Laser-induced nanostructures on shape memory actuators for strain detection

Applying markings to components is essential, since they are required for identification of products, or they serve as markers for optical metrology. Whenever there is a requirement for markings to withstand extreme conditions, as is often the case with medical devices, the laser is the most popular tool for generating those markings. The heat input caused by the laser can be minimized by reducing the duration of the laser pulses. Ultrashort laser pulses in the femtosecond regime enable the generation of nanostructures with almost no melting nor ablation of material. This is particularly important when working with shape memory alloys, as an impact of the laser process on the actuator's function and lifespan must be kept as small as possible.

In this contribution, the nanostructures are created by using an 800 nm Ti:Sa laser. A femtosecond Bessel beam was used to focus the beam onto NiTi actuators. The small and non-diverging focal spot diameter makes the Bessel beam an ideal tool for structuring small confined areas. Its non-diverging nature goes along with an extended focal depth allowing uniform nanostructuring of uneven surfaces - e.g., the rounded shell surface of shape memory wires – without any focus control. The nanostructure pattern applied on the sample surface forms a grating, enabling in situ optical detection of the strain state of the shape memory actuator during a load cycle. Further, scanning electron microscope images were taken to investigate the behaviour of the nanostructures on a micro scale.