JPH0737340B2 - Silicon carbide sintered body and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicon carbide based sintered body having improved oxidation resistance and toughness, and a method for producing the same.

(Prior Art and Problems) Silicon carbide sintered bodies have various excellent properties such as oxidation resistance, corrosion resistance, heat resistance, thermal shock resistance, and high temperature strength. Therefore, high temperature gas turbine parts, automobile engines, etc. It is a suitable material for parts and corrosion resistant members.

However, none of the conventionally provided silicon carbide based sintered bodies has excellent properties in oxidation resistance and toughness. For example, boron (B) and carbon (C) -added systems have excellent toughness of about ³ MN / m ^3/2 even though they have excellent oxidation resistance of about 0.1.
And low.

(Means for Solving the Problem) As a result of research in view of the above points, the present inventor has developed a silicon carbide sintered body having both excellent oxidation resistance and toughness.

That is, in the present invention, a SiC crystal is mainly used, and 1 to 4 wt% of WSi ₂ is contained in a sintered body containing a compound of aluminum (Al) and yttrium (Y) in its grain boundary phase, and the theoretical density ratio is silicon carbide sintered body, characterized in that at least 93%, and Al ₂ O ₃ 1 to 7 wt%, Y ₂ O ₃ 0.1 to 5 wt%, in terms of one or more selected from W compound W alone do it
A method for producing a silicon carbide-based sintered body, which comprises firing a powder compact comprising 0.5 to 3% by weight and the balance SiC at a temperature of 1800 to 1950 ° C in a non-oxidizing atmosphere.

The conversion formula for the simple substance W from the W compound in the above is
The following decoy.

The silicon carbide-based sintered body of the present invention has improved oxidation resistance, and the oxidation weight gain at 1400 ° C. is 0.4 mg / cm ² or less,
The reason is considered as follows.

That is, a liquid phase containing Al and silicate glass as a main component is generated in the grain boundary phase in the process of adding oxides of Al ₂ O ₃ and Y ₂ O _{3 to} SiC and firing, and sintering proceeds, The decomposition and volatilization of this liquid phase proceed, and the metal containing Al and Y and / or their carbides generated by being reduced by SiC are generated together with the oxide liquid phase existing in the grain boundaries of the sintered body.

When the sintered body of the present invention in which the particles of WSi ₂ are dispersed in addition to the above structure is exposed to a high-temperature oxidizing atmosphere, initially, although SiC and grain boundary phase components are oxidized, WSi ₂ is also oxidized. By doing so, a dense protective film is formed on the surface by the oxide of W and silicate, and as the temperature further increases, the volatile acid of the oxide of W progresses, and finally a silica film of high purity remains.

In this way, it can be considered that the surface of the sintered body is covered with the silica film, and as a result, the diffusion rate of oxygen ions is reduced and the oxidation of the sintered body is suppressed.

The sintered body of the present invention has improved toughness and has a critical stress intensity factor K ₁ c (MN / m ^3/2 ) of 6 or more because the thermal expansion of WSi ₂ is higher than that of SiC. The coefficient is relatively large, so it is considered that minute cracks are generated at the interface of WSi ₂ particles in the sintered body, and therefore the mechanism by which the tip of the crack progresses is absorbed by the minute cracks and the stress is dissolved. It is supposed to be.

Further, the production amount of WSi _{2 in} the sintered body produced by the above production method is considered as follows.

That is, the W compound becomes WSi ₂ when mixed with SiC and baked. However, at the same time tungsten oxide (WO ₃ )
It also volatilizes during firing because it also forms. On the other hand, A to SiC
When the oxides of l ₂ O ₃ and Y ₂ O ₃ are added, the liquid phase is decomposed during firing and the weight is reduced. From these facts, the amount of WSi ₂ in the sintered body is slightly higher than the weight ratio at the time of addition.

Further, in the present invention, a sintered body is manufactured at a firing temperature of 1800 ° C to 1950 ° C. This temperature is lower than the firing temperature of the conventional silicon carbide-based sintered body, but the reason for making this possible is that the main component is the silicate glass containing Al mentioned in the section of the reason for improving the oxidation resistance. It is considered that this is due to the formation of a liquid phase that forms and the effect of WSi ₂ .

In the present invention, the weight ratio of each composition is such that Al ₂ O ₃ is 1 to 7% by volume.
Insufficient less the sintering action than 1% had a a no dense sintered body is obtained, and a decomposition of the calcined product exceeds 7 wt% can not be maintained is intense shape, Y ₂ O ₃ is 0.1-5
When the content is less than 0.1%, the sintering does not proceed and a sufficiently dense body cannot be obtained, and when the content is more than 5%, the oxidation resistance of the sintered body deteriorates. That is, it is not possible to withstand use at high temperature due to a large amount of increased oxidation.

In addition, the reason why WSi ₂ is 1 to 4.0% by weight (0.5 to 3% by weight in the sintered body) in the sintered body is that if it is less than 1%, the increase in oxidation is increased, and if it is more than 4.0%, the sintered body becomes dense. However, oxidation resistance and toughness are extremely deteriorated.

Furthermore, in the firing method of the present invention, those having the above composition range
The sintered body of the present invention is obtained by firing at 1800 to 1950 ° C.

Regarding the firing temperature, if the firing temperature is lower than 1800 ° C, the sintering does not proceed to a satisfactory degree, and if the firing temperature exceeds 1950 ° C, decomposition of the fired product becomes severe and voids occur.

Note that firing in a non-oxidizing atmosphere is because firing in an oxidizing atmosphere oxidizes SiC and produces a large amount of SiO ₂ .

Examples Example 1: Silicon carbide (α-SiC, average particle size 0.4μ) powder, alumina (average particle size 0.6μ) powder and yttrium oxide (0.6μ)
Powder, and compound of tungsten (W) (WC, WN, WSi ₂ )
The powders (average particle size 0.8 to 3μ) are shown in Table 1 (Sample No. 1)
~ 17, 19, 20).

This blended powder was wet mixed in a pot mill for 24 hours using an ethanol solvent, and then a molding binder such as polyvinyl alcohol was added to the obtained mixed powder for dry granulation, and the mixture was used as a raw material in a mold. After entering, press molding was performed at a molding pressure of 1 t / cm ² to obtain a molded body.

The molded body was subjected to binder removal treatment and then pressureless firing in an argon gas atmosphere under the firing conditions shown in Table 1.

The theoretical density ratio of the obtained sintered body, the oxidation resistance (oxidization increase at 1400 ° C.), and the toughness value (K ₁ c) obtained by the indentation method were measured and the results are shown in Table 1 as Nos. 1 to 20. It is as follows.

Example 2: Silicon carbide (β-SiC average particle size 0.3μ) powder, alumina (average particle size 0.6μ) powder, yttrium oxide (0.6μ) powder, and tungsten (W) compound (WSi ₂ ) (average particle size) The powder was blended as shown in Table 1 (Sample Nos. 21 to 22).

This blended powder was wet mixed in a pot mill for 24 hours using an ethanol solvent, and then a molding binder such as polyvinyl alcohol was added to the obtained mixed powder for dry granulation, and the mixture was used as a raw material in a mold. A molded body was obtained by press molding with a putting pressure of 1 t / cm ² . The demolded body was subjected to pressureless firing under the firing conditions shown in Table 1 in an argon gas atmosphere.

The theoretical density ratio of the obtained sintered body, the oxidation resistance (oxidization increase at 1400 ° C) and the toughness value (K ₁ c) obtained by the indentation method are shown in Table 1 as No. 21 and 22. It is as follows.

As is clear from the data shown in Table 1, the sample Nos. 7-17 and 21,22 of the present invention showed little increase in oxidation at 1400 ° C. (0.4 mg / cm ² or less) and excellent toughness (MN / m ^{3 / 2} = 6 or more)
ing.

However, as can be seen from the data in Table 1, the samples Nos. 1 to 6 have component compositions that deviate from the scope of the present invention, and the density, high temperature oxidation resistance, toughness or electrical resistivity of the sintered body is bad. Further, in Sample Nos. 19 and 20, the firing temperature was out of the range of the present invention, the sintering was insufficient, and the sintered body was decomposed severely and voids were generated.

The firing in the above case is all performed by pressureless firing, but even if hot press firing is adopted, almost the same result can be obtained.

Incidentally, since SiC is excellent in conductivity and capable of discharge pressurization, precise fine processing is required, for example, it is known as a material suitable for use as an injection nozzle having precise fine holes, A considerable difference occurs in the electrical conductivity (electrical specific resistance) depending on the weight ratio of the added composition or the conditions such as mixing. Therefore, when the electrical resistivity of each of Samples 7 to 8, 21 and 22 was measured, there were some variations in each, but all were 10 Ω.
・ Since it is less than or equal to cm, it has been found that there is a sufficient possibility that electric discharge machining can be performed.

(Effect of the Invention) As described above, according to the present invention, a temperature of 1800, which is relatively low as compared with the conventional example,
The desired sintered body is obtained by firing at ℃ to 1950 ° C, and the obtained sintered body has the following excellent properties, which is unprecedented.

That is, the oxidation resistance is 0.4 mg / cm ² as an increase in oxidation at 1400 ° C.
And the toughness is the critical stress intensity factor K ₁ c (MN / m ^3/2 ).
Is 6 or more.

Regarding conductivity, it was understood that the electrical resistivity was 10 Ω · cm or less, but, by the way, that of a general silicon carbide sintered body using a boron-carbon sintering aid was 10 ⁵ to 10 ⁶ Ω
・ Compared to being cm, there is a marked difference.

Claims (2)

[Claims] 1. A sintered body mainly composed of SiC crystal and containing a compound of aluminum (Al) and yttrium (Y) in its grain boundary phase contains 1 to 4% by weight of WSi _{2 and} has a theoretical density ratio. A silicon carbide based sintered body characterized by being 93% or more. 2. Al ₂ O ₃ 1 to 7% by weight, Y ₂ O ₃ 0.1 to 5% by weight,
0.5 or more converted to W simple substance from one or more selected from W compounds
A method for manufacturing a silicon carbide based sintered body, which comprises firing a powder compact comprising 3 to 3% by weight and the balance SiC at a temperature of 1800 to 1950 ° C in a non-oxidizing atmosphere.