JP2001278666A - Method for manufacturing sintered silicon nitride - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a sintered silicon nitride with a complicated shape whose surface layer is highly hardened because of an added silicon nitride particle. SOLUTION: A molded silicon article containing oxygen is sintered under a condition of carbon-containing nitrogenous atmosphere <=0.1 MPa and temperature <1500 deg.C using a compound consisting of aluminum and/or yttrium and oxygen as an auxiliary for sintering, with the result that a silicon carbide fine particle is formed on the surface layer of silicon nitride. Then the article is heat-treated under the condition of a nitrogenous atmosphere >=0.1 MPa and <=1 MPa and temperature >=1500 deg.C and <=2000 deg.C. The finished compact with a relative density >=97% contains silicon carbide on the surface layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered body containing silicon carbide particles in a surface layer and a method for manufacturing the same, which can be easily manufactured even if the surface has a high hardness and a complicated shape. .

[0002]

BACKGROUND ART heat resistance, thermal shock resistance, wear resistance excellent silicon nitride (Si 3 _{N 4)} sintered body with a high temperature member, the site or the like of occurrence of thermal shock, a general purpose ceramic alumina It has been used as a wear-resistant member instead of (Al ₂ O ₃ ). And, in order to improve the high temperature characteristics and wear resistance characteristics of such a silicon nitride sintered body,
A composite material in which silicon carbide (SiC) is combined with a silicon nitride sintered body has been developed. In order to obtain such a dense sintered body of silicon carbide composite silicon nitride, generally, a method of adding a silicon carbide powder or a silicon carbide precursor to silicon nitride powder to which a sintering aid has been added is adopted. ing.

[0003]

However, since silicon carbide particles added to silicon nitride hinder sintering, hot press (HP) or the like is generally used for producing a silicon carbide composite silicon nitride sintered body. It is necessary to use a pressure sintering method that requires an external force, such as a hot isostatic press (HIP). When these methods are used, for example, only a plate-shaped sintered body can be obtained by hot pressing, and there is a problem that a sintered body of a complicated shape cannot be manufactured, and a hot isostatic press has a high manufacturing cost. Therefore, it is hard to say that it is suitable for the purpose of manufacturing a general-purpose member. Further, it is necessary to use ultrafine powder as silicon carbide to be added to silicon nitride. As such, composite materials produced by conventional methods are expensive.

When a silicon carbide composite silicon nitride sintered body is manufactured as a structural ceramic, a normal pressure sintering method or a 1MP
If the gas pressure sintering method of sintering under a nitrogen atmosphere of a (10 atm) or less can be used, the production cost can be reduced,
In addition, it is considered that a product having a predetermined complicated shape is easily manufactured, which is desirable in practical use.

[0005] The present invention has been made in view of such problems, and the surface layer is made harder by including silicon carbide particles in the surface layer. It is an object of the present invention to provide a silicon nitride sintered body that can be manufactured at a low temperature and a method for manufacturing the same.

[0006]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above-mentioned problems of the prior art, and have found that a two-stage sintering method combining a reaction sintering method and a densification sintering method is used. The present inventors have found that it is possible to generate fine silicon carbide particles on the surface layer of a silicon nitride molded body and to perform sintering by including carbon in a nitrogen atmosphere at the time of reaction sintering, and arrived at the present invention.

That is, according to the present invention, there is provided a silicon nitride sintered body characterized in that the surface layer contains silicon carbide particles and has a relative density of 97% or more. In order to obtain such a silicon nitride sintered body, a compound comprising aluminum and / or yttrium and oxygen is suitably used as a sintering aid.

Further, according to the present invention, there is provided a method for producing a silicon nitride sintered body containing silicon carbide in a surface layer as described above. That is, a compound comprising aluminum and / or yttrium and oxygen is used as a sintering aid, and a silicon compact containing oxygen is reactively sintered at less than 1500 ° C. in a nitrogen atmosphere containing 0.1 MPa or less containing carbon. To form silicon carbide fine particles on the surface layer of silicon nitride.
There is provided a method for producing a silicon nitride sintered body, characterized by heating in a nitrogen atmosphere of MPa to 1 MPa at 1500 ° C. to 2000 ° C. to densify the relative density to 97% or more.

When such a manufacturing method is used, there is no need to use a hot press, a hot isostatic press, or the like. It can be easily manufactured.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Silicon Nitride (Si ₃ N ₄ ) of the present invention
The surface of the sintered body has the properties of a composite material composed of silicon nitride and silicon carbide (SiC) with respect to oxidation resistance, hardness, wear resistance, and the like. Here, even with a general silicon nitride sintered body obtained by molding and sintering silicon nitride powder, green silicon carbide may be generated on the surface depending on the firing atmosphere. Production of silicon is limited to the very surface of the silicon nitride sintered body. However, the surface layer of the silicon nitride sintered body of the present invention refers to a surface layer including the inside from the surface of the sintered body to about 5 mm, and in this regard, conventional silicon carbide is formed on the surface or a very shallow portion of the surface. It is distinguished from a silicon nitride sintered body.

The silicon nitride sintered body of the present invention is characterized in that the amount of silicon carbide increases in a gradient from the inside of the sintered body to the surface, and the intensity of the main peak of silicon carbide and silicon nitride in X-ray diffraction. The silicon carbide content determined from the ratio is, for example, at a depth of 1 mm from the surface of a sintered body having a thickness of 20 mm, the content of silicon carbide is about 5 to 20% by weight. In this case, the content is 5% by weight or less. In the present invention, there is no limitation on the content of silicon carbide to be included in silicon nitride.

[0012] The silicon nitride sintered body of the present invention comprises a heat-resistant member,
When used as a wear-resistant member, the relative density is 97
% Is desirable. Since the relative density of silicon carbide and silicon nitride is almost the same here, the relative density may be indicated as a relative density to the theoretical density of silicon nitride.In the present invention, the relative density is based on the theoretical density of silicon nitride. And If the relative density is less than 97%, there is a problem that the physical properties of the sintered body such as mechanical strength and hardness are significantly reduced.

Next, a method for manufacturing the above-described silicon nitride sintered body of the present invention will be described. Silicon (S
i) The surface of the powder is oxidized in the step of pulverization,
About 3% of oxygen. Further, such a crushed powder is left in an atmosphere containing air or oxygen as particles or a molded body, and is subjected to treatment to promote surface oxidation. In the stage, in the form of oxygen or oxide about 0.5 ~
It comes to contain 5% by weight of oxygen.

[0014] In addition, silicon powder usually contains trace amounts of iron (Fe), aluminum (A1), calcium (C) in the refining stage.
a) and other metal impurities. In the present invention, the content of impurities such as iron is not limited, and depending on the intended use of the manufactured silicon nitride sintered body, impurities such as iron and the like are contained to such an extent that they can be usually used as a raw material for sintering. Silicon powder can be used as appropriate.

Generally, if the purity of the starting material is high, a sintered body having excellent mechanical properties can be obtained. Therefore, it is preferable that the content of such impurities is small, but nitriding is more difficult. There is a problem. In the present invention, the iron content is 0.1% by weight or less and the total amount of metal impurities is 0.4% by weight.
It is possible to use a high purity silicon powder of not more than weight%.

The particle size of the silicon powder used as a starting material is
The average particle size is preferably from 0.5 to 10 μm in order to make the mixing and dispersion of the sintering aid uniform.
It is preferred that it is about. Yttrium oxide (Y ₂ O ₃ ) and aluminum oxide (Al ₂ ) used as sintering aids
The added amount of O ₃ ) is usually 3 to 30% by weight, more preferably 5 to 15% by weight, based on silicon nitride. If the amount of the sintering aid is less than 5%, it will not be sufficiently densified, while if it is more than 30%, the grain boundary phase will increase,
There is a problem that the mechanical properties and the like of silicon nitride deteriorate. Such a sintering aid preferably has an average particle size of about 0.5 to 1 μm for the purpose of uniformly dispersing it in the silicon powder.

The above-mentioned silicon powder and sintering aid are prepared at a predetermined ratio, and are mixed by a wet or dry method using a general mixing means such as a ball mill or a medium stirring mill. The obtained mixed powder can be molded by any molding method such as mold molding, rubber mold molding, injection molding, cast molding, and extrusion molding. It is also preferable to machine the molded body to have a desired shape.

Subsequently, the obtained compact is subjected to reaction sintering (nitriding) in a nitrogen (N ₂ ) atmosphere.
In the step of nitriding the silicon compact at a temperature lower than 1500 ° C., carbon is contained in the atmosphere gas. When the carbon component diffuses from the surface of the silicon molded body toward the inside, a surface layer in which fine particles of silicon carbide are precipitated and distributed in a gradient composition is formed on the surface of the silicon molded body. Here, the nitrogen atmosphere refers to a nitriding gas atmosphere, in the case of only nitrogen gas, and in the case of argon (A) mainly containing nitrogen gas.
r), helium (He), hydrogen (H ₂ ), ammonia (NH ₃ ) and the like in the case of a mixed gas atmosphere in which a small amount is mixed.

As a method for containing carbon in such an atmosphere gas, carbon monoxide (CO), carbon dioxide (CO
₂ ) or a method in which a carbon source that generates carbon monoxide or carbon dioxide by reacting with oxygen released when the silicon molded body is nitrided, for example, silicon carbide or graphite (C) And the like, in which a coexistence with a silicon molded body is made in a firing vessel. In addition, a method in which a ventilation hole is provided in the firing container and supplied from a graphite heater or a silicon carbide heater can be used, but in this case, there is a disadvantage that the service life of the heating element is shortened.

The nitriding temperature is not less than 1200 ° C. and less than 1500 ° C., but 1350 to 1450 ° C. is particularly preferred. In order to effectively volatilize SiO from the silicon molded body, the atmospheric pressure is 0.1 MPa or less, preferably 0.02 MPa or less.
The pressure is preferably set to 5 to 0.1 MPa. Since the nitriding reaction is an exothermic reaction, it is necessary to select an appropriate heating rate so as to avoid rapid heating and prevent the elution of silicon.

As described above, in the present invention, it is important to contain an appropriate amount of a carbon component in a nitrogen atmosphere. However, mixing of an excessive amount of carbon monoxide or carbon dioxide inhibits a nitriding reaction. Amount) is 10% by volume or less, preferably 5% by volume.
It is necessary to be not more than volume%. When graphite, silicon carbide, or the like is used as a carbon source for generating an appropriate amount of a carbon-containing gas in a nitrogen gas, a material that does not elute or vaporize low-boiling impurities during sintering at a high temperature may be used. preferable.

On the other hand, a carbon source which easily reacts with oxygen released when the silicon molded body is nitrided to generate carbon monoxide or carbon dioxide is effective if the carbon source has a large surface area. For this reason, as the graphite or silicon carbide coexisting with the silicon molded body, it is preferable to use a powdery, fibrous, or porous material. However, if the particle size is extremely small in the case of powder, handling properties during production, such as firing preparation, are poor,
On the other hand, if the surface is massive and has a small surface area, a sufficient effect of generating a carbon-containing gas cannot be obtained.

That is, the thickness of the silicon carbide-containing layer formed on the surface of the silicon nitride sintered body depends on the total surface area of the coexisting graphite and silicon carbide, the volume of the sample container, the oxygen content of silicon, the nitrogen atmosphere pressure, the batch furnace If this is the case, it depends on the preparation amount per operation and the like, so it is desirable to determine after appropriately setting conditions.

Next, the silicon nitride body nitrided as described above may be sintered by continuously heating it in the furnace used for nitriding, or may be transferred to another firing furnace and sintered. Is also good.
The sintering temperature is 1500-2000 ° C.
The temperature is more preferably set to 0 to 2000 ° C. 1500
Sintering is not promoted below ℃, while on the other hand, above 2,000 ℃, silicon nitride grains grow violently, silicon nitride particles in the sintered body are coarsened, and there is a problem that mechanical properties and the like deteriorate.

The sintering atmosphere is a pressurized atmosphere with nitrogen. Here, in order to suppress the decomposition of silicon nitride, it is most preferable to set the nitrogen gas atmosphere to a high pressure of 1 MPa or more. However, since the sintering furnace becomes expensive, there is a problem that the production cost rises. Therefore, in the present invention, the pressure is low as long as the decomposition of silicon nitride can be suppressed, that is, 0.1 MPa or more and 1 MPa under nitrogen gas pressurization.
The following is assumed. As a result, the cost of the apparatus can be reduced and the production can be performed safely.

By controlling the sintering temperature, time and sintering atmosphere, a silicon nitride sintered body having a relative density of 97% or more and silicon carbide fine particles dispersed in the surface layer can be obtained. This silicon carbide composite silicon nitride sintered body has a high hardness on the surface of the sintered body due to the silicon carbide particles present in its surface layer, and the proportion of the grain boundary phase in the surface layer of the sintered body decreases. Oxidation resistance and wear resistance are greatly improved because the structure of the sintered body is refined. Such a silicon nitride sintered body of the present invention is suitably used especially as a heat-resistant material and a wear-resistant material.

Next, embodiments of the present invention will be described, but it goes without saying that the present invention is not limited to the following embodiments.

[0028]

EXAMPLES Example 1 To produce the silicon nitride sintered body of Example 1, a silicon (Si) powder having the composition shown in Table 1 and sintering aids (Al ₂ O ₃ and Y ₂ O _{3) were used.} )It was used.
The silicon powder and the sintering aid shown in Table 1 were prepared in the proportions shown in Table 2, and were treated in ethanol by ball milling using silicon nitride balls for about 20 hours. After drying the obtained mixed powder, it was pulverized in an agate mortar and passed through a 200 μm mesh pass. The powder thus obtained was preformed by a uniaxial press, and then was subjected to 98 MPa (1.0 t / c).
m ² ) at a pressure of 50 mm
A silicon molded body of × 50 mm × 5 mm was obtained.

[0029]

[Table 1]

[0030]

[Table 2]

Subsequently, an inner diameter of 200 mmφ and a depth of 50 mm
A silicon carbide (SiC) powder having a purity of 98% or more and an average particle size of 20 μm is placed on the bottom of a reaction sintered silicon nitride container with a lid of
0 g, and the above-mentioned four compacts were placed on the
The sample was placed on a 10 × 5 × 50 mm silicon nitride sample table so as not to touch the powder directly.

The container with the lid was nitrided in a 0.1 MPa nitrogen (purity: 99.99%) atmosphere under the temperature conditions shown in Table 3. In addition, in order to confirm the crystal phase of the sintered body after nitriding, one of the obtained sintered bodies after nitriding was cut, and a silicon carbide-containing layer (referred to as a layer colored green, indicating a surface layer) Synonym.) Was measured. The measurement results are shown in Table 2.

[0033]

[Table 3]

For the remaining nitride, Table 3
The sintering was performed under the sintering conditions shown in (1), the density of the obtained sintered body was measured by the Archimedes method, and the relative density was calculated as a relative density to the theoretical density of silicon nitride. The bending strength was measured by a four-point bending test according to JIS R1601. Further, the hardness of the sintered body is Jls R1610
Hardness test method (indentation pressure 10
N). The measurement results are also shown in Table 2.

The amount of wear of the manufactured sintered body was determined by using white alumina # 70 as the collision particles and setting the collision angle to 30.
°, a flow rate of 70 m / min, a collision amount of 1 kg / 1 hour, and a test time of 2 hours were performed by a sand blast wear test, and the evaluation was performed by reading the maximum wear depth using a surface roughness meter. Further, the oxidation resistance is 1300 in an atmospheric furnace.
The weight increase of the sintered body when heated at 500C for 500 hours was divided by the surface area of the sintered body.

Example 2 The sample of Example 2 is the same as that of Example 1
In place of the SiC powder used in the above, a porous SiC body having a purity of 98% or more, a porosity of 30%, and a shape of 50 mm × 50 mm × 5 mm was used, and the four porous SiC bodies were set in the same firing vessel, and It was produced by nitriding and sintering under the same conditions as in Example 1.

Example 3 The sample of Example 3 is the same as that of Example 1
A graphite (C) powder having a purity of 99% or more and an average particle size of 50 μm was placed on the bottom of the container in place of the SiC powder used in the step (1).
g, and nitriding and sintering were performed under the same conditions as in Example 1 except for the above.

Example 4 The sample of Example 4 is the same as that of Example 1
In place of the SiC powder used in Example 1, a graphite (C) porous body having a purity of 99% or more and a porosity of 35% and a size of 50 mm × 50 mm × 5 mm was used, and the others were nitrided and sintered under the same conditions as in Example 1. It produced by doing.

(Example 5) The sample of Example 5 is the same as that of Example 1
The atmosphere pressure in the nitridation step under the conditions for preparing the sample was 0.
The pressure was set to 08 MPa, and the other conditions were the same as those in Example 1.

(Example 6) The sample of Example 6 is the same as that of Example 1
In the same manner as in Example 1, except that approximately 2% by volume of carbon dioxide (CO ₂ ) gas having a purity of 99% or more was mixed with nitrogen instead of the SiC powder used in Example 1.

(Comparative Examples 1 to 3) The samples of Comparative Example 1 were nitrided under the same conditions as in Example 1 except that the SiC powder used in Example 1 was not used, that is, the carbon source was not used in the nitriding step. And by sintering. The sample of Comparative Example 2 had a purity of 98% when the raw materials were mixed.
As described above, 5% by weight of SiC powder having an average particle size of 0.6 μm was added, and a carbon source was not used in the nitriding step.
The other conditions were the same as in Example 1, and were manufactured by nitriding and sintering. As a sample of Comparative Example 3, a commercially available silicon nitride sintered body was used.

(Test Results) The test results are shown in Table 2. In Comparative Example 1, since no carbon source was present in the nitriding step, a silicon carbide-containing layer was not formed on the surface thereof. Therefore, the sintered body was densely sintered, the relative density was high, and the mechanical strength was large. , The amount of wear in the wear test was large, and the weight increase due to oxidation was also large. In Comparative Example 2, sintering did not proceed because the SiC powder was mixed with the raw material, and as a result, tests such as a strength test could not be performed. The silicon nitride sintered body of Comparative Example 3 exhibited almost the same characteristics as Comparative Example 1, but was inferior in oxidation resistance to Comparative Example 1. It is considered that this is because silicon carbide particles are not contained.

With respect to the test results of Comparative Examples 1 to 3,
All of the silicon nitride sintered bodies of Examples 1 to 6 according to the present invention were densely sintered to a relative density of 97% or more to secure mechanical strength, and a silicon carbide-containing layer of about 2 mm was formed on the surface. , It was confirmed that the surface hardness was high, and that it had good wear resistance. Furthermore, it was confirmed that it was also excellent in oxidation resistance.

[0044]

As described above, according to the silicon nitride sintered body of the present invention and the method for producing the same, carbon is contained in the nitrogen atmosphere during the reaction sintering (nitriding), so that the surface layer of the silicon nitride molded body is formed. It is possible to generate fine silicon carbide particles and sinter them, and the silicon nitride sintered body thus obtained has a high surface hardness depending on the silicon carbide present in the surface layer and has a high wear resistance. And have excellent resistance to oxidation. Further, according to such a manufacturing method, it is possible to easily manufacture a product having a complicated shape, and the cost of the apparatus can be reduced. As described above, the present invention provides a silicon nitride sintered body having excellent mechanical properties such as mechanical strength, hardness, oxidation resistance, and abrasion resistance. It has an excellent effect that it becomes possible.

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Claims (3)

[Claims] 1. A silicon nitride sintered body comprising silicon carbide particles in a surface layer and having a relative density of 97% or more. 2. A sintering aid comprising aluminum and / or
The silicon nitride sintered body according to claim 1, wherein a compound comprising yttrium and oxygen is used. 3. Using a compound comprising aluminum and / or yttrium and oxygen as a sintering aid, subjecting a silicon compact containing oxygen to a reaction sintering at a temperature of less than 1500 ° C. in a nitrogen atmosphere containing carbon of 0.1 MPa or less. To form silicon carbide fine particles on the silicon nitride surface layer.
A method for producing a silicon nitride sintered body, comprising heating in a nitrogen atmosphere of from 1 MPa to 1 MPa at a temperature of from 1500 ° C. to 2000 ° C. to increase the relative density to 97% or more.