Sintered silicon nitride body

Disclosed is a sintered ceramic body having a composition comprising (i) Si.sub.3 N.sub.4, (ii) a combination of an oxide of an element of the group IIIa of the Periodic Table having an ionic radius smaller than 0.97 .ANG. and an oxide of other element of the group IIIa of the Periodic Table, having an ionic radius larger than 0.97.degree. and (iii) an oxide or nitride of at least one of the following: (1) at least one element of group IIa of the Periodic Table; (2) Al; (3) Ti; (4) Cr; (5) Ga; (6) Zr; and (7) Si. This sintered silicon nitride body shows a high oxidation resistance when it is used for a long time at high temperatures, and the sintered body is excellent in the creep characteristics and flexural strength at high temperatures.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a sintered silicon nitride body having a 
novel composition, which is improved in mechanical properties such as 
creep resistance and flexural strength at high temperatures and is 
excellent in the oxidation resistance. 
(2) Description of the Prior Art 
A sintered silicon nitride (Si.sub.3 N.sub.4) body is known as a high 
temperature material or a high abrasion resistance material. However, 
Si.sub.3 N.sub.4 is defective in that the sintering property is bad, and 
as a material for production of a sintered body having high density and 
high strength, there has already proposed a silicon nitride composition in 
which an oxide of an element of the group IIIa of the Periodic Table, such 
as yttria (Y.sub.2 O.sub.3), is incorporated or Al.sub.2 O.sub.3 is 
further incorporated (see, for example, Japanese Patent Publication No. 
21091/74 or No. 3649/77). However, a sintered body obtained from this 
composition is still insufficient in the flexural strength at room 
temperature (about 60 kg/mm.sup.2), and it has been found that when this 
sintered body is used for a long time at a high temperature, especially in 
a high-temperature oxidative atmosphere, the sintered body readily 
undergoes oxidation and such characteristics as the creep resistance, 
flexural strength, dimension precision and shape precision are drastically 
reduced. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the present invention to provide a 
sintered silicon nitride body which is improved in mechanical 
characteristics such as the flexural strength and creep resistance at high 
temperatures. 
Another object of the present invention is to provide a sintered body of 
silicon nitride having a novel composition, which is improved in the 
sintering property of Si.sub.3 N.sub.4 and capable of providing a dense 
molded body and is capable of providing a sintered body in which the 
mechanical characteristics and oxidation resistance are highly improved 
while controlling the growth of crystal grains at high temperatures. 
More specifically, in accordance with the present invention, there is 
provided a sintered silicon nitride body having a composition comprising 
(i) 80 to 99.8 mole% of silicon nitride, (ii) at least one oxide of an 
element of the group IIIa of the Periodic Table having an ionic radius 
smaller than 0.97 .ANG. and (b) at least one oxide of an element of the 
group IIIa of the Periodic Table which is different from said element of 
the group IIIa of the Periodic Table, the total amount of the oxides (a) 
and (b) being 0.2 to 20 mole% and the (a)/(b) molar ratio being within a 
range of from 10/90 to 90/10, and (iii) an oxide or nitride of at least 
one of the following in an amount of 0.1 to 5 parts by weight per 100 
parts by weight of the sum of the components (i) and (ii): 
(1) At least one element of the group IIa of the periodic table; 
(2) Al; 
(3) Ti; 
(4) Cr; 
(5) Ga; 
(6) Zr; and 
(7) Si 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to the present invention, a sintered silicon nitride body is 
prepared from a composition comprising Si.sub.3 N.sub.4, oxides of rare 
earth elements and an oxide or nitride of aluminum or the like. The 
present invention is prominently characterized in that an oxide of an 
element of the group IIIa of the Periodic Table having an ionic radius 
smaller than 0.97 .ANG. and an oxide of other element of the group IIIa of 
the Periodic Table, that is, an element having an ionic radius larger than 
0.97 .ANG., are combined at a specific ratio and used as the rare earth 
element oxide. According to the present invention, by dint of this 
characteristic feature, the sintering property of Si.sub.3 N.sub.4 is 
improved, with the result that a sintered body which is highly densified 
and has an improved flexural strength can be obtained. Namely, a sintered 
Si.sub.3 N.sub.4 body which is excellent in such mechanical 
characteristics as the creep resistance and flexural strength at high 
temperatures and also in the oxidation resistance can be obtained 
according to the present invention. 
It is known that in elements of the lanthanum series among rare earth 
elements, ionic radii are gradually decreased little by little with 
increase of the atomic number. Trivalent ionic radii of rare earth 
elements are shown in Table A below. 
TABLE A 
______________________________________ 
Ionic Radii (A) 
Larger Than 0.97 .ANG. (b) 
Smaller Than 0.97 .ANG. (a) 
______________________________________ 
La (1.06) Ce(Ce.sup.4+) (0.94) 
Pr (1.01) Y (0.89) 
Nd (0.99) Yb (0.86) 
Th (Th.sup.4+) (1.10) 
Sm (0.96) 
Eu (0.95) 
Tb (0.92) 
Dy (0.90) 
Lu (0.84) 
Tm (0.86) 
Ho (0.89) 
Gd (0.93) 
Sc (0.81) 
______________________________________ 
The reason why the above-mentioned excellent functional effects can be 
attained in the present invention by combining oxides of elements of the 
group IIIa of the Periodic Table differing in the ionic radius at a 
specific ratio has not been completely elucidated, but the following can 
be considered. 
Silicon nitride powder is covered with a coating of silica (SiO.sub.2). 
When two kinds of rare earth element (RE) oxides differing in the ionic 
radius are added to silicon nitride powder, reactions of SiO.sub.2 
-(RE).sub.2 O.sub.3 and Si.sub.3 N.sub.4 -(RE).sub.2 O.sub.3 take place, 
and it is considered that phases are formed according to the following 
rule. The rare earth element oxide having a smaller ionic radius is likely 
to react with SiO.sub.2 and such compounds as (RE).sub.2 
O.sub.3.(SiO.sub.2).sub.2 and (RE).sub.2 O.sub.3.(SiO.sub.2) are formed in 
the sintered body. On the other hand, the rare earth element oxide having 
a larger ionic radius is likely to react with Si.sub.3 N.sub.4 and such a 
compound as (RE).sub.2 O.sub.3.Si.sub.3 N.sub.4 is formed in the sintered 
body. In compounds of the type (RE).sub.2 O.sub.3.(SiO.sub.2).sub.2, a 
smaller ionic radius of RE gives a higher melting point and in compounds 
of the type (RE).sub.2 O.sub.3.Si.sub.3 N.sub.4, a larger ionic radius of 
RE gives a higher melting point. However, if two kinds of rare earth 
element (RE) oxides differing in the ionic radius are added to Si.sub.3 
N.sub.4, the grain boundary phase is filled with a high-melting-point 
crystal phase to give an effective influence to improvement of the 
strength at high temperatures. Furthermore, an oxide of RE having a 
smaller ionic radius is highly effective as the sintering aid, and an 
oxide of RE having a larger ionic radius is effective for improving the 
strength at high temperatures. It is deemed that by combining these two RE 
oxides, the sintering property is prominently improved. It is believed 
that the excellent functional effects of the present invention can be 
attained for the above-mentioned reasons. 
In the present invention, Si.sub.3 N.sub.4 having an .alpha.-type or 
.beta.-type crystal structure is used as silicon nitride. 
The elements shown in Table A are used as the element of the group IIIa of 
the Periodic Table having an ionic radius smaller than 0.97 .ANG., and as 
preferred examples of the oxide, there can be mentioned yttria (Y.sub.2 
O.sub.3) and ytterbium oxide (Yb.sub.2 O.sub.3). The elements shown in 
Table A are used as the element of the group IIIa of the Periodic Table 
having an ionic radius larger than 0.97 .ANG., and as preferred examples 
of the oxide, there can be mentioned lanthania (La.sub.2 O.sub.3) and 
neodymium oxide (Nd.sub.2 O.sub.3). 
In the present invention, it is important that Si.sub.3 N.sub.4 should be 
incorporated in an amount of 80 to 99.8 mole%, especially 90 to 99 mole%, 
based on the total amount of Si.sub.3 N.sub.4 and the oxides of the 
elements of the group IIIa of the Periodic Table, and that at least two 
oxides of elements of the group IIIa of the Periodic Table should be 
incorporated in an amount of 0.2 to 20 mole%, especially 1 to 10 mole%, 
based on the total amount of Si.sub.3 N.sub.4 and the oxides of the 
element of the group IIIa of the Periodic Table. If the amount of Si.sub.3 
N.sub.4 is outside the above-mentioned range, the strength at high 
temperatures is drastically degraded and the oxidation resistance is 
somewhat reduced. 
If the total amount of at least two oxides of elements of the group IIIa of 
the Periodic Table is smaller than 0.2 mole%, densification is not caused 
and the strength is degraded, and the bulk specific gravity is reduced and 
the porosity is increased. On the other hand, if the total amount of the 
oxides of the elements of the group IIIa of the Periodic Table exceeds 20 
mole%, the flexural strength at high temperatures is drastically reduced 
and the oxidation resistance is reduced. 
It also is important that (a) an oxide of an element of the group IIIa of 
the Periodic Table having an ionic radius smaller than 0.97 .ANG. and (b) 
an oxide of other element of the group IIIa of the Periodic Table should 
be combined and used at an (a)/(b) molar ratio of from 10/90 to 90/10, 
especially from 30/70 to 70/30. If the molar ratio is outside the 
above-mentioned range, mechanical properties such as the flexural 
strength, especially the creep resistance and flexural strength at high 
temperatures, are degraded. 
According to the present invention, by adding 0.1 to 5 parts by weight of 
at least one member selected from oxides and nitrides of elements of the 
group IIa of the Periodic Table, Al, Ti, Cr, Ga, Zr and Si to 100 parts by 
weight of the main component comprising Si.sub.3 N.sub.4 and appropriate 
amounts of oxides of elements of the group IIIa of the Periodic Table, a 
completely dense, anti-oxidative protecting covering layer is formed. If 
at least one member selected from oxides and nitrides of elements of the 
group IIa of the Periodic Table, Al, Ti, Cr, Ga, Zr and Si is added as the 
additive, there can be attained an effect of densifying a powdery oxide 
formed by oxidation of the composition comprising silicon nitride and rare 
earth element oxides. Accordingly, if a dense oxide covering layer is once 
formed on the surface by oxidation, this covering layer acts as a 
protecting layer for inhibiting diffusion of oxygen in the interior and 
oxidation reaction, and the oxidation resistance is prominently improved 
and the life of the sintered body in a high-temperature oxidative 
atmosphere is prolonged. 
If the amount of the oxide or nitride of an element of the group IIa of the 
Periodic Table, Al, Ti, Cr, Ga, Zr or Si is smaller than 0.1 part by 
weight per 100 parts by weight of the main component comprising Si.sub.3 
N.sub.4 and appropriate amounts of oxides of elements of the group IIIa of 
the Periodic Table, the above-mentioned protecting layer is not 
sufficiently formed and the oxidation degree is increased. If the amount 
of the additive exceeds 5 parts by weight, the creep resistance and 
flexural strength at high temperatures are degraded. 
A sintered silicon nitride body especially preferred for attaining the 
objects of the present invention has a composition comprising (i) 90 to 99 
mole% of Si.sub.3 N.sub.4, (ii) a combination of (a) Yb.sub.2 O.sub.3 and 
(b) Nd.sub.2 O.sub.3, the total amount of (a) and (b) being 1 to 10 mole% 
and the (a)/(b) molar ratio being in the range of from 30/70 to 70/30 and 
(iii) AlN or CrN in an amount of 0.1 to 5 parts by weight per 100 parts by 
weight of the sum of (i) and (ii). 
Powders of the foregoing components are sufficiently mixed and finely 
divided by so-called wet pulverization or the like, and the pulverization 
product is subjected to spray granulation or the like and dried. Molding 
of the resulting composition is accomplished by mixing the composition 
with a binder such as a wax and subjecting the mixture to a known molding 
operation such as compression molding, injection molding or cold isostatic 
pressing. The obtained molded body is preliminarily sintered according to 
need and is then sintered in an inert atmosphere at a high temperature. 
Ordinarily, preliminary sintering is carried out at a temperature of 
1500.degree. to 1800.degree. C. for 0.5 to 5 hours under atmospheric 
pressure, elevated pressure or reduced pressure. It is preferred that the 
sintering treatment be effected by hot isostatic pressing. Hot isostatic 
pressing is accomplished by charging the preliminarily sintered molded 
body in an apparatus comprising a pressure cylinder, a bottom closure, an 
insulator mantle and support arranged in the interior and a heating 
element arranged on the inner side of the insulator mantle, and supplying 
an inert gas under pressure and heating the charged molded body. 
Pressurization of 1500 to 2000 atmospheres (gauge) and heating to 
1500.degree. to 2000.degree. C. are effective for this hot isostatic 
pressing. Nitrogen gas is advantageously used as the inert gas, but other 
gas such as argon can be used. 
The sintered body obtained by the hot isostatic pressing treatment is 
polished by sand blast or the like according to need, and a product is 
thus obtained.

The present invention will now be described in detail with reference to the 
following examples that by no means limit the scope of the invention. 
EXAMPLE 1 
A powdery composition formed by adding an oxide of an element of the group 
IIa, Al, Ti, Cr, Ga, Zr or Si in an amount shown in Table 1 to a mixture 
of Si.sub.3 N.sub.4 and at least two oxides of elements of the group IIIa 
of the Periodic Table was mixed in a ball mill for 24 hours. The obtained 
slurry was dried and granulated and then press-molded, and the binder used 
for molding was removed in vacuo and the molded body was sintered under 
conditions shown in Table 1. Thus, samples of Nos. 1 to 31 were obtained. 
Incidentally, in case of samples Nos. 7 and 9, preliminary sintering was 
carried out by hot pressing, or in case of samples Nos. 8 and 29 through 
31, preliminary sintering was carried out at 1800.degree. C. at 2.0 MPa in 
a nitrogen atmosphere, and then, the HIP treatment was carried out under 
sintering conditions shown in Table 1. In case of other samples, ordinary 
atmospheric sintering was carried out. With respect to each of these 
samples Nos. 1 through 31, the four-point bending flexural strength test 
according to Japanese Industrial Standard R-1601 was carried out at room 
temperature and at 1300.degree. C., and the degree of oxidation was 
examined based on the weight increase (mg/mc.sup.2) after 1000 hours' 
standing at 1300.degree. C. to evaluate the oxidation resistance at high 
temperatures. The obtained results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Composition of Main Component 
Composition of Additive 
Sample 
silicon nitride 
compound of element of 
(part by weight per 100 parts by 
No. (mole %) group IIIa (mole %) 
weight of main component) 
__________________________________________________________________________ 
1 95 Y.sub.2 O.sub.3 
3 Al.sub.2 O.sub.3 
0.1 
La.sub.2 O.sub.3 
2 
2 95 Tb.sub.2 O.sub.3 
3 AlN 1 
La.sub.2 O.sub.3 
2 SiO.sub.2 
2 
3 90 Yb.sub.2 O.sub.3 
5 SiO.sub.2 
2 
Pr.sub.6 O.sub.11 
5 TiO.sub.2 
3 
4 90 CeO.sub.2 
5 Cr.sub.2 O.sub.3 
1 
La.sub.2 O.sub.3 
5 Al.sub.2 O.sub.3 
3 
5 90 Y.sub.2 O.sub.3 
5 Al.sub.2 O.sub.3 
1 
La.sub.2 O.sub.3 
5 Ga.sub.2 O.sub.3 
4 
BaO 1 
6 85 Sm.sub.2 O.sub.3 
5 SiO.sub. 2 
1 
Y.sub.2 O.sub.3 
5 ZrO.sub.2 
2 
CeO.sub.2 
5 
7 95 Y.sub.2 O.sub.3 
2 Al.sub.2 O.sub.3 
0.1 
La.sub.2 O.sub.3 
3 
8 95 Y.sub.2 O.sub.3 
2 Cr.sub.2 O.sub.3 
0.3 
Nd.sub.2 O.sub.3 
3 SrO 1 
9 85 Y.sub.2 O.sub.3 
5 Al.sub.2 O.sub.3 
3 
YN 1 
La.sub.2 O.sub.3 
9 
10* 95 Y.sub.2 O.sub.3 
3 -- 
La.sub.2 O.sub.3 
2 
11* 85 Y.sub.2 O.sub.3 
5 -- 
La.sub.2 O.sub.3 
5 
Sm.sub.2 O.sub.3 
5 
12* 98 Y.sub.2 O.sub.3 
2 -- 
13* 75 Y.sub.2 O.sub.3 
15 Al.sub.2 O.sub.3 
7 
La.sub.2 O.sub.3 
10 
14* 90 Y.sub.2 O.sub.3 
5 Al.sub.2 O.sub.3 
7 
La.sub.2 O.sub.3 
5 
15 96 Nd.sub.2 O.sub.3 
2 AlN 0.5 
Yb.sub.2 O.sub.3 
2 
16 96 La.sub.2 O.sub.3 
2 AlN 0.5 
Yb.sub.2 O.sub.3 
2 
17 96 Nd.sub.2 O.sub.3 
2 AlN 1.5 
Yb.sub.2 O.sub.3 
2 
18 96 Nd.sub.2 O.sub.3 
2 AlN 4.0 
Yb.sub.2 O.sub.3 
2 
19 96 Nd.sub.2 O.sub.3 
2 CrN 0.5 
Yb.sub.2 O.sub.3 
2 
20 96 La.sub.2 O.sub.3 
2 CrN 0.5 
Yb.sub.2 O.sub.3 
2 
21 96 Nd.sub.2 O.sub.3 
2 CrN 4.0 
Yb.sub.2 O.sub.3 
2 
22 96 Nd.sub.2 O.sub.3 
1 AlN 0.5 
Y.sub.2 O.sub.3 
1 
Yb.sub.2 O.sub.3 
2 
23 95.5 La.sub.2 O.sub.3 
1.5 AlN 0.5 
Nd.sub.2 O.sub.3 
1.5 
Yb.sub.2 O.sub.3 
1.5 
24 95.5 La.sub.2 O.sub.3 
1.5 AlN 0.5 
Sm.sub.2 O.sub.3 
1.5 
Yb.sub.2 O.sub.3 
1.5 
25 96 La.sub.2 O.sub.3 
1 AlN 0.5 
Nd.sub.2 O.sub.3 
1 
Sm.sub.2 O.sub.3 
1 
Yb.sub.2 O.sub.3 
1 
26 96 Nd.sub.2 O.sub.3 
2 AlN 0.5 
Y.sub.2 O.sub.3 
1 
Yb.sub.2 O.sub.3 
1 
27 96 Nd.sub.2 O.sub.3 
1 AlN 0.5 
Y.sub.2 O.sub.3 
1 
Yb.sub.2 O.sub.3 
2 
28 95.5 La.sub.2 O.sub.3 
1.5 AlN 0.5 
Nd.sub.2 O.sub.3 
1.5 
Yb.sub.2 O.sub.3 
1.5 
29 95.5 La.sub.2 O.sub.3 
1.5 AlN 0.5 
Sm.sub.2 O.sub.3 
1.5 
Nd.sub.2 O.sub.3 
1.5 
30 96 La.sub.2 O.sub.3 
1 AlN 0.5 
Nd.sub.2 O.sub.3 
1 
Sm.sub.2 O.sub. 3 
1 
Yb.sub.2 O.sub.3 
1 
31 96 La.sub.2 O.sub.3 
2 AlN 0.5 
Yb.sub.2 O.sub.3 
2 
__________________________________________________________________________ 
Properties of Sintered Body 
flexural 
flexural 
Sintering Conditions 
strength 
strength 
nitrogen 
bulk 
at room 
at high weight increase of sintered 
Sample 
tempera- 
pressure 
specific 
temperature 
temperature 
body, 1300.degree. C./1000 hrs 
No. ture (.degree.C.) 
(MPa) 
gravity 
.sigma. b.sub.4 (RT) 
.sigma. b.sub.4 (1300.degree. C.) 
(mg/cm.sup.2) 
__________________________________________________________________________ 
1 1950 10.0 3.26 
115 101 0.50 
2 1950 10.0 3.27 
107 95 0.30 
3 1850 0.1 3.45 
96 82 0.70 
4 1850 2.0 3.47 
87 63 0.90 
5 1850 0.98 3.49 
76 54 0.40 
6 1850 0.98 3.48 
79 53 0.20 
7 1800 0.1 3.29 
123 108 0.50 
(H.P) 
8 2000 200.0 
3.26 
94 86 0.60 
9 1500 0.1 3.58 
76 63 0.90 
(H.P) 
10* 
1950 10.0 3.24 
118 105 15.50 
11* 
1850 0.98 3.44 
83 62 oxidation to the interior 
12* 
2000 20 2.78 
43 40 porosity of 13% 
13* 
1750 0.98 3.56 
74 31 1.30 
14* 
1750 2.0 3.49 
76 25 1.20 
15 1980 0.98 3.38 
96 86 0.19 
16 1980 0.98 3.37 
105 101 0.17 
17 1980 0.98 3.38 
112 104 0.11 
18 1980 0.98 3.37 
105 91 0.10 
19 1980 0.98 3.39 
87 81 0.20 
20 1980 0.98 3.39 
89 86 0.21 
21 1980 0.98 3.44 
90 88 0.19 
22 1980 0.98 3.37 
95 91 0.21 
23 1980 0.98 3.37 
105 92 0.17 
24 1980 0.98 3.37 
103 90 0.16 
25 1980 0.98 3.36 
91 89 0.20 
26 1980 0.98 3.36 
89 83 0.21 
27 1980 0.98 3.34 
87 88 0.23 
28 1750 200.0 
3.38 
124 118 0.09 
(HIP) 
29 1750 200.0 
3.38 
119 110 0.11 
(HIP) 
30 1750 200.0 
3.37 
121 117 0.12 
(HIP) 
31 1750 200.0 
3.38 
126 119 0.10 
(HIP) 
__________________________________________________________________________ 
Note 
*outside the scope of the present invention 
Samples Nos. 1 through 9 and 15 through 31 are within the scope of the 
present invention, and in sintered bodies obtained by sintering a 
composition comprising 80 to 96 mole% of Si.sub.3 N.sub.4 and 4 to 20 
mole% of at least two oxides of elements of the group IIIa as the main 
component and at least one member selected from oxides and nitrides of 
elements of the group IIa, Al, Ti, Cr, Ga, Zr and Si in an amount of 0.1 
to 5 parts by weight per 100 parts by weight of the main component under 
conditions indicated in Table 1, the flexural strength at 1300.degree. C. 
is at least 53 kg/mm.sup.2 and a higher strength is ensured, and the 
weight increase at the oxidation test is smaller than 1.0 mg/cm.sup.2 and 
the oxidation resistance is excellent. It is ordinarily understood that if 
this weight increase is smaller than 1.0 mg/cm.sup.2, an anti-oxidative 
protecting covering layer is sufficiently formed. 
In contrast, it is seen that in samples Nos. 10 through 14, which are 
outside the scope of the present invention, the flexural strength at room 
temperature or high temperatures and the oxidation resistance are 
degraded. In case of samples Nos. 10 and 11 where the oxide or nitride of 
an element of the group IIa, Al, Ti, Cr, Ga, Zr or Si is not added, the 
weight increase of the sintered body is as large as 15.5 mg/cm.sup.2 and 
the oxidation degree is very high or oxidation is advanced even to the 
interior, and it is seen that the oxidation resistance is poor. In sample 
No. 12, only Y.sub.2 O.sub.3 is added as the oxide of the element of the 
group IIIa and the amount added of Y.sub.2 O.sub.3 is 2 mole%, that is, 
outside the scope of the present invention, and the oxide or nitride of an 
element of the group IIa, Al, Ti, Cr, Ga, Zr or Si is not added. In this 
case, sintering is not satisfactory and the density is low, and therefore, 
the flexural strength at room temperature or at high temperatures is 
degraded and the porosity is 13% or higher. Accordingly, the product 
cannot be put into practical use. 
In sample No. 13, Al.sub.2 O.sub.3 is added as the additive in an amount of 
7 parts by weight to 100 parts by weight of the main component comprising 
75 mole% of Si.sub.3 N.sub.4 and 25 mole% of the oxide of the element of 
the group IIa, and the composition is completely outside the scope of the 
present invention. In this sample, the flexural strength at high 
temperatures is drastically degraded and at the oxidation test, the weight 
increase of the sintered body is relatively too large and 1.3 mg/cm.sup.2. 
In sample No. 14, the main component comprising Si.sub.3 N.sub.4 and the 
oxide of the element of the group IIIa is within the scope of the present 
invention, but the amount of the oxide or nitride of the element of the 
group IIa, Al, Ti, Cr, Ga, Zr or Si is 7 parts by weight per 100 parts of 
the main component and exceeds the range specified in the present 
invention. In this sample, the flexural strength at high temperatures is 
degraded and at the oxidation test, the weight increase of the sintered 
body is 1.2 mg/cm.sup.2 and is relatively too large. 
As is apparent from the foregoing description, a sintered body obtained by 
sintering a composition comprising 100 parts by weight of a main component 
formed by adding appropriate amounts of oxides of elements of the group 
IIIa of the Periodic Table to Si.sub.3 N.sub.4 and 0.1 to 5 parts by 
weight of at least one member selected from oxides and nitrides of 
elements of the group IIa of the Periodic Table, Al, Ti, Cr, Ga, Zr and Si 
has a flexural strength at 1300.degree. C. of at least 53 kg/mm.sup.2, 
which is practically sufficient, and the weight increase at the oxidation 
test is smaller than 1.0 mg/cm.sup.2 and the sintered body is excellent in 
the oxidation resistance. 
EXAMPLE 2 
In the same manner as described, molded bodies were prepared by using 
compositions shown in Table 2, and samples Nos. 1 through 11 for the 
flexural test were obtained. With respect to these samples, creep test 
specimens having a thickness of 0.902 mm, a width of 4.01 mm and a length 
of 55 mm were similarly obtained. Samples Nos. 1 through 8 were obtained 
according to the ordinary nitrogen atmosphere sintering method, and 
samples Nos. 9 through 11 were obtained by sintering according to the hot 
isostatic pressing method. 
With respect to each of the creep test specimens, the creep test was 
carried out at 1300.degree. C.for 100 hours under a bending stress of 108 
MN/m.sup.2 according to the four-point bending method in which the upper 
span was 19.1 mm and the lower span was 39.8 mm. The strain quantity 
(.epsilon.) of each specimen was determined according to the following 
formula: 
EQU .epsilon.(strain quantity)=[(4hd)/L.sup.2 ] (1) 
wherein h stands for the thickness of the specimen, L stands for the upper 
span and d stands for the quantity of deflection. 
Furthermore, the flexural test specimens of samples Nos. 1 through 11 were 
subjected to the four-point bending strength test at room temperature and 
at 1300.degree. C., and the degree of oxidation of the sintered body was 
examined based on the weight increase (mg/cm.sup.2) after 1000 hours' 
standing at 1300.degree. C. to evaluate the oxidation resistance at high 
temperatures. 
TABLE 2 
__________________________________________________________________________ 
Composition of Main Component 
Composition of Additive 
Sample silicon nitride 
compound of element of 
(part by weight per 100 parts by 
No. (mole %) group IIIa (mole %) 
weight of main component) 
__________________________________________________________________________ 
1 95.9 Nd.sub.2 O.sub.3 
2.05 AlN 0.5 
Yb.sub.2 O.sub.3 
2.05 
2 98.02 Nd.sub.2 O.sub.3 
0.99 AlN 0.5 
Yb.sub.2 O.sub.3 
0.99 
3 99.61 La.sub.2 O.sub.3 
0.13 AlN 1.5 
Nd.sub.2 O.sub.3 
0.13 
Yb.sub.2 O.sub.3 
0.13 
4 93.6 Nd.sub.2 O.sub.3 
2.5 TiN 0.5 
Sm.sub.2 O.sub.3 
1.5 CrN 0.1 
Yb.sub.2 O.sub.3 
2.4 GaO 0.3 
5 88.1 La.sub.2 O.sub.3 
2.4 BaO 0.5 
Yb.sub.2 O.sub.3 
3.9 SrO 0.1 
Sc.sub.2 O.sub.3 
5.6 ZrO.sub.2 
0.1 
6* 79 La.sub.2 O.sub.3 
7.0 AlN 0.5 
Yb.sub.2 O.sub.3 
7.0 
Sm.sub.2 O.sub.3 
7.0 CrN 0.5 
7* 95.9 Nd.sub.2 O.sub.3 
2.05 Al.sub.2 O.sub.3 
7.0 
Yb.sub.2 O.sub.3 
2.05 
8 97.1 La.sub.2 O.sub.3 
0.99 SiO.sub.2 
4.0 
Nd.sub.2 O.sub.3 
0.95 
Yb.sub.2 O.sub.3 
0.96 Al.sub.2 O.sub.3 
0.1 
9 96 La.sub.2 O.sub.3 
1.0 AlN 0.5 
Nd.sub.2 O.sub.3 
1.0 
CeO.sub.2 
2.0 
10 97.97 La.sub.2 O.sub.3 
0.67 CrN 0.3 
Yb.sub.2 O.sub.3 
0.44 
Gd.sub.2 O.sub.3 
0.92 
11 98.86 Nd.sub.2 O.sub.3 
0.21 SiO.sub.2 
0.9 
Yb.sub.2 O.sub.3 
0.44 
Er.sub.2 O.sub.3 
0.49 
__________________________________________________________________________ 
Properties of Sintered Body 
flexural 
flexural 
Sintering Conditions 
strength 
strength 
weight increase 
nitrogen 
bulk 
at room 
at high of sintered body 
strain quantity 
Sample 
tempera- 
pressure 
specific 
temperature 
temperature 
1300.degree. C./1000 hrs 
(.epsilon.), 1300.degree. C./ 
No. ture (.degree.C.) 
(MPa) 
gravity 
.sigma. b.sub.4 (RT) 
.sigma. b.sub.4 (1300.degree. C.) 
(mg/cm.sup.2) 
100 hrs (.times. 10.sup.-4 
__________________________________________________________________________ 
m/m) 
1 1980 0.98 3.37 
96 86 0.19 5.5 
2 1980 0.98 3.26 
99 91 0.10 4.1 
3 2050 9.80 3.20 
105 93 0.03 3.5 
4 1900 0.98 3.41 
81 77 0.52 7.2 
5 1900 0.98 3.46 
79 69 0.61 8.0 
6* 1900 0.98 3.69 
59 38 2.41 15.2 
7* 1950 0.98 3.12 
89 39 3.92 13.3 
8 1950 0.98 3.29 
91 64 0.72 5.6 
9 1750 200.0 
3.30 
104 98 0.11 5.3 
10 1750 200.0 
3.26 
109 100 0.09 4.9 
11 1750 200.0 
3.24 
87 77 0.07 3.9 
__________________________________________________________________________ 
Note 
*outside the scope of the present invention 
Samples Nos. 1 through 5 and 8 through 11 are within the scope of the 
present invention. With respect to each of sintered bodies obtained by 
sintering a composition comprising 100 parts by weight of a main component 
comprising at least 80 mole% of silicon nitride (Si.sub.3 N.sub.4) and 
oxides of elements of the group IIIa of the Periodic Table, at least two 
of which are selected from lanthanum oxide (La.sub.2 O.sub.3), neodymium 
oxide (Nd.sub.2 O.sub.3) and ytterbium oxide (Yb.sub.2 O.sub.3), and 0.1 
to 5 parts by weight of at least one member selected from oxides and 
nitrides of elements of the group IIa of the Periodic Table, Al, Ti, Cr, 
Ga, Zr and Si under conditions as indicated in Table 2, the flexural 
strength at 1300.degree. C. is at least 64 kg/mm.sup.2 and a higher 
strength is ensured, and the weight increase at the oxidation test is 
smaller than 0.72 mg/cm.sup.2 and the oxidation resistance is excellent. 
It is understood that an anti-oxidative protecting covering layer is 
sufficiently formed. Furthermore, in each of samples Nos. 1 through 5 and 
8 through 11, the strain quantity (.epsilon.) at the creep test is smaller 
than 8.0.times.10.sup.-4 m/m, and it is understood that these samples are 
excellent in the creep characteristics. In samples Nos. 6 and 7, which are 
outside the scope of the present invention, it is seen that the flexural 
strength at room temperature or at high temperatures, the oxidation 
resistance and the creep characteristics are degraded. More specifically, 
in sample No. 6, the content of silicon nitride (Si.sub.3 N.sub.4) is 79 
mole%, which is outside of the scope of the present invention, and the 
flexural strength at room temperatures is 59 kg/mm.sup.2, the flexural 
strength at 1300.degree. C. is 38 kg/mm.sup.2, the weight increase of the 
sintered body at the oxidation test is 2.41 mg/cm.sup.2 and the strain 
quantity (.epsilon.) is 15.2.times.10.sup.-4 m/m. It is seen that these 
properties are inferior to those of samples Nos. 1 through 5 and 8 through 
11. In sample No. 7, 7 parts by weight of Al.sub.2 O.sub.3 is added as the 
additive to 100 parts by weight of a composition comprising 97 mole% of 
silicon nitride and 3 mole% of oxides of elements of the group IIIa. In 
this case, the content of Al.sub.2 O.sub.3 is outside the scope of the 
present invention. The flexural strength at 1300.degree. C. is drastically 
degraded, and the weight increase of the sintered body at the oxidation 
test is as large as 3.92 mg/cm.sup.2. Furthermore, it is seen that the 
strain quantity (.epsilon.) at the creep test is as large as 
13.3.times.10.sup.-4 m/m. 
EXAMPLE 3 
This example is to prove that it is important for the oxidation resistance 
that the additive component should be added in a specific small amount. 
Sintered bodies were prepared in the same manner as described in Example 1 
except that components shown in Table 3 were used. The obtained results 
are shown in Table 3. 
From the results shown in Table 3, it is understood that with increase of 
the amount added of SrO as the additive, the strength at room temperature 
and the strength at high temperatures are increased but the weight 
increase at the oxidation test is extreme and the oxidation resistance is 
degraded. 
TABLE 3 
__________________________________________________________________________ 
Additive 
(parts by weight 
Sintering Strength 
Main Component 
per 100 parts by 
Condition Bulk room Weight 
Sample 
(mole %) weight of main 
tempera- 
pressure 
Specific 
temper- Increase 
No. Si.sub.3 N.sub.4 
(RE).sub.2 O.sub.3 
component) 
ture (.degree.C.) 
(MPa) 
Gravity 
ature 
1300.degree. C. 
(mg/cm.sup.2) 
__________________________________________________________________________ 
1 95 Y.sub.2 O.sub.3 
3 SrO 0.5 1950 10.0 3.27 105 95 0.20 
La.sub.2 O.sub.3 
2 
2 96 Nd.sub.2 O.sub.3 
2 SrO 1 1950 10.0 3.38 108 98 0.20 
Yb.sub.2 O.sub.3 
2 
3 96 La.sub.2 O.sub.3 
2 SrO 3 1950 10.0 3.40 110 99 0.10 
Yb.sub.2 O.sub.3 
2 
4 95 Y.sub.2 O.sub.3 
3 SrO 5 1950 10.0 3.27 112 100 0.30 
La.sub.2 O.sub.3 
1 
5 96 Nd.sub.2 O.sub.3 
2 SrO 8 1950 10.0 3.42 118 103 0.50 
Yb.sub.2 O.sub.3 
2 
6 96 Nd.sub.2 O.sub.3 
2 SrO 10 1950 10.0 3.44 120 110 0.90 
Yb.sub.2 O.sub.3 
2 
__________________________________________________________________________