*3.3. Micro-Roughness*

In the following, the evolution of the micro-roughness of the laser remelted surfaces is presented in detail based on WLI images. The image size is 640 × 480 pixels, which corresponds to a spatial resolution of approximately 0.1 μm. This resolution and image size were chosen, because micro polishing particularly achieves or intends to achieve a reduction of micro-roughness in this range. Remelting has already started at 4 J/cm<sup>2</sup> for Q200 and Q400 (Figure 8g,m), which is evident from the rounded shape of the visible surface structures. In the case of Q100, effective remelting begins at fluences greater than 4 J/cm2. At a fluence of 6 J/cm<sup>2</sup> (Figure 8b), a rounding of surface features can already be seen. However, in addition, crater-like depressions are observed with increasing frequency (Figure 8c,d,g,h). These tend to become deeper and wider with increasing fluence (Figure 8e,i,j). Furthermore, significant micro-porosity is visible at different laser beam dimensions and fluences (Figure 8i,j,o,p).

A comparatively small micro-roughness is observed at fluences of more than 8 J/cm<sup>2</sup> for large beam dimensions Q200 and Q400 (Figure 8j,k,o,p,q). At the same time, as the fluence increases, the stripe structure at the distance of the track offset also becomes more pronounced at increasing fluences (Figure 8k,l,q,r). In all cases, a certain waviness remains on the surface or cannot be smoothed by micro polishing.

In addition to a general overview of the resulting micro-roughness (Figure 8), indications for the origin of crater-like features were found at selected process parameters. Figure 9a shows a WLI image of the surface topography after laser remelting (Q200, *F* = 4 J/cm2) in comparison to a representative SEM image of the surface near chromium carbide distribution in the initial bulk material (Figure 9b).

Even though an unambiguous correlation is difficult, Figure 9 reasonably suggests that the size, agglomeration, and distribution of chromium carbides are most likely the cause and starting point for randomly distributed craters of different sizes after laser remelting.

**Figure 8.** (**<sup>a</sup>**–**<sup>r</sup>**) WLI images of representative surface topographies as a function of laser beam dimension (Q100, Q200, Q400) and six selected laser fluence used for LμP (4, 6, 7, 8, 9, 10, 12 J/cm2).

**Figure 9.** (**a**) WLI image of surface after laser remelting (Q200; *F* = 4 J/cm2) and (**b**) SEM images of size and distribution of chromium carbides of initial surface.
