JPH02311364A - Silicon nitride sintered compact and its production - Google Patents

Abstract

PURPOSE:To obtain the subject high-density sintered compact excellent in both oxidation resistance and breaking strength at elevated temperatures by baking silicon nitride powder with the increased amount of excess oxygen therein and the ratio to a group IIIA element contained in the silicon nitride powder regulated within a specific range. CONSTITUTION:A composition form comprising (A) 89 to 98.9mol% of Si3N4, (B) <1mol% of a Re2O3 (Re is Er, Yb, Tm, Lu or Sc) and (C) 1 to 10mol% (esp. 1.5 to 5mol%) of excess oxygen (in terms of SiO2) with the molar ratio C/B=2 to 10 (esp. 2.3 to 5) is surrounded with e.g. Si/SiO2 to generate SiO to prevent the excess oxygen from volatilization and baked at 1800 to 2000 deg.C under a pressurized nitrogen gas atmosphere of 1.5 to 100atm, thus obtaining the objective Si3N4 sintered compact to be hopefully used for gas turbines, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a silicon nitride sintered body having excellent flexural strength and oxidation resistance at high temperatures, and a method for producing the same.

(Prior Art) Sintered bodies mainly composed of silicon nitride have excellent strength, hardness, and thermal and chemical stability, and are therefore being increasingly applied to gas turbines and the like, especially for heat engines.

In general, since silicon nitride itself is difficult to sinter, a sintering aid mainly composed of acid oxides from group MA of the periodic table is added, and after molding, it is heated in hot press firing or in a nitrogen-containing atmosphere. In some cases, a method of firing while pressurizing the atmosphere is adopted, and in order to make the sintered body dense, it is necessary to add at least 1 mol % or more of the Group Ma oxide to the total amount.

On the other hand, as for acid oxides of group ma of the periodic table, Y2O has traditionally been used.
, is the most used, but Y2O, as a sintering aid, tends to segregate in the sintered body during sintering behavior, resulting in large variations in properties, so Er and Yb, which have relatively small ionic radii, , Sm, , Tb, Dy, l
It has also been proposed to add one or more element oxides selected from +o and o in a proportion of 1 to 5 mol%. (Patent Application No. 124663/1982) (Problems to be Solved by the Invention) According to the conventional sintered bodies as described above, the Group ■a oxide is added in a proportion of 1 mol% or more. In order to
This oxide remains in the grain boundary phase as a large amount of Group IA oxynitride or Group NLA silicon oxynitride.

As described above, when a large amount of compounds containing Group Ma oxides remain in grain boundaries, oxidation resistance and high temperature creep characteristics at high temperatures tend to deteriorate. Therefore, in order to prevent such deterioration of properties, it is desirable to suppress the amount of Group Ma oxide added to less than 1 mol%, but in the conventional method, sintering hardly progresses and a dense sintered body is not obtained. The problem is that it cannot be obtained.

The same thing can be said in the above-mentioned Japanese Patent Application No. 124663/1983, and although this proposal improves the oxidation resistance to some extent, the measurement of oxidation weight gain at 1400°C shows that the weight gain due to oxidation is 0.10 mg/cm2 or more. The characteristics are still insufficient.

(Means for Solving the Problem) As a result of repeated studies regarding the above problems, the present inventors found that even if the amount of a specific Group 111a oxide added is less than 1 mol%, impurities in silicon nitride powder 111a by increasing the amount of excess oxygen determined by the addition of oxygen or 5i02.
When firing with the ratio of group oxides set within a specific range,
It was found that a highly dense body with a theoretical density ratio of 98% or more can be obtained, and at the same time, a sintered body with excellent oxidation resistance and bending strength at a high temperature of 1400°C can be obtained.

That is, the present invention provides Er, Yb, Lu, Tm, Sc
At least one oxide selected from (RE20.)
A molded body containing less than 1 mol %, excess oxygen (SiOz equivalent) 1 to 10 mol %, and silicon nitride 89 to 99 mol %, and in which the molar ratio of excess oxygen/REzOs is in the range of 2 to 10. By firing at a temperature of 1800 to 2000°C in a nitrogen gas atmosphere, the
It is possible to prevent volatilization of i02, to allow sufficient firing to proceed, and finally to obtain a highly dense body with a theoretical density ratio of 98% or more.

The present invention will be explained in detail below.

One feature of the present invention is that oxides of group 1NA of the periodic table (hereinafter simply referred to as RE, 0
RE2 (sometimes abbreviated as 3) reacts with silicon nitride etc. in the sintered body and deteriorates high temperature properties, especially oxidation resistance.
O3 is set to a very small range of less than 1 mol%, especially 0.6 to 0.95 mol%, and RE2O3 is set to a small amount selected from Er, Yb, Tm, Lu, and Sc (both of which are one type of oxide). This is because Y (yttrium) oxide, which has been used conventionally, cannot be expected to have firing stability in a small addition amount range. Since the radius is small, even a system with a small addition amount has sufficient firing stability.Among the above oxides, Er and Yb are particularly preferable in terms of their characteristics.

Although sintering is insufficient in a system containing only a trace amount of the above-mentioned oxide, sintering can be promoted by adding excess oxygen to the system. This excess oxygen refers to oxygen present in the system other than oxygen mixed in as rare earth element oxides, and specifically, it is due to impurity oxygen in silicon nitride powder or addition as SiO□. In the present invention, the excess oxygen is expressed in terms of 5 iOz.

In the present invention, this excess oxygen is reduced to 1 to 10 mol%, particularly 1 to 10 mol%.
．． The composition is set in the range of 5 to 5 mol%, and the ratio with the rare earth mentioned above, that is, the molar ratio expressed as excess oxygen/RE2O3, is 2 to 10, particularly 2.3 to 5. . The reason for setting the amount of excess oxygen in the above range is that if the excess oxygen is less than 1 mol% or the above molar ratio is less than 2, sintering will be difficult and a dense body will not be obtained, and if it exceeds 10 mol% or Even if the above molar ratio exceeds 10, the sinterability decreases, the grain boundary phase increases, and a low melting point glass phase is generated, resulting in a significant decrease in high-temperature strength.

In a system containing a large amount of excess oxygen as described above, 5if2, which is excess oxygen, is a low melting point substance and therefore volatilizes very easily, resulting in compositional fluctuations during firing, which may prevent sintering from proceeding.

Therefore, it is necessary to perform firing while suppressing volatilization of excess oxygen. As a firing method to prevent compositional fluctuations, the molded body is placed in a glass bath and hot isostatically fired under high temperature and high pressure, so-called Glass Seal I.
Although the IIP method is the best, gas pressure calcination, which is excellent in mass production, is most preferably employed because it involves an increase in the size of the equipment and a complicated manufacturing process.

Therefore, this gas pressure firing will be specifically explained as an example. Normally, in gas pressure firing, the molded body is fired at a temperature of 1600 to 2000'C in a nitrogen gas pressurized atmosphere of 1.5 to 100 atmospheres, but in the present invention, as described above, the molded body is fired while preventing the volatilization of 5iOz. Because there is a necessary firing tool, there is no S in the furnace.
By placing i/SiO□ powder or silicon nitride powder, or by burying the molded body in these powders and firing, SiO gas is generated around the molded body, thereby preventing the volatilization of excess oxygen from the molded body. can.

In addition to generating SjO gas, 1300
By setting the heating rate from °C to the firing temperature to 50'C/hr or more, the heating time can be shortened and the amount of volatilization during the heating process can be reduced.

Further, by setting the nitrogen gas pressure to a high pressure of 10 atmospheres or more, it is possible to further prevent excess oxygen from volatilizing.

In addition, in the firing method of the present invention, since the system as a whole is difficult to sinter, it is necessary to perform firing at a high temperature of 1800 to 2000°C, and if it is lower than 1800°C, the theoretical density ratio I can't get a dense body. At the same time, in order to suppress the decomposition of silicon nitride itself at high temperatures of 1800 to 2000°C, it is necessary to set the nitrogen gas pressure to 10 atm or more.

The invention will now be explained with the following examples.

(Example) As raw material powder, specific surface area 14 m27 g, oxygen content 1
．． To 0% by weight of silicon nitride powder, oxides of group 1a of the periodic table (RE2O3) and 5i02 were added so as to have the composition shown in Table 1, and mixed for 72 hours. After drying and granulating the obtained slurry, it was press-molded, the binder was removed in a vacuum, and then it was fired under the firing conditions shown in Table 1 to obtain a sample.

Incidentally, in both cases, the temperature was raised from 1200°C at a rate of 50°C/hr.

In Table 1, except for N014, all the others were buried in silicon nitride powder and fired.

The obtained samples were evaluated for 4-point bending strength at room temperature and 1200°C, oxidation resistance evaluation by oxidation weight increase after 24 hours at 1400"C in the air, and sintering by Archimedes method according to JISR1601. The theoretical density ratio of the body was calculated.In addition, the cut surfaces of 10 samples were observed, and those with color unevenness or smudges on two or more were given a ×, 1
Items below this level were marked with ○ for evaluation. The results are shown in Table 1.

(Left below) B - According to Table 1, N fired without generating SiO gas
In sample o14, volatilization of SiO□ was observed from the molded body,
Firing did not proceed and a dense sintered body could not be obtained. Further, in sample No. 15 in which the amount of excess oxygen exceeded 10% by weight, the high temperature strength decreased significantly. No. 16 containing 1 mol% or more of RE2O3 has poor oxidation resistance. No. 17, which used Y2O as an auxiliary agent, had no problem in strength, but had insufficient oxidation resistance, and the occurrence of color unevenness and stains was observed. Furthermore, with N018 whose firing temperature was lower than 1800° C., a dense body could not be obtained.

On the other hand, the products of the present invention are all homogeneous dense bodies of 98% or more, and have a bending strength of 800 MPa or more at room temperature, 1
600MPa or more at 200℃, oxidation weight increase 0.1mg/
It showed excellent characteristics of less than cm2.

(Effects of the Invention) As detailed above, according to the present invention,
Because high density is achieved as a sintered body despite the small amount of group acid oxides, the
A silicon nitride sintered body is provided that prevents deterioration of high-temperature properties, particularly high-temperature oxidation resistance, due to the formation of group 1a oxynitrides and the like, and has excellent strength and durability at high temperatures.

Claims (2)

[Claims] (1) Silicon nitride 89 to 98.9 mol%, Er, Yb
, Tm, Lu, Sc and less than 1 mol% of at least one oxide (RE_2O_3) and excess oxygen (SiO
Excess oxygen/RE
A silicon nitride sintered body, characterized in that the molar ratio of _2O_3 is in the range of 2 to 10, and the theoretical density ratio is 98% or more. (2) 89 to 98.9 mol% silicon nitride, Er, Yb
, Tm, Lu, Sc and less than 1 mol% of at least one oxide (RE_2O_3) and excess oxygen (SiO
Excess oxygen/RE
A molded body having a molar ratio of _2O_3 in the range of 2 to 10 is heated to 1800 to 20 in a nitrogen gas atmosphere containing SiO gas.
A method for producing a silicon nitride sintered body, the method comprising firing at a temperature of 00°C.