*3.5. Micro-Hardness*

The influence of fluence on micro-hardness was exemplarily investigated for Q400. In this case, the micro-hardness HV0.1 (20s) was investigated by means of nano-indentation (Picodentor HM500, Helmut Fischer GmbH, Sindelfingen, Germany) for fluences ranging from 5 to 12 J/cm<sup>2</sup> (Figure 13). Ten hardness measuring points were examined on each laser-polished surface. The hardness measurements were sorted from the smallest to largest and are shown in Figure 13a. The initial hardness H0 of the initial surface was measured the same way, and a micro-hardness of 420 (±210) HV0.1 was determined (red, dashed line in Figure 13b). The standard deviation on this measurement was very large, depending on whether chromium carbides were completely or partially hit during the measurement (cf. Figure 6b). The maximum micro-hardness on the initial surface was approximately 827 HV0.1 (chromium carbides), while the minimum hardness was only 101 HV0.1 (steel matrix; not shown in Figure 13a).

**Figure 13.** (**a**) Micro-hardness HV0.1 for ten individual measurements, and (**b**) average microhardness HV0.1 of laser polished surfaces for fluences ranging from 5 to 12 J/cm<sup>2</sup> (Q400).

Figure 13b shows the average micro-hardness and standard deviation as a function of laser fluence. At fluences of up to approximately 8 J/cm2, the hardness was thereby increased, and the standard deviation was significantly reduced to approximately ±29 HV0.1. For all laser remelted surfaces, a micro-hardness between approximately 382–464 HV was achieved. The maximum was reached at 464 (±29) HV0.1 (*F* = 8 J/cm2), the minimum average micro-hardness was approximately 382(±36) HV0.1 (*F* = 8 J/cm2). There is a tendency for greater hardness to be achieved on the discretely remelted surfaces (*F* < 9 J/cm2) compared to the presumably continuously remelted surfaces, on which the characteristic remelting stripes were also observed. This might be a result of the different characteristics of the remelting process and higher cooling rates associated with the discrete, pulsed LμP process.
