Full text of DOI:10.1557/JMR.2002.0381

Microstructure and mechanical properties of
Ti–Si–N coatings
W.J. Meng, X.D. Zhang, and B. Shi
Mechanical Engineering Department, Louisiana State University, Baton Rouge, Louisiana 70803
R.C. Tittsworth
Center for Advanced Microstructures and Devices, Louisiana State University,
Baton Rouge, Louisiana 70803
L.E. Rehn and P.M. Baldo
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
(Received 19 February 2002; accepted 17 July 2002)
A series of Ti–Si–N coatings with 0 < Si < 20 at.% were synthesized by inductively
coupled plasma assisted vapor deposition. Coating composition, structure, atomic
short-range order, and mechanical response were characterized by Rutherford
backscattering spectrometry, transmission electron microscopy, x-ray absorption near-edge
structure spectroscopy, and instrumented nanoindentation. These experiments show that the
present series of Ti–Si–N coatings consists of a mixture of nanocrystalline titanium nitride
(TiN) and amorphous silicon nitride (a-Si:N); i.e., they are TiN/a-Si:N ceramic/ceramic
nanocomposites. The hardness of the present series of coatings was found to be less than
32 GPa and to vary smoothly with the Si composition.
I. INTRODUCTION
transmission electron microscopy (TEM), and x-ray ab-
sorption near-edge structure (XANES) spectroscopy. Coat-
ing mechanical response was measured by instrumented
nanoindentation. Our results show that the present series of
Ti–Si–N coatings consists of a nanometer-scale mixture
of crystalline titanium nitride (TiN) and amorphous silicon
nitride (a-Si:N). The hardness of the present series of Ti–
Si–N coatings was found to be less than 32 GPa and to vary
smoothly with the Si composition.
Nanostructured ceramic coatings are being investigated
for a number of macro- and microscale surface engineering
applications.1–3 The mechanical properties and tribological
characteristics of ceramic/ceramic nanocomposite coatings
have been observed to depend systematically on coating
composition, offering the potential of designing coatings for
specific applications.4 Understanding the dependence of the
mechanical response on the nanoscale structure goes to
the heart of coating design. Despite much current activity
in the synthesis and mechanical characterization of nano-
structured ceramic coatings, conflicting results reported in
the literature have yet to be resolved. In the titanium car-
bide/amorphous hydrocarbon (TiC/a-C:H) nanocomposite
coating system, both the presence5 and absence6 of a pro-
nounced hardness peak around 40 at.% Ti have been re-
ported. In the Ti–Si–N coating system, coating hardness
ranging from 40 to 80 GPa has been reported in the com-
position range of 2–20 at.% Si,7,8 significantly exceeding
the hardness of crystalline B1-TiN.9 Whether all ceramic
nanocomposites exhibit such superstrengthening is of sig-
nificant interest, and critical experimental evidence needed
to verify these findings should include detailed structural
and mechanical characterization on the same set of
specimens.
II. EXPERIMENTAL
Deposition of Ti–Si–N coatings was carried out in a
hybrid chemical vapor deposition (CVD)/physical vapor
deposition (PVD) tool. This tool combines a 13.56-MHz
inductively coupled plasma (ICP) with four balanced
magnetron cathodes.10 The two facing Ti (99.99%) cath-
odes were operated at the same current, as were the two
facing Si (99.99%) cathodes. Si(100) wafers of 2-in.
diameter were used as substrates. The Si substrate sur-
face was etched in a pure Ar ICP at a constant bias
voltage of −100 V, followed by immediate deposition of
a Ti–Si–N layer in an Ar (99.999%)/N2 (99.999%) mix-
ture. During deposition, the substrates were rotated con-
tinuously at the center of the deposition zone, and
subjected to a constant bias voltage of −50 V. Ar and N2
gas input flow rates were fixed at 10.0/1.1, and the total
pressure was kept at approximately 1.8 mtorr during all
depositions. By fixing the Ti cathode current at 1.0 A and
systematically raising the Si cathode current, Ti–Si–N
coatings were deposited with increasing Si composition.
This paper reports the results of such an experimental
study on a series of Ti–Si–N coatings with 0–20 at.% Si.
Coating composition, structure, and atomic short-range
order surrounding Ti and Si atoms were probed by com-
bining Rutherford backscattering spectrometry (RBS),
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W.J. Meng et al.: Microstructure and mechanical properties of Ti–Si–N coatings
No intentional substrate cooling or heating was applied
during deposition. Separate substrate temperature meas-
urements made by attaching type K thermocouples to a
sacrificial substrate showed that the substrate tempera-
ture was approximately 250 °C during deposition. A het-
eroepitaxial B1-TiN/Si(111) specimen11 and an a-Si:N/
Si(100) specimen were also prepared for comparison.
Coating composition was measured by RBS. TEM was
performed using a JEOL JEM2010 (Tokyo, Japan) instru-
ment operated at 200 kV. XANES spectroscopy was per-
formed at the Double Crystal Monochromator (DCM)
beamline of the synchrotron facility at the Louisiana State
University Center for Advanced Microstructures and De-
vices (LSU CAMD). Both Ti and Si K-edge spectra of the
Ti–Si–N specimens were recorded in the total electron yield
mode. Photon energy selection was provided by a Ge(220)
and an InSb(111) double-crystal monochromator at the Ti
K-edge and Si K-edge, respectively. Two to five spectra
were acquired consecutively from each specimen and av-
eraged. Further details on RBS, TEM, and XANES tech-
niques have been presented elsewhere.12,13
Instrumented nanoindentation was carried out on a
Hysitron triboscope interfaced to a Digital Instrument
Dimension 3100 atomic force microscope (AFM). A
Berkovich diamond indenter was used. The instrument
load frame compliance and indenter tip area function
were calibrated with a vendor-supplied fused silica speci-
men assuming a contact depth (hc) independent elastic
modulus. The calibration load range was 100–13000 ␮N,
corresponding to an indenter contact depth range of
8 < hc < 220 nm. Indentations on Ti–Si–N/Si specimens
were carried out at multiple loads ranging from 13000
to 200 ␮N, with multiple indents performed at each load.
The Oliver/Pharr analysis procedure was followed to ex-
tract values of the indentation modulus, Eind ס E/(1 −
␯2), where E and ␯ are respectively Young’s modulus
and Poisson’s ratio, and the hardness, H, from the experi-
mental load versus displacement curves.14 Coating surface
profiles were examined in the contact AFM mode with
the Berkovich indenter tip, and the average roughness of
the as-deposited coatings was determined to be approxi-
mately 2 nm, independent of the Si composition.
FIG. 1. Coating composition measurements by RBS: Si and Ti com-
positions of a series of Ti–Si–N coatings in atomic percent as a func-
tion of the Si cathode current.
value of 57 at.% of pure Si3N4. The density of the a-Si:N
coating is approximately 3.3 g/cm3 based on RBS and
crosssectional TEM measurements of coating thickness.
Thickness for the present series of Ti–Si–N coatings ranged
from 110 to 130 nm, according to cross-sectional TEM.
Cross-sectional selected area electron diffraction pat-
terns (SADPs) were taken from the Ti–Si–N coatings
with Si compositions ranging from 0.3 to 19.3 at.%. All
crystalline diffraction signatures from the coatings can
be indexed to a cubic structure with lattice parameters
of 4.22–4.23 Å, close to the 4.24 Å or bulk B1-
TiN.15 Figures 2(a) and 2(b) show cross-sectional high-
resolution images of the coatings. At 0.3 at.% Si
[Fig. 2(a)], the coating consists of columnar-like
nanocrystalline grains, approximately 5 nm in width and
>10 nm in height. Individual grains appear to be dislo-
cation free, and most grain boundaries appear to be
atomically sharp. At 19.3 at.% Si [Fig. 2(b)], the coating
consists of narrower columnar-like nanocrystalline
grains, 2–5 nm in width and >10 nm in height. Amor-
phous regions now separate these nanocrystalline grains.
Again, individual grains appear to be dislocation free,
and the nanocrystalline/amorphous interfaces appear to
be atomically sharp. A plan-view high-resolution micro-
graph of the 19.3 at.% Si coating is shown in Fig. 2(c),
showing 2–5-nm grains separated by amorphous regions
several nanometers in thickness. Dislocations are visible
at boundaries between individual nanocrystalline grains
due to misorientation between grains. All diffraction and
imaging results show that the only crystalline phase pres-
ent within this series of Ti–Si–N coatings is consistent
with B1-TiN and that any other phase present within
these coatings is amorphous in structure.
III. RESULTS AND DISCUSSION
RBS spectra showed compositional uniformity as a
function of depth within the Ti–Si–N layer and impurity
levels below the detection limit of approximately 1 at.%.
The N composition ranged between 52 and 56 at.%. Fig-
ure 1 shows the measured Ti and Si compositions as a
function of the Si cathode current. The average Si com-
position increases monotonically from <1 to approxi-
mately 20 at.% with increasing Si cathode current from
<0.1 to 0.3 A. The composition of the a-Si:N specimen
was measured at 55 5 at.% N, which is close to the
Figures 3(a) and 3(b) show, respectively, Ti and Si
K-edge XANES spectra of a number of the Ti–Si–N
coatings. Pre-edge backgrounds have been removed, and
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W.J. Meng et al.: Microstructure and mechanical properties of Ti–Si–N coatings
FIG. 3. XANES spectra: (a) Ti K-edge spectra obtained from the
Ti–Si–N coatings and from the B1-TiN specimen; (b) Si K-edge spec-
tra from the Ti–Si–N coatings and from the a-Si:N specimen.
all spectra have been normalized to unit edge jump. The
Ti K-edge and Si K-edge spectra of the B1-TiN/Si(111)
and the a-Si:N/Si(100) specimens are also shown in
Figs. 3(a) and 3(b), respectively. At all Si compositions,
the Ti K-edge XANES oscillations exhibit a one-to-one
correspondence with those of the B1-TiN specimen, in-
dicating that the short-range order surrounding the Ti
atoms within the Ti–Si–N coatings is predominantly B1-
TiN like. As the Si composition increases, some ampli-
tude decrease and broadening of the XANES oscillations
are observed, suggesting somewhat increased disorder at
the Si composition increases. The Ti K-edge XANES
spectra are consistent with Ti atoms being incorporated
into the Ti–Si–N coatings predominantly as B1-TiN and
are also consistent with the TEM results showing a
smaller TiN column width at larger Si concentrations.
The Si K-edge XANES spectra display an overall resem-
blance to that of a-Si:N, possessing a pronounced white
line around 1846 eV and one additional broad peak
around 1860 eV. Appreciable differences do exist in the
edge region of the coating spectra as compared to
FIG. 2. High-resolution TEM images of the Ti–Si–N coatings: (a)
cross-sectional image of the coating with 0.3 at.% Si; (b) cross-
sectional image of the coating with 19.3 at.% Si; (c) plan-view image
of the coating with 19.3 at.% Si.
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W.J. Meng et al.: Microstructure and mechanical properties of Ti–Si–N coatings
the a-Si:N specimen. The transition threshold starts at
approximately 1838 eV for the Ti–Si–N coatings com-
pared to approximately 1841 eV for the a-Si:N specimen,
and it becomes more pronounced as the overall Si com-
position decreases. The peak of the white line also shifts
gradually from 1845.3 to 1846.6 eV as the overall Si
concentration decreases. The overall spectral resem-
blance to a-Si:N suggests that Si atoms incorporate into
the Ti–Si–N coatings in an a-Si:N like environment,
while differences in the edge and white line regions as
compared to that of a-Si:N suggest that some Ti atoms
are present within the first few coordination shells of Si,
in addition to Si and N atoms.
Within a nanometer-scale mixture of B1-TiN and a-Si:N,
the proximity of Si and Ti atoms can arise in two different
ways. Some Ti atoms can be dissolved within the a-Si:N
phase, or Ti atoms may preferentially bond to Si atoms
across TiN/a-Si:N interfaces. The dissolution limit of Ti
atoms within the a-Si:N phase under the pres-
ent deposition conditions has not been determined. In the
latter case, with decreasing Si composition, the fraction of
Ti–Si bonds would increase because the surface
to volume ratio of the a-Si:N phase increases. This is con-
sistent with the observed gradual changes seen in the Si
K-edge spectra as the overall Si composition decreases.
Taken together, the Ti and Si K-edge XANES spectra of
these Ti–Si–N coatings corroborate the TEM evidence,
which indicates that they consist of a nanometer-
scale mixture of a B1-TiN phase and an a-Si:N phase;
i.e., they are TiN/a-Si:N nanocomposites. The dispersion
of nanocolumnar B1-TiN grains within an a-Si:N matrix
is reminiscent of metallurgical rod eutectic structures, the
formation of which may involve partitioning of Ti and Si
atoms during growth.16
Figure 4(a) shows hardness H versus indenter contact
depth hc as measured by nanoindentation for a Ti–Si–N
coating with 9.5 at.% Si. The apparent hardness increases as
hc decreases, reaching a maximum between 13 nm < hc <
18 nm, and decreases at smaller contact depths because in-
denter–specimen contact becomes elastic at smaller load
levels. Values of Eind and H obtained from indentations
with hc < 18 nm (approximately 15% of coating thickness),
excluding those corresponding to elastic contact, were
taken as being representative of the Ti–Si–N coating. A set
of indentation data such as that shown in Fig. 4(a) was
collected for each Ti–Si–N coating, and the same procedure
was followed to extract coating modulus and hardness.
Figure 4(b) displays the Eind and H values for the present
series of Ti–Si–N coatings versus the Si composition. H
decreases with increasing Si composition and stays below
32 GPa over the entire composition range of 0–20 at.% Si.
Hardness of the B1-TiN specimen obtained following the
same procedure is approximately 22 GPa. The hardness of
the present series of Ti–Si–N coatings is thus higher than
that of the B1-TiN specimen but lower than the previously
FIG. 4. Instrumented nanoindentation: (a) apparent hardness of a Ti–
Si–N coating with 9.5 at.% Si versus indenter contact depth; (b) hard-
ness and indentation modulus of the Ti–Si–N coatings as a function of
the Si composition; (c) load versus displacement curves for the Ti–
Si–N coating with 6.4 at.% Si and for the B1-TiN specimen.
reported hardness values of 40–80 GPa.7,8 Figure 4(c)
shows load versus displacement curves determined for a
Ti–Si–N coating with 6.4 at.% Si. Load versus displace-
ment curves for the B1-TiN specimen are also shown for
comparison. At 1000-␮N maximum load, the loading curve
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W.J. Meng et al.: Microstructure and mechanical properties of Ti–Si–N coatings
ACKNOWLEDGMENTS
and the depth at peak load are not significantly different for
the Ti–Si–N and B1-TiN specimens. The initial unloading
stiffness for the Ti–Si–N specimen is less than that for
B1-TiN, indicating a lower Eind than B1-TiN. Figure 4(c)
does indicate the presence of increased elastic recovery for
the Ti–Si–N specimen as compared to B1-TiN.
W.J.M. gratefully acknowledges partial project sup-
port from Louisiana Board of Regents through contracts
LEQSF(2000-03)-RD-B-03 and LEQSF(2001-04)-RD-
A-07 and the National Science Foundation through Grant
No. DMI-0124441. The research conducted at LSU has
benefited from support, in part, by the LSU Center for
Applied Information Technology and Learning. The ion
beam analysis work at the Argonne National Laboratory
was supported by the Department of Energy Office of
Science, Basic Energy Sciences, under Contract No.
W-31-109-ENG-38.
The morphology of the present series of Ti–Si–N coat-
ings, nanocolumnar TiN grains along the growth direc-
tion interdispersed within an a-Si:N matrix, is different
from that previously reported, with equiaxed nano-
crystalline TiN grains embedded within an amorphous
matrix.7,8 Whether this morphological difference is re-
sponsible for the difference in observed mechanical
response awaits further clarification. Complete segrega-
tion of B1-TiN and a-Si:N phases within the nanocom-
posite implies little dissolution of Si and Ti atoms within
the TiN and a-Si:N phases, respectively. While the pres-
ent TEM and XANES evidence suggests that few Si
atoms are incorporated within the TiN phase, some
Ti atoms may be dissolved within the a-Si:N phase.
Whether the degree of phase separation would signifi-
cantly influence the mechanical properties of two-phase
ceramic nanocomposites deserves further investigation.
It has been suggested that percolation of the a-Si:N phase
is responsible for the reported enhancement of TiN/
a-Si:N mechanical properties.17 If one takes the density
of the TiN and a-Si:N phases within the Ti–Si–N coat-
ings to be respectively 5.4 and 3.3 g/cm3, the a-Si:N
volume fraction increases approximately linearly with
the Si composition, reaching approximately 0.45 at ap-
proximately 20 at.% Si. The percolation threshold for
a-Si:N depends on the dimensionality of the microstruc-
ture, i.e., on the nanocrystalline TiN grain morphology,
which remains to be determined.
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VI. CONCLUSION
In summary, we have performed a detailed examination
of the microstructure and mechanical properties of a series
of Ti–Si–N coatings with 0–20 at.% Si. Structure and
atomic short-range order studies show that these Ti–Si–N
coatings are TiN/a-Si:N nanocomposites, with nano-
columnar TiN grains aligned in the growth direction
interdispersed within an a-Si:N matrix. Instrumented na-
noindentation measurements show that their mechanical
properties vary smoothly with the Si composition. The
measured hardness is below 32 GPa within the entire com-
position range.
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