JP2708136B2 - Silicon nitride sintered body and method for producing the same - 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 sintered body having excellent strength characteristics from room temperature to a high temperature and particularly used for parts for automobiles and parts for gas turbine engines, and a method for producing the same.

[0002]

2. Description of the Related Art Hitherto, silicon nitride sintered bodies have been applied to engineering ceramics, particularly for heat engines such as turbo rotors, because of their excellent heat resistance, thermal shock resistance and oxidation resistance.

Recently, it has been known to add a rare earth element oxide or aluminum oxide as a sintering aid in order to produce a high-density and high-strength sintered body (JP-B-52-3649, JP-B-58-5190). Is being considered.

Recently, silicon nitride based sintered bodies have
A method of increasing the strength and toughness of a sintered body by adding a compound of a metal such as b, Ta, Ti, W, etc.
No. 87568.

[0005]

However, when yttrium oxide and aluminum oxide are used as sintering aids, their sinterability is enhanced and their densities can be increased, so that the strength at room temperature and high temperature is improved to some extent. However, it is still insufficient for practical use, and further improvement in strength is required.

[0006] In the method of adding Ta, Nb, Ti or the like to the sintered body, improvement in room temperature strength is recognized, but no study has been made on improvement in high temperature strength.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a strength characteristic sufficient for use in automobile parts and gas turbine engine parts from room temperature to high temperature, and particularly to a bending strength from room temperature to high temperature of 1000 ° C. An object of the present invention is to provide an excellent silicon nitride sintered body and a method for producing the same.

[0008]

Means for Solving the Problems In order to enhance the high-temperature characteristics of the sintered body, the present inventors have studied the composition of the sintered body, the shape of the silicon nitride crystal phase in the sintered body, and the silicon nitride crystal. Based on the view that it is important to control the sub-phases present at the grain boundaries of the phase, as a result of repeated investigations, a system in which silicon nitride is the main component, and rare earth element oxides, aluminum oxide and silicon oxide are added thereto Is further added with a Group 5a element oxide of the periodic table, which is formed and fired to precipitate a crystalline grain boundary phase composed of silicon, oxygen and nitrogen in the obtained sintered body. It has been found that a sintered body having excellent high-temperature characteristics can be obtained by precipitating a phase composed of an oxynitride of a Group 5a element of the periodic table.

Hereinafter, the present invention will be described in detail. The silicon nitride-based sintered body of the present invention contains silicon nitride as a main component, and further contains a rare earth element, aluminum, excess oxygen, and a Group 5a element of the periodic table as additional components.

Here, the excess oxygen is defined as a rare earth element oxide, aluminum oxide, or a Group 5a element oxide of the periodic table in the sintering aid based on the total oxygen content of the sintered body. The remaining amount of oxygen excluding the bound oxygen, most of which is contained in the silicon nitride raw material, or
It is mixed as an additive such as SiO ₂ , and is considered in the present invention as being present as all SiO ₂ .

The sintered body of the present invention has a silicon nitride crystal phase as a main phase, and most of the sintered body is composed of β-Si ₃ N ₄ crystals. In some cases, a small amount of aluminum oxide (Al ₂ O _{3) is used.} ) May form a solid solution to form Si-Al-ON (Sialon). The main phase generally has a columnar shape, and has an average crystal grain size (short diameter) of 1.5 μm.
Hereinafter, it is preferable that the aspect ratio is 2 to 15 from the viewpoint of improving strength and toughness.

According to the present invention, a major feature is that a crystal phase composed of silicon, oxygen and nitrogen exists as a sub-phase at a grain boundary of the main phase. This crystal phase is, for example, Si ₂ N
_It consists of a silicon oxynitride crystal phase represented by ₂ O, and the high temperature strength can be increased by the presence of this crystal phase.

According to the present invention, it is also important that an oxynitride phase of a Group 5a element of the periodic table exists in addition to the above-mentioned crystal phase. This crystal phase suppresses abnormal grain growth of the silicon nitride crystal phase and improves strength at room temperature and high temperature.

The above three types of crystal phases are formed in the sintered body of the present invention, and a grain boundary phase composed of silicon, aluminum, oxygen and a rare earth element is formed at the grain boundaries of these crystal phases. . The grain boundary phase may be either an amorphous phase or a crystalline phase.

Further, the sintered body of the present invention has a silicon nitride of 60%.
-93 mol%, especially 68-85 mol%,
The total of the oxide equivalent of the Group 3a element of the periodic table, the oxide equivalent of aluminum and the excess oxygen equivalent of SiO ₂ is 6.0 to 39.9 mol%, particularly 14.2 to 31.8 mol%. And the group 5a element of the periodic table is 0.1% in terms of oxide.
It is also important to have a composition of .about.1.0 mol%, especially 0.2-0.8 mol%.

This is because if the total amount of the rare earth element, aluminum and excess oxygen is less than 6.0 mol%, the liquid phase becomes insufficient during the sintering process, and it is necessary to raise the firing temperature. Particle growth occurs, causing a decrease in strength. If it exceeds 39.9 mol%, the number of grain boundary phases in the sintered body increases, so that the high-temperature strength is reduced and the oxidation resistance at high temperatures is further deteriorated. For the reason, Periodic Table No. 5a
When the group-group element is less than 0.1 mol%, the growth of silicon nitride particles occurs, causing a decrease in strength. When the group element is more than 1.0 mol%, needle-like formation of the silicon nitride particles is suppressed, and the fracture toughness decreases. It is because.

According to the present invention, when the oxide equivalent of Group 3a element of the periodic table is RE ₂ O ₃ and the oxide equivalent of alnium is Al ₂ O ₃ , the excess oxygen is equivalent to SiO _2. The molar ratio expressed by SiO ₂ / RE ₂ O ₃ with respect to the amount is 2 or more, especially 2 to 25, and Al ₂ O ₃ / (SiO ₂ +
It is desirable that the molar ratio represented by RE ₂ O ₃ ) is 1/15 to 5/4, particularly 0.1 to 1.0. This is SiO ₂
When the / RE ₂ O ₃ molar ratio is 2 or less, a crystal phase composed of silicon, oxygen and nitrogen is hardly generated, which causes a decrease in strength. Al ₂ O ₃ / (SiO ₂ + RE ₂ O ₃ )
If the molar ratio is less than 1/15, the sinterability deteriorates, and it is difficult to obtain a high-density sintered body.
_{This is because 2} O ₃ increases the generation of low-melting-point substances and deteriorates the strength and oxidation resistance at high temperatures.

The rare earth element used in the present invention is Y
And lanthanoid elements, among which Y
b ₂ O ₃ , Er ₂ O ₃ , Ho ₂ O ₃ , and Dy ₂ O ₃ are preferable in terms of uniformity and strength of the sintered body.

In addition, as an element of Group 5a of the periodic table, N
Among them, b, V, and Ta are listed, and among these, Nb is the most effective.

According to the method for producing a silicon nitride-based sintered body of the present invention, silicon nitride powder, rare earth element oxide powder, aluminum oxide powder and group 5a element oxide powder of the periodic table are used as raw material powders. In some cases, a silicon oxide powder is further used.

The silicon nitride powder itself is α-Si
₃ N _4, can be used either in β-Si ₃ N _4, it is desirable that their particle size is 0.4～1.2Myuemu.

According to the present invention, silicon nitride (Si ₃ N ₄ ) is used in an amount of 60 to 93 mol%, particularly 68
To 85 mol%, the total amount of rare earth element oxide (RE ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), and excess oxygen (SiO ₂ equivalent) is 6.0 to 39.9 mol%, especially 14. 2 to 31.8 mol%, and an oxide of a Group 5a element of the periodic table in an amount of 0.1 to 3
It is prepared and mixed so as to be 1.0 mol%, especially 0.2 to 0.8 mol%. The excess oxygen (SiO ₂ ) at this time is the total amount of the amount of impurity oxygen contained in the silicon nitride powder in terms of SiO ₂ and the amount of the silicon oxide powder to be added.

The mixed powder thus obtained is formed into a desired shape by a known molding method, for example, press molding, casting molding, extrusion molding, injection molding, cold isostatic pressing and the like.

Next, the obtained molded body is fired by a known firing method,
For example, hot press method, normal pressure firing, nitrogen gas pressure firing, and further, HIP firing after firing, and
It is fired by glass seal HIP firing or the like to obtain a dense sintered body. The firing temperature at this time is 1900 ° C. or less, particularly 1
A non-oxidizing atmosphere containing nitrogen gas at 700 to 1800 ° C. is desirable.

According to this calcination, most of the added components are present at the grain boundaries of the silicon nitride crystal phase, and among these, the Group 5a element of the periodic table has an oxide obtained by partially substituting the oxide with nitrogen. Nitride is produced. On the other hand, other additives form a crystal phase composed of silicon, nitrogen and oxygen, and a grain boundary phase composed of silicon, aluminum, oxygen and nitrogen.
When a crystal phase consisting of silicon, nitrogen and oxygen is not generated depending on the conditions, the obtained sintered body is 1000 to 1500
By treating in a non-oxidizing atmosphere at a temperature of ° C., a crystalline phase can be generated.

[0026]

The silicon nitride crystal grains have an appropriate aspect ratio and size, and in order to obtain high reliability, it is necessary that no abnormally grown grains exist. For that purpose, an appropriate amount of the grain boundary phase is necessary, and the melting point of the liquid phase generated in the sintering process is lowered by adding SiO ₂ in addition to the conventional rare earth element oxide and aluminum oxide. In addition, low-temperature sintering becomes possible, growth of silicon nitride particles can be suppressed, and a fine structure can be formed.

Further, the addition of the Group 5a element oxide of the periodic table changes the oxide into an oxynitride during the sintering process, so that the oxide is not dissolved in the grain boundary phase composed of other components, but as an oxynitride phase. Since the oxynitride phase remains, the oxynitride phase further suppresses the growth of silicon nitride particles, and a fine and uniform structure having a uniform aspect ratio can be formed. Further, silicon in the sintered body,
By forming a crystal phase composed of oxygen and nitrogen, components other than the silicon nitride crystal phase are stabilized at a high temperature, whereby the strength and the toughness of the sintered body at room temperature and high temperature can be increased.

[0028]

EXAMPLES Example 1 Silicon nitride powder (BET specific surface area 8 m ² /
g, α ratio of 98%, oxygen content of 1.2% by weight) and various rare earth element oxide powders, silicon oxide powders, aluminum oxide powders and Group 5a element oxide powders of the periodic table. After mixing and mixing, the mixture was subjected to die press molding at 1 t / cm ² .

[0029]

[Table 1]

The obtained compact was placed in a silicon carbide sagger and fired at 1750 ° C. for 4 hours in a nitrogen gas stream at normal pressure using a carbon heater to obtain a sintered body. .

The obtained sintered body was polished to a shape specified by JIS-R1601 to prepare a sample. The specific gravity of this sample was measured based on the Archimedes method, the average crystal grain size (short diameter) of the silicon nitride crystal and the average aspect ratio thereof were measured from an electron micrograph. A bending strength test was performed, and the fracture toughness was measured. Further, crystals in the sintered body were identified by X-ray diffraction measurement. The results are shown in Table 2.

[0032]

[Table 2]

According to the results shown in Tables 1 and 2, Si ₃ N
₄ is less than 60 mol%, and RE ₂ O ₃ , Al ₂ O ₃ , SiO
_The sintered body of No, 1 in which the total amount of No. ₂ exceeds 39.9 mol% has a low high-temperature strength, and the content of Si ₃ N ₄ exceeds 93 mol%.
The sintered body of No. 10 in which the total amount of RE ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ was less than 6 mol% was insufficient in densification and the strength was reduced. Further, the sintered body of No.6 containing less than 0.1 mol% of the group 5a element has reduced room temperature strength, and the sintered body of No.9 containing more than 1.0 mol% of the group 5a element has a fracture toughness. And the room temperature strength was reduced.

In contrast to these comparative examples, all of the sintered bodies of the present invention exhibited excellent bending strength and fracture toughness. Further, the molar ratio of SiO ₂ / RE ₂ O ₃ is more than 68 and 85
In the following, the Al ₂ O ₃ / (SiO ₂ + RE ₂ O ₃ ) molar ratio is larger than 0.1, and is 1.0 or less.
o, 7, No, 8, No, 11, No, 14, No, 15, No, 1
The sintered compact of No. 6 showed further excellent bending strength and fracture toughness.

Example 2 Of the samples in Table 1 of Example 1, No, 3, No, 10, No,
The sintered bodies No. 12 and No. 13 were further subjected to hot isostatic pressure treatment at 2,000 atm of nitrogen gas-containing argon gas at 1850 ° C. for 1 hour. It should be noted that no composition change was observed during HIP. The same evaluation as in Example 1 was performed on the obtained sintered body, and the results are shown in Table 4.

[0036]

[Table 3]

According to the results shown in Table 3, although the density of the sintered body was improved by the HIP treatment, the strength could not be recovered by the HIP treatment in Sample No. 10 having a small amount of Si ₃ N _4. Was.

[0038]

As described in detail above, according to the present invention, S
To the i ₃ N ₄ —RE ₂ O ₃ —Al ₂ O ₃ —SiO ₂ system, a Group 5a element oxide of the periodic table is added, and a Group 5a oxynitride and silicon, oxygen, By precipitating a crystal phase composed of nitrogen, it is possible to suppress the growth of silicon nitride crystal grains and to stabilize additives such as a sintering aid in a high-temperature region. Strength and toughness can be obtained.

Therefore, it can be used as a structural material used at various room temperatures or high temperatures, including structural materials for heat engines such as gas turbines and turbo rotors.

Claims (5)

(57) [Claims] 1. A silicon nitride crystal phase, a crystalline grain boundary phase comprising silicon, oxygen and nitrogen, a grain boundary phase containing at least a rare earth element and aluminum, and an oxynitride phase of a Group 5a element of the periodic table A silicon nitride-based sintered body characterized by comprising: (2) a total amount of silicon nitride of 60 to 93 mol%, a rare earth element oxide equivalent, aluminum oxide equivalent, and excess oxygen equivalent of SiO ₂ equivalent of 6 to 39.9 mol%;
2. The silicon nitride sintered body according to claim 1, wherein the Group 5a element of the periodic table is present in an amount of 0.1 to 1.0 mol% in terms of oxide. 3. An amount of rare earth element in terms of oxide (RE ₂ O ₃ )
And SiO ₂ / RE ₂ O in terms of SiO _{2 in} terms of excess oxygen and aluminum oxide in terms of oxide (Al ₂ O ₃ ).
_The molar ratio represented by ₃ exceeds 2, and Al ₂ O ₃ / (R
E ₂ O ₃ + SiO ₂ ) in a molar ratio of 1/15 to 5
The silicon nitride sintered body according to claim 1, wherein the ratio is / 4. 4. A total amount of 60 to 93 mol% of silicon nitride and 6 to 93 mol% of rare earth oxide, aluminum oxide and silicon oxide.
39.9% by mole of the Group 5a element oxide in the periodic table
After molding the mixture prepared to be 1.0 mol%, the mixture is sintered in a non-oxidizing atmosphere containing nitrogen, and a silicon nitride crystal phase,
Silicon, a crystalline grain boundary phase composed of oxygen and nitrogen, and a grain boundary phase containing at least a rare earth element and aluminum,
A method for producing a silicon nitride-based sintered body, comprising generating an oxynitride phase of a Group 5a element of the periodic table. 5. The amount of said rare earth element oxide added (RE ₂ O
₃ ), the addition amount of silicon oxide (SiO ₂ ), and the addition amount of aluminum oxide (Al ₂ O ₃ )
The molar ratio represented by O ₂ / RE ₂ O ₃ exceeds 2, and A
l ₂ O ₃ / molar ratio represented by _{(RE 2 O 3 + SiO 2} ) is 1 / 15-5 / 4 in the manufacturing method according to claim 4 silicon nitride sintered body, wherein.