JPH0712980B2 - 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 high mechanical strength and improved 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 sintered bodies has excellent mechanical strength (bending strength) and toughness (critical stress intensity factor K ₁ c). For example, in a boron (B) and carbon (C) addition system, the strength is 40 to 50
Although it has a normal value of Kg / mm ² (4-point bending strength), it has a poor toughness of about ³ MN / m ^3/2 .

(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 high mechanical strength and improved toughness.

That is, the silicon carbide-based sintered body of the present invention is mainly composed of α-SiC and has a grain boundary phase of at least aluminum (A
l), a sintered body composed of an oxide containing yttrium (Y) and silicon (Si), wherein the crystal main body is a fine equiaxed crystal having an average grain size of 10 μm or less in a crystal longitudinal direction and an aspect ratio of 3 or less. As a method for producing such a sintered body, 1 to 7% by weight of aluminum oxide, 0.1 to 5% by weight of yttrium oxide, and 1 to 5 of silicon dioxide are used. % By weight and the balance α-SiC
A powder compact formed by crushing a mixed powder of and having an average particle size of 1.0 μm or less and molding the mixture is fired at 1800 to 1950 ° C. under normal pressure in a non-oxidizing atmosphere. is there.

The silicon carbide-based sintered body of the present invention has high mechanical strength and has a three-point bending strength of 60 kg / mm ² or more at room temperature because the sintered body is mainly composed of α-SiC. Since the crystal grain is mainly composed of fine equiaxed crystals with an average grain size in the longitudinal direction of the crystal of 10 μm or less and an aspect ratio of 3 or less, stress concentration does not occur and therefore bending strength is increased. It is thought to have been done.

Further, 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 5 or more. The reason for this is not clear, but at the grain boundaries. As for, a liquid phase composed of an oxide containing aluminum, yttrium and silicon by the addition of the sintering aid remains. Since cracks selectively extend this grain boundary, energy is consumed by taking branching and zigzag paths. That is, in the present invention, since the aspect ratio is 3 or less and the fine crystal structure is 10 μm or less, the crack extending path becomes a more complicated zigzag path, and the energy consumption increases. As a result, it is considered that the toughness is improved.

Moreover, in the present invention, the firing temperature is considerably low as in the non-pressurized method, but this is considered to be mainly due to the liquid phase forming ability of the contained SiO ₂ , and the average particle diameter of the mixed powder is as fine as 1.0 μm or less. In addition, since the firing temperature can be lowered, grain growth during firing is suppressed. Therefore, it is considered that the main crystal grains in the sintered body are α-SiC composed of fine equiaxed crystals of 10 μm or less.

Next, the components and the range limitation specified in the production method of the present invention will be described.

The weight ratio of the sintering aid component in the starting material in the present invention is set to 1 to 7% by weight of Al ₂ O ₃ because when it is less than 1%, the sintering action is insufficient and a dense sintered body is obtained. If it exceeds 7% by weight, the decomposition of the burned material is so severe that the shape cannot be maintained. The reason why Y ₂ O ₃ is 0.1 to 5% by weight is that if 0.1% is less than 0.1%, sintering does not proceed sufficiently. A dense body is not obtained,
If it is more than%, densification will proceed, but decomposition will be violent and pores will be generated inside the sintered body, which will cause strength deterioration.

Moreover, the reason why the SiO ₂ content is set to 1 to 5% by weight is that if it is more than 5%, a high-strength sintered body in which decomposition during firing is severe and a bore is observed cannot be obtained. %, It is necessary to obtain the sintered body without pressure and at a relatively low temperature as described above.

These sintering aids can be fired at a low temperature by forming a liquid phase having a low melting point during the sintering process. Further, after firing, these liquid phases remain at the grain boundaries of the sintered body as a glassy oxide containing at least aluminum, yttrium and silicon.

Further, in the firing method of the present invention, the compounding ingredients are set in the numerical range as in the gist of the invention because the range is 1800 to
This is because the sintered body of the present invention can be obtained by firing at 1950 ° C. Regarding the firing temperature, sintering does not proceed to a satisfactory degree when performed at a temperature lower than 1800 ° C. As the decomposition becomes more intense and voids occur,
Grain growth of SiC crystals occurs and the above-mentioned fine equiaxed crystals are not generated.

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

Also, the reason why the average particle size of the mixed powder before molding is 1.0 μm or less is that if it is larger than 1 μm, it becomes difficult to sinter at a low temperature, and it is necessary to increase the amount of sintering aid.
This is because decomposition during firing becomes violent, which causes the generation of pores, and the high-strength and high-density body that is the object of the present invention cannot be obtained. The powder having an average particle size of 1.0 μm or less will naturally be 1.0 μm or less after pulverization if powder having an average particle size of 1.0 μm or less is used as a raw material powder for each component. If it exceeds, the crushing time and the like may be appropriately adjusted so that the average particle size becomes 1.0 μm or less by crushing.

Examples Example 1: Silicon carbide (α-SiC, average particle size 0.4 μm) powder, alumina (average particle size 0.6 μm) powder and yttrium oxide (0.6 μm)
m) powder and silicon dioxide (average particle size 0.1 μm) powder are shown in Table 1.
It was compounded as shown in. Regarding silicon dioxide,
Silicon dioxide was externally added so as to have the composition shown in Table 1 including silicon dioxide based on the impurity oxygen that is generally inevitably present in the silicon carbide powder.

This blended powder was subjected to wet mixing powder using an ethanol solvent in a pot mill for 24 hours, and then a molding binder such as polyvinyl alcohol was added to the resulting mixed powder for dry granulation. It was placed in a mold and press-molded 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 at a firing temperature shown in Table 1 to obtain Samples 1 to 19.

The crystal structure of the obtained sample is observed by an electron micrograph, and an equiaxed crystal (aspect ratio of 3 or less) or a columnar crystal (aspect ratio of more than 3) is observed in an average α-SiC crystal form.
And the average grain size in the crystal longitudinal direction was calculated. Further, Table 1 shows the results of measuring the room temperature strength (4-point bending strength), the toughness value (K ₁ c) obtained by the indentation method, and the specific gravity of the sintered body obtained by the Archimedes method.

The data shown in Table 1 represent those within the scope of the present invention and those outside the scope of the present invention, and are outside the scope of the present invention Nos.

As is clear from the data, the sample numbers 7 to 7 of the present invention
Nos. 9 and 13 to 19 are strength (60 kg / mm ² or more) and toughness (MN / m)
^3/2 = 5 or more). However, as can be seen from the data in Table 1, the sample compositions of Sample Nos. 1 to 6 and 10 to 12 had component compositions deviating from the scope of the present invention, and the sintered body had poor strength, toughness or electrical resistivity. In some cases, the sintering is insufficient and the sintered body is decomposed so much that voids are generated.

Example 2: Since SiC has excellent conductivity and is capable of electric discharge machining, precise microfabrication is required. For example, it is known as a material suitable for applications such as an injection nozzle having precise fine holes, but the electric conductivity (electrical resistivity) varies considerably depending on the weight ratio of the additive composition or the conditions such as mixing. Therefore, when the electric resistivity of each of the samples 7 to 9 and 13 to 19 was measured, there were some variations, but all showed a value of 10 Ω · cm or less, and the electric discharge machining was sufficiently possible. I found out.

(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 target sintered body is obtained at a firing temperature of ~ 1950 ° C., and the obtained sintered body has the following excellent properties:
It is unprecedented.

That is, the mechanical strength of JIS 4-point bending strength at room temperature is 60 Kg / m.
m ² or more, and the toughness has a critical stress intensity factor K ₁ c (MN / m
m ^3/2 ) is 5 or more.

Regarding electrical conductivity, it was understood that the electrical resistivity was 10 Ω · cm or less. However, the reason for this was that the general silicon carbide based sintered body using a boron-carbon based sintering aid was 10 ^{5 ~ 6} Ω
There is a marked difference compared to being cm.

Claims (2)

[Claims] 1. A sintered body mainly composed of α-SiC and having a grain boundary phase composed of an oxide containing at least aluminum (A1), yttrium (Y) and silicon (Si), wherein the crystal main body is A silicon carbide-based sintered body characterized by comprising fine equiaxed crystals having an average grain size in the longitudinal direction of the crystal of 10 μm or less and an aspect ratio of 3 or less. 2. Aluminum oxide of 1 to 7% by weight, yttrium oxide of 0.1 to 5% by weight, and silicon dioxide of 1 to 5% by weight.
A powder compact obtained by crushing a mixed powder composed of wt% and the balance of α-SiC to an average particle size of 1.0 μm or less and molding the mixture at 1800 to 1950 ° C under normal pressure in a non-oxidizing atmosphere. A method for producing a silicon carbide-based sintered body, which comprises firing.