**3.3 Surface hardening**

**Figure 3(b)** shows the surface hardness measurement results of the samples. The average surface hardness of the as-SLM sample was approximately 396.4 HV, which increased up to 455.7 and 877.6 HV for the UNSM-25C and UNSM-800C samples, corresponding to a 13.1% and 221.3%, respectively. It is well documented in the literature that the increase in hardness is due to the combination of grain refinement by Hall-Petch expression and increased dislocation density, which are the results of elasto-plastic deformation and S<sup>2</sup> PD took place in the top and subsurface layers [30]. The deformation usually refined the grains, which hinder further deformation and gradually diminishes with the depth of the surface layer. Moreover, Zhang et al. reported that the UNSM-induced work-hardening by plastic strain may also play a major role in increasing the hardness [31]. Moreover, in particular for AM materials, the expelled pores after peening technologies may be contributed to the increase in hardness [32].
