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Cu multilayered thin films with different bilayer thickness
Surface & Coatings Technology 204 (2009) 941–946
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Characteristics of Cr2N/Cu multilayered thin films with different bilayer thickness Chia-Lin Li a, Fan-Bean Wu b, Jyh-Wei Lee a,c,d,⁎, Yan-Zuo Tsai e, Li-Chun Chang f a
Institute of Mechatronic Engineering, Tungnan University, Taipei, Taiwan, ROC Department of Materials Engineering, National United University, Maoli, Taiwan, ROC c Department of Mechanical Engineering, Tungnan University, Taipei, Taiwan, ROC d Research Center for Micro/Nanotechnology, Tungnan University, Taipei, Taiwan, ROC e Department of Materials Science and Engineering, National Tsing Hua University, Taiwan, ROC f Department of Materials Engineering, Mingchi University of Technology, Taipei, Taiwan, ROC b
a r t i c l e
i n f o
Available online 30 May 2009 Keywords: Pulsed DC reactive magnetron sputtering Cr2N/Cu multilayered coatings Bilayer period Pin-on-disk wear test Corrosion test
a b s t r a c t Nanostructured Cr2N/Cu multilayer coatings were deposited periodically by a bipolar asymmetric reactive pulsed DC magnetron sputtering system. The crystalline structure of multilayer coatings was determined by a glancing angle X-ray diffractometer. Microstructures of thin films were examined by an atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. A nanoindenter, a micro Vickers hardness tester and pin-on-disk wear tests were used to evaluate the hardness, fracture toughness and tribological properties of the thin films, respectively. Electrochemical tests in 3.5 wt.% NaCl aqueous solution were performed to evaluate the corrosion resistance of multilayered coatings. The bilayer period of the nanostructured multilayer coatings were controlled within the range from 5 to 40 nm. It was found that the hardness increased with decreasing bilayer period and leveled off at small periods around 6–12 nm. The Cr2N/Cu multilayered coatings with bilayer periods of 5 and 6 nm exhibited a combination of high hardness, good tribological and good anticorrosion performance in this study. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the last decade, many researchers have devoted much effort to TiN-based transition metal nitride/nitride superhard (50 GPa) heterostructured multilayer coatings [1–3] due to their pronounced strength enhancement, optimal hardness/toughness ratios and excellent wear resistance. A different class of metal/transition metal nitride multilayered coatings, such as CrN/Cr [4] and Mo/NbN [5] has also been used to obtain both high hardness and toughness, in which the soft metal and the hard nitride provided ductility and hardness, respectively. The hardness of such metal/transition metal nitride multilayered coatings also increased with decreasing bilayer period and leveled off at small periods. In previous work [6], an unusual hardness enhancement to 21 GPa at maximum and a lower coefficient of friction to 0.13 were observed for a nanocomposite Cr–Cu–N coatings containing 15.7 at.% Cu. A combination of sufficient hardness and good antibacterial properties were achieved in Cr–Cu–N nanocomposite thin films with more than 15.7 at.% Cu. However, a hardness strengthening effect of the Cu contained in Cr2N/Cu [7] and TiN/Cu [8] multilayer coatings was not observed, which might be attributed to
⁎ Corresponding author. Department of Mechanical Engineering, Tungnan University, Taipei, Taiwan, ROC. Tel.: +886 2 86625980; fax: +886 2 86625919. E-mail address: [email protected] (J.-W. Lee). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.05.039
the soft Cu layer [8]. Although much effort has been devoted to study the microstructures and mechanical properties on the transition of metal nitride/Cu multilayered coatings [7–11], a fundamental understanding of the microstructures, mechanical, tribological and corrosion properties of such Cu containing multilayered coating is still lacking. In this study, Cr2N/Cu multilayered thin films were synthesized by a reactive pulsed DC magnetron sputtering system. The influence of bilayer periods on the microstructures, mechanical, toughness, tribological and corrosion properties is discussed in this paper. Suitable deposition parameters to develop a multilayer thin film with adequate mechanical properties and anticorrosion performance are also proposed. 2. Experimental procedure A DC power supply connected with a pulse controller (SPIK 2000A, Shen Chang Electric Co., Taiwan) was employed for the bipolar asymmetry reactive pulsed DC magnetron sputtering. A pulse frequency of 20 kHz was used during sputtering. A pulsing bias of − 100 V was applied to the substrate through a DC power supply and a pulse unit with 20 kHz frequency. No attempt was made to coordinate the pulse cycles. The nanostructured Cr2N/Cu multilayer thin films were deposited on p-type (100) silicon wafer substrates. Cr and Cu targets with 99.99 wt.% purity and 76.2 mm in diameter were used. In this
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work, the pure Cr and Cu targets were in opposite positions and Si substrates were mounted on two sides of a rotating barrel between two targets. Multilayered thin films were deposited by alternately rotating the substrates between the plasma of Cr and Cu targets. The target power values of Cr and Cu were adjusted to achieve a fixed thickness ratio of Cr2N/Cu around 9:1. Various bilayer periods were achieved by controlling the holding time of Si substrates in the plasma stream from Cr or Cu target. The deposition time of each coating was controlled to achieve a fixed thickness of around 1.4 µm. The typical deposition conditions for each coating are listed in Table 1. Chemical compositions of coatings were analyzed with a field emission electron probe microanalyzer (FE-EPMA, JXA-8500F, JEOL, Japan) with a ZAF-corrected program. The surface morphology and surface roughness of each coating were investigated by an AFM (DI 3100, Veeco, USA). The cross-sectional morphologies of coatings were examined with a field emission (FE)-SEM (JSM-6700F, JEOL, Japan) and FE-TEM (Tecnai F30, Philips, Netherlands). A glancing angle X-ray diffractometer (XRD-6000, Shimadzu, Japan) with an incidence angle of 4° was adopted to study the crystal structures of each coating. The nanohardness and elastic modulus of multilayered thin films were investigated by means of a nanoindenter (Triboscope, Hysitron, USA) using a Berkovich 142.3° diamond probe at a maximum applied load of 4 mN. A micro Vickers hardness tester was further used to evaluate the fracture toughness, KIC, of coatings based on the following equation [12]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi E P H c3 = 2
KIC = δ
ð1Þ
where P is the applied indentation load and δ is an indenter geometry constant, equal to 1.53 N and 0.016, respectively, for a Vickers diamond pyramid indenter. E, H and c are elastic modulus, hardness and radial crack length of the coating, respectively. The radial crack length was evaluated using a FE-SEM. A pin-on-disk wear method was used to investigate the wear resistance of coatings. A cemented tungsten carbide (WC + 6 wt.% Co) ball, 5 mm in diameter was adopted as the stationary pin. A normal load of 1 N was applied. The sliding speed was 27.2 mm/s with a wear track diameter of 8 mm. The test temperature was 20 °C, and the relative humidity was kept at 60%. The wear time was 40 min for each test. The wear rate of each coating was determined based on the following equation [13]: t 3t 2 + 4b2 2πr WR =
ð2Þ
6bFn S
where t is the depth of the wear track determined using a surface profilometer, b is the width of the wear track, r is the radius of the wear track, Fn is the normal load and S is the sliding distance.
Dynamic electrochemical polarization tests in 3.5 wt.% sodium chloride aqueous solution were conducted with a Potentiostat (5000 EW, Jiehan, Taiwan) to evaluate the corrosion behavior of coatings. A standard measuring cell with three electrodes was used. The auxiliary and reference electrodes were platinum and saturated calomel (SCE), respectively. The measurements of the polarization in the range of − 300 to 300 mV versus open circuit potential at the scanning rate of 0.5 mV/s were performed. The corrosion current density, Icorr, corrosion potential, Ecorr and polarization resistance, Rp were determined using the Tafel analysis method. 3. Results and discussion 3.1. Composition and phase characterizations of Cr2N/Cu multilayers According to the FE-EPMA analysis, almost the same chemical composition was obtained for each coating. The means of the chemical compositions of the Cr2N/Cu multilayered thin films are 61.3 ± 0.8% Cr-9.2 ± 0.9% Cu-29.5 ± 0.6% N (in at.%). The Cr/N atomic ratio is almost equal to 2 indicating a pure Cr2N phase is achieved. The glancing angle X-ray diffraction patterns of multilayer coatings at different bilayer periods and the bare Si substrate are revealed in Fig. 1. A preferred orientation of Cr2N (111) reflection can be found for each coating. The Cr2N (002) reflection is also observed, whereas the intensity of other Cr2N peaks is too low to be recognized. No XRD peak corresponding to CrN was found due to an Ar/N2 ratio of 2 during sputtering. Meanwhile, the Cu (111) reflection can be observed on each coating. On the other hand, the Cu (200) peaks are also revealed in the L30 and L40 coatings since relatively thick Cu multilayers were produced by holding specimens in the Cu plasma stream for longer time periods. Not any Cu related nitride phase can be recognized in Fig. 1. In addition, the XRD peaks of Si substrate were also observed due to a high incidence angle, 4°, of X-ray used to penetrate the Cr2N/ Cu multilayered coatings. A typical fine granular structure is observed on the surface of each thin film as revealed by AFM. The average surface roughness, Ra, of each multilayered coating is listed in Table 1. A rather low surface roughness with a Ra = 0.73 nm is found on the L5 coating. It can be seen that the surface roughness increases with increasing bilayer period, which is attributed to the larger grain size and higher degree of crystallization of multilayer thin films at higher bilayer periods [9]. Fig. 2(a) and (b) depict the cross-sectional SEM morphologies of L6 and L30 coatings, respectively. A dense and compact structure was found on the L6 coating. The columnar structure of a pure Cr interlayer was also found between the coating and the Si substrate. The thickness values of Cr interlayer and multilayered coatings were found to be around 100 nm and 1316 nm, respectively. Similar crosssectional morphologies are also observed on coatings with bilayer period less than 30 nm, in which the multilayered nanostructure is hard to recognize due to the resolution limits of FE-SEM. The
Table 1 Typical deposition conditions, bilayer period, surface roughness and wear rate for Cr2N/Cu multilayered coatings. Sample
L5
Designed bilayer period (nm) Plasma holding time (s) Pulsed target power Base pressure Plasma etching Interlayer Working pressure Ar:N2 ratio Substrate heating Pulsed substrate bias Surface roughness, Ra (nm) Wear rate x10− 5 (mm3/N/m)
5 6 8 12 5 6 8 12 415 W for Cr and 19 W for Cu targets under 2 kHz pulsed frequency 1.6 × 10− 3 Pa Ar plasma for 10 min at 1.2 Pa under substrate bias − 500 V 100 nm thick pure Cr interlayer 4.0 × 10− 1 Pa 2:1 250 °C − 100 V under 2 kHz pulse frequency 0.73 ± 0.06 0.76 ± 0.06 0.79 ± 0.02 0.78 ± 0.01 0.38 ± 0.08 7.08 ± 3.72 4.04 ± 1.30 11.06 ± 1.47
L6
L8
L12
L20
L30
L40
20 20
30 30
40 40
0.94 ± 0.03 7.65 ± 4.23
1.08 ± 0.10 4.53 ± 1.67
1.15 ± 0.01 6.07 ± 1.70
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Fig. 1. Glancing angle X-ray diffraction patterns of the bare Si substrate and Cr2N/Cu multilayered coatings at different bilayer periods.
nanostructured L30 coating with clear bilayer period features can be recognized in the cleaved cross-sectional image in Fig. 2(b). Ladderlike structures with average bilayer periods of 30.1 nm were achieved. The FE-TEM technique was further used to explore the detailed microstructures of multilayer coatings with less than 20 nm bilayer periods. A low magnification cross-sectional TEM micrograph of an L12 coating is illustrated in Fig. 3. The multilayered nanostructure with an average bilayer period of 12.8 nm can be recognized clearly. Long columnar structures with various widths, ranging from several tens to hundreds of nm, are also observed throughout the whole multilayer coating. The cross-sectional TEM micrograph of L12 coating at higher magnification is also inserted in Fig. 3. Two Cr2N (111) arcs of electron diffraction patterns shown in Fig. 3 indicate a strong preferred orientation of the coating, which is also confirmed by the XRD shown in Fig. 1. It was observed that the interface between the Cr2N and Cu layers is wavy. Although the periodicity of the coating is less clear at higher magnification, the gray and dark regions illustrated in the inserted micrograph are the Cr2N and Cu layers (indicated by arrows), respectively. The average thickness values of Cr2N and Cu layers are 10.8 nm and 2.0 nm, respectively. A nanolayered structure with sequentially alternating Cr2N and Cu layers can be confirmed again in this work.
Fig. 2. The cross-sectional SEM images of (a) L6 (Λ = 6 nm) and (b) L30 (Λ = 30 nm) coatings.
The plastic deformation resistance, H3/E⁎2 of Cr2N/Cu multilayered coatings versus the bilayer periods is also shown on the top of Fig. 4, where E⁎ and H are reduced elastic modulus and hardness of multilayered coatings, respectively. It is found that the same
3.2. Mechanical properties of Cr2N/Cu multilayered thin films The hardness and elastic modulus of Cr2N/Cu multilayered coatings as a function of bilayer period are shown in Fig. 4. Apparently, the hardness increases with decreasing bilayer period and reaches a maximum value at Λ = 12 nm and levels off at Λ = 6–12 nm, followed by a drastic decrease when the bilayer period decreases to 5 nm. A maximum hardness and elastic modulus, which reach 17.3 GPa and 210 GPa, respectively, were observed for the L12 coating with 12 nm bilayer period, whereas a minimum hardness which reached 14.3 GPa was found for the L30 coating with 30 nm bilayer period. It should be pointed out that the hardness of each Cr2N/Cu multilayered coatings is still higher than that of the Si substrate.
Fig. 3. The cross-sectional TEM micrograph of L12 (Λ = 12 nm) coating.
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Fig. 4. The hardness, elastic modulus, plastic deformation resistance and fracture toughness as a function of bilayer period Λ for the Cr2N/Cu multilayered coatings.
tendency is also observed in the relationship between the plastic deformation resistance and bilayer periods. The maximum plastic deformation resistance, 0.12 GPa, was reached when the bilayer period was 12 nm. It is also important to notice that similar values of hardness, elastic modulus and plastic deformation resistance were observed for the multilayer coatings with Λ = 6, 8 and 12 nm. The hardness enhancement and the decrease with further decreasing of the bilayer period are a typical phenomena for multilayered coatings [1]. Several theories including the Koehler effect [14] and Hall–Petch hardening [15,16] can be used to explain the hardening of the coating in a small range of L values of 6–12 nm in this work. According to the Koehler effect [14], the hardness enhancement of the multilayer is caused by the shear modulus, G, a difference between bilayer materials. An additional stress is needed to propagate a
dislocation from a layer with lower G to one with higher G. Since the shear modulus of Cr2N and Cu is 100–145 GPa [17,18] and 40–58 GPa [19,20], respectively, the hardness enhancement effect can be clearly observed in this study. It is also noticed that the hardness values of the Cr2N/Cu multilayered coatings are higher than those reported in the literature [7–11] due to a relatively thinner Cu layer produced in this work. The fracture toughness of the Cr2N/Cu multilayered coatings versus the bilayer periods is also shown in Fig. 4. The surface morphologies of radial cracks on L6 and L30 coatings are also inserted in Fig. 4. It is suggested by Musil and Jirout [21] that cracks in thin films strongly depend on the film thickness and mechanical properties of both the substrate and the coating. The geometrical form of cracks depends on the substrate hardness, Hs. The cracks are radial for hard substrates (Hs N 0.5Hf) [21], where Hf is the hardness of coating. In this work, radial cracks were observed due to the use of a hard Si substrate with a hardness of around 11 GPa. A quite different tendency is observed as compared with other mechanical properties of multilayered coatings. The fracture toughness increases when the bilayer period is reduced from 40 to 30 nm and keeps about the same value at 20 nm and then decreases drastically when the bilayer period is less than 20 nm. The KIC value finally increases when the bilayer period reaches 5 nm. The maximum fracture toughness is obtained for L20 and L30 coatings with lower hardness and plastic deformation resistance. A similar result was also reported by He et al. [10] on TiC/ Cu multilayered coatings. They found that the toughness of TiC/Cu multilayered coatings increased with increasing bilayer period due to a greater deformation tolerance provided by the softer and more ductile Cu metal layers. Analysis of the mechanical properties results shown in Fig. 4 confirms that the bilayer period of the Cr2N/Cu multilayered coatings should be prepared in a moderate range to achieve a high toughness and an adequate hardness, such as the L20 coating with Λ = 20 nm. Fig. 5 illustrates the coefficient of friction (COF) versus the wear length of all thin films. The average COF of each coating is also inserted in Fig. 5. A minimum COF around 0.41 was found for L8. The L6 coating
Fig. 5. The friction coefficients of multilayered coatings against a WC-Co ball as a function of wear length.
C.-L. Li et al. / Surface & Coatings Technology 204 (2009) 941–946
also exhibits a lower COF, 0.64, than other coatings. The wear rate of each coating is listed in Table 1. It is found that the minimum wear rate was obtained on the L5 coating indicating a good wear resistance achieved in this work. A rather low wear rate is also achieved on the L8 coating with the minimum value of COF. The wear scar morphology of L8 coating is shown in Fig. 6. A smooth wear track is found on the L8 coating. No delamination or crack is observed adjacent to the wear track. Similar result is also found on the L5 coating. Smooth wear scar and a few delaminations of coating were also observed on the L6 coatings. On the other hand, severe spallations of multilayered coatings in the wear tracks were found after pin-on-disk wear tests on other coatings. It should be noticed that the bare silicon substrate exhibited the highest COF and the wear rate than the multilayered coatings. Martinez et al. [22] proposed a wear mechanism for the CrN/Cr multilayers deposited onto silicon substrates. They suggested that the wear mechanisms should be more related with the formation of thin oxide tribolayers, which were formed and removed continuously when sliding. This process continued across some nanometers until the coating delamination occurred. This delamination could be produced by the accumulation and propagation of fatigue tensile cracks during the sliding [22]. In this work, the layer by layer wear mechanism is also valid in the Cr2N/Cu multilayered coating system. The surface Cr2N layer begins to wear until fatigue cracks occurs, which can propagate and reach the Cr2N/Cu interface. Therefore, this Cr2N layer is delaminated and the underlying Cu layer is quickly removed. Furthermore, a fresh Cr2N surface starts to wear and the process repeats again. Martinez et al. [22] suggested that the wear resistance will be enhanced by very thin CrN layers because they would not be detached but progressively wear. This may be the case for the L5, L6 and L8 multilayered coatings with bilayer periods of 5, 6 and 8 nm in this study. 3.3. Corrosion properties evaluation of Cr2N/Cu multilayered thin films A family of polarization curves of Cr2N/Cu multilayered coatings and the bare Si substrate in 3.5 wt.% NaCl aqueous solution are shown in Fig. 7. The surface morphology of the corroded surface of L5 coating was also inserted in Fig. 7. The noblest corrosion potential, the lowest corrosion current density and the best corrosion protection properties are achieved for the coating with the lowest bilayer period. The corrosion potential, Ecorr, corrosion current density, Icorr and polarization resistance, Rp of coatings versus the bilayer period is represented in Fig. 8. In general, the corrosion resistance of each coating is better than that of the bare Si substrate. It is obvious that the higher corrosion potential, higher polarization resistance and
Fig. 6. The wear scar morphology of the L8 (Λ = 8 nm) coating after the pin-on-disk wear test.
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Fig. 7. Potentiodynamic polarization curves of multilayered coatings and the bare Si substrate in 3.5% NaCl aqueous solution.
lower corrosion current density are observed as the bilayer period decreases, which corresponds to an increasing number of Cr2N/Cu multilayers in a fixed coating thickness. The relatively high corrosion potential value for coatings with smaller bilayer period indicates that more time is required for the NaCl electrolyte to reach the Si substrate by diffusion through small surface defects [23]. However, an abrupt deviation of the tendency is found on the L8 coating. It is argued that severe thin film defects produced during sputtering are possibly responsible for this degradation. More work is underway to address this problem.
Fig. 8. Electrochemical parameters, Ecorr, Icorr and Rp of coatings in 3.5% NaCl aqueous solution as a function of bilayer period Λ for the Cr2N/Cu multilayers. The corrosion data of the bare Si substrate is also added as a reference.
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Fenker et al. [24] studied the corrosion resistance of CrN/Cr multilayer films and reported that only a slight decrease of the corrosion current density within the passivation range was observed for an increasing number of multilayer periods, i.e. for an increasing number of interfaces. Dobrzański et al. [23] prepared thin multilayer Ti/CrN, Ti/ZrN, Ti/TiAlN, TiAlN/Mo coatings on brass and demonstrated that an increase in the number of layers resulted in a significant increase in the corrosion resistance. Similar results were also achieved in this work. It is believed that the multilayer coating provides better protection to prevent corrosion attacks, such as scratches or crevices. Small pores and cracks in the coating and the effect of big cathode area (coating) versus small anode surface (bottom of pores) will decrease the corrosion protection of coating. Surface defects on a single layer may be further retarded by the successively deposited multilayered coatings [23]. Thus the penetration path for a corrosive electrolyte is extended or blocked. This explains the reason why the L5 coating with the highest number of thin multilayers provides the best anticorrosion performance by blocking the corrosive electrolyte's penetration through coating defects into the substrate material, better than in L30 or L40 coatings with fewer multilayers. 4. Conclusion A series of Cr2N/Cu multilayered thin films with bilayer periods ranging from 5 to 40 nm were prepared by the bipolar asymmetric reactive pulsed DC magnetron sputtering system successfully. It was observed that the hardness, elastic modulus and plastic deformation resistance increased with decreasing bilayer period and reached a maximum at Λ = 6–12 nm and then decreased with decreasing bilayer period. Good fracture toughness was obtained for coatings at Λ = 20–30 nm. For Cr2N/Cu multilayered coatings at 5 and 6 nm bilayer periods, a combination of excellent mechanical, tribological and anticorrosion properties has been achieved in this work.
Acknowledgement The authors gratefully acknowedge the financial support of the National Science Council, Taiwan, ROC through Contract No. NSC972221-E-236-001-MY2. References [1] U. Helmersson, S. Todorova, S.A. Barnett, J.-E. Sundgren, L.C. Markert, J.E. Greene, J. Appl. Phys. 62 (1987) 481. [2] S. Veprek, S. Reiprich, Li Shizhi, Appl. Phys. Lett. 66 (1995) 2640. [3] S. Veprek, M.G.J. Veprek-Heijman, Surf. Coat. Technol. 201 (2007) 6064. [4] A. Madan, X. Chu, S.A. Barnett, Appl. Phys. Lett. 68 (1996) 2198. [5] E. Martínez, J. Romero, A. Lousa, J. Esteve, J. Phys., D, Appl. Phys. 35 (2002) 1880. [6] Y.C. Kuo, J.W. Lee, C.J. Wang, Y.J. Chang, Surf. Coat. Technol. 202 (2007) 854. [7] K.P. Almtoft, M. Kristensen, J. Chevallier, J. Bøttiger, Surf. Coat. Technol. 201 (2007) 4767. [8] Y.Y. Tse, G. Abadias, A. Michel, C. Tromas, M. Jaouen, Mater. Res. Soc. Symp. Proc. 778 (2003) U.6.8.1. [9] Y.J. Kim, T.J. Byun, H.Y. Lee, J.G. Han, Surf. Coat. Technol. 202 (2008) 5508. [10] J.L. He, W.Z. Li, H.D. Li, C.H. Liu, Surf. Coat. Technol. 103–104 (1998) 276. [11] C.B. Wei, X.B. Tian, Y. Yanga, S.Q. Yang, Ricky K.Y. Fu, Paul K. Chu, Surf. Coat. Technol. 202 (2007) 189. [12] P. Jedrzejowski, J.E. Klemberg-Sapieha, L. Martinu, Thin Solid Films 426 (2003) 150. [13] S. Ma, J. Procházka, P. Karvánková, Q. Ma, X. Niu, X. Wang, D. Ma, K. Xu, S. Veprek, Surf. Coat. Technol. 194 (2005) 143. [14] J.S. Koehler, Phys. Rev. B2 (1970) 547. [15] E.O. Hall, Proc. Phys. Soc. London, Sect. B 64 (1951) 747. [16] N.J. Petch, J. Iron Steel Inst., London 174 (1953) 25. [17] S.M. Aouadi, D.M. Schultze, S.L. Rohde, K.C. Wong, K.A.R. Mitchell, Surf. Coat. Technol. 140 (2001) 269. [18] P. Hones, N. Martin, M. Regula, F. Lévy, J. Phys., D, Appl. Phys. 36 (2003) 1023. [19] D. Baral, J.E. Hillard, J.B. Ketterson, K. Miyano, J. Appl. Phys. 53 (1982) 3552. [20] K.P. Almtoft, M. Kristensen, J. Chevallier, J. Bøttiger, Surf. Coat. Technol. 201 (2007) 4767. [21] J. Musil, M. Jirout, Surf. Coat. Technol. 201 (2007) 5148. [22] E. Martinez, J. Romero, A. Lousa, J. Esteve, Surf. Coat. Technol. 163–164 (2003) 571. [23] L.A. Dobrzański, K. Lukaszkowicz, A. Zarychta, L. Cunha, J. Mater. Process. Technol. 164–165 (2005) 816. [24] M. Fenker, M. Balzer, H.A. Jehn, H. Kappl, J.-J. Lee, K.H. Lee, H.S. Park, Surf. Coat. Technol. 150 (2002) 101.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Characteristics of Cr2N/Cu multilayered thin films with different bilayer thickness Chia-Lin Li a, Fan-Bean Wu b, Jyh-Wei Lee a,c,d,⁎, Yan-Zuo Tsai e, Li-Chun Chang f a
Institute of Mechatronic Engineering, Tungnan University, Taipei, Taiwan, ROC Department of Materials Engineering, National United University, Maoli, Taiwan, ROC c Department of Mechanical Engineering, Tungnan University, Taipei, Taiwan, ROC d Research Center for Micro/Nanotechnology, Tungnan University, Taipei, Taiwan, ROC e Department of Materials Science and Engineering, National Tsing Hua University, Taiwan, ROC f Department of Materials Engineering, Mingchi University of Technology, Taipei, Taiwan, ROC b
a r t i c l e
i n f o
Available online 30 May 2009 Keywords: Pulsed DC reactive magnetron sputtering Cr2N/Cu multilayered coatings Bilayer period Pin-on-disk wear test Corrosion test
a b s t r a c t Nanostructured Cr2N/Cu multilayer coatings were deposited periodically by a bipolar asymmetric reactive pulsed DC magnetron sputtering system. The crystalline structure of multilayer coatings was determined by a glancing angle X-ray diffractometer. Microstructures of thin films were examined by an atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. A nanoindenter, a micro Vickers hardness tester and pin-on-disk wear tests were used to evaluate the hardness, fracture toughness and tribological properties of the thin films, respectively. Electrochemical tests in 3.5 wt.% NaCl aqueous solution were performed to evaluate the corrosion resistance of multilayered coatings. The bilayer period of the nanostructured multilayer coatings were controlled within the range from 5 to 40 nm. It was found that the hardness increased with decreasing bilayer period and leveled off at small periods around 6–12 nm. The Cr2N/Cu multilayered coatings with bilayer periods of 5 and 6 nm exhibited a combination of high hardness, good tribological and good anticorrosion performance in this study. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the last decade, many researchers have devoted much effort to TiN-based transition metal nitride/nitride superhard (50 GPa) heterostructured multilayer coatings [1–3] due to their pronounced strength enhancement, optimal hardness/toughness ratios and excellent wear resistance. A different class of metal/transition metal nitride multilayered coatings, such as CrN/Cr [4] and Mo/NbN [5] has also been used to obtain both high hardness and toughness, in which the soft metal and the hard nitride provided ductility and hardness, respectively. The hardness of such metal/transition metal nitride multilayered coatings also increased with decreasing bilayer period and leveled off at small periods. In previous work [6], an unusual hardness enhancement to 21 GPa at maximum and a lower coefficient of friction to 0.13 were observed for a nanocomposite Cr–Cu–N coatings containing 15.7 at.% Cu. A combination of sufficient hardness and good antibacterial properties were achieved in Cr–Cu–N nanocomposite thin films with more than 15.7 at.% Cu. However, a hardness strengthening effect of the Cu contained in Cr2N/Cu [7] and TiN/Cu [8] multilayer coatings was not observed, which might be attributed to
⁎ Corresponding author. Department of Mechanical Engineering, Tungnan University, Taipei, Taiwan, ROC. Tel.: +886 2 86625980; fax: +886 2 86625919. E-mail address: [email protected] (J.-W. Lee). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.05.039
the soft Cu layer [8]. Although much effort has been devoted to study the microstructures and mechanical properties on the transition of metal nitride/Cu multilayered coatings [7–11], a fundamental understanding of the microstructures, mechanical, tribological and corrosion properties of such Cu containing multilayered coating is still lacking. In this study, Cr2N/Cu multilayered thin films were synthesized by a reactive pulsed DC magnetron sputtering system. The influence of bilayer periods on the microstructures, mechanical, toughness, tribological and corrosion properties is discussed in this paper. Suitable deposition parameters to develop a multilayer thin film with adequate mechanical properties and anticorrosion performance are also proposed. 2. Experimental procedure A DC power supply connected with a pulse controller (SPIK 2000A, Shen Chang Electric Co., Taiwan) was employed for the bipolar asymmetry reactive pulsed DC magnetron sputtering. A pulse frequency of 20 kHz was used during sputtering. A pulsing bias of − 100 V was applied to the substrate through a DC power supply and a pulse unit with 20 kHz frequency. No attempt was made to coordinate the pulse cycles. The nanostructured Cr2N/Cu multilayer thin films were deposited on p-type (100) silicon wafer substrates. Cr and Cu targets with 99.99 wt.% purity and 76.2 mm in diameter were used. In this
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work, the pure Cr and Cu targets were in opposite positions and Si substrates were mounted on two sides of a rotating barrel between two targets. Multilayered thin films were deposited by alternately rotating the substrates between the plasma of Cr and Cu targets. The target power values of Cr and Cu were adjusted to achieve a fixed thickness ratio of Cr2N/Cu around 9:1. Various bilayer periods were achieved by controlling the holding time of Si substrates in the plasma stream from Cr or Cu target. The deposition time of each coating was controlled to achieve a fixed thickness of around 1.4 µm. The typical deposition conditions for each coating are listed in Table 1. Chemical compositions of coatings were analyzed with a field emission electron probe microanalyzer (FE-EPMA, JXA-8500F, JEOL, Japan) with a ZAF-corrected program. The surface morphology and surface roughness of each coating were investigated by an AFM (DI 3100, Veeco, USA). The cross-sectional morphologies of coatings were examined with a field emission (FE)-SEM (JSM-6700F, JEOL, Japan) and FE-TEM (Tecnai F30, Philips, Netherlands). A glancing angle X-ray diffractometer (XRD-6000, Shimadzu, Japan) with an incidence angle of 4° was adopted to study the crystal structures of each coating. The nanohardness and elastic modulus of multilayered thin films were investigated by means of a nanoindenter (Triboscope, Hysitron, USA) using a Berkovich 142.3° diamond probe at a maximum applied load of 4 mN. A micro Vickers hardness tester was further used to evaluate the fracture toughness, KIC, of coatings based on the following equation [12]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi E P H c3 = 2
KIC = δ
ð1Þ
where P is the applied indentation load and δ is an indenter geometry constant, equal to 1.53 N and 0.016, respectively, for a Vickers diamond pyramid indenter. E, H and c are elastic modulus, hardness and radial crack length of the coating, respectively. The radial crack length was evaluated using a FE-SEM. A pin-on-disk wear method was used to investigate the wear resistance of coatings. A cemented tungsten carbide (WC + 6 wt.% Co) ball, 5 mm in diameter was adopted as the stationary pin. A normal load of 1 N was applied. The sliding speed was 27.2 mm/s with a wear track diameter of 8 mm. The test temperature was 20 °C, and the relative humidity was kept at 60%. The wear time was 40 min for each test. The wear rate of each coating was determined based on the following equation [13]: t 3t 2 + 4b2 2πr WR =
ð2Þ
6bFn S
where t is the depth of the wear track determined using a surface profilometer, b is the width of the wear track, r is the radius of the wear track, Fn is the normal load and S is the sliding distance.
Dynamic electrochemical polarization tests in 3.5 wt.% sodium chloride aqueous solution were conducted with a Potentiostat (5000 EW, Jiehan, Taiwan) to evaluate the corrosion behavior of coatings. A standard measuring cell with three electrodes was used. The auxiliary and reference electrodes were platinum and saturated calomel (SCE), respectively. The measurements of the polarization in the range of − 300 to 300 mV versus open circuit potential at the scanning rate of 0.5 mV/s were performed. The corrosion current density, Icorr, corrosion potential, Ecorr and polarization resistance, Rp were determined using the Tafel analysis method. 3. Results and discussion 3.1. Composition and phase characterizations of Cr2N/Cu multilayers According to the FE-EPMA analysis, almost the same chemical composition was obtained for each coating. The means of the chemical compositions of the Cr2N/Cu multilayered thin films are 61.3 ± 0.8% Cr-9.2 ± 0.9% Cu-29.5 ± 0.6% N (in at.%). The Cr/N atomic ratio is almost equal to 2 indicating a pure Cr2N phase is achieved. The glancing angle X-ray diffraction patterns of multilayer coatings at different bilayer periods and the bare Si substrate are revealed in Fig. 1. A preferred orientation of Cr2N (111) reflection can be found for each coating. The Cr2N (002) reflection is also observed, whereas the intensity of other Cr2N peaks is too low to be recognized. No XRD peak corresponding to CrN was found due to an Ar/N2 ratio of 2 during sputtering. Meanwhile, the Cu (111) reflection can be observed on each coating. On the other hand, the Cu (200) peaks are also revealed in the L30 and L40 coatings since relatively thick Cu multilayers were produced by holding specimens in the Cu plasma stream for longer time periods. Not any Cu related nitride phase can be recognized in Fig. 1. In addition, the XRD peaks of Si substrate were also observed due to a high incidence angle, 4°, of X-ray used to penetrate the Cr2N/ Cu multilayered coatings. A typical fine granular structure is observed on the surface of each thin film as revealed by AFM. The average surface roughness, Ra, of each multilayered coating is listed in Table 1. A rather low surface roughness with a Ra = 0.73 nm is found on the L5 coating. It can be seen that the surface roughness increases with increasing bilayer period, which is attributed to the larger grain size and higher degree of crystallization of multilayer thin films at higher bilayer periods [9]. Fig. 2(a) and (b) depict the cross-sectional SEM morphologies of L6 and L30 coatings, respectively. A dense and compact structure was found on the L6 coating. The columnar structure of a pure Cr interlayer was also found between the coating and the Si substrate. The thickness values of Cr interlayer and multilayered coatings were found to be around 100 nm and 1316 nm, respectively. Similar crosssectional morphologies are also observed on coatings with bilayer period less than 30 nm, in which the multilayered nanostructure is hard to recognize due to the resolution limits of FE-SEM. The
Table 1 Typical deposition conditions, bilayer period, surface roughness and wear rate for Cr2N/Cu multilayered coatings. Sample
L5
Designed bilayer period (nm) Plasma holding time (s) Pulsed target power Base pressure Plasma etching Interlayer Working pressure Ar:N2 ratio Substrate heating Pulsed substrate bias Surface roughness, Ra (nm) Wear rate x10− 5 (mm3/N/m)
5 6 8 12 5 6 8 12 415 W for Cr and 19 W for Cu targets under 2 kHz pulsed frequency 1.6 × 10− 3 Pa Ar plasma for 10 min at 1.2 Pa under substrate bias − 500 V 100 nm thick pure Cr interlayer 4.0 × 10− 1 Pa 2:1 250 °C − 100 V under 2 kHz pulse frequency 0.73 ± 0.06 0.76 ± 0.06 0.79 ± 0.02 0.78 ± 0.01 0.38 ± 0.08 7.08 ± 3.72 4.04 ± 1.30 11.06 ± 1.47
L6
L8
L12
L20
L30
L40
20 20
30 30
40 40
0.94 ± 0.03 7.65 ± 4.23
1.08 ± 0.10 4.53 ± 1.67
1.15 ± 0.01 6.07 ± 1.70
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Fig. 1. Glancing angle X-ray diffraction patterns of the bare Si substrate and Cr2N/Cu multilayered coatings at different bilayer periods.
nanostructured L30 coating with clear bilayer period features can be recognized in the cleaved cross-sectional image in Fig. 2(b). Ladderlike structures with average bilayer periods of 30.1 nm were achieved. The FE-TEM technique was further used to explore the detailed microstructures of multilayer coatings with less than 20 nm bilayer periods. A low magnification cross-sectional TEM micrograph of an L12 coating is illustrated in Fig. 3. The multilayered nanostructure with an average bilayer period of 12.8 nm can be recognized clearly. Long columnar structures with various widths, ranging from several tens to hundreds of nm, are also observed throughout the whole multilayer coating. The cross-sectional TEM micrograph of L12 coating at higher magnification is also inserted in Fig. 3. Two Cr2N (111) arcs of electron diffraction patterns shown in Fig. 3 indicate a strong preferred orientation of the coating, which is also confirmed by the XRD shown in Fig. 1. It was observed that the interface between the Cr2N and Cu layers is wavy. Although the periodicity of the coating is less clear at higher magnification, the gray and dark regions illustrated in the inserted micrograph are the Cr2N and Cu layers (indicated by arrows), respectively. The average thickness values of Cr2N and Cu layers are 10.8 nm and 2.0 nm, respectively. A nanolayered structure with sequentially alternating Cr2N and Cu layers can be confirmed again in this work.
Fig. 2. The cross-sectional SEM images of (a) L6 (Λ = 6 nm) and (b) L30 (Λ = 30 nm) coatings.
The plastic deformation resistance, H3/E⁎2 of Cr2N/Cu multilayered coatings versus the bilayer periods is also shown on the top of Fig. 4, where E⁎ and H are reduced elastic modulus and hardness of multilayered coatings, respectively. It is found that the same
3.2. Mechanical properties of Cr2N/Cu multilayered thin films The hardness and elastic modulus of Cr2N/Cu multilayered coatings as a function of bilayer period are shown in Fig. 4. Apparently, the hardness increases with decreasing bilayer period and reaches a maximum value at Λ = 12 nm and levels off at Λ = 6–12 nm, followed by a drastic decrease when the bilayer period decreases to 5 nm. A maximum hardness and elastic modulus, which reach 17.3 GPa and 210 GPa, respectively, were observed for the L12 coating with 12 nm bilayer period, whereas a minimum hardness which reached 14.3 GPa was found for the L30 coating with 30 nm bilayer period. It should be pointed out that the hardness of each Cr2N/Cu multilayered coatings is still higher than that of the Si substrate.
Fig. 3. The cross-sectional TEM micrograph of L12 (Λ = 12 nm) coating.
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Fig. 4. The hardness, elastic modulus, plastic deformation resistance and fracture toughness as a function of bilayer period Λ for the Cr2N/Cu multilayered coatings.
tendency is also observed in the relationship between the plastic deformation resistance and bilayer periods. The maximum plastic deformation resistance, 0.12 GPa, was reached when the bilayer period was 12 nm. It is also important to notice that similar values of hardness, elastic modulus and plastic deformation resistance were observed for the multilayer coatings with Λ = 6, 8 and 12 nm. The hardness enhancement and the decrease with further decreasing of the bilayer period are a typical phenomena for multilayered coatings [1]. Several theories including the Koehler effect [14] and Hall–Petch hardening [15,16] can be used to explain the hardening of the coating in a small range of L values of 6–12 nm in this work. According to the Koehler effect [14], the hardness enhancement of the multilayer is caused by the shear modulus, G, a difference between bilayer materials. An additional stress is needed to propagate a
dislocation from a layer with lower G to one with higher G. Since the shear modulus of Cr2N and Cu is 100–145 GPa [17,18] and 40–58 GPa [19,20], respectively, the hardness enhancement effect can be clearly observed in this study. It is also noticed that the hardness values of the Cr2N/Cu multilayered coatings are higher than those reported in the literature [7–11] due to a relatively thinner Cu layer produced in this work. The fracture toughness of the Cr2N/Cu multilayered coatings versus the bilayer periods is also shown in Fig. 4. The surface morphologies of radial cracks on L6 and L30 coatings are also inserted in Fig. 4. It is suggested by Musil and Jirout [21] that cracks in thin films strongly depend on the film thickness and mechanical properties of both the substrate and the coating. The geometrical form of cracks depends on the substrate hardness, Hs. The cracks are radial for hard substrates (Hs N 0.5Hf) [21], where Hf is the hardness of coating. In this work, radial cracks were observed due to the use of a hard Si substrate with a hardness of around 11 GPa. A quite different tendency is observed as compared with other mechanical properties of multilayered coatings. The fracture toughness increases when the bilayer period is reduced from 40 to 30 nm and keeps about the same value at 20 nm and then decreases drastically when the bilayer period is less than 20 nm. The KIC value finally increases when the bilayer period reaches 5 nm. The maximum fracture toughness is obtained for L20 and L30 coatings with lower hardness and plastic deformation resistance. A similar result was also reported by He et al. [10] on TiC/ Cu multilayered coatings. They found that the toughness of TiC/Cu multilayered coatings increased with increasing bilayer period due to a greater deformation tolerance provided by the softer and more ductile Cu metal layers. Analysis of the mechanical properties results shown in Fig. 4 confirms that the bilayer period of the Cr2N/Cu multilayered coatings should be prepared in a moderate range to achieve a high toughness and an adequate hardness, such as the L20 coating with Λ = 20 nm. Fig. 5 illustrates the coefficient of friction (COF) versus the wear length of all thin films. The average COF of each coating is also inserted in Fig. 5. A minimum COF around 0.41 was found for L8. The L6 coating
Fig. 5. The friction coefficients of multilayered coatings against a WC-Co ball as a function of wear length.
C.-L. Li et al. / Surface & Coatings Technology 204 (2009) 941–946
also exhibits a lower COF, 0.64, than other coatings. The wear rate of each coating is listed in Table 1. It is found that the minimum wear rate was obtained on the L5 coating indicating a good wear resistance achieved in this work. A rather low wear rate is also achieved on the L8 coating with the minimum value of COF. The wear scar morphology of L8 coating is shown in Fig. 6. A smooth wear track is found on the L8 coating. No delamination or crack is observed adjacent to the wear track. Similar result is also found on the L5 coating. Smooth wear scar and a few delaminations of coating were also observed on the L6 coatings. On the other hand, severe spallations of multilayered coatings in the wear tracks were found after pin-on-disk wear tests on other coatings. It should be noticed that the bare silicon substrate exhibited the highest COF and the wear rate than the multilayered coatings. Martinez et al. [22] proposed a wear mechanism for the CrN/Cr multilayers deposited onto silicon substrates. They suggested that the wear mechanisms should be more related with the formation of thin oxide tribolayers, which were formed and removed continuously when sliding. This process continued across some nanometers until the coating delamination occurred. This delamination could be produced by the accumulation and propagation of fatigue tensile cracks during the sliding [22]. In this work, the layer by layer wear mechanism is also valid in the Cr2N/Cu multilayered coating system. The surface Cr2N layer begins to wear until fatigue cracks occurs, which can propagate and reach the Cr2N/Cu interface. Therefore, this Cr2N layer is delaminated and the underlying Cu layer is quickly removed. Furthermore, a fresh Cr2N surface starts to wear and the process repeats again. Martinez et al. [22] suggested that the wear resistance will be enhanced by very thin CrN layers because they would not be detached but progressively wear. This may be the case for the L5, L6 and L8 multilayered coatings with bilayer periods of 5, 6 and 8 nm in this study. 3.3. Corrosion properties evaluation of Cr2N/Cu multilayered thin films A family of polarization curves of Cr2N/Cu multilayered coatings and the bare Si substrate in 3.5 wt.% NaCl aqueous solution are shown in Fig. 7. The surface morphology of the corroded surface of L5 coating was also inserted in Fig. 7. The noblest corrosion potential, the lowest corrosion current density and the best corrosion protection properties are achieved for the coating with the lowest bilayer period. The corrosion potential, Ecorr, corrosion current density, Icorr and polarization resistance, Rp of coatings versus the bilayer period is represented in Fig. 8. In general, the corrosion resistance of each coating is better than that of the bare Si substrate. It is obvious that the higher corrosion potential, higher polarization resistance and
Fig. 6. The wear scar morphology of the L8 (Λ = 8 nm) coating after the pin-on-disk wear test.
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Fig. 7. Potentiodynamic polarization curves of multilayered coatings and the bare Si substrate in 3.5% NaCl aqueous solution.
lower corrosion current density are observed as the bilayer period decreases, which corresponds to an increasing number of Cr2N/Cu multilayers in a fixed coating thickness. The relatively high corrosion potential value for coatings with smaller bilayer period indicates that more time is required for the NaCl electrolyte to reach the Si substrate by diffusion through small surface defects [23]. However, an abrupt deviation of the tendency is found on the L8 coating. It is argued that severe thin film defects produced during sputtering are possibly responsible for this degradation. More work is underway to address this problem.
Fig. 8. Electrochemical parameters, Ecorr, Icorr and Rp of coatings in 3.5% NaCl aqueous solution as a function of bilayer period Λ for the Cr2N/Cu multilayers. The corrosion data of the bare Si substrate is also added as a reference.
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Fenker et al. [24] studied the corrosion resistance of CrN/Cr multilayer films and reported that only a slight decrease of the corrosion current density within the passivation range was observed for an increasing number of multilayer periods, i.e. for an increasing number of interfaces. Dobrzański et al. [23] prepared thin multilayer Ti/CrN, Ti/ZrN, Ti/TiAlN, TiAlN/Mo coatings on brass and demonstrated that an increase in the number of layers resulted in a significant increase in the corrosion resistance. Similar results were also achieved in this work. It is believed that the multilayer coating provides better protection to prevent corrosion attacks, such as scratches or crevices. Small pores and cracks in the coating and the effect of big cathode area (coating) versus small anode surface (bottom of pores) will decrease the corrosion protection of coating. Surface defects on a single layer may be further retarded by the successively deposited multilayered coatings [23]. Thus the penetration path for a corrosive electrolyte is extended or blocked. This explains the reason why the L5 coating with the highest number of thin multilayers provides the best anticorrosion performance by blocking the corrosive electrolyte's penetration through coating defects into the substrate material, better than in L30 or L40 coatings with fewer multilayers. 4. Conclusion A series of Cr2N/Cu multilayered thin films with bilayer periods ranging from 5 to 40 nm were prepared by the bipolar asymmetric reactive pulsed DC magnetron sputtering system successfully. It was observed that the hardness, elastic modulus and plastic deformation resistance increased with decreasing bilayer period and reached a maximum at Λ = 6–12 nm and then decreased with decreasing bilayer period. Good fracture toughness was obtained for coatings at Λ = 20–30 nm. For Cr2N/Cu multilayered coatings at 5 and 6 nm bilayer periods, a combination of excellent mechanical, tribological and anticorrosion properties has been achieved in this work.
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