JP3152790B2 - Method for producing silicon nitride based 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 method for producing a silicon nitride sintered body having excellent strength characteristics from room temperature to high temperature and particularly used for parts for automobiles and parts for gas turbine engines.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Because of its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbo rotators. This silicon nitride sintered body is generally made of Y ₂ O ₃ , Al ₂ O ₃
Alternatively, by adding a sintering aid such as MgO, high density and high strength characteristics are obtained. As such silicon nitride sintered bodies are required to be further improved in strength and oxidation resistance at high temperatures when their use conditions are further increased. In response to such demands, various improvements have been attempted, for example, by studying sintering aids and improving firing conditions.

Among them, from the viewpoint that a low melting point oxide such as Al ₂ O ₃ which has been conventionally used as a sintering agent deteriorates high-temperature characteristics, the periodicity of Y ₂ O ₃ or the like with respect to silicon nitride is considered. in the sintered body having a composition of table group 3a element (RE) and a simple ternary system consisting of a silicon oxide _{(Si 3 N 4 -SiO 2 -RE} 2 O 3), the grain boundary of the sintered body Si
It has been proposed to increase the melting point and stabilize the grain boundary by precipitating a crystal phase such as a YAM phase and an apatite phase composed of -RE-ON. Among them, the silicon oxynitride (Si ₂ N ₂ O) phase and the disilicate (RE ₂ Si ₂ O ₇ ) phase are silicon oxide products of silicon nitride.
It is known that O ₂ exists in equilibrium with O _2, and when they are precipitated at grain boundaries, the oxidation resistance of the sintered body is improved.

[0004]

However, by crystallizing the grain boundaries into a silicon oxynitride phase and a disilicate phase, the high-temperature characteristics are improved as compared with the case where the grain boundaries are amorphous. In the case of firing at a high temperature to obtain a high-density sintered body, the outside of the molded body becomes denser than the inside, and pores tend to remain inside, thereby trapping pores and forming voids. I was Also, when firing is performed by lowering the firing temperature by increasing the amount of auxiliary agent,
The grain growth rate of silicon nitride was increased, and voids diffusing at grain boundaries were aggregated to form large voids.

[0005] If such voids are formed in the sintered body, there are problems such as deterioration of room temperature strength, high temperature strength, and deterioration of creep resistance characteristics, and this is a factor that lowers the mechanical reliability of the material. Was. Therefore, in order to put such a sintered body into practical use, characteristics thereof lack reliability and further improvement is required.

Accordingly, the present invention provides a silicon nitride excellent in strength characteristics sufficient for use in automobile parts from room temperature to high temperature, for use in gas turbine engines, and the like, and particularly excellent in bending strength from room temperature to high temperature of 1400 ° C. It is an object of the present invention to provide a method for producing a porous sintered body .

[0007]

Means for Solving the Problems The inventors of the present invention have repeatedly studied the above-mentioned problems of the sintered body, and as a result, when firing a formed body having a predetermined composition, the firing temperature was set at the grain boundary. By setting a specific range in relation to the eutectic temperature with silicon nitride of the crystal phase generated and firing, the generation of voids can be suppressed, and by precipitating a specific crystal phase, The present inventors have found that a highly reliable silicon nitride-based sintered body having high strength and small voids can be obtained, and the present invention has been accomplished.

[0008]

That is, a method for producing the silicon nitride sintered body of the present invention.
The method is mainly composed of silicon nitride, contains a Group 3a element oxide (RE ₂ O ₃ ) of the periodic table and excess oxygen, and contains a Group 3a element oxide of the periodic table in terms of SiO ₂ equivalent amount of the excess oxygen. A method for producing a silicon nitride-based sintered body, comprising: firing a molded body having a composition having a molar ratio of 2 to 10 in a non-oxidizing atmosphere at a temperature of 1600 to 1950 ° C., wherein the firing temperature is the periodic table. Group 3a element (RE) RE ₂ Si ₂ O
Higher than the eutectic temperature of the crystal and silicon nitride represented by ₇ ,
In addition, a difference between the firing temperature and the eutectic temperature is 200 ° C. or less.

Hereinafter, the present invention will be described in detail. With the present invention
The manufactured silicon nitride-based sintered body has a composition of silicon nitride as a main component, and contains, as additional components, a Group 3a element of the periodic table and excess oxygen. Here, the excess oxygen means that when a group 3a element of the periodic table other than Si in the sintered body forms an oxide stoichiometrically from the total amount of oxygen in the sintered body, it binds to the element. Is the amount of oxygen remaining, excluding oxygen, most of which is mixed as oxygen contained in the silicon nitride raw material or added silicon oxide. In the present invention, it is considered that all are present as SiO ₂ .

The silicon nitride-based sintered body manufactured by using the present invention is structurally mainly composed of a silicon nitride crystal phase, most of which is composed of β-Si ₃ N ₄ ,
Present with an average particle size of 10 μm. In addition, at the grain boundaries, at least an element belonging to Group 3a of the periodic table and excess oxygen (which is considered to exist as silicon oxide) are present, and at the grain boundaries, a silicon oxynitride phase (Si ₂ N ₂ O) is present. )
And / or a crystalline phase of a disilicate phase (RE ₂ Si ₂ O ₇ ).

Further, in order to precipitate the crystal phase, the amount of excess oxygen in the sintered body in terms of silicon oxide (SiO ₂ ) is based on the amount in terms of oxides (RE ₂ O ₃ ) of Group 3a elements of the periodic table. The molar ratio represented by SiO ₂ / RE ₂ O ₃ is 2 to 10,
In particular, it is necessary to control the composition to 2 to 4. When this molar ratio is smaller than ₂ , Si ₂ N ₂ O or RE ₂ Si ₂
In addition to O ₇ , a trace amount of a glass phase composed of RE—Si—O—N is likely to be present, lowering high-temperature strength and deteriorating oxidation resistance. In some cases, RE ₁₀ Si ₂ O ₂₃ N ₄ and R
Crystal phases such as an apatite phase described by E ₁₀ (SiO ₄ ) ₆ N ₂ and a YAM phase described by RE ₄ Si ₂ O ₇ N ₂ are precipitated, and properties at high temperatures, particularly oxidation resistance, decrease. I will. Further, when the above ratio exceeds 10, densification is hindered.

The periodic table 3a used in the present invention
Examples of group elements include Y and lanthanoid elements, and among them, Yb, Er, Dy, and Lu are particularly desirable, and the amount thereof is desirably controlled in the range of 3 to 10 mol%, particularly 3 to 7 mol%. If the amount is less than 3 mol%, densification becomes difficult, and if it exceeds 10 mol%, the absolute amount of the grain boundary increases and the high-temperature characteristics are likely to deteriorate.

Furthermore, when a sintered body is usually obtained by liquid phase sintering, it is difficult to completely prevent the formation of voids on the surface or inside thereof even if the number of voids can be reduced. As described above, void formation is remarkable in a composition system having a relatively large SiO ₂ component as represented by the composition. However, according to the present invention, when a dense body having a relative density of 97% or more of the above system is prepared, if the maximum void diameter is controlled to 5 μm or less, particularly 3 μm or less in the sintered body, mechanical properties due to the presence of voids may be reduced. Effect can be reduced, and the original characteristics of the sintered body can be exhibited.
If the maximum void diameter exceeds 5 μm, the voids become a source of fracture, which lowers the strength of the sintered body and causes variations in characteristics.

In the sintered body, in addition to the above composition, metals of Groups 4a, 5a, and 6a of the periodic table and their carbides, nitrides, silicides, SiC, and the like are dispersed particles and whiskers of the present invention. Even if present in the aggregate, the effect of deteriorating the characteristics is small, so that it is naturally possible to improve the characteristics as a composite material by adding an appropriate amount thereof based on a known technique.

However, metals such as Al, Mg and Ca form oxides having a low melting point, which hinders crystallization of grain boundaries and deteriorates high-temperature strength. It is desirable to control it to 0.5% by weight or less.

Next, a method for producing the silicon nitride sintered body of the present invention will be described. According to the present invention, a silicon nitride powder is used as a main component as a starting material, and an oxide of a Group 3a element of the periodic table, and in some cases, a silicon oxide powder are added as an additional component. Further, as an addition form, a compound composed of an oxide of a Group 3a element of the periodic table and silicon oxide, or a compound powder of silicon nitride, an oxide of an element of the Group 3a of the periodic table and silicon oxide can be used.

The silicon nitride powder to be used may be either α-type or β-type, and the particle size is 0.4 to
1.2 μm is appropriate.

According to the present invention, using these powders,
In particular, 70 to 97 mol% of silicon nitride and 3 to 10 mol%, particularly 3 to 7 mol%, of a Group 3a element oxide (RE ₂ O ₃ ) of the periodic table.
Mol% and excess oxygen (equivalent to SiO ₂ )
It is important that the molar ratio represented by O ₂ / RE ₂ O ₃ is 2 or more, especially 2.5 or more in consideration of decomposition during firing.
Here excess oxygen is the total amount of SiO ₂ powder impurities oxygen was added to the amount SiO ₂ in terms contained in the silicon nitride powder.

When the amount of the Group 3a element oxide in the periodic table is less than 3 mol%, the sinterability decreases. When the amount exceeds 10 mol%, the amount of the grain boundary component increases and the high-temperature strength decreases. On the other hand, if the molar ratio is smaller than 2, a trace amount of a glass phase composed of RE-Si-ON is likely to be generated, and a silicon oxynitride (Si ₂ N ₂ O) phase and / or a disilicate (RE ₂ Si ₂ O) ₇ ) Apatite phase other than phase or Y
This is because a crystal phase such as an AM phase is precipitated, and characteristics at high temperatures, particularly, oxidation resistance are reduced.

The mixed powder mixed in the above ratio is formed into an arbitrary shape by a desired molding means, for example, a die press, a casting molding, an extrusion molding, an injection molding, a cold isostatic pressing and the like.

Next, the molded product was heated to 1600
According to the present invention, the sintering temperature is higher than the eutectic temperature of RE ₂ Si ₂ O ₇ and silicon nitride, and the difference between the eutectic temperature and the sintering temperature. Is 200 ° C. or less, preferably 100 to 150 ° C. below the eutectic temperature.
It is important to fire at a high temperature range of ° C. this is,
When the sintering temperature is lower than the eutectic temperature, a liquid phase is not formed and densification is not achieved. When the sintering temperature is higher than 200 ° C. than the eutectic temperature, grain growth is promoted and voids in the sintered body are increased. Are generated, and the voids are aggregated to form a void having a large diameter. Since the eutectic temperature of RE ₂ Si ₂ O ₇ and silicon nitride differs depending on the type of Group 3a element in the periodic table, it is necessary to appropriately set the eutectic temperature depending on the compound type.

Further, the nitrogen pressure at the time when closed pores start to be generated during firing is 10 atm or less, particularly 1.0 atm.
It is also important that it be ~ 5.0 atm. Nitrogen pressure is 10
If the pressure is higher than the atmospheric pressure, when the pores are trapped in the sintered body, the pressure in the closed pores increases, making it difficult to eliminate the closed pores during the sintering process, thereby forming voids having a large diameter.

By the above calcination, the theoretical density ratio is finally 9
Although it is possible to obtain a dense sintered body of 7% or more, according to the present invention, RE at the silicon nitride crystal grain boundaries in the sintered body
_In order to form a crystal phase mainly composed of ₂ Si ₂ O ₇ and / or Si ₂ N ₂ O, a grain boundary is formed by a cooling process in the firing step, a temporary holding in the cooling step, or a heat treatment after the firing step. To precipitate these crystals. An appropriate heat treatment temperature at this time is 1100 to 1500 ° C. Desirably, after once holding in the range of 700 to 1250 ° C., it is desirable to further perform multi-stage heat treatment at a temperature of 1300 to 1600 ° C. This is because such a multi-step heat treatment can reduce the crystal size precipitated at the grain boundary and increase the high-temperature strength.

[0025]

The aforementioned SiO ₂ / RE ₂ O ₃ molar ratio is 2.0.
The grain boundary phase crystal of the silicon nitride sintered body having the above composition is R
A crystal mainly represented by E ₂ Si ₂ O ₇ (RE: an element of Group 3a of the periodic table) and / or Si ₂ N ₂ O is mainly used.

In the composition system of the present invention, S ₃ N ₄ —RE ₂ O ₃ —SiO ₂ reacts in the sintering process to form S
An i-RE-ON-based liquid phase is produced. Sintering proceeds through a liquid phase by a dissolution-reprecipitation process. At this time, pores diffuse out of the grain boundary and are discharged out of the system. However,
The diffusion rate of the pores increases with increasing temperature,
At the same time, the pores remain in the system because the grain growth proceeds and the diffusion is inhibited. Since the grain growth rate in that case is governed by diffusion in the dissolution-reprecipitation process, the lowest temperature at which a liquid phase is generated, that is, the eutectic temperature Tc of RE ₂ Si ₂ O ₇ and silicon nitride, the firing temperature It is governed by the difference from Ts, ΔT = Ts−Tc. Therefore, in order to discharge pores out of the system, Δ
It is necessary to reduce the grain growth rate by reducing T.

According to the present invention, the firing temperature T
By setting s to a temperature slightly higher than the eutectic temperature Tc, the progress of sintering on the surface of the compact can be suppressed and the grain growth rate can be slowed. Trap can be prevented. Accordingly, generation of large voids due to aggregation of voids can be suppressed.

Thus, the number of voids on the surface and inside of the sintered body can be reduced, and the maximum diameter can be reduced, so that the mechanical properties of the sintered body can be improved and the reliability of the properties can be improved. Can be enhanced.

[0029]

EXAMPLES Silicon nitride powder (BET specific surface area 10 m ² / g, α rate 95%, oxygen content 1.2 wt%, metal impurity content 0.03 wt%, direct nitriding raw material) as raw material powder and various periods Table 1 shows the amounts of Si ₃ N ₄ , RE ₂ O ₃ , and SiO ₂ using the oxide powder of the Group 3a element and the silicon oxide powder.
And crushed with a vibration mill for 72 hours using silicon nitride balls with isopropyl alcohol as a solvent, and after drying the slurry, a diameter of 60 mm and a thickness of 10 mm.
Was subjected to rubber press molding at a pressure of 3 t / cm ² .

The sintered body was fired under the conditions shown in Table 1. In order to suppress decomposition during firing, the furnace input during firing was fixed at 300 g / liter. Further, a crystallization treatment was performed in a cooling process after firing.

The obtained sintered body was cut and polished into a test piece of 3 × 4 × 40 mm, and subjected to a four-point bending strength test at room temperature and 1400 ° C. based on JIS-R1601. Regarding the void diameter, the surface of the sample mirror-finished with a diamond paste was observed with an SEM and observed at 400 × 40.
The largest void diameter observed from an area of 0 μm,
The average of the locations was defined as the maximum void diameter.

[0032] Incidentally, RE ₂ Si ₂ O ₇ and the eutectic temperature of the silicon nitride RE ₂ Si ₂ O ₇ with respect to 8 mol% of Si ₃ N
_The temperature at which the mixture to which ₄ was added began to melt in nitrogen at 1.2 atm. Further, the crystal of the grain boundary phase in the sintered body was identified by X-ray diffraction measurement. Further, in order to evaluate the oxidation resistance, the sintered body was kept at 1,300 ° C. for 100 hours in the air, and the weight increase of the sintered body by oxidation was measured. The results are shown in Table 2.

[0033]

[Table 1]

[0034]

[Table 2]

As is clear from the results of Tables 1 and 2, when the firing temperature exceeds 200 ° C. from the eutectic temperature of RE ₂ Si ₂ O ₇ and silicon nitride, the void diameter increases and the strength decreases. However, if the temperature was lower than the eutectic temperature, it could not be densified. If the SiO ₂ / RE ₂ O ₃ molar ratio is lower than 2, the strength is high but the oxidation resistance is reduced. On the other hand, all of the samples satisfying the conditions in the present invention exhibited high strength from room temperature to high temperature and were excellent in oxidation resistance.

[0036]

As described in detail above, according to the present invention,
In addition to suppressing the generation of voids and minimizing the effect of voids on the characteristics, the original characteristics of the sintered body can be exhibited, and while having high strength from room temperature to high temperature,
A silicon nitride-based sintered body having excellent oxidation resistance can be manufactured.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C04B 35/584

Claims (1)

(57) [Claims] An element mainly composed of silicon nitride, containing an oxide of a Group 3a element (RE ₂ O ₃ ) and excess oxygen of the Periodic Table, and an element of Group 3a of the Periodic Table in terms of SiO ₂ equivalent amount of the excess oxygen. A method for producing a silicon nitride-based sintered body, comprising firing a molded body having a composition having a molar ratio to an oxide of 2 to 10 in a nitrogen atmosphere at a temperature of 1600 to 1950 ° C. RE ₂ Si ₂ of Group 3a element (RE)
Nitrogen at a time when the temperature is higher than the eutectic temperature of the crystal represented by O ₇ and silicon nitride, the difference between the sintering temperature and the eutectic temperature is 200 ° C. or less, and the generation of closed pores during sintering starts. A method for producing a silicon nitride-based sintered body, wherein a partial pressure is 10 atm or less.