Investigations on the influence of laser assisted actuation and texturing of NiTi shape memory alloy bimorph for opto- mechatronics applications

Abstract

NiTi SMA/ flexible polyimide bimorph show two-way shape memory effect without training. This property of NiTi bimorph has been used in different opto-mechatronics applications such as micro mirrors, micro-positioning stages etc. The research work focuses on non-contact actuation of NiTi /Kapton polyimide bimorph. NiTi thin film is deposited on flexible substrate kapton polyimide sheet of dimension 5 × 2 cm2 using thermal evaporation process. Ytterbium doped continuous fiber laser is used for non-contact actuation of bimorph by optimizing laser parameters such as laser power, scanning speed, and number of passes. The parameters were optimized to ensure actuation of bimorph without causing damage to the bimorph for as many number of passes as possible. The actuation was carried out laser power of 14 to 16 W and laser scan speed of 13-16 mm/s. The displacement of the bimorph was found to increase with increase in laser power whereas the displacement decrease with increase in scanning speed. A minimum and maximum displacement of 0.35 mm and 3.8 mm was obtained during laser actuation of bimorph. Furthermore, the lifecycle analysis has been performed up to 100 laser passes and the phenomenon for reduction in displacement has been discussed in detail. The temperature experienced by bimorph during laser actuation has been studied using ABAQUS FEM simulation software and has been corroborated experimental values obtained by thermal camera. As the number of laser scans increased, delamination of the thin film from the polyimide was observed. To improve the adhesion between the thin film and the polyimide, laser texturing of kapton polyimide has been carried out. The laser power and scan speed were optimized so that the basic shape does not get distorted. The texturing was carried out at laser power of 13-15 W and Scan speed of 18-36 mm/s. The roughness value of the textured polyimide was measured using surface profilometer. It was found that with increase in laser power, the roughness value increased while it decreased with increase in scan speed. A maximum and minimum roughness value of 1.814 and 0.319 µm has been observed respectively. The deposition was carried out using DC sputtering process and hot plate actuation studies was carried out. A scratch test was done to check the adhesion of thin film to that of the polyimide. The test revealed that the thin film deposited over non-textured polyimide got eliminated at a load of 9 N while those deposited over the textured polyimide withstood the load. Laser actuation of NiTi bimorph was used to show its efficacy as optical function devices such as deformable mirror and optical shutter. Comparison between incident and reflected beam was carried out using MATLAB image processing and it showed that there was no major variation between incident and reflected beam.