Effect of CrYN/TiBN coating on friction performance and corrosion resistance of 316 stainless steel in artificial seawater

CrYN, TiBN, and CrYN/TiBN coatings were successfully deposited on 316 stainless steel substrates via multi-arc ion plating techniques to improve their wear and corrosion resistance properties in marine environments. The morphology, microstructure, friction performance, and corrosion resistance of the three coatings in artificial seawater were systematically studied. X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy investigations confirmed a dominant face-centered cubic CrN structure, accompanied by hexagonal TiB2 and amorphous BN in CrYN/TiBN coatings. The SEM cross-section shows that the TiBN and CrYN/TiBN coatings have a more compact cross-sectional structure than the CrYN monolayer coating. The CrYN/TiBN coatings exhibited the lowest smooth friction coefficient in artificial seawater, and the wear rate was ranked as TiBN < CrYN/TiBN < CrYN. Surface morphological studies performed after tribocorrosion revealed that the protection ability of all three coatings remained acceptable. The electrochemical test showed that the corrosion tendency was CrYN/TiBN < TiBN < CrYN, and the CrYN/TiBN coating had the best performance in the AC impedance spectrum and polarization curve.


Introduction
With the rise of ocean exploration, various material properties, including mechanical and corrosion properties, applied in marine engineering preparation, have gained significant attention [1][2][3]. Owing to its low cost, excellent mechanical properties, and corrosion resistance, 316 stainless steel has been widely used in marine engineering equipment, such as drilling platforms, oil and gas pipelines, and structural components on ships. However, 316 stainless steel used in marine equipment often suffers from corrosion-friction interactions due to exposure to seawater [4][5][6]. Nitrogen (N)-based protective coatings such as TiN, CrN, and ZrN deposited on key components via physical vapor deposition are important to prevent rapid failure of 316 stainless steel in marine environments. CrN coatings are widely used for the surface protection of marine equipment owing to their high oxidation, wear, and corrosion resistance [7,8]. However, the CrN coating exhibits a columnar crystal structure with large grains, and Clions in seawater can easily reach the matrix through the gaps between grains, causing the matrix to corrode [9]. Therefore, in a complex environment, the wear and corrosion resistance of CrN coatings in seawater should be improved.
To improve wear and corrosion resistance of CrN coatings, the ternary or multi-system coatings were constructed by adding metallic elements Ti [10], Al [11], and Y [12] or non-metallic elements Si [13], C [14,15], and B [16,17] to improve the wear and corrosion resistance behaviors.
The addition of Y to the CrN coating could effectively refine grains, improve the coating structure, and improve the mechanical and friction properties of the coatings [18,19]. Wu et al [20] prepared a CrYN coating using a reaction co-sputtering method and found that Y ions replaced Cr ions in the Cr-N lattice to form a solid solution Cr1-xYxN coating, increasing hardness from 12. 9 to 13.7 GPa and improved the adhesion of the coating. Tian et al [21] prepared CrAlYN coatings using the cathodic arc evaporation method and found that the coatings formed a solid solution CrAlYN and achieved grain refinement after Y addition to CrAlN coatings, The addition of B to CrN-based coatings has been extensively studied [16,22,23]. Jahodova et al [24] prepared a CrBN coating using an unbalanced magnetron sputtering system and found that the CrBN coating structure presented irregular and discontinuous crystallite boundaries , compared to the CrN coating and the coating compactness was improved, which could effectively hinder the corrosive medium from penetrating the substrate, showing higher corrosion potential and smaller corrosion current density. Zhou et al [25] prepared a CrBCN coating using a closed field-imbalanced magnetron sputtering system and found that the microstructure became dense with reduced pores and pinholes after B was added to CrCN coating. Moreover, CrBCN coating reacted with seawater to form H 3 BO 3 lubricating layer, thus reducing the friction coefficient and wear rate. Yu et al [26] prepared CrTiBN coatings using a reactive magnetron sputtering system and showed that with the addition of Ti and B elements, the columnar growth and dense structure disappeared, and double face-centered cubic-CrN and amorphous BN structures were formed, the grain size decreased from 33 to 15 nm, the coatings became denser, and the friction and wear properties of the coatings were improved.
The corrosion and tribological properties of CrYN composite TiBN coatings in seawater have not yet been studied. Here, CrYN, TiBN, and CrYN/TiBN coatings were deposited on 316 stainless steel substrates via arc ion plating. The microstructure, mechanical properties, and corrosion resistance of the coatings in artificial seawater were systematically evaluated.

Coating deposition
Single-crystal silicon (111) and 316 stainless steel with dimensions of 20 mm × 20 mm × 3 mm were selected as substrates for deposition. Single-crystal silicon (111) was used for x-ray Powder Diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) tests, and 316 stainless steel was used for the mechanical and anticorrosion tests. The substrates were sequentially ultrasonically cleaned in acetone, deionized water, and absolute ethanol solution for 20 min and then blown dry with nitrogen to be immobilized in a holder inside the coating chamber at a distance of 200 mm. Ar (99.99at%) was used as the working gas and N 2 (99.99 at%) was used as the reaction gas. The cathode target was fabricated using Cr (j100 × 20 mm, 99.9 at. %), CrY (j100 × 20 mm, 99.9 at%, Cr: Y = 95:5), and TiB 2 target (j100 × 20 mm, 99.9 at%, Ti:B = 33:67). A schematic of the deposition apparatus is shown in figure 1. The vacuum of the coated chamber was drawn to 3 × 10 −3 Pa while heating it to 350°C, and the substrate was Ar ion cleaned for 20 min to remove the contaminating layer and oxide on the surface of the substrate. The coatings were deposited under 1 Pa nitrogen pressure for 2 h with a substrate bias of −100 V, duty cycle of 80%, a rotor speed of 1 r min −1 , and target current of 80 A. For CrYN/TiBN coatings, the Cr, CrN, and CrYN layers were prepared prior to depositing the CrYN/TiBN composite to improve the adhesion strength of the coatings. The deposition parameters of all coatings are listed in table 1.

Characterization
To determine the crystal structure of the coating, XRD (MiniFlex II, Rigaku, Japan) was used, a grazing incidence mode was used with a scanning speed of 5°/min and a scanning range from 25°to 75°; to detect the structural components of the coating, XPS (ESCALAB 250Xi, Thermo Fisher, USA) was used, Al-Ka was employed, the radiation source (energy = 1486.68 EV, power = 164 w) with a resolution less than 0.62 EV was calibrated with a C1s binding energy line (284.6 EV). The surface of the coatings, wear traces, friction and side wear traces, and cross-sectional morphologies were observed using a field-emission scanning electron microscope (SEM, JSM-7610F, JEOL, Japan). An energy-dispersive spectrometer (EDS, NORAN System 7, Thermo Fisher Inc., USA) was used to analyze the chemical components. The corrosion resistance of the coating was characterized using an Autolab PGSTAT 100N potentiostat (Autolab PGSTAT 100N, EcoCheime, The Netherlands) in artificial seawater at a scanning rate of 0.5 mV min −1 from −1 V to 1 V. Samples were mounted with a sampler support, and electrochemical measurements were performed in an area of 1.0 cm 2 exposed on the coated side. After 1 h immersion in all samples, they were tested in artificial seawater to obtain a stable OCP; In the alternating impedance test (EIS), the stable OCP was set as the initial voltage. The chemical composition of artificial seawater is listed in table 2 [27]. The friction coefficient of the coating was tested using a ball-on-disk friction and wear tester (CFT-1, Zhongkekaihua Inc., Lanzhou, China). An Al 2 O 3 ball with a diameter of 4 mm was used as the rubbing material, the abrasive diameter was 8 mm, the rotation speed was 0.1 m s −1 , the load was 10 N, and the sliding distance was 500 m. The friction morphology of the coating surface after testing was observed using an Optical Profiler (Bruker Contour GT K 3D, Billerica, USA), and the wear rate was calculated using equation 1 [1,41].
In equation 1, F is the test load (unit: N); L is the friction length (unit m); r indicates the radius of coating abrasion mark (unit: mm); A represents the average wear area of the coating wear marks (mm 2 ), calculated using Origin Software. The wear tests were repeated for three times to obtain the mean values of wear rates.

Results and discussion
3.1. Morphology and structure of the coatings Figure 2 shows the surface morphology and chemical composition of the CrYN, TiBN, and CrYN/TiBN coatings. Some microdroplets, pinholes, and melting craters were distributed on the coating surface with sizes ranging from hundreds of nanometers to several microns on all three coating surfaces. These surface defects are very common in arc ion plating [28]. The CrYN coating surface had the largest particle size among the three coatings, whereas the TiBN coating surface had the smallest particle size. The surface quality of the CrYN/TiBN coating formed by the CrYN composite with TiBN coating was significantly improved compared with that of a single CrYN coating. The size of the large particles was smaller, the molten pit was shallow, and the surface morphology was refined but still inferior to that of the TiBN coating because of the formation of amorphous or nanocrystalline borides on the surface of the TiBN and CrYN/TiBN coatings, which increased the density of the coating [29].
The cross-sectional morphologies of CrYN, TiBN, and CrYN/TiBN coatings are shown in figure 3. The thicknesses of the CrYN, TiBN, and CrYN/TiBN coatings were approximately 2.0, 1.4, and 2.8 μm, respectively. A typical columnar crystal structure with a relatively loose cross-section was observed in the cross-section of the CrYN coating, whereas the TiBN coating had a denser cross-section than the CrYN coating. With the TiBN compound in CrYN, the obtained CrYN/TiBN coating had a more compact cross-section than the single CrYN coating, and the columnar crystal cross-section structure of the CrYN coating was not obvious in the crosssection diagram of the CrYN/TiBN coating. These results indicate that the CrYN/TiBN coating formed by the CrYN composite with TiBN can effectively improve the densification of the CrYN coating. A possible reason is that with the incorporation of element B, part of B exists in the form of a solid solution in CrN and TiN crystals, resulting in lattice distortion, which hinders the growth of columnar crystals to a certain extent [30].
The XRD patterns of CrYN, TiBN, and CrYN/TiBN coatings are shown in figure 4. Compared with the powder diffraction file card, the XRD spectra of CrYN coating presents a face-centered cubic structure correspond to CrN (111) , CrN (200), CrN (220), and TiBN coatings corresponding to TiN (111), TiN (200), TiN  [32] research believed that the addition of B element could form BN amorphous structure and thus refine grains. XPS results also indicated that the B element mainly existed as a BN amorphous structure in the CrYN/TiBN coating ( figure 5).
The B 1s and N 1s XPS core-level spectra of CrYN/TiBN are shown in figure 5 to reveal the chemical binding states of the elements in the coatings. In the B1s spectrum fitting, the binding energy of 191.0 eV corresponds to B-N, 192.5 eV corresponds to B-O, and 189.9 eV corresponds to TiB 2 [33].

Tribological properties
The friction coefficient curves and average steady-state friction coefficients of the CrYN, TiBN, and CrYN/TiBN coatings in artificial seawater sliding with Al 2 O 3 balls are shown in figure 6. The friction coefficient changes sharply during the running-in stage at the beginning of friction. Large particles on the coating surface and other defects lead to high coating roughness and an increased friction coefficient. As friction continues, large abrasive material is destroyed, the friction interface becomes smooth, and the friction coefficient gradually stabilizes [37]. Here, in the friction test of the three types of coatings, the friction coefficient of the CrYN coating reached a stable stage in the shortest time and showed a slowly rising trend over the entire sliding length, with a maximum friction coefficient of 0.48. The friction coefficient of the TiBN coating first increased, then decreased, and finally stabilized at approximately 0.40 after 200 m. Compared with the CrYN and TiBN coatings, the CrYN/TiBN coating reached a stable stage at a shorter distance, showing the lowest friction coefficient (0.38) for the entire sliding length. The CrYN/TiBN coating formed by the composite of CrYN and TiBN effectively reduced the friction coefficient of the CrYN coating likely because CrYN/TiBN coating B mainly exists in the BN phase, the grain size is refined, and the coating compactness and coating defects are effectively improved. Figure 7 shows the wear morphology and surface element distribution of the three coatings after friction in artificial seawater. The wear morphologies of the three coatings were similar, and many friction chips and peeling layers were detected on the surface. There were many furrows in the abrasion marks of the three coatings, but the abrasion marks of the CrYN/TiBN coating were more uniform and flatter than those of the other two coatings. For the CrYN coating, a deeper scratch was observed on the surface of the wear track (1.16 μm) and the middle region was obviously exfoliated, and a clear layer by layer exfoliation could be observed at the edge of wear scar (figures 7(a), (d)). Wear scars were mainly composed of Cr, Y, and O as screened by EDS spectra, which indicated that, during the sliding friction process, wear debris adheres to the friction interface of the coating to form lamellar debris, which increases the roughness of the coating surface and leads to intensified wear, at which point the wear mechanism is adhesive wear [38]. Erdemir et al [39] found that B-containing coatings would oxidize at the wear interface to produce B 2 O 3 , which undergoes a hydration reaction with water to produce H 3 BO 3 with a lamellar structure, which makes the contact interface smooth and flat in favor of hydrodynamic lubrication, resulting in a self-lubricating effect [39][40][41][42].   wear. Overall, the above results indicate that the CrYN composite TiBN coating improves the mechanical defects of wear particle wear of CrYN coating and increases the wear resistance properties of the coating. Figure 8 shows the wear section view of CrYN, TiBN, and CrYN/TiBN coatings after they were tested in artificial seawater. Figure 9 shows  (CrYN/TiBN), respectively, consistent with the structural density of the coatings. After grinding with alumina ball for 500 m in artificial seawater, the trace depths of the three coatings were less than the coating thickness, indicating that the three coatings were not abraded. The study showed that the loosely structured coating was prone to plastic  deformation during the friction process, thus increasing the plowing component of friction and the effective contact area when sliding, leading to severe plastic deformation. Severe plastic deformation would increase the coating surface roughness with the progress of friction. The high roughness will lead to the inhomogeneous deformation in the stress concentration and small range, finally enhancing the coating wear [43].
The wear resistance performance of CrYN/TiBN coatings was improved significantly relative to that of the CrYN coatings. Based on the previous XPS analysis, boron element mainly existed as the BN phase in the CrYN/ TiBN coating, which interrupted the growth of columnar crystals, refined the grain size, improved the compactness of the coating, and decreased the coating defects. Figure 10 shows the electrochemical properties of 316SS substrate, CrYN, TiBN, and CrYN/TiBN coatings in artificial seawater. The capacitive reactance arcs of all four samples ( figure 10(a)) show the characteristics of semicircle capacitance, which indicates that all three coatings and 316SS have good corrosion resistance in artificial seawater. The larger the capacitive arc, the stronger the impedance [44]. Among four capacitance rings,  the radii of TiBN and CrYN/TiBN coatings are close to and larger than that of the CrYN coating, whereas 316SS exhibits the smallest capacitance ring, which indicates that all three coatings have larger impedance values than 316SS. The magnitude of the impedance in the low frequency region in the Bode diagram ( figure 10(b)) is typically used to evaluate the anticorrosion performance of the coating. Impedance Z in the low-frequency region reflects the dielectric properties of the coating: the higher the Z value of the sample in the low-frequency region, the higher corrosion resistance of the coating [45]. The impedance magnitude of all three coatings is similar. The impedance values of the TiBN and CrYN/TiBN coatings are higher than that of the CrYN coating in the entire frequency range, which indicates that the TiBN and CrYN/TiBN coatings show higher corrosion resistance. Figure 9(c) demonstrates the phase angle diagram of the coating. The Bode-impedance in the highand medium-frequency regions reflects the local defects, and that in the low frequency region reflects the charge transfer resistance (Rct) of the coating [46]. The results in figures 10(b) and 10(c) show that the CrYN/TiBN coatings exhibit the highest impedance and phase angle in the low-frequency region. Figure 11 shows the potentiodynamic polarization curve of the coating in artificial seawater, and the electrochemical corrosion parameters obtained via Tafel curve fitting are listed in table 3. The E Corr values of the 316SS substrate, CrYN, TiBN, and CrYN/TiBN coatings in artificial seawater are lower than 0 V, and the corrosion potentials of the TiBN and CrYN/TiBN coatings (−0.145 V and −0.139 V, respectively) are much higher than those of the CrYN coating (−0.325 V) and 316SS (−0.402 V). The I Corr value of the CrYN/TiBN coating in artificial seawater was 0.405 × 10 −8 A cm −2 , which is much lower than that of the CrYN (9.896 × 10 −8 A cm −2 ) and TiBN (3.367 × 10 −8 A cm −2 ) coatings. The R p value of the CrYN/TiBN coating was 7042 kW·cm 2 , which was much higher than those of the CrYN (467 kW ·cm 2 ) and TiBN (1752 kW·cm 2 ) coatings. Previous studies showed that materials with higher corrosion potential and lower corrosion current have higher corrosion resistance [47].

Electrochemical properties in artificial seawater
In terms of the electrochemical performance of the CrYN, TiBN, and CrYN/TiBN coatings, the CrYN/ TiBN coatings showed the best corrosion resistance. The corrosion resistance of the coating is closely related to its microstructure and pore defects. In the CrYN/TiBN coating, B exists in the CrN crystal in the form of a solid solution, which leads to the distortion of the crystal lattice and hinders the growth of columnar crystals, thus refining the grains and improving the coating compactness. Lin et al [48] reported that the columnar crystal structure in their study was relatively loose and easily permeated by a corrosive solution, which corroded the substrate. Thus, improving the compactness of the coating can effectively improve its corrosion resistance [48,49]. Among three coatings, the CrYN/TiBN coating had the largest thickness. In terms of corrosion resistance, the increase in the coating thickness can improve the coating microcracks, pinholes, and other surface defects and enhance corrosion resistance [50][51][52]. The CrYN/TiBN coating showed the best corrosion resistance. The friction test results in artificial seawater showed that the friction coefficients of the CrYN, TiBN, and CrYN/TiBN coatings were 0.48, 0.40, and 0.38, respectively.

Conclusion
The wear width and wear rate of the CrYN coatings were 454.4 μm, 2.05 × 10 −6 mm3/Nm, showing the poorest wear resistance among the three coatings. The wear marks of the TiBN and CrYN/TiBN coatings were smooth, the width of the wear marks were 326.4 and 363.5 μm, and the wear rates were 8.45 × 10 −7 and 1.43 × 10 −6 mm 3 /Nm, respectively.
The electrochemical tests of 316SS and the three types of coatings in artificial seawater showed that all three coatings had better corrosion resistance than 316SS. The TiBN and CrYN/TiBN coatings exhibited better corrosion resistance than the CrYN coating. The CrYN/TiBN coating exhibited the best performance in the AC impedance spectrum and polarization curve.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).