JPH04254473A - Production of ceramic composite sintered body - Google Patents

Abstract

PURPOSE:To obtain a sintered body having high strength and improved sliding property in the surface part by baking a molded body of mixture powder essentially comprising silicon carbide under specified conditions, effecting the reaction of silicon carbide and nitrogen to produce silicon nitride and carbon. CONSTITUTION:To a silicon carbide powder, there are added carbon powder such as carbon black and graphite, phenol resin or coal tar pitch powder which produces carbon by pyrolysis, and boron-contg. compd. such as B4C, by total <=10wt.%. To this mixture powder, a binder, etc., is added and molded, and the molded body is baked at 1000-2200 deg.C under >=500atm. of nitrogen gas pressure. Thereby, the sintered body is highly dense in both inside and surface area, and the reaction above-mentioned proceeds actively in the surface area of the sintetred body. Thereby, the controlling the calcination time, the amt. of carbon is made larger in the surface part than in the inside of the body. The obtd. composite sintered body comprising silicon carbide and silicon nitride as an aggregate has small coefft. of friction in the surface part and higher thermal conductivity and electric conductivity.

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 made of silicon carbide and/or silicon nitride, and relates to a new method for producing a sintered body suitable for, for example, a sliding member.

0002

[Prior Art] Non-oxide ceramics such as silicon carbide and silicon nitride have attracted attention as materials with 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 nitrogen atmosphere or an inert atmosphere, the solid lubricant is uniformly dispersed in a matrix made of silicon nitride or silicon carbide, and the surface of the sintered body is 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 developed a method using silicon carbide as the main component as a starting material without adding a lubricating component to the starting material. After molding the mixed powder into the desired shape,
By firing the compact in a nitrogen pressurized atmosphere and causing silicon carbide and nitrogen to react, silicon nitride and carbon, which itself has solid lubricating properties, can be produced, and the produced carbon It can be present in large amounts in the surface layer of the sintered body, and as a result, it has high strength without inhibiting the sinterability of the system and can improve the sliding properties in the surface layer. It has been discovered that this material can exhibit highly reliable and stable characteristics as a moving member.

[0007] The manufacturing method of the present invention will be explained in more detail below.
First, silicon carbide powder is 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. Additionally, as additives for the silicon carbide powder, carbon powders such as carbon black and graphite, powders such as phenol resins and coal tar pitch that can generate carbon through thermal decomposition, and boron-containing compounds such as B4C are added.
It can be added in a proportion of % by weight or less.

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.

[0009] Next, the molded body obtained as described above is fired, and according to the present invention, this firing is performed 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, 1000 to 2200°C or higher, especially 150°C
By firing at a temperature of 0 to 2200° C. in an atmosphere containing nitrogen gas as an essential component and at a nitrogen gas pressure of 500 atm or more, the reaction shown in Equation 1 above can proceed.

[0010] According to this firing, high densification is achieved in both the interior and the surface layer, and the above-mentioned reaction occurs particularly actively in the surface layer of the sintered body, so by adjusting the firing time etc. A unique sintered body is formed in which the surface layer of the body contains more carbon than the inside. 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.

Further, according to the above method, the quantitative ratio of silicon carbide and silicon nitride, which are aggregates, changes from the surface layer to the inside, and the composition ratio expressed as silicon carbide/(silicon nitride + silicon carbide) changes from the surface layer to the inside. Another major feature is that the silicon nitride crystal grains are large from the inside to the inside, and there is substantially no metal oxide between the generated silicon nitride crystal particles.

The mechanism by which the above-mentioned specific sintered body is formed is not clear, but as the conversion from the surface of silicon carbide particles to silicon nitride progresses in a high-temperature, high-pressure nitrogen atmosphere, volumetric expansion occurs. occurs, and densification progresses to some extent. Once a dense layer is formed in the surface layer, the intrusion of nitrogen gas into the interior of the sintered body is suppressed, resulting in a dense body with a porosity of 10% or less both in the surface layer and inside. It is thought that different tissues are formed almost continuously inside.

[0013] From the viewpoint of sliding properties, it is desirable to control the carbon content in the surface layer of the final sintered body to 1% by volume or more, and if all silicon carbide in the surface layer is converted to silicon nitride In the surface layer, a structure is formed in which silicon nitride is used as an aggregate and carbon is uniformly dispersed at a ratio of about 26% by volume. If you want to make the carbon content more than 26% by volume, you can add carbon powder to the starting material, but if the carbon content in the surface layer exceeds 40% by volume, the strength of the sliding surface will decrease. and undesirable. The content is preferably controlled to 5 to 30% by volume.

[0014] Furthermore, it is desirable that the thickness of the surface layer portion containing at least 20% by volume of carbon is 10 to 2000 μm; if the thickness is thinner 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, it is desirable that the inside of the sintered body is mainly composed of silicon carbide or silicon carbide and silicon nitride, and the composition ratio of silicon carbide/(silicon carbide + silicon nitride) in the inside is 0.2 or more. It is desirable that there be.

[0016]

[Function] According to the method of the present invention, a large amount of carbon can be generated in the sintering process without adding carbon powder to the starting material, so that the sinterability of the system is not inhibited. .

[0017] Furthermore, since a large amount of carbon, which is a solid lubricant in the surface layer, can be present only in the surface layer of the sintered body, the strength of the sintered body as a whole does not decrease, and the strength of the sintered body as a whole is not reduced. Even if a 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, thereby efficiently dissipating the heat generated during sliding. You can also do that. 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.

Furthermore, by completing the above-mentioned reaction in the surface layer, a structure in which carbon is uniformly dispersed as an aggregate of silicon nitride is formed, thereby improving the overall toughness and thermal shock resistance. Since metal oxidation is substantially absent at the grain boundaries of silicon crystals, chemical resistance can be improved.

[0020]

[Example] Example 1 β-SiC powder (average particle size 0.4 μm, oxygen content 0
．． 2% by weight), 10 parts by weight of a 20% resol type phenolic resin solution was added as a molding binder, and an appropriate amount of acetone was further added as a solvent, and after kneading and drying, the mixture was passed through a sieve to obtain molding granules. . The granules were molded into a disc-shaped molded body having an outer diameter of 20 mm and a thickness of 10 mm using a mold press at a molding pressure of 2000 kg/cm 2 .

Next, the molded body is placed in an inert atmosphere (N
2) in an air stream) to carbonize the phenolic resin, and analyzed the composition of the calcined body, which revealed that SiC98.1
% by weight, and carbon content was 1.96% by weight.

[0022] Then, this calcined body was fired by varying the firing temperature and pressure. For the obtained sintered body,
The density of the obtained sintered body was measured by the Archimedes method, and the constituent phases in the surface layer and inside of the sintered body were analyzed by X-ray diffraction. To determine the amount of carbon in the surface layer, cut out the surface layer from the sample, crush it, and then measure the total carbon amount, total nitrogen amount, and total silicon amount using the LECO method. Nitrogen is treated as silicon nitride, and the remaining silicon is treated as silicon carbide. The remaining carbon was calculated as free carbon. At the same time, the constituent phases were examined by powder X-ray diffraction. The results are shown in Table 1.

[0023]

[Table 1]

According to Table 1, samples No. 4, whose temperature is lower than 1000° C. or whose nitrogen pressure is lower than 500 atm.
7, no conversion of silicon carbide to silicon nitride occurred,
No carbon formation was observed.

In addition, the starting materials were changed, the composition of the calcined body was changed to the proportions shown in Table 2, and the firing conditions were changed to 190%
A sintered body was produced in exactly the same manner as described above, except that the sintered body was fired at 0° C. and a nitrogen gas pressure of 2000 atm for 1 hour.

The properties of the obtained sintered body were evaluated in the same manner as described above. In addition, as a mechanical property evaluation, JISR
1601, a four-point bending bending test was conducted.
As shown in Table 2, carbon formation was observed in the surface layer of the sintered bodies in all cases, and the porosity was 1% or less, and the strength was 20 kg/mm2 or more.

[0027]

[Table 2]

[0028]

Effects of the Invention As detailed above, according to the manufacturing method of the present invention, silicon carbide can be produced in the surface layer of a sintered body by forming it during the sintering process without adding carbon powder to the starting materials. A composite material with carbon and/or silicon nitride as an aggregate can be obtained, and a large amount of carbon can be supported without reducing the strength of the sintered body as a whole.

[0029] Therefore, the coefficient of friction in the surface layer portion can be reduced and the thermal conductivity and electrical conductivity can be increased, thereby providing particularly excellent sliding characteristics as a sliding member.

Claims (2)

[Claims] Claim 1: After molding a mixed powder mainly composed of silicon carbide, the molded body is heated at a nitrogen gas pressure of 500 atmospheres or more for 10
1. A method for producing a ceramic composite sintered body, characterized by firing at a temperature of 00 to 2200°C, reacting silicon carbide with nitrogen, and generating silicon nitride and carbon. [Claim 2] The amount of carbon in the surface layer of the sintered body is 5 to 3.
The method for producing a ceramic composite sintered body according to claim 1, wherein the content is 0% by volume.