*3.3. Microhardness Measurements*

In-depth microhardness distributions were obtained with the standard Vickers test method using a Leitz Wetzlar hardness tester with a 200 g load and a 15 s load time. Vickers microhardness was measured at a depth of 2 mm from the LSP-treated surface (Figure 12). The microhardness analysis of the surface layer indicated that, after the LSP, strain hardening occurred. Strain hardening was detected as an increase in microhardness. The microhardness of the heat-treated maraging steel before the LSP was 667 HV0.2. The highest microhardness value after the LSP was measured just below the surface in a range between 730 and 740 HV0.2. The increased hardness was detected in the surface layer. The thickness of the strain-hardened surface was between 0.6 and 1.6 mm, depending on laser PD and laser SD.

Interactions of laser PD and SD have a significant influence on the hardness of an LSP-treated surface, found by a statistical analysis (ANOVA) (*P* < 0.0001) [25]. We fitted the measured values with the quadratic model. The results are presented as contour plots for the whole range of laser SD from 1.5 to 2.5 mm. The contour plots, shown in Figures 13 and 14, indicate that, in general, the maximum microhardness was achieved with a 2.0 mm-diameter laser spot, both at the surface and at a depth of 1.0 mm. Once again, this could not be directly connected with PPD. In the case of a 1.5 mm-diameter laser spot, the PPD was higher than in the case of a 2.0 or 2.5 mm-diameter laser spot. These findings confirm that, for our laser source, with a 2.0 mm-diameter laser spot, the most pronounced mechanical effect was obtained.

**Figure 12.** Micro-hardness measurements in depth for different PDs (i.e., 900, 1600, and 2500 cm<sup>−</sup>2) and different SDs (i.e., 1.5, 2.0, and 2.5 mm).

**Figure 13.** Microhardness at the surface as a function of laser SD for different PDs (i.e., 900, 1600, and 2500 cm<sup>−</sup>2).

**Figure 14.** Microhardness at a depth of 1.0 mm as a function of laser SD for different PDs (i.e., 900, 1600, and 2500 cm<sup>−</sup>2).

Additionally, other researchers found out that higher overlapping rates and multiple LSP impacts tend to cause a hardness increase [4,9]. We have previously shown that hardness increases after LSP is mainly due to the presence of compressive RSs [15,22]. With different LSP parameters, the microhardness decrease from the surface to a depth of 1.0 mm is almost linear. The relationship between microhardness and laser SD for PD = 1600 cm<sup>−</sup><sup>2</sup> is shown in Figure 15.

**Figure 15.** Microhardness at the surface and at a depth of 1.0 mm as a function of laser SD for PD = 1600 cm<sup>−</sup>2.
