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Tribology International 42 (2009) 798–806

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

Structural, mechanical and tribological properties of nanostructuredCNx/TiN multilayers

Anand Vyas a,�, K.Y. Li b, Y.G. Shen b

a Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, Chinab Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China

a r t i c l e i n f o

Article history:

Received 28 February 2008

Received in revised form

18 October 2008

Accepted 24 October 2008Available online 25 November 2008

Keywords:

Hardness

Adhesion

Friction coefficient

9X/$ - see front matter & 2008 Elsevier Ltd. A

016/j.triboint.2008.10.018

esponding author. Tel.: +852 34002611; fax:

ail address: mmavyas@inet.polyu.edu.hk (A. V

a b s t r a c t

Nanostructured CNx/TiN multilayers were deposited onto Si(10 0) and M42 high-speed steel substrates

using closed-filed unbalanced magnetron sputtering. The deposition process was controlled by a closed-

loop optical emission monitor (OEM) to regulate the flow of N2 gas. Different carbon nitride LCNxlayer

thicknesses could be attained by varying the C target current (0.5–2.0 A). OEM settings and bilayer

thickness periods (i.e. LCNx¼ 0:321:2 nm, while LTiN ¼ 3.0 nm) significantly affected the mechanical

and tribological properties of CNx/TiN multilayer films. XPS analyses revealed that the chemical states,

such as TiN, TiC, TiNxOy ???????????and TiO2, existed in a TiN layer. The nanohardness of the film with

bilayer thickness LTiN ¼ 3.0 nm and LCNx¼ 0:9 nm was �41.0 GPa. The residual compressive stress was

found to be between 1.5 and 3.0 GPa. By the scratch test, the critical load value obtained was high �78 N.

Rockwell-C adhesion tests exhibited the best adhesion and cohesive strength for multilayers with

LTiN ¼ 3.0 nm, LCNx¼ 0:3 and 0:6 nm. The friction coefficient of a multilayer was found to be low 0.11.

Lower frictional coefficients and wear rates were the consequences of the increase in LCNx, which

provided a lubricating function in the multilayers.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

There has been a worldwide intensive research in the pastyears in the development of novel engineering materials,especially in the form of thin films. Nanostructured multilayersin which the dimensions of the individual layers are in the rangefrom 1 to 10 nm represent a new class of engineering materials[1–3]. These new materials may have a wide range of industrialand engineering applications, such as high-strength/high-conductivitymaterials, load-bearing components, X-ray optics, thin-film mag-netic heads, recording media, wear-resistant coatings, cutting tools,and micro-electro-mechanical systems. Currently, the deposition ofmultilayer thin films by physical vapor deposition (PVD) ontovarious substrates is the most popular technique [1,4–5]. The mainadvantages include low-temperature processing, extremely lowhydrogen content, use of non-toxic gas and a simple depositionprocess. In addition, PVD also allows greater control of the thicknessand the composition modulation of each layer as compared withother process such as CVD [6].

Up to now, CNx/TiN multilayer thin films have mainly beensynthesized by dc/rf reactive magnetron sputtering in a mixture ofAr and N2 gases [7–11]. The composition, microstructure and

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yas).

mechanical behavior of these multilayers have been studied incomparison with conventional single-layer films of TiN and CNx

films. However, although the structural and mechanical behaviorsof CNx/TiN multilayer thin films are highly important in obtaininga better understanding of their mechanical and tribologicalproperties, they have not yet been well investigated. Therefore,it is the primary purpose of the present study to examinesystematically the effect of deposition conditions on the mechan-ical and tribological properties of CNx/TiN multilayer thin films.

2. Experimental details

2.1. CNx/TiN multilayer deposition technique

CNx/TiN multilayers were synthesized onto Si (10 0) and M42steel substrates using a closed-filed unbalanced magnetronsputtering system UDP-450 manufactured by Teer CoatingsLimited. The base pressure in the chamber was 2�10�7 Torr.The reactive gas nitrogen was controlled by a closed-loop opticalemission monitor (OEM)/// by regulating the reactivity of Ti2+

with the OEM settings between 40% and 60%. To balance thefluctuation of pumping efficiency, the flow rate of reactive gas canbe rapidly and accurately regulated via a set of piezo valves andOEM, where OEM readings reveal the exact proportion of reactivegas consumed by the metal plasma. OEM settings designate the

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A. Vyas et al. / Tribology International 42 (2009) 798–806 799

ratio of the plasma intensity of Ti+2 emitted during the reactivesputtering process with respect to its full emission intensity in aninert gas discharge. Each OEM setting value corresponds to aspecific nitrogen flow rate. The higher OEM value meanscomparatively higher Ti emission intensities and thus lowernitrogen flow rates [12].

The Si (10 0) substrates were taken directly from sealedcontainers and introduced into the deposition chamber withoutfurther cleaning treatment whereas only one side of the M42 steelsubstrates was ground and polished to remove the grindingdamage and any surface irregularities. The root-mean-square(rms)//// surface roughness of the polished surface reached0.01mm and was subsequently ultrasonically cleaned in acetoneand alcohol in sequence and dried in flowing compressed nitrogengas. The substrates were mounted on a turntable between thetwo targets. The target-to-substrate distance was 12 cm. Beforedeposition, the substrates were sputter cleaned at �500 V for10 min. The thickness ratio LTiN=LCNx

, which is crucial for theenhancement of mechanical and tribological properties, wascontrolled by fixing Ti target power and varying C target currentonly. In our multilayer films, a different CNx layer thickness wasachieved by varying the C target current as 0.5, 1.0, 1.5, and 2.0 Aand Ti target current was fixed at 6.0 A. Thus, giving a TiN layerwith a fixed thickness of LTiN ¼ 3.0 nm while the CNx layer LCNx

varied from 0.3 to 1.2 nm. Henceforth, the bilayer thickness will berepresented as LCNx

¼ 0.3, 0.6, 0.9 and 1.2 nm and LTiN ¼ 3.0 nm.The deposition rates of TiN and CNx were 12.4 and 1–5 nm/min,respectively. The bilayer thickness in a multilayer can becalculated using the relationship between the total thickness ofa multilayer film and the substrate rotation speed [13]. The totalthickness increased from 1.6 to 2.1mm due to the increase in ionflux to the target resulting from an increase in plasma density.All the multilayers that were prepared even at different OEMs(with different LCNx

) showed a similar increasing trend. Thesubstrate rotation frequency was fixed at 4 rpm. The dc substratebias voltage was �60 V (250 kHz). A thick Ti buffer layer (500 nm)was deposited onto the substrates prior to the depositionof CNx/TiN multilayer films in order to improve theadhesion and reduce stresses at the interface with the substrate.During the deposition, the substrate temperature was 200 1Cthroughout.

2.2. Characterization methods of CNx/TiN multilayers

The thicknesses of all the CNx/TiN multilayer films prepared atdifferent OEM settings and C target currents were measured by astylus profilometer (Taylor–Hobson). The thickness was in therange of 1.6–2.05mm, which was also confirmed by cross-sectionscanning electron microscope observation. The surface roughnessof the films was evaluated by an atomic force microscopy (AFM) ina contact mode in a scanning area of 1�1mm2 from which theroot-mean-square (rms) roughness could be determined. Thechemical compositions and the bonding states of the films wereevaluated by X-ray photoelectron spectroscopy (XPS) and thecrystallographic structures of the films were determined by X-raydiffraction (XRD). The nanohardness of CNx/TiN films wasmeasured using the nanoindentation technique with a trigonal-shaped Berkovich diamond tip with �150 nm radius. Theindentation depth was less than 10% of the film thickness toreduce the effects of the substrate. Newton’s ring method wasemployed to study the internal stress of the multilayer coatings[14]. The adhesive strength of all the coatings onto M42 steelsubstrates was measured by a scratch tester with a diamond tip(Rockwell-C-type stylus). In scratch testing, a stylus with a radiusof 300 nm was drawn over the sample surface for 1 cm under a

continuously increasing normal force of 1 N/s until the filmbecame detached. The maximum load was 80 N. The test tablewas fixed while a stylus was drawn over the sample surface. Thenormal force causing detachment was defined as the criticalscratching load. Coating detachment was determined by anacoustic emission detector. The morphology of the scratchedchannel was observed by an optical microscope. The Rockwell-Ctest was also done to assess adhesion of the multilayer coatingswith a load of 1500 N. On each sample, five indentations wereperformed and the damage to the film was evaluated on the basisof defined adhesion strength quality classification HF 1–6. HF1–4define sufficient adhesion strength whereas HF 5 and HF 6represent poor adhesion.

Pin-on disc tests were carried out between coated CNx/TiNsteel substrate and tungsten carbide ball with a 5 mm diameterhaving a surface roughness of 0.1mm at room temperature and inair at 60–65% relative humidity. During the tests on all thecoatings, the ball and the disc were cleaned with alcohol to get ridof dust and other contaminants to keep the surface conditions asidentical as possible. The friction and wear tests were carried outat room temperature in dry conditions. The loads used in thisexperiment were 2, 6, and 10 N, with a fixed sliding speed of0.0565 m/s for a total of 10,800 cycles. The wear volume of thefilms was determined from the worn part on the surface mappedby a three-dimensional stylus profilometer (Form Talysurf PGI,Taylor–Hobson). Optical microscopy was employed to observe thewear tracks and specific wear rates were calculated.

3. Results and discussion

3.1. Chemical analysis and microstructure

XPS was used to investigate the composition of the CNx/TiNmultilayer films. After sputtering the multilayer for 5 min,the XPS spectra for TiN were attained. The XPS spectra of TiNlayer in a multilayer that was prepared at OEM 40% and 60%(with bilayer thickness of LTiN ¼ 3.0 nm and LCNx

¼ 0:6 nm)are shown in Figs. 1(a) and (b). Typical, XPS core-level spectraof the TiN layers were fitted using the Gaussian and Lorentzianprofile. In these experiments, a Shirley-type background-subtraction was used [15]. An intricate structure is observedin these spectra. The chemical states corresponding to everypeak in Figs. 1(a) and 2(b) can be concluded with the helpof the published XPS data [16–19]. In Fig. 1(a), the Ti 2pspectra could be fitted well with six components. The two higherposition peaks observed at 454.8 and 460.9 eV are attributedto the Ti 2p3/2 and Ti 2p1/2 electron binding energy of the TiNphase and are in agreement with the previous reports [17,18].The chemical shifts in the core level of the TiN layer relative toelemental Ti reflect a charge transfer from titanium to nitrogenduring the nitridation process. The electron binding energystate of the TiN phase was same in all samples. At higherbinding energies, three more phases co-existed, namely TiC,TiNxOy and TiO2. The appearance of the peak at 455.9 eVclearly shows that titanium has reacted with carbon. Theformation of TiC in both Figs. 1(a) and (b) is possible at theCNx/TiN interface since TiC has the same structure as TiN,except with larger lattice parameters. In Fig. 1(b), the peak at457.3 eV is assigned to Ti2O3 and no chemical bonds of TiNxOy

were present. On the higher binding energy side of the Ti 2p1/2peak, at 462.1 and 462 eV in Figs. 1(a) and (b) ascribed as TiN(loss peak) is also observed. Strydom and Hofmann [19]performed electron energy loss measurements (at 500 eV) andfound a series of characteristic well-defined loss peaks ofsignificant intensity. They assigned these to different intraband

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Fig. 1. Deconvolution of Ti 2p XPS spectra in a multilayer film prepared (a) at OEM

40% and (b) at OEM 60%.

Fig. 2. y–2y X-ray diffraction plots of CNx/TiN multilayer films: (a) Deposited at

OEM 40% with target current C ¼ 0.5 and 2.0 A. (b) Deposited at OEM 60% with

target current C ¼ 0.5 and 2.0 A.

A. Vyas et al. / Tribology International 42 (2009) 798–806800

transitions. In principle, one can still argue that the appearance ofany kind of structure is a consequence of some kind of short-rangestructural compositional change, even if its origin is not yetidentified.

Fig. 2 shows the I–2y diffraction scans (using CuKa radiation at1.54 A) obtained from the TiN/CNx multilayer (with a bilayerthickness of LTiN ¼ 3.0 nm and LCNx

¼ 0:3 and 1:2 nm) coatingsdeposited onto the Si(10 0) substrate at OEM 40% and 60%and their thicknesses being 1.6 and 2.1mm. It can be seenfrom Figs. 2(a) and (b) that TiN has the only prominent feature(111) reflection at 36.61. The peak at 38.41 in Figs. 2(a) and (b)could be assigned to the thick Ti buffer layer. There was noevidence of crystalline peaks corresponding to the CNx layerat a smaller thickness of LCNx

¼ 0:3 nm. As explained previously,if the pseudomorphic growth of b-C3N4 (0 0 0 1) on TiN (111)were to occur, the (0 0 0 2) reflection corresponding to a d-spacingof 1.23 A should appear, which did not in our case. Although theCNx layer should be crystalline when it is sufficiently thin(o1–1.5 nm) [20], this was not detected by X-ray diffraction.Therefore, it is suggested that the CNx layer formed mayhave amorphous structure in majority. Since the depositionrate of the CNx layer is significantly small and the sub-strate rotation speed is high, the lack of sufficient ion bombard-ment may not give enough activation energy to form completecrystalline phases. On the other hand, implantation ofatoms/ions into the surface may result in the amorphousmicrostructure.

3.2. Surface morphology

The variation in the rms surface roughness against the varyingCNx layer thickness for the films prepared at OEM 55% is shown inFig. 3. Although a thicker CNx layer tend to be amorphous andhave a comparatively rougher surface, our results indicate that theroughness decreases with the increase in the CNx layer thickness,which could be due to the corresponding existence of the smallgrains at higher carbon concentration. The multilayer with LCNx

¼

0:3 has the roughness 1.7 nm, which gradually drops to 1.0 nmwhen the CNx layer thickness increases to LCNx

¼ 1:2 nm. Thesefilms are smooth and comparable with previous reports [20,21]. Inthe inset, the AFM images labeled as (a) and (b) are also shown forthe two extreme end results of CNx/TiN multilayer roughness. Asimilar trend has been observed for all the other multilayers thatwere prepared at different OEMs except for the film that wasprepared at OEM 40%, which showed a higher surface roughnessof 2.0 nm.

3.3. Nanoindentation measurements

The nanohardness of CNx/TiN multilayer coatings is drasticallyaffected by OEM settings as well as the LCNx

thickness in themultilayer system. Fig. 4 compares the load–displacement curvesfor the single-layer TiN as well as CNx/TiN multilayer depositedunder OEM 55% and 40% sputtering conditions. The indentationdepth at a maximum load of 5 m N decreases from 111 nm for thesingle-layer TiN film to 80 nm for a CNx/TiN multilayer prepared at

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Fig. 3. Variation in rms surface roughness of the CNx/TiN multilayers prepared at

OEM 55% as a function of varying C target current. Inset shows three-dimensional

AFM images of films prepared with C ¼ 0.5 and 2.0 A.

Fig. 4. The loading and unloading curve of pure TiN prepared at OEM 55% and CNx/

TiN multilayer films prepared at 55% and OEM 40%.

Fig. 5. Variation of the nanohardness of CNx/TiN films with changing C target

current deposited at various OEM %s.

A. Vyas et al. / Tribology International 42 (2009) 798–806 801

OEM 55%. The indenter penetration depth of the TiN single layerwas approximately 111 nm at a hardness of approximately 20 GPa.When compared, the penetration depth in the CNx/TiN multilayerprepared at OEM 40% ðLCNx

¼ 0:9 nmÞ was approximately92 nm and the hardness was 32 GPa whereas the multilayerprepared at OEM 55% ðLCNx

¼ 0:9 nmÞ was approximately 80 nmand the hardness was 41 GPa. The residual indentation wasreduced from 65 to 15 nm, giving an increase of elastic recovery(%R) from 55% to 80%. This result suggests that a large proportionof the deformation of the film deposited under the optimizedconditions is elastic. Thus, these multilayer films have resulted inimproved hardness. As reported earlier [22–24], an increase inthe hardness is because of the increase in shear modulusaccompanying the increase in dislocation line energy at theinterfaces between nanometer order layers in a multilayer system.The hardness and the %R of the film deposited under theoptimized conditions are comparable with other reports [25]and the supermodulus effect has thus been demonstrated in thesemultilayers.

According to the load–unload displacement curves of theseCNx/TiN multilayer films, the respective hardness and Young’smoduli could be found. The hardness of all the films that wereprepared at different OEM settings as well as different C targetcurrents (varied LCNx

thickness) is shown in Fig. 5(a). The

hardness for multilayers prepared at OEM 55% is hardest andfound to be between 31 and 41 GPa. Young’s moduli are between167 and 342 GPa. However, the hardness for multilayers preparedat OEM 40% is found to be between 24 and 29 GPa. On comparingthe results, the nanohardness of these multilayers shows anincreasing trend with the increase in the CNx layer thicknessirrespective of the OEM settings and is found to be maximum at55% OEM setting. The film possesses an enhanced nanohardnessof 41 GPa ðLCNx

¼ 0:9 nmÞ, which is �21 GPa higher than thehardness of a single layer of TiN and CNx films. For a thicker CNx

layer LCNx¼ 1:2 nm, the hardness slightly drops to 39 GPa. OEM

setting lower than 40% shows that the magnetron sputtering isunstable. The relationship between nanohardness–OEMsettings–LCNx

thickness in the CNx/TiN system is obvious. Never-theless, all CNx/TiN coatings demonstrated an improvement innanohardness over the single-layer thin film.

For the case of crystalline multilayers, pronounced variationsin strength and hardness as a function of bilayer thickness arecommonly observed. At smaller bilayer thickness (o2.5 nm), theinterface acts as barriers for dislocation motion, thus leading to asubstantial increase in the system hardness relative to that ofhomogeneous materials, whereas at large-enough interruptinglayer periods (42.5 nm) softening will occur as a result ofdislocation motion within individual layers. Such behavior has

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A. Vyas et al. / Tribology International 42 (2009) 798–806802

been observed in metal and nitride superlattices, e.g. Cu/Ni andTiN/NbN [22,26].

In our films that were prepared at target currents C ¼ 0.5,1.0 A(LCNx

¼ 0:3 and 0:6 nm) instead of improved mechanical proper-ties, they exhibited a reduction of the system hardness to valuesthat were intermediate to those of the constituent layers. Reducedhardness values could be due to the existence of too thin LCNx

¼

0:3 and 0:6 nm layer. At C ¼ 0.5, 1.0 A, the CNx layers seem to bewholly amorphous. Thus we can conclude that the systemresponse for the present multilayers seems to fall in betweenthat of pure amorphous and polycrystalline system. Nevertheless,the present results demonstrate that CNx/TiN films do offeradditional means for mechanical property design includingapparent strengthening over single-layer materials. In addition,the residual compressive stresses in the CNx/TiN multilayers (withLCNx

¼ 0:321:2 nm) were obtained by the well-known Stoney’sequation revised for biaxial stress because it is generally knownthat the hardness of a thin film is increased by the stress in thefilm [27]. The residual compressive stresses as shown in Fig. 5(b)were in the range of 1.5–1.9 GPa for the films prepared atOEMs—40–50% and 60%. The stress increased from 1.8 to3.0 GPa when LCNx

increased from 0.3 to 1.2 nm for the filmsprepared at OEM 55%.

3.4. Adhesion

3.4.1. Scratch test

The critical load LC values of all the CNx/TiN multilayer filmsthat characterize adhesion were conducted by the scratch test (fordetails see Section 2.2). Our analyses are based on the acousticemission that determines the critical load during the scratch test.Both OEM settings and bilayer thickness affect the adhesion

Fig. 6. Optical micrographs of the scratch tracks made at a continuous progressive load

45% at C ¼ 0.5 A, (c) OEM 60% at C ¼ 1.5 A and (d) OEM 55% at C ¼ 1.5 A.

strength of the multilayer coating. Typical optical micrographs ofwear scar for CNx/TiN multilayer films prepared at OEMs 40% and45% (with LCNx

¼ 0:3 nm) and at OEMs 60% and 55% (withLCNx

¼ 0:9 nm) are shown in Fig. 6. The exact load and themeasurement scale are shown as well in the images. It is clearfrom the images in Figs. 6(a) and (b) that at lower OEMs, themultilayers show low adhesion as compared with the coatings athigher OEMs and with larger LCNx

thickness. But these coatings atlower OEMs 40% and 45% (see Figs. 6(a) and (b)) show asubstantial increase in critical load, those being 49 and 58 N ascompared with the single-layer TiN film (17 N in our case). It canbe seen from Figs. 6(c) and (d) that at higher OEMs 60% and 55%,there is a remarkable increase in critical load 73 and 78 N onincreasing the CNx layer thickness ðLCNx

¼ 0:9 nmÞ. The criticalload values for all the films are shown in Fig. 7. In case of lowerOEM films, the increase in critical load with the increase in LCNx

thickness was not significant at all, which suggests that thesefilms contain a higher degree of compressive stress withincreasing layer thickness than do the other coatings at higherOEMs. A maximum critical scratching load of 78 N for TiN/CNx

films at OEM 55% (with LCNx¼ 0:9 nm) was obtained. From our

results, we can say that the CNx interrupting layer as thick asbetween LCNx

¼ 0:621:2 nm gives higher critical load values. Thecomparison of these films by optical evaluation and the frictionalforce measurements and acoustic emission showed that all thefilms were well comparable. For some of the CNx/TiN films, therewere numerous irregular microcracks on the contact site wellbefore the critical load value. The scratch track showed that thepropagation of microcracks was caused by the high compressivestress developed in front of indenter and tensile stress behindthe indenter. These stresses create delamination at the interfacebetween layers of the film, not delamination at the interfacebetween the film and the substrate. Bull and Jones [28] have

for typical CNx/TiN multilayer films prepared at (a) OEM 40% at C ¼ 0.5 A, (b) OEM

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Fig. 7. OEM % effects on the critical load LC of CNx/TiN multilayer films with

different bilayer thickness ðLTiN;LCNxÞ depending on the C target current.

Table 1Grading of adhesion strength and wear performance of all the CNx/TiN multilayers

with different bilayer thickness ðLTiN;LCNxÞ.

OEM (%) Adhesion

strength

Bilayer thickness (nm)

LTiN ¼ 3:0þLCNx

Wear (m3/N m)

40 HF 1 LCNx¼ 0:3 1.2–3.4�0�15

HF 1 LCNx¼ 0:6

HF 3 LCNx¼ 0:9

HF 3 LCNx¼ 1:2

45 HF 1 LCNx¼ 0:3 1.0–3.5�10�15

HF 2 LCNx¼ 0:6

o

HF 2 LCNx¼ 0:9

HF 3 LCNx¼ 1:2

50 HF 1 LCNx¼ 0:3 1.3–4.0�10�15

HF 1 LCNx¼ 0:6

o

HF 2 LCNx¼ 0:9

HF 3 LCNx¼ 1:2

55 HF 1 LCNx¼ 0:3 1.2–2.0�10�16

HF 1 LCNx¼ 0:6

o

HF 1 LCNx¼ 0:9

HF 5 LCNx¼ 1:2

60 HF 1 LCNx¼ 0:3

o1.1–2.0�10�16

HF 1 LCNx¼ 0:6

HF 2 LCNx¼ 0:9

HF 5 LCNx¼ 1:2

A. Vyas et al. / Tribology International 42 (2009) 798–806 803

explained the adhesion properties of multilayer coatings andreported that the cracks could be deflected by the internalinterfaces, which resulted in energy dissipation in the layermaterial without coating failure. However, this reason could haveled to apparent increase in our coating toughness and adhesion.Such damage to the coating remains localized up to a certain load.At higher loads, the lateral cracks originating from the indenteredge are activated, leading to the film’s removal by chipping, thusgiving critical load values.

3.4.2. The. Rockwell-C test

The adhesion of the multilayer film onto the M42 HSS is highlyaffected by OEM settings. Grading of adhesion strength for allsamples can be seen in Table 1. Typical results from the Rockwell-Cadhesion tests on some of our multilayer films prepared at OEM55% and 40% are shown in Figs. 8 and 9. Rockwell-C indentationtests depict significant improvement in film adhesion of CNx/TiN

coatings at OEM 55% (with bilayer thickness LCNx¼ 0:321:2 nm,

LTiN ¼ 3.0 nm) as shown in Fig. 8. Figs. 8(a)–(c) show goodadhesion strength quality HF 1. There are merely some fine cracksat the circumference of the indentations. These multilayer coatingsconform well to the indentation without any noticeable fracture.Fig. 8(d) shows cracks and a circular delamination caused bylateral cracking and piling-up of the substrate. This indentationcan be related to HF 5. In comparison, severe spalling of thecracked film was observed on the single-layer TiN depositedon the M42 HSS substrate due to its low fracture toughness.Tests on films at OEM 60% gave similar results as tests at OEM 55%(see Table 1) in which films with bilayer thickness LCNx

¼ 0:325emand 0:6 nm, LTiN ¼ 3.0 nm and show good adhesion strengthquality HF 1, whereas LCNx

¼ 0:9 and 1:2 nm and LTiN ¼ 3.0 nmshow HF2 and HF5. The advantage of the multilayer structure ofCNx/TiN coating is obvious. However, the results of the indentationon the films prepared at OEM 40% are shown in Fig. 9.Figs. 9(a) and (b) showed good adhesion as well and can beclassified as HF1 except for films (with LCNx

¼ 0:9 and 1:2 nm) inFigs. 9(c) and (d) classified as HF3, which showed layer damageadjacent to the boundary of indentation. All the multilayersirrespective of their preparation condition (in terms of OEMs),the delamination or the deterioration of the film occurs only withincreasing bilayer thickness or hard films.

3.5. Wear of M42 steel substrates

Friction coefficients were determined by ball-on disc trib-ometer with the conditions mentioned in Section 2.2. Figs. 10and 11 show the variation of the friction coefficient as a functionof the number of sliding cycles for selected CNx/TiN multilayers atdifferent OEMs (with bilayer thickness LCNx

¼ 0:321:2 nm andLTiN ¼ 3.0 nm). At OEM 40% in Figs. 10(a)–(d), the frictioncoefficient of the films was between 0.15 and 0.2 at loads 2 and6 N except for the film in Fig. 10(a) (with bilayer thickness LCNx

¼

0:3 nm and LTiN ¼ 3.0 nm), which failed after 6800 cycles at the6 N load. At the load 10 N, the tests showed different failurebehavior for these films. At higher loads 10 N, in Fig. 10(a), the testcould pass only 300 cycles, which increased to 4500 cycles in10(b). On increasing the bilayer thickness, the maximum numberof cycles the film passed shown in Fig. 10(c) was 7200 cycles but in10(d), the film could pass only 4800 cycles, which is 300 cyclesmore than in Fig. 10(b). These failure results at load 10 N dosuggest their dependence on the bilayer thickness. A thin CNx

layer results in a short lifetime of the multilayer and a thicklayer is preferred in this case. The results of the friction coefficienttests on multilayer film at OEM 55% in Fig. 11 showed the besttribological behavior among all the CNx/TiN films tested inthis investigation. The friction coefficient for these films inFigs. 11(a)–(d) was between 0.1 and 0.25 and there was nofailure observed at high loads. This is due to the increased filmhardness, owing to the multilayer structure. The friction coeffi-cient in Fig. 11(c) was remarkably low, 0.11, at 2 and 6 N loads.In this case, at higher loads, 10 N, a thin CNx layer shows improvedfilm durability as compared with the film at OEM 40% (seeFigs. 10(a) and (b)) but caused fluctuations with a somewhathigher friction coefficient. However, with the increase in the CNx

layer thickness the friction coefficient at load 10 N was as low as0.15 in Fig. 11(c) and is 0.19 in Fig. 11(d). It is considered that theimproved tribological property of this film is due to the increasedhardness and lubricity by increase in LCNx

thickness. We foundthat there existed a critical OEM setting (55%) for our films. AtOEMs 60%, the friction coefficient of the films increased by0.1–0.15 at 2, and 6 N loads. Results suggested that below OEM55%, the films had quite a high friction coefficient comparatively

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Fig. 8. Rockwell-C adhesion test images of CNx/TiN films prepared at OEM 55% with increasing C target current from 0.5 to 2.0 A (morphologies a–d).

Fig. 9. Rockwell-C adhesion test images of CNx/TiN film prepared at OEM 40% with increasing C target current from 0.5 to 2.0 A (morphologies a–d).

A. Vyas et al. / Tribology International 42 (2009) 798–806804

and film failure aws observed at 10 N loads for OEMs 40% and 45%only. All CNx/TiN multilayers at different OEMs had frictioncoefficients between 0.1 and 0.25 at 2 and 6 N loads. The resultssuggest that a thick LCNx

¼ 1:0 nm is desirable with the intent toprovide strengthening of the multilayer film. Thus, the currentresults demonstrate that multilayers can be designed to combinehigh hardness along with low friction and optimized tribologicalbehavior.

Fig. 12 shows optical images of wear tracks after the frictiontest for the films in Fig. 11(b) (at OEM 40%) and in Fig. 11(d) (at

OEM 55%) at applied loads of 2, 6 and 10 N. At OEM 40% moredebris and deeper wear tracks were observed at higher loads (seeFig. 12(a)). At OEM 55%, the width of the wear tracks increasedfrom 0.102 mm at 2 N to 0.2 mm at 10 N and not significant debriswas observed (see Fig. 12(b)). On the other hand, the wear rateknown as dimensional wear coefficient is defined as the volume ofthe material removed at a unit applied load and in a unit slidingdistance expressed in cubic millimeters per Newton meters. Thespecific wear rate results for all of our CNx/TiN multilayer filmswere obtained after 10,800 revolutions of the friction test, which

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Fig. 10. Variation of the friction coefficient from (a) to (d) of CNx/TiN films prepared at OEM 40% with different C target current 0.5–2.0 A.

Fig. 11. Variation of the friction coefficient from (a) to (d) of CNx/TiN films prepared at OEM 55% with different C target current 0.5–2.0 A.

A. Vyas et al. / Tribology International 42 (2009) 798–806 805

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Fig. 12. The wear track profile of CNx/TiN coatings ((a) at OEM 40% (C ¼ 1.0 A), (b)

at OEM 55%, C ¼ 2.0 A) against 10,800 cycles after POD tests were performed with

a WC ball (load: 2, 6 and 10 N).

A. Vyas et al. / Tribology International 42 (2009) 798–806806

gave major differences in the wear rates. The specific wear rate ofthe multilayer at OEM 55% and 60% films was very low in therange �1.0–1.2�10�16 m3/N m as compared with films at lowerOEMs, which had higher specific wear rates that showed increaseby 1 order of magnitude �1–4�10�15 m3/N m (see Table 1). Thelow wear rates for the film at OEM 55% could be attributed to thehardness of these films.

4. Conclusions

The structural, mechanical and tribological properties ofCNx/TiN multilayers were evaluated. The multilayers have beenfound to be very dependent on the deposition parameters, thosebeing OEM settings between 40% and 60% and the varying LCNx

¼

0:3 to 1.2 obtained by changing the C target power. By XPS and thedeconvolution of Ti2p spectra showed the various bonding statusthat existed in the film. The rms surface roughness of the filmsdecreased with increasing bilayer thickness ðLCNx

þLTiNÞ that fallsbetween 1 and 1.7 nm only. Nanoindentation results showed thevariation in hardness, depending on OEM settings and bilayer

thickness. The high hardness value 41 GPa was obtained forCNx/TiN multilayer films (at OEM 55%), which is much higher thanthose of TiN (25 GPa) and CNx (23 GPa). Multilayer at OEM 55%(with bilayer thickness LCNx

¼ 0:9 nmþLTiN ¼ 3:0 nm) has aremarkably high critical load of 78 N. Rockwell-C tests showedthat the films deposited at OEMs higher than 50% (bilayerthickness LTiN ¼ 3.0 nm and LCNx

¼ 0:3 to 0.9 nm) fall in HF1and HF2 adhesion strength quality classification. The frictioncoefficient of the CNx/TiN multilayer coatings has been found to bevery dependent on the deposition parameters as well as bilayerthickness ðLCNx

þLTiNÞ. The average friction coefficient of CNx/TiNmultilayer films at OEM 55% is 0.11 (for bilayer thicknessLCNx

¼ 0:9 nmþLTiN ¼ 3:0 nm). The specific wear rates were inthe order of 10�16 m3/N m for the films at OEM 55% and increasedby 1 order of magnitude 10�15 for the films at lower OEM settings.These results suggest that the multilayer films exhibit compara-tively good wear resistance as compared with the respectivesingle-layer TiN film. From the above investigation, we concludethat to some extent the CNx/TiN multilayer may be a goodcandidate for protective coatings on tools.

Acknowledgments

This work was supported by both the HKSAR Research GrantsCouncil (RGC) Grant (Project no. 9040702) and the CityU SRGGrant (Project no. 7002300).

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