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Synthesis and mechanical evaluation of quaternary Ti–Cr–Si–N coatings deposited by a hybrid method of arc ion plating and sputtering techniques
Surface & Coatings Technology 200 (2005) 1489 – 1494 www.elsevier.com/locate/surfcoat
Synthesis and mechanical evaluation of quaternary Ti–Cr–Si–N coatings deposited by a hybrid method of arc ion plating and sputtering techniques Dong Keun Lee a, Dong Shik Kang a, Ju Hyung Suh b, Chan-Gyung Park b, Kwang Ho Kim a,* b
a School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea
Available online 9 September 2005
Abstract New quaternary Ti – Cr – Si – N coatings were synthesized onto steel substrates (AISI D2) and Si wafers using a hybrid method of arc ion plating (AIP) and sputtering techniques. For the syntheses of Ti – Cr – Si – N coatings, the Ti – Cr – N coating process was performed substantially by a multi-cathodic AIP technique using Cr and Ti targets, and Si could be added by sputtering Si target during Ti – Cr – N deposition. In this work, comparative studies on microstructure and evaluation of mechanical properties between Ti – Cr – N and Ti – Cr – Si – N coatings were conducted. As the Si was incorporated into Ti – Cr – N coatings, the Ti – Cr – Si – N coatings showed largely increased hardness value of approximately 42 GPa than one of 28 GPa for Ti – Cr – N coatings. The average friction coefficient of Ti – Cr – N coatings largely decreased from 0.7 to 0.35 with increasing Si content up to 20 at.%. D 2005 Elsevier B.V. All rights reserved. Keywords: Ti – Cr – Si – N; AIP; Sputtering; Microhardness; Wear behavior
1. Introduction Since a few decades, TiN coatings have been widely used for hard coating applications such as drill, cutting tools, etc. [1]. However, TiN coatings are limited for the hightemperature applications above 500 -C, the high-speed dry machining process, due to poor oxidation resistance [2,3]. For this reason, ternary Ti –X – N coating systems, where X = Al, Si, Cr, etc., have been actively investigated [4– 6]. One of these ternary coatings, Ti– Cr– N coatings showed much improved oxidation resistance and better mechanical properties due to the formation of the stable Cr2O3 layer (passive oxide layer) by the migrated Cr atoms to surface region and the solid solution hardening by Cr atoms [7,8]. On the other hand, ternary Ti– Si– N coatings are recently attracting many concerns because of the super hardness above 40 GPa derived from the nanocomposite microstructure, characterized by nanocrystalline TiN and amorphous silicon nitride [9,10].
Recently, some quaternary coatings are designed and explored in order to further improve the characteristics of previous ternary coatings using the nanocomposite concepts. For example, the quaternary Ti– Al– Si– N and Ti –B – C –N coatings [11,12] are reported concerning the nanocomposite concepts based on the introductions of amorphous silicon nitride or amorphous boron nitride into previous Ti– Al –N and Ti– B –C coatings, respectively. And, the quaternary coatings were proved to have much superior properties. In this work, synthesis and characterization of new quaternary Ti– Cr –Si –N coatings were explored using a hybrid method of AIP and sputtering techniques. The effects of Si content on microstructure and mechanical properties of Ti – Cr – Si– N coatings were systematically investigated, compared with previous Ti– Cr –N.
2. Experimental 2.1. Deposition
* Corresponding author. Tel.: +82 51 510 2391; fax: +82 51 510 3660. E-mail address: [email protected] (K.H. Kim). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.023
The Ti –Cr – Si– N coatings were deposited on AISI D2 and Si wafer substrates using a hybrid coating system, where
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D.K. Lee et al. / Surface & Coatings Technology 200 (2005) 1489 – 1494
the AIP method was combined with a magnetron sputtering technique. Arc cathode guns for Cr and Ti sources, a d.c. sputter gun for the Si source, were installed on each side of the chamber wall. A rotational substrate holder was located among the sources. 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. Purities of Ti, Cr and Si targets were 99.99%, respectively. Substrates of the disc type (20 mm in diameter and 3 mm in thickness) were cleaned in an ultrasonic cleaner using 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. Substrates were heated by resistant heaters set inside the chamber, and then Ti– Cr –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 320 -C. Typical deposition conditions for Ti– Cr –Si –N coatings by the hybrid coating system are summarized in Table 1. 2.2. Characterization The coating thickness was measured using a scanning electron microscopy (SEM, Hitach, S-4200) and a stylus (a´-STEP) instrument. Compositional analyses of the coatings to determine the contents of Ti, Cr, Si and N were carried out by electron probe microanalyzer (EPMA, Shimadzu, EPMA 1600). The crystallinity of the Ti– Cr – Si– N coatings was analyzed with X-ray diffractometer (XRD, PHILPS, X’Pert-MPD System) using CuKa´ radiation. X-ray photoelectron spectroscopy (XPS, VG Scientifics, ESCALAB 250) was also performed to observe the bonding status in the Ti –Cr – Si– N coatings. Structural information on the coatings was obtained from the highresolution transmission electron microscopy (HRTEM) using a field emission transmission electron microscope (FE-TEM, JEOL, JEM-2012F) with a 200 kV acceleration voltage. The hardness of coatings was evaluated using a microhardness tester with Knoop indenter (Matsuzawa, MMT-7) under a load of 25 g. The surface topography and root-mean-square (RMS) roughness of coatings were examined with an atomic force microscopy (AFM, PSIA, Table 1 Typical deposition conditions for Ti – Cr – 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 currents for Ti and Cr sources Sputter current for Si source Deposition time Typical coating thickness Rotational velocity of substrate
6.6 10 3 Pa 1.8 10 1 Pa N2/Ar = 5:2 600 V 320 -C 100 V 80 A, 40 A 0 – 2.2 A 60 min ¨2 Am 25 rpm
XE-100). The friction coefficient and wear behavior were evaluated through sliding tests using a conventional ballon-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. Scanning electron microscopy was employed to observe the morphology of the wear track after each sliding experiment.
3. Results and discussion 3.1. Syntheses of Ti– Cr– Si – N coatings Ti –Cr – Si – N coatings with various Si compositions were synthesized on AISI D2 substrates by varying the d.c. sputter current for Si source from 0 to 2.2 A at a fixed Ti (80 A) and Cr (40 A) arc currents. The Si content in Ti– Cr– Si – N coatings increased in the range from 0 to 20 at.%, almost linearly with the d.c. sputter current. The Cr/Ti atomic ratio in the coatings was almost fixed in the deviation range of 41 – 43 at.%. The deviation of Cr/Ti atomic ratio found from our specimens was reported [6] to have little effect on the microstructure and properties of Ti– Cr – N coatings. Fig. 1 shows the X-ray diffraction patterns of Ti– Cr– Si– N coatings with various Si contents, obtained from above deposition condition. The diffraction pattern of Ti – Cr – N coatings exhibited one of B1 NaCl crystal structure with mixed orientations of (111), (200), (220), and (311) crystal planes. Our Ti – Cr – N coatings was observed to form solid solution by substituting Ti atom sites by Cr atoms with smaller atomic radius [8,13], although the Ti– Cr –N coatings were reported to have two phases mixture of cubic (Ti,Cr)N and h-hexagonal (Cr,Ti)2N [14], diffraction peaks of two phases could not be detected. The substitutional solid solution in Ti– Cr –N coatings could be found from the peak shift phenomenon to higher diffraction angle compared to TiN crystal. As the Si was incorporated into Ti– Cr– N coatings, the peak shape was broadened. And, the XRD peaks almost disappeared at the Si content of 20 at.%. Such an XRD peak broadening, in general, originates from the diminution of the grain size or the residual stress induced in the crystal lattice [15]. Similar XRD peak broadening has been reported for other ternary or quaternary coatings with Si addition, and is attributed mainly to the effect of the diminution of crystallite size [16 – 19]. Microstructural changes of Ti– Cr –N coatings with addition of Si were investigated using an HRTEM. Fig. 2 shows the cross-sectional HRTEM images, SADP (selected area diffraction patterns) and dark-field TEM images for Ti –Cr – N, Ti– Cr –Si (8 at.%) –N, Ti– Cr– Si (20 at.%) – N coatings. From our TEM works, the Ti– Cr –N coatings (Fig. 2a and b) was observed to have crystalline phase having a relative columnar structure. A composite
D.K. Lee et al. / Surface & Coatings Technology 200 (2005) 1489 – 1494
Ti TiN
(Ti,Cr)N (220)
(Ti,Cr)N (Ti,Cr)N (200) (111)
1491
(Ti,Cr)N (311)
Ti0.56Cr0.24Si0.20N
Intensity [Arb. U.]
Ti0.62Cr0.26Si0.12N
Ti0.65Cr0.27Si0.08N
Ti0.68Cr0.29Si0.03N
Ti0.71Cr0.29N 20
30
40
50
60
70
80
Diffraction angle [2 theta] Fig. 1. XRD patterns of Ti – Cr – N and Ti – Cr – Si – N coatings with various Si contents.
(a)) (a
(200) (220)
(Ti,Cr)N Crystallites
(111)
(b)
(311)
Growth direction
20 nm
10 nm
(c)
(Ti,Cr)N Crystallites
(d)
Amorphous Si3N4
20 nm
10 nm
(e)
(f)
10 nm
20 nm
Fig. 2. Cross-sectional HRTEM images, selected area diffraction patterns (SADP), and dark-field TEM images. (a, b) Ti – Cr – N, (c, d) Ti – Cr – Si (8 at.%) – N, (e, f) Ti – Cr – Si (20 at.%) – N coatings.
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Fig. 3. Cross-sectional micrographs of Ti – Cr – Si – N coatings with various Si contents. (a) Ti – Cr – N, (b) Ti – Cr – Si (8 at.%) – N, (c) Ti – Cr – Si (20 at.%) – N coatings.
microstructure consisting of crystallites and amorphous phase was found from the Ti– Cr –Si (8 at.%) – N coatings (Fig. 2c and d). The crystalline and amorphous phases could be distinguished from each other by the lattice fringe contrast. In case of Ti– Cr –Si (20 at.%) – N coatings having higher Si content, the average size of crystallites was largely decreased (Fig. 2e and f). In our work, it was found that relatively large columnar microstructure of Ti–Cr – N coatings was refined with Si addition. This microstructural change from large columnar structure to much refined one with Si addition could be confirmed from SEM observation of Fig. 3. The XRD peak broadening phenomena of Fig. 1 could be also explained with the size reduction of Ti– Cr –N crystallites with Si addition. In order to clarify bonding status of the amorphous phase comprising Ti– Cr –Si –N coatings, XPS analyses were performed. Fig. 4 shows XPS spectra near the binding energy of Si 2p for the coatings with various Si contents. The peak corresponding to 101.8 eV, which was in good agreement with that of the Si3N4 compound [20], was observed. The peak intensity increased with the increase of Si content in Ti– Cr –Si –N coatings. In our experiment, the Si3N4 peak was not found from XRD analysis; it was indicated that Si3N4 was not existed as crystal phase. Therefore, Si existed mainly as a phase of amorphous silicon nitride compound in the Ti– Cr –Si –N coatings. Fig. 5 shows the root-mean-square (RMS) roughness of Ti –Cr – Si– N coatings calculated from AFM images of a selected area of 0.5 0.5 Am. The surface morphology of the Ti– Cr –N coatings was relatively rough (RMS, 0.175 nm). As the Si content increased up to 24 at.% in the
Si3N4 (101.8 eV)
Si 2p Al Kα K
Intensity [Arb.U.]
Ti0.56Cr0.24Si0.20N
Ti0.62Cr0.26Si0.12N Ti0.65Cr0.27Si0.08N
Ti0.68Cr0.29Si0.03N Ti0.71Cr0.29N 108
104
100
96
Binding energy [eV] Fig. 4. XPS spectra near the binding energy of Si 2p for the Ti – Cr – Si – N coatings with various Si contents.
0.20
TiN
1.0 0.18
Friction coefficient
Root-mean-square roughness [nm]
D.K. Lee et al. / Surface & Coatings Technology 200 (2005) 1489 – 1494
0.16 0.14 0.12
Ti-Cr-N
0
5
10
15
20
25
0.6 0.4 0.2
0
3.2. Mechanical properties of Ti –Cr – Si –N coatings Fig. 6 shows the microhardness of Ti– Cr– Si –N coatings with various Si contents. As the Si content increased, the hardness of the Ti– Cr –Si –N coatings steeply increased from ¨ 28 GPa of Ti– Cr –N, and reached maximum value of approximately 42 GPa at the Si content of 8 at.%. On the other hand, the hardness decreased with further increase of Si content. The hardness value of Ti– Cr– Si– N coating having Si content of 8 at.% was significantly increased compared to that of Ti– Cr – N coatings. The hardness change of Ti– Cr –Si – N coatings in Fig. 6 is related to the 50 45 40 35
Ti-Cr-Si(8 at. %)-N
0.0 5
1 10
15
20
25 2
30
3
Fig. 5. The average surface roughness (RMS) values of Ti – Cr – Si – N coatings with various Si contents.
coatings, the RMS value continually reduced to 0.118 nm. The Si addition into Ti– Cr –N coatings was found to cause the smoothening of surface morphology of the coatings. As observed and discussed in Figs. 1 – 3, the Si addition induced the microstructure of Ti– Cr– N coatings to be refined. Also, the amorphous phase of silicon nitride was found to be dispersed in the composite of Ti–Cr – Si– N coatings. The two factors, both the refinement of microstructure and the mixture of amorphous phase, reasoned the smoothening of surface morphology with Si addition.
Ti-Cr-Si(3 at. %)-N
0.8
Ti-Cr-Si(20 at. %)-N
0.10
Si content [at. %]
Microhardness [GPa]
1493
Number of cycle [×10 ] Fig. 7. Friction coefficients of TiN, Ti – Cr – N, and various Ti – Cr – Si – N coatings against a steel ball.
microstructural change with Si addition. The hardness increase of Ti– Cr –Si –N coatings must result from the refinement of crystallites and fine composite microstructure. The microstructure of the coatings having the maximum hardness at the Si content of 8 at.% was already proved by our instrumental analyses to be a composite comprising fine crystallites and amorphous Si3N4. Thus, the grain boundary hardening described by Hall-Petch relationship would take place. On the other hand, the hardness reduction with the further increase of Si content after 8 at.% in Fig. 6 was suggested to be due to the increase of volume fraction of amorphous Si3N4 phase. It was reported that the increase in volume fraction of amorphous Si3N4 phase resulted in the hardness reduction [9,21]. This kind of Si effect on the microstructural evolution and the hardness has been reported similarly in other systems [18,19]. Fig. 7 shows the friction coefficient of our Ti– Cr –N, Ti– Cr –Si (3 at.%) – N, Ti– Cr –Si (8 at.%) –N, and Ti–Cr – Si (20 at.%) –N coatings against steel ball. The friction coefficients were also compared in Fig. 7 with that of TiN, which was deposited with the same condition. The average friction coefficient of Ti– Cr –N coatings largely decreased from 0.7 to 0.35 with increasing Si content up to 20 at.%. This result would be caused (i) by the smoothening of surface morphology as mentioned in Fig. 5, (ii) by the tribochemical reaction, which often take place in many ceramics [22,23], e.g. Si3N4 reacts with H2O to produce SiO2 or Si(OH)2 tribo-layer as a self-lubricant [24].
30 25
4. Conclusions
20
Quaternary Ti –Cr – Si– N coatings were synthesized on AISI D2 substrates by the hybrid method, where AIP was combined with a magnetron sputtering technique. From XRD, XPS and HRTEM analyses, it could be suggested that Ti– Cr –Si – N coatings must be a composite consisting of fine Ti – Cr – N crystallites and amorphous Si3N4. The
15
0
5
10
15
20
2 25
Si content [at. %] Fig. 6. Microhardness values of the Ti – Cr – Si – N coatings with various Si contents.
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hardness value of Ti – Cr – Si – N coatings significantly increased from ¨ 28 GPa of Ti– Cr– N coatings to ¨ 42 GPa with Si content of 8 at.% due to the refinement of Ti– Cr – N crystallites and the composite microstructure characteristics. The root-mean-square (RMS) roughness of Ti– Cr – Si– N coatings became greatly smoother than that of Ti– Cr –N coatings. The average friction coefficient of Ti– Cr – N coatings largely decreased from 0.7 to 0.35 with increasing Si content up to 20 at.%.
Acknowledgements This work was performed through NRL project supported by Ministry of Science and Technology of Korea (MOST).
References [1] T. Cselle, A. Barimani, Surf. Coat. Technol. 76 – 77 (1995) 712. [2] M. Witter, J. Noser, H. Melchior, J. Appl. Phys. 52 (11) (1981) 6659. [3] J.-W. He, C.-d. Bai, K.-W. Xu, N.-S. Hu, Sur. Coat. Technol. 74 – 75 (1995) 387. [4] T. Leyendecker, O. Lemmer, S. Esser, Surf. Coat. Technol. 48 (1991) 175. [5] F. Vaz, L. Rebouta, B. Almeida, P. Goudeau, J. Pacaud, J.P. Rivie`re, Surf. Coat. Technol. 102 – 121 (1999) 166. [6] J.J. Nainaparampil, J.S. Zabinski, A. Korenyi-Both, Thin Solid Films 333 (1998) 88. [7] D.B. Lee, M.H. Kim, Y.C. Lee, S.C. Kwon, Surf. Coat. Technol. 141 (2001) 232.
[8] K.H. Lee, C.H. Park, Y.S. Yoon, J.J. Lee, Thin Solid Films 385 (2001) 167. [9] S. Vepøek, S. Reiprich, Thin Solid Films 268 (1995) 64. [10] F. Vaz, L. Rebouta, S. Ramos, M.F. da Silva, J.C. Soares, Surf. Coat. Technol. 108 – 109 (1998) 236. [11] I.-W. Park, S.R. Choi, M.-H. Lee, K.H. Kim, J. Vac. Sci. Technol., A 21 (4) (2003) 895. [12] I.-W. Park, K.H. Kim, O. Kunrath, D. Zhong, J.J. Moore, A.A. Voevodin, E.A. Levashov, J. Vac. Sci. Technol., B 23 (2) (2005) 588. [13] P. Hones, R. Sanjine´s, F. Le´vy, Thin Solid Films 332 (1998) 240. [14] J. Vetter, H.J. Scholl, O. Knotek, Surf. Coat. Technol. 74 – 75 (1995) 286. [15] M. Diserens, J. Patscheider, F. Le´vy, Surf. Coat. Technol. 108 – 109 (1998) 241. [16] S. Veprek, A. Niederhofer, K. Moto, T. Bolom, H.-D. Ma¨nnling, P. Nesladek, G. Dollinger, A. Bergmaier, Surf. Coat. Technol. 133 – 134 (2000) 152. [17] J. Patscheider, T. Zehnder, M. Diserens, Surf. Coat. Technol. 146 – 147 (2001) 201. [18] I.-W. Park, S.R. Choi, J.H. Suh, C.-G. Park, K.H. Kim, Thin Solid Films 447 – 448 (2004) 443. [19] J.-H. Jeon, S.R. Choi, W.S. Chung, K.H. Kim, Surf. Coat. Technol. 188 – 189 (2004) 415. [20] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Minnesota, 1995, p. 238. [21] K.H. Kim, S.-R. Choi, S.-Y. Yoon, Surf. Coat. Technol. 298 (2002) 243. [22] S. Wilson, A.T. Alpas, Wear 245 (2000) 223. [23] M.P. Delplancke-Ogletree, O.R. Monteiro, Surf. Coat. Technol. 108 – 109 (1998) 484. [24] J. Takadoum, H. Houmid-Bennani, D. Mairey, J. Eur. Ceram. Soc. 18 (1998) 553.
Synthesis and mechanical evaluation of quaternary Ti–Cr–Si–N coatings deposited by a hybrid method of arc ion plating and sputtering techniques Dong Keun Lee a, Dong Shik Kang a, Ju Hyung Suh b, Chan-Gyung Park b, Kwang Ho Kim a,* b
a School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea
Available online 9 September 2005
Abstract New quaternary Ti – Cr – Si – N coatings were synthesized onto steel substrates (AISI D2) and Si wafers using a hybrid method of arc ion plating (AIP) and sputtering techniques. For the syntheses of Ti – Cr – Si – N coatings, the Ti – Cr – N coating process was performed substantially by a multi-cathodic AIP technique using Cr and Ti targets, and Si could be added by sputtering Si target during Ti – Cr – N deposition. In this work, comparative studies on microstructure and evaluation of mechanical properties between Ti – Cr – N and Ti – Cr – Si – N coatings were conducted. As the Si was incorporated into Ti – Cr – N coatings, the Ti – Cr – Si – N coatings showed largely increased hardness value of approximately 42 GPa than one of 28 GPa for Ti – Cr – N coatings. The average friction coefficient of Ti – Cr – N coatings largely decreased from 0.7 to 0.35 with increasing Si content up to 20 at.%. D 2005 Elsevier B.V. All rights reserved. Keywords: Ti – Cr – Si – N; AIP; Sputtering; Microhardness; Wear behavior
1. Introduction Since a few decades, TiN coatings have been widely used for hard coating applications such as drill, cutting tools, etc. [1]. However, TiN coatings are limited for the hightemperature applications above 500 -C, the high-speed dry machining process, due to poor oxidation resistance [2,3]. For this reason, ternary Ti –X – N coating systems, where X = Al, Si, Cr, etc., have been actively investigated [4– 6]. One of these ternary coatings, Ti– Cr– N coatings showed much improved oxidation resistance and better mechanical properties due to the formation of the stable Cr2O3 layer (passive oxide layer) by the migrated Cr atoms to surface region and the solid solution hardening by Cr atoms [7,8]. On the other hand, ternary Ti– Si– N coatings are recently attracting many concerns because of the super hardness above 40 GPa derived from the nanocomposite microstructure, characterized by nanocrystalline TiN and amorphous silicon nitride [9,10].
Recently, some quaternary coatings are designed and explored in order to further improve the characteristics of previous ternary coatings using the nanocomposite concepts. For example, the quaternary Ti– Al– Si– N and Ti –B – C –N coatings [11,12] are reported concerning the nanocomposite concepts based on the introductions of amorphous silicon nitride or amorphous boron nitride into previous Ti– Al –N and Ti– B –C coatings, respectively. And, the quaternary coatings were proved to have much superior properties. In this work, synthesis and characterization of new quaternary Ti– Cr –Si –N coatings were explored using a hybrid method of AIP and sputtering techniques. The effects of Si content on microstructure and mechanical properties of Ti – Cr – Si– N coatings were systematically investigated, compared with previous Ti– Cr –N.
2. Experimental 2.1. Deposition
* Corresponding author. Tel.: +82 51 510 2391; fax: +82 51 510 3660. E-mail address: [email protected] (K.H. Kim). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.023
The Ti –Cr – Si– N coatings were deposited on AISI D2 and Si wafer substrates using a hybrid coating system, where
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D.K. Lee et al. / Surface & Coatings Technology 200 (2005) 1489 – 1494
the AIP method was combined with a magnetron sputtering technique. Arc cathode guns for Cr and Ti sources, a d.c. sputter gun for the Si source, were installed on each side of the chamber wall. A rotational substrate holder was located among the sources. 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. Purities of Ti, Cr and Si targets were 99.99%, respectively. Substrates of the disc type (20 mm in diameter and 3 mm in thickness) were cleaned in an ultrasonic cleaner using 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. Substrates were heated by resistant heaters set inside the chamber, and then Ti– Cr –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 320 -C. Typical deposition conditions for Ti– Cr –Si –N coatings by the hybrid coating system are summarized in Table 1. 2.2. Characterization The coating thickness was measured using a scanning electron microscopy (SEM, Hitach, S-4200) and a stylus (a´-STEP) instrument. Compositional analyses of the coatings to determine the contents of Ti, Cr, Si and N were carried out by electron probe microanalyzer (EPMA, Shimadzu, EPMA 1600). The crystallinity of the Ti– Cr – Si– N coatings was analyzed with X-ray diffractometer (XRD, PHILPS, X’Pert-MPD System) using CuKa´ radiation. X-ray photoelectron spectroscopy (XPS, VG Scientifics, ESCALAB 250) was also performed to observe the bonding status in the Ti –Cr – Si– N coatings. Structural information on the coatings was obtained from the highresolution transmission electron microscopy (HRTEM) using a field emission transmission electron microscope (FE-TEM, JEOL, JEM-2012F) with a 200 kV acceleration voltage. The hardness of coatings was evaluated using a microhardness tester with Knoop indenter (Matsuzawa, MMT-7) under a load of 25 g. The surface topography and root-mean-square (RMS) roughness of coatings were examined with an atomic force microscopy (AFM, PSIA, Table 1 Typical deposition conditions for Ti – Cr – 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 currents for Ti and Cr sources Sputter current for Si source Deposition time Typical coating thickness Rotational velocity of substrate
6.6 10 3 Pa 1.8 10 1 Pa N2/Ar = 5:2 600 V 320 -C 100 V 80 A, 40 A 0 – 2.2 A 60 min ¨2 Am 25 rpm
XE-100). The friction coefficient and wear behavior were evaluated through sliding tests using a conventional ballon-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. Scanning electron microscopy was employed to observe the morphology of the wear track after each sliding experiment.
3. Results and discussion 3.1. Syntheses of Ti– Cr– Si – N coatings Ti –Cr – Si – N coatings with various Si compositions were synthesized on AISI D2 substrates by varying the d.c. sputter current for Si source from 0 to 2.2 A at a fixed Ti (80 A) and Cr (40 A) arc currents. The Si content in Ti– Cr– Si – N coatings increased in the range from 0 to 20 at.%, almost linearly with the d.c. sputter current. The Cr/Ti atomic ratio in the coatings was almost fixed in the deviation range of 41 – 43 at.%. The deviation of Cr/Ti atomic ratio found from our specimens was reported [6] to have little effect on the microstructure and properties of Ti– Cr – N coatings. Fig. 1 shows the X-ray diffraction patterns of Ti– Cr– Si– N coatings with various Si contents, obtained from above deposition condition. The diffraction pattern of Ti – Cr – N coatings exhibited one of B1 NaCl crystal structure with mixed orientations of (111), (200), (220), and (311) crystal planes. Our Ti – Cr – N coatings was observed to form solid solution by substituting Ti atom sites by Cr atoms with smaller atomic radius [8,13], although the Ti– Cr –N coatings were reported to have two phases mixture of cubic (Ti,Cr)N and h-hexagonal (Cr,Ti)2N [14], diffraction peaks of two phases could not be detected. The substitutional solid solution in Ti– Cr –N coatings could be found from the peak shift phenomenon to higher diffraction angle compared to TiN crystal. As the Si was incorporated into Ti– Cr– N coatings, the peak shape was broadened. And, the XRD peaks almost disappeared at the Si content of 20 at.%. Such an XRD peak broadening, in general, originates from the diminution of the grain size or the residual stress induced in the crystal lattice [15]. Similar XRD peak broadening has been reported for other ternary or quaternary coatings with Si addition, and is attributed mainly to the effect of the diminution of crystallite size [16 – 19]. Microstructural changes of Ti– Cr –N coatings with addition of Si were investigated using an HRTEM. Fig. 2 shows the cross-sectional HRTEM images, SADP (selected area diffraction patterns) and dark-field TEM images for Ti –Cr – N, Ti– Cr –Si (8 at.%) –N, Ti– Cr– Si (20 at.%) – N coatings. From our TEM works, the Ti– Cr –N coatings (Fig. 2a and b) was observed to have crystalline phase having a relative columnar structure. A composite
D.K. Lee et al. / Surface & Coatings Technology 200 (2005) 1489 – 1494
Ti TiN
(Ti,Cr)N (220)
(Ti,Cr)N (Ti,Cr)N (200) (111)
1491
(Ti,Cr)N (311)
Ti0.56Cr0.24Si0.20N
Intensity [Arb. U.]
Ti0.62Cr0.26Si0.12N
Ti0.65Cr0.27Si0.08N
Ti0.68Cr0.29Si0.03N
Ti0.71Cr0.29N 20
30
40
50
60
70
80
Diffraction angle [2 theta] Fig. 1. XRD patterns of Ti – Cr – N and Ti – Cr – Si – N coatings with various Si contents.
(a)) (a
(200) (220)
(Ti,Cr)N Crystallites
(111)
(b)
(311)
Growth direction
20 nm
10 nm
(c)
(Ti,Cr)N Crystallites
(d)
Amorphous Si3N4
20 nm
10 nm
(e)
(f)
10 nm
20 nm
Fig. 2. Cross-sectional HRTEM images, selected area diffraction patterns (SADP), and dark-field TEM images. (a, b) Ti – Cr – N, (c, d) Ti – Cr – Si (8 at.%) – N, (e, f) Ti – Cr – Si (20 at.%) – N coatings.
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Fig. 3. Cross-sectional micrographs of Ti – Cr – Si – N coatings with various Si contents. (a) Ti – Cr – N, (b) Ti – Cr – Si (8 at.%) – N, (c) Ti – Cr – Si (20 at.%) – N coatings.
microstructure consisting of crystallites and amorphous phase was found from the Ti– Cr –Si (8 at.%) – N coatings (Fig. 2c and d). The crystalline and amorphous phases could be distinguished from each other by the lattice fringe contrast. In case of Ti– Cr –Si (20 at.%) – N coatings having higher Si content, the average size of crystallites was largely decreased (Fig. 2e and f). In our work, it was found that relatively large columnar microstructure of Ti–Cr – N coatings was refined with Si addition. This microstructural change from large columnar structure to much refined one with Si addition could be confirmed from SEM observation of Fig. 3. The XRD peak broadening phenomena of Fig. 1 could be also explained with the size reduction of Ti– Cr –N crystallites with Si addition. In order to clarify bonding status of the amorphous phase comprising Ti– Cr –Si –N coatings, XPS analyses were performed. Fig. 4 shows XPS spectra near the binding energy of Si 2p for the coatings with various Si contents. The peak corresponding to 101.8 eV, which was in good agreement with that of the Si3N4 compound [20], was observed. The peak intensity increased with the increase of Si content in Ti– Cr –Si –N coatings. In our experiment, the Si3N4 peak was not found from XRD analysis; it was indicated that Si3N4 was not existed as crystal phase. Therefore, Si existed mainly as a phase of amorphous silicon nitride compound in the Ti– Cr –Si –N coatings. Fig. 5 shows the root-mean-square (RMS) roughness of Ti –Cr – Si– N coatings calculated from AFM images of a selected area of 0.5 0.5 Am. The surface morphology of the Ti– Cr –N coatings was relatively rough (RMS, 0.175 nm). As the Si content increased up to 24 at.% in the
Si3N4 (101.8 eV)
Si 2p Al Kα K
Intensity [Arb.U.]
Ti0.56Cr0.24Si0.20N
Ti0.62Cr0.26Si0.12N Ti0.65Cr0.27Si0.08N
Ti0.68Cr0.29Si0.03N Ti0.71Cr0.29N 108
104
100
96
Binding energy [eV] Fig. 4. XPS spectra near the binding energy of Si 2p for the Ti – Cr – Si – N coatings with various Si contents.
0.20
TiN
1.0 0.18
Friction coefficient
Root-mean-square roughness [nm]
D.K. Lee et al. / Surface & Coatings Technology 200 (2005) 1489 – 1494
0.16 0.14 0.12
Ti-Cr-N
0
5
10
15
20
25
0.6 0.4 0.2
0
3.2. Mechanical properties of Ti –Cr – Si –N coatings Fig. 6 shows the microhardness of Ti– Cr– Si –N coatings with various Si contents. As the Si content increased, the hardness of the Ti– Cr –Si –N coatings steeply increased from ¨ 28 GPa of Ti– Cr –N, and reached maximum value of approximately 42 GPa at the Si content of 8 at.%. On the other hand, the hardness decreased with further increase of Si content. The hardness value of Ti– Cr– Si– N coating having Si content of 8 at.% was significantly increased compared to that of Ti– Cr – N coatings. The hardness change of Ti– Cr –Si – N coatings in Fig. 6 is related to the 50 45 40 35
Ti-Cr-Si(8 at. %)-N
0.0 5
1 10
15
20
25 2
30
3
Fig. 5. The average surface roughness (RMS) values of Ti – Cr – Si – N coatings with various Si contents.
coatings, the RMS value continually reduced to 0.118 nm. The Si addition into Ti– Cr –N coatings was found to cause the smoothening of surface morphology of the coatings. As observed and discussed in Figs. 1 – 3, the Si addition induced the microstructure of Ti– Cr– N coatings to be refined. Also, the amorphous phase of silicon nitride was found to be dispersed in the composite of Ti–Cr – Si– N coatings. The two factors, both the refinement of microstructure and the mixture of amorphous phase, reasoned the smoothening of surface morphology with Si addition.
Ti-Cr-Si(3 at. %)-N
0.8
Ti-Cr-Si(20 at. %)-N
0.10
Si content [at. %]
Microhardness [GPa]
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Number of cycle [×10 ] Fig. 7. Friction coefficients of TiN, Ti – Cr – N, and various Ti – Cr – Si – N coatings against a steel ball.
microstructural change with Si addition. The hardness increase of Ti– Cr –Si –N coatings must result from the refinement of crystallites and fine composite microstructure. The microstructure of the coatings having the maximum hardness at the Si content of 8 at.% was already proved by our instrumental analyses to be a composite comprising fine crystallites and amorphous Si3N4. Thus, the grain boundary hardening described by Hall-Petch relationship would take place. On the other hand, the hardness reduction with the further increase of Si content after 8 at.% in Fig. 6 was suggested to be due to the increase of volume fraction of amorphous Si3N4 phase. It was reported that the increase in volume fraction of amorphous Si3N4 phase resulted in the hardness reduction [9,21]. This kind of Si effect on the microstructural evolution and the hardness has been reported similarly in other systems [18,19]. Fig. 7 shows the friction coefficient of our Ti– Cr –N, Ti– Cr –Si (3 at.%) – N, Ti– Cr –Si (8 at.%) –N, and Ti–Cr – Si (20 at.%) –N coatings against steel ball. The friction coefficients were also compared in Fig. 7 with that of TiN, which was deposited with the same condition. The average friction coefficient of Ti– Cr –N coatings largely decreased from 0.7 to 0.35 with increasing Si content up to 20 at.%. This result would be caused (i) by the smoothening of surface morphology as mentioned in Fig. 5, (ii) by the tribochemical reaction, which often take place in many ceramics [22,23], e.g. Si3N4 reacts with H2O to produce SiO2 or Si(OH)2 tribo-layer as a self-lubricant [24].
30 25
4. Conclusions
20
Quaternary Ti –Cr – Si– N coatings were synthesized on AISI D2 substrates by the hybrid method, where AIP was combined with a magnetron sputtering technique. From XRD, XPS and HRTEM analyses, it could be suggested that Ti– Cr –Si – N coatings must be a composite consisting of fine Ti – Cr – N crystallites and amorphous Si3N4. The
15
0
5
10
15
20
2 25
Si content [at. %] Fig. 6. Microhardness values of the Ti – Cr – Si – N coatings with various Si contents.
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hardness value of Ti – Cr – Si – N coatings significantly increased from ¨ 28 GPa of Ti– Cr– N coatings to ¨ 42 GPa with Si content of 8 at.% due to the refinement of Ti– Cr – N crystallites and the composite microstructure characteristics. The root-mean-square (RMS) roughness of Ti– Cr – Si– N coatings became greatly smoother than that of Ti– Cr –N coatings. The average friction coefficient of Ti– Cr – N coatings largely decreased from 0.7 to 0.35 with increasing Si content up to 20 at.%.
Acknowledgements This work was performed through NRL project supported by Ministry of Science and Technology of Korea (MOST).
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