NiTi Wires Research Articles

Comprehensive evaluation of the antibacterial and antibiofilm activities of NiTi orthodontic wires coated with silver nanoparticles and nanocomposites: an in vitro study.

Fixed orthodontic appliances act as a niche for microbial growth and colonization. Coating orthodontic wires with antimicrobial silver nanoparticles (AgNPs) and nanocomposite was adopted in this study to augment the biological activity of these wires by increasing their antibacterial and antibiofilm properties and inhibiting bacterial infections that cause white spot lesions and lead to periodontal disease. Three concentrations of biologically synthesized AgNPs were used for coating NiTi wires. The shape, size, and charge of the AgNPs were determined. Six groups of 0.016 × 0.022-inch NiTi orthodontic wires, each with six wires, were used; and coated with AgNPs and nanocomposites. The antimicrobial and antibiofilm activities of these coated wires were tested against normal flora and multidrug-resistant bacteria (Gram-positive and Gram-negative bacterial isolates). The surface topography, roughness, elemental percentile, and ion release were characterized. AgNPs and nanocomposite coated NiTi wires showed significant antimicrobial and antibiofilm activities. The chitosan-silver nanocomposite (CS-Ag) coated wires had the greatest bacterial growth inhibition against both Gram-positive and Gram-negative bacteria. The surface roughness of the coated wires was significantly reduced, impacting the surface topography and with recorded low Ni and Ag ion release rates. NiTi orthodontic wires coated with AgNPs, and nanocomposites have shown increased antimicrobial and antibiofilm activities, with decreased surface roughness, friction resistance and limited- metal ion release.

The Effect of Three Metal Oxide Nanocoatings on the Frictional Resistance of Superelastic Orthodontic Archwires: A Comprehensive In vitro Analysis.

To evaluate and compare the impact of nanocoatings made of oxides of Aluminum, Titanium, and Zirconium, on the frictional resistance on three types of superelastic orthodontics archwires namely; nickel titanium, copper nickel titanium and low hysteresis nickel titanium. There are120 archwire segments of equal dimensions were divided into four groups (n = 30) with 10 samples each of low hysteresis superelastic archwires; NiTi archwires and CuNiTi archwires. While group A were uncoated, other groups were nanocoated with group B: Aluminum oxide; group C: Titanium dioxide; group D: Zirconium oxide respectively. Upper premolar metal brackets MBT 0.022 slot were used for testing. The frictional properties of the archwires were measured using a Universal testing machine equipped with a custom-made jig. Statistical tests including analysis of variance and post hoctests were used for analysis. The least frictional resistance among the three types of archwires was seen with low hysteresis(L&H) NiTi wires coated with ZrO2 (3.1253 ± 0.45822 N) and the highest with uncoated CuNiTi archwires (7.1113 ± 1.29031 N). Among the nanocoatings, the least value was found for ZrO2 nanocoatings followed by TiO2, Al2O3 and highest with uncoated archwires across all three types of archwires. Low hysteresisNiTi have the least frictional resistance compared to CuNiTi and NiTi archwires. The findings also suggest that all the three metal oxide nanocoatings reduce frictional resistance significantly, among which, ZrO2 nanocoatings were the most effective. This study underscores the potential efficacy of metal oxide nanocoatings in reducing archwire friction and, consequently, will improve orthodontic treatment efficiency and patient comfort. How to cite this article: Dilip S, Rajkumar K. The Effect of Three Metal Oxide Nanocoatings on the Frictional Resistance of Superelastic Orthodontic Archwires: A Comprehensive In vitro Analysis. J Contemp Dent Pract 2024;25(7):649-655.

The effect of autoclave cycles on the mechanical properties and surface roughness of NiTi archwires: ex-vivo study

ObjectivesTo evaluate the effect of the universal and rapid autoclave cycles on the mechanical properties and surface roughness of nickel-titanium archwires following clinical use.Material and methodsThirty-six NiTi archwires (0.016 × 0.022 inch) were equally divided into a control group (Group A) and 2 experimental groups (Group B & C). Wires in group A were tested in the “as-received” form. Wires in the two other groups were installed in patients mouth for 4 weeks, and then autoclaved using the rapid-cycle (Group B) or the universal-cycle (Group C). All wires were subjected to 3-point bending test to calculate the elastic limit, modulus of elasticity, spring-back, yield strength, resilience and toughness. Atomic force microscopy (AFM) was used for surface roughness qualitative and quantitative analysis.ResultsGroup B showed significantly higher values of elastic limit, modulus of elasticity, resilience, yield strength and toughness than the other two groups. No significant differences were detected between groups A and C (P > 0.05). Group B showed significantly lower average surface roughness than the other two groups, but no significant differences were detected between groups A and C (P > 0.05).ConclusionsThe mechanical properties and surface roughness of clinically used NiTi wires were less affected by the universal-cycle than the rapid-cycle autoclaving. However, the difference between the effect of both autoclave cycles was diminutive.Clinical relevanceThe mechanical properties and surface roughness of the tested NiTi wires were not notably altered by clinical use and autoclaving.

The Effect of Intraoral Thermal Changes on the Mechanical Behavior of Nickel-Titanium Wires: An In-Vitro Study.

Background The continuous light force with a wide range of activation describes the excellent properties of nickel-titanium (NiTi) archwires. Shape memory is mainly affected by intraoral thermal changes. This study evaluated the effect of three different constant temperatures (i.e., 12°C, 37°C, and 50°C) on the unloading value of three different 0.016 × 0.022 NiTi archwires. Methodology Three types of 0.016 × 0.022-inch diameter NiTi archwires (American Orthodontics®, Sheboygan, Wisconsin, USA) were used. These were the superelastic type (NT3-SE®), the heat-activated type at 25°C (Thermal Ti-D®), and the thermally activated type at 35°C (Thermal Ti-Lite®). The unloading forces of the wires were evaluated using a classic three-point bending test (a universal testing machine: Testometric 350M®, Instron, Lincoln Close, Rochdale, England) under three different constant temperatures (12°C, 37°C, and 50°C). Results All types of wires showed thermal sensitivity; at higher temperatures, the unloading forces increased differentially between small and large deflections, while at lower temperatures, the residual strain increased for all wire types. The most affected type by the thermal changes was thermal Ti-Lite®, followed by thermal Ti-D®, and the superelastic type NT3-SE® showed a behavior similar to thermal wires. At the low temperature (12°C), all wire types showed an incomplete load/deflection curve, whereas no value was measured at unloading points 2, 1, and 0.5 mm. At the normal temperature (37°C), NT3-SE® type and thermal Ti-D® were similar in force level, while significant differences were noted between both previous types and Thermal Ti-Lite®. At the high temperature (50°C), all wire types showed a higher force level, while significant differences between the wire types were inconsistent. In contrast, increasing the temperature from 37°C to 50°C increased the force levels between 40% and 84% for NT3-SE®, between 44% and 64% for the thermal Ti-D®, and between 61% and 268% for the Thermal Ti-Lite®. When comparing the force levels between 12°C and 50°C at 3 mm, the force levels increased by 66% for NT3-SE®, 25% for Thermal Ti-D®, and 109% for thermal Ti-Lite®, while on comparing the forces between 12°C to 37°C, the forces increased between 15% and 95% for NT3-SE®, 20% and 88% for thermal Ti-D®, and 26% and 78% for thermal Ti-Lite®. The value of residual strain was greater at low temperatures and smaller at higher temperatures, while no significant differences were detected between 37°C and 50°C. Conclusions The temperature degree deeply affected the mechanical behavior of all test NiTi wires; the superelastic type behaved similarly to thermal wires. Increasing the temperature degree leads to more unloading forces and less residual strain.

Dissimilar Resistance Welding of NiTi Microwires for High-Performance SMA Bundle Actuators

Shape memory alloys (SMAs) are becoming a more important factor in actuation technology. Due to their unique features, they have the potential to save weight and installation space as well as reduce energy consumption. The system integration of the generally small-diameter NiTi wires is an important cornerstone for the emerging technology. Crimping, a common method for the mechanical and electrical connection of SMA wires, has several drawbacks when it comes to miniaturization and high-force outputs. For high-force applications, for example, multiple SMA wires in parallel are needed to keep actuation frequencies high while scaling up the actuation force. To meet these challenges, the proposed study deals with the development of a resistance-welding process for manufacturing NiTi wire bundles. The wires are welded to a sheet metal substrate, resulting in promising functional properties and high joint strengths. The welding process benefits from low costs, easy-to-control parameters and good automation potential. A method for evaluating the resistance-welding process parameters is presented. With these parameters in place, a manufacturing process for bundled wire actuators is discussed and implemented. The welded joints are examined by peel tests, microscopy and fatigue experiments. The performance of the manufactured bundle actuators is demonstrated by comparison to a single wire with the same accumulated cross-sectional area.

Martensitic transformation induced by cooling NiTi wire under various tensile stresses: Martensite variant microstructures, textures, recoverable strains and plastic strains

Whenever the forward and/or reverse B2 cubic to B19’ monoclinic martensitic transformation in NiTi shape memory alloy wire proceeds under external stress above certain threshold, it generates incremental plastic strains which accumulate during thermomechanical cyclic loading and lead to functional fatigue preventing many promising engineering applications from realization. In this work, unique thermomechanical loading experiments were performed on NiTi shape memory wire with the aim to reveal the mechanism by which the forward martensitic transformation upon cooling under external stress generates plastic strain. Recoverable transformation strains and plastic strains generated by the forward transformation on cooling under various tensile stresses were evaluated, martensite variant microstructures in grains were reconstructed by nanoscale orientation mapping in TEM, martensite textures after cooling at room temperature were evaluated by in-situ synchrotron x-ray diffraction and permanent dislocation defects in martensite were analyzed by TEM. Based on the obtained experimental evidence, it is proposed that the forward martensitic transformation on cooling under stress proceeds via habit plane interfaces between austenite and second order laminate of (001) compound twins in martensite. Depending on the magnitude of the applied stress, the induced martensite reorients, partially detwins and deforms plastically via [100](001) dislocation slip in martensite in extent permitted by the requirement for compatible deformation of grains in nanocrystalline NiTi wire via a single deformation system. During the forward MT upon cooling under highest stresses 600 MPa, the martensite deforms via kwinking deformation enabling generation of large plastic strains 8 %.

Corrosion Behavior of Nickel-Titanium Arch Wires Following the Use of Different Mouthwashes: An In Vivo Study.

The aim of this double-blind in vivo study was to compare the extent of corrosion on the surface of nickel-titanium (NiTi) wires in various mouthwashes. A total of 80 patients who received orthodontic treatment with as-received 0.016x0.022 inch NiTi wires were included in the study, and they were split into 4 groups. The first group used 0.05% of (225ppm F-) sodium fluoride (NaF) (Colgate Plax®) containing mouthwash, 21.6% alcohol (Listerine Cool Mint®) containing mouthwash, and 0.2% clorhexidine (CHX) (Klorhex®) containing mouthwash and the control group used drinking water with melt menthol as mouthwash. After 30 days of using mouthwash, the surfaces of NiTi wires were examined with atomic force microscopy (AFM), and surface roughness values were calculated. Mouthwashes containing fluoride, essential oils, and CHX created higher surface roughness on NiTi wires than the control group. The floride-containing mouthwash group showed less corrosion than the CHX group, whereas there was no difference between the essential oil group. AFM images show supportive data with the results of the clinical study. The results were assessed using a 95% confidence interval and a significance level p<0.05. CHX, essential oil, and floride-containing mouthwashes cause corrosion of NiTi wires. Floride-containing mouthwash can be preferred over CHX mouthwash due to its lesser corrosion effect.

Assessment of Various Archwire Materials and Their Impact on Orthodontic Treatment Outcomes.

Aim Orthodontic treatment relies heavily on the mechanical properties and surface characteristics of archwire materials to achieve optimal outcomes. This study aimed to comprehensively evaluate the mechanical properties, including tensile strength, yield strength, and modulus of elasticity, as well as the surface characteristics, such as surface roughness and frictional properties, of different archwire materials. Methods Four types of archwire materials, stainless steel, nickel-titanium (NiTi), beta-titanium, and esthetic archwires, were subjected to mechanical testing and surface analysis, with 31 in each group. Tensile testing was conducted to determine the maximum tensile strength, yield strength, and elastic modulus of each material. Surface roughness analysis was performed using profilometry techniques, and frictional properties were evaluated using an orthodontic friction testing apparatus. Results Stainless steel exhibited the highest tensile strength (900 N), followed by beta-titanium (850 N), NiTi (800 N), and esthetic archwire (750 N). Stainless steel also demonstrated the highest yield strength (780 N), followed by beta-titanium (740 N), NiTi (710 N), and esthetic archwire (650 N). The modulus of elasticity was the highest for stainless steel (200 GPa), followed by beta-titanium (170 GPa), NiTi (150 GPa), and esthetic archwires (120 GPa). Surface roughness was lowest in stainless steel archwires (mean Ra value of 0.25 µm), leading to reduced frictional resistance, whereas esthetic archwires exhibited the highest surface roughness (mean Ra value of 0.40 µm) and frictional forces. Significant differences in the mechanical properties and surface characteristics were observed among the materials (p < 0.05). Conclusions The choice of archwire material significantly influences orthodontic treatment outcomes by affecting the efficiency and effectiveness of tooth movement. Stainless steel and beta-titanium wires are ideal for high-stress applications, providing the robust mechanical strength necessary for complex movements. In contrast, NiTi wires, with their superelasticity, offer consistent and gentle forces, enhancing patient comfort and accelerating the alignment phase. Esthetic archwires, while visually appealing, often compromise mechanical performance, potentially prolonging treatment duration.

Experimental Investigation of the Impact of Loading Conditions on the Change in Thin NiTi Wire Resistance during Cyclic Stretching.

This paper presents the results of an experimental study designed to evaluate the effect of repeated stretching cycles on the electrical resistance change in a NiTi alloy wire. In particular, tests were carried out to determine the effect of the type of loading on resistance change in the investigated wires. Wires with a diameter of 100 μm were used in the research. The experiment was carried out on a dedicated test stand designed for this purpose. During the test, the samples were subjected to 40 identical tensile cycles. The electrical resistance, sample elongation, and tensile force during successive stretching cycles were measured. The conducted research demonstrated the impact of elongation and reorientation of the structure on the resistance change in NiTi alloy thin wires. The research included a comparison of the effect of two different types of loading on the electrical resistance change in the sample. During cyclic stretching of a NiTi alloy sample with constant displacement, a decrease in electrical resistance was observed after each successive stretching cycle. Alternatively, when stretching with a constant force, the value of electrical resistance increased. In both types of loads, the greatest change in resistance value was observed at the initial cycles.

Adaptive bioinspired morphing surface using temperature-responsive elastomer-SMA composites

The pursuit of "smart" materials, drawing inspiration from biological organisms, has been a significant focal point in the realm of material science and engineering. Shape memory materials, notably Shape Memory Alloys (SMAs), have emerged as promising platforms for the development of adaptive and responsive materials that undergo transformations in response to environmental stimuli. This article explores the creation of a bioinspired morphing surface that capitalizes on the innovative amalgamation of Ecoflex and Nitinol (NiTi) wires. Inspired by biological mechanisms, this morphing surface exemplifies remarkable adaptability, seamlessly transitioning from 2D to 3D shapes with precision. A detailed mechanical characterization underscores pivotal changes in material properties, showcasing a significant reaction force increase from 0.4 N to 1 N in NiTi wires at 20 °C and 50 °C. Concurrently, the embedded NiTi wire within the Ecoflex matrix exhibits a similar force increment from 0.6 N to 1.2 N, reflecting the microstructural alterations dependent on temperature. The study also elucidates the versatility and scalability of this technology, highlighting its potential for diverse applications in aerospace, robotics, medical devices, and adaptive materials. This bioinspired morphing surface offers a versatile foundation for customizable shapes and programmable transformations, paving the way for impactful advancements in a multitude of fields.

Effect of different mouthwash solutions on the surface morphology, nanohardness and flexural modulus of nickel-titanium orthodontic wire.

This laboratory study aimed to evaluate the effect of different mouthwash solutions on the surface and mechanical properties of NiTi arch wire. This experimental study was conducted at the Department of Preventive Dental Sciences, College of Dentistry, Princess Nourah bint Abdulrahman University, Saudi Arabia from September 2023 to November 2023. A 30 mm NiTi wires in length with 0.017 × 0.022" in dimensions were selected and equally divided into four groups: Control (G0) group wires were immersed in distilled water (DW); G1 wires were immersed in antiseptic mouthwash; G2 wires were immersed in fluoridated mouthwash; and G3 wires were immersed in therapeutic mouthwash. Surface morphology, nanohardness, and flexural modulus were evaluated at 24-hours, four weeks, and eight weeks' time points. Data were statistically analyzed using a two-way analysis of variance (P<0.05). The arch wires immersed in DW showed insignificant effects on surface roughness, nanohardness, and flexural modulus at different time points. However, all the experimental groups showed a significant effect of time and immersion solutions on the said properties (P<0.05). At the end of eight weeks, G3 showed the highest surface roughness (0.346 ± 0.032 µm) and the lowest nanohardness (1.350 ± 0.412 GPa) and flexural modulus (3.960 ± 0.140 MPa) compared to other study groups. The efficiency of tooth realignment could be influenced by the detrimental influence of fluoride and hydrogen peroxide mouthwash solutions on superelastic NiTi arch wires.

Nafion/ZrO2 Modified NiTi Orthodontic Wire: Preparation, Material Characterization, and Corrosion Studies

Background: Nickel-titanium (NiTi) orthodontic wires are widely used in dental corrective procedures due to their high mechanical properties and cost-effectiveness. However, they are prone to oral corrosion, leading to mechanical deterioration, aesthetic issues, and potential health concerns. Objective: This study aims to improve the corrosion resistance and durability of NiTi orthodontic wires by employing zirconium dioxide (ZrO2) and Nafion coating with the goal of enhancing wire performance. Methods: Two types of NiTi wires were evaluated: a standard, unmodified wire as a control and another wire treated with electrodeposited ZrO2 film and Nafion (Naf) coating. Surface analysis was conducted using various techniques, including Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDX) analysis. Results: The uncoated NiTi wire exhibited a corrosion rate of 4.436× 10-1 mm/year, whereas the Naf/ZrO2-coated NiTi wire showed a corrosion rate of 4.068× 10-1 mm/year, indicating that the Naf/ZrO2 coating acted as a protective layer. Additionally, the ZrO2 layer provided poor electrical conductivity, resulting in slightly higher impedance compared to bare NiTi. The coating served as a barrier, which significantly enhanced corrosion resistance and improved the wire lifespan. Conclusion: Electro-modification through ZrO2 deposition and Nafion coating significantly improved the corrosion resistance and overall durability of NiTi orthodontic wires, offering a promising advancement for their use in dental orthodontics. This study underscores the potential of ZrO2 and Nafion coating to enhance the corrosion resistance and longevity of NiTi orthodontic wires.

Efficiency of clear aligners with nickel titanium wires for treating mandibular incisor crowding.

Patients who are allergic to nickel, which is present in stainless steel brackets and wires, commonly utilise clear aligners. The objective of this research was to compare the management of mandibular incisor crowding with nickel titanium (NiTi) wires and clear aligners. A random division of forty participants into two groups of twenty samples each was made. Participants in group B received transparent aligners, whereas those in group A received NiTi arch wires. We assessed malocclusion using Little's irregularity index. A survey instrument was utilised to document the degree of discomfort. Patients were routinely recalled every one, two, and three months. The mean score for groups A and B was 2.86 mm and 2.88 mm at baseline, 1.71 mm and 1.52 mm at one month, 1.02 mm and 1.23 mm at two months, and 0.72 mm and 0.48 mm at three months, respectively. The difference wasn't that big (p > 0.05). In groups A and B, the mean change in Little's irregularity index score was 1.19 mm and 1.21 mm at one month, 0.55 mm and 0.51 mm at two months, and 0.26 mm and 0.45 mm at three months, respectively. The difference wasn't that big (p > 0.05). For groups A and B, the corresponding mean discomfort scores were 2.6 and 2.3 at baseline, 2.2 and 1.8 after one month, 1.6 and 1.5 at two months, and 1.1 and 0.9 at three months. Since p > 0.05, the difference was not significant. The results of this study showed that NiTi wires and clear aligners worked just as well to treat cases of mandibular anterior crowding.

Uprighting Mandibular Third Molar Using NiTi Wire

Uprighting Mandibular Third Molar Using NiTi Wire

Influence of distal-end heat treatment in the properties of heat-activated NiTi archwires.

The aim of this study was to evaluate the extent of property changes caused by heating the distal portion of heat-activated nickel-titanium (NiTi) wires. Forty preformed heat-activated NiTi archwires (3MUnitek, Monrovia, CA, USA) with anominal cross-section of 0.018″ were used in this study. The archwires were divided into acontrol group, not submitted to heat treatment and, thus, maintaining the as-received properties, and an experimental group, in which the archwires were submitted to heat treatment for distal bending at one end. Wire segments of control and experimental groups were submitted to differential scanning calorimetry (DSC) and Vickers microhardness measurements. The DSC results suggest local recrystallization and precipitate dissolution at the heat-treated tip, which decreases as the distance to the wire's tip increases. Vickers microhardness tests revealed significant changes for distances between 6and 8 mm from the wire's tip. Heating the distal portion of heat-activated NiTi archwires should be performed with care since this clinical procedure may compromise the performance of these wires to a distance of 8 mm from the archwire end. Heat treatment for distal bending in heat-activated NiTi archwires may be performed, with little impact on the areas adjacent to heat treatment. In cases presenting molars requiring significant orthodontic corrections, it should be preferred to apply other techniques to avoid archwire sliding, such as crimpable stops, or to have flame control to avoid placing aheat-treated section in the tubes of these molars.

Predictors of Curve of Spee Leveling in Class II Division 1 Malocclusion Treated With Reverse Curve of Spee Archwires: An Observational Study.

Introduction Aligning and leveling is the initial stage of comprehensive fixed orthodontic treatment aimed at minimizing the depth of the curve of Spee (COS). Various techniques exist to decrease the magnitude of the curve. This study investigates the skeletal and dental factors that reduce COS in individuals with minor class II malocclusions receiving nitinol (NiTi) wires with a reverse curve of Spee (RCS). Materials and methods The data for this observational study was collected from a sample of 84 patients who had class II molar relations and were sequentially treated with RCS NiTi wires throughout the initial leveling and aligning phase. All patients with class II molar relationships underwent non-extraction procedures during the leveling phase. The COS was determined using digitalized dental models. Skeletal and dental characteristics that could impact COS were identified and quantified using digital lateral cephalograms and orthopantomograms recorded during the pre-treatment (T1) and post-leveling (T2) stages. After calibrating the radiographs and models, we acquired angular and linear data. The data was categorized based on gender, growth pattern, and initial alignment of the teeth. We analyzed the differences between the groups using an independent t-test and an ANOVA. A paired t-test was used to compare the difference in the dimensional values between (T1) and (T2) points. Following the correlation coefficient tests, the study used stepwise multiple linear regression analysis to assess the predictive value of independent factors on the COS. The results were considered statistically significant at p < 0.05. Results The COS decreased by -1.43 ± 0.68 mm, which is statistically significant (<0.001*). There is no significant difference in COS reduction between the categorical variables. Despite statistically significant differences in the parameters between pre- and post-treatment, the linear correlation between most of the variables and COS reduction ranged from very weak (<0.20) to weak (0.20-0.39). Conclusions The vertical extrusion of lower premolars and molars combined with the intrusion of lower incisors contributed to the reduction of the COS by RCS wires. There is a change in the orientation of the occlusal plane with the flattening of the COS.

Stress induced martensitic transformation in NiTi at elevated temperatures: Martensite variant microstructures, recoverable strains and plastic strains

To shed light on the origin of the loss of functional properties of NiTi with temperature increasing above 100 °C, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by thermomechanical tensile testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains by nanoscale orientation mapping in TEM. The stress induced martensitic transformation generating recoverable transformation strain as well as plastic strain is not completed at the end of the upper stress plateau. The higher is the test temperature, the larger is the volume fraction of retained austenite as well as the plastic strain. The martensite variant microstructures in NiTi wire deformed up to the end of the stress plateau at 120 °C contain partially detwinned single domains of (001) compound twin laminate filling entire grains. It is proposed that the stress induced martensitic transformation proceeds via habit plane interface between austenite and second order laminate of (001) compound twins and that the martensite promptly reorients and deforms plastically by dislocation glide in the [100](001) slip system. When the wire is loaded further beyond the end of the stress plateau, the stress induced martensitic transformation continues and the martensite deforms plastically. It is concluded that the observed gradual loss of superelastic functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, as widely assumed in the literature, but that it derives from the loss of resistance of the stress induced martensite to the plastic deformation under increasing stress.

Oxidized Ethylene Glycol/ZrO2-coated NiTi Orthodontic Arch Wires: Surface Characterization and Electrochemical and Corrosion Studies

Background: Orthodontic arch wires, typically made of Nickel Titanium (NiTi), are widely utilized in dental procedures for correcting teeth misalignment and jaw issues due to their favorable mechanical attributes and cost-effectiveness. However, these NiTi wires are prone to corrosion in the oral environment, leading to diminished mechanical stability, compromised aesthetics, and potential health concerns for patients. Objective: There is a growing demand to augment the corrosion resistance and stability of orthodontic wires. Hence, this study aimed to address these issues. Herein, zirconium dioxide (ZrO2) and oxidized ethylene glycol (OEG) films were deposited onto NiTi wires to improve the corrosion resistance and stability. Methods: NiTi wires were modified by a two-step process involving electrodeposition of ZrO2 and oxidized ethylene glycol (OEG) film. The surface characterizations of coated material (OEG/ZrO2/NiTi) were carried out by using Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDS), and Electron Microprobe Analysis (EMap) to confirm the elemental composition of the coated NiTi wire. Results: The OEG/ZrO2/NiTi wire exhibited a potentiodynamic polarization resistance of 547037 Ω and higher stability than the bare NiTi wire (396421 Ω). The corrosion rate for OEG/ZrO2/NiTi wire was found to be 0.040 mm/year, which was comparatively lower than a bare NiTi wire (0.069 mm/year). Due to the formation of OEG/ZrO2 film, NiTi wire became electrically insulative and showed a higher impedance than bare NiTi wire. Conclusion: The bilayer coating of ZrO2 and OEG has proven to significantly improve the corrosion resistance and stability of the wires. Thus, these materials can be considered for coating orthodontic arch wires with improved corrosion stability.

Recoverable and plastic strains generated by forward and reverse martensitic transformations under external stress in NiTi SMA wires

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.

Modeling the interaction between instabilities and functional degradation in shape memory alloys

Localization of the stress-induced martensitic phase transformation plays an important role in the fatigue behavior of shape memory alloys (SMAs). The phenomenon of return-point memory that is observed during the subloop deformation of a partially-transformed SMA is a clear manifestation of the interaction between localized phase transformation and degradation of the functional properties. The present study aims to demonstrate this structure–material interaction in the modeling of return-point memory. It seems that this crucial aspect has been overlooked in previous modeling studies. For this purpose, we developed a gradient-enhanced model of pseudoelasticity that incorporates the degradation of functional properties in its constitutive description. The model is employed to reproduce the hierarchical return-point memory in a pseudoelastic NiTi wire under isothermal uniaxial tension with nested subloops. Additionally, a detailed analysis is carried out for NiTi strip with a more complex transformation pattern. Our study highlights the subtle morphological changes of phase transformation under different loading scenarios and the resulting implications for return-point memory.