**4. Conclusions**

The obtained and summarized results on the effect of different surface engineering techniques on bulk toughness of hard and brittle Cr–V ledeburitic tool steel Vanadis 6 unambiguously indicate that:

Enhanced surface roughness has unfavorable impacts on the toughness of the material, and thereby on the durability of tools, particularly when they are heavily and dynamically loaded. It should be underlined that elevated surface roughness of tools is transmitted to the surfaces of workpieces, which leads to worsened properties of workpieces.

The presence of di ffusion nitrided regions on the steel surface reduces the toughness of the material considerably. The detrimental e ffect of nitrided regions is evident particularly in the cases when either a surface "white" layer is formed or continuous nitrides appear at the boundaries of primary austenite grains.

PVD coating has almost no di ffusion, and hence, its e ffect on the material toughness can be classified as marginal. Hence, the risk of material embrittlement due to the deposition of PVD thin films is minimal.

The abovementioned facts have clear consequences for the designers, heat treaters, and end-users of the tool products. While the presence of thin ceramic films does not influence the mechanical properties of the tools negatively, the enhanced surface roughness as well as the presence of nitrided regions reduces the material toughness. In nitriding, therefore, the heat treaters must inevitably avoid the formation of too thick regions, or reduce the probability of the growth of compound "white" layers on the surface. The tool designers, on the other hand, should keep in mind that the surface finish quality (roughness) influences not only the toughness of the tool itself but also the quality of the workpieces.

**Author Contributions:** Funding acquisition, P.J.; investigation, P.J.; supervision, P.J.; writing—original draft, review, editing, P.J.

**Funding:** The author acknowledges that the paper is a result of experimental works realized within the project VEGA 1/0264/17. Furthermore, this paper is a result of the project implementation "Center for Development and Application of Advanced Diagnostic Methods in Processing of Metallic and Non-Metallic Materials—APRODIMET", ITMS: 26220120014, supported by the Research & Development Operational Programme funded by the ERDF.

**Conflicts of Interest:** The author declares no conflict of interest.
