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Transition metal diboride-based thin films are promising candidates to replace state-of-the-art protective and functional coating materials due to their unique properties. Here, we focus on hexagonal WB2−z , showing that the AlB2 structure is stabilized by B vacancies exhibiting its energetic minima at sub-stoichiometric WB1.5. Nanoindentation reveals super-hardness of 0001 oriented α-WB2−z coatings , linearly decreasing by more than 15 GPa with predominant 1011 orientation. This anisotropy is attributed to differences in the generalized stacking fault energy of basal and pyramidal slip systems, highlighting the feasibility of tuning mechanical properties by crystallographic orientation relations.
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Materials Research Letters
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Anisotropic super-hardness of hexagonal WBz
thin films
C. Fuger, R. Hahn, L. Zauner, T. Wojcik, M. Weiss, A. Limbeck, O. Hunold, P.
Polcik & H. Riedl
To cite this article: C. Fuger, R. Hahn, L. Zauner, T. Wojcik, M. Weiss, A. Limbeck, O. Hunold,
P. Polcik & H. Riedl (2022) Anisotropic super-hardness of hexagonal WBz thin films, Materials
Research Letters, 10:2, 70-77, DOI: 10.1080/21663831.2021.2021308
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2022, VOL. 10, NO. 2, 70–77
Anisotropic super-hardness of hexagonal WB2±zthin films
C. Fuger a,R.Hahn a,L.Zauner a,T.Wojcik a,M.Weiss b,A.Limbeck b, O. Hunoldc,P.Polcik
H. Riedl a,e
aChristian Doppler Laboratory for Surface Engineering of high-performance Components, TU Wien, Wien, Austria; bInstitute of Chemical
Technologies and Analytics, TU Wien, Wien, Austria; cOerlikon Balzers, Oerlikon Surface Solutions AG, Balzers, Liechtenstein; dPlansee
Composite Materials GmbH, Lechbruck am See, Germany; eInstitute of Materials Science and Technology, TU Wien, Wien, Austria
Transition metal diboride-based thin films are promising candidates to replace state-of-the-art pro-
tective and functional coating materials due to their unique properties. Here, we focus on hexagonal
WB2z, showing that the AlB2structure is stabilized by B vacancies exhibiting its energetic minima at
sub-stoichiometric WB1.5. Nanoindentation reveals super-hardness of 0001 oriented α-WB2zcoat-
ings, linearly decreasing by more than 15 GPa with predominant 10¯
11 orientation. This anisotropy is
attributed to differences in the generalized stacking fault energy of basal and pyramidal slip systems,
highlighting the feasibility of tuning mechanical properties by crystallographic orientation relations.
First report of an anisotropic elastoplastic behaviour in super-hard PVD AlB2structured WB2z. Theo-
retical and experimental verification of thermodynamically most stable sub-stoichiometric α-WB2z
coatings by structural and mechanical analysis.
Received 20 September 2021
WB2; physical vapour
deposition; DFT; structural
defects; anisotropy;
Transition metal diborides (TMB2)exhibitatremen-
dous potential to be applied in various applications
ranging from wear- and corrosion resistant coatings,
superconductive thin lms, up to extremely stable pro-
tective layers [14]. Diborides of group 2–5 prefer to
crystallize in the hexagonal AlB2structure (α,space
group 191—P6/mmm), while TMB2of group 6 and
higher typically reveal the so-called W2B5based struc-
ture (ω, space group 194—P63/mmc) [5,6]– except for
the pretended Mo2B5and W2B5are Mo2B4and W2B4,
whereas only non-stoichiometric compounds TMB2±z
prefer to crystallize in the α-structure [9]. Nevertheless,
due to strongly limited kinetics during physical vapour
deposition (PVD), WB2zis stabilized in its metastable
α-phase rather than in the thermodynamically preferred
Supplemental data for this article can be accessed here.
ωstructure [1012]. Structural defects—being predom-
inant in PVD materials—are presumed to stabilize the
rial system, where sub-stoichiometric TiB1.9 is stabilized
by the absence of B between Ti-planes locally relaxing the
structure [13].
In general, the enhanced hardness of hexagonal mate-
rial systems is related to limited slip systems (slip plane
and -direction)—compared to cubic systems—impeding
dislocation movement and thus, plastic deformation [14].
Furthermore, in hexagonal SiC and GaN the dislo-
cation motion and activated slip systems have been
described and anisotropic mechanical properties were
shown [1519]. The experimental observations of SiC
single crystal polytypes revealed higher hardness of basal
indentations compared to prismatic indentations [15].
Moreover, Huang et al. indicated specic slip planes being
© 2022 The Author(s). Published by Informa UK Limited, trading as Taylor& Francis Group
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
active during plastic deformation of GaN, suggesting
that anisotropic elastoplastic mechanical properties cor-
relate with plastic deformation [17]. In contrast, no dis-
tinct orientation-dependent fracture toughness could be
detected so far [15,18].
Moreover, WB2±zis known for its enhanced material
ture toughness, and therefore often consulted as a base
system forming ternary TMITMIIB2±z[10,11,20,21].
In previous studies, we successfully showed the posi-
tive eect of Ta alloying on the mechanical properties
(H, KIC), thermal stability, and oxidation resistance of
WB2±zthin lms [10,21,22].
The focus of this study is to obtain new insights into
the elastoplastic behaviour of WB2±zthin lms concern-
ing their detailed structural constitution using dierent
structural as well as micro-mechanical characterization
techniques. Furthermore, theoretical investigations using
ab initio methods should clarify structural uncertainties
of the AlB2type WB2±z.
Computational details
DFT coded VASP [23,24] calculations using the projector
augmented waves method within the generalized gra-
dient approximation (PAW-PBE) [25]wereappliedto
investigate the energy of formation (Ef)andelasticcon-
stants of α-andω-structured WBxcells (x =2±z). The
perfect, as well as the defected structures, were gener-
ated using the SQS special quasirandom structure (SQS)
approach. The elastic constants were calculated using the
stress/strain method [26], further details are described in
the Supplementary Material.
Thin lm synthesis
To oer a broad spectrum of deposition parameters
beyond temperature, pressure, and bias, we used three
dierent deposition systems. An in-house developed
magnetron sputtering system (for details see Supple-
mentary) an AJA International Orion 5 magnetron
sputtering system, and an Oerlikon Balzers Innova depo-
sition system providing diverse sputter congurations
with dierent target-substrate distances and angles. The
coating facilities have been equipped with six-inch and
three-inch powder-metallurgically prepared W2B4tar-
gets—manufactured by Plansee Composite Materials
GmbH—exhibiting the ω- structure. The deposition pro-
cesses were carried out in pure argon with a rotating
substrate holder. For detailed information to the in-
house developed system and on the variated parameter
settings on each deposition system, see Table S1–3 in the
Characterization methods
For detailed structural characterization of all coat-
ings deposited, a Philips XPERT diractometer in
endstation (DESY Petra III), were used (selected
Hardness (H), and Young’s modulus (E), of all lms,
were investigated by an Ultra Micro-Indentation System
(UMIS), equipped with a Berkovich diamond tip. The
resulted loading and unloading curves were evaluated
afterOliverandPharr[27] to gain H and E, respectively.
The elemental composition of selected lms on Si
substrates was analyzed by liquid inductively coupled
OES measurements were carried out on an iCAP 6500
RAD (Thermo Fisher Scientic, USA), with an ASX-
520 autosampler (CETAC Technologies, USA) using an
HF resistant sample introduction kit, consisting of a
Miramist nebulizer (Burger Research, USA), a PTFE
spray chamber and a ceramic injector tube. The WB2z
coatings were acid digested with the method presented
and validated in [2830]–describedindetailintheSup-
plementary Material.
In addition, one selected sample was surveyed by
transmission electron microscopy (TEM FEI TECNAI,
F20, acceleration voltage of 200kV). Detailed struc-
diraction (SAED). The sample preparation was done
by focused ion beam (FIB, Quanta 200 3D DualBeam),
applying a standard lift-out technique [31].
In situ micromechanical bending tests of substrate-
free coating cantilevers are conducted to obtain the frac-
ture toughness of selected coatings. The experiments
were performed within an SEM equipped with a Hysitron
PI-85 SEM pico-indenter whose spherical diamond tip
was pressed onto the top of the pre-notched cantilever (in
the growth direction of the coatings) until fracture [32].
The calculation of the fracture toughness was per-
formed after Matoy et al. [33]—see also Supplementary
Results and discussion
DFT calculations revealed the energy of formation, Ef,of
perfect and defected α-andω-structuredWB
In Figure 1(a) the development of Efwith increasing B
vacancies (blue lled squares for α, red lled triangles for
ω), W vacancies (blue half-lled squares for α,redhalf-
lled triangles for ω) and Schottky defects (blue empty
Figure 1. (a) Efvalues of fully converged α-WBx(2×2×4) and ω-WBx(2×2×1) supercells (x =1.25–4.0) as a function of chemical
composition represented by the value of x =2±zin WBx.Figure1b gives the evolution of lattice constant a (orange triangles), and c
(blue squares) of α-WBx. Additionally, the elastic constant C44 (c), the theoretical hardness Htheo (d), and the theoretical Young’s modulus
Etheo (e) of all mechanically stable structures are illustrated. The change in valence electron concentration (VEC) of defected WBxcrystals
is depicted in the abscissa on top of Figure 1a–e.
squares for α,redemptytrianglesforω), is indicated,
respectively. The vertical grey line represents stoichio-
metric WB2expressing the value of x =2.0 in WBx.
With increasing B vacancy population x is decreasing,
while an increase of W vacancies leads to an increase of x
(indicated on the bottom abscissa of Figure 1(a)). More-
over, an increasing vacancy concentration is decreasing
the valence electron concentration (VEC) of the material
system, highlighted on the top abscissa of Figure 1(a).
Generally, Efraises by adding vacancies to the ω-
lattice, having its minima around the perfect W2B4/WB2
stoichiometry [9] and lowered with increasing number
of vacancies within the α-lattice. The α-structured cell is
thermodynamically preferred compared to ω—meaning
Efof α-WB2zis below ω-WB2z—at a vacancy con-
centration >6%. Boron defected α-andω-structured
cells energetically intersect at WB1.70 (Ef=−0.23 eV/at)
followed by a thermodynamic minimum at WB1.50
(Ef=−0.28 eV/at) for the α-lattice. The atomic concen-
tration of the α-cell at the Efminima leads to 40 at.%
W and 60 at.% B, hence matching the experimentally
measured compositions obtained by ICP-OES for var-
ious WB2zlms deposited on the dierent routes
(WB2_01: W =40.74 ±0.91 at.%, B =59.26 ±0.91
at.%; WB2_20: W =39.14 ±1.67 at.%, B =60.86 ±
1.67 at.%; WB2_21: W =40.41 ±2.04 at.%, B =59.59
±2.04 at.%). Moreover, investigations on the evolu-
tion of α-WB2±zlattice constants revealed convergence
of α-WB1.5 (a =3.0488 Å, b =3.0483 Å, c =3.0683
Å, V =24.74 Å3) during DFT calculations with our
experimentally obtained values of WB1.47 (a =3.0168 Å,
c=3.0608 Å) using nanobeam diraction on powdered
coating material (stars in Figure 1(b))—for details see
Supplementary. Due to correlating lattice parameters
from our α-WB1.47 with those reported from Woods
et al. (a =b=3.0200 Å, c =3.0500 Å, V =24.09
Å3)[34], we would suggest a sub-stoichiometric compo-
sition also for their structure. In addition, Hayami et al.
theoretically determined lattice constants for WB1.625 of
a=b=3.072 Å, c =3.117 Å [35].
All defected lattice congurations (considered in
Figure 1(a)) have been consulted for calculating the
stiness tensor C. To ensure validity of the resulted
elastic constants Cij, the following criteria have to be ful-
lled in hexagonal crystals: C11 >|C12|; 2C2
13 <C33 ·
(C11 +C12);C
44 >0; C66 >0. All data points presented
in Figure 1(b–d) satisfy the above-mentioned stability
conditions, thus revealing mechanically stable structures.
For the quantication of mechanical stability of per-
fect and defected structures, the elastic constant C44 is
highlighted in Figure 1(c). The data points reveal val-
ues of C44 =131 GPa for stochiometric αas well as
C44 =221 GPa for ω, indicating enhanced mechanical
stability for ω. This trend is inverted by introducing B
vacancies to the crystal structures leading to a maxi-
mum C44 =250 GPa at x =1.5 and VEC =10.5 for α
structured WB1.5.Moreover,theC
44 maximum of the
α-lattice is correlating with the Efminima (Figure 1(a)),
revealing highest thermodynamic stability is also lead-
ing to the highest mechanical stability of this crystal.
Furthermore, theoretical hardness values, Htheo,have
been evaluated using a widely established model [36],
and bulk modulus, respectively. Figure 1(d) illustrates
Htheo, showing a maximum value of Htheo =28 GPa
for αstructured WB1.5 and maximum Htheo =31 GPa
for ω-WB1.93. The same trend was experienced for the
theoretical Young’s moduli (E =9·B·G/(3B+G)) with
a maximum Etheo =546 GPa for αstructured WB1.5
and Etheo =565 GPa for ω-WB1.93, as indicated in
Figure 1(e).
Although the used target material is of ω-WB2struc-
ture, the deposition of ω-WB2coatings points out to be
very challenging, since α-structured WB2zis prefer-
entially formed within magnetron sputtering techniques
(see XRD patterns in the Supplementary Material). The
broad parameter variation on 3 dierent deposition sys-
tems revealed always sub-stoichiometric α-WB2zstruc-
tured thin lms but in various crystal orientations. The
sity especially on the non-metal sublattice (dislocations,
vacancies) due to the extreme cooling rates during con-
densation from the vapour phase to the solid state. Fur-
thermore, the dierence in mass between light (B) and
heavy (W) elements promotes scattering eects dur-
ing sputtering leading to sub-stoichiometric composi-
tions. A highly 0001 oriented α-WB2zcoating (WB1.45)
has been investigated using TEM (see Figure 2). The
coating exhibits a columnar and defected morphology,
see Figure 2(a,b). Section 2a also represents the area
for the recorded SAED pattern, depicted in the inset
a-i. SAED exhibits highly oriented crystals in [11¯
zone axis. Additionally, the inset a-i contains a VESTA
model [37]oftheα-structured WB2unit cell—the W
and B atoms are represented in red and blue, respec-
tively—oriented as obtained from SAED. Figure 2(b)
shows a high-resolution TEM image of the investigated
coating, emphasizing defected zones within the highly-
oriented crystal. However, the FFT image, depicted
in section c, conrms the same crystal orientation as
already revealed from SAED –masking regions for the
white dashed circles. The sections d-f show ltered TEM
images (for technical details see [38]) from the same
region marked in b overlaid with a masked IFFT in
the depicted directions ((1000), (0001), and (10¯
11) for
d, e, and f respectively). Conducting this procedure,
defect/strain-rich domains can be highlighted, conrm-
ing the structural stabilization in the α-phase of the
chemically sub-stoichiometric WB1.45 lm. In corre-
spondence to [13], structural defects (i.e. Boron vacan-
cies) seem to compensate for the sub-stoichiometry of the
In Figure 3(a) experimentally determined hardness H
(blue open squares) and Young’s modulus E (red open
triangles) are plotted as a function of increasing frac-
tion of 0001 lattice orientation of the various α-WB2z
coatings. H is increasing from 25 GPa for coatings
without any 0001 orientation up to 40 GPa for purely
0001 oriented coatings. Thus, the dataset reveals a lin-
ear dependency of H with an increasing 0001 ratio (see
linear t with 95% condence limit in Figure 3(a)). On
the other hand, the evaluation of the 10¯
11 ratio shows
a contrary picture. Figure 3(b) depicts the experimen-
tally determined H as a function of increasing 10¯
11 ratio
(blue open squares), revealing a decrease of H with an
increasing 10¯
11 orientation (blue dashed line and blue
mechanical property of α-WB2zcoatings, revealing the
highest hardness when 0001 oriented by simultaneously
11 orientation. In comparison, the sto-
chiometric α-structured WB2ispredictedtoobtaina
Htheo =15 GPa, whereas the boron defected α-WB1.5
reveals Htheo =28 GPa coinciding with the experimen-
theo neither
anisotropic eects nor hardening due to a Hall-Petch
eect is considered.
The observed anisotropy in hardness can be related
to aggravated dislocation movement due to energeti-
cally less preferred slip systems. Through DFT, Hunter
et al. [39] showed that various slip systems in hexagonal
ZrB2(AlB2prototype, SG191, α) reveal dierent general-
ized stacking fault energies (GSFE). The <a>type basal
slip 0001 is the easiest a-type slip system to activate, ener-
getically followed by pyramidal <1¯
210 >10¯
prismatic <1120 >10 ¯
10 slip systems. Additionally, the
Figure 2. TEM analysis of the WB1.45 coating. Section a presents a cross-sectional BF image of the WB1.45 lamella, pointing out the area
for the recorded SAED (white dashed circle) displayed in the inset a–i. An FFT cut out of the HR-TEM in b (region of interest) is depicted
in section c, furthermore marking the masking regions for the IFFT as white dashed circles. Section d-f show defect/strain rich domains
corresponding to the indicated directions (based on IFFT).
Figure 3. Hardness H (blue open squares) and Young’s modulus E (red open triangles) of various α-WB2zthin films deposited. The
dataset presents the mechanical properties as a function of the 0001 (a) and 10¯
11 lattice plane (b) orientation factor, determined from
XRD data (see Supplementary Material). The dashed lines give a linear fit from H (blue dashed line) and E (red dashed line). The blue and
red shaded areas represent a 95% confidence limit.
Figure 4. The hexagonal α-WB2structure is illustrated in 0001 (a)
and 10¯
11 (b) orientation. W and B atoms are depicted in green
and grey, respectively. Indentation experiments leading to a nor-
mal force F (grey arrow) which is appearing perpendicular to the
(0001) plane (a) (basal slip plane, orange area) or (10¯
11) plane (b)
(pyramidal slip plane, red area).
calculated GSFE values are correlating with interplanar
spacing, meaning the closer the planes are spaced, the
larger the GSFE value becomes. Due to structural corre-
lations of α-ZrB2and α-WB2z, we can assume a similar
behaviour for both hexagonal material systems. Thus, a
hardness increase for [0001] crystals can be explained by
a larger GSFE value of the pyramidal slip system (10¯
slip plane)—compared to the basal slip system—which
experiences the maximum shear stress τmax whenaforce
(during indentation) is applied in [0001] direction (see
Figure 4(a)). However, for a 10¯
11 crystal, the basal slip
system (0001 slip plane) is preferentially activated due
to τmax appearing at a 45° angle to the force vector F
(Schmid’s law), directing in [10¯
11] in this scenario (see
Figure 4(b)).
In contrast to the hardness results, indentation exper-
iments revealed relatively constant Young’s moduli in
the range of 500 GPa, depicted in Figure 3(a,b). The
data set emphasizes that the 0001 and 10¯
11 crystal ori-
by the linear t; red dashed line). For comparison, the
DFT calculations exhibit a Young’s modulus of 408 GPa
for perfect α-WB2structured cells. Only after intro-
ducing B vacancies, the theoretical Young’s modulus
increases to Etheo =546 GPa for α-WB1.5 approaching
the experimentally observed data. Moreover, by evalu-
ating the spatial dependency of the Young’s modulus of
perfect α-WB2and defected α-WB1.5 acleardecrease
of the anisotropy for the α-WB1.5 could be observed
(see Figure S4 and S5 in the Appendix). The anisotropy
reduces from 1.804 for α-WB2to 1.151 for α-WB1.5
structure within our WB2zthin lms.
In addition to the anisotropy of the hardness, we also
evaluated the fracture toughness of selected coatings to
gain a deeper insight into a possible orientation-related
fracture behaviour. Micromechanical cantilever bending
experiments of selected α-WB2zcoatings,revealedK
values ranging from 2.89 ±0.26 MPam(WB
1.45, 0001-
ratio: 0.80), 3.23 ±0.19 MPam(WB
1.87, 0001-ratio:
0.99), to 3.65 ±0.26 MPam(WB
1.55, 0001-ratio: 0.04),
revealing no perceptible inuence of the lm orientation
(for further details see Table S4 in the Supplementary
Material). The results indicate that the fracture tough-
ness of our thin lm materials is not aected by dislo-
cation movement, hence other mechanisms govern the
observed variation in KIC values. Such mechanisms can
be for example: the cohesive grain boundary strength
(inuenced by the growth conditions or the introduc-
tion of column boundary ane elements), introduction
of third order residual stresses by i.e. precipitation tough-
ening or unwanted impurities such as oxygen, or eects
of the microstructure on the fracture behaviour.
In summary, the AlB2structure formation of WB2±zwas
investigated by DFT. The calculations indicate the stabi-
lization of hexagonal α-WB2zby B vacancies, compared
to a thermodynamic minimum of perfect, stoichiometric
ω-WB2. This theoretical result is experimentally under-
lined by: matching lattice parameters, mechanical prop-
erties, and chemical compositions of physical vapour
deposited α-WB2zthin lms. Moreover, nanoindenta-
tion of the synthesized coatings revealed anisotropy in
the elastoplastic behaviour. Super-hardness was deter-
mined for 0001 oriented lms, linearly decreasing by
11 orientation.
In contrast, no impact of the crystal orientation on KIC
could be detected.
We also thank for the nancial support of Plansee SE, Plansee
Composite Materials GmbH, and Oerlikon Balzers, Oerlikon
centre (XRC) of TU Wien for beam time as well as the electron
microscopy centre—USTEM TU Wien—for using the SEM
liothek for nancial support through its Open Access Funding
Programme. The computational results presented have been
achieved using the Vienna Scientic Cluster (VSC). We fur-
ther acknowledge the granted use of the Nanofocus Endstation
of the Beamline P03 of PETRA III at DESY, a member of the
Helmholtz Association (HGF).
Disclosure statement
No potential conict of interest was reported by the author(s).
The nancial support by the Austrian Federal Ministry for Dig-
ital and Economic Aairs, the 10.13039/100007224 National
Foundation for Research, Technology and Development and
the Christian Doppler Research Association is gratefully
acknowledged (Christian Doppler Laboratory ‘Surface Engi-
neering of high-performance Components’).
C. Fuger
R. Hahn
L. Zauner
T. Wojcik
M. Weiss
A. Limbeck
H. Riedl
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... In a recent study, physical vapor deposited (PVD) silicon alloyed transition metal (TM) diboride thin films (TM-Si-B 2±z , TM= Ti, Cr, Hf, Ta, W) are suggested as highly durable protective coating materials, sustaining temperatures beyond 1200 • C in oxidative atmospheres [6]. This group of materials combines the outstanding thermal stability and mechanical properties of TM-diborides [7] with the excellent oxide former Si, being a well-established approach in the field of TM-diboride bulk ceramics (addition of SiC or metal silicides) [8][9][10]. Previous studies also highlighted the potential of silicon alloyed ZrB 2 PVD coatings [11,12]. ...
... Additional Si was added to the thin films by evenly placing single crystalline Si plates (7×7× 0.38 mm) on the target racetrack. By varying the number of pieces (5)(6)(7)(8), the Si content is adjusted within the thin films. The thin films were deposited on sapphire (10×10×0.53 ...
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Within physical vapor deposited Hf-Si-B2±z thin films, selective diffusion-driven oxidation of Si is identified to cause outstanding oxidation resistance at temperatures up to 1500 °C. After 60 h at 1200 °C, the initially 2.47 µm thin Hf0.20Si0.23B0.57 thin film exhibits a dense oxide scale of only 1.56 µm. The thermally induced decomposition of metastable Hf-Si-B2±z leads not only to the formation of Si precipitates within the remaining thin film (related to a non-homogenous Si distribution after the deposition) but also to pure Si layers on top and bottom of the Hf-Si-B2±z coatings next to the excellent adherend SiO2 based scales.
... CrB 2 may possess a significant ductile character -in combination with a strong, covalently bound 2D boron network [69] -thus explaining the high fracture toughness value. Quantitatively grasping these differences in bond strength is, however, extremely difficult. ...
Planar defect structures appearing in transition metal diboride (TMB2) thin films, grown by different magnetron sputtering-deposition approaches over a wide compositional and elemental range, were systematically investigated. Atomically resolved scanning transmission electron microscopy (STEM) imaging, electron energy loss spectroscopy (EELS) elemental mapping, and first principles calculations have been applied to elucidate the atomic structures of the observed defects. Two distinct types of antiphase boundary (APB) defects reside on the {1-100} planes. These defects are without (named APB-1) or with (APB-2) local deviation from stoichiometry. APB-2 defects, in turn, appear in different variants. It is found that APB-2 defects are governed by the film's composition, while APB-1 defects are endemic. The characteristic structures, interconnections, and circumstances leading to the formation of these APB-defects, together with their formation energies, are presented.
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CrBx thin films with 1.90 ≤ x ≤ 2.08 have been deposited by direct-current magnetron sputtering (DCMS) from a stoichiometric CrB2 target at 5 and 20 mTorr (0.67 and 2.67 Pa) Ar pressure onto sapphire (0001) substrates. All films, irrespective of deposition conditions, exhibit a (0001) texture. Attesting to the achievement of close-to-stoichiometric composition, epitaxial film growth is observed at 900°C, while film growth at 500 °C yields (0001) fiber texture. Film composition does not depend on substrate temperature but exhibits slightly reduced B content with increasing pressure for samples deposited at 900°C. Excess B in the overstoichiometric epitaxial CrB2.08 films segregates to form B-rich inclusions. Understoichiometry in CrB1.90 films is accommodated by Cr-rich stacking faults on {11¯00} prismatic planes.
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The atomic structure and local composition of high quality epitaxial substoichiometric titanium diboride (TiB1.9) thin film, deposited by unbalanced magnetron sputtering, were studied using analytical high-resolution scanning transmission electron microscopy, density functional theory, and image simulations. The unpaired Ti is pinpointed to inclusion of Ti-based stacking faults within a few atomic layers, which terminates the {11¯00} prismatic planes of the crystal structure and attributed to the absence of B between Ti planes that locally relaxes the structure. This mechanism allows the line compound to accommodate off-stoichiometry and remain a line compound between defects. The planar defects are embedded in otherwise stoichiometric TiB2 and are delineated by insertion of dislocations. An accompanied decrease in Ti-Ti bond lengths along and across the faults is observed.
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Ternary W 1−x Ta x B 2−z is a promising protective coating material possessing enhanced ductile character and phase stability compared to closely related binaries. Here, the oxidation resistance of W 1−x Ta x B 2−z thin films was experimentally investigated at temperatures up to 700 °C. Ta alloying in sputter deposited WB 2−z coatings led to decelerated oxide scale growth and a changed growth mode from paralinear to a more linear (but retarded) behavior with increasing Ta content. The corresponding rate constants decrease from k* p = 6.3 ⋅ 10 −4 µm 2 /s for WB 2−z , to k* p = 1.1 ⋅ 10 −4 µm 2 /s for W 0.66 Ta 0.34 B 2−z as well as k l = 2.6 ⋅ 10 −5 µm/s for TaB 2−z , underlined by decreasing scale thicknesses ranging from 1170 nm (WB 2−z), over 610 nm (W 0.66 Ta 0.34 B 2−z) to 320 nm (TaB 2−z) after 10 min at 700 °C. Dense and adherent scales exhibit an increased tantalum content (columnar oxides), which suppresses the volatile character of tungsten-rich as well as boron oxides, hence being a key-factor for enhanced oxidation resistance. Thus, adding Ta (in the range of x = 0.2-0.3) to α-structured WB 2−z does not only positively influence the ductile character and thermal stability but also drastically increases the oxidation resistance.
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The room-temperature plastic deformation behavior of 6H–SiC single crystals has been investigated by uniaxial compression of micropillar specimens as a function of crystal orientation and specimen size. Plastic flow is observed even at room temperature by basal and prism slip, latter of which have never been observed in the bulk. The CRSS values for basal and prism slip are as high as above 5 and 6 GPa at the specimen size of 5 μm, respectively, each of which increases with decreasing specimen size, following an inverse power-law relationship with a relatively small power-law exponent of ∼0.10 and ∼0.21, respectively. The CRSS values for basal slip are not virtually affected by the existence of basal dislocations introduced at 1300 °C prior to micropillar compression tests at room temperature. The majority of basal dislocations observed after micropillar compression are perfect (undissociated) screw dislocations, and they are considered to be introduced in the shuffle-set plane during micropillar testing, unlike widely dissociated dislocations introduced in the glide-set plane in the bulk during high-temperature deformation. Prism dislocations are observed also to glide as perfect (undissociated) dislocations and tend to align strongly along their screw orientation. The fracture toughness values are estimated to be 1.37 ± 0.13 and 1.57 ± 0.13 MPa m1/2 by three-point bend tests for chevron-notched single crystalline specimens with a notch plane being parallel to (0001) and {011¯0} planes, respectively.
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By combining experimental and theoretical methods, we have conducted a detailed study of the ternary diboride system (W1 xAlx)1-yB2(1-z). Tungsten rich solid solutions of (W1 xAlx)1-yB2(1-z) were synthesized by physical vapor deposition and subsequently investigated for structure, mechanical properties and thermal stability. All crystalline films show hardness values above 35 GPa, while the highest thermal stability was found for low Al contents. In this context, the impact of point defects on the stabilization of the AlB2 structure type is investigated, by means of ab initio methods. Most notably, we are able to show that vacancies on the boron sublattice are detrimental for the formation of Al-rich (W1 xAlx)1-yB2(1-z), thus providing an explanation why only tungsten rich phases are crystalline.
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Based on density functional theory, we recently suggested that metastable α-WB2 is a promising candidate combining very high hardness with high toughness. These calculations further suggested that the addition of Tantalum supports the crystallization of α-structured W1-xTaxB2-z, with only minor reduction in toughness. Thus, various Ta containing WB2-based coatings have been synthesized using physical vapor deposition. With increasing Ta content, the hardness increases from ∼41 GPa (WB2) to ∼45 GPa (W0.74Ta0.26B2). In situ micromechanical cantilever bending tests exhibit fracture toughness KIC values of 3.7 to 3.0 MPa√m for increasing Ta content (single-phased up to 26 at.% Ta).
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The demand to discover new materials is scientifically as well as industrially a continuously present topic, covering all different fields of application. The recent scientific work on thin film materials has shown, that especially for nitride-based protective coatings, computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments. In this study, semi-automated density functional theory (DFT) calculations were used, to sweep across transition metal diborides in order to characterize their structure, phase stability and mechanical properties. We show that early transition metal diborides (TiB2, VB2, etc.) tend to be chemically more stable in the AlB2 structure type, whereas late transition metal diborides (WB2, ReB2, etc.) are preferably stabilized in the W2B5−x structure type. Closely related, we could prove that point defects such as vacancies significantly influence the phase stability and even can reverse the preference for the AlB2 or W2B5−x structure. Furthermore, investigations on the brittle-ductile behavior of the various diborides reveal, that the metastable structures are more ductile than their stable counterparts (WB2, TcB2, etc.). To design thin film materials, e.g. ternary or layered systems, this study is important for application oriented coating development to focus experimental studies on the most perspective systems.
We review the thin film growth, chemistry, and physical properties of Group 4–6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
The present study investigates the performance of four selected PVD coating materials in application near hot-forming procedure of Al-Si coated 22MnB5 steel sheets. These are: a Ti0.57Al0.43N/Mo-Si-B multilayer, a TiN base layer having a Mo-Si-B top layer, a DLC, and an Al-Cr-O coating. We determined the amount of adherent material (after the hot-forming procedure) by 3D topographical analysis and characterised the build-up volume as well as the coating/build-up interface constitutions with respect to the diffusivity of the participating elements. The smallest build-up volume could be identified for the Ti0.57Al0.43N/Mo-Si-B multilayer as well as the DLC coating, whereas the highest adhesive wear peak-value during the whole testing sequence was detected for the Al-Cr-O coating. Based on detailed TEM analysis of the diffusion zones (between build-ups and coating materials), we can conclude that the smallest adhesive wear is obtained, if the interface formation (between build-up and coatings) is narrow and Fe-free or at least Fe-lean.