Nanomechanical Properties of M-Wire and Superelastic Wires for Rotary Instruments

Fundamental materials science studies are needed to understand reasons for differences in mechanical properties of M-Wire and conventional superelastic (SE) NiTi wire. Rotary endodontic instruments fabricated from M-Wire have superior laboratory fatigue properties compared to those fabricated from SE wire, which should result in improved clinical performance. Objective: Employ a nanoindenter to compare mechanical properties of these two NiTi wires. Methods: Segments (SportsWire LLC) from two batches of M-Wire and one batch of SE wire (Maillefer) were evaluated. Nanoindentation testing was carried out at 25°C (ENT-1100a, ELIONIX, Tokyo, Japan), using a 50 mN load. Five indentations were made for each wire batch, and the maximum depth of indentation, hardness and elastic modulus were calculated. Mean values for hardness and elastic modulus were compared using ANOVA and the Tukey multiple range test, with P < 0.05 for statistical significance. Results: Mean maximum indentation depths were 1510 nm for SE wire, and 1681 and 2315 nm for the batches of M-Wire. Mean hardness was 2.52 GPa for SE wire, and 1.55 and 1.79 GPa for the batches of M-Wire. Mean elastic modulus was 12.41 GPa for SE wire, and 5.09 GPa and 10.41 GPa for the batches of M-Wire. The hardness and elastic modulus were significantly higher for SE wire than M-Wire, and the elastic modulus for the two batches of M-Wire were significantly different. Conclusions: The significant differences in nanomechanical properties for SE wire and M-Wire should have relevance to differences in macroscopic mechanical properties obtained from the tensile test and cyclic fatigue performance for instruments fabricated from the two NiTi alloys. Differences in mechanical properties are consequences of the thermomechanical processing for the alloys. Further research is needed to determine the effects of varying the processing procedures on mechanical properties for different batches of M-Wire.