Electrochemical corrosion and tribological properties of CrMoCN coatings sliding against Al2O3 balls in artificial seawater
Introduction
Titanium alloys have the characteristics of low density, high strength and high specific strength, and are widely used in marine engineering, such as light marine equipment. The high specific strength of titanium alloys can ensure the miniaturization and light weight of marine equipment, thereby helping to increase the speed and effective load of submersible. In addition, the titanium alloys have excellent self-passivation advantages. When they suffer a certain degree of damage, they can be quickly repaired by surface oxide film or passivation film, and then exhibit good corrosion resistance in seawater. Therefore, the titanium alloys are indispensable key materials in the field of marine engineering and have been used in the transmission system and protective shell of civil cruise ships and submersibles. If the titanium alloys are used in the transmission system and fasteners of marine equipment, their low hardness will cause serious wear of transmission parts [1,2]. In fact, the wear of gears, bearings, transmission shafts and propellers in the transmission system of marine engineering will reduce the overall structural system reliability. In addition, in the actual marine environment, the strong corrosive salts and conductive media, the long-term reciprocating pressures of low-frequency waves, and the adhesion, metabolism and reproduction of microorganisms directly or indirectly accelerate the surface corrosion of metal materials [[3], [4], [5], [6], [7]]. If the hard coatings with good corrosion- and wear-resistant characteristics are deposited on the surfaces of titanium alloys, the titanium alloys can obtain excellent corrosion and wear resistances by being directly isolated from salt and microorganisms in seawater.
Actually, the transition metal nitride coatings have been paid more attention due to their good protective properties against wear and corrosion. Among them, the CrN-based coatings are well known for their high ductility and toughness, good wear- and corrosion-resistances. Lin et al. prepared four different kinds of CrN coatings using dc magnetron sputtering (dcMS), middle frequency pulsed dc magnetron sputtering (PMS), and modulated pulse power (MPP) magnetron sputtering plasmas [8]. Their hardness, friction coefficients and wear rates were in the ranges of 16.0– 26.0 GPa, 0.33– 0.58 and 2.40 × 10−6– 8.75 × 10−6 mm3/Nm, respectively. It was seen that the hardness of CrN coatings were relatively low and their wear resistances were poor as compared with CrCN and CrMoN coatings in refs. [[9], [10], [11]]. In addition, Kong et al. compared the tribological properties and corrosion resistances of single CrN coating and multi-layer coatings with different oxide interlayers in seawater [12]. The results showed that the friction coefficients and wear rates of CrN coating were always the highest among all coatings, which were 0.38 and 7.22 × 10−6 mm3/Nm (sliding against Al2O3 ball) as well as 0.33 and 7.19 × 10−6 mm3/Nm (sliding against SiC ball). Besides, the icorr of CrN coating was the highest at 105.9 ± 8.3 nA·cm−2 in seawater. Its Rp and Rct were the lowest at 494.3 ± 7.9 kΩ·cm2 and 6.1 ± 0.50 MΩ·cm2, respectively [12]. Li et al. compared the corrosion resistances of CrN coating and NiCrN coatings with different doses of Ni ions in 3.5 wt% NaCl solution [13]. The results showed that the Ecorr and Rp of CrN coating were always the lowest at −119 mV and 2.02 kΩ·cm2 among all coatings, and then its Icorr was the highest at 42.9 μA/cm2 [13]. To sum up, the CrN coatings could not meet the application requirements because of their low hardness, poor corrosion and wear resistances. In comparison to CrN coatings (Rct = 3.22 × 106 Ω·cm2, icorr = 35.2 nA·cm−2), the CrCN coatings presented the higher charge transfer resistances (Rct > 8.06 × 106 Ω·cm2) and lower corrosion current densities (icorr < 1.79 nA·cm−2) in simulated body fluid, which inhibited the corrosion of substrate [14]. Similarly, Ye et al. found that the CrCN coatings prepared at the acetylene flow rates less than 20 sccm all presented lower friction coefficients (<0.23) and lower wear rates (<9 × 10−7 mm3/Nm) than CrN coatings (μ = 0.31, K = 1.25 × 10−6 mm3/Nm) in artificial seawater [9]. If the CrCN coating was deposited at the C2H2 flow rate of 15 sccm, it exhibited the lowest icorr (7.27 × 10−7 A/cm2). This revealed that the doped carbon improved the structural compactness of CrCN coating and effectively prevent Cl− ion from penetrating into the surface of substrate [10], thus the CrCN coatings provided superior protection for marine equipment.
In fact, after Mo incorporation into CrN coating, the hardness of CrMoN coating was greatly improved due to the strengthening effect of (Cr, Mo)N solid solution [11]. When the Mo content in the coating was 21 at.%, the hardness of CrMoN coating was the highest at 34 GPa, which was much higher than that of CrN coating (18 GPa) [11]. Also, the addition of Mo element into transition metal nitride coatings enhanced the corrosion and wear resistances [[15], [16], [17], [18], [19], [20], [21], [22], [23]]. Jin et al. compared the corrosion resistances of CrN coating and CrMoN coatings with various Mo contents in acid solution (0.5 M H2SO4, 5 ppm HF) [16]. The results showed that as the Mo content increased from 0 at.% to 20.02 at.%, the CrMoN coatings showed more denser cross-section morphologies than CrN coating because of grain refinement caused by Mo incorporation. The Rp (Rp at anode > 7.87 × 104 Ω and Rp at cathode > 1.60 × 105 Ω) and Rct (Rct > 50,536 Ω·cm2) of CrMoN coatings were much higher than those of CrN coating (Rp at anode = 3839 Ω and Rp at cathode = 5834 Ω; Rct = 35,296 Ω·cm2). Thereby, the corrosion resistances of CrMoN coatings were much better than that of CrN coating [16]. The further research implied that when the applied potentials were 0.8, 1.1 and 1.6 VSCE, the corrosion current densities of CrMoN coating at the Mo target current of 4 A were the lowest among all coatings [21]. For CrMoSiCN coating with Mo doping, the MoO3 passivation film formed in the polarization process also acted as a physical barrier to inhibit the seawater penetration, which improved the corrosion resistance of coating [24]. The increase in the Mo content reduced the coating porosity and made the coating structure dense. As a result, the corrosion potential of TiN/TiAlN/(TiAl)1−xMoxN coating increased from −507 mV to −330 mV and the icorr decreased slightly. At the same time, the passivation region was extended to a wider potential range in NaCl solution [17].
On the other hand, as reported by Lu et al., the tribolayer containing MoO2 and MoO3 played a critical role in lubricated and wear-reduced properties of CrMoN/MoS2 coatings at 25 °C–700 °C [22]. As the Mo content in the coating increased from 2.3 at.% to 37.2 at.%, the wear rates of CrMoN/MoS2 coatings decreased continuously from 1.5 × 10−5 mm3/Nm to 3.2 × 10−6 mm3/Nm at 500 °C [22]. Also, the friction coefficient of CrMoN coating/steel ball dropped obviously from 0.49 to 0.37 in humid air due to the tribochemical product of lubricated MoO3 [11,23]. Furthermore, the anti-wear ability of Mo-NiCrBSi coating with 30 at.% Mo content was improved by about 95% as compared with NiCrBSi coating under oil lubricated condition because the formation of MoS2 tribo-film effectively inhibited the peeling off of abrasive particles [25]. It implied that the Mo element played a significant role in improving the anti-wear properties of coating in the liquid lubrication environment. Interestingly, as reported by Fu et al., when the Mo content in the coating was higher than 10.1 at.%, the friction coefficient of CrMoSiCN coating/Al2O3 ball tribopair dropped from 0.38 to 0.37 in seawater because of MoO3 lubrication but the wear rate of coating rose sharply from 1.50 × 10−6 mm3/Nm to 1.95 × 10−6 mm3/Nm due to the accelerated wear from tribochemical reaction [26].
If the Mo element was doped into CrCN coatings, the CrMoCN coatings were expected to possess the improved wear and corrosion resistances. However, to the best of our knowledge, there is no relevant report on the electrochemical performance of CrMoCN coating and wear characteristic of CrMoCN coating/Al2O3 tribopair in artificial seawater. As a result, in order to investigate the effect of Mo content on corrosion and wear resistances of coating and further obtain optimal coated protection for Ti-6Al-4V substrate, CrMoCN coatings with different Mo contents were deposited on Ti-6Al-4V, and then their corrosion resistances, long-term anti-corrosion abilities, mechanical properties after prolonged immersion and tribological behaviors sliding against Al2O3 balls were further studied in artificial seawater in detail.
Section snippets
Preparation and characterization of CrMoCN coatings
The CrMoCN coatings were prepared on Ti-6Al-4V disks (Φ30 mm × 4 mm) using closed unbalanced magnetron sputtering (UDP-650, Teer Coatings Limited). Their deposition details and parameters were described and listed in our previous work [27]. When the Mo target current varied in the range of 0.5 A–3 A, the corresponding CrMoCN coatings were sequentially marked as CrMoCN-0.5~CrMoCN-3 at 0.5 A intervals. The chemical compositions, surface and cross-sectional morphologies of CrMoCN coatings were
Chemical compositions, surface and cross-sectional morphologies as well as mechanical properties of CrMoCN coatings
Table 2, Table 3 displayed the chemical compositions of titanium alloy and CrMoCN coatings, respectively. It was obvious that the Mo content of CrMoCN coating increased from 1.4 at.% to 9.8 at.% when the Mo target current raised from 0.5 A to 3 A. Fig. 1 showed the surface morphologies, surface roughness and cross-sectional morphologies of CrMoCN coatings. It was seen from Fig. 1a that the grain separation occurred in the CrMoCN-0.5 coating and showed a very small inter-crystalline gap. As
Discussion
In Table 5, Table 6, the corrosion resistance of coating was not linearly related to the Mo content. The change of Mo content in the coating not only affected the internal grain structure, but also affected the columnar grain growth morphology and surface roughness. It was seen from Fig. 1 that the CrMoCN coatings with different Mo contents had different surface grain sizes and surface roughness. These characteristics would affect the corrosion resistance of coating. It was seen from Fig. 1e
Conclusions
- (1)
CrMoCN coatings all showed more excellent corrosion resistances than Ti-6Al-4V substrate in artificial seawater including the improvements in the open circuit potentials, electrochemical and polarization resistances.
- (2)
CrMoCN-2.5 coating exhibited the best electrochemical impedance characteristics that were associated with its compact structural features organized by refined surface grains with the minimum surface roughness.
- (3)
Due to long-term corrosion of seawater, the surface material loss of
CRediT authorship contribution statement
Maoda Zhang: Conceptualization, Investigation, Formal analysis, Writing – original draft. Fei Zhou: Supervision, Conceptualization, Formal analysis, Writing – review & editing. Yongqiang Fu: Investigation, Formal analysis. Qianzhi Wang: Validation, Formal analysis. Jizhou Kong: Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
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. HTL-A-19G04). We would like to acknowledge for their financial support.
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