Oxide Formation, Morphology, and Nanohardness of Laser-Patterned Steel Surfaces

A large variety of tribological research studies dealt with the effect of laser induced surface textures and the respective parameters such as depth, feature diameter, and area density on the tribological efficiency. However, only few studies have examined chemical and microstructural changes induced by the laser-patterning process, which can have a significant impact on the tribological properties. The aim of this study is to analyze the surface chemistry of laser-induced line-like patterns with a structural wavelength of 18 mm on steel surfaces (AISI 304), i.e., oxide scale thickness and morphology, prior to and after laser processing. X-ray photoelectron spectroscopy combined with an Argon ion sputtering facility was used to measure depth profiles of the iron- and chromium oxide layer. A subsequent analysis performed by atom probe tomography demonstrated that the laser interference patterning leads to an increased iron- and chromium oxide layer by a factor of 8 compared to the polished unpatterned reference surface and to a change in the oxide morphology from a solid solution in the reference to a layered structure in the laser-patterned state. Nanoindentation experiments revealed that the nano-hardness in the laser-intensity maximum position is two times larger than in the reference position. Finally, hardness calculations assuming a bilayer model correlate the observed nano-hardness with the measured surface chemistry.