JP2734755B2 - Method for producing high-temperature high-strength silicon nitride sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a silicon nitride sintered body, and more particularly, to a method for producing a silicon nitride sintered body having excellent strength at high temperatures. INDUSTRIAL APPLICABILITY The present invention can be used, for example, when manufacturing a silicon nitride gas turbine engine.

[Prior Art] For example, as disclosed in JP-A-59-131579, 70-94% by weight of silicon nitride powder, 2% by weight or more of yttrium oxide powder and 2% by weight or more of spinel powder and yttrium oxide A method is known in which a mixed powder in which the total of the powder and the spinel powder is 6 to 30% by weight is molded and then heated in a non-oxidizing atmosphere. According to this method, since liquid phase sintering is performed and spinel is absorbed as a solid solution in silicon nitride, grain boundaries are strengthened and a high-strength sintered body is obtained.

Japanese Patent Publication No. 88-5661 discloses a method for producing a silicon nitride sintered body from a mixed powder containing yttrium oxide and spinel in amounts of 0.5 to 1.8% by weight or less. According to this method, since the amount of the sintering aid is small, a reduction in strength at high temperatures can be prevented.

[Problems to be Solved by the Invention] In the above-mentioned conventional manufacturing method, there is an advantage that spinel has a function of lowering a liquid phase sintering start temperature to 1500 to 1600 ° C during sintering, and sintering becomes easy. However, in order to obtain such advantages, it is necessary to use yttrium oxide and spinel at a weight ratio of about 1: 1. In this case, spinel is added in an amount more than necessary for sintering. For,
The grain boundaries of the sintered body tend to be amorphous. For this reason, at a temperature of 1200 ° C. or more, there has been a problem that the high-temperature strength sharply decreases. Therefore, it has conventionally been difficult to use silicon nitride ceramics for gas turbine engine parts, which can reach 1300 to 1400 ° C.

The present invention is intended to solve such a problem, and has an object to produce a silicon nitride sintered body having high strength at a high temperature.

[Means for Solving the Problems] The present inventors have studied diligently the compounding ratio of a rare earth element oxide and spinel as a sintering aid. And when the compounding amount of the spinel is in a limited range of a trace amount less than the conventional common sense, the oxidation resistance of the sintered body is remarkably improved, and it has been discovered that the present invention is excellent in high-temperature strength, thereby completing the present invention. It is.

That is, the method for producing a high-temperature and high-strength silicon nitride sintered body of the present invention comprises a silicon nitride (Si ₃ N ₄ ) powder of 91.5 to 96.99% by weight, a rare earth element oxide powder of 3 to 8% by weight and spinel (Mg
Al ₂ O ₄ ) powder is mixed with 0.01 to 0.5% by weight, and after forming, is sintered under normal pressure at 1700 to 1850 ° C. in a non-oxidizing atmosphere.

As the rare earth element oxide, any one that can function as a sintering aid for silicon nitride can be used, and yttrium oxide (Y ₂ O ₃ ), scandium oxide (Sc ₂ O ₃ ), and ytterbium oxide (Yb ₂ O ₃ ) can be used.
Columnar crystals grow by the oxide of the rare earth element, and β-Si ₃ N ₄ having a large aspect ratio is generated.

This rare earth element oxide powder accounts for 3 to 8% by weight of the whole.
Be blended. If the content is less than 3% by weight, dense sintering becomes difficult, and the density and strength of the sintered body decrease. If the content is more than 8% by weight, the content becomes excessive and the high-temperature strength decreases.

Atmospheric pressure sintering is difficult only with rare earth element oxides. Therefore, a small amount of spinel (MgAl ₂ O ₄ ) is added. As a result, liquid phase sintering is performed, and sintering can be performed at normal pressure. Here, the spinel is blended in a total amount of 0.01 to 0.5% by weight. At less than 0.01% by weight, normal pressure sintering becomes difficult,
If it is added in excess of weight%, the oxidation resistance of the sintered body is reduced, and the high-temperature strength is deteriorated.

The rare earth oxide powder and the spinel powder are 91.5%
~ 96.99% by weight of silicon nitride powder is mixed and formed into a desired shape. As the molding method, molding can be performed by a conventionally used molding method such as hydrostatic pressure molding, injection molding, and slurry casting.

The obtained compact is dried, if necessary, and then sintered under normal pressure at 1700 to 1850 ° C. in a non-oxidizing atmosphere. Since sintering is not performed in an oxidizing atmosphere, it is necessary to mold under an inert gas atmosphere or a vacuum condition. This sintering is performed at normal pressure, and the sintering temperature is 1700-1850 ° C. If the normal-pressure sintering temperature is lower than 1700 ° C., the densification of the sintered body does not sufficiently proceed, resulting in a decrease in mechanical properties. On the other hand, when the temperature is higher than 1850 ° C., decomposition of silicon nitride or the like occurs, which is not preferable.

[Function] In the manufacturing method of the present invention, β-Si ₃ N ₄ having a large aspect ratio is generated by the oxide of the rare earth element, so that the sintered body becomes dense and the strength is improved. Then, liquid phase sintering is performed by a small amount of spinel, and normal pressure sintering becomes possible. Further, since the spinel is very small, the amorphous portion formed at the grain boundary is small. Therefore, it is assumed that the oxidation resistance of the amorphous portion is improved and the high-temperature strength is also improved.

[Examples] Hereinafter, specific examples will be described.

Example 1 As shown in Table 1, 96.8% by weight of high-purity silicon nitride (Si ₃ N ₄ ) powder having an average particle diameter of 0.2 μm was used, and the average particle diameter was 0.1 μm.
Yttrium oxide (Y ₂ O ₃ ) powder was mixed at 3.0% by weight and spinel (MgAl ₂ O ₄ ) powder having an average particle size of 0.05 μm at 0.2% by weight. It was shaped into a square bar. Then, normal pressure sintering was performed at a temperature of 1820 ° C. in a nitrogen gas atmosphere.

The obtained sintered body was processed into a bending test piece shape according to JIS standard, and the four-point bending strength (JISR1601) was measured in air at three levels of room temperature, 1300 ° C. and 1400 ° C. The number of samples is 10. The density was measured by the Archimedes method, and the results are shown in Table 1.

(Examples 2 to 9 and Comparative Examples 1 to 4) Scandium oxide (Sc ₂ O ₃ ) and ytterbium oxide (Yb ₂ O ₃ ) were also selected as oxides of rare earth elements, and the type, blending ratio and Except that the sintering temperature was variously changed as shown in Table 1, the same as in Example 1,
Each sintered body was manufactured and tested in the same manner, and the results are shown in Table 1.

(Test Example) The sintered bodies obtained in Example 4 and Comparative Example 4 in Table 1 were each subjected to a static oxidation test in which they were heated at 1400 ° C. for 100 hours in the atmosphere, and the weight after heating and before heating were measured. Was determined from the difference in the weight of the samples. As a result, while the weight gain of the sintered body of Comparative Example 4 was increased by 15 mg / cm ² , the weight gain of oxidation was reduced to 1 mg / cm ² and 1/15 in the sintered body of Example 4. This is presumably because the amount of spinel is extremely small as compared with yttrium oxide, and the oxidation resistance is improved by the reduced thickness of the grain boundary.

(Evaluation) The sintered bodies obtained in Examples 1 to 9 have a high bending strength of 500 MPa or more even at 1400 ° C. It can also be said that there is almost no difference between the types of rare earth element oxides.

However, in the sintered bodies obtained in Comparative Examples 1 and 4 in which the amount of yttrium oxide is small, dense sintering is not performed, the sintered body density is small, and the strength is small. Furthermore, since the sintered body obtained in Comparative Example 4 has a large amount of spinel, the high-temperature strength is further reduced as compared with Comparative Example 1. Further, in the sintered body obtained in Comparative Example 2, yttrium oxide and spinel contained The high-temperature strength is reduced due to too much, and the sintered body obtained in Comparative Example 3 has insufficient spinel, resulting in reduced sinterability,
Density and strength are reduced.

[Effects of the Invention] That is, according to the method for manufacturing a silicon nitride sintered body of the present invention, the number of steps is the same as that of the conventional method, the oxidation resistance is excellent, and 1300 to
A sintered body having excellent high-temperature strength at 1400 ° C. can be reliably produced.

Claims (1)

(57) [Claims] 1. Silicon nitride (Si ₃ N ₄ ) powder 91.5 to 96.99% by weight
If, oxide powder 3-8 wt% and spinel of rare earth elements (MgAl ₂ O ₄₎ to a 0.01 to 0.5 wt% powder mixture, to pressureless sintering under from 1,700 to 1,850 ° C. a non-oxidizing atmosphere after forming A method for producing a high-temperature high-strength silicon nitride sintered body, characterized by the following.