magnetron sputtering of nanocomposite (ti,cr)cn/dlc coatings

Surface and Coatings Technology 162(2002) 42–48

0257-8972/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00561-3

Magnetron sputtering of nanocomposite(Ti,Cr)CNyDLC coatings

Sam Zhang *, Yongqing Fu , Hejun Du , X.T. Zeng , Y.C. Liua, a a b b

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

Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singaporeb

Received 12 February 2002; accepted in revised form 19 August 2002

Abstract

Superhard nanocrystalline(Ti, Cr)CNyDLC coatings were prepared through co-sputtering of Ti, Cr and graphite targets in anargonynitrogen atmosphere. Results from both transmission electron microscopy(TEM) and grazing incident X-ray diffraction(GIXRD) indicated that the grain size of(TiCr)C N crystals was approximately 10–20 nm. X-ray photoelectron spectroscopicx y

studies confirmed that an increase in the sputtering power at the Ti target not only increased the Ti composition in the film butalso brought about an increase in sp bonding in DLC matrix, in agreement with the raising hardness with Ti sputtering power.3

Film hardness and elastic modulus were measured with a nano-indenter, and film hardness reached 40 GPa. Tribological behaviorsof the films were evaluated using a ball-on-disk tribometer, and the films demonstrated properties of low-friction and good wearresistance.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Nanocomposite coating; Diamond-like carbon;(Ti,Cr)N; Sputtering; Tribology; Wear; Coefficient of friction

1. Introduction

Nanostructured coatings have recently attractedincreasing interest because of the possibilities of synthe-sizing materials with unique physical–chemical proper-ties, e.g. superior hardness, toughness, chemical stability,low friction and wear-resistancew1–4x. Highly sophisti-cated surface related properties, such as superplasticity,optical, magnetic, electronic and catalytic properties canbe obtained by advanced nanostructured coatings, mak-ing them attractive for industrial applications in high-speed machining, tooling, biomedical, automotive,optical applications and magnetic storage devicesw5–10x. There are many types of design models for nanos-tructured coatings, such as nanocomposite coatings,nano-scale multilayer coating, superlattice coating, nano-scale-graded coatings, etc. Among them, superhard nan-ocomposite coatings attracted more interest due to theendless possibilities of the synthesizing materials ofunique propertiesw11–21x. Designing of nanocomposite

*Corresponding author. Tel.:q65-790-4400; fax:q65-791-1859.E-mail address: [email protected](S. Zhang).

coating needs consideration of many factors, for exam-ples, the interface volume, grain size, single layerthickness, surface and interfacial energy, texture, epitax-ial stress and strain, etc., all of which depend signifi-cantly on the materials selection, deposition methodsand parametersw22–27x.Grain boundary hardening is one of the possibilities

to control the microstructure in order to increase coatinghardness. With the decrease in crystal size, the hardnessof materials increases according to the ‘Hall–Petch’relationship, especially for crystal size down to tens ofnanometer. However, a new deformation mechanism,called grain boundary sliding, replaces the dislocationactivity that dominates deformation process in conven-tional materials. Softening caused by the grain boundarysliding is mainly attributed to the large amount of defectsin the grain boundaries, which allows fast diffusion ofatoms and vacancies with the applying of stress. Afurther increase of the strength and hardness withdecreasing crystallite size can be achieved only if grainboundary sliding is blocked by appropriate coatingdesign and materials selection.

43S. Zhang et al. / Surface and Coatings Technology 162 (2002) 42–48

Fig. 1. AFM morphology of the film deposited at Ti target power density of 4.5 Wycm and nitrogen gas flow ratio of(a) 33% (b) 100%.2

There are many design ideas for nanocomposite coat-ings, and embedding nanocrystalline phases in amor-phous phase matrix is quite often applied which can befulfilled easily by physical vapor deposition(PVD) andchemical vapor deposition(CVD) methods. Diamond-like carbon(DLC), amorphous carbon nitride and otherhard amorphous materials(with high hardness andelastic modulus) have been recognized as the primarycandidates for the amorphous matrix while nano-sizedrefractory nitrides, such as TiN, Si N , AlN, BN, etc.,3 4

could be used as strengthening phasesw28x. For thiscoating design, the size and the distribution of thesenanocrystals in the amorphous structures need to beoptimized to obtain high hardness. These nanocrystallinegrains should have random orientation(high angle grain

boundaries) to minimize grain incoherence strain, andhigh toughness could be achieved by strain release viananocrystals sliding in the DLC matrix. Termination ofnanocracks by deflection at grain boundaries and byenergy loss within the amorphous matrix can dramati-cally improve the toughness.Carbon is one of the most important and widely used

materials in many areas and diamond-like carbon(DLC)has been recognized as one of the primary candidatesused for wear resistant and solid lubricating coating.TiN (or TiC) and CrN coatings prepared using sputteringhave been applied in industry for wear protective appli-cations. Following the above design idea, in this study,we combine these two types of coatings to synthesize ananocomposite structure. In film design, DLC acts as

44 S. Zhang et al. / Surface and Coatings Technology 162 (2002) 42–48

Fig. 2. GIXD analysis of the films deposited under 300 W Ti targetpower with AryN s2:1.2

Fig. 3. TEM photos of deposited nanocomposite thin films.

hard, tough and lubricating matrix, while nano-particlesact as reinforcing crystallites to improve hardness andmechanical properties.

2. Experimental

Nanocomposite(TiCr)C N yDLC coatings were pre-x y

pared through co-sputtering of graphite, Ti and Cr targets

in argonynitrogen atmosphere in a magnetron sputteringsystem. Silicon wafers and WC discs were used as thesubstrates. WC discs with a diameter of 50 mm and athickness of 5 mm were mechanically ground andpolished, then ultrasonically rinsed in acetone. Thesubstrate holder rotated during the deposition for unifor-mity. In this study, nitrogen gas flow ratiowN y(N q2 2

Ar)x was controlled at 33 and 100%, with Ti targetpower density of 4.5 and 6.5 Wycm (d.c.). The power2

density of chromium and graphite target were controlledat 4.5 Wycm (d.c.) and 6.5 Wycm (r.f.), respectively.2 2

Deposition was performed at a low temperature of 1508C for 2 h. The substrate-to-target distance was 100mm.Surface and cross-section morphologies of the coat-

ings were investigated using an atomic force microscopy(AFM) and a JEOL scanning electron microscope. Thecrystalline structure was obtained by grazing incidenceX-ray diffraction (GIXD) method. The nanocompositecoating was also deposited on potassium bromide(KBr)pellets for 20 min, and then the pellets were dissolvedin water to float off the film for TEM study. The thinfilm thus obtained was examined using a JEOL 200 kVTEM. X-ray photoelectron spectroscopy(XPS) analysiswas performed on film surface using a Kratos AXISspectrometer with monochromatic Al Ka (1486.6 eV)


Fig. 4. Load-displacement curve during load and unloading processfor nano-indentation.

Fig. 5. XPS depth profile of the film deposited at Ti power densityof (a) 4.5 Wycm and(b) 6.5 Wycm .2 2

Table 1Film bonding structure distribution(%) of C 1s and N 1s peaks

Bonding structure %, at Ti power at Ti powerdensity of 4.5 density of 6.5Wycm2 Wycm2

C 1s284.7 eV(C–C bonding) 45.53 57.17285.7 eV(sp bonding)2 36.46 16.04286.6 eV(sp bonding)3 7.57 15.99288.3 eV(C–O bonding) 10.44 10.79

N 1s398.3 eV(N–C sp bonding)3 42.30 61.64399.1 eV(N–C sp bonding)2 40.43 25.44401 eV(N–O bonding) 17.27 12.92

X-ray radiation. Film hardness and elastic recovery wereevaluated using a nano-indentation tester with a pene-trating depth of 100 nm.Load bearing capacity of the nanocomposite coatings

on WC substrate was assessed using a scratch tester. Adiamond stylus was driven across the coating at aconstant speed of 1 mmys and a continuously increasedload rate of 1.5 Nys. Tribological behaviors of nanocom-posite coatings were evaluated using a ball-on-disktribometer under dry sliding conditions at room temper-ature. Alumina balls(with a diameter of 9.5 mm and asurface roughness better thanR s0.05mm) were useda

as the counterface materials. The normal loads were 10,20 and 50 g. All the tests were run in laboratory air(258C and relatively humidity of 65"3%) with a slidingdistance of 300 m and a sliding speed of 0.2 mys.Coefficient of friction was recorded during each test.Wear volumes were calculated from the wear tracksmeasured with a laser profilometer.

3. Results and discussions

Cross-section of the coating is dense and featurelesswith a thickness of approximately 1mm. Fig. 1a showsAFM morphologies of the coating deposited at a powerdensity of 4.5 Wycm on the Ti target with nitrogen to2

argon gas flow ratio of 1:3. The surface features a wavymorphology or hilly humps of approximately 30–40 nmin width and 6–8 nm in height. In the case of depositionunder a pure nitrogen gas, the coating roughness increas-es significantly(see Fig. 1b). This is probably owing tothe rapid formation and growth of nitrides, and theformation of polymeric C–N based phasew29x.Fig. 2 shows the GIXRD profile of the crystalline

structures for the film deposited at Ti target power

density of 6.5 Wycm with nitrogen to argon gas flow2

ratio of 1:3. There are some broad peaks of(TiCr)CNcrystalline phases. As a first degree approximation, theaverage crystallite size can be estimated by the Debye–Scherrer formula(shown in Eq.(1)) w30x:


Fig. 6. Typical XPS high-resolution carbon 1s and nitrogen 1s core level spectra.

Fig. 7. Scratch profile of the film deposited at Ti target power density of 6.5 Wycm and nitrogen flow ratio of 33%. The insets show SEM2

morphology of the scratch track at different loading stage.


Fig. 8. Wear curves for the film deposited at Ti target power densityof 6.5 Wycm and nitrogen gas flow ratio of 33% at increasing normal2

load: (a) 10 g, (b) 20 g, (c) 50 g.

Table 2Average coefficient of friction for specimens prepared under different Ti contents and normal loads

10 N 20 N 50 N

COF Wear volume COF Wear volume COF Wear volume

4.5 Wycm2 0.03 4.06 0.02 0.61 0.01 2.026.5 Wycm2 0.02 not appreciable 0.03 not appreciable 0.03 not appreciable

DsKlybcosu (1)

Where K is a constant(Ks0.91), D is the meancrystallite dimension normal to diffracting planes,l isthe X-ray wavelength(ls0.15406 nm for Cu target),b in rad is the peak width at half-maximum peak heightandu is the Bragg angle. The calculated average grainsize of(Ti,Cr)CN crystals is approximately 20 nm. Forall the films in this study, the calculated grain size of(Ti,Cr)CN crystals are within 10–20 nm.High magnification bright-field TEM photo of the

films is shown in Fig. 3a. In an amorphous DLC matrix,many tiny crystals can be observed with dimensionsapproximately 8–12 nm. These nanocrystalline crystalshave different orientations and lattice spacing. It shouldbe pointed out that these crystals form only after 20-min deposition, while those in the coating deposited for2 h may be larger than those shown in Fig. 3a due tograin growth, as is illustrated in the AFM morphologyin Fig. 1. Fig. 3b shows the dark-field TEM photos,clearly revealing these nano-size crystals. Fig. 3c showsthe diffraction ring patterns, indicating these nanocrys-talline phases of TiCrCN.Fig. 4 is the nanoindentation load-displacement profile

of the film deposited at Ti target power density of 6.5Wycm . The calculation of hardness and elastic modulus2

is performed using Oliver and Pharr methodw31x. Themeasured film hardness is approximately 40 GPa. TheYoung’s modulus of the film is approximately 300 GPa.With the decrease of Ti power density to 4.5 W/cm ,2

the film hardness decreases to approximately 25 GPa.Fig. 5 gives the atomic composition of the films

obtained from XPS analysis. XPS results revealed thatwith the increase of Ti target power density 4.5–6.5 Wycm , Ti in the film increases from 12.3 to 22 at.%,2

while the contents of other elements only change slight-ly. Therefore, one possible reason for the hardnessincreasing with Ti target power could come from theincrease in the amount of nano-sized(TiCr)CN phasesin the film. With the increase in the nitrogen flow ratiofrom 33 to 100%, film hardness decreases dramaticallyfrom 40 to 18 GPa. The high content of polymeric C–N based phase formed under high content of nitrogengas could be detrimental for the film hardness.Fig. 6 shows typical high-resolution carbon 1s and

nitrogen 1s core level spectra. The peaks were fittedusing the binding energy interval values from the liter-aturew32,33x. A Gaussian decomposition of C 1s spectra


gives rise to the following peaks: 284.7 eV(C–Cbonding), 285.7 eV (sp bonding), 286.6 eV (sp2 3

bonding) and 288.3 eV(C–O bonding). Similar analysison N 1s spectra yields 398.3 eV(N–C sp bonding),3

399.1 eV(N–C sp bonding) and 401 eV(N–O bond-2

ing). The concentrations of the different bonding struc-tures of C 1s and N 1s peaks are calculated and theresults are given in Table 1. With the increase of Titarget power density, the concentration of sp bonding3

structure increased for both C 1s and N 1s peaks, andat the same time, sp decreased, which supports the2

significant increase in film hardness as discussed earlier.Scratch test was used to evaluate the dynamic load

bearing capacity and friction properties of the coating-substrate system. The ‘lower critical load’ or the normalload at which the first damage or a sharp increase infriction coefficient is observed is widely used as ameasure of the load bearing capacity. Fig. 7 is such ascratch profile which clearly shows the coating fails ata normal load of approximately 60 N, indicating a goodadhesion and load bearing capacity of nanocompositecoatings on WC–Co substrate. The coefficient of frictionremains at a very low value of 0.05, indicating goodfriction properties. SEM micrographs for the differentstages during scratch testing are also shown as inset inFig. 7. The critical load slightly decreased to 53 N atTi target power density 4.5 Wycm . With the increase2

of Ti target power, there is no significant change incoefficient of friction.The coefficient of friction remains at an extremely

low value of 0.01–0.03 within the 300-m wear distanceunder different normal loads(Fig. 8). With the increaseof normal load, the coefficient of friction decreasesslightly, which can probably be explained by the easiergraphitization of wear debris and generating of lubricat-ing surface layer with the increase of normal load. Table2 lists the average coefficient of friction and wearvolume for specimens prepared under different Ti targetpowers and normal loads. The long-term coefficient offriction remains a low and stable value(less than 0.03).With the increase of Ti target power density from 4.5to 6 Wycm , the wear volume decreases so much that2

the surface profilometry does not reveal noticeable weartrack.

4. Conclusions

Superhard nanocrystalline(Ti, Cr)CNyDLC coatingswere prepared through co-sputtering of Ti, Cr andgraphite targets in an argonynitrogen atmosphere.Results from both TEM and GIXRD indicated that thegrain size of the(TiCr)C N crystals were approximatelyx y

10–20 nm. XPS results confirmed that an increase inthe sputtering power at the Ti target not only increasedthe Ti composition in the film but also brought aboutincrease in sp bonding in the DLC matrix, in agreement3

with the raising hardness with Ti sputtering power. Filmhardness and elastic modulus were measured with anano-indenter. Under the experiment conditions, the filmhardness fell within 18–40 GPa. The tribological behav-iors of the films were evaluated using a ball-on-disktribometer, and the films demonstrated properties oflow-friction and good wear resistance.

References

w1x P. Holubar, M. Jilek, M. Sima, Surf. Coat. Technol. 133–134(2000) 145.

w2x R.A. Andrievski, Mater. Trans. 42(2001) 1471.w3x J. Musil, Surf. Coat. Technol. 125(2000) 322.w4x S. Veprek, A.S. Argon, J Vac. Sci. Technol. B 20(2002) 650.w5x S. Veprek, S. Reiprich, Thin Solid Films 268(1995) 64.w6x V. Provenzano, R.L. Holtz, Mater. Sci. Eng. A 204(1995)

125.w7x F. Mazaleyrat, L.K. Varga, J. Magn. Magnetic Mater. 215–216

(2000) 253.w8x B. Cantor, C.M. Allen, Scr. Mater. 44(2001) 2055.w9x H. Zeng, M.L. Yan, Y. Liu, J. Appl. Phys. 89(2001) 810.

w10x J.L. Solis, S. Saukko, L. Kish, Thin Solid Films 391(2001)255.

w11x S. Veprek, A.S. Argon, Surf. Coat. Technol. 146(2001) 175.w12x S. Veprek, Thin Solid Films 317(1998) 449.w13x J. Musil, F. Kunc, H. Zeman, Surf. Coat. Technol. 154(2002)

304.w14x J. Musil, J. Vlcek, Surf. Coat. Technol. 142(2001) 557.w15x M.A. Baker, S. Klose, C. Rebholz, Surf. Coat. Technol. 151

(2002) 338.w16x E. Ribeiro, A. Malczyk, S. Carvalho, Surf. Coat. Technol. 151

(2002) 515.w17x D.V. Shtansky, K. Kaneko, Y. Ikuhara, Surf. Coat. Technol.

148 (2001) 206.w18x L. Rebouta, C.J. Tavares, Surf. Coat. Technol. 133–134(2000)

234.w19x S. Carvalho, L. Rebouta, Thin Solid Films 398–399(2001)

391.w20x M. Misina, J. Musil, Surf. Coat. Technol. 110(1998) 168.w21x J. Musil, H. Polakova, Surf. Coat. Technol. 127(2000) 99.w22x J. Musil, H. Hruby, Thin Solid Films 36(2000) 104.w23x J. Musil, P. Karvankova, J. Kasl, Surf. Coat. Technol. 139

(2001) 101.w24x A.A. Voevodin, J.P. O’Neill, J.S. Zabinski, Surf. Coat.Technol.

116–119(1999) 36.w25x S. Veprek, A. Niederhofer, K. Moto, T. Bolom, H.D. Mannling,

P. Nesladek, G. Dollinger, A. Bergmaier, Surf. Coat.Technol.133–134(2000) 152.

w26x F. Vaz, L. Rebouta, P. Goudeau, Surf. Coat.Technol. 146(2001)274.

w27x J. Patscheider, T. Zehnder, M. Diserens, Surf. Coat. Technol.146 (2001) 201.

w28x H. Holleck, in: A. Kumar, Y.W. Chung, J.J. Moore, J.E.Smugeresky(Eds.), Surface Engineering: Science and Tech-nology I, The Minerals, Metals and Materials Society, 1999,pp. 207–231.

w29x A. Stanishevsky, L. Khriachtchev, I. Akula, Diamond Relat.Mater. 7(1998) 1190–1195.

w30x M.V. Zdujic, O.B. Milosevic, L.C. Karanovic, Mater Lett. 13(1992) 125–128.

w31x T. Berlind, N. Hellgren, Surf. Coat. Technol. 141(2001) 145.w32x E.G. Wang, Prog. Mater. Sci. 41(1997) 241–298.w33x K. Sharma, J. Narayan, Int. Mater. Rev. 42(1997) 137–192.

magnetron sputtering of nanocomposite (ti,cr)cn/dlc coatings

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