Surface and Coatings Technology 167(2003) 137–142

0257-8972/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(02)00900-3

Magnetron sputtered hard a-C coatings of very high toughness

Sam Zhang*, Xuan Lam Bui, Yongqing Fu

School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

Abstract

Hydrogen-free amorphous carbon coatings of high hardness(f30 GPa) and toughness(plasticity from 50 to 60%) weredeposited on 440C steel substrates by DC magnetron sputtering at target power density of 10.5 Wycm in the bias range fromy2

20 to y150 V. The surface topography, hardness and tribological behavior of the coatings were investigated. With the increaseof bias voltage, coating hardness and surface smoothness increased at expense of some adhesion strength and an increase ofcoefficient of friction. All coatings showed low friction in humid air and graphitization was observed after a high number ofrotation cycles. The graphitization adds more benefit aside from reducing friction: the graphite layer can considerably reduce theadhesive wear since it prevents the asperities of the two surfaces to be adhered to each other.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Diamond-like carbon; Tribology; Adhesion strength; Graphitization

1. Introduction

Amorphous carbon coatings have been intensivelystudied for three decades. In 1971, Aisenberg depositedhard carbon coating by low energy carbon ion beamsand called it diamond-like carbon(DLC) w1x. DLCcoatings do not have a dominant crystalline latticestructure, but rather an amorphous phase consisting of amixture of sp and sp structures. DLC can be produced3 2

by various techniques and from various sources ofcarbonw2x. Comparing to hydrogenated DLC, in whichhydrocarbon gases were employed as the source ofcarbon, hydrogen-free DLC shows advantages such ashigher hardness, elastic modulus, and thus better wearresistance, lower coefficient of friction in humid envi-ronment, higher thermal stability, etc. The only advan-tage when employing hydrocarbon gases is that highdeposition rate can be obtainedw3x. However, recently,PVD technique such as pulsed laser depositionw4x orfiltered cathodic vacuum arcw5x were employed todeposit hydrogen-free DLC with high deposition rate.

In this study, we deposited thick and hard hydrogen-free DLC coatings by magnetron sputtering at highpower density for high deposition rate. The adhesionstrength and tribological properties of the coatings wereinvestigated as a function of substrate bias.

*Corresponding author. Tel.:q65-67938521; fax:q65-67922779.

2. Experimental details

2.1. Preparation of DLC coatings

The DLC coatings were deposited using E303A mag-netron sputtering system(Penta Vacuum—Singapore).The schematic diagram of the system is shown in Fig.1. The graphite targets(99.999%) locate at approxi-mately 100 mm above the substrate holder, which canbe heated, rotated and biased. Co-sputtering of twotargets was carried out. The base pressure was alwayspumped to 10 Torr and the process pressure was kepty7

constant at 3 mTorr with 50 sccm Ar flow. The coatingswere deposited on silicon wafers and 440C steel discs(with a diameter of 55 mm and thickness of 5.5 mm)polished to the surface roughness ofR s60 nm. Thea

power density was chosen at 10.5 Wycm for a depo-2

sition rate of 1mmyh. Substrate bias voltage was variedfrom y20 to y150 V. Before sputtering, the substrateswere ultrasonic cleaned for 20 min in acetone followedby 10 min ultrasonic cleaning in ethanol. After loading,the substrates were heated to and maintained at 1008Cfor 20 min before plasma cleaning for 30 min at RFinduced bias voltage of 300 V to remove surface oxidesand contaminants. After plasma cleaning, the surfaceroughness increased to 66 nm.

2.2. Coating characterization

Coating thickness was measured using profilometer(Dektak 3SJ) through a sharp step created by masking.

138 S. Zhang et al. / Surface and Coatings Technology 167 (2003) 137–142

Fig. 1. Schematic diagram of E303A magnetron sputtering system.

Fig. 2. Raman spectrum ofy60 V bias DLC coating.

Fig. 3. Substrate bias voltage vs.I yI ratio.d g

Surface topography was investigated with an AtomicForce Microscope(Shimadzu SPM-9500J2). The sur-face roughness was measured over an area of 2=2mm . Coating hardness measurements were conducted2

at a Nanoindenter(XP) with a Berkovich diamondindenter. The hardness was determined by continuousstiffness measurement techniquew6x. The indentationdepths were set not to exceed 10% of the coatingthickness to insure no interference from the substratew7x. On each sample, six indentations were made atrandom location and six hardness values calculated foran average. From the indentation load and unload curve,the plasticity is estimated by dividing the minimumdisplacement(the displacement after complete unload-ing) by the maximum displacementw8x. Structure of thecoating was investigated with a Renishaw Raman spec-troscope at 633 nm line excited with a He–Ne laser.The adhesion strength was studied with a micro scratchtester(Shimadzu SST-101) where a diamond tip stylusof 15 mm radius was dragged on the coating with agradually increased load. The scanning amplitude wasset at 50mm at scratching speed of 10mmys for alltests. Tribological behavior was characterized by CSEMtribometer with ball-on-disk configuration at applied

load of 5 N in ambient environment(75% humidity, 228C). Alumina ball of 6 mm in diameter was chosen asthe counter part.

3. Experimental results

3.1. Raman spectroscopy

Fig. 2 shows the Raman spectrum of DLC coatingsproduced withy60 V bias on steel substrate. TheRaman profile was a typical DLC spectrum—a broadsingle peak centered approximately 1530ycm with asmall shoulder at approximately 1350ycm w9x. Thespectrum was deconvoluted into two peaks termed D-band(at 1350ycm) and G-band(at 1530ycm). However,as can be seen in Fig. 2, highly biased coatings hadanother small peak at a wave number of approximately700ycm that was not observed at lower or non-biasedcoatings. The variation ofI yI ratio with substrate biasd g

voltage as shown in Fig. 3 indicated that coatingsdeposited at higher bias have lowerI yI ratio, and thusd g

higher sp fractionw9x. We believe that higher bias gives3

the coatings better Raman spectroscopic transparencydue to higher sp fraction.3

139S. Zhang et al. / Surface and Coatings Technology 167 (2003) 137–142

Fig. 4. Surface roughness of DLC coatings under different biasvoltage. Fig. 6. Hardness as a function of substrate bias voltage.

Fig. 5. AFM image of a-C coatings biased at(a) y20 V and(b) y60 V. Fig. 7. Load and unload curve ofy140 V bias coating.

3.2. Surface roughness

For coatings prepared at low bias voltage, the surfaceroughness(R ) is relatively high as revealed by AFM.a

With increase of bias voltage fromy20 toy100 V, the

coating roughness decreases significantly. As bias volt-age goes overy120 V, the surface roughness remainsconstant at approximately 3.4 nm(Fig. 4). The AFMimages of the surface roughness are presented in Fig. 5where higher bias corresponds to smoother surface as aresult of increased bombarding effect.

3.3. Coating hardness

The coating hardness as a function of bias voltage isshown in Fig. 6. The hardness increases with the biasvoltage. The highest hardness obtained under this studyis 28 GPa aty140 V. Fig. 7 shows the load and unloadcurve of the coating deposited at a bias voltage ofy140 V. The best plasticity obtained is 59.3% aty60 V.As bias increases fromy60, y100,y120 toy140 V,the plasticity decreases from 59.3, 58.5, 53.5 and downto 52%. These values are much higher than the 10% ofsuper hard DLC coating produced by pulsed laserdepositionw4x or the 40% plasticity of super tough TiCya-C coatingw8x.

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Fig. 8. Critical loads of scratch test of DLC coatings at different biasvoltages.

Fig. 9. Coefficient of friction of DLC coatings sliding against aluminaball at normal load of 5 N and speed of 10 cmys.

3.4. Adhesion to the substrate

Scratch tests were carried out to evaluate the adhesivebehavior(scratch toughness) of the coatings. The normalload at which an appreciable coating damage starts orcoefficient of friction suddenly rises is often referred toas the lower critical load. The lower critical load usuallyis used as a measure of the adhesion strength of acoating. As the indenter completely plows through thecoating, the normal load is called the upper critical load.The lower critical load decreases with increasing sub-strate bias voltage as shown in Fig. 8. When the substratebias reachesy150 V, the adhesion of the coatingplunges. Adhesion strength of the coating is stronglyinfluenced by residual stress. In PVD coatings, thegrowth-induced stress contributes much to total residualstress. This stress is proportional to the energy of theions coming to the substratew10x. The energy of incidentions, in turn, is proportional to the negative bias voltagew2x. Therefore, the poor adhesion at high bias may beowing mainly to residual stresses exerted during depo-sition. Stress cracks are observed in the highly biasedcoatings. It should be noted that highly biased coatingshave relatively high hardness and low surface roughnesscompared to that deposited at low bias power.

3.5. Coefficient of friction

The coefficient of friction of DLC coatings slidingagainst alumina ball is shown in Fig. 9. It is clear thatan increase of substrate bias voltage causes significantincrease of coefficient of friction. All worn coatingsurface did not show peeling off or spallation. Anotherobservation is that the coefficient of friction for coatingsprepared at lower biases(y60 or y100 V) becomesstable after approximately 0.6 km of sliding distancewhereas that at very high bias(y120 andy140 Vcoatings) still increases after that distance. To see how

much it can go, further sliding was done for they140V coating by increasing the sliding distance to 2.5 kmand the rotation speed to 20 cmys. In this case, thecoefficient of friction reached a maximum of 0.27 at1.15 km of sliding distance and then decreased andleveled off at a value of 0.24. After the 2.5 km sliding,the coating was still intact, i.e., no sign of peeling offwas observed on the wear track(cf. Fig. 10). On theother hand, the wear scar is noticeable on the aluminaball with debris accumulated nearby(cf. Fig. 11).

4. Discussion

From the experimental results, we have seen that thesurface roughness of sputtered DLC coatings decreasedwith increasing bias power. This is the consequence ofhigh-energy ion bombardment. The sputtered ion energyis proportional to substrate bias voltagew2x:

VbEA 1y2p

where,E, V and p are the ion energy, bias voltage andprocess pressure, respectively. It is clear that with highbias voltage, the carbon ions obtain high energy. If theenergy exceeds the critical value for atomic displacementin the structure then the ions can penetrate deep into theinterior of the structure. The ion energy tends to bedissipated into the volume nearby. Consequently, adenser, smoother, more compressive stress, and highersp content coating is formed. In case the ion energy is3

not high enough(as in the non-biased or low biasedcoatings), diffusion in surface layers takes place sincethe ions cannot penetrate into the interior structure.Diffusion tends to generate ordered clusters with highsp content. Surface diffusion is promoted by the energy2

associated with coming ions, which remain on thesurface. The coatings, therefore, have higher content ofsp , rougher and lower residual stress, as were also2

supported by Raman and scratch test results.

141S. Zhang et al. / Surface and Coatings Technology 167 (2003) 137–142

Fig. 10. Wear track ofy140 V bias coating after 2.5 km sliding of tribo-test.

Fig. 11. Wear scar with debris accumulated on alumina ball.

Coating hardness depends on sp fraction. It is evident3

that lowerI yI (cf. Fig. 3) at higher bias power givesd g

rise to higher sp fraction and thus higher hardness(cf.3

Fig. 6). At high bias power, back sputtering also takesplace w11x. In graphite structures thep bonds are veryweakw12x and carbon atoms in those are easily sputteredoff. Therefore, the formation of graphite structure is

restrained at high bias. Our coatings not only haverelatively high hardness compared to sputtered coatingsin previous reportsw4x but also have high plasticityduring indentation deformation(52% for y140 V biascoating and nearly 60% fory60 V bias coating). Thisis beneficial in reducing chances of brittle fracture athigh loads.

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Fig. 12. Raman spectrum of debris area on alumina ball.

The coefficient of friction is low for coatings depos-ited at low bias although the surface roughness of thoseis higher. At low bias voltage, many ordered clusterswith high sp content are formed. The structure of these2

clusters is close to thermodynamically stable graphite.Such soft structures can be easily scratched and plays alubricating role(especially in high humid air) to com-pensate the effect of surface roughness. This observationhas been reported in previous studyw13x. The graphiti-zation, which causes the decrease of coefficient offriction, is observed after a long sliding distance approx-imately 1.2 km. From Raman spectrum(Fig. 12), thedebris accumulated near the wear scar of alumina ballis determined as graphite-rich phase. The graphitizationis probably induced by stresses produced by frictioncontact rather than by friction-induced heat.

5. Conclusion

Hydrogen-free a-C coatings deposited by magnetronsputtering show high hardness, very high toughness(interms of plasticity) and low friction in humid environ-

ment. The adhesion strength of coatings is dependenton the substrate bias power, but all of coatings depositedin the bias range ofy60 andy140 V withstood a ball-on-disk wear test under 5 N even for 2.5 km withoutobvious damage. High adhesion strength at low bias andhigh hardness at high bias may combine to producehigh adhesion and high hardness coatings to suit specificapplications through a bias-graded deposition(graduallyincrease the bias power as deposition proceeds). Therange of hardness from 18 to 28 GPa is adequate formost tribological applications. The graphitization addsmore benefit aside from reducing friction: the graphitelayer can considerably reduce the adhesive wear sinceit prevents the asperities of the two surfaces to beadhered to each other.

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