JPH07187792A - Production of silicon nitride-based sintered compact - Google Patents

Abstract

PURPOSE:To obtain a uniform sintered compact not having difference in properties between inside and outside of the compact. CONSTITUTION:After forming a mixture consisting of a silicon nitride as a main component and containing at least 0.5-10mol% one oxide of 3a group elements of the Periodic Table into a prescribed shape, the formed body is embedded in a silicon nitride powder containing 0.1-0.5wt.% of oxygen and calcinated in an unoxidizing atmosphere containing nitrogen at 1700-2000 deg.C.

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 silicon nitride-based sintered body suitable for a structural material for a heat engine such as a gas turbine, and more particularly, to a homogeneous sintering having no difference in mechanical properties between inside and outside. A method for making a body.

[0002]

2. Description of the Related Art Sintered silicon nitride materials have been used as high-temperature strength materials and are now being applied to various engineering ceramics as high-temperature materials. This silicon nitride, since it is itself flame sinter, oxides and Al ₂ of the Periodic Table Group 3a elements, such as conventionally Y ₂ O ₃
Various metal oxides such as O ₃ and MgO are added and fired.

Recently, the operating temperature of a silicon nitride sintered body is expanding to a high temperature range of 1400 ° C. or higher, and along with this, improvement of high temperature characteristics of the sintered body is desired. Therefore,
From the viewpoint that the high temperature characteristics greatly change depending on the composition of the grain boundary of the silicon nitride sintered body and the melting point of the grain boundary due to the composition,
It has been proposed to suppress the addition of a low-melting substance such as Al ₂ O ₃ and to add Y ₂ O ₃ or an oxide of a rare earth element, or a sintered body of a system to which silicon oxide or the like is added, and to further crystallize a grain boundary phase. It has been proposed to increase the melting point of the grain boundaries by increasing the melting point.

Further, firing in the case of producing a silicon nitride sintered body is generally carried out under normal pressure firing, hot press firing,
It is carried out in a non-oxidizing atmosphere containing nitrogen in order to suppress the decomposition of silicon nitride itself by a method such as nitrogen gas pressure firing, but in a compact to be fired, it is decomposed at a low temperature.
Components such as O ₂ are also included, and fine control of the atmosphere is required. For example, the molded body is embedded in silicon nitride powder and the atmosphere is controlled to perform firing.

[0005]

However, when a molded body is embedded in silicon nitride powder and fired to control the atmosphere during firing, it is unavoidable in the silicon nitride powder used as the filler. Containing oxygen as an impurity. It is considered that the unavoidable impurities are present as approximately SiO ₂ , but usually commercially available silicon nitride powder contains about 0.7 to 2.0% by weight of oxygen. When such a silicon nitride powder is used as an embedding powder and the molded body is embedded and fired, the SiO ₂ component in the embedded powder is generated as SiO gas to suppress decomposition and volatilization of SiO ₂ in the molded body. A compositional change occurs on the surface of the sintered body thus fired, which causes a problem that a difference occurs in mechanical properties between the surface portion and the central portion of the sintered body.

As a result of studying this phenomenon, the SiO gas generated from the embedding powder penetrates and diffuses into the compact, and the amount of oxygen on the surface of the compact increases compared to the central part. It was found that the surplus oxide components that were not consumed even when the chemical treatment was performed increased, and this formed an amorphous phase and deteriorated the high temperature characteristics of the surface portion.

On the other hand, there are 1
As one of the methods, a method in which carbon is contained in the atmosphere to improve the partial pressure of CO gas in the atmosphere is conceivable, but in such a method, an unstable oxide having a small oxygen content due to the reducing action of CO gas is generated. The formed and decomposed layers and the crystalline phase having poor oxidation resistance such as melilite tended to be deposited on the surface of the sintered body and deteriorate the high temperature characteristics of the sintered body.

[0008]

The inventors of the present invention have made extensive studies on such problems, and as a result, although silicon nitride powder is used as the filling powder for burying the molded body, the silicon nitride powder is used. When firing is performed using a substance in which the amount of oxygen in impurities is reduced to a specific amount, the decomposition and volatilization of SiO _{2 in} the compact is suppressed, and the SiO gas generated from the embedded powder enters the compact. The inventors have found that the above can be suppressed, and that a sintered body having a homogeneous composition can be obtained without changing the composition of the surface portion and the center portion of the sintered body, and the present invention has been accomplished.

That is, the method for producing a silicon nitride-based sintered body of the present invention contains silicon nitride as a main component and contains at least the third periodic table.
Silicon nitride obtained by molding a mixture containing a group a element oxide in a proportion of 0.5 to 10 mol% into a predetermined shape, and then reducing the oxygen content of the molded body to 0.1 to 0.5% by weight. It is embedded in powder and is placed in a non-oxidizing atmosphere containing nitrogen.
It is characterized by firing at a temperature of 00 to 2000 ° C.

The present invention will be described in detail below. According to the production method of the present invention, the starting materials are silicon nitride powder as a main component and at least a Group 3a element oxide powder of the periodic table as a sintering aid component. To use. As the silicon nitride powder, α type,
Beta type can be used and average particle size is 0.4 to 1.2
About μm is appropriate. The Group 3a element oxide of the periodic table is added in an amount of 0.5 to 10 mol% in order to promote sintering. This is because if it is less than 0.5 mol%, a sufficiently dense body cannot be obtained, and if it exceeds 10 mol%, mechanical properties such as high temperature strength are deteriorated. The elements of Group 3a of the periodic table include Y, Er, Sc, Ce, Yb and L.
Examples thereof include u, Dy, Tb and Ho, and among these, Yb, Er and Lu are most desirable.

These starting materials are mixed in the above-mentioned predetermined proportions and then molded into a predetermined shape by a known method, specifically, press molding, extrusion molding, injection molding, cast molding, cold isostatic molding or the like. After obtaining a molded body having a relative density of 50% or more, it is fired.

As another method for producing a molded body as described above, a molded body is prepared by substituting silicon powder for a part or all of silicon nitride in the starting material, and then using this method.
Nitriding is performed at a temperature of 0 to 1500 ° C. to nitrid the silicon in the compact into silicon nitride, and then firing.

The firing is performed at 1700 to 2000 ° C. in a non-oxidizing atmosphere such as nitrogen by a firing means such as atmospheric pressure firing, nitrogen gas pressure firing, hot press firing, etc. According to the present invention, at this time, , The molded body has an oxygen content of 0.1 to 0.
It is a great feature that it is embedded in 5% by weight, particularly 0.2 to 0.4% by weight of silicon nitride powder and fired. As a result, the atmosphere can be controlled without entering SiO gas from the embedded powder into the molded body. That is, when the oxygen content in the embedding powder is less than 0.1% by weight, Si contained in the compact as an unavoidable impurity or as a compounded SiO ₂ powder.
0.5% by weight cannot be suppressed due to decomposition and volatilization of O _2.
If it exceeds, a large amount of SiO gas is generated from the embedded powder, which enters the molded body and changes the composition of the surface of the molded body,
A non-uniform sintered body having internal and external differences in characteristics is obtained. Further, if firing is performed without filling the molded body, the volatilization and decomposition of the SiO ₂ component in the molded body cannot be suppressed, the amount of oxygen in the surface portion is reduced, and the particles in the central portion and the surface portion There is a problem that the crystal phase of the field is different or, if it is remarkable, a decomposition layer is formed on the surface.

The embedding powder used in the present invention is usually obtained by subjecting a commercially available silicon nitride powder having an oxygen content of about 0.7 to 2.0% by weight to a heat treatment in a nitrogen gas atmosphere at 1400 to 1800 ° C. It is obtained by reducing the amount of impurity oxygen by releasing the impurity oxygen in the system to the outside of the system. However, if the silicon nitride powder whose oxygen content is reduced by this treatment is left in an oxidizing atmosphere, the amount of oxygen increases due to oxidation by oxygen in the atmosphere, so it is necessary to store it in nitrogen.

In the firing, when the molded body is embedded in the embedded powder, the molded body is embedded so as to be completely immersed in the embedded powder, and when a plurality of molded bodies are simultaneously embedded, the adjacent molded bodies are It is desirable to install so that sufficient embedding powder is interposed between them.

The silicon nitride-based sintered body obtained as described above is homogeneous in composition and characteristics in the surface portion and the central portion, but from the viewpoint of mechanical characteristics of the sintered body, The sintered body has silicon nitride crystal grains and grain boundaries of Si ₃ N ₄ -group 3a element oxide (RE ₂ O ₃ ) -SiO ₂ and Si _{3 of the} periodic table.
It is desirable that a high melting point crystal phase such as YAM, apatite, wollastonite, silicon oxynitride, or disilicate having a composition of N ₄ —SiO ₂ or RE ₂ O ₃ —SiO ₂ system is deposited. Si ₃ N ₄ -RE ₂
Since the melilite phase composed of O ₃ composition is inferior in oxidation resistance at high temperature, it is preferable to prevent it from being substantially detected by X-ray diffraction or the like. Also, compositionally, Al, Ca, Mg
Etc. form a low melting point substance and hinder crystallization, so these amounts are preferably reduced to 0.5% by weight or less.
However, as components that do not inhibit crystallization, Ti, Zr,
Periodic table 4a, 5a, 6a of Nb, Ta, Mo, W, etc.
Carbides, oxides, and nitrides of group metals may be added in a proportion of 10% by weight or less.

[0017]

According to the method for manufacturing a silicon nitride sintered body of the present invention, when the molded body is embedded in silicon nitride powder and fired, the oxygen content in the silicon nitride powder used as the embedded powder is 0.1%. By using the powder reduced to 0.5 wt%, it is possible to suppress the invasion of SiO gas generated from the embedded powder into the molded body, so that the oxygen amount in the molded body does not fluctuate and the surface It is possible to produce a uniform sintered body having no internal or external difference in composition or mechanical properties in the central part and the central part.

The method of the present invention is suitable for crystallizing the SiO ₂ component in the compact, that is, the amount of oxygen inevitably contained in the silicon nitride raw material powder in terms of SiO ₂ and the grain boundaries. It is effective for firing a molded body having a composition such that the total amount of the added SiO ₂ powder is about 1.5 to 20 mol%.

[0019]

EXAMPLE As a starting material, a silicon nitride material (oxygen content: 1.
2%, BET specific surface area of 2 to 10 m ² / g, α ratio of 98% or more) and various powders of various Group 3a element oxides (purity of 99% or more) of the periodic table as shown in Table 1 as a sintering aid. Were weighed in the proportions shown in Tables 1 and 2, and then placed in a polypot and mixed with a silicon nitride ball for 72 hours using methanol as a solvent. The obtained mixture was dried, granulated, and press-molded into a size of 80 mm × 45 mm × 5 mm.
The amount of SiO ₂ component was measured for the obtained molded body.
For the measurement, the total amount of oxygen in the molded body was measured, and the remaining amount of oxygen was converted to SiO ₂ by subtracting the amount of stoichiometric oxygen mixed as an oxide of a Group 3a element of the periodic table from silicon nitride. Is.

On the other hand, the silicon nitride powder was used as a filling powder.
Several kinds of embedding powders having different oxygen contents were prepared by treating under the treatment conditions shown in.

Then, using the embedding powder shown in Table 2, 1800 ° C.
Nitrogen gas pressure is 9 atm for 5 hours, and the cooling process is 1
Crystallization treatment was performed by holding at 400 ° C. for 5 hours.

A sintered surface (as-
JISR from the surface part including the fire surface) and the center part
A bending test piece based on 1601 was cut out, and the 4-point bending bending strength at 1400 ° C. was measured using this. Further, the surface of the sintered body was measured by X-ray diffraction to identify crystal phases other than the silicon nitride crystal.

[0023]

[Table 1]

[0024]

[Table 2]

According to Tables 1 and 2, the amount of oxygen is 0.5.
Sample No. baked by using embedding powders E and F in excess of weight%.
In all of 6, 7, 12, and 17, the difference between the inside and the outside of the characteristics occurred between the surface portion and the central portion of the sintered body. In addition, in samples No. 8 and 18 in which carbon-containing silicon nitride powder was used as a filling powder,
In each case, a decomposed layer was observed on the surface, the melilite phase was detected, and the high temperature strength was low. Further, in sample No. 1 which was performed without using any embedding powder, there was a difference in characteristics between the central portion and the surface portion.

On the other hand, the samples of the present invention have a small difference in characteristics between the central portion and the surface portion and have a bending strength of ±.
Uniform within 10 kg / mm ² and 1400 ° C strength 580
An excellent sintered body having a pressure of MPa or more was obtained.

[0027]

As described in detail above, according to the present invention, it is possible to obtain a uniform sintered body having no difference between internal and external characteristics by firing a molded body using silicon nitride powder having a small amount of oxygen as a filling powder. You can

Claims (1)

[Claims] 1. A mixture containing silicon nitride as a main component and containing at least 0.5 to 10 mol% of an oxide of a Group 3a element of the periodic table in a predetermined shape, and the formed body containing oxygen. Embedded in a silicon nitride powder in an amount of 0.1 to 0.5% by weight, and 1700 to 2 in a non-oxidizing atmosphere containing nitrogen.
A method for producing a silicon nitride-based sintered body, which comprises firing at a temperature of 000 ° C.