JP2902796B2 - Ceramic composite sintered body and sliding member using the same - 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 present inventors have studied the above problems and found that a composite of silicon carbide and silicon nitride is used as an aggregate and dispersed and contained as a solid lubricant. A large amount of carbon is present only in the surface layer of the sintered body, and the amount of carbon in the interior is reduced, thereby reducing the strength of the sintered body itself and ensuring a stable and highly reliable sliding member in the surface layer. It has been found that characteristics can be exhibited.

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. Based on the idea that it works, the solid lubricant added to greatly improve the sliding characteristics is shown in FIG.
As shown in (1), the amount of the solid lubricant from the surface layer portion to the inside of the sintered body is reduced.

[0008] The aggregate made of the sintered body of the present invention is made of a composite of silicon carbide and silicon nitride, and has a high strength per se and a sliding property as compared with other ceramics. It has excellent properties.

Further, the solid lubricant must have a volume ratio of 5 to 30% in the surface layer portion of the sintered body. If the solid lubricant is less than 5%, desired sliding characteristics are obtained. If it is not obtained and exceeds 30%, the strength at the surface layer is reduced, so that the sliding surface is likely to be damaged.

On the other hand, the interior of the sintered body may not substantially contain a solid lubricant from the viewpoint that it does not affect the sliding characteristics, and may be made of silicon carbide or silicon nitride which is an aggregate component. Good.

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

The method for obtaining the ceramic composite sintered body of the present invention will be described. In the conventional method of mixing and firing a solid lubricant powder with an aggregate component such as silicon carbide or silicon nitride, 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. As an additive to the silicon carbide powder, as an additive, a carbon powder such as carbon black or graphite, a powder such as a phenol resin, coal tar pitch, or sucrose capable of generating carbon by thermal decomposition, or a boron such as B ₄ C The contained compound can be added at a ratio of 10% by weight or less.

After sufficiently adding and mixing the above-mentioned silicon carbide powder and the above-mentioned additives in some cases, a binder or the like is added to the above-mentioned powder, and a known molding method, for example, press molding, extrusion molding, casting molding, cold isostatic pressing is performed. It is formed into a desired shape by molding or the like. If desired, the molded body can be calcined at 200 to 800 ° C. to generate carbon from a carbon-generating compound such as a phenol resin.

Next, the molded body obtained as described above is fired. According to the present invention, this firing is carried out by the following equation (1).

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 Formula 1 can proceed by baking under an atmosphere containing nitrogen gas as an essential component and pressurizing the nitrogen gas at a pressure of 500 atm or more, particularly 1000 atm or more.

According to this sintering, high densification is achieved in both the inside and the surface layer, and the above-mentioned reaction particularly occurs actively in the surface layer of the sintered body, so that carbon is more present in the surface layer of the sintered body than in the inside. A specific sintered body that increases is formed.

Although the mechanism of this sintering is not clear, the reaction of silicon carbide particles into 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, and as a result, a certain density When the formation progresses and a dense layer is once formed on the surface portion, the entry of nitrogen gas into the sintered body is suppressed, and as a result, the surface layer portion and the inside become a dense body with a porosity of 10% or less. It is considered that a different structure is formed almost continuously between the surface layer and the inside.

Therefore, according to the above firing, the amount of carbon generated in the surface layer portion can be increased by controlling the firing temperature, the firing time, and the like, and the generation of carbon in the sintered body can be suppressed. Desirably, the surface portion of the sintered body is allowed to completely react and is composed of silicon nitride and carbon. In this case, a structure in which silicon nitride is used as an aggregate and carbon is uniformly dispersed at a ratio of about 26% by volume is formed in the surface layer.

Another characteristic of this sintered body is that the ratio of the amounts of silicon carbide and silicon nitride, which are aggregates, changes from the surface layer portion to the inside, and the ratio is silicon carbide / (silicon nitride + silicon carbide). ) Increases from the surface layer portion to the inside. According to such a configuration, the surface layer has silicon nitride-like characteristics, that is, characteristics excellent in thermal shock resistance and toughness. According to a normal silicon nitride-based sintered body, the metal oxide used as a sintering aid exists as a grain boundary phase between silicon nitride crystal grains. Another major feature is that the metal oxide does not substantially exist between the silicon crystal particles, so that the chemical resistance can be improved and the application range as a sliding member can be expanded.

The surface portion having a carbon content of at least 20% by volume preferably has a thickness of 10 to 2000 μm. If the thickness is less than 10 μm, the long-term stability of the sliding characteristics is lacking. The strength of the portion is reduced, and chipping or the like is likely to occur.

On the other hand, the interior of the sintered body is preferably composed mainly of silicon carbide or silicon carbide and silicon nitride. In the interior, the composition ratio of silicon carbide / (silicon carbide + silicon nitride) is 0.2 or more. Desirably.

[0022]

According to the present invention, 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, and the surface layer portion is not deteriorated. In this case, even if a relatively large amount of carbon is present, since the strength inside is high, stable sliding characteristics can be exhibited even as a sliding member. 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 a large amount of carbon can be present in the surface layer, the thermal conductivity of the sintered body itself can be increased, and the heat generated during sliding can be efficiently radiated. it can. 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.

The thermal shock resistance can be enhanced by forming the surface layer of the silicon nitride aggregate as the aggregate.

[0025]

EXAMPLE To a β-SiC powder (average particle size 0.4 μm, oxygen content 0.1% by weight), an appropriate amount of a 20% solution of a resol type phenol resin 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 molded using a mold press at a molding pressure of 2000 kg / cm ² to form a disk-shaped molded body having an outer diameter of 20 mm and a thickness of 10 mm.

Next, the compact was placed in an inert atmosphere at 600 ° C. (N
_After calcination in ( ₂ air currents) and carbonizing the phenolic resin,
The composition of the calcined body was analyzed. Then, this calcined body is N
_{2 The} pressure, firing temperature, and firing time were set to the conditions shown in Table 1, and firing was performed.

The density of the obtained sintered body was measured by the Archimedes method, and cut out from the surface layer and the inside of the sintered body. After pulverization, total carbon, total nitrogen and total silicon were measured by the LECO method. Was calculated as silicon nitride, the remaining silicon was determined as being present as silicon carbide, bond carbon was determined, and the remainder was calculated as free carbon. The constituent phases of the aggregate were analyzed by X-ray analysis. In addition, in the sample to which B ₄ C was added as an additive, B ₄ C was also nitrided and became BN, but was not detected by X-ray diffraction measurement because the added amount was very small.

As a characteristic evaluation, a four-point bending strength was measured based on JISR1601. Further, the volume resistivity was measured by a four-terminal method using a bending test piece. Further, as a chemical resistance test, an 8 mm square die-shaped sample was cut out, and the state after immersion in a 20% hydrochloric acid solution, a 60% nitric acid solution, and a 90% sulfuric acid solution for 3 days was observed.

As an evaluation of sliding characteristics, a SUSII steel ball was brought into contact with a disk of φ50 mm having a lap-finished surface as a fixed pin, the sample disk was rotated, and the contact load and frictional force were measured to measure the wear coefficient. I asked.

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 is prepared in the same manner as described above, and then calcined in argon at 2050 ° C. for 1 hour to produce a dense silicon carbide sintered body. Was measured and evaluated (in the table, sample No,
1).

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

According to Tables 1 to 3, in the case of a conventional sintered body made of silicon carbide, the coefficient of friction as a sliding characteristic is 0.4 to 0.4.
In contrast to 0.5, the products of the present invention all reached a coefficient of friction of 0.2 to 0.3. Further, the strength of the sintered body itself was at least 40 kg / mm ² .

[0035]

As described above, according to the present invention,
By using ceramics as the aggregate and having a large amount of solid lubricant on the surface of the sintered body compared to the inside, excellent sliding characteristics can be obtained without lowering the strength of the entire sintered body. Can be In addition, the thermal conductivity and electrical conductivity of the sintered body itself can be increased by allowing a large amount of carbon as a solid lubricant to be present in the surface layer, thereby efficiently radiating heat generated during sliding. And electrical discharge machining can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in carbon content with respect to a depth from a surface layer of a ceramic composite sintered body of the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-138376 (JP, A) JP-A-62-182178 (JP, A) JP-A-63-236782 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) C04B 35/565-35/577 C04B 35/584-35/596 C10M 111/00

Claims (2)

(57) [Claims] 1. A sintered body mainly composed of silicon carbide and silicon nitride, wherein carbon is dispersed and contained, and the surface layer portion of the sintered body contains the carbon in an amount of 5 to 30% by volume which is larger than that of the inside. A ceramic composite sintered body characterized in that: 2. A sintered body mainly composed of silicon carbide and silicon nitride, wherein carbon is dispersed and contained, and the surface layer portion of the sintered body contains the carbon in an amount of 5 to 30% by volume which is larger than that of the inside. A sliding member characterized by the above-mentioned.