Understanding the role of oxide layers on color generation and surface characteristics in nanosecond laser color marking of stainless steel

Abstract

This study conducts an experimental analysis of Laser Color Marking (LCM) on Stainless Steel, aiming to elucidate the role of oxides in color formation. Employing a nanosecond pulsed fiber laser, we systematically varied process parameters to successfully generate a diverse array of distinctive colors. Remarkably, a myriad of colors became evident at the micro-scale, offering a striking contrast to the macroscopic view dominated by a singular prominent color. These colors exhibited a notable increase in oxygen composition, reaching levels as high as 24.5%, coupled with the formation of a thin oxide film spanning thicknesses from 220 to 850 nm. The prevalence of chromium oxides resulted in greenish or bluish shades, whereas iron oxides’ dominance produced brownish and reddish hues. Particularly intriguing was the revelation of a multi-layer oxide film in samples like brown. Furthermore, the nature of oxides was also found to have an influence on surface characteristics. Chromium oxide-dominated colors exhibited ablation depth, while iron oxide-dominated colors resulted in surface overgrowth due to their porous nature. Correspondingly, Vickers hardness of samples dominated by chromium oxides showcased a significant enhancement in microhardness compared to those with iron oxide dominance. Lastly, we observed minimal degradation in surface roughness characteristics after LCM, with variations ranging from 262 nm to 463 nm, compared to 150 nm for untreated samples. These comprehensive findings provide valuable insights for optimizing the LCM process for stainless-steel color markings.