Nanoindentation for NiTi Instruments Modified by Plasma Immersion Ion Implantation

Although NiTi rotary endodontic instruments have widespread use, there is need for improved clinical performance. Purpose: Utilize nanoindenter to investigate surface hardness and elastic modulus for NiTi instruments modified by a new Plasma Immersion Ion Implantation (PIII) technique. Methods: Three as-received ProTaper F2 rotary instruments (Dentsply/Maillefer) were cross-sectioned by low-speed saw, with surfaces polished through 0.05 µm alumina. Two specimens were treated (Institute of Nuclear Energy Research, Taiwan) at 20 kV target voltage and incident nitrogen ion dose of 5x10¹⁷ cm^–2 for 1 hour (group I) and 2 hours (group II); the third instrument served as the control. Resin-mounted specimens were placed in the nanoindenter (MTS Systems Corporation), and all tests were performed within 500 nm depth. Means of hardness or elastic modulus of experimental instruments were compared (ANOVA, Tukey) to the control instrument for 16 measurements. Results: Instruments had thin TiN surface layers after PIII treatment. Surface hardness and elastic modulus of the experimental groups were significantly increased compared to the control group. Mean values of both properties for PIII-treated instruments decreased gradually from the surface to the NiTi matrix with increased nanoindenter displacement. Maximum hardness values were 7.11 and 7.37 GPa, which occurred at 12 and 18 nm depth for groups I and II instruments, respectively. Mean hardness was 3.69 GPa for the NiTi matrix. Both groups of PIII instruments had increased hardness within 120 nm from the surface. Maximum elastic modulus values were 102.8 and 103.6 GPa for groups I and II instruments, respectively, and occurred at 3 nm depth. Mean elastic modulus was 59.8 GPa for the NiTi matrix. Both PIII groups had increased elastic modulus within 60 nm from the surface. Conclusions: PIII is a promising surface modification technique to improve surface hardness and elastic modulus of NiTi instruments.