Study of the electrochemical dissolution behavior of Nitinol shape memory alloy in different electrolytes for micro-ECM process

Abstract

Nickel-titanium alloy (Nitinol) is an excellent shape memory alloy (SMA) for microelectro-mechanical systems (MEMS) particularly in biomedical applications owing to its three excellent features shape memory effect (SME), superelasticity, and biocompatibility. The temperature-dependent material transformation properties of Nitinol SMAs make conventional machining difficult. Micro-ECM, a non-conventional machining process for conductive materials regardless of their strength and hardness, has the potential to fabricate microfeatures on Nitinol. This study presents the investigation on the electrochemical dissolution behavior of Nitinol in different electrolytes for micro-ECM. The influence of electrolytes on the nature of dissolution of Nitinol was studied by fabricating microchannels in three levels of parameters containing applied voltage and electrolyte concentration. The first three electrolytes were all aqueous neutral electrolytes, i.e., NaCl, NaNO3, and NaBr. The aqueous NaNO3 was successful in fabricating microchannels at all levels of process parameters. However, aqueous electrolytes form relatively a huge amount of sludge on the machined surface reducing both dissolution efficiency and machining accuracy. Thus, ethylene glycol-based NaNO3 was used to fabricate microchannels with lower depth overcut (DOC), width overcut (WOC), and length overcut (LOC) with respect to aqueous NaNO3 electrolyte. The potentiodynamic polarization (PDP) tests of Nitinol through cyclic voltammetry (CV) shows passive dissolution in aqueous electrolyte and active dissolution in non-aqueous electrolyte.