Laser-assisted generation of self-assembled microstructures on stainless steel

Utilising a Nd:YVO4 laser (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) and a Nd:YAG laser (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz), it was found that during the pulsed laser ablation of metal targets, such as stainless steel, periodic nodular microstructures (microcones) with average periods ranging from ∼30 to ∼50 μm were formed. This period depends on the number of accumulated laser pulses and is independent of the laser wavelength. It was found that the formation of microcones could occur after as little as 1500 pulses/spot (a lower number than previously reported) are fired onto a target surface location at laser fluence of ∼12 J/cm2, intensity of ∼1.5 GW/cm2. The initial feedback mechanism required for the formation of structures is attributed to the hydrodynamic instabilities of the melt. In addition to this, it has been shown that the structures grow along the optical axis of the incoming laser radiation. We demonstrate that highly regular structures can be produced at various angles, something not satisfactorily presented on metallic surfaces previously. The affecting factors such as incident angle of the laser beam and the structures that can be formed when varying the manner in which the laser beam is scanned over the target surface have also been investigated.