Electrochemical and tribocorrosion performances of CrMoSiCN coating on Ti-6Al-4V titanium alloy in artificial seawater

Journal Pre-proof Electrochemical and tribocorrosion performances of CrMoSiCN coating on Ti-6Al-4V titanium alloy in artificial seawater Yongqiang Fu, Fei Zhou, Qianzhi Wang, Maoda Zhang, Zhifeng Zhou

PII:

S0010-938X(19)31587-2

DOI:

https://doi.org/10.1016/j.corsci.2019.108385

Reference:

CS 108385

To appear in:

Corrosion Science

Received Date:

30 July 2019

Revised Date:

7 December 2019

Accepted Date:

9 December 2019

Please cite this article as: Fu Y, Zhou F, Wang Q, Zhang M, Zhou Z, Electrochemical and tribocorrosion performances of CrMoSiCN coating on Ti-6Al-4V titanium alloy in artificial seawater, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108385

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Electrochemical and tribocorrosion performances of CrMoSiCN coating on Ti-6Al-4V titanium alloy in artificial seawater

Yongqiang Fua,b, Fei Zhoua,b*, Qianzhi Wanga,b**, Maoda Zhanga,b, Zhifeng Zhouc

a

State Key Laboratory of Mechanics and Control of Mechanical Structure, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China

b

Key Laboratory of Helicopter Transmission Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China

Advanced Coatings Applied Research Laboratory, Department of Mechanical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China

**

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Corresponding author: E-mail address: [email protected] (F. Zhou)

Corresponding author: E-mail address: [email protected] (Q.Z. Wang)

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*

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Highlights

CrMoSiCN coatings were applied to Ti-6Al-4V titanium alloy substrates.

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The passive film containing MoO3 and Cr2O3 could improve the coatings’ corrosion resistance.

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The high Mo content could decrease friction coefficient but increase the material loss.

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CrMoSiCN-2 coating possessed the best electrochemical and tribocorrosion properties.

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ABSTRACT: CrMoSiCN coatings were applied to Ti-6Al-4V titanium alloy substrates, and their electrochemical and tribocorrosion performances in seawater were studied systematically. When the coating

polarization was conducted without sliding contact, the passive oxide film with MoO3 and Cr2O3 was formed on the surface of CrMoSiCN coatings to prevent the penetration of seawater. During tribocorrosion testing, the friction coefficient decreased but the material loss of coatings increased with increasing Mo content. This indicated that the tribocorrosion behaviors of CrMoSiCN coatings were closely related to their mechanical properties and electrochemical performance. Among all CrMoSiCN coatings, the CrMoSiCN-2 coating possessed the best corrosion resistance.

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Keywords: CrMoSiCN coating; Electrochemical; Tribocorrosion; Synergistic effect; Seawater.

1. Introduction

Due to the favorable mechanical properties, high specific strength and good anticorrosion

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ability, Ti-6Al-4V titanium alloys have been extensively used in the aviation and marine industries

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However, seawater was a strong corrosive medium and its wave could produce the low-frequency reciprocating stress which impacted on the metallic component, and then the metallic corrosion

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would be accelerated. In order to improve the reliability and service life of titanium alloy parts for

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helicopter transmission systems in corrosive marine environment, the surface of metallic parts would be coated with hard coatings, which possessed the good anti-wear and anti-corrosion

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properties [1-2]. Currently, the multi-element CrN-based coatings have been extensively studied due to their favorable corrosion resistance [3-7]. The corrosion resistance of CrSiN coatings in the

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various environments had been studied in detail, it was found that the CrSiN coatings with Si contents of 16.0 at.%-18.4 at.% exhibited excellent corrosion resistance both in acidic and alkaline environments due to their small grain size and compact nanostructure [8-10]. Besides, Shan et al. [11] found that the corrosion resistance of CrSiN coatings was better than that of CrN coatings in seawater, because the passive layer contained N, O, Si and Cr elements was formed on the surface of CrSiN coating and filled the invisible cracks and pinholes during the polarization process.

Moreover, Ref. [12] manifested that CrCN coatings with 15.4 at.% carbon exhibited favorable corrosion resistance in simulated body fluid due to the formation of amorphous CNx in the coatings. Of course, the formation of chromium oxide on the surface of CrCN coating could restrain the occurrence of pitting corrosion. When the carbon and silicon elements were codoped into CrN coating, the growth of columnar structure could be interrupted, some amorphous phases such as amorphous carbon(a-C) or carbon nitride (a-CNx), a-Si3N4, a-SiC and/or a-SiCxNy were formed in

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the CrSiCN coatings[13, 14], and then the microstructure of CrSiCN coatings became dense with low porosity. If such CrSiCN coatings were immerged in artificial seawater, their corrosion resistance could be improved [13, 14]. Cai et al. [15, 16] studied the electrochemical properties of

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CrSiCN coatings in 3.5% NaCl solution, and indicated that the CrSiCN coatings with 1.3 at.%-1.4 at.% Si possessed small grain size and dense microstructure, and presented high electrochemical

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impedance and resistance to electrochemical corrosion. The aforementioned results indicated that

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the corrosion resistance of hard coatings was related to coating microstructure in a coating-substrate system. Dense coating microstructures resulted in high electrochemical impedance and resistance to

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electrochemical corrosion due to low effective diffusion coefficients and thick effective diffusion layer.

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In addition, Mo element was proved to be a quite effective alloying element to enhance the

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comprehensive properties of hard coatings [17-25]. If the Mo element was added into CrN coatings, the (Cr, Mo)N substitutional solid solution was formed in the CrMoN coatings, which contributed to the dense microstructure and superior mechanical properties [17]. The similar result was found by Zhang et al. [19], the CrMoCN coating with 8.1 at.% Mo content exhibited the compact microstructure and the high hardness of 23.6 GPa. Furthermore, Mo could improve the wear resistance of CrMoSiN coating due to the formation of molybdenum oxide during the friction

process [20, 24]. In addition, Mo element also played a vital role in the anticorrosion field [26-28]. For example, Saidi et al. [29] reported that the 304L stainless steel coated with Mo film exhibited the excellent corrosion resistance because the passive film with semi-conducting molybdenum oxide was formed during the polarization process. Besides, the repassivation performance of Ni-Cr alloys was enhanced with the 6 wt.% doping Mo content [30]. Ref. [31] manifested that the Mo also contributed to the increasing resistance of passive film to breakdown when the Mo was doped into

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austenitic stainless steel. Moreover, the MoxN film was also studied as an anode for micro-supercapacitors [32]. In conclusion, Mo could be the alternative doping element to enhance the corrosion resistance of hard coating.

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Generally, in the actual humid marine environment, the material loss suffering from corrosion and friction simultaneously was always higher than the sum of material loss generated by corrosion

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and friction separately, thus the synergistic effect of friction and corrosion behaviors could not be

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ignored [33-34]. Namely, it was insufficient to evaluate the comprehensive performance of coatings through the separate experimental data obtained from the electrochemical and tribological tests. As

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an alternative, to well simulate the actual working conditions with friction and corrosion, tribocorrosion test had been used to consider the synergistic effect of friction and corrosion on the

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tribo-corrosion properties of coatings [35-38]. For instance, Shan et al. [35] studied the

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tribocorrosion behaviors of PVD CrN coatings, and indicated that the synergistic effect contribution of friction and corrosion increased due to the emergence of pitting corrosion as the applied potential increased from -0.5 V to 1 V. For the tribocorrosion behavior of multilayer DLC coatings, the mechanical wear was dominated in the material loss of multilayer DLC coatings. But as applied potential increased during tribocorrosion test, the material loss caused by corrosion-induced wear increased [36]. The similar result was found by Lee et al. [37]. When the tribocorrosion behaviors

of Ni-Mo coatings were investigated at different applied potentials, the contributions of pure corrosion and the synergistic effect of friction and corrosion to material loss were both increased with an increase in the applied potential. Meanwhile, the corrosion products consisting of NiO and MoOx contributed to the low friction coefficient. The contributions of tribocorrosion components to the material loss of quaternary TiSiCN coatings in seawater were studied in Ref. [38], it was obvious that the proportion of material loss caused by synergistic effect increased to the maximum

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value of 18.0% for TiSiCN coating with 11.9 at.% carbon. This indicated that the synergy between friction and corrosion played the key role in the degradation of TiSiCN coatings.

Under the harsh working conditions, the multicomponent coatings with good electrochemical

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and tribocorrosion performances are highly demanded. Recently our group had reported that the CrMoSiCN coatings possessed superior mechanical and tribological properties [39]. However, the

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electrochemical and tribocorrosion properties of CrMoSiCN coatings in seawater have not yet been

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investigated.

In here, the CrMoSiCN coatings with various Mo contents have been prepared using closed

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field unbalanced magnetron sputtering system. The effect of Mo content on their electrochemical and tribocorrosion behaviors in seawater was analyzed in details. At the meantime, the

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electrochemical corrosion and tribo-corrosion mechanism of CrMoSiCN coatings were elaborated.

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2. Experimental details

CrMoSiCN coatings were fabricated on Ti-6Al-4V disks and Si (100) wafers using closed field

unbalance magnetron sputtering system (UDP-650, Teer Coatings Limited, UK). The chemical composition and corresponding mechanical properties of Ti6Al4V substrate were presented in Table 1. CrMoSiCN coatings with different Mo contents were fabricated by controlling the Mo target current to 0.5 A, 1 A, 1.5 A, 2 A, 2.5 A and 3 A, respectively. According to the sputtering current of

Mo target, the samples were labeled as CrMoSiCN-0.5, CrMoSiCN-1, CrMoSiCN-1.5, CrMoSiCN-2, CrMoSiCN-2.5 and CrMoSiCN-3, respectively. The detailed parameters of deposition were listed in the Ref. [39]. The hardness (H) and elastic modulus (E) of coatings were measured using nanoindentation tester (NanoIndenterG200, KLATencor Co. Ltd., US) with a fixed displacement into surface of 150 nm. The transmission electron microscope (TEM, TECNAI G2 S-TWIN F20, USA) was applied to obtain the structural information of CrMoSiCN coatings at the

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accelerating voltage of 200 KV. The scanning electron microscope (SEM, Regulus 8100, Japan) was used to detect the surface of CrMoSiCN coatings after corrosion tests. The chemical composition and functional group of CrMoSiCN coatings before and after static electrochemical

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test were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientic) with a monochromatic Al Kα radiation source (energy 1468.7 eV) at 164 W.

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The electrochemical test was conducted using the CHI660E electrochemical workstation with

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a conventional three electrode system. In order to simulate the natural seawater environment, according to the standard of ASTM D1141-98, the artificial seawater with the chemical composition

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listed in Table 2 was prepared, and its pH value was 8.2 ± 0.1. All the electrochemical tests were conducted in the aerated artificial seawater. The samples with 1 cm2 exposed area were used as the

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working electrodes, platinum foil was used as auxiliary electrode and the saturated Ag/AgCl

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electrode was used as the reference electrode. Firstly, the samples were immersed into artificial seawater for 1 h to obtain the stable open circuit potential (OCP) value. After that, the EIS test was conducted at the frequency range of 1 mHz -100 kHz under the OCP condition with 10 mV AC excitation. Finally, the potentiodynamic polarization test under no sliding contact condition was performed at the voltage range of -0.6 V to 0.6 V with the scanning rate of 20 mV/min. After the polarization test, the morphology and bonding structure of CrMoSiCN coatings were observed

using SEM and XPS, respectively. The

tribocorrosion

performance

of

CrMoSiCN

coatings

was

evaluated

using

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linear-reciprocating module of tribometer (MFT-EC4000, China) with a three-electrode system in the aerated artificial seawater. All the tribocorrosion tests were executed at the following condition: the applied load was 3 N, the sliding frequency was 4.16 Hz (0.05 m/s) and the sliding stroke was 6 mm. The ball-to-disk mode of tribocorrosion test was carried out at room temperature for 1h. The

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SiC balls with the diameter of 8 mm were used as counterparts, and their hardness, elastic modulus and Poisson’s ratio were 27.5 GPa, 410 GPa and 0.14, respectively. Before the tribocorrosion test, each sample was immersed into seawater for 1 h to stabilize the OCP. After that, the samples were

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mounted in a cell contained seawater with the exposed area of 6.2 cm2. During the tribocorrosion test in OCP condition, the OCP values of CrMoSiCN coatings with no sliding contact were

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continuously recorded for 10 min to ensure the potential stable. Then the normal load of 3 N was

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applied to the samples, the friction coefficient and OCP were both recorded. The OCP was continuously measured for 10 min after the sliding process finished. Finally, the friction tests of

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CrMoSiCN coatings with different Mo contents sliding against SiC balls under potentiodynamic condition were carried out. The voltage range was set as -0.9 V to 0.3 V and the scanning rate was

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20 mV/min. The corresponding morphology and cross sectional profiles of wear tracks after

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tribocorrosion testing were observed and measured using SEM and 3D profilometer (Nanomap1000WLI, USA), respectively. 3. Results and discussion 3.1 Characteristics of CrMoSiCN coatings The chemical composition of CrMoSiCN coatings was listed in Table 3, the Mo content increased gradually from 1.4 at.% to 16.3 at.% as the Mo target current increased from 0.5 A to 3 A.

It indicated that the Mo content was sensitive to the Mo target current during the deposition process. Table 4 showed the hardness and elastic modulus of CrMoSiCN coatings with different Mo contents. It was clear that the hardness of CrMoSiCN coatings increased to the maximum value of 24.8 GPa at the Mo target current of 2 A and then decreased when the Mo target current further increased. The CrMoSiCN-2 coating presented the superior mechanical properties with the highest hardness of 24.8 GPa, H/E of 0.077 and H3/E2 of 0.147 GPa, respectively. This was due to its compact

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microstructure and the high content of (Cr, Mo)N substitutional solid solution contained in the coating [39].

The TEM, HRTEM and SEAD images for CrMoSiCN-2 coatings were depicted in Fig. 1. It

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was obvious that the Cr interlayer was successfully fabricated on the substrate with the thickness of 250 nm, and the microstructure of CrMoSiCN coatings was compact with no obvious defects (Fig.

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1a). As seen in Fig. 1b, the lattice fringe distance of 0.161 nm was corresponded to substitutional

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solid solution (Cr, Mo)N, while the lattice fringes distance of 0.204 nm, 0.142 nm and 0.148 nm were corresponded to CrN (200), (220) and Mo2N (220) planes, respectively. The characteristic

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diffraction rings shown in the SEAD pattern (Fig. 1a) corresponded to the (111), (200) and (220) planes of CrN (Mo2N) phase. Because the lattice fringe distance of CrN and Mo2N phases was quite

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close, it was hard to distinguish their diffraction rings at SEAD image. It could be seen from the

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TEM results that the CrMoSiCN coatings possessed homogeneous and dense microstructure. The TEM results were in agreement with the XRD analysis results of CrMoSiCN coatings [39]. According to our previous work [39], the Si and C elements mainly existed as the amorphous phases of SiNx, Si-C-N and a-C. Thus, it could be concluded that the crystal phases such as CrN, Mo2N phases and (Cr, Mo)N solid solution were embedded into the amorphous phases to form the composite nanostructure of CrMoSiCN coatings.

3.2 Electrochemical properties of CrMoSiCN coatings in artificial seawater 3.2.1 EIS test of CrMoSiCN coatings Fig. 2 showed the OCP curves for CrMoSiCN coatings in artificial seawater. All the coatings reached the stable OCP value after 1200 s. The OCP of CrMoSiCN coatings increased to the maximum value of 0.2 V at the Mo target current of 2 A, but then decreased with the increase of Mo target current. In general, the high OCP value of coatings was indicative of the good corrosion

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resistance of coatings. All OCP values of CrMoSiCN coatings were higher than those of Ti6Al4V. This indicated that the CrMoSiCN coatings possessed good corrosion resistance in artificial seawater. Furthermore, the CrMoSiCN coatings deposited at the Mo target currents of 1.5 A and 2 A

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presented the highest OCP values among all coatings. This indicated that the CrMoSiCN coatings deposited at the Mo target current of 1.5~2 A possessed better corrosion resistance.

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Fig. 3 illustrated the Nyquist plots and Bode plots of CrMoSiCN coatings in artificial seawater.

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As seen in Fig. 3a, all the Nyquist plots presented the incomplete capacitive resistance arcs. It was clear that the arcs’ diameters of CrMoSiCN-0.5~2 coatings and Ti6Al4V substrate were higher than

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those of CrMoSiCN-2.5 and CrMoSiCN-3 coatings, this meant the CrMoSiCN coatings fabricated at the Mo target current of 0.5 A to 2 A possessed the larger impedance values. Generally, the phase

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angle in Bode plot is closer to 90°, the capacitive response characteristics of coating is stronger. As

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seen in Fig. 3b, all the CrMoSiCN coatings presented the high phase angle (> 70°) at the broad range of 10-2-103 Hz, which presented the wider frequency range than Ti6Al4V substrate did. It suggested that CrMoSiCN coatings had the stronger capacitive response than Ti6-Al4-V substrate. In contrast, the phase angle of CrMoSiCN-2 coating was higher than that of other coatings. It meant that the capacitive response of CrMoSiCN-2 coating was stronger than others. Additionally, the phase angle presented negative value at high frequency, which was the evidence of the inductance

behavior due to experimental artefact links mainly to the reference electrode. As seen in Fig. 3c, all the impedance spectra presented the similar trends. Generally, the impedance (|Z|) at high frequency region represented the response characteristics of samples, while |Z| at low frequency region indicated the dielectric properties of the interface between coatings and substrate. The CrMoSiCN-2 coating had the highest |Z| value at low frequency, which reflected the highest corrosion resistance. The linear slope was closed to -1 for all coatings in the frequency range from 10-2 to 103 Hz. This

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was related to the capacitive response characteristics of electrical double layer [40]. In fact, the defects and pores in CrMoSiCN coatings were inevitable during deposition. When the coatings were exposed in seawater, the seawater could permeate into CrMoSiCN coatings

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through the defects or pores, and contacted with the substrate material, and then local corrosion occurred. As read our previous report [39], it was clear that the sample structure was consisted of

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CrMoSiCN coating, Cr interlayer and Ti6Al4V substrate, and their contributions to the

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electrochemical results should be taken into account. Hence, the equivalent circuit models with one time constant (Fig.3d) and three time constants (Fig.3e) were used to fitting the EIS spectra of bare

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Ti6Al4V substrate and CrMoSiCN coatings. In here, Rs was the electrolyte resistance which was related to the ohmic distribution between the working and reference electrode, L was the inductance,

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which was due to experimental artefact links mainly to the reference electrode. Rpo was pore

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resistance which reflected the resistance of ionic current through the paths formed by the defects of CrMoSiCN coatings. CPEpo was the capacitance characteristics of CrMoSiCN coatings. (Rpo-CPEpo) time constant described the electrochemical characteristics of CrMoSiCN coatings. Ril was related to the resistance of ionic current through the Cr interlayer, and CPEil was the related capacitance response of Cr interlayer. Ril paralleled with CPEil was the other time constant. Finally, Rct was the charge transfer resistance which was related to the electrochemical reaction at the interface of

titanium alloy/seawater, CPEdl was the capacitance characteristics of double layer formed at the interface of titanium alloy/seawater. The last time constant, Rct paralleled with CPEdl, described the electrochemical characteristics of the interface between titanium alloy and seawater. Specially, the constant phase element (CPE) was used to describe the capacitive properties of coating and substrate, and the CPE constant (n) was proposed because the real areas of coating and substrate were larger than those contacted with the seawater. The function in the form of impedance could be

ZCPE=1/[Yo1(ωj)n]

(1)

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expressed as:

where Yo1 was the admittance (Fcm-2s-n), ω was frequency (rd/s) and n was the CPE constant which

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depicted the deviation degree from pure capacitor (0 ≤ n ≤ 1). If n=1, the CPE represented the pure capacitor. The fitting data were listed in Table 5. All the fitting errors (2) were controlled within the

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magnitude order of 10-4~10-3. The pore resistance of CrMoSiCN coating (Rpo) was the important

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parameter which reflected the resistance of corrosive species through the pores, cracks and pinholes existed in the coatings. It was obvious that the Rpo values for CrMoSiCN coatings fabricated at 2 A

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was higher than that of other coatings, This indicated that the high resistance for corrosive seawater and oxygen diffusion passed through the coating due to its compact microstructure. Furthermore,

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the corrosion resistance could be estimated by the Rct value. The Rct value of Ti6Al4V substrate was

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2.038×106 Ωcm2, lower than those of CrMoSiCN coatings. This pointed out the poor corrosion resistance of bare Ti6Al4V alloy. Additionally, the Rct value increased gradually as the Mo target current increased from 0.5 A to 2 A, and then decreased. The Rct value of CrMoSiCN-2 coating was 7.116×107 Ωcm2, highest among the coatings. This suggested that the CrMoSiCN-2 exhibited the superior corrosion resistance. Actually, the CrMoSiCN coatings tended to dissolve in the electrolyte as a result of anodic reaction, and then replaced by other reaction products which possessed

low-conductivity, herein proving itself as a capacitive layer [40]. In order to obtain the effective capacitance values of CrMoSiCN coatings (Cpo) from CPE components, the contributions of both ohmic resistance and capacitive components to the coating should be considered. The formula (2) proposed by Hsu and Mansfeld [41] and further developed by Hirschorn [42] was used: 𝐶𝑝𝑜 = (𝐶𝑃𝐸−𝑌𝑜 )𝑝𝑜

1 (𝐶𝑃𝐸−𝑛)𝑝𝑜

𝑅𝑝𝑜

1−(𝐶𝑃𝐸−𝑛)𝑝𝑜 (𝐶𝑃𝐸−𝑛)𝑝𝑜

(2)

In addition, to calculate the double layer capacitance values (Cdl) of a distributed time-constant

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process using EIS data, it was essential to take both electrolyte resistance (Rs) and charge transfer resistance (Rct) for consideration [43]. The capacitance value of double layer was calculated using Eqn. (3) [44].

𝐶𝑑𝑙 = (𝐶𝑃𝐸 − 𝑌𝑜 )𝑑𝑙

1

1

𝑅𝑠

𝑅𝑐𝑡

( +

(𝐶𝑃𝐸−𝑛)𝑑𝑙 −1 (𝐶𝑃𝐸−𝑛)𝑑𝑙

)

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1 (𝐶𝑃𝐸−𝑛)𝑑𝑙

(3)

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According to Eqns. (2) and (3), the coating capacitance and double layer capacitance were calculated and illustrated in Fig. 4. It was obvious that the effective capacitance values of coatings

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varied in the range of 3.35~7.49×10-8 Fcm-2. The capacitance difference for each coating was

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probably due to the accumulation of corrosion product and the thickness decrease of CrMoSiCN coating, which caused by the corrosive decomposition in artificial seawater. Furthermore, the

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capacitance values of CrMoSiCN coatings (3.35~7.49×10-8 Fcm-2) were much lower than those of their double layers (5.09×10-6~3.96×10-5 Fcm-2). The more corrosion products with semiconducting

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properties formed on the surface of coatings could generate the active electrochemical area [45]. The CrMoSiCN-2 coating possessed the lowest double layer capacitance value of 5.09×10-6 Fcm-2. The lowest capacitance value of double layers and the highest charge transfer resistance for CrMoSiCN-2 coating demonstrated that it could be the optimum candidate coating for protecting the Ti6Al4V substrate in artificial seawater. 3.2.2 The potentiodynamic polarization tests of CrMoSiCN coatings

Fig. 5 showed the potentiodynamic polarization curves of CrMoSiCN coatings with different Mo target current in artificial seawater. The corresponding corrosion current density (icorr), corrosion potential (Ecorr), cathodic slope (βc) and anodic slope (βa) were obtained from the polarization curves with Tafel extrapolation method, and then the polarization resistances (Rp) of CrMoSiCN coatings could be calculated. As seen in Table 6, the CrMoSiCN coatings fabricated at the Mo target current of 1~3 A presented lower icorr value and higher Rp value than Ti6Al4V substrate’s. This

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indicated that the CrMoSiCN coatings exhibited better corrosion resistance than titanium alloy in seawater. Moreover, it was obvious that when the Mo target current increased, the icorr decreased to the minimum value of 2.57×10-8 A/cm2 at 2 A and then increased, while the Rp values of

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CrMoSiCN coatings first increased to the maximum value of 1.64×106 Ωcm2 at 2 A and then decreased. This indicated that the CrMoSiCN-2 coating displayed lowest corrosion rate, and

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possessed the best corrosion resistance among all coatings, which was attributed to its dense

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structure. As shown in Fig. 5, the polarization curves exhibited the stable anodic current density at the -0.1~0.18, 0~0.12, -0.2~0.2, -0.12~0.26, -0.18~0.18 and -0.08~0.08 V potential range for

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CrMoSiCN-0.5, CrMoSiCN-1, CrMoSiCN-1.5, CrMoSiCN-2, CrMoSiCN-2.5 and CrMoSiCN-3 coating, respectively. Because the potentiodynamic polarization tests were conducted at the aerated

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condition, the metal elements in the CrMoSiCN coating could be oxidized by the oxygen atoms

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dissolved in seawater. If the oxide film was formed on the coating surface, the pinholes existed in the coating were blocked by the polarized products. Accordingly, the oxygen diffusion was lowered and the current density remained stable. In a word, the formation of passive film on the coating surface during the polarization process could slow down the dissolving speed of coating. However, Ti6Al4V substrate had no obvious passive region in the polarization curve in Fig. 5. Table 6 also showed the average current density of passive region (ipass) for each sample. The ipass values

decreased from 1.29×10-7 A/cm2 for CrMoSiCN-0.5 coating to the minimum value of 4.56×10-8 A/cm2 for CrMoSiCN-2 coating, and then increased to 1.29×10-7 A/cm2 for CrMoSiCN-3 coating. In other words, the lowest ipass value for CrMoSiCN-2 coating indicated that the CrMoSiCN-2 coating combined with the passive film could provide the best protection for the substrate material in the passivation potential region. As seen in Fig. 5, the corrosion current density started to increase slowly in the transpassive region, and the transpassive potential (Etrans) was shown in Table

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6. The sudden rise of current density in the transpassive region suggested that the anodic dissolution of material was occurred. It indicated that the passive film started to dissolve at the high anodic potential, and then the coating underneath the passive film was partial destroyed by the corrosive

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seawater as the anodic potential increased. In addition, no obvious signal of pitting potential was observed during the polarization process. This pointed out that the CrMoSiCN coatings exhibited

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favorable anti-pitting corrosion properties at the scan rate of 20 mV/min.

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3.2.3 Corrosion mechanism of CrMoSiCN coatings in seawater Due to low effective diffusion coefficients and thick effective diffusion layer, the dense coating

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microstructures resulted in high electrochemical impedance and resistance to electrochemical corrosion. As was known, CrN-based coatings had column-like morphology and loose

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microstructure [46-48]. Unfortunately, the invisible defects and micro-pores as well as the columnar

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grain boundary in coatings could act as the penetrating path of the corrosive electrolyte to degrade the CrMoSiCN coating [46, 49-50]. For the CrMoSiCN coatings, because crystalline phases were embedded into amorphous phase to compose the nanocomposite microstructure, hence, the dense microstructure with fewer defects could effectively prevent the seawater from penetrating into the coating. Generally, the coating with compact microstructure always has high pore resistance. As seen in Table 5, the pore resistance of the CrMoSiCN coating deposited at 2 A was higher than those

of other coatings. Actually, the microstructure of CrMoSiCN-2 coating was denser than other coatings [39], and then possessed the high resistance for ionic current through the corresponding coatings. Among all the CrMoSiCN coatings, it was clear that the CrMoSiCN-2 coating had the highest OCP value of 0.2 V and the highest Rct value of 7.116×107 Ωcm2. Additionally, the lowest double layer capacitance (Cdl) value (5.09×10-6 Fcm-2) of CrMoSiCN-2 coating indicated that it had the fastest capacitive response speed. Furthermore, the CrMoSiCN-2 coating had the lowest icorr

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value (2.57×10-8 A/cm2), indicating that the CrMoSiCN-2 coating had the greatest resistance during the corrosion process. In a word, the CrMoSiCN coating possessed better corrosion resistance than Ti6Al4V substrate in artificial seawater, especially the CrMoSiCN coatings deposited at the Mo

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target current of 2 A.

Fig. 6 showed the surface SEM images of CrMoSiCN coatings after polarization. As seen in

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Fig. 6a-6c, it was obvious that the coating surface was covered with the semitransparent film. The

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cauliflower-like morphology underneath the semitransparent film was remained (Fib. 6d-e). This indicated that the coatings were well protected. Besides, the EDS analyses were conducted on the

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semitransparent film (point A) and coating (point B) in Fig. 6d. The EDS results showed that the concentration of oxygen element on the semitransparent film (point A) was almost three times

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higher than that on the coating (point B). Namely, the semitransparent film was an oxide film,

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which could separate seawater from CrMoSiCN coatings. It could effectively prevent the seawater from corroding the coating.

In order to confirm the composition of semitransparent oxide film,

XPS analyses were conducted on the CrMoSiCN coatings before and after polarization tests. It is necessary to deconvolute the Mo 3d spectra to estimate the oxidation. As a consequence of deconvolution, two valence states of Mo element were found in Mo 3d spectra before polarization for CrMoSiCN coatings. The binding energies of 228.5 eV and 231.7 eV were assigned to Mo-N

(Mox+, 0＜x＜+4) bonds while those located at 230.0 eV and 233.2 eV were corresponded to Mo-O (Mo4+) bonds [32, 51] in Fig. 7a. The Mo-O (Mo4+) bonds were caused by the formation of MoO2 on the coatings surface due to the exposure to air. However, after the polarization test, two new bonds were detected at 232.5 eV and 235.6 eV, which were corresponded to Mo-O (Mo6+) bonds from MoO3 [51]. It was obvious that the valence of Mo increased from 0＜x≤+4 to +6 during the polarization and the MoO3 was formed on the CrMoSiCN coatings surface, which was coincided

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with the result of SEM observation of coatings after polarization (Fig. 6a-c). Thus, it was inferred that the passive film was considered as MoO3, which could prevent the penetration of seawater and reduce the coating corrosion rate in seawater. The similar result was found in Ref. [52], in which

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MoO3 could act as a corrosion inhibitor. Furthermore, the Cr 2p spectra were composed of six peaks. The peaks located at 576.0 eV and 585.5 eV were corresponded to Cr-N bonds. The remained bonds

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were attributed to Cr-O bonds [53]. If the deconvolution results of Cr 2p before and after

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polarization tests in Fig. 7c-d were compared, the contents of binding energy of 588.4 eV and 579.3 eV corresponded to Cr-O bonds, increased slightly. This indicated that more Cr element was

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oxidized during the polarization. Consequently, the semitransparent passive film containing MoO3

process.

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and Cr2O3 acted as the physical barrier to separate the seawater from coating during the corrosion

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Nevertheless, the polarized product of MoO3 was insufficient to form the complete passive film during the polarization test due to the low Mo content (1.4 at.%) in CrMoSiCN-1 coating. As a result, the broken semitransparent film was observed on the coating surface in Fig. 6d, which could lead to the direct contact between seawater and coating. According to this theory, CrMoSiCN-3 coating should present better corrosion resistance than that of CrMoSiCN-2 coating due to a thick passivation film resulting from high Mo content (16.3 at%). However, the corrosion resistance of

CrMoSiCN-3 coating was even worse than that of CrMoSiCN-2 coating. This result can be explained as: (1) Many micro-cracks were found on the surface of CrMoSiCN-3 coating (Fig. 6f), which were caused by the inhomogeneity of stress distribution on passivation film with an increase in its thickness. Hence, it created the direct diffusion channels for oxygen, and then the oxygen diffusion speed was enlarged. (2) The more consumption of Mo element contributed to the fast degradation of coating during the polarization process. (3) The loose microstructure resulted in the

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fast diffusion speed of oxygen, more of the oxygen and corrosive species could contact with the interface of coating/substrate. Consequently, the corrosion resistance of CrMoSiCN-3 coating was weakened. In contrast, the CrMoSiCN-2 coating exhibited the lowest corrosion current value and

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highest Rct and Rpo values among all coatings. This showed that the intact passivation film and compact microstructure of CrMoSiCN-2 coating contributed to the improvement of its corrosion

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resistance. Fig. 8 showed a schematic diagram of corrosion for CrMoSiCN coatings. During the

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corrosion, the passive film was formed on the CrMoSiCN coatings surface (Fig. 8a). With the local rupture of the passivation film, the seawater was easy to penetrate into coating through the defects

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and column grain boundary existed in CrMoSiCN-1 and CrMoSiCN-3 coatings. Then, the coating was degraded fast and the substrate was experienced the intrusion of seawater. For CrMoSiCN-2

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coating, the compact microstructure and intact passive films effectively prevented the seawater and

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oxygen from penetrating the coating to corrode titanium alloy (Fig.8b). Thus, the corrosion resistance of CrMoSiCN-2 coating was enhanced. According to the above analyses, it was clear that the CrMoSiCN coatings could ensure Ti6Al4V alloy be safe against corrosion in seawater. 3.3 Tribocorrosion behavior of CrMoSiCN coatings in artificial seawater 3.3.1 Effect of sliding contact on OCP values According to the aforementioned electrochemical results, CrMoSiCN-1, CrMoSiCN-2 and

CrMoSiCN-3 coatings were selected to study the tribocorrosion behavior in artificial seawater. After 1 h immersion in seawater, the OCP values of coatings were recorded continuously before, during and after the tribocorrosion process. As shown in Fig. 9, before tribocorrosion test (in the first 10 min), the stable OCP values indicated a stable electrochemical state at the coating surface. During the tribocorrosion test (in the middle 60 min), the OCP values for all CrMoSiCN coatings and Ti6Al4V substrate dropped to the negative value once the sliding started. The sudden drop of

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OCP could be ascribed to the removal of the passive film, and then the contact area in seawater increased [54-55]. In addition, the OCP values for all coatings stabilized rapidly during the tribocorrosion test, which was due to the balance between the remove of old passive film and the

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generation of new passivation film [36]. It indicated that the CrMoSiCN coatings had a relatively stable tribocorrosion behavior in seawater. Unfortunately, the OCP value for Ti6Al4V substrate

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decreased sharply and fluctuated around -0.8 V (Fig. 9). This pointed out that Ti6Al4V substrate

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displayed the poor corrosion resistance under the sliding condition. The CrMoSiCN-2 coating had the highest OCP value about -0.34 V during the sliding process. The results demonstrated the

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improvement of tribocorrosion tendency for CrMoSiCN-2 coating as compared with other coatings. All the friction coefficients of CrMoSiCN coatings were lower than that of Ti6-Al4-V substrate

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(about 0.2), and exhibited the favorable tribological behaviors under OCP condition. The friction

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coefficient decreased as the Mo target current increased during the tribocorrosion process. The CrMoSiCN-3 coating had the minimum friction coefficient of 0.13 due to the formation of more MoO3 [39]. Once the friction sliding ceased (the last 10 min), the OCP values increased rapidly for all CrMoSiCN coatings. This indicated a gradual passivation of the fresh exposed wear track. The OCP values of CrMoSiCN-2 coating before, during and after sliding were highest, which indicated the excellent tribocorrosion characteristics under OCP condition.

In comparison to the nanocomposite structure coatings with crystalline phase and amorphous phase, the coating with crystalline phase would suffer more material loss. Thus, the composite nanostructure of CrMoSiCN coatings contributed to optimizing their tribocorrosion properties. Among all CrMoSiCN coatings, the high hardness of CrMoSiCN-2 coating could effectively prevent the mechanical destroy of the coatings during the sliding. The superior corrosion resistance of CrMoSiCN-2 coating was another vital factor which contributed to alleviating the material loss

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caused by corrosion. Actually, the OCP value was related to the electrochemical performance of PVD coatings in the electrolyte. For CrMoSiCN-1 and CrMoSiCN-2 coatings, their OCP values decreased slightly during sliding. This indicated that the removal rate of passive film was higher

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than that of re-passivation process, and then the friction coefficients of CrMoSiCN-1 and CrMoSiCN-2 coatings slightly increased. In contrast, CrMoSiCN-3 coating had a stable OCP value

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during friction sliding. This meant that the passive film removal speed reached the balance with that

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of repassivation process, and then the friction of CrMoSiCN-3 coating remained stable as well. Besides, the OCP curves for all CrMoSiCN coatings fluctuated slightly in Fig. 9, which was caused

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by the destruction and rebuilding of passive film.

3.3.2 Tribocorrosion behavior under polarization condition

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Fig. 10a showed the potentiodynamic polarization curves of the CrMoSiCN coatings with

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different Mo contents as sliding under polarization condition. It was found that the current density of anodic polarization curves for Ti6Al4V substrate fluctuated sharply, while the CrMoSiCN coatings presented the slight fluctuation trend. These fluctuations were due to the change of contact areas when the samples slid against the SiC mating balls, thus the current density fluctuated at the small range [56]. Additionally, the obvious passivation region was observed in anodic polarization curves for CrMoSiCN coating. It indicated that the passive layer was formed during the

tribocorrosion. Tafel extrapolation method was employed to obtain the experimental data from polarization curve, and the data were listed in Table 7. Ti6Al4V substrate showed the more negative value of Ecorr (-0.53 V) and higher icorr value (1.86×10-5 A/cm2) than those of CrMoSiCN coating, and possessed its bad tribocorrosion performance in seawater. As compared with the data obtained from polarization test conducted at the non-contact sliding condition, all the CrMoSiCN coatings under sliding condition presented the more negative corrosion potential and higher corrosion density.

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It was suggested that the friction behavior could accelerate the corrosion for CrMoSiCN coatings. In fact, the mechanical remove of passive film during sliding process could deteriorate the electrochemical properties of coatings, and then the corrosion potential for the CrMoSiCN coatings

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tended to decrease toward negative value under sliding condition [57]. Moreover, the increase of corrosion density under sliding condition was because the passive films were continuously

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destroyed and rebuilt during friction sliding, thus, the material loss of coating was accelerated [36].

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Among all the coatings, the CrMoSiCN-2 coating presented the lowest current density of 2.56×10-8 A/cm2 at non-contact sliding condition (Table 6) and 9.37×10-8 A/cm2 at contact sliding condition

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(Table 7). This demonstrated that the CrMoSiCN-2 coating had the best corrosion resistance owing to its superior mechanical and electrochemical properties. In contrast, the CrMoSiCN-3 coating

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showed the highest current density of 4.61×10-8 A/cm2 at non-contact sliding condition and 2.22×

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10-7 A/cm2 at contact sliding condition, respectively. The current density of CrMoSiCN-3 coating was twice that of CrMoSiCN-2 coating during tribocorrosion test. It could be concluded that the tribocorrosion performance for the CrMoSiCN coatings reduced as the Mo target current increased to 3 A. Fig. 10b shows the friction coefficient of CrMoSiCN coatings sliding against SiC balls under the polarization condition in artificial seawater. It was obvious that the friction coefficient for

CrMoSiCN coatings fluctuated at a small range and decreased with an increase in the Mo target current, lower than that of Ti6-Al4-V (about 0.25) substrate. As a result, the CrMoSiCN-3 coating exhibited the lowest friction coefficient of 0.12. Besides, the material loss volume first decreased to the minimum value of 8.76×10-4 mm3 for CrMoSiCN-2 coating, and then increased to 1.31×10-3 mm3 for CrMoSiCN-3 coating in Fig. 11. There was a 50.4% increase of materials loss volume when the Mo target current increased from 2 A to 3 A. Fig. 12 shows the profiles of wear tracks for

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CrMoSiCN coatings. The depths of wear tracks were 0.54 μm, 0.42 μm and 0.62 μm for CrMoSiCN-1, CrMoSiCN-2 and CrMoSiCN-3 coatings, respectively. The CrMoSiCN-2 coating presented the minimum wear depth of 0.42 μm. The widths of wear tracks on CrMoSiCN coatings

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varied at the range of 400-450 μm. In brief, CrMoSiCN-2 coating showed superior tribocorrosion performance in artificial seawater.

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3.3.3 Contribution of tribocorrosion components to material loss

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Actually, the friction behavior could accelerate corrosion during the tribocorrosion test, and vice versa. The material loss volume was not simply the sum of loss caused by friction and loss

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caused by corrosion. Therefore, the synergistic effect of friction and corrosion cannot be ignored [58-59]. Waston et al. [60] studied the synergistic effect of friction and corrosion and proposed the

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following formula to evaluate the role of synergistic effect during the tribocorrosion process: (4)

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Wt=Wf + Wc + ΔWs

The total material loss volume of coatings (Wt) is defined as the sum of material loss during the pure friction test (Wf) and material loss during the pure corrosion test (Wc) as well as material loss caused by synergistic effect (ΔWs). The ΔWs is the sum of the material loss of corrosion caused by friction (ΔWcf) and the material loss of friction caused by corrosion (ΔWfc). Therefore, the ΔWs could be defined as:

ΔWs= ΔWfc + ΔWcf

(5)

After the tribocorrosion test, the material loss Wt and Wf of CrMoSiCN coatings could be determined by the cross sectional wear track profiles in the tribocorrosion test and pure friction test conditions, respectively. According to the Faraday’s law, material loss of Wc and ΔWcf were calculated using Eq. (6): 𝑊=

ItM

(6)

nFρ

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where W is the material loss caused by the pure corrosion or the corrosion caused by friction, I is the corresponding average current during the test, t is the test time, M is the relative atomic mass of material, n is the change of valence of element during corrosion process, F is the Faraday’s constant

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and ρ is the materials density. After that, the material loss of friction caused by corrosion (ΔWfc) could be calculated using Eqns. (4) and (5).

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The contributions of tribocorrosion components to the material loss during the tribocorrosion

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test conducted at polarization condition were illustrated in Fig. 13. It was clear that the proportions of Wf were 79.91%, 68.84% and 62.90%, respectively. It indicated that the material loss was

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dominated by the pure friction behavior. The proportions of synergistic effect ΔWs were 19.92%, 27.44% and 36.95%, respectively, the material loss of friction caused by corrosion (ΔWfc) was

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dominated in the synergistic effect for all the CrMoSiCN coatings. When the Mo target current

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increased, the proportion of synergistic effect and ΔWfc increased. Additionally, the material loss volume caused by pure corrosion (Wc) and corrosion caused by friction (ΔWcf) accounted for a small proportion. As a result, the friction behavior mainly determined the degradation of the CrMoSiCN coating during tribocorrosion process. In order to further analyze the tribocorrosion mechanism of CrMoSiCN coatings sliding against SiC balls under the polarization condition, the SEM images of wear tracks on CrMoSiCN

coatings were illustrated in Fig. 14. For the CrMoSiCN-1 coating, the wear track was clear and smooth, but some black wear debris adhered to the wear track. It could be seen from the local magnification of the wear track that many micro-scratches were observed but no obvious peeling off or pitting corrosion was detected. As could be easily understood, high contact stress generated when SiC balls contacted with the coating during the tribocorrosion process. Peeling off occurred on the coating and the solid particles existed in the interface between mating balls and coatings.

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Thus, the two-body/three-body abrasion mechanism was formed, which accounted for the scratches on the wear track. According to the EDS analysis of the selected point of A, only 14.1 at.% of O content was detected (Fig. 14a). It was clear that the tribolayers containing Cr2O3 and MoO3 were

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formed on the wear tracks during the tribocorrosion, and provided a good lubrication effect. Unfortunately, the insufficient lubrication failed to effectively alleviate the abrasive wear due to the

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low Mo content in CrMoSiCN-1 coating. Thus, the scratches were observed on the wear track, and

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the wear mechanism was abrasion wear and tribochemical wear. For the CrMoSiCN-2 coating, the wear track was smooth with no obvious micro-scratches but some wear debris agglomerated on the

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wear track in Fig. 14b. The EDS analysis on the selected point of B detected huge amount of O content in Fig. 14b, this indicated that the surface of wear track was oxidized during tribocorrosion

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test. It proved that the oxides consisting of Cr2O3 and MoO3 were generated during both wear and

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polarization test. Meanwhile, the more MoO3 product generated during the tribocorrosion test, the better lubrication condition was formed. This may be the reason that the material loss caused by synergistic effect increased from 19.92% to 27.44% in Fig. 13. In addition, the amorphous of SiNx and Si-C-N in CrMoSiCN coatings could react with H2O to generate the Si(OH)4 gel, which contributed to form the well lubrication as well [11, 13]. Consequently, the abrasion wear was eliminated, and the wear mechanism transferred into tribochemical wear. For the CrMoSiCN-3

coating, the severe local peeling-off occurred on the wear track in Fig. 14c. According to the EDS analysis on the peeling-off region (point C), 37.6 at.% of O element was detected, suggesting the huge amount oxides generated. Moreover, elements of Cl, Br and Mg were also detected as well which came from the artificial seawater. The friction coefficient decreased as the Mo target current increased. It could be inferred that the more generation of MoO3 contributed to decreasing the friction coefficient during tribocorrosion process. However, the more generation of MoO3 meant the

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more Mo content consumption on the coating surface. Thus, the surface of wear track was easy to be worn-off, and then the peeling-off occurred on the wear track of CrMoSiCN-3 coating due to its low hardness and H/E value. Hence, the seawater entered the peeling-off region to form the local

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corrosion and the material loss volume was exaggerated due to the synergistic effect of friction and corrosion [56]. As a result, the CrMoSiCN-3 coating experienced severe abrasion, which agreed

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well with the results of total material loss volume for CrMoSiCN coatings in Fig. 11. The dominant

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wear mechanism was tribochemical wear and corrosive wear for CrMoSiCN-3 coating. From the above, this would explain why the proportion of material loss caused by synergistic effect of

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friction-corrosion was the largest (36.95%) in Fig. 13. To sum up, the tribocorrosion performance of CrMoSiCN coatings was largely dependent on their mechanical and electrochemical properties. The

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CrMoSiCN-2 coating possessed low friction coefficient, low material loss volume, relative flat wear

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track and low current density during the tribocorrosion test, which were the proof of the excellent tribocorrosion properties in seawater. 4. Conclusions

(1) The crystalline phases (CrN and Mo2N) and substitutional solid solution of (Cr, Mo)N were embedded into amorphous matrix of SiNx, Si-C-N and a-C to form the nanocomposite structures in CrMoSiCN coatings.

(2) The semitransparent passive film containing MoO3 and Cr2O3 acted as physical barrier to separate the seawater from coating during the electrochemical corrosion process. The substrate was well protected by both the semitransparent passive film and the nanocomposite structures of CrMoSiCN coatings in seawater. CrMoSiCN-2 coating possessed the best corrosion resistance due to its intact passive film and compact microstructure. (3) During the tribocorrosion test, the friction behaviors were the main reason for the degradation of

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CrMoSiCN coatings. The more Mo content containing in coating contributed to decreasing the friction coefficient but increasing the proportion of material loss caused by synergistic effect. CrMoSiCN-2 coating exhibited the best tribocorrosion resistance due to its superior mechanical

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properties and favorable corrosion resistance. Data availability

Conflict of Interests

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also forms part of an ongoing study.

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The raw/processed data required to reproduce these finds cannot be shared at this time as the data

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The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

Authors’ statement

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Yongqiang Fu: Conceptualization, Investigation, Formal analysis, Writing – original draft; Fei Zhou: Supervision, Conceptualization, Formal analysis, Writing – Review and Editing. Qianzhi Wang: Validation, Formal analysis. Maoda Zhang: Investigation, Formal analysis. Zhifeng Zhou: Resources.

Acknowledgements This work has been supported by National Natural Science Foundation of China (Grant No. 51775271) and Key Laboratory Project of Helicopter Transmission Technology (Grant No.

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HTL-A-19G04). We would like to thanks them for the financial support.

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[47] Q. Ma, F. Zhou, Q.Z. Wang, Z.W. Wu, K.M. Chen, F.Z. Zhou, L.K.Y. Li, Influence of CrB2 target current on the microstructure, mechanical and tribological properties of Cr–B–C–N coatings in water, RSC Adv. 6 (53) (2016) 47698–711. [48] F.L. Ma, J.L. Li, Z.Z. Zeng, Y.M. Gao, Structural, mechanical and tribocorrosion behavior in artificial

seawater of CrN/AlN nano-multilayer coatings on F690 steel substrates, Appl. Surf. Sci. 428 (2018) 404-414. [49] D.M. Marulanda, J.J. Olaya, U. Piratoba, A. Marino, E. Camps, The effect of bilayer period and degree of unbalancing on magnetron sputtered Cr/CrN nano-multilayer wear and corrosion, Thin Solid Films 519 (6) (2011) 1886–93. [50] H.J. Zhao, F.X. Ye, Effect of Si-incorporation on the structure, mechanical, tribological and corrosion properties of WSiN coatings, Appl. Surf. Sci. 356 (2015) 958-966.

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Phys. 129 (2011) 138–147.

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[56] Y. Wang, J.L. Li, C.Q. Dang, Y.X. Wang, Y.J. Zhu, Influence of carbon contents on the structure and tribocorrosion properties of TiSiCN coatings on Ti6Al4V, Tribol. Int. 109 (2017) 285-296.

[57] M. Salasi, G.B. Stachowiak, G.W. Stachowiak, New experimental rig to investigate abrasive-corrosive characteristics of metals in aqueous media, Tribol. Lett. 40 (2010) 71–84 [58] H.B. Lee, D.S. Wuu, C.S. Lin, C.Y. Lee, Synergy between corrosion and wear of electrodeposited Ni-P coating in NaCl solution, Tribol. Int. 44 (2011) 1603–1609.

[59] H.B. Lee, M.Y. Wu, A study on the corrosion and wear behavior of electrodeposited Ni-W-P coating, Metall. Mater. Trans. A 48 (2017) 4667–4680. [60] S. W. Watson, F. J. Friedersdorf, B. W. Madsen, S. D. Cramer, Methods of measuring wear-corrosion

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synergism, Wear 181 (1995) 476-484.

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Fig. 1 (a) TEM, SEAD and (b) HRTEM images of CrMoSiCN coating.

0.5

Ti6Al4V CrMoSiCN-0.5 CrMoSiCN-1 CrMoSiCN-1.5 CrMoSiCN-2 CrMoSiCN-2.5 CrMoSiCN-3

Environment: seawater Reference electrode:Ag/AgCl

0.4

E vs SCE(V)

0.3 0.2 0.1 0.0 -0.1 -0.2 0

600

1200

1800

2400

3000

3600

Time(s)

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Fig. 2 Open circuit potential for Ti6Al4V and CrMoSiCN coatings in seawater.

2.5x106

100

(a)

(b)

2.0x104

1000 mHz

80

-Z(im)(ohm/cm2)

1778 mHz

1.5x10

6

1.0x106

1.5x104 1 Hz

60

1.0x104

5.0x103

100 KHz

0.0 0

0.0

100 KHz

0.0

1x103

2x103

Ti6-Al4-V CrMoSiCN-0.5 CrMoSiCN-1 CrMoSiCN-1.5

5.0x105

5.0x105

-Phase(deg)

-Z(im)(ohm/cm2)

2.0x106

1.0x106

3x103

4x103

5x103

Z(re)(ohm/cm2)

40

0

CrMoSiCN-2 CrMoSiCN-2.5 CrMoSiCN-3

-20

2.0x106

-40 -3

1.5x106

Ti6-Al4-V CrMoSiCN-0.5 CrMoSiCN-1 CrMoSiCN-1.5 CrMoSiCN-2 CrMoSiCN-2.5 CrMoSiCN-3

20

2.5x106

2

-2

Z(re)(ohm/cm )

(c)

Ti6-Al4-V CrMoSiCN-0.5 CrMoSiCN-1 CrMoSiCN-1.5 CrMoSiCN-2 CrMoSiCN-2.5 CrMoSiCN-3

6

4 3 2 1 0 -3

Environment:seawater Reference electrode: Ag/AgCl High frequency: 100000 Hz Low frequency: 0.001 Hz -2

-1

0

1

2

3

4

1

Log f(Hz)

2

3

4

5

5

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Log f(Hz)

-p

Log|Z|(Ohm cm2)

5

0

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7

-1

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Fig. 3 Nyquist plots (a), Bode phase angle plots (b), Bode impedance magnitude plots (c) of Ti6Al4V and CrMoSiCN coatings in seawater and the corresponding equivalent circuit models for Ti6-Al4-V alloy (d) and CrMoSiCN coatings (e).

1.0x10-7

1x10-4 Cpo Cdl

8x10-5

6.0x10-8

6x10-5

4.0x10-8

4x10-5

2.0x10-8

2x10-5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Cdl(F/cm2)

Cpo(F/cm2)

8.0x10-8

0

Mo target current(A)

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Fig. 4 Variations of capacitance (Cpo) and double layers (Cdl) for CrMoSiCN coatings with Mo target current in seawater.

-3 -4

CrMoSiCN-2 CrMoSiCN-2.5 CrMoSiCN-3

Ti6-Al4-V CrMoSiCN-0.5 CrMoSiCN-1 CrMoSiCN-1.5

Log(i A/cm2)

-5

Transpassive region

-6 -7 -8

Passive region

-9 -10 -0.6

Etrans -0.4

-0.2

0.0

0.2

0.4

0.6

Potential(V vs SCE)

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Fig. 5 Polarization plots of Ti6Al4V and CrMoSiCN coatings under no sliding contact condition in seawater.

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Fig. 6 SEM images of CrMoSiCN coatings after polarization tests under no sliding contact condition: (a), (d) CrMoSiCN-1 coating, (b), (e) CrMoSiCN-2 coating and (c), (f) CrMoSiCN-3 coating.

(a)

Mo3d 5/2

Mo3d 3/2 Mox+

Mox+ Mo

6+

236

Mo4+

Mo6+

234

Mox+ Mo4+

232

230

Cr-N

226

Cr-O

(d) Cr-O

228

Cr-N

Cr-O

Cr-O Cr-O

(b)

238

Mox+

Intensity(a.u.)

Intensity(a.u.)

Mo

Cr2p 1/2

Mo4+

4+

Cr2p

Cr2p 3/2

(c)

Mo3d

592

Cr-O

Cr-N

Cr-O

Cr-O

590

588

Binding energy(eV)

586

584

582

Cr-N

580

578

576

574

Binding energy(eV)

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Fig. 7 Mo 3d and Cr 2p XPS for CrMoSiCN coatings before (a), (c) and after (b), (d) polarization test.

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Fig. 8 Schematic diagram of anticorrosion mechanism for CrMoSiCN coatings.

Ti6-Al-4V CrMoSiCN-1 Wear start

0.0 -0.2

CrMoSiCN-2 CrMoSiCN-3

0.5

Wear end

-0.4

Potential(V)

0.6

0.4

-0.6 0.3

-0.8 -1.0

0.2

-1.2 0.1

-1.4

-1.8

Wear end

Wear start

-1.6

Friction coefficient(m)

0.2

0.0 0

10

20

30

40

50

60

70

80

Time(min)

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Fig. 9 OCP measurements and respective friction coefficient curves of Ti6Al4V and CrMoSiCN coatings sliding against SiC balls in artificial seawater.

-3

0.40

(a)

Ti6-Al4-V CrMoSiCN-1 CrMoSiCN-2 CrMoSiCN-3

(b)

Log(i A/cm2)

-4 -5 -6 -7

Ti6-Al4-V CrMoSiCN-1 CrMoSiCN-2 CrMoSiCN-3

-8 -9 -0.8

-0.6

-0.4

-0.2

0.0

0.2

Friction coefficient(m)

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

0

10

20

30

40

50

60

Time(min)

Potential (V)

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Fig. 10 Tribocorrosion behaviors of Ti6Al4V and CrMoSiCN coatings sliding against SiC balls under polarization condition: (a) polarization curves and (b) variation of friction coefficient.

Material loss volume(mm3)

1.3x10-3 1.2x10-3 1.1x10-3 1.0x10-3 9.0x10-4 8.0x10-4

CrMoSiCN-1

CrMoSiCN-2

CrMoSiCN-3

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Fig. 11 Material loss volume for different CrMoSiCN coatings sliding against SiC balls under polarization condition.

(a)

(b)

CrMoSiCN-1

0.2

0.0

0.0

0.0

Depth(mm)

0.2

-0.2

-0.2 -0.4

-0.4

-0.6

-0.6

-0.6

0

500

1000

1500

2000

2500

-0.8

-0.8

0

Width(mm)

500

1000

1500

2000

2500

CrMoSiCN-3

-0.2

-0.4

-0.8

(c)

CrMoSiCN-2

0.2

Depth(mm)

Depth(mm)

0.4

0.4

0.4

0

Width(mm)

500

1000

1500

2000

2500

Width(mm)

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Fig. 12 Cross-sectional profiles of wear tracks for CrMoSiCN coatings sliding against SiC balls under polarization condition.

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Fig. 13 Tribocorrosion components for different CrMoSiCN coatings sliding against SiC balls under polarization condition in seawater.

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Fig. 14 SEM images of wear tracks for CrMoSiCN coatings sliding against SiC balls under polarization condition and the corresponding EDS results of selected points.

Table 1 Chemical composition and mechanical properties of Ti6Al4V substrate. Chemical composition (at.%)

Mechanical properties

Al

V

Fe

O

C

N

H

Hardness

Elastic modulus

89.62

5.92

4.14

0.146

0.15

0.015

0.008

0.001

32 HRC

112 GPa

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Ti

Table 2 Chemical composition of artificial seawater Compound

Concentration (g/L)

MgCl2·6H2O

555.6

CaCl2

57.1 2.1

SrCl2·6H2O

69.5 20.1 10.0 2.7 0.3 24.5 4.094

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KCl NaHCO3 KBr H3BO3 NaF NaCl Na2SO4

Table 3 Chemical composition of CrMoSiCN coatings. Cr (at.%)

Mo (at.%)

Si (at.%)

C (at.%)

N (at.%)

CrMoSiCN-0.5

41.6

1.4

4.8

15.3

36.9

CrMoSiCN-1

40.2

2.9

4.2

16.1

36.6

CrMoSiCN-1.5

41.6

6.9

4.1

17.0

30.4

CrMoSiCN-2

35.1

10.1

4.3

22.0

28.5

CrMoSiCN-2.5

36.3

12.6

4.2

17.6

29.3

CrMoSiCN-3

35.6

16.3

3.5

17.0

27.6

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Coatings

Table 4 Hardness, elastic modulus, H/E and H3/E2 for CrMoSiCN coatings at different Mo target currents. Hardness (GPa)

Elastic modulus (GPa)

H/E

H3/E2 (GPa)

CrMoSiCN-0.5

21.0±1.7

287.5±12.2

0.072

0.112

CrMoSiCN-1

21.9±3.1

299.7±27.7

0.073

0.117

CrMoSiCN-1.5

22.2±3.1

294.0±26.2

0.075

0.126

CrMoSiCN-2

24.8±1.9

321.7±12.0

0.077

0.147

CrMoSiCN-2.5

21.2±2.6

275.0±18.7

0.076

0.125

CrMoSiCN-3

22.5±3.0

305.4±24.8

0.073

0.121

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Coatings

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Table 5 Fitted results of EIS spectra for Ti6Al4V and CrMoSiCN coatings in artificial seawater.

Coatings

Rs (Ωcm2)

(CPE-Yo)po (Fcm-2s-n)

Rpo (Ωcm2)

Ti6Al4V

30.340

-

-

-

-

CrMoSiCN-0.5

3.384

8.09×10-8

0.8964

3.951×104

CrMoSiCN-1

7.975

1.651×10-7

0.8777

CrMoSiCN-1.5

2.043

7.744×10-8

0.8630

CrMoSiCN-2

2.437

1.110×10-7

0.8622

CrMoSiCN-2.5

3.160

9.134×10-8

0.9292

CrMoSiCN-3

1.033

1.017×10-7

0.9400

(CPE-n)il

Ril (Ωcm2)

(CPE-Yo)dl (Fcm-2s-n)

(CPE-n)dl

Rct (Ωcm2)

L (Hcm2)

2

-

5.888×10-5

0.8230

2.038×106

-

7.7×10-3

6.648×10-6

0.9373

4.931×105

1.458×10-5

0.9807

1.196×107

5.523×10-6

2.1×10-3

1.382×104

5.061×10-6

0.7410

1.622×105

7.090×10-5

0.8423

1.198×107

1.135×10-6

6.4×10-4

6.608×104

1.275×10-5

0.7961

3.604×105

5.179×10-5

0.9073

6.120×107

4.454×10-6

2.8×10-3

pr

-

Pr 8.482×104

6.756×10-6

0.9119

3.947×105

1.545×10-5

0.9017

7.116×107

3.272×10-6

2.7×10-3

1.030×104

3.161×10-6

0.8123

1.533×105

8.824×10-5

0.9108

1.458×107

8.831×10-6

4.3×10-3

8.098×104

2.028×10-6

0.8789

2.309×105

8.146×10-5

0.8048

1.170×107

4.376×10-6

4.2×10-3

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(CPE-Yo)il (Fcm-2s-n)

e-

(CPE-n)po

Table 6 Potentiodynamic polarization results of Ti6Al4V and CrMoSiCN coatings under no sliding contact condition in artificial seawater. Coatings Ecorr(V) icorr(A/cm2) βa(V) βc(V) Rp(Ωcm2) ipass(A/cm2) Etrans(V)

CrMoSiCN-0.5

-0.23

CrMoSiCN-1

-0.13

CrMoSiCN-1.5

-0.36

CrMoSiCN-2

-0.20

CrMoSiCN-2.5

-0.27

CrMoSiCN-3

-0.23

4.98×10-8 5.45×10-8 3.62×10-8 3.13×10-8 2.57×10-8 4.85×10-8 4.61×10-8

0.39

0.09

0.36

0.12

0.26

0.12

0.38

0.13

0.42

0.13

0.38

0.16

0.32

0.12

7.23×105 6.99×105 9.89×105 1.31×106 1.64×106 8.92×105 8.35×105

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-

-

1.29×10-7

0.18

7.29×10-8

0.12

7.13×10-8

0.20

4.56×10-8

0.26

1.50×10-7

0.18

1.29×10-7

0.12

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-0.30

-p

Ti6Al4V

Table 7 Potentiodynamic polarization results of Ti6Al4V and CrMoSiCN coatings under sliding contact condition in artificial seawater. Ecorr (V)

icorr (A/cm2)

Ti6Al4V

-0.53

1.86×10-5

CrMoSiCN-1

-0.28

1.81×10-7

CrMoSiCN-2

-0.38

9.37×10-8

CrMoSiCN-3

-0.30

2.22×10-7

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Coatings

55