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Tribology International 42 (2009) 1373–1379

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Tribology International

0301-67

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Univers

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Fretting wear behavior of nitrogen ion implanted titanium alloysin bovine serum lubrication

Yong Luo �, Shirong Ge

Institute of Tribology and Reliability Engineering, School of Material Science and Engineering, China University of Mining & Technology, Xuzhou 221116, PR China

a r t i c l e i n f o

Available online 16 April 2009

Keywords:

Fretting testing

Titanium alloys

Nitrogen ion implantation

9X/$ - see front matter & 2009 Elsevier Ltd. A

016/j.triboint.2009.04.009

esponding author at: School of Material Sci

ity of Mining & Technology, Xuzhou 221116, P

+86 516 83591916.

ail addresses: sulyflying@cumt.edu.cn, sulyfly

a b s t r a c t

Nitrogen ion implantation was performed on biomedical titanium alloys by using of the PBII technology

to improve the surface mechanical properties for the application of artificial joints. The titanium nitride

phase was characterized with X-ray photoelectron spectroscopy (XPS). The nanohardness of the

titanium alloys and implanted samples were measured by using of in-situ nano-mechanical testing

system (TriboIndenter). Then, the fretting wear of nitrogen ion implanted titanium alloys was done on

the universal multifunctional tester (UMT) with ball-on-flat fretting style in bovine serum lubrication.

The fretting wear mechanism was investigated with scanning electron microscopy (SEM) and 3D surface

profiler. The XPS analysis results indicate that nitrogen diffuses into the titanium alloy and forms a hard

TiN layer on the Ti6Al4V alloys. The nanohardness increases from 6.40 to 7.7 GPa at the normal load of

2 mN, which reveals that nitrogen ion implantation is an effective way to enhance the surface hardness

of Ti6Al4V. The coefficients of friction for Ti6Al4V alloy in bovine serum are obviously lower than that in

dry friction, but the coefficients of friction for nitrogen ion implanted Ti6Al4V alloy in bovine serum are

higher than that in dry friction. Fatigue wear controls the fretting failure mechanism of nitrogen ion

implanted Ti6Al4V alloy fretting in bovine serum. The testing results in this paper prove that nitrogen

ion implantation can effectively increase the fretting wear resistance for Ti6Al4V alloy in dry friction,

and has a considerable improvement for Ti6Al4V alloy in bovine serum lubrication.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Titanium alloys are generally widely applied for dental andnon-cemented orthopedic implants because of their similarmodulus to human bone, superior biocompatibility and corrosionresistance. Recently, there are an increasing number of biodevicesbeing made of titanium alloys. However, small amplitudeoscillatory sliding motion on the titanium alloy implants willresult in crack initiation, wear and adhesion of these implants,which are the main failure of titanium alloy implants. It isreported that about 6% of the hip total prostheses will be replacedafter 9 years due to aseptic loosening. One of the main causes ofthe aseptic loosening may be attributed to fretting corrosion onthe interfaces of titanium stems and the bone cement. Numerousinvestigations have shown that the most detrimental damage onthe titanium artificial joint components is usually observed in thesmall alternated sliding condition. Therefore, some efforts havebeen made to enhance the surface property of titanium alloy forthe application as heavy-duty implants. He et al. [1] investigated

ll rights reserved.

ence and Engineering, China

R China.

ing@yahoo.com (Y. Luo).

the fretting-corrosion behavior of three biomaterials and con-cluded that material degradation was determined by the syner-gistic effect between corrosion and wear. In particular theyconcluded that corrosion phenomena promote the mechanicaldamage of the materials. Palliative materials and coatings havebeen proposed to improve the fretting resistance for a givenapplication. Vadiraj et al. [2] has paid attention to the frettingwear resistance of uncoated, plasma nitrided and laser nitridedbiomedical titanium alloys (Ti–6Al–7Nb) in air and Ringer fluid.According to their results, laser nitrided specimen has shownsuperior performance with minimum surface damage and wearrate despite high friction coefficient in air (0.6) compared touncoated and plasma nitride specimens. Then, friction coefficientis high for uncoated (0.8) and plasma nitrided alloys (0.6) in air aswell as Ringer fluid. They suggested the fretting inducedelectrochemical dissolution was responsible for higher wear ratesin uncoated and plasma nitrided specimens. Wang et al. [3]investigated the fretting wear behavior of microarc oxidationcoatings formed on titanium alloy against steel in unlubricationand oil lubrication and found that the friction coefficient of themodified titanium alloy under oil lubrication was significantlyreduced, favorable stable at 0.15. The results indicated that thecoatings with oil lubricated reduced the shear and adhesivestresses between contact surfaces, consequently alleviating the

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Z-carriage

Srain gaugesensor

Si3N4 ball d=4 mm

Ti6A14V samples

Normal load 49.6 N

Bovine serum lubricant

Fig. 1. Schematic sketch of fretting wear machine.

Y. Luo, S. Ge / Tribology International 42 (2009) 1373–13791374

possibility of initiation and propagation of cracks in the innerlayer of the coating or titanium alloy substrate. Ren et al. [4]evaluated four coatings for their potential towards improvingfretting fatigue on Ti–6Al–4V substrate. They found that thecoefficients of friction might increase or decrease during thefretting fatigue due to generation of coating debris, andthe decrease in coefficients of friction could be attributed tolubrication by coating debris produced during fretting fatigue.Further, they suggested that the coating degradation should bereduced or eliminated to improve the fretting fatigue life. Caronet al. investigated the fretting fatigue of coatings on Ti6Al4V, andthey revealed that the hard coatings TiN PVD and plasma-nitriding treatment have beneficial effect on fretting behavior.

Nitrogen ion implantation is effective to improve the surfaceproperty and tribological behavior of metals and polymers [5–7].This surface modification technology has also been applied toimprove the surface hardness and wear resistance of titaniumalloys for sliding friction conditions, which showed results thatnitrogen ion implanted titanium alloys performed better in termsof lower friction and wear. Ueda et al. [8] researched thetribological properties of plasma immersion nitrogen implantedTi6Al4V, and in their reports it was shown that a 50 nm surfacelayer with 40% concentration of nitrogen peak was formed, whichresulted in a reduction of friction coefficients and improvement ofwear resistance. But, the effect of nitrogen ion implanted titaniumalloys for improving the fretting behavior in biolubricationcondition is unknown.

In this paper, the fretting wear behavior of nitrogen ionimplanted titanium alloys was studied. The fretting behavior inbovine serum lubrication is particularly focused in this work, whichsimulates some fretting wear conditions in artificial joints. Thenano-mechanical properties of titanium alloy were also investigated.

2. Experimental

The testing specimens in this paper were obtained from thetitanium alloy (Ti6Al4V), which were machined in square shape of15 mm�15 mm, and 5 mm in thickness. Before performing implan-tation, Ti6Al4V samples were polished, and then they wereultrasonically cleaned in acetone for 15 min. The nitrogen ionimplantation was made in a multifunction ion implanting system,with pressure of 10�5 Pa and accelerating energy of 40 keV, and anitrogen doses of 2.1�1017 ions/cm2 was applied in ion implantation.

The implanted titanium samples were examined by X-rayphotoelectron spectroscopy (XPS) by using a VG Thermo ESCALAB250 spectrometer fitted with monochromatic Al-Ka X-ray radia-tion (150 W, hn ¼ 1486.6 eV). Analysis spectra were collected withthe pass energy of 550 eV in an increment of 1 eV, but high-resolution spectra were collected at 20 eV pass energy in anincrement of 0.05 eV. Element measurements were carried out at20 depth levels, which were generated by intermittent sputteringfor 100 s with argon ions at 3 keV and current intensity of 5mA.The base pressure in the chamber was 3.1�10�8 Pa, and thediameter of the analysis spot was 500mm. Sputtering rate wasapproximately on the order of 10 nm per level on the basis of thetitanium-sputtering rate.

The nano-indentation on Ti6Al4V sample and nitrogen ionimplanted samples was done on an in-situ nano-mechanics testingsystem (TriboIndenter, Hysitron) with a peak load of 2 mN, whichcan monitor and record the applied load and displacement of theindenter with a force resolution of 1 nN and displacementresolution of 0.04 nm. A three-sided pyramid diamond Berkovichindenter with normal angle of 65.31 was used in nano-indentationtests. Three-stage mode of loading and unloading on indent tip wasapplied in nano-indentation tests of Ti6Al4V alloys.

The fretting wear of nitrogen ion implanted titanium alloyswas performed on a universal multifunctional tester (UMT)developed at CETR (Campbell, California) with ball-on-flat frettingstyle in bovine serum lubrication and dry friction condition,respectively. This tester is able to measure tribological parametersincluding friction force, in-situ wear depth, contact acousticemission, etc. In order to focus the fretting wear behavior oftitanium alloys and the nitrogen implanted samples, Si3N4 ballwas chosen for the wear testing counterparts because of its superwear resistance compared to titanium alloy. The schematic ofcontact between Si3N4 ball and titanium alloy plate is shown inFig. 1. The titanium alloy plate was fixed to the specimen holder inY direction, and the Si3N4 ball with 4 mm diameter is mounted ona holder connecting to a 3D force sensor. New ball specimens wereused for each test. According to our previous experience, it is hardto observe a wear scar for both the titanium alloys and implantedalloys with a normal stress of the joint for a long time. In order toproduce a visible scar to compare the fretting behaviors of thesamples, a larger normal load of 49.6 N, equivalent stress of2.68 GPa was applied to the fretting experiment to accelerate thefretting failure of the samples. A reciprocating system based onmotor was designed to induce fretting oscillation of desiredamplitude. Si3N4 ball oscillated in a linear velocity of 8 mm/s andreciprocating amplitude of 100mm. The fretting frequency wasappropriately 40 Hz. The tangential force and the coefficient offriction were digitally recorded throughout the 25-h test with aload transducer to explain the contact behavior during fretting.

Bovine serum was diluted to the concentration of 25%. Thetemperature was maintained at 37 1C during the fretting processto simulate the physiological medium around the contact area ofthe prostheses within the body.

The wear volume of fretting scars on titanium alloys wasmeasured by a 3D surface profiler, and the worn surfaces wereobserved and analyzed by means of a scanning electron micro-scopy (SEM) (Hitachi S-3000N) with energy disperse spectrum.

3. Results and discussion

3.1. XPS analysis

The XPS peak positions [electron binding energies (BE) forspecific atomic levels] are characteristics of chemical states oftitanium alloys. Fig. 2 shows the typical X-ray photoelectron spectraof Ti6Al4V surfaces implanted with 2.1�1017 ions/cm2 nitrogendose. Fig. 2a provides XPS surveys between binding energies 0 and550 eV at surface and 1000 s sputtering depth, in which a series of

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Fig. 2. The XPS spectra of nitrogen implanted Ti6Al4V alloy at the dose of 2.1�1017 ions/cm2 on the surface and 1000 s sputtering depth regarding: (a) whole spectra, (b)

titanium spectrum, (c) oxygen spectrum and (d) nitrogen spectrum.

Y. Luo, S. Ge / Tribology International 42 (2009) 1373–1379 1375

N1s, Ti2p, O1s, Al2p, and V2p spectra are observed. According toNIST X-ray photoelectron spectroscopy database, the peak at bindingenergy of 454.9 and 460.7 eV in Fig. 2b corresponds to Ti2p3/2 andTi2p1/2 components, which attributed to TiN phase. N1s peak at397.2 eV in Fig. 2d also corresponds to the formation of TiN layer.Ti3p peak at 35 eV and Ti2p3/2 peak at 458 eV can be interpreted asTiO2 component with the evidence of O1s peak at 531.3 eV in Fig. 2c,which results from the contamination on sample surface duringatmospheric exposure [9–11]. In addition, the Ar2p peak atapproximate 120 eV present in Fig. 2a is due to the Ar+ ion sputtercleaning process. After 1000 s sputtering, the XPS spectra in Fig. 2shows apparent reduction of Ti2p3/2 peak at 458 eV, and O1s peakbecomes very weak, which suggests that TiO2 phase disappears atcertain layer depth. However, the strong peak of N1s at 397.2 eVproves the abundant TiN formation.

3.2. Nano-indentation

The nanohardness values of nitrogen ion implanted titaniumalloys and unimplanted Ti6Al4V samples under the applied loads

of 2 mN are shown in Fig. 3. The nanohardness is determined byload-displacement curves according to Oliver and Pharr methods[12]. It is shown that the nanohardness value increases from6.4 GPa of titanium alloy to 7.7 GPa of nitrogen ion implantedtitanium alloys with the dose of 2.1�1017 ions/cm2, which resultsin 20% growth in nanohardness. Such nanohardness enhancementcan attribute to higher stiffness of the TiN hard layer formed on N+

implanted Ti6Al4V substrates.

3.3. Fretting wear behavior

Fig. 4 shows the variation of the coefficients of friction of thetitanium alloy specimens and nitrogen ion implanted specimensin bovine serum lubrication and dry friction conditions. It isobserved that the bovine serum lubrication has effect reducingthe coefficients of friction of unimplanted Ti6Al4V fretting, whichresults in the lowest values about 0.056 compared to thecoefficients of friction around 0.065 for the unimplantedTi6Al4V fretting in dry friction condition. For the nitrogen ionimplanted Ti6Al4V specimens, it is found from that bovine serum

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Fig. 4. Friction coefficient curves for fretting wear test conducted in dry friction

and bovine serum for unmodified and nitrogen ion implanted titanium alloys.

Fig. 5. The line distribution of oxygen element on the fretting worn scar of Ti6Al4V

alloy in dry friction.

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Fig. 6. The variation in tangential load of Ti6Al4V and nitrogen ion implanted

Ti6Al4V in response to displacement during fretting in dry friction and bovine

serum.

Y. Luo, S. Ge / Tribology International 42 (2009) 1373–13791376

lubrication increases the coefficients of friction about 10% inmean values, which is opposite to the effect on unimplantedTi6Al4V specimens. Such results suggest that nitrogen ionimplanted on Ti6Al4V is possibly beneficial for its application inbiological environmental fretting if the lower friction force isstressed. The reason for higher friction force of nitrogen ionimplanted Ti6Al4V under bovine serum lubrication is possibly dueto protein deposition on fretting surfaces, because nitrogen ionimplantation will increase the wettability of Ti6Al4V to biologicalsolutions, which provides more adhesion of protein on the rubbingsurfaces.

The coefficients of friction of unimplanted Ti6Al4V fretting indry friction condition increase at late fretting time, which is quitedifferent from nitrogen ion implanted samples fretting. Fig. 5shows the line distribution of oxygen element across the frettingscar of Ti6Al4V alloy in dry friction condition. The strong peaks ofoxygen element on the fretting area indicate the existence oftitanium oxides. It suggests that the titanium oxides are formedon the fretting area due to the frictional heat accumulation duringthe fretting wear. So, the coefficients of friction for Ti6Al4V alloyfretting in dry friction condition increase at late phase of fretting

testing. However, the bovine serum lubrication can prevent theexposure of fretting scar to air, which decreases the oxide rate ofthe titanium and then maintain the steady values of coefficients offriction in fretting process. Beside of the lubrication effect, thenitrogen ion implantation resulting in surface hardnessenhancement will reduce the adhesion force, which leads tolower values of coefficients of friction [13].

The variation of tangential load in the fretting stroke ofTi6Al4V and nitrogen ion implanted Ti6Al4V in response toreciprocating displacement is shown in Fig. 6. The elliptical non-symmetrical fretting loop indicates a gross slip mechanism. Theenvelope area of each fretting loop corresponds to the dissipatedenergy produced during the fretting circle, which is a very usefulparameter to describe the fretting wear mechanism [14]. It isfound that the fretting loop of Ti6Al4V in dry friction has thelargest envelope area, which reveals a severe adhesionmechanism. The smaller envelope area for the nitrogen ionimplanted Ti6Al4V fretting indicates the reduction in adhesionand oxidation wear.

SEM micrographs of fretting scar in dry friction and bovineserum lubrication for nitrogen ion implanted Ti6Al4V and

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Fig. 7. SEM micrographs of fretting scar profiles for: (a) Ti6Al4V in dry friction, (b) implanted Ti6Al4V in dry friction, (c) Ti6Al4V in bovine serum lubrication and (d)

implanted Ti6Al4V in bovine serum lubrication.

Y. Luo, S. Ge / Tribology International 42 (2009) 1373–1379 1377

unimplanted Ti6Al4V alloy are shown in Fig. 7. The fretting scarsshow plough and scratch damage evidence on the fretting area.We take the scar diameter, which is defined as the average of themaximum length in horizontal and vertical direction, as acharacterization parameter of fretting scar size. The scardiameters for Ti6Al4V in dry friction and bovine serumlubrication are 212 and 211mm, respectively. For the implantedTi6Al4V in dry friction and bovine serum lubrication, the frettingscar diameter increases to 235 and 237mm. These results indicatethat the bovine serum lubrication has little effect on the variationof fretting scar size for both of Ti6Al4V and nitrogen ion implantedTi6Al4V. However, the nitrogen ion implantation results in theincreasing of fretting scar size for Ti6Al4V both in dry friction andbovine serum lubrication.

Fig. 8 shows the characteristics of the wear damage, which canexplain the fretting regime for the Ti6Al4V and implanted Ti6Al4Valloy in dry friction and bovine serum lubrication. Ti6Al4V alloyfretting in dry friction causes a severe plough, scratch and localfracture, which suggests an adhesion and oxidation mechanismfor the fretting damage [15]. In contrast, a slight scratch and localfracture of the nitrogen ion implanted Ti6Al4V alloy fretting in dryfriction shows some improvement of wear resistance in dryfriction. At the same time, the crack nucleation as well as crackpropagation of the nitrogen ion implanted Ti6Al4V in bovineserum lubrication indicates a fatigue and corrosion mechanism.

All of the evidences suggest that nitrogen ion implantation is aneffective way to enhance the surface of Ti6Al4V alloy so as todecrease the wear damage both in dry friction and bovine serumlubrication.

The wear volumes of the Ti6Al4V and nitrogen ion implantedTi6Al4V fretting in dry friction and bovine serum are shown inFig. 9, which are obtained by the 3D measurements of wear scar.It is seen that the nitrogen ion implantation decreases thewear volume of Ti6Al4V alloy in dry friction from 6.3�104 to3.3�104mm3, with a reduction about 50%. Also, nitrogenion implantation enhances the wear resistance of Ti6Al4V alloyin bovine serum with an increase of 37%. These results indicatethat nitrogen ion implantation is effective for fretting wearresistance improvement both in dry friction and bovine serumlubrication.

The 3D topographies of the fretting scars tested in dry frictionand bovine serum lubrication for nitrogen ion implanted Ti6Al4Vand unimplanted Ti6Al4V alloy are shown in Fig. 10. It is foundthat the fretting scar of nitrogen ion implanted Ti6Al4V in dryfriction is more uniform than the other three cases. Nitrogen ionimplanted Ti6Al4V samples has lighter damage than those ofunimplanted Ti6Al4V alloy both in dry friction and bovine serum.These results suggest that nitrogen ion implantation reduces thesurface damage of Ti6Al4V alloy both in dry friction and bovineserum lubrication. However, the nitrogen ion implanted Ti6Al4V

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Fig. 8. Local SEM micrographs of the fretting scars for: (a) Ti6Al4V in dry friction, (b) implanted Ti6Al4V in dry friction, (c) Ti6Al4V in bovine serum lubrication and (d)

implanted Ti6Al4V in bovine serum lubrication.

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Fig. 9. Wear volume for fretting wear test conducted in dry friction and bovine

serum for unmodified and nitrogen ion implanted titanium alloys.

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alloy does not show fretting wear resistance in bovine serumlubrication as good as that in dry friction, which is consistent withthe variations of coefficients of friction.

4. Conclusions

Nitrogen ion implantation was performed on biomedicaltitanium alloys, and fretting wear behavior of Ti6Al4V alloy andnitrogen ion implanted Ti6Al4V alloy has been individuallyinvestigated and compared to understand the nature of weardamage in dry friction and bovine serum lubrication. The resultsindicate that nitrogen diffuses into the titanium alloy and forms ahard TiN layer on the Ti6Al4V alloys. The nanohardness isincreased from 6.40 to 7.7 GPa at the normal load of 2 mN, whichsuggests that nitrogen ion implantation is an effective way toenhance the surface hardness of Ti6Al4V. The coefficient offriction for Ti6Al4V alloy in bovine serum is obviously lower thanthat in dry friction, but the coefficient of friction for nitrogen ionimplanted Ti6Al4V alloy in bovine serum is a bit higher thanthat in dry friction. The wear volume results indicate that nitrogenion implantation enhances the Ti6Al4V alloy surface and improvethe fretting wear resistance both in dry friction and bovineserum lubrication. The fretting damage mechanism of nitrogenion implanted Ti6Al4V alloy fretting in bovine serum isfatigue, while adhesion and oxidation mechanisms cause thedamage of Ti6Al4V alloy fretting in dry friction. It is proved thatnitrogen ion implantation can effectively improve the frettingwear resistance for Ti6Al4V alloy in dry friction and bovine serumlubrication.

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implanted Ti6Al4V in bovine serum lubrication.

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Acknowledgments

The authors wish to thank National Nature Science Foundationof China (Grant no. 50535050) and Project 2007CB607605supported by the Vital Foundational 973 Program of China.

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