Method for manufacturing a sintered body of silicon nitride

A method for manufacturing a sintered body is provided, comprising the steps of dispersing powdery silicon nitride and a sintering assistant into a first dispersion medium composed mainly of substance capable of being extracted by supercritical fluid to form a first slurry and casting the first slurry to form a compact; dispersing powdery material into a second medium composed mainly of substance capable of being extracted by the supercritical fluid to form a second slurry and coating the second slurry on the surface of the compact to form a film layer; dipping the compact covered with the film layer in the supercritical fluid to extract the first dispersion medium and the second dispersion medium, respectively from the compact and the film layer; heating the compact from which the first and second dispersion mediums have been removed to form the film layer into a film layer impermeable to gas; and sintering the compact covered with the film layer impermeable.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for manufacturing a sintered body 
of silicon nitride by sintering a compact of silicon nitride. 
2. Description of the Prior arts 
Silicon nitride is widely known as material for sintering, but silicon 
nitride alone is hard to sinter. Powders of Y.sub.2 O.sub.3, Al.sub.2 
O.sub.3, MgO, La.sub.2 O.sub.3, AlN, BeSiN.sub.2 and BeAl.sub.2 O.sub.4 
are added, as a sintering assistant, to silicon nitride powders to prepare 
powder material. From this powder material a compact of a complicated 
shape is comparatively easy to be formed by a method of slip casting. 
In the method of slip casting, the powder material is dispersed into a 
liquefied dispersion medium to be formed into slurry and this slurry is 
cast into a mold having a desired cavity shape. Depending on methods of 
solidifying slurry cast, the method for slip casting is classified into 
the following two methods: 
(a) a mold having a feature by absorbing liquid which is made of gypsum, 
porous resin or the like is used. This mold absorbs a part of water 
content in the slurry to form a feature of maintaining the shape of a 
compact once molded. And then, the mold is demolded to obtain a compact of 
a desired shape. In this method, water and alcohol are used as a 
dispersion medium to disperse the powder material. 
(b) The slurry material is cast in a mold which is cooled down upto the 
melting point temperature of a dispersion medium or less, or the mold is 
cooled upto the same temperature after the powder material is cast therein 
to solidify the dispersion medium, thereby maintaining the shape of a 
molded compact. And then, the mold is demolded to obtain a compact of the 
desired shape. In this method, paraffin is used as the dispersion medium. 
The compact thus prepared is sent to the process wherein the dispersion 
medium is removed. This removal of the dispersion medium is carried out 
through air drying or vaporization by heating when the dispersion medium 
is water and alcohol, and through the vaporization and thermal 
decomposition of paraffin by means of heating it at 500.degree. C. when it 
is used as the dispersion of medium. 
The compact from which the dispersion medium has been removed is 
transferred to the process of sintering, and the compact is buried into 
powdery fillers filled in a crucible in advance of the sintering. 
This is because of reducing decomposition of silicon nitride and avoiding 
direct contact of the compact with the graphite crucible which impairs 
sintering. The powdery fillers are those which Si.sub.3 N.sub.4, 
SiO.sub.2, Al.sub.2 O.sub.3, AlN, MgO and BN are appropriately mixed into. 
Sintering operation is applied to the compact covered with the powdery 
fillers The operation is carried out at 1,600.degree. to 2,000.degree. C. 
and in the atmosphere of nitrogen having the pressure or more at which 
silicon nitride is decomposed so as to prevent the thermal decomposition 
thereof The pressure of the nitrogen atmosphere is required to be raised 
in proportion to the rise of the temperature, but within the 
aforementioned range of the temperature, 0 to 10kg/cm.sup.2 .multidot.G is 
ordinarily adopted. In addition, it is known that the dispersion medium 
remaining in the compact after the removal is perfectly removed by 
degassing, for example, at 10.sup.-2 Torr and 1,000.degree. C. 
On the other hand, there is a method wherein a density of silicon nitride 
compact is increased by hot isostatic pressing process Namely, the process 
is applied to the compact at 1,600.degree. to 1,900.degree. C. and in the 
atmosphere of the gas such as N.sub.2 and Ar having 1,000 to 1,300 
kg/cm.sup.2 .multidot.G. In this method, a film layer impermeable to gas 
is formed on the surface of the compact and heat and pressure is applied 
thereto from the outside of the film layer. Those methods are disclosed, 
for example, in a Japanese Examined Patent Publication No. 35870/84 and a 
West German Patent No. 3,403,917. 
Said Japanese Examined Patent Publication mentions a method wherein: 
(a) a pre-compact is dipped in slurry of powder material for forming a film 
layer thereof; 
(b) the film layer formed by said dipping is dried; 
(c) the step of said dipping and drying is repeatedly carried out, and 
through this repetition, an inner porous film layer comprising a high 
melting glass forming substance or a high melting metal forming substance, 
and an outer porous film layer comprising a low melting glass forming 
substance or a low melting metal forming substance are formed; and 
(d) the outer porous film layer is made to be impermeable by means of 
heating, and subsequently the inner porous film layer to be impermeable. 
Said West German Patent discloses another method repeating the step of said 
dipping and drying to form porous film layers as shown in said Japanese 
Examined Patent Publication. But in this method, as an inner porous film 
layer, a substance having no sintering assistant, and as an outer porous 
film layer, a substance having a sintering assistant, respectively are 
used. By means of heating, sintering of the outer film layer is promoted 
thereby to change it into a fine impermeable one. 
In case of silicon nitride, pressure and temperature to which hot isostatic 
pressing treatment contributes is at least 100 kg/cm.sup.2 .multidot.G and 
1,600.degree. C., although it depends on a feature of a sintering 
assistant. The above-mentioned method using a film layer impermeable to 
gas, as a pressure medium, is applicable at the critical pressure or more 
and the critical temperature or more. 
Those prior art methods of sintering silicon nitride have the following 
disadvantages: 
(a) when sintering silicon nitride is carried out without powdery fillers, 
decomposition of silicon nitride is unavoidable; 
(b) even if the sintering is done with powdery fillers wherein the 
pre-compact is buried, the perfect exclusion of decomposition of silicon 
nitride cannot be attained, and a layer of deterioration in quality is 
formed on the surface of the compact; and 
(c) because the heat conduction of the powder fillers is very bad, it takes 
a long time for the sintering when they are used. 
In addition, the prior art process of forming an impermeable film layer 
comprises many steps such as removing a dispersion medium from a compact, 
coating slurry of a porous film layer forming substance, and drying, and 
the pre-process for sintering, thus, becomes intricate. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for 
manufacturing a quality sintered body silicon nitride through a 
comparatively simple process 
To attain the object, in accordance with the present invention, a method is 
provided for manufacturing a sintered body of silicon nitride comprising 
the steps of dispersing powdery silicon nitride and a sintering assistant 
into a first dispersion medium composed mainly of substance capable of 
being extracted by supercritical fluid to form a first slurry and casting 
the first slurry to form a compact; dispersing a powdery material into a 
second dispersion medium composed mainly of substance capable of being 
extracted by the supercritical fluid to form a second slurry and coating 
the second slurry on the surface of the compact to form a film layer; 
dipping the compact covered with the film layer in the supercritical fluid 
to extract and remove the first dispersion medium included in the compact 
and the second dispersion medium included in the film layer; heating the 
compact from which the first medium and second dispersion mediums have 
been removed to change the film layer into a film layer impermeable to 
gas; and sintering the compact covered with the film layer impermeable. 
The object and other objects and advantages will become apparent from the 
detailed description to follow, taken in conjunction with the appended 
drawing

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A method for manufacturing a sintered body of silicon nitride comprises the 
steps of preparing a compact, forming a film layer, extracting and 
removing dispersion mediums, forming a film layer impermeable to gas and 
sintering. 
Step of Preparing a Compact 
Powdery silicon nitride and a sintering assistant are dispersed into a 
first dispersion medium composed mainly of substance capable of being 
extracted by supercritical fluid to form a first slurry and the first 
slurry is cast to form a compact. 
Step of Forming a Film Layer 
Powdery material is dispersed into a second dispersion medium composed 
mainly of substance capable of being extracted by means of supercritical 
fluid to form a second slurry and the prepared compact is coated with the 
second slurry to form a film layer. In this step, means for coating the 
second slurry can be either dipping the compact in the second slurry or 
brushing over the compact The film layer formed by coating is desired to 
be thicker with view to procure reliability of gas impermeability, but the 
excessive thickness is easy to cause cracking of the film layer in the 
step of removing the dispersion mediums after coating. The thickness of 
the film layer ranges preferably 0.2 to 4.0 mm. The powdery material to be 
dispersed into the second dispersion medium can be preferably selected 
from glass powder and non-oxide ceramic powder to which a sintering 
assistant is added As the glass powder, silica and borosilicate glass are 
desirable As the non-oxide ceramic powder, silicon nitride and silicon 
carbide are desirable 
The coated film layer becomes impermeable to gas through the steps of 
extracting and removing the dispersion mediums and of forming the film 
layer impermeable to gas hereinafter mentioned. The impermeable film layer 
can be of multilayer structure to improve its impermeablity to gas. In 
addition, to improve separability of the compact and the impermeable film 
layer, it is recommendable to form an inactive layer. The inactive layer 
can be any one selected from the group consisting of high melting metal 
powder and non-oxide ceramic powder. As the high melting metal powder, 
tungsten, molybdenum, tantalum and niobium, and as the non-oxide powder, 
boron nitride, aluminum nitride,di-boron titanium, di-boron zirconium, 
boron carbide, silicon nitride and silicon carbide, are desirable. 
Step of Extracting and Removing the Dispersion Mediums 
The compact covered with the film layer is dipped in supercritical fluid to 
extract and remove the first dispersion medium included in the compact and 
the second medium in the film layer This step features removing the 
dispersion mediums respectively included in the compact and the film layer 
through a single process by using the supercritical fluid as a solvent. 
The supercritical fluid used in this step is preferably selected from the 
group consisting of carbon dioxide, ethane, ethylene and 
monochlorotrifluoromethane, having their critical temperature at the 
vicinity of room temperature This is because there is fear of the compact 
being broken unless its expansion and contraction is made as small as 
possible. Such expansion and contraction is produced by the change of the 
temperature caused due to the heating it from room temperature to the 
supercritical temperature and the cooling it immediately after the 
heating. 
Step of Forming an Impermeable Film Layer 
The compact from which the first and second dispersion mediums have been 
removed is heated to form the film layer into a film layer impermeable to 
gas. The heating temperature in this process is one necessary to soften 
and densify substance forming a gas impermeable film layer. The process is 
preferably carried out in the vacuum atmosphere or in the nitrogen gas 
atmosphere having gauge pressure of 0 to 10 kg/cm.sup.2. In the vacuum 
atmosphere, the difference of the gas pressures at the inside and outside 
of the gas impermeable film layer is so small that the film layer is hard 
to break. If the gauge pressure is over 10 kg/cm.sup.2, the difference of 
the gas pressures becomes large enough to have the film layer broken 
Step of Sintering 
The step of sintering the compact can be carried out either in continuation 
immediately or with an interval after the step of forming an impermeable 
film layer. 
It is, however, preferable that pressure of gas occupying pores existing 
inside the gas impermeable film layer is no more than the outside pressure 
after the gas impermeable film layer has been formed. This is because 
there is fear of destroying the gas impermeable film layer due to the gas 
bursting out of the inside unless the pressure limit is observed. 
When a sintered body of silicon nitride is manufactured in compliance with 
the above-mentioned steps, there are no intricate steps as found in the 
prior art methods In addition, time for sintering is shortened, compared 
to the prior art methods. Furthermore, no decomposition of silicon nitride 
composing the sintered body occurs, and consequently, no change in quality 
takes place on the surface of the sintered body. 
Example-1 
To material powders consisting of 92 wt.% Si.sub.3 N.sub.4, 6 wt.% Y.sub.2 
O.sub.3 and 2 wt.% Al.sub.2 O.sub.3, 22 wt.% paraffin having a melting 
point of 42.degree. to 44.degree. C. and 2.5 wt% oleric acid were added to 
form a mixture, and the mixture was stirred and mixed at 80.degree. C. 
being maintained for 3 hours to prepare a first slurry. In the meantime, a 
mold having a cavity of a 10.times.50.times.70 mm rectangular 
parallelopiped shape with an aluminum water-cooling conduit attached 
thereto was prepared The first slurry was given the pressure of 3 
kg/cm.sup.2 .multidot.G and was cast in the mold with the water cooling 
conduit to form a compact Subsequently, being taken out of the mold, the 
compact was dipped into a second slurry of 35 wt.% boron nitride and 65 
wt.% t-butyl alcohol. The compact, taken out of the second slurry, was 
left released in the air for 10 minutes. This operation of the dipping and 
leaving the compact in the air was repeated five times. 
Following the aforementioned process, the compact was dipped in a third 
slurry powders consisting of a 35 wt.% powdery mixture consisting of 80 
wt.% Si.sub.3 N.sub.4, 14 wt.% Y.sub.2 O.sub.3 and 6 wt.% Al.sub.2 O.sub.3 
and 65 wt.% t-butyl alcohol After being taken out of the third slurry, the 
compact was left released in the air for 10 minutes In this manner, a film 
layer of duplex film structure amounting to 1.5 mm in thickness in total 
was formed 
Then, extraction by the carbon dioxide having 100 kg/cm.sup.2 .multidot.G 
and 35.degree. C. was applied to the compact covered with the film layer 
for an hour and a half, and it was ascertained that the compact thus 
extracted was nearly equal to the weight of the starting powdery material 
After the extraction was completed, the compact was put into a sintering 
furnace and the steps of vacuum degassing, forming an impermeable film 
layer and pressure sintering was applied to the compact. FIG. 1 is a 
graphic representation showing a sintering pattern of an example of the 
present invention. The vacuum degassing was carried out in the atmosphere 
of 1,000.degree. C. and 10.sup.-3 Torr. The formation of the porous film 
layer impermeable to gas was performed by keeping the compact in the 
atmosphere of nitrogen gas having 1,800.degree. C. and 1 kg/cm.sup.2 
.multidot.G for 20 minutes. The pressure sintering was done at 
1850.degree. C. and in the atmosphere of nitrogen gas having 9 kg/cm.sup.2 
.multidot.G for 2 hours. After cooling the compact, the impermeable film 
layer was taken off by sand blasting. 
The sintered body thus obtained, attained 98.8% of the theoretical density. 
Reduction of the weight was small enough to be of 0.1%. Furthermore, no 
change in quality on the surface of the sintered body was found. 
Example-2 
To material powders consisting of 92 wt.% Si.sub.3 N.sub.4, 6 wt.% Y.sub.2 
O.sub.3 and 2 wt.% Al.sub.2 O.sub.3, 22 wt.% paraffin having a melting 
point of 42.degree. to 44.degree. C. and 2.5 wt.% were added to form a 
mixture, and the mixture was stirred and mixed for 3 hours while the 
temperature was kept at 80.degree. C. Thus, a first slurry was formed out 
of the mixture. In the meantime, a mold having a cavity of 10 mm.times.50 
mm.times.70 mm rectangular parallelopiped shape with an aluminum conduit 
for water cooling attached thereto was prepared. The pressure of 3 
kg/cm.sup.2 .multidot.G was applied to the first slurry, and the first 
slurry was cast into the mold with the water conduit attached thereto 
Subsequently, taken out of the mold, a compact formed out of the first 
slurry was dipped into a second slurry of 35 wt.% powdery mixture 
consisting of 80 wt.% Si.sub.3 N.sub.4, 14 wt% Y.sub.2 O.sub.3 and 6 wt% 
Al.sub.2 O.sub.3 and 65 wt.% t-butyl alcohol. The compact, was taken out 
of the second slurry, and then, it was left released in the air for 10 
minutes In this manner, a film layer of duplex film structure amounting to 
1.0 mm in thickness in total was formed. 
And then, the compact covered with the film layer was extracted by carbon 
dioxide having a pressure of 100 kg/cm.sup.2 .multidot.G and temperature 
of 35.degree. C. for an hour and a half, and it was ascertained that the 
compact thus extracted was nearly equal to the starting powder material in 
weight. 
After the extraction was completed, the compact was put into a sintering 
furnace and vacuum degassing, formation of a gas impermeable porous film 
layer and pressure sintering were applied to the compact. The pattern of 
sintering was the same as that of Example-1 was applied. The vacuum 
degassing was performed in the atmosphere of 1000.degree. C. and 10.sup.-3 
Torr. The formation of the gas impermeable porous film layer was carried 
out by keeping the compact in the nitrogen atmosphere having a temperature 
of 1,800.degree. C. and a pressure of 1 kg/cm.sup.2 .multidot.G for two 
hours. The pressure sintering was done in the nitrogen atmosphere having a 
temperature of 1,850.degree. C. and a pressure of 9 kg/cm.sup.2 
.multidot.G being kept for two hours. After cooling the compact, the 
compact was taken out and the film layer thereon was taken off by sand 
blasting. 
The density of the sintered body thus obtained attained 98.8% of the 
theoretical density. The reduction of the weight was small enough to mark 
0.1%. The removal of the gas impermeable film layer was incomplete because 
the sintering proceeded between the gas impermeable film layer and the 
sintered body inside the impermeable film layer.