www.elsevier.com/locate/surfcoat
Surface & Coatings Technolog
Vickers and Knoop hardness of electron beam deposited ZrC and
HfC thin films on titanium
D. Ferroa, S.M. Barinovb,T, J.V. Raua,c, A. Latinid, R. Scandurrad, B. Brunettid
aCNR Istituto per lo Studio dei Materiali Nanostrutturati, P.le Aldo Moro 7-00185 Roma, ItalybInstitute for Physical Chemistry of Ceramics RAS, Ozernaya 48, 119361 Moscow, Russia
cM.V.Lomonosov Moscow State University, 119899, Moscow, RussiadUniversita’ di Roma bLa SapienzaQ, P.le Aldo Moro 7-00185 Roma, Italy
Received 4 November 2004; accepted in revised form 17 February 2005
Available online 21 February 2006
Abstract
Submicron thickness films of zirconium and hafnium carbides were electron beam deposited on the sandblasted surface of a metallic
titanium substrate. The films were characterized by scanning electron microscopy, and the hardness of the composite film–substrate systems
was measured by both Vickers and Knoop microindentation methods in the wide indentation loading range. The intrinsic hardness of the film
was separated out from the composite hardness using an area law-of-mixtures approach and taking into account the indentation size effect.
Measured with Vickers (Hf0V) and Knoop (Hf0K) indenters, the hardness was evaluated to be of about 26 and 24 GPa, respectively, for ZrC,
and 19 and 21 GPa, respectively, for HfC films, being somewhat higher compared to published data for bulk carbides.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Thin films; Carbides; Electron beam deposition; Hardness
1. Introduction
Titanium and titanium-base alloys have found wide
applications in orthopedics due to their enhanced bio- and
mechanical compatibility with the bone tissue [1]. One of
the major problems to be solved is the protection of titanium
against oxidation at the harsh conditions of the body fluids
surrounding the implanted device. Some transient titanium
oxides can be formed on the titanium surface being low
resistant to wear. The wear debris accumulation can result in
an adverse cellular response and implant loosening. To
protect titanium against oxidation and to improve wear
resistance of implant, the surface modification by coating is
widely used. Electron beam deposited titanium carbide
coating was demonstrated to be effective to increase the
surface hardness of titanium [2]. However, diffusion of
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2005.02.150
T Corresponding author. Tel.: +7 95 4379892; fax: +7 95 4379893.
E-mail address: [email protected] (S.M. Barinov).
carbon from the film into pre-heated titanium substrate
during the deposition decreases the carbide film hardness
down to 9–20 GPa compared to 30 GPa for bulk TiC [3,4].
Alternatively, other bio-inactive refractory carbides might
be deposited on titanium surface, e.g. zirconium and
hafnium carbides. Hardness of bulk ZrC is about 20–22
GPa and that of HfC is about 18–20 GPa [3,4]. Thin films of
HfC deposited by pulsed laser ablation on silicon substrate
have the intrinsic hardness varying in the range from 19 to
29 GPa, being independent on the silicon substrate pre-
heating temperature [5]. The present study was aimed at the
electron beam deposition (EBD) of ZrC and HfC films on
titanium substrate and the comparative evaluation of their
hardness by Vickers and Knoop microindentation techni-
ques. In spite of the widespread use of nanoindenters to
evaluate hardness of thin solid films, the Vickers/Knoop
method has some merit due to the following reasons. The
nanoindentation contact scales are generally less than the
film thickness. The apparent hardness value varies with
contact scale in this range, and the calculated hardness
y 200 (2006) 4701–4707
D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–47074702
increases with a decrease in load. This effect is known as the
indentation size effect [6–10]. Therefore, if the hardness is
used as a material selection criterion, it is clearly insufficient
to quote a single hardness value obtained from the nano-
indentation test. Besides, it could be difficult to obtain
reproducible and reliable nanoindentation data for rough
enough films, or for the films deposited on rough substrate,
whereas the microindentation allows the averaging of the
surface roughness effect. Other problems arising with the
nanoindentation techniques are pointed out by A. Iost [10].
The Knoop indentation method is rather rarely used to
evaluate the intrinsic hardness of thin films in spite of its
advantage such as low indenter penetration depth compared
to that of the Vickers indenter, at equal loads.
2. Materials and methods
To prepare the targets for EBD, the ZrC and HfC
powders (Aldrich, 98% pure) were uniaxially hot pressed
into 18 mm diameter and 10 mm thickness pellets. The
pellets were placed into a crucible made of titanium
diboride/boron nitride composite (Advanced Ceramics
Corp. Europe, UK). The crucible was then inserted into a
water-cooled electron beam gun (EVI-8, Ferrotec, Ger-
many), which has been positioned into a stainless-steel
chamber evacuated by a turbomolecular pump supported by
a rotary pump. Distance between the pellet and the gun was
0.25 m. The accelerating voltage in the range between
�3.05 and �10 kV, the shape, pattern and position of the
beam were controlled. The maximum operation power was
5 kW. The gun has a magnetic lens system that allows a
2708 deflection of the beam for avoiding contamination of
the evaporating material with tungsten from the emitting
filament.
The pure titanium substrates were sandblasted with a 60-
grid SiC abrasive powder to provide surface roughness of
approximately Ra 1.6 d 10�9 m necessary for best cell
adhesion to the surface of implanted device. Before the
deposition started, the substrates were heated under vacuum
of 5 d 10�5 mbar in the chamber with a high-power halogen
lamp. The deposition process was performed at an EB
accelerating voltage �3.5 kV and the emission current of
130–200 mA. The pattern of the electron beam was circular
and slow, to ensure uniform consumption of the evaporating
material. The deposition was performed at the substrate
preheating temperature either 200 8C for ZrC or 500 8C for
HfC. The temperature was measured with a thermocouple
(errorF10 8C). Deposition rate was 1–5 2/s. No significant
spitting of the target was observed, there was a light
degassing only at the beginning of the process. The coated
samples were cooled to room temperature in a nitrogen
atmosphere.
The thickness of film was evaluated by two independent
techniques. The former was scanning electron microscopy
(SEM) observation of the cross-sections of the samples (a
LEO 1450 VP SEM apparatus, Carl Zeiss, Germany). An
absolute error of the thickness measurement was F10 nm.
Alternatively, the thickness of the growing films was real-
time monitored by a quartz microbalance coupled with the
measurement of acoustic impedance of the deposited
material. The thickness measurement error in this case
was F50 nm. SEM apparatus was coupled with an energy-
dispersive X-ray (EDX) analysis system. X-ray diffraction
measurements were performed with a Philips X’Pert PRO
diffractometer (Cu Ka irradiation 1.54056 2, graphite
monochromator).
The hardness was measured with a Leica VMHT
apparatus (Leica GmbH, Germany) equipped with a stand-
ard Vickers pyramidal indenter (square-based diamond
pyramid of face angle 1368) and with a Knoop indenter of
face angles a =1720830V and b =13008. The loading and
unloading speed was 5 d 10�6 m/s, time under the peak load
being 15 s. On each sample indentations were made with 5–
7 loads ranging from 0.098 to 19.6 N. Standard deviation of
the diagonal measurements was dependent on the inden-
tation load ranging from about 17% to 4% with an increase
of the diagonal length. The measured hardness was that of
the film–substrate composite system.
To separate the hardness of the film–substrate system on
its constituents from the film and the substrate, a model
based on an area law-of-mixtures approach [11] was
applied:
Hf ¼ Hs þ Hc � Hsð Þ= 2c t=dð Þ � c2 t=dð Þ2h i
ð1Þ
where c is a constant dependent on indenter geometry and
on materials behavior (brittle or ductile); d is indentation
depth. The model is applicable at the indenter penetration
depths where the surface displacement is more than thick-
ness of the film [12]. Therefore, special attention was paid to
choose correctly the range of indentation depths where the
model is adequate. According to Ref. [12], a trend in the
composite film/substrate hardness change with an increase
of the indentation depth can schematically be represented as
is shown in Fig. 1. Region 1a corresponds to highly
localized coating elasto-plasticity and fully elastic substrate
response before the generation of an elasto-plastic enclave
in the substrate (Region 1b). In Region 2a, the substrate
deformation occurs followed by the coating deformation
sufficient to begin circumferential through-thickness coating
fracture. Region 2b corresponds to the fracture of the film
which is bent to conform to the plastically deformed
substrate. In Region 3, true-substrate-only behavior occurs
where the energy absorbed by the film is insignificant
compared to the amount dissipated by substrate. The
loading conditions corresponding to the Region 2b where
the model above is adequate have to be determined. For HfC
films deposited on silicon by pulsed laser ablation technique
this region was estimated to be in the range d / t from 1 to
about 5 [5].
Further, the indentation size effect (ISE) was taken into
account [6–10]. The reasonable expression for the Hc based
Co
mp
osi
te h
ard
nes
s
1a1b
2a
2b
3
Relative indentation depth, d/t
Fig. 1. A scheme of the composite film–substrate response in dependence
on the relative indentation depth.
D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–4707 4703
on an area law-of-mixtures approach and accounting for
ISE is
Hc ¼ Hs0 þ Bs þ 2c1t Hf0 � Hs0ð Þ½ �=D ð2Þ
where c1i0.5 for Vickers indenter [11] and c1i2.9 for
long diagonal of Knoop indenter [10], respectively, in the
case of brittle hard film on softer substrate; Hf0 and Hs0 are
intrinsic hardness of the film and substrate, respectively; Bs
is a coefficient; D is imprint diagonal. Eq. (2) has been
widely used in experiments with Vickers pyramid inden-
tation. It is not evident either the equation results in
reasonable value for Knoop indenter geometry, or not.
According to Ref. [10], Eq. (2) holds only when 1bD c1t b2, where D =(Hf0�Hs0) /Bs (variation of film hardness
with the thickness was neglected [10]). At the first
approximation, D can be evaluated to be equal to about
0.25 d 10�6 m (Hf0i22 GPa, Hs0=2.1 GPa, Bs=76.7 d 106
GPa/m), therefore at the film thickness of about 0.5 d 10�6
m, the product D c1 t equals to approximately 1.25, being
within the Iost’s criteria range from 1 to 2. So, there is no
need to use a more complicated third order equation [10].
Fig. 2. SEM plane view of ZrC (a) and HfC (b) films; cross-sectional view
of HfC film (c).
3. Results and discussion
Scanning electron microscopy observations revealed that
the films of both ZrC and HfC carbides have a compact
microstructure, as it is shown in Fig. 2. The surface of film
is rough, partly due to the roughness of the target
sandblasted surface. There are some droplets of about 1–5
Am size on the surface resulted from the expulsion of the
target by electron beam (Fig. 2b). The film is relative
uniform in its thickness (Fig. 2c). Interface between the film
and the substrate seems to be very clear, and no diffuse
boundary was revealed by SEM even using the atomic
number contrast technique. Cross-sectional SEM measure-
ment of the film thickness resulted in its averaged value of
about 510F20 nm for ZrC and 500F34 nm for HfC films.
Independent thickness evaluation by acoustic impedance
technique gave the same result, the film thickness equals to
approximately 500 nm. Mean grain size is less than 200 nm
for ZrC and 300 nm for HfC films. Measured by energy-
dispersive X-ray analysis (EDX) metal / carbon ratio at the
surface of the film indicates the carbides retain their
stoichiometry close to Me /C=1 in the film. The content
D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–47074704
of metals and carbon in the films was evaluated to be as
follows:
Film Hf, wt.% Zr, wt.% C, wt.%
ZrC – 76.8 23.2
HfC 94.3 – 5.8
According to these semi-quantitative data, the composi-
tion of zirconium carbide film locates in the two-phase
region ZrC+C of the phase diagram with free carbon
content of about 14 wt.%, whereas the composition of
hafnium carbide film corresponds approximately to the
HfC0.92 formula.
38,89
53,0
3
40,0
7
38,4
1
35,0
333
,49
31,4
9
28,4
3
0
50
100
150
200
250
300
350
20 30 40 50
2Th
Inte
nsi
ty, a
.u.
b
HfC HfC
35,0
5
33,2
9
31,2
9
53,0
9
40,1
1
38,4
1
0
50
100
150
200
250
300
20 30 40 50
2Th
Inte
nsi
ty, a
.u.
a
ZrC ZrC
Fig. 3. XRD pattern of ZrC (a)
Shown in Fig. 3a,b are the XRD patterns which were
recorded from the surface of the films and compared with
the JCPDS cards #05-0682 (Ti), #11-0110 (ZrC), and #09-
0368 (HfC). Reflections due to both the titanium substrate
and the film are present. Main peaks (100) at 2Theta 33.298and (220) at 55.778 for ZrC, and (111) at 33.498 and (220) at56.218. The peaks are significantly broadened indicating the
nanocrystallinity and/or the disordering of the carbide
structure.
Fig. 4a,b show measured indentation diagonal versus
indentation load and relative indentation depth versus
indentation load, respectively, for Vickers and Knoop
indenters applied to the ZrC/Ti composite system, as an
66,9
7
56,2
1
77,5
176
,43
70,6
7
62,8
9
60 70 80 90
eta
HfC
66,3
5
55,7
7
82,3
3
77,3
7
76,1
370,6
5
63,0
9
60 70 80 90
eta
ZrC ZrC
and HfC (b) films on Ti.
Indentation load (N)0 5 10 15 20 25
Impr
int d
iago
nal (
10-6
m)
0
50
100
150
200
250
300
350
VickersKnoop
a ZrC/Ti
Indentation load (N)
0 5 10 15 20 25
Rel
ativ
e in
dent
atio
n de
pth,
d/t
0
5
10
15
20
25
VickersKnoop
b ZrC/Ti
Fig. 4. Imprint diagonal versus indentation load (a) and relative penetration
depth versus indentation load (b) for ZrC/Ti composite system.
Relative indentation depth, d/t0 5 10 15 20 25
Com
posi
te h
ardn
ess,
Hc
(GP
a)
2
3
4
5
6
7
8
VickersKnoop
a
Relative indentation depth, d/t
0 5 10 15 20 25 30
Com
posi
te h
ardn
ess,
Hc
(GP
a)
2
3
4
5
6
7
8
VickersKnoop
HfC/Tib
Fig. 5. Composite hardness versus indenter penetration depth for ZrC/Ti (a)
and HfC/Ti (b) systems.
D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–4707 4705
example. The advantages of the Knoop indentation over the
Vickers method is that the former allows to reach the same
relative penetration depths in the much wider range of
indentation loads, and higher loads can be applied to get the
same indentation diagonal as using the Vickers technique.
Besides, the measured Knoop indentation diagonal is greater
allowing thus to decrease the relative error of the diagonal
measurement. For example, the relative error of the Knoop
indenter diagonal measurement decreases from 6.4% at the
load 0.49 N down to 4.4% at the load 9.8 N, whereas the
relative error of the Vickers diagonal measurement at the
applied load of 0.098 N was 17.2% due to the limited
optical system resolution.
Shown in Fig. 5a,b is the hardness of the composite
systems ZrC/Ti and HfC/Ti versus relative indentation depth
for both the Vickers and the Knoop indenters. The
composite hardness decreases markedly with an increase
of the d / t. The loading range corresponds to the Region 2b
of the scheme in Fig. 1.
The experimental plots Hc versus 1 /D for substrates and
all coated samples were approximated well by a linear
regression for both the Vickers and the Knoop indentations.
Fig. 6a,b show the experimental data plots, regression lines,
99% confidence intervals and correlation coefficients for
ZrC/Ti and HfC/Ti systems, respectively. Correlation
coefficients values for both the Vickers and the Knoop in-
dentations are very close each other indicating the methods
provide approximately the same accuracy of the hardness
determination. A least-squares fit of the plots to the Eq. (2)
results in the slope Bc=Bs+2c1t(Hf0�Hs0). The values of
Hs0 and Bs, which are needed to calculate the intrinsic
hardness of the film, were obtained by separate experiments
with substrate only. In the case of the Vickers method those
were estimated as equal to 1.84F0.6 GPa and 5.38 d 10�6
GPa m, respectively, whereas for the Knoop indentation Hs0
was equal to 2.10F0.4 GPa and Bs=76.69 d 10�6 GPa m.
The difference in Hs0 values is in the error range and
is supposed to result probably from different surface
conditions of the sandblasted uncoated samples. A signifi-
cant difference between values of Bs is due to indenter
geometry. Using the values of Bs, Bc and Hs0, the intrinsic
Inverse imprint diagonal, 1/D (104 m)
0 2 4 6 8 10 12 14 16
Com
posi
te h
ardn
ess,
Hc
(GP
a)
2
3
4
5
6
7
VickersKnoopRegression99% Confidencer = 0.94
r = 0.96
ZrC/Tia
Inverse indentation diagonal, 1/D (104 m-1)
0 2 4 6 8 10 12 14 16 18
Com
posi
te h
ardn
ess,
Hc
(GP
a)
1
2
3
4
5
6
7
8
VickersKnoopRegression99% Confidence
HfC/Ti
r = 0.92
r = 0.91
b
Fig. 6. Composite hardness versus inverse imprint diagonal for ZrC/Ti (a)
and HfC/Ti (b) systems; r is the correlation coefficient.
Table 1
Intrinsic hardness of refractory carbide films on Ti substrate
Film TiC [2] ZrC HfC
Thickness, nm 700F10 1500F100 510F20 500F34
Vickers
hardness, GPaT19.5 10.0 26 (r =0.96) 19 (r =0.92)
Knoop
hardness, GPaT– – 24 (r =0.94) 22 (r =0.91)
T Correlation coefficient is given in the brackets.
D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–47074706
hardness of the films was calculated. Calculated intrinsic
hardness values for both systems under study and, also, for
electron beam deposited TiC films on titanium are give in
Table 1.
It could be pointed out that the difference between
Vickers and Knoop hardness evaluations is small being
within the range of the measurement error. The ZrC film is
somewhat harder compared to the HfC film, at equal
thickness. This is in accordance with the behavior of bulk
carbides hardness, which decreases with an increase of
valence electrons number and, respectively, of molar
weight of the carbide [3]. A reduced intrinsic hardness
of TiC film compared to that for both the bulk titanium
carbide (about 30 GPa [4]) and the pulsed laser ablation
(PLA) deposited on silicon TiC film of about 700 nm
thickness (32 GPa [13]) has been explained as the result of
the diffusion of carbon into the metallic titanium substrate
during the deposition process, which has been carried out
at the substrate pre-heating temperature as high as 800 8C[2]. A decrease in carbon content is known to lower the
hardness of refractory carbides. Particularly, a decrease in
carbon content in titanium carbide lowers the hardness
down to 11.76 GPa in the bulk sintered ceramics of TiC0.58
composition [3]. Vickers hardness values for bulk Zr and
Hf carbides are 20–22 and 18–20 GPa, respectively [3].
Therefore, the diffusion of carbon from the ZrC and HfC
films is expected to be not so significant as it was revealed
for TiC, probably due to the activation energy of carbon
self-diffusion is higher in these carbides than in titanium
carbide: 399 kJ/mol for TiC, 473 kJ/mol for ZrC and 545
kJ/mol for HfC [3]. Besides, the substrate pre-heating
temperature to deposit low-hardness, 1500 nm thickness
TiC film has been 800 8C, much higher than in the case of
ZrC and HfC deposition (200 and 500 8C, respectively),furthering the carbon diffusion into the substrate. Hardness
of HfC film deposited on silicon by pulsed laser ablation
(PLA) method, i.e. in the case when diffusion of carbon
from the film is negligible, has been evaluated to be of
about 29 GPa at the film thickness of 650 nm [5,14].
Hardness of ZrC PLA-deposited film on Si substrate was
evaluated to be of 31 GPa at the film thickness of about
500 nm [15]. Enhanced hardness of PLA-deposited
refractory carbides films on Si has been related to their
ultrafine nanocrystalline structure: evaluated from X-ray
diffraction data, the grain size in the TiC films was in the
range 25 to approximately 30 nm, as an example [16].
Hardness of the electron beam deposited thin films is
somewhat higher than that of bulk carbides. The hardness
of very thin films is known to rise generally with
decreasing the thickness due to reduced degrees of
freedom, which results in lowered stress relaxation by
means of plastic deformation [15]. This behavior is similar
to the yield stress variation with the thickness of very thin
crystals or fibers [17]. The following reasons may be
responsible for such lowered stress relaxation capacity in
thin films: confined dislocation mobility and/or crack
propagation by the film–substrate interface, and nano-
crystallinity of the film resulting in effective dislocation
pinning by the crystallite boundaries [18]. It could be
keeping in mind that the structure of the films which is
forming during the EB deposition is in non-equilibrium
state. Deposition of the films by evaporation in vacuum
results always in tensile stress imposed on the film due to
the densification of the disordered to some extent structure
that is bfrozingQ during the deposition [19]. These tensile
stress reduce the measured hardness promoting cracking
under the indentation.
D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–4707 4707
Thus, submicron thickness films of refractory Zr and Hf
carbides were successfully deposited on titanium substrate
by electron beam deposition method and their intrinsic
hardness was revealed to exceed that of respective bulk
carbides, as evaluated independently by both Vickers and
Knoop microindentation techniques.
4. Conclusions
The following conclusions can be drawn from the results
of the study.
1. Electron beam deposition method allows the fabrication
of submicron thickness, dense continuous ZrC and HfC
films on rough sandblasted surface of metallic titanium
substrate.
2. The intrinsic film hardness was separated out from the
composite film–substrate hardness measured by both the
Vickers and the Knoop methods. The calculated
intrinsic hardness values for both the ZrC film (Hf0V
26 GPa and Hf0K 24 GPa) and the HfC film (Hf0V 19
GPa and Hf0K 21 GPa) are somewhat higher compared
to those reported for bulk carbides. Vickers and Knoop
methods resulted in similar hardness value estimations,
however an advantage of the Knoop method is the
extended indentation loading range, that is useful in
practice.
References
[1] M. Long, H.J. Rack, Biomaterials 19 (1998) 1621.
[2] D. Ferro, R. Scandurra, A. Latini, J.V. Rau, S.M. Barinov, J. Mater.
Sci. 39 (2004) 329.
[3] R.A. Andrievski, I.I. Spivak, Strength of Refractory Compounds and
Related Materials, Metallurgy, Tcheljabinsk, 1989.
[4] A. Krajewski, L. D’Alessio, G. De Maria, Cryst. Res. Technol. 33
(1998) 34.
[5] S.M. Barinov, D. Ferro, C. Bartuli, L. D’Alessio, J. Mater. Sci. Lett.
20 (2001) 1485.
[6] H. Bqckle, in: J.H. Westbrook, H. Conrad (Eds.), Science of Hardness
Testing and Its Research Applications, ASM Publ., Metals Park, 1973,
p. 453.
[7] H. Li, R.C. Bradt, J. Mater. Sci. 28 (1993) 917.
[8] A. Iost, R. Bigot, Surf. Coat. Technol. 80 (1996) 117.
[9] F. Frfhlich, P. Grau, W. Grellman, Phys. Status Solidi, A Appl. Res.
42 (1977) 79.
[10] A. Iost, Scripta. Mater. 39 (1998) 231.
[11] B. Jfnsson, S. Hogmark, Thin Solid Films 114 (1984) 257.
[12] A.M. Korsunsky, M.R. McGurk, S.J. Bull, T.F. Page, Surf. Coat.
Technol. 99 (1998) 171.
[13] G. De Maria, D. Ferro, L. D’Alessio, R. Teghil, S.M. Barinov, J. Mater.
Sci. 36 (2001) 929.
[14] R. Teghil, A. Santagata, M. Zaccagnino, S.M. Barinov, V. Marotta, G.
De Maria, Surf. Coat. Technol. 151–152 (2002) 531.
[15] D. Ferro, R. Teghil, S.M. Barinov, L. D’Alessio, G. De Maria, Mater.
Chem. Phys. 87 (2004) 233.
[16] L. D’Alessio, A.M. Salvi, R. Teghil, V. Marotta, A. Santagata, B.
Brunetti, D. Ferro, G. De Maria, Appl. Surf. Sci. 134 (1998) 53.
[17] A. Kelly, Strong Solids, Clayderon, Oxford, 1973.
[18] R.A. Andrievski, Int. J. Refract. Met. Hard Mater. 19 (2001) 447.
[19] J.A. Thornton, D.W. Hoffman, Thin Solid Films 171 (1989) 5.
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