High density ceramic sinters are manufactured by coating premolded or presintered porous ceramics with a sealing material of high SiO2 porous glass or nitride glass and then sintering by hot isostatic pressing. The ceramics have excellent abrasion and corrosion resistances. Thus LC-10 (Si3N2 powder) and Y2O3-Al2O3 type sintering were mixed and molded to give a premolded porous ceramic (porosity 37%, relative bulk density 63%). The ceramic was dipped in a slurry containing high SiO2 porous glass and an alcohol solution of cellulose acetate and dried. The coated ceramic was treated in a nitrogen atmosphere and then sintered by hot isostatic pressing to give a dense ceramic sinter

Hibata, Y.

English

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NASA TECHNICAL MEMORANDUM NASA TM-77925
MANUFACTURE OF HIGH-DENSITY CERAMIC SINTERS
Hibata, Y.
Translation of Japan Patent No. 59-116178, July 4, 1984,
Tokyo Japan, pp. 1-6
{NASA-TM-77925) B A N U F A C T U H E OP HIGH-DENSITY
CEBAHIC SINTEBS (National Aeronautics and
Space Administration) 10 p HC A02/HF A01
CSCL 11B
N86-21678
Onelas
G3/27 05692
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON D.C. 20546 OCTOBER 1985
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MANUFACTURE OF HIGH-DENSITY CERAMIC SINTERS
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Translation of Japan Patent No. 59-116178, July 4, 1984,
Tokyo Japan, pp. 1-6
ORIGINAL PAGE IS
OF POOR QUALITY
I A.
i-,
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High-d. ceramic sinters are manufd. by coating premolded or presintered
porous ceramics with a sealing material of^ high-Si02 porous glass or
nitride glass and then sintering by hot JSOStatJC press'ing. The _ _
ceramics have excellent abrasion and 'corrosion^ resistances^  Thus, LC-10
(Si3N, powder) and Y,>0_-A1,?0^ type sintering _ were mixed and molded
to give a premolded porous ceramic (porosity 37%, relative bulk d. 63%)
which was dipped in a slurry contg. high-S-i09 porous glass and an ale.
soln. of cellulose acetate to give a coating and dried. The coated
ceramic was treated in a N atm. and then sintered by hot isostatic
pressing to give a dense ceramic sinter.
17. (MwtM fey A«ttMrM|
CUttif.
unclassified - Unlimited
CU«*lf. (•« At* Mftf
10
(19) Japan Patent Bureau (JP)
(12) Open Patent Bulletin
(51) Int. Cl. Classification (43) -Published July 4, 1984
C 04 B 35/64 7158-4G
No. of invention: 1
Judgement: applied.
(54) Manufacturing method of high-density sinters.
(21) No. S57-231678
 v
\
(22) Applied date: Dec. 22 1982
(72) Inventor: Y. Hibata, 1-15-4-713 Mikuni Honcho, Yodogawa, Osaka.
(72) Inventor: K. Eguch, 2-1-72 Kiyowadai Nishi, Kawanishi.
(72) Inventor: M. Kinoshita, 3-4-8 Satsukigaoka, Ikeda.
(71) Applicant: President of Kogyo Gijutsuin.
(74) Agent: Director of Kogyo Gijutsuin Osaka Laboratory.
Detailed statement.
Name of the invention: Manufacturing method of high-density ceramic sin-
ters.
Range of the patent application:
l) High-density ceramic sinters are manufactured by coating pre-molded or
pre-sintered porous ceramics with a sealing material of high-SiO2 porous glass or
its nitride glass.
Detailed explanation of the invention:
This invention was made on a new manufacturing method of high-density
ceramic sinters from porous ceramics using hot isostatic press (HIP).
In recent years there has been much R&D on various ceramic sinters because
ceramic is the only material that can be used under severe enviroments such as
high temperature, high vacuum, high stress and high corrosiveness. Particularly,
in order to improve thermal efficiency of various heat engines,inlet temperature
must be increased considerably and, therefore, high temperature materials with
low heat dissipation have been investigated. Since conventional heat-resisting
metallic alloys do not have satisfactory thermal and mechanical strength, there
has been a growing expectation lof ceramics asmaterialsto replace metals. Ce-
ramic ' is recognized as the material for various die casts or nozzles because of its
good abrasion resistance.
To manufacture such ceramic sinters, hot press is usually used. It is difficult
to manufacture complex shaped products such as gas turbine'_blades jjr_mechani-
icalgears,because ceramic raw material is sintered in a rather simply shaped
mold. Furthermore, since graphite or aluminum is used for the mold, pressure
is limited to several hundred atm which is a large technical limitation to manuf act\-
'^uring ceramic sinters.A new method has been recently proposed utilizing hot
isostaticpressing tosinterhighdensityceramics from pre-molded or pre-sintered
ceramics in high temperature and high pressure Argon or Nitrogen gas. Namely,
pre-molded or pre-sintered ceramics'areenclosed in a high pressure furnace and
sintered isotropically under 500-3000 atm of high pressure and 1300-2500°C of
high temperature.
Since this HIP method uses Ar or N2 gas as a pressure medium, the sur-
face of pre-molded or pre-sintered ceramics must be gas-impenetrable. The following
three methods have been known in applying HIP on ceramics; 1) Ceramics are
pre-molded or pre-sintered up to more than 90% density, or desirably more than
95%, to achieve gas-impenetrability, and then HIP is applied for further densi-
fication. 2) If ceramic density is less than 93%, the material is enclosed in a
gas-impenetrable capsule, and high temperature and high pressure are applied
on the capsule. 3) Instead of a capsule, gas-impenetrable material such as glass
is coated on the surface for gas sealing.
However, each of these methods has its drawback. Namely, method l) re-
quires two steps of sintering at relatively high temperature and, therefore, the
final product becomes expensive, limiting this method only to valuable ceramics.
Since method 2) needs only one sintering, its product can be less expensive. Yet
complex shaped ceramics aredifficult to manufacture because a capsule is neces-
sary. Method 3) is the most promising because materials close to the final shape
can be used. Presently Pyrex glass, quartz glass or Vycor glass (high-SiO2 glass)
are known as the high pressure gas sealing material. However, these materials
are not appropriate for gas sealing of, the above mentioned ceramics such as heat
enginesbecause more than 1500°C is required to manufacture these ceramics. For
example, the viscosity of Pyrex glass is low at high temperatures due to 4% al-
kali contamination. Although the coating of Pyrex glass onto porous ceramics
is simple, penetration of the glass into the pores is significant and the quality of
';the final ceramic product after HIP process is seriously affected.Quartz glass and.
Vycor glass have higher viscosity even at high temperatures in contrast to Pyrex
glass. It is, however, difficult to use these glasses because crystalization of these
glasses starts at around 1200-1300°C, whereas more than 1500°C is necessary to
form a glass layer.
We have been investigating new sealing materials which do not have the above
drawbacks, and found high-SiC>2 porous glass and its nitride glass as ideal sealing
materials. The invention was made of a manufacturing method of high-density
ceramic sinters from porous ceramics using HIP characterized by using high SiOz
porous glass or its nitride as a sealing material.
The high-SiOj porous glass becomes glass at the relatively low temperature of
1100-1200°C and has high viscosity as quartz glass. Therefore, a gas impenetrable
layer can be easily formed on the ceramic surf ace and, at the same time the quality
of the final product after HIP process will not be affected by sealing because
this sealing material does not penetrate into ceramics. HIP can be applied on
ceramics as near to the final shape as possible to manufacture high density sinters.
Thereareno restrictions on pre-molded or pre-sintered porous ceramics as long
as II, III, IV group oxide, carbide, nitride or boride is the major component of
the ceramics. Pre-molded ceramics made by usual metal molding, rubber press
or extrusion, and pre-sintered ceramics made by hot press or normal pressure
sintering can be used. Thereareno restrictions on various binders or sintering
aids which are sometimes added during molding or sintering. Porous ceramics
with less than 93% density is acceptable and, of course, materials with more than
93% density can be processed without the present invention.
High-SiO2 porous glass is made from borosilicate glass by separating into
Na2O, 6263 phases and SiC>2 phase through thermal treatment and by extracting
soluble Na2<3 and 6203 by acid. Chemical composition of the glass is SiC>2
94-98%, B2O3 0.5-5%, A12O3 0-2% and Na2O 0.01%. Pore size is 40-2000A,
pore volume 0.3-lcc/g and pore surface area 50-600m2/g. This glass has good
reactivity with large surface ratio. This material melts into uniform glass at
1000-1200°C.
The above described glass is coated on pre-molded or pre-sintered porous
ceramics. \The coating method is by dipping in a slurry containing the glassand.water
1 ' -I
or alcohol, or spraying or brushing the slurry. If 0.1-0.2% of cellulose acetate
or PVA is added to the slurry, the layer quality after coating can be improved.
Thickness of the coating should be about lOyum or desirably more than 100/mi.
Thickness of more thanaimm is not necessary.
When porous ceramics coated with the glass powderare\sintered at morethan
900°C, or desirably at 1000-1200°C, a gas-impenetrable glass layer is formed
on the surface, thus enabling the HIP process. HIP process can be applied in
any gas environment. For example, in the air or inert gas, high-SiC>2 glass is
formed, and in N2 or ammonia gas, it becomes its nitride glass. Since these
glasses have higher viscosity than quartz glass or Vycor glass, glass penetration
into porous ceramics is reduced and\ a gas-impenetrable seal is formed only on the
surface. Since quartz glass has lower reactivity, it does not form nitride glass via the
above mentioned treatment. Instead of coating high-SiO2 porous glass powder
directly, this powder can be made into its nitride first, and this nitride glass
powder is coated on the porous ceramic surface. Since the glass sealing and HIP
process proceed at the same time, the whole process can be very simplified. Gas-
impenetrable glass seal can be made by dipping porous ceramics into powdered
high-SiC>2 glass or its nitride glass, and by hot-pressing it at 800-1100°C with a
moderate pressure which does not' break ceramics.
Table 1 shows melting point temperature and viscosity of major glasses.
fl 5 ZtKt t
e ft a
CO
820
1650
1500
« fl fits
1000 ~c
io 3-'
IO
101
IO
1500'C
10
10
IO
1) Glass; 2) Melting point; 3) High temperature viscosity; 4) Pyrex glass; 5)
Quartz glass; 6) High-SiC>2 glass (Vycor); 7) Nitride glass.
Although there is no particular restriction on HIP conditions, more than
1300°C and 500-3000 atm pressure are recommended.
The nitride glass sealing material made from high-SiOz porous glass holds the
gas-impenetrability even at HIP condition and has higher viscosity than quartz
glass (which loses gas-impenetrability at above 1300°C). It is the best sealing
material for HIP treatment of nitride, carbide and boride group ceramics. Fur-
thermore, since sealing is possible for any shape of ceramics ,VHIP can be applied
on materials close to the final products.
Examples and comparisons are described as follows;
Example 1:
Si4N4 powder (Stark LC-10) and Y2O3-A12O3 type sintering aid (8 wt%)
were mixed and molded via rubber press at 3000 atm pressure. Its relative bulk
density was 63% and porosity was 37%. This pre-molded ceramic was dipped in
a slurry containing high-SiOa porous glass (pore volume 0.3cc/g, average pore
radius 150A, pore surface area 180m2/g and average grain radius O.lmm) and an
alcohol solution containing 1% of cellulose-acetate to form 1mm thick layer on
the surface. After drying, the coated ceramic was treated in N2 gas (flow rate
50cc/min) for 30 min at 1200°C. HIP process was applied under the conditions
shown in Fig. 1. Final HIP condition was 1800°C, 60 min and 2000 atm pressure
in Ar gas. Results are shown in Table 2.
Comparison 1:
Porous ceramic as made in Example 1 was coated by quartz glass powder
slurry. After treating the material at 1300° for 30 min, HIP process was applied
under the same conditions as Example 1. As shown in Table 2, densification is
not achieved for this case.
Example 2:
SiC powder (Fujimi #8000) was pre-molded by rubber press as described in
Example 1. The relative bulk density was 58%. A slurry made from high-SiC>2
porous glass was coated by a brush. After drying, HIP was applied in N2 gas
(flow rate 50cc/min) at 1200° under conditions shown in Fig. 2 . The results are
shown in Table 2 together with the results obtained by coatingi with quartz powder.
Example 3:
SiaN4 powder and Y2O3-A12O3 type sintering aid (12 wt%) were mixed as
described in Example 1. After rubber pressing, the material was sintered for one hour
under 9.8 atm N2 gas at 1800°C. The relative bulk density was 91%. One sample
was made by coating high-SiO2 porous glass powder as described in Example 1.
Another sample was made by dipping into high-SiO2 porous glass powder in hot
press and pressing for 10 min. at 1000°C and 100kg/cm2. These samples were
treated by HIP under the same conditions as Example 1. The results are shown
in Table 2 together with the results obtained without glass sealing.
ORIGINAL PAGE, IS
OF POOR QUALITY
Example 4:
ZrC and ZrB (Cerac) were pre-molded by hot-press for 10 min. at 1600°C
and 200kg/cm2. The densities were 77% and 83%, respectively. Table 2 shows
the results obtained under the same condition as Example 1.
SiC
ZrC
i")
4 L 7)
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q)
fl 5 ^( * -}
_
\2
fl =5 ^
<«>• -(•«**•«.*.
15)
63
63
63
91
91
58
58
77
83
61.5
98.2
62.2
87.5
99
96
58
97
99
1) SisN4 raw material; 2) Sealing material; 3) Density; 4) Before HIP; 5) After
HIP; 6) Rubber press; 7) Without sealing; 8) High SiOa porous glass (dipping);
9) Quartz glass (dipping); 10) (pre-sintered); 11) Without sealing; 12) High SiC>2
porous glass (hot press); 13) High SiO2 porous glass (dipping); 14) Quartz glass
(dipping); 15) High SiC>2 porous glass (brushing).
ORIGINAL
Fig. 1 HIP schedule for SisN4 ceramics.
Fig. 2 HIPschedulef orSiC ceramics.
1600-
'1200-
>£
8.
2000-
M200-
-2000
7
IV.
T
tf
8f
1) Temperature; 2) Ar gas pressure (atm.); 3) Time (min.).


Manufacture of high-density ceramic sinters

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