Nanosecond pulsed laser irradiation of stainless steel 304L: Oxide growth and effects on underlying metal

Nanosecond-pulsed, infrared (1064 nm) laser irradiation was used to create metal oxide coatings on the surface of polished stainless steel 304L austenite for application as color markings and unique tags/identifiers. By rastering a Gaussian-shaped, focused laser beam across a specimen in air, continuous metal oxide coatings were grown to thicknesses in the range of 20 to ~ 500 nm. Oxide coating thickness generally increased with laser fluence. However, for large accumulated fluences in excess of ~ 600–800 J/cm2, oxide growth was affected by evaporation and particle ejection resulting in a decreased coating thickness. Transmission electron microscopy and X-ray diffraction revealed that oxide coatings developed a polycrystalline, spinel structure having a lattice constant = 8.4 Å (consistent with MnCr2O4 and Fe3O4). Pulsed laser irradiation and oxide growth modified the composition of stainless steel substrates by reducing the Cr and Mn concentration within the melt zone. The reflectance and chromaticity of laser-fabricated oxide coatings were characterized using spectrophotometry. These optical properties are described in the context of measured oxide thicknesses.

► Laser-stimulated surface oxidation of stainless steel
► Formation of color markings
► Characterization of oxide thickness, microstructure, phase and composition
► Optical properties including reflectance and chromaticity
► Correlation of optical properties with oxide thickness