Degradation and fracture of Ni-Ti superelastic wire in an oral cavity

Corrosion Behavior of TiNi Alloy Fabricated by Selective Laser Melting in Simulated Saliva

In this work, TiNi samples were prepared by Selective Laser Melting (SLM) technology, and the influence of microstructure, fluoride ion, and pH value on corrosion behavior in a saline environment was investigated and compared with TiNi alloy fabricated by traditional forging technology. The results indicated that the corrosion resistance of the SLM sample was slightly superior to that of the wrought sample in a saline environment due to the uniform and dense oxide film formed on the SLM sample surface. However, in acidic Artificial Saliva Solution (ASS) containing fluoride ions, the corrosion current density of the SLM sample increased from 9.85 × 10−2 to 13.9 μA/cm2 because of the presence of F−. Fluorine ions disrupted the passive film on the surface, and the Ti-F compound formed in the film, which deteriorated the corrosion resistance of the SLM sample. The increase in fluoride concentration and the decrease in pH value could accelerate the corrosion of the SLM sample.

Tribocorrosion Susceptibility and Mechanical Characteristics of As-Received and Long-Term In-Vivo Aged Nickel-Titanium and Stainless-Steel Archwires

To evaluate the effect of long-term in-vivo aging on orthodontic archwires, we aimed to assess the triboelectrochemical and mechanical characteristics of as-received and in-vivo aged nickel-titanium (NiTi) and stainless-steel (SS) orthodontic archwires. Four consecutive tribocorrosion cycles on six NiTi and six SS archwires, as-received and in-vivo aged, were performed on a reciprocal tribometer. Electrochemical noise and friction coefficient measurements, three-dimensional surface profiling, and hardness measurements were performed. Repassivation times of as-received archwires were longer than of the in-vivo aged; however, were shorter for NiTi. Friction coefficients were higher for NiTi than for SS archwires. Sudden major current drops concomitant with inverse potential shifts and friction coefficients’ fluctuations, were seen for as-received (last cycle) and in-vivo aged (last three cycles) NiTi archwires. More pronounced tribocorrosion damage was observed on in-vivo aged NiTi than on other archwires. Hardness was generally higher inside the wear track of archwires. Long-term in-vivo exposure decreases the corrosion susceptibility of archwires, more evidently for the NiTi ones. Sudden major fluctuations in electrochemical current, potential, and friction coefficient detected for NiTi archwires, might be related to localized residual parts of the oxide layer persisting due to increased surface roughness or to phase transformations of the alloy’s crystal structure.

Structure and Phase State of Ti49.4Ni50.6 (at%) Hydrogenated in Normal Saline

The paper analyzes the surface structure and phase state of Ti_49.4Ni_50.6 (at%) hydrogenated at 295 K in normal saline (0.9% NaCl aqueous solution with pH = 5.7) at 20 A/m² for 0.5–6 h. The analysis shows that the average hydrogen concentration in the alloy increases with the hydrogenation time t_H as follows: slowly to 50 ppm at t_H = 0.5–1.5 h, steeply to 150 ppm at t_H = 1.2–2 h, and linearly to 300 ppm at t_H = 2–6 h. According to Bragg–Brentano X-ray diffraction data (θ–2 θ, 2 θ ≤ 50°, CoKα radiation), the alloy in its scanned surface layer of thickness ~5.6 µm reveals a TiNiH_x phase with x = 0.64 and x = 0.54 after hydrogenation for 4 and 6 h, respectively. The structure of this phase is identifiable as an orthorhombic hydride similar to β₁–TiFeH_0.94 (space group Pmcm), rather than as a tetragonal TiNiH_x hydride with x = 0.30–1.0 (space group I4/mmm). Time curves are presented to trace the lattice parameters and volume change during the formation of such an orthorhombic phase from the initial cubic B2 phase in Ti_49.4Ni_50.6 (at%).

Qualitative and Quantitative Evaluation of Different Types of Orthodontic Brackets and Archwires by Optical Microscopy and X-ray Fluorescence Spectroscopy

The wear behaviour and chemical composition of orthodontic components influence the mechanical characteristics of a fixed orthodontic treatment. The purpose of this paper is to evaluate the surface alterations of different types of brackets (aesthetic, metallic, and conventional self-ligating) and archwires (superelastic and thermal) subjected to wear tests through optical microscopy and, subsequently, to identify the chemical elements of accessories by X-ray fluorescence. The cycles (5000 for each bracket and 10,000 for each wire) of the tribological test were carried out in dry conditions inside a machine that allows alternating sliding. The results of the study highlighted different wear behaviours even within the same type of brackets and archwires. The monocrystalline sapphire brackets maintain their aesthetic properties despite traces of wear inside the slots and contain minimal amounts of nickel. Superelastic NiTi archwires have a better overall rating than thermal wires, as they do not show significant surface wear alterations.

Corrosion induced fracture of NiTi wires in simulated oral environments

This work aims to assess the influence of corrosion on fracture of nickel titanium (NiTi) superelastic wires in physiological solutions (9 g/l NaCl) with and without addition of 1 g/l NaF. The electrochemical cell was coupled to a Hounsfield Tensiometer tensile machine commonly used for corrosion investigation of alloys under stress and strain. Corrosion tests were performed on unstrained and strained conditions up to 4% total strain. This strain limit corresponds to 50% of the total elongation achieved into the superelastic stress plateau of the alloy. All wire specimens were analyzed after testing by scanning electron microscopy (SEM). The results showed that localized corrosion occurred for NiTi wires in solution containing fluoride, while no corrosion attack was detected in NaCl 9 g/l solution. There was no significant difference between the corrosion resistance of unstrained and strained wires. However, brittle like fracture occurred in NaCl + NaF solution within the superelastic domain of the material. The most relevant conclusion achieved is that the use of superelastically strained NiTi in oral environments in the presence of fluoride is followed by significant risk of corrosion induced fracture.

Evolution, clinical applications, and prospects of nickel-titanium alloys for orthodontic purposes

This review article presents an evolution of the nickel-titanium wires for orthodontics, following their introduction by the pioneering studies of Andreasen. The original nonsuperelastic wires were followed by the introduction of superelastic Japanese NiTi wire by Miura and colleagues and Chinese NiTi wire by Burstone and colleagues. Subsequently, new nickel-titanium wires with true shape memory in the oral environment were introduced. Manufacturers have marketed special heat-treated wires with variable force delivery at different positions along the archwire. Ion implantation and other surface modification techniques have been used by manufacturers to reduce in vivo nickel release from the nickel-titanium wires, provide a more esthetic appearance, decrease friction, and improve corrosion resistance. The use of several research techniques to provide supporting information about the structures and transformations, mechanical properties, and clinical failure for the different types of the nickel-titanium wires are summarized. The evolution of the ADA/ISO standard for evaluation of these wires is also described. The closing section focuses on the use of surface modification and special coatings for the nickel-titanium wires, a major recent and ongoing area of active research.

Surface Analysis of Wire-Electrical-Discharge-Machining-Processed Shape-Memory Alloys

Shape-memory alloys such as nitinol are gaining popularity as advanced materials in the aerospace, medical, and automobile sectors. However, nitinol is a difficult-to-cut material because of its versatile specific properties such as the shape-memory effect, superelasticity, high specific strength, high wear and corrosion resistance, and severe strain hardening. Anunconventional machining process like wire-electrical-discharge-machining (WEDM) can be effectively and efficiently used for the machining of such alloys, although the WEDM-induced surface integrity of nitinol hassignificant impact on material performance. Therefore, this work investigated the surface integrity of WEDM-processed nitinol samples using digital microscopy imaging, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. Three-dimensional analysis of the surfaces was carried out in two different patterns (along the periphery and the vertical plane of the machined surface) andrevealed that surface roughness was maximalat the point where the surface was largely exposed to the WEDM dielectric fluid. To attain the desired surface roughness, appropriate discharge energy is required that, in turn, requires the appropriate parameter settings of the WEDM process. Different SEM image analyses showed a reduction in microcracks and pores, and in globule-density size at optimized parameters. EDX analysis revealed the absence of wire material on the machined surface

The Effect of Hydrogen on Martensite Transformations and the State of Hydrogen Atoms in Binary TiNi-Based Alloy with Different Grain Sizes

The analysis presented here shows that in B2-phase of Ti49.1Ni50.9 (at%) alloy, hydrogenation with further aging at room temperature decreases the temperatures of martensite transformations and then causes their suppression, due to hydrogen diffusion from the surface layer of specimens deep into its bulk. When hydrogen is charged, it first suppresses the transformations B2↔B19' and R↔B19' in the surface layer, and when its distribution over the volume becomes uniform, such transformations are suppressed throughout the material. The kinetics of hydrogen redistribution is determined by the hydrogen diffusion coefficient DH, which depends on the grain size. In nanocrystalline Ti49.1Ni50.9 (at%) specimens, DH is three times greater than its value in coarse-grained ones, which is likely due to the larger free volume and larger contribution of hydrogen diffusion along grain boundaries in the nanocrystalline material. According to thermal desorption spectroscopy, two states of hydrogen atoms with low and high activation energies of desorption exist in freshly hydrogenated Ti49.1Ni50.9 (at%) alloy irrespective of the grain size. On aging at room temperature, the low-energy states disappear entirely. Estimates by the Kissinger method are presented for the binding energy of hydrogen in the two states, and the nature of these states in binary hydrogenated TiNi-based alloys is discussed.

Evaluating the effect of clinical usage and autoclave sterilization on the load deflection properties of three different orthodontic wires: Ex-vivo study

OBJECTIVE

The aim of this study was to evaluate the effect of intraoral aging and sterilization on the physical properties of rectangular Nickel-Titanium (NiTi), Beta-Titanium and Cooper NiTi (Cu-NiTi) arch wires.

METHODS

Three types of preformed 0.018×0.025 inch wires: super elastic NiTi wire, Beta-Titanium wire and Cu-NiTi wire (20 of each type) were divided into 4 groups: as-received (T0), autoclave (T1), intra oral aging after sterilization (T2) and intra oral aging (T3). Specimens in T2 and T3 groups were used in oral environment of 30 participants for 8 weeks. In the next step a length of 30mm was cut from both ends of each arch wire, and 120 straight specimens were achieved and tested by Instron for evaluating their load deflection properties. Data were analysed by means of One-way ANOVA and Tukey's (honestly significant difference) HSD tests.

RESULTS

In NiTi wire, all conditions led to a significant decrease in deactivation mean load compared with control in most deflections (P=0.000). In Cu-NiTi wire, all conditions led also to a significant decrease in deactivation mean load compared with the control (P=0.000). In Beta-Titanium wire, sterilization had no significant effect on the load deflection properties; but significant increase was observed in T2 (in all deflections) and T3 (in 1.8-1mm) compared with the control.

CONCLUSIONS

After all conditions, NiTi wire in spite of reduction in stiffness presented a mean load which stayed in category of heavy force. The Cu-NiTi wires saw an improvement in light physiologic force. In contrast, the Beta-Titanium stiffness increased after clinical usage, and the force level remained in the range of heavy force.

Crystallographic character of grain boundaries resistant to hydrogen-assisted fracture in Ni-base alloy 725

Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries. Yet, despite over a century of research, the physical mechanisms of HE are too poorly understood to predict HE-induced failures with confidence. We use non-destructive, synchrotron-based techniques to investigate the relationship between the crystallographic character of grain boundaries and their susceptibility to hydrogen-assisted fracture in a nickel superalloy. Our data lead us to identify a class of grain boundaries with striking resistance to hydrogen-assisted crack propagation: boundaries with low-index planes (BLIPs). BLIPs are boundaries where at least one of the neighboring grains has a low Miller index facet-{001}, {011}, or {111}-along the grain boundary plane. These boundaries deflect propagating cracks, toughening the material and improving its HE resistance. Our finding paves the way to improved predictions of HE based on the density and distribution of BLIPs in metal microstructures.

Do Mechanical and Physicochemical Properties of Orthodontic NiTi Wires Remain Stable In Vivo?

Introduction and Aim. Exceptional properties of the NiTi archwires may be jeopardized by the oral cavity; thus its long-term effect on the mechanical and physiochemical properties of NiTi archwires was the aim of work. Material and Methods. Study group comprised sixty 0.016 × 0.022 NiTi archwires from the same manufacturer evaluated (group A) after the first 12 weeks of orthodontic treatment. 30 mm long pieces cut off from each wire prior to insertion formed the control group B. Obeying the strict rules of randomization, all samples were subjected to microscopic evaluation and nanoindentation test. Results. Both groups displayed substantial presence of nonmetallic inclusions. Heterogeneity of the structure and its alteration after usage were found in groups B and A, respectively. Conclusions. Long-term, reliable prediction of biomechanics of NiTi wires in vivo is impossible, especially new archwires from the same vendor display different physiochemical properties. Moreover, manufacturers have to decrease contamination in the production process in order to minimize risk of mutual negative influence of nickel-titanium archwires and oral environment.

Do the NiTi low and constant force levels remain stable in vivo?

OBJECTIVE

The aim was to evaluate the alteration of the deactivation forces of the most commonly used nickel-titanium wires under long-lasting oral cavity environmental influence.

MATERIALS AND METHODS

Randomized in vitro and in vivo trials of 540 pieces of orthodontic archwires, NeoSentalloy®, Copper NiTi® 35°C and Titanol Superelastic(®), round (0.016 inch), and rectangular (0.016 × 0.022 inch), were carried out. Randomization and blinding was achieved with 12-colour system that ensured encoding of key information on the tested specimens. Each of 270 patients (females, 18-20 years old, in the finishing stage of orthodontic treatment) received the piece of NiTi wire ligated piggyback, for a period of 4-6 weeks. Eventually, all samples were subjected to a three-point bending test. Data were statistically analysed at a 5 per cent significance level.

RESULTS

In the group of the round used wires, when compared with the new ones, the deactivation force (F dav) values increased significantly for Titanol Superelastic®; the NeoSentalloy® and Copper NiTi® 35°C wires did not change their values of F dav. In the group of rectangular wires, F dav decreased for Titanol Superelastic®, but increased for NeoSentalloy® wires. The F dav values of the Copper NiTi® 35°C used wires practically did not change, but a remarkable increase of the standard deviation was noted.

LIMITATIONS

No calculations concerning effectiveness of in vivo aligning of analysed wires were made.

CONCLUSIONS

For the purpose of a 4-6-week aligning stage, round NeoSentalloy® with a diameter of 0.016 inches seems to be the wire of choice because of the low level of F dav.

Effect of time and pH on physical-chemical properties of orthodontic brackets and wires

OBJECTIVE

To test the hypothesis that treatment time, debris/biofilm, and oral pH have an influence on the physical-chemical properties of orthodontic brackets and arch wires.

MATERIALS AND METHODS

One hundred twenty metal brackets were evaluated. They were divided into four groups (n  =  30) according to treatment time: group C (control) and groups T12, T24, and T36 (brackets recovered after 12, 24, and 36 months of treatment, respectively). Rectangular stainless-steel arch wires that remained in the oral cavity for 12 to 24 months were also analyzed. Dimensional stability, surface morphology, composition of brackets, resistance to sliding of the bracket-wire set, surface roughness of wires, and oral pH were analyzed. One-way analysis of variance, followed by a Tukey multiple comparisons test, was used for statistical analysis (P < .05).

RESULTS

Carbon and oxygen were shown to be elements that increased expressively and in direct proportion to time, and there was a progressive increase in the coefficient of friction and roughness of wires as a function of time of clinical use after 36 months. Oral pH showed a significant difference between group T36 and its control (P  =  .014).

CONCLUSIONS

The hypothesis was partially accepted: treatment time and biofilm and debris accumulation in bracket slots were shown to have more influence on the degradation process and frictional force of these devices than did oral pH.

Surface characterization of nickel titanium orthodontic arch wires

BACKGROUND

Surface roughness of nickel titanium orthodontic arch wires poses several clinical challenges. Surface modification with aesthetic/metallic/non metallic materials is therefore a recent innovation, with clinical efficacy yet to be comprehensively evaluated.

METHODS

One conventional and five types of surface modified nickel titanium arch wires were surface characterized with scanning electron microscopy, energy dispersive analysis, Raman spectroscopy, Atomic force microscopy and 3D profilometry. Root mean square roughness values were analyzed by one way analysis of variance and post hoc Duncan's multiple range tests.

RESULTS

Study groups demonstrated considerable reduction in roughness values from conventional in a material specific pattern: Group I; conventional (578.56 nm) > Group V; Teflon (365.33 nm) > Group III; nitride (301.51 nm) > Group VI (i); rhodium (290.64 nm) > Group VI (ii); silver (252.22 nm) > Group IV; titanium (229.51 nm) > Group II; resin (158.60 nm). It also showed the defects with aesthetic (resin/Teflon) and nitride surfaces and smooth topography achieved with metals; titanium/silver/rhodium.

CONCLUSIONS

Resin, Teflon, titanium, silver, rhodium and nitrides were effective in decreasing surface roughness of nickel titanium arch wires albeit; certain flaws. Findings have clinical implications, considering their potential in lessening biofilm adhesion, reducing friction, improving corrosion resistance and preventing nickel leach and allergic reactions.

An in vivo study on the incidence and location of fracture in round orthodontic archwires

OBJECTIVE

The main objective of this in vivo study was to determine the incidence and location of fracture in round nickel-titanium (NiTi) and round stainless steel orthodontic archwires, both commonly used in orthodontics. Secondarily, this study sought to determine if there is any correlation between archwire fracture and gender, diameter of the archwire, arch type (maxillary/mandibular) or bracket used.

DESIGN

In vivo study.

MATERIALS AND METHODS

One thousand orthodontic patients (1434 archwires) were evaluated during regular treatment visits to assess archwire fracture and location. The patient's gender, age, type of archwire (round NiTi and round stainless steel), diameter of the archwire, arch type, location of fracture (anterior or posterior) and period of service before fracture were recorded.

STATISTICAL ANALYSIS

Chi-square statistical test was utilized to address the frequency and the correlation between the different variables. Level of statistical significance (α) was set at 0.05.

RESULTS

Twenty-five archwire failures were reported (1.7%) of the total sample size. All fractured archwires were NiTi, and 76% of the fractures were located in the posterior region. No statistical significance was found between archwire fracture and gender, arch type (maxillary/mandibular), archwire diameter or bracket type.

CONCLUSION

The frequency of archwire fracture during regular orthodontic visits is very low. The most common archwire fracture site is the posterior region. NiTi wires are the most commonly fractured archwire. No statistically significant correlation exists between archwire fracture and gender, arch type, bracket type or diameter of archwire.

Corrosion resistance of surface modified nickel titanium archwires

OBJECTIVE

To compare the corrosion behavior of commercially available surface modified nickel titanium (NiTi) arch wires with respect to a conventional NiTi and to evaluate its association with surface characteristics.

MATERIALS AND METHODS

Five types of surface modified arch wires and a conventional NiTi arch wire, all from different manufacturers, were evaluated for their corrosion resistance from breakdown potential in an anodic polarization scan in Ringer's solution. Surface characteristics were determined from scanning electron microscopy, atomic force microscopy, and energy dispersive analysis. One-way analysis of variance and post hoc Duncan's multiple range tests were used to evaluate statistical significance.

RESULTS

Surface modified NiTi wires showed significant improvement in corrosion resistance and reduction in surface roughness values. Breakdown potentials increased in the order of group 6 (conventional; 204 mV) < group 1 (nitride; 333 mV) < group 5 (epoxy resin; 346mV) < group 3 (oxide; 523 mV) < group 2 (gold; 872 mV) < group 4 (Teflon; 1181 mV), but root mean square (RMS) roughness values, which indicated surface roughness, followed a different pattern: group 3 (oxide; 74.12 nm) < group 1 (nitride; 221.651 nm) < group 4 (Teflon; 278.523 nm) < group 2 (gold; 317.894 nm) < group 5 (epoxy resin; 344.236 nm) < group 6 (conventional; 578.555 nm).

CONCLUSIONS

Surface modification of NiTi wires proved to be effective in improving its corrosion resistance and decreasing surface roughness. However, neither factor could maintain a direct, one-to-one relationship. It meant that the type and nature of coating material can effectively influence the anticorrosive features of NiTi wires, compared with its surface roughness values.

Assessment of nickel titanium and beta titanium corrosion resistance behavior in fluoride and chloride environments

Objective:

To assess the influence of fluoride concentration on the corrosion behavior of nickel titanium (NiTi) superelastic wire and to compare the corrosion resistance of NiTi with that of beta titanium alloy in physiological solution with and without addition of fluoride.

Materials and Methods:

NiTi corrosion resistance was investigated through electrochemical impedance spectroscopy and anodic polarization in sodium chloride (NaCl 0.15 M) with and without addition of 0.02 M sodium fluoride (NaF), and the results were compared with those associated with beta titanium. The influence of fluoride concentration on NiTi corrosion behavior was assessed in NaCl (0.15 M) with and without 0.02, 0.04, 0.05, 0.07, and 0.12 M NaF solution. Galvanic corrosion between NiTi and beta titanium were investigated. All samples were characterized by scanning electron microscopy.

Results:

Polarization resistance decreased when NaF concentration was increased, and, depending on NaF concentration, NiTi can suffer localized or generalized corrosion. In NaCl solution with 0.02 M NaF, NiTi suffer localized corrosion, while beta titanium alloys remained passive. Current values near zero were observed by galvanic coupling of NiTi and beta titanium.

Conclusions:

There is a decrease in NiTi corrosion resistance in the presence of fluoride. The corrosion behavior of NiTi alloy depends on fluoride concentration. When 0.02 and 0.04 M of NaF were added to the NaCl solution, NiTi presented localized corrosion. When NaF concentration increased to 0.05, 0.07, and 0.12 M, the alloy presented general corrosion. NiTi corrosion resistance behavior is lower than that of beta titanium. Galvanic coupling of these alloys does not increase corrosion rates.

Effect of soft drinks on the physical and chemical features of nickel-titanium-based orthodontic wires

OBJECTIVE

The purpose of this study was to evaluate the effect of three popular soft drinks on the Young's modulus, hardness, surface topography and chemical composition of widely used nickel-titanium-based orthodontic wires.

MATERIALS AND METHODS

Thirty-two specimens (20 mm in length) were cut from the straight portion of pre-formed 0.019 × 0.025 inch Nitinol Heat-Activated archwires and randomly divided into four groups of eight specimens each: Group A1 (Coca Cola(®) regular); Group A2 (Santal(®) orange juice); Group A3 (Gatorade(®)); Group B (distilled, deionized water; dH(2)O). Each specimen was immersed in 10 ml of one of the soft drinks or dH(2)O, control, for 60 min, at 37°C. At the end of the soaking time, the Young's modulus and hardness were determined using a nanoindenter. Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) was used to characterize the effects on the topography and chemical composition of the wires.

RESULTS

No statistically significant differences were found between the groups either in the Young's modulus or in hardness after the selected soaking protocol. Besides some surface colour changes, the topography and the chemical composition of the wires were not affected by the immersion in any of the chosen soft drinks.

CONCLUSIONS

These in-vitro results suggest that the consumption of soft drinks cannot be acknowledged as one possible reason for the degradation of the physical and chemical properties of heat activated nickel titanium orthodontic wires in patients undergoing fixed orthodontic treatment.

Influence of fluoride and chloride on corrosion behavior of NiTi orthodontic wires

The influence of fluoride and chloride ions on the corrosion behavior of nearly equiatomic nickel-titanium orthodontic wires was studied using conventional electrochemical measurement methods, including corrosion potential, potentiodynamic and cyclic potentiodynamic polarization measurements. In addition, scanning electron microscopy was employed to observe the surface morphology before and after the test. All the electrochemical parameters are analyzed based on the sample standard deviations. The results indicated that NiTi alloy is primarily susceptible to localized corrosion when exposed to a solution containing chloride, while it is susceptible to general corrosion when subjected to a solution containing fluoride. Furthermore, the synergistic interaction of fluoride and chloride on corrosion of NiTi alloy is associated with their respective molar concentrations.

Why do nickel-titanium archwires fracture intraorally? Fractographic analysis and failure mechanism of in-vivo fractured wires

INTRODUCTION

The aim of this study was to characterize intraorally fractured nickel-titanium (Ni-Ti) archwires, determine the type of fracture, assess changes in the alloy's hardness and structure, and propose a mechanism of failure.

METHODS

Eleven Ni-Ti SE 200 and 19 copper-Ni-Ti (both, Ormco, Glendora, Calif) intraorally fractured archwires were collected. The location of fracture (anterior or posterior), wire type, cross section, and period of service before fracture were recorded. The retrieved wires and brand-, type-, and size-matched specimens of unused wires were subjected to scanning electron microscopy to assess the fracture type and morphological variation of fracture site of retrieved specimens, and to Vickers hardness (HV200) testing to investigate the hardness of as-received and in-vivo fractured specimens. Fracture site distribution was statistically analyzed with the chi-square test (alpha = 0.05), whereas the results of the hardness testing were analyzed with 2-way ANOVA with state (control vs in-vivo fractured) and composition (Ni-Ti SE vs copper-Ni-Ti) serving as discriminating variables and the Student-Newman-Keuls test at the 95% confidence level.

RESULTS

The fracture site distribution showed a preferential location at the midspan between the premolar and the molar, suggesting that masticatory forces and complex loading during engagement of the wire to the bracket slot and potential intraoral aging might account for fracture incidence. All retrieved wires had the distinct features of brittle fracture without plastic deformation or crack propagation, whereas no increase in hardness was observed for the retrieved specimens.

CONCLUSIONS

Most fractures sites were in the posterior region of the arch, probably because of the high-magnitude masticatory forces. Brittle fracture without plastic deformation was observed in most Ni-Ti wires regardless of archwire composition. There was no increase in the hardness of the intraorally exposed specimens regardless of wire type. This contradicts previous in-vitro studies and rules out hydrogen embrittlement as the cause of fracture.

Evaluation of the effect of fluoride-containing acetic acid on NiTi wires

The possibility of the formation of hydrofluoric acid by the reaction of fluoride with acetic acid seems natural in the oral cavity. The effect of an acidic fluoride solution on NiTi wires was investigated by testing microhardness and color changes on wires. For aesthetic reasons, the color change on wires was also evaluated. Wires were immersed in four different test solutions for 1 or 3 days. As-received wires were used as a reference. After immersion for 3 days, the microhardness of the tested wires increased 1.8% to 10.4% compared to that of their as-received state. Wires immersed in a higher NaF concentration, lower pH solution with longer immersion yielded a more corroded surface than those of the counter cases. Wires showed a different color after immersion. However, after 3 days in solutions of pH 4, wires showed an appreciable color change regardless of the products. In test solutions, 3M wires showed the highest volumetric and percentage (0.59 for 0.05%; 1.19 for 0.2% solution) weight loss and G&H wires showed the least volumetric and percentage (0.43 for 0.05%; 1.05 for 0.2% solution) weight loss among tested wires. In pH 6 solutions, wires lost weight and were under the detection limit of the testing machine.

Effect of acidic fluoride solution on ? titanium alloy wire

The interaction between acidic fluoride solution and beta titanium alloy was investigated to explore the changes that occur in beta titanium alloy by fluoride-containing acetic acid solutions. For this, alloy crystal structure, tensile strength, and elements released from the alloy wires were determined using four solutions (0.05%/pH 6, 0.05%/pH 4, 0.2%/pH 6, and 0.2%/pH 4) for 1 or 3 days. The immersed wire did not form any identifiable new crystal structure compared with the as-received wire. The tensile strength of the immersed wires was significantly reduced compared to the as-received wires in the test solutions if the period of immersion increased from as-received to 3 days. The fractured area of the immersed wire was reduced compared to the as-received one. The dimple pattern at the inner part and a cup-cone morphology at the outer part of the fractured wires were similar in both as-received and immersed wires. After a 3-day immersion, the amount of the released Ti and Mo has much increased for higher NaF concentration and lower pH value. During the long-period orthodontic treatment, both patient and clinical doctor should carefully use the fluoride-containing products to minimize unexpected damage on orthodontic wires.

Hydrogen embrittlement of work-hardened Ni–Ti alloy in fluoride solutions

Hydrogen embrittlement of work-hardened Ni-Ti alloy has been examined in acidulated phosphate fluoride (APF) solutions. Upon immersion in a 2.0% APF solution with a pH of 5.0, tensile strength decreased markedly with immersion time. Moreover, the fracture mode changed from ductile to brittle due to brittle layer formation at the peripheral part of the cross section of the specimen. The amount of absorbed hydrogen increased linearly with immersion time, and it reached above 5000 mass ppm after 24 h. The hydrogen desorption temperature of the immersed specimens shifted from 450 degrees C to a lower temperature with immersion time. As the amount of absorbed hydrogen was larger than 500 mass ppm, the degradation of mechanical properties was recognized. Although the tensile properties and fracture mode scarcely change in a 0.2% APF solution, the slight reduction in hardness and hydrogen absorption of several hundreds mass ppm were observed. The results of the present study imply that work-hardened Ni-Ti alloy is less sensitive to hydrogen embrittlement compared with Ni-Ti superelastic alloy.

Delayed fracture of Ni-Ti superelastic alloys in acidic and neutral fluoride solutions

Hydrogen-related degradation of the mechanical properties of a Ni-Ti superelastic alloy has been examined by means of delayed fracture tests in acidic and neutral fluoride solutions and hydrogen thermal desorption analysis. Delayed fracture took place in both solutions; the time to fracture was shorter in the acidic solutions than in the neutral solutions with the same fluoride concentration. The time to fracture was reduced in both solutions when applied stress exceeded the critical stress for martensite transformation. In the acidic solutions, Ni-Ti superelastic alloy underwent general corrosion and absorbed substantial amounts of hydrogen. Fractographic features suggested that the delayed fracture in the acidic solutions was attributable to hydrogen embrittlement, whereas in the neutral solutions, a different fracture mode appeared associated with localized corrosion only in the vicinity of the fracture initiation area. In the neutral solutions, the amount of absorbed hydrogen was much less than that in the acidic solutions, and the delayed fracture was likely to be induced by active path corrosion accompanying hydrogen absorption. The results of the present study imply that the hydrogen-related degradation of performance of Ni-Ti superelastic alloys occurs in the presence of fluoride.

Comparative corrosion performance of black oxide, sandblasted, and fine-drawn nitinol wires in potentiodynamic and potentiostatic tests: Effects of chemical etching and electropolishing

The corrosion performance of sandblasted (SB) and smooth fine-drawn (FD) medical-use nitinol wires was compared with the performance of wires with black oxide (BO) formed in air during their manufacture. Potentiodynamic and ASTM F746 potentiostatic tests in a 0.9 % NaCl solution were conducted on wires in their as-received, chemically etched, aged in boiling water, and electropolished states. As-received wires with various surface finishes revealed breakdown potentials in the range from -100 mV to +500 mV; similar passive current density, 10(-6) A/cm(2); and a wide hysteresis on the reverse scan, demonstrating strong susceptibility to localized corrosion. Chemically etched wires with original black oxide displayed consistent corrosion performance and surpassed, in corrosion resistance, electropolished wires that showed significantly lower breakdown (400-700 mV) and localized corrosion potentials ( approximately -50 to +113 mV). Sandblasted and fine-drawn wires exhibited rather inconsistent corrosion behavior. In potentiodynamic tests these wires could perform with equal probability either on the level of pretreated BO wires or rather similar to as-received wires. Both SB and FD wires revealed low breakdown potentials in the PS regime. SEM analysis performed before tests indicated that sandblasting was not efficient for the complete removal of the original scaling, and fine drawing aggravated the situation, resulting in a persistent scaling that contributed to the inferior corrosion performance. Inclusions (oxides, carbides, and oxidized carbides) inherited from the bulk and retained on electropolished surfaces are the cause of their inferior performance compared to chemically etched surfaces. In electropolished wires corrosion was initiated around inclusions.

Hydrogen embrittlement of Ni-Ti superelastic alloy in fluoride solution

Hydrogen embrittlement of Ni-Ti superelastic alloy in a fluoride solution (0.2% APF) has been investigated by means of a tensile test (after immersion) and hydrogen thermal desorption analysis. Upon immersion, the tensile strength of the alloy decreased to the critical stress level of martensite transformation. Hydrogen desorption of the immersed specimens appeared with a peak at around 500 degrees C. The amount of absorbed hydrogen in the alloy ranged from 100 to 1000 mass ppm when immersed in the fluoride solution for 2 to 24 h. The immersion in the fluoride solution led to the degradation of mechanical properties due to hydrogen embrittlement. The results of the present study imply that one reason that Ti and its alloys fracture in the oral cavity is the fact that hydrogen is absorbed in a fluoride solution, such as prophylactic agents.

Delayed fracture of beta titanium orthodontic wire in fluoride aqueous solutions

Hydrogen embrittlement of a beta titanium orthodontic wire has been examined by means of a delayed-fracture test in acid and neutral fluoride aqueous solutions and hydrogen thermal desorption analysis. The time to fracture increased with decreasing applied stress in 2.0% and 0.2% acidulated phosphate fluoride (APF) solutions. The fracture mode changed from ductile to brittle when the applied stress was lower than 500MPa in 2.0% APF solution. On the other hand, the delayed fracture did not occur within 1000h in neutral NaF solutions, although general corrosion was also observed similar to that in APF solutions. Hydrogen desorption of the delayed-fracture-tested specimens was observed with a peak at approximately 500 degrees C. The amount of absorbed hydrogen was 5000-6500 mass ppm under an applied stress in 2.0% APF solution for 24h. It is concluded that the immersion in fluoride solutions leads to the degradation of the mechanical properties and fracture of beta titanium alloy associated with hydrogen absorption.

Fracture associated with hydrogen absorption of sustained tensile-loaded titanium in acid and neutral fluoride solutions

The fracture of commercial pure titanium in acid and neutral fluoride solutions has been examined by a sustained tensile-loading test and hydrogen thermal desorption analysis. It was found that the fracture of titanium occurred in neutral 2.0% NaF solution as well as in 2.0% acidulated phosphate fluoride (APF) solution. The time to fracture decreased with increasing applied stress in both 2.0% APF and 2.0% NaF solutions. In the case of the same applied stress, the time to fracture in the 2.0% APF solution was shorter than that in the 2.0% NaF solution. General corrosion was exhibited on the side surface of the tested specimens. The formation of sodium titanium fluoride was observed on the surface of the immersed specimens in the 2.0% APF solution. Hydrogen desorption of the tested specimen in the 2.0% APF solution was observed with a peak at approximately 600 degrees C. The amount of absorbed hydrogen was >300 mass ppm in the 2.0% APF solution under an applied stress for 24 h. The results of the present study imply that applying stress to titanium by immersing in fluoride solutions leads to the degradation of its mechanical properties.

Fracture mechanisms of retrieved titanium screw thread in dental implant

Titanium and its alloy are increasingly attracting attention for use as biomaterials. However, delayed fracture of titanium dental implants has been reported, and factors affecting the acceleration of corrosion and fatigue have to be determined. The fractured surface of a retrieved titanium screw and metallurgical structures of a dental implant system were analyzed. The outer surface of the retrieved screw had a structure different from that of the as-received screw. It was confirmed that a shear crack initiated at the root of the thread and propagated into the inner section of the screw. Gas chromatography revealed that the retrieved screw had absorbed a higher amount of hydrogen than the as-received sample. The grain structure of a titanium screw, immersed in a solution known to induce hydrogen absorption, showed features similar to those of the retrieved screw. It was concluded that titanium in a biological environment absorbs hydrogen and this may be the reason for delayed fracture of a titanium implant.