Effect of Different Heat Treatments and Surface Treatments on the Mechanical Properties of Nickel-Titanium Rotary Files

This study aimed to compare the fatigue resistance of files made from different heat treatment methods and surface treatment. Four prototype files were created through heat treatment and titanium coating surface treatment (AT, DT, ER, EN; named arbitrarily by the manufacturer) at different times and temperatures. Artificial canals with curvatures of 45- and 90-degree were used for the fatigue testing. The files were operated at the speed of 500 rpm at 37 degrees C, and the time until fracture incurred by a 4-mm dynamic pecking motion at a speed of 8 mm/s was measured, and the number of cycles to failure (NCF) was calculated by applying rotation speed and time. The length of the fractured fragment was measured. The fractured specimens were observed under the SEM to compare the characteristics of fatigue fracture patterns. Differential scanning calorimetry analysis was performed to estimate the phase transformation temperature. One-way ANOVA with Duncan's post-hoc comparison, the Kruskal-Wallis test, and Mann-Whitney U were applied to compare the fatigue resistance among the prototypes at a significance level of 95%. Regardless of the canal angle, the EN showed the highest fatigue resistance (p < 0.05). AT had the lowest NCF at the 90-degree canal (p < 0.05). ER had a higher NCF than the DT at 45 degrees (p < 0.05), but there was no difference at 90 degrees. DSC analysis revealed that the ER and EN groups exhibited two austenite peaks above 40 degrees C. In conclusion, the file that underwent a specific temperature heat treatment with titanium coating surface treatment showed the highest fatigue resistance.