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Subsurface Understanding of the Formation of Hierarchical Surface Structures Produced with Femtosecond Laser Pulses
Femtosecond lasers were first utilized for precision cutting/drilling and spectroscopy, but as peak laser power increased, so did the number of applications, including surface functionalization. Femtosecond laser pulses can create self-organized micro/nanostructures on the surface of most materials by controlling processing parameters such as laser fluence, number of pulses per area, and environment. Researchers primarily study the hierarchical structures by analyzing surface topology and chemistry, although, the complex processes that lead to structure formation cannot be understood solely with surface sensitive techniques. In this dissertation, self-organized structures are cross-sectioned and subsurface features are investigated to explain their formation mechanisms. A newly developed technique utilizing layers of aluminum and low carbon steel is presented. Visualizing the redistribution of material by each formation mechanism is possible by taking advantage of the chemical contrast of the different metals in energy dispersive x-ray spectroscopy maps and line scans. This work differentiates between areas that developed through each of the structure formation mechanisms: preferential ablation, redeposition of ablated materials, fluid flow, and resolidification of melted/sintered materials. Potential applications of multi-material, multi-layered structures are briefly explored. In addition, the effect of other processing parameters is investigated including processing gas, pressure, and laser polarization. Detailed studies of aluminum processed in nitrogen-rich gases is compared to processing in air. With the proper conditions, aluminum nitride is incorporated into the self-organized surface structures which has applications in heat transfer.
Electrical engineering|Materials science
Tsubaki, Alfred Tooru, "Subsurface Understanding of the Formation of Hierarchical Surface Structures Produced with Femtosecond Laser Pulses" (2020). ETD collection for University of Nebraska - Lincoln. AAI28027157.