Surface structure and roughness of Nickel-Titanium wires

Nickel-Titanium wires not only show the well known shape memory effect, but also pseudoelastic behaviour. This means, as the material is bent in a wide range of tension, the stress in the material stays constant. This special behaviour allows the improvement of orthodontic devices. The investigation of wires for this application reveals great differences in the surface roughness of the wires from various suppliers. The roughness causes a loss of spring energy for the orthodontic wire by frictional effects. The investigation of the mechanical behaviour of these materials leeds to differences compared with the ideal behaviour expected for the stress/strain curve. Obviously these differences are caused by surface effects of the wires. To evaluate these questions intensive roughness measurements with profilometry, laser spectroscopy and scanning force microscopy have been performed. Further more defined bending and tensile experiments have been carried out and corresponding roughness and structure analysis with the scanning force microscope has been taken into account. By etching the wires significant surface effects could be found.