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 (Af) 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.