Surface oxidation of NiTi shape memory alloy

Mechanically polished NiTi alloy (50 at% Ni) was subjected to heat treatment in air in the temperature range 300-800 degreesC and characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Thermogravimetry measurements were carried out to investigate the kinetics of oxidation. The results of thermodynamic calculations were compared to the experimental observations. It was found that NiTi alloy exhibits different oxidation behaviour at temperatures below and above 500 degreesC. A Ni-free zone was found in the oxide layer for oxidation temperatures of 500 degreesC and 600 degreesC. The oxidation at 500 degreesC produces a smooth protective nickel-free oxide layer with a relatively small amount of Ni species at the air/ oxide interface, which is in favour of good biocompatibility of NiTi implants. The oxidation mechanism for the NiTi shape memory alloy is discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.