JP2631108B2 - Manufacturing method of silicon nitride sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a silicon nitride sintered body used for a gas turbine or the like.

(Background technology)

Sintered bodies made of silicon nitride are mainly composed of covalent bonds in the bonding mode of atoms, and have excellent properties in strength, hardness, and thermal and chemical stability. Application to is being promoted.

(Prior art)

In recent years, it has been predicted that the operating temperature of a heat engine will be 1400 ° C. or higher as the efficiency of the heat engine increases, and materials that can be used under these conditions are desired. As a material usable for such a heat engine, a silicon nitride sintered body is most expected.

Conventionally, as a method for producing a silicon nitride-based sintered body, a silicon nitride is added with a Group IIIa oxide of the periodic table, such as Y ₂ O ₃ , or Al ₂ O ₃ as a sintering aid. This is fired in a nitrogen atmosphere. Further, in order to enhance the high-temperature characteristics of the silicon nitride-based sintered body, oxides such as Al ₂ O ₃ are reduced, and silicon nitride—a group IIIa oxide of the periodic table—SiO ₂
A sintered body in which a system crystal is generated at a grain boundary has also been proposed.
Instead of hot press method and nitrogen gas pressure firing method, 1000 ~
A hot isostatic press sintering method in which sintering is performed under an ultra-high pressure of 3000 atm is known.

According to this hot water pressure press firing method, the relative density is 90%
As described above, hot isostatic pressing is often used for the primary fired sintered body. However, in such a method, it is necessary to strictly control the atmosphere around the sintered body, and the There is a problem that it is difficult to control the crystal phase and stabilize the characteristics.

On the other hand, a so-called glass seal HIP method is known in which an object to be processed is placed in a glass container or a glass bath, and sintering is performed by applying an ultra-high pressure using glass as a pressure medium. This method has the advantage that a high-strength sintered body with a fine structure can be obtained because firing is performed at a relatively low temperature without requiring control of the atmosphere in order to completely block the workpiece from the atmosphere and firing. Having. For example, in Japanese Patent Application Laid-Open No. 60-155576, a silicon nitride-based sintered body having high density and excellent toughness is obtained by calcination and glass sealing HIP of an Al ₂ O ₃ -group IIIa oxide-added molded body. A method is provided for making a compact.

[Problems to be solved by the invention]

However, in the glass seal HIP method, although a high-strength sintered body can be manufactured with a fine structure, there is a problem that the toughness is lower than that of a sintered body obtained by a normal method. Further, there is a problem that the sintered body described in Japanese Patent Application Laid-Open No. 155576/1985 contains Al ₂ O ₃ , resulting in poor strength and oxidation resistance at a high temperature of 1400 ° C. or higher. Therefore, an object of the present invention is to prepare a sintered body having excellent toughness after HIP and excellent strength and oxidation resistance even in a high temperature range of 1400 ° C. or higher.

[Means for solving the problem]

According to the present invention, silicon nitride powder is 85 to 95 on a three-component basis.
Mol%, and a compound of at least one element (M) selected from Group IIIa elements of the periodic table in terms of oxide (M ₂ O ₃ )
A mixed powder containing 0.5 to 5 mol% and excess oxygen and having a ratio of (mol number of excess oxygen in terms of SiO ₂ / mol number of M ₂ O ₃ ) of 1 or more is molded as a starting material, and the molded body is formed. 1550 ℃ ~
After performing a heat treatment in the range of 1850 ° C. to grow needle-like crystals, the heat-treated compact was further heated to 1650 ° C. to 2000 ° C., 10
A method for producing a silicon nitride-based sintered body, characterized in that hot isostatic press sintering is performed using glass as a pressure medium in the range of 00 to 3000 atm.

That is, when producing the sintered body of the present invention, first, Si ₃ N
_{4 The} powder may be either α-type or β-type, and it is particularly desirable to use a powder having a BET specific surface area of 10 m ² / g or more and an oxygen content of 3% by weight or less.

Above the Si ₃ N ₄ powder and periodic table III a group β-Si ₃ N ₄ in the sintered body in a compound of a metal selected from the elements 85 to 95
The metal element compound is weighed and mixed so as to be 0.5 to 5 mol% in terms of oxide in terms of oxide. As the metal compound used in the preparation, compounds such as oxides, carbides, nitrides, oxynitrides, borides, nitrates, carbonates, and oxalates can be used. preferable.

After blending, mix a suitable binder etc. into the mixed powder,
After molding into a desired shape by a known molding method, for example, press molding, casting molding, extrusion molding, injection molding or the like, heat treatment is performed.

The heat treatment is performed in the range of 1550 to 1850 ° C. If the temperature is lower than 1550 ° C, silicon nitride does not develop into needle-like crystals, and the toughness after HIP is low. If the temperature is higher than 1850 ° C, the specific gravity after HIP does not increase.

Further, as a requirement required for producing a good needle-like structure in the heat-treated material, it is necessary that the density of the heat-treated material is in the range of 30 to 75% of the theoretical density. This means that a space enough to extend like a needle is kept in the molded body. If it is lower than 30%, acicularization can be achieved, but deformation and shrinkage during subsequent sintering are large, making it difficult to control dimensions and the like. On the other hand, if the density is 75% or more, needle formation during heat treatment is not sufficient.

The heat-treated body is then subjected to hot isostatic press sintering at 1650 ° C. to 2000 ° C. and 1000 to 3000 atm using glass as a pressure medium. The processing temperature is 1650 ° C. to 2000 ° C. When the temperature is 1650 ° C. or higher, the specific gravity of the sintered body does not increase, and when the temperature is 2000 ° C. or higher, the strength deteriorates due to grain growth.

Further, (molar ratio of excess oxygen in terms of SiO ₂ / mol number of M ₂ O ₃ ) (molar ratio) is 1 or more, and if it is less than 1, the oxidation resistance deteriorates.

Here, excess oxygen is defined as the sintering aid in the periodic table
When a group IIIa element oxide is added, it is the remaining oxygen obtained by subtracting the oxygen bound to the group IIIa oxide from the total oxygen content in the sintered body.

〔Example〕

Α-Si ₃ N with a specific surface area of 14 m ² / g and an oxygen content of 2% by weight or less
₄ Powder oxide of a metal selected from Group IIIa elements, SiO ₂ for excess oxygen adjustment, and Al ₂ O ₃ as a comparative example were appropriately blended, and these were mixed for 24 hours in a ball mill. .
The obtained slurry is dried and granulated, press-formed, debindered in a vacuum, and then heat-treated under the conditions shown in Table 1 to grow needle-like crystals. Sintering was performed under the conditions shown in Table 1 to obtain sintered bodies having the compositions of Sample Nos. 1 to 17 in Table 1.

JIS-R at room temperature and 1400 ℃ for each sample obtained
1601 Four-point flexural strength test and increase of oxidized weight of sintered body by weight increase (mg / cm ² ) after 1400 ° C × 24 hours, that is, oxidation resistance at high temperature was measured. Was.

The amount of excess oxygen was measured by first measuring the total oxygen content using an oxygen and nitrogen simultaneous analyzer from LECO, and then measuring the Group IIIa compound in the sintered body by ICP analysis. The amount was calculated and calculated by subtracting the oxygen amount from the total oxygen amount.

As can be seen from Table 1, sample no.
Since the oxides of group elements are as large as 7 mol% and the SiO ₂ / M ₂ O ₃ (molar ratio) is as small as 0.85, the toughness is deteriorated and the oxidation weight at high temperature is increased, and the oxidation resistance is deteriorated. Sample No. 14
Has an inferior high-temperature flexural strength of 35 kg / mm ² and a toughness of 5.7 MPa · m ^1/2 because the oxide of the Group IIIa element is as small as 0.4 mol%. Further, in sample No. 17, 1 mol% of Al ₂ O ₃ was added, so that high-temperature bending strength and oxidation resistance at high temperature were significantly deteriorated.

Furthermore, as can be understood from Sample Nos. 15 and 16, the fracture toughness of the sintered body after HIP is improved by adding a further heat treatment step, and the processing temperature must be 1550 ° C. or higher. Is understood.

〔The invention's effect〕

In the present invention, a silicon nitride sintered body having excellent high-temperature strength and high-temperature oxidation resistance and high toughness can be obtained.

Claims (2)

(57) [Claims] (1) 85-95 mol% of silicon nitride powder based on three components
And at least one selected from Group IIIa elements of the periodic table
The compound of the element (M) is 0.5 to 0.5 in oxide equivalent (M ₂ O ₃ ).
5 mol% and excess oxygen, and (excess oxygen SiO ₂
The mixed powder having a ratio of (converted mole number / M ₂ O ₃ mole number) of 1 or more is molded as a starting material, and the molded body is formed at 1550 ° C. to 1850 ° C.
After growing a needle-shaped crystal by performing a heat treatment in the range of 1650 ℃ ~ 2000 ℃, further heat-treated molded body, 1000 ~ 3000
A method for producing a silicon nitride-based sintered body, wherein hot isostatic press sintering is performed using glass as a pressure medium in a range of atmospheric pressure. 2. The method according to claim 2, wherein said Group IIIa element is Sc, Er, Yb, Ho and D.
2. The method for producing a silicon nitride-based sintered body according to claim 1, which is selected from the group of y.