Effect of martensitic transformation on the performance of coated NiTi surfaces

The comparative mechanical and corrosion performances of electropolished, uncoated NiTi surfaces, NiTi surfaces coated with diamond-like carbon, nitrided NiTi surfaces and NiTi surfaces coated with Ti were evaluated. To understand the effect of strain on coatings, the patterns of the deformation (local versus total strain) and alteration of surface topography induced by stress activated transformation in the substrate were examined using light, scanning electron and confocal laser scanning microscopy. Samples were strained in tension mode within the stress–strain plateau (1%, ∼3%, and 5.6% local strain) and above it at ∼8% local strain. In contrast to the substrate material that retained surface integrity, coated surfaces started cracking at very low strains. First cracks were observed at 1% for nitrided and at ∼3% local strain for surfaces coated with diamond-like carbon and Ti, respectively. Cracks developed mostly at the locations of impurity-related particles. A corrosion study conducted in Ringer Solution showed that coated strain-free NiTi surfaces might not have any passivity and material under 8% local strain exhibited deterioration in corrosion resistance.