JPH0524926A - Production of silicon nitride-silicon carbide combined sintered compact - Google Patents

Abstract

PURPOSE:To produce a sintered compact minimal in the shrinkage factor and amount of deformation of a sintered compact, excellent in demensional accuracy, and applicable to a rotor for a heat engine, etc., requiring high dimensional accuracy. CONSTITUTION:A green compact, in which 1-150 pts.wt. of silicon carbide is added to 100 pts.wt. of component consisting of 0.2-10mole% oxide of group IIIa element of the periodic table, such as Er2O3, Yb2O3, Ho2O3, and Dy2O3, and the balance silicon or silicon and silicon nitride, is prepared. Heat treatment is applied to the green compact in a nitrogen-containing atmosphere of 800-1500 deg.C to nitride the silicon in the green compact and increases the density of the green compact and then sintering is performed in the nitrogen-containing nonoxidizing atmosphere of 1600-2000 deg.C, by which a silicon nitride - silicon carbide combined sintered compact can be obtained.

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-silicon carbide composite sintered body mainly composed of silicon nitride and silicon carbide, and more specifically, suitable for high-temperature structural materials,
The present invention relates to a method for manufacturing a sintered body suitable for manufacturing a gas turbine component or the like that requires high dimensional accuracy.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been applied to engineering ceramics, especially as materials for heat engine structures, because they are excellent in strength, hardness and thermal / chemical stability. As a method for obtaining such a silicon nitride sintered body, a sintering aid such as a Group 3a element oxide of the periodic table is added to and mixed with silicon nitride powder, and after molding,
It is obtained by firing at a temperature of 1500 to 2000 ° C. in a non-oxidizing atmosphere.

However, the silicon nitride sintered material has excellent characteristics, but on the other hand, it has a problem that strength and the like decrease at high temperatures. The problem of deterioration of high temperature strength has been improved so far by improving the sintering aid and changing the firing atmosphere and firing pattern.
The current situation is that no definitive measures have been taken. on the other hand,
Although the silicon carbide based sintered body has an absolute strength lower than that of the above-mentioned silicon nitride based sintered body, it has a characteristic that the strength is hardly deteriorated at a high temperature.

Therefore, recently, a composite sintered body obtained by adding silicon carbide to silicon nitride and firing it has been proposed. Since this composite sintered body has the excellent characteristics that the absolute strength is higher than that of the silicon nitride sintered body and the strength deterioration at high temperature is small, the sintered body is used as a material for a heat engine. Practical application is under study.

[0005]

However, the above-mentioned composite sintered body is largely deformed during firing and can be applied to a gas turbine component which requires high dimensional accuracy even though it has high temperature characteristics. It is difficult and requires a large working margin when manufacturing a sintered body. Therefore, although improvements have been made by making uniform and high-density compacts by improving the raw materials and the molding method that have been conventionally used, the definitive measures have not yet been reached.

It is considered that the main cause of the deformation is that the shrinkage during firing is caused by the stress due to its own weight and the like, so that the shrinkage cannot be performed uniformly. The above-mentioned sintered body has a large density difference between the green body and the sintered body, and since the contraction is large, the deformation tends to be large. Shrinkage during such sintering
Deformation is a factor that inevitably occurs when silicon nitride and silicon carbide are compounded, because their respective sintering behaviors are different, and such a compound sintered body is required to have high dimensional accuracy such as a gas turbine. However, it was difficult to apply it to various products.

[0007]

Means for Solving the Problems As a result of a detailed study on the above problems, the present inventors have found that when a composite sintered body of silicon nitride and silicon carbide is produced, a sintering aid is used. After adding an oxide of a Group 3a element of the periodic table as described above, silicon powder is blended in the molded body, and this is heat-treated in a low temperature range to nitride the silicon powder to suppress shrinkage and improve density. It has been found that by firing in a high temperature range and densifying, a composite sintered body with small deformation and high dimensional accuracy can be obtained without impairing the conventional characteristics.

That is, in the method for producing a silicon nitride-silicon carbide composite sintered body of the present invention, first, a Group 3a element oxide of the periodic table is contained in a proportion of 0.2 to 10 mol%, and the balance is silicon. Alternatively, a molded body is prepared by adding silicon carbide in an amount of 1 to 150 parts by weight to 100 parts by weight of a component composed of silicon and silicon nitride, and the molded body is placed in a nitrogen-containing atmosphere at 800 to 1500 ° C. After nitriding the silicon by heat treatment at 1
It is characterized in that it is densified by firing in a non-oxidizing atmosphere at 600 to 2000 ° C.

The production method of the present invention will be described in detail below. First, silicon powder, silicon carbide powder as a starting material, a Group 3a element oxide of the periodic table as a sintering aid, and optionally a silicon nitride powder, Silicon oxide powder is used.

The silicon powder used is preferably fine particles having an average particle size of 10 μm or less, particularly 3 μm or less, in order to facilitate nitriding. Further, as the silicon carbide powder, α
Type and β type can be used, and from the viewpoint of grain growth suppressing effect on silicon nitride due to dispersion thereof, fine particles having an average particle size of 1 μm or less and an impurity oxygen amount of 2% by weight or less are preferable. Further, as the silicon nitride powder, since silicon carbide is contained in this system and the sinterability of the whole is lowered due to the nitriding of silicon, α-type silicon nitride is contained in a proportion of 95% or more, and the average A particle size of 1 μm or less and an impurity oxygen amount of 2% by weight or less are used. When the silicon nitride powder is contained, the silicon nitride powder and the silicon carbide powder are present as individual powders, or a powder obtained by compounding silicon nitride and silicon carbide at a predetermined ratio can be used.

Next, these raw material powders are mixed in a predetermined ratio to form a molded body. According to the present invention, the composition of the molded body contains the oxide of the Group 3a group 3a element of the periodic table in a proportion of 0.5 to 10 mol%, particularly 1 to 7 mol%, and the balance being silicon powder.
Alternatively, 1 to 100 parts by weight of a silicon carbide component may be added to 100 parts by weight of a silicon nitride component composed of a mixed powder of silicon powder and silicon nitride powder.
It is mixed so that the ratio is 150 parts by weight.

The amount of the Group 3a element oxide of the periodic table in the above-mentioned composition of the compact is limited to the above range because the sinterability is improved because the system itself contains silicon carbide as described later. Since it is low, the amount of oxide of Group 3a element of the periodic table is 0.
If it is less than 5 mol%, a high-density sintered body cannot be obtained, and conversely 1
This is because if it exceeds 0 mol%, the high temperature characteristics of the final sintered body deteriorate.

Further, the amount of silicon carbide added is limited to the above range because when the amount is less than 1 part by weight with respect to 100 parts by weight of the silicon nitride component, the effect of suppressing the grain growth of the silicon nitride crystal is small and the silicon nitride is reduced. As a result, the bending strength and creep properties are the same as those of the conventional silicon nitride sintered body, and if the amount exceeds 100 parts by weight, the sinterability of the entire system deteriorates. However, it becomes difficult to obtain a dense sintered body, and the strength of the sintered body decreases. From the viewpoint of each characteristic, the amount of the silicon carbide component is preferably 30 to 70 parts by weight with respect to 100 parts by weight of the silicon nitride component.

If the silicon nitride component in the compact contains a large amount of silicon powder, it will be difficult to convert all of the added silicon powder into silicon nitride in the nitriding step described later, and conversely the characteristics will be poor. According to the present invention, it is desirable to add an appropriate amount of silicon nitride powder because it may decrease.

A known method is adopted as a molding means used for producing the above-mentioned molded body, and for example, a desired molding method such as press molding, injection molding, extrusion molding, cast molding, cold isostatic molding or the like is used. To shape.

Next, the molded body obtained as described above is heat-treated in a nitrogen atmosphere at a temperature of 800 to 1500 ° C. to nitride the silicon powder contained in the molded body to generate silicon nitride. Upon conversion into silicon nitride, there is no dimensional change and the weight increases, so the density of the molded body improves. It is desirable to control such that the ratio of the theoretical density of the molded body after the nitriding treatment is 60% or more. In this nitriding treatment, in order to completely nitrid the contained silicon, it is desirable to gradually raise the temperature within the above temperature range while raising the temperature in multiple stages. May not be possible.

Next, the molded body obtained by the above method is fired at a temperature of 1600 to 2000 ° C. or less, particularly 1600 to 1
Firing is performed at a temperature of 900 ° C. in a non-oxidizing atmosphere containing nitrogen set to be equal to or higher than the decomposition equilibrium pressure of silicon nitride at the firing temperature. Further, as the firing means, normal pressure firing, hot press firing, nitrogen gas pressure firing (GPS firing), hot isostatic pressure firing (HIP firing), etc. are adopted. As a combination of these firing methods, for example, after obtaining a sintered body having a theoretical density ratio of 90% or more by atmospheric pressure firing or GPS firing, N _{2 at} 1600 to 1900 ° C. or Ar gas pressure of 50 MPa by HIP method. The densification can be achieved by firing under the above pressure or HIP firing the molded body under the same conditions as described above through the glass film.

Further, according to the method for manufacturing the composite sintered body of the present invention, Al ₂ O ₃ , MgO, CaO, etc. easily form a low melting point substance in the sintered body in view of the high temperature characteristics, and the high temperature range is high. In some cases, since the sintered body may be deformed in each step, each element of Al, Mg, and Ca contained in the molded body is adjusted to 0.5% by weight or less in total amount in terms of oxide. It is desirable to avoid mixing of these elements. In particular,
It is necessary to consider using raw materials having a small content of these elements or using balls having a small content of these elements used in mixing, for example, in a ball mill.

The sintered body obtained as described above is heat-treated in a non-oxidizing atmosphere at 1100 to 1700 ° C. to crystallize the grain boundaries of the sintered body, for example, Si ₃ N ₄ -RE ₂ O _3: by precipitating (RE periodic table group 3a elements) -SiO ₂ system known crystalline phase can also be improved high temperature properties.

The sintered body thus obtained is composed of a silicon nitride component and a silicon carbide component in terms of composition. The silicon carbide component basically consists of silicon carbide, while the silicon nitride component consists of a system containing silicon nitride and a sintering aid component in the sintered body. This silicon carbide component is present in the grains of the silicon nitride crystal and / or in the grain boundaries, and thus suppresses grain growth during sintering of the silicon nitride crystal and makes the sintered body a microstructured structure.

Further, according to the composite sintered body of the present invention, it is desirable that both the silicon nitride crystal and the silicon carbide crystal are present as fine particles. The silicon nitride crystal itself has a needle-like shape due to grain growth, has an average grain size (minor axis) of 1 μm or less, particularly 0.8 μm or less, and has an average aspect ratio represented by major axis / minor axis of 2 to It is present in the particle form of 10, especially 3-9. Most of this silicon nitride crystal is β
-Si ₃ N ₄ exists as α-Si in the sintered body due to the effect of suppressing the transition of α-type to β-type of silicon nitride by silicon carbide.
The presence of —Si ₃ N ₄ is desirable.

On the other hand, the silicon carbide crystal itself has a granular shape, and is present as a particle having an average particle size of 1 μm or less, particularly 0.8 μm or less, in the grain boundary of the silicon nitride crystal or in the silicon nitride crystal grain. Most of the silicon carbide crystals are β type, but α type may be present depending on the case.

The elements of Group 3a of the periodic table used in the present invention are Y, Sc, Er, Yb, Ho,
Dy may be mentioned, but among these, Er and Yb are particularly useful because they are excellent in uniform dispersibility in the sintered body.

[0024]

According to the manufacturing method of the present invention, silicon powder is further added to the system containing the sintering aid and the silicon carbide powder, and the powder is nitrided to increase the density of the compact itself before firing. it can. As a result, it is possible to suppress shrinkage of the compact in the firing process with respect to the initial compact size, and thus it is possible to obtain a sintered compact with high dimensional accuracy. Further, even in the deformation in the sintering process, the maximum deformation amount can be reduced because the shrinkage rate becomes small.

[0025]

Example: Silicon powder having an average particle size of 3 μm as a raw material powder,
Silicon carbide powder with an average particle size of 0.3 μm, average particle size 0.5
μm of Y ₂ O ₃ , Er ₂ O ₃ , Yb ₂ O ₃ , Ho
₂ O ₃ , Dy ₂ O ₃ , and average particle size 0.3 μm, α
-Si ₃ N ₄ content of 98% and oxygen content of 1.3% by weight of silicon nitride powder were weighed and mixed so that their composition was in the ratio shown in Table 1, and mixed and pulverized in methanol. .. A binder and a solvent are added to this to prepare a slurry,
The rotor was molded by the casting method.

The obtained molded body was nitrided under the conditions shown in Table 1. At this time, it was confirmed from the rate of weight increase that all the added silicon was nitrided in all the samples.

Thereafter, the molded body thus obtained was subjected to nitrogen gas pressure 1
Firing was performed under the conditions of Table 1 under 0 atm. The dimensions of the obtained sintered body were measured, and the shrinkage ratio with respect to the dimensions of the initial compact was measured. In addition, the blade shape of the rotor was measured to measure the maximum deformation amount from the design value.

In addition, as an evaluation of mechanical properties, JISR
A bending test piece based on 1601 was cut out and the 4-point bending bending strength at 1400 ° C. was measured. The results are shown in Table 1.

[0029]

[Table 1]

According to Table 1, in the samples No, 13 in which the amount of silicon carbide component exceeds 150 parts by weight with respect to the silicon nitride component,
The sinterability was greatly reduced and a dense body could not be obtained. In addition, Sample No. 17 to which silicon carbide was not added had a low bending strength.

Further, in samples No. 11, 11 and 12 to which silicon powder was not added as a raw material, the shrinkage rate was as large as 19%, and in particular, the amount of deformation exceeded 100 μm.

Sample No. 1 having a high firing temperature after nitriding
In No. 9, shrinkage and deformation were small, but the high temperature characteristics deteriorated. Furthermore, the sample No. 1 containing less than 0.2 mol% of the sintering aid component
No. 9 could not be densified, and sample No. 20 and 20 exceeding 10 mol% suffered deterioration of high temperature strength.

In contrast to these comparative examples, the sintered bodies produced according to the production method of the present invention each have a high temperature strength of 80.
0 MPa or more is maintained, the shrinkage rate is 15% or less,
The amount of deformation was 100 μm or less, which was extremely excellent in dimensional accuracy.

[0034]

As described in detail above, according to the present invention,
Shrinkage and deformation in the sintering process of silicon nitride-silicon carbide can be reduced, and a sintered body with high dimensional accuracy can be manufactured. As a result, the composite sintered body can be put into practical use as a heat engine structure such as a gas turbine or a turbo rotor in which high dimensional accuracy is required, or as another heat resistant material, and its application can be expanded. ..

Claims (1)

Claim: What is claimed is: 1. An oxide of Group 3a element of the Periodic Table of 0.2 to
1 to 15 parts by weight of silicon carbide with respect to 100 parts by weight of 10 mol% and the balance of silicon, or a component consisting of silicon and silicon nitride.
A molded body added at a ratio of 0 parts by weight is 800 to 150.
A silicon nitride-silicon carbide composite sintered body characterized by being heat-treated in a nitrogen-containing atmosphere at 0 ° C. to nitride the silicon, and then sintered in a non-oxidizing atmosphere at 1600 to 2000 ° C. to be densified. Manufacturing method.