Method for production of sliding member

A method for production of a sliding member exhibiting excellent mechanical strength, resistance to thermal shock, and resistance to corrosion at elevated temperature is produced by a process of manufacture which comprises firing a shaped mass of silicon nitride powder obtained by the silica reduction method and cutting the resultant sintered ceramic mass in a prescribed shape by machining.

FIELD OF THE INVENTION 
This invention relates to method for production of a sliding member made of 
a sintered ceramic material consisting preponderantly of silicon nitride 
(Si.sub.3 N.sub.4) and exhibiting excellence in mechanical strength, 
resistance to thermal shock, and resistance to corrosion at elevated 
temperatures. It has previously been proposed to use a sintered ceramic 
material exhibiting excellent mechanical properties at elevated 
temperatures in bearings destined to serve in atmospheres of elevated 
temperatures. 
The conventional sintered ceramic material, however, has the disadvantage 
that the bearings made of such material, when used in liquids such as 
fused metals which are held at elevated temperatures and are highly 
corrosive, are deficient in mechanical strength, resistance to thermal 
shock, and resistance to corrosion and undergo breakage early. 
BACKGROUND OF THE INVENTION 
Various attempts have been made to solve these problems mentioned above. 
For example, F. Strasser et al disclose in their invention (U.S. Pat. No. 
4,410,285) a sliding member formed of an outer metal ring and an inner 
bearing of a ceramic material, and a ring of a metal felt located between 
the metal ring and the ceramic material ring. 
F. R. Marrison et al give the results from their studies (Lubrication 
Engineering, vol. 40.3, 153-159) wherein the fatigue property of ceramic 
balls of Si.sub.3 N.sub.4 and steel bearings was measured in detail. 
The above disclose the use of ceramics in combination with a metal. 
J. Lammer et al disclose in their invention (U.S. Pat. No. 4,522,453) a 
sliding member formed of a member coated with TiC and a member coated with 
TiN. 
Yamamoto et al disclose in their invention (Japanese Patent No. S55-100421) 
that a sliding member comprises a member of sintered Si.sub.3 N.sub.4 and 
a member of sintered SiC, is capable of using in non-lubricated condition 
and less wearing. They are the combination of ceramics. 
Silicon nitride type sintered ceramic materials have found popular 
recognition as sintered ceramic materials of large mechanical strength. 
Unfortunately, the silicon nitride type sintered ceramic materials 
produced by the metal silicon nitriding method, the imide method, etc. and 
offered generally in the market fail to satisfy the levels of mechanical 
strength, resistance to thermal shock, and resistance to corrosion which 
sliding members destined to serve in atmospheres of elevated temperatures. 
OBJECT AND SUMMARY OF THE INVENTION 
The inventors continued a diligent study directed to the elimination of the 
drawbacks suffered by the conventional sintered ceramic materials as 
described above. They have, consequently, found that sliding members 
produced by using silicon nitride powder obtained by the silica reduction 
method offer satisfactory serviceability for an amply long period even in 
highly corrosive liquids under harsh thermal conditions. 
This invention, therefore, aims to provide a method for production of 
sliding members made preponderantly of a sintered ceramic material 
exhibiting excellence in mechanical strength, resistance to thermal shock, 
and resistance to corrosion at elevated temperatures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention concerns a method for making sliding members of 
sintered ceramic pieces produced by molding in prescribed shapes the 
silicon nitride powder obtained by the silica reduction method and firing 
the molded masses of silicon nitride powder. 
The silicon nitride powder to be used in this invention is manufactured by 
the silica reduction method to be described below. 
Specifically, the silica reduction method comprises mixing silicon dioxide 
powder and carbon powder of an excess amount relative to the reaction 
equivalent weight thereof (preferably such that the C/SiO.sub.2 ratio will 
fall in the range of 0.4 to 4) in conjunction with a small amount of a 
reaction catalyst selected from among silicon powder, silicon nitride 
powder, and silicon carbide powder and roasting the resultant mixture in 
an atmosphere of nitrogen at a temperature in the range of 1,350.degree. 
to 1,600.degree. C. (preferably 1,400.degree. to 1,500.degree. C.). In 
this method, silicon nitride powder containing carbon powder is obtained 
in consequence of the reaction shown by the following formula. 
EQU 3SiO.sub.2 +C (excess)+2N.sub.2 +Si.sub.3 N.sub.4 (small 
amount).fwdarw.Si.sub.3 N.sub.4 +3CO.sub.2 +C (about 5 to 15% by weight) 
Generally, as the starting components for the aforementioned reaction, 60 
to 70% by weight of SiO.sub.2 and 25 to 35% by weight of carbon powder are 
used as additionally mixed with 2 to 10% by weight of Si.sub.3 N.sub.4 
powder. 
If the reaction temperature is lower than 1,350.degree. C., the reaction 
velocity is low and the yield is insufficient. If it exceeds 1,600.degree. 
C., the reaction entails formation of SiC. Thus, any deviation from the 
aforementioned temperature range is undesirable. 
Adjustment of the amount of carbon powder remaining during the course of 
roasting can be easily effected by adjusting the roasting time. And the 
roasting is carried out by heating at a temperature in the range of 
600.degree. to 800.degree. C. in an oxidative atmosphere. 
The silicon nitride powder produced by the silica reduction method and used 
in the present invention advantageously has a carbon content in the range 
of 0.1 to 3% by weight. 
The silicon nitride powder obtained as described above is such that 
.alpha.-phase silicon nitride accounts for at least 95%, whereas the 
silicon nitride powder obtained by the method such as metal silicon 
nitriding, etc. consists of about 90% of .alpha.-phase silicon nitride and 
the balance of .beta.-phase silicon nitride. The silicon nitride powder is 
also characterized by having an average particle diameter less than 1 
.mu.m, whereas the silicon nitride powder obtained by the conventional 
method has an average particle diameter more than 1 .mu.m. 
This silicon nitride powder is comminuted, caused to contain therein not 
more than 10% by weight of such a sintering aid as yttrium oxide (Y.sub.2 
O.sub.3), aluminum oxide (Al.sub.2 O.sub.3), or aluminum nitride (AlN), 
for example, 0.5 to 10% by weight of yttrium oxide (Y.sub.2 O.sub.3), 0.1 
to 10% by weight of aluminum oxide (Al.sub.2 O.sub.3) or 0.1 to 10% by 
weight of aluminum nitride (AlN), and further mixed with an organic 
binder. The resultant mixture is molded in a prescribed shape, calcined at 
a temperature in the range of 1400.degree. to 1600.degree. C. and sintered 
at a temperature in the range of 1,600.degree. to 1,800.degree. C. The 
molding may be made in a complete shape which a final product is expected 
to assume. 
Otherwise, it may be molded in the shape of a simple raw material such as a 
plate or round bar suitable for yielding a final product by machining. 
Then, the molded mass is fired and then cut in the final shape by 
machining. 
The firing may be carried out in an inert atmosphere under atmospheric 
pressure. The method which fires the molded mass in a nitrogen-containing 
atmosphere under pressure of 1.5 to 50 kg/cm.sup.2 or the method which 
sinters the molded mass with a hot press, however, proves advantageous. 
Optionally, the characteristic properties of the sintered mass obtained by 
the method described above can be further improved by placing the sintered 
mass in an alumina container and heating it at a temperature of about 
1,700.degree. C. in an inert gas such as argon or nitrogen compressed to 
the level of about 1,000 kg/cm.sup.2 thereby decreasing the spaces between 
the silica nitride particles. 
The sliding member formed in a prescribed shape by machining is enabled to 
acquire improvement in mechanical strength, resistance to thermal shock, 
resistance to corrosion, etc. by being subjected to a heat treatment at a 
temperature of 800.degree. to 1,200.degree. C. (preferably 900.degree. to 
1,100.degree. C.) in an oxidative atmosphere for one to 100 hours 
(preferably 2 to 50 hours). 
The sliding member of this invention is particularly suitable for use in a 
bearing. When it is used in a bearing, this bearing may be wholly formed 
of sintered ceramic material. Optionally, only the balls or the rollers 
serving as rotating members in the bearing may be formed of sintered 
ceramic material. When the rotating members of the bearing are formed of 
sintered ceramic material as described above, they are less susceptible to 
the effect of centrifugal force because the sintered ceramic material is 
light. Thus, the properties of the rotating members during a high-speed 
rotation are improved. 
EXAMPLE 1 
A mixture was obtained by using 63% by weight of silicon dioxide powder 
having an average particle diameter of about 0.05 .mu.m, 31% by weight of 
carbon powder having an average particle diameter of 0.03 .mu.m, and 6% by 
weight of silicon nitride (.alpha.-phase Si.sub.3 N.sub.4) having an 
average particle diameter of about 0.8 .mu.m was heated under a flow of 
nitrogen gas at about 1,500.degree. C. for about three hours, to effect 
reduction of the silicon dioxide powder. 
The product of this reduction was silicon nitride powder (.alpha.-phase 
content more than 95%) having an average particle diameter of about 0.8 
.mu.m and containing carbon powder. The carbon powder content in the 
mixture was about 10% by weight. This mixture was heated in an 
oxygen-containing atmosphere at about 700.degree. C. for one hour to 
adjust the carbon content to 1.75% by weight. 
Then, the powder was combined with ytrrium oxide, aluminum oxide, and 
aluminum nitride in such proper proportions that the resultant mixture 
consisted of 91% by weight of silicon nitride, 5% by weight of yttrium 
oxide, 2% by weight of aluminum oxide, and 2% by weight of aluminum 
nitride. The mixture was thoroughly blended, then caused to contain 
therein paraffin as a caking agent, and molded in a metal die under 
pressure of about 400 kg/cm.sup.2 to produce a plate 40.times.40.times.8 
mm. This plate was calcined in nitrogen gas at about 1,550.degree. C. for 
about one hour and thereafter sintered in nitrogen gas at about 
1,750.degree. C. for about 2 hours. 
The sintered silicon nitride plate was cut into square bars 
3.times.3.times.35 mm and tested for breaking strength under the 
conditions of 0.5 mm/min. of crosshead speed, 20 mm of span, and a varying 
temperature of normal room temperature, 1,000.degree. C., or 1,200.degree. 
C. The results (breaking strength in kg/mm.sup.2) are shown in the Table. 
COMATIVE EXPERIMENT 
Proceess was same as Example 1, except for the silicon powder adjusted by 
the metal silicon nitriding method which was not contained carbon. 
Obtained sintered silicon nitride article was tested as same as Example 1. 
Results are shown in the Table. 
EXAMPLE 2 
The carbon powder containing silicon nitride powder obtained in Example 1 
was thoroughly mixed with yttrium oxide, aluminum oxide, and aluminum 
nitride in proportions such that the resultant mixture consisted of 91% by 
weight of silicon nitride, 5% by weight of yttrium oxide, 2% by weight of 
aluminum oxide, and 2% by weight of aluminum nitride. The resultant 
mixture and parrafin added thereto as a caking agent were molded in a 
metal die under pressure of about 300 kg/cm.sup.2 to produce a plate 
40.times.40.times.8 mm. This plate was pressed under hydro-static pressure 
of about 1 ton/cm.sup.2 and then fired in nitrogen gas under the pressure 
of about 3 kg/cm.sup.2 at about 1,6000 C. for about 2 hours. 
Breaking strength of this working example was measured as same as Example 
1. Results are shown in the Table. 
EXAMPLE 3 
The carbon powder containing silica nitride powder obtained in Example 1 
was thoroughly mixed with yttrium oxide and aluminum oxide in proportions 
such that the resultant mixture consisted of 93% by weight of silicon 
nitride, 5% by weight of yttrium oxide, and 2% by weight of aluminum 
oxide. The resultant mixture and paraffin added thereto as a caking agent 
were molded in a metal die under pressure of about 300 kg/cm.sup.2 to 
produce a plate 40.times.40.times.8 mm. This plate was pre-sintered in 
nitrogen gas at about 1,750 C. for about one hour and a half and then 
subjected to hot-press sintering in a carbon container using aluminum 
nitride as a packing powder at about 1,750 C. under pressure of about 500 
kg/cm.sup.2 for about 2 hours. Breaking strength of this working example 
was measured as same as Example 1. Results are shown in the Table. 
PRACTICAL EXAMINATION 
A bearing for each practical examination was produced by assembling 
rotating members obtained in Example 1, 2, 3 and comparative experiment by 
cutting the sintered silicon nitride material by machining and an inner 
race, an outer race and a retainer prepared separately for each rotating 
member in the same composition by the same method. 
In the diagram, 1 stands for the inner race, 2 for the outer race, 3 for 
the rotating members and 4 for the retainer. Each bearing in the practical 
examination (except for the bearing in the comparative experiment) 
possessed outstanding mechanical strength even in fused plating metal and 
exhibited excellent resistance to thermal shock and to corrosion. 
EXAMPLE 4 
The bearing produced by assembling a rotating member, an inner race, an 
outer race and a retainer obtained by cutting the sintered silicon nitride 
in Example 1 was subjected to a heat treatment in an oxidative atmosphere 
at about 1,000.degree. C. for 50 hours. 
In consequence of this heat treatment, the characteristic properties of 
this bearing were improved and the dispersion in the properties was 
reduced. 
TABLE 
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Examples Comparative 
1 2 3 Experiment 
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Density (gr/cm.sup.2) 
3.23 3.23 3.24 3.22 
Breaking Strength 
(kg/mm.sup.2) 
Normal 100 110 120 90 
Temperature 
1000.degree. C. 
95 100 120 80 
1200.degree. C. 
80 80 85 50 
Practical Examination 
good good good poor 
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