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CrN coatings synthesized by a cathodic-arc deposition process
Surface & Coatings Technology 201 (2006) 4209 – 4214 www.elsevier.com/locate/surfcoat
Structural and mechanical properties of AlTiN/CrN coatings synthesized by a cathodic-arc deposition process Yin-Yu Chang ⁎, Shun-Jan Yang, Da-Yung Wang Institute of Materials and Systems Engineering, Mingdao University, Peetow, Changhua 52342, Taiwan, ROC Available online 7 November 2006
Abstract Monolayered AlTiN and Multilayered AlTiN/CrN coatings were synthesized by a cathodic-arc deposition process, using TiAl (with 50/50 and 33/67 at.%) and Cr elemental cathodes. The atomic ratio of Al/(Ti + Al) in the AlTiN coatings was reduced to 0.44 and 0.61, respectively, compared with the corresponding Ti50Al50 and Ti33Al67 cathode materials. The multilayered AlTiN/CrN films showed smaller crystallite size, larger lattice strain, higher hardness, higher residual stress, and better adhesion strength as well than the monolayered AlTi films. The multilayered Al0.35Ti0.22N0.43/CrN coating exhibited the highest hardness of about 38 GPa and the highest H3/E*2 ratio value of 0.188 GPa, indicating the best resistance to plastic deformation, among all the coatings studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Multilayer; Cathodic-arc evaporation; Mechanical properties
1. Introduction Multicomponent AlTiCrN coatings synthesized by physical vapor deposition (PVD) exhibit improved mechanical performance due to its advanced tribological properties and high temperature oxidation resistance [1,2]. Such coatings can be produced by different PVD techniques, among them the cathodic arc ion plating process is well known of high ionization level in the plasma which leads to a dense coating structure [3,4]. The incorporation of aluminum in the cubic fcc TiN structure effectively enhances the coating thermal stability and hardness [5]. And the oxidation resistance of the AlTiN coating for dry cutting applications can be further improved by the incorporation of chromium to form Cr based nitrides which are known for their excellent corrosion and tooling performance [6,7]. This improved high temperature oxidation resistance results in the outperformance of the TiAlCrN coated cutting tools over the AlTiN coated cutting tools [8,9]. In the present study, a cathodic arc ion plating process with TiAl (with 50/50 and 33/67 at.%) and Cr elemental cathodes was used for the deposition of AlTiN, and multilayered AlTiN/CrN coatings. The effect of different coating composition, resulting from different cathode materials used, on the microstructure and ⁎ Corresponding author. 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.062
mechanical properties (hardness, elastic modulus, and adhesion strength) of the as-deposited coatings were studied. 2. Experimental details AlTiN, and multilayered AlTiN/CrN coatings were deposited on polished silicon and SKH-51 tool steels (HRc 62) by using a cathodic arc evaporation (CAE) system. Ar and reactive gas (N2) were introduced through a conducting duct around the target to enhance the reaction of the plasma and reduce the droplet on the deposited coatings. The layout of the deposition system was Table 1 Operating parameters of AlxTi1−x/CrN, and multilayered AlxTi1−x/CrN coatings Parameters
Values
CAE target
Cr, Ti50Al50, Ti33Al67 (100 mm in diameter) 80 180 1.0 × 10− 3 3.0 (N2) 40 − 1000 − 100 250–300 2
Cathode current (A) Distance from cathode to substrate (mm) Base pressure (Pa) Reactive gas pressure (Pa) Deposition time (min) Bias voltage at ion cleaning stage (V) Bias voltage at coating stage (V) Substrate temperature (°C) Rotational speed of the substrate (rpm)
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revealed in our previous study [10]. Chromium and Ti/Al (50/50 at.%) or Ti/Al (33/67 at.%) cathodes were arranged on opposite sides of the chamber to deposit the multilayered AlTiN/CrN coatings. Optical emission spectra of the plasma in front of the TiAl and Cr cathodes were recorded in the range from 200 nm to 500 nm using an in-situ optical emission spectrometer (OES, SD2048DL Spectrograph) equipped with a SpectraView application software Version 2.44. A fiber-optic cable, positioned at a distance of 80 mm from the cathode surface, was used for light collection. The samples were mounted on the rotational substrate
Fig. 2. X-Ray diffraction spectra of the as-deposited monolayered and multilayered Al0.26Ti0.33N0.41/CrN coatings (a), monolayered and multilayered Al0.35Ti0.22N0.43/CrN coatings (b), and the (200) diffraction peaks of multilayered AlTiN/CrN coatings(c).
Fig. 1. Optical emission spectra of the cathodic arc plasma obtained in front of the (a) Ti50Al50, (b) Ti33Al67 and (c) Cr cathodes ranging from 260 to 460 nm during the deposition of the multilayered AlTiN/CrN coating.
holder for the deposition of the AlTiN and multilayered AlTiN/ CrN coatings. The experimental parameters of the deposition process are shown in Table 1. The monolayered AlTiN and multilayered AlTiN/CrN coatings were deposited at a N2 pressure of 3.0 Pa and substrate bias voltage of −100 V. For the deposition of multilayered AlTiN/CrN coatings, TiAl and Cr was deposited as an interlayer, and the rotational speed of the substrate holder was controlled at 2 rpm. The monolayered AlTiN coatings were deposited with a TiAl interlayer. Cross-sectional structures of the as-deposited multilayered AlTiN/CrN coatings were examined in a Joel JSM-6700 F high resolution field emission scanning electron microscope equipped
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Fig. 3. Cross-sectional secondary and backscattered SEM micrographs showing the structure of the deposited monolayered Al0.26Ti0.33N0.41 (a), multilayered Al0.26Ti0.33N0.41/CrN (b), monolayered Al0.35Ti0.22N0.43 (c) and multilayered Al0.35Ti0.22N0.43/CrN coatings (d).
with secondary electron imaging (SEI) and backscattered electron imaging (BEI) detectors. Chemical composition of the deposited films were identified by using an X-ray photoelectron spectro-
scope (PHI1600 XPS) with monochromatized Mg Kα radiation. Glancing angle X-ray diffractometer (PANalytical X'pert Pro) with a high resolution ψ goniometer and Cu Kα radiation was
Table 2 Crystallite size, lattice strain, hardness, elastic modulus, H3/E*2, residual stress, and adhesion of the monolayered AlTiN and multilayered AlTiN/CrN coatings deposited on SKH-51 tool steels Coatings
Al0.26Ti0.33N0.41
Al0.35Ti0.22N0.43
Al0.26Ti0.33N0.41/CrN
Al0.35Ti0.22N0.43/CrN
Crystallite size (nm) Lattice strain (nm) Elastic modulus (GPa) Hardness (GPa) H3/E*2 (GPa) Residual stress (GPa) Adhesion
42 4.4 × 10− 3 510 ± 30 31 ± 2 0.114 ± 0.009 −6.9 ± 0.3 HF2
32 3.9 × 10− 3 560 ± 40 35 ± 2 0.137 ± 0.004 − 9.8 ± 0.4 HF3
31 5.7 × 10− 3 520 ± 35 36 ± 2 0.173 ± 0.006 − 11.3 ± 0.4 HF1
25 5.5 × 10− 3 540 ± 40 38 ± 2 0.188 ± 0.002 − 12.1 ± 0.4 HF1
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employed for phase identification and residual stress analysis. According to Williamson–Hall plot method [11–13], contribution of crystallite size and lattice strain to the diffraction line broadening can be calculated. The residual stresses of the deposited coatings were calculated by the sin2ψ method [14]. Adhesion of the deposited monolayered AlTiN and multilayered AlTiN/CrN coatings on SKH-51 tool steel substrates was evaluated using a Rockwell indentation tester according to VDI 3198 standard. Hardness and Young's modulus of the films were obtained using XP-MTS nanoindentation with a Berkovich indenter, under load–unloading condition, and measured as a function of indenter displacement using continuous stiffness measurement method. The maximum penetration depth is controlled at 150 nm, therefore, the influence of substrate on the measured hardness is negligible. 3. Results and discussion 3.1. Emission spectra of the cathodic arc plasma during the AlTiN/CrN deposition process Representative Ti–Al–N2 and Cr–N2 emission spectra obtained from the AlTiN/CrN deposition process at arc current of 80 A and bias voltage of − 100 V are shown in Fig. 1. During the whole deposition process, titanium and chromium appeared in both the ionic and excited atomic states, and nitrogen also existed in both the ionic and excited molecular (N2+ and N2) states, but aluminum only appeared as excited neutral atom. The intensity of titanium and aluminum atomic emission lines revealed the regularity of the chemical composition in the AlTiN [15–17]. Periodic AlTiN and CrN were deposited on the substrate depending on the substrate rotation during the multilayered AlTiN/CrN coating process. 3.2. Microstructure analysis From the XPS analyses, the composition of the CrN coating deposited from the Cr cathode materials was 58.3 at.% of Cr, 40.6 at.% of N, and 1.1 at.% of oxygen. The composition of the Al0.26Ti0.33N0.41 coating deposited from the Ti50Al50 alloy cathode was 32.5 at.% of Ti, 25.9 at.% of Al, 40.1 at.% of N, and 1.5 at.% of oxygen. The composition of the Al0.35Ti0.22N0.43 coating deposited from the Ti33Al67 alloy was 22.2 at.% of Ti, 34.6 at.% of Al, 42.0 at.% of N, and 1.2 at.% of oxygen. An atomic ratio of Al/(Ti + Al) in the AlTiN film was reduced to 0.44 and 0.61 compared with the Ti50Al50 and Ti33Al67 cathode material, respectively. It can be related to the re-sputtering effect on the deposited coatings and the lower atomic mass of Al that suffers higher scattering in the collisions with nitrogen, and leads to a lower volume density in the vapor [18,19]. Typical glancing angle X-ray diffraction spectra from the monolayered AlTiN and multilayered AlTiN/CrN coatings are shown in Fig. 2. All of the four coatings crystallized in the B1NaCl structure and no softer hcp-AlTiN phase was found [9]. Except for the monolayered Al0.26Ti0.33N0.41 displaying a preferred (200) orientation parallel to the substrate surface, all the other three coatings showed a preferred (111) orientation. This
indicated that the Al/(Ti + Al) content ratio of the film would influence the texture development in the AlTiN coatings deposited from different TiAl alloy cathode materials. As shown in Fig. 2(c), the (200) peak position shift toward higher Bragg angles as the Al content increases, indicating a contraction of lattice because of the smaller atomic radius of Al (0.143 nm) than that of Ti (0.146 nm). The peak positions represented the weighted mean of the individual reflections from the AlTiN and CrN phases. The (200) diffraction peak was asymmetric because of the presence of AlTiN and CrN components. Cross-sectional secondary and backscattered SEM micrographs of the monolayered AlTiN and multilayered AlTiN/CrN coatings are shown in Fig. 3. The film thickness was 1.5 μm, and the deposition rate was about 0.04 μm/min. The deposited monolayered AlTiN coatings had a dense columnar structure typical of the zone T according to the zone classification proposed by Thornton [20]. A stepwise columnar structure was found in the multilayered AlTiN/CrN coatings. Cross-sectional SEM micrograph of the multilayer obtained with backscattered electron detector emphasized the dissimilarity between AlTiN and CrN, as shown in Fig 3 (d). CrN produced more backscattered electrons, and was displayed as layers with brighter contrast. The backscattered electron image revealed the periodic thickness of 20 nm in the multilayered Al0.35Ti0.22N0.43/CrN coatings deposited at 2 rpm. The crystallite size and lattice strain of the deposited films are shown in Table 2. The crystallite size of monolayered Al0.26Ti0.33N0.41 is 42 nm. Replacement of Ti atoms with smaller Al atoms in TiN may affect the crystallite size. The Al0.35Ti0.22N0.43 coating with higher Al content possesses smaller crystallite size of 32 nm. The incorporation of Al atoms in TiN lattice leads to a decrease in crystallite size, showing a higher density of grain boundaries [21]. With the multiple cathode arrangement, the multilayered AlTiN/CrN films had smaller crystallite size than the monolayered AlTiN films. The smaller crystallite size was controlled by the interface phenomena [22]. The multilayered Al0.35Ti0.22N0.43/CrN coating possessed the smallest crystallite size of 25 nm among the deposited coatings. Substrate rotation during the deposition process was used to control the time that the substrate is under ion bombardment from different cathodes (Cr and TiAl). In this manner nanolayers can be obtained with different mechanical properties, which may enhance the film hardness. The value of lattice strain was larger in multilayered AlTiN/CrN coatings (5.5 × 10− 3 nm–5.7 × 10− 3 nm) than in monolayered AlTiN coatings (3.9 × 10− 3 nm–4.4 × 10− 3 nm). It indicates a larger structural anisotropy at the atomic level in AlTiN/CrN multilayered layers, probably connected with the nano-scale multilayered columnar texture. 3.3. Mechanical properties Hardness and Elastic modulus of the monolayered and multilayered AlTiN/CrN coatings on SKH-51 tool steels are also shown in Table 2. The hardness of the Al0.26Ti0.33N0.41 coating is 31 ± 2 GPa. The Al0.35Ti0.22N0.43 with higher Al content possesses a higher hardness of 35 ± 2 GPa due to the
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solid solution hardening effect and the effect of grain boundary hardening by smaller crystallite size [21]. The multilayered AlTiN/CrN coatings exhibited a higher hardness of 36–38 ± 2 GPa. The multilayered Al0.35Ti0.22N0.43/CrN coating with higher Al content possessed the highest hardness of 38 ± 2 GPa among the deposited coatings. The presence of the nanolayered structure of the AlTiN/CrN coatings can increase the hardness based on the dislocation blocking by layer interfaces and Hall– Petch effect [22–26]. In order to increase the resistance to plastic deformation, it is desirable to obtain materials that possess high hardness but lower elastic modulus values. With higher hardness and lower elastic modulus, the plastic deformation is lower, because the external load is distributed over a larger area and the stress distribution is more uniform. This behavior is then expressed by the H3/E*2 ratio, where H and E* are the hardness and effective modulus of the coating. E* could be further expressed as E* = E / (1 − v2), where E is the Young's modulus, and v is the Poisson ratio (∼ 0.25) [27–29]. As also shown in Table 2, it is noted that the multilayered AlTiN/CrN coatings exhibited higher H3/E*2 ratio than that of monolayered AlTiN coatings. The H3/E*2 ratio showed the highest value of 0.188 ± 0.002 GPa for the multilayered Al0.35Ti0.22N0.43/CrN coating among the studied coatings. The H3/E*2 ratio was enhanced by the interface contribution of the multilayered AlTiN/CrN. This provides highest resistance to plastic deformation of the multilayered Al0.35Ti0.22N0.43/CrN with periodic thickness of 20 nm. The residual stresses of the monolayered AlTiN and multilayered AlTiN/CrN coatings, which were determined by the sin2ψ method and the measured elastic modulus, are also revealed in Table 2. The difference in magnitude was related to the residual stress which composed of an intrinsic component resulted from the growth process and an extrinsic component (thermal stress) due to the presence of different thermal expansion coefficients between the coating and the substrate. The residual stress of the monolayered Al0.26Ti0.33N0.41 and Al0.35Ti0.22N0.43 coatings is −6.9 ± 0.3 GPa and −9.8 ± 0.4 GPa, respectively. The incorporated Al atoms may serve as centers of structure contraction, resulting in a rise of compressive stress field and coating hardening effect as well. The higher compressive residual stress was found in the multilayered AlTiN/CrN coating with higher hardness. The multilayered Al0.35Ti0.22N0.43/CrN coating possessed the highest residual stress (−12.1 ± 0.4 GPa). As revealed in Table 2, the result of Rockwell indentation tests showed the monolayered Al0.26Ti0.33N0.41 and Al0.35Ti0.22N0.43 coatings possessed lower adhesion strength of HF2 and HF3 fracture modes, respectively. The multilayered Al0.35Ti0.22N0.43/CrN showed a criteria HF1 fracture mode, and demonstrated the excellent film adhesion of the multilayered AlTiN/CrN nanocrystalline coatings. Higher adhesion strength can be obtained by the deposition of CrN and multilayered AlTiN/CrN coatings synthesized by the cathodic arc deposition process which was used in this study. 4. Conclusions 1. In this study, multilayered AlTiN/CrN and monolayered AlTiN coatings were synthesized by cathodic-arc evapora-
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tion. Optical emission spectra revealed that titanium and chromium appeared in both ionic and excited atomic states, and nitrogen also existed in both the ionic and excited molecular (N2+ and N2) states, but aluminum only appeared as excited neutral atom. 2. All of the monolayered AlTiN and multilayered AlTiN/CrN coatings exhibited the B1-NaCl crystal structure. Except for the Al0.26Ti0.33N0.41 coatings that showed a preferred (200) orientation parallel to the substrate surface, all the other three coatings showed a (111) preferential orientation. The multilayered Al0.35Ti0.22N0.43/CrN coating showed the smallest crystallite size among all the coatings studied in this work. 3. The multilayered AlTiN/CrN coatings possessed higher hardness of 36–38 ± 2 GPa than that of the monolayered AlTiN coatings (31–35 GPa). The nanolayered structure of the AlTiN/CrN coatings is responsible for the hardness enhancement due to the smaller crystallite size based on the Hall–Petch effect. The highest H3/E*2 ratio of 0.188 ± 0.002 GPa, indicating the best resistance to plastic deformation and the best adhesion strength were also obtained in the Al0.35Ti0.22N0.43/CrN multilayered coating. Acknowledgements The Authors wish to thank Mr. Shein-Chen Liu from Surftech Corp. for generously providing the CAE deposition system to accomplish all the experiments. In addition, the authors would like to thank Mr. Yu-Fong Lu in National Chung-Hsing University for providing FESEM analysis. The funding in part from the National Science Council of Taiwan under the contract NSC94-2622-E-451-002-CC3 is sincerely appreciated. References [1] S.G. Harris, E.D. Doyle, A.C. Vlasveld, J. Audy, D. Quick, Wear 254 (2003) 723. [2] S. Paldey, S.C. Deevi, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 342 (2003) 58. [3] W.D. Münz, L.A. Donohue, P. Eh. Hovsepian, Surf. Coat. Technol. 125 (2000) 269. [4] F. Weber, F. Fontaine, M. Scheib, W. Bock, Surf. Coat. Technol. 177–178 (2004) 227. [5] H.G. Prengel, A.T. Santhanam, R.M. Penich, P.C. Jindal, K.H. Wendt, Surf. Coat. Technol. 94–95 (1997) 597. [6] I. Wadsworth, I.J. Smith, L.A. Donohue, W.D. Münz, Surf. Coat. Technol. 94–95 (1997) 315. [7] M.I. Lembke, D.B. Lewis, W.D. Münz, Surf. Coat. Technol. 125 (2000) 263. [8] K. Yamamoto, T. Sato, K. Takahara, K. Hanaguri, Surf. Coat. Technol. 174–175 (2003) 620. [9] A.E. Santana, A. Karimi, V.H. Derflinger, A. Schutze, Surf. Coat. Technol. 177–178 (2004) 334. [10] Y.Y. Chang, D.Y. Wang, C.Y. Hung, Surf. Coat. Technol. 200 (2005) 1702. [11] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [12] V. Valvoda, R. Kužel Jr., R. Černý, Thin Solid Films 156 (1988) 53. [13] B.S. Yau, J.L. Huang, D.F. Lii, P. Sajgalik, Surf. Coat. Technol. 177–178 (2004) 209. [14] C.H. Ma, J.H. Huang, Haydn Chen, Thin Solid Films 418 (2002) 73. [15] A. Raveh, M. Weiss, R. Shneck, Surf. Coat. Technol. 111 (1999) 263. [16] R.L. Boxman, D.M. Sanders, P.J. Martin, J.M. Lafferty, Handbook of Vacuum Arc Science and Technology, Noyes, New Jersey, 1995. [17] J. Bujak, J. Walkowicz, J. Kusinski, Surf. Coat. Technol. 180–181 (2004) 150.
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[18] R. Wuhrer, W.Y. Yeung, Scr. Mater. 49 (2003) 199. [19] J.M. Lackner, W. Waldhauser, R. Ebner, R.J. Bakker, Appl. Phys., A 79 (2004) 1469. [20] J.A. Thornton, Annu. Rev. Mater. Sci. 7 (1977) 239. [21] Z.J. Liu, P.W. Shum, Y.G. Shen, Thin Solid Films 468 (2004) 161. [22] P.C. Yashar, W.D. Sproul, Vacuum 55 (1999) 179. [23] S.J. Bull, A.M. Jones, Surf. Coat. Technol. 78 (1996) 173. [24] X. Chu, Scott A. Barnett, J. Appl. Phys. 77 (1995) 4403.
[25] M. Shinn, S.A. Barnett, Appl. Phys. Lett. 64 (1994) 61. [26] H. Holleck, V. Schier, Surf. Coat. Technol. 76–77 (1995) 328. [27] T.Y. Tsui, G.M. Pharr, W.C. Oliver, C.S. Bhatia, R.L. White, S. Anders, A. Anders, I.G. Brown, Mater. Res. Soc. Symp. Proc. 383 (1995) 447. [28] J. Musil, F. Kunc, H. Zeman, H. Polakova, Surf. Coat. Technol. 154 (2002) 304–313. [29] A. Leyland, A. Matthews, Wear 246 (2000) 1.
Structural and mechanical properties of AlTiN/CrN coatings synthesized by a cathodic-arc deposition process Yin-Yu Chang ⁎, Shun-Jan Yang, Da-Yung Wang Institute of Materials and Systems Engineering, Mingdao University, Peetow, Changhua 52342, Taiwan, ROC Available online 7 November 2006
Abstract Monolayered AlTiN and Multilayered AlTiN/CrN coatings were synthesized by a cathodic-arc deposition process, using TiAl (with 50/50 and 33/67 at.%) and Cr elemental cathodes. The atomic ratio of Al/(Ti + Al) in the AlTiN coatings was reduced to 0.44 and 0.61, respectively, compared with the corresponding Ti50Al50 and Ti33Al67 cathode materials. The multilayered AlTiN/CrN films showed smaller crystallite size, larger lattice strain, higher hardness, higher residual stress, and better adhesion strength as well than the monolayered AlTi films. The multilayered Al0.35Ti0.22N0.43/CrN coating exhibited the highest hardness of about 38 GPa and the highest H3/E*2 ratio value of 0.188 GPa, indicating the best resistance to plastic deformation, among all the coatings studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Multilayer; Cathodic-arc evaporation; Mechanical properties
1. Introduction Multicomponent AlTiCrN coatings synthesized by physical vapor deposition (PVD) exhibit improved mechanical performance due to its advanced tribological properties and high temperature oxidation resistance [1,2]. Such coatings can be produced by different PVD techniques, among them the cathodic arc ion plating process is well known of high ionization level in the plasma which leads to a dense coating structure [3,4]. The incorporation of aluminum in the cubic fcc TiN structure effectively enhances the coating thermal stability and hardness [5]. And the oxidation resistance of the AlTiN coating for dry cutting applications can be further improved by the incorporation of chromium to form Cr based nitrides which are known for their excellent corrosion and tooling performance [6,7]. This improved high temperature oxidation resistance results in the outperformance of the TiAlCrN coated cutting tools over the AlTiN coated cutting tools [8,9]. In the present study, a cathodic arc ion plating process with TiAl (with 50/50 and 33/67 at.%) and Cr elemental cathodes was used for the deposition of AlTiN, and multilayered AlTiN/CrN coatings. The effect of different coating composition, resulting from different cathode materials used, on the microstructure and ⁎ Corresponding author. 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.062
mechanical properties (hardness, elastic modulus, and adhesion strength) of the as-deposited coatings were studied. 2. Experimental details AlTiN, and multilayered AlTiN/CrN coatings were deposited on polished silicon and SKH-51 tool steels (HRc 62) by using a cathodic arc evaporation (CAE) system. Ar and reactive gas (N2) were introduced through a conducting duct around the target to enhance the reaction of the plasma and reduce the droplet on the deposited coatings. The layout of the deposition system was Table 1 Operating parameters of AlxTi1−x/CrN, and multilayered AlxTi1−x/CrN coatings Parameters
Values
CAE target
Cr, Ti50Al50, Ti33Al67 (100 mm in diameter) 80 180 1.0 × 10− 3 3.0 (N2) 40 − 1000 − 100 250–300 2
Cathode current (A) Distance from cathode to substrate (mm) Base pressure (Pa) Reactive gas pressure (Pa) Deposition time (min) Bias voltage at ion cleaning stage (V) Bias voltage at coating stage (V) Substrate temperature (°C) Rotational speed of the substrate (rpm)
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revealed in our previous study [10]. Chromium and Ti/Al (50/50 at.%) or Ti/Al (33/67 at.%) cathodes were arranged on opposite sides of the chamber to deposit the multilayered AlTiN/CrN coatings. Optical emission spectra of the plasma in front of the TiAl and Cr cathodes were recorded in the range from 200 nm to 500 nm using an in-situ optical emission spectrometer (OES, SD2048DL Spectrograph) equipped with a SpectraView application software Version 2.44. A fiber-optic cable, positioned at a distance of 80 mm from the cathode surface, was used for light collection. The samples were mounted on the rotational substrate
Fig. 2. X-Ray diffraction spectra of the as-deposited monolayered and multilayered Al0.26Ti0.33N0.41/CrN coatings (a), monolayered and multilayered Al0.35Ti0.22N0.43/CrN coatings (b), and the (200) diffraction peaks of multilayered AlTiN/CrN coatings(c).
Fig. 1. Optical emission spectra of the cathodic arc plasma obtained in front of the (a) Ti50Al50, (b) Ti33Al67 and (c) Cr cathodes ranging from 260 to 460 nm during the deposition of the multilayered AlTiN/CrN coating.
holder for the deposition of the AlTiN and multilayered AlTiN/ CrN coatings. The experimental parameters of the deposition process are shown in Table 1. The monolayered AlTiN and multilayered AlTiN/CrN coatings were deposited at a N2 pressure of 3.0 Pa and substrate bias voltage of −100 V. For the deposition of multilayered AlTiN/CrN coatings, TiAl and Cr was deposited as an interlayer, and the rotational speed of the substrate holder was controlled at 2 rpm. The monolayered AlTiN coatings were deposited with a TiAl interlayer. Cross-sectional structures of the as-deposited multilayered AlTiN/CrN coatings were examined in a Joel JSM-6700 F high resolution field emission scanning electron microscope equipped
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Fig. 3. Cross-sectional secondary and backscattered SEM micrographs showing the structure of the deposited monolayered Al0.26Ti0.33N0.41 (a), multilayered Al0.26Ti0.33N0.41/CrN (b), monolayered Al0.35Ti0.22N0.43 (c) and multilayered Al0.35Ti0.22N0.43/CrN coatings (d).
with secondary electron imaging (SEI) and backscattered electron imaging (BEI) detectors. Chemical composition of the deposited films were identified by using an X-ray photoelectron spectro-
scope (PHI1600 XPS) with monochromatized Mg Kα radiation. Glancing angle X-ray diffractometer (PANalytical X'pert Pro) with a high resolution ψ goniometer and Cu Kα radiation was
Table 2 Crystallite size, lattice strain, hardness, elastic modulus, H3/E*2, residual stress, and adhesion of the monolayered AlTiN and multilayered AlTiN/CrN coatings deposited on SKH-51 tool steels Coatings
Al0.26Ti0.33N0.41
Al0.35Ti0.22N0.43
Al0.26Ti0.33N0.41/CrN
Al0.35Ti0.22N0.43/CrN
Crystallite size (nm) Lattice strain (nm) Elastic modulus (GPa) Hardness (GPa) H3/E*2 (GPa) Residual stress (GPa) Adhesion
42 4.4 × 10− 3 510 ± 30 31 ± 2 0.114 ± 0.009 −6.9 ± 0.3 HF2
32 3.9 × 10− 3 560 ± 40 35 ± 2 0.137 ± 0.004 − 9.8 ± 0.4 HF3
31 5.7 × 10− 3 520 ± 35 36 ± 2 0.173 ± 0.006 − 11.3 ± 0.4 HF1
25 5.5 × 10− 3 540 ± 40 38 ± 2 0.188 ± 0.002 − 12.1 ± 0.4 HF1
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employed for phase identification and residual stress analysis. According to Williamson–Hall plot method [11–13], contribution of crystallite size and lattice strain to the diffraction line broadening can be calculated. The residual stresses of the deposited coatings were calculated by the sin2ψ method [14]. Adhesion of the deposited monolayered AlTiN and multilayered AlTiN/CrN coatings on SKH-51 tool steel substrates was evaluated using a Rockwell indentation tester according to VDI 3198 standard. Hardness and Young's modulus of the films were obtained using XP-MTS nanoindentation with a Berkovich indenter, under load–unloading condition, and measured as a function of indenter displacement using continuous stiffness measurement method. The maximum penetration depth is controlled at 150 nm, therefore, the influence of substrate on the measured hardness is negligible. 3. Results and discussion 3.1. Emission spectra of the cathodic arc plasma during the AlTiN/CrN deposition process Representative Ti–Al–N2 and Cr–N2 emission spectra obtained from the AlTiN/CrN deposition process at arc current of 80 A and bias voltage of − 100 V are shown in Fig. 1. During the whole deposition process, titanium and chromium appeared in both the ionic and excited atomic states, and nitrogen also existed in both the ionic and excited molecular (N2+ and N2) states, but aluminum only appeared as excited neutral atom. The intensity of titanium and aluminum atomic emission lines revealed the regularity of the chemical composition in the AlTiN [15–17]. Periodic AlTiN and CrN were deposited on the substrate depending on the substrate rotation during the multilayered AlTiN/CrN coating process. 3.2. Microstructure analysis From the XPS analyses, the composition of the CrN coating deposited from the Cr cathode materials was 58.3 at.% of Cr, 40.6 at.% of N, and 1.1 at.% of oxygen. The composition of the Al0.26Ti0.33N0.41 coating deposited from the Ti50Al50 alloy cathode was 32.5 at.% of Ti, 25.9 at.% of Al, 40.1 at.% of N, and 1.5 at.% of oxygen. The composition of the Al0.35Ti0.22N0.43 coating deposited from the Ti33Al67 alloy was 22.2 at.% of Ti, 34.6 at.% of Al, 42.0 at.% of N, and 1.2 at.% of oxygen. An atomic ratio of Al/(Ti + Al) in the AlTiN film was reduced to 0.44 and 0.61 compared with the Ti50Al50 and Ti33Al67 cathode material, respectively. It can be related to the re-sputtering effect on the deposited coatings and the lower atomic mass of Al that suffers higher scattering in the collisions with nitrogen, and leads to a lower volume density in the vapor [18,19]. Typical glancing angle X-ray diffraction spectra from the monolayered AlTiN and multilayered AlTiN/CrN coatings are shown in Fig. 2. All of the four coatings crystallized in the B1NaCl structure and no softer hcp-AlTiN phase was found [9]. Except for the monolayered Al0.26Ti0.33N0.41 displaying a preferred (200) orientation parallel to the substrate surface, all the other three coatings showed a preferred (111) orientation. This
indicated that the Al/(Ti + Al) content ratio of the film would influence the texture development in the AlTiN coatings deposited from different TiAl alloy cathode materials. As shown in Fig. 2(c), the (200) peak position shift toward higher Bragg angles as the Al content increases, indicating a contraction of lattice because of the smaller atomic radius of Al (0.143 nm) than that of Ti (0.146 nm). The peak positions represented the weighted mean of the individual reflections from the AlTiN and CrN phases. The (200) diffraction peak was asymmetric because of the presence of AlTiN and CrN components. Cross-sectional secondary and backscattered SEM micrographs of the monolayered AlTiN and multilayered AlTiN/CrN coatings are shown in Fig. 3. The film thickness was 1.5 μm, and the deposition rate was about 0.04 μm/min. The deposited monolayered AlTiN coatings had a dense columnar structure typical of the zone T according to the zone classification proposed by Thornton [20]. A stepwise columnar structure was found in the multilayered AlTiN/CrN coatings. Cross-sectional SEM micrograph of the multilayer obtained with backscattered electron detector emphasized the dissimilarity between AlTiN and CrN, as shown in Fig 3 (d). CrN produced more backscattered electrons, and was displayed as layers with brighter contrast. The backscattered electron image revealed the periodic thickness of 20 nm in the multilayered Al0.35Ti0.22N0.43/CrN coatings deposited at 2 rpm. The crystallite size and lattice strain of the deposited films are shown in Table 2. The crystallite size of monolayered Al0.26Ti0.33N0.41 is 42 nm. Replacement of Ti atoms with smaller Al atoms in TiN may affect the crystallite size. The Al0.35Ti0.22N0.43 coating with higher Al content possesses smaller crystallite size of 32 nm. The incorporation of Al atoms in TiN lattice leads to a decrease in crystallite size, showing a higher density of grain boundaries [21]. With the multiple cathode arrangement, the multilayered AlTiN/CrN films had smaller crystallite size than the monolayered AlTiN films. The smaller crystallite size was controlled by the interface phenomena [22]. The multilayered Al0.35Ti0.22N0.43/CrN coating possessed the smallest crystallite size of 25 nm among the deposited coatings. Substrate rotation during the deposition process was used to control the time that the substrate is under ion bombardment from different cathodes (Cr and TiAl). In this manner nanolayers can be obtained with different mechanical properties, which may enhance the film hardness. The value of lattice strain was larger in multilayered AlTiN/CrN coatings (5.5 × 10− 3 nm–5.7 × 10− 3 nm) than in monolayered AlTiN coatings (3.9 × 10− 3 nm–4.4 × 10− 3 nm). It indicates a larger structural anisotropy at the atomic level in AlTiN/CrN multilayered layers, probably connected with the nano-scale multilayered columnar texture. 3.3. Mechanical properties Hardness and Elastic modulus of the monolayered and multilayered AlTiN/CrN coatings on SKH-51 tool steels are also shown in Table 2. The hardness of the Al0.26Ti0.33N0.41 coating is 31 ± 2 GPa. The Al0.35Ti0.22N0.43 with higher Al content possesses a higher hardness of 35 ± 2 GPa due to the
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solid solution hardening effect and the effect of grain boundary hardening by smaller crystallite size [21]. The multilayered AlTiN/CrN coatings exhibited a higher hardness of 36–38 ± 2 GPa. The multilayered Al0.35Ti0.22N0.43/CrN coating with higher Al content possessed the highest hardness of 38 ± 2 GPa among the deposited coatings. The presence of the nanolayered structure of the AlTiN/CrN coatings can increase the hardness based on the dislocation blocking by layer interfaces and Hall– Petch effect [22–26]. In order to increase the resistance to plastic deformation, it is desirable to obtain materials that possess high hardness but lower elastic modulus values. With higher hardness and lower elastic modulus, the plastic deformation is lower, because the external load is distributed over a larger area and the stress distribution is more uniform. This behavior is then expressed by the H3/E*2 ratio, where H and E* are the hardness and effective modulus of the coating. E* could be further expressed as E* = E / (1 − v2), where E is the Young's modulus, and v is the Poisson ratio (∼ 0.25) [27–29]. As also shown in Table 2, it is noted that the multilayered AlTiN/CrN coatings exhibited higher H3/E*2 ratio than that of monolayered AlTiN coatings. The H3/E*2 ratio showed the highest value of 0.188 ± 0.002 GPa for the multilayered Al0.35Ti0.22N0.43/CrN coating among the studied coatings. The H3/E*2 ratio was enhanced by the interface contribution of the multilayered AlTiN/CrN. This provides highest resistance to plastic deformation of the multilayered Al0.35Ti0.22N0.43/CrN with periodic thickness of 20 nm. The residual stresses of the monolayered AlTiN and multilayered AlTiN/CrN coatings, which were determined by the sin2ψ method and the measured elastic modulus, are also revealed in Table 2. The difference in magnitude was related to the residual stress which composed of an intrinsic component resulted from the growth process and an extrinsic component (thermal stress) due to the presence of different thermal expansion coefficients between the coating and the substrate. The residual stress of the monolayered Al0.26Ti0.33N0.41 and Al0.35Ti0.22N0.43 coatings is −6.9 ± 0.3 GPa and −9.8 ± 0.4 GPa, respectively. The incorporated Al atoms may serve as centers of structure contraction, resulting in a rise of compressive stress field and coating hardening effect as well. The higher compressive residual stress was found in the multilayered AlTiN/CrN coating with higher hardness. The multilayered Al0.35Ti0.22N0.43/CrN coating possessed the highest residual stress (−12.1 ± 0.4 GPa). As revealed in Table 2, the result of Rockwell indentation tests showed the monolayered Al0.26Ti0.33N0.41 and Al0.35Ti0.22N0.43 coatings possessed lower adhesion strength of HF2 and HF3 fracture modes, respectively. The multilayered Al0.35Ti0.22N0.43/CrN showed a criteria HF1 fracture mode, and demonstrated the excellent film adhesion of the multilayered AlTiN/CrN nanocrystalline coatings. Higher adhesion strength can be obtained by the deposition of CrN and multilayered AlTiN/CrN coatings synthesized by the cathodic arc deposition process which was used in this study. 4. Conclusions 1. In this study, multilayered AlTiN/CrN and monolayered AlTiN coatings were synthesized by cathodic-arc evapora-
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tion. Optical emission spectra revealed that titanium and chromium appeared in both ionic and excited atomic states, and nitrogen also existed in both the ionic and excited molecular (N2+ and N2) states, but aluminum only appeared as excited neutral atom. 2. All of the monolayered AlTiN and multilayered AlTiN/CrN coatings exhibited the B1-NaCl crystal structure. Except for the Al0.26Ti0.33N0.41 coatings that showed a preferred (200) orientation parallel to the substrate surface, all the other three coatings showed a (111) preferential orientation. The multilayered Al0.35Ti0.22N0.43/CrN coating showed the smallest crystallite size among all the coatings studied in this work. 3. The multilayered AlTiN/CrN coatings possessed higher hardness of 36–38 ± 2 GPa than that of the monolayered AlTiN coatings (31–35 GPa). The nanolayered structure of the AlTiN/CrN coatings is responsible for the hardness enhancement due to the smaller crystallite size based on the Hall–Petch effect. The highest H3/E*2 ratio of 0.188 ± 0.002 GPa, indicating the best resistance to plastic deformation and the best adhesion strength were also obtained in the Al0.35Ti0.22N0.43/CrN multilayered coating. Acknowledgements The Authors wish to thank Mr. Shein-Chen Liu from Surftech Corp. for generously providing the CAE deposition system to accomplish all the experiments. In addition, the authors would like to thank Mr. Yu-Fong Lu in National Chung-Hsing University for providing FESEM analysis. The funding in part from the National Science Council of Taiwan under the contract NSC94-2622-E-451-002-CC3 is sincerely appreciated. References [1] S.G. Harris, E.D. Doyle, A.C. Vlasveld, J. Audy, D. Quick, Wear 254 (2003) 723. [2] S. Paldey, S.C. Deevi, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 342 (2003) 58. [3] W.D. Münz, L.A. Donohue, P. Eh. Hovsepian, Surf. Coat. Technol. 125 (2000) 269. [4] F. Weber, F. Fontaine, M. Scheib, W. Bock, Surf. Coat. Technol. 177–178 (2004) 227. [5] H.G. Prengel, A.T. Santhanam, R.M. Penich, P.C. Jindal, K.H. Wendt, Surf. Coat. Technol. 94–95 (1997) 597. [6] I. Wadsworth, I.J. Smith, L.A. Donohue, W.D. Münz, Surf. Coat. Technol. 94–95 (1997) 315. [7] M.I. Lembke, D.B. Lewis, W.D. Münz, Surf. Coat. Technol. 125 (2000) 263. [8] K. Yamamoto, T. Sato, K. Takahara, K. Hanaguri, Surf. Coat. Technol. 174–175 (2003) 620. [9] A.E. Santana, A. Karimi, V.H. Derflinger, A. Schutze, Surf. Coat. Technol. 177–178 (2004) 334. [10] Y.Y. Chang, D.Y. Wang, C.Y. Hung, Surf. Coat. Technol. 200 (2005) 1702. [11] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [12] V. Valvoda, R. Kužel Jr., R. Černý, Thin Solid Films 156 (1988) 53. [13] B.S. Yau, J.L. Huang, D.F. Lii, P. Sajgalik, Surf. Coat. Technol. 177–178 (2004) 209. [14] C.H. Ma, J.H. Huang, Haydn Chen, Thin Solid Films 418 (2002) 73. [15] A. Raveh, M. Weiss, R. Shneck, Surf. Coat. Technol. 111 (1999) 263. [16] R.L. Boxman, D.M. Sanders, P.J. Martin, J.M. Lafferty, Handbook of Vacuum Arc Science and Technology, Noyes, New Jersey, 1995. [17] J. Bujak, J. Walkowicz, J. Kusinski, Surf. Coat. Technol. 180–181 (2004) 150.
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