The Effects of Temperature and Time of Heat Treatment on Thermo-Mechanical Properties of Custom-Made NiTi Orthodontic Closed Coil Springs

Abstract

Nickel-Titanium (NiTi) springs have been increasingly used in orthodontics; however, no optimum condition of heat treatment has been reported. Therefore, this research was conducted to determine the optimum heat-treatment temperature and duration for the fabrication of NiTi-closed coil springs by investigating their effects on thermo-mechanical properties. As-drawn straight NiTi wires of 0.2 mm diameter were used to fabricate closed coil springs of 0.9 mm lumen diameter. The springs were heat-treated at three different temperatures (400, 450, and 500 °C) with three different durations (20, 40, and 60 min). Electron Probe Micro-Analysis (EPMA) and Differential Scanning Calorimetry (DSC) were used to investigate element composition and thermo-mechanical properties, respectively. Custom-made NiTi closed coil springs composed of 49.41%-Ti and 50.57%-Ni by atomic weight, where their DSC curves of 500 °C presented the obvious endothermic and exothermic peaks, and the austenite finish temperature (A_f) were approximately 25 °C. With increasing temperature, deactivation curves presented decreased plateau slopes generating higher superelastic ratios (SE ratios). At 500 °C, closed coil springs showed superelastic tendency with lower stress hysteresis. The thermo-mechanical properties were significantly influenced by heat-treatment temperature rather than duration. The optimum parameter appeared to be 500 °C for 40 min to produce appropriate force delivery levels, relatively low plateau slope, and lower hysteresis for orthodontic use.

Keywords: closed coil springs; nickel-titanium; orthodontics; superelasticity; thermo-mechanical properties.

Figure 1

A custom-made NiTi closed coil spring with a middle 3 mm span of active length attached with acrylic blocks on both ends.

Figure 2

A load-extension curve illustrating loading and unloading curve, unloading plateau/slope, and final slope of deactivation curve, as well as stress hysteresis.

Figure 3

Cooling (as solid lines) and heating (as dash lines) curves of custom-made and commercial NiTi closed coil springs under different temperatures and times of heat treatment (comparable with oral temperature 37 °C); M = Martensitic phase, R = Rhombohedral phase, A_s = Austenite start temperature, A_f = Austenite finish temperature, RA_s = Rhombohedral reverse start temperature.

Figure 4

Superimpose of load-extension curves of custom-made closed coil springs under different heat treatment conditions with commercial NiTi closed coil springs (TOMY^®): (a) at a constant temperature (400 °C) with various durations; (b) at a constant temperature (450 °C) with various durations; (c) temperature (500 °C) with various durations.

Figure 5

Superimpose of load-extension curves of custom-made NiTi closed coil springs under different heat treatment conditions with commercial NiTi closed coil springs (TOMY^®); (a) at a constant time (20 min) with various temperatures; (b) at a constant time (40 min) with various temperatures; (c) at a constant time (60 min) with various temperatures.

Figure 5

Superimpose of load-extension curves of custom-made NiTi closed coil springs under different heat treatment conditions with commercial NiTi closed coil springs (TOMY^®); (a) at a constant time (20 min) with various temperatures; (b) at a constant time (40 min) with various temperatures; (c) at a constant time (60 min) with various temperatures.