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Role of Si segregation in the structural, mechanical, and compositional evolution of high-temperature oxidation resistant Cr-Si-B2±z thin films


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This work investigates the influence of Si-alloying up to 17 at.% on the structural, mechanical, and oxidation properties of magnetron sputtered CrB2±z-based thin films. Density-functional theory calculations combined with atom probe tomography reveal the preferred Si occupation of Cr-lattice sites and an effective solubility limit between 3 to 4 at.% in AlB2-structured solid solutions. The addition of Si results in refinement of the columnar morphology, accompanied by enhanced segregation of excess Si along grain boundaries. The microstructural separation leads to a decrease in both film hardness and Young’s modulus from H ~ 24 to 17 GPa and E ~ 300 to 240 GPa, respectively, dominated by the inferior mechanical properties of the intergranular Si-rich regions. Dynamic thermogravimetry up to 1400 °C reveals a significant increase in oxidation onset temperature from 600 to 1100 °C above a Si content of 8 at.%. In-situ X-ray diffraction correlates the protective mechanism with thermally activated precipitation of Si from the Cr-Si-B2±z solid solution at 600 °C, enabling the formation of a stable, nanometer-sized SiO2¬¬-based scale. Moreover, high-resolution TEM analysis exposes the scale architecture after dynamic oxidation to 1200 °C (10 K/min heating rate) – consisting only of ~20 nm amorphous SiO2 beneath ~200 nm of nanocrystalline Cr2O3. In summary, the study provides detailed guidelines connecting the chemical composition with the respective thin film properties of high-temperature oxidation resistant Cr-Si-B2±z coatings.
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Journal of Alloys and Compounds
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Research article
Role of Si segregation in the structural, mechanical, and compositional
evolution of high-temperature oxidation resistant Cr-Si-B
2 ± z
thin films
L. Zauner
, A. Steiner
, T. Glechner
, A. Bahr
, B. Ott
, R. Hahn
, T. Wojcik
, O. Hunold
J. Ramm
, S. Kolozsvári
, P. Polcik
, P. Felfer
, H. Riedl
Christian Doppler Laboratory for Surface Engineering of high-performance Components, TU Wien, Austria
Department of Materials Science, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
Institute of Materials Science and Technology, TU Wien, Austria
Oerlikon Balzers, Oerlikon Surface Solutions AG, Liechtenstein
Plansee Composite Materials GmbH, Germany
article info
Article history:
Received 13 December 2022
Received in revised form 2 February 2023
Accepted 6 February 2023
Available online 7 February 2023
Thin films
Si alloying
Oxidation resistance
Phase stability
Mechanical properties
This work investigates the influence of Si-alloying up to 17 at.% on the structural, mechanical, and oxidation
properties of magnetron sputtered CrB
2 ± z
-based thin films. Density-functional theory calculations com-
bined with atom probe tomography reveal the preferred Si occupation of Cr-lattice sites and an effective
solubility limit between 3 to 4 at.% in AlB
-structured solid solutions. The addition of Si results in refine-
ment of the columnar morphology, accompanied by enhanced segregation of excess Si along grain
boundaries. The microstructural separation leads to a decrease in both film hardness and Young’s modulus
from H 24 to 17 GPa and E 300 to 240 GPa, respectively, dominated by the inferior mechanical prop-
erties of the intergranular Si-rich regions. Dynamic thermogravimetry up to 1400 °C reveals a significant
increase in oxidation onset temperature from 600 to 1100 °C above a Si content of 8 at.%. In-situ X-ray
diffraction correlates the protective mechanism with thermally activated precipitation of Si from the Cr-Si-
2 ± z
solid solution at 600 °C, enabling the formation of a stable, nanometer-sized SiO
-based scale.
Moreover, high-resolution TEM analysis exposes the scale architecture after dynamic oxidation to 1200 °C
(10 K/min heating rate) – consisting only of 20 nm amorphous SiO
beneath 200 nm of nanocrystalline
. In summary, the study provides detailed guidelines connecting the chemical composition with the
respective thin film properties of high-temperature oxidation resistant Cr-Si-B
2 ± z
© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
1. Introduction
Transition metal diboride (TMB
) thin films are promising can-
didates to replace state-of-the-art functional and protective coating
materials in a wide range of applications [1–8]. TMB
s typically
feature a high melting temperature, excellent thermal stability, as
well as high hardness and strength, thus providing a strong incentive
for ultra-high temperature applications [9–13]. However, this out-
standing property spectrum is usually confined to inert atmospheres
due to the consecutive/competitive formation of both TM- and B-
based oxide scales, both usually incapable of forming a fully pro-
tective layer at temperatures beyond 600–700 °C [14–18]. More
drastically, above 1100 °C linear mass gain kinetics are regularly
observed, which coincides with the evaporation of the glassy-like
boria (B
) embedded within the non-protective, porous metal
oxide [18,19].
Different alloying concepts have been studied and implemented
successfully to address the poor oxidation resistance of TMB
and thin film materials. Adding Si-containing compounds such as
SiC, MoSi
, Si
, or Ta
is the most commonly used method for
bulk diboride materials and improves the oxidation resistance by
forming a stable, amorphous (boro-)silicate surface layer [19–21].
For instance, Fahrenholtz et al. demonstrated that adding SiC to ZrB
and HfB
permits drastically decreased oxidation rates up to
1600 °C [19].
Regarding TMB
-based thin film materials, several ternary al-
loying routes, e.g., the addition of Al(B
), TaB
, or CrB
, have been
explored to improve the oxidation resistance [14–16,22,23]. Bakhit
et al. [15] demonstrated that Al alloying into TiB
-based thin films
Journal of Alloys and Compounds 944 (2023) 169203
0925-8388/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (
Correspondence to: Christian Doppler Laboratory for Surface Engineering of high-
performance Components, TU Wien, Getreidemarkt 9, 1060 Wien, Austria.
E-mail address: (L. Zauner).
ORCID-ID: 0000–0002-8373–6552
significantly retards the oxide scale growth at temperatures up to
800 °C based on the formation of a dense Al-oxide surface layer.
Moreover, Kashani et al. [17] showed that the Ti-Al-B
2 ± z
system even
outperforms the corresponding nitride system at 700 °C, especially
for stoichiometric compositions close to B/TM ratios of 2. Indeed,
this necessity for tailoring the B/TM-ratio within TiB
2 ± z
thin films to
optimize oxidation properties is highlighted in several works and
rationalized by the fast-track oxidation pathway created through
excess B-rich phases preferentially located at column and grain
boundaries [24–26]. However, this effect appears specific to the
material system and/or annealing treatment conditions, since a
stable, protective boria surface layer was observed for HfB
films up to 900 °C [27].
A seemingly universal alloying route for improved oxidation re-
sistance was recently published by Glechner et al. [28,29], showing
that co-sputtering of pure Si to various TMB
(TM = Ti, Cr, Hf, Ta, W)
drastically improves the oxidation resistance in all materials, with
the onset of oxidation elevated to 1200 °C specifically for Cr-Si-B
and Hf-Si-B
. Thereby, the protective mechanism relies on the for-
mation of a stable Si-rich oxide scale above the formed TM-Si-B
solid solution.
Inevitably, the ensemble of available tools to improve the oxi-
dation resistance within TMB
thin films influences the property
spectrum of the initial binary alloy. While strategies involving
ternary TM
2 ± z
thin films proved successful only below
800 °C, their mechanical properties, including hardness and fracture
toughness, can often be preserved or even improved over the cor-
responding binary constituents [13–15]. Contrary, Si-based protec-
tive mechanisms can result in superior high-temperature
performance of TM-Si-B
thin films, however, typically at the ex-
pense of reduced mechanical properties at higher Si contents
[28,30]. Consequently, finding the optimum alloying content to
achieve the desired oxidation resistance while maintaining good
mechanical properties is a vital prerequisite for an industrial appli-
cation of the entire TM-Si-B
material family.
Therefore, within this work we systematically study the influ-
ence of Si-alloying on the structural evolution, phase stability, as
well as the mechanical and oxidation properties of magnetron
sputtered Cr-Si-B
2 ± z
thin films. This novel high-temperature ceramic
is modelled by density functional theory calculations to reveal the
energetically preferred lattice occupation of the alloying atom
within various AlB
-structured compositions. Furthermore, limita-
tions in the accessible alloying range to yield solid solutions are
discussed in conjunction with detailed atom probe tomography,
thereby spanning a clear connection to the observed thin film
growth and mechanical properties. The mechanism leading to the
drastically increased oxidation resistance is revisited through dy-
namic oxidation, in-situ X-ray diffraction, and transmission electron
microscopy, thus providing an in-depth correlation to the thin film
properties towards finding an optimum Si alloying composition.
2. Experimental
2 ± z
thin films were synthesized from a 3-inch CrB
(Plansee Composite Materials GmbH, 99.3% purity) in a pure Ar at-
mosphere (99.999% purity) using direct current magnetron sputtering
in an in-house developed deposition system (base pressure below
1.0 × 10
Pa). The Si content was adjusted by placing 0, 2, 4, 6, 8, 12, or
16 Si platelets (3.5 × 3.5 × 0.38 mm) on the target racetrack. The ro-
tating substrate holder (0.25 Hz) was positioned at a target-to-sub-
strate distance of 90 mm. All thin films were grown on Si ((100)-
oriented, 20 × 7 × 0.38 mm), single crystalline Al
10 × 10 × 0.53 mm), and poly-crystalline Al
(20 × 7 × 0.38 mm)
substrates, which were ultrasonically pre-cleaned in acetone and iso-
propanol, respectively. Following a heating sequence to a substrate
temperature of 550 °C, an Ar-ion etching step was performed at a total
pressure of 5 Pa and an applied substrate bias potential of −800 V for
10 min. The target and Si alloying platelets were sputter-cleaned for
3 min prior to all depositions to reduce oxygen contamination. The thin
films were then grown at a total Ar pressure of 0.7 Pa, a target current
of 0.4 A (corresponds to a power density of 5 W/cm
), and a bias
potential of −40 V. The synthesis conditions resulted in deposition rates
of 16.1 and 18.1 nm/min for 0 and 16 Si platelets on the target surface.
Aiming for a consistent coating thickness of 3 µm, deposition times of
190 and 170 min were selected, respectively. Deposition times for in-
termediate Si compositions were calculated from linear interpolation,
resulting in a total thickness variation of ± 0.1 µm between all samples.
The overall chemistry of the Cr-Si-B
2 ± z
coatings was obtained
using liquid inductively coupled plasma-optical emission spectro-
scopy (ICP-OES). A detailed explanation of this methodology is given
in Ref. [28]. Structural analysis was performed by X-ray diffraction
on a PANalytical XPert Pro MPD equipped with a Cu-K
source (wave-length λ = 1.54 Å, operated at 45 kV and 40 mA) in
Bragg-Brentano geometry. The cross-sectional growth morphology
was further investigated by scanning-electron microscopy (ZEISS
Sigma 500VP, operated at 5 kV) on coated Si substrates.
The hardness and elastic modulus of all coatings was studied
using instrumented nanoindentation (ultra-micro indentation
system, UMIS) with a minimum of 30 load-displacement curves
evaluated according to Ref. [31] for each coating. Increasing in-
dentation loads ranging from 5 to 22 mN (steps of 0.5 mN), with
additional measurements up to 45 mN to probe for any substrate
influence, were applied. Moreover, the obtained E values were fitted
over the indentation depth using a power law function and extra-
polated to the sample surface to receive the film-only modulus [32].
Macro-stresses in the coatings were additionally analyzed through
curvature measurements using optical profilometry (PS50, Nanovea)
and the modified Stoney equation [33,34]. All mechanical properties
were determined on coated sapphire substrates.
The oxidation behavior of the Cr-Si-B
2 ± z
thin films was de-
termined from thermogravimetric analysis (TGA, Netzsch STA 449
F1, equipped with a Rhodium furnace) performed on coated poly-
crystalline Al
substrates. The substrates were weighed before and
after deposition to determine the coating-only mass. This value
serves as a reference during dynamic oxidation experiments up to
1400 °C (heating rate of 10 °C/min) in a synthetic air environment
(50 ml/min flow rate). Any oxidation-related mass change was re-
corded at a resolution of 0.1 µg. Pre-tests on uncoated Al
strates additionally proved their inertness during the oxidation
treatment [28].
Additional oxidation experiments combined with in-situ X-ray
diffraction analysis were carried out on a PANalytical XPert Pro MPD
radiation source, wave-length λ = 1.54 Å, operated at 45 kV
and 40 mA) in Bragg-Brentano geometry using an Anton Paar high-
temperature furnace chamber (HTK 1200 N). Measurements were
taken in a lab-air environment (0.3 l/min flow rate) at room-tem-
perature and from 400 to 1200 °C in 50 °C steps. The sample was
heated at a rate of 50 °C/min between the individual temperature
steps, with each diffraction measurement taking 21 min
Furthermore, detailed microstructural and chemical analysis on
selected oxidized samples was performed using transmission elec-
tron microscopy (TEM, FEI TECNAI F20, operated at 200 kV). Bright-
field (BF) and high-angle annular dark field (HAADF) imaging are
utilized to gain information on the microstructure and oxide scale
growth. In addition, energy dispersive X-ray spectroscopy (EDX)
performed in scanning TEM (STEM) revealed the chemical compo-
sition of the entire coating cross-section as well as the oxide layer.
Density functional theory (DFT) coded VASP [35,36] calculations
(projector augmented waves method within the generalized gra-
dient approximation [37]) were performed to study the preferred
atomic configuration for silicon alloying atoms within various AlB
structured Cr-Si-B
compositions. Moreover, the influence of
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
increasing Si content as well as the impact of various vacancy con-
figurations on the phase stability were investigated. The influence of
vacancies was studied up to a content of 2 Si atoms within the su-
percell (corresponds to 4 at.%), with 2 vacancies introduced either
on the Cr-sublattice, the B-sublattice, or as a Schottky defect. All
2 × 2 × 4 supercell structures (16 metal and 32 boron atoms) were
generated using the special quasi-random structure (SQS) approach
[38]. Values for the formation energy were only extracted from fully
converged supercells. A plane wave cut-off energy of 600 eV and an
automated k-point mesh (length = 60) were chosen to provide a total
energy accuracy of about 10
eV/at. All calculations were conducted
without considering the paramagnetic states of Cr.
Finally, atom probe tomography (APT) analysis was performed on
an exemplary coating in the as-deposited state to reveal the initial
elemental distribution. Sample preparation involved milling of an
initial coating pillar and sharpening to a tip using a focused ion beam
microscope FEI Scios 2 DualBeam operated at 30 kV and stepwise
decreasing milling currents. Final tip sharpening was performed at
50 pA, with a subsequent clean-up step at 5 kV and 28 pA to
minimize possible Ga
ion-induced damage. Subsequent APT ana-
lysis was carried out on a CAMECA LEAP 4000X HR in pulsed laser
mode with a set pulse energy of 50 pJ. The system uses a 355 nm UV
laser equipped with a reflection lens, resulting in a detection effi-
ciency of 37%. The sample was cooled to a constant temperature of
44 K. Experiments were performed with a target evaporation rate of
1% and pulse repetition rate of 200 kHz. Data analysis was conducted
using an open-source Matlab Toolbox for APT data evaluation [39].
3. Results & discussion
3.1. Computational phase formation & stability boundaries
A regular requirement for alloying strategies to successfully im-
prove the oxidation resistance of a coating material involves un-
altered phase stability for the host structure to maintain a distinct
property profile. This necessity is demonstrated by the well-studied
N system, where the oxidation resistance of rock-salt struc-
tured TiN scales with the AlN alloying fraction [40]. However, upon
exceeding the Al solubility threshold on the metal sublattice (x
67% for DCMS deposited thin films), precipitation of the thermo-
dynamically favored wurtzite-structured Al
N phase occurs,
thus deteriorating both the thermal stability and mechanical prop-
erties. Analogously, to probe the effect of an increasing Si alloying
content on the phase stability of prototypical Cr-Si-B
thin films from
a theoretical point of view, Fig. 1a presents ab initio calculated for-
mation energies for various AlB
structured compositions. This
evaluation allows to assess the influence of Si-addition on the phase
stability but also provides information on the energetically preferred
lattice occupation within the unit cell and hence a guideline for the
Si solubility limit.
The model assumes that up to 8 Si atoms are either placed in-
terstitially or substitutionally within the AlB
-structured Cr-Si-B
Over the entire compositional range, DFT calculations associate the
formation of all possible alloying configurations with an increasing
compared to the binary CrB
composition. This implies an overall
reduced stability of the hexagonal structure with increasing Si
content. In more detail, all structures where Si is placed interstitially
either within B- or Cr-planes show the most substantial increase in
, already leading to positive formation energies upon alloying 2 Si
atoms (equals 4 at.%). Hence, these structures are energetically
unstable, and all substitutional configurations are significantly more
The structures where Si is placed in substitution for B/ Cr/ or
equally on both sublattices exhibit very similar formation energies in
the low alloying regime and thus can co-exist without any preferred
atomic position of Si. However, upon increasing the alloying content
beyond 3 atoms (equals values ≥6 at.%), structures with Si sub-
stituting solely B atoms and later also upon replacing both B and Cr
equally become unstable and undergo a phase transformation (i.e.,
converge into a different structure type) – see red and grey crosses in
Fig. 1a. Only calculations where Si replaces Cr exclusively within the
supercell yield negative formation energies up to 4 Si atoms (equals
8 at.%), while maintaining the hexagonal configuration. In fact,
compositions with up to 8 Si alloying atoms on the Cr sublattice
relax in the AlB
-type structure, although thermodynamically un-
stable due to positive E
. Consequently, these predictions also in-
dicate a theoretical Si solubility limit within AlB
-structured CrB
above 4 Si atoms, which equals an overall alloying content of about
8 at.%.
Another factor to consider is the presence of point defects, as
they are typically related to PVD synthesized films, which can
strongly influence the phase stability criteria compared to the
thermodynamic equilibrium [41]. Therefore, three different vacancy
configurations either two Cr vacancies, two B vacancies, or one
Schottky defect – were analyzed for Cr-Si-B
structures with up to 2
alloyed Si atoms. The differences in energy of formation between the
defected and the corresponding prototypical structure
=E E E
) are presented in Fig. 1b. In perfect agreement
with the findings of Moraes et al. [42], the binary CrB
favors the formation of boron and boron-containing vacancies over
Fig. 1. (a) DFT-calculated E
per atom for prototypical Cr-Si-B
structures (AlB
with various Si contents. The lower x-axis gives the number of Si atoms within the
employed supercell, whereas the upper x-axis gives the corresponding atomic con-
centration. The Si atoms are either positioned interstitially on Cr- or B-planes, or in
substitution for Cr, B, or an equal fraction of both Cr and B atoms. Crossed data points
indicate alloying-induced deviations from the prototype structure during supercell
relaxation. (b) DFT-evaluated formation energy differences per atom between the
prototypical Cr-Si-B
structures in (a), with up to two Si atoms replacing either Cr
(plane bars) or B (striped bars) atoms in defected structures that hold either two Cr
vacancies (blue bars)/ two B vacancies (red bars)/ or one Schottky defect (grey bars),
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
Cr point defects. The calculations further confirm this trend for both
Si alloyed compositions, again indicating the preferred incorporation
of B over Cr vacancies, except for the case of two B atoms exchanging
Si. There, both vacancy types contribute to increased stability by
slightly lowering E
. Overall, the DFT calculations suggest the pre-
ferred incorporation of synthesis induces point defects on the non-
metal sublattice for all compositions, with only minor negative in-
fluence from transition-metal vacancies.
3.2. Structural & morphological properties
The chemical composition of all Cr-Si-B
2 ± z
thin films is presented
within a ternary phase diagram in Fig. 2. The diagram is extended
with guidelines connecting stoichiometric CrB
with single-phased
Si and CrSi
(endpoints not visible due to reduced axis ranges),
corresponding to narrow two-phase fields according to the equili-
brium phase diagram [43]. The chemical analysis revealed an in-
creasing silicon content of 0, 1, 3, 8, 11, and 17 at.% for the alloyed thin
films with an increasing number of Si platelets placed on the target
racetrack, respectively. The synthesis approach allowed for a pre-
dictable and linear adjustment of the Si content within the resulting
thin film compositions. The coating prepared with two Si platelets
on the target surface obtains an effective Si content below the de-
tection limit of the employed ICP-OES method, thus the coating is
referenced with a content of 0 at.% Si (Cr
). Nevertheless,
compared to the unalloyed coating, a minute fraction of Si is still
expected within this thin film.
During the PVD deposition of (ternary) compound materials,
coatings usually become enriched or depleted in specific con-
stituents due to their preferred sputtering or scattering behavior
within the plasma [26]. Interestingly for the Cr-Si-B
2 ± z
increasing the Si content in the thin films leads to a stoichiometric
(B:Cr = 2:1) replacement of the CrB
mole fraction. This is also in-
dicated by the direct overlap of all data points with the connecting
line between stoichiometric CrB
and pure Si in Fig. 2. Moreover, the
Si-free coating Cr
obtains an almost nominal stoichiometry
with a B:Cr-ratio close to 2:1, which is consequently preserved for all
further Si-containing depositions. Nevertheless, with increasing Si
content in the Cr-Si-B
2 ± z
thin films, the overall B content decreases
from 66 at.% down to 55 at.% for the coating with the highest Si-
The X-ray diffractograms depicted in Fig. 3 demonstrate that all
2 ± z
thin films, regardless of their chemical composition,
adopt the hexagonal AlB
-type structure (space group 191). More-
over, within the accuracy of the employed method, no additional
phases could be determined for any coating. All thin films obtain a
polycrystalline growth, with slightly preferred orientations notice-
able for coatings with a Si content below 8 at.%. Within these sam-
ples, the preferred orientation shifts from (101) for the Si-free
coating, towards (001) for Cr
, to (100)-or-
iented for both the Cr
and Cr
thin films,
respectively. Further increasing the Si content results in equally or-
iented grains and causes a reduction of the diffracted intensities,
hinting towards a concomitant decrease in the coherently diffracting
domains (i.e., a reducing grain size). This structural evolution cor-
relates well with the DFT calculated Si solubility threshold close to
8 at.% (see Fig. 1), thus suggesting that the excess alloying fraction
preferably occupies grain boundary sites while also rationalizing
their suggested increase in volume fraction due to smaller grains.
The data further reveals that incorporating Si into the CrB
structure leads to a slight decrease of the lattice parameter c in
(001)-direction from 3.00 to 2.97 Å, meaning that the bond distance
between adjacent B and Cr lattice planes is reduced. On the other
hand, the lattice parameter a remains relatively unchanged at 2.97 Å,
hence lateral bond distances between similar atoms are maintained.
Further correlating the phase formation with the chemical
composition of all coatings shows that the decrease in B content
with increasing Si fraction takes no influence on the stability of the
hexagonal CrB
structure in the as deposited state. Considering that
only minor quantities of Si are chemically stable when located on the
B-sublattice, the missing B-fraction is likely accommodated by in-
troducing vacancies on the non-metal sublattice during the de-
position process (compare with Cr-rich/B-deficient planar defects
previously observed in CrB
[46]). This is also in excellent agree-
ment with the above DFT calculations, where enhanced thermo-
dynamic stability is indicated for all structures containing B-
Fig. 2. Ternary phase diagram showing the chemical composition of all synthesized
2 ± z
coatings. The dashed lines connecting CrB
with Si and CrSi
narrow two-phase fields according to the equilibrium phase diagram [43]. The axis
ranges are reduced for improved separation of the data points.
Fig. 3. X-ray diffractograms of all Cr-Si-B
2 ± z
coatings, arranged with increasing Si
content from bottom to top. The diffractographs are correlated with standardized
reference patterns for hexagonal CrB
(space group 191, AlB
prototype, [44]) and
cubic silicon (substrate material, [45]). The corresponding coating chemistry for each
diffractograph is included on the left side.
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
vacancies – especially pronounced when Si is introduced on the Cr-
The influence of Si on the growth mode of Cr-Si-B
2 ± z
thin films,
specifically the decreasing columnar crystallite size, is further illu-
strated in Fig. 4 through selected SEM fracture cross-sectional stu-
dies. The non- and low-alloyed Cr
and Cr
coatings show a pronounced columnar structure with grains ex-
tending throughout the entire cross-section. The coating with
11 at.% Si shows denser and increasingly more fibrous crystal col-
umns – see Fig. 4c. With the highest alloying content of 17 at.%, the
growth morphology appears featureless, with limited indications for
individual columnar structures remaining.
3.3. Atom probe tomography
To obtain an improved view of the distribution of Si atoms within
the Cr-Si-B
2 ± z
thin films – especially at concentrations close to the
proposed solubility limit – an additional coating with a composition
of Cr
was analogously prepared and investigated using
detailed atom probe tomography. Fig. 5 shows reconstructions of the
atomic positions for Cr, Si, and B recorded within the tip volume. A
random distribution is observed for both Cr and B atoms in the
entire volume, although slight clustering of Cr atoms is noticeable in
certain regions. In contrast, local chemical analysis of the Si dis-
tribution reveals the formation of Si-enriched regions at defect sites,
identified as grain boundaries and triple junctions within the as-
deposited thin film. A concentration profile in the grain interior (see
Fig. 5i and Supplementary) shows an entirely homogeneous dis-
tribution of the constitutional elements within the undisturbed,
crystalline region. The calculated average chemistry reveals a com-
position close to Cr
, thus indicating that the actual Si
solubility within the CrB
-structure could be even lower than the
DFT calculated limitation. The corresponding concentration profile
taken at a grain boundary location (see Fig. 5ii and Supplementary)
depicts an increased fluctuation of all elements and a drastically
increased Si content of up to 30 at.% in specific locations. The latter
findings clearly underline the preferred segregation of surplus Si
during the deposition process.
Overall, these findings experimentally underpin the DFT calcu-
lated solubility threshold above 8 at.% Si for the analyzed sample
composition. Evidently, the conducted calculations neglect the
possible impact of temperature and can only incorporate the che-
mical as well as kinetic limitations during PVD synthesis to a limited
extent, thus rationalizing the deviation from the experimentally
observed solubility threshold of 4 at.%. Nevertheless, within the
accuracy of the conducted analysis, the agreement between the
chemical composition and DFT calculations is clearly given.
Moreover, the proposed influence of Si segregation to promote grain
refinement, as evidenced in XRD analysis above an alloying content
of 3 at.%, is additionally confirmed.
3.4. Mechanical properties
Fig. 6 presents the mechanical properties of all Cr-Si-B
2 ± z
films deposited. The residual stress state, film hardness, and Young’s
modulus are plotted as function of the Si alloying content. The Si-
free Cr
coating obtains a compressive residual stress state
with σ ∼ − 0.5 GPa and corresponding hardness and Young’s modulus
values of H = 23.5 ± 2.7 GPa and E = 295 ± 18 GPa, respectively.
When compared to other TMB
materials, such as TiB
[47,48] or
[49], CrB
typically features a reduced hardness and a relatively
low elastic modulus [50]. Nevertheless, related works have also re-
ported vastly higher hardness values for this material systems using
similar deposition techniques, yet the origin of the observed varia-
tion remains unresolved [51–53]. Alloying a minute fraction of Si
into CrB
shows a reversed residual stress state, with Cr
revealing a tensile stress of σ 0.9 GPa. Interestingly, despite the
adverse effect of tensile stresses on the measurable hardness, this
coating shows identical nanoindentation results with H = 23.9 ± 1.1
and E = 291 ± 6 GPa. The maintained properties are related to the
preferred orientation rather than the influence of Si alloying per se.
In line with a work by Fuger et al. [54], the preserved hardness is
explained by the pronounced orientation towards the (001)-direc-
tion (see Fig. 3), which was demonstrated to yield the highest
hardness for TMB
thin films in general. Thus, the anisotropy effect
balances the negative impact of the tensile stress state. Conse-
quently, even higher hardness values could be expected for this
material system by tailoring the residual stress state towards the
compressive regime. The actual shift in the residual stress state
between the Cr
and Cr
coating may also be re-
lated to the preferred orientation. CrB
obtains a significant aniso-
tropy in the thermal expansion (
= 10.8
= 6.3
, [55]), therefore higher in-plane tensile stresses are to be
expected for (001)-textured thin film (a-direction parallel to coating-
substrate interface) when grown on sapphire substrate (
Al O
2 3
, [56]). With a further increase in Si, the residual
tensile stress is gradually reduced from σ 0.9 GPa for
down to σ ∼ 0.3 GPa for Cr
, before again
increasing in the compressive regime to σ ∼ − 0.5 for Cr
Concomitantly, with the above observed increase in grain boundary
volume – i.e., an increase in regions that are less strongly bound than
the surrounding crystal a linear decrease in the elastic modulus
down to E = 238 ± 7 GPa for the highest Si content of 17 at.% was
recorded. Interestingly, Si alloying did not result in any solid solution
hardening effect during nanoindentation. Upon introducing more
than 1 at.% Si into CrB
, the hardness gradually decreases from H
= 21.6 ± 1.1 GPa for Cr
, down to a constant value of H
17 GPa for all coatings having a Si content ≥ 8 at.%. In relation to the
Si segregations observed on grain boundaries for higher alloying
compositions (see Fig. 5), the measured hardness of these thin films
is likely dominated by the inferior mechanical properties of the Si-
rich regions.
3.5. Dynamic oxidation
Dynamic oxidation experiments were conducted in a TGA system
to revisit [28] the impact of Si alloying on the high-temperature
oxidation resistance of CrB
-based thin films, and to determine the
minimum alloying quantity necessary to yield enhanced protective
properties. Fig. 7 presents the mass change of the coating material,
deposited onto pre-weighed polycrystalline Al
substrate (inert in
the temperature rage up to 1400 °C, [28]), with respect to the an-
nealing temperature T. Up to a range of T 600 °C a constant mass
Fig. 4. SEM images depicting the growth morphology of selected Cr-Si-B
2 ± z
deposited on Si substrate including the corresponding chemical composition.
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
signal is recorded for all Cr-Si-B
2 ± z
thin films, indicating no pro-
gressive oxide scale formation. Only the Cr
thin film
shows an earlier onset temperature at T 500 °C (blue dotted line),
visible by the already occurring mass gain due to oxide scale growth.
Upon reaching the oxidation onset temperature, all Cr-Si-B
2 ± z
coatings up to a Si content of 3 at.% show a stepwise increase in the
mass signal until reaching a maximum value at T 1200 °C, in-
dicating the fully oxidized state. Beyond this temperature, a de-
creasing sample mass is recorded, which is correlated with the
evaporation of B
-based oxides. The mass signal evolution shows
intermediate plateaus for all these coatings between
600 < T < 1200 °C, hinting towards a competitive formation of B
and Cr
-based scales and thus limited protection against con-
tinued oxidation. Moreover, with already small alloying fractions
(e.g., Cr
), both the slope of the increase and the overall
Fig. 5. Atom probe tomography determined chemical composition of a Cr
thin film. Reconstructed positions of Cr, Si, and B atoms are presented. Insets (i) and (ii)
depict concentration profiles of the grain interior and grain boundary, respectively. Both profiles were collected in the corresponding regions of interest marked in the Si atom
distribution. The supplementary material contains animations of the Si distribution to provide an improved view on both regions of interest.
Fig. 6. (a) Residual stress state of all Cr-Si-B
2 ± z
thin films versus the Si alloying
content. (b) Corresponding hardness and Young’s modulus data. All mechanical
properties were determined on coated sapphire substrates.
Fig. 7. Mass change of all Cr-Si-B
2 ± z
coatings as function of the annealing tempera-
ture, recorded during dynamic oxidation in a TGA system (10 K/min heating rate) in
synthetic air environment. The coatings were deposited on pre-weighed Al
strates, which are inert over the entire temperature range.
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
mass gain up to T 1100 °C are significantly reduced, already
pointing towards the effectiveness of the employed alloying routine.
Increasing the Si content within the Cr-Si-B
2 ± z
thin films beyond
8 at.% leads to a fully preserved coating mass up to T 1100 °C due to
the formation of a stable, protective oxide scale preventing any
oxidative attack of the underlying coating material. Only at
T > 1100 °C, these higher alloyed thin films show a slight increase in
the overall mass signal up to 1400 °C. Unlike the low-alloyed coat-
ings, the protective mechanism relies on the formation of a con-
tinuous, dense SiO
-based scale enabled by a sufficient Si diffusion
provided through the Cr-Si-B
2 ± z
thin film. Moreover, as known for a
coating [28], the formed oxide scale after annealing
at T = 1400 °C should in fact be comprised of a layered amorphous
-based phase with a crystalline Cr
scale on top (discussed in
more detail in Section 3.7). Overall, these results highlight that
achieving high-temperature oxidation resistance for Cr-Si-B
2 ± z
films involves a minimum alloying content close to 8 at.% Si to ac-
tivate the protective mechanism.
3.6. In-situ X-ray diffraction
Comparative in-situ X-ray diffraction studies were performed
during the oxidation of Cr
and Cr
in lab-air
environment, to reveal the underlying mechanism causing the
drastically improved oxidation resistance above a distinct Si content.
Fig. 8 depicts the diffractographs taken at room temperature (RT) as
well as from 400 to 1200 °C in steps of 50 °C, with the corresponding
annealing temperature included on the right axis. The data for an
“insufficiently” alloyed Cr
coating (see Fig. 8a) reveal an
unaltered crystal structure up to a temperature of T = 550 °C, de-
picting a preferred (100)-orientation as shown in Fig. 3. With the
oxidation onset at T = 600 °C (see also Fig. 7), initial indications to-
wards a boron depleted CrB phase are formed (e.g., 2θ ∼ 32.2°, 38.5°,
44.9°, etc.), increasing in intensity up to T = 1100 °C. This suggests the
partial decomposition of the Cr-Si-B
structure to form an un-
protective B
scale, which is in line with the mass gain to an initial
plateau observed during the TGA measurements. In the temperature
regime beyond T = 750–800 °C, additional recrystallization of the
remaining Cr-Si-B
2 ± z
solid solution is observed, as indicated by the
decreasing peak width and increase in diffracted intensities (e.g., 2θ
45.8°). Similar behavior was previously reported for amorphous
Cr-Al-Si-B-(N) coatings, experiencing crystallization of the CrB
phase at T = 800 °C [57]. At T = 1050 °C the diffraction signals for the
structure diminish, pointing towards a full decomposition of
the diboride phase. Finally, at T = 900 °C, an additional Cr
scale is
formed compare with the second mass gain plateau observed in
Fig. 7 which subsequently consumes the entire coating at
T = 1200 °C. Over the entire temperature range, no Si-based phase is
A direct comparison to results obtained for a “sufficiently” al-
loyed Cr
(Si content 8 at.%) coating is illustrated in
Fig. 8b. In the as-deposited state at RT, the data shows an analogous
diffraction result as depicted in Fig. 3, revealing a hexagonal struc-
tured CrB
-based coating with polycrystalline grain distribution.
Moreover, no additional Cr-Si- or Si-B-based phase is detected,
suggesting that Si is dissolved up to the solubility limit of 3–4 at.%
Fig. 8. In-situ X-ray diffractographs recorded during subsequent annealing treatments in lab-air environment of (a) Cr
and (b) Cr
thin films deposited on
polycrystalline Al
. Standardized reference patterns for hexagonal CrB
(blue hexagon, [44]), cubic Si (light blue square, [45]), hexagonal CrSi
(dark yellow triangle, [58]),
rhombohedral Cr
(green diamond, [59]), orthorhombic CrB (dark red circle, [60]) and rhombohedral Al
(grey star, [61]) are included. The sample temperature corresponding
to each diffraction experiment is added on the right axis.
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
within the CrB
structure – note that excess Si is located at grain
boundaries as shown in Fig. 5. The structure is fully preserved up to a
temperature of T = 600 °C. A further increase to T = 650 °C leads to
first indications for crystalline Si precipitates (e.g., 2θ ∼ 28.7°, 47.5°,
56.3°). Analogously to Cr
, recrystallization of the CrB
structure is observed beyond T = 750–800 °C (e.g., 2θ ∼ 29.0°, 34.7°,
45.8°, etc.), being more pronounced due to the absence of a CrB
phase. Furthermore, in accordance with the three-phase field of
-Si in Fig. 2 (area between the dashed lines), the inter-
mediate formation of a CrSi
phase is suggested between T = 750 and
1050 °C by a set of low intensity reflexes (e.g., 2θ 26.8°, 42.3°,
49.6°). However, it should be noted that the deviation from the in-
dexed peak positions is significant, so that additional high-resolution
analysis would be required for confirmation. Recrystallization of the
2 ± z
matrix and Si precipitation continue up to T = 1200 °C,
resulting in sharp peaks for both phases. In addition, several in-
dications for a Cr
oxide layer emerge after the annealing ex-
periments above T ≥ 1100 °C (e.g., 2θ 33.5°, 50.0°, 54.5°), yet no
diffraction peaks pointing towards a B
or the more important
-based structure occur. Thus, in line with previous findings,
especially the latter phase is expected to be in an amorphous state.
Overall, pronounced diffraction peaks indicate that the original Cr-
2 ± z
structure is still intact at T = 1200 °C, highlighting the ex-
cellent oxidation resistance of this coating and confirming the pre-
sence of a stable oxide scale protecting the underlying coating
When discussed in relation to the dynamic oxidation experi-
ments (see Fig. 7), the precipitation of Si at T > 600 °C in this “suf-
ficiently” alloyed coating correlates well with the oxidation onset
temperature of the “insufficiently” alloyed samples. Consequently,
this provides a strong indication that Si precipitates within the
coating either derived from excess Si on grain
boundaries or the surrounding Cr-Si-B
2 ± z
solid solution (discussed
in more detail in Section 3.7) are the primary source for the in-
creased oxidation resistance. In addition, the formation of crystalline
above T = 1100 °C is in excellent agreement with the previous
TGA analysis, rationalizing the mass gain for all coatings with Si
content above 8 at.% in the same temperature regime.
Regarding the thermally activated precipitation of Si from the
coating, a possible explanation is seen in the con-
tinuous increase in the DFT calculated E
for the AlB
structured Cr-
compositions over the binary CrB
with increasing Si content.
Theoretically comparing the difference in energy of formation be-
tween a Cr
structure (see Fig. 1) with its corresponding
decomposition products of CrB
and Si according to:
= + +
E x E x E x E E
[(1 ) 2 ] ,
with 0. 06
Si f
Cr Si B
20.27 0. 06 0.67
a significant energetic benefit of
= −1.34 eV/at. towards the de-
composed constituents is attained. The DFT calculated formation
energies of elemental Si (cubic, −5.41 eV/at.) and B (rhombohedral,
−6.67 eV/at.) correspond to their stable configuration at room-tem-
perature and ambient pressure.
Analogous results are obtained for all other theoretically and
experimentally studied compositions of Cr-Si-B
2 ± z
, thus underlining
that the precipitation follows the thermodynamically prescribed
equilibrium condition. Finally, the precipitation is believed to be
further supported during recrystallization of the Cr-Si-B
2 ± z
above T = 750–800 °C, allowing for even enhanced Si diffusion.
Considering a melting temperature of T
= 2200 °C [62] for pure
, the recrystallization process occurs at a typical homologous
temperature of T
Fig. 9. (a) bright-field and (b) high-angle annular dark field TEM micrographs of a Cr
coating on polycrystalline Al
substrate, gradually oxidized up to 1200 °C (see
Fig. 8). (c) elemental EDX mapping of the entire sample cross-section according to the insert in (a). (d) detailed scanning-TEM image of the oxide scale indicated in (a) including
the elemental EDX mapping.
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
3.7. Structural & chemical analysis post annealing
In order to complete the established viewpoint on the mor-
phological evolution and especially the oxide scale growth during
high-temperature oxidation of Cr-Si-B
2 ± z
thin films, com-
plementary detailed TEM analysis (see Fig. 9) is performed on the
thin film used during the in-situ X-ray studies (see
Fig. 8b). Fig. 9a and b depict bright-field and high-angle annular
dark field micrographs of the entire sample cross-section, including
the interface to the polycrystalline Al
substrate and the formed
oxide scale, respectively. Both images immediately visualize the
pronounced recrystallization of the Cr-Si-B
2 ± z
thin film, revealing
large globular grains throughout the cross-section. Given the
atomic number contrast in the HAADF image, regions of different
elemental compositions indicated by lighter and darker grey
areas can be identified next to several black appearing voids.
When combined with the elemental mapping in Fig. 9c, bright
areas can be correlated with a Cr- and B-rich phase (i.e., CrB
2 ± z
whereas darker regions correspond exclusively to pure Si. Note the
superposition of Cr and O signals during EDX analysis, thus creating
a slight, artificial O signal (see Fig. 9c-iv) overlapping with all
2 ± z
regions. Furthermore, also Si and W overlap in the EDX
spectrum, resulting in a misinterpretation of the W protection layer
with an artificial Si region in the top area of Fig. 9c-ii. Several
conclusions can be drawn from these results: (i) Silicon precipita-
tion is not restricted to grain boundary sites already holding excess
Si in the as-deposited state. (ii) Upon thermal activation, the
2 ± z
solid solution fully decomposes into large globular
phase regions containing solely CrB
2 ± z
or Si. (iii) Globular Si pre-
cipitates are formed throughout the coating cross-section in addi-
tion to a continuous surface layer. As a result, several voids are
formed in the thin film volume to compensate for the Si surface
diffusion (note, certain voids may also originate from focus-ion
beam milling preparation of the TEM lamella, resulting from the
weak connection between the individual recrystallized grains). Fi-
nally, the elemental distributions of Al and O show that no inter-
action between the coating and substrate material occurred during
the entire oxidation treatment.
Fig. 9a and b also clearly show a thin, dense oxide scale formed
on the sample surface. Using detailed STEM imaging combined with
EDX analysis (see Fig. 9d), the oxide reveals a defined, layered ar-
chitecture composed of a thin amorphous SiO
layer on the coating-
oxide interface and a nanocrystalline Cr
top layer. Similar to the
elemental distribution in Fig. 9c, no intermixing of Cr- and B-rich
sites with Si can be observed in the oxide layer and the unaffected
material below. However, it has to be considered that the employed
chemical analysis is not suitable for tracing minimum quantities of
light elements such as B within, e.g., the SiO
layer. Interestingly,
despite the extended annealing time at temperatures above
T > 1000 °C, the SiO
layer features a thickness in the range of only
20–40 nm, whereas the Cr
top layer extends over 200–250 nm.
Regarding the temporal sequence of forming the highly pro-
tective oxide scale, the primary mechanism is seen in the pre-
cipitation of Si – especially towards the coating surface – allowing
for the initial growth of a stable SiO
layer in the temperature range
from T = 650–1100 °C. Due to the minimal thickness of this layer,
even at T = 1200 °C, no mass gain is visible in the TGA signal.
Furthermore, in line with the TGA and in-situ X-ray diffraction
analysis, the additional Cr
surface layer is subsequently formed
in the temperature regime above T = 1100 °C. Unlike many TM-
oxides, Cr-cations primarily diffuse outwards on grain boundaries
within the Cr
oxide layer, thus allowing for a scale growth on
top of the oxide surface rather than the oxide-coating interface
[63]. Also, resulting from the vastly increased layer thickness, the
formation is clearly relatable to the mass gain signal shown
in Fig. 7.
4. Conclusion
Si alloying was proven a successful concept to significantly en-
hance the oxidation resistance of transition-metal diboride-based
thin films. In this work, DC magnetron sputtered Cr-Si-B
2 ± z
with Si content up to 17 at.% were analyzed to reveal the impact of
the alloying element on the structural and mechanical properties of
the AlB
-type thin films. In addition, the mechanisms leading to the
enhanced oxidation resistance were investigated to deepen the
knowledge on optimized chemical compositions.
DFT calculations performed on various stoichiometric and de-
fected AlB
-structured Cr-Si-B
compositions indicate the en-
ergetically favored incorporation of Si on the Cr-sublattice over a
wide alloying range. Contrary, already limited occupation of the B-
sublattice destabilizes the hexagonal cell. Detailed APT analysis of a
thin film revealed Si segregation towards grain
boundaries in the as-deposited state, while the grain interior holds
up to 4 at.% Si, being in line with a DFT-calculated solubility limit.
Despite a concomitant increase in B under-stoichiometry with
increasing Si content, all synthesized Cr-Si-B
2 ± z
coatings obtain the
hexagonal AlB
structure, irrespective of the chemical composition.
Moreover, increasing the Si content is accompanied by a variation of
the preferred growth orientation and a gradual reduction in the
average columnar grain size. Nanoindentation measurements
showed a direct correlation between the morphological features and
the mechanical properties. The highest film hardness was recorded
for low Si alloyed, (001)-oriented coatings at H 24 GPa, whereas an
increased alloying content of Si ≥ 8 at.% resulted in H 17 GPa due to
mechanically weak Si grain boundary segregates.
Thermogravimetric analysis proofed the excellent oxidation re-
sistance of Cr-Si-B
2 ± z
thin films with Si content 8 at.% up to T
1400 °C, whereas lower alloyed coatings suffer from stepwise oxi-
dation above T 600 °C related to a non-protecting Cr- and B-based
oxide scale. The enhanced oxidation resistance could be linked to
thermally activated precipitation of Si and the subsequent re-
crystallization of the Cr-Si-B
2 ± z
solid solution, thereby creating a
continuous Si-based surface layer. This layer allows for a dense,
amorphous SiO
-based scale (20 nm at 1200 °C) in the temperature
range between T = 650–1100 °C, beyond which an additional nano-
crystalline Cr
top layer (200 nm at 1200 °C) is formed due to
increased Cr-outward diffusion.
In summary, the results underpin the promising capabilities of
2 ± z
coatings applied in high-temperature oxidative environ-
ments and provide detailed guidelines to connect the chemical
composition with resulting thin film properties.
CRediT authorship contribution statement
L. Zauner: Conceptualization, Investigation, Visualization,
Writing – original draft. A. Steiner: Investigation, Writing – review &
editing. T. Glechner: Investigation, Writing review & editing. A.
Bahr: Investigation, Writing – review & editing. B. Ott: Investigation,
Writing – review & editing. R. Hahn: Investigation, Writing – review
& editing. T. Wojcik: Investigation, Writing – review & editing. O.
Hunold: Project administration, Writing review & editing. J.
Ramm: Conceptualization, Project administration, Writing – review
& editing. S. Kolozsvári: Project administration, Writing – review &
editing. P. Polcik: Conceptualization, Project administration, Writing
review & editing. P. Felfer: Investigation, Writing review &
editing. H. Riedl: Supervision, Conceptualization, Project adminis-
tration, Writing review & editing.
Data Availability
Data will be made available on request.
L. Zauner, A. Steiner, T. Glechner et al. Journal of Alloys and Compounds 944 (2023) 169203
Declaration of Competing Interest
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
The financial support by the Austrian Federal Ministry for Digital
and Economic Affairs, the National Foundation for Research,
Technology and Development and the Christian Doppler Research
Association is gratefully acknowledged (Christian Doppler
Laboratory "Surface Engineering of high-performance
Components"). We also thank for the financial support of Plansee SE,
Plansee Composite Materials GmbH, and Oerlikon Balzers, Oerlikon
Surface Solutions AG. In addition, we want to thank the X-ray center
(XRC) of TU Wien for beam time as well as the electron microscopy
center - USTEM TU Wien - for providing the SEM and TEM facilities.
We also thank Dr. M. Weiss and Prof. A. Limbeck from the Institute of
Chemical Technologies and Analytics, TU Wien, for their support
with chemical analysis of our samples. The authors acknowledge TU
Wien Bibliothek for financial support through its Open Access
Funding Programme.
Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2023.169203.
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... The poor oxidation resistance of binary TMB 2±z coatings stimulated studies on the development of alloying strategies to design novel ternary transition metal diborides (TM-X-B 2±z ) with enhanced high-temperature oxidation resistance. So far, several attempts have been carried out to enhance the oxidation resistance of TMB 2±z films based on alloying with elements that are capable of forming protective oxide scale at high temperatures such as Al [8,17], Ta [16], and Si [15,[18][19][20]. Compared to other alloying routes, the Si alloying of TMB 2 ±z provides superior oxidation resistance, especially in the hightemperature regime (> 1000 • C), due to the formation of highly protective Si-based scales [15]. ...
... Moreover, an optimum Si-content of ~8 at. % is required to improve the oxidation resistance of Cr-Si-B 2±z coatings [20]. The reported hightemperature oxidation resistance is attributed to the formation of protective Si-based scales. ...
... The reported hightemperature oxidation resistance is attributed to the formation of protective Si-based scales. However, the outward diffusion of excess Si towards the surface leaves a high degree of porosity within the coating [20]. Additionally, high Si contents decrease the hardness of Cr-Si-B 2±z coatings compared to binary CrB 2±z [15,20]. ...
Full-text available
The Si-based alloying of transition metal diborides is a promising strategy to improve their limited oxidation resistance in high-temperature environments. In this study, we investigate the oxidation resistance of ternary and quaternary Cr-(Mo)-Si-B2-z coatings sputter-deposited from alloyed CrB2/TMSi2 targets (TM = Cr or Mo). The as-deposited Cr-(Mo)-Si-B2-z coatings are stabilized in the single-phased hexagonal AlB2-structure, except the high-Si containing Cr0.26Mo0.11Si0.24B0.39 presenting amorphous character. The Mo-containing Cr-Mo-Si-B2-z films exhibit relatively high hardness compared to their ternary Cr-Si-B2-z counterparts, obtaining up to 26 GPa due to the formation of (Cr,Mo)B2 solid solutions. The Si-alloying in ternary and quaternary coatings provides oxidation resistance up to 1200 °C, owing to the formation of highly protective double-layered scales consisting of SiO2 with a Cr2O3 layer on top, inhibiting oxygen inward diffusion. The quaternary Cr0.31Mo0.07Si0.15B0.47 coating is distinguished by superior oxidation resistance with lower porosity and void formation compared to the ternary Cr0.37Si0.16B0.47. Mo proved to be the key element for the higher stability and enhanced oxidation resistance due to the evolution of the MoSi2 phase at ~600 °C. This phase formation controls the Si diffusion and mobility within the microstructure, thus reducing the porosity and governing the Si supply to form SiO2 scale. The quaternary Cr0.31Mo0.07Si0.15B0.47 coating maintained an oxidation resistance up to 30 h at 1200 °C by forming a 2.5 μm dense amorphous Si-based oxide scale with a thin Cr2O3 on top.
... Following this alloying strategy, recent studies highlighted the viability of using Si as a similarly potent oxide former in TM-Si-B 2based coatings, revealing oxidation rates three orders of magnitude lower compared to TMB 2 /SiC compounds at 1100 °C [26,27]. Especially Cr-Si-B 2 -based coatings portrayed outstanding oxidation properties, exhibiting minimum mass gain and oxide scale growth rates even up to 1400 °C [28]. ...
... Considering the complex bonding nature of AlB 2 -structured diborides, the limited research covering compositional and synthesis variations so far suggests a negative impact of Si on the room temperature hardness for several TM-Si-B 2 [27]. Besides the known orientation dependency of the mechanical properties [34], this effect could be linked to Si grain boundary decorations formed during film growth upon exceeding a TMB 2 specific solubility limit [28]. In consequence, it seems imperative to investigate altered synthesis conditions, expanding the current viewpoint of the relationship between deposition parameters and the thermo-physical properties of TM-Si-B 2 thin films. ...
... It is imperative to mention that the overstoichiometry is inherently dependent on the actual lattice occupation of the Si atoms as well as the individual contribution of the constitutional elements to the phase formation. As previously revealed by detailed DFT calculations, the Cr-Si-B 2 system prefers the incorporation of substitutional Si atoms on the Cr-and B-lattice for lower alloying fractions, whereas for higher concentrations only Cr replacement yields chemically stable configurations [28]. Moreover, a theoretically proposed Si solubility limit close to 8 at.% in the AlB 2 -structure, which was experimentally determined with a lower value of ∼3-4 at.%, suggests that all coatings with an alloying fraction higher than Cr 0.26 Si 0.03 B 0.71 obtain Si grain boundary segregations in the asdeposited state. ...
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The corrosion resistance of cathodic arc evaporated Al0.7Cr0.3-xVxN coatings with a vanadium content up to 22.3 at.% has been electrochemically tested in a 0.1 M NaCl-solution. Significant improvement in the open porosity and corrosion rate was observed for coatings with higher V-contents, due to a denser and more refined coating morphology. Further reduction in the open porosity rate was achieved through an annealing step in air at 700 °C. Here, the formation of an AlVO4 top-oxide and underlying oxygen-rich V-depletion zone provides additional sealing of the coating surface, whilst reducing the corrosion current density to a final 1.59×10-9 A/cm2.
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Hexagonal transition metal diborides embody promising material systems for the purpose of protective thin films. Here, we focus on DC magnetron sputtered TiB2+z coating materials, comprehensively revisiting the impact of the stoichiometry on the structure-mechanical properties, from nearly stoichiometric TiB2.07 (B: 67 at. %) up to super-stoichiometric TiB4.42 (B: 82 at. %). The structural analysis confirmed the apparent correlation between the deposition pressure and the preferred {0001} orientation, which is essential to gain super-hardness (>40 GPa). In contrast, the hardness decreases for >10 GPa for 101¯1 and 1000 oriented thin films, underlining the pronounced anisotropy of TiB2+z. The broad stoichiometry variation revealed no predominant hardness effect based on a B-rich tissue phase. The excess B contributes to a decreasing column size correlating with a decreasing hardness of ≈ 7 GPa (B/Ti ratios >2.5) due to column boundary sliding events. Micro-cantilever bending experiments proved a declining fracture toughness from 3.02 ± 0.13 MPa√m for TiB2.43 to 2.51 ± 0.14 MPa√m for TiB4.42 to be column size dependent.
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Fatigue failure through sustained loading of ductile materials manifests in irreversible motion of dislocations, followed by crack initiation and growth. This contrasts with the mechanisms associated with brittle ceramics, such as nanostructured physical vapor deposited thin films, where inhibited dislocation mobility typically leads to interface-controlled damage. Hence, understanding the fatigue response of thin films from a fundamental viewpoint – including altered atomic bonds, crystal structures, and deformation mechanisms – holds the key to improved durability of coated engineering components. Here, a novel method utilizing quasi-static and cyclic-bending of pre-notched, unstrained microcantilever beams coupled with in situ synchrotron X-ray diffraction is presented to study the fracture toughness and fatigue properties of thin films under various loading conditions. Investigating a model system of sputter-deposited Cr and Cr-based ceramic compounds (CrN, CrB2, and Cr2O3) demonstrates that the fatigue resistance of such thin films is limited by the inherent fracture toughness. In fact, cantilever cycling close to the critical stress intensity is sustained up to 10⁷ load cycles on all materials, without inducing noticeable material damage, structural or stress-state changes. The observed variation in fracture toughness is put into context with linear-elastic fracture theory and complementary micro-pillar compression, thereby elucidating the wide range of values from as low as 1.6±0.2 MPa√m for Cr1.79O3 up to 4.3±0.3 MPa√m for Cr1.03B2, respectively. Moreover, possible mechanisms governing the elastic-plastic deformation response of all coatings, both in quasi-static and cyclic-loading conditions, are discussed. Our findings contribute key-insights into the underlying mechanisms dictating the damage tolerance of PVD coated components by relating fatigue strength limits to fundamental material properties.
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We systematically study oxidation properties of sputter-deposited TiB2.5 coatings up to 700 °C. Oxide-scale thickness dox increases linearly with time ta for 300, 400, 500, and 700 °C, while an oxidation-protective behavior occurs with at 600 °C. Oxide-layer’s structure changes from amorphous to rutile/anatase-TiO2 at temperatures ≥ 500 °C. Abnormally low oxidation rate at 600 °C is attributed to a highly dense columnar TiO2-sublayer growing near oxide/film interface with a top-amorphous thin layer, suppressing oxygen diffusion. A model is proposed to explain oxide-scale evolution at 600 °C. Decreasing heating rate to 1.0 °C/min plays noticeable role in TiB2.5 oxidation.
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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.
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The concept of Si alloyed transition metal (TM) diborides – well explored for bulk ceramics – is studied for five different physical vapor deposited TM-Si-B2±z (TM = Ti, Cr, Hf, Ta, W) coatings, focusing on the oxidation behavior up to 1200 °C. In their as deposited state, all coatings exhibit single phased AlB2 prototype structures, whereby the addition of Si results in dense, refined morphologies with no additional phases visible in the X-ray diffractograms. With already low amounts of Si, the slope of the mass increase during dynamic oxidation flattens, especially for Ti-Si-B2±z, Cr-Si-B2±z, and Hf-Si-B2±z. Above distinct Si contents, the formation of a steady state region exhibiting no further mass increase is promoted (starting at around 1000 to 1100 °C). Best results are obtained for Hf0.21Si0.18B0.61 and Cr0.26Si0.16B0.58 (both around 2.4 μm thick in the as deposited sate), revealing drastically retarded oxidation kinetics forming 400 nm thin oxide scales after 3 h at 1200 °C in ambient air (significantly lower compared to bulk ceramics). This highly protective oxidation mechanism is attributed to the formation of an amorphous Si rich oxide scale. The Si content needed to form these oxide scales largely differs between the TM-Si-B2±z coatings investigated, also diversifying the prevalent oxidation mechanism, especially for Cr-Si-B2±z.
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Direct-current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) were used to deposit understoichiometric Ti1-xAlxB2-y diboride coatings by sputtering from a segmented TiB2-AlB2 target using Ar and Kr as sputtering gas. For films with a fixed Al/(Ti + Al) ratio of x = 0.1 (Ti0.9Al0.1B2-y), the B content was varied with y ∈ (0.1, 0.6 and 0.7). For films with a fixed y = 0.7 (Ti1-xAlxB1.3), the Al content was varied with x ∈ (0.1, 0.4 and 0.7). Evaluation of the mechanical properties of the Ti1-xAlxB1.3 samples showed a reduction in both hardness and elastic modulus with increasing Al concentration, while the Ti0.9Al0.1B2-y samples showed a hardness increase with decreasing B content. Thus, Ti0.9Al0.1B1.3 films exhibited a superior hardness of 46.2 ± 1.1 GPa and an elastic modulus of 523 ± 7 GPa, compared to the values for Ti0.9Al0.1B1.4 and Ti0.9Al0.1B1.9, showing a hardness of 44 ± 1 GPa and 36 ± 1 GPa, and an elastic modulus of 569 ± 7 GPa and 493 ± 6 GPa, respectively. The oxidation behavior of the mechanically most promising Ti0.9Al0.1B2-y sample series was investigated through air-annealing at 600 °C for durations from 1 h to 10 h. All films formed a mixed non-conformal Al2O3-TiO2 oxide scale which acts as an inward and outward diffusion barrier, significantly reducing the oxidation rate compared to TiBz films, which form an oxide scale consisting of porous TiO2. The thinnest oxide scale after 10 h was found in the B-deficient samples, Ti0.9Al0.1B1.3 and Ti0.9Al0.1B1.4, at ~200 nm, which is significantly below that for Ti0.9Al0.1B1.9 at 320 nm. The enhanced oxidation resistance of highly understoichiometric films is due to the elimination of the B-rich tissue phase that is present at the grain boundaries for higher B content, where the latter has been shown to enhance the rate of oxidation in borides.
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The effect of B concentration on phase formation and oxidation resistance of (Ti0.35Al0.65)By coatings with y = 1.7, 2.0, 2.4 was investigated. Elemental B targets in radio frequency mode and a compound Ti0.4Al0.6 target in direct current mode were sputtered. The B concentration was varied systematically by adjusting the applied power to the respective magnetrons while keeping the power supplied to the magnetron with the Ti0.4Al0.6 target constant. Measured lattice parameters and elastic properties are consistent with ab initio predictions. The oxidation resistance at 700 °C in air for up to 8 h was compared to a cathodic arc evaporated (Ti0.37Al0.63)0.49N0.51 coating with an Al/Ti ratio of 1.69 ± 0.20 which is very similar to 1.84 ± 0.40 for the boride coatings. Scanning transmission electron microscopy imaging revealed oxide scale thicknesses of 39 ± 7 and 101 ± 25 nm for (Ti0.35Al0.65)B2.0 and (Ti0.37Al0.63)0.49N0.51 after 8 h, respectively. Hence, the close to stoichiometric diboride outperforms the nitride coating. This behavior can be understood based on composition and structure analysis of the oxide scales: While the protective layer on the diboride is primarily composed of Al and O, the porous oxide layer on the nitride coating contains Ti, Al and O.
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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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The influence of the non-metal species on the oxidation resistance of transition metal ceramic based thin films is still unclear. For this purpose, we thoroughly investigated the oxide scale formation of a metal (Hf), carbide (HfC0.96), nitride (HfN1.5), and boride (HfB2.3) coating grown by physical vapor deposition. The non-metal species decisively affect the onset temperature of oxidation, ranging between 550 °C for HfC0.96 to 840 °C for HfN1.5. HfB2.3 and HfN1.5 obtain the slowest oxide scale kinetic following a parabolic law with kp values of 4.97∙10⁻¹⁰ and 5.66∙10⁻¹¹ kg² m⁻⁴ s⁻¹ at 840 °C, respectively. A characteristic feature for the oxide scale on Hf coatings, is a columnar morphology and a substantial oxygen inward diffusion. HfC0.96 reveals an ineffective oxycarbide based scale, whereas HfN1.5 features a scale with globular HfO2 grains. HfB2.3 exhibits a layered scale with a porous boron rich region on top, followed by a highly dense and crystalline HfO2 beneath. Furthermore, HfB2.3 presents a hardness of 47.7 ± 2.7 GPa next to an exceptional low inward diffusion of oxygen during oxidation. This study showcases the strong influence of the non-metallic bonding partner despite the same metallic basis, as well as the huge potential for HfB2 based coatings also for oxidative environments.
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.