JPH0543331A - Production of highly tough silicon nitride sintered compact - Google Patents

Abstract

PURPOSE:To prevent micro-cracks from forming in a silicon nitride sintered compact and remarkably improve the toughness value. CONSTITUTION:Si3N4 powder containing oxide-based sintering assistant powder or Si3N4 powder containing the oxide-based superconducting sintering assistant powder and Cr3C2 powder is formed and the resultant compact is then pressed and sintered to provide a sintered compact. In the process, the compact is cooled within the temperature region from the maximum sintering temperature to 1200 deg.C at <=10 deg.C/min rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high toughness silicon nitride sintered body which is applied as engineering ceramics to various machine parts, automobile parts and the like.

[0002]

2. Description of the Related Art Reinforcement by whisker fibers can be cited as an effective means of increasing the toughness of a silicon nitride sintered body. However, there are technical problems such as dispersibility and formability. Attempts have also been made to grow the columnar particles themselves to increase the toughness. This method is a method in which matrix silicon nitride particles are grown up to several times the usual size by firing at high temperature under a pressure condition to increase the toughness by the particle size effect.
5958, JP-A-63-310771, "Diversification of Si ₃ N ₄ ceramics by controlling the structure: Silicon nitride ceramics 2", Uchida Otsukaku, 135-146 (1990) and the like.

[0003]

However, the fracture toughness value K _IC of the sintered body obtained by the conventional method is at most 9 to 10 MPa · m ^1/2.
There is no known way to improve this significantly. Therefore, the inventors of the present invention have conducted extensive studies to clarify the cause, and as a result, the silicon nitride particles of the sintered body are cooled in an abnormally grown state, so that microcracks occur in the sintered body. , It was found that it causes a decrease in toughness value. An object of the present invention is to provide a silicon nitride sintered body having excellent toughness based on such findings.

[0004]

The feature of the present invention resides in that the cooling rate in the firing step is controlled in order to prevent the generation of microcracks. The present invention, after the Si ₃ N ₄ powder containing the oxide-based sintering aid powder is molded, the molded body is pressure-fired to produce a sintered body. Provided is a method for producing a high toughness silicon nitride sintered body, which comprises cooling the zone at a rate of 10 ° C./minute or less. Further, the present invention, after the Si ₃ N ₄ powder containing the oxide-based sintering aid powder and Cr ₃ C ₂ powder is molded, the molded body is pressure-fired to produce a sintered body. Provided is a method for producing a high toughness silicon nitride sintered body, which comprises cooling a temperature range from the maximum temperature to 1200 ° C at a rate of 10 ° C / minute or less.

The present invention will be described in more detail below.
The silicon nitride powder used is not particularly limited, but the α fraction of the silicon nitride powder is preferably 90% or more, particularly preferably 95% or more. The reason is that if the α fraction is low, the aspect ratio of the silicon nitride particles in the sintered body will be small, and the toughness value will not be improved so much. The average particle size is 3μ
It is preferably m or less, and particularly preferably 1 μm or less. The reason is that if the particle size is too large, the densification is hindered and the strength is apt to decrease, and the mechanical properties important as a material are impaired.

Examples of the oxide-based sintering aid include rare earth oxides such as yttria alone, or rare earth oxides and alumina-based materials, or systems containing magnesia-based oxides such as magnesia and spinel. The silicon nitride powder and the oxide-based sintering aid are mixed in a desired ratio and molded by an appropriate molding method. Examples of the molding method used include press molding, CIP molding, cast molding, injection molding and extrusion molding.

The obtained molded body is subjected to a pressure of 1 to 2000 under a pressure of 1800
Heat to ~ 2000 ° C and hold for several hours. In this case, in order to obtain a dense sintered body, it is necessary to heat at a low temperature or a low pressure to appropriately densify and then gradually pressurize. The holding time is the time required for the matrix silicon nitride particles to grow sufficiently, and is generally at 1800 ° C for 5 to 10 hours and 2000 ° C.
It takes 1-2 hours.

After maintaining the maximum firing temperature, up to 1200 ° C
It is cooled at a rate of 10 ° C / min or less, and is naturally cooled from 1200 ° C or less to obtain a sintered body. At this time, if the cooling rate is 10 ° C./minute or more, microcracks are likely to occur in the obtained sintered body and toughness is not improved, which is not preferable. The reason for limiting the cooling temperature to 1200 ° C is that if it exceeds 1200 ° C,
This is also because microcracks are likely to occur and the toughness is not improved. As a means for controlling the cooling rate, for example, a temperature program controller controlling the temperature of the sintering furnace is used.

The present invention is sufficiently effective even when using a monolithic powder which is a Si ₃ N ₄ powder containing the above-mentioned oxide-based sintering aid powder.
When r ₃ C ₂ powder is added, the improvement in toughness value is more remarkable than in the case of monolithic powder. The cause of this is not clear, but it is considered that one factor is the particle size effect caused by the further growth of the silicon nitride particles as compared with the case of monolithic powder. In this case, the amount of Cr ₃ C ₂ added is preferably 1 to 10% by weight, particularly 2 to 5% by weight, based on the raw material powder. If it is less than 1 wt%, the effect of grain growth of silicon nitride is small, and the toughness value is not much different from that of monolithic powder,
When it exceeds wt%, the reaction between Cr ₃ C ₂ and silicon nitride becomes vigorous, and the produced sintered body is locally deformed, so that the desired shape cannot be obtained.

[0010]

EXAMPLES The present invention will be specifically described below with reference to examples. Examples 1 to 7 and Comparative Examples 1 to 4 93 wt% Si ₃ N ₄ powder (average particle size 0.4 μm, α fraction 97%)
, 5 wt% Y ₂ O ₃ and 2 wt% as oxide-based sintering aids
Al ₂ O ₃ was placed in a Si ₃ N ₄ pot, mixed in 1,1,1-trichloroethane using Si ₃ N ₄ balls for 20 hours, and then dried. The obtained powder was molded into a predetermined shape with a mold, then CIP molded, and then fired under the conditions shown in Table 1 to obtain a sintered body. In the firing conditions, the temperature rising rate from 1000 ° C was
The pressure pattern was 10 ° C./minute, the pressure pattern was 15 MPa up to 1400 ° C., the pressure was gradually increased to the maximum firing temperature, and finally 40 MPa. The density, the three-point bending strength according to JIS R1601, and the fracture toughness according to the SEPB method according to JIS R1607 of the obtained sintered body were measured.

The results are shown in Table 1. From the results in Table 1, Si ₃
When fired at a temperature of 1800 ° C or higher, which is effective for grain growth of N ₄ , compared to Comparative Examples 1 to 4 in which natural cooling or cooling temperature was 1400 ° C, 1200 ° C at a cooling rate of 10 ° C / min or less. It was confirmed that high toughness values were obtained in the sintered bodies of Examples 1 to 7 that had been cooled to.

[0012]

[Table 1]

Examples 8-14 and Comparative Examples 5-8 Cr ₃ having an average particle size of 4.3 μm was used under the firing conditions shown in Table 2.
Sintered bodies were manufactured and measured in the same manner as in Examples 1 to 7 except that 2 wt% of C ₂ powder was added. The results are shown in Table 2. From the results shown in Table 2, even in the system containing Cr ₃ C ₂ ,
When firing at a sintering temperature of 1800 ° C or higher, which is effective for grain growth of Si ₃ N ₄ , it is possible to cool naturally or at a cooling rate of 10 ° C / min.
Compared to Comparative Examples 5 to 8 cooled to 1400 ° C. or 1200 ° C. at a cooling rate of 20 ° C./min, 1200 at a cooling rate of 10 ° C./min or less
It was confirmed that in the sintered bodies of Examples 8 to 14 that were cooled to 0 ° C, a high toughness value was obtained, and the toughness value was higher than that of the monolithic system shown in Table 1.

1 and 2 are scanning electron micrographs showing the crystal structures of the polished surfaces of the sintered body obtained in Example 9 and the sintered body obtained in Comparative Example 5 at a magnification of 2000. Microcracks were observed in the naturally cooled sintered body of Comparative Example 5 shown in FIG. 2, but microcracks were not observed in the sintered body of Example 9 shown in FIG. confirmed.

[0015]

[Table 2]

Examples 15 to 17 Sinters were manufactured and measured in the same manner as in Example 9 except that the final maximum pressure was changed from 40 MPa to the conditions shown in Table 3. The results are shown in Table 3. From the results of Table 3, it was confirmed that even if the final maximum pressure was changed, there was no great difference in the toughness values, and high toughness values were obtained. However, when the final maximum pressure was 30 MPa, which was not much different from the intermediate pressure, it was difficult to densify.

[0017]

Examples 18 to 21 Sintered bodies were manufactured and measured in the same manner as in Example 9 except that the powder having the powder characteristics shown in Table 4 was used instead of the Si ₃ N ₄ powder. The results are shown in Table 4. From the results of Table 4, it was confirmed that a high toughness value was obtained under the condition that the α fraction of the Si ₃ N ₄ powder was 90% or more and the average particle size was 2.8 μm or less.

[0019]

Examples 22-25 Oxide-based sintering aids 5 wt% Y ₂ O ₃ and 2 wt% Al ₂ O ₃
A sintered body was manufactured in the same manner as in Example 19 except that the auxiliaries shown in Table 5 were used, and the measurements were performed. The results are shown in Table 5. From the results in Table 5, it was confirmed that a high toughness value can be obtained with other oxide-based auxiliary agents.

[0021]

Examples 26 to 29 Sintered bodies were manufactured and measured in the same manner as in Example 9 except that the Cr ₃ C ₂ addition amount was changed from 2 wt% to the addition amount shown in Table 6. .. The results are shown in Table 6. From the results in Table 6, Cr ₃ C ₂
It was confirmed that a high toughness value was obtained when the addition amount was in the range of 1 to 10 wt%.

[0023]

[0024]

According to the method of the present invention, a silicon nitride sintered body having high fracture toughness can be obtained without impairing the strength.

[Brief description of drawings]

FIG. 1 is a scanning electron micrograph (magnification: 2000) showing a crystal structure of a polished surface of a sintered body of Example 9 obtained by the method of the present invention.

FIG. 2 is a scanning electron micrograph at a magnification of 2000 showing a crystal structure of a polished surface of a sintered body of Comparative Example 5 obtained by a conventional method.

Claims (2)

[Claims] 1. When a Si ₃ N ₄ powder containing an oxide-based sintering aid powder is molded and then the molded body is pressure-fired to produce a sintered body, the maximum firing temperature up to 1200 ° C. A method for producing a high toughness silicon nitride sintered body, which comprises cooling the temperature range at a rate of 10 ° C / minute or less. 2. A sintered body is manufactured by molding a Si ₃ N ₄ powder containing an oxide-based sintering aid powder and a Cr ₃ C ₂ powder, and then sintering the molded body under pressure to produce a sintered body. A method for producing a high toughness silicon nitride sintered body, which comprises cooling a temperature range from temperature to 1200 ° C at a rate of 10 ° C / minute or less.