Date of Award

5-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Committee Chair/Advisor

Dr. Xin Zhao

Committee Member

Dr. Hongseok Choi

Committee Member

Dr. Chenning Tong

Committee Member

Dr. Gang Li

Abstract

Surface structures are crucial for regulating surface functions, including self-cleaning, surface coloration, and anti-fouling, which are of significance in various applications such as prolonging the lifetime of the solar cells, anti-counterfeiting marking, and enhancing the vessel’s performance. Ultrafast laser technology emerges as a revolutionary approach for surface structuring, distinguished by its exceptional precision, high throughput, versatility across materials, and remarkable processing efficiency. Despite the intensive study in the ultrafast laser surface structuring area, the fundamental mechanisms accounting for the ultrafast laser-induced surface structures are still not well understood. The correlation among laser processing parameters, surface structures, and surface functions remains unclear, hindering the applications of this advanced technique. This study aims to advance the fundamental understanding of the formation behaviors of ultrafast laser-induced surface structures on metals and dielectrics, materials critical for diverse applications, as well as establish the process-structure-property relationship for various potential applications. By manipulating laser parameters such as laser fluence, overlapping ratio, and repetition rate, this study has successfully generated four distinct surface structures on stainless steel - low spatial frequency laser-induced periodic surface structure (LSFL), high spatial frequency laser-induced periodic surface structure-1 (HSFL-1), HSFL-2, and microgrooves - as well as seven unique surface structures on fused silica. These include micro-dots array, micro-dots array with laser-induced periodic surface structures (LIPSSs), LIPSSs, microgrooves with LIPSSs, microgrooves with irregular nanostructures, micro-channels, and micro-channels with LIPSSs, all fabricated using a femtosecond laser. The formation behaviors of these structures have been systematically explored. By integrating experimental analyses based on single and two-spot irradiation with advanced electromagnetic (EM) simulations employing the Finite-Difference Time-Domain (FDTD) method, the inter-pulse formation behavior of LSFL on fused silica has been unveiled, demonstrating a strong alignment with the numerically simulated laser energy redistribution on the surface. This supports the EM-based mechanism for LSFL formation. Moreover, for the first time, the sensitivity to scanning direction in ultrafast laser surface structuring has been discovered, showing that switching the scanning direction can create two distinct surface structures, a novel phenomenon attributed to non-thermal (stress) accumulation. In addition, applications of ultrafast laser surface structuring on controlling surface wettability have been evaluated on stainless steel and fused silica, aiming to correlate laser-induced surface structures with changes in wettability. A detailed investigation into the long-term ultrafast laserinduced surface wetting performance reveals a complicated interplay mechanism between surface structures and surface chemistry. Furthermore, a new approach (double-scan method) for creating periodic nanostructures that simplifies optical system requirements has been developed, facilitating advanced surface coloration applications. This research not only seeks to deepen our understanding of the mechanisms behind surface structure formation but also to enhance material functionalities through laser processing innovations, addressing the limitations of current technologies. Through this comprehensive study, I showcase the potential of ultrafast laser processing in advancing material functionalities, contributing significantly to the fields of materials science and laser manufacturing.

Author ORCID Identifier

0000-0003-3401-4796

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