Full text of DOI:10.1557/JMR.2002.0295

Combustion synthesis of mechanically activated powders in
the Nb–Si system
Filippo Maglia, Chiara Milanese, and Umberto Anselmi-Tamburinia)
Department of Physical Chemistry and C.S.T.E./CNR, University of Pavia, V. le Taramelli 16,
I-27100 Pavia, Italy
Stefania Doppiu and Giorgio Cocco
Department of Chemistry, Via Vienna 2, I-07100 Sassari, Italy
(Received 28 January 2002; accepted 13 May 2002)
The effect of the mechanical activation of the reactants on the self-propagating
high-temperature synthesis (SHS) of niobium silicides was investigated. SHS
experiments were performed on reactant powder blends of composition Nb:Si ס 1:2
and Nb:Si ס 5:3 pretreated for selected milling times. A self-sustaining reaction could
be initiated when a sufficiently long milling time was employed. At short milling
times, the reactions self-extinguished or propagated in an unsteady mode. Combustion
peak temperature, wave velocity, and product composition were markedly influenced
by the length of the milling treatment. Single-phase products could be obtained for
sufficiently long milling times. Observation of microstructural evolution in quenched
reactions together with isothermal experiments allowed clarification of the mechanism
of the combustion process and the role played by the mechanical activation of the
reactants.
I. INTRODUCTION
synthesis through SHS resulted to be quite problematic.
Despite the fairly high adiabatic temperatures (1879 K
for NbSi2 and 2513 K for Nb5Si3), it has been necessary
to preheat the reacting mixtures at temperatures between
523 and 723 K to obtain a self-propagating reaction. Fur-
thermore, for both stoichiometries the final product con-
tained multiple phases. For the Nb:Si ס 1:2 mixture
NbSi2 and Si were identified, and for the Nb:Si ס 5:3
mixture NbSi2, ␣–Nb5Si3, ␤–Nb5Si3, and Nb were iden-
tified. Using the FACS method Gedevanishvily and Mu-
nir9 were able to produce a self-propagating reaction in
the Nb:Si ס 5:3 mixture imposing fields above a thresh-
old value of 7.4 V/cm. The mode of wave propagation
and the composition of the products depended largely on
the magnitude of the applied field. At low fields pulsat-
ing propagation was observed and the product was mul-
tiphase, while at higher fields the combustion was steady
and the product contained only the low and high tem-
perature modification of Nb5Si3. On the other hand,
FACS was ineffective in the synthesis of NbSi2 because
the mixture Nb:Si ס 1:2 shows a very low electrical
conductivity.
The excellent thermal and mechanical properties of
niobium silicides have resulted in them being considered
for high-temperature applications. For example, NbSi2
has been considered as a potential substitute for super-
alloys in high-temperature structural applications, as it
has one of the highest melting points among the silicides
and a density less than that of nickel-based superalloys.1
Nb5Si3/Nb has been reported to show excellent thermo-
chemical stability and resistance to coarsening up to
1500 °C,2,3 together with high bond strength and high
fracture toughness at room temperature.4,5 Due to their
high melting points, the synthesis of niobium silicides
has been attempted using nonconventional techniques,
including mechanical alloying (MA),6,7 self-propagating
high-temperature synthesis (SHS),8 and field-activated
combustion synthesis (FACS).9,10 Mechanical alloying
allows pure NbSi2 to be obtained from the elements. The
process, however, is not characterized by the gradual
transition from reactants to products typical of mechani-
cal alloying: a fast, spontaneous, combustion-like phe-
nomena occur after few hours of milling and drives
within few seconds to the final product. Surprisingly, the
Recently, it has been reported that combustion reac-
tions that are thermodynamically or kinetically hindered
can be activated if preceded by mechanical treatments of
the reactants through prolonged milling, a process re-
ferred to as mechanically activated SHS or MASHS.11–13
Mechanical activation of the reactants, in fact, has been
a)Address all correspondence to this author.
e-mail: tau@chifis.unipv.it
1992
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F. Maglia et al.: Combustion synthesis of mechanically activated powders in the Nb–Si system
shown to be able to influence the macrokinetic charac-
reported elsewhere.23 We just note that an impact energy
of approximately 0.1 J/hit was used, corresponding to a
reduced milling intensity of about 0.4 W/g.
teristics (reaction temperatures and propagation rates) of
the combustion reaction.14 Intensive or prolonged mill-
ing, however, results in some systems in a spontane-
ous ignition of the reacting mixture.15–20 This process
has been referred to as mechanically induced self-
propagating reaction, or MSR.21
The aim of this work is to further our understanding of
the fundamental aspects of the mechanical activated SHS
process as applied to the synthesis of intermetallic com-
pounds in the binary system Nb–Si.
The binary system Nb–Si contains three intermetallic
compounds: NbSi2, Nb5Si3, Nb3Si.22 Nb5Si3 has been
reported to exist in three different crystallographic forms,
a tetragonal, low-temperature form (␣–Nb5Si3) and a hexa-
gonal, high temperature form (␤–Nb5Si3), with a transi-
tion temperature lying between 1900 and 2100 °C. A
second tetragonal, carbon-stabilized form was also re-
ported. The heats of formation as well as the calculated
adiabatic combustion temperatures of formation from the
elements, Tad, are reported in Table I.9,22
After milling, powders were cold-pressed to form cy-
lindrical pellets 8 mm in diameter and 10 mm in height.
The pellets were then ignited inside a stainless steel re-
actor by using an electrically heated tungsten coil placed
1–2 mm away from the top of the sample. To ensure
identical ignition condition in all the experiments and to
reduce the ignition time and sample preheating, 0.5 g of
thermite mixture (Zr + Fe2O3) was pressed above the re-
acting pellet. The thermite mixture ignites almost instan-
taneously and in most cases favors the formation of a flat
and even reaction front inside the Nb–Si mixture. All
experiments were performed in a high-purity argon
(99.998%) atmosphere. Temperature profiles were meas-
ured with a two-color pyrometer, with a resolution time
of 0.01 s, focused on the middle of the sample, while the
propagation rates were determined from video recordings
of the reaction front.
Structural and microstructural characterization of re-
actants and products were made by optical microscopy,
scanning electron microscopy (SEM), an energy disper-
sion electron microprobe (EDS), and powder x-ray dif-
fraction (XRPD). A Zeiss Axioplan optical microscope
(Oberkochen, Germany) and a Cambridge SEM Stereo-
scan 200 (Cambridge, United Kingdom) operated at
30 kV and equipped with a back-scattered electron de-
tector and a Link microprobe were used. XRPD analysis
was performed using a diffractometer Philips 1710
(Eindhoven, The Netherlands) equipped with a copper
anode (operated at 40 kV and 35 mA), graphite-curved
monochromator on the diffracted beam, and a propor-
tional counter.
Bulk diffusion couples were prepared using circular
wafers of Nb and Si with a diameter of 5 mm and a
thickness of 1–2 mm. Prior to assembly, the mating sur-
faces were prepared by polishing first with silicon car-
bide abrasive paper and then with 1-␮m diamond paste
and finally rinsed with acetone. The cleaned Nb and Si
wafers were then pressed tightly together between spring
loaded alumina rods inside an alumina tube, and the re-
sulting assembly was placed in a furnace under a high-
purity argon flux (99.998%). The diffusion couples were
held at temperatures ranging from 1200 to 1350 °C for
times ranging from 2 to 16 h. The temperature was meas-
ured with a thermocouple in contact with a thin alumina
layer (1 mm) placed behind the Si slice. After cooling,
the samples were embedded in epoxy resin, sectioned,
polished, and examined by optical and electron micros-
copy. The composition of the reacted zone was deter-
mined by EDS.
II. EXPERIMENTAL MATERIALS AND METHODS
Appropriate amounts of niobium and silicon powders
were milled in a hardened steel vial using a Spex Mixer-
Mill (Spex Certiprep, Metuchen, NJ) (model 8000)
equipped with a 230 V–15 Hz motor operating at
1425 rpm, corresponding (in its standard configuration)
to 875 cycles/min of the vial. One hardened steel grind-
ing ball of 13.7 g and 1.5 cm in diameter and constant
powder batches of 8 g were used. Milling runs were car-
ried out in continuous mode. The vial temperature was
continuously monitored during milling runs with a thin
lamella-shaped Pt-resistance thermometer fixed on the
external side of the vial. Samples were prepared using
−325 mesh (<45 ␮m) silicon powder of 99.5% purity
and −325 mesh niobium powder of 99.8% purity sup-
plied by Alfa (Karlsruhe, Germany). Powder handling
and milling were done under an atmosphere of highly
purified argon in a glovebox. The levels of the residual
humidity, oxygen, and nitrogen are controlled to the ppm
level. More details concerning powder treatment are
TABLE I. Crystal structures, enthalpies of formation, and adiabatic
combustion temperatures for the silicides of niobium.
Crystal
structure
⌬Hf,298
Compounds
(kJ/mol of atoms)16
Tad (K), T0 ס 298
NbSi2
␣–Nb5Si3
␤–Nb5Si3
P6222
−46.0
−60.7
−60.7
−60.7
1879
2513
2513
2513
I4/mcm
P63/mcm
I4/mcm
a
C-Nb5Si3
Interactions between Nb solid and Si liquid were stud-
ied by immersion tests performed in isothermal condi-
tions at temperatures ranging between 1450 and 1600 °C
aTetragonal metastable form. Stabilized by the presence of small amounts
of C, O, and N.
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F. Maglia et al.: Combustion synthesis of mechanically activated powders in the Nb–Si system
for times ranging between 2 and 16 min. A 0.65-g
amount of Si powders was melted in an alumina crucible
(10 mm in diameter), under a pure argon atmosphere,
inside a graphite resistance furnace. The temperature was
measured with a thermocouple in contact with the bottom
surface of the crucible. The Nb sample, in the form of a
2.5-mm rod, was attached to a sample holder fixed at the
top of the furnace, its long axis parallel to the crucible axis.
Once the desired temperature was attained, the Nb sample
was dipped into the molten Si. At the end of the reaction
time, the system was air-quenched by quickly pushing the
crucible outside the hot zone of the furnace. More details
about the experimental setup are reported elsewhere.24
III. RESULTS
A. Mechanically activated self-propagating
high-temperature synthesis (MASHS)
MASHS experiments were performed on samples with
stoichiometries corresponding to Nb:Si ס 1:2 and
Nb:Si ס 5:3. Both compositions undergo spontaneous
combustion during the milling process. The occurrence
of the mechanically induced self-propagating reaction is
observed well before the appearance of any product
phase in the milled powders. The corresponding milling
times (hereafter tig) resulted to be equal to approximately
553 min for the Nb:Si ס 1:2 composition and approxi-
mately 295 min for the Nb:Si ס 5:3 composition.
Upon milling, the powder morphology changed con-
siderably. An example of these modifications is shown in
Fig. 1. At the beginning of the milling process
[Fig. 1(a)], the reactant powders appear as irregular par-
ticles several micrometers in diameter. Upon milling the
evolution of the microstructure appears to be controlled
by the elastic and mechanical properties of the reactants
[Figs. 1(b) and 1(c)]. The ductile Nb is, in fact, promptly
deformed to give lamellar particles [Fig. 1(b)], while the
brittleness of silicon leads to a refinement of particles
mainly by fracture processes. The fine Si particles result
to be packed between the metallic Nb lamellae. At longer
milling times the lamellae width is reduced to a value of
few micrometers and the mixture appears more homoge-
neous when observed at low magnification [Fig. 1(c)].
The milled mixture appears in the form of round shaped
agglomerates of diameter up to several hundreds of mi-
crometers made of tightly packed reactants. The dimen-
sions of the coherent diffraction domains, ꢀLhklꢁ, which
express the subgrain sizes, and the corresponding strain
content, ꢀ⑀ꢁ, of niobium and silicon in the course of the
alloying process were assessed from the line profile analy-
sis of the x-ray diffraction patterns by resorting to Rietveld
refinement methods. Data are collected in Table II as a
function of the milling time. ꢀLhklꢁ for silicon, decreasing by
almost 1 order of magnitude, highlights its higher brittle-
ness, as it is also apparent from the distinguishing ꢀ⑀ꢁ trend.
FIG. 1. Scanning electron micrographs of Nb:Si ס 1:2 powder par-
ticles undergoing ball milling: (a) after 60 min of milling; (b) after
180 min of milling; (c) after 540 min of milling. The SEM backscat-
tered image is on the polished section.
The SHS experiments were performed on samples
which underwent increasing milling periods up to the
self-ignition time. The macrokinetic characteristics of
the combustion process (combustion mode, temperature
profile, and propagation rate) were found to be largely
dependent on the extent of the milling time for both
compositions (Figs. 2 and 3). When unmilled powders of
1994
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F. Maglia et al.: Combustion synthesis of mechanically activated powders in the Nb–Si system
composition Nb:Si ס 1:2 were employed, a pulsed front
the Si-rich eutectic melting point (1385 °C).22 When pre-
treated powders were employed, the Nb:Si ס 1:2 blends
produced stable, stationary fronts whose peak tempera-
ture and propagation rates increased monotonically with
milling time.
that self-extinguished before reaching the opposite end
of the sample was observed. The measured peak tem-
perature was intermediate between the Si (1414 °C) and
Similar results were found for the composition
Nb:Si ס 5:3 (Fig. 3). In this case, the ignition of un-
milled powders is even more difficult and the front ex-
tinction occurs almost immediately, when the front
moves outside the zone heated by the ignition coil. Un-
stable propagation (either pulsating or spin mode) was
observed for milling times up to 120 min. However,
when powders milled for longer times are used, the
propagation of the combustion front becomes stationary.
Also for this composition, an increase in peak combus-
tion temperature and propagation rates was observed as a
function of milling time (hereafter m.t.). It must be no-
ticed however that, at long milling times, the peak tem-
perature stabilizes at a value around 1900 °C, while at
m.t. ס 80 min the temperature is just above the Si melt-
ing point. Also for this composition the front velocity
increases more than 1 order of magnitude as an effect of
ball-milling treatments.
TABLE II. Effect of milling on lattice parameters, crystallite size, and
strain.
Milling time
(min)
ꢀLhkl Nb
(nm)
ꢁ
ꢀLhkl Si
(nm)
ꢁ
aNb
⑀
aSi
⑀
Si
Nb
Nb:Si ס 1:2
0
60
180
360
540
3.299
3.302
3.302
3.303
3.306
42
4
2
2
2
1
0.0017 5.424 136 14
3E − 5
1.0E − 4
1.1E − 4
1.5E − 4
3.3E − 4
19
18
17
13
0.0063 5.427
0.0075 5.428
0.0077 5.429
0.0081 5.431
45
31
20
11
4
3
2
1
Nb:Si ס 5:3
2
80
120
240
285
3.300
3.301
3.302
3.302
3.303
53
18
17
15
11
5
2
2
1
1
8E − 4 5.426 112 11
4E − 5
1.0E − 4
1.6E − 4
9.5E − 4
0.0036
0.0063 5.426
0.0065 5.426
0.0061 5.424
0.0065 5.423
45
28
12
12
4
3
1
1
For both compositions not only the peak temperature
but also the shape of the entire temperature profile was
dependent on m.t. Figures 4 and 5 show the temperature
profiles recorded during the front propagation, with the
pyrometer spot focused on the middle portion of the sam-
ple. As the leading edge of the combustion front enters
FIG. 2. Dependence of the maximum (peak) temperature (filled sym-
bols) and wave velocity (hollow symbols) on milling time for the
composition Nb:Si ס 1:2.
FIG. 3. Dependence of the maximum (peak) temperature (filled sym-
bols) and wave velocity (hollow symbols) on milling time for the
composition Nb:Si ס 5:3.
FIG. 4. Temperature profiles for the composition Nb:Si ס 1:2, for the
following milling times (min): (a) 2; (b) 60; (c) 180; (d) 360; (e) 540.
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F. Maglia et al.: Combustion synthesis of mechanically activated powders in the Nb–Si system
FIG. 5. Temperature profiles for the composition Nb:Si ס 5:3, for the
following milling times (min): (a) 80; (b) 120; (c) 240; (d) 285.
the spot area (1 mm in diameter) an abrupt increase in
temperature is detected. For sake of comparison all the
profiles have being aligned as to have the leading edge of
the combustion front entering the measure spot at the
same time lag (1 s). The temperature profiles at short m.t.
appear broader, and a second large peak is present at
temperatures slightly lower than the first one. As m.t. is
increased, the profile sharpens and the second peak oc-
curs at shorter time lags and finally disappears, giving,
for the longest milling times, a very sharp single peak
profile. These results suggest that the time required to
complete the reaction decreases when finer powders with
enlarged surface contact area are employed. This is con-
firmed in the composition of the products which is pre-
sented later.
The final products show a microstructure character-
ized by a fairly large porosity (65–70%) regardless of
m.t. This is a common feature of SHS processes, deriv-
ing in part from the original porosity of the green pellet
and in part from other sources active during the process
itself,25 like decrease in molar volume, gas evolution due
to the expulsion of volatile impurities, and thermal mi-
gration resulting from the extremely steep thermal gra-
dients. However, pore size and distribution change
considerably with m.t. As can be observed in Fig. 6(a) at
low milling times, the pores appear smaller and more
uniformly distributed. For long milling times [Fig. 6(b)],
FIG. 6. Microstructure of the end product corresponding to the start-
ing composition Nb:Si ס 1:2: (a) m.t. ס 2; (b) m.t. ס 360 min. The
SEM backscattered image is on the polished section.
the products preserve the microstructure characterized by
large compact blocks already present in the reacting
powders and developed during the milling process.
The composition of the reaction products and the rela-
tive amounts (Table III) were evaluated by Rietveld
refinement analysis26,27 of the relevant x-ray diffrac-
tion data. In agreement with literature, the composition
Nb:Si ס 1:2 produced, whatever the m.t., NbSi2 plus
traces of the reactants whose quantity decreases with the
increase in the milling time. At the longest m.t., a small
amount of Nb5Si3 appears in the products, probably be-
cause of Si losses due to evaporation.
The composition Nb:Si ס 5:3 gave, at short milling
times, products characterized by a large amount of unre-
acted Nb and NbSi2. The scarce conversion degree is the
obvious main reason for the low tendency, showed by
this composition, to sustain the combustion wave. The
amount of Nb and NbSi2 in final products decreases con-
siderably at m.t. equal to 2 h, and both disappear com-
pletely in the samples characterized by the two longest
1996
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F. Maglia et al.: Combustion synthesis of mechanically activated powders in the Nb–Si system
m.t. All three crystallographic forms of Nb5Si3 have been
(dotted line). In the region ahead of the front limit, only
grains of the reactants can be discerned and no indication
of any product formation can be detected. At the front
position, the dark Si grains disappear due to their melt-
ing. The molten Si spreads around the Nb grains leading
to the formation of NbSi2. A part of the product surrounds
the residual Nb grains, while most of it is present in the
form of small precipitates. It must be noted that, at least
for this short m.t., the reaction is not complete at the front
but a considerable amount of Nb, in the form of the larger
grains core, is present well behind the front zone. This is
in agreement with the broad temperature profiles previ-
ously shown.
found including the one usually referred as “carbon sta-
bilized”, (hereafter C-Nb5Si3). This phase, however, can
also be stress stabilized, which is likely to occur in this
case, due to the rapid cooling experienced by SHS
samples. However, increasing the milling time increases
the amount of the ␤–Nb5Si3 and of the “carbon-
stabilized” form with respect to the ␣–Nb5Si3 one.
The SEM analysis of the samples characterized by the
shortest m.t. which underwent spontaneous interruption
of the wave propagation allowed observation of the
phase evolution accompanying the wave propagation and
hence of the reaction mechanism. Figure 7 shows a SEM
image of the zone across the extinct front for a sample of
composition Nb:Si ס 1:2 (m.t. ס 2 min). The arrow in-
dicates the direction of propagation of the combustion
front. The sharp change in the microstructure corre-
sponds to the position where the front extinguished
B. Diffusion couples
The solid-state diffusion couple experiments showed
that the interaction between Nb and Si always produces
a double-layer product, as can be seen in Fig. 8. At the
Si-rich side, NbSi2 is formed. This phase was always
the most conspicuous, and its thickness increases para-
bolically with time.24 At the Nb-rich side, a thin Nb5Si3
layer is formed. Also Nb5Si3 grows parabolically with an
integrated diffusion coefficient of more than 1 order of
magnitude smaller than the NbSi2 one in the explored
temperature range (1200–1350 °C). Figure 9 shows the
microstructure resulting from the interaction between Nb
and Si when the temperature is above the melting point of
Si. The product is similar to the one obtained in solid-
solid diffusion couples but with one major difference.
The NbSi2 layer appears to be made out of crystals dis-
persed inside the silicon matrix. However, the dissolution
kinetics obtained from these solid–liquid experiments is
very different from the one observed with other sili-
cides.13,28–30 In this case, in fact, the diffusion of Nb in
Si liquid appears to be extremely slow. A detailed de-
scription of the isothermal reactivity between Nb and Si
is reported elsewhere.24
TABLE III. Effect of milling on product composition.
Milling
time
Product composition (%)
(min) NbSi2
Nb
Si
␣–Nb5Si3 ␤–Nb5Si3 C-Nb5Si3
Nb:Si ס 1:2
0
60
180
360
540
97.76
99.91
99.79
99.77
94.35
0.75
1.48
0
0
0
0
0
0
0
0
0
0
0
0
0.088
0.203
0.25
0
0
0
0
0.163
0.463
0.16
4.85
Nb:Si ס 5:3
0
80
120
240
285
35.02 32.6
15.89 18.06
1.47
8.87E − 4
0
0
0
25.82
41.03
75.41
72.97
48.06
2.81
13.89
13.56
9.32
2.25
11.1
11.02
17.7
28.58
0
0
0
0
0
0
23.34
FIG. 7. Spatial evolution of the microstructure during the formation of
NbSi2: zone a, reactants; zone b, products. The dotted line indicates the
position of the extinguished front. The SEM backscattered image is on
the polished section.
FIG. 8. Backscattered SEM image of a Nb–Si diffusion couple.
T ס 1350 °C, and t ס 16 h. The SEM backscattered image is on the
polished section.
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F. Maglia et al.: Combustion synthesis of mechanically activated powders in the Nb–Si system
starts in the early stages of the interaction. The limited
solubility of Nb solid in Si liquid plays an important role
in reducing the reactivity and in hindering the combus-
tion reaction in this system. In this regard, two main
effects must be considered. First of all, the limited solu-
bility reduces the rate of Nb dissolution in Si. As shown
in detail elsewhere,24 the kinetics of the dissolution proc-
ess of refractory transition metals in liquid silicon de-
pends strongly on their solubilities at temperatures in the
range 1400–1650 °C. Second, and more importantly, it
must be remembered that both NbSi2 and Nb5Si3 melting
points are far higher than combustion temperatures ob-
served in our experiments and once produced they can
only be present as solid phases. This requires that the
much slower solid state reactivity must be involved to
sustain the combustion process. This feature is much
more relevant in the case of the composition Nb:Si ס
5:3, where the small amount of Si liquid present at the
combustion leading edge is rapidly converted into solid
NbSi2 representing the primary product of the interaction
between Nb solid and Si liquid. Any further advancement
of the combustion process requires, per force, the slow
solid-state reaction between the primarily formed NbSi2
and the residual Nb to obtain any further advancement
of the reaction. This explains the well-known difficulty of
obtaining this phase through combustion, despite its con-
siderable heat of formation. A similar situation is ob-
served in the Ta–Si system.32 This is in contrast with
other M–Si systems like Ti–Si, Cr–Si, V–Si, Zr–Si, etc.,
for which a very efficient dissolution–precipitation proc-
ess is observed.17,27–30 The dissolution process is favored
by the large solubility of the corresponding transition
metals in melted Si (in all these three systems at 1800 °C
complete solubility between the metal and silicon is re-
ported)21 and by the relatively lower melting point of
some phases present in these systems. A typical example
is represented by the process that leads to the formation
of Ti5Si3. Despite the fact that the pertinent adiabatic
temperatures are similar to the ones involved in the for-
mation of Nb silicides, the reaction between Ti and Si
generates combustion waves that are stable and ex-
tremely fast even when unmilled powders are used. If we
compare the phase diagrams of the two systems, we can
observe that not only the solubility of Ti in Si is higher
but also the melting point of the primary formed TiSi2 is
lower than the measured combustion temperatures. As a
result, no solid products are present at the combustion
front and appear only in regions behind the front limit.
Within this framework, the mechanical activation step
plays a fundamental role in increasing the surface contact
area of the powders and decreasing the critical diffu-
sional lengths required for completion of the solid-state
reaction, thereby enhancing the overall reactivity of the
Nb–Si system. The effects become apparent when one
considers the dependence of the combustion temperature
FIG. 9. Microstructure of the product obtained from solid Nb and
liquid Si isothermal reaction. T ס 1600 °C, and t ס 2.5 min. The
SEM backscattered image is on the polished section.
IV. DISCUSSION
The main result of this work is represented by the
demonstrated possibility to obtain both NbSi2 and Nb5Si3
through combustion processes when mechanical activa-
tion of the reactant powders is used. It is worth remem-
bering that the synthesis through combustion of both
phases resulted to be problematic using classical SHS,
and none of the two silicides could be obtained without
preheating, despite the relatively large heat of formation
associated with these compounds. Mechanical activa-
tion showed a significant influence on both combustion
front temperature and propagation rate. Moreover, at suf-
ficiently long m.t., the end products are almost com-
pletely free of secondary phases, while it was impossible
to obtain pure products using classical SHS coupled with
a preheating of reactants.8 Gedevanishvili and Munir9
obtained pure Nb5Si3 using the FACS method, but any
attempt to obtain NbSi2 was unsuccessful due to the poor
electrical conductivity of the reacting mixture.
Many of the experimental results reported in this study
can be interpreted on the basis of the characteristic of the
Nb–Si phase diagram and of the behavior shown by this
system when isothermal experiments of reactive diffusion be-
low and above the Si melting point are performed. On this
basis, a coherent microscopic mechanism can be suggested.
As observed in Fig. 7 the formation of NbSi2 at the
leading edge of the reaction front is due to a dissolution
precipitation process.31 This mechanism has been al-
ready recognized to be responsible for the formation of
many other MSi2 phases in the combustion regime
(ZrSi2,28 CrSi2,29 VSi2,30 TiSi2,13,31 and TaSi232). In this
process, following the melting of silicon, the NbSi2 phase
forms as a result of the Nb dissolution in melted Si simi-
lar to that observed in isothermal experiments of inter-
action between Nb and melted Si (Fig. 9). However,
since the Nb solubility in melted Si is limited (at 1800 °C
is only of 15 mol%), the precipitation of the NbSi2 phase
1998
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F. Maglia et al.: Combustion synthesis of mechanically activated powders in the Nb–Si system
and of the front velocity on milling time. In Figs. 2 and 3,
completion of the reaction. Such an effect plays a funda-
mental role in the Nb–Si reactivity due to the slow kinetics
of interdiffusion between the solid elements and the slow
dissolution kinetics of niobium into melted silicon.
a temperature rise of several hundreds degrees is marked
together with an increase of more than 1 order of mag-
nitude in the wave velocity. The rising course of the
maximum combustion temperature relates to an intensi-
fied and faster release of the reaction heat, as clearly
indicated by the sequence of temperature profiles de-
picted in Fig. 5. Progress in m.t. produces sharper tem-
perature traces, the heat release ending in shorter times as
a result of the more rapid microkinetics. Furthermore, the
increase in the heat release produces a higher peak tem-
perature due to the heat evolution being faster than the
dissipation by conduction at the reaction front. Beside
that, microstructure refinement produces higher degrees
of chemical conversion at the front leading edge, also
increasing the amount of heat release.
A confirmation of the critical role played by particle
size on the stability of the combustion front can be found
in the literature relative to the combustion behavior of
thin multilayers. In fact, it has been reported that layered
systems obtained by vacuum or sputter deposition are
able to produce self-propagating combustion processes,
whose characteristics depend strongly on layer thick-
nesses.33 Recently, Reiss et al.34 have shown, as in the
case of the Nb–Si system, multilayers can produce com-
bustion reaction characterized by very high propagation
rates (1–4 m s−1) for layer thickness in the range of
tenths of nanometers. A fourfold increase in propagation
rate has been observed when the thickness of each bilayer
decrease from 310 to 50 nm, due to the decrease in dif-
fusion path required for the reaction. The same mecha-
nism can be considered responsible for the behavior in
milled samples as discussed previously, although the
much more complex microstructure does not allow a di-
rect quantitative comparison.
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V. SUMMARY
Previously reported investigations on the synthesis of
niobium silicides by SHS have shown that, despite the high
reaction heats involved, a self-sustaining process can be
initiated only by preheating the reactants or applying an
electric field, the latter method being limited to the
Nb:Si ס 5:3 composition. Only the field activation leads to
a product with uniform composition, while by preheating
the reactants, multiphase products were obtained.
In this work, the reactivity in the system Nb–Si was
investigated as a function of milling. By prolonged milling
of the reactants’ mixtures, a self-propagating combustion
reaction could be initiated for both compositions without
the need of preheating. Moreover, pure products were ob-
tained for both compositions when sufficiently long mill-
ing treatments were employed. The enhancement of the
reactivity in the Nb–Si system was shown to be determined
by the increased surface contact area of the powders and
the decreased critical diffusional lengths required for
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J. Mater. Res., Vol. 17, No. 8, Aug 2002
1999
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