Microstructure and Corrosion Behavior of Ni-Alloy/CrN Nanolayered Coatings

The Ni-alloy/CrN nanolayered coatings, Ni-Al/CrN and Ni-P/CrN, were deposited on (100) silicon wafer and AISI 420 stainless steel substrates by dual-gun sputtering technique. The influences of the layer microstructure on corrosion behavior of the nanolayered thin films were investigated. The bilayer thickness was controlled approximately 10 nm with a total coating thickness of 1μm. The single-layer Ni-alloy and CrN coatings deposited at 350◦C were also evaluated for comparison. Through phase identification, phases of Ni-P and Ni-Al compounds were observed in the single Ni-alloy layers. On the other hand, the nanolayered Ni-P/CrN and Ni-Al/CrN coatings showed an amorphous/nanocrystalline microstructure. The precipitation of Ni-Al and Ni-P intermetallic compounds was suppressed by the nanolayered configuration of Ni-alloy/CrN coatings. Through Tafel analysis, the Ecorr and Icorr values ranged from –0.64 to –0.33 V and 1.42 × 10−5 to 1.14 × 10−6 A/cm2, respectively, were deduced for various coating assemblies. The corrosion mechanisms and related behaviors of the coatings were compared. The coatings with a nanolayered Ni-alloy/CrN configuration exhibited a superior corrosion resistance to single-layer alloy or nitride coatings.


Introduction
Nanolayered thin film was recognized as a coating with alternating layers of two different materials with dissimilar physical, chemical, and related characteristics.With proper control of layer composition and microstructure, multilayered coatings could yield superior mechanical properties than monolayer ones [1][2][3][4][5].Chromium nitride thin films have been investigated and demonstrated to exhibit good mechanical performance, thermal properties, and antioxidation behavior [6,7].Also, Ni-based alloy coatings were frequently adopted as protective alloy coatings due to their various merits, including corrosion resistance, toughness, and wear resistance [8][9][10].The combination of these two materials systems to form a dual layer composite coating was a possible way to further the coating properties [11][12][13].Several materials systems with nanolayer configuration, such as CrN/AlN, TiN/CrN, Ag/Pd, and Au/Ag, had been developed [1,[14][15][16].However, limited literature on nitride/alloy coating systems were published.The integration of CrN and Ni-alloy in a nanolayered feature was thus of great interest.Moreover, the control of the nanolayered configuration, layer microstructure, and related properties was not fully understood.In this study, Ni-P/CrN and Ni-Al/CrN nanolayered coatings were fabricated by dual-gun sputtering technique.The microstructure and phases of the multilayer coatings were controlled by process temperature during sputtering.The corrosion behavior of the multilayer coatings was investigated through electrochemical potentiodynamic analysis.The relationship between microstructure, phases, and corrosion behavior of the nanolayered Ni-P/CrN and Ni-Al/CrN coatings was discussed.

Experimental Details
The Ni-alloy/CrN nanolayered coatings were deposited on the silicon (100) and 420 stainless substrates by dual-gun sputtering technique.The Ni 70 Al 30 and Ni 75 P 25 (in at.%) sputtering targets of 50.8 mm in diameter were employed for alloy layer fabrication.The CrN coating was deposited using a Cr target with 99.9% in purity and N 2 gas as reactive source.Before deposition, the chamber was evacuated down to 4.0 × 10 −4 Pa.The high purity Ar flow was introduced to a working pressure of 2.7 × 10 −1 Pa.Presputtering was performed for 10 minutes to clean the target surface followed by the fabrication process.The target powers were D.C. 100 W for Ni-alloy and R.F. 200 W for Cr.The working pressure was 2.7 × 10 −1 Pa for Ni-alloy fabrication, while a working pressure of 6.7 × 10 −1 Pa was adopted for CrN due to the coinlet of N 2 gas.The process temperature was fixed at 350 • C for all coating assemblies.Nanolayered Nialloy/CrN coatings were prepared with fixed bilayer period of 10 nm.The deposition time of individual layer of multilayer Ni-alloy/CrN coatings during sequential sputtering were modified from 30 to 120 seconds.The total thickness of the multilayer coating systems was controlled around 1.0 μm.The thickness ratio of Ni-alloy and CrN was fixed at 1.0.An X-ray diffractometer (Shimadzu, XRD-6000, Japan) with continuous θ-2θ scan was used to identify the phases in the coatings.The 2θ scan ranged from 30 • to 65 • with a step width of 0.02 • at a scanning speed of 6 • /min.The microstructure and surface morphology of the coatings were evaluated by FE-SEM (JSM-6500, JEOL, Japan).The compositions of selected areas of corroded coatings were inspected by an energy dispersion spectra inspected analyzer (INCA Energy 350, Oxford, UK).The depth profile was measured by Auger electron nanoscope (PHI 700, ULVAC-PHI, Japan).The corrosion behavior was measured by an electrochemical workstation (Jiehan-5000, Jiehan, Taiwan).The corrosion test was operated in 3.5 M NaCl aqueous solution at room temperature with the saturated calomel electrode (SCE) as the reference electrode.The scanning range was from −0.25 to +0.25 V according to the open circuit potential of each sample.The potentiodyanmic curves were fitted using CorrView software.For convenience, coatings deposited on Si substrates were applied in crosssectional SEM observation, nano-Auger analysis, and XRD phase identification.Corrosion test and related analyses were conducted on coatings deposited on stainless steel substrates.

Results and Discussion
The cross-sectional view of the Ni-P/CrN and Ni-Al/CrN nanolayered coatings was presented in Figure 1.Smooth and dense coating configurations were recognized by sputtering fabrication process.The total thickness of all coatings estimated from FE-SEM images were about 1 μm.In the magnified regions, as indicated in Figure 1, the smooth configuration of the Ni-P and CrN stacking was observed.A bilayer thickness of approximately 10 nm could be measured.To confirm the multilayer feature, the depth analysis of Ni-P/CrN nanolayered coating was conducted by nano-Auger technique.The sputter time was set to be 3 nm/min.The depth profile of Ni, P, Cr, and N showed a sequentially alternating distribution with a bilayer thickness around 10 nm, as shown in Figure 2. A nanolayered structure of the coating was again demonstrated.The X-ray patterns of single and nanolayer coatings were shown in Figure 3.The diffraction peaks of CrN (111), (200), and (220) were found The results of potentiodynamic analysis for various coating systems were indicated in Figure 4.The trend of corrosion resistance in different coatings systems could be discovered.The numberical results of I corr and E corr were listed in Table 1.The E corr values were from −0.64 to −0.33 V.The I corr values ranged from 1.42 × 10 −5 to 1.14 × 10 −6 A/cm 2 .All the coatings showed better corrosion potentials than that of AISI 420 substrate.Firstly, for the  single-layer coatings, the Ni-Al coating showed the most nonnegative E corr value of −0.39 Volts.The Ni-P and Cr-N coatings exhibited the less I corr values of 1.14 × 10 −6 and 1.34 × 10 −6 , respectively.This could be due to different corrosion behavior for the Ni-Al, Ni-P, and CrN coatings.For the Ni-Al coating, the corrosion occurred homogeneously and isotropically on coating surface [10].On the other hand, the Ni-P and CrN showed in-depth pitting corrosion on localized areas [12,17].This could be referred to the corroded surface morphology of Ni-Al, Ni-P, and CrN, as shown in Figures 5(a), 5(b), and 5(c).Large but homogeneously distributed corroded areas were observed for the Ni-Al coating.On the contrary, localized small etching pits were found for Ni-P and CrN coatings, as indicated in Figures 5(b) and 5(c).Secondly, take Ni-Al, CrN, and Ni-Al/CrN coatings for comparison, the E corr and I corr values of these two coatings were similar.Figure 5(d) showed the surface morphology of Ni-Al/CrN coating.The pits were less than other single-layer coatings.However, the large I corr of Ni-Al was not significantly decreased by nanolayered feature.Through EDX analysis, a few Fe concentration were detected in the single Ni-Al coating, indicating some of the Ni-Al was coating etched away.Nevertheless, in the Ni-Al/CrN coating, the substrate was well protected by the coating.The corrosion behavior of Ni-P, CrN, and Ni-P/CrN was also compared.The Ni-P/CrN exhibited a superior corrosion resistance than those of other thin films.It should be noted that the Ni-P coating exhibited a lowest E corr value between all materials systems.The enhancement of nanolayered Ni-P/CrN as compared to CrN coating was evident.The surface morphology of Ni-P/CrN after corrosion test was shown in Figure 5(e).The etching pits were significantly reduced.The corrosion resistance was thus improved effectively for the combination of Ni-P and CrN coatings.
The corrosion behaviors of the nanolayered Ni-P/CrN and Ni-Al/CrN coatings were taken into comparison.Even the E corr of Ni-P was lower than that of Ni-Al coating, as the Ni-P and Ni-Al were deposited with CrN to form nanolayered coatings, the corrosion resistance of Ni-P/CrN was better than that of Ni-Al/CrN nanolayered coating.This was because Ni-Al layer exhibited a severer layer area corrosion mechanism, while similar localized pitting corrosion was found for both Ni-P and CrN layers.Nevertheless, nanolayered coatings exhibited good protection from corrosion attacks.The corroded surface morphologies of two nanolayered coatings were shown in Figure 6.Though

× 10 − 6 *
The test was conducted in a 3.5 M NaCl solution at room temperature.
Nanolayered Ni-alloy/CrN coatings systems were successfully fabricated by dual-gun sputtering technique.The coatings showed smooth and dense structure, as revealed in FE-SEM cross-sectional images.The Ni-P/CrN and Ni-Al/CrN nanolayered layer coatings exhibited a nanocrystalline/amorphous microstructure because of the restricted precipitation by nanolayer feature.For single-layer coatings, Ni-Al exhibited a homogeneously large area corrosion phenomenon, while localized pitting within limited regions was found for NiP and CrN coatings.Through Tafel analysis, the Ni-alloy/CrN nanolayered coating exhibited a superior corrosion resistance than monolayer coatings.Moreover, the Ni-P/CrN coating showed an even greater corrosion resistance due to its localized pitting corrosion behavior for both Ni-P and CrN layers.Through morphology observation, the nanolayered Ni-Al/CrN and Ni-P/CrN coatings maintained a solid integrity after corrosion test.The combination of Ni-P and CrN in a nanolayered coating structure to promote the corrosion resistance was demonstrated.

Figure 6 :
Figure 6: The SEM surface images of corroded regions of (a) NiP/CrN, (b) NiAl/CrN nanolayered coatings on stainless steel substrates.

Table 1 :
The E corr and I corr values of various coating systems.