Enhancement of mechanical properties and corrosion resistance of NiTi alloy by carbon plasma immersion ion implantation

NiTi alloys are used in orthodontic and orthopedic applications but the long-term corrosion resistance and related properties need improvement in order to satisfy the demand by more stringent applications. In this study, carbon plasma immersion ion implantation (C-PIII) is performed to improve the corrosion resistance and mechanical properties of the Ni_50.8Ti_49.2 (at.%) alloy. The structure, morphology, chemical and mechanical properties, and corrosion resistance are investigated systematically. A uniform, smooth, and crack-free surface layer with a thickness of 50 nm is produced after C-PIII for 2 h. C-PIII also changes the roughness, hardness, and elastic modulus from 17.4 nm, 64.31 GPa, and 2.74 GPa for the pristine NiTi alloy to 6.1 nm, 89.27 GPa, and 4.77 GPa of the C-PIII sample, respectively. Electrochemical impedance spectroscopy (EIS) indicates that C-PIII improves the corrosion resistance and hardness of NiTi. No surface defects are observed and the reinforced surface structure improves the corrosion resistance by 85%. Penetration of corrosive ions during immersion in 3.5% NaCl results in failure of the passive layer but subsequent reactions with titanium re-form the passive layer. The passive layer is more stable at the immersion time point of 96 h and C-PIII also increases the pitting or local corrosion resistance.