**5. Conclusions**

The multilayered AlCrN hard coating with the Cr adhesion and the gradient CrN interlayer was deposited on the SS substrate AISI 316L in a dominating cubic CrN structure by LARC PVD. The protective efficiency of the gradient AlCrN PVD coating increases up to 99.8% indicating a passivation layer developed through Al-based reaction products due to the reduced penetration rate of the corrosive media. The presence of surface defects (pinholes, inclusions, etc.) does not significantly affect the failure of coating in static corrosive conditions.

Tribo-corrosion tests performed in 3.5 wt % NaCl solution allows evaluation of the coating reactivity due to the AlN passivation effect combined with the presence of Al-based corrosion products. Change in OCP from −0.18 down to −0.3 V vs. Ag/AgCl after about 2 h-long sliding test under 1 kg load is related to the unprotected layer of Cr rich gradient CrN interlayer due to the partially lost layer of the AlCrN. An appearance and evolution of severe damages in the coating causes OCP shifting down to −0.42 V vs. Ag/AgCl at the beginning of sliding test under 3 kg load, indicating a mechanically initiated reaction with free Al from the AlN and free Cr from the AlCrN and CrN interlayer. The CoF of the coated samples remains about 25% lower (about 0.6) even after partial degradation as compared to the pristine SS under threefold lower load, indicating a reliability of the coating in tribo-contact even after a loss of chemical protection.

**Author Contributions:** Conceptualization, M.A. and I.H.; Methodology, J.B.; Software, T.R.; Validation, M.A., S.B. and I.H.; Formal Analysis, J.B.; Investigation, J.B.; Resources, M.A., S.B. and T.R.; Data Curation, J.B.; Writing-Original Draft Preparation, J.B.; Writing-Review & Editing, M.A. and I.H.; Visualization, J.B.; Supervision, M.A., I.H.; Project Administration, M.A.; Funding Acquisition, M.A. and I.H.

**Funding:** This research was supported by the Estonian Research Council under the personal gran<sup>t</sup> (PUT1063) (I. Hussainova), Estonian Ministry of Higher Education and Research under Projects (IUT19-29 and IUT19-28), the European Union through the European Regional Development Fund, (Project TK141) and TTÜ base finance projects (B54, B56 and SS427) (M. Antonov).

**Acknowledgments:** The authors would like to thank Heinar Vagiström for the help with preparation of AlCrN PVD coatings. The authors would also like to thank Rainer Traksmaa and Arvo Mere for the help with XRD measurements.

**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.
