Silicon carbide-graphite composite material and process for producing same

A silicon carbide-graphite composite material is disclosed. The composite material includes graphite as a secondary phase which are dispersed uniformly in a grain boundary of the silicon carbide. The graphite have an average grain size of not more than 3 .mu.m and are present in a proportion of 1 to 20 vol % based on the volume of the silicon carbide. The composite material has a density greater than 90% of the theoretical density. A process for producing the silicon carbide-graphite composite material is also disclosed. The composite material is a high-density and high-strength material.

FIELD OF THE INVENTION 
The present invention relates to a high-density and high-strength silicon 
carbide-graphite composite material and a process for producing the same. 
BACKGROUND OF THE INVENTION 
Because of its excellent chemical and physical properties, silicon carbide 
has potential applications as structural materials for use in 
high-temperature environments, sliding materials or corrosion-resistant 
materials, which include mechanical seal rings, plungers, bearings, sand 
blast nozzles, tappets and microwave absorbers. 
Silicon carbide sintered product is conventionally produced by reaction 
sintering, hot pressing or pressureless sintering. Of these methods, the 
pressureless sintering is most commonly used. In order to improve the 
sinterability of silicon carbide, various sintering aids have recently 
been developed. 
Two typical methods have been developed for the sintering of silicon 
carbide are hereunder described according to crystal form. 
First, Japanese Patent Application (OPI) No. 148712/76 corresponding to 
U.S. Pat. No. 4,124,667 (the term "OPI" as used herein refers to 
"published unexamined Japanese patent application") describes a method 
wherein .alpha.-silicon carbide is mixed with 0.15 to 3.0 wt% of boron, 
0.5 to 5.0 wt% of a carbonized organic material and up to 1.0 wt% of an 
additional carbon. The mixture is molded into a suitable shape and then 
the molded product is sintered so as to provide a density of at least 2.4 
g/cm.sup.3 which is greater than 75% of theoretical density 
Second, Japanese Patent Application (OPI) No. 78609/75 corresponding to 
U.S. Pat. No. 4,004,934 describes a process for producing sintered silicon 
carbide with a density not smaller than 85% of theoretical density by 
sintering a molded mixture of .beta.-silicon carbide with a boron compound 
corresponding to 0.3 to 3.0 wt% of boron and a carbonaceous additive 
corresponding to 0.1 to 1.0 wt% of carbon. According to the specification, 
the function of carbonaceous additive is to reduce silica which always is 
present in silicon carbide powder in small amounts or which forms on 
heating from oxygen absorbed on the powder surfaces. The other function of 
the carbonaceous additive acts as a getter for free silicon. If this 
additive is incorporated in an amount of more than 1.0 wt%, the unreacted 
excess carbon tends to form voluminous grains in the sintered silicon 
carbide that act much like permanent pores and such excess thereby limits 
the ultimate achievable density and strength. 
However, whether the silicon carbide is in the .alpha.- or .beta.-form, if 
sintering is effected at temperatures higher than 2,000.degree. C. in an 
attempt to obtain a dense product, .beta.-silicon carbide experiences 
grain growth as it is transformed to .alpha.-silicon carbide. Further 
.alpha.-silicon carbide also undergo grain growth although no phase 
transformation occurs. In any event, it has been impossible to obtain a 
dense and microfine sintered product by any of the two prior art 
techniques. 
SUMMARY OF THE INVENTION 
The present invention have determined that carbon is effective in 
inhibiting the grain growth of silicon carbide during the sintering and 
have found that 1 to 20 vol% of carbon black based on the volume of 
silicon carbide incorporated in addition to the sintering aids will 
inhibit the grain growth of silicon carbide during sintering whereas the 
same carbon black is converted into graphite so as to produce a composite 
material wherein the resulting microfine graphite as a secondary phase of 
the composite material are uniformly dispersed through a grain boundary of 
silicon carbide of the composite material. 
This composite material has a dense and microfine structure which exhibits 
high mechanical strength and great thermal shock resistance.

DETAILED DESCRIPTION OF THE INVENTION 
Preferred embodiments of the present invention are: 
(1) A silicon carbide-graphite composite material, comprising: 
a silicon carbide as a primary phase; and 
graphite as a secondary phase having an average grain size of not more than 
3 .mu.m uniformly dispersed through a grain boundary of the silicon 
carbide in the composite material where carbon black present in a 
proportion of 1 to 20 vol% based on the volume of silicon carbide is 
converted into graphite upon producing the composite material, said 
composite material having a density greater than 90% of the theoretical 
density. 
(2) A process for producing a silicon carbide-graphite composite material, 
comprising the steps of: 
adding a sintering aid to silicon carbide of an average grain size less 
than 1 .mu.m, said sintering aid comprising 0.1 to 3.0 wt% of a compound 
selected from the group consisting of boron, a boron compound which 
includes a corresponding amount of boron, aluminum and an aluminum 
compound which includes a corresponding amount of aluminum; and 0.1 to 6.0 
wt% of a compound selected from the group consisting of carbon and a 
carbonacious compound which includes a corresponding amount of carbon each 
based on the weight of silicon carbide; 
further adding carbon black to silicon carbide in an amount of 1 to 20 vol% 
based on the volume of the silicon carbide; 
blending the compositions into an intimate mixture; 
molding the intimate mixture to obtain a molded body; and 
sintering the molded body. 
Specifically, the composite material of the present invention has a bending 
strength at least 20% greater than that of the conventionally sintered 
product and has thermal shock resistance 90.degree. to 200.degree. C. 
higher than that of the conventional product. As a further advantage, the 
carbon black added is converted into graphite during the sintering and 
becomes dispersed through the grain boundary of silicon carbide of the 
composite material. Accordingly, the coefficient of kinetic friction of 
the composite material is reduced by at least 20% compared with the 
conventional sintered silicon carbide. The graphite forming the secondary 
phase of the composite material has such a high resistance to corrosion 
that it will not impair the chemical stability of silicon carbide. The 
present inventors have found that the average grain size of the graphite 
forming the secondary phase of the composite material is not more than 3 
.mu.m. 
The carbon source used in the present invention in addition to the 
sintering aids must be carbon black for the following reasons. If the 
carbon source added is a carbonaceous organic compound such as a phenolic 
resin, a uniform dispersion may be obtained. However, because of the 
inherent activity of the phenolic resin as a binder, the molding and 
subsequent processing of the green body becomes difficult if the phenolic 
resin is added in an amount greater than 10 wt% based on the weight of the 
silicon carbide. Furthermore, because almost half of the phenolic resin 
added evaporates at low temperatures and causes a significant decrease in 
the green density of the calcined product, the formation of dense product 
is prevented. If graphite powder, rather than carbon black, is directly 
added as the carbon source, the effect of inhibiting the grain growth of 
silicon carbide during sintering is smaller than that of the carbon black. 
In addition, there is no increase in the bending strength of the final 
composite material. If less than 1.0 vol% of carbon black based on the 
volume of silicon carbide is added, various desired effects are not 
obtained. If, on the other hand, more than 20 vol% of carbon black based 
on the volume of silicon carbide is used, the sinterability of silicon 
carbide is impaired and its other desired properties are not maintained. 
For the purpose of obtaining a dense composite material, it is most 
effective to use silicon carbide of a grain size not greater than 1 .mu.m. 
By adding 1 to 20 vol%, preferably 1 to 10 vol%, of carbon black based on 
the volume of silicon carbide, the grain growth of silicon carbide can be 
inhibited and effective sintering can be achieved. If the amount of carbon 
black added is less than 1 vol%, the possibility of phase transformation 
or and grain growth is increased. Using more than 20 Vol% of carbon black 
causes a reduction in the sintering efficiency, which eventually leads to 
a product of low quality. 
The present invention is hereunder described in greater detail by working 
examples which are given here for illustrative purposes only and are by no 
means intended to limit the scope of the invention. 
EXAMPLE 1 
A wet-mixture of .alpha.-silicon carbide powder (average particle size: 0.8 
.mu.m), 0.5 wt% boron carbide, 8.0 wt% phenolic resin based on the weight 
of silicon carbide and 5 vol% carbon black based on the volume of silicon 
carbide was prepared while adding water. The mixture was dried, sieved and 
molded into 30 mm.times.10 mm.times.5 mm in this order. The molded body 
were calcined in nitrogen gas at 800.degree. C. for 60 minutes and 
subsequently sintered in argon gas atmosphere at 2,100.degree. C. under 
atmospheric pressure for 60 minutes. 
The calcined (unsintered) samples and those of the sintered composite 
materials were ground into fine particles in an agate mortar. The ground 
samples were analyzed by an X-ray diffract meter and the results are shown 
in FIGS. 1(a) and (b), from which one can see that all the carbon black 
that was initially added was converted into graphite which was distributed 
through the grain boundary of silicon carbide of the composite material. 
EXAMPLE 2 
Composite samples were prepared in the same manner as in Example 1 and 
subjected to grinding. They were then wet-polished with a diamond paste (9 
.mu.m) and observed under optical microscope to check for the dispersion 
of graphite. The polished surface was then etched with Murakami reagent 
having the following composition and observed for the sizes of silicon 
carbide grains and graphite grains disparsed through the grain boundary of 
silicon carbide. 
______________________________________ 
The composition of Murakami reagent 
______________________________________ 
Sodium hydroxide 7 g 
Potassium ferricyanide 10 g 
Water 100 g 
______________________________________ 
The comparative experiment was conducted among composite samples prepared 
in the same manner as in Example 1, sintered samples prepared in the same 
manner as in Example 1 except that carbon black was not contained and 
composite samples prepared in the same manner as in Example 1 except that 
graphite was contained instead of carbon black. The results are shown in 
FIG. 2 by micrographs a; (the sample prepared from a powder mixture 
containing neither carbon black nor graphite), b (the sample prepared from 
a mixture containing graphite), and c (the sample prepared from a mixture 
containing carbon black). In the absence of graphite and carbon black, 
excessive grain growth of silicon carbide occurred. This could be partly 
inhibited by the addition of graphite, but a composite material having a 
finer and denser structure could be obtained by addition of carbon black. 
EXAMPLE 3 
Composite samples were prepared in the same manner as in Example 1 except 
that the amount of carbon black was varied as shown in Table 1. The bulk 
densities of the so prepared samples were measured. The samples were then 
ground into 4 mm.times.8 mm.times.25 mm and subjected to a three-point 
bending test. The results are also shown in Table 1, from which one can 
see that the samples prepared from powder mixtures containing at least 1 
vol% of carbon black had a bending strength at least 20% greater than that 
of the sample containing no carbon black. When the content of carbon black 
exceeded 20 vol%, samples having a density greater than 90% of theoretical 
density could not be obtained. Furthermore, a significant decrease in the 
bending strength was observed. When graphite rather than carbon black was 
added, there was no increase in the bending strength no matter how much 
graphite was added. 
TABLE 1 
______________________________________ 
Bending 
Sample Amount Relative Strength 
No. Additive (Vol %) Density (%) 
(kg/mm.sup.2) 
______________________________________ 
1 Carbon Black 
0 98.5 45 
2 Carbon Black 
1 98.5 55 
3 Carbon Black 
3 98.5 55 
4 Carbon Black 
5 98.0 60 
5 Carbon Black 
10 97.0 50 
6 Carbon Black 
20 90.0 40 
7 Carbon Black 
30 77.0 15 
8 Graphite 1 98.5 45 
9 Graphite 3 98.5 43 
10 Graphite 5 97.5 43 
11 Graphite 10 96.5 40 
12 Graphite 20 88.0 35 
13 Graphite 30 75.0 10 
______________________________________ 
EXAMPLE 4 
Composite samples were prepared in the same manner as in Example 3 and 
ground into 4 mm.times.8 mm.times.25 mm. The thermal shock resistance 
(.DELTA.T) of each sample was measured by the water-quenching method, 
wherein a sample that had been held at a predetermined temperature 
(T.degree.C. ) for 15 minutes was thrown into water (To.degree.C.) to 
determine the critical temperature (.DELTA.T=T-To) that caused no decrease 
in the bending strength of the sample. The results of this test are shown 
in Table 2, from which one can see that the thermal shock resistance 
(.DELTA.T) increased as a function of the content of carbon black. The 
thermal shock resistance of the sample containing 20 vol% of carbon black 
was as much as 200.degree. C. higher than the corresponding value of the 
sample containing no carbon black. 
TABLE 2 
______________________________________ 
Thermal 
Shock 
Sample Amount Relative Resistance 
No. Additive (Vol %) Density (%) 
.DELTA.T (.degree.C.) 
______________________________________ 
14 Carbon Black 
0 98.5 280 
15 Carbon Black 
1 98.5 370 
16 Carbon Black 
3 98.5 370 
17 Carbon Black 
5 98.0 370 
18 Carbon Black 
10 97.0 400 
19 Carbon Black 
20 90.0 480 
20 Carbon Black 
30 77.0 480 
______________________________________ 
EXAMPLE 5 
Composite sintered rings (outside diameter 30 mm, inside diameter 20 mm, 
thickness 5 mm) were prepared in the same manner as in Example 3. After 
grinding their sliding faces, the sliding faces were wet-polishing with a 
diamond paste (9 .mu.m) and subjected to a wet-sliding test under the 
following conditions: 
Tester: Mechanical seal type (ring-on-ring system) 
Lubricant: Water 
Sliding velocity: 100 m/min. 
Surface Pressure: 7 kg/cm.sup.2 
Test period: 100 hr. 
The results of the wet-sliding test are shown in Table 3, from which one 
can see that the samples prepared from powder mixtures containing at least 
1 vol% of carbon black had coefficients of kinetic friction at least 20% 
smaller than the sample containing no carbon black. This reduction in the 
friction coefficient was accompanied by a 50% reduction in the resulting 
wear. However, it has been found that addition of 30 vol% or higher of 
carbon black makes it impossible to sinter up to 80% or higher of 
theoretical density, and causes increases in the coefficient of kinetic 
friction and the wear. 
TABLE 3 
______________________________________ 
Kinetic 
Relative 
Coeffi- 
Wear (.times. 
Sample Amount Density 
cient of 
10.sup.-9 mm.sup.3 / 
No. Additive (Vol %) (%) Friction 
mm Kg) 
______________________________________ 
21 Carbon 0 98.5 0.0050 6 
Black 
22 Carbon 1 98.5 0.0040 3 
Black 
23 Carbon 3 98.5 0.0035 3 
Black 
24 Carbon 5 98.0 0.0030 3 
Black 
25 Carbon 10 97.0 0.0030 3 
Black 
26 Carbon 20 90.0 0.0040 4 
Black 
27 Carbon 30 77.0 0.0200 11 
Black 
______________________________________ 
The foregoing data shows that the present invention provides a silicon 
carbide-graphite composite material having improved friction coefficient, 
bending strength and thermal shock resistance. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apprent to one skilled in the art 
that various changes and modifictions can be made therein without 
departing from the spirit and the scope thereof.