JPH064514B2 - Method for manufacturing silicon nitride sintered body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-density silicon nitride-based sintered body having excellent toughness and high-temperature strength.

Conventional technology Silicon nitride is a substance with a strong covalent bond,
Since it has excellent characteristics in heat resistance, chemical stability, etc., its application to structural members, particularly as a heat engine, for example, gas turbine engine 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 containing yttrium oxide (Y ₂ O ₃ ) and aluminum oxide (Al ₂ O ₃ ) has excellent thermal shock resistance, but 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 a 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
A silicon nitride sintered body in which an O ₃ ) 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 InventionHowever, 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 method for producing a silicon nitride sintered body having particularly high toughness and heat resistance such as a small decrease in mechanical strength even under a high temperature oxidizing atmosphere. is there.

Means for Solving the Problems A method for producing a silicon nitride sintered body according to the present invention comprises: 5 to 15 wt% of yttrium oxide (Y ₂ O ₃ ); zirconium silicide (ZrS
i ₂ ) 0.1 to 5 wt% and the balance silicon nitride (Si ₃ N ₄ ), and the weight ratio range of Y ₂ O ₃ and ZrSi ₂ is 3 ≦ Y ₂ O ₃ / ZrS.
i ₂ ≦ 15 mixed powder is molded, and the molded body 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 a temperature lowering process or reheating As a grain boundary phase, Y ₅ N (SiO ₄ ) ₃ phase or Y _4.67 (SiO ₄ ) ₃
It is characterized by generating an O phase. Here, the Y ₅ N (SiO ₄ ) ₃ phase and the Y _4.67 (SiO ₄ ) ₃ O phase are high melting point crystalline phases.

In the present invention, using Y ₂ O ₃ as a sintering aid but, Y ₂
O ₃ is high temperature by the crystalline phase transition from α phase during sintering the Si ₃ N ₄ to β-phase has a function to promote its melt during further promote the growth of the columnar phase the Si ₃ N ₄ Improves strength and toughness. In addition, in the present invention, Y ₂ O ₃ reacts with Si ₃ N ₄ by a sintering temperature lowering process or a reheating treatment, and Y ₅ N (SiO ₄ ) ₃
Phase or Y _4.67 (SiO ₄ ) ₃ O phase is formed.

If the amount of Y ₂ O ₃ added exceeds 15% by weight, the mechanical strength and oxidation resistance at high temperature of the obtained sintered body will decrease.
It is preferably not more than weight%. 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 5 to 15% by weight.
It is desirable to be in the range of, but it is more preferable to be in the range of 8 to 10% by weight in order to obtain sufficiently high mechanical strength and toughness.

ZrSi ₂ is Si in a melt formed of Y ₂ O ₃ and silicon dioxide (SiO ₂ ) inevitably contained in Si ₃ N ₄ powder during sintering.
_It is considered that ₃ N ₄ acts as a nucleus in the transition from the α phase to the β phase, which promotes the crystal phase transition and contributes to the homogenization of the material.

Further, when ZrSi ₂ is added, the aspect ratio (ratio of major axis and minor axis) of the columnar phase generated in the sintering process becomes large, and the diameter of the minor axis becomes large, so that the toughness is improved. Furthermore, ZrSi ₂ is considered to precipitate in the grain boundary phase of the sintered body as ZrO ₂ or a Zr compound having a high melting point during the cooling process of sintering, and it is expected that excellent high temperature characteristics will be obtained.

In the present invention, ZrSi ₂ is contained as a sintering aid in an amount of 0.1 to 5% by weight, but if it is added in an amount of more than 5% by weight, the mechanical strength at high temperature decreases, and if less than 0.1% by weight, it is sufficient. Aspect ratio cannot be obtained.

Further, in the present invention, Y ₂ O ₃ and ZrSi ₂ are simultaneously added as sintering aids, but in order to achieve high temperature strength and toughness at the same time, the weight ratio of Y ₂ O ₃ and ZrSi ₂ is 3%. It is particularly preferable that the range is -15. If it is larger than 15, it is difficult to obtain a dense sintered body, and if it is smaller than 3, 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 (Si
O ₄ ) ₃ phase or Y _4.67 (SiO ₄ ) ₃ O phase is present, and high high temperature strength can be obtained.

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 and ZrSi ₂ added as a sintering aid are also preferably fine particles having an average particle size of 5 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 acetone or ethanol, and Si ₃ N ₄ or S
Perform on a planetary mixer using iC 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 a plurality of 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
_{Since 4} decomposes 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 ℃
Below, β grains of Si ₃ N ₄ grow insufficiently and high toughness cannot be obtained, and above 2000 ° C, decomposition of Si ₃ N ₄ is remarkable. 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.

As the grain boundary phase, Y ₅ N (SiO ₄ ) ₃ phase or Y _4.67 (SiO ₄ ) ₃ phase is used.
In order to allow the O phase to exist, cooling is performed at a temperature lowering rate of 5 ° C. per minute or less during the temperature lowering process of sintering, or after sintering, 1300 to 1400 ° C. for 2 hours in an atmosphere containing nitrogen. Reheating treatment is preferably performed. The temperature lowering rate when the Y ₅ N (SiO ₄ ) ₃ or Y _4.67 (SiO ₄ ) ₃ O phase is present in the temperature lowering process is preferably 5 ° C./min or less, more preferably 2 ° C./min or less. Y ₅ N when the cooling rate is faster than 5 ℃ per minute
(SiO ₄ ) ₃ phase or Y _4.67 (SiO ₄ ) ₃ O phase is 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 formed.

In addition, pressure is applied at a pressure of 4 MPa or more during sintering, and as a pressing method, a uniaxial pressing method using a mold or an isotropic hydrostatic pressing method using gas pressure can be used. In order to obtain a sufficiently dense sintered body, sintering at a pressure higher than 4 MPa is preferable, and 10 MPa or more is particularly preferable.

According to the method for producing a silicon nitride sintered body of the present invention, Y ₂ O _{3 is} 5 to 15% by weight, ZrSi _{2 is} 0.1 to 5% by weight, and the balance is silicon nitride (Si ₃ N
₄ ) and the weight ratio range of Y ₂ O ₃ and ZrSi ₂ is 3 ≦ Y ₂ O ₃
/ ZrSi ₂ ≦ 15, a mixed powder is molded, and the molded body is heated to 4 MPa at a temperature range of 1800 to 2000 ° C. in an atmosphere containing nitrogen gas.
Pressure sintering is performed at the above pressure, and the temperature is reduced at a rate of 5 ° C / min or less, or reheat treatment is performed in the temperature range of 1300 to 1400 ° C after the sintering temperature is reduced. The task was achieved.

The sintered body obtained according to the present invention has a large average grain size of Si ₃ N ₄ of about 1 to 5 μm and has a structure in which granular crystals having a large aspect ratio are entangled with each other, and the grain boundaries have a high melting point.
Zr and compounds are precipitated, relatively stable Y ₅ N (SiO _{4) 3} phase or Y to a higher temperature a higher melting point as the grain boundary crystal phase
Due to the presence of the _4.67 (SiO ₄ ) ₃ O phase, the toughness value K _IC is 8
It has a high toughness of MPam1 / 2 or more and a high anti-segregation strength of 500 MPa or more at 1400 ° C in the atmosphere.

In order to obtain a sintered body having particularly high toughness and transverse rupture strength, it is preferable to use the hot pressing method. As shown in the examples described later, a sintered body having an extremely high K _IC of 12 MPam1 / 2 or a bending strength at 1400 ° C of 750 MPa.
Is obtained. Further, 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 pressing sintering method. It is desirable to use the hydraulic press sintering method.

As shown in the examples described later, K _IC is 9.6 MPa.
With a sintered body as high as 1/2, the bending strength at 1400 ℃ is 710MPa.
And a high sintered body is 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 of 97% or more) was added to Y ₂ O ₃
Powder (average particle size 0.3 μm) and ZrSi ₂ powder (average particle size 2 μm) were added in the prescribed amounts (% by weight) shown in Table 1, and kneaded for 24 hours in a Si _{3 4} ball mill using acetone as a solvent. did.

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

The characteristics of each sintered body obtained according to the present invention are shown in Table 1 together with the addition amount of the sintering aid and the hot pressing temperature. Regarding strength, in accordance with JIS R1601, at room temperature and 1400 ° C in air, 3
A point bending test was performed and the bending strength was measured. Regarding toughness, SEPB (Single Edge Pre-crack) of JIS R1607 at room temperature
The fracture toughness value K _IC was measured by the ed beam method. Also,
The crystal phase of the sintered body was analyzed using an X-ray diffraction method.

As shown in Table 1, the samples according to the examples of the present invention are excellent in both toughness and transverse rupture strength, but the samples corresponding to the comparative examples are inferior in toughness and high temperature transverse rupture strength. It was confirmed. In the case of the present invention, the Y ₅ N (SiO ₄ ) ₃ phase or the Y _4.67 (SiO ₄ ) ₃ O phase was present in each case, and the Zr compound was also present.

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. The sintered body obtained by gas pressure sintering or hot isostatic pressing was subjected to a reheating treatment at a temperature of 1400 ° C for 2 hours after the temperature of the sintering was lowered, and under a nitrogen gas atmosphere under 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 and the sintering conditions. Similar to Example 1, the properties of the sintered body according to the present invention are excellent in both toughness and transverse rupture strength, but the sample corresponding to the comparative example may be inferior in toughness and high temperature transverse rupture strength as compared with the inventive example. confirmed. Similar to Example 1, in the sintered body according to the present invention, Y ₅ N (SiO ₄ ) ₃ phase or Y _4.67 (SiO ₄ ) ₃ O phase was present together with the Zr compound.

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

Claims (2)

[Claims] 1. Yttrium oxide (Y ₂ O ₃ ) 5 to 15% by weight,
Zirconium silicide (ZrSi ₂ ) 0.1 to 5 wt% and the balance silicon nitride (Si ₃ N ₄ ) and the weight ratio range of Y ₂ O ₃ and ZrSi ₂ is 3 ≦ Y ₂ O ₃ / ZrSi ₂ A mixed powder of ≤15 is molded, and the molded body 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 a rate of 5 ° C or less per minute. A method for manufacturing a silicon nitride-based sintered body, characterized in that the temperature is lowered at. 2. Yttrium oxide (Y ₂ O ₃ ) 5 to 15% by weight,
Zirconium silicide (ZrSi ₂ ) 0.1 to 5% by weight and the balance silicon nitride (Si ₃ N ₄ ) and the weight ratio range of Y ₂ O ₃ and ZrSi ₂ is 3 ≦ Y ₂ O ₃ / ZrSi ₂ A mixed powder of ≤15 is molded, the molded body 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 is lowered to 1300 to 1400 ° C. 2. A method for manufacturing a silicon nitride sintered body, characterized in that the reheating treatment is carried out in the temperature range of 1.