JP2811493B2 - Silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is directed to production of a silicon nitride sintered body excellent in bending strength and oxidation resistance at high temperatures suitable for heat engines such as gas turbines and turbo rotors. About the method.

(Prior Art) Conventionally, silicon nitride-based sintered bodies have attracted attention as engineering ceramics, particularly as materials for heat engines, because of their excellent strength, hardness, and thermochemical stability at high temperatures. Specific heat engines include parts for turbo rotors and gas turbines, and when applied to these, have excellent mechanical properties in the range of room temperature to about 1200 ° C for sintered bodies, and at high temperatures of 1400 ° C for some parts. However, recently, improvement of characteristics at 1500 ° C. has been desired.

Generally, as a method for producing these silicon nitride-based sintered bodies, since silicon nitride itself is difficult to sinter, various sintering aids such as rare earth element oxides are added, and a hot press method and a normal pressure method are used. A firing method, a gas pressure firing method, and the like are employed. Recently, for the purpose of increasing the density and strength, a method of forming an impermeable seal made of glass or the like on the surface of a silicon nitride molded body having a desired composition and firing it under high pressure (hereinafter referred to as seal HIP) Has been studied.

On the other hand, from the viewpoint of composition, in addition to rare earth element oxides such as Y ₂ O ₃ as described above, oxides such as Al ₂ O ₃ and MgO are most commonly used as sintering aids. However, considering the high-temperature characteristics of the sintered body, if Al ₂ O ₃ or MgO is included, a low-melting-point substance is generated at the grain boundaries of the crystalline body, which lowers the high-temperature strength and high-temperature oxidation resistance. Accordingly, a simple ternary composition of Si ₃ N ₄ —RE ₂ O ₃ (rare earth oxide) —SiO ₂ , which does not substantially contain the above oxides, is also being studied.

Also, from the viewpoint of the structure of the sintered body, the grain boundary phase in the sintered body has been attracting attention as a factor that determines the high-temperature characteristics, and the grain boundary phase is intended to improve the strength of the grain boundary phase itself. Attempts have been made to substantially crystallize. Therefore, recently, for the above simple ternary composition, Si ₃ N ₄ was added to the grain boundary by examining the firing conditions or heat treatment of the sintered body.
-RE ₂ O ₃ (rare earth oxide) various crystalline phase consisting -SiO _2, for example melilite, apatite, attempts have been made to improve the high temperature properties by precipitating YAM or wollastonite, and the like.

(Problems to be Solved by the Invention) However, although the sintered body obtained by precipitating the above-mentioned various crystal phases at the grain boundaries has an effect of improving strength at room temperature and high temperature of 1400 ° C. or improving oxidation resistance to some extent, It has the disadvantage that its properties at high temperatures of 1500 ° C., especially its oxidation resistance, are severely degraded, and that it is hardly usable at temperatures of 1500 ° C.

Therefore, the present applicant has made a
As a sintered body having excellent oxidation resistance at a high temperature of 00 ° C., a system containing a large amount of excess oxygen, specifically, 70 to 99 mol% of silicon nitride, 0.1 to 5 mol% of rare earth oxide, and excess oxygen (SiO ₂ equivalent) 25% by mole or less, and the molar ratio of (excess oxygen / rare earth element oxide) is more than 2 and is in the range of 25 or less. Of sintered or precipitated silicon oxynitride crystals has been proposed, but this sintered body lacks the stability of its properties and has the drawback of insufficient strength, especially at 1400 ° C. .

(Object of the Invention) An object of the present invention is to maintain excellent high-temperature oxidation resistance,
It is an object of the present invention to provide a silicon nitride sintered body having improved bending strength at a high temperature of 1400 ° C.

(Means for Solving the Problems) The present inventors have examined the above problems, and as a result, have found that Si ₃ N ₄ -RE ₂ O ₃ (rare earth oxide)-containing a large amount of excess oxygen. SiO ₂ composition, especially 70-99 mol% silicon nitride
And rare earth element oxide 0.1 to 5 mol%, and excess oxygen (SiO 2
(Equivalent to ₂ ) 25 mol% or less, and a composition in which the molar ratio of (excess oxygen / rare earth element oxide) is more than 2 and is in the range of 25 or less, and at the same time, the grain boundaries of silicon nitride crystal particles in the sintered body. It has been found that the high-temperature strength at 1400 ° C. can be greatly improved by allowing the crystal phases of the silicon oxynitride phase and the disilicate phase to be present at the same time and setting the silicon nitride crystal grain size to 10 μm or less.

 Hereinafter, the present invention will be described in detail.

According to the present invention, first, the composition of the sintered body is silicon nitride 70 to 99
Mol%, especially 80-93.5 mol%, and rare earth element oxide 0.1
-5 mol%, especially 0.5-3 mol%, excess oxygen is 25 mol%
In the following, it is important that the molar ratio (excess oxygen / rare earth element oxide) is more than 2 and 25 or less, especially 3 to 20. The excess oxygen is the amount of oxygen obtained by removing the oxygen mixed in a stoichiometric amount as a rare earth element oxide from the total amount of oxygen contained in the entire system of the sintered body, specifically, the amount of oxygen in the silicon nitride raw material. It is composed of impurity oxygen or oxygen added as SiO ₂ , each of which indicates a SiO ₂ equivalent.

The reason for limiting the composition of the sintered body to the above range is that the room temperature strength and the high temperature strength are deteriorated even if any of silicon nitride, rare earth oxide and excess oxygen deviates from the above range, The reason for limiting the molar ratio between excess oxygen and the rare-earth element oxide to the above range is that if the molar ratio is less than 2, it becomes difficult to precipitate crystals of the silicon oxynitride phase or the disilicate phase at the grain boundary, and the temperature becomes 1500 ° C. This is because the oxidation resistance at high temperatures tends to deteriorate, and conversely, if it exceeds 25, a glass having a low melting point is likely to be generated, and the high-temperature characteristics will deteriorate.

According to the present invention, a major feature is that both a silicon oxynitride phase and a disilicate phase are present at grain boundaries of silicon nitride crystal grains of a sintered body having the above composition. The silicon oxynitride phase is a crystal phase composed of silicon, nitrogen and oxygen, and is generally represented by a chemical formula of Si ₂ N ₂ O. On the other hand, the disilicate phase is a crystal phase composed of a rare earth element, silicon and oxygen, and is generally represented by a chemical formula of RE ₂ O ₃ .2SiO ₂ . When the grain boundary contains only the silicon oxynitride phase or only the disilicate phase, the oxidation resistance at 1500 ° C. is excellent, but the high-temperature strength is insufficient, and the object of the present invention is not achieved.

Next, as a specific method for producing the silicon nitride based sintered body of the present invention, first, while comprising the above-described composition,
A sintered body in which a silicon oxynitride phase and a glass phase coexist at the grain boundary is prepared and heat-treated under specific conditions to form a disilicate phase together with the silicon nitride phase at the grain boundary. This method is most suitable from the viewpoint of productivity and stability of properties.

As a method for producing a sintered body having a silicon oxynitride crystal phase at a grain boundary, a silicon nitride powder, a rare earth oxide powder, and optionally a silicon oxide powder are used as raw material powders, and silicon nitride, a rare earth oxide, Excess oxygen (SiO ₂
(Equivalent amount) is weighed and mixed such that the composition contains a large amount of excess oxygen as described above. The silicon nitride powder at this time has a BET specific surface area of 3 to ²⁰ m ² / g and a pregelatinization ratio of 95 to promote crystallinity.
% Is desirable. The silicon nitride powder generally contains an impurity oxygen content of about 0.8 to 1.5% by weight, but the total oxygen content can be arbitrarily adjusted by adding silicon oxide.

After adding the appropriate binder to the above mixed powder and granulating,
Molding. A well-known method can be employed for molding, and specifically, press molding, extrusion molding, casting molding, injection molding or the like can be employed.

The molded body thus obtained is calcined after removing the binder.

The firing is performed in a non-oxidizing atmosphere at 1450 to 2000 ° C., depending on the firing means. As the firing means, a gas pressure firing method, a hot isostatic pressure firing method, or the like is preferable.

According to the gas pressure firing method, the firing temperature is controlled at 1700 to 2000 ° C., and nitrogen gas is introduced into the atmosphere at a gas pressure of 1.5 to 100 atm for firing. Since the composition of the present invention contains a large amount of excess oxygen, SiO ₂ is contained in the furnace in order to suppress the decomposition fluctuation of the excess oxygen amount.
Powder or mixed powder of SiO ₂ and Si ₃ N ₄
Is desirably generated.

On the other hand, according to the hot isostatic firing method, a so-called seal HIP method is preferably employed, in which a sealing material made of glass or the like is formed on the surface of the molded body and fired at a high temperature and a high pressure. As a specific method, first, the molded body obtained by the above-described method prior to firing is subjected to wettability with glass such as BN powder for the purpose of preventing a reaction with glass or the like as a sealing material in the firing step. The bad powder is slurried and applied to the compact, or the slurry is spray applied. The thickness of the BN applied to the surface of the molded body is preferably about 1 to 10 mm.

Next, a glass powder that forms a seal at the time of firing is applied to the surface of the molded body to which BN has been applied, or the molded body is encapsulated in a glass capsule. As another method, the compact can be embedded in a crucible filled with glass powder. After that, it is fired at high temperature and high pressure by the HIP method.

Firing is first at least the softening point of the glass present on the surface of the molded body, and at the same time the temperature is raised to the firing temperature, which is equal to or higher than the decomposition equilibrium pressure of silicon nitride at that temperature 0.01 to 0.2 M
The glass is softened while introducing a nitrogen gas at a pressure as high as Pa to form a gas impermeable film made of glass on the surface of the molded body. After the gas-impermeable membrane is completely formed on the surface of the molded body, under conditions under which the furnace pressure can be sufficiently densified, for example,
Increase to a pressure of 50MPa or more. At this time, an inert gas such as nitrogen or argon is used as the pressure medium. At this stage, a liquid phase is generated by the rare earth oxide, SiO ₂ , and silicon nitride, the firing proceeds, and the densification is almost completed. Thereafter, the temperature and the pressure are both reduced to end the firing.

The firing temperature is 1450 ° C to 1800 ° C, especially 1450 ° C to 1730
Set to ° C.

By the gas pressure firing method or the seal HIP method, it is possible to obtain a sintered body in which a grain boundary is a silicon oxynitride phase alone or a silicon oxynitride crystal phase and an amorphous phase of silicon, a rare earth element, oxygen and nitrogen. it can.

Next, 1300 to the sintered body obtained by the above method
Heat treatment for about 3 to 100 hours in an oxidizing atmosphere at a temperature of ℃ to 1600 ° C or a non-oxidizing atmosphere such as nitrogen to further promote the crystallization of grain boundaries, and precipitate a disilicate phase together with a silicon oxynitride phase at the grain boundaries Can be done. At this time, the grain boundary may include an amorphous phase together with the crystal phase.

If the heat treatment temperature at this time is lower than 1300 ℃, the crystallization of the grain boundary does not progress, so that no disilicate phase is generated,
If the temperature is higher than ℃, the surface of the sintered body tends to decompose and the strength tends to deteriorate.

According to the above-described manufacturing method, a sintered body having a fine structure having an average particle diameter (major axis) of silicon nitride crystal particles of 10 μm or less and an aspect ratio of 3 or more can be obtained. In the hydrostatic firing method, the firing temperature is 1450 to 17
When the temperature is set to a low temperature of 30 ° C., the grain growth of silicon nitride particles is suppressed, so that miniaturization of silicon nitride crystal grains can be promoted. Specifically, a fine structure having an average particle diameter (major axis) of 7 μm or less is used. Can be obtained. Thereby, the temperature strength as well as the room temperature strength of the sintered body can be improved.

According to the present invention, oxides that are present at the grain boundaries and easily form a glass phase to enhance the crystallinity of the grain boundaries in the sintered body, specifically Al ₂ O ₃ , CaO, MgO, Fe ₂ Oxides such as O ₃ are contained in the entire sintered body,
It is desirable that the content be 0.05% by weight or less. As a rare earth element oxide, Y ₂ O ₃ is generally used, but Yb ₂ O ₃ , Er ₂ O _3
3 , It is preferable to use a heavy rare earth element oxide such as Ho ₂ O _{3 in} that the occurrence of color unevenness and the like can be prevented and a sintered body having stable characteristics can be obtained.

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

(Example 1) Silicon nitride powder (BET specific surface area 5 m ² / g,
Using a rare earth oxide or SiO ₂ powder with a pregelatinization ratio of 95% and an impurity oxygen content of 1.0% by weight) and mixing to obtain the composition shown in Table 1, the mixture was press-formed at 1 t / cm ² . .

The obtained compact was fired at a temperature shown in Table 1 under a 50 atm nitrogen gas atmosphere in which a SiO ₂ powder was placed in a furnace.

Next, the obtained crystal was further heat-treated under the conditions shown in Table 1.

For each sintered body after heat treatment, according to JISR1601, room temperature,
A four-point bending strength at 1400 ° C. and an oxidation resistance test at 1500 ° C. for 24 hours were performed, and the increase in oxidized weight after the test was measured.

Further, crystal phases other than silicon nitride were identified from the X-ray diffraction curve of the sintered body after the heat treatment.

 The results are shown in Table 1.

(Example 2) Using the same raw material powder as in Example 1, after mixing and mixing to have the ratio shown in Table ² , press molding at 1 t / cm ² , then 1400
It was calcined at ℃.

A paste of BN powder (particle size: 1 to 5 μm) was applied to the obtained molded body at a thickness of 1 to 10 mm, and a glass containing SiO ₂ as a main component was applied to a thickness of 1 to 10 μm.

The green body thus treated was placed in a hot isostatic firing furnace and fired under various conditions to prepare several types of samples having different characteristics as shown in Table 2.

Next, these samples were heat-treated under the conditions shown in Table 1.

The strength, oxidation resistance and crystal phase of the sintered body after the heat treatment were identified in the same manner as in Example 1.

 The results are shown in Table 1.

According to Table 1, the grain boundary has a silicon oxynitride phase or a sintered body composed of the silicon oxynitride phase and vitreous material (Samples 8, 19)
Are excellent in oxidation resistance at 1500 ° C, but 1400
Low flexural strength at ℃. Further, the molar ratio of (excess oxygen / rare earth element oxide) is 2 or less, and the grain boundary is glass or YA.
The sintered bodies containing M crystals (Samples 11 and 22) had excellent strength at 1400 ° C, but had poor oxidation resistance at 1500 ° C, and no improvement in oxidation resistance was observed even after heat treatment (Samples 12 and 22). Two
3).

When the temperature of the sintered body in which the silicon oxynitride phase is formed at the grain boundaries is low (samples 9 and 20), the formation of the disilicate phase is not recognized, and the high-temperature strength is hardly improved. . When the processing temperature was too high (samples 10 and 21), other crystal phases such as apatite were observed in addition to the silicon oxynitride phase and the disilicate phase, but in each case the surface of the sintered body was decomposed. And the characteristics were not satisfactory.

On the other hand, all of the samples of the present invention have a silicon oxynitride phase and a disilicate phase precipitated, and have excellent oxidation resistance with an oxide weight increase of 0.12 mg / cm ² or less, and have a characteristic of 500 MPa or more. Showed excellent high-temperature strength. The two crystal phases at the grain boundaries in the sample of the present invention both contain silicon oxynitride as a main component, and the average particle diameter (major axis) of the silicon nitride particles in the sintered body is 10 μm. Or less, and the average aspect ratio is 3
As described above, in particular, a sintered body having a fine structure of 7 μm or less was obtained by the hot isostatic firing method shown in Table 2.

The X-ray diffraction chart of Sample No. 2 is shown in FIG.
Shown in the figure.

(Effect of the Invention) As described in detail above, according to the silicon nitride based sintered body of the present invention, a simple ternary of Si ₃ N—RE ₂ O ₃ (rare earth oxide) —SiO ₂ containing a large amount of excess oxygen In the system, by precipitating a crystal phase of a silicon oxynitride phase and a disilicate phase at a grain boundary, high-temperature bending strength can be improved while maintaining oxidation resistance at a high temperature of 1500 ° C.

Therefore, the application of the silicon nitride-based sintered body as an industrial part, particularly as a part for a heat engine can be further expanded.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction chart of the silicon nitride sintered body of the present invention.

────────────────────────────────────────────────── ─── Continued from the front page Examiner Misako Anzai (56) References JP-A-59-169980 (JP, A) JP-A-62-223066 (JP, A) JP-A-58-88174 (JP, A) 58-88175 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C04B 35/587-35/594

Claims (2)

(57) [Claims] 1. An oxide comprising 70 to 99 mol% of silicon nitride, 0.1 to 5 mol% of a rare earth oxide and 25 mol% or less of excess oxygen (equivalent to SiO ₂ ), expressed as excess oxygen / rare earth oxide. A silicon nitride-based sintered body having a molar ratio of greater than 2 and less than or equal to 25, wherein the crystals at the grain boundaries in the sintered body are silicon oxynitride and disilicate. Silicone sintered body. 2. The silicon nitride-based sintered body according to claim 1, wherein the average particle diameter of the silicon nitride crystal particles of the sintered body is 10 μm or less.