Mechanism of laser-chemical process to remove of oxide layer and formation of periodic microstructures for drag reduction

The paper presents a novel method of combining nanosecond (ns) pulse laser ablation (LA) with chemical etching (CE) to prepare controllable periodic microstructures on 316L stainless steel surface, which utilized the intrinsic thermal effect of ns pulse laser in processing materials. The microstructures were generated by LA, and CE removed the surface oxides and residues to make the microstructures appear. The experiments demonstrated that the size and period of microstructures can be easily controlled by adjusting the ns laser average power and scanning interval. Moreover, the grain refinement caused by the thermal effect in the LA process increased the hardness of processed area from 181 HV to 295.7 HV. The surface treated by laser ablation-chemical etching (LACE) process became hydrophobic or even superhydrophobic after being stored in an ambient atmosphere for 5 days without hydrophobic reagent treatment. When the laser average power was 17.5 W and the scanning interval was 80 μm, the contact angle on the surface was 158.6°. Compared with the polished surface, the fluid can achieve an effective slip length of 17 μm on the samples prepared by the LACE process. Hence, the periodic microstructures prepared by LACE has great potential in the self-cleaning, drag reduction application.