JPH07187798A - Production of silicon nitride-based sintered compact - Google Patents

Abstract

PURPOSE:To obtain a silicon nitride-based sintered compact low in deterioration of strength from a room temperature to a high temperature, especially to 1500 deg.C and having excellent mechanical strength. CONSTITUTION:A formed body having 50-65% relative density and having a composition composed of 30-80wt.% of silicon nitride, 19.5-69.5wt.% of silicon and 0.5-20wt.% of one or more kinds of oxides of group IIIa elements in the Periodic Table is subjected to nitriding treatment by carrying out heat treatment in a nitrogen-containing atmosphere so that the content of beta-silicon nitride in total silicon nitride may become <=10wt.%, and then, this nitride is baked in a non-oxidizing atmosphere containing nitrogen.

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 sintered body which is excellent in strength characteristics from room temperature to high temperature and is particularly used for automobile parts, gas turbine engine parts and the like.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have
Due to its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbo rotors. This silicon nitride sintered material is generally used for Y ₂ O ₃ , Al ₂ O _{3 and} silicon nitride.
Alternatively, by adding a sintering aid such as MgO, high density and high strength characteristics are obtained. Further improvement in strength and oxidation resistance at high temperatures is demanded for such silicon nitride sintered bodies when the operating conditions thereof further increase. In order to meet such demands, various improvements have been attempted so far, such as examination of sintering aids and improvement of firing conditions.

On the other hand, in the process of producing a high-density sintered body, when the molded body is sintered, firing contraction is inevitably caused by liquid phase sintering. Therefore, there is a problem in that it is very difficult to control the dimension to be finally set when manufacturing a component requiring dimensional accuracy, and the polishing process becomes complicated. Therefore, as a method of manufacturing a component requiring such dimensional accuracy, the starting material contains silicon powder and is nitrided to increase the density during molding,
A so-called reactive sintering method is known in which this is fired.

[0004]

However, according to the conventional reaction sintering method, the sinterability tends to deteriorate as compared with the general method not including the nitriding step. for that reason,
Since the high-temperature strength of the finally obtained sintered body is inferior to that obtained by the general method in high-temperature strength and acid resistance, it has excellent high-temperature strength and oxidation resistance, and at the same time dimensional accuracy. It was not possible to produce a product excellent in quality. Therefore, the sintered body using the reaction sintering method is still insufficient for practical use, and after improving the dimensional accuracy,
Further improvements in sinterability, strength, and oxidation resistance are required.

The present invention is particularly excellent in sinterability, has excellent resistance to oxidation from low temperature to high temperature, and has sufficient strength property to be used for automobile parts from room temperature to high temperature, gas turbine engine, etc. It is an object of the present invention to provide a method for producing a silicon nitride sintered body which is excellent in bending strength from room temperature to a high temperature of 1500 ° C. and is applied to parts and the like that require high dimensional accuracy.

[0006]

[Means for Solving the Problem] The present inventor has investigated the cause of the characteristics of the sintered body produced by the reaction sintering method being inferior to those obtained by the general method and the nitriding behavior in the nitriding step. As a result of stacking, the structure of the sintered body is more likely to be nonuniform as compared with the general method, and the tendency is more remarkable as the amount of β-silicon nitride in the nitrided body is larger, and β -By finding that the formation of silicon nitride depends on the density of the molded body before nitriding, and controlling the density of the molded body before nitriding and the amount of β-silicon nitride in the nitrided body after nitriding within a specific range, The inventors have found that excellent strength can be obtained without being inferior to that of a sintered body produced by a general method, and completed the present invention.

That is, in the method for manufacturing a silicon nitride sintered body according to the present invention, 30 to 80% by weight of silicon nitride and 19.5 to 59.5 of silicon are used.
69.5% by weight and 1 of Group 3a element oxides of the periodic table
A mixture having a composition of 0.5 to 20% by weight of seeds is molded to have a relative density of 50 to 65%, and the molded body is heat-treated in a nitrogen-containing atmosphere to perform β-nitriding with respect to the total amount of silicon nitride. After the nitriding treatment is performed so that the content of silicon is 10% or less, the nitride is fired in a non-oxidizing atmosphere containing nitrogen.

The present invention will be described in detail below. According to the method for producing a silicon nitride sintered body of the present invention, the raw material powder contains silicon nitride powder and silicon powder as main components, and at least a Group 3a oxide of the periodic table as a sintering aid. The silicon powder used has an average particle size of 10 to facilitate nitriding.
Fine particles having a particle size of less than or equal to μm, particularly 3 μm or less are desirable. In order to suppress the amount of β-silicon nitride in the nitrided silicon nitride powder, it is desirable that α-Si ₃ N ₄ is 95% or more, and the particle size thereof is 0.4 to 1.2 μm. Is appropriate. Examples of the Group 3a element of the periodic table used in the present invention include Y and lanthanoid elements, but Yb, Er and Lu are particularly preferable.

After the mixed powder of these raw material powders is molded into a desired shape by a known molding method such as press molding, cast molding, extrusion molding, injection molding, cold isostatic molding (rubber press), etc. 80 in the containing atmosphere
Heat treatment is performed at a temperature of 0 ° C. to 1500 ° C. to nitrid the silicon contained in the compact to generate silicon nitride. When this silicon is converted into silicon nitride, there is no dimensional change, and since the weight increases, the density of the compact improves.

According to the present invention, the amount of β-silicon nitride contained in the nitrided body at this time is controlled to be 10% or less, particularly 5% or less with respect to the total amount of silicon nitride. This is very important. This is because if the amount of β-silicon nitride exceeds 10%, densification is hindered in the subsequent firing, non-uniform grain growth is caused, and strength properties and oxidation resistance properties are deteriorated. Therefore, by controlling the β ratio in the silicon nitride generated in the nitriding step to be 10% or less, it is possible to enhance the sinterability, the high temperature strength and the oxidation resistance.

As a method for suppressing the formation of β-silicon nitride in the nitride as described above, the relative density with respect to the theoretical density of the compact before nitriding is 50 to 65%, particularly 55 to 6
It is important to control it to be 2%. That is, when the relative density is less than 50%, the nitriding reaction rapidly progresses and the production of β-silicon nitride is promoted, so that the amount of β-silicon nitride in the nitride exceeds 10%.
This is because if the content exceeds%, the nitriding reaction does not proceed and the silicon powder remains. In order to control the compact density within the above range, it can be easily controlled by adjusting the pressure applied to the powder during compaction.

In this nitriding treatment, in order to efficiently proceed the nitriding while suppressing the amount of β-silicon nitride produced, the nitriding temperature is raised in multiple steps within the above temperature range and gradually nitrided. It is desirable that the nitriding treatment at a constant temperature cannot completely nitridize silicon or suppress the formation of β-silicon nitride. The nitrogen gas pressure may be 1 to 30 atm in order to promote nitriding, but nitriding at high temperature and high pressure produces a large amount of heat.
Since the temperature of the molded body rises and the amount of β-silicon nitride produced increases, it is desirable to set it to 5 atm or less in the temperature range of 1300 ° C. or higher.

According to the present invention, the composition of the starting material is such that silicon nitride is 30 to 80% by weight and silicon is 19.5.
˜69.5 wt% and one or more of Group 3a element oxides of the Periodic Table should be prepared so as to have a composition of 0.5 to 20 wt%. That is, when the oxide of the Group 3a element of the Periodic Table exceeds 20% by weight, β-silicon nitride is likely to be formed, and the volume fraction of the grain boundary phase in the sintered body is increased to increase the high temperature strength. If it is less than 0.5% by weight, sufficient sintering does not proceed, a dense body cannot be obtained, and the characteristics of the sintered body deteriorate. If the amount of silicon nitride powder is less than 30% by weight, or if the amount of silicon powder exceeds 69.5% by weight, nitriding of silicon becomes difficult and β-silicon nitride is likely to be produced. On the contrary, if the amount of silicon nitride exceeds 80% by weight or the amount of silicon is less than 19.5% by weight, the dimensional shrinkage becomes large and the predetermined dimensional accuracy cannot be obtained. Desirable ranges are 50 to 70% by weight for silicon nitride, 25 to 45% by weight for silicon, and 5 to 15% by weight for Group 3a element oxide of the periodic table.

Next, the obtained molded body is subjected to a known firing method,
For example, hot press firing, atmospheric pressure firing, nitrogen gas pressure firing, and further hot isostatic firing (HI) after these firings.
P) treatment and glass seal HIP firing to obtain a dense sintered body. If the temperature at this time is too high, the silicon nitride crystal grains grow and the strength decreases.
It is desirable that the temperature is 0 ° C to 2000 ° C, particularly 1650 ° C to 1900 ° C.

[0015]

In general, when silicon is nitrided, β-silicon nitride tends to be produced more easily than α-silicon nitride. Therefore, when a molded body obtained by mixing the silicon powder and α-silicon nitride having excellent sinterability is subjected to nitriding treatment, a molded body in which α-silicon nitride and β-silicon nitride coexist. In this way, α
-When silicon nitride and β-silicon nitride coexist, α-silicon nitride changes to β-silicon nitride during the sintering process, but the sintering progresses, but β-silicon nitride does not undergo phase transformation and is simple. Since the sintering progresses in a simple liquid phase sintering, completely different sintering behaviors simultaneously proceed in one compact. Therefore, a non-uniform structure is formed in the final sintered body due to non-uniform sintering behavior due to abnormal grain growth or the like.

On the other hand, according to the present invention, the starting material composition, the nitriding conditions, etc. are controlled as described above to suppress the amount of β-silicon nitride in the compact after the nitriding treatment to 10% or less. Since the progress of sintering in the subsequent sintering process can be unified with the sintering behavior of α-silicon nitride, the progress of sintering can be made uniform. Thereby, in the final sintered body, it is possible to improve the high temperature strength and the oxidation resistance because the structure can be made uniform.

When the relative density of the green body before nitriding is low, the reaction area between the nitrogen-containing atmosphere and silicon is widened, the nitriding reaction is promoted, and β-silicon nitride is generated by the exothermic reaction during nitriding. To be done. On the other hand, if the relative density is too high, the nitriding reaction does not proceed, and unnitrided parts occur,
Since the sinterability is deteriorated, by setting the relative density to 50 to 65%, the nitriding reaction can be stabilized and a sintered body with high strength can be obtained.

[0018]

Example As a raw material powder, the average particle size is 3 μm, and the amount of oxygen is 1.
1% by weight of silicon powder, silicon nitride powder (BET specific surface area 8
m ² / g, α ratio 98%, oxygen amount 1.2% by weight) and periodic table 3a element oxide powder, silicon oxide powder are used in Table 1.
After mixing and mixing so as to have the composition shown in (1), the pressing pressure was changed by a rubber press to prepare several kinds of molded bodies having different relative densities. The obtained molded body is 1200 ° C., nitrogen pressure is 1
2 hours at 0 atm and 1300 ° C, nitrogen pressure 4 atm for 2 hours,
Nitriding was performed by treating at 1400 ° C. and a nitrogen pressure of 1 atm for 2 hours. At the time of this nitriding treatment, it was confirmed from the weight increase rate of the molded body whether or not silicon remained. Then, a part of the molded body after nitriding was crushed and the amount of β-silicon nitride was obtained by X-ray diffraction measurement. To calculate the amount of β-silicon nitride, (1 of β-Si ₃ N ₄ is used.
01), (210) peak intensities are H (101), H
(210), α-Si ₃ N ₄ (102), (210)
When the peaks of h (102) and h (210) are
[H (101) + H (210)] / Σ [H (101) +
The ratio was calculated from the formula of H (210) + h (102) + h (210)].

Thereafter, the obtained nitride was placed in a jar in silicon carbide, and the atmosphere was controlled in order to reduce the compositional variation, and it was fired in a nitrogen gas stream of 10 atm at 1850 ° C. for 4 hours. .

The dimensions of the obtained sintered body were measured, and the shrinkage ratio with respect to the dimension of the molded body was measured. Then, the sintered body was cut into a shape specified in JIS-R1601, processed, and polished to prepare a sample for characteristic evaluation. Specific gravity measurement based on Archimedes method, JIS
A 4-point bending bending test was carried out at room temperature and 1500 ° C. according to R1601. It should be noted that the relative density was 9 in the specific gravity measurement.
When the content was less than 0%, the shrinkage ratio and the strength were not measured. The results are shown in Table 2.

[0021]

[Table 1]

[0022]

[Table 2]

According to the results of Tables 1 and 2, samples No. 9, 13, and 17 in which the density of the molded body before nitriding was lower than 50% were obtained.
The formation of β-silicon nitride is remarkable, and the amount of β-silicon nitride after nitriding exceeds 10%, and the densification is insufficient and the strength is low. In addition, the sample No. 1 in which the density of the molded body exceeds 65%
With 0 and 20, nitriding is insufficient and silicon remains, resulting in a relative density of 90.
% Or more of the sintered body could not be obtained. Further, in sample No. 1 in which the amount of silicon in the starting material is small, the dimensional shrinkage is large,
Sample No. 5 containing a small amount of silicon nitride and Sample No. 6 containing an oxide of a Group 3a element of the periodic table of less than 0.5% by weight were not sufficiently densified, and a sintered body having satisfactory characteristics could be obtained. There wasn't.

In contrast to these comparative examples, all the products of the present invention showed a small shrinkage ratio of 15% or less, were densified without residual silicon, and had a room temperature strength of 800 MPa or more and 1500
It showed excellent mechanical properties with a strength of 450 MPa or higher.

[0025]

As described in detail above, according to the present invention,
By suppressing the amount of β-silicon nitride produced after nitriding, it is possible to provide an excellent silicon nitride sintered body which is excellent in dimensional accuracy and has little strength deterioration from room temperature to high temperature, particularly 1500 ° C. As a result, the reliability of high-temperature materials such as gas turbine blades, turbine rotors, and nozzles that require high dimensional accuracy can be improved.

Claims (1)

[Claims] 1. Silicon nitride is 30 to 80% by weight, and silicon is 1
A molded product having a relative density of 50 to 65%, which is composed of 9.5 to 69.5% by weight and 0.5 to 20% by weight of one or more kinds of Group 3a element oxides of the periodic table, in a nitrogen-containing atmosphere. After heat treatment to perform nitriding treatment so that the content of β-silicon nitride content with respect to the total silicon nitride content is 10% or less, the nitride is baked in a non-oxidizing atmosphere containing nitrogen. Of manufacturing a high quality sintered body.