JP2724764B2 - Method for producing silicon nitride based sintered body - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for producing a silicon nitride sintered body, and more particularly, to producing a heat engine structural material having a large shape and having no difference between inside and outside. The present invention relates to an improvement for producing a simple sintered body.

(Prior Art) Conventionally, silicon nitride based sintered bodies have been applied to engineering ceramics, particularly to heat engines such as turbo rotors and gas turbines, as materials having excellent strength, hardness and thermochemical stability. .

As a method for obtaining such a silicon nitride sintered body, in addition to Y ₂ O ₃ and a rare earth element oxide as a sintering aid, Al
₂ O ₃ , MgO, SrO, etc. are added, and this is fired in a non-oxidizing atmosphere at 1500 to 2000 ° C. by a method such as normal pressure firing, hot press firing, nitrogen gas pressure firing, hot isostatic pressure firing, etc. It has been obtained by.

On the other hand, a silicon nitride sintered body has excellent characteristics,
At a high temperature exceeding 1200 ° C., the properties tend to deteriorate significantly. This is because the sintered body obtained by the above-described method is composed of silicon nitride crystal grains and a grain boundary phase, and a low melting point glass phase composed of a sintering aid and impurities is generated in the grain boundary phase. It is thought to be.

Therefore, in order to reduce impurity oxygen in the silicon nitride raw material powder, which has been conventionally considered as one of the factors for deteriorating high-temperature strength in order to improve high-temperature characteristics, it is necessary to hold at a low temperature for a long time in the initial stage of firing, Alternatively, a method has been proposed in which a compact is embedded and baked with silicon nitride, boron nitride, or aluminum nitride powder to extract impurity oxygen from the compact.

(Problems to be Solved by the Invention) The above-described method for improving the characteristics has a certain effect when producing a small-sized sintered body, but is effective when producing a large-sized sintered body. In any case, the inside and the surface of the sintered body tended to be heterogeneous in composition or organization. That is, in any of the conventional cases,
Although the impurity oxygen on the surface is removed and a dense layer is formed on the surface of the compact, the impurity oxygen remains inside and the amount of oxygen differs inside and outside, or the sintering aid becomes heterogeneous with this. There was a problem. Due to such a difference between the inside and the surface portion, there is a problem in that the high-temperature characteristics inside the sintered body are greatly deteriorated as compared with the characteristics of the surface portion, as shown in FIG.

(Means for Solving the Problems) The present inventor has repeatedly studied a method of firing a large-sized molded body. In the firing step, the high-temperature firing is performed so that denseness is not formed on the surface portion. After increasing the density to some extent, it is possible to homogenize the composition or organization of the inside and the surface part by holding once at a low temperature for a predetermined time, and then to promote sintering at a high temperature, It has been found that a large silicon nitride sintered body having excellent high-temperature characteristics can be obtained.

That is, according to the method for producing a silicon nitride-based sintered body of the present invention, when firing a molded body containing a Group IIIa element oxide of the periodic table as a sintering aid in a non-oxidizing atmosphere, first, After increasing the density of the molded body to a relative density of 75 to 90% at a temperature of 1600 to 1800 ° C under which no dense layer is formed on the surface, the temperature is once lowered to 1500 to 1750 ° C and maintained for homogenization. Thereafter, the temperature is increased to 1800 to 2000 ° C., and the density is increased to a relative density of 95% or more.

 Hereinafter, the present invention will be described in detail.

A major feature of the production method of the present invention is that, when producing a large-sized sintered body, the sintering step is improved in order to homogenize the surface portion and the inside.

The sintering step in the present invention is mainly composed of three steps.

First, in the first step, a liquid phase is generated by the added sintering aid and impurity oxygen to promote sintering, and a certain degree of densification is achieved to obtain a sintered body having a relative density of about 75 to 90%. If the relative density is less than 75%, it becomes difficult to obtain a high-density sintered body in the subsequent sintering process, and if the density exceeds 90%, the inner and outer Cannot be achieved. Further, according to the present invention, it is important that a dense layer is not generated on the surface of the molded body in the first step. this is,
In the subsequent homogenization step, if a dense phase is formed on the surface in the first step, the atmosphere in contact with the surface part and the inside are different. This is because a difference occurs between the surface and the surface portion, and homogenization cannot be achieved. As a condition in the first step, it is necessary to set the temperature to 1600 to 1800 ° C.
If it is lower, it is difficult to increase the relative density to the above-mentioned predetermined density, and if it exceeds 1800 ° C., a dense layer will be formed on the surface. The atmosphere at this time is a non-oxidizing atmosphere containing nitrogen, and particularly, the nitrogen gas pressure is 1.0 to 5.0.
Atm is desirable. If it is lower than 1.0 atm, silicon nitride is decomposed, and if it is higher than 5.0 atm, high-pressure gas is trapped inside the sintered body and densification is inhibited. Under these conditions, 0.5-3
Fired for hours.

Next, the sintered body obtained in the first step is kept at a lower temperature than in the first step to homogenize the inside and the surface. Specifically, the temperature is kept at 1500 to 1750 ° C. lower than the temperature in the first step. In this step, densification of the sintered body does not substantially proceed, and since the surface and the inside of the sintered body are maintained under the same conditions at the same temperature and atmosphere, impurity oxygen and sintering aids are removed. In order to uniformly diffuse or volatilize uniformly without difference between inside and outside, the composition or composition of the sintered body can be homogenized.
The reason for setting the temperature at this time to the above range is that the holding temperature is
If the temperature is lower than 1500 ° C., diffusion of each element in the sintered body, volatilization of impurity oxygen and the like do not occur, so that the sintered body cannot be homogenized. On the other hand, if the temperature is higher than 1750 ° C., since a dense layer is formed on the surface layer, the surface portion and the inside cannot be maintained under similar conditions, so that homogenization cannot be achieved. In the second step, the nitrogen gas is maintained at 1.0 to 5.0 atm as in the first step. Further, the homogenization in the second step is substantially achieved by keeping the temperature for 3 hours or more.

According to the third step, the sintered body homogenized in the second step is maintained at a temperature of 1800 to 2000 ° C. and a relative density of 95%
Densify to above. If the temperature at this time is lower than 1800 ° C., densification is insufficient, and if it is higher than 2000 ° C., grain growth of silicon nitride or decomposition of silicon nitride occurs, and the strength of the sintered body is deteriorated.
The atmosphere at this time is desirably maintained at a nitrogen gas pressurized atmosphere of 5 to 100 atm for the purpose of suppressing the decomposition of silicon nitride. An appropriate holding time is 1 to 6 hours.

The firing temperature in these first, second and third steps is:
Set the temperature higher in the second, first, and third order, and set the temperature difference to 30 ° C.
It is desirable to set above.

The silicon nitride-based sintered body obtained in this way achieves homogenization in both the inside and the surface, and can prevent deterioration of the internal characteristics in terms of characteristics.

Further, in order to further increase the density, the above sintered body was heated in a high pressure atmosphere of 1000 to 2000 atm by hot isostatic pressure firing to 1600.
It can be fired at 20002000 ° C.

In the present invention, the molded body subjected to the above-described firing is mainly composed of silicon nitride, and as a sintering aid, Y ₂ O ₃ ,
The final firing is made up of the three components of the Group IIIa element oxide (RE ₂ O ₃ ) of the periodic table such as Er ₂ O ₃ , Yb ₂ O ₃ , Dy ₂ O ₃ , Sc ₂ O ₃ and the SiO ₂ component. It is most suitable because it has excellent high-temperature properties of the sintered body. It is desirable that the oxide of the Group IIIa element of the periodic table be prepared at a ratio of 1 to 5 mol% and the SiO ₂ component at a ratio of 2 to 15 mol%. The SiO ₂ component also includes a value obtained by converting the amount of impurity oxygen contained in the silicon nitride raw material powder into SiO ₂ . In addition, the sintering aid component has a molar ratio of SiO ₂ / RE ₂ O ₃ of 2-3.
It is desirable that If the molar ratio is less than 2, a liquid phase required for densification is hardly generated, and a dense body cannot be obtained. If the molar ratio exceeds 3, the amount of SiO ₂ increases and the high-temperature strength deteriorates.

The average particle diameter of the silicon nitride raw material used is 0.1 to
Both 1.0 μm α-type and β-type can be used.

What has been prepared in the above ratio may be molded by known molding means, for example, press molding, extrusion molding, injection molding, casting molding, cold isostatic pressing and the like.

As a final sintered body obtained by firing this molded body by the firing method described above, the molar ratio of SiO ₂ / RE ₂ O _{3 in} the composition is preferably 0.8 to 2.0, particularly preferably 1.3 to 1.9. If the ratio is less than 0.8, the oxidation resistance at high temperatures tends to deteriorate, and if it is more than 2.0, the high temperature strength tends to deteriorate. In addition, it is structurally composed of a silicon nitride crystal phase and a grain boundary phase, and the grain boundary phase includes a YAM, apatite, and wollastonite structure Si ₃ N ₄ —RE ₂ O ₃ —SiO ₂ system crystal phase. It is desirable.

Further, as the sintering aid used in the present invention,
Although it contains the Group IIIa element oxide of the periodic table described above, in addition, Al ₂ O ₃ , MgO, SrO, WC, WO _{3 and the} like are also known, but from the viewpoint of the high-temperature strength of the sintered body Because Al ₂ O ₃ and MgO may form a low-melting glass phase at the grain boundary of the sintered body, a high-melting substance is easily generated by the reaction with silicon nitride. It is desirable to keep it below. In addition, WC and WO ₃ promote crystallization, so 1.0
There is no problem if it is added at a ratio of not more than weight%.

 Hereinafter, the present invention will be described with reference to the following examples.

(Example) As a raw material powder, a silicon nitride raw material powder having an average particle diameter of 0.6 μm, an α conversion of 98%, and an oxygen content of 1.3% by weight was used, and Y ₂ O ₃ and Sc ₂ O having an average particle diameter of 0.5 μm were further used. ₃ , Er ₂ O ₃ , Yb ₂ O ₃ , Ho ₂ O
₃ , Dy ₂ O ₃ powder and SiO ₂ powder were used to prepare a SiO ₂ / RE ₂ O ₃ molar ratio of 1.8 to 3.5, which was mixed and pulverized in methanol with a binder. After the obtained slurry was dried and granulated, it was press-molded and debindered in vacuum, and then a large-sized compact having a diameter of 60 mm and a thickness of 30 mm was prepared by cold isostatic pressing.

 This was fired under the conditions shown in Table 1.

For the obtained sintered body, relative density by Archimedes method, a bending test piece was cut out from the surface part and the inside of each sintered body according to JIS-R1601, and the bending strength at room temperature and 1400 ° C. was measured. .

In calculating the SiO ₂ / RE ₂ O ₃ molar ratio of each sintered body, the total oxygen content was determined by a nitrogen-oxygen simultaneous analyzer (LECO), and each element was quantified by ICP analysis. The remaining oxygen content obtained by subtracting the oxygen content bound as RE ₂ O ₃ (RE: Group IIIa element of the periodic table) from the total oxygen content was calculated as SiO ₂ .

 The results are shown in Table 1.

In Table 1, sample No. 1 (product of the present invention) and sample No. 11
Fig. 1 and Fig. 2 show the firing pattern of the (conventional product), the surface view and the internal room temperature, 1000 ° C, 1200 ° C,
The flexural strength at 00 ° C. is shown in FIGS. 3 and 4.

According to Table 1, in the first firing step, the density of the final sintered body was low in Sample No. 4 where the temperature was low, and the density in the first step was low in Sample No. 5 where the firing temperature was high. Is high,
A dense layer was formed on the surface, and there was a difference in characteristics between the inside and the surface of the final sintered body.

In the second step, in sample No. 6 whose firing temperature is lower than 1500 ° C., the homogenization is insufficient and the sample higher than 1750 ° C.
In No. 7, sintering progressed, and a dense layer was formed on the surface, and there was a difference between inside and outside.

In the third step, in Sample No. 9 in which the firing temperature is lower than 1800 ° C., the densification of the sintered body itself is insufficient, and in Sample No. 10 in which the temperature exceeds 2000 ° C., decomposition of silicon nitride occurs. And a decrease in strength was observed.

Furthermore, in sample No. 11, in which the firing temperature in the second step was higher than the first firing temperature, the densification was insufficient and the strength was reduced.

In contrast to these comparative examples, all of the samples of the present invention can suppress the difference in strength between the internal strength and the surface part to ± 50 MPa or less, and among them, the SiO ₂ / RE ₂ O ₃ ratio in the prepared composition is 2 In the samples of ~ 3, the room temperature strength inside was 800MPa or more, 14
A strength of 500 MPa or more at 00 ° C. was achieved.

(Effects of the Invention) As described in detail above, according to the method for producing a silicon nitride-based sintered body of the present invention, after a certain degree of densification is performed at a predetermined temperature during firing, the temperature is once maintained at a low temperature and homogenized. By performing the treatment and then sintering at a high temperature, a large-sized sintered body can be obtained with a homogeneous sintered body having no difference in properties between the inside and the surface.

This makes it possible to increase the reliability of the sintered body when manufacturing various heat engine parts and the like having a large shape, and to supply a product having stable characteristics.

[Brief description of the drawings]

FIGS. 1 and 2 each show a firing pattern, and FIGS. 3 and 4 show the relationship between the temperature of the inside and the surface of the sintered body and the precipitation strength. , FIGS. 1 and 3
The figure shows the product of the present invention (sample No. 1), FIG. 2 and FIG.
This is for the comparative product (Sample No. 11).

Claims (1)

(57) [Claims] 1. A sintered body mainly composed of silicon nitride and containing a Group IIIa element oxide of the periodic table as a sintering aid in a non-oxidizing atmosphere to obtain a sintered body having a relative density of 95% or more. In the method for producing a silicon nitride-based sintered body, the sintering step includes: (a) maintaining a temperature of 1600 to 1800 ° C. and forming a relative density of 75 to 90% without forming a dense layer on the surface of the molded body; (B) a step of maintaining the temperature at 1500 to 1750 ° C. lower than that of the step (a) to homogenize the structure; and (c) a temperature of 1800 to 2000 ° C. and a relative density of 95% or more. A method for producing a silicon nitride-based sintered body, comprising the steps of: