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Surface & Coatings Technolog

Microstructure and tribological properties of Cu–Zn/TiN multilayersfabricated by dual magnetron sputtering

C.B. Wei a,c, X.B. Tian a,b,c,⁎, Y. Yang a, S.Q. Yang a,b, Ricky K.Y. Fu c, Paul K. Chu c

a State Key Lab. of Advanced Welding Production & Technology, School of Materials Science & Engineering, Harbin Institute of Technology, 150001 Harbin, Chinab Shenzhen key Lab. of Composite Materials, Shenzhen-Tech-Innovation International, 518057 Shenzhen, China

c Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Received 16 December 2006; accepted in revised form 8 May 2007Available online 13 May 2007

Abstract

Metal/nitride multilayers have attracted much interest due to their superior features such as high hardness and good wear resistance. In thiswork, the microstructure and tribological properties of Cu–Zn/TiN multilayers with nm and sub-μm bilayer periods fabricated by magnetronsputtering are investigated. Delamination occurs when the individual TiN layers are too thin and the Cu–Zn sublayers are thick. Cu (111) and TiN(002) growth orientation have been found in the multilayers. The wear results show that Cu–Zn/TiN multilayers can offer good wear resistance.The wear resistance is improved with larger Cu–Zn content and bilayer periods. When the tCu–Zn:tTiN ratios are 6:1, the wear resistance of themultilayers is improved significantly with increasing the periods and the film shows longer punch through time than that with tCu–Zn:tTiN ratios ofabout 1:1. The hardness of multilayers is better than that of the steel substrate, but little influence has been found on the hardness with changingthe tCu–Zn:tTiN ratios or the bilayer periods.© 2007 Elsevier B.V. All rights reserved.

PACS: 68.55.Jk; 68.60.BsKeywords: Cu–Zn/TiN multilayers; Microstructure; Wear resistance; Magnetron sputtering

1. Introduction

In recent years, multilayers have attracted much attentiondue to their superior features such as high hardness and wearresistance. It has been theoretically shown [1,2] that propaga-tion of cracks approaching an interface with materialspossessing different hardness depends on the direction of thecracks. Cracks propagating from a softer material stop at or arediverted from the interface whereas cracks coming from thehard side may cross the interface and enter the softer material.Thus, a relatively thin layer with lower hardness than itssurrounding should be able to impede crack propagation [1,2].Multilayers possessing both high hardness and toughness maybe obtained by using alternating soft and hard thin layers.Previous research [3–5] has indicated that ceramic–metal

⁎ Corresponding author. State Key Lab. of Advanced Welding Production &Technology, School of Materials Science & Engineering, Harbin Institute ofTechnology, 150001 Harbin, China. Tel./fax: +86 451 86418784.

E-mail address: xiubotian@163.com (X.B. Tian).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.05.013

multilayers possess much higher toughness and strength thanmonolithic ceramics and consequently, multilayered structuresconsisting of alternating thin layers of a hard material liketitanium nitride and a softer, more ductile material such as Cuhave many potential applications. TiN/Cu multilayers are ofparticular interest due to the mechanical and structural contrastbetween the two materials: huge misfit (15.9%) between the twofcc lattices (αTiN=4.242 Å and αCu=3.615 Å) and largedifference in shear moduli (GTiN∼192 GPa and GCu∼58 Pa),which are important features from the perspective of enhancedmechanical properties [6]. More importantly, copper is a goodantibacterial agent [7,8] and attempts have been made toimpregnate or compound conventional materials such as stain-less steel with copper [9,10] for uses in chemical and foodprocessing, kitchen ware [11]. In the work reported here, Cu–Zn/TiN multilayers are fabricated on stainless steel. The duplexfilms have good mechanical properties and antibacterial ability.The antibacterial activities have been discussed [12] and thestructure and mechanical properties of the multilayered struc-ture are reported here.

Fig. 1. SEM images of the multilayers: (a) Sample 1 and (b) Sample 4.

190 C.B. Wei et al. / Surface & Coatings Technology 202 (2007) 189–193

2. Experimental details

The composition of 1Cr18Ni9Ti stainless steel alloys was C:≤0.12 wt.%, Si: ≤1.00 wt.%, Mn: ≤2.00 wt.%, P: ≤0.030 wt.%, S: ≤0.035 wt.%, Cr: 17–19 wt.%, Ni: 8–11 wt.%, Ti:∼0.8 wt.% and Fe balanced. Specimens 5 mm thick were cutfrom a rod 20 mm in diameter. The specimens weremechanically polished to a mirror finish and cleaned in acetonebefore loading into the vacuum chamber. Argon plasmabombardment was used to clean the specimens beforedeposition. A mixture of argon and nitrogen whose flow rateratios were controlled by flow meters was subsequently bledinto the vacuum chamber. Cu–Zn/TiN multilayers weredeposited by rotating the samples sequentially to the brassand Ti (99.9 wt.%) targets. The composition of the brass targetwas Cu: 61.5–63.5 wt.%, Pb:b0.08 wt.%, Fe:b0.15 wt.%,other:b0.27 wt.%, and Zn balanced. The Ti target was sputteredby a direct-current (DC) power supply, while a radio-frequency(RF) power supply was used for the brass target. Pure Ti wasfirst sputtered for 5 min to produce a seeding layer before theCu–Zn/TiN multilayers were deposited. The thickness ofindividual layers was controlled by the individual depositiontime and the total thickness of the multilayers was about 2 μm.The top layer of all samples was TiN. The detailed instrumentalparameters are shown in Table 1.

The surface and cross-sectional morphology was investigat-ed by scanning electronic microscopy (SEM). X-ray diffraction(XRD) (Philips) was used to investigate the structure of themultilayers. The Cu Kα line (1.5406 Å) was used and theincident beam angle (α) was fixed at 2°. The hardness of themultilayers was determined by a digital microhardness tester(HVS-1000) under a load of 10 g. At least 5 tracks weremeasured to obtain good statistics. The ball-on-disk friction andwear apparatus was used to evaluate the tribological properties.A hardened GCr15 steel ball 6.35 mm in diameter acted as thecounter body. The specimen was mounted on a rotating tablewith a revolution rate of 50 rpm. The track diameter was 6 mmand the normal load was 50 g.

3. Results and discussion

3.1. Surface and cross-sectional morphology

Fig. 1 shows the surface topography of the multilayers. It isevident that the multilayered structure of sample 1 curls off

Table 1Instrumental parameters during deposition

Parameter Value

Base pressure 6.0×10−3 PaWork pressure 0.7–1.0 PaDistance between target and substate 90 mmDC sputtering current for Ti target 0.6 ARF power for Brass target 300 WTi interface 5 min/Ar 0.7 PaAr/N2 flow 3.6/1.2 sccmBias voltage (pulse) 150 V/25 μs/7.5 kHz

spontaneously. The adhesion of the multilayers is so weak that itpeels off the substrate and destroys absolutely. This may beattributed to the large internal stress which is usual in crystallinefilms deposited by sputtering. For instance, large internal stresshas been reported from nanocomposite TiN/Cu multilayeredthin films [6]. And it reported that TiN layers are under largecompressive stress in the range −7 to −5 GPa whereas the stressin the Cu sublayers is slightly tensile. The failure of the film inthis study may happen particularly when the TiN sublayers aretoo thin (e.g., for the case of tCu–Zn=51 nm and tTiN=8.4 nm) asshown in Fig. 1(a). Poor adhesion has also been found in themultilayer when the Cu–Zn sublayers are too thin (with tCu–Zn=10.2 nm and tTiN=42 nm), which is not presented here. Incontrast, the films with medium-thickness of individualsublayer seldom display the delamination as shown in Fig. 1(b).

Fig. 2. SEM images of the cross-section of sample 2.

Table 2Deposition parameters of multilayers, where Λ is the bilayer period, t is theestimated thickness of individual layers

Sample no. Λ (nm) tCu–Zn (nm) tTiN (nm) Period numbers

1 59.4 51 8.4 302 118.8 102 16.8 153 237.6 204 33.6 74 55.8 30.6 25.2 305 111.6 61.2 50.4 176 223.2 122.4 100.8 7

Fig. 4. Microhardness of multilayers and steel substrate. The top dashed line isthe hardness value of TiN grown under the same deposition conditions.

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Fig. 2 shows the cross-sectional morphology of the multi-layers. The TiN layers appear dark and the Cu–Zn layers arebrighter. The multilayer periods are clearly shown. Thethickness of the multilayers is controlled by the depositiontime and determined from the cross-sectional image. Theestimated deposition rates of Cu–Zn and TiN are 0.51 nm/s and0.42 nm/s, respectively. The estimated individual layerthickness and the numbers of periods are listed in Table 2.The individual layer thicknesses range from nanometer to sub-micrometer.

3.2. XRD diffraction

Fig. 3 depicts the XRD diffraction patterns of the multi-layers. The main diffraction peaks appearing approximately at42.46° and 43.5° correspond to TiN (002) and Cu (111)respectively. The TiN(111) diffraction peaks appear at 36.5° andCu(002) are found at 50.56°. There is no noticeable differencein the diffraction patterns obtained from samples 4, 5, and 6.The lattice spacings are found to be 2.1271 Å for d002(TiN) and2.0786 Å for d111(Cu). These values are comparable to thecorresponding bulk values 2.1207 Å (PDF-2 card: 38-1420) and2.0880 Å (PDF-2 card: 04-0836). This means that the averagelattice parameter of TiN layers is expanded and Cu layers issuppressed along the growth direction, as compared with thebulk value. The difference in the lattice spacing may be ascribedto the residual stress in the TiN and Cu layers, as in the case ofthe data reported by Abadias et al. [6]. It is reported that the TiNlayers are under large compressive stress (in the range −7 to−5 GPa) whereas the stress in the Cu sublayers is slightly tensile

Fig. 3. XRD diffraction patterns of multilayers.

[6]. It should be noted that other mechanisms such as chemicalinter-diffusion or misfit dislocations may also be responsible forthe deviation near the interface [6,13,14]. Zn is not found in theXRD patterns. This may be due to its low fraction in thecoatings and can not be detected by XRD.

3.3. Hardness

Fig. 4 shows the hardness of the multilayers and the steelsubstrate. The multilayers possess higher hardness compared to

Fig. 5. Friction coefficients of multilayers and steel substrate: (a) tCu–Zn:tTiN=6:1,(b) tCu–Zn: tTiN=1:1.

Fig. 7. SEM images after wear tests: (a) Steel substrate, (b) Sample 2, and(c) Sample 5.

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the steel substrate. A larger fraction of hard TiN in themultilayers should result in higher hardness, but in ourexperiments, the hardness does not vary very much when thetCu–Zn:tTiN ratios change from about 6:1 to 1:1. Samples 4 and 5possess higher hardness values when the thickness of theindividual layers is small (either of tCu–zn or tTiN is nanometer).It has been shown that the hardness of nanoscale Cu–Agmetallic multilayers varies linearly with 1 /√h for h≥50 nm[15], which is consistent with the Hall–Petch behavior thatstrength increases with decreasing bilayer periods. The motionof dislocation is greatly curtailed by the microstructure and themultiple interfaces may lead to strengthening of the multilayers[16]. Adding a ductile metal layer to the nitride interface mayincrease the practical work of adhesion of the film due to theability of the metal layer to dissipate energy through plasticdeformation [17–19]. No significant hardness enhancement inTiN/Cu superlattice structure has been observed [18] and in ourexperiments, the thickness of the individual layers also only hasa little influence on the microhardness.

3.4. Tribological properties

Fig. 5 shows the time-dependent friction-coefficient curvesof the steel substrate and multilayers. The wear properties ofsample 1 are not obtained due to the delamination of coatingsright after deposition process. All the rest multilayers show lowfriction coefficients of 0.2–0.3 before punch through. The wearresistance increases with larger bilayer periods. Especially withtCu–Zn:tTiN ratio of 6:1, the enhancement of the punch throughtime is evident. As indicated in Fig. 5(a), the multilayerconsisting bilayer periods of 118.8 nm (sample 2 with 102/16.8 nm for tCu–Zn:tTiN) has low friction coefficients of about0.3 and is punched through at 2800 s. The multilayer consistingof the thickest bilayer periods of 237.6 nm (sample 3 with 204/33.6 nm for tCu–Zn:tTiN) has the best wear resistance among allthe samples with a punch through time of about 8500 s. Whenthe tCu–Zn:tTiN ratios vary from about 6:1 to 1:1, the wearresistance of the multilayers decreases significantly. As shownin Figs. 5(b) and 6, the punch through time observed fromsamples 4, 5, and 6 corresponding to tCu–Zn:tTiN=1:1 are below

Fig. 6. Punch through time in wear tests.

1000 s, that are almost one-tenth of sample 3 (tCu–Zn:tTiN=6:1),as well as lower than that of sample 2.

Fig. 7 displays the sample surface after the wear tests. Thesurface of steel substrate demonstrates the adhesive and metaltransfer wear behavior. The worn surface is plowed with debris.However, the multilayers show relatively smooth surface withlittle plowing and scratches are observed only after a longtesting time. Sample 5 shows more abrasion than the sample 2,though the wear time is shorter. The absence of adhesive wearand little or no third body interaction result in low frictioncoefficients when copper is introduced [20]. Previous reports[21,22] suggest that the soft phase in the hard nanocompositecoating dramatically influences the tribological properties. Ithas been demonstrated that incorporation of copper (b10 at.%)into the TiNx film results in a large decrease in the friction

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coefficients from high values of 0.6 to 0.7 corresponding toTiNx films to very low values of approximately 0.2. A decreaseof friction coefficient has also been found in the Cu–Zn/TiNmultilayers and this may be due to the lubrication effect of Cu–Zn during the wear process.

The tribological results suggest that the multilayers contain-ing thick Cu–Zn individual layers or with high Cu–Zn contentsin the multilayers can substantially prolong the punch throughtime. It was found by Bromark [23] that the erosion ratedecreased with increasing relative amounts of softer metal Ti inthe Ti/TiN coatings. The hard layers are the weak zone whencracks occur or in crack propagation. The introduction of softmetal layers improves the stress distribution in the hard layersand allows extensive plastic deformation at the crack tips.Furthermore, owing to the lower elastic modulus compared tothe hard layer, the cracks are deflected into the plane of thecoating. Thus, with increasing softer metallic layers, the capac-ity of plastic deformation of the coating increases and coatingremoval via the accumulated effect of “cutting” particlebecomes more important [4,23]. The low hardness of the Cu–Zn layers allows the abrasives to penetrate easily to producedeep scratches. However, the high ductility reduces the amountof materials removed relative to the size of the scratch. Close tothe TiN layers, the penetration is limited because of the higherhardness of TiN. This zone shows the beneficial combination ofa small penetration depth and high ductility giving rise to veryhigh wear resistance [24,25].

4. Conclusions

Cu–Zn/TiN multilayers have been successfully fabricatedand the properties have been investigated. The multilayers aremainly composed of (002)TiN and (111)Cu. Delaminationoccurs when the TiN sublayers are too thin and the Cu–Znsublayers are thick. The microhardness of the multilayers ishigher than that of the substrate but shows little difference withchanging the periods or the tCu–Zn to tTiN ratios. The wearresistance of multilayers has been improved effectively com-pared with that of the substrate. The friction coefficient of themultilayers is significantly decreased to 0.2–0.3. The wearresistance increases with larger Cu–Zn contents and bilayerperiods. For the case of the tCu–Zn:tTiN=6:1, the wear resistanceof the multilayers is improved significantly with increasing thebilayer periods and the films demonstrate longer punch throughtime than those with tCu–Zn:tTiN≈1:1.

Acknowledgements

This work was financially supported by Natural ScienceFoundation of China (no. 10575025 and no. 50373007) and CityUniversity of Hong Kong Direct Allocation Grant 9360110.

References

[1] M. Larsson, M. Bromark, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol.76–77 (1995) 202.

[2] S. Suresh, Internal. Rep., Brown University, Providence, RI, USA, 1992.[3] M.L Emiliani, J. Mater. Sci. 28 (1993) 5280.[4] J.L. He, W.Z. Li, H.D. Li, C.H. Liu, Surf. Coat. Technol. 103–104 (1998)

276.[5] S.A. Barnett, A. Madan, Scr. Mater. 50 (2004) 739.[6] G. Abadias, Y.Y. Tse, A. Michel, C. Jaouen, M. Jaouen, Thin Solid Films

433 (2003) 166.[7] E.A. Abou Neel, I. Ahmed, J. Pratten, S.N. Nazhat, J.C. Knowles,

Biomaterials 26 (2005) 2247.[8] S. Knasmüller, E. Gottmann, H. Steinkellner, A. Fomin, C. Pickl, A.

Paschke, R. Göd, M. Kundi, Mutat. Res. 420 (1998) 37.[9] Z.G. Dan, H.W. Ni, B.F. Xu, J. Xiong, P.Y. Xiong, Thin Solid Films 492

(2005) 93.[10] I.T. Hong, C.H. Koo, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct.

Process. 393 (2005) 213.[11] I. Korkut, M. Kasap, I. Ciftci, U. Seker, Mater. Des. 25 (2004) 303.[12] X.B. Tian, Z.M. Wang, S.Q. Yang, R.K.Y. Fu, P.K. Chu, Proceedings 14th

International Conference on Surface Modification of Materials by IonBeams (14th SMMIB), Kusadasi, Turkey, Paper TE-03, September 2005,p. 71.

[13] Y.Y. Tse,D.Babonneau,A.Michel,G.Abadias, Surf. Coat. Technol. 180–181(2004) 470.

[14] H. Ljungcrantz, S. Benhenda, G. Håkansson, I. Ivanov, L. Hultman, J.E.Greene, J.E. Sundgren, Thin Solid Films 287 (1996) 87.

[15] J. McKeown, A. Misra, H. Kung, R.G. Hoagland, M. Nastasi, Scr. Mater.46 (2002) 593.

[16] W.D. Sproul, Science 273 (1996) 889.[17] G. Abadias, S. Dub, R. Shmegera, Surf. Coat. Technol. 200 (2006) 6538.[18] G. Abadias, C. Tromas, Y.Y. Tse, A. Michel, Mater. Res. Soc. Symp. Proc.

875 (2005) 63.[19] D.F. Bahr, J.W. Hoehn, N.R. Moody, W.W. Gerberich, Acta Mater. 45

(1997) 5163.[20] A.S.M.A. Haseeb, J.P. Celis, J.R. Roos, Thin Solid Films 444 (2003) 199.[21] J. Musil, J. Vlček, Surf. Coat. Technol. 142–144 (2001) 557.[22] Z. Soukup, University of West Bohemia, Plzen, Czech Republic, (Ph.D.

Thesis (in czech), June 2000), 2000.[23] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol.

90 (1997) 217.[24] M. Berger, U. Wiklund, M. Eriksson, H. Engqvist, S. Jacobson, Surf. Coat.

Technol. 116 (1999) 1138.[25] M.R. Stoudt, R.C. Cammarata, R.E. Ricker, Scr. Mater. 43 (2000) 491.