ICMCTF 2012! - CD-Lab Application Oriented Coating Development

VxMo1-xN thin films with 0.3≤x≤1, as determined by EDX and RBS, were
deposited on MgO(001) substrates by dual reactive magnetron sputter
epitaxy (MSE) in a 5 mTorr Ar+N2 gas mixture. The substrate temperature
was varied from 100 to 700 °C. For the entire composition range, at Ts =
700 °C, we obtain single-phase cubic-B1-structure VxMo1-xN as determined
by XRD. Cross-sectional HR-TEM and SAED confirms the cubic VxMo1-xN
solid solution viewed along both and zone axes. The lattice
parameter varies from a = 4.149 Å for x = 0.8 to a = 4.167 Å for x = 0.4,
increasing with increasing Mo content. High- resolution XRD reciprocal
space mapping of a V0.4Mo0.6N film grown at 700 ºC shows that the film is
relaxed with a = c = 4.167 Å. At temperatures between 500 and 300 °C,
with x = 0.5, we obtained single phase B1 VxMo1-xN(001) films.
The nanoindentation hardness H and elastic modulus E of the VxMo1-xN
alloy films are H = 14.3 GPa and E = 286 GPa for x = 0.6, and H = 12.3
GPa and E = 243 GPa for x = 0.4.
[1] D.G. Sangiovanni, L. Hultman, V. Chirita, Acta Mater 59 (2011) 2121-
2134
[2] H. Kindlund et al., ICMCTF 2011, Abstract #266
10:00am B7-1-7 Atomistic study of crack formation in strained thin
films, A. Oila (Adrian.Oila@ncl.ac.uk), S.J. Bull, Newcastle University,
UK
We present the results of Kinetic Monte Carlo (KMC) studies on dislocation
and crack formation in strained nanometre thick coatings. The Kinetic
Monte Carlo (KMC) computer program developed (NUKIMOCS) enables
atomistic simulation of the mechanical processes which occur at the
nanoscale in thin films. The code performs off-lattice KMC calculations to
determine the strain in a material at the atomic scale and how this changes
with time. Off lattice KMC allows atoms to occupy any position in space as
in a real atomic lattice with interaction between atoms defined by
interatomic potentials. Therefore, both the elastic and kinetic properties of
an off-lattice KMC model are entirely defined by the interatomic potential.
The underlying physics of strain-induced microstructural evolution can
therefore be presented accurately and on meaningful time and length scales.
The parameters of the interatomic potentials have been determined by
fitting the energy surface obtained from first principles calculations, the
elastic constants and the coefficient of thermal expansion to the unit cell.
Different initial and boundary conditions were applied in order to study the
behaviour of defects such as vacancies and dislocations. The local
relaxation process was performed after every 10 simulation steps within a
radius of three interatomic distances from the location of the event. The
energy barrier between two states - the activation energy - was calculated by
the ‘frozen crystal approximation’. For energy minimisation we employed a
limited-memory BFGS method.
Our KMC modelling shows that an atomically smooth surface under tensile
strain will spontaneously develop a surface roughness to minimise the
energy of the system and crack nuclei can develop from the roughness
profile generated. The most important relaxation processes are dislocation
generation and propagation.
10:20am B7-1-8 Classical Molecular Dynamics Studies of Initial
Nucleation Kinetics during TiN Thin Films Growth, D. Sangiovanni, D.
Edström, V. Chirita (vio@ifm.liu.se), L. Hultman, Linköping University,
Sweden, I. Petrov, J.E. Greene, University of Illinois at Urbana-
Champaign, US
Advancements in Modified Embedded Atom Method (MEAM) formalism
present the opportunity to perform, previously not possible, realistic large
scale classical Molecular Dynamics (MD) simulations of important model
material systems such as TiN. As a preliminary step in achieving this goal,
we report the first MD study of typical processes occurring during the initial
nucleation stages of TiN thin film growth. We use an improved TiN MEAM
parameterization, which reproduces the experimentally observed trends in
the diffusion of single species (Ti, N), Ti-N dimers and TiN2 complexes,
and correctly accounts for the all-important Ehrlich-Schwoebel (ES) stepedge
barriers on most representative, (100) and (111), steps/surfaces for
TiN growth. Simulations totaling hundreds of nanoseconds are carried out
at 1000 K, in statistically independent runs of between 2 and 10 ns, and
concentrate on the diffusion of Ti and N single adatoms, as well as Ti-N
complexes, on the (001) surface and islands. Results show significant
differences in terms of total migration distance and the diffusion
mechanisms between different species. As it will be shown, on the (001)
surface, Ti and N adatoms migrate via single and/or multiple jumps along
different diffusion channels, with Ti recorded as the leading diffusion
species. Diffusion mechanisms are observed to become considerably more
complex for Ti-N dimers and TiN2 complexes. On islands, for single
adatoms, the primary mechanism to overcome the ES step-edge barrier is
that of push-out exchange with island edge-atoms. However, this situation
changes for TiN2, case in which results point to hopping-over the island
edge as the preferred pathway for descent on the terrace. We quantify the
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events observed in terms of total migration distance and residence times on
islands for each species studied, and discuss the potential effects of our
findings on initial nucleation kinetics, which clearly affect TiN thin film
growth modes.
10:40am B7-1-9 Do nitride alloys exhibit Vegard's-like linear
behaviour?, D. Holec (david.holec@unileoben.ac.at), P.H. Mayrhofer,
Montanuniversität Leoben, Austria
Early transition metal nitrides (TMN) and their alloys with Al are widely
used in various protective coatings due to their outstanding mechanical and
thermal stability. The increasing demand on coating performance from the
application-side often requires sophisticated designs of the protective thin
films. The major routes in a knowledge-based materials selection and
design are by architecture (e.g. use of multilayer or patterning) by alloying,
and by the combination of these two.
In the present paper we use Density Functional Theory (DFT) calculations
to address a topic related mostly to alloying, i.e. to assess the extend of
Vegard's-like linear behaviour of ternary TM-Al-N and TM-TM-N, and
quaternary TM-TM-Al-N (TM=Sc, Ti, Zr, Nb, Ta, Hf) alloys. In particular,
we will discuss the compositional dependence of lattice parameters,
energies of formation, bulk moduli and elastic properties.
As an example we will show that although some systems for various
properties exhibit reasonably linear dependence on composition (e.g. lattice
parameter of cubic Ti1–xAlxN), in general it is dangerous to blindly use the
so-called „Vegard's rule“. A consequence of the non-linear dependence of
the lattice constants on e.g., the driving force for decomposing the
metastable c-TM-Al-N phases will be discussed. On another front, detailed
calculations of single crystal elastic constants of ZrN-AlN system reveal
that the elastic responce of the cubic ternary system is very similar to ZrN
up to AlN mole fraction ~40%, only after which the elastic properties
change drastically towards the AlN elastic behaviour.
11:00am B7-1-10 Toughness Enhancement in Transition Metal Nitride
Thin Films by Alloying and Valence Electron Concentration Tuning, D.
Sangiovanni, V. Chirita (vio@ifm.liu.se), L. Hultman, Linköping
University, Sweden
Enhanced toughness in hard and superhard thin films is a primary
requirement for present day ceramic hard coatings, known to be prone to
brittle failure during in-use conditions, in modern applications. Based on
the successful approach and results obtained for TiN- and VN-based ternary
thin films [1,2], we expand our Density Functional Theory (DFT)
investigations to TiAlN-based quarternary thin films. (TiAl)1-xMxN thin
films in the B1 structure, with 0.06 < x < 0.75, are obtained by alloying with
M = V, Nb, Ta, Mo and W, and results show significant ductility
enhancements, hence increased toughness, in these compounds.
Importantly, these thin films are also predicted to be hard/superhard, with
similar and/or increased hardness values, compared to TiAlN. For (TiAl)1xWxN
these results have experimentally been confirmed recently [3]. The
general, electronic mechanism responsible for the ductility increase is
rooted in the enhanced occupancy of d-t2g metallic states, induced by the
valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect
is more pronounced with increasing valence electron concentration (VEC),
and, upon shearing, leads to the formation of a layered electronic structure,
consisting of alternating layers of high and low charge density in the
metallic sublattice. This unique electronic structure allows a selective
response to tetragonal and trigonal deformation: if compressive/tensile
stresses are applied, the structure responds in a “hard” manner by resisting
deformation, while upon the application of shear stresses, the layered
electronic arrangement is formed, bonding is changed accordingly, and the
structure responds in a “ductile/tough” manner as dislocation glide along
the {110} slip system becomes energetically favored [2]. The
findings presented herein open new avenues for the synthesis of hard, yet
tough, ceramic coatings, by tuning the VEC of alloying elements to
optimize the hardness/toughness ratio in relevant applications.
[1] D. G. Sangiovanni et. Al. Phys. Rev. B 81 (2010) 104107.
[2] D. G. Sangiovanni et. Al. Acta Mater. 59 (2011) 2121.
[3] T. Reeswinkel et. Al. Surf. Coat. Technol. 205 (2011) 4821.
11:20am B7-1-11 Bridging atomic structure with properties in III-
Nitride heterostructures, Komninou (komnhnoy@auth.gr), Aristoteles
University of Thessaloniki, Greece INVITED
New III-Nitride technology involves 1 -, 2- and 3-dimensional (nanowires,
quantum wells, quantum dots) nanostructures as the building blocks of
emerging novel photonic and electronic device applications. This
technology, although one of the most “environment-friendly” available in
the market is still far from being mature and hence devices are far from their
intrinsic limits. Much more research efforts are needed to address materials
related issues which are the bottleneck against the rapid advances of these