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Applied Surface Science 370 (2016) 306–311
Contents lists available at ScienceDirect
Applied Surface Science
j ournal homepage : www.elsevier .com/ locate /apsusc
Protection of brittle film against cracking
J. Musil ∗, J. Sklenka, R. ˇ Cerstvý
Department of Physics andNTIS—European Centre of Excellence, University of West Bohemia, Univerzitní 8, 30614 Plzeˇ n, Czech Republic
a r t i c l e i n f o
Article history:
Received 11 November 2015
Received in revised form 8 February 2016
Accepted 13 February 2016
Available online 16 February 2016
Keywords:
Zr Si O film
Mechanical properties
Transparency
Film cracking
Protective film
Magnetron sputtering
a b s t r a c t
This article reports on the protection of the brittle Zr Si O film against cracking in bending by the highly
elastic top film (over-layer). In experiments the Zr Si O films with different elemental composition and
structure were used. Both the brittle and highly elastic films were prepared by magnetron sputtering
using a dual magnetron. The brittle film easily cracks in bending. Onthe other hand, the highly elastic filmexhibits enhanced resistance to cracking in bending. Main characteristic parameters of both the brittle
and highly elastic films are given. Special attention is devoted to the effect of the structure (crystalline,
amorphous) of both the brittle and highly elastic top film onthe resistance of cracking of the brittle film.
It was found that (1) both the X-ray amorphous and crystalline brittle films easily crack in bending, (2)
the highly elastic film can have either X-ray amorphous or crystalline structure and (3) both the X-ray
amorphous and crystalline, highly elastic top films perfectly protect the brittle films against cracking in
bending. The structure, mechanical properties and optical transparency of the brittle and highly elastic
sputtered Zr Si O films are described in detail. At the end of this article, the principle of the low-
temperature formation of the highly elastic films is also explained.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
It is well known that brittle films very easily crack during bend-
ingwhen theyare depositedon the flexible substrates. The cracking
of thebrittle films is their serious drawbackbecause the crackscre-
ated in the film interrupt its function [1–12]. Therefore, it is vitally
important to prevent cracking of the brittle films.
Recently, it was reported that the brittle film can be pro-
tected against cracking by the elastic top film (protective film)
with the high hardness H ≈15 to 25GPa, low elastic modulus
E * =E /(1− 2) which results in a high ratio H /E * ≥0.1, high elastic
recovery W e≥60% and compressive macrostress ( < 0) [13–16];
here E is the Young’s modulus and is the Poisson’s ratio. In the
experiment described in Ref. [17] it was shown that the amor-
phous brittle film over-coated by the crystalline elastic film does
not cracks, i.e. the system of two layers – brittle bottomfilm/elastictop protective film – exhibits an enhanced resistance to cracking.
However, there is a questionif also thecrystalline brittle filmcanbe
protected against cracking by an elastic top film and if this elastic
top film can also have the X-ray amorphous structure. The investi-
gation of the effect of the structure of both the brittle film and the
∗ Corresponding author. Tel.: +420 377632201.
E-mail address: musil@kfy.zcu.cz (J. Musil).
elastic protective film on the resistance of cracking of the brittle
film is the main subject of this article.This article reports on the two-layer coatings composed of brit-
tle bottom film and top elastic film with different compositions of
crystallographic structure of both films: (a) a-+ c-, (b) c-+ c-, (c)
a-+a-, and (d) c-+ a-; here a- and c- denotes the film with amor-
phous andcrystallinestructure, respectively,see Fig. 1. Physicaland
mechanical properties, and optical transparency of individual films
(a- brittle, c- brittle, a- elastic and c- elastic) and couples of these
films (the bottom brittle film + the top elastic film) are reported in
detail.
2. Experimental details
The Zr Si O coatings with different physical and mechanicalproperties were deposited on the Si (10 0) substrate and the Mo
strip (60×10×0.1mm3) by reactive magnetron sputtering using
a dual magnetron equipped with Zr/Si targets of 50mm in diam-
eter composed of a circular Si (10 0) plate (∅ 38mm) fixed to the
magnetron cathode by Zr fixing ring with inner diameter ∅in. The
dual magnetron was operated in ac pulsed bipolar mode gener-
ated by a pulsed power supply DORA MSS-10 with a maximum
output power 10kW. The repetition frequency f r of pulses was
2 kHz and ac frequency of negative half-sinusoids inside pulses
was 112kHz. More details on the deposition system are given in
Ref. [17].
http://dx.doi.org/10.1016/j.apsusc.2016.02.132
0169-4332/© 2016 Elsevier B.V. All rightsreserved.
http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.apsusc.2016.02.132http://www.sciencedirect.com/science/journal/01694332http://www.elsevier.com/locate/apsuscmailto:musil@kfy.zcu.czhttp://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.apsusc.2016.02.132http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.apsusc.2016.02.132mailto:musil@kfy.zcu.czhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.apsusc.2016.02.132&domain=pdfhttp://www.elsevier.com/locate/apsuschttp://www.sciencedirect.com/science/journal/01694332http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.apsusc.2016.02.132
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J. Musil et al. / Applied Surface Science 370 (2016) 306–311 307
Fig. 1. Four two-layer coatings composed of brittle bottom film and top elas-
tic film with different combinations of the structure of bottom and top films.
(a) crystalline/amorphous couple, (b) crystalline/crystalline couple, (c) amor-
phous/amorphous coupleand (d) amorphous/crystalline couple.
The films were sputtered under the following conditions: dis-
charge current I da =1 A averaged over the pulse period T = 1/ f r ,
target power density Sd =P d/S≈25 W/cm2, substrate bias U s =U fl,
the substrate temperature T s =RT (the unheated substrate) and
500 ◦C, substrate-to-target distance ds−t =80mm, partial pressure
of oxygen pO2 = 0 and 0.15Pa for the brittle films and the elas-
tic films, respectively, and total pressure pT = pAr + pO2 = 1Pa;
here S is the whole area of the magnetron target and U fl is
the floating potential. At first, single layer coatings were sput-
tered on the Si (10 0) substrate, glass substrate and the Mo strip(60×10×0.1mm3), their physicaland mechanical propertieswere
measured and their resistance to cracking was assessed by bend-
ing of the coated Mo strip around a fixed cylinder. Physical and
mechanical properties of sputter deposited single layer films and
their elemental composition are given in Table 1. Then, two-layer
coatingswith differentcrystallographic structureof the bottom and
top films (layers) were sputtered. Their physical and mechanical
properties were also measured and their resistance to cracking was
also assessed by bending of the coated Mo strip, see Table 2. Both
films in two-layer coatings were sputtered without an exposure to
the air simply by changingthe partial pressure of oxygen pO2 in the
Ar+O2 sputtering gas mixture.
The structure of sputtered films was characterized by X-ray
diffraction using an XRD diffractometer PANalytical X’pert PROin Bragg-Brentano configuration with CuK radiation. The ele-
mental composition was determined by X-ray Fluorescence (XRF)
spectroscopy with PANalytical XRF Spectrometer MagiX PRO.
Mechanical properties of films were determined from load vs.
displacement curves measured by a microhardness tester Fisch-
erscope H100 with Vicker’s diamond indenter at a load L=20mN.
For all sputtered films the ratio d/h of diamond depth impression
d in the film and the film thickness h was less than 0.1. It indicates
Fig. 2. X-ray (XRD) diffraction patterns of brittle – amorphous (film 1) and crys-
t alline ( film 2) and e last ic – amo rphous (fi lm 3) an d crys tall in e ( film 4) Zr S i O
films with propertiessummarized in Table 1.
that the measured hardness H of sputtered films is not influenced
by the substrate [18,19]. The macrostress and the thickness h of
thefilms were measured usinga stylusprofilometerDEKTAK 8. Themacrostress was determined from the thickness h and curvature
of the films deposited on Si (10 0)strips (35×5×0.64mm3) using
a Stoney’s formula [20]. The resistance of the film to cracking was
tested by a bending test. The coating was deposited on a Mo strip
(80×10×0.1mm3) and the coated Mo strip was bended around a
fixed cylinder of diameter r up to the occurrence of cracks in the
coating surface. More details are given in Refs [13–16]. The trans-
parency of Zr Si O layerswas measured inthe range from 300 to
800nm using a spectrometer Specord M400.
3. Results and discussion
3.1. Mechanical properties and structure of Zr Si O films
Physical and mechanical properties of single-layer Zr Si O
films are summarized in Table 1. From this table it is seen that
(1) the amorphous and crystalline films can be (i) sputtered from
a composed Zr/Si target, see Fig. 2, and (ii) either brittle (eas-
ily crack) or highly elastic (resistant to cracking) films can be
formed, (2) the brittle crystalline and amorphous films exhibit a
low ratio H /E
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308 J. Musil et al. / Applied Surface Science 370 (2016) 306–311
Fig. 3. Surface morphology of one-layer brittle film (a) the amorphous film (thefilm No. 1) and (b) the crystalline film (the film No. 2) after bending of coated Mo
strip (the film/Mo strip couple) around cylinder of radius r =10mm. Physical and
mechanical properties of the film 1 and 2 are given in Table 1.
(3) the elastic amorphous and crystalline films resistant to cracking
exhibit a high ratio H /E * ≥0.1 and high elastic recovery W e >60%,
compressive macrostress ( < 0), high optical transparency in visi-
ble and IR regions and strongly differ in the content of Zr and Si, (4)
the elastic amorphous film is soft and contains more Si compared
withZr (Si/Zr> 1),(5)the elastic crystallinefilm is hard and contains
more Zrcompared with Si(Si/Zr < 0), (6)thebrittleandelasticamor-
phous films exhibit a low compressive macrostress ( < 0), (7) the
brittle crystalline films exhibit tensile macrostress ( >0) and low
hardnessH ≈14GPa, (8) the elastic crystalline films exhibit a com-
pressive macrostress ( < 0), relatively high hardness H ≈17GPa
and almost the same value of the effective Young’s modulus E * as
the brittle crystalline film. These properties are controlled by the
elemental composition of films and their deposition rate aD, i .e.by
the energy delivered to the film during its growth.
The combination of individual films (layers) in the two-layer
Zr Si O coating makes itpossible to find the effect of the crystal-
lographic structure and mechanical properties of the bottom and
top layer on its resistance to cracking in bending.
3.2. Resistance of two-layer coatings to cracking in bending
At first, the brittle amorphous (the film No. 1) and the brit-
tle crystalline (the film No. 2) films, i.e. the single-layer Zr Si O
films, deposited on Mo strip were bended around cylinder of
radius r =10mm. Both coatings cracked in bending due to lowratio
H /E * < 0.1 and low elastic recovery W e < 60%, see Fig. 3 and Table 1.
Cracks are perpendicular to the direction of bending. On the other
hand, the elastic amorphous (the film No. 3) and the elastic crys-
talline (the film No. 4) single-layer films deposited on Mo strip do
not crack and exhibit no cracks after bending around cylinder of
radius r = 10mmdue to highratioH /E * > 0.1 and high elastic recov-
ery W e >60%.
Then, both brittle films (the film Nos. 1 and 2) were overlapped
with the highly elastic top films exhibiting high ratio H /E * ≥0.1,
highelasticrecoveryW e > 60%and compressive macrostress (
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J. Musil et al. / Applied Surface Science 370 (2016) 306–311 309
Fig. 5. Four combinations of the indentation curves of the bottom brittle film and the top elastic film which prevent cracking of the bottom brittle film: (a) the film 1+ the
film 4, (b) the film 2+ the film 4, (c) the film 1+ the film 3 and (d) the film 2+ the film 3. Indentation curvesweremeasured onthe filmssputtered on Si (10 0) substrates.
5. The bottom brittle film with the lower hardness than that of top
filmcan be protected against cracking by the harder topfilm. On
the other hand, the bottom brittle film with the higher hardness
than that of top film can be protected against cracking by the
softer top film.
3.4. Optical transparency
Very interesting is the optical transparency of individual films.
Therefore, the Zr Si O films were also sputtered on glass sub-
strates. The optical transparency was measured in a wide range of
wavelengths from250 to 800 nm. Results of these measurements
are given in Fig. 6. From this figure it is seen that while brittle films
(the film No. 1 and the film No. 2) are opaque, the elastic films are
well transparent. The opacity of the brittle films is due to a large
amount of free Si and Zr atoms in the film. The transparency of
the elastic film with higher content of Si is practically the same as
that of the glass without film and higher than that of the elastic
film with the higher content of Zr. This difference is due to the fact
that the Zr Si O film with higher Si content essentially representsan SiO2 film which almost completely eliminates reflections at the
film/glass substrate interface. This is a big advantage of the elastic
Zr Si O film withhigh Sicontent. Ontheotherhand,thedisadvan-
tage of this filmis itslow hardnessH ≈7 GPa. Onthe other hand, the
elastic Zr Si O film with higher Zr content is the ZrO2 based film
with lower optical transparency due to reflections at the film/glass
substrate interface but with a higher hardness H ≈17GPa.
4. Low-temperature sputtering of flexible nanocomposite
films
At present, there is an urgent need to master the forma-
tion of crystalline or at least nanocrystalline flexible films on
heat sensitive substrates such as polymer foils, textiles, plastics,
Fig. 6. Transmittance ofthe brittle Zr Si O films– amorphous (film 1)and crys-
talline(film2)andtheelasticZr Si O filmswith enhancedresistance tocracking –amorphous(film 3) andcrystalline (film4) sputteredon glasssubstrate as a function
of wavelength of the electromagnetic wave.
polycarbonates, etc., without the substrate heating. It is a very
difficult and vitally important task because the formation of the
flexible films withenhanced resistance to cracking at low substrate
temperatures T s≤100 ◦C is of a great potential for many advanced
applications, namely for the flexible electronics, flat panel displays,
micro-electromechanical (MEMS) systems, formation of the func-
tional films on polymer foils and fabrics, etc.
The formation of the flexible films on unheated substrates is
possible. It is basedon the replacement of the equilibrium substrate
heating (T s/T m) by the non-equilibrium atomic scale heating, i.e.
on the energy E p delivered to the growing film by condensing and
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Fig. 7. Schematic illustration of two dimensional (2D) Thornton’s structural zone
model (SZM) extended into region of low-pressure sputtering. Adapted after Ref.
[22].
bombarding particles; here T m is the melting temperature of the
material of the film. The principle of formation of the flexible films
on unheated substratesis illustrated in Fig.7. This figure represents
the two dimensional (2D) structural zonemodel (SZM) of sputteredfilms developed by Thornton [21] extended to low sputtering gas
pressures p≤0.1Pa [22]. The zone T of the SZM represents dense,
voids-free films in compression ( < 0) with enhancedresistance to
cracking; here is the macrostress in the film.
From Fig. 7 it is clearly seen that just the zone T expands to low
values of the ratio T s/T m→0 with decreasing argon pressure pAr.
This means that the dense, voids-free films with enhanced resis-
tance to cracking in the zone T of the SZM can be formed without
the substrate heating by increasing of the energyE p at low sputter-
ing gas pressures p. The energy E p can be delivered in the growing
filmeither bythe combinedactionof bombarding ions andfastneu-
trals (E p = Ebi + Efn) or bythe fast neutrals only (E p = Efn). The energy
of bombarding ions Ebi is controlled by the substratebiasU s. Onthe
contrary, the energy of condensing fast neutrals Efn is controlled bythe sputtering gas pressure pAr and is increased by decreasing of
the sputtering gas pressure pAr. The whole kinetic energy of these
particlesis transferred to the growing film andstimulatesits densi-
fication and nano-crystallization. This means that the formation of
the flexible, dense, voids-free sputtered films with enhanced resis-
tance to cracking on unheatedsubstrates basedon the replacement
of the conventional heating (T s) with the atomic scale heating (E p)
is possible [22]. There are two key parameters which decide on
the formation of such films: (1) the energy E p and (2) the melting
temperatureT m of created films. More details can be found in Refs.
[13,14,23–26].
In summary, it can be concludedthatthe films with dense,voids-
free microstructure, compressive macrostress ( < 0)and enhanced
resistance to cracking can be formed in the zone T of the Thorn-ton’s SZM also at low substrate temperaturesT s≤100 ◦C,i.e. atlow
values of T s/T m ratio, when sufficient energy E p is delivered to the
growing film by bombarding ions and/or when a low sputtering
gas pressure p is used. Very important role in the low-temperature
sputtering of the flexible films plays also the melting temperature
T m of the film material which (i) decides on the value of T s/T mratio and(ii) canbe controlled by the addition of selected elements
in the film.
5. Conclusions
The investigation reported in this article shows that the brittle
film can be protected against cracking when it is over-coated with
the elastic topfilm. It was found that (i)the brittle films exhibit low
elastic recoveryW e < 60% and low ratio H /E * ≤0.1 and can be either
X-ray amorphous or crystalline, (ii)the elastic top filmsexhibithigh
elastic recovery W e ≥60% and high ratio H /E * ≥0.1 and also can be
either X-ray amorphous or crystalline, and (iii) the cracking of the
brittle filmcan be preventedwhen it is over-coated with the elastic
top film. Thelast finding is of keyimportance notonly for the deep-
ening of the present knowledge in the field of two-layer coatings
but also for many applications. Many functional coatings are brittle
and easily crack when they are (i) deposited on the flexible sub-strates or (ii) bended and thus their function can be interrupted.
Therefore, the overlapping of the brittle film by the elastic top film
(over-layer) is a very efficient tool which ensures the reliable func-
tion of the brittle film even in the case when it is deposited on
the flexible substrate. These conclusions are applicable also to the
films on plastic substrates and the thick multilayer coatings with
alternating brittle/elastic bi-layers.
Acknowledgement
This work was supported by the project LO1506 of the Czech
Ministry of Education, Youth and Sports.
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