JP2000109367A - Heat treatment method for silicon carbide sintered body and heat-treated silicon carbide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the strength and low toughness of a silicon carbide sintered body by a simple and industrial method. SOLUTION: In this heat treatment method, a dense base material of the silicon carbide sintered body which is produced by an atmospheric sintering method and has a relative density of >=90% to the theoretical density, is heat- treated under an oxidative atmosphere at a temp. of >=1,000 to <1,500 deg.C to form silicon oxide having a thickness of 0.5-50 μm (preferably, 10-50 μm) on the surface of the base material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heat treatment method for improving the strength and toughness of a silicon carbide sintered body, and a silicon carbide sintered body manufactured by the method.

[0002]

2. Description of the Related Art Sintered silicon carbide has heat resistance, oxidation resistance,
It is a material excellent in corrosion resistance, abrasion resistance, electrical characteristics, and the like, and is currently used in various fields as a structural material, a heating element, and the like.

In normal pressure sintering of silicon carbide, B ₄ C and C
BC-containing additive system such as Al, Al ₂ O ₃ , AlN
Y ₂ O ₃ , Ce ₂ O, etc. are used as sintering aids such as Al, Be, BeO, etc., and B, II, Be, etc.
₃ , various sintering aids such as a system to which a rare earth element such as Er ₂ O ₃ is added have been developed.

A silicon carbide sintered body using B and C as sintering aids has excellent properties such as light weight, high hardness, and the bending strength does not decrease even at room temperature to a high temperature of 1400 ° C. Therefore, it is widely used not only at room temperature but also in a heating atmosphere.

[0005] A silicon carbide sintered body using an Al-based sintering aid has a higher bending strength at room temperature than a sintered body produced using boron and carbon as sintering aids, and sinters at a lower temperature. There are advantages which are possible.

[0006] The following is a brief description of the conventional example, classified into B + C-based and Al-based + additives.

[B and C systems] JP-A-50-78609 (β-SiC + B, C) JP-A-51-148712 (α-SiC + B, C) JP-A-52-6716 (β-SiC + α-SiC +)
B, C) JP-A-55-116664 (α-SiCorβ-Si
C + B, C) [Al-based + additive] JP-A-53-127512 (AlB ₂ + C; hot press) JP-A-55-20269 (0.15 <B / Al <2, 0.15-0.
JP-A-55-136174 (Solubilized Al + C) JP-A-57-156377 (Al-containing nitride) JP-A-58-140374 (B compound + C + Al
N) JP-A-60-33262 (B compound + C + Al compound: liquid) JP-A-60-81065 (B compound + C + Al compound: colloidal) JP-A-61-26566 (Al compound + Be, B, Al, lV
a-Vla At least one of the elements containing)

[0008]

The silicon carbide produced by the normal pressure sintering method using B and C as sintering aids has a higher density of sintered bodies than those using other sintering aid systems. Although it is easy to obtain, it generally has low toughness and is not suitable for applications in which impact is applied.

The Al-based silicon carbide sintered body obtained by the normal-pressure sintering method has higher toughness than the B- and C-based sintered bodies, but has a low bending strength at high temperatures and is not preferable as a high-temperature structural member. In addition, since sintering involves a liquid phase, there is a problem that it is difficult to control the temperature and atmosphere during sintering.

[0010] Further, silicon carbide obtained by adding a variety of additives to an Al-based compound by a normal pressure sintering method has also been proposed. For example, JP-A-60-33262, JP-A-60-81065
issue. However, in these conventional examples, various types of Al in a liquid state are used.
Since the compound is added, there is a disadvantage that it cannot be manufactured at low cost. Furthermore, since a large amount of an Al compound (up to 8% by mass or 15% by mass in terms of Al) and an organic carbon compound (up to 15% by mass of the residual carbon after pyrolysis) are added, the mechanical properties of B and C are reduced. There are also drawbacks such as a significant reduction compared to the case of using a sintering aid.

A Be-based silicon carbide sintered body obtained by a normal pressure sintering method is difficult to obtain a high-density sintered body as compared with B- and C-based sintered bodies, and therefore has poor mechanical properties. Toxicity is a problem, and handling is a problem. Further, since the electrical resistivity is 10 ⁹ to 10 ¹³ Ω · cm, which is higher than those of the B and C systems, charging is a major problem for the sliding member.

An object of the present invention is to significantly improve the strength and toughness of a silicon carbide sintered body.

[0013]

The present invention is based on the premise that it is manufactured by a normal pressure sintering method. Then, a dense silicon carbide sintered body having a density of 90% or more with respect to the theoretical density is used as a base material. 1000 in an oxidizing atmosphere
The heat treatment is performed at a temperature not lower than 1500 ° C. Thereby, silicon oxide having a thickness of 0.5 μm or more and 50 μm or less is generated on the surface of the base material.

[0014] The silicon carbide sintered body preferably has a bulk density of 3.0 g / cm ³ or more and a three-point bending strength at room temperature of 370 MPa or more. Then, the center line average roughness (Ra) of the surface of the silicon carbide sintered body is less than 10 μm.

The density of the substrate may be 90% or more with respect to the theoretical density, but is preferably 95% or more.

The heat-treated silicon carbide body produced by the above-described method has a surface of 0.5 μm or more and less than 50 μm, preferably 1 μm or less.
It has a silicon oxide film having a thickness of 0 to 50 μm.

The reasons for limiting the numerical values in the claims are described below.

If the heat treatment temperature is lower than 1000 ° C., an oxidation treatment time for obtaining a sufficient action and effect is required for several hundred hours or more, and the efficiency is poor. Becomes

When the thickness of the silicon oxide formed on the surface of the sintered body is less than 0.5 μm, there is no effect of alleviating the surface defects.
Further, when an oxide film of silicon oxide having a thickness of 50 μm or more is formed, the inside of the silicon oxide film becomes a starting point of destruction, and conversely, the strength is reduced to below the substrate. The reason why the thickness is preferably 10 μm or more is that the critical crack (flat crack) length (calculated from the bending strength and the fracture toughness) of the normal pressure sintered SiC is about 1 μm.
This is because the effect of alleviating surface defects is increased because the thickness is 0 μm.

When the bulk density of the silicon carbide sintered body to be subjected to the heat treatment is less than 3.0 g / cm ³ and the three-point bending strength at room temperature is less than 370 MPa, many open pores are formed on the surface of the sintered body. Exists, and a uniform and smooth silicon oxide film cannot be formed, which makes it difficult to reduce the influence of surface defects.

Further, the center line average roughness (R
If a) is 10 μm or more, a uniform and smooth oxide film of silicon oxide cannot be generated due to the influence of unevenness, and even if it can be formed, it is difficult to reduce the influence of surface defects.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, not only a silica film having a predetermined thickness is formed on the surface of a silicon carbide sintered substrate, but also free silicon is not contained in silicon carbide. Silicon carbide is limited to that of normal pressure sintering.

Since the silicon carbide sintered body is heat-treated so as to form a silicon oxide film having a thickness of 0.5 to 50 μm, preferably 10 to 50 μm on the surface of the SiC substrate under normal pressure sintering (self-sintering). is there.

1000 ° C. or more and 1500 ° C. in an oxidizing atmosphere
Heat treatment at less than ℃, silicon oxide on the surface 0.5μ
m and less than 50 μm (preferably 10 to 50 μm).

By such a heat treatment, it is possible to improve mechanical strength and toughness which are disadvantages of the silicon carbide sintered body.

The possibility of strength recovery due to the difference in critical crack length will be described.

Bending strength and fracture toughness are different between SiSiC (Si-impregnated SiC) and normal pressure sintered SiC.

The bending strength of conventional normal pressure sintered SiC is generally about 400 to 600 MPa, and the SiSiC has a bending strength of 200 MPa.
３００300 MPa is common. The fracture toughness value is about ² to 3 MPa · m ^{1/2 for} normal pressure sintered SiC. S
In the case of iSiC, it is about 4 to 5 MPa · m ^1/2 .

There are flat cracks and semi-circular cracks in the critical cracks, but the flat cracks greatly contribute to the decrease in strength. The critical crack length is about 10 μm in normal pressure sintered SiC, In SiSiC, about 10
It is about 100 μm, which is twice as large. In other words, normal pressure sintering S
In the case of iC, if a flat crack of about 10 μm is present, the strength is greatly reduced.
Even if cracks before and after exist, the strength is not significantly reduced. That is, in order to recover strength, S
iSiC has a thicker SiO ₂ than normal pressure sintered SiC.
An oxide film is required. Therefore, the oxide film thickness is at most 0.
If it is less than 5 μm, it does not contribute to the recovery of strength. The oxide film thickness is preferably set to 10 μm or more.

In the case of SiSiC having a low strength level,
Compared with normal pressure sintered SiC, the strength recovery ratio is lower,
The effect is small.

In order to clarify the features of the present invention, a comparison is made with the main conventional example.

Regarding this point, Japanese Patent Application Laid-Open No. 2-172879
The recrystallized SiC described in the above item is porous because it has a porosity of 15 to 25%. Therefore, since everywhere in the material can be a starting point of the breakdown, the strength level is further lower than that of SiSiC (about 100 MPa or less). Therefore, it is irrelevant to the strength recovery, or the strength recovery is negligible.

SiC described in JP-A-64-61376
Alternatively, a silicon carbide substrate such as SiSiC forms an SiO ₂ oxide film on its surface, and further forms a CVD-S
An iC film is formed. This has a different meaning from the present invention. At that time, the normal pressure sintering had a low density, and the strength could not be recovered by forming a CVD-SiC film on the surface.

In the carbon substrate described in JP-A-62-293713, a SiC film is formed on the surface thereof by a CVD method or the like, and a SiO ₂ film is formed on the surface in a thickness of 5 to 1000 Å. In this case, the SiO ₂ film has a maximum thickness of 0.1 μm (1000 angstroms), and the strength is not recovered by such a thin oxide film.

In the present invention, the density of the substrate is also important. For example, if the density is too low, desired characteristics cannot be obtained. The present invention is limited to dense silicon carbide sintered substrates having a density of 90% or more with respect to the theoretical density.
Preferably, a dense silicon carbide sintered substrate having a theoretical density of 95% or more is used.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be described in more detail.

The heat treatment temperature of the silicon carbide sintered body according to the present invention is 1000 ° C. to less than 1500 ° C., preferably 1200 ° C. to less than 1450 ° C. in an oxidizing atmosphere. Although it differs depending on the surface condition and the like, it is long at a low temperature and short at a high temperature, and silicon oxide is generated on the surface at 0.5 μm or more and less than 50 μm.

In general, when a low-density dense silicon carbide sintered body is processed by a diamond grindstone, crystal grains fall off and fine cracks are generated. When an external stress is applied thereto, it is inevitable that these points will be the starting points and breakage.However, the above-mentioned heat treatment generates silicon oxide on the surface of the sintered body, and the theoretical basis is as follows. Although it is not clear, it is possible to reduce the influence of the defects existing on the surface on the breaking strength.

In particular, the silicon carbide sintered body used has a bulk density of not less than 3.0 g / cm ³ and a three-point bending strength at room temperature of not less than 370 Mpa. When a silicon carbide sintered body having a center line average roughness (Ra) of less than 10 μm is used, the effect of increasing the strength is remarkable.

Experimental Example Hereinafter, the effect of the heat treatment of the present invention will be described in more detail with reference to an experimental example.

1) Preparation of a silicon carbide sintered body using a BC-based sintering aid A commercially available SiC having a purity of about 99% and a specific surface area of about 15 m ² / g
B ₄ C having an average particle size of about 2.5 μm as a sintering aid
Was added and 8.5% by mass of a phenol resin (resole type) having a residual carbon amount of about 50% by mass was wet-pulverized and mixed in ethanol for 24 hours. The slurry is dried with a hot air temperature of 80 ° C. using a spray drier having an average particle size of about 85.
After granulating to a thickness of μm, primary molding was performed using a mold at a molding pressure of 30 MPa, and CIP molding was further performed at 147 MPa to obtain a molded body of about 150 × 150 × 30 mm. This molded body was filled in a carbon case,
Sintered at 00 ° C. for 1.5 hours. The atmosphere during sintering was in a vacuum from room temperature to 1400 ° C. in consideration of the thermal decomposition of phenolic resin, and thereafter in a normal pressure argon atmosphere.

2) Production of a silicon carbide sintered body using an Al-based sintering aid Commercially available SiC having a purity of about 99% and a specific surface area of about 15 m ² / g
To the powder, 1.0% by mass of high-purity ultrafine aluminum oxide having a specific surface area of about 50 m ² / g was added as a sintering aid, and 2.0% by mass of PVB was added as a molding binder. Wet pulverized and mixed. The slurry was granulated to an average particle size of about 85 μm with a hot air temperature of 80 ° C. using a spray drier, then subjected to primary molding using a mold at a molding pressure of 30 MPa, and further subjected to CIP molding at 147 MPa, to form about 150 × 150 × A 30 mm compact was obtained. This molded body is filled in a bonbon case,
And sintering at 2200 ° C. for 1.5 hours. The atmosphere during sintering was in a vacuum from room temperature to 1400 ° C., and thereafter in a normal pressure argon atmosphere.

3) Characteristic evaluation of normal product and heat-treated product After the bulk density of the silicon carbide sintered body obtained in the above 1) and 2) was measured in accordance with JIS R1634, JIS R
Thirty bending test pieces (3 × 4 × 40 mm, C0.2) were prepared in accordance with 1601 and JIS R1604, respectively, and the three-point bending strengths at room temperature and 1400 ° C. were measured.

Further, a bending test piece (3 × 4 × 40 mm, C0.2, 30 pieces each) of the obtained silicon carbide sintered body was subjected to a heat treatment in the air, and then similarly to JIS R1601 and JIS R1601.
3 at room temperature and 1400 ° C. according to SR1604
The point bending strength was measured.

The thickness of the silicon oxide formed by this heat treatment was determined by measuring the amount of SiO ₂ using an ICP emission spectrometer and converting the bulk density to an oxide film thickness.

Table 1 summarizes the processing conditions and measurement results.

[0047]

[Table 1] In the table, Examples 1 to 8 fall within the scope of the present invention, and Comparative Examples 1 to 8.
8 does not fall within the scope of the present invention.

4) Superior Difference Test (*) Example 1 and Comparative Example 1 in accordance with JIS Z9048
As a result of performing a significant difference test on the measurement results of the three-point bending strength at room temperature of Example 2, the bending strength of Example 2 in which the heat treatment was performed according to the present invention at a risk factor of 5% was higher than that of Comparative Example 1 of the untreated product. It was recognized as having high strength.

5) Evaluation with sintered bodies having different surface states From the silicon carbide sintered body of Comparative Example 1, several kinds of bending test pieces having different surface roughness were prepared, and room temperature before and after heat treatment was measured.
The point bending strength (JIS R1601, n = 30) was measured. Table 2 shows the results.

[0050]

[Table 2] [Measurement conditions of center line average roughness] Measurement length 4.8 mm, cut-off value 0.8 mm 6) Strength recovery using pre-crack-introduced test piece Using silicon carbide sintered body of Comparative Example 1, JIS R16
According to No. 07, three indentations were formed on one side (3 × 40 mm) of a bending test piece with a load of 10 kgf, a pre-crack was introduced, and the breaking load before and after the heat treatment was measured. Table 3 shows the results.

[0051]

[Table 3] [Indentation preparation conditions] 3 × 40 m of bending test pieces (3 × 4 × 40 mm, C0.2) according to JIS R1601
Three indentations were made at the center of one surface (tensile surface) of m.

The center average roughness of the tensile surface of the bending test piece is 10
heat treatment of those having a diameter of 30 μm or less.
As described above, it was recognized that the bending strength was improved.

By heat-treating the test piece provided with the pre-crack, the breaking load was greatly improved, and it was found that it was very effective for a micro-rack existing on the processed surface such as a pre-crack.

7) Evaluation of actual machine A heat treatment-related product (actual machine product) utilizing the high heat resistance, high oxidation resistance and high corrosion resistance of the silicon carbide sintered body was produced, and the initial stage of use (from the start of use) depending on the presence or absence of heat treatment The damage rate for one week or 1000 h) was evaluated. Table 4 shows the results.

[0055]

[Table 4] The heat treatment of the related products around the burner, which is rapidly heated and cooled, and the shelf products, which require hot strength, significantly reduced the breakage rate at the beginning of use and confirmed the effectiveness.

[0056]

As is clear from the above Examples 1 to 8, the dense silicon carbide sintered body produced by the normal pressure sintering method is subjected to a heat treatment in an oxidizing atmosphere, and silicon oxide is applied to the surface thereof. By generating 0.5 μm or more and less than 50 μm,
Its strength and toughness can be greatly improved. In addition, its effectiveness was confirmed even when applied to actual heat treatment-related products. Moreover, such a heat treatment is an industrially very simple heat treatment.

According to the present invention, in addition to general heat treatment-related members used at high temperatures, it is applicable to various products which have been considered to be inapplicable to silicon carbide sintered bodies due to low toughness and low strength. It becomes possible.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Jun Hikita 30 Soya, Hadano-shi, Kanagawa Pref., Toshiba Ceramics Co., Ltd. (72) Inventor Kazuji Matsuyama 1 Minami Fuji, Ogakie-cho, Kariya-shi, Aichi F-term (reference) 4G001 BA03 BA22 BA23 BB03 BB04 BB22 BB23 BC13 BC56 BC72 BD04 BD07 BD14 BD15 BD16 BD37 BE33 BE35

Claims (7)

[Claims] 1. A dense silicon carbide sintered body made by a normal pressure sintering method and having a density of 90% or more with respect to a theoretical density in an oxidizing atmosphere at 1000 ° C. to 1500 ° C. Less than 0.1% on the surface of the substrate.
A heat treatment method for a silicon carbide sintered body, wherein silicon oxide having a thickness of 5 μm or more and 50 μm or less is generated. 2. The silicon carbide sintered body has a bulk density of 3.0.
g / cm ³ or more, and the three-point bending strength at room temperature is 370M
The heat treatment method for a silicon carbide sintered body according to claim 1, wherein the pressure is Pa or more. 3. The heat treatment method for a silicon carbide sintered body according to claim 1, wherein the center line average roughness (Ra) of the surface of the silicon carbide sintered body is less than 10 μm. 4. The sintered silicon carbide according to claim 1, wherein the substrate is a dense silicon carbide sintered body having a density of 95% or more with respect to a theoretical density. Heat treatment method of the solidified body. 5. The thickness of silicon oxide on the surface of a substrate is 10 μm.
The heat treatment method for a silicon carbide sintered body according to any one of claims 1 to 4, wherein the thickness is not less than 50 µm. 6. A heat-treated silicon carbide produced by the heat treatment method according to claim 1, wherein the heat-treated silicon carbide has a surface
A heat-treated silicon carbide, wherein a silicon oxide film having a thickness of not less than μm and less than 50 μm is present. 7. The thickness of silicon oxide on the surface of a substrate is 10 μm.
The heat-treated silicon carbide according to any one of claims 1 to 6, which is at least 50 µm or less.