Tribological and electrochemical properties of Cr–Si–C–N coatings in artificial seawater

Abstract

Cr–Si–C–N coatings were deposited on Si(100) wafer and stainless steel 316L by using unbalanced magnetron sputtering via adjusting trimethylsilane (Si(CH₃)₃H, or TMS) flow, and their microstructure and hardness were analyzed by using EDS, SEM, XRD, XPS and nano-indenter. The electrochemical behavior of coatings in seawater were measured by using a CHI660D electrochemical workstation, and the tribological properties of coatings sliding against SiC balls in artificial seawater were investigated by using ball-on-disk tribometers. The results indicated that there were nanocrystallites of Cr(C, N) crystal and amorphous phases such as a-Si₃N₄ and a-C(a-CN_x) in the CrSiCN coatings. With an increase in the TMS flow, the hardness of CrSiCN coatings first increased to the maximum value of 21.3 GPa, and then decreased to 13 GPa. As compared with the CrCN coating, the CrSiCN coatings exhibited a lower friction coefficient. The pitting corrosion only appeared on the wear tracks, and therefore the mechanical wear and wear-accelerated corrosion were the main material deterioration mechanisms in seawater. The impedance modulus and phase value became high in the low frequency range with an increase in the TMS flows. Because the amorphous phase of silicide dispersed in the grains could effectively improve the density of the coatings, thus the coatings deposited at higher TMS flows exhibited better corrosion resistance.

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1. Introduction

When exploiting marine resources, some marine equipments such as pumps and hydraulic drive systems have to be exposed in seawater. In order to prolong their service life, the components are required to possess both excellent tribological properties and good corrosion-resistance in seawater.^1–3 Consequently, the ocean tribology as a new research field of tribology is being developed, which has an important significance for ocean utilization.⁴

It is well-known that the CrN-based coatings possess superior resistance to oxidation and corrosion due to the formation of passive films as compared with the TiN-based coatings in aggressive solutions such as simulated body fluid and seawater.^5–7 Thus, many researchers have committed to enhancing the overall quality of CrN-based coatings via using element incorporation. It is clear that the hardness, friction behavior and anti-corrosion performance of CrN-based coatings could be further improved by doping Si or C elements.^8–16 For example, the CrCN or CrSiN coatings showed high hardness (≥24 GPa) due to the formation of fine nanocomposite structure and solid solution effect.^8–12 The friction coefficient of CrCN/Si₃N₄ tribopairs decreased from 0.32 to 0.18 in seawater.¹⁵ When the CrSiN coatings slid against WC balls in seawater, the passive layer contained Cr₂O₃ and SiO₂ could be formed on the wear track, which could fill the cracks to prevent seawater penetrating into the micro-cracks.¹⁷ Furthermore, the CrCN and CrSiN coatings presented the improved corrosion resistance in seawater and 3.5% NaCl solution respectively owing to dense microstructure.^15,16 However, as compared with the CrCN and CrSiN coatings, the lower coefficient of friction was obtained for CrSiCN coatings as sliding against steel balls in air due to tribochemical reaction of amorphous silicon nitride phase.¹⁸ In addition, the solid solution effect of Si elements and the formation of nano-crystalline Cr(C, N) and amorphous SiN_x/SiC_x (a-SiN_x/a-SiC_x) nanocomposite all contributed to the enhancement of hardness of CrSiCN coatings,^18,19 and the favourable elastic recovery and compressive stress endowed CrSiCN (C_Si = 2.05 at%) coatings with a potential to inhibit radial crack.¹⁹ As a result, the CrSiCN coatings exhibited better tribological behavior than CrN and CrCN coatings owing to the synergistic effect of amorphous carbon layer and tribochemical (tribo-oxidation) reaction.^18,20,21 Recently, Zhou's group has reported that the lowest friction coefficient of 0.11 was obtained when the CrSiCN coating slid against SiC ball in deionizer water.²² Moreover, the electrochemical impedance spectroscopy (EIS) of CrSiCN coatings in 3.5% NaCl solution demonstrated that the CrSiCN coatings (C_Si = 1.4 at%) displayed better corrosion resistance than CrN and CrSiCN (C_Si = 3.7 at%) coatings.²¹

Actually, when the friction tests were performed in an aggressive solution, the interaction of wear and corrosion led to a more deterioration of material, namely tribocorrosion.²³ Because the seawater is much more corrosive than deionizer water, thus it is imperative to know the tribological and electrochemical properties of Cr–Si–C–N coatings in seawater. However, until now, the research works related to the electrochemical and tribological properties of Cr–Si–C–N coatings in seawater have not yet been performed. In here, the Cr–Si–C–N coatings were deposited on Si(100) wafer and 316L stainless steel by using closed-field unbalanced magnetron sputtering system. The tribological behavior of Cr–Si–C–N coatings sliding against SiC balls in seawater was investigated, and their electrochemical properties were also evaluated via electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The influences of TMS flow on the microstructure, tribological and electrochemical properties of Cr–Si–C–N coatings were analyzed and discussed.

2. Experimental details

2.1 Deposition of Cr–Si–C–N coatings

The Cr–Si–C–N coatings were deposited on Si(100) wafer and 316L stainless steel by using closed-field unbalanced magnetron sputtering system (UDP-650, Teer Coatings Limited, UK) with a four-target configuration (2Cr + 2graphite targets). During deposition, the silicon elements were derived from the decomposition of trimethylsilane (Si(CH₃)₃H, or TMS), and the partial pressure of N₂ reactive gas was controlled by optical emission monitor (OEM) with rapid feedback. The thickness of CrSiCN coatings and Cr interlayer was about 2 μm and 0.4 μm, respectively. The deposition process has been described in detail in our previous report,²² and the deposition parameters of Cr–Si–C–N coatings are listed in Table 1. According to TMS flows, the CrSiCN coatings in the next section would be named as CrCN, CrSiCN-5, CrSiCN-10, CrSiCN-15, CrSiCN-20, CrSiCN-25, and CrSiCN-30, respectively.

Table 1 Deposition parameters of CrSiCN coatings using closed-field unbalanced magnetron sputtering system

Process parameters	Corresponding values
Base pressure	4 × 10^−4 Pa
Working pressure	0.23 Pa
Purity of Cr target	99.9%
Cr targets (2) current	4.0 A
Purity of C target	99.99%
C targets (4) current	1.0 A
Substrate temperature	150 °C
Substrate bias voltage	−80 V
Rotational velocity of substrate	10 rpm
Typical deposition rate	1.2 μm h^−1
Pressure of nitrogen	OEM = 50%
TMS flows	0, 5, 10, 15, 20, 25, 30 sccm

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2.2 Characterization of Cr–Si–C–N coatings

The surface micrographs of Cr–Si–C–N coatings were observed by a field emission scanning electron microscope (SEM) (JEOL-JSM-7001F) equipped with EDS (Inca Energy 350, Oxford, UK). And the crystal phase of Cr–Si–C–N coatings was characterized using X-ray diffractometer (XRD, Ultima IV, Japan) with Cu K_α radiation source (λ = 0.1542 nm) at a scan rate of 5° per minute from 20° to 80°. The bonding structures of Si2p and C1s were analyzed by X-ray photoelectron spectroscopy (ESCALAB 250, Thermo Scientific). The hardness (H) and Young's modulus (E) of Cr–Si–C–N coatings were measured by using nanoindentation tester (ENT-1100a, Elionix Co. Ltd., Japan) with an indentation penetration depth of 100 nm. To ensure the reliability of the data, the measurement for one sample was repeated for 36 times.

2.3 Friction test in seawater

The friction behaviour of Cr–Si–C–N coatings sliding against SiC balls in seawater was investigated by using a ball-on-disc tribometer. The parameters of SiC balls are shown as: hardness 22 GPa, elastic modulus 430 GPa, surface roughness 88.5 nm, and diameter 8 mm, respectively. According to the American National Standard (D 1141-98), the artificial seawater was prepared and its chemical composition was listed in Table 2. The friction tests were carried out at the normal load of 2 N and the sliding speed of 0.1 m s⁻¹ with total sliding distance of 500 m. The friction tests were done for three times to ensure the reliability of data, and therefore the mean values of friction coefficient and wear rate could be obtained. The wear tracks on coatings and mating balls were observed by SEM and the optical microscope (XJZ-6) separately, while the cross-section area of wear track for the CrSiCN coatings was measured by using Micro-XAM™ non-contact optical profilometer (ADE Phase-Shift, USA). Consequently, the specific wear rates of balls and coatings were calculated.²²

Table 2 Chemical composition of artificial seawater

Compound	Concentration, g L^−1
NaCl	24.53
MgCl2	5.20
Na2SO4	4.09
CaCl2	1.16
NaHCO3	0.201
KCl	0.695
KBr	0.101
H3BO3	0.027
SrCl2	0.025
NaF	0.003

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2.4 Electrochemical tests in seawater

The CrSiCN coatings deposited on Si(100) wafers were used for the electrochemistry test. A copper wire was first connected on coating by conductive carbon tape, and then the sample was enveloped by 704 silicon rubber with an exposure area of 1 × 1 cm². Electrochemical measurement was performed by using a standard three electrode electrochemical cell.²⁴ After samples were immersed in electrolyte for 1 hour, the electrochemical impedance spectrum (EIS) was performed at OCP with an AC excitation of 10 mV over a frequency range from 1 mHz to 100 kHz. To ensure the reliability of data, one test was repeated for three times. Subsequently, the potentiodynamic polarization test was carried out via polarizing specimen in the anodic direction from −1 V to 1 V at a sweep rate of 20 mV min⁻¹.

3. Results and discussion

3.1 Composition, chemical bonding and microstructure of Cr–Si–C–N coatings

As seen in Table 3, the content of Si and C increased from 0 at% to 7.0 at% and from 8.7 at% to 33.4 at% separately due to decomposition of TMS, while that of N decreased from 42.5 at% to 6.1 at%. Similarly, ref. 25 pointed out that the addition of Si caused nitrogen atoms moving out of the solid solution Ti(C, N) crystal lattice, which was due to the relative deficiency of nitrogen source during the deposition, as the partial pressure of silicon source increased under a fixed N₂ partial pressure. As seen the X-ray diffraction patterns of Cr–Si–C–N coatings in Fig. 1, it is clear that three main peaks of (111), (200) and (220) orientations for Cr(C, N) crystal planes were observed, while the crystal phase of silicide was not detected.

Table 3 Chemical composition and the mechanical properties of CrSiCN coatings deposited at different TMS flows

Coatings	TMS flow (sccm)	Cr (at%)	Si (at%)	C (at%)	N (at%)	Hardness (GPa)	Reduced elastic modulus (GPa)
CrCN	0	48.8	0.0	8.7	42.5	18.1 ± 1.0	293.2 ± 9.9
CrSiCN-5	5	44.8	1.0	9.3	45.0	19.6 ± 1.0	282.5 ± 9.8
CrSiCN-10	10	42.5	2.1	13.5	42.0	21.3 ± 0.9	300.1 ± 8.5
CrSiCN-15	15	50.5	3.4	16.2	30.0	19.4 ± 0.6	306.1 ± 5.9
CrSiCN-20	20	52.3	5.4	25.3	17.0	13.6 ± 0.3	244.7 ± 3.8
CrSiCN-25	25	55.6	6.2	29.5	8.7	13.2 ± 0.2	236.3 ± 3.0
CrSiCN-30	30	53.5	7.0	33.4	6.1	13.0 ± 0.2	233.8 ± 2.4

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Fig. 1 X-ray diffraction patterns of Cr–Si–C–N coatings deposited at different TMS flows.

In order to observe the chemical bonding status of Si and C in Cr–Si–C–N coatings, the corresponding XPS spectra were deconvoluted using XPS 4.1 software. The Si2p XPS spectra were deconvoluted into two peaks in Fig. 2a, which were in good agreement with the binding energy of SiCN and Si₃N₄ phase.²² However, the binding energies of Si–C–N shifted to high energy when the Si content was higher than 3.4 at%, and the fraction of Si–N bonds decreased from 51.6% to 20.1% in Fig. 2c. That result indicated that the SiC-like Si–C–N bonds were changed into the Si₃N₄-like Si–N–C bonds. As seen in Fig. 2b, the sp²C–N/sp²C–C bond was detected at 284.4–284.7 eV,²² and the corresponding fraction of bond kept at 65.2–78.0 at% in Fig. 2d. After doping the silicon atoms into the CrCN coatings, the first peak located at 282.2–283 eV, which was attributed to C–Cr/C–Si bond, and the fraction of bond also increased from 3.2 at% to 20.1 at%. According to the analysis of XRD and XPS, silicon was mainly in the presence of a-Si₃N₄, a-SiCN and a-SiC, and the amorphous carbon or amorphous carbon nitride also were formed in CrSiCN coatings.

Fig. 2 XPS spectra and bond fraction of Si2p and C1s.

Fig. 3 showed the surface topography of coatings, the cauliflower-like structure on coatings disappeared and then the surface topography became smooth with an increase of TMS flows. That result was attributed to more amorphous phase formation.⁸ As seen in Table 3, the hardness of Cr–Si–C–N coatings fluctuated in the range of 19.4–21.3 GPa at a low Si content (≤3.4 at%), which was higher than that of CrCN coating (18.1 GPa), while the hardness plummeted to 13 GPa with further increasing Si content (≥5.4 at%). This indicated the corresponding microstructure of the coatings tended to be consistent. Similarly, the hardness of Cr–Si–C–N coatings with low carbon content also decreased to about 13 GPa when the Si content was higher than 5.7 at%, regardless of the change in the content of carbon and nitrogen.²⁶ It is obvious that the silicon content of 5.4 at% was the critical content for the structure and mechanical properties of the Cr–Si–C–N coatings.

Fig. 3 Surface micrographs of Cr–Si–C–N coatings deposited at different TMS flows.

3.2 Tribological properties of Cr–Si–C–N coatings sliding against SiC balls in seawater

The friction behavior of Cr–Si–C–N coatings sliding against SiC balls in seawater is illustrated in Fig. 4. The friction coefficient of CrCN increased gradually to about 0.19 and then reached a steady value after a sliding distance of 250 m. With an increase in the TMS flows, the friction coefficient of Cr–Si–C–N coatings displayed a different variation tendency. The friction coefficient of CrSiCN-5 gradually declined to a steady value of 0.15 after sliding 300 m, while a very low friction coefficient for CrSiCN-30 was obtained during the whole sliding friction. In contrast, the friction coefficient of other coatings tended to fluctuate around 0.18. As seen in Fig. 5a, the mean-steady friction coefficient of CrCN was 0.19. After the silicon atoms were introduced into the CrCN coating, the friction coefficient first decreased to 0.15 for the CrSiCN-5 coating and then increased gradually to 0.19 for the CrSiCN-20 coating. With further increasing the TMS flow to 30 sccm, the friction coefficient decreased to 0.12 eventually. These results indicated that the friction coefficient of CrCN coating could be effectively reduced after the introduction of silicon element. According to XPS analysis in Fig. 2, the amorphous phases, such as a-Si₃N₄, a-SiCN and a-SiC, were formed in the Cr–Si–C–N coatings, the tribochemical reactions of silicide occurred easily during friction in water and then hydrated silica layer was formed, which played a role of lubrication to reduce the friction coefficient. As seen in Fig. 5b, it was obvious that the wear rate of mating balls declined gradually, while the wear rate of CrSiCN coatings first decreased to the lowest value (6.47 × 10⁻⁸ mm³ N⁻¹ m⁻¹), and then increased gradually with an increase in the TMS flow. It was worth noting that the wear rates of coatings were lower than those of their mating balls when the TMS flow was less than 20 sccm. It was obvious that the Cr–Si–C–N coatings deposited at lower TMS flow showed better wear resistance, which was related to higher hardness. But when the TMS flow was beyond 20 sccm, the wear rates of coatings were higher than those of their mating balls. This indicated that the transfer of wear debris from coatings to ball would improve the wear-resistance of mating balls in water.

Fig. 4 Friction behavior of Cr–Si–C–N coatings sliding against SiC balls in seawater.

Fig. 5 Influence of TMS flow on the mean-steady friction coefficient (a) and specific wear rates (b) of CrSiCN/SiC tribopairs.

The SEM images of wear tracks for Cr–Si–C–N coatings are shown in Fig. 6. Some shallow scratch lines were observed on the wear track. Especially, there are obvious traces of corrosion on the wear tracks as compared with the original surface of coatings. This indicated that the friction accelerated corrosion, because the fresh friction surface on the coating appeared after sliding against SiC in water. As seen the corresponding amplified images of wear tracks in Fig. 6h, j, l and n, there were some wear debris adhered to the wear tracks. Especially, some corrosion pits appeared on the wear tracks. In order to analyze the corrosion of seawater on the coatings, the residual seawater on the surface of wear tracks were first cleaned by ethanol, and then the wear tracks were detected by EDS. As seen in Fig. 7, the composition of seawater was not detected on wear tracks, even in the peeling pit in Fig. 7c, which indicated that seawater was not immersed in the coatings. In addition, a small amount of Si element was observed on the wear track of CrCN in Fig. 7a, which came from SiC ball.

Fig. 6 SEM images of wear tracks on Cr–Si–C–N coatings (a), (c), (e), (g), (i), (k), (m), and the corresponding amplified images (b), (d), (f), (h), (j), (l), (n).

Fig. 7 EDS analysis of wear track of CrCN and CrSiCN-15 coatings.

Fig. 8 shows the cross-sectional profiles of wear tracks on coatings. There were some shallow scratches on the CrCN coating. A very smooth wear surface was observed on the CrSiCN-5 coating while the ploughs increased gradually with an increase in the TMS flow, and the corresponding depth increased to maximum value of 0.6 μm for the CrSiCN-25 coating. That indicated the wear resistance became weak, which was attributed to decline of hardness. Fig. 9 shows the optical images of wear scars on SiC balls. The wear debris along the edge of the wear scar could be clearly observed in Fig. 9a–f. Especially, it seems that the wear scar in Fig. 9g was completely covered by a very thin layer of transfer film, which was one of the reasons for the reduction of friction coefficient.

Fig. 8 Cross-sectional profiles of wear track on Cr–Si–C–N coatings.

Fig. 9 Optical photographs of wear scars on SiC balls sliding against Cr–Si–C–N coatings.

3.3 EIS of Cr–Si–C–N coatings in seawater

EIS is an effective method to study the defects related degradation mode of coatings, and to investigate the changes in the resistive-capacitive nature of electrochemical interfaces without destruction.²¹ As seen in Fig. 10a, all samples showed incomplete capacitive reactance arcs in the Nyquist diagrams, and the diagrams all presented a single semicircle. This indicated that only the coatings were corroded without the degradation of substrate. The diameters of capacitive reactance arcs for the CrSiCN coatings became large as the TMS flows increased, especial that of CrSiCN-25 and CrSiCN-30. Generally, the longer the circular arc diameter was, the higher the corrosion resistance of coatings was.²⁷ It was inferred that CrSiCN-25–30 coatings could present better corrosion-proof ability as compared with other coatings in seawater. As seen in Fig. 10b, the modules of impedance (|Z|) of CrSiCN coatings were higher than that of CrCN coatings. The phase angle value of CrCN was higher than 70° in a very broad frequency range from 10⁻³ to 10² Hz in Fig. 10c, while those of CrSiCN-5 and CrSiCN-10 were lower than that of CrCN coating in low frequency of 10⁻³ to 10⁰ Hz and 10⁻³ to 10^1.5 Hz respectively. With an increase in the TMS flows, the phase angle values gradually increased in the low frequency and then became higher than that of CrCN coating. This indicated that the CrSiCN coatings deposited at higher TMS flow would be more likely to perform as ideal capacitor to prevent from electrolyte attacking in a broader frequency range.⁶

Fig. 10 (a) Nyquist plots, Bode plots for coatings: (b) evaluation of log|Z| as a function of log f(Hz) for coatings; (c) evaluation of phase as a function of log f(Hz) for coatings.

According to Nyquist, Bode plots and chi-square values,^6,24 the equivalent circuit (EC) for CrSiCN coatings with two time constants was depicted in Fig. 11. Here, the constant phase element (CPE or Q) represent a non-ideal capacitor to describe the deviation from the actual capacitive behavior,²⁸ and the corresponding impedance is expressed as follows:

Z_Q = 1/[Y_o(jω)ⁿ] (1)

where Y_o is the capacitance (Fs n⁻¹ m⁻²), ω is the angular frequency (rad s⁻¹), and n is the CPE power that represents the degree of deviation from a pure capacitor. As n = 1, Q is an ideal capacitor, while for n < 1, Q is non-ideal. The corresponding values of each component in the equivalent circuit were fitted by ZsimpWin software and were listed in Table 4. The R_s was the electrolyte resistance, which represented the ohmic contribution of the electrolyte solution between the working and reference electrodes, and their fluctuations were caused by a systematic error in the high frequency.²⁹ The R_po was the pore resistance, which reflected the ionic conductivity due to ionic path across the coatings and played a function of hindering the electrolyte penetration.^18,27,30 As seen in Table 4, the R_po value of CrCN coatings was lower than that of CrSiCN coatings. As the TMS flow was below 20 sccm, the R_po values fluctuated in the range of 1.9 × 10⁴ to 8.1 × 10⁴ Ω cm² with the same scale, while that increased to the scale of 10⁵ Ω cm² at the TMS flows beyond 25 sccm. Such result was attributed to the change of microstructure from typical columnar structures to the dense nanocomposite structure of Cr(C, N) embedded in a-Si₃N₄/a-SiC/a-C matrix.²² The R_ct was related to charge transfer resistance due to the formation of a double layer of charge at the substrate/electrolyte interface. The R_ct of CrCN coatings was less than that of CrSiCN coatings. The CrSiCN-25 presented the maximum R_ct value of 5.4 × 10⁷ Ω cm². This indicated that the anti-corrosion ability of CrCN coatings was enhanced.²⁹

Fig. 11 Equivalent circuit model for Cr–Si–C–N coatings.

Table 4 Fitted parameters of equivalent circuits derived from EIS of coatings in seawater

Coatings	Rs (Ω cm^2)	(CPE − Yo)po (F cm^−2)	(CPE − n)po	Rpo (Ω cm^2)	(CPE − Yo)dl (F cm^−2)	(CPE − n)dl	Rct (Ω cm^2)
CrCN	16.2	7.0 × 10^−6	0.99	2.3 × 10^3	2.4 × 10^−5	0.79	4.5 × 10^6
CrSiCN-5	11.8	1.1 × 10^−5	0.87	5.1 × 10^4	9.3 × 10^−6	0.74	9.1 × 10^6
CrSiCN-10	49.8	5.3 × 10^−6	0.83	8.1 × 10^4	1.0 × 10^−5	0.76	1.1 × 10^7
CrSiCN-15	3.0	1.0 × 10^−5	0.88	1.9 × 10^4	7.6 × 10^−6	0.71	3.0 × 10^7
CrSiCN-20	13.0	1.6 × 10^−5	0.87	3.2 × 10^4	8.7 × 10^−7	0.93	6.5 × 10^6
CrSiCN-25	18.1	7.2 × 10^−6	0.89	6.3 × 10^5	1.0 × 10^−6	0.98	5.4 × 10^7
CrSiCN-30	6.3	4.9 × 10^−6	0.91	1.1 × 10^5	3.2 × 10^−6	0.77	3.0 × 10^7

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3.4 Potentiodynamic polarization of Cr–Si–C–N coatings in seawater

Potentiodynamic polarization measurement is a traditional direct current (DC) approach that can provide kinetic information of corrosion performance of a system.²⁸ As seen in Fig. 12, the polarization curves significantly shifted towards low polarization current, this indicated that the polarization current decreased with an increase in the TMS flows. The polarization resistance (R_p) and porosity (P, %) of coatings were calculated by using Stern–Geary eqn (2) and (3)^31,32 and listed in Table 5.

P = (R_ps/R_p) × 10^{−|ΔE_corr|/β_a} (3)

where β_a and β_c are the Tafel anodic and cathodic slopes; the corrosion current density (i_corr) is deduced by the Tafel extrapolation from the curves.^32,33 The R_ps is the polarization resistance of the Si wafer substrate, and ΔE_corr is the difference in corrosion potentials between the coating and substrate.²¹

Fig. 12 Polarization curves of Cr–Si–C–N coatings deposited at different TMS flows.

Table 5 Analysis results of potentiodynamic polarization tests

Coatings	Ecorr (V)	icorr (A cm^−2)	βa (V)	βc (V)	Rp (kΩ cm^2)	Porosity (%)
CrCN	−0.15	2.3 × 10^−8	0.44	0.14	2.0 × 10^3	2.8 × 10^−3
CrSiCN-5	−0.23	1.4 × 10^−8	0.31	0.12	2.8 × 10^3	3.0 × 10^−3
CrSiCN-10	−0.24	1.0 × 10^−8	0.32	0.13	3.9 × 10^3	2.2 × 10^−3
CrSiCN-15	−0.15	1.1 × 10^−8	0.42	0.11	3.7 × 10^3	1.5 × 10^−3
CrSiCN-20	−0.16	1.8 × 10^−8	0.32	0.12	2.1 × 10^3	2.4 × 10^−3
CrSiCN-25	−0.19	5.2 × 10^−9	0.31	0.12	7.2 × 10^3	8.7 × 10^−4
CrSiCN-30	−0.25	2.6 × 10^−9	0.40	0.14	1.7 × 10^4	4.9 × 10^−4

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As seen in Table 5, the corrosion potentials (E_corr) for the CrSiCN coatings were lower than that of CrCN coating except for the CrSiCN-15 coating, while the corrosion current densities (i_corr) were less than that of CrCN coating. This indicated that the corrosion rates of the CrSiCN coatings were lower than that of CrCN coating once corrosion occurred. The R_p values of CrSiCN coatings were higher than that of CrCN coating, and fluctuated in the range of 2.1 × 10³ to 1.7 × 10⁴ kΩ cm² with an increase in the TMS flow, this pointed out that the CrSiCN coatings possessed stronger corrosion resistances than the CrCN coating. In addition, the porosity of the CrSiCN coatings decreased with an increase in the TMS flow. This showed that the compact microstructure of coatings was obtained at the high TMS flow,¹⁶ and then displayed good corrosion resistance.

3.5 Discussion

Due to the strong corrosion of seawater, the friction test in seawater was the synergistic interaction of corrosion and wear. The friction coefficient of Cr–Si–C–N coatings showed a decreasing tendency, especially those deposited at the TMS flow higher than 20 sccm. Besides, the tribo-layers adhered to the balls' wear scar were observed in Fig. 9. Ref. 26 and 34 pointed out that the hydrated silica layers were formed on the friction interface of tribopairs which contains the silicon in water environment, and the hydrated silica layers was beneficial to reducing friction coefficient. According to XPS analysis, the amorphous phases of silicide such as a-Si₃N₄, a-SiCN and a-SiC were formed in the CrSiCN coatings, so that the tribochemical reaction occurred easily to form more silica gel. And therefore the friction coefficient decreased to lowest value 0.12 with an increase in the Si content. The hydration reactions of silicide at the friction surface are expressed as:

Si₃N₄ + H₂O → SiO₂ + NH₃ (4)

SiC + H₂O → SiO₂ + CO₂ + H₂ (5)

SiO₂ + nH₂O → SiO₂·nH₂O (6)

As compared with the wear track in deionized water,²² the grooves effects were relieved and the corresponding depth also decreased in seawater. That was attributed to high viscosity of seawater and the deposition of Mg²⁺ and Ca²⁺ in the form of Mg(OH)₂ and CaCO₃ on the friction interface. Shan et al.¹⁷ have proved that the wear debris on the wear scar of mating balls was Mg(OH)₂ deposition, which showed a layered crystal structure and played an important role in the reduction of friction coefficient. And therefore, the friction coefficient and wear rate of CrSiCN coatings in seawater were lower than those in deionized water. As seen in Fig. 6a, c, e, g, i, k and m, the pit corrosion of coatings in seawater appeared on the wear track, because the fresh wear tracks in seawater were more vulnerable to be corroded, the mechanical wear and wear-accelerated corrosion were therefore the main material deterioration mechanisms.²³ The friction tests in seawater were a typical tribocorrosion system due to the high concentrations of chloride ions.²⁶ However, the Mg(OH)₂ deposition only acted as a lubricant and did not adhere to the wear track. This was attributed to hydration reaction of silica. Ref. 35 and 36 have reported that the silica particles was negative at high pH (around pH 6 or 7) and adsorbed on the grinding marks to form the electric double layer. Ref. 15 and 37 have reported that the wear losses of TiCN, TiN, CrCN, and CrN coatings sliding against WC balls in artificial seawater were higher than those in air and in distilled water, which was due to the corrosion of seawater. But in here, the wear rates of CrSiCN coatings in seawater (6.47 × 10⁻⁸ to 6.25 × 10⁻⁷ mm³ N⁻¹ m⁻¹) were lower than that in deionized water (8.36 × 10⁻⁸ to 8.07 × 10⁻⁶ mm³ N⁻¹ m⁻¹).²² This indicated that the amorphous phases, such as a-Si₃N₄, a-SiCN and a-SiC, could effectively improve the friction property and corrosion resistance of coatings. In fact, ref. 17 reported that the passive layer with Cr, N, Si and O elements could effectively improve the corrosion response during the friction test. It was proposed that the hydrated silica layers were formed by tribochemical reaction and adsorbed on the friction interface to fill the cracks and pinholes during the friction test, which could effectively inhibit the penetration of seawater into coatings. Thus the seawater did not penetrate into CrSiCN coatings.

According to electrochemical analysis, the corrosion resistance of CrSiCN coating was improved with an increase in the TMS flow. That was attributed to formation of more amorphous phase (a-SiN, a-SiCN and a-C) at high TMS flows, which dispersed between grains to form compact microstructure. Actually, the microscopic defect of coatings such as pores, pinholes, inter-columnar voids, droplets, etc. is one of the key factors to their corrosion resistance, which could act as penetration channels for aggressive electrolyte.¹⁶ When the TMS flow was below 20 sccm, the porosity value of coatings fluctuated in a range of 1.5 to 3.0 × 10⁻³%, and then decreased to 4.9 × 10⁻⁴% and 8.7 × 10⁻⁴% at the TMS flows beyond 25 sccm. In this case, the corrosion medium was inhibited to pass the permeable pores, and the galvanic corrosion cells between the substrate and the coating could not be established,²¹ and the corrosion current densities at the lowest scale of 10⁻⁹ A cm⁻² were obtained at the TMS flows beyond 25 sccm.

Though the corrosion resistance of the coatings was improved at the high TMS flows, the corresponding wear resistance was indeed weak. Actually, the wear and corrosion make the interaction in corrosive solution. Li et al.³⁸ have compared the friction in air and tribo-corrosion behavior of a-SiC:H, a-SiN_x:H and a-SiC_xN_y:H films in NaCl 1% solution, only some scratches were observed in dry wear, whilst for the tribo-corrosion test, the delamination and peeling off appeared on the wear track, this indicated that the corrosion accelerated the degradation of films. Similarly, Zhang et al.³⁹ also found that the hardness of wear track on the 304SS steel after friction test was higher than that after tribo-corrosion test. As compared with the SEM images of wear track on the CrSiCN coatings in deionized water,²² some corrosion pits were observed on the wear track in seawater in Fig. 6, this indicated that the corrosion of seawater resulted in the part degradation of the coatings. As far as the wear rate of coatings was concerned, ref. 23 and 40 reported that the wear rate of material in tribo-corrosion was attributed to the mechanical wear (V_mech) and wear-accelerated corrosion (V_chem). In here, the wear rate of CrSiCN/SiC tribopairs decreased obviously in seawater as compared with that in deionized water. This suggested that the corrosion-accelerated wear was insignificant, but the wear-accelerated corrosion could improve the lubrication properties of CrSiCN/SiC tribopairs in seawater, and then the wear rates of tribo-materials all decreased. In addition, as seen in Fig. 8, the depth of grooves on the wear track increased with an increase in the Si content. This indicated that the mechanical properties of coatings deposited at the high TMS flow became poor due to the decrease of H/E* value⁴¹ and the deteriorating resistance to crack of CrSiCN coatings.¹⁹

4. Conclusions

(1) The silicon was mainly in the presence of a-Si₃N₄, a-SiCN and a-SiC in CrSiCN coatings. And the hardness of CrSiCN coatings fluctuated in the range of 19.4–21.3 GPa at the Si content below 3.4 at%.

(2) The friction coefficient of CrSiCN coatings in artificial seawater was lower than that of CrCN coating due to hydration reaction of silicide. Furthermore, the friction coefficient of CrSiCN coatings decreased to 0.12 when the TMS flow increased to 30 sccm.

(3) The wear rates of CrSiCN coatings showed an increase trend with the increase in the TMS flows, and the pitting corrosion only appeared on the wear track. The mechanical wear and wear-accelerated corrosion were the main material deterioration mechanisms in seawater.

(4) The corrosion resistance of Cr–Si–C–N coatings was improved with an increase in the TMS flow.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 51375231); The Research Fund for the Doctoral Program of Higher Education (grant no. 20133218110030). We would like to acknowledge them for their financial support.

References

1. S. D. Yang and Z. Y. Li, Seawater hydraulic drive and its application in ocean exploitation, Ocean Eng., 2000, 18(1), 81–85 , in Chinese.
2. J. Z. Wang, F. Y. Yan and Q. J. Xue, Tribological behavior of PTFE sliding against steel in sea water, Wear, 2009, 267, 1634–1641.
3. G. J. Cui, Q. L. Bi, S. Y. Zhu, J. Yang and W. M. Liu, Tribological properties of bronze–graphite composites under sea water condition, Tribol. Int., 2012, 53, 76–86.
4. J. Z. Wang, F. Y. Yan and Q. J. Xue, Tribological behaviors of some polymeric materials in sea water, Chin. Sci. Bull., 2009, 54, 4541–4548.
5. F. Zhou, C. M. Suh, S. S. Kim and R. Murakami, Sliding wear behavior of TiN- and CrN-coated 2024 aluminum alloy against an Al₂O₃ Ball, Tribol. Lett., 2002, 13(3), 173–178.
6. Q. Z. Wang, F. Zhou, C. D. Wang, M. F. Yuen, M. L. Wang, T. Qian, M. Matsumoto and J. W. Yan, Comparison of tribological and electrochemical properties of TiN, CrN, TiAlN and a-C:H coatings in simulated body fluid, Mater. Chem. Phys., 2015, 158, 74–81.
7. L. Shan, Y. X. Wang, J. L. Li and J. M. Chen, Tribological property of TiN, TiCN and CrN coatings in sea water, China Surf. Eng., 2013, 26(6), 86–92 , in Chinese.
8. Q. Z. Wang, F. Zhou, X. D. Ding, Z. F. Zhou and C. D. Wang, Microstructure and water-lubricated friction and wear properties of CrN(C) coatings with different carbon contents, Appl. Surf. Sci., 2013, 268, 579–587.
9. G. A. Zhang, L. P. Wang, P. X. Yan and Q. J. Xue, Structure and mechanical properties of Reactive sputtering CrSiN films, Appl. Surf. Sci., 2009, 255(8), 4425–4429.
10. J. L. Lin, B. Wang, Y. X. Ou, W. D. Sproul, I. Dahan and J. J. Moore, Structure and properties of CrSiN nanocomposite coatings deposited by hybrid modulated pulsed power and pulsed dc magnetron sputtering, Surf. Coat. Technol., 2013, 216, 251–258.
11. H. Y. Lee, W. S. Jung, J. G. Han, S. M. Seo, J. H. Kim and Y. H. Bae, The synthesis of CrSiN film deposited using magnetron sputtering system, Surf. Coat. Technol., 2005, 200, 1026–1030.
12. J. H. Jeon, C. S. Jang, S. Y. Yoon, B. C. Shin and K. H. Kim, Effects of Si addition on the characteristic evolution and syntheses of nanocomposite Cr–Si–C–N coatings prepared by a hybrid coating system, Surf. Coat. Technol., 2005, 200, 1635–1639.
13. K. Yamamoto, T. Sato and M. Takeda, Structural analysis of (Cr_1−xSi_x)N coatings and tribological property in water environment, Surf. Coat. Technol., 2005, 193, 167–172.
14. Z. G. Geng, H. X. Wang, C. B. Wang, L. P. Wang and G. G. Zhang, Effect of Si content on the tribological properties of CrSiN films in air and water environments, Tribol. Int., 2014, 79, 140–150.
15. Y. W. Ye, Y. X. Wang, C. T. Wang, J. L. Li and Y. R. Yao, An analysis on tribological performance of CrCN coatings with different carbon, Tribol. Int., 2015, 91, 131–139.
16. Y. H. Yoo, J. H. Hong, J. G. Kim, H. Y. Lee and J. G. Han, Effect of Si addition to CrN coatings on the corrosion resistance of CrN/stainless steel coating/substrate system in a deaerated 3.5 wt% NaCl solution, Surf. Coat. Technol., 2007, 201, 9518–9523.
17. L. Shan, Y. R. Zhang, Y. X. Wang, J. L. Li, X. Jiang and J. M. Chen, Corrosion and wear behaviors of PVD CrN and CrSiN coatings in seawater, Trans. Nonferrous Met. Soc. China, 2016, 26, 175–184.
18. J. H. Jeon, C. S. Jang, S. Y. Yoon, B. C. Shin and K. H. Kim, Effects of Si addition on the characteristic evolution and syntheses of nanocomposite CrSiCN coatings prepared by a hybrid coating system, Surf. Coat. Technol., 2005, 200, 1635–1639.
19. Q. Z. Wang, Z. W. Wu, F. Zhou, H. Huang, K. Niitsu and J. W. Yan, Evaluation of crack resistance of CrSiCN coatings as a function of Si concentration via nanoindentation, Surf. Coat. Technol., 2015, 272, 239–245.
20. F. Cai, X. Huang, Q. Yang, R. H. Wei and D. Nagy, Microstructure and tribological properties of CrN and CrSiCN coatings, Surf. Coat. Technol., 2010, 205, 182–188.
21. F. Cai, Q. Yang, X. Huang and R. H. Wei, Microstructure and Corrosion Behavior of CrN and CrSiCN Coatings, J. Mater. Eng. Perform., 2010, 19(5), 721–727.
22. Z. W. Wu, F. Zhou, Q. Z. Wang, Z. F. Zhou, J. W. Yan and L. K. Y. Li, Influence of trimethylsilane flow on the microstructure, mechanical and tribological properties of CrSiCN coatings in water lubrication, Appl. Surf. Sci., 2015, 355, 516–530.
23. S. Mischler, Triboelectrochemical techniques and interpretation methods in tribocorrosion: A comparative evaluation, Tribol. Int., 2008, 41, 573–583.
24. Q. Z. Wang, F. Zhou, Z. F. Zhou, C. D. Wang, W. J. Zhang, L. K.-Y. Li and S.-T. Lee, Effect of titanium or chromium content on the electrochemical properties of amorphous carbon coatings in simulated body fluid, Electrochim. Acta, 2013, 112, 603–611.
25. S. Abraham, E. Y. Choi, N. Kang and K. H. Kim, Microstructure and mechanical properties of Ti–Si–C–N films synthesized by plasma-enhanced chemical vapor deposition, Surf. Coat. Technol., 2007, 202, 915–919.
26. Z. W. Wu, F. Zhou, K. M. Chen, Q. Z. Wang, Z. F. Zhou, J. W. Yan and L. K. Li, Friction and wear properties of CrSiCN coatings with low carbon content as sliding against SiC and steel balls in water, Tribol. Int., 2016, 94, 176–186.
27. G. H. Song, X. P. Yang, G. L. Xiong, Z. Lou and L. J. Chen, The corrosive behavior of Cr/CrN multilayer coatings with different modulation periods, Vacuum, 2013, 89, 136–141.
28. C. Liu, Q. Bi, A. Leyland and A. Matthews, An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part I. Establishment of equivalent circuits for EIS data modeling, Corros. Sci., 2003, 1243–1256.
29. A. Zeng, E. Liu, S. Zhang, S. N. Tan, P. Hing, I. F. Annergren and J. Gao, Impedance study on electrochemical characteristics of sputtered DLC films, Thin Solid Films, 2003, 426, 258–264.
30. X. Y. Wu, M. Suzuki, T. Ohana and A. Tanaka, Characteristics and tribological properties in water of Si-DLC coatings, Diamond Relat. Mater., 2008, 17, 7–12.
31. E. McCafferty, Validation of corrosion rates measured by the Tafel extrapolation method, Corros. Sci., 2005, 47, 3202–3215.
32. D. A. Jones, Principles and Prevention of Corrosion, Macmillan Publishing Company, New York, 1992.
33. B. Matthes, E. Broszeit, J. Aromaa, H. Ronkainen, S. P. Hannula, A. Leyland and A. Matthews, Corrosion Performance of Some Titanium-Based Hard Coatings, Surf. Coat. Technol., 1991, 49, 489–495.
34. N. Liu, J. Z. Wang, B. B. Chen and F. Y. Yan, Tribochemical aspects of silicon nitride ceramic sliding against stainless steel under the lubrication of seawater, Tribol. Int., 2013, 61, 205–213.
35. N. D. Spencer, Aqueous Lubrication Natural and Biomimetic Approaches [M], Word Scientific Publishing, Singapore, 2014, pp. 242–247.
36. J. G. Xu and K. Kato, Formation of tribochemical layer of ceramics sliding in water and its role for low friction, Wear, 2000, 245, 61–75.
37. L. Shan, Y. X. Wang, J. L. Li, H. Li, X. D. Wu and J. M. Chen, Tribological behaviours of PVD TiN and TiCN coatings in artificial seawater, Surf. Coat. Technol., 2013, 226, 40–50.
38. D. Li, S. Guruvenket, M. Azzi, J. A. Szpunar, J. E. Klemberg-Sapieh and L. Martinu, Corrosion and tribo-corrosion behavior of a-SiC_x:H, a-SiN_x:H and a-SiC_xN_y:H coatings on SS301 substrate, Surf. Coat. Technol., 2010, 204, 1616–1622.
39. Y. Zhang, X. Yin, J. Wang and F. Yan, Influence of microstructure evolution on tribocorrosion of 304SS in artificial seawater, Corros. Sci., 2014, 88, 423–433.
40. J. Jiang and M. M. Stack, Modelling sliding wear: From dry to wet environments, Wear, 2006, 261, 954–965.
41. A. Leyland and A. Matthews, On the significance of the H/E ratio in wear control: nanocomposite coating approach to optimized tribological behaviour, Wear, 2000, 246, 1–11.

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