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MoN coatings synthesized by cathodic arc ion-plating
Surface & Coatings Technology 265 (2015) 117–124
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of substrate rotation speed on the structure and mechanical properties of nanocrystalline AlTiN/MoN coatings synthesized by cathodic arc ion-plating M.I. Yousaf a, V.O. Pelenovich a, B. Yang b, C.S. Liu a, D.J. Fu a,⁎ a b
Key Laboratory of Artificial Micro- and Nano-Materials of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China School of Power & Mechanical Engineering, Wuhan University, Wuhan 430072, China
a r t i c l e
i n f o
Article history: Received 7 October 2014 Accepted in revised form 20 January 2015 Available online 28 January 2015 Keywords: Cathodic arc ion-plating Multilayer coatings AlTiN MoN Structural and mechanical properties
a b s t r a c t We have investigated the effects of substrate rotation speed (SRS) on the structural and mechanical properties of AlTiN/MoN multilayer coatings produced by cathodic arc ion-plating using 2-fold rotation technique on Si (100) and cemented carbide substrates. The Al:Ti ratios measured by EDS over all samples are closed to design 2:1 ratio and corresponds to Al0.63Ti0.37 N composition. The bilayer periods are in the range of 11–61 nm depending on rotation speed. The 3 round per minute (RPM) coating exhibits the smoothest surface (Rms = 19 nm) and 1 RPM highest hardness of 44 GPa. Hardness and Young's modulus decrease with increasing RPM. Hardness, Young's modulus, smoothness and friction coefficient of multilayer structures are higher in comparison with single layer counterparts. Wear track and surface morphology experiments have also been analyzed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The great attentions to multilayer thin films of different compounds have been focused because of their abilities to increase the lifetime of a tool significantly as compared to the single layered coatings [1]. Furthermore, multilayer coatings have been found to possess improved mechanical properties such as fracture toughness and hardness as compared to homogenous coatings [2]. Hard nitrides coatings of Group IVB transition metals are widely produced by PVD process and are increasingly used in a wide range of tribological applications to improve performance and extend the life of metal cutting, drilling, and forming tools, as well as bearings and various machine parts. Tribological properties of MoN [3], diffusion barrier [4], catalytic [5], superconductor properties [6], improved surface wear resistance, and self-lubrication over a wide temperature range [7,8] were published in recent years. Other nitride coatings of TiAl are well known in cutting applications to their advanced wear resistance, enhanced hardness, and superior performance under corrosive or high temperature conditions [9]. These metastable hard coatings remain stable in the single-phase cubic structure used for metal working applications up to more than 800 °C [10], and are therefore, suitable for near-dry or dry-high speed cutting operations. The addition of Mo to AlTiN forming AlTiN/MoN multilayers or nanocomposites provides a possible way to decrease the coefficient of friction from 0.8 to 0.9 to around 0.3 to 0.4 at higher temperature. This ⁎ Corresponding author. Tel./fax: +86 27 6875 3587. E-mail address: [email protected] (D.J. Fu).
http://dx.doi.org/10.1016/j.surfcoat.2015.01.049 0257-8972/© 2015 Elsevier B.V. All rights reserved.
is caused by the formation of orthorhombic MenO3n − 1 Magnéli phase oxides which are known to decrease the coefficient of friction due to the presence of easy shareable planes [11]. AlTiN/MoN coating applied on tooling results in a significant tool life improvement under conditions of cutting hard to machine alloys such as Inconel 718 super alloy [12]. Review of recent literature on multilayer hard coatings has revealed that much work has been done on AlTiN/TiN, AlTiN/CrN, and AlTiN/VN as a self-lubricating agent, but quite few studies on multilayer structure composed of MoN. In the present study, a cathodic-arc ion plating system with AlTi and Mo alloy cathodes is used for the deposition of multilayer AlTiN/MoN coatings with different substrate rotation speed (SRS). The effects of the substrate rotation and the multilayer structure on the mechanical properties, microstructure, and tribological performance of AlTiN/MoN coatings are examined. 2. Experimental details The AlTiN/MoN coatings were deposited on polished Si (100) and cemented carbides by cathodic arc ion-plating. Al0.67Ti0.33 and Mo were used as targets. The schematic figure of the cathodic arc ionplating chamber is shown in Fig. 1. The size of the chamber was 54 × 29·5 × 39.5 cm3, the minimum distance between samples and source was 22.5 cm. For 2-fold rotation the ratio between turn table rotation speed (substrate rotation/rotation 1) and sample holder rotation speed (rotation 2) was 1:2. The chamber was pumped before deposition to the base pressure of 2 × 10− 3 Pa. The substrates were cleaned by
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Fig. 1. Schematic diagram of the deposition system.
acetone and degreased ultrasonically in hot alkaline baths for 20 min and rinsed in deionized water. The substrates were also subjected to Ar+ bombardment at − 800 V bias voltage with 80% duty cycle for 30 min. Next the surface was bombarded with Mo+ ions in Ar+ atmosphere at 6 × 10−2 Pa, followed by deposition of pure Mo sub-layer at bias voltage of −200 V for 8 min at pressure of 5 × 10−1 Pa with 70 A current. After this, nitrogen gas was introduced in the chamber to deposit MoN as an interlayer for 10 min. Finally, deposition of AlTiN/ MoN multilayers started at −200 bias voltage. The deposition pressure (N2) was fixed at 2.5 Pa and cathodic target current was 70 A. The substrate temperature was 300 °C during deposition process and the duration of the deposition was 20 min. SRS (turn table rotation, rotation 1) was varied from 1 RPM to 5 RPM with other parameters being the same. The crystal structure of the AlTiN/MoN coatings was characterized by X-ray diffraction (XRD, D8 advanced) with a Cu Kα radiation and high resolution transmission electron microscopy (HRTEM, JEOL JEM 2010). The bilayer period was evaluated from SEM and HRTEM
techniques. The surface topography was analyzed on an atomic force microscope (AFM, Shimadzu SPM-9500J3) operated in the tapping mode with a measuring area of 5 × 5 μm2. The plane surface and cross-sectional micrographs were measured by Sirion FEG scanning electron microscopy (SEM) with EDAX genesis 7000 EDS. X-ray photoelectron spectra (XPS) were collected by Thermo Scientific Escalab 250 Xi spectrometer. Nanohardness and Young's modulus were evaluated by nanoindentation (G200, Agilent technologies, USA) operated in continuous stiffness measurement (CSM) mode, the resolution of the loading system and the displacement system was 50 nN and 0.04 nm, respectively. The maximum indentation depth was set 500 nm and a three sided Berkovich-shaped indenter was used with a 60 nm tip radius. Six indentations were made per sample with the load-displacement data statistically averaged for each sample. The friction and wear were measured on an MS-T3000 ball-on-disk tester, which slide in an ambient air at a temperature of 30 °C and relative humidity of 70%, with Si3N4 ball of 3 mm in diameter being used as the mating materials, and 5 N loads were applied. The average sliding speed was 0.02 m/s for a sliding time of 60 min and the friction coefficients were recorded during the test.
3. Results and discussions Fig. 2 shows XRD patterns of AlTiN/MoN multistructures with Mo and MoN interlayers on Si (100) substrates. No any phases are observed except AlTiN, δ-MoN and Mo. The patterns are dominated by AlTiN alloy peaks at 36.9°, 42.9°, and 61.1° shifted on 0.3–0.4° to higher angles from those corresponded to TiN (Fm-3 m space group, PDF-65-0970). The reduction of crystal parameters proves the substitution of Ti3+ by smaller
Table 1 Elements composition of AlTiN/MoN multistructure coatings and single layers.
Fig. 2. X-Ray diffraction patterns of AlTiN/MoN multilayer coatings.
Elements
1 RPM
2 RPM
3 RPM
4 RPM
5 RPM
AlTiN
MoN
NK Al K Mo L Ti K
40.4 21.4 26.9 11.3
34.6 17.6 35.0 12.8
37.9 17.6 34.2 10.3
36.3 21.7 29.5 12.5
39.5 16.2 34.5 9.8
50.4 33.0 – 16.6
56.5 – 43.5 –
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Fig. 3. XPS spectra of AlTiN/MoN coatings.
Al3+ ions and the stress between AlTiN and MoN layers. The broadened main peak at 43° reveals crystallite size of about 6 nm for AlTiN and peak at 48.3° of MoN shows crystallite size of ~3 nm for 1 RPM sample and less than 3 nm or amorphous nature for other samples, estimated by Debye-Scherrer formula. MoN and Mo peaks are also observed at 36.0° and 40.3°, corresponding to expanded hexagonal MoN (PDF-895024) and metallic Mo (PDF-89-4896), respectively, in agreement with the literature [13]. The XRD result shows that the rotation speed does not influence phase formation but may control particle size of AlTiN layers, leading to better crystallinity at higher speed. Table 1 shows atomic concentration measured by EDS analysis for the samples deposited on cemented tungsten carbide. The data were collected from different areas of each sample and averaged over 5–7 measurements. All multistructures have similar elemental composition of AlTiN with Al:Ti ratio corresponding to Al0.63Ti0.37 N,
which is slightly different of designed 2:1 ratio. It is worth noticing that the determination of Mo:N ratio in the multilayered structure is complicated by the presence of Mo in Mo interlayer as well as N in AlTiN multilayers. In Fig. 3, XPS core level spectra of the coatings on Si substrates are presented, the binding energy values are corrected relatively to the C 1s level at 284.6 eV. Gaussian–Lorentzian functions were used for fitting the spectra. Mo 3d spin-orbital doublet does not show any significant shift in all the spectra and the position of 3d5/2 peak at 228.6 eV corresponds to Mo–N bonding [14,15]. The intensity varies greatly depends on the deposition of the final layer, e.g. for 1 RPM sample the final layer is MoN and for 4 RPM the final layer is AlTiN. Asymmetry of the Mo3d doublet assumes the presence of a peak at 229.1 eV for 3d5/2 peak and corresponds to surface MoO2 [16,17]. Mo 3p3/2 peak is observed near N 1s peak and overlaps it partially at 394.7 eV that match
Fig. 4. SEM surface and cross-section images of 2 RPM sample. On the right figure Mo columnar interlayer, MoN interlayer, and layered AlTiN/MoN multistructure are observed.
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Table 2 Mechanical properties of the multistructure coatings and single layers. SRS, RPM
Roughness (RMS) nm
Friction coef.
Thickness (μm)
Number of Layers
Bilayer period (nm)
Hardness (GPa)
Elastic modulus (GPa)
Wear track
1 2 3 4 5 AlTiN MoN
32.7 22.9 19.3 23.9 21.4 40.5 17.0
0.56 0.52 0.63 0.44 0.41 0.66 0.42
1.3 1.2 1.7 1.3 1.1 1.1 1.3
19 30 58 75 93 – –
58 40 29 17 11
44 ± 2.5 41 ± 2 37 ± 2 35 ± 2.5 32 ± 2.5 27 ± 2 29 ± 2
700 ± 20 665 ± 20 650 ± 20 630 ± 20 615 ± 10 575 ± 30 600 ± 20
Polishing Chipping and delamination Chipping and delamination Polishing Chipping and delamination Abrasion Polishing and abrasion
well with the literature data for MoN [18]. Al 2p3/2 peak is the narrowest and strongest for 4 RPM sample with the AlTiN last layer, positioned at 74.45 eV, and corresponds to Al2O3 [19]. Peaks of the other samples are broadened, suggesting the presence of different bonding such as Al–N, Al–O–Mo, and Al–N–Mo. The Ti 2p 3/2 spectra were deconvolved into three peaks at 455.4 eV, 457.0 eV, and 458.9 eV, which are usually associated with the presence of TiN, Ti2O3 or TiNO, and TiO2, respectively [20,22] suggesting moderate surface oxidation. The N 1s spectra can be de-convolved into three peaks at 396.7 eV, 397.6 eV, and 400.1 eV. The intensity of the 397.6 eV peak correlates well with Mo 3p3/2 one and can be associated with N–Mo bonding. The main peak at 396.7 eV and small high energy peak belong to AlTiN and N–Al–O or N–Ti–O bonds [21,23], respectively. In conclusion, XPS measurement shows the presence of Al–N, Ti–N, and Mo–N bondings corresponding to AlTiN and MoN. In Fig. 4 SEM image clearly shows the existence of micro particles on the surface. The right figure shows the cross-sectional SEM image of the
sample with a multilayer thickness of 1.2 μm. In the image, multilayer morphology of AlTiN and MoN, columnar morphology of Mo interlayer, and MoN interlayer are observed. Thickness, number of layers, and bilayer period of the multilayer structure derived from SEM and HRTEM images are given in Table 2. The surface morphology and roughness of the coatings on Si substrates are studied by AFM with a measured area of 5 × 5 μm2 selected from the random position of the coatings. It is seen that the surface of the coatings is crater like with the minimum of the roughness at 19.3 nm for 3 RPM sample (Fig. 5). The 1 RPM sample has a higher roughness and other shows constant. It is in agreement with Ref. [24], where it is found that the value of bilayer period shows no direct influence on the surface roughness of multilayer coatings. It can also be seen in cross-sectional image (Fig. 4) of the multilayered structure the wavy appearance can be observed down to MoN–Mo interface; hence the origin of the roughness is related to roughness of the columnar structure of metallic Mo.
Fig. 5. Three-dimensional AFM images of AlTiN/MoN coatings deposited at different SRS.
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Fig. 6. HRTEM cross-section images of AlTiN/MoN coatings deposited at SRS of 3 RPM (a and b). EDS spectra of dark layer (c) and bright layer (d). Bottom inset shows interplanar distance of MoN and top inset shows interplanar distance of AlTiN.
Fig. 6a shows a bright-field cross-sectional HRTEM image of AlTiN/ MoN coatings synthesized at SRS of 3 RPM. It is seen a typical nanomultilayer structure in which the bright layers alternate with dark layers, but due to the 2-fold rotation of the samples during synthesis; the thickness of the alternated sub-layers is more modulated [25], in the present study the bilayer periods are composed of 10 nm AlTiN + 10 nm MoN and 3 nm AlTiN + 5 nm MoN sublayers, as seen in Fig. 6b. Thus the total multilayer period (turn table bilayer period) is 28 nm, which is in agreement with SEM image (as seen in Table 2). Fig. 6b shows magnified area at the surface in which layers of different thickness are visible. The EDS microanalysis of bright layer marked with dash line demonstrates AlTiN layers and dark layer marked with dot line reveals the presence of MoN layers. This qualitative analysis reveals the presence of dark MoN layers alternate with bright AlTiN layers, both AlTiN and MoN phases are composed of crystalline and amorphous regions with the average grain size of AlTiN 10 nm and MoN 4 nm, the values are closed to XRD result. Fig. 7 shows low angle X-ray diffraction patterns of the multilayer structure on Si substrates obtained by subtraction of background data from experimental data. Results show complex patterns for all samples. 1 RPM sample does not show any peaks in the studied region due to large bilayer period. Analysis of the peak positions for all samples has
revealed that they belong only to first order maxima. The loss of low angle XRD fringes and the low intensity of the peaks are due to surface roughness, since low angle XRD is extremely sensitive to surface roughness [26,28]. Therefore, the observation of a few low angle peaks for the multilayer structure suggests the presence of sub-layers with different thickness, as seen in the HRTEM image (Fig. 6) of 3 RPM sample. Using modified Bragg's formula [27], the thicknesses of the sub-layers are found as 7.7, 5.1, 4.4 nm (2 RPM); 6.4, 5.7 nm (3 RPM); 6.3, 4.9 nm (4 RPM); and 5.7, 4.4, 3.6 nm (5 RPM). It is seen; on average, the thickness of the sub-layers decreases with increasing SRS. For multilayer coatings, the abruptness of interface and the shear moduli of the individual layers can affect hardness. However, hardness is not only dependent on grain size but also on other factors such as texture, porosity, residual stress etc. which are influenced by the thin film deposition process conditions. The indentation system used for this work, known as frequency specific depth-sensing or continuous stiffness measurement (CSM), continuously measures contact stiffness during indentation [28]. The CSM method is the preferred measurement technique given the undetermined thin film thickness of the nano-indentation samples which were located little bit far or change their direction from target(s) during rotation in PVD system. The traditional fixed depth
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Fig. 7. Low angle X-ray diffraction patterns.
nano-indentation technique may not have yielded good results using the rule of thumb of 7–15% of film thickness [29] in order to minimize substrate artifacts. This restriction would have resulted in indentation depth which is too shallow, on the order of 20–40 nm. Nanohardness, elastic modulus and friction coefficient of AlTiN/MoN multilayers on tungsten carbide substrates as a function of rotation speed as well as with their single layers samples are shown in Table 2. The rapid increase in the hardness and elastic modulus occurs in the first 40–60 nm of the indentation in Fig. 8 is due to the Berkovich
indenter tip radius of about 60 nm is the primary influence that establishes the curve during initial indentation rather than surface roughness or residual effect of the layers. The maximum hardness of 44 GPa is observed at 1 RPM. It is seen from Table 2 that the average values of hardness of multilayer coatings are higher than those of AlTiN (27 GPa) and MoN (29 GPa) deposited under identical experimental conditions. This result is similar to data obtained for multilayer structures; AlTiN/CrN [30], AlTiN/AN (A = Si, W) [31] and AlTiN/AN (A = Ti, Cr) [32]. Nanohardness and elastic modulus results reveal that hardness and elastic modulus decrease with increasing SRS; such result is also confirmed by other studies [30,33]. It is known that the sharpness of the interface between sub-layers controls the hardness of the multilayered structure [34]. Any diffusion process between sub-layers inevitably causes degradation of mechanical properties. In Fig. 6b the diffusion region of 1–2 nm thickness is visible between MoN and AlTiN layers. At higher SRS, the bilayer period is smaller and the ratio of this diffused region to volume of pure materials increases, therefore interface sharpness decreases, which explains the decrease of the hardness at higher SRS [33]. The friction coefficient of AlTiN/MoN coatings is in the range of 0.41–0.56, which is between friction coefficients of AlTiN and MoN single layers. This result could show comparable areas of MON and AlTiN outward layers on the contact surface under Si3N4 ball during the wear process, as well as the presence of Mo–O Magnéli phase on the MoN surface, the last can be proved by moderate oxidation found in XPS measurement. Fig. 9 shows typical SEM images of wear tracks of the AlTiN/MoN coatings. It is found that chipping and delamination of the coatings down to MoN or Mo interlayer occurs for 2, 3, and 5 RPM samples; the result is proved by EDS analysis. The 1 and 4 RPM samples show only polishing of the surface without any notice-able changes in EDS spectra.
Fig. 8. Hardness and elastic modulus as function of displacement (penetration depth) and SRS.
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Fig. 9. SEM morphologies of the wear track of the AlTiN/MoN coating deposited at different SRS and AlTiN single layer.
AlTiN and MoN layers have heavy abrasion and polishing of the surface, respectively. The wear result of 1 and 4 RPM samples is in agreement with enhanced wear resistance for multilayer structures in comparison with single layer of the same materials [34,35]. In summary, the mechanical property examination shows moderate decrease of the hardness and elastic modulus for all multilayered structures with increasing SRS and 1 and 4 RPM multilayer samples shows better wear resistance then single layers. 4. Conclusion An industrial-scale cathodic arc ion-plating system was used to deposit AlTiN/MoN multilayer coatings on Si (100) and cemented carbide substrates using 2-fold rotation at various SRS. XRD has revealed broad peaks of AlTiN and δ-MoN phases with crystallite size of ~ 6 nm and ~ 3 nm, respectively. The averaged Al:Ti ratio over all samples is close to Al0.63Ti0.37 N composition. AFM images have found the smoothest surface with roughness of ~19 nm for SRS 3 RPM. Hardness and elastic modulus decrease with increasing SRS and they are higher than those for single layers of AlTiN and MoN. Wear analysis shows enhanced wear resistance for 1 and 4 RPM AlTiN/MoN multilayer structure in comparison with single layer coatings. Acknowledgements This work was supported by National Natural Science Foundation of China under grants 11275141, 11375133, 11375135 and Basic Research Fund of Central Universities under 2012202020217.
References [1] K. Kawata, H. Sugimura, O. Takai, Thin Solid Films 390 (2001) 64. [2] H. Holbeeck, H. Schulz, Thin Solid Films 153 (1987) 382. [3] M. Urgen, O.L. Eryilmaz, A.F. Cakir, E.S. Kayali, B. Nilufer, Y. Isik, Surf. Coat. Technol. 94–95 (1997) 501. [4] V.P. Anitha, S. Vitta, S. Major, Thin Solid Films 245 (1994) 1. [5] S. Li, J.S. Lee, J. Catal. 178 (1998) 119. [6] M.S. Mudholkar, L.T. Thompson, J. Appl. Phys. 77 (1995) 5138. [7] J.L. Li, D.S. Xiong, H.Y. Wu, J.H. Dai, T. Cui, Wear 271 (2011) 987. [8] A. Erdemir, Tribol. Lett. 8 (2000) 97. [9] Y.C. Chim, X.Z. Ding, X.T. Zeng, S. Zhang, Thin Solid Films 517 (2009) 4845. [10] R. Cremer, M. Witthaut, D. Neuschutz, Value-addition metallurgy: Proceedings of an International Symposium Sponsored by the Process Fundamentals Committee of the Extraction and Processing Division of the Minerals, The Minerals, Metals & Materials Society, 1998. 249. [11] G. Gassner, P.H. Mayrhofer, K. Kutschej, C. Mitterer, M. Kathrein, Surf. Coat. Technol. 201 (2006) 3335. [12] A. Biksa, K. Yamamoto, G. Dosbaeva, S.C. Veldhuis, G.S. Fox-Rabinovich, A. Elfizy, T. Wagg, L.S. Shuster, Tribol. Int. 43 (2010) 1491–1499. [13] M.K. Kazmanli, M. Urgen, A.F. Cakir, Surf. Coat. Technol. 167 (2003) 77. [14] G. Soto, W. de la Cruz, M.H. Farias, J. Electron Spectrosc. Relat. Phenom. 135 (2004) 27. [15] R. Fix, R.G. Gordon, D.M. Hoffman, Thin Solid Films 288 (1996) 116. [16] G.T. Kim, T.K. Park, H. Chung, Y.T. Kim, M.H. Kwon, J.G. Choi, Appl. Surf. Sci. 152 (1999) 35. [17] J.F. Moulder, W.F. Stikle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer Corp., Eden Prairie, Minnesota, 1992. [18] R. Sanjines, P. Hones, F. Levy, Thin Solid Films 332 (1998) 225. [19] J.L. Mo, M.H. Zhu, Tribol. Int. 42 (2009) 1758. [20] I. Bertoti, Surf. Coat. Technol. 151–152 (2002) 194. [21] I. Miloshev, H.H. Strehblow, B. Navinshek, Thin Solid Films 303 (1997) 246. [22] R.J. Rodriguez, J.A. Garcia, A. Medrano, M. Rico, R. Sanchez, R. Martinez, C. Labrugere, M. Lahaye, A. Guette, Vacuum 67 (2002) 559. [23] J.L. Endrino, G.S. Fox-Rabinovich, A. Reiter, S.V. Veldhuis, R. Escobar Galindo, J.M. Albella, et al., Surf. Coat. Technol. 201 (2006) 4505. [24] S.H. Huang, S.F. Chen, Y.C. Kuo, et al., Surf. Coat. Technol. 206 (2011) 1744.
124
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[25] Z. Zhou, W.M. Rainforth, B. Rother, A.P. Ehiasarian, P.Eh. Hovsepian, W.-D. Munz, Surf. Coat. Technol. 183 (2004) 275–282. [26] R.A. Koshy, M.E. Graham, L.D. Marks, Surf. Coat. Technol. 202 (2007) 1123. [27] C. Kim, S.B. Qadri, M.R. Scanlon, R.C. Cammarata, Thin Solid Films 240 (1994) 52. [28] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [29] K.S.A. Butcher, T.L. Tansley, X. Li, Surf. Interface Anal. 25 (1997) 99. [30] C.X. Tian, B. Yang, S.J. Yan, Z.H. Lu, Z.H. Huang, D.J. Fu, Surf. Coat. Technol. 228 (2013) S228.
[31] [32] [33] [34] [35]
K. Yamamoto, S. Kujime, K. Takahara, Surf. Coat. Technol. 200 (2005) 435. C. Ducros, F. Sanchette, Surf. Coat. Technol. 201 (2006) 1045. S.K. Kim, V.V. Le, Surf. Coat. Technol. 204 (2010) 3941. W.D. Sproul, Science 273 (1996) 889. Z.G. Zhang, O. Rapaud, N. Allain, D. Mercs, M. Baraket, C. Dong, C. Coddet, Adv. Eng. Mater. 11 (2009) 667.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of substrate rotation speed on the structure and mechanical properties of nanocrystalline AlTiN/MoN coatings synthesized by cathodic arc ion-plating M.I. Yousaf a, V.O. Pelenovich a, B. Yang b, C.S. Liu a, D.J. Fu a,⁎ a b
Key Laboratory of Artificial Micro- and Nano-Materials of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China School of Power & Mechanical Engineering, Wuhan University, Wuhan 430072, China
a r t i c l e
i n f o
Article history: Received 7 October 2014 Accepted in revised form 20 January 2015 Available online 28 January 2015 Keywords: Cathodic arc ion-plating Multilayer coatings AlTiN MoN Structural and mechanical properties
a b s t r a c t We have investigated the effects of substrate rotation speed (SRS) on the structural and mechanical properties of AlTiN/MoN multilayer coatings produced by cathodic arc ion-plating using 2-fold rotation technique on Si (100) and cemented carbide substrates. The Al:Ti ratios measured by EDS over all samples are closed to design 2:1 ratio and corresponds to Al0.63Ti0.37 N composition. The bilayer periods are in the range of 11–61 nm depending on rotation speed. The 3 round per minute (RPM) coating exhibits the smoothest surface (Rms = 19 nm) and 1 RPM highest hardness of 44 GPa. Hardness and Young's modulus decrease with increasing RPM. Hardness, Young's modulus, smoothness and friction coefficient of multilayer structures are higher in comparison with single layer counterparts. Wear track and surface morphology experiments have also been analyzed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The great attentions to multilayer thin films of different compounds have been focused because of their abilities to increase the lifetime of a tool significantly as compared to the single layered coatings [1]. Furthermore, multilayer coatings have been found to possess improved mechanical properties such as fracture toughness and hardness as compared to homogenous coatings [2]. Hard nitrides coatings of Group IVB transition metals are widely produced by PVD process and are increasingly used in a wide range of tribological applications to improve performance and extend the life of metal cutting, drilling, and forming tools, as well as bearings and various machine parts. Tribological properties of MoN [3], diffusion barrier [4], catalytic [5], superconductor properties [6], improved surface wear resistance, and self-lubrication over a wide temperature range [7,8] were published in recent years. Other nitride coatings of TiAl are well known in cutting applications to their advanced wear resistance, enhanced hardness, and superior performance under corrosive or high temperature conditions [9]. These metastable hard coatings remain stable in the single-phase cubic structure used for metal working applications up to more than 800 °C [10], and are therefore, suitable for near-dry or dry-high speed cutting operations. The addition of Mo to AlTiN forming AlTiN/MoN multilayers or nanocomposites provides a possible way to decrease the coefficient of friction from 0.8 to 0.9 to around 0.3 to 0.4 at higher temperature. This ⁎ Corresponding author. Tel./fax: +86 27 6875 3587. E-mail address: [email protected] (D.J. Fu).
http://dx.doi.org/10.1016/j.surfcoat.2015.01.049 0257-8972/© 2015 Elsevier B.V. All rights reserved.
is caused by the formation of orthorhombic MenO3n − 1 Magnéli phase oxides which are known to decrease the coefficient of friction due to the presence of easy shareable planes [11]. AlTiN/MoN coating applied on tooling results in a significant tool life improvement under conditions of cutting hard to machine alloys such as Inconel 718 super alloy [12]. Review of recent literature on multilayer hard coatings has revealed that much work has been done on AlTiN/TiN, AlTiN/CrN, and AlTiN/VN as a self-lubricating agent, but quite few studies on multilayer structure composed of MoN. In the present study, a cathodic-arc ion plating system with AlTi and Mo alloy cathodes is used for the deposition of multilayer AlTiN/MoN coatings with different substrate rotation speed (SRS). The effects of the substrate rotation and the multilayer structure on the mechanical properties, microstructure, and tribological performance of AlTiN/MoN coatings are examined. 2. Experimental details The AlTiN/MoN coatings were deposited on polished Si (100) and cemented carbides by cathodic arc ion-plating. Al0.67Ti0.33 and Mo were used as targets. The schematic figure of the cathodic arc ionplating chamber is shown in Fig. 1. The size of the chamber was 54 × 29·5 × 39.5 cm3, the minimum distance between samples and source was 22.5 cm. For 2-fold rotation the ratio between turn table rotation speed (substrate rotation/rotation 1) and sample holder rotation speed (rotation 2) was 1:2. The chamber was pumped before deposition to the base pressure of 2 × 10− 3 Pa. The substrates were cleaned by
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Fig. 1. Schematic diagram of the deposition system.
acetone and degreased ultrasonically in hot alkaline baths for 20 min and rinsed in deionized water. The substrates were also subjected to Ar+ bombardment at − 800 V bias voltage with 80% duty cycle for 30 min. Next the surface was bombarded with Mo+ ions in Ar+ atmosphere at 6 × 10−2 Pa, followed by deposition of pure Mo sub-layer at bias voltage of −200 V for 8 min at pressure of 5 × 10−1 Pa with 70 A current. After this, nitrogen gas was introduced in the chamber to deposit MoN as an interlayer for 10 min. Finally, deposition of AlTiN/ MoN multilayers started at −200 bias voltage. The deposition pressure (N2) was fixed at 2.5 Pa and cathodic target current was 70 A. The substrate temperature was 300 °C during deposition process and the duration of the deposition was 20 min. SRS (turn table rotation, rotation 1) was varied from 1 RPM to 5 RPM with other parameters being the same. The crystal structure of the AlTiN/MoN coatings was characterized by X-ray diffraction (XRD, D8 advanced) with a Cu Kα radiation and high resolution transmission electron microscopy (HRTEM, JEOL JEM 2010). The bilayer period was evaluated from SEM and HRTEM
techniques. The surface topography was analyzed on an atomic force microscope (AFM, Shimadzu SPM-9500J3) operated in the tapping mode with a measuring area of 5 × 5 μm2. The plane surface and cross-sectional micrographs were measured by Sirion FEG scanning electron microscopy (SEM) with EDAX genesis 7000 EDS. X-ray photoelectron spectra (XPS) were collected by Thermo Scientific Escalab 250 Xi spectrometer. Nanohardness and Young's modulus were evaluated by nanoindentation (G200, Agilent technologies, USA) operated in continuous stiffness measurement (CSM) mode, the resolution of the loading system and the displacement system was 50 nN and 0.04 nm, respectively. The maximum indentation depth was set 500 nm and a three sided Berkovich-shaped indenter was used with a 60 nm tip radius. Six indentations were made per sample with the load-displacement data statistically averaged for each sample. The friction and wear were measured on an MS-T3000 ball-on-disk tester, which slide in an ambient air at a temperature of 30 °C and relative humidity of 70%, with Si3N4 ball of 3 mm in diameter being used as the mating materials, and 5 N loads were applied. The average sliding speed was 0.02 m/s for a sliding time of 60 min and the friction coefficients were recorded during the test.
3. Results and discussions Fig. 2 shows XRD patterns of AlTiN/MoN multistructures with Mo and MoN interlayers on Si (100) substrates. No any phases are observed except AlTiN, δ-MoN and Mo. The patterns are dominated by AlTiN alloy peaks at 36.9°, 42.9°, and 61.1° shifted on 0.3–0.4° to higher angles from those corresponded to TiN (Fm-3 m space group, PDF-65-0970). The reduction of crystal parameters proves the substitution of Ti3+ by smaller
Table 1 Elements composition of AlTiN/MoN multistructure coatings and single layers.
Fig. 2. X-Ray diffraction patterns of AlTiN/MoN multilayer coatings.
Elements
1 RPM
2 RPM
3 RPM
4 RPM
5 RPM
AlTiN
MoN
NK Al K Mo L Ti K
40.4 21.4 26.9 11.3
34.6 17.6 35.0 12.8
37.9 17.6 34.2 10.3
36.3 21.7 29.5 12.5
39.5 16.2 34.5 9.8
50.4 33.0 – 16.6
56.5 – 43.5 –
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Fig. 3. XPS spectra of AlTiN/MoN coatings.
Al3+ ions and the stress between AlTiN and MoN layers. The broadened main peak at 43° reveals crystallite size of about 6 nm for AlTiN and peak at 48.3° of MoN shows crystallite size of ~3 nm for 1 RPM sample and less than 3 nm or amorphous nature for other samples, estimated by Debye-Scherrer formula. MoN and Mo peaks are also observed at 36.0° and 40.3°, corresponding to expanded hexagonal MoN (PDF-895024) and metallic Mo (PDF-89-4896), respectively, in agreement with the literature [13]. The XRD result shows that the rotation speed does not influence phase formation but may control particle size of AlTiN layers, leading to better crystallinity at higher speed. Table 1 shows atomic concentration measured by EDS analysis for the samples deposited on cemented tungsten carbide. The data were collected from different areas of each sample and averaged over 5–7 measurements. All multistructures have similar elemental composition of AlTiN with Al:Ti ratio corresponding to Al0.63Ti0.37 N,
which is slightly different of designed 2:1 ratio. It is worth noticing that the determination of Mo:N ratio in the multilayered structure is complicated by the presence of Mo in Mo interlayer as well as N in AlTiN multilayers. In Fig. 3, XPS core level spectra of the coatings on Si substrates are presented, the binding energy values are corrected relatively to the C 1s level at 284.6 eV. Gaussian–Lorentzian functions were used for fitting the spectra. Mo 3d spin-orbital doublet does not show any significant shift in all the spectra and the position of 3d5/2 peak at 228.6 eV corresponds to Mo–N bonding [14,15]. The intensity varies greatly depends on the deposition of the final layer, e.g. for 1 RPM sample the final layer is MoN and for 4 RPM the final layer is AlTiN. Asymmetry of the Mo3d doublet assumes the presence of a peak at 229.1 eV for 3d5/2 peak and corresponds to surface MoO2 [16,17]. Mo 3p3/2 peak is observed near N 1s peak and overlaps it partially at 394.7 eV that match
Fig. 4. SEM surface and cross-section images of 2 RPM sample. On the right figure Mo columnar interlayer, MoN interlayer, and layered AlTiN/MoN multistructure are observed.
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Table 2 Mechanical properties of the multistructure coatings and single layers. SRS, RPM
Roughness (RMS) nm
Friction coef.
Thickness (μm)
Number of Layers
Bilayer period (nm)
Hardness (GPa)
Elastic modulus (GPa)
Wear track
1 2 3 4 5 AlTiN MoN
32.7 22.9 19.3 23.9 21.4 40.5 17.0
0.56 0.52 0.63 0.44 0.41 0.66 0.42
1.3 1.2 1.7 1.3 1.1 1.1 1.3
19 30 58 75 93 – –
58 40 29 17 11
44 ± 2.5 41 ± 2 37 ± 2 35 ± 2.5 32 ± 2.5 27 ± 2 29 ± 2
700 ± 20 665 ± 20 650 ± 20 630 ± 20 615 ± 10 575 ± 30 600 ± 20
Polishing Chipping and delamination Chipping and delamination Polishing Chipping and delamination Abrasion Polishing and abrasion
well with the literature data for MoN [18]. Al 2p3/2 peak is the narrowest and strongest for 4 RPM sample with the AlTiN last layer, positioned at 74.45 eV, and corresponds to Al2O3 [19]. Peaks of the other samples are broadened, suggesting the presence of different bonding such as Al–N, Al–O–Mo, and Al–N–Mo. The Ti 2p 3/2 spectra were deconvolved into three peaks at 455.4 eV, 457.0 eV, and 458.9 eV, which are usually associated with the presence of TiN, Ti2O3 or TiNO, and TiO2, respectively [20,22] suggesting moderate surface oxidation. The N 1s spectra can be de-convolved into three peaks at 396.7 eV, 397.6 eV, and 400.1 eV. The intensity of the 397.6 eV peak correlates well with Mo 3p3/2 one and can be associated with N–Mo bonding. The main peak at 396.7 eV and small high energy peak belong to AlTiN and N–Al–O or N–Ti–O bonds [21,23], respectively. In conclusion, XPS measurement shows the presence of Al–N, Ti–N, and Mo–N bondings corresponding to AlTiN and MoN. In Fig. 4 SEM image clearly shows the existence of micro particles on the surface. The right figure shows the cross-sectional SEM image of the
sample with a multilayer thickness of 1.2 μm. In the image, multilayer morphology of AlTiN and MoN, columnar morphology of Mo interlayer, and MoN interlayer are observed. Thickness, number of layers, and bilayer period of the multilayer structure derived from SEM and HRTEM images are given in Table 2. The surface morphology and roughness of the coatings on Si substrates are studied by AFM with a measured area of 5 × 5 μm2 selected from the random position of the coatings. It is seen that the surface of the coatings is crater like with the minimum of the roughness at 19.3 nm for 3 RPM sample (Fig. 5). The 1 RPM sample has a higher roughness and other shows constant. It is in agreement with Ref. [24], where it is found that the value of bilayer period shows no direct influence on the surface roughness of multilayer coatings. It can also be seen in cross-sectional image (Fig. 4) of the multilayered structure the wavy appearance can be observed down to MoN–Mo interface; hence the origin of the roughness is related to roughness of the columnar structure of metallic Mo.
Fig. 5. Three-dimensional AFM images of AlTiN/MoN coatings deposited at different SRS.
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Fig. 6. HRTEM cross-section images of AlTiN/MoN coatings deposited at SRS of 3 RPM (a and b). EDS spectra of dark layer (c) and bright layer (d). Bottom inset shows interplanar distance of MoN and top inset shows interplanar distance of AlTiN.
Fig. 6a shows a bright-field cross-sectional HRTEM image of AlTiN/ MoN coatings synthesized at SRS of 3 RPM. It is seen a typical nanomultilayer structure in which the bright layers alternate with dark layers, but due to the 2-fold rotation of the samples during synthesis; the thickness of the alternated sub-layers is more modulated [25], in the present study the bilayer periods are composed of 10 nm AlTiN + 10 nm MoN and 3 nm AlTiN + 5 nm MoN sublayers, as seen in Fig. 6b. Thus the total multilayer period (turn table bilayer period) is 28 nm, which is in agreement with SEM image (as seen in Table 2). Fig. 6b shows magnified area at the surface in which layers of different thickness are visible. The EDS microanalysis of bright layer marked with dash line demonstrates AlTiN layers and dark layer marked with dot line reveals the presence of MoN layers. This qualitative analysis reveals the presence of dark MoN layers alternate with bright AlTiN layers, both AlTiN and MoN phases are composed of crystalline and amorphous regions with the average grain size of AlTiN 10 nm and MoN 4 nm, the values are closed to XRD result. Fig. 7 shows low angle X-ray diffraction patterns of the multilayer structure on Si substrates obtained by subtraction of background data from experimental data. Results show complex patterns for all samples. 1 RPM sample does not show any peaks in the studied region due to large bilayer period. Analysis of the peak positions for all samples has
revealed that they belong only to first order maxima. The loss of low angle XRD fringes and the low intensity of the peaks are due to surface roughness, since low angle XRD is extremely sensitive to surface roughness [26,28]. Therefore, the observation of a few low angle peaks for the multilayer structure suggests the presence of sub-layers with different thickness, as seen in the HRTEM image (Fig. 6) of 3 RPM sample. Using modified Bragg's formula [27], the thicknesses of the sub-layers are found as 7.7, 5.1, 4.4 nm (2 RPM); 6.4, 5.7 nm (3 RPM); 6.3, 4.9 nm (4 RPM); and 5.7, 4.4, 3.6 nm (5 RPM). It is seen; on average, the thickness of the sub-layers decreases with increasing SRS. For multilayer coatings, the abruptness of interface and the shear moduli of the individual layers can affect hardness. However, hardness is not only dependent on grain size but also on other factors such as texture, porosity, residual stress etc. which are influenced by the thin film deposition process conditions. The indentation system used for this work, known as frequency specific depth-sensing or continuous stiffness measurement (CSM), continuously measures contact stiffness during indentation [28]. The CSM method is the preferred measurement technique given the undetermined thin film thickness of the nano-indentation samples which were located little bit far or change their direction from target(s) during rotation in PVD system. The traditional fixed depth
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Fig. 7. Low angle X-ray diffraction patterns.
nano-indentation technique may not have yielded good results using the rule of thumb of 7–15% of film thickness [29] in order to minimize substrate artifacts. This restriction would have resulted in indentation depth which is too shallow, on the order of 20–40 nm. Nanohardness, elastic modulus and friction coefficient of AlTiN/MoN multilayers on tungsten carbide substrates as a function of rotation speed as well as with their single layers samples are shown in Table 2. The rapid increase in the hardness and elastic modulus occurs in the first 40–60 nm of the indentation in Fig. 8 is due to the Berkovich
indenter tip radius of about 60 nm is the primary influence that establishes the curve during initial indentation rather than surface roughness or residual effect of the layers. The maximum hardness of 44 GPa is observed at 1 RPM. It is seen from Table 2 that the average values of hardness of multilayer coatings are higher than those of AlTiN (27 GPa) and MoN (29 GPa) deposited under identical experimental conditions. This result is similar to data obtained for multilayer structures; AlTiN/CrN [30], AlTiN/AN (A = Si, W) [31] and AlTiN/AN (A = Ti, Cr) [32]. Nanohardness and elastic modulus results reveal that hardness and elastic modulus decrease with increasing SRS; such result is also confirmed by other studies [30,33]. It is known that the sharpness of the interface between sub-layers controls the hardness of the multilayered structure [34]. Any diffusion process between sub-layers inevitably causes degradation of mechanical properties. In Fig. 6b the diffusion region of 1–2 nm thickness is visible between MoN and AlTiN layers. At higher SRS, the bilayer period is smaller and the ratio of this diffused region to volume of pure materials increases, therefore interface sharpness decreases, which explains the decrease of the hardness at higher SRS [33]. The friction coefficient of AlTiN/MoN coatings is in the range of 0.41–0.56, which is between friction coefficients of AlTiN and MoN single layers. This result could show comparable areas of MON and AlTiN outward layers on the contact surface under Si3N4 ball during the wear process, as well as the presence of Mo–O Magnéli phase on the MoN surface, the last can be proved by moderate oxidation found in XPS measurement. Fig. 9 shows typical SEM images of wear tracks of the AlTiN/MoN coatings. It is found that chipping and delamination of the coatings down to MoN or Mo interlayer occurs for 2, 3, and 5 RPM samples; the result is proved by EDS analysis. The 1 and 4 RPM samples show only polishing of the surface without any notice-able changes in EDS spectra.
Fig. 8. Hardness and elastic modulus as function of displacement (penetration depth) and SRS.
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Fig. 9. SEM morphologies of the wear track of the AlTiN/MoN coating deposited at different SRS and AlTiN single layer.
AlTiN and MoN layers have heavy abrasion and polishing of the surface, respectively. The wear result of 1 and 4 RPM samples is in agreement with enhanced wear resistance for multilayer structures in comparison with single layer of the same materials [34,35]. In summary, the mechanical property examination shows moderate decrease of the hardness and elastic modulus for all multilayered structures with increasing SRS and 1 and 4 RPM multilayer samples shows better wear resistance then single layers. 4. Conclusion An industrial-scale cathodic arc ion-plating system was used to deposit AlTiN/MoN multilayer coatings on Si (100) and cemented carbide substrates using 2-fold rotation at various SRS. XRD has revealed broad peaks of AlTiN and δ-MoN phases with crystallite size of ~ 6 nm and ~ 3 nm, respectively. The averaged Al:Ti ratio over all samples is close to Al0.63Ti0.37 N composition. AFM images have found the smoothest surface with roughness of ~19 nm for SRS 3 RPM. Hardness and elastic modulus decrease with increasing SRS and they are higher than those for single layers of AlTiN and MoN. Wear analysis shows enhanced wear resistance for 1 and 4 RPM AlTiN/MoN multilayer structure in comparison with single layer coatings. Acknowledgements This work was supported by National Natural Science Foundation of China under grants 11275141, 11375133, 11375135 and Basic Research Fund of Central Universities under 2012202020217.
References [1] K. Kawata, H. Sugimura, O. Takai, Thin Solid Films 390 (2001) 64. [2] H. Holbeeck, H. Schulz, Thin Solid Films 153 (1987) 382. [3] M. Urgen, O.L. Eryilmaz, A.F. Cakir, E.S. Kayali, B. Nilufer, Y. Isik, Surf. Coat. Technol. 94–95 (1997) 501. [4] V.P. Anitha, S. Vitta, S. Major, Thin Solid Films 245 (1994) 1. [5] S. Li, J.S. Lee, J. Catal. 178 (1998) 119. [6] M.S. Mudholkar, L.T. Thompson, J. Appl. Phys. 77 (1995) 5138. [7] J.L. Li, D.S. Xiong, H.Y. Wu, J.H. Dai, T. Cui, Wear 271 (2011) 987. [8] A. Erdemir, Tribol. Lett. 8 (2000) 97. [9] Y.C. Chim, X.Z. Ding, X.T. Zeng, S. Zhang, Thin Solid Films 517 (2009) 4845. [10] R. Cremer, M. Witthaut, D. Neuschutz, Value-addition metallurgy: Proceedings of an International Symposium Sponsored by the Process Fundamentals Committee of the Extraction and Processing Division of the Minerals, The Minerals, Metals & Materials Society, 1998. 249. [11] G. Gassner, P.H. Mayrhofer, K. Kutschej, C. Mitterer, M. Kathrein, Surf. Coat. Technol. 201 (2006) 3335. [12] A. Biksa, K. Yamamoto, G. Dosbaeva, S.C. Veldhuis, G.S. Fox-Rabinovich, A. Elfizy, T. Wagg, L.S. Shuster, Tribol. Int. 43 (2010) 1491–1499. [13] M.K. Kazmanli, M. Urgen, A.F. Cakir, Surf. Coat. Technol. 167 (2003) 77. [14] G. Soto, W. de la Cruz, M.H. Farias, J. Electron Spectrosc. Relat. Phenom. 135 (2004) 27. [15] R. Fix, R.G. Gordon, D.M. Hoffman, Thin Solid Films 288 (1996) 116. [16] G.T. Kim, T.K. Park, H. Chung, Y.T. Kim, M.H. Kwon, J.G. Choi, Appl. Surf. Sci. 152 (1999) 35. [17] J.F. Moulder, W.F. Stikle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer Corp., Eden Prairie, Minnesota, 1992. [18] R. Sanjines, P. Hones, F. Levy, Thin Solid Films 332 (1998) 225. [19] J.L. Mo, M.H. Zhu, Tribol. Int. 42 (2009) 1758. [20] I. Bertoti, Surf. Coat. Technol. 151–152 (2002) 194. [21] I. Miloshev, H.H. Strehblow, B. Navinshek, Thin Solid Films 303 (1997) 246. [22] R.J. Rodriguez, J.A. Garcia, A. Medrano, M. Rico, R. Sanchez, R. Martinez, C. Labrugere, M. Lahaye, A. Guette, Vacuum 67 (2002) 559. [23] J.L. Endrino, G.S. Fox-Rabinovich, A. Reiter, S.V. Veldhuis, R. Escobar Galindo, J.M. Albella, et al., Surf. Coat. Technol. 201 (2006) 4505. [24] S.H. Huang, S.F. Chen, Y.C. Kuo, et al., Surf. Coat. Technol. 206 (2011) 1744.
124
M.I. Yousaf et al. / Surface & Coatings Technology 265 (2015) 117–124
[25] Z. Zhou, W.M. Rainforth, B. Rother, A.P. Ehiasarian, P.Eh. Hovsepian, W.-D. Munz, Surf. Coat. Technol. 183 (2004) 275–282. [26] R.A. Koshy, M.E. Graham, L.D. Marks, Surf. Coat. Technol. 202 (2007) 1123. [27] C. Kim, S.B. Qadri, M.R. Scanlon, R.C. Cammarata, Thin Solid Films 240 (1994) 52. [28] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [29] K.S.A. Butcher, T.L. Tansley, X. Li, Surf. Interface Anal. 25 (1997) 99. [30] C.X. Tian, B. Yang, S.J. Yan, Z.H. Lu, Z.H. Huang, D.J. Fu, Surf. Coat. Technol. 228 (2013) S228.
[31] [32] [33] [34] [35]
K. Yamamoto, S. Kujime, K. Takahara, Surf. Coat. Technol. 200 (2005) 435. C. Ducros, F. Sanchette, Surf. Coat. Technol. 201 (2006) 1045. S.K. Kim, V.V. Le, Surf. Coat. Technol. 204 (2010) 3941. W.D. Sproul, Science 273 (1996) 889. Z.G. Zhang, O. Rapaud, N. Allain, D. Mercs, M. Baraket, C. Dong, C. Coddet, Adv. Eng. Mater. 11 (2009) 667.