Method for production of sintered article of silicon nitride

As the aluminum nitride component of a sintering aid for powdered silicon nitride, either a spinel type compound having oxygen dissolved in aluminum nitride to form a solid solution or a poly-type aluminum nitride is used. Since the compound is highly stable in water, it can be effectively used in the form of an aqueous slurry mixture. As the sintering aid, this compound is used as effectively as aluminum nitride.

BACKGROUND OF THE INVENTION 
This invention relates to a method for the production of a sintered article 
of silicon nitride. 
The sintered ceramic articles formed preponderantly of silicon nitride 
possess heat resisting property enough to withstand elevated temperatures 
up to 1900.degree. C., exhibit a low thermal expansion coefficient, and 
excel in resistance to thermal shock. Thus, the feasibility of these 
articles in applications to structural parts such as gas turbine blades 
and nozzles which require high strength at elevated temperatures is being 
studied. 
Shaped articles formed solely of silicon nitride powder have a poor 
capacity for sintering. It has been customary, therefore, to produce 
sintered articles of silicon nitride powder by blending the silicon 
nitride powder with a sintering aid such as the oxide of a rare earth 
element like yttrium oxide (Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 
O.sub.3), or aluminum nitride (AlN), molding the resultant mixture in a 
given shape, and sintering the shaped mixture by the hot press method, the 
normal sintering method, or the gas pressure sintering method. 
As means of forming such shaped articles of silicon nitride, numerous 
methods have been known such as the pressure molding method applying of 
pressure to powder, injection molding method using blended materials and 
the slip casting method using slurry blends. Particularly, the molding 
method involving slurry blends is known for its ability to permit such 
slurry blend to be easily and inexpensively molded in a complex shape. 
When silicon nitride powder and an aluminum nitride sintering aid are mixed 
and graded in water to produce a slurry, the aluminum nitride reacts with 
water and undergoes decomposition, converting itself into Al.sub.2 O.sub.3 
with evolution of ammonia. 
When this decomposition occurs, the amount of aluminum nitride present in 
the mixture becomes smaller than the amount initially incorporated 
therein. The sintered article finally obtained, therefore, fails to 
acquire properties as expected and suffers from dispersion of properties. 
Moreover, the offensive odor which emanates from the shaped article 
because of the evolution of ammonia seriously impairs the work 
environment. Consequently, when the blended material of silicon nitride 
containing aluminum nitride is subjected to slip casting, the blend must 
be converted into a slurry using an organic solvent. This unavoidable of 
the organic solvent results in additional cost. 
Aluminum nitride functions effectively as a sintering aid particularly when 
the sintered article of silicon nitride is produced by sintering in a 
furnace. Thus, the desirability of developing a method which effects slip 
casting inexpensively with use of aluminum nitride as the sintering aid 
has been increasingly recognized. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the aforementioned 
disadvantages suffered by the prior art. It has been found that a spinel 
type compound having oxygen dissolved in aluminum nitride to form a solid 
solution [(AlN).sub.X (Al.sub.2 O.sub.3).sub.Y ], or a poly-type aluminum 
nitride (Al-Si-O-N), is highly stable in water. The discovered spinel-type 
compound is stable enough to avoid being decomposed with evolution of 
ammonia during the course of mixing, grading, and slip casting. When this 
compound is used in place of aluminum nitride (AlN), it functions as a 
sintering aid as effectively as does aluminum nitride (AlN). 
It is another object of the present invention to provide a method for the 
production of a sintered article of silicon nitride incorporating therein 
as a sintering aid a compound which avoids being decomposed during the 
course of mixing, grading, and slip casting in water and performs the role 
of a sintering aid as effectively as aluminum nitride. 
In accordance with the above objects, there has been provided a method for 
the production of a sintered article of silicon nitride comprising mixing 
silicon nitride powder with a sintering aid containing either a spinal 
type compound having oxygen dissolved in aluminum nitride to form a solid 
solution [(AlN).sub.X (Al.sub.2 O.sub.3).sub.Y ], (wherein X has a value 
in the range of 0.8 to 1.2 and Y a value in the range of 0.8 to 1.2), or a 
poly-type aluminum nitride (Al-Si-O-N), (wherein the ratio of 
(Al+Si)/(O+N) falls in the range of 5/6 to 1, exclusive of 1), converting 
the resultant powdered mixture into an aqueous slurry mixture, molding the 
slurry mixture in a given shape, and sintering the resultant shaped 
mixture. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the preferred embodiments of the 
invention as illustrated by the accompanying examples. 
The spinel type compound having oxygen dissolved in aluminum nitride to 
form a solid solution according to the present invention is obtained by 
mixing aluminum nitride powder and aluminum oxide powder in a molar ratio 
of about 1:1 and allowing the two compounds to react with each other by 
heating the powdered mixture in a non-oxidative atmosphere at a 
temperature in the range of 1600.degree. to 2000.degree. C. for 0.5 to 4 
hours. 
The spinel type compound obtained by this method is represented by the 
general formula, (AlN).sub.X (Al.sub.2 O.sub.3).sub.Y wherein X and Y each 
generally denote 1. Depending on the conditions of the reaction, the 
values of X and Y are each variable within the range of 0.8 to 1.2. 
The poly-type aluminum nitride is obtained by mixing silicon dioxide powder 
with aluminum nitride powder in an amount calculated to account for 0.1 to 
20% by weight, preferably 1 to 10% by weight, based on the powdered 
mixture and allowing the two compounds to react with each other by heating 
the resultant mixture in a non-oxidative atmosphere for 0.5 to 4 hours, 
for example. If the amount of the silicon dioxide powder in the mixture 
exceeds 20% by weight, the formation of the poly-type aluminum nitride 
becomes hardly feasible. 
The poly-type aluminum nitride used for the present invention is desired to 
be such that the ratio of (Al+Si)/(O+N) falls in the range of 5/6 to 1 
(exclusive of 1). The reason for this specific range is as follows. If 
this range is not met, the poly-type aluminum nitride will not have the 
properties of AlN. Examples of the poly-type aluminum nitride are 27R 
poly-type, 21R poly-type. 
According to the present invention, any of the known sintering aids such as 
oxides of rare earth elements represented by yttrium oxide and aluminum 
oxide, titanium oxide, zirconium oxide, and magnesium oxide can be used in 
connection with the spinel compound mentioned above. The amount of the 
spinel compound having oxygen dissolved in aluminum nitride or the 
poly-type aluminum nitride to be incorporated in the powdered mixture is 
in the range of 0.5 to 15% by weight, preferably 1 to 10% by weight, based 
on the weight of the powdered mixture. When the amount of the compound 
falls within this range, the sintered article of silicon nitride acquires 
outstanding mechanical strength. 
According to the present invention, the oxide of a rare earth element like 
yttrium oxide is desirably incorporated in an amount of 0.5 to 10% by 
weight, preferably 1 to 7% by weight, and/or aluminum oxide is desirably 
incorporated in an amount of not more than 10% by weight, preferably 2 to 
5% by weight in addition to the spinel type compound having oxygen 
dissolved in aluminum nitride to form a solid solution or the poly-type 
aluminum nitride. 
In the present invention, the aforementioned sintering aid is mixed with 
silicon nitride in water. The resultant mixture, after grading as 
required, is converted into a slurry. This slurry mixture is moled in a 
prescribed shape. The proper amount of water to be incorporated in the 
slurry generally falls in the range of 150 to 450 g, preferably around 300 
g, per 1 kg of the powdered mixture. The molding of the slurry is 
desirably carried out by the slip casting method. Otherwise, the molding 
method of spraying may be adopted. 
The shaped article produced by the molding of the slurry mixture is 
sintered in a non-oxidative atmosphere at a temperature in the range of 
1600.degree. C. to 1800.degree. C. 
The sintered article of silicon nitride produced as described above 
acquires properties substantially equal to the properties of the 
conventional sintered article obtained by mixing silicon nitride with AlN 
as a sintering aid, converting the resultant mixture into a slurry using 
an organic solvent, molding the slurry in a prescribed shape, and 
sintering the shaped slurry. Since none of the components of the powdered 
mixture undergoes decomposition during the course of mixing, grading, or 
slip casting in water, the sintered article of silicon produced acquires 
stable properties. Since the shaped article avoids emitting ammonia gas 
during the course of sintering, the production of the sintered article of 
this invention does not impair the work environment. 
Now, the present invention will be described more specifically below with 
reference to working examples. It should be noted, however, that this 
invention is not limited in any sence by these working examples.

EXAMPLE 1 
A powdered mixture consisting of 88% by weight of silicon nitride powder 
containing 92% of .alpha.-phase silicon nitride and having an average 
particle size of 1.2 .mu.m, 5% by weight of yttrium oxide powder having an 
average particle size of 0.7 .mu.m, 2% by weight of aluminum oxide powder 
having an average particle size of 0.3 .mu.m, and 5% by weight of a spinel 
type compound of a composition formula, (AlN).sub.X (Al.sub.2 
O.sub.3).sub.Y, (wherein x=1 and Y=1). The mixture was placed in a ball 
mill (using a pot of Al.sub.2 O.sub.3 with water and balls of Al.sub.2 
O.sub.3), blended therein for 48 hours, and subjected to grading to 
produce a slurry mixture. The aforementioned spinel type compound had been 
produced by mixing 29% by weight of aluminum nitride powder and 71% by 
weight of aluminum oxide powder, heating the resultant powdered mixture in 
a nitrogen atmosphere at 1800.degree. C. for one hour to effect reaction 
of the two compounds, and pulverizing the resultant reaction mixture. 
The aforementioned slurry was mixed with sodium alginate and a surfactant 
added thereto as a binder in an amount of 7.5% by weight based on the 
total weight of the powdered mixture. The resultant mixture was molded by 
the slip casting method in the shape of a square block measuring 37 
mm.times.37 mm.times.10 mm. 
When the spinel type compound in the shaped article was analyzed by X-ray, 
it was confirmed that the spinel structure remained unimpaired. 
Thereafter, the shaped article was dried, degreased in a nitrogen gas 
atmosphere at a maximum temperature of 500.degree. C., and sintered in the 
nitrogen gas atmosphere at 1780.degree. C. for two hours, to afford a 
sintered article of silicon nitride. The sintered article of silicon 
nitride so produced was found to have density of about 3.24 g/cc (average 
for five samples) and flexural strength of 95 kg/mm.sup.2 at ordinary 
temperature and 88 kg/mm.sup.2 (average for five samples) at 1000.degree. 
C. 
Separately, a slurry was obtained by following the procedure described 
above, except that aluminum nitride (AlN) was used in place of the spinel 
type compound. When this latter slurry was left standing for one day, the 
aluminum nitride (AlN) underwent hydrolysis with evolution of ammonia. 
The high-temperature strength exhibited by the sintered article of silicon 
nitride obtained in the present example was equal to that of a sintered 
article of silicon nitride obtained by repeating the procedure described 
above using aluminum nitride in place of the spinel type compound. 
EXAMPLE 2 
Silicon nitride power containing 92% of .alpha.-phase silicon nitride and 
having an average particle size of 1.2 .mu.m, yttrium oxide power having 
an average particle size of 0.7 .mu.m, aluminum oxide powder having an 
average particle size of 0.3 .mu.m, and 27R poly-type AlN weighed out in 
amounts meeting a varying percentage composition shown in the following 
table and water were mixed in a ball mill (using a pot of Al.sub.2 O.sub.3 
and balls of Al.sub.2 O.sub.3) for 48 hours and subjected to grading, to 
afford a slurry mixture. The aforementioned 27R poly-type AlN had been 
produced in advance by mixing 95% by weight of aluminum nitride powder and 
5% by weight of silicon oxide powder, heating the resultant powdered 
mixture in a nitrogen atmosphere at 1950.degree. C. for about one hour, 
and pulverizing the resultant reaction mixture. 
The aforementioned slurry was mixed with a sodium alginate type binder of 
an amount of 7.0% by weight based on the total amount of the powdered 
mixture and then molded by the slip casting method in the shape of a 
square block measuring 37 mm.times.37 mm.times.10 mm. 
The 27R poly-type AlN found in the shaped article, upon X-ray analysis, was 
found to retain its original structure. 
Thereafter, the shaped article was dried and degreased in a nitrogen 
atmosphere at a maximum temperature of 500.degree. C. and sintered in a 
furnace filled with nitrogen gas at 1780.degree. C. for two hours, to 
afford a sintered article of silicon nitride. The properties (average for 
five samples) of the sintered article of silicon nitride so produced were 
as shown in the following table. 
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Composition 
(% by weight) Three-point 
Poly-type Density flexural strength 
Y.sub.2 O.sub.3 
Al.sub.2 O.sub.3 
AlN (g/cc) (kg/mm.sup.2) 
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5 2 4 3.26 85 
7 2 2 3.27 96 
7 3 3 3.27 94 
5 3 3 3.26 98 
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Separately, a slurry was obtained by following the procedure of this 
example, except that aluminum nitride (AlN) was used in place of the 27R 
poly-type AlN. When this slurry was left standing for one day, the 
aluminum nitride (AlN) underwent hydrolysis with evolution of ammonia. 
The flexural strength exhibited by the sintered article of silicon nitride 
obtained in this example was equal to that manifested by a sintered 
article of silicon nitride obtained by following the procedure of the 
present example and using aluminum nitride in place of the 27R poly-type 
AlN. 
It is noted from the examples cited above that the present invention 
permits a notable saving in the cost of molding of the sintered article of 
silicon nitride ceramic using aluminum nitride as a sintering aid, because 
the aluminum nitride does not undergo decomposition even when the powdered 
mixture is converted into an aqueous slurry.