Study of the Mechanical Properties of NiTi Modified by Carbon Plasma Immersion Ion Implantation Using Nano-Indentation Test and Finite Element Method Simulation

Plasma immersion ion implantation is one of the common methods to enhance the corrosion resistance, mechanical properties, and biological characteristics of NiTi alloys. Herein, the nano-mechanical behavior and distribution of compressive and tensile stress of NiTi alloy samples modified by carbon plasma immersion ion implantation (C-PIII) are analyzed by atomic force microscopy (AFM), nano-indentation test, and finite element method (FEM) simulation. C-PIII produces a modified layer with a thickness of 48 nm. The average surface roughness decreases from 34.023 to 25.180 nm and the hardness and Young's modulus increase to 80.7 and 21.8%, respectively, after C-PIII. FEM simulation reveals that elastic and plastic deformation similar to that observed experimentally arises from the increase in strength and changes in the surface phases stemming from C-PIII. The larger Young's modulus and hardness of the C-PIII sample reduces the penetration depth and plastic strain during nano-indentation resulting in a decrease of the compressive residual stress by 46% from -0.619 to -0.313 GPa at the tip of the indenter. Our results show that the durability and service lifetime of NiTi alloys can be improved by C-PIII thus boding well for biomedical applications in harsh environments such as blood vessel stents and bone joints.