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Applied Surface Science 280 (2013) 482– 488

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com/ locate /apsusc

urface properties of Mo-implanted PVD TiN coatings usingEVVA source

in Tiana,b, Wen Yuea,b,∗, Zhiqiang Fua, Yanhong Guc, Chengbiao Wanga, Jiajun Liud

School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, China

Beijing Institute of Petrochemical Technology, Beijing 102617, ChinaMechanical Engineering Department, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

rticle history:eceived 25 January 2013eceived in revised form 2 May 2013ccepted 3 May 2013vailable online 13 May 2013

eywords:

a b s t r a c t

To further improve the tribological properties of TiN coatings used on mechanical parts, Mo ions wereimplanted into PVD TiN coatings with Metal Vapor Vacuum Arc (MEVVA) source at the implantation doseas high as 1 × 1018 ions/cm2. Surface morphology, microstructures, and nano-hardness of TiN coatingswere investigated by optical profilometer, X-ray diffraction (XRD), X-ray photoelectron spectroscope(XPS), and Nano Indenter System. The tribological properties were investigated on a ball-on-disk fric-tion and wear tester. The XRD results demonstrated that the diffraction peak of Ti2N appeared in the

riction coefficienton implantation

olybdenumiN coating

Mo-implanted TiN coatings. However, there was obvious decrease of nano-hardness due to the softMolybdenum phase and its oxides. It was approved that Mo-implanted TiN coatings could greatlyimprove their tribological properties and that the implantation at dose of 1 × 1018 ions/cm2 could resultin much lower friction coefficient. The existence of soft molybdenum, lubricious molybdenum oxidesand titanium oxides resulted in the remarkable reducing of the friction coefficient of TiN coatings withMo-implantation.

. Introduction

Titanium nitride (TiN) coatings have been used as wear resistantoatings for cutting tools and dies owing to their high hardness,ood wear and corrosion resistance. In recent years, TiN coatingstarted to be used on machine parts like automobile parts andn some industries which required low friction [1–5]. As a result,ow friction coefficient becomes essential to TiN coatings. There-ore, some studies have been carried out to investigate frictionoefficients of TiN coatings. Azushima et al. [2,3] studied the influ-nces of preferred grain orientations on friction coefficients of TiNoatings with different counter ball materials under dry condition.hey revealed that a (1 1 1) preferred grain orientation could obtain

lower friction coefficient than a (2 0 0) preferred grain orienta-ion for TiN coatings when the counter ball materials are SUJ2,US304, SUS440C and SWRM10. And they proposed that titaniumxide film on the contact surface of counter balls for TiN coatings

ith the (1 1 1) preferred grain orientations resulted in lower fric-

ion coefficients. On the other hand, lots of ions like C, N, Al, Vnd Zr [6–10] were implanted into TiN coatings and showed obvi-

∗ Corresponding author at: No. 29, Xueyuan Road, Haidian District, Beijing, China.el.: +86 10 82320255; fax: +86 10 82322624.

E-mail address: cugbyw@163.com (W. Yue).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.05.014

© 2013 Elsevier B.V. All rights reserved.

ous effects on improving tribological performance of TiN coatings.As a result, ion implantation has been proved to be an effectiveway to improve the tribological properties of TiN coatings. How-ever, there were few studies of Mo ion implantation on tribologicalproperties of TiN coatings. Deng, et al. [11] investigated the effectsof Mo and Mo+C ion implantations on PVD TiN coatings. Theyreported that nano fiber phase rich in Mo was found in TiN coatingsafter Mo implantations and that higher implantation dose couldenhance the hardness, and decrease the wear rate and frictioncoefficient. Thus, it is possible to obtain much lower friction coef-ficient of TiN coatings by further increasing Mo ion implantationdose. This is because besides forming some special microstructure,higher implantation dose may provide more Mo element whichmay produce more molybdenum oxides with Magneli phase struc-ture [12,13] as lubricant under dry friction. However, the dose ofMo ion implantation in the former study [11] was no more than4.5 × 1017 ions/cm2. Moreover, there were lack of further studieson the surface morphology and worn surfaces, which were verycritical for TiN coatings with Mo implantation applied on machineparts and in relevant industries.

In this paper, Mo ions with dose as high as 1 × 1018 ions/cm2

are implanted into PVD TiN coatings with MEVVA source. Themicrostructure, surface morphology and mechanical properties ofTiN coatings are investigated. Ball-on-disk sliding friction and weartests are carried out to investigate their tribological properties,

ace Science 280 (2013) 482– 488 483

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B. Tian et al. / Applied Surf

specially Mo implantation effects on friction coefficients and wornurfaces. The wear mechanism of the Mo-implanted TiN coatingss also proposed.

. Experimental methods

All substrate samples were made from 316L stainless steel withimensions of 70 mm × 30 mm × 1 mm with Ra of 4.13 nm and theardness of 150 HV0.2. After ultrasonically cleaned for 15 min withcetone and alcohol, respectively, about 1.6 �m thick PVD TiNoatings were deposited on the surface of substrate samples by anIP-10-800 Multi Arc Ion Plating. The parameters of TiN deposi-

ion included a nitrogen pressure of 0.6 Pa, an arc current of 60 A, auty cycle of 30%, a temperature of 200 ◦C, a substrate bias of 200 V,nd TiN deposition for 90 min after Ti deposition for 5 min. Beforeo ion implantation, the TiN coating samples were again ultrason-

cally cleaned for 15 min with acetone and alcohol, respectively.hen a MEVVA II A-H source, made by Beijing Normal University,hina, was utilized to implant Mo ions into the TiN coatings. Thearameters of Mo-implantation included a vacuum of 2 × 10−4 Pa,n average ion energy of 80 keV, a current density of 24 �A/cm2,nd an accelerating voltage of 26 kV. The implantation tempera-ure was room temperature at the beginning and without heatingn the whole process. To further control the surface temperatureower than 100 ◦C during the whole implantation, the sample sur-aces of TiN coatings were kept out of implantations for 10 miny a sample shutter every 20 min. And the two doses of Mo ion

mplantation were 3 × 1017 ions/cm2 and 1 × 1018 ions/cm2 (corre-ponding to the 3E17 and 1E18 in the following part of the paper,espectively).

NanoMap-D optical profilometer was used to measure surfaceorphology of TiN coatings and their roughness. PHI-710 scanninguger microprobe (SAM) was adopted to detect the Mo elementrofile on the TiN coating surface and it was found that the implan-ation depth was about 200 nm. ESCALAB 250 X-ray photoelectronpectroscope (XPS) was employed to analyze the chemical states ofhe elements of the coating surfaces after sputter for 10 nm by Arons. D/max-2500 X-ray diffraction (XRD) with 2◦ incidence anglef fixed omega was utilized to characterize the phases of the sur-ace coatings. Nano-hardness and elastic modulus of the samplesere surveyed by MTS XP Nano Indenter System with a Berkovich

ndenter tip in the Continuous Stiffness Mode (CSM).Friction and wear tests were conducted on an MS-T 3000 ball-

n-disk friction and wear tester with a 4 mm diameter Si3N4 ballliding on the TiN samples under an applied load of 1.96 N. Theall was static while TiN coating disk was rotating during the dryriction driven by a motor. The tests were carried out at a slidingelocity of 0.126 m/s with the wear track radius of 3 mm for 60 minnder dry friction in air. The room temperature was 20 ± 3 ◦C andhe relative humidity was 45 ± 5%. The volumes of wear tracks were

easured by the NanoMap-D optical profilometer and the Scan-ing Probe Image Processor (SPIP) software, and calculated by the

ollowing formula:

= V1

L1× � × d

here V is the total wear volume of the wear tracks (�m3); L1 is therc length of a part of the wear tracks (�m); V1is the wear volumef the part of wear track with the arc length of L1 (�m3); and d ishe wear track diameter (�m).

Then the wear volumes were calculated into wear rates.

EOL JSM-7001F scanning electron microscope (SEM) and Energy-ispersive X-ray Spectrometer (EDS) were used to analyze theorphologies and element distribution of the worn surfaces of

amples.

Fig. 1. Morphology of the TiN coating surfaces with different Mo implantation doses(a) un-implanted; (b) 3E17; and (c) 1E18.

3. Results and discussion

3.1. Characterization

Mo implantations can obviously change the surface color of TiNcoatings from golden yellow to silvery white with a little light yel-low for TiN coatings with Mo dose of 3E17 and to bright silverywhite for TiN coatings with Mo dose of 1E18, by naked-eye obser-vation. Fig. 1 shows the three-dimension morphology of the TiNcoating surfaces with different Mo implantation doses. It can beseen that there are several droplets and pits on all TiN coatings.However, droplets are a little smaller after Mo implantation dueto the sputter effect on PVD TiN surfaces during the implantationprocess. Furthermore, their roughness with a measuring area of0.5 mm × 0.5 mm is listed in Table 1. The results show that Moimplantations do not cause increase of surface roughness of TiN

coatings. And from the results of morphology and roughness, itcan be seen that Mo implantation may not result in higher fric-tion coefficients and is beneficial to TiN coatings used in machineparts.

484 B. Tian et al. / Applied Surface Science 280 (2013) 482– 488

Table 1Surface roughness of TiN coatings with different Mo implantation doses.

Roughness (nm) TiN TiN-Mo-3E17 TiN-Mo-1E18

Ra 9.95 9.58 8.00Rq 62.67 58.60 47.86

30 40 50 60 70 80 90

TiN-Mo-1 E18

TiN-Mo-3E17

TiN

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Ti2O3 2p3/2

TiN 2p3/2

Ti2N 2p

3/2

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Binding En ergy (eV)

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TiO2 2p3/2

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3/2

(b)

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Binding En ergy (eV)

(c)

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(a

.u.)

MoO2 3d

3/2

Mo 3d3/2

MoO2 3d

5/2

Mo 3d5/2

2-theta (de gree s)

ig. 2. X-ray diffractograms of TiN films with Mo implantation at different doses.

Phase structures of TiN films with Mo implantation are analyzedy XRD (Fig. 2). It can be seen that there is only TiN phase on theiN coating samples, while the TiN coatings after Mo-implantationt dose of both 3E17 and 1E18 ions/cm2 show similar structures ofi2N (2 0 0) besides TiN [14,15]. It suggests that Mo-implantationay generate high energy and activate elements in TiN coatings

o cause new Ti2N structures. To further investigate the influencef Mo ion implantation on microstructures of TiN coatings, the XPStudies of TiN coatings and Mo-implanted TiN coatings at 1E18 doseere carried out after surface etching for 10 nm. The results are

hown in Fig. 3 and Table 2. It reveals that Mo implantation doesot cause any change of the chemical states of Ti element of TiNoatings, and that Ti element on the surfaces of TiN coatings bothn-implanted and implanted is composed of Ti2N, TiN, Ti2O3, andiO2. However, the chemical state of Ti2N should exist only at a veryuperficial layer for the un-implanted TiN coating according to theole TiN phase of its XRD result. Moreover, after Mo-implantation,iN coating surfaces display the existence of Mo and MoO2, whichs helpful to decrease the friction coefficients of TiN coatings.

Fig. 4 displays the nano-hardness and elastic modulus of the un-mplanted and Mo-implanted TiN surfaces at 1E18 dose. It can beeen that Mo ion implantations could obviously lower mechanicalroperties of TiN coatings. Taking 50 nm to 150 nm displacement

nto surface as the steady part of the curves, the average values ofiN, TiN-Mo-3E17 and TiN-Mo-1E18 are shown in Table 3. It verifieshat Mo implantation could decrease the nano-hardness and elas-

ic modulus of TiN coatings, while higher Mo implantation dosesan obtain much lower average nano-hardness and elastic modu-us, with dropping range of 21.17% and 15.2% respectively for TiNoatings after Mo implantation at 1E18 dose. Also, it proves that

able 2hemical states of Ti and Mo of TiN coatings with and without Mo-implantation.

Samples Binding energy (eV) Chemical state

Ti 2p3/2 Mo 3d5/2

TiN 454.63, 455.8,457.2, 458.45

– Ti2N, TiN, Ti2O3,TiO2

TiN-Mo-1E18 454.21, 455.95,457.6, 458.4

227.5, 228.6 Ti2N, TiN, Ti2O3,TiO2, Mo, MoO2

Fig. 3. XPS spectra of elements on the TiN surfaces of Mo un-implanted andimplanted at 1E18 dose (a) Ti 2p, un-implanted; (b) Ti 2p, Mo-implanted; and (c)Mo 3d, Mo-implanted.

the existence of soft Mo and its oxides lead to the distinct hardnessdecrease although there is hard Ti2N phase in the TiN coatings.

3.2. Friction and wear performance

Fig. 5 demonstrates the wear rates and friction coefficients ofthe un-implanted and Mo-implanted TiN surfaces along with time.As shown in Fig. 5a, the average wear rates of TiN, TiN-Mo-3E17and TiN-Mo-1E18 coatings are 20.120 × 10−6, 13.807 × 10−6 and12.982 × 10−6 mm3/Nm, respectively. The Mo-implanted TiNcoatings can obviously decrease wear rates of TiN coatings by

31.38% for TiN-Mo-3E17 and by 35.48% for TiN-Mo-1E18. Andhigher dose of Mo implantation does not further display remarkablereducing of wear rates comparing with the lower dose. In Fig. 5b,it can be seen that the friction coefficient curve of TiN samples

Table 3Average nano-hardness and elastic modulus of TiN coatings with different Moimplantation doses.

Mechanical properties (GPa) TiN TiN-Mo-3E17 TiN-Mo-1E18

Average nano-hardness 35.53 33.04 28.01Average modulus 491.38 463 416.69

B. Tian et al. / Applied Surface Science 280 (2013) 482– 488 485

0 20 40 60 80 10 0 120 1400

5

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20

25

30

35

40

45

Displ acement Into Surface (n m)

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TiN

TiN-Mo-3E17

TiN-Mo-1E18

(a)

0 20 40 60 80 10 0 120 1400

100

200

300

400

500

600

700

Displace ment Into Surfa ce (nm)

Mo

du

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TiN

TiN -Mo -3E17

TiN -Mo -1E18

(b)

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Fig. 5. Wear rates and friction coefficients of the TiN coating surfaces with Mo

ig. 4. Mechanical properties of the TiN surfaces of un-implanted and Mo-implantedt 1E18 dose (a) nano-hardness; and (b) elastic modulus.

ithout implantation is entirely above that of the TiN samples witho-implantation. Taking the parts after the sharp increasing at the

eginning of the friction coefficients curves as the steady stages,he average friction coefficients of TiN, TiN-Mo-3E17 and TiN-Mo-E18 at the steady stages are 1.05, 0.63 and 0.43, respectively. Itan be seen that the friction coefficients of TiN coatings decreasedy 40% for TiN-Mo-3E17 and by 59.05% for TiN-Mo-1E18. In addi-ion, the friction coefficient curve for TiN coating fluctuates moreidely and begins to rise after wear for 50 min compared with therevious steady stage. However, the friction coefficient curves foro-implanted TiN coatings are stable without obvious increase.oreover, the TiN-Mo-1E18 samples notably reduce the time of

he sharp increasing of friction coefficients at the beginning of theear. It suggests that Mo-implanted TiN coatings can reduce fric-

ion effectively and higher dose is greatly beneficial to the decreasef its friction coefficient. It is supposed that hard Ti2N phase andoft phase Mo and lubricious phase MoO2 play important roles innti-wear and reducing friction during the dry friction process.

.3. Worn surfaces analysis

The morphology of wear tracks and their typical cross-sectionrofiles are displayed in Fig. 6. To examine the details of the wearracks, two typical parts of flat area (red curves in Fig. 6d–f) and pitrea (green curves in Fig. 6d–f) are analyzed respectively. It is foundhat all wear tracks have pits and furrows. The pits are similar to

ach other and all extend to the 316L steel substrates, while theurrows on the wear tracks show distinct differences. As seen inig. 6a, there are obvious furrows along the friction direction onhe wear track for TiN coating with the width of 0.7 mm and the

implantation at different doses (a) wear rates; and (b) friction coefficients.

depth of near 2.0 �m (Fig. 6d). Some furrows and the bottom ofthe flat area of TiN coatings reach the 316L substrate which resultin the wide fluctuation and rising of friction coefficient after wearfor 50 min. After Mo-implantation at dose of 3E17, the wear trackbecomes narrower and shallower with the width of 0.6 mm andthe depth of 1.0 �m (Fig. 6b and e). However, there are still lotsof furrows on the wear tracks of the Mo-implanted TiN coatings at3E17 dose. Furthermore, on the wear track of the Mo-implanted TiNcoatings at 1E18 dose, the furrows are very few and much shallower(Fig. 6c and f), and the width sharply decreases to 0.4 mm with thedepth of about 1.0 �m (Fig. 6f). It reveals that Mo implantation cangreatly decrease severe degree of the friction and abrasive wearduring dry friction.

SEM images of wear tracks in Fig. 7 display the similar resultsas Fig. 6. After Mo-implantation at dose of 1E18, the wear trackbecomes much narrower and shallower with fewer furrows andsmaller pits (Fig. 7a and b) which are further proved by their rel-evant EDS results of element distribution of Ti and Fe on the weartracks. Three points marked as 1, 2 and 3 (Fig. 7c and d) repre-sent the areas of substrate, flat area and pit area of the wear tracks,respectively. It can be seen that the morphology of the substratesof both TiN and TiN-Mo-1E18 coatings is similar, while the weartracks are quite different. First, there are much deeper furrows onthe wear tracks of TiN coatings while there are no obvious furrowsafter Mo implantation, which reveals that severe abrasive wear isremarkably reduced by Mo implantation. Second, there are muchbigger dark oxides pieces above the pits area of TiN coatings with

many cracking on the oxides surfaces, while the smaller dark oxidespieces mainly disperse on the edge of pits and above flat area after

486 B. Tian et al. / Applied Surface Science 280 (2013) 482– 488

Fig. 6. Morphology of wear tracks of the TiN coatings with and without Mo implantation (a) TiN; (b) 3E17 ions/cm2; and (c) 1E18 ions/cm2 and its typical cross-section profile( 2 2 ces to

Mr

3

oflttattofAimmu

d) TiN; (e) 3E17 ions/cm ; and (f) 1E18 ions/cm . (For interpretation of the referen

o implantation, which reveals that Mo implantation could alsoemarkably reduce the severe adhesive wear.

.4. Discussion

Lubricious tribofilms like titanium oxides and molybdenumxides could be formed and used as good lubricants during dryriction condition. They have the Magneli-phase structures withow shearing strength and could isolate friction surfaces effectivelyo reduce the wear and friction of surfaces [12,13,16]. However,he titanium oxide film begins to deteriorate over about 400 ◦Cnd increase its friction coefficient. As seen in Fig. 5b, the fric-ion coefficient of TiN coatings shows a longer increase period athe beginning of the wear, which may be due to the deteriorationf Ti oxides. On the contrary, Mo implantation could decrease theriction coefficients of TiN coatings and keep it much more stable.fter Mo implantation, soft phase Mo of TiN coatings could be eas-

ly cut and adhere to worn surfaces during the wear process, whichay effectively decrease the friction coefficient. Also Mo elementay diffuse into deeper place of the worn surface of TiN coating

nder the high temperature during wear process. At the same time,

color in text, the reader is referred to the web version of this article.)

a part of molybdenum can be gradually transformed to molybde-num oxides under high temperature of friction process. Besides theoriginal MoO2 on the surfaces, there could continuously producelubricious molybdenum oxides on the worn surfaces of TiN coatingswith Mo implantation during the dry friction process. Therefore,besides titanium oxides, there are Mo and molybdenum oxides asgood lubricants which are beneficial to get smoother worn sur-faces and maintain lower friction coefficients. Also, the existenceof Mo and its oxides could be able to delay the temperature risingof the friction surfaces and prolong the valid lubrication time of thetitanium oxides. In addition, TiN coatings with Mo implantationcould remarkably improve their anti-wear properties. Firstly, thehard substrate phases of TiN can provide good load supports for thelubricious phases of titanium oxides, Mo and molybdenum oxides,which can greatly reduce their abrasive wear and adhesive wear formuch longer time. Secondly, hard phases of Ti2N may disperse onthe friction surfaces and further enhance its wear resistance after

Mo implantation.

Owing to its higher dose of Mo implantation, TiN coating withMo-implantation at dose of 1E18 could get much more molybde-num on its surface than that at dose of 3E17. Also, it can provide

B. Tian et al. / Applied Surface Science 280 (2013) 482– 488 487

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ig. 7. SEM images of wear tracks of the TiN coatings with and without Mo implagnification; and (d) 1E18 ions/cm2, high magnification.

uch more molybdenum oxides produced during the wear process,long with the titanium oxides, to get much better lubrication effector a much longer time. As a result, TiN coatings with Mo implan-ation at 1E18 dose can obtain the lowest friction coefficient, andemarkably improve the surface morphology of wear tracks. How-ver, the wear resistance of TiN coatings with Mo implantation atE18 dose could not be further distinctly improved comparing tohe 3E17 dose due to the obvious decreasing of surface hardness.

. Conclusions

Mo ions were implanted into PVD TiN coatings at doses of × 1017 ions/cm2 and 1 × 1018 ions/cm2 by MEVVA source at roomemperature. From the XRD results, it was found that Mo implan-ations could produce the diffraction peak of hard phase Ti2Nt the two doses. And soft phases of Mo and MoO2 were alsoetected by XPS which obviously reduced the nano-hardness andlastic modulus of TiN coatings after Mo implantation. The tribo-ogical properties of Mo-implanted TiN coatings, against Si3N4 ball

ere remarkably improved. The wear rates and friction coefficientsecreased by 31.38% and 40% respectively for Mo-implanted TiNoatings at dose of 3E17, while that of the Mo-implanted TiNoatings at dose of 1E18 decreased by 35.48% and 59.05% respec-ively. The wear mechanism of TiN coatings un-implanted andmplanted was both abrasive wear and adhesive wear. However,

o implantation could remarkably reduce the abrasive wear anddhesive wear of TiN coatings resulting from its lubricious oxidesnd hard phases of Ti2N and TiN. Furthermore, the TiN coating

ith Mo-implantation at 1E18 dose could obtain the lowest fric-

ion coefficient and the narrowest wear tracks, due to more softolybdenum and molybdenum oxides as good lubricant and longer

ubrication time of titanium oxides during wear process.

n (a) TiN, low magnification; (b) 1E18 ions/cm2, low magnification; (c) TiN, high

Acknowledgments

The authors would like to thank the Beijing Natural ScienceFoundation (3132023), the National Natural Science Foundation ofChina (51275494, 51005218), the Fundamental Research Funds forthe Central Universities, the Program for Key International Scienceand Technology Cooperation Project of China (2010DFR50070) andthe Tribology Science Fund of State Key Laboratory of Tribology(SKLTKF11B04) for their financial supports.

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