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Current Applied Physics 12 (2012) 247e253

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Current Applied Physics

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Nanoindentation testing on copper/diamond-like carbon bi-layer films

Neeraj Dwivedi a,b, Sushil Kumar a,*a Physics of Energy Harvesting Division, National Physical Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi 110012, IndiabDepartment of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India

a r t i c l e i n f o

Article history:Received 21 April 2011Received in revised form14 June 2011Accepted 15 June 2011Available online 28 June 2011

Keywords:Plasma depositionNanoindentationCu/DLC bi-layer

* Corresponding author. Tel.: þ91 11 45608650; faxE-mail address: skumar@nplindia.org (S. Kumar).

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2011.06.013

a b s t r a c t

In the present work, the effect of indentation load on nano-mechanical properties of copper/diamond-like carbon (Cu/DLC) bi-layer films was explored. In addition, effect of Cu interlayer and influence ofself bias on residual stress and various other nano-mechanical properties such as hardness (H) and elasticmodulus (E) of Cu/DLC bi-layer films were also discussed. These Cu/DLC bi-layer films were deposited,using hybrid system involving radio frequency (RF)-plasma enhanced chemical vapor deposition and RF-sputtering units, under varied self biases from �125 to �225 V. The effect of penetration depth withvaried load from 5 to 20 mN on H and E of these Cu/DLC bi-layer films was also analyzed.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Over the last 30 years, diamond-like carbon (DLC) or hydroge-nated amorphous carbon (a-C:H) thin films have attractedconsiderable scientific and industrial attention of researchersacross the world due to its distinguishable mechanical and tribo-logical properties, together with its excellent biocompatibility,chemical inertness, electrical and optical properties, which resultsin its wide spread industrial applications including cutting tools,automobile parts, knee implants, solar cells and IR devices etc[1e5]. However, DLC films have big drawback of high residualstress, which restrict its potential industrial applications. Kumaret al. have discussed the problem of residual stress and suggestedvarious possible solutions for its reduction [6]. Recently, we madean effort to minimize residual stress of DLC films by nitrogenincorporation but it also reduces the hardness of the films [1]. Thus,there is keen interest to minimize residual stress without affectingits hardness. Presently, metal incorporation in DLC films hasevolved as an alternative approach to minimize residual stresswithout scarifying its hardness [7]. But the metal incorporation inDLC films using hybrid system combining radio frequency-plasmaenhanced chemical vapor deposition (RF-PECVD) and RF-sputtering techniques was found to be quite complex due to“target poison problem” [8]. Among some DLC based bi-layer (1interlayerþ 1 DLC layer form 1 bi-layer) structures, Ti/DLC and TiN/

: þ91 11 45609310.

All rights reserved.

DLC bi-layer structures with improved mechanical properties havealready been studied [9,10]. Recently, we deposited Cu/DLC multi-layer structure and obtain minimum stress and excellent electronicproperty in multilayer structure having 1 bi-layer [11]. Thus,detailed study on mechanical properties of Cu/DLC bi-layer struc-ture can also be very interesting.

The characterization of mechanical properties of DLC films wasalso found to be a subject of discussion. Generally indentation ormicroindentation is used to measure mechanical properties of DLCfilms, in which information about penetration depth cannot beobtained. Simple indentation is found to be suitable for character-ization of bulk samples and microindentation for characterizationof very thick films. However, presently due to shrinkage in size ofdevices like from micro electromechanical system (MEMS) to nanoelectromechanical system (NEMS) and rapid enhancement in theuse of very thin DLC films (thickness of the order of few nanome-ters) such as hard and protective coating on magnetic storagemedia, solar cells and on NiTi alloys for MEMS, it became verynecessary to characterize these thin films using high resolution anddepth sensitive indentation [12e14]. Nanoindentation possesses allthe required features such as high resolution and depth sensingability and hence it provides accurate nano-mechanical properties.The nano-mechanical properties such as hardness of thin filmsstrongly depend on penetration depth and it decrease withincreasing the penetration depth. According to Hook’s lawdisplacement (penetration depth) varies proportional to force.Therefore, indentation load was also found to be an importantparameter (which control the penetration depth) to figure outactual nano-mechanical properties of DLC thin films.

N. Dwivedi, S. Kumar / Current Applied Physics 12 (2012) 247e253248

It is wondering despite very challenging parameter, very fewreports pertained to effect of indentation load on the mechanicalproperties of DLC thin films was found in literature. In this paper,we report the effect of indentation load on the nano-mechanicalproperties of copper/diamond-like carbon (Cu/DLC) bi-layer films.In addition, the effect of Cu interlayer and influence of self bias onresidual stress, hardness, elastic modulus and other nano-mechanical parameters of Cu/DLC bi-layer films have also beendiscussed.

2. Experimental details

Hybrid system combining radio frequency-plasma enhancedchemical vapor deposition (RF-PECVD) and RF-sputtering units wasused to grow Cu/DLC bilayer films, on well cleaned Si<100 > substrate, at a base pressure 1 � 10�5 Torr. The details ofdeposition unit used for growth of these Cu/DLC bi-layer films canbe found elsewhere [11]. Prior to deposition of Cu/DLC bi-layerfilms, Si substrates were cleaned by argon plasma to eliminateoxygen and any other contamination from the surface. In thedeposition of Cu/DLC bi-layer films, first the Cu interlayer wasgrown using RF-sputtering technique at a self bias of �400 V andworking Ar gas pressure of 20mTorr. It is important tomention thatin sputtering requires a negative bias on the target to create thesputter plasma by ionizing the inert gas like Ar and draw (accel-erate) the ions to the target. Sputter yields depend on ion energy(bias voltage). In case of RF-sputtering process the self biasdevelops at the electrode. It is to be noted that the parameters usedfor the growth of Cu interlayer were kept same in all the films thatresults in its constant contribution to all Cu/DLC bi-layer films.Further, DLC layer was grown on Cu layer under varied self biasesof �125, �150, �175 and �225 V with maintaining constantworking pressure of 1 mTorr that was achieved by feeding C2H2 gasinto the process chamber. It is to be noted that pure DLC film havingno interlayer was also deposited at �125 V to visualize the role ofCu on residual stress and nano-mechanical properties of Cu/DLC bi-layer films.

The residual stress of Cu/DLC bi-layer films deposited on Sisubstrate was determined from change in radius of curvature of thesubstrate, before and after deposition, using 500 TC temperaturecontrolled film stress measurement system (M/s FSM FrontierSemiconductor, USA) by Stoney formula given by Eq. (1)

s ¼ Esd2s6ð1� nsÞdf

1Rf

� 1R0

!(1)

where Es, ns and ds are Young’s modulus, Poisson ratio and thicknessof the substrate, respectively and R0 and Rf are the radius ofcurvature of substrate before and after film deposition. The nano-mechanical properties of Cu/DLC bi-layer films was measuredusing IBIS nanoindentation (M/s Fisher-Cripps laboratories Pvt.Limited, Australia) having triangular pyramid diamond Berkovichindenter with tip radius of w150 nm and normal angle of 65.3

�

between the tip axis and the faces of triangular pyramid. The nano-mechanical properties were studied at different indentation loadsof 5, 10, 15 and 20 mN. The schematic representation of nano-indentation equipment is shown in Fig. 1. Nanoindentation unitconsist of piezoelectric translator (made of PZT), carriage, springsand force and depth sensors. The force and depth sensors are linearvariable differential transformers (LVDT). PZT has property that itexpands or contract under applying the voltage and vice versa.Thus, under certain voltage PZT drives the carriage that deflects thespring and generates the indentation load. The LVDT sense the forceas well as depth and provides the load versus displacement curves.In order to get accurate results from nanoindentation, the

measurements must be performed in noise free environment andmust avoid the vibrations. The thicknesses of these Cu/DLC bilayerfilms were measured using Taylor-Hobson Talystep instrument andfound to be w337, w348, w357 and w376 nm of these filmsdeposited at self bias �125, �150, �175 and �225 V, respectively.The thickness of single DLC layer grown at �125 V was found to bew310 nm.

3. Results and discussion

3.1. Residual stress

High level of residual stresses (s) that results in detachment offilm from the substratewas found to be a big drawback of DLC films.To overcome this problem, Cu/DLC bi-layer films were prepared inwhich interlayer of Cu act as an adhesive layer for successive DLClayer. Fig. 2 is the plot of residual stress versus self bias for Cu/DLCbi-layer films. Pure DLC film grown at �125 V was also studied tovisualize the effect of Cu interlayer on the residual stress and othernano-mechanical properties of Cu/DLC bi-layer films. From figure, itis evident that residual stress was drastically decreased as soon asCu interlayer was placed between substrate Si and DLC layer. Thevalues of residual stress in pure DLC and Cu/DLC bi-layer filmsgrown at self bias �125 V were found to be 1.8 and 0.95 GPa,respectively. In DLC films the interfacial mismatch was found to bea main cause of residual stress [15]. However, residual stress wasminimized considerably in Cu/DLC bi-layer films, which mayascribe to following facts (i) Interfacial relaxation and (ii) Cu doesnot form carbide and not form any other compound with Si, soavoid formation of anymixed layer at interface. Thus, Cu layer act asan excellent adhesive layer for successive DLC layer. However,further increase in self bias for DLC layer deposition in Cu/DLC bi-layer corresponds to nominal increase in its residual stress due toincrease in energy of bombardment of carbon atoms/ions, whichcan enhance the roughness and therefore changes the radius ofcurvature of the films.

3.2. Nanoindentation testing

High resolution nanoindentationwas used tomeasure the nano-mechanical properties of Cu/DLC bi-layer films. Load versusdisplacement curves were employed to estimate various mechan-ical parameters including hardness (H) and elastic modulus (E). Theload versus displacement curves at 5 mN for Cu/DLC bi-layer filmsgrown under self bias from �125 to �225 V is shown in Fig. 3(a).The depth of penetrationwas found to increase with increasing selfbias from �125 to �225 V due to initiation of graphite-like sp2

bonding. The load versus displacement curve of pure DLC filmgrown at �125 V was also drawn and found to be almost identicalwith Cu/DLC bi-layer film grown at �125 V. Since penetrationdepths of these films at 5 mNwere considerable lower thereforewediscussed the effect of self bias on H and E at this load.

The variation of H versus indentation load as well as self bias forCu/DLC bi-layer films is depicted in Fig. 4. H can be defined as ratioof maximum load applied in loading process to projection areacreated at maximum load. In other words, hardness of materialrepresents resistance to surface penetration under applied load. Hstrongly depends on nano- to microstructural defect present in thenetwork, and it should be related to the bonding between theatoms and to the ability of the bonds to withstand deformationstemming from compression, extension, bending or breaking. It isevident from figure that the value of H of Cu/DLC bi-layer films wasfound to decrease from 26.2 to 18.1 GPa at (5 mN) with increase inself bias from�125 to�225 V. The variation of E versus indentationload as well as self bias for Cu/DLC bi-layer films is depicted in Fig. 5.

Fig. 1. Schematic of nanoindentation used for measuring the nano-mechanical prop-erties of DLC and Cu/DLC bi-layer films at different indentation loads of 5, 10, 15 and20 mN.

-120 -140 -160 -180 -200 -220

0.8

1.0

1.2

1.4

1.6

1.8

5

4

3

2

1

(G

Pa)

Self Bias (V)

1- DLC (-125 V)

2- Cu/DLC (-125 V)

3- Cu/DLC (-150 V)

4- Cu/DLC (-175 V)

5- Cu/DLC (-225 V)

Fig. 2. Variation of residual stress versus self bias for different Cu/DLC bi-layer films.

N. Dwivedi, S. Kumar / Current Applied Physics 12 (2012) 247e253 249

E basically depends on the slope of harmonic interatomic potential.The value of E (at 5 mN) of Cu/DLC bi-layer films followed similartrend and found to decrease from 280.9 to 219 GPawith increase inself bias from �125 to �225 V. The value of H and E (at 5 mN) ofpure DLC film grown at�125 Vwere also estimated and found to be25.1 and 257.2 GPa, respectively. This can be seen clearly thatinterfacial Cu layer reduces the stress of DLC film without affectingbi-layer film hardness, somewhat there is improvement of H and Evalues. The decrease in H and Ewith increase in self bias was founddue to following two facts: (i) increase in energy of bombardingcarbon atoms/ions which may enhance surface roughness and (ii)change in bondings from diamond-like sp3 C to graphite-like sp2 C.The latter case i.e. bondings was found to be most important fact todiscuss while explaining nano-mechanical properties of DLC andCu/DLC bi-layer films. Self bias possesses linear relation with ionenergy; therefore role of ion energy can also be expressed in term ofself bias. On the basis of ion energy (or self bias) Erdemir andDonnet [16] categorized hydrogenated amorphous carbon filmsinto three levels, which are: (i) polymer-like carbon for lower selfbias (ii) diamond-like carbon for intermediate self bias and (iii)graphite-like carbon for higher self bias. In the lower self bias (orion energy) hydrocarbon precursor insufficiently decomposed andcontain high amount of bound and unbound hydrogen resulting inpolymer-like carbon soft films. In intermediate self bias, hydro-carbon precursor sufficiently decomposed with loss of significantamount of hydrogen as well as formation of high dense structureresulting in diamond-like carbon hard film. However, higher selfbias causes increase in sp2 induced disorder resulting in graphite-like carbon films. Fallon et al. [17] reported maximum sp3Cbonding at w 100 eV in filtered cathodic vacuum arc growntetrahedral amorphous carbon films, beyond which the initiation ofgraphite-like bonding was observed. Singh et al. [18] observedmaximum sp3 C bonding with maximum H at �150 V. Recently, wehave also observed diamond-like properties at self biasbetween �100 and �150 V [1,4,19]. Thus, self bias between �100and �150 V seems to be ideal for obtaining diamond-like coatings.However, range of self bias for the deposition of diamond likecoatings may depend upon system geometry also.

The load versus displacement curves at 10, 15 and 20 mN ofdifferent Cu/DLC bi-layer films are also shown in Fig. 3(b), (c) and(d), respectively. The penetration depth was found to increase withincreasing the load towards higher value and thus, reduces H and E.From Figs. 4 and 5, it is confirmed that H and E continuouslydecrease with increasing load from 5 to 20mN. The values of H at 5,10, 15 and 20mN for Cu/DLC bi-layer film deposited at�125 Vwere

found to be 26.2, 18.7, 14.8 and 14.2 GPa, respectively. Similarly, thevalues of E at 5, 10, 15 and 20 mN for Cu/DLC bi-layer film depositedat �125 V were found to be 280.9, 223.9, 211.5 and 209 GPa,respectively. Other Cu/DLC bi-layer films deposited at �150, �175and �225 V also exhibited same trend of H and E with increase inload from 5 to 20 mN. Chen et al. [20] systematically discussed theeffect of substrate on mechanical properties of thin films. Theysuggested that if hard film grown on soft substrate then H ofcomposite substrate/film is continuously decreased with increasingpenetration depth. However, if soft film grown on hard substratethen H of composite substrate/film is increased with increasingpenetration depth. Yu et al. [21] explained the effect of substrate onelastic properties of films with considering boundary value prob-lems. Saha et al. also [22] explained the effect of substrate onmechanical properties of thin films. They performed series ofmeasurements such as hard film on soft substrate and soft film onhard substrates and then analyzed the results. Lin et al. [23] alsoobserved reduction in H and Ewith increase in penetration depths.Therefore, we also plotted H and E against penetration depth fordifferent Cu/DLC bi-layer films which as shown in Fig. 6 (a) and (b),respectively. In the present study DLC and Cu/DLC bi-layer films aregrown on Si substrate. It is to be noted that DLC and Cu/DLC bi-layerfilms are harder than Si as discussed earlier in this section. Thus, themodel of hard film on soft substrate is applicable here. This can beseen from figures that initially with increase in penetration depththe H and E decreases drastically as represented in region- I (ReI).Further increase in penetration depths led to further decrease in Hand E values due to increase in influence of substrate as representedin R- II. However, the values of H and E saturate (as represented inR-III) at higher penetration depths due to completely dominance ofsubstrate effect. In addition, H and E also expressed as a function ofdepth/film thickness (h/t). The figures in inset of Fig. 6(a) and (b),show the variations of H and E versus depth/film thickness. Thevalues of H and E were found to be maximum between 0.28 < h/t < 0.33 due to negligible substrate effect. The H and E weredecreased slightly between 0.36 < h/t < 0.5 due to initiation ofsubstrate effect. However, when h/t > 0.5 then H and E values werereduces significantly due to more influence of substrate.

Plasticity index (H/E) was found to be an important parameter toexplain elastic-plastic and wear resistance properties of thin filmsdue to combined effect. The H/E ratio showed physical response ofan atomic lattice to an external force and related to the bulk

0.00 0.02 0.04 0.06 0.08 0.10 0.12

0

1

2

3

a

4

5 Load = 5 mN

Lo

ad

(m

N)

Displacement ( µm)

-125 V

-150 V

-175 V

-225 V

DLC

0.00 0.04 0.08 0.12 0.16

0

2

4

6

8

10 Load = 10 mN

Lo

ad

(m

N)

Displacement ( µm)

-125 V

-150 V

-175 V

-225 V

0.00 0.05 0.10 0.15 0.20 0.25

0

5

10

15 Load = 15 mN

Lo

ad

(m

N)

-125 V

-150 V

-175 V

-225 V

0.00 0.05 0.10 0.15 0.20 0.25 0.30

0

5

10

15

20 Load = 20 mN

Lo

ad

(m

N)

-125 V

-150 V

-175 V

-225 V

b

c d

Fig. 3. Load versus displacement curves of different Cu/DLC bi-layer films taken at (a) 5, (b) 10, (c) 15 and (d) 20 mN.

N. Dwivedi, S. Kumar / Current Applied Physics 12 (2012) 247e253250

fracture strength. The domain of validity of H/E generally varied inthe range from 0 to 0.1 for the DLC coatings (natural diamond withH w 100 GPa, E w 1000 GPa exhibited H/E w0.1). The upper andlower limits showed their elastic and elastic-plastic behavior,respectively. For high wear resistance coatings the parameter H/Emust be very high. The variation of H/E versus load for different Cu/

5

10

15

20

0

7

14

21

28

510

1520

-225 V

-175 V

-150 V

-125 V

H(G

Pa)

Load (m

N)

Fig. 4. Variation of H versus indentation load for different Cu/DLC bi-layer films.

DLC bi-layer films is depicted in Fig. 7. H/E rapidly decreases withincrease in load from 5 to 10 mN. Beyond 10 mN saturation in thevalues of H/E was observed. Similarly, H/E was also found tocontinuously decreases with the increase in self bias from �125to �225 V. Beyond 5 mN (till 20 mN) and �125 V (till �225 V)observed decrease in H/E exhibited more fraction of work

5

10

15

20

0

70

140

210

280

-175 V

-225 V

-150 V

-125 V

E(G

Pa)

Load

(mN

)

Fig. 5. Variation of E versus indentation load for different Cu/DLC bi-layer films.

100 150 200 250 300 350

14

21

28

35

0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8

12

16

20

24

28

H (G

Pa)

Depth / Film thickness

-125 V

-150 V

-175 V

-225 V

R - III R- II R- I

H (G

Pa)

Depth (nm)

-125 V

-150 V

-175 V

-225 V

100 150 200 250 300 350

200

240

280

320

0.2 0.3 0.4 0.5 0.6 0.7 0.8

200

220

240

260

280

E (

GP

a)

Depth / Film thickness

-125 V

-150 V

-175 V

-225 V

R - IIIR - IIR - I

E (

GP

a)

Depth (nm)

-125 V

-150 V

-175 V

-225 V

a

b

Fig. 6. Variations of (a) H and (b) E versus penetration depths for different Cu/DLC bi-layer films. Insets of Fig. 6(a) and (b) show the variations of H and E versus depth/filmthickness for different Cu/DLC bi-layer films.

5 10 15 20

55

60

65

70

75

)%

( R

E

Load (mN)

-125 V

-150 V

-175 V

-225 V

Fig. 8. Plot of ER versus indentation load for various Cu/DLC bi-layer films.

N. Dwivedi, S. Kumar / Current Applied Physics 12 (2012) 247e253 251

consumed in a plastic deformation and large plastic strain isexpected when contacting a material. It is to be noted that withchange in load from 5 to 20 mN observed enhancement in plasticdeformationwas due to substrate effect. On the other hand increase

5 10 15 20

0.063

0.072

0.081

0.090

H/E

Load (mN)

-125 V

-150 V

-175 V

-225 V

Fig. 7. Variation of H/E versus indentation load for different Cu/DLC bi-layer films.

in plastic deformation with increase in self bias from �125to �225 V was due to material properties because with increase inself bias the initiation of graphite-like sp2 C bonding takes place. Inaddition, interfacial mismatch as well as surface roughness alsoreduces H/E in Cu/DLC bi-layer films. Nonetheless, observed higherH/E (maximum H/E w 0.093) in Cu/DLC bi-layer films made it anexcellent wear resistance coating for various applications. Analysisof elastic recovery (ER) has also been performed to elucidate moreinformation about elastic properties [1,4]. The values of ER werecalculated by employing a relation given in Eq. (2)

%ER ¼ ðhmax � hresÞhmax

� 100 (2)

where hmax and hres are the displacement at the maximum load andresidual displacement after load removal, respectively. The varia-tion of % ER versus load for various Cu/DLC bi-layer films depositedat �125, �150, �175 and �225 V is shown in Fig. 8. It is clear fromfigure that with increase in self bias from �125 to �225 V thevalues of ER was found to continuously decrease from 76 to 68.7 %

5 10 15 20

0.25

0.30

0.35

0.40

0.45

hs

er

h/x

am

Load (mN)

-125 V

-150 V

-175 V

-225 V

Fig. 9. Variation of hres/hmax against indentation load for different Cu/DLC bi-layerfilms.

5 10 15 20

8

12

16

20

24

28

H/

Load (mN)

-125 V

-150 V

-175 V

-225 V

Fig. 12. Variation of H/s versus indentation load for different Cu/DLC bi-layer films.

5 10 15 20

0.0

9.0x10-10

1.8x10-9

2.7x10-9

3.6x10-9

Ur

)J

(

Load (mN)

-125 V

-150 V

-175 V

-225 V

Fig. 10. Plot of Ur versus indentation load for various Cu/DLC bi-layer films.

N. Dwivedi, S. Kumar / Current Applied Physics 12 (2012) 247e253252

(at 5 mN). However, at 10, 15 and 20 mN the variation of ER withself bias was not inversely but it was oscillating in nature. However,actual ER of bi-layer filmswas observed at 5mN. Observed decreasein ERwith increase in self bias was due increase in bonding inducedplastic deformation whereas decrease in ER with increase of loadwas due to increase in substrate induced plastic deformation thatreduces recovery ability. Further, the hres/hmax is another parameterthat provides information similar to that of ER with differentdomain of validity. It varies inversely with ER. The domain of val-idity of hres/hmax varies in the range from 0 to 1 inwhich lower limiti.e. 0 corresponds to fully elastic and upper limit i.e. 1 correspond toelastic-plastic behaviour. The variation of hres/hmax versus load fordifferent Cu/DLC bi-layer films is shown in Fig. 9. From figure it isevident that trend of hres/hmax was found to be exactly inverselywith ER. It has also been found that the results of hres/hmax werefound to be good in agreements not only with ER results but alsowith H, E and H/E results. The elastic and plastic deformation wasalso expressed in term of energy and for this the parameter plasticdeformation energy (Ur) [1,23] was also estimated at differentloads, which as shown in Fig. 10. From figure, it is evident that thevalue of Ur was found to continuously increase with increasing loadfrom 5 to 20 mN. The increase in load enhances the plastic

5 10 15 20

0.12

0.16

0.20

0.24

0.28

Sx

am

/µ

N(

)m

Load (mN)

-125 V

-150 V

-175 V

-225 V

Fig. 11. Variation of Smax against indentation load for different Cu/DLC bi-layer films.

deformation. It is also to be noted that with increase in self biasfrom -125 to -225 V the values of Ur were increased due to loweringof H values because at higher self biases the initiation of graphite-like sp2 bonding take place. Ur values at 5 mN were found to bequite lower of the order of 10�10 J, which increases to 10�9 J withfurther increase in load upto 20mN. Thus, observed lower Ur valuesagain confirmed highly elastic behaviour of Cu/DLC bi-layer films.In addition, the variation of unloading stiffness at maximum load(Smax) versus load for different Cu/DLC bi-layer films is shown inFig. 11. The Smax was estimated by taking the slope of unloadingcurve. It is evident from figure that with increase in load from 5 to20 mN the values of Smax in all the films continuously increasingexcept Cu/DLC film deposited at �125 V. In this film the value ofSmax was found to decrease beyond 15 mN. It is worth noting thatactual Smax was observed at 5 mN and beyond this load substrate/film composite was contributed to Smax. Further, the estimation ofparameter hardness per unit stress (H/s) can be very useful tofigure out the quality of Cu/DLC bi-layer films. The variation of H/sversus load for different Cu/DLC bi-layer films is plotted in Fig. 12. Itis to be noted that higher the H and lower the s resulting in higher

5 10 15 20

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Load (mN)

-125 V

-150 V

-175 V

-225 V

Fig. 13. Variation of (H/E)/s versus indentation load for different Cu/DLC bi-layer films.

N. Dwivedi, S. Kumar / Current Applied Physics 12 (2012) 247e253 253

H/s and thus good quality of DLC and Cu/DLC bi-layer coatings.From Fig. 12, it is evident that with increase in load from 5 to 20mNthe values of H/s were continuously decreased. Observed decreasein H/s was due to increase of substrate effect which becomesdominant at higher load say 20 mN. Thus, actual H/s of Cu/DLCfilms seem to be obtained at 5 mN and beyond this load obtainedH/s showed composite substrate/film effect. From figure it can also beseen that the values of H/swere decreased with increasing self biasfrom�125 to�225 V because increase in self bias reduce the H dueto initiation of soft graphite-like sp2 bonding. Thus, it has beenrealized that better quality Cu/DLC bi-layer film with highlydiamond-like character was obtained at �125 V. Due to combinedeffect, the plasticity index per unit stress [(H/E)/s] was found to beanother important parameter to discuss. The variation of (H/E)/sversus load for Cu/DLC bi-layer films is shown in Fig. 13. It is to benoted that higher the H/E and lower the s correspond to higher (H/E)/s, resulting in good quality wear resistance coating. Observedvalues of (H/E)/s followed similar trend and found to decrease withincrease in load from 5 to 20 mN. Increase in self bias also loweredthe values of (H/E)/s values. Thus, from Fig. 13 it was concluded thatfilm deposited at �125 V also showed excellent wear resistancecapability.

4. Conclusions

The effect of indentation load on nano-mechanical properties ofCu/DLC bi-layer films studied systematically. Cu/DLC bi-layer filmswere grown; using hybrid system involving RF-PECVD and RF-sputtering units, under varied self biases from �125 to �225 V.The nano-mechanical properties of each Cu/DLC bi-layer films weremeasured at 5, 10, 15 and 20 mN. With increase in load from 5 to10 mN, the penetration depth increase and nano-mechanicalproperties drastically degrade due to initiation of influence of softsubstrate. However, beyond 10 mN (between 15 and 20 mN i.e. R-III) where higher penetration depth observed, the nano-mechanicalproperties saturates due to completely dominance of substrateeffect. In addition with increase in self bias from �125 to �225 Vthe nano-mechanical properties were also decreases due to initia-tion of graphite-like sp2 C bonding as well as increase of surfaceroughness. The addition of Cu interlayer in DLC films reduces notonly residual stress but also result in nominal increase of itshardness and elastic modulus. Lower (even negligible) substrateeffect was encountered at 5 mN, which therefore provided actualnano-mechanical properties of Cu/DLC bi-layer films. Thus, due to

lower stress and excellent other nano-mechanical properties suchas H, E, H/E and ER etc. these Cu/DLC bi-layer films seems to be idealfor hard and protective coating on cutting tools, automobile parts,solar cells and MEMS.

Acknowledgements

The authors are grateful to the Director, National PhysicalLaboratory, New Delhi (India) for his kind support. Authors alsowish to thank Mr. C. M. S. Rauthan and Dr. O. S. Panwar for theirkind support. One of author ND acknowledges CSIR, Govt. of Indiafor providing SRF fellowship. We acknowledge CSIR, Govt. of Indiafor sponsoring network project NWP-0027 and for their financialsupport.

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