*4.2. Formation of Craters and Dimples*

The formation of crater-like features in the surface was observed for many sets of process parameters but was particularly pronounced for Q100. As the initial analysis of microstructure regarding the size and distribution of chromium carbides indicates, it can be assumed that these chromium carbides are the source or at least the initiation side for these craters. The number of craters seems to correlate approximately with the carbides near the surface (cf. Figure 6). The formation of crater-like features is not unknown in LμP, but it was intensely investigated by Liebing [52] for bearing steel 100Cr6. However, Liebing [52] primarily identified oxides and sulfides as sources for melt pool disturbances or evaporation that lead to pronounced crater formation. According to the combined works of Tolochko et al. [53] and Boley et al. [54], the conclusion can be drawn that the absorption coefficient of chromium carbides at the laser wavelength is significantly larger than that of the surrounding steel matrix. Additionally, the intensity distribution for Q100 shows a significant intensity peak in the center of the distribution (cf. Figure 2a). Both effects in combination are assumed to result in the increased number of craters after laser remelting. This demonstrates that a top-hat-shaped intensity distribution or at least an intensity distribution without pronounced peaks is preferable for laser micro polishing. Additionally, it can be assumed that the chromium carbides also influence the surface topography evolution for the larger laser beam sizes. However, instead of pronounced craters, preferably long-wave dimples or depressions were created. Due to larger laser beam dimensions and thus larger melt pools, the influence of an evaporating chromium carbide of a certain size is relatively smaller than for a smaller melt pool. Additionally, larger melt pools lead to longer melt durations and increase the available time for the effective damping of capillary surface waves [28]. Capillary surface waves may result from disturbances of the melt pool such as localized evaporation [55]. On the other hand, larger melt pools lead to the remelting of more chromium carbides at the same time. However, an evolution mechanism of similar features was discussed by Nüsser et al. [43], who assume that these dimples result from macro ripple formation and the repeated remelting of these ripples. An additional effect might result from a low pulse stability of the laser beam source, which might lead to additional, undesired surface features with the approximate dimensions of the laser beam [19]. Furthermore, Spranger and Hilgenberg [56] found that a significant dissolution of carbides was a result of pulsed laser remelting of AISI D2. In sum, we assume that all these effects play a role in the formation of long-wave dimples. The partial evaporation and partial dissolution of chromium carbides are assumed to be the key reasons for disturbances of the melt pool volume, changes in melt pool dynamics, and deformation of the melt pool surface. These lead to the formation of craters at small laser beam dimensions and to long wave dimples when remelting with larger laser beams. Multiple remelting cycles (approximately 100 times per spot) lead to a partial smoothing and directional homogenization of the specific spatial roughness frequencies, as it is shown in the roughness spectra (Figures 10–12).

### *4.3. Transition to Continous Remelting Process and Micro-Hardness*

This stripe-like structure with continuous boundary lines between the individual tracks (Figure 7l,q) is typically a feature of a continuous remelting process [27]. This is to be expected for long pulse duration, for large track and pulse overlap, and for high laser fluences. This effect is essentially due to heat accumulation (Weber et al. [57,58]), so that the molten pool no longer solidifies before the following laser pulse impinges on the surface. This is an effect that was observed in LμP particularly by Temmler et al. [18] at high pulse repetition frequencies for ns laser pulses. The authors defined an empirical formula for an estimation of a threshold scan speed *vscan,th* as a function of laser beam diameter *dL*, pulse duration *tP*, and pulse repetition frequency *frep*, at which the discrete pulsed remelting process changes to a continuous one (Equation (2)).

$$\left| \upsilon\_{scan,th} \le d\_L \cdot t\_P \cdot f\_{rep} \right|^2 \tag{2}$$

Although the criterion from Equation (2) is not fulfilled for the process parameters used, the threshold scan speed (*vscan, th* = 192 mm/s; *tP* = 1.2 μs) is just smaller than the used scan speed by a factor of approximately 4. Additionally, it must be added that the introduced criterion was established on an empirical basis and should be valid for a quasicontinuous remelting process at laser polishing fluence without material ablation. However, as already determined from the microscope images and visual observations during the process, a continuous remelting process occurs only at a fluence level, where significant material evaporation is already clearly observable without optical aids. Therefore, it is reasonable to assume that the stripes are a clear indication for a continuous remelting process. This would particularly help to explain that almost no craters were visible anymore for a small laser beam size Q100 and very high fluences (*F* = 12 J/cm2).

The effect of surface ripple formation (cf. Figure 7q,r) is probably due on the one hand to the formation of a continuous melt pool and on the other hand to the irradiation of the surface in reoccurring, discrete time intervals of 50 μs (20 kHz). This leads to periodic fluctuations of the melt pool volume, which results in a periodic structuring of the surface. This principle is typically known from the WaveShape process, where it is specifically used to generate mostly periodic surface structures [7,59,60]. Furthermore, a low pulse stability of the laser beam source probably causes the generated ripples to fluctuate in shape and structure. A similar effect was specifically used by Pfefferkorn and Morrow [61] to achieve surface structuring by the spatial and temporal control of laser fluence.

The transition to the continuous remelting process also tends to be reflected in the results of the micro-hardness measurements (Figure 13). A dissolution and/or partial evaporation of chromium carbides is assumed to lead to a significant reduction of standard deviation in the micro-hardness measurements. This is presumably because only fewer, smaller, or no chromium carbides were hit in the micro-hardness measurements. Thus, the largest micro-hardness measured was approximately 550 HV instead of approximately 830 HV0.1 on the initial surface. However, at the same time, no areas of the comparatively soft steel matrix were hit any more (≈100 HV 0.1), but the minimal micro-hardness in the laser-polished fields was approximately 320 HV0.1. This indicates that carbon from the chromium carbides was increasingly dissolved in the steel matrix. It is assumed that the entire surface boundary layer was hardened through the formation of martensite and achieved an average micro-hardness of up to approximately 464 HV0.1 with significantly smaller standard deviations (partially < 30 HV0.1). Additionally, the micro-hardness of the discretely, remelted LμP surface tends to be higher than for the continuously remelted surface. This is presumably due to the higher cooling rates in the LμP process in comparison to the continuous remelting process. A similar effect has been observed in earlier works on other steels e.g., H11 (Preußner et al. [62]), or S7 (Morrow and Pfefferkorn [13]). However, Maharjan et al. [33] found for 50CrMo4 steel that the most pronounced hardening effect was achieved for longer pulse duration and cw laser radiation, i.e., in principle for smaller cooling rates. The reason for these contradictory results is unclear, but it is most likely a

consequence of the different chemical composition of these steels or the specifics of the temperature evolution during the laser treatment.
