Corrosion resistance of CrN thin films produced by dc magnetron sputtering

Applied Surface Science 270 (2013) 150–156

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Corrosion resistance of CrN thin films produced by dc magnetron sputtering A. Ruden a,b,c , E. Restrepo-Parra a,∗ , A.U. Paladines b , F. Sequeda b a

Laboratorio de Física del Plasma, Universidad Nacional de Colombia Sede Manizales, Km. 9 vía al Magdalena, Manizales, Colombia Laboratorio de Recubrimientos Duros y Aplicaciones Industriales–RDAI, Universidad del Valle, Calle 13 N◦ 100-00 Ciudadela Meléndez, Cali, Colombia c Departamento de matemáticas, Universidad Tecnológica de Pereira, Pereira, Colombia b

a r t i c l e

i n f o

Article history: Received 27 March 2012 Received in revised form 28 December 2012 Accepted 29 December 2012 Available online 4 January 2013 Keywords: Corrosion EIS Pressure Tafel curves XRD

a b s t r a c t In this study, the electrochemical behavior of chromium nitride (CrN) coatings deposited on two steel substrates, AISI 304 and AISI 1440, was investigated. The CrN coatings were prepared using a reactive d.c. magnetron sputtering deposition technique at two different pressures (P1 = 0.4 Pa and P2 = 4 Pa) with a mixture of N2 –Ar (1.5-10). The microstructure and crystallinity of the CrN coatings were investigated using X-ray diffraction. The aqueous corrosion behavior of the coatings was evaluated using two methods. The polarization resistance (Tafel curves) and electrochemical impedance spectra (EIS) in a saline (3.5% NaCl solution) environment were measured in terms of the open-circuit potentials and polarization resistance (Rp ). The results indicated that the CrN coatings present better corrosion resistance and Rp values than do the uncoated steel substrates, especially for the coatings produced on the AISI 304 substrates, which exhibited a strong enhancement in the corrosion resistance. Furthermore, better behavior was observed for the coatings produced at lower pressures (0.4 Pa) than those grown at 4 Pa. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Ceramic coatings composed of transition metal nitrides, such as TiN, TiAlN, CrN and NbN, have been widely applied in industrial applications because they provide wear protection, heat and corrosion resistance, good adhesion and a high level of hardness [1,2]. In addition to TiN, CrN is another popular coating because it is a tough and superior coating for wear and corrosion resistance applications, especially for soft substrates used in plastic extrusion [3] and die casting molds [4,5]. CrN coatings as monolayers and multilayers combined with other compounds have been produced using different techniques, and the most commonly used techniques for producing these coatings are cathodic vacuum arc and magnetron sputtering [6–8]. Moreover, one of the most interesting applications for CrN is as a corrosion-resistant protective coating. The corrosion of hard coatings on steel produced using plasmaassisted physical vapor deposition usually takes a localized form due to the establishment of an electropotential difference between the coating material and the less noble steel substrate [9]. Based upon this electrochemical corrosion characteristic, the factors that affect the corrosion resistance of coatings, such as CrN, on steel have been investigated for many years with respect to pit initiation and further development at through-coating defects in PVD

∗ Corresponding author. Tel.: +57 6 8879495; fax: +57 6 8879495. E-mail address: [email protected] (E. Restrepo-Parra). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.148

(physical vapor deposition) hard coating systems [10,11]. The primary advantage for depositing CrN films is the low deposition temperature, and this characteristic makes it suitable for deposition onto temperature-sensitive materials and metals with low melting points [12,13]. However, it is well-known that the deposition parameters, such as the pressure and substrate material, have a considerable influence on the film properties; subsequently, the different functions of a component are performed by different layers, including the substrate, the substrate-coating interface and the coating layer. The purpose of a coating is generally related to wear resistance, protection against corrosion, thermal and electrical insulation, and the optical appearance of the work-piece. The substrate-coating interface is important for adhesion and providing a diffusion barrier, whereas the substrate can impart strength, stiffness and weight [14]. Although there are many reports on studies of corrosion resistance in CrN coatings [15–17], we are interested in investigating the differences in the behavior of coatings grown on two widely used steels that contain different percentages of Cr under two values of a N2 flux. There are many studies in the literature concerning the influence of the substrate on the coating characteristics; nevertheless, we will focus our attention on the differences in the structural and corrosion resistance behavior of coatings when they are grown on two different Cr-based steels, which provides a possible synergistic relationship between the coating-substrate. Furthermore, it was demonstrated that although the substrate is highly suitable for growing the coatings, other deposition conditions, such as the working pressure, can influence the corrosion resistance of

A. Ruden et al. / Applied Surface Science 270 (2013) 150–156

151

the coatings. Therefore, to enhance the behavior of the coatings, it is necessary to combine the investigation of the effects of several parameters, such as the substrate, pressure as in this study, and other parameters, such as the temperature, bias voltage and concentration of gases. In this work, an investigation of the corrosion resistance of CrN coatings produced by d.c. magnetron sputtering on two steel substrates at two different working pressure is presented. The corrosion resistance was analyzed using polarization resistance (Tafel curves) and electrochemical impedance spectroscopy (EIS) measurements.

2. Experimental setup Two substrates were selected to produce the coatings. The austenitic AISI 304 steel has one of the highest corrosion resistances compared to other Cr–Ni (carbon stabilized) based steels when it is exposed to several harmful environments [18]. A common misconception concerning stainless steel is that it is not affected by corrosion. While misleading, the phenomenal success of the metal makes this common belief understandable. When held in the temperature range between 800 and 1650 ◦ F, austenitic stainless steels may undergo a change that renders them susceptible to intergranular corrosion when exposed to a number of corrosives, including certain corrosives that may otherwise only have a slight effect on them. One of New York City’s most impressive landmarks is the stainless steel-clad peak of the Chrysler Building, which was built in 1930 of 302 stainless steel. A recent inspection of this landmark revealed no signs of corrosion or loss of thickness. The tallest manmade monument in the US, the St. Louis Arch, is entirely clad in 304 stainless steel plates; nevertheless, all austenitic stainless steels contain a small amount of carbon. At extremely high temperatures, such as those experienced during welding, the carbon forces the local chrome to form chromium carbide around it, which consequently starves the adjacent areas of the chrome required for its own corrosion protection. When welding, it is recommended to consider using low-carbon stainless steel, such as 304 and 316 [19,20]. The AISI-SAE 4140 steel is a Cr–Mo alloyed steel that has high stability up to 400 ◦ C and is suitable for resisting stress and torsion [21]. The dimensions of the sample were 1.25 cm in diameter and 4 mm in thickness. The samples were polished using abrasive silicon carbide paper, and they subsequently were thoroughly cleaned using an ultrasonic cube in acetone for 15 min to eliminate oil, dust and any contamination. A d.c. magnetron sputtering system placed in a class 1000 clean-room was employed, and the base vacuum pressure was 5 × 10−5 Pa. The system is composed of multisource equipment (ATC 1500) from AJA International. The coatings were grown using the following conditions: a target of Cr (99.99%), room temperature (RT), −300 V as a bias voltage, inter-electrode distance of 10 cm, laboratory temperature of 24 ◦ C and a relative humidity of 55%. Furthermore, two working pressures (P1 = 0.4 Pa and P2 = 4 Pa), a gas mixture of N2 –Ar (1.5-10 sccm) and a power of 8 W/m2 for 90 min were employed. The XRD analysis was performed using a Rigaku Ultima III with Cu K␣ radiation, which had a wavelength of  = 0.1540 nm and used 40 kV with 44 mA for the source power. The Rigaku Ultima III software package was employed. The polarization resistance (Tafel curves) and electrochemical impedance spectra (EIS) in a saline (3.5% NaCl solution) environment were obtained in terms of open-circuit potentials. The Tafel curves were obtained using a PG TEXCORR 4.1 Potentiostat-Galvanostat system at room temperature with a cell composed of the working electrode, which had an exposed area of 1 cm2 . The curves were obtained using a Ag/AgCl reference electrode and a platinum wire as a counter-electrode

Fig. 1. X- Ray diffraction patterns of the CrN coatings grown on AISI 304 substrates at 0.4 Pa while varying the Ar–N2 gas mixture.

with a scanning step of 1 mV/s. The Tafel diagrams were obtained using a scan speed of 0.126 mV/s over a voltage range between −0.25 and 1 with an exposure area of 1 cm2 in an electrochemical solution of 3.5% p/p of NaCl. 3. Results and discussion 3.1. XRD analysis Fig. 1 presents the X ray diffraction patterns of the CrN coatings produced on AISI 304 stainless steel at 4 Pa for different mixtures of Ar–N2 . Diffraction peaks corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes for the cubic CrN structure were identified. These orientations are based on the JCPDF 00-011-0065 file for the CrN ICDD cards. These results are consistent with studies performed by Hones et al. [22]. The diffraction patterns reveal a preferential orientation in the (1 1 1) direction, which is characteristic of the fcc-CrN phase [23,24]. The identification of the peaks was performed using the ICSD crystal structure database [25]. A more detailed analysis of the crystallographic orientation of the ␣CrN phase is presented in Fig. 2 (a)–(d), in which the peaks that correspond to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes for nitrogen flows of 1.5, 2 and 5 sccm are magnified. Based on the peaks with the highest intensities (Fig. 2(a) and (c)), it can be concluded that the peaks tend to shift toward lower diffraction angles as the flow of nitrogen increases. This behavior is difficult to observe in the other peaks (Fig. 2(b) and (d)) because of their low intensity and the noise present in the diffraction patterns. This tendency is attributed to the insertion of nitrogen into the CrN structure, which produces an increase in the interplanar distance and in the lattice parameter. Subsequently, an increase in the nitrogen concentration generates an increase in the movement of the surface atoms on the substrate. This effect increases the energetic state of the atoms, which generates a structural distribution where there is a lower Gibbs energy. This distribution produces an increase in the density and in the interatomic distance [26] that causes a compressive stress; next, at low values of nitrogen flux, coatings with less intrinsic stress were

152

A. Ruden et al. / Applied Surface Science 270 (2013) 150–156

Fig. 2. Magnification of the peaks that correspond to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of CrN coatings grown on AISI 304 substrates for nitrogen gas flow rates of 1.5, 2 and 5 sccm.

produced, which are structurally suitable for industrial applications [27]. Other authors have recognized that excessive ion bombardment caused by high pressures will induce an accumulation of the residual stress and point/line defects in the growing coatings, which is detrimental to the toughness and adhesion of the coating [28,29]. Based on the XRD analysis, the gas flux ratio that results in structural properties more similar to the non-stressed CrN is 10-1.5 of Ar–N2 . This result is specifically observed for coatings grown at low pressures, which form the ␣-CrN phase. Theoretically, this phase has the plane with the greatest sliding in the (1 1 1) direction [30]. According to the literature, coatings with preferential orientation in the (1 1 1) direction not only exhibit better mechanical and tribological properties but also good anticorrosive behavior. Therefore, a corrosion resistance study of CrN coatings grown at two different pressures (0.4 and 4 Pa, named P1 and P2, respectively) with a flux ratio of 10-1.5 (Ar–N2 ) on AISI 304 and AISI 4140 steel substrates will be performed.

nevertheless, changes caused by the pressure were observed. A lower thickness was obtained at the higher pressure because the energy of the atoms that arrive at the substrate is lower (caused by a greater quantity of collisions), which produces a decrease in the island coalescence. At the lower pressure, nucleation is favored [31]. This result can also be associated with the deposition rate. Lin et al. [32] reported that the deposition rate decreased as the concentration of nitrogen increased in CrN coatings grown in both dc and pulsed conditions. The lower deposition rate of CrN coatings produced under a greater N2 flux can be corroborated with the results presented in Table 1. This phenomenon can be explained by the poor N2 sputtering capability compared to Ar (reduced Ar in the chamber) and also by the target poisoning effect (formation of nitride on the target surface) when the concentration of N2 in the system was increased.

3.3. Polarization curves (Tafel) 3.2. Thickness measurement using profilometry The thicknesses of the films deposited on two substrates (AISI304 and AISI4140) at P1 and P2 were determined using profilometry, and the results are shown in Table 1. No influence of the substrate type on the thicknesses of the films was observed; Table 1 Thicknesses of the CrN coatings. System

P1

P2

Thickness (nm) AISI304/CrN AISI4140/CrN

896 ± 6 890 ± 4

549 ± 5 583 ± 7

Tafel diagrams were constructed for the materials investigated in this study. Figs. 3–5 present the Tafel diagrams of the substrates (Fig. 3), AISI 304 with coatings produced at P1 and P2 (Fig. 4(a)) and AISI 4140 with coatings produced at P1 and P2 (Fig. 5(a)), in which regions of anodic oxidation and cathodic reduction can be identified with respect to the activation polarization corrosion potential (Ecorr ). According to Fig. 3, a greater Ecorr and lower current density (Icorr ) are observed for AISI 304. This result has a direct relationship with the corrosion velocity because it has a higher percentage of Cr compared to AISI 4140. AISI 304 has greater resistance to environments that have a high concentration of chlorides [33]. Table 1 presents the results of Ecorr , Icorr and corrosion velocity (measured in meters per year-mpy).

A. Ruden et al. / Applied Surface Science 270 (2013) 150–156

153

the stationary current density on the order of 10−2 A/cm2 . For the coating synthesized at P1, no pitting potential was observed, and only an active corrosion region can be identified [34]. Table 2 presents the corrosion resistance results for coatings grown at P1 and P2 on AISI 304 substrates. Fig. 5(a) presents the Tafel curves of the AISI 4140 substrate with and without a coating of CrN grown at P1 and P2. A lower current density was observed for the substrates coated at P1 than those synthesized at P2. 3.4. SEM analysis

Fig. 3. Potential versus current density (Tafel curves) of the AISI 304 and AISI 4140 substrates.

Fig. 4(a) presents the Tafel curves of the AISI 304 substrate coated with CrN grown at P1 and P2. An improvement of the substrate corrosion resistance after the deposition of the coating is observed because it presents lower current densities; thus, the corrosion velocity is lower. The coating produced at P2 exhibits a pitting potential of −0.4 V, and it presents a degree of passivation that can be identified from

Scanning electron microscopy (SEM) analyses were performed on the surfaces of the coatings immediately after finishing the electrochemical measurements. The images, which were recorded under identical magnification conditions (50×), clearly reveal degradation of the surface due to chemical attack by the NaCl solution for the uncoated AISI 304 (Fig. 4(b)) and AISI 4140 (Fig. 5(b)) steel substrates, AISI 304/CrN grown at P1 and P2 (Fig. 4(c) and (d)) and AISI 40140/CrN grown at P1 and P2 (Fig. 5(c) and (d)). Less damage was observed for the coated substrates. This result indicates that materials producing passivating layers tend to undergo local or generalized corrosion more slowly, especially in environments that contain active ions, such as chloride ions, which present reduction action [35]. The level of degradation evidently depends on the working pressure, which indicates that for this study, the coating produced at the lower pressure (P1) is the most appropriate coating for reducing corrosion processes in steel. Moreover, the corrosion damage

Fig. 4. (a) Tafel curves of AISI 304 and AISI 304/CrN grown at P1 and P2, 50X SEM micrographs of (b) the AISI 304 substrate, (c) AISI 304/CrN grown at P1 and (d) AISI 304/CrN grown at P2.

154

A. Ruden et al. / Applied Surface Science 270 (2013) 150–156

Fig. 5. (a) Tafel curves of AISI 4140 and AISI 4140/CrN grown at P1 and P2, 50X SEM micrographs of (b) the AISI 4140 substrate, (c) AISI 4140/CrN grown at P1 and (d) AISI 4140/CrN grown at P2.

is greater for coatings produced on the AISI 4141 substrate than those grown on the AISI 304 substrate according to the Tafel curves because of the high content of Cr. The corrosion resistance behavior also depends on the growth pressure. At P1, the coatings exhibit less damage for both the AISI 4140 and AISI 304 substrates; moreover, the coatings grown on the AISI 304 substrate at P2 exhibit pitting corrosion. In contrast, the coatings grown at P1 exhibit generalized electrochemical etching [36]. The working pressure affects the structure of the coating. A greater number of surface defects appear at higher pressures. The porosity caused by the greater number of ions hitting the surface produces greater corrosion due to the creation of direct routes for the saline environment on the substrate and helping the galvanic corrosion process. The galvanic corrosion process is caused by the potential difference that is

produced between the two metals immersed in the solution [37]. These results are consistent with those reported by Xu et al. [25]. The authors produced single layers of CrN using electron beam PAPVD (EBPAPVD), and they observed that the corrosion is directly dependent on the structure and the stoichiometry of the nitride. These researchers stated that the density of the CrN films decreases with an increase of the gas flows (and pressure) at room temperature. The generally low corrosion resistance of tool steels is due to the galvanic couple between the matrix and the hard carbides. In contrast, chromium nitride exhibits a considerably improved performance with respect to the uncoated specimens. First, CrN presents a more noble open circuit potential, and it describes a wide passive stage. At higher potentials, the curve describes the breakdown potential, which indicates the formation of pitting. However,

Table 2 Corrosion parameters (Ecorr , Icorr and Vcorr ) of the AISI 304 and AISI 4140 substrates with and without CrN coatings obtained from the Tafel curves and EIS measurements. AISI304

AISI4140

AISI304/CrN

AISI4140/CrN

Pressure (Pa)

–

–

P1

P2

P1

P2

Tafel curves Ecorr (mV) Icorr (␮A/m2 ) Vcorr (mpy)

−501 6.950 3.174

−593 19.80 9.044

−348 0.222 101.5 × 10−3

−166 0.37 17.22 × 10−3

−566 15.80 7.231

−414 9.70 4.99

EIS results Rp (-cm2 ) ˇa (V/decade)

5.45 × 1010 9.18 × 10−4

1.06 × 106 7.18 × 10−4

3.81 × 1011 9.18 × 10−4

7.99 × 1010

5.2 × 1011 7.18 × 10−4

2.6 × 106

Porosity (%)

–

–

32.07 × 10−6

0.0146

2.03 × 10−6

0.171

A. Ruden et al. / Applied Surface Science 270 (2013) 150–156

155

inversely proportional to the Vcorr and is inversely proportional to the pressure. The polarization resistance is used to detect the tendency of the coating to passivity, and it is less affected by environmental factors, such as air or water. The polarization resistance involves a shielding outer layer of corrosion [41]. This layer subsequently decreases the corrosion velocity. Comparing the results presented in Figs. 4 and 5, the coatings produced on the AISI 304 substrate exhibited greater polarization resistance than those on the AISI 4140 substrate because of the greater concentration of Cr and the other alloying elements [42]. Moreover, the AISI304/CrN coatings deposited at P1 exhibited a higher reactance compared to those produced at P2; the last one presented a higher Vcorr . ˇa is the parameter that represents the anodic Tafel slope of the base metal, which, in this case, corresponds to the AISI 304 and AISI 4140 substrates. A special characteristic of the Nyquist diagrams is the different shape of the curves that is dependent on the substrate. For the coatings grown on AISI 304 substrates, the curves tend to increase, indicating that the polarization resistance (Rp ), which is inversely proportional to the corrosion velocity, increases. In contrast, for a loop similar to a semicircle (AISI 4140 substrates), the Rp decreases, which indicates that the corrosion velocity increases. Finally, the calculated porosity factor that was associated with the different coatings was consistent with the results reported by Tato and Landolt [43,44]. The porosity factor corresponds to the ratio between the polarization resistance of the uncoated substrates (Rp , u ) and the coated substrates (Rp , r−u ), as shown in the following equation: Por =

Fig. 6. Nyquist diagrams of (a) AISI 304 and AISI 304/CrN grown at P1 and P2 and (b) AISI 4140 and AISI 4140/CrN grown at P1 and P2.

the sample presented a notably low corrosion current density [38]. For instance, Zhou [39] presented the electrochemical behavior of H13 steel coated with duplex and gradient CrNx coatings deposited by cathodic arc deposition. The authors varied the nitrogen pressure from 0.05 to 2.0 Pa. Clearly, Ecorr decreased with an increasing pressure of nitrogen. The porosity was calculated using the EIS analysis. The results of the corrosion velocity (Vcorr ) in meters by year (mby) for both AISI 4140/CrN and AISI 3l4/CrN at P1 and P2 are listed in Table 1. Vcorr exhibits a strong dependence on the substrate materials. The coatings grown on the AISI 304 substrate exhibit a decrease in the Vcorr value for P1 and P2 of 18432.05% and 3127.09%, respectively. For the coatings grown on the AISI/4140 substrate, no significant improvement in Vcorr was observed because it remained of the same order of magnitude (4 mpy and 9 mpy). 3.5. Electrochemical impedance spectroscopy (EIS) Fig. 6 presents Nyquist diagrams that correspond to the uncoated and coated AISI 304 and AISI 4140 steel substrates and a single CrN layer at P1 and P2. The results of RP and the porosity for the substrates with and without coatings are presented in Table 2. The polarization resistance (Rp ) can be considered to be an indicator of the corrosion resistance of the material, in which a higher value represents a highly corrosion-resistant material. At higher pressures, the RP value tends to decrease [40]. The RP value varies

Rp,u −(Ecorr /ˇa ) e Rp,r−u

(1)

The appearance of corrosion is closely related to the quality of the surface, the morphology of the coating and defects. These defects form direct paths between the corrosive environment and the substrate. The formation of defects is due to imperfections within the coating, e.g., microcracks, pores, pinholes and droplets [45]. The porosity of the coatings was determined, and the values are listed in Table 2. From these results, it was possible to conclude that the coatings with the best behavior are those produced on the AISI 304 substrates at P1 (lower pressure) because they present lower corrosion damage and greater corrosion resistance. This result is directly related to the low porosity and high density of the CrN films because the pores provide possible paths by which the corrosive media could reach the less noble substrate [46]. 4. Conclusions The structural and corrosion behavior of chromium nitride (CrN) coatings deposited on two steel substrates (AISI 304 and AISI 1440) have been investigated at two pressures (P1 = 0.4 and P2 = 4 Pa). From the XRD analysis, a study that varied the nitrogen flow allowed the conclusion that as the nitrogen flow increases, the diffraction peaks tended to shift toward lower diffraction angles, which produced an increase in the interplanar distance in the lattice parameter and the compressive stress. Moreover, coatings grown with a flux ratio of 10-1.5 (Ar–N2 ) presented the highest preferential orientation in the (1 1 1) direction; therefore, this condition was chosen for the production of the coatings. From the profilometry studies, no influence of the substrate type on the films thickness was observed; nevertheless, lower thicknesses were obtained at a greater pressure because the energy of the atoms that arrive at the substrate is lower. A corrosion resistance study of CrN coatings grown at two pressures (P1 = 0.4 and P2 = 4 Pa).Tafel diagrams were measured, which exhibited regions with anodic oxidation and cathodic

156

A. Ruden et al. / Applied Surface Science 270 (2013) 150–156

reduction. A higher Ecorr and lower density of current (Icorr ) were observed for the AISI 304 substrate, and they have a relationship with the corrosion velocity because it has a higher percentage of Cr compared to the AISI 4140 substrate. An improvement of the substrate corrosion resistance after the coating deposition is observed because of lower current densities and corrosion velocities. For the coating synthesized at P1, no pitting potential was observed; only a corrosion active region can be identified. Nyquist diagrams corresponding to the uncoated and coated steel AISI 304 and AISI 4140 substrates and CrN single layer at P1 and P2 revealed that the polarization resistance (RP ) and porosity for the substrates with and without coatings at higher pressure, Rp , tend to decrease. Coatings produced on the AISI 304 substrate exhibited a higher polarization resistance than those on the AISI 4140 substrate because of the greater concentration of Cr and the other alloying elements. Moreover, the AISI304/CrN coatings deposited at P1 exhibited a higher reactance than those produced at P2; the last coating subsequently presented a higher Vcorr . Acknowledgment The authors gratefully acknowledge the financial support of the Instituto Colombiano para el Desarrollo de la Ciencia y la Tecnología (Colciencias). References [1] C.M. Suh, B.W. Hwang, R.I. Murakami, Materials Science and Engineering A 343 (2003) 1–7. [2] F.R. Lamastra, F. Leonardi, R. Montanari, F. Casadei, T. Valente, G. Gusmano, Surface and Coatings Technology 200 (2006) 6172. [3] E. Bienk, J.H. Reitz, N.J. Mikkelsen, Surface and Coatings Technology 76–77 (1995) 475. [4] B. Navinsek, P. Panjan, Surface and Coatings Technology 74–75 (1995) 919. [5] W.H. Zhanga, J.H. Hsieh, Surface and Coatings Technology 130 (2000) 240. [6] V. Ezirmik, E. Senel, K. Kazmanli, A. Erdemir b, M. Ürgen, Surface and Coatings Technology 202 (2007) 866. [7] Eh.P. Hovsepian, D.B. Lewis, W.D. Münz, S.B. Lyon, M. Tomlinson, Surface and Coatings Technology 120–121 (1999) 535. [8] B. Warcholinski, A. Gilewicz, Z. Kuklinski, P. Myslinski, Surface and Coatings Technology 204 (2010) 2289. [9] C. Liu, A. Leyland, Q. Bi, A. Matthews, Surface and Coatings Technology 141 (2001) 164. [10] L. Cunha, M. Andritschky, L. Rebouta, K. Pischow, Surface and Coatings Technology 116–119 (1999) 1152. [11] S. Rudenja, C. Leygraf, J. Pan, P. Kulu, E. Tallimets, V. Mikli, Surface and Coatings Technology 114 (1999) 129. [12] W.D. Münz, J. Göbel, Surface Engineering 3 (1987) 47. [13] Y.H. Yoo, J.H. Hong, J.G. Kim, H.Y. Lee, J.G. Han, Surface and Coatings Technology 201 (2007) 9518–9523.

[14] M. Rosso, Journal of Achievements in Materials and Manufacturing Engineering 19 (2006) 35. [15] L. Wang, D.O. Northwood, X. Nie, J. Housden, E. Spain, A. Leyland, A. Matthews, Journal of Power Sources 195 (2010) 3814. [16] J. Barranco, F. Barreras, A. Lozano, M. Maza, Journal of Power Sources 196 (2011) 4283. [17] Z.-y. Chen, Z.-q. Li, X.-h. Meng, Applied Surface Science 255 (2009) 7408. [18] M. Nalbant, Y. Yildiz, Transactions of Nonferrous Metals Society of China 21 (2011) 72. [19] D. Warren, Corrosion 15 (1959) 213t. [20] W.O. Binder, C.M. Brown, R. Franks, Transactions of American Society for Metals 41 (1949) 1301–1370. [21] H. Holzapfel, A. Wick, V. Schulze, O. Vöhringer, Härterei Technische Mitteilung 53 (1998) 155. [22] P. Hones, R. Sanjines, F. Lévy, Surface and Coatings Technology 94–95 (1997) 398. [23] A. Barata, L. Cunha, C. Moura, Thin Solid Films 398–399 (2001) 501. [24] Z. Han, J. Tian, Q. Lai, X. Yu, G. Li, Surface and Coatings Technology 162 (2003) 189. [25] J. Xu, H. Umehara, I. Kojima, Applied Surface Science 201 (2002) 208. [26] H.-S. Park, H. Kappl, K.H. Lee, J.-J. Lee, H.A. Jehn, M. Fenker, Surface and Coatings Technology 133–134 (2000) 176. [27] Z. Zong, J. Lou, M. El-Mustaffa, O. Adewoye, F. Hammad, W.O. Soboyejo, Materials Science and Engineering: A A434 (2006) 178. [28] E. Forniés, R. Escobar Galindo, O. Sánchez, J.M. Albella, Surface and Coatings Technology 200 (2006) 6047. [29] J. Lin, J.J. Moore, B. Mishra, M. Pinks, W.D. Sproul, J.A. Rees, Surface and Coatings Technology 202 (2008) 1418. [30] R. Lamni, R. Sanjinés, M. Parlinska-Wojtan, A. Karimi, F. Lévy, Journal of Vacuum Science & Technology A 23 (593) (2005) 593. [31] J. Lou, Mechanical characterization of thin film materials for MEMS devices, in: W. Soboyejo (Ed.), Advanced Structural Materials: Properties, Design Optimization and Applications, CRC Press, Boca Raton, FL, 2007, pp. 35–63. [32] J. Lin, Z.L. Wu, X.H. Zhang, B. Mishra, J.J. Moore, W.D. Sproul, Thin Solid Films 517 (2009) 1887. [33] Jia-Hong Huang, Fan-Yi Ouyang, Yu. Ge-Ping, Surface and Coatings Technology 201 (2007) 7043. [34] J. Piippo, B. Elsener, H. Bohni, Surface and Coatings Technology 61 (1993) 43–46. [35] A.S. Mogoda, Thin Solid Films 357 (1999) 202–207. [36] J. Creus, H. Idrissi, H. Mazille, F. Sanchette, P. Jacquot, Surface and Coatings Technology 107 (1998) 183. [37] C. Liu, A. Leyland, Q. Bi, A. Matthew, Surface and Coatings Technology 76–77 (1995) 615. [38] A. Conde, C. Navas, A.B. Cristóbal, J. Housden, J. de Damborenea, Surface and Coatings Technology 201 (2006) 2690. [39] Q.G. Zhoua, X.D. Baia, X.W. Chena, D.Q. Penga, Y.H. Linga, D.R. Wang, Applied Surface Science 211 (2003) 293. [40] K.-L. Chang, S.-C. Chung, S.-H. Lai, C. Han, Applied Surface Science 236 (2004) 406. [41] W.-M. Zhao, Y. Wang, T. Han, K.-Y. Wu, J. Xue, Surface and Coatings Technology 183 (2004) 118. [42] V.K. William Grips, C. Harish, V. Barshilia, Ezhil Selvi, K.S. Kalavati, Rajam, Thin Solid Films 514 (2006) 204. [43] W. Tato, D. Landolt, Journal of The Electrochemical Society 145 (1998) 4173. [44] B. Matthews, E. Broszeit, J. Aromaa, H. Ronkainen, S.P. Hannula, A. Leyland, A. Matthews, Surface and Coatings Technology 49 (1991) 489. [45] M. Fenker, M. Balzer, H.A. Jehn, H. Kappl, J.-J. Lee, K.-H. Lee, H.-S. Park, Surface and Coatings Technology 150 (2002) 101. [46] J.H. Lee, S.H. Ahn, J.G. Kim, Surface and Coatings Technology 190 (2005) 417.