Microstructures of M-Wire and Superelastic NiTi Wires for Rotary Instruments

Rotary endodontic instruments fabricated from M-Wire have recently become commercially available (Dentsply Tulsa Dental). M-Wire is processed by a proprietary thermomechanical procedure that yields significantly improved mechanical properties compared to superelastic (SE) NiTi wire that has been used to fabricate the instruments. At the recent 2008 AADR meeting, our group presented results of studies using scanning transmission electron microscopy (Paper #101127), Micro-X-ray diffraction (Paper #101192), and temperature-modulated differential scanning calorimetry (Paper #100962) to compare M-Wire with conventional SE NiTi wire. Objective: Develop an etching procedure to reveal the microstructures of M-Wire and conventional SE Wire (Maillefer). Methods: Segments of sample wires of both NiTi alloys (provided by SportsWire LLC) were resin-mounted and polished with a sequence of abrasives following standard metallographic procedures. After preliminary experiments, an aqueous etchant containing hydrofluoric, nitric and acetic acids yielded high-quality microstructures. Photomicrographs were obtained with an optical microscope having digital image capture capability. Results: M-Wire and SE wire had room-temperature microstructures consisting of colonies of lenticular features reminiscent of a martensite structure. While unambiguous phase identification of these features is not possible from microscopy solely, the features were much coarser for M-Wire compared to SE wire. Our previous Micro-XRD and TMDSC studies indicate that at room temperature SE wire has the austenite structure and M-Wire appears to be predominantly austenite with some martensite and R–phase. Conclusions: The microstructures of both M-Wire and SE wire show evidence of a pronounced martensite structure, which Micro-XRD and TMDSC analyses in our previous studies show has undergone phase transformation. The observed difference in mechanical properties for these two wires is consistent with their microstructures. These optical microscope photographs should correlate with features on the fracture surfaces of the two NiTi alloys for endodontic instruments, which will be reported in a separate study.