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Quantitative Depth Profiling Using Online-Laser Ablation of Solid Samples in Liquid (LASIL) to Investigate the Oxidation Behavior of Transition Metal Borides

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The increased demand for sustainability requires, among others, the development of new materials with enhanced corrosion resistance. Transition metal diborides are exceptional candidates, as they exhibit fascinating mechanical and thermal properties. However, at elevated temperatures and oxidizing atmospheres, their use is limited due to the fact of their inadequate oxidation resistance. Recently, it was found that chromium diboride doped with silicon can overcome this limitation. Further improvement of this protective coating requires detailed knowledge regarding the composition of the forming oxide layer and the change in the composition of the remaining thin film. In this work, an analytical method for the quantitative measurement of depth profiles without using matrix-matched reference materials was developed. Using this approach, based on the recently introduced online-LASIL technique, it was possible to achieve a depth resolution of 240 nm. A further decrease in the ablation rate is possible but demands a more sensitive detection of silicon. Two chromium diboride samples with different Si contents suffering an oxidation treatment were used to demonstrate the capabilities of this technique. The concentration profiles resembled the pathway of the formed oxidation layers as monitored with transmission electron microscopy. The stoichiometry of the oxidation layers differed strongly between the samples, suggesting different processes were taking place. The validity of the LASIL results was cross-checked with several other analytical techniques.
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Citation: Weiss, M.; Glechner, T.;
Weiss, V.U.; Riedl, H.; Limbeck, A.
Quantitative Depth Profiling Using
Online-Laser Ablation of Solid
Samples in Liquid (LASIL) to
Investigate the Oxidation Behavior of
Transition Metal Borides. Molecules
2022,27, 3221. https://doi.org/
10.3390/molecules27103221
Academic Editor: Ioana Demetrescu
Received: 4 April 2022
Accepted: 15 May 2022
Published: 18 May 2022
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molecules
Article
Quantitative Depth Profiling Using Online-Laser Ablation of
Solid Samples in Liquid (LASIL) to Investigate the Oxidation
Behavior of Transition Metal Borides
Maximilian Weiss 1,* , Thomas Glechner 2, Victor U. Weiss 1, Helmut Riedl 2,3 and Andreas Limbeck 1 ,*
1Institute of Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9, 1060 Wien, Austria;
victor.weiss@tuwien.ac.at
2Christian Doppler Laboratory for Surface Engineering of High-Performance Components, TU Wien,
Getreidemarkt 9, 1060 Wien, Austria; thomas.glechner@tuwien.ac.at (T.G.); helmut.riedl@tuwien.ac.at (H.R.)
3Institute of Materials Science and Technology, TU Wien, Getreidemarkt 9, 1060 Wien, Austria
*Correspondence: maximilian.weiss@tuwien.ac.at (M.W.); andreas.limbeck@tuwien.ac.at (A.L.)
Abstract:
The increased demand for sustainability requires, among others, the development of new
materials with enhanced corrosion resistance. Transition metal diborides are exceptional candidates,
as they exhibit fascinating mechanical and thermal properties. However, at elevated temperatures
and oxidizing atmospheres, their use is limited due to the fact of their inadequate oxidation resistance.
Recently, it was found that chromium diboride doped with silicon can overcome this limitation.
Further improvement of this protective coating requires detailed knowledge regarding the compo-
sition of the forming oxide layer and the change in the composition of the remaining thin film. In
this work, an analytical method for the quantitative measurement of depth profiles without using
matrix-matched reference materials was developed. Using this approach, based on the recently
introduced online-LASIL technique, it was possible to achieve a depth resolution of 240 nm. A
further decrease in the ablation rate is possible but demands a more sensitive detection of silicon.
Two chromium diboride samples with different Si contents suffering an oxidation treatment were
used to demonstrate the capabilities of this technique. The concentration profiles resembled the
pathway of the formed oxidation layers as monitored with transmission electron microscopy. The
stoichiometry of the oxidation layers differed strongly between the samples, suggesting different
processes were taking place. The validity of the LASIL results was cross-checked with several other
analytical techniques.
Keywords:
online-LASIL; quantitative depth profiling; boride thin films; high-temperature oxidation
1. Introduction
Corrosion can quickly deteriorate the mechanical properties of a material, especially
when exposed to elevated temperatures. The resulting shortening of the material’s lifetime
and the limitation of its application range can be addressed by deposition of protective
coatings. In this context, transition metal diborides are among the most promising can-
didates that have gained attention in recent years [
1
3
]. The reason for this behaviour is
the boron–metal bond allowing for a favorable combination of metal–ceramics properties
such as high strength elastic modulus with high thermal and electrical conductivity [
4
,
5
].
Additionally, the extremely high melting points (over 3000
C) of these materials makes
them suitable for future high-temperature applications. Such environments are relevant for
cutting tools or the aerospace industry [
6
]. However, in practice, their application is limited
through their poor oxidation resistance [
7
,
8
]. Pure borides tend to form a B
2
O
3
layer upon
oxidation, which evaporates at elevated temperatures, a process further enhanced by the
presence of water vapor by forming the more volatile boric acid, thus limiting the oxidation
resistance [
9
11
]. The high-temperature performance can be enhanced by alloying with
Molecules 2022,27, 3221. https://doi.org/10.3390/molecules27103221 https://www.mdpi.com/journal/molecules
Molecules 2022,27, 3221 2 of 12
elements forming highly stable oxides, such as Si and Al, as these are known to yield low
parabolic rate constants for the oxidation reaction [
8
,
12
]. Silicon is especially of interest,
as its oxidation on the surface of borides gives rise to a protective borosilicate glass-like
film [13,14].
Transition metal borides can exist over a wide stoichiometric range, and systems with
a plethora of transition metals are under investigation. Through their synthesis via sputter
deposition, the amount of the alloying elements, such as silicon, can easily be varied over
a wide range [
2
]. To tune the properties of a material, it is of fundamental interest to
understand the processes during the oxidation reaction. Hence, a wide range of methods
are employed in this field such as thermogravimetry to measure the reaction kinetics, X-ray
diffraction for the resulting phase composition, and electron microscopy for its microstruc-
ture [
15
]. Of particular interest is the composition of the oxide layer and the change in the
underlying thin film. A method to characterize these needs to give quantitative results and
a sufficient depth resolution to separate the layers. There are several frequently employed
methods for the analysis of boride thin films: SIMS (secondary ion mass spectrometry) [
16
],
GD-OES/MS (glow discharge optical emission/mass spectrometry) [
17
,
18
], XPS (X-ray
photoelectron spectrometry) [
19
], LA-ICP-MS (laser ablation inductively coupled plasma
mass spectrometry) [
20
], and X-ray-based methods, such as XRF (X-ray fluorescence) and
SEM-EDX (scanning electron microscopy with energy-dispersive X-ray analysis), with their
specific strengths and drawbacks [
18
,
21
]. Of those SIMS, GD-OES/MS and LA-ICP-MS
are commonly used for depth profiling. A general limitation of direct solid-state anal-
ysis, especially for depth profiling, is the availability of suitable reference materials for
quantitative analysis. The availability of matrix-matched standards and certified reference
materials (CRMs) is limited, especially for novel classes of materials such as borides [
22
].
A reference material must match the chemical and physical properties of the sample as
closely as possible. In the case of thin films undergoing oxidation, the oxide layer is signifi-
cantly different in properties compared to the boride-based material; therefore, at least two
reference materials are needed to precisely determine the stoichiometry throughout the
depth profile.
A promising solution to this limitation is the recently introduced online-laser ablation
of solids in liquids (online-LASIL) technique [
23
], as it can circumvent several limitations
of other techniques. In online-LASIL, tiny portions of the samples are removed by laser ab-
lation. The ablation is performed in a continuous flow of carrier solution, which transports
the removed material into a hyphenated instrument to derive the sample’s composition.
The main advantage of online-LASIL is that it is possible to use ready-to-hand liquid
standards to determine the composition of the particle suspension produced in the LASIL
process. It is well known from slurry analysis and single-particle ICP-MS (spICP-MS) in
the ICP plasma, that particles below a specific size behave like ions and can therefore be
quantified with liquid standards [
24
,
25
]. Through this, the use of matrix-matched standards
or certified reference materials (CRMs) can be circumvented. Laser ablation in liquid has
been successfully applied to several material science samples in the past [
26
,
27
]. The flexible
preparation of customized liquid standards allows for quantitative investigations of a wide
range of materials not accessible with more conventional techniques. The second main
advantage of online-LASIL is the improved depth resolution. Whereas with conventional
nanosecond laser ablation, depth resolutions in the order of some hundred nm are possi-
ble [
18
], previously published online-LASIL applications report values below 50 nm [
23
,
28
].
With these characteristics, online-LASIL is a promising candidate to perform quantitative,
depth-resolved analysis to understand the oxidation behavior of transition metal diboride.
In a preceding work [
15
], the oxidation behavior of several Si-alloyed transition metal
diborides was investigated. As the systems based on chromium diboride doped with
silicon showed an exceptionally good oxidation resistance, forming oxide scales with an
average thickness of only 400 nm, they are of particular interest. For this work, two samples
with different silicon contents were chosen to observe the influence of silicon on oxide scale
formation. A methodology based on online-LASIL was developed to perform quantitative
Molecules 2022,27, 3221 3 of 12
depth profile measurements of these materials, enabling the determination of the oxide
scale and changes in the stoichiometry of the underlying thin film. Classical acid digestion
with subsequent liquid ICP-OES was performed to obtain the bulk stoichiometry before
the oxidation. With the data from the rigorous characterization of the thin films, including
X-Ray diffraction (XRD), thermogravimetry (TGA), and transmission electron microscopy
(TEM), the information of the online-LASIL depth profiles could be validated and put into
a material science context.
2. Results and Discussion
2.1. Optimization of Carrier Solution
From conventional liquid sample analysis, it is well known that analytes, primarily
metal ions, can become lost during sample storage or analysis due to the fact of precipitation
or adsorption to the walls of the sample container or the analysis system. Therefore, in
classical trace elemental analysis, samples and standards are prepared in a diluted acidic
solution, usually 1% (v/v) HNO
3
or HCl, to stabilize the dissolved ions and prevent
possible losses during storage and measurement. When performing laser ablation in
liquid, the ablated material can be present in the form of suspended particles but also as
dissolved ions. For online-LASIL, this is an especially critical issue as the use of a liquid
carrier for the transport of the reaction products from the ablation site to the detection unit
might result in fractionation effects preventing an accurate determination of the sample
stoichiometry. Possible reasons for the loss of dissolved species have been mentioned
above. However, particles can also become lost, for example, through agglomeration and
subsequent sedimentation. Thus, to avoid fractionation effects, stabilization of both forms,
particles and solvated species, is necessary to prevent losses of these ablation products.
For this purpose, the design of the online-LASIL cell has been refined [
29
] to include two
inlets for liquid solutions: one for the carrier solution, which flows over the sample surface,
and one for the make-up solution, which is mixed with the carrier solution right after
the ablation process. As the flow of the carrier solution was chosen to be higher than the
make-up solution, it was ensured that the make-up solution could not be directed into the
sample cavity, impeding contact with the sample in the LASIL cell. This concept allows
for the use of more concentrated acids to prevent a loss of analytes, as the concentrated
acid solution does not come into contact with the sample, which is a precondition for the
investigation of acid-sensitive samples.
To find an optimal combination for the composition of carrier and make-up solution,
the approach presented in Weiss et al. (2021) [
28
] was applied as exemplified in Figure 1. A
series of standard solutions of the elements of interest were prepared in different candidate
make-up solutions. The candidate make-up solution and a candidate carrier solution
were flown through the cell until a stable background signal was reached. Then, a defined
amount of a spiking material was added to the make-up solution, causing a sudden increase
in the respective ICP-MS signals. After the signals in the ICP-MS reached a plateau for
all elements, the spike solution was exchanged to the pure make-up solution again. After
the signal stabilized again, a washing step with 10% HCl was performed (flown through
both inlets). If the analyte was adsorbed onto the walls of the system, in this washing step,
a strong peak in the signal would appear; if not, the analyte signal would remain at the
baseline level. The lowest acid concentration in the make-up and carrier solutions able to
achieve this behavior was considered optimal. Based on the experience reported in [
28
], for
the carrier solution, a NH
4
Cl/HCl buffer with EDTA added was used. A concentration of
910 mmol NH
4
Cl and 30 mmol/L HCl resulting in a pH of 5 and an EDTA concentration of
4.17 mg/L for the make-up solution and a mixture of 2% (v/v) HCl and 0.2% (v/v) HF for
the make-up solution were found to give optimal results, as HF is known to be necessary
to stabilize transition elements [30].
Molecules 2022,27, 3221 4 of 12
Molecules 2022, 27, x FOR PEER REVIEW 4 of 13
HCl and 0.2% (v/v) HF for the make-up solution were found to give optimal results, as HF
is known to be necessary to stabilize transition elements [30].
Figure 1. Demonstration of the optimization process for the make-up and carrier solutions: If the
analyte was not stabilized enough, it was absorbed onto the walls, and it appeared as a peak if it
was desorbed by concentrated acid.
2.2. Quantification of LASIL Measurements
In online-LASIL measurements, the material was at least partially ablated in the form
of particles. To use liquid standards, it is a prerequisite that these particles behave in the
plasma the same way as the liquid standards. This assumption is also routinely used in
single-particle ICP-MS (spICP-MS), where liquid standards are used to infer the size of
nanoparticles typically below 100 nm, which have gained a broad spectrum of applica-
tions over the last years [25,31]. On the upper end, it was found that particles with a di-
ameter of up to 3 µm can be quantified with liquid standards in slurry analysis [24]. To
ensure that the particles generated with online-LASIL are small enough to be fully ionized
in the plasma and fulfill the requirements for the application of liquid standard solutions,
measurement of the size distribution was necessary. For this purpose, the flow out of the
online-LASIL cell was collected in a tube during the ablation. The solution containing the
generated nanoparticles was analyzed in a ZetaView particle tracker after dilution with
water to provide a signal in the working range of the instrument. Particles in the range
between 65 and 200 nm were found with a median of 114 nm, which is the size range
typically studied in spICP-MS and one order of magnitude below the size limit reported
for slurry samples. In Supplementary Materials Figure S1, the size distribution graph is
shown. Thus, the use of liquid standards is a valid approach for quantification of the par-
ticle suspensions produced with online-LASIL.
Online-LASIL measurements were quantified using the standard addition approach
[32] by adding defined amounts of the investigated analytes to the make-up solution. In
Figure 2, a typical time-resolved signal for
52
Cr of a depth profile measurement is demon-
strated.
Figure 1.
Demonstration of the optimization process for the make-up and carrier solutions: If the
analyte was not stabilized enough, it was absorbed onto the walls, and it appeared as a peak if it was
desorbed by concentrated acid.
2.2. Quantification of LASIL Measurements
In online-LASIL measurements, the material was at least partially ablated in the form
of particles. To use liquid standards, it is a prerequisite that these particles behave in the
plasma the same way as the liquid standards. This assumption is also routinely used in
single-particle ICP-MS (spICP-MS), where liquid standards are used to infer the size of
nanoparticles typically below 100 nm, which have gained a broad spectrum of applications
over the last years [
25
,
31
]. On the upper end, it was found that particles with a diameter of
up to 3
µ
m can be quantified with liquid standards in slurry analysis [
24
]. To ensure that the
particles generated with online-LASIL are small enough to be fully ionized in the plasma
and fulfill the requirements for the application of liquid standard solutions, measurement
of the size distribution was necessary. For this purpose, the flow out of the online-LASIL
cell was collected in a tube during the ablation. The solution containing the generated
nanoparticles was analyzed in a ZetaView particle tracker after dilution with water to
provide a signal in the working range of the instrument. Particles in the range between
65 and 200 nm were found with a median of 114 nm, which is the size range typically
studied in spICP-MS and one order of magnitude below the size limit reported for slurry
samples. In Supplementary Materials Figure S1, the size distribution graph is shown. Thus,
the use of liquid standards is a valid approach for quantification of the particle suspensions
produced with online-LASIL.
Online-LASIL measurements were quantified using the standard addition approach [
32
]
by adding defined amounts of the investigated analytes to the make-up solution. In Figure 2,
a typical time-resolved signal for 52Cr of a depth profile measurement is demonstrated.
The concentration of the elements in the ablated material can be calculated by the
following formula:
cabl =cs
S(AS)(1)
where
cabl
is the concentration of the respective element in the flow from ablation;
cS
is the
concentration of the spike; Ais the integrated area of the ablation peak; Sis the integrated
area from the spike. Note that the integration regions Aand Shave the same duration.
To exclude that material is removed from the fused silica window of the LASIL cell
during ablation, which would contribute to the silicon signal detected with ICP-MS, and
measurements with pure Al
2
O
3
substrates were performed. In these experiments, no
increase in the silicon signal was observed during sample ablation, indicating that the use
of a silicon window did not influence the analysis of Si.
Molecules 2022,27, 3221 5 of 12
Molecules 2022, 27, x FOR PEER REVIEW 5 of 13
Figure 2. Illustration of the signal during a depth profile measurement with standard addition.
The concentration of the elements in the ablated material can be calculated by the
following formula:
𝑐 𝑐
𝑆
𝐴
𝑆 (1)
where 𝑐 is the concentration of the respective element in the flow from ablation; 𝑐 is
the concentration of the spike; A is the integrated area of the ablation peak; S is the inte-
grated area from the spike. Note that the integration regions A and S have the same dura-
tion.
To exclude that material is removed from the fused silica window of the LASIL cell
during ablation, which would contribute to the silicon signal detected with ICP-MS, and
measurements with pure Al
2
O
3
substrates were performed. In these experiments, no in-
crease in the silicon signal was observed during sample ablation, indicating that the use
of a silicon window did not influence the analysis of Si.
2.3. Optimization of Depth Profile Measurements
Measurement of silicon with ICP-MS is challenging due to the high ionization energy
of 8.151 eV. Moreover, the main isotopes
28
Si and
29
Si are interfered with by several poly-
atomic ions arising from the ICP plasma [33]. Therefore, the detection limit for silicon with
ICP-MS is orders of magnitude worse than for most other elements [34]. With the advent
of collision-reaction cell technology, the intensities of interfering polyatomic ions could be
significantly reduced; thus, the signal ratio of analyte to the background can be improved
[35]. However, the sensitivity achieved for silicon is still lower compared to the other ele-
ments examined in this study. With the use of liquid calibration standards which were
introduced via the make-up solution, detection limits (LODs) for the applied ICP-MS pro-
cedure were determined. Silicon had, as expected, the highest LOD of 69 ng/g, compared
to 2 ng/g for B and 0.08 ng/g for Cr, and is, therefore, the limiting factor in the analysis of
the samples.
In the used online-LASIL setup, the main parameter influencing the amount of sam-
ple material introduced into the detection unit was the ablation rate, which is determined
by the applied laser energy [36,37]. The ablation energy cannot be arbitrarily reduced, as
the ablation process only onsets over a certain energy threshold [38]. For optimization a
Figure 2. Illustration of the signal during a depth profile measurement with standard addition.
2.3. Optimization of Depth Profile Measurements
Measurement of silicon with ICP-MS is challenging due to the high ionization energy
of 8.151 eV. Moreover, the main isotopes
28
Si and
29
Si are interfered with by several
polyatomic ions arising from the ICP plasma [
33
]. Therefore, the detection limit for silicon
with ICP-MS is orders of magnitude worse than for most other elements [
34
]. With the
advent of collision-reaction cell technology, the intensities of interfering polyatomic ions
could be significantly reduced; thus, the signal ratio of analyte to the background can be
improved [
35
]. However, the sensitivity achieved for silicon is still lower compared to
the other elements examined in this study. With the use of liquid calibration standards
which were introduced via the make-up solution, detection limits (LODs) for the applied
ICP-MS procedure were determined. Silicon had, as expected, the highest LOD of 69 ng/g,
compared to 2 ng/g for B and 0.08 ng/g for Cr, and is, therefore, the limiting factor in the
analysis of the samples.
In the used online-LASIL setup, the main parameter influencing the amount of sample
material introduced into the detection unit was the ablation rate, which is determined
by the applied laser energy [
36
,
37
]. The ablation energy cannot be arbitrarily reduced, as
the ablation process only onsets over a certain energy threshold [
38
]. For optimization a
defined laser pattern was ablated using different laser energies, and the peak area of the
derived transient signals was integrated. The lowest investigated laser energy to achieve a
measurable
28
Si signal was 0.17 mJ (Figure 3). At this energy, the boride film could be fully
ablated with ten layers, indicated by a sudden drop in the total signal observed for the
measured elements when the Al
2
O
3
substrate was reached. With further ablation passes
at this energy, it was impossible to observe an
27
Al signal, indicating that the energy was
below the ablation threshold for Al
2
O
3
. These findings were confirmed by a profilometric
scan of the ablation crater, which suggests that, in total, approximately 2.4
µ
m of the sample
had been ablated, resulting in a depth resolution of ~240 nm per layer at this laser energy.
For experiments with a reduced laser energy of 0.068 mJ, an ablation rate of approximately
85 nm was observed. A further reduction in the laser energy to 0.034 mJ resulted in a
decreased ablation rate of 54 nm per layer. However, with these conditions the amount of
Si introduced into the ICP-MS was insufficient for quantitative measurements.
Molecules 2022,27, 3221 6 of 12
Molecules 2022, 27, x FOR PEER REVIEW 6 of 13
defined laser pattern was ablated using different laser energies, and the peak area of the
derived transient signals was integrated. The lowest investigated laser energy to achieve
a measurable 28Si signal was 0.17 mJ (Figure 3). At this energy, the boride film could be
fully ablated with ten layers, indicated by a sudden drop in the total signal observed for
the measured elements when the Al2O3 substrate was reached. With further ablation
passes at this energy, it was impossible to observe an 27Al signal, indicating that the energy
was below the ablation threshold for Al2O3. These findings were confirmed by a profilo-
metric scan of the ablation crater, which suggests that, in total, approximately 2.4 µm of
the sample had been ablated, resulting in a depth resolution of ~240 nm per layer at this
laser energy. For experiments with a reduced laser energy of 0.068 mJ, an ablation rate of
approximately 85 nm was observed. A further reduction in the laser energy to 0.034 mJ
resulted in a decreased ablation rate of 54 nm per layer. However, with these conditions
the amount of Si introduced into the ICP-MS was insufficient for quantitative measure-
ments.
Figure 3. Dependence of the ablation rate and the ICP-MS signal from the applied laser energy.
This outcome indicates that the sensitivity of the silicon analysis was the limiting
factor, as the amount of ablated material decreased concomitantly with the reduction in
the thickness of an ablation layer. Nevertheless, for elements with an enhanced sensitivity
in ICP-MS analysis, investigations with a better depth resolution were possible. In the case
of B, even for the lowest laser energy, excellent signals were obtained (for details see Fig-
ure 3), enabling measurements with an ablation rate of 54 nm only. This is in accordance
with previous works [23,28], where a similar depth resolution could be achieved.
On the upper bound, the laser energy was limited by the mechanical strength of the
fused silica window, which can burst through the generated cavitation bubble in the car-
rier solution at approximately 1 mJ laser energy. Further, it was observed that the boride
thin films delaminated from the substrate if the laser energy was too high.
Other factors influencing the ablation were the spot size and the number of shots per
sample location. To reduce cratering effects due to the Gaussian profile of the laser beam,
overlapping spots were chosen where the laser hit each ablation spot two times by a stage-
velocity of 0.5 mm/s and a laser frequency of 10 Hz.
Figure 3. Dependence of the ablation rate and the ICP-MS signal from the applied laser energy.
This outcome indicates that the sensitivity of the silicon analysis was the limiting
factor, as the amount of ablated material decreased concomitantly with the reduction in the
thickness of an ablation layer. Nevertheless, for elements with an enhanced sensitivity in
ICP-MS analysis, investigations with a better depth resolution were possible. In the case of
B, even for the lowest laser energy, excellent signals were obtained (for details see Figure 3),
enabling measurements with an ablation rate of 54 nm only. This is in accordance with
previous works [23,28], where a similar depth resolution could be achieved.
On the upper bound, the laser energy was limited by the mechanical strength of the
fused silica window, which can burst through the generated cavitation bubble in the carrier
solution at approximately 1 mJ laser energy. Further, it was observed that the boride thin
films delaminated from the substrate if the laser energy was too high.
Other factors influencing the ablation were the spot size and the number of shots
per sample location. To reduce cratering effects due to the Gaussian profile of the laser
beam, overlapping spots were chosen where the laser hit each ablation spot two times by a
stage-velocity of 0.5 mm/s and a laser frequency of 10 Hz.
2.4. Measurement of Oxidized Samples
Two silicon-alloyed chromium diboride samples, as stated before, designated as A
and B, with different silicon doping levels were investigated within this study and used for
the depth profile measurements. Of the as-deposited samples, the bulk stoichiometry was
determined with liquid ICP-OES; the values are listed in Table 1, and the standard deviation
was derived from three replicates of the digestion. The obtained bulk stoichiometry agreed
with the composition expected from the production of thin film.
In the first step, to validate that online-LASIL yields accurate values for the stoichiome-
try of the samples, measurements with an enhanced laser energy of 0.51 mJ were performed.
This high laser energy was not suitable for the depth profile analysis, but the increased
ablation rate resulted in higher analyte concentrations in the carrier solution and, thus,
improved signal-to-noise ratios for subsequent ICP-MS analysis. As can be seen in Table 1
the findings derived for the native, non-oxidized samples were in good agreement with
the values obtained from ICP-OES measurements, in particular when considering that in
the case of online-LASIL only tiny sample areas of approximately 0.1 mm
2
were used for
analysis, whereas for the ICP-OES measurements, 5
×
5 mm large pieces were used. The
presented averages and standard deviations were determined from three ablation passes at
Molecules 2022,27, 3221 7 of 12
different sample positions. This outcome demonstrates the applicability of the proposed
online-LASIL procedure, but it also confirms that the optimized composition of the carrier
solution prevented the fractionation effects of the analyte.
Table 1.
Comparison of the results of the stoichiometry determination between the liquid ICP-OES
(n= 3) and the online-LASIL analysis performed on the native samples with high laser energy
(0.51 mJ). The cations were normalized to 100%.
Sample Measurement Cr at% Si at% B at%
Sample A ICP-OES 27.1 ±0.1 8.9 ±0.3 64.0 ±0.3
LASIL bulk analysis 25.0 ±1.8 7.7 ±3.2 67.3 ±1.5
Sample B ICP-OES 25.6 ±0.2 15.8 ±0.1 58.6 ±0.1
LASIL bulk analysis 22.8 ±1.0 18.8 ±1.5 58.4 ±0.6
In Figure 4, the quantitative online-LASIL depth profiles of the two oxidized samples
with BF-TEM cross-sections of the same samples are shown. In the TEM cross-section of
sample A, two layers separated by a sharp interface can be seen; the upper corresponded
to the formed oxide scale with a thickness of approximately 500 nm, and the lower was the
remaining boride film.
Molecules 2022, 27, x FOR PEER REVIEW 8 of 13
Figure 4. LASIL depth profiles with corresponding bright-field TEM cross-sections of the samples:
(a) sample A (native stoichiometry Cr
0
.
27
Si
0
.
9
B
0
.
64
); (b) sample B ,(native stoichiometry
Cr
0
.
26
Si
0
.
16
B
0
.
58
). The scale bar represents 1000 nm, and the images were rotated to correspond to the
depth profiles. The thin red line in the TEM image of sample B indicates the boundary of the oxide
and was determined with high-angle annular dark-field imaging (HAAFD) STEM (scanning transi-
tion electron microscopy).
The performed online-LASIL measurement revealed a very similar outcome, show-
ing for the first ~500 nm a distinctly different composition when compared to the rest of
the sample. The first ablation layer exhibited a stoichiometry of Cr
0
.
04
Si
0
.
62
B
0
.
34
, indicating
an enrichment of silicon in the oxide thin film. With the fourth ablation layer, the sample
nearly reached its native bulk composition.
The TEM image of sample B shows a rough interface between the oxide scale and the
base boride coating, with a thickness varying between 180 and 750 nm with 400 nm in
mean. Further investigations [15] indicate that the oxide film consists of an outer Cr
2
O
3
layer and an inner silicon-rich layer. The irregular interface and the large grains stem from
recrystallization processes during the heat treatment and are influenced through the
higher temperature and the higher silicon content compared to sample A. X-ray diffracto-
grams [15] obtained from the sample show that Cr
2
O
3
and Si were present as phases along-
side the base material. This was confirmed through the online-LASIL measurements. The
first two ablation layers were highly enriched in chromium, showing no boron signal. At
a depth of approximately 500 nm, the boron signal started to in increase and reached its
bulk value at approximately 1500 nm. Silicon was enriched in the top layers, which agrees
with the large grains of silicon visible in the TEM, and reached the nominal sample con-
centration at a depth of 1500 nm. Interestingly, the composition of the third ablation layer
Figure 4.
LASIL depth profiles with corresponding bright-field TEM cross-sections of the samples:
(
a
) sample A (native stoichiometry Cr
0.27
Si
0.9
B
0.64
); (
b
) sample B (native stoichiometry
Cr
0.26
Si
0.16
B
0.58
). The scale bar represents 1000 nm, and the images were rotated to correspond
to the depth profiles. The thin red line in the TEM image of sample B indicates the boundary of the
oxide and was determined with high-angle annular dark-field imaging (HAAFD) STEM (scanning
transition electron microscopy).
Molecules 2022,27, 3221 8 of 12
The performed online-LASIL measurement revealed a very similar outcome, showing
for the first ~500 nm a distinctly different composition when compared to the rest of the
sample. The first ablation layer exhibited a stoichiometry of Cr
0.04
Si
0.62
B
0.34
, indicating
an enrichment of silicon in the oxide thin film. With the fourth ablation layer, the sample
nearly reached its native bulk composition.
The TEM image of sample B shows a rough interface between the oxide scale and
the base boride coating, with a thickness varying between 180 and 750 nm with 400 nm in
mean. Further investigations [
15
] indicate that the oxide film consists of an outer Cr
2
O
3
layer and an inner silicon-rich layer. The irregular interface and the large grains stem from
recrystallization processes during the heat treatment and are influenced through the higher
temperature and the higher silicon content compared to sample A. X-ray diffractograms [
15
]
obtained from the sample show that Cr
2
O
3
and Si were present as phases alongside the
base material. This was confirmed through the online-LASIL measurements. The first two
ablation layers were highly enriched in chromium, showing no boron signal. At a depth of
approximately 500 nm, the boron signal started to in increase and reached its bulk value at
approximately 1500 nm. Silicon was enriched in the top layers, which agrees with the large
grains of silicon visible in the TEM, and reached the nominal sample concentration at a
depth of 1500 nm. Interestingly, the composition of the third ablation layer neither matched
with the top layer nor with the underlying layer and seemed to be a mixture of the formed
oxide and the bulk material, which agrees with the coarse interface visible in the TEM.
The results derived from online-LASIL measurements indicate that no significant
change in the overall stoichiometry of the whole film occurred during the oxidation.
3. Material and Methods
3.1. Reagents and Instrumentation
High-purity (18.2 M
resistivity at 25
C) water was obtained from a Barnstead
EASYPURE II system (Thermo Fisher Scientific, Waltham, MA, USA). Acids and all other
chemicals not otherwise mentioned and certified ICP liquid standards were purchased
from Merck (Darmstadt, Germany) in at least analytical quality. Laser ablation was per-
formed with a J200 Tandem (Applied Spectra Inc., Sacramento, CA, USA) equipped with
a 266 nm Nd:YAG laser with a pulse duration of 5 ns. For the ICP-MS measurements, an
iCAP Qc, (Thermo Fisher Scientific, Bremen, Germany) equipped with an HF (hydrofluoric
acid)-resistant sample introduction kit (alumina injector tube, a perfluoroalkoxy alkane
(PFA) cyclonic spray chamber, and a PFA concentric nebulizer) was used. Liquid ICP-OES
measurements were performed on an iCAP 6500 RAD (Thermo Fisher Scientific, USA) cou-
pled to an ASX-520 autosampler (CETAC Technologies, Omaha, NE, USA) equipped with
an HF-resistant sample introduction kit, which included a Miramist nebulizer (Burgener
Research, Mississauga, ON, Canada), a Teflon spray chamber, and an Al
2
O
3
injector tube.
TEM measurements were performed on an FEI TECNAI F20 (Thermo Fisher Scientific,
Bremen, Germany). The depth of ablation craters was recorded on a Dektak XT (Bruker,
Billerica, MA, USA) stylus profilometer. Particle size distribution of the ablated material was
conducted on a ZetaView particle tracker (Particle Metrix, Inning am Ammersee, Germany).
Data analysis was performed in Excel (Microsoft Cooperation, Redmond, MA, USA) and
OriginPro 2020 (OriginLab Corporation, Northampton, MA, USA).
3.2. Deposition and Treatment of Samples
The deposition of the coatings, the oxidation treatment, and rigorous characterization
of the samples are described in detail by Glechner et al. [
15
]. The two samples, designated as
A and B in the following, were deposited in an in-house built magnetron sputter device [
39
]
using a CrB
2
target (Plansee Composite Materials GmbH, Lechbruck am See, Germany)
on which Si wafer plates (CrysTec GmbH, Berlin, Germany) were placed on the racetrack
to alloy Si. Through changing the number of silicon plates, the amount of silicon in the
samples varied. Polycrystalline Al
2
O
3
(CrysTec GmbH) platelets were used as substrate.
The thickness of the deposited films was approximately 2.4
µ
m. Oxidation treatment of the
Molecules 2022,27, 3221 9 of 12
samples was performed in a chamber box furnace under ambient air. The temperatures
used were 1100 C for sample A or 1200 C for sample B for 3 h.
3.3. LASIL Setup
For the online-LASIL measurements, specimens were put in a liquid-tight in-house made
cell, which formed the flow path. The cell design was similar to previous works [
28
,
29
,
32
]. The
LASIL cell (Figure 5) consisted of a PEEK (polyether ether ketone) body with a 5
×
5
×
0.5 mm
pocket to contain the sample. The employed LASIL cell design contained two inlets and
one outlet for fluids. The cell was sealed by a PDMS (polydimethylsiloxane) film (500
µ
m
thick) containing the 500
µ
m wide flow path to guide the liquid over the sample. A fused
silica window, transparent to the UV laser wavelength, was placed on top and aligned to
the body by a counterpart made of PEEK. A peristaltic pump (Perimax 12, SPETEC, Erding,
Germany) was used to transport the required fluid flows through the system. All tubing
was made out of PFA with an inner diameter of 0.5 mm on the input side and 0.25 mm
between the LASIL cell and the ICP-MS instrument.
Molecules 2022, 27, x FOR PEER REVIEW 10 of 13
Figure 5. Left: CAD exploded drawing of the LASIL cell. The gray parts represent PEEK, the PFA
tubing pushed in the PEEK body is depicted in red, the PDMS spacer in black, and the fused silica
window in green. Right: Cross-section of the LASIL cell showing the fluid path and the flows
through the sample cell. The LASIL cells consisted of a PEEK body with a cavity for the sample.
Through the body, three tubes pushed the flow for two inlets and one outlet. The carrier flowed
over the sample, the make-up solution was added behind the sample in the fluid path, so it did not
come into contact with the sample. The two flows mixed just before they left the LASIL cell at the
outlet to the ICP-MS instrument. The cell was sealed by a PDMS spacer and a fused silica window.
The LASIL cell was positioned on the movable XYZ sample stage of the J200 laser
ablation platform. To analyze the ablated material, the generated particle suspension was
purged with a carrier solution into a quadruple ICP-MS. The instrument was tuned daily
for a maximum 115In signal intensity and a minimum 140Ce16O/140Ce oxide ratio. The ICP-
MS instrument was operated in the KED (kinetic energy discrimination) mode using 7%
H2 in He as collision gas. The ICP-MS measurement parameters are listed in Table 2. The
instrument software (Qtegra version 2.10) was used for data collection and evaluation.
Table 2. ICP-MS measurement parameters for online-LASIL.
Parameter Value
RF power 1550 W
Auxiliary gas flow (Ar) 1.0 L/min
Cooling gas flow (Ar) 14 L/min
Nebulizer gas flow (Ar) 0.8 L/min
CCT bias 21 V
Pole bias 18 V
KED gas flow (7% H2 in He) 5 mL/min
Monitored ions 11B, 27Al, 28Si, 52Cr, 53Cr, 115In,
Dwell time 0.01 s for 27Al, 52Cr, 53Cr, 115In
0.05 s for 11B and 28Si
For LASIL measurements, substrates were broken into 5 × 5 mm pieces by scratching
with a diamond cutter to fit tightly into the pocket of the LASIL cell. The current LASIL
setup allowed for the use of two fluid flows: a carrier flow and a make-up flow (for details,
see Figure 5); both contained 10 ng/g of indium as an internal standard to monitor poten-
tial signal drifts during measurements. The isotopes selected for ICP-MS analysis, as well
as the applied instrumental parameters, are compiled in Table 2. Laser ablation was per-
formed in the line scan mode, and the parameters are stated in Table 3.
Figure 5.
(
Left
): CAD exploded drawing of the LASIL cell. The gray parts represent PEEK, the PFA
tubing pushed in the PEEK body is depicted in red, the PDMS spacer in black, and the fused silica
window in green. (
Right
): Cross-section of the LASIL cell showing the fluid path and the flows
through the sample cell. The LASIL cells consisted of a PEEK body with a cavity for the sample.
Through the body, three tubes pushed the flow for two inlets and one outlet. The carrier flowed over
the sample, the make-up solution was added behind the sample in the fluid path, so it did not come
into contact with the sample. The two flows mixed just before they left the LASIL cell at the outlet to
the ICP-MS instrument. The cell was sealed by a PDMS spacer and a fused silica window.
The LASIL cell was positioned on the movable XYZ sample stage of the J200 laser
ablation platform. To analyze the ablated material, the generated particle suspension was
purged with a carrier solution into a quadruple ICP-MS. The instrument was tuned daily
for a maximum
115
In signal intensity and a minimum
140
Ce
16
O/
140
Ce oxide ratio. The
ICP-MS instrument was operated in the KED (kinetic energy discrimination) mode using
7% H
2
in He as collision gas. The ICP-MS measurement parameters are listed in Table 2.
The instrument software (Qtegra version 2.10) was used for data collection and evaluation.
For LASIL measurements, substrates were broken into 5
×
5 mm pieces by scratching
with a diamond cutter to fit tightly into the pocket of the LASIL cell. The current LASIL
setup allowed for the use of two fluid flows: a carrier flow and a make-up flow (for details,
see Figure 5); both contained 10 ng/g of indium as an internal standard to monitor potential
signal drifts during measurements. The isotopes selected for ICP-MS analysis, as well as the
applied instrumental parameters, are compiled in Table 2. Laser ablation was performed in
the line scan mode, and the parameters are stated in Table 3.
Molecules 2022,27, 3221 10 of 12
Table 2. ICP-MS measurement parameters for online-LASIL.
Parameter Value
RF power 1550 W
Auxiliary gas flow (Ar) 1.0 L/min
Cooling gas flow (Ar) 14 L/min
Nebulizer gas flow (Ar) 0.8 L/min
CCT bias 21 V
Pole bias 18 V
KED gas flow (7% H2in He) 5 mL/min
Monitored ions 11B, 27Al, 28Si, 52Cr, 53Cr, 115In,
Dwell time 0.01 s for 27Al, 52Cr, 53Cr, 115In
0.05 s for 11B and 28Si
Table 3. Laser parameters for the online-LASIL measurements.
Parameter Value
Laser energy depth profile 0.17 mJ
Laser energy survey run 0.51 mJ
Spot size 100 µm
Scan speed 500 µm/s
Carrier solution flow 0.53 mL/min
Makeup solution flow 0.28 mL/min
Repetition rate 10 Hz
Investigated sample area 0.1 mm2
3.4. ICP-OES Reference Measurements
To obtain reference values for the samples, an aliquot of the native, unoxidized samples
was converted into a solution and measured with conventional liquid ICP-OES. For this
purpose, samples were broken into pieces of approximately 5
×
5 mm and digested in
triplicates in metal-free falcon tubes with a mixture of 0.25 mL nitric acid and 0.25 mL
hydrofluoric acid at a temperature of 80
C for 10 min. Derived solutions were diluted
to a final volume of 20 mL with ultrapure water, and europium was added as an internal
standard with a final concentration of 1
µ
g/g. External calibration with matrix-adjusted
standards was used for quantification. Two emission lines were observed per element,
one used for quantification one for quality control; for further details, see Supplementary
Materials Table S1. The applicability of this procedure was recently demonstrated [
6
,
20
,
29
].
4. Conclusions
In this work, the oxidation behavior of two samples of chromium diboride doped
with different levels of silicon was investigated. TEM images revealed a clear difference in
the oxidation resistance of the two coating materials. Online-LASIL has been applied for
the determination of sample stoichiometry but also for the measurement of quantitative
depth profiles. Further improvement of the cell design and careful optimization of the
measurement conditions enabled, for the first-time, quantitative measurements of depth
profiles without the use of matrix-matched standards.
The results obtained for the native, unoxidized samples were found to be in good
agreement with ICP-OES reference measurements, demonstrating the suitability of the
proposed online-LASIL approach for the analysis of Si-alloyed transition metal diborides.
The derived depth profiles showed a good correlation with TEM images of the samples.
Moreover, with online-LASIL, it was possible to gain insight into the exact stoichiometry
of the oxide layer and the change in the bulk sample below. It could be shown that the
oxide layers of the two samples had a very different composition. This information is
not easily accessible with other techniques capable of depth-resolved measurements, as
matrix-matched standards are required for most methods. In this case, where the sample
consisted of two different materials (i.e., the oxide layer and the native boride), at least two
Molecules 2022,27, 3221 11 of 12
CRMs would be needed for quantitative investigations. This is a difficult task since, for
many materials, it remains challenging to find even one suitable CRM.
As the capabilities of online-LASIL for the assessment of depth-resolved changes in
thin film stoichiometry have been shown and validated by several reference techniques, it
is intended to extend the investigations to a larger number of samples in the future. These
should cover a broader range of sample compositions and treatment conditions, providing
more insights into the fundamental processes of high-temperature corrosion.
Supplementary Materials:
The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/molecules27103221/s1, Figure S1: Particle size distribution;
Table S1: ICP-OES parameters for liquid reference measurements.
Author Contributions:
Conceptualization, M.W. and A.L.; Data curation, M.W.; Funding acquisition,
H.R. and A.L.; Investigation, M.W., T.G. and V.U.W.; Methodology, M.W.; Project administration, H.R.;
Resources, V.U.W., H.R. and A.L.; Supervision, H.R. and A.L.; Visualization, M.W.; Writing—original
draft, M.W.; Writing—review and editing, T.G., V.U.W., H.R. and A.L. All authors have read and
agreed to the published version of the manuscript.
Funding:
Open Access Funding by the Austrian Science Fund (FWF)—(grant number: P31165-N37).
The financial support from the Austrian Federal Ministry for Digital and Economic Affairs; the
National Foundation for Research, Technology, and Development; the Christian Doppler Research
Association is gratefully acknowledged (Christian Doppler Laboratory, “Surface Engineering of
High-Performance Components”). We are also thankful for the financial support from Plansee SE,
Plansee Composite Materials GmbH, and Oerlikon Balzers, Oerlikon Surface Solutions AG.
Data Availability Statement:
The data presented in this study are available upon request from the
corresponding author.
Acknowledgments:
We want to thank the University Service Centre for Transmission Electron Mi-
croscopy of the TU Wien (USTEM) for providing the TEM measurements. The authors acknowledge
TU Wien Bibliothek for financial support through its Open Access Funding Program.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Not available.
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