eects of ti/tin multilayer on corrosion resistance of ... · eects of ti/tin multilayer on...

Effects of Ti/TiN multilayer on corrosionresistance of nickel–titanium orthodontic brackets

in artificial saliva

Chenglong Liu a,b, Paul K. Chu a,*, Guoqiang Lin b, Dazhi Yang b

a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue,

Kowloon, Hong Kong, Chinab State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams,

Dalian University of Technology, Dalian 116024, China

Received 26 April 2006; accepted 27 March 2007Available online 23 May 2007

Abstract

The effects of multilayered Ti/TiN or single-layered TiN films deposited by pulse-biased arc ionplating (PBAIP) on the corrosion behavior of NiTi orthodontic brackets in artificial saliva are inves-tigated. The multilayered Ti/TiN coating is found to exhibit a greater free corrosion potential, muchlower passive current density, and no breakdown up to 1.5 V. Moreover, electrochemical impedancespectroscopy (EIS) results indicate that the multilayered Ti/TiN coating has a larger impedance andlower porosity which is believed to be responsible for the exceedingly low metal ion release rate dur-ing 720 h exposure in the test solution. Visual inspection of the surfaces reveals different corrosionprocesses for the TiN and multilayered Ti/TiN coatings.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Multilayered Ti/TiN coating; B. Metal ion release; B. Artificial saliva; C. Corrosion resistance

1. Introduction

Since the shape memory effect was first introduced in 1963, nickel–titanium (NiTi)alloys have been used in many biomedical applications such as surgical devices including

0010-938X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2007.03.041

* Corresponding author. Tel.: +852 27887724; fax: +852 27889549.E-mail address: [email protected] (P.K. Chu).

Corrosion Science 49 (2007) 3783–3796

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self-expanding vascular and urological stents, bone fracture fixation plates and nails,orthodontic and dental implants, and so on [1]. Dental implants are in direct contact withthe aggressive oral environment, and the interactions between the host tissue and foreignimplants play an important role in their long-term stability and lifetime. Good surfaceproperties and corrosion resistance are required when the implants are subjected to corro-sive physiological fluids in vivo. NiTi implants have been pre-processed (inactive, quies-cent) to form polycrystalline [2] or amorphous oxides [3] on the surface, but the resultsare not satisfactory. Su et al. found that NiTi cardiovascular stents with polycrystallineoxides had less corrosion resistance both in vitro [4] and in vivo [5]. In contrast, due tothe absence of crystalline defects such as grain boundaries, crystalline particles, and segre-gation, amorphous oxide layers fabricated on NiTi stent wires can impede chloride pene-tration in physiological fluids and retard electrochemical breakdown. However, the oxidelayers are typically too thin, fragile, and vulnerable to delamination [6]. If the film isdestroyed or broken through, ‘‘self-healing’’ of the passive film has been reported to beslow and difficult [7]. As a result, much effort is being made to improve the corrosion resis-tance and impede nickel release from NiTi orthodontic implants [8]. The cytotoxicity ofNiTi stent wires has been demonstrated using smooth muscle cells. However, if the con-centration of nickel leached from NiTi is higher than 9 ppm, proliferation of the musclecells is inhibited during the transition from the G0–G1 phase to S phase [9]. Jia et al. [8]have also indicated that nickel intake from repeated soft-tissue trauma may be the primesource of nickel-induced reactions in orthodontic applications.

Since the corrosion of dental NiTi implants is crucial to their long-term reliability andlifetime, a stable and corrosion-resistant passive layer is needed. Metal and non-metal ionimplantation [10,11], gas nitriding [12], as well as TiN and TiO2 coatings [6,10,13] havehitherto been employed to improve the surface properties of NiTi alloys. The TiN coatingspossess favorable characteristics with regard to load bearing, friction coefficients, wearresistance, chemical stability, electrical properties, and biocompatibility [14]. It also hasa gold color that is considered attractive to some orthodontic patients. However, the cor-rosion resistance of TiN films varies depending on the fabrication methods. It has beenfound that TiN coatings are highly effective in preventing corrosion provided that theyare thick enough to produce complete coverage [15]. TiN coatings deposited by arc ionplating are prone to localized attack in artificial saliva due to coating defects. The TiNcoating shows improved corrosion resistance in the low-potential region but suffers fromincreased pitting corrosion at potentials above 500 mV in 0.9% NaCl solution [12,14].Hence, the ability to produce a reasonably thick and defect-free protective coating on NiTiorthodontic brackets is the key to the improvement of the corrosion resistance.

Multilayered coatings have many virtues, especially the compact microstructure andgood mechanical properties. Therefore, they may be suitable materials for NiTi orthodon-tic materials. In the work described here, we produce Ti/TiN multilayered and single-layered TiN films by pulse-biased arc ion plating (PBAIP) on NiTi orthodontic bracketsand investigate their corrosion resistance in artificial saliva.

2. Materials and methods

Surgical orthodontic plates and brackets made of NiTi (50 at% Ni and 50 at% Ti) wereused in our experiments. The samples were electrochemically polished to obtain a smoothsurface in a (HF + HNO3) solution and then cleaned ultrasonically in acetone and etha-

3784 C. Liu et al. / Corrosion Science 49 (2007) 3783–3796

nol. The single-layered TiN and multilayered Ti/TiN coatings were deposited using aBulat-6 AIP pulse-biased arc ion plating (PBAIP) system [16]. The distance between thesamples and cathodic arc targets was about 400 mm. Prior to deposition, the substrate sur-face was cleaned by argon and titanium ion sputtering (900 V, 30% duty cycle, and40 kHz) for 5 min at a vacuum of about 0.4 Pa. Voltage pulses of �600 V with a duty cycleof 40% and frequency of 40 kHz were used for film deposition. Argon and nitrogen wereintroduced into the vacuum chamber to deposit the single-layered TiN film. For the depo-sition of the multilayered Ti/TiN coatings, an argon plasma was used to deposit the Ti lay-ers and deposition of the TiN layers was performed by using a mixture of argon andnitrogen. Deposition was performed at a nitrogen partial pressure of 0.34 Pa and argonpartial pressure of 0.5 Pa. The change from Ti deposition to TiN deposition was achievedby controlling the nitrogen flow without plasma interruption. The multilayered Ti/TiNspecimen is composed of 22 layers of Ti and 22 layers TiN. The total thickness of bothtypes of coatings is about 2 lm.

The phases of the coatings were identified by X-ray diffraction (XRD) using a Shima-dzu XRD-6000 diffractometer and CuKa irradiation (k = 0.154060 nm). The morphologyof the coatings was characterized by scanning electron microscopy (SEM) using a JEOL-JSM-5600LV. A Philip Tecnai G2-20 transmission electron microscope was used to studythe cross-sectional microstructure of the coatings.

The corrosion and electrochemical behavior of the uncoated and coated specimens werestudied in artificial saliva at a temperature of 37 ± 1 �C [14], and the compositions of theartificial saliva are listed in Table 1. A copper sheet was attached onto the uncoated side ofthe specimen which was cold-mounted by olefin with a monitored area of 1 cm2. The testswere carried out in a 0.5 l capped electrochemical cell under nitrogen in order to simulatethe oral environment. The experimental setup consisted of a conventional three-electrodecell comprising a working electrode, saturated calomel electrode (SCE), and platinumsheet as the counter electrode. The tests were conducted with a potentiostat M273EG&G. Changes in the free corrosion potential (Ecorr) as a function of time during anexposure time of abut 16 h were recorded.

After 0.5 h immersion in the test solution, a fairly stable potential could be achieved, andthen potentiodynamic polarization tests were conducted employing a scanning rate of1 mV/s. The initial potential was 100 mV below Ecorr. The corrosion current density (icorr)was estimated by a linear fit and Taffel extrapolation to the cathodic part of the polarizationcurve. Afterwards, the specimens were examined by SEM to detect surface pitting.

Electrochemical impedance spectroscopy (EIS) measurements were carried out by useof GAMRY PCI4/300 at a stable open circuit potential. The perturbing signal had an

Table 1Content of the artificial saliva

Content Concentration (mg/100 ml)

Sodium chloride 0.844Potassium chloride 1.2Calcium chloride anhydrous 0.146Potassium phosphate dibasic 0.34Sorbitol solution 70% 60Methyl paraben 2Hydroxyethyl cellulose 3.5Hydrochloric acid To control the pH value

C. Liu et al. / Corrosion Science 49 (2007) 3783–3796 3785

AC amplitude of 15 mV and frequency range from 3 · 104 Hz to 10 mHz. The data wasanalyzed through the software Gamry Echem Analyst.

In order to study the effects of the coatings on metal ion release, the coated anduncoated NiTi specimens were placed in 100 ml of artificial saliva at 37 ± 1 �C. The sur-face area of each specimen was approximately 10 cm2. Evaporation of the test solutionwas compensated by the addition of distilled water. After 720 h, the amount of Ti andNi ions leached into the solution was measured utilizing graphite-furnace atomic absorp-tion spectrometry (Shimadzu AA6650).

3. Results

Fig. 1 exhibits the Ti/TiN or TiN coated NiTi orthodontic brackets with a brightgolden color. The coatings appear to be quite uniform and the thickness is about 2 lm.The cross-sectional electron diffraction patterns acquired from the multilayered Ti/TiNfilms are shown in Fig. 2(c). The multilayered films show polycrystalline patterns thatcan be indexed as FCC structures composed of pure Ti and TiN phases with the latticeconstants that can be calculated from the X-ray diffraction (XRD) patterns depicted inFig. 3. The XTEM micrograph of the multilayered Ti/TiN film in Fig. 2(b) shows distinctalternating layers of Ti and TiN. Compared to the TiN microstructure in Fig. 2(a), thestructure of the multilayered film is less columnar and more compact and has fewer defectssuch as pinholes.

Fig. 4 displays the Ecorr versus time curves obtained in the in vitro simulated oral envi-ronment. Different corrosion processes occur in the coated and uncoated specimens. Themultilayered Ti/TiN coated specimen shows a stable free corrosion potential that is themost positive during the 15 h exposure in the test solution. Throughout the entire immer-sion period, the corrosion process pertaining to the TiN coated specimen comprises twostages. The first stage which lasts from the beginning to about 9 h is characterized by fluc-tuating Ecorr values. The second stage which is from 9 h to the end of the test shows morestable Ecorr values that decrease monotonically. The Ecorr of the TiN coated specimen after

Fig. 1. Visual morphology of the multilayered Ti/TiN coated NiTi bracket with bright golden color. (Forinterpretation of the references in colour in this figure legend, the reader is referred to the web version of thisarticle.)


about 16 h of immersion is less than that of the uncoated NiTi alloy. The Ecorr values after16 h immersion are displayed in Table 2.

Typical potentiodynamic polarization curves obtained from the coated and uncoatedspecimens are presented in Fig. 5. The multilayered specimen is observed to be spontane-ously passive in the artificial saliva, whereas the uncoated and TiN coated specimens are inthe active state of Ecorr in the anodic environment. The current density within the passiverange for a potential of 0.4 V is higher for the uncoated specimen than the coated one.Following further polarization towards the anodic direction, the breakdown potential(Ebrk) determined from the uncoated NiTi alloy is approximately 0.489 V. When thepolarization potential is increased to about 0.818 V, the TiN coated NiTi alloy shows apitting behavior as manifested by the rapid increase in the corrosion current density. In

Fig. 2. XTEM morphology of: (a) TiN, (b) multilayered Ti/TiN, and (c) SAED of the multilayered Ti/TiN films.The arrows show the columnar structure.

Fig. 3. XRD patterns obtained from the two coatings produced by PBAIP.


comparison, the multilayered Ti/TiN coated specimen is not broken at a polarizationpotential up to 1.5 V. In addition, a large hysteresis loop appears in the uncoated andTiN coated specimens, indicating that they suffer intense corrosion attack and have alower tendency for repassivation in the artificial saliva, which is also proven by the vari-ation of Eback. These results impart that the multilayered Ti/TiN coating enhances the cor-rosion resistance of the NiTi alloy significantly.

Figs. 6 and 7 show the Bode plots of the EIS spectra generated from the coated samplesin artificial saliva. In the multilayered Ti/TiN coated specimen, logjZj is linear with log f,which is symptomatic of a predominantly capacitive behavior at the electrode/solutioninterface [17]. In the frequency range of 102–10�2 Hz, h has values close to 80�. At a highfrequency, the phase angle shift is close to 0�, and ohmic resistance dominates the imped-ance. Hence, it can be concluded that the multilayered Ti/TiN coated sample is in the pas-sive state in the simulated oral environment [12], while the TiN coated specimen shows anactive behavior. Before 24 h exposure in the test solution, the EIS spectrum acquired fromthe TiN coating shows only one time constant. However, after 24 h, a second time con-stant related to the solution/substrate interface via pinholes can be resolved. To studythe efficacy of the coatings against out-diffusion, the amounts of nickel and titanium

Fig. 4. Variation of the corrosion potential (Ecorr vs. SCE) as a function of time in artificial saliva.

Table 2Electrochemical parameters obtained from the uncoated and coated specimens in artificial saliva

Samples Ecorr, V (vs. SCE) Ebrk, V (vs. SCE) Eback, V (vs. SCE) ia at 0.4 V, lA cm�2

Uncoated �0.519 0.489 �0.356 1.709TiN �0.545 0.818 0.349 0.622Ti/TiN �0.049 – – 0.081

Notes: Ecorr: free corrosion potential after 16 h; Ebrk: breakdown potential; Eback: potential for passivation fromthe transpassive into the passive region (cathodic polarization).


leached from the coated and uncoated brackets after 720 h of immersion are determinedfrom the solution and listed in Fig. 8.

4. Discussion

Our objective is to conduct the corrosion tests in an environment that resembles that inthe mouth so that useful data can be obtained from the coated and uncoated brackets for

Fig. 5. Potentiodynamic polarization curves of the uncoated and coated specimens in artificial saliva.

10-2 10-1 100 101 102 103 104 105

101

102

103

104

105

1 h 6 h 24 h 120 h 720 h

|Z|,

ohm

.cm

2

Frequency, Hz

-Phase Angle, deg

0

20

40

60

80

100

Fig. 6. Bode plots of the multilayered Ti/TiN coated specimen in artificial saliva after different time.


comparison. Materials which behave in a truly passive manner are usually expected toshow increases in the free corrosion potential with immersion time as the passive filmrestrains the reaction between the electrolyte and materials and any sharp decrease andincrease in the potential usually indicate film breakdown and repair. The potentialsobserved in the multilayered Ti/TiN specimen exhibit a general trend in which the poten-tials rise slightly in the first few hours and then stabilize. In contrast, sudden or sharpchanges in the potentials are observed from the TiN coated sample, and the fluctuationsare even more pronounced than those observed from the untreated control specimen.

10-3 10-2 10-1 100 101 102 103 104 105

101

102

103

104

105

1 h 6 h 24 h 120 h

Frequency, Hz

|Z|,

ohm

.cm

2

-20

0

20

40

60

80

100

-Phase Angle, deg

Fig. 7. Bode plots for the TiN coated specimen in artificial saliva after different time.

Fig. 8. Release rate of metal ions into artificial saliva after 720 h immersion at 37 ± 1�C (ND = not detectable).


Similar fluctuations in the potentials with time in saline solutions have been reported forindented CrN coated Co–Cr–Mo alloys [15]. The fluctuations indicate that the single-layered TiN coating cannot provide adequate protection in the artificial saliva. Further-more, defects such as micro-metal droplets and micropoles can result in the penetrationof the solution leading to localized corrosion of the NiTi alloy.

The TiN coated specimen shows a greater degree of metal ion release than the uncoatedspecimens (Fig. 8). Our results are consistent with those previously reported [14]. The poorcorrosion protection may be contributed to the columnar microstructure, micro-particles,and pinholes in the coatings that are observed in Fig. 2(a). In contrast, almost undetect-able amounts of Ti and Ni (Ti, 0.05 mg/cm2 h; Ni, not detectable) are observed from themultilayered Ti/TiN coating after 720 h immersion in artificial saliva. The Ni release levelis far lower than that of the dietary intake [18] and appears to be safe for clinical use. Thelower metal ion release rate corresponds to the stable and higher free corrosion potential.Thus, the coating can effectively suppress leaching which can cause galvanic corrosion inthe coating or between the coating and substrate.

Compared to the uncoated specimen, the positive scan of the curves obtained from thecoated specimens show a wider region of passivity that is characterized by lower corrosioncurrent densities at the same over-potential. In Table 2, the corrosion current density (at0.4 V) of the TiN coated sample is about one order of magnitude smaller than that of theuncoated specimen, whereas that of the multilayered Ti/TiN coating is approximately 1/21of that of the uncoated specimen. After changing the polarization direction, the Eback

value of the TiN coated specimen is greater than that of the uncoated specimen. In theTiN coating, at intermediate potentials in the range of �0.2 V to 1.3 V, electrochemicaloxidation takes place leading to the formation of titanium oxynitride, oxide, and nitride[19]. Consequently, the electrochemical nitrogen dissolution reaction occurs and it canbe represented as

½N� þ 4Hþ þ 3e! NHþ4

The formation of ammonium ions, NHþ4 , consumes protons and thereby increases the pHin the pits and promotes metallic repassivation [20]. The data indicate improved corrosionresistance of the coated samples which can be further verified by long-term corrosion testsusing electrochemical impedance spectroscopy.

The EIS spectra should reveal the impedance information in the coated systems exposedto the test solution, and the variations in the impedance and phase angle indicate theresponse from both coatings and substrate areas to coating defects. The EIS spectra ofthe coated specimens in Figs. 6 and 7 are usually interpreted by using a simple model con-sisting of a constant phase element (CPE) in parallel with the polarization resistance, i.e.Rs(RpQ) [21]. The CPE is used to account for the non-ideal capacitive response instead of acapacitance. The impedance of CPE is given by the relationship: ZCPE = 1/[Y0(j-)g], whereY0 is the admittance magnitude of the CPE and g is the power index number. It is wellknown that the CPE is affected from the properties of the coating such as surface rough-ness, porosity, and so on [21,22]. Fig. 9 shows the fitted results obtained from the aboveequivalent circuit from the coated specimens in the test solution. The Y0 values of the TiNcoated specimen increase with time in the initial 96 h indicating that the exposed coatingarea increases due to uncovering of porosity. However, the values decrease after 120 himmersion because rust begins to block the pinholes [22] leading to some loss in the cor-rosion protection. Meanwhile, the changes of the porosity ratio (a) with time also accounts


for it. Creus described that the porosity of the protective coatings was an important factorfor effective corrosion protection [23]. During initial immersion (for example, 1 h), a isequal to 0.923, which is close to 1, suggesting a strong capacitive response from the solu-tion/coating. There exists obvious decrease in the a values as time increases, implying thatthe capacitive interface is wakened to a great extent. Due to the more abundant pinholes,the NiTi substrate suffers from more attacks at the coating/substrate interface. Comparedto the TiN coated specimen, the variation in the Y0 and a values determined from the mul-tilayered coating with time is relatively consistent. The small decrease in the a values from

Fig. 9. EIS fitted results for the coated specimens in artificial saliva: (a) TiN and (b) multilayered Ti/TiN.


about 0.943 during the initial immersion may be due to the interaction between the coatingand test solution [17,22]. Besides, after exposure of the multilayered Ti/TiN coating for720 h, the a value is still larger than 0.88, suggesting that the coated specimen undergoesinsignificant changes during immersion. It can be inferred that the multilayered Ti/TiN

Fig. 10. SEM images of: (a) TiN, and (b) multilayered Ti/TiN after 720 h immersion in artificial saliva (localizedcorrosion is indicated by arrows).


specimen has higher corrosion resistance in artificial saliva. The results are consistent withthe Ecorr and potentiodynamic polarization tests results described in the previousparagraphs.

Fig. 10 depicts the SEM pictures after the EIS tests. Coating defects such as macro-particles possessing a spindle or round shape, which are characteristic for the AIP method[16], can be observed in the coatings but no defects that propagate through the entire thick-ness can be found. Fig. 10(a) indicates severe localized corrosion around the macro-parti-cles causing cracking of the TiN coating. In comparison, no cracking is observed in themultilayered Ti/TiN coating after 720 h immersion, but several developing pits manifestingas black points (indicated by arrows in Fig. 10b) emerge. The EDS results obtained fromthe black points show the existence of titanium and oxygen, indicating that after 720 hexposure, there is no pitting corrosion in the NiTi substrate coated by the multilayeredTi/TiN structure.

Our electrochemical measurements indicate that the corrosion process in the TiNcoated samples has different stages. When the test solution reaches the pinholes in thecoatings, because of the difference in the binding energy and chemical compositionbetween the coating matrix and droplet or substrate, galvanic corrosion cells are estab-lished [22]. With the development of localized corrosion, corrosion products includingoxide compounds [24] and calcium compounds could block the micropores temporarily.This process retards corrosion, as proven by the increase in a at about 100 h. However,the natural characteristics of the TiN coating cause corrosion failure mainly caused bythe autocatalytic effect in the test solution [25].

Compared to the TiN coating, the columnar structure in the multilayered Ti/TiN coat-ing is disordered with fewer defects such as micropores and pinholes (Fig. 2). The multi-layered structure can redistribute the current flow [26] to eliminate current concentrationat the small pinholes and prevent rapid galvanic attack at the pits. In addition, the alter-nating structure suppresses penetration of the test solution, and therefore, the long-termelectrochemical stability of the coated samples is improved. Moreover, the compact struc-ture allows the corrosion rust to plug the micro-corrosion holes more efficiently. Thus,based on our results, the main reasons why the multilayered Ti/TiN coating offers bettercorrosion resistance in a simulated oral environment are: (1) The compact structure makesthe coating less permeable to the corrosive solution inhibiting oxygen diffusion and theformation of occluded cells. (2) The multilayered structure causes the redistribution of cor-rosion current due to mild interphase corrosion, and so corrosion current concentration atsmall pinholes is not as severe.

5. Conclusion

The effects of a multilayered Ti/TiN coating on the corrosion resistance of orthodonticbrackets made of NiTi are investigated in artificial saliva. The cross-sectional morphologyshows that the multilayered Ti/TiN coating has a more compact structure compared tothat of the single-layered TiN coating. In the potentiodynamic polarization tests, nobreakdown can be observed from the multilayered Ti/TiN coated specimen in the simu-lated oral environment, suggesting that the coating has greater corrosion resistance andpassivation stability. The corrosion tests indicate that the multilayered Ti/TiN coatinghas a higher electrochemical stability than the TiN coating in the test solution. However,with increasing immersion time, the changes in the Ecorr, Y0, and a values differ for the two


coatings leading to different corrosion processes. The first stage is characterized by the cor-rosive solution penetrating the pinholes of the coating causing localized corrosion,whereas the second stage is the ‘‘plugging’’ of the pinholes by rusts.

Acknowledgments

The project is supported by the National Natural Science Foundation of China underContract Nos. 50081001, 5039060 and NAMCC 863 (No. 2002AA326010) as well as CityUniversity of Hong Kong Applied Research Grant (ARG) No. 9667002.

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