Influence of Heat Accumulation on Morphology Debris Deposition and Wetting of LIPSS on Steel upon High Repetition Rate Femtosecond Pulses Irradiation

Abstract

The fabrication of laser-induced periodic surface structures (LIPSS) over extended areas at high processing speeds requires the use of high repetition rate femtosecond lasers. It is known that industrially relevant materials such as steel experience heat accumulation when irradiated at repetition rates above some hundreds of kHz, and significant debris redeposition can take place. However, there are few studies on how the laser repetition rate influences both the debris deposition and the final LIPSS morphology. In this work, we present a study of fs laser-induced fabrication of low spatial frequency LIPSS (LSFL), with pulse repetition rates ranging from 10 kHz to 2 MHz on commercially available steel. The morphology of the laser-structured areas as well as the redeposited debris was characterized by scanning electron microscopy (SEM) and µ-Raman spectroscopy. To identify repetition rate ranges where heat accumulation is present during the irradiations, we developed a simple heat accumulation model that solves the heat equation in 1 dimension implementing a Forward differencing in Time and Central differencing in Space (FTCS) scheme. Contact angle measurements with water demonstrated the influence of heat accumulation and debris on the functional wetting behavior. The findings are directly relevant for the processing of metals using high repetition rate femtosecond lasers, enabling the identification of optimum conditions in terms of desired morphology, functionality, and throughput.

Black marking for medical industry with ultrashort-pulsed lasers at different laser pulse durations

“…This structure is superimposed on the remaining ripples. A possible explanation is given by C. Florian et al [14], which indicates that steel substrates irradiated by femtosecond laser pulses are often covered by debris redeposition in the form of nanoparticle agglomerates. At 8,000 fs the ripples structure disappears and the structure of beads gets dominant.…”

“…Parameters to consider are the laser parameters pulse energy, pulse frequency, pulse duration, spot radius of the laser spot, polarization of the laser, the marking strategy, line distance of the hatch and the marking speed. This leads to the necessity to include the heat accumulation process into the studies, especially when working with high repetition rates [14]. Additional dependencies are the characteristics of the processed material like the material composition and surface treatment.…”

Control of processing effects under high-frequency femtosecond laser irradiation on three commercial polymers with different thermal properties

“…For higher frequencies, time between pulses is reduced, so it is more difficult for the material to diffuse the induced heat on its surface, obtaining deeper modifications at high frequency processing. High frequency irradiations also provide considerably reduced processing times and present further favorable results for ablative purposes, as higher homogeneity and lower amount of debris in the surroundings of the processed areas [11][12][13].…”