JPS6265973A - Silicon carbide/graphite composite sintered body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-density, high-strength silicon carbide/graphite composite sintered body.

Silicon carbide has extremely excellent chemical and physical properties, so it is attracting attention as a sliding material, a high-temperature structural material, or a corrosion-resistant material.

For example, it is a useful material that can be used in many applications such as mechanical seal rings, plungers, bearings, sandblasting nozzles, tappets, and radio wave absorbers.

Generally, methods for manufacturing silicon carbide sintered bodies include reaction sintering method,
The hot press method and the non-pressure sintering method are known, and among these methods, the pressureless sintering method is the most advantageous, but silicon carbide is originally a material that is difficult to sinter. Therefore, it has conventionally been considered difficult to sinter sufficiently without applying pressure.

However, recently various sintering aids have been developed and attempts have been made to overcome the above-mentioned difficulties, and the following are typical examples of them for crystal shaping of silicon carbide.

That is, according to JP-A-51-148712, α-5i
C with 0.15-3.0% by weight of boron and 0.5-5.0%
A carbonaceous organic material corresponding to carbon is mixed and molded with up to 1.0% by weight of additional carbon, and the molded body is then sintered to a density of at least 2.4 g/cm' or 75% of the theoretical density. A method is disclosed.

On the other hand, according to JP-A-50-78609, β-5iC
A boron-containing compound in an amount corresponding to 0.3 to 3.0 weight % of boron and a carbonaceous additive in an amount corresponding to 0.1 to 1.0 weight % of carbon are mixed and molded, and at least theoretically 85% of density
A method for obtaining a silicon carbide sintered body having It acts as if it were being formed, reducing silica and acting as a getter for free silicon, and when added at 1.0% by weight or more, the unreacted excess carbon acts very similar to permanent pores. It is stated that it limits the density and strength that can ultimately be reached.

According to such conventional methods, when sintering is carried out at a high temperature of 2000°C or higher to obtain a dense sintered body, β-5iC is converted into α-5iC, regardless of the crystal form of silicon carbide. Grain growth occurs with transformation, and although phase transformation does not occur in α-3iC, grain growth still occurs.
This hinders obtaining a fine and dense sintered body.

In view of this situation, as a result of various studies, the inventors focused on the fact that carbon has the effect of suppressing the grain growth of SiC in the sintering of α-silicon carbide, and developed carbon black in addition to the sintering aid. If 1 to 5% by volume is added to α-silicon carbide, it not only suppresses the grain growth of SiC during sintering, but also forms a fine and uniform second phase component at the grain boundaries of SiC after sintering. Unexpectedly, it was found to exist as graphite, and we succeeded in obtaining a fine and dense silicon carbide/graphite composite sintered body with excellent thermal shock resistance.

It was discovered that although graphitization of amorphous carbon such as carbon black normally requires high temperature heating of 2500 to 3000 DEG C., the above-mentioned graphitization occurs during the sintering process of α-5iC.

The silicon carbide/graphite sintered body obtained in this way has a mechanical bending strength that is more than 20% higher than that of a SiC sintered body in which grain growth is forced according to the conventional method, and a thermal shock temperature that is lower than that of the conventional one. It was also confirmed that the temperature rose by 90 to 200°C.

The addition of carbon black causes graphitization during sintering of αSiC, and its presence as graphite at the SiC grain boundaries in the silicon carbide/graphite composite sintered body reduces the coefficient of dynamic friction by 20% compared to conventional silicon carbide sintered bodies. % or more, and since the graphite present in the grain boundaries as a second phase has excellent corrosion resistance, it does not deteriorate the chemical stability of silicon carbide in any way.

Note that the average grain size of graphite present as a second phase in this αSiC grain boundary is 3 μm or less.

The necessity of adding carbon black in this invention is as follows.

In other words, if a carbonaceous organic compound such as a phenolic resin is used in an increased amount to replace the addition of carbon black, even if it is possible to disperse and mix it uniformly, the phenolic resin itself has properties as a binder. For example, if it is added in an amount of 10% by weight or more based on α-silicon carbide, it becomes difficult to perform raw processing such as molding into an arbitrary shape, and approximately 50% of the added phenolic resin
% scatters at low temperatures, resulting in an extremely low green density during calcination, which impedes sinterability. Furthermore, when graphite powder was added from the beginning instead of carbon black, compared to the addition of carbon black, there was virtually no effect of suppressing the growth of αSiC particles during sintering, and no increase in bending strength was observed.

When the addition of carbon black is less than 1% by volume based on α-silicon carbide, the addition effect is insufficient, while when it is added above 5% by volume, it reduces the sinterability of SiC and improves the sintering characteristics of SiC. damage.

In the present invention, if a powder of 1 μm or less is used as α-silicon carbide, a dense sintered body can be obtained most effectively, and in particular, the crystal growth of silicon carbide can be suppressed by adding 1 to 5% by volume of carbon black. However, if the amount of carbon black is outside the specified range, the risk of crystal growth increases accordingly.
Furthermore, if the amount exceeds the predetermined range, the sintering efficiency will decrease due to an unnecessary increase in the amount added, and the quality may also deteriorate.

Next, an example will be described.

Example 1 α・SiC powder with an average particle size of 0.8 μm and
．． 5% by volume of carbon black was added to a mixture of 5% by weight of boron carbide and 8.0% by weight of phenolic resin, wet mixed in water, dried, and passed through a sieve to form a 30mm x 10mm product. It was molded to a size of 5 mm.

Next, it was calcined in nitrogen at 800℃ for 60 minutes, and then
Normal pressure sintering was performed in an argon atmosphere at 100°C for 60 minutes.

The calcined body and the sintered body were each crushed into powder in an agate mortar and diffracted using an X-ray diffraction device.
As shown in Figures (a) and (b), it was found that all of the added carbon black was graphitized and present in the sintered body.

Example 2 A sintered body obtained in the same manner as in Example 1 was surface-ground and then wet-polished with a 9 μm diamond, and the state of dispersion of graphite present in the sintered body was observed using an optical microscope. After that, the polished surface was etched with Murakami's reagent, and S
The iC grain size and the graphite grain size existing at the grain boundaries of SiC were observed.

For comparison, a sintered body treated in the same manner without the addition of carbon black and a sintered body with graphite powder added instead of carbon black were made and tested. The results are shown in the micrograph in Figure 2 (No. 11). As is clear from the comparison of the additive-free sintered body, the graphite-added sintered body of (2), and the carbon black-added sintered body according to the present invention shown in (3), in the case of no additive, the grain growth of SiC is Although the crystal growth is severely suppressed to some extent by the addition of graphite, the addition of carbon black yields a finer and denser sintered body than in the case of the addition of graphite.

Example 3 After measuring the bulk density of the sintered body obtained by changing the amount of carbon black added in Example 1 as shown in Table 1,
Surface grind to dimensions of 4n+mX 8mmX25mm 3
Point bending strength was measured. The results are shown in Table 1. From these results, it can be seen that when the amount of carbon black added is 1% to 5% by volume, the bending strength increases by about 20% compared to the product without the addition of carbon black.

Table 1 Example 4 A sintered body obtained in the same manner as Example 3 was
The surface was ground to a size of 25 mm and the thermal shock temperature (
ΔT) was measured. The measurement method uses an underwater quenching method, and after holding the sample at various temperatures (T"C) for 15 minutes, it is dropped into water (TO"C), and then the limit temperature at which no deterioration of the bending strength of the sample is observed is determined. ΔT=T−To was measured.

The results are shown in Table 2.

As a result, it was found that as the amount of carbon black added increases, the thermal shock resistance increases, and when 5% by volume is added, the thermal shock temperature (ΔT) increases by almost 100°C.

Table 2 Example 5 A ring with an outer diameter of 30, an inner diameter of 20, and a fin thickness of 5 was sintered in the same manner as in Example 3, and the sliding surface was surface-ground and then wet-polished with a 9 μm diamond, under the following conditions. A wet sliding test was conducted. The results are shown in Table 3.

These results show that when 1% or more of carbon black is added, this carbon black converts into graphite, and the coefficient of dynamic friction of the sintered body existing at the SiC grain boundaries is 20% higher than that of a product without additives.
It can be seen that the amount of wear has been reduced by half, and the amount of wear has been reduced by half.

However, it was found that when a large amount of 30% by volume was added, the sintering did not reach even 80% of the theoretical density, and the coefficient of dynamic friction and amount of wear increased.

The sliding conditions are as follows.

Testing machine Mechanical seal type (ring-on-ring method) % formula % As is clear from the above comparative tests, the present invention uses silicon carbide, which has excellent properties such as friction coefficient, bending strength, and thermal shock temperature.
A graphite composite sintered body can be manufactured through a relatively simple process.

[Brief explanation of drawings]

Figures 1 (a) and (b) are X'fa diffraction graphs for the calcined body and sintered body before sintering, and Figure 2 is for the sintered body without carbon black, the sintered body with graphite added, and It is a micrograph of a carbon black added sintered compact. Figure 1 (a) (Before sintering 2 (ill) 2θ (de >) Procedure Amendment Book July 16, 1986 Commissioner of the Patent Office Black 1) Akio Tono 1, Indication of Case 1986 Patent Application No. 96399 2, Name of the invention Silicon carbide/graphite composite sintered body 3, Relationship to the case of the person making the amendment Patent applicant (454) Nippon Spark Plug Co., Ltd. 4, Sub-agent address ■100 Chiyoda-ku, Tokyo Kasumigaseki 2-2-4 Kasumiyama Building 7th floor Telephone number (581) 2241 (representative) (revised) Specification 1, Title of invention Silicon carbide/graphite composite sintered body 2, Claim 1, Alpha carbonization It is a pressureless sintered body of silicon, in which graphite with an average particle size of 3 μm or less is formed as a second phase at SiC grain boundaries by binding carbon black to 1 to 7 times the amount of silicon carbide. Uniformly dispersed in a proportion of 5% by volume,
A silicon carbide/graphite composite sintered body characterized by having a density of 90% or more of the theoretical density of Σ-Ω. 3. Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a high-density, high-strength silicon carbide/graphite composite sintered body. Silicon carbide has extremely excellent chemical and physical properties, so it is attracting attention as a sliding material, a high-temperature structural material, or a corrosion-resistant material. For example, it can be used in many ways, such as mechanical seal rings, plungers, bearings, sandblasting nozzles, tappets, or radio wave absorbers (prior technology). Sintering methods, hot pressing methods, and non-pressure (pressureless) sintering methods are known. Among them, the pressureless sintering method is the most advantageous, but silicon carbide is inherently difficult to use. Since it is a sinterable material, it has conventionally been considered difficult to sinter it sufficiently without applying pressure. However, recently various sintering aids have been developed and attempts have been made to overcome the above-mentioned difficulties. Typical examples of this are as follows when looking at crystal shaping of silicon carbide. That is, according to JP-A-51-148712, 0.15 to 3.0% by weight of boron and 0.5 to 5% by weight of boron are added to α silicon carbide.
a carbonaceous organic material corresponding to 0% by weight of carbon; and 1.
Additional carbon up to 0 wt.
% or more is disclosed. On the other hand, according to JP-A No. 50-78609, a β silicon carbide-containing compound and a carbonaceous additive in an amount corresponding to 0.1 to 1.0% by weight of carbon are mixed and molded to form a compound with a theoretical density of at least 85% by weight. A method for obtaining a silicon carbide sintered body having a Reduction of silica and getter of free silicon as if formed from
However, when added in excess of 1.0% by weight, the unreacted excess carbon acts much like permanent pores, limiting the density and strength that can ultimately be achieved; etc. are stated. (Problems to be Solved by the Invention) According to such conventional methods, regardless of the crystal form of silicon carbide, when sintering is performed at a high temperature of 2000°C or higher to obtain a dense sintered body, , β-silicon carbide causes grain growth as it transforms into α-silicon carbide, and although α-silicon carbide does not undergo phase transformation, grain growth still occurs.
It is pointed out as a problem that must be solved that it is an obstacle to obtaining a fine and dense sintered body. (Means for solving the problem) In view of this situation, as a result of various studies, the inventors focused on the fact that carbon has the effect of suppressing the grain growth of SiC especially in the sintering of α-silicon carbide, and developed the above-mentioned method. By the way, 3% by weight or less of boron or a boron compound (in this case, preferably 0.1 to 3% by weight in terms of boron, particularly preferably 0.1 to 3% by weight).
Apart from the sintering aid (for example, 1 to 1.0% by weight), carbon black is used in an amount of 1 to 5% by volume relative to alpha silicon carbide.
If added, it not only suppresses the growth of SiC grains during sintering, but also unexpectedly exists as graphite at SiC grain boundaries in a fine and uniform state as a second phase component after sintering. We succeeded in obtaining a fine and dense silicon carbide/graphite composite sintered body with excellent thermal shock resistance. It has been discovered that although graphitization of amorphous carbon such as carbon black usually requires high temperature heating of 2500 to 3000°C, the above-mentioned graphitization occurs during the sintering process of α-silicon carbide. That is, the present invention is an atmospheric pressure sintered body of α silicon carbide,
As a second phase at SiC grain boundaries, graphite with an average particle size of 3 μm or less, which is graphitized during sintering using carbon black as a starting material, is uniformly dispersed at a ratio of 1 to 5% by volume based on the amount of silicon carbide. It is a silicon carbide/graphite composite sintered body characterized by having a density of 90% or more of the theoretical density of SiC. The silicon carbide/graphite sintered body obtained in this manner has a higher
It was confirmed that the mechanical bending strength increased by more than 20%, and the thermal shock temperature increased by 90 to 200°C compared to the conventional one. The addition of carbon black causes graphitization during sintering of α-silicon carbide, and by existing as graphite at the SiC grain boundaries in the silicon carbide/graphite composite sintered body, compared to conventional silicon carbide sintered bodies, Graphite, which lowers the coefficient of dynamic friction by 20% or more and is present in the SiC grain boundaries as a second phase, has excellent corrosion resistance, so it does not deteriorate the chemical stability of silicon carbide in any way. (Function) In this invention, the importance of carbon black is as follows. In other words, carbonaceous organic compounds such as phenolic resin,
However, if the amount is increased so that it can be used as a substitute for adding carbon black, even if it is possible to disperse and mix it uniformly, the phenol resin itself has properties as a binder.
For example, if it is added in an amount of 10% by weight or more based on α-silicon carbide, it becomes difficult to perform raw processing such as molding into an arbitrary shape, and approximately 5% of the added phenolic resin
Since 0% scatters at low temperatures, the green density is extremely reduced during calcination, impeding sinterability. Furthermore, when graphite powder is blended from the beginning instead of carbon black, compared to the addition of carbon black, the effect of suppressing the grain growth of α silicon carbide particles during sintering is not effectively stopped, and an increase in flexural strength is also observed. I couldn't. When carbon black is added at less than 1% by volume relative to α-silicon carbide, the effect of adding it is insufficient, while when it is added to α-silicon carbide at 5% by volume, it reduces the sinterability of silicon carbide and impairs its excellent properties. cormorant. In the present invention, using powder of 1 μm or less as α-silicon carbide,
A dense sintered body can be obtained most effectively, especially 1
By adding up to 5% by volume of carbon black, effective sintering can be performed while suppressing the crystal growth of silicon carbide, and if the amount of carbon black is outside the specified range and the amount is small, the risk of crystal growth increases accordingly. In addition, if the amount exceeds the specified range, the sintering efficiency will decrease due to an unnecessary increase in the amount added, and it may also lead to quality deterioration. The average particle size of the graphite present as a phase must be less than 3 μm.
This is because graphite having a particle size larger than m actually deteriorates the sinterability and strength of α-silicon carbide. (Example) Example 1 α silicon carbide powder with an average particle size of 0.8 μm and
．． 5% by weight of boron carbide and 8.0% by weight of phenolic resin, in addition to this, 5% by volume of carbon black.
After adding and wet-mixing in water, it was dried, passed through a sieve, and then molded into a size of 30 mm x 10 mm x 5 mm. Next, it was calcined in nitrogen at 800℃ for 60 minutes, and then
Normal pressure sintering was performed in an argon atmosphere at 100°C for 60 minutes. The calcined body and the sintered body were each crushed into powder in an agate mortar and diffracted using an X-ray diffraction device.
As shown in Figures (a) and (b), it was found that all of the added carbon black was graphitized and present in the sintered body. Example 2 A sintered body obtained in the same manner as in Example 1 was surface-ground and then wet-polished with a 9 μm diamond, and the state of dispersion of graphite present in the sintered body was observed using an optical microscope. After that, the polished surface was etched with Murakami's reagent, and S
The particle size of graphite present within iC grains and at SiC grain boundaries was observed. For comparison, a sintered body treated in the same manner without the addition of carbon black and a sintered body with graphite powder added instead of carbon black were made and tested.The results are shown in the micrograph in Figure 2 +11. As is clear from the comparison of the additive-free sintered body in (2), the graphite-added sintered body in (2), and the carbon black-added sintered body according to the present invention shown in (3), in the case of no additive, SiC grain growth Although the crystal growth is slightly suppressed by the addition of graphite, the addition of carbon black yields a finer and denser sintered body than in the case of addition of graphite. Example 3 After measuring the bulk density of the sintered body obtained by changing the amount of carbon black added in Example 1 as shown in Table 1,
Surface grinding was performed to a size of 4 mm x 8 mm x 25 mm, and the three-point bending strength was measured. The results are shown in Table 1. From this result, when the amount of carbon black added is 1% to 5% by volume, the bending strength is 2.30% higher than that of a product without additives.
This is in contrast to graphite addition, which shows a tendency to decrease. Table 1 Example 4 A sintered body obtained in the same manner as in Example 3 was also treated with 4Ryu
The surface was ground to a size of 3 mm x 25++n, and the thermal shock temperature (ΔT) of each was measured. The measurement method uses an underwater quenching method, and after holding the sample at various temperatures (T"C) for 15 minutes, it is dropped into water (T, °C), and then the limit temperature at which no deterioration of the bending strength of the sample is observed is determined. ΔT=T−T was measured. The results are shown in Table 2. From this result, the increase in thermal shock resistance due to the addition of carbon black was found to be such that the thermal shock temperature (ΔT) was approximately 100°C when 1 to 5% by volume was added. Table 2 Example 5 A ring with an outer diameter of 30 mm and an inner diameter of 20 mm and a thickness of 5 mm was sintered in the same manner as in Example 3, the sliding surface was surface ground, and then wet-grinded with a 9 μm diamond. After polishing, a wet sliding test was conducted under the following conditions.The results are shown in Table 3.The results show that when 1% by volume or more of carbon black is added, this carbon black becomes graphite and exists at the SiC grain boundaries. The coefficient of dynamic friction of the sintered body is 20% compared to additive-free products.
It can be seen that the amount of wear has been reduced by half, and the amount of wear has been reduced by half. However, it was found that when a large amount of addition of more than 5% by volume, especially as much as 30% by volume, was not sintered to even 80% of the theoretical density, the coefficient of dynamic friction and amount of wear increased on the contrary. The sliding conditions are as follows. Test machine Mechanical seal type (ring-on-ring method) % formula % In Example 1, carbon black and graphite were added with different particle sizes, and the sintered bodies were sintered in the same manner as in Example 1. After etching in the same manner as in 2, the average grain size of graphite present at SiC grain boundaries was measured using a scanning electron microscope (SEM). Next, the density and bending strength of the sintered body were measured in the same manner as in Example 3, and the relationship with the grain size of graphite present at the SiC grain boundaries was investigated. The results are shown in Table 4. This result shows that the presence of graphite with a particle size larger than about 3 μm actually reduces the sinterability and strength of α-silicon carbide under normal pressure, and from a comparison of sample numbers 18 and 19, the use of carbon black shows that graphite ( It is also known that the grain growth inhibiting effect is more remarkable than in the case where the grain size is 2.5 μm (the lower limit grain size). Table 4 □□□= In each of the above examples, the test pieces were distinguished by the amounts of carbon black and graphite added for simplicity, but the actual amount of graphite present at the SiC grain boundaries in the sintered body can be quantified by crushing the sintered body into a powder state, then burning it in an oxygen stream at 850°C and detecting the amount of CO or CO□ gas generated by combustion of graphite. A specific example will be shown below. Example 7 Each of the sintered bodies obtained in Example 3 was ground into powder in an agate mortar, and then each sample was burned at 850°C for 5 minutes in an oxygen stream to cause the graphite to burn and emit light. The amount of CO2 gas produced was measured by absorbing NaOt+ warm solution, and the amount of graphite present in the sintered body was quantified. For this measurement, a sintered body to which no carbon black was added was ground into a powder state, a blank test was conducted, and each measurement result was corrected. The results are shown in Table 5. From this result, it was found that an amount of graphite approximately equal to the amount of added carbon black was present in the sintered body. Table 5 Note: Cases in which the amount of residual graphite exceeds the amount added are considered to be errors caused by the addition of phenolic resin in both blanks and test samples. (Effects of the Invention) As is clear from the above comparative tests, the present invention is a silicon carbide/graphite composite sintered body that is excellent in properties such as friction coefficient, flexural strength, and thermal shock temperature. 4. Brief explanation of the drawings Figures 1 (a) and (b) are X-ray diffraction graphs of the calcined body and sintered body before sintering, Figure 2 is the sintered body without carbon black added, 1 is a micrograph of a graphite-added sintered body and a carbon black-added sintered body. Patent applicant: NGK SPARK PLUG Co., Ltd. Patent attorney: Hide Sugimura
Amendment (method) % formula % 1. Indication of the case Patent Application No. 96399 of 1985 2. Name of the invention Silicon carbide/graphite composite sintered body 3. Person making the amendment Relationship to the case Name of the patent applicant ( 454) NGK Spark Plug Co., Ltd. 4, sub-agent threshold 5, date of amendment order July 29, 1988 1,
The column "Brief explanation of 4 drawings" on page 12 of the specification is corrected as follows. 719 Brief explanation of drawings Figures 1 (a) and (b) are X-ray diffraction graphs of the calcined body and sintered body before sintering,

Claims (1)

[Claims] 1. A sintered body of α-silicon carbide, in which graphite with an average grain size of 3 μm or less is contained as a second phase at SiC grain boundaries at a ratio of 1 to 5% by volume based on the amount of silicon carbide. A silicon carbide/graphite composite sintered body, which is uniformly dispersed and has a density of 90% or more of the theoretical density. 2. Claim 1, wherein the second phase graphite is graphitized during sintering using carbon black as a starting material.
The silicon carbide/graphite composite sintered body described in .