JP2976534B2 - 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 silicon nitride sintered body and a method for producing the same, and more particularly, to a silicon nitride sintered body excellent in fracture toughness and a method for producing the same.

[0002]

2. Description of the Related Art Silicon nitride is the most widely used heat-resistant structural material, and is a material which is being researched and developed. Since silicon nitride is difficult to sinter, it is generally Y ₂ O ₃ , Al ₂ O
_3. Add sintering aid such as MgO and sinter at 1700-1800 ℃. In particular, as a method for improving the toughness of a silicon nitride sintered body, a method of adding a silicon carbide whisker is known.

[0003] Although the purpose is different from that of the present invention, as a related technique, silicon nitride, group IIIa of the periodic table, L ₁ ,
A raw material powder to which an oxide of an element selected from the group consisting of Ca and Mg is added is molded, and the molded body is pre-sintered in a low-pressure nitrogen atmosphere, and then main-sintered in a high-pressure nitrogen atmosphere. Japanese Patent Laid-open No. Sho 62
No. -207769.

[0004]

The silicon nitride sintered body to which silicon carbide whiskers are added has improved fracture toughness, but it is not easy to uniformly disperse the whiskers in the matrix, and the thermal expansion of the whiskers and the matrix is also difficult. There are drawbacks such as internal defects easily occurring due to differences in coefficients.

Therefore, an object of the present invention is to improve the fracture toughness of a silicon nitride sintered body without such a problem.

[0006]

The present invention achieves the above object.
To form granular crystals having an average particle size of 1 μm or less.
Β-Si_ThreeN_FourThe matrix has an average diameter of 1-3 μm, average
Β-Si with a length of 10-30 μm_ThreeN_FourStrong by anisotropic spontaneous particles
Provided is a silicon nitride sintered body characterized by a modified structure.
Similarly, Sc was added as a sintering aid._TwoO_Three2-6% by weight
And Y_TwoO_Three, Yb_TwoO_Three, Al_TwoO_Three, MgAl_TwoO_FourOne or more of Sc_Two
O_ThreeAnd 3 to 8% by weight in total, with the balance being Si _ThreeN_FourAnd mixed
The powder is molded and the molded body is heated to a temperature in the range of 1530 to 1650 ° C.
And pre-sintered, then at a temperature within the temperature range
The pressure is increased to 1500 atmospheres or more, and the main sintering is performed.
It is characterized by heat treatment at a temperature below 1900 ° C from the temperature range
To provide a method for producing a silicon nitride sintered body.

The silicon nitride sintered body of the present invention has an average particle size of 1 μm.
β-Si observed in granular crystals below m _ThreeN_FourIn the matrix
Β-Si with an average diameter of 1-3 μm and an average length of 10-30 μm_ThreeN
_FourAnisotropic particles grow naturally, as if the average diameter is 1-3 μm
m, like adding whiskers with an average length of 10-30 μm
Microstructure and improved fracture toughness values. Moreover, reinforcement
Both the anisotropic particles and the matrix that act as β-Si_ThreeN_Fourso
There is no difference in thermal expansion coefficient, and anisotropic particles
The organization is extremely uniform
You.

The matrix β-Si ₃ N ₄ has an average particle size of 1 μm.
m or less, more preferably 0.6 μm or less. This is 1μ
If it is larger than m, the fracture toughness value and the bending strength decrease. On the other hand, β-Si ₃ N ₄ of anisotropic particles has an average diameter of 1 to ₃ μm, preferably 2 μm, and an average length of 10 to 30 μm, preferably 20 to
30 μm. The density of the sintered body is preferably at least 99% of the theoretical density, more preferably at least 99.5%. Such a high density in a tissue like the present invention was made possible by the manufacturing method of the present invention.

The production of such a silicon nitride sintered body is as follows. Si ₃ N ₄ as starting material and sintering aid have particle size
It is 0.6 μm or less, more preferably 0.1 to 0.4 μm. If the particle size of the starting Si ₃ N ₄ is large, the matrix β
Particle size of -Si ₃ N ₄ increases undesirably. If the particle size of the starting sintering aid is large, uniform mixing is difficult and dense sintering is difficult, which is not preferable. In addition, the purity is Si ₃ N ₄ having a total amount of metal impurities of 100 ppm or less and a sintering aid of 99.9% or more. If the amount of impurities is more than this, the softening temperature of the grain boundary glass phase decreases, and the strength at high temperatures decreases.

[0010] Si_ThreeN_FourThe amount of sintering aid added to
Sc of the mixture_TwoO_Three2-6% by weight, more preferred
Or 2.5-5% by weight, Y_TwoO_Three, Yb_TwoO_Three, Al_TwoO_Three, MgAl_TwoO
_FourOne or more of Sc_TwoO_ThreeAnd 3 to 8% by weight
I do. Sc_TwoO_ThreeIs essential to promote self-generation of anisotropic particles
It is. Sc_TwoO_ThreeIf the added amount is small, the spontaneous anisotropic β-Si_ThreeN _Four
Becomes insufficiently grown. Sc_TwoO_ThreeIf too much is added,
Dense sintering is difficult. Y_TwoO_Three, Yb_TwoO_Three, Al_TwoO_Three, MgAl_TwoO_Four
At least one of them is Si_ThreeN_FourIs sintered densely and
It is necessary to prevent the growth of particles. But this
If these are too large, the matrix particles will grow large.

The mixing and molding of the starting powder can be carried out by a conventional method. The sintering is performed by heating the molded body to a temperature in the range of 1530 to 1650 ° C. to perform preliminary sintering, and then performing main sintering by increasing the pressure to 1500 atm or more at a temperature in the temperature range. The sintering atmosphere is a nitrogen atmosphere. That is, in the present invention, the sintering temperature is a temperature in the range of 1530 to 1650 ° C. This is the conventional normal-pressure sintering temperature of 1700 to 1800 ° C., or 1700 to 1900 ° C. of JP-B-62-13310, and JP-A-62-207.
The sintering temperature is lower than the HIP temperature of 1900 ° C. in the 7769 publication. Conventionally, at 1700 to 1800 ° C. sintered body is not sufficiently densified, it is not possible to obtain a dense sintered body, in order to sufficiently sintered at high temperatures, the thermal decomposition the Si ₃ N ₄ A method of firing under high pressure (HIP) is used to suppress this. On the other hand, the present invention, even at a low temperature of 1530-1650 ℃,
It is based on the discovery that sintering can proceed using high pressures of 1500 atmospheres or more. By this sintering at a low temperature, a dense sintered body can be obtained while suppressing the grain growth of the β-Si ₃ N ₄ particles of the matrix. Sintering temperature
If it exceeds 1650 ° C., the particle size of the matrix particles becomes coarse, and the room temperature strength and the 1400 ° C. strength decrease. On the other hand, if it is lower than 1530 ° C., it does not sinter densely. Conventionally, there were able to effect a "1600 ° C. or higher" as a general sintering temperature of silicon nitride is mentioned, not actually example of manufacturing the Si ₃ N ₄ sintered body at 1600 ° C., the In JP-A-62-207769, 1900 ° C. is used, and in JP-B-62-13310, 1700-1900 ° C. is actually used. Further, the HIP pressure of the present invention needs to be 1500 atm or more. At a pressure lower than this, the sintered body density decreases. There is no particular upper limit on the pressure, and it is adopted according to facility restrictions. Typically between 1500 and 2500 atmospheres.

The purpose of pre-sintering is to sinter under low pressure until closed porosity progresses in order to obtain the effect of the HIP treatment. The pressure may be normal pressure, but the next pressurizable furnace (HIP
The pressure rises somewhat (several tens of atmospheres) due to the
In short, the pressure may be substantially lower than 1500 atm, especially around normal pressure. As a tentative method for closed pore formation, the density is about 90% of the theoretical density. Next, main sintering is performed at a high pressure. This main sintering is performed at least until the density becomes 95% or more of the theoretical density, more preferably 98% or more. If densification is not sufficiently performed at this stage, matrix particles become coarse when heat treatment is performed at a high temperature next time.

[0013] Typically, the pre-sintering is performed at about 1 atm N 2
₂ Under an atmosphere, at a heating rate of about 0.5 to 10 ° C / min, 1530 to 16
The temperature is raised to a temperature in the range of 50 ° C. The heating profile can be changed as desired. When the temperature reaches 1530 to 1650 ° C., the pressure is increased to 1500 atm or more at a rate of 5 to 20 atm / min, and main sintering is performed while maintaining the pressure. The pressure for the main sintering is at least 1500 atm as described above, typically from 1500 to 2500 atm.

Next, heat treatment is performed at a temperature lower than 1900 ° C. from the sintering temperature. By this heat treatment, elongated anisotropic particles grow in matrix particles of 1 μm or less. Without this heat treatment, only a structure in which small columnar crystals exist in a fine matrix can be obtained. On the other hand, when heat treatment is performed at 1900 ° C. or more, the structure becomes a columnar crystal that has grown to the size of matrix particles. This heat treatment is performed in a nitrogen atmosphere under a pressure of 1500 atm or more. If the pressure is lower than 1500 atm, a dense sintered body cannot be obtained.

The structure of the silicon nitride sintered body thus obtained is schematically shown in FIG. In other words, it is a structure in which elongated anisotropic particles of β-Si ₃ N ₄ naturally grown in a fine β-Si ₃ N ₄ matrix are dispersed and reinforced by whiskers.

[0016]

[Action] into fine β-Si ₃ N ₄ matrix particle diameter 1~3μm, β-Si ₃ N ₄ of anisotropic particles of length 10~30μm
Are naturally entangled with each other three-dimensionally, so that the structure becomes as if strengthened by adding whiskers, and the fracture toughness value is improved. In addition, since the matrix particles are dense and the anisotropic particles acting as a reinforcing material are the same β-Si ₃ N ₄ as the matrix, the room temperature strength,
Both toughness and strength are high.

[0017]

EXAMPLES Si ₃ N ₄ powder (average particle size 0.2 [mu] m, the metal impurities total 30 ppm, a rate approximately 100%) Sc ₂ O ₃ powder as a first component of the sintering agent (average particle diameter 0. 6μm, purity 99.9%)
With Y ₂ O ₃ powder (average particle size 0.3 μm, purity as a second component)
99.9%), Yb ₂ O ₃ powder (average particle size 0.1 μm, purity 99.9)
%), Al ₂ O ₃ powder (average particle size 0.1 μm, purity 99.9%) or MgAl ₂ O ₄ powder (average particle size 0.3 μm, purity 99.9%). Various powders mixed (ball mill made of Si ₃ N ₄ ) with the composition as shown in the table are pressed and molded at a pressure of 200 atm.
Pressure was applied at a pressure of 3000 atm by IP (hydrostatic pressing).

Thereafter, the molded body was subjected to N 2 under the conditions shown in Table 1.
It was sintered in a furnace in _two atmospheres. Heating rate is 5 ℃ / min
Under an atmosphere of N _{2 at} 1 atm until the maximum temperature is reached, and after reaching the maximum temperature, up to the conditions shown in Table 1 at 15 kgf / min.
Pressurization was performed at a pressure rise rate of cm ² . The holding time at the highest temperature was 4 hours. Thereafter, heat treatment was further performed under the conditions shown in Table 1 while pressurizing in the same furnace.

FIG. 2 shows the above firing temperature and pressure profiles. The fracture toughness values of these sintered bodies (JIS R 1607,
SEPB method), room temperature 4-point bending strength (JIS R 1601) and high temperature 4
The point bending strength (JIS R 1604, in air) was measured, and the results shown in Table 2 were obtained. The relative density of the sintered body is a value obtained by dividing the bulk density obtained by the n-butanol substitution method by the theoretical density.
The remainder is the porosity, which was supported by the observation result of the mirror-polished surface with an optical microscope.

[0020]

[Table 1]

[0021]

[Table 2]

Comparative Example After molding in the same manner as in the example, this molded body was sintered in a furnace in an N ₂ atmosphere under the conditions shown in Table 3 to obtain the results shown in Table 4.

[0023]

[Table 3]

[0024]

[Table 4]

All of the silicon nitride sintered bodies in the examples
μm or less β-Si ₃ N ₄ matrix, diameter 1-3μ
m, anisotropic particles of β-Si ₃ N _{4 having} a length of 10 to 30 μm grew naturally, and had a structure as if a whisker having a diameter of 1 to 3 μm and a length of 10 to 30 μm was added. As a result, it is considered that the fracture toughness value was improved without significantly lowering the room temperature bending strength and the high temperature bending strength.

On the contrary, Nos. 52, 53, 54,
The 55 silicon nitride sintered body had a structure in which small columnar crystals were present in a very fine matrix. NO.56, 5
The 7, 60, and 61 silicon nitride sintered bodies had a structure in which the matrix particles also grew into large columnar crystals. Maximum temperature during sintering
At a temperature of 1650 ° C or higher, the particle size of the matrix particles becomes coarse, and the fracture toughness, room temperature strength and 1400 ° C strength decrease as shown in Comparative Sample Nos. 60 and 61. As shown in the sample Nos. 58 and 59, it was not densely sintered.

When the pressure is set to 1500 atm or less, the comparative sample N
As shown in O.62 and 63, although the sintered body had an anisotropic structure, the density of the sintered body was reduced and the fracture toughness, room temperature strength and 1400 ° C strength were reduced. The addition of Sc ₂ O ₃ is essential for anisotropic organization, and Comparative Samples No. 52 and 53 without addition of Sc ₂ O ₃
Now the fine matrix grew evenly.

When the heat treatment was not performed, the comparison sample No. 54,
As shown in Fig. 55, the structure was such that small columnar crystals existed in a fine matrix. When the heat treatment temperature was higher than 1900 ° C, the matrix particles had a structure in which the matrix particles grew into large columnar crystals as in Comparative Sample Nos. 56 and 57.

[0029]

According to the present invention, the fine and densely fired β-Si ₃ N ₄ matrix particles have an average diameter of 1 to 3
μm, β-Si ₃ N ₄ of anisotropic particles with an average length of 10 to 30 μm grow naturally, and the anisotropic particles function as whiskers of whisker reinforced ceramics, that is, high toughness due to the action of debonding and deflection Is expressed.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the structure of a high toughness silicon nitride sintered body of the present invention.

FIG. 2 is a diagram showing a temperature and pressure profile of an example.

[Explanation of symbols]

 1: Matrix particles 2: Self-generated anisotropic particles

Claims (2)

(57) [Claims] 1. Anisotropic spontaneous β-Si ₃ N ₄ particles having an average diameter of 1 to 3 μm and an average length of 10 to 30 μm in a β-Si ₃ N ₄ matrix observed in granular crystals having an average particle diameter of 1 μm or less. A silicon nitride sintered body characterized by having a structure strengthened by: 2. A sintering aid comprising 2 to 6% by weight of Sc ₂ O ₃ and Y _{2
O 3, Yb 2 O 3,} Al 2 O 3, MgAl 2 O 1 or more of Sc ₂ O ₃ of ₄
And a mixed powder containing 3 to 8% by weight in total and Si ₃ N _{4 as} the balance, and presintering the molded body by heating to a temperature in the range of 1530 to 1650 ° C., and then in the temperature range. The method for producing a silicon nitride sintered body, wherein the pressure is raised to 1500 atm or more at the temperature of the above, the main sintering is performed, and the heat treatment is performed at a temperature lower than 1900 ° C. from the temperature range.