Effect of ceramic conversion treatments on the surface damage and nickel ion release of NiTi alloys under fretting corrosion conditions.

Recent researches have demonstrated that surface modification can improve the fretting wear resistance of NiTi alloys in air or enhance their aqueous corrosion resistance without fretting. However, little is known about the behaviour of surface engineered NiTi under fretting corrosion conditions. This is important for such body implants as orthodontic arch wires and orthopedic bone fixation devices because they need to withstand the combined attack of corrosion from body fluid and mechanical fretting. In this study, a NiTi alloy was ceramic conversion (CC) treated at 400 and 650 degrees C. The effect of the surface treatment on the fretting corrosion behaviour of NiTi alloy was investigated using fretting corrosion tests in the Ringer's solution. The experimental results have shown that the CC treatment can convert the surface of NiTi into a TiO2 layer, which can effectively improve the fretting corrosion resistance of NiTi alloy and significantly reduce Ni ion release into the Ringer's solution. Detailed SEM observations revealed that the untreated samples were severely damaged by adhesion and delamination; the high temperature (HT) (650 degrees C/1 h) treated samples were damaged mainly by spallation and adhesion; and the low temperature (LT) (400 degrees C/50 h) treated samples were characterised by mild abrasion. Mild oxidation and corrosion were also observed for all three types of samples tested under fretting corrosion conditions.