Roughness studies of NiTi shape memory alloy treated by nitrogen plasma based ion implantation at high temperatures

Abstract

The NiTi shape memory alloys (SMA) are employed in various applications, thanks to its specific mechanical properties. However, for many applications, it is interesting to improve its surface mechanical resistance to tribological or chemical attacks. This work aims to analyze the effects of nitrogen plasma based ion implantation (PBII) technique on the surface of NiTi produced by electron beam melting (EBM). The samples were treated for 120min (550°C and 770°C) and 360min (550°C), with 16kV high voltage pulses. The surfaces were analysed by X-ray diffraction (XRD), white light interferometry (WLI) and glow discharge optical emission spectrometry (GDOES). In addition to martensitic and austenitic NiTi, the XRD analysis shows the appearance of titanium nitride TiN and Ni3Ti phase in the PBII treated samples. Whereas the untreated specimen presents a RMS roughness RRMS around 16nm, the PBII treatments were shown to increase it, from 44nm, at 550°C, to 82nm at 770°C. The nitrogen incorporation ranges from 150nm (at 550°C, whatever the duration), up to more than 500nm (770°C) thanks to temperature activated diffusion. The increase of RRMS may be explained by the sputtering effect but it may be linked to the presence of a buried oxi-nitride phase grown by diffusion at higher temperature.