Influence of Dual Heat Treatment on the Metallurgy and Mechanical Behaviour of Diamond‐Like Carbon–Coated NiTi Rotary Systems: A Multimethod Investigation

TL;DR

This study shows how dual heat treatment affects the mechanical properties of nickel-titanium dental instruments, with different outcomes in flexibility and strength.

Contribution

The study introduces a multimethod approach to evaluate the impact of dual heat treatment on NiTi instruments' metallurgy and mechanical behavior.

Findings

Rainbow Ultra showed higher cyclic fatigue resistance and cutting efficiency due to dual heat treatment.

Rainbow One had greater torsional strength and buckling resistance, indicating stiffer behavior.

Dual heat treatment altered phase transformation temperatures and mechanical performance in NiTi instruments.

Abstract

To investigate how the application of dual heat treatment during the manufacturing process influences the mechanical performance (flexibility, cyclic fatigue resistance, torsional strength, bending, buckling and cutting efficiency) of rotary nickel‐titanium endodontic instruments.

Rainbow One (n = 58) and Rainbow Ultra (n = 58) instruments (size 25/0.06, 25 mm length) were evaluated. Geometric analysis was performed using optical 3D scanning and SEM to assess cutting edges, tip design and surface features. Metallurgical characterisation included EDS for elemental composition and DSC for phase transformation temperatures, with segments obtained from the tip, middle and coronal blade regions. Mechanical testing comprised cyclic fatigue, torsional strength (torque to failure, maximum angle of rotation), bending resistance, buckling strength and cutting efficiency. Standardised testing protocols were applied, including ASTM and ISO guidelines for mechanical assessments. Statistical analyses were conducted using Shapiro–Wilk and Levene tests to assess assumptions, followed by independent Student's t‐tests, with significance set at 5%.

Both instruments shared equivalent geometries, with deviations below 100 μm, and similar tip designs and surface characteristics. EDS confirmed near‐equiatomic NiTi composition for both systems. DSC revealed R‐phase crystallographic structure at room temperature for both instruments; however, Rainbow Ultra exhibited higher phase transformation temperatures (Rs 35°C vs. 25°C; Rf 45°C vs. 31°C; As 30°C vs. 24°C; Af 50°C vs. 38°C) and a double austenitic transformation peak, compared with a single peak for Rainbow One. Mechanically, Rainbow Ultra demonstrated greater cyclic fatigue resistance (95.5° ± 7 vs. 79.1 ± 6 s), higher maximum rotation angle (485° ± 49° vs. 402° ± 50°), and improved cutting efficiency (p < 0.05). Rainbow One exhibited higher torque to failure (1.8° ± 0.2 vs. 1.4° ± 0.1 N·cm), greater bending load (369 ± 34 vs. 336 ± 14 gf), and higher buckling resistance (339 ± 32 vs. 273 ± 24 gf), indicating stiffer behaviour.

The application of dual heat treatment during manufacturing significantly influenced the mechanical performance of the tested rotary NiTi instruments. Rainbow Ultra, subjected to sequential heat treatments, exhibited higher flexibility, cyclic fatigue resistance and cutting efficiency, whereas Rainbow One demonstrated greater torsional strength and buckling resistance.

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