Influence of Mechanical Deformation On Corrosion Resistance in NiTi Alloys After Glow Discharge Nitriding or Nitriding/Oxidizing Process

Application of NiTi shape memory alloys for long-term implants still remains controversial. It is due to the presence of nickel, which is cancerogenic and allergic element.  In order to limit corrosion and simultaneously increase biocompatibility, protective layers are deposited on surface. The layer can be composed form titanium oxides and/or nitrides. Apart from biological properties, ability to mechanical deformation of the layers is the most important factor which determines shape memory effect in NiTi alloy. The main goal of the present work was to study response of deposited layer on NiTi to mechanical deformation caused by strain. The layers were formed from titanium oxide and/or nitride using low temperature glow discharge process. Degree of the damage, as a result of deformation after shape recovery, was determined from measurement of corrosion resistance. Experiments were carried out in Tyrode’s solution. The optimum properties were obtained for layer deposited at 300^oC and 30 minutes. Covered NiTi alloy deformed to 6.5% does not reveal significant changes in corrosion resistance (E_cor= - 0,25V,  E_br = 2.44V). Insignificant lowering of resistance occurred after several thermo-mechanical cycles.