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Effect of nitriding surface treatment on the corrosion resistance of dental nickel–titanium files in 5.25% sodium hypochlorite solution
Journal of Alloys and Compounds 475 (2009) 789–793
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of nitriding surface treatment on the corrosion resistance of dental nickel–titanium files in 5.25% sodium hypochlorite solution Jeng-Fen Liu a,b , Mau-Chin Lin a,c , Ming-Lun Hsu a , Uei-Ming Li d , Chun-Pin Lin e , Wen-Fa Tsai f , Chi-Fong Ai f , Li-Kai Chen g , Her-Hsiung Huang a,g,h,∗ a
Department of Dentistry, National Yang-Ming University, Taipei, Taiwan Department of Dentistry, Taichung Veterans General Hospital, Taichung, Taiwan c Department of Dental Laboratory Technology, Central Taiwan University of Science and Technology, Taichung, Taiwan d Dental Department, Cardinal Tien Hospital, Hsintien, Taiwan e Department of Dentistry, National Taiwan University, Taipei, Taiwan f Institute of Nuclear Energy Research, Atomic Energy Council, Taoyuan, Taiwan g Department of Dentistry, Taipei City Hospital, Taipei, Taiwan h Institute of Oral Biology, National Yang-Ming University, Taipei, Taiwan b
a r t i c l e
i n f o
Article history: Received 11 March 2008 Received in revised form 1 August 2008 Accepted 7 August 2008 Available online 11 October 2008 Keywords: Dental alloy Nitriding Nickel–titanium file Sodium hypochlorite Corrosion resistance
a b s t r a c t This study investigated the effect of nitriding surface treatment on the corrosion resistance of commercial dental alloy, in the form of helical nickel–titanium (Ni–Ti) files, when treated with 5.25% sodium hypochlorite (NaOCl) solution. The surface of dental helical Ni–Ti files was modified using nitriding treatment at 200 ◦ C, 250 ◦ C and 300 ◦ C in an NH3 -containing environment. The surface morphology and chemical composition of the Ni–Ti files were analyzed using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The corrosion resistance of the Ni–Ti files when treated with a clinical solution of 5.25% NaOCl was evaluated using the linear polarization method and by potentiodynamic polarization curve measurement. The nitriding treatments at different temperatures created titanium nitride (TiN) on the surface of the helical Ni–Ti files. The Ni–Ti files nitrided at 200 ◦ C and 250 ◦ C showed higher polarization resistance and higher passive film breakdown potential together with a lower passive current than untreated files. The presence of TiN on dental Ni–Ti files significantly increased the corrosion resistance of the files in the presence of 5.25% NaOCl solution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In dental clinical applications, the advent of engine-driven nickel–titanium (Ni–Ti) alloy files with a super-elastic property has altered dental root canal treatment by decreasing operator fatigue, clinical preparation time, and procedural errors related to hand operation [1]. During Ni–Ti instrument use throughout dental root canal treatment, corrosive sodium hypochlorite (NaOCl) is widely used as a root canal irrigant and lubricant. Some previous research has investigated corrosion of commercial dental Ni–Ti files in the presence of NaOCl solution [2–4]. Some researchers have reported
∗ Corresponding author at: Department of Dentistry, School of Dentistry, National Yang-Ming University, No. 155, Sec. 2, Li-Nong Street, Taipei 112, Taiwan. Tel.: +886 2 2826 7068; fax: +886 2 2826 4053. E-mail address: [email protected] (H.-H. Huang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.025
that pitting or crevice corrosion is not likely to occur when Ni–Ti files are treated with 5.25% NaOCl irrigation solution [2]. On the other hand, other researchers found that the corrosion resistance of different Ni–Ti files in 5.25% NaOCl solution varied significantly between brands [3] and that the corrosion of Ni–Ti files can be decreased by lowering the NaOCl solution’s pH to about 10 [4]. Recently, the influence of NaOCl on the overall mechanical properties of Ni–Ti files has been reported [5–8]. One study investigated the effect of cleaning procedures, including immersion in NaOCl solution, on the fracture properties and corrosion of Ni–Ti files. The results showed that files immersed in 1% NaOCl overnight display a variety of corrosion patterns although NaOCl does not significantly reduce the torque at fracture [5]. However, it has been found that immersion in 5% NaOCl solution significantly increases the incidence of Ni–Ti file fracture from cyclic fatigue [6]; additionally, it has been found that Ni–Ti files show reduced resistance to cyclic fatigue after contact with 5.25% NaOCl solution [8]. Therefore, our
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J.-F. Liu et al. / Journal of Alloys and Compounds 475 (2009) 789–793
Fig. 1. Appearance of the commercial helical dental Ni–Ti file used in this study.
hypothesis is that a surface treatment that significantly decreases file corrosion has the potential to lower the incidence of Ni–Ti file fracture during dental clinical use. It is well known that corrosion may be caused by an electrochemical reaction that occurs between the metal surface and corrosive environment. Therefore, from the clinical point of view, a surface treatment has the potential to improve the corrosion resistance of a Ni–Ti file surface without decreasing the superelastic character of file itself. Lately, extensive studies have focused on the effects of different surface treatments on the mechanical properties of commercial dental Ni–Ti files and these properties include wear resistance, cutting efficiency and fracture behavior [9–17]. However, related information concerning the surface effects of such treatments on commercial dental Ni–Ti files and whether this improves the corrosion resistance in clinical irrigation solution is limited from the literature. In a previous study, nitriding treatment has been proved to be a promising method of increasing wear resistance and cutting efficiency of dental Ni–Ti files [17]. In this study, nitriding surface treatment at various temperatures was applied to commercial dental Ni–Ti files and the corrosion resistance of the resulting nitrided Ni–Ti files in clinical irrigation solution, 5.25% NaOCl, was quantitatively evaluated.
assisted design (CAD) software (Pro/ENGINEER Wildfire 3.0, Parametric Technology Corporation, Needham, MA, USA). In this study, the exposed area (i.e. the cutting flute region) of file surface for corrosion tests was calculated as 0.33629751 cm2 . The corrosion resistance of the test specimens was evaluated using their linear polarization curve from −10 to +10 mV (versus corrosion potential) with a scan rate of 0.1 mV/s after dipping the files into the test electrolyte for one hour. The polariza-
2. Experimental procedures Commercial helical ProFile® (0.04 taper with #25) Ni–Ti dental alloy files (Dentsply Mailleffer, Ballaigues, Switzerland) (Fig. 1) were chosen for the nitriding treatment, with a NH3 flow rate of 100 ml/min, at controlled temperatures for four hours. Details of the nitriding procedures have been described elsewhere [17,18]. Three nitriding groups, obtained at 200 ◦ C, 250 ◦ C and 300 ◦ C, were designated as group-200, group-250 and group-300, respectively. The untreated commercial helical Ni–Ti files were designated as group-un. The surface morphology and chemical analysis of the Ni–Ti files before and after nitriding treatments were analyzed using scanning electron microscopy (SEM) (JSM-6500F, JEOL, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS) (Microlab 350, VG Scientific, Oxford, UK), respectively. The surface analyses and following corrosion tests of the files have been carried out in less than three weeks after nitriding treatments. A potentiostat (AUTOLAB PGSTAT 30, Eco Chemie BV, Utrecht, Netherlands) was used to perform the corrosion tests, which consisted of linear polarization curve and potentiodynamic polarization curve measurements, which are explained later in this section. The nitrided surface, that is the whole cutting flute surface of Ni–Ti files, was chosen as the corrosion test region. During the corrosion tests, the untested surface, that is the surface excluding the cutting flute surface of Ni–Ti files, was isolated using alkali-resistant epoxy resin. A saturated calomel electrode (SCE) and a platinum sheet were used as the reference electrode and counter electrode, respectively. A clinical irrigation solution of 5.25% NaOCl at pH 12 was used as the corrosion test electrolyte and maintained at 37 ◦ C. The exposed area of file surface for corrosion tests is described as follows. The cutting flute region of Ni–Ti file (Fig. 1) was scanned at 18-m interval in a microcomputed tomography (Micro-CT) machine (Skyscan 1076, Micro Photonics Inc., Allentown, PA, USA) to attain a real three-dimensional geometry of the file. Then a three-dimensional model of cutting flute region of file was reproduced in computer-
Fig. 2. X-ray photoelectron spectroscopy analysis results for the Ni–Ti files with and without nitriding treatments: (a) full spectra; (b) spectra for N1s (as TiN).
J.-F. Liu et al. / Journal of Alloys and Compounds 475 (2009) 789–793
791
Fig. 3. Scanning electron microscope observations of the tested Ni–Ti files before the corrosion test: (a) group-un; (b) group-200; (c) group-250; (d) group-300.
tion resistance (Rp ), which is inversely proportional to the corrosion rate, is defined as the slope of the potential versus the current near the corrosion potential in the linear polarization curves [19]. The properties of surface passive film on the metal file play an important role in the corrosion resistance of metal. Therefore, the properties of surface passive film on the tested files were also evaluated in this study using potentiodynamic polarization curve measurements. This is able to provide two corrosion resistance parameters, namely the passive film breakdown potential (Ebd ) and passive current (Ipass ) [19]. The potentiodynamic polarization curves were measured from −0.3 V (SCE) in the anodic direction with a scan rate of 1 mV/s after dipping the files into the electrolyte for one hour. The scanning potential was stopped on reaching an anodic current of 10−3 A/cm2 . When the applied potential is higher than the Ebd , the protective passive film breaks down and as a result the anodic current increases significantly. The number of specimens for group-un, group-200, group-250 and group-300, was 12, respectively. For each nitriding condition (including group-un), the number of specimens for linear polarization curve and potentiodynamic polarization curve measurements was five, respectively.
3. Results and discussion 3.1. Surface characteristics Fig. 2 shows the X-ray photoelectron spectroscopy analysis results of a Ni–Ti file with and without nitriding treatments. It showed that TiO2 and small amount of NiO were the main surface oxides of the untreated dental Ni–Ti file (Fig. 2(a)). Similar results have been reported in other study: TiO2 and NiO (trace amount) are the main surface oxides in Ni–Ti alloys [20]. For the nitrided Ni–Ti files, an additional compound, titanium nitride (TiN), could be detected on the file’s surface along with the TiO2 and trace amount of NiO as shown in Fig. 2(b). Furthermore, the presence of TiN on the surface of the Ni–Ti files gave the file surface a light golden-yellow color. As the nitriding temperature increased, this led to a deeper color for the files. However, the surface morphology of the tested Ni–Ti files with or without nitriding treatments showed no signif-
icant difference when analyzed by scanning electron microscopy (Fig. 3), which implies that the nitriding treatment did not change the surface morphology of these commercial dental Ni–Ti files. Table 1 shows the X-ray photoelectron spectroscopy quantitative analysis results, demonstrating the atomic percentage of Ti, Ni, N, and O (as balance) and the atomic ratio of N/Ti ratio on the surface of the Ni–Ti files with and without nitriding treatments. For all the tested Ni–Ti files, the Ni content on files’ surface was nearly undetectable. For nitrided Ni–Ti files, the Ti content on files’ surface was approximately the same, but a higher N content was detected as the nitriding temperature increased. Therefore, the N/Ti ratio increased with increasing the nitriding temperature. This may imply that a higher content ratio of TiN to TiO2 was obtained on the nitrided Ni–Ti files with a higher nitriding temperature. During the nitriding treatment of the three groups of files, the N atoms diffused into the files reacting with Ti atoms on the file’s surface. This atomic diffusion and interaction process should not change the file’s surface morphology. This is able to explain the fact that the TiN formed on the outermost surface of the Ni–Ti files after the nitriding treatment did not result in any detectable morphological changes. Furthermore, it has been also reported that Ni–N compounds are unstable as compared to TiN. Thus, the formation
Table 1 X-ray photoelectron spectroscopy analysis results for the Ni–Ti files with and without nitriding treatments, showing the atomic percentage of four major elements (Ti, Ni, N and O as balance) and atomic ratio of N/Ti
Group-un Group-200 Group-250 Group-300
Ti
Ni
N
O
N/Ti ratio
35 32 31 31
<1 <1 <1 <1
0 6 9 10
Balance Balance Balance Balance
0 0.19 0.29 0.32
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J.-F. Liu et al. / Journal of Alloys and Compounds 475 (2009) 789–793
Table 2 Polarization resistance (Rp , ), obtained from the linear polarization curves of the Ni–Ti files tested in 5.25% NaOCl solution (note: standard deviations are given in parentheses)
Rp
Group-un
Group-200
Group-250
Group-300
6.24 × 10 (6301)
6.41 × 10 (7017)
1.09 × 10 (1204)
1.35 × 105 (2431)
4
4
of TiN is thermodynamically easier than that of Ni–N compounds [21]. This may partially explain why TiN instead of Ni–N compound was detected on Ni–Ti file surface after nitriding treatments. For nitriding procedure using NH3 atmosphere in this study, there is no divergent interface between the bulk substrate and the treated surface layer. This is a very crucial point for Ni–Ti file in dental clinical applications because the modified surface layer must have potential to follow the transformation of the bulk materials and prevent from the occurrence of delamination [17,18]. Furthermore, the use of NH3 atmosphere for nitriding treatment is also a simpler and cheaper nitriding procedure as compared to some other nitriding procedures [22–25]. However, the difference in the surface microstructure and composition of the Ni–Ti file after different nitriding procedures is another important topic but not the purpose of this study. Therefore, this needs further investigations. 3.2. Corrosion tests Table 2 shows the polarization resistance (Rp , ) obtained from the linear polarization curves of the tested Ni–Ti files in 5.25% NaOCl solution. The Rp ranking was as follows: group-300 (1.35 × 105 ) > group-250 (1.09 × 105 ) > group-200 (6.41 × 104 ) > group-un (6.24 × 104 ). Linear polarization curve measurement is a fast and simple corrosion test method that can provide a quantitative corrosion parameter Rp , which is inversely proportional to the corrosion rate [19]. A higher Rp represents a better corrosion resistance. In this study, the nitrided files all had a better corrosion resistance than the untreated file in terms of Rp , which was related to the presence of TiN on the nitrided files. It should be noted that the nitrided files with a higher N/Ti ratio showed a better corrosion resistance. This implies that a higher
Fig. 4. Potentiodynamic polarization curves of the tested Ni–Ti files, with and without nitriding treatments, in 5.25% NaOCl solution.
5
content ratio of TiN to TiO2 on the nitrided files was beneficial to the corrosion resistance, in terms of Rp , in 5.25% NaOCl solution. Fig. 4 shows the potentiodynamic polarization curves of the tested Ni–Ti files in 5.25% NaOCl solution. Regardless of the nitriding treatment condition, all the tested Ni–Ti files in 5.25% NaOCl solution revealed passive and transpassive regions. As compared to the untreated Ni–Ti file, all the nitrided Ni–Ti files had a lower passive current, but only the Ni–Ti files with lower nitriding temperatures (200 ◦ C and 250 ◦ C) had a higher passive file breakdown potential. This implies that using suitable nitriding treatment conditions could increase the corrosion resistance, in terms of passive film properties, of commercial dental Ni–Ti files in simulated clinical environment. A more detailed discussion on the parameters for passive file properties is described below. Table 3 shows the passive film breakdown potential (Ebd , mV) and passive current (Ipass , A/cm2 ) obtained from potentiodynamic polarization curves of the tested Ni–Ti files in 5.25% NaOCl solution. The Ebd ranking was as follows: group-200 (664 mV) > group250 (597 mV) > group-un (552 mV) > group-300 (513 mV). The Ipass ranking was as follows: group-un (5.56 A/cm2 ) > group200 (5.05 A/cm2 ) > group-250 (4.04 A/cm2 ) > group-300 (2.11 A/cm2 ). The properties of surface passive film on metal file’s surface play an important role in the corrosion resistance of metals. A higher Ebd represents better passive film stability. The Ipass is related to the charge transfer resistance through the surface passive film that forms spontaneously on corrosion resistant metal. A lower Ipass represents higher charge transfer resistance through the passive film. Basically, a protective passive film should have higher Ebd and lower Ipass . In this study, the nitrided group-200 and group-250 files, with the main surface compounds of TiN and TiO2 , had better passive film stability and a higher charge transfer resistance as compared to the untreated file, with the main surface compound of TiO2 only. The fact that group-300 file had the lowest Ipass (or higher charge transfer resistance) among the tested Ni–Ti files was believed to be related to the presence of higher TiN content on file surface. On the other hand, the stability (in terms of Ebd ) of group-300 file was the worst, which was probably due to the presence of surface defects formed at a higher nitriding temperature. However, this needs further investigations. From clinical application point of view, it is necessary that the influence in a clinical environment of any surface treatment on the corrosion resistance of Ni–Ti files be understood. Different surface treatment techniques have been used to improve the cutting efficiency, wear resistance and/or fatigue resistance of dental Ni–Ti files [9–17]. However, up to now, very limited information has been available in the literature concerning the corrosion resistance of surface-treated commercial Ni–Ti files in clinical environments. A previous study found that nitrided Ni–Ti files have better wear resistance and cutting efficiency than the untreated ones [17]. In this study, a further attempt was made to investigate the effect of nitriding treatment on the corrosion resistance of Ni–Ti files in a clinical environment. It was clear that the presence of TiN on commercial Ni–Ti files after nitriding treatments increased the files’ corrosion resistance, which is indicated by higher Rp and Ebd values and lower Ipass values, as compared to files without nitriding. Furthermore, the nitriding treatment used in this study did not microscopically change the surface morphology of commercial dental Ni–Ti files. Although the group-300 file had the highest Rp
J.-F. Liu et al. / Journal of Alloys and Compounds 475 (2009) 789–793 Table 3 Passive film breakdown potential (Ebd , mV) and passive current (Ipass , A/cm2 ), obtained from the potentiodynamic polarization curves of Ni–Ti files tested in 5.25% NaOCl solution (note: standard deviations are given in parentheses)
Ipass Ebd
Group-un
Group-200
Group-250
Group-300
5.56 (0.23) 552 (7)
5.05 (0.89) 664 (12)
4.04 (0.25) 597 (11)
2.11 (0.59) 513 (15)
and lowest Ipass , its clinical application is not suggested because Ni–Ti files are thought to lose their super-elastic character when the nitriding temperature is set at 300 ◦ C [17]. It has been previously reported that the TiN layer improves the corrosion resistance of Ti and/or Ti alloys in corrosive environments [26,27]. However, information on the corrosion resistance of TiN-coated Ni–Ti alloy in biological environments is very limited in the literature. Researchers have reported that a TiN layer improves the corrosion resistance of Ni–Ti surgical alloy in a neutral simulated body environment (Ringer’s solution) [28,29]. In dental applications, NaOCl, a widely used root canal irrigant, is corrosive to metallic instruments. Ni–Ti files that have been immersed in 1% NaOCl overnight display a variety of corrosion patterns [5]. Furthermore, the early fracture of Ni–Ti files may be attributable to corrosion when the file is immersed in 5% NaOCl solution [6]. Therefore, we believed that a high NaOCl concentration might have a highly significant influence on the corrosion of Ni–Ti files. In this study, the nitrided Ni–Ti files had better corrosion resistance in 5.25% NaOCl solution than the untreated Ni–Ti files. It was obvious that the TiN layer on the outermost surface of Ni–Ti files is able to provide the files with a protective barrier against the corrosive irrigation chemical, 5.25% NaOCl. As a result, it is expected that nitrided Ni–Ti files ought to have a better early fracture resistance in clinical NaOCl solution when compared to untreated files. This, however, needs further investigations in the future. 4. Conclusions The presence of a TiN layer on commercial dental Ni–Ti files created using nitriding treatment at 200 ◦ C and 250 ◦ C was found to significantly increase the corrosion resistance of files placed in a clinical treatment solution of 5.25% NaOCl. Although the 300 ◦ Cnitrided file showed the highest polarization resistance and lowest passive current, its clinical application is not recommended as there may be the loss of the file’s super-elastic character after this nitriding treatment. Therefore, the 250 ◦ C-nitrided file, which was placed second in terms of corrosion resistance among the nitrided files, should be the optimal candidate in terms of the corrosion resistance when used clinically.
793
Acknowledgements The authors thank the National Science Council, Taiwan (NSC 962628-B-010-032-MY3) and Taipei City Hospital, Taiwan (95003-62117) for financial support. Thanks are also due to Prof. Yong-Chwang Chen, National Taiwan University and Prof. Chun-Li Lin, National Yang-Ming University, Taiwan, for the assistance with the nitriding treatment and Micro-CT/CAD analysis, respectively. References [1] O.A. Peters, J. Endod. 35 (2004) 559–567. [2] M. Darabara, L. Bourithis, S. Zinelis, G.D. Papadimitriou, Int. Endod. J. 37 (2004) 705–710. [3] O.W. Stokes, P.M. Fiore, J.T. Barss, A. Koerber, J.L. Gilbert, E.P. Lautenschlager, J. Endod. 25 (1999) 17–20. ˜ A. Collazo, A. Macías-Luaces, [4] X.R. Nóvoa, B. Martin-Biedma, P. Varela-Patino, ˜ G. Cantatore, M.C. Pérez, F. Magán-Munoz, Int. Endod. J. 40 (2007) 36– 44. [5] P.Y.Z. O’Hoy, H.H. Messer, J.E.A. Palamara, Int. Endod. J. 36 (2003) 724–732. [6] E. Berutti, E. Angelini, M. Rigolone, G. Migliaretti, D. Pasqualini, Int. Endod. J 39 (2006) 693–699. [7] R. de Castro Martins, M.G.A. Bahia, V.T.L. Buono, Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod. 102 (2006) e99–e105. [8] O.A. Peters, J.O. Roehlike, A.B. Michael, J. Endod. 33 (2007) 589–593. [9] T.B. Bui, J.C. Mitchell, J.C. Baumgartner, J. Endod. 34 (2008) 190–193. [10] E. Rapisarda, A. Bonaccorso, T.R. Tripi, G.G. Condorelli, L. Torrisi, J. Endod. 27 (2001) 588–592. [11] E. Schäfer, J. Endod. 28 (2002) 800–803. [12] T.R. Tripi, A. Bonaccorso, G.G. Condorelli, J. Endod. 29 (2003) 132–134. [13] T.R. Tripi, A. Bonaccorso, G.G. Condorelli, Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod. 102 (2006) e106–e114. [14] T.R. Tripi, A. Bonaccorso, E. Rapisarda, V. Tripi, G.G. Condorelli, R. Marino, I. Fragalà, J. Endod. 28 (2002) 497–500. [15] E. Rapisarda, A. Bonaccorso, T.R. Tripi, I. Fragalk, G.G. Condorelli, Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod. 89 (2000) 363–368. [16] E. Schäfer, M. Oitzinger, J. Endod. 34 (2008) 198–200. [17] U.M. Li, Y.C. Chiang, W.H. Chang, C.M. Lu, Y.C. Chen, T.M. Lai, C.P. Lin, J. Dent. Sci. 1 (2006) 53–58. [18] U.M. Li, Development of Nondestructive Testing and Application of Surface Treatment in Nickel Titanium Rotary Instruments, Ph.D. thesis, National Taiwan University, Taipei, June 2007. [19] D.A. Jones, Principles and Prevention of Corrosion, second ed., Prentice-Hall Inc., 1996, pp. 116–167. [20] H.H. Huang, J. Biomed. Mater. Res. A 74A (2005) 629–639. [21] K. Yokota, K. Nakamura, T. Kasuya, S. Tamura, T. Sugimoto, K. Akamatsu, K. Nakao, F. Miyashita, Surf. Coat. Technol. 158–159 (2002) 568–572. [22] M. Yamada, T. Yasui, M. Fukumoto, K. Takahashi, Thin Solid Films 515 (2007) 4166–4171. [23] L. Nosei, S. Farina, M. Ávalos, L. Náchez, B.J. Gómez, J. Feugeas, Thin Solid Films 516 (2008) 1044–1050. [24] J. Lutz, A. Lehmann, S. Mändl, Surf. Coat. Technol. 202 (2008) 3747–3753. [25] P. Kochmanski, J. Nowacki, Surf. Coat. Technol. 202 (2008) 4834–4838. [26] A. Shenhar, I. Gotman, S. Radin, P. Ducheyne, E.Y. Gutmanas, Surf. Coat. Technol. 126 (2000) 210–218. [27] H.H. Huang, C.H. Hsu, J.L. He, S.J. Pan, C.C. Chen, Appl. Surf. Sci. 244 (2005) 252–256. [28] D. Starosvetsky, I. Gotman, Surf. Coat. Technol. 148 (2001) 268–276. [29] D. Starosvetsky, I. Gotman, Biomaterials 22 (2001) 1853–1859.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of nitriding surface treatment on the corrosion resistance of dental nickel–titanium files in 5.25% sodium hypochlorite solution Jeng-Fen Liu a,b , Mau-Chin Lin a,c , Ming-Lun Hsu a , Uei-Ming Li d , Chun-Pin Lin e , Wen-Fa Tsai f , Chi-Fong Ai f , Li-Kai Chen g , Her-Hsiung Huang a,g,h,∗ a
Department of Dentistry, National Yang-Ming University, Taipei, Taiwan Department of Dentistry, Taichung Veterans General Hospital, Taichung, Taiwan c Department of Dental Laboratory Technology, Central Taiwan University of Science and Technology, Taichung, Taiwan d Dental Department, Cardinal Tien Hospital, Hsintien, Taiwan e Department of Dentistry, National Taiwan University, Taipei, Taiwan f Institute of Nuclear Energy Research, Atomic Energy Council, Taoyuan, Taiwan g Department of Dentistry, Taipei City Hospital, Taipei, Taiwan h Institute of Oral Biology, National Yang-Ming University, Taipei, Taiwan b
a r t i c l e
i n f o
Article history: Received 11 March 2008 Received in revised form 1 August 2008 Accepted 7 August 2008 Available online 11 October 2008 Keywords: Dental alloy Nitriding Nickel–titanium file Sodium hypochlorite Corrosion resistance
a b s t r a c t This study investigated the effect of nitriding surface treatment on the corrosion resistance of commercial dental alloy, in the form of helical nickel–titanium (Ni–Ti) files, when treated with 5.25% sodium hypochlorite (NaOCl) solution. The surface of dental helical Ni–Ti files was modified using nitriding treatment at 200 ◦ C, 250 ◦ C and 300 ◦ C in an NH3 -containing environment. The surface morphology and chemical composition of the Ni–Ti files were analyzed using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The corrosion resistance of the Ni–Ti files when treated with a clinical solution of 5.25% NaOCl was evaluated using the linear polarization method and by potentiodynamic polarization curve measurement. The nitriding treatments at different temperatures created titanium nitride (TiN) on the surface of the helical Ni–Ti files. The Ni–Ti files nitrided at 200 ◦ C and 250 ◦ C showed higher polarization resistance and higher passive film breakdown potential together with a lower passive current than untreated files. The presence of TiN on dental Ni–Ti files significantly increased the corrosion resistance of the files in the presence of 5.25% NaOCl solution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In dental clinical applications, the advent of engine-driven nickel–titanium (Ni–Ti) alloy files with a super-elastic property has altered dental root canal treatment by decreasing operator fatigue, clinical preparation time, and procedural errors related to hand operation [1]. During Ni–Ti instrument use throughout dental root canal treatment, corrosive sodium hypochlorite (NaOCl) is widely used as a root canal irrigant and lubricant. Some previous research has investigated corrosion of commercial dental Ni–Ti files in the presence of NaOCl solution [2–4]. Some researchers have reported
∗ Corresponding author at: Department of Dentistry, School of Dentistry, National Yang-Ming University, No. 155, Sec. 2, Li-Nong Street, Taipei 112, Taiwan. Tel.: +886 2 2826 7068; fax: +886 2 2826 4053. E-mail address: [email protected] (H.-H. Huang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.025
that pitting or crevice corrosion is not likely to occur when Ni–Ti files are treated with 5.25% NaOCl irrigation solution [2]. On the other hand, other researchers found that the corrosion resistance of different Ni–Ti files in 5.25% NaOCl solution varied significantly between brands [3] and that the corrosion of Ni–Ti files can be decreased by lowering the NaOCl solution’s pH to about 10 [4]. Recently, the influence of NaOCl on the overall mechanical properties of Ni–Ti files has been reported [5–8]. One study investigated the effect of cleaning procedures, including immersion in NaOCl solution, on the fracture properties and corrosion of Ni–Ti files. The results showed that files immersed in 1% NaOCl overnight display a variety of corrosion patterns although NaOCl does not significantly reduce the torque at fracture [5]. However, it has been found that immersion in 5% NaOCl solution significantly increases the incidence of Ni–Ti file fracture from cyclic fatigue [6]; additionally, it has been found that Ni–Ti files show reduced resistance to cyclic fatigue after contact with 5.25% NaOCl solution [8]. Therefore, our
790
J.-F. Liu et al. / Journal of Alloys and Compounds 475 (2009) 789–793
Fig. 1. Appearance of the commercial helical dental Ni–Ti file used in this study.
hypothesis is that a surface treatment that significantly decreases file corrosion has the potential to lower the incidence of Ni–Ti file fracture during dental clinical use. It is well known that corrosion may be caused by an electrochemical reaction that occurs between the metal surface and corrosive environment. Therefore, from the clinical point of view, a surface treatment has the potential to improve the corrosion resistance of a Ni–Ti file surface without decreasing the superelastic character of file itself. Lately, extensive studies have focused on the effects of different surface treatments on the mechanical properties of commercial dental Ni–Ti files and these properties include wear resistance, cutting efficiency and fracture behavior [9–17]. However, related information concerning the surface effects of such treatments on commercial dental Ni–Ti files and whether this improves the corrosion resistance in clinical irrigation solution is limited from the literature. In a previous study, nitriding treatment has been proved to be a promising method of increasing wear resistance and cutting efficiency of dental Ni–Ti files [17]. In this study, nitriding surface treatment at various temperatures was applied to commercial dental Ni–Ti files and the corrosion resistance of the resulting nitrided Ni–Ti files in clinical irrigation solution, 5.25% NaOCl, was quantitatively evaluated.
assisted design (CAD) software (Pro/ENGINEER Wildfire 3.0, Parametric Technology Corporation, Needham, MA, USA). In this study, the exposed area (i.e. the cutting flute region) of file surface for corrosion tests was calculated as 0.33629751 cm2 . The corrosion resistance of the test specimens was evaluated using their linear polarization curve from −10 to +10 mV (versus corrosion potential) with a scan rate of 0.1 mV/s after dipping the files into the test electrolyte for one hour. The polariza-
2. Experimental procedures Commercial helical ProFile® (0.04 taper with #25) Ni–Ti dental alloy files (Dentsply Mailleffer, Ballaigues, Switzerland) (Fig. 1) were chosen for the nitriding treatment, with a NH3 flow rate of 100 ml/min, at controlled temperatures for four hours. Details of the nitriding procedures have been described elsewhere [17,18]. Three nitriding groups, obtained at 200 ◦ C, 250 ◦ C and 300 ◦ C, were designated as group-200, group-250 and group-300, respectively. The untreated commercial helical Ni–Ti files were designated as group-un. The surface morphology and chemical analysis of the Ni–Ti files before and after nitriding treatments were analyzed using scanning electron microscopy (SEM) (JSM-6500F, JEOL, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS) (Microlab 350, VG Scientific, Oxford, UK), respectively. The surface analyses and following corrosion tests of the files have been carried out in less than three weeks after nitriding treatments. A potentiostat (AUTOLAB PGSTAT 30, Eco Chemie BV, Utrecht, Netherlands) was used to perform the corrosion tests, which consisted of linear polarization curve and potentiodynamic polarization curve measurements, which are explained later in this section. The nitrided surface, that is the whole cutting flute surface of Ni–Ti files, was chosen as the corrosion test region. During the corrosion tests, the untested surface, that is the surface excluding the cutting flute surface of Ni–Ti files, was isolated using alkali-resistant epoxy resin. A saturated calomel electrode (SCE) and a platinum sheet were used as the reference electrode and counter electrode, respectively. A clinical irrigation solution of 5.25% NaOCl at pH 12 was used as the corrosion test electrolyte and maintained at 37 ◦ C. The exposed area of file surface for corrosion tests is described as follows. The cutting flute region of Ni–Ti file (Fig. 1) was scanned at 18-m interval in a microcomputed tomography (Micro-CT) machine (Skyscan 1076, Micro Photonics Inc., Allentown, PA, USA) to attain a real three-dimensional geometry of the file. Then a three-dimensional model of cutting flute region of file was reproduced in computer-
Fig. 2. X-ray photoelectron spectroscopy analysis results for the Ni–Ti files with and without nitriding treatments: (a) full spectra; (b) spectra for N1s (as TiN).
J.-F. Liu et al. / Journal of Alloys and Compounds 475 (2009) 789–793
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Fig. 3. Scanning electron microscope observations of the tested Ni–Ti files before the corrosion test: (a) group-un; (b) group-200; (c) group-250; (d) group-300.
tion resistance (Rp ), which is inversely proportional to the corrosion rate, is defined as the slope of the potential versus the current near the corrosion potential in the linear polarization curves [19]. The properties of surface passive film on the metal file play an important role in the corrosion resistance of metal. Therefore, the properties of surface passive film on the tested files were also evaluated in this study using potentiodynamic polarization curve measurements. This is able to provide two corrosion resistance parameters, namely the passive film breakdown potential (Ebd ) and passive current (Ipass ) [19]. The potentiodynamic polarization curves were measured from −0.3 V (SCE) in the anodic direction with a scan rate of 1 mV/s after dipping the files into the electrolyte for one hour. The scanning potential was stopped on reaching an anodic current of 10−3 A/cm2 . When the applied potential is higher than the Ebd , the protective passive film breaks down and as a result the anodic current increases significantly. The number of specimens for group-un, group-200, group-250 and group-300, was 12, respectively. For each nitriding condition (including group-un), the number of specimens for linear polarization curve and potentiodynamic polarization curve measurements was five, respectively.
3. Results and discussion 3.1. Surface characteristics Fig. 2 shows the X-ray photoelectron spectroscopy analysis results of a Ni–Ti file with and without nitriding treatments. It showed that TiO2 and small amount of NiO were the main surface oxides of the untreated dental Ni–Ti file (Fig. 2(a)). Similar results have been reported in other study: TiO2 and NiO (trace amount) are the main surface oxides in Ni–Ti alloys [20]. For the nitrided Ni–Ti files, an additional compound, titanium nitride (TiN), could be detected on the file’s surface along with the TiO2 and trace amount of NiO as shown in Fig. 2(b). Furthermore, the presence of TiN on the surface of the Ni–Ti files gave the file surface a light golden-yellow color. As the nitriding temperature increased, this led to a deeper color for the files. However, the surface morphology of the tested Ni–Ti files with or without nitriding treatments showed no signif-
icant difference when analyzed by scanning electron microscopy (Fig. 3), which implies that the nitriding treatment did not change the surface morphology of these commercial dental Ni–Ti files. Table 1 shows the X-ray photoelectron spectroscopy quantitative analysis results, demonstrating the atomic percentage of Ti, Ni, N, and O (as balance) and the atomic ratio of N/Ti ratio on the surface of the Ni–Ti files with and without nitriding treatments. For all the tested Ni–Ti files, the Ni content on files’ surface was nearly undetectable. For nitrided Ni–Ti files, the Ti content on files’ surface was approximately the same, but a higher N content was detected as the nitriding temperature increased. Therefore, the N/Ti ratio increased with increasing the nitriding temperature. This may imply that a higher content ratio of TiN to TiO2 was obtained on the nitrided Ni–Ti files with a higher nitriding temperature. During the nitriding treatment of the three groups of files, the N atoms diffused into the files reacting with Ti atoms on the file’s surface. This atomic diffusion and interaction process should not change the file’s surface morphology. This is able to explain the fact that the TiN formed on the outermost surface of the Ni–Ti files after the nitriding treatment did not result in any detectable morphological changes. Furthermore, it has been also reported that Ni–N compounds are unstable as compared to TiN. Thus, the formation
Table 1 X-ray photoelectron spectroscopy analysis results for the Ni–Ti files with and without nitriding treatments, showing the atomic percentage of four major elements (Ti, Ni, N and O as balance) and atomic ratio of N/Ti
Group-un Group-200 Group-250 Group-300
Ti
Ni
N
O
N/Ti ratio
35 32 31 31
<1 <1 <1 <1
0 6 9 10
Balance Balance Balance Balance
0 0.19 0.29 0.32
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Table 2 Polarization resistance (Rp , ), obtained from the linear polarization curves of the Ni–Ti files tested in 5.25% NaOCl solution (note: standard deviations are given in parentheses)
Rp
Group-un
Group-200
Group-250
Group-300
6.24 × 10 (6301)
6.41 × 10 (7017)
1.09 × 10 (1204)
1.35 × 105 (2431)
4
4
of TiN is thermodynamically easier than that of Ni–N compounds [21]. This may partially explain why TiN instead of Ni–N compound was detected on Ni–Ti file surface after nitriding treatments. For nitriding procedure using NH3 atmosphere in this study, there is no divergent interface between the bulk substrate and the treated surface layer. This is a very crucial point for Ni–Ti file in dental clinical applications because the modified surface layer must have potential to follow the transformation of the bulk materials and prevent from the occurrence of delamination [17,18]. Furthermore, the use of NH3 atmosphere for nitriding treatment is also a simpler and cheaper nitriding procedure as compared to some other nitriding procedures [22–25]. However, the difference in the surface microstructure and composition of the Ni–Ti file after different nitriding procedures is another important topic but not the purpose of this study. Therefore, this needs further investigations. 3.2. Corrosion tests Table 2 shows the polarization resistance (Rp , ) obtained from the linear polarization curves of the tested Ni–Ti files in 5.25% NaOCl solution. The Rp ranking was as follows: group-300 (1.35 × 105 ) > group-250 (1.09 × 105 ) > group-200 (6.41 × 104 ) > group-un (6.24 × 104 ). Linear polarization curve measurement is a fast and simple corrosion test method that can provide a quantitative corrosion parameter Rp , which is inversely proportional to the corrosion rate [19]. A higher Rp represents a better corrosion resistance. In this study, the nitrided files all had a better corrosion resistance than the untreated file in terms of Rp , which was related to the presence of TiN on the nitrided files. It should be noted that the nitrided files with a higher N/Ti ratio showed a better corrosion resistance. This implies that a higher
Fig. 4. Potentiodynamic polarization curves of the tested Ni–Ti files, with and without nitriding treatments, in 5.25% NaOCl solution.
5
content ratio of TiN to TiO2 on the nitrided files was beneficial to the corrosion resistance, in terms of Rp , in 5.25% NaOCl solution. Fig. 4 shows the potentiodynamic polarization curves of the tested Ni–Ti files in 5.25% NaOCl solution. Regardless of the nitriding treatment condition, all the tested Ni–Ti files in 5.25% NaOCl solution revealed passive and transpassive regions. As compared to the untreated Ni–Ti file, all the nitrided Ni–Ti files had a lower passive current, but only the Ni–Ti files with lower nitriding temperatures (200 ◦ C and 250 ◦ C) had a higher passive file breakdown potential. This implies that using suitable nitriding treatment conditions could increase the corrosion resistance, in terms of passive film properties, of commercial dental Ni–Ti files in simulated clinical environment. A more detailed discussion on the parameters for passive file properties is described below. Table 3 shows the passive film breakdown potential (Ebd , mV) and passive current (Ipass , A/cm2 ) obtained from potentiodynamic polarization curves of the tested Ni–Ti files in 5.25% NaOCl solution. The Ebd ranking was as follows: group-200 (664 mV) > group250 (597 mV) > group-un (552 mV) > group-300 (513 mV). The Ipass ranking was as follows: group-un (5.56 A/cm2 ) > group200 (5.05 A/cm2 ) > group-250 (4.04 A/cm2 ) > group-300 (2.11 A/cm2 ). The properties of surface passive film on metal file’s surface play an important role in the corrosion resistance of metals. A higher Ebd represents better passive film stability. The Ipass is related to the charge transfer resistance through the surface passive film that forms spontaneously on corrosion resistant metal. A lower Ipass represents higher charge transfer resistance through the passive film. Basically, a protective passive film should have higher Ebd and lower Ipass . In this study, the nitrided group-200 and group-250 files, with the main surface compounds of TiN and TiO2 , had better passive film stability and a higher charge transfer resistance as compared to the untreated file, with the main surface compound of TiO2 only. The fact that group-300 file had the lowest Ipass (or higher charge transfer resistance) among the tested Ni–Ti files was believed to be related to the presence of higher TiN content on file surface. On the other hand, the stability (in terms of Ebd ) of group-300 file was the worst, which was probably due to the presence of surface defects formed at a higher nitriding temperature. However, this needs further investigations. From clinical application point of view, it is necessary that the influence in a clinical environment of any surface treatment on the corrosion resistance of Ni–Ti files be understood. Different surface treatment techniques have been used to improve the cutting efficiency, wear resistance and/or fatigue resistance of dental Ni–Ti files [9–17]. However, up to now, very limited information has been available in the literature concerning the corrosion resistance of surface-treated commercial Ni–Ti files in clinical environments. A previous study found that nitrided Ni–Ti files have better wear resistance and cutting efficiency than the untreated ones [17]. In this study, a further attempt was made to investigate the effect of nitriding treatment on the corrosion resistance of Ni–Ti files in a clinical environment. It was clear that the presence of TiN on commercial Ni–Ti files after nitriding treatments increased the files’ corrosion resistance, which is indicated by higher Rp and Ebd values and lower Ipass values, as compared to files without nitriding. Furthermore, the nitriding treatment used in this study did not microscopically change the surface morphology of commercial dental Ni–Ti files. Although the group-300 file had the highest Rp
J.-F. Liu et al. / Journal of Alloys and Compounds 475 (2009) 789–793 Table 3 Passive film breakdown potential (Ebd , mV) and passive current (Ipass , A/cm2 ), obtained from the potentiodynamic polarization curves of Ni–Ti files tested in 5.25% NaOCl solution (note: standard deviations are given in parentheses)
Ipass Ebd
Group-un
Group-200
Group-250
Group-300
5.56 (0.23) 552 (7)
5.05 (0.89) 664 (12)
4.04 (0.25) 597 (11)
2.11 (0.59) 513 (15)
and lowest Ipass , its clinical application is not suggested because Ni–Ti files are thought to lose their super-elastic character when the nitriding temperature is set at 300 ◦ C [17]. It has been previously reported that the TiN layer improves the corrosion resistance of Ti and/or Ti alloys in corrosive environments [26,27]. However, information on the corrosion resistance of TiN-coated Ni–Ti alloy in biological environments is very limited in the literature. Researchers have reported that a TiN layer improves the corrosion resistance of Ni–Ti surgical alloy in a neutral simulated body environment (Ringer’s solution) [28,29]. In dental applications, NaOCl, a widely used root canal irrigant, is corrosive to metallic instruments. Ni–Ti files that have been immersed in 1% NaOCl overnight display a variety of corrosion patterns [5]. Furthermore, the early fracture of Ni–Ti files may be attributable to corrosion when the file is immersed in 5% NaOCl solution [6]. Therefore, we believed that a high NaOCl concentration might have a highly significant influence on the corrosion of Ni–Ti files. In this study, the nitrided Ni–Ti files had better corrosion resistance in 5.25% NaOCl solution than the untreated Ni–Ti files. It was obvious that the TiN layer on the outermost surface of Ni–Ti files is able to provide the files with a protective barrier against the corrosive irrigation chemical, 5.25% NaOCl. As a result, it is expected that nitrided Ni–Ti files ought to have a better early fracture resistance in clinical NaOCl solution when compared to untreated files. This, however, needs further investigations in the future. 4. Conclusions The presence of a TiN layer on commercial dental Ni–Ti files created using nitriding treatment at 200 ◦ C and 250 ◦ C was found to significantly increase the corrosion resistance of files placed in a clinical treatment solution of 5.25% NaOCl. Although the 300 ◦ Cnitrided file showed the highest polarization resistance and lowest passive current, its clinical application is not recommended as there may be the loss of the file’s super-elastic character after this nitriding treatment. Therefore, the 250 ◦ C-nitrided file, which was placed second in terms of corrosion resistance among the nitrided files, should be the optimal candidate in terms of the corrosion resistance when used clinically.
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