JPH0669905B2 - Silicon nitride sintered body and method for manufacturing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a high-density silicon nitride sintered body excellent in high-temperature strength and toughness, and a method for producing the same.

Conventional technology Silicon nitride is a substance with a strong covalent bond,
Since it has excellent properties in heat resistance, chemical stability, etc., its application to structural members, particularly as a heat engine, such as gas turbine members, is being studied. As the engine becomes more efficient, it is expected to be used at a temperature of 1400 ° C or higher, and a material having high strength and high toughness that can be used under these conditions is desired.

Since it is difficult to sinter silicon nitride alone, it is generally sintered by adding various additives. For example, a system in which aluminum oxide (Al ₂ O ₃ ) is added with yttrium oxide (Y ₂ O ₃ ) has excellent thermal shock resistance, but it is inferior in heat resistance, mechanical strength, and toughness. There was a case.

For the purpose of improving heat resistance, JP-A-62-2028
Zirconium oxide (ZrO ₂ ) + yttrium oxide (Y ₂ O ₃ ) + silicon oxide (SiO ₂ ) disclosed in JP-A-64-
A silicon nitride sintered body in which ZrO ₂ is precipitated in the sintered body has been tried, and it is known to be effective in improving high temperature strength and the like.

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

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, although the above materials have excellent high temperature strength and oxidation resistance, they have not been able to dramatically improve toughness while maintaining high temperature strength. When applied to a structural member in which particles collide with each other in a combustion flame, there are problems such as lack of reliability. Therefore, it is desired to improve the toughness in addition to the high temperature strength.

The present invention has been made to solve the above 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 there.

Means for Solving the Problems The silicon nitride sintered body of the present invention is yttrium oxide (Y
₂ O ₃ ) 5 to 15% by weight, zirconium oxide (ZrO ₂ ) 0.1 to 3
% By weight, zirconium silicide (ZrSi ₂ ) 0.1 to 3% by weight, and the balance silicon nitride (Si ₃ N ₄ ), Y ₂ O ₃ , ZrO ₂ , ZrSi ₂
The weight ratio range of 1 ≦ Y ₂ O ₃ / (ZrO ₂ + ZrSi ₂ ) ≦ 12 is a sintered body formed by molding and sintering, and containing at least a Zr compound. _There is at least one crystalline phase selected from the ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase, and the bending strength is 600 MPa or more at 1400 ° C. in the atmosphere and the toughness value K at room temperature. It is characterized by an _IC of 7MPam1 / 2 or more.

As its manufacturing method, yttrium oxide (Y ₂ O ₃ ) 5 to
15% by weight, zirconium oxide (ZrO ₂ ) 0.1 to 3% by weight,
Zirconium silicide (ZrSi ₂ ) 0.1 to 3% by weight and the balance silicon nitride (Si ₃ N ₄ ) and the weight ratio range of Y ₂ O ₃ , ZrO ₂ , and ZrSi ₂ is 1 ≦ Y ₂ O ₃ / ( ZrO ₂ + ZrSi ₂ ) ≦ 12 mixed powder is molded, and the compact is pressure-sintered at a pressure of 4 MPa or more in a temperature range of 1800 to 2000 ° C. in an atmosphere containing nitrogen gas, and the temperature lowering process Alternatively, it is characterized in that at least one kind of crystallinity selected from the Y ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase is generated as the grain boundary phase by the reheating treatment.

As the grain boundary phase of the sintered body in the present invention, only a Zr compound and at least one crystal phase selected from Y ₅ N (SiO ₄ ) ₃ phase and Y _4.67 (SiO ₄ ) ₃ O phase may exist. However, a Y ₂ Si ₂ O ₇ phase or a glass phase may exist, but at least a Zr compound and at least a Y ₅ N (SiO ₄ ) ₃ phase and a Y _4.67 (SiO ₄ ) ₃ O phase are selected. It is preferable that one kind of crystal phase exists as a grain boundary phase.

Here, the Zr compound is zirconium oxide (ZrO ₂ ), zirconium nitride (ZrN), zirconium silicide (ZrSi ₂ ) or
Although it is a compound selected from Y ₆ ZrO ₁₁ , it is preferable that ZrO ₂ having an orthorhombic structure has a crystal structure that is relatively stable particularly at high temperatures. In addition, Y ₅ N (SiO ₄ ) ₃ phase and Y _4.67 (Si
The O ₄ ) ₃ O phase is a crystal phase with a high separation point.

Keep present invention uses a Y ₂ O ₃ as a sintering aid but, Y ₂ O ₃
Has the function of promoting the crystalline phase transition from the α phase to the β phase in the melt during the sintering of Si ₃ N ₄ , and by further promoting the growth of the columnar phase of Si ₃ N ₄ , the high temperature strength and Improve toughness. Further, in the present invention, Y ₂ O ₃ reacts with Si ₃ N ₄ by a sintering temperature lowering process or a reheating process to generate a Y ₅ N (SiO ₄ ) ₃ phase or a Y _4.67 (SiO ₄ ) ₃ O phase. .

If the amount of addition of Y ₂ O ₃ exceeds 15% by weight, the mechanical strength of the obtained sintered body at high temperature decreases, so it is preferably 15% by weight or less. Further, if it is less than 5% by weight, the melt is insufficient and sufficient densification cannot be achieved, which is not preferable. Therefore, the amount added is preferably in the range of 5 to 15% by weight, but more preferably in the range of 8 to 10% by weight in order to obtain sufficiently high mechanical strength and toughness.

ZrO ₂ forms a liquid phase at the time of sintering together with Si ₃ N ₄ , but by precipitating in the grain boundary phase of the sintered body as a Zr compound having a high melting point in the cooling process of sintering, excellent high temperature characteristics can be obtained. Obtainable. Further, the addition of ZrO ₂ promotes the production of Y ₅ N (SiO ₄ ) ₃ phase and Y _4.67 (SiO ₄ ) ₃ O phase in the grain boundary phase.

In the present invention, ZrO ₂ is contained as a sintering aid in an amount of 0.1 to 3% by weight, but if it exceeds 3% by weight, sufficient oxidation resistance cannot be obtained, and if it is less than 0.1% by weight, sufficient high temperature characteristics cannot be obtained. .

ZrSi ₂ is considered to act as a nucleus when Si ₃ N ₄ transitions from α phase to β phase in the melt formed by Y ₂ O ₃ and ZrO ₂ during sintering, and promotes the crystal phase transition. It is considered that this also contributes to homogenization of the material.

In addition, when ZrSi ₂ is added, the aspect ratio (ratio of major axis and minor axis) of the columnar phase of Si ₃ N ₄ generated in the sintering process increases,
Moreover, since the diameter of the minor axis is large, the toughness can be improved. In addition, since a Zr compound having a high melting point precipitates in the grain boundary phase of the sintered body during the cooling process of sintering, the strength is maintained up to a high temperature. By adding ZrSi ₂ , high toughness can be obtained while maintaining high temperature strength.

In the present invention, ZrSi ₂ is contained as a sintering aid in an amount of 0.1 to 3% by weight, but if it is added in an amount of more than 3% by weight, the mechanical strength at high temperature decreases, and if it is less than 0.1% by weight, a Si ₃ N ₄ columnar phase is formed. No sufficient grain growth can be obtained.

Further, in the present invention, ZrO ₂ and ZrSi ₂ are added at the same time in addition to Y ₂ O ₃ as a sintering aid, but in order to achieve high high temperature strength and toughness at the same time, the weight of Y ₂ O ₃ and ZrO 2 ₂ and
The total weight ratio of ZrSi ₂ should be in the range of 1-12. If it is more than 12, it is difficult to obtain a dense sintered body, and if it is less than 1, the high temperature strength in air and the oxidation resistance are deteriorated.
Further, in the grain boundaries of the sintered body sintered under the conditions of the present invention in a weight ratio within this range, with the high melting point Zr compound, the melting point is high, in the grain boundary crystal phase relatively stable up to high temperatures. Yes Y ₅ N (S
Since iO ₄ ) ₃ phase or Y _4.67 (SiO ₄ ) ₃ O phase is present, high high temperature strength can be obtained. Further, it is easy to obtain an orthorhombic phase ZrO _{2 which} is relatively stable at high temperature as a Zr compound.

Si ₃ N ₄ powder used in the present invention, Si ₃ N ₄ powder having an α-type crystal structure is preferable from the viewpoint of sintering property, contains β-type or amorphous Si ₃ N ₄ powder It doesn't matter. In order to obtain a sufficiently high bulk density during sintering, fine particles having an average particle diameter of 2 μm or less are preferable. Y ₂ O ₃ , ZrO ₂ , and ZrSi ₂ added as sintering aids are also preferably fine particles having an average particle diameter of 5 μm or less in order to obtain a homogeneous and high-density sintered body.

The silicon nitride-based sintered body of the present invention is yttrium oxide (Y
₂ O ₃ ) 5 to 15 wt%, zirconium oxide (ZrO ₂ ) 0.1 to 3
% By weight, zirconium silicide (ZrSi ₂ ) 0.1 to 3% by weight, and the balance silicon nitride (Si ₃ N ₄ ), Y ₂ O ₃ , ZrO ₂ , ZrSi ₂
The weight ratio range of 1 ≦ Y ₂ O ₃ / (ZrO ₂ + ZrSi ₂ ) ≦ 12 is formed and sintered, and at least a Zr compound as a grain boundary phase and Y ₅ N (SiO ₄ ) _Three- phase and Y _4.67 (S
At least one crystalline phase selected from the iO ₄ ) ₃ O phase is present, but due to the combination of these conditions, heat resistance such as a small decrease in mechanical strength under a high-temperature oxidizing atmosphere, which is the subject, Achieving both high toughness, toughness value K _IC with an anti-segregation strength of 600 MPa or more at 1400 ° C in air and room temperature.
It was possible to obtain a silicon nitride-based sintered body having a value of 7 MPa m ^1/2 or more.

In the present invention, these components are mixed in a planetary mixer using a solvent such as acetone or ethanol and using Si ₃ N ₄ or SiC pots and balls. The mixed powder thus adjusted is pressure-molded to obtain a molded product having a predetermined shape.

This molded body is 1800 to 2000 ° C in an atmosphere containing nitrogen gas.
Sintering is performed by pressure sintering at a pressure of 4 MPa or more in the temperature range of. As a sintering method, a hot press sintering method, a gas pressure sintering method, a hot isostatic pressing sintering method can be used, and it is also possible to combine one or more sintering methods. .

The atmosphere during sintering is preferably an atmosphere containing nitrogen gas in order to suppress decomposition of Si ₃ N ₄ at high temperature. Si ₃ N ₄
Since decomposition occurs at about 1800 ° C. or higher under 1 atmosphere of nitrogen gas, it is preferable to set the nitrogen gas pressure to be equal to or higher than the critical decomposition pressure of Si ₃ N _{4 at} the sintering temperature.

Sintering is performed in the temperature range of 1800-2000 ℃, but 1800 ℃
In the following, the growth of β grains of Si ₃ N ₄ is insufficient and high toughness cannot be obtained, and Y ₅ N (SiO ₄ ) ₃ phase and Y _4.67 (grain boundary phase are used as grain boundary phases during the temperature lowering process or reheating treatment. SiO ₄ ) ₃ O phase is difficult to obtain. Decomposition of Si ₃ N ₄ is remarkable above 2000 ° C.

During sintering, Si ₃ N ₄ is dissolved and re-precipitated in the liquid phase consisting of a sintering aid, which causes a crystal phase transition and causes densification and progress of sintering. Since there is a solid solution limit of Si ₃ N ₄ in the melt during the precipitation process, holding for 30 minutes or longer is preferable.

Further, as a grain boundary phase, Y ₅ N (SiO ₄ ) ₃ phase or Y _4.67 (S
In order to allow the iO ₄ ) ₃ O phase to exist, it is necessary to add 5
℃ or less after cooling per minute, or after sintering,
It is preferable to perform reheating treatment at 1300 to 1400 ° C. for about 2 hours in an atmosphere containing nitrogen.

Y ₅ N (SiO ₄ ) ₃ phase or Y _4.67 (SiO ₄ ) ₃ O during cooling process
The temperature lowering rate for precipitating the phase is preferably 5 ° C./min or less, and more preferably 2 ° C./min or less.

If the cooling rate is higher than 5 ° C / min, the Y ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase are not sufficiently formed. Also, when the temperature at the time of reheating treatment is 1300 ° C. or lower and 1400 ° C. or higher, similarly, Y ₅ N (SiO ₄ ) ₃ phase and Y _4.67 (SiO ₄ ) ₃ O phase are not sufficiently generated.

In addition, pressure is applied at a pressure of 4 MPa or more during sintering, and a uniaxial pressing method using a mold or an isotropic hydrostatic pressing method using gas pressure can be used as a pressing method. In order to obtain a sufficiently dense sintered body, it is preferable to sinter at a pressure higher than 4 MPa, particularly preferably 10 MPa or more. Further, by sintering at a high pressure, it becomes easy to obtain an orthorhombic ZrO ₂ which is a Zr compound stable at a high pressure.

The method for manufacturing a silicon nitride sintered body according to the present invention comprises a raw material powder of silicon nitride and yttrium oxide (Y ₂ O ₃ ) 5 to 15 as a sintering aid.
%, Zirconium oxide (ZrO ₂ ) 0.1 to 3% by weight, zirconium silicide (ZrSi ₂ ) 0.1 to 3% by weight, Y ₂ O ₃ ,
The weight ratio range of ZrO ₂ and ZrSi ₂ is 1 ≦ Y ₂ O ₃ / (ZrO ₂ + ZrSi ₂ ) ≦ 12, a mixed powder is molded, and the molded body is 1800 to 2000 in an atmosphere containing nitrogen gas. Pressure sintering at a pressure of 4MPa or more in the temperature range of ℃, and a temperature decrease rate of 5 ℃ per minute or less in the temperature decrease process, or reheat treatment in the temperature range of 1300 to 1400 ℃ in an atmosphere containing nitrogen. As a result, a Zr compound and at least one crystal phase selected from the Y ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase are produced as the grain boundary phase. The present invention provides a method for manufacturing a silicon nitride sintered body, which has heat resistance such as a small decrease in mechanical strength even under a high-temperature oxidizing atmosphere, and exhibits high toughness, which achieves the object. .

Action The sintered body obtained by the present invention has a large average crystal grain size of Si ₃ N ₄ of about 1 to 5 μm and exhibits a structure in which columnar crystal grains are entangled with each other, and a high melting point Zr compound is precipitated at grain boundaries. In addition, the presence of at least one crystal phase selected from the Y ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase having a high melting point and relatively stable at high temperatures as a grain boundary crystal phase, Has high toughness while maintaining high strength up to high temperature,
Flexural a high strength of at least 600MPa is at 1400 ° C. in air strength and toughness K _IC has 7MPam1 / 2 or more high toughness.

In order to obtain a sintered body having particularly high bending strength and toughness, it is preferable to use the hot pressing method. As shown in Examples described later, a sintered body having a flexural strength of 800 MPa at 1400 ° C. or a sintered body having an extremely high K _IC of 11 MPa1 / 2 was obtained.

Further, in order to obtain a sintered body having a plurality of shapes, a gas pressure sintering method,
It is preferable to use the hot isostatic pressing sintering method, but it is preferable to use the hot isostatic pressing sintering method in order to obtain a sintered body having particularly high characteristics. As shown in Examples described later, the bending strength at 1400 ° C is 750 MPa.
Thus, a sintered body with a high K _IC of 9.0MPam1 / 2 was obtained.

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

Examples Example 1 Si ₃ N ₄ (average particle size 0.5 μm, α conversion rate 97% or more), Y ₂ O ₃ powder (average particle size 0.3 μm), ZrO ₂ powder (average particle size 0.2 μm),
And ZrSi ₂ powder (average particle size 2 μm) were added in the predetermined amount (% by weight) shown in Table 1, and acetone was used as a solvent to produce Si ₃ N.
Kneading was carried out for 24 hours with a _4- made ball mill.

Then, the obtained mixed powder was dried and hot-press sintered. The hot press conditions were a graphite die in a nitrogen gas atmosphere, a pressure of 40 MPa, and a holding time of 2 hours. The temperature lowering rate during sintering was 5 ° C. per minute.

The characteristics of each sintered body obtained according to the present invention are as follows: the amount of sintering aid added, hot pressing temperature, Y ₅ N (SiO ₄ ) ₃ phase, Y _4.67 (SiO 2
₄ ) Table 1 shows the presence or absence of ₃ O phase. As a comparative example, Z
Table 1 also shows the case where SiO ₂ is added instead of rSi ₂ . Regarding the strength, a three-point bending test was performed at room temperature and in the atmosphere at 1400 ° C in accordance with JIS R1601, and the bending strength was measured. Regarding toughness, SEPB of JIS R1607 at room temperature
The fracture toughness value K _IC was measured by the (Single Edge Pre-cracked Beam) method. The crystal phase of the sintered body was analyzed by the X-ray diffraction method.

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 are particularly inferior in high temperature bending strength as compared with the examples of the present invention. Was confirmed. In the case of the present invention, at least one crystal phase selected from the Y ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase is present in each case, and the Zr compound is an orthorhombic phase.
ZrO ₂ (Ortho-ZrO ₂ ) was present. FIG. 1 shows an example of the X-ray diffraction result of the sintered body according to the present invention.

In the comparative example, Y ₅ N (SiO ₄ ) ₃ phase or Y _4.67 (SiO 2
₄ ) The amount of ZrO ₂ added is large, even when the ₃ O phase is present,
Alternatively, when the weight ratio of Y ₂ O ₃ and ZrO ₂ + ZrSi ₂ is large, sufficient high temperature strength is not obtained.

On the other hand, in the system in which SiO ₂ is added instead of ZrSi ₂ , Y ₅ N (Si
The O ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase were absent.

Example 2 A mixed powder was prepared in the same manner as in Example 1, and after molding, gas pressure sintering or hot isostatic pressing sintering was performed. Molding conditions were a uniaxial mold forming pressure of 100 MPa and a cold hydrostatic pressure of 700 MPa to obtain a plate-like body of 50 mm × 50 mm × 10 mm. In the case of gas pressure sintering, it was performed under the conditions of a pressure of 4 MPa in a nitrogen gas atmosphere, a temperature of 2000 ° C., and a holding time of 2 hours. Further, in the case of hot isostatic press sintering, it was carried out under the conditions of a pressure of 100 MPa in a nitrogen gas atmosphere, a temperature of 1800 ° C. and a holding time of 1 hour. After sintering, reheating treatment was performed at 1400 ° C. for 2 hours in a nitrogen atmosphere at atmospheric pressure.

As in Example 1, the characteristics of the sintered body are shown in Table 2 together with the addition amount of the sintering aid, the sintering conditions, the presence or absence of the Y ₅ N (SiO ₄ ) ₃ phase, and the Y _4.67 (SiO ₄ ) ₃ O phase. Show. As in Example 1, SiO instead of ZrSi _2
The system to which ₂ is added is also shown in Table 2. Similar to Example 1, the sintered body according to the present invention has excellent bending strength and toughness, but the sample corresponding to the Comparative Example is inferior in high temperature bending strength and toughness as compared with the Examples of the present invention. Was confirmed. Further, in the case of the present invention, as in the case of the first embodiment, each of Y ₅ N (SiO 2
₄ ) ₃ phases or Y _4.67 (SiO ₄ ) ₃ O phase exists,
Further, ZrO ₂ was present as a Zr compound.

In the comparative example, Y ₅ N (SiO ₄ ) ₃ phase or Y _4.67 (SiO 2
₄ ) ZrSi ₂ is not added and ZrO ₂ is added even if the ₃ O phase is present.
When the addition amount is large or the weight ratio of Y ₂ O ₃ and ZrO ₂ + ZrSi ₂ is large, sufficient high temperature strength and toughness are not obtained.

On the other hand, as in Example 1, in the system in which SiO ₂ was added instead of ZrSi ₂ , the Y ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase did not exist at the grain boundaries.

EFFECTS OF THE INVENTION According to the present invention, it has become possible to further improve the mechanical strength and toughness of a silicon nitride sintered body having sufficient heat resistance as described above. This makes it possible to produce a silicon nitride sintered body with extremely excellent reliability,
Its industrial utility is enormous.

[Brief description of drawings]

FIG. 1 is a diagram showing an X-ray diffraction result of a sintered body according to the present invention.

Claims (7)

[Claims] 1. Yttrium oxide (Y ₂ O ₃ ) 5 to 15% by weight,
Zirconium oxide (ZrO ₂ ) 0.1 to 3 wt%, zirconium silicide (ZrSi ₂ ) 0.1 to 3 wt% and the balance silicon nitride (Si
₃ N ₄ ), and the weight ratio range of Y ₂ O ₃ , ZrO ₂ , and ZrSi ₂ is 1 ≦ Y ₂ O ₃ / (ZrO ₂ + ZrSi ₂ ) ≦ 12. And at least a Zr compound as a grain boundary phase, a Y ₅ N (SiO ₄ ) ₃ phase and a Y _4.67 (S
A silicon nitride-based sintered body characterized by the presence of at least one crystal phase selected from the iO ₄ ) ₃ O phase. 2. The silicon nitride sintered body according to claim 1, wherein the crystal structure of zirconium oxide (ZrO ₂ ) is an orthorhombic phase. 3. The method according to claim 1 or 2, wherein the transverse rupture strength is 600 MPa or more at 1400 ° C. in air and the toughness value K _IC at room temperature is 7 MPam ^1/2 or more. The described silicon nitride sintered body. 4. A raw material powder of silicon nitride and 5 to 15% by weight of yttrium oxide (Y ₂ O ₃ ) as a sintering aid, 0.1 to 3% by weight of zirconium oxide (ZrO ₂ ) and zirconium silicide (ZrSi ₂ ) 0.
A mixed powder containing 1 to 3 wt% and having a weight ratio range of Y ₂ O ₃ , ZrO ₂ , and ZrSi ₂ of 1 ≦ Y ₂ O ₃ / (ZrO ₂ + ZrSi ₂ ) ≦ 12 is molded, and the molding is performed. The body is pressure-sintered in an atmosphere containing nitrogen gas, and a Zr compound as a grain boundary phase, a Y ₅ N (SiO ₄ ) ³ phase and a Y _4.67 (SiO ₄ ) ₃ O phase are obtained by a temperature lowering process or a reheating process. A method for producing a silicon nitride sintered body, which comprises producing at least one crystal phase selected from the following. 5. The method for producing a silicon nitride sintered body according to claim 4, wherein the pressure sintering condition is a temperature range of 1800 to 2000 ° C. and a pressure of 4 MPa or more. . 6. The method for producing a silicon nitride sintered body according to claim 4 or 5, wherein the temperature lowering rate in the temperature lowering process of sintering is 5 ° C./min or less. 7. The reheating treatment condition is an atmosphere containing nitrogen,
The method for producing a silicon nitride sintered body according to claim 4, 5, or 6, characterized in that the temperature is in the range of 1300 to 1400 ° C.