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 . - . _

/ .. _ I

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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