Production of silicon nitride sintered body

A process for manufacturing a silicon nitride sintered body comprises molding a mixed powder of a silicon nitride starting material and a sintering aid and firing a thus obtained molding. The firing is carried out in an N.sub.2 atmosphere or a mixed atmosphere of N.sub.2 and an inert gas to which CO.sub.2 or a mixed gas of CO.sub.2 and CO is added. Thereby, an O.sub.2 partial pressure is increased to restrain the evaporation of SiO.sub.2 and nitriding of SiO.sub.2 during the firing. The silicon nitride sintered body suffers almost no deterioration of the fired surface resulting from the evaporation of SiO.sub.2 and the nitriding of SiO.sub.2, and exhibits substantially equal four point bending strength and oxidation resistance with respect to the fired surface and the inside thereof.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a process suitable for manufacturing a 
silicon nitride sintered body. 
(2) Related Art Statement 
According to a conventional process for producing a silicon nitride 
sintered body, a sintering aid which forms a liquid phase in grain 
boundaries, such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO, etc., is added 
to a raw material powder of silicon nitride, and a molding obtained from a 
resulting mixture is fired in an N.sub.2 atmosphere or a mixed atmosphere 
of N.sub.2 and an inert gas or in such an atmosphere under pressure. For 
instance, Japanese Patent Publication No. 58-49,509 discloses a process 
for performing firing in an N.sub.2 atmosphere under pressure or in a 
mixed atmosphere of N.sub.2 and an inert gas under pressure. 
In these cases, a carbonaceous heater or a firing jig is ordinarily used, 
and the atmosphere is an N.sub.2 atmosphere or a mixed atmosphere with a 
low O.sub.2 partial pressure in which O.sub.2 contained as an impurity in 
N.sub.2 gas is reduced. 
By the way, SiO.sub.2 which is inherently contained in an oxide additive 
and a silicon nitride raw material and serves as a sintering aid forms 
glass in grain boundaries through reaction and effectively performs 
densification of a structure and formation of a fine structure. However, 
in the above-mentioned conventional processes, when the silicon nitride 
molding is fired in N.sub.2 atmosphere or N2pressurizing atmosphere with a 
low O.sub.2 partial pressure, as shown in the following expressions (1) 
and (2), the oxide additive and SiO.sub.2 evaporate from the glass phase, 
or are nitrided. Thereby, the ratio between O and N in the glass phase 
varies so that the composition of the glass phase at the grain boundary 
changes. 
EQU Evaporation reaction of SiO.sub.2 :SiO.sub.2 .revreaction.SiO+1/2O.sub.2 ( 
1) 
EQU Nitriding reaction of SiO.sub.2 :3SiO.sub.2 +2N.sub.2 .revreaction.Si.sub.3 
N.sub.4 +3O.sub.2 ( 2) 
For this reason, the conventional process has the drawback that a 
sufficiently densified silicon nitride cannot be obtained or difference 
occurs in fine structure between the surface at which evaporation easily 
takes place and the inside portion at which the evaporation is difficult, 
thereby deteriorating the properties of the fired face. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to eliminate the 
above-mentioned problems and to provide a process for producing a silicon 
nitride sintered body which suffers almost no deterioration of a fired 
surface resulting from the evaporation and nitriding reaction of 
SiO.sub.2. 
According to the present invention, there is a provision of a process for 
producing a silicon nitride sintered body, which comprises molding a mixed 
powder of a silicon nitride starting material powder and a sintering aid, 
and firing a resulting molding in an N.sub.2 atmosphere or a mixed 
atmosphere of N.sub.2 and an inert gas in which CO.sub.2 or a mixed gas of 
CO.sub.2 and CO is added so that an O.sub.2 partial pressure is increased 
to restrain the evaporation of SiO.sub.2 and the nitriding of SiO.sub.2 
when the molding is fired. 
By so doing, SiO.sub.2 is prevented from evaporating and being nitrided by 
increasing the partial pressure of O.sub.2 in the N.sub.2 atmosphere or 
the mixed atmosphere of N.sub.2 and the inert gas. 
These and other objects, features and advantages of the present invention 
will be well understood from the following description of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
A preferable O.sub.2 partial pressure can be selectively determined 
depending upon an equilibrium O.sub.2 partial pressure, an evaporation 
speed, and a nitriding speed in the evaporation reaction of SiO.sub.2 and 
the nitriding reaction of SiO.sub.2, and a firing time at a firing 
temperature. 
As a way of increasing this O.sub.2 partial pressure, CO.sub.2 or a mixed 
gas of CO.sub.2 and CO is further mixed into the N.sub.2 atmosphere or the 
mixed atmosphere of N.sub.2 and the inert gas. Thereby, the O.sub.2 
partial pressure is increased by utilizing the equilibrium O.sub.2 partial 
pressure resulting from the CO.sub.2 dissociation reaction shown by the 
following expressions (3) and (4). 
Equilibrium reactions of CO.sub.2 : 
EQU CO.sub.2 .revreaction.CO+1/2O.sub.2 (3) 
EQU CO.sub.2 .revreaction.C+O.sub.2 (4) 
The reason why CO.sub.2 or the mixed gas of CO.sub.2 or CO is selected as 
an addition gas to enhance the O.sub.2 partial pressure in the present 
invention is that by controlling the O.sub.2 partial pressure through the 
CO.sub.2 dissociation reactions, damages of a carbonaceous heater, a jig, 
etc. frequently used for firing the silicon nitride sinterable body are 
reduced and that a range of the O.sub.2 partial pressure is appropriately 
selected and easy to be controlled. 
The reason why 0.001% or more of CO.sub.2 is favorably mixed into the 
N.sub.2 atmosphere or the mixed atmosphere of N.sub.2 and the inert gas is 
that no effect is recognized in the case of less than 0.001% because less 
than 0.001% becomes lower than the content of impurities usually contained 
in N.sub.2 gas. 
On the other hand, if the mixing amount of CO.sub.2 is too much, the 
O.sub.2 partial pressure becomes too high so that the oxidation reaction 
of Si.sub.3 N.sub.4 unfavorably becomes conspicuous. The reason why the 
total pressure of the N.sub.2 atmosphere or the mixed atmosphere is 
favorably set at not less than 1 atm. is because the oxidation reaction of 
Si.sub.3 N.sub.4 can be restrained and firing can be done by increasing 
the N.sub.2 partial pressure even when the O.sub.2 partial pressure is 
high. Thus, the evaporation of the SiO.sub.2 can be effectively 
restrained. 
In summarizing the above, the O.sub.2 partial pressure is raised to 
restrain the evaporation of SiO.sub.2. The O.sub.2 partial pressure is 
increased and balanced with the N.sub.2 partial pressure to restrain the 
nitriding of SiO.sub.2. Further, the N.sub.2 partial pressure is increased 
and balanced with the O.sub.2 partial pressure to restrain the oxidation 
of Si.sub.3 N.sub.4. 
Further, when CO is mixed into the firing atmosphere together with 
CO.sub.2, a CO formation reaction shown in the following formula (5) is 
made difficult to take place, and a consumed amount of carbon (C) is 
decreased to reduce damages of the carbonaceous heater, jig, etc. 
frequently used in the firing of the silicon nitride sinterable body. 
CO formation reaction: 
EQU CO.sub.2 +C.fwdarw.2CO (5) 
It is preferable that a mixing rate of CO is greater than that of CO.sub.2. 
However, if CO is in a pressure higher than an equilibrium partial 
pressure of CO in the reaction shown by the formula (5), a formation 
reaction of CO.sub.2 and C reverse to the reaction shown in the formula 
(5) takes place so that a produced C deposits on the silicon nitride 
sintered body or reacts with SiO.sub.2 in the silicon nitride sinterable 
body. Thus, the pressure higher than the CO equilibrium partial pressure 
is unfavorable. 
In addition, in order to reduce damages of the carbonaceous heater, jig, 
etc. in the firing furnace for the silicon nitride sinterable body and 
control the O.sub.2 partial pressure in the firing furnace to an 
appropriate range, it may be that an N.sub.2 gas or a mixed gas of N.sub.2 
and an inert gas which contains CO and CO.sub.2 is prepared by passing 
N.sub.2 gas or a mixed gas of N.sub.2 and the inert gas which is mixed 
with gases of O.sub.2, H.sub.2 O, air, CO.sub.2, etc. through a heater 
placing a consumable carbon source therein, and is then introduced into 
the firing furnace for the silicon nitride sinterable body. As a matter of 
course, it may be that a gas containing CO and CO.sub.2 is prepared by 
passing a gas comprising O.sub.2, H.sub.2 O, air, CO.sub.2, etc. through 
the heater placing the carbon source therein, and mixed into an N.sub.2 
gas or a mixed gas of N.sub.2 and an inert gas, which is then introduced 
into the silicon nitride sinterable body-firing furnace. 
Moreover, in order to reduce damages of the carbonaceous heater, jig, etc. 
in the firing furnace for the silicon nitride sinterable body and control 
the O.sub.2 partial pressure in the firing furnace to the appropriate 
range, it may be that an N.sub.2 gas or a mixed gas of N.sub.2 and an 
inert gas containing CO and CO.sub.2 is prepared by reacting an N.sub.2 
gas or a mixed gas of N.sub.2 and the inert gas which contains gases of 
O.sub.2, H.sub.2 O, air, CO.sub.2, etc. with a consumable carbon source at 
an initial stage location of a gas introducing path of the firing furnace, 
and then introduced into a certain location of the heater of the jig in 
the firing furnace. 
The process for manufacturing the silicon nitride sintered body according 
to the present invention will be explained in more detail. 
First, the silicon nitride powdery starting material is prepared. The 
silicon nitride powdery starting material is composed of a formulated 
powder of a silicon nitride raw material powder and a sintering aid. 
Y.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3, etc. is added as a sintering aid 
as it is or in a form of an aqueous solution. 
Next, the above silicon nitride starting material powder is crushed and 
mixed by means of a mill using media. The mills of a wet type and a dry 
type may be both used. For instance, a ball mill, an attrition mill, a 
vibration mill, etc. may be used. Then, a thus obtained molding powder is 
molded by a dry type press, an injection molding, a slip casting, etc., 
thereby, obtaining a desired molding. 
The thus obtained molding is fired in an N.sub.2 atmosphere or a mixed 
atmosphere of N.sub.2 and an inert gas into which CO.sub.2 or a mixed gas 
of CO.sub.2 and CO is added. The firing temperature is preferably in a 
temperature range from 1,600.degree. C. to 2,000.degree. C. The addition 
amount of CO.sub.2 to N.sub.2 is preferably in a range not less than 
0.001%. It is preferable that the total pressure of the N.sub.2 atmosphere 
or the mixed atmosphere is not less than 1 atm. The desired silicon 
nitride sintered body can be obtained by the above-mentioned process. 
The present invention will be explained below in more detail based on the 
following examples, which are merely given in illustration thereof, but 
should never be interpreted to limit the scope of the invention. 
EXAMPLE 1 
A preparation powder was formulated by adding a sintering aid to a silicon 
nitride powdery starting material of 97.1% by weight in purity with a 
content of oxygen being 1.5% by weight at a rate shown in Table 1. After 
the preparation powder was mixed and crushed by means of a water wet type 
ball mill, the powder was dried and granulated, thereby obtaining a 
molding powder. Then, the molding powder was preliminarily molded, and 
molded by a hydrostatic press under a pressure of 3 ton/cm.sup.2, thereby 
preparing a planar molding of 60.times.60.times.6 mm. Such moldings were 
fired in an atmosphere and at a temperature given in Table 1, thereby 
obtaining silicon nitride sintered body Nos. 1-12 according to the 
manufacturing process of the present invention. On the other hand, such 
moldings were fired in an atmosphere outside of the restricted range of 
the manufacturing process according to the present invention given in 
Table 1, thereby obtaining silicon nitride sintered body Nos. 13-24 as 
comparative examples. The firing atmosphere given in Table 1 were 
controlled by supplying N.sub.2, CO.sub.2 and CO raw gases to a firing 
furnace at rates given in Table 1. 
With respect to the silicon nitride sintered body Nos. 1-24, Table 1 shows 
a bulk density, an oxygen content, a four point bending strength when a 
fired face or an inside worked face was employed as a tensile face, and 
increased amounts of oxidation per unit area in the fired face and the 
inside worked face when heated at 1,200.degree. C. in air for 100 hours. 
The bulk density and the four point bending strength were measured 
according to an Archimedes method and "a fine ceramics bending strength 
testing method" of JIS R-1601, respectively, with respect to a fired face 
and an inside worked face worked as tensile face at a depth deeper than 1 
mm from the surface. An increased amount of oxidation was determined from 
a weight increase and a surface area with respect to a sample having the 
whole surface fired and a sample having the whole surface which was an 
inside worked face worked at a depth not shallower than 1 mm from the 
surface after being heated in air. 
TABLE 1 
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Four point 
Increased amount 
Firing conditions bending strength 
of oxidation 
Atmosphere (MPa) (mg/cm.sup.2) 
Addition Total 
Temper- 
Bulk Oxygen Inside Inside 
composition 
CO.sub.2 
CO Balance 
pressure 
ature 
density 
content 
Fired 
worked 
Fired 
worked 
No. (wt %) 
(%) (%) 
(%) (atm) 
(.degree.C.) 
(g/cm.sup.3) 
(wt %) 
face 
face face 
face 
__________________________________________________________________________ 
Present 
invention 
1 MgO(5) 
0.001 
0 N.sub.2 
1 1750 3.18 4.2 680 710 0.8 0.8 
2 MgO(3) 
0.1 0 N.sub.2 
20 1900 3.20 3.3 820 880 0.6 0.6 
3 Al.sub.2 O.sub.3 (5) 
10 1 N.sub.2 
10 1850 3.16 3.8 650 700 0.1 0.1 
4 Al.sub.2 O.sub.3 (3) 
20 2 N.sub.2 
50 1900 3.15 3.0 720 800 0.1 0.1 
5 Y.sub.2 O.sub.3 (5) 
0.1 0 N.sub. 2 
10 1900 3.26 3.4 870 920 0.2 0.2 
6 Y.sub.2 O.sub.3 (3) 
1 0.2 
N.sub.2 
100 2000 3.24 2.8 900 950 0.2 0.2 
7 Y.sub.2 O.sub.3 (5), 
0.01 
0 N.sub.2 
1 1700 3.26 5.3 820 860 0.3 0.3 
MgO(5) 
8 Y.sub.2 O.sub.3 (3), 
1.5 0.1 
N.sub.2 
10 1850 3.28 4.4 970 1020 0.2 0.2 
MgO(3) 
9 Y.sub.2 O.sub.3 (3), 
50 0 N.sub.2 (50), 
50 1900 3.29 4.2 900 990 0.2 0.2 
MgO(3) Ar(50) 
10 Y.sub.2 O.sub.3 (5), 
0.001 
0 N.sub.2 
1 1750 3.28 5.2 750 790 0.4 0.4 
Al.sub.2 O.sub.3 (5) 
11 Y.sub.2 O.sub.3 (3), 
0.1 0.01 
N.sub.2 
300 1950 3.25 4.3 820 900 0.3 0.2 
Al.sub.2 O.sub.3 (3) 
12 Y.sub.2 O.sub.3 (3), 
0.01 
0 N.sub.2 (50), 
10 1850 3.24 4.5 830 840 0.2 0.2 
Al.sub.2 O.sub.3 (3) 
Ar(50) 
Comparative 
example 
13 MgO(5) 
0 0 N.sub.2 
1 1750 3.10 3.3 480 650 1.8 1.2 
14 MgO(3) 
0 0 N.sub.2 
20 1900 3.08 2.4 420 550 2.0 1.9 
15 Al.sub.2 O.sub.3 (5) 
0 0 N.sub.2 
10 1850 3.10 2.9 400 550 0.7 0.2 
16 Al.sub.2 O.sub.3 (3) 
0 0 N.sub.2 
50 1900 3.08 2.4 380 560 0.7 0.3 
17 Y.sub.2 O.sub.3 (5) 
0 0 N.sub.2 
10 1900 3.16 2.6 420 620 0.9 0.5 
18 Y.sub.2 O.sub.3 (3) 
0 0 N.sub.2 
100 2000 3.07 2.0 360 480 1.0 1.3 
19 Y.sub.2 O.sub.3 (5), 
0 0 N.sub.2 
1 1700 3.17 4.2 580 630 0.5 0.3 
MgO(5) 
20 Y.sub.2 O.sub.3 (3), 
0 0 N.sub.2 
10 1850 3.19 3.4 560 680 0.9 0.2 
MgO(3) 
21 Y.sub.2 O.sub.3 (3), 
0 0 N.sub.2 (50), 
50 1900 3.20 2.9 510 700 0.6 1.2 
MgO(3) Ar(50) 
22 Y.sub.2 O.sub.3 (5), 
0 0 N.sub.2 
1 1750 3.22 4.0 470 700 0.8 0.6 
Al.sub.2 O.sub.3 (5) 
23 Y.sub.2 O.sub.3 (3), 
0 0 N.sub.2 
300 1950 3.19 2.8 480 670 1.1 0.8 
Al.sub.2 O.sub.3 (3) 
24 Y.sub.2 O.sub. 3 (3), 
0 0 N.sub.2 (50), 
10 1850 3.19 2.9 510 650 0.9 0.4 
Al.sub.2 O.sub.3 (3) 
Ar(50) 
__________________________________________________________________________ 
As evident from Table 1, as compared with the conventional processes, the 
sintered bodies having the same composition being denser and exhibiting 
the equal four point bending strength and the oxidation resistance both 
with respect to the fired surface and inside can be obtained by firing in 
the N.sub.2 gas atmosphere or the mixed atmosphere of N.sub.2 and an inert 
gas to which CO.sub.2 or a mixed gas of CO.sub.2 and CO is added. 
The same moldings as in Sample No. 5 of the present invention in Example 1 
were fired at 1900.degree. C. in respective atmospheres containing CO and 
CO.sub.2 shown in Table 2. As a result, sintered bodies each having 
approximately the same four point bending strength and increased amount of 
oxidation as in Sample No. 5 of the present invention with respect to the 
fired face and the inside worked face were obtained. At that time, a 
carbon pellet of 20 mm in diameter and 10 mm in height was placed and 
fired together with the silicon nitride sinterable body in the firing 
furnace. Reduced weights of carbon pellets due to the firing were shown in 
Table 2. 
As understood from Table 2, the weight reducing of the carbon pellet in the 
firing is reduced by mixing CO to the firing atmosphere. That is, the 
damages of the carbonaceous heater or jig in the firing furnace can be 
reduced by mixing of CO. 
TABLE 2 
______________________________________ 
Firing atmosphere Reduced 
Total weight of 
CO.sub.2 
CO Balance 
pressure 
carbon 
No. (%) (%) (%) (atm) pellet 
______________________________________ 
Present 
25 0.1 0 N.sub.2 
10 0.5 
invention 
Present 
26 0.1 0.05 N.sub.2 
10 0.4 
invention 
Present 
27 0.1 0.1 N.sub.2 
10 0.15 
invention 
Present 
28 0.1 1 N.sub.2 
10 0.05 
invention 
______________________________________ 
As obvious from the foregoing explanation, according to the silicon 
nitride-manufacturing process of the present invention, the silicon 
nitride sintered body which suffers almost no deterioration of the fired 
surface resulting from the evaporation and nitriding reaction of 
SiO.sub.2, is dense, and exhibits the equal four bending strength and 
oxidation resistance with respect to the fired surface and the inside can 
be obtained by firing the molding in the N.sub.2 atmosphere or the mixed 
atmosphere of N.sub.2 and an inert gas to which CO.sub.2 or a mixed gas of 
CO.sub.2 and CO is added. 
While there has been described preferred examples of the invention, obvious 
modifications and variations are possible in light of the above teachings. 
It is therefore to be understood that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically 
described.