CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
EFFECT OF SHOCK-ACTIVATION ON POST-SHOCK REACTION
SYNTHESIS OF TERNARY CERAMICS
Jennifer L. Jordan and Naresh N. Thadhani
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245
Abstract. The effects of shock-compression of powder precursors on subsequent reaction synthesis
and formation of Ti3SiC2 and Ti2AlN ternary ceramics were investigated in this study. Mixtures of the
powder precursors were shock-densified at different pressures using an 80-mm diameter gas gun and
the double tube cylindrical implosion technique. Characterization of the shock-densified compacts
showed an intimately mixed state of powders with high retained strain in unreacted compacts and little
or no reaction at low pressures. The high pressure compact showed formation of non-stoichiometric
TiC. The subsequent reaction behavior of the shock-densified compacts resulting in the formation of
Ti3SiC2 was studied via heat treatments and differential thermal analysis (DTA). A non-stoichiometric
TiCx phase was observed as an intermediate phase prior to the formation of Ti3SiC2. This paper will
present the results of the effects of shock compression on the the reaction mechanisms and kinetics of
reactions leading to the formation of the ternary ceramics in the shock-densified precursor powders.
INTRODUCTION
the solid state, has been found to be essential for the
synthesis of pure Ti3SiC2 and Ti2AlN [1,2].
The rationale for the proposed work was
therefore to use shock compression to activate
powder precursors for subsequent reactions
occurring in the solid state and with activated
kinetics. Hence, in this work, the reaction behavior
of precursor powders was investigated by
characterizing the as shock-compacted state of
powder mixtures and determining their reaction
kinetics. The high strain rate deformation behavior
of the ceramics is currently being studied and will
be presented in a later publication.
Ti-based complex ternary ceramics called 312
and H-phases, including Ti3SiC2 and Ti2AlN, are
unique materials having high stiffness (-320 GPa
for Ti3SiC2) but low hardness (4.5 GPa for Ti3SiC2
and 4.3 GPa for Ti2AlN) [1,2]. They possess metallike properties including electrical conductivity,
thermal conductivity, and easy machinability, while
demonstrating oxidation resistance, refractory
behavior, and insusceptibility to thermal shock,
typical of ceramics. Recent equation of state
studies on Ti3SiC2 have shown that its bulk
modulus is ~ 64% of the elastic modulus [3].
Hence, while its Young's modulus is similar to
molybdenum metal, it is more compressible than
Mo, but less than Si and oc-Ti [3].
The
compressibility and deformation response of these
ternary ceramics makes them interesting candidates
for damage tolerant armor applications.
Barsoum, et al. [1,4] have developed a hot
pressing process for producing pure, bulk Ti3SiC2
starting with a mixture of Ti, SiC, and graphite. A
controlled rate of heating, which ensures reaction in
EXPERIMENTAL PROCEDURE
The precursor powders used for shock
densification and subsequent reaction synthesis for
Ti3SiC2 were titanium (Alfa Aesar), silicon carbide
(Superior Graphite Company and Performance
Ceramics Company), and graphite (Cerac, Inc. and
Aldrich Chemical Company). One batch of the
precursor powder ("as blended") was prepared by
1097
TABLE 1. Characteristics of recovered shock
compressed Ti + SiC + Graphite samples before and after
reaction synthesis
10 °C/min to
Sample
As Shocked
1600 °C
Hold 4 hours
As Blended Powder N/A
Ti3SiC2
TiC (0.430)*
combining Ti, SiC, and graphite in the
stoichiometric ratio and mixing in a V blender. The
other batch of precursor powders was prepared by
ball milling small quantities Ti and SiC powders for
2 hours in a Spex mill ("ball milled") or roller
milling larger quantities for 6 hours ("roller
milled"). Graphite was then added to both ball and
roller milled mixtures, and the final mixing of the
powders was performed using a V-blender which
was run overnight.
The precursor powders used for shock
densification and reaction synthesis for Ti2AlN
formation were titanium (-325 mesh, Alfa Aesar)
and A1N (Alfa Aesar). The powders were blended
in a V blender overnight.
The shock compression experiments were
performed using a three capsule recovery fixture
with the single-stage 80-mm diameter gas gun at
Georgia Tech and the double-tube cylindrical
implosion fixtures at the Energetic Materials
Research and Testing Center in Socorro, NM. For
the three capsule fixture, the powders were pressed
in the steel capsules at -65% theoretical maximum
density and shock compressed at calculated peak
pressures of 5 and 9 GPa. For the double-tube
cylindrical implosion experiments, powders were
packed to -55% TMD in 1 inch diameter tubes.
The powder containment fixtures were placed in 6
inch diameter PVC, which was packed with ANFO
or ANFOIL explosive.
The corresponding
calculated maximum peak pressures are ~ 4 and 6
GPa, respectively.
590 m/s , ~ 5 GPa** Reactants (Ti,
TiC (0.431)*
As Blended
SiC, and graphite)
870 m/s , 9 GPa** Reactants (Ti,
TiC (0.433)*
As Blended
SiC, and graphite)
and TiCx (0.432)*
As Ball Milled
TiC (0.428)*
Ti3SiC2
N/A
Implosion Cylinder Reactants (Ti,
4 GPa**
SiC, graphite)
Rolling Mill 6 h
870 m/s, 9 GPa** TiC (0.430)*
Ball Milled 2 h
Ti3SiC2
TiC (0.432)*
Ti3SiC2
TiC (0.429)*
** Pressures calculated from Autodyn-2D [5]; * lattice
parameter in parenthesis
retained strain in the shock densified compacts is of
the same order of magnitude as the powders ball
milled in a Spex or roller mill. At higher pressures
(~9 GPa), the recovered powder compact of the as
blended Ti + SiC + graphite powder showed
evidence of partial reaction, forming TiCx, while
the compact of the ball milled precursor showed
almost complete reaction, forming TiC. For the
reaction product observed in the reacted compact,
the TiC has a lattice parameter of 0.430 nm
corresponding to a non-stoichiometric TiCx phase.
No reaction was observed in either the low (~ 5
GPa) or higher (9 GPa) pressure experiments in the
case of Ti + A1N precursors.
RESULTS AND DISCUSSION
Table 1 summarizes the results of the compacts
in the as shocked state and that following reaction
synthesis of shocked precursors for Ti3SiC2
experiments.
Reaction Behavior of Shock-Densified Compacts
Titanium - Silicon Carbide, ThSiC?
Shock Densified State
The as blended and as Spex milled powders
and sections of the recovered, shock densified
compacts were heat treated in a tube furnace to
1600 °C at 10 °C/minute, with a hold time of four
hours, and subsequently characterized by XRD
analysis. Reaction synthesis of the as blended,
shock densified precursor powder compacts showed
formation of TiC with a lattice parameter the same
The recovered shock compressed compacts
from low pressure (< 6 GPa) experiments showed
retention of reactants in both Ti + SiC + graphite
mixtures. XRD line broadening analysis showed
extensive residual microstrain (e ~ 10~2) retained in
all of the precursor powders. The magnitude of
1098
as that of the stoichiometric compound for the 9
GPa sample and non-stoichiometric compound for
the 5 GPa sample. No Ti3SiC2 phase was observed
to be found in either shocked sample. Formation of
the stoichiometric TiC phase appears to be due to a
self-sustained SHS-type combustion reaction in this
as blended powder mixture, which in turn inhibits
the formation of the ternary carbide phase.
Reaction synthesis of the unshocked, as milled
powder and the Spex and roller milled, shock
densified powder compacts showed the formation
of both Ti3SiC2 and TiC products. The TiC formed
in conjunction has a lattice parameter less than that
of the stoichiometric (0.433 nm) value.
Thus, while reaction synthesis of the as
blended, shock densified compacts reveals a
tendency to form stoichiometric TiC and no ternary
carbide, the milled and shock densified compacts
yield the ternary phase along with TiC.
Furthermore, the non-stoichiometric TiCx phase
formed during the reaction synthesis of milled,
shocked compacts appears to be a TiC - Si solid
solution having possibly formed by silicon
diffusion into TiC. The TiC - Si solid solution
could be an intermediate phase prior to the
formation of the ternary phase. The intimate
mixing during milling and the dense packed, highly
activated state attained during shock compaction,
appear to aid the solid state diffusion of carbon into
titanium and subsequently silicon into TiCx, thereby
resulting in Ti3SiC2 formation.
To further evaluate the effect of shock
compression on reaction synthesis and formation of
Ti3SiC2, kinetic studies were conducted to
determine the activation energy for reaction by
heating the powders in a DTA. At low heating rates
(10 °C/min), a single broad DTA peak was
observed characteristic of a solid state diffusion
reaction. At higher heating rates (40 °C/min), two
DTA peaks became obvious - a low temperature
peak corresponding to solid state diffusion and a
higher temperature peak from an SHS reaction,
which initiates as the rate of heat release exceeds
the rate of heat dissipation. Hence, depending on
the activation induced by the shock compression
process, changes in both the peak reaction
temperature and the degree of reaction by solid
state and SHS mechanisms were manifested by the
exotherms observed in the DTA traces. The peak
400
350-300
250E 200-
J
150-
Temperature (C)
FIGURE 1. Sample DTA trace, from the Ti + SiC
+ graphite compact showing one peak at low
heating rates and two peaks at high heating rates.
temperatures corresponding to the solid state
reaction ranged from 700 °C - 920 °C in as blended,
680 °C - 930 °C in as ball milled and 800 °C - 1024
°C in the 4 GPa imploding cylinder sample. The
corresponding peak temperature for the selfsustained combustion type reaction remained
constant over a much narrower range of 1320 °C 1485 °C in all samples. The 9 GPa shocked sample,
which contained TiC product that had formed
during shock compression, showed further solid
state reaction in the temperature range of 720 °C 1010 °C, but with no subsequent combustion
reaction.
The modified Kissinger method [6, 7] was used
to determine the activation energy from the peak
temperature (Tt) of the solid state reaction exotherm
obtained from the DTA traces. The activation
energy, E, is obtained from the slope of the linear
plot of heating rate (<))) over peak temperature
according to the following equation:
where R is the gas constant. The results illustrate
that the activation energy of the solid state reaction
decreases from 80 kJ/mole for the as blended state
to 71 kJ/mole for the ball milled state, and 56
kJ/mole in the case of the powder shock
compressed using the cylindrical implosion
geometry, and 68 kJ/mole for the 9 GPa gas gun
sample.
1099
Titanium-aluminum Nitride, Ti?AlN
activates powder precursors and promotes the
formation of the Ti3SiC2 phase via solid state
diffusion.
Shock activated reaction synthesis of Ti + A1N
also showed the formation of Ti2AlN. However,
the amount of phase formed decreased with
increasing shock pressure indicating that there is an
optimum window of shock compression pressure in
which shock activation can be beneficially used for
the formation of Ti2AlN.
In general, the results of the present work on
reaction synthesis of shock densified powder
precursors illustrate that shock compression does
activate the powder precursors thereby favoring the
formation of Ti3SiC2 and Ti2AlN ternary ceramics.
However, formation of the ternary phases is not
complete with the reaction treatments investigated.
In most cases, formation of TiC (in the case of the
ternary carbide) or TiN (in the case of the ternary
nitride) by combustion-type reactions inhibits the
completion of reaction via solid-state mechanisms.
Modeling of the reaction behavior of shockdensifled precursor powders will be conducted to
predict the reaction treatment coupled with the
degree of shock activation desired to ensure
complete formation of the ternary phases by solid
state diffusion reactions.
Reaction synthesis experiments on Ti + A1N
have also been performed using a heat treatment
similar to that used for the formation of Ti3SiC2.
Reaction heat treatment of the 4 GPa samples
showed higher XRD peak intensities of Ti2AlN
phase compared to the as blended, reacted sample.
However, the peak intensities of the Ti2AlN phase
decreased from the 4 GPa to the 6 GPa sample
indicating that there might be an optimum pressure
window for shock activation. In the as blended and
4 and 9 GPa shocked samples, TiN was present in
addition to Ti2AlN. Reaction synthesis of the high
pressure (9 GPa) sample showed formation of TiN
and Ti3Al2N2, which is a ternary phase with a
stacking sequence of ABABACBC and is typically
observed to be formed in a narrow temperature
range (1200-1300 °C) [8].
Kinetic studies were, also, performed on the Ti
+ A1N compacts. These samples showed evidence
of only a single broad peak indicative of a solid
state reaction. The activation energy for this solid
state reaction for the as blended sample was 97
kJ/mole, for the shocked, 4 GPa sample was 16
kJ/mole, and the 6 GPa and 9 GPa samples had
activation energies of 58 kJ/mole. The activation
energy decrease for the 4 GPa samples agrees with
the increased amount of Ti2AlN formed during
reaction synthesis.
ACKNOWLEDGEMENTS
This work is funded by DOD/ASSERT program through
the Army Research Office, contract number DAAG5598-1-0161.
DISCUSSION AND SUMMARY
Shock compression of Ti, SiC, and graphite
powders at - 4 - 5 GPa showed formation of dense
packed highly activated state of reactants, while
compression at a high pressure (—9 GPa) resulted in
TiC formation in the recovered shock-densified
compacts. Reaction heat treatment showed the
formation of Ti3SiC2 and a TiC phase. The lattice
parameter of the TiC phase was different from that
of the stoichiometric value, suggesting that the TiC
phase may be a TiCx + Si solid solution, i.e. an
intermediate state prior to Ti3SiC2 formation. In
addition, the fraction of Ti3SiC2 in the compacts
was higher in those densifled at a higher shock
pressure. These results, along with those of the
reaction kinetic studies to determine the activation
energies of solid state and combustion type
reactions, illustrate that shock compression
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1100
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