JPS6128626B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a sintered body of silicon carbide, and particularly to a sintered body of high strength silicon carbide.

Silicon carbide has high strength at high temperatures, is a chemically extremely stable material, and has characteristics such as excellent corrosion resistance and oxidation resistance. Due to these characteristics of silicon carbide, its application as a high-temperature structural material is attracting attention and being studied. Along with this, there is a desire to establish a method for manufacturing high-strength sintered bodies.

Conventionally, hot pressing methods have been mainly studied in order to obtain high-strength silicon carbide. As a result, a silicon carbide sintered body having a room temperature bending strength of 10 Kg/mm ² to 100 Kg/mm ² has been obtained by adding a metal or a metal compound to silicon carbide powder and hot pressing at around 2000°C.

Silicon carbide is a very hard material that exhibits little creep and is prone to brittle fracture. If there is a defect on the surface of the material, stress will be concentrated in the defective area, and the material will break with a much lower force than expected. This tendency is particularly seen in high-strength silicon carbide sintered bodies. This is simply due to sufficient sintering, which strengthens the bonds between silicon carbide particles and makes surface defects more noticeable. As a countermeasure for this, 1μ
It is necessary to completely eliminate deep and sharp surface scratches of m or more.

Mirror polishing is a conventional method, but it is difficult to remove deep scratches by polishing. It is even more difficult for objects with complex shapes.

An object of the present invention is to easily eliminate surface defects in silicon carbide sintered bodies.

The present invention aims to eliminate defects such as bite of jigs during sintering and scratches caused by machining that occur on the surface of a silicon carbide sintered body by coating it with an equivalent material, thereby improving mechanical strength with good reproducibility. .

The present invention can be easily realized by the method described below.

The first is a gas phase reaction method. A silicon carbide sintered body that has been sintered or machined after sintering is placed in a reaction vessel, and a raw material gas is introduced into the vessel and reacted to adhere and coat the sintered body. A feature of this method is that it is possible to produce a uniform film.

The second method is to attach an organic silane compound to a sintered body by coating, impregnating, etc., and then firing it. Examples of organic silane compounds include ethyl silicate and polycarboxysilane. A feature of this method is that a film of sufficient thickness can be easily obtained.

Furthermore, the same effect can be obtained by coating the sintered body with a substance that decomposes at high temperatures to produce silicon or silicon carbide, such as various silicone binders, silicone resins, or organic silane compounds, and heating it in a hydrocarbon atmosphere. is obtained.

The method of attaching fine silicon carbide powder to a slurry and firing it requires a high temperature of 1800° C. or higher for firing, and the surface of the product has large irregularities, so that the desired effect of the present invention cannot be obtained.

Further, the silicon carbide sintered body to which the present invention is applied is required to have a certain degree of high density. Even if the present invention is applied to a sintered body which is incompletely sintered and whose relative density does not reach 80%, the effects of the present invention cannot be obtained. The present invention is effective in eliminating surface defects.

The details of the present invention will be explained below using specific examples.

Example 1 2% by weight of aluminum nitride powder with an average particle size of 2 μm was added to silicon carbide powder with an average particle size of 2 μm, and 10% by weight of silicone varnish was added,
The mixture was kneaded for 1 hour using a milling machine to obtain a silicon carbide powder composition. When the above-mentioned silicone varnish is thermally decomposed, it is almost equivalent to adding 2% by weight of silicon. The above silicon carbide powder composition was heated at 1×10 ^-4 torr.
1 in the following vacuum at 2000℃ and 200Kgf/cm ²
By hot pressing for a period of time, a sintered body having a relative density of 98% of the theoretical density of silicon carbide was obtained. This sintered body was cut into a rectangular parallelepiped of 2 mm x 2 mm x 50 mm to prepare a sample (A) for strength evaluation.
Next, one of the 2 mm x 50 mm surfaces was left with the sintered surface, and the other three surfaces were finished with a mirror finish.
(B) was prepared, tensile stress was applied to the surface as it was at the time of sintering, and the bending strength was measured, and it was 80 to 105 Kgf/mm ² .

Next, mirror finish the 3 sides in the same way as above, and
The bending strength of the sample (C), which had an uneven surface of about 10 μm formed by polishing, was measured by applying tensile stress to the uneven surface, and it was 25 to 35 kg.
It was f/ ^mm2 . As is clear from this, the strength of the silicon carbide sintered body depends on the degree of surface finish. In other words, since it is affected by scratches and defects on the surface, in order to obtain a reliable sintered body, it is desirable that there be no defects on the surface.

When all four sides of the sample (A) were mirror-finished and the bending strength was measured in the same manner as above, it was 72 to 103 Kgf/mm ² . The maximum value is for the sample with an as-sintered surface (B)
It is almost the same as in the case of , but the minimum value is lower.
This is thought to be due to the fact that, depending on the sample, non-negligible micro defects were formed by polishing.

The sample (A) was placed in a reaction tube and fixed on a graphite jig. Carbon tetrachloride and silicon tetrachloride were introduced into a reaction tube using hydrogen gas as a carrier. Synthesize silicon carbide by keeping it at 1300℃~1350℃ using high frequency heating.
It was deposited on the sample to a thickness of about 20 μm. In this case, the reaction rate decreases significantly below 1300℃,
At 1200°C, the yield was poor and it was not practical. Also
When the temperature exceeds 13,500°C, crystal growth tends to accelerate and the surface roughness tends to increase.

Next, the surface on which silicon carbide was deposited was removed, and the other three surfaces were mirror-polished to obtain a sample (D). Regarding this, the bending strength was measured by applying tensile stress to the silicon carbide precipitation surface, and the result was 101 to 113 Kgf/mm ² .

Example 2 Sample (A) of Example 1 was placed in a reaction tube and fixed on a graphite jig, and monosilane and carbon dioxide were introduced.
The sample was kept at approximately 1500°C by high-frequency heating, and silicon carbide was synthesized from monosilane and carbon dioxide and deposited on the sample.

Next, the surface other than the surface on which silicon carbide was precipitated was polished in the same manner as sample (D), and the bending strength was measured by applying tensile stress to the surface on which silicon carbide was precipitated. The result is 98~
It was 114Kgf/ ^mm2 .

Example 3 Ethyl silicate was applied to the as-fired surface of sample (B) of Example 1, and the sample was gradually heated in a reducing atmosphere and held at a maximum temperature of 1500°C for 30 minutes. Thereafter, it was heated at 900° C. in an oxygen-added atmosphere to oxidize and remove the residual carbon exposed on the surface, producing sample (E). The bending strength was measured by applying tensile stress to the ethyl silicate coated surface. The result was 89-102Kgf/ ^mm2 .

Example 4 Polycarboxysilane (molecular weight approximately 1500) dissolved in petroleum ether was coated on the as-fired surface of sample (B) of Example 1, and was fired and measured in the same manner as in Example 3. As a result, a strength of 89 to 105 Kgf/mm ² was obtained.

Example 5 When silicon carbide produced from carbon tetrachloride, silicon tetrachloride, and hydrogen is precipitated on sample (C), its strength is 99~
It became 110Kgf/mm ² .

Example 6 When silicon carbide produced from monosilane and carbon dioxide was deposited on sample (C), the strength was 89 to 113 Kgf/mm ² .

Example 7 When sample (C) was coated with ethyl silicate and baked, the strength was 92 to 107 Kgf/mm ² .

Example 8 When sample (C) was impregnated with ethyl silicate and baked, the strength was 94 to 113 Kgf/mm ² .

Example 9 When sample (C) was coated with polycarbosilane and baked, the strength was 82 to 109 Kgf/mm ² .

As is clear from the above examples, when the surface of the silicon carbide sintered body is coated with silicon carbide, stress concentration on surface defects is alleviated and the strength is improved. Furthermore, it is effective for recovering strength loss after machining. Furthermore, since a coating of several micrometers to several tens of micrometers is effective, the accuracy after processing can be maintained.

Almost the same effect was obtained in an experiment using a hot-pressed silicon carbide sintered body containing boron oxide.

Claims (1)

[Claims] 1. A silicon carbide sintered body, characterized in that the silicon carbide sintered body has, on its surface, one of a silicon carbide film formed by a gas phase reaction method and a silicon carbide film formed by firing an organic silicon compound.