Process for producing compact silicon nitride ceramics

A process for producing a compact silicon nitride ceramic product which comprises molding a powder mixture of Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5 and Al.sub.2 O.sub.3 in proportions defined by the region A, B, C, D in the FIGURE within the ranges of 65.0 to 96.0% by volume Si.sub.3 N.sub.4, 1.0 to 32.0% by volume Ta.sub.2 O.sub.5 and 3.0 to 20.0% by volume Al.sub.2 O.sub.3, and then firing the powder mixture in a non-oxidizing atmosphere containing a carbon compound; or hot-pressing the powder mixture; in a graphite mold.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for producing compact silicon nitride 
ceramics. 
2. Description of the Prior Art 
Silicon nitride (Si.sub.3 N.sub.4) ceramics have a unique low degree of 
thermal expansion, high mechanical strength and superior thermal 
stability, and therefore, have attracted attention as various thermally 
stable materials of high strength. However, ceramic products thereof have 
unsatisfactory sinterability, and various efforts have been made to 
improve this property. Products which have emerged from these efforts are 
still unsatisfactory; particularly, bubbling occurs during manufacture at 
high temperature in air, which is an inherent defect of silicon nitride 
ceramics. Therefore, elimination of this defect has been strongly desired. 
SUMMARY OF THE INVENTION 
An object of this invention to provide a process for producing on a 
mass-production basis silicon nitride ceramics having superior properties 
hitherto not attainable, which are free from the defects of the prior art. 
Accordingly this invention provides a process for producing a compact 
silicon nitride ceramic product which comprises molding a powder mixture 
of Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5 and Al.sub.2 O.sub.3 in proportions 
defined by the region A, B, C and D in the attached Figure, the points A, 
B, C and D, respectively representing the compositions set forth below in 
volume %: 
______________________________________ 
Si.sub.3 N.sub.4 
Ta.sub.2 O.sub.5 
Al.sub.2 O.sub.3 
______________________________________ 
A 65.0% 32.0% 3.0% 
B 65.0% 15.0% 20.0% 
C 79.0% 1.0% 20.0% 
D 96.0% 1.0% 3.0% 
______________________________________ 
and then firing the powder mixture in a non-oxidizing atmosphere containing 
a carbon compound; or hot-pressing the powder mixture; in a graphite mold.

DETAILED DESCRIPTION OF THE INVENTION 
The Si.sub.3 N.sub.4, the Ta.sub.2 O.sub.5 and the Al.sub.2 O.sub.3 powders 
used as starting materials are all commercially available materials. A 
suitable particle size for these powders is an average particle size of 
about 3.mu. or less, preferably 1.mu. or less, for the Si.sub.3 N.sub.4, 
about 2.mu. or less, preferably 1.mu. or less, for the Ta.sub.2 O.sub.5 
and about 2.mu. or less, preferably 1.mu. or less, for the Al.sub.2 
O.sub.3. 
In preparation of the compact silicon nitride ceramic product of this 
invention, a powder mixture of Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5 and 
Al.sub.2 O.sub.3 within the proportions defined by the region A, B, C and 
D in the attached FIGURE is prepared and then the molded powder mixture is 
fired in a non-oxidizing atmosphere containing a carbon compound. A 
suitable firing temperature for the powder mixture is in the range of 
about 1600.degree. C. to about 1850.degree. C., preferably 1650.degree. C. 
to 1800.degree. C. If the temperature is less than about 1600.degree. C., 
sinterability is degraded, and if the temperature is above about 
850.degree. C., silicon nitride is decomposed to an excessive extent. 
Suitable non-oxidizing atmospheres which can be used include those 
atmospheres where oxygen is present in an amount of about 5 volume percent 
or less, preferably 2 volume percent or less. The carbon compound present 
generally is carbon monoxide contained in the graphite mold, or carbon 
monoxide generated and obtained by reaction with oxygen. A suitable amount 
of the carbon compound is less than about 30% by volume (with at least 
some carbon compound being present) using hot-pressing or normal pressure 
sintering. 
Alternatively, the molded powder mixture can be hot-pressed in a graphite 
mold. A suitable hot-pressing temperature is in the range of about 
1550.degree. C. to about 1850.degree. C. preferably 1600.degree. C. to 
1800.degree. C. A suitable pressure in the hot-pressing is a pressure of 
more than about 100 kg/cm.sup.2 (with higher, pressures being preferred). 
In general, a suitable pressure in operation is preferably 200 to 400 
kg/cm.sup.2. Where the hot pressing temperature is low, the hot pressing 
period preferably is increased and a higher pressure preferably is used. 
In general, a hot pressing period of about 30 to 60 minutes is suitable. 
In hot pressing, since a graphite mold is used, oxygen in the interior of 
the mold reacts with carbon to produce carbon monoxide, even if hot 
pressing is conducted in the air. Thus, a non-oxidizing atmosphere is 
obtained in the mold, and therefore, it is not necessary to provide a 
non-oxidizing atmosphere around the graphite mold where hot-pressing is 
employed. 
The following Examples are given to illustrate the present invention more 
specifically. Unless otherwise indicated, all parts percents, ratios and 
the like are by weight. 
EXAMPLE 1 
Si.sub.3 N.sub.4 powder (a product of Advanced Material Engineering; 
purity: 98 weight %; containing more than 80 weight % .alpha.-type silicon 
nitride) having an average particle diameter of 1 micron, Ta.sub.2 O.sub.5 
powder (a product of Helman Schtalk; purity: 99.5 weight %) having an 
average particle diameter of 1.5 microns, and Al.sub.2 O.sub.3 powder 
(A-16; a product of Alcoa) having an average particle diameter of 1 
micron, all commercially available materials, were mixed in the various 
proportions shown in Table 1 below, and an organic binder (parafin) was 
added thereto. The starting powder was press-formed under a pressure of 
2,000 kg/cm.sup.2 into a size of 5.times.10.times.30 mm, first heated to 
500.degree. C. in a nitrogen atmosphere to decompose and remove the 
organic binder, and then placed in a graphite muffle furnace and fired. 
Thus, Sample Nos. 1 to 16 were produced. Furthermore, each of the starting 
powder mixtures having the compositions corresponding to Samples Nos. 6 to 
8 was charged into a graphite mold, and hot-pressed at 1750.degree. C. at 
200 kg/cm.sup.2 for 20 minutes to produce Sample Nos. 6' to 8'. 
The various properties shown in Table 1 below of these samples were 
measured, and the results obtained are shown in Table 1. 
Table 1 
__________________________________________________________________________ 
Composition (vol. %) Properties 
Main Coefficient 
Ther- 
Com- Firing Modulus of 
of Thermal 
mal 
po- Subsidiary 
Temper- 
Density 
Rupture 
Expansion** 
Sta- 
Sample 
nent 
Components 
ature 
(T.D. 
Strength* 
(R.T.-1200.degree. C. 
bility 
No. Si.sub.3 N.sub.4 
Ta.sub.2 O.sub.5 
Al.sub.2 O.sub.3 
(.degree.C.) 
ratio,%) 
(kg/mm.sup.2) 
1.degree. C.) 
(.degree.C.) 
Remarks 
__________________________________________________________________________ 
1 65.0 
32.0 3.0 1700 94.0 44 3.1 .times. 10.sup.-6 
1250 
Point A in the FIG. 
2 65.0 
15.0 20.0 1650 94.1 49 3.3 1250 
Point B in the FIG. 
3 79.0 
1.0 20.0 1650 94.3 48 3.4 &gt;1300 
Point C in the FIG. 
4 96.0 
1.0 3.0 1750 94.8 47 3.2 &gt;1300 
Point D in the FIG. 
5 65.0 
22.5 12.5 1700 95.6 48 3.0 1250 
6 70.0 
17.5 12.5 1700 96.8 54 3.1 &gt;1300 
7 75.0 
12.5 12.5 1700 96.9 56 3.1 &gt;1300 
8 80.0 
7.5 12.5 1650 96.2 58 3.2 &gt;1300 
9 86.5 
1.0 12.5 1650 96.0 49 3.2 &gt;1300 
10 85.0 
12.0 3.0 1700 95.5 47 3.2 1250 
11 75.0 
22.0 3.0 1700 95.2 46 3.2 1250 
12 72.5 
7.5 20.0 1650 96.4 45 3.4 &gt;1300 
6' 70.0 
17.5 12.5 1750 99.4 85 3.1 &gt;1300 
Hot-pressed (1750.degree. 
C.; 200 kg/cm.sup.2) 
7' 75.0 
12.5 12.5 1750 99.6 83 3.1 &gt;1300 
Hot-pressed (1750.degree. 
C.; 200 kg/cm.sup.2) 
8' 80.0 
7.5 12.5 1750 99.5 80 3.2 &gt;1300 
Hot-pressed (1750.degree. 
C.; 200 kg/cm.sup.2) 
13 60.0 
27.5 12.5 1700 94.2 37 3.2 1200 
Outside the present 
invention 
14 68.0 
7.0 25.0 1700 93.3 30 4.5 1250 
Outside the present 
invention 
15 87.0 
-- 12.5 1800 90.1 28 3.5 1250 
Outside the present 
invention 
16 80.0 
20.0 -- 1800 87.7 25 3.2 1200 
Outside the present 
invention 
__________________________________________________________________________ 
*JIS B4104 
**JIS C2141 
Note 
The TD ratio, % was calculated using the following relationship: 
##STR1## 
The coefficient of thermal expansion is that from room temperature (R.T.) 
to 1200.degree. C. per .degree. C. 
The thermal stability was measured as follows: The surface of a sample was 
rapidly heated with a burner, and the temperature at which bubbling 
occurred at the surface due to the vitrification of silicon nitride was 
measured using a photo high-temperature meter. Thus, the silicon nitride 
ceramic bodies are stable up to the temperatures listed in Table 1 above. 
It can be clearly seen from the results in Table 1 above that Sample Nos. 1 
to 12 including Sample Nos. 6' to 8' produced by hot-pressing had 
increased thermal stability and modulus of rupture strength and a reduced 
coefficient of thermal expansion despite the fact that the firing 
temperatures for these samples were considerably lower than the firing 
temperatures used for producing Sample Nos. 13 to 16 which were outside 
the scope of the present invention. 
The high thermal stability and other superior properties were obtained in 
this Example by the addition of specific proportions of Ta.sub.2 O.sub.5 
and Al.sub.2 O.sub.3 as subsidiary components to Si.sub.3 N.sub.4 as a 
main component. This is believed to be due to the fact that the Ta.sub.2 
O.sub.5 and Al.sub.2 O.sub.3 in the composition first form a solid 
solution which then reacts with the Si.sub.3 N.sub.4 to form a compact 
sintered body; thereafter, the Ta.sub.2 O.sub.5 is reduced by carbon in 
the atmosphere to form TaC; the thermal stability of the sintered body is 
increased, and bubbling of the sintered body is inhibited, by the high 
melting point and the high thermal conductivity of TaC; and the high 
mechanical strength of the TaC associated with the high mechanical 
strength of the Al.sub.2 O.sub.3 contributes to the increase of the 
modulus of rupture strength of the final ceramic product. 
In the starting material used in this invention, the proportions of 
Si.sub.3 N.sub.4 as a main component and of Ta.sub.2 O.sub.5 and Al.sub.2 
O.sub.3 as subsidiary components are limited to the region connecting 
points A, B, D and D in the attached FIGURE within the range of 65.0 to 
96.0% by volume Si.sub.3 N.sub.4, 1.0 to 32.0% by volume Ta.sub.2 O.sub.5 
and 3.0 to 20.0% by volume Al.sub.2 O.sub.3. This is because as shown by 
Sample Nos. 1 to 12 and 6' to 8' in Table 1, compositions having 
proportions within the above-specified range can be formed into products 
having superior properties including high thermal stability despite the 
fact that sintering occurs at a temperature of 1700.degree. C. or less, 
whereas Sample Nos. 13, 14, 15 and 16 obtained from materials having 
proportions outside the above-specified range have a markedly low modulus 
of rupture strength, and are not useful in practical application. 
Addition of a small amount of an alkaline earth metal oxide such as MgO, 
CaO and BaO or a compound convertible to an alkaline earth metal oxide to 
the composition defined by the region A, B, C, D described above reduces 
the viscosity of a molten liquid of Ta.sub.2 O.sub.5 and Al.sub.2 O.sub.3 
as subsidiary components and free SiO.sub.2 present in Si.sub.3 N.sub.4 as 
a main component, decreases the sintering temperature, and promotes 
compaction. Suitable compounds convertible to alkaline earth metal oxides 
during firing which can be used include for example, carbonates, sulfates, 
chlorides, nitrates, fluorides, and hydroxides of alkaline earth metals. A 
suitable average particle size of less than about 2.mu.; preferably less 
than 1.mu., can be used for a powder mixture containing Si.sub.3 N.sub.4, 
Ta.sub.2 O.sub.5 and Al.sub.2 O.sub.3 plus the above described alkaline 
earth metal oxide. Excessive amounts of such an alkaline earth metal oxide 
cause a reduction in the modulus of rupture strength and thermal 
stability, and an increase in the coefficient of thermal expansion. 
Considering all factors, a suitable amount of the alkaline earth metal 
oxide that can be added for practical purposes is up to about 5 parts by 
weight per 100 parts by weight of the starting powder of Si.sub.3 N.sub.4 
as a main component and Ta.sub.2 O.sub.5 and Al.sub.2 O.sub.3 as secondary 
components. This is demonstrated by the results obtained in Example 2 
below. 
EXAMPLE 2 
The procedures of Example 1 were repeated except that each of the alkaline 
earth metal oxides shown in Table 2 below was added in the amounts 
indicated to 100 parts by weight of a powder mixture of the composition 
used for Sample No. 7 (this composition being centrally positioned in the 
region A, B, C, D in the attached FIGURE) comprising Si.sub.3 N.sub.4 as a 
main component and Ta.sub.2 O.sub.5 and Al.sub.2 O.sub.3 as secondary 
components. The properties of the resulting samples were measured, and the 
results obtained are shown in Table 2 below. 
Table 2 
__________________________________________________________________________ 
Fir- 
Basic Components* ing 
Den- Coefficient 
Ther- 
Main Tem- 
sity 
Modulus of 
of Thermal 
mal 
Sam- 
Com- 
Subsidiary Additive** per- 
(T.D. 
Rupture 
Expansion 
Stab- 
ple 
ponent 
Components 
To- To- 
ature 
ratio, 
Strength 
(RT-1200.degree. C. 
ility 
No. 
Si.sub.3 N.sub.4 
Ta.sub.2 O.sub.5 
Al.sub.2 O.sub.3 
tal 
MgO 
CaO 
BaO 
SrO 
tal 
(.degree.C.) 
%) (kg/mm.sup.2) 
1.degree. C.) 
(.degree.C.) 
Remarks 
__________________________________________________________________________ 
7 75.0 
12.5 
12.5 
100 
-- -- -- -- -- 1700 
96.9 
56 3.1 &gt;1300 
Repro- 
duced 
from 
Table 1 
7a " " " " 0.5 
-- -- -- 0.5 
1700 
97.0 
56 3.1 &gt;1300 
7b " " " " 2.0 
-- -- -- 2.0 
1670 
97.2 
57 3.2 1300 
7c " " " " 3.0 
-- -- -- 3.0 
1650 
97.3 
56 3.4 1250 
7d " " " " 3.5 
-- -- -- 3.5 
1630 
97.1 
53 3.6 1150 
7j " " " " 5.0 
-- -- -- 5.0 
1630 
97.0 
53 3.6 1150 
7k " " " " 6.0 
-- -- -- 6.0 
1600 
97.1 
50 3.9 1000 
Amount 
of additive 
outside 
specified 
range 
7e " " " " -- 2.0 
-- -- 2.0 
1650 
97.2 
55 3.3 1250 
7f " " " " -- -- 2.0 
-- 2.0 
1650 
96.5 
54 3.5 1250 
7g " " " " -- -- -- 2.0 
2.0 
1670 
97.2 
55 3.4 1300 
7h " " " " 1.0 
-- 1.0 
-- 2.0 
1670 
97.0 
56 3.3 1250 
7i " " " " -- 1.0 
-- 1.0 
2.0 
1670 
97.1 
54 3.4 1250 
__________________________________________________________________________ 
*Parts by volume 
**Parts by weight per 100 parts by weight of the basic components 
In Examples 1 and 2, oxides are directly used as starting subsidiary 
components and additive components. Other compounds convertible to oxides 
by heating in a customary manner, such as salts, can also be used in the 
invention. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.