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Syntheses and mechanical properties of quaternary Cr-Mo-Si-N coatings by a hybrid coating system
Materials Science and Engineering A 487 (2008) 586–590
Syntheses and mechanical properties of quaternary Cr-Mo-Si-N coatings by a hybrid coating system Seung Gyun Hong a , Dong-Woo Shin b , Kwang Ho Kim a,∗ a
School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea b Division of Nano and Advanced Materials Engineering, ERI, Gyeongsang National University, Kyeong-nam 660-701, South Korea Received 5 June 2007; received in revised form 18 October 2007; accepted 18 October 2007
Abstract Quaternary Cr-Mo-Si-N coatings have been deposited on AISI D2 steel and Si wafers by a hybrid coating system of arc ion plating (AIP) and dc magnetron sputtering techniques. A Cr3 Mo AIP target with the fixed arc current of 55 A and Si sputtering target with the varying current of 0–1.4 A were utilized in Ar/N2 mixed atmosphere during Cr-Mo-Si-N deposition. Microstructure characteristics and mechanical properties of the Cr-Mo-Si-N coatings by this kind of hybrid coating technique were investigated in this paper. The results showed that the microstructure of CrMo-Si-N coatings is nanocomposite consisting of Cr-Mo-N crystallites and amorphous Si3 N4 . The hardness of the Cr-Mo-Si-N coatings increased from 33 GPa of Cr-Mo-N coating to the peak value of about 50 GPa with Si content of 12.1 at.%, and then decreased with further increasing Si content. The hardness variation of Cr-Mo-Si-N coatings was related to the microstructure evolution in the coatings with increasing the Si contents. The average friction coefficient of Cr-Mo-Si-N coatings gradually decreased with increasing the Si content in the coatings. © 2007 Elsevier B.V. All rights reserved. Keywords: Cr-Mo-Si-N; Hybrid coating system; Microstructure; Mechanical properties
1. Introduction CrN coatings synthesized by various physical vapor deposition techniques such as sputtering, cathodic arc ion plating, ion beam sputtering, etc., are popularly used for forming applications (drawing dies, molds, etc.) for their attractive properties, such as high hardness, good wear resistance, excellent oxidation resistance, and good adhesion to steels [1–4]. Lately, ternary Cr-X-N (X = Ti, Al, Si, Mo, C, B, Ta, Nb, Ni) [5–17] coating systems have been explored to improve the properties of CrN coatings. One of the above ternary coatings, Cr-Mo-N coating [10], whose microstructure can be described as a substitutional solid solution (Cr,Mo)N, showed higher hardness and improved tribological behavior compared with CrN coating by solution strengthening of Mo atoms in the CrN crystal
∗
Corresponding author. Tel.: +82 51 510 2391; fax: +82 51 510 3660. E-mail address: [email protected] (K.H. Kim).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.049
lattice. Another kind of ternary coatings, Cr-Si-N coatings [18,19], whose microstructure was found to be a nanocomposite consisting of CrN crystallites and amorphous Si3 N4 , have shown much enhanced hardness and improved high-temperature oxidation resistance. This kind of nanocomposite coatings represents the new-generation superhard wear-resistant coatings for high speed and dry machining applications. It is possible that quaternary Cr-Mo-Si-N coatings can show superior hybrid properties of two ternary coatings of Cr-Mo-N and Cr-Si-N if the quaternary microstructure were properly tailored from two kinds of ternary microstructures. However, the quaternary Cr-Mo-Si-N system was never reported until now. In this paper, we will investigate on this kind of coatings. A hybrid coating system, combining the arc ion plating (AIP) and magnetron sputtering techniques, has been successfully used for fabricating Cr-Mo-N [10] and Cr-Si-N [19] coatings before. In this paper, we utilized this kind of technique to synthesize the quaternary Cr-Mo-Si-N coatings on AISI D2 steel and Si wafer substrates. The microstructure, mechanical, and tribo-
S.G. Hong et al. / Materials Science and Engineering A 487 (2008) 586–590
logical properties of Cr-Mo-Si-N coatings were systematically investigated. 2. Experimental 2.1. Deposition The Cr-Mo-Si-N coatings were deposited on AISI D2 steel (for friction coefficient measurements) and Si wafer (for microstructure characterization and hardness experiments) substrates using a hybrid coating system, where an arc ion plating method was combined with a magnetron sputtering technique. An arc cathode gun for the Cr3 Mo (at.%) source and a dc sputter gun for the Si (99.99%) source were installed on each side of the chamber wall. A 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. The substrates of a disc type (20 mm in diameter and 3 mm in thickness) were cleaned in an ultrasonic bath cleaner using an acetone and alcohol for 20 min. The substrates were cleaned again by ion bombardment using a bias voltage of −600 V under Ar atmosphere of 32 Pa for 15 min. The substrates were heated by a resistance heater set inside the chamber, and then Cr-Mo-Si-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-Si-N coatings by the hybrid coating system are summarized in Table 1. 2.2. Characterization The coating thickness was measured using a stylus (␣-STEP) instrument. Compositional analyses of the coatings to determine the contents of Cr, Mo, Si and N were carried out by electron probe microanalyzer (EPMA, Shimadzu, EPMA 1600). The phase structure of Cr-Mo-Si-N coatings was analyzed with X-ray diffractometer (XRD, BRUKER axs, D8 Advanced LYNXEYE Detector) using Cu K␣ radiation with the glancing angle of 7◦ . X-ray photoelectron spectroscopy (XPS, VG Scientifics, ESCALAB 250) was also performed to observe the chemical bonding status in the Cr-Mo-Si-N coatings. The XPS spectra were obtained after removing the surface adatoms on the
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samples by sputtering with Ar+ ion for 360 s and the spectra were calibrated for the value of carbon peak C 1s at 284.5 eV. Structural information on the coatings was obtained from the high-resolution transmission electron microscopy (HRTEM) using a field emission transmission electron microscope (FETEM, JEOL-400FX) with a 400 kV acceleration voltage. The hardness of coatings was evaluated using a micro-hardness tester with Knoop indenter (Matsuzawa, MMT-7) under a load of 25 g. The friction coefficient was 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 counterpart material. The sliding tests were conducted with a sliding speed of 0.157 m/s under a load of 1 N at ambient temperature (around 20 ◦ C) and relative humidity (25–30% RH) condition. 3. Results and discussion 3.1. Syntheses of Cr-Mo-Si-N coatings Fig. 1 shows the variations of Cr, Mo, Si, and N in the coatings as a function of the dc sputter current collected by the Si target at a fixed arc current of 55 A for Cr3 Mo target. As the dc sputter current increased from 0 to 1.4 A, the Si content in the CrMo-Si-N coating increased almost linearly from 0 to 16.9 at.%, whereas Cr and Mo content in Cr-Mo-Si-N coatings decreased linearly in the ranges from 29.3 to 18.6 at.% for Cr element and from 19 to 11.6 at.% for Mo element. On the other hand, the nitrogen content in Cr-Mo-Si-N coatings was almost constant (about 50 at.%) in the coatings. Fig. 2 shows the X-ray diffraction patterns of Cr-Mo-N and Cr-Mo-Si-N coatings with various Si contents. The diffraction pattern of Cr-Mo-N coating showed face-centered-cubic (Cr,Mo)N phase with almost random orientations of (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystal planes. No XRD peaks corresponding to other crystalline phase were observed. As Si content in the Cr-Mo-Si-N coating increased, the diffraction patterns of Cr-Mo-Si-N coatings showed a weak (2 0 0) orientation. And the peaks almost disappeared and became X-ray amorphous at
Table 1 Typical deposition conditions for Cr-Mo-Si-N coatings prepared by hybrid coating system Base pressure Working pressure Working gas ratio Ion Bom. bias voltage Substrate temperature Substrate bias voltage Arc current for Cr3 Mo source Sputter current for Si 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 −50 V 55 A 0–1.4 A 60 min ∼2.5 m 25 rpm
Fig. 1. Compositional changes of Cr, Mo, Si, and N in the coating as a function of the dc sputter current collected by the Si target at a fixed arc current of 55 A for Cr3 Mo target.
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Fig. 4. Binding energies of Si 2p for the Cr-Mo-Si-N coatings with various Si contents. Fig. 2. X-ray diffraction patterns of Cr-Mo-N and Cr-Mo-Si-N coatings with various Si contents.
the highest Si content of 16.9 at.%. In addition, a peak broadening phenomenon took place with increasing Si content. Such an XRD peak broadening is, in general, originated from the diminution of the grain size. The change of diffraction patterns of Cr-Mo-Si-N coating with increasing Si content in Fig. 2 are similar to the case of Si addition into Ti-Al-N [20,21]. In addition, the diffraction peak position of Cr-Mo-Si-N coatings was a little shifted to the higher angle with increasing Si content. This result indicated that some amount of Si existed as the dissolved atoms in Cr-Mo-N lattice. Fig. 3 shows the interplanar distance, d2 0 0 , of Cr-Mo-N (2 0 0) crystal plane calculated from the peak shifting phenomenon of Fig. 2 as a function of Si content. For comparison, d value of (2 0 0) crystal plane for standard CrN powder sourced from the Joint Committee for ˚ The CrPowder Diffraction Studies (JCPDS) [22] is 2.0680 A. Mo-N coatings had a higher interplanar spacing value than that of standard CrN phase. This higher d-value of Cr-Mo-N coatings was probably derived from the substitutional replacement of larger Mo atoms for Cr sites in the CrN crystal site [10]. The d2 0 0 value of Cr-Mo-Si-N coatings continually decreased with
Fig. 3. Interplanar distance, d2 0 0 , of Cr-Mo-N (2 0 0) crystal plane as a function of Si content calculated from Fig. 2.
increasing Si content, and then exhibited a minimum value at the Si content of approximately 12.1 at.%. The d-value change is similar to the case of Si addition into CrN coating [19]. The decreased d2 0 0 value with increasing Si content indicates that the silicon atoms were dissolved in the Cr-Mo-N crystal where smaller Si atoms were substituted for Cr and Mo atoms. The d2 0 0 value, however, started to rebound with further increasing the Si content above 12.1 at.% as shown in Fig. 3. This rebounded phenomenon is related to the dilatation of the lattice constant that increases when crystallite size decrease to nano-scale [23]. In order to clarify chemical bonding status of Cr, Mo, Si and N in the coatings, X-ray photoelectron spectroscopy (XPS) was carried out on Cr-Mo-Si-N coatings. Fig. 4 shows the binding energies of Si 2p on the Cr-Mo-Si-N coatings with various Si contents. The peak corresponding to 101.8 eV, which is in good agreement with the binding energy of Si3 N4 phase [24], was clearly observed, and the peak intensity increased with increasing Si content. This XPS analyses combining with the XRD data of Fig. 2 indicates that amorphous Si3 N4 phase formed together with the fine Cr-Mo-N crystallites in the coatings. As the Si content increased above 8.2 at.% in Fig. 4, another peak corresponding to 99.3 eV which is in agreement with Si–Si bondings [25,26] appeared together with Si3 N4 peak. The appearance of Si–Si bindings can be attributed to the relative deficiency of nitrogen source during the deposition as the partial pressure of silicon source increased under a fixed N2 partial pressure [20,25,26]. Microstructural changes of Cr-Mo-N coatings with addition of Si were investigated by using an HRTEM. Fig. 5 shows darkfield TEM image, SADP (selected area diffraction patterns), and cross-sectional HRTEM images for Cr-Mo-N, Cr-Mo-Si (12.1 at.%)-N and Cr-Mo-Si (16.9 at.%)-N coatings. From our TEM results of Fig. 5, the Cr-Mo-N coating had crystalline phase of columnar structure (Fig. 5a). And, large columnar microstructure of Cr-Mo-N was modified into refined grains with increasing Si contents (Fig. 5b). In case of Cr-Mo-Si (16.9 at.%)-N coatings having high Si content, it mostly consisted of amorphous phase (Fig. 5c). A nanocomposite microstructure consisting of
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Fig. 5. Dark-field TEM images for (a) Cr-Mo-N, (b) Cr-Mo-Si (12.1 at.%)-N, (c) Cr-Mo-Si (16.9 at.%)-N, and (d) HRTEM image for Cr-Mo-Si (12.1 at.%)-N coatings. Inserts are the selected area diffraction patterns (SADP).
crystallites and amorphous phase was found from the Cr-Mo-Si (12.1 at.%)-N coating (Fig. 5d). The crystalline and amorphous phases could be distinguished from each other by the lattice fringe contrast. In our work, it was found that relatively large columnar microstructure of Cr-Mo-N coating was refined with Si addition. The XRD peak broadening phenomena of Fig. 2 could be also explained with the size reduction of Cr-Mo-N crystallites with Si addition.
coatings. On the other hand, the hardness reduction with further increase of Si content after maximum hardness can be explained to be due to the increase of volume fraction of the soft amorphous Si3 N4 phase. For comparison, the microhardness values of the Cr-Si-N coatings [19] are also presented in Fig. 6. It can be
3.2. Mechanical properties of Cr-Mo-Si-N coatings Fig. 6 presents the microhardness of Cr-Mo-Si-N coatings as a function of Si content. As the Si content increased, the hardness of the Cr-Mo-Si-N coatings gradually increased from 33 GPa for Cr-Mo-N to maximum value of approximately 50 GPa at the Si content of 12.1 at.%, and then steeply decreased again with further increase of Si content. The large increase of hardness at the Si content of 12.1 at.% is explained with grain boundary hardening, created by the strong cohesive energy of inter-phase boundaries [27] and by the Hall–Petch relation derived from crystal size refinement [28]. Both of these phenomena can be generated by percolation of amorphous Si3 N4 into Cr-Mo-N
Fig. 6. Microhardness of Cr-Mo-Si-N and Cr-Si-N coatings [19] as a function of Si content.
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enabled to form SiO2 or Si(OH)2 tribo-layer playing a role as self-lubricant. Acknowledgements 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
Fig. 7. Friction coefficients of Cr-Mo-Si-N coatings with various Si contents.
seen that coatings with much higher hardness values can be obtained by adding Mo in the Cr-Si-N coatings. Therefore, the quaternary Cr-Mo-Si-N coatings can be tailored to possess much better mechanical properties than both Cr-Mo-N and Cr-Si-N coatings. To see the Si effect on the wear behavior of coatings, five kinds of coatings, such as Cr-Mo-N, Cr-Mo-Si (8.2 at.%)-N, Cr-Mo-Si (10.4 at.%)-N, Cr-Mo-Si (12.1 at.%)-N and Cr-Mo-Si (16.9 at.%)-N coatings were prepared. Fig. 7 shows the friction coefficient of Cr-Mo-Si-N coatings with various Si contents against a steel ball. The friction coefficient of coatings decreased from 0.49 to 0.30 with increasing the Si content. The decrease in friction coefficient with increase of Si content would be caused by a smoother surface due to formation of the amorphous phase and a tribo-chemical reaction, which often takes place in many ceramics, e.g., Si3 N4 reacts with H2 O to produce SiO2 or Si(OH)2 tribo-layer. These products of SiO2 and Si(OH)2 are known to a function as a self-lubricating layer [29,30]. The formation of tribo-layer would be more promoted with increasing Si content. 4. Conclusions Quaternary Cr-Mo-Si-N coatings were synthesized on AISI D2 steel and Si wafer substrates by the hybrid coating method, where AIP was combined with a magnetron sputtering technique. From XRD, XPS and HRTEM analyses, it was concluded that Cr-Mo-Si-N coatings must be a composite consisting of fine Cr-Mo-N crystallites and amorphous Si3 N4 . As the Si content increased, the hardness value of Cr-Mo-Si-N coatings significantly increased from 33 GPa of Cr-Mo-N coatings to 50 GPa with Si content of 12.1 at.% due to the refinement of Cr-Mo-N crystallites and the composite microstructure characteristics. The average friction coefficient of Cr-Mo-N coatings largely decreased from 0.49 to 0.30 with increasing Si content up to 16.9 at.%. This behavior would be attributed to the tribo-chemical reaction between Si and ambient humidity, which
[1] C. Rebholz, H. Ziegele, A. Leyland, A. Matthew, Surf. Coat. Technol. 115 (1999) 222. [2] J. Creus, H. Idrissi, H. Mazille, F. Sanchette, P. Jacquot, Surf. Coat. Technol. 107 (1998) 183. [3] G. Bertrand, H. Mahdjoub, C. Meunier, Surf. Coat. Technol. 126 (2000) 199. [4] P. Hones, C. Zakri, P.E. Schmid, F. Levy, O.R. Shojaei, Appl. Phys. Lett. 76 (22) (2000) 3194. [5] 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. [6] J.J. Nainaparampil, J.S. Zabinski, A. Korenyi-Both, Thin Solid Films 333 (1998) 88. [7] S. Ulrich, H. Holleck, J. Ye, H. Leiste, R. Loos, M. Stuber, P. Pesch, S. Sattel, Thin Solid Films 437 (2003) 164. [8] J. Vetter, E. Lugscheider, S.S. Guerreiro, Surf. Coat. Technol. 98 (1998) 1233. [9] E. Martinez, R. Sanjines, A. Karimi, J. Esteve, F. Levy, Surf. Coat. Technol. 180/181 (2004) 570. [10] K.H. Kim, E.Y. Choi, S.G. Hong, B.G. Park, J.H. Yoon, J.H. Yong, Surf. Coat. Technol. 201 (2006) 4068. [11] J. Almer, M. Oden, G. Hakansson, Thin Solid Films 385 (2001) 190. [12] S.H. Yao, Y.L. Su, Wear 212 (1997) 85. [13] B. Rother, H. Kappl, Surf. Coat. Technol. 96 (1997) 163. [14] M. Cekada, P. Panjan, B. Navinsek, F. Cvelbar, Vacuum 52 (1999) 461. [15] R. Saha, R.B. Inturi, J.A. Barnard, Surf. Coat. Technol. 82 (1996) 42. [16] J.N. Tan, J.H. Hsieh, Surf. Coat. Technol. 167 (2003) 154. [17] F. Regent, J. Musil, Surf. Coat. Technol. 142–144 (2001) 146. [18] D. Mercs, N. Bonasso, S. Naamane, J.-M.C. Bordes, C. Coddet, Surf. Coat. Technol. 200 (2005) 403. [19] J.H. Park, W.S. Chung, Y.R. Cho, K.H. Kim, Surf. Coat. Technol. 188/189 (2004) 425. [20] I.W. Park, S.R. Choi, M.H. Lee, K.H. Kim, J. Vac. Sci. Technol. A 21 (4) (2003) 895. [21] I.W. Park, S.R. Choi, J.H. Suh, C.G. Park, K.H. Kim, Thin Solid Film 447/448 (2004) 443–448. [22] JCPDS, X-ray Index Cards, 11-0065. [23] J. Prochazka, P. Karvankova, M.G.J. Veprek-Heijman, S. Veprek, Mater. Sci. Eng. A 384 (2004) 102. [24] J.F. Mouleder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Minnesota, 1995, p. 56. [25] J.B. Choi, K. Cho, M.-H. Lee, K.H. Kim, Thin Solid Films 447/448 (2004) 365. [26] M. Diserens, J. Patscheider, F. L´evy, Surf. Coat. Technol. 108/109 (1998) 241. [27] S. Veprek, S. Reiprich, L. Shizhi, Appl. Phys. Lett. 66 (1995) 2640. [28] A. Lasalmonie, J.L. Strudel, J. Mater. Sci. 21 (1986) 1837. [29] J. Takadoum, H. Houmid-Bennani, D. Mairey, J. Eur. Ceram. Soc. 18 (1998) 553. [30] J. Patscheider, MRS Bull. 28 (March (3)) (2003) 180–183.
Syntheses and mechanical properties of quaternary Cr-Mo-Si-N coatings by a hybrid coating system Seung Gyun Hong a , Dong-Woo Shin b , Kwang Ho Kim a,∗ a
School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea b Division of Nano and Advanced Materials Engineering, ERI, Gyeongsang National University, Kyeong-nam 660-701, South Korea Received 5 June 2007; received in revised form 18 October 2007; accepted 18 October 2007
Abstract Quaternary Cr-Mo-Si-N coatings have been deposited on AISI D2 steel and Si wafers by a hybrid coating system of arc ion plating (AIP) and dc magnetron sputtering techniques. A Cr3 Mo AIP target with the fixed arc current of 55 A and Si sputtering target with the varying current of 0–1.4 A were utilized in Ar/N2 mixed atmosphere during Cr-Mo-Si-N deposition. Microstructure characteristics and mechanical properties of the Cr-Mo-Si-N coatings by this kind of hybrid coating technique were investigated in this paper. The results showed that the microstructure of CrMo-Si-N coatings is nanocomposite consisting of Cr-Mo-N crystallites and amorphous Si3 N4 . The hardness of the Cr-Mo-Si-N coatings increased from 33 GPa of Cr-Mo-N coating to the peak value of about 50 GPa with Si content of 12.1 at.%, and then decreased with further increasing Si content. The hardness variation of Cr-Mo-Si-N coatings was related to the microstructure evolution in the coatings with increasing the Si contents. The average friction coefficient of Cr-Mo-Si-N coatings gradually decreased with increasing the Si content in the coatings. © 2007 Elsevier B.V. All rights reserved. Keywords: Cr-Mo-Si-N; Hybrid coating system; Microstructure; Mechanical properties
1. Introduction CrN coatings synthesized by various physical vapor deposition techniques such as sputtering, cathodic arc ion plating, ion beam sputtering, etc., are popularly used for forming applications (drawing dies, molds, etc.) for their attractive properties, such as high hardness, good wear resistance, excellent oxidation resistance, and good adhesion to steels [1–4]. Lately, ternary Cr-X-N (X = Ti, Al, Si, Mo, C, B, Ta, Nb, Ni) [5–17] coating systems have been explored to improve the properties of CrN coatings. One of the above ternary coatings, Cr-Mo-N coating [10], whose microstructure can be described as a substitutional solid solution (Cr,Mo)N, showed higher hardness and improved tribological behavior compared with CrN coating by solution strengthening of Mo atoms in the CrN crystal
∗
Corresponding author. Tel.: +82 51 510 2391; fax: +82 51 510 3660. E-mail address: [email protected] (K.H. Kim).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.049
lattice. Another kind of ternary coatings, Cr-Si-N coatings [18,19], whose microstructure was found to be a nanocomposite consisting of CrN crystallites and amorphous Si3 N4 , have shown much enhanced hardness and improved high-temperature oxidation resistance. This kind of nanocomposite coatings represents the new-generation superhard wear-resistant coatings for high speed and dry machining applications. It is possible that quaternary Cr-Mo-Si-N coatings can show superior hybrid properties of two ternary coatings of Cr-Mo-N and Cr-Si-N if the quaternary microstructure were properly tailored from two kinds of ternary microstructures. However, the quaternary Cr-Mo-Si-N system was never reported until now. In this paper, we will investigate on this kind of coatings. A hybrid coating system, combining the arc ion plating (AIP) and magnetron sputtering techniques, has been successfully used for fabricating Cr-Mo-N [10] and Cr-Si-N [19] coatings before. In this paper, we utilized this kind of technique to synthesize the quaternary Cr-Mo-Si-N coatings on AISI D2 steel and Si wafer substrates. The microstructure, mechanical, and tribo-
S.G. Hong et al. / Materials Science and Engineering A 487 (2008) 586–590
logical properties of Cr-Mo-Si-N coatings were systematically investigated. 2. Experimental 2.1. Deposition The Cr-Mo-Si-N coatings were deposited on AISI D2 steel (for friction coefficient measurements) and Si wafer (for microstructure characterization and hardness experiments) substrates using a hybrid coating system, where an arc ion plating method was combined with a magnetron sputtering technique. An arc cathode gun for the Cr3 Mo (at.%) source and a dc sputter gun for the Si (99.99%) source were installed on each side of the chamber wall. A 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. The substrates of a disc type (20 mm in diameter and 3 mm in thickness) were cleaned in an ultrasonic bath cleaner using an acetone and alcohol for 20 min. The substrates were cleaned again by ion bombardment using a bias voltage of −600 V under Ar atmosphere of 32 Pa for 15 min. The substrates were heated by a resistance heater set inside the chamber, and then Cr-Mo-Si-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-Si-N coatings by the hybrid coating system are summarized in Table 1. 2.2. Characterization The coating thickness was measured using a stylus (␣-STEP) instrument. Compositional analyses of the coatings to determine the contents of Cr, Mo, Si and N were carried out by electron probe microanalyzer (EPMA, Shimadzu, EPMA 1600). The phase structure of Cr-Mo-Si-N coatings was analyzed with X-ray diffractometer (XRD, BRUKER axs, D8 Advanced LYNXEYE Detector) using Cu K␣ radiation with the glancing angle of 7◦ . X-ray photoelectron spectroscopy (XPS, VG Scientifics, ESCALAB 250) was also performed to observe the chemical bonding status in the Cr-Mo-Si-N coatings. The XPS spectra were obtained after removing the surface adatoms on the
587
samples by sputtering with Ar+ ion for 360 s and the spectra were calibrated for the value of carbon peak C 1s at 284.5 eV. Structural information on the coatings was obtained from the high-resolution transmission electron microscopy (HRTEM) using a field emission transmission electron microscope (FETEM, JEOL-400FX) with a 400 kV acceleration voltage. The hardness of coatings was evaluated using a micro-hardness tester with Knoop indenter (Matsuzawa, MMT-7) under a load of 25 g. The friction coefficient was 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 counterpart material. The sliding tests were conducted with a sliding speed of 0.157 m/s under a load of 1 N at ambient temperature (around 20 ◦ C) and relative humidity (25–30% RH) condition. 3. Results and discussion 3.1. Syntheses of Cr-Mo-Si-N coatings Fig. 1 shows the variations of Cr, Mo, Si, and N in the coatings as a function of the dc sputter current collected by the Si target at a fixed arc current of 55 A for Cr3 Mo target. As the dc sputter current increased from 0 to 1.4 A, the Si content in the CrMo-Si-N coating increased almost linearly from 0 to 16.9 at.%, whereas Cr and Mo content in Cr-Mo-Si-N coatings decreased linearly in the ranges from 29.3 to 18.6 at.% for Cr element and from 19 to 11.6 at.% for Mo element. On the other hand, the nitrogen content in Cr-Mo-Si-N coatings was almost constant (about 50 at.%) in the coatings. Fig. 2 shows the X-ray diffraction patterns of Cr-Mo-N and Cr-Mo-Si-N coatings with various Si contents. The diffraction pattern of Cr-Mo-N coating showed face-centered-cubic (Cr,Mo)N phase with almost random orientations of (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystal planes. No XRD peaks corresponding to other crystalline phase were observed. As Si content in the Cr-Mo-Si-N coating increased, the diffraction patterns of Cr-Mo-Si-N coatings showed a weak (2 0 0) orientation. And the peaks almost disappeared and became X-ray amorphous at
Table 1 Typical deposition conditions for Cr-Mo-Si-N coatings prepared by hybrid coating system Base pressure Working pressure Working gas ratio Ion Bom. bias voltage Substrate temperature Substrate bias voltage Arc current for Cr3 Mo source Sputter current for Si 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 −50 V 55 A 0–1.4 A 60 min ∼2.5 m 25 rpm
Fig. 1. Compositional changes of Cr, Mo, Si, and N in the coating as a function of the dc sputter current collected by the Si target at a fixed arc current of 55 A for Cr3 Mo target.
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Fig. 4. Binding energies of Si 2p for the Cr-Mo-Si-N coatings with various Si contents. Fig. 2. X-ray diffraction patterns of Cr-Mo-N and Cr-Mo-Si-N coatings with various Si contents.
the highest Si content of 16.9 at.%. In addition, a peak broadening phenomenon took place with increasing Si content. Such an XRD peak broadening is, in general, originated from the diminution of the grain size. The change of diffraction patterns of Cr-Mo-Si-N coating with increasing Si content in Fig. 2 are similar to the case of Si addition into Ti-Al-N [20,21]. In addition, the diffraction peak position of Cr-Mo-Si-N coatings was a little shifted to the higher angle with increasing Si content. This result indicated that some amount of Si existed as the dissolved atoms in Cr-Mo-N lattice. Fig. 3 shows the interplanar distance, d2 0 0 , of Cr-Mo-N (2 0 0) crystal plane calculated from the peak shifting phenomenon of Fig. 2 as a function of Si content. For comparison, d value of (2 0 0) crystal plane for standard CrN powder sourced from the Joint Committee for ˚ The CrPowder Diffraction Studies (JCPDS) [22] is 2.0680 A. Mo-N coatings had a higher interplanar spacing value than that of standard CrN phase. This higher d-value of Cr-Mo-N coatings was probably derived from the substitutional replacement of larger Mo atoms for Cr sites in the CrN crystal site [10]. The d2 0 0 value of Cr-Mo-Si-N coatings continually decreased with
Fig. 3. Interplanar distance, d2 0 0 , of Cr-Mo-N (2 0 0) crystal plane as a function of Si content calculated from Fig. 2.
increasing Si content, and then exhibited a minimum value at the Si content of approximately 12.1 at.%. The d-value change is similar to the case of Si addition into CrN coating [19]. The decreased d2 0 0 value with increasing Si content indicates that the silicon atoms were dissolved in the Cr-Mo-N crystal where smaller Si atoms were substituted for Cr and Mo atoms. The d2 0 0 value, however, started to rebound with further increasing the Si content above 12.1 at.% as shown in Fig. 3. This rebounded phenomenon is related to the dilatation of the lattice constant that increases when crystallite size decrease to nano-scale [23]. In order to clarify chemical bonding status of Cr, Mo, Si and N in the coatings, X-ray photoelectron spectroscopy (XPS) was carried out on Cr-Mo-Si-N coatings. Fig. 4 shows the binding energies of Si 2p on the Cr-Mo-Si-N coatings with various Si contents. The peak corresponding to 101.8 eV, which is in good agreement with the binding energy of Si3 N4 phase [24], was clearly observed, and the peak intensity increased with increasing Si content. This XPS analyses combining with the XRD data of Fig. 2 indicates that amorphous Si3 N4 phase formed together with the fine Cr-Mo-N crystallites in the coatings. As the Si content increased above 8.2 at.% in Fig. 4, another peak corresponding to 99.3 eV which is in agreement with Si–Si bondings [25,26] appeared together with Si3 N4 peak. The appearance of Si–Si bindings can be attributed to the relative deficiency of nitrogen source during the deposition as the partial pressure of silicon source increased under a fixed N2 partial pressure [20,25,26]. Microstructural changes of Cr-Mo-N coatings with addition of Si were investigated by using an HRTEM. Fig. 5 shows darkfield TEM image, SADP (selected area diffraction patterns), and cross-sectional HRTEM images for Cr-Mo-N, Cr-Mo-Si (12.1 at.%)-N and Cr-Mo-Si (16.9 at.%)-N coatings. From our TEM results of Fig. 5, the Cr-Mo-N coating had crystalline phase of columnar structure (Fig. 5a). And, large columnar microstructure of Cr-Mo-N was modified into refined grains with increasing Si contents (Fig. 5b). In case of Cr-Mo-Si (16.9 at.%)-N coatings having high Si content, it mostly consisted of amorphous phase (Fig. 5c). A nanocomposite microstructure consisting of
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Fig. 5. Dark-field TEM images for (a) Cr-Mo-N, (b) Cr-Mo-Si (12.1 at.%)-N, (c) Cr-Mo-Si (16.9 at.%)-N, and (d) HRTEM image for Cr-Mo-Si (12.1 at.%)-N coatings. Inserts are the selected area diffraction patterns (SADP).
crystallites and amorphous phase was found from the Cr-Mo-Si (12.1 at.%)-N coating (Fig. 5d). The crystalline and amorphous phases could be distinguished from each other by the lattice fringe contrast. In our work, it was found that relatively large columnar microstructure of Cr-Mo-N coating was refined with Si addition. The XRD peak broadening phenomena of Fig. 2 could be also explained with the size reduction of Cr-Mo-N crystallites with Si addition.
coatings. On the other hand, the hardness reduction with further increase of Si content after maximum hardness can be explained to be due to the increase of volume fraction of the soft amorphous Si3 N4 phase. For comparison, the microhardness values of the Cr-Si-N coatings [19] are also presented in Fig. 6. It can be
3.2. Mechanical properties of Cr-Mo-Si-N coatings Fig. 6 presents the microhardness of Cr-Mo-Si-N coatings as a function of Si content. As the Si content increased, the hardness of the Cr-Mo-Si-N coatings gradually increased from 33 GPa for Cr-Mo-N to maximum value of approximately 50 GPa at the Si content of 12.1 at.%, and then steeply decreased again with further increase of Si content. The large increase of hardness at the Si content of 12.1 at.% is explained with grain boundary hardening, created by the strong cohesive energy of inter-phase boundaries [27] and by the Hall–Petch relation derived from crystal size refinement [28]. Both of these phenomena can be generated by percolation of amorphous Si3 N4 into Cr-Mo-N
Fig. 6. Microhardness of Cr-Mo-Si-N and Cr-Si-N coatings [19] as a function of Si content.
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enabled to form SiO2 or Si(OH)2 tribo-layer playing a role as self-lubricant. Acknowledgements 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
Fig. 7. Friction coefficients of Cr-Mo-Si-N coatings with various Si contents.
seen that coatings with much higher hardness values can be obtained by adding Mo in the Cr-Si-N coatings. Therefore, the quaternary Cr-Mo-Si-N coatings can be tailored to possess much better mechanical properties than both Cr-Mo-N and Cr-Si-N coatings. To see the Si effect on the wear behavior of coatings, five kinds of coatings, such as Cr-Mo-N, Cr-Mo-Si (8.2 at.%)-N, Cr-Mo-Si (10.4 at.%)-N, Cr-Mo-Si (12.1 at.%)-N and Cr-Mo-Si (16.9 at.%)-N coatings were prepared. Fig. 7 shows the friction coefficient of Cr-Mo-Si-N coatings with various Si contents against a steel ball. The friction coefficient of coatings decreased from 0.49 to 0.30 with increasing the Si content. The decrease in friction coefficient with increase of Si content would be caused by a smoother surface due to formation of the amorphous phase and a tribo-chemical reaction, which often takes place in many ceramics, e.g., Si3 N4 reacts with H2 O to produce SiO2 or Si(OH)2 tribo-layer. These products of SiO2 and Si(OH)2 are known to a function as a self-lubricating layer [29,30]. The formation of tribo-layer would be more promoted with increasing Si content. 4. Conclusions Quaternary Cr-Mo-Si-N coatings were synthesized on AISI D2 steel and Si wafer substrates by the hybrid coating method, where AIP was combined with a magnetron sputtering technique. From XRD, XPS and HRTEM analyses, it was concluded that Cr-Mo-Si-N coatings must be a composite consisting of fine Cr-Mo-N crystallites and amorphous Si3 N4 . As the Si content increased, the hardness value of Cr-Mo-Si-N coatings significantly increased from 33 GPa of Cr-Mo-N coatings to 50 GPa with Si content of 12.1 at.% due to the refinement of Cr-Mo-N crystallites and the composite microstructure characteristics. The average friction coefficient of Cr-Mo-N coatings largely decreased from 0.49 to 0.30 with increasing Si content up to 16.9 at.%. This behavior would be attributed to the tribo-chemical reaction between Si and ambient humidity, which
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