Characteristics of NiTi in the Vicinity of Laser-Induced Periodic Surface Structures

Laser systems are essential for microfabrication of NiTi components. Additional to cutting, femtosecond laser systems are capable of periodically structuring a material's surface. Such laser-induced periodic surface structures (LIPSS) bear potential for locally optimizing a surface with regard to biocompatibility or resistance to abrasion, and to generate unique labeling for documentation. In the scientific literature, a few cases of formation of LIPSS on NiTi are reported. However, little is known regarding the characteristics of NiTi in the vicinity of the region affect by the laser after treatment for LIPSS.

In the present work, LIPSS were successfully generated on NiTi using a femtosecond laser system. The integral distribution of Ni, Ti and O from the surface towards the depth were evaluated using glow-discharge optical emission spectroscopy. Local assessment of elemental distribution and features of the microstructure in high resolution were carried out using transmission electron microscopy. It is shown, that the application of the femtosecond laser results in a thin amorphous layer, likely a consequence of melting and rapid solidification during the laser pulses. The formation of Ti-rich oxides and a Ni-rich region is recognizable, but limited due to the brief interaction of the NiTi with the laser and the subsequent rapid cooling. Potential benefits and limitations will be discussed.