Bimodal microstructure and reaction mechanism of Ti2SnC synthesized by a high-temperature reaction using Ti/Sn/C and Ti/Sn/TiC powder compacts

Article abstract of DOI:10.1111/j.1551-2916.2006.01275.x

High purity of titanium tin carbide (Ti₂SnC) powder was fabricated by pressureless sintering two types of mixtures of Ti/Sn/C and Ti/Sn/TiC powders under different conditions. A bimodal microstructure of Ti₂SnC with plate-like and rod-like forms was first observed, which is determined by the grain growth rate in different planes, the C particle's size, and the growth environment. Based on the microstructure observation, a reaction model was proposed to understand the reaction mechanism for the formation of Ti₂SnC. Further investigation of the thermal stability of Ti₂SnC demonstrates that Ti₂SnC decomposes to TiC and Sn in vacuum atmosphere at 1250 C.

Full text of DOI:10.1111/j.1551-2916.2006.01275.x

J. Am. Ceram. Soc., 89 [12] 3617–3623 (2006)
DOI: 10.1111/j.1551-2916.2006.01275.x
r 2006 The American Ceramic Society
ournal
J
Bimodal Microstructure and Reaction Mechanism of Ti2SnC
Synthesized by a High-Temperature Reaction Using
Ti/Sn/C and Ti/Sn/TiC Powder Compacts
Shi-Bo Li,w Guo-Ping Bei, Hong-Xiang Zhai, and Yang Zhou
School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China
High purity of titanium tin carbide (Ti2SnC) powder was fabri-
cated by pressureless sintering two types of mixtures of Ti/Sn/C
and Ti/Sn/TiC powders under different conditions. A bimodal
microstructure of Ti2SnC with plate-like and rod-like forms was
first observed, which is determined by the grain growth rate in
different planes, the C particle’s size, and the growth environ-
ment. Based on the microstructure observation, a reaction model
was proposed to understand the reaction mechanism for the for-
mation of Ti2SnC. Further investigation of the thermal stability
of Ti2SnC demonstrates that Ti2SnC decomposes to TiC and Sn
in vacuum atmosphere at 12501C.
action mechanisms for the formation of Ti2SnC were proposed
by Vincent et al.6 and Zhou et al.,7 respectively, there are still
some differences and unclear points. Furthermore, only two-di-
mensional Ti2SnC platelets have been observed, while other
shapes were not reported. According to the hexagonal structure
of Ti2SnC, it should possess different morphologies such as rod-
like, plate-like, and equiaxed shapes, just like those for Ti3SiC2.8
In the present study, the main purposes are to fabricate the
high purity of the Ti2SnC powder from different mixture pow-
ders and to explain the reaction mechanism of the formation
of Ti2SnC from the microstructure observation and analysis
results. A bimodal microstructure of Ti2SnC is first reported,
and its thermal stability in vacuum is also investigated.
I. Introduction
II. Experimental Procedures
ITANIUM tin carbide (Ti2SnC) is one of the most interesting
materials in the family of the layered ternary compounds
n11AXn (n 5 1, 2, 3), where M is a transition metal, A is a
T
Commercial powders of Ti (ꢂ325 mesh, 499.2% purity),
Sn (ꢂ200 mesh, 499.5% purity), C (graphite, ꢂ325 mesh,
499.0% purity), and TiC (average particle size: 4 mm,
498.0% purity) were used in the present study. All of the
powders were from General Research Institute for Nonferrous
Metals (GRINM), China.
Two types of powder mixtures containing Ti/Sn/C and Ti/Sn/
TiC with different mole ratios were mixed in a polypropylene
bottle for 10 h. The mixed powders were cold pressed to form
compacts with a diameter of B20 mm and a height of B5 mm.
The compacts were placed in a graphite crucible and then pres-
sureless sintered at different temperatures for 1–6 h in a vacuum
atmosphere. A differential scanning calorimetry (DSC) test on
the powder of Ti/Sn/C was performed in a thermal analysis
instrument (Netzsch STA409C, Selb, Germany) at a heating rate
of 101C/min, under flowing Ar in the temperature range of 251–
12501C. The sintered samples were characterized by X-ray dif-
M
group IIIA or IVA element, and X is C or N.1
Ti2SnC has been proved to exhibit a number of excellent
properties.1–5 It has low hardness (B3.5 GPa), high electrical
conductivity (B14 ꢀ 106 (O ꢁ m)ꢂ1), self-lubricity, machinability,
etc. In addition, it is damage tolerant at room temperature,
resistant to corrosion, and stable up to at least 12001C. The
combination of these properties makes this material a promising
candidate in diverse applications in reinforcements for electrical
composites, rotating electrical contacts, bearings, and other
components for the chemical and petrochemical industries.
Hence, it has attracted much worldwide attention.
Up to now, only hot isostatic pressing (HIP),3,4 hot pressing
(HP),3 and pressureless sintering techniques6,7 have been adopt-
ed to fabricate Ti2SnC powders from a stoichiometric mixture of
Ti/Sn/C powders. However, high purity of the Ti2SnC powder is
difficult to obtain by adopting the above-mentioned methods
and the mixture, because impurities such as TiC, Ti–Sn com-
pounds, or Sn always accompany Ti2SnC. The disadvantages of
the existing impurities, especially with the presence of TiC, de-
teriorate some properties of Ti2SnC. Purification of the Ti2SnC
powder is an unambiguous task. Systematic investigation of the
influences of different parameters such as sintering atmosphere,
starting mixtures as well as the ratios of starting composition on
the high purity of the Ti2SnC powder is necessary. To make
clear the reason why the impurities are always present, it is also
needed to understand the reaction mechanism of the formation
of Ti2SnC in the Ti–Sn–C system.
fraction (XRD) using
a D/Max 2200PC diffractometer
(Japan) at 40 kV and 40 mA with CuKa radiation. The micro-
structure of the sintered samples was observed by scanning
electron microscopy (SEM, model: Hitachi S3500N, Japan,
and Stereoscan 360, Cambridge, UK, respectively) equipped
with energy-dispersive spectroscopy (EDS).
III. Results and Discussion
(1) Synthesis and Characterization of Ti2SnC
Figure 1 shows the XRD patterns of the samples made of the
Ti/Sn/C powders with a mole ratio of 2:1:1 (denoted as 2Ti–Sn–C)
after sintering at different temperatures. The major peaks shown
in Fig. 1(a) belong to Ti2SnC, indicating that the Ti2SnC phase
should be formed below 10501C. Ti–Sn compounds (Ti6Sn5 and
Ti5Sn3) and Sn peaks can also be found in Fig. 1(a), however,
with weak peaks. With increasing temperature in the range of
10501–12001C, the content of Ti2SnC increases. At 11501C,
Ti2SnC peaks are dominant, only with weak peaks of Sn
(Fig. 1(b)). A pure Ti2SnC powder was obtained at 12001C
(Fig. 1(c)). The present results, including the synthesis temper-
ature and the powder purity, are different from those reported in
However, little information on systematically studying the
influences of parameters, microstructures, and reaction mecha-
nisms of Ti2SnC was available until now. Although relative re-
T. Besmann—contributing editor
Manuscript No. 21662. Received April 3, 2006; approved June 26, 2006.
Supported by National Science Foundation of China (NSFC) under Grant No.
50472045, and Science Developing Foundation of Beijing Jiaotong University.
wAuthor to whom correspondence should be addressed. e-mail: shibo-li@sohu.com
3617

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Journal of the American Ceramic Society—Li et al.
Vol. 89, No. 12
So it confirmed that vacuum atmosphere improves the Ti2SnC
powder’s purity.
Further increase in temperature results in a decrease in the
content of Ti2SnC. As the temperature increases to 12501C for 1
h, TiC and Sn appear in the XRD pattern (Fig. 1(d)). The main
reason is the decomposition of Ti2SnC, which has been analyzed
in the section on ‘‘Thermal Stability of Ti2SnC in Vacuum At-
mosphere.’’ Figure 2 shows the morphologies of the sample after
sintering at 11501C. A few C particles identified by EDS
(Fig. 3(a)) were found in the sample, shown in Figs. 2(a) and
(b). However, these unreacted or residual C particles with a size
less than 20 mm were not detected by XRD analysis (Fig. 1(a)),
mainly owing to the lower amount that exceeds the limit of de-
tection. Around the residual C particles, the Ti2SnC grains with
smooth surfaces cluster together. They have a growth tendency
that is perpendicular to the surfaces of the C particles (Fig. 2(a)).
From careful observations from Figs. 2(a) and (b), it can be seen
that C particles are covered by a double layer containing an in-
ner layer and an outer layer, which was analyzed by EDS,
shown in Fig. 3. The inner layer is composed of TiC (Fig. 3(b)),
which is thinner than 1 mm. The outer layer identified by EDS
(Fig. 3(c)) is composed of Ti, Sn, and C and the atomic ratio of
Ti/Sn is 2:1, indicating the Ti2SnC phase.
Figure 4(a) shows the morphology of the Ti2SnC grains taken
from the fracture surface of the sample after sintering at 12001C
for 1 h. These plate-like grains are less than 8 mm in size, with
thickness in the range of 0.3–1.3 mm. All of the grains exhibit a
smooth surface, indicating that they grow and develop in the
liquid.
However, another typical feature of the Ti2SnC grains has
been found on the surface at the same sample. A large amount
of Ti2SnC clusters with a rod-like shape and a smooth surface,
Fig. 1. X-ray diffraction patterns of the samples made of 2Ti–Sn–C
powders after sintering at different temperatures for 1 h in a vacuum
atmosphere, (a) 10501C, (b) 11501C, (c) 12001C, and (d) 12501C.
references.3–7 One of the main reasons could be attributed to the
sintering atmosphere, because the previously reported Ti2SnC
powders containing Sn or TiC were always synthesized by sin-
tering a stoichiometric mixture of Ti/Sn/C in an Ar atmo-
sphere.3–7 We have also found that the Ti2SnC powder was
always contaminated by Sn and TiC after sintering the mixture
of Ti/Sn/C in an Ar atmosphere in the range of 10501–12001C.
Fig. 2. Scanning electron micrographs of the fracture surfaces of the
sample made of 2Ti–Sn–C powders after sintering at 11501C for 1 h
shown in (a) and (b). (a) A lower magnification micrograph; the marked
‘‘C’’ particles are graphite, and (b) a higher magnification micrograph.

December 2006
Bimodal Microstructure and Reaction Mechanism of Ti2SnC
3619
Fig. 4. Scanning electron micrographs for the sample made of 2Ti–Sn–
C powders after sintering at 12001C for 1 h. (a) Fracture surface, and (b)
surface of the sample.
Fig. 3. Energy-dispersive spectroscopy spectra from the marked A, B,
and C areas in Fig. 2(b) shown in (a), (b) and (c), respectively.
(2) Reaction Mechanism for Formation of Ti2SnC
From the above observations, it is necessary to clarify the mech-
anism for the formation of Ti2SnC.
identified by EDS, is shown in Fig. 4(b). The rod-like grains
cluster together densely and mainly grow toward the same
orientation and its tip is round and smooth. These grains are
B10 mm in length and B1 mm in diameter.
Figure 6 shows the DSC curve of a stoichiometric mixture of
Ti, Sn, and C powders. The sharp endothermic peak at 2311C
was observed, corresponding to the melting of Sn. At 4981C, the
first broad exothermic peak appeared, which should be attrib-
uted to a reaction between Ti and Sn to form Ti6Sn5. The second
exothermic peak at 7561C was associated with a reaction be-
tween Ti and Ti6Sn5 to form Ti5Sn3. The above results have
been further confirmed by XRD analysis (XRD patterns not
On carefully comparing the observations from Figs. 2 and 4,
it can be deduced that the clusters must grow around the C
particles. The shape of the Ti2SnC particles shown in Fig. 4 is
different, although taken from the same specimen, but not the
same location. The difference mainly results from the growth
environment and the size of the C particles. The images shown
in Fig. 2 are taken from the inner area of the sample, in which
the growth of the Ti2SnC grains is restrained by adjacent par-
ticles. However, the Ti2SnC grains at the outer surface will grow
free from restriction, so these Ti2SnC grains exhibit a long rod
feature. From the above observations, it can be seen that Ti2SnC
has an anisotropic growth behavior, similar to 312 phases
(Ti3SiC2, Ti3AlC2, etc.).
Furthermore, sufficient liquid phase benefits the rapid growth
of Ti2SnC, especially at the simple surfaces where sufficient liq-
uid phase and minimum restriction both favor the Ti2SnC grains
to grow rapidly and develop a rod-like microstructure. This
supposition was confirmed by additional evidence in which ex-
cess Sn in the Ti–Sn–C reaction system benefits the rapid growth
of the Ti2SnC rod grains. A starting composition with an extra
10% mol Sn (2Ti–1.1Sn–C) is used and sintered under the same
condition as that for 2Ti–Sn—C. The results show that more
Ti2SnC rods appear both on the surface and in the inner area of
the sample. Figure 5, taken from the inner area of sample, shows
that Ti2SnC rods B10 mm in length are clustered around the
residual C particles marked by arrows. The residual C particles
are B2 mm in size. The above-formed feature is discussed in the
following section.
Fig. 5. Scanning electron micrograph of the fracture surface taken
from the inner area of the sample made of 2Ti–1.1Sn–C and sintered
at 12001C for 1 h. Residual C particles are marked by white arrows.

3620
Journal of the American Ceramic Society—Li et al.
Vol. 89, No. 12
formation of TiC. The last exothermic peak with a broad and a
weak shape around 9861C corresponded to the formation of
Ti2SnC from a reaction between Ti and Sn compounds (Ti6Sn5
and Ti5Sn3) and TiC.
Thus, the following reactions occur in an orderly manner
during the sintering of the mixture of Ti, Sn, and C powders:
around 232ꢃC
SnðsÞ ————ꢂ! SnðlÞ
(1)
(2)
(3)
(4)
around 498ꢃC
5SnðlÞ þ 6Ti ðsÞ ————ꢂ! Ti6Sn5 ðsÞ
around 756ꢃC
3Ti6Sn5 ðsÞ þ 7Ti ðsÞ ————ꢂ! 5Ti5Sn3 ðsÞ
Fig. 6. Differential scanning calorimetry curve of a stoichiometric
mixture of Ti, Sn, and C powders.
around 786ꢃC
Ti ðsÞ þ C ðsÞ ————ꢂ! TiC ðsÞ
around 986ꢃC
Ti ꢂ Sn comp: ðsÞ þ TiC ðsÞ ————ꢂ! Ti2SnC ðsÞ (5)
shown). Only a new phase of Ti6Sn5, except for Ti, Sn, and C
peaks, was found in the XRD pattern after sintering the stoic-
hiometric mixture of Ti/Sn/C powders at 5001C in an Ar at-
mosphere for 1 h, while both Ti6Sn5 and Ti5Sn3 were found at
7301C. The third exothermic peak at 7861C was related to the
Based on the DSC, XRD, and SEM analyses, a model shown in
Fig. 7 has been proposed to further explain the growth mech-
anism as follows:
Fig. 7. Proposed growth mechanism for the formation of Ti2SnC with a bimodal microstructure shown in (a)–(e).

December 2006
Bimodal Microstructure and Reaction Mechanism of Ti2SnC
3621
At first stage, the powders of Ti, Sn, and C are mixed homo-
geneously (Fig. 7(a)). As the temperature increases to above
Sn melting point (2321C), Sn starts to melt. With increasing
temperature, molten Sn spreads everywhere; under this environ-
ment, Ti and C particles are easily rearranged and are close to
each other (Fig. 7(b)). Ti atoms diffuse toward and accumulate
in the grain boundaries of C that result in the formation of a
thinner Ti–C layer (not TiC) owing to the strong chemical af-
finity of Ti to C. When the temperature increases to 5001C, Ti
reacts with Sn to form Ti–Sn intermetallics, which includes four
phases in the Ti–Sn binary system such as Ti6Sn5, Ti5Sn3, Ti2Sn,
and Ti3Sn. However, the Ti6Sn5 compound was first formed
according to XRD and DSC analyses. With increasing tem-
perature, Ti continues to react with Ti6Sn5 to form Ti5Sn3. The
reactions producing Ti6Sn5 and Ti5Sn3 are exothermic, which
induces a reaction occurrence in the previously formed Ti–C
layer. So the thin TiC layer forms around the C particles
(Fig. 7(c)). On increasing the temperature to 9801C, a reaction
producing Ti2SnC takes place between the Ti–Sn compounds
and TiC. Nuclei of Ti2SnC precipitate and grow on the surface
of the TiC layer (Fig. 7(d)). All of the above reactions are exo-
thermic and may occur in the liquid-Sn system, in which Ti2SnC
grows fast and develops on the TiC layer.
Fig. 8. X-ray diffraction pattern of the sample made of Ti–0.8Sn–
0.9TiC and sintered at 12001C for 15 min.
result does confirm that Ti2SnC forms between the Ti–Sn com-
pounds and TiC. Furthermore, the rod-like feature of Ti2SnC
was not found in these specimens, further illustrating that the
size of the C particles influences the microstructural feature of
Ti2SnC.
With repetition of the above reactions, the smaller C particles
are consumed quickly and nuclei of Ti2SnC develop into plate-
lets. Some medium-size C particles are consumed gradually, and
the Ti2SnC grains formed grow around the C particles until they
are exhausted, leaving Ti2SnC clusters (Fig. 7(e)). However, if
the C particles were larger and other Ti–Sn compounds were
consumed completely, the residual C cores would be left and
covered with the inner layer of TiC and the outer layer of
Ti2SnC clusters, as shown in Fig. 7(e).
The effect of the C particle size on the microstructural fea-
tures of Ti2SnC has been confirmed by previous work in which
mechanically alloyed Ti/Sn/C powders consisting of sub-mi-
crometer or nano-sized C particles were used to synthesize
Ti2SnC.9 It was found that the Ti2SnC grains were platelet,
not rod like, whether present on the surface or in the inner area
of the sample.
The formation of the bimodal microstructure of Ti2SnC has
relation not only to the C particle size, liquid phase, and location
but also to the grain growth rate, because crystal morphology is
dependent on the grain growth rate in different planes. The
slowly growing face plays an important role in determining the
crystal morphology.10 For the hexagonal crystal structure of
Ti2SnC, the crystal feature is determined by the relative growth
rate on (101), (002), and (100) faces, just like Ti3SiC2.8 If the
growth rate on the (101) face is the fastest, the (002) and (100)
faces exhibit a relatively slow growth rate, and the rod shape will
be formed, as shown in Figs. 4(b) and 5. If the growth rate on
the (100) face is the fastest, and the (002) and (101) faces have a
relatively slow growth rate, the thin plate-like shape will be
formed as shown in Fig. 4(a).
(3) Thermal Stability of Ti2SnC in a Vacuum Atmosphere
Barsoum et al.2 reported that Ti2SnC might decompose at
132517251C in an Ar atmosphere. Dong et al.5 reported, ac-
cording to DTA and TGA analyses, that the decomposition of
Ti2SnC was stable up to at least 12001C in an Ar atmosphere.
Confirming the decomposition temperature in a different at-
mosphere is of importance for Ti2SnC in versatile applications.
However, in the present study, the decomposition tempera-
ture of Ti2SnC in vacuum is 12501C on the basis of the results
given in Figs. 1 and 9.
As the holding time at 12501C increased to 6 h, a large
amount of TiC was formed and Sn also appeared in the XRD
patterns (Fig. 9). But no Ti2SnC was found, indicating that it
has completely converted to the former two phases. In addition,
a large amount of spherical Sn granules with metal luster and
less than 3 mm in size were found on the walls of the graphite
crucible. The main reason is that Sn evaporates from the sample
at a high temperature and then condenses on the walls as the
ambient temperature decreases. Hence, Ti2SnC dissociates
according to the following equation:
Ti2SnC ! Ti2C þ Sn
(7)
Figure 10 shows the micrographs of the fracture surface of the
sample made of 2Ti–1.1Sn–C and sintered at 12501C for 4–6 h.
Clusters with large sizes were found, and a large amount of TiC
particles less than 3 mm in size agglomerated in the root of the
From the above observations and discussion, it is further
confirmed that TiC plays an important role in the formation of
Ti2SnC, although Vincent et al.5 deduced that TiC and a Sn–Ti
alloy are in thermodynamic equilibrium. But in the present
study, the reaction between TiC and the Ti–Sn compounds to
form Ti2SnC is unambiguous.
A further investigation for the formation of Ti2SnC by sin-
tering a mixture of Ti/Sn/TiC powders has been performed ac-
cording to the following equation:
Ti þ Sn þ TiC ! Ti2SnC
(6)
A large amount of Ti2SnC has been obtained by sintering the
above mixture at 12001C in a vacuum atmosphere, while a small
amount of TiC and Sn as impurities accompanies Ti2SnC. High
purity of Ti2SnC is obtained after sintering the mixture of Ti, Sn,
and TiC with a mole ratio of 1:0.8:0.9 (Ti–0.8Sn–0.9TiC) at
12001C for 15 min (Fig. 8). The Ti2SnC grains obtained also
possess a plate-like shape and are smaller than 4 mm in size. This
Fig. 9. X-ray diffraction pattern of the sample made of 2Ti–Sn–C
powders after sintering at 12501C for 6 h.

3622
Journal of the American Ceramic Society—Li et al.
Vol. 89, No. 12
It can be clearly seen from the inset in Fig. 10(c) that the TiC
particles formed are of a homogeneous size of about 0.8 mm.
To understand properly the conversion of Ti2SnC to TiC and
Sn, it is necessary to analyze the crystal structure of Ti2SnC,
which consists of layers of Sn atoms connected with octahedral
Ti6C. In the layered structure, each Sn atom links two chains
formed by Ti–C–Ti atoms with a covalent bond. The bonding in
the Ti–C–Ti bond chain is strong, while that linking the Ti–C–Ti
chain and Sn atom is relatively weak.11 So at the decomposition
temperature, Sn atoms easily diffuse from Ti2SnC, leaving Ti2C
block layers that finally form a TiCx structure with ordered C
vacancies, which is similar to the decomposition of Ti3SiC2.12
It has been proved that ‘‘A’’ element in the Mn11AXn com-
pound is easily induced to diffuse outward from the layered
structure under certain conditions, for example, when Ti3SiC2
was placed in an environment such as cryolite,12 chlorine,13 or
molten Al,14 in which the Si atoms were induced to migrate
outward, leaving the TiCx layers behind. Additional evidence
also confirmed that Ti3AlC2 decomposed at 14501C with Al
diffusion outward, with the formation of larger TiC particles,
which exhibit a polyhedral shape or a layered feature.15
IV. Conclusion
High-purity Ti2SnC powder has been obtained from mixtures of
Ti/Sn/C with a mole ratio of 2:1:1 and Ti/Sn/TiC with 1:0.8:0.9
after pressureless sintering at 12001C in vacuum for 1 h and 15
min, respectively. Ti2SnC with a bimodal microstructure of
plate-like and rod-like shapes appeared in the samples after
pressureless sintering the Ti/Sn/C mixture. The bimodal micro-
structure mainly resulted from the C particle size, growth envi-
ronment, and the grain growth on the different planes. In
addition, liquid Sn content also plays an important role in the
above microstructure formation. The mechanism for the forma-
tion of Ti2SnC was analyzed and a reaction model for explaining
the mechanism was proposed. With increasing temperature, Sn
melted and formed a liquid environment and particle rearrange-
ment occurred. Ti atoms diffuse toward the surfaces of C par-
ticles, around which a thin Ti–C layer forms due to the strong
chemical affinity of Ti to C. With increasing temperature, Ti and
Sn first react to form Ti–Sn intermetallic compounds, and then,
TiC forms in the Ti–C layer. Lastly, Ti2SnC forms from a re-
action between the Ti–Sn compounds and TiC at a certain tem-
perature. The last reaction to form Ti2SnC has been further
proved by sintering Ti/Sn/TiC powders at 12001C.
The thermal stability of Ti2SnC is lower than 12501C in vac-
uum. Above 12501C, Ti2SnC decomposes to TiC and Sn. An-
alyzing the crystal structure can lead to a proper understanding
of the decomposition of Ti2SnC. At a high temperature, Sn
easily diffuses outward from the weak links between Sn and the
Ti–C–Ti chains, leaving Ti2C block layers that finally formed
a TiCx structure with ordered C vacancies.
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Fig. 10. Scanning electron micrographs of the fracture surface of the
sample made of 2Ti–1.1Sn–C and sintered at 12501C for 4 h shown in (a)
and (b), and for 6 h shown in (c). (a) Elongated Ti2SnC laths and smaller
TiC particles, (b) triangle structure of TiC formed on an elongated
Ti2SnC lath. (c) Microstructure of the decomposed sample. The inset
shows an enlarged micrograph taken from the marked area in (c).
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