A Nanoscale Characterization with Electron
Microscopy of Multilayered CrAlYN Coatings:
A Singular Functional Nanostructure
Teresa C. Rojas,1,* Santiago Domínguez-Meister,1 Marta Brizuela,2 Alberto García-Luis,2
Asunción Fernández,1 and Juan Carlos Sánchez-López1
1Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Avda. Américo Vespucio 49, 41092-Sevilla, Spain
2TECNALIA, Mikeletegui Pasealekua, 20009 Donostia-San Sebastián, Spain
Abstract: A combination of transmission electron microscopy techniques and spatially resolved microanalysis
is used to investigate the nanostructure, constituting phases, and chemical elemental distribution in CrAlYN
multilayered coatings. The location of the metallic elements and their chemical state are needed to understand
their functional properties. Samples were prepared with variable Al ~4–12 at%! and Y ~2–5 at%! contents by
direct current reactive magnetron sputtering on silicon substrates using metallic targets and Ar/N2 mixtures
under different deposition parameters ~power applied to the target and rotation speed of the sample holder!.
The changes produced in the nanostructure and chemical distribution were investigated. Nanoscale resolution
electron microscopy analysis has shown that these coatings present a singular nanostructure formed by
multilayers containing at a certain periodicity nanovoids filled with molecular nitrogen. Spatially resolved
energy dispersive spectroscopy and electron energy loss elemental mappings and profiles showed that the
chromium, aluminum, and yttrium atoms are distributed in a sequential way following the position of
the targets inside the deposition chamber. Analysis of the different atomic distribution and phases formed at
the nanoscale is discussed depending on the deposition parameters.
Key words: EELS, high angle annular dark field ~HAADF!, phase composition, sputtering, nanoscale chemical
mapping, N2-filled nanopores
INTRODUCTION
Magnetron sputtered chromium aluminum nitride ~CrAlN!
films are excellent candidates for advanced machining and
protection for high temperature applications ~Reiter et al.,
2005; Endrino et al., 2007; Wang et al., 2008!. The funda-
mental reasons are the increased thermal stability ~Will-
mann et al., 2006; Barshilia et al., 2009!, mechanical
properties ~Hasegawa et al., 2005; Barshilia et al., 2006!, and
oxidation resistance ~Banakh et al., 2003; Kawate et al.,
2003; Brizuela et al., 2005; Braun et al., 2010; Escobar
Galindo et al., 2010! in comparison to the binary CrN
coating. CrN experiences a fast oxidation over 7008C, where
the properties of hardness and corrosion resistance decrease
rapidly ~Mège-Revil et al., 2009!. The oxidation resistance
can be enhanced above 800–9008C with reasonably minor
amounts of aluminum ~Al! additions into the Cr1xAlxN
coating ~Sánchez-López et al., 2005a; Rojas et al., 2012!
although optimizing its content can yield a further im-
proved oxidation ~Lin et al., 2008; Mayrhofer et al., 2008!.
The upper critical limit is found around x  0.6–0.8
~depending on the deposition conditions!, where the struc-
ture of Cr1xAlxN changes from the face-centered cubic B1
~NaCl! to the hexagonal closed-packed B4 ~wurtzite!, which
is accompanied by a decrease in hardness and wear perfor-
mance. The protective effect against oxidation lies in the
formation of complex Al and chromium ~Cr! oxide scales,
which eventually suppress the oxygen diffusion into the
bulk and the outward diffusion of metallic cations ~Sánchez-
López et al., 2005a; Lin et al., 2008; Mayrhofer et al., 2008;
Mège-Revil et al., 2009; Braun et al., 2010; Sánchez et al.,
2010; Rojas et al., 2012!.
Recently, much attention has been paid to the incorpo-
ration of yttrium ~Y! to ternary nitride films. Adding Y as a
reactive element has been demonstrated to decrease the
oxidation rate and to modify the oxide growth mechanism.
The main effect of Y on transport processes may be the
inhibition of cation diffusion along oxide grain boundaries
~Prescott & Graham, 1992!. Moreover, it is suggested that Y
favors the scale adhesion because the oxides protrude into
the film. Y-containing cubic Cr1xAlxN films were shown to
exhibit improved oxidation resistance up to temperatures
exceeding 1,0008C when the Y content did not exceed
around 1 at% ~2 mol% YN! ~Rovere et al., 2008; Brizuela
et al., 2009!. However, the location and chemical state of the
Y atoms in the complex nanostructured and nanocomposite
films are still open questions.
Another important parameter is tailoring architecture
to combine the benefits of advanced ternary or quaternary
metal nitrides with the enhanced performance obtained by
controlled modulation at the nanoscale. Nanocomposite
and multi-layered coatings have been demonstrated to
provide superior hardness, toughness, tribological, and ox-
idation resistance thanks to phase separation and defined
Received May 17, 2013; accepted December 4, 2013
*Corresponding author. E-mail: tcrojas@icmse.csic.es
Microsc. Microanal. 20, 14–24, 2014
doi:10.1017/S1431927613013962 MicroscopyAND
Microanalysis
© MICROSCOPY SOCIETY OF AMERICA 2014
interfaces at the nanoscale level ~Hovsepian et al., 2000,
2006; Musil et al., 2008; Steyer et al., 2008; Veprek &
Veprek-Heijman Maritza, 2008!. This increased overall per-
formance proceeds by different mechanisms such as the
Hall–Petch effect, the numbers of interfaces that act as
obstacles for the crack propagation, and the inward and
outward diffusions of atomic species between the layers,
among other factors. Different CrN-based multilayer sys-
tems like CrN/AlN ~Tien et al., 2010; Cabrera et al., 2011!,
WC/CrAlN ~Lee et al., 2002!, CrN/CrAlN ~Barshilia et al.,
2007!, TiAlN/CrAlN ~Barshilia et al., 2009!, and TiAlSiN/
CrAlN ~Fukumoto et al., 2009! have been deposited show-
ing improvement of mechanical properties and oxidation
resistance compared with the single layer CrN and CrAlN
coatings. The influence of the location of the metallic
elements inside the ternary or quaternary nitrides and their
chemical state may have a significant influence on the
oxidation behavior and protective mechanism.
In some cases, although the formation of a multilayer
structure was not sought on purpose, it appeared as a result
of the sequential exposure of the substrate to the material
fluxes generated by magnetrons during rotation ~Panjan
et al., 2008, 2009!, especially in semi-industrial chambers
with multiple rotation. This is particularly important when
dealing with scaling-up for industrial processing. Multilay-
ered nanostructure has been previously reported ~Ross et al.,
2010! in CrAlYN layers where the observed contrast is
because of alternating CrAlYN/CrN bilayers. In our previ-
ous paper ~Rojas et al., 2012! we also showed a multilayered
nanostructure in CrAlYN coatings and further studies were
needed to assess the nanostructure and chemical distribu-
tion of the CrAlYN components in these coatings. This
work reports on the investigation of two series of CrAlYN
coatings deposited by the magnetron sputtering technique
using different deposition parameters aiming to elucidate
the influence of the synthesis conditions on the microstruc-
ture and chemical composition. The location and chemical
state of Al and Y atoms in these nanostructured coatings are
not well known and are a great experimental challenge
owing to their low concentration. Several transmission elec-
tron microscopy ~TEM! techniques have been used in order
to fully characterize and correlate the layered nanostructure,
the present phases, and the elemental distribution with
nanoscale resolution, in particular, for the dopant elements
Al and Y, as a function of the deposition parameters used.
The role that this singular nanostructure can play in the
demonstrated high-temperature oxidation resistance of these
coatings is proposed.
MATERIALS AND METHODS
Sample Deposition
CrAlYN coatings were prepared on Si ~100! substrates by
direct current magnetron sputtering using Ar/N2 mixtures
in commercial equipment ~CemeCon CC800/8! provided
with four rectangular targets ~200  88  5 mm!. A sche-
matic diagram of the deposition system is shown in Fig-
ure 1. Cr ~99.9% purity!, Al ~99.5% purity!, and Y ~99.5%
purity! targets were used. The sputtering power was set at
3,000 W for Cr and Al targets whilst 1,500 or 3,000 W was
employed for the Y target. The target configuration in the
chamber, the applied power, and the rotation speeds of the
prepared samples ~labelled as A, B, C, D, and E! are summa-
rized in Table 1. Two series of samples were prepared: ~set I!
A, B, and C deposited using two Cr, one Al, and one Y
targets; ~set II!, samples D and E, where a Cr target was
replaced by Al in order to increase the Al content. The base
Figure 1. Scheme of the target distribution in the deposition
chamber and planetary sample holder.
Table 1. Synthesis Conditions of the CrAlYN Coatings Prepared by Magnetron Sputtering: Target Materials, Power
and Rotation Speeds ~v1: Turntable; v2: Tower! Applied to the Planetary Sample Holder.
Sample Set C1
Power
~W! C2
Power
~W! C3
Power
~W! C4
Power
~W!
v1
~rpm!
v2
~rpm!
A I Cr 3,000 Y 1,500 Cr 3,000 Al 3,000 0.75 2.07
B Cr 3,000 Y 1,500 Cr 3,000 Al 3,000 1.20 3.33
C Cr 3,000 Y 3,000 Cr 3,000 Al 3,000 1.20 3.33
D II Cr 3,000 Al 3,000 Y 1,500 Al 3,000 1.20 3.33
E Cr 3,000 Al 3,000 Y 3,000 Al 3,000 1.20 3.33
Nanoscale Characterization with Electron Microscopy of Multilayered CrAlYN Coatings 15
pressure of the vacuum chamber was ;1104 Pa and the
working pressure 1 Pa, with an Ar/N2 ratio of 1.5. The sam-
ple holder was negatively biased in the range of 110–120 V
and the measured temperature span from 200 to 4008C. The
film thickness values typically vary in the 2–3 mm range.
Sample Characterization
Chemical composition of the samples was obtained by
electron probe microanalysis ~EPMA!. The EPMA equip-
ment was a JEOL JXA-8200 SuperProbe instrument equipped
with four wavelength-dispersive detectors and one energy-
dispersive X-ray ~EDS! spectrometer.
For TEM characterization cross-sectional specimens
were prepared in the conventional manner by mechanical
polishing followed by Ar ion milling to electron transpar-
ency using an ion etching milling system from Fischione
Instruments ~model 1010!. A Zeiss Auriga focused ion beam
was used to prepare a cross-section lamella of sample C. To
get information about the microstructure, the chemical
composition, the phases formed, and the bonding types of
the coatings, several TEMs were used: A Philips CM200
operating at 200 kV, with a parallel spectrometer from
Gatan ~model 766-2k!. Electron energy loss spectra were
recorded in diffraction mode with a spectrometer collection
angle of 1.45 mrad, a camera length of 470 nm and a 2 mm
spectrometer entrance aperture. Under these conditions the
energy resolution of the coupled microscope/spectrometer
system was ;1.2 eV. After experimental acquisition the
electron energy loss spectroscopy ~EELS! spectra were pro-
cessed using Gatan EL/P software for background subtrac-
tion, plural scattering deconvolution, and normalization to
the continuum level after the core-loss edges of interest.
Selected area electron diffraction ~SAED! patterns of the
samples were also recorded.
A JEOL JEM 2010F field emission gun scanning trans-
mission electron microscope ~STEM! operating at 200 kV
with a high angle annular dark field ~HAADF! detector and
a Gatan imaging filter ~GIF-2000! was also used. EELS
spectra were measured in STEM mode where a 0.5 nm
beam with a current of 0.1–0.3 nA scanned several lines
along the sample C. The HAADF signal was also simulta-
neously collected at each point within the lines scanned.
A Zeiss LIBRA 200HR STEM with an HAADF, an
Omega filter, and an EDS ~Oxford Instruments! detector
was used to acquire series of energy-filtered TEM ~EFTEM!
images and a series of EDS spectra in scanning mode of
sample C. EFTEM elemental maps using the three-windows
method were acquired at Cr L2,3-edge, O K-edge, and N
K-edge.
A JEOL JEM 3100F UHR STEM operating at 300 kV,
with HAADF and EDS ~Oxford Instruments! detectors was
used to do high-resolution TEM ~HRTEM! and EDS area
scans of Al, Cr, and Y of sample C.
The Gatan Digital Micrograph software was used to
measure lattice spacings and to calculate digital diffracto-
grams. The program Eje-Z ~Perez-Omil, 1994! was used to
analyze digital diffraction patterns ~DDP! and to identify
different phases.
RESULTS AND DISCUSSION
Morphology and Nanostructure of Coatings
In a previous TEM study of coating A, shown in Rojas et al.
~2012!, we demonstrated that this coating grows forming a
polycrystalline columnar microstructure constituted by the
cubic Cr~Al, Y!N phase. In Figure 2a, a bright field ~BF!
TEM image of sample A shows the multilayered nano-
structure inside the columns. Layers of 25–30 nm thick
appear separated by two thin layers of 1.5–2 nm in thick-
ness with brighter contrast. The TEM study of the remain-
ing samples ~B–E! revealed the same multilayer nanostructure
as sample A. Figure 2b shows a BF TEM image correspond-
ing to sample B. A decrease of the layer thickness ~15–
20 nm! is observed because of the increase of the rotation
speeds ~v1: turntable; v2: tower!. This thickness decrease
was also observed in the remaining samples.
To further investigate the origin of this layered con-
trast, the HAADF/STEM technique was used. The images
were formed from the very high angle incoherently scat-
tered electrons that have suffered a Rutherford scattering.
The pixel intensity in the HAADF images is proportional to
the atomic number Z and/or local density ~often named as
Z-contrast images!. In Figure 3a, a HAADF image of a very
thin area of sample C is shown. Looking at this picture,
Figure 2. High magnification bright-field transmission electron microscopy images showing the multilayer structure of
samples A ~a! and B ~b!.
16 Teresa C. Rojas et al.
layers with dark contrast ~bright contrast in BF image! of
two alternating thicknesses can be observed. Thicker layers
are formed by aligned nanospheres of around 6–8 nm and
the thinner one of around 1–2 nm. The profile of the
HAADF signal ~cf. Fig. 3b! obtained along the marked line
also shows that the intensity decreases to very low levels on
the spherical features ~1 and 4! suggesting that the nano-
spheres are actually pores. A high magnification BF TEM
image of this layer ~inset in Fig. 3a! reveals that is formed by
two thin lines containing small pores of 1–2 nm.
Figures 4a and 4b show BF TEM images of the samples
D and E ~set II!, respectively, which were synthesized using
different Y target powers. In Figure 4a, a bright double layer
with an approximate thickness of 2 nm each is repeated
every 15–18 nm, and in some points, spherical features can
be identified. Likewise, in sample E, the bright thin layers
are slightly thicker, around 3–4 nm each, with the same
periodicity of sample D. Similar morphological features
were observed in samples B and C ~set I!, which indicates
that by increasing the Y target power the thickness of the
bright thin bilayers increases.
Chemical and Phase Composition
Several spectroscopy and microscopy techniques have been
used to study the chemical composition, elemental distribu-
tion, and phases present at different scales in the coatings.
Table 2 summarizes the atomic composition measured by
EPMA of all samples and shows that the increase in number
of Al targets and the increase of power applied to the Y
target lead to an increment of Al and Y concentration,
respectively. In Figure 5 we have included the SAED mea-
sured for the samples under study. The patterns reveal the
existence in all the samples of diffraction rings correspond-
ing to the ~111!, ~200!, and ~220! planes of the c-Cr~Al!N
phase. A preferred orientation of the crystals is noticed in
the case of the ~200! diffraction ring. In the case of the
samples C and E with much higher Y content, diffraction
rings also appear at 2.8 and 1.7 Å, that can be assigned to
the ~111! and ~220! planes of c-YN. This phase also presents
a ring at 2.4 Å assigned to the ~200! planes that overlap with
that of the c-CrN phase.
EELS spectroscopy can be used to assess the local
crystal/electronic structure as well as the local composition
of a sample. In particular, we have done electron energy-loss
near-edge structure to determine the phase, in which an
Figure 3. a: High angle annular dark field ~HAADF! image of sample C revealing the ordered pore layers. Four marked
positions 1, 2, 3, and 4 have been selected along a profile perpendicular to the multilayer structure; the inset shows a
high magnification bright-field transmission electron microscopy image of the marked area. b: HAADF signal variation
and Cr and N relative chemical composition along the marked profile.
Figure 4. High magnification bright-field transmission electron
microscopy images of samples D ~a! and E ~b!.
Nanoscale Characterization with Electron Microscopy of Multilayered CrAlYN Coatings 17
element is present ~Mackenzie et al., 2005; Sánchez-López
et al., 2005b; Abad et al., 2011; Holec et al., 2011; Godinho
et al., 2012b!.
In Figure 6 we have depicted for each sample A to E the
experimental N-K edge EELS spectra normalized to the
edge jump and after aligning the first peak at 400 eV. We
used as ELNES references or “fingerprints” the spectrum
measured on a sample of CrN prepared by magnetron
sputtering ~CrN-MS! and the spectrum of bulk crystalline
c-AlN presented in Mackenzie et al. ~2005!. Figure 6 shows
that the intensities of the first and second peak of the
spectra at 400 and'410 eV, respectively, suffer small changes.
A decrease in the intensity of the first peak is associated
with the presence of N-Al bonding, either forming a c-AlN
phase or a c-CrAlN phase ~Ross et al., 2010!. In addition, an
energy shift of the second peak is observed that can be
related with a change in crystal lattice parameter ~Craven,
1995!. It is expected that a shift to lower energies when the
lattice parameter decreases ~i.e., more Al incorporation! will
occur. To make the comparison between EELS spectra easier
we have calculated the ratio of intensities DI I1/I2, where
I1 is the maximum intensity of the first peak at 400 eV and
I2 is the intensity of the second around 410 eV. Likewise, we
can define DE E2 E1 as the measured energy difference
between these two intensity maxima for all the spectra.
The obtained values are summarized in Table 2. We
observe a variation of DI from 0.94 ~CrN-MS! to 0.73 for
the film containing the maximum Al content ~sample D!.
Using the measured values DI for all the samples, and the
spectra of CrN and c-AlN references we can estimate the
fraction of Al present in cubic environments ~hereafter b!.
The obtained values are included in Table 2 and follow in
good agreement the Al contents measured by EPMA. Sam-
ples from set II exhibit a strong decrease of the intensity of
the first peak, as expected by the higher Al contents, but also
a widening of the peaks, which can be related with a
decrease in the crystalline order and/or to the presence of
amorphous nitride phases. Regarding the change of DE a
decrease from 9.4–9.8 ~from set I! to 8.4–8.8 ~from set II! is
observed when the Al content is increased from 4–5 ~set I!
to ;12 at% ~set II!. In the case of sample E, where the Al
content is similar to sample D, but with higher Y concentra-
Table 2. Elemental Chemical Composition of the CrAlYN Coatings Measured by EPMA and DI, DE Parameters
Obtained from N-K EELS Spectra.
Sample Set Cr ~at%! Al ~at%! N ~at%! Y ~at%! DI DE b
A I 38.6 5.1 54.6 1.7 0.82 9.4 6.6
B 40.1 4.0 54.3 1.6 0.84 9.7 5.2
C 39.3 3.6 53.4 3.6 0.85 9.8 5.1
D II 32.7 11.8 53.4 2.2 0.73 8.4 12.6
E 31.5 11.5 52.0 5.0 0.74 8.8 12.7
CrN-MS Ref. 45.6 — 54.4 — 0.94 10.8 —
Figure 5. Selected area electron diffraction pattern of all CrAlYN coatings.
18 Teresa C. Rojas et al.
tion, the same value of DI and a bigger value of DE are
measured. This difference can be explained for an increase
in the lattice parameter, by the presence of YN phase, as has
been detected by SAED analysis. The same occurs in sample
C in comparison to B.
At this point we can propose that Al atoms are forming
part of an amorphous AlN phase or substituting Cr atoms
in the c-CrN lattice, forming a CrAlN solid solution, as
proposed in previous publications ~Hasegawa et al., 2005;
Willmann et al., 2006; Lin et al., 2008; Mayrhofer et al.,
2008; Rovere et al., 2008; Rojas et al., 2012!. In addition, Y
atoms form a CrYN solid solution and a crystalline c-YN
phase in samples C and E.
To further investigate the chemical elemental distribu-
tion a combination of spatially resolved microanalysis tech-
niques was employed. N and Cr elemental maps were
obtained from EFTEM images. Figures 7a and 7b show the
Cr and N elemental maps, respectively, from a thin area of
sample C shown in Figure 3a. The multilayer structure is
clearly observed with areas concurrently rich in Cr and N,
indicative of the presence of a CrN phase in the bright
layers, separated by thinner lines where these elements are
not dominant. Focussing on the Cr image, two types of dark
lines can be observed. The larger is also coincident with N
mapping and is congruent with the porous regions detected
in BF images, but the thinner layer is not clearly seen in the
N image. In order to establish the location of Al and Y
atoms STEM-based EDS was used. EDS spectra line scans
were taken perpendicular to the stack of layers with an
electron probe size of 2 nm ~shown in Fig. 7c!. Al and Y
profiles display alternating maxima and minima. Starting
from the thicker dark layer in the Cr map ~see marked
arrow in Fig. 7c!, the Y signal reaches its maximum intensity
while the Al peak the least. The opposite situation is noticed
when crossing over the second and thinner dark layer and
so repeatedly. Therefore, it can be concluded that thicker
dark bands correspond to regions richer in Y, while the
thinner dark layers are richer in Al.
To get chemical and spectroscopic information with
higher spatial resolution the STEM mode was used to
measure an EELS line scan across the multilayer structure in
sample C using a probe of less than 1 nm. In Figure 3b we
have included the relative Cr and N composition along the
depicted line together with the HAADF intensity profile.
The Cr concentration follows a similar shape to the HAADF
intensity profile. Interestingly, the N concentration fluctu-
ates from average values of about 50% ~yielding Cr/N ratio
'1! to nearly 100% coincident with the porous region
~points 1 and 4!. The N-K edge EELS spectra measured in
points 1, 2, and 3 marked in Figure 3a are depicted in
Figures 8a, 8b, and 8c, respectively. A significant change
in the N signal is observed. In the pore region ~1!, the N
signal has a characteristic signature of molecular nitrogen.
A small signal of Cr-L2,3 is still detected as can be noticed in
the region above 570 eV. Outside the pore, point 2, the
spectrum shows a pattern characteristic of a CrN phase.
Comparing point 2 with 3, the intensity of the first peak of
the N-K edge experiences a clear decrease from DI 0.96 to
0.61 and the value of DE decreases from 10.41 to 8.95 eV.
The changes of these two parameters indicate the presence
of more Al-N bonding in point 3. In fact, this area corre-
sponds to the zones rich in Al detected by the EDS line scan.
Therefore, this change is attributed to the presence of a
c-AlN phase or CrAlN phase richer in Al. The calculated b
value 21.5 in this point supports this assumption.
Putting together all of these results, the complex layered
architecture for samples from set I can be defined as the
stacking of @~CrYN/YN! N2//CrN//~CrAlN/AlN! N2#
sequences. This finding is crucial for understanding the
chemical and microstructure characteristics of this multilay-
ered system affecting the thermal and oxidation stability of
this nanostructured system.
In the case of set II ~samples D and E!, the number of
Al targets was doubled to increase the Al concentration in
the coatings and placed between Y and Cr targets ~cf. Fig. 1!.
The chemical composition and distribution across the multi-
layers are expected to change accordingly. The Al, Cr, and Y
elemental mappings obtained by the STEM-based EDS area
scan, using a probe size of around 0.5 nm, of a selected area
of sample D, are shown in Figure 9. A HAADF image of the
scanned area is also included where the dark areas corre-
spond to the pore zones. In the EDS chemical elemental
maps a layered distribution is observed alternating areas
rich in Cr with regions where Y and Al atoms are mixed.
Comparing with the HAADF image, the thick and bright
bands are due to the presence of Cr from CrN while the
dark pore regions are enriched in Al and Y atoms. With the
aim of obtaining further information about the crystalline
phases present and their location, a HRTEM study was
taken across these interfaces.
Figure 6. N K-edge electron energy loss spectroscopy spectra of
CrAlYN coatings under study. The spectra measured for CrN
prepared by magnetron sputtering ~CrN-MS! and c-AlN reference
compounds are included as fingerprints.
Nanoscale Characterization with Electron Microscopy of Multilayered CrAlYN Coatings 19
A HRTEM image of sample D is shown in Figure 10a.
Two layers of higher contrast are denoted with some nano-
crystals embedded whose measured d-spacing of the lattice
fringes ~2.3 and 2 Å! agrees with the ~111! and ~200! planes
of the cubic CrN phase. Most of the interplanar distances
correspond to ~200! planes in agreement with the preferen-
tial growth mentioned before. Between these two dark nano-
crystalline layers is an amorphous region composed of two
brighter layers with a darker region in the middle of around
2 nm. In this last thin layer some small nanocrystals of
1–2 nm are found. The measured d-spacing of these lattice
fringes are 1.9 Å that can be assigned to ~200! planes of
cubic-AlN phase or CrN phase, but taking into account the
previous EDS elemental mappings, this region was richer in
Al ~mixed with Y in lower quantity! so we can conclude that
they correspond to c-AlN or c-CrAlN. In the upper part of
Figure 10a, a d-spacing of 2.4 Å has been measured in the
lighter layer taking into account the EDS mapping, could be
assigned to a nanocrystal of c-YN.
In Figure 10b, a high magnification HRTEM image of
the interface region ~see marked area in Fig. 10a! is shown.
The insets in Figure 10 correspond to the DDP obtained
from two marked crystalline areas. The values of 2.3, 2.0,
and 1.4 Å measured in the DDP’s can be assigned to the
~111!, ~200!, and ~220! planes, respectively, of the c-CrN
crystals oriented along @001# and @011# zone axis for the A
and B insets, respectively. The EELS spectra measured in the
porous layers of this coating ~not shown! also demonstrated
the presence of molecular nitrogen embedded in these areas.
The results of the general N-K EELS shown in Figure 6
manifested a decrease of the first peak and peak widening,
both phenomena are in agreement with the increment of the
Al content and the presence of amorphous phases, respec-
tively. In summary, for the highest Al contents ~set II! the
assembly of HREM, EDS mapping, and EELS results allows
concluding that the CrN nanocrystalline layers are separated
by a heterogeneous layer where some nanocrystals of c-AlN,
c-YN, and pores filled with N2 molecules are embedded in
an amorphous matrix mainly constituted by AlN phase. Tak-
ing into account the assembly of results obtained for Sets I
and II, in Figure 11 we have depicted a schematic drawing
summarizing the nanostructure of these sets of coatings.
Figure 7. Cr ~a! and N ~b!
energy-filtered transmission
electron microscopy elemental
mapping distribution obtained
from sample C. Al and Y
distribution along the marked
profile perpendicular to the
columnar growth obtained by
energy-dispersive X-ray
spectrometer ~c!.
Figure 8. Electron energy loss spectroscopy spectra taken at 1, 2, and 3, marked positions in Figure 3a.
20 Teresa C. Rojas et al.
Figure 9. Chemical distribution mapping of Cr, Al, and Y elements obtained by energy-dispersive X-ray spectrometer
analysis for sample D.
Figure 10. Left: High-resolution transmission electron microscopy image displaying the complex multi-layered struc-
ture of sample D. The d-spacings for several identified phases are marked in the micrograph. Right: Detail at higher
magnification of the crystalline/amorphous interface.
Nanoscale Characterization with Electron Microscopy of Multilayered CrAlYN Coatings 21
Embedded nitrogen-filled nanovoids are consequently
found periodically in our CrAlYN coatings prepared by
magnetron sputtering. The presence of molecular nitrogen
trapped in disordered and amorphous films has already
been pointed out ~Romero-Gomez et al., 2010; Godinho
et al., 2012a! although the distribution was not periodic as
observed here. This induced periodicity is likely a conse-
quence of the position of the targets and the rotation of the
samples. The reasons for the formation of these nitrogen
bubbles can be found in the diffusion of nitrogen released
from point-defects and dislocations generated during growth
~Kucheyev et al., 2000; Ruck et al., 2004!. The N atoms can
migrate through these easy pathways and nucleates into the
molecular form when they are stopped by crystalline or
amorphous nitride interfaces that are formed with Al and Y
targets. These embedded N2-filled nanovoid barriers may
hinder ion diffusion during oxidation and maintain an inert
environment at the CrN layer boundaries.
The changes in the deposition parameters have yielded
to different Al and Y distribution and crystalline/amorphous
interfaces. The thermal stability and oxidation resistance of
sample A ~set I!, where the Al and Y are forming alternative
periodic layers, was studied under heating up to 1,0008C
~Rojas et al., 2012!. The multilayer structure appeared
preserved, proving the efficient blocking effect of the singu-
lar nanostructure found in this paper, which contains voids
embedded in CrAlN and CrYN periodic layers. Further
investigations are currently underway to correlate the influ-
ence of nanostructure and chemical distribution of the
remaining samples on the oxidation rate and protectiveness.
CONCLUSIONS
The structure and chemical composition of complex multi-
layered CrAlYN coatings for high temperature applications
are investigated using a variety of state-of-the-art TEM
techniques. The multilayered structure is the result of the
sequential exposure of the substrates to different target and
plasma compositions. The main feature is the alternation of
crystalline CrN layers with amorphous or crystalline re-
gions where Al and/or Y are present. These elements appear
mainly in the form of AlN and YN compounds although
partial substitution in CrN phases cannot be ruled out.
Ordered layers of nanovoids filled with molecular nitrogen
appear embedded in this matrix. The stacking sequence
varied depending on the nature, position of the targets, and
the rotation speeds controlling the chemical composition
and architecture of the multilayered system. The repartition
of Al and Y nitrides in alternative periodic layers in samples
from set I is suggested to play an important role in the
demonstrated high temperature oxidation resistance of these
coatings as they can act as a sum of barrier layers to oxygen
diffusion inwards. Moreover, the presence of periodic arrays
of nitrogen bubbles may help to block ion transport and to
maintain an inert environment at the CrN layer boundaries
during oxidation.
ACKNOWLEDGMENTS
The Spanish MINECO ~projects No. MAT2011-29074-C02-
01/-02, CONSOLIDER FUNCOAT CSD2008-00023!, CSIC
~20160I041!, and European Union ~CT-REGPOT-2011-1-
285895 AL-NANOFUNC! are acknowledged for financial
support. The contributions of Dr. Alexander Orchowisk and
Dr. Katja Etzel from Application Carl Zeiss NTS Gmbh, Mr.
Cailler from JEOL ~Europe! SAS, and Mr. Aoki from JEOL
USA, are gratefully acknowledged.
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