*4.3. Raman Spectroscopy Analysis*

The disordering of the crystallographic structure is also validated by Raman spectroscopy measurements on the surface of the laser textured areas using the micro Raman microscope LabRam HR VIS (Horiba Scientific). The excitation wavelength of 473 nm was deliberately chosen to measure with a shallow depth of penetration and thus to determine the superficially changes in the material properties. Figure 8 (left) illustrates the investigated microscopic features by high-resolution field emission scanning electron microscopy (FESEM) images, such as: a) untreated steel surface for reference purposes, as well as ripples and CLP´s produced with *H*<sup>0</sup> = 1.7 J/cm<sup>2</sup> and b) *pd* = 2 μm*, ld* = 10 μm, 5 scans; c) *pd* = 2 μm, *ld* = 10 μm, 20 scans; and d) *pd* = 2 μm, *ld* = 2 μm, 20 scans. The corresponding Raman signals detected from the laser excitation of the substrate surfaces are presented in Figure 8 (right, e–h). The reference spectrum in Figure 8e shows two sharp bands below 800 cm-1 which are characteristic for the original (untreated) steel material. On the laser processed surfaces, however, these bands merge and become wider with increasing total energy input. Further information on this signal broadening can be found in the literature reporting broad spectral features appearing on furnace heat treated Fe-Cr-Ni alloy [49]. The spectra broadening was related to spinel-like chromium oxide, disordered FexCr3-xO4 spinel or an amorphous oxide layer forming at the substrate surface at temperatures ranging between 300 ◦C and 500 ◦C. From this it can be concluded, the crystal structure of the laser textured surfaces is no longer ordered and becomes more amorphous due to potential deformation or melting during the laser processing.

In Figure 8, the spectral features shown in the range from 600 cm−<sup>1</sup> to 800 cm−<sup>1</sup> matches pretty well with FeCr2O4 and Cr2O3 Raman spectra published in the RRUFF™ database [50]. The slight shift of the spectral peaks, for instance 683.8 cm−<sup>1</sup> versus 679.5 cm−<sup>1</sup> for the untreated and laser textured substrates, gives a hint for compressive or tensile stresses induced by the laser process. The weak features appearing in the Raman spectra between 1250 cm−<sup>1</sup> and 1500 cm−<sup>1</sup> might result from the carbon layer developed on the metal alloy surface without significant effect from the laser texturing. However, the detected Raman signals could not unambiguously be assigned to specific chemical compounds due to overlapping peak positions. In consequence, the Raman analysis carried out here is more suitable for the qualitative assessment of the laser textured substrates than for the exact determination of the chemical composition of the laser treated (sub)surface region.

**Figure 8.** Raman analysis carried out on untreated stainless steel surface for reference purposes (**a**) as well as laser made ripple (**b**) and CLP (**c**,**d**) surface textures. The corresponding Raman signals detected following 473 nm laser excitation are presented on the right side of the figure (**e**–**h**).
