JPH03218974A - Silicon nitride sintered body and production thereof - Google Patents

Abstract

PURPOSE:To obtain a sintered body having a prescribed microstructure, superior thermal shock resistance, prescribed heat conductivity and strength by mixing powdery starting materials having the same prescribed purity and alpha-silicon nitride content but different average particle sizes in a prescribed ratio and firing a molded body of the mixture under prescribed conditions. CONSTITUTION:Silicon nitride powder of 0.1-0.5mum average grain size (D1) as starting material (1) and silicon nitride powder of prepd. so that the ratio of D2 to D1 is regulated to 2-6. Both the powders have >=99wt.% purity and >=95wt.% alpha-silicon nitride content. A mixture of 20-60wt.% of the starting material 1 with 80-40wt.% of the starting material 2 is blended with a sintering aid, molded and fired at 1,800-1,900 deg.C under pressure to obtain a sintered body having a microstructure contg. 20-50% grains of >=10mum length and 20-50% grains of <=3mum length. This sintered body has 0.13-0.16Cal/cm.sec. deg.C heat conductivity and >=800 MPa bending strength at four points.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a silicon nitride sintered body suitable as a high-temperature structural material that has high strength, high thermal conductivity, and excellent thermal shock resistance, and a method for manufacturing the same. It is.

(Prior Art) Silicon nitride has long been expected to be used as a high-temperature structural material, and various sintered bodies have been developed. For example, in Japanese Patent Publication No. 55-46997, in order to obtain a silicon nitride sintered body with good mechanical strength and thermal shock properties, Bed. MgO. S
A silicon nitride sintered body comprising a predetermined amount of rO and a rare earth element oxide is disclosed.

(Problems to be Solved by the Invention) The silicon nitride sintered body described in Japanese Patent Publication No. 55-46997 mentioned above has a high strength of, for example, 650 to 700 MPa and a strength of 0.5 MPa. 06 ~0. 09 cal/am-see
- Although it is possible to obtain the thermal conductivity of "C", its thermal conductivity is lower than that of good ceramic materials such as SiC, and there are problems with its thermal shock properties, making it unsuitable for use as a high-temperature structural material. there were.

An object of the present invention is to solve the above-mentioned problems and provide a silicon nitride sintered body having high thermal conductivity and improved thermal shock characteristics as a result, and a method for manufacturing the same.

(Means for Solving the Problems) In the silicon nitride sintered body of the present invention, the proportion of particles having a grain length of 10 μm or more is 20 to 50%, and the proportion of particles having a grain length of 3 μm or less is 20 to 50%. It has a microstructure that is ~50% and a thermal conductivity of 0.13-0.16 cal/c
m "SeC""C, 4-point bending strength or 80 at room temperature
It is characterized by having a property of 0 MPa or more.

Further, the method for producing a silicon nitride sintered body of the present invention has a purity of 99%.
Prepare raw material 1 and raw material 2, which are silicon nitride powder raw materials with wt% or more and alpha conversion rate of 95 wt% or more, and have different particle sizes as follows, (1) Raw material 1 average particle diameter D1 (0.1 ~ 0.5 μm) (2) Raw material 2 average particle diameter D2 (0.3 to 1.0 μm) (3) Relationship between raw material 1 and raw material 2 2≦D2/D,≦6 Prepared raw material 1 and raw material 2 , 20 to 60% by weight of raw material 1 and 80 to 40% by weight of raw material 2 are mixed, a predetermined amount of auxiliary agent is added to the mixed raw material, blended and molded, 180% by weight.
It is characterized by heating and firing at a temperature of 0 to 1900°C.

(Function) In the above-mentioned configuration, the auxiliary agent group used or the
The sintered body is similar to the sintered body disclosed in Publication No. 46997, or in the present invention, the sintered body is obtained by mixing a raw material with a large particle size and a raw material with a small particle size and obtaining a sintered body by a predetermined manufacturing method. The microstructure has a predetermined ratio of large particles with a length of 10 μm or more and small particles with a grain length of 3 μm or less, achieving high strength, high toughness, and high thermal conductivity that could not be obtained with conventional silicon nitride sintered bodies. be able to.

That is, in the product of the present invention, needle-like crystals of β-SiaNi are developed, and the maximum diameter exceeds 20 μm, while small needle-like crystals of 3 μm or less are also observed, indicating a wide particle size distribution.

The conventional product has relatively uniform particles in addition to poorly developed needle-shaped crystals, and this may be due to the difference in microstructure between the present product and the conventional product in terms of strength, toughness, and thermal conductivity. This is reflected in the difference between

Further, according to the manufacturing method of the present invention, it is possible to obtain a fired body having a well-balanced range of strength, toughness, and thermal conductivity. That is, as the firing temperature increases, the values of toughness and thermal conductivity increase, or the strength sharply decreases. On the other hand, lowering the firing temperature lowers the toughness and thermal conductivity. Therefore, the composition and blending ratio of raw materials should be within the range of the production method of the present invention, or within a well-balanced range.

In addition, as is clear from the examples described later, the proportion of particles having a particle length of 10 μm or more is 30 to 40%, and 3
It is preferable that the proportion of particles having a particle length of .mu.m or less is 30 to 40% because various properties become even better.

(Example) An actual example will be explained below.

Example 1 A mixed powder of either of the two types of silicon nitride powder raw material 1 and raw material 2 (both 99.9% purity) with different particle sizes listed in Table 1 and an additive was nitrided in an attritor mill. A slurry was obtained by wet grinding for 24 hours using silicon boulders. A molding aid such as polyvinyl alcohol was added to the obtained slurry and spray-dried to obtain spherical granulated powder with an average particle diameter of 70 μm.

After this powder was subjected to isostatic pressing at a pressure of 5000 kg/ad, binder removal treatment was performed to obtain a square plate sample of 60 x 60 x 5 mm. The square plate sample was fired for 1 hour at the firing temperature shown in Table 1 in a nitrogen gas atmosphere to obtain a fired body.

β conversion rate, thermal conductivity, room temperature and 800% of the obtained fired body
In addition to measuring the four-point bending strength, fracture toughness value (KIc), and thermal shock resistance coefficient ratio at °C, we also measured the microstructure of the proportion of particles with a grain length of IO μm or more and grains of 3 μm or less in the fired body. The proportion of particles with long length was determined. The results are also shown in Table 1.

The β conversion rate was measured by X-ray diffraction.
The diffraction intensity α (210) α of the (210) and (201) planes of
(201), (101) and (210) of β-Si*N4
) surface diffraction intensity β(101) and β(210) using the following equation.

β conversion rate = [β (101) + β (210)) / (α(
210) + α (201) + β (101) 10 β (210)
) The microstructure of the XIOO silicon nitride sintered body was measured by numerical processing according to the following method. First, the sample was lapped with diamond abrasive grains, and then treated with a hydrofluoric acid stock solution at 50°C for 4 hours to etch the grain boundary phase. This sample was photographed using a scanning electron microscope at a magnification of 5,000 times, and the long axis (maximum length) and particle area of the photograph for each sample 10 field of view were determined using an image analyzer and statistical processing was performed. Therefore, the grain length here is the major axis (maximum length) of the grains in the cross section of the sample, and the ratio represents the grain area ratio.

Thermal conductivity was measured using a laser flash method thermal constant measuring device manufactured by Rigaku Corporation.

The 4-point bending strength is measured at room temperature and at
Measurements were taken at 0°C.

The fracture toughness value (K.) was measured by the SEPB method,
The sample shape was 3 x 4 x 20 mm, and a four-point bending method was used with an internal span of 5 mm and an external span of 15 mm. The crosshead speed at this time was 0.05 ■/win.

The ratio with the Nal7 test specimen of Example 1 was calculated.

Here, S is four-point bending strength, ν is Boisson's ratio, E is Young's modulus, and α is linear thermal expansion coefficient.

From the results in Table 1, it can be seen that each predetermined average particle diameter D. ，
Examples Arashi 1 to 7 produced using raw material 1 and raw material 2 having D2 and the ratio between them being a predetermined value are comparative examples that do not meet the requirements of the present invention in any respect. klO
It can be seen that it has higher thermal conductivity, higher 4-point bending strength, higher fracture toughness value (K, c), and higher thermal shock resistance coefficient ratio compared to No. 17. Also, focusing on the microstructure of the sintered body, in Examples NcL1 to NcL9, the amount of particles having a grain length of 10 μm or more and the amount of particles having a grain length of 3 μm or less is a predetermined value, while in Comparative Examples NcL1 to It can be seen that No. 17 does not meet the requirements of the present invention in some respects.

Figures 1 and 2 show the relationship between the four-point bending strength and thermal conductivity at room temperature and the value of D2/D.

From the results shown in FIGS. 1 and 2, it can be seen that by selecting an appropriate particle size ratio, it is possible to obtain a sintered body with high strength and high thermal conductivity.

Example 2 In order to investigate the influence of the mixing ratio of two types of raw material 1 and raw material 2 with different particle sizes, raw material 1 and raw material 2 were mixed at various mixing ratios as shown in Table 2, and Example 1 was prepared. Molding and firing were carried out in the same manner as in Example Nα2l~38 and Comparative Example No.39.
~50 fired bodies were obtained.

The obtained fired body was measured for β conversion rate, thermal conductivity, four-point bending strength at room temperature and 800°C, fracture resistance value, and thermal shock resistance coefficient ratio in the same manner as in Example 1. The structure was also measured in the same way. The results are also shown in Table 2.

From the results in Table 2, Examples Nos. 121 to 38 in which raw materials 1 and 2 having predetermined average particle diameters were mixed at a ratio of 20 to 60% by weight of raw material 1 and 80 to 40% by weight of raw material 2.
is a comparative example fish 39 to 50 that does not meet the mixing ratio or the above range.
In comparison, it can be seen that it has high thermal conductivity, high four-point bending strength, high fracture toughness value (K,.), and high thermal shock resistance coefficient ratio. Also, if we focus on the microstructure of the sintered body,
In Examples Nα21 to 38, the amount of particles having a particle length of 10 μm or more and the amount of particles having a particle length of 3 μm or less is a predetermined value, whereas Comparative Examples Arashi 39 to 50 meet the requirements of the present invention in any respect. I understand that it does not meet the requirements.

FIG. 3 and FIG. 4 show the relationship between the four-point bending strength and thermal conductivity at room temperature and the mixing ratio of raw material 1. By selecting an appropriate mixing ratio based on the results shown in Figures 3 and 4,
It can be seen that it is possible to obtain a sintered body with high strength and high thermal conductivity.

Example 3 In order to investigate the influence of changes in firing temperature in the examples of the present invention, raw materials 1 and 2 shown in Table 3 were used and fired in the same manner as in Example 1 at the firing temperatures shown in Table 3. Example Nα5
Fired bodies of Na1-59 and Comparative Example Na60-65 were obtained.

The obtained fired body was measured for β conversion rate, thermal conductivity, 4-point bending strength at room temperature and soo''c, fracture toughness value, and thermal shock resistance coefficient ratio in the same manner as in Example 1. The structure was also measured in the same manner.The results are also shown in Table 3.

From the results in Table 3, it can be seen that the lower the firing temperature, the higher the strength and the lower the thermal conductivity, whereas the higher the firing temperature, the higher the thermal conductivity, but the lower the strength. As shown in Figure 6, it is possible to obtain a sintered body with well-balanced high strength and high thermal conductivity by selecting the four-point bending strength and thermal conductivity at room temperature and the firing temperature.

Example 4 In order to investigate the effect of changing the auxiliary agent content in the examples of the present invention, raw materials 11 and 2 shown in Table 4 were used, and the same procedure as in Example 1 was carried out at the auxiliary ratio shown in Table 4. Example N after firing
α71-73 and Comparative Example 74. 75 fired bodies were obtained.

The obtained fired body was measured for β conversion rate, thermal conductivity, four-point bending strength at room temperature and 800°C, fracture resistance value, and thermal shock resistance coefficient ratio in the same manner as in Example 1. The structure was also measured in the same way. The results are also shown in Table 4.

From the results in Table 4, it can be seen that even if the auxiliary agent content changes somewhat, those within the range of the present invention exhibit high strength and high thermal conductivity.

(Effects of the Invention) As is clear from the above description, according to the present invention, a raw material with a large particle size and a raw material with a small particle size are mixed and a sintered body is obtained by a predetermined manufacturing method. The microstructure has a predetermined ratio of large particles with a length of 10 μm or more and small particles with a grain length of 3 μm or less, achieving high strength, high toughness, and high thermal conductivity that cannot be obtained with conventional silicon nitride sintered bodies. I can do it.

Therefore, the silicon nitride sintered body of the present invention is more suitable as a high-temperature structural material than conventional ceramics, and reduces thermal stress when used in ceramic rotors, secondary combustion chambers, valves, piston heads, plates, etc. , the reliability of ceramics can be improved.

[Brief explanation of drawings]

Figure 1 shows room temperature strength and D2/D,
FIG. 2 is a graph showing the relationship between the thermal conductivity and D2/D in the example of the present invention,
FIG. 3 is a graph showing the relationship between room temperature strength and W1 in the example of the present invention, and FIG. 4 is a graph showing the relationship between the thermal conductivity and W1 in the example of the present invention. Figure 5 is a graph showing the relationship between room temperature strength and firing temperature in an example of the present invention. Figure 6 is a graph showing the relationship between thermal conductivity and firing temperature in an example of the present invention. It is. Figure 1 Figure 3 Figure 4 Wl (w1%)

Claims (1)

[Claims] 1. The proportion of particles with a particle length of 10 μm or more is 20 ~
50%, the proportion of particles with a grain length of 3 μm or less is 20-50%, and the thermal conductivity is 0.13-0.16 cal/cm・sec・℃, 4-point bending. A silicon nitride sintered body characterized by having a strength of 800 MPa or more at room temperature. 2. Prepare raw material 1 and raw material 2, which are silicon nitride powder raw materials with a purity of 99 wt% or more and a gelatinization rate of 95 wt% or more, and have different particle sizes as shown below. (1) Raw material 1 Average particle size D_1 (0.1 ~0.5 μm) (2) Raw material 2 average particle diameter D_2 (0.3 to 1.0 μm) (3) Relationship between raw material 1 and raw material 2 2≦D_2/D_1≦6 Prepared raw material 1 and raw material 2, Mix 20 to 60% by weight of raw material 1 and 80 to 40% by weight of raw material 2, add a predetermined amount of auxiliary agent to the mixed raw material, prepare and mold,
A method for producing a silicon nitride sintered body, which comprises performing pressure firing at a temperature of 1800 to 1900°C.