Picosecond laser-induced shock waves patterning on shape memory alloys

An advanced direct imprinting method with low cost, quick, and minimal environmental impact to create a thermally controllable surface pattern using nanosecond and picosecond laser pulses is reported. Patterned micro indents were generated on shape memory alloys (SMA) and aluminum using nanosecond and picosecond laser operating at various wavelengths combined with suitable transparent overlay, a sacrificial layer of graphite, and copper grid. Laser pulses at different energy densities which generate pressure pulses up to a few GPA on the surface were focused through the confinement medium, ablating the copper grid to create plasma and transferring the grid pattern onto the surface. Scanning electron microscope (SEM), atomic force microscope (AFM), and optical microscope images show that various patterns were obtained on the surface with high fidelity. Optical profile analysis indicates that the depth of the patterned sample initially increases with the laser energy and later levels off. Our simulations of the laser irradiation process also confirm that high temperature and high pressure (up to 10 GPA) could be generated when laser energy of 2 J/cm2 is used. Experimental data is in good agreement with a theoretical simulation of laser-induced shock wave propagation inside the material. Stress wave closely followed the rise time of the laser pulse to its peak values and initial decay. Ongoing experiments on a different wavelength and confinement medium conditions and recovery ratio (ratio of the depth of cold indent to the depth of the initial indent) will also be presented.