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Correlation between anionic substitution and structural properties in AlCr(OxN1−x) coatings deposited by lateral rotating cathode arc PVD
Thin Solid Films 520 (2011) 1597–1602
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Correlation between anionic substitution and structural properties in AlCr(OxN1−x) coatings deposited by lateral rotating cathode arc PVD H. Najafi a,⁎, A. Karimi a, P. Dessarzin b, M. Morstein b a b
Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Lausanne, Switzerland PLATIT AG, Advanced Coating Systems, CH-2545 Selzach, Switzerland
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Available online 25 August 2011 Keywords: Alumina Oxynitride Pin-on-disk test Rotating arc cathodes TEM α-(Al,Cr)2O3
a b s t r a c t The influence of oxygen content on the properties of cathodic arc-deposited AlCr(OxN1−x) coatings has been studied. All samples were prepared in a nitrogen-rich mixture of N2 and O2 at 550 °C using lateral rotating arc cathodes (LARC) technology together with a pulsed bias voltage. The obtained coatings were characterized by various techniques including XRD, EPMA, TEM, pin-on-disk wear tests and nanoindentation. The results obtained allow to classify the coatings into three groups with respect to their microstructure, mechanical properties and oxygen content, x. For the first group of samples with x ≤ 0.6, single-phase films of (Al,Cr) OxN1-x with fcc lattice were obtained, with well-developed columnar structure and a hardness of 30 to 33 GPa. In the second group, a diffuse columnar structure was observed while the fcc lattice was still present despite the large proportion of oxygen, 0.6 b x ≤ 0.97, and the observed hardness decreased to 25 GPa. No amorphous phase was detected in this group as confirmed by TEM. The simulation of XRD patterns of nitride lattices with oxygen incorporation allowed to suggest the formation of cation vacancies in the structure of the investigated oxynitride coatings. The third group is formed by coatings with x N 0.97, where a well-crystalline α-(Al,Cr)2O3 corundum phase was observed and the hardness increased again to 28 GPa. Our results indicate that the second group of coatings is metastable and after heat treatment transforms to a composite of cubic oxynitride and corundum oxide. Both friction and wear of samples from the entire investigated compositional range were studied at room temperature and 600 °C. The low wear rates observed for the oxynitride coatings underline their potential for use in turning and milling applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transition metal nitride coatings have found numerous applications in different industrial areas [1–8]. The effects of the elemental substitution on structure, physical and chemical properties of nitride films have been studied intensively since decades [9–15], where most of this work has focused on cationic substitutions [16,17]. However, the anionic substitutions, i.e. the formal replacement of nitrogen by other anions, can also strongly affect the properties of such compounds due to different charges and differences in the nature of metal-anion bonds [18]. In this regard, metal oxynitride films have recently received considerable attention due to their particular physical and chemical properties, which make them interesting candidates for new applications [19–26]. The incorporation of oxygen into the nitride lattice leads to strong changes in the electronic structure and physical properties of nitride films. In
⁎ Corresponding author. Tel.: + 41 21 693 5414; fax: +41 21 693 4470. E-mail address: hossein.najafi@epfl.ch (H. Najafi). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.08.075
order to maintain electroneutrality of the network, two nitrogen atoms need to be replaced by three oxygen atoms (2 N 3− – 3 O 2−), thereby generating interstitial anions. An alternative method of oxygen insertion is to compensate the reduced negative charge by cation n− vacancies (N3− + M n+ = O 2− + xVM + (1−x)Mn+) [18]. One interesting task is therefore to explore new synthesis methods that allow preparation of transition metal oxynitrides with controlled physical properties. As has been shown by Safi [27], this can be achieved by addition of oxygen into the argon–nitrogen atmosphere typically used in physical vapor deposition (PVD) methods. Such oxynitrides normally demonstrate a variety of microstructural changes by varying the O/N in the films [28–30]. In this concept, AlCr-based hard coatings are attracting much attention as a replacement to TiAl-based films due to their higher solubility of phases in comparison with the Ti-Al system [31–33]. Only little information is available on the AlCr-based oxynitrides, in the form of a singlephase (Al,Cr)(O,N) or mixed phases of (Al,Cr)N–(Al,Cr)2O3. Therefore, the main purpose of this study is to grow single-layer AlCrbased oxynitrides and to investigate the relationship between their composition and structure. The influence of post-deposition heat treatment on thermal stability will also be considered.
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Table 1 Elemental composition of the coatings calculated by Electron Probe Micro Analysis (EPMA, systematical error for the main elements b ± 1 at%). Sample
Elemental composition [at.%]
ðAlþCr Þ ðOþN Þ
O ðOþN Þ
Al ðAlþCrÞ
Number
Al
Cr
O
N
(± 0.04)
[%] ± 3%
[%]
1 2 3 4 5 6 7 8
25 24.6 22 21.6 18.2 20.6 21.1 23
24.7 23.9 23 21 20 20.4 17.2 17.9
0.5 10.5 29 41.8 52.2 53.1 60.1 58.6
49.8 41 26 15.6 9.6 5.9 1.6 0.5
0.99 0.94 0.82 0.74 0.62 0.69 0.62 0.69
1 20 53 73 84 90 97 99.2
50.3 50.7 48.8 50.7 47.6 50.2 55 56
2. Experimental details AlCr-based oxynitrides were deposited using a Platit π 300 rotating cathodes arc system, using nitrogen-rich N2/O2 reactant gas mixtures at a total working pressure of 4–5 Pa together with a MF pulsed substrate bias of -35 V. The substrate temperature was set to 550 °C, and the coating thickness was kept at 3–4 μm. Polished HSS disks (Böhler S390) and WC-10% Co disks (Extramet, EMT210) with a diameter of 32 mm each were used as substrates for coating deposition, along with dia. 50 mm cemented carbide plates for high-temperature tribology tests (Extramet, EMT100, 6% Co). To investigate the effect of heat treatment on the structural properties, selected samples were annealed for 1 h at 1000 °C in an argon environment. The chemical composition of the films was quantitatively analyzed by Electron Probe Micro Analysis (EPMA, by WDX). The crystal structure was analyzed by X-ray diffraction (XRD) using a Rigaku X-ray diffractometer (CuKα radiation, 40 kV, 30 mA) at a grazing incidence (θ=4°) scan mode. The diffraction patterns of oxynitrides were simulated using the Rietveld, Crystalmaker and Crystal Impact Diamond software packages [34–36], which allow investigation of the influence of oxygen distribution in the nitride unit cell on the diffraction pattern. The microstructure of samples was observed by means of transmission electron microscope (TEM), using a Philips CM-20 equipment working at 200 kV. To prepare TEM foils, the coated specimens were first cut by a diamond wire saw to obtain slices with thickness of 600 μm.
The slices were subsequently thinned by mechanical polishing on diamond pads up to ≅40 μm, followed by ion milling for final thinning up to transparency to the electron beam. The Young's modulus and hardness of samples were determined by the nanoindentor XP with a Berkovitch-type pyramidal diamond tip, indenting the coatings under continuous stiffness measurement method to a maximum depth of 1000 nm; however, the values obtained at a penetration depth of 300–400 nm were assigned as film hardness. The Oliver and Pharr method [37] was employed to determine the Young's modulus of the coatings where the Poisson ratio was kept constant at ν = 0.25. Friction and wear behavior of the coatings were studied by CSM pin-on-disk tribometers both at room temperature (25°C) and 600°C. Settings for the room temperature tests were: 20 cm/s sliding speed against dia. 6 mm Si3N4 balls at 10 N load for a total distance of 2000 m, under dry air atmosphere (approx. 5% rel. humidity). Tribotests at 600 °C used the same sliding speed but Al2O3 balls, only 5 N load and a reduced sliding distance of between 200 and 1000 m, under ambient air. Wear rates were evaluated by means of a TaylorHobson Talysurf Series 2 50i stylus profilometer. 3. Results and discussion 3.1. Chemical composition The elemental composition of the investigated coatings is presented in Table 1. The pure nitride film (sample 1) without addition of oxygen to the reactive gas had an oxygen impurity level of about O = 0.5 at.% and a 1:1 stoichiometric ratio of metals to nitrogen. The intentional addition of oxygen to the system then decreased the cation/anion ratio from 0.98 (sample 1) to 0.62 (sample 7). This can be attributed to the formation of cation vacancies or the presence of an excess anion content in the cubic lattice, which is covered in greater detail in the next section. The pure oxide coatings (sample 8) displayed a cation/anion ratio of 0.69, which was found to be practically stoichiometric. It is noticeable that the Al/Cr ratio, which is around 1 at lower oxygen fractions, increases to 1.3 as the oxygen content of the coatings is raised (sample 7, 8). This could be related to complex poisoning and phase formation processes at the surface of the powder-metallurgical target, that has been previously
Fig. 1. Grazing incidence XRD patterns of as-deposited oxynitride coatings for different concentrations of oxygen. The symbols ■ and * are cubic-(Al,Cr)(O,N) and α-(Al,Cr)2O3
respectively, and ● refers to substrate (WC).
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described for Al-metal composite arc PVD targets especially at high oxygen flows [38]. 3.2. Phase content and crystalline structure Fig. 1 illustrates the XRD patterns of the as deposited coatings. The oxygen-free coating (sample 1) is crystallized in a rocksalt-type cubic structure with peaks at around 2θ = 37.5°, 43.6° and 63.5° assigned to (111), (200) and (220), respectively (pdf n°11-0065). The intensity of all these three diffraction peaks, and in particular the (111) reflection, decreases with increasing the oxygen content, due to alternation of crystallinity or refinement of grain size (samples 2–7). Development of the (200) orientation could arise from minimization of the surface energy γ of the crystal planes. Castaldi et al. [39] reported that γ (111) increases as a function of oxygen content since the introduction of oxygen would enhance the polarization due to its higher electronegativity. As a result the (200) planes have the lowest surface energy. Moreover, the peaks are shifted toward higher angles, indicating a decrease of lattice parameter caused by the formation of metal vacancies or replacement of nitrogen atoms by the smaller oxygen atoms. In fact, the transition from (Al,Cr)N to (Al,Cr)2O3 requires vacancies in the metal sublattice to maintain electroneutrality of the network. In general, for the incorporation of each oxygen atom in the nitride lattice, one third of the metal vacancies are created as confirmed by simulated XRD patterns in the next section. It is noticeable in sample 7 that the fcc lattice survives despite the large proportion of oxygen. This can be explained by the high values of replacement of N by O in the nitride lattice. While the kinetic aspect of coating growth is presumably dominated by the nitrogen-rich atmosphere leading to the formation of the cubic lattice, the thermodynamic aspect is suspected to play a key role for the preferential formation of metal\oxygen bonds resulting in replacement of N−3 by O−2. It is consistent with the recent results obtained by Stueber et al. [40] and Khatibi et al. [41] who reported the formation of Al–Cr–O–N and Al–Cr–O thin films in fcc structure, respectively. The pure oxide coatings (sample 8) exhibit pronounced peaks corresponding to (012), (104), (110) and (116) of α-(Al,Cr)2O3 (pdf n°43-1484). The appearance of α phase at 550°C is favored by the presence of Cr2O3 which promotes the formation of solid solution of (Al,Cr)2O3 as shown by Ramm et al. [42] and other research groups [43–45]. All samples heat-treated for 1 h at 1000 °C in argon display enhanced crystallinity and keep their initial structure except sample 7, where a huge difference between asdeposited and heat-treated states was observed. The results revealed
Fig. 2. Grazing incidence XRD patterns of sample 7 (O/(O + N) = 97%) in as-deposited and heat-treated (1000°C, Ar) states.
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that the single-phase oxynitride formed in sample 7 is metastable and after heat treatment converts to a composite of cubic and corundum phases (Fig. 2). This behavior is attributed to the presence of a sufficient quantity of oxygen in the coatings to enable local formation of the oxide lattice during annealing. A simulation of XRD patterns was carried out in order to study the effect of oxygen incorporation into the nitride lattice. The simulation was initiated by building rock salt cubic (Fm −3 m-Fig. 3a and b) and trigonal (R −3C — Fig. 3c) cells using the Crystal Maker package [35]. To construct a rocksalt cell, cations (Al + 3, Cr + 3) and anions (O −2, N −3) are distributed on the 4a (000) and 4b (1/2,1/2,1/2) positions of the Fm −3 m space group respectively. The structure of α-(Al, Cr)2O3 is built of corundum lattice, where the cations and anions are located on the 12c (0,0,0.35) and 18e (0.69,0,.25), respectively. In order to match with chemical compositions, the built cells can be corrected for implantation of the metal vacancies or interstitial anions. In fact, two possibilities were considered for decreasing the cation/anion ratio (see Table 1). A first possibility can be attributed to the presence of cation vacancies while the second one is the presence of an excess anion content in the interstitial positions (1/4,1/4,1/4) of the lattice. At this point, we did not yet consider the combination of both possibilities. The constructed unit cells were imported to Crystal Impact Diamond package [36] to obtain their diffraction pattern. The correction factors for preferred orientation (Pk), polarization and stress were considered in the simulation of XRD patterns using Rietveld method [34], which allowed the obtained patterns to be different solely in vacancy or interstitial anion insertion. Simulated XRD patterns for either vacancy or interstitial anion implantation of selected samples are compared to experimental diffractograms in Fig. 4a and b, respectively. Evidently, the cation vacancy concept simulates more closely the experimental patterns than the interstitial anion consideration. In addition, the (111) peak intensity is very sensitive to the oxygen content. This is in good agreement with the experimental patterns despite a generally observed higher intensity of simulated patterns. This difference can be attributed to a random distribution of vacancies in the lattice in simulated patterns, whereas in experimental patterns, apparently more vacancies are located on (111) orientation. Considering these points, the cation vacancies model rather than the interstitial anions model is suggested to explain the structure of the investigated oxynitride coatings. 3.3. Microstructure Cross-sectional TEM images obtained from samples with various ratios of O/(O + N) are shown in Fig. 5. A well-developed columnar microstructure is observed in sample 3 (O/(O + N) = 53%) and the diffraction pattern shows intense (200) and (220) reflections from the rock salt cubic structure, Fig. 5a. The mean diameter of the columns is around 100 nm. In contrast, the coatings with higher oxygen content (0.6 b x ≤ 0.97) do not have such a pronounced columnar microstructure but instead, rather diffuse boundaries (Fig. 5b). However, the fcc lattice is still present. The (200) and (220) reflections are visible on the selected area electron diffraction (SAED) pattern with weak intensity indicating a significant reduction of grain size. Moreover, the electron diffraction pattern does not show any broad ring, as would be expected for an amorphous phase. In contrast, recently we have prepared similar oxynitride coatings by reactive magnetron sputtering which displayed nanocrystalline grains and an amorphous phase in the range of 0.6 b O/(O + N) ≤ 0.95 (not shown here). The presence of this amorphous phase in the sputtered coatings can be explained by the much higher ion-to-neutral ratio in the LARC with respect to magnetron sputtering. For the oxide coatings (sample 8, x N 97%), the structure becomes again columnar although the column size is decreased drastically to 50 nm. The SAED pattern exhibits mainly the (012), (104) and (110) crystal orientations of the
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Fig. 3. Unit cells of a) cubic oxynitride with insertion of metal vacancy b) cubic oxynitride with insertion of interstitial anion and c) (Al,Cr)2O3 with corundum structure.
corundum structure and simultaneously, diffraction spots of individual crystallites, indicating some tendency to the formation of a preferred crystallographic orientation (see Fig. 5c). As mentioned before in the XRD section, there are significant differences between as-deposited and heat-treated states in sample 7. The TEM micrograph of sample 7 heat-treated for 1 h/1000 °C (Fig. 6) demonstrates a very smooth structure without any detectable growth features, indicating a fine-scale microstructure. The corresponding selected area diffraction pattern shows both cubic and corundum arrangements of the diffraction rings, which confirm the co-existence of both phases and the formation of a composite made of cubic oxynitride and
Fig. 4. Calculated XRD patterns (dashed line) of selected coatings (Samples 3, 7 and 8) compared to those of experimental (solid line) considering a) metal vacancy and b) interstitial anion in the structure.
corundum oxide. As discussed earlier, a sufficient quantity of oxygen in the coatings enables local formation of corundum phase during heat treatment at sufficiently high temperatures.
Fig. 5. Cross-sectional TEM micrographs showing the microstructure and diffraction patterns of coatings with different ratios of O/(O + N: a) Sample 3, O/(O + N) = 53 at.%, fcc structure; b) Sample 7, O/(O + N) = 97 at.%, fcc structure c) Sample 8, O/(O + N) = 99.2%, α-corundum structure.
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Fig. 6. Cross-sectional TEM micrograph and diffraction pattern of the sample 7 (O/(O + N) = 97%) after heat treatment for 1 h at 1000 °C.
3.4. Mechanical properties The variation of nanohardness and Young's modulus (E) of the coatings as a function of O/(O+ N) is reported in Fig. 7. As the oxygen fraction increases from 1 to 60 at.%, both hardness and modulus of coatings decrease steadily. When the oxygen fraction reaches 0.6 b x ≤ 0.97, a sharp decline of hardness from 30 to 33 GPa to 25–26 GPa was observed. This can be related to the change in the microstructure of coatings as discussed above. Furthermore, the anion substitution when changing the anion from nitrogen to oxygen together with the bonding from covalency in the direction of greater ionic can lead to the hardness variations [46]. The low values of mechanical properties for sample 7 can be attributed to the maximum microstructure evolution and formation of metal vacancies (33%) in the fcc lattice as also reported by Khatibi et al. [41]. It is noteworthy that in comparable oxynitride films (0.6b x ≤0.97) prepared by sputtering, hardness is collapsing to no more than 17–18 GPa, due to the appearance of amorphous phase. In the pure oxide coatings, corresponding to the formation of corundum structure, the hardness grows back to higher values (28 GPa). Fig. 8 shows the tribological properties of the coatings measured at both ambient and elevated temperatures. At room temperature (RT), the oxygen content of the coatings has only minor effect on the friction coefficient (0.83 to 0.98). Also at 600 °C, the influence of the oxygen content on the friction coefficient is rather small, although the friction appears to be at its minimum for the intermediate samples (sample 6 and 7). The room temperature wear rates for all coatings are very low and remain in the range of 5 × 10−17 to 1.6 × 10−16 m3N−1 m−1 at 10 N load against Si3N4 balls. As a comparison, the typical wear rates reported for TiAlN and TiAlSiN coatings under similar conditions are about 5 × 10 −15 to 5 × 10−14 m3N−1 m−1 [33,47], thus the wear resistance of AlCr-based oxynitride coatings is 2–3 orders of magnitude higher. There is however no systematic trend of the wear rate versus oxygen concentration: at first, the wear rate decreases until an intermediate oxygen content of O/(O+ N)≤ 0.6 is reached, then increases again
Fig. 8. Friction (●-RT, ▴-600 °C) and wear behavior (■-RT, 10 N, against Si3N4 balls) of coatings as a function of oxygen fraction.
for higher oxygen concentrations 0.6 b O/(O+ N) ≤ 0.97. For the pure oxide coatings (O/(O+ N) N 0.97), the wear rate is again lower. The tribo-tests at 600 °C did not yield any valid wear rates for all samples investigated, however; friction coefficients could be extracted from those experiments. As shown in Fig. 8, the high-temperature friction coefficients are systematically lower that those obtained at RT, however please note that different materials were used as the static friction partner at both conditions. Recently, using optimized samples together with slightly modified experimental parameters, we were able to obtain valid high-temperature wear rates. For these new samples, practically no coating wear was created, the wear rates at 600 °C ranging typically in the order of 1 × 10−16 m 3N −1 m −1 at 5 N load versus dia. 6 mm alumina balls. 4. Summary A series of AlCr(OxN1−x) coatings with fixed nominal metallic composition and variable oxygen content 0 b x ≤ 1 was deposited at 550 °C by a reactive arc evaporation process, using MF-pulsed substrate bias. The coatings with lowest oxygen content, x ≤ 0.6, exhibit a cubic (fcc) lattice with nanohardness values ranging from 30 to 33 GPa, and a well-developed columnar structure. The incorporation of oxygen into the nitride lattice results in a decrease of the cubic lattice parameter a, which allows to suggest the formation of a substitutional AlCr(OxN1−x) solid solution containing metal vacancies. In the range of oxygen content from 0.6 b x ≤ 0.97, coatings with diffuse columnar structure and high values of metal vacancies were formed. However, the fcc lattice survives despite the large proportion of oxygen. The hardness decreased to about 25–26 GPa, which can be explained by the formation of vacancies, accompanied by changes of chemical bonding states and in the coating microstructure. No amorphous phase was detected in these films. Coatings with nitrogen contents close to or even below the detection limit of EPMA analysis, x ≥ 0.97, consisted of a solid solution of α-(Al,Cr)2O3 with corundum lattice and finer columnar structure; coating hardness increased again to 28 GPa. Oxynitrides in the range of 0.6 b x ≤ 0.97 were shown to be metastable and after heat treatment to 1000 °C transformed to a dualphase composite of cubic oxynitride and corundum oxide. This behavior was attributed to the presence of a sufficient quantity of oxygen in the coatings to enable local formation of corundum lattice during annealing. All investigated oxynitride coatings exhibit low wear rates, which, together with their chemical inertness, qualify them for coating of tools used in severe cutting conditions. Acknowledgments
Fig. 7. Nanohardness and Young's modulus of the coatings as a function of oxygen fraction (O/(O+ N)) in the coatings.
Financial support from the Swiss Commission for Technology and Innovation (CTI) under grant No 8873.2 PFNM-NM is highly
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acknowledged. The authors would also like to thank CIME-EPFL for providing electron microscopy facilities, CSM Instruments, Peseux, Switzerland for their support as well as C. Steinberg and K. Schiffmann from Fraunhofer IST, Braunschweig, Germany for EPMA analysis. References [1] J.L. Endrino, G.S. Fox-Rabinovich, C. Gey, Surf. Coat. Technol. 200 (2006) 6840. [2] A.E. Reiter, B. Brunner, M. Ante, J. Rechberger, Surf. Coat. Technol. 200 (2006) 5532. [3] R. Rodr´ıguez-Baracaldo, J.A. Benito, E.S. Puchi-Cabrera, M.H. Staia, Wear 262 (2007) 380. [4] S. Veprek, R.F. Zhang, M.G.J. Veprek-Heijman, S.H. Sheng, A.S. Argon, Surf. Coat. Technol. 204 (2010) 1898. [5] A. Horling, L. Hultman, M. Oden, J. Sjolen, L. Karlsson, J. Vac, Sci. Technol. A: Vac. Surf. Films 20 (2002) 1815. [6] R. Kaindl, R. Franz, J. Soldan, A. Reiter, P. Polcik, C. Mitterer, B. Sartory, R. Tessadri, M. O'Sullivan, Thin Solid Films 515 (2006) 2197. [7] H. Hasegawa, M. Kawate, T. Suzuki, Surf. Coat. Technol. 200 (2005) 2409. [8] T.I. Selinder, D.J. Miller, K.E. Gray, M.R. Sardela, L. Hultman, Vacuum 46 (1995) 1401. [9] A.E. Santana, A. Karimi, V. Derflinger, A. Schutze, Surf. Coat. Technol. 177–178 (2004) 334. [10] F. Rovere, D. Music, J.M. Schneider, P.H. Mayrhofer, Acta Mater. 58 (2010) 2708. [11] K. Ichijo, H. Hasegawa, T. Suzuki, Surf. Coat. Technol. 201 (2007) 5477. [12] I. Park, S.R. Choi, M. Lee, K.H. Kim, J. Vac. Sci. Technol. A 21 (2003) 895. [13] E. Martinez, R. Sanjines, O. Banakh, F. Levy, Thin Solid Films 447–448 (2004) 332. [14] P.E. Hovsepian, C. Reinhard, A.P. Ehiasarian, Surf. Coat. Technol. 201 (2006) 4105. [15] F. Rovere, P.H. Mayrhofer, A. Reinholdt, J. Mayer, J.M. Schneider, Surf. Coat. Technol. 202 (2008) 5875. [16] J.L. Endrino, V. Derflinger, Surf. Coat. Technol. 200 (2005) 988. [17] H. Hasegawa, T. Suzuki, Surf. Coat. Technol. 188–189 (2004) 234. [18] S.G. Ebbinghaus, H.P. Abicht, R. Dronskowski, T. Müller, A. Reller, A. Weidenkaff, Prog. Solid State Chem. 37 (2009) 173. [19] J. Sjölén, L. Karlsson, S. Braun, R. Murdey, A. Hörling, L. Hultman, Surf. Coat. Technol. 201 (2007) 6392. [20] J. Ye, S. Ulrich, C. Ziebert, M. Stüber, Thin Solid Films 517 (2008) 1151. [21] M. Hirai, T. Suzuki, H. Suematsu, W. Jiang, K. Yatsui, J. Vac. Sci. Technol. A 21 (2003) 947.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Correlation between anionic substitution and structural properties in AlCr(OxN1−x) coatings deposited by lateral rotating cathode arc PVD H. Najafi a,⁎, A. Karimi a, P. Dessarzin b, M. Morstein b a b
Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Lausanne, Switzerland PLATIT AG, Advanced Coating Systems, CH-2545 Selzach, Switzerland
a r t i c l e
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Available online 25 August 2011 Keywords: Alumina Oxynitride Pin-on-disk test Rotating arc cathodes TEM α-(Al,Cr)2O3
a b s t r a c t The influence of oxygen content on the properties of cathodic arc-deposited AlCr(OxN1−x) coatings has been studied. All samples were prepared in a nitrogen-rich mixture of N2 and O2 at 550 °C using lateral rotating arc cathodes (LARC) technology together with a pulsed bias voltage. The obtained coatings were characterized by various techniques including XRD, EPMA, TEM, pin-on-disk wear tests and nanoindentation. The results obtained allow to classify the coatings into three groups with respect to their microstructure, mechanical properties and oxygen content, x. For the first group of samples with x ≤ 0.6, single-phase films of (Al,Cr) OxN1-x with fcc lattice were obtained, with well-developed columnar structure and a hardness of 30 to 33 GPa. In the second group, a diffuse columnar structure was observed while the fcc lattice was still present despite the large proportion of oxygen, 0.6 b x ≤ 0.97, and the observed hardness decreased to 25 GPa. No amorphous phase was detected in this group as confirmed by TEM. The simulation of XRD patterns of nitride lattices with oxygen incorporation allowed to suggest the formation of cation vacancies in the structure of the investigated oxynitride coatings. The third group is formed by coatings with x N 0.97, where a well-crystalline α-(Al,Cr)2O3 corundum phase was observed and the hardness increased again to 28 GPa. Our results indicate that the second group of coatings is metastable and after heat treatment transforms to a composite of cubic oxynitride and corundum oxide. Both friction and wear of samples from the entire investigated compositional range were studied at room temperature and 600 °C. The low wear rates observed for the oxynitride coatings underline their potential for use in turning and milling applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transition metal nitride coatings have found numerous applications in different industrial areas [1–8]. The effects of the elemental substitution on structure, physical and chemical properties of nitride films have been studied intensively since decades [9–15], where most of this work has focused on cationic substitutions [16,17]. However, the anionic substitutions, i.e. the formal replacement of nitrogen by other anions, can also strongly affect the properties of such compounds due to different charges and differences in the nature of metal-anion bonds [18]. In this regard, metal oxynitride films have recently received considerable attention due to their particular physical and chemical properties, which make them interesting candidates for new applications [19–26]. The incorporation of oxygen into the nitride lattice leads to strong changes in the electronic structure and physical properties of nitride films. In
⁎ Corresponding author. Tel.: + 41 21 693 5414; fax: +41 21 693 4470. E-mail address: hossein.najafi@epfl.ch (H. Najafi). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.08.075
order to maintain electroneutrality of the network, two nitrogen atoms need to be replaced by three oxygen atoms (2 N 3− – 3 O 2−), thereby generating interstitial anions. An alternative method of oxygen insertion is to compensate the reduced negative charge by cation n− vacancies (N3− + M n+ = O 2− + xVM + (1−x)Mn+) [18]. One interesting task is therefore to explore new synthesis methods that allow preparation of transition metal oxynitrides with controlled physical properties. As has been shown by Safi [27], this can be achieved by addition of oxygen into the argon–nitrogen atmosphere typically used in physical vapor deposition (PVD) methods. Such oxynitrides normally demonstrate a variety of microstructural changes by varying the O/N in the films [28–30]. In this concept, AlCr-based hard coatings are attracting much attention as a replacement to TiAl-based films due to their higher solubility of phases in comparison with the Ti-Al system [31–33]. Only little information is available on the AlCr-based oxynitrides, in the form of a singlephase (Al,Cr)(O,N) or mixed phases of (Al,Cr)N–(Al,Cr)2O3. Therefore, the main purpose of this study is to grow single-layer AlCrbased oxynitrides and to investigate the relationship between their composition and structure. The influence of post-deposition heat treatment on thermal stability will also be considered.
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Table 1 Elemental composition of the coatings calculated by Electron Probe Micro Analysis (EPMA, systematical error for the main elements b ± 1 at%). Sample
Elemental composition [at.%]
ðAlþCr Þ ðOþN Þ
O ðOþN Þ
Al ðAlþCrÞ
Number
Al
Cr
O
N
(± 0.04)
[%] ± 3%
[%]
1 2 3 4 5 6 7 8
25 24.6 22 21.6 18.2 20.6 21.1 23
24.7 23.9 23 21 20 20.4 17.2 17.9
0.5 10.5 29 41.8 52.2 53.1 60.1 58.6
49.8 41 26 15.6 9.6 5.9 1.6 0.5
0.99 0.94 0.82 0.74 0.62 0.69 0.62 0.69
1 20 53 73 84 90 97 99.2
50.3 50.7 48.8 50.7 47.6 50.2 55 56
2. Experimental details AlCr-based oxynitrides were deposited using a Platit π 300 rotating cathodes arc system, using nitrogen-rich N2/O2 reactant gas mixtures at a total working pressure of 4–5 Pa together with a MF pulsed substrate bias of -35 V. The substrate temperature was set to 550 °C, and the coating thickness was kept at 3–4 μm. Polished HSS disks (Böhler S390) and WC-10% Co disks (Extramet, EMT210) with a diameter of 32 mm each were used as substrates for coating deposition, along with dia. 50 mm cemented carbide plates for high-temperature tribology tests (Extramet, EMT100, 6% Co). To investigate the effect of heat treatment on the structural properties, selected samples were annealed for 1 h at 1000 °C in an argon environment. The chemical composition of the films was quantitatively analyzed by Electron Probe Micro Analysis (EPMA, by WDX). The crystal structure was analyzed by X-ray diffraction (XRD) using a Rigaku X-ray diffractometer (CuKα radiation, 40 kV, 30 mA) at a grazing incidence (θ=4°) scan mode. The diffraction patterns of oxynitrides were simulated using the Rietveld, Crystalmaker and Crystal Impact Diamond software packages [34–36], which allow investigation of the influence of oxygen distribution in the nitride unit cell on the diffraction pattern. The microstructure of samples was observed by means of transmission electron microscope (TEM), using a Philips CM-20 equipment working at 200 kV. To prepare TEM foils, the coated specimens were first cut by a diamond wire saw to obtain slices with thickness of 600 μm.
The slices were subsequently thinned by mechanical polishing on diamond pads up to ≅40 μm, followed by ion milling for final thinning up to transparency to the electron beam. The Young's modulus and hardness of samples were determined by the nanoindentor XP with a Berkovitch-type pyramidal diamond tip, indenting the coatings under continuous stiffness measurement method to a maximum depth of 1000 nm; however, the values obtained at a penetration depth of 300–400 nm were assigned as film hardness. The Oliver and Pharr method [37] was employed to determine the Young's modulus of the coatings where the Poisson ratio was kept constant at ν = 0.25. Friction and wear behavior of the coatings were studied by CSM pin-on-disk tribometers both at room temperature (25°C) and 600°C. Settings for the room temperature tests were: 20 cm/s sliding speed against dia. 6 mm Si3N4 balls at 10 N load for a total distance of 2000 m, under dry air atmosphere (approx. 5% rel. humidity). Tribotests at 600 °C used the same sliding speed but Al2O3 balls, only 5 N load and a reduced sliding distance of between 200 and 1000 m, under ambient air. Wear rates were evaluated by means of a TaylorHobson Talysurf Series 2 50i stylus profilometer. 3. Results and discussion 3.1. Chemical composition The elemental composition of the investigated coatings is presented in Table 1. The pure nitride film (sample 1) without addition of oxygen to the reactive gas had an oxygen impurity level of about O = 0.5 at.% and a 1:1 stoichiometric ratio of metals to nitrogen. The intentional addition of oxygen to the system then decreased the cation/anion ratio from 0.98 (sample 1) to 0.62 (sample 7). This can be attributed to the formation of cation vacancies or the presence of an excess anion content in the cubic lattice, which is covered in greater detail in the next section. The pure oxide coatings (sample 8) displayed a cation/anion ratio of 0.69, which was found to be practically stoichiometric. It is noticeable that the Al/Cr ratio, which is around 1 at lower oxygen fractions, increases to 1.3 as the oxygen content of the coatings is raised (sample 7, 8). This could be related to complex poisoning and phase formation processes at the surface of the powder-metallurgical target, that has been previously
Fig. 1. Grazing incidence XRD patterns of as-deposited oxynitride coatings for different concentrations of oxygen. The symbols ■ and * are cubic-(Al,Cr)(O,N) and α-(Al,Cr)2O3
respectively, and ● refers to substrate (WC).
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described for Al-metal composite arc PVD targets especially at high oxygen flows [38]. 3.2. Phase content and crystalline structure Fig. 1 illustrates the XRD patterns of the as deposited coatings. The oxygen-free coating (sample 1) is crystallized in a rocksalt-type cubic structure with peaks at around 2θ = 37.5°, 43.6° and 63.5° assigned to (111), (200) and (220), respectively (pdf n°11-0065). The intensity of all these three diffraction peaks, and in particular the (111) reflection, decreases with increasing the oxygen content, due to alternation of crystallinity or refinement of grain size (samples 2–7). Development of the (200) orientation could arise from minimization of the surface energy γ of the crystal planes. Castaldi et al. [39] reported that γ (111) increases as a function of oxygen content since the introduction of oxygen would enhance the polarization due to its higher electronegativity. As a result the (200) planes have the lowest surface energy. Moreover, the peaks are shifted toward higher angles, indicating a decrease of lattice parameter caused by the formation of metal vacancies or replacement of nitrogen atoms by the smaller oxygen atoms. In fact, the transition from (Al,Cr)N to (Al,Cr)2O3 requires vacancies in the metal sublattice to maintain electroneutrality of the network. In general, for the incorporation of each oxygen atom in the nitride lattice, one third of the metal vacancies are created as confirmed by simulated XRD patterns in the next section. It is noticeable in sample 7 that the fcc lattice survives despite the large proportion of oxygen. This can be explained by the high values of replacement of N by O in the nitride lattice. While the kinetic aspect of coating growth is presumably dominated by the nitrogen-rich atmosphere leading to the formation of the cubic lattice, the thermodynamic aspect is suspected to play a key role for the preferential formation of metal\oxygen bonds resulting in replacement of N−3 by O−2. It is consistent with the recent results obtained by Stueber et al. [40] and Khatibi et al. [41] who reported the formation of Al–Cr–O–N and Al–Cr–O thin films in fcc structure, respectively. The pure oxide coatings (sample 8) exhibit pronounced peaks corresponding to (012), (104), (110) and (116) of α-(Al,Cr)2O3 (pdf n°43-1484). The appearance of α phase at 550°C is favored by the presence of Cr2O3 which promotes the formation of solid solution of (Al,Cr)2O3 as shown by Ramm et al. [42] and other research groups [43–45]. All samples heat-treated for 1 h at 1000 °C in argon display enhanced crystallinity and keep their initial structure except sample 7, where a huge difference between asdeposited and heat-treated states was observed. The results revealed
Fig. 2. Grazing incidence XRD patterns of sample 7 (O/(O + N) = 97%) in as-deposited and heat-treated (1000°C, Ar) states.
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that the single-phase oxynitride formed in sample 7 is metastable and after heat treatment converts to a composite of cubic and corundum phases (Fig. 2). This behavior is attributed to the presence of a sufficient quantity of oxygen in the coatings to enable local formation of the oxide lattice during annealing. A simulation of XRD patterns was carried out in order to study the effect of oxygen incorporation into the nitride lattice. The simulation was initiated by building rock salt cubic (Fm −3 m-Fig. 3a and b) and trigonal (R −3C — Fig. 3c) cells using the Crystal Maker package [35]. To construct a rocksalt cell, cations (Al + 3, Cr + 3) and anions (O −2, N −3) are distributed on the 4a (000) and 4b (1/2,1/2,1/2) positions of the Fm −3 m space group respectively. The structure of α-(Al, Cr)2O3 is built of corundum lattice, where the cations and anions are located on the 12c (0,0,0.35) and 18e (0.69,0,.25), respectively. In order to match with chemical compositions, the built cells can be corrected for implantation of the metal vacancies or interstitial anions. In fact, two possibilities were considered for decreasing the cation/anion ratio (see Table 1). A first possibility can be attributed to the presence of cation vacancies while the second one is the presence of an excess anion content in the interstitial positions (1/4,1/4,1/4) of the lattice. At this point, we did not yet consider the combination of both possibilities. The constructed unit cells were imported to Crystal Impact Diamond package [36] to obtain their diffraction pattern. The correction factors for preferred orientation (Pk), polarization and stress were considered in the simulation of XRD patterns using Rietveld method [34], which allowed the obtained patterns to be different solely in vacancy or interstitial anion insertion. Simulated XRD patterns for either vacancy or interstitial anion implantation of selected samples are compared to experimental diffractograms in Fig. 4a and b, respectively. Evidently, the cation vacancy concept simulates more closely the experimental patterns than the interstitial anion consideration. In addition, the (111) peak intensity is very sensitive to the oxygen content. This is in good agreement with the experimental patterns despite a generally observed higher intensity of simulated patterns. This difference can be attributed to a random distribution of vacancies in the lattice in simulated patterns, whereas in experimental patterns, apparently more vacancies are located on (111) orientation. Considering these points, the cation vacancies model rather than the interstitial anions model is suggested to explain the structure of the investigated oxynitride coatings. 3.3. Microstructure Cross-sectional TEM images obtained from samples with various ratios of O/(O + N) are shown in Fig. 5. A well-developed columnar microstructure is observed in sample 3 (O/(O + N) = 53%) and the diffraction pattern shows intense (200) and (220) reflections from the rock salt cubic structure, Fig. 5a. The mean diameter of the columns is around 100 nm. In contrast, the coatings with higher oxygen content (0.6 b x ≤ 0.97) do not have such a pronounced columnar microstructure but instead, rather diffuse boundaries (Fig. 5b). However, the fcc lattice is still present. The (200) and (220) reflections are visible on the selected area electron diffraction (SAED) pattern with weak intensity indicating a significant reduction of grain size. Moreover, the electron diffraction pattern does not show any broad ring, as would be expected for an amorphous phase. In contrast, recently we have prepared similar oxynitride coatings by reactive magnetron sputtering which displayed nanocrystalline grains and an amorphous phase in the range of 0.6 b O/(O + N) ≤ 0.95 (not shown here). The presence of this amorphous phase in the sputtered coatings can be explained by the much higher ion-to-neutral ratio in the LARC with respect to magnetron sputtering. For the oxide coatings (sample 8, x N 97%), the structure becomes again columnar although the column size is decreased drastically to 50 nm. The SAED pattern exhibits mainly the (012), (104) and (110) crystal orientations of the
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Fig. 3. Unit cells of a) cubic oxynitride with insertion of metal vacancy b) cubic oxynitride with insertion of interstitial anion and c) (Al,Cr)2O3 with corundum structure.
corundum structure and simultaneously, diffraction spots of individual crystallites, indicating some tendency to the formation of a preferred crystallographic orientation (see Fig. 5c). As mentioned before in the XRD section, there are significant differences between as-deposited and heat-treated states in sample 7. The TEM micrograph of sample 7 heat-treated for 1 h/1000 °C (Fig. 6) demonstrates a very smooth structure without any detectable growth features, indicating a fine-scale microstructure. The corresponding selected area diffraction pattern shows both cubic and corundum arrangements of the diffraction rings, which confirm the co-existence of both phases and the formation of a composite made of cubic oxynitride and
Fig. 4. Calculated XRD patterns (dashed line) of selected coatings (Samples 3, 7 and 8) compared to those of experimental (solid line) considering a) metal vacancy and b) interstitial anion in the structure.
corundum oxide. As discussed earlier, a sufficient quantity of oxygen in the coatings enables local formation of corundum phase during heat treatment at sufficiently high temperatures.
Fig. 5. Cross-sectional TEM micrographs showing the microstructure and diffraction patterns of coatings with different ratios of O/(O + N: a) Sample 3, O/(O + N) = 53 at.%, fcc structure; b) Sample 7, O/(O + N) = 97 at.%, fcc structure c) Sample 8, O/(O + N) = 99.2%, α-corundum structure.
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Fig. 6. Cross-sectional TEM micrograph and diffraction pattern of the sample 7 (O/(O + N) = 97%) after heat treatment for 1 h at 1000 °C.
3.4. Mechanical properties The variation of nanohardness and Young's modulus (E) of the coatings as a function of O/(O+ N) is reported in Fig. 7. As the oxygen fraction increases from 1 to 60 at.%, both hardness and modulus of coatings decrease steadily. When the oxygen fraction reaches 0.6 b x ≤ 0.97, a sharp decline of hardness from 30 to 33 GPa to 25–26 GPa was observed. This can be related to the change in the microstructure of coatings as discussed above. Furthermore, the anion substitution when changing the anion from nitrogen to oxygen together with the bonding from covalency in the direction of greater ionic can lead to the hardness variations [46]. The low values of mechanical properties for sample 7 can be attributed to the maximum microstructure evolution and formation of metal vacancies (33%) in the fcc lattice as also reported by Khatibi et al. [41]. It is noteworthy that in comparable oxynitride films (0.6b x ≤0.97) prepared by sputtering, hardness is collapsing to no more than 17–18 GPa, due to the appearance of amorphous phase. In the pure oxide coatings, corresponding to the formation of corundum structure, the hardness grows back to higher values (28 GPa). Fig. 8 shows the tribological properties of the coatings measured at both ambient and elevated temperatures. At room temperature (RT), the oxygen content of the coatings has only minor effect on the friction coefficient (0.83 to 0.98). Also at 600 °C, the influence of the oxygen content on the friction coefficient is rather small, although the friction appears to be at its minimum for the intermediate samples (sample 6 and 7). The room temperature wear rates for all coatings are very low and remain in the range of 5 × 10−17 to 1.6 × 10−16 m3N−1 m−1 at 10 N load against Si3N4 balls. As a comparison, the typical wear rates reported for TiAlN and TiAlSiN coatings under similar conditions are about 5 × 10 −15 to 5 × 10−14 m3N−1 m−1 [33,47], thus the wear resistance of AlCr-based oxynitride coatings is 2–3 orders of magnitude higher. There is however no systematic trend of the wear rate versus oxygen concentration: at first, the wear rate decreases until an intermediate oxygen content of O/(O+ N)≤ 0.6 is reached, then increases again
Fig. 8. Friction (●-RT, ▴-600 °C) and wear behavior (■-RT, 10 N, against Si3N4 balls) of coatings as a function of oxygen fraction.
for higher oxygen concentrations 0.6 b O/(O+ N) ≤ 0.97. For the pure oxide coatings (O/(O+ N) N 0.97), the wear rate is again lower. The tribo-tests at 600 °C did not yield any valid wear rates for all samples investigated, however; friction coefficients could be extracted from those experiments. As shown in Fig. 8, the high-temperature friction coefficients are systematically lower that those obtained at RT, however please note that different materials were used as the static friction partner at both conditions. Recently, using optimized samples together with slightly modified experimental parameters, we were able to obtain valid high-temperature wear rates. For these new samples, practically no coating wear was created, the wear rates at 600 °C ranging typically in the order of 1 × 10−16 m 3N −1 m −1 at 5 N load versus dia. 6 mm alumina balls. 4. Summary A series of AlCr(OxN1−x) coatings with fixed nominal metallic composition and variable oxygen content 0 b x ≤ 1 was deposited at 550 °C by a reactive arc evaporation process, using MF-pulsed substrate bias. The coatings with lowest oxygen content, x ≤ 0.6, exhibit a cubic (fcc) lattice with nanohardness values ranging from 30 to 33 GPa, and a well-developed columnar structure. The incorporation of oxygen into the nitride lattice results in a decrease of the cubic lattice parameter a, which allows to suggest the formation of a substitutional AlCr(OxN1−x) solid solution containing metal vacancies. In the range of oxygen content from 0.6 b x ≤ 0.97, coatings with diffuse columnar structure and high values of metal vacancies were formed. However, the fcc lattice survives despite the large proportion of oxygen. The hardness decreased to about 25–26 GPa, which can be explained by the formation of vacancies, accompanied by changes of chemical bonding states and in the coating microstructure. No amorphous phase was detected in these films. Coatings with nitrogen contents close to or even below the detection limit of EPMA analysis, x ≥ 0.97, consisted of a solid solution of α-(Al,Cr)2O3 with corundum lattice and finer columnar structure; coating hardness increased again to 28 GPa. Oxynitrides in the range of 0.6 b x ≤ 0.97 were shown to be metastable and after heat treatment to 1000 °C transformed to a dualphase composite of cubic oxynitride and corundum oxide. This behavior was attributed to the presence of a sufficient quantity of oxygen in the coatings to enable local formation of corundum lattice during annealing. All investigated oxynitride coatings exhibit low wear rates, which, together with their chemical inertness, qualify them for coating of tools used in severe cutting conditions. Acknowledgments
Fig. 7. Nanohardness and Young's modulus of the coatings as a function of oxygen fraction (O/(O+ N)) in the coatings.
Financial support from the Swiss Commission for Technology and Innovation (CTI) under grant No 8873.2 PFNM-NM is highly
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acknowledged. The authors would also like to thank CIME-EPFL for providing electron microscopy facilities, CSM Instruments, Peseux, Switzerland for their support as well as C. Steinberg and K. Schiffmann from Fraunhofer IST, Braunschweig, Germany for EPMA analysis. References [1] J.L. Endrino, G.S. Fox-Rabinovich, C. Gey, Surf. Coat. Technol. 200 (2006) 6840. [2] A.E. Reiter, B. Brunner, M. Ante, J. Rechberger, Surf. Coat. Technol. 200 (2006) 5532. [3] R. Rodr´ıguez-Baracaldo, J.A. Benito, E.S. Puchi-Cabrera, M.H. Staia, Wear 262 (2007) 380. [4] S. Veprek, R.F. Zhang, M.G.J. Veprek-Heijman, S.H. Sheng, A.S. Argon, Surf. Coat. Technol. 204 (2010) 1898. [5] A. Horling, L. Hultman, M. Oden, J. Sjolen, L. Karlsson, J. Vac, Sci. Technol. A: Vac. Surf. Films 20 (2002) 1815. [6] R. Kaindl, R. Franz, J. Soldan, A. Reiter, P. Polcik, C. Mitterer, B. Sartory, R. Tessadri, M. O'Sullivan, Thin Solid Films 515 (2006) 2197. [7] H. Hasegawa, M. Kawate, T. Suzuki, Surf. Coat. Technol. 200 (2005) 2409. [8] T.I. Selinder, D.J. Miller, K.E. Gray, M.R. Sardela, L. Hultman, Vacuum 46 (1995) 1401. [9] A.E. Santana, A. Karimi, V. Derflinger, A. Schutze, Surf. Coat. Technol. 177–178 (2004) 334. [10] F. Rovere, D. Music, J.M. Schneider, P.H. Mayrhofer, Acta Mater. 58 (2010) 2708. [11] K. Ichijo, H. Hasegawa, T. Suzuki, Surf. Coat. Technol. 201 (2007) 5477. [12] I. Park, S.R. Choi, M. Lee, K.H. Kim, J. Vac. Sci. Technol. A 21 (2003) 895. [13] E. Martinez, R. Sanjines, O. Banakh, F. Levy, Thin Solid Films 447–448 (2004) 332. [14] P.E. Hovsepian, C. Reinhard, A.P. Ehiasarian, Surf. Coat. Technol. 201 (2006) 4105. [15] F. Rovere, P.H. Mayrhofer, A. Reinholdt, J. Mayer, J.M. Schneider, Surf. Coat. Technol. 202 (2008) 5875. [16] J.L. Endrino, V. Derflinger, Surf. Coat. Technol. 200 (2005) 988. [17] H. Hasegawa, T. Suzuki, Surf. Coat. Technol. 188–189 (2004) 234. [18] S.G. Ebbinghaus, H.P. Abicht, R. Dronskowski, T. Müller, A. Reller, A. Weidenkaff, Prog. Solid State Chem. 37 (2009) 173. [19] J. Sjölén, L. Karlsson, S. Braun, R. Murdey, A. Hörling, L. Hultman, Surf. Coat. Technol. 201 (2007) 6392. [20] J. Ye, S. Ulrich, C. Ziebert, M. Stüber, Thin Solid Films 517 (2008) 1151. [21] M. Hirai, T. Suzuki, H. Suematsu, W. Jiang, K. Yatsui, J. Vac. Sci. Technol. A 21 (2003) 947.
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