JP2691294B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicon nitride sintered body excellent in bending strength and oxidation resistance at high temperatures and a method for producing the same.

(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.

Since silicon nitride itself is covalently bonded and therefore difficult to sinter, rare earth element oxides, alkaline earth metal oxides, Al ₂ O ₃ and various other sintering aids are added and molded, and It is obtained by firing at a temperature of 1600 to 2000 ° C. by a firing means such as pressure firing, hot press firing, gas pressure firing, hot isostatic firing.

According to such a silicon nitride sintered body, the grain boundary phase of the sintered body has attracted attention as a factor that determines the high temperature characteristics, and attempts have been made to improve the strength of the grain boundary phase. Therefore, recently, by subjecting the sintered body to a heat treatment or the like, the grain boundaries are changed to silicon nitride (Si ₃ N ₄ ), rare earth element oxide (RE ₂
Various crystal phases such as O ₃ ) and SiO ₂ such as melilite, apatite, YAM and wollastonite have been deposited.

[Problems to be solved by the invention]

However, although there is some improvement in high-temperature strength by crystallizing the grain boundaries, it is difficult to stably generate a desired crystal phase because the composition of the grain boundaries changes depending on various conditions, and thus it is generated. Since the characteristics differ depending on the type of crystal phase, the stability is poor and it is difficult to crystallize all the grain boundaries.In addition to the crystal phase, a glass phase with a low melting point is generated. Therefore, there is a problem that desired high temperature characteristics cannot be obtained.

[Object of the invention]

It is an object of the present invention to solve the above problems and provide a silicon nitride sintered body having excellent stability and excellent high temperature characteristics, and a method for producing the same.

[Means for solving the problem]

The present invention, with respect to the above problems, as a result of repeated studies, SiO ₂ was excessively added as an additive component, and this was baked at a low temperature,
By rapidly cooling the grain boundaries to a high melting point glass phase composed of silicon, a rare earth element, oxygen, and nitrogen, it is possible to form a fine crystal structure and to form the grain boundary phase from a uniform structure. It was found that the high temperature strength and the oxidation resistance are improved while the stability is excellent.

That is, the present invention uses silicon nitride (hereinafter referred to as SiN ₄ ) 70 to 99
Mol% rare earth oxide 0.1 to 5 mol% and excess oxygen 25
From mol% or less to excess oxygen / rare earth oxide (molar ratio)
Is a sintered body having a composition range of more than 2 and 25 or less, and the Si ₃ N ₄ crystal has an average particle size of 7 μm or less and its grain boundary phase is composed of silicon, a rare earth element, oxygen and nitrogen. It is made into an amorphous body, and as its manufacturing method, a gas-impermeable seal is provided on the surface of the molded body having the above composition, and after firing at a temperature of 1400 to 1800 ° C under high pressure, 1000゜ / h
It is obtained by quenching at a rate of r or higher.

 Hereinafter, the present invention will be described in detail.

A major object of the present invention is to form a grain boundary as a uniform single phase in order to eliminate the conventional non-uniformity of the grain boundary phase caused by crystallization, and to improve the grain boundary by making it amorphous. It is intended to increase the viscosity.

As a basic composition for this, Si ₃ N ₄ is 70 to 99 mol%, especially 80 to 93.5 mol%, and a rare earth element oxide 0.1 to 5
Mol%, especially 0.5 to 4 mol%, and a proportion of less than 25 mol% excess oxygen, especially 6 to 20 mol%. It should be noted that the excess oxygen is the amount of oxygen excluding the oxygen mixed in as a rare earth element oxide from the total amount of oxygen contained in the entire system, specifically, Si ₃ N
₄ oxygen impurity in the raw material, or is intended to be composed of added oxygen as SiO _2, in the present invention shows the SiO ₂ equivalent amount. A major feature of the composition of the present invention is that the excess oxygen / rare earth element oxide (molar ratio) is more than 2 and is 25 or less, particularly 3 to 20, that is,
This means that there is a large amount of excess oxygen, which is an important factor in forming the grain boundaries as amorphous. The reason for limiting this molar ratio to the above range is that if the molar ratio is 2 or less, crystals tend to precipitate at the grain boundaries, lacking stability, and the object of the present invention cannot be achieved. This is because the glass having the melting point is easily formed, and the melting point of the grain boundary is lowered too much, so that the high temperature strength of the sintered body is lowered. Further, in the above composition range,
Even if Si ₃ N _{4 or} a rare earth element oxide deviates from the above range, the strength decreases.

In the grain boundary of the sintered body of the present invention, the rare earth element oxygen and excess oxygen as additive components, and Si ₃ N ₄ are partially solid-dissolved, and thus substantially consist of silicon, rare earth element, oxygen, and nitrogen. Since it does not have amorphous precipitation as in the conventional case, a uniform grain boundary is formed.

One of the specific judgments of such amorphization of the grain boundary phase is whether or not there is a silicon oxynitride phase (Si ₂ N ₂ O phase), which is the crystal phase most likely to precipitate in the composition system described above. It can be evaluated, and in this case, Si ₂ N in the X-ray diffraction chart
₂ O (020) plane peak intensity / (α-Si ₃ N ₄ (210) plane peak intensity + β-Si ₃ N ₄ (210) plane peak intensity) x 100 (%) at a level of 30% or less It is said that those in which the precipitation of the Si ₂ N ₂ O phase is suppressed have the grain boundaries substantially amorphized.

According to the present invention, in order to raise the melting point of this grain boundary, it is desirable to use an element capable of forming a high melting point silicate as a rare earth element. Such elements include Yb, Er, D
y, Ho can be mentioned.

Further, according to the sintered body of the present invention, it is important that the Si ₃ N ₄ crystal grains have an average grain size of 7 μm or less, and particularly 5 μm or less. By forming such a fine crystal structure structure, The mechanical strength can be improved.

According to the method for producing a silicon nitride-based sintered body of the present invention, silicon nitride powder, rare earth element oxide powder, and optionally SiO ₂ powder are used as the raw material powder, but silicon nitride powder is used to promote sinterability. It is desirable that the BET specific surface area is 3 to ²⁰ m ² / g and the α conversion rate is 95% or more.

70-99 mol% of silicon nitride, especially 80
~ 93.5 mol%, rare earth oxide 0.1 ~ 5 mol%, especially
0.5 to 4 mol%, excess oxygen (equivalent to SiO ₂ ) 25 mol% or less, especially 6 to 20 mol% composition and excess oxygen /
The rare earth element oxide (molar ratio) is prepared and mixed so as to be more than 2 and 25 or less, particularly in the range of 3 to 20.

A known molding method of the mixed powder thus obtained,
For example, it is formed into a desired shape by press molding, cast molding, extrusion molding, ink injection molding, etc., and then transferred to firing.

According to the present invention, since the above composition contains a large amount of a component that is easily volatilized by firing, that is, excess oxygen, it is necessary to select a firing method that sufficiently suppresses such vaporization.

Therefore, according to the present invention, firing is performed under a high pressure gas in a state where the surface of the molded body is completely sealed with a gas impermeable seal made of glass or the like. According to this method, the composition of the molded body does not change due to the presence of the sealing material between the firing atmosphere and the molded body, and the composition of the molded body is almost the same as that of the sintered body. It also has the advantage of being excellent in sex. Specifically, if desired, a mold release agent such as BN is applied to the surface of the molded body, and then glass is further applied. The compact is placed in, for example, a hot isostatic firing furnace to raise the temperature, and after the glass seal on the surface of the compact is completed, the temperature is further raised and pressure is applied by a gas such as N ₂ or argon. Final firing temperature 1450-1800
Hold at a pressure of 500-2000 atm at a low temperature of ℃, especially 1500-1750 ℃. After firing, cooling rate is 1000 ℃ / hr or more, especially 1100 ℃ /
The grain boundaries are made amorphous by setting the time to be more than hr.

In addition, as another method, a molded article coated with a release agent or the like may be dipped in a glass bath and fired by the same method, pressure and temperature settings as described above.

According to the present invention, under the above conditions, the cooling rate is particularly important for making the grain boundaries amorphous, and the cooling rate is 1000 ° C / hr.
If it is below the range, the crystal phase tends to precipitate at the grain boundary, and the object of the present invention cannot be achieved.

Incidentally, when employing the sintering method as described above, generally enters the glass component in the green body, it affects the properties of the sintered body in question, the composition of the present invention is SiO ₂
Since it contains a large amount of, it also has a peculiar property that the characteristics of the sintered body are hardly affected even if the glass component invades. This has the advantage that almost no various devices for preventing glass intrusion are required.

Furthermore, the feature of the production method of the present invention is that since the entire system is easily sinterable due to containing a large amount of excess oxygen in the composition, the firing time can be shortened, and 1450 to The point is that it can be fired at a relatively low temperature of 1800 ° C. According to such low temperature firing, α
Since grain growth accompanying the transition of -Si ₃ N ₄ to β-Si ₃ N ₄ is suppressed, a fine structure can be obtained as a sintered body structure,
Strengthening is achieved by setting the average particle size of the Si ₃ N ₄ crystal to 7 μm or less. At the same time, because the firing temperature is low, the transition of α-Si ₃ N ₄ to β-Si ₃ N ₄ is less likely to occur, so that the proportion of α-Si ₃ N ₄ in the sintered body is α-Si ₃ N _{4. 3} N ₄
It is also a great feature that the ratio of / α-Si ₃ N ₄ + β-Si ₃ N ₄ is 5% or more, and the room temperature strength can be increased by increasing α-Si ₃ N ₄ .

Further, the silicon nitride sintered body according to the present invention has the characteristics as described above. For the purpose of improving the toughness of the sintered body, for example, an inorganic fibrous structure (whisker) is sintered. It can also be dispersed throughout the body.

Specifically, silicon carbide whiskers or silicon nitride whiskers having a length of 20 μm or less are uniformly dispersed at a ratio of 5 to 30% by volume, and the toughness can be improved by the whisker extraction effect or the deflection of cracks by the whiskers.

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

〔Example〕

Silicon nitride powder as raw material powder (BET specific surface area 5 m ² / g, α
(95% conversion rate, oxygen content 1.0% by weight) and various rare earth oxide powders or SiO ₂ powders are mixed and mixed to have a composition shown in Table 1, and then press-molded at 1 t / cm ^2. It was calcined at 1400 ° C.

After applying the BN powder with a thickness of 1 to 10 mm to the obtained molded body, and further applying the glass with a thickness of 1 to 10 mm,
The impurities were removed by heat treatment at 1350 ° C. under reduced pressure.

The compact thus treated is placed in a hot isostatic firing furnace to raise the temperature up to the softening temperature of the glass under 1 atmosphere of N ₂ gas atmosphere, and thereafter, the temperature is raised and raised to a predetermined value shown in Table 1. Hot isostatic firing was performed under the conditions.

After that, the sintered body obtained by removing the glass on the surface was subjected to JIS R1601 at room temperature, 1200 ° C, 1400 ° C, four-point bending bending strength, and 1000 ° C × 24 hours, and 1400 ° C for 24 hours. I checked.

Further, in the X-ray diffraction curve, the amorphization of the grain boundary phase was observed in the Si ₂ N ₂ O (020) plane peak / (α-Si ₃ N ₄ (21
The strength ratio represented by (0) plane peak) × 100 (%) was judged, and those having a value of 30 (%) or less were determined to be the products of the present invention. Furthermore, from the content of α-Si ₃ N ₄ and the SEM photograph, Si ₃ N 4
₄ The average particle size of the crystal particles was measured.

As is clear from Table 1, in Nos. 10 and 16 having an excess oxygen / rare earth element oxide ratio of 2 or less, the room temperature strength is low and the strength at 1400 ° C. is also low. On the other hand, if this ratio exceeds 25 N
In o.6, a large amount of Si ₂ N ₂ O phase precipitation is observed, and the room temperature strength is high, but the high temperature strength is poor.

Further, in Nos. 12, 13, and 17 where the HIP firing temperature exceeded 1800 ° C, the average grain size was large due to the grain growth of Si ₃ N ₄ , and therefore, the bending strength was low in all cases.

Further, in Nos. 11 and 18 in which the cooling rate was slower than 1000 ° C./hr, the apatite crystal phase was precipitated in both cases, and the object of the present invention was not achieved.

In contrast to these comparative examples, the other invention products have room temperature strength of 10
50MPa or more, 1200 ℃ strength 800MPa or more, 1400 ℃ strength 600MPa
The above was achieved, and the increase in the weight of oxidation was 0.1 mg / cm ² at 1000 ° C.
Below, it showed excellent characteristics of 0.1 mg / cm ² or less at 1400 ° C.
Further, as a result of TEM analysis of grain boundaries for these products of the present invention, silicon, rare earth elements, oxygen, and nitrogen were all detected.

〔The invention's effect〕

As described in detail above, the silicon nitride sintered body of the present invention has a fine grain structure structure and grain boundaries are amorphous, so that the grain boundaries are formed uniformly and have a high melting point. It is possible to obtain a sintered body that is excellent in high temperature strength and oxidation resistance.

Claims (2)

(57) [Claims] 1. Silicon nitride 70 to 99 mol%, rare earth element oxide 0.1 to 5 mol% and excess oxygen 25 mol% or less, and excess oxygen / rare earth element oxide (molar ratio) is larger than 2,
A silicon nitride sintered body in the range of 25 or less, wherein the average particle diameter of the silicon nitride crystals of the sintered body is 7 μm or less and the grain boundary phase is substantially composed of silicon, a rare earth element, oxygen and nitrogen. And a silicon nitride sintered body. 2. 70 to 99 mol% of silicon nitride, a rare earth element oxide
0.1-5 mol% and excess oxygen (SiO ₂ ) 25 mol% or less, and excess oxygen / rare earth element oxide (molar ratio) of more than 2, and the composition of the composition within the range of 25 or less does not cause gas on the surface. Providing a permeable seal, firing at a temperature of 1450 to 1800 ℃ under high pressure gas, cooling at a rate of 1000 ℃ / hr or more, the grain boundary is substantially composed of silicon, rare earth elements, oxygen, nitrogen A method of manufacturing a silicon nitride sintered body, which is characterized by being made amorphous.