# Formation Mechanisms of Micro-Nano Structures on Steels by Strong-Field Femtosecond Laser Filament Processing

## TL;DR

This paper explores how femtosecond laser filaments create micro-nano structures on steel surfaces and identifies the mechanisms behind their formation.

## Contribution

The study reveals the core–reservoir energy distribution and dynamic interplay of ablation, molten flow, and vapor condensation in forming micro-nano structures.

## Key findings

- Filament-induced structures on steel evolve through sequential morphological transitions.
- Ablation, molten metal flow, and vapor condensation jointly drive structure evolution.
- The core–reservoir energy distribution is critical in determining surface structure formation.

## Abstract

Functional steel surfaces engineered through tailored micro-nano structures are increasingly vital for various applications such as high-performance aerospace components, energy conversion systems and defense equipment. Femtosecond laser filament processing is a recently proposed remote fabrication technique, showing the capability of fabricating micro-nano structures on irregular and large-area surfaces without the need of tight focusing. Nevertheless, the mechanisms underlying the formation of filament-induced structures remain not fully understood. Here we systematically investigate the formation mechanisms of filament-induced micro-nano structures on stainless steel surfaces by processing stainless steel in three manners: point, line, and area. We clarify the decisive role of the unique core–reservoir energy distribution of the filament in the formation of filament-induced micro-nano structures, and reveal that ablation, molten metal flow, and metal vapor condensation jointly drive the structure evolution through a dynamic interplay of competition and coupling, giving rise to the sequential morphological transitions of surface structures, from laser-induced periodic surface structures to ripple-like, crater-like, honeycomb-like, and ultimately taro-leaf-like structures. Our work not only clarifies the mechanisms of femtosecond laser filament processed morphological structures on steels but also provides insights onto intelligent manufacturing and design of advanced functional steel materials.