Full text of DOI:10.1002/1527-2648(200105)3:5<333::AID-ADEM333>3.0.CO;2-7

unsymmetrical. Such field-controlled shape memory effect
can be contributed to the driving force from the magnetic
field to the twin boundaries of the variants based on the mag-
netic anisotropy of the martensite.
amorphous carbon nitride system (CN
x
), is also interesting
with respect to various applications in the hard-disk and mi-
[
1±4]
crosystems industries.
The typical film thicknesses for
these applications range from a few nanometers up to a cou-
ple of hundred nanometers.
Received: September 14, 2000
Final version: January 03, 2001
The influence of the substrate in the case of a mechanical
characterization of thin films is well known but not fully
understood. Different approaches are described in the litera-
[
5±10]
[
[
[
1] P. J. Webster, K. R. A. Ziebeck, S. L. Town, M. S. Peak,
Philos. Mag. B 1984, 49, 295.
2] V. A. Chernenko, E. Cesari, V. V. Kokorin, I. N. Vitenko,
Scr. Metall. Mater. 1995, 33, 1239.
3] V. V. Kokorin, V. V. Martynov, V. A. Chernenko, Scr.
Metall. Mater. 1992, 26, 175.
ture.
It is reasonable that the substrate should also have
an effect on the tribological response of the film/substrate
system. In this communication, examples of the influence of
film thickness and substrate on the mechanical and tribologi-
cal properties of thin CN and DLC films in the low-load
x
regime are analyzed and discussed.
x
As already mentioned, DLC and CN are both candidates
for hard-disk and microsystems industries. Both coatings are
widely used today due to their high tribological potential.
Therefore, these two systems were chosen for a study of the
mechanical and tribological response of a film/substrate sys-
tem on the nanometer scale.
[
[
4] V. V. Martynov, V. V. Kokorin, J. Phys. III 1992, 2, 739.
5] K. Ullakko, J. K. Huang, C. Kantner, R. C. O'Handley,
V. V. Kokorin, Appl. Phys. Lett. 1996, 69, 1966.
6] G. H. Wu, C. H. Yu, L. Q. Meng, J. L. Chen, F. M. Yang,
S. R. Qi, W. S. Zhan, Z. Wang, Y. F. Zheng, L. C. Zhao,
Appl. Phys. Lett. 1999, 75, 2990.
[
The DLC films characterized in this communication were
deposited via a plasma-enhanced chemical vapor deposition
[
[
[
7] R. Tickle, R. D. James, T. Shield, M. Wutting, V. V.
Kokorin, IEEE Trans. Magn. 1999, 35, 4301.
8] S. J. Murray, M. Marioni, S. M. Allen, R. C. O'Handley,
T. A. Lograsso, Appl. Phys. Lett. 2000, 77, 886.
(
PECVD) process on Si(100). Details of the deposition condi-
[
11]
tions are reported elsewhere.
Aside from the film thick-
ness, the influence of bias voltage on the mechanical and
tribological behavior is of interest. That is why two sample
series were prepared: One with a fixed film thickness (250±
9] R. D. James, M. Wuttig, Philos. Mag. A 1998, 77, 1273.
[
[
[
10] R. C. O'Handley, J. Appl. Phys. 1998, 83, 3263.
11] R. Tickle, R. D. James, J. Magn. Magn. Mater. 1999, 195, 627.
12] W. H. Wang, G. H. Wu, J. L. Chen, S. X. Gao, W. S.
Zhan, Z. Wang, Y. F. Zhang, L. C. Zhao, Appl. Phys. Lett.
3
8
50 nm) at different substrate bias voltages between 300 and
00 V, and one series at a constant bias voltage of 500 V and
film thicknesses between 19 and 400 nm.
The CN films analyzed in this work were deposited via
reactive DC-magnetron sputtering on Si(100). Details of the
2
000, 77, 3247.
[
13] Y. Suzuki, in Shape Memory Materials (Eds: K. Otsuka,
C. M. Wayman), Cambridge University Press, Cam-
bridge 1998, Ch. 6.
x
[
12]
process conditions are reported elsewhere.
For the CN
x
system the two parameters of interest were the nitrogen con-
centration of the film and the film thickness. Therefore two
series were prepared. One series had different nitrogen con-
centrations between 0 and 20 at.-% at a constant film thick-
ness (200±250 nm). The other series contains different film
thicknesses (6±226 nm) at a constant nitrogen concentration
of about 9 at.-%.
The nanomechanical properties of the substrate and the
coated systems were determined by nanoindentation using a
Hysitron Nanoindenter/TriboScope in combination with a
Park Universal atomic force microscope (AFM). The sample
surface is loaded by a diamond indenter (Berkovich indenter;
[
14] H. D. Chopra, C. Ji, V. V. Kokorin, Phys. Rev. B 2000, 61,
R14 914.
[
15] B. Wedel, M. Suzuki, Y. Murakami, C. Wedel, T. Suzuki,
D. Shindo, K. Itagaki, J. Alloys Compd. 1999, 290, 137.
Micromechanical and
Microtribological Properties
of Thin CN and DLC Coatings
x
1
00 nm tip radius) and, after holding at a constant load, the
By Thorsten Staedler* and Kirsten I. Schiffmann
load is reduced again. During the whole process the load and
indenter position are measured resulting in a load±displace-
ment curve. The unloading segment of these curves is ana-
lyzed with the model proposed by Oliver and Pharr in order
to obtain the mechanical properties.
Diamond-like carbon (DLC) can be used as a tribological
coating in mechanical engineering to reduce friction and wear
(
for drilling and cutting tools). This system, as well as the
[13]
Beside these single
indents multiple indents were carried out. During a multiple
indentation there are unloading segments at 10, 20, 30 % of
the maximum load down to a load of about 50 % of the actual
load. By this means it is possible to determine the mechanical
properties, hardness, and Young's modulus for various in-
[
*] T. Staedler, Dr. K. Schiffmann
Fraunhofer Institute for Surface Engineering and Thin Films
Bienroder Weg 54E, D-38108 Braunschweig (Germany)
E-mail: staedler@ist.fhg.de
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Staedler, Schiffmann/Micromechanical and Microtribological Properties of Thin CN and DLC Coatings
dentation depths with only one measurement. These experi-
ments were carried out with three different final loads lead-
ing to three different loading rates, 200, 400, and 1000 lN/s
respectively. The experimental results for the materials pre-
sented in this work show no significant deviation between
single- and multiple indentation or different loading rates.
For film thicknesses below 100 nm and a typical influence
of the tip radius (100 nm) up to indentation depth of about
for all samples and measurements is interpreted as a homoge-
neous hardness over the considered film thickness range of
19±400 nm. The hardness of the DLC layers deposited at
500 V substrate bias is 27 ± 1 GPa.
The fact of a substrate influence also applies to the Young's
modulus. The measured values of Young's modulus are well
described by Equation 2. An analysis of samples from the
bias voltage variation shows only a very small influence on
the mechanical properties in the considered bias range of
300±800 V. Hardness and Young's modulus both have a
slight maximum at 500 V bias: 27 and 260 GPa respectively.
These results are in good agreement with other reported
hardness values of PECVD DLC coatings (for an overview
1
5±20 nm the substrate should have a significant influence on
the measured hardness and Young's modulus of the film/
substrate systems. To take this effect into account the film
hardness is calculated according to Equation 1. This equation
[
9]
follows a proposal given earlier and is modified to satisfy
the case of a hard film on a soft substrate. It is therefore simi-
lar to the basic approach given for the substrate influence on
[
16]
see Bhushan ).
[
17]
Recent publications
show that it is difficult to describe
[
10]
the measured Young's modulus.
Hmeasured, Hsubstrate, and
load±displacement curves of CN by a power law like the
x
H
film are the measured, the substrate, and the film hardness
Oliver and Pharr model due to the very strong elastic recov-
ery of these films. In this case it is convenient to introduce a
respectively; j = h
thickness dfilm; a is a fitting parameter.
c
/dfilm is the ratio of contact depth h
c
to film
(1)
recovery R = (h
± h )/h
to compare different CN coat-
max
f
max x
[
18,19]
ings:
hmax and h are the maximum displacement under
f
Hmeasured = Hfilm + (Hsubstrate ± Hfilm)e±
aj
load and the final displacement after unloading respectively.
Our samples with a relatively low nitrogen concentration of
9 at.-% (for the film thickness variation) show no fullerene-
like phase and provide a recovery of about 74 % compared
The Young's modulus is calculated via an empirical ap-
proach based on the model of Doerner and Nix[ : Equa-
tion 2. Emeasured, Esubstrate, and Efilm are the measured, the
substrate, and the film Young's modulus respectively.
14]
[
18,19]
to 84 and 90 % reported elsewhere.
Therefore problems
of the Oliver±Pharr model associated with the strong elastic
recovery do not have a significant influence on the analysis
here and all experimental data are well described by the
model.
1/Emeasured) = (1/Esubstrate) + [(1/Efilm) ± (1/Esubstrate)]e± (2)
aj
(
For both models a hardness of 12 GPa and a Young's mod-
x
The measured hardness of the CN samples treated in this
ulus of 160 GPa is assumed for Si(100). These values result
from measurements of uncoated Si(100) substrates and are
constant in the contact depth regime of 20±120 nm.
work shows an unusual behavior (Fig. 2) with respect to the
ratio of contact depth to film thickness. At first, as in the case
of the DLC coatings, the measured hardness increases with
decreasing j: a behavior expected for a coating being harder
then the substrate material underneath. Thereafter the mea-
sured hardness decreases with decreasing j (j < 0.5). This
behavior could be taken as a non-homogeneous hardness dis-
tribution with film thickness but a closer look at the topogra-
phy around the indents reveals pile-up. This material behav-
ior is described in the literature and in this case occurs
because of a confinement of the plastic zone inside the film
because of the larger Young's modulus of the silicon substrate
compared to CNx.
The results of the film thickness variation are presented in
Figure 1. In good agreement with the Bückle law a significant
influence of the substrate on the measured hardness for con-
[
15]
tact depth exceeding 10 % of the film thickness is observed.
The measured values are well described by Equation 1. As
the absolute contact depth of all indentations range some-
where between 20 and 120 nm the regime of larger ratios, j
represents measurements taken on thinner films and the
regime of small j values corresponds to indentation measure-
ments of thicker coatings. The existence of one fitting curve
[
8]
Fig. 1. Hardness results of nanoindentation measurements of DLC films on Si(100)
Fig. 2. Unusual behavior of the measured hardness (nanoindentation) for the CN
thickness variation (for details see text).
x
(
sample series of film thickness variation) are well described by Equation 1.
334
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Staedler, Schiffmann/Micromechanical and Microtribological Properties of Thin CN and DLC Coatings
As this pile-up is not taken into account by the evaluation
of the Oliver and Pharr model the estimated contact area is
smaller then the actual one leading to an over-rated hardness.
In this special case (9 at.-% nitrogen films) the error is about
0
.8 GPa. In the further treatment this effect was taken into
account.
An analysis of the CN
x
sample series shows a significant
influence of the nitrogen concentration on the mechanical
properties. The hardness decreases in a linear way from 16 to
1
0 GPa with increasing nitrogen concentration (0±20 at.-%).
Analogously the Young's modulus also decreases with
increasing nitrogen concentration (140±90 GPa). These obser-
vations are in good agreement with values reported by Li et
[
4]
al.
The tribological tests were performed with the same
experimental setup as in the case of the nanomechanical mea-
surements. The only exception is the conical diamond inden-
ter (cone angle 90 ꢀ, tip radius 560 nm) used here instead of
the Berkovich diamond. Two different methods were used.
On the one hand a single scratch with linear increasing load
over a length of 10 lm was applied to study the load depen-
dence of the friction coefficient (final loads 0.5, 1, and 5 mN,
scratch velocity 0.67 lm/s). On the other hand an oscillating
wear test over a length of 4 lm at a constant load was used to
analyze the time dependence (1, 2, 3, 4, 5, 10, and 25 cycles) of
wear at different loads (0.5, 1, and 3 mN) at a scratch velocity
of 4 lm/s. In both tests the topography of the scratched sur-
face was mapped with the same tip before (pre-scan) and
after loading (post-scan) at a contact load of 20 lN. The dif-
ference between these two topographic measurements leads
to an information about the remaining wear depth after the
test.
Fig. 3. Different phases during a single scratch test of a thin film: a) 14 nm CN
b) 19 nm DLC film on Si(100) (details are given in the text).
x
,
irreversible deformation may occur in form of film cracking,
material compression, or yield, whereas it is difficult to dis-
tinguish between those forms by measurements on the nano-
meter scale. Due to an increased growth of the contact area
between indenter and surface the friction coefficient rises
during this phase (Phase II). The main change compared to
Phase I is the additional contact area component perpendicu-
lar to the scratch direction, which adds a new (ploughing)
[
22]
term to the lateral force. This evolution proceeds until the
indenter contacts the substrate. The different slopes of Pha-
se II and Phase III are due to a friction contrast between film
and substrate material. During Phase III a gradually increas-
ing pressure is induced into the film substrate interface until
finally complete film failure occurs (Phase IV).
The microtribological tests (single scratch and oscillating
test) were carried out with two samples of the CN
at.-% nitrogen concentration (226 and 14 nm film thickness)
and the DLC series of film thickness variation.
x
series at
9
Depending on the mechanical properties and the film
thickness of the coated samples as well as the final load, two
or more phases are observed during one single scratch. In the
case of the softer CN system, four different scratch phases
x
could be found, which are particularly pronounced in the
For the single scratch test of the 19 nm thick DLC coating
on Si(100) only three different phases are observed (see
Fig. 3b). The maximum scratch load of 5 mN is not enough to
lead to a complete failure of the film. Possibly this behavior is
caused by the so-called plate bending effect described else-
friction signal. In Figure 3a the friction coefficient l during
[
23]
the scratch of the 14 nm thick CN film on Si(100) is plotted
where. The floes of the cracked hard film on the softer sub-
strate still function as a kind of protective quasi-coating.
It is obvious that aside from the absolute values of the fric-
tion coefficient the different transition loads between the sin-
gle scratch phases are particularly suitable to describe the
scratch process of the coated substrates. In Figure 4 the criti-
cal load of the transition between Phase I and II is plotted
versus actual film thickness. A significant influence of the
substrate on the critical load is observed. The critical load
decreases with decreasing film thickness from the value of
quasi-bulk DLC (film thickness > 250 nm) to the value
obtained for the uncoated silicon substrate, 595 and 170 lN
respectively. A similar behavior was reported for much thin-
x
against the applied normal load.
At low loads (Phase I) there is pure elastic deformation of
the sample surface. In this case the friction coefficient
decreases according to Equation 3, which is derived from
[
20,21]
Hertzian contact theory.
Fload and Ffriction are the normal
load and friction force respectively.
2
/3
Ffriction µ Fload
±
1/3
l = Ffriction/Fload µ Fload
(3)
If the load exceeds a critical value there is an onset of irre-
versible deformation underneath the indenter (Phase II). This
[
3]
ner films. In the case of the thinner DLC films the onset of
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Staedler, Schiffmann/Micromechanical and Microtribological Properties of Thin CN and DLC Coatings
that of the thick one. This corresponds to the pile-up effect on
the measured hardness reported above. In Figure 5b the time
dependence of wear is shown for the 19 and 210 nm thick
DLC coatings. The remaining wear depth of the 210 nm thick
film at 1 mN is almost neglectable. This is not true for the
1
9 nm thick film. The remaining wear depth for this thinner
film starts at the same small values at low cycle numbers
cycle number < 5), but afterwards there is a significant onset
(
of wear leading finally to a complete film failure. This behav-
ior is possibly caused by fatigue due to a bending of the hard
film on the softer Si(100) substrate. In this case the substrate
acts like a soft spring underneath the hard coating and tensile
stress is induced in the film leading to cracks.
Fig. 4. Dependence of the critical load for a transition between scratch Phase I and II
elastic to non-elastic contact) during a single scratch test of a DLC film on Si(100)
substrate with respect to the film thickness (for details see text).
(
The results reported here show the influence of the me-
chanical properties of the film (CN
bological behavior of a film/substrate system. Due to the
poorer mechanical properties of CN the critical number of
cycles for film failure (oscillating wear test/load 1 mN) is
smaller compared to the DLC coating. For the single scratch
x
and DLC) on the nanotri-
irreversible deformation starts underneath the film in the
substrate resulting in the decrease of the critical load of tran-
sition from Phase I to II with decreasing film thickness.
In Figure 5 results of the oscillating wear tests for four
x
samples are presented, two CN films (14 and 226 nm film
x
test the sample coated with 14 nm CN shows a complete film
thickness) and two DLC coatings (19 and 210 nm film thick-
ness). In this illustration the remaining wear depth (the differ-
ence between pre- and post-scan topography) is plotted ver-
sus the cycle number. For the thicker films of both systems
x
failure at a load of 5 mN whereas the sample coated with
1
9 nm DLC shows no failure at all.
Besides the influence of the film material it was shown that
the film thickness has also a significant influence on the tribo-
logical response of the film/substrate system. At a given load
a decreasing film thickness leads to a growing influence of
the substrate on the mechanical and tribological response of
the system. The critical load for a transition between elastic
and non-elastic phase of a single scratch test is significantly
influenced by the substrate. For a hard DLC coating on a
Si(100) substrate a decreasing critical load with decreasing
film thickness is observed. This behavior indicates an onset of
non-elastic processes inside the substrate underneath the film
at smaller film thicknesses.
x
the wear behavior of CN and DLC is observed. In contrast to
this behavior the two thin films show fatigue and a complete
film failure after a certain number of cycle. The critical num-
ber of cycles to film failure at a given load of 1 mN is smaller
for the thin CN film due the poorer mechanical properties
x
compared to DLC. But aside from this both materials show
interesting features in their wear behavior.
x
In Figure 5a, the CN films, the remaining wear depth at
low cycle numbers is smaller for the thinner film compared to
Received: October 31, 2000
Final version: January 12, 2001
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and thick CN
x
(normal load 0.5 and 1 mN); b) DLC films (normal load 1 and 3 mN)
with respect to cycle number.
336
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