**4. Conclusions**

In this study, the AlCrSiN hard coatings were deposited on the plasma nitrided SKH9 steel using a duplex treatment combing plasma nitriding and cathodic-arc evaporation PVD technology. The deposited AlCrSiN had bottom interlayers of CrN and AlCrN and a transition layer (AlCrSiN/AlCrN). From SEM and XRD evaluation, it was observed that the AlCrSiN coating with gradient layer structure was a dense structure of nanocrystalline B1-NaCl, and it showed that the coating had obvious columnar bottom layers of AlCrN and CrN structure. AlCrSiN-coated samples (without PN pretreatment) possessed hardness of 2285 HV25g. The PN + AlCrSiN had the highest hardness of 3256 HV25g among all the samples. This significant change occurred due to the support of the modified layer formed as a result of the nitriding process and the reduction of plastic deformation under the applied load.

Impact fatigue tests for AlCrSiN- and PN + AlCrSiN-coated samples at room temperature and at high temperature (500 ◦C) were conducted. Both the AlCrSiN and duplex-treated samples possessed good impact fatigue performance at room temperature. The PN + AlCrSiN duplex-treated sample could resist 400k impacts at a high temperature of 500 ◦C, where only small area of iron signal from the substrate was exposed and no cohesive failure was observed. The PN + AlCrSiN duplex-treated tool steel had high hardness, and it possessed good impact fatigue performance at room temperature and at high temperature.

**Author Contributions:** Conceptualization: Y.-Y.C., S.A.; Methodology: Y.-Y.C., S.A.; Validation: Y.-Y.C.; Formal Analysis: Y.-Y.C., S.A.; Investigation: Y.-Y.C., S.A.; Resources: Y.-Y.C.; Data Curation: Y.-Y.C., S.A.; Writing—Original Draft Preparation: Y.-Y.C., S.A.; Writing—Review and Editing: Y.-Y.C.; Visualization: Y.-Y.C.; Supervision: Y.-Y.C.; Project Administration: Y.-Y.C.; Funding Acquisition: Y.-Y.C.

**Funding:** This research was supported by the Ministry of Science and Technology (MOST 107-2218-E-131 -001 and MOST 107-2221-E-150-003) of Taiwan.

**Acknowledgments:** The instrumental assistance from the Common Lab. for Micro/Nano Sci. and Tech. of National Formosa University is sincerely appreciated.

**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.
