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Tribology International 43 (2010) 1245–1252

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

� Corr

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

Evaluation of fretting corrosion behaviour of CP-Ti for orthopaedicimplant applications

Satendra Kumar a, T.S.N. Sankara Narayanan a,�, S. Ganesh Sundara Raman b, S.K. Seshadri b

a National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, Indiab Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

a r t i c l e i n f o

Article history:

Received 6 May 2009

Received in revised form

27 October 2009

Accepted 10 December 2009Available online 21 December 2009

Keywords:

Fretting corrosion

CP-Ti

Implant application

Repassivation

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

016/j.triboint.2009.12.007

esponding author. Tel.: +91 44 2254 2077; fa

ail address: tsnsn@rediffmail.com (T.S.N. Sank

a b s t r a c t

The fretting corrosion behaviour of CP-Ti in Ringer’s solution was studied as a function of normal load,

frequency and number of fretting cycles. The restoration ability of CP-Ti after the passive film is

damaged due to fretting and as a function of the on-time/off-time ratio (intermittent fretting) was also

evaluated. The change in free corrosion potential measured before the onset of fretting, with the onset

of fretting, during fretting and after the fretting motion ceases, as a function of time, was used to

evaluate the fretting corrosion behaviour. The restoration ability of CP-Ti after the passive film is

damaged was ascertained by performing regression analysis of the potential data measured during

repassivation. The morphological features of the fretted zone were assessed using scanning electron

microscopy. Energy dispersive X-ray analysis was performed at the centre and edge regions of the

fretted zone to identify their chemical nature. The study reveals that the excellent corrosion resistance

and biocompatibility of CP-Ti are nullified under fretting conditions. Once the passive oxide layer is

damaged due to fretting, repassivation is not instantaneous. The significant time delay in reaching the

steady state potential implies that CP-Ti remains active and susceptible for corrosion. The difficulty in

the instantaneous formation of passive film after the fretting induced damage of the passive film,

dissolution of bare Ti from the damaged areas and the possible accumulation of the debris generated

during fretting in the surrounding tissues raises concern on the safer use of CP-Ti as an implant

material.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Titanium and its alloys are widely used as orthopaedic anddental implants due to their low density, better mechanicalproperties, very high strength-to-weight ratio (specific strength),excellent corrosion resistance and biocompatibility [1–5]. One ofthe important reasons for choosing these materials is their abilityto develop a naturally formed passive oxide layer, typically of4–6 nm thickness, which is comprised of either amorphous orpoorly crystallized non-stoichiometric TiO2 [6]. This passive oxidelayer is highly stable and its neutral behavior in corrosive mediumprovides excellent corrosion protection. However, the stability ofthe passive oxide layer could be altered under in vivo conditions.Analysis of retrieved implants reveals discolouration and accu-mulation of metal ions on tissues adjacent to the implant. Themetal ions could have originated due to the fracture of the passivelayer following fretting/sliding wear, resulting in the generationof the wear debris and subsequent dissolution of the debris and/or

ll rights reserved.

x: +91 44 2254 1027.

ara Narayanan).

metal in the body fluid. In fact, such an attribute seems to be validas the ratios of Ti, Al and V in the surrounding tissues are similarto those in the Ti-6Al-4V alloy. The inferior mechanical propertiesof the native forms of TiO2 layer that can be disrupted at very lowshear stresses, even by rubbing against soft tissues, is consideredresponsible for such an occurrence [7]. Due to the inherentproperty of the titanium and its alloys, the passive oxide layercould subsequently form upon reaction with the local environ-ment. However, the capability for restoration and how quickly itwill restore become the main criteria for assessing the suitabilityof Ti and its alloys as an implant material.

Fretting corrosion is the deterioration of a material that occursat the interface between two contacting surfaces due to a smalloscillatory movement between them and the corrosivity of themedium. Orthopaedic implants, particularly hip and kneeimplants, exposed to the physiological medium, suffer a lot dueto fretting corrosion that leads to a reduction in the life time ofsuch prosthesis [8–11]. The modular interfaces of total jointprostheses, mainly at the fixation of the implant stem and bone orcement, are subjected to micro motion (o100mm) that couldresult in fretting corrosion [12]. Consequently, the release ofcorrosion products and particulate debris could cause adverse

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S. Kumar et al. / Tribology International 43 (2010) 1245–12521246

biological reactions. The particulate debris could enter into thecrevices and participate as a third-body or it could migrateoutside the crevice and interact with the tissues surrounding theimplant. The abrasive nature of the particulate debris couldaccelerate wear at the articulation interface, which results inloosening and eventual failure of the implant, thus causingsuffering to the patients and warrants re-surgery [13]. Hence,in vitro studies on fretting corrosion are essential to understandthe nature of corrosion damage and the particulate debrisgenerated during fretting of a modular interface. Moreover, sucha study will provide useful information about the stability of thepassive film and corrosion resistance of the material underfretting conditions [14,15].

The objectives of the present study are: (i) to study the frettingcorrosion behaviour of CP-Ti in Ringer’s solution as a function ofnormal load, frequency and number of fretting cycles; (ii) toidentify the factors influencing the ability of CP-Ti to restore to itsoriginal condition after the passive film is partially or completelydamaged due to fretting; and (iii) to evaluate the influence of on-time/off-time ratio (intermittent fretting) on the restorationability of CP-Ti.

2. Experimental details

CP-Ti (grade-2) (chemical composition in wt. %: N: 0.01; C:0.03; H: 0.01; Fe: 0.20; O: 0.18 and Ti: Balance) discs of 20 mmdiameter and 2 mm thickness were used as the substrate. Theywere mechanically polished using various grades of SiC paperfollowed by 0.3mm diamond paste to a mirror finish, rinsed withdeionized water and dried using a stream of compressed air.Fretting corrosion experiments were performed using a frettingcorrosion test assembly (Fig. 1) (Wear and Friction Tech, Chennai).A ball-on-flat contact configuration that involves an 8 mm +alumina ball moving against a stationary CP-Ti flat was chosen sothat large contact stresses could be achieved under very lowloads. The alumina balls used in the present study were of G 10grade. The alumina balls were procured from Salem Specialty BallCompany Inc., Canton, CT, U.S.A. The choice of alumina ball as thecounterface material was made because of its high hardness(1365 HV), high wear resistance, chemical inertness, and electricalinsulating properties. Since the present study aims to analyze theelectrochemical response of CP-Ti under static and frettingconditions, electrochemical stability of the counterface material

N

Computer

Potentiostat/Galvanostat

CounterElectrode

ReferenceElectrode

Fig. 1. Schematic diagram of the fr

is very important. The displacement of the counterface wascontrolled by a variable frequency drive motor. The appliednormal force was varied using a spring loaded assembly, whichwas measured by a load cell. The tangential force was measuredusing another load cell. The whole system was mounted on a largesteel base to get a better stability and some degree of damping.

Normal loads of 3, 5, 7 and 10 N, an oscillating frequencies of 5and 10 Hz and a linear peak-to-peak displacement amplitude of180mm were used as the fretting corrosion test parameters. TheHertzian contact pressure for the loads used (3 to 10 N) will bearound 500 to 1200 MPa. The tests were performed for 1500,3000, 4500, 9000, 18,000 and 36,000 fretting cycles. The testparameters employed in this study imply a gross slip condition.The number of cycles, the tangential force, the normal force, thedisplacement amplitude, and the coefficient of friction wereacquired automatically using a National Instruments data acqui-sition card (6008) and Lab View software at equally spaced timeincrements over the whole test duration. Ringer’s solution, havinga chemical composition (in g/l) of 9 NaCl, 0.24 CaCl2, 0.43 KCl and0.2 NaHCO3 (pH: 7.8) at 3771 1C that chemically simulates thephysiological medium of the human body, was used as theelectrolyte solution.

CP-Ti discs (20 mm diameter and 2 mm thick) subjected tofretting corrosion formed the working electrode while a saturatedcalomel electrode (SCE) and a graphite rod served as the referenceand auxiliary electrodes, respectively. These electrodes were placedin the fretting corrosion cell in such a way that only 2 cm2 area ofthe working electrode was exposed to the Ringer’s solution. Thealumina ball/CP-Ti flat contact was arranged in such a way thatthey were totally immersed in the Ringer’s solution. The frettingcorrosion cell was connected to a potentiostat/galvanostat/fre-quency response analyzer of ACM instruments (model: Gill AC) tomeasure the free corrosion potential (FCP) of CP-Ti as a function oftime. Before performing the FCP measurement, the CP-Ti was keptin Ringer’s solution and allowed to stabilize for 1 h.

The changes in FCP of CP-Ti, before the onset of fretting, withthe onset of fretting, during fretting and after the fretting motionceases, were monitored as a function of time. The FCP was oftenused as a qualitative indicator of the corrosion regime (active orpassive) in which a metal resides and such a measurement will bemuch useful to evaluate the performance of CP-Ti under frettingconditions. The FCP measurement was repeated at least 3 times soas to ensure reproducibility of the test results. Regression analysiswas performed on the repassivation region to ascertain the

ormal Load

Spring

Deldrin Container

Teflon Sleeve

Alumina Ball

Untreated/TO CP-Ti

Working Electrode

Ringer’s Solution

TribometerControlPanel

etting corrosion test assembly.

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S. Kumar et al. / Tribology International 43 (2010) 1245–1252 1247

restoration ability of CP-Ti after the passive film is damaged. Themorphological features of the fretted zone were assessed usingscanning electron microscopy (SEM). Energy dispersive X-rayanalysis (EDX) was performed at the centre and edge regions ofthe fretted zone to identify the chemical nature.

Fig. 2. Change in free corrosion potential (FCP) (a) and current (b) of CP-Ti as a

function of time [Conditions: Load: 10 N; Frequency: 10 Hz; Amplitude: 180mm;

Number of fretting cycles: 36 000].

3. Results and discussion

3.1. Free corrosion potential (FCP) measurement

The change in FCP and the anodic current of CP-Ti (Conditions:Load: 10 N; Frequency: 10 Hz; Amplitude: 180mm; Number offretting cycles: 36 000), measured before the onset of fretting, withthe onset of fretting, during fretting and after the fretting motion isstopped, as a function of time, are shown in Figs. 2(a) and (b),respectively. With the onset of fretting, a sudden drop (cathodicshift) in FCP (Fig. 2(a)) with a consequent increase in anodic current(Fig. 2(b)) is observed. Similar observations have been made earlierby Galliano et al. [16], Geringer et al. [17], Azzi and Szpunar [18] fortitanium in Ringer’s solution, Barill et al. [19] for Ti–6Al–4V alloy in0.9% NaCl, Tang et al. [20] for untreated and Mo–N modifiedTi–6Al–4V alloy in Hank’s as well as in Na3PO4 solutions, Berradjaet al. [21] for stainless steel in Ringer’s solution and Xulin et al. [22]for stainless steel in artificial physiological solution. In fact, thedrop in FCP and increase in anodic current have been observed notonly during fretting, but also during slurry impingement,scratching, sliding [23–25]. It has been established that thepotential of an electrode shifts in the noble direction when apassive film grows on the surface with a consequent decrease inanodic current. On the contrary, the potential shifts in the negativedirection upon damage and, partial or complete removal of thepassive film with a consequent increase in the anodic current [26].Hence, the sudden drop in FCP (in about 30 s) (Fig. 2(a)) and theconsequent increase in anodic current observed for CP-Ti (Fig. 2(b))are due to the removal of the passive oxide layer induced byfretting, suggesting an increase in susceptibility of CP-Ti forcorrosion. Ponthiaux et al. [27] have reported that the FCP oftitanium during corrosion-wear test is quite close to the freshlyground material in the electrolyte. The extent of cathodic shift inFCP of CP-Ti observed in the present study further confirms thatthe removal of the passive layer with the onset of fretting increasesthe corrosion susceptibility of CP-Ti in Ringer’s solution.

During fretting, some fluctuations in the FCP of CP-Ti areobserved. This is due to the periodic removal (depassivation) andgrowth (repassivation) of the passive film in the fretted zone.Similar observations were made earlier by Tang et al. [20] for bothuntreated and Mo–N modified Ti–6Al–4V alloy during fretting inHank’s as well as in Na3PO4 solutions, by Quan et al. [26] for TiNcoated AISI 316 stainless steel during fretting in H2SO4, by Wu andCelis [28] for stainless steel during fretting in 0.5 M H2SO4, 0.5 MNaCl, and Na3PO4 and by Berradja et al. [21] for stainless steelduring sliding in Ringer’s solution. According to Quan et al. [26], ata fretting frequency of 10 Hz, breakdown of passive film is notexpected to occur during each cycle and such an occurrence ispossible only after the passive film reaches a critical thickness.The oxidational wear mechanism proposed by Quinn [29], whichwas further developed by Abd-El-Kader and El-Raghy [30] alsosuggests that the oxide film grows gradually until a thresholdthickness is reached before it is removed by rubbing or by othermeans of removal. Since the fretting frequencies used in thepresent study are 5 and 10 Hz, a similar phenomenon observed byQuan et al. [30] is expected and the fluctuations in the FCP ofCP-Ti further confirms this. According to Tang et al. [20], Berradjaet al. [21] and Quan et al. [26], the fluctuations in the FCPobserved during fretting are due to the establishment of a

dynamic equilibrium between depassivation and repassivation.Other possible explanations have also been proposed to accountfor the occurrence of fluctuations in FCP during fretting. Accordingto Tritschler et al. [31], during fretting corrosion of stainless steelin Ringer’s solution, the debris generated from poly(methyl-methacrylate) (PMMA) counterface could provide a lubricationeffect during the initial fretting motion, which enables an increasein FCP whereas removal of the PMMA debris from the fretted zoneto the edges leads to a drop in FCP. According to Varenberg et al.[32], the debris generated during fretting enables the build up of ahigh electrical resistance which results in an increase in FCP.However, when the debris falls to a nearby pore and acts as a porefill material in the process of wear, the electrical resistance isreduced, which results in a drop in FCP.

When the fretting motion is stopped, the FCP of CP-Ti exhibitsan anodic shift, suggesting the occurrence of repassivation of theactive area in the fretted zone. During repassivation, twoimportant factors, namely, the ability of the material to returnback to the steady state potential and the time required for suchan occurrence should be considered. Ideally, the potential shouldreach the initial steady state value before the onset of fretting.CP-Ti tends to reach the steady state potential. However, therepassivation is not instantaneous and the initial steady statepotential is reached only after a long duration of time.

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Fig. 3. Secondary electron (a, c) and backscattered electron (b) images of the fretted zone of CP-Ti after subjected to fretting corrosion in Ringer’s solution (Fretting

direction is indicated by double headed arrow) (Debris particles are indicated by arrow marks) [Conditions: Load: 3 N; Frequency: 5 Hz; Amplitude: 180mm; Number of

fretting cycles: 18 000].

Fig. 4. SEM image of the central region of the fretted zone of CP-Ti after subjected

it to fretting corrosion in Ringer’s solution (Fretting direction is marked by double

sided arrow; Debris particles are marked by arrow marks) [Conditions: Load: 3 N;

Frequency: 5 Hz; Amplitude: 180mm; Number of fretting cycles: 18 000].

S. Kumar et al. / Tribology International 43 (2010) 1245–12521248

3.2. Surface analytical characterization of the fretted zone of CP-Ti

The secondary electron and backscattered electron (BSE)images of the fretted zone of CP-Ti, after subjecting it to frettingcorrosion in Ringer’s solution at 3 N, 5 Hz, 180mm for 18 000fretting cycles, are shown in Figs. 3(a) and (b), respectively. The

fretted zone has experienced severe damage due to the extensiveshear deformation and the ploughing action of the alumina ball.The surrounding area of the fretted zone is smoother in which thedebris is smeared all around the fretted zone (indicated by arrowmarks in Fig. 3(c)). The morphological features of the centralregion of the fretted zone (Fig. 4) indicate transfer of materialfrom CP-Ti to the alumina ball. This type of morphological feature,commonly called as ‘‘prows’’ has been observed for systems thatinvolves adhesive wear failure. Hence, it is evident that adhesivegalling is the predominant wear mechanism of CP-Ti when frettedagainst the alumina ball. Micro-welding between surfaceasperities, which occurs during the initial stages gets shearedand plucked away in the subsequent stages. The re-deposition ofthe removed material, confirmed by the presence of debris withinthe fretted zone (Fig. 4), enables an increase in roughness andfurther increases the rate of wear. The presence of Ti (63.09 at%), O(30.86 at%) and Al (06.05 at%) in the EDS spectrum taken at thecentral region of the fretted zone (Fig. 5(a)) suggest that it ispredominately oxides of titanium with a smaller fraction of oxidesof aluminium, the latter should gave originated from the aluminaball. The presence of Ti (25.18 at%), O (51.31 at%) and Al(04.19 at%) in the EDS spectrum taken at the debris particlescollected at the receding edge of the fretted zone (Fig. 5(b))indicates that they are also rich in oxides of titanium with somefractions of oxides of aluminium. A comparison of the elementalcomposition of the central region of the fretted zone and thedebris accumulated at the edges reveals that the debris particlesare rich in oxides. The presence of Cl and Na (Fig. 5(b)) could bedue to re-crystallized salts from the Ringer’s solution. Evaluationof the wear scars on the alumina ball reveals material transferwhen fretted against CP-Ti (Fig. 6).

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Fig. 5. EDX pattern taken at the central region of the fretted zone (a); and at the

debris particles collected at the receding edge of the fretted zone (b) of CP-Ti.

Fig. 6. Wear scar on the alumina ball fretted against CP-Ti [Conditions: Load: 3 N;

Frequency: 5 Hz; Amplitude: 180mm; Number of fretting cycles: 18 000].

S. Kumar et al. / Tribology International 43 (2010) 1245–1252 1249

3.3. Restoration ability of CP-Ti after the passive film is partially or

completely damaged due to fretting

Ennoblement of potential is a good indicator to study thecombined influence of fretting wear and corrosion on therestoration ability of CP-Ti after the passive film is partially orcompletely damaged due to fretting. It is evident from Fig. 2(a)that the ennoblement of potential is not instantaneous and theinitial potential is reached only after a longer duration of time.While correlating the restoration ability with the ennoblement ofpotential, two important factors, namely, the extent and the rateof ennoblement of potential should be considered. The extent ofennoblement of potential is related to the extent of repair of thedamaged (active) area while the rate of ennoblement of potentialis related to the competition between rate of corrosion and rate ofpassive film formation. Calculation of the time to reach the initialsteady state potential and extent of ennoblement of potentialindicate no reasonable correlation with the experimental vari-ables. It might probably be due to the variation in the potentialrange for different experiments, which warrants normalization ofpotential. The normalization of the potential was performed using

the following equation:

Y 0t ¼ ðYt�YINPÞ=DE

where Y0t is the normalized potential; Yt is the measured potentialat a given time; YINP is the Initial Natural Potential; Y0 is thepotential measured at the time where fretting motion ceases; andDE=YINP�Y0.

Analysis of normalized potential vs. time curves obtainedusing different combination of experimental conditions revealsthat several factors influence the ability of CP-Ti to restore to itsinitial steady state potential after the passive film is damaged.Among the experimental variables such as, normal load,frequency and number of fretting cycles, each one of them hasits own influence. The extent of damage of the fretted zone due tothe combined action of fretting wear and corrosion is one of theimportant factors that decide the restoration ability. A compar-ison of the extent of ennoblement of FCP obtained after 3000 and36 000 fretting cycles at 3 N and 10 Hz are shown in Fig. 7(a) andthe corresponding curves obtained at 10 N and 10 Hz are shown inFig. 7(b). When the number of fretting cycles is limited to 3000,the extent of ennoblement of potential after the first 100 s couldreach 86.50 and 63.90% at 3 and 10 N, respectively. However,when the number of fretting cycles is increased from 3000 to36 000, the extent of ennoblement of FCP during the first 40–50 sis limited only to 10%. This is due to the higher extent of damageof the fretted zone after 36 000 cycles. Hence, it is evident that ifthe extent of damage of the fretted zone is higher, which is mostlikely at higher frequency and at higher fretting cycles, the extentof ennoblement of potential will be lower. This implies that alonger time period would be required to repair the damage andcorrosion of the fretted zone would continue until the damage isrepaired, which in turn will affect the rate of ennoblement.

During fretting, the debris generated might get trappedbetween the CP-Ti and the alumina ball, which is most likely tooccur under higher normal loads. Until their removal, the trappeddebris could influence the extent of damage of the fretted zone.

The damage of the passive film on CP-Ti due to fretting isrestricted only to a limited area (amplitude: 180mm) while in theremaining area the passive film is intact. This condition would

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-1

-0.9

-0.8

-0.7

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-0.4

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0

0Time (sec.)

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CP-Ti 3 N 10 Hz 3000 cycles

CP-Ti 3 N 10 Hz 36000 cycles

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CP-Ti 10 N 10 Hz 3000 cycles

CP-Ti 10 N 10 Hz 36000 cycles

10 20 30 40 50 60 70 80 90 100

10 20 30 40 50 60 70 80 90 100

Fig. 7. Comparison of the extent of ennoblement of FCP of CP-Ti in Ringer’s

solution obtained after 3000 and 36 000 fretting cycles at (a) 3 N and 10 Hz; and

(b) 10 N and 10 Hz.

S. Kumar et al. / Tribology International 43 (2010) 1245–12521250

lead to the formation of a galvanic cell between the ‘active’ frettedzone and the ‘passive’ unworn area. The cathodic to anodic arearatio and the potential difference between them would induce astrong influence on the extent of corrosion of titanium from thefretted zone. During repassivation, a dynamic increase in thecathodic to anodic area ratio and a decrease in the potentialdifference between them (due to the repair of the damaged area)would occur and this phenomenon will continue until thedamaged area is completely repaired. Hence, it is evident thathigher extent of damage and formation of galvanic cells wouldcause a deleterious effect on the restoration ability of CP-Tiwhereas trapping of debris would favour the restoration ability ofCP-Ti.

Restoration of the passive film in the damaged area dependson the extent of damage and corrosion of the titanium acceleratedby the formation of galvanic cell. During the initial period of

ennoblement of potential, a dynamic equilibrium exists betweenthe extent of corrosion, which would cause a cathodic shift inpotential and, formation of passive film, which would enable ananodic shift in potential. Once the damage is repaired, the growthof the passive film will occur very fast while its subsequentthickening will occur at a relatively lower rate.

3.4. Effect of on-time/off-time ratio (intermittent fretting) on the

restoration ability of CP-Ti

The fretting corrosion experiments performed using differentcombination of experimental conditions in which the frettingmotion is continuous, reveal that the extent of damage of thefretted zone due to the combined action of fretting wear andcorrosion determines the restoration ability of CP-Ti. Hence, it willbe of much interest to study the restoration ability of CP-Ti, if thefretting motion is intermittent. A series of experiments wereperformed with a constant on-time (30 s) and varying off-times(60 and 300 s) with on-time/off-time ratios of 1:2 and 1:10. Theexperiments were performed at 3 N and 5 Hz, 3 N and 10 Hz and10 N and 10 Hz. The total number of on/off cycles in all theexperiments was kept constant at 10. The change in FCP measuredas a function of time is shown in Fig. 8. It is evident that anincrease in on-time/off-time ratio from 1:2 to 1:10 enables aneasy restoration of the passive film. This can be explained on thebasis of the extent of damage of the passive film and fretted zoneduring the on-time and, extent of repassivation and thickening ofthe passive film during the off-time. In all the intermittent frettingcorrosion experiments, during the first on-time of 30 s, the passivefilm that has been allowed to grow for a period of 1 h getsdamaged, causing a cathodic shift in FCP. During the first off-time,depending up on the available time period (60 or 300 s) forrestoration, both repassivation and thickening of the passive filmwould occur; higher the off-time greater the chance forthickening of the passive film. During subsequent on-times(every 30 s), only a partial removal of the passive film occurswhich gets repassivated during the subsequent off-times. Hence,when the off-time is increased from 60 to 300 s (increase in on-time/off-time ratio from 1:2 to 1:10), the thickening of the passivefilm during off-time and the partial removal of the passive filmduring on-time enables an easy restoration of CP-Ti. Experimentsperformed at 10 N and 10 Hz (Figs. 8(e) and (f)) exhibit an easyrestoration of the passive film at both on-time/off-time ratios of1:2 to 1:10. This may be due to the trapping of debris between theCP-Ti and the alumina ball. The involvement of an adhesive wearmechanism would lead to a decrease in the extent of damage andpromote an easy restoration. Further studies are required to get abetter insight on the fretting corrosion behaviour of CP-Ti underintermittent fretting conditions. The relevance of intermittentfretting to real-life situation and the possibility of developing atest protocol will be quite challenging.

4. Conclusions

The excellent corrosion resistance and biocompatibility ofCP-Ti are nullified under fretting conditions due to the removal ofthe naturally formed passive oxide layer by rubbing against thehard alumina counterface during fretting motion. The fracture ofthis passive oxide film induced by the fretting motion has resultedin a cathodic shift in FCP of CP-Ti. The extent of cathodic shift inFCP depends on the area of contact, load and frequency. Therepassivation is not instantaneous and the significant time delayin reaching the steady state potential implies that CP-Ti remainsactive and susceptible for corrosion. The extent of damage andformation of galvanic corrosion cell determines the restoration

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Fig. 8. Effect of on-time/off-time ratio on the restoration ability of CP-Ti conditions: (a) Normal load: 3 N; Frequency: 5 Hz; Total number of fretting cycles: 1500; On time:

30 s; Off time: 60 s; On time/off time ratio: 1:2. (b) Normal load: 3 N; Frequency: 5 Hz; Total number of fretting cycles: 1500; On time: 30 s; Off time: 300 s; On time/off

time ratio: 1:10. (c) Normal load: 3 N; Frequency: 10 Hz; Total number of fretting cycles: 3000; On time: 30 s; Off time: 60 s; On time/off time ratio: 1:2. (d) Normal load:

3 N; Frequency: 10 Hz; Total number of fretting cycles: 3000; On time: 30 s; Off time: 300 s; On time/off time ratio: 1:10. (e) Normal load: 10 N; Frequency: 10 Hz; Total

number of fretting cycles: 3000; On time: 30 s; Off time: 60 s; On time/off time ratio: 1:2. (f) Normal load: 10 N; Frequency: 10 Hz; Total number of fretting cycles: 3000; On

time: 30 s; Off time: 300 s; On time/off time ratio: 1:10.

S. Kumar et al. / Tribology International 43 (2010) 1245–1252 1251

ability of CP-Ti. If fretting is intermittent, increase in on-time/off-time ratio enables an easy restoration of the passive film. Thefretting corrosion behaviour of CP-Ti raises concern on its saferuse as an implant material.

Acknowledgements

The authors express their sincere thanks to professor S.P.Mehrotra, Director, National Metallurgical Laboratory, Jamshed-pur, for his keen interest and permission to publish this paper, Mr.

B. Sivakumar, CSIR Research Intern, for performing some of thefretting corrosion experiments and Mr. T. Raghavaiah, SeniorTechnical Assistant, IIT Madras, Chennai for his help in SEM-EDXanalysis.

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