Abstract
AIM: This study aimed to evaluate the impact of diamond-like carbon (DLC) surface treatment on the mechanical properties of nickel-titanium (NiTi) rotary instruments. METHODOLOGY: One hundred and ten nickel-titanium instruments with a size of 25/.06 and a length of 25 mm, both with (n = 55) and without (n = 55) a DLC coating were selected and compared regarding their design (stereomicroscopy, scanning electron microscopy), metallurgy (energy-dispersive X-ray spectroscopy, differential scanning calorimetry), and mechanical performance (time to fracture, bending strength, buckling strength, cutting efficiency, and microhardness). Data were analysed using Mann-Whitney and independent Student's t-test (α = 5%). RESULTS: The design analysis confirmed that both instruments had identical geometric features, similar spiral and tip designs, with DLC-coated instruments showing fewer surface irregularities and a multi-coloured appearance. Metallurgical analysis revealed identical transformation temperatures for both groups, with the R-phase starting at ~32°C, finishing at ~25°C during cooling, and the austenitic finish occurring at ~35°C. DLC-coated instruments demonstrated significantly superior cyclic fatigue resistance (p = 0.0028), lower bending load (p = 0.0294), lower cutting efficiency (p < 0.0001), and higher microhardness (p = 0.0019), whilst no difference was observed in terms of buckling strength (p = 0.3569). CONCLUSIONS: Diamond-like carbon surface treatment significantly enhanced cyclic fatigue resistance, flexibility, and microhardness of NiTi rotary instruments without compromising their structural integrity.