JP2784280B2 - Ceramic composite sintered body, method for producing the same, and sliding member - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite sintered body having high strength and excellent sliding characteristics and suitable for sliding members such as mechanical seals and bearings, and a sliding member using the same.

[0002]

2. Description of the Related Art Non-oxide ceramics represented by silicon carbide and silicon nitride have attracted attention as materials having excellent hardness, strength, toughness, chemical stability and the like as compared with other ceramics and metals. Mechanical seal parts,
It is used as bearing parts and valve members for chemicals.

However, since silicon nitride and silicon carbide alone cannot provide sufficient sliding characteristics, Al ₂ O 3 is used as a sintering aid for silicon nitride powder or silicon carbide powder.
By adding a solid lubricant such as graphite or BN at the same time as adding the element oxide of group ₃ or the element of group 3a of the periodic table, or carbon and B ₄ C, and baking this in a vacuum or in an inert atmosphere, It has been practiced to uniformly disperse the solid lubricant in a matrix made of silicon nitride or silicon carbide, thereby improving the sliding characteristics on the surface of the sintered body.

[0004]

In order to enhance the sliding characteristics, it is desirable that the amount of the solid lubricant in the surface layer of the sintered body is large. And the strength of the ceramic itself as a so-called aggregate is reduced, so that the sliding member is liable to be cracked or chipped. Therefore, the amount of the solid lubricant added was naturally limited.

[0005] In addition, it is necessary to uniformly disperse the solid lubricant itself in the manufacturing method, and in some cases, the solid lubricant inside the sintered body acts as a destruction source of the sintered body, resulting in a problem that the strength is reduced. Moreover, a sintered body dispersed as a solid lubricant containing silicon nitride as a matrix has poor chemical resistance due to the presence of a metal oxide added as a sintering aid at the grain boundaries of the silicon nitride crystal. There is also a problem that is limited.

[0006]

Means for Solving the Problems The inventors of the present invention have studied the above problems, and as a result, have found that in a system using silicon carbide as an aggregate, a three-dimensional network structure is formed by silicon carbide particles. By forming a porous structure of the silicon carbide particles and filling the voids of the silicon carbide particles with silicon nitride and carbon, excellent mechanical strength and sliding characteristics are obtained. Even in the surface layer portion of the sintered body, it is possible to improve the slidability in the surface layer portion without reducing the strength of the sintered body itself by reducing the amount of the solid lubricant inside the sintered body, thereby It has been found that various sliding members can exhibit highly reliable and stable characteristics.

According to the present invention, the sliding characteristics of a ceramic sintered body are characteristics that are governed by the structure and structure of the surface layer portion of the sintered body. From the viewpoint of acting, the aggregate part holding carbon and the like, which is a component improving the slidability, is formed of a porous material having a three-dimensional network structure mainly composed of silicon carbide crystals, and the void part is formed in the void part. It is characterized by having a structure that holds a solid lubricant such as carbon.
This aggregate component is composed of silicon carbide or a composite of silicon carbide and silicon nitride.
As such, it has high strength and excellent properties in terms of sliding characteristics as compared with other ceramics.

Further, according to the present invention, as shown in FIG. 1, the amount of the solid lubricant added from the surface to the inside of the sintered body is reduced as shown in FIG. It is characterized by doing so. In particular, from the viewpoint that the inside of the sintered body does not contribute to the sliding characteristics, a solid lubricant may not be substantially contained, and it is preferable that the sintered body be made of silicon carbide or silicon nitride which is an aggregate component.

However, if the composition or the structure changes suddenly from the surface layer portion of the sintered body to the inside, stress is likely to be generated due to a difference in characteristics at the boundary portion, and cracks or chips may occur. As shown in FIG. 1, it is preferable that the amount of the solid lubricant gradually decreases toward the inside.

The method of obtaining a ceramic composite sintered body according to the present invention will be described. In the conventional method of mixing a solid lubricant powder with an aggregate component such as silicon carbide or silicon nitride and firing it, a uniform structure is obtained. A structure having different solid lubricant contents is not formed between the surface layer portion and the inside according to the present invention.

Therefore, according to the present invention, first, silicon carbide powder is prepared as a raw material powder. As the silicon carbide powder to be used, any of α-SiC and β-SiC, or a mixture thereof can be used. The average particle size of the silicon carbide powder is suitably from 0.1 to 2 μm. A sintering aid is added to the silicon carbide powder. Examples of the sintering aid include commonly known sintering aids for silicon carbide, such as boron and boron-containing compounds such as B ₄ C, aluminum compounds such as AlN, carbon and organic compounds capable of producing carbon by firing. Agents can be used.

After the above-mentioned silicon carbide powder and, if necessary, the above-mentioned sintering aid are added and thoroughly mixed, a binder or the like is added to the above-mentioned powder, and a known molding method, for example, press molding,
It is formed into a desired shape by extrusion, casting, cold isostatic pressing, or the like. The molded body is calcined in an inert atmosphere and in a vacuum to form a porous body having a high open porosity. This porous body has a network structure in which silicon carbide crystals are connected three-dimensionally.

Next, the porous body obtained as described above is fired. According to the present invention, this firing is carried out by the following chemical formula 1.

[0014]

Embedded image As shown by 3SiC + 2N ₂ → Si ₃ N ₄ + 3C, firing is performed in an atmosphere in which silicon nitride and carbon can be generated by a reaction between silicon carbide and nitrogen. Specifically, at a temperature of 1000 ° C. or more, particularly 1500 ° C. or more,
The reaction of Chemical Formula 1 can be advanced by including nitrogen gas as an essential component in the atmosphere and firing at a nitrogen gas pressure of 50 atm or more.

In such a production method, the porous body before firing preferably has an open porosity of about 5 to 25%. This is because, if the open porosity is less than 5%, the production of silicon nitride and carbon is reduced, and the sliding characteristics are not sufficient.
%, The mechanical strength of the sintered body is reduced due to the low strength as the aggregate, and the voids are not sufficiently filled by the formation of silicon nitride and carbon by the reaction represented by the above formula,
Pore remains and sliding characteristics deteriorate.

According to this baking, the reaction of the above chemical formula 1 proceeds on the surface of the silicon carbide in contact with the nitrogen gas, and the voids of the porous body are filled with silicon nitride and carbon generated by the reaction, thereby densifying the surface. Advances.

In addition, since the above-mentioned reaction is particularly active in the surface layer of the sintered body, the reaction product of the surface layer of the sintered body, ie, carbon and silicon nitride, are more likely to be present in the surface layer than in the interior. , A specific sintered body is formed. As a result, the amount ratio of silicon carbide and silicon nitride as the aggregate changes from the surface layer portion to the inside, and the composition ratio expressed as silicon carbide / (silicon nitride + silicon carbide) changes from the surface layer portion to the inside. growing.

The mechanism of the sintering is not clear, but the reaction of silicon carbide particles to silicon nitride proceeds from the surface of the silicon carbide particles in a nitrogen atmosphere under high temperature and high pressure, resulting in a volume expansion, thereby increasing the density to some extent. When the formation progresses and a dense layer is once formed in the surface layer portion, the entry of nitrogen gas into the inside of the sintered body is suppressed, and as a result, the surface layer portion and the inside become a dense body with a porosity of 5% or less, It is considered that a different structure is formed almost continuously between the surface layer and the inside.

[0019]

According to the present invention, the strength of the entire sintered body can be increased by forming the aggregate holding the solid lubricant such as carbon from a three-dimensional network structure made of silicon carbide. Moreover, by increasing the amount of carbon, which is a solid lubricant in the surface layer portion, only in the surface layer portion of the sintered body, the strength of the entire sintered body is not reduced,
Even if a relatively large amount of carbon is present in the surface layer portion, the strength inside is high, so that the sliding member can exhibit stable sliding characteristics. Moreover, since the structural change from the surface layer portion to the inside is formed almost continuously, the stress generated due to the difference in the characteristics in the sintered body can be reduced.

In addition, since the structure has a structure in which silicon carbide particles, which are aggregate components, are bonded to each other, the thermal conductivity of the sintered body itself can be increased, so that heat generated during sliding can be efficiently radiated. You can also. Furthermore, the electric resistance of the entire sintered body can be reduced by allowing an appropriate amount of carbon to be present in the inside, whereby electric discharge machining can be performed.

Further, in the surface layer portion of the sintered body, the reaction shown in Chemical Formula 1 is advanced to form a large amount of silicon nitride and carbon in the porous void portion made of the aggregate of silicon carbide, so that the thermal shock resistance and A surface layer having excellent toughness is formed.

According to a normal silicon nitride sintered body,
The metal oxide used as a sintering aid exists between the silicon nitride crystal grains as a grain boundary phase, but in the surface layer portion of the sintered body, the metal oxide substantially exists between the silicon nitride and the silicon carbide crystal grains. Is also a major feature of the present invention, whereby the chemical resistance can be increased and the range of application as a sliding member can be expanded.

[0023]

EXAMPLE To a β-SiC powder (average particle size 0.4 μm, oxygen content 0.1% by weight), an appropriate amount of a resol type phenol resin 20% solution was added as a molding binder,
Further, an appropriate amount of acetone is added as a solvent, and after kneading and drying,
The granules for molding were obtained through a sieve. The granules were formed into a disk-shaped molded body having an outer diameter of 60 mm and a thickness of 10 mm at a molding pressure of 2000 kg / cm ² using a mold press.

Next, the green body was green-processed into a predetermined shape, and then fired at 2000 ° C. for 1 hour in an inert atmosphere (in an Ar gas stream). The calcined body was fired by setting the N ₂ pressure, firing temperature and firing time shown in Table 1.

With respect to the obtained sintered body, the density and the open porosity of the sintered body were measured by the Archimedes method, and the constituent phases of the surface layer were analyzed by X-ray diffraction analysis. Further, the slidability of the surface which was cut down by 0.2 mm from the baked surface was evaluated. Sliding characteristics evaluation: φ50m with surface lap finished
The SUSII steel ball was brought into contact with the m-shaped disc as a fixing pin, the sample disc was rotated, and the contact load and the frictional force were measured to determine the wear coefficient. After the test, Vickers hardness was measured using the surface of the sample. Further, the structure of the obtained sintered body was observed with an electron microscope to confirm the presence or absence of a three-dimensional network structure. In addition, the strength of the sintered body
1601. The results are shown in Table 2.

Next, as a comparative example, 0.4 parts of B ₄ C powder was added to 100 parts by weight of β-SiC powder (average particle size: 0.4 μm).
Except for adding and mixing parts by weight, a molded body was prepared in exactly the same manner as described above, and then fired at 2050 ° C. for 1 hour in a vacuum to produce a dense silicon carbide sintered body. The various properties were measured and the sliding was evaluated (in the table,
Sample No, 1).

In the table, the composition distribution from the surface to the inside of the sample No. 3 was measured, and the composition distribution and the hardness distribution of silicon carbide and silicon nitride are shown in FIG. 2
It was shown to.

[0028]

[Table 1]

[0029]

[Table 2]

According to Tables 1 and 2, in the case of a conventional sintered body made of silicon carbide, the coefficient of friction as a sliding property is 0.4.
On the other hand, the friction coefficient of any of the products of the present invention was 0.2 or less, and the bending strength according to JISR1601 was measured. / Mm ² or higher.

FIGS. 1 and 2 show that carbon and silicon nitride are present in a larger amount on the surface of the sintered body than on the inside. According to the hardness distribution, the hardness of the surface of the sintered body is slightly lower than that of the inside and 0.5% lower than the surface.
It can be seen that there is a portion having a maximum hardness near mm.

[0032]

As described in detail above, the ceramic composite sintered body of the present invention has a high strength and a very small coefficient of friction on its surface. When applied to a moving member, excellent sliding characteristics can be exhibited, and the life of the member can be extended.

[Brief description of the drawings]

FIG. 1 is a diagram showing a composition distribution and a hardness distribution of silicon carbide and silicon nitride with respect to a depth from a surface layer portion of a ceramic composite sintered body of the present invention.

FIG. 2 is a view showing a carbon distribution with respect to a depth from a surface layer portion of the ceramic composite sintered body of the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-235767 (JP, A) JP-A-4-270174 (JP, A) JP-A-4-254473 (JP, A) JP-A-2- 44085 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C04B 35/565

Claims (4)

(57) [Claims] 1. A porous structure comprising at least silicon carbide, silicon nitride and free carbon, wherein said silicon carbide forms a three-dimensionally connected porous structure, and a void portion of said porous structure is occupied by silicon nitride and free carbon. A ceramic composite sintered body characterized in that: 2. The ceramic composite sintered body according to claim 1, wherein the amount of silicon nitride and free carbon in the surface layer portion of the sintered body has a composition larger than that of the inside. 3. A sintered body comprising silicon carbide powder as a main component is sintered in an inert atmosphere to obtain a silicon carbide sintered body having a porous structure. A method for producing a ceramic composite sintered body, comprising performing a pressure heat treatment under a gas pressure to precipitate silicon nitride and free carbon in voids of the porous structure. 4. A composition comprising silicon carbide and silicon nitride as main components,
A carbide and / or nitride of boron and / or aluminum and free carbon dispersed therein, and the silicon carbide-based crystal forms a three-dimensionally connected porous structure;
A sliding member comprising a ceramic composite sintered body in which voids of the porous structure are occupied by silicon nitride and free carbon.