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Applied Surface Science
j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc
icrostructure and surface properties of chromium-dopediamond-like carbon thin films fabricated by high power pulsedagnetron sputtering
hongzhen Wua,b, Xiubo Tiana,∗, Gang Guia, Chunzhi Gonga, Shiqin Yanga, Paul K. Chuc
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, ChinaSchool of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen, ChinaDepartment of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
r t i c l e i n f o
rticle history:eceived 9 October 2012eceived in revised form 20 February 2013ccepted 22 February 2013vailable online xxx
ACS:8.55.Jk
a b s t r a c t
High power pulsed magnetron sputtering (HPPMS) has attracted much interest due to the large plasmadensity and high ionization rate of sputtered materials. It is expected to produce a highly ionized C fluxfrom a graphite target but unfortunately, the ionization rate of carbon is still very small and the dischargeon a solid carbon target is unstable as well. In this work, a stable discharged chromium target is usedin the preparation of chromium-doped diamond-like carbon (Cr-DLC) films in HPPMS in reactive C2H2
gas, but the unstable graphite. The chromium concentration in the Cr-DLC films is limited by surfacepoisoning due to reactive gas. Less than 2% of Cr is incorporated into the DLC films at C2H2 flow rate of
1.15.Cd
eywords:igh power pulsed magnetron sputteringr-DLCicrostructure
urface properties
5 sccm or higher. However, as a result of the high ionization rate of the reactive gas in HPPMS, intense ionbombardment of the substrate is realized. The films show a smooth surface and a dense structure witha large sp3 concentration. As the C2H2 flow increase, the sp3 fraction increase and the sp3 to sp2 ratioincrease to 0.75 at a C2H2 flow rate of 10 sccm. Compared to the substrate, the Cr-DLC films have lowerfriction and exhibit excellent corrosion resistance.
DLC films have widespread applications as protective coatingsn cutting tools, molds, optical windows, magnetic storage disks,nd micro-electromechanical devices (MEMs) due to their highechanical hardness, chemical inertness, and wide band gap. [1–3]ost of the key properties of DLC stem from the sp3 componenthich can be enhanced by ion bombardment. [2,4] Thus, a highly
onized and energetic carbon flux should bode well for the deposi-ion of DLC films with the desirable properties.
High power pulsed magnetron sputtering is considered a highlyonized physical vapor deposition technique [5,6] which can pro-uce highly ionized fluxes similar to those produced by arcvaporation sources but without excessive heating and droplet for-ation. [7,8] Therefore, HPPMS has been utilized to get highly
onized carbon fluxes from pure graphite target. For example,
Please cite this article in press as: Z. Wu, et al., Microstructure and surffabricated by high power pulsed magnetron sputtering, Appl. Surf. Sci. (20
ünz, et al. [9] fabricated several kinds of DLC films using a devicequipped with the HPPMS cathode. Schmidt, et al. [10] and Lat-emann, et al. [11] also tried to prepare DLC films by HPPMS, but
unfortunately, the ionization rate of carbon from a graphite cathodewas very low and the discharge on the C cathode was also unsta-ble. [12] In fact, carbon contained gases such as CH4 and C2H2 canbe ionized much more easily than a graphite cathode in HPPMS.[13] Besides, HPPMS can produce an ion flux with significant largeramounts of ionized film-forming species in a reactive gas atmo-sphere. [14] Therefore, carbon containing gases may be a goodchoice in the preparation of DLC by HPPMS.
To achieve a steady discharge containing excited carbon species,a metal magnetron target, which is easily ignited, ought to beemployed and this will result in metal doped DLC films. Therehave been many reports stating that the adhesion and otherproperties of DLC can be improved by doping with metallic ornonmetallic elements, such as Ti, [15,16] W, [17,18] Cu, [19]Cr, [20,21] C, [22] N, [23] Ar, [24] and so on, and among theseelements, chromium shows the most intense and steady dis-charge in HPPMS configuration. [12] In this paper, a Cr cathodeis employed to stabilize the discharge in a mixed atmosphereof Ar and C2H2 to deposit DLC films by HPPMS. By controlling
ace properties of chromium-doped diamond-like carbon thin films13), http://dx.doi.org/10.1016/j.apsusc.2013.02.104
the C2H2 flow rate, only a small content of Cr is introducedto the DLC films. The surface micrography, microstructure andother surface properties of the Cr-doped DLC (Cr-DLC) films areinvestigated.
2 Z. Wu et al. / Applied Surface Science xxx (2013) xxx– xxx
2
apptotnp(ausewibsdunwp
cewHamfi
-200 -100 0 100 200 300 400-1000
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Targ
et voltage / V
-20
0
20
40
60
80T
arg
et c
urre
nt / A
TI
Fig. 1. Schematic diagram of the HPPMS setup.
. Experimental details
The experiments were performed in a vacuum chamber with diameter of 40 cm and height of 40 cm evacuated by a molecularump to a base pressure of 3 × 10−3 Pa. The carrier gas (Ar, 99.9997%ure) and reactive gas (C2H2, 99.998% pure) were introducedhrough a mass flow meter. Fig. 1 shows the schematic diagramf the HPPMS equipment. A Cr target (50 mm in diameter, 6 mm inhickness, and 99.9% pure) was mounted on an unbalanced mag-etron cathode. The magnetron cathode was driven by a hybridulsed power supply developed in our laboratory. [25] Silicon1 0 0) and SU304 stainless steel samples (30 × 30 mm2) were useds substrates. Prior to loading into the chamber, the substrates wereltrasonically cleaned in ethanol and acetone for 20 min. The sub-trates were placed at a distance of 12 cm from the target and noxternal heating was applied during the process. Plasma etchingas performed using a high-voltage self-excited glow discharge
n argon. Afterwards, a Cr layer was deposited on the substratesy HPPMS in argon to increase the adhesion between the film andubstrate. C2H2 was gradually introduced into the chamber to pro-uce the Cr/CrCx/Cr-DLC multilayered structure for good adhesionntil the required ratio of Ar to C2H2 was reached. The thick-esses of inter Cr layer, gradient CrCx layer, and outer Cr-DLC layerere 194 nm, 130 nm and 1.74 �m, respectively. The importantrocessing parameters are listed in Table 1.
A digital oscilloscope was used to monitor the target voltage andurrent. The microstructure of the films was examined by scanninglectron microscopy (SEM, Hitachi S4800). Chemical informationas obtained on a micro Raman scattering instrument (LabRAM
Please cite this article in press as: Z. Wu, et al., Microstructure and surffabricated by high power pulsed magnetron sputtering, Appl. Surf. Sci. (20
R800, HORIBA Jobin Yvon) between 300 cm−1 and 3000 cm−1
nd an X-ray photoelectron spectroscope (XPS, PHI-5802) withonochromatic Al K� radiation. The phase composition of the
lms was determined by X-ray diffraction (XRD, D8 ANVANCE) in
able 1nstrumental parameters of the deposition of Cr-DLC films by HPPMS using different Ar/C
Step Process Parameters
1 Plasma etching Ultimate vacuum = 1.0 × 10−3 Pa, High voltage pupressure = 1.0 Pa, 20 min
2 Cr interlayer HPPMS pulse = 900 V, Frequency = 50 Hz, width = 2Bias = −100 V
3 Cr-DLC film HPPMS pulse = 900 V, Frequency = 50 Hz, width = 2Working pressure = 0.5 Pa, T = 50 min, Bias = −100
Time / s
Fig. 2. I–V curves of Cr target in the HPPMS modes.
the Bragg-Brentano geometry. The wear resistance of the sampleswas evaluated by a home-made ball-on-disk tester under ambi-ent conditions (relative humidity of 25 ± 1 RH% and temperatureof 20 ± 1 ◦C) by sliding against a � 6 mm GCr15 ball at a load of100 g, speed of 50 r/min, and a wear radius of 2 mm. The corro-sion resistance of the films was investigated by potentiodynamicpolarization tests in a 3% NaCl solution using a CHI604C system.
3. Results and discussion
Fig. 2 shows the I–V curves of the Cr target in the HPPMS mode.After the onset of the pulse, the current of the target increasesrapidly to about 50 A at about 100 �s after the initial current riseand then decreases slowly due to the gas rarefaction effect. [26]After the pulse, the discharge current drops to zero within 20 �s.The discharge is stable and reproducible in the experiments.
The Cr-DLC films have a bright black color. Fig. 3(a) and (b) depictthe surface and cross-sectional micrographs of the Cr-DLC filmsproduced at Ar to C2H2 ratio of 10 to 10. The film surface is dense,uniform, and free of holes or micro-particles. The cross-sectionalSi/Cr/CrCx/Cr-DLC structure displayed in Fig. 2(b) suggests effectiverelease of residual stress and good adhesion between the film andsubstrate. [15,19,20,27]
As described in Section I, the Cr content in the as-depositedDLC films is controlled by introducing C2H2 which poisoned thechromium target. Fig. 4 shows the composition of the top surfaceof Cr-DLC films prepared by HPPMS with different Ar/C2H2 ratiosdetermined by XPS. The Cr concentrations decrease with increasingC2H2 flow due to the surface reaction on the Cr target in the reac-tive atmosphere. The C and Cr concentrations in the films changeslightly when the C2H2 flow is changed to 5 sccm or larger butdeceased to 1.72% at a C2H2 flow rate of 10 sccm. Besides, thereis about 18% oxygen in the films as a result of contamination fromthe residual vacuum.
ace properties of chromium-doped diamond-like carbon thin films13), http://dx.doi.org/10.1016/j.apsusc.2013.02.104
Raman scattering is a popular and effective tool to characterizethe carbon bonding in DLC films. Fig. 5 depicts the Raman spec-tra of the Cr-DLC films as a function of Ar to C2H2 gas flow ratiosbetween 1000 and 1800 cm−1. All the spectra exhibit asymmetric
2H2 ratios.
lse = −10 kV, Frequency = 50 Hz, width = 200 �s, Ar flow rate = 10 sccm, Working
00 �s, DC = 0.2 A, Ar flow rate = 10 sccm, Working pressure = 0.5 Pa, T = 5 min,
00 �s, DC = 0.2 A, Ar flow rate = 10 sccm, C2H2 flow rate = 2.5, 5, 7.5, 10 sccm, V
Z. Wu et al. / Applied Surface Science xxx (2013) xxx– xxx 3
Fa
dapto
v
Fd
800 1000 1200 1400 1600 1800
0
600
1200
1800
2400
3000
G-band
Ar/C2H
2=10/2.5
Ar/C2H
2=10 /5.0
Ar/C2H
2=10 /7.5
Ar/C2H
2=10/10 .0
Inte
nsity /a
rb.u
nit
Raman shift /cm-1
D-band
bond, and 285.5 eV for sp3. [33,34] The relative contents of the3 2
ig. 3. Surface and cross-sectional micrographs of Cr-DLC films prepared by HPPMSt Ar/C2H2 = 10/10.
ispersion indicative of DLC. The G peak at around 1580–1600 cm−1
nd D peak at around 1350 cm−1 are usually assigned to zone centerhonons of the E2g symmetry and K-point phonons of A1g symme-
Please cite this article in press as: Z. Wu, et al., Microstructure and surffabricated by high power pulsed magnetron sputtering, Appl. Surf. Sci. (20
ry, respectively. [28,29] With increasing C2H2 flow, the intensitiesf the Raman peaks are enhanced obviously.
To investigate the film properties, the Raman spectra are decon-oluted into two Gaussian peaks denoted as G and D. [30] Fig. 6
10/2.5 10/5.0 10/7.5 10/10
0
20
40
60
80
100
Conte
nts
/ a
t%
Ar/C2H
2
C
Cr
O
ig. 4. Composition of the top surface of Cr-DLC films prepared by HPPMS usingifferent Ar/C2H2 ratios.
Fig. 5. Raman spectra of Cr-DLC films produced by HPPMS using different Ar/C2H2
ratios.
shows the relative intensity of the D-band to the G-band togetherwith the locations of D peak and G peak in the Raman spectraof the Cr-DLC films. For the film deposited at a C2H2 flow rateof 2.5 sccm, the G peak is centered at 1558.4 cm−1 and shifts tosmaller wave numbers in a small range when the C2H2 flow rateis increased from 2.5 to 10 sccm. The G peak shifts considerablyfrom 1580 cm−1 in pure graphite to 1550 cm−1 in DLC, and thelocations of the G peak depends on the sp2 content in the DLCfilms. [31] Therefore, the smaller wave numbers of the G peakand shift suggest that the sp3 content in the DLC films is largeand it increases with larger C2H2 flow rates. The ratio of the areaunder the D-peak to that of the G-peak is small and it decreaseswith the increasing C2H2 flow rates, thus indicating a larger sp3
component. [32]Fig. 7 displays the high-resolution C1s XPS spectra of Cr-DLC
films produced by HPPMS using different Ar/C2H2 ratios. As theC2H2 flow rates go up, the C1s peak shifts from 284.47 eV toward284.56 eV implying increased sp3 content. All the spectra showbinding energies of 282.8 eV for the Cr3C2 bond, 284.15 eV for sp2
ace properties of chromium-doped diamond-like carbon thin films13), http://dx.doi.org/10.1016/j.apsusc.2013.02.104
three types of bonds and sp /sp ratio determined by integratingthe related peak intensity in the C1s spectra are shown in Fig. 8.
10/2.5 10 /5.0 10/ 7.5 10 /10
1.5
2.0
2.5
3.0
3.5
137 5
138 0
138 5
139 0
155 2
155 6
156 0
(c)
I D/I
G
(a)
(b)
D p
ostio
n
Ar/C2H
2
G p
ostio
n
Fig. 6. (a) D peak, (b) G peak and (c) Relative intensity ID/IG according to the Ramanspectra acquired from the Cr-DLC films produced by HPPMS using different Ar/C2H2
4 Z. Wu et al. / Applied Surface Science xxx (2013) xxx– xxx
288 286 284 282 280
10/10
Binding energy /eV
10/7.5
10/5
10/2.5
Cr3C
2
sp3
sp2
Inte
nsity /a.u
.C1s
Fr
Aflror
irt[sHiCaCCa
FiA
10 20 30 40 50 60 70 80
Si(40 0)
Cr3C
2(215 )
Cr3C
2(204)
Cr2O
5(312)
10/10
10/7.5
10/5Inte
nsity /a
.u.
2 /d eg.
10/2.5
ig. 7. XPS spectra of Cr-DLC films produced by HPPMS using different Ar/C2H2
atios.
small amount of Cr3C2 exists in the DLC film prepared at a C2H2ow rate of 2.5 sccm but almost disappears when the C2H2 flowate is larger than 5 sccm. With increasing C2H2 flow, the fractionf sp3 increases and the sp3/sp2 ratio is up to 0.75 at a C2H2 flowate of 10 sccm.
The large sp3 fraction in the Cr-DLC films stem from the highlyonized plasma in HPPMS and energetic ion bombardment. Withegard to the latter, various mechanisms such as preferential sput-ering, [35] shock wave effect, [36] subplantation, [37] and so on,38,39] synergistic enhances the formation of the sp3 bonds. Fig. 9hows the XRD spectra acquired from the Cr-DLC films prepared byPPMS using different Ar/C2H2 ratios. In addition to the peak aris-
ng from the Si substrate, several weak peaks such as Cr3C2 (2 0 4),r3C2 (2 1 5) and Cr2O5 (3 1 2) can be observed [40,41] and they
Please cite this article in press as: Z. Wu, et al., Microstructure and surffabricated by high power pulsed magnetron sputtering, Appl. Surf. Sci. (20
rise from carbonization and oxidation of chromium in the films.hromium oxide exists in the chemical form of Cr2O5 instead ofr2O3 [42] possibly due to the intense discharge, high ion energy,nd local high temperature during HPPMS. [41,43] As the C2H2 flow
10/2. 5 10/5.0 10/7. 5 10/100.4
0.5
0.6
0.7
0.8
sp3/sp
2
Cr3C
2
sp3
Ar/C2H
2
Ratio o
f sp
3 t
o s
p2
sp2
0
20
40
60
80
100
Fra
ctio
n o
f the p
eaks in
XP
S
ig. 8. Relative contents of sp2, sp3, and Cr3C2 bonds and ratios of sp3 to sp2 accord-ng to XPS spectra acquired from the Cr-DLC films prepared by HPPMS using differentr/C2H2 ratios.
Fig. 9. XRD of Cr-DLC films prepared by HPPMS using different Ar/C2H2 ratios.
rate is increased, the intensity of the carbide peaks decrease andalmost the peaks also vanish at C2H2 flow rates higher than 5 sccm.This is consistent with the XPS results shown in Fig. 7. No purecarbon peak can be observed, indicating that nearly the carbon isin the amorphous state in the films. [44] Furthermore, the oxidedetected by XRD may be due to the small oxidation potential of Crand surface contamination.
Fig. 10 shows the hardness of the Cr-DLC films produced byHPPMS using different Ar/C2H2 ratios. The hardness of the Cr-DLC films increased to 26 GPa as the Ar/C2H2 flow ratios changefrom 10/2.5 to 10/10. The hardness is about 5 times that of thestainless steel substrate. The wear resistance of the films is eval-uated on a ball-on-disk tester. The friction coefficients and thewear traces acquired from the film produced using an Ar/C2H2ratio of 10/10 and substrate (for comparison) are shown in Fig. 11.The friction coefficient of the film is quite small and about 0.1throughout the 9-hour test. Compared to SU304 stainless steel, thewear trace of the film is narrow and shallow. The average frictioncoefficients of the samples are listed in Table 2 which shows thatas the C2H2 flow rate is increased, the average friction coefficientdiminishes.
The corrosion resistance is determined by monitoring the corro-sion potentials and currents and the results are displayed in Table 2.HPPMS is expected to yield dense and smooth films with improvedcorrosion resistance. [45] Compared to the SU304 stainless steelsubstrate, the coated samples exhibit higher corrosion potentialsand smaller corrosion currents and so excellent corrosion resis-tance is revealed. In fact, the corrosion resistance improves with
ace properties of chromium-doped diamond-like carbon thin films13), http://dx.doi.org/10.1016/j.apsusc.2013.02.104
increasing the C2H2 flow.
0 100 200 300 400 500 600 700
0
5
10
15
20
25
30
Ha
rdn
ess(G
Pa
)
Displacement into surface (nm)
Con tro l
10 /2.5
10 /5.0
10 /7.5
10 /10
Fig. 10. Micro-hardness of Cr-DLC films prepared by HPPMS using different Ar/C2H2
Cr-doped DLC films are prepared by HPPMS using a Cr targetn mixed atmosphere of Ar and C2H2. The Cr-DLC films have amooth surface and dense structure with large sp3 to sp2 ratio dueo high ionization rate of the reaction gas in HPPMS and intenseon bombardment of the substrate. Gradual interfacial transition isbserved from the cross-sectional micrographs and Cr3C2 is formedy adjusting the C2H2 flow rate which in turn dictates the poison-
ng effects on the Cr target. The DLC films are doped with 1.75% Crtoms in the top surface and as the C2H2 flow rate is increase, thep3 fraction goes up. Compared to the substrate, the coated sam-les exhibited a smaller friction coefficients and better corrosionesistance.
cknowledgments
This work was jointly financially supported by Natural Scienceoundation of China (No. 10905013, No. 10975041), State Key Labf Advanced Welding and Joining, Harbin Institute of TechnologyNo. AWJ-M13-13), Hong Kong Research Grants Council (RGC) Gen-ral Research Funds (GRC) No. CityU 112212, and City University ofong Kong Applied Research Grants (ARG) No. 9667066.
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