus the treatment of Comparative Example 3 was stopped at that point. <Second heat treatment step> The sintered body of Comparative Example 4 was heat-treated for 4 hours under the conditions shown in Table 1 in the same manner as in Examples 1 to 8. <Test> X-ray diffraction analysis was performed on the obtained sintered body,
Table 2 shows the α conversion and β-Si ₃ N ₄ columnar crystal orientation ratio.
Table 2 shows the results of measuring the four-point bending strength at room temperature after processing according to JIS R-1601 and the results of measuring the fracture toughness after processing according to JIS R-1607. (Evaluation) In Comparative Example 1, since the first heat treatment step was performed by giving a temperature distribution by the carbon heaters 2a to 2d, the orientation ratio was as low as 8% as compared with the embodiment using microwaves. Therefore, although the strength is equivalent to that of Example 6, no improvement in the fracture toughness value is observed.

In Comparative Example 2, the α-form of the sintered body after the firing step was 0%, and at that time, β-Si ₃ N ₄ columnar crystals were all precipitated in three-dimensional random directions. Therefore, the orientation ratio is as low as 4%, and as a result, the bending strength and the fracture toughness value are lower than those of the examples. In Comparative Example 3, since the relative density of the sintered body after the firing step was as low as 65.9%, deformation occurred in the first heat treatment step. Further, in Comparative Example 4, the α-formation ratio of the sintered body after the firing step was as low as 2%, and almost all β-Si ₃ N ₄ columnar crystals were precipitated in a three-dimensional random direction at that time as in Comparative Example 2. ing. Therefore, even if the second heat treatment step was performed, the β-Si ₃ N ₄ columnar crystals could not be oriented, the orientation ratio was 3%, and the bending strength and the fracture toughness were the same as in Comparative Example 2. It is a low value compared to.

On the other hand, each of the sintered bodies of the examples shows high bending strength and fracture toughness, and has high strength and high toughness. This is based on the high orientation ratio. From the comparison between Examples 1 to 8 and Comparative Example 1, it is clear that a high orientation ratio cannot be obtained unless microwave heating is used. When the four-point bending strength of the sintered bodies of Example 2 and Comparative Example 2 in the air at 1000 ° C. was measured, each was 17%.
It was 60 MPa and 1280 MPa, and it was clear that the sintered body of the example was superior to the conventional sintered body even in high-temperature strength.

[0021]

[Table 1]

[0022]

[Table 2] Example 9 <Molding Step> Si ₃ N ₄ powder (average particle size 0.2 μm, α
6% by weight of Y ₂ as a sintering aid.
O ₃ powder (average particle size 0.3 μm, purity 99.9%),
4 wt% MgAl ₂ O ₄ powder (average particle size 0.3 μm,
(Pure 99.9%) in ethanol, dried and pulverized, packed in a silicon tube, and pressurized with a pressure of 800 kgf / cm ² to form C.
IP molded. The molded body was packed in a thin rubber bag, vacuum-encapsulated, and then pressurized again with CIP at a pressure of 3000 kgf / cm ² ,
It was then subjected to lathe processing to obtain a piston pin-shaped cylindrical body having a diameter of 24 mm and a length of 100 mm. <Firing step> The above compact was placed in a furnace in a nitrogen atmosphere for 1400
Sintered at ℃. The rate of temperature rise is 5 ° C./minute, and 1400
The pressure was kept at 1 atm until the temperature reached ° C. After reaching 1400 ° C., the pressure was increased to 2000 atm at a pressure increase rate of 15 atm / min. The holding time at the maximum temperature and pressure is 4
Time.

The dimensions of the obtained sintered body are φ19 mm × length
80 mm, phase determined by the n-butanol substitution method
The pair density is 99.3%, α-Si determined by X-ray diffraction
_ThreeN _FourThe fraction was 55%. <First heat treatment step> The sintered body was gyrotron (28GH)
Microwave using high frequency high power oscillation tube
Place it on the heating device and rotate it around
The heat treatment was performed under a high pressure nitrogen atmosphere. At this time, micro
As shown in FIG. 3, the waves are applied along the longitudinal direction of the sintered body 3.
The output decreases from one end 30 to the other end 31.
Power distribution is provided so that the maximum temperature at one end 30 is 1
The temperature was set to 600 ° C. And the output of microwave is time and
After 30 minutes, the other end 31 also reaches 1600 ° C.
Was adjusted to <Second heat treatment step> Then, in the same furnace as the above-mentioned baking step
At 1600 ° C under a nitrogen atmosphere of 2000 atm for 4 hours
During the heat treatment.

When the obtained sintered body was analyzed by X-ray diffraction, it was found that the α-conversion rate was 0% and that all of the sintered body was converted to β-Si ₃ N ₄ . Further, the β-Si ₃ N ₄ columnar crystal was entirely oriented in parallel with the longitudinal direction, and the whole was the oriented part 33. Comparative Example 5 A compact formed in the same manner as in Example 9 was sintered at 1600 ° C. in a furnace in a nitrogen atmosphere. The rate of temperature rise was 5 ° C./minute, and 1 atm until reaching 1600 ° C. After the temperature reached 1600 ° C., the pressure was increased to 2,000 at a pressure of 15 atm / minute. 1600 ° C
And the holding time at 2000 atmospheres is 4 hours.

The relative density of the obtained sintered body determined by the n-butanol substitution method was 99.3%. (Evaluation) The piston pin-shaped sintered bodies of Example 9 and Comparative Example 5 were machined to φ18 mm × length 40 mm and surface roughness 0.2Z, respectively, and the sintered body 3 was broken by the apparatus shown in FIG. The load was measured. As a result, Example 9: Breaking load of 245 kN (average of n = 3, stress of 1600 MPa) Comparative Example 5: Breaking load of 160 kN (average of n = 3, stress of 1045 MPa), and Example 9 is Comparative Example 5 The value was significantly higher than that of. (Example 10) <Molding step> In the same manner as in Example 9, stem diameter φ12 mm × length 150 mm, umbrella part outer diameter 60 by CIP molding.
mm was formed into a bulb-shaped compact. <Firing Step> The molded body was sintered in the same manner as in Example 9.
As shown in FIG. 5, the shape of the obtained sintered body 4 was 9.5 mm in diameter of the stem portion 40 × 120 mm in length, and the outer diameter of the umbrella portion 41 was 48 mm. The relative density determined by the n-butanol substitution method was 99.2%, and the degree of pregelatinization determined by the X-ray diffraction method was 55%. <First heat treatment step> The sintered body was gyrotron (28GH)
z, 15 kW high-frequency high-power oscillation tube), and heat-treated in a nitrogen atmosphere at 1 atm while rotating about a shaft 42. This heat treatment was performed separately for each of the stem portion 40 and the umbrella portion 41.

As shown in FIG. 5, the stem portion 40 is provided with an output distribution such that the output decreases from one end 40a to the other end 40b as in the ninth embodiment.
The maximum temperature of a was 1600 ° C. Then, the output of the microwave is changed with time, and after 30 minutes, the other end 40
b was also adjusted to 1600 ° C. Umbrella part 41 is shown in FIG.
As shown in the figure, the microwave is irradiated from a direction inclined with respect to the axial direction of the stem portion 40, and an output distribution is provided in a ridge direction on a plane including the axial direction of the stem portion 40, and
The maximum temperature on the 1a side was 1600 ° C. Then, the output of the microwave was changed with time, and the base 41b side was adjusted to 1600 ° C. after 30 minutes. <Second heat treatment step> Thereafter, heat treatment was performed for 4 hours at 1600 ° C under a nitrogen atmosphere of 2000 atm in the same furnace as the above-mentioned baking step.

When the obtained sintered body was analyzed by X-ray diffraction, it was found that the α-form was 0% and all of the sintered body was transformed into β-Si ₃ N ₄ . The β-Si ₃ N ₄ columnar crystal has a stem portion 40.
In the umbrella part 41, it was oriented radially in the ridge line direction. That is, the orientation portions 43 and 44 are formed on the stem portion 40 and the umbrella portion 41, respectively. (Comparative Example 6) A molded article formed in the same manner as in Example 10
Sintering was performed at 1600 ° C. in a furnace in a nitrogen atmosphere. The rate of temperature rise was 5 ° C./minute, and 1 atm until reaching 1600 ° C. After the temperature reached 1600 ° C., the pressure was increased to 2,000 at a pressure of 15 atm / minute. Also 1600
The retention time at 2000C and 2000C is 4 hours.

The relative density of the obtained sintered body determined by the n-butanol substitution method was 99.3%. (Evaluation) The valve-shaped sintered bodies of Example 10 and Comparative Example 6 were machined to a stem of φ8 mm × length of 120 mm, an umbrella outer shape of φ30 mm, and a surface roughness of 0.2Z.
The breaking load of the sintered body 4 was measured using the apparatus shown in FIG. As a result, Example 10: Breaking load 16.4 kN (average of n = 3, stress of 1600 MPa) Comparative Example 6: Breaking load of 9.9 kN (average of n = 3, stress of 1045 MPa) Showed a significantly higher value than Comparative Example 6. (Example 11) <Molding step> Si ₃ N ₄ powder (average particle size 0.2 μm, α
6% by weight of Y ₂ O as a sintering aid.
₃ powder (average particle diameter 0.3 μm, purity 99.9%) and 4 wt% MgAl ₂ O ₄ powder (average particle diameter 0.3 μm, purity 99.9%) are mixed in ethanol, dried and pulverized. A raw material having a composition consisting of 100 parts by weight of powder, 5 parts by weight of ethylene-vinyl acetate copolymer, 5 parts by weight of polybutyl methacrylate, 5 parts by weight of modified wax, and 1 part by weight of dibutyl phthalate was mixed using a pressure kneader, Injection molded.

After degreased in a nitrogen atmosphere at 450 to 500 ° C., a film is formed from liquid rubber, dried, and then dried by CIP.
It was pressurized at a pressure of 00 kgf / cm ^{2 to} obtain a molded body for a radial turbo rotor. <Firing Step> The molded body was sintered in the same manner as in Example 9.
The shape of the obtained sintered body is an inlet diameter φ 68 mm × an outlet diameter φ
It was 52 mm. The relative density determined by the n-butanol substitution method was 99.2%, and the degree of pregelatinization determined by the X-ray diffraction method was 55%. <First heat treatment step> The sintered body 5 was gyrotron (28G
(Hz, 15 kW high-frequency high-output oscillation tube), and heat-treated in a nitrogen atmosphere at 1 atm while rotating about a shaft. This heat treatment was performed separately on the inlet-side hub rear surface 50 and the outlet-side wing root portion 51.

At the inlet-side hub rear surface 50 and the outlet-side wing root portion 51, as shown in FIG. 7, the focus of the microwave was narrowed to 40 mm, and an output distribution was provided from the outer peripheral side toward the axial center. Then, the maximum temperature on the outer peripheral side was set to 1600 ° C., and the output of the microwave was changed with time, and the axial center side was adjusted to 1600 ° C. after 30 minutes. <Second heat treatment step> Thereafter, heat treatment was performed for 4 hours at 1600 ° C under a nitrogen atmosphere of 2000 atm in the same furnace as the above-mentioned baking step.

When the obtained sintered body was analyzed by X-ray diffraction, it was found that the α-conversion rate was 0% and that all of the sintered body was converted to β-Si ₃ N ₄ . Further, the β-Si ₃ N ₄ columnar crystal was oriented from the outer periphery toward the axial center from the outer periphery of the inlet-side hub rear surface 50 and the outlet-side wing root portion 51, and oriented portions 52 and 53 were formed, respectively. (Comparative Example 7) A molded body formed in the same manner as in Example 11 was
Sintering was performed at 1600 ° C. in a furnace in a nitrogen atmosphere. The rate of temperature rise was 5 ° C./minute, and 1 atm until reaching 1600 ° C. After the temperature reached 1600 ° C., the pressure was increased to 2,000 at a pressure of 15 atm / minute. Also 1600
The retention time at 2000C and 2000C is 4 hours.

The relative density of the obtained sintered body determined by the n-butanol substitution method was 99.3%. (Evaluation) The rear surfaces of the hubs of the turbo rotors of Example 11 and Comparative Example 7 were machined to a surface roughness of 3.2Z, a metal shaft having a diameter of 6 mm was joined by means of a soldering fit, and a cold spin test was performed. The numbers were each measured. As a result, Example 11: Breaking rotation speed 1720,000 rotations / minute (average of n = 3) Comparative Example 7: Breaking rotation speed 1370,000 rotations / minute (average of n = 3) Example 11 showed a markedly higher value than Comparative Example 7.

[0033]

That is, according to the turbo rotor of the present invention, both the high strength and high toughness, which have been considered difficult in the past, can be obtained.
Provided, the field of use will be expanded more and more. According to the manufacturing method of the present invention, a silicon nitride sintered body having high strength and high toughness can be easily and reliably manufactured.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a state in which a first heat treatment step is performed in a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a state in which a first heat treatment step is performed in a first comparative example.

FIG. 3 is an explanatory view showing a state in which a first heat treatment step is performed in a ninth embodiment of the present invention.

FIG. 4 is an explanatory view showing a state in which a breaking strength test is performed in a ninth embodiment of the present invention.

FIG. 5 is an explanatory view showing a state in which a first heat treatment step is performed in a tenth embodiment of the present invention.

FIG. 6 is an explanatory view showing a state in which a breaking strength test is performed in a tenth embodiment of the present invention.

FIG. 7 is an explanatory view showing a state in which a first heat treatment step is performed in an eleventh embodiment of the present invention.

[Explanation of symbols]

1,3,4,5: Sintered body 12,33,43,4
4, 52, 53: orientation part

Claims (2)

(57) [Claims] 1. A step of preparing a silicon nitride sintered body containing 20% by volume or more of α-Si ₃ N ₄ and having a relative density of 98% or more, and applying a temperature gradient to the silicon nitride sintered body at 1500 ° C.
A first heat treatment step in which the heat treatment is performed as described above, and β-Si ₃ N ₄ columnar crystals are one-dimensionally oriented in a certain direction and precipitated by α-β transition; and the silicon nitride sintered body after the first heat treatment step is 1500
A high-strength and high-toughness silicon nitride sintered body, characterized by sequentially performing a second heat treatment step of maintaining the temperature at not less than 0 ° C. or more and growing the precipitated β-Si ₃ N ₄ columnar crystals in grains. 2. An oriented part substantially consisting of β-Si ₃ N ₄ columnar crystals, wherein the β-Si ₃ N ₄ columnar crystals are one-dimensionally oriented in a predetermined direction on the back surface of the hub and at the root of the outlet side wing. A turbo rotor, wherein at least 20% by volume of β-Si ₃ N ₄ columnar crystals are one-dimensionally oriented in a fixed direction from the outer peripheral side toward the axial direction.