Appl Phys A (2011) 102: 225–230DOI 10.1007/s00339-010-5908-5

Nano indentation measurements on nitrogen incorporateddiamond-like carbon coatings

Neeraj Dwivedi · Sushil Kumar · C.M.S. Rauthan ·O.S. Panwar

Received: 1 December 2009 / Accepted: 17 June 2010 / Published online: 21 July 2010© Springer-Verlag 2010

Abstract Nanoindentation testing was performed on ni-trogen (N2) incorporated diamond-like carbon (N-DLC)films and deposited using radio-frequency plasma-enhancedchemical vapor deposition technique, with varied percent-age of nitrogen partial pressures of 0, 44.4, 66.6, and 76.1%.The values of nanohardness (H ) and elastic modulus (E)of these films were obtained from 38 to 22 GPa and 462to 330 GPa, respectively, as the partial pressure of N2 in-creases from 0 to 76.1%. Further, these films were stud-ied for % elastic recovery, ratio between residual displace-ment after load removal and displacement at maximum load(dres/dmax), plastic deformation energy and plasticity indexparameter (H/E). Both hardness per unit stress and plas-ticity index per unit stress were found to be maximum at N2

partial pressure of 76.1%. X-ray photoelectron spectroscopymeasurements confirmed the presence of N2 in these films.

1 Introduction

The plasma-produced nitrogen incorporated diamonds likecarbon (N-DLC) thin films have attracted considerable at-tention due to its versatile properties, quite closer to that ofnatural diamond. Depending upon nitrogen (N2) concentra-tion, the properties of N-DLC films can be tailored for elec-trical, optical, and mechanical applications [1–3]. Incorpo-ration of small amount of N2 corresponds to the significantimprovement in the electrical and optical properties and re-duces the residual stress as well, whereas incorporation of

N. Dwivedi · S. Kumar (�) · C.M.S. Rauthan · O.S. PanwarPlasma Processed Materials Group, National Physical Laboratory,Dr. K.S. Krishnan Road, New Delhi 110 012, Indiae-mail: skumar@nplindia.orgFax: +91-11-45609310

high amount of N2 may lead to the formation of theoreti-cally predicted β-C3N4 phases [4, 5]. However, its practicalconfirmation is still awaited. Nevertheless, N2 incorporationin DLC promotes the formation of nanocrystalline structure;therefore, there could be a possibility of creation of someβ-C3N4 nanocrystallites in N-DLC films. Charitidis et al.[6] have reported in their recent review that PECVD-grownDLC film exhibits low hardness (H ∼ 22 GPa) and elasticmodulus (E), compared to that of filtered cathodic vacuumarc deposited hydrogen free tetrahedral amorphous carbon(ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H) films. Chowdhury et al. [7] and Hao et al. [8] have alsoreported mechanical properties of carbon nitride films withthe maximum value of H as 24 and 19.8 GPa, respectively.However, Lejeune et al. [9] reported very low values of H

(maximum H ∼ 2.3 GPa) and E (maximum E ∼ 27 GPa)for a-CN films deposited using radio frequency-sputteringtechnique. Compared with hydrogen-free DLC, a-C:H filmsare relatively soft but exhibit some of the lowest frictionand wear coefficients. The coefficient of friction reducedas the hydrogen content of the DLC (a-C:H) coating is in-creased. The hydrogen content in these films is primarilyindependent variable that can be differ considerably depend-ing on deposition method, hydrocarbon source gas, and de-position parameters used, which determine the structure andhence the properties of hydrogenated DLC. Incorporationof hydrogen in these films deposited by PECVD or sputter-ing techniques is required in order to obtain “diamond-like”properties and stabilizing the diamond structure by passivat-ing dangling bonds through maintaining the sp3 hybridiza-tion configuration. This is what said for DLC is also truefor N-DLC (a-C:N:H). N-DLC films possess the impres-sive mechanical, tribological, and corrosion properties thatare desirable in biomedical and magnetic disc applications.Because of correlation of biocompatibility and coefficient

226 N. Dwivedi et al.

Fig. 1 Schematic ofnanoindentation used formeasuring the mechanicalproperties of N-DLC filmsunder applying maximumindentation load of 10 mN

of friction with hydrogen in carbon coating on bioimplantsparts in a human body and magnetic disc, respectively, N-DLC films require more H to enhance the surface propertiesand for its biomedical and mechanical applications. Thus,the key issue is achieving super hardness values (>40 GPa)in hydrogenated DLC (a-C:H) based coatings such as N-DLC (a-C:N:H). The study of indentation properties of suchcoating with thickness about 100 nm is very important.

This paper reports the nanoindentation studies of N-DLC films grown under varied % N2 partial pressure(% N2 partial pressure = N2/(N2 + C2H2)) from 0 to76.1%. Using nanohardness (H ), E, and load-displacementcurves, further investigation of % elastic recovery (% ER),ratio between residual displacement after load removal anddisplacement at maximum load (dres/dmax), plastic defor-mation energy (Ur), plasticity index parameter (H/E) havebeen performed. These films were also characterized forstress (S), hardness per unit stress (H/S), and plasticityindex per unit stress [(H/E)/S]. Two N-DLC films werecharacterized for X-ray photoelectron spectroscopy (XPS),to ensure the incorporation of N2 in the carbon network.

2 Experimental details

Nano indentation is a very advance technique to study themechanical properties of hard coatings such as H and E,due to its ability to estimate the loading–unloading responsewith very high resolution. The H basically is defined as theratio of maximum load applied in the loading process to theprojection area created at the instant maximum load is ap-plied. The mechanical properties of N-DLC films have beenmeasured using IBIS nano indentation (Fisher-Cripps labo-ratories Pvt. Limited, Australia) having triangular pyramiddiamond Berkovich indenter with normal angle of 65.3° be-tween the tip axis and the faces of triangular pyramid and thecurvature of 150–200 nm at the tip. Figure 1 is the schematic

of nanoindentation setup. The piezoelectric translator (PZT)drives the carriage that deflects the spring and generates theindentation load. The force and depth sensors are linear vari-able differential transformers (LVDT). These devices arevery sensitive to sense the force and the depth. Maximumload of 10 mN was applied in all the nanoindentation mea-surements. Fully automatic software controlled IBIS nanoindentation provided load-displacement curve, H and E ofN-DLC films. These results were further used to investigatethe % ER, dres/dmax, Ur, and H/E values.

N-DLC films were deposited on well-cleaned Si 〈100〉wafer and corning 7059 glass under varied N2 partial pres-sures of 0, 44.4, 66.6, and 76.1%. However, C2H2 gas pres-sure and negative self bias of 100 V were kept constant inall the depositions. These films were deposited in a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) system having relatively high base pressure of∼1 × 10−3 Torr, as vacuum in the chamber was obtainedusing root pump backed by rotary pump. Prior to film depo-sition, Ar plasma treatment for 10 min was applied to elimi-nate any contamination and moisture effect from the surfaceof the substrates. Four sets of films of different thicknessesranging from 162 to 276 nm were prepared. The thicknessesof these films were measured using Taylor–Hobsson talystepinstrument. XPS measurements of these N-DLC films wereperformed using Perkin Elmer 1257 instrument.

3 Results and discussions

Figure 2 shows the variation of deposition rate as a func-tion of N2 partial pressure for as grown N-DLC films. It isevident from the figure that deposition rate decreases from42.5 nm/min. to 25 nm/min., with increasing N2 partial pres-sure from 0 to 76.1%. The observed trend is similar to thatof reported result [10, 11]. The decrease in deposition ratewith the increase in N2 partial pressure can be attributed

Nano indentation measurements on nitrogen incorporated diamond-like carbon coatings 227

Fig. 2 The variation of deposition rate versus N2 partial pressure

Fig. 3 XPS spectra of N-DLC films deposited at N2 partial pressuresof (a) 0% and (b) 76.1%

to the fact that nitrogen species in carbon–nitrogen plasmaincreases with increasing N2 partial pressure. Decrease ofdeposition rate means that there is a competition betweendeposition and etching of film due to ionic bombardmenton growing film surface. Also reduction in deposition ratecould be attribute to the deposition from locally N2 enrichedregions during the film growth process. Consequently, in-crease in N2 partial pressure hinders the transport of carbonspecies with nitrogen and therefore, reduces the depositionrate.

XPS measurements were performed on two representa-tive films to confirm the incorporation of N2 in the carbonnetwork of DLC films. Figures 3(a) and 3(b) show the XPSspectra of two N-DLC films deposited at N2 partial pres-sures of 0 and 76.1%, respectively. In addition to carbon andnitrogen, these spectra also exhibit the presence of oxygen.The presence of oxygen is due to the fact that (i) the deposi-

Fig. 4 Load-displacement curves of N-DLC films deposited at N2 par-tial pressures of 0, 44.4, 66.6, and 76.1%

tion of these films were carried out in a low vacuum systemhaving base pressure of 1 × 10−3 Torr and to (ii) surfacecontaminations during the exposure of films in ambient airbefore the analysis. O 1s peak in both the films was foundnear 532 eV, whereas N 1s peak of film deposited at N2 par-tial pressure of 76.1% obtained at 399.2 eV. Petrov et al.[12] also obtained N 1s peak at 399.2. This peak may ac-company with sufficient sp3 C–N bonding in the structure.However, N-DLC film deposited at 0% N2 partial pressureshows the C 1s peak at 285 eV, which changes to 284 eVfor N-DLC film deposited at N2 partial pressure of 76.1%.Therefore, the shifting of C 1s peak from higher binding en-ergy to lower binding energy due to incorporation of N2 inthe structure, which resulted in decrease of strong sp3 bond-ing and increase of soft graphite like sp2 bonding in the N-DLC films [12, 13].

Figure 4 shows the load-displacement curves obtained bynano indentation on N-DLC films, deposited at N2 partialpressures of 0, 44.4, 66.6, and 76.1%. The maximum load of10 mN was applied in the present study due to the fact thatbeyond 10 mN, indenter may penetrate the substrate consid-ering the thickness of the film less than 300 nm. The pen-etration depth for N-DLC films was varied in the range of∼110–141 nm. Load-displacement curves were employedto estimate the percentage of elastic recovery (% ER) usingthe relation [9] given by

% ER = [(dmax − dres)/dmax] × 100 (1)

where dmax and dres are the displacement at the maximumload and residual displacement after load removal, respec-tively. The variation of % ER versus N2 partial pressure isshown in Fig. 5(a). Depending upon N2 partial pressure, ERin N2-DLC films were found to be in the range of 76 to81.3%. Maximum ER of 81.3% was obtained for the filmdeposited without N2 gas incorporation, which changed to80.2% for the film grown at N2 partial pressure of 44.4%.

228 N. Dwivedi et al.

Fig. 5 The variation of (a) % ER, (b) dres/dmax ratio, and (c) Ur versusN2 partial pressure

ER further decreased to 79 and 76% for the film depositedat N2 partial pressures of 66.6 and 76.1%, respectively. Thedres/dmax ratios were also estimated to identify more aboutmechanical behavior of N-DLC films. The dres/dmax ratiogives the information similar to that of % ER with differ-ent domains of validity. Typically, the domain of validity ofdres/dmax ratios is 0 to 1 [7]. The lower limit corresponds tothe fully elastic and upper limit corresponds to the elastic-plastic behavior, respectively. Figure 5(b) shows the varia-tion of dres/dmax ratio versus % N2 partial pressure, for var-ious N-DLC films. The ratios were found to be in the rangefrom 0.19 to 0.24, which revealed the highly elastic behav-ior of these films. However, increase in N2 partial pressure,changes elastic behavior into elastic-plastic behavior due tothe fact that increase in N2 partial pressure enhance the softgraphite like sp2 clusters in the carbon network. Highly elas-tic characteristic of these films is also confirmed by % ER.

The concept of plastic deformation energy (Ur) devel-oped by Sakai et al. [14] is found to be an important parame-ter to study the mechanical properties of thin films. Gener-ally, Ur of the film is estimated by area surrounded by theloading–unloading curve in the load-displacement profile.Ur possesses a relation with H , which is given by [15]

Ur = P 3 / 2/3H 1/ 2[(wo tan2 ψ)−1/2] (2)

where wo is the geometry constant and attains the value of1.3 for pyramid indenter, P is the load, and ψ is the half an-gle of Berkovich indenter and has the value 65.3◦. Variationof Ur as a function of % N2 partial pressure is depicted inFig. 5(c). These films exhibited very low values of Ur, whichincreased slightly with increasing the N2 partial pressure, re-vealed highly rigid and elastic behavior. The film deposited

Fig. 6 Variation of (a) H , (b) E, and (c) H/E versus N2 partial pres-sure

without N2 incorporation exhibited the lowest value of Ur

of 6.9 × 10−10 joule, increased to 7.6 × 10−10, 8.7 × 10−10,and 9.0 × 10−10 joule for the films deposited with N2 par-tial pressures of 44.4, 66.6, and 76.1%, respectively. The in-crease in Ur, with the increase in N2 partial pressures, inthese films may be attributed due to the fact that Ur variesinversely proportional to the H , and increase in N2 partialpressure corresponds to the decrease in H values by mod-ifying the bonding structure. Brittle material exhibit lowervalue of Ur, while ductile material shows comparativelyhigh values of Ur. The increase in Ur with increase in N2

partial pressure reveals the slight transition of brittle phaseof material into ductile phase.

The variation in the evaluated experimental H values ver-sus % N2 partial pressure for the N-DLC films is presentedin Fig. 6(a). It is evident from the figure that H in thesefilms varied inversely proportional to the % N2 partial pres-sure and estimated values were found to be in the range from22 to 38 GPa. Maximum H of 38 GPa was obtained for thefilm deposited without N2 gas incorporation, which changesto 31, 24, and 22 GPa for the films deposited at N2 partialpressures of 44.4, 66.6, and 76.1%, respectively. Two im-portant factors to illustrate the H of DLC and N-DLC filmsare ion energy and different bondings which take place inthe structure of the films. Erdemir et al. [16] have proposedthat at low ion energy, the hydrocarbon precursor is not suf-ficiently decomposed, which results in very soft polymer-like features in the films. At intermediate ion energy, hydro-gen content is reduced, and hydrocarbon precursor is suffi-ciently decomposed, deposited films possesses high densityand reveal diamond-like structure. At very high ion energy,graphite like bonding is dominant due to the increase in sp2

induced disorder. The ion energy, which is a function of ap-

Nano indentation measurements on nitrogen incorporated diamond-like carbon coatings 229

plied RF power and varied proportionally to negative selfbias, possesses a relation given by

Negative self bias = RF power/(pressure)1/2 (3)

Fallon et al. [17] reported that maximum sp3 bonding intetrahedral amorphous carbon films deposited by filtered ca-thodic vacuum arc process is obtained at around negativeself bias of 100 V, and beyond 100 V, the graphitization ofthe films starts. Another approach to explain the H in thesefilms is the changes in the bonding and the structure due toN2 incorporation. Generally, N2 incorporation in DLC takesplace on the expenses of C from strong C–C sp3 bondingand promotes the N–H and C–N bonding which reduce thecoordination of the films. N2 incorporation in DLC also pro-motes the strong C–H debonding and finally, reduces the H

of overall structure [2, 3]. However, high values of H in theN-DLC films may be accompanied to the formation of someβ-C3N4 nanocrystallites embedded in the amorphous DLCnetwork, but this needs to be confirmed in the present inves-tigation. The variation of E versus % N2 partial pressure fordifferent N-DLC films is presented in the Fig. 6(b). The fig-ure clearly demonstrates that the values of E in these filmswere found to be very high (462 to 330 GPa), which de-creases with increasing N2 partial pressure. The N-DLC filmdeposited without N2 gas incorporation exhibited maximumE of 462 GPa, which decreased to 418, 360.5, and 330 GPafor the film deposited at N2 partial pressure of 44.4, 66.6,and 76.1%, respectively.

The plasticity index parameter (H/E) [6] is an anotherimportant parameter to differentiate between the elastic andelastic-plastic behavior. For protective coating over mag-netic hard disk or for good wear resistance coating over that,the H/E must be very high. Even if less hard films possesshigh H/E value is accepted for such application. The vari-ation of H/E as a function of N2 partial pressure is pre-sented in Fig. 6(c). It is to be noted that H/E values in theseN-DLC films were found to be high, which decrease withincreasing N2 partial pressure. High value of H/E meansthat these films are highly resistance to plastic deformation.In other words, these films have to undergo a high elastic de-formation, which is also confirmed by % ER and thus resultsin high values of H .

H strongly depends on the presence of nano- to mi-crostructural defect present in the network, and it should berelated to the bonding between the atoms and to the abil-ity of the bonds to withstand deformation stemming fromcompression, extension, bending, or breaking, whereas E

depends on the slope of harmonic interatomic potential [6].The H versus E estimated for N-DLC films is demonstratedin Fig. 7. Generally, H varied linearly to the E and pos-sessed the relation H = (1/10)E [6]. It may be noted thatcrystalline diamond exhibits H ∼ 100 GPa and E ∼1000–1100 GPa. However, in these films, in the beginning it fol-lows linear path, but after certain values (H > 23 GPa and

Fig. 7 Variation of H versus E for different N-DLC films

Fig. 8 Variation of (a) S, (b) H/S, and (c) (H/E)/S versus N2 partialpressure

E > 350 GPa) their linear path deviates and shows H ≥(1/10)E. Thus, one can see that instead of the conventionallinear behavior (represented by dash line in the figure), thesefilms exhibited superlinear behavior (represented by darkline) after certain values, which provides more toughness tothe structures as revealed from H versus E curve.

High level of residual stress (S) limits the growth of thickDLC films to less than micron and restricts its wide spreadindustrial applications. Figure 8(a) shows the variation of S

as a function of N2 partial pressure for as deposited N-DLCfilms. It is evident from the figure that film deposited with-out N2 gas incorporation exhibits considerable large valueof S of 1.9 GPa, which decreases to 1.3 GPa for the N-DLCfilm deposited at N2 partial pressure of 44.4%. On furtherincreasing the N2 partial pressure to 66.6%, S in N-DLCfilms further decreases to 1.0 GPa. However, the minimum

230 N. Dwivedi et al.

value of S of 0.8 GPa was evaluated for the N-DLC filmdeposited at maximum N2 partial pressure of 76.1%. It canbe seen that S in these N-DLC films varies inversely to theN2 partial pressure. The possible explanations for the reduc-tion of residual stress in N-DLC films are (i) N2 incorpo-ration in DLC enhance the soft graphite-like sp2 bonding,(ii) promote hard sp3 C–C debonding, and (iii) reduced av-erage coordination number [3]. High hardness in DLC andN-DLC films is always accompanied by high residual stress.Therefore, estimation of hardness per unit stress (H/S) andplasticity index per unit stress [(H/E)/S] gives completeinformation about mechanical properties due to combinedeffect. Higher the values of H/S and [(H/E)/S] are theindicator of better films quality, because higher values ofthese parameter mean lower the stress and higher the hard-ness and plastic resistance parameter. Variations of H/S

and (H/E)/S versus N2 partial pressure are depicted inFigs. 8(b) and 8(c). It is evident from the figure that boththese parameters increase with increasing N2 partial pres-sure, which reveal the importance of increase of N2 partialpressure in these N-DLC films.

4 Conclusions

Nitrogen-incorporated diamond-like carbon (N-DLC) filmswere deposited using RF-PECVD technique at various N2

partial pressures. XPS measurement shows the presence ofN2 in the carbon network. XPS spectra also reveal that dueto N2 incorporation, the C 1s peak shift toward lower bind-ing energy, which corresponds to the increase of sp2 clus-tering in the carbon network. Nanoindentation techniquewas used to study the mechanical properties at micro- tonanoscale due to its high resolution characteristics. Thesefilms exhibited very high nano hardness (H ) and elasticmodulus (E), which were in the range 22 to 38 GPa and 330to 462 GPa, respectively. The values of H and E were foundto decrease with increasing N2 partial pressure. Besides hav-ing high H and E values, these films exhibited high elasticnature and lower plastic deformation energy. S in these filmswere found to be very low, decreasing from 1.9 to 0.8 GPa asN2 partial pressure increases from 0 to 76.1%, respectively.The trend of increase of H/S and (H/E)/S with increasethe N2 partial pressure were observed, which is quite impor-tant for mechanical applications of these films.

Acknowledgements The authors are grateful to the Director, Na-tional Physical Laboratory, New Delhi (India) for his kind permissionto publish this paper. The authors also wish to thank Dr. Govind forproviding XPS measurement facility. We acknowledge CSIR, Govt. ofIndia for sponsoring Network Project NWP-0027 and for their finan-cial support.

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