Surface & Coatings Technology 445 (2022) 128749

Contents lists available at ScienceDirect

Surface & Coatings Technology

journal homepage: www.elsevier.com/locate/surfcoat

Influence of positive pulse voltages on structure, mechanical, and corrosion

inhibition characteristics of Si/DLC coatings
Saad M. Fayed a, b, Haodong Wu a, Dongxu Chen a, *, Shengli Li a, *, Yanwen Zhou a,
Hongbin Wang a, M.M. Sadawy b
a
School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China
b
Al-Azhar University, Faculty of Engineering, Mining and Pet. Dept., Nasr City 11371, Cairo, Egypt

A R T I C L E I N F O A B S T R A C T

Keywords: In this study, Si/DLC films were effectively deposited on 2024-Al alloy using a plasma-enhanced chemical vapor
Plasma enhanced chemical vapor deposition method at different pulse voltages. Scanning electron microscopy, X-ray diffraction spectroscopy, 2D optical
Si/DLC coatings profilometer, Raman spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectros­
Pulse voltage
copy were used to characterize the structure of the DLC coatings. The adhesion strength was performed using
Adhesion strength
Scratch Tester. Further, the Nano Indenter G200 apparatus was used to measure the hardness and Young's
Mechanical properties
Corrosion performance modulus of coatings. The Ball-on-Disk Tester was used to measure the friction coefficient of the specimens. A
2024-Al alloy Potentiostat/Galvanostat electrochemical workstation was used to examine the corrosion performance and
passive stability of DLC coatings in 3.5 wt% NaCl solution. The findings demonstrated that the thickness of the
coatings increased with increasing the pulse voltage to 1800 V, causing an improvement in mechanical properties
and corrosion resistance. This is due to an increase in the amount of sp3 hybrid in the coating, which reduces the
electrical conductivity of DLC and limits electron transfer and electrical charge exchange at the coating surface.
However, the corrosion rate of the coatings increased when the pulse voltage increased to 2200 V.

1. Introduction deposition method [11]. PECVD is regarded as a promising approach for

depositing Si/DLC on Al substrates due to working at low temperatures;
2XXX series aluminum alloys have superior mechanical properties, thus, it does not affect the microstructure of the substrate [10]. Unfor­
and for this reason, it is widely used in the aerospace, automotive, tunately, the thinness and imperfections existing in the films act as
offshore application, and machinery industries [1,2]. However, these immediate channels between the film and the substrate, allowing
alloys are susceptible to corrosion attacks in neutral and highly moisture, oxygen, and corrosive species to infiltrate the substrate and
conductive environments due to microstructure heterogeneity [3,4]. cause substrate deterioration [10,12]. In addition, mechanical proper­
Diamond-like carbon (DLC) coatings are one of the most efficient ties and thermal expansion coefficient mismatch among the substrate
surface treatments for enhancing the corrosion resistance of the Al al­ and coating. All these reasons are considered the main trouble of lower
loys. DLC is an amorphous carbon film with a network structure that anticorrosion resistance and limits the usage of the DLC films in
includes a high proportion of sp3 bonds (diamond structure) and sp2 aggressive environments [13].
characters (graphite structure) [5]. Consequently, DLC coatings have Lately, many attempts have indicated that incorporate different
operated well in different industries owing to their extraordinary me­ metallic or nonmetallic constituents, like Ti [14], Cr [15], W [16], Cu
chanical characteristics, tribological capabilities, chemical steadiness, [17], Si [18], N [19], F [20], etc., to decrease the residual stress in the
and anti-corrosion performance [6]. Numerous methods have been DLC coatings and improve the films corrosion behavior. Deng et al. [21]
utilized to create DLC coating such as filtered cathodic vacuum arc noticed that inserting the Si component into DLC film lowered corrosion
(FCVA) [7], pulsed laser deposition (PLD) [8], magnetron sputtering current density and increased electrode potential. Cui et al. [22] utilized
(MS) [9], plasma-enhanced chemical vapor deposition (PECVD) [10]. It the PECVD method to create a single/multi-layer DLC coating. They
is worth noting that the DLC film structure is highly reliant on the indicated that the multi-layer coating had a lesser corrosion current than

* Corresponding authors.
E-mail addresses: dxchen11b@alum.imr.ac.cn (D. Chen), ustl_Li@yeah.net (S. Li).

https://doi.org/10.1016/j.surfcoat.2022.128749
Received 4 May 2022; Received in revised form 19 July 2022; Accepted 21 July 2022
Available online 25 July 2022
0257-8972/© 2022 Elsevier B.V. All rights reserved.
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

the single DLC layer owing to fewer defects and more interfaces in 2.3. Characterization of the coatings
coatings. Ye et al. [23] used unbalanced magnetron sputtering to create
thick and thin DLC films and informed that the thick DLC membrane had The micrographs and cross-section of the DLC coatings were assessed
a higher impedance modulus than the thin film. using a scanning electron microscope (SEM) model (Quanta FEG-250).
Recently, most of the current studies have focused on adjusting An energy dispersive spectrometer (EDS) connected to an SEM was
manufacturing parameters to control the structural defects and charac­ used to identify the chemical constituents of the test specimens. The
teristics of DLC coatings. Numerous scholars [10,24–27] have examined structure of the DLC coating was investigated using an X-ray diffraction
the effect of negative bias voltage [10], gas flow [24], temperature [25], spectrometer (XRD). The diffraction pattern was surveyed in the range
deposition time [26], and pulse voltage [27] on the characteristics of (2ϴ) from 20◦ to 90◦ . The thickness of the Si/DLC coatings was deter­
DLC coatings. They found that altering the manufacturing factors mined utilizing a two-dimensional optical profilometer type (Alpha-Step
changes the ratio of sp2 and sp3 fraction in a network structure, fewer D-500), attached with a pen of 25 μm radius. The vertical movements of
coating defects, and improved bonding configuration within the DLC the pen are measured by a digital transducer and then recorded those
coating. Zhang et al. [27] used a chemical vapor deposition approach at values in the memory for later plot manipulation. Therefore, the dif­
various pulse voltages to create DLC films on the surface of 13Cr super ference in the thickness between the substrate and the coating was
martensitic stainless steel. They found that raising the pulse voltage determined. Furthermore, a Raman spectrometer (XploRA PLUS) was
increases the corrosion resistance of the DLC coatings in high Cl− utilized to identify the properties of DLC coatings in the 500 to 2500
environment. cm− 1 spectral range. The quantitative analysis and carbon‑carbon
In our previous work [10], the main purpose was to study the effect bonding configurations of coatings were determined using X-ray
of DC voltage applied to the sample on the bonding configuration, photoelectron spectroscopy (XPS, K-alpha system). The functional group
microstructure, and properties of the DLC thin coatings. But in the of Si/DLC coatings spectra was acquired using a Fourier transform
present work, what we study is the mechanism of the effect of positive infrared spectrometer (FTIR, Cary 630) linked to a DTGS detector and
pulse voltages on the thin coating. The two kinds of voltage are applied monitored using MicroLab software.
in different positions, in different forms and in different sizes (DC is 100
V, 200 V, and 300 V; AC pulse bias is 1400 V–2200 V). The influence
mechanisms of these two kinds of voltage on plasma ionization rate, 2.4. Mechanical properties
plasma energy and plasma growth during the PECVD process are
different. Therefore, the mechanism of the effect of different positive The adhesion strength between the Si/DLC coating and the substrate
pulse voltages on the properties of Si/DLC films on aluminum alloy was determined using a scratch tester (MFT-4000) with a loading speed
surfaces needs to be studied. of 100 N/min and a maximum load of 100 N. The occasional load and
acoustic emission signals were collected and recorded throughout the
2. Materials and methodologies experiment. An optical microscope (OM) was utilized to inspect the
scratch track to determine the films' critical load. The hardness and
2.1. Materials Young's modulus of investigated specimens were assessed using a
Nanoindenter tester G200 connected with a Berkovich diamond
Disk specimens were chopped from 2024 Al ingots with a size of (Φ indenter. Indentation depths were set to <10 % of the whole coating
20 mm × 10 mm) using an electric saw. The chemical constituents of the thickness to eliminate substrate influence on the hardness. The friction
Al substrate are described in Table 1. Before treatment, all specimens coefficients of the studied specimens were evaluated utilizing a rotary
were ground with SiC abrasive paper in the range of 240 to 3000 grit and tribometer (MS-T3001) ball-on-disk tester under atmospheric condi­
polished with diamond paste according to ASTM E3-95 [28]. The tions. Experiments were performed under a normal load of 1 N and a
specimens were then washed in acetone for 20 min to eliminate any rotating speed of 200 rpm for 30 min. After the tribological tests, the
remaining contamination on the substrate surfaces. Then, the specimens worn tracks were examined for Si/DLC coatings using an optical
were withdrawn and desiccated in atmospheric conditions. microscope.

2.2. Fabrication of Si/DLC film

2.5. Electrochemical measurements
After placing specimens in the deposition chamber, the mechanical
and molecular pumps were utilized to lower the chamber vacuum inside All corrosion experiments were carried out using an electrochemical
the PECVD to 0.02 Pa. In order to eliminate the residual air, the room workstation (M273, PerkinElmer). The corrosion cell has a reference
was fed with 100 sccm of Ar gases for 10 min; afterward, 50 sccm of Ar electrode (Ag/AgCl), a working electrode (specimens), and a counter
was introduced also for 20 min. Firstly, the bare surface was cleaned for electrode (Pt sheet). The corrosion techniques were carried out in a 3.5
30 min with Ar+ sputtering at 1800 V pulse voltage to remove undesired wt% NaCl environment. The test specimens were immersed in the so­
oxides and contaminants. After that, a silicon transition layer is depos­ lution for roughly 30 min to examine their open-circuit potentials (Eoc).
ited by filling the PECVD chamber with tetramethylsilane and argon for Electrochemical impedance spectroscopy (EIS) trials were performed at
40 min. The substrate temperature was fixed to 100 ◦ C during the 30 min Eoc in the frequency region of 105 to 0.01 Hz with an amplitude
deposition procedure. After that, a gradient layer of (Si/DLC) was in­ of 10 mV peak-to-peak. Potentiodynamic polarization was performed at
tegrated using tetramethylsilane and acetylene (C2H2) gases at the same a scanning rate of 0.16 mV/s. The electrochemical corrosion data ob­
flow rate of 20 sccm. Finally, the top layer was deposited from DLC using tained from potentiodynamic polarization were calculated by extrapo­
C2H2 gas for 120 min. The manufacturing factors for Si/DLC films are lating Tafel slope branches according to the Stern–Geary equation.
demonstrated in Table 2. The deposition was performed at various pulse Furthermore, the corrosion rate (CR) in mm y− 1 can be determined
voltages (1400–2200 V) with a constant frequency of 600 Hz. utilizing Eq. (1) [29]:

Table 1
Chemical composition of 2024-T3 aluminum alloy in (wt%).
Elementals Cu Mg Fe Mn Si Cr Ti Zn Al

wt% 3.81 1.4 0.4 0.4 ˂0.06 ˂0.05 ˂0.05 ˂0.14 Balance

2
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

Table 2
The operational parameters of Si/DLC coating deposition.
Process Time, (min) Temperature, (◦ C) Pulse width, (μs) Pulse frequency, (Hz) Pressure, (Pa) Gases flow rate, (sccm)

Ar SiH4 C2H2

Vacuum chamber – 110 – – – – – –

Cleaning stage 30 100 20 800 0.45 40 – –
Deposition multilayer films Si-interlayer 40 100 20 600 2 20 40 –
Si-DLC 20 100 20 600 2 20 20 20
Pure DLC film 120 100 20 600 2 20 – 60

Micorr increments from 1400 V to 2200 V, the carbon level augments from
CR = 0.00327 (1)
ρ 85.62 at. % to around 91.49 at. %, while the Si content increases from
1.95 at. % to 2.65 at. %. Further, it is worth noting that as the pulse
where 0.00327 represents the mm/year constant, M signifies the voltage increased, the oxygen level in the coating decreased slightly
equivalent weight (g/equiv.), icorr is corrosion current density (μA/cm2), from 3.01 at. % to 0.76 at. %. This suggests reducing the pores in the DLC
and ρ is the corroding metal density in (g/cm3). Furthermore, after membranes with increasing pulse voltages.
completing the potentiodynamic polarization test, the corroded surface The phase analysis of Si/DLC coating applied at 1800 V was per­
morphologies of all specimens were examined utilizing the SEM-EDS formed using X-ray diffraction (XRD), as shown in Fig. 2. It is evident
device. The potentiostatic technique was conducted for the specimens from the XRD profile that the crystalline peaks at about 2ϴ ≈ 31.45◦ and
vs. soaking time for roughly 900 s to measure coating stability. After 34.7◦ , match the graphite (graphitic form). While the other heap peaks
that, the corroded surface of the specimens was inspected after poten­
tiostatic measurements using XPS.

Graphite
3. Results and discussion
Intensity, (Arbitrary unit)
3.1. Coating evaluation

Graphite
3.1.1. Coating structural
Fig. 1 shows Si/DLC coating specimens deposited on the 2024-Al
substrate at various pulse voltages. It is clear that all micrographs
Heap peaks
show similar morphologies. However, the pores decreased as the pulse
voltage increased to 1800 V. This may be attributed to increasing the
pulse voltage from 1400 V to 1800 V, causing the gradual increase in
atomic and molecular ion energies of Ar and C2H2 gas, establishing a
smooth surface. However, beyond 1800 V more pores are found due to a
reduction in connectivity degree of carbon network structure and
20 30 40 50 60 70 80 90
increasing hydrogen content in the film [27]. The EDS spectra and
chemical components for zones highlighted in SEM images of the DLC 2θ,(deg.)
coatings are also shown in Fig. 1. According to the findings, all coating
Fig. 2. XRD spectrum of Si/DLC coating deposited at 1800 V pulse voltage.
structures are primarily made up of Si and C. When the pulse voltage

C Elements At.% C Elements At.% Elements At.% Elements At.%

C C
EDS1 C 85.62 EDS2 C 88.27 C 92.17 C 91.49
EDS4 Al
O 3.01 Al O 2.85 EDS3 O 1.33 O 0.76
Al
CPS/eV

CPS/eV

CPS/eV
CPS/eV

Al Mg 0.33 Mg 0.37 Mg 0.14 Mg 0.17

Si Al 5.09 Si Al 6.57 Al 3.59 Al 4.93
Si
Si 1.95 Si 1.94 Si 2.77 Si 2.65
O Mg O
O Si O

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Energy, (KeV) Energy, (KeV) Energy, (KeV) Energy, (KeV)

Fig. 1. SEM-EDS micrographs of the Si/DLC coatings samples at various pulse voltages. [(a) 1400 V, (b) 1600 V, (c) 1800 V, and (d) 2200 V].

3
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

detected at 2ϴ ≈ 47.155◦ , 61.97◦ , and 67.77◦ signifies typical amor­ described in Table 3. The parameters consist of the location of G peak,
phous carbon structure. This suggests that the formed Si/DLC coating at the full width at half maximum of G peak (GFWHM), position of D peak,
1800 V shows an amorphous structure. Moreover, the results show that the full width at half maximum of D peak (DFWHM), the intensity ratio of
the α-Al peaks in the Si/DLC coating at various 2ϴ entirely disappeared D to G peak (ID/IG), and the hydrogen content (%). According to Table 3,
owing to the reflection of the X-ray beams are parallel to the coating the ID/IG ratios of Si/DLC coatings were approximately 1.62, 1.43, 1.35,
without penetrating the substrate. Thus, this justifies the absence of α-Al and 1.42 for films deposited at 1400, 1600, 1800, and 2200 V, respec­
planes in the broad spectrum of the coating [12]. tively. This suggests that as the pulse voltages increases, the value of ID/
IG for DLC coatings decreases to 1800 V; after this value, the ID/IG in­
3.1.2. Film thickness creases slightly again. The lower value of (ID/IG) for film prepared at
The coating thickness was determined utilizing a 2D profilometer 1800 V is due to a high quantity of sp3 hybrid in DLC coatings. Also, the
device. Fig. 3 depicts the thickness of the Si/DLC coatings formed at G peak position shifts to a lower wavenumber with increased pulse
various pulse voltages, including a cross-section image for film prepared voltages until it reaches 1800 V. This behavior is attributed to the silicon
at 1800 V. The thickness of the DLC film gradually increases with layer, which serves as an activator that decreases the inner stress and
increment of the pulse voltage. The cross-section image shows that the impedes the creation of the C-sp2 phase. Nevertheless, the increase in
coating consists of three layers (Si interlayer, Si/DLC, and pure DLC), GFWHM reflects an increment in carbon disorder level caused by
respectively. The thickness of the coatings ranges from (1.12 ± 0.01) to increased C-sp3 bonds in carbon coating [34]. Further, Table 3 reveals
(1.61 ± 0.1) μm when pulse voltage changes from 1400 to 2200 V, that the amount of hydrogen % decreases with increasing pulse voltage
respectively. This behavior is due to high pulse voltages improving ion until it reaches 1800 V and then increases for film 2200 V. This behavior
bombardment and ionization of sputtered Ar+ and carbon species, is attributed to increasing the hydrogen content of the Si/DLC coating.
promoting the incorporation of ions into the growing film and raising
the deposition rate [30]. 3.1.4. XPS spectrometry
Fig. 5 indicates the XPS spectrum of the Si/DLC film created at 1800
3.1.3. Raman spectroscopy V pulse voltage. The spectrum elucidates the existence of peaks of sili­
Fig. 4 represents the Raman spectrums of Si/DLC films at different con, carbon, and oxygen into the coating matrix. The peak found at
pulse voltages. Clearly, Raman graphs comprise a broad and asymmetric binding energy (B.E) ≈ 100.91 eV, suggests SiC 2p, as seen in Fig. 5(b).
peak in the range of 1000–1700 cm− 1, which is the distinctive peak of Therefore, the presence of Si–C bond in the XPS spectrum indicates a
DLC films. Using the Gaussian function, it can be deconvoluted into two high degree of adhesion between Si/DLC coating and the bare alloy
sub-peaks: D bands (1350 cm− 1) is associated with the symmetric [35]. The peak observed at B.E ≈ 285 eV belongs to the C 1s. The C1s
breathing vibration of C-sp2 bonds only in rings. While G bands (1580 peak was deconvoluted using the Gaussian function into three parts at
cm− 1) is correlated with the stretching vibrations of all pairs (C-sp2) in roughly 283.65 eV, 284.79 eV, and 286.1 eV, which are denoted to C-
aromatic rings and chains. Casiraghi et al. [31]. found that the photo­ sp2, C-sp3, and C–– O respectively, as illustrated in Fig. 5(c). This suggests
luminescence (PL) background of the visible Raman spectra was strong the primary functional groups on the surface of a-C:H coatings, as shown
for DLC containing high hydrogen (H), and the normalized PL slope, m/ in Fig. 5. Furthermore, the existing oxygen peaks in the spectrum of Si/
IG, rose exponentially with hydrogen content. The ratio m/IG of the slope DLC coatings relate to ambient pollution. Table 4 summarizes the
of the fitted linear background to the intensity of the G peak can be used chemical composition and fitted parameters derived from the XPS
empirically to calculate the bonded H content. Ferrari et al. [32] pro­ spectra of the film [36].
posed an empirical Eq. (2) to determine the H content in the film [33]:
{ ( ) } 3.1.5. Fourier transform infrared spectroscopy (FT-IR)
m
H[at.%] = 21.7 + 16.6 log [μm] (2) Fig. 6 reveals the excitation wavelength (cm− 1) vs. absorbance (a.u)
IG
for Si/DLC films at various pulse voltages. The IR absorption bands were
The Gaussian parameters acquired from Raman curves (Fig. 4) are measured in the range of 400–4000 cm− 1. The wide peak that appeared
in the region of 720–1000 cm− 1 corresponds to (Si-CH) vibrational
modes. The intensity of these peaks increases as the silicon concentra­
tion in the coating increases. Further, the observed peaks at approxi­
1.7 mately 1200 cm− 1 belong to the C–C vibration arrangement, which is
1.61 consistent with Raman spectral data (Fig. 6). The signal found at 1624
1.6 cm− 1 is attributed to the carbon double bond (C– – C) of sp2 hybridiza­
tion. Also, the wide peaks arise in the range of 2800–3000 cm− 1, indi­
cating the presence of carbon hybridization in form of sp3 bonds. This
Thickness, (µm)

1.5
1.42 implies the formation of tetrahedral carbon (a-C) configuration as well
as the embedded hydrogen in the DLC films. Further, the spectra
1.4 revealed that the peak intensity of (C–H, sp3) increases as the pulse
voltage increases to 1800 V; beyond this value, the peak intensity de­
1.28
1.3 creases due to the increase of hydrogen percentage, as discussed before
in Section 3.1.3. Table 5 describes the outcomes of stretching vibration
1.2 modes derived from FT-IR spectra [37].
1.12
3.2. Mechanical measurements
1.1

3.2.1. Adhesion strength

1.0 It is widely recognized that adhesion strength plays a vital role in
1400V 1600V 1800V 2200V mechanical and corrosion performance. The adhesion strengths were
Samples investigated in this study by combining the friction coefficient curve, the
acoustic emission spectra, and the scratch track images [38]. Fig. 7
Fig. 3. Thickness of Si/DLC coatings at different pulse voltages, including depicts the recorded friction coefficient curve, acoustic emission spectra,
cross-section image for film prepared at 1800 V. and scratch track image. The scratch images show that the scratch track

4
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

Intensity, (Arbitrary Unit)

Intensity, (Arbitrary Unit)
1400V (b) 1600V
(a) Gaussian curve fit Gaussian curve fit
G-peak G-peak
D-peak ID/IG=1.43 D-peak
ID/IG=1.62

G G
D D

500 1000 1500 2000 2500 500 1000 1500 2000 2500
-1 -1
Raman Shift, (cm ) Raman Shift, (cm )
Intensity, (Arbitrary Unit)

Intensity, (Arbitrary Unit)

1800V 2200V
(c) (d) Gaussian curve fit
Gaussian curve fit
G- peak G-peak
D-peak D-peak
ID/IG=1.42
ID/IG=1.35
G G
D
D

500 1000 1500 2000 2500 500 1000 1500 2000 2500
-1 -1
Raman Shift, (cm ) Raman Shift, (cm )
Fig. 4. Raman spectra of the Si/DLC coatings performed at different pulse voltages.

and an inflection point appears in the acoustic emission spectra as an

Table 3
indication of the critical load of the coatings. It is worth noting that the
Gaussian fitting results of Raman spectroscopy data of Si/DLC coatings at
critical loads derived from acoustic emission spectral, friction co­
various pulse voltages.
efficients graphs, and scratch track images have a high degree of con­
Samples D peak (cm− 1) G peak (cm− 1) ID/ H, %
sistency. The scratch direction obtained was from left to right for all the
IG
D peak DFWHM G peak GFWHM scratch images. Furthermore, the critical loads were defined using the
(cm− 1) (cm− 1) (cm− 1) (cm− 1) scratch profile and scratch track images. The adhesion strengths for Si/
1400 V 1356.21 355.92 1552.9 173.44 1.62 31.30 DLC coatings range from 6.5 to 22.1 N when changed pulse voltage from
1600 V 1351.41 353.62 1550.04 175.81 1.43 29.69 1400 to 2200 V. The adhesion strength of DLC coatings was about 6.5,
1800 V 1341.5 362.05 1546.31 181.05 1.35 28.73
13.36, 24.1, and 22.2 N for films deposited at 1400, 1600, 1800, and
2200 V 1349.01 363.20 1549.8 175.01 1.42 29.14
2200 V, respectively. The findings also reveal that the DLC coating
prepared at 1800 V (Fig. 7(c)) has the best adhesion strength. The results
becomes wider and deeper as the load increases. Obviously, the scratch demonstrated that plasma intensity increases as the pulse voltages in­
track is smooth at low loads, but as the load increases the coating is crease and boost the Si doping into DLC coating. As a result, the reducing
peeled from the bare substrate, and fragile spalling occurs in the DLC mismatch between the DLC film's thermal expansion coefficient and the
coating. Meanwhile, the acoustic emission spectrum suddenly increases, mechanical characteristic of the substrate decreases internal stress and

SiC 2p C1s
(a) C 1s (b)
Cumulative Fit peak
(c) C 1s
Cumulative Fit peak
Intensity, (Arbitrary Unit)
Intensity, (Arbitrary Unit)
Intensity, (Arbitrary Unit)

C-sp3
C-sp2
O1s
C-sp2 C=O

C-sp3

OKLL SiC 2p
C=O
Si 2p

0 200 400 600 800 1000 1200 96 98 100 102 104 106 108 278 280 282 284 286 288 290 292
Binding Energy, (eV) Binding Energy, (eV) Binding Energy, (eV)

Fig. 5. XPS spectra of Si/DLC coating formed at 1800 pulse voltage [(a) broad scan spectrum and (b) high-resolution XPS regions of SiC 2p, and (c) high-resolution
XPS regions of C1s].

5
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

Table 4
Chemical composition and fitted parameters derived from the XPS spectra of the film prepared at 1800 V pulse voltage.
Samples Si O C C XPS (at.%)
XPS (at.%) XPS (at.%) XPS (at.%)
Binding energy of sp2/eV Area of sp2 peak% Binding energy of sp3/eV Area of sp3 peak%

1800 V 1.51 9.43 89.06 283.65 24,083.81 284.79 24,507.2

Si-CH3 Si-CH3 C-C 1600V

Absorbance,(Arbitrary Unit)

Absorbance,(Arbitrary Unit)
C-H,sp3
C C
C C

1400V C C C-H,sp3

500 1000 1500 2000 2500 3000 3500 4000 500 1000 1500 2000 2500 3000 3500 4000
Wavenumber, (cm-1) Wavenumber, (cm-1)
Si-CH3 1800V Si-CH3 2200V
Absorbance,(Arbitrary Unit)

Absorbance,(Arbitrary Unit)

C C
C-H,sp3
C-H,sp3
C C C C

500 1000 1500 2000 2500 3000 3500 4000 500 1000 1500 2000 2500 3000 3500 4000

Wavenumber, (cm-1) Wavenumber, (cm-1)

Fig. 6. FT-IR spectra of Si/DLC coatings synthesized at different pulse voltages.

110.2, and 114.9 GPa. This signifies that the Si/DLC coating enhances
Table 5
the surface hardness of the substrate by about fourfold. The reason for
Stretching vibration peaks obtained from FT-IR spectra of Si/DLC coatings at
this is due to controlling the ion energy bombardment, which results in
different pulse voltages.
more ordered carbon structures, as discussed before in the Raman
Samples Wavenumber (cm− 1) spectrum analysis Section 3.1.3. Furthermore, increasing the pulse
Si-CH3 (C–C) alkane (C–
–C) (C–
–C)
– (C–H), sp3 stretching voltage to 1800 V increases the energy of the C+ species enough to
1400 V 740 1384 1554 2105 2850, 2917 infiltrate the subsurface film. Consequently, it increases the density of
1600 V 870 1239 – 2115 2903 the membranes and grows the creation of sp3 bonds [30]. According to
1800 V 868 – 1437 2114 2922 Damasceno et al. [42], the hardness of the membranes is reliant not only
2200 V 748 2132 2899
– –
on the sp3/sp2 proportion but also on bonding properties, notably the
hydrogenation of the sp3 matrix. On the other hand, Fig. 8 shows that the
improves film adherence [39–41]. H and E values decrease with increment of pulse voltage beyond 1800 V.
The behavior is attributed to the C–H bond being one of the most bonds
3.2.2. Surface hardness in the sp3 configuration within the hydrogen-containing DLC thin
Fig. 8(a) shows (load-displacement) graphs of Si/DLC coatings at coating. More microvoids were incorporated into the DLC thin coating
various pulse voltages. The graphs clearly indicate that the loading because of the hydrogenation process. This leads to a reduction in the
behavior remains relatively stable, with changes in peak load. Similarly, connectivity degree of the carbon network structure. As a result, the
Fig. 8(b) displays the hardness (H) and Young's modulus (E) of the compactness of the thin coating has deteriorated, and the mechanical
specimens as a function of pluses voltages. The H and E values were properties of the thin coating were decreased.
determined from nanoindentation trials, as seen in Fig. 8(a). The The hardness-to-Young's modulus ratio (H/E) is regarded as a sig­
outcome reveals that the H and E values of the substrate are approxi­ nificant characteristic for the mechanical and tribological behavior of
mately 2.27 GPa and 89.3 GPa, respectively. On the one hand, the H DLC coatings. Fig. 9 describes the H3/E3 and H/E ratios of membranes
values of Si/DLC film monotonously increase from 12.59, 13.37, and prepared at various pulse voltages. The H/E ratio gradually rises from
14.21 GPa with rising pulse voltages from 1400 V, 1600 V, and 1800 V, 0.025 with bare alloy to 0.123 with 1800 V positive pulse, then falls to
respectively. While the corresponding E values increased from 108.34, 0.122 at 2200 V. Similarly, the mean value of H3/E2 increased from

6
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

(a) (b)

Acoustic signals (%)

Acoustic signals (%)

0.4 0.4

Friction coefficient

Friction coefficient
0.2 0.2

6.5N 13.36N
0.0 0.0

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
Loading force (N) Loading force (N)
0.4
(c) (d)

Acoustic signals (%)

Acoustic signals (%)

0.4

Friction coefficient
Friction coefficient
0.3

0.2
0.2

0.1 22.2N
24.1N

0.0
0.0

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
Loading force (N) Loading force (N)

Fig. 7. The acoustic emission spectra, friction coefficients curve, and scratching track morphologies of the Si/DLC coatings at different pulse voltages. [(a) 1400 V,
(b) 1600 V, (c) 1800 V, and (d) 2200 V].

16
120 (a) (b) 115
14 Hardness, (GPa)
young's modulus, (GPa)

young's modulus, (GPa)

100 110
substrate 12
Hardness, (GPa)
Load, (mN)

80 1400V
10 105
1600V
1800V
60 2200V 8 100

40 6 95

4
20 90
2
0 85
0 200 400 600 800 1000 1200 1400 1600 substrate 1400V 1600V 1800 2200
Penetration depth, (µm) samples

Fig. 8. (a) Load-penetration depth and (b) hardness and Young's modulus of Si/DLC coatings at different pulse voltages.

0.001 with the bare alloy to 0.217 with a film prepared at 1800 V, before of sliding time at various pulse voltages. The tribology test was per­
decreasing to 0.210 with a film prepared at 2200 V. The improvement in formed in an ambient environment. The findings show that all FC of
H/E ratio is attributable to reducing the region among the loading and specimens reach the upper limit at the start of the trials due to Hertzian
unloading curves and increasing the part under the unloading curve contact, then stabilizes after smoothing and matching the sliding route.
[43]. The area between the loading and unloading curves indicates the Furthermore, the curves show that all coating specimens exhibit the
energy dispersed in the DLC coating owing to plastic deformation; in same trend. As in Fig. 10, the substrate has the maximum FC and re­
contrast, the part under the unloading curve signifies the elastic energy mains almost stable over the test. This behavior is attributable to the
for deformation. As a result, increasing the pulse voltage to 1800 V creation of oxide tribo-layers at the ball/coating contact. On another
improves the H3/E2 ratio in the films, indicating an increase in me­ hand, the values of FC for Si/DLC coating are roughly dropped from 0.21
chanical and tribological properties, particularly wear resistance [44]. to 0.1 when the applied voltage is increased from 1400 to 2200 V,
respectively. The oscillations in the FC curves during the runtime are
3.2.3. Friction coefficient (FC) inherent to the test itself [45]. Fayed et al. [10] found the decline in FC
Fig. 10 represents the friction coefficients of specimens as a function during measurement is attributable to direct mechanical contact at the

7
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

0.14 0.25 oxide and then fluctuates during the runtime. This behavior is caused by
H/E the passive film breakdown, which depends on the activity of different
0.12
3 2
H /E impurities in the Al matrix [48]. From another aspect, the Eoc of the
0.20 coatings shows an increase in a positive trend comparing the bare alloy.
After 30 min, the Eoc values of the Si/DLC coatings are about − 632,
0.10
− 640, − 589, and –605 mVAg/AgCl for film prepared at 1400 V, 1600 V,
0.15
1800 V, and 2200 V, respectively. As the pulse voltage rises, the Eoc rises

H3/E2
0.08
to a positive value until it reaches 1800 V and drops to lower values. The
H/E

0.10 decrease in Eoc for films deposited <1800 V is due to the low pulse
0.06 voltage, which results in a decreased ionization rate of gases and a low
secondary electrons number given by ionization. Consequently, a low
0.04 0.05 floating potential is generated on the substrate surface. The large-sized
species of C2H+ 2 have inadequate energy, which binds and grows on
0.02 the substrate surface. Consequently, the Si/DLC film reveals the
0.00
morphological structure with open pores owing to the inadequate ki­
substrate 1400V 1600V 1800 2200 netic energy of the diffusion of the (C2H+2 ) interactive species. At 1800 V,
Samples the atomic and molecular ion energies of Ar and C2H2 gases increase
steadily with further ionization. According to Eq. (3), the dissociation
Fig. 9. H/E and H3 /E2 ratios for the substrate and Si/DLC coatings. and recombination reactions of acetylene ions and electrons in plasma
occur.

0.7 C2 H2+ + e− →C2 H + H

substrate →CH + CH (3)
1400V
0.6 1600V →CH 2 + C
1800V The floating potentials on the substrate increased as the sizes of the
Friction Coefficient

0.5 2200V
attached particles decreased. At the initial deposition stage, the attached
particles overcome the cluster barriers and fill in the gaps between the
0.4 columns by exchanging energy with the dense and high energy Ar+ ions
attracted by the floating potentials. Furthermore, large particle clusters
could be bombarded away from the surface of growing films, causing an
0.3
etching effect [49]. Consequently, the film surfaces become smooth, and
the particle sizes decrease as the increases of the pulse voltages. Addi­
0.2 tionally, Fig. 12 illustrates that Eoc shifts again to more negative value
(− 605 mVAg/AgCl) after 30 min for the film deposited at 2200 V. This is
0.1 ascribed to the pores observed in the microstructure (see Fig. 1). This
occurs by increasing the hydrogenation process, which reduces the
0.0 connectivity degree of the carbon network structure.
0 5 10 15 20 25 30
Sliding time, (min) 3.3.2. Electrochemical impedance spectroscopy
Fig. 13(a) represents the Nyquist diagrams of the Si/DLC coatings,
Fig. 10. Friction coefficients curves of the Al substrate and Si/DLC coatings at after Eoc monitoring for 30 min in aerated 3.5 wt% NaCl solution. Fig. 13
different pulse voltages. demonstrates that all the Nyquist plots exhibit similar features. All
coated specimens show two semicircles over the full frequency range.
interfaces or (most likely) local heat that causes the sp3 bonds to The first capacitive arc at high frequencies is associated with the pores
transform into a graphite structure that operates as a solid lubricant. The (defective) of the coatings, and the capacitive loop at low frequencies is
low frictional properties are ascribed to creating a transfer layer in both linked to the double layer of charge transfer. In general, the greater
DLC and metal-doped DLC [46]. radius of capacitive loop is correlated to the surface stability in corrosive
Fig. 11(a–d) represents the optical image of the wear track for Si/ environments [50]. It is worth noting that the radius of a semicircle of
DLC coatings at different pulse voltages. It is obvious from (Fig. 11(a, b)) the coating rises monotonously with the increment of pulse voltage.
that the films deposited at 1400 V and 1600 V are fractured and peeled Fig. 13(b, c) shows bode-phase plots of the investigated specimens
off from the substrate along their worn surface due to a decrease in the over the whole frequency range. The bode curves show two-time con­
adhesion strength of the films, as seen in Fig. 7(a, b). However, the stants at high and low frequencies regions. The first one is linked to the
coating formed at 1800 and 2200 V (Fig. 11(c, d)) is smooth, with creation of the passive film, whereas the second one is linked to the
limited wear debris visible along the worn surface. The film formed at charge transfer process. Moreover, an increase in phase angle in the
1800 and 2200 V has shallow abrasive grooves than the other speci­ high-frequency part may be attributable to the accumulated corrosion
mens, implying the ratio of H3/E2 is higher than other coating speci­ products on the coated surface, hindering the passage of corrosive
mens; hence, it has higher wear resistance [47]. environmental species to the bare substrate. At low frequencies parts,
the phase angle tends to decrease in value with immersion time due to
some defects that may gradually generates and accumulate in the pas­
3.3. Corrosion measurements sive films [51].
Fig. 14 demonstrates the corresponding electrical equivalent circuits
3.3.1. Open circuit potential (Eoc) (EECs) employed to analyze the EIS data of specimens. The fitted vari­
Fig. 12 illustrates Eoc evolution with soaking time for both the sub­ ables of EIS data are summarized in Table 6. Where Rs signifies the
strate and Si/DLC coatings as a function of pulse voltages in 3.5 wt% resistance of the NaCl solution. Rpore and CPEpore denote resistance and
NaCl solution. The substrate turns in a positive direction at the begin­ constant phase element of the pores, respectively. Rct, CPEct signify
ning of the test due to the creation of the passive layer from aluminum

8
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

Fig. 11. Worn surface images of Si/DLC coatings at different pulse voltages [(a) 1400 V, (b) 1600 V, (c) 1800 V, and (d) 2200 V].

represents the imaginary part, ω is the angular frequencies, and n is the

coefficient of dispersion correlated with inhomogeneous surfaces.
-400 Table 6 revealed an increment in the value of solution resistance with
increasing pulse voltages. Furthermore, the outcomes exhibit that an
Potential, (mVAg/AgCl)

Substrate 1400V 1600V 1800V 2200V

increase in capacitive radius indicates an increase in film stability (i.e.,
-684 -632 -640 -589 -605
-500 mVAg/AgCl mVAg/AgCl mVAg/AgCl mVAg/AgCl mVAg/AgCl excellent corrosion resistance) and vice versa. The results also showed
that the polarization resistance (Rpore) of Si/DLC coatings increases with
increasing pulse voltage up to 1800 V; beyond this value, the polariza­
-600 tion resistance decreases (see Table 6). Similarly, the CPE value de­
creases with increasing pulse voltage. This means that Si/DLC coating
could enhance the anti-corrosion property of the substrate. This phe­
-700 nomenon may be caused by two factors. The first is the morphology of
Si/DLC film, which has a thicker surface and less faulty structure due to
the higher pulse voltage, which accelerates the ionization rate of the
-800 Substrate 1400V 1600V 1800V 2200V reactive gases and produces more secondary electrons. As a result, C2H2
ions receive enough energy to adhere to and grow on the DLC texture,
0 200 400 600 800 1000 1200 1400 1600 1800 2000
resulting in fewer open pores in the morphological structure of the DLC
membranes and increased corrosion resistance. The second reason is SiC
Time, (sec) particles, which can prevent corrosive environmental species from
penetrating DLC coatings and degrading the substrate [13,52].
Fig. 12. Potential vs. time graphs of the substrate and coated samples at
different pulse voltages in 3.5 wt% NaCl aqueous solution.
3.3.3. Potentiodynamic polarization (PP)
Fig. 15 represents PP of Si/DLC coatings at different pulse voltages.
resistance and constant phase element of charge transfer at interface,
The outcomes suggest that the corrosion potentials (Ecorr) of the coatings
respectively. Due to deviation from ideal conditions, the phase constant
shifts to a positive value in comparison to the bare substrate. As the pulse
was used in the EECs instead of pure capacitance. The impedance of ZCPE
voltage increases, the Ecorr value progressively turns to a greater positive
is computed from the following Eq. (4) [10].
value, indicating an improvement in corrosion resistance. This behavior
1 can be ascribed to reducing defects in the coating structure, which closes
ZCPE = (jω)− n
(4)
Y0 direct paths between aggressive species and bare alloys. Consequently,
this inhibits the oxidation and reduction responses that happen at the
where ZCPE is the value of CPE, Y0 signifies capacitance value, j coating/solution contact, increasing the anti-corrosion property of the

9
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

Fig. 13. Nyquist plots and Bode-phase plots of the coated samples at different pulse voltages after exposure to 3.5 wt% NaCl solution.

when the pulse voltage increases from 1400 V to 1800 V. The results also
demonstrated that increasing the pulse voltage to 1800 V decreases the
corrosion rate and then slightly increases again for the film made at
2200 V. This demonstrates that DLC layers improve the anticorrosion
properties of Al alloys [27]. Moreover, Fig. 15 shows that the corrosion
current for the film deposited at 2200 V increases compared to the
specimen treated at 1800 V. This is due to the pores observed in the
microstructure (see Fig. 1). Add to this, increasing the pulse voltages
increase the kinetic energy of the carbon types, and the sp3 cross-linking
structure forms among each sp2 cluster. Hence, this prevents electrons
from migrating and increases the electrode potential of the specimens.
Thus, the films can operate as a barrier film to inhibit corrosive ions from
assaulting the Al substrate [52]. Further, the protective efficiency % (Pi)
of Si/DLC membranes can be computed utilizing Eq. (8) [55]:
( )
icorr
Pi = 1 − o × 100 (8)
icorr
Fig. 14. Equivalent electrical circuit model for EIS spectra of Si/DLC coatings
at different pulse voltages. where icorr and iocorr refer to the coated and uncoated corrosion current
densities, respectively. Notably, the protective efficiency (%) was
substrate. The reactions that happen at the cathodic locations can be significantly augmented with the increment of pulse voltage and ach­
determined by Eq. (5) [27]: ieved the highest value at 1800 V (≈ 93 %).

O2 + 2H2 O + 4e− →4OH − (5)

3.3.3.1. Morphologies of corroded surface. Fig. 16 shows SEM-EDS
From another aspect, the reactions that occur in anodic sites can be morphologies of the corroded surface for the film deposited at
described as follows Eq. (6) [27]: different pulse voltages after potentiodynamic measurements. The SEM
morphologies reveal that all coatings have several pits filled with
C − 2e− →C2+ (6)
corrosion outputs. Further, the corroded surface morphologies exhibit
The corrosion parameters derived from the PP plots (Fig. 15) are the presence of black and white particles distributed randomly on the
described in Table 7. The parameters included corrosion potential coated surface. This indicates that the corroded surface confirmed the
(Ecorr), corrosion current density (icorr), and polarization resistance (Rp). results of the potentiodynamic polarization test. Moreover, the mor­
The polarization resistance of the substrate and the coated film was phologies demonstrate that the coatings flaked off the substrate for film
determined using Eq. (7) [53,54]: prepared at 1400 and 1600 V, suggesting that Si/DLC film possesses
poor corrosion resistance. The weakening and flaking of the Si/DLC
βc βa
Rp = (7) coatings are generated by the pores and lack of bonding force between
2.3(βc + βa )icorr
the substrate and DLC coating, leading to voids forming during the test.
Consequently, the poorly adhesive areas of the DLC deforms under so­
where (βa), (βc) represents anodic and cathodic line slopes, respectively,
lution pressure, causing the formation of pits. Then the aggressive ions
and (icorr) corrosion current density. The outcomes reveal that the
enter the cavities and assault the substrate. Furthermore, the
corrosion potentials shift from − 636 to − 562 mVAg/AgCl, and the
morphology revealed that the degree of damage reduces as the pulse
corrosion current decline progressively from 8.46 to 0.919 μA⋅cm− 2
voltage rises. This demeanor is ascribed to the rising sp3 fraction in the

Table 6
Equivalent circuit parameters of Si/DLC coatings at different pulse voltages.
Samples Rs (Ω⋅cm2) Rpore (Ω⋅cm2) CPEpore (F⋅cm− 2) n1 Rct (Ω⋅cm2) CPEct (F⋅cm− 2) n2

1400 V 18.2 9.47 × 103 7.09 × 10− 6

0.81 1.23 × 104 2.24 × 10− 7
0.85
1600 V 23.51 1.47 × 103 7.17 × 10− 6
0.82 1.85 × 104 2.31 × 10− 7
0.86
1800 V 55.01 4.35 × 104 1.29 × 10− 6
0.85 2.27 × 104 6.41 × 10− 8
0.91
2200 V 42.9 6.06 × 103 1.52 × 10− 6
0.81 4.96 × 104 4.86 × 10− 7
0.84

10
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

-100 corrosion products are in the form of aluminum oxide and aluminum
substrate chloride. Moreover, the EDS outcomes exhibit a decrease in carbon level
-200 1400V in the DLC coating from 88.2 to 69.17 % with decreased pulse voltage
1600V from 2200 to1400 V, indicating the degradation of the carbon-based
Potential, (mVAg/AgCl)

-300 coating. The reason for this is the presence of pores in the configura­
1800V
tion of the DLC coating, as discussed before in Section 3.1.
-400
2200V
3.3.4. Potentiostatic measurements
-500 Potentiostatic tests were used to assess the electrochemical stability
of the passive layers produced on the Si/DLC coatings. Fig. 17 displays
-600 the current density (A/cm2) as a function of immersion time at different
pulse voltages. The specimens were immersed in a 3.5 wt% NaCl
-700 aqueous solution at an anodic potential of − 400 mVAg/AgCl. At first, the
current density of all specimens declines rapidly due to the formation of
-800 passive layers on the electrode's surface. Then, all investigated speci­
mens in the second stage exhibit a steady-state current density during
-900 the runtime of the test due to the complete growth of the passive layer.
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 Further, Fig. 17 shows that the substrate has a larger current density
Current density, (A.cm ) -2
than the other coating specimens. In contrast, the current density of the
Si/DLC coatings declines with increased pulse voltage up to 1800 V. The
Fig. 15. Potentiodynamic polarization plots of investigated samples in 3.5 wt% current densities are about 1.37, 0.12, 0.005, and 0.013 mA/cm2 for film
NaCl solution at various pulse voltages. coated at 1400 V, 1600 V, 1800 V, and 2200 V, respectively. These re­
sults agree with the polarization observations in the previous section
film, which limits the electrical properties of the layers and hinders the (Section 3.3.3). This demeanor is related to the increased thickness and
ions exchange of corrosive ions on the coating's surface [55]. content of sp3 bonds in coating structure and the diminished defects,
All corroded surfaces of DLC coatings at the selected regions in SEM operating as a good dielectric barrier between the substrate and
images (Fig. 16) are mainly composed of C, Si, Al, Cu, Mg, O, and high aggressive ions, preventing ions exchange on the coating surface and
content from Cl− ions. The EDS results also reveal that the oxygen inhibiting the electrochemical reaction. Furthermore, the Si/DLC film
content of the corroded surface decreases from 13.47 to 0.81 at. % with creates a passive layer of SiO2, leading to the deterrent of corrosive ion
the pulse voltage increase from 1400 to 2200 V. This means that penetration. Consequently, a small anodic current is sufficient to

Table 7
Electrochemical corrosion data of the Si/DLC coatings at different pulse voltages.
Samples Ecorr (mV) icorr (A/cm2) Rp, (Ω⋅cm2) − βc (V/dec) βa (V/dec) Corrosion rate, (mmy− 1) Pi, (%)
− 6 3
Substrate − 692 12.3 × 10 1.99 × 10 0.103 0.122 0.13 –
1400 V − 636 8.466 × 10− 6 7.71 × 103 0.270 0.339 0.092 31.17
1600 V − 620 1.693 × 10− 6 2.17 × 104 0.112 0.346 0.018 86.23
1800 V − 562 8.799 × 10− 7 4.94 × 105 0.092 0.315 0.009 92.84
2200 V − 586 9.19 × 10− 7 4.12 × 104 0.120 0.320 0.01 92.52

Al (c) O
(a) Al Elements At.% (b) Elements At.% Elements At.% (d) Al Elements At.%
C 69.17 C 74.26 C EDS3 C 88.39 C 88.21
EDS1 13.47 1.80 0.76 EDS4
O EDS2 O O O 0.81
5.56 0.35 Al Si 0.30
Na Na Na Na 0.56
CPS/eV
CPS/eV

CPS/eV

CPS/eV

Mg 0.20 C Mg 0.46 Si Mg 2.65 C Cl Mg 0.16

6.08 18.87 Mg Cu
Al Al Cl Al 7.93 Al 7.41
Cu Cl Si
Si 2.09 O Mg Si 4.49 Na Si 2.17 Mg 2.52
O Si
C Cu 7.97 Cu Si Cl Cl 0.12 7.97 O
Mg Si
Cl Cl Cl 0.33
Na Na
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Energy, (KeV) Energy, (KeV) Energy, (KeV) Energy, (KeV)

Fig. 16. SEM-EDS morphologies of the corroded Si/DLC coating samples at various pulse voltages. [(a) 1400 V, (b) 1600 V, (c) 1800 V, and (d) 2200 V].

11
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

Current density, (A.cm-2)

Current density, (A.cm-2)

1.4x10-1 Subsatrate 2x10-3 1400 V

1.2x10-1 2x10-3

1.0x10-1 2x10-3

1x10-3
8.0x10-2

0 200 400 600 800 1000 0 200 400 600 800 1000
Time, (sec) Time, (sec)
Current density, (A.cm-2)

Current density, (A.cm-2)

2.0x10-4 1600 V 2x10-5 1800 V
-4
1.8x10 2x10-5

1.6x10-4 1x10-5

9x10-6
1.4x10-4
6x10-6
1.2x10-4
3x10-6
0 200 400 600 800 1000 0 200 400 600 800 1000
Time, (sec) Time, (sec)
-5
4.2x10
Current density, (A.cm-2)

2200V
3.6x10-5

3.0x10-5

2.4x10-5

1.8x10-5

1.2x10-5
0 200 400 600 800 1000
Time, (sec)

Fig. 17. Potentiostatic-time curves of the investigated samples at an anodic potential of − 400 mVAg/AgCl in 3.5 wt% NaCl solutions.

passivate the 2024 Al substrate surface. However, the value of passive signifies the Na 1s [13,58]. Table 8 summarized the chemical compo­
current density increases again for the film deposited at 2200 V due to sitions and fitted parameters of XPS spectra after potentiostatic
increased hydrogen content and decreased network structure connec­ measurements.
tivity in coating films [56,57].
4. Conclusions
3.3.4.1. XPS of the corroded surface. Fig. 18(a–f) depicts XPS spectra of
the Si/DLC coating after potentiostatic measurement for film prepared In this study, Si/DLC coatings were fabricated on Al substrates at
at 1800 V. The results showed that the main constituents found in the different positive pulse voltages using the PECVD technique from acet­
spectra are Al, Si, Cl, C, O2, and Na. The chemical components and fitted ylene and tetramethyl silane as precursor gases. The main points of the
parameters of XPS spectra are described in Table 8. The peak at B.E of research are summarized as follows:
72.5 eV (Fig. 18b) matches the Al 2p. While the peak obtained at B.E ≈
102 eV in (Fig. 18c) correspond to the SiO2 2p. The detected peak at B.E • The Raman spectrometer proves that the pulse voltage significantly
≈ 199 eV agrees with the Cl 2p, as seen in Fig. 18(d). This peak was fitted affects the bonding configuration of the DLC coatings. As the pulse
to sub-peaks at roughly 197.9 and 200.5, which correspond to CuCl2, Cl voltage increased from 1400 to 1800 V, the ID/IG ratio of the coatings
2p respectively. This means that Cl− ions penetrate the coated surface decreased, and the sp2 bonds declined in network structure; beyond
and assault the intermetallic precipitates. The core-level XPS spectra of C this value, the sp2 augmented again.
1s was divided into three sub-peaks at 284.7 eV, 285.2 eV, and 287.8 eV, • The electrochemical data revealed that as pulse voltages increased
which match with (C–C (sp2)), (C–C (sp3)), and (C– – O), respectively, from 1400 to 1800 V, the corrosion potential of the coatings shifted
as shown in Fig. 18(e). The O 1s peak at B.E ≈ 530 eV was split up into to positive values, and the corrosion current density dropped from
three sub-peaks at roughly 531.2 eV, 532.06 eV, and 535.5 eV, which 8.46 × 10− 6 A⋅cm− 2 to 8.79 × 10− 7 A⋅cm− 2.
matches the Al2(CO3)3, Al(OH)3, and C–O–O–C, respectively, as • The Si/DLC film deposited at pulse voltage 1800 V provided good
shown in Fig. 18(f). The presence of these compounds in corroded sur­ structural quality, better adhesion strength, high mechanical prop­
faces is a good indication of the entry of corrosive ions into the coated erties, and higher corrosion resistance in 3.5 wt% NaCl solution
film through the pores that exist in the structure and the attacking of the compared with other coatings.
carbon layer and degraded it. The peak located at B.E ≈ 1071 eV

12
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

Fig. 18. XPS spectra of corroded surface for film prepared at 1800 V after potentiostatic test and their high-resolution XPS regions of [(a) broad scan spectrums, (b)
Al 2p, (c) SiO2 2p, (d) Cl 2p, (e) C 1s, and (f) O 1s respectively].

Table 8
The chemical compositions and fitted parameters of XPS spectra of the corroded film made at 1800 V after potentiostatic test in 3.5 wt% NaCl solution.
Sample Si XPS (at. O XPS (at. C XPS (at. Cl2 XPS (at. Na 1 s XPS Al XPS (at. C
%) %) %) %) (at.%) %)
Binding energy of Area of sp2 Binding energy of Area of sp3
sp2/ev peak sp3/ev peak

1800 V 1.94 17.75 53.09 13.38 12.2 1.64 284.7 7193.69 285.2 10,840.51

• The passivate current of Si/DLC coatings reduced as the pulse voltage Acknowledgments
increased up to 1800 V. However, above this value, the passive
current increases caused by decreased network structure connectiv­ This study was supported by the Liaoning Provincial Key Research
ity in coating films. and Development Project (2020JH1/10100001), China's National Nat­
ural Science Foundation (51874172 and 51972155), and the Province
CRediT authorship contribution statement Education Foundation of the Liaoning (LJKZ 0286).

Saad M. Fayed: Methodology, Formal analysis, Investigation, References

Writing – original draft, Visualization, Project administration. Haodong
Wu: Resources, Writing – review & editing. Dongxu Chen: Resources, [1] H. Schäfer, H.R. Stock, Improving the corrosion protection of aluminium alloys
using reactive magnetron sputtering, Corros. Sci. 47 (2005) 953–964, https://doi.
Writing – review & editing, Supervision, Project administration, Fund­ org/10.1016/j.corsci.2003.06.002.
ing acquisition. Shengli Li: Resources, Writing – review & editing, Su­ [2] H.D. Johansen, C.M.A. Brett, A.J. Motheo, Corrosion protection of aluminium alloy
pervision, Project administration, Funding acquisition. Yanwen Zhou: by cerium conversion and conducting polymer duplex coatings, Corros. Sci. 63
(2012) 342–350, https://doi.org/10.1016/j.corsci.2012.06.020.
Writing – review & editing. Hongbin Wang: Writing – review & editing. [3] A. Covelo, S. Rodil, X.R. Nóvoa, M. Hernández, Development and characterization
M.M. Sadawy: Resources, Writing – review & editing. of sealed anodizing as a corrosion protection for AA2024-T3 in saline media,
Materials Today Communications. 31 (2022), https://doi.org/10.1016/j.
mtcomm.2022.103468.
Declaration of competing interest [4] H.H. Zhang, Y.W. Liu, H. Bian, Y. Zhang, Z.N. Yang, Z. Zhang, Y. Chen,
Electrodeposition of silane/reduced graphene oxide nanocomposite on AA2024-T3
There seem to be no declared conflicts. alloy with enhanced corrosion protection, chemical and mechanical stability,
J. Alloys Compd. 911 (2022), 165058, https://doi.org/10.1016/j.
jallcom.2022.165058.
Data availability [5] C. Anandan, L. Mohan, P.D. Babu, Electrochemical studies and growth of apatite on
molybdenum doped DLC coatings on titanium alloy β-21S, Appl. Surf. Sci. 296
No data was used for the research described in the article. (2014) 86–94, https://doi.org/10.1016/j.apsusc.2014.01.049.
[6] E. Marin, A. Lanzutti, M. Nakamura, M. Zanocco, W. Zhu, G. Pezzotti, F. Andreatta,
Corrosion and scratch resistance of DLC coatings applied on chromium
molybdenum steel, Surf. Coat. Technol. 378 (2019), 124944, https://doi.org/
10.1016/j.surfcoat.2019.124944.

13
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

[7] A.O. Volkhonskii, A.A. Vereshchaka, I.V. Blinkov, A.S. Vereshchaka, A.D. Batako, [31] C. Casiraghi, A.C. Ferrari, J. Robertson, Raman spectroscopy of hydrogenated
Filtered cathodic vacuum arc deposition of nano-layered composite coatings for amorphous carbons, Phys. Rev. B: Condens. Matter Mater. Phys. 72 (2005) 1–14,
machining hard-to-cut materials, Int. J. Adv. Manuf. Technol. 84 (2016) https://doi.org/10.1103/PhysRevB.72.085401.
1647–1660, https://doi.org/10.1007/s00170-015-7821-8. [32] A.C. Ferrari, S.E. Rodil, J. Robertson, S.E. Rodil, J. Robertson, Interpretation of
[8] H.-U. Krebs, M. Weisheit, J. Faupel, E. Süske, T. Scharf, C. Fuhse, M. Störmer, infrared and Raman spectra of amorphous carbon nitrides, Phys. Rev. B: Condens.
K. Sturm, M. Seibt, H. Kijewski, D. Nelke, E. Panchenko, M. Buback, in: Pulsed Matter Mater. Phys. 67 (2003) 1–20, https://doi.org/10.1103/
Laser Deposition (PLD) – A Versatile Thin Film Technique, 2003, pp. 505–518, PhysRevB.67.155306.
https://doi.org/10.1007/978-3-540-44838-9_36. [33] C. Casiraghi, F. Piazza, A.C. Ferrari, D. Grambole, J. Robertson, Bonding in
[9] B.C. Yeldose, B. Ramamoorthy, Characterization of DC magnetron sputtered hydrogenated diamond-like carbon by raman spectroscopy, Diam. Relat. Mater. 14
diamond-like carbon (DLC) nano coating, Int. J. Adv. Manuf. Technol. 38 (2008) (2005) 1098–1102, https://doi.org/10.1016/j.diamond.2004.10.030.
705–717, https://doi.org/10.1007/s00170-007-1131-8. [34] J. Wang, J. Pu, G. Zhang, L. Wang, Tailoring the structure and property of silicon-
[10] S.M. Fayed, D. Chen, S. Li, Y. Zhou, H. Wang, Effect of bias voltage on doped diamond-like carbon films by controlling the silicon content, Surf. Coat.
characteristics of multilayer si-DLC film coated on AA6061 aluminum alloy, Technol. 235 (2013) 326–332, https://doi.org/10.1016/j.surfcoat.2013.07.061.
J. Mater. Eng. Perform. 30 (2021) 743–759, https://doi.org/10.1007/s11665-020- [35] B. Han, M. Yan, D. Ju, M. Chai, S. Sato, Chemical composition and corrosion
05397-2. behavior of a-c:H/dlc film-coated titanium substrate in simulated pemfc
[11] F. Sanchette, M. El Garah, S. Achache, F. Schuster, C. Chouquet, C. Ducros, environment, Coatings 11 (2021), https://doi.org/10.3390/coatings11070820.
A. Billard, Dlc-based coatings obtained by low-frequency plasma-enhanced [36] I. Bouabibsa, S. Lamri, F. Sanchette, Structure, mechanical and tribological
chemical vapor deposition (Lfpecvd) in cyclohexane, principle and examples, properties of me-doped diamond-like carbon (DLC) (Me = Al, Ti, or Nb)
Coatings 11 (2021), https://doi.org/10.3390/coatings11101225. hydrogenated amorphous carbon coatings, Coatings 8 (2018), https://doi.org/
[12] S.M. Fayed, P. Gao, D. Chen, S. Li, Y. Zhou, H. Wang, M.M. Sadawy, Corrosion 10.3390/COATINGS8100370.
inhibition characteristics of multilayer si-DLC, phosphating and anodizing coatings [37] S.R. Mousavi, H. Sereshti, H. Rashidi Nodeh, A. Foroumadi, A novel and reusable
deposited on 2024 Al alloy: a comparative study, Diam. Relat. Mater. 117 (2021), magnetic nanocatalyst developed based on graphene oxide incorporated strontium
108460, https://doi.org/10.1016/j.diamond.2021.108460. nanoparticles for the facial synthesis of β-enamino ketones under solvent-free
[13] S.M. Fayed, D. Chen, S. Li, Y. Zhou, H. Wang, M.M. Sadawy, Corrosion behavior conditions, Appl. Organomet. Chem. 33 (2019) 1–9, https://doi.org/10.1002/
and passive stability of multilayer DLC-si coatings, Surf. Coat. Technol. 431 (2021), aoc.4644.
128001, https://doi.org/10.1016/j.surfcoat.2021.128001. [38] H. Zhang, Z. Li, W. He, B. Liao, G. He, X. Cao, Y. Li, Damage evolution and
[14] K. Wang, H. Zhou, K. Zhang, X. Liu, X. Feng, Y. Zhang, G. Chen, Y. Zheng, Effects of mechanism of TiN/Ti multilayer coatings in sand erosion condition, Surf. Coat.
ti interlayer on adhesion property of DLC films: a first principle study, Diam. Relat. Technol. 353 (2018) 210–220, https://doi.org/10.1016/j.surfcoat.2018.08.062.
Mater. 111 (2021), 108188, https://doi.org/10.1016/j.diamond.2020.108188. [39] X. Wang, X. Sui, S. Zhang, M. Yan, Y. Lu, J. Hao, W. Liu, Impacts of the a-si: H
[15] F. Shahsavari, M. Ehteshamzadeh, M.R. Naimi-Jamal, A. Irannejad, interlayer nanostructure on the adhesion of the thick DLC coatings prepared by
Nanoindentation and nanoscratch behaviors of DLC films growth on different PECVD, Surf. Sci. 565 (2021), 150539, https://doi.org/10.1016/j.
thickness of cr nanolayers, Diam. Relat. Mater. 70 (2016) 76–82, https://doi.org/ apsusc.2021.150539.
10.1016/j.diamond.2016.10.003. [40] M. Tomáš, M. Džupon, E. Evin, E. Spišák, Surface analysis of uncoated and PVD
[16] L. Cao, J. Liu, Y. Wan, J. Pu, Corrosion and tribocorrosion behavior of W doped coated punch at the hole-flanging process, Metals. 8 (2018) 1–15, https://doi.org/
DLC coating in artificial seawater, Diam. Relat. Mater. 109 (2020), 108019, 10.3390/met8040218.
https://doi.org/10.1016/j.diamond.2020.108019. [41] G. Ma, G. Lin, G. Sun, H. Zhang, H. Wu, Characteristics of DLC containing ti and zr
[17] N. Dwivedi, S. Kumar, H.K. Malik, C. Sreekumar, S. Dayal, C.M.S. Rauthan, O. films deposited by reactive magnetron sputtering, Phys. Procedia 18 (2011) 9–15,
S. Panwar, Investigation of properties of cu containing DLC films produced by https://doi.org/10.1016/j.phpro.2011.06.049.
PECVD process, J. Phys. Chem. Solids 73 (2012) 308–316, https://doi.org/ [42] J.C. Damasceno, S.S. Camargo, F.L. Freire, R. Carius, Deposition of si-DLC films
10.1016/j.jpcs.2011.10.019. with high hardness, low stress and high deposition rates, Surf. Coat. Technol.
[18] M. Ikeyama, S. Nakao, Y. Miyagawa, S. Miyagawa, Effects of si content in DLC films 133–134 (2000) 247–252, https://doi.org/10.1016/S0257-8972(00)00932-4.
on their friction and wear properties, Surf. Coat. Technol. 191 (2005) 38–42, [43] M. Wu, X. Tian, M. Li, C. Gong, R. Wei, Effect of additional sample bias in Meshed
https://doi.org/10.1016/j.surfcoat.2004.08.075. plasma immersion ion deposition (MPIID) on microstructural, surface and
[19] Z. Seker, H. Ozdamar, M. Esen, R. Esen, H. Kavak, The effect of nitrogen mechanical properties of si-DLC films, Appl. Surf. Sci. 376 (2016) 26–33, https://
incorporation in DLC films deposited by ECR microwave plasma CVD, Appl. Surf. doi.org/10.1016/j.apsusc.2016.02.127.
Sci. 314 (2014) 46–51, https://doi.org/10.1016/j.apsusc.2014.06.137. [44] B. Zhou, B. Xu, Y. Xu, S. Yu, Y. Wu, Y. Wu, Z. Liu, Selective bonding effect on
[20] A. Jiang, J. Xiao, X. Li, Z. Wang, Effect of structure, composition, and microstructure and mechanical properties of (Al, N)-DLC composite films by ion
micromorphology on the hydrophobic property of F-DLC film, J. Nanomater. 2013 beam-assisted cathode arc evaporation, Appl. Phys. A 125 (2019) 1–15, https://
(2013), https://doi.org/10.1155/2013/690180. doi.org/10.1007/s00339-018-2320-z.
[21] H. Deng, D. Chen, Y. Wang, Y. Zhou, P. Gao, Effects of silicon on microstructure [45] S. Miyake, T. Shindo, M. Miyake, Regression analysis of the effect of bias voltage on
and corrosion resistance of diamond-like-carbon film prepared on 2024 aluminum nano- and macrotribological properties of diamond-like carbon films deposited by
alloy by plasma-enhanced chemical vapor deposition, Diam. Relat. Mater. 110 a filtered cathodic vacuum arc ion-plating method, J. Nanomater. 2014 (2014),
(2020), 108144, https://doi.org/10.1016/j.diamond.2020.108144. https://doi.org/10.1155/2014/657619.
[22] M. Cui, J. Pu, J. Liang, L. Wang, G. Zhang, Q. Xue, Corrosion and tribocorrosion [46] A. Li, Q. Chen, G. Wu, Y. Wang, Z. Lu, G. Zhang, Probing the lubrication
performance of multilayer diamond-like carbon film in NaCl solution, RSC Adv. 5 mechanism of multilayered Si-DLC coatings in water and air environments, Diam.
(2015) 104829–104840, https://doi.org/10.1039/c5ra21207c. Relat. Mater. 105 (2020), https://doi.org/10.1016/j.diamond.2020.107772.
[23] Y. Ye, S. Jia, D. Zhang, W. Liu, H. Zhao, A study for anticorrosion and tribological [47] N.W. Khun, E. Liu, Tribological properties of platinum/ruthenium/nitrogen doped
behaviors of thin/thick diamond-like carbon films in seawater, Surf. Topography diamond-like carbon thin films deposited with different negative substrate biases,
Metrol. Properties 6 (2018), https://doi.org/10.1088/2051-672X/aaab9a. Friction. 2 (2014) 317–329, https://doi.org/10.1007/s40544-014-0059-x.
[24] X. Wang, H. Zhou, S. Zhang, M. Yan, Y. Lu, J. Hao, W. Liu, The effect of acetylene [48] R. Zhang, L. Wang, W. Shi, Variable corrosion behavior of a thick amorphous
flow rate on the uniform deposition of thick DLC coatings on the inner surface of carbon coating in NaCl solution, RSC Adv. 5 (2015) 95750–95763, https://doi.org/
pipes with different draw ratios, Vacuum 196 (2022), 110720, https://doi.org/ 10.1039/c5ra18966g.
10.1016/j.vacuum.2021.110720. [49] K. Yamamoto, K. Matsukado, Effect of hydrogenated DLC coating hardness on the
[25] K. Liu, J.Jie Kang, G.An Zhang, Z. bin Lu, W. Yue, Effect of temperature and mating tribological properties under water lubrication, Tribol. Int. 39 (2006) 1609–1614,
pair on tribological properties of DLC and GLC coatings under high pressure https://doi.org/10.1016/j.triboint.2006.01.004.
lubricated by MoDTC and ZDDP, Friction 9 (2021) 1390–1405, https://doi.org/ [50] L. Joska, J. Fojt, L. Cvrcek, V. Brezina, Properties of titanium-alloyed DLC layers for
10.1007/s40544-020-0420-1. medical applications, Biomatter. 4 (2014) 37–41, https://doi.org/10.4161/
[26] H. Zhang, C. Luo, S. Duo, Q. Liu, R. Wen, M. Yuan, Y. Wu, Effect of deposition time biom.29505.
on hardness and bonding strength of magnetron sputtered TiN coatings on 316 [51] H. Zeng, Y. Yang, L. Liu, M. Li, Pitting and crevice corrosion evolution
stainless steel, Key Engineering Materials. 726 (2017) 303–307, https://doi.org/ characteristics of 2205 duplex stainless steel in hot concentrated seawater, J. Solid
10.4028/www.scientific.net/KEM.726.303. KEM. State Electrochem. 25 (2021) 1555–1565, https://doi.org/10.1007/s10008-021-
[27] Y. Zhang, P. Gao, D. Chen, Y.W. Zhou, Structure related corrosion behavior of DLC 04935-9.
films in high cl-environment, Corros. Rev. 39 (2021) 465–476, https://doi.org/ [52] X. Wei, S. Shi, C. Ning, Z. Lu, G. Zhang, Si-DLC films deposited by a novel method
10.1515/corrrev-2021-0004. equipped with a co-potential auxiliary cathode for anti-corrosion and anti-wear
[28] E3-95, in: Standard Practice for Preparation of Metallographic Specimens 82, application, J. Mater. Sci. Technol. 109 (2022) 114–128, https://doi.org/10.1016/
ASTM International, 2016, pp. 1–15. http://www.astm.org/cgi-bin/resolver.cgi? j.jmst.2021.08.077.
E140%0Ahttp://www.astm.org/Standards/E8.htm. [53] P. Wongpanya, N. Pintitraratibodee, K. Thumanu, C. Euaruksakul, Improvement of
[29] Z. Hamidi, S.Y. Mosavian, N. Sabbaghi, M.A. Karimi Zarchi, M. Noroozifar, Cross- corrosion resistance and biocompatibility of 316L stainless steel for joint
linked poly(N-alkyl-4-vinylpyridinium) iodides as new eco-friendly inhibitors for replacement application by ti-doped and ti-interlayered DLC films, Surf. Coat.
corrosion study of st-37 steel in 1 M H2SO4, Iranian Polym. J. (Engl. Ed.) 29 (2020) Technol. 425 (2021), 127734, https://doi.org/10.1016/j.surfcoat.2021.127734.
225–239, https://doi.org/10.1007/s13726-020-00787-8. [54] T. Rabizadeh, S.K. Asl, Casein as a natural protein to inhibit the corrosion of mild
[30] J.A. Santiago, I. Fernández-Martínez, T. Kozák, J. Capek, A. Wennberg, J. steel in HCl solution, J. Mol. Liq. 276 (2019) 694–704, https://doi.org/10.1016/j.
M. Molina-Aldareguia, V. Bellido-González, R. González-Arrabal, M.A. Monclús, molliq.2018.11.162.
The influence of positive pulses on HiPIMS deposition of hard DLC coatings, Surf. [55] H.G. Kim, S.H. Ahn, J.G. Kim, S.J. Park, K.R. Lee, Corrosion performance of
Coat. Technol. 358 (2019) 43–49, https://doi.org/10.1016/j. diamond-like carbon (DLC)-coated Ti alloy in the simulated body fluid
surfcoat.2018.11.001.

14
S.M. Fayed et al. Surface & Coatings Technology 445 (2022) 128749

environment, Diam. Relat. Mater. 14 (2005) 35–41, https://doi.org/10.1016/j. bipolar plates, Appl. Sci. (Switzerland). 9 (2019), https://doi.org/10.3390/
diamond.2004.06.034. app9122568.
[56] J. Wei, H. Li, L. Liu, P. Guo, P. Ke, A. Wang, Enhanced tribological and corrosion [58] P.R. Riley, P. Joshi, S.A. Machekposhti, R. Sachan, J. Narayan, R.J. Narayan,
properties of multilayer Ta-C films via alternating sp3 content, Elsevier B.V. Enhanced vapor transmission barrier properties via silicon-incorporated diamond-
(2019), https://doi.org/10.1016/j.surfcoat.2019.05.087. like carbon coating, Polymers 13 (2021) 1–12, https://doi.org/10.3390/
[57] K. Shi, X. Li, Y. Zhao, W.W. Li, S.B. Wang, X.F. Xie, L. Yao, J.O. Jensen, Q.F. Li, polym13203543.
Corrosion behavior and conductivity of TiNb and TiNbN coated steel for metallic

15