Investigating and understanding the effects of multiple femtosecond laser scans on the surface topography of metallic specimens

TL;DR

This study explores how multiple femtosecond laser scans affect metallic surface topographies, revealing two regimes with distinct features and growth behaviors, advancing understanding of laser-material interactions.

Contribution

It provides a comprehensive analysis of surface topography evolution under multiple femtosecond laser scans across different fluence regimes, highlighting new insights into surface pattern formation.

Findings

Ellipsoidal cones form at low fluence, composed of unablated steel with reduced fluence.

High fluence induces stable surface patterns like columnar and chaotic structures.

Ellipsoidal cones grow with repeated scans, unlike high-fluence patterns that remain unchanged.

Abstract

The majority of studies performed on the formation of surface features by femtosecond laser radiation focuses on single scan procedures, i.e. solely manipulating the laser beam once over the target area to fabricate different surface topographies. In this work, the effect of scanning stainless steel 304 multiple times with femtosecond laser pulses is thoroughly investigated over a wide range of fluences. The resultant laser-induced surface topographies can be categorized into two different regimes. In the low fluence regime $(F_{{\Sigma}line,max} < 130 ~J/cm^2)$, ellipsoidal cones (randomly distributed surface protrusions covered by several layers of nanoparticles) are formed. Based on chemical, crystallographic, and topographical analyses, we conclude that these ellipsoidal cones are composed of unablated steel whose conical geometry offers a significant degree of fluence reduction (35-52%). Therefore, the rest of the irradiated area is preferentially ablated at a higher rate than the ellipsoidal cones. The second, or high fluence regime $(F_{{\Sigma}line,max} > 130~ J/cm^2)$ consists of laser-induced surface patterns such as columnar and chaotic structures. Here, the surface topography showed little to no change even when the target was scanned repeatedly. This is in stark contrast to the ellipsoidal cones in the first regime, which evolve and grow continuously as more laser passes are applied.