Interfacial structure of V2AlC thin films deposited on -sapphire

Available online at www.sciencedirect.com
Scripta Materialia 64 (2011) 347–350
<strong>Interfacial</strong> <strong>structure</strong> <strong>of</strong> V 2AlC <strong>thin</strong> <strong>films</strong> <strong>deposited</strong>
on ð1120Þ-sapphire
Darwin P. Sigumonrong, a Jie Zhang, a,b,⇑ Yanchun Zhou, b Denis Music, a Jens Emmerlich, a
Joachim Mayer c and Jochen M. Schneider a
a Materials Chemistry, RWTH Aachen University, Mies-van-der-Rohe-Str. 10, 52074 Aachen, Germany
b Shenyang National Laboratory for Materials Science, Institute <strong>of</strong> Metal Research, Chinese Academy <strong>of</strong> Sciences,
Shenyang 110016, China
c Central Facility for Electron Microscopy, RWTH Aachen University, Ahornstr. 55, D-52074 Aachen, Germany
Received 14 October 2010; accepted 24 October 2010
Available online 30 October 2010
Local epitaxy between <strong>V2AlC</strong> and sapphire without intentionally or spontaneously formed seed layers was observed by transmission
electron microscopy. Our ab initio calculations suggest that the most stable interfacial <strong>structure</strong> is characterized by the stacking
sequence ...C–V–Al–V//O–Al..., exhibiting the largest work <strong>of</strong> separation for the configurations studied and hence strong interfacial
bonding. It is proposed that a small misfit accompanied by strong interfacial bonding enable the local epitaxial growth <strong>of</strong> <strong>V2AlC</strong>
on ð1120Þ-sapphire.
Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: MAX-phase <strong>thin</strong> film; TEM; Epitaxial growth; Ab initio calculation
MAX-phases are a family <strong>of</strong> ternary compounds
with the formula <strong>of</strong> Mn+1AXn, where M is a transition
metal, A is an A group element (mostly IIIA and
IVA), X is either C or N, and n = 1–3. These phases exhibit
remarkable properties which are usually associated
either with ceramics or with metals [1–4]. Due to the unique
combination <strong>of</strong> properties, MAX-phases are promising
candidates for both structural components [5] and
protective <strong>thin</strong> <strong>films</strong> [6]. Recently, oxidation induced
self-healing has been reported by Song et al. [7] in
Ti3AlC2 at 1100 °C. By preferential oxidation <strong>of</strong> Al-containing
MAX-phases during high-temperature oxidation,
crack healing was observed by the formation <strong>of</strong>
a-Al2O3. Theoretical and experimental investigations
<strong>of</strong> the interface <strong>structure</strong> between MAX-phase and
a-Al2O3 are therefore <strong>of</strong> importance from a high-temperature
application prospective as well as from a <strong>thin</strong> film
physics point <strong>of</strong> view. Epitaxial growth <strong>of</strong> MAX-phase
<strong>thin</strong> <strong>films</strong> on (0 0 0 1)-sapphire substrates [8–11] as well
as on the binary carbide seed layer [12–16] was reported,
but no stacking sequence proposal has been communi-
⇑ Corresponding author at: Materials Chemistry, RWTH Aachen
University, Mies-van-der-Rohe-Str. 10, 52074 Aachen, Germany.
Fax: +49 241 80 22295; e-mail: zhang@mch.rwth-aachen.de
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.scriptamat.2010.10.035
www.elsevier.com/locate/scriptamat
cated so far. The stacking sequence defines the interfacial
strength and therefore not only affects the coating adhesion
but also has the ability to self-heal, since one <strong>of</strong>
the requirements for self-healing materials, according to
van der Zwaag [17], is that the healed material exhibits
properties superior or equal to the pristine material.
Clearly the stacking sequence defining the interfacial
strength is one <strong>of</strong> the key quantities determining whether
the strength <strong>of</strong> the healed material is superior to that
<strong>of</strong> the pristine material. Hence, the methodology <strong>of</strong> the
interface investigation communicated here provides a
pathway to improve self-healing materials by probing
the interfacial bonding.
<strong>V2AlC</strong> (space group P63/mmc, prototype Cr2AlC) was
<strong>deposited</strong> on ð1120Þ-sapphire from elemental targets.
Local epitaxial growth was observed on ð1120Þ-sapphire
substrate by high-resolution transmission electron microscopy
(HRTEM) with the orientation relationship <strong>of</strong>
<strong>V2AlC</strong> (0 0 0 1)//a-Al2O3 ð1120Þ and <strong>V2AlC</strong> ½1210Š//
a-Al 2O 3 [0 0 0 1]. Furthermore, ab initio calculations
based on density functional theory (DFT) have been
performed to study the interface with respect to the
stacking sequence.
The <strong>V2AlC</strong> <strong>films</strong> studied in this work were grown on
polished ð1120Þ a-Al2O3 substrates by magnetron
sputtering, using an experimental apparatus and setup

348 D. P. Sigumonrong et al. / Scripta Materialia 64 (2011) 347–350
described in detail elsewhere [16]. During deposition, the
substrate was kept at floating potential and heated to
750 °C. The film was grown to a thickness <strong>of</strong> approximately
1.5 lm (±5%), as determined by scanning electron
microscopy. The chemical composition <strong>of</strong> the film
was quantified using energy-dispersive X-ray analysis
with an EDAX Genesis 2000 system. The atomic concentration
<strong>of</strong> each element deviates by less than 4% from
the stoichiometric value. Hence, the composition <strong>of</strong> the
film reported here is close to stoichiometric. Phase analysis
was carried out by X-ray diffraction (XRD) with a
Bruker D5000 diffractometer in Bragg–Brentano geometry
and a Cu X-ray source. Figure 1 shows the X-ray
diffractogram <strong>of</strong> the <strong>V2AlC</strong> film grown in this work.
All major XRD peaks can be assigned to <strong>V2AlC</strong> except
for the peak indicated by the arrow with an integrated
intensity (area under the peak) <strong>of</strong> 1% <strong>of</strong> the total diffracted
intensity. Based on XRD, the <strong>V2AlC</strong> film studied
in this work is 99% phase pure.
Structural investigation <strong>of</strong> the interface was carried
out by TEM using a 300 kV Tecnai G 2 F30. Fast Fourier
transformation (FFT) was carried out using the
Digital Micrograph package. A low-magnification
cross-sectional image <strong>of</strong> the film is depicted in Figure 2a,
which reveals that the <strong>V2AlC</strong> film is polycrystalline. At
the interface, the orientation relationship between the
film and the substrate is determined by the corresponding
selected area diffraction (SAED) pattern (shown in
Fig. 2b) as follows:
– <strong>V2AlC</strong> (0 0 0 1)//a-Al2O3 ð1120Þ,
<strong>V2AlC</strong> ð2020Þ//a-Al2O3 ð3300Þ,
and <strong>V2AlC</strong> ½1210Š//a-Al2O3 [0001]
The small deviation <strong>of</strong> the two-zone axis is most likely
due to the finite size effect <strong>of</strong> the SAED aperture used.
The lattice mismatch between the film and substrate
can be calculated by f =(as af)/af; in which as and af
are the d-spacings <strong>of</strong> the Al2O3 and <strong>V2AlC</strong> planes which
are perpendicular to the interface [18]. The d-spacings <strong>of</strong>
Al2O3 ð3300Þ and <strong>V2AlC</strong> ð2020Þ employed are the
stress-free lattice parameters <strong>of</strong> <strong>V2AlC</strong> and Al2O3. The
interfacial mismatch is calculated to be 8.16%.
Figure 2(c) shows an HRTEM image obtained from
the interface, indicating (local) epitaxial growth <strong>of</strong>
<strong>V2AlC</strong> <strong>thin</strong> film on ð1120Þ a-Al2O3. In Figure 2(c),
the ð2110Þ plane <strong>of</strong> Al2O3 transfers continously into
Figure 1. X-ray diffractogram <strong>of</strong> the V 2AlC film grown on ð1120Þ
a-Al2O3 substrate at 750 °C.
the ð1013Þ plane <strong>of</strong> <strong>V2AlC</strong>, both <strong>of</strong> which are inclined
to the interface. This feature <strong>of</strong> the interfacial micro<strong>structure</strong>
is more clearly revealed by Figure 2d, in which
the processed image obtained by calculating the FFT
after masking both the Al2O3 {2110} and <strong>V2AlC</strong>
{1013} reflections on the opposite side <strong>of</strong> the unscattered
beam, as shown in the inset image in Figure 2d.
For the <strong>V2AlC</strong> film grown on a-Al2O3 substrates, a
semicoherent interface forms which is characterized by
coherent regions separated by misfit dislocations. Thus
the lattice mismatch at the interface is accommodated
by the misfit dislocations. In Figure 2d, the misfit dislocations
are evenly distributed in V 2AlC near the interface.
This is expected since <strong>V2AlC</strong> is less stiff than
Al2O3 [19]. Hence, epitaxial growth <strong>of</strong> <strong>V2AlC</strong> <strong>films</strong> on
ð1120Þ Al 2O 3 substrates may at least in part be caused
by small lattice mismatch, compensated by misfit dislocations.
It is important to note that the thickness <strong>of</strong> the
interfacial region where local epitaxy observed is on the
order <strong>of</strong> 100 nm. It may be speculated that the relatively
high deposition rate employed here hinders global epitaxy.
The growth rate, at 37.5 nm min 1 , was one order
<strong>of</strong> magnitude larger than the rates reported in the literature
resulting in the formation <strong>of</strong> epitaxial Ti2AlC <strong>thin</strong>
<strong>films</strong> [12,13].
In order to investigate the origin <strong>of</strong> the experimentally
observed local epitaxy, we applied ab initio calculations
to probe the atomistic and electronic nature <strong>of</strong> the
V 2AlC//a-Al 2O 3 interface. These ab initio calculations
were performed using the OpenMX code [20], based
on DFT [21] and basis functions in the form <strong>of</strong> linear
combinations <strong>of</strong> localized pseudoatomic orbitals [22].
The electronic potentials were fully relativistic pseudopotentials
with partial core correction [23,24], and a local
density approximation was applied [25]. The basis
functions used were generated based on a confinement
scheme [26] and specified as follows: Al6.0-s2p2d1,
O5.0-s2p2, V7.5-s2p2d2 and C4.5-s2p2. The energy cut<strong>of</strong>f
(150 Ryd) and k-point grid (1 1 1) wi<strong>thin</strong> the real
space grid technique [27] were adjusted to reach an accuracy
<strong>of</strong> 10 6 H atom 1 . Spin polarization was not considered
since the total energy differences between spin
polarized and non-polarized <strong>V2AlC</strong> configurations are
negligible [28]. The interface was described using a slab
model where periodic boundary conditions are broken
in one crystallographic direction by inserting a vacuum
layer (here thickness = 10 A ˚ ).
Two interface configurations have been studied in this
work. V 2AlC (0001)//a-Al 2O 3 ð1120Þ and V 2AlC
½1210Š//a-Al2O3 [0 0 0 1], which is identical to the one
observed by TEM; as well as <strong>V2AlC</strong> (0 0 0 1)//a-Al2O3
ð1120Þ and V 2AlC ½1100Š//a-Al 2O 3 [0 0 0 1] each consist
<strong>of</strong> 60 atoms corresponding to the substrate and 128 and 94
atoms, respectively, corresponding to the film. The most
stable surface termination <strong>of</strong> ð1120Þ a-Al 2O 3 reported
in the literature is oxygen [29,30]; consequently, this termination
was adopted here. In both configurations, the
substrate is stoichiometric a-Al2O3 with six atomic layers
in thickness and the film is stoichiometric V 2AlC with
eight atomic layers in thickness. Two layers at the bottom
<strong>of</strong> the interface belonging to a-Al2O3 were fixed to represent
the (infinite) bulk. For each <strong>of</strong> these interfacial
configurations, the following stacking sequences were

studied: ...V–C–V–Al//O–Al..., ...C–V–Al–V//O–Al
... and ...Al–V–C–V//O–Al.... The stacking sequences
...V–Al–V–C//O–Al...have been excluded because convergence
could not be obtained. The most stable interface
configuration is characterized by the largest work <strong>of</strong> separation,
calculated from the energy change per unit area
corresponding to the introduction <strong>of</strong> the interface in comparison
to two separated slabs [31]. For the <strong>V2AlC</strong>
(0 0 0 1)//a-Al2O3 ð1120Þ and <strong>V2AlC</strong> ½1210Š//a-Al2O3
[0 0 0 1] interface, the calculated work <strong>of</strong> separation
values are 2.86, 2.73 and 1.90 J m 2 , corresponding to
the ...C–V–Al–V//O–Al..., ...Al–V–C–V//O–Al...
and ...V–C–V–Al//O–Al... stacking sequences, respectively.
Furthermore, for the <strong>V2AlC</strong> (0 0 0 1)//a-Al2O3
ð1120Þ and <strong>V2AlC</strong> ½1100Š//a-Al2O3 [0 0 0 1] interface,
the calculated work <strong>of</strong> separation values are 1.72, 1.60
and 1.40 J m –2 , corresponding to ...C–V–Al–V//O–Al
..., ...Al–V–C–V//O–Al... and ...V–C–V–Al//O–Al...
stacking, respectively.
Based on our calculation results, the strongest interfacial
bonding and also the most stable interface are
obtained for ...C–V–Al–V//O–Al... stacking. The electron
density distribution in Figure 3 shows that the bulk
part <strong>of</strong> a-Al2O3 exhibits mostly ionic and covalent bonding
since electrons are transferred from Al to O and
some charge is shared. In V–C slabs <strong>of</strong> V 2AlC, electrons
are shared and partly transferred, which is consistent
with a mixture <strong>of</strong> covalent and ionic bonding. The bonding
between V and Al is characterized by more uniform
electron density as well as some shared charge. This is
consistent with metallic bonding with some covalent
D. P. Sigumonrong et al. / Scripta Materialia 64 (2011) 347–350 349
Figure 2. (a) Cross-sectional TEM micrograph <strong>of</strong> the V 2AlC film. (b) SAED pattern corresponding to the V 2AlC/a-Al 2O 3 interface. (c) HRTEM
image <strong>of</strong> the interface recorded with the incident beam parallel to the ½1210Š direction <strong>of</strong> V 2AlC and [0 0 0 1] <strong>of</strong> a-Al 2O 3. (d) One-dimensional
Fourier-filtered image <strong>of</strong> (c) by a pair <strong>of</strong> <strong>V2AlC</strong> as well as a-Al2O3 reflections, which are shown in the inset pattern.
Figure 3. The interface configuration studied in this work with the
largest work <strong>of</strong> separation value, overlapped with the corresponding
electron density distribution.
contributions, in agreement with the literature [32,33].
At the interface, namely between V and O, charge transfer
occurs and some charge is shared. These bonds are <strong>of</strong>
an ionic–covalent nature and are expected to be strong,
resulting in a very stable interface, which may enhance
the self-healing ability <strong>of</strong> MAX-phase at high
temperatures.
In conclusion, the growth <strong>of</strong> <strong>V2AlC</strong> <strong>films</strong> on ð1120Þ
a-Al2O3 was reported. Local epitaxy without intentionally
or spontaneously formed seed layers was observed
by transmission electron microscopy. Based on the
experimental and ab initio data presented, it is proposed

350 D. P. Sigumonrong et al. / Scripta Materialia 64 (2011) 347–350
that the stable interfacial bonding in conjunction with
the small lattice mismatch compensated by misfit dislocations
enable the local epitaxial growth <strong>of</strong> V 2AlC on
ð1120Þ a-Al2O3. Therefore, interfacial design based on
misfit minimization and interface stabilization by strong
interfacial bonding may enable local epitaxial growth.
The methodology <strong>of</strong> the theoretical interface investigation
communicated here provides a pathway to improving
self-healing materials by probing the interfacial
bonding.
The authors gratefully acknowledge the financial
support by the Deutsche Forschungsgemeinschaft
(DFG) wi<strong>thin</strong> the Collaborative Research Center 561
“Thermally highly loaded, porous, and cooled multilayer
systems for combined cycle power plants”.
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