A mixture containing glass 70 to 95 and BN or B4C powder (0.1-10 microns) 5 to 30 vol. % is used as a secondary pressure medium in hot isostatic pressing of ceramics. Thus, Pyrex beads were mixed with 15% vol. BN powder (average diameter 2 microns), fused at 1400 deg for 2 h, cooled, crushed, and put into a graphite crucible. A Si3N4 sintered body was embedded in the powder, heated in vacuum at 1200 deg for 2 h, treated in a hot isostatic press furnace at 1700 deg and 1000 atm. for 1 h, and cooled to give a Si3N4 ceramic. It was easily separated from the crucible

Honma, K.

English

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https://ntrs.nasa.gov/search.jsp?R=19860005927 2020-03-20T16:40:22+00:00Z
NASA TECHNICAL MEMORANDUM 	 NASA TM-77931
,2345
HOT ISOSTATIC PRESSING OF CERAMICS
	 0>
JA^V, 1985
RECEIVED'
NASA S11 FAQ u
Houma, K.	 >,,
Translation of Japan Patent No. 58-84713, May 20, 1983,
Tokyo Japan, pp. 1-5
(NA,SA-T8-77931) HOT ISOSTATIC PRESSING OF
CESAeICS (National Aeronautics and Space
Administration) 10 p HC A021Rf A01 CSCL 11B
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON D.C. 20546	 OCTOBER 1985
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A mixt. contg. glass 70-95 and BN or B4C powder (0.1-10 p)
5-30 vol. % is used as a secondary pressure medium in hot isostatic
pressing of ceramics. 	 Thus, Pyrex beads were mixed_ with 15% vol.
BN powder (ay . diam. 2 u), fused at 1400 ° for 2 h, cooled, crushed,
and put into a graphite crucible.	 A S13N4	 sintered body was em-s
bedded in the powder, headed in vacuum at 1200 ° for 2 h, treated .
in a hot isostatic press furnace at 1700° and-1000 atm for 1 h,
+	 and cooled to give a Si jN,. ceramic.	 It wan ?asily sepd. from the
crucible.
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(19) Japan Patent Bureau (JP)
(12) Open Patent Bulletin
(51) Int. Cl.	 Classification (43) Published May 20. 1983
C 04 B 35/14	 6417-4G 6375-4G
No. of invention: i
Judgement: not applied.
(54) Hot isostatic pressing method of ceramics.
(21)No. S56-182735°
(22) Applied date: Nov. 13 1981	 I
(72) Inventor: K. Honma, 2-14-5 Takaoka, Okubocho, Akashi-Ku, Obe.
(72) Inventor: T. Tachino, 3-13-12 Shinwadai, Tarumi-Ku, Kobe.
(72) Inventor: H. Okada, 1-3-21 Mukonoshyo, Amagasaki.
(71) Applicant: Kobe Seiko Co., 1-3-18 Wakihama-Cho, Chuo-Ku, Kobe.
(74) Agent: T. Miyamoto.
Detailed statement.
1. Name of the invention: Hot isostatic press (HIP) of ceramics.
2. Range of the patent application:
1) As a secondary pressure medium in HIP process of ceramics, a mixture of
silicate glass and 5-30 vol.% of a powdered material (which does not react with
the silicate glass and remains in a solid phase under HIP temperature) is used .
2) Silicate glass is selected from silica glass, Vycor glass or Pyrex glass.
3) Added material is BN powder.
4) Added material is B 4 C powder.
5) Diameter of the added powder is 0.1-10µm.
1	 I ~.
6) Pro aund ceramics is pre-molded powder of Si 3N4, SiC, Al203, TiC-A1:O3.
3. Detailed explanation:
This invention is on a HIP method of ceramics using glass as a secondary
pressure medium.
In order to achieve greater hat efficiency, fuel economy, pollution reduction
and weight reduction, many developments have been under way in the recent
years on machines operated at high temperatures such as high temperature gas
turbine, diesel engine and MHD generator. Hat resisting metals do not have sat-
isfactory mechanical strength and natural resources of these metals are limited.
Therefore, utilization of ceramics whichareabundant on the earth has been ex-
panding. Fepecially, Si3N4 and SIC are the most recognized materials because of
the strength, chemical stability and hat shock resistance at high temperatures.
In order to manufacture Si3N4 products, Si3N4 is sintered by HIP process. A
typical example is described in Tokkyo Koho 51-702081, where Si 3N4 powder or
its pre-molded material is enclosed in a glass capsule and processed by HIP.
However, in the HIP process using glass as a capsule or a secondary pressure
medium, the glass tends to penetrate into sintered materials during the process,
forming a reaction layer between Si3N4
 and glass, and causing cracks due to
difference of thermal expansion coefficients of these materials.
In order to prevent this problem, mold separation material such as BN powder
is usually coated on the surface of pre-molded ceramics before enclosing into a glass
capsule or glass bath. But it is difficult to obtain thick and uniform coating
of BN powder, especially for complex shaped ceramics, causing distortions or
density variations in the final sinters. Other methods have been also tried such
as spraying the powder or coating the powder mixed in a solvent. The coating
often comes off during the later process, or ceramics and glass react with each other
at thinly coated areas, thus making it difficult to remove the glass capsule. Since
the glass penetration depth may reach up to 0.5-1mm depending on the kinds
of glass, temperature or pressure of HIP process, the coating thickness requires
at least lmm. It is difficult to achieve such thickness »sing the above mentioned
methods.
We previously proposed a method in which presintered ceramics are dipped in
BN powder, made into as impleblockby pressing, covered by glass and processed
by HIP. Yet the same problems have been foundf o rcoating the BN powder such
as difficulties wi ,.h removing glass layer, because the glass and BN powder form
separate layers.
The present invention was made to improve HIP process of ceramic sinter and to
further develop our previous proposal mentioned above. A mixture of BN powder
and glass is used as a pressure medium. Capsule separation has become easier
because ofthe difference in thermal expansion rates of these two materials. Material
such as BN powder which does not react with glass and remains in solid phase
under HIP temperature is mixed with a glass capsule material or filled in a glass
bath in an amount of 5-30 wt%. HIP is applied on ceramics surrounded by this
mixture.
Detailed explanations will be described as follows:
Pre-molded or pre-sintered ceramics made from Si3N4 or SiC powder are en-
closed in a glass capsule or glass bath, and HIP is applied using this glass as a
secondary pressure medium. The glass serves as a secondary pressure medium of
HIP process and, therefore, it must surround the ceramic body.
Si3N4 powder pre-sintering is made by a conventional method such as hot
press or Nz gas sintering. When sinteringa ids such as Y203 i Al2 03 or MgO is
added, the amount should be less than 10 wt% relative to the Si 3 N4
 powder.
Relative bulk density does not have to be so high and porous material with le:;s
than 92% density is acceptable. Al 203 , TiC-Al 2 03
 can be also used as ceramic
raw material.
Silicate glass with high melting point temperature such as silica glass, Vycor
glass, Pyrex glass or their mixture can be used.
3
The glass can be made into a glass capsule and a work piece is enclosed as
shown in Fig. 1. The glass can be also filled in a graphite crucible and serves as
a secondary pressure medium for the work piece as shown in Fig. 2.
The uniqueness of the present invention is in the point that the glass is mixed
with powder such as BN or B!C which does not react with the glass and remains
in solid phase at HIP temperature. SiC powder can be also used as a mixing
material. Such powders do not mix with glass well and have different thermal
expansion rate than glass. Therefore, after HIP process, cracks starts in the glass
easily, making it easier to remove the capsule.
Mixing rate of the powder is 5-30 vol.Qi. desirably 5-20 vol.%. For less than
5%, glass adhesion with ceramics is unavoidable, and for :Her s
 than 30%, the mix-
ture no longer works effectively as a secondary pressure medium and satisfactory
densification will not be achieved.
Since BN powder does not mix well with glass, the powder usually accumu-
lates around the ceramics during HIP process, making it easier to remove the
capsule.
Coating BN powder on ceramics by spraying also works well. Acceptable
powder grain radius is 0.1-10µm, or practically 1-5µm.
It is usually practical to mix powder and glass when making the glass capsule or
glass bath. Therefore, powder is usually added to glass beads. This powder-glass
mixture is fused into a uniform glass ingot at high temperature, for example, in
case of Pyrex glass at 1400°C for 2 hours, then crushed by a conventional method
such as ball mill and filled in a glass capsule or a graphite crucible.
In case of aglassbath,the glass is heated up above melting point by a vacuum
furnace to effectively surround the ceramics.
Once the mixture of BN powder and glass surrounds the ceramics, HIP is
applied. In the HIP process, pressure and temperature are increased according
to operating conditions in an inert gas atmosphere such as Ar or N2, and uniform
4
pressurization is made.
Although HIP temperature varies with materials, ceramics usually require
higher temperature. Therefore, glass must be selected considering this tempera-
ture. In case of Si3N4, HIP temperature is usually more than 1400°C, desirably
1600-19WC. Depending on the material, temperature can be increased up to
2000°C. But, of course, it must be lower than the decomposition temperature
of ceramics. Since the decomposition temperature increases with the HIP pres-
sure, temperature should be selected at least 100°C lower than the decomposition
temperature of the operating HIP pressure.
HIP pressure should be higher than 500 atm. If pressure is lower than 500
atm, longer HIP process time is necessary. Furthermore, since amount of reacting
Si3N4 increases with the process time, weight reduction of ceramics is caused and
densification itself is affected. Therefore, HIP pressure must be higher than 500
atm, desirably higher than 700 atm.
Although higher HIP pressure reduces the reaction rate of Si2 N4 and den-
sification becomes easier, longer pressurization time is necessary and, therefore, a
larger compressor and HIP equipment are required. Practically, HIP should be
applied under 2500 atm. HIP process time is 20 min - 5 hon*s.
Fig. 3 and Fig. 4 show BN powder cohesion situation during HIP process. 	 •;^
Although the glass melts at HIP temperature, the powder remains in solid phase
and does not react with the glass. As a result, a BN powder layer (5) is formed
separated from glass-BN powder mixture (1) and glass (6), and the ceramics can
be separated easily.
After HIP process, the glass capsule or graphite crucible is taken out of the
furnace, cooled and the glass is removed. Because of the difference of thermal
expansion rates of BN powder and glass, small cracks are formed during cooling
and the mold can be easily separated.
According to the present invention, mold removing process can be simplified
without sacrificing the quality of the final products.
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More detailed explanations will be described using some ex.anples as follows:
Example 1:
Pyrex beads with grain diameter 1mm (melting point 320 °C) mixed with 15
vol % of BN powder withgraindiameter 2µm were fused at 1400 °C for 2 hours to
make auniformBNmixedglass ingot and crushed into powdered glass by a ball
mill. Then, this BN mixed glass powder was filled into a graphite crucible and the
Si3N4 pre-sintered work piece was embedded in the powder. The crucible was
heated in a vacuum furnace at 1200°C for 2 hours to get as1 3N 4body surrounded
by a continuous glass layer. HIP process was applied at 1700'C, 1000 atm. for one
hour. After the process, the crucible was removed from the furnace and cooled.
During the cooling, many small cracks were recognized in the glass capsule and
the ceramic was easily separated. The surface of Si 3 N4 sinter was good.
Example 2-8:
Various glasses and powders have been tried and the results are shown in
Table.
0	 -13	 1)	 41	 r^ nt '1V
1) Example; 2) Glass; 3) Powder amount; 4) Ceramics; 5) HIP condition;
6
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ORIGWAL ' PH iZ. iS
OF POOR QUALITY
6) Mold separation; 7) Comment; 8) Pyrex; 9) Vol.%; 10) Silica; 11) Vycor;
12) Pyrex; 13) Comparison 1; 14) Glass adhesion; 15) Comparison 2; 16) No
densification.
These examples show that the powder-glass mixture achieved satisfactory
mold separation as well as HIP densification.
Fig. 1 Capsule method used to enclose work piece.
Fig. 2 Glass bath method used to embed work piece.
Fig. 3 and 4. Powder cohesion situation. Fig. 4 Enlarged view of area A.
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Hot isostatic pressing of ceramics

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