Efeito do tratamento térmico nas propriedades físicas e mecânicas de fios e instrumentos endodônticos de NiTi

Abstract

Effect of heat treatment on mechanical and physical properties of NiTi wires and endodontic instruments. The aim of this work was to compare chemical, physical, and mechanical properties of conventional NiTi wire (CW) and thermomechanically treated (MW) used to the manufacture of endodontic instruments from two different companies (Dentsply Tulsa Dental Specialties, Tulsa - OK, USA and Dentsply Malleifer - Ballaigues, Switzerland). Torque and apical force required for novel ProTaper Next endodontic instruments were also evaluated. Chemical composition was determined by energy-dispersive X-ray spectroscopy, phase constitution by X-ray diffraction and transformation temperatures by differential scanning calorimetry. Tensile loading/ unloading, three-point bending, rotating-bending fatigue tests and Vickers microhardness measurements were performed to assess the wires mechanical behavior. Fracture surfaces were observed by scanning electron microscopy and the non-deformed microstructures by transmission electron microscopy. Analysis of variance with 95% of confidence level was applied to the results. The two wires showed approximately the same chemical composition, close to the 1:1 atomic ratio. The phases austenite ( phase), B19 martensite and the R-phase were found in MW in agreement with the higher transformation temperatures found in this specimen. On the other hand, CW wire showed only austenite and transformation temperatures below room temperature. Average Vickers microhardness values were similar for MW and CW (p = 0.91) of the first manufacturer. However, MW wires from Dentsply Maillefer presented higher Vickers microhardness values than CW wire. The analysis of the tensile load-unload curves showed lower stress and greater uniformity in the transformation plateau for MW wire, which also showed the smallest stress hysteresis and apparent elastic modulus. The tensile tests to failure resulted in higher ultimate tensile stress and total elongation for MW wire. The wire MW presented flexibility and fatigue resistance higher than CW, In general, it was observed that the termomechanical treatment to which MW was subjected to resulted in improved physical and mechanical properties. The evaluation of torque and apical force during the shaping of artificial canals for ProTaper Next instruments (PTN) manufactured with MW showed that X2 instrument required higher torque values. The highest positive forces were obtained with X5 instruments and the highest peaks of negative force were reached by the instruments X1 and X2. The use of PTN under 350 revolutions per minute and four insertion movements resulted in lower levels of torque as well as apical positive force and negative force.