Passivation and oxygen ion implantation double surface treatment on porous NiTi shape memory alloys and its Ni suppression performance

The surfaces of porous NiTi shape memory alloys (SMAs) were modified by double treatment of passivation (PA) in HNO3 solution and oxygen plasma immersion ion implantation (O-PIII) methods. SEM and XPS were used to characterize the modified surface. It has been found that a protective film consists of three layers formed on the surface after modification. From the outmost surface towards inside, they are a 10 nm thick layer consisting of rutile and anatase TiO2 with pure rutile TiO2 on the surface and their ratio varying in gradient, a 45 nm thick layer containing anatase TiO2 and NiTi with constant ratios and a 40 nm thick layer of TiO2, TiO and NiTi with their ratio varying in gradient. The immersion tests in simulated human body fluid demonstrated that the modified porous NiTi samples exhibit a good Ni suppression performance, approaching to that of the untreated dense NiTi samples. The Ni ion content released from the porous NiTi SMAs double treated by PA and O-PIII is one magnitude lower than that from the untreated porous samples. Moreover, the released Ni ion content after 8 weeks can be reduced into the safe range acceptable to the human body.