Effect of Al additions in WC-(Cr1−xAlx)N coatings on the corrosion resistance of coated AISI D2 steel in a deaerated 3.5 wt.% NaCl solution

Surface & Coatings Technology 190 (2005) 417 – 427 www.elsevier.com/locate/surfcoat

Effect of Al additions in WC-(Cr1
xAlx)N coatings on the corrosion resistance of coated AISI D2 steel in a deaerated 3.5 wt.% NaCl solution J.H. Lee, S.H. Ahn, J.G. Kim * Department of Advanced Materials Engineering, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea Received 11 September 2003; accepted 26 March 2004 Available online 11 June 2004

Abstract Multilayered WC-(Cr1
xAlx)N coatings were deposited on AISI D2 steel using cathodic arc ion plating (CAIP) process. Five kinds of WC-(Cr1
xAlx)N coatings were prepared: WC-Cr0.6Al0.4N, WC-Cr0.57Al0.43N, WC-Cr0.53Al0.47N, WC-Cr0.48Al0.52N and WC-Cr0.45Al0.55N. The Al concentration could be controlled by using evaporation source for Al targets and fixing the evaporation rate of the other metals (WC alloy and Cr). In this study, the corrosion behavior in deaerated 3.5 wt.% NaCl solution was investigated by electrochemical corrosion tests (potentiodynamic polarization test, galvanic corrosion test, electrochemical impedance spectroscopy (EIS)) and surface analyses (glow discharge optical emission spectroscopy, X-ray diffractometry, scratch adhesion test, scanning electron microscopy, electron probe microanalyzer). The results of potentiodynamic polarization test showed that the WC-Cr0.48Al0.52N coating with lower porosity exhibited the lower corrosion current density. The galvanic corrosion current between the coating and the substrate showed low values. In EIS measurements, the charge transfer resistance (Rct) value of WC-Cr0.48Al0.52N coating only increased with the immersion time, when compared to the other coatings. It can be due to the corrosion products plugging the pores and increasing the pathway resistance. D 2004 Elsevier B.V. All rights reserved. Keywords: Multilayered coating; CAIP; Potentiodynamic polarization test; Galvanic corrosion test; EIS; Porosity; EPMA

1. Introduction Electroplated hard coatings have been used in many industrial applications for several years. However, there have been some efforts for several years to replace electrodeposited methods by other deposition methods with fewer environmental problems. Thus, the development of ‘clean’ technologies in all spheres of industrial manufacturing is an essential task and an important role for improving the lifetime and performance of tools [1]. In recent years, the PVD techniques are well established to protect materials surfaces. The multilayered WC-(Cr1
xAlx)N coatings were deposited on AISI D2 steel by cathodic arc ion plating process. The deposition of multilayered WC-(Cr1
xAlx)N coatings at different Al contents leads to the formation, such as WCCr0.6Al0.4N, WC-Cr0.57Al0.43N, WC-Cr0.53Al0.47N, WCCr0.48Al0.52N and WC-Cr0.45Al0.55N. Multilayered coatings based on a stacking arrangement of interlayer, buffer layer and multilayer have been developed. The benefits of these * Corresponding author. Tel.: +82-31-290-7360; fax: +82-31-290-7371. E-mail address: [email protected] (J.G. Kim). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.054

materials include a high wear resistance, strength and chemical stability. For example, WC combines favorable properties, such as high hardness, lubrication and a certain amount of plasticity. The benefits of Cr and Al materials presented the characteristics of corrosion resistance, oxidation resistance and lubrication. Despite these good chemical properties, coated parts were often severely corroded, especially if the PVD coatings were deposited directly to less noble substrates and then exposed to a corrosive environment. This corrosive attack is due to the ejection of melt metal droplets from the cathode sources. These defects are very much dependent on such parameters as the cathode current or power [2,3]. Even though there are some investigation on the wear-resistant characteristics of WC-(Cr1
xAlx)N coatings, little information exists on their corrosion properties. The corrosion behavior of these materials was investigated by potentiodynamic polarization test, galvanic corrosion test and electrochemical impedance spectroscopy test in deaerated 3.5 wt.% NaCl solution at room temperature. The purpose of this paper is to evaluate the influence of Al content on the corrosion resistance of multilayered WC-(Cr1
xAlx)N coatings deposited by CAIP.

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2. Experimental 2.1. Coating deposition High speed steel (AISI D2) and Si-wafer were used as substrate materials. AISI D2 steel of size 25  25 mm was cut from a 5 mm thick sheet. The specimen was finished by grinding on 2000 grit SiC for the final step. AISI D2 steel has the nominal composition in weight percent of 5.0 Cr, 1.3 Mo, 1.0 Si, 1.0 V, 0.5 Mn, 0.37 C, 0.03 P, 0.03 S and 90.77 Fe. All samples were cleaned using Ar gas for 5 min in chamber. Three circular Cr targets were installed on one side of chamber wall and three circular WC targets were attached on the other side. Two Al targets were installed between Cr targets. A rotation substrate holder was used to obtain a layered structure and the composition modulation wavelength of the crystal structure was controlled by the rotation speed. The interlayer deposition (Cr/CrN) was done using a negative substrate bias of 400 V (Cr) and 200 V (Cr/N), respectively. A dense Cr and CrN interlayer of about 0.1 Am thickness was deposited onto the steel substrate prior to multilayer formation. The interlayer was deposited primarily to control and set to a residual stress gradient between the multilayer film and the steel substrate [4]. Buffer layer deposition (WC/Cr) was done using a negative substrate bias voltage of 200 V. The multilayered WC-(Cr1
xAlx)N coatings on AISI D2 steel substrate were deposited with a negative substrate bias voltage of 200 V. The coatings should also be deposited at 300 jC. The conditions of coating deposited are listed in Table 1. A total coating thickness of about 2.1 Am was controlled at all cases. The film thickness was analyzed by SEM. 2.2. Electrochemical behavior A conventional three-electrode electrochemical cell was used. The working electrode was the substrate or coated sample. The test samples were masked with Amercoat 90 epoxy in order to expose a constant surface area of 0.25 cm2. A saturated calomel electrode (SCE) as a reference electrode

Table 1 Conditions of coating deposited Plasma pre-cleaning WC-(Cr1
xAlx)N deposition

Gas pressure

Ar 1  10
2 Torr

Time

5 min

Target Target power density Base pressure Working pressure Bias voltage Temperature Motor rotation speed Coating thickness

Eight targets: WC(3), Cr(3), Al(2) WC: 53 W/cm2, Cr: 33 W/cm2 Al (31, 32, 34, 35, 37 W/cm2) 3  10
5 Torr 1  10
2 Torr
200 V 300 jC 4 rpm 2.1 Am

and a carbon counter electrode were utilized. All measurements were performed in a 3.5 wt.% NaCl solution, saturated with high purity nitrogen with a flow rate of 25 cm3/min. For each material and electrolyte combination, the sample was allowed to stabilize for 4 h in the electrolyte. This potential then was taken as the open-circuit potential (OCP). The potential of the electrode was swept at a scan rate of 0.166 mV/s from the initial potential of
250 mV vs. OCP to the final potential of 1000 mV vs. SCE. The results of potentiodynamic polarization test were analyzed by the EG&G PAR Model 263A and the software M352 ParCal program. A Gamry PC3/750 instrument equipped with a zero resistance ammeter (ZRA) was used for galvanic corrosion tests during 12 h. The galvanic current density was measured as a function of the immersion time. An IM6e impedance analyzer was employed to measure the electrochemical impedance. The impedance data were analyzed by the IM6e analysis software (Thales) program which uses a nonlinear least square (NLLS) fitting. Sinusoidal potentials of 10F10 mV vs. the OCP, with a frequency range of 10 kHz to 1 mHz were applied to the samples. The impedance data were measured at an interval of 24 h over an exposure period of 168 h. 2.3. Porosity The corrosion performance of the deposited coating is not only due to the intrinsic corrosion behavior of coating itself, but also results from small structural defects such as pores, pinholes and microcracks formed during or after the deposition process. These coatings do not assure total insulation from the surrounding corrosive environment, and do not totally inhibit the diffusion of aggressive agents through the coating, which leads to the formation of localized corrosion, galvanic corrosion, and blistering of the coating. Electrochemical techniques have been applied to the characterization of the corrosion behavior and porosity of PVD coatings. It is possible to estimate the porosity of these coatings [5– 7]. The porosity corresponds to the ratio of the polarization resistance of the uncoated and the coated substrates. The polarization resistance can be experimentally determined from the DC polarization curve. Tato and Landolt [8] established an empirical equation (Eq. (1)) to estimate the porosity ( P) of coating: P¼

Rp;u Rp;r
u

ð1Þ

where P is the total coating porosity, Rp,u is the polarization resistance of substrate, and Rp,r – u the polarization resistance of coating – substrate system. 2.4. Scratch adhesion test To evaluate the adhesion of coatings, a specially designed scratch tester was used to determine critical loads

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419

Table 2 Al concentration (x) changes for various Al target power densities Al target power density (W/cm2)

31

32

34

35

37

x = Al/(Cr + Al)

0.4

0.43

0.47

0.52

0.55

(Lc). The tester was equipped with acoustic emission and friction force measurement. All the tests were performed employing a continuous increase in the normal load from 0 to 100 N at a loading rate of 100 N/min. Each scratch had a length of 1 cm. 2.5. Surface analyses XRD analysis was to evaluate the crystal structure and compounds formation behavior. After the immersion test was completed, the surface features of the coated samples were examined using SEM and EPMA.

3. Results and discussion 3.1. Coating characteristics The chemical compositions of the coating were obtained using GDOES. The average contents of WC-(Cr1
xAlx)N coatings with various Al (x = Al/(Cr + Al) at.%) concentration (x) are listed in Table 2.

Fig. 2. Results of scratch adhesion tests for multilayered WC-(Cr1
xAlx)N coatings.

Fig. 1 shows the XRD patterns of WC-(Cr1
xAlx)N coatings. The crystal orientations of the WC-(Cr1
xAlx)N coatings were identified to be a mixture of CrAlN (111), CrN (200), CrN (220), and WC(1
x) (200), Cr2N (111) with B1 NaCl type fcc structure. The crystal orientations of the multilayered WC-(Cr 1
xAl x)N coatings depend on the amount of Al. Most coatings exhibit a pronounced CrAlN (111) texture and the breadth of the diffraction peak increases with Al content. It was found that the intensity was very weak due to the presence of very small nanometer size grains with

Fig. 1. XRD patterns for WC-(Cr1
xAlx)N films.

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Fig. 3. Potentiodynamic polarization curves for WC-(Cr1
xAlx)N coatings in a 3.5 wt.% solution.

Al content in the coatings. The reason is that Cr atoms in the CrN lattice were substituted by Al atoms [9– 11]. 3.2. Scratch adhesion test The results of scratch adhesion test (SAT) of the WC(Cr1
xAlx)N coatings with the Al addition are shown in Fig. 2. During each test, the normal load was increased from zero until severe coating failure occurred. The results of critical adhesion loads were measured to be in a range of 35– 45 N. Except for WC-Cr0.57Al0.43N and WC-Cr0.53Al0.47N coatings, most of the coatings had comparably good adhesion to the substrate. 3.3. Electrochemical properties 3.3.1. Potentiodynamic polarization test The results of potentiodynamic polarization test for the multilayered WC-(Cr1
xAlx)N coatings are presented in Fig. 3. The corrosion potentials of all the samples were around
430 to
550 mV. From the polarization curves, all samples presented a similar behavior, i.e., active corrosion. The corrosion current density of 3.023 AA/

cm2 for the WC-Cr0.48Al0.52N is lower than that of the other samples, as shown in Table 3. The polarization resistance (Rp) can be considered as an indicator of the corrosion resistance of the material, with a higher value denoting a highly corrosion-resistant material. The WCCr0.48Al0.52N has polarization resistance of 13.77  103 V cm2, which is higher than the other coatings. The corrosion appearance is closely related to the surface quality, coated morphology and defects. These defects form direct paths between the corrosive environment and the substrate. It is due to imperfections within the coating, e.g., microcracks, pores, pinholes and droplets [12,13]. They open possible paths for the corrosive media to reach the less noble substrate. An important shift of the anodic curves towards higher current values was observed in samples having a higher porosity. The porosity corresponds to the ratio of the polarization resistance of the substrate and the coating. A combination of Eq. (1) with the electrochemical determinations gives a porosity of 0.983 for WC-Cr0.6Al0.4N, 0.698 for WCCr0.57Al0.43N, 0.563 for WC-Cr0.53Al0.47N, 0.144 for WCCr0.48Al0.52N and 0.868 for WC-Cr0.45Al0.55N. Fig. 4 shows the porosity of WC-(Cr1
xAlx)N coatings as a function of Al

Table 3 Results of potentiodynamic polarization tests Specimen

Ecorr (mV)

icorr (AA/cm2)

ba (V/decade)

bc (V/decade)

Rp (  103 V cm2)

Porosity

Substrate WC-Cr0.6Al0.4N WC-Cr0.57Al0.43N WC-Cr0.53Al0.47N WC-Cr0.48Al0.52N WC-Cr0.45Al0.55N


541.9
515.1
546.8
430.8
536.9
533.3

14.69 23.93 13.25 8.856 3.023 27.17

0.096 0.1443 0.1005 0.1197 0.1193 0.1500

0.2231 0.4857 0.6262 0.1791 0.4866 3.0250

1.986 2.021 2.840 3.522 13.77 2.286

– 0.983 0.698 0.563 0.144 0.868

J.H. Lee et al. / Surface & Coatings Technology 190 (2005) 417–427

Fig. 4. Porosity of WC-(Cr1
xAlx)N coating as a function of Al content.

content. The porosity is lower in the WC-Cr0.48Al0.52N coating than the other coatings. 3.3.2. Galvanic corrosion test Galvanic corrosion tests were accomplished using the galvanic couple between the substrate and the WC(Cr1
xAlx)N coating. Fig. 5 shows the galvanic current densities vs. immersion time. In general, the multilayered coating systems composed of a substrate and coating layer are sensitive to galvanically induced localized corrosion. The galvanic effect is an important factor in causing the localized corrosion. Nevertheless, all the samples showed that the galvanic current density is very low.

421

3.3.3. EIS measurement Fig. 6 shows the Bode plots for the samples as a function of time. The curves were drawn for frequencies between 1 mHz and 10 kHz after 1, 24, 48, 72, 96, 120, 144 and 168 h. The two time constants related to the electrolyte/coating interface via pinholes or pores had been resolved by exposure time. This suggests that the solution had penetrated the coating through defects. Initially, one time constant was clearly distinguished. For the WC-Cr0.6Al0.4N, WCCr0.57Al0.43N and WC-Cr0.45Al0.55N, EIS spectra indicate one time constant during 168 h. However, as exposure time increased, it tended to exhibit two time constants after 120 h (WC-Cr0.53Al0.47N) and 24 h (WC-Cr0.48Al0.52N) as presented in Table 4. However, it showed typically two time constants which were better resolved at longer exposure times. This effect was accompanied by a continuous and slight decrease in the absolute values of the impedance, more noticeable in the low frequency limit of the spectra. The total impedance can be represented in terms of the equivalent circuit of Fig. 7 [14]. In the case of one time constant (Fig. 7a), the equivalent circuit consists of the following elements; Rs corresponds to the solution resistance of the test electrolyte between the working electrode and reference electrode, CPE1 (Ccoat) is the capacitance of the multilayer coatings, Rct is the charge transfer resistance of the coating/electrolyte interface. For two time constants (Fig. 7b), the equivalent circuit consists of the following elements; Rs, CPE1 (Cpore) is the capacitance of the multilayer coating including the defects, Rpore is the pore resistance resulting from the formation of ionically conducting

Fig. 5. Results of galvanic corrosion tests for multilayered WC-(Cr1
xAlx)N coatings.

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Fig. 6. Bode plots for EIS data of WC-(Cr1
xAlx)N coatings for different exposure times.

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423

Table 4 Electrochemical values obtained by equivalent circuit simulation Exposure time (h)

Power densitya (W/cm2)

Rs (V cm2)

1

31 32 34 35 37 31 32 34 35 37 31 32 34 35 37 31 32 34 35 37 31 32 34 35 37

13.9 9.95 8.355 8.853 12.24 15.93 13.12 7.414 6.626 16.72 14.08 12.39 7.090 10.59 18.21 15.59 12.44 9.823 10.91 18.31 13.75 11.78 8.471 11.29 17.23

24

72

120

168

CPE1 Cpore, Ccoatb (AF/cm2)

n (0 – 1)

1.795 3.625 5.344 3.827 2.633 7.829 8.403 4.343 2.843 8.136 4.971 7.184 4.868 2.763 10.38 6.018 6.629 3.954 3.042 10.28 6.834 5.977 3.983 3.166 8.283

0.855 0.708 0.762 0.710 0.829 0.744 0.693 0.777 0.685 0.649 0.696 0.716 0.776 0.903 0.767 0.680 0.705 0.894 0.917 0.764 0.674 0.693 0.902 0.916 0.722

Rpore, Rctb (V cm2)

111,500 26,380 69,910 62,330 42,360 81,860 36,180 23,970 3663 139,500 72,210 30,350 33,090 119.8 74,950 92,790 18,070 152.3 145 58,180 66,800 27,180 124.4 157.6 52,870

Rct (V cm2)

CPE2 Ccoat (AF/m2)

WSSc

n (0 – 1)

7.919

0.630

19,950

2.326

0.534

96,680

2.546 3.408

0.688 0.566

26,430 115,500

2.266 3.793

0.643 0.552

20,120 158,700

3.1 0.8 2.3 1.4 1.2 0.9 0.5 0.9 2.2 1.3 0.9 0.5 0.8 1.0 0.8 0.9 0.7 0.4 0.8 1.1 0.8 1.1 0.6 0.7 0.9

a

31 W/cm2(WC-Cr0.6Al0.4N), 32 W/cm2(WC-Cr0.57Al0.43N), 34 W/cm2 (WC-Cr0.53Al0.47N), 35 W/cm2 (WC-Cr0.48Al0.52N), 37 W/cm2 (WC-Cr0.45Al0.55N). Applied for WC-Cr0.6Al0.4N, WC-Cr0.57Al0.43N, and WC-Cr0.45Al0.55N. c Weighted sum squares. b

paths across the coating, CPE2 (Ccoat) is the capacitance of the multilayer coatings within the pit [15]. The measurement of constant phase element (CPE) is defined by Eq. (2). It is defined as a CPE that accounts for deviations from the ideal dielectric behavior related to surface inhomogeneities. ZCPE ¼ Z0 =ðj xÞn

ð2Þ

where Z0 is the adjustable parameter used in the nonlinear least squares fitting, x is angular frequency and the factor n, defined as a CPE power, is the adjustable parameter that always lies between 0.5 and 1, which can be obtained from the slope of [Z] on the Bode plot. When n = 0.5, the CPE represents a Warburg impedance with diffusional character. The low n values obtained for the coated samples should be noticed, which indicated a rough surface of the coating [16,17]. For the WC-Cr0.53Al0.47N and the WC-Cr0.48Al0.52N coatings, the Bode plots represent two time constants; one at high frequencies corresponding to corrosion of the coating through the outer porous layer, and the second at low frequencies corresponding to the corrosion-product layer in the pores [18]. Fig. 8 presents the charge transfer resistance (Rct) of EIS measurements in the low-frequency range, which is inversely proportional to the corrosion rate of substrate. The Rct

measurement tended to decrease with the immersion time. It was due to localized corrosion through the coating defect. After 96 h of immersion time, the Rct value of WCCr0.48Al0.52N coating is higher than other coatings. This is probably due to corrosion products plugging the pores and increasing the pathway resistance [19]. 3.4. Surface analyses Fig. 9a – e show the SEM surface images of the WC(Cr1
xAlx)N coatings on the substrate. It was observed that most multilayered coatings had the droplets distributed on surface. In addition, the droplets can be compositionally inhomogeneous, as compared to adjacent surface. Thus, they can form a local galvanic couple between the coating and substrate. The interfacial bonding between the coating matrix and the droplets is obviously weakened due to shrinkage [20 –22]. Consequently, localized corrosion can occur at the sites of micro-defects that reach the substrate in the coating films, and may cause significant undercutting of the coating. SEM cross-sectional morphologies of multilayered coating (Figs. 9f– j) clearly show that multilayered WC-(Cr1
xAlx)N coatings have a dense microcolumnar morphology. The coating was quite uniform in thickness, with defect-embedding observed some way through the coatings (Figs. 9f– g). The coating had a dense mircrostructure (Fig. 9h).

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Figs. 10f – j shows the EPMA image of the corroded surfaces after 168 h immersion. All samples presented the Cr and Al all over region on surface. However, from the EPMA observation on surface, the corrosion products of the WC-Cr0.48Al0.52N plugged the defects in coating. Such mechanism is in agreement with the EIS behavior. Specially, Figs. 10d and i shows very small pitting and corrosion products on pits around.

4. Conclusions The examination of multilayered WC-(Cr1
xAlx)N coatings using CAIP shows the following results:

Fig. 7. Equivalent circuits for WC-(Cr1
xAlx)N coating systems; (a) most coatings, (b) WC-Cr0.53Al0.47N (after 120 h), WC-Cr0.48Al0.52N (after 24 h) coatings.

Figs. 10a –e shows the corroded surface morphologies after 168 h immersion time. Figs. 10a – c, and e shows the localized corrosion on coating surface. It was extensive surface attack of the coating and substrate. However, Fig. 10d shows a small pitting and corrosion product formed in pitting circumference. This characteristic of EIS evolution can be interpreted as the ‘‘blocking’’ of the pitting by corrosion products between coating/electrolyte interfaces [23]. After certain period of exposure time, the rust generated from coating/electrolyte interface had plugged through pitting. The process of the pitting will be extremely fast for coated systems due to the high current density in the defect as a result of the large ratio between the surface areas of the cathodic outer surface and the anodic defects. Then, galvanic effect takes place by the potential difference, which leads to an accelerated corrosion. It means that corrosion behavior could be explained by the corrosion products underneath the coating and the hydrogen evolution according to reaction (3) [24]: 2H2 O þ 2e
! H2 þ 2OH


(1) Multilayered WC-(Cr1
xAlx)N coatings using CAIP with increasing Al content presented the broad diffraction peak. The reason is that Cr atoms in the CrN lattice were substituted by Al atoms. (2) From potentiodynamic polarization test, the corrosion current density (icorr) of 3.023 AA/cm2 for WCCr0.48Al0.52N coating was lower than that of the other samples. Also, the WC-Cr0.48Al0.52N coating showed the higher polarization resistance and lower porosity. From the results of galvanic corrosion test, the multilayered WC-(Cr1
xAlx)N coatings showed that the galvanic current density is low. From the results of EIS test, the Rct value of WC-Cr0.48Al0.52N coating increased after 96 h immersion time. WC-Cr0.53Al0.47N and WCCr0.48Al0.52N show two time constants after 120 and 24 h, respectively. (3) All samples show various forms of pitting corrosion. However, the WC-Cr0.48Al0.52N coating shows a very small pitting corrosion. From the results of EPMA, Cr and Al compounds with surface of all samples were found to prevent passive film from pitting corrosion.

ð3Þ

It shows the SEM image (Figs. 10a –e) that a portion of the WC-(Cr1
xAlx)N coatings was missing where the pit was formed. This cracking portion was probably delaminated by hydrogen gas evolution originating from inside the pit. This corrosive attack is due to imperfections within the coating, e.g., pinholes and pores.

Fig. 8. Calculated charge transfer resistance for the WC-(Cr1
xAlx)N coatings as a function of exposure time.

J.H. Lee et al. / Surface & Coatings Technology 190 (2005) 417–427

425

Fig. 9. SEM images showing surface and cross-sectional morphologies deposited at various Al contents: (a, f) WC-Cr0.6Al0.4N, (b, g) WC-Cr0.57Al0.43N, (c, h) WC-Cr0.53Al0.47N, (d, i) WC-Cr0.48Al0.52N, (e, f) WC-Cr0.45Al0.55N.

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Fig. 10. EPMA images of multilayered WC-(Cr1
xAlx)N coatings after 168-h immersion in a 3.5 wt.% NaCl solution: (a, f) WC-Cr0.6Al0.4N, (b, g) WCCr0.57Al0.43N, (c, h) WC-Cr0.53Al0.47N, (d, i) WC-Cr0.48Al0.52N, (e, j) WC-Cr0.45Al0.55N.

J.H. Lee et al. / Surface & Coatings Technology 190 (2005) 417–427

(4) Finally, optimized deposition conditions by CAIP process are given for WC-Cr0.48Al0.52N coating from the result of corrosion resistance.

Acknowledgements The authors are grateful for the financial support provided by the Korea Science and Engineering Foundation through the Center for Advanced Plasma Surface Technology (CAPST) at the Sungkyunkwan University.

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