Simultaneous synthesis and consolidation of nanostructured NbSi2-Si3N4 composite from mechanically activated powders by high-frequency induction-heated combustion

Article abstract of DOI:10.1016/j.jallcom.2007.07.065

Dense nanostructured 4NbSi₂-Si₃N₄ composite was synthesized by high-frequency induction-heated combustion synthesis (HFIHCS) method within 1 min in one step from mechanically activated powders of NbN and Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense 4NbSi₂-Si₃N₄ composite with relative density of up to 98% was produced under simultaneous application of a 60 MPa pressure and the induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.

Full text of DOI:10.1016/j.jallcom.2007.07.065

Journal of Alloys and Compounds 461 (2008) 560–564
Simultaneous synthesis and consolidation of nanostructured
NbSi2–Si3N4 composite from mechanically activated
powders by high-frequency induction-heated combustion
Hyun-Kuk Parka, In-Jin Shona,∗, Jin-Kook Yoonb, Jung-Mann Dohb,
In-Yong Koa, Z.A. Munirc
a Department of Materials Engineering, Research Center of Advanced Material Development, Chonbuk National University,
664-14 Deokjin-dong 1-ga, Deokjin-gu, Jeonju, Jeonbuk 561-756, South Korea
b Metal Processing Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang,
Seoul 136-791, South Korea
c Facility for Advanced Combustion Synthesis, Department of Chemical Engineering and Materials Science,
University of California, Davis, CA 95616, USA
Received 31 May 2007; received in revised form 2 July 2007; accepted 14 July 2007
Available online 25 July 2007
Abstract
Dense nanostructured 4NbSi2–Si3N4 composite was synthesized by high-frequency induction-heated combustion synthesis (HFIHCS) method
within 1 min in one step from mechanically activated powders of NbN and Si. Simultaneous combustion synthesis and densification were accom-
plished under the combined effects of an induced current and mechanical pressure. Highly dense 4NbSi2–Si3N4 composite with relative density
of up to 98% was produced under simultaneous application of a 60 MPa pressure and the induced current. The average grain size and mechanical
properties (hardness and fracture toughness) of the composite were investigated.
© 2007 Elsevier B.V. All rights reserved.
Keywords: High-frequency induction-heated combustion; Composite materials; Intermetallic; Sintering; Nanophase; Mechanical properties; NbSi2–Si3N4
1. Introduction
due to its low thermal expansion coefficient and a good resis-
tance to oxidation when compared to other structural materials
[13,14]. The isothermal oxidation resistance of NbSi2-40 vol.%
Si3N4 composite prepared by spark plasma sintering (SPS) pro-
cess in dry air at 1300 ◦C was superior to that of monolithic
NbSi2 compact since the composite contained a larger amount
of Si, which made it easier to form dense SiO2 scale [15]. There-
fore, Si3N4 may be the most promising additive as a reinforcing
material for NbSi2-based composites.
Many similar high-temperature dense composites are usually
prepared in a multi-step process [16,17]. However, the method of
field-activated and pressure-assisted combustion synthesis has
been successfully employed to synthesize and densify materi-
als from the elements in one step in a relatively shorter period
of time. This method has been used to synthesize a variety of
ceramics and composites, including MoSi2–ZrO2, Ti5Si3 and
its composites, WSi2 and its composites, and WC–Co hard
materials [18–23]. These materials, which are generally char-
acterized by low adiabatic combustion temperature, cannot be
Interest in refractory metal silicides has increased signifi-
cantly in recent years because of their potential application as
high-temperature structural materials [1]. This class of materi-
als has an attractive combination of properties including high
melting temperature, high modulus, high oxidation resistance in
air, and a relatively low density [2,3]. However, as in the case
of many intermetallic compounds, the current concern about
these materials focuses on their low fracture toughness below the
ductile–brittle transition temperature [4–6]. To improve on their
mechanicalproperties, theapproachcommonlyutilizedhasbeen
the addition of a second phase to form composites [7–12]. An
example is the addition of Si3N4 to NbSi2 to improve the latter’s
properties. Silicon nitride has a high thermal shock resistance,
∗
Corresponding author. Tel.: +82 63 270 2381; fax: +82 63 270 2386.
E-mail address: ijshon@chonbuk.ac.kr (I.-J. Shon).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.07.065

H.-K. Park et al. / Journal of Alloys and Compounds 461 (2008) 560–564
561
Fig. 1. Temperature dependence of the Gibbs free energy for the interaction
between NbN and silicon.
synthesized directly by the self-propagating high-temperature
synthesis (SHS) method. More recently, a new approach has
been developed in which synthesis and densification can be
affected simultaneously. This new process, referred to as the
high-frequency induction-heated combustion synthesis (HFI-
HCS), has been successfully used to synthesize and densify
materials in one step in a relatively shorter period of time (2 min)
[24–26].
The objective of this study is to investigate the preparation
of dense nanophase 4NbSi2–Si3N4 composite by the HFIHCS
method starting from a mixture of mechanically activated NbN
and Si powders. The interaction between these phases, i.e.,
Fig. 2. Scanning electron microscope images of raw materials: (a) niobium
nitride and (b) silicon.
The FWHM of the milled powder is greater than that of the raw powders due
to internal strain and grain size reduction. The average grain sizes of the milled
NbN and Si powders were determined as 75 and 22 nm, respectively.
After milling, the mixed powders were placed in a graphite die (outside
diameter, 45 mm; inside diameter, 20 mm; height, 40 mm) and then introduced
into the high-frequency induction-heated combustion system [28]. Following
the introduction of the die into the apparatus, the system was evacuated and a
uniaxial pressure of 60 MPa was applied. An induced current (frequency of about
50 kHz) was then activated and maintained until densification was attained as
indicated by a linear gauge measuring the shrinkage of the sample. Temperatures
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. The process
was carried out under a vacuum of 40 mTorr.
The relative densities of the synthesized sample were measured by the
Archimedes method. Microstructural characterization was made on product
samples which had been polished and etched using a solution of HF (10 vol.%),
HNO3 (30 vol.%), and H2O (60 vol.%) for 1 min at room temperature. Compo-
sitional and microstructural analyses of the products were made through X-ray
diffraction (XRD) and scanning electron microscopy (SEM) with energy dis-
persive X-ray analysis (EDAX). Vickers hardness was measured by performing
indentations at a load of 10 kg and a dwell time of 15 s.
4NbN + 11Si → 4NbSi2 + Si3N4
(1)
is thermodynamically feasible, as can be seen from Fig. 1.
2. Experimental procedure
Powders of 99.97% pure niobium nitride (−325 mesh, Alfa Products, Ward
Hill, MA) and 99.5% pure silicon (−325 mesh, Alfa Products, Ward Hill, MA)
wereused asstarting materials. Fig. 2showstheSEMimagesoftherawmaterials
used. Powder mixtures of NbN and Si in the molar proportion of 4:11 were first
milled in a high-energy ball mill (Pulverisette-5, planetary mill) at 250 rpm for
10 h. Tungsten carbide balls (5 mm in diameter) were used in a sealed cylindrical
stainless steel vial under argon atmosphere. The weight ratio of ball-to-powder
was 30:1. Milling resulted in a significant reduction of grain size. The grain size
and the internal strain were calculated by Strokes and Wilson’s formula [27]:
kλ
b = bd + be =
d cos θ + 4ε tan θ
where b is the full width at half-maximum (FWHM) of the diffraction peak
after instrument correction, bd and be are FWHM caused by small grain size and
internal stress, respectively, k the constant (with a value of 0.9), λ the wavelength
of the X-ray radiation, d and ε are grain size and internal stress, respectively, and
θ is the Bragg angle. The parameters b and bs follow Cauchy’s form with the
relationship: B0 = b + bs, where B0 and bs are FWHM of the broadened Bragg
peaks and the standard sample’s Bragg peaks, respectively. Fig. 3 shows the
XRD patterns of the raw powders and the milled 4NbN + 11Si powder mixture.
3. Results and discussion
The variations in shrinkage displacement and temperature
with heating time during the processing of 4NbN + 11Si system

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H.-K. Park et al. / Journal of Alloys and Compounds 461 (2008) 560–564
Fig. 4. Variations of temperature and shrinkage displacement with heating time
during high-frequency induction-heated combustion synthesis and densification
of 4NbSi2–Si3N4 composite (under 60 MPa and 90% output of total power
capacity).
milling, (b) sample heated to 800 ◦C, and (c) sample heated to
1200 ◦C, respectively. Fig. 5(a and b) shows the presence of the
reactants as separate phases. X-ray diffraction results, shown
in Fig. 6(a and b) exhibit only peaks pertaining to the reac-
tants NbN and Si. However, when the temperature was raised
to 1200 ◦C, the starting powders reacted producing highly dense
products (Fig. 4(c)). SEM image of an etched surface of the sam-
ples heated to 1200 ◦C under a pressure of 60 MPa is shown in
Fig. 5(c). A complete reaction between NbN and Si took place
under these conditions. X-ray diffraction analyses of this sample
showed peaks of only NbSi2 and Si3N4, as indicated in Fig. 6(c).
The abrupt increase in the shrinkage displacement at the ignition
temperature is due to the increase in density as a result of molar
volume change associated with the formation of 4NbSi2–Si3N4
from the reactants (NbN and Si) and the 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 different than the values in the middle of the sample. Thus,
the onset of the reaction to form the composite (and the con-
comitant rapid shrinkage) may be at a higher temperature than
the observed value of 850 ◦C. As the imposition of an induced
electric field increases the combustion temperature for 4NbN
and 11Si system, it is expected that Si would be molten dur-
ing the continuation of the reaction. Once the liquid is formed,
its further formation and spreading over the NbN particles are
fast, due to surface energy. As soon as the liquid penetrates the
particles boundaries, a tremendous capillary force is developed
which leads to particle rearrangement and pore filling for closer
packing in the presence of Si liquid phase [29]. In this stage,
NbN continuously reacts with Si into NbSi2 and Si3N4. And
then grain growth and densification occur simultaneously under
high pressure. This is most likely the reason for the significant
Fig. 3. XRD patterns of raw materials: (a) NbN, (b) Si, and (c) milled
4NbN + 11Si.
are shown in Fig. 4. As the induced current was applied, the
reactant powders (Fig. 4(a)) shows initially a small (thermal)
expansion and the shrinkage displacement increased gradually
with temperature up to 800 ◦C (Fig. 4(b)), but then abruptly
increased at about 850 ◦C. When the reactant mixture of
4NbN + 11Si was heated under 60 MPa pressure to 800 ◦C, no
reaction took place and no significant shrinkage displacement
as judged by subsequent XRD and SEM analyses. Fig. 5(a–c)
shows the SEM (secondary electron) images of (a) powder after

H.-K. Park et al. / Journal of Alloys and Compounds 461 (2008) 560–564
563
Fig. 6. XRD patterns of the 4NbN + 11Si system: (a) after milling, (b) before
combustion synthesis, and (c) after combustion synthesis.
age of five measurements, of the NbSi2–Si3N4 composite is
680 kg mm−2. Indentations with large enough loads produced
median cracks around the indent. The length of these cracks
permits an estimation of the fracture toughness of the materials
by means of the expression [30]:
ꢀ ꢁ−3/2
c
KIC = 0.204
Hva1/2
a
where c is the trace length of the crack measured from the cen-
ter of the indentation, a the half of average length of two indent
diagonals, and Hv is the hardness. A typical indentation pattern
for the 4NbSi2–Si3N4 composite is shown in Fig. 7(a). Typi-
cally, one to three additional cracks were observed to propagate
from the indentation corner. The calculated fracture toughness
value for the NbSi2–Si3N4 composite is about 3.1 MPa m1/2 As
in the case of the hardness value, the toughness value is the
average of measurements on five measurements. Higher magni-
fication view of the indentation median crack in the composite
is shown in Fig. 7(b). This shows the crack propagates along
phase boundary of NbSi2 and Si3N4.
The absence of reported values for hardness and toughness
on NbSi2–Si3N4 precludes making direct comparison to the
results obtained in this work to show the influence of grain size.
Similarly, an absence of corresponding data on NbSi2 makes it
difficult to show the effect of the addition of Si3N4. The only
related study reported in the literature is that we are cognizant
by Guille and Matini [31]. They investigated the microhardness
and fracture toughness of silicide coatings on metal substrates.
Coatings of thicknesses ranging from 30 to 40 ␮m were formed.
In the case of Nb substrates, the coating was composed of a layer
NbSi2 only. It was found that the hardness of NbSi2 depended on
theappliedload, beingconstant, atavalueofabout10 GPa, inthe
load range of 0.15–0.6 N, but increased linearly at higher loads.
The calculated fracture toughness, however, was independent
of load, being about 2 MPa m1/2. Although a direct comparison
cannot be made, but the hardness of our composite is signifi-
cantly lower than the lowest value of about 10 GPa at 0.15–0.6 N
in the reported literature account results. Likewise, the frac-
Fig. 5. Scanning electron microscope images of 4NbN + 11Si system: (a) after
milling, (b) before combustion synthesis, and (c) after combustion synthesis.
increase in shrinkage beyond the value dictated by the molar vol-
ume change. The relative density of the product is about 98%.
This result is thought to be due to the formation of the liquid
phase during combustion synthesis, which can easily reduce the
porosity under mechanical pressure.
The average grain sizes of NbSi2 and Si3N4 calculated by
the Strokes–Wilson formula [27] were about 90 and 25 nm. The
Si3N4 particles were well distributed in matrix, as can bee seen
from the SEM image, Fig. 5(c).
Vickers hardness measurements were made on polished sec-
tions of the NbSi2–Si3N4 composite using a 10 kg load and 15 s
dwell time. The calculated hardness value, based on an aver-

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H.-K. Park et al. / Journal of Alloys and Compounds 461 (2008) 560–564
approximate comparison shows that the present results exhibit
a lower hardness and higher toughness.
Acknowledgments
We are grateful for the financial support from the Korea Insti-
tute of Science and Technology, which was provided through
the program for study on Technologies for Formation and Eval-
uation of Nanostructured Composite Coatings. The support of
the US Army Research Office (ARO) to one of us (ZAM) is
acknowledged.
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4. Summary
Using the high-frequency induction-heated combustion
method, the simultaneous synthesis and densification of nanos-
tructured 4NbSi2–SiC composite was accomplished using
powders of NbN and Si. Complete synthesis and densification
can be achieved in one step within 1 min. The relative density of
the composite was 98% under an applied pressure of 60 MPa and
the induced current. The average grain sizes of NbSi2 and Si3N4
phases in the composite were about 90 and 25 nm, respectively.
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