Comparative Analysis of Cr–B Coatings Deposited by Magnetron Sputtering in DC and HIPIMS Modes

Abstract

Surface coatings of the Cr–B system have been obtained by magnetron sputtering in the DC and
high power impulse (HIPIMS) regimes. It is established that the passage from the DC regime to HIPIMS
leads to suppression of the columnar grain growth and a twofold increase in the resistance of coatings to plastic deformation, while the plasticity index and hardness of coatings increase by 29 and 18%, respectively.

Full text

ISSN 10637850, Technical Physics Letters, 2014, Vol. 40, No. 7, pp. 614–617. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © Ph.V. KiryukhantsevKorneev, D. Horwat, J.F. Pierson, E.A. Levashov, 2014, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2014, Vol. 40, No. 14,
pp. 63–70.
614
The method of magnetron sputtering (MS) in
highfrequency (HF) or direct current (DC) regimes
is widely used in practice for depositing thinfilm
coatings with a broad range of compositions in a num
ber of applications. In recent years, a modification of
this technique called highpower impulse magnetron
sputtering (HIPIMS) has been extensively developed
[1–4]. A specific feature of HIPIMS is the supply of
very high power in short pulses on magnetron, which
prevents the target material from overheating and
melting. Magnetron sputtering at powers within
10 kW–10 MW (corresponding to a specific power
density of 0.1–5 kW/cm
2
versus 1–50 W/cm
2
in the
conventional DC regime) ensures a significant
increase in the plasma density: from ~10
10
ion/cm
3
for DCMS to 10
13
–10
14
ion/cm
3
for HIPIMS [1, 5].
In this case, sputtered atoms are intensely ionized
during passage through the plasma and the flux to
the substrate consists predominantly of ions, rather
than atoms as in the case of conventional DCMS.
An increase in the ion/atom ratio in the flux, which
is inherent in HIPIMS, leads to a significant
improvement of the adhesion strength of deposited
coatings due to the formation of pseudodiffusion lay
ers and ion implantation effects at the stage of prelim
inary etching of the substrate [2]. The mechanical
properties and wear resistance of coatings are also
improved due to increased density of their structure [3,
4, 6]. At present, the use of HIPIMS is restricted to the
deposition of coating based on pure metals and com
pounds of the MeO
x
and MeN types (Me = Ti, Cr, Al,
Zr, V, etc.) [1–7]. To the best of our knowledge, no
data have been reported to date on the HIPIMS depo
sition of transition metal borides. As a rule, HIPIMS
regimes are employed with targets of pure metals or
alloys [2–7].
The present work is devoted to a comparative study
of the structure and properties of Cr–B coatings
deposited by DCMS and HIPIMS using a composite
boride target.
A boride target was prepared by the method of self
propagating hightemperature synthesis (SHS) [8].
According to the Xray diffraction (XRD) data, the
phase composition of the target corresponded to
61.5% CrB
2
+ 21.0% CrB + 17.5% Cr
3
B
4
. The disk
cathode had a diameter of 50 mm and a thickness of
5 mm. The coatings were deposited in a vacuum setup
comprising a 40 L vacuum chamber with three magne
trons and Adixen Alcatel pumping stage based on Pas
cal 2021 SD forevacuum pump and Act 600TH turbo
molecule pump. The distance from target to substrate
was 70 mm. The magnetic induction at the center of
the target surface was 61 mT. The target was sputtered
in argon and the area of a sputtered zone was about
onethird of the target area. The gas supply was con
trolled by a Flow View VI.17 program. Coatings in the
DCMS regime were deposited using a Pinnacle+
(Advanced Energy, United States) source at a power of
350 W and a gas pressure of 1 Pa. The HIPIMS regime
was realized using an SPIK 2000A source (Melec,
Germany) at voltage amplitude of up to 600 V, pulse
duration of 3 ms, and duty cycle of 13.33%, respec
tively. The working gas pressures were 1 and 1.7 Pa,
which allowed the peak currents of 7.5 and 20 A,
respectively, to be reached.
The waveforms of process parameters were
recorded using a Tektronix TDS 2024B oscilloscope.
Figure 1 shows the typical current waveforms mea
sured in the HIPIMS regime, which reveals two
Comparative Analysis of Cr–B Coatings Deposited
by Magnetron Sputtering in DC and HIPIMS Modes
Ph. V. KiryukhantsevKorneev*, D. Horwat, J. F. Pierson, and E. A. Levashov
National University of Science and Technology, Moscow, 119049 Russia
Institut Jean Lamour, Université de Lorraine, UMR 7198CNRS, Nancy F54000, France
*email: kiruhancevkorneev@yandex.ru
Received December 27, 2013
Abstract
—Surface coatings of the Cr–B system have been obtained by magnetron sputtering in the DC and
highpower impulse (HIPIMS) regimes. It is established that the passage from the DC regime to HIPIMS
leads to suppression of the columnar grain growth and a twofold increase in the resistance of coatings to plas
tic deformation, while the plasticity index and hardness of coatings increase by 29 and 18%, respectively.
DOI:
10.1134/S1063785014070219

TECHNICAL PHYSICS LETTERS Vol. 40 No. 7 2014
COMPARATIVE ANALYSIS OF Cr–B COATINGS 615
extrema. The first (within 0–150
μ
s) is related to rar
efaction on the passage from argon plasma to that
based on the target components [7], while the second
(within 250–350
μ
s) is related to reflection of the ion
flux from chamber walls back to the target, as was
demonstrated in [9]. The substrates had the form of
disks made of cemented carbide VK6M with a diame
ter of 30 mm and a thickness of 5 mm. Prior to coat
ing, the substrates were ground and ultrasonically
treated in isopropyl alcohol. Using Thermax indicator
strips, it was established that the substrate temperature
in DCMS and HIPIMS experiments did not exceed
100
°
C. The elemental concentration–depth profiles
were obtained using the method of glowdischarge
optical emission spectroscopy (GDOES) using a Pro
filer 2 instrument (Horiba JobinYvon, France). In
addition, the structure of coating was studied by scan
ning electron microscopy (SEM) on a Hitachi S4800
instrument operating at an accelerating voltage of 15
kV. The Xray diffraction (XRD) analysis was per
formed on Difray 401 NP (Scientific Instruments,
Russia) and Thermo Scientific diffractometers using
Cr
K
α
and Co
K
α
radiations. The values of hardness,
elastic modulus, and elastic recovery of coatings were
determined using a Nano Hardness Tester (CSM
Instruments) at an indenter load of 4 mN.
According to the data of GDOES, chromium and
boron were uniformly distributed in depth of coatings.
Averaged concentrations of these elements in the coat
ings and their thicknesses determined from elemental
depth profiles are presented in the table for both
DCMS and HIPIMS regimes. As can be seen from
these data, the B/Cr atomic ratio decreases from 1.78
to 1.44 with increasing specific power density. In
addition to the main elements, the coatings also con
tained small amounts of oxygen (1–1.5 at %) and
carbon (0.2–0.6 at %), which is related to penetration
of these elements from the working gas and SHS target
material. The coating thickness in the DCMS regime
was 3.8
μ
m, while that in the HIPIMS (1 Pa) regime
was much smaller (0.4
μ
m) and only reached about 3.3
μ
m at a gas pressure increased to 1.7 Pa. The coating
growth rate estimated from data on their thicknesses
was 14, 1.5, and 12 nm/min for the DCMS, HIPIMS
(1 Pa), and HIPIMS (1.7 Pa) regimes, respectively.
The XRD patterns of DCMS coatings (Fig. 2a) dis
played intense WC peaks from the substrate and a peak
at 2
θ
= 54.4
°
, which corresponds to the (101) reflec
tion from a hexagonal CrB
2
phase (JCPDS card 89
3533). The calculated values of the interplanar dis
tance and average crystallite size were 0.19592 and
9 nm, respectively. In the case of HIPIMS coatings,
the reflections from WC were supplemented by
strongly broadened lines at 2
θ
= 51.7
°
and 53.7
°
(Fig. 2b). The first of these peaks is probably related to
the formation of phases of the Cr
2
B, CrB, and Cr
3
B
4
types (JCPDS cards 894876, 893587, and 760188),
while the second peak is due to the presence of CrB
2
phase. Thus, the XRD data agree well with the results
of chemical analyses, according to which the passage
from DCMS to HIPIMS regime leads to an increase
in the Cr/B ratio. This effect can be explained by the
preferential sputtering of boron from deposited coat
ings under the action of ion bombardment. In the case
of HIPIMS coatings, the intensification of ion bom
bardment leads to a decrease in the CrB
2
phase crys
tallite size (6.5 nm) as compared to that in DCMS
coatings. SEM examination of the crosssection frac
ture of samples showed that Cr–B coatings obtained
in the DCMS regime possess a wellpronounced
columnar structure (Figs. 3a and 3b) with an average
−
20
−
15
−
10
−
5
0
5
−
100 0 100 200 300 400 500
Time,
μ
s
Discharge current, A
1 Pa
1.7 Pa
Fig. 1.
Waveforms of current in the HIPIMS regimes at
a gas pressure of 1 and 1.7 Pa.
Deposition parameters and characteristics of coatings
Sputtering regime Specific
power,
kW/cm
2
h
,
μ
mComposition, at %
H
, GPa
E
, GPa
H
/
EH
3
/
E
2
,
GPa
W
, %
Cr B
DCMS 1 Pa 0.047 3.8 36 64 28 330 0.085 0.202 51
HIPIMS 1 Pa 0.6* 0.4 39 61 21 260 0.081 0.137 47
HIPIMS 1.7 Pa 1.6* 3.3 41 59 33 300 0.110 0.399 58
* Per pulse.

616
TECHNICAL PHYSICS LETTERS Vol. 40 No. 7 2014
KIRYUKHANTSEVKORNEEV et al.
diameter of columnar grains about 70 nm, which
implies that each of these crystallites consists of
smaller ones with dimensions below 10 nm. HIPIMS
coatings possessed a structure of higher density and
contained no columnar elements (Fig. 3c). It should
be also noted that a columnar structure is unfavorable
from the standpoint of mechanical properties, wear
resistance, and corrosion resistance of coatings.
The results of measurements of the hardness (
H
),
elastic modulus (
E
), elastic recovery (
W
), index of
plasticity (
H
/
E
), and resistance to plastic deformation
(
H
3
/
E
2
) are presented in the table. In DCMS coatings,
the hardness, elastic modulus, and elastic recovery
reached values of
H
= 28 GPa,
E
= 330 GPa, and
W
=
51%, respectively. After the sputtering at 1 Pa in the
HIPIMS regime, the
H
and
E
values decreased by 20–
25%, which was probably related to a small thickness
of these coatings and the influence of a hardalloy sub
strate with
H
~ 20 GPa. The HIPIMS coatings depos
ited at 1.7 Pa had
H
= 33 GPa,
E
= 300 GPa, and
W
=
58%. Note that the passage to HIPIMS not only
increases the hardness of coatings, but also improves
their elasticplastic characteristics, including the
index of plasticity (
H
/
E
= 0.110) and resistance to
plastic deformation (
H
3
/
E
2
= 0.399), which also
directly influence the wear resistance and local defor
mation of the material [10]. The observed change in
the properties of coatings on the passage to HIPIMS
8300
8800
9300
9800
10300
10800
11300
11800
Intensity, a. u.
40 42 44 46 48 50 52 54 56 58 60
2
θ
, deg
6000
6500
40 42 44 46 48 50 52 54 56 58 60
2
θ
, deg
7000
7500
8000
8500
9000
(a)
(b)
Fig. 2.
XRD patterns of coatings obtained in the (a)
DCMS and (b) HIPIMS (1.7 Pa) regimes.
3
µ
m3
µ
m
(b)
(a) (c)
200 nm
Fig. 3.
SEM images of the crosssectio n fracture of C r–B coatings d eposited in th e (a, b) DCMS and (c) H IPIMS (1.7 Pa) regimes .

TECHNICAL PHYSICS LETTERS Vol. 40 No. 7 2014
COMPARATIVE ANALYSIS OF Cr–B COATINGS 617
may be related to an increase both in the density of
coatings and in the level of internal stresses. In order to
elucidate the main factor, the characteristics of coat
ings obtained in the DCMS and HIPIMS (1.7 Pa)
regimes were repeatedly measured after keeping the
samples for 6 months under normal conditions, during
which the internal stresses must be partly or com
pletely relaxed. The coatings of both types exhibited
close values of
H
= 24–25 GPa, while the values of
W
and
E
remained unchanged. Therefore, this experi
ment showed that the main contribution to increasing
H
is related to the increased level of internal stresses
achieved due the intensification of ion bombardment
in the HIPIMS regime.
Acknowledgments.
The authors are grateful to
N.V. Shvyndina and A. Kozlov for their help in struc
tural investigations. The authors gratefully acknowl
edge the financial support of the Ministry of Educa
tion and Science of the Russian Federation in the
framework of Increase Competitiveness Program of
NUST “MISiS” (no. K22014012).
REFERENCES
1. U. Helmersson, M. Lattemann, J. Bohlmark, A. P. Ehi
asarian, and J. T. Gudmundsson, Thin Solid Films
513
,
1 (2006).
2. M. Lattemann, A. P. Ehiasarian, J. Bohlmark, P. A. O. Pers
son, and U. Helmersson, Surf. Coat. Technol.
200
,
6495 (2006).
3. E. Lewin, D. Loch, A. Montagne, A. P. Ehiasarian, and
J. Patscheider, Surf. Coat. Technol.
232
, 680 (2013).
4. A. P. Ehiasarian, P. Eh. Hovsepian, L. Hultman, and
U. Helmersson, Thin Solid Films
457
, 270 (2004).
5. A. V. Kozyrev, N. S. Sochugov, K. V. Oskomov, A. N. Za
kharov, and A. N. Odivanova, Plasma Phys. Rep.
37
,
621 (2011).
6. V. Sittinger, O. Lenck, M. Vergöhl, B. Szyszka, and
G. Brauer, Thin Solid Films
548
, 18 (2013).
7. D. Horwat and A. Anders, J. Appl. Phys.
108
, 123306
(2010).
8. V. V. Kurbatkina, E. A. Levashov, E. I. Patsera, N. A. Ko
chetov, and A. S. Rogachev, Int. J. SHS
17
, 189 (2008).
9. J. Alami, J. T. Gudmundsson, J. Bohlmark, J. Birch,
and U. Helmersson, Plasma Sources Sci. Technol.
14
,
525 (2005).
10. D. V. Shtansky, S. A. Kulinich, E. A. Levashov, A. N. She
veiko, F. V. Kiriuhancev, and J. J. Moore, Thin Solid
Films
420–421
, 330 (2002).
Translated by P. Pozdeev