Effect of heat treatment on transformation temperatures and bending properties of nickel-titanium endodontic instruments.
OBJECTIVE
To investigate the effect of heat treatment on the bending properties of nickel-titanium endodontic instruments in relation to their transformation behaviour.


METHODS
Nickel-titanium super-elastic alloy wire (1.00 mm Ø) was processed into a conical shape with a 0.30 mm diameter tip and 0.06 taper. The heat treatment temperature was set at 440 or 500 degrees C for a period of 10 or 30 min. Nonheat-treated specimens were used as controls. The phase transformation behaviour was examined using differential scanning calorimetry. A cantilever-bending test was used to evaluate the bending properties of the specimens. Data were analyzed by ANOVA and the Tukey-Kramer test (P = 0.05).


RESULTS
The transformation temperature was higher for each heat treatment condition compared with the control. Two clear thermal peaks were observed for the heat treatment at 440 degrees C. The specimen heated at 440 degrees C for 30 min exhibited the highest temperatures for M(s) and A(f), with subsequently lower temperatures observed for specimens heated at 440 degrees C for 10 min, 500 degrees C for 30 min, 500 degrees C for 10 min, and control specimens. The sample heated at 440 degrees C for 30 min had the lowest bending load values (P < 0.05), both in the elastic range (0.5 mm deflection) and in the super-elastic range (2.0 mm deflection). The influence of heat treatment time was less than that of heat treatment temperature.


CONCLUSIONS
Change in the transformation behaviour by heat treatment may be effective in increasing the flexibility of nickel-titanium endodontic instruments.
Yahata
Y
Y
Pulp Biology and Endodontics, Department of Restorative Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan. y.yahata.endo@tmd.ac.jp
Yoneyama
T
T
Hayashi
Y
Y
Ebihara
A
A
Doi
H
H
Hanawa
T
T
Suda
H
H
D002152:Calorimetry, Differential Scanning; D003722:Dental Alloys / Q000737:chemistry*; D055119:Elastic Modulus; D004548:Elasticity; D004867:Equipment Design; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D009532:Nickel / Q000737:chemistry*; D018583:Pliability; D018915:Root Canal Preparation / Q000295:instrumentation*; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*
D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium