Dense WSi2 and WSi2-20 vol.% ZrO2 composite synthesized by pressure-assisted field-activated combustion

Article abstract of DOI:10.1016/S0925-8388(01)01167-7

The simultaneous synthesis and consolidation of WSi₂ and WSi₂-20 vol.% ZrO₂ from elemental powders of W, Si, and ZrO₂ additive was investigated. The synthesis was carried out under the combined effect of an electric field and uniaxial pressure. The final density of the products increased nearly linearly with the applied pressure in the range 10 to 60 MPa. Highly dense WSi₂ and WSi₂-20 vol.% ZrO₂ with relative densities of up to 98.0% were produced with the application of a 60 MPa pressure and a 3000 A DC current. The percentages of the total shrinkage occurring before and during the synthesis reaction were 17.5 and 82.5% for the case of WSi₂, and 25.8 and 74.2% for the case of WSi₂-20 vol.% ZrO₂, respectively. The respective Vickers microhardness values for these materials were 8.2 and 10.6 GPa. From indentation crack measurements, the fracture toughness values for WSi₂ and WSi₂-20 vol.% ZrO₂ were calculated to be 3.2 and 9.4 MPa m^1/2, respectively.

Full text of DOI:10.1016/S0925-8388(01)01167-7

Journal of Alloys and Compounds 322 (2001) 120–126
L
www.elsevier.com/locate/jallcom
Dense WSi2 and WSi2 –20 vol.% ZrO2 composite synthesized by
pressure-assisted field-activated combustion
I.J. Shona, D.H. Rhoa, H.C. Kima, Z.A. Munirb ,
*
aDepartment of Materials Engineering, Research Institute of Advanced Materials Development, Chonbuk National University, Chonbuk 560-757,
South Korea
bDepartment of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA
Received 16 October 2000; received in revised form 20 February 2001; accepted 21 February 2001
Abstract
The simultaneous synthesis and consolidation of WSi2 and WSi2 –20 vol.% ZrO2 from elemental powders of W, Si, and ZrO2 additive
was investigated. The synthesis was carried out under the combined effect of an electric field and uniaxial pressure. The final density of
the products increased nearly linearly with the applied pressure in the range 10 to 60 MPa. Highly dense WSi2 and WSi2 –20 vol.% ZrO2
with relative densities of up to 98.0% were produced with the application of a 60 MPa pressure and a 3000 A DC current. The
percentages of the total shrinkage occurring before and during the synthesis reaction were 17.5 and 82.5% for the case of WSi2, and 25.8
and 74.2% for the case of WSi2 –20 vol.% ZrO2, respectively. The respective Vickers microhardness values for these materials were 8.2
and 10.6 GPa. From indentation crack measurements, the fracture toughness values for WSi2 and WSi2 –20 vol.% ZrO2 were calculated to
be 3.2 and 9.4 MPa m1 / 2, respectively.
2001 Elsevier Science B.V. All rights reserved.
Keywords: Combustion synthesis; Densification; Tungsten silicides; Field activation
1. Introduction
mechanical properties [4]. Thus, it is anticipated that the
mechanical properties of WSi2 will be improved by
forming composites with appropriate reinforcements of
ZrO2.
The attractive high-temperature properties of transition
metal silicides have generated considerable interest in this
class of materials [1]. Their stability in oxidizing environ-
ments has been cited as a major attractive characteristic.
Some of these silicides have been identified as possible
replacements for nickel alloys in the next generation of
high-temperature gas turbines [2]. However, since these
silicides possess low ductility at low temperatures, in-
vestigations on the improvement of their mechanical
properties have been of special interest [3]. Attempts to
accomplish this goal have included microalloying or the
addition of a second phase to increase fracture toughness
[4,5]. The addition of ZrO2 to silicide phases has been
investigated. Its martensitic transformation from the meta-
stable tetragonal zirconia to the monoclinic modification,
either spontaneously upon cooling or in the vicinity of
crack-tip stress fields, provides the improvement in the
As in the case of many similar high-temperature com-
pounds, dense WSi2 is usually prepared in a multi-step
process [6,7]. The first step is to react mixed stoichiometric
powders of W and Si at high temperature to form WSi2.
This is followed by the milling of the product to produce
fine powders. Finally, the powders are sintered at high
temperatures under pressure to obtain dense WSi2. A
recently developed method utilizing field activation with
pressure application has been successfully employed to
simultaneously synthesize and densify materials in one
step [8–11]. The method has recently been modified to
prepare dense nanomaterials [12,13]. In this work we apply
this method to produce dense WSi2 and WSi2 –20 vol.%
ZrO2 composite.
2. Experimental procedures
*Corresponding author. Tel.: 11-916-752-0400; fax: 11-916-752-
1031.
Powders of 99.9% pure tungsten (1–5 mm), 99.5% pure
E-mail address: zamunir@ucdavis.edu (Z.A. Munir).
silicon, and 99.5% pure (unstabilized) zirconia were
0925-8388/01/$ – see front matter
PII: S0925-8388(01)01167-7
2001 Elsevier Science B.V. All rights reserved.

I.J. Shon et al. / Journal of Alloys and Compounds 322 (2001) 120–126
121
Table 1
obtained from Alfa Products (Ward Hill, MA, USA). The
latter two powders had a sieve classification of 2325. Two
different compositions were investigated for this study.
The first was aimed at producing monolithic WSi2 and the
second was made to produce the composite WSi2120
vol.% ZrO2. A schematic diagram of the experimental
setup of the field-activated and pressure-assisted combus-
tion is shown in Fig. 1. Details of this experimental
method are provided in previous publications [9–11,14].
The mixed powders were placed in a graphite die (outside
diameter, 45 mm; inside diameter, 20 mm; height, 40 mm)
and introduced into the system. The system was first
evacuated and a uniaxial pressure (ranging from 10 to 60
MPa) was applied. A DC current was then activated and
maintained until densification was attained, as indicated by
a linear gauge measuring the sample shrinkage. Tempera-
tures were measured by a pyrometer focused on the surface
of the graphite die. At the end of the process, the sample
was cooled to room temperature at a fixed rate of 6008C
min21. Typical parameters for the process are presented in
Table 1. The process was carried out under a vacuum of
231022 Torr.
The relative density of the synthesized sample was
measured by the Archimedes method. Microstructural
information was obtained from product samples which
were polished and etched using a solution of HF (60
vol%), HNO3 (30 vol%), and H2O (10 vol.%). Com-
positional and microstructural analyses of the products
were made through X-ray diffraction (XRD) and scanning
electron microscopy (SEM) with energy dispersive X-ray
analysis (EDX). Vickers microhardness measurements
Processing parameters for the field-activated pressure-assisted synthesis
of WSi2 and WSi2 –20 vol% ZrO2
Parameter
Applied value
Vacuum level
Applied pressure
Voltage
231022 Torr
10–60 MPa
18 V
Current
3000 A
Heating time
Heating rate
Maximum temperature
Cooling rate
1 min
12008C min21
12008C
6008C
(under a 10 kg load) were made on the synthesized
samples.
3. Results and discussion
The variations of shrinkage displacement rate and
temperature of the die surface with heating time during the
processing of W12Si and W12Si120 vol.% ZrO2 are
shown in Fig. 2a,b, respectively. For both cases, the
shrinkage displacement increased gradually with tempera-
ture up to about 11708C and then abruptly at about this
temperature. When mixtures of W12Si were heated under
60 MPa pressure to 11708C, no reaction took place and no
significant displacement was observed. An SEM (sec-
ondary electron) image of a sample heated to 11708C under
60 MPa, shown in Fig. 3a, indicates the presence of the
two reactants as separate phases. X-ray diffraction results,
shown in Fig. 4a, exhibit only peaks pertaining to the
reactants. However, when the temperature was raised to
12208C, the starting powders reacted producing highly
dense products. A secondary electron image of an etched
surface of a sample heated to this temperature under a
pressure of 60 MPa is shown in Fig. 3b. It was determined
from an EDX analysis that a complete reaction between
these elements has taken place under these conditions.
These conclusions were supported by X-ray diffraction
analyses in which only peaks of the product phase, WSi2,
were evident as indicated in Fig. 4b. Fig. 3b shows the
presence of porosity within the grains and at the grain
boundaries. Since the latter can come about, in part, from
the removal of minor phases during etching, unetched
samples were also analyzed. The SEM/EDX analyses
showed the presence of WSi2 only.
The abrupt increase in the shrinkage displacement at the
ignition temperature (see Fig. 2a) is due to the increase in
density as a result of the molar volume change associated
with the formation of WSi2 and the further consolidation
of the product. It should be recalled that the measured
temperatures are those of the surface of the die and are,
therefore, likely to be lower than the values in the middle
of the sample. Thus, the onset of the reaction to form the
silicide (and the concomitant rapid shrinkage) may be at a
Fig. 1. Schematic diagram of apparatus for field-activated and pressure-
assisted combustion.

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I.J. Shon et al. / Journal of Alloys and Compounds 322 (2001) 120–126
Fig. 3. (a) SEM image of W12Si compact heated to 11708C. (b) SEM
image of synthesized WSi2.
It is believed that this enhanced densification by our
current method may be due to the presence of a liquid
phase during the synthesis process or due to reported
benefits of this method. These latter include the presence
of a plasma and the enhancement of mass transport by
electromigration [16,17]. Table 2 shows values of the
density, sample volume, and volume change at different
stages in the synthesis and densification of WSi2. These
results show that 17.5% of the total volume shrinkage
occurred prior to ignition and 82.5% occurred during the
synthesis stage.
When mixtures of W12Si120 vol.% ZrO2 were heated
to 11508C, no reaction took place. A secondary electron
image of a sample heated to 11508C (under 60 MPa) is
shown in Fig. 5a. The distribution of W and Si is not
uniform, indicating the absence of a significant reaction.
X-ray analysis confirmed this conclusion, shown in Fig.
6a. However, when the reaction temperature was raised to
12508C, the powders reacted, producing the desired prod-
uct in a dense form. A secondary electron image of such a
sample is shown in Fig. 5b. The image shows a two-phase
microstructure (silicide and zirconia). X-ray diffraction
results confirmed the presence of WSi2 and ZrO2 only
(Fig. 6b). Table 3 shows the density, sample volume and
Fig. 2. Variation of shrinkage displacement rate and temperature with
heating time during synthesis under a pressure of 60 MPa and a current of
3000 A: (a) WSi2 and (b) WSi2 –20 vol.% ZrO2.
higher temperature than the observed value of 11708C.
Since the imposition of a field increases the combustion
temperature for WSi2, it is expected that Si would be
molten during the continuation of the reaction. This is
most likely the reason for the significant increase in
shrinkage beyond the value dictated by the molar volume
change. The relative density of the synthesized WSi2 phase
was determined as 97.8.%. This is significantly higher than
the reported value of 81% obtained for samples which
were made by hot-isostatic pressing of WSi2 powders [15].

I.J. Shon et al. / Journal of Alloys and Compounds 322 (2001) 120–126
123
Fig. 4. XRD patterns of the W12Si system heated to: (a) 11708C and (b)
12208C.
volume change at different stages in the synthesis and
densification of WSi2 –20 vol.% ZrO2 composite. The
results show that 25.8% of the total volume shrinkage
occurred prior to the ignition of the reaction, with the
remaining 74.2% occurring during and subsequent to
synthesis. Comparing the microstructures of the single
phase samples (Fig. 3b) and the composite samples (Fig.
5b), it is evident that the grain size of WSi2 in the
monolithic samples is significantly larger than in the
composites. This observation suggests that the presence of
zirconia limits grain growth of the silicide phase in the
composites.
Fig. 5. (a) SEM image of W12Si120 vol.% ZrO2 compact heated to
11508C. (b) SEM image of synthesized WSi2 –20 vol.% ZrO2 (heated to
12508C).
pressure was varied from 10 to 60 MPa. The effect of the
applied pressure on the final relative density of WSi2 and
WSi2 –20 vol.% ZrO2 is shown in Fig. 7. The relative
density of the composite material increased nearly linearly
from 93.0 to 97.5% as the pressure was changed over this
range. The corresponding change for WSi2 is from 92.0 to
98.0%.
As indicated in the experimental section, the applied
Vickers microhardness measurements were made on
polished sections of the 20 vol.% ZrO2 –WSi2 matrix
composite and the pure WSi2 samples using a 10 kg load.
The calculated hardness values of WSi2 and the WSi2 –20
vol.% ZrO2 composite were 8.2 and 10.6 GPa, respective-
ly. These values represent averages of measurements on 10
samples. Indentation fracture toughness (Kc) values for
these samples were calculated using the following expres-
sion [18,19]:
Table 2
Density, volume, and volume change during field-activated pressure-
assisted synthesis of WSi2
Initial
sample
Before
ignition
Reactant
(theor.)
Product
Exp.
Theor.
Density
(g cm23
)
2.58
3.26
1.68
0.00
0.00
3.12
2.69
5.32
1.58
9.66
0.87
9.86
0.85
Sample volume
(cm3)
Kc 5 0.203[c/a]23 / 2 ? HV ? a1 / 2
(1)
Pore volume
(cm3)
Volume change
(%)
Incremental volume
change (%)
1.11
0.00
0.02
0.00
where c is half of the average crack length, a is half of the
average length of two indent diagonals, and HV is the
hardness. Typical hardness indentations for WSi2 and
WSi2 –20 vol.% ZrO2 are shown in Fig. 8. It is evident
17.48
17.48
51.53
34.05
73.31
21.78
73.92
0.61

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I.J. Shon et al. / Journal of Alloys and Compounds 322 (2001) 120–126
Fig. 7. Variation of the relative density of WSi2 and WSi2 –20 vol.%
ZrO2 composite with applied pressure.
Fig. 6. XRD patterns of the W12Si120 vol.% ZrO2 system heated to:
(a) 11508C and (b) 12508C.
ing might be playing an important role in the toughening
behavior, and that ZrO2 acts as crack-deflecting and
branching agent in this composite.
from these micrographs that at 10 kg indentation load, the
radial crack pattern for pure WSi2 is significantly larger
than that for the composite. The calculated fracture tough-
ness values of WSi2 and WSi2 –20 vol.% ZrO2 composite
are about 3.2 and 9.4 MPa m1 / 2, respectively. As in the
case of microhardness values, each of the toughness values
is the average of measurements on 10 samples. The results
show a factor of 3 improvement in the fracture toughness
as ZrO2 is added to the silicide to make a composite.
A higher magnification view of one of the indentation
radial cracks in the composite is shown in Fig. 9. Crack
segments can be seen to run both along the ZrO2 –WSi2
phase boundaries and through the ZrO2 phase itself. Crack
bridging of the ZrO2 phase is also evident in Fig. 9. The
preliminary SEM observation indicates that crack branch-
4. Summary
Using the field-activated pressure-assisted method, the
simultaneous synthesis and densification of WSi2 and
WSi2 –20 vol.% ZrO2 composites was accomplished using
elemental reactants (with ZrO2 added for the case of the
composite). Complete synthesis and densification can be
achieved in one step within about 1 min. The final density
of the synthesized materials depended in a nearly linear
fashion on the applied pressure. The density of the single
phase varied from 92.0 to 98.0% as the applied pressure
increased from 10 to 60 MPa. For the composite the
density changed from 93.0 to 97.5% over the same
Table 3
Density, volume, and volume change during field-activated pressure-assisted synthesis of WSi2 –20 vol.% ZrO2
Initial
sample
Before
ignition
Reactant
(theor.)
Product
Exp.
Theor.
Density
(g cm23
)
2.41
3.57
1.72
0.00
0.00
3.25
2.65
4.64
1.85
8.82
0.97
9.05
0.95
Sample volume
(cm3)
Pore volume
(cm3)
Volume change
(%)
Incremental
volume change (%)
1.10
0.00
0.02
0.00
25.77
25.77
48.18
22.40
72.81
24.65
73.39
0.56

I.J. Shon et al. / Journal of Alloys and Compounds 322 (2001) 120–126
125
pressure range. Shrinkage displacement during the process-
ing of W12Si and W12Si 120 vol.% ZrO2 increased
gradually with heating temperature up to the ignition
temperature and then abruptly increased. In the case of the
WSi2 –20 vol.% ZrO2 composite synthesized under a
pressure of 60 MPa, it was shown that 17.5% of the total
shrinkage took place prior to the ignition of the reaction
and the remaining 8.5% took place during synthesis. The
corresponding values for the case of WSi2 are 25.8 and
74.2%, respectively. The Vickers microhardness of the
pure silicide and its composite were determined as 8.2 and
10.6 GPa, respectively. The fracture toughness of the
silicide was improved by a factor of about 3 when ZrO2 is
incorporated as a second phase in a composite. The
fracture toughness values calculated for samples synthes-
ized and densified in this work are 3.2 and 9.4 MPa m1 / 2
for WSi2 and WSi2 –20 vol.% ZrO2, respectively.
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