JP2534214B2 - Silicon nitride sintered body and method for manufacturing 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 excellent in 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.

[0006] For the purpose of improving heat resistance,
In the sintered body disclosed in Japanese Patent Laid-Open No. 56-59674
Melilite mineral phase (Y₂O₃・ Si₃N_FourCompound)
Sintered silicon nitride sintered body, and JP-A-62-20286
Zirconium oxide (Zr
O₂) + Yttrium oxide (Y₂O₃) + Silicon oxide (Si
O₂) Is added, and ZrO is added to the sintered body. ₂Deposited silicon nitride
An elemental sintered body has been tried, and it is effective in improving high temperature strength.
It is known to be recognized.

Further, a J-phase (Si ₂ N ₂ O.2Y ₂ O ₃ ) solid solution is formed in the grain boundary phase in a sintered body containing the rare earth oxide ZrO ₂ disclosed in JP-A-62-246865. The existing silicon nitride sintered bodies have been tried, and it is known that they are effective in improving heat resistance, oxidation resistance and static fatigue characteristics.

Zircon (Z) as a sintering aid of silicon nitride is used together with yttrium oxide (Y ₂ O ₃ ) for the purpose of improving the oxidation resistance of a sintered body using ZrO ₂ as a sintering aid.
A system to which rSiO ₄ ) is added is disclosed in Japanese Patent Application Laid-Open No. 2-107566.

However, although the above materials have excellent high-temperature immediate rupture strength, the toughness and oxidation resistance have not been dramatically improved while maintaining the high-temperature strength.

For example, in a system using zirconium oxide (ZrO ₂ ) as a sintering aid, Zr added during the sintering process
A part of O ₂ reacts with Si ₃ N _4, and remains as zirconium nitride (ZrN) in the sintered body.

This ZrN changes from ZrN to Zr in a high temperature atmosphere.
It is known that about 30% volume expansion occurs in the process of oxidation to O ₂ , causing cracks on the surface of the sintered body and significantly deteriorating the strength characteristics.

Therefore, the sintered body containing ZrN has a problem that it lacks reliability as a high temperature structural material. However, it was difficult to completely suppress the residual ZrN.

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to provide a silicon nitride sintered body which has heat resistance such as a small decrease in mechanical strength even in a high temperature oxidizing atmosphere and has high toughness. And to provide a manufacturing method thereof.

[0014]

The silicon nitride-based sintered body of the present invention contains silicon nitride (Si ₃ N ₄ ) as a main component and has a Zr ₃ Er ₄ O ₁₂ crystal phase as a grain boundary phase. It is a feature.

The manufacturing method is as follows: erbium oxide (Er ₂ O ₃ ) 5 to 15% by weight, zircon (ZrSi
O ₄ ) 1 to 20% by weight, and the balance silicon nitride (Si
₃ N ₄ ) mixed powder is molded, the molded body is sintered in a nitrogen gas atmosphere in a temperature range of 1700 to 2000 ° C., and Z is used as a grain boundary phase by at least one of the following means.
It is characterized in that an r ₃ Er ₄ O ₁₂ crystal phase is produced.

In order to crystallize the Zr ₃ Er ₄ O ₁₂ phase as a grain boundary phase, cooling is performed at a temperature lowering rate of 5 ° C./min or less in the temperature lowering process of sintering, or 1500 to 1,700 in the temperature lowering process.
After the heat treatment of holding at 2 ° C. for 2 hours or more, or after the sintering, at least one treatment of 1500 to 1700 ° C. and the reheating treatment of holding for 2 hours or more is performed.

As the grain boundary phase of the sintered body in the present invention,
It is preferred that substantially only the Zr ₃ Er ₄ O ₁₂ crystalline phase be present.

Here, the Zr ₃ Er ₄ O ₁₂ crystal phase has the same type of diffraction line as 3ZrO ₂ .2Er ₂ O ₃ identified by the powder X-ray diffraction method and shown in JCDPS card 30-539, and has a high temperature oxidizing atmosphere. It is a crystalline phase with a high melting point that is stable inside.

The above-mentioned problems can be solved by suppressing the formation of the ZrN phase and by converting Zr into a stable compound Zr ₃ Er ₄ O ₁₂ by the crystallization treatment after sintering.

In the present invention, Er _{2 is} used as a sintering aid.
The O ₃ is used but, Er ₂ O ₃ is a crystal phase transition from Si ₃ N alpha phase during sintering of ₄ to β-phase has a function to promote its melt during further columnar phase the Si ₃ N ₄ The high temperature strength and toughness are improved by promoting the growth of.

Further, in the present invention, Er₂O₃Is sintered
Zr during cooling or reheating SiO_FourReact with
And Zr₃Er_FourO₁₂Generate phases.

If the amount of addition of Er ₂ O ₃ exceeds 15% by weight, the mechanical strength and oxidation resistance of the obtained sintered body at high temperature are deteriorated, so the content is preferably 15% by weight or less.

On the other hand, if the amount is less than 5% by weight, the melt is insufficient, and the relative density is 95% or less, which is not sufficiently densified, which is not preferable.

Therefore, the addition amount is 5 to 15% by weight.
It is often in the range of, especially high mechanical strength,
In order to obtain toughness, the range is more preferably 7 to 12% by weight.

ZrSiO ₄ forms a liquid phase during sintering together with Er ₂ O ₃ , but it has more oxygen atoms than ZrO _2, and therefore ZrN is less likely to be produced.

Even if ZrO ₂ and SiO ₂ are added as a sintering aid, the same molar ratio can be obtained, but ZrO ₂
In order to suppress the nitriding, it is necessary that each auxiliary agent be uniformly dispersed during kneading.

ZrSiO ₄ , which is already present as an oxygen-rich compound at the auxiliary stage, is more advantageous for suppressing the formation of ZrN.

Further, the ZrSiO ₄ phase reacts with Er ₂ O ₃ during the cooling process of sintering or the reheating process, and the Zr ₃ Er ₄ O ₁₂ phase having a high melting point and stable in a high temperature oxidizing atmosphere is sintered. By precipitating in the grain boundary phase of, excellent high temperature characteristics can be obtained.

In the present invention, 1 to 20% by weight of ZrSiO ₄ is contained as a sintering aid, but if it is less than 1% by weight, it is difficult to obtain a sufficiently dense sintered body, and if it is more than 20% by weight, a sufficiently high temperature is obtained. No strength can 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 does not matter if it is 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.

Er ₂ O ₃ and ZrS added as sintering aids
In order to obtain a homogeneous and high density sintered body, iO _{4 is} also preferably fine particles having an average particle diameter of 2 μm or less.

In the method of the present invention, these components are mixed with each other 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. Do it. The mixed powder thus adjusted is subjected to pressure molding to obtain a molded body having a desired shape.

This molded body was heated 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, an atmospheric pressure sintering method, a gas pressure sintering method, a hot isostatic pressing sintering method, a hot pressing sintering method can be used, and one or more firings can be further performed. It is also possible to combine the methods.

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

Si ₃ N ₄ is about 185 under 1 atmosphere 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 other inert gas such as Ar may be contained.

Sintering is carried out in the temperature range of 1700 to 2000 ° C., but if it is less than 1700 ° C., the growth of β grains of Si ₃ N ₄ is insufficient and high toughness cannot be obtained. No density can be obtained. Β-S generated above 2000 ° C
i ₃ N ₄ Needle-shaped grains grow remarkably and the strength decreases.

Further, during sintering, Si ₃ N ₄ is dissolved and reprecipitated in a liquid phase composed of a sintering aid to cause a crystal phase transition, resulting in densification and progress of sintering. During this dissolution / reprecipitation process, since there is a solid solution limit of Si ₃ N ₄ in the melt, holding for 30 minutes or more is preferable.

In order to crystallize the Zr ₃ Er ₄ O ₁₂ phase as a grain boundary phase, cooling is performed at a temperature lowering rate of 5 ° C./min or less during the temperature lowering process of sintering, or 1500 to 1700 during the temperature lowering process.
At least one means of applying heat treatment at 2 ° C. for 2 hours or more, or performing reheating treatment at 1500 to 1700 ° C. for 2 hours or more in a nitrogen atmosphere after sintering is applied.

When the Zr ₃ Er ₄ O ₁₂ phase is precipitated in the temperature lowering process, the temperature lowering rate is preferably 5 ° C./min or less, more preferably 2 ° C./min or less.

When the temperature lowering rate is faster than 5 ° C./minute, Zr ₃
Er ₄ O ₁₂ phase is not sufficiently formed. In addition, the holding temperature during the temperature lowering process and the temperature during the reheating treatment are 1500 ° C.
If the temperature is less than 1,700 ° C., the Zr ₃ Er ₄ O ₁₂ phase is not sufficiently formed.

The method for producing a silicon nitride sintered body of the present invention is as follows:
Er ₂ O ₃ 5 to 15 wt%, ZrSiO ₄ 1 to 20 wt%, and the balance by molding a mixed powder consisting of Si ₃ N _4, a molded article in the temperature range of 1700-2000 ° C. in a nitrogen gas atmosphere The Zr ₃ Er ₄ O ₁₂ crystal phase is formed as a grain boundary phase by sintering and a temperature lowering process or a reheating treatment, but this problem was achieved only by combining these conditions.

[0045]

The sintered body obtained according to the present invention has a large average grain size of Si ₃ N ₄ of about 1 to ₃ μm and exhibits a structure in which columnar crystal grains are entangled with each other, and further has a high melting point as a grain boundary crystal phase and high temperature. Due to the existence of a stable Zr ₃ Er ₄ O ₁₂ crystal phase in an oxidizing atmosphere, it has a high toughness while maintaining a high strength in a high temperature atmosphere, and a flexural strength of 1400 in the atmosphere.
High strength of 500 MPa or more at ℃ and toughness value K _IC of 5
It has a high toughness of at least MPam ^1/2 .

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
Er ₂ O ₃ powder (average particle size 0.8 μm) and ZrSiO ₄ powder (average particle size 0.3 μm) were added in a predetermined amount (% by weight) shown in Table 1 to 7% or more), and acetone was used as a solvent. It was kneaded for 24 hours with a ball mill made of Si ₃ N ₄ .

Next, using the obtained mixed powder, after molding,
Pressureless sintering was performed. Molding conditions include a mold 1 axis molding pressure 1
50 MPa, cold hydrostatic pressure 500 MPa,
A plate-like body having a size of 50 mm × 50 mm × 10 mm was obtained.

As the atmospheric pressure sintering conditions, the temperature was maintained at 1700 to 1800 ° C. for 6 hours in a nitrogen atmosphere of 1 atm.

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. /
If it is less than a minute and is kept in the temperature lowering process, 1550 to 1
The temperature was held at 650 ° C. for 6 hours, and when reheating treatment was performed after sintering, the temperature was held at 1500 to 1700 ° C. for 12 hours.

The characteristics of each of the sintered bodies obtained according to the present invention are shown by the amount of sintering aid added, the normal pressure sintering temperature, the crystallization conditions, and Zr ₃ E.
Table 1 shows the presence or absence of the r ₄ O ₁₂ crystalline 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., in consideration of the oxidative deterioration in the air, a test piece preliminarily kept in the air at 1400 ° C. for 24 hours was used.

Regarding toughness, JIS R1607 was used at room temperature.
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 bending strength and toughness, but the samples corresponding to the comparative examples have particularly high temperature bending strength as compared with the examples of the present invention. Was confirmed to be inferior.

In each case of the present invention, the presence of the Zr ₃ Er ₄ O ₁₂ crystal phase was 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, after producing a sintered body by the atmospheric pressure sintering method as in Example 1, the temperature was 1700 to 2000 ° C. under a pressure of 4 MPa in a nitrogen gas atmosphere.
Resintering was performed under the condition that the holding time was 4 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.

When any of the pressure sintering methods is used, after sintering, the temperature is 1550 ° C. and the holding time is 12 at the atmospheric pressure in a nitrogen atmosphere.
It was reheated for an hour.

Similar to Example 1, the characteristic values of the sintered body produced under the conditions outside the range of the present invention are shown in Table 2 as a comparative example.

As in Example 1, the characteristics of the sintered body are shown in Table 2 together with the amount of the sintering aid added, the sintering conditions, and the presence or absence of the Zr ₃ Er ₄ O ₁₂ crystal phase.

Similar to Example 1, the sintered body according to the present invention is excellent in both bending strength and toughness. However, the samples corresponding to Comparative Examples have particularly high high temperature bending strength and toughness as compared with the Examples of the present invention. Was confirmed to be inferior.

In the case of the present invention, as in Example 1, the existence of the Zr ₃ Er ₄ O ₁₂ phase was confirmed by the X-ray diffraction method.

[0075]

[Table 4]

[0076]

[Table 5]

[0077]

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

As a result, it becomes possible to produce a silicon nitride-based sintered body having a very high reliability, and its industrial utility is very large.

Claims (2)

(57) [Claims] 1. A silicon nitride-based sintered body comprising silicon nitride (Si ₃ N ₄ ) as a main component and having a Zr ₃ Er ₄ O ₁₂ crystal phase as a grain boundary phase. 2. A mixed powder composed of 5 to 15% by weight of erbium oxide (Er ₂ O ₃ ), 1 to 20% by weight of zircon (ZrSiO ₄ ) and the balance of silicon nitride (Si ₃ N ₄ ) is molded, 1700 to 200 in a nitrogen gas atmosphere
A method for producing a silicon nitride sintered body, which comprises sintering in a temperature range of 0 ° C. and producing a Zr ₃ Er ₄ O ₁₂ crystal phase as a grain boundary phase by at least one of the following means. The temperature lowering rate in the temperature lowering process of sintering shall be 5 ° C / min or less.
It 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, reheating treatment is performed for 2 hours or more in a temperature range of 1500 to 1700 ° C. in a nitrogen atmosphere.