*4.2. Microstructure Characterization*

The laser made microscopic surfaces features were investigated regarding their microstructural characteristics in order to assess whether or not the metallographic structure underneath the surface was affected by the laser processing. Therefore, the ion beam slope cutting (IBSC) technique [46,47] was applied with help of the EM TIC 020 Milling System (Leica Microsystems) to prepare the laser textured regions for cross-sectional scanning electron microscope (SEM) and electron back scatter diffraction (EBSD) inspections. The areas of interest were exposed at an angle of 90◦ to create a slope plane for the representative view on the laser-treated near-surface region. The advantage of the IBSC preparation method over common mechanical preparation techniques was that no additional deformations were introduced to the material [47] and thus the original metallographic structure of the microscopic features was exposed as developed during laser processing.

In the SEM analysis, two detector modes were used for providing different information for the ion beam cut slope planes on the laser processed surfaces. First, the signal detected from the secondary electrons (SE detector mode) is carrying more detailed information from the laser treated surface as it originated from the inelastic interaction between the electron beam and the sample atoms. And second, the signal received from the backscattered electrons (BSE detector mode) resulted from the inelastic interactions between the electron beam and the sample providing in depth information from deeper regions of the laser processed areas. In addition, the EBSD method was applied for a more precise insight on the grain size, crystallographic structure and orientation of the laser affected subsurface regions.

The cross-section SEM micrographs of Figure 7 show two laser textures at different magnifications: LSFL-ripples produced on the steel sample at 1.7 J/cm<sup>2</sup> and 5 scan passes (left, a–e) and CLP formations covering the steel surface that were made with pulses of same laser peak fluence but 20 scan passes (right, f–j). On the one hand, there is a clear difference in the height and period of the developed laser texture features. From Figure 7 left, the height of the LSFL can be estimated of about 0.4 μm and the period is somewhat less than the wavelength of the impinging laser beam of 1.03 μm which is consistent to our previous findings for near-infrared ultrashort laser beams [48]. The height of the CLP structures

can be seen in the range between 10 μm and 20 μm and their period of about 10 μm is almost one order of magnitude greater than the ripple period. On the other hand, there is a grain structure variation between the near surface region and the base material thus indicating the laser affected subsurface region. This can clearly be seen in Figure 7c,d as well as in Figure 7h,i where the grain size is much smaller in the molten and re-solidified transition layer than in the bulk metal. According to this, the depth of the laser affected region is approximated by the thickness of the transformed subsurface layer, which is in the range between 2 μm and 5 μm for the ripple textures and from 20 μm up to 30 μm for the CLPs. The formation of such a transition layer is also confirmed in Figure 7e,j showing the results of the EBSD measurements carried out on the laser-treated near-surface region. The re-solidified areas formerly melted by the laser irradiations become visible in the EBSD mappings by slight orientation changes within the larger grains.

**Figure 7.** SEM micrographs recorded with the SE (**a**,**f**) and BSE detector (**b**,**g**); detailed micrographs (**c**,**d**,**h**,**i**) and electron back scatter diffraction (EBSD) mappings (**e,j**) to illustrate the depth of the laser affected zone.
