JP2746761B2 - Method for producing silicon nitride-silicon carbide composite sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride-silicon carbide composite sintered body mainly composed of silicon nitride and silicon carbide, and a method for producing the same. And a method for producing a sintered body having excellent high-temperature strength and high-temperature creep characteristics.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been promoted as engineering ceramics, particularly as heat engine structural materials, because of their excellent strength, high degree of heat, and excellent thermal and chemical stability.

[0003] As a method of obtaining such a silicon nitride sintered body, a sintering aid such as an oxide of an element of Group 3a of the periodic table is added to silicon nitride powder, mixed, molded, and then non-oxidized. It is obtained by firing at a temperature of 1500 to 2000 ° C. in an atmosphere.

[0004] However, while the silicon nitride sintered body has excellent characteristics, it has a problem that its strength and the like are reduced at high temperatures. Up to now, the problem of deterioration of the high-temperature strength has been improved by improving the sintering aid and changing the firing atmosphere and the firing pattern.
At present, no definitive measures have been taken. Also,
The silicon carbide-based sintered body has a characteristic that although the absolute strength is lower than that of the silicon nitride-based sintered body, the strength is hardly deteriorated at a high temperature.

Therefore, recently, a composite sintered body in which silicon carbide is added to silicon nitride and fired has been proposed.
Since the sinterability of the composite sintered body is reduced by adding silicon carbide, the composite sintered body is generally improved by adding Al ₂ O ₃ etc. in addition to rare earth element oxides such as Y ₂ O _3. We are trying to increase the density.

[0006]

According to the above prior art, deterioration of strength at high temperature by adding silicon carbide to silicon nitride is reduced as compared with a silicon nitride sintered body. Can. However, the present inventors have studied the high-temperature characteristics of the above-mentioned sintered body in more detail. It turned out that there was a problem that the characteristics were low. This characteristic becomes a factor that greatly hinders practical use because it becomes difficult to operate the sintered body for a long time when applied to a turbine rotor, for example. Although this creep characteristic is considered to be largely due to the state of the grain boundary in the sintered body, it has not been specifically studied, and the effect of improving the characteristic by combining silicon nitride and silicon carbide is sufficiently exhibited. It was not at the moment.

[0007]

Means for Solving the Problems The present inventors have studied the optimum firing conditions for the above-mentioned system in which silicon carbide is added to silicon nitride, and as a result, at least as a sintering aid, The molded body to which a predetermined amount of the Group 3a element oxide of the periodic table is added is fired once at a relatively low temperature, and then fired at a temperature higher than the firing temperature, whereby the silicon nitride crystal and the silicon carbide crystal are uniformly and finely divided. It has been found that a sintered body interdispersed as fine particles can be obtained, thereby improving the transverse rupture strength and creep characteristics of the sintered body at room temperature and high temperature.

That is, in the method for producing a silicon nitride-silicon carbide composite sintered body of the present invention, 90 to 99.5 mol% of silicon nitride powder containing impurity oxygen having an average particle diameter of 1 μm or less is obtained. Silicon carbide powder having an average particle diameter of 1 μm or less is dispersed and contained at a ratio of 1 to 100 parts by weight with respect to 100 parts by weight of a silicon nitride component containing a Group 3a oxide at a ratio of 0.5 to 10 mol%. And forming a compact having a total amount of oxides of Al, Ca, and Mg of 0.5% by weight or less, and baking the compact in a nitrogen atmosphere at 1400 to 1800 ° C. It is characterized by comprising a first firing step and a second firing step of firing in a nitrogen-containing atmosphere at 1500 to 2000 ° C. higher than the temperature in the first firing step.

According to the method for producing a silicon nitride-silicon carbide composite sintered body of the present invention, first, silicon nitride powder, silicon carbide powder, an oxide of an element of Group 3a of the periodic table, and in some cases, oxidized as starting materials. Use silicon powder.

The silicon nitride powder may be either α-type or β-type, and has an average particle size of 1 μm or less and an impurity oxygen content of 2 μm or less.
Those having a weight percent or less are preferably used. As the silicon carbide powder, any of α-type and β-type can be used, and a powder having an average particle diameter of 1 μm or less and an impurity oxygen amount of 2% by weight or less is used. These silicon nitride powder and silicon carbide powder may be present as individual powders, or may be a powder obtained by compounding silicon nitride and silicon carbide at a predetermined ratio.

Next, 90 to 99.5 mol%, particularly 93 to 99 mol% of silicon nitride containing impurity oxygen and 0.5 to 10 mol of the oxide of a Group 3a element of the periodic table using the above powder. %, Especially 1 to 7 mol% of silicon nitride component 1
1 to 100 parts by weight of the silicon carbide component with respect to 00 parts by weight,
In particular, the mixture is weighed to 30 to 70 parts by weight and then thoroughly mixed.

The oxide of the Group 3a element of the periodic table in the silicon nitride component is an essential component for improving the sinterability of the entire system. If the amount is less than 0.5 mol%, the sinterability will be poor. When the density is reduced, a dense sintered body cannot be obtained, and the properties are deteriorated. When the content exceeds 10 mol%, the high-temperature strength is deteriorated. Further, the amount of the silicon carbide component is limited to the above range. When the amount of the silicon carbide component is less than 1 part by weight, the effect of improving the high-temperature characteristics by combining silicon nitride and silicon carbide cannot be expected, and exceeds 100 parts by weight. This is because sinterability decreases and strength deteriorates.

The silicon nitride inevitably contains impurity oxygen. The impurity oxygen is contained in the third element of the periodic table.
It is a substance that greatly contributes to sinterability, like the group a element oxide. From this viewpoint, the amount of impurity oxygen (amount in terms of SiO ₂ ) is desirably 2 to 20 mol% in the aforementioned silicon nitride component, and the amount in terms of an oxide of a Group 3a element of the periodic table (RE ₂ O). ₃ ) the molar ratio of impurity oxygen (SiO ₂ ) expressed as SiO ₂ / RE ₂ O ₃ is 0.5 to 1;
It is desirable to adjust the composition by adding silicon oxide powder in some cases so as to be 0, particularly 1 to 7.

[0014] Also, Al ₂ O ₃ , CaO, MgO and the like are conventionally known as components for improving sinterability, but these are solid-solved in grain boundaries in the final sintered body or partially in silicon nitride crystals. When these components are present at the grain boundaries, the melting point of the grain boundaries is lowered and the viscosity at high temperatures is also reduced, so that the creep characteristics are deteriorated together with the high-temperature strength. Therefore,
It is desirable to control the metal elements of Al, Ca, and Mg in consideration of the material of the ball at the time of mixing so as to be 0.5% by weight or less, particularly 0.3% by weight or less in the total amount in terms of oxide. .

The elements of Group 3a of the Periodic Table used in the present invention include Y, Sc, Er, Yb, Ho,
Dy is mentioned, but Er and Yb are particularly desirable from the viewpoint of uniform dispersibility in the sintered body.

Next, the above mixture is formed into a desired shape by a well-known molding method such as press molding, injection molding, extrusion molding, casting molding, cold isostatic pressing and the like. According to the present invention, the molded body obtained as described above is used for 1450 to 20
Firing is performed in a nitrogen-containing atmosphere at 00 ° C. At this time, as a first firing step, the temperature is temporarily maintained at 1450 to 1800 ° C. for about 0.5 to 10 hours, and then the temperature is raised to 1550 to 1550 ° C.
It is important to raise the temperature to 2000 ° C. and bake for about 0.5 to 10 hours. Naturally, the atmosphere at this time must be set to be equal to or higher than the decomposition equilibrium pressure of silicon nitride at each holding temperature.

As firing means, normal pressure firing, hot press firing, nitrogen gas pressure firing (GPS firing), hot isostatic pressure firing (HIP firing) and the like are employed. And hot press firing. The first and second set firing temperatures are also changed by these firing means, and the hot press method, the GPS
In the method, the first firing temperature is 1450-1800 ° C., the second firing temperature is 1650-2000 ° C., and in the hot isostatic pressing method, the first firing temperature is 1450-1800 ° C. and the second firing temperature is 1550-1900. It is desirable to set each to ° C.

In the present invention, the sinterability may be reduced when a low melting point substance such as Al ₂ O ₃ is not contained.
Baking under the above conditions by S baking, theoretical density ratio 90%
After obtaining the above sintered body, 1600 to 1
Densification can be achieved by firing at 900 ° C. under a nitrogen or argon gas pressure of 50 MPa or more, or by HIP firing the molded body by multi-stage firing as described above via a glass film.

Further, the obtained sintered body is
The grain boundary of the sintered body is crystallized by heat treatment in a non-oxidizing atmosphere at 0 to 1700 ° C., for example, Si ₃ N ₄ —R
High temperature properties can be improved by precipitating a known crystal phase of E ₂ O ₃ (RE: Group 3a element of the periodic table) -SiO ₂ system.

In the sintered body thus obtained, both the silicon nitride crystal and the silicon carbide crystal are present as fine particles, and the silicon nitride crystal itself has a needle-like shape by grain growth. The particles have an average particle diameter (minor diameter) of 1 μm or less, particularly 0.8 μm or less, and have an average particle diameter of 2 to 10, especially 3 to 10, having an aspect ratio expressed by major axis / minor axis. Most of this silicon nitride crystal is β-
Although present as Si ₃ N ₄ , α-Si ₃ N ₄ may be present depending on the case where the firing temperature is low.

On the other hand, the silicon carbide crystal itself has a granular shape, and is present as particles having an average particle size of 1 μm or less, particularly 0.8 μm or less in the grain boundaries of the silicon nitride crystal or in the silicon nitride crystal grains. Most of the silicon carbide crystals are β-type, but α-type may exist in some cases.

[0022]

According to the present invention, it is possible to obtain a sintered body having excellent high-temperature strength and high-temperature creep characteristics, as is clear from the examples described later, by temporarily holding the molded body at a relatively low temperature. . The reason is not clear, but is considered as follows. By holding once at a low temperature during firing, fine silicon carbide powder is uniformly dissolved in a liquid phase composed of silicon nitride and a sintering aid.
In the calcination step, the uniformly dispersed silicon carbide particles act as nuclei when phase transition is made from α-Si ₃ N ₄ to β-Si ₃ N ₄ , so that the fine and needle-like nitrides are finally formed. This is probably because a silicon crystal can be grown.

Therefore, if the temperature in the first firing step is lower than 1450 ° C., uniform dissolution of silicon carbide into the liquid phase will not occur, and if it exceeds 1800 ° C., α-Si will not be uniformly dispersed without silicon carbide being uniformly dispersed. from _{3 N 4 β-Si 3 N} 4
As the dislocation proceeds, large particles due to abnormal grain growth partially exist, and the strength and creep characteristics deteriorate. Further, the temperature in the second firing step is 1550.
If the temperature is lower than ℃, dislocation from α-Si ₃ N ₄ to β-Si ₃ N ₄ does not occur, and the properties are lowered due to insufficient densification. Is promoted to generate silicon nitride crystals having a large grain size, and the characteristics are similarly deteriorated.

[0024]

EXAMPLES The raw material powder had an average particle size of 0.6 μm and α-S
a silicon nitride powder having an i ₃ N ₄ content of 98% and an oxygen content of 1.0 to 1.5% by weight, a silicon carbide powder having an average particle size of 0.3 μm, and Er ₂ having an average particle size of 0.5 μm O ₃ , Yb
Each powder of ₂ O ₃ , Ho ₂ O ₃ and Dy ₂ O ₃ and silicon oxide powder were weighed and mixed such that the composition became the ratio shown in Table 1, and this was mixed and pulverized in methanol with a binder. . After drying and granulating the obtained slurry, it was press-molded at a pressure of 1 ton / cm ² .

In a nitrogen atmosphere, the obtained molded body is
Firing was performed under the conditions shown in Table 1 by applying a nitrogen gas pressure of 10 atm and a mechanical pressure of 333 kg / cm ² . Sample N
For samples o and 16 to 19, a glass layer was formed on the surface of the molded body, and hot isostatic firing was performed at the temperature shown in Table 1.

For the obtained sintered body, the relative density was determined by the Archimedes method, the four-point bending strength at room temperature and 1400 ° C. according to JISR1601, the fracture toughness by the IF method, and the silicon nitride from the electron micrograph. The average grain size of the crystal and the silicon carbide crystal was measured. Also, 13
Each was held at a temperature of 71 ° C. under a load of 80 ksi and 90 ksi for a maximum of 100 hours, and the time until breakage was measured.

[0027]

[Table 1]

[0028]

[Table 2]

According to the results shown in Tables 1 and 2, firing was performed for 1
Sample No. 11 performed at 800 ° C. had poor high-temperature characteristics, and Sample No. 12 fired at 1700 ° C. could not obtain a sintered body having satisfactory characteristics. Similarly, even in the case where the temperature deviates from the range of the present invention in multi-stage firing, satisfactory characteristics cannot be obtained. Further, in Sample Nos. 15 and 15 in which Al ₂ O ₃ is contained in the system, the high temperature strength is deteriorated as compared with the product of the present invention.

On the other hand, all of the samples of the present invention were 1
A 400 ° C strength of 750 MPa or more and a creep characteristic of 50 hr or more under 80 ksi were achieved.

[0031]

As described in detail above, according to the present invention,
In producing a composite sintered body of silicon nitride and silicon carbide, by performing multi-stage firing at a predetermined temperature, each of the silicon nitride crystal and the silicon carbide crystal can be inter-dispersed as fine particles, whereby Reduces the high-temperature strength degradation and exhibits excellent creep characteristics, and promotes its practical use as a composite sintered body for heat engine structures such as gas turbines and turbo rotors, or as other heat-resistant materials. At the same time, its use can be expanded.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koichi Tanaka 1-4-4 Yamashitacho, Kokubu-shi, Kagoshima Inside Kyocera Research Institute (72) Inventor Hideki Uchimura 1-4-4 Yamashitacho, Kokubu-shi, Kagoshima Kyocera Corporation Examiner in the Research Institute Yoichi Gotani (56) References JP-A-64-76969 (JP, A) JP-A-3-223169 (JP, A)

Claims (1)

(57) [Claims] 1. The method according to claim 1, wherein said silicon nitride powder containing impurity oxygen having an average particle diameter of 1 μm or less is 90 to 99.5 mol%,
The average particle size is 1 part by weight based on 100 parts by weight of the silicon nitride component containing the group a oxide in a ratio of 0.5 to 10 mol%.
a step of producing a molded body containing silicon carbide powder having a particle size of 1 μm or less dispersed in a proportion of 1 to 100 parts by weight, and having a total amount of 0.5% by weight or less in terms of oxides of Al, Ca, and Mg; A first firing step of firing the molded body in a nitrogen atmosphere at 1400 to 1800 ° C., and a second firing step of firing in a nitrogen-containing atmosphere at 1500 to 2000 ° C. higher than the temperature in the first firing step. A method for producing a silicon nitride-silicon carbide composite sintered body, comprising: