Corrosion Behavior of Aluminum Doped Diamond-Like Carbon ... NW_JNN2010-2.pdf · Diamond-like carbon (DLC) lms can have high hardness, dielectric strength, biocompatibility and optical

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Copyright copy 2010 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 10 4767ndash4772 2010

Corrosion Behavior of Aluminum Doped Diamond-LikeCarbon Thin Films in NaCl Aqueous Solution

N W Khun and E Liulowast

School of Mechanical and Aerospace Engineering Nanyang Technological University50 Nanyang Avenue Singapore 639798 Singapore

Aluminum doped diamond-like carbon (DLCAl) thin films were deposited on n-Si(100) substratesby co-sputtering a graphite target under a fixed DC power (650 W) and an aluminum target undervarying DC power (10ndash90 W) at room temperature The structure adhesion strength and surfacemorphology of the DLCAl films were characterized by X-ray photoelectron spectroscopy (XPS)micro-scratch testing and atomic force microscopy (AFM) respectively The corrosion performanceof the DLCAl films was investigated by means of potentiodynamic polarization testing in a 06 MNaCl aqueous solution The results showed that the polarization resistance of the DLCAl filmsincreased from about 18 to 307 k though the corrosion potentials of the films shifted to morenegative values with increased Al content in the films

Keywords DLCAl Thin Film Magnetron Sputtering NaCl Solution Corrosion PotentiodynamicPolarization

1 INTRODUCTION

Diamond-like carbon (DLC) films can have high hardnessdielectric strength biocompatibility and optical trans-parency as well as low wear and friction These proper-ties are strongly dependant on the fractions of sp3 andsp2 hybridized carbon bonding in the films DLC filmscan also possess outstanding chemical properties in vari-ous environments The corrosion resistance of DLC-coatedmaterials is closely related to the structure surface mor-phology thickness and defect density of the DLC films aswell as the interface condition between film and substrate1

DLC films can be synthesized by different techniquessuch as magnetron sputtering laser ablation cathodicarc and various chemical vapor deposition (CVD)processes2ndash4 The impinging energy of sputtered carbonspecies during film deposition is an important parameterto get a dense packing of carbon atoms in DLC filmsHowever a high impinging energy tends to induce a highcompressive stress due to the distortions of bond lengthand bond angle of sp3 carbon bonds Thus a major con-straint of DLC films is a poor adhesion of the films totheir substrates caused by high residual stresses Someefforts have been made to improve the adhesion of DLCfilms by doping metals or non metals into the films5ndash9

Zhang et al10 reported that doping Al into DLC films

lowastAuthor to whom correspondence should be addressed

drastically decreased the residual stress of the films Liuet al11 reported that the adhesion strength of DLC filmshad a significant influence on the effectiveness of cor-rosion protection A sufficient adhesive strength of DLCfilms can lessen undermining effects in corrosive mediaand promote their corrosion resistance However due todifferent electrochemical potentials between metals andDLC films introduction of metals into DLC films coulddegrade the corrosion properties of the films in chemicalenvironments Detailed mechanisms of Al incorporationduring magnetron sputtering deposition still needs to befully understood because the structures of DLCAl filmsare rather different from those of undoped DLC filmsIn this paper the effect of Al doping on the bonding

structure surface morphology adhesive strength and cor-rosion resistance of DLCAl films deposited on n-Si sub-strates with DC magnetron co-sputtering was investigatedin terms of DC sputtering power applied to Al target

2 EXPERIMENTAL DETAILS

Aluminum doped diamond like carbon (DLCAl) thinfilms were grown on highly conductive n-Si (100)(1ndash5times10minus3 cm) substrates by DC magnetron co-sputtering deposition using a pure graphite target(99995 C) and a pure aluminum target (99995 Al)of 4 inch in diameter The substrate rotation speed andbias applied during all the film depositions were 33 rpm

J Nanosci Nanotechnol 2010 Vol 10 No 7 1533-48802010104767006 doi101166jnn20101682 4767


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Corrosion Behavior of Aluminum Doped Diamond-Like Carbon Thin Films in NaCl Aqueous Solution Khun and Liu

and minus50 V respectively The Si substrates were cleanedby ethanol in an ultrasonic container for 20 min followedby distilled water and then dried with dry air Prior tothe film deposition the substrates were pre-sputtered withAr+ plasma at a substrate bias of minus250 V for 20 minto remove the oxide and contamination layers on the sur-faces The DLCAl film depositions were conducted at aworking pressure of 35 mTorr for 120 min An Al inter-layer between the DLCAl films and the Si substrates wasdeposited by sputtering the same Al target with a fixedDC power of 100 W for 1 min to promote the adhesionof the films to the substrates and make an ohmic contactbetween the films and substrates Though all the deposi-tions were conducted with a fixed DC power of 650 W onthe graphite target at room temperature the maximum tem-perature reached during the film depositions was approxi-mately 62 C The DC power applied to the Al target wasvaried from 10 to 90 WThe film chemical composition and bonding configura-

tion were investigated by means of X-ray photoelectronspectroscopy (XPS) (Kratos Axis Ultra) using pass ener-gies of 40 eV for C 1s Al 2p and O 1s core level spectraand 160 eV for wide scans with a monochromatic Al K

X-ray radiation (h = 148671 eV) XPS depth profilingof the DLCAl films was conducted using an etching rateof 65 nmmin calibrated with a standard SiO2 film

The bonding structure of the DLCAl films was alsoinvestigated with confocal micro-Raman spectroscopy(Renishaw RM 1000) using a HendashNe 632 nm laserThe surface morphology of the DLCAl films was

measured using scanning electron microscopy (SEMJEOL-JSM-5600LV) and atomic force microscopy (AFMDigital Instruments S-3000) and the surface roughness(Ra) was measured using AFM with five measurements oneach sample and an average roughness value being takenThe adhesion strength of the DLCAl films was mea-

sured with a micro-scratch tester (Shimadzu SST-101) hav-ing a diamond stylus of 15 m in radius dragged downthe films under a progressive loading condition at roomtemperature The scan amplitude frequency scratch rateand down speed for all the tests were set as 50 m 30 Hz2 ms and 2 ms respectively Five measurements oneach sample were performed and an average value of thecritical load was takenPotentiodynamic polarization measurements were car-

ried out using a potentiostatgalvanostat station (EGampG263A) having a three-electrode flat cell kit at a scan rateof 08 mVs at room temperature The electrolyte usedfor all the measurements was a deaerated and unstirred06 M NaCl solution For all the electrochemical mea-surements the DLCAl film coated samples were cut into2 cmtimes2 cm square pieces and a gold layer was depositedon the backsides of the Si substrates to make the testingsamples in good electrical connection during the polariza-tion measurements The testing area on the films was a

circle of 1 cm in diameter A saturated calomel referenceelectrode (SCE) (244 mV vs SHE at 25 C) and a plat-inum counter electrode were used during the polarizationtests

3 RESULTS AND DISCUSSION

Figure 1(a) shows the AlC and OC atomic ratios in termsof DC sputtering power applied to the Al target where theOC ratio increases from 006 to 070 and the AlC ratiolinearly increases from 0 to 054 when the DC power isincreased from 10 to 90 W An increase in surface oxy-gen percentage with increased Al content in the DLCAlfilms results from a greater difference in electronegativitiesbetween Al (sim161 pauling scale) and O (sim344) com-pared to the difference between Al and C (sim255) Theconsistently higher OC ratios than the AlC ones indicatethat Al on the surfaces of the DLCAl films may be fullyoxidized10

The depth profile of the DLCAl film deposited with20 W applied to the Al target shows that about 43 atoxygen is incorporated into the film though a higher oxy-gen content of about 1211 at is observed at the initial

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Fig 1 (a) AlC and OC atomic ratios of DLCAl films as a functionof DC sputtering power applied to Al target and (b) depth profile of aDLCAl sample deposited with 20 W on Al target

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Khun and Liu Corrosion Behavior of Aluminum Doped Diamond-Like Carbon Thin Films in NaCl Aqueous Solution

stage till about 60 s due to exposure to air after ventingthe deposition chamber as shown in Figure 1(b)Figure 2(a) shows the XPS Al 2p spectra of the DLCAl

films as a function of DC power on the Al target The Al2p peaks located at approximately 748 eV are attributedto AlndashO bonds12 The XPS C 1s spectra of the DLCAlfilms versus the DC power on the Al target are shownin Figure 2(b) where the C 1s peaks at approximately2848 eV shift to a slightly lower binding energy of about2846 eV with increased Al content indicating that thereare more sp2 bonds in the films For all the samples usedin this study there is no evidence of the formation of AlndashCbonds since no peaks are found at around 2815 eV in allthe C 1s spectra Therefore it can be deduced that thedoped Al may exist as O bonded or pure elemental formin the carbon matrix10 From the fitted Al 2p peaks usingGaussian functions it is found that the calculated area ratioof the AlndashO band over the AlndashAl band is about 1017 forthe DLCAl film deposited with 20 W on the Al target AnAlndashOAlndashAl ratio of about 1692 is found when the poweris increased to 90 WThe Raman spectra of the DLCAl films deposited with

varying DC power applied to the Al target from 10 to 90 Ware shown in Figure 3 where the improved symmetry

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of the Raman spectra with increased Al content in thefilms indicates a decrease in the level of disorder13 TheRaman spectra of the DLCAl films were deconvolutedusing a Gaussian function for G peaks and a Lorentzianfunction for D peaks to investigate the effect of Al dop-ing on the bonding structure of the films The increasedAl in the films causes downshifts of both the G bandfrom 1538 to 1444 cmminus1 and the D band from 1359 to1303 cmminus1 as shown in Figure 4(a)14 The full-widths-at-half-maximum (FWHMs) of the D bands decrease from486 to 363 cmminus1 with increased sputtering power on theAl target (Fig 4(b)) indicating an increase in ring order15

(a)

(b)

Fig 4 Results determined from fitted Raman spectra shown inFigure 3 (a) positions of G and D peaks and (b) FWHMs of D peaksand IDIG ratios with respect to DC sputtering power on Al target

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An intensity ratio between D and G peaks IDIG gives theinformation about carbon structure such as graphitic clus-tering and structure disorder1617 It is found that the IDIGratios of the DLCAl films increase from about 16 to 312with increased DC power as shown in Figure 4(b)17 Theintroduction of Al into the DLC films promotes the clus-tering of the sp2 bonds which in turn increases the IDIGratio The local increase in sp2 bonds in the amorphouscarbon matrix is caused by metal-induced graphitizationwhich the metal species within the C matrix can act as acatalyst to promote the formation of sp2 sites18ndash23 In addi-tion the incorporation of Al into the DLC films can alsoconvert stress-induced sp3 bonds to sp2 bonds during thefilm depositionFrom the micro-scratch tests it is observed that the crit-

ical load of the DLCAl films increases from about 248to 343 mN when the sputtering power applied to the Altarget is increased from 10 to 90 W as shown in Figure 5It is well known that the adhesion strength of DLC filmsis apparently affected by the residual stress in the films Ahigher sp2sp3 ratio can reduce the residual stress in a DLCfilm as the sp2 bonds are shorter than the sp3 bonds24ndash26

Therefore an increased sp2-bonded fraction with increasedAl content in the DLC films promotes the adhesion of thefilms to the substratesThe insets of Figure 5 show the SEM micrographs of

the surface morphologies of the scratched DLCAl filmsdeposited with 10 and 90 W applied to the Al targetThe observed brittle fracture of the DLCAl film depositedwith 10 W may be due to stress-induced interfacial cracksOne way of improving interfacial bond strength betweenfilm and substrate would be to increase metal dopinglevel in the film in order to reduce the residual stressin the film The increased critical load of the DLCAlfilms with increased Al content in the films indicate animproved interfacial bond strength between the films andthe substrates However a heavier Al incorporation intothe DLCAl film deposited with 90 W applied to theAl target causes a flaky feature of the scratched film atthe critical load which is probably due to the interfaces

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Crit

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load

(m

N)

(a)

(b)

Fig 5 Critical loads of DLCAl films as a function of DC power onAl target The insets show SEM micrographs of scratched DLCAl filmsdeposited with (a) 10 and (b) 90 W

between the Al species and the C matrix that may degradethe cohesive strength of the filmThe surface roughness (Ra) of the DLCAl films

decreases from about 079 to 016 nm with increased sput-tering power from 10 to 90 W applied to the Al targetas shown in Figure 6(a) which cannot be correlated tothe increased sp2 fraction with increased Al content inthe films A possible mechanism proposed by Corbella23

and Peng27 is that a higher ion bombardment energy

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(a)

Ra

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)

(b)

(c)

Fig 6 (a) Surface roughness of DLCAl films as a function of DCpower on Al target and (b) and (c) AFM images of DLCAl filmsdeposited with 10 and 90 W on Al target respectively



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could smoothen a film surface by preferentially removingmaterials from protruding regions on the film surfaceSmoothening of the DLCAl films by this proposed mech-anism is pronounced by increased bombardment energyof heavier Al ions induced by the higher sputtering pow-ers applied to the Al target during the film depositionsTherefore larger asperities are found on the surface of theDLCAl film deposited with 10 W (Fig 6(b)) as well asfiner asperities are found on the surface of the DLCAlfilm deposited with 90 W (Fig 6(c))From the potentiodynamic polarization curves of the

DLCAl films obtained in the 06 M NaCl solution asshown in Figure 7(a) the corrosion parameters such ascorrosion potential (Ecorr) and current (Icorr) are analyzedusing the Tafel technique The polarization resistance (Rp)values of the DLCAl films are calculated from the anodic(a) and cathodic (c) Tafel slopes and Icorr according tothe following formula28

Rp = a timesc23 Icorra +c (1)

with Rp in k a and c in V I-decade and Icorr in A

(a)

(b)

Fig 7 (a) Potentiodynamic polarization curves of DLCAl films as afunction of DC power on Al target and (b) SEM micrograph showing thecorroded area of a DLCAl sample (90 W on Al target) after polarizationtest

Table I Results determined from potentiodynamic polarization curvesof DLCAl films as shown in Figure 7(a)

DC power onAl target (W) Ecorr (mV) Icorr (A) Rp (k)

10 minus2327 488 179820 minus2400 495 181240 minus2423 542 197155 minus2669 599 174170 minus2844 622 252990 minus2988 513 3071

The corrosion test results obtained are summarized inTable I The Ecorr of the DLCAl films shifts from minus2327to minus2988 mV versus SCE when the sputtering power onthe Al target is increased from 10 to 90 W Due to the dif-ferent electrochemical potentials between the Al species onthe film surface and the background C matrix they behaveas tiny anodes and cathodes on the film surface induc-ing anodic and cathodic current flows between them andresulting in the dissolution of the films It is found fromTable I that the Icorr of the DLCAl films first increasesfrom 488 to 622 A with increased power from 10 to70 W and then turns to decrease to 513 A with furtherincreased power up to 90 W The increased sp2 fraction inthe DLCAl films with increased sputtering power appliedto the Al target which is confirmed by the increased IDIGratios (Fig 4(b)) is one of the reasons for the increasedIcorr In addition the increase of the Icorr can also beattributed to the doping of Al into the C matrix since theAl species also degrade the sp3-bonded cross-linking struc-ture Moreover the resulted interfacial bonds between theAl species and C matrix can be easily attacked by electro-chemically active species eg water molecules and Clminus

ions in the electrolyte Therefore these effects becomepronounced when the Al content in the films is increasedby increasing the sputtering power on the Al target How-ever the decreased Icorr of the DLCAl film deposited with90 W can be probably related to the development of anoxide layer on the film surface with increased Al con-tent in the film A possible reason is that the oxide layerwould slow down the dissolution of the film by preventinga direct access of the electrolyte to the active Al sites onthe film surface as well as retarding electrochemical dis-solution reactions via hindering electron transfer processthrough itFrom the XPS results shown in Figure 1(a) the increase

in the surface oxygen with increased sputtering poweron the Al target can be correlated to the growth of theoxide layer Besides during the polarization measure-ments refreshed Al sites produced by dissolving the oxidelayer on the film surface as the applied potential is shiftedto higher positive values react with water molecules toform a new oxide layer through the following reaction

2Al+3H2Orarr Al2O3+6H++6eminus



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This process would replenish the dissolved oxide layerthrough the anodic dissolution in the NaCl solution andconsequently make the dissolution of the film difficult dur-ing the measurement The replenishing process becomesmore significant with increased Al content resulting inmore difficult polarization processes Therefore the appar-ently increased Rp of the DLCAl films from 1798 to3071 k with increased sputtering power on the Al targetfrom 10 to 90 W confirms that the increased Al content inthe films makes the polarization processes more difficultvia the growth and replenishment of the oxide layers onthe film surfacesAfter the polarization tests the surface morphologies of

the corroded DLCAl films are characterized with SEMFigure 7(b) shows the corroded area of the DLCAl filmdeposited with 90 W applied to the Al target in whichthe film surface is covered with corrosion products prob-ably produced from the reaction of the film with theelectrolyte

4 CONCLUSIONS

The effect of Al doping on the bonding structure surfaceroughness adhesion strength and corrosion resistance ofDLCAl films was investigated in terms of DC sputteringpower applied to Al target during film depositions It wasfound that increased Al content in the DLCAl films withincreased DC power on the Al target caused an increase inadsorbed surface oxygen Raman results revealed that theincorporation of Al in the DLC films promoted the for-mation of sp2 bonds Higher critical loads with increasedsputtering power on the Al target were attributed to theincreased metal-induced sp2 bonds as well as the elementalAl dispersed in the films It was found that the polariza-tion resistance of the DLCAl films increased though theircorrosion potentials shifted to more negative values withincreased Al incorporation in the films

Acknowledgments This work was supported by theresearch project (EWI-0601-IRIS-035-00) from the Envi-ronment and Water Industry Development Council (EWI)Singapore N W Khun is grateful for the PhD scholar-ship from the Nanyang Technological University (NTU)Singapore

References and Notes

1 Z H Liu P Lemoine J F Zhao D M Zhou S Mailley E TMcAdams P Maguire and J McLaughlin Diam Relat Mater7 1059 (1998)

2 S Aisenberg and R Chabot J Appl Phys 42 2953 (1971)3 D S Whitmell and R Williamson Thin Solid Films 35 255

(1976)4 Y Lifshitz Diam Relat Mater 5 388 (1996)5 T I T Okpalugo P D Maguire A A Ogwu and J A D

McLaughlin Diam Relat Mater 13 1549 (2004)6 C Corbella E Pascual G Oncins C Canal J L Andujar and

E Bertran Thin Solid Films 482 293 (2005)7 D Y Wang Y Y Chang C L Chang and Y W Huang Surf Coat

Tech 200 2175 (2005)8 G K Burkat T Fujimura V Y Dolmatov E A Orlova and M V

Veretennikova Diam Relat Mater 14 1761 (2005)9 I G Gonzalez J D Jesus D A Tryk G Morell and C R Cabrera

Diam Relat Mater 15 221 (2006)10 S Zhang Y Q Fu X L Bui and H J Du Inter J Nanosci 3 797

(2004)11 C L Liu D P Hu J Xu D Z Yang and M Qi Thin Solid Films

496 457 (2006)12 P Zhang B K Tay G Q Yu S P Lau and Y Q Fu Diam Relat

Mater 13 459 (2004)13 C C Chen and F C N Hong Appl Surf Sci 242 261 (2005)14 S Zhang H Du S E Ong K N Aung H C Too and X Miao

Thin Solid Films 515 66 (2006)15 A C Ferrari and J Robertson Phys Rev B 61 14095 (2000)16 M A S Oliveira A K Vieira and M Massi Diam Relat Mater

12 2136 (2003)17 B K Tay and P Zhang Thin Solid Films 420ndash421 177 (2002)18 C S Lee T Y Kim K R Lee and K H Yoon Thin Solid Films

447ndash448 169 (2004)19 H Hofsass H Feldermann R Merk M Sebastian and C Ronning

Appl Phys A 153ndash181 66 (1998)20 A Oya and S Otani Carbon 17 131 (1979)21 G J Qi S Zhang T T Tang J F Li X W Sun and X T Zeng

Surf Coat Tech 198 300 (2005)22 K Bewilogua R Wittorf H Thomsen and M Weber Thin Solid

Films 447ndash448 142 (2004)23 C Corbella E Bertran M C Polo E Pascual and J L Andujar

Diam Relat Mater 16 1828 (2007)24 S Zhang X L Bui and Y Fu Surf Coat Tech 167 137 (2003)25 J P Sullivan T A Friedmann and A G Baca J Electro Mater

26 1021 (1997)26 N W Khun E Liu and H W Guo Electroanalysis 20 1851

(2008)27 X L Peng Z H Barber and T W Clyne Surf Coat Tech 138 23

(2001)28 B Bhushan Tribology and Mechanics of Magnetic Storage Devices

2nd edn Springer New York (1996)

Received 20 July 2008 Accepted 20 January 2009



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and minus50 V respectively The Si substrates were cleanedby ethanol in an ultrasonic container for 20 min followedby distilled water and then dried with dry air Prior tothe film deposition the substrates were pre-sputtered withAr+ plasma at a substrate bias of minus250 V for 20 minto remove the oxide and contamination layers on the sur-faces The DLCAl film depositions were conducted at aworking pressure of 35 mTorr for 120 min An Al inter-layer between the DLCAl films and the Si substrates wasdeposited by sputtering the same Al target with a fixedDC power of 100 W for 1 min to promote the adhesionof the films to the substrates and make an ohmic contactbetween the films and substrates Though all the deposi-tions were conducted with a fixed DC power of 650 W onthe graphite target at room temperature the maximum tem-perature reached during the film depositions was approxi-mately 62 C The DC power applied to the Al target wasvaried from 10 to 90 WThe film chemical composition and bonding configura-

tion were investigated by means of X-ray photoelectronspectroscopy (XPS) (Kratos Axis Ultra) using pass ener-gies of 40 eV for C 1s Al 2p and O 1s core level spectraand 160 eV for wide scans with a monochromatic Al K

X-ray radiation (h = 148671 eV) XPS depth profilingof the DLCAl films was conducted using an etching rateof 65 nmmin calibrated with a standard SiO2 film

The bonding structure of the DLCAl films was alsoinvestigated with confocal micro-Raman spectroscopy(Renishaw RM 1000) using a HendashNe 632 nm laserThe surface morphology of the DLCAl films was

measured using scanning electron microscopy (SEMJEOL-JSM-5600LV) and atomic force microscopy (AFMDigital Instruments S-3000) and the surface roughness(Ra) was measured using AFM with five measurements oneach sample and an average roughness value being takenThe adhesion strength of the DLCAl films was mea-

sured with a micro-scratch tester (Shimadzu SST-101) hav-ing a diamond stylus of 15 m in radius dragged downthe films under a progressive loading condition at roomtemperature The scan amplitude frequency scratch rateand down speed for all the tests were set as 50 m 30 Hz2 ms and 2 ms respectively Five measurements oneach sample were performed and an average value of thecritical load was takenPotentiodynamic polarization measurements were car-

ried out using a potentiostatgalvanostat station (EGampG263A) having a three-electrode flat cell kit at a scan rateof 08 mVs at room temperature The electrolyte usedfor all the measurements was a deaerated and unstirred06 M NaCl solution For all the electrochemical mea-surements the DLCAl film coated samples were cut into2 cmtimes2 cm square pieces and a gold layer was depositedon the backsides of the Si substrates to make the testingsamples in good electrical connection during the polariza-tion measurements The testing area on the films was a

circle of 1 cm in diameter A saturated calomel referenceelectrode (SCE) (244 mV vs SHE at 25 C) and a plat-inum counter electrode were used during the polarizationtests

3 RESULTS AND DISCUSSION

Figure 1(a) shows the AlC and OC atomic ratios in termsof DC sputtering power applied to the Al target where theOC ratio increases from 006 to 070 and the AlC ratiolinearly increases from 0 to 054 when the DC power isincreased from 10 to 90 W An increase in surface oxy-gen percentage with increased Al content in the DLCAlfilms results from a greater difference in electronegativitiesbetween Al (sim161 pauling scale) and O (sim344) com-pared to the difference between Al and C (sim255) Theconsistently higher OC ratios than the AlC ones indicatethat Al on the surfaces of the DLCAl films may be fullyoxidized10

The depth profile of the DLCAl film deposited with20 W applied to the Al target shows that about 43 atoxygen is incorporated into the film though a higher oxy-gen content of about 1211 at is observed at the initial

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(b)

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Fig 1 (a) AlC and OC atomic ratios of DLCAl films as a functionof DC sputtering power applied to Al target and (b) depth profile of aDLCAl sample deposited with 20 W on Al target



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4 CONCLUSIONS





























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(a)

(b)











IP 1556944Mon 01 Aug 2011 070740

RESEARCH

ARTIC

LE




4 CONCLUSIONS





























IP 1556944Mon 01 Aug 2011 070740

RESEARCH

ARTIC

LE






240

260

280

300

320

340

360

0 10 20 30 40 50 60 70 80 90 100


Crit

ical

load

(m

N)

(a)

(b)





0

01

02

03

04

05

06

07

08

09

0 20 40 60 80 100


(a)

Ra

(nm

)

(b)

(c)




IP 1556944Mon 01 Aug 2011 070740

RESEARCH

ARTIC

LE






(a)

(b)











IP 1556944Mon 01 Aug 2011 070740

RESEARCH

ARTIC

LE




4 CONCLUSIONS





























IP 1556944Mon 01 Aug 2011 070740

RESEARCH

ARTIC

LE






(a)

(b)











IP 1556944Mon 01 Aug 2011 070740

RESEARCH

ARTIC

LE




4 CONCLUSIONS





























IP 1556944Mon 01 Aug 2011 070740

RESEARCH

ARTIC

LE




4 CONCLUSIONS




























Documents

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