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J. Am.
Cerom
SOC 73 151 1440-42 19901
Effect of Aspect Ratio and liquid Phase Content on
Densification of Alumina Silicon Carb ide Whisker Composites
Terry
N,
Tiegs*
and
D.
Matt
Dillard
Metals and Ceramics Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37831-6069
Densification of alumina-Sic whisker composites by pres-
sureless sintering is inhibited a s a result of whisker interfer-
ence with particle rearrangement and composite shrinkage.
Reduction of the aspect ratio improves densification by im-
proving particlelwhisker packing for increased green densi-
ties and enhances the ability of the whiskers to rearrange
themselves during sintering. Increasing the am ount of liq-
uid phases present during sintering also improves densifica-
tion by aiding whisker rearrangement. [Key words: densi-
fication, liquid phase, whiskers, composities, sintering.]
I. Introduction
NI T I AL
research on the mechanical properties of alumina-
I
i c whisker composites showed that these materials have
significantly improved fracture toughness and strength as
compared with monolithic All of the composite
materials in these early studies were fabricated by hot-press-
ing and had test samples machined from large billets. Since
hot-pressing is limited to relatively simple shapes, th e ability
to pressureless sinter whisker-containing composites was in-
vestigated as a means to den sify material into ne ar-net, com-
plex shapes. T he initial study on p ressureless sintering of the
alumina-S ic whisker materials showed that densification was
severly inhibited by the Sic
whisker^.^
The authors found
tha t, at whisker volume contents greater than about 10 vol%,
densities were limited to <95% of theoretical density (T D). A
minimum of 95% T D was considered necessary for closed
porosity,
so
that complex-shaped articles could be subse-
quently hot isostatically pressed t o full density without encap-
sulation. Other studies of ceramic composites have also shown
inhibition of densification with Sic-whisker additiom6-I2Al-
though property improvements are achieved at contents of
10 vol%, volume contents on the order of 20 vol% or higher
are desirable for significant mechanical property improve-
ments. The purpose of th e present work was to investigate the
sintering of composites with these higher whisker contents by
altering the whisker aspect ratio and varying the amount
of
liquid phases.
During sintering, the densification
of
the composite is lim-
ited because the whiskers interfere with particle rearrange-
ment and shrinkage. To minimize the amount of shrinkage
necessary during sintering, the green densities need to be as
high as possible. Improvements in green densities result in di-
rect increases in sintered densities? P article and rod-packing
M. D. Sacks-contributing editor
Manuscript No. 198156. Received August 28, 1989; approved Janua ry
3,
1990.
Supported by the U.S. Depar tment of Energy, Assistant Secretary for
Conservation and Renewable Energy. Office of Transportation Systems, as
part
o
the Ceramic Technology for Advanced Heat Engines Project
of
the
Advanced Materials Development Program, under Contract No. DE-ACOS-
84OR21400 with Martin Marietta Energy Systems, Inc.
*Member, American Ceramic Societ .
*Georgia Institute
of
Technology, Atra nta, GA, student; present address
is
Washington, DC.
theory shows that the whisker aspect ratio and the ceramic-
matrix owder size can be tailored to maximize the green
density In th e present study, the effect of whiske r aspect
ratio was investigated by holding the ceram ic-matrix powder
constant and a ltering th e whisker lengths by milling. Th e ma-
trix powder chosen had given the best green and fired densi-
ties in a previous study: Becau se toughe ning is mainly
dependent on crack bridging and deflection in this composite
s y ~ t e m , ' ~ong whisker pullout lengths a re not required for good
mechanical properties in the alum ina-Sic whisker system.
Another approach to promote densification in ceramics is
by the formation of liquid phases, which is widely used for
Si3N4-basedmaterials. Liquid-phase sintering using
Y2O3
and
MgO aided the densification of alumina-Sic whisker com-
posites: In the p resent study, composites employing up to
30 wt%
Y z 0 3
were fabricated and fired at various tempera-
tures to obtain a range of liquid contents.
11.
Experimental Procedure
As-received S ic whiskers may contain extrane ous ma-
terials, such as Si c particulate and unrea cted rice hulls, which
can vary significantly from one manufacturer to another.I5
Much of this material can be removed by simple sedimenta-
tion
technique^.'^^
The as-received whiskerst in this study
were "cleaned" by dispersing them in deionized water (with a
pH of
10
adjusted with N H 4 0 H and a solids content of
1 ~ 0 1 % ) nd allowing the slurry to settle for
16
h. The
whiskers would settle to th e bottom, whereas the fine particu-
late and the rice hulls would not. The liquid above the
whiskers was then decanted
off,
an additional amount of
deionized water (no pH adjustment) was added to obtain a
1
vol% solids slurry, and the slurry was stirred by hand and
allowed to settle an additional
16
h. Th is process of decanting
and reslurrying was repeated for a total of five times, and
then the whiskers were dried. A total of
22% of
extraneous
material, consisting
of
fine particulate and very low aspect
ratio whiskers, was removed in this mann er.
Composite powders for dry pressing were prepared by m ix-
ing the appropriate quantities of alumina: cleaned Si c
whiskers, and Y203" in an isopropyl alcohol slurry. T he
sus-
pension was mixed and the powders were deagglomerated
using a high-shear mixer.n Th e slurry was then milled for
various lengths of time in a polyurethane-lined mill jar con-
taining Sic rods. The solids loading of the slurry during
milling was approximately 5% (1 whiskers). Aft er milling,
2 wt% poly(ethy1ene glycol)** (for a pressing lubricant) and
magnesium acetate (to yield 0.5 wt% MgO) were added, and
the mixture was dried. Th e friable, homogeneous powder was
broken up with a boron carbide mortar and pestle and
screened to
- 00
mesh.
'Grade SC-9, AR C 0 Chemical (now Advanced Composites Materials
&ade RC-HP DBM, Reynolds Chemicals, Richmond, VA.
:99.99
purity, Code 5600, Molycorp, Louviers, CO.
Model PT 45/80 Brinkmann Instrument, Westbury, NY.
"Carbowax PEG 8000, Union Carbide, Indianapolis, IN.
Cor
),
Greer, SC.
1440
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May
1990
Communications of th American Ceramic Society
1441
Table I. Whisker-Length Distributions as a F unction of Milling Time
Whisker leneth urn)
O h 4 h 8 h
16
h 32 h
Mean 26.9 15.2 12.0
11.0
10.3
Mean whisker aspect ratiof
45 25
20 18 17
Median*
21-25 11-15 11-15
6-10 6-10
Modet
16-20 11-15 6-10
6-10 6-10
Fraction of whiskers
( )
Size range pm) O h 4 h 8 h 16 h 32 h
0-5
2.0 10.8 22.4 26.5 27.3
5-10
12.5 21.3 27.1 30.7 33.6
10-20
20-30
30-40
40-50
50-60
31.5 40.5 36.5 29.0 30.4
21 0 18.9 8.3 9.2 6.2
11.5 5.4 3.6 3.3 1.6
10.0 1o
1.2 0.9 0.8
3.5 1.0
1.2 0.4 0
60-70 5.0 1.0
0 0
0
>
70 3.0
0 0 0
0
*Defined as observation a t
50
of popula t ion. +D efined as observation which occurs most f requently. 'Length to diam eter ra t io; whiskers have d iameters
of
~ 0 . 6m
The composite powder was formed into bars by a uniaxial
press at
140
MPa
(20
ksi) and then isopressed at
420
MPa
(60
ksi). After binder burnout, the green samples were loaded
into alumina crucibles containing excess Si c whiskers as pack-
ing material. Sintering was performed in a water-cooled tube
furnace heated by a graphite resistance element under a flow-
ing argon atmosphere. The furnace temperature was linearly
increased to the sintering temperature in
150
min and held
5
min at the desired temperature. Densities were measured by
the Archimedes technique. For calculating the theoretical
densities after sintering, all of the
Yz 3
present in the samples
was assumed to form yttrium aluminum garnet
(3Y203-
5A1203) with a density of
4.55
g/cm3.
To determine the change in aspect ratio during milling,
whiskers were milled alone at the same whisker volume load-
ing
(1 )
used in the composite milling. For identical milling
times, the aspect ratio distributions were assumed to be com-
parable with those in the composite materials. Without
the matrix powders present, some difference between these
whiskers and those in the composites may result, but the dif-
ference is believed to be insignificant. Scanning electron mi-
croscopy micrographs of the as-received and milled whiskers
were examined manually to produce the distributions of
whisker lengths. Approximately
400
well-separated whiskers
in at least 10 different fields of view were measured for each
size distribution generated.
111.
Effect of Whisker Aspect Ratio on Sintering
Previous studies showed very little densification in dry-
pressed alumina-Sic whisker composites using as-received
whiskers and low amounts of liquid-phase sintering aids.5
Consequently, some modification
of
the aspect ratio was re-
quired t o obtain high fired densities. For this series of tests,
the composites were fabricated with a low-sintering-aid con-
tent
(2
wt Y 2 0 s and
0.5
wt MgO) to help distinguish the
effects of varying the whisker aspect ratio from the effect
of
liquid-phase volume content on sintering.
The resulting whisker-length distributions obtained by the
milling procedure are given in Table I. As shown, the whisker
lengths (average, median, and mode) are reduced considerably
during milling. For the composites, as the milling time in-
creases and the whisker lengths are reduced, the composite
green densities increase slightly, but not dramatically for both
the
10
and
20
vol composites (Table 11 . However, when the
same composites are sintered, the effect
of
the lower aspect
ratio is clearly evident, especially for the composites contain-
ing
20
vol whisker.
As
shown, the sintered densities are im-
proved with lower aspect ratios at all sintering temperatures,
indicating that inhibition to particle rearrangement and com-
posite shrinkage is reduced as the whisker aspect ratio is low-
ered. In addition, an effect of increasing the whisker volume
content on the rearrangement ability of the whiskers is appar-
ent. This is reflected in the fact that high densities with
whisker contents
of
10 vol were attainable with very little
aspect ratio modification, and they did not change apprecia-
tively as the aspect ratio was lowered. However, significant
aspect ratio reduction was necessary to achieve dense com-
posites with 20 vol whiskers, and the effect of lowering the
aspect ratio was more dramatic.
IV.
Effect of Liquid-Phase Sintering Aids
The ability to sinter material with 10 vol Sic whiskers
with low amounts of liquid phase has been demonstrated pre-
viously? Consequently, we investigated the sintering
of
com-
Table
11.
Green and Sintered Densities of Alumina-SiC Wh isker Composites atVarious Milling Times
Green densi ty ( TD)
Sintered density ( TD)
Mill ing t ime
(h)
10
vol% whisker
20
vol% whisker
10
vol% whisker 20 vol% whisker
1850°C
4 59.7
56.6 95.8 80.7
8 59.5
56.9 96.0 82.5
16 59.6 57.6 96.5 87.7
32 59.0 89.7
4 59.7 56.6 95.2 78.2
8 59.5
56.9 95.3 78.9
16 59.6 57.6 95.6 82.7
4 59.7
56.6 91.3 66.6
8 59.5
56.9 93.3 67.0
16 59.6 57.6 94.3 74.5
1800°C
1700°C
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1442
Communications
of
the merican Ceramic Society
posites with various liquid-phase contents only with 20 vol
whiskers. Specimens were prepared with increasing amounts
of YzO3 to create more liquid phase at the sintering tempera-
ture. The whiskers were milled 32 h to produce a whisker-size
distribution similar to that in Table I for the same mill time
and having a mean aspect ratio of about 17:l.
The results for the liquid-phase sintering are summarized
in Fig. 1.
As
shown, the sintered densities generally increase
with both the temperature and Y203 content. However, at
the higher temperatures and Yz03 contents (i.e.,
>10
wt
Y2O3 and >1750”C) the densities remain constant or slightly
decrease.
The most important effect of varying the temperarture and
the YzO, content is to change the amount of liquid phase
present during sintering. Using the phase diagramI7 for the
AI203-Y2O3system, the amount of matrix liquid phase at the
sintering temperatures was estimated.** A good correlation
between the calculated quantity of liquid phase in the matrix
and the final density is shown in Fig. 2. As indicated, the
range
of
liquid phase can be quite large with quantities up to
approximately
80
wt .
As
the liquid-phase contents are in-
creased, however, no fur ther improvements in density are ob-
served with liquid contents greater than about 30 . It is
thought that volatilization or decomposition of the liquid
phases was responsible for the decreasing densities at the high
liquid contents. Accurate weight losses could not be deter-
mined for the high-Y203-contentsamples because
of
surface
reactions with the packing powders.
V
Conclusions
It is evident from prior studies and the present results that
Sic whiskers inhibit sintering
of
alumina-matrix composites
by interfering with particle rearrangement and composite
shrinkage. Reduction of the whisker aspect ratio improves
the sintering by two mechanisms. As-received whiskers have
median aspect ratios normally >40:1, but during processing
the ratio can easily approach 20:l. The first effect observed of
lower aspect ratios is that the packing efficiency of the SiC-
whisker and the alumina-particle mixtures is increased which
results in higher green densities. This small improvement in
green density also corresponds to an increase in the sintered
density because
less
shrinkage needs to occur for comparable
densities. However, the most important effect of reducing the
aspect ratio is
to
allow the whiskers and alumina particles to
rearrange more easily during sintering and permit shrinkage
of the composite to occur.
The densification behavior is also a function
of
the amount
of
liquid phase present at the sintering temperature. Gener-
“Th e solidus tern eratur e on the a lumina-r ich port ion of the phase dia-
gram was es t imatecf to be 1700°C due t o the M gO addit ions and th e
l to
2 w t% S i 0 2 associa ted w ith t he S ic whiskers .
70
10 20 30
40
Yttria Content of Matrix wt. )
Fig.
1.
Sintered densities for alumina-
20
vol SIC whisker composites at
various Y203 additions and sintering
tempera tures 0) 700”, (A) 750”, and
0)
8OWC). Whiske rs milled 32 h .
100 7
Vol. 73, No. 5
5
F
.-
-
Lc
0 1 0 20
30 4 0 5 0 6 0 7 0 80
Estimated Matr ix Liquid Phase
(9 of
Matrix)
Fig. 2 Sintered densities for alumina-
20 vol SIC whisker composites (with 2 to
30
w t % Y20 as the s in te r ing a id , f i red a t
1700” to
1850°C,
with a whisker aspect ratio of
=17:1) as a func t ion of estimated liquid-phase
content a t the s in ter ing tempera ture .
ally, the densification improves with increasing liquid-phase
content. However, as the amount of liquid phase reaches
about 30 , no further improvement in densification is ob-
served because of decomposition
of
the liquid phases.
Acknowled merits- Th e authors acknowledge J .W. Geer for composite
fabrica t ion an 5 H. Keak ng for preparing tes t specimens.
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