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Syntheses and mechanical properties of Cr–Mo–N coatings by a hybrid coating system
Surface & Coatings Technology 201 (2006) 4068 – 4072 www.elsevier.com/locate/surfcoat
Syntheses and mechanical properties of Cr–Mo–N coatings by a hybrid coating system Kwang Ho Kim a,⁎, Eun Young Choi a , Seung Gyun Hong a , Bong Gyu Park b , Jae Hong Yoon b , Jeong Hae Yong c b
a Division of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea Department of Nano and Advaced Materials Engineering, 9 Sarim-dong, Changwon, Kyungnam 641-773, South Korea c Department of Materials Science and Engineering, Pukyong National University, Busan 608-739, South Korea
Available online 20 September 2006
Abstract Effects of Mo content up to 30.4 at.% on the microstructure and mechanical properties of CrN coatings are reported in this study. Ternary Cr–Mo– N coatings were deposited onto steel substrates (AISI D2) using a hybrid coating method of arc ion plating (AIP) using Cr target and DC magnetron sputtering technique using Mo target in N2/Ar gaseous mixture. The synthesized Cr–Mo–N coatings formed a substitutional solid solution of (Cr, Mo)N where larger Mo atoms replaced Cr in CrN crystal. The Cr–Mo–N coatings showed increased hardness value of approximately 34 GPa at 21 at. % Mo, compared with 18 GPa for pure CrN. The friction coefficient decreased from 0.49 for pure CrN coating to 0.37 for Cr–Mo–N with 30.4 at.% Mo. This result is believed to be due to tribo-layer formation of MoO3 which is known to function as a solid lubricant. © 2006 Elsevier B.V. All rights reserved. Keywords: Cr–Mo–N; Arc ion plating; Sputtering techniques; Mechanical properties
1. Introduction CrN coatings has excellent corrosion resistance under severe environment conditions [1,2], superior oxidation resistance to 700 °C [2,3], and show good wear resistance [1,4,5]. Therefore, CrN coatings have been widely used as protective coatings for various forming and casting applications such as drawing dies, molds, etc [6,7]. CrN coatings are usually synthesized by various physical vapor deposition (PVD) techniques such as sputtering, cathodic arc evaporation, ion beam sputtering, etc [8–11]. On the other hand, MoN coatings have been found to have many unique physical and mechanical properties such as high hardness and low solubility in non-ferrous alloys [12]. In addition, MoN coatings exhibit a good adherence with steel substrates because of the solubility of molybdenum in iron-based materials [13]. Also, MoN coatings showed the low friction behavior due to the formation of MoO3 as a solid lubricant [14–17]. Therefore, MoN coatings appear to be a good candidate as hard coating material layer for tribological applications [13]. ⁎ Corresponding author. Tel.: +82 51 510 2391; fax: +82 51 510 3660. E-mail address: [email protected] (K. Ho Kim). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.098
Ternary Cr–Mo–N coatings can have superior properties tailored through appropriate combinations of CrN and MoN. Although other ternary Cr–X–N (X = Ti, Al, Si, C, B, Ta, Nb, Ni.) [18–29] coating systems have been recently explored in order to improve properties of CrN coatings, Cr–Mo–N coatings were scarcely studied. In this paper, ternary Cr–Mo–N coatings were deposited on AISI D2 substrate using a hybrid system of arc ion plating (AIP) and DC magnetron sputtering techniques. The microstructure and mechanical properties of Cr–Mo–N coatings were systematically investigated. 2. Experimental 2.1. Deposition Cr–Mo–N coatings were deposited on AISI D2 steel and Si wafer substrates using a hybrid coating system, where an arc ion plating (AIP) method was combined with a magnetron sputtering technique. An arc cathode gun for the Cr source and a DC sputter gun for the Mo source was installed on each side of the chamber wall. Purities of Cr and Mo targets were 99.9%, respectively. A
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Table 1 Typical deposition conditions for Cr–Mo–N coatings prepared by hybrid coating system Base pressure Working pressure Working gas ratio Ion Bom. Bias voltage Substrate temperature Arc current for Cr source Sputter current for Mo source Deposition time Typical coating thickness Rotational velocity of substrate
6.3 × 10− 3 Pa 1.8 × 10− 1 Pa N2/Ar = 2:1 − 600 V 300 °C 55 A Mo (0–2.2 A) 60 min ~2 μm 25 rpm
rotational substrate holder was located among the sources. The rotational speed of the substrate was 25 rpm. Ar gas (99.999%) was introduced into the sputter target holder to increase the sputtering rate and N2 gas (99.999%) was injected near the substrate holder. Disk substrates (20 mm in diameter and 3 mm in thickness) were cleaned in an ultrasonic bath cleaner using an acetone and alcohol for 20 min. Substrates were cleaned again by ion bombardment using a bias voltage of −600 V under Ar atmosphere of 32 Pa for 15 min and were heated by a resistance heater inside the chamber. Cr–Mo–N coatings were deposited from arc and sputter sources at a working pressure of 1.8 × 10− 1 Pa. The deposition temperature was fixed at 300 °C. Typical deposition conditions for Cr–Mo–N coatings by the hybrid coating system are summarized in Table 1. 2.2. Characterization
Fig. 2. XRD patterns of CrN, γ-Mo2N, and Cr–Mo–N coatings with various Mo contents.
hardness tester with a Knoop indenter (Matsuzawa, MMT-7) under a load of 10 g. The residual stress was obtained from the curvature of coatings/Si-substrate composite using Stoney equation [30]. The friction coefficient and wear behavior were evaluated through sliding tests using a conventional ball-on-disc wear apparatus. A steel ball (diameter 6.34 mm, 700 Hv0.2) was used as a counterface material. Sliding tests were conducted with a sliding speed of 0.157 m/s under a load of 1 N at ambient temperature (around 23 °C) and relative humidity of 25–30% RH.
The coating thickness was measured using a scanning electron microscopy (SEM, Hitach, S-4200) and a stylus (α-STEP) instrument. Compositional analyses of the coatings to determine the contents of Cr, Mo, and N were carried out by electron probe microanalyzer (EPMA, Shimadzu, EPMA 1600). The crystallinity of Cr–Mo–N coatings was analyzed with X-ray diffractometer (XRD, PHILPS, X'Pert–MPD System) using CuKα radiation. X-ray photoelectron spectroscopy (XPS, VG Scientifics, ESCALAB 250) was also performed to observe the bonding states in their coatings. The hardness of coatings was evaluated using a micro-
3. Results and discussion
Fig. 1. Compositional changes of Cr–Mo–N coatings as a function of DC sputter current at a fixed Cr arc current of 55 A.
Fig. 3. Interplanar distance, d200, of CrN (200) crystal plane as a function of Mo content.
3.1. Syntheses of Cr–Mo–N coatings Fig. 1 shows the compositional changes of Cr, Mo, and N in the coating as a function of the DC sputter current collected by the Mo target at a fixed Cr arc current of 55 A. As the DC sputter current increased from 0 to 2.2 A, the Mo content in the Cr–Mo– N coating increased almost linearly from 0 to 30.4 at.%, with a corresponding decrease of Cr from 50 to 15.6 at.%. The nitrogen
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content was almost constant (about 50 at.%) in the coating. In our experiments, the maximum Mo content in Cr–Mo–N coatings was confined to 30.4 at.%. Pure Mo–N coating was prepared as a reference by sputtering without the Cr arc source. Fig. 2 shows X-ray diffraction patterns of Cr–Mo–N coatings at various Mo concentrations. The diffraction pattern of pure Cr–N coating synthesized without Mo source by the cathodic arc ion plating technique showed multiple orientations of (111), (200),
Fig. 5. Micro-hardness value and residual stress of Cr–Mo–N coatings as a function of Mo content.
(220), and (311) crystal planes of face-centered-cubic (f.c.c) CrN crystal structure. On the other hand, the Mo–N coating synthesized without Cr source by the sputtering technique showed multiple orientations of (111), (200), (220), and (311) crystal planes of f.c.c. γ-Mo2N crystal structure. As the Mo content in the Cr–Mo–N coating increased, the diffraction peaks of Cr–Mo–N coatings were gradually shifted from those of CrN coating to those of γMo2N coating in Fig. 2. Fig. 3 shows the interplanar distance, d200, of CrN (200) crystal plane as a function of Mo content, obtained from Fig. 2. The d200 value of CrN coating (d200 = 2.0680 Å for the CrN powder from JCPDS No. 11-0065 [31]) increased with Mo content, due to the larger atomic size Mo. The peak shift phenomenon due to Mo addition toward lower angle reflects that our Cr– Mo–N coatings were substitutional solid solutions of (Cr,Mo)N. In order to investigate the bonding states of Cr, Mo, and N in Cr–Mo–N coatings, X-ray photoelectron spectroscopy (XPS) was carried out. Fig. 4(a) shows the binding energies of N 1s as a function of Mo concentration in the coating. The peak at 396.4 eV corresponds to the N 1s binding energy of the chromium nitride [32]. With increasing Mo concentration, the peak position shifted toward 397.2 eV, the N 1s binding energy in γ-Mo2N phase [33].
Fig. 4. XPS spectra near the binding energy of (a) N 1s, (b) Mo 3d, and (c) Cr 2p for the Cr–Mo–N coatings with various Mo contents.
Fig. 6. Friction coefficients with sliding of Cr–Mo–N coatings with various Mo contents.
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The peak at 394 eV, due to Mo 3p 3/2 [34], was observed in Fig. 4 (a), where the peak intensity increased with Mo content. This peak is believed to originate from molybdenum nitride rather than metallic molybdenum because no XRD peak due to metallic Mo was found in Fig. 2. Fig. 4(b) shows evolution of the Mo 3d the binding energy as a function of Mo content. Peak at 231.4 eVand 228.1 eV, correspond to Mo 3d 3/2 and 3d 5/2 in γ-Mo2N [35]. The peak intensities increased, but did not shift with increase of Mo content. Fig. 4(c) shows the Cr 2p peaks in the coatings. Their positions correspond to CrN [36,37]. Peak intensities decreased, but did not shift with increase of Mo content. It was concluded from our analyses of Figs. 2,3 and 4 that the Cr–Mo–N coatings having the Mo content less than 30.4 at.% must be a substitutional solid solution of (Cr,Mo)N.
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4. Conclusions The following summaries the effect of Mo addition into CrN coatings obtained from our experimental results: (1) Cr–Mo–N coatings with Mo content less than 30.4 at.% is a substitutional solid solution of (Cr,Mo)N. (2) The microhardness of Cr–Mo–N coatings shows a maximum value of 34 GPa at an optimum Mo content of 21 at.%. (3) With increasing Mo content up to 30.4 at.% in Cr–Mo–N coatings, the friction coefficient decreased from 0.49 for CrN coating to 0.37. This result is believed due to a tribolayer formation of MoO3 which is known to function as a solid lubricant.
3.2. Mechanical properties of Cr–Mo–N coatings Acknowledgements Fig. 5 shows the microhardness value and the residual stress of Cr–Mo–N coatings as a function of Mo content. The hardness values were obtained from samples having thickness of about 2 μm and the indent depths were approximately one-tenth of the film thickness. As the Mo content in the coatings increased, the microhardness of Cr–Mo–N coatings increased from 18 GPa for CrN, and then reached a maximum value of approximately 34 GPa for the Cr–Mo–N coatings having Mo content of 21 at. %. The microhardness dropped to 27 GPa with further increase of the Mo content beyond 21 at.%. Compared with the coatings by chemical vapor deposition (CVD) method, the coatings prepared by physical vapor deposition (PVD) methods such as arc ion plating and sputtering usually accumulate considerable amount of residual stress in the coatings. The residual stress is, in general, proportional to the amount of defects accumulated in the coatings. The residual stress in our Cr–Mo–N coatings was compressive ranging from − 0.5 to − 4 GPa as shown in Fig. 5, probably due to defects accumulated in there coatings. The defects might be solid solution, dislocation, and interfacial defects, which will hinder the dislocation movement. In Fig. 5, the hardness was found to correlate with the absolute value of residual stress. Therefore, the effective hardening mechanism of Cr–Mo–N coatings is believed to be due to defects playing as effective barrier against dislocation propagation. Fig. 6 shows the friction coefficient of Cr–Mo–N coatings with various Mo contents against a steel ball. The friction coefficient of coatings decreased from 0.49 for CrN to 0.37 with increasing the Mo content up to 30.4 at.%. For reference, the friction coefficient of our γ-Mo2N coatings seen in Fig. 2 was 0.43. The friction coefficient of CrN coatings was improved by addition of Mo. The decrease in friction coefficient with increasing the Mo content in Cr–Mo–N coatings can be explained by tribo-chemical reaction, where the coating layer reacts with ambient H2O to produce MoO3 thin layer during sliding process. Such a tribo-layer can function as a solid lubricant [14–17]. The formation of a MoO3 tribo-layer should be easier with increasing Mo content. The reason for the lower friction coefficient of Cr– Mo(30.4 at.%)–N coatings compared with that of pure γ-Mo2N coating cannot be solely explained by the tribo-chemical reaction. Other mechanisms must to be involved.
This work was supported by a grant from the National Core Research Center (NCRC) Program (R15-2006-022-01002-0) funded by KOSEF and MOST. References [1] P. Hones, R. Sanjines, F. Levy, Surf. Coat. Technol. 94–95 (1997) 398. [2] B. Navinsek, P. Panjan, I. Milosev, Surf. Coat. Technol. 97 (1997) 182. [3] Jung Wook Kim, Kwang Ho Kim, D.B. Lee, J.J. Moore, Surf. Coat. Technol. 200 (2006) 6702. [4] C. Rebholz, H. Ziegele, A. Leyland, A. Matthews, Surf. Coat. Technol. 115 (1999) 222. [5] X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 133–134 (2000) 331. [6] P.H. Mayrhofer, H. Willmann, C. Mitterer, Surf. Coat. Technol. 146–147 (2001) 222. [7] J. Creus, H. Idrissi, H. Mazille, F. Sanchette, P. Jacquot, Surf. Coat. Technol. 107 (1998) 183. [8] P.H. Mayrhofer, G. Tischler, C. Mitterer, Surf. Coat. Technol. 142–144 (2001) 78. [9] Ghislaine Bertrand, Catherine Savall, Cathy Meunier, Surf. Coat. Technol. 96 (1997) 323. [10] C. Gautier, H. Moussaoui, F. Elstner, J. Machet, Surf. Coat. Tehchnol. 86–87 (1996) 254. [11] J.D. Demaree, C.G. Fountzoulas, J.K. Hirvonen, Surf. Coat. Technol. 86–87 (1996) 309. [12] Yimin Wang, Ray Y. Lin, Mater. Sci. Eng., B 112 (2004) 42. [13] M.K. Kazmanli, M. Urgen, A.F. Cakir, Surf. Coat. Technol. 167 (2003) 77. [14] S.F. Murray, S.J. Calabrese, Lubr. Eng. 49 (1992) 955. [15] Tomasz Suszko, Witold Gulbinski, Jacek Jagielski, Surf. Coat. Technol. 194 (2005) 319. [16] In-Woong Lyo, Hyo-Sok Ahn, Dae-Soon Lim, Surf. Coat. Technol. 163–164 (2003) 413. [17] J. Valli, U. Makela, H.T.G. Hentzell, J. Vac. Sci. Technol., A 4 (6) (1986) 2850. [18] D.H. Jung, H.S. Park, H.D. Na, J.W. Lim, J.J. Lee, J.H. Joo, Surf. Coat. Technol. 169–170 (2003) 424. [19] J.J. Nainaparampil, J.S. Zabinski, A. Korenyi-Both, Thin Solid Films 333 (1998) 88. [20] S. Ulrich, H. Holleck, J. Ye, H. Leiste, R. Loos, M. Stuber, P. Pesch, S. Sattel, Thin Solid Films 437 (2003) 164. [21] J. Vetter, E. Lugscheider, S.S. Guerreiro, Surf. Coat. Technol. 98 (1998) 1233. [22] E. Martinez, R. Sanjines, A. Karimi, J. Esteve, F. Levy, Surf. Coat. Technol. 180–181 (2004) 570. [23] J. Almer, M. Oden, G. Hakansson, Thin Solid Films 385 (2001) 190. [24] S.H. Yao, Y.L. Su, Wear 212 (1997) 85. [25] B. Rother, H. Kappl, Surf. Coat. Technol. 96 (1997) 163.
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[26] M. Cekada, P. Panjan, B. Navinsek, F. Cvelbar, Vacuum 52 (1999) 461. [27] Ranjana Saha, Rama B. Inturi, John A. Barnard, Surf. Coat. Technol. 82 (1996) 42. [28] J.N. Tan, J.H. Hsieh, Surf. Coat. Technol. 167 (2003) 154. [29] F. Regent, J. Musil, Surf. Coat. Technol. 142–144 (2001) 146. [30] S.A. Brenner, S. Senderoff, J. Res. Bur. Stand. 42 (1949) 105. [31] JCPDS, X-ray Index Cards, 11-0065. [32] Nishimura, K. Yabe, M. lwaki, J. Electron Spectrosc. Relat. Phemon. 49 (1989) 335. [33] Shuwu Yang, Yongxue Li, Chunxin Ji, Can Li, Qin Xin, J. Catal. 174 (1998) 34.
[34] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Minnesota, 1995, p. 112. [35] Y.G. Shen, Mater. Sci. Eng., A 359 (2003) 158. [36] Q.G. Zhou, X.D. Bai, X.W. Chen, D.Q. Peng, Y.H. Ling, D.R Wang, Appl. Surf. Sci. 211 (2003) 293. [37] Peijun Cai, Wanqun Zhang, Luyang Chen, Yitai Qian, J. Alloy. Compd. 414 (2006) 221.
Syntheses and mechanical properties of Cr–Mo–N coatings by a hybrid coating system Kwang Ho Kim a,⁎, Eun Young Choi a , Seung Gyun Hong a , Bong Gyu Park b , Jae Hong Yoon b , Jeong Hae Yong c b
a Division of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea Department of Nano and Advaced Materials Engineering, 9 Sarim-dong, Changwon, Kyungnam 641-773, South Korea c Department of Materials Science and Engineering, Pukyong National University, Busan 608-739, South Korea
Available online 20 September 2006
Abstract Effects of Mo content up to 30.4 at.% on the microstructure and mechanical properties of CrN coatings are reported in this study. Ternary Cr–Mo– N coatings were deposited onto steel substrates (AISI D2) using a hybrid coating method of arc ion plating (AIP) using Cr target and DC magnetron sputtering technique using Mo target in N2/Ar gaseous mixture. The synthesized Cr–Mo–N coatings formed a substitutional solid solution of (Cr, Mo)N where larger Mo atoms replaced Cr in CrN crystal. The Cr–Mo–N coatings showed increased hardness value of approximately 34 GPa at 21 at. % Mo, compared with 18 GPa for pure CrN. The friction coefficient decreased from 0.49 for pure CrN coating to 0.37 for Cr–Mo–N with 30.4 at.% Mo. This result is believed to be due to tribo-layer formation of MoO3 which is known to function as a solid lubricant. © 2006 Elsevier B.V. All rights reserved. Keywords: Cr–Mo–N; Arc ion plating; Sputtering techniques; Mechanical properties
1. Introduction CrN coatings has excellent corrosion resistance under severe environment conditions [1,2], superior oxidation resistance to 700 °C [2,3], and show good wear resistance [1,4,5]. Therefore, CrN coatings have been widely used as protective coatings for various forming and casting applications such as drawing dies, molds, etc [6,7]. CrN coatings are usually synthesized by various physical vapor deposition (PVD) techniques such as sputtering, cathodic arc evaporation, ion beam sputtering, etc [8–11]. On the other hand, MoN coatings have been found to have many unique physical and mechanical properties such as high hardness and low solubility in non-ferrous alloys [12]. In addition, MoN coatings exhibit a good adherence with steel substrates because of the solubility of molybdenum in iron-based materials [13]. Also, MoN coatings showed the low friction behavior due to the formation of MoO3 as a solid lubricant [14–17]. Therefore, MoN coatings appear to be a good candidate as hard coating material layer for tribological applications [13]. ⁎ Corresponding author. Tel.: +82 51 510 2391; fax: +82 51 510 3660. E-mail address: [email protected] (K. Ho Kim). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.098
Ternary Cr–Mo–N coatings can have superior properties tailored through appropriate combinations of CrN and MoN. Although other ternary Cr–X–N (X = Ti, Al, Si, C, B, Ta, Nb, Ni.) [18–29] coating systems have been recently explored in order to improve properties of CrN coatings, Cr–Mo–N coatings were scarcely studied. In this paper, ternary Cr–Mo–N coatings were deposited on AISI D2 substrate using a hybrid system of arc ion plating (AIP) and DC magnetron sputtering techniques. The microstructure and mechanical properties of Cr–Mo–N coatings were systematically investigated. 2. Experimental 2.1. Deposition Cr–Mo–N coatings were deposited on AISI D2 steel and Si wafer substrates using a hybrid coating system, where an arc ion plating (AIP) method was combined with a magnetron sputtering technique. An arc cathode gun for the Cr source and a DC sputter gun for the Mo source was installed on each side of the chamber wall. Purities of Cr and Mo targets were 99.9%, respectively. A
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Table 1 Typical deposition conditions for Cr–Mo–N coatings prepared by hybrid coating system Base pressure Working pressure Working gas ratio Ion Bom. Bias voltage Substrate temperature Arc current for Cr source Sputter current for Mo source Deposition time Typical coating thickness Rotational velocity of substrate
6.3 × 10− 3 Pa 1.8 × 10− 1 Pa N2/Ar = 2:1 − 600 V 300 °C 55 A Mo (0–2.2 A) 60 min ~2 μm 25 rpm
rotational substrate holder was located among the sources. The rotational speed of the substrate was 25 rpm. Ar gas (99.999%) was introduced into the sputter target holder to increase the sputtering rate and N2 gas (99.999%) was injected near the substrate holder. Disk substrates (20 mm in diameter and 3 mm in thickness) were cleaned in an ultrasonic bath cleaner using an acetone and alcohol for 20 min. Substrates were cleaned again by ion bombardment using a bias voltage of −600 V under Ar atmosphere of 32 Pa for 15 min and were heated by a resistance heater inside the chamber. Cr–Mo–N coatings were deposited from arc and sputter sources at a working pressure of 1.8 × 10− 1 Pa. The deposition temperature was fixed at 300 °C. Typical deposition conditions for Cr–Mo–N coatings by the hybrid coating system are summarized in Table 1. 2.2. Characterization
Fig. 2. XRD patterns of CrN, γ-Mo2N, and Cr–Mo–N coatings with various Mo contents.
hardness tester with a Knoop indenter (Matsuzawa, MMT-7) under a load of 10 g. The residual stress was obtained from the curvature of coatings/Si-substrate composite using Stoney equation [30]. The friction coefficient and wear behavior were evaluated through sliding tests using a conventional ball-on-disc wear apparatus. A steel ball (diameter 6.34 mm, 700 Hv0.2) was used as a counterface material. Sliding tests were conducted with a sliding speed of 0.157 m/s under a load of 1 N at ambient temperature (around 23 °C) and relative humidity of 25–30% RH.
The coating thickness was measured using a scanning electron microscopy (SEM, Hitach, S-4200) and a stylus (α-STEP) instrument. Compositional analyses of the coatings to determine the contents of Cr, Mo, and N were carried out by electron probe microanalyzer (EPMA, Shimadzu, EPMA 1600). The crystallinity of Cr–Mo–N coatings was analyzed with X-ray diffractometer (XRD, PHILPS, X'Pert–MPD System) using CuKα radiation. X-ray photoelectron spectroscopy (XPS, VG Scientifics, ESCALAB 250) was also performed to observe the bonding states in their coatings. The hardness of coatings was evaluated using a micro-
3. Results and discussion
Fig. 1. Compositional changes of Cr–Mo–N coatings as a function of DC sputter current at a fixed Cr arc current of 55 A.
Fig. 3. Interplanar distance, d200, of CrN (200) crystal plane as a function of Mo content.
3.1. Syntheses of Cr–Mo–N coatings Fig. 1 shows the compositional changes of Cr, Mo, and N in the coating as a function of the DC sputter current collected by the Mo target at a fixed Cr arc current of 55 A. As the DC sputter current increased from 0 to 2.2 A, the Mo content in the Cr–Mo– N coating increased almost linearly from 0 to 30.4 at.%, with a corresponding decrease of Cr from 50 to 15.6 at.%. The nitrogen
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content was almost constant (about 50 at.%) in the coating. In our experiments, the maximum Mo content in Cr–Mo–N coatings was confined to 30.4 at.%. Pure Mo–N coating was prepared as a reference by sputtering without the Cr arc source. Fig. 2 shows X-ray diffraction patterns of Cr–Mo–N coatings at various Mo concentrations. The diffraction pattern of pure Cr–N coating synthesized without Mo source by the cathodic arc ion plating technique showed multiple orientations of (111), (200),
Fig. 5. Micro-hardness value and residual stress of Cr–Mo–N coatings as a function of Mo content.
(220), and (311) crystal planes of face-centered-cubic (f.c.c) CrN crystal structure. On the other hand, the Mo–N coating synthesized without Cr source by the sputtering technique showed multiple orientations of (111), (200), (220), and (311) crystal planes of f.c.c. γ-Mo2N crystal structure. As the Mo content in the Cr–Mo–N coating increased, the diffraction peaks of Cr–Mo–N coatings were gradually shifted from those of CrN coating to those of γMo2N coating in Fig. 2. Fig. 3 shows the interplanar distance, d200, of CrN (200) crystal plane as a function of Mo content, obtained from Fig. 2. The d200 value of CrN coating (d200 = 2.0680 Å for the CrN powder from JCPDS No. 11-0065 [31]) increased with Mo content, due to the larger atomic size Mo. The peak shift phenomenon due to Mo addition toward lower angle reflects that our Cr– Mo–N coatings were substitutional solid solutions of (Cr,Mo)N. In order to investigate the bonding states of Cr, Mo, and N in Cr–Mo–N coatings, X-ray photoelectron spectroscopy (XPS) was carried out. Fig. 4(a) shows the binding energies of N 1s as a function of Mo concentration in the coating. The peak at 396.4 eV corresponds to the N 1s binding energy of the chromium nitride [32]. With increasing Mo concentration, the peak position shifted toward 397.2 eV, the N 1s binding energy in γ-Mo2N phase [33].
Fig. 4. XPS spectra near the binding energy of (a) N 1s, (b) Mo 3d, and (c) Cr 2p for the Cr–Mo–N coatings with various Mo contents.
Fig. 6. Friction coefficients with sliding of Cr–Mo–N coatings with various Mo contents.
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The peak at 394 eV, due to Mo 3p 3/2 [34], was observed in Fig. 4 (a), where the peak intensity increased with Mo content. This peak is believed to originate from molybdenum nitride rather than metallic molybdenum because no XRD peak due to metallic Mo was found in Fig. 2. Fig. 4(b) shows evolution of the Mo 3d the binding energy as a function of Mo content. Peak at 231.4 eVand 228.1 eV, correspond to Mo 3d 3/2 and 3d 5/2 in γ-Mo2N [35]. The peak intensities increased, but did not shift with increase of Mo content. Fig. 4(c) shows the Cr 2p peaks in the coatings. Their positions correspond to CrN [36,37]. Peak intensities decreased, but did not shift with increase of Mo content. It was concluded from our analyses of Figs. 2,3 and 4 that the Cr–Mo–N coatings having the Mo content less than 30.4 at.% must be a substitutional solid solution of (Cr,Mo)N.
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4. Conclusions The following summaries the effect of Mo addition into CrN coatings obtained from our experimental results: (1) Cr–Mo–N coatings with Mo content less than 30.4 at.% is a substitutional solid solution of (Cr,Mo)N. (2) The microhardness of Cr–Mo–N coatings shows a maximum value of 34 GPa at an optimum Mo content of 21 at.%. (3) With increasing Mo content up to 30.4 at.% in Cr–Mo–N coatings, the friction coefficient decreased from 0.49 for CrN coating to 0.37. This result is believed due to a tribolayer formation of MoO3 which is known to function as a solid lubricant.
3.2. Mechanical properties of Cr–Mo–N coatings Acknowledgements Fig. 5 shows the microhardness value and the residual stress of Cr–Mo–N coatings as a function of Mo content. The hardness values were obtained from samples having thickness of about 2 μm and the indent depths were approximately one-tenth of the film thickness. As the Mo content in the coatings increased, the microhardness of Cr–Mo–N coatings increased from 18 GPa for CrN, and then reached a maximum value of approximately 34 GPa for the Cr–Mo–N coatings having Mo content of 21 at. %. The microhardness dropped to 27 GPa with further increase of the Mo content beyond 21 at.%. Compared with the coatings by chemical vapor deposition (CVD) method, the coatings prepared by physical vapor deposition (PVD) methods such as arc ion plating and sputtering usually accumulate considerable amount of residual stress in the coatings. The residual stress is, in general, proportional to the amount of defects accumulated in the coatings. The residual stress in our Cr–Mo–N coatings was compressive ranging from − 0.5 to − 4 GPa as shown in Fig. 5, probably due to defects accumulated in there coatings. The defects might be solid solution, dislocation, and interfacial defects, which will hinder the dislocation movement. In Fig. 5, the hardness was found to correlate with the absolute value of residual stress. Therefore, the effective hardening mechanism of Cr–Mo–N coatings is believed to be due to defects playing as effective barrier against dislocation propagation. Fig. 6 shows the friction coefficient of Cr–Mo–N coatings with various Mo contents against a steel ball. The friction coefficient of coatings decreased from 0.49 for CrN to 0.37 with increasing the Mo content up to 30.4 at.%. For reference, the friction coefficient of our γ-Mo2N coatings seen in Fig. 2 was 0.43. The friction coefficient of CrN coatings was improved by addition of Mo. The decrease in friction coefficient with increasing the Mo content in Cr–Mo–N coatings can be explained by tribo-chemical reaction, where the coating layer reacts with ambient H2O to produce MoO3 thin layer during sliding process. Such a tribo-layer can function as a solid lubricant [14–17]. The formation of a MoO3 tribo-layer should be easier with increasing Mo content. The reason for the lower friction coefficient of Cr– Mo(30.4 at.%)–N coatings compared with that of pure γ-Mo2N coating cannot be solely explained by the tribo-chemical reaction. Other mechanisms must to be involved.
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