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Applied Surface Science 288 (2014) 727– 735

Contents lists available at ScienceDirect

Applied Surface Science

j ourna l ho me page: www.elsev ier .com/ locate /apsusc

he tribocorrosion behaviour of NiTi alloy

adeja Kosec ∗, Petra Mocnik, Andraz Legatlovenian National Building and Civil Engineering Institute, Dimiceva 12, SI–1000 Ljubljana, Slovenia

r t i c l e i n f o

rticle history:eceived 25 July 2013eceived in revised form 18 October 2013ccepted 19 October 2013vailable online 28 October 2013

a b s t r a c t

In biomedical applications, NiTi alloys are used mainly because of their favourable shape memory andsuperelastic properties. However, in many applications the tribocorrosion properties of these alloys canbe of critical concern. For this reason the electrochemical and tribocorrosion properties of superelasticNiTi sheet and orthodontic archwire were studied, taking into account their microstructures and the effectof different surface finishes. In the case of the electrochemical tests, samples were tested in artificial saliva,

eywords:iTiribocorrosionicrostructure

assive film

whereas in the tribocorrosion tests the experiments were performed in ambient air, distilled water, andartificial saliva, the latter as a corrosive medium. In these tests, the total wear rate of the alloy samples wasdetermined, together with the corresponding chemical and tribological contributions. It was confirmedthat the microstructure of the investigated alloys had a significant effect on the measured electrochemicaland tribocorrosion properties.

imulated saliva

. Introduction

Nickel titanium alloy has received a considerable amount of sci-ntific attention due to its shape memory properties. The researcherformed so far can be roughly divided into the following fields:asic corrosion and electrochemical studies [1–6], mechanical andaterial property studies [7–11], and surface treatment studies

12–19]. During extensive corrosion studies on nickel titaniumlloy [1–6], Rondelli made a comparison between NiTi and othermplant materials in body simulating fluids [1]. He showed thathe passive films which are formed on NiTi are inferior to those oniAlV alloys, but are comparable to those formed on stainless steels1]. Pitting potentials lower than 250 mV have been measured oniTi alloy in simulated body fluids at 37 ◦C. They depend on the

urface modification procedures, by means of which the pittingotential can be increased to values exceeding 800 mV [3]. Figueirat al. have recently studied NiTi, and compared it to Ti, Al, and 316Ln Hank’s solution and in Eagle’s minimum essential medium, alsot 37 ◦C [4]. It was found that NiTi is subject to crevice corrosion,nd that the passive films are thinner in an amino acid containingnvironment [4]. Among the various drawbacks of the use of Nitinols the fact that it is subject to pitting corrosion [3], and failures haveeen reported such as stress corrosion cracking in orthodontic NiTirchwires [8], as well as extensive Ni release [7]. Since nickel can

ause hypersensitive reactions in the human body, a large numberf surface treatments have been proposed and implemented inrder to reduce the content of nickel in the surface oxide film [12],

∗ Corresponding author. Tel.: +386 1 2804547; fax: +386 1 2804484.E-mail address: tadeja.kosec@zag.si (T. Kosec).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.10.116

© 2013 Elsevier B.V. All rights reserved.

including ion implantation [12], mechanical polishing, chemicaletching and selective oxidation [13,19]. It was found that the cor-rosion properties are improved by chemical etching, which resultsin a depletion of nickel in the surface compact oxide film, whereasoxidation at 530 ◦C as a part of the shape setting procedure canhave a negative effect on corrosion performance [13]. The highercorrosion resistance of an austenitic grown oxide film has also beenobserved [5]. In a study of the possibilities of alloying with Nb andCo, thus altering the content of Ni in NiTi, the corrosion propertiesof the alloy were improved, as well as its hardness [14]. Many stud-ies involve the evaluation of different coatings on NiTi alloy, fromhydroxyapatite [17] to hard coatings such as TiN [15,16,20]. Thecorrosion resistance of NiTi was significantly improved by almost60 times by the electro-deposition of a hydroxyapatite/zirconiacomposite coating. The drawback of the latter is that it cannotbe used in load-bearing applications [17]. Plasma immersionion implantation on a NiTi surface has been shown to enhancethe corrosion resistance of such an oxide film [18]. The surfaceof NiTi has also been modified by the implementation of N+and/or Ar+ ions in order to produce a nickel depleted surface withimproved biocompatibility [12]. The success of this method wasdemonstrated by the results of cell culture experiments.

Concerning the tribological behaviour of NiTi alloys, severalstudies have been performed in dry conditions [9–11], but thereis a great lack of tribocorrosion studies on NiTi alloys [6]. Thetribological behaviour of NiTi alloy, against that of steel andtungsten carbide, was studied at elevated temperatures in order to

obtain data about the tribological behaviour, where superelasticityand the shape memory effect might be lost [9]. At elevated tem-peratures a reduction in the wear rate at the NiTi/steel contact wasobserved, which was attributed to a tribological layer on the wear

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urface of the NiTi [9]. Another study by the same authors showedhat, in its austenitic state, NiTi has a higher wear resistanceompared to the martensitic state, and a higher coefficient ofriction was measured as a result of the lower strength of thelloy in its martensitic state [10]. The sliding wear of superelasticiTi has also been studied in dry conditions [11], where it was

hown that this alloy’s wear resistance was about 30 and 10 timesigher than that of pure Ti and Ni, respectively. The dominant wearechanisms observed in the case of NiTi consisted of abrasion

nd the delamination of sub-surface cracks as a result of theyclic loading. Moreover, dry tribological tests were performed onlasma source ion implantated NiTi samples in the austenitic asell as the martensitic state [6], where it was shown that wear was

educed by 38% in the case of the austenitic state in dry conditions,nd by as much as 79% in lubricated conditions. A study of an NiTilloy, which was the closest to being a tribocorrosion study, waserformed by Rondelli [1]. The results of localized corrosion testshere the passive film was abruptly damaged showed that the

haracteristics of the passive film on the NiTi were inferior to thosef a corresponding film on TiAlV, and in some cases even to that toe found on stainless steels [1].

Most tribological studies of NiTi alloys have been performedn ambient air, so that only the effect of mechanical wear haseen more thoroughly studied, but there is a great demand for theecognition of the favourable properties of NiTi alloy in the case ofimultaneous wear and corrosion in aggressive media. The aim ofhe present study was to evaluate and compare the electrochemi-al and tribo-electrochemical behaviour of nickel titanium alloys inimulated saliva, in the case of two microstructurally and dimen-ionally differing materials: nickel titanium sheet and orthodonticrchwire.

. Experimental

.1. Preparation of the samples

Samples prepared from the following two superelastic NiTilloys were studied:

(1) “NiTi sheet coupon, alloy S (superelastic)”, thickness 2 mm,at annealed, 55.8 wt% Ni, Af = 0 ◦C (Memry GmbH, Germany), (2)Nitinol Super-Elastic Archwire”, 0.48 mm × 0.64 mm, 54.5–57 wt%i), Af = 5–25 ◦C (3 M Unitek, USA).

The samples (i.e. working electrodes) for the electrochemicalests were cut from the sheet in the form of discs with a diameter of5 mm, and from the archwire along the straight part to a full lengthf 50 mm. The samples for the tribo-electrochemical tests were cutrom the sheet by means of electro discharge machining with ahape of 50 × 10 mm. The electrical contact was attached, and theack and sides of the sample, as well as the electrical contact, wererotected by epoxy coating, so that the exposed surface had an areaf 4.5 cm2. The samples for the tribo-electrochemical tests maderom archwire were prepared from the flat part of the wire, with aotal length of 40 mm, and were moulded in an acrylate mould withhe electrical contact attached. They were subsequently grindednd polished in order to get a surface that would be free of oxides.

Samples with two different surface finishes were studied: therst were those without an oxide film, which were prepared byrinding and polishing in order to investigate the effect of theicrostructure. The second set of samples consisted of those whichere in “as received” state with an oxide film according to the sup-lier’s finish, in order to investigate the actual material which is

sed in a real environment (i.e. archwire in saliva). For the removalf the oxide film, the samples were abraded with 800 and 1200 gridiC paper. The samples were ultrasonically cleaned in ethanol for

min, and then well dried.

ience 288 (2014) 727– 735

2.2. Micro-structural examination

Samples for metallographic investigation were etched in a solu-tion of 10 vol% HF, 40 vol% HNO3 and 50 vol% H2O for 2 min. Shortlyafterwards, an optical microscopy study was conducted at differentmagnifications. The number of inclusions was estimated at scanswith a magnification of 20×.

2.3. Electrochemical measurements

A suitable artificial saliva solution was chosen by testing nickeltitanium alloy in solutions with various compositions. The onethat most closely represented natural saliva was chosen for fur-ther experiments, similarly as in the case of the salivas which weretested by Duffo et al. [21]. Thus, the electrochemical measurementswere performed in a solution of 0.60 g/L NaCl, 0.72 g/L KCl, 0.22 g/LCaCl2·2 H2O, 0.68 g/L KH2PO4, 0.856 g/L Na2HPO4·12 H2O, 0.060 g/LKSCN, 1.5 g/L KHCO3 and 0.03 g/L citric acid, pH = 6.5, which wasdesignated “simulated saliva”.

Due to the flatness of the disc electrode, a three-electrode corro-sion cell was used, with a volume of 350 cm3. The working electrodewas embedded in a Teflon holder, and had an exposed area of0.785 cm2. In the case of the working electrodes for the testedarchwire, the area exposed to the solution was 1.0 cm2 for longitu-dinal exposure and 0.003 cm2 for the cross section. Autolab PGStat100, Floating version equipment was used, which was expanded byNova software 1.8.

The electrochemical tests were performed as follows: 2-h sta-bilization at open circuit potential (OCP), with linear polarizationmeasurements at ± 20 mV vs. OCP at a scan rate 0.1 mV/s. Thepotentiodynamic measurements were then performed startingfrom −0.25 V vs. OCP, and progressing in the anodic direction upto +2.0 V at a scan rate of 1 mV/s. In the case of the cyclic polariza-tion measurements, the reverse potential was chosen at a potentialof 1.4 V in order to provide a definite unknown for all the testedsamples. All potentials are reported with respect to the SCE scale.

At least three measurements were performed in order to fulfilthe statistical requirements for electrochemical testing [22]. Afterestimating the mean values of the logarithmic results of corrosionresistance, the measurement that had the closest value to the meanvalue from the set was chosen to be presented in the graphs.

2.4. Tribo-electrochemical measurements

The wear tests were performed on a reciprocal tribometer (Pinon disc and reciprocating tribometer Tribotechnic, 2009, France)with a 6 mm Al2O3 ball as a counter-body. The tribocorrosion set-upand the working electrodes are presented in Fig. 1.

The normal loads in the tests varied from 1 N to 5 N (with acontact pressure within the range 850–1450 MPa), with an aver-age sliding speed of 5 mm/s. Each measurement was repeatedat least twice, and very similar results were obtained. The wearlength was 10 mm. In all of the experiments the friction force andthe coefficient of friction (COF) were monitored against time. Thetribo-electrochemical three-electrode cell, which was made fromTeflon, consisted of a working electrode with an electrical contact,a platinum archwire counter-electrode, and an Ag/AgCl referenceelectrode.

The electrochemical and tribocorrosion experiments wereperformed at room temperature, since, due to its design, the tribo-corrosion set-up did not permit testing at elevated temperatures.

Wear mechanisms were studied by a comparison of wear in

ambient air and wear in simulated saliva as a corrosive environ-ment. The wear rate was estimated from the tribocorrosion wearin the simulated saliva and in a non-corrosive environment, forwhich distilled water was used.

T. Kosec et al. / Applied Surface Science 288 (2014) 727– 735 729

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Fig. 1. (a) Schematic presentation of the tribocorrosio

.5. SEM/EDS, profilometer and hardness analysis

A low-vacuum scanning electron microscope (JEOL 5500 LV,apan), equipped with energy dispersive spectroscopy (EDX)xford Inca (Oxford Instrument Analytical, UK), was used tobserve and analyse the contact surfaces and the formed electro-hemical and tribocorrosion products using an accelerating voltagef 20 kV. A Taylor Hobson profilometer (Taylor Hobson Precision,urtronic 25 France, 2010) was used for the wear track analysis,

nd Vickers Hardness was determined by means of a model 38542rank Finotest tester, using an indenter at a test load of 9.8 N, andicroscopic enlargements of 40×.

ig. 2. Microstructural examination of the investigated NiTi sheet in (a) cross-sectionalongitudinal view.

p and (b) samples for the tribocorrosion experiments.

3. Results and discussion

3.1. Metallographic examination

Both of the samples, i.e. the NiTi sheet and the NiTi archwire,were metallographically examined both in cross-section and inthe longitudinal direction. The results are presented in Fig. 2. TheNiTi sheet (Fig. 2a and b) had a martensitic structure. The poorlydefined crystal grains had sizes of 40–50 �m in the cross-sectional

and longitudinal views. The grains were uniformly oriented in alldirections due to thermal processing. There were a small numberof larger inclusions in the longitudinal direction (Fig. 2a, showing

and (b) longitudinal view, and of the NiTi archwire in (c) cross-sectional and (d)

730 T. Kosec et al. / Applied Surface Science 288 (2014) 727– 735

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Fig. 3. Cyclic polarization scans for the NiTi sheet and archwire: (a) without a s

he exposed working surface during the electrochemical and tribo-orrosion experiments), and several larger inclusions of diameter–5 �m were found in the cross-sectional view (Fig. 2b). The aver-ge area of all the inclusions in the NiTi sheet was 1.34% in theongitudinal view, and 2.34% in the cross-sectional view.

It was very difficult to see the martensitic microstructure ofhe NiTi archwire in the longitudinal and cross-sectional viewsFig. 2c and d). In cross-sectional view the crystal grain size wasery small (Fig. 2c), but was elongated in the longitudinal directionFig. 2d, showing the exposed surface for electrochemical testing)ue to the manufacturing procedure, which includes cold draw-

ng. There are many small inclusions (less than 1 �m in size) in theicrostructure of the archwire. These occupied 4.2% of the total

rea in cross-section, and 2.45% of the total area in the longitudi-al view. The inclusions were elongated in the drawing direction.hey had lengths of between 10 and 200 �m. The inclusions werenvestigated by SEM microscopy coupled with EDS. Their composi-ion was determined to be TiC and Ti2NiOx, as reported elsewhere23,24].

.2. Electrochemical measurements

In order to estimate pitting susceptibility, cyclic polarizationCP) measurements were performed, at a scan rate of 1 mV/s, onhe polished samples of NiTi sheet and archwire, as well as on theiTi sheet and archwire samples with surface oxide films in as-

eceived state. The repassivation properties of both the NiTi sheetnd the archwire were determined.

In the case of the NiTi sheet and archwire without surface oxide

lms, it was clearly shown that NiTi is not susceptible to pittingorrosion, since a negative hysteresis was observed in the reversednodic scan in simulated saliva (Fig. 3). In the case of NiTi sheet,he corrosion current density was 74 nA/cm2. The forward scan, in

able 1lectrochemical parameters for the investigated NiTi dental wire and sheet (Ecorr, jcorr, Eb, E

= 0.1 mV/s).

Sheet Sheet cross-section Wire

Ecorr [V] −0.29 (±0.04) −0.21 (±0.03) −0.27 (±0.04)

jcorr [A/cm2] 74 (±8) × 10−9 313 (±62) × 10−9 46 (±3) × 10−9

Rp [M� cm2] 0.82 (±0.06) 0.25 (±0.07) 1.2 (±0.1)

Eb [V] 1.1 (±0.002) 1.1 (±0.005) 1.2 (±0.003)

Erp [V] 0.9 (±0.2) 0.051 (±0.04) 0.86 (±0.03)

e oxide film, (b) with an oxide film, in simulated saliva at a scan rate of 1 mV/s.

which a low passive current density of approx. 3 �A/cm2 was mea-sured, was consistent with passive behaviour. A sharp increase inthe current density due to transpassivity marked the breakdownpotential, Eb, at 1.1 V in the forward scan. The potential appliedto the electrode was then reversed, and scanned in the reversedirection at a potential of 1.4 V, when a current of approximately1 mA/cm2 was reached. The reverse scan showed currents whichwere smaller than those obtained in the passive forward scan. Theresults showed that, under the studied conditions, the polished NiTielectrode could successfully repassivate.

In the case of the NiTi archwire without an oxide film, the shapeof the CP curve was similar to that of the NiTi sheet, whereas theforward corrosion current density was lower (46 nA/cm2), and Ecorr

was slightly more positive (−0.267 V) than in the case of the NiTisheet. The passive current in the forward scan was similar to thatobtained for the NiTi sheet, whereas in the reversed scan it wassmaller than for the NiTi archwire. The breakdown potential wasobserved at 1.2 V (Table 1). The repassivation potentials for bothsamples were relatively high, i.e. 0.9 V for the sheet, and 0.86 V forthe archwire. Also, the polarization resistance, Rp, as a measure forcorrosion susceptibility, where the scan rates are lower (the curvesare not shown), was smaller in the case of the NiTi sheet without asurface oxide film, i.e. 0.824 M� cm2, than that obtained forthe NiTiarchwire without a surface oxide film, i.e. 1.19 M� cm2. Compar-ing together the two different samples without surface oxide films,it can be concluded that the archwire exhibited better corrosionproperties than the NiTi sheet.

For a realistic evaluation of the corrosion properties of a metalalloy, it is important to study its characteristics in the “as received”

state, with a surface oxide film present (Fig. 3b, Table 1). In the caseof the NiTi sheet with a surface oxide film, the corrosion potentialwas more negative (−0.21 V) than in the case of the NiTi archwirewith an oxide film (−0.14 V), although both were more positive

rp estimated from CP curves, � = 1 mV/s, Rp from linear polarization measurements,

Wire cross-section NiTi sheet withsurface oxide film

NiTi wire withsurface oxide film

−0.29 (±0.08) −0.21 (±0.01) −0.14 (±0.05)77 (±33) × 10−9 15 (±0.8) × 10−9 44 (±0.5) × 10−9

0.79 (±0.19) 2.6 (±0.2) 1.4 (±0.09)1.0 (±0.09) 1.3 (±0.005) 1.1 (±0.004)0.87 (±0.09) 0.32 (±0.02) 0.06 (±0.06)

T. Kosec et al. / Applied Surface Science 288 (2014) 727– 735 731

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ig. 4. Linear polarization measurements for the investigated NiTi sheet and archw

han the values obtained in the case of the samples without oxidelms, as described above. In the case of the NiTi sheet with a surfacexide film, the corrosion current density was as low as 15 nA/cm2

see the values in Table 1). The passive current densities are compa-able, whereas in the case of the NiTi sheet with a surface oxide film,he breakdown potential was much higher, at 1.3 V. Scanning in theeverse direction showed a negative hysteresis, which indicated theood repassivation properties of the investigated samples in thetudied environment and experimental conditions. In this case theepassivation potentials were lower (0.32 V for the NiTi sheet, and.06 V for the NiTi archwire) when compared to the samples with-ut an oxide film. The reverse current density obtained in the casef the archwire exhibited numerous oscillations in the anodic cur-ent path, possibly indicating a metastable passive state. It can beoncluded that the investigated NiTi sheet with a surface oxide filmhowed better corrosion properties than the NiTi archwire with anxide film. The results of the polarization resistance measurementsonfirmed this observation, the Rp of the NiTi sheet with a surfacexide film amounting to 2.57 M� cm2, and 1.38 M� cm2 in the casef the NiTi archwire with a surface oxide film.

Contrary to our results, higher values for the anodic current den-ities and a positive hysteresis in the reverse scan were reported byziz Kerrzo et al. [3], in the case of a NiTi archwire electrode. In thisase the cross-section of the investigated Ti–45Ni surface exhib-ted a pitting potential as low as 250 mV (vs. SCE) in a phosphateuffer solution at 37 ◦C. In another study by Rondelly, however,

ower pitting potentials (800 mV) were measured in a de-aeratedhysiological solution in the case of a flat NiTi specimen [1]. Theross sections of rods with two different dimensions (2 and 8 mm)ere tested, and it was found that the smaller diameter electrode

xperienced the onset of crevice corrosion at potentials of 250 mVs. SCE, and 1000 mV in the case of the larger specimens, both inank’s solution at 37 ◦C. The possible explanations for the differ-nt results obtained in the case of the NiTi alloy observed in [1,3]re the following: a higher experimental temperatures of 37 ◦C, aigher content of chlorides in the phosphate buffer solution (with

pH of 7.4), which contained 8.77 g/L NaCl, 1.42 g/L Na2HPO4 and.72 g/L KH2PO4), and in the cross-sectional surface of the exposedolished NiTi archwire used in [3]. The surface modification tech-iques appeared not to influence the repassivation kinetics, which

s contrary to the results obtained in our experiments where, in aimulated saliva containing 0.123 M of chlorides, good repassiva-ion characteristics were obtained in the case of both the polishediTi sheet and the polished archwire.

cross-section and longitudinally, in simulated saliva at a scan rate of 0.1 mV/s.

Besides susceptibility to crevice corrosion in the case of sam-ples mounted in resin and the susceptibility of NiTi alloy to pittingcorrosion [1], it is possible that microstructure could have a signif-icant effect on the corrosion properties. As observed in the case ofsteel wires in pore water [25], the delicate edges of microstructuralgrains, exposed in cross-section, are, from the point of view of cor-rosion, more susceptible than the elongated grains. For this reasonadditional experiments were performed, where the effect of thedifferent microstructures on the electrochemical properties of theused NiTi sheet and archwire was studied (Fig. 4).

Each of the curves presented in Fig. 4 represents six measure-ments that were performed on the cross-sectional and longitudinalsurfaces. It was shown that the surface polarization resistance of thearchwire was, in cross-sectional direction, smaller (0.79 M� cm2)than in the longitudinal direction (1.20 M� cm2). The polarizationresistance of the NiTi sheet was, in cross-section, 0.25 M� cm2, and0.82 M� cm2 in the case of the longitudinally exposed surface. Fromthe results of the metallographic observation it was shown that thenumber of inclusions in the longitudinal direction was smaller thanin cross-section, for both samples, i.e. the sheet and the archwire.These results indicate that, in cross-section, both the NiTi sheet andthe NiTi archwire have poorer corrosion characteristics than in thelongitudinal direction.

3.3. Tribocorrosion tests

Since, during use, NiTi alloy undergoes different types of wear,it is very important to study the alloy’s corrosion properties con-currently with its wear properties. In situ information about thesurface state of an NiTi alloy at open circuit potential can beobtained by combining wear and corrosion information, when testsare performed in a simulated saliva solution. The results of thecoefficient of friction (COF) and corrosion potential measurements(Ecorr) are shown, for the investigated NiTi specimens that weretested in simulated saliva at a sliding speed of 5 mm/s, and at dif-ferent normal loads, in Fig. 5.

In the case of the tribocorrosion experiments on the NiTi sheetwithout a surface oxide film (Fig. 5a), more negative changes in thecorrosion potentials were observed during sliding, as the normalload was increased from 1 to 5 N. As has been reported, lowering of

the open circuit potential is frequently experienced during slidingand wear, which is amplified by increasing normal load and/or slid-ing velocity [26]. Repassivation depends on the wear regime, andthe repassivation time is affected by the exposed, newly uncovered

732 T. Kosec et al. / Applied Surface Sc

Fig. 5. OCP potential and coefficient of friction vs. time for the NiTi sheet (a) withoutawa

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surface oxide film, and (b) with the surface oxide film, and for the archwire (c)ithout a surface oxide film against a Al2O3 ball, in simulated saliva, at different

pplied normal loads.

urface after wear. It was estimated to amount to 30 s at a normaload of 1 N, to 60 s at a load of 2 N, and to 100 s at a load of 5 N.

Similar friction behaviour was observed in the case of all theoads (Fig. 5a). The COF curves show two friction regimes; a run-ineriod, which is indicated by a rapid increase in the measured valuef the coefficient of friction, followed by a steady-state period. Inhe case of the NiTi sheet without a surface oxide film, which was

ested in the simulated saliva solution, the steady state friction coef-cient at a 1 N load was 0.8, and at higher loads 0.7. Furthermore, itas found that the higher the load was, the shorter was the run-ineriod.

ience 288 (2014) 727– 735

Fig. 5b shows the decrease in the measured corrosion potentialsfor the NiTi sheet with an as received oxide film rubbed againstan Al2O3 ball in the simulated saliva solution. The higher the nor-mal load, the larger was the decrease in the measured corrosionpotential during sliding. The initially lower measured OCP poten-tial for the sample tested at a 1 N normal load decreased by approx.200 mV, but relatively large fluctuations in the measured poten-tial were observed. It can be assumed that, at a normal load of1 N, sliding did not lead to damage of the oxide film. Once slid-ing had stopped, the open circuit potential returned to the initialequilibrium electrochemical potential in 20 s. In terms of reachingthe initial OCP potential, the period was the shortest in the case ofthe least severe contact conditions, when a normal load of 1 N (20 s)was applied, followed by a load of 2 N (70 s) and then 5 N (100 s).

The coefficient of friction was at the same level (0.7) as observedin the case of the NiTi sheet without an oxide layer, when the normalload was 2 or 5 N. However, the run-in period was shorter in thecase of the as received NiTi sheet samples with an initial oxide film(Fig. 5a and b).

Different friction behaviour was observed when a low normalload of 1 N was used. The coefficient of friction increased in thefirst cycles of sliding, then remained at a level of 0.6 for about 500 s,and then decreased again after ∼600 s of sliding. At the same time,an increase in the measured corrosion potential was detected. Thesame behaviour was observed in all of the repeated tests. Theseresults suggest that the applied normal load of 1 N does not lead toa complete removal of the surface oxide film, which can be restoredduring sliding, as indicated by the decrease in the coefficient offriction and the increase in electrochemical potential during sliding.

Due to problems with the fixing of the samples, only the NiTiarchwire without a surface oxide film was tested, using normalloads of 1 and 2 N. The results of the tribocorrosion study are pre-sented in Fig. 5c. During sliding, the friction behaviour and thecoefficient of friction were the same as those observed in the caseof the NiTi sheet, with a coefficient of 0.8 for the 1 N normal load,and 0.7 for a 2 N load. On the other hand, the oscillation of theelectrochemical potential during sliding was much smoother whencompared to the sheet samples, with the electrochemical potentialbeing lowered by 250 mV for the 1 N normal load, and 300 mV forthe 2 N load. The passive film recovered in 30 s after the sliding wasstopped.

3.4. Wear track analysis

The wear tracks were analysed by SEM, whereas the wear vol-ume loss was determined by profilometry measurement of thewear track (Figs. 6 and 7). Depending on the type and surface con-dition of the samples, the type of media (ambient air or simulatedsaliva), and the normal load, different wear tracks were observed.

Analysis of the wear tracks showed an increased wear trackwidth and depth as the applied normal load was increased. Accord-ing to the Hertz theory, the elastic point contact, the applied loadsof 1 N, 2 N and 5 N resulted in a calculated contact pressure pHand a calculated contact radius aH of about 765 MPa and 25.0 �m,965 MPa and 31.5 �m, and 1310 MPa and 42.7 �m, respectively.These calculated contact pressures are all below the yield strengthof the investigated NiTi alloy (1100 MPa for the NiTi sheet, and1960 MPa for the NiTi archwire). During tribological loading, nophase transformations were expected, nor were any deformationsthat would affect the estimated wear volume anticipated.

Two wear mechanisms, one being abrasion and the other delam-ination, were observed. During sliding the harder Al2O3 ball causes

abrasive wear in the form of parallel grooves, whereas repeatedloading leads to the formation and propagation of micro-cracks andto delamination of the NiTi surface. The hardness measurements ofthe as received NiTi plate outside (302 ± 4 HV) and inside the wear

T. Kosec et al. / Applied Surface Science 288 (2014) 727– 735 733

e film

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Fig. 6. Wear track corresponding to the NiTi sheet without an oxid

rack (340 ± 8 HV) confirmed that work hardening had taken placenside the wear track, as has been observed in the case of other

aterials [27]. With an increased load, the size of the delamina-ion wear particles increases, reaching sizes of up to 150 �m at aoad of 5 N (the results are not shown for all the loads). Althoughhe same wear mechanism was observed in the case of the tribo-orrosion tests and the tests performed in ambient air, the amount

f wear debris in the wear track was larger in the case of ambi-nt air (Fig. 5a). In the case of the NiTi archwire without a surfacexide film, a high average hardness of 403 ± 18 HV was measured,esulting in a shallower wear profile.

Fig. 7. Wear track corresponding to the NiTi sheet with an oxide film a

after sliding (FN = 5 N) in (a) ambient air and (b) simulated saliva.

Similar wear mechanisms and damage types have beenobserved in TiNi alloys which were investigated by means of slidingwear experiments in ambient air [11]. The results of EDS analysisof the investigated samples, inside and outside the wear track, areshown in Table 2. Only the results for the tribocorrosion tests per-formed on the NiTi sheet in simulated saliva, with and without asurface oxide film, are presented and discussed.

Outside the wear track, the surface with an oxide film consistedof Ti and Ni almost in atomic equilibrium, and 24.0% of oxygen(Table 2, Fig. 7b). Inside the wear track, the amount of oxygen wasincreased, and some traces of aluminium were also found, with the

fter sliding (FN = 5 N) in (a) ambient air and (b) simulated saliva.

734 T. Kosec et al. / Applied Surface Science 288 (2014) 727– 735

Fig. 8. The contribution of mechanical and corrosion wear to total wear in

Table 2EDS analysis at different spots after tribocorrosion testing in simulated saliva, nor-mal load 5 N (Figs. 6a and 7a).

Ti Ni O Al

NiTi sheet without surface oxide filmOutside the wear track – no. 1 50 50Inside the wear track – no. 2 42.5 43.0 13.8 0.7Wear particle – no. 3 18.8 18.7 52.5 10.0

NiTi sheet with surface oxide filmOutside the wear track – no. 1 38.8 37.2 24.0

batcotF

[a

V

wwtva

tvlfaTedae(tsavcl

Inside the wear track – no. 2 33.6 33.8 30.1 2.5Wear Particle – no. 3 19.0 17.9 54.1 9.0

iggest wear particles showing very high amounts of oxygen andluminium. This shows that wear of the Al2O3 counterbody alsoakes place during sliding. Similar observations were made in thease of the tests carried out at different loads, as well as in the casef the polished samples without an oxide film, the difference beinghat no oxides could be detected outside the wear track (Table 2,ig. 6b).

The wear rate was determined by measuring the wear volume28], so that the total wear is expressed as the sum of the mechanicalnd corrosion wear:

tot = Vmech + Vcorr (1)

The total wear is defined by the tribocorrosion, with the totalear volume being measured after the tribocorrosion experiments,hereas the mechanical wear volume is defined by tribological

ests in the absence of a corrosive environment. The corrosion wearolume was calculated from Eq. (1), and the observed wear ratesre presented in Fig. 8.

The corrosion wear, Vcorr, results in a loss in volume due tohe corrosion process in the simulated saliva, and in a loss inolume of the alloy due to synergistic effects during wear. Theoss of volume due to the corrosion process only was calculatedrom the electrochemical parameters, and amounted to values ofbout 0.1–1 × 10−13 cm3/s at different loads and surface conditions.his clearly demonstrates the fact that, during the tribocorrosionxperiments, the corrosion contribution to the total wear is pre-ominantly due to synergistic effects during the wear of the NiTilloy. As well as this, the effect of the microstructure in the tribo-lectrochemical experiments was minimal. At a low normal load1 N), the component of the corrosion wear was negligible, withhe wear rate in distilled water being equal to the wear rate inimulated saliva. At higher loads the mechanical wear still made

considerable contribution to the total wear rate, probably due toery good repassivation properties of the studied alloy. The highestontribution of chemical wear was detected in the case of a normaload of 5 N (64%) for the sample without a surface oxide film, and

the case of different loads and surface conditions of the NiTi plate.

at a load of 2 N in the case of the sample with an oxide film (45%),see Fig. 8. Thus, at higher loads corrosion wear predominates inthe case of the samples with an oxide film, whereas mechanicalwear predominates in the case of the samples without an oxidefilm. However, for all loads the total wear rate of the NiTi sheet washigher when a surface oxide film was present. Similar behaviourwas found in the case of 316L steel, where due to a large accumu-lation of strain at the passive potential, and the facilitation of crackformation due to microstructural defects, the wear rate was greaterthan in the case of the polished samples [29]. Thus, the wear rate isof great concern in the case of passive materials, such as NiTi alloyat open circuit potentials. This is in agreement with observationsmade on other passive materials [29], where higher wear rates havebeen observed at passive potential than at cathodic potentials. Theresults of the experiments described in this paper have shown theseobservations.

4. Conclusions

Metallographic, electrochemical, and tribocorrosion experi-ments, as well as surface analyses, were performed on twomicrostructurally different NiTi alloys with and without surfaceoxide films, in order to study the possible effect of the microstruc-ture on the chemical and physical properties of these alloys inartificial saliva. The results of cyclic polarization tests showed thatthe electrochemical parameters of the NiTi sheet with a surfaceoxide film showed better corrosion properties than those of theNiTi archwire, whereas the results of linear polarization measure-ments, where a particular study of the effect of the microstructurewas performed, showed that the cross-sections of both the wire andthe sheet were more susceptible to corrosion than the correspond-ing longitudinal surfaces. Further, tribocorrosion experiments onthe NiTi alloys showed that corrosion is accelerated when com-bined with mechanical wear, thus resulting in a higher total wearrate of the NiTi alloy in simulated saliva than in the absence ofa corrosive environment. In the total tribocorrosion wear of theNiTi alloy, the mechanical contribution prevailed in the case ofsurfaces with an oxide film, whereas the corrosion contributionprevailed in the case of surfaces without an oxide film. It can alsobe concluded that, if archwire is damages during handling, its cross-section would probably show poorer corrosion properties, leadingto more rapid corrosion and earlier mechanical failure. This finding,however, needs to be confirmed by additional studies.

Acknowledgments

The work described in this paper was sponsored by the Slove-nian Research Agency, Grant No Z2-2298.

The help of the laboratory staff, Viljem Kuhar and Monika Pros-enc, is hereby gratefully acknowledged. Thanks are also due to dr.

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