Applied Surface Science 254 (2008) 5760–5765

Contents l is ts ava i lab le at ScienceDirec t

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

Optimization of plasma parameters for high rate deposition of titaniumnitride films as protective coating on bell-metal by reactive sputtering incylindrical magnetron device

Sankar Moni Borah, Arup Ratan Pal, Heremba Bailung, Joyanti Chutia *

Plasma Physics Laboratory, Material Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Guwahati 781035, Assam, India

A R T I C L E I N F O

Article history:

Received 28 December 2007

Received in revised form 12 March 2008

Accepted 12 March 2008

Available online 18 March 2008

PACS:

52.77.Dq

52.80.Sm

Keywords:

Nano-structured titanium nitride thin film

Reactive sputtering

Cylindrical magnetron

Bell-metal

Anti-corrosive

A B S T R A C T

Nano-structured titanium nitride (TiN) thin film coating is deposited by reactive sputtering in cylindrical

magnetron device in argon and nitrogen gas mixtures at low temperature. This method of deposition

using DC cylindrical magnetron configuration provides high uniform yield of film coating over large

substrate area of different shapes desirous for various technological applications. The influence of

nitrogen gas on the properties of TiN thin film as suitable surface protective coating on bell-metal has

been studied. Structural morphological study of the deposited thin film carried out by employing X-ray

diffraction exhibits a strong (2 0 0) lattice texture corresponding to TiN in single phase. The surface

morphology of the film coating is studied using scanning electron microscope and atomic force

microscope techniques. The optimized condition for the deposition of good quality TiN film coating is

found to be at Ar:N2 gas partial pressure ratio of 1:1. This coating of TiN serves a dual purpose of providing

an anti-corrosive and hard protective layer over the bell-metal surface which is used for various

commercial applications. The TiN film’s radiant golden colour at proper deposition condition makes it a

very suitable candidate for decorative applications.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, rapid developments in the field of nanotech-nology have been witnessed due to the existing as well as potentialapplications of nanomaterials in diverse technological areas suchas electronics, catalysis, ceramics, magnetic data storage andstructural components. The reduction in size leads to increasedmechanical strength, enhanced diffusivity, higher specific heat andelectrical resistivity as compared to their coarse grained counter-parts [1].

Titanium nitride is a rocksalt structure (NaCl) compoundconsisting of titanium (Ti) atoms filled in fcc based lattice with alloctahedral sites filled with nitrogen atoms. Some of its physicalproperties are as follows: lattice parameter (a) 0.424 nm; meltingpoint: 3173 K; thermal stability (DH298): 336.6 kJ mol�1; Young’smodulus: 450 GPa [2]. Titanium nitride is a suitable candidate forindustrial applications because of its unique properties like high

* Corresponding author. Tel.: +91 361 2740659; fax: +91 361 2740659.

E-mail address: joyanti_c@sify.com (J. Chutia).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.03.047

hardness, good wear and corrosion resistance [3,4]. Due to theseproperties, TiN has been used as a coating for cutting tools or as ananti-corrosive coating for turbines as in aerospatial industries.Further, it has good electrical, thermal, mechanical and chemicalproperties. In micro-electronics instrumentation, it finds applica-tion for its electrical characteristics and for its diffusion barrierproperties [5]. Study of TiN deposition has generated a lot ofinterest among the scientific community because of its lustrousgolden yellow coloured film [6] which has been used for decorativeapplications. The golden colour of the film is due to the highreflectance of TiN at the red end of the visible spectrum with lowreflectance near the ultraviolet region [7–12].

Numerous methods have been employed till date for thedeposition of titanium nitride thin film which includes bothphysical vapour deposition (PVD) and chemical vapour deposition(CVD) [13]. Other methods used in the preparation of TiN thin filmincludes electrochemical [14] and chemical nitridation of eithermetallic Ti (pure or alloy) [15] or Ti compounds [16]. The formationprocess of TiN thin film by reactive sputtering has been studied bymany researchers over the years [17–23]. In the case of reactivelysputtered TiN film coatings, nitrogen partial pressure plays an

S.M. Borah et al. / Applied Surface Science 254 (2008) 5760–5765 5761

important role in determining the mechanical properties [24–26].Deposition of thin films by reactive magnetron sputtering is a verysuitable method for the surface protection of materials. Reactivemagnetron sputtering is used for the elaborate study of titaniumnitride thin films [27].

Crystallographic texture has been observed in a number of TiNfilms and coatings and can be considered as a typical feature ofmagnetron sputtered coatings as coating formation occurs instrongly non-equilibrium conditions. A lot of work has been carriedout to investigate the preferred orientation of TiN under differentdeposition conditions [28–34]. The mechanical properties of TiNare strongly related to its orientation [3,35]. It has been reportedthat TiN film with (1 1 1) preferred orientation possesses thehighest hardness [36,37]. During the PVD deposition of a thin film,the packing density and preferred orientation of the film normallychanges with the increasing film thickness. Therefore, filmthickness is an important parameter that affects the preferredorientation and hardness of the coating.

In this paper, we have studied the morphology of TiN thin filmcoatings on bell-metal produced in a DC magnetron discharge atdifferent partial pressures of nitrogen gas. The correspondingpartial pressure of argon gas has also been changed but the overallgas pressure has been maintained at a constant value. The noveltyof DC cylindrical magnetron sputtering as compared to the otherdeposition system configurations lies in the fact that high yield ofuniform coating is possible over large area of the substrates ofdifferent shapes. Moreover, the deposition time required is alsoless. TiN thin films have been deposited by reactive sputtering inargon–nitrogen (Ar–N2) plasma on static bell-metal (an alloy ofcopper and tin) substrates. Bell-metal has been effectively used formaking household utensils and various other decorative items. Themain drawback of using bell-metal for such applications is that thealloy gets oxidised when exposed to the environment over a periodof time. Thin film of titanium nitride deposited over bell-metalprovides a hard protective coating and also protects the alloy bothfrom corrosion and oxidation. Qualitative study of the film byscanning electron microscope (SEM) and atomic force microscope(AFM) techniques shows the formation of titanium nitride nano-structures. X-ray diffraction (XRD) analysis has also been carriedout on the thin film samples. Hardness study has been done with

Fig. 1. Schematic diagram of the experimental set up: E – electric field, B – magnetic field, E

power supply.

the help of the nanoindentation technique. To determine the anti-corrosive property of the TiN thin film, copper acetic acid salt spray(CASS) test is done. The main purpose of these qualitativecharacterizations is to correlate them and thereby determinethe optimization condition for good quality TiN thin film coatingfor proper surface protection of bell-metal. To the best of ourknowledge, surface protection of bell-metal by TiN coating usingcylindrical reactive magnetron sputtering system has not beenreported.

This paper is structured as follows: Section 2 containsexperimental set up. In Section 3 the experimental results anddiscussion are presented and Section 4 contains the conclusion.

2. Experimental set up

The experimental magnetron device is a stainless steelcylindrical chamber having dimensions of 30 cm diameter and100 cm length. A small titanium cylinder is placed co-axially insidethe chamber which acts as the cathode. The length of the cathode is25 cm and its outer diameter is 3.25 cm. A schematic diagram ofthe experimental set up is shown in Fig. 1. For generation of asteady axial magnetic field, two coils are placed around the body ofthe chamber. Each coil is mounted over rails and fitted with castorwheels so that it can be easily moved along the axis of the chamberfor necessary adjustment of the distance between the coils. Eachcoil consists of enamel coated copper wire and contains 1500numbers of turns. Direct current is passed through both the coils inthe same direction which produces an axial magnetic field parallelto the cathode surface that is uniform within a length of�30 cm atthe central region of the chamber. One ampere current through thecoils generates a magnetic field of 0.0025 T at the central region ofthe plasma chamber.

The vacuum system consists of a rotary pump having adisplacement capacity of 350 l/min and a diffusion pump with aneffective pumping speed of 700 l/s. The base pressure of thechamber is of the order of 10�6 Torr and working gas pressure is ofthe order of 10�3 Torr. A Pirani gauge and an ionization gauge areused for the measurement of pressure inside the chamber. Thedischarge power is supplied from a stabilized DC power supply(1500 V, 5 A) working in the voltage-regulated mode. The working

R – end reflectors, Lp – Langmuir probe, MM – magnetic field coils, PS – DC discharge

Table 1Thickness values of deposited TiN thin films at different Ar:N2 partial pressures

Ar:N2 partial

pressure

Dm (g) Area, A (cm2) Density, r (cm3) Thickness, t (cm) Thickness, t (mm)

3:1 11.31 � 10�4 72.25 � 10�2 5.22 3 � 10�4 3

1:1 18.86 � 10�4 72.25 � 10�2 5.22 5 � 10�4 5

1:3 1.89 � 10�4 72.25 � 10�2 5.22 0.5 � 10�4 0.5

Fig. 2. TiN film deposition rate for different Ar:N2 partial pressure ratios

(Ar:N2 � 3:1, Ar:N2 � 1:1, Ar:N2 � 1:3 and N2 only) at fixed total pressure

2 � 10�3 Torr. Discharge voltage is 600 V and magnetic field is 0.01 T.

S.M. Borah et al. / Applied Surface Science 254 (2008) 5760–57655762

gas environment inside the magnetron chamber consists of amixture of argon and nitrogen gases in different partial pressureratios. The gases are injected to the chamber to raise the neutralpressure up to 10�3 Torr by using a double valve system consistingof a stop valve and a needle valve. An external magnetic field of0.01 T is applied.

The bell-metal substrates (dimension 0.85 cm in length and0.85 cm in breadth) used for TiN film deposition are mirrorpolished using a variable speed polishing machine having amaximum rpm of 2000. The final polish is done using 0.25 mmdiamond paste. The mirror polished samples are ultrasonicallycleaned with propan-2-ol organic solvent and properly dried. Thesamples are then suitably placed atop the substrate holder at adistance of 6 cm below the titanium cathode. Ti target (99.99%purity) and substrate are sputter cleaned prior to the actualdeposition of the film for 10 min to remove oxide and any othercontaminant layer existing on their surfaces. Typical dischargeparameters for producing plasma are as follows – dischargevoltage is 600 V and discharge current is in the range of (50–250) mA. The substrates are kept floating. The substrate tempera-ture is maintained at (180–200) 8C during the sputter depositionprocess. Plasma parameters measured with the help of a cylindricalLangmuir probe are – density�109 cm�3 and electron temperature�(2–4) eV. An axial magnetic field (B) of strength 0.01 T is appliedexternally which is parallel to the target surface and perpendicularto the sheath electric field (E). The crossed electric and magnetic(E � B) fields confine the plasma near the cathode target. Theelectrons confined by the magnetic field, ionize neutral gas atomsby gaining energy from the cathode sheath and it creates a regionof intense ionization adjacent to the cathode. The plasma sheathelectric field then accelerates ions born in the electron trap regiontoward the cathode target. The ions impact on the cathode targetwith several hundred electron volts of energy, sputter atoms fromthe target as well as cause secondary electron emission. Thesecondary electrons are accelerated back into the magnetic trapregion, helping to sustain the discharge. The sputtered atoms getdeposited on the bell-metal substrates to form the desired thin filmcoating. The deposition is done for time duration of 75 min undercontrolled discharge parameters. The structural characteristics ofthe TiN thin films are studied by X-ray diffractometer (Bruker AXSD8, Cu Ka). Surface morphology of the thin films is studied usingscanning electron microscope (LEO-1430VP) and atomic forcemicroscope (SMENA – B). The instrument used for hardness studyof the films is the Nanohardness tester (Triboscope by NanoInstruments).

3. Experimental results and discussion

3.1. Variation of deposition rates of TiN coatings at different Ar:N2

partial pressure ratio

The corresponding deposition rates for three different Ar:N2

partial pressure ratios (Ar:N2 � 3:1, Ar:N2 � 1:1 and Ar:N2 � 1:3)in the gas mixture have been measured. The deposition conditionof the TiN films is as follows – discharge voltage is 600 V, magneticfield is 0.01 T and total gas pressure is 2 � 10�3 Torr. The thickness

(t) of the deposited film is calculated from the relation given below[38]:

t ¼ Dm

r� A(1)

where Dm, r and A are the deposited mass measured, bulkdensity of TiN and surface area of the substrate respectively. Thecorresponding thickness values of the deposited TiN thin films atdifferent Ar:N2 partial pressure ratios are shown in Table 1.Keeping in mind the fact that the deposited film thickness valuesshown in the above table has been obtained by considering thetheoretical bulk density of TiN, we have also determined thethickness of the film coatings using SEM. The thickness of the filmsmeasured using SEM at the three different Ar:N2 partial pressureratios as mentioned above are 2.947 mm, 4.860 mm and 0.507 mm,respectively. These values are comparable to the theoreticallyobtained values. Fig. 2 shows the deposition rates for the differentAr:N2 partial pressure ratios (Ar:N2 � 3:1, Ar:N2 � 1:1 andAr:N2 � 1:3) in the gas mixture. The deposition rate has beencalculated from the amount of deposited mass of the film per hourover the substrate having a surface area of 1 cm2. It has beenobserved that the deposition rate increases with the increase ofnitrogen partial pressure from 0.5 � 10�3 Torr to 1 � 10�3 Torr.The rate of formation of TiN depends on the amount of Ti atomsbeing sputtered from the target and the rate at which they getnitrided by reacting with nitrogen. There is a gradual increase inthe reaction rate between Ti and nitrogen atoms with increase ofnitrogen gas and the simultaneous decrease of argon gas in the gasmixture, which results in the increase of the rate of deposition. Ithas been observed that Ar:N2 � 1:1 in the gas mixture is the mostsuitable condition for high-rate TiN film deposition among thethree conditions. At this particular condition, the sputtering ratebalances the reaction rate between Ti and nitrogen atoms. As aresult, almost all the sputtered Ti atoms from the target react withthe nitrogen gas atoms to form TiN and the deposition ratebecomes maximum at this condition. Further increase of nitrogen

S.M. Borah et al. / Applied Surface Science 254 (2008) 5760–5765 5763

partial pressure to 1.5 � 10�3 Torr leads to a significant decrease inthe argon partial pressure ratio which decreases the argon iondensity in the discharge. Argon being heavier than nitrogen, argonions is mainly responsible for the sputtering mechanism. Thedecrease in argon ions due to the decrease in the argon ratio in thegas mixture greatly reduces the ionization resulting in thereduction in plasma density and increase in the sheath thickness.Due to the increase of sheath thickness, the sheath electric fieldbecomes weaker and the energetic ions striking the target looseenergy. As a result, there is less number of sputtered atoms whichleads to the decrease of the deposition rate.

3.2. Lattice structure and crystallite dimension of TiN coatings

X-ray diffraction study of the deposited titanium nitride thinfilms at different Ar:N2 partial pressure ratios [(a) – Ar:N2 � 3:1, (b) –Ar:N2 � 1:1, (c) – Ar:N2 � 1:3 and (d) –N2 only] in the gas mixturemaintained at a fixed total pressure of 2 � 10�3 Torr, dischargevoltage of 600 V and magnetic field of 0.01 T is shown in Fig. 3(i). Ithas been found that single phase TiN thin films have been deposited

Fig. 3. (i) XRD patterns of TiN for different Ar:N2 partial pressure ratios [(a)

Ar:N2 � 3:1, (b) Ar:N2 � 1:1, (c) Ar:N2 � 1:3 and (d) N2 only] at fixed total pressure

2 � 10�3 Torr, discharge voltage 600 V and magnetic field 0.01 T. (ii) TiN crystallite

average size plot for different Ar:N2 partial pressure ratios.

over the bell-metal substrates for each of the different depositionconditions. A clear sharp and intense peak of TiN corresponding to(2 0 0) lattice texture has been observed. The intensity variation ofthe (2 0 0) peak obtained for each of the different Ar:N2 partialpressure ratios is in arbitrary units. The metallic Cu(1 1 1) andCu(2 0 0) peaks seen corresponds to the bell-metal substrate. Nophase change is observed in the lattice structure. The preferredgrowth of (2 0 0) TiN lattice texture has been well-defined by the‘Preferential sputtering’ model [39,40] and the ‘surface energyminimisation’ model [41–43]. The average crystallite dimension (D)measurement, along a line perpendicular to the (2 0 0) lattice planeis obtained from the Scherrer formula [44].

The results of the average TiN crystallite dimension deposited atdifferent Ar:N2 partial pressure ratios in the gas mixture is shown inFig. 3(ii). Although, not much significant change in the D value isobtained, the crystallite dimension of TiN deposited at Ar:N2 � 1:1partial pressure in the gas mixture is found to be higher than the rest.Thisresultcan beattributedtothe fact thatatthisconditioninthe gasmixture the proportion of almost saturated bonds of nitrogen atomswith Ti atoms [3] contained in the film is maximum in comparison tothe other nitrogen gas partial pressures in the mixture.

3.3. Surface morphology of TiN coatings by SEM and AFM

Scanning electron microscope analysis of TiN thin film coatingdeposited on bell-metal substrate shows the formation of circularTiN structures distributed throughout the scanned film surface.The measured dimensions of these structures reveal that they arenano-sized. The measured average diameter of these nano-structures is 65.2 nm which is nearly equal to that obtained fromXRD crystallite dimension analysis. Absence of inter-metallicphase peaks is confirmed using SEM EDX.

The surface morphology of the TiN thin films has also beenstudied using the atomic force microscope. AFM images of the filmsdeposited at different Ar:N2 partial pressures (Ar:N2 � 3:1,Ar:N2 � 1:1, Ar:N2 � 1:3 and N2 only) show that the film growthover the bell-metal substrate maintains a uniform pattern through-out. Roughness values of the deposited films show that TiN filmdepositedat Ar:N2 � 1:1 partial pressureratiohas minimumvalue ofroughness. AFM image of TiN thin film on bell-metal for Ar:N2 � 1:1partial pressure ratio in the gas mixture is shown in Fig. 4. Theaverage roughness of this film is found to be of the order of 20 nm.The films are found to exhibit dense columnar structure.

3.4. Adhesion property of the deposited TiN coatings

Proper adhesion of the deposited film coating to the substratematerial is an important factor for providing better protection to

Fig. 4. AFM image of TiN thin film on bell-metal for Ar:N2 � 1:1 partial pressure

ratio in the gas mixture.

Fig. 5. (i) Image showing the profile of an indent obtained on TiN sample by

nanoindentation technique; (ii) dual plots of hardness and Young’s modulus values

of the films deposited at different Ar:N2 partial pressure ratios in the gas mixture.

The value of hardness and Young’s modulus of uncoated bell-metal substrate is also

shown corresponding to zero on the X-axis scale.

S.M. Borah et al. / Applied Surface Science 254 (2008) 5760–57655764

the material. The adhesion test is done by scotch tape according tothe standard tape test method [45] using scotch tape (3 M). For theadhesion test, the scotch tape is fixed on the coated surface andpulled off to realize the adhesion quality of the coating. Applicationof negative bias voltages of 50 V and 100 V to the substratesresulted in the decrease in the film’s adhesion to the substrateunder the above deposition conditions. So, all the films weredeposited without applying any external bias voltage to thesubstrate. This is an important result of our experimentaldeposition which has significant characteristics differing withdeposition on other metal substrates. This result is due to the factthat the bell-metal substrates used contain some amount ofimpurity elements in it. Applying a bias voltage to the substrateactivates these impurity sites present within it which in turnincreases the lattice distortions and thereby hinders the surfaceadhesion of the impinging atoms. We have used these bell-metalsubstrates of less purity to facilitate the application of these resultsdirectly to the similar commercially used materials. Floatingpotential of the substrate is within the range of � 30 V to �40 V. Itis observed that increasing the substrate temperature above 200 8Clead to the decrease in the adhesive property of the deposited TiNfilm coatings. So, the films were deposited at substrate tempera-ture maintained below 200 8C. All the films show good adhesionproperty and the coating does not peel off in the test. Almost nosurface defects are observed in the films after the test, whichsignifies good quality adhesion of the films.

3.5. Mechanical properties of the TiN coatings

Mechanical properties of the deposited thin film coatings, likehardness and Young’s modulus have been studied using thenanoindentation technique. The intrinsic hardness of thin filmsbecomes meaningful only when the influence of the substratematerial is eliminated [46]. Thus, for obtaining proper hardness,the indentor’s depth of penetration should be about one-tenth ofthe deposited film’s thickness. This is necessary so that thecalculated value provides a true measurement of the coatinghardness. An indentation depth profile for hardness measurementdone by the nanoindenter on the film coating is shown in Fig. 5(i).

The hardness, H of the deposited TiN thin films at differentAr:N2 partial pressure ratios (Ar:N2 � 3:1, Ar:N2 � 1:1 andAr:N2 � 1:3) in the gas mixture along with that of the uncoatedbell-metal substrate has been calculated using the method ofOliver and Pharr (1992), according to the relation:

H ¼ Pmax

A(2)

where Pmax is the peak indentation load and A is the indentationcontact area, determined from the indenter shape function.Generally, loading data are influenced more by the material’splastic properties while the unloading data by the elastic proper-ties. Hardness is calculated from the loading data of the curves. Thehardness values determined by nanoindentation method arefunctions of surface roughness, chemical state of the surface layerand indenter size effect. Proper care has been taken to minimizethe influence of these effects on the hardness value. In Fig. 5(ii), thevariation of hardness and the Young’s modulus at differentnitrogen partial pressures in the gas mixture is shown. Thehighest hardness of TiN coating is obtained at the condition wherethe Young’s modulus value is maximum. It is found that the highesthardness of the film is at Ar:N2 � 1:1 in the gas mixture. It shouldbe stressed again that the TiN film coatings with the highesthardness contain the maximum number of nitrogen atoms whichhave almost completely filled bonds with Ti atoms as reportedearlier by Arnell et al. [3] The average crystallite dimension value,

already shown in Fig. 3(ii), is also found to be the highest at thiscondition. From this observation, it can be said that the hardnessand the corresponding average crystallite dimension of the TiNsamples follow the well established negative Hall-Petch relation-ship, where it is found that with increasing crystallite dimension,the material hardness also increases [47]. The TiN film coating atthis condition increases the hardness of bell-metal by approxi-mately 13 times.

3.6. Corrosion study of the TiN coatings

The copper acetic acid salt spray test is done to determine theanti-corrosive property of the TiN thin films. The CASS test is nowcovered by BS 5466 Part III: 1977 (ISO 3770 and ASTM B368) [48].The samples of TiN thin film on bell-metal substrates have beenproperly cleaned and then dipped into the test solution. The testsolution is prepared using 5% NaCl solution and 0.026% CuCl2

solution, the PH of which is found to be close to 7. Required amountof glacial acetic acid is used to get the standardized test solutionhaving PH value of 3. All the samples [(a) – Ar:N2 � 3:1, (b) –Ar:N2 � 1:1, (c) – Ar:N2 � 1:3 and (d) –N2 only] have been visually

S.M. Borah et al. / Applied Surface Science 254 (2008) 5760–5765 5765

examined over a period of time to determine their capability toprotect the bell-metal alloy from corrosive attack. Each of the filmsamples under observation at different Ar:N2 partial pressuresexhibited no change in their surface morphology for 48 h. Whenobserved after 48 h, minute cracks have been seen to develop onthe film surface of sample (d). The cracks on the surface can beattributed due to the permeation of the salt solution. The filmsurfaces of samples (a), (b) and (c) did not have any cracks. Thisindicates that the TiN coating is able to shield the bell-metalsurface from the severe and hostile environment and thus, providea good protective cover to it. Titanium nitride itself has an inherentproperty of high internal resistance to corrosion which arises fromthe instant formation of an oxide layer on the surface at roomtemperature that is nearly immune to corrosion from salt-water.

4. Conclusion

Titanium nitride thin films have been deposited on bell-metalby reactive sputtering in cylindrical magnetron device in argon andnitrogen gas mixtures at low temperature. Nitrogen gas percentagein the mixture plays a significant role in the quality of thedeposited film. XRD analysis shows that single phase TiN filmscorresponding to (2 0 0) lattice texture has been formed under theabove mentioned deposition condition. SEM study shows theformation of nano-structured films. The optimum depositioncondition for the best quality titanium nitride thin film withmaximum hardness is found to be at Ar:N2 � 1:1 in the gasmixture. The TiN film coating at this condition increases thehardness of bell-metal by nearly 13 times. The titanium nitridethin films provide very good protection to the bell-metal substratefrom corrosion. Apart from its anti-corrosive property, the radiantgolden colour of these films makes it suitable to be usedcommercially for decorative applications.

Acknowledgements

This work is supported by a grant from the Department ofScience and Technology, Government of India. The authorsacknowledge the facilities provided by the Centre for Nanotech-nology, the Central Instrumentation Facility of Indian Institute ofTechnology (IIT) Guwahati and Prof. A. Khare, Head, Department ofPhysics, IIT Guwahati for XRD, SEM and AFM analysis of thesamples. Acknowledgement is also due to Prof. D. Ghosh of SahaInstitute of Nuclear Physics, Kolkata for providing the nanoinden-tation facility.

References

[1] R. Chandra, A.K. Chawla, D. Kaur, P. Ayyub, Nanotechnology 16 (2005) 3053.[2] Xuan-qian Xu, H. Ye, T. Zou, J. Zhejiang Univ. Sci. A 7 (3) (2006) 472.

[3] R.D. Arnell, J.S. Colligon, K.F. Minnebaev, V.E. Yurasova, Vacuum 47 (5) (1996) 425.[4] M. Diesselberg, H.-R. Stock, P. Mayr, Surf. Coat. Technol. 177–178 (2004) 399.[5] A. Gicquel, N. Laidani, P. Saillard, J. Amouroux, Pure Appl. Chem. 62 (9) (1990)

1743.[6] B. Szikora, Vacuum 50 (3–4) (1998) 273.[7] R.C. Glass, L.M. Spellman, S. Tanaka, R.F. Davis, J. Vac. Sci. Technol. A 10 (1992)

1625.[8] S.P. Murarka, R.J. Gutmann, A.E. Kaloyeros, W.A. Lanford, Thin Solid Films 236

(1993) 257.[9] A. Tarniowy, R. Mania, M. Rekas, Thin Solid Films 311 (1997) 93.

[10] C.A. Dimitriadis, J.I. Lee, P. Patsalas, S. Logothetidis, D.H. Tassis, J. Brini, G.Kamarinos, J. Appl. Phys. 85 (1999) 4238.

[11] P. Patsalas, C. Charitidis, S. Logothetidis, C.A. Dimitriadis, O. Valassiades, J. Appl.Phys. 86 (1999) 5296.

[12] S. Niyomsoan, W. Grant, D.L. Olson, B. Mishra, Thin Solid Films 415 (2002) 187.[13] S. Chatterjee, S. Chanrashekhar, T.S. Sudarshan, J. Mater. Sci. 27 (1992) 3409.[14] J. Schreckenbach, F. Schlottig, D. Dietrich, A. Hofmann, G. Merx, J. Mater. Sci. 14

(1995) 1344.[15] S.W. Russell, M.J. Rack, D. Adams, T.L. Alford, J. Electrochem. Soc. 143 (1996) 2349.[16] C. Jimenez, M. Langlet, Surf. Coat. Technol. 68–69 (1994) 249.[17] Y. Igasaki, H. Mitsuhashi, Thin Solid Films 70 (1980) 17.[18] M. Wittmer, H. Melchior, Thin Solid Films 93 (1982) 397.[19] M.K. Hibbs, J.-E. Sundgren, B.E. Jaccobson, B.-O. Johansson, Thin Solid Films 107

(1983) 149.[20] N. Kumar, K. Pourrezaei, M. Fissel, T. Begley, B. Lee, E.C. Douglas, J. Vac. Sci.

Technol. 5A (1987) 1778.[21] H. Joswig, A. Kohlhase, P. Kucher, Thin Solid Films 175 (1989) 17.[22] W. Tsai, J. Fair, D. Hodul, J. Electrochem. Soc. 139 (1992) 2004.[23] M. Kawamura, Y. Abe, H. Yanagisawa, K. Sasaki, Thin Solid Films 287 (1996) 115.[24] J. Musil, L. Bardos, A. Rajsky, J. Vyskocil, B. Dolezal, G. Loncar, K. Dadourek, V.

Kubicek, Thin Solid Films 136 (1986) 229.[25] W.D. Sproul, P.J. Rudnik, M.E. Graham, Surf. Coat. Technol. 39–40 (1989) 355.[26] S. Yang, D.B. Lewis, I. Wadsworth, J. Cawley, J.S. Brooks, W.D. Munz, Surf. Coat.

Technol. 131 (2000) 228.[27] M. Diesselberg, H.-R. Stock, R. Cremer, H.-G. Fuss, Surf. Coat. Technol. 200 (2005)

1604.[28] M. Ahern, M.S.J. Hashmi, Surf. Coat. Technol. 50 (1992) 249.[29] K. Baba, R. Hatada, Surf. Coat. Technol. 66 (1994) 368.[30] H. Jiang, K. Tao, H. Li, Thin Solid Films 258 (1995) 51.[31] J.E. Greene, J.-E. Sundgren, L. Hultman, I. Petrov, D.B. Bergstrom, Appl. Phys. Lett.

67 (20) (1995) 2928.[32] L. Hultman, J.-E. Sundgren, J.E. Greene, D.B. Bergstrom, I. Petrov, J. Appl. Phys. 78

(9) (1995) 5395.[33] R. Banerjee, R. Chandra, P. Ayyub, Thin Solid Films 405 (2002) 64.[34] I. Iordanova, P.J. Kelly, R. Mirchev, V. Antonov, Vacuum 81 (2007) 830.[35] F. Richter, H. Kupfer, G. Schaarschmidt, F. Scholze, F. Elstner, G. Hecht, Surf. Coat.

Technol. 54–55 (1992) 338.[36] H. Harma, T. Inoue, S. Nakashima, H. Okumura, Y. Ihasida, S. Yoshida, T. Koizumi,

H. Grille, F. Bechsted, Appl. Phys. Lett. 74 (1999) 191.[37] F. Hohl, H.-R. Stok, P. Mayr, Surf. Coat. Technol. 54–55 (1992) 160.[38] J.S. Bhat, K.I. Maddani, A.M. Karguppikar, Bull. Mater. Sci. 29 (3) (2006) 331.[39] R.M. Bradley, J.M.E. Harper, D.A. Smith, J. Appl. Phys. 60 (1986) 4160.[40] R.M. Bradley, J.M.E. Harper, D.A. Smith, J. Vac. Sci. Technol. A 5 (1987) 1792.[41] L. Hultman, W.-D. Munz, J. Musil, K. Kadlec, I. Petrov, J.E. Greene, J. Vac. Sci.

Technol. A 9 (1991) 434.[42] G. Knuyt, C. Quaeyhaegens, J. D’Haen, L.M. Stals, Thin Solid Films 258 (1995) 159.[43] G. Knuyt, C. Quaeyhaegens, J. D‘Haen, L.M. Stals, Surf. Coat. Technol. 76–77 (1995)

311.[44] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley Publishing Co.,

Reading, MA, 1978.[45] S.K. Nema, P.M. Raole, S. Mukherjee, P. Kikani, P.I. John, Surf. Coat. Technol. 179

(2004) 193.[46] N.X. Randall, C.J. Schmutz, J.M. Soro, Surf. Coat. Technol. 108–109 (1998) 489.[47] K.Y. Wang, T.D. Shen, M.X. Quan, W.D. Wei, J. Mater. Sci. Lett. 12 (23) (1993) 1818.[48] V.E. Carter, Corrosion Testing for Metal Finishing, vol. 53, Institute of Metal

Finishing, Butterworth Scientific, Birmingham, 1982.