Silicon nitride sintered bodies

Silicon nitride sintered bodies are disclosed which contain silicon carbide therein and in which intergranular phases between silicon nitride particles are substantially crystallized. Further, a manufacturing method of the sintered bodies is disclosed, in which a silicon carbide powdery raw material is used as an additive when preparing raw powders and the intergranular phases are crystallized during a temperature descending stage following a firing. Silicon carbide effectively promotes densification of the structure of the sintered body and crystallization of the intergranular phases, thereby making it possible to provide the sintered bodies having intergranular phases with little glass phases uncrystallized and excellent high-temperature strengths.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to silicon nitride sintered bodies having 
excellent mechanical strengths at high temperatures and to a method of 
manufacturing the same. 
2. Related Art Statement 
Until recently, as for silicon nitride sintered bodies containing oxides of 
IVa group elements including rare earth elements as additives, for 
example, in Japanese Patent Publication No. 48-7486, a manufacturing 
method of the sintered body is disclosed which comprises preparing a mixed 
powder consisting essentially of at least 85 mol % of silicon nitride 
powdery raw material and less than 15 mol % of at least one oxide selected 
from the oxides of IIIa group elements, shaping the thus prepared powder, 
and subjecting the resulting shaped-body to a firing under an inert 
atmosphere. In Japanese Patent Publication No. 49-21091, the silicon 
nitride sintered body is also disclosed which consists of at least 50 wt % 
of Si.sub.3 N.sub.4, less than 50 wt % of at least one oxide selected from 
Y.sub.2 o.sub.3 and the oxides of La group elements, and 0.01.about.20 wt 
% of Al.sub.2 O.sub.3. 
However, it is unlikely to obtain a sintered body having excellent 
high-temperature strength when adding only rare earth elements to the 
silicon nitride powdery raw material. On the other hand, when the sintered 
body contains Al.sub.2 O.sub.3 as an additive, structure of the sintered 
body is progressively densified, however, the softening point of its 
intergranular phase is lowered to considerably degrade the 
high-temperature strengths of the sintered body. 
To obtain a sintered body having excellent high-temperature strength, the 
applicant of the present invention, in Japanese Patent Publication No. 
63-100067, discloses a technique which provides the sintered body with 
excellent high-temperature strength by adding rare earth elements, having 
a predetermined composition and a predetermined weight ratio, to the 
silicon nitride powdery raw material and by specifying the crystal phase 
of the sintered body. 
In the silicon nitride sintered body disclosed in the Japanese Patent 
Publication No. 63-100067, it is possible to improve the high-temperature 
strength of the sintered body to a certain extent, which is still lower 
than a room-temperature strength thereof. The result is interpreted as 
follows. Even performing crystallization of grain boundaries of the 
sintered body still leaves some amount of uncrystallized glass phase when 
adopting the composition disclosed in the publication. For lowering a 
residual amount of the uncrystallized glass phase, it is possible to 
propose a manufacturing method comprising a powdery raw material in which 
little glass phase remains in the grain boundaries by enlarging a molecule 
ratio of the added oxides of the rare earth elements with respect to an 
amount of SiO.sub.2, to which a whole amount of oxygen contained in the 
silicon nitride powdery raw material is converted. However, the method 
makes it difficult to sufficiently densify the structure of the sintered 
body. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the drawbacks 
mentioned above, and to provide silicon nitride sintered bodies having 
substantially the same excellent strength at high temperatures as that at 
room temperatures and a method of manufacturing the same. 
The present invention provides silicon nitride sintered bodies, which are 
consisting essentially of silicon nitride, at least one compound of each 
rare earth element and silicon carbide, and in which intergranular phases 
between silicon nitride particles contained in the sintered bodies are 
substantially composed of crystal phases. 
Further, the present invention provides a method of manufacturing silicon 
nitride sintered bodies which comprises: 
preparing a powder consisting of powdery raw materials of silicon nitride, 
at least one oxide of each rare earth element and silicon carbide, 
shaping the thus prepared powder to obtain a shaped-body, 
subsequently subjecting the shaped-body to a firing under an N.sub.2 
atmosphere, and 
substantially crystallizing intergranular phases between silicon nitride 
particles during a temperature-descending stage following the firing. 
In the above mentioned construction, the following effects are found: 
Namely, the present invention, which comprises adding a silicon carbide 
powder to the silicon nitride powdery raw material containing the oxides 
of selected rare earth element, shaping the resulting powder and firing 
the thus obtained shaped-body under the N.sub.2 atmosphere to crystallize 
the shaped-body, makes it possible to provide the sintered body containing 
silicon carbide with intergranular phases between silicon nitride 
particles substantially forming crystal phases and to substantially 
eliminate residual glass phases in grain boundaries in the silicon nitride 
sintered body. Thereby it becomes possible to provide the sintered body 
with excellent high-temperature strength substantially equal to the 
room-temperature strength thereof. 
Namely, if an amount of the oxides of rare earth elements added to the 
silicon nitride powdery raw material, which contains the silicon carbide 
powder as an additive and a certain amount of oxygen, is larger than a 
predetermined amount of the oxides thereof, which provides the resulting 
sintered body with the most excellent high-temperature strength when the 
powdery raw material contains no silicon carbide powder and the above 
certain amount of oxygen, it is possible to sufficiently densify the 
structure of the sintered body by adding the silicon carbide powder, which 
also results in effective crystallization of the grain boundaries. 
Consequently, it is possible to obtain the silicon nitride sintered body, 
having the crystallized intergranular phase with very little glass phase 
and the excellent high-temperature strength. Further, the certain amount 
of oxygen contained in the silicon nitride powdery raw material is 
indicated in its SiO.sub.2 amount. Moreover, the above preconverted 
determined amount of the oxides of rare earth elements, which provides the 
sintered body with the most excellent high-temperature strength when no 
silicon carbide powder is added, differs according to the silicon nitride 
powdery raw material used. If the amount of the oxides of rare earth 
elements added to the powdery raw material is not larger than the 
predetermined amount of the oxides thereof, it is possible to proceed with 
the densification of the structure of the sintered body without adding the 
silicon carbide powder. However, it becomes possible to promote 
crystallization of grain boundaries, and consequently to obtain the 
silicon nitride sintered body having intergranular phases with little 
glass phases and the excellent high-temperature strength, by adding the 
silicon carbide powder. 
An amount of oxygen in the silicon nitride powdery raw material is 
preferably 1.about.3 wt %. The amount of oxygen can be controlled by 
oxidizing the silicon nitride powdery raw material, or by adding a silicon 
oxide powder thereto. 
A total amount of oxides of rare earth elements as additives is preferably 
2.7.about.10 mol %. The reason is as follows: If the total amount is lower 
than 2.7 mol %, it is impossible to obtain a liquid phase to sufficiently 
cause the densification. If the total amount is higher than 10 mol %, it 
exhibits a tendency to make the densification difficult even when adding 
the silicon carbide powder. Further, it is possible to use Lu.sub.2 
O.sub.3, Tm.sub.2 O.sub.3, Er.sub.2 O.sub.3, etc. as the oxides of rare 
earth elements other than Y.sub.2 O.sub.3, Yb.sub.2 O.sub.3 to obtain 
substantially the same effects. An amount of rare earth elements contained 
in the resulting sintered body is the same as that of the elements 
contained in the starting powdery raw material. The amount of the oxides 
of rare earth elements (indicated by mol %) is calculated as (the amount 
of the oxides of rare earth elements indicated by mol)/(a sum of the 
amount of the oxides of rare earth elements and silicon nitride, both 
indicated by mol). 
The amount of the silicon carbide powder as an additive is preferably 
0.1.about.11 wt % with respect to an amount of a formulated powder 
consisting of silicon nitride and the oxides of rare earth elements. When 
the amount of the silicon carbide powder is smaller than 0.1 wt %, it is 
impossible to sufficiently densify the structure of the sintered body and 
effectively promote crystallization. On the other hand when the amount is 
larger than 11 wt %, silicon carbide may occasionally inhibit the 
densification. The amount of the silicon carbide powder is more preferably 
0.5.about.7 wt %. The amount of silicon carbide contained in the sintered 
body may be reduced a little compared to the amount of the silicon carbide 
powder formulated into the raw material. Moreover, .alpha.-type, 
.beta.-type and amorphous silicon carbide may be used as appropriate, 
respectively. 
In the method of manufacturing silicon nitride sintered bodies according to 
the invention, first the formulated powdery raw material is prepared by 
mixing the silicon nitride powder, the powder of the oxide of each rare 
earth element and the silicon carbide powder. Next, the thus prepared 
mixture is shaped to a predetermined shape to produce the shaped-body. 
Subsequently, the resulting shaped-body is subjected to firing at a 
temperature of 1700.degree..about.2100.degree. C., preferably 
1900.degree..about.2000.degree. C., under N.sub.2 atmosphere at a normal 
pressure or a high pressure according to the firing temperature, and the 
intergranular phases are substantially crystallized during the following 
temperature-descending step. Consequently, the silicon nitride sintered 
body according to the invention having the intergranular phases between 
silicon nitride particles, which is substantially crystallized and 
contains very little glass phase.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, actual embodiments according to the invention will be 
described. 
A silicon nitride powder having a purity of 97 weight %, an oxygen content 
of 2.2 weight %, an average grain diameter of 0.6 .mu.m and BET specific 
surface area of 17 m.sup.2 /g, additives described in Table 1, each 
additive having a purity of 99.9 weight %, an average grain diameter of 
0.3.about.2.5 .mu.m, and a silicon carbide powder having a purity of 99 
weight %, an average grain diameter of 0.4 .mu.m and BET specific surface 
area of 20 m.sup.2 /g are mixed and formulated in the proportions as 
described in Table 1. Then, by using media made of silicon nitride 
porcelain and a nylon resin vessel having an inner volume of 1.2 l, 1.8 kg 
of media and 300 ml of water were added to 200 g of the formulation raw 
material, which was ground by a vibration mill at a vibration rate of 1200 
times/min for 3 hours. Subsequently, the water was evaporated to obtain 
dry powders, which were further granulated to have a grain diameter of 150 
.mu.m. The thus obtained powders were isostatic-pressed under a pressure 
of 7 ton/cm.sup.2 to obtain shaped bodies having dimensions of 
50.times.40.times.6 mm, which were then fired according to the firing 
conditions as described in Table 1 to provide silicon nitride sintered 
bodies Nos. 1.about.27 according to the invention. Besides, by using the 
same silicon nitride raw materials as described above, formulated powdery 
raw materials were obtained by adopting additives and formulated ratios as 
described in Table 1. Then, the resulting formulated powdery raw materials 
were ground, granulated and shaped as described above, and the resulting 
shaped bodies were then subjected to firing in the firing conditions as 
described in Table 1 to obtain the sintered bodies according to 
comparative embodiment Nos. 28.about.31. Besides, in the comparative 
example Nos. 29 and 30, the crystallization was performed by a reheating 
treatment. 
Bulk densities, four point bending strengths at room temperature and 
1400.degree. C. of the sintered body were measured and the intergranular 
crystal phases thereof were detected. The experimental results were shown 
in Table 1, in which the bulk densities of the sintered bodies were 
measured by Archimedes method and indicated a values relative to 
theoretical densities calculated from the compositions and densities of 
the formulated powders. The densities of the formulated powders were 
calculated by adopting the following values; Si.sub.3 N.sub.4 : 3.2 
g/cm.sup.3, Y.sub.2 O.sub.3 : 5.0 g/cm.sup.3, Yb.sub.2 O.sub.3 : 9.2 
g/cm.sup.3, Tm.sub.2 O.sub.3 : 8.8 g/cm.sup.3, Lu.sub.2 O.sub.3 : 9.4 
g/cm.sup.3, Er.sub.2 O.sub.3 : 8.6 g/cm.sup.3, SiC: 3.2 g/cm.sup.3. The 
four point bending strengths were measured according to "a testing method 
of bending strengths of fine ceramic materials (JIS R-1601)". The 
intergranular crystal phases were detected by performing X-ray diffraction 
test using CuK.alpha.-ray. In Table 1, J is used to mean a crystal phase 
having a caspidine structure, which have the same type diffraction curve 
as that of Si.sub.3 N.sub.4.4Y.sub.2 O.sub.3.SiO.sub.2 shown in JCPDS card 
No. 32-1451 and in which the crystallographic position of Y can be 
replaced or occupied by the other rare earth elements. H is used to mean a 
crystal phase having an apatite structure, which have the same type 
diffraction curve as that of Si.sub.3 N.sub.4.10Y.sub.2 O.sub.3.9SiO.sub.2 
shown in JCPDS card No. 30-1462 and in which the crystallographic position 
of Y can be replaced or occupied by the other rare earth elements. K is 
used to mean a crystal phase having an wollastonite structure, which have 
the same type diffraction curve as that of 2Y.sub.2 
O.sub.3.SiO.sub.2.Si.sub.3 N.sub.4 shown in JCPDS card No. 31-1462 and in 
which the crystallographic position of Y can be replaced or occupied by 
the other rare earth elements. L is used to mean a crystal phase indicated 
as Re.sub.2 SiO.sub.5 (Re: rare earth elements), which have the same type 
diffraction curve as that shown in JCPDS card No. 21-1456, 21-1458, 
21-1461, 22-992 or 36-1476. S is used to mean a crystal phase indicated as 
Re.sub.2 Si.sub.2 O.sub.7 (Re: rare earth elements), which have the same 
type diffraction curve as that shown in JCPDS Card 20-1416, 21-1457, 
21-1459, 21-1460, 22-994 or 22-1103. 
Moreover, in Table 1, ratios of intergranular crystal phases were also 
shown, and they were detected by an integrated value of the strength peak 
of respective intergranular phases other than .beta.-Si.sub.3 N.sub.4. 
Further, in FIG. 1, a SEM photograph of the sintered body No. 4 according 
to the invention was shown. In FIG. 1, A shows .beta.-Si.sub.3 N.sub.4 
particles, B shows intergranular phases and C shows SiC particles. 
TABLE 1 
__________________________________________________________________________ 
Total 
amount Rela- 
Room- 
of the tive 
temper- 
Strength 
Oxides of rare earth 
oxides Temper- Pres- 
den- 
ature 
at 
elements (wt %) (mol 
SiC ature 
Time 
sure 
sity 
strength 
1400.degree. C. 
Intergranular 
No. Y.sub.2 O.sub.3 
Yb.sub.2 O.sub.3 
The other 
%) (wt %) 
(.degree.C.) 
(hr) 
(atm) 
(%) (MPa) 
(MPa) 
crystal 
__________________________________________________________________________ 
phase 
Present 
invention 
1 3.4 
14 7.8 0.1 
1900 2 10 97 770 750 J:H = 50:50 
2 3.4 
14 7.8 0.5 
1900 2 10 99 800 800 J:H = 60:40 
3 3.4 
14 7.8 1 1900 2 10 99 810 810 J:H = 70:30 
4 3.4 
14 7.8 5 1900 2 10 99 810 810 J = 100 
5 3.4 
14 7.8 7 1900 2 10 98 800 800 J = 100 
6 3.4 
14 7.8 11 1900 2 10 97 770 760 J = 100 
7 2 9 4.7 0.5 
1900 2 10 99 780 770 H = 100 
8 2 9 4.7 1 1900 2 10 99 790 780 H = 100 
9 2 9 4.7 3 1900 2 10 99 790 790 H:J = 60:40 
10 2 9 4.7 7 1900 2 10 98 780 780 J:H = 60:40 
11 2 9 4.7 1 2100 2 100 99 780 750 J:H:K = 50:40:10 
12 2 7 3.9 1 1900 2 10 98 730 700 H:S = 70:30 
13 2 7 3.9 3 1900 2 10 98 710 700 H = 100 
14 2 4 2.7 1 1900 2 10 97 680 670 L:S = 70:30 
15 0 15 5.9 3 1700 3 1 97 780 770 J = 100 
16 10 0 6.4 1 1950 2 50 98 800 800 J:H = 80:20 
17 4.2 
17 10 2 1900 2 10 97 760 760 J = 100 
18 3.8 
15 8.8 2 1900 2 10 98 800 800 J = 100 
19 2.6 
11 5.8 1 1900 2 10 98 800 790 J:H = 60:40 
20 0 19 7.8 1 1900 2 10 99 810 800 J = 100 
21 5.8 
10 7.8 1 1900 2 10 98 800 790 J = 100 
22 3.4 
14 7.8 1 2000 2 100 99 810 800 J = 100 
23 3.4 
14 7.8 1 1950 2 15 99 810 810 J = 100 
24 3.4 
14 7.8 1 1800 3 10 97 770 760 J:H = 80:20 
25 2 0 Tm.sub.2 O.sub.3 9 
4.7 7 1950 2 20 98 760 760 H:L = 60:40 
26 0 7 Lu.sub.2 O.sub.3 7 
4.6 3 1900 2 10 98 780 780 J = 100 
27 2 0 Er.sub.2 O.sub.3 13 
6.6 1 1900 2 10 99 800 800 J = 100 
Compara- 
tive 
examples 
28 3.4 
14 7.8 0 1900 6 10 90 400 300 J:H:L = 40:40:20** 
29 2 9 4.7 0 1900 2 10 98 700 650 H:S = 70:30* 
30 2 7 3.9 0 1900 2 10 98 730 620 H:S = 60:40* 
31 4.2 
17 10 0 1900 6 10 80 -- -- J = 100** 
__________________________________________________________________________ 
(Note) 
*Intergranular phases were crystallized by performing the reheating 
treatment. 
**not sufficiently densified 
J: Caspidine structure 
H: Apatite structure 
K: Wollastonite structure 
L: Re.sub.2 SiO.sub.5 (Re: rare earth elements) 
S: Re.sub.2 Si.sub.2 O.sub.7 (Re: rare earth elements) 
As clearly shown in Table 1, the sample Nos. 1.about.6 according to the 
invention, which contain relatively large amount of the oxides of rare 
earth elements and further silicon carbide as additives, had high relative 
densities of more than 97% and high strengths at high temperature, which 
was only a little different from the room-temperature strengths. On the 
contrary, the structure of the comparative sample No. 28 containing no 
silicon carbide as an additive was not sufficiently densified. The results 
demonstrate that the silicon carbide additive promotes densification of 
the structure effectively. 
For example, sample Nos. 8, 12 according to the invention, in which the 
amounts of the oxides of rare earth elements as additives were relatively 
low and the grain boundaries were crystallized to principally form H 
phases by adding silicon carbide, had higher strengths at the high 
temperature compared to that of the comparative sample Nos. 29, 30, in 
which no silicon carbide was added and the grain boundaries were 
crystallized by performing the reheating treatment. In this case, silicon 
carbide as an additive effectively proceeds the crystallization of the 
grain boundaries to effectively reduce the residual glass phase therein 
rather than the densification of the structure of the sintered body. 
As can be seen from the above explanation, in the silicon nitride sintered 
body and the manufacturing method thereof according to the present 
invention, because silicon carbide is added to the silicon nitride powder 
containing predetermined oxides of each rare earth element and the thus 
obtained formulated powdery raw material is shaped to form the shaped body 
which is then subjected to the firing under the N.sub.2 atmosphere and 
crystallized, it is possible to provide the sintered body containing 
silicon carbide, in which the intergranular phases between silicon nitride 
particles are substantially crystallized and the high-temperature strength 
of the sintered body is comparable to the room-temperature strength 
thereof.