Micrograph and structure of CrN films prepared by plasma immersion ion implantation and deposition using HPPMS plasma source

Surface & Coatings Technology 229 (2013) 210–216

Contents lists available at SciVerse ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Micrograph and structure of CrN films prepared by plasma immersion ion implantation and deposition using HPPMS plasma source Wu Zhongzhen a, Tian Xiubo a,⁎, Gong Chunzhi a, Yang Shiqin a, Paul K. Chu b a b

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, China Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

a r t i c l e

i n f o

Available online 11 April 2012 Keywords: Plasma Base ion implantation and deposition High power pulsed magnetron sputtering CrN Micro-structure Surface properties

a b s t r a c t In order to achieve excellent adhesion between coatings and substrates, PBII&D technique was proposed and used more and more widely to produce hard coatings. Among the metal plasma source, one based on HPPMS technique (HPPMS-PIID) proposed recently seems to have great prospects. The present study aims to develop the difference of CrN films deposited on Si (100) and SU201 stainless steel substrates with PBII&D technique using different metal plasma sources of HPPMS, FCA and DCMS. In contrast to the other two samples, the sample based on HPPMS appears to have dense knitting-like surface and discontinuous columnar structure. The highest intensity of CrN(200) preferential orientation, best adhesion and hardness is obtained by HPPMS plasma source for the highly energetic ion implantation and bombardment, but the C fraction is a little higher since the sputtering of highly energetic ions. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In coatings upgrading, adhesion deterioration has to be considered more for the increasing difference of physical properties between coatings and substrates. Another, the inherently line-of-sight restrictions in beam-linearly preparative techniques are unexpected in industrial production. Consequently, Plasma-Based Ion Implantation and Deposition (PBII&D) was established in the 1980s and was considered a suitable technology for surface modification and thin film deposition gradually [1,2]. For the surrounding plasma and high negative-voltage pulse biased to the substrate, not only a near-surface alloy of graded composition without an abrupt interface with the substrate will form [3], but also the line-of-sight restrictions will be circumvented. To extend the applications, alternative techniques featuring nongaseous elements such as metallic elements and carbon had been developed [3–5]. Generally, the metal plasma with high ionization fraction was provided by many techniques, including filtered cathodic arc plasma source (FCA) [5], magnetron sputtering plasma source (MS) [6], and some hybrid magnetron sputtering or evaporating method with inductively coupled plasma source (ICP-MS) [7,8], or electron cyclotron resonance plasma source (ECR-MS) [9,10], etc. However, many personal shortcomings of these plasma sources were discovered in using, such as macroparticles or similar defects for FCA, low metal ionization fraction for DCMS, and the complexity of the hybrid devices. Considering that, we proposed a novel PBII&D technique recently based on high power pulsed magnetron discharge [11,12]. It had been

⁎ Corresponding author. Tel.: + 86 451 86418791; fax: + 86 451 86418784. E-mail address: [email protected] (X. Tian). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.04.012

reported that HPPMS could provide a high density plasma flux with high metal ionization fraction but metal droplets as cathodic arc [13,14]. For the “clean” metal plasma flux and simple device, the PBII&D technique using HPPMS plasma source is considered to have great prospects. In this paper, CrN films were prepared on Si (100) and SU201 stainless steel substrates with PBII&D technique using different metal plasma sources of HPPMS, FCA and DCMS. Morphology, structure and properties of the films were compared, and aspects of film growth processes and their effects on film properties were discussed.

2. Experimental The experiments were performed in turbo-molecular pumped highvacuum chamber with a diameter of 40 cm and a height of 80 cm, as shown in Fig. 1. The chamber was evacuated to a base pressure of 3 × 10− 3 Pa prior to the depositions. The inert gas (Ar, 99.9997% purity) and reactive gas (N2, 99.998% purity) were introduced through a leak valve under the chamber. A φ50 mm × 6 mm Cr target (99.9% purity) was mounted on an unbalanced magnetron cathode. The magnetron cathode was driven by a hybrid pulsed power supply developed in our lab [15]. The substrate was biased by a pulsed power supply delivering pulses with a maximum voltage of 35 kV at a repetition frequency of 20–200 Hz and a pulse width of 20–500 μs. Silicon (100) and SU201 stainless steel disks (30 × 30 × 1 mm3) were used as substrate materials polished to final mirror finish using a 1 μm diamond paste. After ultrasonically cleaning in ethanol and acetone for 20 min, the substrates were placed at a distance of 16 cm from the target and no external heating was used during the processes.

Z. Wu et al. / Surface & Coatings Technology 229 (2013) 210–216

211

Fig. 1. Schematic diagram of the HPPMS-PIID device.

Four stages were followed in the experiments. First, a plasma etching by high voltage self-excited glow discharge was applied to the substrate with a high-voltage pulse amplitude of −10 kV, frequency of 50 Hz and width of 200 μs in the inert atmospheres. The work pressure was kept 1.0 Pa for 15 min. Second, a Cr layer was prepared by PBII&D technique using different metal plasma sources of HPPMS, FCA and DCMS in inert atmospheres. Third, CrN thin films were deposited by PBII&D technique using different metal plasma sources of HPPMS, FCA and DCMS in mixed atmosphere. The details of the surface treatment for the samples discussed in this work can be found in Table 1. Fourth, the samples cooled down in the vacuum with the work pressure of 3 × 10− 3 Pa for 30 min. The microstructure of the films was analyzed by scan electron microscopy (SEM) using a 30 kV Hitachi S4800 instrument. X-ray diffraction (XRD) was used to investigate the phase composition of the films in the Bragg–Brentano geometry. The composition and chemical bonding of elements were tested by X-ray photoelectron spectroscopy (XPS). The adhesion were evaluated by scratch test performed in a MFT-4000 scratch tester with a diamond tip of radius 200 μm and indentation test using TH320 Rockwell hardness tester equipped with Table 1 The instrumental parameters of the CrN films deposited by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C. Sample

Cr interlayer

CrN film

A

High voltage pulse = − 18 kV, frequency = 50 Hz, width = 50 μs, phase = 0 μs, HPPMS pulse = 900 V, frequency = 50 Hz, width = 200 μs, DC = 0.2A, Ar flow rate = 10 sccm, working pressure = 0.5 Pa, 5 min

B

High voltage pulse = − 18 kV, frequency = 50 Hz, width = 50 μs, ICA = 50A, Ar flow rate = 10 sccm, working pressure = 0.5 Pa, 5 min

C

High voltage pulse = − 18 kV, frequency = 50 Hz, width = 50 μs, IDC = 0.5A, Ar flow rate = 10 sccm, working pressure = 0.5 Pa, 5 min

High voltage pulse = − 18 kV, frequency = 50 Hz, width = 50 μs, phase = 0 μs, HPPMS pulse = 900 V, frequency = 50 Hz, width = 200 μs, DC = 0.2A, Ar flow rate = 10 sccm, N2 flow rate = 4 sccm, working pressure = 0.5 Pa, 60 min High voltage pulse = − 18 kV, frequency = 50 Hz, width = 50 μs, ICA = 50A, Ar flow rate = 10 sccm, N2 flow rate = 4 sccm, working pressure = 0.5 Pa, 30 min High voltage pulse = − 18 kV, frequency = 50 Hz, width = 50 μs, IDC = 0.5A, Ar flow rate = 10 sccm, N2 flow rate = 4 sccm, working pressure = 0.5 Pa, 60 min

a ϕ1.5875 mm GCr15 ball with the load of 45 kgf. An MTS nanoindenter XPII assembled with a Berkovich diamond indenter was used to measure the hardness and Young's modulus of the films. 3. Results and discussion SEM surface morphology of CrN films deposited on Si (100) substrates with PBII&D techniques using different metal plasma sources of HPPMS (Sample A), FCA (Sample B) and DCMS (Sample C) are displayed in Fig. 2(a), (b), (c), respectively. It was noticeable that the change of the metal plasma source of HPPMS, FCA and DCMS resulted in a different coating morphology. A dense and smooth knitting-like surface structure with many micro-pits distributed on the film surface homogeneously was achieved on Sample A. In contrast, the surface of Sample B exhibited a dense granular microstructure with some random distribution of larger-particles and macro-pits. The image obtained from Sample C was a smooth and structure-less morphology and the morphology could not be resolved with the conventional SEM. The difference that happened among the three samples was related to the different characteristics of the three metal plasma sources. HPPMS was proposed to possess a high plasma density of more than 1018 m− 3 and ionization rate of about 90%, so it can bring a “clean” ion flux with high metal ionization fraction and without metal droplets [16,17]. Ions in the high density plasma were accelerated by the substrate negative high-voltage at the way from the target to the substrate. The film surface was bombarded and implanted by the accelerated ions, which induced the formation of surface bombardment damages, mainly micro-pits [18]. The distribution of the micro-pits is homogeneous for the uniform distribution and little size difference of the bombarded ions. Although high density metal plasma can be produced by FCA, numbers of charged droplets produced the cathodic arc spots [5], that led to the formation of larger particles and damages on the film surface [19]. Compared with HPPMS and FCA, DCMS produced a plasma with a very low metal ionization fraction and mainly neutral atoms were included. They can't be accelerated by the negative high-voltage, and consequently a smooth surface and few bombarded defects were formed. The cross-sectional SEM micrographs of the CrN coatings deposited on Si (100) substrates by PBII&D technique using different metal plasma sources of HPPMS (Sample A), FCA (Sample B) and DCMS (Sample C) are shown in Fig. 3(a), (b), (c), respectively. Sample A exhibited a well defined densely packed columnar structure. Dense granular grains

212

Z. Wu et al. / Surface & Coatings Technology 229 (2013) 210–216

Fig. 2. SEM of the surfaces of CrN films deposited on Si (100) substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C.

formed near the substrate at the initial nucleation stages during film formation. Subsequently, the grains demonstrated competitive growth adjacent to the granular grains and formed typical columnar structure. Most of the columnar structures were broken and exhibited a discontinuous morphology. And the top columns seemed to form on the broken columns quickly due to a re-nucleation mechanism. The interface provided a direct and closed contact between the coating and the substrate and thus promoted the formation of the metallic, covalent, or ionic bonds as opposed to the significantly weaker van der Waals bonds. These features showed that the coating growth occurred with highly energetic (about 18 keV when the ions arrived at the substrate) and highly ionized implantation and bombardment and consequent sufficient thermal diffusion [20].

Fig. 3. Cross-sectional SEM of the CrN films deposited on Si (100) substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C.

An obviously underdense columnar structure composed by largesized grains was predominated in Sample B. Most of columnar grains were continuous from the substrate to the surface of the film. Additionally, a well defined boundary and an abrupt interface appeared between the SU201 stainless steel substrate and the CrN film, indicating a bad adhesion, which may be attributed to the high deposition rate and the presence of macro-particles. A similar columnar structure with Sample B was observed on Sample C. And the difference was that much smaller and looser packed columnar grains were achieved in Sample C. The interface between the SU201 stainless steel substrate and the CrN film also possessed a well

Z. Wu et al. / Surface & Coatings Technology 229 (2013) 210–216

defined boundary. But the reason to these structures, different with Sample B, is the low ionized fraction of metal plasma produced by the DCMS technique. It was widely established that a typical columnar structure would be formed at the condition of low energetic deposition for low adatom mobility [21,22]. Fig. 4 shows the XRD of CrN films deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma source of HPPMS (Sample A), FCA (Sample B) and DCMS (Sample C). It can be seen that all the samples were highly textured with a preferential orientation in the CrN(200) direction with the corresponding 2θ of ~43.3°, while the intensities of other orientation were very low. Generally, polycrystalline films are expected to show orientations which minimize total energy [23]. And to fcc CrN films, the highly CrN(200) preferential orientation means the very low intrinsic stress that may be released by atomic mobility and thermal diffusion induced by highly energetic bombardment and implantation [24]. The relative intensity of the preferential orientation in the CrN(200) decreased rapidly from Sample A to Sample C, indicating a variation of the oriented grains in number [25]. These characteristics may be attributed to the different extent of the ion bombardment. In fact, the current of the substrate will be up to 40 mA for HPPMS, but only 5 mA for DCMS, which induce a great difference in substrate temperature. A high temperature is considered to be related to the high crystalline [26]. Additionally, some oxide and carbide detected in the XRD data are related with the low oxidation potential of Cr and surface contamination. Table 2 shows the components of the CrN films deposited by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C. All the film surfaces were contaminated by carbohydrate and oxidation. The contents of Cr and O of the three samples were comparable. But the C content of Sample A was about 10% higher than the other two samples with the loss of N fraction. The C was speculated coming from the ion sputtering from the vacuum contamination. The XPS Cr2p 3/2, N1s, C1s and O1s lines of the CrN films deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C are shown in Fig. 5, respectively. The films examined in the experimental condition showed carbon and oxygen peaks along with Cr and N peaks. The XPS spectra were calibrated by taking the carbon C1s peak (284.6 eV) as a reference. Table 3 shows the fraction of all peaks fixing from the XPS of the three samples. The electron binding energy of Cr2p 3/2 peak showed a broad peak at approximately 575.2 eV that was too high to be metallic 574 eV. This peak consisted of five components that correspond to Cr metal at 574 eV, Cr3C2 at

Intensity/(a.u.)

CrN(200)

Cr2N(002)

Substrate

a

CrN(111)

Substrate

b c

20

40

60

80

100

2θ/(deg.) Fig. 4. XRD of the CrN films deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C.

213

Table 2 The components of the CrN films deposited by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C. Sample

Cr (at.%)

N (at.%)

C (at.%)

O (at.%)

A B C

39.54 43.89 39.96

10.35 17.89 19.29

25.89 13.04 15.84

24.23 25.18 24.91

574.6 eV, CrN at 575.8 eV, Cr2N at 576.8 eV and Cr2O5 at 577.6 eV [27,28]. The Cr2N component may also comprise Cr2O3 for which the binding energy is very close to that of Cr2N [29,30], but the peak intensity was very low for all the three samples. Due to thermal oxidation, Cr2O5 oxide layers were formed on top of nitride coatings, but Cr2O3 seen more frequently in previous reports [29–31]. The main phase of the three samples was CrN with the content of 44.6%, 53.3% and 52.7% in Samples A, B and C, respectively. The N1s peak consisted of the three peaks of different intensities that corresponded to CrN and Cr2N centered at 396.7 eV, and 397.4 eV and sp3 C\N at 398.1 eV [28]. Only a little N assumed to exist in the form of Cr2N, that consisted with the results of XRD. The C1s spectra may also be described by three components with peaks of Cr3C2 at 282.8 eV, polymeric hydrocarbons at 284.6 eV, and amorphous C\N at 286.1 eV, respectively [30,32]. The interest was that lots of Cr3C2 formed in Sample A, whereas few in the other two samples. It was an indication that some C was induced and interacted with Cr atoms in the process, which explained the higher C fraction in Sample A shown in Table 2. The O1s peak was also shown with the two peaks of different intensities that correspond to lattice oxygen and H\OH at 530.7 eV and 533.2 eV [27]. The appearance of oxide was related with the low oxidation potential of Cr. The adhesions of the coatings deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma source of HPPMS (Sample A), FCA (Sample B) and DCMS (Sample C) were evaluated by the indentation test. And the SEM micrographs of the imprints are shown in Fig. 6. Least cracks or detachment signs appeared on the imprints of Sample A, indicating a best film adhesion among the three samples. From Sample A to Sample C, more delamination happened surrounding the indentation, which means the worse adhesions. The adhesions of the coatings deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma source of HPPMS (Sample A), FCA (Sample B) and DCMS (Sample C) were also measured by the scratch test, from which we could achieved the quantifiable adhesions. And the optical micrographs of the scratches are shown in Fig. 7. The critical load Lc used as an index of adhesion between the film and the substrate was determined by the first failure point in the scratch channel. Sample A exhibited an excellent adhesion with the critical load Lc of 61.7 N. In contrast, Sample B and Sample C processed a weak adhesion with the critical loads of only 41.2 N and 28.8 N. The result was consistent with that of the interface micrographs between the films and substrates and the indentation tests. According to our previous investigate [12], the high critical load of Sample A achieved on stainless steel without any plasma pretreatment and extra heating originates from the high-voltage ion implantation effect. An implanted layer formed in the near surface of the substrate, which may reduce the stress induced by the different physical properties [33]. However, weak adhesions of the other two samples may originate by the formation of macroparticles and less ion bombardment, respectively. The hardness and Young's modulus of the CrN coatings deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma sources of HPPMS (Sample A), FCA (Sample B) and DCMS (Sample C) are shown in Fig. 8. The hardness of Sample A was up to 21.9 GPa, a factor of 4.4 compared to that of the SU201 stainless steel substrates. Sample B exhibited a little lower hardness of 18.2 GPa. And the lowest hardness happened on Sample C, only 12.7 GPa. The

214

Z. Wu et al. / Surface & Coatings Technology 229 (2013) 210–216

Fig. 5. The XPS Cr2p3/2, N1s, C1s and O1s lines of the CrN films deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C.

results are considered to relate to the film structures and phase composition. Generally, a denser structure and a higher crystallinity will improve the mechanical properties [34]. For the high intensity ion Table 3 The fractions of all the peaks fixing from the XPS of the CrN films deposited by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C. Sample Cr2p3/2

N1s

C1s

O1s

CrN Cr2C3 Cr Cr2O5 Cr2N CrN Cr2N C\N C Cr2C3 CN O2 − H2O

A (%)

B (%)

C (%)

44.6 33.8 14.1 7.2 0.3 45.7 24.8 29.5 52.2 27.5 20.3 85.6 14.4

53.3 24.3 7.6 10.9 3.9 70.9 25.9 3.2 69.3 26 4.7 90.7 9.3

52.7 28.1 6.1 10.2 2.9 69.9 26.7 3.4 45.1 32.4 22.5 77.7 22.3

bombardment, Sample A formed the densest microstructure and highest crystallinity among the three samples, while the microstructure and crystallinity of Sample C were opposite. The hardness of Sample B was influenced by macroparticles more or less.

4. Conclusions A comparative study of the structure, chemical, hardness and corrosion resistance of CrN films fabricated with PBII&D technique using different metal plasma sources of HPPMS, FCA and DCMS was presented. Compared with FCA and DCMS, the sample deposited with HPPMS was shown to have a densest microstructure and highest crystallinity because of the highly energetic (about 18 keV) ion bombardment and implantation from a high density “clean” plasma flux without droplets. Consequently, the adhesion and hardness of the sample deposited with HPPMS showed better than the other two samples. Considering the various processing issues, the PBII&D technique using the metal plasma source based on HPPMS seems to have great prospects in preparing films with a dense structure, high crystallinity, and good properties. However, the other two metal plasma sources, FCA and DCMS, are

Z. Wu et al. / Surface & Coatings Technology 229 (2013) 210–216

215

35

320

30

280

25

240

20

200

15

160

Modulus (GPa)

Hardness (GPa)

Fig. 7. Micrographs obtained from the scratches on the CrN films deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C.

Hardness

10

120 Modulus

5

80 0

Control

(a)

(b)

(c)

Methods Fig. 8. Hardness and Young's modulus of the CrN films deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C.

References [1] [2] [3] [4] [5] [6] [7] Fig. 6. Micrographs obtained from the indentation on the CrN films deposited on SU201 stainless steel substrates by PBII&D technique using different metal plasma sources of HPPMS: (a) Sample A, FCA; (b) Sample B and DCMS; (c) Sample C.

[8] [9] [10] [11] [12]

considered to have disadvantages of macroparticles and low ionization fraction, respectively.

[13] [14]

Acknowledgments

[15] [16]

This work was jointly financially supported by Natural Science Foundation of China (No. 10905013, No. 10975041), and Hong Kong Research Grants Council (RGC) Special Equipment Grant No. SEG CityU05.

[17] [18] [19]

J.R. Conrad, J.L. Radtke, J. Appl. Phys. 62 (1987) 4591. I.G. Brown, X. Godechot, K.M. Yu, Appl. Phys. Lett. 58 (1991) 1392. Q. Sun, L. Xia, X. Ma, M. Sun, Appl. Surf. Sci. 206 (2003) 53. A. Anders, S. Anders, I.G. Brown, M.R. Dickinson, R.A. MacGill, J. Vac. Sci. Technol., B 12 (1994) 815. A. Anders, Surf. Coat. Technol. 93 (1997) 158. L. F. Xia, China Patent. CN, 92113717.6, 01 (1996). K. Nakamura, H. Yoshinaga, K. Yukimura, Nucl. Instrum. Methods Phys. Res. B 242 (2006) 315. S.A. Nikiforov, K.W. Urm, G.H. Kim, G.H. Rim, S.H. Lee, Surf. Coat. Technol. 171 (2002) 106. M. Tomida, Y. Kato, T. Asaji, Nucl. Instrum. Methods Phys. Res. B 237 (2005) 83. Lei Zhang, L.Q. Shi, Z.J. He, B. Zhang, Y.F. Lu, A. Liu, B.Y. Wang, Surf. Coat. Technol. 203 (2009) 3356. X. B. Tian, Z. Z. Wu, C. Z. Gong, China Patent. CN, 201010213894.4, 06 (2010). Z.Z. Wu, X.B. Tian, J.W. Shi, C.Z. Gong, S.Q. Yang, P.K. Chu, Rev. Sci. Instrum. 82 (2011) 033511. V. Kouznetsov, K. Macák, J.M. Schneider, U. Helmersson, I. Petrov, Surf. Coat. Technol. 122 (1999) 290. U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Thin Solid Films 513 (2006) 1. X.B. Tian, Z.Z. Wu, J.W. Shi, C.Z. Gong, S.Q. Yang, Chin. Vac. 47 (2010) 44. J. Bohlmark, J.T. Gudmundsson, Member, IEEE, J. Alami, M. Latteman, U. Helmersson, IEEE Trans. Plasma Sci. 33 (2005) 346. J. Bohlmark, J. Alami, C. Christou, A.P. Ehiasarian, U. Helmersson, J. Vac. Sci. Technol., A 23 (2005) 18. Y. Cheng, Y.F. Zheng, Thin Solid Films 515 (2006) 1358. M. Kumagai, K. Yukimura, E. Kuze, T. Maruyama, M. Kohata, K. Numata, H. Saito, X. Ma, Surf. Coat. Technol. 169–170 (2003) 401.

216

Z. Wu et al. / Surface & Coatings Technology 229 (2013) 210–216

[20] Y.P. Sharkeev, B.P. Gritsenko, S.V. Fortuna, A.J. Perry, Vacuum 52 (1999) 247. [21] C. Gautier, H. Moussaoui, F. Elstner, J. Machet, Surf. Coat. Technol. 86–87 (1996) 254. [22] W. Ernst, J. Neidhardt, H. Willmann, B. Sartory, P.H. Mayrhofer, C. Mitterer, Thin Solid Films 517 (2008) 568. [23] D.R. McKenzie, Y. Yin, W.D. McFall, N.H. Hoang, J. Phys. Condens. Matter 8 (1996) 5883. [24] Y.J. Zhang, Z.G. Wu, W.W. Zhang, X. Li, P.X. Yan, W.M. Liu, Q.J. Xue, Chin. J. Nonferrous Met. 14 (2004) 1264. [25] C. Mendibide, P. Steyer, J. Fontaine, P. Goudeau, Surf. Coat. Technol. 201 (2006) 4119. [26] J. Xu, H. Umehara, K. Isao, Appl. Surf. Sci. 201 (2002) 208.

[27] A. Conde, A.B. Cristóbal, G. Fuentes, T. Tate, J. de Damborene, Surf. Coat. Technol. 201 (2006) 3588. [28] A. Vyas, Y.G. Shen, Z.F. Zhou, K.Y. Li, Compos. Sci. Technol. 68 (2008) 2922. [29] O. Nishimura, K. Yabe, M. Iwaki, J. Electron. Spectrosc. Relat. Phenom. 49 (1989) 335. [30] Y. Shi, S. Long, L. Fang, S. Yang, F. Pan, Appl. Surf. Sci. 254 (2008) 5861. [31] K.L. Chang, S.C. Chung, S.H. Lai, H.C. Shih, Appl. Surf. Sci. 236 (2004) 406. [32] G.M. Wilson, S.O. Saied, S.K. Field, Thin Solid Films 515 (2007) 7820. [33] E.G. Gerstner, D.R. McKenzie, M.K. Puchert, P.Y. Timbrell, J. Zou, J. Vac. Sci. Technol., A 13 (1995) 406. [34] J. Lin, J.J. Moore, W.D. Sproul, B. Mishra, Z. Wu, J. Wang, Surf. Coat. Technol. 204 (2010) 2230.