Influence of Laser Annealing of Kapton Polyimide Substrate on Non-Contact-Based Actuation of NiTi Shape Memory Alloy Bimorph Based Smart Actuator

Abstract

The NiTi/Kapton polyimide bimorph-based structure has grabbed a considerable attention of the researchers for the actuator applications. The devices such as microgrippers, micropumps, microrobots etc. have been explored owing to the flexibility and high power-to-weight ratio of the structure. However, the bimorph structure deals with the limitations of limited actuation and low substrate-to-film adhesion. Laser annealing can enhance the crystallinity and wettability of the polyimide surface which is favorable for better film deposition and improves the adhesion. This work investigated the laser annealing process with 355 nm Nd: YAG laser for its fluence to achieve better polyimide surface properties. The Kapton polyimide substrate of size 75 mm x 44 mm was annealed using a 355 nm Nd: YAG laser at fluences of 1.42 J/cm2, 1.63 J/cm2 and 1.83 J/cm2. The spot diameter was 2.5 mm and a constant pulse overlap of 95 % was adopted for the annealing operation. NiTi thin film was deposited on the untreated and laser annealed Kapton polyimide substrate using thermal evaporation technique. The vapor pressure of nickel is less than that of titanium and this results in deposition of Ni-rich film. To compensate this loss of Ti, the source material utilized for deposition was added with excess Ti. The source material composition was NiTi: Ti = 70:30 (by weight). The laser annealing induced roughness to the polyimide surface. The laser fluence up to 1630 mJ/cm2 results in increase in the roughness of the substrate. Laser fluence above 1630 mJ/cm2 reduced the substrate roughness. The change in surface roughness alters the contact angle of the polyimide which can quantify the degree of deposited film to the substrate. The contact angle of Kapton polyimide reduced from 82° for pristine sample to 62.9° after laser annealing at a fluence of 1.63 J/. The non-contact-based actuation of the fabricated actuators has been studied using a 1064 nm fiber laser. The NiTi SMA bimorph actuator of size 4 cm x 1.5 cm was fixed in a cantilever arrangement and supplied with laser power. The laser energy was supplied in form of a linear scan of the laser beam and the actuation was recorded using a laser displacement sensor. The actuation was studied at 15 W laser power with scan speed of 15 mm/s. The tip displacement of the actuator increased from 0.60 mm for bimorph with untreated substrate to 0.78 mm for the bimorph fabricated on laser treated substrate at 1.63 J/cm2. The actuation life cycle has improved by 71% for bimorph fabricated on laser annealed polyimide with laser fluence of 1.63 J/cm2 compared to the untreated sample. © © 2025 by ASME.