JPH04254471A - Ceramic composite sintered body and sliding member using the same - Google Patents

Abstract

PURPOSE:To improve the sliding property of the surface layer part without decreasing the strength of a sintered body itself using silicon carbide and silicon nitride as an aggregate by dispersing carbon in a manner that a larger amt. of carbon exists in the surface layer of the sintered body. CONSTITUTION:The above ceramic composite body is a sintered body essentially comprising silicon carbide and silicon nitride and contains dispersion of carbon. The amt. of carbon in the surface layer is larger than that in the inside. It is preferable that the carbon amt. is 5-30vol.% in the surface layer. If the amt. is less than 5vol.%, the desired sliding properties can not be obtd., and if it exceeds 30%, strength in the surface layer decreases. On the other hand, the inner area of the sintered body does not affect the sliding property, so that no carbon may be included. However, if the compsn. or structure of the sintered body from the surface to inside abruptly changes, the interfacial part suffers from stress due to the difference of characteristics and causes cracks or chipping. Therefore, the amt. of carbon is gradually decreased as shown in the figure.

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 which has high strength and excellent sliding properties and is suitable for sliding members such as mechanical seals and bearings, and a sliding member using the same.

0002

[Prior Art] Non-oxide ceramics such as silicon carbide and silicon nitride have attracted attention as materials that have superior hardness, strength, toughness, and chemical stability compared to other ceramics and metals. Used as mechanical seal parts, bearing parts, and chemical valve parts.

However, silicon nitride and silicon carbide alone cannot provide sufficient sliding properties, so Al2 is used as a sintering aid for silicon nitride powder and silicon carbide powder.
At the same time as adding O3, oxides of Group 3a elements of the periodic table, or carbon and B4C, graphite and BN are added.
By adding a solid lubricant such as and firing it in a vacuum or an inert atmosphere, the solid lubricant is uniformly dispersed in a matrix made of silicon nitride or silicon carbide, and this causes the surface of the sintered body to be uniformly dispersed. Efforts are being made to improve the sliding characteristics of

0004

[Problems to be Solved by the Invention] In order to improve the sliding properties, it is desirable to have a large amount of solid lubricant on the surface layer of the sintered body, but if a large amount of solid lubricant is added, the sintered body itself There is a problem in that the densification of ceramics is inhibited, and the strength of the ceramic itself as so-called aggregate becomes low, making it easy for sliding members to crack or chip. For this reason, there was naturally a limit to the amount of solid lubricant added.

[0005] Furthermore, due to the manufacturing method, it is necessary to uniformly disperse the solid lubricant itself, and in some cases, there is a problem that the solid lubricant inside the sintered body becomes a source of destruction of the sintered body, reducing its strength. Moreover, sintered bodies in which silicon nitride is dispersed as a solid lubricant with a matrix have poor chemical resistance due to the presence of metal oxides added as sintering aids at the grain boundaries of the silicon nitride crystals. There is also the problem that it is limited.

[0006]

[Means for Solving the Problems] As a result of repeated studies on the above problems, the present inventors have devised a system that uses silicon carbide and/or silicon nitride as aggregates to disperse and contain carbon, etc. By making a large amount of solid lubricant exist only in the surface layer of the sintered body and reducing the amount of solid lubricant inside, it is possible to improve the sliding properties in the surface layer without reducing the strength of the sintered body itself. The inventors have discovered that this makes it possible to exhibit highly reliable and stable characteristics as various sliding members.

The present invention provides that the sliding characteristics of a ceramic sintered body are controlled by the structure and organization of the surface layer of the sintered body, and that the inside of the sintered body is a so-called support member for sliding. Figure 1 shows solid lubricants added to greatly improve sliding properties.
As shown in Fig. 2, the sintered body is characterized in that the amount of solid lubricant from the surface layer to the inside of the sintered body is reduced.

The aggregate of the sintered body of the present invention may be silicon carbide, silicon nitride, or a composite of silicon carbide and silicon nitride, and these ceramics themselves have high strength and are easy to slide. It has superior properties compared to other ceramics.

[0009] Furthermore, it is desirable that the solid lubricant exists in the surface layer of the sintered body at a volume ratio of about 5 to 30%; if it is less than 5%, the desired sliding characteristics cannot be obtained. First, if it exceeds 30%, the strength of the surface layer portion decreases, making it easy for damage to the sliding surface to occur.

On the other hand, the interior of the sintered body does not need to contain substantially any solid lubricant from the viewpoint that it does not affect the sliding properties, and may be composed of silicon carbide or silicon nitride, which are aggregate components. good.

However, if the composition or structure of the sintered body suddenly changes from the surface layer to the inside, stress is likely to occur at the boundary due to the difference in characteristics, which may cause cracks or chips. As shown in FIG. 1, the amount of solid lubricant is preferably configured to gradually decrease toward the inside.

To explain the method for obtaining the ceramic composite sintered body of the present invention, the conventional method of mixing solid lubricant powder with aggregate components such as silicon carbide and silicon nitride and firing the mixture results in a uniform structure. According to the structure of the present invention, a structure in which the solid lubricant content differs between the surface layer portion and the interior portion is not formed.

According to the present invention, silicon carbide powder is first prepared as a raw material powder. The silicon carbide powder used may be either α-SiC or β-SiC, or a mixture thereof. The average particle size of the silicon carbide powder is suitably 0.1 to 2 μm. Additives to the silicon carbide powder mentioned above include carbon powders such as carbon black and graphite, powders such as phenol resins that can generate carbon through thermal decomposition, coal tar pitch, and sucrose, and boron-containing materials such as B4C. 10% by weight of compound
It can be added in the following proportions.

After the silicon carbide powder and optionally the additives are sufficiently added and mixed, a binder or the like is added to the powder, and the powder is subjected to a known molding method such as press molding, extrusion molding, casting molding, or cold isostatic pressing. Form into a desired shape by molding or the like. The molded body can be calcined at 200 to 800°C, if desired, to generate carbon from a carbon-generating compound such as a phenol resin.

Next, the molded body obtained as described above is fired, and according to the present invention, this firing is carried out according to the following equation 1.

[Math. 1] 3SiC+2N2 → Si3 N
4 +3C, firing is carried out in an atmosphere in which silicon nitride and carbon can be produced by a reaction between silicon carbide and nitrogen. Specifically, at a temperature of 1000°C or higher, especially 1500°C or higher, the atmosphere contains nitrogen gas as an essential component and the nitrogen gas pressure is 500 atm or higher, especially 1000 °C or higher.
By firing under pressure equal to or higher than atmospheric pressure, the reaction shown in Equation 1 can proceed.

[0016] According to this firing, high densification is achieved both in the interior and the surface layer, and the above reaction occurs particularly actively in the surface layer of the sintered body, so that carbon is more concentrated in the surface layer of the sintered body than in the inside. A specific sintered body is formed that increases in number.

Although the mechanism of this sintering is not clear, in a nitrogen atmosphere under high temperature and high pressure, a reaction progresses from the surface of silicon carbide particles to silicon nitride, resulting in volumetric expansion, which results in some degree of densification. As the sintering progresses and once a dense layer is formed in the surface layer, the entry of nitrogen gas into the interior of the sintered body is suppressed, resulting in a dense body with a porosity of 10% or less in both the surface layer and the interior. It is thought that different structures are formed almost continuously between the surface layer and the inside.

Therefore, according to the above-described firing, by controlling the firing temperature, firing time, etc., it is possible to increase the amount of carbon produced in the surface layer portion and to suppress the production of carbon inside the sintered body. Preferably, the reaction proceeds completely in the surface layer of the sintered body, and it is desirable that the surface layer is composed of silicon nitride and carbon. In this case, a structure is formed in the surface layer in which silicon nitride is used as aggregate and carbon is uniformly dispersed at a ratio of about 26% by volume.

Another feature of this sintered body is that the quantitative ratio of silicon carbide and silicon nitride, which are aggregates, changes from the surface layer to the inside, and the ratio of silicon carbide/(silicon nitride + silicon carbide) changes. The composition ratio expressed by ) increases from the surface layer to the inside. According to such a structure, the surface layer portion has characteristics similar to silicon nitride, that is, characteristics excellent in thermal shock resistance and toughness. Furthermore, in a normal silicon nitride sintered body, a metal oxide used as a sintering aid exists between silicon nitride crystal grains as a grain boundary phase, but in the surface layer of this sintered body, nitride Another major feature is that there is substantially no metal oxide between silicon crystal particles, which improves chemical resistance and expands the range of applications for sliding members.

[0020] It is preferable that the thickness of the surface layer containing at least 20% by volume of carbon is 10 to 2000 μm; if the thickness is less than 10 μm, the long-term stability of the sliding properties will be lacking, and if it is thicker than 2000 μm, the surface layer will be damaged. The strength of the parts decreases and chips are more 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, and the composition ratio of silicon carbide/(silicon carbide + silicon nitride) is 0.2 or more in the interior. It is desirable that there be.

[0022]

[Function] According to the present invention, by making a large amount of carbon, which is a solid lubricant in the surface layer, exist only in the surface layer of the sintered body, the strength of the sintered body as a whole is not reduced, and the surface layer Even if a relatively large amount of carbon is present, the internal strength is high, so it can exhibit stable sliding characteristics as a sliding member. Moreover, since the structural changes from the surface layer to the inside are almost continuous, it is possible to reduce stress caused by differences in characteristics within the sintered body.

Furthermore, 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 thereby the heat generated during sliding can be efficiently dissipated. can. Furthermore, by allowing an appropriate amount of carbon to exist inside the sintered body, the electrical resistance of the entire sintered body can be reduced, thereby allowing electric discharge machining to be performed.

Thermal shock resistance can be improved by forming the surface layer of silicon nitride aggregate as the aggregate.

[0025]

[Example] To β-SiC powder (average particle size 0.4 μm, oxygen content 0.1% by weight), an appropriate amount of a 20% solution of resol type phenolic resin was added as a molding binder, and acetone was added as a solvent. After adding an appropriate amount and kneading and drying, the mixture was passed through a sieve to obtain moldable granules. These granules were molded using a mold press at a molding pressure of 2000 kg/cm2 to an outer diameter of 20 mm.
A disc-shaped molded body with a thickness of 10 mm was prepared.

Next, the molded body is placed in an inert atmosphere (N
2 in an air stream) to carbonize the phenol resin, the composition of the calcined body was analyzed. Then, this calcined body was fired under the conditions shown in Table 1: N2 pressure, firing temperature, and firing time.

The density of the obtained sintered body was measured by the Archimedes method, and the surface layer and the inside of the sintered body were cut out and crushed, and the total carbon, total nitrogen, and total silicon were measured by the LECO method. was calculated as silicon nitride, the remaining silicon was calculated as bonded carbon assuming that it existed as silicon carbide, and the rest was calculated as free carbon. In addition, the constituent phases of the aggregate were analyzed by X-ray analysis. In addition, in the sample to which B4 C was added as an additive, B4 C was also nitrided and became BN, but the amount added was so small that it was not detected by X-ray diffraction measurement.

As a characteristic evaluation, four-point bending strength was measured based on JISR1601. Further, the volume resistivity was measured using a 4-terminal method using a bending test piece. Further, as a chemical resistance test, a dice-shaped sample of 8 mm square was cut out, and the state was observed after immersing it in a 20% hydrochloric acid solution, a 60% nitric acid solution, and a 90% sulfuric acid solution for 3 days.

In addition, as a sliding property evaluation, a SUS II steel ball was brought into contact with a φ50 mm disk whose surface was lapped as a fixed pin, the sample disk was rotated, and the contact load and friction force were measured to determine the wear coefficient. I asked for

Next, as a comparative example, B4C powder was added to 100 parts by weight of β-SiC powder (average particle size 0.4 μm).
．． A molded body was created in the same manner as above except that 4 parts by weight was added and mixed, and then fired at 2050°C in argon for 1 hour to create a dense silicon carbide sintered body. Characteristics were measured and evaluated (in the table, sample N
o, 1).

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

According to Tables 1 to 3, conventional sintered bodies made of silicon carbide have a friction coefficient of 0.4 to 0.4 as sliding characteristics.
0.5, all of the products of the present invention reached a level of friction coefficient of 0.2 to 0.3. Furthermore, the strength of the sintered body itself was 40 kg/mm2 or more.

[0035]

[Effects of the Invention] As detailed above, according to the present invention,
By using ceramics as aggregate and having a larger amount of solid lubricant present in the surface layer of the sintered body than inside, excellent sliding properties can be obtained without reducing the strength of the sintered body as a whole. It will be done. In addition, by having a large amount of carbon as a solid lubricant present in the surface layer, the thermal conductivity and electrical conductivity of the sintered body itself can be increased, thereby efficiently dissipating the heat generated during sliding. It is also possible to perform electrical discharge machining.

[Brief explanation of the drawing]

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

Claims (5)

[Claims] 1. A ceramic composite sintered body mainly composed of silicon carbide and/or silicon nitride and containing carbon dispersed therein, characterized in that the amount of carbon in the surface layer of the sintered body is larger than in the inside. Concretion. [Claim 2] The amount of carbon in the surface layer of the sintered body is 5 to 3.
The ceramic composite sintered body according to claim 1, wherein the content is 0% by volume. 3. A sliding member containing silicon carbide and/or silicon nitride as an aggregate and a solid lubricant dispersed therein, characterized in that the content of the solid lubricant is higher in the surface layer than in the interior. moving parts. 4. The sliding member according to claim 3, wherein the solid lubricant is carbon. 5. The sliding member according to claim 3, wherein the amount of carbon in the surface layer portion is 5 to 30% by volume.