Micro-XRD and temperature-modulated DSC investigation of nickel-titanium rotary endodontic instruments

Objectives: Employ Micro-X-ray diffraction and temperature-modulated differential scanning calorimetry to investigate microstructural phases, phase transformations, and effects of heat treatment for rotary nickel-titanium instruments. Methods: Representative as-received and clinically used ProFile GT and ProTaper instruments were principally studied. Micro-XRD analyses (Cu Kα X-rays) were performed at 25 °C on areas of approximately 50 μm diameter near the tip and up to 9 mm from the tip. TMDSC analyses were performed from -80 to 100 °C and back to -80 °C on segments cut from instruments, using a linear heating and cooling rate of 2 °C/min, sinusoidal oscillation of 0.318 °C, and period of 60 s. Instruments were also heat treated 15 min in a nitrogen atmosphere at 400, 500, 600 and 850 °C, and analyzed. Results: At all Micro-XRD analysis regions the strongest peak occurred near 42°, indicating that instruments were mostly austenite, with perhaps some R-phase and martensite. Tip and adjacent regions had smallest peak intensities, indicative of greater work hardening, and the intensity at other sites depended on the instrument. TMDSC heating and cooling curves had single peaks for transformations between martensite and austenite. Austenite-finish (A_f) temperatures and enthalpy changes were similar for as-received and used instruments. Heat treatments at 400, 500 and 600 °C raised the A_f temperature to 45-50 °C, and heat treatment at 850 °C caused drastic changes in transformation behavior. Significance: Micro-XRD provides novel information about NiTi phases at different positions on instruments. TMDSC indicates that heat treatment might yield instruments with substantial martensite and improved clinical performance.