Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments

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1 doi: /j x Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments E. S. J. Pereira 1, I. F. C. Peixoto 1,A.C.D.Viana 1, I. I. Oliveira 2, B. M. Gonzalez 2, V. T. L. Buono 2 & M. G. A. Bahia 1 1 Department of Restorative Dentistry, Faculty of Dentistry, Universidade Federal de Minas Gerais, Belo Horizonte-MG, Brazil; and 2 Department of Metallurgical and Materials Engineering, Engineering School, Universidade Federal de Minas Gerais, Belo Horizonte-MG, Brazil Abstract Pereira ESJ, Peixoto IFC, Viana ACD, Oliveira II, Gonzalez BM, Buono VTL, Bahia MGA. Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments. International Endodontic Journal, 45, , Aim To compare physical and mechanical properties of one conventional and one thermomechanically treated nickel titanium (NiTi) wire used to manufacture rotary endodontic instruments. Methodology Two NiTi wires 1.0 mm in diameter were characterized; one of them, C-wire (CW), was processed in the conventional manner, and the other, termed M-Wire (MW), received an additional heat treatment according to the manufacturer. Chemical composition was determined by energy-dispersive X- ray spectroscopy, phase constitution by XRD and the transformation temperatures by DSC. Tensile loading/ unloading tests and Vickers microhardness measurements were performed to assess the mechanical behaviour. Data were analysed using analysis of variance (a = 0.05). Results The two wires showed approximately the same chemical composition, close to the 1 : 1 atomic ratio, and the b-phase was the predominant phase present. B19 martensite and the R-phase were found in MW, in agreement with the higher transformation temperatures found in this wire compared with CW, whose transformation temperatures were below room temperature. Average Vickers microhardness values were similar for MW and CW (P = 0.91). The stress at the transformation plateau in the tensile load unload curves was lower and more uniform in the M-Wire, which also showed the smallest stress hysteresis and apparent elastic modulus. Conclusions The M-Wire had physical and mechanical properties that can render endodontic instruments more flexible and fatigue resistant than those made with conventionally processed NiTi wires. Keywords: endodontic instruments, fatigue, mechanical properties, M-Wire, nickel titanium, thermomechanical treatment, transformation temperatures. Received 1 October 2011; accepted 29 November 2011 Introduction Nickel titanium (NiTi) alloys allowed the development of rotary endodontic instruments because of their Correspondence: Professor Vicente T. L. Buono, Department of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627 Bloco 2 room 2640, Belo Horizonte, MG , Brazil ( vbuono@ demet.ufmg.br). superelasticity and low elastic modulus, which improved several features of chemo-mechanical root canal preparation. Superelasticity is associated with the occurrence of a martensitic phase transformation of the alloy upon the application of a certain amount of stress to the austenite phase and the spontaneous reversion of the stress-induced martensite when the stress is released, causing the material to recover its original shape (Otsuka & Wayman 1998, Thompson 2000). The superelasticity of NiTi alloys can be improved by ª 2011 International Endodontic Journal International Endodontic Journal, 45, ,

2 Properties of a thermomechanically treated NiTi wire Pereira et al. using special thermomechanical treatments. The mechanism for this improvement is to suppress slip during stress-induced martensitic transformation by raising the critical stress for slip (Miyazaki et al. 1982). Two mechanisms are available to raise the critical stress for slip in Ni-rich NiTi alloys: (i) precipitation hardening and (ii) hardening because of a high density of thermally rearranged dislocations (Miyazaki et al. 1982, Saburi et al. 1982). Despite the advantages of the rotary NiTi technique, unexpected instrument fracture is not uncommon and represents a major concern in its clinical use. Besides failure by torsional overload (Sattapan et al. 2000), metal fatigue has been identified as an important reason for the fracture of endodontic instruments (Pruett et al. 1997, Bahia & Buono 2005, Al-Hadlaq et al. 2010). Attempting to improve instrument performance, manufacturers concentrated on changing the geometric and dimensional characteristics, such as tip sizing, taper, cross-section design, helical angle and pitch length (Al-Sudani & Al-Shahrani 2006, Schäfer & Oitzinger 2008, Viana et al. 2010). More recently, a proprietary thermomechanical procedure has been applied to nitinol with the objective of producing superelastic NiTi wires containing martensite, which can be used to produce endodontic instruments with improved fatigue resistance (Berendt 2007). This wire has been termed M-Wire NiTi (Dentsply Tulsa Dental Specialties), and subsequent studies suggested that commercial instruments made with M-Wire NiTi exhibited enhanced flexibility and fatigue resistance (Johnson et al. 2008, Kell et al. 2009, Larsen et al. 2009, Al-Hadlaq et al. 2010, Gao et al. 2010, Peixoto et al. 2010). Typical processing of superelastic NiTi-based wires includes vacuum casting of an ingot, hot forging, rolling and cold drawing followed by appropriate heat treating (Thompson 2000). There are reports in the literature suggesting that the transformation temperatures of NiTi instruments can influence their flexibility (Miyai et al. 2006, Hayashi et al. 2007). Other authors claim that heat treating these instruments can improve their flexibility (Yahata et al. 2009, Ebihara et al. 2011). Although the exact thermomechanical history of M-Wire is proprietary, the mechanism indicated as responsible for its properties is somewhat different than what could result from a simple heat treatment (Berendt 2007). In this work, the physical and mechanical properties of M-Wire were assessed and compared with those of a conventional wire used for manufacturing rotary NiTi instruments, in an attempt to provide a better understanding of this new trend in the development of endodontic instruments. The null hypothesis was that M-Wire had the same physical and mechanical properties as a conventional NiTi wire. Materials and methods The conventional NiTi wire used for the experiments (CW) was provided by Tulsa (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA) and was processed in a conventional way by drawing and annealing at low temperature. The other wire, M-Wire NiTi (MW), was also provided by Tulsa and is trained by thermally cycling at low temperatures under strain (Berendt 2007). Both types of wires were 1.0 mm in diameter. X-ray energy-dispersive spectroscopy (EDX) with a Noran TN-M3055 spectrometer (Noran, Middleton, WI, USA) was used to determine the average amounts of the nickel and titanium in the wires. Ten small areas were analysed in each type of wire. To identify the crystallographic phases present, ten segments of each material that were 12 mm in length were glued side by side, forming a specimen of mm in area. These specimens were analysed by X-ray diffraction (XRD) with a PANalytical PW1710 diffractometer (PANalytical, Almelo, the Netherlands) using Cu-Ka radiation at 40 kv and 30 ma and a graphite monochromator. Before characterization by EDX and XRD, oxide layers in the wire surfaces were removed by etching in an aqueous solution of hydrofluoric, acetic and nitric acids at a ratio of 1 : 5 : 5. Transformation temperatures were determined as the beginning and end of exothermic/endothermic peaks on the heating and cooling curves recorded by differential scanning calorimetry (DSC) with a Shimadzu DSC 60 calorimeter (Shimadzu, Kyoto, Japan). Three tests were performed with different samples of 25 mg of each wire and consisted of heating the sample to 80 C and then cooling it down to )80 C at a heating and cooling rate of 10 C min )1. Vickers microhardness measurements were performed with a Leica Durimet II tester (Leica, Weltzlar, Germany) using a 100 gf load. Three 15-mm-long specimens of each wire were embedded in acrylic resin, ground to half their diameter and then polished with diamond paste. Twenty indentations were measured for each type of wire. The load/unload behaviour was assessed in tensile tests made with an Instron 5580 testing machine (Instron, Norwood, MA, USA). Specimens 100 mm in length were tensile loaded up to 6% 470 International Endodontic Journal, 45, , 2012 ª 2011 International Endodontic Journal

3 Pereira et al. Properties of a thermomechanically treated NiTi wire Table 1 Average values of the transformation temperatures, Ms, Mf, As and Af, transformation stress, r A-M, and Vickers microhardness, MHV, of the wires studied Wires Transformation temperatures ( C) r A M MHV Ms Mf As Af (MPa) (kgf mm )2 ) MW CW 14 )15 ) Figure 1 XRD patterns of the wires in the region of the main peaks of austenite, B19 martensite and the Rphase. elongation and then unloaded to zero stress. The tests were performed in triplicate. Results The data obtained on each wire by EDX were 50.0% Ni- 50.0% Ti for MW and 50.3% Ni-49.7% Ti for CW, and the standard deviations were <0.5%. Comparison of these contents using anova (a = 0.05) showed no statistical difference, meaning that both type of wires had essentially the same average Ni and Ti contents, close to the equiatomic ratio of 1 : 1. The results of the XRD analysis showed that austenite, which is designated as the b-phase in the NiTi system, was the predominant phase in both wires. Figure 1 shows the main X-ray peaks of austenite at approximately 42.5, 61.5 and 77.6 in the diffractogram for both wires. For the M-Wire, there were also peaks at 44.4 and 54.2, which belong to the B19 martensitic phase, and another peak at 51.6, indicative of the presence of the R-phase. The presence of the R-phase also caused broadening of the first and the third b-phase peaks in the diffractogram of the M-Wire. The average transformation temperatures determined by DSC are shown in Table 1. Ms and Mf indicate the beginning and end of the formation of martensite during cooling, whilst As and Af are the corresponding temperatures for the reverse transformation taking place upon heating the wires; the standard deviations were <3 C. The Af temperature of the CW wire was around room temperature, whilst the same temperature in MW was well above it. Typical tensile load/unload curves for the two wires are shown in Fig. 2, with dotted lines indicating their Figure 2 Tensile loading and unloading curves of the wires studied. The difference between the stresses for direct and reverse transformations represents the stress hysteresis. initial apparent elastic modulus and the slope of the transformation plateau. For the CW specimen, the initial portion of the loading curve (E CW ) was approximately linear until the transformation stress was attained, whilst in MW, it was composed of two linear portions, the first (E 1-MW ) with a lower inclination and the second (E 2-MW ) with an inclination similar to that of CW. Both curves showed the transformation plateau, which was lower for MW, with the load level almost constant up to the maximum elongation of 6%. The mean values of the slope of the transformation plateau, calculated between 2% and 6% strains, were S MW = 8 MPa and S CW = 16 MPa for MW and CW, respectively (maximum standard error smaller than 3%). During the unloading process, the decrease in load corresponding to the reverse transformation was followed by elastic unloading, leaving behind a small permanent residual elongation of approximately 0.5% in both wires. The average values of the stress at the loading plateau (transformation stress) are given in Table 1, which also shows the average values of Vickers microhardness, were similar for the MW and CW wires (P = 0.91). ª 2011 International Endodontic Journal International Endodontic Journal, 45, ,

4 Properties of a thermomechanically treated NiTi wire Pereira et al. Discussion This study evaluated the physical and mechanical properties of two wires used for manufacturing rotary NiTi instruments, one conventional and the other submitted to a proprietary thermomechanical treatment. NiTi alloys with a small excess of Ni in relation to the equiatomic ratio of 1:1 respond well to heat treatment at lower temperatures because coherent, submicroscopic Ti 3 Ni 4 precipitates can form in the b- phase matrix (Miyazaki et al. 1982). Because of their small size and coherence with the matrix, these precipitates increase the mechanical strength of austenite, helping to prevent plastic deformation upon straining, and thus favouring the occurrence of stressinduced martensitic transformation that is paramount to the superelasticity of the alloy. As the Ti 3 Ni 4 precipitates contain more Ni than Ti, their formation decreases the amount of Ni in solid solution and increase the transformation temperatures, which are very sensitive to the Ni content (Tang 1997). The semiquantitative chemical analysis of the wires indicated that they have appropriate amounts of Ni and Ti to be sensitive to the low-temperature heat treatments that improve their superelasticity, and it is reasonable to assume that both the CW and MW wires were submitted to such a kind of treatment. The results of the XRD analysis (Fig. 1) revealed that austenite, the b-phase, was the predominant phase in the two wires evaluated. Figure 1 also shows peaks of B19 martensite and of the R-phase in the diffractogram of the M-Wire, whose relatively low intensity indicates that, in addition to the austenite, a small amount of these phases were also present in the MW specimen. These observations are in agreement with those of Johnson et al. (2008) and Alapati et al. (2009) about M-Wire and are compatible with the higher transformation temperatures found by DSC in MW (Table 1). To understand why such a microstructure would form in MW, it is necessary to examine the description of the proprietary thermomechanical treatment applied to it. First of all, it is said that this treatment is to be performed in conventional nitinol wire after cold drawing in the martensitic phase to the finished diameter and straightening annealed in the temperature interval C (Berendt 2007). The wire is then subjected to thermal cycling between a low temperature bath (below 10 C) and a high temperature one (up to 180 C) 3 5 times under a strain in the superelastic regime. Although the effect of this thermomechanical cycling is complex, it is known that thermal cycling under load has the potential to raise the starting temperature for the formation of B19 martensite, but has little effect on the R-phase starting temperature (Miyazaki et al. 1986). This should be the reason why both B19 and the R-phase coexist in MW at room temperature (Fig. 1). It has been reported that NiTi instruments with higher transformation temperatures are more flexible (Miyai et al. 2006, Hayashi et al. 2007) and that heat treating these instruments can increase the transformation temperatures (Yahata et al. 2009) and improve this property (Yahata et al. 2009, Ebihara et al. 2011). However, the greater flexibility of the instruments made with M-Wire should be attributed to their lower initial apparent elastic modulus (E 1-MW, Fig. 2), which can be associated with the stress-induced reorientation of the R- phase and B19 martensite (Miyazaki & Otsuka 1986). Thermal cycling under stress is also a form of training treatment that makes it easy for the same types of martensite variants to form upon loading (Otsuka & Ren 2005). This is responsible for the observation that stressinduced transformation of austenite in MW can be achieved at lower stress levels and has another important consequence. It produces an additional factor contributing to improve the flexibility of the instruments made with this wire. On the other hand, the heat treatments applied to the instruments in the works cited earlier were conducted in the temperature range C, and the observed improvement in flexibility should be the result of softening associated with dislocation annealing and coalescence of precipitates (Reed- Hill et al. 2008). In this respect, it is important to observe that the average values of Vickers microhardness obtained here (Table 1) showed that MW was as hard as CW. The importance of this finding lies in the fact that instruments made with M-Wire should possess a torsional resistance comparable with that of the instruments made with conventionally treated NiTi wires (Johnson et al. 2008, Kramkowski & Bahcall 2009). According to the literature (Johnson et al. 2008, Kell et al. 2009, Larsen et al. 2009, Al-Hadlaq et al. 2010, Gao et al. 2010, Peixoto et al. 2010), experimental and commercial instruments made with M-Wire exhibited better fatigue properties than instruments conventionally produced. The results of the load unload tests presented in Fig. 2 are important to demonstrate this difference in behaviour. As commented before, the superelastic plateau of the MW specimen was achieved under a lower tensile stress (Table 1). Additionally, the stress at the plateau remained relatively constant until the 6% elongation was attained (S MW = 8 MPa), whilst 472 International Endodontic Journal, 45, , 2012 ª 2011 International Endodontic Journal

5 Pereira et al. Properties of a thermomechanically treated NiTi wire this stress in CW tended to increase in the same region (S CW = 16 MPa). This increase is the result of work hardening, which can take place in the superelastic region associated with a certain amount of plastic deformation (Saburi 1998, Otsuka & Ren 2005). This concurrent plastic deformation is observed to take place when superelastic NiTi alloys are strained at temperatures somewhat higher than Af, and the amount of slip deformation increases as the test temperature rises (Saburi et al. 1982). This should be the case with CW, which possessed lower transformation temperatures than MW. The occurrence of slip deformation during superelastic straining is also responsible for the higher stress hysteresis exhibited by CW compared with MW (area under the load unload curve in Fig. 2). This is because more lattice defects were being introduced in CW than in MW when the wires suffered the load/ unload cycle. When this reasoning is applied to the rotary endodontic instruments made from these wires, it is easy to understand why those in which MW was used presented higher fatigue resistance (Johnson et al. 2008, Kell et al. 2009, Larsen et al. 2009, Al-Hadlaq et al. 2010, Gao et al. 2010, Peixoto et al. 2010). The reason is that the thermomechanical treatment applied to MW led to a more efficient superelastic behaviour with less generation and accumulation of lattice defects in each load unload cycle. Conclusions Compared with a conventionally treated NiTi wire employed for the manufacture of NiTi instruments, M-Wire showed higher transformation temperatures, contained small amounts of B19 martensite and R-phase, exhibited the same Vickers hardness, but a lower apparent elastic modulus and smaller transformation stress and mechanical hysteresis. These physical and mechanical properties can render endodontic instruments made with this wire more flexible and fatigue resistant than those made with conventionally processed NiTi wires. Acknowledgements The authors thank Greg Phillips of Dentsply Tulsa Dental Specialties for his cooperation in providing trial NiTi wires. This work was partially supported by Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Belo Horizonte, MG, Brazil, and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brasília, DF, Brazil. References Alapati SB, Brantley WA, Iijima M et al. (2009) Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. Journal of Endodontics 35, Al-Hadlaq SMS, Aljarbou FA, Althumairy RI (2010) Evaluation of cyclic flexural fatigue of M-Wire nickel-titanium rotary instruments. 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