Sintered silicon carbide articles

The invention relates to a method for producing a sintered silicon carbide article having a sintered density of at least 90% of the theoretical density and a mechanical flexural strength of 50 kg/mm.sup.2 or higher. This is achieved by adding to a finely divided .alpha.-type silicon carbide powder, a tar pitch in such amounts as to provide 4.2-6 parts by weight of carbon after carbonization and a boron compound such as to correspond to a boron content of 0.005-0.15 parts by weight as densification aids, mixing and shaping the resulting mixture, and then sintering the thus-shaped article in an inert atmosphere at a temperature of from 1950 .degree. to 2300.degree. C.

The present invention relates to a method for producing a sintered silicon 
carbide article high in sintered density, mechanical strength and 
corrosion resistance, and more particularly to a method for producing a 
sintered silicon carbide article having a high sintered density, high 
mechanical strength and high corrosion resistance by mixing a finely 
divided silicon carbide powder with a definite amount of a specific 
carbon-containing material and a boron compound as densification aids, 
shaping and then sintering said shaped article under an inert atmosphere. 
A sintered silicon carbide article has excellent physical and chemical 
properties, and, in particular, has high hardness and excellent corrosion 
resistance and its mechanical properties do not change at a high 
temperature as compared with those at a normal temperature. Therefore, a 
sintered silicon carbide article has been regarded as promising as a wear 
resistant material and as a high-temperature structural material. However, 
as silicon carbide is hard to sinter, it is difficult to sinter it into a 
sintered article having a high sintered density by a usual method. 
Therefore, a sintering method by a hot press method and a sintering method 
by using densification aids have been proposed. 
For example, in Japanese Patent Laid-Open No. 148712/1976, it was disclosed 
that a sintered silicon carbide article having a high sintered density is 
produced by mixing 91-99.35 parts by weight of .alpha.-type silicon 
carbide powder having a specific surface area of 1-100 m.sup.2 /g with 
0.67-20 parts by weight of a carbonizable, organic solvent-soluble organic 
material having a carbonization ratio of 25-75% by weight, a definite 
amount of a boron compound containing 0.15-3.0 parts by weight of boron 
component and 5-15 parts by weight of a binding agent which is consumed 
under sintering conditions, and then the resulting mixture is sintered. 
Further, in U.S. Pat. No. 4,123,286, there is disclosed a silicon carbide 
powder containing: 
(a) from about 5 to about 100% by weight of alpha crystalline phase silicon 
carbide, 
(b) a maximum of the following components in % by weight: 
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SiO.sub.2 2.00 
free silicon 0.25 
iron 0.50 
alkali and alkaline 
0.50 
earth metal 
total metal oxides 
3.75 
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(c) the particles in said powder having an average particle size from about 
0.10 to about 2.50 microns, 
(d) said powder characterized by the property of being sinterable under 
substantially pressureless conditions at a temperature between about 
1950.degree. C. and about 2500.degree. C. when mixed with between about 
0.05 and about 4.0% by weight of combinable carbon and from about 0.03 to 
about 3.0% by weight of a densification aid, 
and it is mentioned that such powder can be sintered by usual sintering 
operations to obtain a high density, high strength product. 
However, although a sintered silicon carbide article having a high sintered 
density is obtained by any of the above mentioned methods, the mechanical 
strength of the sintered article is not always satisfactory and, for 
example, a sintered silicon carbide article having a flexural strength 
exceeding 50 kg/mm.sup.2 could not be obtained by the known methods. 
Under these circumstances, the present inventors have made extensive 
studies to find a method for producing a sintered silicon carbide article 
having a high sintered density and which is excellent in mechanical 
properties. As a result, it has been found that, a sintered silicon 
carbide article satisfying all the above-mentioned requirements can be 
obtained by mixing a finely divided silicon carbide powder with a definite 
amount of a special carbonaceous component combined with a boron compound 
in an amount less than the amount of boron component conventionally 
required to increase the sintered density of a sintered silicon carbide 
article, shaping and then sintering the shaped article. That is, according 
to the present invention, a sintered silicon carbide article having 
excellent mechanical properties and corrosion resistance can be obtained 
without lowering the desirable sintered density of the article. 
In accordance with the present invention, there is provided a method for 
producing a sintered silicon carbide article excellent in mechanical 
properties and corrosion resistance and having a sintered density of at 
least 90% of the theoretical density and a flexural length of at least 50 
kg/mm.sup.2 which comprises the steps of adding a tar pitch in such an 
amount as to provide 4.2-6 parts by weight of carbon after being 
carbonized and a boron compound in such an amount as to correspond to 
0.03-0.15 parts by weight of boron content as densification aids to 100 
parts by weight of a finely divided .alpha.-silicon carbide powder. The 
mixture is then shaped and sintered in an inert atmosphere at a 
temperature of from 1900.degree. to 2300.degree. C. 
The present invention will be explained in detail hereinafter. 
In the present invention, as a finely divided silicon carbide powder, it is 
suitable to employ a silicon carbide powder having an average particle 
size of 1 micron or less and consisting essentially of silicon carbide 
having mainly .alpha. crystals of noncubic polytypes. This silicon carbide 
generally contains 0.2-2% by weight of free carbon, but the starting 
material of the present invention is also the same. 
In the present invention, a tar pitch in such an amount as to provide 4.2-6 
parts by weight of carbon after being carbonized and a boron compound in 
such an amount corresponding to 0.03-0.15 parts, preferably 0.05-0.14 
parts, more preferably 0.05-0.13 parts most preferably 0.08-0.13 parts by 
weight of boron content are added to and mixed with 100 parts by weight of 
a silicon carbide powder. If the amount of tar pitch is less than 4.2 
parts by weight as carbon after being carbonized, it is impossible to 
obtain a sintered silicon carbide article having a high sintered density 
and mechanical strength required for desired use such as for mechanical 
parts. On the other hand, if the amount of a tar pitch exceeds the upper 
limit, the mechanical strength of the sintered article lowers, so that an 
improvement in mechanical properties can not be attained. 
Further, if the amount of boron compound is less the amount corresponding 
to 0.03 parts by weight of boron content based on 100 parts by weight of a 
silicon carbide powder, it is impossible to obtain a sintered article 
having a high sintered density, so that such an amount is not suitable. On 
the other hand, if the amount of boron compound exceeds the upper limit, 
it is possible to obtain a sintered article having a high sintered 
density, but the mechanical strength and corrosion resistance of the 
sintered article are lowered unfavorally, so that the object of the 
present invention can not be attained. 
The tar pitch used in the invention is a commercial coal or oil pitch coke 
and preferred to use an organic solvent-soluble coal tar pitch or oil tar 
pitch having a carbonization ratio of 40-60% by weight. Further, the boron 
compound used in the invention is not specified but it is preferred to use 
such a boron compound as to be able to be stable up to a sintering 
temperature for the present invention and, to be concrete, as preferred 
boron compounds, boron, boron carbide, and the like are exemplified. 
In the present invention, the above mentioned amounts of silicon carbide 
powder, boron compound and tar pitch are mixed uniformly using an organic 
solvent such as benzene, quinoline, anthracene, or the like or water and 
then the resulting mixture is shaped by a known slip casting method, or 
the resulting mixture is spray dried to obtain the mixture granules, which 
are then press-molded into the objective article by a known method. As the 
other molding method, the starting materials comprising a silicon carbide 
powder, a boron compound and a tar pitch are admixed uniformly with an 
organic binder or water and then the admixture is molded into a shaped 
article by a known extrusion molding, injection molding or the like. If 
desired, the thus shaped article is subjected to a machining step or to 
treatment to remove the binder. As regards the sintering conditions, the 
article shaped to a desired form is sintered in an inert atmosphere such 
as argon, helium, nitrogen or the like at a temperature of from 
1900.degree.-2300.degree. C., preferably 2050.degree.-2300.degree. C., 
most preferably 2050.degree.-2250.degree. C. for a period of time from 10 
minutes to 10 hours. When the sintering temperature is lower than 
1900.degree. C., particularly below 2050.degree. C. the resulting sintered 
article has a low sintered density, and when the sintering temperature 
exceeds 2300.degree. C., the evaporation of silicon carbide and the coarse 
growth of crystal grains occurs and the resulting sintered article has a 
low mechanical strength, so that such a high sintering temperature is not 
preferred. 
It is not fully understood why it is possible to obtain a sintered article 
having a high sintered density and excellent mechanical strength. However, 
as a result of investigating sintered articles by analytical experiments, 
it has been found that the tar pitch used in the invention is converted 
into carbon in the process of sintering, although a high molecular 
aromatic compound such as a phenol resin, a polyphenylene resin or the 
like, or a carbon-containing organic compound such as an aromatic 
hydrocarbon is, in general, converted through a solid phase state into a 
structurally unoriented carbon. It is assumed that, due to the different 
characteristics of tar pitch as mentioned above, the growth of crystal 
grains of silicon carbide is restrained and the oxide layer on the surface 
of the shaped article is removed smoothly in the process of calcining at a 
temperature of 1200.degree. C. or higher. As the result, the sintering 
takes effect in the presence of smaller amount of boron as a densification 
aid than that in the conventional method when the shaped article of 
silicon carbide is sintered at a sintering temperature of 1900.degree. C. 
or higher, so that the improvement in mechanical strength without causing 
adverse affect in respect to the sintering density. It is also believed 
that, by decreasing the amount of boron, there is obtained an effect of 
improving the corrosion resistance. 
In the above, according to the present invention, it has become possible to 
produce a high-density, high-strength and high corrosion-resistant 
sintered silicon carbide article having a sintered density of at least 
90%, preferably 95% or more of the theoretical density of the sintered 
article and a mechanical strength (flexural strength) of 50 kg/mm.sup.2 or 
higher, preferably 55 kg/mm.sup.2 or higher by using tar pitch and boron 
compound in the specified amounts as densification aids. Thus, the present 
invention has great industrial significance for the production of 
materials suitable as wear-resistant material for producing mechanical 
seals, bearings, etc. and as heat-resistant material for producing gas 
turbins, heat-exchangers, etc.

The present invention is further described in detail below according to an 
example, which is not, however, limitative of the present invention. 
EXAMPLE 1 
After 10 g. of coal tar pitch (having a carbon yield of 45% (by weight 
after being carbonized)) was dissolved in 15 g. of quinoline, 200 g. of 
benzene was added to the solution and was mixed sufficiently. To the 
solution, 100 g. of .alpha.-type silicon carbide having a silicon carbide 
content of 96% by weight and a BET specific surface area of 9 m.sup.2 /g. 
and 0.15 g. of boron carbide powder passing through 1200 mesh were added 
and they were mixed with and dispersed in the solution using a plastics 
ball mill for 3 hours. The dispersion was dried at 60.degree. C. in a 
nitrogen gas stream and the resulting powder was pulverized and then 
sieved with a 180 mesh screen. Then, after the thus obtained pulverized 
mixed powder was cold pressed, it was charged into a rubber mold and 
subjected to hydrostatic pressure press compacting under compacting 
pressure of 2 tons/cm.sup.2 to prepare a green shaped article having 
dimensions of 50.times.30.times.4 mm. 
After the said article was calcined at 600.degree. C. for 3 hours in an 
argon gas stream, it was further sintered at 2050.degree. C. for 30 
minutes in an argon gas atmosphere. The thus obtained sintered article had 
a sintered density of 3.14 g/cm.sup.3 and 3 point flexural strength of 60 
kg/mm.sup.2. 
COMATIVE EXAMPLE 1 
9 g. of novolak type phenol resin (having a carbon yield of 50% by weight) 
was dissolved in 200 g. of benzene. Then 100 g. of .alpha.-type silicon 
carbide having a silicon carbide content of 96% by weight and a BET 
specific surface area of 9 m.sup.2 /g. and 0.15 g. of boron carbide powder 
passing through 1200 a mesh were added to the solution, and the materials 
were mixed with and dispersed in the solution for 3 hours. The dispersion 
was dried at 60.degree. C. in a nitrogen gas stream and after the dried 
powder was pulverized, it was sieved with a 180 mesh screen. After the 
obtained pulverized mixed powder was cold pressed, it was charged in a 
rubber mold and then subjected to hydrostatic pressure press compacting 
under compacting pressure of 2 tons/cm.sup.2 to prepare a green article 
having dimensions of 50.times.30.times.4 mm. 
After the green article was calcined at 600.degree. C. for 3 hours in an 
argon gas stream, it was further sintered at 2050.degree. C. for 30 
minutes in an argon gas atmosphere. The thus obtained sintered article had 
a sintered density of 2.90 g/cm.sup.3 and 3 point frexural strength of 30 
kg/mm.sup.2. 
EXAMPLE 2 
The procedure of Example 1 was repeated except that the amount of boron 
carbide was varied. The results of sintered density and flexural strength 
measurements and microstructure observation of the resulting article are 
shown in Table 1. 
COMATIVE EXAMPLES 2-7 
The procedure of Example 1 was repeated except that the amount of boron 
carbide, amount of coal tar pitch and/or sintering temperature were varied 
as indicated in Table 1. The results of sintered density and flexural 
strength measurements and microstructure observation of the resulting 
articles are shown in Table 1. 
EXAMPLE 3 
100% by weight of .alpha.-type silicon carbide powder (Lonza UF-10) having 
a BET specific surface area of 10 m.sup.2 /g and a purity of about 99 wt. 
% (free carbon 0.34 wt. %. Si 0.05 wt. %, other oxides 0.6 wt. %), 0.15 
parts by weight of boron carbide B.sub.4 C powder (Denki Kagaku) having an 
average particle diameter of about 3.mu., a quinoline solution of a coal 
tar pitch (a char yield of 53% by weight) and 200% by weight of a 0.25 wt. 
% aqueous solution of polyvinyl alcohol were mixed in a plastic ball mill 
for 5 hours and the suspension was spray-dried. The mixture was shaped 
into an article of 50.times.35.times.5 mm by isostatic pressing under a 
pressure of 1.5 ton/cm.sup.2. The shaped article was sintered in a 
graphite resistance furnace, wherein the temperature was increased up to 
1800.degree. C. at a rate of 450.degree. C./hr. under a reduced pressure 
of 0.01 Torr, further from 1800.degree. C. to 2125.degree. C. at a rate of 
50.degree. C./hr. in an argon atmosphere (1 atm.) and finally the 
temperature of 2125.degree. C. was maintained for 2 hours. The sintered 
density and flexural strength of the thus obtained sintered article are 
shown in Table 1. 
EXAMPLES 4-9 
The procedure of Example 3 was repeated except that the amount of boron 
carbide, amount of coal tar pitch and/or sintering temperature were varied 
as indicated in Table 1. The results of sintered density and flexural 
strength measurements and microstructure observation of the resulting 
sintered articles are shown in Table 1. 
COMATIVE EXAMPLES 8-13 
The procedure of Example 3 was repeated except that the amount of boron 
carbide, amount of coal tar pitch and/or sintering temperature were varied 
as indicated in Table 1. The results of sintered density and flexural 
strength measurements and microstructure observation of the resulting 
sintered articles are shown in Table 1. 
TABLE 1 
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Silicon carbide 
Boron carbide 
Coal tar pitch 
Sintering 
Sintered density 
Flexural 
Micro-th 
BET(m.sup.2 /g) 
(parts by wt.) 
(parts by wt.) 
conditions 
(g/cm.sup.3) 
(kg/mm.sup.2) 
structure 
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Example 2 
9 0.1 10 2050.degree. C. .times. 0.5 
3.14 65 Normal 
grain growth 
Comparative 
9 0.2 10 2050.degree. C. .times. 0.5 
3.13 40 Normal 
Example 2 grain growth 
Comparative 
9 0.02 10 2050.degree. C. .times. 0.5 
2.80 30 Normal 
Example 3 grain growth 
Comparative 
9 0.1 14 2050.degree. C. .times. 0.5 
3.08 45 Normal 
Example 4 grain growth 
Comparative 
9 0.1 3 2050.degree. C. .times. 0.5 
2.60 25 -- 
Example 5 
Comparative 
9 0.1 10 1800.degree. C. .times. 0.5 
2.40 20 -- 
Example 6 
Comparative 
9 0.1 10 2350.degree. C. .times. 0.5 
3.00 30 Exaggerated 
Example 7 grain growth 
Example 3 
10 0.15 8.5 2125.degree. C. .times. 2 
3.17 63 Normal 
grain growth 
Example 4 
10 0.15 8.5 2200.degree. C. .times. 2 
3.17 63 Normal 
grain growth 
Example 5 
10 0.15 9 2125.degree. C. .times. 2 
3.17 64 Normal 
grain growth 
Example 6 
10 0.15 11 2125.degree. C. .times. 2 
3.15 53 Normal 
grain growth 
Example 7 
10 0.15 8.5 1950.degree. C. .times. 2 
2.95 50 Normal 
grain growth 
Example 8 
16 0.15 8.5 2125.degree. C. .times. 2 
3.13 63 Normal 
grain growth 
Example 9 
16 0.15 8.5 2200.degree. C. .times. 2 
3.15 62 Normal 
grain growth 
Comparative 
10 0.15 3.9 2125.degree. C. .times. 2 
1.75 7 -- 
Example 8 
Comparative 
10 0.15 4.8 2125.degree. C. .times. 2 
1.75 7 -- 
Example 9 
Comparative 
10 0.15 5.5 2125.degree. C. .times. 2 
2.40 21 -- 
Example 10 
Comparative 
10 0.15 6.5 2125.degree. C. .times. 2 
3.08 47 Exaggerated 
Example 11 grain growth 
Comparative 
10 0.5 8.5 2125.degree. C. .times. 2 
3.17 44 Normal 
Example 12 grain growth 
Comparative 
10 0.5 8.5 2200.degree. C. .times. 2 
3.16 35 Exaggerated 
Example 13 grain 
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growth 
COMATIVE EXAMPLE 14 
Thus 96.7 parts by weight of .beta.-type silicon carbide powder (IBIDEN 
Ultra Fine) of a purity of about 99.2% (SiO.sub.2 0.23 wt. %, free carbon 
0.42 wt. %, Al 0.03 wt. %, Fe 0.06 wt. %, othe impurities less than 0.06 
wt. %) and average particle diameter 0.28 .mu.m, 0.2 wt. part of boron 
powder of an average particle diameter of 0.2 .mu.m, 3.0 wt. parts of 
carbon black (ASAHI THERMAL) of an average particle diameter of 0.1 .mu.m 
and 0.1 wt. part of polyvinyl alcohol (as binder) are mixed and then added 
with 150 wt. parts of water. The whole was mixed in a plastic ball mill 
for 60 minutes and then dried at 80.degree. C. for 24 hours. The dried 
product was pulverized by the use of a mortar and the powder was shaped 
into an article of 20 mm.times.4 mm.sup.t by isostatic pressing under a 
pressure of 1.5 ton/cm.sup.2. The article was sintered in a graphite 
resistance furnace, wherein the temperature was increased up to 
1800.degree. C. at a rate of 450.degree. C./hr. under a reduced pressure 
of 0.01 Torr, further up to 2100.degree. C. at a rate of 50.degree. C./hr. 
in an argon atmosphere (1 atm.) and the temperature of 2100.degree. C. was 
maintained for 2 hours. 
The resulting sintered article has a density of 2.80 g/cm.sup.2 (about 87% 
of theoretical density), flexural strength of 35 kg/mm.sup.2 and boron 
content of 0.16 wt. %. 
COMATIVE EXAMPLE 15 
The procedure of Comparative Example 14 was repeated except that the amount 
of the silicon carbide powder was 96.78 parts by weight and the amount of 
boron powder was 0.12 parts by weight. 
The resulting sintered article had a density of 2.56 g/cm.sup.3 (about 80% 
of theoretical density), flexural strength of 29 kg/mm.sup.2 and boron 
content of 0.11 wt. %. 
EXAMPLE 10 
In the flasks containing the solutions shown in Table 2 the sintered 
articles obtained in Example 3 and Comparative Example 12 were immersed, 
and each flask was left stand in a thermostatic vessel at 80.degree. C. 
for 83 hours, and then the weight loss of the sintered article was 
measured to evaluate the corrosion-resistance. The results are shown in 
Table 2. 
TABLE 2 
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50% NaOH 98% H.sub.2 SO.sub.4 
Sample (mg/dm.sup.2.day) 
(mg/dm.sup.2.day) 
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Example 3 0.36 0.18 
Comparative 0.86 0.42 
Example 12 
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As shown in Table 1 and 2, it is clear that silicon carbide sintered 
articles prepared by the method of the invention have high mechanical 
strength, high corrosion resistance and high sintered density as compared 
with the sintered articles by the conventional method.