Effects of oxygen plasma source ion implantation on microstructure evolution and mechanical properties of nickel-titanium shape memory alloy.

摘要

Near-equiatomic NiTi is an important shape memory alloy used in both medical and non-medical applications, which are dependent upon the surface characteristics of NiTi. The work presented here is the first use of plasma source ion implantation with oxygen as the incident species to modify the surface structure of NiTi shape memory alloy. Two levels of voltage bias and three levels of ion dose were employed to investigate the effect of processing parameters on surface microstructure and surface-related properties.; Several surface analytical techniques, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM), were employed to analyze the effects of the surface modification on surface characteristics including oxide thickness, oxide constitution, phase distribution, morphology and topography. A two-layer surface structure consisting of an oxide layer and a precipitate accommodation layer was observed on modified NiTi. The surface morphology, roughness and hydrophilicity, which are considered to play important roles in affecting protein adsorption behavior, were found to be altered by surface modification. The effects of surface modification on surface-related properties including corrosion resistance, hardness and wear resistance were evaluated by cyclic potentiodynamic polarization tests, Knoop hardness microindentation and fretting wear tests, respectively. The optimum corrosion and wear resistance of NiTi were achieved with ion implantation under high bias and moderate dose. Archard's equation was modified by incorporating the pseudoelasticity effect on wear resistance in addition to hardness. The modified Archard's equation better describes the fretting wear resistance of NiTi. A combination of nanoindentation and AES was employed to understand the relationship between mechanical properties and composition of the modified material.