JP2652936B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-density silicon nitride sintered body having excellent strength and toughness in a high-temperature atmosphere and a method for producing the same.

[0002]

2. Description of the Related Art Silicon nitride is a substance having a strong covalent bond and has excellent properties such as strength, hardness, heat resistance and chemical stability. Application to such applications is under consideration.

[0003] With the increase in the efficiency of the engine, utilization at a temperature of 1400 ° C or higher is expected, and a material having high strength, high toughness and high oxidation resistance that can be used under these conditions is desired. .

Since it is difficult to sinter silicon nitride alone, it is generally sintered with various additives.

For example, in a system to which yttrium oxide (Y ₂ O ₃ ) and aluminum oxide (Al ₂ O ₃ ) are added, excellent heat shock resistance is obtained, but heat resistance, mechanical strength and toughness are obtained. Was sometimes inferior.

For the purpose of improving heat resistance, a melilite mineral phase (Y ₂ O ₃ .Si ₃ N ₄ compound) was formed in a sintered body disclosed in Japanese Patent Application Laid-Open No. 56-59674. Silicon nitride sintered body and Japanese Patent Laid-Open No. 62-20286
No. 4 discloses a zirconium oxide (Zr
O ₂ ) + yttrium oxide (Y ₂ O ₃ ) + silicon oxide (Si
O ₂ ) has been added, and a silicon nitride sintered body in which ZrO ₂ is precipitated in the sintered body has been tried, and it is known that the effect of improving the high-temperature strength is recognized.

Furthermore, rare earth oxides as disclosed in JP-A-62-246865, J phase in the grain boundary phase in the sintered body containing _{Zr0 2 (Si 2 N 2 O} · 2Y 2 O 3) is a solid solution An existing silicon nitride sintered body has been tried, and it is known that the effect is recognized in improving heat resistance, oxidation resistance, and static fatigue properties.

In Japanese Patent Application Laid-Open No. 3-153574, HfO ₂ is added as a sialon sintering aid and Y ₂ H is used as a grain boundary phase in order to further improve the high temperature strength characteristics.
Disclosed are α'-β 'sialons that produced f ₂ O ₇ .

[0009] However, although the above materials have excellent high-temperature instantaneous breaking strength, they have not been able to dramatically improve toughness and oxidation resistance while maintaining high-temperature strength. When applied to a structural member in which collision of particles or the like occurs, there is a problem that reliability is lacking.

Therefore, there is a need for a material having improved oxidation resistance and toughness in addition to improved high-temperature strength.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems.

An object of the present invention is to provide a silicon nitride sintered body having heat resistance such as a small decrease in mechanical strength even in a high-temperature oxidizing atmosphere and having high toughness, and a method for producing the same. Is to do.

[0013]

The silicon nitride sintered body of the present invention contains silicon nitride (Si ₃ N ₄ ) as a main component, and has a Y ₂ grain boundary.
Hf ₂ O ₇ phase and Y ₅ N (SiO ₄ ) ₃ phase are present.

The manufacturing method is as follows: yttrium oxide (Y ₂ O ₃ ) 3 to 10% by weight, hafnium oxide (Hf
O ₂ ) is formed into a mixed powder consisting of 0.1 to 3% by weight, silicon oxide (SiO ₂ ) 1 to 5% by weight and the balance silicon nitride (Si ₃ N ₄ ), and the formed body is placed in a nitrogen gas atmosphere. 1700
Sintered in a temperature range of 2000 ° C., and as a grain boundary phase, a Y ₂ Hf ₂ O ₇ phase and a Y ₅ N
(SiO ₄ ) It is characterized in that _three phases are crystallized.

As a grain boundary phase, a Y ₂ Hf ₂ O ₇ phase and a Y ₅ N
In order to crystallize the (SiO ₄ ) ₃ phase, cooling is performed at a cooling rate of 5 ° C./min or less during the cooling step of sintering, or at 1500 to 1700 ° C. for 2 hours or more during the cooling step. After sintering, in a nitrogen atmosphere 1500 to 17
At least one of a reheating treatment at 00 ° C. for 2 hours or more.
To perform two processes.

As the grain boundary phase of the sintered body in the present invention,
From the viewpoint of toughness and oxidation resistance, Y_TwoHf_TwoO
₇Phase and Y_FiveN ( SiO_Four)_ThreePhase exists as grain boundary phase
Is preferred.

Not only the Y ₂ Hf ₂ O ₇ phase and the Y ₅ N (SiO ₄ ) ₃ phase exist, but also the Y ₂ Si ₂ O ₇ phase and the Y ₂ Si ₃ N ₄ phase.
Some amount of O ₃ or glass phase may be present.

Here, the Y ₂ Hf ₂ O ₇ crystal phase has the same type of diffraction line as Y ₂ O ₃ .2HfO ₂ identified by the powder X-ray diffraction method and indicated by JCPDS card 24-1406.
A crystalline phase that is stable in a high-temperature atmosphere.

The Y ₅ N (SiO ₄ ) ₃ phase is JCPD
10Y ₂ O ₃ · 9Si represented by the S card 30-1462
It has the same type of X-ray diffraction lines as O ₂ .Si ₃ N ₄ and is a high melting point N-apatite crystal phase that is stable in a high-temperature oxidizing atmosphere.

In the present invention, Y ₂ O _{3 is} used as a sintering aid.
Y ₂ O ₃ is converted from α phase to β during sintering of Si ₃ N _4.
It has the function of promoting the crystal phase transition to the phase in the melt, and further enhances the high-temperature strength and toughness by promoting the growth of the columnar phase of Si ₃ N ₄ .

[0021] In the present invention, Y ₂ O ₃ is in the cooling process or reheat treatment after sintering Si ₃ N ₄ and Si
Reacts with O ₂ to form Y ₅ N (SiO ₄ ) ₃ phase.

When the addition amount of Y ₂ O ₃ exceeds 10% by weight,
Since the mechanical strength and oxidation resistance at a high temperature of the obtained sintered body are reduced, the content is set to 10% by weight or less.

If the content is less than 3% by weight, the melt is insufficient, and the relative density is not more than 95%, which is not preferable because sufficient densification cannot be achieved.

Therefore, the amount of addition is 3 to 10% by weight.
In order to obtain a sufficiently high mechanical strength and toughness, the range is more preferably 4 to 7% by weight.

HfO ₂ forms a liquid phase together with Y ₂ O ₃ during sintering.
By precipitating in the grain boundary phase of the sintered body as a ₂ Hf ₂ O ₇ phase, excellent high-temperature characteristics can be obtained.

The addition of HfO ₂ has the effect of promoting the formation of a Y ₅ N (SiO ₄ ) ₃ phase in the grain boundary phase.

In the present invention, HfO ₂ is used as a sintering additive in an amount of 0.1%.
If the content is less than 0.1% by weight or more than 3% by weight, sufficient high-temperature strength cannot be obtained.

SiO ₂ is formed by sintering the above Y ₂ O ₃ and HfO ₂
Has the effect of lowering the melting point of the melt formed by the above, promotes densification, and reacts with Si ₃ N ₄ and Y ₂ O _{3 in} the cooling step or reheating treatment after sintering, and , A stable high-melting point Y ₅ N (SiO ₄ ) ₃ phase is generated, so that the strength is maintained up to a high temperature.

In the present invention, SiO ₂ is used as a sintering aid in an amount of 1 to ₂ .
Although the content is 5% by weight, if it is added more than 5% by weight, the mechanical strength at high temperature decreases, and if it is less than 1% by weight, a sufficiently dense sintered body cannot be obtained.

The Si ₃ N ₄ powder used in the present invention is Si ₃ N ₄ powder having an α-type crystal structure is preferable from the viewpoint of sintering property, beta type or amorphous Si ₃ N ₄ powder It may be included.

In order to obtain a sufficiently high density during sintering,
Fine particles having an average particle diameter of 1 μm or less are desirable.

Y ₂ O ₃ , Hf added as a sintering aid
O ₂ and SiO ₂ are also preferably fine particles having an average particle size of 2 μm or less in order to obtain a homogeneous and high-density sintered body.

In the method of the present invention, these components are mixed using a solvent such as purified water, acetone or ethanol, and a planetary mixer or a pot mill mixer using Si ₃ N ₄ or SiC pots and balls. Perform at The mixed powder thus adjusted is subjected to pressure molding to obtain a molded body having a desired shape.

The molded body was cooled to 170 in a nitrogen gas atmosphere.
Sintering is performed in a temperature range of 0 to 2000 ° C. to obtain a sintered body. As the sintering method, it is possible to use a normal pressure sintering method, a gas pressure sintering method, a hot isostatic press sintering method, a hot press sintering method, and further combine one or more sintering methods. It is also possible.

The atmosphere during sintering is preferably a nitrogen gas atmosphere in order to suppress the decomposition of Si ₃ N ₄ at high temperatures.

Si ₃ N ₄ is approximately 185 at 1 atm of nitrogen gas.
Since decomposition occurs at 0 ° C. or higher, when sintering at 1850 ° C. or higher, the nitrogen gas pressure is set to Si ₃ N _{4 at the} sintering temperature.
Is set to be equal to or higher than the critical decomposition pressure.

Here, the nitrogen gas is substantially N ₂ gas, but may contain other inert gas such as Ar.

The sintering is performed at a temperature in the range of 1700 to 2000 ° C., but if the temperature is lower than 1700 ° C., the growth of β grains of Si ₃ N ₄ is insufficient and high toughness cannot be obtained. Y ₅ N (SiO ₄ ) as grain boundary phase during heat treatment
_It is difficult to obtain _three phases.

When the temperature exceeds 2000 ° C., the β-Si ₃ N ₄ needle-like grains formed grow remarkably, and the strength decreases.

In sintering, Si ₃ N ₄ is dissolved and re-precipitated in a liquid phase composed of a sintering aid, thereby causing a crystal phase transition and densification and sintering. Dissolution
In the reprecipitation process, since there is a limit of solid solution of Si ₃ N ₄ in the melt, it is preferable to hold for 30 minutes or more.

In order to crystallize the Y ₂ Hf ₂ O ₇ phase and the Y ₅ N (SiO ₄ ) ₃ phase as the grain boundary phase, the sintering is cooled at a cooling rate of 5 ° C./min or less during the cooling process. Heat treatment at 1500 to 1700 ° C. for 2 hours or more during the temperature decreasing process, or sintering, and then 1500 to 17 ° C. in a nitrogen atmosphere.
At least one of reheating treatments held at 00 ° C. for 2 hours or more is performed.

During the cooling process, the Y ₂ Hf ₂ O ₇ phase and the Y ₅ N (S
iO ₄ ) When the _three phases are precipitated, the temperature decreasing rate is preferably 5 ° C / min or less, more preferably 2 ° C / min or less.

When the cooling rate is higher than 5 ° C./min, Y ₅ N
(SiO ₄ ) _Three phases are not sufficiently formed. Further, the holding temperature in the temperature decreasing process and the temperature in the reheating process are set to 1500.
When the temperature is lower than 1 ° C. and higher than 1700 ° C., Y ₅ N (SiO ₄ )
Insufficient formation of _three phases.

The method for producing a silicon nitride based sintered body of the present invention comprises:
Y_TwoO_Three3 to 10% by weight, HfO_Two0.1-3% by weight, S
iO_Two1 ~ 5% by weight and the balance is Si_ThreeN_FourMixed powder consisting of
A powder is formed, and the formed body is placed in a nitrogen gas atmosphere at 1700 to 1700.
Sintering in the temperature range of 2000 ° C, cooling down or reheating
Y as grain boundary phase by heat treatment_TwoHf_TwoO₇Phase and Y_FiveN
(SiO_Four)_ThreeIt produces a crystalline phase.
This task was achieved for the first time by a combination of conditions.

[0045]

The sintered body obtained according to the present invention has a structure in which the average crystal grain size of Si ₃ N ₄ is as large as about 1 to ₃ μm and columnar crystal grains are entangled, and a high melting point Y ₂ Hf ₂ O
_Seven phases are precipitated, and the coexistence of a stable Y ₅ N (SiO ₄ ) ₃ crystal phase in a high-temperature oxidizing atmosphere with a high melting point as a grain boundary crystal phase enables high toughness while maintaining high strength up to high temperatures. Having a flexural strength of 5 at 1400 ° C. in the atmosphere.
High strength of 00MPa or more and toughness value K _IC of 5MPam
^Has high toughness of ^1/2 or more.

In order to obtain a sintered body having particularly high bending strength and toughness, it is necessary to use a gas pressure sintering method, a hot isostatic press sintering method, or a pressure sintering method of a hot press method. preferable.

As shown in the embodiment described later, 14
A sintered body having a bending strength at 700C of 700 MPa;
Alternatively, a sintered body having an extremely high K _IC of 7 MPam ^1/2 is obtained.

In order to obtain a sintered body having a complicated shape, it is preferable to use a gas pressure sintering method or a hot isostatic press sintering method.

Next, examples of the present invention will be described together with comparative examples.

[0050]

【Example】

[0051]

Example 1 Si ₃ N ₄ (average particle size 0.5 μm, α conversion 9
7% or more), Y ₂ O ₃ powder (average particle size 0.3 μm), Hf
O ₂ powder (average particle diameter 1 μm) and SiO ₂ powder (average particle diameter 0.1 μm) were added in predetermined amounts (% by weight) shown in Table 1, and the mixture was mixed with a ball mill made of Si ₃ N ₄ using acetone as a solvent. Kneaded for 24 hours.

Next, using the obtained mixed powder, after forming, normal pressure sintering was performed. The molding conditions were a uniaxial molding pressure of 100 MPa and a pressure of 200 MPa by cold isostatic pressure to obtain a 50 mm × 50 mm × 10 mm plate.

The normal-pressure sintering was performed at 1700 to 1800 ° C. for 4 hours in a 1-atmosphere nitrogen atmosphere.

As a condition for crystallization of the grain boundary phase, when slow cooling in the cooling step after sintering is used, the cooling rate is 5 ° C. /
Minutes or less, and 1550 to 1 when the temperature is maintained during the cooling process.
The temperature was held at 650 ° C. for 4 hours, and when sintering was performed, the temperature was held at 1500 to 1700 ° C. for 8 hours.

The characteristics of each sintered body obtained according to the present invention were determined by the amount of a sintering aid added, normal pressure sintering temperature, crystallization conditions, Y ₂ Hf ₂
Table 1 shows the presence or absence of the O ₇ phase and the Y ₅ N (SiO ₄ ) ₃ phase.

The strength was measured at room temperature and atmospheric pressure in accordance with JIS R1601 and JIS R1604.
A three-point bending test was performed at 0 ° C., and the bending strength was measured.

At the time of the test at 1400 ° C., a test piece previously held in the air at 1400 ° C. for 24 hours was used in consideration of the oxidative deterioration in the air.

Regarding toughness, JIS R160 at room temperature
7, the fracture toughness value K _IC was measured by the SEPB method. The crystal phase of the sintered body was analyzed using an X-ray diffraction method.

Table 1 also shows, as comparative examples, the characteristic values of the sintered bodies produced under conditions outside the scope of the present invention.

As shown in Table 1, the samples according to the examples of the present invention are excellent in both the bending strength and the toughness, but the samples corresponding to the comparative examples are particularly higher in the high-temperature bending strength than the examples of the present invention. Was confirmed to be inferior.

In the case of the present invention, the presence of the Y ₂ Hf ₂ O ₇ phase and the Y ₅ N (SiO ₄ ) ₃ crystal phase were confirmed by the X-ray diffraction method.

[0062]

[Table 1]

[0063]

[Table 2]

[0064]

[Table 3]

[0065]

Example 2 After forming a mixed powder in the same manner as in Example 1,
A sintered body was produced by pressure sintering.

For pressure sintering, a gas pressure sintering method, a hot isostatic pressing method and a hot pressing method were used.

In the case of gas pressure sintering, a sintered body is prepared by the normal pressure sintering method in the same manner as in the first embodiment, and then at a temperature of 1700 to 1950 ° C. under a pressure of 4 MPa in a nitrogen gas atmosphere.
It was re-sintered under the condition of a holding time of 2 hours.

In the case of hot isostatic press sintering, a sintered body produced by a normal pressure sintering method is used in a nitrogen gas atmosphere.
It was re-sintered under the pressure of 00 MPa at a temperature of 1800 to 2000 ° C. and a holding time of 1 hour.

In the case of hot press sintering, the mixed powder is charged into a graphite die, and the mixture is placed in a 1 atm nitrogen gas atmosphere at 40 M
Sintering was performed under the conditions of 1750 to 1850 ° C. and a holding time of 2 hours under a pressure of Pa.

In any of the pressure sintering methods, after sintering, a reheating treatment was performed in a nitrogen atmosphere at atmospheric pressure at 1600 ° C. for a holding time of 8 hours.

The characteristic values of the sintered bodies produced under the conditions outside the scope of the present invention as in Example 1 are shown in Table 2 as comparative examples.

In the same manner as in Example 1, the characteristics of the sintered body were determined by adding the sintering aid, sintering conditions, Y ₂ Hf ₂ O ₇ phase, Y ₅ N (Si
Table 2 shows the presence or absence of O ₄ ) ₃ phases.

As in Example 1, the characteristics of the sintered body according to the present invention are excellent in both the bending strength and the toughness, but the samples corresponding to the comparative examples are particularly higher in the high-temperature bending strength and the toughness than the examples of the present invention. Was confirmed to be inferior.

In the case of the present invention, the presence of the Y ₂ Hf ₂ O ₇ phase and the Y ₅ N (SiO ₄ ) ₃ phase was confirmed by the X-ray diffraction method as in Example 1.

[0075]

[Table 4]

[0076]

[Table 5]

[0077]

[Table 6]

[0078]

According to the present invention, a silicon nitride sintered body having sufficient heat resistance as described above can have higher mechanical strength and toughness.

This makes it possible to produce a highly reliable silicon nitride sintered body, and its industrial utility is extremely large.

Claims (2)

(57) [Claims] 1. A silicon nitride-based sintering method comprising silicon nitride (Si ₃ N ₄ ) as a main component and Y ₂ Hf ₂ O ₇ phase and Y ₅ N (SiO ₄ ) ₃ phase at grain boundaries. Union. 2. Yttrium oxide (Y ₂ O ₃ ) 3 to 10% by weight, hafnium oxide (HfO ₂ ) 0.1 to 3% by weight,
A mixed powder consisting of 1 to 5% by weight of silicon oxide (SiO ₂ ) and the balance silicon nitride (Si ₃ N ₄ ) is formed, and the formed body is sintered in a nitrogen gas atmosphere at a temperature of 1700 to 2000 ° C. and, following Y ₂ Hf ₂ O ₇ phase and Y ₅ N production method of (SiO _{4) 3} phase, characterized in that to produce silicon nitride sintered body as a grain boundary phase by at least one means. The cooling rate in the cooling step of sintering is set to 5 ° C./min or less. In the process of decreasing the temperature of the sintering, the temperature is kept in a temperature range of 1500 to 1700 ° C. for 2 hours or more. After the sintering, a reheating treatment is performed in a nitrogen atmosphere in a temperature range of 1500 to 1700 ° C. for 2 hours or more.